Chemistry of amido radicals: intramolecular hydrogen abstraction as related to amido radical configurations

Abstract

Model bromamides were photolysed to investigate the reactivity of intramolecular C-5 hydrogen abstraction (from the nitrogen atom) of the corresponding amidyl radicals and of the cyclization of the resulting C-bromides. The observed results are discussed in terms of stereoelectronic as well as energetic considerations. In the former C-5 hydrogen abstraction, the reactivity difference between semi-rigid trans and cis-oriented reaction centers is only slightly in favor of the latter; the small difference is interpreted as due to the planar ground state Π amidyl radical structure with a low N—CO twisting barrier (4-5 kcal/mol). This interpretation is supported by the efficient C-5 hydrogen abstraction in N-bromo-N-alkyltoluamides, since a rigid planar amidyl radical does not allow the methyl C—H bond to colinearly approach the Π orbital. In marked contrast, there are large reactivity differences in the cyclization of C-bromides derived from these intramolecular H—Br exchanges; these arise from the requirement of colinearity of the approaching and departing bonds. Thus this requires the coplanar orientation of the C—CH 3 and C—CO bonds and this is the source of the reactivity differences.