Photochemical behavior of thioketenes in solution: reaction from S2

Abstract

The higher excited-state reactions of sterically encumbered thioketenes la-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 $S_2$. The reactive state is identified to be S2 (\pi \pi^*)$. 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.