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We have performed some preliminary mechanistic inves- tigations of the arylation. The qualitative arylation mecha- nism is presented in Scheme 1. The deuterium isotope effect for the arylation of benzothiazole was found to be 1.3 (Scheme 1). Isotope effect values of this magnitude are often observed for aromatic electrophilic substitution reactions. Additionally, the regiochemistry of 1-n- butylimidazole ary-lation is consistent with that observed in electrophilic substitution.As a consequence, we favor the electrophilic substitution mechanism proposed by Miura for the arylation of thiazoles and imidazoles;an analogous mechanism was proposed by Gevorgyan for the arylation of indolizines. The observed counterion effect is also interesting. The reactions are the fastest for triflates. Somewhat slower reactions are observed for chlorides; bromides and iodides react the slowest. Because the oxidative additions to Pd(0) are normally the fastest for iodides, it is clear that the oxidative addition of the C-Cl bond is not the rate-determining step. Additionally, the leaving group is unlikely to act as a base in proton removal because compounds containing the weakest base, triflate, react the fastest. It has to be stated that the arylation of such different heterocycles does not necessarily proceed by exactly the same mechanism. Certain Rh-catalyzed heterocycle arylations proceed via Rh carbene intermediates. |
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| 我们已经进行了一些芳基化机理的初步研究。定性的芳基化机理是烈在表1 。氘同位素对芳香化芳基噻唑的影响如1.3 （图1）所示。这种程度的同位素效应常在芳香亲电取代观察到。此外，1-N-丁基咪唑的芳香化和亲电取代的区域选择性是一致的，因此，我们认同由 Miura 提出的芳香化噻唑和咪唑的机理是电取代机理，由Gevorgyan提出的氮茚衍生物的芳香化机理与此类似。观测到的反离子的效果也很有趣。三氟甲磺酸盐的反应最快。氯化反应有点慢，溴化和碘化反应是最慢的。由于钯（0）上增加氧碘化最快，所以很显然的是 C-Cl键的氧化加成不是速率决定步骤。此外，去除质子中离去基团不可能充当碱，因为含有最弱碱即三氟甲磺酸的化合物的反应最快。必须说的是，这些不同的杂环芳基化不一定完全以相同机制的进行。某种铑催化杂环芳香化是通过铑卡宾中间体进行的。 |
2楼2012-04-29 18:03:27
