The decomposition of benzenesulfonyl azide: a matrix isolation and computational study

The thermal-decomposition and photo-decomposition of benzenesulfonyl azide, PhS(O)₂N₃, have been studied by combining matrix-isolation IR spectroscopy and quantum chemical calculations. Upon flash vacuum pyrolysis at 800 K, the azide splits off molecular nitrogen and exclusively furnishes phenylnitrene (PhN) and SO₂ in the gas phase. In contrast, the azide favors stepwise photodecomposition in solid Ar and Ne matrices at 2.8 K. Specifically, the UV laser photolysis (193 and 266 nm) of PhS(O)₂N₃ results in the formation of the key nitrene intermediate PhS(O)₂N in the triplet ground state, which undergoes pseudo-Curtius rearrangement into N-sulfonyl imine PhNSO₂ under subsequent visible light irradiation (380-450 nm). Further fragmentation of PhNSO₂ into SO₂ and PhN followed by ring-expansion to didehydroazepine also occurs upon visible light irradiation. The preference of the stepwise mechanism for the decomposition of PhS(O)₂N₃ is supported by quantum chemical calculations using DFT B3LYP/6-311++G(3df,3pd) and CBS-QB3 methods.