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 Cl 2 C: (Carbene) R 2 BH (Hydroboration) CH 2 I 2 Zn/Cu (Carbenoid) O 3 (Ozonolysis) H-metal (Catalytic Hydrogenation) R-metal (Metathesis, Polymerization) RC (Epoxidation) OOHOOH O OsO 4 or MnO 4 - (Dihydroxylation)

OsO 4 and Permanganate Os analogue of cyclic acetal H-O-H OsO 4 is poisonous and expen$ive! Use as a 1% catalyst by adding oxidant. H 2 O 2 (1936) “NMO” ( Upjohn) Chiral Amine Ligand e.g. J&F Sec. 10.5c p. 443 Osmate Ester H H C C H3CH3C CH 3 H H C C H3CH3C OO O O Os O O OO O HH Sharpless Asymmetric Dihydroxylation (1988) SAD C C K+K+ O-O- O Mn OO 97% ee C C OO O-O- O Mn O HH H2OH2O K+K+ 85% yield KMnO 4 MeOH / H 2 O NaOH, 20°C all syn (S,S) + (R,R) syn addition to trans 2-butene

H  * LUMO  HOMO orthogonal Catalytic Hydrogenation HOMO/LUMO : Concerted H  * LUMO  HOMO CC H CC H H CCCC  * LUMO  HOMO (“works” with metal catalysts!) HOMO-HOMO repulsive empty Pd e.g. J&F Sections Sec 4.9A, 168ff., 10.2a ( ), (452)

Orbital Variety from Metals

Pd HOMO (4d ) Ethylene LUMO (   ) HOMO (  ) HOMO-4 Ethylene-Pd Complex …(4d) 10 (5s) 0 (5p) 0 13%   40% 4d xy 47%  C-H x 2 -y 2 z2z2 xy xzyz

HOMO (  ) Pd HOMO (4d) Ethylene UMO ( 5s ) UMO ( 5p ) (4d) 10 (5s) 0 (5p) 0 HOMO Ethylene-Pd Complex + 6% 5s 5% 5p 15% 4d z 2 67% 

Sigma Bond Analogue “Oxidative” Addition (crummy PM3 calculation) H-H + Pd kcal/mole H 2 dissociates on bulk Pd surface, then hydrides move. (entropy help) bonding H 2 to Pd splitting H 2

kcal/mole

H Catalytic Hydrogenation  “ oxidative addition”  “ oxidative addition” Pd CC C C HH H H “ reductive elimination” Pd H CC H H C C H Pd addition concerted (syn) Pd H C C H H C C Experts discuss the extent of bonding in this “  -complex” H atoms replace Pd frontside  syn hydrogenation product

Catalytic Hydrogenation Stereochemistry syn addition e.g. J&F pp. 412

Stereochemistry No yields specified! No literature reference! A general elementary text e.g. Loudon, Sec. 7.9 E p. 313

pp of H. O. House Modern Synthetic Chemistry (1972) (a graduate-level text)

J. Chem. Soc., 1354 (1948) H 2 / Pt R’ = Ac allylic isomers

Suppose there is an allylic H in the alkene: can lead to allylic rearrangement H Catalytic Hydrogenation Pd H H C C H C C H H C C H C C CHCH H H C C CHCH C C alkene isomerized symmetric C C C H H H

?? VIIVIII

Alkene Metathesis metallacyclobutane CC Grubbs Catalyst Ru C C C C C C C C C C C C C Nobel Prize 2005 a metal alkylidene complex

Tall Prof. F. Ziegler (not Prof. Karl Ziegler) with Prof. R. Grubbs Tourists Ziegler Grubbs Host Prof. S.-I. Murahashi

ROMP Ring-Opening Metathesis Polymerization Ru C C C n n metathesis metatheses

Catalytic Hydrogenation Ti R CC R C C R C C H Pd H C C H C C H C C H H C C H 25 x 10 6 tons (2004) -(CH 2 -CH 2 ) n - n = ,000 Ziegler-Natta Polymerization 45 x 10 6 tons (2007) -(CH 2 -CH) n - CH 3 n up to 10 5 isotactic Heterogeneous Catalyst hard to study mechanism R RR Et 3 Al+ TiCl 4

Stereochemistry: Tacticity All head-to-tail, but stereorandom (atactic) All head-to-tail, and stereoregular (isotactic) All head-to-tail, and stereoregular (syndiotactic) How do you know which is which? NMR (coming soon) How do you control what you make?

R 2 B-H C=C-CH 3 R’ R C-C-CH 3 Stereochemistry: Tacticity All head-to-tail, but stereorandom (atactic) All head-to-tail, and stereoregular (isotactic) All head-to-tail, and stereoregular (syndiotactic) axis Homogeneous “Kaminsky” catalysts activated by MAO (“methyaluminoxane”) homotopic faces mirror enantiotopic faces achiral faces C=C-CH 3 R’ C-C-CH 3 R R C=C-CH 3 R’ C-C-CH 3 + Alkenes approach from alternate faces

Radical Polymerization (e.g. J&F Sec 11.5 pp ) R H Occasional butyl side-chains inhibit close packing.

ClCCl 3 RCl Controlling Polymer Chain Length CCl 4 is a “Chain-Transfer Agent” shortens polymer molecules without terminating chain reaction Properties like viscosity and melting point depend on chain length. etc. Cl k transfer /k polymerization ~ 0.01 for styrene polymerization        When other termination is negligible, molecular length ~ k p [styrene] / k t [CCl 4 ] “dispersity”

Alkene/Diene Oligomerization and Polymerization Using Carbon Electrophiles R+R+ (S N 1) R-LR-L ** (S N 2) (“oligo”, a few)

CH 3 H2CH2CC R + Electrophile in Formation of 2,2,4-Trimethylpentane, “Isooctane” CH 3 CH H2CH2CC CCH 2 CH 3 HC H 2 SO 4 + CH 3 C + C + CH 2 C H + (defined as “100 octane”) inter molecular hydride shift (Bartlett, 1944) chain poly(isobutylene) “butyl rubber” air-tight + CH 3 CH 2 C CH 3 C CH 2 C etc. CH 3 H2CH2CC H C +

e.g. J&F Sec pp R-L and R + Electrophiles in ** Terpene/Steroid Biogenesis

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