Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1<i>H</i>‑2,1-Benzazaboroles) at Ruthenium

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 BN cycle. The corresponding benzazaboryl complex [Ru­{(η5-C­(H)­N­(H)­B­(NiPr2)­(C6H4)}­{(η3-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.我们近期披露了一种在温和氢气（H₂）氛围下合成BN分子的催化转化路径。本文报道一项深入的机理研究，旨在阐明同时带有氰基（C≡N）与硼氢（B–H）两种不同官能团的底物——2-氰基苯基(二异丙基氨基)硼烷HB(NiPr₂)C₆H₄(CN)（2）——如何转化为BN杂环1H-2,1-苯并氮杂硼咯（3）。这类复杂的转化过程与三类核心重要的反应路径直接相关：腈类加氢反应、极性键硼氢化反应以及B–N键的构建。通过结合催化反应的原位监测、化学计量比实验、变温多核核磁共振（Nuclear Magnetic Resonance, NMR）以及密度泛函理论（Density Functional Theory, DFT）研究，我们成功解析了该催化循环。我们证实，催化前体[RuH₂(η²-H₂)₂(三环己基膦（PCy₃）)₂]（1）在转化过程结束后得以再生。我们捕捉到了起始底物2的转化中间体：由生成的BN环与金属中心配位形成的1H-2,1-苯并氮杂硼咯基配体。我们通过独立合成得到了对应的苯并氮杂硼咯基配合物[Ru{(η⁵-C(H)N(H)B(NiPr₂)(C₆H₄)}{(η³-C₆H₈)二环己基膦（PCy₂）}]（9），并通过X射线衍射与多核核磁共振（NMR）对其进行了全面表征。我们还证实，配合物9会经历分步加氢过程，随后发生配位数迁移重排（haptotropic rearrangement），最终释放出目标产物3并再生催化前体1。我们得以较为清晰地阐明C≡N与B–H键的活化过程。截至目前，学界普遍认为金属氢化物介导的腈类硼氢化反应以腈的还原作为起始步骤，但后续反应路径高度依赖具体反应体系。在本研究体系中，首次氢转移至腈基后，硼-氮相互作用会显著优先发生，而B–H键的断裂则在后续步骤中进行。该领域仍需开展更多研究以推动BN分子的应用开发。未来催化领域的发展应鼓励探索包含至少两种不同分子内反应官能团的反应体系。