Ligand effect on nickle-catalyzed reductive alkyne-aldehyde coupling reactions: a computational study

Density functional theory (DFT) calculations were employed to study Ni(0)-catalyzed reductive coupling of alkyne and aldehyde.The mechanism of that reaction is determined to consist of oxidative cyclization, transmetalation and reductive elimination.Theoretical calculations indicate that, no matter IMes or SIPr were used as ligand, the regioselectivity would be controlled by the oxidative cyclization step and the rate-limiting step is the transmetalation of silane, which is different from previous theoretical and experimental studies.In our computational results, 2-methyl-3-phenyl-allyl silyl ether 5 is confirmed to be the main product when IMes is used as ligand, whereas allyl-3-methyl-2-phenyl-silyl ether 4 is calculated to be main product using SIPr as ligand.Distortion-interaction energy analysis along the whole reaction pathways indicate that when IMes is used as the ligand, the oxidative cyclization transition state 8-ts has lower distortion energy due to the smaller distortional angle of alkyne substituents.Moreover, 2D contour maps analysis was used to investigate the difference of regioselectivity with IMes and SIPr ligands.The computational results suggest that the smaller bulkiness of IMes can tolerate phenyl around it, thus decreasing the repulsion of phenyl in alkyne with that in aldehyde.On the other hand, when the more bulky ligand SIPr was used, the formation of 2-methyl-3-phenyl-allyl silyl ether 5 would be unfavored due to the large steric repulsion.