Intermolecular triplet-sensitized photolysis of alkyl azides: Trapping of triplet alkyl nitrenes

We report the first observation that the intermolecular triplet sensitization of alkyl azides leads to bimolecular reactivity, by forming triplet alkyl nitrenes. The intermolecular triplet-sensitized photolysis of 1-azidoadamantane with acetone, acetophenone and benzophenone leads to the formation of di-(adamantan-1-yl)-1,2-diazene as the major product via dimerization of triplet adamantan-1-yl nitrene. The triplet alkyl nitrene also abstracts a H-atom from the solvent to form adamantan-1-yl amine, adamantan-1-yl-benzyl amine and adamantan-1-yl benzylidene amine. The rates of the energy transfer from acetophenone and benzophenone to 1-azidodamantane are 1×108 and 5×106 M−1 s−1, respectively. Triplet-sensitized photolysis of benzyl azide with acetophenone gave methylene phenyl amine, dibenzyl amine and tribenzyl amine as the major products. Thus triplet benzyl nitrene abstracts a H-atom from the solvent to form dibenzyl amine and tribenzyl amine and rearranges to form methylene phenyl amine. The energy transfer rate between acetophenone and benzyl azide is diffusion controlled, or 2×109 M−1 s−1. Sensitized photolysis of benzyl azide with benzophenone yielded only benzylideneamine. Presumably, benzophenone abstracts a H-atom from benzyl azide and the resulting radical rearranges into benzylideneamine. The rate of the chemical quenching of benzophenone with benzyl azide is 5×107 M−1 s−1.