Mechanism, Reactivity, and Selectivity of Nickel-Catalyzed [4 + 4 + 2] Cycloadditions of Dienes and Alkynes

Density functional theory (DFT) calculations with B3LYP and M06 functionals elucidated the reactivities of alkynes and <i>Z</i>/<i>E</i> selectivity of cyclodecatriene products in the Ni-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes. The Ni-mediated oxidative cyclization of butadienes determines the <i>Z</i>/<i>E</i> selectivity. Only the oxidative cyclization of one <i>s-cis</i> to one <i>s-trans</i> butadiene is facile and exergonic, leading to the observed 1<i>Z</i>,4<i>Z</i>,8<i>E</i>-cyclodecatriene product. The same step with two <i>s-cis</i> or <i>s-trans</i> butadienes is either kinetically or thermodynamically unfavorable, and the 1<i>Z</i>,4<i>E</i>,8<i>E</i>- and 1<i>Z</i>,4<i>Z</i>,8<i>Z</i>-cyclodecatriene isomers are not observed in experiments. In addition, the competition between the desired cooligomerization and [2 + 2 + 2] cycloadditions of alkynes depends on the coordination of alkynes. With either electron-deficient alkynes or alkynes with free hydroxyl groups, the coordination of alkynes is stronger than that of dienes, and alkyne trimerization prevails. With alkyl-substituted alkynes, the generation of alkyne-coordinated nickel complex is much less favorable, and the [4 + 4 + 2] cycloaddition occurs