Reaction mechanism Peterson olefination

1 reaction mechanism

1.1 basic elimination
1.2 acidic elimination
1.3 alkyl substituents
1.4 electron-withdrawing substituents

reaction mechanism

one attractive feature of peterson olefination can used prepare either cis- or trans-alkenes same β-hydroxysilane. treatment of β-hydroxysilane acid yield 1 alkene, while treatment of same β-hydroxysilane base yield alkene of opposite stereochemistry.

basic elimination

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

potassium alkoxides eliminate quickly, while sodium alkoxides require heating. magnesium alkoxides eliminate in extreme conditions. order of reactivity of alkoxides, k > na >> mg, consistent higher electron density on oxygen, hence increasing alkoxide nucleophilicity.

acidic elimination

the treatment of β-hydroxysilane 1 acid results in protonation , anti elimination form desired alkene.

alkyl substituents

when α-silyl carbanion contains alkyl, hydrogen, or electron-donating substituents, stereochemical outcome of peterson olefination can controlled, because @ low temperature elimination slow , intermediate β-hydroxysilane can isolated.

once isolated, diastereomeric β-hydroxysilanes separated. 1 diastereomer treated acid, while other treated base, converted material alkene required stereochemistry.

electron-withdrawing substituents

when α-silyl carbanion contains electron-withdrawing substituents, peterson olefination directly forms alkene. intermediate β-hydroxysilane cannot isolated eliminates in-situ. basic elimination pathway has been postulated in these cases.