chem 1140; catalysis general principles ziegler-natta olefin polymerization mechanism of...
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Chem 1140; Catalysis
• General Principles• Ziegler-Natta Olefin Polymerization• Mechanism of Hydrogenation with Wilkinson’s Catalyst• Asymmetric Hydrogenation
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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
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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
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Activation energy
• A catalyst lowers the energy of activation by:– Forcing molecules into
conformations that favor the reaction• I.e. the catalyst may re-orientate
molecules
• Change in free energy is identical to uncatalyzed reaction: the catalyst does not change the thermodynamic equilibrium!
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Activation energy
• Sometimes catalysts cause one large energy barrier to be replaced by two smaller ones– Reaction passes
through intermediate stage
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How do you correlate rate constants to activation barriers?
Arrhenius Equation
k (rate constant) = A e(-E/RT)
where A = “frequency factor”, and e(-E/RT) = activation energy
Eyring Absolute Rate Theory
k (rate constant) = [kbT/h]e(-G*/RT) = [kbT/h]e(S*/RT) e(-H*/RT)
Energy and Time
G‡
reactant
transition state
product
Greleased
kforward
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Ziegler-Natta Catalysis of Ziegler-Natta Catalysis of Alkene PolymerizationAlkene Polymerization
A typical Ziegler-Natta catalyst is a combination A typical Ziegler-Natta catalyst is a combination of TiClof TiCl44 and (CH and (CH33CHCH22))22AlCl, or TiClAlCl, or TiCl33 and and
(CH(CH33CHCH22))33Al.Al.
Many Ziegler-Natta catalyst combinations Many Ziegler-Natta catalyst combinations include a metallocene.include a metallocene.
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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 Ni100 atm110 C
CH3CH2CH=CH2 + +AlCl(Et)2
+ Ni(AcAc) Same result
+ Cr(acac) White Ppt. (Not reported by Holzkamp)
+ Zr(acac) White Ppt. (Eureka! reported by Breil)
TiCl4 1 atm20-70 C
Al(Et)3 + CH2CH2"linear"
Mw = 10,000 - 2,000,000
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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
Al(Et)2Cl
CH3 CH3 CH3 CH3
CH3
VCl4
Al(iBu)2Cl
CH3 CH3
O inCH3
- 78 CCH3
CH3
Isotactic
Syndiotactic
Ziegler and Natta won Nobel Prize in 1963
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
Al(CHAl(CH22CHCH33))33 ++ TiClTiCl44 ClAl(CHClAl(CH22CHCH33))22
++
CHCH33CHCH22TiClTiCl33
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
Al(CHAl(CH22CHCH33))33 ++ TiClTiCl44 ClAl(CHClAl(CH22CHCH33))22
++
CHCH33CHCH22TiClTiCl33
HH22CC CHCH22CHCH33CHCH22TiClTiCl33 ++
CHCH33CHCH22TiClTiCl33
HH22CC CHCH22
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
CHCH33CHCH22TiClTiCl33
HH22CC CHCH22
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22
HH22CC CHCH22
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22
HH22CC CHCH22
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22CHCH22CHCH22
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Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization
TiClTiCl33
CHCH33CHCH22CHCH22CHCH22CHCH22CHCH22
HH22CC CHCH22
etc.etc.
