DFT Studies on the Carboxylation of Arylboronate Esters with CO2 Catalyzed by Copper(I) Complexes

DFT calculations have been carried out to study the carboxylation reactions of arylboronate esters with CO2 catalyzed by (NHC)Cu(I) complexes (NHC = N-heterocyclic carbene). The results affirm the basic mechanistic proposal that (1) the transmetalation of the aryl group of the boronic ester to the Cu(I) metal center occurs first; (2) CO, then inserts into the resulting Cu-Ar bond to give a carboxylate complex; and (3) displacement or the carboxylate by the base adduct of the arylboronic ester substrate completes the catalytic cycle and affords the product ArCOOK. Transmetalation was shown to proceed via a B-O-Cu-bridged intermediate that is formed from the KOR adduct of the arylboronic acid, rather than by the CuOR complex, although both pathways leading to the bridged intermediate are extremely facile. Insertion or CO2 into the Cu-Ar bond is the rate-determining step, in which nucleophilic attack of the aryl ligand on the electron-deficient carbon atom of CO2 leads to the formation of the new C-C bond. Substituents on the aryl group modify its nucleophilicity, affecting the insertion barrier. We predict that alkylboronic esters, but not alkynylboronic esters, Would be suitable substrates for the reaction.