Kinetic and mechanistic studies of the transformation of the catalyst, tris (pentafluorophenyl)borane, in the presence of silyl and germyl hydrides

Tris(pentafluorophenyl)borane catalyzed Si-H bond activation opens the door to numerous transition metal -free reduction processes and is widely used in organic and polymer chemistry. However, chemical stability of B(C6F5)(3) in the presence of silyl hydrides is limited, which can strongly affect its catalytic activity. Transformations of B(C6F5)(3) in the presence of phenyldimethylsilane, triethylsilane and triethylgermane were studied by F-19 NMR and UV spectroscopy, GC/MS and quantum-mechanical calculations. The observed exchange of pentafluorophenyl group attached to boron to hydrogen results in the formation of bis(pentafluorophenyl)borane, which has a strongly reduced ability to activate the Si-H bond. The substitution kinetics were studied by following the disappearance of absorption of the B(C6F5)(3) charge transfer peak in the UV spectrum. Complementary quantum mechanical calculations allowed us to propose a mechanism of the ligand exchange reaction, which involves electrophilic substitution of the pentafluorophenyl group through a four-center transition state. (C) 2019 Elsevier Inc. All rights reserved.