Mechanistic study of nickel-catalyzed intramolecular [4 + 2] cycloaddition of cyclobutanone with allene: origin of selectivity and ligand effect

Abstract

The transition metal catalyzed cycloaddition reaction is one of the most powerful tools for the construction of carbon- and heterocycles. Comprehensive computational studies are employed to investigate the mechanism and the origins of the regio-, chemo-, Z/E-, and enantioselectivities, as well as the ligand effect in the Ni-catalyzed intramolecular [4 + 2] cycloaddition of cyclobutanone with allene. The reaction is found to follow the oxidative addition/migratory insertion pathway instead of the originally proposed cyclometallation/β-carbon elimination pathway, in which the oxidative addition is the rate- and enantioselectivity-determining step. Detailed electronic and geometric analysis reveals that the regio- and chemoselectivities are controlled by the electronic effect, while the Z/E selectivity is primarily determined by the steric effect. Further distortion/interaction analysis shows that the (S,S)-enantioselectivity originated predominantly due to the lower distortion energy of the Ni-chiral ligand fragment in the C–C bond oxidative addition transition state. Finally, the calculations show that it is critical to produce a free coordination site by phosphine ligand dissociation during the reaction pathway. This requisite phosphine ligand dissociation is available for monodentate phosphine ligands but highly disfavored for bidentate phosphine ligands because of the chelation effect, which agrees well with the experimentally observed decreased reactivity of bidentate phosphine ligands.