Fürmeier Sven, Griep-Raming Jens, Hayen Ahlke, Metzger Jürgen O
Institute of Chemistry, University of Kassel, Heinrich-Plett-Strasse 40, 34132 Kassel, Germany.
Chemistry. 2005 Sep 19;11(19):5545-54. doi: 10.1002/chem.200500188.
Electrospray ionization mass spectrometry (ESI-MS) is a novel tool for the investigation of chemical reactions in solution and for the direct detection and identification of reactive intermediates. The tributyltin hydride mediated addition of tert-butyl iodide to dimethyl 2-cyclohexyl-4-methyleneglutarate (2) in the presence of Lewis acids was investigated by ESI-MS using a microreactor coupled on-line to an ESI mass spectrometer. For the first time we have been able to show that transient radicals in radical chain reactions can be detected unambiguously under steady-state conditions in the reaction solution and can be characterized by ESI-MS/MS and accurate mass determination. The detection of different heterodimer radical complexes by ESI-MS/MS has provided new insights into the mechanism of Lewis acid controlled radical chain reactions. Dimeric chelate complexes of glutarates, such as 2 and 3, and Lewis acids, like Sc(OTf)3, MgBr2OEt2 and LiClO4, were observed as well as higher aggregates with additional equivalents of Lewis acid. Evidence for a dynamic equilibrium of the complexes in solution was found by NMR spectroscopy. The ESI-MS investigation of the chelation of glutarate 2 with various Lewis acids has led to the conclusion that the tendency for Lewis acids to form dimeric chelate complexes and higher aggregates has an important effect on the stereoselective outcome of the radical reactions.
电喷雾电离质谱（ESI-MS）是一种用于研究溶液中化学反应以及直接检测和鉴定反应中间体的新型工具。通过使用与ESI质谱仪在线连接的微反应器，利用ESI-MS研究了在路易斯酸存在下，三丁基氢化锡介导的叔丁基碘与2-环己基-4-亚甲基戊二酸二甲酯（2）的加成反应。首次能够证明，在反应溶液的稳态条件下，可以明确检测到自由基链反应中的瞬态自由基，并且可以通过ESI-MS/MS和精确质量测定对其进行表征。通过ESI-MS/MS检测不同的异二聚体自由基络合物，为路易斯酸控制的自由基链反应机理提供了新的见解。观察到了戊二酸酯（如2和3）与路易斯酸（如Sc(OTf)3、MgBr2OEt2和LiClO4）的二聚螯合物以及含有额外当量路易斯酸的更高聚集体。通过核磁共振光谱发现了溶液中络合物动态平衡的证据。对戊二酸酯2与各种路易斯酸螯合的ESI-MS研究得出结论，路易斯酸形成二聚螯合物和更高聚集体的倾向对自由基反应的立体选择性结果有重要影响。
