A value

Abstract: The anomeric effect (the tendency of heteroatomic substituents adjacent to a heteroatom within the cyclohexane ring to prefer the axial orientation instead of the sterically less hindered equatorial position) is traditionally explained through either the dipole moment repulsion or the hyperconjugation effect. In this work, by employing our recent work in density functional steric analysis, we provide a novel two-component explanation, which is consistent with the common belief in chemistry that the effect has a stereoelectronic origin. With α-D-glucopyranose as the prototype, we systematically explore its conformational space and generate 32 isomers, leading to a total of 80 axial-equatorial conformation pairs. The energy difference analysis of these pairs shows that while statistically speaking the tendency is valid, the anomeric effect is not always true and can be violated. Three energy components, exchange-correlation, classical electrostatic, and density functional steric, are found to be directly proportional to the total energy difference between axial and equatorial isomers. We also found that the total dipole moment change, not the hyperconjugation effect, is a reasonable indicator of the total energy difference. However, all these correlations alone are not strong enough to provide a compellingly convincing explanation for the general validity of the effect. With the help of strong correlations between energy components, an explanation with two energy components, steric and electrostatic, was proposed in this work. We show that the axial-equatorial energy difference in general, with the anomeric effect as a special case, is dictated by two factors of the stereoelectronic origin, steric hindrance and classical electrostatic interactions, synchronously working together. Another explanation in terms of exchange-correlation and electrostatic interactions has also been obtained in this work.

Abstract: A quantitative analysis of the steric effect of aliphatic groups was carried out from first principles. An intuitive framework is proposed that allows the separation and straightforward interpretation of two contributors to the steric effect: steric strain and steric shielding (hindrance). When a sterically demanding group is introduced near a reactive center, deformation of its reactive zone will occur. By quantifying this deformation, a convincing correlation was established with Taft's steric parameters for groups of typical size, supporting the intuitive image of steric shielding; bulky groups slow down the reaction by limiting the accessibility of the reactive centre. On the other hand, the strong initial repulsion between the reactant and the substrate by means of the filled-filled orbital interaction results in the deformation of the substrate as well as a less stabilizing reaction zone, which are the manifestations of the steric strain. We thus conclude that both steric strain and steric hindrance can be derived from the Pauli repulsion evolving between the reactants in the course of the reaction.

Abstract: An NMR study of the diaxial/diequatorial chair equilibrium in a range of silylated derivatives of trans-1,4- and trans-1,2-dihydroxycyclohexane is reported and discussed with a view to explaining unusually large populations of chair conformations with axial substituents, noted previously for some monosilyloxycyclohexanes and in some silylated sugars. X-ray diffraction studies of three bis-triphenylsilyloxycyclohexanes are reported and show both axial and equatorial silyloxy groups with the exocyclic bonds eclipsed. Eclipsing is also suggested by molecular mechanics (MM3) calculations on such derivatives. Both axial and equatorial tertiary silyl groups have 1,3-repulsive interactions with whatever substituents or hydrogen atoms are at the two adjacent equatorial positions, and these are relieved by rotation toward the eclipsed conformation of the exocyclic C−O bond. The three substituents on silicon interact attractively with the nine atoms at the 3, 4, and 5-positions of the cyclohexane ring and calculati...