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. 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. The Corey-Winter olefination is a
stereospecific reaction: a
trans-diol gives a
trans-alkene, while a
cis-diol gives a
cis-alkene as the product. For instance,
cis- and
trans-1,2-cyclodecanediol gives the respective
cis- and
trans-
cyclodecene. ==References==