Enantiodifferentiating photocyclodimerization of phenyl vinyl ethers and 4-methoxystyrene sensitized by chiral aromatic esters

Abstract

Enantiodifferentiating [2+2] photocyclodimerizations of phenyl vinyl ethers ( 1 and 2) and 4-methoxystyrene ( 3) were performed in acetonitrile at 25 °C in the presence of optically active menthyl or 8-phenylmenthyl arene(poly)carboxylate sensitizers ( 7– 13) to give the corresponding cyclodimers ( 4– 6) in low to good chemical yields. The optical yields of 4t and the specific rotations of 5t and 6t were quite low: less than 1% enantiomeric excess (ee) and less than 0.2° respectively. These photosensitizations with chiral sensitizers, although not sufficiently enantiodifferentiating, provide mechanistic insights into the intermediate radical ion pairs and indicate that the monomer and/or dimer radical cations involved in the cyclodimerization process are not located within reach of the stereochemical control of the counter radical anions of the chiral sensitizers.