A chiral bis(oxazoline) ligand embedded into polysiloxane gel: application to a reusable copper catalyst for asymmetric cyclopropanation.
A chiral bis(oxazoline) ligand embedded into polysiloxane gel: application to a reusable copper catalyst for asymmetric cyclopropanation.
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DOI：
10.1002/asia.201000527
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发表时间：
2011-01
期刊：
影响因子：
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通讯作者：
Yukihiro Motoyama;T. Nishikata;H. Nagashima
中科院分区：
文献类型：
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作者：
Yukihiro Motoyama;T. Nishikata;H. Nagashima
金属催化的不对称反应是制备光学活性分子最有效的方法之一。[1]通过合理设计手性配体和选择合适的过渡金属，可以高产率、高对映选择性地合成目标化合物。然而，考虑到催化剂的成本，手性配体的分离、回收和再利用是有待解决的问题，而从环境的观点来看，来自产物的金属物质的分离、回收和再利用是重要的。将手性过渡金属配合物固定到聚合物载体或固体表面上是解决这些问题的方法之一。[2]其中，交联聚二甲基硅氧烷（PDMS）由于其热稳定性好、有机分子在硅氧烷基质中的快速传输和催化活性物种的有效稳定等优点，被认为是物理包埋过渡金属配合物的有效载体材料。[3]然而，PDMS树脂负载的分子催化剂在有机合成中还没有得到充分的研究。一个主要的问题是将包藏的金属物质浸出到有机溶剂中;反应通常在水或醇介质中进行以避免金属浸出。我们最近建立了一种方便有效的方法，通过金属催化交联聚甲基氢硅氧烷（PMHS）来合成含有过渡金属物种[M]@Si的聚硅氧烷凝胶。[4，5]我们的新方法与制备金属包藏PDMS的常规方法相比具有几个优点。首先，通过过渡金属催化的α，ω-二烯的氢化硅烷化或α，ω-二醇的氢化硅烷化容易地实现PMHS的交联;交联伴随着催化剂物质（M1）的有效捕获。[M1]@Si，其中M1 = Pt和Ru，可用于可重复使用的催化剂，用于促进烯烃异构化[5a]和用于硝基芳烃氢化[5b]，而无需在普通有机溶剂中浸出金属。其次，各种α，ω-二烯和α，ω-二醇可用作交联剂;这使得可以引入官能团，所述官能团潜在地充当作为凝胶组分的第二过渡金属（M2）的配体。我们最近合成了在交联剂[bipy][Pt]@Si中包含2，2'-联吡啶单元的凝胶，其随后用CuCl处理以形成[CuClACHTUNGTRENNUNG（bipy）][Pt]@Si。[5c]该凝胶中的铜（I）物种催化N-烯丙基三氯乙酰胺的原子转移自由基环化反应而不浸出Cu和Pt，并且催化剂[CuClACHTUNGTRENNUNG（bipy）][Pt]@Si可重复使用。使用[CuClACHTUNGTRENNUNG（bipy）][Pt]@Si的成功促使我们研究将手性胺引入交联剂及其在不对称合成中的应用。在此，我们希望报告沿着这条线的沿着第一结果，其中将2，2 '-亚甲基双-ACHTUNGTRENNUNG（恶唑啉）单元[6]（Box;方案1）引入交联剂以形成[Box][Pt]@Si。用CuACHTUNGTRENNUNG（OTf）2处理所得凝胶[Box][Pt]@Si，得到[CuACHTUNGTRENNUNG（OTf）2ACHTUNGTRENNUNG（Box）][Pt]@Si。[CuACHTUNGTRENNUNG（OTf）2ACHTUNGTRENNUNG（Box）][Pt]@Si的效用通过Cu催化的烯烃与重氮乙酸1-薄荷酯的不对称环丙烷化来证明。[7]在氢化钠存在下，（S，S）-tBu2-Box（4，4 ′-位带有叔丁基的亚甲基双恶唑啉）与烯丙基溴反应，得到桥碳上带有二烯丙基的（S，S）-tBu2-Box配体（1：（S，S）-tBu2-DABox）。[7d制备凝胶以得到[（S，S）-tBu2-DABox][Pt.
