Remote polar substituent effects on ¹³C NMR spectra of olefinic and aromatic carbons

Abstract

Chemical shifts of the olefinic carbons of o-substituted 1-pentenes and 1-butenes and the aromatic carbons of phenylethanes and phenylpropanes have been measured. Correlations with various substituent constants, electronegativities, NMR data from related chemical systems and calculated electrical fields were evaluated. Although the pattern of results is quite consistent with expectations for a field-induced shift of =-electron density, simple correlations of high statistical precision are not found. The general problem of correlating substituent effects with reactivity and NMR shifts is discussed.In the course of other studies, we observed that the chemical shifts of the olefinic carbons of 5-bromo-lpentene differ from those of its Grignard reagent by several parts per million. Since the position of substitution is 8 and E to the affected carbons, the substituent effects appeared to be rather larger than expected. We therefore studied a number of related compounds. We have concluded that the magnitude of the substituent effects is indeed 'normal', resulting from .rr-electron polarization by the polar substituent groups, and does not represent a specific interaction unique to these particular substituents. A number of other recent studies of NMR substituent effects also support the concept of bond electron polarization by polar substituents.'-'6 In this paper we report the chemical shifts of the olefinic carbons of a number of w-substituted 1-butenes and 1-pentenes (1, n = 2, 3), the aromatic carbons of the phenyl analogs (2, n = 2, 3) and olefinic carbons of some related cyclic compounds, and our attempts to correlate these shifts with measures of polar substituent effects. RESULTSIn Tables 1 and 2, chemical shifts are reported for series 1 and 2, respectively. Spectra were run mostly at concentrations of 10-15% by volume in diethyl ether. Shifts for some compounds were concentrationdependent at higher concentrations, but little change appeared to result from further dilution. Omitted from correlations because of poor fit to plot of 6(C-1) vs 6(C-2).

Sulphur-33 nuclear magnetic resonance spectroscopy of sulphones

“…Such Harnrnett-type correlations have been reported on for carbon-13 nuclei, in general concentrating on the study of substituted aromatic compounds (20,21). Relatively few studies on aliphatic compounds appear to have been made (22); these have focused on the Taft polar substituent constant, a * (23).…”

“…The correlations between 6-S and 6-C or 6 -0 are superior to the correlations between 6-S or 6-C and a * , but this is perhaps to be expected since in the former case correlations are being made between similar parameters whereas in the latter the correlation is between a chemical shift and a parameter derived from acid equilibrium constants. As has been mentioned previously (21), it may be unwise to attempt correlations between chemical shifts and reactivity of molecules. However, there does seem to be a better correlation between 6-S and a * compared to 6-C vs. a * , which may be indicative of the more polar nature of the sulphonyl group.…”

Tethered 1,4-Benzoquinones and Their DCNQI Derivatives: Syntheses, Electronic Interactions, Redox Properties, Charge-Transfer Complexes, and Copper Salts

“…The sensitivity of aromatic 13 C-NMR shifts towards the yielded the pentamethylene and hexamethylene tethers in chain lengths of alkyl substituents is well docu-14 and 15, respectively, via the diketones 12 and 13 after mented, [15] [16] together with their correlation with calcucatalytic reduction (Scheme 2). lated charge densities.…”

“…Ϫ [a] Assignments interchange-30 min, the mixture was extracted with DCM and the organic able. Ϫ C 17 H 16 [a] , 136.75 (C-6) [a] , 147.46 (C-2), 187.06 184.60 (s, C-1). Ϫ C 15 H 10 Nr 2 O 4 (414.0): calcd.…”

Interpretation of substituent‐induced chemical shifts in 13C NMR spectra of 2‐substituted norbornadienes. Influence of homoconjugation

“…The chemical shift difference between C-6 and C-5, 6c-6 -Bc-, = SCS,-,, may be considered to be a semiquantitative measure of the transmission of the substituent effect across the NBD skeleton. 34 One may assume that the parameter SCS, -, reflects more or less quantitatively the polarization of the n-electrons of the C-5-C-6 double bond. Then we can see that this estimator is strongly correlated with the n-electronic effect at the C-2-C-3 double bond as represented by SCS,-,.…”