Introducing A Podand Motif to Alkyne Metathesis Catalyst Design: A Highly Active Multidentate Molybdenum(VI) Catalyst that Resists Alkyne Polymerization
Introducing A Podand Motif to Alkyne Metathesis Catalyst Design: A Highly Active Multidentate Molybdenum(VI) Catalyst that Resists Alkyne Polymerization
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DOI：
10.1002/anie.201007559
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Zhang, Wei
中科院分区：
文献类型：
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作者：
Jyothish, Kuthanapillil;Zhang, Wei
近年来，人们对过渡金属催化的烯烃和炔烃的反应越来越感兴趣然而，炔复分解的合成潜力却很少被探索，尽管它们在制备芳炔-乙炔聚合物、[2]大环、[3]和天然产物合成方面显示出巨大的潜力。[1c， 4]通常，用于炔烃复分解的金属烷基炔催化剂含有钨或钼碳三键和醇氧/酰胺配体，[1-5]并且它们的催化活性可以通过合理的配体设计来调节。[1-9]众所周知，小分子，特别是2-丁炔（一种常见的复分解副产物）与六价钼烷基炔配合物的配位是一种干扰反应，并导致不期望的炔聚合（通过环膨胀机制，这需要两个开放的底物结合位点）以及非生产性反应途径多面体低聚硅氧烷（POSS）和二氧化硅是迄今为止唯一报道的可以克服这个长期存在的问题的配体。[9a， 11]然而，基于硅氧烷的方法在催化剂结构上缺乏可调性，因此难以研究催化剂的构效关系和调节其活性。我们目前的研究旨在设计一种多齿有机配体，该配体可以阻断钼中心的一个亚策略结合位点，以抑制不期望的炔聚合，同时保持结构的可调节性，以引入可定制的吸电子取代基，以提高复分解活性和官能团耐受性。利用三取代胺有利的三角金字塔几何结构，我们设计了三酚胺配体L1（方案1），它可以使三个苯酚部分与钼有效配合，三个亚甲基单元阻断金属中心的一个底物结合位点。从相应的甲基保护水杨醛开始，然后进行还原胺化和脱保护，以较好的收率合成了多齿三酚胺配体（L1）（方案1）。用含有硝基苯和四氯化碳的溶剂体系，将钼（VI）丙炔前驱体和L1以1:1的比例混合得到配合物1的晶体单晶x射线结构分析显示，配合物1的二聚体由苯氧化合物桥接，在每个金属中心周围具有八面体配位几何结构（图1）。有趣的是，三酚胺配体的三角锥体几何结构使中心氮与钼配合，从而有效地阻断复合物的一个开放结合位点。这些有趣的特性有望使催化剂1抵抗干扰炔聚合，多齿配体的强螯合作用将显著提高催化剂的稳定性和活性。方案1。以钼（VI）前驱体[(3,5 - c6h3 (tBu) N) 3Mo (CEt)]为原料合成多齿配体L1及生成炔烃转化催化剂[L1Mo (CEt)](1)和[(L2) 3Mo (CEt)](2)。条件：a) NaBH (OAc) 3, NH4OAc， THF， RT, 69%；b) LiI，喹啉，1708C, 87%。
There has been significantly growing interest in recent years in the transition-metal-catalyzed metathesis of alkenes and alkynes.[1] The synthetic potential of alkyne metathesis, however, is much less explored though they have shown enormous potential in the preparation arylene ethynylene polymers,[2] macrocycles,[3] and in natural product synthesis.[1c, 4] Typically, the metal alkylidyne catalysts for alkyne metathesis contain a tungsten or molybdenum–carbon triple bond and alkoxide/amide ligands,[1–5] and their catalytic activity can be tuned by judicious ligand design.[1–9] Coordination of small molecules, and in particular 2-butyne (a common metathesis byproduct), to the hexavalent molybdenum alkylidyne complex is known to be an interfering reaction and leads to undesired alkyne polymerization (through the ring-expansion mechanism, which requires two open substrate-binding sites) as well as nonproductive reaction pathways.[10] Polyhedral oligomeric silsesquioxane (POSS) and silica are the only reported ligands to date that can overcome this long-standing problem.[9a, 11] However, the siloxane-based approach lacks tunability in the catalyst structure, thus making it difficult to study the structure–activity relationship of the catalyst and tune its activity. Our present study is aimed at the design of a multidentate organic ligand that can block one substratebinding site of the molybdenum center to inhibit the undesired alkyne polymerization while also keeping the structural tunability for introducing customizable electronwithdrawing substituents to improve both the metathesis activity and functional group tolerance. Taking advantage of the favorable trigonal pyramid geometry of trisubstituted amines,[12] we designed the triphenolamine ligand L1 (Scheme 1) that would allow the effective coordination of the three phenol moieties to molybdenum, with the three methylene units blocking one substrate-binding site of the metal center. The synthesis of the multidentate triphenolamine ligand (L1) was achieved in good yield starting from the corresponding methyl-protected salicylaldehyde followed by reductive amination and deprotection (Scheme 1). A crystal of the complex 1 was obtained from a 1: 1 mixture of the molybdenum (VI) propylidyne precursor and L1 using a solvent system comprising nitrobenzene and carbon tetrachloride.[13] The single-crystal X-ray structure analysis showed a phenoxide-bridged dimer of complex 1 with an octahedral coordination geometry around each metal center (Figure 1). Interestingly, the trigonal-pyramidal geometry of the triphenolamine ligand enables the coordination of the central nitrogen to molybdenum, thus efficiently blocking one open binding site of the complex. These interesting features are anticipated to make the catalyst 1 resistant to the interfering alkyne polymerization, and the strong chelating effect of the multidentate ligand should significantly enhance the catalyst stability and its activity.Scheme 1. Synthesis of the multidentate ligand L1 and the generation of the alkyne metathesis catalysts [L1Mo (CEt)](1) and [(L2) 3Mo (CEt)](2) from the molybdenum (VI) precursor [(3, 5-C6H3 (tBu) N) 3Mo (CEt)]. Conditions: a) NaBH (OAc) 3, NH4OAc, THF, RT, 69%; b) LiI, quinoline, 1708C, 87%.