Catalyst rhodium/ruthenium

Monometallic ruthenium, bimetallic cobalt-ruthenium and rhodium-ruthenium catalysts coupled with iodide promoters have been recognized as the most active and selective systems for the hydrogenation steps of homologation processes (carbonylation + hydrogenation) of oxygenated substrates alcohols, ethers, esters and carboxylic acids (1,2). 

Catalytic hydrogenation of tnfluoroacetic acid gives tnfluoroethanol in high yield [73], but higherperfluorocarboxybc acids and their anhydndes are reduced much more slowly over rhodium, iridium, platinum, or ruthenium catalysts [7J 74] (equation 61) Homogeneous catalysis efficiently produces tnfluoroethanol from tnfluoroacetate esters [75] (equation 61)... 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

Nitrene addition to alkenes can be aided by the nse of a transition metal, such as copper, rhodium, ruthenium, iron, cobalt, etc. NHC-Cu catalysts have been used in nitrene addition. For example [Cu(DBM)(IPr)] 147 (DBM = dibenzoyl-methane) was successfully employed in the aziridination of aliphatic alkenes 144 in presence of trichloroethylsulfamate ester 145 and iodosobenzene 146 (Scheme 5.38) [43]. 

Several S/N ligands have also been investigated for the asymmetric hydrogenation of prochiral olefins. Thus, asymmetric enamide hydrogenations have been performed in the presence of S/N ligands and rhodium or ruthenium catalysts by Lemaire et al., giving enantioselectivities of up to 70% ee. Two... 

In recent years, the asymmetric hydrogenation of prochiral olefins have been developed in the presence of various chiral sulfur-containing ligands combined with rhodium, iridium or more rarely ruthenium catalysts. The best results have been obtained by using S/P ligands, with enantioselectivities of up to 99% ee in... 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Important by-products are urea derivatives (ArNHC(0)NHAr) and azo compounds (Ar-N=N-Ar). The reaction is highly exothermic (—128kcalmol-1) and it is surprising that still such low rates are obtained (several hundred turnovers per hour) and high temperatures are required (130 °C and 60 bar of CO) to obtain acceptable conversions.533 Up to 2002, no commercial application of the new catalysts has been announced. Therefore, it seems important to study the mechanism of this reaction in detail aiming at a catalyst that is sufficiently stable, selective, and active. Three catalysts have received a great deal of attention those based on rhodium, ruthenium, and palladium. Many excellent reviews,534"537 have appeared and for the discussion of the mechanism and the older literature the reader is referred to those. Here we concentrate on the coordination compounds identified in relation to the catalytic studies.534-539... 

Intermolecular cyclopropanation of olefins poses two stereochemical problems enantioface selection and diastereoselection (trans-cis selection). In general, for stereochemical reasons, the formation of /ra ,v-cyclopropane is kinetically more favored than that of cis-cyclopropane, and the asymmetric cyclopropanation so far developed is mostly /ram-selective, except for a few examples. Copper, rhodium, ruthenium, and cobalt complexes have mainly been used as the catalysts for asymmetric intermolecular cyclopropanation. 

Both the rhodium and ruthenium catalysts have been used to successively hydrogenate the terpene geraniol (3) to citronellol (4) and 3,7-dimethyl-octanol (J08) ... 

Catalysts and Catalyst Concentration. The most active catalyst for benzaldehyde reduction appears to be rhodium [Rh6(C0)i6 precursor], but iron [as Fe3(C0)i2] and ruthenium [as Ru3(C0)12] were also examined. The results of these experiments are shown in Table 1. Consistent with earlier results (12), the rhodium catalyst is by far the most active of the metals investigated and the ruthenium catalyst has almost zero activity. The latter is consistent with the fact that ruthenium produces only aldehydes during hydroformylation. Note that a synergistic effect of mixed metals does not appear to be present in aldehyde reduction as contrasted with the noticeable effects observed for the water-gas shift reaction (WGSR) and related reactions (13). 

The initial steep rise can be attributed simply to activation of the catalyst precursor. The amount of base corresponds approximately to one equivalent of KOH for the ruthenium catalyst and two equivalents for the rhodium catalyst. Activation could result from hydroxide attack as in (5) and (6) for rhodium and (10) for ruthenium ... 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

There is much current excitement and activity in the field of homogeneous hydrogenation using ruthenium catalysts. This is reflected in the recent, explosive increase in the number of research publications in this area, now rivaling those for rhodium catalysts (Fig. 3.1). Meanwhile, the price of rhodium metal has risen dramatically, becoming about ten times that of ruthenium, on a molar basis. The number of reports on the use of osmium catalysts has remained low, partly because of the higher price of osmium compounds - about ten times that of ruthenium - and partly because the activity of osmium catalysts is often lower. 

Hsu et al. [15] applied a bimetallic catalyst comprising rhodium and ruthenium for the hydrogenation to combine the high selectivity of the rhodium complex with the lower cost of the ruthenium complex. When the amount of each metal is identical, the catalytic activity of the bimetallic complex catalyst system was similar to that of the single rhodium-complex catalyst, containing... 

Adsorption is commonly used for catalyst removal/recovery. The process involves treating the polymer solution with suitable materials which adsorb the catalyst residue and are then removed by filtration. Panster et al. [105] proposed a method involving adsorbers made from organosiloxane copolycondensates to recover rhodium and ruthenium catalysts from solutions of HNBR. These authors claimed that the residual rhodium could be reduced to less than 5 ppm, based on the HNBR content which had a hydrogenation conversion of over... 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

Enantioselective hydrogenation of functionalized alkenes is a well-developed field. A wide variety of rhodium and ruthenium catalysts and substrates are available for this purpose (see Chapters 23 to 28), and the reaction is widely used as a common synthetic tool in both academia and industry. 

With most rhodium and ruthenium catalysts 4 and 5 (Fig. 30.2), only low en-antioselectivities were obtained (Table 30.1, entries 1-6) [2-6]. However, good results were reported by Noyori and coworkers, who used DuPHOS with potassium tert-buloxide activation to hydrogenate substrate 1 in 86% ee (Table 30.1, entry 6) [6], as well as hydrogenating a range of other 1,1-disubstituted alkenes (see Section 30.2.2). 

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