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Reactivity of Boronic Acids toward Catechols in Aqueous Solution
Yota Suzuki1, Daisuke Kusuyama1, Tomoaki Sugaya2
1Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
The Journal of Organic Chemistry
|February 1, 2020
Summary
This study resolves "proton ambiguity" in boronic acid and boronate ion reactivity studies. Researchers found that boronic acid reactivity decreases while boronate ion reactivity increases with higher acidity, revealing reversed reactivity at high pKa.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Supramolecular Chemistry
Background:
- Understanding boronic acid reactivity with catechols in aqueous solutions is crucial across various scientific fields.
- Previous studies faced challenges due to "proton ambiguity," preventing separate determination of rate constants for boronic acids and their conjugate boronate ions.
Purpose of the Study:
- To establish reaction systems that eliminate "proton ambiguity" for accurate kinetic and equilibrium studies of boronic acid-catechol interactions.
- To investigate the influence of boronic acid pKa on the reactivity of both the neutral boronic acid and its anionic boronate form.
Main Methods:
- Developed two novel reaction systems utilizing Alizarin Red S and Tiron, respectively, to study interactions with various boronic acids.
- Conducted kinetic and equilibrium studies on these systems to determine rate constants and formation constants.
- Analyzed the relationship between boronic acid pKa and the reactivity of both boronic acid and boronate ion species.
Main Results:
- Demonstrated a linear inverse correlation between the pKa of boronic acids and the rate constants of the neutral boronic acid species (RB(OH)2).
- Showed a linear positive correlation between the pKa of boronic acids and the rate constants of the conjugate boronate ion species (RB(OH)3-).
- Observed a reversal in reactivity between boronic acid and boronate ion at higher pKa values.
- Found that bulky ortho-substituents on phenylboronic acids significantly retard backward reactions, enhancing the stability of boronic acid-catechol esters.
Conclusions:
- Successfully circumvented "proton ambiguity" by employing specific reaction systems, enabling precise characterization of boronic acid and boronate ion reactivities.
- The study provides fundamental insights into how acidity influences the reaction kinetics and equilibrium of boronic acid-catechol complexation.
- These findings are critical for the rational design and application of boronic acid derivatives in diverse chemical and biological contexts.
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