The acid dissociation constants (
Ka) of 4-pyridylboronic acid derivatives were determined spectrophotometrically, and the p
Ka assignment was performed based on
11B NMR spectroscopy. It was found that both of the boron centers in 4-pyridylboronic acid (4-PyB(OH)
2) and the
N-methylated derivative [4-(
N-Me)Py
+B(OH)
2] have strong acidities (p
KaB = 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)
2 and 4-(
N-Me)Py
+B(OH)
2 reacted 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 (p
KaB = 3.96), was still higher than the phenyl and pyridyl boronic acid derivatives having p
KaB = 4.40 – 10.74, which we have examined so far.
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