Complexing properties of two benzocrown-ether moieties arranged at a cyclobutane ring system

Biscrown ethers and arranged at a cyclobutane ring were prepared by intermolecular [2+2] photocycloaddition of vinylated benzocrown ethers. The complexing behavior of toward alkali metal cations was evaluated by ESI-MS analysis, liquid–liquid extraction, and the comparison of complexing stability constant. An intramolecular sandwich-type 1:1 (host/guest) complexation was observed by ESI-MS analysis in the competitive system where -Na+, -K+, and -Cs+ were formed selectively. In the liquid–liquid extraction, however, hardly extracted any cation, while both and efficiently extracted larger cations such as K+, Rb+, and Cs+. It was found that the complexing stability constant of -Na+ is lower than that of benzo-15-crown-5-Na+ though extraordinarily high values were obtained for -K+ and -Cs+ complexes compared with those of 18-crown-6-K+ and dibenzo-24-crown-8-Cs+ complexes, respectively. Hence, the excellent complexing ability was achieved by using the cyclobutane ring, which strongly preorganized two benzocrown-ether moieties for the larger alkali metal cations.