Combined Brønsted Base-Promoted CO₂ Fixation into Benzylic C–H Bonds of Alkylarenes

Abstract

Interest in developing methods for direct CO2 fixation into readily available unfunctionalized C–H bonds in organic substances has recently surged. In contrast to the well-studied carboxylations of alkynyl C(sp)–H and aromatic C(sp2)–H bonds, carboxylation of benzylic C(sp3)–H bonds to produce 2-arylacetic acids is limited to photoirradiation reactions and continues to be a challenging issue because of the low chemical reactivity. We herein describe that a combined Brønsted base (i.e., LiO-t-Bu/CsF and LiOCEt3/CsF) achieves benzylic carboxylation of electron-deficient, -neutral, and -rich alkylarenes and enables various functionalities, including fragile ones such as bromide, alkene, alkyne, and carbonyl moieties. Dicarboxylation at the benzylic position is also established. Cs-alkoxide generated in situ acts as a reactive base, as demonstrated in experiments with independently prepared CsO-t-Bu and by 133Cs nuclear magnetic resonance studies.