Solvent and structure effects on rotamerization in 3,4‐alkylenedioxy‐ and 3,4‐dialkoxy‐2,5‐bis[di(<i>tert</i>‐butyl) hydroxymethyl]thiophenes: an NMR, IR, molecular mechanics and single‐crystal x‐ray study

Hydrazone derivatives of β-dicarbonyl compounds illustrate an integrated configurational and conformational changes, which make them be considered as probe in disclosing H-bonding importance in hydrogen transfer and switching. The ethyl-3-oxo-butanoate derivatized hydrazones were synthesized by Japp-Klingemann reaction and were characterized in view of chromatographic and spectroscopic behavior employing thin layer chromatography, infrared (IR), ultraviolet-visible (UV-vis), and 1 H and 13 C nuclear magnetic resonance (NMR) techniques. Conformational isomerization between two possible rotamers along with their tautomeric forms was identified and investigated in detail regarding to resonance-assisted H-bonding (RAHB) concept by the referred spectroscopic measurements. Furthermore, substituent impacts were assessed using the acquired spectroscopic data and evaluated for structureproperty correlation on the base of Hammett relationships. All of the hydrazone derivatives were found to equilibrate in acetone solution, and more of them existed in one rotameric form in solid state. The aryl substituents influence the intermolecular and intramolecular interactions differently, because the derivatives could be isolated as two distinct crystals only in a few members.Moreover, the RAHB concept was found to stabilize the conformational isomers of the hydrazone derivatives to different degrees leading to distinct equilibrium constant. Quantum chemical calculations (DFT/B3LYP) were also used to characterize the nature of the RAHB and examine the rotameric transformation process and substituent impacts on the rotamerization equilibrium.

Section: Solution Aspects Of the Arylhydrazone Derivatives Ahd 1-9mentioning

confidence: 99%

Rotamerism and tautomerism in hydrazone derivatives of ethylacetoacetate: Spectroscopic features, Hammett relationships, and computational calculations

Zamanloo

Salmanzadeh

Esrafili

et al. 2023

Self‐association in {2‐[3,4‐alkylenedioxy‐5‐(3‐pyridyl)]‐thienyl}alkanols: an NMR, IR, and single‐crystal X‐ray study

Lomas

Cordier

Adenier

et al. 2007

syn‐2,2,4,4‐Tetramethyl‐3‐{2‐[3,4‐alkylenedioxy‐5‐(3‐pyridyl)]thienyl}pentan‐3‐ols self‐associate both in the solid state and in solution. Single‐crystal X‐ray diffraction study of the 3,4‐ethylenedioxythiophene (EDOT) derivative shows that it exists as a centrosymmetric head‐to‐tail, syn dimer in the solid state. The IR spectra of the solids display only a broad OH absorption around 3300 cm−1, corresponding to a hydrogen‐bonded species. 1H Nuclear Overhauser Effect Spectroscopy (NOESY) NMR experiments in benzene reveal interactions between the tert‐butyl groups and the H2 and H6 protons of the pyridyl group. Two approaches have been used to determine association constants of the EDOT derivative by NMR titration, based on the concentration dependence of (i) the syn/anti ratio and (ii) the OH proton shift of the syn rotamer. Reasonably concordant results are obtained from 298 to 323 K (3.6 and 3.9 M−1, respectively, at 298 K). Similar values are obtained from the syn OH proton shift variation for the 3,4‐methylenedioxythiophene (MDOT) derivative. Concentration‐dependent variation of the anti OH proton shift in the latter suggests that the anti isomer associates in the form of an open, singly hydrogen‐bonded dimer, with a much smaller association constant than the syn rotamer. Self‐association constants for 3‐pyridyl‐EDOT‐alkanols with smaller substituents vary by a factor of 4 from (i‐Pr)2 up to (CD3)2, while the hetero‐association constants for the same compounds with pyridine vary slightly less. Copyright © 2007 John Wiley & Sons, Ltd.

Direct NMR measurement of rotation rates: solvent effects on rotation barriers

Lomas

2007

Abstract2,2,4,4‐Tetramethyl‐3‐{2‐[3,4‐dialkoxy‐5‐(3‐pyridyl)]thienyl}pentan‐3‐ols self‐associate both in the solid state and in solution. The IR spectra of the solids display a broad OH absorption at 3320 cm−1, corresponding to an intermolecularly hydrogen‐bonded syn rotamer, probably a dimer, as well as absorptions around 3500 cm−1 of the intramolecularly hydrogen‐bonded anti form. Well‐crystallized samples of these derivatives go into solution in the syn form but undergo rotation to the anti rotamer at a rate which can be measured directly by proton Nuclear Magnetic Resonance (NMR) spectroscopy. The diethoxy derivative was studied in a wide variety of solvents. The activation energy for syn→anti rotation is practically solvent‐independent, whereas that of the reverse reaction falls in hydrogen‐bonding solvents, by more than 2 kcal mol−1 on going from chloroform or benzene to dimethylsulfoxide (DMSO). By combining direct measurements at low temperature and Dynamic Nuclear Magnetic Resonance (DNMR) results at high temperature, rotation rates were evaluated over a range of more than 100 K, and significantly large negative activation entropies determined. Copyright © 2007 John Wiley & Sons, Ltd.