Metal-catalyzed asymmetric reactions are one of the most efficient tools for the production of optically active molecules.[1] Rational design of chiral ligands and proper choice of transition metals enable one to produce desired molecules in high yields with high enantioselectivity. However, the separation, recovery, and reuse of chiral ligands are problems to be solved in considering the cost of the catalyst, while those of metallic species from products are important from an environmental point of view. Immobilization of chiral transition-metal complexes to polymer supports or on a solid surface is one of the solutions to these problems.[2] Among them, cross-linked polydimethylsiloxanes (PDMS) are known to behave as efficient carrier materials for physically entrapped transition-metal complexes owing to their advantages on thermal stability, rapid transportation of organic molecules through the siloxane matrix, and effective stabilization of catalitically active species.[3] However, molecular catalysts supported on the PDMS resin have not been fully investigated in organic synthesis. A major problem is leaching of the occluded metal species to organic solvents; reactions are usually carried out in aqueous or alcoholic media to avoid the metal leaching. We have recently established a convenient and effective method to synthesize polysiloxane gels containing transition-metal species,[M]@ Si, by metal-catalyzed cross-linking of polymethylhydrosiloxane (PMHS).[4, 5] Our new method has several advantages over the conventional way to prepare metal-occluding PDMS. First, cross-linking of PMHS is achieved with ease by transition-metal-catalyzed hydrosilylation of a, w-dienes or dehydrogenative silylation of a, w-diols; the cross-linking is accompanied by effective entrapment of the catalyst species (M1). The [M1]@ Si, where M1= Pt and Ru, is useful for the reusable catalysts for promoting isomerization of alkenes [5a] and for hydrogenation of nitroarenes [5b] without metal leaching in common organic solvents. Second, various a, w-dienes and a, w-diols can be used as a cross-linker; this makes it possible to introduce functional groups which potentially act as a ligand for the second transition metal (M2) as a component of the gel. We have recently synthesized a gel including a 2, 2’-bipyridine unit in the crosslinker,[bipy][Pt]@ Si, which is followed by treatment with CuCl to form [CuClACHTUNGTRENNUNG (bipy)][Pt]@ Si.[5c] The copper (I) species in the gel catalyzed the atom-transfer radical cyclization of N-allyltrichloroacetamides without leaching of both Cu and Pt, and the catalyst,[CuClACHTUNGTRENNUNG (bipy)][Pt]@ Si, was reusable. Success of using [CuClACHTUNGTRENNUNG (bipy)][Pt]@ Si prompted us to examine introduction of chiral amines to the cross-linker and its application to asymmetric synthesis. Herein, we wish to report the first results along this line, in which a 2, 2’-methylenebis-ACHTUNGTRENNUNG (oxazoline) unit [6](Box; Scheme1) is introduced to the cross-linker to form [Box][Pt]@ Si. The resulting gel,[Box][Pt]@ Si, is treated with CuACHTUNGTRENNUNG (OTf) 2 to give [CuACHTUNGTRENNUNG (OTf) 2ACHTUNGTRENNUNG (Box)][Pt]@ Si. Utility of [CuACHTUNGTRENNUNG (OTf) 2ACHTUNGTRENNUNG (Box)][Pt]@ Si is demonstrated by Cu-catalyzed asymmetric cyclopropanation of alkenes with l-menthyl diazoacetate.[7]The reaction of (S, S)-tBu2-Box, methylenebis (oxazoline) bearing tert-butyl groups at the 4, 4’-positions, with allyl bromide in the presence of sodium hydride afforded a (S, S)-tBu2-Box ligand with diallyl groups on the bridge carbon (1:(S, S)-tBu2-DABox).[7d, 8] Preparation of a gel to give [(S, S)-tBu2-DABox][Pt …