Peterson reaction

The Peterson olefination (also called the Peterson reaction) is the chemical reaction of α-silyl carbanions 1 with ketones (or aldehydes) to form a β-hydroxysilane 2 which eliminates to form alkenes 3.

Several reviews have been published.

One attractive feature of the Peterson olefination is that it can be used to prepare either cis- or trans-alkenes from the same β-hydroxysilane. Treatment of the β-hydroxysilane with acid will yield one alkene, while treatment of the same β-hydroxysilane with base will yield the alkene of opposite stereochemistry.

The action of base upon a β-hydroxysilane 1 results in a concerted syn elimination of 2 or 3 to form the desired alkene. The penta-coordinate silicate intermediate 3 is postulated, but no proof exists to date.

Potassium alkoxides eliminate quickly, while sodium alkoxides generally require heating. Magnesium alkoxides only eliminate in extreme conditions. The order of reactivity of alkoxides, K > Na >> Mg, is consistent with higher electron density on oxygen, hence increasing the alkoxide nucleophilicity.

The treatment of the β-hydroxysilane 1 with acid results in protonation and an anti elimination to form the desired alkene.

When the α-silyl carbanion contains only alkyl, hydrogen, or electron-donating substituents, the stereochemical outcome of the Peterson olefination can be controlled, because at low temperature the elimination is slow and the intermediate β-hydroxysilane can be isolated.

Once isolated, the diastereomeric β-hydroxysilanes are separated. One diastereomer is treated with acid, while the other is treated with base, thus converted the material to an alkene with the required stereochemistry.

When the α-silyl carbanion contains electron-withdrawing substituents, the Peterson olefination directly forms the alkene. The intermediate β-hydroxysilane cannot be isolated as it eliminates in-situ. The basic elimination pathway has been postulated in these cases.

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