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

摘要

A series of primary-face-or secondary-face-modified beta-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 H-1 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 beta-CD. The combined yields of slipped cyclodimers 5 and 6 were greatly enhanced in the presence of modified beta-CDs. In contrast to native beta-CD and other beta-CD derivatives, host 9, in which a phenyltriazolyl group was introduced on the primary face of beta-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.