Ruthenium based alkylidene catalyst

Gompared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. Reactivity in RuGl2(PR3)2(CHPh) may efficiently be tuned by the use of different phosphanes rather than by the nature of the alkylidene moiety or by substitution of the chlorides by other, more electron-withdrawing groups. The widely accepted dissociative mechanism is shovm in Scheme 3. 

The two most commonly used single-site catalysts for ADMET today are (1) Schrock s alkylidene catalysts of the type M(CHR )(NAr )(OR)2 where M = W or Mo, AC = 2, 6-C6H3-/-Pr2, R = CMe2Ph, and R = CMe(CF3)2 (14)7 and (2) Grubbs ruthenium-based catalyst, RuCl2(=CHPh)(PCy3)2 (12) where Cy = cyclohexyl.9 While both catalysts meet the requirements to be successful in ADMET, they are markedly different in their reactivity and in die results each can produce. 

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. 

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... 

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. 

Alternatively, Dorta and co-workers used SINap ligands in order to improve the activity and stability of second-generation ruthenium metathesis catalysts (14). Effects of NHC backbone substitution on imidazolinylidene-derived ligands were studied by Grubbs et who also replaced the imidazole-based N-heterocycles with their thiazole analogues.Because these modifications were also applied to oxygen-chelated alkylidene catalysts, they are discussed in more details in the next Section. 

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