Chemistry 125: Lecture 52 February 16, 2011 Transition Metal Catalysis: Hydrogenation & Polymerization Additions by Radicals & Electrophilic Carbon; Isoprenoids; Tuning Polymer Properties This For copyright notice see final page of this file Other “Simultaneous” Reagents RC O Cl2C: (Carbene) R2BH (Hydroboration) CH2I2 Zn/Cu (Carbenoid) (Epoxidation) O3 (Ozonolysis) OOH OsO4 or MnO4 (Dihydroxylation) H-metal (Catalytic Hydrogenation) R-metal (Metathesis, Polymerization) e.g. J&F Sec. 10.5c p. 443 OsO4 and Permanganate “NMO” (1976 - Upjohn) H2O2 (1936) H-O-H O O Chiral Amine Ligand O O K+ O- O to trans 2-butene (1988) OsO4 is poisonous and expen$ive! Use as a 1% catalyst by adding oxidant. O Mn O Os Os analogue O O HO O OH of C cyclic acetal H CH3 C CH C C H C(S,S) 3 H H 3C H3C + (R,R) H syn addition Sharpless Osmate Asymmetric Dihydroxylation Ester Os K+ O- Mn O H2O HO O C C OH C KMnO4 MeOH / H2O NaOH, 20°C OH 85% yield SAD OH 97% ee all syn Catalytic Hydrogenation HOMO/LUMO : Concerted (“works” with metal catalysts!) *HOMO LUMO * LUMO HOMO H H Pd H H H H orthogonal H H HOMO-HOMO repulsive C C C C C C HOMO empty C C *LUMO e.g. J&F Sections Sec 4.9A, 168ff., 10.2a (410-413), 10.10 (452) Orbital Variety from Metals 47% C-H Ethylene Ethylene-Pd Complex Pd …(4d)10 (5s) 0 (5p)0 HOMO LUMO ( ()) 13% HOMO-4 40% 4dxy HOMO (4dxyz 22-y2) xy zxz 67% Ethylene HOMO () Ethylene-Pd Complex HOMO Pd 0 (5p)0 (4d)10 (5s) 15% 4dz2 + UMO (5p) HOMO (4d) UMO (5s) 6% 5s 5% 5p Sigma Bond Analogue “Oxidative” Addition (crummy PM3 calculation) H-H + Pd 10 splitting H2 5 0 H2 dissociates on bulk Pd surface, then hydrides move. (entropy help) kcal/mole bonding H2 to Pd kcal/mole Catalytic Hydrogenation “oxidative addition” C C C Pd “oxidative addition” C H Pd H H Pd “reductive elimination” H Pd “reductive elimination” Experts discuss the extent of bonding in this “-complex” H H Pd H H H Pd addition concerted (syn) Pd H H H Pd Pd H atoms replace Pd frontside syn hydrogenation product Catalytic Hydrogenation Stereochemistry syn addition e.g. J&F pp. 412 Stereochemistry A general elementary text e.g. Loudon, Sec. 7.9 E p. 313 No yields specified! No literature reference! pp. 20-22 of H. O. House Modern Synthetic Chemistry (1972) (a graduate-level text) J. Chem. Soc., 1354 (1948) allylic isomers H2 / Pt R’ = Ac R’ = Ac Catalytic Hydrogenation Suppose there an allylic H in the alkene: can lead to isallylic rearrangement HC HC C CH H C H C Pd symmetric H C Pd alkene isomerized CH C H C Pd H CH C C Pd H H H Pd H Pd 4 3 2 VII 1 10 9 5 8 4 6 3 7 2 5 10 1 9 6 7 8 VIII ?? Alkene Metathesis C C Ru Grubbs Catalyst a metal alkylidene complex Nobel Prize 2005 Ru C C C Ru metallacyclobutane Ru Ru Tourists Ziegler Grubbs Host Prof. S.-I. Murahashi Tall Prof. F. Ziegler (not Prof. Karl Ziegler) with Prof. R. Grubbs ROMP Ring-Opening Metathesis Polymerization metathesis Ru C C Ru metatheses n C Ru n isotactic CH3 -(CH -CH ) - Hydrogenation Catalytic -(CH -CH) 2 2 n 2 n = 800-250,000 H H Pd 106 25 x tons (2004)Pd Et3Al + TiCl4 H n n up to 105 H 6 tons 45 x 10 H (2007) Pd H H Pd hard to study mechanism Heterogeneous Catalyst Ziegler-Natta Polymerization R Ti R Ti R Ti R Stereochemistry: Tacticity How do you How do you know which is which? what you make? NMR (coming soon) All head-to-tail, and stereoregular (isotactic) All head-to-tail, and stereoregular (syndiotactic) All head-to-tail, but stereorandom (atactic) Stereochemistry: Tacticity Alkenes approach from alternate faces achiral faces mirror enantiotopic faces axis Homogeneous “Kaminsky” catalysts homotopic activatedfaces by MAO + R2B-H C=C-CH C=C-CH 33 C -C-CH3 R C-C-CH R R’ C-C-CH R’ 3 (“methyaluminoxane”) All head-to-tail, and stereoregular (isotactic) All head-to-tail, and stereoregular (syndiotactic) All head-to-tail, but stereorandom (atactic) Radical Polymerization (e.g. J&F Sec 11.5 pp.487-489) R H Occasional butyl side-chains inhibit close packing. Controlling Polymer Chain Length CCl4 is a “Chain-Transfer Agent” shortens polymer molecules without terminating chain reaction ktransfer/kpolymerization ~ 0.01 for styrene polymerization R When other termination is negligible, molecular length ~ kp[styrene] / kt[CCl4] “dispersity” Cl Cl CCl3 Properties like viscosity and melting point depend on chain length. etc. (“oligo”, a few) Alkene/Diene Oligomerization and Polymerization Using Carbon Electrophiles R-L (SN2) + R (SN1) * + R Electrophile in Formation of 2,2,4-Trimethylpentane, “Isooctane” (defined as “100 octane”) H+ H CH3 CH CH33 CH3 C CH H + H22C C C C 3 H CH3 H2SO4 CH CH33 CH3 CH3 CH3 C CH2 C H CH3 CH3 inter molecular hydride shift (Bartlett, 1944) CH3 CH3 CH3 C + CH3 poly(isobutylene) “butyl rubber” air-tight etc. etc. CH3 CH3 C CH2 C+ CH3 CH3 C CH2 CH3 CH3 CH3 CH3 chain C CH2 C+ CH3 CH3 CH3 + R-L and * + R Electrophiles in Terpene/Steroid Biogenesis e.g. J&F Sec. 12.13 pp. 554-562 End of Lecture 52 February 16, 2011 Copyright © J. 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