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MATERIALS WITH SUPRAMOLECULAR CHIRALITY:
LIQUID CRYSTALS AND POLYMERS FOR CATALYSIS
'ARCIHIv
BY
MASSACHUSErS
IS
OF TECHNOLOGY
KAREN VILLAZOR MARTIN
MAR 2 5 2005
B.S., Chemistry, cum laude, 1999
Boston College, Chestnut Hill, MA
LIBRARIES
Submitted to the Department of Chemistry in Partial
Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS
INSTITUTE OF TECHONOLOGY
February, 2005
© Massachusetts Institute of Technology, 2004. All Rights Reserved.
Signature of Author:
7)
Department of Chemistry
October 27, 2004
Certified by:
...
0
Timothy M. Swager
Thesis Supervisor
Accepted by:
Robert W. Field
Chairman, Departmental Committee on Graduate Studies
.
This doctoral thesis has been examined by a Committee of the Department of Chemistry
as follows:
Professor Timothy F. Jamison:
2'
Chairman
4-
Professor Daniel S. Kemp:
Department of Chemistry
'1
Professor Timothy M. Swager:
\
2
-J
Thesis Advisor
With much love and gratitude,
I dedicate this thesis to my family and friends,
who all came along for the ride.
3
MATERIALS WITH SUPRAMOLECULAR CHIRALITY:
LIQUID CRYSTALS AND POLYMERS FOR CATALYSIS
By
KAREN VILLAZOR MARTIN
Submitted to the Department of Chemistry, February, 2005
In partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Chemistry
ABSTRACT
Mesomorphic organizations provide a powerful and efficient method for the
preorganization of molecules to create synthetic materials with controlled supramolecular
architectures. Incorporation of polymerizable groups within a liquid crystalline template
can set the stage for the synthesis of anisotropic molecular networks. This dissertation
details the synthesis and characterization of chiral liquid crystals and crosslinked polymer
networks, with an eye toward applications in asymmetric catalysis.
Chapter One gives an introduction to the study of liquid crystals and their phases.
Chapters Two and Three describe the incorporation of terminal olefins as polymerizable
groups within a columnar liquid crystalline template as an effective method for the
synthesis of robust, anisotropic polymeric materials. Upon in situ acyclic diene
metathesis (ADMET) polymerization, the original mesophase order is retained. Chapter
Two involves the room temperature polymerization of iron(III) tris(diketonate) liquid
crystals, resulting in densely crosslinked materials. The focus of Chapter Three is the
polymerization of dioxomolydenum-based liquid crystals, performed at high temperature,
and their potential to serve as catalysts for asymmetric epoxidation. In Chapter Four, a
different approach towards the synthesis of catalytically active anisotropic materials is
taken, incorporating well-established, transition metal catalysts within a liquid crystalline
framework. Progress towards the formation of liquid crystal phases containing C2symmetric bis(oxazoline) and pincer ligands is detailed. Finally, Chapter Five describes
the immobilization of chiral monodentate oxazoline ligands for use as catalysts in
asymmetric cyclopropanation. Preliminary results indicate that the heterogeneous system
gives higher enantioselectivities than the analogous homogeneous system.
Thesis Supervisor: Timothy M. Swager
Title: Professor of Chemistry
4
Table of Contents
Dedication.........
Abstract.........
.................................................................................
...................................................................................
Table of Contents.........
.......................................................................
3
4
5
Table of Figures ..........
7.............................
Chapter 1: An Introduction to Liquid Crystals .........................................................
10
11
1.1
The Liquid Crystal Phase...................................
..
... ................ 12
1.2
Classification of Liquid Crystals
1.3. Metallomesogens ........................................................................................... 17
18
1.4
Chirality in Liquid Crystals .........................................
21
1.5. Characterization of Liquid Crystals .........................................
22
Polarized Microscopy ............................................
1.5.1
....
22
1.5.2
Differential Scanning Calorimetry ........................................
23
1.5.3
X-Ray Diffraction ............................................
....
26
1.6. Outlook on Future Applications ........................................
References .........................................................
27
Chapter 2: In Situ Polymerization of Columnar Liquid Crystals using Acyclic Diene
Metathesis Polymerization: Iron(III) Diketonate Complexes ................................... 29
2.1.
Introduction ...................................................................................................
2.2.
Results and Discussion ..............................................
2.3.
Concluding Remarks ..............................................
Experimental Section.....................................................................................
2.4.
References ................................................................................................................
30
37
47
48
55
Chapter 3: In Situ Polymerization of Columnar Liquid Crystals using Acyclic Diene
Metathesis Polymerization: Dioxomolybdenum
Complexes .....................................
3.1.
Introduction .............................................
3.2.
Results and Discussion .............................................
3.3.
Concluding Remarks .....................................................................................
3.4.
Experimental Section .............................................
References .................................................. ..............................................................
5
59
60
62
68
69
77
Chapter 4: Liquid Crystals containing Catalytic Ligands.......................................... 78
4.1.
Introduction ..........................................
4.2.
Pyridine bis(oxazoline) Ligands..........................................
4.2.1.
Background ..........................................
4.2.2.
Results and Discussion..........................................
4.3.
Pincer Liquid Crystals ..........................................
4.3.1.
Background ..........................................
4.3.2.
Results and Discussion..........................................
4.4.
Concluding Remarks ..........................................
4.5.
Experimental Section..........................................
References ..............................................................................................................
79
80
80
83
89
89
92
93
94
105
Chapter 5: Immobilized Chiral Monodentate Oxazolines: Heterogeneous Catalysis
within an Organic Polymer Network ..........................................
107
5.1.
Introduction .................................................................................................
108
5.2.
Results and Discussion ................................................................................
111
4.3.
Concluding Remarks ........................................
4.4.
Experimental Section.................................
References ..............................................................................................................
119
121
127
Appendix 1: 1H and 13C NMR Spectra for Chapter 2 ......................................
129
Appendix 2: 1H and 13C NMR Spectra for Chapter 3 .....................................
138
Appendix 3: 1H and 13C NMR Spectra for Chapter 4............
Appendix 4: 1 H and
13C
....................... 151
NMR Spectra for Chapter 5 ......................................
167
CurriculumVitae........................................
172
Acknowledgements ..................................................................................
175
6
Table of Figures
Figures
Figure 1.1. Examples of lyotropic liquid crystals ...............................................
12
Figure 1.2. Micellar aggregates and phases formed by lyotropic liquid crystals..
.........
13
Figure 1.3. Examples of calamitic liquid crystals ...............................................
14
Figure 1.4. Some phases formed by calamitic liquid crystals.................................
15
Figure 1.5. Examples of discotic liquid crystals ...............................................
16
Figure 1.6. Phases formed by discotic liquid crystals ..........................................
17
Figure 1.7. Some chiral mesophases formed by calamitic mesogens ........................
19
Figure 1.8. Frustrated chiral phases ...............................................................
20
Figure 1.9. Examples of helical arrangements in columnar phases ...........................
21
Figure 1.10. Low (1)and wide (w) angle maxima for calamities and discotics ............. 23
Figure 1.11. Periodicities within the hexagonal lattice which give rise to low angle
peaks. The lattice constant a corresponds to the distance between neighboring columns,
while the distance d corresponds to planes of columns ........................................
24
Figure 1.12. Order (a) and disorder (b) within a column ......................................
26
Figure 2.1. Schematic representations of in situ crosslinking of an aligned smectic C*
phase and an inverted hexagonal phase ..........................................................
31
Figure 2.2. Acyclic diene metathesis polymerization of terminal olefins ...................
32
Figure 2.3. Grubbs' "first generation" catalyst (a) and "second generation" catalyst (b) for
olefin metathesis ...................................................................
34
Figure 2.4. Iron(III) octahedral complexes ......................................................
35
Figure 2.5. In situ polymerization of columnar hexagonal liquid crystals ..................
36
Figure 2.6. Microphotographs of the columnar hexagonal texture of 6a. Samples were
sandwiched between untreated glass slides and viewed through crossed polarizers ....... 39
7
Figure 2.7. Top: X-ray diffraction profiles of 6a on aluminum plates (a) before
polymerization and (b) after polymerization. Bottom: X-ray diffraction profiles of 6b on
aluminum plates (a) before polymerization and (b) after polymerization ................... 42
Figure 2.8. Circular dichroism of 6a on aluminum plates (a) before polymerization and
(b) after polymerization .................................................................
43
Figure 2.9. Circular dichroism of mixtures of 6b with chiral dopants, lb or lc............ 44
Figure 2.10. Left: X-ray diffraction profiles of 6b with 30% chiral dopant on aluminum
plates (a) before polymerization and (b) after polymerization Right: CD spectra of
crosslinked films of 6 on aluminum plates with (a) 30% chiral dopant and (b) 0% chiral
dopant..........
..............................................
46
Figure 2.11. Guest chromophores for porous networks........................................
47
Figure 3.1. Dioxomolybdenum liquid crystals ..................................
60
Figure 3.2. Tapered columnar phase formed by dioxomolydenum complexes............. 61
Figure 3.3. X-ray diffraction pattern of crosslinked film of 7a ...............................
66
Figure 3.4. Phase behavior of mixtures of 7a with 7b(S) as chiral dopant..................
67
Figure 4.1. C2-symmetric pyridine bis(oxazoline) ligand.........
.............................
80
Figure 4.2. Previously studied chiral oxazoline liquid crystals .........
................. 82
Figure 4.3. Microphotographs of the columnar hexagonal texture of CuDOS(7a).
Samples were sandwiched between untreated glass slides and viewed through crossed
polarizers.........
....................................................................................
Figure 4.4. a-Ketone solvents used as additives for CuDOS(7a) .........
85
................ 86
Figure 4.5. Microphotographs of the columnar hexagonal texture of CuDOS(7a).
Samples were sandwiched between untreated glass slides and viewed through crossed
polarizers ........................................................
87
Figure 4. 6. General structure of pincer ligands..................................................
89
Figure 4.7. Bimetallic pincer catalyst (a) and grafted onto silica support (b) where R =
phenyl,t-butyl........................................................
.........
Figure 4.8. Previously studied pincer liquid crystals ............................................
Figure 5.1. Swelling behavior of polymer films obtained by Route A .....................
8
90
91
115
Schemes
Scheme 2.1. ADMET mechanism ..........................................................
33
Scheme 2.2. Synthesis of iron(III) complexes ...................................................
37
Scheme 3.1. Peroxomolybdenum-catalyzed olefin epoxidation..............................
62
Scheme 3.2. Synthesis of dioxomolydenum complexes .......................................
63
Scheme 3.3. Attempted epoxidation of crotyl alcohol ..........................................
68
Scheme 4.1. Example of bimetallic catalysis observed in system using pybox ligands...81
Scheme 4.2. Synthesis of pyridine bis(oxazoline) ligand ......................................
83
Scheme 4.3. Synthesis of Cu(DOS)(7) ..........................................................
84
Scheme 4.4. Synthesis of Pincer Complexes..................
.................. 92
Scheme 4.5. Addition of 4'-Pentyl-4-biphenyl-carbonitrile to 12 ............................
93
Scheme 5.1. Copper-catalyzed cyclopropanation of styrene with ethyl diazoacetate....109
Scheme 5.2. Immobilization of bis(oxazoline)s ..............................................
110
Scheme 5.3. Synthesis of chiral oxazoline monomer.........................................
111
Scheme 5.4. Preparation of polymeric copper oxazoline catalysts .........................
112
Scheme 5.5. Polymerization of monomer 5 and the potential linkages present in the
resulting polymer network ........................................................................
114
Tables
Table 2.1. Phase Behavior of 6a and 6b.........................................................
38
Table 2.2. X-ray Diffraction Data for 6a and 6b Before and After Crosslinking ........... 41
Table 3.1. Phase Behavior of 7a-b..........................................................
64
Table 3.2. X-ray diffraction data for 7a-b .....................................................
65
Table 5.1. Results of cyclopropanation reactions ...................................
9
117
Chapter
1
An Introduction to Liquid Crystals
1.1
The Liquid Crystal Phase
The phases of matter can be characterized by the degree of molecular order
present within a given phase. Molecules in the solid phase are highly ordered, possessing
both positional order, wherein molecules occupy a specific site in a crystal lattice, and
orientational order, wherein the molecular axes are pointed in a specific direction. On the
other hand, molecules in the liquid phase possess neither positional nor orientational
order, resulting in a highly disordered, fluid phase.
A discrete phase exists in which the molecular order is intermediate between a
three-dimensionally ordered crystalline state and a disordered liquid state. Such phases
are often referred to as mesophases and can be separated into two broad categories:
plastic crystals and liquid crystals. Plastic crystals are formed when molecules in a
crystal phase lose orientational order while retaining positional order, allowing molecules
to freely rotate while remaining in their original position in the crystal lattice. Solid
methane is an example of a plastic crystal. When positional order is lost and orientational
order is retained, a liquid crystal phase is formed. Molecules in a liquid crystal phase
possess the orientational order of a crystal phase, as the molecular axes tend to point
along a preferred direction (called the director n), but also freely diffuse throughout the
sample, retaining the fluidity of a liquid phase.'
As a consequence of the orientational order present, liquid crystals exhibit
anisotropic behavior. That is, measurements having to do with elastic, electric, magnetic,
and optical properties of the material will give different results depending on the
direction along which it is measured.
Examples of such properties are index of
refraction, magnetic susceptibility, and dielectric constant. Contrastly, liquid phases
11
exhibit isotropic behavior, where the lack of molecular order allows such measurements
to be equivalent from any direction. The combined crystal-like anisotropy and liquid-like
fluidity of liquid crystals allows them to be oriented in the presence of electric and
magnetic fields, which is the basis of a large number of practical applications.2
The formation of a mesophase requires a delicate balance between attractive and
dispersive forces between neighboring molecules. While there is no way to definitively
predict whether or not a molecule will exhibit a liquid crystal phase, there are certain
structural and electronic guidelines often followed to ensure that there is sufficient
interaction between neighboring molecules. Such factors include the geometrical shape,
rigidity, polarity, and polarizability of a molecule. In general, a mesogen will possess
some rigid, structural core responsible for the stabilizing, attractive forces, as well as
aliphatic chains that are responsible for introducing dispersive forces.
1.2
Classification of Liquid Crystals
There are two broad classifications for liquid crystalline phases: lyotropics and
thermotropics. Lyotropic liquid crystals3 form anisotropic aggregates when combined
with a solvent, typically water, and the phase behavior is dependent on the concentration
Figure 1.1. Examples of lyotropic liquid crystals. Pictured are (a) sodium stearate or
soap and (b) a phospholipid.
O
C6r0
=
C15H31.o-
C 17H35
Na(D
(a Na®
(
O
0O
C17H35-o)
OO0O~ ,
(b)S~O'N0
(a)
(b)
12
I
and polarity of solvent and temperature. Molecules which form lyotropic phases are
usually amphiphilic, having non-polar, hydrophobic "tails" at one end with a polar,
hydrophilic "head" at the other end. Some examples are sodium stearate (soap) and
phospholipids. (Figure 1.1) The concentration of material in the solvent and the response
of the amphiphile to the solvent environment dictate the type of lyotropic phase formed.
For example, in a polar solvent like water, micelles are formed in which the hydrophobic
tails assemble together and the hydrophilic heads groups are presented to the solvent.
(Figure 1.2a) When combined with a non-polar solvent such as hexane, an inverse
micelle is formed where the hydrophobic tails shield the hydrophilic head groups from
the non-polar environment.
(Figure 1.2b) Under certain conditions, these micelles
Figure 1.2. Micellar aggregates and phases formed by Iyotropic liquid crystals.
jib
a.) micelle
b.) inverse micelle
c. ) lamellar
4?0
I
7'W"IF
4?_16
0""'weir
4?10
"I"saw
d.) hexagonal phase (Hi)
e.) inverse hexagonal phase (H2 )
13
further aggregate to form more complicated assemblies, such as lamellar and hexagonal
phases, which generate lyotropic liquid crystal phases. (Figure 1.2c-e) Lamellar phases
are particularly significant as they form the structural basis for biological membranes.
In thermotropic liquid crystals, the mesophase exists only within a certain
temperature range. When a thermotropic liquid crystal phase is observed upon both
heating and cooling processes, the phases are thermodynamically stable and the behavior
is referred to as enantiotropic. Thermodynamically unstable, kinetically formed phases
that only appear upon cooling and are referred to as monotropic. Molecules which form
thermotropic liquid crystals typically have large shape anisotropy (or high aspect ratio)
and consist of some rigid, aromatic core to provide dipolar attractive forces and pendant
aliphatic sidechains to provide highly dynamic motion and fluidity.
The most common thermotropic liquid crystals are formed by calamitic or rodshaped molecules. Calamitic mesogens4 typically consist of some rigid, elongated,
linearly-linked ring system that provides the shape anisotropy needed to produce
interactions that favor alignment. Usually, a number of alkyl or alkoxy sidechains are
Figure 1.3. Examples of calamitic liquid crystals.
C4H
XG/CN
C5H1
C
CH
C5H N_-CH1
NC
~-oC8H17
O
14
CN
-CN
placed at either or both ends of the mesogen to provide dispersive forces.
Some
examples of calamitic mesogens are shown in Figure 1.3. There are two types of phases
formed by calamities: nematic and smectic (or lamellar) mesophases. The nematic phase
is the simplest and least ordered thermotropic phase, as molecules freely diffuse
throughout the sample but, on average, align their long axes in the same direction.
(Figure 1.4a) Nematics are named for the "thread-like" features when viewed through a
polarizing microscope.
Smectic mesophases show a higher degree of order than
nematics, as the molecules are not only aligned in one direction, but are further organized
into layers. Smectic phases exhibit polymorphism, with each phase differing in the
degree of order present within and between layers. For example, in more fluid smectic
phases, the director n may lie perpendicular to the layer plane as in smectic A phases, or
it may be tilted with respect to the layer plane, as in smectic C phases. (Figure 4b-c)
Higher order smectic phases also exist, wherein molecules have more restricted mobility
and three-dimensional order is present.
Discotics5 are another type of thermotropic liquid crystal. Discotic mesogens
traditionally involve molecules with a flat, rigid, symmetrical, disc-shaped aromatic core
Figure 1.4. Some phases formed by calamitic liquid crystals.
a.) nematic
(N)
b.) smectic A (SA)
15
c .) smectic C (Sc)
surrounded by a periphery of aliphatic chains. Some examples of discotic mesogens are
show in Figure 1.5. Discotics can form either nematic or columnar mesophases. The
discotic nematic phase is analogous to the nematic phase formed by calamitics in that
molecules freely diffuse throughout the sample, yet the short axes of the molecules have
a preferred orientation along a single direction.
(Figure 1.6a) However, the most
commonly found discotic phases are columnar phases, wherein molecules aggregate in
columns that further organize to give different two-dimensional columnar assemblies. In
the nematic columnar phase (Figure 1.6b), columns mimic calamitic mesogens, aligning
the long axes of the columns along the same average direction. Some other examples of
columnar phases include rectangular, hexagonal, and tetragonal phases, based on the
symmetry of the two-dimensional lattice of columns. (Figure 1.6c-e)
In recent years, liquid crystal research has expanded beyond small, purely organic
molecules to include polymers,6 organometallic complexes (further discussed in section
1.5), and hydrogen-bonded supramolecular assemblies.7 Also, there has been an
increasing number of mesogens reported having molecular shapes that do not adhere to
Figure 1.5. Examples of discotic liquid crystals.
C7 H, 5
C 7H 1 5
H15
H1 5
16
Figure 1.6. Phases formed by discotic liquid crystals.
w
A
4EW- 4
4 40 4p4
A
n
4
1
a.) nematic
c.) rectangular
b.) nematic columnar
d.) hexagonal
e.) tetragonal
the classic calamitic or discotic model. Some of these structural motifs include cyclic
compounds and cyclophanes, swallow-tailed compounds, calamitic-discotic dimers,
epitaxygens, bowlic compounds, dendrimers, and bent-core liquid crystals.8 Many new
classes of liquid crystals have been created, each revealing new insights into ways in
which mesogens can interact and aggregate to support a liquid crystal phase.
1.3.
Metallomesogens
Metallomesogens, or metal-containing liquid crystals, combine the properties of
liquid crystals (fluidity, anisotropy) with those of metal atoms (magnetic, electrical,
optical, electro-optical properties).9 The metal centers can serve to induce, modify, or
enhance the liquid crystalline behavior of the free organic ligand. Metallomesogens that
17
mimic calamitics and discotics in shape anisotropy and phase behavior have been
described, as well as ones that largely deviate from the classic rod-shaped and diskshaped prototypes. The diverse array of coordination geometries and polydentate ligands
available has allowed researchers to study new types of molecular organization
previously inaccessible with purely organic mesogens. Additionally, metallomesogens
provide a reliable method for the ordered aggregation of metal centers coupled with the
long range orientation and ease of alignment in the mesophase, making them attractive
candidates for technologically useful materials.
1.4
Chirality in Liquid Crystals
When a liquid crystal phase contains molecules having one or more stereogenic
centers, the molecular chirality is translated to chirality of the macroscopic mesophase,
forming a helical, chiral assembly.
The pitch of the formed helix is temperature
dependent, and the handedness of the helical structure will depend on the stereogenic
center present, as one enantiomer generates a left-handed helix and the other enantiomer
generates a right-handed helix. Introduction of chirality into a mesophase results in a
reduction in the symmetry when compared to analogous achiral phases. In general, chiral
mesophases have reduced phase stability and lower clearing points (temperature at which
the transition from mesophase to isotropic phase occurs), often due to the steric effects
caused by the chiral center.
Chiral phases are most often formed by thermotropic liquid crystals. A chiral
mesophase can be formed in two ways. First, the phase can be composed of only chiral
molecules. That is, the mesogen itself has one or more stereogenic centers, found either
18
along the terminal chain of the mesogen or in the central core. Most often, the chiral
center is found in the terminal chain of the molecule due to relative ease of synthesis and
the number of commercially available, chiral alkyl chains. Second, a chiral dopant can
be added to an otherwise achiral phase. Although the chiral dopant need not be liquid
crystalline itself, ideally it will have a mesogenic-like structure, preferably similar to the
host phase in order to preserve the properties of the original mesophase.
Chiral mesophases formed by rod-shaped mesogens are analogous to their achiral,
calamitic counterparts. (Figure 1.7) The chiral nematic (or cholesteric) phase is much
like the achiral nematic phase, except that the presence of the chiral unit causes a gradual
rotation of the director n in the form of a helix along the long molecular axis. Helical
structures are also formed by several chiral smectic phases, but the most commonly found
phase is the chiral smectic C phase (Sc*).l° As in the achiral Sc phase, molecules within a
given layer are tilted with respect to the layer plane, yet, in the Sc* phase, there is a
gradual change in tilt direction from layer to layer in the form of a helix. The reduction
in phase symmetry causes a spontaneous polarization of molecules within each layer, but
Figure 1.7. Some chiral mesophases formed by calamitic mesogens.
)"""I\\
)
I,,,,
a.) chiral nematic (N*)
b.) smectic C* (Sc*)
19
due to the helical arrangement of the layers, the polarization direction is rotated from
layer to layer and the bulk polarization of the material is zero.
Frustrated chiral phases are formed when competition between different structural
features of the mesogens prevents a continuous phase from forming, giving rise to a
periodic array of defects. Blue phases form when molecules adopt double twist helices
which pack in a cubic manner. (Figure 1.8a) Twist grain boundary phases also exist,
where blocks of smectic phases (smectic A for TGBA*, smectic C for TGBC*) are
arranged in a helical fashion, broken by screw dislocations which abruptly twist the
director of the next block. (Figure 1.8b) Frustrated phases such as these exist only in
very narrow temperature ranges.
Discotic molecules also form chiral phases. Analogous to the structure of the
Figure 1.8. Frustrated chiral phases.
a.) blue phase
screw dislocation
where blocks of
smectic A meet
bloceks of smectic A
4
zI
zI
=1
b.) twist grain boundary phase
20
calamitic chiral nematic phase, the chiral discotic nematic phase has a gradual rotation of
the molecular director in the form of a helix. However, there are only a few examples of
the chiral discotic nematic, as chiral columnar phases are more commonly found. The
chirality of a columnar phase can be defined by the chirality within a given column and
within the lattice of columns. In either case, the loss of mirror symmetry can arise from a
helical twist in the molecular director (Figure 1.9a), a spiraling of molecular position
(Figure 1.9b), or the introduction of tilt and polarization in the molecules, analogous to
the smectic C* phase (Figure 1.9c).
Figure 1.9. Examples of chirality in columnar phases.
(a)
1.5.
(b)
(c)
Characterization of Liquid Crystals
Liquid crystal phases are typically characterized using three techniques: polarized
microscopy, differential scanning calorimetry, and X-ray diffraction. Other techniques
include miscibility studies with materials with known mesophases, neutron scattering
studies (usually of partially deuterated systems), and NMR studies (useful for studying
lyotropic systems), but will not be discussed here.
21
1.5.1
Polarized Microscopy
When an isotropic liquid is placed between polarizers crossed at 90° to each other,
the polarized light is unaffected by the sample and no light passes through the second
polarizer. However, when an anisotropic, birefringent medium such as a liquid crystal is
present, light interacts with the medium and a complex pattern or texture is observed.
Analysis of the defects and deformations in the texture can give information relating to
the molecular arrangement of the mesophase.1 ' 12 Typically, a thin sample of material is
sandwiched between a glass microscope slide and a glass cover slip and placed on a
temperature-controlled heating stage between two polarizers, and the mesophase behavior
is observed upon several cycles of heating and cooling.
Nematics normally give rise to schlieren textures, identified by black bands or
"brushes" that meet at point singularities or disclinations. Smectics can give a variety of
textures including focal conic fans, mosaic, schlieren, and homeotropic. Fan textures,
linear birefringent defects, and large areas of uniform extinction are common for
columnar hexagonal phases, while rectangular phases typically show wedge-shaped
domains.
1.5.2
Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) detects the presence of a liquid crystal
phase by measuring the enthalpy change associated with a phase transition. A DSC
instrument measures the energy absorbed or released by a sample as it is heated or
cooled, indicating at which temperatures endothermic melting processes and exothermic
crystallization processes occur. The magnitude of the enthalpy change is proportional to
22
the change in structural ordering. As such, solid to liquid phase transitions are relatively
drastic in terms of structural change, as reflected by high enthalpy values. Liquid crystal
to liquid crystal and liquid crystal to liquid phase transitions are subtle, as evidenced by
the relatively small enthalpy changes. DSC alone cannot identify the exact nature of the
phase present, but can indicate the degree of molecular order within the phase 3 and
should be used in combination with other methods like optical microscopy and X-ray
diffraction.
1.5.3
X-Ray Diffraction
X-ray diffraction (XRD) is the technique most often used for unambiguous
characterization of liquid crystal phases. Reflected X-rays can be carried out on either a
"powder" sample, consisting of polydomains with random director orientation, or aligned
samples, usually obtained by application of an electric or magnetic field or mechanical
shearing of the viscous mesophase.
Only "powder" or unaligned samples will be
discussed in the following text.
