Benzylidene ruthenium complex Grubbs

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 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. 

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)] °... 

Thanks to the development of the Grubbs benzylidene catalyst (2) and other related ruthenium complexes, olefin metathesis has experienced spectacular advances over the past 10 years. The various incarnations of the reaction (acyclic diene metathesis, ring-closing metathesis, ring-opening metathesis polymerization, etc.) have now acquired first rank importance in synthesis. Clearly, the emergence of a similar, generic, efficient catalytic system for con-... 

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). 

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. 

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]. 

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]. 

Figure 9.1 Representative examples of ruthenium indenylidene precatalysts (32-36) versus the well-studied Grubbs benzylidene complexes 1, 2, and 26.

Metal vinylidenes easily form by isomerization of metal-alkynecomplexes, which is important for instance in alkyne polymerization by the metathesis mechanism. Synthesis of Grubbs ruthenium vinylidene complexes [Ru(PR3)2Cl2(=C=CHPh)] originally proceeded by reaction of the ruthenium precursor with diphenyl-cyclopropene, but a more modern method leading to the parent vinylidene complex involves benzylidene exchange for the vinylidene in the metathesis with 1,3-butadiene ... 

Simal a al. first reported that a series of Grubbs benzy-lidene complexes (Ru-34) can also mediate living radical polymerizations of MMA and styrene to afford controlled polymers with narrow MWDs (M /Mn 1.2). Ruthenium alkylidene complexes with the NHG ligand are so-called second-generation catalysts in the metathesis chemistry. The benzylidene complexes (Ru-35 and Ru-36) with phosphine or NHG ligands were also employed for the radical polymerization of MMA and styrene to give controlled molecular weights but broader MWDs than Ru-35. 

Halogen bonding between bromide anion and bromoimidazolium moieties was used to template the formation of catenane Br PFe through ring-closing metathesis (Scheme 1) [56]. The combination of precursors 16 PFs and 16 Br in the presence of the Grubbs second-generation ruthenium benzylidene complex led to the formation of the catenated structure in 24% yield. In the absence of the... 

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