A Functional-Group-Tolerant Catalytic trans Hydrogenation of Alkynes

摘要

The catalytic semihydrogenation of alkynes to Z alkenes is widely practiced textbook knowledge. Amongst the various heterogeneous or homogenous catalysts capable of effecting this transformation, the use of palladium on CaCO3 doped with lead is particularly popular, and is commonly called Lindlar catalyst after its discoverer; variants using BaSO4 as the support material and quinoline as additive are also popular. The major role of the catalyst poisons is to prevent over-reduction of the alkene primarily formed, whereas the Z selectivity is an intrinsic feature: transition metal catalysts, be they homogenous or heterogeneous, usually deliver the two hydrogen atoms of H2 suprafacially to the π system of the substrate by a sequence of hydrometalation/reductive elimination as the elementary step (although the exact processes on a metallic surface, where multimetallic sites may be engaged, are still not understood in full detail).