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Photoenols, formed through photoinduced intra-molecular H atom abstraction in o-alkyl-substituted arylketones,typically have limited utility as reactive intermediates owing to fastreversion to the starting material. Herein, we introduced an azidogroup on the o-alkyl substituent to render the photoreactionirreversible. Irradiation of 2-azidomethylbenzophenone (1) inmethanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchangedand that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet andsinglet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2 

Abstract Although alkyl azides are known to typically form imines under direct irradiation, the product formation mechanism remains ambiguous as some alkyl azides also yield the corresponding triplet alkylnitrenes at cryogenic temperatures. The photoreactivity of 3‐azido‐3‐phenyl‐3H‐isobenzofuran‐1‐one (1) was investigated in solution and in cryogenic matrices. Irradiation (λ = 254 nm) of azide 1 in acetonitrile yielded a mixture of imines 2 and 3. Monitoring of the reaction progress using UV‐Vis absorption spectroscopy revealed an isosbestic point at 210 nm, indicating that the reaction proceeded cleanly. Similar results were observed for the photoreactivity of azide 1 in a frozen 2‐methyltetrahydrofuran (mTHF) matrix. Irradiation of azide 1 in an argon matrix at 6 K resulted in the disappearance of its IR bands with the concurrent appearance of IR bands corresponding to imines 2 and 3. Thus, it was theorized that azide 1 forms imines 2 and 3 via a concerted mechanism from its singlet excited state or through singlet alkylnitrene¹1N, which does not intersystem cross to its triplet configuration. This proposal was supported by CASPT2 calculations on a model system, which suggested that the energy gap between the singlet and triplet configurations of alkylnitrene 1N is 33 kcal/mol, thus making intersystem crossing inefficient.