Sequential Generation and Utilization of Radical and Anionic Species with a Novel Manganese-Lead Reducing Agent. Three-Component Coupling Reactions of Alkyl Iodides, Electron-Deficient Olefins, and Carbonyl Compounds.
Sequential Generation and Utilization of Radical and Anionic Species with a Novel Manganese-Lead Reducing Agent. Three-Component Coupling Reactions of Alkyl Iodides, Electron-Deficient Olefins, and Carbonyl Compounds.
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使用新型锰铅还原剂连续生成和利用自由基和阴离子物质。
DOI：
10.1021/jo9617530
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
T. Moriwake
中科院分区：
文献类型：
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作者：
K. Takai;T. Ueda;N. Ikeda;T. Moriwake
烷基通常通过链法、共价键的均解裂解或基于氧化还原反应的非链法制备。1后者中的一种通过用还原剂还原卤代烷(RX)来访问自由基；然而，这种方法不那么受欢迎，因为在还原条件下，导致烷基阴离子(R-)的初始自由基(R·)的进一步还原通常比第一种自由基的形成进行得更快。3虽然在阴离子反应之前观察到了分子内自由基环化，2a，3由于上述限制，分子间环化的例子很少。4，5对于连接两个反应的合适的还原剂有两个要求：(1)初始的径向(R·)不易还原为R-，并且具有足够的寿命进行分子间反应。(2)最终的自由基(R‘·)容易被单电子还原为R’-。因此，所需的还原剂应该足够弱，以便能够区分两个基团R·和R‘·。考虑到这些考虑，这里使用了一种新型的锰铅还原剂，并将上述概念实现为三组分耦合反应。与锌粉相比，锰粉对有机化合物的反应性较小，因为其表面有一层致密的锰氧化物。最近，人们发现，用Me3SiCl7、8处理可以有效地去除这种金属氧化物，而且特别是通过添加催化量的PbCl2来激活金属锰。7用活化的金属锰处理烷基碘1，得到4个化合物，这表明用锰体系还原碘化物形成了烷基自由基(式1)。在质子溶剂中，R，β-不饱和酯与分子间1，4-加成，在温和的条件下以极好的产率(式2)。9因此，导致烷基锰化合物的第二次单电子还原很可能比使用该锰系的第一次还原慢。
Alkyl radicals are usually prepared either by chain methods, the homolytic cleavage of covalent bonds, or by nonchain methods based on redox reactions. 1 One of the latter accesses radicals by reduction of alkyl halides (RX) with reducing agents; however, the method is not so popular as further reduction of the initial radicals (R•) leading to alkyl anions (R-) normally proceeds faster than the first radical formation under the reduction conditions. 3 Although intramolecular radical cyclization before anionic reactions has been observed in several cases, 2a, 3 there are few examples of the intermolecular version due to the above restriction. 4, 5 Two requirements exist for a suitable reducing agent to connect the two reactions:(1) The initial radial (R•) is not easily reduced to R-and has a sufficient lifetime to undergo an intermolecular reaction.(2) The final radical (R′•) is easily subjected by oneelectron reduction to R′-. Therefore, the desired reductant should be weak enough to be able to discriminate between the two radicals R• and R′•. With these considerations in mind, here, a novel manganese-lead reducing agent was utilized, and the above concept was realized as a three-component coupling reaction. Manganese powder6 is less reactive toward organic compounds than zinc powder due to a tight layer of manganese oxide on its surface. Recently, it was found that such metal oxide is effectively removed by treatment with Me3SiCl; 7, 8 and moreover, the manganese metal is especially activated by addition of a catalytic amount of PbCl2. 7 Treatment of alkyl iodide 1 with the activated manganese metal afforded four compounds, which suggests the formation of an alkyl radical by reduction of the iodide with the manganese system (eq 1).Intermolecular 1, 4-addition to an R, β-unsaturated ester in a protic solvent was achieved in an excellent yield under mild conditions (eq 2). 9 It is likely, therefore, that the second one-electron reduction leading to an alkylmanganese compound is slower than the first reduction with this manganese system.