The photodecomposition of cyclic N-bromo imides: evidence for stereoelectronic control in intramolecular hydrogen transfer in imidyl radicals

Abstract

In order to examine the reactivities and configurations of imidyl radicals, several cyclic N-bromo imides possessing five- or six-membered imide rings have been photolytically decomposed in the presence of 1,1-dichloroethene in methylene chloride in order to scavenge bromine atoms and bromine. The five-membered N-bromo imides and (±)-N-bromocamphorimide undergo ring opening to afford products derived from C-bromo acyl isocyanates. While the corresponding imidyl radicals abstract a hydrogen atom from methylene chloride, none of them undergo intramolecular H-atom abstraction to effect an intramolecular hydrogen–bromine exchange reaction, even when there are suitably located C–H bonds at a fifth (or sixth) position that can overlap the π-orbital of the imidyl radicals. This phenomenon clearly reveals the presence of Stereoelectronic controls on the intramolecular H-abstraction of imidyl radicals, and indicates that the reactive state of imidyl radicals does not possess the Π-electronic configuration. We conclude that imidyl radicals possess the ε-electronic configuration in their reactive state (which is probably the ground state). The intramolecular addition of a C-radical centre to an isocyanate group is established; this indicates that the reversibility of the ring-opening reaction of imidyl radicals is controlled by steric strains among other factors. Because of the presence of reactive allylic hydrogens, the N-bromo imide (10) upon photolysis forms the C-bromo compounds (11) efficiently by a bromine atom chain process which is retarded by the presence of a better Br˙ scavenging alkene.