Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology , Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan.
J Org Chem. 2014 May 16;79(10):4484-91. doi: 10.1021/jo500514e. Epub 2014 May 2.
The first tail-to-tail dimerization of methacrylonitrile (MAN) has been realized by the cooperative use of N-heterocyclic carbene (NHC) and Brønsted acid catalysts, producing 2,5-dimethylhex-2-enedinitrile with the E/Z ratio of 24:76. Although the NHC alone was not effective for the catalysis, the addition of alcohols resulted in the significant increase of the dimer yield up to 82% in the presence of 5 mol % NHC. Detailed experimental studies including the ESI-MS analysis of the intermediates, stoichiometric (co)dimerizations, and deuterium-labeling experiments revealed the mechanistic aspects of the proton transfer, isomerization, umpolung, and rate-limiting steps, allowing us to observe several mechanistic differences between the dimerization of MAN and that of methyl methacrylate. The stoichiometric reactions in the presence and absence of an alcohol suggest that the alcohol additives play a role in promoting the intermolecular proton transfers from the deoxy-Breslow intermediate to the regenerated NHC in the second half of the catalytic cycle. In addition, the codimerizations of MAN with n-butyl methacrylate (n-BuMA) have been studied. While the dimerization of n-BuMA was sluggish in the presence of an alcohol, the catalytic activity for the codimerization was enhanced by the cooperative systems.
首次通过 N-杂环卡宾（NHC）和布朗斯特酸催化剂的协同作用实现了丙烯腈（MAN）的头尾二聚反应，生成 E/Z 比为 24:76 的 2,5-二甲基-2-己烯二腈。尽管 NHC 单独对催化没有效果，但在添加醇时，在 5mol%NHC 的存在下，二聚产物的产率显著增加至 82%。详细的实验研究包括中间体的 ESI-MS 分析、计量（共）二聚化和氘标记实验，揭示了质子转移、异构化、反转和限速步骤的机理方面，使我们能够观察到 MAN 的二聚化和甲基丙烯酸甲酯的二聚化之间的几个机理差异。有和没有醇存在时的计量反应表明，醇添加剂在促进催化循环后半部分脱氧 Breslow 中间体到再生 NHC 的分子间质子转移方面发挥作用。此外，还研究了 MAN 与正丁基甲基丙烯酸酯（n-BuMA）的共聚反应。虽然在醇存在下 n-BuMA 的二聚化反应缓慢，但协同体系增强了共聚反应的催化活性。
