12.18:
Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism
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.
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.
