Conformational Dynamics of Alkoxydiarylboranes¹

The low-temperature ¹H-nmr spectra of several alkoxydiarylboranes are shown to be consistent with three alternatives: the molecules adopt propeller conformations, and stereoisomerization is slow on the nmr time scale; the molecules adopt propeller conformations with rapid stereoisomerization by means of one-flip pathways; the molecules adopt perpendicular conformations with slow stereoisomerization. Stereoisomerization phenomena observed at elevated temperatures are discussed in terms of flip mechanisms, and pathways which involve the flipping of aryl groups only are found to have lower activation energies than those which also involve flipping of the alkoxy group. An analysis of the isomerization processes reveals restricted rotation about the B-O bond, and arguments are presented which suggest that conjugative effects between boron and oxygen play a significant role in the stereochemistry of alkoxyarylboranes.