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

Abstract

Density functional theory calculations have been performed to address the detailed mechanism of phosphine-catalyzed 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.