Photoredox and cobalt co-catalyzed C(sp 2 )–H functionalization/C–O bond formation for synthesis of lactones under oxidant- and acceptor-free conditions

A practical photoredox and cobalt co-catalyzed C(sp 2 )–H functionalization/C–O bond formation is described herein, with dihydrogen as the only by-product. A variety of lactone products can be successfully constructed under oxidant- and acceptor-free conditions with good functional group tolerance and high yield. This protocol also provides a facile route to remote hydroxylated arenes and chromene.