Theoretical study on a new active species for the Pd(II)-catalyzed Mizoroki-Heck reaction

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• 作者：Michinori Sumimoto^a ; ^{sumimoto@yamaguchi-u.ac.jp} ; Takaaki Kuroda^a ; Daisuke Yokogawa^b ; Hidetoshi Yamamoto^a ; Kenji Hori^a ; ^{kenji@yamaguchi-u.ac.jp}
• 关键词：Density functional theory ; Mizoroki&ndash ; Heck reaction ; Catalytic cycle ; Active species ; Palladium
• 刊名：Journal of Organometallic Chemistry
• 出版年：2012
• 期刊代码：64_0022328x
• 类别：ch
• 出版时间：1 July, 2012
• 卷：710
• 期：Complete
• 页码：26-35
• 文件大小：1890 K

摘要

The formation process of an active species for the Pd(II)-catalyzed Mizoroki-Heck reaction between bromobenzene and ethylene was investigated theoretically using the DFT(B3PW91) method. In this mechanism, one need not consider the dissociation of the chloride ion from the PdCl₂(dppe) complex. It was confirmed that the ethylene insertion into the Pd-Cl bond (Path A) occurs more easily than the C-Br oxidative addition (Path B) and the Cl^{鈭?/sup> dissociation (Path Z) for the PdCl₂(dppe) complex. In the next step, the C₆H₅Br approaches the Pd center of the complex with ethylene via the C-Br oxidative addition, followed by the ethylene insertion into the Pd-Cl bond (Path D) to form a six-coordinate complex, PdBrCl(C₂H₄Cl)(C₆H₅)(dppe). In the final step, a C-Cl elimination proceeds from the complex to give a four-coordinate complex, PdBr(C₆H₅)(dppe). The rate-determining step of Path D is the C-Cl reductive elimination, and the energy difference between reactant and TS was calculated to be 30.4聽kcal/mol. Similar mechanisms releasing Cl-C₄H₈-Cl (Path E and F) are also acceptable, since those barrier heights were calculated to be 32.3 and 17.6聽kcal/mol. Path D, E and F are plausible candidates to form the PdBr(C₆H₅)(dppe) complex as a starting complex of the catalytic cycle. The rate-determining step of the entire catalytic reaction using the PdBr(C₆H₅)(dppe) complex is the 尾-H abstraction about the electronic energy of solute in solution and the free energy in solution, and those barrier heights were calculated to be 26.4 and 24.9聽kcal/mol, respectively. As a result, the first step of the catalytic reaction mechanism is not the Cl鈭?/sup> dissociation from the PdCl₂(dppe) complex. Instead, the ethylene insertion into the Pd-Cl bond first proceeds to produce an active species, and next the PdBr(C₆H₅)(dppe) complex forms through Path D, E or F. It is concluded that the catalytic cycle of the Mizoroki-Heck reaction starts from PdBr(C₆H₅)(dppe).}