Reactivity of Boronic Acids toward Catechols in Aqueous Solution

Fundamental information on the reactivities of boronic acids toward catechols in aqueous solution is required in all the fields dealing with boronic acid. However, comprehensive studies on reactivity are often hindered by so-called "proton ambiguity," which makes it impossible for the rate constants of boronic acid and boronate ion to be determined separately. Herein, we set up two reaction systems without proton ambiguity: (1) Alizarin Red S and (2) Tiron with several boronic acids (RB(OH)₂) with different pK_as and performed kinetic and equilibrium studies on the reaction systems. It was shown that the logarithms of the rate constants of RB(OH)₂ and its conjugate boronate ion (RB(OH)₃ ^-) decreased and increased linearly, respectively, with increasing pK_a of RB(OH)₂ for both systems. Consequently, the reactivities of RB(OH)₂ and RB(OH)₃ ^- were reversed at high RB(OH)₂ pK_a. It was also shown that the bulky o^- substituents of phenylboronic acids retarded the backward reactions, resulting in enhancement of the formation constants of boronic acid-catechol esters.