Enantiodifferentiating photocyclodimerization of cyclohexene sensitized bychiral benzenecarboxylates

The photosensitized cyclodimerization of (Z)-cyclohexene (1Z) was performed over a range of temperatures in the presence of chiral benzene(poly)carbox ylate 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 obtai ned optically active with enantiomeric excesses as high as 68.3% at -78 deg rees C, whereas 3 was consistently racemic under various reaction condition s employed. The detailed reaction mechanism and the origin of enantiodiffer entiation have been elucidated and involve the initial enantiodifferentiati ng photoisomerization of 1Z to the highly reactive (E)-isomer (1E) and the subsequent stereospecific concerted cyclodimerization with 1Z giving optica lly active 2 which is competing with the non-stereospecific stepwise cyclod imerization to racemic 2 and 3.