Chemistry of amido radicals: Stereochemical controls in II radical reactions

Abstract

The acyclic amido radicals generated from the corresponding N-bromo-N-tert-alkylamides reacted like N-radicals, rather than O-radicals, and preferentially abstracted a δ-hydrogen of the alkyl chain, rather than a γ-hydrogen of the acyl chain. On the basis that, in these amido radicals, the conformation of the transition state of the hydrogen transfer is a five-membered ring and the stereoelectronic requirement for the transfer demands a colinear arrangement of the radical containing orbital and the C—H bond, the results are satisfactorily rationalized by the II electron configuration of the amido radicals. It is also established that the chain length of these N-bromoamide photodecompositions increases as the intramolecular hydrogen transfer of the corresponding amido radicals becomes easier; the hydrogen abstraction step must be the rate determining step in the chain reaction. The spontaneous formations of γ-iminolactones from γ-bromoamides indicate that the intramolecular nucleophilic displacement by the O-center of the amido group is facilitated since its transition state conformation readily accommodates the stereoelectronic requirements for the nucleophilic substitution. A colinear approach of the p-orbital of the more nucleophilic N-center to the rear side of the γ-C—Br bond is shown to be prohibitively strained.