The acid dissociation constants (Ka) of 4-pyridylboronic acid derivatives were determined spectrophotometrically, and the pKaassignment was performed based on 11B NMR spectroscopy. It was found that both of the boron centers in 4-pyridylboronic acid (4-PyB(OH2) and the N-methylated derivative [4-(N-Me)Py+B(OH)2] have strong acidities (pKaB= 4.00 and 3.96, respectively), indicating that the introduction of a methyl group into the nitrogen atom of the pyridine ring in 4-pyridylboronic acid has no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 4-(N-X)Py+B(OH)2(X = H, Me) with hinokitiol (Hipt) were carried out under pseudo first-order conditions, where the total concentration of boronic acid was much larger than that of Hipt. Both 4-HPy+B(OH)2and 4-(N-Me)Py+B(OH)2reacted with Hipt much faster than their conjugate boronate ions (4-HPy+B(OH)3-and 4-(N-X)Py+B(OH)3-, respectively), which is consistent with our recent results. The rate constants (k1) for the reactions of the boronic acids with Hipt increase with an increase of the acidity of boronic acid. The reactivity of 4-(N-Me)Py+B(OH)2, having the strongest acidity (pKaB= 3.96), was still higher than the phenyl and pyridyl boronic acid derivatives having pKaB= /4.40 - 10.74, which we have examined so far.
ASJC Scopus subject areas
- Analytical Chemistry