Ruthenium catalysts carboxylic acid-promoted

Very similar reactions using a ruthenium catalyst, carboxylic acid solvent, and a slightly different promoter system have been reported (197) to give increased amounts of ethyl ester product (Table XVI, Expts. 4 and 5). Most examples show the use of Ru02 H20 as the catalyst precursor in a carboxylic... 

Although related reactions have also been done under low pressures/ very low rates of product formation are observed (8/10/11). We have found/ however, that a ruthenium carbonyl catalyst is quite active for converting H2/CO to methanol under moderate pressures (below 340 atm). More significantly, we also discovered that an ethylene glycol product could be obtained from this catalyst by use of carboxylic acid promoters or solvents (12) This remarkable and intriguing promoter effect deserved, we felt, further mechanistic investigation... 

IV. Unpromoted and Carboxylic Acid-Promoted Ruthenium Catalysts... 

The scope and mechanism of carboxylic acid homologation is examined here in relation to the structure of the carboxylic acid substrate, the concentrations and composition of the ruthenium catalyst precursor and iodide promoter, synthesis gas ratios, as well as 13C labelling studies and the spectral identification of ruthenium iodocarbonyl intermediates. 

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

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). 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Initial studies showed that Ru3(CO)i2 and [Ru(CO)2(02CCH3)]n were able to promote the addition of carboxylic acids to diphenylacetylene at 145 °C in toluene [29, 30]. Subsequently, a number of catalytic systems based on ruthenium catalysts have been discovered, and these have made possible - under mild conditions - the Markovnikov addition of carboxylic acids to terminal alkynes according to Scheme 8.14 to produce enol esters used as acylating reagents. 

Group Vin metals are well established oxidising agents for the functionalisation of olefinic compounds. Ruthenium and osmium catalysts promote the oxidative cleavage of the double bond in olefinic compounds to aldehydes or ketones which may undergo further reaction to form carboxylic acids in some cases. However selective epoxidation is influenced by the nature of the ligands which... 

Synthesis of Carboxylic Acid Esters. The preparation of aliphatic carboxylic acid esters from synthesis gas and the corresponding acid is illustrated by the experimental data summarized in Table VI. Here propionic acid is the coreactant (eq. 21) and CO hydrogenation yields substantial quantities of Cx C4 alkyl propionates. Some 14 catalyst combinations of ruthenium with quaternary Group 5B and alkali metal promoters have been considered. The important features of the catalysis are as follows ... 

In addition to ruthenium-catalyzed reactions, a range of other transition metal catalysts have shown activity toward the addition reaction. A series of air-stable gold compounds promoted the addition of carboxylic acids to alkynes (Scheme 2.93) [138]. A variety of gold and silver compounds were screened as catalysts for the reaction, and the most effective pair under the mildest conditions was (Ph3P)AuCl and AgPF. Under the reactions conditions, the reaction was highly selective for the formation of the Markovnikov addition product, and minimal or none of the anti-Markovnikov products were observed. 

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