A generalized exo‐anomeric effect. Substituent and solvent effects on the conformational equilibria of 2‐(arylseleno)cyclohexanones
Authors: Mcleod Ronald G. , Johnston Blair D. , Pinto B. Mario 
Published: April 2004

AbstractThe effects of substitution and solvent on the conformational equilibria of 2‐[(4‐R‐substituted‐phenyl)seleno]cyclohexanones are described. The conformational equilibria were determined by comparison of the linewidths of the H‐2 resonances in the 1H NMR spectra of the conformationally averaged systems with those of the anancomeric (highly biased) 4‐isopropyl‐2‐substituted cyclohexanones. The substituent (R = NMe2, OMe, Me, H, F, Cl, CF3, NO2) and solvent ((CD3)2CO, CD3CN, CD2Cl2, CDCl3) effects are discussed in terms of electrostatic effects and the possible stabilizing orbital interactions. The values of Keq (axial‐equatorial) increase as the substituent becomes more electron withdrawing, in agreement with the dominance of nSe → π*C=O or σC‐Se → π*C=O orbital interactions in the axial conformers. The increase in the proportion of the equatorial isomers in more polar solvents for a given substituent suggests a damping of the dipolar interactions in the equatorial isomers. However, the proportion of the equatorial isomers in a given solvent increases as the substituent becomes more electron withdrawing, indicating that electrostatic interactions do not dominate in controlling the conformational equilibria. Analysis of the equilibrium data by means of a dual substituent parameter approach indicates the best correlation with σI and σ+R substituent constants in CD2Cl2 and with σI and σ°R substituent constants in CD3CN, with similar sensitivities to the resonance and polar effects. The correlations are interpreted in terms of accommodation of effective positive charge on the selenium atom in the axial isomers in CD2Cl2, and a lesser sensitivity to the buildup of positive charge in the more polar solvent CD3CN. Comparison of the IR μCO‐stretching frequencies for the axial and equatorial ArSe‐substituted anancomeric systems (R = NO2, NMe2) indicates a higher stretching frequency for the NO2‐substituted isomers. In the case of the NMe2‐substituted compounds, μCO appears at a higher frequency in the equatorial isomer, whereas in the case of the NO2‐substituted compounds, μCO is less sensitive to the axial or equatorial orientation of the substituent. The results are consistent with the operation of nse → π*c=0 or σC‐Se → π*C=O orbital interactions in the axial isomers. The JC2‐H2 values in the axially‐substituted anancomeric isomers are of greater magnitude than those in the equatorially‐substituted isomers, which is also consistent with the operation of the orbital interactions described above. There is, however, no marked substituent effect on the JC2–H2 values within the series of axial or equatorial isomers. We argue that this does not support the dominance of σC‐Se → π*C=O orbital interactions. Examination of crystal structures reported in the literature for related compounds indicates a particular gauche orientation about the C2–Se bond, which lends further support to the operation of an nSe → π*C=O orbital interaction. We suggest that the latter interaction is a manifestation of a generalized exo‐anomeric effect.