Photoracemization and Deracemization of Biaryl Compounds

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In this study, we have synthesized blestriarene C (2,2',7,7'-tetrahydroxy-4,4'-dimethoxy-1,1'-biphenanthrene) and its analogs by using an ester-mediated nucleophilic aromatic substitution as a key step and investigated their photoracemization under fluorescent light exposure. Racemization of 1,1'-binaphthalenes and deracemization of blestriarene C were also examined.Blestriarene C was found to racemize by exposing its solution to the light near the ^1L_b absorption band. Removal of oxygen from the solution significantly retarded the reaction, suggesting intermediary of oxygen in the racemization. Blestriarene C analogs lacking 4,4'-dimethoxy and/or 7,7'-dihydroxy groups did not racemize under the conditions, while the 7,7'-diisopropoxy derivative, as well as the unsymmetrical biphenanthrene lacking 4'-methoxy and 7'-hydroxy groups, easily racemized. The tendency toward the racemization was found to strongly correlate with the redox potential of the compounds by cyclic voltammetry (CV) experiments. Thus, the lower redox potential the compound has, the higher racemization activity it shows. Based on these observations, we proposed a feasible reaction mechanism, which involves one-electron oxidation of blestriarene C by oxygen with the aid of the fluorescent light to form a radical cation intermediate. 1,1'-Binaphthalenes having two or three hydroxy and/or methoxy groups in total on each naphthalene half did not racemize under the same conditions. These compounds did not show reversible redox behavior on the CV spectra, which should be crucial in the photoracemization.