A -Heterocyclic carbene ligands

Since the successful exploration of silver(i) oxide usage as a multifunctional precursor for the synthesis of silver(i) A-heterocyclic carbene complexes, there has been an increasing number of reports related to silver(i) A-heterocyclic carbene chemistry. Silver(i) oxide can act as a weak base to deprotonate imidazolium salts, generating the free A-heterocyclic carbene ligands in situ, which then forms the silver(i) carbene complexes readily. This reaction can take place in the presence of air and moisture, and as a result, no special treatment in regard to the solvents has to be undertaken. More importantly, its basicity is rather specific toward the deprotonation at the G2 position of the imidazole moiety. Exploration of using silver(i) carbonate as a milder precursor in place of silver(i) oxide has also been pursued, but longer reaction times are usually required. 

Imine-functionalized mono-A-heterocyclic carbene ligands were complexed to the silver(i) center to form the corresponding [Ag(carbene)2][AgX2] complexes 41-43 (Figure 18).88-90... 

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Sames et al. have reported the intramolecular cyclization of alkene-amide substrates catalyzed by [Ir(COE)2Cl]2 and the A-heterocyclic carbene ligand, N, A -bis-(2,6-diisopropylphenyl)-imidazolyl, via olefin insertion following oxidative addition of an sp C-H bond (Scheme 11) [117]. 

Title Alkylidene Complexes of Ruthenium Containing A-Heterocyclic Carbene Ligands Use as Highly Active, Selective Catalysts for Olefin Metathesis... 

Monomethyl pyrazolato complexes, with Ti(IV), 4, 335 Mono(A-heterocyclic carbene) ligands, in silver(I) complexes, 2, 210... 

Altenhoff G, Goddard R, Lehmann CW, Glorius F (2004) Sterically demanding, bisoxazoline-derived A-heterocyclic carbene ligands with restricted flexibility for catalysis. J Am Chem Soc 126 15195-15201... 

Glorius F, Altenhoff G, Goddard R, Lehmann C (2002) Oxazolines as chiral building blocks for imidazolium salts and A-heterocyclic carbene ligands. Chem. Comm 2704-2705... 

The first examples of compounds with an A-heterocyclic carbene ligand coordinated to a low-valent uranium center were reported in 2004. The synthetic procedures are illustrated in Schemes 7 and 8. Notably, the silylamide derivative (Scheme 8) was isolated in the form of dark blue crystals in excellent yield (>90%). DFT studies indicated a significant degree of 7t-bonding in the U(m) carbene entity.32... 

Kiindig and coworkers have studied a variety of chiral A-heterocyclic carbene ligands, e.g., 49, for the synthesis of all-carbon or heteroatom containing (methoxy or amino) quaternary centers at oxindole C3 [33, 34]. For example, oxindole 50 was prepared via asymmetric intramolecular cyclization of the corresponding o-bromoanilide 48 in 94% ee (Scheme 14). 

Nickel-catalyzed Kumada-Gorriu cross-coupling reactions of a variety of monofluorinated aromatics have been aceomplished using nickel(O) complexes bearing the A-heterocyclic carbene ligand l,3-di(2 6 -diisopropylphenyl)imi-dazolin-2-ylidene (Scheme 42). The catalysts can also be prepared in situ from [Ni(acac>2] and an imidazolium salt. 

Thus, we developed successfully a stable well-defined catalytic hybrid nanomaterial that is highly active in the metathesis reaction of functionalized olefins. This material was achieved by generating well-defined ruthenium-A -heterocyclic carbene ligands (Ru-NHC) supported systems perfectly dispersed in a mesoporous hybrid material and farther stabilized by surface ligands (Scheme 12.20). 

Equally significant are the last two catalyst systems shown in Scheme 6. As mentioned above, strongly basic ligands promote the activity of ruthenium-based metathesis systems of the type defined by 13 as such, the discovery by Herrmann and co-workers in 1998 that a modified variant of 13 bearing more strongly a-donat-ing A(-heterocyclic carbene ligands (15, Scheme 6) could initiate metathesis is quite important. " Despite this potentially beneficial feature, however, the overall metathesis activity of 15 is only a... 

Lee and coworkers also prepared beads with A-heterocyclic carbene ligands located on the outer shell. In this preparation, they used beads formed in a suspension polymerization [8, 9]. 

The related cycloaddition of diynes with carbon dioxide has also been developed as an efficient entry to pyrones (Scheme 3-29). As observed with isocyanate couplings, initial reports described the use of phosphine complexes of Ni(0), " but more recent work with A-heterocyclic carbene ligands demonstrated considerably milder conditions and broader scope. " Additionally, the cycloaddition of diynes with nitriles efficiently proceeds to generate pyridine derivatives under similar conditions. ... 

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