N-heterocyclic carbene ligands and

Second-Generation Ruthenium Indenylidene Catalysts Bearing a N-Heterocyclic Carbene Ligand and a Phosphite Ligand... 

M. R. Chaulagain, G. J. Sormunene, J. Montgomery, J. Am. Chem. Soc. 2007, 129, 9568-9569. New N-heterocyclic carbene ligand and its application in asymmetric nickel-catalyzed aldehyde/alkyne reductive couplings. 

Bonnet LG, Douthwaite RE, Hodgson R. Synthesis of constrained-geometry chiral di-N-heterocyclic carbene ligands and their silver(I) and paUadium(II) complexes. 

Sluijter SN, Warsink S, Lutz M, Elsevier CJ. Synthesis ofpaUadium(0) and -(II) complexes with chelating bis (N-heterocyclic carbene) ligands and their application in semi hydrogenation. Dalton Trans. 2013 42 7365-7372. 

Zanardi A, MataJA, Peris E. A simple route to chelating, structurally different triazole-based bis(N-heterocyclic carbene) ligands and their coordination to Pt . Eur J Inorg Chem. 2011 2011 416-421. 

It has been speculated in the past that it might be possible to isolate the first Au(I) fluoride LAuF [182], if disproportionation into metallic gold and Au( 111) can be avoided by stabilizing ligands L, such as (PR3)3AuF [264]. This has just been achieved. Laitar et al. [185] were able to isolate a compound with an N-heterocyclic carbene ligand... 

A vast number of derivatives of these general types have been prepared by similar routes for catalytic applications, and at this point we can do no more than provide a series of recent references some have P-donor ligands [24], some have N-heterocyclic carbenes [25], and others have mixed donors [26]. 

Tejel C, Ciriano MA (2007) Catalysis and Organometallic Chemistry of Rhodium and Iridium in the Oxidation of Organic Substrates. 22 97-124 Tekavec TN, Louie J (2006) Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions). 21 159-192 Tesevic V, see Gladysz JA (2008) 23 67-89... 

Iridium(l) precursors [lr(cod)(L)] with bidentate N-heterocyclic carbene ligands L3 appeared slightly less active in the hydrosilylation of acetophenone with diphe-nylsilane than did the similar rhodium complexes, giving respectively yields of 85% of I and 15% of II for the Pr substituent, and 83% of I and 17% of II for the benzyl moiety, after 2 h reaction at room temperature [47]. However, when carbene ligands of type L3 were used a significant increase in the ee-value of the sec-phenethyl alcohol R isomer of up to 60% was observed. 

An N-heterocyclic carbene ligand, formed from l,3-bis-(2,4,6-trimethylphenyl)-3//-imidazol-l-ium chloride and cesium carbonate, with dipalladium tris(dibenzylideneacetone) gave excellent yields (93-96% yields) in the Suzuki coupling of 2-chloropurines and arylboronic acids in anhydrous dioxane <2001TL8751>. The combination of an imidazolium-carbene and nickel(O) bis(cyclooctadiene) formed a catalyst capable of insertion into the C-F bond of 6-fluoropurine nucleosides (Scheme 35) <20050L1149>. 

In 2001, Grubbs and co-workers reported a class of Ru catalysts [(cf. 90, Eq. (2)] [30] that bear a chiral monodentate N-heterocyclic carbene ligand [31], The re-... 

Scarborough et al. developed a number of seven-membered N-heterocyclic carbene ligands (based on 2,2 -diaminobiphenyl) and characterized several of the transition metal complexes formed with these ligands via X-ray crystallography <2005JOM6143, 2005AGE5269>. Furthermore, numerous papers presented the X-ray structures as proof for the formation of a type of compound without going into an in-depth discussion of the structural features of the compound itself. 

In a report by Ozdemir et al. <2005SL2394>, tetrahydrodiazepinium salts 63 (Equation 8) are used as precursors to carbene ligands to be used in Suzuki reactions with aryl chlorides in an aqueous media. In order to prepare these N-heterocyclic carbene ligands, the diamines 62 were reacted with ammonium chloride and triethyl orthoformate. The catalysts prepared from these ligands are stable to air. 

Grubbs and co-workers have recently reported a new class of Ru catalysts (83, Eq 2) [29] that bear a chiral monodentate N-heterocyclic carbene ligand [30]. The reactions illustrated in Eq 2 include the highest ee reported (13-90% ee) asymmetric induction is clearly dependent on the degree of olefin substitution (cf. Schemes 18 and 4 for comparison with the Mo-catalyzed reactions of the same substrates). As is the case with nearly catalytic enantioselective reactions [4], the identity of the optimal catalyst depends on the substrate a number of chiral... 

Adlhart and Chen [126] reported a QM/MM study of the olefin-metathesis reaction catalyzed by [L2(Cl)2Ru=CHPh] complexes L are phosphane groups in the case of the first-generation catalyst, whereas for the second-generation catalysts an N-heterocyclic carbene ligand replaces one of the phosphanes. A study of first- and second-generation metathesis catalysts... 

Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions)... 

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

The case studies below will provide selected examples of calorimetric studies carried out on metal-ligand exchange reactions of phosphorous-based ligands (such as phosphines, phosphite, A -pyrrolyl-phosphines) and N-heterocyclic carbene ligands (NHCs) as well as addition reactions relevant to catalytic transformations, involving metals such as ruthenium, rhodium,platinum,and molybdenum. ... 

The chemistry of zerovalent nickel complexes will be discussed in Section 4. One of the most exciting developments in this area is the recent introduction of N-heterocyclic carbene ligands. Ni complexes of these hgands have been shown to be important catalysts and reagents for a number of important organic transformations. This section wiU describe briefly the practical methods for preparing Ni-carbene complexes and will outline some of the catalyhc reactions promoted by these complexes. This section wih also describe the preparation and reactivihes of other types of Ni(0) complexes. 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

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