Temperature-Dependent Photoreactivity of 2-Azidomethylbenzophenone: Insights into the Triplet Imine Biradical Pathway.

Abstract

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