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The dependence of the 1,2-disubstituted propanes X-C1H12-C2H2(Y)-C3H33 methyl protons (C3H33) chemical shifts on the nature of substituents X in the 1H NMR spectra. An anomeric effect action?
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1  Ukrainian Academy of Printing


In our previous communication (see ECSOC-18) we have observed an interesting phenomenon. In 1H NMR spectra of 1,2-disubstituted 1-arylpropanes of general formula Ar-C1H1(X)-C2H2(Y)-C3H33 the presence of such electron withdrawing substituents X as X = -OR, and X = -NR2 at the carbon atom C1 leads to upfield shift of the methyl protons signals in the fragment -C3H33, compared to the same signal in unsubstituted analog Ar-C1H12-C2H2(Y)-C3H33 (where X = H).

In order to establish the causes of this phenomenon, we have undertaken the study 1H NMR spectra of the simpler objects – the series of 1-Х,2-Y-disubstituted propanes of general formula Х-C1H12-C2H2(Y)-C3H33. This class of compounds, not contain the 1-aryl moiety Ar in the unaltered propyl skeleton of the molecule (C1-C2-C3). This should simplify the explanation of the observed phenomenon. For this research we selected a fairly limited number of compounds with substituents Х and Y, the spectra of which we consider to be the most reliable. We considered 12 series of compounds, wherein the row number is determined by substituent Y. Each of these twelve series includes from 3 to 16 compounds with different substituents Х.

All substituents Х and Y are divided into four groups depending on the nature of the atom in the substituent that is bound to a propane skeleton of molecule. The hydrogen atom is not included in any group, and is the "arbitration" substituent. In the first two groups such atom is a carbon atom; in a first group this carbon atom included in the saturated alkyl moiety (Alk); in the second group it is bonded with a double bond with one of the remaining atoms of the substituent. In the third and fourth groups such atom is heteroatom. In the third group - this is the atom of the second period - nitrogen or oxygen; in a fourth group – it is an atom of third (or fourth) periods: sulfur, chlorine, bromine, iodine.

The spectral parameters δНСН3 , ie, the chemical shift of three methyl protons, for 97 substances (of 192 possible) of all 16 types of combinations of the substituents Х and Y are studied in details. In 15 of 16 cases we observed the predicted changes of parameter δНСН3, depending on the nature of the substituents Х with any type of substituent Y. For example, the substituents Х of the first group (alkyl fragments Alk) cause upfield shift of parameters δНСН3 in the compounds Alk-C1H12-C2H2(Y)-C3H33 for all four types of substituents Y, compared with the arbitration substituent - a hydrogen atom in Alk-C1H12-C2H22-C3H33. On the contrary, the substituents Х of the fourth group (heteroatoms of third or fourth periods, for example in Hal-C1H12-C2H2(Y)-C3H33) cause downfield shift of δНСН3.

The only exception is the case when both substituents Х and Y are the substituents of the third group (the heteroatoms of the second period - atoms N and O). In this case, instead of the expected downfield shift of δНСН3 (such as in the case of substituents Y in the fourth group - S, Cl, Br, I) occurs upfield shift of the parameter δНСН3 in all eight studied examples, the same as abovementioned for Ar-C1H1(X)-C2H2(Y)-C3H33.

To explain this phenomenon, we hypothesized the certain action of the anomeric effect, apparently occurring also for parameters of 1H NMR spectra.

Keywords: 1,2-disubstituted propanes, chemical shift of methyl protons δНСН3, substituents Х and Y, four groups of substituents.