Acid-catalyzed photoreduction of dialkyl sulfoxides by an acid-stable NADH analogue

Photoreduction of dialkyl sulfoxides (R₂SO) by an acid-stable NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH₂), proceeds in the presence of perchloric acid in acetonitrile containing H₂O (0.50 M) to yield 10-methylacridinium ion (AcrH⁺) and the corresponding dialkyl sulfides (R₂S). No photoreduction of R₂SO by AcrH₂ has occurred in the absence of perchloric acid. In the presence of perchloric acid the protonation of dialkyl sulfoxides occurs to increase the oxidizing ability of the sulfoxides significantly, when the singlet excited state (¹AcrH₂*) is quenched efficiently by the protonated sulfoxides (R₂SOH⁺). The ¹AcrH₂* is also quenched by HClO₄. The dependence of quantum yields on [R₂SOH⁺] indicates that the photoreduction of R₂SO by AcrH₂ proceeds via electron transfer from ¹AcrH₂* to R₂SOH⁺, followed by the hydrogen transfer from AcrH₂^•+ to R₂SOH^•, accompanied by the concomitant dehydration to yield AcrH⁺ and R₂S.