Chem 1140; Catalysis General Principles Ziegler-Natta Olefin Polymerization Mechanism of Hydrogenation with Wilkinson ’ s Catalyst Asymmetric Hydrogenation

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 re-orientate

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

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

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.

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

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

Mechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination PolymerizationMechanism of Coordination Polymerization

Al(CHAl(CH22CHCH33))33 ++ TiClTiCl44 ClAl(CHClAl(CH22CHCH33))22

++

CHCH33CHCH22TiClTiCl33


Al(CHAl(CH22CHCH33))33 ++ TiClTiCl44 ClAl(CHClAl(CH22CHCH33))22

++


HH22CC CHCH22CHCH33CHCH22TiClTiCl33 ++


HH22CC CHCH22



HH22CC CHCH22

TiClTiCl33

CHCH33CHCH22CHCH22CHCH22


TiClTiCl33


TiClTiCl33


HH22CC CHCH22


TiClTiCl33


HH22CC CHCH22

TiClTiCl33

CHCH33CHCH22CHCH22CHCH22CHCH22CHCH22


TiClTiCl33

CHCH33CHCH22CHCH22CHCH22CHCH22CHCH22

HH22CC CHCH22

etc.etc.

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

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)

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

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

Alkene Hydrogenation with Wilkinson’s Catalyst

CO2Me

H2

cat. RhCl(PPh3)3

H2

cat. PtO2

CO2Me CO2Me

96:4

49:26

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'

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

Asymmetric Hydrogenation

CO2Me

NHAcR'

RH2

Me BPE Rh or

DuPHOS RhMe90 psi, PhH

96-99% ee

CO2Me

R

R'

NHAc

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

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'


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



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


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

Documents

Chem 1140; Catalysis General Principles Ziegler-Natta Olefin Polymerization Mechanism of Hydrogenation with Wilkinson ’ s Catalyst Asymmetric Hydrogenation