Lecture 20 February 18, 2011
Transition metals:Pd and Pt
Nature of the Chemical Bond
with applications to catalysis, materials
science, nanotechnology, surface science,
bioinorganic chemistry, and energy
William A. Goddard, III, wag@wag.caltech.edu
316 Beckman Institute, x3093
Charles and Mary Ferkel Professor of Chemistry,
Materials Science, and Applied Physics,
California Institute of Technology
Teaching Assistants: Wei-Guang Liu <wgliu@wag.caltech.edu>
Caitlin Scott <cescott@caltech.edu>
Ch120a-Goddard-L20
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Last time
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Pt goes from s1d9 to d10 upon reductive elimination
thus product stability is DECREASED by 12 kcal/mol
Using numbers
from QM
Ch120a-Goddard-L20
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Pd goes from s1d9 to d10 upon reductive elimination
thus product stability is INCREASED by 20 kcal/mol
Using numbers
from QM
Pd and Pt would be ~ same
Ch120a-Goddard-L20
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Thus reductive elimination from Pd is stabilized by an extra 32
kcal/mol than for Pt due to the ATOMIC nature of the states
The dramatic stabilization of the product by 35 kcal/mol
reduces the barrier from ~ 41 (Pt) to ~ 10 (Pd)
This converts a forbidden reaction to allowed
Ch120a-Goddard-L20
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Summary energetics
Conclusion the atomic
character of the metal can
control the chemistry
Ch120a-Goddard-L20
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Examine bonding to all three rows of transition metals
Use MH+ as model because a positive metal is more
representative of organometallic and inorganic complexes
M0 usually has two electrons in ns orbitals or else one
M+ generally has one electron in ns orbitals or else zero
M2+ never has electrons in ns orbitals
Ch120a-Goddard-L20
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Ground states of neutral atoms
Sc
(4s)2(3d)
Sc+
(4s)1(3d)1
Ti
(4s)2(3d)2
Ti+
(4s)1(3d)2
V
(4s)2(3d)3
V+
(4s)0(3d)3
Cr
(4s)1(3d)5
Cr+
(4s)0(3d)5
Mn
(4s)2(3d)5
Mn+
(4s)1(3d)5
Fe
(4s)2(3d)6
Fe+
(4s)1(3d)6
Co
(4s)2(3d)7
Co+
(4s)0(3d)7
Ni
(4s)2(3d)8
Ni+
(4s)0(3d)8
Cu
(4s)1(3d)10 Cu+
Ch120a-Goddard-L20
(4s)0(3d)10
Sc++
Ti ++
V ++
Cr ++
Mn ++
Fe ++
Co ++
Ni ++
Cu++
© copyright 2011 William A. Goddard III, all rights reserved
(3d)1
(3d)2
(3d)3
(3d)4
(3d)5
(3d)6
(3d)7
(3d)8
(3d)10
8
Bond energies MH+
Re
Mo
Au
Cr
Cu
Ag
Ch120a-Goddard-L20
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Exchange energies:
Mn+: s1d5
For high spin (S=3)
A[(d1a)(d2a)(d3a)(d4a)(d5a)(sa)]
Get 6*5/2=15 exchange terms
5Ksd + 10 Kdd
Responsible for Hund’s rule
Ksd Kdd
Mn+ 4.8
19.8 kcal/mol
Tc+ 8.3
15.3
Re+ 11.9 14.1
Form bond to H, must lose half
the exchange stabilization for
the orbital bonded to the H
A{(d1a)(d2a)(d3a)(d4a)(sdba)[(sdb)H+H(sdb)](ab-ba)}
sdb is a half the time and b half the time
Ch120a-Goddard-L20
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Ground state of M+ metals
Mostly s1dn-1
Exceptions:
1st row: V, Cr-Cu
2nd row: Nb-Mo, Ru-Ag
3rd row: La, Pt, Au
Ch120a-Goddard-L20
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Size of atomic orbitals, M+
Valence s similar for all
three rows,
5s biggest
Big decrease from
La(an 57) to Hf(an 72
Valence d very small
for 3d
Ch120a-Goddard-L20
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Charge transfer in MH+ bonds
electropositive
1st row all
electropositive
2nd row:
Ru,Rh,Pd
electronegative
3rd row:
Pt, Au, Hg
electronegative
electronegative
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Ch120a-Goddard-L20
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1st row
Ch120a-Goddard-L20
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Schilling
Ch120a-Goddard-L20
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Steigerwald
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2nd row
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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3rd row
Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Ch120a-Goddard-L20
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Physics behind Woodward-Hoffman Rules
For a reaction to be allowed, the number of bonds must be
conserved. Consider H2 + D2
2 bonds
TS ? bonds
2 bonds
To be allowed must have 2 bonds at TS
How assess number of bonds at the TS. What do the
dots mean? Consider first the fragment
Have 3 electrons, 3 MO’s
Have 1 bond. Next
consider 4th atom, can
nonbonding
Bonding
antibonding
we Ch120a-Goddard-L20
get 2 bonds?
