NUCLEAR MAGNETIC RESONANCE STUDY OF AMINOBORANES AND RELATED NITROGEN-BORON COMPOUNDS

Proton and boron magnetic resonance spectroscopy has been applied to the study of a number of aminoboranes in which restriction of rotation about the nitrogen-boron bond is expected to be present. The magnetic anisotropy of the cyclopropyl and vinyl groups has been shown to lead to differential shielding of vicinal substituents when restriction is present. In a number of B-vinyl compounds, the results can be explained by conjugation and restricted rotation about both the N-B and the B-Cvinyl bonds with only one isomer present. In (cyclopropylamino )dimethylborane and in (methylphenylamino)-phenylvinylborane, but not in the other B-vinyl compounds, the nonequivalence of the methyl substituents could be removed either by raising the temperature or by using dimethyl sulfoxide as the solvent. The lack of temperature and solvent effects indicates an increased barrier to rotation about the N-B bond contributed by the vinyl substituent. In a number of aminoboranes and related nitrogen-boron compounds, the 11B chemical shifts can be described fairly well in terms of a set of additive substituent contributions. These contributions depend on the mesomeric effect of the substituent rather than on its electronegativity. Steric effects seem to be important insofar as they influence the amount of double-bond character between the boron and the substituents. © 1969, American Chemical Society. All rights reserved.