Synthesis and complexing property of four-bridged crownopaddlanes

A new series of crown compounds crownopaddlanes 3a-c bearing three cyclobutane rings were prepared by means of intramolecular 2+2] photocycloaddition of styrene derivatives. The yield of crownopaddlane 3b possessing five ethereal oxygen atoms was remarkably high 52% with the addition of sodium borofluoride in the photoreaction system. As this template effect suggests, 3b showed extraordinarily high Na⁺-selectivity with high efficiency on the liquid-liquid extraction of alkali metal picrates, though 3a having four ethereal oxygen atoms did not extracted any cations in this system. The high Na⁺-selectivity of 3b was further clarified by the equilibrium stability constants (log K_a) for Na⁺ (5.85) and K⁺ (2.91) in acetonitrile solution. The complexation of 3b to Na⁺ cation was also examined by X-ray crystallography. Crownopaddlane 3c bearing six ethereal oxygen atoms also efficiently and selectively extracted alkali metal cations, compared with conventional 18-crown-6 derivatives.