Ruthenium-based organometallic catalysts

Many different metal catalysts have been explored for racemization of secondary alcohols. Among them, ruthenium-based organometallic complexes have been most intensively tested as the racemization catalyst (Figure 1.1). 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

The outstanding performances of five-membered NHC ligands in organometallic chemistry and catalysis prompted Grubbs and co-workers to develop a novel stable four-membered NHC [64]. Following their interest in developing new ruthenium olefin metathesis catalysts, they synthesised and fully characterised complex 51 to study the impact of the architecturally unique NHC ligand on the activity of the Ru-based catalyst [65] (Fig. 3.20). In the RCM of 1 at 40°C in CH Cl with 51 (5 mol% catalyst), the reaction reached completion within 20 min, whereas less than 10 min are required for standard catalysts 14 and 16. It should be noted that catalysts 14 and 16 are able to complete the RCM of 1 with only 1 mol% catalyst at 30°C. 

Ionic liquids display a limited nuscibility with various polar and nonpolar organic substrates, as well as organic and inorganic solvents, and they usually dissolve organometallic catalyst precursors based on rhodium, ruthenium, palladium, uickel, cobalt and iron complexes [18]. 

Organometallic compounds often show unique catalytic properties that may also allow their use as potential drug candidates. The cyclopentadienyl-ruthenium carbonyl catalyst 75, that bears a quinoline-based bidentate ligand, was found to be a potent inhibitor of certain protein kinases <2006AGE1504>. 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum > rhodium > ruthenium = iridium > osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267]... 

Dimethyleneoctahydronaphthalene has been polymerized by a great variety of Ziegler-Natta and ROMP catalysts based on transition metal salts of titanium, zirconium, vanadium, molybdenum, tungsten, ruthenium, iridium, osmium, platinum or palladium and organometallic compounds [159]. Depending on the catalyst employed, addition or ring-opened polymers were preferentially formed [Eqs. (99) and (100)]. 

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