Corey–Winter olefin synthesis

The reaction mechanism involves the formation of a cyclic thiocarbonate from the diol and thiophosgene.

The second step involves treatment with trimethyl phosphite, which attacks the sulfur atom, producing S=P(OMe)3 (driven by the formation of a strong P=S double bond) and leaving a carbene.

[6] This carbene collapses with loss of carbon dioxide to give the olefin.

An alternative mechanism does not involve a free carbene intermediate, but rather involves attack of the carbanion by a second molecule of trimethylphosphite with concomitant cleavage of the sulfur-carbon bond.

The phosphorus stabilized carbanion then undergoes an elimination to give the alkene, along with an acyl phosphite, which then decarboxylates.