Ruthenium alkylidene metathesis catalysts

Figure 3.12 The preparation of bis-hydroxy telechelic PBD using a ruthenium alkylidene metathesis catalyst in combination with an allylic functionalized CTA [34].

NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis... 

Scheme 8.5 Ruthenium phosphonium alkylidene metathesis catalysts (22a-c) reported by...

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... 

More recently, a new metathesis catalyst involving a ruthenium-alkylidene complex with a sterically bulky and electron-rich phosphine ligand has been synthesized and applied to RCM in aqueous media (Figure 3.5).197 This catalyst has the benefit of being soluble in almost... 

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. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Metathesis of 1-octene leads cleanly to ethene and 7-tetradecene, but as the reaction proceeds also 2-octene is formed and metathesis products derived from the isomerisation reaction. It was found that after prolonged reaction times decomposition of the ruthenium alkylidene catalyst occurs. At least eight different products were formed and several of them have been identified [37], Figure 16.22 shows the identified compounds derived from Grubbs 1st generation catalyst (the 2nd generation gives basically the same result [38]). 

The in situ preparation of a ruthenium-alkylidene catalyst for olefin metathesis is the first step for extending this high-throughput approach toward other catalytic transformations and opens up the way to the screening of azolium salt libraries for olefin metathesis reactions. ... 

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. 

As already mentioned, the development of metathesis catalysts that can be easily accessed from simple precursors is necessary if a large-scale application is desired. With this in mind, Forman et al. developed a robust ruthenium-based phoban-indenylidene complex through a simple and relatively inexpensive procedure, if compared to the preparation of C3 [40]. This mthenium alkylidene was tested in the bulk SM of methyl oleate. As a result, they could reach up to 50% conversion with 0.005 mol% catalyst at 50°C. 

A tandem RCM-alkene isomerization sequence to form 5-, 6-, and 7-membered enol ethers was reported by Snapper and co-workers <02JA13390> (Scheme 36). In this process the RCM reaction is run under an atmosphere of 95 5 N2.TI2 to convert the intermediate ruthenium alkylidene into an olefin-isomerization catalyst. Note that alkene migration can convert isomeric metathesis products into the same 2,3-enol ether. A single example of the formation of a 6-membered tosyl enamide was reported in this manuscript. 

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