Microenvironmental polarity control of electron-transfer photochirogenesis. Enantiodifferentiating polar addition of 1,1-diphenyl-1-alkenes photosensitized by saccharide naphthalenecarboxylates.

Abstract

Enantiodifferentiating polar photoaddition of alcohol to 1,1-diphenylpropene and 1,1-diphenyl-1-butene sensitized by saccharide naphthalene(di)carboxylates was performed in nonpolar to polar solvents containing methanol, ethanol, or 2-propanol as the nucleophile to give the corresponding anti-Markovnikov alcohol adduct, that is, 1,1-diphenyl-2-alkoxy-propane and -butane in low-to-good chemical yields, depending on the sensitizer, chiral auxiliary, alcohol, solvent, and temperature employed. The excited state and intermediate involved, the reaction and enantiodifferentiation mechanism operating, and the factors controlling chemical and optical yields were elucidated from the photochemical and stereochemical outcomes under various conditions and also from the sensitizer and exciplex fluorescence quenching experiments and the molecular orbital calculations. A new strategy was developed to overcome the normally accepted tradeoff between the chemical and optical yields. This is made possible by employing protected saccharides as chiral auxiliaries and running the photoreactions not in a nonpolar but in a low-polarity solvent such as diethyl ether, which jointly enhance the "microenvironmental" polarity around the sensitizer to facilitate electron transfer, keeping the intimate interactions between the chiral sensitizer and substrate within the exciplex intermediate. By optimizing these factors, we obtained the photoadduct in enantiomeric excesses of up to 58%, which is the highest ever reported for a photosensitized bimolecular enantiodifferentiating reaction.