Experimental and Theoretical Evidence of an SN2-Type Mechanism for Dissociation of B−N Coordination Bonds in 2,6-Bis((dimethylamino)methyl)phenylborane Derivatives1

To distinguish between dissociation of a B−N coordination bond by SN1- and SN2-type mechanisms, two series of 1,3,2-dioxaborolanes (boronates) and BEt2 (borane) complexes carrying a 2,6-bis((dimethylamino)methyl)phenyl group as a third substituent were synthesized by the reaction of the corresponding organolithium compound with an appropriate boron reagent. In the solid state, the boronate complex exhibits a structure in which only one NMe2 group is coordinated to a tetracoordinated boron atom according to the X-ray analysis and the solid-state NMR. In solution there is a rapid exchange between the coordinated and uncoordinated amine ligands. The barriers to B−N dissociation in the boronate and borane complexes are lower by >3.4 and 6.6 kcal/mol than in the corresponding monoamino complexes, respectively, which is due to electronic assistance in an SN2-type mechanism. This observation is supported by ab initio calculations for the system of NH3 and BH3. The dynamic process observed in the boronate complex with 4,4-diphenyl substituents is also discussed.