Benzophenothiazine/Boronic Acid Cooperative Photocatalysis Enables the Synthesis of γ-Lactones via the [3 + 2] Cycloaddition of α,β-Unsaturated Carboxylic Acids with Olefins

The radical-mediated [3 + 2] cycloaddition between α-carboxy radicals and olefins is an efficient method for the synthesis of γ-lactones. Here, we report a [3 + 2]-type lactonization via the reductive single-electron transfer (SET) and subsequent protonation of α,β-unsaturated carboxylic acids (UCAs), which are ideal α-carboxy radical precursors in terms of atom economy. The cooperative catalysis of benzophenothiazine and boronic acid efficiently promotes the formation of α-carboxy radicals from UCAs in the presence of appropriate Brønsted acids such as benzoic acid, leading to a practical synthetic method without the need for strong acids or reductants. The chemoselective activation of UCAs provides access to a wide range of alkenes, including α,β-unsaturated amides, to be used as radical acceptors. Mechanistic studies revealed that the thermodynamic stability of the α-carboxy radicals and the charge distribution of the radical anion intermediates have a significant impact on the reaction rate and regioselectivity of the protonation.