ISSN:
0030-4921
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 1H 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 -1.87 ppm for 3-COR, 3-(2′-quinoxalyl) and 3-No in CDCl3], but also when electron withdrawal must be minimal: -1.10 ppm when X is an sp3 center [3-CH(OH)CCl3 in CDCl3], -1.0 ppm when the 3-substituent is negatively charged (3-CO2 -Na+ in D2O), and -1.32 to -1.46 ppm when the ring system is protonated (3-COR and 3-NO2 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 desielding is attributed primarily to a through-space electric field effect.
Additional Material:
1 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1270140112
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