Photochemical behavior of thioketenes in solution: Reaction from S₂

The higher excited-state reactions of sterically encumbered thioketenes 1a-d in solution were investigated. Substituted thiiranes, thio esters, thioacetophenones, and 1,2-diones are formed on photoexcitation of thioketenes in hydroxylic and nonhydroxylic solvents. Thioketenes while unreactive upon excitation to S₁ produce thiiranylidene carbene and zwitterionic intermediates upon excitation to S₂. The reactive state is identified to be S₂ (ππ*). The excited thioketene resists carbon monosulfide elimination but undergoes rearrangement. The photobehavior of thioketenes, established in solution for the first time, differs significantly from that of ketenes, and it cannot be extrapolated from that of structurally analogous ketenes and allenes.