Abstract
On resonance interactions in methyl 4-X-benzoates and the effect of 2,6-dimethyl substitution. 13C or 17O NMR chemical shifts as effective indicators of π-electron-density distribution?
Highlights
Craik, Brownlee and coworkers, in their pioneering and extensive study on 13C NMR chemical shifts in 1,4-disubstituted benzenes,[1] pointed out that in closely related series of compounds chemical-shift changes at a probe carbon induced by remote substituents can be related to πelectron-density variations, as most unwanted effects cancel out
They stressed that the Substituent Chemical Shift (SCS = δ4-X – δ4-H) values on the carbonyl carbon of, e.g., 4-Xsubstituted benzaldehydes, acetophenones, ethyl benzoates and benzamides can be dissected into a polar and a resonance component [dual substituent parameter (DSP) model]2 according to the equation
Chemical shift variations at the methoxy carbon exhibit a good correlation with the electronic effect of 4-substituents, as well as the previously studied variations of the chemical shift of the carbonyl carbon3c and of the one-bond C(α)-C(1) coupling constant.[6]
Summary
Brownlee and coworkers, in their pioneering and extensive study on 13C NMR chemical shifts in 1,4-disubstituted benzenes,[1] pointed out that in closely related series of compounds chemical-shift changes at a probe carbon induced by remote substituents can be related to πelectron-density variations, as most unwanted effects cancel out. They stressed that the Substituent Chemical Shift (SCS = δ4-X – δ4-H) values on the carbonyl carbon of, e.g., 4-Xsubstituted benzaldehydes, acetophenones, ethyl benzoates and benzamides can be dissected into a polar and a resonance component [dual substituent parameter (DSP) model]2 according to the equation
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