Conformational Enantiomerism in Chiral Organic Salts Containing Crystallographically Independent Anion−Cation Pairs
Conformational Enantiomerism in Chiral Organic Salts Containing Crystallographically Independent Anion−Cation Pairs
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DOI：
10.1021/ja002256a
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发表时间：
2000-11
影响因子：
15
通讯作者：
E. Cheung;T. Kang;M. Netherton;A. Scheffer;J. Trotter
中科院分区：
文献类型：
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作者：
E. Cheung;T. Kang;M. Netherton;A. Scheffer;J. Trotter
在研究结晶状态下通过光化学反应的不对称诱导过程中，我们有机会合成并获得许多有机盐的晶体结构。1在大多数情况下，它们由光学纯的铵离子和光反应性的前手性羧酸根阴离子组成。这种不对称合成方法采用离子手性助剂，并利用了这样的事实，即由于存在光学纯离子，所得到的盐需要在手性空间群中结晶。因此，在溶液中为非手性的离子在固态中采用手性构象。正是这种构象手性的组合，加上晶格的各向异性，导致在固态下光解时观察到的不对称诱导。我们经常能够获得晶体结构的盐，提供高的光产物对映体过量，并将此结构数据与基板的观察到的反应性。1对于产生低ee的盐，难以获得适合于X射线分析的晶体。尽管如此，我们已经能够获得足够的数据来确定在这些情况下操作的两个结构基序。第一种机制源于晶体中的无序，其中两种或更多种反应离子的构象随机分布在整个晶体中（静态无序）2或热相互转换（动态无序）。3，4第二个主题是这项工作的主题，涉及在不对称单元中存在等量的两个晶体学上独立的离子对。值得注意的是，两个独立的光不稳定的羧酸根阴离子被发现存在作为接近完美的构象对映异构体，其反应产生相反的产物对映异构体，从而占低ee的观察。5这种现象的一个最显著的例子涉及盐1，它可以作为两种晶体变体存在，针状或片状。6针状二晶型物含有一个独立的离子对，在光解至低转化率和重氮甲烷后处理后，产生光学纯度为97%的环丁醇光产物2（方案1）。在这个杨光环化反应中，羰基部分在构象上只与两个中的一个反应，
In the course of investigating asymmetric induction via photochemical reactions in the crystalline state, we have had occasion to synthesize and obtain crystal structures of numerous organic salts. 1 For the most part, these consist of an optically pure ammonium ion and a photoreactive, prochiral carboxylate anion. This approach to asymmetric synthesis employs ionic chiral auxiliaries, and exploits the fact that, owing to the presence of an optically pure ion, the resulting salt is required to crystallize in a chiral space group. Consequently, the ion that was achiral in solution adopts a chiral conformation in the solid state. It is the combination of this conformational chirality, coupled with the anisotropy of the crystal lattice, that gives rise to the asymmetric induction observed upon photolysis in the solid state. We have frequently been able to obtain crystal structures of salts that afford high photoproduct enantiomeric excesses, and to correlate this structural data with the substrate’s observed reactivity. 1 Crystals suitable for X-ray analysis have been difficult to obtain for salts that give rise to low ee’s. Nevertheless, we have been able to acquire enough data to identify two structural motifs operating in these instances. The first mechanism arises from disorder in the crystals in which two or more conformations of the reactive ion are distributed randomly throughout the crystal (static disorder) 2 or are thermally interconverting (dynamic disorder). 3, 4 The second motif, which is the subject of this work, involves the presence of equal amounts of two crystallographically independent ion pairs in the asymmetric unit. Remarkably, the two independent photolabile carboxylate anions are found to exist as near-perfect conformational enantiomers, reaction of which gives rise to opposite product enantiomers, thus accounting for the low ee’s observed. 5One of the most striking examples of this phenomenon involves salt 1, which can exist as two crystal modifications, needles or plates. 6 The needle dimorph contains one independent ion pair and yields cyclobutanol photoproduct 2 in 97% optical purity after photolysis to low conversion and diazomethane workup (Scheme 1). In this Yang photocyclization reaction, 7 the carbonyl moiety is conformationally poised to react with only one of two