Dipartimento di Chimica Organica e Industriale, Università degli Studi di Parma, Parco Area delle Scienze, 17/A, 43124 Parma, Italy.
J Am Chem Soc. 2011 Jun 8;133(22):8574-85. doi: 10.1021/ja110988p. Epub 2011 May 12.
Mechanistic questions concerning palladium and norbornene catalyzed aryl-aryl coupling reactions are treated in this paper: how aryl halides react with the intermediate palladacycles, formed by interaction of the two catalysts with an aryl halide, and what is the rational explanation of the "ortho effect" (caused by an ortho substituent in the starting aryl halide), which leads to aryl-aryl coupling with a second molecule of aryl halide rather than to aryl-norbornyl coupling. Two possible pathways have been proposed, one involving aryl halide oxidative addition to the palladacycle, the other passing through a palladium(II) transmetalation, also involving the palladacycle, as previously proposed by Cardenas and Echavarren. Our DFT calculations using M06 show that, in palladium-catalyzed reaction of aryl halides, not containing ortho substituents, and norbornene, the intermediate palladacycle formed has a good probability to undergo transmetalation, energetically favored over the oxidative addition leading to Pd(IV). The unselective sp(2)-sp(2) and sp(2)-sp(3) coupling, experimentally observed in this case, can be explained in the framework of the transmetalation pathway since the energetic difference between aryl attack onto the aryl or norbornyl carbon of the palladacycle intermediate is quite small. On the other hand, according to the experimentally observed "ortho effect", selective aryl-aryl coupling only occurs in the reactions of ortho-substituted metallacycles. The present work offers the first possible rationalization of this finding. These in situ formed palladacycles containing an ortho substituent could more easily undergo oxidative addition of an aryl halide rather than reductive elimination from the transmetalation intermediate as a result of a steric clash in the transition state of the latter. The now energetically accessible Pd(IV) intermediate, featuring a Y-distorted trigonal bipyramidal structure, can account for the reported selective aryl-aryl coupling through a reductive elimination which is easier than aryl-norbornyl coupling. Thus, the steric effect represents the main factor that dictates the energetic convenience of the system to follow the Pd(IV) or the transmetalation pathway. Ortho substituents cause a higher energy transition state for reductive elimination from the transmetalation intermediate than for oxidative addition to the metallacycle palladium(II) and the pathway based on the latter predominates.
本文探讨了钯和降冰片烯催化的芳基-芳基偶联反应的机理问题：芳基卤化物如何与由两种催化剂与芳基卤化物相互作用形成的中间体钯环反应，以及如何合理解释“邻位效应”（由起始芳基卤化物中的邻位取代基引起），该效应导致芳基-芳基与第二个芳基卤化物偶联，而不是芳基-降冰片烯偶联。已经提出了两种可能的途径，一种涉及芳基卤化物对钯环的氧化加成，另一种途径是通过钯（II）的转金属化，这也涉及到钯环，正如 Cardenas 和 Echavarren 之前提出的那样。我们使用 M06 的 DFT 计算表明，在钯催化的不含邻位取代基的芳基卤化物和降冰片烯反应中，形成的中间体钯环很有可能经历转金属化，这在能量上有利于导致 Pd(IV)的氧化加成。在这种情况下，实验中观察到的无选择性的 sp(2)-sp(2)和 sp(2)-sp(3)偶联，可以在转金属化途径的框架内得到解释，因为芳基攻击钯环的芳基或降冰片基碳的中间体的能量差异很小。另一方面，根据实验观察到的“邻位效应”，只有在邻位取代的金属环反应中才会发生选择性的芳基-芳基偶联。本工作首次对这一发现提供了合理的解释。这些原位形成的含有邻位取代基的钯环更容易发生芳基卤化物的氧化加成，而不是从转金属化中间体进行还原消除，这是由于后者的过渡态中存在空间位阻。现在能量上可及的 Pd(IV)中间体具有 Y-扭曲的三角双锥结构，可以通过比芳基-降冰片烯偶联更容易的还原消除来解释报道的选择性芳基-芳基偶联。因此，空间位阻效应代表了决定系统遵循 Pd(IV)或转金属化途径的能量便利性的主要因素。邻位取代基使从转金属化中间体进行还原消除的能量过渡态高于氧化加成到钯(II)金属环的能量过渡态，因此基于后者的途径占主导地位。
