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Mechanistic study of cobalt(I)‐catalyzed asymmetric coupling of ethylene and enynes to functionalized cyclobutanes

Density functional theory (DFT) calculations have been performed to gain insight into the reaction mechanism of the Co(I)‐catalyzed asymmetric [2 + 2] cycloaddition reaction of enyne 1a with ethylene 2 to give the functionalized cyclobutene E‐4a possessing a chiral, all‐carbon quaternary center in the ring framework (Science, 361, 68–72). This study reveals that the whole catalysis can be characterized via three stages: (i) oxidative dimerization followed by reductive elimination gives the intermediate IM3, (ii) the alkenyl‐Co(III) metallacycloheptene IM6 formation with the addition of another equivalent ethylene via an oxidative dimerization process, (iii) β‐Hydrogen elimination and reductive elimination from IM6 to result in the final product E‐4a and regenerate the active speices IM1 for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible for experimental realization under mild conditions, and the formation of the alkenyl‐Co(III) metallacycloheptene IM6, with a barrier of 27.2 kcal mol−1 (i.e., IM2 → TS4), should be the rate‐determining step (RDS) during the whole catalysis. In addition, the origins of enantioselectivity and regioselectivity of the product are discussed.