1 elect
2 2011
elect
© copyright
William A. Goddard
III, all rights reserved 0 elect 36
Can we have 2s + 2s reactions for transition
metals?
2s + 2s forbidden for organics
X
2s + 2s forbidden for organometallics?
?
Cl2Ti
Cl2Ti
Me
Me
Ch120a-Goddard-L20
Cl2Ti
Cl2Ti
Me
?
Me
Cl2Ti
Cl2Ti
Me
© copyright 2011 William A. Goddard III, all rights reserved
Me
37
Physics behind Woodward-Hoffman Rules
Bonding
2 elect
nonbonding
1 elect
antibonding
0 elect
Have 1 bond. Question, when add 4th atom, can we get 2 bonds?
Can it bond to the nonbonding orbital?
Answer: NO. The two orbitals are orthogonal in the TS, thus the
reaction is forbidden
38
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
Now consider a TM case: Cl2TiH+ + D2
Orbitals of reactants
GVB orbitals
of TiH bond
for Cl2TiH+
GVB orbitals
of D2
Ch120a-Goddard-L20
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39
Is Cl2TiH+ + D2  Cl2TiD+ + HD allowed?
Bonding
2 elect
nonbonding
1 elect
antibonding
0 elect
when add Ti 4th atom, can we get 2 bonds?
Now the bonding orbital on Ti is d-like. Thus at TS have
Answer: YES. The two orbitals can have high overlap at the TS
orthogonal
in the TS,©thus
the reaction is allowed
Ch120a-Goddard-L20
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40
GVB orbitals at the TS for
Cl2TiH+ + D2  Cl2TiD+ + HD
Ch120a-Goddard-L20
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GVB orbitals for the Cl2TiD+ + HD product
Note get phase change
for both orbitals
Ch120a-Goddard-L20
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Follow the D2
bond as it
evolves into the
HD bond
Ch120a-Goddard-L20
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43
Follow the TiH
bond as it
evolves into the
TiD bond
Ch120a-Goddard-L20
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44
Barriers small, thus allowed
Increased d character in
bond  smaller barrier
Ch120a-Goddard-L20
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Are all MH reactions with D2 allowed? No
Example: ClMn-H + D2
Here the Mn-Cl bond is very polar
Mn(4s-4pz) lobe orbital with Cl:3pz
This leaves the Mn: (3d)5(4s+4pz), S=3 state to bond to the H
But spin pairing to a d orbital would lose
4*Kdd/2+Ksd/2= (40+2.5) = 42.5 kcal/mol
whereas bonding to the (4s+4pz) orbital loses
5*Ksd/2 = 12.5 kcal/mol
As a result the H bonds to (4s+4pz), leaving a high spin d5.
Now the exchange reaction is forbidden
Ch120a-Goddard-L20
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46
Show reaction for ClMnH + D2
Show example reactions
Ch120a-Goddard-L20
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47
Olefin Metathesis
2+2 metal-carbocycle reactions
Diego Benitez, Ekaterina Tkatchouk, Sheng Ding
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
48
OLEFIN METATHESIS
Catalytically make and break double bonds!
R1
R1
+
R2
2
R2
R1
R2
Mechanism: actual catalyst is a metal-alkylidene
R2
M
R1
Ch120a-Goddard-L20
R2
R2
M
M
R3
R1
R3
R1
R3
© copyright 2011 William A. Goddard III, all rights reserved
49
Ru Olefin Metathesis Basics
Ch120a-Goddard-L20
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Applications of the olefin metathesis reaction
Small scale synthesis
to industrial polymers
Acc. Chem. Res. 2001, 34, 18-29
Ch120a-Goddard-L20
bulletproof resin
http://www.pslc.ws/macrog/pdcpd.htm
51
© copyright 2011 William A. Goddard III,
all rights reserved
History of Olefin Metathesis Catalysts
Ch120a-Goddard-L20
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52
Common Olefin Metathesis Catalysts
Ch120a-Goddard-L20
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Well-defined metathesis catalysts
R
iPr
N
(F3C)2MeCO
Mo
(F3C)2MeCO
iPr
Ph
CH 3
CH 3
1
Schrock 1991
alkoxy imido
molybdenum
complexa
Bazan, G. C.; Oskam, J. H.;
Cho, H. N.; Park, L. Y.;
Schrock, R. R. J. Am.
Chem. Soc. 1991, 113,
6899-6907
Ch120a-Goddard-L20
Cl PCy3 Ph
Ru
Cl
PCy3
Mes N
Cl
Cl
R
Ru
N Mes
Ph
PCy3
R=H, Cl
2
Grubbs 1991
ruthenium
benzylidene
complexb
Wagener, K. B.;
Boncella, J. M.; Nel,
J. G. Macromolecules
1991, 24, 2649-2657
3
Grubbs 1999
1,3-dimesityl-imidazole-2-ylidenes
P(Cy)3 mixed ligand system”c
Scholl, M.; Trnka, T. M.; Morgan,
J. P.; Grubbs, R. H. Tetrahedron
Lett. 1999, 40, 2247-2250.
