Advanced Synthesis & Catalysis ( IF 4.4 ) Pub Date : 2024-07-03 , DOI: 10.1002/adsc.202400593 Juan Carlos Pérez-Sánchez 1 , Raquel Herrera 2 , M. Concepción Gimeno 1
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Introduction
Gold homogeneous catalysis has become an indispensable tool in modern organic synthesis, garnering considerable attention for its unique reactivity and exceptional selectivity in multiple transformations.1 The use of a diverse range of ligands to fine-tune the properties of these gold(I) complexes has expanded their synthetic versatility, facilitating the customization of their catalytic performance.2 Moreover, homogeneous catalysis seems to stand incomplete without the incorporation of ferrocene. Its unique properties such as electron density, aromaticity, and reversible redox characteristics, make ferrocene a privileged scaffold.3
Despite the significant advancement of gold catalysis,4 the synergistic interplay between gold and ferrocene remains relatively underexplored.5 The incorporation of the ferrocenyl core within gold catalysts offers several potential advantages, including: a) the robustness and stability of the ligands, b) facile functionalization of the metallocene core, c) high conformational flexibility, d) facile access to chiral ferrocenyl ligands, and e) the reversible redox properties of ferrocene, enabling redox-switchable catalysis (RSC).6 The concept of RSC, pioneered by Wrighton and colleagues in 1995,7 relies on adjusting (“switching on and off”) the catalytic activity of a transition metal complex, by manipulating the electron-donating or withdrawing characteristics of a coordinated ligand. Given ferrocene versatility in exhibiting a reversible oxidation process, it frequently serves as the redox-active species in catalytic systems.6
In this context, the groups of Sarkar,8 Heinze,9 Peris10 and Hey-Hawkins11 have described a small number of gold(I) complexes bearing pendant ferrocenyl groups, demonstrating their potential as redox switchable catalysts (Scheme 1a). Two possible activation mechanisms behind this concept has been reported: a) The oxidation of the ferrocene moiety, resulting in the generation of a more electron-withdrawing ligand, thereby enhancing the electrophilicity and catalytic activity of the Au(I) center7 and b) the formation of a coordinatively unsaturated Fe(II)/Au(II) electromer, which act as a catalytically active putative Au(II) complex (Scheme 1b).9a Moreover, the described complexes are based on ferrocenyl triazole mesoionic carbenes,7 Fischer or N-heterocyclic carbenes,9, 10 and tris(ferrocenyl)arene trisphosphane cores.11 However, to the best of our knowledge the utilization of ferrocenyl imines remains unexplored (Scheme 1c). Ferrocenyl imines are considered “non-innocent” ligands, capable of inducing strong charge delocalization upon connection by π-linkers. This delocalization facilitates one-electron redox processes, wherein the single electron is delocalized throughout the entire aromatic system.12
a) Representative examples of ferrocenyl gold(I) redox switchable catalysts, b) an example of an activation mode and c) this work.
In this regard, triarylaminium cations, derived from the oxidation of triarylamines, have found extensive use as one-electron chemical oxidants due to their strong preference for simple outer-sphere single electron-transfer (SET) reactions.13 Thus, the hexachloridoantimonate salt of the tris(4-bromophenyl)aminium radical cation, known as “Magic Blue” (Scheme 1c),14 has been widely adopted both as a stoichiometric and catalytic oxidant because of its stability, ease of preparation,15 commercial availability, and reasonable oxidizing power (Ered=0.70 V vs. Fc+/Fc).16 Nonetheless, its potential applications beyond its role as an oxidant have yet to be explored.
Herein, we present the synthesis of a chloride gold(I) ferrocenyl imine-phosphane complex as another class of gold(I) redox-switchable catalyst. The efficacy of this catalyst has been evaluated, employing “Magic Blue” as an oxidant. Intriguingly, we also explored an additional role for “Magic Blue” highlighting its potential ability to activate the Au−Cl bond. To elucidate the comparative activation mechanisms of gold complexes via oxidation or halide abstraction, we contrasted the catalytic performance of this ferrocene complex, activated by oxidation, against that of the ubiquitous [AuCl(JohnPhos)] in conventional gold(I) homogeneous catalysis, using “Magic Blue” as the initiator.
