Carbonyl sulfides as possible intermediates in the photolysis of oxathiiranes

Diphenyl oxathiirane, formed by irradiation of thiobenzophenone S-oxide at 77 K, is photochemically converted into a blue, thermally unstable compound which decomposes at ca 100–110 K (λ_max 550 nm, (? ca 11,000). Lack of change in magnetic susceptibility during the light induced conversion of sulfine to ketone via the oxathiirane and the subsequent blue intermediate implies the absence of triplet and biradical singlet transients. The unknown carbonyl sulfide functionality, R₂COS, thereby emerges as a strong candidate for producing the visible absorption.Comparison of the wave functions for CH₂SO and CH₂OS arising from MNDO limited CI geometry optimizations leads to the conclusion that the carbonyl sulfide structure is best described as a zwitterion rather than as a singlet biradical. The failure to observe cycloaddition products between the blue species and several dipolarophiles is rationlized in terms of a labile carbonyl suffide intermediate capable of facile sulfur extrusion from a long, weak O-S bond. Finally, the electronic absorption spectra of a series of para-substituted benzaldehyde O-sulfide model system have been calculated with CNDO/S-CI and correlated with the λ_max's of the corresponding series of diaryl blue substrates.The sum of the available experimental and theoretical data is consistent with the existence of closed shell carbonyl sulfides as observable, though labile, intermediates from the photolysis of oxathinanes.