Murarka Sandip, Möbus Juri, Erker Gerhard, Mück-Lichtenfeld Christian, Studer Armido
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany.
Org Biomol Chem. 2015 Mar 7;13(9):2762-7. doi: 10.1039/c4ob02689f.
The mechanism of the TEMPO mediated oxidative homo-coupling of aryl Grignard reagents is investigated in detail by experimental and computational studies. Experimental data reveal that the nitroxide-mediated homocoupling reaction of aryl Grignard reagents does not occur via free aryl radicals. Evidence for the presence of biaryl radical anions as intermediates in the coupling reaction is provided. It is also shown that PhMgPh under bromide free conditions in the presence of TEMPO does not undergo homocoupling. However, upon addition of MgBr2, C-C bond formation smoothly proceeds documenting the important role of the bromide anions in the oxidative homocoupling. DFT calculations show that an intramolecular electron transfer to a Mg-complexed TEMPO ligand with subsequent biaryl formation in a dimeric complex is viable and in agreement with experimental reaction conditions.
通过实验和计算研究，详细探究了TEMPO介导的芳基格氏试剂氧化均偶联反应的机理。实验数据表明，芳基格氏试剂的氮氧化物介导的均偶联反应不是通过游离芳基自由基发生的。提供了在偶联反应中存在联芳基自由基阴离子作为中间体的证据。还表明，在无溴条件下，在TEMPO存在下，PhMgPh不发生均偶联。然而，加入MgBr2后，C-C键的形成顺利进行，证明了溴离子在氧化均偶联中的重要作用。密度泛函理论（DFT）计算表明，分子内电子转移至镁络合的TEMPO配体，随后在二聚体络合物中形成联芳基是可行的，并且与实验反应条件一致。
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