Catalytic Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylic Acid Mediated by a Non-Sensitizing Chiral Metallosupramolecular Host
Catalytic Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylic Acid Mediated by a Non-Sensitizing Chiral Metallosupramolecular Host
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DOI：
10.1002/anie.200902911
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Inoue, Yoshihisa
中科院分区：
文献类型：
--
作者：
Ke, Chenfeng;Yang, Cheng;Inoue, Yoshihisa
与基态不对称合成的最新进展形成鲜明对比的是，其在光化学中的对应物，或“光手性”，仍然是化学家的挑战，主要是因为激发态中存在短暂的弱相互作用。[1]传统不对称合成的成功很大程度上归功于手性过渡金属催化剂的使用。[2]一个类似的方法似乎适用于光化学不对称合成，但手性金属配合物很少被用于手性光化学，除了少数尝试。[3]这可能是由于配合物中金属与配体或底物之间发生了光诱导电子转移，导致受激底物猝灭或手性配体解离所致。实现催化光手性的唯一方法是使用手性敏化系统。[4，5]因此，开发一种新的非敏化，但催化，手性化合物的光化学途径应大大扩大光手性的范围。在此，我们报道了第一个由金属超分子主体介导的催化对映体分化光反应。该体系不仅使我们能够严格控制容纳在手性主体中的底物的取向和对映体选择性，而且使我们能够用催化量的主体加速光反应，从而为实现光催化手性合成提供了一种简便的策略，而不需要使用传统的手性光敏化技术。2-蒽甲酸（AC）的对映体分化超分子[4+ 4]光环二聚反应已知由g-环糊精（CD）衍生物介导的手性顺式-头-尾（syn-HT）二聚体2和反式-头-尾（anti-HH）二聚体3以良好的对映体过量（ee）产生。[6]因此，由本机G-CD介导的光环化二聚反应得到2在51%ee，而同样的反应介导的二甲氨基乙氨基-G-CD提供3在41%ee。尽管在这些超分子光致手性反应中获得良好的ee值，过量的手性主机必须使用，以达到最高的ee值，通过最大限度地减少人口和光反应的游离AC在散装溶液中。本论文以6-O-对甲苯磺酰化的g-CD为手性主体，与相应的胺反应，合成了一系列g-CD衍生物5-8。[7]引入到g-CD上的游离和金属配位的二氨基侧链预期通过静电相互作用或连接到二价金属阳离子来增强AC的络合并控制其在CD腔中的取向和对映选择性。因此，这种络合应加速随后的光环化二聚的AC容纳在空腔中，最终实现催化的光手性与提高的化学和光学产率。AC（0.2 mm）在208 ℃下溶于甲醇和缓冲水溶液（pH 5）的1：1混合物中，但在不存在主体的情况下在较低温度下形成聚集体，这通过AC吸收（0-0带）从386 nm向429 nm的红移显示（支持信息，图S1）。然而，在g-CD宿主的存在下，即使在508 ℃也没有AC聚集体形成。的
In remarkable contrast to the recent progress in asymmetric syntheses in the ground state, its counterpart in photochemistry, or “photochirogenesis”, is still a challenge for chemists, mostly because of the short-lived weak interactions available in the excited state.[1] The success in conventional asymmetric synthesis owes largely to the use of chiral transition metal catalysts.[2] A similar approach seems to be applicable to the photochemical asymmetric synthesis, but chiral metal complexes have rarely been employed in chiral photochemistry, with the exception of a few attempts.[3] The lack of success is probably due to the photoinduced electron transfer occurring between metal and ligand or substrate in the complex, resulting in the quenching of excited substrate or dissociation of chiral ligand. The only method to achieve catalytic photochirogenesis is the use of a chiral sensitizing system.[4, 5] Therefore, the development of a novel nonsensitizing, yet catalytic, photochemical route to chiral compounds should greatly expand the range of photochirogenesis. Herein, we report the first catalytic enantiodifferentiating photoreaction mediated by a metallosupramolecular host. This system enables us not only to critically control the orientation and enantioface selectivity of substrate accommodated in a chiral host, but also to accelerate the photoreaction with a catalytic amount of host, thus providing a convenient strategy to achieve the catalytic photochirogenesis without using the conventional chiral photosensitization.Enantiodifferentiating supramolecular [4+ 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC)(Scheme 1) mediated by g-cyclodextrin (CD) derivatives is known to give chiral syn-head-to-tail (syn-HT) dimer 2 and anti-head-tohead (anti-HH) dimer 3 in good enantiomeric excess (ee).[6] Thus, the photocyclodimerization mediated by native g-CD gives 2 in 51% ee, whereas the same reaction mediated by dimethylaminoethylamino-g-CD affords 3 in 41% ee. In spite of the good ee values obtained in these supramolecular photochirogenic reactions, an excess amount of chiral host has to be used to attain the highest ee values by minimizing the population and photoreaction of free AC in the bulk solution. In the present study, we synthesized a series of g-CD derivatives 5–8 as chiral hosts by the reactions of 6-O-tosylated g-CD with the corresponding amines.[7] The free and metal-coordinated diamino side chains introduced onto g-CD are expected to enhance the complexation of AC and control its orientation and enantioface selectivity in the CD cavity through electrostatic interaction or ligation to a divalent metal cation. Therefore, this complexation should accelerate the subsequent photocyclodimerization of ACs accommodated in the cavity, eventually achieving the catalytic photochirogenesis with enhanced chemical and optical yields. AC (0.2 mm) is soluble in a 1: 1 mixture of methanol and an aqueous buffer solution (pH5) at 208C, but forms aggregates at lower temperatures in the absence of host, which was revealed by a bathochromic shift of the AC absorption (0-0 band) from 386 nm to 429 nm (Supporting Information, Figure S1). However, in the presence of g-CD hosts, no AC aggregate was formed even at À508C. The