Can triorganoboroxins exist in a “monomeric” R---B=O form? MNDO calculations and ebulliometric molecular weight determination

MNDO calculations were made for triethylboroxin (EtBO)₃ and triphenylboroxin (PhBO)₃ using both X-ray determined and optimized geometry of these molecules. The results were compared with hypothetical “monomeric” molecules R---B=O. Calculated energies of trimerization are about −200 kJ mol⁻¹ for both compounds and confirm the much higher stability of the “trimer”. Ebulliometric determination of molecular weight of triphenylboroxin in 2-pentanone confirms its trimeric character.