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Relevant academic research and scientific papers

Photosensitized oxygenation of benzyl ethyl sulfide

Singlet oxygen adds to benzyl ethyl sulfide (5, total quenching rate ca. 1 x 107 M-1 s-1, little dependent on the solvent) to ultimately give benzaldehyde (6) and a small amount of the sulfone (8) in aprotic media (rate of the chemical reaction in benzene 5.5 x 106 M-1 s-1) and mainly the sulfoxide (7) in protic media (1.1 x 107 M-1 s-1 in methanol). In the presence 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 solvents. The first one is an exciplex or a syn persulfoxide. It undergoes intramolecular hydrogen abstraction to give a ylide and finally benzaldehyde. Such rearrangement is either a concerted or a radicalic process and not a proton transfer (as indicated by the deuterium effect observed with the α-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 usually associated with the persulfoxide (possibly it is the anti rotamer). This species gives some sulfoxide but mainly decays to the unreacted sulfide; it can be trapped intermolecularly, however, both by acids and by diphenyl sulfoxide (in the latter case it gives more of sulfone 8 than of sulfoxide 7). The relative rates of protonation of both the first and the second intermediate (determined in benzene doped with protic additives) correlate with the gas-phase acidity of such additives. As for the reaction in neat alcohols and in benzene doped with acids, a single intermediate intervenes. This is better described as a S-hydroperoxy cation rather than a neutral hydroperoxysulfurane and is trapped by both diphenyl sulfoxide and sulfone at rates close to those measured for the photo-oxidation of diethyl sulfide.

The photooxygenation of benzyl, heteroarylmethyl, and allyl sulfides

The photosensitised oxidation of benzyl ethyl sulfides in aprotic solvents (benzene or acetonitrile) gives the corresponding aldehydes under mild conditions. This is a general reaction which applies to benzyl derivatives containing either electron-donating or electron-withdrawing substituents and furthermore to hereto analogues such as 2-pyridinylmethyl sulfide (not to the 3-indolylmethyl sulfide, since reaction at the heterocycle moiety competes) as well as to allyl sulfides. In a protic solvent (methanol) these sulfides give the sulfoxides instead (except for the nitrobenzyl derivatives, where the aldehyde remains the major product). Among the α-substituted sulfides tested, the α-phenylbenzyl and the 3- cyclohexenyl sulfide give the corresponding ketone (the latter in a low yield), but the α-methylbenzyl sulfide gives the sulfoxide as the main product. The rate for singlet oxygen quenching and for chemical reaction have been measured for representative benzyl sulfides. The reaction is discussed in the frame of the currently accepted mechanism for sulfide photooxygenation. The key step for oxidative C-S bond cleavage appears to be hydrogen transfer from the activated α position in the first formed intermediate, the persulfoxide. This reaction is inhibited in methanol where the persulfoxide is hydrogen-bonded.