Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-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 H-2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (eta(2)-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 pi-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 H-2 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 H-2, 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.