Curtius-Type Rearrangement of Sulfinyl Azides: A Matrix Isolation and Computational Study.

Abstract

The highly unstable methyl sulfinyl azide, CH3S(O)N3, has been synthesized and characterized for the first time. In the gas phase, CH3S(O)N3 decomposes quickly at room temperature (300 K) with an estimated half-life (t1/2) of 7 min. Upon irradiation at 266 nm in cryogenic Ar (10 K) and Ne (3 K) matrices, the azide extrudes molecular nitrogen by yielding the novel sulfinyl nitrene intermediate CH3S(O)N in the closed-shell singlet ground state, which has been characterized with matrix-isolation IR and UV-vis spectroscopy. Prolonged irradiation at 266 nm causes Curtius rearrangement of the nitrene to form N-sulfinylamine CH3NSO and S-nitrosothiol CH3SNO. By high-vacuum flash pyrolysis (HVFP) at 800 K, CH3S(O)N3 also decomposes and furnishes CH3S(O)N with minor fragmentation products HNSO and CH2 in the gas phase. A similar photo-induced Curtius-type rearrangement of trifluoromethyl sulfinyl azide CF3S(O)N3 to CF3NSO and CF3SNO has also been observed in matrices. According to the theoretical calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G (3df,3pd) level of theory, the rearrangement of CH3S(O)N3 prefers a stepwise pathway by initial formation of the nitrene intermediate CH3S(O)N. In line with the thermal persistence of CH3S(O)N in the gas phase, the barriers for its subsequent rearrangement are higher than 30 kcal mol-1.