The Peterson Reaction allows the preparation of alkenes from α-silylcarbanions. The intermediate β-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric β-hydroxy silanes and subsequently performing two different eliminations.
Mechanism of the Peterson Olefination
In the first step of the Peterson Olefination, addition of the silylcarbanion to a carbonyl compound and subsequent aqueous work up leads to diastereomeric adducts.
Some of these reactions are stereoselective and may be rationalized with simple models: The reaction of benzaldehyde and a silylcarbanion gives the threo-product if the silyl group is small. This implies that in the transition state, the two sterically demanding groups are anti. As the silyl group becomes more sterically demanding than trimethylsilyl, the selectivity shifts towards the erythro-isomer.
Acidic hydrolysis proceeds via an anti-elimination:
In contrast, the base-catalyzed elimination may proceed via a 1,3-shift of the silyl group after deprotonation, or with the formation of a pentacoordinate 1,2-oxasiletanide that subsequently undergoes cycloreversion:
The use of α-silyl organomagnesium compounds is helpful for the isolation of the intermediate β-hydroxysilanes, because magnesium strongly binds with oxygen, making the immediate elimination impossible. If excess organolithium or lithium amide base is used to generate the α-silyl carbanion, this base can effect the deprotonation as well, and since the lithium-oxygen bond is not as strong as magnesium-oxygen, the reaction leads directly to the alkene. Some reactions proceed with good diastereoselectivity, so the direct conversion can be an attractive option.