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Chemistry with Electrochemically Generated N-Centered Radicals.
Accounts of Chemical Research ( IF 18.3 ) Pub Date : 2019-11-27 , DOI: 10.1021/acs.accounts.9b00472
Peng Xiong 1 , Hai-Chao Xu 1
Affiliation  

N-centered radicals are versatile reaction intermediates that can react with various π systems to construct C-N bonds. Current methods for generating N-centered radicals usually involve the cleavage of an N-heteroatom bond; however, similar strategies that are applicable to N-H bonds prove to be more challenging to develop and therefore are attracting increasing attention. In this Account, we summarize our recent efforts in the development of electrochemical methods for the generation and synthetic utilization of N-centered radicals. In our studies, N-aryl amidyl radical, amidinyl radical and iminyl radical cation intermediates are generated from N-H precursors through direct electrolysis or indirect electrolysis assisted by a redox catalyst. In addition, an electrocatalytic method that converts oximes to iminoxyl radicals has also been developed. The electrophilic amidyl radical intermediates can participate in 5-exo or 6-exo cyclization with alkenes and alkynes to afford C-centered radicals, which can then undergo various transformations such as H atom abstraction, single-electron transfer oxidation to a carbocation, cyclization, or aromatic substitution, leading to a diverse range of N-heterocyclic products. Furthermore, amidinyl radicals, iminyl radical cations, and iminoxyl radicals can undergo intramolecular aromatic substitution to afford various N-heteroaromatic compounds. Importantly, the electrochemical reaction can be channeled toward a specific product despite the presence of other competing pathways. For a successful electrosynthesis, it is important to take into consideration of both the electron transfer steps associated with the electrode and the nonelectrode related processes. A unique feature of electrochemistry is the simultaneous occurrence of anodic oxidation and cathodic reduction, which, as this Account demonstrates, allows the dehydrogenative transformations to proceed through H2 evolution without the need for chemical oxidants. In addition, cathodic solvent reduction can continuously generate a low concentration of base, which facilitates anodic substrate oxidation. Such a mechanistic paradigm obviates the need for stoichiometric strong bases and avoids base-promoted decomposition of sensitive substrates or products. Furthermore, electrode materials can also be adjusted to control the reaction outcome, as demonstrated by the synthesis of N-heteroaromatics and the corresponding N-oxides from biaryl ketoximes.

中文翻译:

化学与电化学产生的N中心自由基。

N中心自由基是通用的反应中间体,可以与各种π系统反应以构建CN键。当前产生N中心自由基的方法通常涉及N-杂原子键的断裂;例如,N-杂原子键的断裂。但是,适用于NH键的类似策略被证明在开发方面更具挑战性,因此越来越引起人们的关注。在此报告中,我们总结了最近在开发用于生成和合成利用以N为中心的自由基的电化学方法方面所做的努力。在我们的研究中,N-芳基a基自由基,a基自由基和亚氨基自由基阳离子中间体是由NH前驱体通过直接电解或氧化还原催化剂辅助的间接电解生成的。另外,还已经开发了将肟转化为亚氨基氧基的电催化方法。亲电子a基自由基中间体可以参与烯烃与炔烃的5-exo或6-exo环化反应,得到C中心的自由基,然后可以进行各种转变,例如H原子抽象,单电子转移氧化成碳正离子化,环化,或芳族取代,从而导致N杂环产品种类繁多。此外,a基,亚胺基阳离子和亚氨基氧基可经历分子内芳族取代以提供各种N-杂芳族化合物。重要的是,尽管存在其他竞争途径,电化学反应仍可引导至特定产物。为了成功进行电合成,重要的是要考虑与电极有关的电子转移步骤和与电极无关的过程。电化学的一个独特特征是同时发生阳极氧化和阴极还原,正如该帐户所证明的,这使得脱氢转化可以通过H2的释放而进行,而无需化学氧化剂。另外,阴极溶剂的还原可连续产生低浓度的碱,这有助于阳极底物的氧化。这样的机械范例消除了对化学计量的强碱的需要,并且避免了碱促进的敏感底物或产物的分解。此外,还可以调节电极材料以控制反应结果,
更新日期:2019-11-28
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