Oxime rearrangements: ab initio calculations and reactions in the solid state

Abstract

Rearrangements of oximes in reactions with strong Lewis acids may include Beckmann alkyl group shift or hydride migration. These reaction mechanisms were studied by FT-IR spectroscopy in solid superacid cryogenic matrices and supported by ab initio calculations. It was found that cyclic oximes with smaller rings (cyclobutanone oxime and cyclopentanone oxime) undergo hydride migration rather than alkyl group shift. Resulting aminoallyl cations were isolated in superacid cryogenic matrix and characterized spectroscopically. Cyclohexanone oxime rearranges by a classical Beckmann mechanism to a cyclic seven-membered nitrylium cation, which was also observed in the matrix. The ion is found to be persistent in solid superacid medium despite the highly strained non-linear carbon–nitrogen triple bond. It seems that hydrogen bonds with halogens formed in the Lewis acid–oxime complex could play an important role in the hydrogen migrations.