Theoretical Studies of Rhodium-Catalyzed Borylation of Nitriles through Cleavage of Carbon-Cyano Bonds

Rhodium(I)-catalyzed borylation of nitriles is investigated theoretically, using the density functional theory method, to clarify the reaction mechanism, including the formation process of the catalytically active species, carbon-carbon bond cleavage, and the effect of an amine additive. The initial step in this reaction is the formation of a borylrhodium(I) species, in which the rhodium center carries a significant negative charge. The most energetically favorable pathway for carbon-carbon bond cleavage involves the insertion of a cyano group into borylrhodium(I) to form an iminoacyl intermediate, followed by extrusion of boryl isocyanide (iminoacyl mechanism). The calculation suggests that DABCO can react with coproduced reactive boron species, such as boryl chloride and boryl isocyanide, to form stable adducts, lowering the energy of the entire reaction system. In addition, the chemoselectivities among C-CN, C-Br, and C-Cl bonds observed in experimental studies are in good agreement with the calculated activation energies required for these bond activation processes.