Rhodium complexes oxidation catalysts

The use of a catalyst such as cadmium oxide increases the yield of dibasic acids to about 51% of theoretical. The composition of the mixed acids is about 75% C-11 and 25% C-12 dibasic acids (73). Reaction of undecylenic acid with carbon monoxide using a triphenylphosphine—rhodium complex as catalyst gives 11-formylundecanoic acid, which, upon reaction with oxygen in the presence of Co(II) salts, gives 1,12-dodecanedioic acid in 70% yield (74). 

At present, the main industrial catalyst of ammonia oxidation is platinum and its alloys with aluminium and rhodium. Taking into account the deficit and high cost of platinum metals, the dcCTcasing of the consumption and losses of platinum metals is an urgent problem. Therefore, several compositions of complex oxide catalysts have been developed with iron (111), cobalt and chromium oxides as an active component. Complex oxides with perovskite structure are used as new catalysts they provide selective oxidation of ammonia with an yield not less than 90 %. The authors of [33] proposed to use perovskite powders LaMeOj, where Me=Fe, Co, Ni, Cr, Mn, and La,.,Sr,Me03, where Me=Co, Mn and x=0.25-0.75. To prepare these compounds, they used the precipitation by tetraethyl ammonia from diluted nitrate solutions taken at necessary ratios. The powders as prepared are poorly molded as in the form of honeycomb stractures as well as in the form of simple granules. 

Hydrogenation of ketones generally proceeds with more difficulty than reduction of olefins. This is due in part to lesser stability of complexes with ketones (which activate hydrogen) compared to olefin complexes and to the ability to coordinate of the resulting alcohols in contrast to the alkanes. Moreover, the resulting secondary alcohols show a tendency to oxidize themselves back to ketones. Some rhodium complexes oxidize secondary alcohols to ketones. Wilkinson s complexes RhX(PR3)3 do not represent suitable catalysts for reduction of aldehydes, because decarbonylation of the substrate and the formation of compounds of the type RhCl(CO)(PR3)2 takes place these compounds are not catalytically active. However, hydrogenation of ketones is effectively accelerated by complexes such as [Rh(NBD)(PR3) ] CIO (w = 2, 3). The... 

Pd(OAc)2 in AcOH at 120 C for 2 h] (62) were not suited for this preparation. However, the compound 131 was obtained in 74-85% yield when the reaction was carried out in die presence of sodium hydrogoi carlwnaie and chloranil as an oxidizing agent in an aprotic solvent, such as 1,2-dkhloroethane or 1,2,4-trichlorobenzene. Asymmetric reduction in the presence of a rhodium-complex as catalyst afforded the 4-bromotryptophan 133 (Scheme 51). 

The most recent catalysts that operate under thermal conditions were then based on the premise that a Cp M fragment with ligands that dissociate under thermal conditions could be a catalyst for alkane borylation. After a brief study of Cp IrH4 and Cp Ir(ethylene)2, Dr. Chen studied related rhodium complexes. Ultimately, he proposed that the Cp Rh(ri" -C6Me6) complex would dissociate CeMce as an iimocent side product, and that Cp Rh(Bpin)2 from oxidative addition of pinBBpin (pin=pinacolate) would be the active catalyst. The overall catalytic... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

The bluish white, hard, yet ductile, metal is inert to all acids and highly non-abrasive. Used for heavy-duty parts in electrical contacts and spinning jets. Reflectors are prepared from the mirror-smooth surfaces (e.g. head mirrors in medicine). Thin coatings provide a corrosion-resistant protective layer, for example, for jewelry, watches, and spectacle frames. The metal is a constituent of three-way catalysts. Rhodium complexes are used with great success in carbonylations (reactions with CO) and oxidations (nitric acid) in industry. Platinum-rhodium alloys are suitable thermocouples. 

It was recognized during the development of propene hydroformylation that propene provided some stabilization for the catalyst. In the absence of the alkene, but in the presence of carbon monoxide and hydrogen, the catalyst can undergo what has been termed intrinsic deactivation. [3 3] Apparently after oxidative addition of triphenyl-phosphine to rhodium, diphenylphosphido bridged rhodium complexes are formed. 

Butyne-l,4-diol has been hydrogenated to the 2-butene-diol (97), mesityl oxide to methylisobutylketone (98), and epoxides to alcohols (98a). The rhodium complex and a related solvated complex, RhCl(solvent)(dppb), where dppb = l,4-bis(diphenylphosphino)butane, have been used to hydrogenate the ketone group in pyruvates to give lactates (99) [Eq. (15)], and in situ catalysts formed from rhodium(I) precursors with phosphines can also catalyze the hydrogenation of the imine bond in Schiff bases (100) (see also Section III,A,3). 

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

Scheme 65 outlined a proposed mechanism in which rhodium served as catalyst for the conjugate addition, and the C-Bi bond was proposed to be oxidatively added to the Rh complex. 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The two-substituted-Quinazolinap-derived rhodium complexes proved extremely efficient catalysts for the hydro-boration-oxidation of vinylarenes (Table 6). For styrene derivatives, in most cases quantitative conversions were obtained after just 2 h at the relevant temperature (entries 1-6). Higher enantioselectivities were afforded with a 4-methoxy substituent (up to 95% ee, entry 3) compared to the 4-chloro or unsubstituted styrene analogs (entries 5 and 1), a trend also observed in hydroboration with rhodium complexes of QUINAP 60. This highlights that both the electronic nature of the substrate combined with the inherent steric properties of the catalyst are important for high asymmetric induction. It is noteworthy that in most cases, optimum enantioselectivities were afforded by the... 

The first step consists of the substitution of one of the ligands (L) of 18 by dioxane (39) in an oxidative addition (a) (Scheme 20.16). / -Elimination of 40 releases 2,3-dihydro-dioxine (41) and the 16-electron dihydrogen rhodium complex (42) (b). Alkene 43 coordinates to the vacant site of 42 (c) to give complex 44. A hydride insertion then takes place (d), affording complex 45. After a reductive elimination (e) of the product 46, the coordination of a ligand reconstitutes the Wilkinson-type catalyst (18). 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

The rate-determining step in this process is the oxidative addition of methyl iodide to 1. Within the operating window of the process the reaction rate is independent of the carbon monoxide pressure and independent of the concentration of methanol. The methyl species 2 formed in reaction (2) cannot be observed under the reaction conditions. The methyl iodide intermediate enables the formation of a methyl rhodium complex methanol is not sufficiently electrophilic to carry out this reaction. As for other nucleophiles, the reaction is much slower with methyl bromide or methyl chloride as the catalyst component. 

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