Enhanced irregular photodimers and switched enantioselectivity by solvent and temperature in the photocyclodimerization of 2-anthracenecarboxylate with modified β-cyclodextrins

Abstract

A series of primary-face- or secondary-face-modified β-cyclodextrins (CDs) 7-11 were prepared for mediating the enantiodifferentiating [4 + 4] photocyclodimerization reactions of 2-anthracenecarboxylic acid (AC). The complexation behavior of these hosts with AC was studied by 1H NMR, UV–vis, circular dichroism spectroscopy and ITC titration. The product distribution and product’s ee turned out to be a critical function of the skeleton and periphery modifications of β-CD. The combined yields of slipped cyclodimers 5 and 6 were greatly enhanced in the presence of modified β-CDs. In contrast to native β-CD and other β-CD derivatives, host 9, in which a phenyltriazolyl group was introduced on the primary face of β-CD, afforded antipodal cyclodimer 6 in a much higher enantioselectivity of 46% ee. The influence of temperature and salt additives were investigated in depth. Intriguingly, product chirality was switched by temperature in the presence of 6 M CsCl. Under an optimized condition, host 8 gave slipped cyclodimer 6 in 70% ee, which is the highest enantioselectivity ever reported for the slipped cyclodimer.