Synthesis, structure (NMR and mass spectrometry) and conformational analysis of heterocyclic analogues of dibenzo[a,e]cycloocta-1,5-diene: 5,6,12,13-tetrahydrobispyrazolo[1,2-a:1′,2′-e][1,2,5,6]tetraazocinediium dihalides

Abstract

Several 5,6,12,13-tetrahydrobispyrazolo[1,2-a:1′,2′-e][1,2,5,6] tetraazocinediium dihalides 4a–d and 8 are prepared from pyrazole, 3,5-dimethylpyrazole, 4-(1-adamantyl)pyrazole and campho[2,3-c]pyrazole by stepwise alkylation with 1,2-dibromoethane or 1,2-dichloroethane. Their structural characterization has been achieved by NMR and mass spectrometry. Dynamic NMR spectroscopy allowed the measurement of the barrier for the chair–chair interconversion in the case of the parent compound 4a and the 1,3,8,10-tetramethyl derivative 4b. These barriers as well as the preferred chair conformation are rationalized through semi-empirical and molecular mechanics calculations with regard to dibenzo[a,e]cycloocta-1,5-diene. The study of doubly charged bispyrazolium salts allows demonstration of their reduction by addition of a hydride ion [C⁺⁺+ H^–→(C + H)⁺] during FABMS experiments.