Photoracemization and Deracemization of Biaryl Compounds

本论文以酯基芳香亲核取代反应为关键步骤，合成了2，2 ′，7，7 ′-四羟基-4，4 ′-二甲氧基-1，1 ′-联菲（blestriarene C）及其类似物，并研究了它们在荧光灯下的光外消旋反应。研究了1，1 '-联萘的外消旋作用和Blestriarene C的去外消旋作用，发现Blestriarene C在^1L_b吸收带附近的光照射下发生外消旋反应。从溶液中除去氧显著地延缓了反应，表明氧在外消旋化中起中介作用。缺少4，4 '-二甲氧基和/或7，7'-二羟基的Blestriarene C类似物在该条件下不外消旋，而7，7 '-二异丙氧基衍生物以及缺少4'-甲氧基和7 '-羟基的不对称联菲容易外消旋。通过循环伏安法（CV）实验发现，外消旋化的趋势与化合物的氧化还原电位密切相关。因此，化合物具有的氧化还原电位越低，其显示的外消旋活性越高。基于这些观察结果，我们提出了一个可行的反应机理，它涉及单电子氧化的blestriarene C的氧气与荧光的帮助下，形成一个自由基阳离子中间体。在相同条件下，在每个萘半部上总共具有两个或三个羟基和/或甲氧基的1，1 ′-联萘不发生外消旋化。这些化合物在CV光谱上没有表现出可逆的氧化还原行为，这在光外消旋中应该是至关重要的。

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.