Chem 1140; Catalysis
• General Principles
• Ziegler-Natta Olefin Polymerization
• Mechanism of Hydrogenation with Wilkinson’s
Catalyst
• Asymmetric Hydrogenation
Catalysis
• Catalysts increase reaction rate without
themselves being changed
• Can accelerate a reaction in both directions
• Do not affect the state of equilibrium of reaction
– simply allow equilibrium to be reached faster
Activation energy
• Molecules must be
activated before they
can undergo a reaction
– Reactants must absorb
enough energy from
surroundings to
destabilize chemical
bonds (energy of
activation)
• Transition state
– Intermediate stage in
reaction where the
reactant molecule is
strained or distorted but
the reaction has not yet
occurred
Activation energy
• A catalyst lowers the
energy of activation by:
– Forcing molecules into
conformations that favor
the reaction
• I.e. the catalyst may reorientate molecules
• Change in free energy is
identical to uncatalyzed
reaction: the catalyst does
not change the
thermodynamic
equilibrium!
Activation energy
• Sometimes catalysts
cause one large
energy barrier to be
replaced by two
smaller ones
– Reaction passes
through intermediate
stage
Energy and Time
How do you correlate rate constants to activation barriers? transition state
Arrhenius Equation
k (rate constant) = A e(-E/RT)
kforward
reactant
where A = “frequency factor”, and
e(-E/RT) = activation energy
DG‡
DGreleased
Eyring Absolute Rate Theory
k (rate constant) = [kbT/h]e(-DG*/RT) = [kbT/h]e(DS*/RT) e(-DH*/RT)
product
Ziegler-Natta Catalysis of
Alkene Polymerization
A typical Ziegler-Natta catalyst is a combination
of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and
(CH3CH2)3Al.
Many Ziegler-Natta catalyst combinations
include a metallocene.
Ziegler’s Discovery
• 1953 K. Ziegler, E. Holzkamp, H. Breil & H. Martin
• Angew. Chem. 67, 426, 541 (1955); 76, 545 (1964).
Al(Et)3 + NiCl2
CH3CH2CH=CH2 + Ni + AlCl(Et)2
100 atm
110 C
+ Ni(AcAc)
+ Cr(acac)
+ Zr(acac)
Same result
White Ppt. (Not reported by Holzkamp)
White Ppt. (Eureka! reported by Breil)
Al(Et)3 + TiCl4
CH2CH2
1 atm
"linear"
20-70 C Mw = 10,000 - 2,000,000
Natta’s Discovery
•
•
•
1954 Giulio Natta, P. Pino, P. Corradini, and F. Danusso
J. Am. Chem. Soc. 77, 1708 (1955) Crystallographic Data on PP
J. Polym. Sci. 16, 143 (1955) Polymerization described in French
CH3
TiCl3
CH3
Al(Et)2Cl
CH3
VCl4 - 78 C
Al(iBu)2Cl
O
in
CH3
CH3
CH3
Isotactic
CH3 CH3
CH3
CH3
Syndiotactic
Ziegler and Natta won Nobel Prize in 1963
CH3
Mechanism of Coordination Polymerization
Al(CH2CH3)3 + TiCl4
ClAl(CH2CH3)2
+
CH3CH2TiCl3
Mechanism of Coordination Polymerization
Al(CH2CH3)3 + TiCl4
ClAl(CH2CH3)2
+
CH3CH2TiCl3
CH3CH2TiCl3 + H2C
CH2
CH3CH2TiCl3
H2C
CH2
Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
CH3CH2TiCl3
H2C
CH2
Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
H2C
CH2
TiCl3
CH3CH2CH2CH2
Mechanism of Coordination Polymerization
CH3CH2CH2CH2CH2CH2
TiCl3
H2C
CH2
TiCl3
CH3CH2CH2CH2
Mechanism of Coordination Polymerization
CH3CH2CH2CH2CH2CH2
TiCl3
H2C
etc.
