Dissociation or cyclization : options for a triad of radicals released from oxime carbamates

Abstract

A set of oxime carbamates, having N-alkyl- and N,N-dialkyl-substituents, was prepared via carbonyldiimidazole intermediates. It was shown by EPR spectroscopy that they underwent clean homolysis of their NO bonds on UV photolysis. During photolysis of acetophenone O-allylcarbamoyl oxime the corresponding oxazolidin-2-onylmethyl radical was detected by EPR spectroscopy, providing the first evidence that N-mono-substituted carbamoyloxyl radicals can hold their structural integrity. N,N-Di-substituted carbamoyloxyl radicals dissociated rapidly at the lowest accessible temperatures. Above room temperature both types of oxime carbamate acted as selective new precursors for aminyl and iminyl radicals. Rate parameters were measured for 5-exo-cyclization of N-benzyl-N-pent-4-enylaminyl radicals; the rate constant was smaller than for C-centered and O-centered analogs. Oxime carbamates derived from the volatile diethylamine afforded aryl-iminyl radicals that proved convenient for phenanthridine preparations.