Figure 1.10. Low (I) and wide (w) angle maxima for a.) calamities and b.) discotics.
ik
A
I
I
4
I
n
11111 III
I~lr
M
W
W
a.) calamities
b.) discotics
23
Low angle maxima correspond to long distances between molecules (tens of
Angstroms) while wide angle maxima correspond to short distances (between 3-6
Angstroms). Periodic distances d are calculated from these maxima using Bragg's law:
nX = 2dsin 0
For calamitics, low angle maxima are measured along the director, and d roughly
corresponds to molecular length (or interlayer spacing). (Figure 1.10a) Wide angle
maxima, on the other hand, are measured perpendicular to director, and correspond to
roughly the molecular width. For nematic phases, low angle maxima are diffuse since
there is no periodic structure and positional order is short range. Wide angle maxima are
also broad since the phase is liquid-like in the direction perpendicular to the director. In
smectics, sharp low angle peaks (Bragg peaks) are observed in the scattered intensity due
to periodic arrangement of layers. Wide angle maxima are diffuse for smectic A and C,
where molecular packing perpendicular to the director is liquid-like, whereas they are
Figure 1.11. Periodicities within the hexagonal lattice which give rise to low angle
peaks. The lattice constant a corresponds to the distance between neighboring
columns, while the distance d corresponds to planes of columns.
(1
(100)
24
sharp for smectics other than A and C, in which there is two-dimensional order within the
layer.
For discotics, low angle maxima are measured perpendicular to the director and d
roughly corresponds to the molecular diameter, while wide angle maxima are measured
along the director and correspond to molecular thickness. (Figure 1.10b) In columnar
phases, Bragg peaks are observed in the scattered intensity due to periodic arrangement
of columns. The spacing ratio of the low angle maxima reflects the type of columnar
packing present.
Hexagonal phases, for example, show spacing with ratios of 1: 3: N4 for the
(100): (110): (200) reflections. (Figure 1.11) Typically, a hexagonal phase has a strong,
sharp (100) peak and two weak peaks related to the (110) and (200) reflections, as well as
a broad peak around 4.5 angstroms due to the diffuse scattering from the flexible, alkyl
side changes. While the (100) peak is always observed, the (110) and (200) peaks may
not be present if the columnar lattice is sufficiently disordered. Rectangular phases
typically have two sharp, low angle peaks relating to the (100) and (200) reflections, and,
like the hexagonal phase, a broad halo at 4.5 angstroms. Additional mid-angle peaks are
needed to determine which type of rectangular symmetry is present.
The lattice constant a, which corresponds to the separation between nearest
neighboring columns, is calculated based on the symmetry of the two-dimensional lattice
of columns. The lattice constant for hexagonal phases, for example, can be calculated
from the distance d, which corresponds to the separation between planes of columns,
using the equation, a = d/cos30 ° = d213, where d is calculated from the (100) peak using
Bragg's law.
25
Finally, columnar phases can be ordered or disordered with respect to molecules
within a given column, depending on the length scale of molecular correlations within the
columns. (Figure 1.12) Ordered columnar phases will exhibit an additional broad peak at
3.3-3.6 A corresponding to the distance between neighboring cores within the individual
column caused by dense packing of molecules within the columns.
Figure 1.12. Order (a) and disorder (b) within a column.
b
4
(b)
(a)
1.6. Outlook on Future Applications
Until now, the hallmark application for liquid crystals has been restricted to the
field of displays, in large part since, historically, the majority of known liquid crystal
phases involved solely calamitic mesogens, which can be aligned in the presence of an
electric or magnetic field. It was in the 1970s that discotic mesogens and their phases
began to receive considerable attention, introducing more diverse modes of molecular
organization and providing new direction towards a wide range of potential technological
applications. In particular, columnar phases have been suggested to be useful as sensors,
charge transport materials, and other conducting materials. With an array of chiral
mesomorphic assemblies at our disposal, this thesis investigates the prospect of utilizing
chiral columnar liquid crystals and polymers as asymmetric heterogeneous catalysts,
26
exploring the influence that supramolecular chirality may have on the stereochemical
outcome of a chemical reaction. Such materials would have a tremendous impact on the
fields of liquid crystals, polymers, and catalysis.
References
1
Collings, P.J. and Hird, M.
Introduction to Liquid Crystals: Chemistry and Physics
Taylor and Francis: Philadelphia, 1997; pp 1-16.
2
Collings, P. J. Liquid Crystals: Nature's Delicate Phase of Matter; Princeton University
Press: Princeton, 1990; pp 35-55.
3
Collings, P.J. and Hird, M.
Introduction to Liquid Crystals: Chemistry and Physics
;
Taylor and Francis: Philadelphia, 1997; pp 133-146.
4
Collings, P.J. and Hird, M.
Introduction to Liquid Crystals: Chemistry and Physics
;
Taylor and Francis: Philadelphia, 1997; pp 43-77.
5 Collings, P.J. and Hird, M.
Introduction to Liquid Crystals: Chemistry and Physics
;
Taylor and Francis: Philadelphia, 1997; pp 79-110.
6
Collings, P.J. and Hird, M.
Introduction to Liquid Crystals: Chemistry and Physics
;
Taylor and Francis: Philadelphia, 1997; pp 93-110.
7
Some recent examples include: (a) Lee, K.-M.; Lee, Y.-T.; Lin, I. J. B.; J. Mater. Chem.
2003, 13(5), 1079. (b) Song, X.; Li, J.; Zhang, S.; Liq. Cryst. 2003, 30(3), 331. (c) Li, M.;
Guo, C.; Wu, Y.; Liq. Cryst. 2002, 29(8), 1031. (d) Chen, D.; Wan, L.; Fang, J.; Yu, X.;
Chem. Lett. 2001, 11, 1156. (d) Lee, H.-K.; Lee, K.; Ko, Y. H.; Chang, Y. J.; Oh, N.-K.;
Zin, W.-C.; Kim, K.; Angew. Chem., Int. Ed. 2001, 40, 2669.
27
8Demus, D. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G.W., Spiess,
H.-W., Vill, V. Eds.; Wiley-VCH: Weinheim, 1998; Vol 1, pp 153-176.
9 For reviews of metallomesogens,
please see: (a)
Metallomesogens: Synthesis,
Properties, and Applications; Serrano, J. L., Ed.; VCH: New York, 1996. (b) Donnio, B.;
Bruce, D. W.; Liquid Crystals II, Vol. 95: Berlin, 1999; pp 193-247. (c) Hudson, S. A.;
Maitlis, P. M.; Chem. Rev. 1993, 93, 861. (d) Espinet, P.; Esteruelas, M. A.; Oro, L. A.;
Serrano, J. L.; Sola, E.; Coord. Chem. Rev. 1992, 117, 215. (e) Inorganic Materials, 2nd
ed.; Bruce, D. W., O'Hare, D., Ed.; John Wiley & Sons: New York, 1992. (f) GiroudGodquin, A. M.; Maitlis, P. M.; Angew. Chem., Int. Ed. Engl. 1991, 30, 375.
l0Gray, G.W. and Goodby, J.W.G.
Smectic Liquid Crystals: Textures and Structures
Leonard Hill: Glasgow and London, 1984; pp 61-64.
" Demus, D.; Richter, L. Textures of Liquid Crystals; Verlag Chemie, Weinheim, 1978.
12
Gray, G.W.; Goodby, J.W. Smectic Liquid Crystals: Textures and Structures ; Leonard
Hill, Glasgow, 1984.
13
Collings, P.J. and Hird, M. Introduction to Liquid Crystals: Chemistry and Physics
Taylor and Francis: Philadelphia, 1997; pp. 1 9 1 - 1 9 3 .
28
Chapter 2
In Situ Polymerization of Columnar Liquid Crystals using Acyclic
Diene Metathesis Polymerization: Iron(III) Diketonate Complexes
Adapted from:
Villazor, K. R.; Swager, T. M. Mol. Cryst. Liq. Cryst. 2004, 410, 775-781
2.1.
Introduction
Mesomorphic organizations represent the most powerful and efficient method for
the preorganization of molecules to create nanometer-scale ordered synthetic systems.' 2
The incorporation of polymerizable groups within liquid crystals is effective for the
synthesis of anisotropic molecular networks by in situ polymerization, wherein reactive
monomers are crosslinked in an ordered mesomorphic state with retention of molecular
order. The use of such polymerizable liquid crystals as self-assembling building blocks
provides a versatile method for processing anisotropic polymeric films with control over
both the order and symmetry of the material. Using the appropriate liquid crystal phase
as a template, "designer" organic materials can be tailored to suit a specific function. For
example, aligned smectic C* phases have been crosslinked in order to make
noncentrosymmetric polymer networks, either using a chiral polymerizable mesogen3 or
mixtures containing achiral polymerizable mesogens and chiral dopants.4 (Figure 2.la)
Such materials have been found to exhibit pyroelectric, piezoelectric, and nonlinear
optical properties.3 g' 4Also,
lyotropic liquid crystals exhibiting inverse hexagonal phases
have been crosslinked to produce nanoporous structures with hexagonally ordered,
hydrophilic pores.2 (Figure 2. lb) Gin and co-workers have applied such materials as
heterogeneous Lewis acid5 and Br0nsted acid6 catalysts, as well as molecular filters.2a
In the crosslinking of liquid crystal phases, the polymerizable group should
undergo rapid and efficient crosslinking with minimal perturbation of the liquid crystal
phase. Ideally, the reactive group should be synthetically accessible and stable to a wide
range of reaction conditions. Photopolymerization of acrylate-containing mesogens has
been the most common method of crosslinking with retention of the original mesophase.7
30
Figure 2.1. Schematic representations of in situ crosslinking of (a) an aligned
smectic C* phase where p = direction of polarization, and (b) an inverted
hexagonal phase.
a.)
%P
P
%%%%
cross-linked network with
bulk C2 symmetry
aligned smectic
phase
b.)
crosslink
C
>
cross-linked network with
hydrophilic pores
inverse hexagonal
phase
Typically, a photoinitiator and thermal inhibitor are added to decouple the polymerization
event from temperature, allowing for the ordering of the mesophase prior to irradiation.
While an efficient and reliable method for polymerization, the use of acrylate groups
presents certain drawbacks as well. A highly reactive functional group, acrylates are
typically introduced at the final step of a given synthesis, a limitation that can be
problematic for mesogens with more complex syntheses. Furthermore, introduction of
the polar and sterically bulky acrylate groups often precludes formation of the
mesophase.8 The addition of the branched functionality and intermolecular dipolar
interactions effectively destabilizes the mesophase, preventing the side chains from
31
efficiently filling space in the liquid crystal phase when compared to mesogens
containing only aliphatic side chains.
In order to circumvent this problem, Gin and co-workers have had success
employing 1,3-dienes, 9 and to a lesser degree, styrene 'Oand isoprene " groups, within the
sides chains of lyotropic monomers. Photopolymerization of the terminal dienes in an
inverted hexagonal phase proceeded with little perturbation to the liquid crystalline
order.9 However, while eliminating the steric bulk and polarity found in acrylate groups,
the syntheses of 1,3-diene-containing monomers still require several additional steps to
incorporate the polymerizable functional groups within the mesogen.
Acyclic Diene Metathesis Polymerization
Acyclic diene metathesis polymerization 1 2 (ADMET) is a step-growth
polycondensation reaction in which the production and expulsion of ethylene gas drives
the polymerization. An application of olefin metathesis, ADMET has proven to be a
powerful synthetic route to high molecular weight unsaturated polymers through the
polymerization of terminal olefins.'3 (Figure 2.2) The general mechanism involves two
metallocyclobutane intermediates in the reaction cycle, as show in Scheme 2.1, and the
Figure 2.2. Acyclic diene metathesis polymerization of terminal olefins.
ADMET
- C2H 4
32
Scheme 2.1. Mechanism of ADMET polymerization.
R'
VR
iIW===Nft=
\I
L,==
LnM=\
R'
L'M
R 1%_
0.
1
LnM=\
R
L-MRl
R
R 11
LM= A
R
active metal species is released from the polymer chain during each propagation step.
Similar to catalysts for ring-opening metathesis polymerization (ROMP), catalysts
for ADMET include ruthenium-based Grubbs-type carbenes, as well as tungsten-based
and molybdenum-based Schrock-type alkylidenes, pictured in Figure 2.3. Resembling
the general reaction conditions of other polycondensation reactions, ADMET is often
carried out in neat monomer to maximize monomer concentration and drive the reaction
towards polymer formation. Additionally, the reaction is carried out under reduced
pressure to remove the generated ethylene, again to shift the equilibrium irreversibly
towards polymer formation and to accelerate monomer conversion.
33
Figure 2.3. Grubbs' "first generation" catalyst (a), "second generation" catalyst (b),
and Shrock's molybdenum alkylidene (c) for olefin metathesis.
LDr
Er
Ci.
CI
r
0I
1-rI
II
PCy 3
I
H3 C(F3 C)2 C -O-Mo%
(CH3)2
:
I
Ph
PCy3
(a)
I
N
P
h
C(CF3)2CH3
(b)
(c)
To date, there have been no examples of the utilization of ADMET for the in situ
crosslinking of liquid crystal phases with retention of mesophase order. However, there
are limited examples of the use of ADMET in the polymerization of liquid crystals to
make main-chain liquid crystalline oligomers and polymers. 4 In ease case, terminal
olefins were easily incorporated into the side chains of the mesogen using commercially
available bromoalkenes. Herein we describe the incorporation of terminal olefins within
a metal-containing liquid crystalline monomer and the use of ADMET polymerization to
crosslink with retention of the original mesophase order. This approach is particularly
attractive since the olefin crosslinking groups more closely resemble typical alkyl side
chains of the mesogens in size, hydrophobicity, and thermal stability, yet are reactive
towards olefin metathesis. Another advantage of this method is that terminal olefins do
not require additional synthetic steps as bromoalkenes of various chain lengths are
commercially available.
Iron(III) Diketonate Complexes
Previous work in the Swager group focused on octahedral iron(III) diketonate
complex 1.15(Figure 2.4) In the liquid crystal phase, these low aspect ratio complexes
34
Figure 2.4. Iron(III) octahedral complexes.
la
R = (CH2)nH, n = 6, 12, 15, 18
lb R==
1c R=
I
align in columnar arrangements with hexagonal and rectangular packing of columns.
When the side chains are chiral (lb-c),'
6
the complexes resolve into single optical
isomers and segregate into microdomains of net chirality.' 7 This allows for
interdigitation of the aromatic rings of nearest neighbors within a column and the most
efficient packing arrangement.
In the isotropic phase, 1 is fluxional, rapidly
interconverting between optical isomers (A and A). However, in the case of lb-c,
examination of the circular dichroism (CD) spectra as a function of temperature shows
that the chiral sidechains provide enough perturbation in the mesophase to favor one
optical isomer and to effectively induce helicity within a given column. No CD signal
was observed for complexes with achiral sidechains.
Using polymerizable analogues of these iron(III) tris(diketonate) complexes, we
have developed a method to create robust, polymeric materials from columnar liquid
crystals. As demonstrated by Gin and co-workers, columnar phases can act as templates
for ordered, porous materials having a variety of potential functions such as ion transport,
35
molecular filtration, and catalysis. By employing ADMET as a means of crosslinking,
we have prepared anisotropic materials using polymerizable columnar hexagonal liquid
crystals with the retention of the original mesophase. (Figure 2.5)
We have also
employed chiral columnar hexagonal phases to synthesize materials having bulk chirality,
potentially for use in asymmetric catalysis and chiral separation technologies.
Figure 2.5. In situ polymerization of columnar hexagonal liquid crystals.
MEUU*
36
2.2. Results and Discussion
The liquid crystalline monomers were synthesized following similar procedures
as complex 1.14 (Scheme 2.2.)
Compound 2 was synthesized via a Williamson
etherification of ethyl 3,4-dihydroxybenzoate and the appropriate alkyl bromide.
Subsequent hydrolysis of the ethyl benzoate to carboxylic acid salt 3 followed by
treatment with >2.0 equivalents of CH3Li gave the methyl ketone 4. C-acylation of 4
with the appropriate ethyl benzoate using NaH in anhydrous THF gave the -diketone 5.
The iron tris([-diketonate) complexes were synthesized from reaction of the appropriate
ligand with Fe(acac)3.
Scheme 2.2. Synthesis of iron(III) complexes.
O
0
0
ii
j
OEt
·
OH
HO
RO
RO
OH
ii
%e%~
OEt
i,
-O
OR
OR
2a,b
3a,b
F,,e
O
O
OH
v
iv
RO
RO
[
OR
4a,b
OR'
OR
RO
OR'
R
5a,b
OR'
OR'
OR
6a,b
2a,3a,4a: R=
5a, 6a: R=m
2b,3b,4b: R = (CH )92
R' = (CH2)9
5b, 6b: R = R' = (CH 29) "
(i) RBr, K 2CO3, KI, 2-butanone, 90-100%; (ii) KOH, EtOH/H 20, 97-99%; (iii) CH 3 Li, THF, 0°C,
90%; (iv) 2a or 2b, NaH, THF, 68-72%; (v) Fe(acac) 3, benzene, 50-55%.
37
The phase behavior is summarized in Table 1. Complexes 6a and 6b exhibit
enantiotropic columnar hexagonal (Colh) mesophases as identified by polarized
microscopy and X-ray diffraction.
When viewed by polarized microscopy, these
compounds exhibited linear birefringent defects and large areas of uniform extinction,
characteristic of columnar phases. (Figure 2.6) The Colh phases were characterized by
the observation of sharp (100) peaks in the low angle region of X-ray diffraction patterns.
The wide-angle regions all display broad halos at approximately 4.5 A, which correspond
to the distance between liquid-like sidechains, confirming that the phases are indeed
liquid crystalline as opposed to crystal or plastic phases.
Table 2.1. Phase Behavior of 6a and 6b. The phase behavior for complexes lb-c
has been previously reported [ref 16] and is included here for clarity. The
transition temperatures and the enthalpies (in parentheses) are given in *C and
kcal/mol, respectively, and were determined by differential scanning calorimetry
(10 *C / min).
Phase Behavior
lb
Colh
81.0 (2.1)
_
I
-
73.8 (-2.1)
lc
Colh
83.8 (2.4)
I
_-
76.5 (-2.4)
6a
Colh
62.4(1.1)
I
54.0 (-1.1)
6b
Colh
_
68.8 (7.5)
I
-
61.0 (-7.6)
38
Figure 2.6. Microphotographs of the columnar hexagonal texture of 6a. Samples
were sandwiched between untreated glass slides and viewed through crossed
polarizers.
39
For the in situ crosslinking of polymerizable mesogens 6a and 6b, Grubbs'
"second-generation" catalyst (Figure 2.3b) was chosen for its stability over a wide range
of temperatures and high degree of tolerance for a wide variety of functional groups.12
Also, an induction period has been observed for the second-generation catalyst, making it
relatively slow at the initial stage of the reaction, attributed to the slower rate of
phosphine dissociation. g This induction period is particularly attractive for crosslinking
liquid crystals, as it allows the mesophase to form before polymerization occurs.
Crosslinking can be performed at any given temperature within range of the mesophase.
For this particular system, the mesophase is conveniently accessible at ambient
temperature.
Polymerization studies were carried out on thin films of the appropriate
mesogenic monomer and catalyst at room temperature. Film preparation was performed
in a glove box under nitrogen atmosphere. Hexane solutions of 6a or 6b containing 1.0
mol % catalyst were drop cast on aluminum plates for X-ray diffraction measurements
and spin cast onto a quartz plates for circular dichroism measurements.9 The films were
then annealed to 90 oC to order the mesophase and cooled to room temperature. The
annealed films were then placed under vacuum at room temperature for 24 hours to drive
the polymerization, resulting in heavily crosslinked free-standing films which were rinsed
with hexanes to remove any un-crosslinked material.
Table 2.2 lists the XRD lattice constants of the hexagonal phases for 6a and 6b,
before and after cross-linking, and the XRD patterns are pictured in Figure 2.7. The
XRD patterns of the polymerized films show slightly reduced peak intensities but closely
resemble the XRD patterns for the unpolymerized films, indicating that the columnar
hexagonal organization remains intact upon crosslinking. Also, circular dichroism (CD)
40
confirms that the post-polymerization film of 6a retains its chiral structure (Figure 2.8).
The achiral complex 6b showed no CD signal, verifying that the CD signal for 6a was
not a result of the chiral ruthenium catalyst, but from the bulk chirality of the mesophase.
Table 2.2. X-ray Diffraction Data for 6a and 6b Before and After Crosslinking.
Lattice
o
Constant (A)
6a (Before crosslinking)
26.9
Spacing
observed ()
23.3
13.5
4.49
6a (After crosslinking
and extraction)
6b (Before crosslinking)
6b (After crosslinking
26.1
13.4
4.45
28;3
2813
and extraction)
6b + chiral dopant (30 mol %)
29.4
(Before crosslinking)
6b + chiral dopant (30 mol %)
22.6
27.8
(After crosslinking and
extraction)
41
Miller
indices
(100)
(110)
halo
(100)
(110)
halo
24.5
4.18
(100)
24.5
4.29
(100)
25.5
4.44
(100)
24.1
4.39
(100)
halo
halo
halo
halo
Figure 2.7. Top: X-ray diffraction profiles of 6a on aluminum plates (a) before
polymerization and (b) after polymerization. Bottom: X-ray diffraction profiles of
6b on aluminum plates (a) before polymerization and (b) after polymerization.
, a nS5
L.
IV
5
2 10
1.5 105
1105
5 104
0
4
8
12
16
20
24
28
2 theta
o
0
n4
UIV
4
7 10
6 104
4
5 10
0
4 104
3 104
2 104
1 104
0
8
12
16
2 theta
42
20
24
28
In an attempt to introduce porosity into the materials, the achiral complex 6b was
combined with varying amounts of lb or lc as a chiral dopant. It is well known that
small chiral perturbations in liquid crystalline phases, often in the form of a chiral dopant,
can induce a strong, cooperative chiral response in the mesophase.20 Here we attempt to
synthesize chiral porous materials by using a chiral dopant to form a chiral hexagonal
phase with 6b, then extracting the dopant upon crosslinking.
Figure 2.8. Circular dichroism of 6a on aluminum plates (a) before polymerization and
(b) after polymerization.
10
i 5
0
-5
-10
300
350
400
450
500
550
Wavelength(nm)
43
600
650
700
Figure 2.9. Circular dichroism of mixtures of 6b with chiral dopants, (a) lb or (b)
Ic.
10
5
9
.3
0
'0
.R
.
-5
-10
350
400
450
500
550
600
650
700 350
400
450
500
550
600
650
700
Wavelength (nm)
12
E
10
0%
00
Cn
8
8
9
t
cu
~--
10% dopant
11
ae
0
.0
0%dopant
6
4
(,' Oclopatll
0
2
0
0
20
40
60
80
Mole % chiral dopant
44
-
50%dopant
----
70%dopant
100
90%dopant
Before crosslinking, the CD spectra of thin films of mixtures of 6b with chiral
dopant were measured and a non-linear dependence on the concentration of dopant was
observed, suggesting cooperative chiral induction in the mesophase.
(Figure 2.9)
Interestingly, addition of chiral (S)-3,7-dimethyloctyl bromide as a chiral dopant does not
induce chirality in the mesophase, indicating the importance of a covalent linkage
between the mesogenic core and the chiral alkyl chain.
The mixtures of achiral 6b and chiral dopant (lb or c) were crosslinked as
described above and rinsed with hexanes to wash away the chiral dopant. XRD profiles
show that the peak intensity is slightly diminished upon crosslinking, yet the hexagonal
phase is retained. Table 2.2 lists representative XRD spacing of the un-crosslinked and
crosslinked hexagonal phases of chirally-doped 6b, and the XRD patterns are pictured in
Figure 2.10. Circular dichroism confirmed the chirality of the doped polymerized
networks in contrast to the lack of chirality in the undoped network. (Figure 2.10)
Attempts were made to determine the extent to which the chiral dopants were
successfully extracted from the crosslinked films. UV measurements of crosslinked films
before and after hexane extraction of the chiral dopant were made, but the observed
decrease in optical density did not correspond to the amount of chiral dopant that was
presumed to be extracted. Also, attempts were made to incorporate a guest chromophore
within the crosslinked films following extraction of the chiral dopant. To maximize film
porosity, mixtures containing 70% chiral dopant or higher were crosslinked as previously
described and rinsed with hexanes to remove the dopant. The films were immersed in
solutions containing either chromophore 7 or 8 to allow the guest chromophore to diffuse
into the porous networks.
The chromophores are relatively small molecules with
potential ability to diffuse within the pores of the crosslinked film. The hope was that the
45
addition of these chromophores within the crosslinked films would be observable in the
UV spectrum and, ideally, would give rise to a new CD signal if the chromophores were
affected by the bulk chirality of the films. However, upon examination of the UV and
CD spectra, no change was observed to denote the inclusion of the chromophores within
the networks. These results were attributed to the densely crosslinked nature of the films,
wherein each monomer bears at least six polymerizable groups in three-dimensions,
making it difficult to extract any guest molecules trapped within the crosslinked network.
Figure 2.10. Left: X-ray diffraction profiles of 6b with 30% chiral dopant on
aluminum plates (a) before polymerization and (b) after polymerization Right: CD
spectra of crosslinked films of 6b on aluminum plates with (a) 30% chiral dopant
and (b) 0% chiral dopant.
610-
on
10
5 104
4
4 10
0
.
1 3104
-10
2 104
M-20
1 104
-30
-40
300
0
4
8
12
16
20
24
28
2 theta
350
400
450
500
550
Wavelength(m)
46
600
650
700
Figure 2.11. Guest chromophores for porous networks.
,N
7
/~NSO
2F
8
2.5. Concluding Remarks
In summary, we have demonstrated that the use of ADMET polymerization is a
viable and attractive route towards the synthesis of supramolecular polymeric materials.
Incorporation of terminal olefins within the side chains of mesogenic monomers is
synthetically straightforward using commercially available bromoalkenes. The addition
of the olefin groups within iron(III) tris(diketonate) complexes does not impede
mesophase formation, and the liquid crystalline monomers exhibit columnar hexagonal
phases over a wide temperature range.
Upon in situ crosslinking using ADMET
polymerization, retention of the original liquid crystal phase order is achieved. This
method has been used to synthesize both achiral and chiral polymer networks. Attempts
to synthesize porous networks were unsuccessful due to the densely crosslinked nature of
the films.
47
Experimental Section
General Methods. Tetrahydrofuran was dried by passing through activated alumina
columns. (S)-(+)-Citronellyl bromide and (R)-(+)-citronellyl bromide were purchased
from Aldrich (>99% purity) and hydrogenated to give (S)-3,7-dimethyloctyl bromide and
(R)-3,7-dimethyloctyl bromide, respectively, using literature procedure.21 All other
chemicals were of reagent grade and were used as received, unless otherwise specified.
The dialkoxy ethyl benzoate derivatives acetophenone derivatives,22 as well as complexes
lb and c, were synthesized using modified literature procedures. 'H and ' 3C NMR
spectra were obtained on Varian Inova-500 spectrometers.
