Suo Weiqun, Qi Jian-Qing, Liu Jing, Sun Songtao, Jiao Lei, Guo Xingwei
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China.
J Am Chem Soc. 2024 Sep 18;146(37):25860-25869. doi: 10.1021/jacs.4c09792. Epub 2024 Sep 4.
Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on α-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between α-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of α-aminoalkyl radicals in XAT reactions is 10 to 10 times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that α-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of α-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for α-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.
卤原子转移（XAT）是一种生成碳自由基的通用方法。最近的研究兴趣集中在α-氨基烷基自由基作为潜在的XAT试剂上，此前报道其表现出与锡自由基相当的反应活性。利用先进的时间分辨电子顺磁共振（EPR）技术，研究了α-氨基烷基自由基与有机卤化物之间的XAT反应，通过EPR光谱直接观察该过程并分析自由基动力学。测定了这些反应的二级速率常数，其中一些通过瞬态吸收光谱进行了验证。关键发现是，α-氨基烷基自由基在XAT反应中的反应活性比锡和硅自由基低10到10倍，仅略高于烷基自由基。这对α-氨基烷基自由基与锡自由基反应活性相同的观点提出了挑战。对溶剂效应的研究表明，α-氨基烷基自由基的XAT反应不涉及高度极化的过渡态，这表明该XAT过程中的动力学极化效应不如先前认为的那么显著。本研究为α-氨基烷基自由基提供了可靠的XAT反应活性标度，这对于设计XAT反应及其机理理解至关重要。
