Shekhar Shashank, Hartwig John F
Contribution from the Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA.
J Am Chem Soc. 2004 Oct 13;126(40):13016-27. doi: 10.1021/ja0480365.
Symmetrical bis-aryl platinum complexes (DPPF)Pt(C(6)H(4)-4-R)(2) (R = NMe(2), OMe, CH(3), H, Cl, CF(3)) and electronically unsymmetrical bis-aryl platinum complexes (DPPF)Pt(C(6)H(4)-4-R)(C(6)H(4)-4-X) (R = CH(3), X = NMe(2), OMe, H, Cl, F, CF(3); R = OMe, X = NMe(2), H, Cl, F, CF(3); R = CF(3), X = H, Cl, NMe(2); and R = NMe(2), X = H, Cl) were prepared, and the rates of reductive elimination of these complexes in the presence of excess PPh(3) are reported. The platinum complexes reductively eliminated biaryl compounds in quantitative yields with first-order rate constants that were independent of the concentration of PPh(3). Plots of Log(k(obs)/k(obs(H))) vs Hammett substituent constants (sigma) of the para substituents R and X showed that the rates of reductive elimination reactions depended on two different electronic properties. The reductive elimination from symmetrical bis-aryl platinum complexes occurred faster from complexes with more electron-donating para substituents R. However, reductive elimination from a series of electronically unsymmetrical bis-aryl complexes was not faster from complexes with the more electron-donating substituents. Instead, reductive elimination was faster from complexes with a larger difference in the electronic properties of the substituents on the two platinum-bound aryl groups. The two electronic effects can complement or cancel each other. Thus, this combination of electronic effects gives rise to complex, but now more interpretable, free energy relationships for reductive elimination.
制备了对称双芳基铂配合物（DPPF）Pt（C₆H₄ - 4 - R）₂（R = NMe₂、OMe、CH₃、H、Cl、CF₃）和电子不对称双芳基铂配合物（DPPF）Pt（C₆H₄ - 4 - R）（C₆H₄ - 4 - X）（R = CH₃，X = NMe₂、OMe、H、Cl、F、CF₃；R = OMe，X = NMe₂、H、Cl、F、CF₃；R = CF₃，X = H、Cl、NMe₂；以及R = NMe₂，X = H、Cl），并报道了这些配合物在过量PPh₃存在下的还原消除速率。铂配合物以定量产率还原消除联芳基化合物，其一级速率常数与PPh₃的浓度无关。Log（k(obs)/k(obs(H))）对对位取代基R和X的哈米特取代基常数（σ）作图表明，还原消除反应的速率取决于两种不同的电子性质。对称双芳基铂配合物的还原消除在具有更多给电子对位取代基R的配合物中发生得更快。然而，一系列电子不对称双芳基配合物的还原消除在具有更多给电子取代基的配合物中并非更快。相反，还原消除在两个与铂相连的芳基上取代基电子性质差异更大的配合物中更快。这两种电子效应可以相互补充或抵消。因此，这种电子效应的组合产生了复杂但现在更易于解释的还原消除自由能关系。
