Theoretical Determination of Molecular Structure and Conformation. XI. The Puckering of Oxolanes

Structural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3-dioxolane (3), 1,2-dioxolane (4), 1,2,4-trioxolane (5), 1,2,3-trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6-31G* basis set at the Hartree-Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentials V of compounds 1–8 are evaluated as a function of the puckering amplitude q and the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O-O bonds but decrease with the number of ether bridges. Puckered C₂-symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for the C₂-symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers < RT) or large amplitudes of pseudolibration. These results are rationalized in terms of the rotor potentials of appropriate reference compounds (C₂H₆, CH₃OH, H₂O₂). A more elegant approach leads to a simple π electron count and an analysis of bonding and antibonding overlap in the π-type HOMO's. In this way the effects of substituents can be predicted and conformational preferences of the furanose ring in nucleotides, nucleosides and carbohydrates explained. The relative stability of the oxolanes is analyzed by calculating O-O bond energies, bond-bond interactions and ring strain for each compound. The lability of the higher oxolanes is traced back to increased ring strain. A new method of the conformational analysis of ring compounds is outlined.