External Oxidant-Free Dehydrogenative Lactonization of 2-Arylbenzoic Acids via Visible-Light Photocatalysis.

An external oxidant-free C-H functionalization/C-O bond formation reaction for constructing benzo-3,4-coumarins accompanied by quantitative H2 evolution has been developed. High functional group tolerance and excellent reaction efficiency are shown in this transformation. Meanwhile, the substrates containing heterocyclic substituents such as thienyl-, pyridinyl-, and pyrrolylbenzoic acids displayed good performance. Importantly, this reaction can be performed with good efficiency on a gram scale. A cyclic voltammetry study and density functional theory calculations could provide insight into the mechanism of this reaction.