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

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…Even quadrupolar nuclei 14 N, 17 O, and 33 S can be employed. If the tautomeric exchange is still fast on the NMR timescale, and only weighted averages of the NMR parameters are on hand, the NMR parameters of the frozen tautomers (if not available from solid-state NMR spectra) can be computed at bench without major problems (Chapter 13).…”

“…Also the ring-chain tautomerism of 2-formylphenylboronic acids 5 (Scheme 5.3) was studied by variable-temperature 1 H NMR spectroscopy to determine the equilibrium constants as well as enthalpies and entropies of tautomerization in a mixed solvent (acetone-d 6 /D 2 O = 95 : 5) [17]. The ring-chain tautomerism of polyfluoroacylmethylenenaphthalides 6 (Scheme 5.4) could be readily assigned by 19 The ring-ring tautomerism (Scheme 5.6) of 5-hydrazino-2 -isoxazoline (8a) 5-hydroxyamino-2 -pyrazoline type (8b) was studied both in solution and in the solid state [20].…”

“…The tautomerism of slow keto-enol and fast enol-enol tautomeric equilibria of a number of 1-(2-hydroxyphenyl)-3-naphthylpropane-1,3-diones can be easily monitored also by 17 As an alternative, the spatial magnetic properties of the tautomers were examined [38,39] (see Figure 5.3), which unequivocally represent the aromaticity of the central ring in the keto (18a) and the quinoid π-electron distribution in 18b [40][41][42]. Employing the same methodology [38,39], the preferred quinoid type structure of a bridged bisphenazine type molecule 19 could be assigned [43] (Scheme 5.13).…”

“…15 N chemical shifts can easily distinguish benzohydroxamic acid (δ = −200 to −220 ppm) and the corresponding hydroximic acid derivatives (δ = −110 to −150 ppm) as well [73]. Also, the 17 [74]. The tautomerism of 15 N-labeled isocytosine was reinvestigated in solution and in the solid state (Scheme 5.28) [75].…”

“…The alkylidene carbon 13 C signals are characteristically different: 76a: δ( 13 C) = 358.8 ppm ( 2 J P,C = 14.5 Hz) and in 76b: δ( 13 C) = 256.4 ppm ( 1 J C,H = 123.5, 2 J P,C = 11.8 Hz) and 254.7 ppm ( 1 J C,H = 102.6 Hz, 2 J P,C = 12.6 Hz), respectively, at −50 • C in toluene-d 8 [143]. Quantum chemical calculation of chemical shifts of the quadrupolar nuclei 14 N, 17 O, and 33 S and comparison with experimental δ-values allowed reliable prediction of the deprotonation sites of hydantoin, urazile, and N-hydroxy urea, as well as the effect of solvation, especially hydration, on the relative stability of the thus generated ions [144].…”