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首页> 外文期刊>European journal of organic chemistry >Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface
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Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

机译：自由基钟的结构效应和格氏试剂形成机理：当选择性的十字路口在金属/溶液界面时，自由基钟中苯基取代基的特殊作用

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A large class of radical clocks is based on the intramolecular trapping of a reactive radical by a suitably located unsaturated system. Depending on the substituents present on this unsaturated system, the rate of cyclisation may vary drastically. This property has been repeatedly used to diagnose the participation of very short-lived radicals in the mechanisms of a wide variety of reactions. For reactions occurring in homogeneous solution, a phenyl substituent capable of stabilizing the radical formed during the act of trapping has been one of the most widely used tools of this type. During study of the mechanisms of formation of Grignard reagents _ reactions that occur at the interface of the metal and the solution _ the phenyl substituent displayed a specific new behaviour pattern. Besides its stabilizing role, it was also able to play the role of mediator in redox catalysis of electron transfer. In this case, the first events on the pathway to the Grignard reagents involve a cascade of three (one intermolecular followed by two intramolecular) electron transfers. Introduction of a p-methoxy substituent on the phenyl ring, making the phenyl group a poorer electron acceptor, suppresses this specific second role. Applied to the mechanism of Grignard reagent formation, this p-methoxy effect is consistent with a triggering mechanistic act of electron transfer from the metal to the aryl halide rather than with a concerted oxidative addition. A similar change in selectivity is observed when a pmethoxy group is introduced onto a phenyl group that also bears a halogen, but its origin is different: this effect is associated with the shortening of the lifetime of the radical anion formed by the triggering electron transfer. These observations reemphasise our earlier proposals to use concepts originating from electrochemical kinetics to explain the selectivities of reactions occurring at metal/solution interfaces. This conjecture could possibly hold for any interface where the diffusion of reactive species plays a role in the settling of selectivity. These concepts emphasise the necessity to consider, for each reactive species, their average distance of diffusion away from the metal/solution interface.
机译：自由基时钟的一大类是基于通过适当定位的不饱和体系对反应性自由基的分子内捕获。取决于存在于该不饱和体系上的取代基，环化的速率可以急剧变化。此属性已被反复用于诊断非常短暂的自由基参与各种反应机制。对于在均相溶液中发生的反应，能够稳定在捕集过程中形成的自由基的苯基取代基已成为这种类型中使用最广泛的工具之一。在格氏试剂形成机理的研究中，发生在金属与溶液界面的反应是苯基取代基，表现出特定的新行为模式。除了稳定作用外，它还可以在电子转移的氧化还原催化中发挥介体的作用。在这种情况下，格氏试剂通路上的第一个事件涉及三个（一个分子间，然后两个分子内）电子转移的级联。在苯环上引入对甲氧基取代基，使苯基成为较弱的电子受体，抑制了这种特定的第二作用。应用到格氏试剂形成的机理中，这种对甲氧基作用与电子从金属转移到芳基卤化物的触发机制作用一致，而不是协同的氧化加成作用。当将对甲氧基基团引入还带有卤素的苯基时，会观察到类似的选择性变化，但是其来源不同：这种作用与触发电子转移形成的自由基阴离子的寿命缩短有关。这些观察结果再次强调了我们先前的建议，即使用源自电化学动力学的概念来解释在金属/溶液界面发生的反应的选择性。这种猜想可能适用于反应物种扩散在选择性沉降中起作用的任何界面。这些概念强调了对于每种反应物种而言，必须考虑其远离金属/溶液界面的平均扩散距离。

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