The ability to photoinduce enantiomeric excess from the chirality of circularly polarized light (CPL) is pertinent to the study of the origin of homochirality in biomolecules. Such CPL-induced reactions, including both chirality generation and formation of partial enantiomeric imbalance, from nonchiral starting compounds have been known, however, only for the conversion of diarylolefins into chiral helicenes. In this study we synthesized three different prochiral molecules, each featuring a pair of photoisomerizable phenylazo moieties arranged symmetrically upon the phenyl rings of an sp3-hybridized carbon atom (>1), the phenyl rings of [2.2]paracyclophane (>2), and the ortho positions of a phenyl ring bearing a naphthyl unit (>3), and then investigated the possibility of photoinducing enantiomeric excess under CPL. Irradiation of >1–3 with light induced E ↔ Z photoisomerizations of their azobenzene moieties, giving mixtures of their EE, EZ, and ZZ isomers in the photostationary state (PSS). Among these regioisomers, the EZ forms are chiral and existed as racemic mixtures of R and S stereoisomers. Upon CPL irradiation of >3, circular dichroism (CD) revealed enantiomeric enrichment of one of the EZ stereoisomers; furthermore, irradiation with r- or l-CPL gave CD signals opposite in sign, but with equal intensity, in the PSS. In contrast, >1 and >2 did not give any detectable induced CD upon CPL irradiation. These experimental results can be explained by considering the different Kuhn anisotropy factors (g) of the (R)-EZ and (S)-EZ stereoisomers of >1–3, assuming that the origin of the enantiomeric excess is the enantio-differentiating photoisomerization from EZ stereoisomers to nonchiral EE or ZZ regioisomers by r- or l-CPL. In short, we demonstrate the simultaneous induction of chirality and enantiomeric excess from a prochiral azobenzene dimer via a chiral regioisomer formed in situ upon CPL irradiation.
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机译：从圆偏振光（CPL）的手性中光诱导对映体过量的能力与生物分子中手性性起源的研究有关。然而，由非手性起始化合物引起的这种CPL诱导的反应，包括手性的产生和部分对映异构体不平衡的形成，都是已知的，但是仅用于将二芳基烯烃转化为手性螺旋烯。在这项研究中，我们合成了三个不同的前手性分子，每个分子均具有一对对称分布在sp 3 杂化碳原子（> 1 ）的苯环上的可光异构化的苯基偶氮部分， [2.2]对环环烷（> 2 ）的苯环和带有萘基单元（> 3 ）的苯环的邻位，然后研究了光诱导对映体的可能性CPL下的超额费用。用光诱导> 1-3 对其偶氮苯部分进行E↔Z光异构化，得到其EE，EZ和ZZ异构体的混合物为光平稳态（PSS）。在这些区域异构体中，EZ形式是手性的，以R和S立体异构体的外消旋混合物形式存在。 CPL照射> 3 后，圆二色性（CD）显示EZ立体异构体之一的对映体富集；此外，用r-或l-CPL照射在PSS中产生符号相反但强度相同的CD信号。相比之下，> 1 和> 2 在CPL照射后未提供任何可检测到的诱导CD。这些实验结果可以通过考虑> 1-3 的（R）-EZ和（S）- EZ 立体异构体的不同库恩各向异性因子（g）来解释对映体过量的起源是从 EZ 立体异构体到非手性 EE 或 ZZ 区域异构体的对映体分化型光异构。通过 r -或 l -CPL。简而言之，我们证明了通过CPL辐射原位形成的手性区域异构体同时诱导前手性偶氮苯二聚物手性和对映体过量。
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