The anomeric effect in substituted cyclohexanes. II. The role of hyperconjugative interactions and steric effect in 1,4-disubstituted cyclohexanes

The conformational equilibria of the cis/trans isomers of some 1,4-di-substituted cyclohexanes (X=OH, OMe, Me, OCOCH3, OCOC(CH3)3, OCOCCl3, OCOCF3) were calculated at several levels of theory; the best correlation between calculated and experimentally available ΔG0s refers to the MP2/6-311+G*//MP2/6-311G* results. In addition, the hyperconjugative effect of the substituents was studied with the NBO options included in the gaussian-98 package; a number of interactions between filled NBOs and antibonding orbitals could be considered as most representative for delocalization along the molecules studied. The effect of the substituents on the molecular geometry of the substituted cyclohexanes as well as the partitioning of both hyperconjugative and steric substituent effects on the present conformational equilibria is critically evaluated. Our model [E. Kleinpeter, F. Taddei, J. Mol. Struct. (THEOCHEM) 683 (2004) 29] for interpreting the relative stability of conformers of substituted cyclohexanes could be further verified and its reliability assessed.