Conformational Analysis of Triarylboranes

An X-ray structure determination shows that trimesitylborane assumes a propeller conformation in the crystal lattice. The low-temperature ¹H-nmr spectra of diverse triarylboranes which have substituents other than hydrogen in all six ortho positions are consistent with this geometry in solution. At higher temperatures, the spectra reflect rapid stereoisomerizations, which are interpreted in terms of two-ring flip mechanisms. These processes have activation energies of ca. 14-16 kcal/mol, and separation of stereoisomers at ambient temperatures is therefore precluded.