Peterson Olefination
The Peterson olefination is the chemical reaction of alpha-silyl carbanions and carbonyl compounds for the preparation of alkenes. It is considered to be the silicon-variant of the Wittig reaction.
General features:
1. The α-silyl carbanions can be prepared via metal-halogen exchange of the α-halogenated alkylsilanes or by direct deprotonation of alkylsilanes at the α-position. 2. The addition of the α-silyl carbanions to carbonyl compounds gives rise to β-silylcarbinols that upon treatment with a base (NaH, KH, t-BuOK) undergo a stereospecific syn-elimination, while treatment with dilute acid or a Lewis acid (AcOH, H2SO4, BF3·OEt2) results in a stereospecific anti-elimination. 3. Either the (E) or (Z)-alkene can be obtained from a diastereomerically pure β-silylcarbinol by choosing acidic or basic conditions. Therefore, the stereoselectivity of the reaction depends on the availability of the diastereomerically pure β-silylcarbinol.
Peterson Olefination
Reaction Mechanism
1. Formation of the silyl-lithium and 1,2-addition to the aldehyde delivers an alpha-hydroxysilane. 2. Apple reaction converts the alcohol into the alkyl chloride. 3. Formation of the Grignard reagent, 1,2-addition to the aldehyde and hydrolysis. 4. Treatment of the beta-hydroxysilane with acid or a base yields the corresponding alkene.
Experimental Procedure
To a solution of the ketone (3.0 mmol, 1.0 eq) in diethyl ether (15 mL) under argon was added (trimethylsilyl)methyllithium (TMSCH2Li; 0.56 M in hexanes, 4.0 eq) at 25 °C, and the resulting mixture was stirred for 30 min. Methanol (100 mL), p-toluenesulfonic acid (10.0 eq) were added, and the mixture was stirred for 2 h. The mixture was quenched with saturated aqueous sodium bicarbonate and extracted with ethyl acetate. The combined organic layers were dried over Na2SO4, filtered and concentrated in vacuo. The crude residue was purified by silica gel column chromatography to afford the olefin (86% yield).
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