Substituent Effects on Configurational Stabilities at Tetrahedral Boron Atoms in Intramolecular Borane–Amine Complexes: Structures, Enantiomeric Resolution, and Rates of Enantiomerization of [2-(Dimethylaminomethyl)phenyl]phenylboranes

Abstract

Abstract The structures and stereochemistries of the five borane–amine complexes, (N–B)-{2-(Me2NCH2)C6H4}BPhX (X = Cl, F, Me, –OCOCF3, and –OCOC2F5), were studied by some spectroscopic methods. The Cl complex prepared by the standard method was converted to each of the other complexes by the treatment with an appropriate reagent (MeLi or AgX). The enantiomers of the two perfluoroacyloxy complexes were resolved by chiral HPLC as a novel intramolecular borane–amine complex with a chiral center at the tetrahedral boron atom, and their chiroptical properties were investigated by optical rotations and CD spectra. The classical kinetics or the saturation transfer measurements revealed that the barriers to enantiomerization via the dissociation of the B–N coordination bond were enhanced from 94 to 116 kJ mol−1 in the following order of X: F < Me < –OCOCF3 ≈ Cl < –OCOC2F5. The substituent effects on the configurational stability, which is closely related to the Lewis acidity of boron atoms, are discussed on the basis of the X-ray structures of the three complexes.