Dioxygen concentration-dependent selective hydroxysulfonylation of olefins by rose bengal-sensitized photocatalysis

Abstract

The synthesis of β-hydroxysulfones selectively in preference to β-ketosulfones by dye-sensitized photocatalysis is demonstrated by employing inexpensive and readily available olefins and sulfonyl hydrazides/N-hydroxy sulfonamides in the presence of oxygen. The facile hydroxysulfonylation reaction, which involves the use of rose bengal (RB) as a photocatalyst and dioxygen as an oxidant, permits access selectively to secondary and tertiary β-hydroxysulfones in good to excellent isolated yields at rt and is compatible with aryl, heteroaryl and alkyl sulfonyl hydrazides. Mechanistically, sulfonyl radicals are generated by a cascade of redox reactions, set off by the photocatalyst RB, between sulfonyl hydrazide and dioxygen. Attack of the sulfonyl radicals on olefins followed by oxygenation of the radical intermediates leads to hydroxysulfonylation products selectively without the requirement of a metal catalyst/additive/reductant/base, conforming to the tenets of sustainable chemistry. It is shown that the formations of β-hydroxysulfones and β-ketosulfones proceed concurrently through two different routes and that the outcome is crucially dependent on oxygen concentration with the former being formed predominantly under an oxygen atmosphere, a feature that is heretofore unprecedented to the best of our knowledge.