20.26:
Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction
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.
20.26:
Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction
The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.
The reaction is a two-step process. The mechanism is still under study, but for some reagent combinations, the first step is similar to the pinacol coupling reaction, involving a single electron transfer from titanium to the carbonyl group to form a ketyl, eventually forming a diol. The second step involves deoxygenation of the diol via binding on the surface of titanium metal particles or coordinating to titanium complexes to yield an alkene. The McMurry coupling reaction involving two same carbonyl compounds gives symmetrical alkenes.
The intramolecular McMurry reaction affords cycloalkenes with eight or more carbon atoms in the ring.