Mechanistic insight into anti-carbometalation of an alkyne via η2-vinyl-nickel type Z/E isomerization

Abstract

Alkenyl metal Z/E isomerization involving the use of an alkyne-tethered electrophile is an important topic in transition-metal catalyzed alkyne functionalization. In this report, we present computational evidence that supports a mechanism in which alkyne coordination to the nickel center as a η²-type ligand assists the Z/E isomerization. This η²-vinyl-nickel type Z/E isomerization is found to be lower in energy than carbonyl-assisted Z/E isomerization from a syn-alkenylnickel intermediate, providing new insights into the mechanism of Ni-catalyzed anti-carbometallative cyclization of alkyne. The Newman projection analysis was conducted to characterize the key transition states of the carbonyl insertion process and elucidate the origin of diastereoselectivity in this type of reaction. Theoretical calculations further revealed that the Z/E isomerization mode of alkenyl nickel is more inclined to rotate through the carbon–carbon double bond, which is also confirmed by the NBO charge and Wiberg bond indices (WBI). We anticipate that the novel η²-vinyl-nickel type Z/E isomerization mode could be extended to other alkyne functionalization processes explaining the unique effectiveness of this reaction mode in many instances.