Chemistry 125: Lecture 52 February 16, 2011 Transition Metal Catalysis: Hydrogenation & Polymerization Additions by Radicals & Electrophilic Carbon; Isoprenoids; Tuning Polymer Properties Preliminary 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

Orbital Variety from Metals

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 Pt/C Catalyst!) HOMO-HOMO repulsive empty Pd e.g. J&F Sections Sec 4.9A, 168ff., 10.2a ( ), (452)

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 debate 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?

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

Isopentenyl Pyrophosphate Dimethylallyl Pyrophosphate Adjacent unsaturation apparently speeds S N 2 (as well as S N 1) Cl I benzyl 250 Cl allyl 90 Cl n-propyl k rel for rxn with I - in acetone [1] Cl

Isopentenyl Pyrophosphate Geranyl Pyrophosphate C5C5 C 10

Geranyl Pyrophosphate cis trans Neryl Pyrophosphate Limonene -H +  -Pinene -H + +H 2 O [Ox] Camphor "Terpene" essential oils C 10 Markovnikov anti-Markovnikov

Geranyl Pyrophosphate Farnesyl Pyrophosphate "head-to-head reductive dimerization" Squalene (shark liver oil) new bond C 15 “sesquiterpenes” C 30 “triterpenes” e.g. caryophyllene (clove, hemp, rosemary)

+ Squalene H HO O Markovnikov Anti- Markovnikov Enzyme makes “O” selective among many trisubstituted alkene groups.

Squalene HO H H H CH 3 H H H + H3CH3C H3CH3C H3CH3C Lanosterol (source of cholesterol & steroid hormones) Not this time! (enzyme control) C 30 “triterpenes” 3°

Squalene HO H H H CH 3 H H H + H3CH3C H3CH3C H3CH3C Lanosterol (source of cholesterol & steroid hormones) Not this time! (enzyme control) C 30 “triterpenes” 3° Cute Story Is it True? (Wait for NMR)

2 Isoprenes Isoprene H OH Geraniol “dimer” H OH

Isoprene OH 2 Isoprenes Menthol “dimer” OH

Isoprene O 4 Isoprenes Retinal “tetramer” O

Latex “polymer” Isoprene 30,000 Isoprenes

Axel Boldt Hevea braziliensis

Latex to Caoutchouc

Gooey in heat Brittle in cold Thomas Hancock (England -1820) “Masticator” Goodyear (1839) Vulcanization Charles Macintosh (Scotland ) Sandwiched rubber between cloth layers for waterproof garments

The occurrence did not at the time seem to them to be worthy of notice; it was considered as one of the frequent appeals that he was in the habit of making, in behalf of some new experiment.” He endeavoured to call the attention of his brother, as well as some other individuals who were present, and who were acquainted with the manufacture of gum-elastic, to this effect, as remarkable, and unlike any before known, since gum-elastic always melted when exposed to a high degree of heat. “He was surprised to find that the specimen, being carelessly brought into contact with a hot stove, charred like leather. Discovery of Vulcanization from Goodyear’s Autobiographical “Gum-Elastic” (1855) 1839

Silliman consult “Having seen experiments made, and also performed them myself, with the India rubber prepared by Mr. Charles Goodyear, I can state that it does not melt, but rather chars, by heat, and that it does not stiffen by cold, but retains its flexibility with cold, even when laid between cakes of ice.” B. Silliman October 14, 1839

U.S. Pavilion Crystal Palace (1851)

Goodyear’s Vulcanite Court India Rubber Desk Mattatuck Museum, Waterbury

Somehow Vulcanization joins adjacent chains with sulfur “cross-links” S Latex polymer Radical Addition and Allylic Substitution? H ?

Vulcanization and the Physical Properties of Polymers

Gough

wordsworth “ No floweret blooms Throughout the lofty range of these rough hills, Nor in the woods, that could from him conceal Its birth-place; none whose figure did not live Upon his touch.” Wordsworth “Excursion” (1813) ( )

John Gough Heating rubber makes it expand (more than H 2 O).

Heating tightly stretched rubber makes it contract !

If stretching rubber generates heat, what should letting it contract do? A) If heat comes from internal friction, contraction should also cause friction and generate heat. B) If heat comes from some other cause, contraction may do the opposite and absorb heat (“generate cold”).

Why?

Goodyear Plot

Goodyear Inventor

Goodyear to Gibbs

Gibbs Mathematical Physics

Gibbs to Onsager

Kirkwood & Onsager

Polymer Statistical Mechanics

Statistics Contracts a Stretched Chain etc. only one arrangement of maximum lengthmany arrangement of shorter length

Near maximum extension there is local Crystallization Stretching Rigidity Contributes Rigidity Releases Heat Fixed, irregular cross-links between adjacent chains prevents crystallization (and brittleness) in the cold. Warming “melts” the crystalline regions, and allows statistics to make the material contract. Absorbing heat “melts” the crystalline regions, and allows statistics to make the material contract.

Lengthwise Motion by “Reptation” Change shape by snaking along a tunnel through the tangled neighbors. How to make a tangle flow?

Sulfur Cross-Links Stop Reptation Vulcanization (no flow when hot) and inhibit crystallization. (not brittle when cold)

Vulcanization in the Home

Hair before Permanent Wave “Reduce” disulfide cross links with excess basic RSH S S S S S S S S RS - - H SR - H RS-SR - H H H H H H (pK a ~11) + NH 4 HS CO 2 HS CO 2 OH or H SR - H Curl

Permanent Wave H H H H H H BDE kcal/mole HO-OH 52 RS-SR ~ 64 RS-H 87 RO-H 105 S S S S S S S S H H Curl “Oxidize” thiols back to disulfide with HOOH

Synthetic Rubber

Thermoplastic Ionomers Malleable cross links

Julius Nieuwland Cl Neoprene

Natural Rubber vs. Synthetics

Radical Polymerization Poly(styrene) Regiochemistry R           R   head-to-tail random ~ 13 kcal/mole more stable than

Radical Polymerization Poly(propylene) Tacticity CH 3 H H H H H H H H H H H H H H H H H H H H H H H H H H H Isotactic (Radical) (Ziegler-Natta) Syndiotactic Atactic

Radical Copolymerization     CO 2 CH 3 Block CO 2 CH 3 Methyl Methacrylate  Styrene CO 2 CH  [1]2 k relative      CO 2 CH 3 Alternating ? fastest

Anti-Hammond Copolymerization     ~ 20 kcal/mole CO 2 CH 3    not as stable but twice as fast!

Radical Copolymerization CO 2 CH 3      C=O gives unusually low LUMO. Good when SOMO is not low. “Ionic resonance structure stabilizes transition state.” COCH 3 - O  + - O N.B. This special stability applies in TS only,not in the radical product!

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