Conformational behaviors of 1,7-dioxa-spiro[5,5]undecane and its dithia and diselena analogs in relation to the anomeric effect: A hybrid-DFT, ab initio MO study and NBO interpretation

NBO analysis, hybrid density functional theory (B3LYP/6-311+G**//B3LYP/6-311+G**) and ab initio molecular orbital (MP2/6-311+G**//B3LYP/6-311+G**) based methods were used to study the impacts of the anomeric effects (AE) associated with electron delocalization, total dipole differences and steric repulsion effects on the conformational properties of 1,7-dioxa-spiro[5,5]undecane (1), 1,7-dithia-spiro[5,5]undecane (2) and 1,7-diselena-spiro[5,5]undecane (3). Both methods showed the greater stability of conformations A (in which two heteroatoms having each an electron pair oriented antiperiplanar to the carbon–heteroatom bond) compared to their corresponding conformations B (with only one electron pair oriented antiperiplanar to the carbon–heteroatom bond) and C (without electron pair oriented antiperiplanar to the carbon–heteroatom bond). B3LYP/6-311+G** method showed that total Gibbs free energy difference (GC − GA and GB − GA) values (i.e. ΔGC–A and ΔGB–A) between conformations A, B and C decrease from compound 1 to compound 3. The NBO analysis of donor–acceptor (LP→σ*) showed that the AE is in favor of conformations A. The AEC–B, AEB–A and AEC–A values calculated (i.e. AEC–AEB, AEB–AEA, AEC–AEA) decrease from compound 1 to compound 3. The calculated total dipole moment values decrease from conformation A to conformation C. The calculated total dipole moment difference (μC − μB, μB − μA, μC − μA) values between conformations C, B and A increase from conformations C–B to C–A (i.e. ΔμC–B < ΔμB–A < ΔμC–A). However, the variations of the calculated ΔμC–B, ΔμB–A and ΔμC–A values are not in the same trend observed for the corresponding ΔG values. Therefore, the total dipole moment differences do not seem to be sufficient to account for conformation A preferences in compounds 1–3. Although total steric exchange energy (TSEE) values in conformations A of compounds 1–3 are smaller than those of their corresponding conformations B and C, the calculated Δ(TSEE) values between conformations A, B and C cannot explain the variations of the total energy differences (e.g., ΔGC–A and ΔGB–A) from compound 1 to compound 3. These findings led to the proposal that the AE, due to LP ax M 1 → σ C 2 – M 7 ∗ hyperconjugation effect, is a reasonable descriptor of the total energy differences between the various conformations of compounds 1–3 compared to the total dipole moment differences and steric effects.