Higher reactivity of 3‐pyridinium boronic acid compared with 3‐pyridinium boronate ion toward 4‐isopropyltropolone in acidic aqueous solution: fundamental reaction analyses for an effective organoboron‐based chemosensor

Abstract

The p K a s of 3‐pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two p K a s assignable to the boron center and pyridine moiety. The p K a assignment performed by 11 B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3‐pyridylboronic acid [3‐PyB(OH) 2 ] and the N ‐methylated derivative [3‐( N ‐Me)Py + B(OH) 2 ] have strong acidities (p K a = 4.4 for both). It was found that introduction of a substituent to pyridine‐ C atom in 3‐pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine‐ N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3‐pyridinium boronic acid [3‐HPy + B(OH) 2 ] with 4‐isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3‐HPy + B(OH) 2 reacts with Hipt and protonated H 2 ipt + , and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3‐HPy + B(OH) 2 reacts with Hipt faster than its conjugate boronate [3‐HPy + B(OH) 3 – ], which is consistent with our recent results. The reactivity of 3‐( N ‐Me)Py + B(OH) 2 towards Hipt was also examined kinetically; the reactivities of 3‐( N ‐Me)Py + B(OH) 2 and 3‐( N ‐Me)Py + B(OH) 3 – are almost the same as those of their original 3‐HPy + B(OH) 2 and 3‐HPy + B(OH) 3 – , respectively. Copyright © 2012 John Wiley & Sons, Ltd.