Electron impact induced rearrangements of organic ions. Part 111. A Beckmann-like aryl migration in substituted isoxazol-5(4H)-one molecular ions

Abstract

A novel unimolecular reaction leading to primary amine radical ions occurs in the gas phase decomposition of ions generated from substituted isoxazol-5(4H)-ones, upon electron impact. The proposed mechanism implies aryl migration as the first step, and this is unambiguously demonstrated by ¹⁵N and ²H labelling experiments, metastable transition studies, and exact mass measurements. This [1,2] sigmatropic shift is a fast process, as evidenced by a primary kinetic isotope effect on competing metastable transitions. Energetic and electronic factors involved in the process suggest striking similarities to the Beckmann rearrangement in the condensed phase.