12.18 : Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism

The Wittig reaction, which converts aldehydes or ketones to alkenes using phosphorus ylides, proceeds through a nucleophilic addition‒elimination process.

The reaction begins with the nucleophilic addition between a phosphorus ylide and the carbonyl compound. Due to its carbanionic character,  phosphorus ylide acts as a strong nucleophile and attacks the electrophilic carbonyl group. This generates a charge-separated dipolar intermediate called betaine. The negatively charged oxygen atom and the positively charged phosphorus atom in betaine undergo a ring-closure reaction to generate the four-membered oxaphosphetane ring.

In some cases, a concerted [2 + 2] cycloaddition between the Wittig reagent and the carbonyl compound results in the oxaphosphetane intermediate.

The unstable oxaphosphetane ring undergoes fragmentation to yield the desired alkene molecule along with a stable triphenylphosphine oxide as the by-product. The driving force for the Wittig reaction is the formation of a strong P＝O bond in the phosphine oxide molecule.