Ru-carbene complex

Table 2 Ru- carbene complexes by exchange of the carbene ligand in 9...

Insertion can also be carried out on the C-H bonds of heteroaromatics. Masahiro Murakami of Kyoto University has described (J. Am. Chem. Soc. 2003,125,4720) a Ru catalyst that will effect rearrangement of a silyl alkyne such as 10 into the vinylidene carbene. The intermediate Ru carbene complex is then electrophilic enough to insert into the aromatic C-H bond. The insertion is highly regioselective. The Au and the Ru alkylidene insertions are geometrically complementary, as Ru gives the E-alkcne. 

Complexes of Pd, Pt and Ru catalyse enyne metathesis, giving similar products to those obtained by the Ru-carbene complex 22 as described in this chapter. These enyne metatheses are discussed in Section 7.2.6. Other mechanisms, without involving carbene complexes as intermediates, have been proposed. 

Scheme 13. Cationic Ru carbene complexes observed from solutions containing alkali metal chlorides. H2lMes= 1.3-Bis(2.4.6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, M = Li. Na. K. Cs 73. ...

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

Knox and co-workers (190) reported that irradiation of the Fe-and Ru-carbene complexes 157 in the presence of alkynes results in loss of CO and insertion of the alkyne into a metal-carbene bond to give 158 [Eq. (150)]. A variety of alkynes (R"=H, CH3, or C02Me) and... 

In contrast to other alkenyl-substituted organosilicon compounds, a metathetical conversion of divinylsilyl derivatives in the ADMET polymerization as well as the RCM process does not occur in the presence of well-defined W, Mo, and Ru carbene complexes because of stereoelectronic effects of the silyl group stimulating non-productive cleavage of the disilyhnetallacyclobutane intermediate [16],... 

Much work is still in progress to improve the Ru-carbene complexes. Considerable attention has recently also been directed at efforts to immobilize a Ru-based metathesis catalyst on solid supports, which could make the metathesis reaction even more attractive for industrial applications. 

Recent years have seen rapid advancements in development of W, Mo, and Ru carbene complexes that serve not only as catalysts for metathesis of small molecules (Section 11-1-3) but also as ROMP catalysts. Grubbs use of a Ru(III) catalyst for ROMP in aqueous medium helped pave the way for development of his first- and second-generation Ru alkylidenes, which also catalyze ROMP. The Schrock catalysts that we have encountered already (compounds 15... 

Historically one of the first asymmetric methods to be explored, cyclopropanation came of age32 with box and salen ligands on Cu(I). Diazo compounds, particularly diazoesters 138, react with Cu(I) to give carbene complexes 140 that add to alkenes, particularly electron-rich alkenes to give cyclopropanes 141. The reaction is stereospecific with respect to the alkene -1runs alkenes giving trans cyclopropanes - and reasonably stereoselective as far as the third centre is concerned. Any enantioselectivity comes from the chiral ligand L. You have already seen the Ru carbene complexes are intermediates in olefin metathesis (chapter 15). 

Metathetic ring closure. Five-, six-, and seven-membered carbocycles as well as N and O heterocycles are constructed from dienes in the presence of a Ru-carbene complex. For unhindered amines, prior protonation is necessary. Fused bicyclic structures are accessible from dienynes. ... 

It is interesting that a Mo-carbene complex and a Ru-carbene complex can show different stereoselective patterns in ring-closing olefin metathesis. ... 

Yao described stable and soluble poly(ethylene glycol)-bormd Ru catalysts (10) for the ring-closing metathesis (RCM) of a variety of dienes. Bidentate nonphosphine ligands are in fact promising candidates for the recycling of Ru-carbene complexes, since the formation of chelates is entropicaUy favored at the end of the reaction [Eq. (8)] [20]. 

ROMP and ROM are the two most frequently used methods in aqueous media. It was shown that the activity of the Ru catalyst is increased in the presence of small amount of water in the ROMP of 7-oxanorbornene derivatives. The most effective precursor was the aqua complex [Ru(H20)6](tos)2. After the polymerization was completed the aqueous catalyst phase could be reused and the catalytic activity increased in the subsequent reaction (Figure 26). An aqua-ruthenium (ii) olefin complex was isolated with higher catalytic activity than the adduct formed from the [Ru(H20)6](tos)2-complex and the 7-oxanorbornene derivative it was suggested that the Ru-carbene complex was formed from this compound. 

Olefin metathesis reaction that reorganizes carbon-carbon double bonds provides fundamentally new strategies for natural product synthesis and polymer chemistry. Hilvert and coworkers built up an artificial metalloenzyme by covalently tethering a Grubbs-Hoveyda-type Ru complex to a protein scaffold [78]. An /V-heterocyclic carbene (NHC) ligand, which has been reported as a suitable ligand for a number of water-soluble ruthenium-based metathesis catalysts, was derivatized with an electrophilic bromoacetamide. The Ru carbene complex (27 in Figure 10.16) was then attached by site-selective alkylation of the cysteine... 

Fig. 20. Formation of a porphyrin-Ru carbene complex by reaction with a diazo compound.

Fig. 26. Addition of chloroform to olefins by a radical reaction catalyzed by a Ru carbene complex.

It is noteworthy that the Grubbs-Hoveyda catalyst can polymerize monosub-stituted and diphenylacetylenes [67]. The Grubbs-Hoveyda catalyst and a series of Ru carbene complexes catalyze the polymerization of o-substituted PhAs, as represented by (o-isopropoxy)phenylacetylene [68]. The substituents at the ortho position of monomers are assumed to serve as supportive ligands that maintain and prolong the life of unstable propagating carbene species. 

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