© copyright 2011 William A. Goddard III, all rights reserved
54
Examples
of Common
Second
Generation
Examples
2nd Generation
Grubbs
Metathesis Grubbs-type
Catalysts
Metathesis Catalysts and Mechanism Overview
Mes N
N Mes
Cl
Mes N
Ru
Cl
PCy3
N Mes
Cl
Mes N
Ru
Cl
Ph
Ru
O
Cl
i-Pr
slow initiating catalyst
N Mes
Cl
fast-initiating catalyst
Py
Ph
ultra-fast-initiating catalyst
General mechanism of Metathesis
IMes
Ru
Cl
Cl
Ph
L
IMes
Cl
Ru
Cl
IMes
Cl
Ru
R3
R1
L
R2
Initiation
R
Cl
IMes
IMes
Cl R
Ru
Cl
R3
R2
Cl
Propagation
Ru
Cl
R3
+
R1
R2
Ch120a-Goddard-L20
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55
Schrock and Grubbs catalysts make olefin metathesis practical
Schrock catalyst –
very active, but destroys
many functional groups
Grubbs catalyst –
very stable, high functional
group tolerance, but not as
reactive as Schrock
Catalysts contain many years of evolutionary improvements
Ch120a-Goddard-L20
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new
Ch120a-Goddard-L20
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57
Structure Grubbs Carbene Catalyst
Ccarbene
RuCl2
Calkylidene
PCy3 or P(iPr)3
experimental structurea
of 3 (with R=H)
predicted structure of 5 (a model of 3)
from QM (DFT-B3LYP).
(a) For R=H: Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett.
1999, 40, 2247-2250. (b) For R=Cl: Ding, S; Scholl, M.; Grubbs, R. H. unpublished
results.
Ch120a-Goddard-L20
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58
Structure Grubbs Carbene
Catalyst
Ru-Carbene 2.109
CH2-Ru-Carb 100.5 º
CH2
Cl(1)-Ru-Cl(2) 174.5º
Ru-CH2 1.813
P(iPr)3
Ch120a-Goddard-L20
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59
Compare QM and (Xray)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Bond Lengths (Å)
Ru-CH2
1.813 (1.841) Ru-P
2.506 (2.419)
Ru-Carbene 2.109 (2.069) Ru-Cl(2)
2.471 (2.383)
Ru-Cl(1)
2.467 (2.393) C(1)-N(1) 1.370 (1.366)
Carb-N(2) 1.370 (1.354) C(2)-C(3) 1.351 (1.296)
Bond Angles (deg)
CH2-Ru-Carb 100.5 (99.2) CH2-Ru-Cl(2) 90.0 (87.1)
Carb-Ru-Cl(2) 87.8 (86.9) CH2-Ru-Cl(1) 94.3 (104.3)
Cl(1)-Ru-Cl(2) 174.5 (168.6) CH2-Ru-P
93.9 (97.1)
Carb-Ru-P
165.6 (163.2) Cl(1)-Ru-P
89.4 (89.9)
Carb-N(1)-C(2) 111.2 (112.1) N(1)-C(1)-N(2) 104.0 (101.0)
Important Torsion Angles (deg)
Cl(1)-Ru-CH2-H
177.3 N(1)-Carb-Ru-Cl
75.7
Carb-Ru-CH2-H
88.6
N(1)-Carb-Ru-CH2 169.7
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
60
Ru-Methylidene Double Bond
z
x
Cz=Cpp
Ruxz
Ru dxz-C pzRu-C Pi bond
Cs
3B
1
CH2
Ru2xx-yy-zz
Ru dx2 - C sp2 Ru-C Sigma bond
CH2 is triplet state with singly occupied s and p orbitals get
spin pairing s bond to© Ru
dx2 and
p bond to Ruxz III, all rights reserved
Ch120a-Goddard-L20
copyright
2011 William A. Goddard
61
Ru-Methylidene Double Bond
CH2 is triplet state with singly occupied s and
p orbitals get spin pairing s bond to Ru dx2
and p bond to Ruxz
z
x
Ru-C Sigma bond (covalent)
Ru dx2 - C sp2
Ru-C Pi bond (covalent)
Ru dxz - C pz
Bond dist. Theory Experiment
Ru-CH2
1.813 1.841
Ru-Carbene
2.109 2.069
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
62
Carbene sp2-Ru dz2 Don-Accep Bond
Planar N with 3
s bonds and 2 e
in pp orbital
Planar N with 3
s bonds and 2 e
in pp orbital
Singlet methylene or carbene with
2 s bonds to C and 2 electrons in
Cs lone pair but empty pp orbital
Ru-Carbene Sigma donor bond
(Lewis base-Lewis acid)
C sp2
Ru dz2
Singlet Carbene (Casey
Carbene or Fisher carbene
Bond dist. Theory Experiment
stablized by donation of N p
Ru-CH2
1.813 1.841
lone
pairs, leads to LUMO
63
2.109
Ch120a-Goddard-L20
© copyright 2011 William A.Ru-Carbene
Goddard III, all rights
reserved 2.069
Carbene sp2-Ru dz2 Don-Accep Bond
Ru-Carbene Sigma donor bond
(Lewis base-Lewis acid)
C sp2
Ru dz2
Carbene p-p LUMO)
Antibonding to N p lone pairs
Ch120a-Goddard-L20
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64
Ru-dyz - Carbene py Don-Accep Bond
Ru dyz Lewis Base
to Carbene py pi acid
stabilizes the RuCH2
in the xy plane
This aligns RuCH2 to
overlap incoming olefin
Ru dyz Lone Pair (Lewis base-Lewis acid)
Ru dyz
Carbene py LUMO
Carbene p-p LUMO)
Antibonding to N p lone pairs
