Mechanistic insight into cobalt-catalyzed stereodivergent semihydrogenation of alkynes: The story of selectivity control &z.star;

Xiaotian Qi, Xufang Liu, Ling-Bo Qu, Qiang Liu, Yu Lan

Index: 10.1016/j.jcat.2018.03.016

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Abstract

Selectivity control is a challenging and important subject in semihydrogenation of alkynes. Here, a combined theoretical and experimental study was performed to reveal the origin of the chemo- and stereoselectivity in cobalt-catalyzed stereodivergent semihydrogenation of alkynes. Three NNP and PNP type pincer ligands were considered in calculation. The computational results show that over-reduction of the alkene is forbidden in this catalytic system because the alkylcobalt(I) intermediate formed by alkene insertion prefers to undergo β-H elimination rather than protonation of the Co C bond. Distortion–interaction analysis along the reaction coordinate suggests that the higher distortion energy during methanol-mediated protonation of the alkylcobalt(I) species suppresses formation of the side product, thereby determined the chemoselectivity. Mechanistic investigation reveals that the active cobalt(I) hydride species formed by pre-catalyst [C] has strong catalytic activity for alkene isomerization. Subsequent control experiments combined with free energy comparison confirms this conclusion and reveals that fast deactivation of catalyst and the weaker reactivity of the cis-alkene intermediate in the presence of the alkyne substrate prevents Z/E alkene isomerization using pre-catalyst [C]. This results in the divergent stereoselectivity in the presence of different cobalt pincer complexes.