Glutarimidyl Chemistry: Substitution Reactions. Mechanism of "Ziegler Brominations"

Article abstract of DOI:10.1021/jo00361a026

Glutarimidyl (G-radical) radicals are generated in liquid-phase chain reactions by halogen atom abstractions from N-haloglutarimides by alkyl radicals.These reactions are carried out in the presence of small amounts of alkenes which act as halogen scavengers to eliminate halogen atom chains.The distinguishing characteristics of glutarimidyl radicals are (1) a constant hydrogen abstraction ratio (k_{neo-C5H12^/kCH2Cl2⁾H} = 5.3 at 15 deg C, over a wide range of reaction conditions, (2) no ring opening with glutarimidyls lacking 2-substituents, and (3) ring opening to make 4-bromoalkanoyl isocyanates with N-bromoglutarimides substituted by methyl(s) in the 2-position.Glutarimidyl radical hydrogen abstraction selectivities are characterized by early transition states for a variety of substrates, with behavior similar to that shown by chlorine atoms and by succinimidyl radicals.With adequate scavenging of bromine, using 1,3-butadiene or norbornene, brominations of benzylic hydrogen take place with the G-radical carrier, with selectivities similar to those obtained with Cl-radical, thus providing definitive proof that "Ziegler brominations" are not attributable to G-radical hydrogen abstractions.