Origin of regioselectivity of Ni-catalyzed N-butenyl-substituted imidazolium salt annulation reaction: A theoretical study

Density function calculations were carried out to investigate the mechanism of the Ni-catalyzed N-butenyl-substituted imidazolium salt annulation reaction. The results indicate that the reaction proceeds along the proposed oxidative addition–insertion-reductive elimination mechanism while some steps could be combined in specific pathways. The most favored pathway starts with oxidative addition, followed by coordination of the CC double bond to Ni(II), and finishes by direct reductive elimination to generate the 5-membered fused-ring product. The direct reductive elimination step is rate limiting. The origin of the regioselectivity is attributed to the coordination of the CC double bond to Ni(II) after oxidative addition which is the initial key divergence to give different products. The intermediate ready for insertion and reductive elimination to form the 6-membered fused-ring product has a significantly higher energy than that ready for direct reductive elimination to form the 5-membered ring, leading to the lack of the necessary intermediate for the formation of the 6-memebered ring product.