Solvent effects on internal rotation about the BN bond in aminoboranes as studied by proton magnetic resonance spectroscopy

Proton magnetic resonance spectra of two series of aminoboranes, Me 2 N BRPh (R = Cl, Me, Et, i ‐Pr, i ‐Bu, t ‐Bu and CCMe) and PhRN BMe 2 (R = Me, Et and i ‐Pr), have been examined in various solvents to show that the potential barrier to internal rotation about their BN bond is lowered in some electron‐pair donor solvents. The barrier in an aminoborane decreases with increasing electron‐donating power of the solvent. The barrier in alkylaminoboranes in an electronpair donor solvent was found to increase with increasing size of the substituent alkyl group, owing to the steric repulsion between the alkyl and the solvent molecule. The solvent effects of benzene resulted in downfield shifts of the proton signals due to B ‐substituents and in upfield shifts of the signals due to N ‐substituents, as expected. The formation of a 1:1 adduct of Me 2 N BPh( t ‐Bu) with carbon disulphide was found.