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 C C 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 C C 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.