The willgerodt-kindler reactions. 6. Isomerization of the carbonyl group in alkanones and cycloalkanones

The most unusual feature of the Willgerodt-Kindler Reactions is the facile isomerization of the carbonyl function along a chain of unbranched methylene groups, or around a cycloaliphatic ring containing several connected methylene groups. We have demonstrated that the first step in the Kindler process is the formation of enamines by reaction of the carbonyl function with secondary aliphatic amines, followed by reaction of the enamine with certain sulfur-amine catalysts to form reactive heterocyclic sulfur intermediates that facilitate the elimination-readdition of the amines reversibly along the chain. It was shown that compounds of the type R₂N-S-S-NR₂ are effective catalysts but not compounds of the type R₂N-S-NR₂. Some cyclohexanone derivatives undergo aromatization, with anomalous results in certain cases.