Photosensitized enantiodifferentiating cis-trans isomerizations of 1,2-diphenylcyclopropane (1), 1,2bis(4-methoxyphenyl)cyclopropane (2), and 2,3-diphenyloxirane (3) were performed in the presence of optically active alkyl arenecarboxylates as electron-accepting photosensitizers. Several photosensitizers possessing different chromophores and chiral groups were examined in polar and nonpolar solvents, and optical purities (op) up to 10% for It and 1% for 2t and 3t were obtained. The product's op varied drastically with solvent polarity and, in an extreme case, typical polar and nonpolar solvents gave antipodal products. Mechanistic investigations revealed that the enantiodifferentiation occurs in the quenching of the excited chiral sensitizer by racemic substrates and that full equilibrium is not established between the enantiomeric ambident intermediates, probably owing to their short lifetimes.Considerable effort has been devoted to the study of enantiodifferentiating photosensitized isomerizations1-10 ever since the first such isomerization, that of trons-1,2diphenylcyclopropane (1), was reported by Hammond and Cole.3 4® They employed (+)-(A)-l-(l-(JV-acetylamino)-
Enantiodifferentiating cis-trans photoisomerization of 1,2-diphenylcyclopropane (1) in the presence of (−)-tetramenthyl 1,2,4,5-benzenetetracarboxylate as a chiral sensitizer gave (+)-(1S,2S)-1t in enantiomeric excesses (e.e.) of 10.4 and 2.8% in pentane and acetonitrile, respectively, while the photosensitization with the corresponding (−)-tetrabornyl ester afforded (+)-(1S,2S)-1t (3.8% e.e.) in pentane and, notably, the antipode (−)-(1R,2R)-1t (2.4% e.e.) in acetonitrile.
Enantiodifferentiating cis-trans Photoisomerization of 1,2-Diarylcyclopropanes and 2,3-Diphenyloxirane Sensitized by Chiral Aromatic Esters. -Symmetrically cis-1,2-disubstituted 3-membered ring compounds such as the 1,2-diphenylcyclopropane (I) undergo photoisomerization in the presence of a chiral sentisizer to yield the trans-isomers (+)-(II) and (-)-(II) with up to 10% optical purity. The effect strongly depends on the sentisizer and the solvent employed. -(INOUE, Y.; YAMASAKI, N.; SHIMOYAMA, H.; TAI, A.; J.
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