Enantioselective Lewis Acid Catalysis in Intramolecular [2+2] Photocycloaddition Reactions of Coumarins
Enantioselective Lewis Acid Catalysis in Intramolecular [2+2] Photocycloaddition Reactions of Coumarins
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DOI：
10.1002/anie.201003619
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Bach, Thorsten
中科院分区：
文献类型：
--
作者：
Guo, Hao;Herdtweck, Eberhardt;Bach, Thorsten
尽管近年来的研究工作不断增加，但通过光化学方法对映选择性地形成手性化合物的新方法仍然很少。激发的光底物的高能量含量导致进一步转化的低活化势垒和快速弛豫途径。因此，通过催化剂活化光激发的化合物本质上是困难的。通过使用圆偏振光来获得显著的对映选择性的尝试几乎没有成功。[1]目前，最常用的方法来实现在溶液中的催化对映选择性的光化学反应依赖于手性敏化剂，其通过电子或能量转移工作。[2，3]在此，我们提出了一个尚未探索的概念，对映体选择性光化学反应，这是基于使用手性刘易斯酸。经过一些优化，我们现在已经发现了一种手性阳离子恶唑硼烷催化剂，它能够实现4-烯基取代的香豆素的对映选择性分子内[2+ 2]光环加成[4]反应。1989年，刘易斯和Barancyk报道了2，3-二甲基-2-丁烯与香豆素的[2+ 2]光环化加成反应，[5]在没有添加剂的情况下是无效的，但BF 3·OEt 2促进了该反应。[6]当使用50mol%的刘易斯酸时，在Pyrex装置中用中压汞灯照射5小时，产物2的产率为57%。类似的反应进行了与其他烯烃和光化学和光物理数据的基础上，增强的反应性的络合比未络合的香豆素是由于其增加的单重态寿命和亲电性。刘易斯酸在[2+ 2]光环加成反应中的催化活性的观察激发了使用手性刘易斯酸实现对映选择性过程的想法。为了寻找最佳的刘易斯酸催化剂，[7]在2，3-二甲基-2-丁烯的存在下，在CH 2Cl 2中用不同的刘易斯酸在λ= 366 nm下进行香豆素的辐照（方案1）。在没有刘易斯酸的情况下，5小时后没有观察到反应，而50摩尔%的
Despite increasing efforts in recent years, new methods for the enantioselective formation of chiral compounds by photochemical methods remain rare. The high energy content of an excited photosubstrate results in low activation barriers for further transformations and in rapid relaxation pathways. It is therefore intrinsically difficult to activate a photoexcited compound by a catalyst. Attempts to attain significant enantioselectivities by the use of circularly polarized light have seen little success.[1] Currently, the most frequently used method to achieve catalytic enantioselective photochemical reactions in solution relies on chiral sensitizers, which work by electron or energy transfer.[2, 3] Herein we present an as yet unexplored concept for enantioselective photochemical reactions, which is based on the use of chiral Lewis acids. After some optimization we have now found a chiral cationic oxazaborolidine catalyst, which enables enantioselective intramolecular [2+ 2] photocycloaddition [4] reactions of 4-alkenyl-substituted coumarins. Our preliminary results are disclosed herein.In 1989, Lewis and Barancyk reported that the [2+ 2] photocycloaddition of 2, 3-dimethyl-2-butene to coumarin (1),[5] which is inefficient in the absence of an additive, was promoted by BF3· OEt2.[6] By employing 50mol% of the Lewis acid, product 2 was obtained in 57% yield upon irradiation in a Pyrex apparatus with a mercury mediumpressure lamp for five hours. Similar reactions were conducted with other alkenes and—based on photochemical and photophysical data—the enhanced reactivity of complexed over uncomplexed coumarin was attributed to its increased singlet-state lifetime and electrophilicity. The observation of catalytic Lewis acid activity in the [2+ 2] photocycloaddition stimulated the idea to use chiral Lewis acids to achieve an enantioselective process. In search for an optimum Lewis acid catalyst,[7] the irradiation of coumarin in the presence of 2, 3-dimethyl-2-butene was performed in CH2Cl2 with different Lewis acids at λ= 366 nm (Scheme 1). Without a Lewis acid no reaction was observed after five hours, while 50 mol% of