Ch120a-Goddard-L20
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65
Ru LP and Ru-CH2 Acceptor Orbitals
Ru dxy Lone Pair
Want perpendicular to C-Ru-C plane
Avoid overlap with NCN bonds
Orients Methylidene
Perpendicular to Plane
Ru-CH2 p* (antibonding) LUMO
Acceptor for olefin p bond
Orients Olefin Perpendicular to plane
Ch120a-Goddard-L20
Because RuCH2 is
perpendicular to
plane, the empty
antibonding orbital
overlaps the
bonding pi orbital
of the incoming
olefin
© copyright 2011 William A. Goddard III, all rights reserved
66
Ru Electronic Configuration
Z
Ru(CH2)Cl2(phosphine)(carbene)
Ru-Cl bonds partially ionic (50% charge transfer),
consider as RuII (Cl-)2
H
H
H
H
II
1
1
2
2
0
Ru : (dxz) (dx2) (dxy) (dyz) (dz2)
Ru (dx2)1 covalent sigma bond to
Mes N
N Mes
Mes N
N Mes
singly-occupied sp2s orbital of CH2
H
Cl
H
Cl
X
C
Ru
1
C Ru
Ru (dxz) covalent pi bond to
H
Cl
H
Cl
H
H
singly-occupied ppz orbital of CH2
PCy3
C C
H
H
( the CH2 is in the triplet or methylidene form)
5'
5
2
Ru (dxy) nonbonding
Ru (dyz)2 overlaps empty carbene py orbital stabilizing RuCH2 in xy plane
Ru (dz2)0 stabilizes the carbene and phosphine s donor orbitals
RuCH2 p* (antibonding) LUMO overlaps the p bonding orbital of incoming olefin
stabilizing it in the confirmation required for metallacycle formation.
Ch120a-Goddard-L20
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67
Generally Accepted Mechanism
E or Z olefin
products
Ch120a-Goddard-L20
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68
Originally Postulated Mechanisms
Ch120a-Goddard-L20
Tetrahedron 2004, 60, 7117-7140
© copyright 2011 William A. Goddard III, all rights reserved
69
Why Ruthenium Metathesis?
• One of the simplest, most general and widely used
C=C forming reactions.
• Exceptional functional group tolerance.
• Excellent catalyst stability and bench-top handling
ease.
• Large catalyst family allows for reaction
optimization.
• Clean reactions with minimal waste and byproducts.
Ch120a-Goddard-L20
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70
Ch120a-Goddard-L20
(C&E news, 80(51) 2002)
© copyright 2011 William A. Goddard III, all rights reserved
71
Chauvin mechanism most consistent with experiment
Chauvin nonpairwise model
pairwise model
Tetrahedron 60
(2004) 717-7140
experiment
Postulates metallocyclobutane intermediate
Ch120a-Goddard-L20
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72
Metal [2+2] cycloaddition is thermally allowed
All-carbon [2+2]
cycloaddition is
forbidden
H
H
H
H
H
H
HOMO
LUMO
d orbital has different
phase overlaps; other
orbitals available
(more details to follow in
upcoming lectures!)
Woodward-Hoffman rules still apply, but d-orbitals now participate
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
73
Design of the 2nd generation Grubbs catalyst
Enchancing
dissociation
works wonders
s donor makes trans-phosphine
more labile
bulky groups block
bimolecular decomposition
JACS 123(27) 2001 6549
Ch120a-Goddard-L20
falls off more easily
to accomodate olefin
© copyright 2011 William A. Goddard III, all rights reserved
74
More active catalyst makes functionalized cross-metathesis possible
R
+
R
EWG
EWG
High E/Z selectivity:
EWG
R
minor product
“These findings further demonstrate
the high activity and functional group
compatability of [the new catalyst],
which significantly expands the range of
olefins that can participate in the olefin
metathesis reaction”
Can join even more reactive
functional groups together
Ch120a-Goddard-L20
JACS 2000 122 3783-3784
75
© copyright 2011 William A. Goddard III, all rights reserved
Simplified Models
In order to gauge the implications of potential steric and
electronic effects, we performed calculations of the following
model systems:
1. Methatesis of propene with ethylidene
2. Methatesis of 3-buten-2-ol with 2-hydroxy
propylidene
Ch120a-Goddard-L20
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76
2-Butene Formation from Propene
20
18.7
18
15.8
16
15.7
∆G (kcal/mol)
14
13.1
12
A
9.9
10
C
B
TSAB
14.8
13.6
E
13.6
TSBC
Z
12.1
9.8
8
6
Slight kinetic and thermodynamic
preference for E isomer.
