Direct C─H Lactonization of Carboxylic Acids Enabled by LMCT Photoactivation

Photoinduced ligand-to-metal charge-transfer (LMCT) activation of carboxylic acids has increasingly become recognized as a versatile platform for the development of synthetically useful new reactions. When the metal species is also capable of mediating an oxidative radical coupling process, this approach has been shown to be a powerful strategy for decarboxylative coupling of carboxylate feedstocks with diverse nucleophilic reaction partners. LMCT photoreactions that could engage the acyloxy radical intermediate in other canonical reactions of heteroatom-centered radicals such as 1,5-hydrogen atom transfer (HAT) would broaden the scope of possible reactions. The rapid intrinsic rate of decarboxylation, however, presents a formidable challenge. Herein, we report the LMCT-promoted C─H lactonization of benzoic and aliphatic carboxylic acids. Mechanistic investigations suggest that under the optimized reaction conditions, the key acyloxy radical intermediate can be stabilized by the metal center, enabling 1,5-zHAT to outcompete decarboxylation.