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Mechanism of Palladium-Catalyzed Alkylation of Aryl Halides with Alkyl Halides through C–H Activation: A Computational Study
Organometallics ( IF 2.8 ) Pub Date : 2018-07-10 , DOI: 10.1021/acs.organomet.8b00185
Ling Zhu 1 , Yuan-Ye Jiang 1 , Xia Fan 1 , Peng Liu 1 , Bao-Ping Ling 1 , Siwei Bi 1
Affiliation  

Pd-catalyzed C(sp3)–H activation/alkylation of 2-tert-butylaryl halides with alkyl halides and CH2Br2 represents an advantageous strategy for the C–H functionalization with halogens as traceless directing groups. Several possible mechanisms were proposed for the reactions, but no further evidence was available to judge their relative feasibilities. Herein, a mechanistic study was performed with the aid of density functional theory (DFT) methods. Calculations indicate that the coupling of aryl bromides with alkyl chlorides is likely to generate alkylated benzocyclobutenes via aryl–Br oxidative addition on Pd(0) catalysts, C(sp3)–H activation, alkyl–Cl oxidative addition, aryl–alkyl reductive elimination, aryl–H activation, and aryl–C(sp3) reductive elimination. The coupling of aryl iodides with CH2Br2 is likely to generate indane derivatives via aryl–I oxidative addition, C(sp3)–H activation, alkyl–Br oxidative addition, aryl–CH2Br reductive elimination, alkyl–Br oxidative addition, C(sp3)–alkyl reductive elimination, and reduction of palladium dibromide complexes by amines. By comparison, the metathesis of alkyl chlorides on Pd(II) intermediates and the pathway involving palladium carbene intermediates are found to be less favored. Meanwhile, the coordination of in situ generated salts KI, KBr, and KHCO3 to palladium complexes, which has been less considered in previous mechanistic studies, is found to lead to more energetically favored pathways in most of the steps. Finally, the oxidative addition of alkyl halides generating Pd(IV) intermediates or the reduction of palladium dibromide complexes by amines, rather than the previously proposed C(sp3)–H activation, is found to be the rate-determining step in the two types of coupling reactions. This result does not go against the reported primary kinetic isotope effect (KIE) based on intramolecular competition reactions because the C(sp3)–H activation is irreversible according to our calculations.

中文翻译:

钯通过C–H活化催化卤代芳烃与卤代烷的烷基化机理:计算研究

Pd催化的2-叔丁基芳基卤化物与烷基卤化物和CH 2 Br 2的C(sp 3)–H活化/烷基化代表了一种以卤素作为无痕导向基团进行C–H官能化的有利策略。提出了几种可能的反应机理,但尚无进一步的证据来判断其相对可行性。在此,借助密度泛函理论(DFT)方法进行了机械研究。计算表明,芳基溴化物与烷基氯化物的偶联很可能通过在Pd(0)催化剂C(sp 3上)芳基-溴氧化加成而生成烷基化的苯并环丁烯。)–H活化,烷基–Cl氧化加成,芳基–烷基还原消除,芳基–H活化和芳基–C(sp 3)还原消除。芳基碘化物与CH 2 Br 2的偶联 可能通过芳基-I氧化加成,C(sp 3)-H活化,烷基-Br氧化加成,芳基-CH 2 Br还原消除,烷基-Br氧化生成茚满衍生物。此外,C(sp 3)-烷基还原消除,并通过胺还原二溴化钯配合物。相比之下,发现烷基氯化物在Pd(II)中间体上的易位以及涉及钯卡宾中间体的途径较不受欢迎。同时,就地协调生成的KI,KBr和KHCO 3盐与钯配合物的盐,在以前的机理研究中很少考虑,发现在大多数步骤中会导致能量上更有利的途径。最后,发现烷基卤化物的氧化加成生成Pd(IV)中间体或胺还原二溴化钯络合物,而不是先前提出的C(sp 3)-H活化,是这两者中的决定速率的步骤。偶联反应的类型。由于我们的计算结果表明C(sp 3)–H的活化是不可逆的,因此该结果与基于分子内竞争反应所报道的主要动力学同位素效应(KIE)并不相符。
更新日期:2018-07-12
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