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 (J. Am. Chem. Soc. 2012, 134, 115). Although an unconventional beta-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, beta-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.