Bimetallic [Ge-Rh] complex catalyzed alkyne semihydrogenation: Theoretical investigations on the germanium promoted mechanism and selectivity

Abstract

Transitional metal catalyzed alkynes functionalization is a very important method to obtain the organic intermediates. In this work, the mechanism of PhCCPh hydrogenation catalyzed by [Ge-Rh] bimetallic catalyst has been studied by means of density functional theory. The whole reaction consists of two cycles, cis-semihydrogenation of alkyne to give Z-alkene and trans-isomerization to get E-alkene. The rate-determining step is the hydrogen complexation in cis-semihydrogenation. Because the isomerization from Z-alkene to E-alkene is easy, while the reverse transformation is difficult, E-alkene is main product of alkyne semihydrogenation. The semihydrogenation would be facilitated by replacing the terphenyl ligands in the [Ge-Rh] bimetallic catalyst with the polar N-heterocyclic ligands. The addition of H2 is crucial in controlling the chemoselectivity of the reaction. The Ge acts as the Lewis acid to reduce the electron density on Rh, making Rh center becomes more electropositive than that in the monorhodium catalysts, thus increasing the selectivity of the [Ge-Rh] bimetallic catalyst. Our work elucidates the detailed mechanism of PhCCPh semihydrogenation catalyzed by the [Ge-Rh] catalyst and provides explanations for the experimental results.