Computational Studies of Reactions Involving Acyl Radicals

Acyl radicals prefer to attack at the nitrogen end of an imine (C=N) group although they are commonly thought to be nucleophilic. We have been interested in this intriguing selectivity of acyl radicals and sought recourse in ab-initio and DFT calculations to shed light on the chemistry of these radicals. In this paper, the origin of the N-philicity of acyl radicals is discussed using (1) geometry of the transition states (TS) involved in the addition reaction, (2) energy barriers for the reaction, (3) the multi-orbital interactions and (4) the NBO analysis of the TS. Calculations revealed that alpha,beta-unsaturated acyl radicals are even more N-philic, supported by experimental results in which alpha,beta-unsaturated acyl radicals react with imines to give 4-8 membered rings exclusively. Our computational investigations successfully established a reasonable mechanistic pathway for the reaction of alpha,beta-unsaturated acyl radicals with amines involving 1,4-hydrogen shift and homolytic substitution reaction at nitrogen. Calculations of other types of reactions involving acyl radicals, such as homolytic substitution at group XIV atoms, halogen atom transfer, and reduction with borohydride, are also described.