Detailed Reaction Mechanisms of 4-Pyridylboronic Acid and (N-Methyl)-4-Pyridinium Boronic Acid with D-Sorbitol in Aqueous Solution

Kinetic measurements of the reactions of (N-methyl)-4-pyridinium boronic acid [(N−Me)-4-Py ⁺ B(OH) ₂ ] and 4-pyridinium boronic acid [4-HPy ⁺ B(OH) ₂ ] with D-sorbitol in aqueous solution were performed, and the reaction mechanisms were clarified in detail. Both reactions proceeded in one step; i. e., the intermolecular rate-determining step forming the 2,3-bicoordinate product was followed by the fast intramolecular step from the bicoordinate product to the 2,3,5-tricoordinate product. N-Methylation of 4-HPy ⁺ B(OH) ₂ was found to dramatically reduce the reactivity of the N-methylated-4-pyridinium boronate ion [(N−Me)-4-Py ⁺ B(OH) ₃ ⁻ ], although it had little effect on the reactivity of (N−Me)-4-Py ⁺ B(OH) ₂ . The reaction rates were enhanced by ion-pair formation between the boronate ion and the protonated buffer cation.