N-Heterocyclic Carbene Complexes in C–H Activation Reactions

Abstract

In this contribution, we provide a comprehensive overview of C−H activation methods promoted by NHC−transition metal complexes, covering the literature since 2002 (the year of the first report on metal−NHC-catalyzed C−H activation) through June 2019, focusing on both NHC ligands and C−H activation methods. This review covers C−H activation reactions catalyzed by group 8 to 11 NHC−metal complexes. Through discussing the role of NHC ligands in promoting challenging C−H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon−carbon and carbon−heteroatom bonds by directly engaging ubiquitous C−H bonds.

Suzuki–Miyaura Cross-Coupling of Amides Using Well-Defined, Air- and Moisture-Stable Nickel/NHC (NHC = N-Heterocyclic Carbene) Complexes

“…Reaction profile studies demonstrated that these reactions are complete in < 1 h at 80 • C. In a broader context, the present method should be evaluated in comparison with other known approaches to biaryl ketones from amides [3][4][5][6][7][8][9][10] and acyl electrophiles [15]. The use of Ni catalysis [1-3] and the beneficial performance of Ni-NHC complexes [25][26][27][28][29] may accelerate the development of new approaches to activating amide bonds. Considering the utility of nickel catalysis in activation of unreactive bonds, we anticipate that [CpNi(NHC)Cl] complexes will be of interest in activation of bench-stable acyl electrophiles.…”

“…In this Special Issue on N-Heterocyclic Carbenes and Their Complexes in Catalysis, we report the first example of Suzuki-Miyaura cross-coupling of amides catalyzed by well-defined, air-and moisture-stable nickel/NHC (NHC = N-heterocyclic carbene) complexes ( Figure 1). We were attracted to the recent elegant advances made in the design of half-sandwich, cyclopentadienyl [CpNi(NHC)X] practical [CpNi(NHC)Cl] precatalysts [25][26][27][28][29][30][31] are capable of selective activation of amide N-C(O) bonds. The following features of our study are noteworthy: (1) The reaction represents, to the best of our knowledge, the first example of acyl-type cross-coupling achieved by half-sandwich [CpNi(NHC)X] complexes.…”

Towards the Preparation of Stable Cyclic Amino(ylide)Carbenes

“…Since the isolation of the first stable singlet carbenes by Bertrand and Arduengo [1] 30 years ago, these divalent carbon ligands have led to a revolution in coordination chemistry and produced most remarkable developments in various fields of chemistry. Owing to their high donor strength, they are applied as potent ligands in transition metal catalysis [2][3][4][5] (e.g., in the second generation Grubbs catalyst [6]), in organo catalysis [2,[7][8][9][10], or main group chemistry [2,11,12], such as for the stabilization of reactive [2,12], electron-deficient [2,12], or low-valent species [2,12]. The success of carbenes is mostly based on their thermal stability and high donor capacity, which usually surpasses that of phosphines or amines.…”

“…This has led to drastic changes in the carbene properties and hence to a myriad of new fields of applications. For example, in a landmark report, Bertrand and coworkers demonstrated that the replacement of one nitrogen substituent in an NHC by a sp 3 -carbon substituent to form acyclic and cyclic alkyl(amino)carbenes (CAAC, B) significantly increases the donor and acceptor properties of these compounds [18]. This results in a sufficiently small HOMO-LUMO (highest molecular orbital, lowest molecular orbital, respectively) gap to allow for straightforward dihydrogen activation [19].…”

Rate-Limiting Steps in the Intramolecular C–H Activation of Ruthenium N-Heterocyclic Carbene Complexes

“… 27 The phenomenon of C–H activation is primarily characteristic for NHC ligands, during which the electronic properties and catalytic activity of the NHCs are altered. 28 , 29 The advantage of a catalyst performing subsequent hydrogenation and C–H activation is the possibility to use it in more complex tandem reactions. In the case of ruthenium complexes, the best known example of such activity is the tandem alcohol dehydrogenation/Wittig reaction/alkene hydrogenation reaction performed by the Williams catalyst.…”