Fairlamb Ian J S, Kapdi Anant R, Lee Adam F, McGlacken Gerard P, Weissburger Felix, de Vries André H M, Schmieder-van de Vondervoort Lizette
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
Chemistry. 2006 Nov 24;12(34):8750-61. doi: 10.1002/chem.200600473.
The reactivity of palladium(0) complexes, [Pd(0) (2)(dba-n,n'-Z)(3)] (n,n'-Z=4,4'-F; 4,4'-CF(3); 4,4'-H; 4,4'-MeO) and [Pd(0)(dba-n,n'-Z)(2)] (n,n'-Z=4,4'-CF(3); 4,4'-H; 3,3',5,5'-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dba-n,n'-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L(n)Pd(0)-eta(2)-dba interaction (in dpi-pi* synergic bonding, n=1 or 2), which ultimately increases the global concentration of catalytically active L(n)Pd(0) available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n'-Z ligand stabilizes the Pd(0)-eta(2)-dba interaction, reducing catalytic activity. The specific type of dba-n,n'-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L(n)Pd(0) concentration (and the associated catalytic activity) and catalyst longevity. Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dba-n,n'-Z ligands in Heck arylation is more convoluted and dependent on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n'-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n'-Z with amine and/or base effectively lessens its interference in the catalytic cycle.
钯(0)配合物[Pd(0)(2)(dba - n,n'-Z)(3)]（n,n'-Z = 4,4'-F；4,4'-CF(3)；4,4'-H；4,4'-MeO）和[Pd(0)(dba - n,n'-Z)(2)]（n,n'-Z = 4,4'-CF(3)；4,4'-H；3,3',5,5'-OMe）与合适的给体配体（如膦、N-杂环卡宾）用作前体催化剂，已在多个钯(0)介导的交叉偶联过程中建立了相关性。在大多数此类过程中，增加dba - n,n'-Z配体芳基部分的电子密度会提高整体催化活性。这种效应主要源于L(n)Pd(0)-η(2)-dba相互作用（在dpi - π*协同键合中，n = 1或2）的不稳定，这最终会增加在一般催化循环（氧化加成）的第一个关键步骤中可用于与芳基卤化物反应的催化活性L(n)Pd(0)的整体浓度。降低dba - n,n'-Z配体芳基部分的电子密度会稳定Pd(0)-η(2)-dba相互作用，从而降低催化活性。dba - n,n'-Z配体的特定类型似乎在催化循环中也起到稳定作用，防止钯团聚，并提高催化剂寿命。因此，在L(n)Pd(0)浓度（以及相关的催化活性）和催化剂寿命之间存在微妙的平衡。改变dba - n,n'-Z配体的类型会控制L(n)Pd(0)的浓度和氧化加成步骤的速率，而不是催化循环中的其他内部步骤，例如，转金属化（或碳钯化）和还原消除。dba - n,n'-Z配体在Heck芳基化中的作用更为复杂，并且取决于所使用的烯烃底物，尽管已经出现了一些趋势。dba - n,n'-Z结构的变化对Buchwald - Hartwig芳基胺化过程影响最小。dba - n,n'-Z与胺和/或碱的二级迈克尔反应有效地减少了其对催化循环的干扰。
