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

The pK _as of 3-pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pK _as assignable to the boron center and pyridine moiety. The pK _a assignment performed by ¹¹B 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 (pK _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 ₂ipt ⁺, 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.