Supramolecular catalysis of the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate by gamma-cyclodextrin.

2-Anthracenecarboxylic acid (AC) makes a very stable 1:2 inclusion complex with gamma-cyclodextrin (gamma-CDx) (K(1) = 161 +/- 25 M(-1), K(2) = 38 500 +/- 3300 M(-1) at 25 degrees C). The formation of the 1:2 inclusion complex accelerated the photocyclodimerization of AC. The 1:2 inclusion could be clearly verified by UV-vis, CD, and (1)H NMR spectroscopies. Although these spectroscopies provide little information about the structural isomers of the inclusion complex, there should be several structural isomers of the 1:2 inclusion complex which have a different longitudinal orientation of the guest molecules in the cavity. The isomer distribution of the photodimerization product primarily depends on the population of these orientational isomers of the 1:2 inclusion complex in the ground state before photoreaction, because, in the lifetime of the excited singlet state, exchanging the orientation is impossible. The enantioselectivity of the photodimerization originates from the difference in the stability of the diastereomeric pair of orientational isomers of the 1:2 inclusion complex in the ground state, which are the precursors of the enantiomers of a specific chiral cyclodimer. The ee of a chiral cyclodimer 2 was 32% at 25 degrees C and was enhanced by lowering the temperature to 41% at 0 degrees C. This is the highest value reported for the asymmetric photodimerization in solution.