Ruthenium benzylidene

With the development of an analogous ruthenium benzylidene catalyst 17 by Grubbs and co-workers in 1995, a ruthenium carbene catalyst suitable for the cross-metathesis reaction was in place [34]. Benzylidene 17 exhibited the same impressive tolerance of air and moisture, and the same stability towards functional groups as its predecessor 4, but benefited from easier preparation [35,36] and much improved initiation rates. 

Although the Grubbs ruthenium benzylidene 17 has a significant advantage over the Schrock catalyst 3 in terms of its ease of use, the molybdenum alkylidene is still far superior for the cross-metathesis of certain substrates. Acrylonitrile is one example [28] and allyl stannanes were recently reported to be another. In the presence of the ruthenium catalyst, allyl stannanes were found to be unreactive. They were successfully cross-metathesised with a variety of alkenes, however, using the molybdenum catalyst [39] (for example Eq. 20). 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

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)] °... 

Ring-closing metathesis, which has proved to be a popular route to the marine toxins, has found a further application as the key step in the synthesis of the pheromone (-)- and ( )-frontalin <99TL1425>. The precursor in this reaction is a mixture of the syn- and anri-isomers 39. Ring closure in the presence of a ruthenium benzylidene catalyst occurs within minutes at room temperature when only the syn-isomer cyclises to 40. The unreacted anri-isomer can be re-equilibrated for a further cyclisation. 

Metathesis of conjugated enyne-enes has been carried out using bispyridine-substituted ruthenium benzylidene catalyst li. An intramolecular version with conjugated enynes affords novel butadienyl cycloalkenes (Equation (8)). The reaction does not proceed with Ic or Ig. 

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>. 

Ruthenacyclopentane 331 has been postulated as an intermediate in the ruthenium-catalyzed cycloisomerization of lactones <2003TL2157>. Cycloisomerization of phenylsulfonylallenes to the cyclohexane derivatives catalyzed by a ruthenium benzylidene complex might proceed through the ruthenacyclopentane intermediate 332 <2006TL3971>. The [( 7 -Cp")RuCl( -COD)]-catalyzed cyclotrimerization of 1-octyne with dimethyl acetylenedi-carboxylate proceeds via a ruthenacyclopentadiene <2004JMO(209)35>. 

Although the NHC ligands are carbenes, the metal-carbon bond of the NHC is completely different than that of the ruthenium benzylidene bond, displayed in... 

Scheme 7. Homobimetallic ruthenium-benzylidene complexes bearing a Schiff...

Scheme 11. Reaction ofhomobimetallic ruthenium-benzylidene 17 and indenylidene 18 complexes with ethylene...

A ruthenium benzylidene complex has been incorporated into G1-G4 phosphine-substituted dendrimers by Astruc... 

Because the key to control of molecular weight distribution depends on the relative rates for initiation and propagation, studies to control these relative rates have been conducted. Studies with the Grubbs-type ruthenium carbene complexes have shown that the ruthenium benzylidene complexes undergo faster initiation than vinyl alkylidene... 

In a recent paper, we reported on the exceptional efficiency and versatility of new catalysts based on RuCl2(p-cymene)(PR3) (4) for promoting the ATRP of vinyl monomers (p-cymene is 4-isopropyltoluene) [25]. Since the best ruthenium-based catalysts for ROMP were also the most efficient ones for ATRP, switching from olefin metathesis to ATRP became of interest in order to prepare block and graft copolymers and to establish the relationship between these two related modes of olefin polymerisation. Model studies for this purpose included the conversion of Grubbs ruthenium benzylidene complexes,... 

The Grubbs ruthenium benzylidene complexes [RuCl2(=CHPh)-(PR3)2] (4), have had a tremendous impact in olefin metathesis [4, 5,16]. 

Olefin Metathesis versus ATRP Catalysed by Ruthenium Benzylidene Complexes... 

An exactly opposite trend was observed in olefin metathesis for which the mechanistic scheme for complexes 4 and 5 postulates the dissociation of a phosphine ligand from the metal centre as the key step in the dominant reaction pathway [23]. For instance, ruthenium benzylidene complexes 5 bearing only one /V-heterocyclic carbene entity were found to be significantly more active than those incorporating two of them (6) and the original Grubbs complex (4). This was clearly substantiated for the conversion of the suitable dienes into dihydropyrrole (Scheme 6, Table 2) [20d], and polyhydroxylated cyclohexene rings [24]. 

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