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Downfield 1 H NMR shifts induced by electron‐rich substituents
Author(s) -
Hand E. Smakula,
Paudler W. W.
Publication year - 1980
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270140112
Subject(s) - chemistry , substituent , protonation , lone pair , ring (chemistry) , crystallography , chemical shift , atom (system on chip) , proton , electron , resonance (particle physics) , stereochemistry , medicinal chemistry , ion , molecule , atomic physics , physics , organic chemistry , quantum mechanics , computer science , embedded system
The 1 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 −1.87 ppm for 3‐COR, 3‐(2′‐quinoxalyl) and 3‐No in CDCl 3 ], but also when electron withdrawal must be minimal: −1.10 ppm when X is an sp 3 center [3‐CH(OH)CCl 3 in CDCl 3 ], −1.0 ppm when the 3‐substituent is negatively charged (3‐CO 2 − Na + in D 2 O), and −1.32 to −1.46 ppm when the ring system is protonated (3‐COR and 3‐NO 2 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.