Guest inding drives reversible atropisomerism in cavitand hosts

A novel class of cavitand bowl structures carrying rim substituents were developed that can reversibly switch from inwardly- to outwardly-directed orientations. It was found that a preference for either form can be established through purely physical means resulting in measuring relative solvent guest binding affinities without recourse to the hydrophobic effect. It was also found that the complementary fit of the ethyl and n-propyl substituents in the cavitand bowl not only affects the carrier towards atropisomerism, but also sufficiently strong to override the usual thermodynamic preference for the outside form in toluene. This unique system offered a very convenient method for the measurement of relative solvent-guest binding strengths of organic solvents in an operationally straightforward manner, that is, by simply measuring atropisomer ratios, and without recourse to the hydrophobic effect.