Photosensitized oxygenation of benzyl ethyl sulfide

Singlet oxygen adds to benzyl ethyl sulfide (5, total quenching rate ca. 1 x 10(7) M-1 s(-1), little dependent on the solvent) to ultimately give benz aldehyde (6) and a small amount of the sulfone (8) in aprotic media (rate o f the chemical reaction in benzene 5.5 x 10(6) M-1 s(-1)) and mainly the su lfoxide (7) in protic media (1.1 x 10(7) M-1 s(-1) in methanol). In the pre sence of small amounts (0.002-0.3 M) of protic additives (alcohols, phenol, carboxylic acids), the sulfoxide becomes the main product in benzene also. Various evidence support the formation of two intermediates in aprotic sol vents. The first one is an exciplex or a syn persulfoxide. It undergoes int ramolecular hydrogen abstraction to give a ylide and finally benzaldehyde. Such rearrangement is either a concerted or a radicalic process and not a p roton transfer (as indicated by the deuterium effect observed with the alph a-d benzyl sulfide and the occurrence of the process with the p-nitro and p -methoxy derivatives, 5' and 5 "). This intermediate is not quenched except under relatively strong acidic conditions. A second intermediate, arising either from the first one or through a parallel path, has the properties us ually associated with the persulfoxide (possibly it is the anti rotamer). T his species gives some sulfoxide but mainly decays to the unreacted sulfide ; it can be trapped intermolecularly, however, both by acids and by dipheny l sulfoxide (in the latter case it gives more of sulfone 8 than of sulfoxid e 7). The relative rates of protonation of both the first and the second in termediate (determined in benzene doped with protic additives) correlate wi th the gas-phase acidity of such additives. As for the reaction in neat alc ohols and in benzene doped with acids, a single intermediate intervenes. Th is is better described as a S-hydroperoxy cation rather than a neutral hydr operoxysulfurane and is trapped by both diphenyl sulfoxide and sulfone at r ates close to those measured for the photo-oxidation of diethyl sulfide.