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 |
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| Authors: | Satoshi Iwatsuki Yuki Kanamitsu Hidetaka Ohara Eisuke Watanabe Koji Ishihara |
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| Institution: | 1. Department of Chemistry, Graduate School of Natural Science, Konan University, , Kobe, 658‐8501 Japan;2. Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, , Tokyo, 169‐8555 Japan |
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| Abstract: | The pKas of 3‐pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pKas assignable to the boron center and pyridine moiety. The pKa 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 (pKa = 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 H2ipt+, 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. |
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| Keywords: | acid dissociation boronate ion boronic acid kinetics mechanism |
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