12.18 : 醛和酮转化为烯烃：Wittig 反应机理

The Wittig reaction converts aldehydes or ketones to alkenes using phosphorus ylides.

The reaction occurs via a nucleophilic addition‒elimination process generating a new C＝C bond in the product.

Nucleophilic addition begins with the reaction between phosphorus ylide and the carbonyl compound.

Phosphorus ylide is a strong nucleophile that attacks the electrophilic carbonyl group to generate a charge-separated dipolar intermediate — a betaine.

In the intermediate, the negatively charged oxygen atom and the positively charged phosphorus atom undergo a ring-closure reaction to give the four-membered oxaphosphetane ring.

In some instances, the oxaphosphetane intermediate is formed via a concerted [2 + 2] cycloaddition between the Wittig reagent and the carbonyl compound.

The unstable oxaphosphetane ring undergoes fragmentation to yield the desired alkene along with the stable by-product triphenylphosphine oxide.

The formation of the strong P＝O bond in the by-product is the driving force for the Wittig reaction.