Photosensitized electron transfer oxidation of sulfides. A steady-state study

The photosensitized electron-transfer oxidation of a series of ethyl sulfides RSEt (1, R = C12H25; 2, PhCH2CH2; 3, PhCH2; 4, PhCMe2; 5, Ph2CH) has been examined in acetonitrile and the product distribution discussed on the basis of the mechanisms proposed. In nitrogen-flushed solutions, cleaved alcohols and alkenes are formed, whereas under oxygen, in reactions that are 10-70 times faster, sulfoxides and cleaved aldehydes and ketones are formed in addition to the aforementioned products. Two sensitizers are compared, 9,10-di-cyanoanthracene (DCA) and 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+BF4-), the former giving a higher proportion of the sulfoxide, the latter of cleaved carbonyls. The sulfoxidation is due to the contribution of the singlet oxygen path with DCA. Oxidative cleavage, on the other hand, occurs both with DCA and with TPP+ which is known to produce neither singlet oxygen nor the superoxide anion. This process involves deprotonation from the a position of the sulfide radical cation, but the TPP+ results suggest that O-2(-) is not necessarily involved and non-activated oxygen forms a weak adduct with the radical cation promoting a-hydrogen transfer, particularly with benzylic derivatives. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).