Rotational Isomers of N-Alkyl-N-(o-acylphenyl)acetamides

We have studied molecular structure of N-alkyl-N-(o-acylphenyl)acetamides 1–3 by 1H NMR spectroscopy. The 1H NMR spectra clearly displayed that two rotational isomers are present in solution and two geminal N–CH2 protons are magnetically nonequivalent. This was explained by restricted rotations about both the amide N–CO bond and the aryl-nitrogen bond. The ratios of exo (benzene ring trans to carbonyl) and endo rotamers were estimated to be 6–8:1 from the 1H NMR spectra. The X-ray analysis of N-ethyl-N-(o-benzoylphenyl)acetamide 1 showed that the crystal has exo-form. The rotations about both the amide N–CO bond and the aryl-nitrogen bond were slower on the NMR time scale.