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

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 1 H 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.