Mechanism of Cobalt-Catalyzed Heterodimerization of Acrylates and 1,3-Dienes. A Potential Role of Cationic Cobalt(I) Intermediates
Mechanism of Cobalt-Catalyzed Heterodimerization of Acrylates and 1,3-Dienes. A Potential Role of Cationic Cobalt(I) Intermediates
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DOI：
10.1021/acscatal.9b05455
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发表时间：
2020-04-03
期刊：
影响因子：
12.9
通讯作者：
RajanBabu, T., V
中科院分区：
文献类型：
--
作者：
Gray, Montgomery;Hines, Michael T.;RajanBabu, T., V
原料烯烃的偶联反应是很有前途的，但这些反应很少在工业上实施。尽管合成方法学的最新进展已经在1，3-二烯和丙烯酸酯之间的二聚反应中产生了优异的区域选择性和对映选择性，但通过催化剂中的周转数（TON）测量的效率仍然适中。通过结合反应进程动力学分析（RPKA）的一个原型的二聚反应和表征的孤立的低价钴催化剂前体参与，这个反应的机理出现了几个重要的细节。(i)原型反应具有诱导期，其需要至少2小时的搅拌时间以产生活性催化剂。(ii)Co（II）络合物还原为Co（I）络合物和随后产生阳离子[Co-（I）]（+）物质是造成这种延迟的原因。(iii)通过使用原位红外光谱的RPKA，相同的过量实验揭示了反应结束时产物的抑制作用，并且只要介质中存在二烯，就没有观察到催化剂失活。观察到的低TON最有可能是催化反应的推定阳离子Co（I）物质的固有不稳定性的结果。(iv)不同的过量实验表明，反应是一级的二烯和零级的丙烯酸酯。(v)催化剂负载实验表明，催化剂的负载量为一级反应。通过变时间归一化分析进一步证实了各种试剂中的顺序。(vi)提出了基于氧化二聚[通过Co（I）/Co（III）-循环]的机理。基于该研究的结果，通过在增加的起始材料（特别是二烯）浓度下进行反应，可以将TON增加10倍，这似乎使催化物质稳定。
Coupling reactions of feedstock alkenes are promising, but few of these reactions are practiced industrially. Even though recent advances in the synthetic methodology have led to excellent regio- and enantioselectivies in the dimerization reactions between 1,3-dienes and acrylates, the efficiency as measured by the turnover numbers (TONs) in the catalyst has remained modest. Through a combination of reaction progress kinetic analysis (RPKA) of a prototypical dimerization reaction and characterization of isolated low-valent cobalt catalyst precursors involved, several important details of the mechanism of this reaction have emerged. (i) The prototypical reaction has an induction period that requires at least 2 h of stir time to generate the competent catalyst. (ii) Reduction of a Co(II) complex to a Co(I) complex and subsequent generation of a cationic [Co-(I)](+) species are responsible for this delay. (iii) Through RPKA using in situ infrared spectroscopy, same excess experiments reveal inhibition by the product toward the end of the reaction, and no catalyst deactivation is observed as long as the diene is present in the medium. The low TON observed is most likely the result of the inherent instability of the putative cationic Co(I) species that catalyzes the reaction. (iv) Different excess experiments suggest that the reaction is first order in the diene and zero order in the acrylate. (v) Catalyst loading experiments show that the catalyst is first order. The orders in the various regents were further confirmed by variable time normalization analysis. (vi) A mechanism based on oxidative dimerization [via Co(I)/Co(III)-cycle] is proposed. Based on the results of this study, it is possible to increase the TON by a factor of 10 by conducting the reaction at an increased concentration of the starting materials, especially the diene, which seems to stabilize the catalytic species.