γ-Cyclodextrin modulates the chemical reactivity by multiple complexation

Abstract

Multiple complexation by γ-CD has been studied by self-diffusion coefficients (DOSY) and chemical kinetics experiments in which 4-methoxybenzenesulfonyl chloride (MBSC) solvolysis was used as a chemical probe. The addition of a surfactant as a third component to the reaction mixture induced a very complex reactivity pattern that was explained on the basis of multiple complexation phenomena and surfactant self-assembly to form micelles. A cooperative effect that yielded a ternary complex formed by cyclodextrin–surfactant–MBSC was observed. The larger cavity of γ-CD in comparison with β-CD is responsible for the change from the competitive complexation mechanism predominant with β-CD to a cooperative/competitive mixed mechanism operating for the larger derivative. The cavity size in γ-CD is large enough to bind two surfactant alkyl chains with a cooperative effect. Water molecules released by the formation of 1 : 1 host–guest complexes made the cavity more hydrophobic and promoted further inclusion. A reduction in the available volume of the cavity should be considered on binding a second guest.