Radiolytic studies of the reductive cyclization of 2‐nitroarylamides: Cyclization via hydroxylamine intermediates

The reductive cyclization of several 2‐nitroarylamides was studied by radiolytic reduction, examining the effects of substituents on the nitrophenyl ring and on the leaving aniline and variations in the nature of the link between the nitrophenyl ring and the leaving aniline. The stoichiometry of the reduction and the identification of N‐hydroxylactam and aniline products suggest that the major initial products of such a reduction of the nitroamides are the corresponding hydroxylamines. Under anaerobic conditions, cyclization via the hydroxylamines was considerably faster (up to 160‐fold) than via the corresponding amines under comparable conditions, but was similarly influenced by changes in geometry. Unlike cyclization via the amines, rates of cyclization via the hydroxylamines were sensitive to substitution on the leaving aniline, being accelerated by electron‐withdrawing groups. The rate‐determining step in the cyclization of the hydroxylamines is proposed to be breakdown of the tetrahedral intermediate.