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首页> 外文期刊>Chemistry: A European journal >Mechanistic origin of ligand-controlled regioselectivity in Pd-catalyzed C-H activation/arylation of thiophenes
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Mechanistic origin of ligand-controlled regioselectivity in Pd-catalyzed C-H activation/arylation of thiophenes

机译：Pd催化噻吩的C-H活化/芳基化中配体控制的区域选择性的机制起源

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The use of ligands to control regioselectivity in transition-metal- catalyzed C-H activation/functionalization is a highly desirable but challenging task. Recently, Itami et al. reported an important finding relating to Pd-catalyzed ligand-controlled α/β-selective C-H arylation of thiophenes. Specifically, the use of the 2,2′-bipyridyl ligand resulted in α-arylation, whereas the use of the bulky fluorinated phosphine ligand P[OCH(CF_3)_2]_3 resulted in β-arylation. Understanding of this surprising ligand-controlled α/β-selectivity could provide important insights into the development of more efficient catalyst systems for selective C-H arylation, and so we carried out a detailed computational study on the problem with use of density functional theory methods. Three mechanistic possibilities-S_EAr and migration, metalation/deprotonation, and Heck-type arylation mechanisms-were examined. The results showed that the S_EAr and migration mechanism might not be plausible, because the key Wheland intermediates could not be obtained. On the other hand, our study indicated that the metalation/deprotonation and Heck-type arylation mechanisms were both involved in Itami's reactions. In the metalation/deprotonation pathway the α-selective product (C5-product) was preferred, whereas in the Heck-type arylation mechanism the β-selective product (C4-product) was favored. The ligands played crucial roles in tuning the relative barriers of the two different pathways. In the 2,2′-bipyridyl- assisted system, the metalation/deprotonation pathway was energetically advantageous, leading to α-selectivity. In the P[OCH(CF_3) _2]_3-assisted system, on the other hand, the Heck-type arylation mechanism was kinetically favored, leading to β-selectivity. An interesting finding was that P[OCH(CF_3)_2]_3 could produce a C-H?O hydrogen bond in the catalyst system, which was crucial for stabilization of the Heck-type transition state. In comparison, this C-H?O hydrogen bond was absent with the other phosphine ligands [i.e., P(OMe)_3, PPh_3, PCy_3] and these phosphine ligands therefore favored the metalation/deprotonation pathway leading to α-selectivity. Furthermore, in this study we have provided theoretical evidence showing that the Heck-type arylation reaction could proceed through an anti-β-hydride elimination process.
机译：在过渡金属催化的C-H活化/官能化中使用配体来控制区域选择性是一项非常理想但具有挑战性的任务。最近，伊丹等人。报道了与Pd催化的噻吩的α/β-选择性C-H芳基化有关的重要发现。具体而言，使用2,2'-联吡啶基配体导致α-芳基化，而使用大分子氟化膦配体P [OCH（CF_3）_2] _3导致β-芳基化。了解这种令人惊讶的配体控制的α/β选择性可以为开发更有效的选择性C-H芳基化催化剂体系提供重要的见识，因此，我们使用密度泛函理论方法对该问题进行了详细的计算研究。研究了三种机械可能性-S_EAr和迁移，金属化/去质子化以及Heck型芳基化机制。结果表明，由于无法获得关键的Wheland中间体，S_EAr和迁移机制可能不合理。另一方面，我们的研究表明，金属化/去质子化和Heck型芳基化机制均与Itami的反应有关。在金属化/去质子化途径中，首选α-选择性产物（C5-产物），而在Heck型芳构化机理中，首选β-选择性产物（C4-产物）。配体在调节两种不同途径的相对壁垒中起关键作用。在2,2'-联吡啶辅助系统中，金属化/去质子化途径在能量上是有利的，从而导致α选择性。另一方面，在P [OCH（CF_3）_2] _3辅助系统中，动力学上偏爱Heck型芳基化机制，从而导致β选择性。有趣的发现是P [OCH（CF_3）_2] _3可以在催化剂体系中产生C-H2O氢键，这对于稳定Heck型过渡态至关重要。相比之下，该C-H 2 O氢键与其他膦配体[即P（OMe）_3，PPh_3，PCy_3]不存在，因此这些膦配体有利于金属化/去质子化途径，从而导致α-选择性。此外，在这项研究中，我们提供了理论证据，表明Heck型芳基化反应可通过抗β-氢化物消除过程进行。

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