Ackermann Lutz
Institut für Organische und Biomolekulare Chemie , Georg-August-Universität , Tammannstrasse 2 , 37077 Göttingen , Germany.
Acc Chem Res. 2020 Jan 21;53(1):84-104. doi: 10.1021/acs.accounts.9b00510. Epub 2019 Dec 19.
To improve the efficacy of molecular syntheses, researchers wish to capitalize upon the selective modification of otherwise inert C-H bonds. The past two decades have witnessed considerable advances in coordination chemistry that have set the stage for transformative tools for C-H functionalizations. Particularly, oxidative C-H/C-H and C-H/Het-H transformations have gained major attention because they avoid all elements of substrate prefunctionalization. Despite considerable advances, oxidative C-H activations have been dominated by precious transition metal catalysts based on palladium, ruthenium, iridium, and rhodium, thus compromising the sustainable nature of the overall C-H activation approach. The same holds true for the predominant use of stoichiometric chemical oxidants for the regeneration of the active catalyst, prominently featuring hypervalent iodine(III), copper(II), and silver(I) oxidants. Thereby, stoichiometric quantities of undesired byproducts are generated, which are preventive for applications of C-H activation on scale. In contrast, the elegant merger of homogeneous metal-catalyzed C-H activation with molecular electrosynthesis bears the unique power to achieve outstanding levels of oxidant and resource economy. Thus, in contrast to classical electrosyntheses by substrate control, metalla-electrocatalysis holds huge and largely untapped potential for oxidative C-H activations with unmet site selectivities by means of catalyst control. While indirect electrolysis using precious palladium complexes has been realized, less toxic and less expensive base metal catalysts feature distinct beneficial assets toward sustainable resource economy. In this Account, I summarize the emergence of electrocatalyzed C-H activation by earth-abundant 3d base metals and beyond, with a topical focus on contributions from our laboratories through November 2019. Thus, cobalt electrocatalysis was identified as a particularly powerful platform for a wealth of C-H transformations, including C-H oxygenations and C-H nitrogenations as well as C-H activations with alkynes, alkenes, allenes, isocyanides, and carbon monoxide, among others. As complementary tools, catalysts based on nickel, copper, and very recently iron have been devised for metalla-electrocatalyzed C-H activations. Key to success were detailed mechanistic insights, prominently featuring oxidation-induced reductive elimination scenarios. Likewise, the development of methods that make use of weak -coordination benefited from crucial insights into the catalyst's modes of action by experiment, in operando spectroscopy, and computation. Overall, metalla-electrocatalyzed C-H activations have thereby set the stage for molecular syntheses with unique levels of resource economy. These electrooxidative C-H transformations overall avoid the use of chemical oxidants and are frequently characterized by improved chemoselectivities. Hence, the ability to dial in the redox potential at the minimum level required for the desired transformation renders electrocatalysis an ideal platform for the functionalization of structurally complex molecules with sensitive functional groups. This strategy was, inter alia, successfully applied to scale-up by continuous flow and the step-economical assembly of polycyclic aromatic hydrocarbons.
为提高分子合成的效率,研究人员希望利用对原本惰性的C-H键进行选择性修饰。在过去二十年中,配位化学取得了长足进展,为C-H官能化的变革性工具奠定了基础。特别是,氧化C-H/C-H和C-H/Het-H转化受到了广泛关注,因为它们避免了底物预官能化的所有环节。尽管取得了显著进展,但氧化C-H活化一直由基于钯、钌、铱和铑的贵金属过渡金属催化剂主导,这损害了整体C-H活化方法的可持续性。对于使用化学计量的化学氧化剂来再生活性催化剂的主要情况也是如此,其中高价碘(III)、铜(II)和银(I)氧化剂最为突出。由此产生了化学计量的不需要的副产物,这阻碍了C-H活化在大规模应用中的使用。相比之下,均相金属催化的C-H活化与分子电合成的巧妙结合具有实现出色的氧化剂和资源经济性的独特能力。因此,与通过底物控制的经典电合成不同,金属电催化在通过催化剂控制实现具有未满足的位点选择性的氧化C-H活化方面具有巨大且尚未充分开发的潜力。虽然已经实现了使用贵金属钯配合物的间接电解,但毒性较小且成本较低的贱金属催化剂在实现可持续资源经济性方面具有明显的优势。在本综述中,我总结了由储量丰富的3d贱金属及其他金属实现的电催化C-H活化的出现,特别关注截至2019年11月我们实验室的贡献。因此,钴电催化被认为是一个特别强大的平台,可用于多种C-H转化,包括C-H氧化、C-H氮化以及与炔烃、烯烃、丙二烯、异腈和一氧化碳等的C-H活化等。作为补充工具,已经设计了基于镍、铜以及最近的铁的催化剂用于金属电催化的C-H活化。成功的关键在于详细的机理见解,其中氧化诱导的还原消除情况最为突出。同样,利用弱配位的方法的发展得益于通过实验、原位光谱和计算对催化剂作用模式的关键见解。总体而言,金属电催化的C-H活化从而为具有独特资源经济性水平的分子合成奠定了基础。这些电氧化C-H转化总体上避免了化学氧化剂的使用,并且通常具有提高的化学选择性。因此,能够将氧化还原电位调节到所需转化所需的最低水平,使电催化成为用于对具有敏感官能团的结构复杂分子进行官能化的理想平台。该策略尤其成功地应用于通过连续流动进行放大以及多环芳烃的步骤经济性组装。