 The reaction mechanism for the Johnson – Corey – Chaykovsky reaction consists of nucleophilic addition of the ylide to the <unk> or <unk> group . A negative charge is transferred to the <unk> and because the sulfonium cation is a good leaving group it gets expelled forming the ring . In the related <unk> reaction , the formation of the much stronger phosphorus @-@ oxygen double bond prevents <unk> formation and instead , <unk> takes place through a 4 @-@ membered cyclic intermediate . 
