Effect of ligand coordination on the mechanism and regioselectivity of cobalt-catalyzed hydroboration/cyclization of 1,6-enynes

Abstract

The 3d transition metal-catalyzed cyclization of enynes provides a sustainable and useful strategy for the construction of cyclic compounds. Controlling the regioselectivity is a central challenge for the hydrofunctionalization of multiply unsaturated substrates such as 1,6-enynes. Herein, we report a computational mechanistic study of the cobalt-catalyzed hydroboration/cyclization of 1,6-enynes in the presence of three representative ligands including bidentate nitrogen ligand and phosphine ligand as well as tridentate nitrogen-based ligand. A detailed analysis is carried out on the ligand-substrate interactions to understand the effect of ligand coordination on the mechanism and regioselectivity. Our results indicate that although the initial catalyst-substrate complexes adopt the pseudo-trigonal bipyramidal geometries for the two nitrogen-based ligands, there is a remarkable difference in the free energy surfaces. The steric repulsions and hydrogen bonding interactions between the substrate and the ligand play important roles in regulating the competing migration insertion. Energy decomposition analyses are further performed for the subsequent σ-bond metathesis to illustrate the impacts of steric and electronic effects on the regioselectivity and the electrostatics and orbital interactions are disclosed to be the dominant factors. This work provides a deeper understanding of the specific structural and electronic properties of the cobalt complexes and their potential influences on the reactivity and selectivity.