Enantiodifferentiating photocyclodimerization of cyclohexa-1,3-diene sensitized by chiral arenecarboxylates

The enantiodifferentiating photosensitized cyclodimerization of cyclohexa-1,3-diene 1 was performed over a range of temperatures in the presence of chiral arene(poly)carboxylates, giving endo- and exo-[4 + 2] cyclodimers (2a, 2b) and anti- and syn-[2 + 2] cyclodimers (3a, 3b). Among the three chiral cyclodimers (2a, 2b, 3a), only 2b was obtained as an optically active species with an enantiomeric excess (ee) of up to 8.2%. The detailed reaction mechanism and the origin of the enantiodifferentiation have been elucidated, and the crucial role played by the 'microenvironmental polarity' around the chromophore in determining the photoreactivity and the ee of the product is also discussed.