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—CH3 and C—CO bonds and this is the source of the reactivity differences.