The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals
The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals
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DOI：
10.1002/chem.201700144
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发表时间：
2017-05-02
影响因子：
4.3
通讯作者：
Bertrand, Guy
中科院分区：
文献类型：
--
作者：
Mahoney, Janell K.;Jazzar, Rodolphe;Bertrand, Guy
一个环和一个无环二（氨基）卡宾以及一个环和一个无环（烷基）（氨基）卡宾与苯甲酰氯干净地反应，分别得到相应的加合物1（环）（+）、1（环）（+）、2（环）（+）和2（环）（+）。由环碳烯衍生的1(cyc)（+）和2(cyc)（+）还原得到相应的自由基1（cyc）和2(cyc)，在室温下稳定。相反，自由基1（acy）和2(acy)，衍生自无环碳，不能分离。结果表明，1（acy）与1（cyc）一样具有热力学稳定性，但其不稳定性是由于氮取代基上的-氢，以及C-N键周围增强的柔韧性，这使得H-成为可能。-migration-elimination过程。自由基2（acy）在热力学上是不稳定的，它会歧化成相应的最小2(acy)（+）和烯酸2(acy)（-）。这是由于过度的空间位阻，阻止了NCCO片段上的电子离域，从而阻止了俘获性稳定。这项工作提出了设计高持久性（氨基）（羧基）自由基的一般指导方针，特别是通过强调循环模式的关键优势。
A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1(cyc)(+), 1(acy)(+), 2(cyc)(+), and 2(acy)(+), respectively. The reduction of 1(cyc)(+) and 2(cyc)(+) derived from cyclic carbenes affords the corresponding radicals 1(cyc) and 2(cyc) that are stable at room temperature. In contrast, radicals 1(acy) and 2(acy), derived from acyclic carbenes, cannot be isolated. It is shown that 1(acy) is as thermodynamically stabilized as its cyclic counterpart 1(cyc), but its instability is the result of -hydrogens of the nitrogen substituent, along with the enhanced flexibility around C-N bonds, which allow for a H-.-migration-elimination process. Radical 2(acy) is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2(acy)(+) and enolate 2(acy)(-). This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.