Predicted E/Z ratio ~6:1
4
2
0
Ru + S
0.0
Ch120a-Goddard-L20
Ru + P
4.5
3.4
77
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
3-Hexene-2,5-diol Formation from 3-Buten-2-ol
20
TSAB
18
16.0
16
TSBC
E
15.3
Z
14
B
∆ G (kcal/mol)
12
A
8.3
10
C
7.4
8
5.6
6
4
2
8.9
4.9
4.6
Ru + S
0
0.0
-2
-4Ch120a-Goddard-L20 -3.2
2.0
0.7 kcal thermodynamic preference
for E isomer
Predicted E/Z ratio ~4:1
0.1
Ru + P
-1.2
78
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
Sterics and Electronics
TSBC
E R=OH
TSAB
18.7
Z R=OH
16.0
15.3
E R=CH3
20
18
16
15.8
14
A
∆ G (kcal/mol)
12
15.7
14.8
Z R=CH3
13.6
9.8
8
8.9
8.3
7.4
6
5.6
4.5
4
2
13.1
C
13.6
12.1
9.9
10
B
4.9
4.6
Ru + S
3.4
2.0
0.1
0
Ru + P
0.0
-2
-4Ch120a-Goddard-L20 -3.2
-1.2
79
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
Sterics and Electronics
TSAB E
TSAB Z
H-bonding lowers energy of only specific isomers
(E or Z) with the correct geometry.
H-bonding could be used
selectivity in the reaction.
Ch120a-Goddard-L20
to
induce
some
80
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
RCM Experimental Results
Nonenolides are important antimalarial drug precursors. Both isomers
are needed separately. A selective synthesis avoids $$ separations.
After extensive cooking, a selective synthetic strategy was finally found.
PMB= p-methoxy-benzoate (COC6H4OMe)
Mohapatra, Ramesh, Giardello,
Chorghade,
Gurjar,
Grubbs
Letters
48, 2007,
2621–2625.
Ch120a-Goddard-L20
© copyright
2011
William
A. Tetrahedron
Goddard III,
all rights
reserved
81
Ring Closing Metathesis of Desmethyl Nonenolides
30
25
19.6
20
15.5
14.7
∆ G (kcal/mol)
15
9.0
10
10.5
10.0
TSAB
5
8.2
B
TSBC
4.1
1.1
0
-5
-10
3.6
0.0
0.8
Ru + S
C
-8.2
Ch120a-Goddard-L20 A
Ru + P
E
Z
Ring preorganization and H-bonding to the catalyst (Ru-Cl∙∙∙∙HO) in the E isomer raises the barriers of the E pathway, while
intramolecular O-H∙∙∙∙O-H stabilize the intermediates in the Z
isomer.
Predicted E/Z ratio 1:221
82
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
Transition State Comparison
TSAB E
TSBC E
2.13
2.12
2.21
2.17
TSAB Z
TSBC Z
2.08
2.09
2.15
2.1
Lower Energy TS
Ch120a-Goddard-L20
TSAB Z is 5.4 kcal/mol lower in energy
than TSAB E and TSBC Z is 4.2 kcal/mol
lower in energy than TSBC E as a
consequence of the double H-bond in
83
the 2011
E metallacycle
(B E). III, all rights reserved
B3LYP/LACVP**
© copyright
William A. Goddard
Acetate Protected Nonenolide RCM
30
E
28.2
26.7
Z
25
25.8
24.1
TSBC
TSAB
∆ G (kcal/mol)
20
18.7
C
18.3
15
15.2
15.1
A
10.0
10.6
Ru+P
10
10.0
7.5
B
5
Ru + S
0
Ch120a-Goddard-L20
0.0
Transition state energies are very close, as well as
the stability of the intermediates. Therefore, a 1:1
84
mixture
is2011
expected.
B3LYP/LACVP**
© copyright
William A. Goddard III, all rights reserved
Acetate Protected Transition States
TSBC E
TSAB E
2.13
2.19
2.27
2.13
TSAB Z
TSBC Z
2.08
2.16
2.10
2.05
Slight steric encumbrance between the protecting group and the catalyst
destabilizes both isomers in similar relative amounts.
Ch120a-Goddard-L20
85
B3LYP/LACVP**
© copyright 2011 William A. Goddard III, all rights reserved
Method Comparison: Acrylonitrile XM H2IMesRu
35
30
25
∆G (kcal/mol)
20
B3LYP E
15
B3LYP Z
CH2Cl2 E
10
CH2Cl2 Z
MO6 E
5
MO6 Z
0
-5
1:2 E/Z Observed Experimentally
-10
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
86
Method Comparison: H2IMes Ru Phenylallylidene
35
30
25
∆G (kcal/mol)
20
B3LYP E
15
B3LYP Z
CH2Cl2 E
10
CH2Cl2 Z
MO6 E
5
MO6 Z
0
-5
Ch120a-Goddard-L20
-10
6:1 E/Z Observed Experimentally
© copyright 2011 William A. Goddard III, all rights reserved
87
Ligand Comparison: Thiazole Ru Phenylallylidene
35
30
25
B3LYP E
∆G (kcal/mol)
20
B3LYP Z
CH2Cl2 E
15
CH2Cl2 Z
10
MO6 E
MO6 Z
5
0
-5
3:1 E/Z Observed Experimentally
-10
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
88
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
89
Olefin metathesis reactions are attractive transformations in organic in
synthesis for their functional group tolerance (acids, amines, alcohols, etc.),
extremely mild conditions, bench stable and commercially available catalysts
:
N-Heterocyclic Carbene Ligand
Ch120a-Goddard-L20
Thiazol-2-ylidene Ligands
© copyright 2011 William A. Goddard III, all rights reserved
90
cis-1,4-diacetoxy-2-butene
1-acetoxy-4-phenyl-2-butene
Experimental ratio E/Z : 10/1 * Standard Cross Metathesis
(XM) substrate
Acrylonitrile
Experimental ratio E/Z: 1/2†
* Organometallics 2006, 25, 5740-5745
† Eur.
J. Org. Chem. 2003, 2225© copyright 2011 William A. Goddard III, all rights reserved
Ch120a-Goddard-L20
91
cis-1,4-diacetoxy-2butene
1-acetoxy-4-phenyl-2-butene
Experimental ratio* E/Z 4/1
Acrylonitrile
* J. Am.
Chem. Soc; 2008; 130(7);
2234-2245.
Ch120a-Goddard-L20
© copyright
2011 William A. Goddard III, all rights reserved
92
Kinetic Ratio
Thermodynamic Ratio
Ar
True Thermod. Ratio
isallnot
J. Am.
Chem. Soc; 2008; 130(7);
2234-2245.
Ch120a-Goddard-L20
© copyright
2011 William A. Goddard III,
rightsreached
reserved
93
Calculated ratio E/Z
B3LYP 26:1
M06 18:1
Calculated ratio E/Z
B3LYP 3:1
M06 1.5:1
It is believed that metathesis is a thermodynamically
controlled reaction, however, if this were a true, different
catalysts would produce the exact same E/Z Ratio.
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
94
Coordination of olefin to
Ru alkylidene
The activated
species is 14e4-coordinate
(highly coord.
unsaturated)
Ch120a-Goddard-L20
Formation of metallacycle
intermediate.
[2+2] cycloaddition of an
olefin double bond
[2+2]
Retrocyclization
© copyright 2011 William A. Goddard III, all rights reserved
95
10
Product-Substrate exchange is rate
determining step
[Ru]+P
6
ΔG‡ (Kcal/mol)
2
-2
5.2
3.5
[Ru]+S
0.0
-2.4
TSAB
-6
TSBC
-5.7
-8.2
-10
B
A
-12.7
-13.3
-10.9
-10.2
-12.5
-14
-14.6
-18.8
-18
-22
-16.2
-21.9
-21.6
-23.1
-13.7
-15.0
-19.2
-20.2
-24.9
C
-21.4
-21.8
E B3LYP
Z B3LYP
E MO6
Z MO6
-26
Ch120a-Goddard-L20
-30
-28.1
© copyright 2011 William A. Goddard III, all rights reserved
96
8
TSAB
TSBC
6.4
6
4.3
ΔG‡ (Kcal/mol)
4
A
B
4.3
3.8
C
3.7
1.9
2
2.1
1.3
1.1
2.9
1.7
0.8
1.5
1.3
0
-0.4
0.0
-1.7
-2
-1.9
-4
C2 symmetric
ligand
-5.1
Potential Energy Surface for metallacycle formation
-6
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
E B3LYP
Z B3LYP
E M06
Z M06
97
Exp. E/Z = 10:1
cis-1,4diacetoxy-2butene
Rate Limiting TS
E/Z
Ratio
Intermediate
B3LYP
M06
A
24:2
7:1
B
139:1 223:1
C
8:1
2:1
TSBC E product
E
90:1
12:1
a
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
98
More Z-Selective Ligand
Ru+P
20
TSBC
16
15.0
13.3
13.2
12
TSAB
C
ΔG‡ (Kcal/mol)
8
4
0
12.0
A
-0.8
-4.3
0.3
0.2
-4
-8
3.6
2.5
