Ruthenium-NHC catalysts

Fig. 4 NHC-ruthenium catalysts with chelating benzylidene ligand...

A variety of NHC-ruthenium catalysts exhibiting different coordination spheres were synthesized, for example Schiff base (see complex 28) [77], arene (see complex 29) [78], and metallic moieties (see complexes 30) [7,79, 80] (Fig. 5) and show activity in RCM and ROMP. 

Fig. 8 Water-soluble and dichloroimidazole-derived NHC-ruthenium catalysts...

The unique power of Hoveyda s recyclable ruthenium catalyst D in RCM with electron-deficient and sterically demanding substrates is illustrated in Honda s total synthesis of the simple marine lactone (-)-malyngolide (54), which contains a chiral quaternary carbon center (Scheme 10) [35]. Attempted RCM of diene 52 with 5 mol% of NHC catalyst C for 15 h produced the desired... 

The CM reaction between 2-methyl-2-butene (a gera-disubstituted olefin that served in this case also as solvent) and the allylated compound 300, possessing the bicyclo[3.3.1]nonane core of the potential Alzheimer therapeutic garsubellin A (302) [137], underlines the increased activity of the second-generation ruthenium catalysts (Scheme 58). In the presence of 10 mol% of NHC catalyst C, the prenylated compound 301 was formed after only 2 h in 88% yield. 

Allylic alkylations of cinnamyl carbonate by sodium malonate have been studied with a series of ruthenium catalysts, obtained from the azohum salts 126-128 and the ruthenium complex 129 (Scheme 2.25) in MeCN or THF to give moderate yields of mixtures of alkylated products in the allylic and ipi o-carbons (90 10 to 65 35). The observed regioselectivity is inferior to similar ruthenium systems with non-NHC co-ligands. The stereoelectronic factors which govern the observed regioselectivity were not apparent [102]. 

Fig. 3.10 Ruthenium catalysts with mono-ortho-substituted NHCs...

Unfortunately, complexes 39 and 40 are still more prone to decomposition than catalyst 16. Therefore, Grubbs sought to investigate a series of new ruthenium catalysts bearing NHCs with varying degrees of iV-heterocyclic backbone and aryl side chain substitution, and catalysts 16 and 30a were chosen as basic catalyst structures [57]. In 2009, complexes 41a-c and 42a-c were prepared to attempt to understand how the degree of substitution on the backbone influences catalyst activity and lifetime (Fig. 3.15). 

Nitrene addition to alkenes can be aided by the nse of a transition metal, such as copper, rhodium, ruthenium, iron, cobalt, etc. NHC-Cu catalysts have been used in nitrene addition. For example [Cu(DBM)(IPr)] 147 (DBM = dibenzoyl-methane) was successfully employed in the aziridination of aliphatic alkenes 144 in presence of trichloroethylsulfamate ester 145 and iodosobenzene 146 (Scheme 5.38) [43]. 

Since the discovery of catalysts 2 and 3 containing one NHC ligand, the attractive family of NHC-ruthenium complexes has been rapidly expanded. In the following section, the different structural modifications of complexes 2 and 3 reported in the literature will be presented (phosphine and halide ligands, benzylidene ligand, NHC ligand). 

Fig. 6 Ruthenium catalysts with different sterically demanding aryl groups on the NHC...

Fig. 9 Ruthenium catalysts with six- and four-membered ring NHCs 6...

Scheme 8.23 Preparation of Herrmann s bis(NHC) ruthenium(II) complexes 84 and the mono(NHC) complexes 85 and 86 from the first generation Grubbs catalyst 83b (X = H) as the precursor...

Anon. NHC ruthenium complexes as second generation Grubbs catalysts. Synlett 2003, 423-424. 

Several groups reported the reachons of catalyst 1 with a variety of NHC ligands [96-99]. The use of derivatives containing relahvely small A-substituents (cyclohexyl, isopropyl) led to the formation of both the mono and bis(NHC) ruthenium products (Fig. 4.36a) [97]. However, the reaction of an excess of 1,3-... 

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