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 <i>E</i>-alkenes are formed by <i>trans</i>-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 <i>E</i>-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 <i>gem</i>-hydrogenation of a π-bond, which has hardly any precedent. The barriers for <i>trans</i>-hydrogenation and <i>gem</i>-hydrogenation are similar: whereas DFT predicts a preference for <i>trans</i>-hydrogenation, CCSD­(T) finds <i>gem</i>-hydrogenation slightly more facile. The carbene, once formed, will bind a second H₂ molecule and evolve to the desired <i>E</i>-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with <i>trans</i>-hydrogenation. The computed scenario concurs with <i>para</i>-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct <i>trans</i>-delivery of H₂, the formation of carbene intermediates by <i>gem</i>-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 <i>trans</i>-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by <i>gem</i>-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions