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The interaction of pinanediol with 2-fluorophenylboronic acid and several other substituted phenylboronic acids was studied in 40% vol. aqueous acetonitrile by 1H and 11B NMR, potentiometric and spectrophotometric titrations at variable pH values. The experimental results reveal the formation of a very stable trigonal ester (Ktrig ≈ 2 × 104 M-1) and a significantly less stable tetrahedral hydroxocomplex (Ktet ≈ 5 × 103 M-1) in contrast to the traditionally observed inverted order of stabilities KtrigKtet. Comparison of the crystal structure of the trigonal ester isolated from aqueous acetonitrile with the DFT simulated structure of the respective hydroxocomplex shows that an unusual order of stabilities KtrigKtet is observed in spite of the existence of the usual strain release effect in the O-B-O angle considered responsible for the typically observed increased stability of the tetrahedral hydroxocomplex. A complementary study of the stability of the six-membered cyclic boronate esters of chromotropic acid demonstrated the order Ktrig ≪ Ktet although the strain was absent in these esters. The results for m-, p-substituted phenylboronic acids show that the stability of both five- and six-membered trigonal esters formed with pinanediol and chromotropic acid, respectively, is insensitive to electronic effects but the electron accepting substituents stabilize the hydroxocomplexes. It follows from the whole set of results that Ktet can be much larger than Ktrig in the absence of the strain, but with a sufficiently acidic diol, and that the presence of the strain does not necessarily make Ktet larger than Ktrig for a less acidic diol with a purely saturated hydrocarbon backbone. Thus, the electronic effects manifested in the acidity of the diol appear to be more significant than the strain release effect in determining the Ktet/Ktrig ratio.

Interactions of 2-, 3- and 4-formyl phenylboronic acids (FPBAs) with sugars, amino sugars, aminoglycosides and various poly- and monoamines have been studied by UV-vis, (1)H and (11)B NMR titrations in water at variable pH. Behavior of 2-FPBA was anomalous in several aspects. Transformation of the acid into its conjugate base was slow in NMR time scale and was accompanied by intramolecular cyclization affording the respective benzoboroxole. The equilibrium constants for imine formation (K(imine)) between 2-FPBA and simple monoamines including amino sugars were ca. 2 orders of magnitude larger than those with other isomers. Still one order of magnitude larger K(imine) values were observed for 2-FPBA with aminoglycosides (kanamycin, amikacin, gentamicin, neomycin) and polyamines (spermine, spermidine). The examination of UV-vis and (11)B NMR spectra of imines formed with 2-FPBA showed that formally neutral Schiff bases in fact were zwitterionic species containing a protonated imine group and an anionic B(OH)(3)(-) group. The enhanced stability of imines with monoamines can therefore be attributed to the electrostatic stabilization provided by the zwitterionic structure and further increased stability of imines with antibiotics and polyamines is explicable by additional stabilization of the borate anionic group by ion paring with ammonium groups not involved in Schiff base formation. Thanks to high molar absorptivity of protonated imines interaction of 2-FPBA with aminoglycosides allows detecting them spectrophotometrically in a μM concentration range in neutral aqueous solutions in the presence of sugars, amino sugars and amino acids.