Computational Exploration of Intramolecular Cycloaddition of Cyclobutanone with Alkene Catalyzed by Rhodium Versus Nickel: The Origins of Selectivities

Density functional theory (DFT) calculations are employed to elucidate the mechanisms behind the rhodium- and nickel-catalyzed intramolecular [4 + 2] cycloaddition of cyclobutanone with alkene, and uncover the origins of various selectivities. The major findings are as follows: (1) the previously proposed cyclometalation pathway for the nickel system is energetically unfavorable due to large bridge ring strain, and low electron density of Ni-center; (2) both catalytic systems follow a similar reaction pathway, including C-C(acyl) bond oxidative addition, C═C bond insertion, and C-C bond reductive elimination, but the insertion mode of C═C bond differs; (3) C═C bond insertion is a regio- and chemoselectivity determination step. Due to spatial configuration of the metal complex and orbital interaction, in the Rh system, the C═C bond undergoes 1,2-insertion into the Rh-C bond to form [3.2.1] bicycle product 2, while in the Ni system, it inserts into the Ni-C(acyl) bond to afford [2.2.2] bicycle product 3; (4) in the Rh system, the preferred generation of (S,R)-2a is attributed to low catalyst twist energy during the C-C(acyl) bond oxidative addition step. Conversely, in the Ni system, the minimal steric repulsion between substrate and catalyst during the C═C bond insertion step leads to forming (R,R)-3b.