Cyclophanes. II. Preparation and conformational properties of 5,14-bis(4-bromophenyl)diimidazolo[32]metacyclophane and a higher homolog.

Abstract

5, 14-Bis(4-bromophenyl)diimidazolo[32]metacyclophane (12) and 5, 14, 29, 38-tetrakis(4-bromophenyl)tetraimidazolo[34]metacyclophane (13) as a higher homolog were synthesized by the one-pot coupling reaction of 1, 3-bis(2-isocyano-2-tosylethyl)benzene (3b) with 1, 3-bis(4-bromophenyliminomethyl)benzene (11). At -5°C, the proton nuclear magnetic resonance (1H-NMR) spectrum of 12 strongly suggests the existence of two fixed conformers, i.e., syn and anti forms, on the NMR time scale.Moreover, on the basis of the variable-temperature (VT)-NMR spectra of 12 at various temperatures between 25°C and 100°C, the coalescence temperature (Tc) of the methylene proton signals is 80°C and the energy barrier (ΔG〓) of the conformational change is determined to be 69.3kJ/mol, which is higher than those of the parent[32]metacyclophane (4a) and dioxazolo [32]metacyclophane (6a). The conformational rigidity of 12 is attributed to the further substitutions of bulky 4-bromophenyl groups on the imidazole moieties in addition to the annelations of two imidazole rings to the two methylene bridges of the [32]metacyclophane ring.