Studies on the s-cis–trans isomerism for some furan derivatives through IR and NMR spectroscopies and theoretical calculations

The s-cis–trans isomerism of two furan derivatives [2-acetyl- (AF) and 2-acetyl-5-methylfuran, (AMF)] was analyzed, using data from the deconvolution of their carbonyl absorption band in two solvents (CH2Cl2 and CH3CN). These infrared data showed that the O,O-trans conformers predominate in the less polar solvent (CH2Cl2), but these equilibria change in a more polar solvent (CH3CN) leading to a slight predominance of the O,O-cis conformers, in agreement with the theoretical calculations. The later results were obtained using B3LYP-IEFPCM/6-31++g(3df,3p) level of theory, which taking into account the solvent effects at IEFPCM (Integral Equation Formalism Polarizable Continuum Model). Low temperature 13C NMR spectra in CD2Cl2 (ca. −75 °C) showed pairs of signals for each carbon, due to the known high energy barrier for the cis–trans interconversion leading to a large predominance of the trans isomers, which decreases in acetone-d6. This was confirmed by their 1H NMR spectra at the same temperatures. Moreover, despite the larger hyperconjugative interactions for the O,O-cis isomers, obtained from NBO data, these isomers are destabilized by the their Lewis energy.

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► s-cis/s-trans equilibrium of two 2-acetylfurans is fully described.
► Infrared data showed these equilibria were sensitive to solvent polarity.
► Theoretical calculations were performed in the isolated phase and in solution.
► Low temperature 13C NMR spectral data were also in agreement with infrared data.