Computational study on the reaction mechanism of phosphine-catalyzed hydroboration of propiolonitriles: With cyano group or not?

Density functional theory calculations have been performed to address the detailed mechanism of phosphinecatalyzed hydroboration of propiolonitriles. The mechanism with the cyano participation was hindered by an inaccessible free-energy barrier of 42.3 kcal/mol, while the conventional one without the cyano participation was identified to be plausible. The involved elementary steps include nucleophilic addition of PBu3, 1,2-hydride transfer, 1,2-boron migration and removal of PBu3 in succession, with the rate-determining free-energy barrier of 21.4 kcal/mol. The anti-addition stereoselectivity originated from the steric effect in the 1,2-boron migration step, and the complete regioselectivity was governed by the positional preference of PBu3 attack. Lastly, the strongly electron-withdrawing effect of cyano was found to facilitate the desired hydroboration at room temperature by stabilizing some zwitterionic intermediates and transition states. This study provided a preciser understanding on the mechanism of phosphine-catalyzed hydroboration and unveiled factors governing the reactivity and selectivity.