Page 199 - Advanced Organic Chemistry Part B - Reactions & Synthesis

The unique feature of the Horner-Wittig reaction is that the addition intermediate 171
can be isolated and purified, which provides a means for control of the reaction’s
stereochemistry. It is possible to separate the two diastereomeric adducts in order to SECTION 2.4
prepare the pure alkenes. The elimination process is syn, so the stereochemistry of the Olefination Reactions of
Stabilized Carbon
alkene that is formed depends on the stereochemistry of the adduct. Usually the anti Nucleophiles
adduct is the major product, so it is the Z-alkene that is favored. The syn adduct is
most easily obtained by reduction of -ketophosphine oxides. 269

O O
Ph PCHCH CH Ph PhCH CH 2 CH 3
2
2
2
2
CH 3 1) BuLi NaBH 4 Ph P CH 3
2
2) CH CH O HO O
3
H
OH H
2
PhCH CH 2 + PhCH CH 2
2
CH 3 separate CH 3
CH Ph P CH 3
Ph P 3 2
2
O NaH O
NaH
CH 2 CH 2 Ph
CH 3 CH 3 CH CH Ph
2
2
H CH 3 CH 3 H
2.4.2. Reactions of -Trimethylsilylcarbanions with Carbonyl Compounds
Trialkylsilyl groups have a modest stabilizing effect on adjacent carbanions (see
Part A, Section 3.4.2). Reaction of the carbanions with carbonyl compounds gives
-hydroxyalkylsilanes. -Hydroxyalkylsilanes are converted to alkenes by either acid
or base. 270 These eliminations provide the basis for a synthesis of alkenes. The reaction
is sometimes called the Peterson reaction. 271 For example, the Grignard reagent derived
from chloromethyltrimethylsilane adds to an aldehyde or ketone and the intermediate
can be converted to a terminal alkene by acid or base. 272

Mg or Li R C O acid
2
or
(CH ) SiCH X (CH ) SiCH M (CH ) SiCH CR CH 2 CR 2
3 3
2
2
3 3
3 3
2
2
base
M = Li or MgX OH
Alternatively, organolithium reagents of the type CH SiCH Li Z, where Z is a
3 3
carbanion-stabilizing substituent, can be prepared by deprotonation of CH SiCH Z
2
3 3
with n-butyllithium.
R C O
n-BuLi 2
2
(CH ) SiCH Z (CH ) SiCHZ R C CHZ
3 3
2
3 3
Li
269 A. D. Buss and S. Warren, J. Chem. Soc., Perkin Trans. 1, 2307 (1985).
270
P. F. Hudrlik and D. Peterson, J. Am. Chem. Soc., 97, 1464 (1975).
271 For reviews, see D. J. Ager, Org. React., 38, 1 (1990); D. J. Ager, Synthesis, 384 (1984); A. G. M. Barrett,
J. M. Hill, E. M. Wallace, and J. A. Flygare, Synlett, 764 (1991).
272
D. J. Peterson, J. Org. Chem., 33, 780 (1968).