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General Composition of Catalyst SystemGroup I – III Metals
Transition Metals Additives
AlEt3 TiCl4 H2
Et2AlCl
EtAlCl2
TiCl3
MgCl2 Support O2, H2O
i-Bu3Al VCl3, VoCL3,
V(AcAc)3
R-OH
Phenols
Et2Mg
Et2Zn
Titanocene dichloride
Ti(OiBu)4
R3N, R2O, R3P
Aryl esters
Et4Pb (Mo, Cr, Zr, W, Mn, Ni)
HMPA, DMF
R C CH
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MeX
X
+ Al O
CH3
* *n
CH3
Al:Zr = 1000
Me = Ti, Zr, Hf
Linear HD PE
Activity = 107 g/mol Zr
Atactic polypropylene
Activity = 106 g/mol Zr
Kaminsky Catalyst SystemW. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390,
(1980); Angew. Chem. 97, 507 (1985)
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Methylaluminoxane: the Key Cocatalyst
Al(CH3)3 + H2Otoluene
0 C Al O
CH3
* *n
n = 10-20
O
Al
AlAl
CH3
OO
O
Al
OAl
OAl
AlCH3
CH3
Proposed structure
MAO
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Nature of active catalyst
Transition metal alkylation
Ionization to form active sites
MAO
Noncoordinating Anion, NCA
Cp2MeX
X+ Al O
CH3
* *n
Cp2MeCH3
X+ Al O
CH3
Al
X
Om
Cp2MeCH3
+Al O
CH3
Al
X
Om
X
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Alkene Hydrogenation with Wilkinson’s Catalyst
CO2Me
H2
cat. RhCl(PPh3)3
H2
cat. PtO2
CO2Me CO2Me
96:4
49:26
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Mechanism PPh3
Rh H
PPh3Cl
H
PPh3
Rh H
PPh3
Cl
H
R
RH
H
coordination
R
migratoryinsertion
reductiveelimination
oxidativeaddition
-PPh3
+PPh3
[RhCl(PPh3)2] RhCl(PPh3)3
H H
PPh3
Rh H
PPh3
HCl
R
R'
R'
R'
R'
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Enantiomerically Enriched Phosphines
PPh2
PPh2
HO
OH
DIOP
**
PPh2
PPh2
**
CHIRAPHOS DIPAMP
PH
PPhOMePh
Ph *
*N
PPh2
PPh2
O O
BPPM
**
PPh2
PPh2
BINAP
P P
R
RR
R
DuPHOS
PP
R
R
R
R
BPE
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Asymmetric Hydrogenation
CO2Me
NHAcR'
RH2
Me BPE Rh or
DuPHOS RhMe90 psi, PhH
96-99% ee
CO2Me
R
R'
NHAc
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CO2H
R1R3
R2
H2
96-99% ee
CO2H
R3
R2
R1
CO2H
MeO
97% ee (Naproxen)
NHO
CO2H
R3SiOH H
74% de (Thienamycin)
Me Me H 91
H Me 87
H Me Ph 85Ph H H 92H HOCH2 Me 93H CH3 COOCH2CMe 95
R1 R2 R3 ee
Ru(OCOR)2 (binap)
Asymmetric Hydrogenation
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Mechanism: Halpern, J. Science 1982, 217, 401-407.
PRh
S
P S NH
OPh
MeO2C
equilibriummust be fast for high ee
majork'
k'-1
MeO2C NH
ORh
LL Ph
minor
<5%
diastereoisomers
fastH2k2
rate limitingstep
very slow
H2k'2
NH
CO2Me
ORh
LL Ph
>95%
k'-1
k'
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Mechanism: Halpern, J. Science 1982, 217, 401-407.
major
MeO2C NH
ORh
LL Ph
minor
<5%
diastereoisomers
fastH2k2
rate limitingstep
very slow
H2k'2
NH
CO2Me
ORh
LL Ph
>95%
MeO2C HN
RhLL
Ph
H
O
k2 > k'2 ≈>103
NH CO2Me
HRh
LLPh
O
H H
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Mechanism: Halpern, J. Science 1982, 217, 401-407.
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Mechanism: Halpern, J. Science 1982, 217, 401-407.
MeO2C HN
RhLL
Ph
H
ONH CO2Me
HRh
LLPh
O
k3k'3
NHMeO2C
Ph
O
RhH
SL L
CO2MeHNO
Rh PhH
SL L
H H
HH
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Mechanism: Halpern, J. Science 1982, 217, 401-407.
NHMeO2C
Ph
O
RhH
SL L
k4
Ph
NH
MeO2C
H O
(R) > 98%
O
NH
CO2Me
Ph
H
(S) < 2%
k'4
CO2MeHNO
Rh PhH
SL L
ee lower at high H2 pressure - k'2 increasedlower atlow temp - equilibrationdecreased. Majordiast. accumulates
HH