Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1H-2,1-Benzazaboroles) at Ruthenium

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We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H2 atmosphere under mild conditions. We now report an in-depth mechanistic study to understand how a substrate featuring two different functional groups, C≡N and B–H, namely the 2-cyanophenyl(amino)borane HB(NiPr2)C6H4(CN) (2), can be transformed into the BN heterocycle 1H-2,1-benzazaborole (3). Such a complex transformation has direct links with three key important processes: hydrogenation of nitriles, hydroboration of polar bonds, and B–N bond formation. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and variable-temperature multinuclear NMR and DFT studies allowed us to decipher the catalytic cycle. We show that the catalyst precursor [RuH2(η2-H2)2(PCy3)2] (1) is regenerated at the end of the transformation. We intercepted the transformation of the starting substrate 2, in the form of a 1H-2,1-benzazaborolyl ligand coordinated to the metal center by the formed B...