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 (boronates) and BEt 2 (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 NMe 2 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 S N 2-type mechanism. This observation is supported by ab initio calculations for the system of NH 3 and BH 3 . The dynamic process observed in the boronate complex with 4,4-diphenyl substituents is also discussed.

The molecular structures of a set of intramolecular boron-amine complexes, 9-[2-(dialkylaminomethyl)phenyl]-9-borabicyclo[3.3.1]nonanes (alkyl = Me and Et) and 2-[2-(dimethylaminomethyl)phenyl]-4,4-diphenyl-1,3,2-dioxaborolane, were determined by the X-ray analyses. The boron atom has a tetrahedral geometry and the five-membered ring is puckered with the nitrogen atom out of the plane in every complex. The distances of the N–B coordination bonds, which are in the range of 1.74–1.77Å, are not always correlated to the barrier to dissociation of the N–B bonds. In order to correlate the molecular structures to the strength of the N–B coordination bonds, the tetrahedral character, which is calculated from bond angles at a boron atom, is proposed. The usefulness as well as the limitation of this parameter are discussed.

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In some esters of formic acid which carry bulky alkyl groups (e .g, t-butyl formate, 1,1-diethylpropyl formate, and triphenylmethyl formate), the presence of s-cis conformer has been confirmed by various spectroscopic techniques. The dipole-moment measurements also support the conclusion drawn from the spectroscopic data.

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