Ruthenium catalyst precursors

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. 

Effect of Catalyst Composition. Where acetic is the typical acid substrate, effective ruthenium catalyst precursors include ruthenium(IV) oxide, hydrate, ruthenium(III) acetyl-acetonate, triruthenium dodecacarbonyl, as well as ruthenium hydrocarbonyls, in combination with iodide-containing promoters like HI and alkyl iodides. Highest yields of these higher MW acids are achieved with the Ru02-Mel combination,... 

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). 

An unusual effect of the catalyst concentration on product rates has been observed. Increasing the concentrations of both the iodide promoter and the ruthenium catalyst precursor while holding their ratio constant leads to... 

Ammonium carbamates are readily and reversibly produced on reaction of secondary amines with carbon dioxide. In the presence of a ruthenium catalyst precursors such as Ru3(CO)12 [3], (arene)RuCl2(PR3) [4] or Ru(methallyl)2(dppe) [5] (dppe=bis(diphenylphosphino)ethane) complexes, the three-component combination of a secondary amine, a terminal alkyne, and carbon dioxide selectively provides vinylcarbamates resulting from addition of carbamate to the terminal carbon of the triple bond (Scheme 2). 

Intramolecular addition of a hydroxy group to the terminal sp-carbon of pent-4-yn-l-ols, leading to the corresponding cycloisomerization dihydropyrans, has been successfully achieved with a similar ruthenium catalyst precursor containing the electron-deficient tris(p-fluorophenyl)phosphine ligand, excess phosphine, and sodium N-hydroxysuccinimide as additives (Scheme 9) [20]. 

In the presence of appropriate ruthenium catalyst precursors, diallyl and allyl homoallyl ethers do not lead to the expected metathesis or cycloisomerization products, but undergo first isomerization to form allyl vinyl ethers, and then a Claisen rearrangement which gives unsaturated aide-... 

In a number of publications, propanol is employed as a co-solvent, apparently serving a two-fold purpose To decrease the viscosity of the ionic liquid and to assist in the catalyst-forming step. With common ruthenium catalyst precursors of the type Ru(OAc)2(PP) (PP = chelate diphosphine), a reaction sequence as shown in Scheme 3.4 has been proposed to account for catalyst activation.1241... 

Rhodium-chiraphos cations also hydrogenate ketone and epoxide functionalities, albeit with low optical yields, and are, therefore, not synthetically useful. While this rhodium system seems somewhat limited to the preparation of amino acids, other rhodium and ruthenium catalyst precursors are currently available which show enhanced activity and selectivity for a much broader group of hydrogenation substrates. 

Recently, new types of ruthenium catalyst precursors that perform the Markovnikov addition of carboxylic acids to terminal alkynes have been developed. The most representative examples are [RuCl2(p-cymene)]2/P(furyl)3/base [50], Ru-vinylidene complexes such as RuCl2(PCy3)2(=C=CHt-Bu), RuCl2(PCy3)(bis(mesityl)imidazolyli-dene)(=C=CHf-Bu), [RuCl(L)2(=C=CHt-Bu)]BF4 [51], and the ruthenium complexes shown in Figure 8.1 [52-54]. 

The catalytic hydrogenation of arenes with an arene ruthenium catalyst precursor has also been performed in ionic liquids and the products separated from the ionic solution by distillation under high vacuum, allowing the same batch of ionic liquid to be used repeatedly for the catalytic hydrogenation of several different arenes.Interestingly, colloidal rhodium catalysts can hydrogenate arenes in aqueous/sc fluid biphasic media whereas the reactions do not work in [BMIM][BF4]. ... 

Table IX illustrates the generation of N,N-dimethylformamide (DMF) and N-methylformamide (MMF), plus formamide, using different ruthenium catalyst precursors dispersed in tetrabutylphosphonium bromide and iodide. In the first entry, treatment of the Ru3(C0)x2 Bu4PI dispersion with CO/H2/NH3 at 220 C for 4 h yielded a liquid product comprising 24% DMF and 43% MMF. The liquid yield increase was 112%. Entry 2...

Scheme 1 Hydroalkoxylation of olefins in the presence of ruthenium catalyst precursors...

Primary and secondary alcohols have been regioselectively added to functionalized olefins such as acrylonitrile, crotonitrile, methacrylonitrile and other unsaturated nitriles in the presence of a ruthenium catalyst precursor containing an amido ligand (Scheme 3) [15,16]. It is assumed that this Michael addition is facilitated by coordination of the nitrile group to the ruthenium centre. 

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