Temperature-DependentPhotoreactivity of 2‑Azidomethylbenzophenone:Insights into the Triplet Imine Biradical Pathway

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