Fast trigonal/tetragonal interconversion followed by slow chelate-ring closure in the complexation of boronic acids

Which of the step is rate determining in the complexation of boronic acid with bidentate ligands, the first trigonal/tetragonal interconversion step or the following chelate-ring closure step ? No one could have ever answered this simple question clearly. In this study, we for the first time gave an explicit answer to this question. Reaction systems were devised for kinetic measurements to answer this simple question. At first, pKa values of 15 boronic acids were determined spectrophotometrically at I = 1.0 M and 25 °C: pKa = 6.40 ± 0.06 for 3,5-Cl2PhB(OH)2, 6.62 ± 0.03 for 3,5-(CF3)2PhB(OH)2, 6.95 ± 0.09 for 3,5-Br2PhB(OH)2, 7.04 ± 0.03 for 3-NO2PhB(OH)2, 7.08 ± 0.03 for 3,5-F2PhB(OH)2, 7.12 ± 0.02 for 2,4-F2PhB(OH)2, 7.39 ± 0.02 for 4-CF3PhB(OH)2, 7.50 ± 0.02 for 3-FPhB(OH)2, 7.83 ± 0.02 for 2-FPhB(OH)2, 7.87 ± 0.01 for 3-CF3PhB(OH)2, 8.11 ± 0.01 for 2-ClPhB(OH)2, 8.14 ± 0.03 for 3-(COOH)PhB(OH)2, 8.15 ± 0.03 for 3-(CH3CO)Ph(OH)2, 8.20 ± 0.01 for 3-ClPhB(OH)2, and 8.66 ± 0.05 for 4-FPhB(OH)2. The kinetic results for the reactions of some of these boronic acids with 4-isopropyltropolone clearly show that the rate-determining step is not the interconversion step but the chelate-ring closure step.