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首页> 外文期刊>Chemistry: A European journal >Formation of Transient Anionic Metal Clusters in Palladium/Diene- Catalyzed Cross-Coupling Reactions
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Formation of Transient Anionic Metal Clusters in Palladium/Diene- Catalyzed Cross-Coupling Reactions

机译：钯/二烯催化交叉偶联反应中瞬时阴离子金属簇的形成

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摘要

Despite their considerable practical value, palladium/ 1,3-diene-catalyzed cross-coupling reactions between Grignard reagents RMgCl and alkyl halides AlkylX remain mechanistically poorly understood. Herein, we probe the intermediates formed in these reactions by a combination of electrospray-ionization mass spectrometry, UV/Vis spectroscopy, and NMR spectroscopy. According to our results and in line with previous hypotheses, the first step of the catalytic cycle brings about transmetalation to afford organopalladate anions. These organopalladate anions apparently undergo S_N2-type reactions with the AlkylX coupling partner. The resulting neutral complexes then release the cross-coupling products by reductive elimination. In gas-phase fragmentation experiments, the occurrence of reductive eliminations was observed for anionic analogues of the neutral complexes. Although the actual catalytic cycle is supposed to involve chiefly mononuclear palladium species, anionic palladium nanoclusters [Pd_nR(DE)_n]~-, (n=2, 4, 6; DE=diene) were also observed. At short reaction times, the dinuclear complexes usually predominated, whereas at longer times the tetra- and hexanuclear clusters became relatively more abundant. In parallel, the formation of palladium black pointed to continued aggregation processes. Thus, the present study directly shows dynamic behavior of the palladium/ diene catalyst system and degradation of the active catalyst with increasing reaction time.
机译：尽管其相当大的实用价值，但格氏试剂RMGCL和烷基卤化物之间的钯/ 1,3-二烯催化的交叉偶联反应仍然理解机械地差不多。在此，我们通过电喷雾电离质谱，UV / VI光谱和NMR光谱的组合探测在这些反应中形成的中间体。根据我们的结果和符合先前的假设，催化循环的第一步带来了透射率，得到有机玻璃阴离子。这些有机铝基团阴离子显然经过与烷基偶联伴侣的S_N2型反应。然后通过还原的消除释放所得的中性复合物。在气相破碎实验中，观察到中性复合物的阴离子类似物的未经化消除的发生。尽管应该观察到实际催化循环涉及主要单核钯物种，阴离子钯纳米团簇[PD_NR（DE）_N]〜 - ，（n = 2,4,6; de =二烯）。在短时间内，通常占主导地位的核络合物，而在较长的时间越来越多地变得相对较高。平行，钯黑的形成指向持续的聚集过程。因此，本研究直接显示钯/二烯催化剂体系的动态行为，并随着反应时间的增加，活性催化剂的降解。

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