17O NMR studies on substituted N‐arylacetamides and aryl acetates: Torsion angle and electronic effects

Abstract 17O NMR spectroscopic data at natural abundance in acetonitrile at 75°C were obtained for sixteen N‐arylacetamides, three N‐methyl‐N‐arylacetamides and seven aryl acetates. The 17O chemical shifts of the N‐arylacetamides are significantly influenced by both electronic and steric factors. A Hammett plot of the data for 4‐substituted N‐arylacetamides gives a slope of 22 ppm. Ortho substituents result in significant shielding effects, in contrast to previously reported results for conjugated ketnoes. A quantitative relationship between 17O chemical shift data and torsion angle was found for the sterically hindered arylacetamides. Electronic effects on the aryl acetate 17O chemical shifts are, in comparison, greatly reduced. Steric factors for aryl acetates show even smaller effects on the 17O chemical shifts.