Solvents ruthenium catalysis

The current research areas with ruthenium chemistry include the effective asymmetric hydrogenation of other substrates such as imines and epoxides, the synthesis of more chemoselective and enantioselective catalysts, COz hydrogenation and utilization, new methods for recovering and recycling homogeneous catalysts, new solvent systems, catalysis in two or three phases, and the replace-... 

Here we describe a new route to the synthesis of N,N-dimethylform-amide using synthesis gas and ammonia as the only chemical building blocks (eq. 26). Once again this involves an adaption of ruthenium melt catalysis (13). At about the time this work was published, Marsella and Fez of Air Products, disclosed a similar synthesis using solvent-solubilized ruthenium catalysis (79). 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Solutions of ruthenium carbonyl complexes in acetic acid solvent under 340 atm of 1 1 H2/CO are stable at temperatures up to about 265°C (166). Reactions at higher temperatures can lead to the precipitation of ruthenium metal and the formation of hydrocarbon products. Bradley has found that soluble ruthenium carbonyl complexes are unstable toward metallization at 271°C under 272 atm of 3 2 H2/CO [109 atm CO partial pressure (165)]. Solutions under these conditions form both methanol and alkanes, products of homogeneous and heterogeneous catalysis, respectively. Reactions followed with time exhibited an increasing rate of alkane formation corresponding to the decreasing concentration of soluble ruthenium and methanol formation rate. Nevertheless, solutions at temperatures as high as 290°C appear to be stable under 1300 atm of 3 2 H2/CO. 

The observed ratio of [HRu3(CO)n] to [Ru(CO)3I3] in solutions after catalysis is sometimes found to vary from the 2 1 ratio shown by (59). This may be expected if acids or bases (e.g., a basic solvent) are involved in oxidation or reduction processes, which can interconvert the two such equilibria can change with pressure (193). Nevertheless, these two species are normally observed to be stable under catalytic conditions, and a combination of the two is found to provide the optimum catalytic rates (e.g., see Fig. 21). Catalyst solutions derived from nonhalide salt promoters are presumed to contain [HRu3(CO)n] and an oxidized ruthenium species analogous to [Ru(CO)3I3]-, although no detailed studies of such systems have been reported. 

Dihydropyrroles have recently become readily available by ring-closing metathesis. For this purpose, N-acylated or N-sulfonylated bis(allyl)amines are treated with catalytic amounts of a ruthenium carbene complex, whereupon cyclization to the dihydropyrrole occurs (Entries 6 and 7, Table 15.3 [30,31]). Catalysis by carbene complexes is most efficient in aprotic, non-nucleophilic solvents, and can also be conducted on hydrophobic supports such as cross-linked polystyrene. Free amines or other soft nucleophiles might, however, compete with the alkene for electrophilic attack by the catalyst, and should therefore be avoided. 

Ford and co-workers have also recently developed a homogeneous catalyst system for the water-gas shift reaction (95). Their system consists of ruthenium carbonyl, Ru3(CO)12, in an ethoxyethanol solvent containing KOH and H20 under a CD atmosphere. Experiments have been conducted from 100-120°C. The identity of the H2 and CD2 products has been confirmed, and catalysis by both metal complex and base has been verified since the total amount of H2 and COz produced exceeds the initial amounts of both ruthenium carbonyl and KOH. The authors point out that catalysis by base in this system depends on the instability of KHC03 in ethoxyethanol solution under the reaction conditions (95). Normally the hydroxide is consumed stoichiometrically to produce carbonate, and this represents a major reason why a water-gas shift catalyst system has not been developed previously under basic conditions. As has been noted above, coordinated carbonyl does not have to be greatly activated in order for it to undergo attack by the strongly nucleophilic hydroxide ion. Because of the instability of KHC03... 

It is mostly complexes of ruthenium and rhodium that have been used to conduct hydrogenation reactions in ionic liquids and little attention has so far been paid to modifying the employed catalysts to improve their performance in the ionic environment. The majority of the catalysts used are identical to those employed in conventional homogeneous catalysis conducted in molecular solvents like, for example, RhCl(PPh3)3 and RuCl2(PPh3)3. 

One of the areas gamering attention in catalysis research has been the development of green or enviromnentally benign catalytic systems. For olefin metathesis, the trend has been to develop catalytic systems that can be efficiently recycled. Success in this area has multiple implications for OM processes. First, a recyclable catalyst will give overall more turnovers per catalyst molecule, and thereby be more economical. Second, a catalyst that can be efficiently recycled (low loss of activity over repeated uses) leaches less Ruthenium into the product and thus less expensive processing costs. To this end inunobihzation of the olefin metathesis catalysts on a variety of sohd supports and utilization of nonorganic solvent systems have been explored. 

The copper and palladium transition metal catalysts noted in Table 18 proved to be superior to nickel, ruthenium and rhodium catalysts. The nature of the reacting species has not been unequivocally defined, but the following experimental observations may provide some insight (i) tetrahydrofuran solvent is essential for the palladium-mediated reactions, since complex reaction mixtures (presumably containing carbinols) were observed when the reactions were performed in either benzene or methylene chloride (ii) the reaction is truly catalytic with respect to palladium (2 mmol alkylaluminum, 0.05 mmol of Pd(PPh3)4), whereas the copper catdyst is stoichiometric and (iii) in the case where a direct comparison may be made (entries 1-8, Table 18), the copper-based system is superior to palladium catalysis with regard to overall yield. 

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