17 O NMR spectroscopy of heterocycles. Steric effects for N ‐oxides

The 17 O chemical shift data for a series of 2‐ and 4‐alkyi‐ and ‐aryl‐substituted pyridine N ‐oxides, as well as those for quinoline N ‐oxide (13), benzo[ f ]quinoline N ‐oxide (14) and benzo [ h ]quinoline N ‐oxide (15) at natural abundance were measured in acctonitrile at 75°C. 2‐Alkyl groups cause a deshielding effect on the N ‐oxide chemical shifts compared with those of the 4‐alkyl isomers. The largest effects are noted for 2‐ tert ‐butylpyridine N ‐oxide (8) and 2,6‐dimethylpyridine N ‐oxide (12). Similarly, the 17 O chemical shift of 15 is deshielded by 18 ppm compared with that of 13 and 14. 17 O chemical shift data were obtained for 2‐and 4‐ tert ‐butylanisoles and compared with those of 8 and 4‐ tert ‐butylpyridine N ‐oxide (9). The sensitivity of the N ‐oxide 17 O chemical shifts was substantially greater than that for the anisoles, an effect that was also observed in an analysis of the 13 CNMR data of the tert ‐butyl groups of 8 and 9. Steric inhibition of resonance is noted for 3‐methyl‐4‐nitropyridine N ‐oxide; the N ‐oxide 17 O signal was shielded and the NO 2 17 O signal was deshielded relative to those for 4‐nitropyridine N ‐oxide.