中文翻译：
探索二茂铁基亚胺-膦配合物在金(I)氧化还原可切换催化中的作用以及“幻蓝”氧化剂的作用
介绍
金均相催化已成为现代有机合成中不可或缺的工具，因其独特的反应活性和多重转化中卓越的选择性而受到广泛关注。 1使用多种配体来微调这些金 (I) 配合物的性能，扩大了其合成多功能性，促进了其催化性能的定制。 2此外，如果不加入二茂铁，均相催化似乎就不完整。其独特的性质，如电子密度、芳香性和可逆氧化还原特性，使二茂铁成为一种特殊的支架。 3
尽管金催化取得了显着进步， 4金和二茂铁之间的协同相互作用仍然相对未得到充分研究。 5在金催化剂中加入二茂铁基核心具有多种潜在优势，包括：a) 配体的稳健性和稳定性，b) 茂金属核心易于官能化，c) 高构象灵活性，d) 易于获得手性二茂铁基配体，e）二茂铁的可逆氧化还原特性，实现氧化还原可切换催化（RSC）。 6 RSC 概念由 Wrighton 及其同事于 1995 年首创， 7依赖于通过操纵配位配体的供电子或吸电子特性来调整（“打开和关闭”）过渡金属络合物的催化活性。鉴于二茂铁在可逆氧化过程中的多功能性，它经常充当催化系统中的氧化还原活性物质。 6
在这方面，Sarkar、 8 Heinze、 9 Peris 10和 Hey-Hawkins 11等小组描述了少量带有二茂铁基侧基的金 (I) 配合物，证明了它们作为氧化还原可转换催化剂的潜力（方案 1a）。据报道，这一概念背后有两种可能的激活机制：a) 二茂铁部分的氧化，导致产生更多吸电子配体，从而增强 Au(I) 中心 7 的亲电性和催化活性7和 b)配位不饱和 Fe(II)/Au(II) 静电体的形成，其充当催化活性的推定 Au(II) 络合物（方案 1b）。 9a此外，所述配合物基于二茂铁基三唑介离子卡宾、 7 Fischer或N-杂环卡宾、 9、10和三(二茂铁基)芳烃三磷烷核。 11然而，据我们所知，二茂铁亚胺的利用仍未得到探索（方案 1c）。二茂铁基亚胺被认为是“非无辜”配体，能够在通过 π 连接体连接时诱导强电荷离域。这种离域促进了单电子氧化还原过程，其中单个电子在整个芳香系统中离域。 12
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a）二茂铁金（I）氧化还原可切换催化剂的代表性示例，b）活化模式的示例以及c）这项工作。
在这方面，源自三芳胺氧化的三芳基胺阳离子由于其对简单外球单电子转移（SET）反应的强烈偏好而被广泛用作单电子化学氧化剂。 13因此，三（4-溴苯基）铵自由基阳离子的六氯锑酸盐，称为“幻蓝”（方案 1c）， 14由于其稳定性、易于制备而被广泛用作化学计量和催化氧化剂， 15商业可用性和合理的氧化能力（E red =0.70 V vs. Fc + /Fc）。 16尽管如此，其作为氧化剂以外的潜在应用仍有待探索。
在此，我们提出了氯化金（I）二茂铁基亚胺-膦络合物的合成，作为另一类金（I）氧化还原可切换的催化剂。使用“魔幻蓝”作为氧化剂对该催化剂的功效进行了评估。有趣的是，我们还探索了“幻蓝”的另一个作用，强调了它激活 Au−Cl 键的潜在能力。为了阐明金络合物通过氧化或卤化物提取的比较活化机制，我们将这种通过氧化活化的二茂铁络合物的催化性能与传统金(I)均相催化中普遍存在的[AuCl(JohnPhos)]的催化性能进行了对比，使用“魔幻蓝”作为发起者。
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