Mechanism of Alkaline Earth Metal Catalyzed Hydroboration of Carbodiimides: A Theoretical Study

Density functional theory (DFT) calculations are employed to study the mechanism of alkaline earth metal catalyzed hydroboration of carbodiimides. Our theoretical study revealed that the active catalytic species is a hydridemagnesium complex when magnesium is used as catalyst. The catalytic cycle starts with a C=N bond insertion into Mg-H bond followed by a B-N bond formation. A hydride transfer from boron to magnesium regenerates the active catalytic species and yields the hydroboration product. This process is considered to be the rate-determining step. Moreover, the mechanism of calcium or strontium catalyzed corresponding reactions was also studied theoretically. Alternatively, DFT calculations showed that the active catalytic species is amide-metal complex, which could be generated by the carbodiimide insertion into metal- hydride bond. In this catalytic cycle, amide-metal complex reacts with borane to form a B-N bond. After the coordination of another molecular carbodiimide, a concerted hydride transfer takes place from boron to carbon, which yields the final product and regenerates the active species amide-metal. The different reaction pathway with calcium or strontium catalyzed corresponding reactions could be attributed to that the radius of calcium or strontium is larger than that of magnesium significantly. Thus, those two metals would be coordinated with an extra carbodiimide molecule, which is the precursor for the concerted hydride transfer. The DFT calculations showed that the activation free energy for the rate-determining step with calcium or strontium catalyst is much lower than that with magnesium catalyst. Therefore, a mild reaction condition might be found with calcium or strontium as catalyst for corresponding reactions.