In the past two decades, NHCs (NHC = N-heterocyclic carbenes) have emerged as one of the most important classes of ligands in transition metal catalysis. Although historically 5-membered carbenes, such as imidazol-2-ylidenes, have been the dominant class of NHC ligands in transition metal catalysis, recently, ring expanded carbenes on 6-, 7-, 8-, 9- and 10-membered ring systems have been shown to provide significant advantages in numerous catalytic reactions embracing various metals, transformations and mechanisms. In this comprehensive review, we provide a survey of the applications of ring-expanded N-heterocyclic carbenes (reNHCs) in transition metal catalysis. Spanning from the initial reports on ring-expanded NHCs through June 2023, the review centers on the types of ring-expanded NHCs, their organometallic complexes and applications in catalytic processes. By delving into the impact of ring-expanded NHC ligands, the review aims to provide the reader with an understanding of the significance of this ligand class and its pivotal role in transition metal catalysis and organometallic chemistry.

中文翻译：

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扩环 N-杂环卡宾 (reNHC) 配合物：在过渡金属催化中的应用

在过去的二十年中，NHC（NHC = N-杂环卡宾）已成为过渡金属催化中最重要的配体类别之一。尽管历史上 5 元卡宾（例如咪唑-2-亚基）一直是过渡金属催化中 NHC 配体的主要类别，但最近，在 6-、7-、8-、9- 和 10- 元环上扩环卡宾系统已被证明在包括各种金属、转化和机制的众多催化反应中提供了显着的优势。在这篇综合综述中，我们对扩环 N-杂环卡宾 (reNHC) 在过渡金属催化中的应用进行了调查。从扩环 NHC 的初步报告到 2023 年 6 月，审查的重点是扩环 NHC 的类型、其有机金属配合物以及在催化过程中的应用。通过深入研究扩环 NHC 配体的影响，该综述旨在让读者了解此类配体的重要性及其在过渡金属催化和有机金属化学中的关键作用。