Mechanisms of the Reactions of B-Substituted Amine Boranes with THF·BH3

Article abstract of DOI:10.1002/ejic.201900847

The reactions of NH₃BH₂R (R = Me, Ph and Cl) with THFBH₃ have been investigated and it was found that different substituents on the B atom help to proceed the reactions in different ways. The expected doubly-bridged B-substituted aminodiborane products, similar to aminodiborane (ADB, BH₂(μ-H)(μ-NH₂)BH₂) via the reaction of ammonia borane (AB, NH₃BH₃) and tetrahydrofuran borane (THFBH₃), are not obtained. Two competitive reactions occurred with the change of R = Me or Ph. When R is a Me group, an “open“ version of B-substituted μ-aminodiborane, THFBH(Me)(μ-NH₂)BH₃, is formed as a major product; when R is a Ph group, AB and THFBH₂Ph are formed as main products via the intermolecular NH₃–THF exchange reaction. However, if R is Cl, then NH₃BH₂Cl reacts with THFBH₃ through reversible intermolecular Cl–H exchange mechanism. Furthermore, DFT calculations are performed to elucidate the formation mechanism of THFBH(Me)(μ-NH₂)BH₃ via the reaction of NH₃BH₂Me and THFBH₃ as well as the exchange mechanism of Cl–H in the reaction of NH₃BH₂Cl and THFBH₃.