Page 185 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1) NaH Zn Hg 157
PhCOCHSOCH PhCOCH CH
PhCOCH 2 SOCH 3 3 2 3
2) CH 3 I
SECTION 2.4
CH 3
Olefination Reactions of
Ref. 231 Stabilized Carbon
These reactions accomplish the same overall synthetic transformation as the acylation Nucleophiles
of ester enolates, but use desulfurization rather than decarboxylation to remove
the anion-stabilizing group. Dimethyl sulfone can be subjected to similar reaction
sequences. 232

2.4. Olefination Reactions of Stabilized Carbon Nucleophiles

This section deals with reactions that correspond to Pathway C, defined earlier
(p. 64), that lead to formation of alkenes. The reactions discussed include those of
phosphorus-stabilized nucleophiles (Wittig and related reactions), -silyl (Peterson
reaction)and -sulfonylcarbanions(Juliaolefination)withaldehydesandketones.These
important rections can be used to convert a carbonyl group to an alkene by reaction
with a carbon nucleophile. In each case, the addition step is followed by an elimination.

R EWG O – R
–
C + O
R R
R R
EWG
A crucial issue for these reactions is the stereoselectivity for formation of E-or
Z-alkene. This is determined by the mechanisms of the reactions and, as we will
see, can be controlled in some cases by the choice of particular reagents and reaction
conditions.

2.4.1. The Wittig and Related Reactions of Phosphorus-Stabilized
Carbon Nucleophiles

The Wittig reaction involves phosphonium ylides as the nucleophilic carbon
species. 233 An ylide is a molecule that has a contributing resonance structure with
opposite charges on adjacent atoms, each of which has an octet of electrons. Although
this definition includes other classes of compounds, the discussion here is limited
to ylides having the negative charge on the carbon. Phosphonium ylides are stable,
but quite reactive, compounds. They can be represented by two limiting resonance
structures, which are referred to as the ylide and ylene forms.

+
(CH ) P CH 2 – (CH ) P CH 2
3 3
3 3
ylide ylene
231
P. G. Gassman and G. D. Richmond, J. Org. Chem., 31, 2355 (1966).
232 H. O. House and J. K. Larson, J. Org. Chem., 33, 61 (1968).
233
For general reviews of the Wittig reaction, see A. Maercker, Org. React., 14, 270 (1965); I. Gosney and
A. G. Rowley, in Organophosphorus Reagents in Organic Synthesis, J. I. G. Cadogan, ed., Academic
Press, London, 1979, pp. 17–153; B. A. Maryanoff and A. B. Reitz, Chem. Rev., 89, 863 (1989);
A. W. Johnson, Ylides and Imines of Phosphorus, John Wiley, New York, 1993; N. J. Lawrence,
in Preparation of Alkenes, Oxford University Press, Oxford, 1996, pp. 19–58; K. C. Nicolaou,
M. W. Harter, J. L. Gunzer, and A. Nadin, Liebigs Ann. Chem., 1283 (1997).