The stereochemistry of the stable conformational diastereomers in substituted dihydrodibenzo[ef,kl]heptalenes, the doubly bridged biphenyls. Synthesis, structural elucidation and barrier to conformational diastereomerism

Abstract

The doubly bridged substituted biphenyls are prepared and separated into the diastereomers by normal chromatography. While in 5,11-dichloro-4,10-dimethoxy-4,10-dihydrodibenzo[ef,kl]heptalene, 1A, all the three possible diastereomers, i.e.endo–endo; endo–exo; and exo–exo, are formed; in 5,11-dichloro-4,12-dimethoxy-4,12-dihydrodibenzo[ef,kl] heptalene, 2A, only exo–exo and endo–exo diastereomers could be detected. The diethoxy substituted compounds behave like the methoxy ones, but for the propoxy group, two diastereomers for 1C and two diastereomers for 2C are observed. The endo–endo diastereomer in 1A could be converted to the exo–exo by a mechanism involving rotation around the pivot bond and a double ring inversion but the reverse process is not observed. The barrier to 1A_endo–endo → 1A_exo–exo conversion is found to be ΔG^#₄₀₃ = 30 ± 0.3 kcal mol⁻¹ with ΔH^# of 24.4 ± 0.2 kcal mol⁻¹ and ΔS^# of −14 ± 4 e.u. The structures of the 1A_endo–exo and 1A_exo–exo isomers and the structure of 2A_endo–exo were determined by single crystal X-ray crystallography. Allylic coupling constants are derived for all compounds and compared to the calculated ones. All derivatives having an ethoxy or propoxy group show the CH₂ protons attached to oxygen, diastereotopics.