EXPERIMENTAL AND THEORETICAL EVIDENCE OF AN S(N)2-TYPE MECHANISM FOR DISSOCIATION OF B-N COORDINATION BONDS IN 2,6-BIS((DIMETHYLAMINO)METHYL)PHENYLBORANE DERIVATIVES

To distinguish between dissociation of a B-N coordination bond by S(N) 1- and S(N)2-type mechanisms, two series of 1,3,2-dioxaborolanes (boro nates) and BEt2 (borane) complexes carrying a 2,6-bis((dimethylamino)m ethyl)phenyl group as a third substituent were synthesized by the reac tion of the corresponding organolithium compound with an appropriate b oron reagent. In the solid state, the boronate complex exhibits a stru cture in which only one NMe2 group is coordinated to a tetracoordinate d boron atom according to the X-ray analysis and the solid-state NMR. In solution there is a rapid exchange between the coordinated and unco ordinated amine ligands. The barriers to B-N dissociation in the boron ate and borane complexes are lower by >3.4 and 6.6 kcal/mol than in th e corresponding monoamino complexes, respectively, which is due to ele ctronic assistance in an S(N)2-type mechanism. This observation is sup ported by ab initio calculations for the system of NH3 and BH3. The dy namic process observed in the boronate complex with 4,4-diphenyl subst ituents is also discussed.