Ruthenium alkylidene metathesis-active

In current reports of mechanochemically latent ruthenium alkylidenes, metathesis activity is known only in solution, in which access to mechanical force is achieved through solvodynamic shear stresses during ultrasonication [115, 116]. These catalysts are touted as being apphcable to self-healing polymers, but several refinements toward self-healing applications are necessary. First, mechanochemically induced ligand dissociation must be demonstrated through macroscopic stresses on a bulk polymer. Second, once catalyst initiation... 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

A Grubbs-type ruthenium complex and a Hoveyda ruthenium complex were compared under similar conditions for recycled activity. Both the reference catalysts showed a large drop in metathesis activity in the subsequent tests. For example, a Grubbs-type ruthenium alkylidene catalyst showed a drop of nearly 50% conversion in the second run. 

Research Focus Identification of high-activity ruthenium alkylidene catalysts for ringopening and ring-closing metathesis reactions. 

Furthermore, a new metathesis-active ruthenium alkylidene with a sterically bulky and electron-rich phosphine ligand has been synthesized and applied to RCM in aqueous media (Eq. 24) [54]. 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

These ruthenium complexes react rapidly and quantitatively with ethyl vinyl ether to form a Fischer carbene that is only weakly metathesis active at elevated temperatures [86, 87]. This property can be employed to end-cap ROMP and ADMET polymers and to ensure that there are no polymeric ruthenium alkyhdenes present. Since ruthenium alkylidenes are relatively robust complexes they could survive workup procedures, although experimental evidence has yet to confirm this notion. Treatment of an ADMET polymer with ethyl vinyl ether gives the polymer well-defined terminal olefinic endgroups and should prevent backbiting metathesis upon dilution of the polymer (Scheme 6.22). 

In the 1980s, well-defined metal alkylidenes were introduced as catalyst precursors for olefin metathesis [99, 109-111]. Especially for aqueous ROMP, ruthenium alkylidenes represent readily activated, well-defined, easy to handle catalyst precursors respectively initiators (for a living ROMP without chain-transfer, the term initiator appears more appropriate). Whereas in initial work vinyl-substituted carbenes (cf. 16a) were employed [112], more straightforward routes to aryl-substituted carbenes (16b) were soon developed [113]. Today, vinyl-substituted carbenes are also accessible in one-pot procedures [114], and 16a and 16b are both commercially available. 

Although a number of systems are now available that will allow some metathesis reactions to take place in aqueous solution, the generality and activity of these systems is not yet sufficient to carry out many RCM and cross metathesis reactions with the desired catalyst efficiency. The instability in water of the ruthenium alkylidenes known to date has not allowed alkene metathesis processes in aqueous media to reach the level of utility that is possible in organic solvents. 

In the 1980s, well-defined metal alkylidenes were introduced as catalyst precursors for alkene metathesis [48, 51-53]. For aqueous ROMP especially, ruthenium alkylidenes represent readily activated, well-defined, easy to handle catalyst precur-... 

The cycle outlined in Fig. 4.32 indicates that the overall metathesis activity of the catalysts is determined by the relative magnitudes of several rate constants (i) the rate constant for phosphine dissociahon ( i) dictates the rate at which the 16-electron pre-catalyst complex enters the catalytic cycle, (ii) the ratio of k- to k2 controls the rate of catalyst deachvation (by re-coordination of phosphine) versus catalyhc turnover (by coordination of olehnic substrate and subsequent steps), and (iii) the rate constant for metallacycle formahon k ) determines the rate of the carbon-carbon bond formation. High olefin metathesis activity is expected when a ruthenium alkylidene catalyst exhibits fast initiation (a large value of k ), high selectivity for binding olefins relative to phosphines (a small value of k-x/k ), and fast metallacyclobutane formation (a large value of k ). 

Grubbs-type ruthenium alkylidenes have been shown to be catalytically active for ring-closing olefin metathesis in ILs such as [G4GiIm]PE6 and [G4GiIm]GF3S03, but their catalytic reactivity rapidly vanished in subsequent recycling runs and reuse. " ... 

By 1992, Grubbs and co-workers had discovered an alternative catalyst that overcame many of these shortcomings. Indeed, although ruthenium alkylidene 12 (Scheme 5) displays a lower metathesis activity than Schrock s molybdenum systems, it importantly demonstrated air stability and the ability to initiate metathesis in the presence of alcohols, water, and carboxylic acids. Thus, 12 represents the first true catalyst for general bench top olefin metathesis reactions, and over time has been optimized to 13 (Scheme 6), which has proven far easier to prepare than the parent structure 12 and constitutes the current gold standard with which all new catalyst systems are compared. Without question, this... 

Metathesis reactions are reported in some reviews [72-75], a handbook [76], etc. [77-82]. The first well-defined metathesis-active mthenium-alkylidene complex is a compound, (Ph3P)2Cl2 Ru=CH-CH=CPh2), but Hoveyda-Grubbs first- and second-generation ruthenium-based catalysts are ruthenium-alkylidene five-membered ring compounds (see Chap. 1). 

Scheme 5.3 illustrates the most commonly used ruthenium-based olefin metathesis catalysts (I) the first well-defined, metathesis-active ruthenium alkylidene complex... 

Ruthenium alkylidene compounds bearing two imidazol-2-ylidene ligands (6) have also been prepared and, in olefin metathesis, they exhibit a catalytic activity comparable to that of the... 

After having observed that the most active ruthenium-based catalyst systems for olefin metathesis also displayed a high efficiency in atom transfer radical polymerisation, we then became interested in comparing the role of the catalyst in those two different reaction pathways. Ruthenium alkylidene complexes 4-6 are unsaturated 16-electron species which formally allow carbon-halogen bond activation to form a 17-electron ruthenium(III) intermediate. Our preliminary results indicate that polymerisations occur through a pathway in which both tricyclohexylphosphine and/or imidazolin-2-ylidene ligands remain bound to the metal centre. 

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