Conformational Properties of (Z,Z)-, (E,Z)-, and (E,E)-Cycloocta-1,4-dienes

Abstract

Ab initio calculations at the HF/6-31G* level of theory for geometry optimization and the MP2/6-31G*//HF/6-31G* level for a single point total energy calculation are reported for (Z,Z)-, (E,Z)-, and (E,E)-cycloocta-1,4-dienes. The C 2-symmetric twist-boat conformation of (Z,Z)-cycloocta-1,4-diene was calculated to be by 3.6 kJ·mol−1 more stable than the C S-symmetric boat-chair form; the calculated energy barrier for ring inversion of the twist-boat conformation via the C S-symmetric boat-boat geometry is 19.1 kJ·mol−1. Interconversion between twist-boat and boat-chair conformations takes place via a half-chair (C 1) transition state which is 43.5 kJ·mol−1 above the twist-boat form. The unsymmetrical twist-boat-chair conformation of (E,Z)-cycloocta-1,4-diene was calculated to be by 18.7 kJ·mol−1 more stable than the unsymmetrical boat-chair form. The calculated energy barrier for the interconversion of twist-boat-chair and boat-chair is 69.5 kJ·mol−1, whereas the barrier for swiveling of the trans-double bond through the bridge is 172.6 kJ·mol−1. The C S symmetric crown conformation of the parallel family of (E,E)-cycloocta-1,4-diene was calculated to be by 16.5 kJ·mol−1 more stable than the C S-symmetric boat-chair form. Interconversion of crown and boat-chair takes place via a chair (C S) transition state which is 37.2 kJ·mol−1 above the crown conformation. The axial- symmetrical twist geometry of the crossed family of (E,E)-cycloocta-1,4-diene is 5.9 kJ·mol−1 less stable than the crown conformation.