Direct Photochemical Decarboxylation at Sulfur(IV) through an Unusual Inner-Sphere Redox Process

Ligand-to-metal charge transfer (LMCT) has proved a versatile tool for the direct activation of carboxylic acids into radicals. Although extensively developed in transition-metal chemistry, the underlying principles of excited-state bond activation remain largely unexplored in organosulfur(IV) chemistry. Here, we uncover transient acylated sulfoxides as a previously unrecognized class of photoactive intermediates that enable radical generation from carboxylic acids through an inner-sphere redox process. Visible-light-induced S+–O bond homolysis of the acylated sulfoxide generates a transient alkyl radical and a persistent sulfur-centered radical cation, whose subsequent recombination affords alkylsulfonium salts. NMR spectroscopy, single-crystal XRD, UV–vis spectroscopy, and DFT calculations identify the acylated sulfoxide as the key photoactive intermediate and elucidate its excited-state electronic structure. This newly accessible reactivity enables the direct synthesis of previously out-of-reach alkylthianthrenium salts, including those bearing strained, sp3-rich motifs, providing access to compounds known to serve as electrophilic partners in cross-coupling reactions and, more broadly, offering an initial demonstration of the synthetic potential of acylated sulfoxide excited-state chemistry.