20.30: 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.