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. pi-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.