Peterson olefination reaction stereochemistry

Chapters 1 and 2 focus on enolates and other carbon nucleophiles in synthesis. Chapter 1 discusses enolate formation and alkylation. Chapter 2 broadens the discussion to other carbon nucleophiles in the context of the generalized aldol reaction, which includes the Wittig, Peterson, and Julia olefination reactions. The chapter and considers the stereochemistry of the aldol reaction in some detail, including the use of chiral auxiliaries and enantioselective catalysts. 

The Peterson olefination is a two-step process for the formation of alkenes from an a-silylcarbanion and an aldehyde or ketone. The first step is an addition reaction that affords both syn and anti p-hydroxysilanes. The stereochemistry is then controlled during the elimination step by using either an acid or a base. Reviews Kano, N. Kawashima, T. In Modem Carbonyl Olefination-Methods and Applications Takeda, T. Ed. Wiley-VCH Weinheim, 2004 Chapter 2 The Peterson and Related Reactions, pp. 18-103. (b) Kelly, S. E. In Comprehensive Organic Synthesis Trost, B. M. Fleming, 1., Eds. Pergamon Oxford, 1991 Vol. 1, Chapter 3.1 Alkene Synthesis, pp. 731-737. (c) Ager, D. J. Org. React. 1990, 38, 1-223. (d) Ager, D. J. Synthesis 1984, 384-398. 

The stereochemistry of the Peterson reaction has been investigated. When unsymmet-rically substituted a-silylcarbanions react with aldehydes or unsymmetric ketones, E or Z olefins are produced. In many cases the E Z ratio is 1 1, however, some workers have reported a predominance of cis olefins when aldehydes are employed. Typical results are given in Table 15. Unlike the Wittig reaction the stereochemical outcome of the Peterson reaction is insensitive to counterion, solvent, added salts and temperature255. Stereochemical control of the Wittig reaction usually depends upon the reversibility of the first step. However, as discussed earlier, the first step of the Peterson reaction is irreversible. Thus the stereochemical outcome is determined solely by the relative rates of formation of threo and erythro 0-silyl alkoxides (/ct and k in Scheme 5). 

---