Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols
Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols
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DOI：
10.1021/ja031936l
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发表时间：
2004-06-02
影响因子：
15
通讯作者：
Tsuji, Y
中科院分区：
文献类型：
--
作者：
Iwasawa, T;Tokunaga, M;Tsuji, Y
在均相催化剂体系中，金属聚集和沉淀导致催化剂分解和催化活性损失的问题一直存在。Pd黑色地层就是一个典型的例子。众所周知，钯催化剂容易聚集并形成钯黑，尽管它们在有机合成中实现了各种有用的反应。为了克服均相钯催化的固有问题，我们以醇的好氧氧化为探针反应，探索了一类新的钯催化剂。在此，我们报道了一种新的催化剂体系，即使在空气中，在高底物与催化剂的摩尔比（S/C：大于1000）下，也能抑制醇氧化中Pd黑的形成。该新型吡啶衍生物具有2,3,4,5-四苯基苯基取代基及其在吡啶环3位的更高枝状单元，在80℃的甲苯中钯催化空气（气球）氧化醇时，与Pd(OAc)2配体良好。与结构相关的吡啶配体比较发现，在吡啶环3位引入2,3,4,5-四苯基苯基取代基可以有效抑制Pd黑的生成，长时间保持催化活性，高产出醛类或酮类产物。
In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 °C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields.