JV-Heterocyclic carbenes

Table 3 Ruthenium alkylidene complexes with JV-heterocyclic carbene ligands...

An jV-heterocyclic carbene 110 catalyzed the rearrangement of the 0-acyl carbonates 109 into their corresponding C-acylated isomers 111, generating a C-C bond and a quaternary stereocenter with high efficiency <06OL3785> The same reaction can be performed enantioselectively using TADMAP 112 <06JA925>. 

In the initial study, which focused on palladium-catalyzed cross-couplings of primary alkyl bromides with vinylstannanes, use of P(t-Bu)2Me provided the best yields (solvent=THF) [27]. PCy3 was slightly less effective, whereas a variety of other ligands, including JV-heterocyclic carbenes, were essentially ineffective. The choice of activator (Me4NF) and the presence of molecular sieves were important for the success of these reactions. Some examples of Pd/P(t-Bu)2Me-catalyzed Stille couplings of alkyl bromides with vinylstannanes are provided in entries 1-3 of Table 6. 

The Heck reaction performed in ILs has various advantages compared with all previously described molecular solvents, such as catalytic efficiency for the vinylation of chloroarenes, improved thermal catalyst stability and lifetime during the reaction. In the reactions performed in 1,3-dialkylimidazolium ILs, jV-heterocyclic carbene complexes of palladium can be formed in situ. These palladium-carbene complexes are formed from the deprotonation of the imidazolium cation in the presence of the catalyst precursor. ... 

Chiral iV-heterocyclic ligands have been employed in the design and development of new stereoselective carbene catalysts with applications in organic synthesis [35]. During recent years, several medicinal applications have also been reported for JV-heterocyclic carbenes (NHC) (Figure 3.2c) [36-39] including anticancer therapy, although few studies have been carried out on the antitumor activity of chirally pure isomers. 

It has been eight years since the first monograph on JV-heterocyclic carbenes (NHC) appeared [1]. In this rather short timespan the uses of NHCs in their many incarnations have contributed to advances in numerous areas of synthetic chemistry. These initial curiosities [2] have become workhorses in synthesis and homogeneous catalysis [3]. Catalytic studies nowadays almost always include the testing of a NHC or NHC precursor as part of a ligand/catalyst screening. 

In order to increase the binding between the ancillary ligand and platinum(O), we turned our attention to an emerging class of more robust o-donor ligands -the JV-heterocyclic carbenes. In analogy to the preparation of the phosphine derivatives 13, the addition of various NHC carbenes (generated in situ by deprotonation of the corresponding imidazolium salts) to the Karstedt catalyst (1) enabled the preparation of a new family of platinum(O) complexes 15 (Table 5.2) [14,15]. Unlike the Karstedt catalyst (1) or the phosphine complexes 13, these readily available crystalline compounds are insensitive to air and moisture, are bench-stable for extended periods of time, and are easy to handle. 

JV-Heterocyclic carbene-metal complexes are powerful synthetic tools. The development of a catalyst can be problematic, and different methodologies to prepare such complexes have been reported. The most common methodology requires the generation of the free carbene followed by binding to a metal center via ligand substitution or dimer scission reaction. A direct and straightforward process via NHC transfer can be accomplished using silver salts. Indeed, the... 

Over the past 10 years, studies on early-transition metal complexes comprising JV-heterocyclic carbene ligands have certainly established this class of compounds as a stable and viable species. While the coordination and knowledge of the structural patterns of such derivatives have witnessed significant developments, the potential applications of early-transition metal NHC complexes remain in their infancy. Most strikingly, the use of such compoimds in homogeneous... 

With Organocatalysts Gaunt s group [75] in 2013 reported an organocatalytic C-H bond arylation of aldehydes with diaryliodonium salts using commercial NHC (JV-heterocyclic carbene) catalysts... 

Scheme 2.10 Reaction of ethyl trans-4-oxo-2-butenoate with unactivated imines catalysed by a combination of a chiral JV-heterocyclic carbene and o-chlorobenzoic acid.

Scheme 2.35 Three-component tandem reaction catalysed by chiral diphenylpro-linol trimethylsilyl ether and an JV-heterocyclic carbene.

Scheme 7.14 Annulation reaction catalysed by chiral JV-heterocyclic carbene catalysis and titanium catalysis.

Scheme 3.45 Stereoselective protonation of catalytically generated chiral enolates by means of chiral JV-heterocyclic carbenes...

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