Ketones react with dimethylsulfonium methylide to yield epoxides. Suggest a mechanism for the reaction.

The epoxide is a three-member cyclic ring containing an oxygen atom. So we can say that epoxide contains an oxygen atom and two carbon atoms in the ring structure. As the three-member ring is highly strained so it is very reactive. Epoxides generally favor ring-opening reactions.

In this reaction dimethylsulfonium, methylidene is reacted with a ketone to give epoxide. Dimethylsulfonium is a sulfur ylide. The mechanism involves a nucleophilic addition reaction.

Mechanism

In the above mechanism carbon of sulfur, ylide attacks the electrophilic carbon atom of the carbonyl group. Simply it is an addition of sulfur ylide to a carbonyl group. In the next step, intramolecular substitution takes place in which negatively charge oxygen attacks the carbon and $\text{-S}{\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}$remove as a leaving group.