All chemical shifts are
referenced to residual CHC13 (7.27 ppm for H, 77.23 ppm for 13C). Multiplicities are
indicated as s (singlet), d (doublet), t (triplet), and m (multiplet). DSC investigations were
carried out on a Perkin Elmer DSC-7. Optical microscopy was performed on a Leica
polarizing microscope in combination with a Mettler FP 80HT/FB 82HT hot stage. Spin
cast films were made on quartz plates using a Laurell Spin Processor WS-400-6NPPLITE at 500 rpm. X-ray diffraction studies were carried out on unoriented samples on
aluminum plates with an INEL diffractometer with a 2kW Cu K-a X-ray source fitted
with an INEL CPS-120 positive-sensitive detector. The detector was calibrated using a
silver behenate standard which was produced by Eastman Kodak and supplied by The
Gem Dugout.
3,4-Di-[(S)-3,7-dimethyloctyloxy]-benzoic acid ethyl ester (2a). (S)-3,7-dimethyloctyl
bromide (17.6 g, 95.7 mmol) and K2CO3 (19.7 g, 198 mmol) were added to a 2-butanone
solution (135 mL) of 3,4-dihydroxybenzoic acid ethyl ester (5.28 g, 38.1 mmol)
containing a catalytic amount of potassium iodide. The mixture was heated to reflux at
48
80°C under an argon atmosphere for three days. The excess salts were removed by
filtration, and the filtrate was washed successively with 0.5M NaOH (aq), water, and
brine, and extracted with dichloromethane. The organic fraction was dried over MgSO4
and the solvents were removed by rotary evaporation to give a yellow-tinted oil. Excess
alkyl bromide was removed by vacuum distillation, and the remaining residue was further
purified by filtering through a plug of silica gel (1% ethyl acetate/hexane) to afford the
product (13.4 g, 100%) as a clear oil. 1H NMR (CDC13, 500 MHz) 6: 0.87 (d, J = 6.5 Hz,
12H, CH 3), 0.96 (dd, J = 6.5, 3.0, 6H, CH3 ), 1.14-1.36 (m, 12H, CH2 ), 1.39 (t, J = 7.0 Hz,
3H, CH3 ), 1.49-1.57 (m, 2H, CH2 ), 1.60-1.73 (m, 4H, CH, CH2 ), 1.86-1.92 (m, 2H, CH),
4.04-4.12 (m, 4H, OCH 2 ), 4.35 (q, J = 14.0; 7.0, Hz, 2H, CH2 ), 6.87 (d, J = 8.0 Hz, 1H,
Ar-H), 7.55 (d, J = 2.0 Hz, 1H, Ar-H), 7.65 (dd, J = 8.5, 2.0 Hz, 1H). 13CNMR (CDC13,
500 MHz) 8: 14.60, 19.87, 19.91, 22.79, 22.89, 24.90, 24.92, 28.16, 30.09, 30.10, 36.16,
36.30, 37.49, 37.52, 39.41, 39.42, 60.87, 67.50, 67.72, 111.9, 114.2, 122.9, 123.6, 148.6,
153.2, 166.7. HRMS-ESI (m/z): [M+H]+ calcd for C29H50 04 463.3782, found 463.3767.
3,4-Di-(10-undecen-l-ol-oxy) benzoic acid ethyl ester (2b). The title compound was
prepared using the same procedure as above except that
-bromoundec-1-ene was used
instead of (S)-3,7-dimethyloctyl bromide (90%). 1H NMR (CDC13,500 MHz) 6: 1.331.40 (m, 20H, (CH 2 )5 ), 1.38 (dd, J = 7.0, 1.5Hz, CH 3 ), 1.45-1.49 (m, 4H, CH 2 ), 1.81-1.90
(m, 4H, CH2 ), 2.03-2.07 (m, 4H CH2 ), 4.05 (t, J = 6.5 Hz, 4H, OCH2 ), 4.35 (q, J = 14.0,
7.0, Hz, 2H, CH 2 ), 4.94 (dd, J = 10.0, 1.0 Hz, 2H, CHCH 2), 5.00 (dd, J = 17.0, 1.5 Hz,
2H, CHCH 2 ), 5.78-5.86 (m, 2H, CH), 6.87 (d, J = 8.5 Hz, 1H, Ar-H), 7.55 (s, 1H, Ar-H),
7.65 (dd, J = 8.5, 1.5 Hz, 1H).
13C
NMR (CDC1 3, 500 MHz)
49
: 14.61, 26.14, 26.18,
29.12, 29.13, 29.24, 29.33, 29.35, 29.36, 29.55, 29.57, 29.62, 29.63, 29.73, 29.75, 34.01,
60.89, 69.12, 69.39, 112.0, 114.31, 114.33, 122.9, 123.6, 139.4, 148.6, 153.2, 166.7.
HRMS-ESI (m/z): [M+H]+ calcd for C31H50 04 487.3782, found 487.3783.
3,4-Di-[(S)-3,7-dimethyloctyloxy]-benzoic acid (3a). A solution of 2a (2.01 g, 4.36
mmol) and potassium hydroxide (1.31 g, 23.4 mmol) in ethanol (15 mL) and deionized
water (15 mL) was heated to reflux at 80 °C for four hours. The solution was then poured
into 100 mL of 1N HC1to form a white precipitate, which was filtered and washed with
ethanol to afford the product (1.85 g, 97% yield) as a white solid.
H NMR (CDC13, 500
MHz) 8: 0.88 (dd, J = 6.5, 1.0 Hz, 12H, CH 3), 0.97 (dd, J = 6.5, 1.0 Hz, 6H, CH 3), 1.151.21 (m, 4H, CH 2), 1.25-1.36 (m, 8H, CH 2), 1.50-1.57 (m, 2H, CH 2), 1.62-1.71 (m, 4H,
CH, CH2), 1.87-1.93 (m, 2H, CH), 4.06-4.15 (m, 4H, OCH 2), 6.91 (d, J = 8.5 Hz, 1H, ArH), 7.61 (d, J = 2.0 Hz, 1H), Ar-H), 7.75 (dd, J = 8.5, 2.0 Hz, 1H, Ar-H).
(CDC13,500 MHz)
3C
NMR
: 19.91, 19.94, 22.83, 22.92, 22.93, 24.93, 24.95, 28.20, 28.21,
30.13, 36.12, 36.26, 37.51, 37.53, 39.43, 39.45, 67.59, 67.75, 111.9, 114.4, 121.5, 124.7,
148.7, 154.1, 172.5. HRMS-ESI (m/z): [M+Na]+ calcd for C27H460 4 457.3288, found
457.3291.
3,4-Di-(10-undecen-l-ol-oxy)
benzoic acid (3b). The title compound was prepared
using the same procedure as above except that 2b was used instead of 2a (99%). H NMR
(CDC13, 500 MHz) 6: 1.31-1.34 (m, 20H, (CH 2) 5) 1.46-1.50 (m, 4H, CH 2), 1.82-1.89 (m,
4H, CH2 ), 2.05 (q, J = 7.0 Hz, 4H, CH2 ), 4.07 (q, J = 7.0, 6.5 Hz, 4H, OCH 2 ), 4.94 (d, J=
1.0 Hz, 2H, CHCH 2 ), 5.00 (d, J = 17.0 Hz, 2H, CHCH 2), 5.78-5.86 (m, 2H, CH), 6.90 (d,
50
J = 8.5 Hz, 1H, Ar-H), 7.60 (d, J = 1.5 Hz, Ar-H), 7.74 (dd, J = 8.5, 2.0, 1H, Ar-H). 13C
NMR (CDC13, 500 MHz) 6: 26.15, 26.19, 29.14, 29.15, 29.21, 29.33, 29.35, 29.37, 29.56,
29.59, 29.64, 29.66, 29.74, 29.77, 34.03, 34.04, 69.19, 69.41, 112.0, 114.3, 114.6, 121.5,
124.7, 139.4, 148.7, 154.1, 172.2. HRMS-ESI (m/z): [M-H]- calcd for C 29H 460 4 457.3312,
found 457.3304.
3,4-Di-[(S)-3,7-dimethyloctyloxy]-acetophenone (4a). A solution of 3a (1.35 g, 3.11
mmol) and dry THF (30 mL) was cooled to 0°C under an argon atmosphere. A 1.4M
solution of methyllithium in ether (7.0 mL) was added dropwise via syringe, and the
solution was allowed to stir overnight, warming to room temperature. After being poured
into 100 mL of N HC1 and extracted with dichloromethane, the solution was dried over
MgSO4 and the solvents were removed by rotary evaporation. The remaining residue
was purified by column chromatography using 5% ethyl acetate/hexane as the eluant to
afford the product (1.22 g, 91%) as a clear oil. H NMR (CDC13, 500 MHz)
: 0.86 (d, J
= 6.5 Hz, 12H, CH 3), 0.95 (d, J = 6.5 Hz, 6H, CH 3), 1.13-1.17 (m, 4H, CH 2), 1.23-1.36
(m, 8H, CH2), 1.48-1.56 (m, 2H, CH2 ), 1.60-1.70 (m, 4H, CH, CH2 ), 1.85-1.92 (m, 2H,
CH), 2.55 (s, 3H, CH 3), 4.05-4.11 (m, 4H, OCH 2), 6.86 (d, J = 8.0 Hz, 1H, Ar-H), 7.52 (s,
1H, Ar-H), 7.53 (d, J = 8.5 Hz, 1H, Ar-H). 13C NMR (CDC13, 500 MHz) 6: 19.84, 19.87,
22.76, 22.85, 22.86, 24.86, 24.88, 26.36, 28.13, 30.05, 30.06, 36.08, 36.24, 37.45, 37.46,
39.36, 39.38, 67.49, 67.62, 111.5, 112.2, 123.3, 130.3, 149.0, 153.6, 197.0. HRMS-ESI
(mlz): [M+Na]+ calcd for C28H48 03 455.3496, found 455.3493.
51
3,4-Di-(10-undecen-l-ol-oxy)-acetophenone. (4b). The title compound was prepared
using the same procedure as above except that 3b was used instead of 3a (90%). 'H NMR
(CDC13, 500 MHz) 6: 1.30-1.38 (m, 20H, (CH2) 5) 1.45-1.50 (m, 4H, CH 2), 1.80-1.88 (m,
4H, CH 2), 1.80-1.88 (m, 4H, CH 2), 2.02-2.06 (m, 4H, CH 2), 2.55 (s, 3H, CH 3), 4.07 (q, J
= 6.5, 6.0 Hz, 4H, OCH 2), 4.93 (d, J = 10.0 Hz, 2H, CHCH 2), 5.00 (dd, J = 17.0, 1.5 Hz,
2H, CHCH2), 5.77-5.85 (m, 2H, CH), 6.86 (d, J = 8.5 Hz, 1H, Ar-H), 7.52 (d, J = 2.0 Hz,
Ar-H), 7.54 (dd, J = 8.5, 2.0, 1H, Ar-H).
13
C NMR (CDC13,500 MHz) 8: 26.11, 26.15,
26.40, 29.10, 29.11, 29.19, 29.32, 29.52, 29.55, 29.60, 29.61, 29.70, 29.73, 33.98, 33.99,
69.15, 69.33, 111.6, 112.4, 114.3, 123.4, 130.4, 139.3, 149.0, 153.6, 197.1. HRMS-ESI
(m/z): [M+H]+ calcd for C30H48 03 457.3676, found 457.3664.
1-[3',4'-((S)-3,7-Dimethyloctyloxy)phenyl]-3-[3",4"-(10-undecen-1-ol-oxy)phenyl]-
propan-1,3-dione (5a). A solution of 2b (1.67 g, 3.53 mmol) and 4a (1.02 g, 2.35
mmol) in anhydrous THF (10 mL) was added via cannula to a 3-neck flask containing
sodium hydride (0.624 g, 26.0 mmol) and 20 mL dry THF at 0°C under an argon
atmosphere and was stirred for two hours, warming to room temperature, then heated to
reflux at 80C for four hours. The resulting dark orange solution was cooled to room
temperature, and water was added to quench excess NaH. The diketone was neutralized
using N HC1 and was extracted with dichloromethane and dried over MgSO4 . The
solvents were removed by rotary evaporation to give a dark orange oil, which was
purified by column chromatography using 5% ethyl acetate as the eluant to afford the
product (2.68 g, 74.0%) as a bright yellow oil. H NMR (CDC13500 MHz) 6: 0.97 (dd, J
= 6.5, 2.0 Hz, 12H, CH3 ), 1.06 (dd, J = 6.5, 2.0 Hz, 6H, CH 3), 1.25-2.02 (m, XH, CH 2),
52
2.12-2.16 (m, 4H CH2), 4.09-4.24 (m, 8H, OCH2), 5.03 (dd, J = 10.0, 1.0 Hz, 2H,
CHCH2), 5.07-5.11 (m, 2H, CHCH2), 5.87-5.95 (m, 2H, CH), 6.78 (s, 1H, CH), 7.02 (d, J
= 8.0 Hz, 2H, Ar-H), 7.36 (s, 2H, Ar-H), 7.66 (dd, J = 7.5, 2.0 Hz, 2H, Ar-H). 3C NMR
(CDC13 500 MHz) 8: 19.91, 19.96, 22.83, 22.93, 24.94, 24.96, 26.27, 26.30, 28.20, 20.16,
29.19, 29.37, 29.41, 29.60, 29.68, 29.76, 29.77, 29.80, 29.88, 30.14, 30.16, 30.55, 34.04,
34.06, 36.19, 36.32, 36.36, 37.53, 37.56, 39.44, 39.46, 67.65, 67.99, 69.62, 73.80, 112.2,
114.3, 121.4, 128.4, 130.7, 139.4, 142.3, 149.2, 153.1, 184.4, 184.8. HRMS-ESI (m/z):
[M-H]-calcd for C57H920 6 871.6810, found 871.6816.
1,3-Bis[3',4'-(10-undecen-1-ol-oxy)phenyl]-propan-1,3-dione (5b). The
title
compound was prepared using the same procedure as above except that 4b was used
instead of 4a (68%). 'H NMR (CDC13 500 MHz) 8: 1.31-1.39 (m, 40H, (CH2 )5), 1.50 (m,
8H, CH2), 1.83-1.89 (m, 8H, CH2), 2.03-2.07 (m, 8H CH2), 4.06-4.10 (m, 8H, OCH2),
4.94 (dd, J= 10.0, 1.0 Hz, 4H, CHCH2), 5.00 (d, J= 17.0 Hz, 4H, CHCH2), 5.78-5.86 (m,
4H, CH), 6.73 (s, 1H, CH), 6.92 (d, J = 8.5 Hz, 2H, Ar-H), 7.55 (s, 2H, Ar-H), 7.57 (d, J
= 8.5 Hz, 2H, Ar-H). ' 3C NMR (CDC13,500 MHz) : 26.18, 26.21, 29.15, 29.28, 29.35,
29.37, 29.43, 29.58, 29.61, 29.64, 29.66, 29.75, 29.78, 34.03, 69.23, 69.58, 91.86, 112.3,
114.3, 121.2, 128.4, 139.4, 149.1, 153.1, 184.8. HRMS-ESI (m/z): [M-H]- calcd for
C59H92 06 895.6810, found 895.6816.
Tris[1-[3',4'-((S)-3,7-dimethyloctyloxy)phenyl]-3-[3",4"-(1-undecen-1-ol-oxy)phenyl]-propanedionato]iron(III). (6a) A suspension of 5a (1.05g, 1.20 mmol), iron
trisacetylacetonate (0.14, 0.40 mmol), and 5 ml benzene was heated to reflux at 100 °C
53
for 12 hours under argon. The solvent was removed using vacuum rotary evaporation,
affording a red oil. To purify, the red oil was dissolved in hot acetone and placed in the
refrigerator until red solids form at the bottom of the flask. The solvent was decanted
away from the red solids, which was then redissolved in hot acetone, and the purification
procedure was repeated again to yield the product as a waxy, red solid in 50% yield.
HRMS-MALDI (m/z): [M+Na]+ calcd for C1 77H273FeO18 2766.9735, found 2766.9729.
Tris[1,3-di-(3,4-di-10-undecenoxyphenyl)-propanedionatoliron(III). (6b) The
title
compound was prepared using the same procedure as above except that 5b was used
instead of 5a (55%). HRMS-MALDI (mlz): [M+Na]+ calcd for C171H273FeO18 2694.9735,
found 2694.973 1.
54
References
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6 XU,
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Keum, C.-D.; Kanazawa, A.; Ikeda, T.; Adv. Mater. 2001, 13(5), 321. (d) Sanchez, C.;
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8
(a) Dwer, M. J., Goldberg, R. S.; J. Am. Chem. Soc. 1970, 92, 1582. (b) Takenaka, S.;
Morita, H.; Iwano, M.; Sakurai, Y.; Ikemoto, T.; Kusabayashi, S.; Mol. Cryst. Liq. Cryst.
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9 (a)
Pindzola, B. A.; Hoag, B. P.; Gin, D. L.;
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10Reppy,
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56
J. Am. Chem.
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12 For
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'3 (a) Lindmark-Hamberg, M.; Wagener, K. B.;
Macromolecules
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14
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'5 Zheng, H.; Swager, T. M.; J. Am. Chem. Soc. 1994, 116(2), 761-2.
16
Trzaska, S. T.; Hsu, H. F.; Swager, T. M.; J. Am. Chem. Soc. 1999, 121, 4518.
17
Recent studies of discoid molecules have also established the preference for a single
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1
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'9 Circular dichroism is measured on spin-case films with an optical density ranging from
0.1-0.5.
57
9 Goodby,
J. W. In Handbook of Liquid Crystals: Fundamentals; Demus, D.; Goodby, J.
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21
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22
Zheng, H.; Xu, B.; Swager, T. M.; Chem. Mater. 1996, 8, 907.
58
Chapter 3
In Situ Polymerization of Columnar Liquid Crystals using Acyclic
Diene Metathesis Polymerization: Dioxomolybdenum Complexes
3.1.
Introduction
In the previous chapter, in situ crosslinking of a columnar hexagonal phase was
successfully carried out using acyclic diene metathesis polymerization (ADMET). The
mesophase was essentially "frozen" in place, affording a free-standing, densely
crosslinked, anistropic film.
The octahedral iron tris(diketonate) system studied,
however, exhibited a wide phase range and was liquid crystalline at room temperature,
two advantages not typically available with other mesophases. Also, the monomers
contained terminal olefins within each of its twelve alkyl chains, a relatively high number
of polymerizable groups per monomer, resulting a densely crosslinked film. In order to
explore the applicability of this method to other columnar liquid crystals, systems with
different phase behavior and number of polymerizable groups required investigation.
Previous work in the Swager group focused on the dioxomolybdenum pyridine
2,6-dimethanolate complex 1, which forms a tapered columnar phase.' (Figure 3.1)
While the free ligand is not itself mesogenic, the polymeric (--Mo=O--Mo=O--)n
interaction of complex 1 provides the organizational force needed to direct the mesophase
assembly. The polymeric metal-oxygen coordination lies within the central axis of a
column, with the ligands tapered around columnar core in order to effectively fill space
and stabilize the columnar structure. (Figure 3.2) When the sidechains are sufficiently
Figure 3.1. Dioxomolybdenum liquid crystals.
0O
IR(CH)Hn=1O,
RO
ROO
60
12,14
long (n a 10), the complexes exhibit enantiotropic liquid crystal behavior at temperatures
higher than - 80 C.
Our interest in this particular molybdenum system stemmed from not only from
its ability to form columnar phases, but also for its potential to become a heterogeneous
catalyst upon crosslinking of the mesophase. Dioxomolybdenum complexes have been
well-established as catalysts for olefin epoxidation when treated with hydrogen peroxide
or t-BuOOH
to form peroxomolydenum.2 (Scheme 3.1) Activation of the
dioxomolybdenum metal center within a chiral columnar phase would result in a chiral
arrangement of catalytic metal centers within the axis of a column, providing a potential
medium for the asymmetric epoxidation of olefins.
Figure 3.2. Tapered columnar phase formed by dioxomolydenum complexes.
RO
OR
O
-Mo:O
RO
Y
RO
I
RO
RO
!
RO
OR
:,
View
RSide
Top View
Side View
61
Utilizing the methods developed to achieve the in situ crosslinking of liquid
crystals, we have synthesized ordered networks from the columnar hexagonal phase
formed by dioxomolybdenum complexes. Incorporation of terminal olefins within the
side chains of the mesogenic monomers affords a monomer with three polymerizable
groups, anchoring the mesogens at the periphery of the column upon polymerization.
Also, the effectiveness of the dioxomolybdenum mesogens in the epoxidation of olefins
is evaluated.
Scheme 3.1. Peroxomolybdenum-catalyzed
LnMo
4?
olefin epoxidation.
t-BuOOH
LnMo
RoR/
0
3.2.
Results and Discussion
Variations on the general synthetic route previously developed by Swager and
Serrette afforded the target mesogens 7a-b. As depicted in Scheme 3.2, Williamson
etherification of methyl 3,4,5-trihydroxybenzoate with the appropriate alkyl bromide
gave methyl benzoate 2. Reduction of 2 with lithium aluminum hydride or diisobutyl
aluminum hydride followed by treatment with phosphorus tribromide gave 4 in 91%
yield. Williamson etherification of 4-hydroxypyridine-2,6-dicarboxylic acid dimethyl
62
ester3 with benzylic bromide 4 followed by reduction of the ester groups at the C-2 and
C-6 positions afforded ligand 6. Finally, complexes 7a-b were synthesized by reaction of
the appropriate ligand 6 with MoO 2(acac) 2.
Scheme 3.2. Synthesis of dioxomolydenum complexes.
O
o
OCH 3
OCH3
~~~~~~i
HO-OH
iii
RO
OH
iv
OR
OR
OR
2a,b
4a,b
i
3a,b -
.OCH3
RO
vi
RO
5a,b
6a,b
7a R = (CH2)
7b(R) R =
RO
RO
7b(S) R =
(i) RBr, K 2 C03 , KI, MEK, 94-98%; (ii) DIBAL-H or LAH, THF, 0 C, 93-97%; (iii) PBr3 , toluene,
acid methyl ester, K 2 C03 , KI, MEK, 78%; (v) NaBH4,
91%; (iv) 4-hydroxy-pyridine-2,6-dicarboxylic
CHCI3/CH 3 0H, 64-87%; (vi) MoO 2(acac)2 , CHCI3/CH 30H, 65-80%.
63
The phase behavior is summarized in Table 3.1. Mesogens 7a-b display
enantiotropic liquid crystalline behavior, with phases appearing at temperatures above 80
°C. The optical textures viewed through a polarizing microscope are characteristic of
hexagonal columnar phases and display linear birefringent defects and large areas of
uniform extinction. The mesophase-to-isotropic transition enthalpies are small (1.0 to 1.3
kcal mol'), in accordance with a highly disordered phase. When compared to the
mesogens with unbranched, achiral, aliphatic chains, the introduction of either terminal
olefins or chiral branching groups in the side chains of the mesogens destabilizes the
phase and results in lower clearing points and narrower phase ranges. Examination of the
X-ray diffraction (XRD) patterns reveals low angle peaks corresponding to the (100) and
(110) reflections of a hexagonal lattice. A broad halo at wide angle is also observed,
indicating the presence of weak, liquid-like interactions between alkyl chains. (Table 3.2)
Table 3.1. Phase Behavior of 7a-b. The transition temperatures and the enthalpies
(in parentheses) are given in given in C and kcal/mol, respectively, and were
determined by differential scanning calorimetry (10 °C / min).
Phase Behavior
94.3 (15.5)
7a
100.6 (1.2)
K
Colh
84.8 (-15.1)
K
107.7 (1.0)
-
Colh
84.6 (-14.1)
K
I
103.6 (-1.2)
92.6 (14.4)
7b (S)
I
98.3 (-1.3)
91.1 (14.3)
7b (R)
-
108.3 (1.0)
Colh
87.9 (-14.2)
64
105.5 (-1.3)
I
Table 3.2. X-ray diffraction data for 7a-b.
Lattice
o
Constant (A)
7a (at 95 °C)
7a (at 25 °C, after
crosslinking and
extraction)
7b (R) (at 95 °C)
7b (S) (at 95 °C)
34.3
32.4
32.5
32.5
Spacing
.
observed (A)
29.7
16.6
Miller
indices
4.55
(100)
(110)
halo
28.0
(100)
16.4
14.3
(110)
4.55
halo
28.1
16.2
4.18
(100)
(110)
28.1
16.2
4.18
(100)
(110)
(200)
halo
halo
Crosslinking this liquid crystal system required different conditions than in the
case of the octahedral complexes (Chapter 1). The mesophase is only available at higher
temperatures, requiring that polymerization be performed above at least 85 C. Also, a
greater amount of Grubbs' catalyst is needed, as initial attempts to crosslink neat films of
7a using 1 mole % catalyst 95 oC were not successful. Therefore, under nitrogen
atmosphere, a solution of 7a and 5 mole % Grubbs' catalyst in dichloromethane is drop
cast onto an aluminum plate. The film is placed on a hot plate at 95 oCfor 24 hours,
resulting in a heavily crosslinked free-standing film, which is then rinsed with
65
Figure 3.3. X-ray diffraction pattern of crosslinked film of 7a.
"A'lo
1.2 105
k liU)
O.oJ A
1 105
1
,
8104
6104
4104
(1 10)16.4A
(200) 14.3A
2 104
)
4.55A
~~~~~~~~~~~~~~~~~~I
I
2
5
10
15
20
25
2theta
dichloromethane to remove any un-crosslinked material. Table 3.2 lists the XRD lattice
constants of the hexagonal phases, before and after cross-linking, while Figure 3.3 shows
the X-ray diffraction pattern of the crosslinked material. Upon polymerization, the
mesogens are locked in place at the periphery of the columns, resulting in retention of the
columnar hexagonal organization. The crosslinked material displays the (100), (110),
and (200) reflections of a hexagonal lattice, as well as a broad halo at wide angle.
Chiral phases were formed using mixtures of 7a and various amounts of 7b(R) or
7b(S) as a chiral dopant. DSC measurements of the mixtures are shown in Figure 3.4.
With addition 50 mole % or less of chiral dopant, the clearing and crystallization points
66
Figure 3.4. Phase behavior of mixtures of 7a with 7b(S) as chiral dopant.
*
Columnar hexagonal phase
03
a
E
aE
0
10
30
50
Percent Chiral Dopant
70
90
both occur at a lower temperatures, indicating destabilization of the mesophase.
However, with the addition of 50 mole % or more, the clearing points are raised and the
phase ranges are broadened, signifying a higher degree of stabilization.
The mixtures were crosslinked as described above and the resulting anisotropic
materials again retain the original columnar mesophase order. Attempts to obtain circular
dichroism spectra on the films proved to be challenging due to the lack of homogeneity in
the drop cast films. Also, at higher temperatures the mesophase is highly fluid, resulting
in highly irreproducible spectra. Spin cast films of sufficient thickness and homogeneity
were difficult to obtain.
Initial evaluation of 7b(S) in the solution phase epoxidation of crotyl alcohol with
t-BuOOH4 indicated that the monomeric mesogen was not catalytically active (Scheme
67
3.3) and only starting materials were recovered. In an effort to enhance the reactivity of
the molybdenum center, complex 8 was synthesized by hydrolysis of 5b(S), followed by
complexation to MoO2. However, the replacement of hydroxymethyl groups with more
electronegative carboxylate groups did not provide sufficient activation for the metal
center and complex 8 was found to be catalytically inactive in the epoxidation of crotyl
alcohol.
Scheme 3.3. Attempted epoxidation of crotyl alcohol.
