Mechanism and Regioselectivity of the Iron-Catalyzed Hydroboration of N-Heteroarenes: A Computational Study

Iron complex Cp*(Ph2PC6H4S)Fe was recently reported to regioselectively catalyze the hydroboration of N-heteroarenes with pinacolborane (HBpin). Density functional calculations were performed to elucidate the reaction mechanism and to rationalize the regioselectivity. Three different reaction pathways were considered, and the hydrogen atom transfer pathway was found to be the most favorable one. The reaction started with a hydrogen atom transfer from pyridine–HBpin complex to the metal center of the catalyst. This leads to the generation of a pyridine–Bpin radical and an iron(III)-hydride intermediate. Then, the hydrogen atom can be transferred to either ortho- or para-position of the borylated N-heteroarenes, affording the corresponding products. The origin of the regioselectivity was found to be interaction-controlled from distortion/interaction analysis. The suggested mechanism can rationalize very well the regioselectivity of all other N-heteroarene substrate.