Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis

Abstract

Chiral ligands play a central role in enantioselective transition-metal catalysis. The success of achiral N-heterocyclic carbenes (NHCs) as stable electron-rich neutral ligands in homogeneous catalysis led to the development of a manifold of chiral NHCs as stereodirecting ancillary ligands for various enantioselective transformations. Due to the modular design of NHCs and the ease of access to their azolium salt precursors, tailor-made NHCs are readily available. Many chiral NHC scaffolds have been synthesised and tested in catalysis. Herein, we highlight only those NHC structures which have enabled high degrees of enantioselectivity in transition-metal catalysis. Following a brief introduction to the field of chiral NHCs, this tutorial review introduces different categories of chiral NHCs and provides a guide to the structural fine-tuning of ligand requirements and stereochemical models.

Helicenes as Chirality‐Inducing Groups in Transition‐Metal Catalysis: The First Helically Chiral Olefin Metathesis Catalyst

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Helical chirality is an ovel enantioselectivity-inducing property in transition-metal-catalyzed transformations. [3,4] The former approacho ffersn ot only the advantage of ac onvenient ligand synthesis from homochiral primary amines, but also the opportunityfor tuning the electronic ligandproperties through backbone substituents. This work reports the synthesis of the first helically chiral Ru-NHC alkylidene complex from an aminohelicene-derivedi midazolium salt, which was ligated to the first generation Hoveyda-Grubbs catalyst.

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Nickel(II) Complexes with Tritopic NimineCNHCNamine Pincer Ligands

“…In particular, nickel complexes with functional NHC ligands have been widely used as pre‐catalysts for a range of reactions Although the donor groups in the side arms of the pincer architecture may not always bind firmly to the metal center because of hemilability, they can affect the course of reactions and provide anchimeric assistance.…”

A Bulky Chiral N‐Heterocyclic Carbene Nickel Catalyst Enables Enantioselective C−H Functionalizations of Indoles and Pyrroles

“…[12g-h, 15, 16] In this context, asymmetric [Ni 0 ]NHC-catalyzed processes [17] -aided in several cases by Lewis acid co-activation [18] -have enabled selective C À Hfunctionalizations of pyridones, [19] pyridines, [20] and benzimidazoles. [22] We recently reported [19,23] achiral NHC [24] family that offers large modulation opportunities based on Gawleysc arbene. Nakao,H artwig, and co-workers reported an undirected C2-functionalization of electron-rich heterocycles using an achiral nickel-NHC catalyst system for anti-Markovnikov hydroarylations.…”