Enantiodifferentiating photocyclodimerization of cyclohexene sensitized by chiral benzenecarboxylates

The photosensitized cyclodimerization of (Z)-cyclohexene (1Z) was performed over a range of temperatures in the presence of chiral benzene(poly)carboxylate sensitizers, giving trans-anti-trans-, cis-trans- and cis-anti-cis[2 + 2]- cyclodimers 2-4. Of the two chiral cyclodimers (2, 3), only 2 was obtained optically active with enantiomeric excesses as high as 68.3% at -78°C, whereas 3 was consistently racemic under various reaction conditions employed. The detailed reaction mechanism and the origin of enantiodifferentiation have been elucidated and involve the initial enantiodifferentiating photoisomerization of 1Z to the highly reactive (E)-isomer (1E) and the subsequent stereospecific concerted cyclodimerization with 1Z giving optically active 2 which is competing with the non-stereospecific stepwise cyclodimerization to racemic 2 and 3.