Ruthenium-carbene intermediate

However, at that time the results obtained did not permit distinction between the reaction mechanism involving ruthenium carbene intermediates, being classical catalysts of the metathesis, and the non-metallacarbene mechanism. 

There has also been a renewal of interest in reactions catalyzed by ru-thenium(II) porphyrin complexes, simultaneously with the development of new chiral ruthenium porphyrins [175-178]. Although these reactions focus mainly on asymmetric epoxidation of olefins [179,180], in some cases asymmetric cyclopropanations were very successful As a recent example, the intermolecular cyclopropanation of styrene and its derivatives with ethyl diazoacetate afforded the corresponding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up to 36 and extremely high catalyst turnovers of up to 1.1 X 10 [140]. The structure of the metalloporphyrin is given in Fig. 2. Asymmetric intramolecular cyclopropanations were also reported with the same catalyst [140]. hi this case, the decomposition of a series of aUyhc diazoacetates afforded the cyclopropyl lactones in up to 85% ee. Both the inter-and intramolecular cyclopropanation were proposed to proceed via a reactive chiral ruthenium carbene intermediate. The enantioselectivities in these processes were rationahzed on the basis of the X-ray crystal structures of closely related stable chiral carbene complexes obtained from the reaction of the chiral complex with N2CPh2 and N2C(Ph)C02CH2CH = CH2. 

Figure 8.3 Additives, such as LiCI, can be introduced to facilitate the polymerization of monomers that aggregate in solution or chelate to ruthenium carbene intermediates.

While metalloporphyrin carbene complexes are well established for ruthenium and osmium, they are less well known for rhodium. Cationic rhodium porphyrin carbene intermediates were implicated in a report by Callot et al. in w- hich... 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

One-pot RCM-CM reaction was carried out by Royer et al. (Scheme 18). The RCM of 49 followed by CM with methyl acrylate gives cyclic compound 50 using ruthenium-carbene complex 2,423 intermediate 51 with... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

A theoretical study of the mechanism of ruthenium-catalyzed formation of pyran-2-one and the corresponding sulfur and selenium analogues 8 from acetylene and CX2 (X = O, S, Se) has been reported (Equation 3) <2004NJC153>. This cyclotrimerization reaction has been experimentally carried out using carbon disulfide as a substrate <2002JA28>. The proposed mechanism involves formation of a bicyclic metal carbene intermediate. Formation of this intermediate seems to be particularly unfavorable energetically in the case of carbon diselenide. 

Scheme 8.21 Preparation of the carbene ruthenium(II) complex 81, the first representative of the first generation Grubbs catalysts", from 79 and 3,3-diphenylcyclopropene via the related ruthenium carbene 80 as the intermediate...

The enyne metathesis generates the first ring and an intermediate ruthenium carbene species, which undergoes a second RCM process to produce the fused [n.m.0]bicyclic product (Scheme 2). 

Catalysts include copper pyrazolylborates (TpJCutQHt), Ru11 porphyrins, and Rh11 carboxylates.98 Metal carbene intermediates are likely such species are also present in the ruthenium-catalyzed stereoselective coupling of a-diazocarbonyl compounds 99... 

The olefin metathesis story started with the advent of well-defined catalysts, most notably the Grubbs ruthenium catalysts.These are ruthenium carbenes that undergo a [2 + 2] cycloaddition to olefins to produce a metallacyclobutane intermediate. Cycloreversion of the metallacyclobutane in the opposite sense leads to olefin metathesis (Scheme 1.35). 

A ruthenium carbene complex in the presence of a chiral ligand is capable of catalyzing the formation of optically active cyclopropane derivatives from alkenes and diazo compounds in high enantiomeric excess [177]. A mixture of [RuCl2(/ -cymene)] in the presence of pybox-(5,5)-/ catalyzes the asymmetric cyclopropanation of styrene (eq (48)). The key intermediate is proposed to be a dichloro(pybox)ruthenium carbene complex. 

In this example, and in many before it, the formation of the carbene is initiated by a metal— often copper, rhodium, or silver. The carbene intermediates in these reactions are formed as reactive complexes with those metals, but in other cases the complexes are extremely stable. For example, decomposition of phenyldiazomethane in the presence of a ruthenium(II) complex gives a carbene complex stable enough to be isolated and stored for months. This complex, and a family of related Ru complexes, are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or olefin) metathesis. 

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