HOJ
MoO2 L, tBuOOH
HO
P
MoO2 L =
7b(S)
8
3.3. Concluding Remarks
In summary, we have demonstrated that the use of ADMET polymerization can
be used in the in situ crosslinking of liquid crystals at higher temperature ranges and with
fewer number of polymerizable groups per monomer. The addition of terminal olefins or
chiral, branched sidechains within dioxomolybdenum complexes does not impede
68
mesophase formation, although the phase ranges are narrowed relative to the complexes
with aliphatic side chains. Retention of the original liquid crystal phase order is achieved
indicating that the weak, polymeric (--Mo=O--Mo=O--)n interaction remains intact. An
initial evaluation of the catalytic activity of the mesogens indicated the metal complex is
not catalytically active in the epoxidation of crotyl alcohol.
3.4. Experimentals
General Methods. (S)-(+)-Citronellyl bromide and (R)-(-)-Citronellyl bromide were
purchased from Aldrich (>99% purity) and were hydrogenated using literature
procedure.5 All other chemicals were of reagent grade and were used as received, unless
otherwise specified. 'H and ' 3C NMR spectra were obtained on Varian Inova-500
spectrometers. All chemical shifts are referenced to residual CHC13 (7.27 ppm for 'H,
77.3 ppm for
3 C).
Multiplicities are indicated as s (singlet), d (doublet), t (triplet), and m
(multiplet). DSC investigations were carried out on a Perkin Elmer DSC-7. Optical
microscopy was performed on a Leica polarizing microscope in combination with a
Mettler FP 80HT/FB 82HT hot stage. X-ray diffraction studies were carried out on
unoriented samples on aluminum plates with an INEL diffractometer with a 2kW Cu K-a
X-ray source fitted with an INEL CPS-120 positive-sensitive detector. The detector was
calibrated using a silver behenate standard which was produced by Eastman Kodak and
supplied by The Gem Dugout.
69
3,4,5-Tri-[3,7-dimethyloctyloxy]-benzoic acid methyl ester, (R) and (S). (2b) 3,4,5Trihydroxybenzoic acid methyl ester (2.22 g, 12.1 mmol), (R)- or (S)-3,7-dimethyloctyl
bromide (10.7 g, 48.3 mmol), potassium carbonate (6.68 g, 48.3 mmol), and a catalytic
amount of potassium iodide were combined in 60 mL of 2-butanone and the mixture was
heated to reflux at 800 C under an argon atmosphere for 72 hours. The excess salts were
removed by filtration, and the filtrate was washed successively with 0.5M NaOH (aq),
water, and brine, and extracted with dichloromethane. The organic fraction was dried
over MgSO4 and the solvents were removed by rotary evaporation to give a yellow-tinted
oil. Excess alkyl bromide was removed by vacuum distillation, and the remaining
residue was further purified by filtering through a plug of silica gel (5% ethyl
acetate/hexane) to afford the product (7.30 g, 94.0%) as a clear oil. 'H NMR (CDC13,500
MHz)
: 0.87 (dd, J = 6.5, 2.0 Hz, 18H, CH 3), 0.95 (d, J = 6.5 Hz, 9H, CH 3), 1.22-1.29
(m, 6H, CH2 ), 1.31-1.38 (m, 12H, CH 2), 1.49-1.57 (m, 4H, CH 2), 1.59-1.64 (m, 2H, CH2),
1.71-1.72 (m, 3H, CH), 1.81-1.90 (m, 3H, CH), 3.89 (s, 3H, OCH3), 4.02-4.10 (m, 6H,
OCH 2 ), 7.27 (s, 2H, Ar-H). ' 3C NMR (CDC13, 500 MHz) 8: 19.73, 19.76, 22.79, 22.81,
22.90, 24.90, 24.93, 28.17, 29.79, 29.98, 36.47, 37.52, 37.67, 39.44, 39.53, 52.28, 67.56,
71.84, 108.0, 124.8, 142.4, 153.0, 167.1. HRMS-ESI (m/z): [M+H] + calcd for C3 8H68 0 5
605.5140, found 605.5146.
3,4,5-Tri-(10-undecen-l-ol-oxy)-benzoic acid methyl ester. (2a) The title compound
was prepared using the same procedure as above using 11-bromoundec-1-ene instead of
(R)- or (S)-3,7-dimethyloctyl bromide (98.1%). H NMR (CDC13, 500 MHz) 6:1.27-1.44
(m, 30H, (CH2)5), 1.44-1.48 (m, 6H, CH2 ), 1.77-1.82 (m, 6H, CH2), 2.03-2.07 (m, 6H,
70
CH2 ), 3.89 (s, 3H, CH 3), 4.00-4.04 (m, 6H, OCH 2), 4.92-4.95 (m, 3H, CHCH(H)), 4.97-
5.02 (m, 3H, CHCH(H)), 5.77-5.85 (m, 3H, CH), 7.26 (s, 2H, Ar-H). 13C NMR (CDC13,
500 MHz) 6: 26.23, 26.25, 29.13, 29.15, 29.34, 29.38, 29.47, 29.55, 29.64, 29.68, 29.73,
29.76, 29.80, 29.85, 30.50, 31.79, 34.01, 34.02, 52.26, 69.27, 73.61, 108.1, 114.3, 124.8,
139.3, 142.4, 153.0, 167.1. HRMS-ESI
(m/z): [M+H] + calcd for C41H6 80 5 641.5140,
found 641.5146.
3,4,5-Tri-[3,7-dimethyloctyloxy]-benzyl alcohol, (R) and (S). (3b) A solution of 3a
(4.39 g, 7.26 mmol) and dry THF (25 mL) was added to a stirring solution of LAH (0.69
g, 18.2 mmol) and THF (50 mL) at 0°C. The suspension was allowed to stir for four
hours, warming to room temperature, then cooled back to 0° C. A small amount of cold
distilled water was added slowly to quench the excess LAH, then 1.OMHC1was added to
break up the formed aluminum salts.
The white solution was extracted with
dichloromethane, and the organic layer was dried over MgSO4 . The solvents were
removed by rotary evaporation to yield the product (4.20 g, 97.4%) as a white solid. H
NMR (CDC13, 500 MHz) 8: 0.88 (d, J = 6.5 Hz, 18H, CH3 ), 0.93 (t, J = 6.5 Hz, 9H, CH3 ),
1.12-1.20 (m, 6H, CH2 ), 1.24-1.38 (m, 12H, CH 2 ),1.50-1.63 (m, 6H, CH2 ), 1.70-1.71 (m,
3H, CH), 1.80-1.89 (m, 3H, CH), 3.92-4.04 (m, 6H, OCH 2), 4.59 (s, 2H, CH2), 6.56 (s,
2H, Ar-H).
13C
NMR (CDC13, 500 MHz) : 19.77, 22.80, 22.83, 22.91, 24.91, 24.94,
28.18, 29.88, 30.00, 36.60, 37.51, 37.54, 37.73, 39.47, 39.56, 65.81, 67.49, 71.84, 105.3,
136.3, 137.6, 153.4. HRMS-ESI (m/z): [M+Na]+ calcd for C3 7H 68 0 4 599.5010, found
599.5011.
71
3,4,5-Tri-(10-undecen-1-ol-oxy)-benzyl
alcohol. (3a) The title compound was prepared
using the same procedure as above using 2a instead of 2b (92.5%).
H NMR (CDC13,
500 MHz) 6: 1.31-1.40 (m, 30H, (CH 2) 5), 1.44-1.48 (m, 6H, CH2 ), 1.77-1.83 (m, 6H,
CH 2), 2.03-2.07 (m, 6H, CH 2), 3.92-3.98 (mn,6H, OCH 2), 4.58 (d, J = 6.0 Hz, 2H, CH2 ),
4.93-4.95 (m, 3H, CHCH(H)), 4.98-5.02 (m, 3H, CHCH(H)), 5.78-5.86 (m, 3H, CH),
6.55 (s, 2H, Ar-H).
3C
NMR (CDC13,500 MHz) : 26.27, 26.31, 29.12, 29.15, 29.34,
29.39, 29.58, 29.65, 29.73, 29.77, 29.86, 30.39, 34.00, 34.02, 65.75, 69.19, 73.58, 105.4,
114.3, 136.3, 137.6, 139.4, 153.4. HRMS-ESI (m/z): [M+H]+ calcd for C4OH
680 4
613.5190, found 613.5185.
3,4,5-Tri-[3,7-dimethyloctyloxy]-benzyl bromide, (R) and (S). (4b) To a solution of
3b (3.71 mmol) in toluene (20 mL) was added phosphorus tribromide (0.35 mL) and the
solution was heated to reflux at 100 C for three hours. The clear solution was decanted
away from the inorganic salts and was poured slowly into 100 mL of water. The organic
layer was extracted with dichloromethane and dried over MgSO4 and the solvents were
removed in vacuo to yield the product (95%) as a clear oil. The product was used
without further purification. For analytical measurements, the material was purified by
filtration through silica gel (5:95 ethyl acetete: hexane). 1H NMR (CDC13,500 MHz) :
0.90 (d, J = 6.5 Hz, 18H, CH3 ), 0.95 (t, J = 6.5 Hz, 9H, CH3), 1.14-1.22 (m, 6H, CH2 ),
1.26-1.40 (m, 12H, CH 2),1.50-1.73 (m, 6H, CH2 ), 1.76-1.82 (m, 3H, CH), 1.84-1.92 (m,
3H, CH), 3.95-4.06 (m, 6H, OCH 2 ), 4.45 (s, 2H, CH 2), 6.61 (s, 2H, Ar-H).
13C
NMR
(CDC13,500 MHz) : 19.74, 22.78, 22.80, 22.89, 24.89, 24.92, 28.16, 29.86, 29.98,
72
36.59, 37.49, 37.53, 37.70, 39.45, 39.54, 53.66, 67.45, 71.82, 107.3, 133.3, 138.5, 153.4.
HRMS-ESI (m/z): [M+H]+calcd for C37H68BrO3 639.4346, found 639.4348.
3,4,5-Tri-(10-undecen-1l-ol-oxy)-benzyl bromide. (4a) The title compound was
prepared using the same procedure as above using 3a instead of 3b. 'H NMR (CDC13,
500 MHz) 6: 1.31-1.41 (m, 30H, (CH2 )5), 1.45-1.51 (m, 6H, CH2 ), 1.71-1.84 (m, 6H,
CH2 ), 2.04-2.08 (m, 6H, CH2 ), 3.94-3.98 (m, 6H, OCH2 ), 4.44 (d, J = 6.0 Hz, 2H, CH2),
4.93-4.95 (m, 3H, CHCH(H)), 4.98-5.03 (m, 3H, CHCH(H)), 5.78-5.86 (m, 3H, CH),
6.55 (s, 2H, Ar-H).
13
C NMR (CDC13, 500 MHz) 6: 26.22, 26.24, 29.08, 29.11, 29.30,
29.34, 29.49, 29.53, 29.60, 29.62, 29.65, 29,69, 29.72, 29.82, 30.46, 33.96, 33.98, 34.74,
53.60, 69.16, 73.52, 107.5, 114.3, 132.7, 138.4, 139.3, 153.2. HRMS-ESI (m/z): [M+H] +
calcd for 675.4346, found 675.4344.
4-[3,4,5-Tri-(3,7-dimethyloctyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid
methyl ester, (R) and (S). (5b) A mixture of 4b (2.46 g, 3.84 mmol), 4-hydroxypyridine-2,6-dicarboxlyic acid methyl ester (0.65 g, 3.08 mmol), K2CO3 (0.82 g, 5.93
mmol) and a catalytic amount of KI in 2-butanone was heated to reflux at 80 C for 12
hours. The mixture was poured into 0.5M NaOH (aq) and extracted with ethyl acetate.
The combined organic layers were dried over MgSO4 and the solvents were removed in
vacuo. Purification by column chromatography in 1:1 ethyl acetate:hexane gave the
product (2.30 g, 78.0%) as a clear oil. 'H NMR (CDC13, 500 MHz) : 0.86 (d, J = 6.5 Hz,
18H, CH3 ), 0.93 (t, J = 7.0 Hz, 9H, CH3 ), 1.11-1.37 (m, 18H, CH2 ), 1.47-1.63 (m, 6H,
CH2 ), 1.71-1.72 (m, 3H, CH), 1.80-1.90 (m, 3H, CH), 3.89 (s, 3H, OCH3 ), 3.97-4.09 (m,
73
6H, OCH 2 ), 4.01 (s, 6H, OCH 3), 5.12 (s, 2H, OCH 2), 6.62 (s, 2H, Ar-H), 7.91 (s, 2H, Ar-
H).
13 C
NMR (CDC13, 500 MHz) 6: 19.76, 22.79, 22.81, 22.90, 22.91, 24.91, 24.92,
28.17, 29.86, 29.97, 31.13, 36.55, 37.51, 37.53, 37.70, 39.44, 39.54, 53.47, 67.63, 71.43,
71.87, 106.4, 115.0, 129.6, 138.6, 150.0, 153.7, 165.3, 166.9. HRMS-ESI (m/z): [M+H] +
calcd for 770.5565, found 770.5562.
4-[3,4,5-Tri-(10-undecen-1-ol-oxy)-benzyloxy]-pyridine-2,6-dicarboxylic
acid
methyl ester. (5a) The title compound was prepared using the same procedure as above
using 4a instead of 4b. H NMR (CDC13,500MHz) 6: 1.28-1.45 (m, 30H, (CH2 )5), 1.431.46 (m, 6H, CH2 ), 1.70-1.82 (m, 6H, CH2), 2.00-2.04 (m, 6H, CH2), 3.93-3.97 (m, 6H,
OCH2), 3.99 (s, 6H, OCH3 ), 4.89-4.92 (m, 3H, CHCH(H)),
4.95-4.99 (m, 3H,
CHCH(H)), 5.11 (s, 2H, OCH 2 ), 5.75-5.83 (m, 3H, CH), 6.60 (s, 2H, Ar-H), 7.88 (s, 2H,
Ar-H). ' 3C NMR (CDC13 , 500 MHz) 6: 26.20, 26.22, 29.05, 29.08, 29.27, 29.32, 29.48,
20.50, 29.57, 29.66, 29.69, 29.79, 30.44, 33.93, 33.96, 53.39, 69.25, 71.31, 73.53, 106.3,
114.3, 115.0, 129.6, 138.5, 139.3, 149.9, 153.6, 165.2, 166.8. [M+H] + calcd 806.5565,
found 806.5532.
{6-Hydroxymethyl-4-[3,4,5-tri-(3,7-dimethyloctyloxy]-pyridin-2-yl}methanol,
(R)
and (S). (6b) Sodium borohydride (0.40 g, 10.6 mmol) was added in small portions to a
(1:1) (15 mL) at 0 °C. The solution
solution of 5b (0.82 g, 1.06 mmol) in CHC13:CH OH
3
was then stirred for 1 hour, warming to room temperature, and heated to reflux at 70 °C
for 72 hours. The solution was then cooled to room temperature, and water was added to
quench excess NaBH4 . The mixture was extracted with dichloromethane and washed
74
successively with 1.OM HC1(aq), water, and brine, and the organic layer was dried over
MgSO4 . The solvents were removed in vacuo to yield the product as a white solid,
which was recrystallized in CHC13/MeOH to yield the pure product in 74% yield. 'H
NMR (CDC13, 500 MHz) 6: 0.86 (d, J = 6.5 Hz, 18H, CH 3), 0.93 (t, J = 7.0 Hz, 9H, CH3),
1.14-1.35 (m, 18H, CH 2 ), 1.47-1.63 (m, 6H, CH 2 ), 1.71-1.72 (m, 3H, CH), 1.80-1.90 (m,
3H, CH), 3.94-4.04 (m, 6H, OCH2), 4.98 (s, 2H, OCH2 ), 6.60 (s, 2H, Ar-H), 6.80 (s, 2H,
Ar-H). 13C NMR (CDC13,500 MHz) 6: 19.74, 22.78, 22.80, 22.89, 24.92, 28.16, 29.87,
29.96, 36.56, 37.51, 37.54, 37.70, 39.44, 39:53, 64.61, 67.59, 70.60, 71.87, 106.1, 106.2,
130.6, 138.3, 153.6, 160.9, 166.4. HRMS-ESI (m/z): [M+H]+ calcd 714.5667, found
714.5655.
{6-Hydroxymethyl-4-[3,4,5-tri-(10-undecen-1-ol-oxy)]-pyridin-2-yl}methanol.
(6a)
The title compound was prepared using the same procedure as above using 5a instead of
5b. (87%)
IH NMR (CDC13, 500MHz) 6: 1.29-1.39 (m, 30H, (CH2 ) 5 ), 1.43-1.47 (m, 6H,
CH2), 1.71-1.82 (m, 6H, CH2 ), 2.02-2.06 (m, 6H, CH2), 3.94-3.98 (m, 6H, OCH2 ), 4.70 (s,
4H, CH2 ), 4.93 (d, J = 10.0 Hz, 3H, CHCH(H)), 4.99 (dd, J = 17.0, 2.0 Hz, 3H,
CHCH(H)), 4.99 (s, 2H, OCH 2), 5.77-5.85 (m, 3H, CH), 6.58 (s, 2H, Ar-H), 6.79 (s, 2H,
Ar-H). ' 3C NMR (CDC13,500 MHz) 6: 26.27, 26.29, 29.12, 29.15, 29.34, 29.39, 29.43,
29.56, 29.64, 29.68, 29.73, 29.76, 29.80, 29.86, 30.50, 32.79, 34.00, 34.02, 64.59, 69.32,
70.60, 73.63, 106.0, 106.3, 114.3, 130.6, 138.3, 139.4, 153.6, 160.7, 166.4. HRMS-ESI
(mlz): [M+H]+ calcd 750.5673, found 750.5562.
75
MoO 2(6b), (R) and (S). (7b) Bis(acetylacetonato)dioxomolybdenum(VI)
(0.51 g, 1.57
mmol) was added to a stirring solution of 6b (1.12 g, 1.57 mmol) in CHCl3:MeOH (1:1)
at room temperature. The mixture was heated to reflux at 80 °C for 12 hours, during
which the orange suspension becomes a clear solution.
After cooling to room
temperature, the solvents were removed in vacuo, and the product (80%) was
1H NMR
recrystallized in CHCl 3:MeOH.
(CDC13 , 500 MHz) 6: 0.87 (d, J = 6.5 Hz, 18H,
CH 3), 0.94 (t, J = 6.5 Hz, 9H, CH3 ), 1.12-1.20 (m, 12H, CH 2), 1.22-1.38 (m, 6H, CH2),
1.49-1.57 (m, 4H, CH2 ), 1.57-1.63 (m, 2H, CH2), 1.70-1.71 (m, 3H, CH), 1.81-1.91 (m,
3H, CH), 3.95-4.06 (m, 6H, OCH 2), 5.15 (s, 2H, OCH 2), 5.86 (s, 4H, OCH 2), 6.61 (s, 2H,
Ar-H), 6.96 (s, 2H, Ar-H). 13C NMR (CDC13, 500 MHz) 8: 19.77, 22.82, 22.84, 22.93,
24.93, 24.95, 28.20, 29.89, 30.00, 36.57, 37.54, 37.58, 37.72, 39.47, 39.56, 67.74, 71.94,
72.48, 80.95, 104.9, 106.5, 128.9, 139.0, 153.8, 168.0, 170.6. HRMS-ESI (m/z): [M+H] +
calcd 836.4447, found 836.4456.
MoO2 (6a). (7a) The title compound was prepared using the same procedure as above
using 6a instead of 6b. (65%) H NMR (CDC13,500MHz) 6: 1.30-1.37 (m, 30H, (CH2) 5),
1.45-1.48 (m, 6H, CH2), 1.72-1.83 (m, 6H, CH2), 2.02-2.07 (m, 6H, CH2), 3.95-3.99 (m,
6H, OCH2), 4.2-4.95 (m, 3H, CHCH(H)), 5.00 (dd, J = 17.0, 2.0 Hz, 3H, CHCH(H)),
5.14 (s, 2H, OCH2), 5.77-5.85 (m, 3H, CH), 5.86 (s, 4H, CH2), 6.58 (s, 2H, Ar-H), 6.94
(s, 2H, Ar-H).
13C
NMR (CDC13, 500 MHz) : 26.30, 29.14, 29.18, 29.35, 29.41, 29.46,
29.58, 29.61, 29.67, 29.76, 29.78, 29.89, 30.54, 34.02, 34.05, 69.49, 72.43, 73.71, 80.02,
104.9, 106.6, 114.4, 128.8, 131.8, 139.4, 153.8, 168.0, 170.5. HRMS-ESI (mlz): [M+Na]+
calcd 894.4296, found 894.4300.
76
References
Serrette, A. G.; Swager, T. M.; Angew. Chem., Int. Ed. Engl. 1994, 33, 2342.
2
(a) Rossiter, B. E.; Verhoeven, T. R.; Sharpless, K. B.; Tetrahedron Lett. 1979, 4733.
(b) Arcoria, A.; Ballistreri, F. P.; Tomaselli, G. A.; Di Furia, F.; Modena, G.; J. Org.
Chem. 1986, 51, 2374. (c) Sharpless, K. B.; Michaelson, R. C.; J. Am. Chem. Soc. 1973,
95, 6136.
3
4-hydroxypyridine-2,6-dicarboxylic acid dimethyl ester is obtained from acid-catalyzed
esterification of commercially available chelidamic acid monohydrate.
C. R.; J. Org. Chem. 1999, 64, 3699.
4
Adam, W.; Mitchell, C. M.; Saha-Mller,
5
Trzaska, S. T.; Zheng, H.; Swager, T. M.; Chem. Mater. 1999, 11, 130-134.
77
Chapter 4
Liquid Crystals containing Catalytic Ligands
4.1.
Introduction
The success of an asymmetric catalyst relies on its ability to enhance the
enantioselectivity for a chemical reaction. While the focus has primarily been on the
clever design and choice of chiral ligands, researchers have also continued to explore
other methods of enhancing asymmetric induction by undertaking a more supramolecular
approach. In certain cases, the aggregation of multiple catalytic moieties has been shown
to facilitate a bimetallic cooperative mechanism of catalysis,l wherein the catalyst plays a
dual role in the catalytic cycle, such as simultaneous activation of both nucleophile and
electrophile. As such, researchers have attempted to bring multiple metal centers in close
proximity to each other by incorporation of chiral, catalytic complexes within dimeric,
oligomeric, and dendritic frameworks.2 Such efforts have resulted in significant
enhancements in both rate and enantioselectivity when compared to the monomeric
counterparts.
The observed improvements in stereochemical communication are
attributed in part to the greater effective molarity of catalytic units, as well as their
relative orientation to one another.
Metallomesogens or metal-containing liquid crystals represent a unique
supramolecular platform for the controlled aggregation of metal complexes. In chiral
columnar liquid crystal phases formed by metallomesogens, the metal centers can be
distributed in a helical fashion along the columnar axis. This type of supramolecular
array of catalytic metal complexes could potentially facilitate a cooperative mechanism
of catalysis and exhibit improved reactivity relative to the monomeric complex. In this
chapter we describe our efforts towards the incorporation of well-known chiral catalytic
79
ligands into liquid crystal phases. Systems under investigation include chiral pyridine
bis(oxazoline) ligands as well as pincer ligands.
4.2.
Pyridine bis(oxazoline) Ligands
4.2.1. Background
Chiral, C2 -symmetric pyridine bis(oxazoline) or "pybox" ligands are one of the
most versatile and widely used systems in asymmetric catalysis.3 (Figure 4.1) First
synthesized by Nishiyama in 1989,4 these tridentate ligands are conformationally
constrained once chelated to a metal, minimizing the number of possible substratecatalyst arrangements and transition states in a particular reaction.5 Substituents at the
stereogenic centers of the oxazoline rings are in close proximity to the metal center,
directly influencing the stereochemical outcome of the reaction. The chirality in pybox is
derived from a wide variety of commercially available optically active natural and
unnatural amino alcohols, allowing for flexibility in ligand design as well as availability
of both enantiomeric forms.
Pybox ligands have been utilized in a vast array of asymmetric reactions and have
been the subject of numerous reviews.3
Some examples include various additions to
C=O and C=N double bonds (aldol, reduction of ketones, silylcyanations) and formation
Figure 4.1. C2 -symmetric pyridine bis(oxazoline) ligand.
.O N
RI
"
R
80
Scheme 4.1. Example of bimetallic catalysis observed in system using pybox ligands.
pyboxligand, M = YbC13
TMSCN, CHCI3
YbLy(pybox)
Lx
° R0
.
X s (pybox)Yb/
CN
R
TMSO
R
PN
R
of three-membered rings from olefins and imines (cyclopropanation, aziridination,
epoxidation). Also, several pericyclic reactions have been investigated (Diels-Alder and
hetero Diels-Alder, 1,3-dipolar cycloadditions, [2,3]-sigmatropic rearrangements).
In
certain cases, cooperative bimetallic catalysis has been observed as evidenced by a
positive nonlinear effect and a second-order dependence on catalyst.
Studies on the
asymmetric ring opening of meso epoxides with cyanide, for example, indicate that the
pybox catalyst performs the dual tasks of nucleophilic delivery and activation of the
electrophile via Lewis acidity.6 (Scheme 4.1)
The incorporation of chiral pybox ligands within supramolecular platforms has
been mainly restricted to immobilization on polymeric supports.7 To date, there have
been no reports of a mesomorphic system containing chiral pybox ligands, but there are
limited examples of systems containing a chiral oxazoline ring within the mesogenic
core. In their studies on helical columnar superstructures, Serrano and co-workers have
investigated mesogens containing a chiral, methyl-substituted oxazoline ring. Ligands
containing three alkoxy side chains were complexed to either a copper(II) or
palladium(II) metal center to give phasmid-shaped complexes which did not exhibit
liquid crystalline phases unless combined in certain proportions with an electron acceptor
such as trinitrofluorenone (TNF).8(Figure 4.2a) Typically, placing the stereogenic center
81
on the central core as opposed to the peripheral side chains introduces steric repulsions
between neighboring molecules and often precludes formation of a liquid crystal phase.
Without the addition of TNF, the methyl groups at the stereogenic centers of the
oxazoline rings provide sufficient steric bulk to prevent the aromatic cores from
assembling into a stable columnar phase. However, when the ligand has an expanded
aromatic core and six alkoxy chains, the negative steric effect of the methyl groups is
overcome and a columnar mesophase is formed without need for any additive. (Figure
4.2b) Columns within the phase have a helical organization and a 60° rotation between
neighboring molecules, an arrangement that allows for the even distribution of the alkyl
side chains around the central core of the column.
Figure 4.2. Previously studied chiral oxazoline liquid crystals.
(a)
NO2
02N
-
N02
l
PI
nematic and smectic phasesformed
(b)
82
4.2.2. Results and Discussion
Building on the ligand design previously utilized for dioxomolybdenum
complexes (Chapter 3), the target pybox ligands append chiral oxazoline rings at the C-2
and C-6 positions of a pyridine ring, with a trialkoxy-substituted benzyl group at the C-4
position. (Bis)methyl esters 1 and 2 were synthesized using methods developed for our
studies with dioxomolybdenum complexes.9 Direct condensation of the aminoalcohol
with diester
or 2 in refluxing xylenes gave the bis(hydroxy)amides
3 and 4,
respectively. Conversion of the hydroxy groups to chlorides with SOC12was performed
in the presence of 2,6-di-t-butyl-4-methylpyridine as a proton scavenger in order to
prevent cleavage of the benzylic bond.
