Photochemistry of Matrix Isolated (Trifluoromethyl)sulfonyl Azide, CF3SO2N3

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

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 (t_1/2) 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.

Sulfonyl Nitrene and Amidyl Radical: Structure and Reactivity

Photocatalytic generation of nitrenes and radicals can be used to tune or even control their reactivity. Photocatalytic activation of sulfonyl azides leads to the elimination of N₂ and the resulting reactive species initiate C−H activations and amide formation reactions. Here, we present reactive radicals that are generated from sulfonyl azides: sulfonyl nitrene radical anion, sulfonyl nitrene and sulfonyl amidyl radical, and test their gas phase reactivity in C−H activation reactions. The sulfonyl nitrene radical anion is the least reactive and its reactivity is governed by the proton coupled electron transfer mechanism. In contrast, sulfonyl nitrene and sulfonyl amidyl radicals react via hydrogen atom transfer pathways. These reactivities and detailed characterization of the radicals with vibrational spectroscopy and with DFT calculations provide information necessary for taking control over the reactivity of these intermediates.

Photochemistry of HNSO2 in cryogenic matrices: spectroscopic identification of the intermediates and mechanism

Small molecules solely consisting of H, N, O, and S are highly relevant intermediates in atmospheric chemistry and biology. Even though several isomers of [HNO2S] have been computationally predicted, only the IR spectra for the two lowest-energy isomers HNSO2 and syn-syn HONSO have been previously reported. Herein, the photochemistry (193 nm laser) of HNSO2 in N2-, Ne-, and Ar-matrices (≤15 K) has been studied. Aside from syn-syn HONSO, several new isomers including anti-syn HONSO, gauche-syn HOSNO, syn HOS(O)N, anti HOS(O)N, syn HS(O)NO, anti HN(O)SO, gauche-syn HSONO, and an elusive caged-radical pair HOS˙˙NO have been identified. Additionally, the formation of fragments HONO, HO˙, ˙NO, and ˙NO2 has also been observed. The characterization of these species with matrix-isolation IR and UV/Vis spectroscopy is supported by 15N-labeling and quantum chemical computations at the B3LYP/6-311++G(3df,3pd) level. Furthermore, the photo-induced isomerization reactions, including the conformational conversion of syn-syn HONSO → anti-syn HONSO and reversible isomerization of HOSNO ↔ anti-syn HONSO, syn-syn HONSO ↔ HN(O)SO, HSONO ↔ HS(O)NO, and HOS˙˙NO ↔ HOSNO have also been observed, and the underlying mechanism is discussed.

Heterocumulenic carbene nitric oxide radical OCCNO˙

The elusive heterocumulenic radical OCCNO˙ and its isotopologues OC13CNO˙ and OCC15NO˙ have been prepared by reacting photolytically generated unsaturated carbene OCC/OC13C with ˙NO/15˙NO in cryogenic N2-, Ar-, and Ne-matrices. Upon UV-light (365 nm) irradiation, the C-C bond in OCCNO˙ breaks and yields a long-sought ground-state radical CNO˙ (X2Π), which has also been identified with matrix-isolation infrared spectroscopy.

Photodecomposition of Thienylsulfonyl Azides: Generation and Spectroscopic Characterization of Triplet Thienylsulfonyl Nitrenes and 3-Thienylnitrene

The photochemistry of 2-thienylsulfonyl azide (1) and 3-thienylsulfonyl azide (6) has been disclosed by combining matrix-isolation spectroscopy with quantum chemical calculations. Two novel heteroaryl sulfonyl nitrenes, 2-thienylsulfonyl nitrene (2) and 3-thienylsulfonyl nitrene (7), have been generated during the 266 nm laser photolysis of 1 and 6, respectively. Both nitrenes in the triplet ground state have been characterized with matrix-isolation IR (¹⁵N-labeling) in solid Ar (10.0 K), N₂ (10.0 K), and Ne (2.8 K) matrixes and EPR spectroscopy (2, |D/hc| = 1.452 cm^-1 and |E/hc| = 0.0058 cm^-1; 7, |D/hc| = 1.492 cm^-1 and |E/hc| = 0.0060 cm^-1) in solid toluene at 5.0 K. Upon subsequent UV-light irradiation (365 nm), no Curtius rearrangement but decomposition of 2 occurs by SO₂-elimination and the concurrent formation of ring-opening product (s-Z)-4-thioxo-2-butenenitrile (3) via the intermediacy of the putative 2-thienylnitrene (4). In contrast, violet-light irradiation (400 ± 20 nm) of 7 causes SO₂-elimination to yield triplet 3-thienylnitrene (8), for which the IR spectroscopic identification is supported by quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level. 3-Thienylnitrene is highly reactive, since it not only combines with SO₂ to furnish 3-thienyl-N-sulfonylamine (9) but also undergoes ring-opening to (s-E)-4-thioxo-2-butenenitrile (10) under the UV-light irradiation (365 nm).

