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Downfield 1H NMR shifts induced by electron-rich substituents

✍ Scribed by E. Smakula Hand; W. W. Paudler

Publisher: John Wiley and Sons
Year: 1980
Tongue: English
Weight: 286 KB
Volume: 14
Category: Article
ISSN: 0749-1581
DOI: 10.1002/mrc.1270140112

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✦ Synopsis

The 'H NMR signal due to the peri proton H-5 in imidazo [1,2-a]pyridines exhibits large downfield shifts when the 3-position bears a substituent (-XY) containing an electron-rich atom (Y) two bonds removed from C-3. Such shifts are observed not only when the ring electron density can be decreased by resonance interaction [from -1.29 to -1.87ppm for 3-COR, 3-(2'-quinoxalyl) and 3-NO in CDCI,], but also when electron Withdrawal must be minimal: -1.10ppm when X is an sp3 center [3-CH(OH)CC13 in CDCl,],

-1.0 ppm when the 3-substituent is negatively charged (3-C02-Na' in DzO), and -1.32 to -1.46 ppm when the ring system is protonated (3-COR and 3-N02 in TFAA). The effect is largest in those compounds for which orientation of lone-pair electrons on atom Y toward H-5 can be invoked. The deshielding is attributed primarily to a through-space electric field effect.

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