Vicinal coupling constants electronegativity

TABLE 2.5. Vicinal Coupling Constants as a Function of Multiple Electronegative Substituents... 

With decrease in electronegativity of the heteroatom in the ring, the chemical shifts of /3-protons in the parent compounds move downfield and vicinal coupling constants - 3,4) increase. Irregularities in chemical shifts of... 

The average He e value for a freely rotating carbon-carbon bond is approximately 7-8 Hz. Vicinal coupling constants are reduced (ca. less than 1 Hz) by electronegative substituents and reduced as... 

The values of /hs, h6 and h6 observed for I, which were 5.7 and 7.5 Hz, respectively, did not appear at first glance to be consistent with the theoretical values of approximately 3 Hz and 10 Hz normally observed for gauche and antiparallel conformations of vicinal protons. However electronegative substituents modify the ma itude of vicinal coupling constants (26), In cyclic compounds, such as steroids (27) or 4-t-butylcyclohexanols (28), coupling values of 5.5 1 Hz vs. 2.5-3.2 Hz are possible for protons separated by identical dihedral angles of 60°. 

NMR spectroscopy is the most widely used method to investigate both the configuration and conformation of Knoevenagel products. The configuration of acrylic or cinnamic acids obtained by condensation of aldehydes with malonic acid and other 1,2-disubstituted alkenes can be determined from the value of the vicinal coupling constant Vh,h (between the protons of the double bond) on the basis of the relationship > Vh.h-hi. If only one isomer is available, the electronegativity of the substituents has... 

The vicinal coupling constants (/2t3 and /]<4) vary in the congener series systematically with the Pauling electronegativity of the heteroatoms and with the bond angles at C(2) and C(3) (Figure 1). 

From the data in Table 14 it further can be seen that the same is also caused by the electronegative atoms 0(2) and N(6) on the vicinal coupling constants between the bridgehead H-atoms and the corresponding H-atoms H—0(4) and H—C(8) geminal to an acetoxy group, independent of the latter substituents. 

The same relationship between torsional angle and vicinal coupling constant holds for substituted alkanes if appropriate values are used for J and These limiting values depend on the electronegativity and orientation of substituents, the hybridization of carbon atoms, bond lengths, and bond angles. 

Vicinal coupling constants ( J) decrease if an electronegative substituent is attached to the H—C—C—H moiety. The relationship between the electronegativity of a substituent X and the coupling constant between vicinal... 

In cyclic systems the effect of the electronegative substituent X on the vicinal coupling constant is dependent on the steric disposition of X with respect to the coupled vicinal protons. The maximum effect of the substituent in reducing the vicinal coupling constant is observed when the substituent and a proton on the vicinal carbon are trans-coplanar. In Figure 2.52 the substituent X is in an equatorial disposition and is not trans-coplanar with respect to any of the coupled protons. However, in Figure 2.53 the substituent X and the axial proton attached to C-2 are trans-coplanar. Thus, while the dihedral angles between the protons (connected with thick lines) are about 60° in both cases, J,e is 5.5 Hz ( 1 Hz) when the substituent X is equatorial... 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

---