Half-Sandwich Ruthenium Carbene Complexes Link <i>trans</i>-Hydrogenation and <i>gem</i>-Hydrogenation of Internal Alkynes

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η2-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD­(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H2 molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with trans-hydrogenation. The computed scenario concurs with para-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct trans-delivery of H2, the formation of carbene intermediates by gem-hydrogenation, and their evolution into product and side products alike. Propargylic −OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective trans-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by gem-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions.以[Cp*Ru]基催化剂（[Cp*Ru]-based catalysts）催化内炔烃的氢化反应，其立体化学路径别具一格：氢气（H₂）的两个氢原子经反式加成生成E型烯烃。本研究结合实验与计算手段，构建了完整的反应机理图景：反应首先生成金属环丙烯（metallacyclopropene），即η²-乙烯基配合物（η2-vinyl complex）；该中间体既可通过协同过程直接转化为E型烯烃，亦可进一步反应生成半夹心型钌卡宾配合物（half-sandwich ruthenium carbene）——此时起始炔烃的一个碳原子会被转化为亚甲基。该转化属于π键的形式上双位氢化（gem-hydrogenation）反应，此类反应此前几乎无先例。反式氢化与双位氢化的反应能垒相近：密度泛函理论（DFT）计算结果显示反式氢化更具优势，而耦合簇理论CCSD(T)则表明双位氢化的能垒略低。生成的卡宾配合物一旦形成，即可结合第二个H₂分子，继而转化为目标E型烯烃、烯烃位置异构体或对应的烷烃；该缔合反应路径解释了为何双键异构化与过度还原会与反式氢化形成竞争。计算得到的反应机理与顺磁氢诱导极化转移（para-hydrogen-induced polarization transfer, PHIP）核磁共振（NMR）实验数据相符，后者证实了H₂的直接反式加成、双位氢化生成卡宾中间体，以及卡宾中间体向产物与副产物的转化过程。炔丙基-OR（R=H、Me）基团具有显著的导向与稳定作用，使得多种卡宾中间体可被分离并通过X射线衍射（X-ray diffraction）进行结构表征。本研究获得的信息推动了多项制备化学领域的重要进展：其一，我们报道了炔丙醇的高选择性反式氢化反应，该反应兼容诸多其他可还原官能团；其二，通过双位氢化生成金属卡宾的路径，为非经典氢化环丙烷化、环扩张与环加成反应开辟了新途径。