Popp Brian V, Thorman Joseph L, Morales Christine M, Landis Clark R, Stahl Shannon S
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA.
J Am Chem Soc. 2004 Nov 17;126(45):14832-42. doi: 10.1021/ja0459734.
The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine-palladium(0) complexes bearing trans-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (3(X)), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3). Density functional theory calculations of model alkene-substitution reactions at a diimine-palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty pi orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an "inverse-electron-demand" ligand-substitution reaction.
人们对钯（0）上烯烃取代的机理进行了研究，研究结果为深入了解富电子后过渡金属的基本反应活性提供了独特的见解。制备并表征了一系列带有反式-β-硝基苯乙烯配体（ns(X) = X-C(6)H(4)CH=CHNO(2)；X = OCH(3)、CH(3)、H、Br、CF(3)）的浴铜灵-钯（0）配合物，即(bc)Pd(0)ns(X) (3(X))，并发现这些配合物的烯烃取代反应按缔合机理进行。在(bc)Pd(ns(CH)()3)与ns(X)（X = OCH(3)、H、Br、CF(3)）的交叉反应中，电子缺乏程度更高的烯烃反应更快（相对速率：ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3）。对二亚胺-钯（0）中心处模型烯烃取代反应的密度泛函理论计算表明，钯中心通过基于金属的孤对电子进攻进入烯烃的空π轨道而作为亲核试剂发生反应。这种轨道图景与传统配体取代反应的不同，在传统配体取代反应中，进入的配体将电子密度提供给金属上的受体轨道。基于这些结果，钯（0）上的烯烃取代被归类为“反电子需求”配体取代反应。
