A New Synthesis of .alpha.-Asymmetric Nitroxide Radicals
A New Synthesis of .alpha.-Asymmetric Nitroxide Radicals
复制标题
α-不对称氮氧自由基的新合成
DOI：
10.1021/jo00126a035
复制
发表时间：
1995
影响因子：
3.6
通讯作者：
Y. Ishii
中科院分区：
文献类型：
--
作者：
R. Tamura;S. Susuki;N. Azuma;A. Matsumoto;F. Toda;Y. Ishii
描述了一种合成不对称双环硝基氧的新方法。在50℃下，用3.0当量的Sml2在THF或THF-Et2O中还原某些高烯丙基硝烯酮，得到稳定的硝基氧自由基-烯醇化物中间体，一种自由基阴离子物种。随后在约 50°C 下添加亲电试剂（例如酰氯）导致烯醇部分发生 O-酰化，形成相应的中性硝基氧自由基。还原过程对底物结构、Sml2 量、溶剂组成和反应温度非常敏感。确定了该两步反应的最佳反应条件。当被吸电子基团取代的苯甲酰氯用作 O-酰化的亲电子试剂时，可以以良好到中等的产率分离出分析纯的稳定硝基氧自由基。尽管使用有机甲硅烷基氯和烷基化试剂作为亲电子试剂似乎在反应混合物中提供了相应的自由基产物，但没有获得纯的自由基产物。通过 EPR 光谱和产物分析研究了氮氧自由基-烯醇化物中间体的形成机理。
A new methodology for the synthesis of-asymmetric bicyclic nitroxides is described. Reduction of certain homoallylic nitroenone with 3.0 equiv of Sml2 in THF or THF-Et20 at-50 C gave a stable nitroxide radical-enolate intermediate, a kind of radical-anion species. The subsequent addition of electrophiles such as acyl chlorides at-50 C resulted in the O-acylation of the enolate moiety to form the corresponding neutral nitroxide radical. The reduction process was quite sensitive to the substrate structure, amount of Sml2, solvent composition, and reaction temperature. The optimum reaction conditions for this two-step reaction were determined. Analytically pure stable nitroxide radicals were isolated in good to fair yields when benzoyl chlorides substituted by electron-withdrawing groups were used as the electrophile for O-acylation. Although the use of organosilyl chlorides and alkylating reagents as the electrophile seemed to provide the corresponding radical products in the reaction mixture, pure radical products were not obtained. The mechanism of formation of the nitroxide radical-enolate intermediate was investigated by EPR spectroscopy and the product analysis.