Induction of Point Chirality by E/Z Photoisomerization
Induction of Point Chirality by E/Z Photoisomerization
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DOI：
10.1002/anie.201104614
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Tamaoki, Nobuyuki
中科院分区：
文献类型：
--
作者：
Hashim, P. K.;Tamaoki, Nobuyuki
点手性起源于有四个不同的取代基键合到一个中心原子，使非平面分子具有不可重叠的镜像。[1]在分子系统中点手性的诱导与自然界中同手性的起源有关，如在氨基酸中所见。[2-4]我们的兴趣是提出一个新的概念，引入点手性，并了解如何在中心碳原子上的取代基的微小差异可以导致可检测的结构中的不对称性。以前的工作表明，取代基的微小差异，如吡啶基的位置异构性[5]或不同同位素的存在[6]，可以导致分子中可检测到的不对称性。然而，化学键需要被打破，使即使是这样小的差异，在取代基，这一观察启发我们设想的分子系统中，不对称性可以引入光异构化的取代基没有任何键cracking. In本文中，一类新的偶氮苯为基础的前手性分子和开/关开关的点手性报道。我们设计了一种分子，该分子由具有两个可光异构化的偶氮苯部分的碳原子以及甲基和苯基组成（方案1）。由偶氮苯部分之一的E/Z光异构化引起的构象差异被成功地用于在分子中产生点手性。据我们所知，这是诱导点手性的第一个例子，其中sp3碳原子周围的两个取代基是几何异构体。1在乙酸乙酯中的溶液表现出偶氮苯衍生物的典型吸收光谱;该光谱在327 nm处具有强的π-π* 跃迁带，在400- 500 nm处具有弱而宽的n-π* 跃迁带。在366 nm处照射溶液导致π-π* 跃迁带的强度逐渐降低，由于偶氮苯部分的光化学E/Z异构化，n-π* 跃迁带的强度显著增加。在436 nm处照射时观察到相反的光谱变化。在366和436 nm光下处于光稳态（PSS）的溶液显示约15%和80%，
Point chirality originates from having four different substituents bonded to a central atom to give nonplanar molecules with nonsuperimposable mirror images.[1] The induction of point chirality in molecular systems is of particular interest in connection with the origin of homochirality in nature, as seen in amino acids.[2–4] Our interest is to propose a new concept for the introduction of point chirality and also to understand how small differences in the substituents on the central carbon atom can cause detectable asymmetry in the structure. Previous work has shown that a small difference in the substituents such as position isomerism of pyridyl groups [5] or the presence of different isotopes [6] can cause detectable asymmetry in a molecule. However, chemical bonds need to be broken and made to make even such small differences in the substituents; this observation inspired us to envisage a molecular system in which the asymmetry can be introduced by photoisomerization of the substituents without any bond cleavage.Herein, a new class of azobenzene-based prochiral molecules and the on/off switching of point chirality is reported. We designed a molecule consisting of a carbon atom having two photoisomerizable azobenzene moieties and a methyl and a benzene group (Scheme 1). The conformational difference caused by the E/Z photoisomerization of one of the azobenzene moieties was successfully utilized for the generation of point chirality in the molecule. To the best of our knowledge this is the first example of the induction of point chirality in which two of the substituents around an sp3 carbon atom are geometric isomers. A solution of 1 in ethyl acetate exhibits a typical absorption spectrum for azobenzene derivatives; this spectrum features an intense π–π* transition band at 327 nm and a weak and broad n–π* transition band at 400–500nm. Irradiation of the solution at 366nm caused a gradual decrease in the intensity of the π–π* transition band, with a notable increase in the n–π* transition band owing to photochemical E/Z isomerization of the azobenzene moieties. The reverse spectral changes were observed upon irradiation at 436 nm. The solutions at the photostationary state (PSS) under 366 and 436nm light showed about 15 and 80%,