Ruthenium benzylidene carbene complex

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)] °... 

In conclusion, the disappearance of the benzylidene fragment during the ATRP of methyl methacrylate could be explained by the reaction of the ruthenium benzylidene with the monomer, giving rise to highly unstable ruthenium ester-carbene complexes, and it is possible that these species then quickly decompose. In addition, the absence of [Ru=CH2] is also most probably indicative of the decomposition of these ruthenium carbene species, since [Ru=CH2] are presumed to be the propagating species in RCM and related ruthenium methylidene derivatives have a quite long lifetime in olefin metathesis. Until now, the exact nature of the inorganic decomposition products is not known. 

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. 

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). ... 

Olefin metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis, and the reaction has been generally accepted to proceed through a series of metallacyclobutanes and carbene complexe intermediates [40-43]. For this type of reaction, the most widely used catalysts include an alkoxyl imido molybdenum complex (Schrock catalyst) [44] and a benzylidene ruthenium complex (Grubbs catalyst) [43]. 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. 

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. 

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... 

A very efficient group of catalysts are the 2-pyridylethanyl substituted ruthenium carbene complexes 9 and 10. Also this new class of catalyst can be easily prepared, either via the reaction of Grubbs benzylidene catalyst with a 2-(3-butenyl)pyridine or directly via a one-pot procedure for the synthesis of ruthenium carbenes starting from [RuCl2(l,5-cyclooctadiene)] via a ruthenium hydride species, see Scheme 5b. 

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]. 

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). 

Particularly noteworthy, in the ATRP of methyl methacrylate (Figure 6), the observed thermolytic half-lives of the mixed ruthenium benzylidene complex 5 (R = Cy, R = (5)-CHMePh fvi = 5 min), the bis-A-heterocyclic carbene complex 6 (R =Cy t = 1,5 min), and RuCl2(=CHPh)(PCy3)2 4 (f% = 16.5 min) approximate the relative reactivity of the complexes with (meth)acrylates. This observation could be indicative that a metathesis reaction takes place between those ruthenium benzylidene complexes and methyl methacrylate. At this stage, however, one can object that the so far reported inertness of RuCl2(=CHPh)(PCy3)2 towards methacrylates [35] is inconsistent with its decomposition over the course of ATRP through a metathetical pathway. 

A series of benzylidene and low-valent alkylidene complexes have also been prepared from sulfur ylides. Deprotonation of benzyl or alkyl diphenylsulfonium salts with a strong base, such as KN(SiMe3)2, KNPr j or LiNPr generates the corresponding sulfur ylide. Reaction of this ylide with low-valent ruthenium, osmium, rhodium, or iridium complexes containing a labile ligand leads to transfer of the carbene unit from sulfur to... 

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