A tautomeric equilibrium between functionalized 2-formylphenylboronic acids and corresponding 1,3-dihydro-1,3-dihydroxybenzo [c][2,1]oxaboroles

Functionalized 2-formylphenylboronic acids undergo an unprecedented tautomeric rearrangement in solution to form corresponding 1,3-dihydro-1,3-dihydroxybenzo[c][2,1] oxaboroles. X-Ray analyses of selected examples revealed diverse solid-state molecular structures from a planar open form with a hydrogen-bonded carbonyl group (X=3-F) through a twisted conformer showing a weak carbonyl-boron interaction (X=3,5-Br-2) to a cyclic oxaborole derivative (X=3-Br). Variable-temperature H-1 NMR spectroscopy was used to determine equilibrium constants as well as enthalpies and entropies of tautomerization in a mixed solvent [D-6] acetone-D2O (95:5). A computational approach to the process by DFT (B3LYP) and MP2 methods has also been performed.