Chemistry 125: Lecture 51 February 15, 2010 More Addition to Alkenes: Organometallic Reagents and Catalysts This For copyright notice see final page of this file

Mechanism for Acid-Catalyzed Hydrolysis of Acetal RO CH 2 + H HOH : : RO CH 2 + HROH RO-CH 2 + HO RO CH 2 + H First remove RO, and replace it by HO. HO RO CH 2 Now remove second RO, then H (from HO) + H : HO RO CH 2 + H RO=CH 2 + cation unusually stable; thus easily formed ROH H-O-CH 2 + O=CH 2 ROH RO CH 2 O H H  : Overall Transformation: H 2 O + Acetal Carbonyl + 2 ROH H+H+ (pp ) (hemiacetal) ? ? SN1SN1 E1

HOHO O=CH 2 CH 2 HO-O O H H O-O O H2CH2C O H H H H H 2 C=O Ozonide is a Double Acetal So Double Hydrolysis and hydrogen peroxide Gives Two Carbonyl Compounds which oxidizes aldehydes to carboxylic acids!

Sec. 10.5b pp Add a reducing agent like (CH 3 ) 2 S (or Zn) to destroy HOOH and save RCH=O. Or go with the flow and add more HOOH to obtain a good yield of RCOOH.

3-membered ring with O-O bond is even worse. What Happens to HOOH + RCHO? O C H R O OH - O C H R O - O C H R O - HOH - O CR O R B R R O OH - Cf. Problem: Try drawing an analogous acid-catalyzed mechanism in which HOOH attacks the protonated carbonyl, then H + is lost from one O of the HOOH fragment in the product and added to the other before rearrangement. OH OH - is a bad leaving group from C, but O-O bond is very weak. Hydride Shift

“Nucleophilic” Addition to C=O

The nucleophilic addition of methyl lithium to carbonyl groups* is formally quite different from these additions of electrophiles to alkenes, but the following transition state analysis reveals a marked mechanistic similarity. * which will be discussed in more detail later.

Transition State Motion Li-CH 3 O=CH 2 Li CH 3 O CH 2

Transition State Orbital Mixing Li-CH 3 O=CH 2 HOMOLUMO+2  * LUMO  HOMO

Orbital Variety from Metals

overlaps with alkene  *overlaps with alkene  LUMOHOMO OsO 4 / Permanganate Sec. 10.5c p. 443 Os or Mn - Os analogue of cyclic acetal H2OH2O Diol + O 2 Os=O OsO 4 is poisonous and expen$ive! Use as a 1% catalyst by adding oxidant. H 2 O 2 (1936) “NMO” (1976) Chiral Amine Ligand Sharpless (1988) R R R R

H  * LUMO  HOMO orthogonal Sections 10.2a ( ), (452) Catalytic Hydrogenation HOMO/LUMO : Concerted H  * LUMO  HOMO CC H CC H H CCCC  * LUMO  HOMO (“works” with Pt/C Catalyst! Sec 4.9A, 168ff ) HOMO-HOMO repulsive empty Pd

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

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” Insertion (crummy PM3 calculation) H-H + Pd kcal/mole H 2 dissociates on bulk Pd surface (and moves and dissolves) (entropy help)

H Catalytic Hydrogenation  “ oxidative insertion”  “ oxidative insertion” Pd CC C C HH H H “ reductive elimination” Pd H CC H C C H C C H H C C H Pd addition concerted; H replaces Pd twice  syn addition

Catalytic Hydrogenation Stereochemistry syn addition (p. 412)

Stereochemistry (Loudon, Sec. 7.9 E p. 313 ) No yields specified! No literature reference!

pp of H. O. House Modern Synthetic Chemistry (1972)

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

H Catalytic Hydrogenation Pd H H C C H C C H H C C H C C C C H HH H C C C H H C C C H H C C C H CC alkene isomerized

?? VIIVIII

Alkene Metathesis   CC Grubbs Catalyst Ru C C C C C C C C C C C C C Nobel Prize 2005

Tall Fred Ziegler (not Karl Ziegler) with Robt. Grubbs Tourists Ziegler Grubbs Host

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 = Ziegler-Natta Polymerization 45 x 10 6 tons (2007) -(CH 2 -CH) n - CH 3 n up to 10 5 isotactic All head-to-tail, but stereorandom (atactic)All head-to-tail, and stereoregular (isotactic)

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