Treatment of bis(chloromethyl)amides 5 and 6
Scheme 4.2. Synthesis of pyridine bis(oxazoline) ligand.
X
O
O
x
x
H
iii
1 R = C14 H29
2 R = (CH2) 9CHCH2
For Y = OH:
3a X = CH3, R = C14 H2 9
3b X = H, R = C1 4 H29
4a X = CH3, R = (CH2 ) 9CHCH2
4b X = H, R = (CH2 ) 9CHCH2
7a X = CH3, R = C14 H29
7bX = H, R= C 41H2 9
8a X = CH3, R = (CH2 )9 CHCH2
8b X = H, R = (CH2 ) 9CHCH2
For Y = CI:
5a X = CH3, R = C 14 H2 9
5b X = H, R = C14 H29
6a X = CH3 , R = (CH2 ) 9CHCH2
6b X = H, R = (CH2 )9 CHCH2
(i) (S)-alaninol or ethanolamine, xylenes, 57-64%; (ii) SOCI2, 2,6-di-t-butyl-4-methylpyridine,
THF, 60 C, 42-86%; (iii) NaH, THF, 1 hour, 76-94%.
83
with base gave the oxazoline ligands 7 and 8.
Ligands 7 and 8 did not exhibit mesomorphic behavior. As such, a metal center
was needed to provide a sufficient driving force for mesophase formation. Copper and
palladium pybox complexes are two of the most common metal-pybox complexes
utilized and have been shown to catalyze a wide variety of reactions.
However,
complexation of ligands 7a-b to CuC12 , Cu(BF4)2, CuOAc2 , CuOTf, PdC12 , and
Pd[CH3 CN]4 (BF4 )2 resulted in non-mesomorphic materials which melted directly into the
isotropic phase. However, CuCl(7a) and CuCl(7b) showed bright birefringence when
viewed under crossed polarizers, but no clear mesophase was evident. To promote
mesomorphic behavior, dodecylsulfate (DOS) chains were introduced into the fourth
coordination site of the metal center. Previous work by Bruce and co-workers with silver
complexes of stilbazoles found that the replacement of BF4-,NO3, or CF3SO3 anions with
DOS anions resulted in stabilized mesophases, lower clearing points and broadened
temperature range of the phase. ° Thus, ligands 7a and 7b were first complexed to CuCl,
followed by treatment with AgDOS in the absence of light to extract the chloride,
affording CuDOS(7a) and CuDOS(7b), respectively. (Scheme 4.3)
Scheme 4.3. Synthesis of Cu(DOS)(7).
1. CuCI, EtOH/CH2CI2
CuDOS(7a): X = CH3
CuDOS(7b): X = H
7a, 7b
2. AgDOS, CH2C12
84
When viewed under a polarized microscope, achiral CuDOS(7b) showed
birefringence and appeared to be forming a columnar phase. However, upon heating, a
dark reddish brown color began to appear at temperatures above 200 °C and the material
decomposed before reaching the clearing point. In the case of CuDOS(7a), the chiral
methyl groups significantly lower the melting point of the complex. When viewed
through crossed polarizers, regions of a columnar phase were observed, yet, upon
repeated heating and cooling cycles, the regions disappeared and only disordered
birefringence was observed. (Figure 4.3)
We therefore investigated the effect that the addition of TNF would have on the
phase behavior of CuDOS(7a). As described above, the "sandwiching" of an electron
acceptor such as TNF between neighboring mesogens has been known to stabilize liquid
Figure 4.3. Microphotographs of the columnar hexagonal texture of CuDOS(7a).
Samples were sandwiched between untreated glass slides and viewed through
crossed polarizers.
85
crystal phases, creating a supramolecular array of charge transfer complexes. Contact
preparations were performed, in which the two materials are placed on the same glass
slide and the phase behavior in the region where the two materials meet is observed.
CuDOS(7a) forms a charge transfer complex with TNF, as evidenced by the reddish
brown color change in the contact region in the isotropic phase. However, upon cooling,
the two materials separate as TNF crystallizes and no mesophase is observed under
polarized microscopy.
Interestingly, it was observed that the columnar phase seemed to be stabilized by
the presence of coordinating solvents like acetone. (Figure 4.5a) Large regions of
columnar phase were observed and persisted upon numerous cycles of heating and
cooling. Still, the phase was not uniform throughout the sample. (Figure 4.5b) In an
attempt to uniformly stabilize the phase, several coordinating a-ketones were added. The
a-ketone solvents contained long, aliphatic chains to lower the volatility of the solvent
and to also facilitate formation of the mesophase. (Figure 4.4) Contact preparations were
performed, with the best results obtained in the mixture between (R)-4,8-dimethyl-2nonanone and CuDOS(7a). (Figure 4.5c-d) Clear columnar phases were observed by
polarized microscopy as characterized by fan shaped textures and large domains of
uniform extinction. However, subsequent contact preparations with the same material
Figure 4.4. a-Ketone solvents used as additives for CuDOS(7a).
10-decanone
(R)-DMN
(S)-DMN
DMN = 4,8,-dimethyl-2-nonanone
86
gave inconsistent results and mesophase formation was not observed. Upon the study of
specific ratios of CuDOS(7a):solvent, inconsistent phase behavior was again observed.
Multiple samples from the same mixture showed drastically different behavior, often
displaying no mesophase.
It became apparent that the material was in some way decomposing, as evidenced
by broad melting transitions observed by DSC. One possible explanation for the
inconsistency of the columnar phase could be the instability of the Cu(I) metal center,
Figure 4.5. Microphotographs of the columnar hexagonal texture of CuDOS(7a).
Samples were sandwiched between untreated glass slides and viewed through
crossed polarizers.
a.)
b.)
c.)
d.)
87
which readily oxidizes to Cu(II) at high temperatures. In order to determine whether or
not oxidation was indeed occurring, Cu(DOS) 2 (7a) was synthesized and its phase
behavior was observed. Cu(DOS)2 (7a) was an isotropic liquid at room temperature and
no birefringence was observed when viewed through crossed polarizers, indicating that
oxidation of the metal center to form Cu(DOS)2(7a) was unlikely. Another possibility for
decay of the columnar phase is metal leaching, as catalytic copper(I) complexes of
tridentate pybox ligands are typically generated in situ and not isolated.l'
The proton NMR of CuDOS(7a) only showed peaks for the aliphatic side chains
and the 3,4,5-alkoxy substituted phenyl ring and none for the pybox ring. This could be
explained by oxidation of Cu(I) to some Cu(II) species, which can have a strong
disproportionate effect, even in small quantities, dampening the NMR signals of protons
in close proximity to the paramagnetic metal center by electron transfer between Cu(I)
and Cu(II) centers. Another explanation would involve cleavage of the weak benzylic
linkage, given the introduction of electron-deficient oxazoline substituents on the
pyridine ring and the electron-rich nature of the trialkoxy-substituted benzyl group. In
fact, the instability of the benzylic linkage was observed in separate attempts to
polymerize the free ligand 7a via ADMET polymerization with Grubbs' second
generation catalyst.
No polymerization occurred, as proton NMR revealed
decomposition of the ligand by cleavage of the benzylic bond. The ruthenium catalyst
likely binds the tridentate pybox moiety, effectively diminishing catalyst activity and
catalyzing cleavage of the benzylic linkage.
The difficulties experienced with this particular system stemmed not from the
chiral oxazoline groups, but largely from the instability of both the benzylic linkage and
88
the Cu(I) metal center with the tridentate pybox ligand. Nonetheless, a metastable
columnar phase was observed, indicating that there is potential for C2-symmetric pyboxes
to form stable mesophases. Modification of the linkage between the pybox entity and the
trialkoxy-substituted appendage should stabilize the ligand and facilitate formation of a
stable, columnar phase. Furthermore, other metal centers forming more stable complexes
with pybox ligands should be explored.
4.3.
Pincer Liquid Crystals
4.3.1. Background
"Pincer" complexes are comprised of an aryl anion bound to a metal center via a
metal-carbon o bond, with substituents ortho to the metal-carbon bond that coordinate to
the metal.12 (Figure 4.6) Chelation of the two neutral donor groups (E) results in the
formation of two five-membered metallacycles.
The rigid terdentate binding mode
substantially stabilizes the metal-carbon bond, preventing metal leaching. Pincers are
stable to air and moisture, and are thermally robust. Also, a range of steric and electronic
modifications to the pincer framework can be achieved without substantially affecting the
properties of the metal site. This excellent stability of the pincer complex has made it
particularly attractive for various applications in materials science. Pincer complexes
have been incorporated into various supramolecular platforms, including polymers,
Figure 4. 6. General structure of pincer ligands.
RAII·
R IiXnLn
89
macrocycles, dendrimers, sensors, and switches, and immobilized on gold surfaces,
fullerenes, silica, and various soluble and insoluble polymeric supports.l2a
The unique and highly protective environment for the metal site coupled with the
ability to fine tune electron density around the metal has made pincers ideal system for
catalysis. The stability of the metal-carbon a bond prevents metal leaching, a problem
common to many catalysts in which the metal is coordinated solely to heteroatoms.
Pincers have found application in a wide array of reactions, including Heck, Suzuki,
transfer hydrogenation, alkane dehydrogenation, asymmetric aldol, Michael addition,
allylation of aldehydes. Also, chiral modifications can be made on the benzylic positions
or on the donor atoms to produce chiral catalysts.
Previous work in the Swager group took an unconventional approach to creating a
chiral pincer catalyst.'3 As shown in Figure 4.7a, two
C2-symmetric pincer units were
linked using a chiral spacer derived from O-isopropylidene-L-threitol.
While the
stereogenic center is remote from the active site, it was hoped that interaction of a
substrate with both metal centers would create a highly chiral pocket and influence the
Figure 4.7. Bimetallic pincer catalyst (a) and grafted onto silica support (b) where R =
phenyl, t-butyl.
3F4 ) 2
(a)
(b)
90
enantioselectivity. Studies on the aldol condensation of isocyanoacetates and aldehydes
or ketones proceeded with <1% ee, suggesting that the catalytic centers behave
independently and no cooperative catalysis occurs. In a further attempt to promote
interaction of adjacent pincer moieties, the bimetallic pincer compounds were grafted
onto silica support in order to achieve a densely packed array of catalysts with constricted
conformational mobility. (Figure 4.7b) However, only enantiomeric excesses of 2-3%
were observed.
Herein we describe our initial efforts towards the synthesis of pincer liquid
crystals. In order to achieve the desired aggregation of pincer complexes, we sought to
incorporate the pincer moiety into a liquid crystalline columnar phase, which would give
rise to a one-dimensional array of metal centers. The high level of stability of the pincer
ligand with substitution in the 4-position makes it a better candidate for appendage with
an electron rich, alkoxy-substituted benzyl group. The only example of mesomorphic
pincers are based on pyridine-2,6-bis(carboxylate) and pyridine-2,6-bis(thiocarboxylate)
Figure 4.8. Previously studied pincer liquid crystals.
(a)
1r~(sO)
~~
O
I
OCoH2
H
10 2
'NLO
~
(b)
91
C
O
OH
12
1
ligands, which formed smectic, nematic, and in one case, columnar, phases when another
mesomorphic ligand is placed in the fourth coordinate site of the metal center. (Figure
4.8)
4.3.2. Results and Discussion
The design for the target pincer ligands places a trialkoxy-substituted benzyl
group at the 4-position of the pincer ring. 3,5-Bis-thiophenylmethyl-anisole 9 was
prepared in a series of steps that has been described in literature starting from 5hydroxyisophthalate, with the exception that a methyl group was used to protect the
phenol.14 Treatment with BBr 3 gave phenol 10. Alkylation of 10 via a Williamson ether
synthesis gives ligand 11, which was then metallated with Pd(CH3CN)4(BF 4) 2 in
anhydrous acetonitrile to give complex 12.
Scheme 4.4. Synthesis of Pincer Complexes.
CO2 CH3
Ho
j jj
-SPh
iii, iv
H3C
H3CO
-~
/SPh
-HO
CO2 CH3
SPh
SPh
9
10
SPh
SPh
RO
I
RO
RO
A
vI
o0
O
I
SPh
-NCC-NCC
H
3 (BF4 )
SPh
RO
RO
11 R = C4 H2 9
12 R = C 14 H29
(i) CH31, K2CO3 , KI, 2-butanone, reflux 4 hrs; (ii) LAH, THF, 0C; (iii) PBr3, toluene, reflux 3 hours; (iv)
thiophenol, ADOGEN ® 464, toluene/H 20, 96%; (v) BBr3, THF, 0°C, 94%; (vi) 3,4,5-tritetradecyloxybenzyl
bromide, K 2 CO3 , KI, 2-butanone, reflux 4 hrs, 80%; (vii) Pd(CH 3CN)4(BF 4) 2 , CH 3 CN, reflux 14 hours.
92
No mesophase is observed for ligand 11 or complex 12, upon both heating and
cooling. Due to the C2 symmetry of the pincer complex, the phenyl substituents are in a
trans conformation with respect to the square planar palladium coordination plane,
causing a large degree of steric repulsion between neighboring complexes and preventing
effective aggregation of the mesogens. Replacement of the acetonitrile ligand with a
mesogenic alkyl-substituted cyanobiphenyl ligand, as shown in Scheme 4.5, indicated no
improvement of mesomorphic behavior. Based on these preliminary results, future work
should include the attachment of smaller substituents on the donating groups to facilitate
mesogen aggregation.
Scheme 4.5. Addition of 4'-Pentyl-4-biphenyl-carbonitrile
to 12.
Ph
NC
'CH1
I
I-PhNC
11
CH 2CI 2
4.4.
RO
C5H,
Ph
Concluding Remarks
Incorporation of catalytic moieties within supramolecular platforms has been
shown to enhance catalytic behavior, prompting the investigation of metallomesogens
containing well-established catalytic moieties. Chiral bis(oxazoline) ligands displayed a
metastable columnar phase when complexed to Cu(I) center. However, while instability
of the metal-ligand complex as well as the benzylic linkage of the ligand caused the
phase to readily decompose, there is still potential for stable mesophase formation. By
93
appropriate modification of the ligand. Pincer ligands were also investigated as a system
offering a more robust metal-ligand sigma bond. Preliminary results indicate that phenyl
groups on the donating sulfur groups provide significant steric repulsion, precluding
formation of a liquid crystal phase.
The incorporation of C 2-symmetric ligands within liquid crystal phases remains
promising. From our studies, it was observed that while the phenyl substituents on the
pincer ligand caused significant steric repulsion between mesogens, the methyl groups on
the pybox ligand did not impede formation of a metastable mesophase. Optimization of
these systems requires tailoring the substituents at the stereogenic centers as well as
determining a suitable metal center for complexation. Modifications such as these can
lead to successful formation of a liquid crystalline phase containing catalytic moieties.
4.5.
Experimental Section
General Methods.
Compounds 3-8 were synthesized using modified literature
procedures.'5 Tetrahydrofuran was dried by passing through activated alumina columns.
AgDOS was synthesized following literature procedures.'6 All chemicals were of reagent
grade and were used as received, unless otherwise specified. H and
13C
NMR spectra
were obtained on Varian Inova-500 spectrometers. All chemical shifts are referenced to
residual CHC13 (7.27 ppm for 'H, 77.23 ppm for ' 3 C). Multiplicities are indicated as s
(singlet), d (doublet), t (triplet), and m (multiplet). High resolution mass spectra were
obtained at the MIT Department of Chemistry Instrumentation Facility (DCIF) on a
Finnigan MAT 820 or on a Bruker Daltonics Apex II 3T FT-ICR MS.
94
DSC
investigations were carried out on a TA Instruments DSC-Q10. Optical microscopy was
performed on a Leica polarizing microscope in combination with a Mettler FP 80HT/FB
82HT hot stage.
Pybox Coumpounds:
4-[3,4,5-Tri-tetradecyloxy-benzyloxy]-pyridine-2,6-dicarboxylic
acid bis-[(2'-(R)-
hydroxy-l'-methyl-ethyl)-amide]. (3a) (S)-alaninol (0.46 mL, 5.90 mmol) was added
to a stirring solution of 1 (1.85 g, 1.97 mmol) in xylenes (14 mL) and the solution was
heated at 100 °C for three hours under argon atmosphere. The solvents were removed by
rotary evaporation and the resulting material was purified by column chromatography
(10:90 methanol:ethyl acetate) to afford the product (1.30 g, 64.0%) as a white solid. 'H
NMR (CDC13, 500 MHz) 8: 0.86-0.89 (m, 9H, CH 3), 1.26-1.33 (m, 60H, (CH 2), 0), 1.30
(d, J = 6.5 Hz, 6H, CH 3 ), 1.44-1.50 (m, 6H, CH 2), 1.71-1.82 (m, 6H, CH 2), 3.69 (dd, J =
10.5, 5.0, Hz, 2H, CH(H)), 3.79-3.81 (m, 2H, CH(H)), 3.93-3.98 (m, 6H, OCH2), 4.224.25 (m, 2H, CH), 5.06 (s, 2H, OCH 2), 6.59 (s, 2H, Ar-H), 7.85 (s, 2H, Ar-H), 8.13 (d, J
= 8.0 Hz, 2H, NH). 13C NMR (CDC13, 500 MHz) 8: 14.33, 17.18, 22.90, 26.32, 29.58,
29.60, 29.66, 29.84, 29.87, 29.88, 29.90, 29.93, 29.97, 29.98, 30.54, 32.13, 47.91, 66.44,
69.31, 71.16, 73.64, 106.2, 111.6, 129.9, 138.4, 150.9, 153.6, 163.8, 167.6. HRMS-ESI
(m/z): [M+H]+ calcd for C 62 H, 09N 3 0 8 1024.8287, found 1024.8285.
4-[3,4,5-Tri-(undec-10-enyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid bis-[(2'(R)-hydroxy-l'-methyl-ethyl)-amide]. (4a) The title compound was prepared using the
same procedure as above except that 2 was used instead of 1 (57.0%). 'H NMR (CDC13,
95
500 MHz) 6: 1.30-1.42 (m, 30H, (CH2 )5), 1.32 (d, J = 7.0 Hz, 6H, CH 3 ), 1.47 (m, 6H,
CH 2), 1.70-1.83 (m, 6H, CH 2), 2.02-2.06 (m, 6H, CH 2), 3.68-3.71 (m, 2H, CH(H)), 3.80-
3.83 (m, 2H, CH(H)), 3.94-3.98 (m, 6H, OCH2), 4.25 (m, 2H, CH), 4.93 (d, J = 10.0 Hz,
3H CHCH(H)), 4.99 (d, 3H, CHCH(H)), 4.98-5.02 (m, 3H, CHCH(H)), 5.08 (s, 2H,
OCH2), 5.77-5.85 (m, 3H, CHCH2), 6.60 (s, 2H, Ar-H), 7.87 (s, 2H, Ar-H), 8.11 (d, J=
8.0 Hz, 2H, NH).
3C
NMR (CDC13, 500 MHz) 8: 26.29, 26.31, 29.14, 29.17, 29.36,
29.40, 29.45, 20.58, 29.60, 29.66, 29.75, 29.79, 30.52, 32.12, 32.81, 34.01, 47.91, 66.58,
69.32, 71.16, 73.63, 106.2, 111.7, 114.3, 124.7, 130.0, 131.8, 138.4, 139.4, 150.9, 153.6,
1653.8, 167.6. HRMS-ESI (m/z): [M+H]+ calcd for C53H85N3 0 8 892.6409, found
894.6517.
4-(3,4,5-Tri-tetradecyloxy-benzyloxy)-pyridine-2,6-dicarboxylic acid bis-[(2'hydroxyethyl)-amide]. (3b) Ethanolamine (0.06 mL, 0.99 mmol) was added to a stirring
solution of 4-[3,4,5-tri-(tetradecyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid methyl
ester (0.33 g, 0.35 mmol) of xylenes (3.5 mL) and the solution was heated at 100 °C for
three hours under argon atmosphere. The solvents were removed by rotary evaporation
and the resulting material was purified by column chromatography (10:90 methanol:ethyl
acetate) to afford the product (0.20 g, 57.4%) as a white solid. 'H NMR (CDC13,500
MHz) 6: 0.87-0.90 (m, 9H, CH3 ), 1.26-1.33 (m, 60H, (CH2 ), 0), 1.44-1.47 (m, 6H, CH 2),
1.72-1.80 (m, 6H, CH 2), 3.66 (q, J = 5.0 Hz, 4H, CH 2), 3.86 (t, J = 5.0 Hz, 4H, CH 2)
3.93-3.98 (m, 6H, OCH2), 5.07 (s, 2H, OCH2), 6.61 (s, 2H, Ar-H), 7.83 (s, 2H, Ar-H),
8.46 (t, J = 6.0 Hz, 2H, NH). ' 3C NMR (CDC13, 500 MHz) 6: 14.32, 22.90, 26.35, 29.58,
29.60, 29.62, 29.69, 29.86, 29.88, 29.94, 29.96, 29.97, 29.99, 30.56, 32.13, 32.14, 42.33,
96
61.81, 69.30, 71.20, 73.65, 106.3, 111.4, 129.9, 138.4, 150.6, 153.6, 164.4, 167.5.
HRMS-ESI (m/z): [M+H]+ calcd for C60H05N30 8 996.7974, found 996.7977.
4-[3,4,5-Tri-(undec-10-enyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid bis-[(2'hydroxyethyl)-amide]. (4b) The title compound was prepared using the same procedure
as above except that 4-[3,4,5-tri-(undec- 10-enyloxy)-benzyloxy]-pyridine-2,6dicarboxylic acid methyl ester was used instead of 4-[3,4,5-tri-(tetradecyloxy)benzyloxy]-pyridine-2,6-dicarboxylic acid methyl ester (59.6%). H NMR (CDC13,500
MHz) : 1.30-1.47 (m, 36H, (CH2 )6), 1.78-1.81 (m, 6H, CH2 ), 2.02-2.06 (m, 6H, CH2),
3.66 (q, J = 5.0 Hz, 4H, CH2), 3.86 (t, J = 5.0 Hz, 4H, CH2), 3.93-3.98 (m, 6H, OCH2),
4.93 (d, J = 10.0 Hz, 3H, CH(H)), 4.99 (dt, J = 17.0, 2.5 Hz, 3H, CH(H)), 5.07 (s, 2H,
OCH2 ), 5.77-5.84 (m, 3H, CH), 6.61 (s, 2H, Ar-H), 7.83 (s, 2H, Ar-H), 8.47 (t, J = 6.0
Hz, 2H, NH). 3 C NMR (CDC13 , 500 MHz) 8: 26.32, 29.15, 29.18, 29.38, 29.42, 29.61,
29.63, 29.68, 29.77, 29.81, 29.90, 30.55, 34.03, 34.05, 42.40, 62.07, 69.32, 71.21, 73.64,
106.3, 111.6, 114.3, 130.0, 138.4, 139.4, 150.7, 153.6, 164.5, 167.5. HRMS-ESI (m/z):
[M+H]+calcd for C51H8 lN30 8 864.6096, found 864.6078.
4-(3,4,5-Tri-tetradecyloxy-benzyloxy)-pyridine-2,6-dicarboxylic
acid bis-[(2'-
chloro-1'-methyl-ethyl)-amide]. (5a) SOC12(0.04 mL, 0.55 mmol) was added dropwise
to a stirring solution of 3a (0.205 g, 0.20 mmol) and di-t-butyl-methylpyridine (0.13 g,
0.70 mmol) in THF (4 mL) at room temperature under argon atmosphere. The solution
was heated to 60 °C for four hours and then poured into water. The aqueous phase was
extracted with dichloromethane and dried over MgSO4.
97
Purification by column
chromatography (30:70 ethyl acetate:hexane) afforded the product in 66.0% yield.
1H
NMR (CDC13,500 MHz) : 0.85-0.88 (m, 9H, CH3), 1.25-1.34 (m, 60H, (CH2)10), 1.39
(d, J = 7.0 Hz, 6H, CH 3), 1.43-1.49 (m, 6H, CH 2), 1.70-1.82 (m, 6H, CH2), 3.70 (dd, J =
11.0, 3.0 Hz, 2H, CH(H)), 3.81 (dd, J = 11.0, 4.0 Hz, 2H, CH(H)), 3.93-3.98 (m, 6H,
OCH2), 4.51-4.58 (m, 2H, CH), 5.09 (s, 2H, OCH2), 6.59 (s, 2H, Ar-H), 7.89 (s, 2H, ArH), 8.06 (d, J = 9.0 Hz, 2H, NH). ' 3C NMR (CDC13, 500 MHz) 6: 14.35, 18.31, 22.92,
26.32, 26.34, 29.60, 29.61, 29.65, 29.84, 29.87, 29.89, 29.92, 29.94, 29.97, 29.99, 30.55,
32.15, 45.51, 49.97, 69.34, 71.23, 73.65, 106.2, 111.7, 129.9, 138.4, 150.6, 153.6, 162.7,
167.8. HRMS-ESI (m/z): [M+H] + calcd for C 62HI0 7C12N 3 06 1060.7610, found 1060.7628.
4-[3,4,5-Tri-(undec-10-enyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid bis-[(2'chloro-l'-methyl-ethyl)-amide]. (6a) The title compound was prepared using the same
procedure as above except that 4a was used instead of 3a (42.0%). H NMR (CDC13,500
MHz)
: 1.30-1.37 (m, 30H, (CH 2) 5), 1.42 (d, J = 6.5 Hz, 6H, CH3), 1.44-1.47 (m, 6H,
CH2 ), 1.70-1.83 (m, 6H, CH2 ), 2.02-2.06 (m; 6H, CH2), 3.72-3.75 (m, 2H, CH(H)), 3.833.86 (m, 2H, CH(H)), 3.94-3.99 (m, 6H, OCH2), 4.56-4.60 (m, 2H, CH), 4.93 (dd, J =
10.0, 1.5 Hz, 3H CHCH(H)),
4.98-5.01 (m, 3H, CHCH(H)), 5.13 (s, 2H, OCH 2), 5.78-
5.86 (m, 3H, CHCH2), 6.61 (s, 2H, Ar-H), 7.91 (s, 2H, Ar-H), 8.02 (d, J = 8.5 Hz, 2H,
NH). 3C NMR (CDC13,500 MHz) : 26.28, 26.32, 29.15, 29.18, 29.37, 29.41, 29.57,
20.61, 29.67, 29.76, 29.79, 29.89, 30.53, 34.03, 34.06, 45.51, 49.96, 69.32, 71.22, 73.62,
106.2, 111.7, 114.3, 129.9, 139.4, 150.6, 153.6, 162.7, 167.8.
1H
NMR (CDC13, 500
MHz) a: 3.73 (dd, J = 11.0, 3.0, 2H, CH(H)), 6.61 (s, 2H, Ar-H), 7.91 (s, 2H, Ar-H), 8.02
98
(d, J = 8.5 Hz, 2H, NH), HRMS-ESI (m/z): [M+H]+ calcd for C53H 83N 30 6 928.5732,
found 928.5744.
