Mechanistic Study of Borylation of Nitriles Catalyzed by Rh–B and Ir–B Complexes via C–CN Bond Activation

Recently the Chatani group reported the Rh­(I)-catalyzed borylation of nitriles, which provided an efficient protocol for transformation of the C–CN bond to the C–B bond (<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 115). Although an unconventional β-carbon elimination mechanism was proposed in their study, the other previously proposed mechanisms, i.e., oxidative addition, deinsertion, and initial C–H bond activation, cannot be excluded. To clarify the dominant mechanism of this reaction, a density functional theory study on borylation of PhCN and BnCN catalyzed by [Rh­(XantPhos)­(B­(nep))] (nep = neopentylglycolate, XantPhos = 4,5-Bis­(diphenylphosphino)-9,9-dimethylxanthene) was conducted. The computational results indicated that the deinsertion mechanism (2,1-insertion of the Rh–B bond into the CN bond occurs first, followed by the insertion of the metal center into C–CN bond) is favored over oxidative addition, β-carbon elimination, and the initial C–H bond activation mechanism within all the investigated reactions. The activation of the C–CN bond is a facile step in the deinsertion mechanism, and the oxidative addition of the diboron reagent is the rate-determining step. On this basis, the mechanism of borylation of PhCN catalyzed by a similar Ir–B complex ([Ir­(XantPhos)­(B­(nep))]) was also examined. The deinsertion mechanism was found to be the most favorable. The overall energy barrier of the Ir–B complex-catalyzed borylation of benzonitriles was slightly higher than that of the same Rh–B complex-catalyzed reaction (by 1.1 kcal/mol), indicating that [Ir­(XantPhos)­(B­(nep))] could act as an alternative catalyst for borylation of nitriles