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

We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H-2 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((NPr2)-Pr-i)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(eta(2)-H-2)(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 BN cycle. The corresponding benzazaboryl complex [Ru{(eta(5)-C(H)N(H)B((NPr2)-Pr-i)(C6H4)}{(eta(3)-C6H8)PCy2}] (9) was independently prepared and fully characterized by X-ray diffraction and multinuclear NMR. We also showed that complex 9 undergoes stepwise hydrogenation, followed by haptotropic rearrangement before release of the final product 3 and regeneration of the catalyst precursor 1. We were able to provide a fairly good view of the activation of the C N and B-H bonds. So far, it appears that nitrile hydroboration with metal hydrides starts with nitrile reduction but subsequent steps are highly dependent on the system. In our case, after the first hydrogen transfer to the nitrile, a boron-nitrogen interaction is highly favored, B-H bond cleavage occurring at a later stage. This field needs further investigation for promising developments of BN molecules. Prospects on reactions involving at least two different intramolecular reactive functions should be encouraged for future development in catalysis.