Stannane-mediated radical addition to arenes. Generation of cyclohexadienyl radicals and increased propagation efficiency in the presence of catalytic benzeneselenol
Stannane-mediated radical addition to arenes. Generation of cyclohexadienyl radicals and increased propagation efficiency in the presence of catalytic benzeneselenol
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DOI：
10.1021/jo972197s
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发表时间：
1998-04-17
影响因子：
3.6
通讯作者：
Hwang, JT
中科院分区：
文献类型：
--
作者：
Crich, D;Hwang, JT
在制备自由基化学中，产率最高、最干净的反应通常是链序列，其中每个单独的繁殖步骤都是快速的。1坚持这种模式可以使自由基浓度保持在最低限度，从而减少削弱自由基反应的可能性。在给定的链序列中，任何一个增殖步骤的效率低下都会导致自由基浓度的增加，从而形成二聚化和歧化产物，并缩短动力学链的长度。后一种现象反过来要求使用异常高量的链引发剂，如果底物被完全消耗。一个恰当的例子，也是本研究的重点，是锡烷介导的芳烃自由基的加成。在这里（方案1），从锡烷中抽氢生成了不愿传播链的环己二烯基自由基。在这种化学中，环己二烯自由基的最终命运通常是重新芳构化成取代芳烃2,3，但这一氧化步骤发生的机制并不是很清楚，在文献中是争论的主题。被锡烷捕获得到环己二烯的区域异构体混合物，这些环己二烯在工作或色谱上被氧化。然而，从芳烃的桦木型还原中很容易形成和分离出环己二烯，这至少证明了环己二烯是主要途径。这种反应的传播能力差，动力学链长短，因此需要不成比例的大量引发剂，这使得人们认为，环己二烯基自由基可能被引发剂（通常是AIBN）或引发剂衍生的自由基氧化。对这一假设的一些支持可以从恩格尔的工作中得到，6在他的工作中证明了苯羟基自由基可以减少偶氮化合物。Rosa等人最近对标记化合物的研究减轻了这种机制，至少对AIBN来说是这样。7 Bowman和他的同事们提出了一个更进一步的假设，与SRN1型反应有关，在SRN1型反应中，加合物自由基经历去质子化，产生一个阴离子自由基，阴离子又将一个电子转移给烷基卤化物，从而获得芳香性。正如鲍曼所写的，这种机制，已经找到了一些支持，9,10使用锡烷作为碱来进行质子提取，导致分子氢气和锡基自由基的形成。
In preparative free radical chemistry, the highest yielding, cleanest reactions are typically chain sequences in which each of the individual propagation steps is rapid. 1 Adherance to this paradigm enables radical concentration to be maintained at a minimum which in turn reduces the possibility of debilitating radicalradical reactions. Inefficiency in any one of the several propagation steps in a given chain sequence will lead to a build up of radical concentration, the consequent formation of dimerization and disproportionation products, and a shortening of the kinetic chain length. This latter phenomenon in turn requires the use of abnormally high amounts of chain initiator if the substrate is to be fully consumed. A case in point, and the focus of this study, is the stannane-mediated addition of radicals to arenes. Here (Scheme 1), a cyclohexadienyl radical is generated which is reluctant to propagate the chain by hydrogen abstraction from the stannane. In this chemistry the ultimate fate of the cyclohexadienyl radical is usually rearomatization to a substituted arene, 2, 3 but the mechanism by which this oxidation step takes place is not at all well understood and is the subject of debate in the literature. Capture by the stannane to give regiosiomeric mixtures of cyclohexadienes which are oxidized on work up or chromatography is often assumed. However, the ease of formation and isolation of cyclohexadienes from Birch type reductions4 of arenes belies this argument, at least as the major pathway. The poor propagation and short kinetic chain lengths of such reactions with the consequent need for disproportionately large amounts of initiator have led to the suggestion that the cyclohexadienyl radicals may be oxidized by the initiator, 5 usually AIBN, or an initiator-derived radical.Some support for this hypothesis may be drawn from the work of Engel, 6 in which it is demonstrated that benzhydryl radicals reduce azo compounds. More recent work by Rosa et al. with labeled compounds mitigates against this mechanism, at least for AIBN. 7 Bowman and coworkers have advanced a further hypothesis, related to the SRN1 type reaction, in which the adduct radical undergoes deprotonation to give a radical anion which, in turn, transfers an electron to the alkyl halide and so achieves aromaticity. 8 As written by Bowman, this mechanism, which has found some support, 9, 10 uses the stannane as a base to perform the proton abstraction leading to the formation of molecular hydrogen gas and a stannyl radical.