†Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
‡Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
Acc Chem Res. 2015 Jun 16;48(6):1746-55. doi: 10.1021/acs.accounts.5b00061. Epub 2015 May 8.
Chemists no longer doubt the importance of a methodology that could activate and utilize aldehydes in organic syntheses since many products prepared from them support our daily life. Tremendous effort has been devoted to the development of these methods using main-group elements and transition metals. Thus, many organic chemists have used an activator-(aldehyde oxygen) interaction, namely, η(1) coordination, whereby a Lewis or Brønsted acid activates an aldehyde. In the field of coordination chemistry, η(2) coordination of aldehydes to transition metals by coordination of a carbon-oxygen double bond has been well-studied; this activation mode, however, is rarely found in transition-metal catalysis. In view of the distinctive reactivity of an η(2)-aldehyde complex, unprecedented reactions via this intermediate are a distinct possibility. In this Account, we summarize our recent results dealing with nickel(0)-catalyzed transformations of aldehydes via η(2)-aldehyde nickel and oxanickelacycle intermediates. The combination of electron-rich nickel(0) and strong electron-donating N-heterocyclic carbene (NHC) ligands adequately form η(2)-aldehyde complexes in which the aldehyde is highly activated by back-bonding. With Ni(0)/NHC catalysts, processes involving intramolecular hydroacylation of alkenes and homo/cross-dimerization of aldehydes (the Tishchenko reaction) have been developed, and both proceed via the simultaneous η(2) coordination of aldehydes and other π components (alkenes or aldehydes). The results of the mechanistic studies are consistent with a reaction pathway that proceeds via an oxanickelacycle intermediate generated by the oxidative cyclization with a nickel(0) complex. In addition, we have used the η(2)-aldehyde nickel complex as an effective activator for an organosilane in order to generate a silicate reactant. These reactions show 100% atom efficiency, generate no wastes, and are conducted under mild conditions.
化学家们不再怀疑一种能够在有机合成中激活和利用醛的方法的重要性，因为许多从醛制备的产物都支持我们的日常生活。人们已经投入了巨大的努力来开发使用主族元素和过渡金属的这些方法。因此，许多有机化学家已经使用了一种活化剂-（醛氧）相互作用，即 η（1）配位，其中路易斯酸或布朗斯特酸激活醛。在配位化学领域，醛通过碳-氧双键的配位与过渡金属的 η（2）配位已经得到了很好的研究；然而，这种活化模式在过渡金属催化中很少见。鉴于 η（2）-醛络合物的独特反应性，通过这种中间体进行前所未有的反应是很有可能的。在本综述中，我们总结了我们最近在镍（0）催化的通过 η（2）-醛镍和氧杂镍环中间体的醛转化方面的研究结果。富电子镍（0）和强供电子 N-杂环卡宾（NHC）配体的组合可以充分形成 η（2）-醛络合物，其中醛通过反馈键高度活化。使用 Ni（0）/NHC 催化剂，已经开发了涉及烯烃的分子内氢酰化和醛的同/交叉二聚化（蒂希切恩反应）的过程，这两种过程都通过醛和其他 π 组分（烯烃或醛）的同时 η（2）配位进行。机理研究的结果与通过镍（0）配合物的氧化环化生成氧杂镍环中间体的反应途径一致。此外，我们还将 η（2）-醛镍络合物用作有效的有机硅烷活化剂，以生成硅酸盐反应物。这些反应具有 100%的原子效率，不产生废物，并在温和条件下进行。
