INTRAMOLECULAR [2 + 2] PHOTOCYCLOADDITION .10. CONFORMATIONALLY STABLE SYN-[2.2]METACYCLOPHANES

Three isomerized 1,2:9,10-diethano-syn-[2.2]metacyclophanes 5, 6, and 7 were prepared from cis-1,2-bis(m-vinylphenyl)cyclobutane by photocycloaddition. Their yields were 10, 5, and 3%, respectively. H-1 NMR chemical shifts of aromatic protons for cyclophane 5 clearly suggest the syn conformation. It has a rigid structure, according to its VT NMR spectra. The structure was thoroughly analyzed by X-ray crystallography. It became clear that the aromatic nuclei of this syn conformer are not arranged parallel but tilted by ca. 31-34-degrees toward each other. Since the MM2 calculation for parent syn-[2.2]metacyclophane (1), as well as compounds 5, 6, and 7, gives the tilted structure, the cyclobutane ring systems are not responsible for tilting. Therefore, these cyclobutane-fused compounds are concluded to be good models for syn-[2.2]-metacyclophane (1), whose structure has not been fully explored in detail yet. Cyclophanes 6 and 7 are also concluded to have syn conformation. Their structures were determined by NMR experiments and UV spectroscopy. Their conformations are stable in a solid state, yet in a solution (in CDCl3, benzene, etc.) they slowly interconverted to each other. The initial rates were 0.29 X 10(-6) and 0.44 X 10(-6) s-1 at 20-degrees-C for 6 and 7, respectively. At 20-degrees-C, the isomer ratio of 6 to 7 was 60:40 at equilibrium. The conformational change should occur through an unstable, probably short-lived anti conformer that could not have been detected yet. The mechanisms for the interconversion and also for the photocycloaddition are proposed.