Unusual in situ ligand modification to generate a catalyst for room temperature aromatic C-O bond formation
Unusual in situ ligand modification to generate a catalyst for room temperature aromatic C-O bond formation
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DOI：
10.1021/ja002543e
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发表时间：
2000-11-01
影响因子：
15
通讯作者：
Hartwig, J
中科院分区：
文献类型：
--
作者：
Shelby, Q;Kataoka, N;Hartwig, J
催化剂的寿命通常由其分解途径控制，例如配体降解。一般来说，人们试图识别这些分解途径，然后防止它们。我们报告了一个不寻常的例子，相反的scanario：一个令人惊讶的原位结构变化，转换膦连接，过渡金属络合物显示低催化活性到另一个系统表现出高活性。鉴定的改性催化剂和独立的合成，它导致了一系列的室温偶联的芳基溴与酚盐，醇盐，和carnide，包括环化反应，形成含氧杂环。我们的研究结果强调，许多因素的基础表观催化剂的结构反应性的关系，包括潜在的形成意想不到的复合物显示高活性。温和的芳族碳-氧键的形成是一种困难的转化。对于未活化的芳基卤化物的反应，直接的、未催化的取代和铜介导的偶联通常需要100 ℃或更高的温度。1-3重氮化和用氧亲核试剂置换通常限于苯酚合成，并使用化学计量量的最温和形式的铜。[4]我们最近报道了第一批用于由未活化的芳基卤化物形成二芳基和烷基芳基醚的钯催化剂。[5]然而，我们的系统以及Buchwald最近报道的系统所需的温度与铜介导的过程相似。1-3，7，8我们认为深入了解催化剂结构如何影响反应性可以改善这一过程。两个结果表明，我们最近研究的系统比预期的更复杂，它将作为一个说明性的例子，如何在原位结构的变化可以影响催化剂的活性。首先，由Pd（dba）2和PFc（t-Bu）2生成的络合物催化的芳基卤化物与苯酚钠的反应以高产率发生，但从假定的芳基苯酚钯中间体的化学计量还原消除以低得多的产率发生（方案1）。第二，仔细监测催化反应（图1）表明，芳基卤最初被消耗而没有形成二芳基醚，并且该产物形成的时间依赖性是S形的。这些结果表明存在诱导期。测试了几种机制假设来解释这些观察结果中的一个或两个。首先，我们考虑了分离的二聚体的裂解是否比直接分解形成二芳基醚以外的产物慢，这是一个可以解释产率差异的假设。于是，我们进行了
The lifetime of a catalyst is generally controlled by its decomposition pathways, such as ligand degradation. Generally, one seeks to identify these decomposition pathways and to then prevent them. We report an unusual example of the opposite scanario: a surprising in situ structural change that transforms a phosphine-ligated, transition-metal complex displaying low catalytic activity into another system exhibiting high activity. Identification of the modified catalyst and independent synthesis of it has led to a series of room-temperature couplings of aryl bromides with phenoxides, alkoxides, and siloxides, including cyclizations to form oxygenated heterocycles. Our results emphasize that many factors underlie apparent catalyst structurereactivity relationships, including the potential to form unexpected complexes displaying high activity. Mild, aromatic carbon-oxygen bond formation is a difficult transformation. For reactions of unactivated aryl halides, direct, uncatalyzed substitutions and copper-mediated couplings typically require temperatures of 100 C or greater. 1-3 Diazotization and displacement with oxygen nucleophiles is generally limited to phenol synthesis and uses stoichiometric amounts of copper in its mildest form. 4 We recently reported the first palladium catalysts for the formation of diaryl and alkyl aryl ethers from unactivated aryl halides. 5 Yet, our system, as well as that reported recently by Buchwald, 6 required temperatures similar to those for coppermediated processes. 1-3, 7, 8We felt that a deep-seated understanding of how catalyst structure affects reactivity could improve this process. Two results suggested that the system we recently studied was more complex than expected and that it would serve as an illustrative example of how in situ changes in structure can affect catalyst activity. First, reactions of aryl halides with sodium phenoxides catalyzed by complexes generated from Pd (dba) 2 and PFc (t-Bu) 2 occurred in high yields, but stoichiometric reductive elimination from the presumed arylpalladium phenoxide intermediate occurred in much lower yields (Scheme 1). 5 Second, careful monitoring of the catalytic reactions (Figure 1) showed that aryl halide was consumed initially without formation of diaryl ether and that the time dependence for formation of this product was sigmoidal. These results suggested the presence of an induction period. Several mechanistic hypotheses were tested to explain one or both of these observations. First, we considered whether cleavage of the isolated dimer was slower than direct decomposition to form products other than diaryl ether, a hypothesis that would explain the discrepancy in yields. Thus, we conducted the