4-(3,4,5-Tri-tetradecyloxy-benzyloxy)-pyridine-2,6-dicarboxylic acid bis-[2'-chloroethyl)-amide]. (5b) The title compound was prepared using the same procedure as above
except that 3b was used instead of 3a (73.6%). 'H NMR (CDC13,500 MHz) 8: 0.86-0.89
(m, 9H, CH3 ), 1.25-1.33 (m, 60H, (CH2 ) 0 ); 1.43-1.49 (m, 6H, CH2), 1.71-1.82 (m, 6H,
CH 2), 3.75 (t, J = 5.5 Hz, 4H, CH2 ), 3.85 (q, J = 6.0 Hz, 4H, CH2 ) 3.93-3.98 (m, 6H,
OCH 2 ), 5.11 (s, 2H, OCH2 ), 6.60 (s, 2H, Ar-H), 7.93 (s, 2H, Ar-H), 8.26 (t, J = 6.0 Hz,
2H, NH). ' 3C NMR (CDC13,500 MHz) 8: 14.33, 22.90, 26.31, 26.33, 29.58, 29.59, 29.64,
29.83, 29.88, 29.90, 29.92, 29.95, 29.97, 30;54, 32.13, 32.14, 41.42, 44.08, 69.36, 71.25,
73.65, 106.3, 112.0, 129.9, 138.5, 150.7, 153.6, 163.7, 167.8. HRMS-ESI (m/z): [M+H] +
calcd for C60H0 Cl2N
3
30 6
1032.7297, found 1032.7317.
4-[3,4,5-Tri-(undec-10-enyloxy)-benzyloxy]-pyridine-2,6-dicarboxylic acid bis-[(2'chloro-ethyl)-amide]. (6b) The title compound was prepared using the same procedure
as above except that 4b was used instead of 3a (86.0%). H NMR (CDC13,500 MHz) 6:
1.30-1.49 (m, 36H, (CH2 ) 6 ), 1.78-1.83 (m, 6H, CH2 ), 2.02-2.06 (m, 6H, CH2 ), 3.77 (t, J =
5.5 Hz, 4H, CH 2 ), 3.87 (q, J = 5.5 Hz, 4H, CH2), 3.94-3.99 (m, 6H, OCH 2 ), 4.93 (d, J =
10.0 Hz, 3H, CH(H)), 4.99 (dd, J= 17.5, 1.5 Hz, 3H, CH(H)), 5.13 (s, 2H, OCH 2), 5.785.86 (m, 3H, CH), 6.61 (s, 2H, Ar-H), 7.94 (s, 2H, Ar-H), 8.18 (t, J = 6.0 Hz, 2H, NH).
'3 C NMR (CDC13, 500 MHz) : 26.32, 29.15, 29.19, 29.37, 29.42, 29.58, 29.61, 29.67,
29.76, 29.79, 29.89, 30.53, 34.03, 34.06, 41.40, 44.15, 69.34, 71.25, 73.63, 106.2, 112.0,
99
114.3, 129.9, 138.5, 139.4, 150.6, 153.6, 163.6, 167.8. HRMS-ESI (m/z): [M+H] + calcd
for C51H79C12N3 06 900.5419, found 900.5445.
4-[3,4,5-Tris-(tetradecyloxy)-benzyloxy]-2,6-bis-[(4-methyl-4,5-dihydro-oxazol-2-
yl)]-pyridine. (7a) To a suspension of NaH (10.4 mmol) in anhydrous THF (5 mL) was
added a solution containing 5a (.76 mmol) and 5 mL of THF. The mixture was stirred at
room temperature under argon atmosphere for 1 hour and was then poured over ice to
quench excess NaH. The residue was extracted with dichloromethane and dried over
MgSO4. Concentration of the organic layer afforded the product (93.5%) as a white
solid. 'H NMR (CDC13 , 500 MHz) : 0.86-0.88 (m, 9H, CH3 ), 1.25-1.41 (m, 30H,
(CH2), 0), 1.36 (d, J = 6.0 Hz, 6H, CH3 ), 1.44-1.48 (m, 6H, CH2 ), 1.71-1.82 (m, 6H, CH2),
3.95 (q, J = 6.5 Hz, 6H, OCH2 ), 4.06 (t, J = 8.0 Hz, 2H, CH(H)), 4.41-4.44 (m, 2H,
CH(H)), 4.58-4.62 (m, 2H, CH(H)), 5.09 (d, J = 1.5 Hz, 2H, CH 2), 6.58 (s, 2H, Ar-H),
7.78 (s, 2H, Ar-H). 13 C NMR (CDCl3 , 500 MHz) : 14.30, 21.56, 22.87, 26.28, 26.30,
29.55, 29.57, 29.60, 29.79, 29.82, 29.85, 29,87, 29.89, 29.93, 29.95, 30.51, 32.11, 62.35,
69.27, 70.87, 73.60, 74.93, 106.0, 112.5, 130.2, 138.3, 148.6, 153.5, 162.5, 165.8.
HRMS-ESI (m/z): [M+H]+ calcd for C62H,05N3 06 988.8076, found 988.8069.
4-[3,4,5-Tri-(undec-10-enyloxy)-benzyloxy]-2,6-bis-[(4-methyl-4,5-dihydro-oxazol-2-
yl)]-pyridine. (8a) The title compound was prepared using the same procedure as above
except that 6a was used instead of 5a (94.0%). 'H NMR (CDC13 , 500 MHz) 6: 1.29-1.47
(m, 36H, (CH 2) 6), 1.35 (d, J = 7.0 Hz, 6H, CH3), 1.71-1.81 (m, 6H, CH 2), 2.02-2.04 (m,
6H, CH2), 3.93-3.97 (m, 6H, OCH2), 4.06 (t, J = 8.0 Hz, 2H, CH(H)), 4.40-4.44 (m, 2H,
100
CH), 4.58-4.62 (m, 2H, CH(H)), 4.92 (dd, J = 10.0, 1.0 Hz, 3H, CHCH2), 4.98 (dd, J =
17.0, 1.0 Hz, 3H, CHCH 2), 5.08 (s, 2H, CH 2 ), 5.76-5.84 (m, 3H, CHCH 2), 6.58 (s, 2H,
Ar-H), 7.77 (s, 2H, Ar-H).
13C
NMR (CDC13, 500 MHz) 8: 21.53, 26.23, 26.26, 29.09,
29.12, 29.31, 29.35, 29.52, 29.54, 29.61, 29.70, 29.73, 29.83, 30.47, 33.97, 33.99, 62.32,
69.25, 70.86, 73.56, 74.91, 106.0, 112.5, 114.26, 114.27, 130.2, 138.3, 139.33, 139.34,
148.5, 153.5, 162.5, 165.8. HRMS-ESI (m/z): [M+H] + calcd for C 53 H8 N 30 6 856.6198,
found 856.6184.
2,6-Bis-(4,5-dihydro-oxazol-2-yl)-4-[3,4,5-tri-(tetradecyloxy)-benzyloxy]-pyridine.
(7b) The title compound was prepared using the same procedure as above except that 5b
was used instead of 5a (76.0%). 'H NMR (CDC13, 500 MHz) 8: 0.87-8.89 (m, 9H, CH3 ),
1.26-1.33 (m, 30H (CH2) 10 ), 1.44-1.47 (m, 6H, CH2 ), 1.72-1.83 (m, 6H, CH2 ), 3.94-3.98
(m, 6H, OCH 2), 4.11 (t, J = 10.0 Hz, 4H, CH 2), 4.53 (t, J = 9.5 Hz, 4H, CH 2), 5.09 (s, 2H,
OCH 2), 6.60 (s, 2H, Ar-H), 7.77 (s, 2H, Ar-H).
3C
NMR (CDC13, 500 MHz) 6: 14.34,
22.91, 26.31, 26.33, 29.58, 29.60, 29.64, 29.83, 29.64, 29.88, 29.90, 29.93, 29.96, 29.98,
30.54, 32.14, 55.18, 68.59, 69.34, 70.98, 73.65, 106.2, 112.5, 130.1, 138.4, 148.4, 153.6,
163.7, 165.9. HRMS-ESI (m/z): [M+H]+ calcd for C60H
1 ON306 960.7763, found
960.7784.
2,6-Bis-(4,5-dihydro-oxazol-2-yl)-4-[3,4,5-tri-(undec-10-enyloxy)-benzyloxy]-
pyridine. (8b) The title compound was prepared using the same procedure as above
except that 6b was used instead of 5a (93.2%). 'H NMR (CDC13,500 MHz) 6: 1.29-1.47
(m, 36H, (CH2 ) 6), 1.77-1.83 (m, 6H, CH2 ), 2.01-2.06 (m, 6H, CH2 ), 3.94-3.98 (m, 6H,
101
OCH 2 ), 4.11 (t, J = 10.0 Hz, 4H, CH 2), 4.53 (t, J = 9.5 Hz, 4H, CH2 ), 4.92 (d, J = 10.0
Hz, 3H, CH(H)), 4.99 (dd, J = 17.0, 1.0 Hz, 3H, CH(H)), 5.08 (s, 2H, OCH 2 ), 5.77-5.85
(m, 3H, CH), 6.59 (s, 2H, Ar-H), 7.75 (s, 2H, Ar-H). 3C NMR (CDC13, 500 MHz) 6:
26.25, 26.28, 29.11, 29.14, 29.33, 29.38, 29.53, 29.56, 29.63, 29.72, 29.76, 29.85, 30.32,
30.49, 33.99, 34.01, 55.12, 68.57, 69.28, 70.96, 73.60, 106.2, 112.5, 114.3, 130.1, 138.3,
139.4, 148.3, 153.5, 163.7, 167.9. HRMS-ESI (m/z): [M+H] + calcd for C sH
5
7 7N30 6
828.5885, found 828.5873.
Pincer Compounds:
3,5-Bis-thiophenylmethyl-anisole. (9) A solution of NaOH (0.81 g, 20.3 mmol) in 25
mL of water was added to a solution of 3,5-bis-bromomethyl-anisole (1.50 g, 5.10 mmol)
in 75 mL of toluene and a catalytic amount of ADOGEN 464. Thiophenol (1.50 mL,
14.6 mmol) was added, and the solution was heated to reflux for 2 hours under argon.
The solution was cooled to room temperature and poured into dichloromethane, washed
three times with 0.5M NaOH(aq) to remove excess thiophenol, and extracted with ether.
The organic layer was dried over MgSO4, and the solvents were removed. Purification
by column chromatography on silica gel using ethyl acetate/hexane (5/95) as eluant
yielded the product as a white solid in 95.7% yield. H NMR (CDC13, 500 MHz) 6: 3.65
(s, 3H, CH 3), 4.00 (s, 4H, CH2 ), 6.68 (d, J = 1.5 Hz, 2H, Ar-H), 6.81 (s, 1H, Ar-H), 7.12-
7.16 (m, 2H, Ar-H), 7.18-7.22 (m, 4H, Ar-H), 7.24-7.27 (m, 4H, Ar-H).
13
C NMR
(CDC13, 500 MHz) 6: 38.93, 55.21, 113.3, 121.8, 126.5, 129.0, 129.9, 136.4, 139.2,
159.8. HRMS-ESI (m/z): [M+H]+ calcd for C21H20OS2 353.1028, found 353.1030.
102
3,5-Bis-thiophenylmethyl-phenol.
(10) A 1.OM solution of BBr 3 in dichloromethane (2
mL) was added slowly via syringe to a stirring solution of 3,5-bis-thiophenylmethylanisole (1.07g, 3.31 mmol) and dichloromethane (50 mL) at -78°C. After stirring
overnight and warming to room temperature, the solution was slowly poured over ice to
quench excess BBr3. The organic layer was extracted with ether. was dried over MgSO4 ,
and the solvents were removed to afford the product in 93.5% yield. Characterization of
the title compound agrees with reported literature.13
3,5-Bis-thiophenylmethyl-1-[3,4,5-tri-tetradecyloxy-benzyloxy]-benzene.
(11) A
mixture of 10 (0.35 g, 1.03 mmol) 3,4,5-tri-tetradecyloxy-benzyl bromide (0.92 g, 1.14
mmol), K2CO 3 (0.21 g, 1.50 mmol), and a catalytic amount of KI in 2-butanone (25 mL)
was heated to reflux at 80 C for 12 hours. The mixture was poured into 0.5M NaOH
(aq) and extracted with ethyl acetate. The solution was washed successively with water
and brine, and the combined organic layers were dried over MgSO4. The solvents were
removed in vacuo, followed by purification by column chromatography in 5:95 ethyl
acetate:hexane to give the product (80%) as a clear oil. H NMR (CDC13,500 MHz) b:
0.88-0.90 (m, 9H, CH3), 1.27 (m, 60H, (CH2)10 ),1.47(m, 6H, CH2), 1.75-1.82 (m, 6H,
CH2 ), 3.93-3.99 (m, 6H, OCH 2 ), 4.04 (s, 4H, SCH2 ), 4.85 (s, 2H, OCH 2), 6.59 (s, 2H, ArH), 6.82 (s, 2H, Ar-H), 6.84 (s, 1H, Ar-H), 7.18-7.20 (m, 2H, Ar-H), 7.23-7.29 (m, 8H,
Ar-H).
13C NMR
(CDC13, 500 MHz) 6: 14.30, 26.33, 29.59, 29.61, 29.62, 29.66, 29.89,
29.91, 29.94, 29.97, 29.99, 30.55, 32.15, 39.19, 55.29, 69.03, 70.57, 73.63, 106.3, 114.3,
122.2, 126.6, 129.1, 130.0, 131.8, 136.4, 137.6, 139.3, 153.5, 159.8. HRMS-ESI (m/z):
[M+H]+ calcd for C6 9HI 08 04 S2 1065.7765, found 1065.7763.
103
Pd(ll)(CH 3 CN). (12) A solution of 11 (0.05 g, in dichloromethane (2 mL) is added
dropwise to a stirring solution of commercially available palladiumtetrakis(acetonitrile)
bis(tetrafluoroborate) in anhydrous acetonitrile. The yellow solution was heated at 60 °C
for 12 hours and was then filtered through a pad of Celite. The solution was diluted with
dichloromethane and washed with water. The organic layer was dried over MgSO4, and
the solvents were removed in vacuo. The material was purified by precipitation in ether.
'H NMR (CDC13,500 MHz) 8: 0.88-0.91 (m, 9H, CH3 ), 1.26 (m, 60H, (CH2) 0O),1.49(m,
6H, CH 2), 1.77-1.83 (m, 6H, CH 2), 3.94-3.99 (m, 6H, OCH 2), 4.55 (broad s, 4H, SCH 2),
4.88 (s, 2H, OCH 2 ), 6.57 (s, 2H, Ar-H), 6.69 (s, 2H, Ar-H), 7.52 (m, 8H, Ar-H), 7.81 (m,
2H, Ar-H).
13C
NMR (CDC13, 500 MHz)
: 14.35, 26.33, 29.59, 29.61, 29.62, 29.66,
29.89, 29.91, 29.94, 29.97, 29.99, 30.55, 32.15, 39.19, 55.29, 69.03, 70.57, 73.63, 106.3,
111.4, 118.9, 131.3, 132.4, 132.8, 133.1, 136.4, 137.6, 152.8, 153.5, 159.8.
104
References
1
Girard, C.; Kagan, H. B.; Angew. Chem. Int. Ed. 1998, 37, 2922.
2
a) ( a) White, D. E.; Jacobsen, E. N.;
Tetrahedron: Asymmetry 2003, 14 , 3633. (b)
Ready, J. M.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2002, 41, 1374. (c) Ready, J. M.;
Jacobsen, E. N.; J. Am. Chem. Soc. 2001 ,123, 2687. (d) Breinbauer, R.; Jacobsen, E. N.;
Angew. Chem., Int. Ed. 2000, 39, 3604-3607. (e) Konsler, R. G.; Karl, J.; Jacobsen, E. N.;
J. Am. Chem. Soc. 1998, 120, 10780.
3
For recent reviews on bis(oxazoline) ligands in catalysis, please see: (a) McManus, H.
A.; Guiry, P. J.; Chem. Rev. 2004, 104, 4151. (b) Ghosh, A. K.; Mathivanan, P.;
Cappiello, J.; Tetrahedron: Asymmetry 1998, 9, 1. (c) Johnson, J. S.; Evans, D. A.; Acc.
Chem. Res. 2000, 33, 325-335.
4
Nishiyama, H.; Sakaguchi, H.; Nakamura, T.; Horihata, M.; Kondo, M.; Itoh, K.;
Organometallics 1989, 8, 846.
5 Whitesell,
J. K.; Chem. Rev. 1989, 89, 1581.
6
Schaus, S. E.; Jacobsen, E. N.; Org. Lett. 2000, 2, 1001.
7
(a) Cornejo, A.; Fraile, J. M.; Garcia, J. I.; Garcia-Verdugo, E.; Gil, M. J.; Legarreta, G.;
Luis, S. V.; Martinez-Merino, V.; Mayoral, J. A.; Org. Lett. 2002, 4, 3927. (b) Lundgren,
S.; Lutsenko, S.; Jonsson, C.; Moberg, C.; Org. Lett. 2003, 5, 3663.
8
Lehmann, M.; Sierra, T.; Barbera, J.; Serrano, J. L.; Parker, R.;
12, 1342.
9 See Chapter 3.
'0 Bruce, D. W.; Acc. Chem. Res. 2000, 33, 831.
105
J. Mater. Chem. 2002,
11 (a)
Doyle, M. P.; Protopopova, M. N.; Tetrahedron 1998, 54, 7919. (b) Ghosh, A. K.;
Mathivanan, P.; Cappiello, J.; Tetrahedron: Asymmetry 1998, 9, 1. (c) Wei C.; Li C.-J.;
J. Am. Chem. Soc. 2002, 124, 5638.
12
For reviews on pincer ligands, please see: (a) Albrecht, M.; van Koten, G.; Ang. Chem,
Int. Ed. 2001, 40, 3750. (b) Singleton, J. T.; Tetrahedron 2003, 59, 1837. (c) Gossage, R.
A.; van de Kuil, L. A.; van Koten, G.; Acc. Chem. Res. 1998, 31, 423-431.
13
14
Gimenez, R.; Swager, T. M.; J. Mol. Catal. A 2001, 166, 265.
Huck, W. T. S.; van Veggel, F. C. J. M.; Kropman, B. L.; Blank, D. H. A.; Keim, E. G.;
Smithers, M. M. A.; Reinhoudt, D. N.; J. Am. Chem. Soc. 1995, 117, 8293.
5Motoyama, Y.; Kurihara, O.; Murata, K.; Aoki, K.; Nishiyama, H. Organometallics,
2000, 19, 1025-1034.
16
Aqueous solutions of AgNO 3 and NaDOS were combined in the absence of light until
AgDOS precipitated as a white crystalline solid, which was filtered and dried under
vacuum for 48 hours in the absence of light. Bruce, D.W.; Dunmur, D. A.; Hudson, S.
A.; Lalinde, E.; Maitlis, P. M.; McDonald, M. P.; Orr, R.; Styring, P.; Cherodian, A. S.;
Richardson, R. M.; Feijoo, J. L; Ungar, G.; Mol. Cryst., Liq. Cryst. 1991, 206, 79.
106
Chapter 5
Immobilized Chiral Monodentate Oxazolines: Heterogeneous Catalysis
within an Organic Polymer Network
5.1.
Introduction
Although homogeneous asymmetric catalysts offer more versatility and viability
in the synthesis of chiral molecules,l achiral heterogeneous catalysts are still the most
widely used systems in the industrial production of fine chemicals.2 Heterogeneous
systems are appealing for a number of practical reasons such as ease of separation,
handling and recovery, and a higher potential for regeneration and reuse. As such, the
heterogenization of chiral homogeneous catalysts has been an active area of research,
though the levels of activity, enantioselectivity, and recyclability that are required for
application in industry have yet to be reached.
Various methods exist for the
immobilization of homogeneous catalysts including covalent or non-covalent bonding to
organic and inorganic polymers, and soluble and insoluble polymers. The work relevant
to this chapter involves covalent bonding to insoluble, organic supports.
The strategy for the preparation of chiral heterogeneous catalysts typically begins
with a well-established and efficient homogenous system. Structural modifications are
then made in order to allow for incorporation into a polymer matrix and, ideally, the
catalytic activity of the homogeneous system is preserved upon immobilization and after
numerous catalytic cycles.
In such a process, it is often hoped that the polymeric
backbone will serve as an inert matrix to which the active catalytic moiety is merely
attached. However, the nature of the support and the method of immobilization have
been shown to have profound influence on the activity, stability, and selectivity of the
catalyst. More often than not, the polymeric support is seen as having a deleterious effect
on catalytic behavior. Examination of ligand systems whose catalytic behavior could
108
Scheme 5.1. Copper-catalyzed cyclopropanation of styrene with ethyl diazoacetate.
Ph~
+
N'COE 2 t
Ph
Cu catalyst
H... A.CO 2Et
Hi AH
Ph
Ph
H
trans(IS,2S)
*~
A
Phi
H
H
C0 2Et
trans(1R,2R)
CO2 Et
cis(1R,2S)
Ph,A"Co
H
2Et
H
cis(IS,2R)
potentially benefit from the presence of a polymeric support would lead to improved
design and use of heterogeneous catalysts.
One of the most studied and well-established catalytic systems is the coppercatalyzed cyclopropanation reaction between styrene and ethyl diazoacetate.3 (Scheme
5.1) The postulated mechanism involves formation of a copper(I) carbene complex,
followed by cyclopropanation of the double-bonded substrate. First reported in 1966, this
reaction represents the first use of a chiral catalyst to impart control over the
stereochemical outcome of a reaction.4 Pfaltz and co-workers then pioneered the use of
Cu(II)-semicorrin complexes for enantioselective cyclopropanation, determining that the
Cu(I) oxidation state was responsible for catalytic activity.5 Structurally related yet more
synthetically accessible bis(oxazoline) ligands later emerged as excellent catalysts for
asymmetric cyclopropanation and have been extensively studied by Masamune,6 Evans, 7
and Pfaltz.8 Evans first described direct access to the catalytically active species by
in
situ mixing of the bis(oxazoline) ligand with a stoichiometric amount of CuOTf,
achieving enantiomeric excesses of >99%.7
109
As such, the immobilization of bis(oxazoline) cyclopropanation catalysts onto
various heterogeneous supports has received considerable attention.9 Heterogenization
methods used have included grafting chiral ligands onto commercially available resins or
direct polymerization of a monomer containing the chiral ligand. One recent example
from Mayoral and co-workers involved the homopolymerization
of a styrene-
functionalized bis(oxazoline) monomer followed by Cu(OTf)2 loading. 0° (Scheme 5.2)
Enantioselectivities approaching and even slightly surpassing those of the homogeneous
parent bis(oxazoline) ligand were achieved. Furthermore, enantioselectivities were only
slightly reduced in the second cycle of catalyst use.
Scheme 5.2. Immobilization of bis(oxazoline)s.
1. homopolymerization
2. excess Cu(OTf)2
3. washing with CH3 0H
In the interests of exploring the effects of heterogenization on catalytic behavior,
we examined the immobilization of a chiral, monodentate oxazoline ring on an insoluble,
organic polymer.
Monodentate nitrogen ligands are generally less effective as
asymmetric catalysts when compared with the bidentate and tridentate counterparts in
asymmetric cyclopropanation. However, incorporation of such ligands within a crosslinked polymeric network could create a sufficiently chiral microenvironment to
110
favorably impact the enantioselectivity of the cyclopropanation reaction. A flexible,
swellable polymer system is required for any reaction to occur within a polymeric
support. Incorporation of a long, flexible spacer between the oxazoline ring and the
polymerizable group would provide a more solution-like availability of the chiral ligands.
In this chapter we describe the synthesis of a chiral oxazoline monomer and its
polymerization using ruthenium-catalyzed acyclic diene metathesis. l The catalytic
behavior of both the homogeneous system and the heterogeneous system in the
cyclopropanation of styrene with ethyl diazoacetate is examined.
5.2.
Results and Discussion
The synthesis of the chiral oxazoline monomer is shown in Scheme 5.3. The
target monomer possesses long, aliphatic chains as the spacer between the oxazoline ring
and the polymerizable functionality to allow for a more flexible polymer network.
Terminal olefins provide the polymerizable functionality needed for ADMET
Scheme 5.3. Synthesis of chiral oxazoline monomer.
RO
RO
RO
X
-
RO
*
RO
RO
HN*
RO
1 X=OH
3Y=OH
'
RO
RO
YRO
5
ii
2X=CI
.
iii
ii
4Y=CI
(i) SOCI2, CHC13; (ii) (S)-alaninol, NEt3, CH2CI2, 85%; (iii) SOCI2, THF, reflux 4 hours, 78%; (iv)
NaH, THF, 0 °C, 81%.
R = (CH 2 )9 CHCH
2
111
Scheme 5.4. Preparation of polymeric copper oxazoline catalysts.
f\N
Ms-N NMs
1.
2. CuOTf
*
Route A
l
Route B
Route B
T
2.
1. CuOTf
I'
MsNyN.Ms
CIV,. -
polymerization. Acid 1 is synthesized by the hydrolysis of the corresponding methyl
benzoate, whose synthesis is described in Chapter 3. Condensation of the chiral
aminoalcohol with acid chloride 2 gives the bis(hydroxy)amide 3. Treatment of the
amide with thionyl chloride affords the chloride 4, which affords the oxazoline monomer
5 when treated with base.
Two methods to generate the polymeric copper catalyst from monomer 5 were
explored. (Scheme 5.4)
Following Route A, films of the chiral monomer were
polymerized via ADMET polymerization, followed by treatment with CuOTf. Due to the
presence of three terminal olefins per monomer, there is a wide range of possible
112
intermolecular and intramolecular linkages that can be made between monomers.
(Scheme 5.5) The resulting free-standing film was easily peeled off the glass slide and
was slightly discolored by a dark green tint due to degradation of the ruthenium catalyst.
Insolubility of the films indicated that the polymers possess a sufficient degree of
crosslinking to anchor the polymer chains and prevent the configurational entropy
required to form a solution.
The swelling capability of the polymer was investigated to estimate the degree of
cross-linking and to ensure good mass transport such that the CuOTf and subsequent
substrates could readily diffuse through the polymer network. As the degree of crosslinking increases, the polymer becomes more rigid and swelling becomes difficult. For
example, styrene-divinylbenzene co-polymers which contain 20% or higher degree of
crosslinking exhibit no swelling with toluene. l2 Systems with relatively dilute
crosslinking (>1.0%) can give expanded polymer networks having reduced chain
entanglements resulting in higher mobility and good swelling capability, but mechanical
stability is often sacrificed.
The films were examined in solvents relevant to the conditions for the
cyclopropanation of styrene with ethyl diazoacetate: styrene, the solvent in which
cyclopropanation is typically performed, and toluene, whose structures and properties are
similar to that of styrene. (Figure 5.1) In both styrene and toluene, the polymer size
increased to 150% of its original size, indicating an estimated 5-10% degree of
crosslinking.l3 This indicates the presence of a large amount of linear polymer within the
network, allowing for good swelling properties, yet the polymer network is sufficiently
crosslinked to maintain good mechanical stability. Other hydrophobic solvents such as
113
hexane also cause the films to swell, but to a lesser degree, while in polar solvents such
as methanol, the polymer does not swell.
