Direct reversible decarboxylation from stable organic acids in dimethylformamide solution

The loss of carbon dioxide (CO 2 ) from carboxylic acids is a common reaction in both biochemical and synthetic contexts, but it has generally involved catalysis or prolonged heating. Kong et al. now report that certain polar solvents, such as dimethylformamide, promote reversible CO 2 loss all by themselves from carboxylates bridged by one carbon to aromatic rings. With electron-withdrawing substituents on the ring, isotopically labeled CO 2 can be efficiently swapped in even at room temperature. Alternatively, reaction with aldehydes leads to alcohol formation. Science , this issue p. 557 Certain polar solvents labilize CO 2 bound at benzylic sites under surprisingly mild conditions. Many classical and emerging methodologies in organic chemistry rely on carbon dioxide (CO 2 ) extrusion to generate reactive intermediates for bond-forming events. Synthetic reactions that involve the microscopic reverse—the carboxylation of reactive intermediates—have conventionally been undertaken using very different conditions. We report that chemically stable C(sp 3 ) carboxylates, such as arylacetic acids and malonate half-esters, undergo uncatalyzed reversible decarboxylation in dimethylformamide solution. Decarboxylation-carboxylation occurs with substrates resistant to protodecarboxylation by Brønsted acids under otherwise identical conditions. Isotopically labeled carboxylic acids can be prepared in high chemical and isotopic yield by simply supplying an atmosphere of 13 CO 2 to carboxylate salts in polar aprotic solvents. An understanding of carboxylate reactivity in solution enables conditions for the trapping of aldehydes, ketones, and α,β-unsaturated esters.