Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines

Abstract

Tris(pentafluorophenyl)borane, [B(C₆F₅)₃], has been used as an efficient catalyst in the guanylation reaction of amines with carbodiimide under mild conditions. A combined approach involving NMR spectroscopy and DFT calculations was employed to gain a better insight into the mechanistic features of this process. The results allowed us to propose a new Lewis acid-assisted Brønsted acidic pathway for the guanylation reaction. The process starts with the interaction of tris(pentafluorphenyl)borane and the amine to form the corresponding adduct, [(C₆F₅)₃B-NRH₂] 1, followed by a straightforward proton transfer to one of the nitrogen atoms of the carbodiimide, ⁱPrNCNⁱPr, to produce, in two consequent steps, a guanidine–borane adduct, [(C₆F₅)₃B-NRC(NⁱPrH)₂] 2. The rupture of this adduct liberates the guanidine product RNC(NⁱPrH)₂3 and interaction with additional amine restarts the catalytic cycle. DFT studies have been carried out in order to study the thermodynamic characteristics of the proposed pathway. Significant borane adducts with amines and guanidines have been isolated and characterized by multinuclear NMR in order to study the N–B interaction and to propose the existence of possible Frustrated Lewis Pairs. Additionally, the molecular structures of significant components of the catalytic cycle, namely 4-tert-butylaniline-[B(C₆F₅)₃] adduct 1b and both free and [B(C₆F₅)₃]-bonded 1-(phenyl)-2,3-diisopropylguanidine, 2a and 3a respectively, have been established by X-ray diffraction.