Silyl-Wittig reaction Peterson olefination

Peterson Olefination Reaction (Peterson Elimination, Silyl-Wittig Reaction) The Reaction ... 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

Peterson olefination, a silicon variant of the Wittig reaction, has been used to convert a-silyl benzyl carbamates (78) into trisubstituted vinyl carbamates (79) in moderate-to-good yields and with some ii/Z-selectivity. ... 

Gilman, H., Tomasi, R. A. a-Silyl-substituted ylides. Tetraphenylallene via the Wittig reaction. J. Org. Chem. 1962, 27, 3647-3650. Peterson, D. J. Carbonyl olefination reaction using silyl-substituted organometallic compounds. J. Org. Chem. 1968, 33, 780-784. 

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). 

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), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. 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. 

Wittig olefination reaction ( the phosphorus way ) has been a very popular reaction in organic synthesis. However, it is now in competition with Peterson/Chan olefination reaction327 ( the silicon way ). Formally, this latter involves the formation of a (3-silyl heteroatomic anion, which in the absence of an electrophile undergoes a (3-shift of the silyl moiety to the heteroatom (usually oxygen) with final elimination of silylated heteroatomic anion and formation of the olefin. 

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