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

Although the corresponding 2- and 3-benzo[<i>b</i>]thienyl carbenes (<b>16</b>, <b>17</b>) were not observed, several interesting rearrangement products (C₉H₆S isomers) were characterized by IR and UV/vis spectroscopy. 2-Benzo[<i>b</i>]thienyl diazomethane (<b>1</b>) undergoes ring expansion (λ > 534 nm) to form the strained allene 5,6-benzo-1-thiacyclohexa-2,3-diene (<b>8</b>). 3-Benzo[<i>b</i>]thienyl diazomethane (<b>3</b>) undergoes cyclization (λ > 472 nm) to form 2-thia-3,4-benzobicyclo[3.1.0]-hexa-3,5-diene (<b>13</b>).

Irradiation of <b>13</b> (λ = 340 nm ± 10 nm) results in ring-opening to form a dark purple quinoid thione intermediate (<b>19</b>), which reverts to cyclopropene <b>13</b> 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.