Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions.
Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions.
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DOI：
10.1021/acs.accounts.1c00122
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发表时间：
2021-05-04
影响因子：
18.3
通讯作者：
Bäckvall JE
中科院分区：
文献类型：
--
作者：
Li MB;Bäckvall JE
涉及级联过程的钯催化氧化提供了一个通用平台，可以将简单原料简化转化为具有高原子和步骤经济性的功能分子。然而，在许多情况下，由于活性钯物质聚集成钯黑以及每个成键步骤中可能发生的副反应，在钯催化的氧化级联反应中实现高钯效率和选择性仍然具有挑战性。目前解决这些问题的两种解决方案是利用氧化剂稳定的配体或使用电子转移介体（ETM）。前一种解决方案包括使用胺、吡啶、亚砜和卡宾衍生物，在催化循环过程中抑制 Pd0 的聚集，而后一种解决方案则促进 Pd0 再氧化为 PdII，从而提高活性和选择性。遵循我们对钯催化氧化的长期兴趣，最近我们开发了多相催化剂来解决氧化级联反应中的上述问题。非均相钯催化剂（Pd-AmP-MCF或Pd-AmP-CNC）由固定在氨基功能化硅质介孔泡沫（MCF）或结晶纳米纤维素（CNC）上的钯纳米团簇（1-2 nm）组成，具有高活性、选择性和优异的回收能力。在本篇文章中，我们将讨论多相钯催化剂的合成和表征，以及它们的催化行为和反应机制。这些催化剂在氧化反应中的一个重要方面是在非均相内生成活性 Pd(II) 物质。我们近期研究的典型氧化级联反应包括氧化碳环化-羰基化、氧化碳环化-硼基化、氧化炔基化-环化、氧化羰基化-环化和氧化碳环化-炔基化。这些反应提供了在药物化学和功能材料中有吸引力的重要化合物，例如基于γ-内酯/γ-内酰胺的多环、环丁烯醇、高度取代的呋喃和氧杂硼杂环戊烯。在这些过程中，多相催化剂表现出比均相催化剂（例如 Pd(OAc)2）更高的周转数 (TON) 以及均相钯催化剂无法实现的独特选择性。还研究了多相催化剂的高效率和独特选择性的起源。基于这些非均相过程中优异的选择性，实现了用于构建光学纯化合物的不对称合成。动力学研究表明，在回收过程中反应的速率和产率基本保持不变，这表明 Pd-AmP-MCF 和 Pd-AmP-CNC 在这些氧化级联反应中具有稳健性和高活性。此外，电感耦合等离子体发射光谱（ICP-OES）分析和热过滤测试表明这些过程很可能通过异质途径进行。我们课题组的最新进展表明，通过添加Ag+生成阳离子Pd(II)，可以进一步提高Pd-AmP-MCF和Pd-AmP-CNC的活性。此外，在 Pd-AmP-MCF 催化的氧化级联过程中观察到了有趣的溶剂效应，并基于催化剂的这一特性开发了溶剂控制的化学选择性转化。该帐户的多相策略为 Pd(II) 催化的氧化级联反应中的钯失活和选择性问题提供了解决方案，并实现了催化剂的高效回收，这将为氧化级联反应开辟新的机遇。
Palladium-catalyzed oxidations involving cascade processes provide a versatile platform for streamlined conversion of simple feedstocks into functional molecules with high atom and step economy. However, the achievement of high palladium efficiency and selectivity in Pd-catalyzed oxidative cascade reactions is still challenging in many cases, as a result of the aggregation of active palladium species to Pd black and the possible side reactions during each bond-forming step. The two current solutions for addressing these issues are either to utilize oxidant-stable ligands or to use electron transfer mediators (ETMs). The former solution, which includes the use of amines, pyridines, sulfoxides, and carbene derivatives, inhibits aggregation of Pd0 during the catalytic cycle, while the latter solution facilitates reoxidation of Pd0 to PdII to improve the activity and selectivity. Following our long-standing interest in Pd-catalyzed oxidations, very recently we developed heterogeneous catalysts to resolve the issues mentioned above in oxidative cascade reactions. The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprise palladium nanoclusters (1–2 nm) immobilized on amino-functionalized siliceous mesocellular foam (MCF) or on crystalline nanocellulose (CNC), exhibiting high activity, selectivity as well as excellent recycling ability. In this Account, we will discuss the synthesis and characterizations of the heterogeneous palladium catalysts, as well as their catalytic behaviors, and the mechanisms involved in their reactions. An important aspect of these catalysts in oxidation reactions is the generation of active Pd(II) species within the heterogeneous phase. Typical oxidative cascade reactions of our recent research on this topic include oxidative carbocyclization-carbonylation, oxidative carbocyclization-borylation, oxidative alkynylation-cyclization, oxidative carbonylation-cyclization, and oxidative carbocyclization-alkynylation. These reactions provide access to important compounds attractive in medicinal chemistry and functional materials, such as γ-lactone/γ-lactam-based poly rings, cyclobutenols, highly substituted furans, and oxaboroles. During these processes, the heterogeneous catalysts exhibited much higher turnover numbers (TONs) than their homogeneous counterparts (e.g., Pd(OAc)2) as well as unique selectivity that cannot be achieved by homogeneous palladium catalysts. The origin of the high efficiency and unique selectivity of the heterogeneous catalysts was also investigated. Asymmetric syntheses for the construction of optically pure compounds were realized based on the excellent selectivity in these heterogeneous processes. Kinetic studies revealed that the rate and yield of the reactions were essentially maintained during recycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robust and highly active in these oxidative cascade reactions. In addition, inductively coupled plasma optical emisson spectroscopy (ICP-OES) analysis and hot filtration test suggest that these processes most likely proceed via a heterogeneous pathway. Recent progress in our group has shown that the activity of Pd-AmP-MCF and Pd-AmP-CNC could be improved even further by the addition of Ag+ to generate cationic Pd(II). Furthermore, intriguing solvent effects were observed in a Pd-AmP-MCF-catalyzed oxidative cascade process, and solvent-controlled chemoselective transformations were developed based on this property of the catalyst. The heterogeneous strategy of this Account provides solutions to palladium deactivation and selectivity issues in Pd(II)-catalyzed oxidative cascade reactions and enables efficient catalyst recycling, which will open up new opportunities in oxidative cascade reactions.
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影响因子：
62.1
作者：
He J;Wasa M;Chan KSL;Shao Q;Yu JQ
通讯作者：
Yu JQ
影响因子：
18.2
作者：
Hartwig JF;Larsen MA
通讯作者：
Larsen MA
影响因子：
16.6
作者：
Li, Jiawei;Liao, Jianhua;Jiang, Huanfeng
通讯作者：
Jiang, Huanfeng
影响因子：
16.6
作者：
Li, Man-Bo;Yang, Ying;Backvall, Jan-E
通讯作者：
Backvall, Jan-E
影响因子：
16.6
作者：
Engstrom, Karin;Johnston, Eric V.;Verho, Oscar;Gustafson, Karl P. J.;Shakeri, Mozaffar;Tai, Cheuk-Wai;Backvall, Jan-E.
通讯作者：
Backvall, Jan-E.