Poly(ethylene glycol)-supported nitroxyls: branched catalysts for the selective oxidation of alcohols.
Poly(ethylene glycol)-supported nitroxyls: branched catalysts for the selective oxidation of alcohols.
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DOI：
10.1021/jo0490494
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发表时间：
2004-09
期刊：
影响因子：
--
通讯作者：
P. Ferreira;E. Phillips;D. Rippon;S. Tsang;W. Hayes
中科院分区：
文献类型：
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作者：
P. Ferreira;E. Phillips;D. Rippon;S. Tsang;W. Hayes
氮氧自由基如2，2，6，6-四甲基哌啶-1-氧基（克里思）是将伯醇转化为相应醛的高选择性氧化催化剂。在这项研究中，克里思单元到线性聚（乙二醇）（PEG）的直接拴系提供了大分子催化剂，表现出在水和有机溶剂中的溶解性。溶解的聚合物负载的催化剂的回收已经通过用合适的溶剂如乙醚沉淀来进行。传统上与硝酰基介导的醇氧化相关的高催化剂活性和选择性通过一系列“无接头”线性PEG-TEMPO催化剂保留，其中克里思部分直接偶联到PEG载体。尽管选择性保持不变，但在无连接基的聚合物负载的催化剂再循环时，需要延长的反应时间以保持所需羰基化合物的高产率。或者，两个硝酰基自由基通过5-羟基戊酸接头连接到每个官能化的PEG链末端上提供支化的聚合物负载的催化剂。与无连接基催化剂形成鲜明对比的是，这些支化硝酰基在类似条件下表现出比单独的4-甲氧基-TEMPO高多达五倍的催化活性。此外，在通过溶剂诱导沉淀再循环这些支化大分子催化剂时观察到催化活性的最小降低。高催化活性和保存这些支化系统的回收后的活性是由于增强再生的硝酰基物种作为一个结果的分子内syn-proportionation。
Nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) are highly selective oxidation catalysts for the conversion of primary alcohols into the corresponding aldehydes. In this study, direct tethering of TEMPO units onto linear poly(ethylene glycol) (PEG) has afforded macromolecular catalysts that exhibit solubility in both aqueous and organic solvents. Recovery of the dissolved polymer-supported catalyst has been carried out by precipitation with a suitable solvent such as diethyl ether. The high catalyst activities and selectivities associated traditionally with nitroxyl-mediated oxidations of alcohols are retained by the series of "linker-less" linear PEG-TEMPO catalysts in which the TEMPO moiety is coupled directly to the PEG support. Although the selectivity remains unaltered, upon recycling of the linker-less polymer-supported catalysts, extended reaction times are required to maintain high yields of the desired carbonyl compounds. Alternatively, attachment of two nitroxyl radicals onto each functionalized PEG chain terminus via a 5-hydroxyisophthalic acid linker affords branched polymer-supported catalysts. In stark contrast to the linker-less catalysts, these branched nitroxyls exhibit catalytic activities up to five times greater than 4-methoxy-TEMPO alone under similar conditions. In addition, minimal decrease in catalytic activity is observed upon recycling of these branched macromolecular catalysts via solvent-induced precipitation. The high catalytic activities and preservation of activity upon recycling of these branched systems is attributed to enhanced regeneration of the nitroxyl species as a result of intramolecular syn-proportionation.