CH2
General Composition of Catalyst System
Group I –
III Metals
AlEt3
Et2AlCl
EtAlCl2
i-Bu3Al
Et2Mg
Et2Zn
Et4Pb
Transition Metals
Additives
TiCl4
a,g, d TiCl3
MgCl2 Support
VCl3, VoCL3,
V(AcAc)3
Titanocene dichloride
Ti(OiBu)4
H2
O2, H2O
(Mo, Cr, Zr, W, Mn,
Ni)
HMPA, DMF
R C CH
R-OH
Phenols
R3N, R2O, R3P
Aryl esters
Kaminsky Catalyst System
W. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390,
(1980); Angew. Chem. 97, 507 (1985)
Me
X +
X
CH3
*
Al O
*
n
Al:Zr = 1000
Linear HD PE
Activity = 107 g/mol Zr
Me = Ti, Zr, Hf
CH3
Atactic polypropylene
Activity = 106 g/mol Zr
Methylaluminoxane: the Key Cocatalyst
CH3
toluene
Al(CH3)3 + H2O
0C
*
Al O
n = 10-20
CH3
Al
O
Al
O
O
MAO
O
Al
Al
O
O
Al
Al
Al
CH3
CH3
Proposed structure
*
n
Nature of active catalyst
X
Cp2Me
CH3
+
*
X
Al O
*
n
MAO
Cp2Me
CH3
CH3
+
Al O
X
CH3
Cp2Me
CH3
+
Transition metal
alkylation
X
Al
m
O
X X
Al O Al O
m
Ionization to
form active sites
Noncoordinating Anion, NCA
Alkene Hydrogenation with Wilkinson’s Catalyst
H2
cat. RhCl(PPh3)3
CO2Me
CO2Me
CO2Me
H2
96:4
cat. PtO2
49:26
Mechanism
oxidative
addition
PPh3
H Rh H
Cl
PPh3
H H
coordination
R'
R
-PPh3
[RhCl(PPh3)2]
RhCl(PPh3)3
+PPh3
R' H
R
reductive
elimination
PPh3
R
R'
H
H
R
H
R'
Cl Rh H
PPh3
PPh3
Cl
Rh H
PPh3
migratory
insertion
Enantiomerically Enriched Phosphines
PPh2
Ph *
PH
H
O
*
*
O
PPh2
PPh2
H
DIOP
*
*
Ph
PPh2
R
R
P
R
BINAP
P*
PhOMe
O
DIPAMP
CHIRAPHOS
PPh2
PPh2
*
N *
PPh2
P
R
DuPHOS
O
BPPM
R
R
P
P
R
R
BPE
PPh2
Asymmetric Hydrogenation
R
R'
CO2Me
NHAc
H2
Me BPE Rh or
Me DuPHOS Rh
90 psi, PhH
R'
CO2Me
R
NHAc
96-99% ee
Asymmetric Hydrogenation
R2
CO2H
R3
R1
Ru(OCOR)2 (binap)
H2
R2
CO2H
R3
R1
96-99% ee
CO2H
MeO
97% ee (Naproxen)
R3SiO
R1
R2
R3
ee
Me
Me
H
91
Me
87
Ph
H
Me
COOCH2CMe
85
92
93
95
H
H H
CO2H
NH
O
74% de (Thienamycin)
H
Ph
H
H
Me
H
HOCH2
CH3
Mechanism:
P
P
Rh
Halpern, J. Science 1982, 217, 401-407.
Ph
O
S
S
MeO2C
k'
minor
L
MeO2C
fast
Ph L
Rh
O
N
H
H2
k2
N
H
k'
k'-1
k'-1
diastereoisomers
>95%
<5%
equilibrium
must be
fast for high ee
major
L Ph L
Rh
O
N
H
rate limiting
step
H2
k'2
CO2Me
very
slow
Mechanism:
Halpern, J. Science 1982, 217, 401-407.
major
minor
Ph L
Rh
O
L
MeO2C
fast
N
H
H2
k2
H
Rh
MeO2C HN
>95%
<5%
N
H
CO2Me
H2
k'2
rate limiting
step
very
slow
H
Ph
L
L
L Ph L
Rh
O
diastereoisomers
H
O
k2 > k'2  >103
L
Ph
H
O
L
Rh
NH
CO2Me
Mechanism:
Halpern, J. Science 1982, 217, 401-407.
Mechanism:
H
Halpern, J. Science 1982, 217, 401-407.
H
Ph
L
L
Rh
MeO2C HN
L
H
O
Ph
H
L
Rh
O
NH
k'3
k3
S
Ph
H
MeO2C
CO2Me
L
L
Rh
L
H
NH
O
S
H Rh
O
HN
L
Ph
H
CO2Me
Mechanism:
S
Ph
H
MeO2C
Ph
MeO2C
L
L
Rh
Halpern, J. Science 1982, 217, 401-407.
L
H
S
O
k4
ee lower at high H2 pressure - k'2 increased
lower at
low temp - equilibration
decreased. Major
diast. accumulates
N
H
(R) > 98%
Ph
H
CO2Me
H Rh
NH
H O
L
O
HN
k'4
Ph
O
N
H
H
CO2Me
(S) < 2%