On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes
On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes
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DOI：
10.1021/ja052099l
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发表时间：
2005-07-27
影响因子：
15
通讯作者：
Myers, AG
中科院分区：
文献类型：
--
作者：
Tanaka, D;Romeril, SP;Myers, AG
提出了钯介导的芳烃羧酸脱羧烯化的机理研究，为所提出的逐步过程中的中间体提供了光谱学证据，并在两个例子中提供了晶体学证据。随后，所提出的途径涉及双（三氟乙酸）钯（II）和芳烃羧酸底物之间的羧基交换，限速脱羧形成芳基钯（II）三氟乙酸中间体（含有两个反式配置的S-结合二甲亚砜配体，以晶体学特征的形式），然后烯烃插入和β-氢化物消除。由于三氟乙酸芳基钯(II)中间体的独特生成模式，相对于之前研究的含膦芳基钯(II)络合物，该中间体被认为是基本上缺电子的，因此有可能对 Heck 反应常见的步骤（即烯烃插入和 β-氢化物消除）获得新的见解。目前的结果表明，脱羧钯化生成的芳基钯(II)中间体与传统 Heck 反应生成的芳基钯(II)中间体之间的反应活性存在显着差异。具体来说，我们发现更多的富电子烯烃优先与脱羧钯化形成的三氟乙酸芳基钯(II)中间体反应，而在传统的 Heck 反应中发现了相反的趋势。此外，我们发现，与相应的膦连接的芳烷基钯(II)配合物相比，本研究中烯烃插入时形成的芳烷基钯(II)三氟乙酸盐中间体在β-氢化物消除方面是稳定的。我们还对芳烷基钯(II)三氟乙酸酯中间体进行了晶体学表征，该中间体衍生自芳基钯(II)插入降冰片烯，并且该结构也包含S-结合的二甲亚砜配体；转移芳基的内碳和三氟乙酸酯在观察到的扭曲方形平面钯(II)配合物中充当第三和第四配体。
Mechanistic studies of a palladium-mediated decarboxylative olefination of arene carboxylic acids are presented, providing spectroscopic and, in two instances, crystallographic evidence for intermediates in a proposed stepwise process. Sequentially, the proposed pathway involves carboxyl exchange between palladium(II) bis(trifluoroacetate) and an arene carboxylic acid substrate, rate-determining decarboxylation to form an arylpalladium(II) trifluoroacetate intermediate (containing two trans-disposed S-bound dimethyl sulfoxide ligands in a crystallographically characterized form), then olefin insertion and beta-hydride elimination. Because of the unique mode of generation of the arylpalladium(II) trifluoroacetate intermediate, a species believed to be substantially electron-deficient relative to phosphine-containing arylpalladium(II) complexes previously studied, it has been possible to gain new insights into those steps that are common to the Heck reaction, namely, olefin insertion and beta-hydride elimination. The present results show that there are notable differences in reactivity between arylpalladium(II) intermediates generated by decarboxylative palladation and those produced in conventional Heck reactions. Specifically, we have found that more electron-rich alkenes react preferentially with an arylpalladium(II) trifluoroacetate intermediate formed by decarboxylative palladation, whereas an opposite trend is found in conventional Heck reactions. In addition, we have found that the aralkylpalladium(II) trifluoroacetate intermediates that are formed upon olefin insertion in the present study are stabilized with respect to beta-hydride elimination as compared to the corresponding phosphine-ligated aralkylpalladium(II) complexes. We have also crystallographically characterized an aralkylpalladium(II) trifluoroacetate intermediate derived from arylpalladium(II) insertion into norbornene, and this structure, too, contains an S-bound dimethyl sulfoxide ligand; the ipso-carbon of the transferred aryl group and trifluoroacetate function as the third and fourth ligands in the observed distorted square-planar palladium(II) complex.