trans-Hydrogenation, gem-Hydrogenation, and trans-Hydrometalation of Alkynes: An Interim Report on an Unorthodox Reactivity Paradigm.

cis-Delivery of H2 to the π-system of an unsaturated substrate is the canonical course of metal catalyzed hydrogenation reactions. The semireduction of internal alkynes with the aid of [Cp*Ru]-based catalysts violates this rule and affords E-alkenes by direct trans-hydrogenation. A pathway involving σ-complexes and metallacyclopropenes accounts for this unconventional outcome. Connected to this process is an even more striking reactivity mode, in which both H atoms of H2 are delivered to one and the same C atom. Such gem-hydrogenation of stable carbogenic compunds is a fundamentally new transformation that leads to the formation of discrete metal carbene complexes. Computational studies suggest that the trans- and the gem-pathway have similar barriers, but polar substituents in the vicinity of the reacting triple bond provide opportunities for imposing selectivity and control. Moreover, it is shown that catalytic trans-hydrogenation is by no means a singularity: rather, the underlying principle is also manifest in trans-hydroboration, trans-hydrosilylation, trans-hydrogermylation, and trans-hydrostannation, which are equally paradigm-changing processes. These reactions are robust and distinguished by excellent compatibility with many (reducible) functional groups and have already stood the test of natural product synthesis in a number of demanding cases.