Self-assembly via hydrogen bonding of bis(18-crown-6)-1,3-distyrylbenzene and [2 + 2] photocycloaddition: Stereoselective synthesis of [2.2]metacyclophanes and butterfly-type thermal isomerization

The photochemistry of bis(18-crown-6)-containing 1,3-distyrylbenzene and its 2 : 2 bis-pseudo-sandwich complex with ethane-1,2-diammonium was studied. The UV irradiation of the supramolecular complex resulted in the formation of three geometric isomers of tetracrown-containing dicyclobutano[2.2]metacyclopane, differing in the orientation of the cyclobutane moieties, with the unsymmetrical isomer (>50 %) predominating. The diammonium cations used as the template could be removed after photolysis by simple extraction, and the product was obtained in high yield (82 %). The ¹H NMR signals of a mixture of geometric isomers of [2.2]metacyclophane derivative were assigned resorting to quantum chemistry methods (DFT). The kinetic and thermodynamic parameters of thermal isomerization between the endo,endo- and exo,exo-isomers were measured. Tetracrown-containing dicyclobutano[2.2]metacyclophanes are homotetratopic ligands that contain four binding sites for metal and ammonium cations, which is of interest for the design of photo- and thermo-switchable supramolecular devices with a variable complex formation behavior and supramolecular machines owing to the butterfly-type thermal isomerization.