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

2. 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

3. 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

4. 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!

5. Activation energy
Sometimes catalysts cause one large energy barrier to be replaced by two smaller ones
Reaction passes through intermediate stage

6. Energy and Time
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(-DG*/RT) = [kbT/h]e(DS*/RT) e(-DH*/RT)
transition state
kforward
reactant
DG‡
DGreleased
product

7. 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.

8. Ziegler’s Discovery 1953 K. Ziegler, E. Holzkamp, H. Breil & H. Martin
Angew. Chem. 67, 426, 541 (1955); 76, 545 (1964).
+ Ni(AcAc) Same result
+ Cr(acac) White Ppt. (Not reported by Holzkamp)
+ Zr(acac) White Ppt. (Eureka! reported by Breil)

9. Natta’s Discovery Isotactic Syndiotactic
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
Isotactic
Syndiotactic
Ziegler and Natta won Nobel Prize in 1963

10. Mechanism of Coordination Polymerization
Al(CH2CH3)3 +
TiCl4
ClAl(CH2CH3)2 +
CH3CH2TiCl3

11. Mechanism of Coordination Polymerization
Al(CH2CH3)3 +
TiCl4
ClAl(CH2CH3)2 +
CH3CH2TiCl3
CH3CH2TiCl3 +
H2C
CH2
CH3CH2TiCl3
H2C
CH2

12. Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
CH3CH2TiCl3
H2C
CH2

13. Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
H2C
CH2
TiCl3
CH3CH2CH2CH2

14. Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2CH2CH2
H2C
CH2
TiCl3
CH3CH2CH2CH2

15. Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2CH2CH2
H2C
CH2
etc.

16. General Composition of Catalyst System
Group I – III Metals
Transition Metals
Additives
AlEt3
TiCl4 H2
Et2AlCl
EtAlCl2
a,g, d 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

17. Kaminsky Catalyst System W. Kaminsky et. al. Angew. Chem. Eng. Ed
Kaminsky Catalyst System W. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390, (1980); Angew. Chem. 97, 507 (1985)
Linear HD PE
Al:Zr = 1000
Activity = 107 g/mol Zr
Me = Ti, Zr, Hf
Atactic polypropylene
Activity = 106 g/mol Zr

18. Methylaluminoxane: the Key Cocatalyst
MAO
Proposed structure

19. Nature of active catalyst
Transition metal alkylation
MAO
Ionization to form active sites
Noncoordinating Anion, NCA

20. Alkene Hydrogenation with Wilkinson’s Catalyst

21. Mechanism

22. Enantiomerically Enriched Phosphines

23. Asymmetric Hydrogenation

24. Asymmetric Hydrogenation

25. Mechanism: Halpern, J. Science 1982, 217, 401-407.

26. Mechanism: Halpern, J. Science 1982, 217, 401-407.

27. Mechanism: Halpern, J. Science 1982, 217, 401-407.

28. Mechanism: Halpern, J. Science 1982, 217, 401-407.

29. Mechanism: Halpern, J. Science 1982, 217, 401-407.