Photochemical Synthesis of Thioesters from Aryl Halides and Carboxylic Acids

Thioesters are important in synthesis, materials science, and biology, and their preparation traditionally relies on the use of disagreeable thiols. Here, we report a thiol‐free protocol that stitches together widespread carboxylic acids and aryl halides, producing a diverse array of thioesters. Crucial to this strategy is the discovery that tetramethylthiourea can serve as both a sulfur source and, upon direct excitation by purple light, as a strong reductant, suitable for activating aryl halides via single‐electron transfer. Coupling of the resulting aryl radicals provides an isothiouronium ion intermediate, which can be attacked by carboxylic acids via a polar pathway, affording the thioester products under mild conditions. A photochemical method for synthesizing thioesters from readily available aryl halides and carboxylic acids is here disclosed. This thiol‐free protocol hinges on the direct excitation of 1,1,3,3‐tetramethylthiourea and its multifaceted ability to act as the sulfur source, generate aryl radicals upon SET activation of halides, and trap them to in situ produce reactive isothiouronium ions.