Multimodal Acridine Photocatalysis Enables Direct Access to Thiols from Carboxylic Acids and Elemental Sulfur

Development of photocatalytic systems that facilitate
mechanistically
divergent steps in complex catalytic manifolds by distinct activation
modes can enable previously inaccessible synthetic transformations.
However, multimodal photocatalytic systems remain understudied, impeding
their implementation in catalytic methodology. We report herein a
photocatalytic access to thiols that directly merges the structural
diversity of carboxylic acids with the ready availability of elemental
sulfur without substrate preactivation. The photocatalytic transformation
provides a direct radical-mediated segue to one of the most biologically
important and synthetically versatile organosulfur functionalities,
whose synthetic accessibility remains largely dominated by two-electron-mediated
processes based on toxic and uneconomical reagents and precursors.
The two-phase radical process is facilitated by a multimodal catalytic
reactivity of acridine photocatalysis that enables both singlet excited
state PCET-mediated decarboxylative carbon–sulfur bond formation
and previously unknown radical reductive disulfur bond cleavage by
a photoinduced hydrogen atom transfer process in the silane–triplet
acridine system. The study points to a significant potential of multimodal
photocatalytic systems in providing unexplored directions to previously
inaccessible transformations.