Calculation of gauche coupling constants from substituent electronegativities

Abstract

Abstract-The calculation of gauche coupling constants from substituent electronegativities by a method which takes into account the orientation of the substituents is described. This correlation has been derived from the coupling constants of disubstituted ethanes and has been applied to the calculation of coupling constants in cyclohexanes and 6-membered heterocycles. The detection of angular distortion in 6-membered rings from the difference between calculated and observed coupling constants is also described.THE lH nuclear magnetic resonance parameter most frequently employed in configurational and conformational analyses is the vicinal coupling constant. A relationship between the dihedral angle and coupling constant was proposed by Karplusl and several modifications of the original equation have been employed in calculating dihedral angles from coupling constants. The electronegativity of the substituents attached to the atoms bearing the coupled protons also has a substantial effect which is angle dependent, the electronegative substituent having the maximum influence when trans to a coupled p r o t~n .~,~ Abraham and Gatti4 have determined the coupling constants for a variety of disubstituted ethanes and established a linear correlation between the electronegativities of substituents and coupling constants. It has been noted5r6 that these correlations are limited in their ability to predict gauche coupling constants because they do not take into account the orientation of the represented by the Eqns. (1) From their experimental results Abraham and Gatti were unable to measure the individual values of JHxg and JHy', and therefore could only correlate the average value with the sum of the substituent electronegativities. For substituents of substantially different electronegativities, the values of these two coupling constants are quite different2s3 and should not be equated to the average.In those ethane derivatives which have the same substituent on each carbon (Fig. 1 , X = Y) JHxg and Jayg will be the same and will be equal to the observed coupling Jgg. The magnitudes of J H H g and J H X g for several symmetrically substituted ethanes are given in Table 1. If it is assumed that the difference between which we wish to describe here.In a disubstituted ethane such as illustrated in Fig. 1, there are four different gauche constants JHX' and J E H g is due for the most part to the difference designated as J H H '~ J H X '~ JHY' and J x~g , in which the in orientation of the electronegative substituent, and that superscript refers to the gauche orientation of the coupled a linear relationship exists between coupling constants protons and the subscript refers to the atoms which are and substituent electronegativity, then the ratio of trans to the coupled protons.J& to J H H g may be expressed as shown in the Eqn. From the series of disubstituted ethanes analysed (4),* by Abraham and Gatti the linear relationships between gauche couplings and substituent electronegativities JHE' x--AEx where AE, is the elec...

A highly stereoselective intramolecular aldol condensation. Part I. Nuclear magnetic resonance spectroscopic investigation of the stereochemistry of the products derived from the reaction of 2,2′-O-methylene-bis-D-glycerose with base

“…The signal for H-5 appears as a broadened singlet, signifying a gauche relationship of H-5 and H-6 in Isomer 2. The small coupling between these two protons may be attributed to the antiperiplanar arrangement of these protons with vicinal electronegative oxygen atoms (that is, H-5 with 0-1; H-6 with 0-5) since this is known (7)(8)(9) to reduce the magnitude of vicinal coupling constants markedly. The nondeuterated derivative of Isomer 2 is, therefore, identified as being 2-C-acetoxymethyl-l,3,5-tri-0-acetyl-2,4-0-methylene-D,L-threo-pentitol (12).…”

“…As was the case with Isomer 1, the acetoxymethyl group that is tentatively assigned to the equatorial orientation bears the deuterium label; this suggests that the formyl group in the aldehyde precursor, obtained from the aldol condensation reaction, is also equatorially oriented. The minor product of the aldol condensation reaction is, therefore, tentatively identified as being 2-C-hydroxymethyl-2,4-0-methylene-D,L-xylose (8).…”

Conformations of Sugars: Proton Dihedral Angles from Gauche Coupling Constants

“…As well as the number and electronegativities of substituents, the orientation of the substituents relative to the coupled protons is extremely important, the substituent having maximum effect when antiperiplanar to a coupled proton (6,7 negativities of substituents and their orientation relative to the coupled protons (8). The method consists of the modification of a base value of the coupling constant by two correction factors, one of which is angle independent and for substituents with electronegativities greater than hydrogen, increases the size of the constant, and another which is angle dependent and decreases the size of the coupling constant, as indicated in eq.…”