Oxidation of sulfides and disulfides under electron transfer or singlet oxygen photosensitization using soluble or grafted sensitizers

Two different photosensitizers, 9,10-dicyanoanthracene (DCA) and benzophenone (BzO) or a silica bound derivative (BzO-Si) have been compared for the photooxidation of di-n-butyl sulfide and di-n-butyl disulfide. With either photosensitizer, sulfide photooxidation in acetonitrile leads very efficiently to sulfoxide, with sulfone and disulfides as by-products. Although an electron transfer mechanism has previously been established starting with DCA, our results are indicative of two competitive mechanisms using BzO as the photo sensitizer, instead of singlet oxygen addition and electron transfer. The more sluggish photooxidation of disulfides leads to a complex mixture of products, among which n-butyl butanethiosulfonate and strong acids (alkylsulfonic and sulfuric) are the major ones. The relative ratio thiosulfonate : acids depends, among other factors, on the medium polarity with acid formation favored starting with BzO or BzO-Si in a methanol-water mixture. An electron transfer mechanism only can account for the observed products. Superoxide anion, the formation of which is much easier starting from BzO than from DCA, is suggested to play a crucial role in this oxidative radical pathway. Starting from disulfides, grafted benzophenone is more efficient for acid formation than its soluble counterpart. As this photosensitizer can easily be recycled, an easy and smooth way to acid formation is thus available, provided that the reaction solvent is properly chosen.