Studies on the Enantioselective Iminium Ion Trapping of Radicals Triggered by an Electron-Relay Mechanism.
Studies on the Enantioselective Iminium Ion Trapping of Radicals Triggered by an Electron-Relay Mechanism.
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DOI：
10.1021/jacs.7b01446
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发表时间：
2017-03-29
影响因子：
15
通讯作者：
Melchiorre P
中科院分区：
文献类型：
--
作者：
Bahamonde A;Murphy JJ;Savarese M;Brémond É;Cavalli A;Melchiorre P
电化学、光谱、计算和动力学研究的结合已被用来阐明先前报道的光化学产生的碳中心自由基的对映选择性亚胺离子捕获的关键机制方面。该过程，它提供了一种直接的方法来锻造具有高保真度的四元立体中心，依赖于两个不同的催化循环的相互作用：aminocatalytic电子中继系统，它触发立体选择性自由基陷阱后亚胺离子形成，和光氧化还原循环，在温和的条件下产生自由基。反应发展的关键是使用带有氧化还原活性咔唑单元的手性胺催化剂，该催化剂可以快速减少在自由基拦截时产生的高度反应性和不稳定的中间体。然而，咔唑单元还参与电子中继机制的另一个步骤：瞬时产生的咔唑自由基阳离子作为氧化剂使光催化剂返回到原始状态。通过动力学和光谱研究，我们已经确定了最后的氧化还原事件作为整个过程的营业额限制步骤。这种机制框架是证实了由线性之间的相关性的反应速率和还原电位的咔唑单元拴在氨基催化剂。因此，可以合理地调节咔唑单元的氧化还原性质以提高催化活性。这些知识可能为下一代电子继电器催化剂的机械驱动设计开辟道路。
A combination of electrochemical, spectroscopic, computational, and kinetic studies has been used to elucidate the key mechanistic aspects of the previously reported enantioselective iminium ion trapping of photochemically generated carbon-centered radicals. The process, which provides a direct way to forge quaternary stereocenters with high fidelity, relies on the interplay of two distinct catalytic cycles: the aminocatalytic electron-relay system, which triggers the stereoselective radical trap upon iminium ion formation, and the photoredox cycle, which generates radicals under mild conditions. Critical to reaction development was the use of a chiral amine catalyst, bearing a redox-active carbazole unit, which could rapidly reduce the highly reactive and unstable intermediate generated upon radical interception. The carbazole unit, however, is also involved in another step of the electron-relay mechanism: the transiently generated carbazole radical cation acts as an oxidant to return the photocatalyst into the original state. By means of kinetic and spectroscopic studies, we have identified the last redox event as being the turnover-limiting step of the overall process. This mechanistic framework is corroborated by the linear correlation between the reaction rate and the reduction potential of the carbazole unit tethered to the aminocatalyst. The redox properties of the carbazole unit can thus be rationally tuned to improve catalytic activity. This knowledge may open a path for the mechanistically driven design of the next generation of electron-relay catalysts.