Visible-Light-Promoted Stereoselective Alkylation by Combining Heterogeneous Photocatalysis with Organocatalysis
Visible-Light-Promoted Stereoselective Alkylation by Combining Heterogeneous Photocatalysis with Organocatalysis
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DOI：
10.1002/anie.201108721
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Koenig, Burkhard
中科院分区：
文献类型：
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作者：
Cherevatskaya, Maria;Neumann, Matthias;Koenig, Burkhard
应用敏化剂以利用可见光进行化学反应是一种既定的方法。[1]几个最近的出版物[2]令人印象深刻地证明了可见光在各种转化中的多用途，例如醇转化为烷基卤化物，[3]和[2+ 2]，[4][3+ 2]，[5]和[4+ 2][6]环加成以及碳-碳[7]和碳-杂原子键的形成。[8]有机催化与使用钌或铱配合物[9]或有机染料[9d]作为光催化剂的可见光光氧化还原催化的组合允许扩展到对映选择性反应。[10]虽然无机半导体，如二氧化钛，已被广泛用于有机废物的光催化降解[11]，但它们在有机合成中光催化键形成的例子仍然有限。[12]Kisch及其同事[13]探索了CdS介导的键形成，并且已知二氧化钛的氧化CCl 3C偶联反应[14]。然而，非均相光催化剂上的键形成通常在不控制立体化学的情况下进行，并且获得异构体的混合物。[15我们在此证明了立体选择性有机催化与可见光多相光氧化还原催化的组合以良好的选择性和产率促进碳-碳键的立体选择性形成。该方法结合了非均相催化的优点（稳定，简单，易于分离的催化剂材料）与均相有机催化中实现的立体选择性。[17，18] MacMillan等人开发的醛的对映选择性α-烷基化。[9a]作为试验反应，应用无机非均相光催化剂（表1）。使用了五种半导体：市售白色TiO 2（1），[19]用Phos-Texas Red染料共价表面改性的相同材料，其增加可见光的吸收（Phos-Texas-Red-TiO 2，2），黄色PbBiO 2 Br，其吸收蓝光，和PbBiO 2 Br作为本体材料（3）和纳米晶形式（4）。平均粒径为21 nm的TiO 2（1）是一种稳定且廉价的半导体，带隙为3.2 eV，但由于缺陷和表面沉积物，未改性的粉末仅在高达405 nm处有微弱的吸收。[20]其吸收范围可以通过结构修饰[21]或用染料进行表面修饰而扩展到可见光范围。[22德克萨斯红衍生染料10 [24]（方案1）共价锚定在TiO 2上，产生2，其在560 nm处吸收（参见10的合成和2的表征的支持信息）。PbBiO 2Br 3和4通过不同的合成路线制备，导致半导体的不同颗粒尺寸：采用高温固相合成法制备了带隙为2.47 eV的PbBiO 2Br块体材料3，[25]而纳米晶体材料4是由在水溶液中合成获得的，导致平均计算粒度为（28 μ 6）。nm和2.56 eV的光学带隙。黄色CdS（5）具有2.4eV的带隙，并且如先前报道的那样制备。[26日]
The application of sensitizers to utilize visible light for chemical reactions is an established method.[1] Several recent publications [2] have impressively demonstrated the versatile use of visible light for various transformations, such as the conversion of alcohols to alkyl halides,[3] and [2+ 2],[4][3+ 2],[5] and [4+ 2][6] cycloadditions as well as carbon–carbon [7] and carbon–heteroatom bond formations.[8] The combination of organocatalysis with visible-light photoredox catalysis using ruthenium or iridium complexes [9] or organic dyes [9d] as photocatalysts allows for an expansion to enantioselective reactions.[10] Although inorganic semiconductors, such as titanium dioxide, have been widely used for the photocatalytic degradation of organic waste,[11] the number of examples in which they photocatalyze bond formation in organic synthesis is still limited.[12] Kisch and co-workers [13] explored CdS-mediated bond formations, and oxidative CÀC coupling reactions with titanium dioxide [14] are known. However, bond formations on heterogeneous photocatalysts typically proceed without control of the stereochemistry and mixtures of isomers are obtained.[15, 16] We demonstrate herein that the combination of stereoselective organocatalysis with visible-light heterogeneous photoredox catalysis promotes the stereoselective formation of carbon–carbon bonds in good selectivity and yield. The approach combines the advantages of heterogeneous catalysis (robust, simple, and easy-toseparate catalyst material) with the stereoselectivity achieved in homogeneous organocatalysis.[17, 18] The enantioselective a-alkylation of aldehydes developed by MacMillan et al.[9a] was selected as a test reaction to apply inorganic heterogeneous photocatalysts (Table 1). Five semiconductors were used: commercially available white TiO2 (1),[19] the same material surface-modified covalently with a Phos-Texas Red dye increasing the absorption of visible light (Phos-Texas-Red-TiO2, 2), yellow PbBiO2Br, which absorbs blue light, and PbBiO2Br as bulk material (3) and in nanocrystalline form (4). TiO2 (1) with an average particle size of 21 nm is a stable and inexpensive semiconductor with a band gap of 3.2 eV, but the unmodified powder absorbs only weakly up to 405 nm as a result of to defects and surface deposits.[20] Its absorption range can be extended into the visible range by structure modification [21] or surface modification with dyes.[22, 23] The Texas Red derived dye 10 [24](Scheme1) was covalently anchored on TiO2 yielding 2, which absorbs at 560 nm (see the Supporting Information for the synthesis of 10 and the characterization of 2). PbBiO2Br 3 and 4 were prepared by different synthetic routes leading to different particle sizes of the semiconductors: PbBiO2Br bulk material 3 with a band gap of 2.47 eV was prepared by hightemperature solid-phase synthesis,[25] while the nanocrystalline material 4 was obtained from synthesis in aqueous solution leading to an average calculated particle size of (28 Æ 6) nm and an optical band gap of 2.56 eV. Yellow CdS (5) has a band gap of 2.4 eV and was prepared as previously reported.[26]