N-Heterocyclic Carbenes NHCs

Carbenes in general are discussed in Chapter 11, but NHCs (4.15) are covered here because they have attained equal importance with phosphines as spectator ligands, particularly in catalysis Compared with phosphines, the range of accessible steric parameters is equally wide, but most NHCs are much stronger donors, best quantified in DFT calculations NHCs also seem to be modest tt acceptors  

NHCs are most commonly derived from A,A -disubstituted imidazo-lium salts (4.16) by deprotonation at C2 to give the free NHC, 4.15. This acts as a very powerful 2e donor, binding to a variety of ML fragments to give NHC complexes (4.17). These are often seen represented in one of two ways depending on whether we want to emphasize the carbene character of the product (4.17a) or else the alternative picture of a metal substituted arenium ring (4.17b). 

Because the M-NHC bond is so strong, the NHC does not normally dissociate from the metal. This causes problems in the case of potentially chelating NHCs, where 2 1 complexes like 4.18 can easily be formed as kinetic products in attempts to make chelates. In a similar diphosphine case, reversible dissociation/association would soon convert this 2 1 complex into the thermodynamically favored chelate, but NHCs do not rearrange if they initially form the wrong complex.  

NHC complexes can be synthesized in a wide variety of ways from a metal precursor and the imidazohum salt (Eq. 4.19 and Eq. 4.20) a free NHC (Eq. 4.21) or by transmetallation from the silver NHC complex, often conveniently available under mild conditions from the 

Other Ligand Systems 5.3.4.1 N-Heterocyclic Carbenes (NHCs) 

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Ray, L., Shaikh, M.M. and Ghosh, P. (2008) Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric (NHC)MC1 2 (M=Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand. Inorganic Chemistry, 47, 230-240. 

Addition of acyl anions generated from acylsilanes to a,(3-unsaturated ketones using N-heterocyclic carbenes (NHCs) derived from thiazolium salts as catalyst produced 1,4-diketones, which cyclized to form the corresponding furans in good yields under an acidic condition <06JOC5715>. 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

Steric effects were also found to be important for determining the reactivity of rhodium complexes containing N-heterocyclic carbene (NHC) ligands [47] (Scheme 10), which have been the subject of intense in-... 

The KR of aryl alkyl sec-alcohols using various chiral nucleophilic N-heterocyclic carbenes (NHCs) has also recently been achieved by the groups of Suzuki [125, 126] and Maruolca [127]. These studies build on an emerging body of information showing that achiral NHCs are extremely efficient nucleophilic organocatalysts for transesterification [128]. The levels of selectivity achieved by Maruoka using the Cj-symmetric NHC 29b for the KR of aryl alkyl sec-alcohols (and two allylic alcohols) lie in the range 16 to 80 (Scheme 8.8). 

Instead, it is the goal of this chapter to highlight some conceptually novel developments in the field of HAT reactions. We will concentrate on complexes of borane with N-heterocyclic carbenes (NHCs), on organometallic hydrogen atom donors in stoichiometric and catalytic reactions, and on the activation of water and alcohols for HAT. 

Abstract N-Heterocyclic carbene (NHC) has become a major ligand class and has proven to be more than just a phosphine mimic . Some important features like electronic and steric properties are discussed and typical examples of NHC are given herein. 

Abstract N-heterocyclic carbenes (NHCs) have attracted increasing attention since their discovery. Notably, they have allowed for major advances in palladium-catalyzed reactions. Mainly known for their application in cross-coupling reactions, this review intends to provide a broader overview of (NHC)-palladium systems in organic transformations. 

N-heterocyclic carbenes (NHC) were first reported by Wanzlick [5,6] in the 1960s and (NHC)-transition metal complexes have been known since 1968 [7-10]. But it was not until the early 1990s that Arduengo and co-... 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

Due to their topological and electronical versatility, N-heterocyclic carbenes (NHCs) are an increasingly useful type of ligand in transition metal compounds. The first NHC complexes were described by Ofele and Wanzlick in... 

With the application of N-heterocyclic carbene (NHC) ligands, the number of transition metal-catalyzed reactions has grown considerably in the past decade. The replacement of traditional amine or phosphine ligands with electron-rich NHC ligands has led to a substantial enhancement in catalytic activity. This chapter summarizes the recent impact that the use of NHC ligands has had in furthering the field of transition metal-mediated catalysis. 

Neutral or cationic Grubbs-type complexes bearing an alkylidene fragment and either phosphine, N-heterocyclic carbene (NHC), or Schiff base ligands. 

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