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

Abstract

AbstractKinetic 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.