Geometries, torsional barriers, and dipole moments have been computed for mono-, di-, tri-, and tetra-substituted five-membered cyclic carbonates and for some substituted cyclopentanones, oxolanes, 4-butyrolactones, and 1,3-dioxolanes. With respect to cyclic carbonates it was deduced that the potential curve increases moderately with T3456 (torsion angle) and for most compounds a 1 kcal difference between 4T5 or 5T4 conformers and planar form corresponds to a torsional angle of ± 14-16°. The minimum is, in general, flattened and close to planar. Additional modes of ring puckering such as 5E, E 5, 4E, E4 and 2E, E2 were also examined. Between the enantiomeric structures with opposite distortions, the quasi equatorial conformation of the alkyl group is preferred. endo and exo anomeric effects in carbonate systems have been studied by introducing strong electron-withdrawing substituents, in the carbonate 4-position. The exo anomeric effect was observed with many of the polar substituents, but the role of the anomeric effect in ring puckering (endo effect) was found to be small. For other five-membered ring systems MNDO predicts that the energy barrier to interconversion of planar ↔ twist is in the following order, carbonates > 1,3-dioxolanes > butyrolactones > oxolanes > cyclopentanones. N.m.r data are also included as well as the single-crystal X-ray diffraction study of 4-chlorophenyloxymethyl-1,3-dioxolan-2-one.
|Number of pages
|Journal of the Chemical Society, Perkin Transactions 2
|Published - 1989