REDOX CATALYSIS INVOLVING SUBSTRATE PHOTOOXIDATION WITH CATALYST REGENERATION BY SUBSTRATE REDUCTION - SIMULTANEOUS OXIDATIVE C-H BOND-CLEAVAGE AND REDUCTIVE C-S BOND-CLEAVAGE IN THIOETHERS CATALYZED BY W10O32(4-)

Several experiments examining the photooxidation of thioethers by polyoxotungstates have established that the less oxidizing photochemically active complex W10O324- but not the more oxidizing photochemically active heteropolytungstates such as α-PW12O403- can be used to effect both substrate photooxidation and reduction in the same reaction. Under anaerobic conditions in solution at ambient temperature, the excited state of W10O324- reacts with thioethers in high selectivity by α-hydrogen abstraction to form the α-carbon radicals. Neither photooxidation by electron transfer to generate the thioether cation radicals nor production of the conventional oxygenated products, sulfoxide and sulfone, is seen in these reactions. The principal reduced form of the catalyst, W10O326-, reduces thioethers to effect C-S bond cleavage; thus, the net observed products in anaerobic photooxidation of these substrates catalyzed by W10O324- are the dimers resulting from coupling of the α-carbon radicals and C-S bond cleavage products including the hydrocarbon resulting from complete desulfurization of the thioether. The only important processes exhibited by the excited state, W10O324-⋆, under conditions where background photooxidation of CH3CN solvent is not significant ([thioether] > 25 mM) are attack on thioether substrate, k, and unimolecular radiationless decay, krd. The rate law for production of the readily monitored inorganic product, W10O326- (rate = KΦI[R2S]/k[R2S] + krd), is consistent with the kinetically competent excited state, W10O324-⋆, being produced by absorption of one photon of near-UV light and rate-limiting attack of this species on thioether. Under aerobic conditions, W10O324-, catalyzes the complete oxidative conversion of thioethers principally to sulfoxides and sulfones by a complicated process that involves, in part, radical-chain autoxidation. Sulfoxides readily quench the photoredox-active excited state of W10O324- (kqτ = 9.4 M-1). © 1990, American Chemical Society. All rights reserved.