| Sumario: | The detailed reaction mechanism for the reduction of CO to CO catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition state structures. Our DFT calculations show that the reaction occurs through CO insertion into the CuB bond to give a CuOC(O)-boryl species (i.e., containing CuO and CB bonds), and subsequent boryl migration from C to O, followed by o-bond metathesis between pinB-Bpin (Bpin, pin = pinacolate = OCMeCMeO) and (NHC)Cu(OBpin). The overall reaction is exergonic by 38.0 kcal/mol. It is the nucleophilicity of the CuB bond, a function of the very strong σ-donor properties of the boryl ligand, rather than the oxophilicity of boron, which determines the direction of the CO insertion process. The boryl migration from C to O, which releases the product CO, is the rate-determining step and involves the ‘vacant’ orbital orbital on boron. The (NHC)Cu(boryl) complexes show unique activity in the catalytic process. For the analogous (NHC)Cu(alkyl) complexes, the CO insertion into the CuC bond giving a copper acetate intermediate occurs with a readily achievable barrier. However, the elimination of CO from the acetate intermediate through a methyl migration from C to O is energetically inaccessible.
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