Enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate catalyzed by 6 A,6 X-diamino-6 A,6 X-dideoxy-γ-cyclodextrins: Manipulation of product chirality by electrostatic interaction, temperature and solvent in supramolecular photochirogenesis

Abstract

6 A,6 X-Dideoxy-6 A,6 X-diamino-γ-cyclodextrins (X = B, C, D and E) 5a– d were synthesized as chiral hosts for catalyzing the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (ACA). The electrostatic interaction between 5a– d and ACA efficiently affected the preorganization of two ACA molecules within the γ-CD cavity, and improved the yields of head-to-head cyclodimers. By lowering the reaction temperature or solvent polarity, the electrostatic interaction was further enhanced. The anti-to-syn ratio of the head-to-head isomers gradually increased by changing the host from 5a to 5d (with increasing distance between the two amino groups on the CD rim), demonstrating a good structure–function relationship in this supramolecular photoreaction system. The chiral sense and enantiomeric excess of the photoproducts obtained are significantly affected, and even inverted, by solvent composition and reaction temperature. This temperature- and solvent-controlled chirality switching behavior is proven to originate from the contribution of non-zero differential entropy (ΔΔ S ‡) in the enantiodifferentiating process. This finding is the first example of a chirality inversion driven by entropy-related factors, such as solvent and temperature, in a non-sensitized asymmetric photoreaction. The sign of ΔΔ S ‡ was switched by the composition of solvent and exhibited an excellent compensatory relationship against the differential enthalpy (ΔΔ H ‡), revealing that the photoenantiodifferentiation is governed not only by enthalpy but also by entropy, and also that the enantiodifferentiation mechanism does not vary throughout the whole system, irrespective of the condition changes.