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

Abstract

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.

Acridine photocatalysis enables tricomponent direct decarboxylative amine construction

“…3A), pointing to the radical decarboxylative pathway, in line with the reactivity previously observed in other acridine-catalyzed reactions. 11,16,17…”

“…Acridine photocatalysis has recently emerged as a new catalytic platform for direct decarboxylative functionalization that has demonstrated broad scope and compatibility with a variety of easily oxidizable functionalities. 15–17 The chemoselectivity of the acridine-catalyzed direct decarboxylation is due to the directional character of the decarboxylative process within the acridine–carboxylic acid complex that undergoes photoinduced proton-coupled electron transfer (PCET). Despite the progress, the scope of the functionalizations that can be facilitated by acridine photocatalysis remains ill-defined.…”