Catalysis Principles and Applications
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This document discusses several topics related to catalysis: 1. It provides an overview of general principles of catalysis such as catalysts increasing reaction rate without changing themselves and lowering the activation energy of reactions. 2. It describes Ziegler-Natta catalysis for alkene polymerization using titanium and aluminum catalyst systems to produce polymers like polyethylene. 3. It outlines the mechanism of asymmetric hydrogenation using chiral phosphine ligands and rhodium catalysts to produce enantiomerically enriched products.
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Catalysis Principles and Applications
- 1. 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. 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) Energy and Time DG‡ reactant transition state product DGreleased kforward
- 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). Al(Et)3 +NiCl2 Ni 100 atm 110 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 atm 20-70 C Al(Et)3 + CH2CH2 "linear" Mw = 10,000 - 2,000,000
- 9. 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 in CH3 - 78 C CH3 CH3 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 H2C CH2CH3CH2TiCl3 + CH3CH2TiCl3 H2C CH2
- 12. Mechanism of Coordination Polymerization CH3CH2TiCl3 H2C CH2 TiCl3 CH3CH2CH2CH2
- 13. Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2 TiCl3 CH3CH2CH2CH2 H2C CH2
- 14. Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2 H2C CH2 TiCl3 CH3CH2CH2CH2CH2CH2
- 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 R C CH
- 17. Me X 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 System W. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390, (1980); Angew. Chem. 97, 507 (1985)
- 18. Methylaluminoxane: the Key Cocatalyst Al(CH3)3 + H2O toluene 0 C Al O CH3 * * n n = 10-20 O Al AlAl CH3 OO O Al O Al O Al Al CH3 CH3 Proposed structure MAO
- 19. Nature of active catalyst Transition metal alkylation Ionization to form active sites MAO Noncoordinating Anion, NCA Cp2Me X X + Al O CH3 * * n Cp2Me CH3 X + Al O CH3 Al X Om Cp2Me CH3 + Al O CH3 Al X Om X
- 20. Alkene Hydrogenation with Wilkinson’s Catalyst CO2Me H2 cat. RhCl(PPh3)3 H2 cat. PtO2 CO2Me CO2Me 96:4 49:26
- 21. Mechanism PPh3 Rh H PPh3 Cl H PPh3 Rh H PPh3 Cl H R R H H coordination R migratory insertion reductive elimination oxidative addition -PPh3 +PPh3 [RhCl(PPh3)2] RhCl(PPh3)3 H H PPh3 Rh H PPh3 H Cl R R' R' R' R'
- 22. Enantiomerically Enriched Phosphines PPh2 PPh2 H O O H DIOP * * PPh2 PPh2 * * CHIRAPHOS DIPAMP PH P PhOMePh Ph * * N PPh2 PPh2 O O BPPM * * PPh2 PPh2 BINAP P P R RR R DuPHOS PP R R R R BPE
- 23. Asymmetric Hydrogenation CO2Me NHAcR' R H2 Me BPE Rh or DuPHOS RhMe 90 psi, PhH 96-99% ee CO2Me R R' NHAc
- 24. CO2H R1R3 R2 H2 96-99% ee CO2H R3 R2 R1 CO2H MeO 97% ee (Naproxen) NH O CO2H R3SiO H H 74% de (Thienamycin) Me Me H 91 H Me 87 H Me Ph 85 Ph H H 92 H HOCH2 Me 93 H CH3 COOCH2CMe 95 R1 R2 R3 ee Ru(OCOR)2 (binap) Asymmetric Hydrogenation
- 25. Mechanism: P Rh S P S N H O Ph MeO2C equilibrium must be fast for high ee major k' k'-1 MeO2C N H O Rh LL Ph minor <5% diastereoisomers fast H2 k2 rate limiting step very slow H2 k'2 N H CO2Me O Rh LL Ph >95% k'-1 k'
- 26. Mechanism major MeO2C N H O Rh LL Ph minor <5% diastereoisomers fast H2 k2 rate limiting step very slow H2 k'2 N H CO2Me O Rh LL Ph >95% MeO2C HN Rh LL Ph H O k2 > k'2 >103 NH CO2Me H Rh LL Ph O H H
- 27. Mechanism:
- 28. Mechanism: NHMeO2C Ph O Rh H S L L k4 Ph N H MeO2C H O (R) > 98% O N H CO2Me Ph H (S) < 2% k'4 CO2MeHNO Rh Ph H S L L ee lower at high H2 pressure - k'2 increased lower at low temp - equilibration decreased. Major diast. accumulates H H
- 29. Mechanism: MeO2C HN Rh LL Ph H O NH CO2Me H Rh LL Ph O k3 k'3 NHMeO2C Ph O Rh H S L L CO2MeHNO Rh Ph H S L L H H H H