12.17 : 醛和酮转化为烯烃：维蒂希反应概述

The Wittig reaction converts aldehydes or ketones to alkenes using phosphorus ylide, or the Wittig reagent.

Phosphorus ylide is a neutral molecule with a negatively charged carbon and a positively charged phosphorus atom, stabilized by resonance. It is synthesized from unhindered alkyl halides in two steps.

First, triphenylphosphine attacks the alkyl halide via an S_N2 process, forming a phosphonium salt.

The salt reacts with a strong base that deprotonates the weakly acidic α hydrogen, generating the carbanionic ylide nucleophile.

Wittig reactions are regioselective, as the new C=C bond is formed explicitly at the carbonyl position.

The reactions are also stereoselective depending on the ylide.

Phosphorus ylides with electron-withdrawing groups-stabilized by additional resonance structure-predominantly generate E alkenes. Reagents with simple alkyl groups primarily form Z alkenes.

Steric crowding around the carbonyl group affects the yield of the products formed: Sterically hindered ketones give poor yields compared to unhindered aldehydes.