The reaction mechanism for the Johnson – Corey – Chaykovsky reaction consist of nucleophilic addition of the ylide to the carbonyl or imine group . A negative charge is transferred to the heteroatom and because the sulfonium cation is a good leaving group it get expelled forming the ring . In the related Wittig reaction , the formation of the much stronger phosphorus @-@ oxygen double bond prevent oxirane formation and instead , olefination take place through a 4 @-@ membered cyclic intermediate .