Comparing nickel- and palladium-catalyzed Heck reactions
Comparing nickel- and palladium-catalyzed Heck reactions
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DOI：
10.1021/om034067h
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发表时间：
2004-04-26
期刊：
影响因子：
2.8
通讯作者：
Guo, QX
中科院分区：
文献类型：
--
作者：
Lin, BL;Liu, L;Guo, QX
用密度泛函理论方法研究了Ni在Heck反应中的可能应用。结果表明，Ni和Pd催化的Heck反应的机理非常相似。然而，氧化加成和烯烃插入在Ni系统中发生时的势垒比在Pd系统中要低。由于β-氢化物在Pd体系中的消除比在Ni体系中更有效，因此在Ni体系中对Michael加成反应的选择性比在Pd体系中要差。此外，通过脱除HX来再生催化剂，Ni系统比Pd系统更难实现。因此，要么应该使用非常强的碱来催化镍，要么应该设计还原途径来从镍络合物中脱除HX。与Pd体系相比，Ni体系中烯基或芳基氯的氧化加成并不比烯基或芳基碘的氧化加成难多少。因此，镍催化的Heck反应可以相对容易地应用于烯基或芳基氯。此外，由于β-氢化物在Ni体系中的消除比在Pd体系中更难，因此Ni催化的Heck反应可以应用于脂肪族卤化物。对于具有给电子取代基的烯烃，如果空间效应不明显，镍催化的偶联反应应该稍微有利于Markovnikov类型的产物。对于具有吸电子取代基的烯烃，镍催化的偶联反应应以反马尔可夫尼科夫型产物为主。此外，还发现膦和吡啶配体可以很好地降低HX脱除步骤中的自由能。因此，它们似乎是镍催化Heck反应的很有前途的配体。最后，我们发现Heck反应中的溶剂化效应、阳离子途径和阴离子途径并没有改变Ni和Pd催化剂的反应活性的总体趋势。
The possible use of Ni in Heck reactions was investigated with use of the density function theory method. It was found that the mechanisms of the Ni- and Pd-catalyzed Heck reactions are quite similar to each other. Nevertheless, oxidative addition and olefin insertion occur with lower energy barriers in the Ni system than in the Pd system. Because beta-hydride elimination is more efficient in the Pd system than in the Ni system, there is a poorer selectivity to vinylation over Michael addition in the Ni system than in the Pd system. In addition, catalyst regeneration through HX removal is considerably harder to achieve with the Ni system than with the Pd system. Therefore, either a very strong base should be used for the Ni catalysis or a reductive pathway should be designed to remove HX from the Ni complex. Compared to the Pd system, oxidative addition of an alkenyl or aryl chloride is not much harder than oxidative addition of an alkenyl or aryl iodide in the Ni system. Therefore, the Ni-catalyzed Heck reaction may be applied to alkenyl or aryl chloride relatively easily. Also, because beta-hydride elimination is more difficult in the Ni system than in the Pd system, the Ni-catalyzed Heck reaction may be applied to aliphatic halides. For an olefin with an electron-donating substituent, the Ni-catalyzed coupling should slightly favor the Markovnikov-type product, if the steric effect is not significant. For an olefin with an electron-withdrawing substituent, the Ni-catalyzed coupling should provide the anti-Markovnikov-type product as the major product. In addition, it was found that phosphine and pyridine ligands can reasonably well reduce the free energy in the HX removal step. Therefore, they appear to be promising ligands for the Ni-catalyzed Heck reactions. Finally, we found that the solvation effects, cation pathway, and anionic pathway in the Heck reactions did not change the general trends for the reactivities of the Ni and Pd catalysts.