Ru+S
0.0
B
-3.5
-1.2
-12
-15.6
-16
-14.2
-7.0
-7.5
-6.1
-7.1
-16.9
-15.5
-19.7
-20
-24
Ch120a-Goddard-L20
-19.8
-17.4
E
Z
E MO6
Z MO6
-22.9
© copyright 2011 William A. Goddard III, all rights reserved
99
C1 symmetric ligand
TSBC
14.4
14
13.2
12
10
8.6
ΔG‡ (Kcal/mol)
8
A
TSAB
B
8.2
E
C
Z
6
3.6
E MO6
4
Z MO6
2
0
1.4
1.4
0.0
-1.3
0.1
-2
-3.1
-4
-6
0.4
-2.3
-4.1
-4.1
-8
Ch120a-Goddard-L20
-1.8
-4.9
-6.0
-7.2
© copyright 2011 William A. Goddard III, all rights reserved
100
Exp. E/Z = 4:1
cis-1,4diacetoxy-2butene
Rate Limiting TS
E/Z Ratio
Intermediate
B3LYP M06
A
1:194 1:1065
B
90:1 208:1
C
5:1
1:18
TSBC E product
E
7:1
2:1
a
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
101
TSAB
TSBC
Acrylonitrile
Exp. E/Z = 1:2
E/Z Ratio
Intermediate
B3LYP M06
A
1:2
1:2
B
6:1
3:1
C
1:2
1:3
Ea
12:1
1:7
Ch120a-Goddard-L20
TSAB Z
© copyright 2011 William A. Goddard III, all rights reserved
102
8
TSBC
6
6.0
TSAB
4
ΔG‡ (Kcal/mol)
2
0
A
Ru + S
0.0
2.1
B
1.8
0.4
-0.4
-2
-6.1
-9.4
-9.2
-11.5
-12
-14
-10.3
-13.1
-13.5
-13.9
-10.7
-15.4
-16
-18
Ch120a-Goddard-L20
2.1
1.8
1.1
0.4
-5.0
-3.2
-8
-10
C
-1.6
-4
-6
Ru+ P
5.0
-16.4
© copyright 2011 William A. Goddard III, all rights reserved
E
Z
E MO6
Z MO6
103
Acrylonitrile
14
A
B
E/Z Ratio
B3LYP M06
13430:
1:26
1
31:1 4:1
12
TSAB
10
8.3
ΔG‡ (kclal/mol)
Intermediate
TSBC
8
A
6
5.6
8.2
7.9
12.1
B
6.5
E
Z
E MO6
Z MO6
11.1
6.5
4.5
4
2
0
-2
0.9
1.3
0.0
-1.9
-1.6
-2.4
-4
C
1:1.5
5:1
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
-6
C
-3.1
-3.4
-3.9
-4.9
104
2 plausible intermediates for Ruthenium
Metathesis
Trans
Cis
Trans is direct product of initiation.
All previous mechanistic studies have assumed Trans.
Either could explain propagation
Trans
Cis
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
105
Previous mechanisms have assumed that the Ru-Cl bonds remain trans
throughout the reaction  “trans” products
To probe the mechanism
designed a ligand
of the cis-trans
ChlorideGrubbs
Isomerization
Equillibriu
that could go into either cis or trans Cl structure
IMes
IMes
Cl
Ru
Cl N
trans
0
Ru
N Cl
Cl
K = 3.5 *
G = -0.78 Kcal mol-1
For this constrained
ligand, cisexperimental
is more
cis
stable than trans by 0.8 kcal/mol
But cis initiates more rapidly than trans
6.7
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
106
Use DFT QM to determine Structures and
Energetics for Isomerization between cis and trans
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
107
Validation of DFT calculations
Cl
N
L Cl
Ru
Cl
L Cl
Ru
L Cl
Ru
Cl
N
4
4d
G (kcal/mol)
N
L Cl
Ru
N Cl
5d
5
Gas phase
631G**
6311G**++
0
0
14.95
14.64
23.03
22.07
6.78
8.17
Solvent phase
631G**
6311G**++
0
0
13.55
11.67
18.83
17.62
-1.12
-0.70
CH2Cl2:
ε=9.1,
R0=2.4A
Experiment: K=3.5  ΔG = -0.78 kcal/mol
Theory: ΔG = -0.70 kcal/mol
Experiment: benzene solvent only observe trans  ΔG > 2 kcal/mol
Theory: ΔG = 2.2 kcal/mol (ε=2.3, R0=2.6A)
Theory: polar solvent (ε>20) leads to 100% cis
Thus can tune stereochemistry of product by solvent polarity
NotCh120a-Goddard-L20
tested experimentally
© copyright 2011 William A. Goddard III, all rights reserved
108
Method Comparison in the Prediction of Stable
Isomers of Ru Olefin Metathesis Catalysts in Solution
Geometry
SP Energy
Structure
B3LYP B3LYP
M06-L
B3LYP
M06
M06
Relative Energy (kcal mol−1)
B3LYP B3LYP M06-L
B3LYP
M06
M06
Relative Abundance
Experiment
1H-NMR
5a
0.0
0.0
0.0
9.8
15.9
95.9
10
5b
0.36
0.44
2.21
5.4
7.6
2.3
4
5c
0.29
0.78
2.82
6.0
4.3
0.8
2
5d
1.35
1.64
2.70
1.0
1.0
1.0
1
5e
0.25
0.02
4.88
6.5
15.4
0.0
N.O.
5f
1.67
1.98
5.61
0.6
0.6
0.0
N.O.
5g
1.70
2.57
7.76
0.6
0.2
0.0
N.O.
