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

✍ Scribed by D. W. Boykin; P. Balakrishnan; A. L. Baumstark

Publisher
John Wiley and Sons
Year
1985
Tongue
English
Weight
333 KB
Volume
23
Category
Article
ISSN
0749-1581

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✦ Synopsis

The 1 7 0 chemical shift data for a series of 2and 4-alkyl-and -aryl-substituted pyridine N-oxides, as well as those for quinoline N-oxide (13), benzocflquinoline N-oxide (14) and benzo[h]qninoline N-oxide (15) at natural abundance were measured in acetonitrile 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-fert-butylpyridine N-oxide (8) and 2,6-dimethylpyridine N-oxide (12). Similarly, the 170 chemical shift of 15 is deshielded by 18 ppm compared with that of 13 and 14. I7O chemical shift data were obtained for 2and 4-tert-butylanisoles and compared with those of 8 and 4-tert-butylpyridme N-oxide (9). The sensitivity of the N-oxide "0 chemical shifts was substantially greater than that for the anisoles, an effect that was also observed in an analysis of the -CNMR data of the terr-butyl groups of 8 and 9. Steric inhibition of resonance is noted for 3-methyl-4-nitropyridine N-oxide; the N-oxide 1 7 0 signal was shielded and the NO, 0 signal was deshielded relative to those for 4-nitropyridme N-oxide.

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