A generalized exo‐anomeric effect. Substituent and solvent effects on the conformational equilibria of 2‐(arylseleno)cyclohexanones

Abstract

The 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 1 H NMR spectra of the conformationally averaged systems with those of the anancomeric (highly biased) 4-isopropyl-2-substituted cyclohexanones. The substituent (R = NMe 2 , OMe, Me, H, F, Cl, CF 3 , NO 2 ) and solvent ((CD 3 ) 2 CO, CD 3 CN, CD 2 Cl 2 , CDCl 3 ) effects are discussed in terms of electrostatic effects and the possible stabilizing orbital interactions. The values of K eq (axial-equatorial) increase as the substituent becomes more electron withdrawing, in agreement with the dominance of n Se →π* C=O or σ C-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 σ I and σ + R substituent constants in CD 2 Cl 2 and with σ I and σ o R substituent constants in CD 3 CN, 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 CD 2 Cl 2 , and a lesser sensitivity to the buildup of positive charge in the more polar solvent CD 3 CN. Comparison of the IR ν CO -stretching frequencies for the axial and equatorial ArSe-substituted anancomeric systems (R = NO 2 , NMe 2 ) indicates a higher stretching frequency for the NO 2 -substituted isomers. In the case of the NMe 2 -substituted compounds, ν CO appears at a higher frequency in the equatorial isomer, whereas in the case of the NO 2 -substituted compounds, ν CO is less sensitive to the axial or equatorial orientation of the substituent. The results are consistent with the operation of n Se →π* C=O or σ C-Se →π* C=O orbital interactions in the axial isomers. The J C2-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 J C2-H2 values within the series of axial or equatorial isomers. We argue that this does not support the dominance of σ C-Se →π* C=O orbital interactions. Examination of crystal structures reported in the literature for related compounds indicates a particular gauche orientation about the C 2 -Se bond, which lends further support to the operation of an n Se →π* C=O orbital interaction. We suggest that the latter interacti...

Orbital Interactions in Selenomethyl-Substituted Pyridinium Ions and Carbenium Ions with Higher Electron Demand

“…The preference for the gauche conformation in 11a may have its origins in intramolecular π−π interactions; however, this is not the case for the remainder of the structures. We considered that this conformation may be favored for one or both of the following reasons (Figure ): (i) it allows for the p-type lone pair on the selenium to interact with the CH 2 −Ar antibonding orbital (analogous to the anomeric effect) and/or (ii) it allows for a through-space interaction between the selenium p-type lone-pair and the low-lying π-system of the positively charged pyridinium ringa similar type of through space orbital interaction (n Se −π* CO ) has been used to explain the conformational preferences of α-phenylselenylcyclohexanones. , It is compelling to propose the presence of the n Se −σ* CC as this may help to explain the unexpected effects on the Se−CH 2 coupling constants (Table ) which increased upon methylation of the pyridine precursors ( 12a − c and 13a − c ), this interaction that would strengthen the Se−CH 2 bond may offset the expected bond weakening effects of σ C−Se −π* hyperconjugation.…”

Modes of Neighbouring Group Participation by the Methyl Selenyl Substituent in β-Methylselenylmethyl-substituted 1-Phenylethyl Carbenium Ions

“…The orbital interactions include σ Se-C -π* hyperconjugation (Figure 3A), an anomeric effect (Figure 3B) and a through-space interaction between the selenium p-type lone pair orbital and the electron deficient pyridinium ion π system (Figure 3C) which represents the early stages of the bridging interaction as represented by structure 3 above. The latter two interactions explain the preferred gauche dihedral angle about the Se-C bond in these structures, a conformation, which is also preferred in α-phenylselenyl ketones, where similar orbital interactions are plausible [12,13].…”