Chloro- and Dichloro-methylsulfonyl Nitrenes: Spectroscopic Characterization, Photoisomerization, and Thermal Decomposition

Chloro- and dichloro-methylsulfonyl nitrenes, CH₂ClS(O)₂N and CHCl₂S(O)₂N, have been generated from UV laser photolysis (193 and 266 nm) of the corresponding sulfonyl azides CH₂ClS(O)₂N₃ and CHCl₂S(O)₂N₃, respectively. Both nitrenes have been characterized with matrix-isolation IR and EPR spectroscopy in solid N₂ (10 K) and glassy toluene (5 K) matrices. Triplet ground-state multiplicity of CH₂ClS(O)₂N (|D/hc| = 1.57 cm^-1 and |E/hc| = 0.0026 cm^-1) and CHCl₂S(O)₂N (|D/hc| = 1.56 cm^-1 and |E/hc| = 0.0042 cm^-1) has been confirmed. In addition, dichloromethylnitrene CHCl₂N (|D/hc| = 1.57 cm^-1 and |E/hc| = 0 cm^-1), formed from SO₂-elimination in CHCl₂S(O)₂N, has also been identified for the first time. Upon UV light irradiation (365 nm), the two sulfonyl nitrenes R⁻S(O)₂N (R = CH₂Cl and CHCl₂) undergo concomitant 1,2-R shift to N-sulfonlyamines R⁻NSO₂ and 1,2-oxygen shift to S-nitroso compounds R⁻S(O)NO, respectively. The identification of these new species with IR spectroscopy is supported by ¹⁵N labeling experiments and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level. In contrast, the thermally-generated sulfonyl nitrenes CH₂ClS(O)₂N (600 K) and CHCl₂S(O)₂N (700 K) dissociate completely in the gas phase, and in both cases, HCN, SO₂, HCl, HNSO, and CO form. Additionally, ClCN, OCCl₂, HNSO₂, •NSO₂, and the atmospherically relevant radical •CHCl₂ are also identified among the fragmentation products of CHCl₂S(O)₂N. The underlying mechanisms for the rearrangement and decomposition of CH₂ClS(O)₂N and CHCl₂S(O)₂N are discussed based on the experimentally-observed products and the calculated potential energy profile.

Sulfamoyl nitrenes: singlet or triplet ground state?

Two prototypical sulfamoyl nitrenes R₂NS(O)₂–N (R = H and Me) in the triplet state were generated via the closed-shell singlet state by passing a low-energy minimum energy crossing point (MECP).

Thermal Rearrangement of Sulfamoyl Azides: Reactivity and Mechanistic Study

The rearrangement of sulfamoyl azides under thermal conditions to form a C-C bond while breaking two C-N bonds is reported. Mechanistic study shows that this reaction goes through a Curtius-type rearrangement to form a 1,1-diazene, then which rearranges possibly through both a concerted rearrangement process and a stepwise radical process. This rearrangement could be used in the synthesis of complex biologically active molecules, such as sterols, and piperine derivatives.

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

The stepwise Curtius-rearrangement of benzenesulfonyl azide, PhS(O)₂N₃, has been proven experimentally by the direct observation of the novel intermediates sulfonyl nitrene PhS(O)₂N and N-sulfonyl imine PhNSO₂ and theoretically by quantum chemical calculations.

Exploring photochemistry of p-bromophenylsulfonyl, p-tolylsulfonyl and methylsulfonyl azides by ultrafast UV-pump–IR-probe spectroscopy and computations

The photochemistry of three sulfonylazides was studied by femtosecond time-resolved infrared (TRIR) spectroscopy and quantum chemical computations.