"Cone" - "Partial-cone" Isomerism in Tetramethoxycalix[4]arenes. Influence of Solvent Polarity and Metal Ions

It was found that the "cone" - "partial-cone" equilibrium in 25,26,27,28-tetramethoxycalix[4]arene derivatives is sensitively affected by the solvent polarity: the cone% increases with the increase in the solvent polarity. The novel finding was rationalized in terms of the difference in the dipole moment: cone conformers with four dipole moments orientating into the same direction are more polar than partial-cone conformers with one inverted phenyl unit and therefore are more stabilized in polar solvents. Alkali metal cations such as Li⁺ and Na⁺ strongly interact with four methoxy oxygens, resulting in the shift of the equilibrium to cone.