Scheme 5.5. Polymerization of monomer 5 and the potential linkages present in the
resulting polymer network.
~~,,,,_ossNsr
Ms-N
N-Ms
CIT
C"
CIU
=Rue
Ph
PCy3
(5 mol %)
100 °C, 24 hrs
~NX""""0"
0
n-bw
0 N
#%~NO\/
I
n
0
(
= crosslinked polymer network
114
d,
Figure 5.1. Swelling behavior of polymer films obtained by Route A.
a.)
swollen with toluene
non-swollen polymer
b.)
non-swollen polymer
swollen with toluene
c.)
non-swollen polymer
swollen with styrene
115
The oxazoline polymer was then converted into a copper catalyst by treatment
with CuOTf. First, the polymer was ground to a fine powder and suspended in toluene
with an excess of CuOTf and stirred for 24 hours under argon atmosphere. The insoluble
material was filtered and washed successively with toluene, dichloromethane, and
methanol to remove any excess copper triflate, and dried under vacuum. Elemental
analysis confirmed the presence of copper ions within the polymer network.
The second strategy for generating a polymeric copper catalyst involved
formation of the metal complex first, followed by polymerization. (Route B) The
presence of the CuOTf seemed to inhibit metathesis polymerization, and a longer reaction
time was required. The resulting films were dark brown and brittle, and elemental
analysis confirmed that copper ions were included in the polymer network.
The copper catalysts were then tested in the cyclopropanation of styrene with
ethyl diazoacetate and the results are summarized in Table 5.1. For catalysts prepared via
Route A (Table 5.1, Entries 4-7), the method of copper loading proved to be a significant
factor in the success of the catalyst. When methanol was used as the solvent during
copper loading, the polymer network does not swell and the copper ions are prevented
from diffusing into the polymer network. As a result, the catalyst is not active and only
starting materials are recovered. (Table 5.1, Entry 4) However, when toluene is used
during copper loading, the polymer swells significantly and a sufficient amount of copper
ion is bound within the polymer and catalytic activity is exhibited. (Table 5.1, Entries 47) Additionally, slow diffusion of CuOTf is needed. Sonication of the polymer network
with CuOTf in toluene for 30 minutes did not result in sufficient copper loading, and the
cyclopropanation reaction did not proceed. (Table 5.1, Entry 5)
116
Table 5.1. Results of cyclopropanation reactions.
Cis
Trans
(%ee)
(%ee)
<1
<1
78/22
3
8
Yield
Time
trans/cis
CuOTf alone
(%)
60
(h)
20
69/31
2
5 + CuOTf
58
20
3
Polymer
Entry
Catalyst
1
Catalyst Prep
20
alone
4
5
Polymer
(Route A)
Stir in MeOH
Polymer
Sonicate in
toluene with
(Route A)
20
with CuOTf
20
CuOTf, 30 min
6a
Polymer,
Run
6b
1
Stir in toluene
with CuOTf,
(Route A)
24 hours
Polymer,
Reuse catalyst
from Entry 6a
Run 2
26
20
64/36
16
2
16
20
45/55
15
4
15
20
78/22
8
2
(Route A)
7
Polymer
(Route B)
117
Enantiomeric excesses of the products were determined by gas chromatography as
previously described.'
For the homogeneous systems studied (Table 5.1, Entries 1-2),
the presence of monomer 5 increases the trans/cis ratio and the enantioselectivities of
both trans and cis isomers, more significantly in the case of the trans isomer. As is
typical for heterogeneous catalysts, lower yields are observed for the polymeric catalysts
when compared to the homogeneous ligand 5. Also, the trans/cis ratio is generally
decreased in the heterogeneous systems. However, the enantioselectivities for the cis
products are higher for heterogeneous systems than the homogeneous system, (Table 5.1,
Entry 2 vs. Entries 6-7) demonstrating a favorable effect of the immobilization on this
particular selectivity. The polymeric support clearly exerts influence on the energy of the
four diastereomeric transition states, although elucidation of such effects requires further
investigation. One possibility for the enhanced stereoselectivity is the spatial constraint
induced by the polymer matrix, which may increase the influence of the chiral ligands.
In homogeneous reactions, the chiral directing ligand and the catalytic metal center
influence the transition state of a reaction.
In heterogeneous reactions, the spatial
confinement of the immobilized catalyst adds additional parameter that often enhances
the selectivity of the reaction.
An interesting factor that calls for further investigation is the effect of copper
loading on the stereochemical outcome of the reaction. Interaction of multiple Cu(I)
catalytic centers often has a detrimental effect on activity and enantioselectivity,9 and the
polymeric catalysts used in this study possessed high copper loading.
However,
complexation of multiple chiral monodentate oxazoline ligands to a single Cu(I) center
has been shown to increase enantioselectivities when compared to a 1:1 metal:ligand
118
complex. 4 Decreasing the copper loading of the polymer catalysts may encourage
interaction of multiple chiral ligands with a single metal center, creating a local chiral
environment in which the reaction can take place.
Interestingly, upon reuse of the catalyst (Table 5.1, Entry 6b), the level of
enantioselectivity remains essentially the same, although the trans/cis ratio and yield are
decreased, possibly due to coordination of impurities or metal leaching. Another possible
alteration of the original catalyst is cyclopropanation of the polymer backbone, although
the reaction is performed in excess styrene to prevent such side reactions. With the
polymer obtained by Route B, the translcis ratio obtained is the same as in the
homogeneous case, but lower levels of enantioselectivity of the cis isomer are observed.
5.3.
Concluding Remarks
The preliminary results presented here open the way for the design of
heterogeneous catalysts based on chiral monodentate oxazoline ligands. Immobilization
of chiral oxazoline ligands using ADMET polymerization gives flexible polymer
networks that swell in hydrophobic solvents such as toluene and styrene. When charged
with copper(I) triflate, the polymers catalyze the cyclopropanation of styrene with ethyl
diazoacetate with higher enantioselectivity than the corresponding homogeneous phase
reaction.
Elucidation of the nature of the effects of the polymeric support and methods to
benefit from such effects require further study, involving investigation of the effects of
copper loading on the catalytic behavior of the polymeric material. Also, co-monomers
119
can be employed to control the amount of chiral ligand within the material, as well as to
explore the effects of polymer morphology on the system. Furthermore, the use of
Cu(OTf)2 to form a supported precatalyst should also be explored, with in situ reduction
of the metal center to attain the catalytically active Cu(I) species. Studies such as these
would provide useful information for the optimization of oxazoline-containing polymers
as catalysts for asymmetric cyclopropanation.
120
5.4.
Experimental Section
General Methods.
All chemicals were of reagent grade and were used as received,
unless otherwise specified. H and 13C NMR spectra were obtained on Varian Inova-500
spectrometers. All chemical shifts are referenced to residual CHC13 (7.27 ppm for 'H,
77.23 ppm for 13C). Multiplicities are indicated as s (singlet), d (doublet), t (triplet), and
m (multiplet). High resolution mass spectra were obtained at the MIT Department of
Chemistry Instrumentation Facility (DCIF) on a Finnigan MAT 820 or on a Bruker
Daltonics Apex II 3T FT-ICR MS.
GC analysis was performed on a Varian CP-3800
gas chromatograph equipped with FID detector and Cyclodex B capillary columns.
3,4,5-Tri-(10-undecen-1-ol-oxy)-benzoic acid (1). A solution of 3,4,5-tri-(10-undecen1-ol-oxy)-benzoic acid methyl ester 5 (5.01 g, 7.82 mmol) and potassium hydroxide (2.19
g, 39.1 mmol) in ethanol (80 mL) and deionized water (40 mL) was heated to reflux at 80
°C for four hours. The solution was then poured into 100 mL of 1N HC1 to form a white
precipitate, which was filtered and washed with ethanol to afford the product (4.80 g,
98% yield) as a white solid.
H NMR (CDC13, 500 MHz) 8: 1.31-1.39 (m, 30H, (CH 2) 5),
1.46-1.50 (m, 6H, CH2), 1.73-1.86 (m, 6H, CH2), 2.03-2.07 (m, 6H, CH2), 4.02-4.07 (m,
6H, OCH2), 4.93-4.95 (m, 3H, CHCH(H)), 4.98-5.03 (m, 3H, CHCH(H)), 5.78-5.86 (m,
3H, CH), 7.34 (s, 2H, Ar-H).
13
C NMR (CDC13, 500 MHz) 8: 26.23, 26.27, 29.14, 29.17,
29.36, 29.41, 29.46, 29.57, 29.67, 29.74, 29.78, 29.86, 30.52, 34.03, 34.05, 69.33, 73.73,
108.7, 114.3, 123.9,139.4,
143.3, 153.0, 172.1. HRMS-ESI
C40H66 0 5 625.4915, found 628.4900.
121
(m/z): [M-H]- calcd for
N-(2'-(R)-Hydroxy-1 '-methyl-ethyl)-3,4,5-tri-(undec-10-enyloxy)-benzamide.
(3) To
a solution of 1 (0.55 g, 0.88 mmol) in anhydrous THF (8.0 mL) was added SOC12(1.0
mL), and the solution was heated to reflux for two hours. The solution was cooled to
room temperature, and then excess SOC12 and solvent were removed by vacuum
distillation. The resulting acid chloride was dissolved in anhydrous CH2C12(4.0 mL) and
was added slowly via cannula to a stirring solution of (S)-alaninol (0.20 mL), a catalytic
amount of NEt3, and anhydrous CH2C12(4.0 mL) at 0 °C under argon atmosphere. The
solution was allowed to stir overnight, warming to room temperature. Water was added
to quench any excess acid chloride, and the reaction was extracted with CH2C12and the
organic layer was dried over MgSO4. The solvents were removed in vacuo, and the
product (85% yield) was taken onto the next step without further purification.
For
analytical purposes, a small amount of material was purified by column chromatography
using 50:50 ethyl acetate: hexane as eluant, affording the product as a white solid. ' H
NMR (CDC13, 500 MHz) 8: 1.7 (d, J = 6.5 Hz, 3H, CH 3), 1.29-1.39 (m, 30H, (CH2) 5),
1.42-1.47 (m, 6H, CH2), 1.71-1.80 (m, 6H, CH2), 2.01-2.06 (m, 6H, CH2), 3.62 (dd, J =
11.0, 6.0 Hz, 1H, CH(H)), 3.75 (dd, J = 11i.0,3.5 Hz, 1H, CH(H)), 3.96-3.99 (m, 6H,
OCH2), 4.20-4.25 (m, 1H, CH), 4.91-5.01 (m, 3H, CHCH(H)), 4.99 (dd, J = 17.0, 1.5
Hz, 3H, CHCH(H)), 5.77-5.85 (m, 3H, CHCH2), 6.37 (d, J = 7.0 Hz, 2H, NH), 6.95 (s,
2H, Ar-H).
13 C
NMR (CDC13,500 MHz) : 26.24, 26.29, 29.11, 29.14, 29.29, 29.33,
29.37, 29.53, 29.56, 29.59, 29.71, 29.73, 29.75, 29.80, 30.47, 31.11, 33.99, 48.41, 67.12,
69.52, 73.14, 73.64, 105.9, 114.3, 129.4, 139.3, 141.4, 153.2, 168.2. HRMS-ESI (m/z):
[M+H]+ calcd for C43H73NO, 684.5567, found 684.5535.
122
N-(2'-(R)-Chloro-l'-methyl-ethyl)-3,4,5-tri-(undec-10-enyloxy)-benzamide.
(4)
Thionyl chloride (0.10 mL) was added dropwise to a stirring solution 3 (0.31 g, 0.45
mmol) in THF (5 mL) at 0 °C under argon atmosphere. The solution was heated to reflux
at 75 °C for four hours and was then poured into water. The mixture was extracted with
dichloromethane, and washed successively with 0.5M NaOH (aq), water, and brine. The
organic layer was separated and dried over MgSO4.
Purification by column
chromatography (30:70 ethyl acetate:hexane) afforded the product as a white solid in
78% yield. 'H NMR (CDC13, 500 MHz) 8: 1.30-1.39 (m, 30H, (CH2) 5), 1.37 (d, J = 6.5
Hz, 3H, CH 3), 1.44-1.50 (m, 6H, CH2 ), 1.71-1.84 (m, 6H, CH 2), 2.02-2.07 (m, 6H, CH 2),
3.67 (dd, J = 11.5, 3.5 Hz, 1H, CH(H)), 3.83 (dd, J = 11.0, 4.0 Hz, 1H, CH(H)), 3.99-
4.03 (m, 6H, OCH2), 4.53-4.57 (m, 1H, CH), 4.92-4.94 (m, 3H, CHCH(H)), 4.98-5.02
(m, 3H, CHCH(H)), 5.77-5.86 (m, 3H, CHCH 2), 6.17 (d, J = 8.5 Hz, 2H, NH), 6.95 (s,
2H, Ar-H). ' 3C NMR (CDC13, 500 MHz) 6: 18.20, 26.26, 29.13, 29.16, 29.35, 29.39,
29.54, 29.57, 29.65, 29.73, 29.74, 29.77, 29.85, 30.49, 34.01, 34.03, 45.92, 49.79, 69.57,
73.68, 105.9, 114.3, 129.4, 139.4, 141.5, 153.3, 167.0. HRMS-ESI (mlz): [M+H] + calcd
for C43 H72 CINO4 702.5534, found 701.5531.
4-(R)-Methyl-2-[3,4,5-tri-(undec-10-enyloxy)-phenyl)]-4,5-dihydro-oxazole. (5) To a
suspension of NaH (0.06g, 2.46 mmol) in THF (5 mL) was added a solution containing 4
(0.15 g, 0.21 mmol) and 5 mL of THF. The mixture was stirred at room temperature
under argon atmosphere for 1 hour and was then poured over ice to quench excess NaH.
The residue was extracted with dichloromethane, and washed successively with 0.5M
NaOH (aq), water, and brine. The organic layer was separated and dried over MgSO4.
123
Removal of the solvents in vacuo afforded. the product (81%) as a clear oil. 'H NMR
(CDC13, 500 MHz) 6: 1.29-1.38 (m, 30H, (CH 2) 5), 1.34 (d, J = 6.5 Hz, 3H, CH 3), 1.421.48 (m, 6H, CH 2), 1.70-1.82 (m, 6H, CH 2), 2.01-2.05 (m, 6H, CH 2), 3.93 (t, J= 15.5 Hz,
1H, CH(H)), 3.97-4.01 (m, 6H, OCH2), 4.31-4.39 (m, 1H, CH), 4.49 (t, J = 17.0 Hz, 1H,
CH(H)), 4.92 (dd, J = 10.0, 1.5 Hz, 3H, CHCH(H)), 4.98 (dd, J = 17.5, 1.5 Hz, 3H,
CHCH(H)), 5.76-5.84 (m, 3H, CHCH2), 7.15 (s, 2H, Ar-H). ' 3C NMR (CDC13,500
MHz) : 21.63, 26.20, 26.21, 29.09, 29.12, 29.30, 29.35, 29.46, 29.51, 29.61, 29.69,
29.70, 29.72, 29.81, 30.45, 33.97, 33.99, 62.07, 69.22, 73.56, 74.23, 106.7, 114.3, 122.6,
139.3, 140.9, 153.0, 163.6. HRMS-ESI (m/z): [M+H] + calcd for C 43H 71NO 4 666.5456,
found 666.5452.
Polymerization Procedure (Route A)
A solution of monomer 5 (0.01 g, 0.02 mmol) and 5 mole % of Grubbs' catalyst'6
(0.15 mL, 5.0 x 10-3 M in CH 2Cl 2) is drop cast onto a glass slide under argon atmosphere.
The film is placed on a hot plate at 100 °C.for 12 hours, and the resulting material was
washed with dichloromethane. The green-tinted, free-standing film was easily peeled off
the glass slide and was dried under vacuum overnight.
Preparationof Catalyst(RouteA)
The Cu complexes were prepared by adding the polymer to a solution of excess
Cu(I)OTfotoluene,2 in toluene. The suspension was stirred at room temperature for 24
hours. The solid was filtered, washed successively with toluene, methylene chloride, and
124
methanol, and dried under vacuum overnight. Anal. Cald. 7.57% Cu, Found 7.15%, 94%
loading.
Polymerization Procedure (Route B)
To a solution of Cu(I)OTftoluenel,2 (0.01 g, 0.05 mmol) in a 1:1 mixture of
CHC13:CH3OH was added monomer 5 (0.04 g, 0.05 mmol). The solution was stirred at
room temperature for 30 minutes, and the solvents were removed by rotary evaporation.
To the resulting material was added a solution of Grubbs' catalyst (0.55 mL, 5.0 x 10-3M
in CH2C12), and the solution was drop cast onto a glass slide under argon atmosphere.
The film was placed on a hot plate for 60 hours at 100 °C. The resulting polymer was
washed with methylene chloride and dried under vacuum overnight. Anal. Cald. 7.57%
Cu, Found 5.86%, 77% loading.
GeneralProcedurefor AsymmetricCyclopropanation
Styrene (1000 eq.) was added to a stirring suspension of the appropriate catalyst
(1.0 eq.) in dry CH2C12 (1 mL) at 0
C under argon. A CH2C12 solution (3.5 mL) of ethyl
diazoacetate (100 eq.) was added slowly over 4 hours using a syringe pump. The
reaction mixture was allowed to warm to room temperature and was stirred an additional
16 hours, and then was quenched with a 10% aqueous solution of NH4C1 (10 mL). For
reactions with polymeric catalysts, the solution was filtered to remove heterogeneous
materials. The solution was diluted with diethyl ether, washed with water, brine, and
dried over MgSO4. The solvents were removed by rotary evaporation, affording the
crude product as a mixture of cis and trans. For solution phase reactions, the crude
125
mixture was purified by flash chromatography (5% ethyl acetate in hexane). The ratio of
trans to cis product was determined by H NMR spectroscopy. Enantiomeric excess was
determined by gas chromatography using a Cyclodex B column, 15 m x 0.25 mm x 0.25
Gm, hydrogen as carrier gas, 20 p.s.i, injector temperature 200 °C; oven temperature 200
°C; 100 C isotherm, retention times (min) 60.7 (1S,2R), 62.5 (R, 2S), 76.1 (1R, 2R),
77.7 (S,2S).
126
References
1
(a) Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds
Catalysis,
Springer-Verlag: Berlin-Heidelberg,
.; Comprehensive Asymmetric
1999. (b) Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley: New.York, 1994.
2
De Vos, D. E.; Vankelecom, I. F. J.; Jacobs, P. A., Eds .; Chiral Catalyst Immobilization
and Recycling, Wiley-VCH: Weinheim, 2000.
3
(a) Doyle, M. P.; Protopopova, M. N.;
Tetrahedron 1998, 54, 7919. (b) Ghosh, A. K.;
Mathivanan, P.; Cappiello, J.; Tetrahedron: Asymmetry 1998, 9, 1.
4
S
Nozaki, H.; Morituri, S.; Takaya, H.; Noyori, R.; Tet. Lett. 1966, 5239.
Fritschi, H.; Leutenegger, U.; Pfaltz, A.;
Helv. Chim. Acta 1988, 71, 1553. (b) Pfaltz,
A.; Acc. Chem. Res. 1993, 26, 339.
6
Lowenthal, R. E.; Abiko, A.; Masamune, S.; Tet. Lett. 1990, 31, 6005.
7
(a) Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul, M. M.; J. Am. Chem. Soc.
1991, 113(2), 726-728. (b) Evans, D. A.; Woerpel, K. A.; Scott, M. J.; Angew. Chem. Int.
Ed. Engl. 1992, 31, 430.
8
Muller, D.; Umbricht, G.; Weber, B.; Pfaltz, A.; Helv. Chim. Acta 1991, 74, 232.
9 For papers
discussing immobilization of copper-mediated asymmetric cyclopropanation
catalysts, please see: (a) Rechavi, D.; Lemaire, M.; Chem. Rev. 2002, 102, 3467, and
references therein. (b) Cornejo, A.; Fraile, J. M.; Garcia, J. I.; Gil, M. J.; Luis, S. V.;
Martinez-Merino, V.; Mayoral, J. A.; C. R. Chimie 2004, 7, 161, and references therein.
(c) Altava, B.; Burguete, M. I.; Garcia-Verdugo, E.; Luis, S. V.; Vicent, M. J.; Mayoral,
J. A.; Reactive & Functional Polymers 2001, 48, 25.
127
10 Burguete,
M. I.; Fraile, J. M.; Garcia, J. I.; Garcia-Verdugo, E.; Herrerias, C. I.; Luis,
S. V.; Mayoral, J. A.; J. Org. Chem. 2001, 66, 8893.
2 Grubbs' second generation catalyst is used. See Chapters 2 and 3.
1
2 Davankov, V. A.;
13
Pastukhov, A. V.; Tsyurupa, M. P.; J. Polym. Sci. B 2000, 38, 1553.
Estimated by comparison with swelling behavior of polystyrene-divinylbenzene
polymers in toluene. Rana, S.; White, P.; Bradley, M.; J. Comb. Chem. 2001, 3, 9.
'4 Dakovic, S.; Liscic-Tumir, L.; Kirin, S. I.; Vinkovic, V.; Raza, Z.; Suste, A.; SunJic,
V.; J. Mol. Catal. A 1997, 118, 27.
'5
See 2a in Chapter 3.
16
Grubbs' second generation catalyst = 1,3-(Bis(mesityl)-2-imidazolidinylidene)dichloro-
(phenylmethylene)(tricyclohexyl-phosphine)ruthenium.
128
Appendix 1: 1H and
13 C
NMR Spectra for Chapter 2
Chapter 2 NMR Spectra
__
.
.
.
1
_
.
I
8
I
7
6
I
- - - - -I I - -I - .' I .I ' . .. . . . . . . . . . . . .
5
4
2
3
1
pp
-20
ppa
'H NMR of 2a (500 MHz, CDC13 )
240
13C NMR
220
200
180
160
140
120
100
of 2a (500 MHz, CDC13)
130
80
60
40
20
0
Chapter 2 NMR Spectra
:H3
8
7
6
4
5
3
2
1
ppm
-20
pa
1H NMR of 2b (500 MHz, CDC13)
240
220
200
180
160
140
120
100
'3 C NMR of 2b (500 MHz, CDC13)
131
80
60
40
20
0
Chapter 2 NMR Spectra
0
I
L
I
8
7
6
5
IVA AM
V_
4
3
K
2
pP
1
'H NMR of 3a (500 MHz, CDC13)
i-l/ill//i/il I ---·im
Wr
W-q·r·
Wl,, r, ·s-w!~-1. lr~Bqml
, ,.~ .i
-, .. ll .
240
220
i·-
200
i
--
1 Fl
180
w mY ~lwl-I
s--v · I
160
.
III Jlfll41
,
140
.
I -----I IiiNii1III
i
I I
r11~l
Ill
I
120
I
I'l
.
I....
-
100
' 3C NMR of 3a (500 MHz, CDC13)
132
I....
,
I
80
I
,
J ..., I,l
I
60
.
...
I'll.
.
40
I... I ---~1l.l
20
.
..
· -··-·-···-·
[llll
0
l ..
lJ .
.
.
-20
l
llllllIll
.
I
pp
Chapter 2 NMR Spectra
I
1 11
A
· · · I·······___
I . .
I .
____
............................
p 'kf
X
$I
.X
7
I
7
6
5
4
3
..
2
.
1
.
pI
'H NMR of 3b (500 MHz, CDC13)
I
240
13C
,
220
.
.
a
·
I
I
,
,
.
.
~~Y~~' ~~~~""'"""~~~~~"'"
~~WII"'
200
180
160
140
.
..
120
100
NMR of 3b (500 MHz, CDC13)
133
.'
.
.
80
·
..
.
1WI
I-.,
.... .,...i
.
60
'
'1'
40
.
20
0
-20
ppM
Chapter2 NMR Spectra
'
II -
Li
I
·_
I . . .
8
7
I . '. '. ' T .
6
5
··
--
. '. Il
4
AA -
I '
3
2
L-- - .
I I 7I '
I I
v
-
1
ppm
-20
ppm
tH NMR of 4a (500 MHz, CDC13 )
240
13C NMR
220
200
180
160
140
120
.00
of 4a (500 MHz, CDC13 )
134
80
60
40
20
0
Chapter2 NMR Spectra
0
I
7
7
8
..
I.
6
6
l'
S
.I
4
2 ...
2
3
1
pp
]PI=
'H NMR of 4b (500 MHz, CDC13)
II
.
1.
YI·Y-.·-rn--I··-----·-- ~L·rn-YI--Y
240
13C
220
200
180
160
140
120
I
·_~ w r-mm
11-.
100
NMR of 4b (500 MHz, CDC1 3 )
135
o80
60
I
nI..
I -....Y.-.....
40
20
.--c...,I.L1.
0
-20
1.1..
ppM
Chapter 2 NMR Spectra
ii
I!
.
:
Z
z.
:i
:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lI ,
H~~~~~~~~~~~~~~~~~
-
.--
.------
-------------------------_ _ _ _ __C_~_
A-------O--s-------~-~----_----------~-----~~~-~~-~~--------;
7
7
6
5
-_~
-_r---m--_T----_r----r~-or-
---
-
3
4
i
It-i
U
~~
_
- -___r~
_-- _r---r-------_____r---r-
1
2
pIm
'H NMR of 5a (500 MHz, CDC13)
180
13C
160
140
120
100
NMR of 5a (500 MHz, CDC13)
136
80
60
40
20
ppm
Chapter 2 NMR Spectra
OH
I .,I
O
I
i 7
7
a
6
d
4
4
b
3
3
2
2
1
Nlmllllf I -uuc--
u·uulurcur-,.L.-··-··
]
ppm
ppMr
'H NMR of 5b (500 MHz, CDC13)
-~·r~u~u
uLI~IP
·Y-IY~II
.1'..I YLNI
ll1~I
A,
i
;
.. . ;I ; . 1.
,
240
220
I T l_.
'- ,"
200
. :·;.'
·
.;-
,III.
180o
II-1
160
TT.'
I
I I l;...... -
140
4
,.
120
-'-"-;
·--'-:
-· ·
100
13CNMR of 5b (500 MHz, CDC1 3)
137
''
TL
.
'
o80
T-j...
; ::
60o
.
; ._
-;
40
I
"
20
I
.. .
.I
0
I'--
.
.
-20
r .V-7
!J
I
ppm
Appendix 2: 1H and
13C NMR
Spectra for Chapter 3
Chapter 3 NMR Spectra
-- OCH,
0
I
I
L
. .
.
8
. . -
_
-e
. . . . . .
. .I . .
. .
6
7
5
5
I
i
i
. . . . .I .
. . . .
,
. . .I .
3
4
.
Pm
'H NMR of 2a (500 MHz, CDC13 )
1
.1. 160
160
.. ........
1
140
. ..
I2 .....
120
....
I
.....
....
....
I
80
100
13
C NMR of 2a (500 MHz, CDC13)
139
...
.....
I
60
....
I
.....
0.
40
....
I
....
