Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

The McMurry reaction converts aldehydes or ketones into alkenes through a two-step process involving titanium. Unlike pinacol coupling, which produces vicinal diols at low temperatures, the McMurry reaction proceeds further as titanium reacts with the diol intermediate to eliminate oxygen and form a carbon-carbon double bond.

The first step involves single-electron transfer from reduced titanium to carbonyl groups, forming radicals that couple to produce a diol intermediate. The second step is slow deoxygenation, where oxygen atoms bind to titanium species or coordinate to titanium complexes, yielding the final alkene product.

Titanium's unusual behavior stems from its ability to continue reacting after forming the initial diol product. At low temperatures, diols can be isolated, but titanium proceeds to deoxygenate the diol and form alkenes, making it uniquely suited for the McMurry reaction compared to other reducing metals.

The intermolecular McMurry reaction typically uses two identical carbonyl compounds to produce tetrasubstituted symmetrical alkenes in good yield. This selectivity for symmetrical products makes it a reliable method for synthesizing specific alkene structures from simple starting materials and carbonyl precursors.

The intramolecular McMurry reaction works exceptionally well for forming large cycloalkenes with eight or more carbon atoms in the ring. This approach is demonstrated by the cyclization of a 15-keto-aldehyde to flexibilene, a natural product containing a 15-membered ring, showcasing the reaction's utility in complex ring synthesis.

Single-electron transfer from reduced titanium species to carbonyl oxygens initiates the McMurry reaction by generating ketyl radicals. These radicals undergo radical-radical coupling between carbonyl carbons to form the diol intermediate, which then undergoes deoxygenation to yield the final alkene product.

Deoxygenation is slow because it requires oxygen atoms to bind to titanium metal particles or coordinate to titanium complexes on the metal surface. This slow step determines the overall reaction rate and is essential for converting the diol intermediate into the desired alkene product.