Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
Education Center, Faculty of Engineering, Chiba Institute of Technology, Narashino, Chiba 275-0023, Japan.
J Org Chem. 2020 Apr 17;85(8):5255-5264. doi: 10.1021/acs.joc.9b03326. Epub 2020 Jan 31.
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)) with different ps and performed kinetic and equilibrium studies on the reaction systems. It was shown that the logarithms of the rate constants of RB(OH) and its conjugate boronate ion (RB(OH)) decreased and increased linearly, respectively, with increasing p of RB(OH) for both systems. Consequently, the reactivities of RB(OH) and RB(OH) were reversed at high RB(OH) p. It was also shown that the bulky substituents of phenylboronic acids retarded the backward reactions, resulting in enhancement of the formation constants of boronic acid-catechol esters.
硼酸与儿茶酚在水溶液中的反应性的基本信息在涉及硼酸的所有领域都是必需的。然而，反应性的综合研究通常受到所谓的“质子模糊性”的阻碍，这使得硼酸和硼酸根离子的速率常数无法分开确定。在此，我们建立了两个没有质子模糊性的反应系统：（1）茜素红 S 和（2）Tiron 与几种具有不同 ps 的硼酸（RB(OH)），并对反应系统进行了动力学和平衡研究。结果表明，对于这两个系统，RB(OH)和其共轭硼酸根离子（RB(OH))的速率常数的对数分别随 RB(OH)的 p 值的增加而线性减小和增加。因此，在高 RB(OH) p 下，RB(OH)的反应性发生逆转。还表明，苯硼酸的大位阻取代基会阻碍逆反应，从而增强硼酸-儿茶酚酯的形成常数。
