Mechanistic Insights into Copper-Catalyzed Carboxylations

The copper-NHC-catalyzed carboxylation of organoboranes with CO2 was investigated using computational and experimental methods. The DFT and DLPNO-CCSD­(T) results indicate that nonbenzylic substrates are converted via an inner-sphere carboxylation of an organocopper intermediate, whereas benzylic substrates may simultaneously proceed along both inner- and outer-sphere CO2 insertion pathways. Interestingly, the computations predict that two conceptually different carboxylation mechanisms are possible for benzylic organoboranes, one being copper-catalyzed and one being mediated by the reaction additive CsF. Our experimental evaluation of the computed reactions confirms that carboxylation of nonbenzylic substrates requires copper catalysis, whereas benzylic substrates can be carboxylated with and without copper.