Ruthenium catalysts reactivation

In an effort to apply the cooperative principles of metalloenzyme reactivity, involving a combination of metal-ligand and hydrogen bonding, we have reported a ruthenium catalyst incorporating imidazolyl phosphine ligands that efficiently and selectively hydrates terminal alkynes (5). We subsequently found that application of pyridyl phosphines to the reaction resulted in a >10-fold rate enhancement and complete anti-Markovnikov selectivity, even in the... 

Polymeric carbon refers to chains of carbon monomers (surface carbide) that are connected by covalent bonds. It has been shown recently47 that the barrier for C-C coupling on flat surfaces (1.22 eV) is half that for a step site (2.43 eV), and may indicate that the growth of these polymeric species is favored on terraces. Polymeric carbon may also refer to carbon chains that contain hydrogen. In the case of CO hydrogenation on ruthenium catalysts, polymeric carbon has been identified as a less reactive carbon that forms from polymerization of CHX and has an alkyl group structure.48... 

In addition to the successful hydrogenation of the two fluorinated esters, this report describes the hydrogenation of dimethyl oxalate. Using the reactive anionic ruthenium catalyst, a 70% conversion to methyl glycolate could be achieved (TON = 235 TOF 12 h-1) (Scheme 15.19, Table 15.14, final entry). 

The most widely used catalysts for RCM are Grubbs ruthenium catalyst 9 and its second generation analogue 10, as well as first and second generation Hoveyda-Grubbs catalysts 11 and 12 (Fig. 6) [38]. The latter have superior stability and reactivity, expanding the applicability of the method considerably. Schrock molybdenum catalyst 13 has also been described for macrocy-clization [38]. 

Hoveyda synthesized recyclable ruthenium catalyst 104b for enantioselective olefin metathesis. This catalyst is very effective for AROM-CM and can be recovered after chromatography [Eqs. (6.74)-(6.76)]. The recovered catalyst can be reused without significant loss of enantioselectivity and with similar reactivity ... 

In addition to allylsilanes, CM can also be applied to allylstannanes, which serve as valuable reagents for nucleophilic additions and radical reactions.To date, only eatalyst 1 has been shown to demonstrate CM reactivity in the preparation of 1,2-disubstituted allylstannanes, as ruthenium catalysts were found to be inactive in the presence of this substrate class.Poor stereoselectivities were generally observed, with the exeeption of one instance of >20 1 Z-selectivity in the reaction of allyltributylstannane with an acetyl-protected allyl gluco-side. 

Ring-closing metathesis involving a sulfoximine 25 was utilized to access the novel 1,3-thiazepine 26, which incorporates a sulfoximine moiety (Equation 4) <2005S1421> the more reactive Grubbs II ruthenium catalyst was utilized. 

For both reactivity and regioselectivity, however, a compromise must be found between the bulkiness of the reagents (alkyne and carboxylic acid) and the steric hindrance of the diphosphine ligand, all of which are present in the coordination sphere of the ruthenium center during the catalytic process. Thus, with the more bulky trimethylsilylacetylene, the less hindered bis(diphenylphosphino)ethane ligand provides an efficient ruthenium catalyst (Ru(methallyl)2(dppe)) for producing silylated enol esters. Better reactivity is also observed with Ru(methal-lyl)2(dppe) as catalyst precursor when propargylic ethers are used as acetylenic substrates (Scheme 5) [10]. 

This ligand, MeO-BIPHEP (96a), has shown similar reactivities and enantioselectivities to catalysts that contain BINAP.117 Ruthenium catalysts that contain MeO-BIPHEP have been used in several asymmetric hydrogenations from bench scale to multi-ton scale, which include the large-scale preparation of a P-keto ester, an aryl ketone, allylic alcohol, and several oc,P-unsaturated carboxylic acid substrates, which are shown in Figure 12.5. 

Most of the recent synthetic applications of M-RCM involve one of the above catalysts, particularly G1 or G2, chosen as a function of its own reactivity profile, generally after preliminary reaction assays on the genuine substrate or specific model compounds. The sensitivity of the RCM reaction to steric hindrance is well established. These ruthenium catalysts exhibit high affinity for carbon-carbon double bonds and are compatible with the presence of many functional groups, even the presence of free polar hydroxyl or amino groups. Their use does not require special conditions such as glove boxes, which are required when using Schrock s molybdenum catalyst. 

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