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

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

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). + Ni(AcAc) Same result + Cr(acac) White Ppt. (Not reported by Holzkamp) + Zr(acac) White Ppt. (Eureka! reported by Breil)

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

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 TiCl3 CH3CH2CH2CH2CH2CH2 H2C CH2 TiCl3 CH3CH2CH2CH2

Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2CH2CH2 H2C CH2 etc.

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

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

Methylaluminoxane: the Key Cocatalyst MAO Proposed structure

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

Alkene Hydrogenation with Wilkinson’s Catalyst

Mechanism

Enantiomerically Enriched Phosphines

Asymmetric Hydrogenation

Asymmetric Hydrogenation

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

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

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

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

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