20
....
pp
Chapter 3 NMR Spectra
7
6
5
4
3
2
i
pp
'H NMR of 2b (500 MHz, CDC13 )
160
140
120
100
80
' 3C NMR of 2b (500 MHz, CDC13 )
140
60
40
20
ppm
Chapter3 NMR Spectra
CH2OH
H2CHC(H
2 C)Oy O(CH2 )CHCH 2
6(CH2 )9 CHCH2
ii
I
_
n
in
1
II
w-
1I
iJL'
7I
7
6
5
4
2
3
1
pO
'H NMR of 3a (500 MHz, CDC13)
-____
-_~~·
160
13CNMR
~
$woo"_
140
I
IN .; _.~~
No
t..! [[Ld Irr all
. - - . . . . . . . . . .---I . . . . . . . ......... - . . . . . ..... . .60. . . . . . . . .40...
... . ..120
.... . . . . . . . ..... . . .100
-
Irlr
mr
120
l
..
-~~
...
100
80
of 3a (500 MHz, CDC13 )
141
-
L
60
40
-
20
-
-
II,"
ppm
Chapter3 NMR Spectra
OH
0
I
A
I
-
7
1H NMR
6
5
j hI
AAA
V
v
/
l
v
_
_-
I
3
4
I.
.
-
-
-
I
I
2
-
Pp
of 3b (500 MHz, CDC13 )
t
i :
i
: tf
: t
I
i :
I......
......
lI
-rl··W1YL
i-L·.r- -u ·L·i.ulc--r -rrr y, . l·L --LI-·.l
·
r-···· {wals-mmomm""
-----1--
..T
-
'-.
160
13C NMR
14
...
140
:
120
1.. i
.'
''0
100
''
80
of 3b (500 MHz, CDC13 )
142
Y·_-. -·
I-V'''...
...
60
II
II [I
I---·---~
I
'
40
ow
I
-
---
-
':
20
ppm
Chapter 3 NMR Spectra
rBr
H2 CHC(H2
CO)gO O(CC
O(CH 2)gCI
niC
L_J I
4
. . . . . . . . . . . . . . .i . . . . . . . . . . . . . . .3I8
7
6
-. .. . . .. .. . ..I . . .p .
5
4
3
2
1
p
100
80
60
40
20
ppl
'H NMR of 4a (500 MHz, CDC13 )
160
13
140
120
C NMR of 4a (500 MHz, CDC13 )
143
Chapter 3 NMR Spectra
I
I
I
-'·s
1
7
6
5
AAA
V
3
4
I
3
1
2
'H NMR of 4b (500 MHz, CDC13 )
-
-
--,-----'
ii.-I
.
160
. ,,.I
, I
140
--
-·--
I I I I
i,, ' '
--- I-
r-
---
'
---I-I----
I
120
100
I,
0o
13CNMR of 4b (500 MHz, CDC13)
144
I
'-· --·----
m ·
----- ---
I
. .
60
. .
woo
-N ---I
40
I --I
~~
ki l WANORMONVANNOO
l
-
I
--
r~-
.,,,
20
pp
Chapter 3 NMR Spectra
CO2 CH3
H2 CHC(H
2 C)O/
H2CHC(H
~
2 C)O
0'N
H2 CHC(H
2 C)O
9
02 CH3
i
S
i
A
. . . . . . . 3. . . . . . . . . . .I . . .
. . . . . . . . . .7 . . . . .I . . . . 5. .
8
7
6
I
JALAJ
5
4
I.
2
3
pp=
'H NMR of 5a (500 MHz, CDC13)
S.
a'
i!
z
:1
l
L
z
Ii
I
I
,
-
. '---'-
-
160
-
'
'-'
.....'
""'
140
"
"*
120
T'".
-'-T
........--.''."----t--'
-T
' .....--7-
100
'-'
--
80
' 3C NMR of 5a (500 MHz, CDC13)
145
'' '"'-''--,'-:"T-.-'--r-?-r-:
.---
60
-1..''.-.----.-'- -'--T
...
..
40
.
.
-- -
'-' '-r--v
20
--
ppM
Chapter 3 NMR Spectra
CO2 CH3
C02CH3
orc
I
--
-
.
IJ
I
-~
........................................
7
8I
7
/
AAJJl v
Ivv
i
4
6
5
6
I
I
2
3
1
Pa
'H NMR of 5b (500 MHz, CDC13)
·· _L·_L· -·- ·· I
Y'- -
Ad ....
I^rU
Ill..
160
13
I
~Aal
I
Y····_l··
.k
k~
i. & in
140
120
Ill_.Y.IY.-··IU·
~J i
r__
100
80
C NMR of 5b (500 MHz, CDC13 )
146
.
·1 1·····--I
1111 _
|d .
60
LIn_
_"
_IL ill
I
40
.,
U..
I I II
II I I YI1 L -·I_·1_···1-I
Has_
I
Ace
___
20
En
.,
it
pa
Chapter 3 NMR Spectra
CH20H
H2 CHC(H2 C)9 ,0
,
H2 CHC(
I
i , I , --~~I
7
i
iii
, i
6
5
·~~~~~~~~~~~~~~~~
i!
~ , I I4 ,
.
iL
7
6
.4
5
,
.
3
,
I I
2
I
I
1
I
I m
PPa
'H NMR of 6a (500 MHz, CDC13)
·
~r.~ ~r~..
-,~~
160
3C
.~yr~~~. ..
140
i.,
120
I·II
t,,,
YL
100
-
80
NMR of 6a (500 MHz, CDC13)
147
.
..
zi
I .
I.~-···rl
.·L.'
I1
60
40
20
ppm
Chapter 3 NMR Spectra
·
L
-_
7
_
_
Jl
S
6
AXk
.~~~~
..vx
1A ~-~~
I.
4
. - ---..
,
,~-
.-
,
j
---- ----~ - -
,
-
-
3
-~`
,
,
.
I
1
1
.
2
,
,
,
I
pp
'H NMR of 6b (500 MHz, CDC13)
1
--l-----·--Y--·--·--·--L-IY-------YL·_r
-VYCI·LY·Y·--I·-I-~" " -" ---'"'~lrl--·yyll
I-·?m-r--,,-,,-r
,,
..
1I0
13C NMR of
t.
1 0
140
120
100
6b (500 MHz, CDC13 )
148
,,,,,-,,
LI--
80
.1
.i
&i.i
.
I
_~·UY·-·
[
[
sO
.
.,,,
mI1L
__
_'""-
__P
POP
-
H2CHC(H
2 C)
Chapter 3 NMR Spectra
'
_ -
H2CHC(H2 C)90
H2 CHC(H2C)9
°
.i
,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.
I]
i
I
i
i
I
i~~~~
i
i
1
I
.-
'*
.- I .
.
.
..-'--
T"
-
-
T
---------
----
5
''-"---'
4
3
-- T-----------
r------------------
2
ppi
'H NMR of 7a (500 MHz, CDC13)
i
I
i
i
I
1..,I.....
160
140
z.Ii
Ii:
I:
z
II.1.Ii
-.''I
120
100
80
13
C NMR of 7a (500 MHz, CDC13)
149
60
40
20
ppm
Chapter 3 NMR Spectra
I
7
r
I 5I
I
T
7
6
I
5
'
I
I
4
I I
I
I
T
3
I
2
I
I
I
1
PI
'H NMR of 7b (500 MHz, CDC13)
· W·
;I
-'-!
-r--r·
·-..
60
:- 140
---120
160
140
120
13C NMR of
r.L·rru·
-.
.
100
ri
·*I~,-rW--~.
. ..-
80
7b (500 MHz, CDC13)
150
I .
60
Ii
.·i-~Ly
iWri~l~l-l~~
40
20
ppl
Appendix 3: 1H and
13 C NMR
Spectra for Chapter 4
Chapter 4 NMR Spectra
0
HOJ.N
NOH
H
0
H2gCI40
0C14H2
OC14 H2 9
Al
S
I
,
I
a
7I I
I I
I
I6
I
I
I
I
A)
AL
I
4
4
5
.1 -I
3
32 I
-
I
-
I1I
I 2I
I I. .
II
'H NMR of 3a (500 MHz, CDC13)
.. . 160 . . 140
. . . . 120
....
160
13
140
120
...
...
100
. ..
C NMR of 3a (500 MHz, CDC13)
152
so
60
60
40
.. ..
40
..
20
. ppm...
...
P
Chapter4 NMR Spectra
0
0
HO-NH,
N-,,OH
HI1
H
H2 9C14 O OC4H
29
0C 1 4 H 2 9
I~ I
1
illl
'
5
4
|
. - . . . . . . . . . . . . . . . . . . . .I . . . ...4
.
.
I.
8
.
.
Il
.
.
7
l.
l.
l.
l.X
6
IA) \ IC
L
-
3
3
. . .I . . . .
2
2
PPm
'H NMR of 3b (500 MHz, CDC13)
i
1iI
-..
'
.4. .
.
.
'-
160
i
i
- '
-
I
~L ·
·· --~· -_·_.~1_-I
~--_L-·_·-~
-Y--I--_--LL_-_I_
-_I-·~-·.-·_·~I--·
.
'
..... -- - -" ---
140
T
-
'-
120
'-T
-
1-0 ,
.
_---_·
.
100
~I
-.-
153
l
ld
'
' 20'
'
..
J.
r-
-
n
-
80
' 3C NMR of 3b (500 MHz, CDC13)
i
I--iL._ILI· · -··_~__.-·I---- --- I I
1-,- .
60
------:---.
..-0--
40
r-
..............
-
20
T !pp ---
P
Chapter 4 NMR Spectra
OH
8
7
6
5
4
3
2
1
pp
'H NMR of 4a (500 MHz, CDC13 )
i
i
i
i
i
I
I
160
13C
140
120
100
80
NMR of 4a (500 MHz, CDC13 )
154
60
I
40
20
ppm
Chapter4 NMR Spectra
OH
HN
0
t~O
N
HN
OH
I;
L,
i!
u:
1
I
' I
-
8
1H NMR
6.
6
7
--3
44
5
--3
-
I'\
) `
2
r-- r- --
-- p--r
1
ppm
of 4b (500 MHz, CDC13 )
I
i
II I
I
I
1
. I .1I
1.
1.,1,
I
.I
1.
.
"ujduI~IIiUUI~iLIujt LrYhIlW~kL",WA· YlU·iw
I . . .I . . . .
.
.
.
.
13C NMR
....
I . . . .'..
160
.
,
.-
. . ..
.
.1
.-
...
. . . .'
140
.
.
...
..
.I
120
I
I.
. . . .
100
155
ii
.
I
II
.
"
,
I
..
Ill]
II
,
,
.
I
...... .. .lillAlhL~~h"Ly
I . . . . . . . .'.
80
of 4b (500 MHz, CDC13 )
II
I ..
60
.
.'
'
..
40
. . I
..
. . . . . .
I . .. . " . '
20
ppm
I
Chapter 4 NMR Spectra
0 :
0
H
H
0o o
H2g140 OC14H29
I
I
__
.1
-4H29
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7
8I
6
4
S
I
2
3
ppM
'H NMR of 5a (500 MHz, CDC13 )
.
1. inmL··r
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I TT
T
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160
.1
I
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m
Y·L-L
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120
r r 100 r
140
120
100
80
Lr
'3 C NMR of 5a (500 MHz, CDC13)
156
I'YY
· ·--
60
·-
n·rr
I
_ m mm.
m_
I0I
40
20
ppm
Chapter4 NMR Spectra
o
o
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A
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3
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ppm
)
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.
13C NMR
_
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a
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120
80
100
of 5b (500 MHz, CDC13)
157
Chapter4 NMR Spectra
HN
N 0~~~~~~~•
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158
.
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Chapter 4 NMR Spectra
CI
HN
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:
i,
Ti
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8
6
7
_____
4
5
3
2
1
60
40
20
pp
'H NMR of 6b (500 MHz, CDC13)
t11.i
1,.
160
13 C NMR
140
120
80
100
of 6b (500 MHz, CDC13)
159
pp
Chapter4 NMR Spectra
*'N'
No
H29C140O
OC14H
OC14 H29
2
9
...................................
t
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'H NMR of 7a (500 MHz, CDC13 )
___
_
___
___
_
I
I
I
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160
140
120
I.
C________
.
100
80
' 3C NMR of 7a (500 MHz, CDC13 )
160
. . . . . .
______
.6 .0 . . .
60
l.aL .
1.-
. . . . . . . . . . . . . . . . . . . : .
40
20
ppm
Chapter4 NMR Spectra
N-
N
H29C
OC14H
C140
29
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8
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II
2
.
.
pPa
'H NMR of 7b (500 MHz, CDC13 )
z
.I
160
13
140
120
80
100
C NMR of 7b (500 MHz, CDC13)
161
60
40
i
20
ppm
Chapter4 NMR Spectra
ll
0
a
7
5
6
4
3
2
1
pm
'H NMR of 8a (500 MHz, CDC13)
-1 J,f
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r·LJ~Lyrr~r:,
;
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80
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162
..
l l~r·~.·Lluy-l
um-msIn
-
60
40
II I
.
lialfMs
Tv,5st-r~·l
-y-mr
20
A
ppm
Chapter4 NMR Spectra
0
-
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0
~
k~~
i,, 1I
7I
7
I
I
'6
6
I
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~~~
I
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,
,
I
2
I
1
ppm
'H NMR of 8b (500 MHz, CDC13)
z1.
160
,.
140
120
100
80
13
C NMR of 8b (500 MHz, CDC13 )
163
60
40
20
paP
Chapter 4 NMR Spectra
O's
sIO
OCH3
I
J
L.1
Y
L
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. .
16
1H
14
12
10
8
6
4
2
-0
pl
20
ppn
NMR of 9 (500 MHz, CDC13)
I
160
140
. .
120
I
100
80
13C NMR of 9 (500 MHz, CDC13 )
164
-1
60
40
Chapter 4 NMR Spectra
,-SC6H5
C14H290
C14H290-~ '-~SC
H
6 5
C14H29 0
l
.A
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l
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.
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4
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---
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+LI'J
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2
1
pp
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i
I
ii
I
It
1.
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,
i ~ f
160
.
I. I .
....
N
.
.
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l.
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.
120
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.
11
.
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I
100
.
.
- -
r
. . ..
80
'3C NMR of 11 (500 MHz, CDC13 )
165
a "''
inmm..,·..ulimi
'""
- - - - -- - -~ ~
.~
.
-
. . .I
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60
.
lp t -' r"w1--
.t .
.
I
40
I
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r
r_
miu
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rvf""
i
20
ppI
Chapter4 NMR Spectra
C,.H,.O
CSH
C6H5
l1
12
1s4
2
4
10
ppa
-0
'H NMR of 12 (500 MHz, CDC13)
i
I
t
iALL
1II
.
i:
1
J
1
-L-_--L·----_LI_--_-Y_-L·-I-·-·YY-YI---YL_-I-·_L..LI· YILY_·---·-I- rY i-: X
:___
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, .
...
160
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Ii
. -
...... ......-
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140
.
........
120
,-.
.
, ..._...
............. '
_._.'
,
80
100
NMR of 12 (500 MHz, CDC13)
166
..
''~~~~~~~~~~~~~
60
''
m
-.
...............
!-
-- -
-----------
40
I,
_
- T'
......-
--
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;=
...---
20
-
_
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.....
ppm
Appendix 4: 1H and
13C
NMR Spectra for Chapter 5
Chapter 5 NMR Spectra
-~,,~~
8
7
6
x-
5
4
n
3
2
2
ppm
'H NMR of 1 (500 MHz, CDC13)
YLLIIII·L--
-C·jJ Yir
160
13C NMR
IIIILYIIL·1Y(IL-
I 1 -. --
O
al-N
·-UILCLIUIWL-I·
~
--.--. 11---,r
--, - - I-- --. - - .·1r-'------
140
120
100
80
of 1 (500 MHz, CDC13)
168
l
_t
_
_"
wimmmomms"
Y LlrrlYIYU· II
T--,Tr.,,-~--,-..T'.,--,,-,
1--l~--,,--,--,-,T-r~·.--~-160
40
40
20
20
ppm
p
Chapter5 NMR Spectra
-
HN '
i
JLJ . V
. .
I
I
_I
- - - - -. -I -. - - --
^A
F,
J%.
7
1H
6
6
I-b
ll
YV,
.. . . . . . . . . . . . . . . .
,
A
5
I,
-
2
.
3
4
. . . . .
2
pa
NMR of 3 (500 MHz, CDC13)
-
-s
a
Ift"
lE
-
1
I
I Io
omr w.
.
.
.
.
.
..... . . . .80. . .
60 -- -- -- -.... . .I20
.
..... . . . . . . . 00
.
.
.
.
.
.
.
.
.
----- --
160
140
120
100
.
' 3C NMR of 3 (500 MHz, CDC13)
169
80
I
low
%mom-
..
. . .60. . . . . . . . . .40.....
60
40
Iw 4 u-41L~
......
M . ..
20
~rl~
..
ii
pI=
Chapter 5 NMR Spectra
- --
-
.
I-------
.
l
L
I I Ii
I I
7
i
' · L
,--~--·I
.
A
IJ
I,
I
. . . . . I I . . . . . . . . . I . . . . . .
6
5
4
3
2
1
ppm
20
ppm
'H NMR of 4 (500 MHz, CDC13)
160
140
120
100
80
'3 C NMR of 4 (500 MHz, CDC13)
170
60
40
Chapter 5 NMR Spectra
A
1
I 'I
6511'II
I I'
7
6
I
I
I
I
1I
I '
5
I
3
I
I
I
I
2
1
NM=
'H NMR of 5 (500 MHz, CDC13)
I
,-,.-._,.
.....
.....
13C
-l.Y,..-_..
"W"~LII
' , ' .. .
160
1m
.. _,,_
_I-L_--
· -Ly--l
IiN
.. .........................................................
1.0
10
100
80
so
1.40
1.20
100
NMR of 5 (500 MHz, CDC13)
171
mm
-
. 60
.0..
60
- ..
"-O ..,......
I wI.....,.....
a-msmorv NM
·.- ·
-------- ·.-----------
.. i.
.i .. . . ..20. .
40
PPa
Curriculum Vitae
Karen Villazor Martin
ACADEMIC INTERESTS
Organic chemistry, liquid crystals and liquid crystalline materials,
supramolecular chemistry, chiral materials, heterogeneous catalysis
EDUCATION
1999-2004
Massachusetts Institute of Technology
Candidate for Ph.D., Organic Chemistry
Advisor: Professor Timothy M. Swager
1995-1999
Boston College
Cambridge, MA
Chestnut Hill, MA
B.S., Chemistry, cum laude
Advisor: Professor Lawrence T. Scott
Thesis title: "Synthesis of [8]-Circulene"
RESEARCH EXPERIENCE
1999-2004
Massachusetts Institute of Technology
Cambridge, MA
Graduate Research Assistant
*Designed and synthesized several series of organometallic liquid crystals
for potential applications in chiral separation technology and asymmetric catalysis
* Applied a variety of techniques for the study of new liquid crystal phases
*Developed method to polymerize liquid crystal phases in situ using metathesis
2001-2002
Massachusetts Institute of Technology
Cambridge, MA
Chemistry Outreach Volunteer
*Conducted science experiments for local high school students
* Designed chemistry demonstrations to promote interest in scientific careers
1997-1999
Boston College
ChestnutHill, MA
Undergraduate Research Assistant
* Designed syntheses of curved polycyclic aromatic hydrocarbons
*Synthesized fragments of C60using flash vacuum pyrolysis
TEACHING EXPERIENCE
1999-2000
Teaching Assistant, Massachusetts Institute of Technology
*Lectured recitation sections for introductory organic chemistry
*Prepared recitation materials and graded problem sets and exams
172
TECHNICAL PROFICIENCIES
Synthetic organic chemist: small molecule purification and characterization
Materials design, synthesis, and characterization
Liquid crystal design, synthesis, and characterization
Variable temperature X-ray diffraction
Polarized microscopy of liquid crystal defect textures
Inert atmosphere and Schlenk techniques
PRESENTATIONS AND PUBLICATIONS
Villazor, K. R.; Swager, T. M. "Chiral Supramolecular Materials from Columnar Liquid
Crystals" Mol. Cryst. Liq. Cryst. 2004, 410, 775-781.
K. R. Villazor and T. M. Swager, "Materials with Supramolecular Chirality," presented at
the Gordon Research Conference on Liquid Crystals, New London, NH (16-20 June
2003).
K. R. Villazor and T. M. Swager, "Chiral Supramolecular Materials: Polymerization of
Chiral Columnar Liquid Crystals with Retention of Mesophase Order," presented at the
Materials Research Symposium, Boston, MA (2-6 December 2002).
K. R. Villazor and T. M. Swager, "Chiral Supramolecular Materials from Columnar
Liquid Crystals," presented to the Organic Division of the American Chemical Society at
the 224h Nation Meeting, Boston, MA (18-22 August 2002)
K. R. Villazor and T. M. Swager, "Incorporating Catalytic Function into Chiral
Supramolecular Materials," presented at the 19"h International Liquid Crystals
Conference, Edinburgh UK (30 June - 5 July 2002).
K. R. Villazor and T. M. Swager, "Chiral Supramolecular Materials from Columnar
Liquid Crystals," presented at the 19" International Liquid Crystals Conference,
Edinburgh UK (30 June - 5 July 2002).
K. R. Villazor and T. M. Swager, "Chiral Supramolecular Materials from Columnar
Liquid Crystals," presented at the Gordon Research Conference on Liquid Crystals, New
London, NH (24-29 June 2001).
K. R. Villazor and T. M. Swager, "Chiral Supramolecular Materials from Columnar
Liquid Crystals," presented at the 7" International Symposium on Metallomesogens,
Nagano, Japan (6-9 June 2001).
173
AWARDS AND HONORS
Wyeth Scholar, MIT (2003)
International Liquid Crystal Conference Poster Winner (2002)
Copithorne Scholar, Boston College (2002)
Scholar of the College, Boston College (1999)
Golden Key National Honor Society (1998)
AFFILIATIONS
Member, Materials Research Society
Member, American Chemical Society
174
Acknowledgements
First and foremost, I would like to thank my advisor, Tim Swager, for spoiling me
with innumerable acts of kindness over the past five years. From funding me when there
was no funding, to sending me to conferences all over the world, to giving me the
freedom to work on my own terms, his faith in me, and in all of his students, is both
relentless and inspiring. A true mentor and educator, he empowers his students to
develop their skills in ways that are unique to them, rather than some prescribed model.
He pursues science with levels of enthusiasm, imagination and fearlessness that never
cease to amaze, and yet, despite all his achievements and accolades, he begs for a level of
irreverence that his group members so gladly grant him. His unassuming, good-natured,
and humorous demeanor provides the foundation for a relaxed, open, and collaborative
atmosphere in the lab. I could not imagine experiencing (or surviving) graduate school in
any other environment. Tim is also someone who shows appreciation for the nontangible contributions of his students, those achievements that cannot be published in
literature nor itemized on a resume. I have received a compliment or even a bottle of
wine for simple group tasks that would have gone unacknowledged in any other setting.
For the past five years, I have felt privileged and immensely proud to be part of the
Swager group and have been grateful for every opportunity I had to represent Tim, and I
thank him for that. I can only hope that our paths will continue to cross.
Over the years, I have had the opportunity to work and play with so many
wonderful people in the Swager lab. I could not have asked for a warmer, friendlier, and
more welcoming environment to spend five years. We laughed together, teased each
other, harassed each other, shamelessly inhaled PPST pizza together, and battled mice
together. Oh, and we also did some research occasionally. I would like to thank a
number of former group members for giving me a lot of guidance back in the day, when
we had no windows in the lab but a big lunchroom table and lots of time to kill. Tyler
McQuade, my personal favorite, always pushed me to do better, to try harder, and to
think bigger that I had ever thought possible. Never have I enjoyed debating someone as
much as him, and I am forever grateful for his friendship.
J. D. Tovar always seemed to
put things in perspective for me in his own unique way, typically through the immortal
words of Snoop or Dre. Zhengguo ("ZZ") Zhu offered me a lot of synthetic advice, as
well as a freakishly strong arm to open various bottles and pry loose any of my stuck
glassware. Vance ("Wance") Williams made me feel right at home in the lab and helped
me get acclimated quickly. I am convinced that Hindy Bronstein followed me from BC
for the sole purpose of looking after me those first few years. Steffen Zahn happily
showed me how to use the CD and always had time to discuss my research, and in return
received much heckling for his red cargo pants and special VS deliveries. Shige
Yamaguchi had to endure working with me in such close quarters in old 18-126, but
generously acted as my personal travel agent for my trip to Japan.
I especially thank Alex Paraskos, who taught me everything about liquid crystals,
DSC, X-ray diffraction, optical microscopy, and Minesweeper, and answered every
question I had no matter how many times I asked them. I think our first conversations
consisted of my timid questions about the microscope, but over the years evolved into
cynical wisecracks and pestering comments about his lovely wardrobe. Alex eventually
became a permanent fixture in the "Somerville Shuttle" that was my car, and, oddly
175
enough, became just one of the girls. I wish him, Stacie, and their beautiful daughter
Georgia all the best, and hope they find reasons to make their way back to Boston.
And to Phoebe Kwan... what can I say?? We barely knew each other before we
joined the lab, but for the past five years, there is hardly an MIT memory that she is not a
huge part of. Although we have finally broken free, I will miss horrifying the Dali
waiters with how much food we can eat, taking "coffee breaks" at Legal Seafoods, and
sharing an inappropriate joke and a devious laugh every now and then. The lab may be a
little more politically correct upon our departure, but nonetheless, I am proud of the
impact that we have made.
Also, thanks to Gigi Bailey for her friendship and support, and for always making
sure my French was pronounced properly (Au Bon Pain?). Although I am confident that
my leaving the group does not equate to a goodbye by any means, I wish her all the best
for the remainder of her MIT run, and I remind her that "being nice is overrated." To
Juan (and all her heavenly baked goods), Paul (P-Diddy, no more broken bones), Craig
(what could have been...), "Professor" Sam (I'd join your group in a heartbeat), John
(Provo, Spain?), Andrew (don't worry, I'll still hit on you), Dahui (thanks for the free
therapy sessions), Hyuna, Scott, and the rest of the current Swager group, thanks for all
the laughs and memories.
I'm not sure I would have survived my first year at MIT without Aimee Crombie.
Attached at the hip from day one, we took comfort in the fact that we both felt entirely
clueless and were able to find humor in all situations. Although our careers have taken us
in different directions, I do not doubt that we will remain strong friends.
Thanks to Dave and Mark, and to Li Li for their help with all the DCIF
instruments. Also, thank you to our lab manager, Becky Bjork, for keeping the lab
running smoothly and for helping me with the random essentials that popped up over the
course of the past year.
Finally, there are a number of people outside of MIT who have helped me stay
afloat throughout this experience. Dr. Larry Scott at Boston College has had an indelible
impact on my life, from the day he pulled me out of his class and stuck me in front of a
hood. He made applying to MIT sound like a given rather than a reach for me. I owe a
special thank you to Jolynn, Janet, and Tara, for being a great group of cheerleaders who
kept the lab phone number on speed dial. To my parents, Rodney, Rose, Reese, Paul,
Gail, and Nick, I thank them for not knowing a blessed thing about chemistry but for
caring the most about what I did each day in lab. Finally, and most notably, thank you to
my husband Chris, who sees me at my best and at my very worst, but who sees me
through it all. Thanks for never, ever letting me quit. I love you and know I could not
have made it here without you. And now, onto the next chapter...
176