Rhodium-ruthenium catalysts product selectivity

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... 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

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). 

The application of various ruthenium compounds in acetonitrile330 or rhodium and ruthenium catalysts or their mixtures331 was found to show significant improvements (high product selectivities, high linearities) in the aminomethylation of terminal alkenes to produce tertiary amines. 

Oxygen-containing Cl and C2 molecules can be efficiently synthesized from CO and H2 (syngas) using cobalt, rhodium, and ruthenium catalysts. Among these catalysts, ruthenium is very efficient and selectively provides products with less than C2 units [8,9] this is in contrast to the Rh and Co catalysts, which produce byproducts with more than C3 units (Eq. 11.2). 

This approach should be useful in determining the direction of hydrogenation for molecules in which the carbinol group is replaced by carbon-carbon or carbon-nitrogen double bonds. With an alkene, though, the simple conformational model would have to be used and the hydrogenation should be run under conditions that do not promote double bond isomerization, that is, not with palladium or nickel catalysts. With carbonyl compounds the preferred eonditions for selective reaction involve platinum, rhodium or ruthenium catalysts imder non-diffusion control conditions. The use of nickel catalysts, especially Raney nickel, with its basic components, can cause an equilibration of the alcohol product. 

When an enantiomerically pure catalyst is used in the directed hydrogenation of a racemic reactant then the enantiomers will react at intrinsically different rates since the transition states are diastereomeric. With some rhodium and ruthenium catalysts the difference [expressed as a selectivity factor S = k(fast)/k(slow)] is > 10 which makes the process synthetically useful. This is the case for all the x-hydroxyalkylacrylates described in Section 2.5.1.1.1. when complexes based on rhodium(DiPAMP)+ are employed as catalysts. The procedure is operationally simple in that the reaction is run to the point where the enantiomeric excess of recovered reactant is ca. 95% (57-65 % reaction) and the hydrogenation is then stopped. The starting material and product arc separated after removal of the catalyst. Similar results are obtained for a-hydro-xyalkyl-39, x-amidoalkyl-40 and a-carboxyalkylacrylatcs41 (entries 1-3, in the table below). 

Rhodium and ruthenium catalysts may alternatively be used and sometimes show useful selective properties. Rhodium allows hydrogenation of alkenes without concomitant hydrogenolysis of an oxygen function. For example, hydrogenation of the plant toxin, toxol 5 over rhodium-alumina gave the dihydro compound 6 (7.6) with platinum or palladium catalysts, however, extensive hydrogenolysis took place and a mixture of products was formed. 

Wanat et al. investigated methanol partial oxidation over various rhodium containing catalysts on ceramic monoliths, namely rhodium/alumina, rhodium/ceria, rhodium/ruthenium and rhodium/cobalt catalysts [195]. The rhodium/ceria sample performed best. Full methanol conversion was achieved at reaction temperatures exceeding 550 °C and with O/C ratios of from 0.66 to 1.0. Owing to the high reaction temperature, carbon monoxide selectivity was high, exceeding 70%. No by-products were observed except for methane. 

Formation of carbamates or ureas by carbonylation reactions of nitrosoarenes is not an important reaction. Only in one case has the synthesis of phenylcarbamates been reported to be catalysed by a rhodium complex and the selectivity was very poor (< 33 %) [169]. RhCl(COD)(PhNO) was the best catalyst, but several ruthenium, palladium and platinum complexes were also tested. Since the main product of these reactions was almost always azoxybenzene, they are discussed in Chapter 4. 

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