THE INFLUENCE OF ALTERED AMIDIC RESONANCE ON THE INFRARED AND C-13 AND N-15 NMR SPECTROSCOPIC CHARACTERISTICS AND BARRIERS TO ROTATION ABOUT THE N-C(O) BOND IN SOME ANILIDES AND TOLUAMIDES

In order to provide physiochemical evidence for or against amidic resonance, three series of anilides and toluamides have been investigated by infrared spectroscopy and C-13 and N-15 NMR, and barriers to rotation for several toluamides have been determined. Each series contains amides that are predisposed in one or more ways to disfavor resonance. These include rotation about the N-C(O) bond in a series of bicyclic anilides, N-pyramidalization in a series of N-toluoyl cyclic amines, and removal of N electron density in some toluamides by inductive means or by occupying the N lone pair in an aromatic sextet. The structures of the N-toluoyl derivatives of azetidine, pyrrole, and 2,5-dimethylpyrrole were determined by X-ray diffraction. The physicochemical data are generally consistent with the resonance model, but there are anomalies indicating that other factors are also important. When resonance is favored, the infrared nu-C = O and nu-C-N bands are respectively found at lower and higher values than in amides where resonance is disfavored. The C-13 NMR chemical shift for a wide variety of toluamides and anilides with different resonance abilities consistently appears between delta-167 and 172 ppm, except for the distorted bicyclic anilides (delta-180-200 ppm) or N-toluoylaziridine (delta-178 ppm). Since the sets include amides of considerable structural diversity, the C-13 = O chemical shifts are compared with the C-13 chemical shift of the corresponding N-methylamine: the DELTA-delta(C-13)(amide-amine) values span a range of approximately 122-143 ppm, with amides having inhibited resonance lying at the higher values. N-15 chemical shifts for amides for which resonance can be invoked appear to be approximately 80 ppm downfield from their corresponding N-methylated tertiary amines. The N-15 chemical shifts of amides in which resonance is impaired by rotation about the N-C(O) bond, by N-pyramidalization, or by occupying the N lone pair in an aromatic sextet move upfield by approximately 30-40 ppm, but are still 45-50 ppm downfield from their corresponding tertiary amines. The DELTA-G double-ended dagger for rotation about the N-C(O) bond in a series of toluamides appears to be relatively insensitive to the ability of the N to support (+)-charge as gauged by the gas-phase basicity of the corresponding amines. Where structural and DELTA-G double-ended dagger rotational data can be compared, those amides with short N-C(O) bonds (1.34-1.37 angstrom) have high rotational barriers (14.5-16.5 kcal/mol), while those amides with longer N-C(O) bonds (1.40-1.42 angstrom) have reduced barriers to rotation (< 6-8 kcal/mol).