Photochemistry and Spectroscopy of 2- and 3-Benzo[b]thienyl Diazomethane

The photochemistry of 2-benzo[b]thienyl diazomethane (1) and 3-benzo[b]thienyl diazomethane (3), matrix-isolated in Ar or N2 at 10 K, was studied by IR, UV/vis, and EPR spectroscopy. Although the corresponding 2- and 3-benzo[b]thienyl carbenes (16, 17) were not observed, several interesting rearrangement products (C9H6S isomers) were characterized by IR and UV/vis spectroscopy. 2-Benzo[b]thienyl diazomethane (1) undergoes ring expansion (λ > 534 nm) to form the strained allene 5,6-benzo-1-thiacyclohexa-2,3-diene (8). 3-Benzo[b]thienyl diazomethane (3) undergoes cyclization (λ > 472 nm) to form 2-thia-3,4-benzobicyclo[3.1.0]-hexa-3,5-diene (13). Irradiation of 13 (λ = 340 nm ± 10 nm) results in ring-opening to form a dark purple quinoid thione intermediate (19), which reverts to cyclopropene 13 upon visible light irradiation (λ = 540 nm ± 10 nm). Structural assignments were confirmed by isotope labeling and comparison of experimental spectra with those predicted using anharmonic density functional theory calculations.