Synthesis and complexing property of four-bridged crownopaddlanes

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