M06 leads to slightly better relative free energies (G298) (by 2 to 3 kcal/mol)
and relative abundances of isomers of 5 in CH2Cl2 at 298K than B3LYP
109
Ch120a-Goddard-L20
© O'Leary,
copyright
2011 William
A. Goddard
III,J.allAm.
rights
reserved
Stewart, Benitez,
Tkatchouk,
Day, Goddard,
Grubbs,
Chem.
Soc., 2009, 131, 1931–1938.
Method Comparison in the Prediction of Stable
Isomers of Ru Olefin Metathesis Catalysts in Solution
Geometry
B3LYP
B3LYP
M06-L
B3LYP
B3LYP
M06-L
SP Energy
B3LYP
M06
M06
B3LYP
M06
M06
Structure
Relative Energy (kcal mol−1)
3a
0.13
0.0
0.0
3c
0.0
0.37
0.45
3b
0.75
0.66
1.15
3d
0.40
0.04
0.95
Experiment
1H-NMR
Relative Abundance
2.9
1.2
7.0
6.7 (syn)
1
1
1
1 (anti)
Benitez, Tkatchouk,
Goddard
2009, 28, 2643–2645.
M06 leads to slightly better (0.5 kcal/mol)
relative
freeOrganometallics
energies (G
298) and
relative abundances of isomers in CH2Cl2 at 298K than B3LYP
110
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
Analysis
of results
nalysis of the cis-trans
Chloride
Isomerization Mechanism
alysis of the cis-trans Chloride Isomerization Mechanism
Trans
Cis
N N Mes
Mes
N
N
N N Mes
Mes Cl
Mes
Mes
N
N
Cl
Mes
Mes
Cl
Cl
Ru Ru is a much
is afaster
much
faster
initiator
than
initiator than
Ru
Ru
Cl N
Cl N
N Cl
N Cl
The strong dependence on solvent polarity results from the enormous
differenceL in
the dipoleL moment
from Lthe
wavefunctions of the
Cl
Cl
Cl
L Cl
L Cl
L
L Cl
Cl
Ru
Ru
Ru
L C
complexes (in methylene
chloride)
Ru
Ru
Ru
Cl
Cl
Ru
Cl N
Ru
N
NCl
and 12.4 Debye
for cisN Cl
Cl N 1.5 DebyeClfor trans
N Cl
N
N
This difference arises from the polarity in the Ru-Cl bonds, which
cancel in the trans geometry.
0 difference15in polarity translates
23
7 solvation
This marked
to very different
14
methane
0
19
-1
energies
calculated
15
ase
0
23
7
adius
= 2.4A
14.8 kcal for14trans and 22.7 kcal
for cis,
oromethane
0
19
-1
Energy
14 Kcal
20 Kcal
ent radius = 2.4A
which dramatically increases the relative stability of the cis chloride
111
Ch120a-Goddard-L20 14 Kcal
© copyright 2011 William A. Goddard III, all rights reserved
nstructure.
Energy
20 Kcal
Analysis of cis-trans Cl isomerization
Chloride Isomerization
Mechanism
Rates
of
metathesis
initiation
Isomerization
Equilibrium Between
cis
and trans Chloride
Analysis
of the cis-t
Ruthenium Olefin Metathesis Catalysts from Quantum
Mechanics Calculations
Cis
Trans
nalysis of the cis-trans NChloride
Isomerization Mechanism
N
Mes
Mes
Mes
Cl
initiates much slower than
h experimentally
faster initiator than
Mes N
Cl
N Mes
Cl
Ru
N
romethane
t radius = 2.4A
L Cl
Ru
Cl
L Cl
Ru
Mes N N N Mes
Cl
L Cl than
is a much faster initiator
Ru
Ru
N Cl
Cl
N
Cl
E
Cl
N Mes
Cl
Ru
N
i
N
L Cl
Ru
N Cl
Cl
se
Ru
N Cl
N
L Cl
N Ru
Cl N
0
L Cl
Ru
Cl
Cl
trans
L Cl
Ru
N
L Cl
Ru
L Cl
RuN Cl
Cl
N
N
11.7
15
23
0
Trans 11.7 barrier
14
0
19
L Cl
Ru
cis
Cl N
17.7
23
19
7
-1
L Cl
Ru
N Cl
-0.7 kcal/mol0
Gas phase
PBF/Dichloromethane
7
Cis 18.4 barrier
-1
Initiation Energy
 = 9.1, solvent radius = 2.4A
0
Thus expect cis initiation should be much slower than trans:
n Energy
14 Kcal
20 Kcal
agrees
with
experiment
Ch120a-Goddard-L20
©
copyright
2011
William
A.
Goddard
III, all rights reserved
20 Kcal
14 Kc
112
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
113
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
114
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
115
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
116
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
117
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
118
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
119
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
120
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
121
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
122
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
123
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
124
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
125
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
126
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
127
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
128
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
129
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
130
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
131
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
132
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
133
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
134
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
135
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
136
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
137
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
138
Ch120a-Goddard-L20
© copyright 2011 William A. Goddard III, all rights reserved
139
Ch120a-Goddard-L20
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