Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-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 (η 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 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 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.