Photochemistry of 2‑Iodobenzothiazole and Iodine-Triggered 2‑Isocyanophenylthiyl Radical Cyclization in Argon Matrices

The photochemistry of 2-iodobenzothiazole (IBT) was investigated in an argon matrix at 4 K. Irradiating matrix-isolated IBT at λ = 254 nm led to the formation of 2-isocyanophenylthiyl radical 3 and benzothiirene 4; surprisingly, the 2-dehydrobenzothiazole radical 1 was not detected. A subsequent 365 nm irradiation or annealing of matrix containing 3 and iodine atom (at 30 K) reverted the precursor IBT, indicating an iodine-triggered thiyl radical-based cyclization, which was confirmed by computations using density functional theory. For gaining mechanistic insights, similar investigations were performed using benzothiazole BT as a precursor. Once again, the 254 nm light-induced C–S scission accompanied by H-shift led to 2-isocyanothiophenol 7. A conformational analysis of 2-isocyanothiophenol 7 and its photochemistry was also carried out with the help of IR spectroscopy under cryogenic conditions and computations. Considering the importance of thiyl radicals in biomolecular pathways, the investigations on its photochemical generation and a rare, reversible phenomenon of iodine-mediated cyclization under cryogenic conditions are insightful for fundamental photochemistry and sulfur chemistry.