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Understanding electronic effects on carboxylate-assisted C-H activation at ruthenium: the importance of kinetic and thermodynamic control.
Faraday Discussions ( IF 3.4 ) Pub Date : 2019-12-02 , DOI: 10.1039/c9fd00063a
Raed A Alharis 1 , Claire L McMullin , David L Davies , Kuldip Singh , Stuart A Macgregor
Affiliation  

Meta- and para-substituted 1-phenylpyrazoles (R-phpyz-H) react with [RuCl2(p-cymene)]2 in the presence of NaOAc to form cyclometallated complexes [M(R-phpyz)Cl(p-cymene)] (where R = NMe2, OMe, Me, H, F, CF3 and NO2). Experimental and DFT studies indicate that product formation can be reversible or irreversible depending on the substituents and the reaction conditions. Competition experiments show that the kinetic selectivity favours electron-donating substituents and correlate well with the Hammett parameter, giving a negative slope (ρ = -2.4) that is consistent with a cationic transition state. However, surprisingly, the thermodynamic selectivity is completely opposite, with substrates featuring electron-withdrawing groups being favoured. These trends are reproduced with DFT calculations that locate a rate-limiting transition state dominated by Ru-O bond dissociation and minimal C-H bond elongation. Detailed computational analysis of these transition states shows that C-H activation proceeds by an AMLA/CMD mechanism through a synergic combination of a C-H→Ru agostic interaction and C-HO H-bonding. NBO calculations also highlight a syndetic bonding term, and the relative weights of these three components vary in a complementary fashion depending on the nature of the substituent. With meta-substituted ligands H/D exchange experiments signal kinetically accessible ortho-C-H activation when R = NMe2, OMe and Me. This is also modelled computationally and the calculations highlight the kinetic relevance of the HOAc/Cl exchange that occurs post C-H bond cleavage, in particular with the bulkier NMe2 and Me substituents. Our study highlights that the experimental substituent effects are dependent on the reaction conditions and so using such studies to assign the mechanism of C-H activation in either stoichiometric or catalytic reactions may be misleading.

中文翻译:

了解钌上羧酸盐对CH活化的电子效应:动力学和热力学控制的重要性。

在NaOAc存在下,间位和对位取代的1-苯基吡唑(R-phpyz-H)与[RuCl2(p-cymene)] 2反应形成环金属化的配合物[M(R-phpyz)Cl(p-cymene)] (其中R = NMe2,OMe,Me,H,F,CF3和NO2)。实验和DFT研究表明,取决于取代基和反应条件,产物的形成可以是可逆的或不可逆的。竞争实验表明,动力学选择性有利于给电子取代基,并且与Hammett参数具有良好的相关性,从而给出了与阳离子过渡态一致的负斜率(ρ= -2.4)。然而,令人惊讶的是,热力学选择性完全相反,具有吸电子基团的底物受到青睐。这些趋势可通过DFT计算重现,该计算确定了以Ru-O键解离和最小CH键伸长为主导的限速过渡态。对这些过渡态的详细计算分析表明,CHLA激态相互作用与C-HO H键的协同结合,通过AMLA / CMD机制进行CH活化。NBO计算也突出了一个合成键术语,这三个组分的相对重量根据取代基的性质以互补的方式变化。当R = NMe2,OMe和Me时,使用间位取代的配体进行H / D交换实验,信号表明动力学上可及的邻位CH活化。这也是通过计算建模的,计算结果突出显示了CH键断裂后发生的HOAc / Cl交换的动力学相关性,特别是具有较大的NMe 2和Me取代基。我们的研究强调,实验性取代基的作用取决于反应条件,因此使用此类研究来确定化学计量或催化反应中CH活化的机理可能会产生误导。
更新日期:2019-12-30
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