Ligands N-heterocyclic carbene

Ruthenium Precatalysts with N-Heterocyclic Carbene Ligands.238... 

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 mononuclear nickel hydride complex with three N-heterocyclic carbene ligands has been reported the compound was formed by oxidative addition of an imidazolium salt to the Ni(0) bis(carbene) complex [19]. The hydride signal of this nickel(II) complex appears at -15 ppm. 

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... 

The catalytic activity of Cp Ir(III) complexes in the Oppenauer-type oxidation of alcohols was considerably enhanced by the introduction of N-heterocyclic carbene ligands. Here, high turnover numbers (TONs) of up to 950 were achieved in the oxidation of secondary alcohols [40]. 

In asymmetric ketone hydrosilylation, axially chiral bidentate N-heterocyclic carbene ligands derived from BINAM proved to be moderately effective when bound to iridium, but less so with rhodium (Scheme 3.19) [44]. 

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. 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

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>. 

Alkylidene Complexes of Ruthenium Containing N-Heterocyclic Carbene Ligands... 

The polymerization of cyclooctene shows a pronounced dependence of the N-heterocyclic carbene ligand, due to steric effects. 

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... 

Fig-i Basic structural unit of N-heterocyclic carbene ligands bearing chiral N-sub-stituents... 

Finally, two chiral monodentate N-heterocyclic carbene ligands that contain an oxazoline unit have been reported. Glorius et al. reported the synthesis of the imidazolium salts 76 by cyclizing the corresponding bisoxazolines 75 (Scheme 50) [133]. 

Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands 171... 

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