Ruthenium complexes, reactions catalytic activity

In 2007, synthesis and complexation of a PSiP-pincer hgand, in which a sihcon atom and two phosphorus atoms are tethered by a phenylene group, was first reported by Turculet and coworkers [11-19]. They reported that the PSiP-ruthenium complex exhibited catalytic activity for transfer hydrogenation of ketones [11], and the PSiP-platinum and -palladium complexes efficiently catalyzed reduction of COj to methane by silanes [18[. Shortly after the Turculefs first report in 2007, we reported the first example of utilization of the phenylene-bridged PSiP-pincer complex in carbon-carbon bond formation reactions of unsaturated hydrocarbons [20[. 

We have demonstrated the ACP reaction catalyzed by Ru Pybox complexes. The catalytic activity of ruthenium complexes is commonly not strong. Nevertheless, ruthenium catalysts activated by newly designed ligands have recently received much attention not only for ACP but also for the nonasymmetric version in terms of coordination chemistry and also industrial curiosity because of high stereoselectivity. We believe that further improvement of the ruthenium catalysts will be in environmental interest to realize industrially applicable process. 

The development of ruthenium complexes for other applications in radical chemistry is still in its infancy, but seems well suited to future expansion, thanks to the versatility of ruthenium as a catalytically active center. Large avenues have not been explored yet and remain open to research. For instance, the development of methodologies for the asymmetric functionalization of C-H bonds remains a challenge. The Kharasch-Sosnovsky reaction [51,52],in which the allylic carbon of an alkene is acyloxylated, its asymmetric counterpart, and the asymmetric version of the Kharasch reaction itself are practically terra incognita to ruthenium chemistry, and await the discovery of improved catalysts. 

For aldol and Michael reaction of nitriles, cyclopentadienyl ruthenium enolate complexes shows catalytic activity. The reaction of 2-methylphenylacetonitrile with ethyl acrylate gave the corresponding Michael addition product in 99% yield (Eq. 9.58) [79]... 

Transition-metal-mediated C-O bond cleavage reactions are interesting in view of environmentally benign halogen-free chemical processes [59]. Zerovalent ruthenium complexes are also active toward C-O bond-deavage reactions, and a number of catalytic processes have been developed in this respect. For example, Ru(l,5-COD)(l,3,5-COT) catalyzes allylic alkylation of carbon nucleophiles with allylic carbonates in basic solvent (Scheme 14.24) [60]. 

Ruthenium Catalysts. The catalytic activity of Ru complexes for olefin oligomerization is considerably lower than that of Rh systems . Therefore, reactions that are... 

Chaudret and coworkers synthesized an ortho-ruthenated acetophenone complex (26) having axial tricyclohexylphosphine ligands. Complex 26 showed almost no catalytic activity, and on the basis of this observation and the activity of 22, they proposed that the binding of the CO Hgand to the ruthenium suppresses the catalytic activity of the ruthenium complex. Fogg and coworkers prepared ortho-ruthenated benzophenone complex 27, which showed only low catalytic activity and was proposed to be a catalytic sink in the alkylation of aromatic ketones. Weber and coworkers synthesized a unique zero-valent ruthenium complex (23), which was effective for the alkylation of aromatic ketones. Subsequently, Whittlesey and coworkers synthesized complex 25, which did not catalyze the hydroarylation. However, the authors stated it was highly Hkely that alternative isomers of 25 could be involved in the catalytic pathways. Further hints toward this end came with the characterization of the two N,0-coordinated acetylpyrrolyl complexes 24 and 28. Complex 24 was found to be an active catalyst of the reaction but was shown to isomerize to its inactive isomer 28 at 80 °C. 

Although not as frequent as in iridium or ruthenium complexes, C-H activation of Af-aryl substituents in palladium-NHC complexes is also known to occur and have an influence on catalytic reactions. Danopoulos and coAvorkers showed that NHC complexes in which the Af-aryl moiety contained only one substituent underwent metalation at the aromatic C-H bond. Depending on the reaction conditions, one or both of the NHC ligands would be metalated. " Even iron complexes, which are not generally susceptible to ortAo-metalation, were shown to undergo C-H activation of the NHC ligand. ... 

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

In 2009, Buchmeiser and co-workers reported the synthesis of a novel ruthenium complex 54 based on a seven-membered NHC ligand [68] (Fig. 3.22). To examine the catalytic activity of complex 54 in the RCM reaction, the authors subjected the complex to a series of typical RCM reactions by using substrates 1, 3, and 5. Pre-catalyst 54 showed only moderate reactivity with 1 and 3 and no reaction occurred with 5. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

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