Ruthenium benzylidene complexes

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

Second-generation ruthenium-carbene complex, (tricyclohexylphosphine-[l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium (IV) dichloride),4e was purchased from Strem Chemicals. 

Later Grubbs discovered ruthenium carbene complex and used it for a metathesis reaction to synthesize cyclic compounds 5a-d [Eqs. (6.4) and (6.5)]. In 1995, Grubbs found that ruthenium benzylidene carbene complex Ic," which is now commercially available, has the same reactivity as that of lb. Many researchers have therefore used this complex for olefin metathesis, and this reaction has been useful for the synthesis of carbo- and heterocyclic compounds and fused bicyclic compounds [Eq. [6.6)] °... 

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). 

Thanks to the development of the Grubbs benzylidene catalyst (2) and other related ruthenium complexes, olefin metathesis has experienced spectacular advances over the past 10 years. The various incarnations of the reaction (acyclic diene metathesis, ring-closing metathesis, ring-opening metathesis polymerization, etc.) have now acquired first rank importance in synthesis. Clearly, the emergence of a similar, generic, efficient catalytic system for con-... 

Enyne derived from ditosyl o-phenylenediamine 257 formed in the presence of benzylidene ruthenium carbene complex a nine-membered ring 258 in 5% yield (Equation 30) <20000L543, 2001S654>. Dimerization was a major by-process (22% yield) along with formation of a small amount of 259 (5% yield), which was explained by /3-hydride elimination from the intermediary ruthenacyclobutane. 

The fate of the benzylidene fragment in these ruthenium complexes is another matter of debate [20a, 25]. The thermal stability of benzylidene complexes 4-6 was tested at 85 °C, under conditions mimicking polymerisation of methyl methacrylate. As monitored by H NMR, complete disappearance of the benzylidene fragment of the mixed phosphine/Af-heterocyclic carbene complex 5 (R = Cy, R = (5j-CHMePh) was observed within 20 min, whereas the Grubbs complex, RuCl2(=CHPh)(PCy3)2, showed only 55 % decomposition, and the bis-A/ -heterocyclic carbene ruthenium complex 6 (R = Cy) 88 % decomposition over the same time interval (Figure 6). 

DFT calculations have shown that the experimentally observed decomposition pathway likely occurs through insertion of the benzylidene into the chelating Ru-C bond. The computed free-energy profile for the decomposition of complex 22 is shown in Figure 7.21. Insertion of the alkylidene into the chelating ruthenium-carbon(adamantyl) bond required 29.7 kcalmoC and formed alkyl ruthenium complex 28. Complex 28 then underwent facile fi-hydride elimination to form ruthenium hydride 30, which then converted to the q -bound olefin complex 32 and eventually the dimer complex 26. The a-hydride ehmination pathway from intermediate 28 via 33-ts required much higher activation energy than the fi-hydride elimination. 

Binuclear complexes with bridging trimethyltriazoldiylidene ligands were also used in TH and p-alkylation reactions. Thus, complexes 76 and 77 were applied for TH of ketones and imines in i-PrOH/KOH (Figure 13.11). The reductions of ketones and A -benzylidene anilines were complete within 1 h. Related 78 and 79 ruthenium complexes were investigated as catalysts in the p-alkylation of secondary alcohols with primary alcohols. Such catalytic systems were very selective and more active than the previously reported catalysts. ... 

Benzyl alcohol, oxidation, by ruthenium oxo complexes, 39 287-289 Benzylazide, NMR of, 4 263 Benzylidene diacetate, nitration of, 6 114 Berkelium... 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

Mono- and bicyclic phosphorus heterocycles 199, 200, 202, and 203 were synthesized starting from the bifunctional phosphorylating agent bis(diisopropylamino)ethynyl phosphine 195 via ring-closing enyne metathesis using 4,5-dihydroimidazol-2-ylidene ruthenium benzylidene complex, as a catalyst. Bicyclic phosphorus oxides 199 were obtained in 66-83% yield, whereas phosphorus borane derivative 202 was isolated in 74% yield <2001TL8231>. 

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