20.26 : Alkene durch reduktive Kupplung von Aldehyden oder Ketonen: McMurry-Reaktion

The pinacol coupling reaction involves the radical dimerization of ketones or aldehydes to give vicinal diols.

Among the different metals used as the source of electrons for the reaction, titanium is unusual.

If the reaction is performed at a low temperature, diols can be isolated. Otherwise, titanium reacts further to form alkenes. This forms the basis of the McMurry reaction.

The McMurry reaction proceeds in two steps.

The first step may be analogous to pinacol coupling, where single-electron interactions between the carbonyl oxygens and a reduced titanium species lead to radical–radical coupling between the carbonyl carbons.

The second step is slow deoxygenation, which leaves the oxygen atoms with the titanium species to yield an alkene.

The intermolecular McMurry reaction typically uses two of the same carbonyl compounds to make tetrasubstituted symmetrical alkenes, often in good yield.

Interestingly, the intramolecular McMurry reaction works well for making large cycloalkenes, like the cyclization of a 15-keto-aldehyde to flexibilene, a natural product with a 15-membered ring.