Iminium salt-catalyzed liquid-phase Beckmann rearrangement of cyclohexanone oxime

Alkoxy-methylene-N,N-dimethyliminium salt, a kind of Meerwein salt, effectively catalyzed the liquid-phase Beckmann rearrangement of cyclohexanone oxime to E-caprolactam under mild reaction conditions (50-60 degrees C, atmospheric pressure). The iminium salt was easily prepared in situ either by O-alkylation of N,N-dimethylformamide (DMF) with dimethyl sulfate or by reacting DMF with epichlorohydrin in the presence of a strong acid. The product lactam poisoned the iminium catalyst, though reversibly. In the reaction employing a low concentration of the oxime (0.29 M), turnover of the iminium catalyst derived from heteropolyacid attained 200. A possible mechanism which involves intramolecular migration of the hydroxy group of oxime was proposed.