Triplet m-nitrophenyl nitrene reacts with frozen polycrystalline toluene at the benzylic position to give appreciable yields of a formal CH insertion adduct. This reaction does not occur on photolysis of m-nitrophenyl azide in toluene solution. Photoaffinity labelling (PAL) is a widely used technique in biochemistry and aryl azides are undoubtedly the most popular of all labelling reagents.2 In the PAL experiment one appends an azide to a natural ligand of a biological receptor. Photolysis of the receptorligand complex should produce a reactive intermediate capable of forming a covalent bond to the biomolecular host. In an ideal case the reactive intermediate will undergo a formal insertion into an alkyl CH bond. Unfortunately, this is an extremely rare reaction for aryl nitrenes.3 Photolysis of phenyl azide in alkanes or in toluene leads mainly to tar accompanied by small amounts of azobenzene, aniline, and virtually zero insertion product.3 A few examples of formal nitrene insertion into aryl CH bonds have been reported.4 The only known example of aryl nitrene insertion into an alkyl CH bond was reported by Lindley who performed triplet sensitized photolyses of phenyl azides at high (>lOO') temperature.5 Although very interesting the conditions of high temperatures and photosensitization are inappropriate for PAL. In this letter we report a set of experimental conditions which do lead to formal nitrene CH insertion that may be compatible with PAL. MOSS,~ Tomioka,' and Platz8 have found that carbenes experience markedly enhanced yields of formal CH insertion products when they are generated in low temperature solids. Accordingly the photochemistry of phenyl azide was studied as a function of temperature and phase.9 Leyva found that photolysis of phenyl azide at low temperature gave high yields of the triplet phenyl nitrene derived products aniline and azobenzene, in marked contrast to the near exclusive formation of singlet derived products in solution at room temperature. No products of formal triplet phenyl nitrene CH insertion with toluene or cyclohexane were observed at low temperature, however. Reiser and Leyshon have shown that electron withdrawing groups increase the reactivity of triplet aryl nitrenes.l' Accordingly we repeated our analysis of the temperature and phase dependent photochemistry of aryl azides with m-nitrophenyl azide 1.