Publication Date:
2011-08-06
Description:
The effect of the intramolecular H-bonding of the primary amide group on the spectral properties and reactivity of this group towards electrophiles has been studied in systematic rows of 1,2,5,6,7,8-hexahydro-7,7-dimethyl-2,5-dioxo-1-R-quinoline-3-carboxamides and 2-aryliminocoumarin-3-carboxamides using 1 H and 15 N NMR spectroscopy and the kinetics of model reactions. The upfield signal of the amide proton that is not intramolecularly H-bonded (H a ) depends on external factors such as solvent nature and concentration. At the same time, the downfield chemical shift of the H b proton (bonded by the intramolecular hydrogen bond) depends mostly on the strength of the intramolecular H-bond, which is affected by such internal factor as electron nature of substituent R. The substituent's influence on the H b proton's chemical shift is more effective in deuterochloroform medium than in DMSO- d 6 where the intramolecular hydrogen bond is less stable. The value Δδ(H) = δ(H b ) − δ(H a ) is suggested as a simple comparative spectral index of the intramolecular hydrogen bond strength in these and similar compounds. By contrast, the effect of R on the 15 N NMR chemical shift of the amide nitrogen has turned out to be too small to estimate changes of the electron density at the nitrogen. The effect of the intramolecular H-bond on the reactivity of the amide group is twofold. When the cleavage of the H-bond occurs on the rate limiting step it dramatically reduces the reaction rate. In the other case, the strengthening of the H-bond favors the reaction rate because of the increase of the electron density at the amide nitrogen. Copyright © 2011 John Wiley & Sons, Ltd. The origins of the amide proton signals nonequivalence are discussed. The value Δδ(H) = δ(H a ) − δ(H b ) is suggested as a simple comparative spectral index of the intramolecular hydrogen bond strength in these and similar compounds. The effect of the intramolecular H-bond on the reactivity of the amide group towards electrophiles is studied.
Print ISSN:
0894-3230
Electronic ISSN:
1099-1395
Topics:
Chemistry and Pharmacology
,
Physics
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