Microenvironmental control of enantiodifferentiating photocyclization of 5-hydroxy-1,1-diphenylpentene through...

For mechanistic elucidation of the photosensitized cyclization of 5-hydroxy-1,1-diphenylpentene (1), its methyl ether (4) was synthesized as an unreactive "dummy" substrate and used as a quencher of the sensitizer fluorescence to reveal the intervention of an exciplex intermediate that was unable to detect when reactive substrate 1 was used as a quencher/reactant In the enantiodifferentiating photocyclization of 1 to 2-(diphenylmethyl)tetrahydrofuran (2) sensitized by a chiral saccharide ester of 1,4-naphthalenedicarboxylate (3), the enantiomeric excess (ee) of chiral product 2 obtained in methylcyclohexane (MCH) at 25 °C was significantly enhanced from 20% to 35% upon 10-fold dilution of the sample solution by MCH, for which the reduced solvent polarity, discouraging dissociation of the intervening radical ionic exciplex, is likely to be responsible. Further attempts to microenvironmentally control the photochirogenic reaction and enhance the product's ee through selective solvation of polar cosolvent to the diastereomeric exciplex pair in nonpolar solvent were not successful probably due to the inherently high local polarity around the exciplex of saccharide-appended 3 with alcoholic substrate 1.