Formation of Para-Substituted Triphenylboroxines: A Computational Study

Density functional theory (B3LYP//6-311+G*) calculations including Poisson−Boltzmann (PB) implicit solvent
were applied to study the relative stability of triphenylboroxine (PhBO)3 with respect to its phenylboronic
acid monomers. In solution, formation of (PhBO)3 is thermodynamically unfavorable; however, addition of
an amine base results in the formation of stable 1:1 adducts of (PhBO)3 and amine. Formation of 1:2 adducts
is energetically unfavorable. We find that adduct formation is more exothermic than cleavage of the boroxine
ring back to its monomers. π-Electron-withdrawing groups in the para-position of the phenyl ring destabilize
the boroxine ring with respect to its monomers. However, para-substituents that are net electron-withdrawing
are found to stabilize formation of the 1:1 adduct.