Reaction systems of boronic acid (RB(OH2), R = phenyl or 3-fluorophenyl) with diols and no proton ambiguity were elaborately set up, and kinetic measurements were conducted to elucidate the relative reactivities of RB(OH)2 and RB(OH)3−. In the reactions of phenylboronic and 3-fluorophenylboronic acids with propylene glycol, the reactivity order was: RB(OH)2 >> RB(OH)3−, whereas in the reactions of 3-pyridylboronic acid with Tiron and 2,2′-biphenol, the reactivity of RB(OH)2 was comparable to that of RB(OH)3−. These results are in contrast to those that have been previously reported, and widely accepted for over thirty years, that concluded that the reactivity of RB(OH)3− is several orders of magnitude higher than that of RB(OH)2. The reactivity of Tiron with 3-pyridylboronic acid is affected by the protonation of one of its sulfonate groups.
