Choudary Boyapati M, Madhi Sateesh, Chowdari Naidu S, Kantam Mannepalli L, Sreedhar Bojja
Indian Institute of Chemical Technology, Hyderabad 500 007, India.
J Am Chem Soc. 2002 Nov 27;124(47):14127-36. doi: 10.1021/ja026975w.
Layered double hydroxide and Merrifield resin supported nanopalladium(0) catalysts are prepared by an exchange of PdCl(4)(2-) followed by reduction and well characterized for the first time. The ligand-free heterogeneous layered double hydroxide supported nanopalladium (LDH-Pd(0)) catalyst using the basic LDH in place of basic ligands indeed exhibits higher activity and selectivity in the Heck olefination of electron-poor and electron-rich chloroarenes in nonaqueous ionic liquids (NAIL) over the homogeneous PdCl(2) system. Using microwave irradiation, the rate of the Heck olefination reaction is accelerated, manifold with the highest turnover frequency ever recorded in the case of both electron-poor and electron-rich chloroarenes. The basic LDH-Pd(0) shows a superior activity over a range of supported catalysts, from acidic to weakly basic Pd/C, Pd/SiO(2,) Pd/Al(2)O(3), and resin-PdCl(4)(2-) in the Heck olefination of deactivated electron-rich 4-chloroanisole. The use of LDH-Pd(0) is extended to the Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes in an effort to understand the scope and utility of the reaction. The catalyst is quantitatively recovered from the reaction by a simple filtration and reused for a number of cycles with almost consistent activity in all the coupling reactions. The heterogeneity studies provide an insight into mechanistic aspects of the Heck olefination reaction and evidence that the reaction proceeds on the surface of the nanopalladium particles of the heterogeneous catalyst. TEM images of the fresh and used catalyst indeed show that the nanostructured palladium supported on LDH remains unchanged at the end of the reaction, while the XPS and evolved gas detection by TGA-MS of the used catalyst identify ArPdX species on the heterogeneous surface. Thus, the ligand-free nanopalladium supported on LDH, synthesized by the simple protocol, displays superior activity over the other heterogeneous catalysts inclusive of nanopalladium in the C-C coupling reactions of chloroarenes.
层状双氢氧化物和 Merrifield 树脂负载的纳米钯(0)催化剂是通过 PdCl(4)(2-)的交换然后还原制备的，并首次得到了充分表征。使用碱性层状双氢氧化物代替碱性配体的无配体多相层状双氢氧化物负载纳米钯(LDH-Pd(0))催化剂，在非水离子液体(NAIL)中，对于贫电子和富电子氯代芳烃的 Heck 烯基化反应，确实比均相 PdCl(2)体系表现出更高的活性和选择性。使用微波辐射，Heck 烯基化反应的速率加快，在贫电子和富电子氯代芳烃的情况下，周转频率达到了有史以来的最高值。在失活的富电子 4-氯苯甲醚的 Heck 烯基化反应中，碱性 LDH-Pd(0)比一系列负载型催化剂表现出更高的活性，这些催化剂包括酸性到弱碱性的 Pd/C、Pd/SiO(2)、Pd/Al(2)O(3)以及树脂-PdCl(4)(2-)。将 LDH-Pd(0)的应用扩展到氯代芳烃的 Suzuki-、Sonogashira-和 Stille-型偶联反应，以了解该反应的范围和实用性。通过简单过滤可从反应中定量回收催化剂，并在所有偶联反应中以几乎一致的活性重复使用多个循环。多相性研究深入了解了 Heck 烯基化反应的机理方面，并证明该反应在多相催化剂的纳米钯颗粒表面进行。新鲜和使用过的催化剂的 TEM 图像确实表明，负载在 LDH 上的纳米结构钯在反应结束时保持不变，而使用过的催化剂的 XPS 和 TGA-MS 对逸出气体的检测确定了多相表面上的 ArPdX 物种。因此，通过简单方法合成的负载在 LDH 上的无配体纳米钯在氯代芳烃的 C-C 偶联反应中比包括纳米钯在内的其他多相催化剂表现出更高的活性。
