Rhodium carbonyl, reactions with

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. 

In summary, we have summarized representative examples of transition-metal-catalyzed carbonylative domino reactions. In the area of carbonylations, palladium, rhodium, and cobalt are still the main actors. The abihty of palladium catalysts in carbonylative cross-coupling, rhodium catalysts in carbonylative C-H activation, and cobalt catalyst in carbonylative reactions with unsaturated bonds is impressive. 

Shortly after the report of the reaction of a rhodium carbonyl complex with O2 to give coordinated CO2, the catalytic oxidation of CO to CO2 was reported in a similar system [193]. Kiji and Furukawa reported that both [RhCl(COXPPh3)2] and [RhCl(COXMe2SO)2] catalyzed the oxidation of CO in benzene, ethanol, or dimethylsulfoxide. However, the rate was slow and the catalytic efficiency was poor. Reactions catalyzed by [RhCl(COXMe2SO)2] in benzene gave approximately 11m moles of CO2 per m mole of metal complex. The authors describe the reaction as involving (1) coordination of molecular oxygen, (2) reaction with CO in the coordination sphere, and (3) displacement of the product by unreacted CO. 

The organosilicon reagents also participate in the rhodium-catalyzed reaction with a./S-unsaturated carbonyl compounds, imines, and alkynes (eq 4). The use of optically active diene ligands allows the rhodium-catalyzed transformation to proceed in an enantioselective manner. The reaction can be performed on a gram scale, and the cyclic silyl ether can also be recovered by distillation of a crude product. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Rhodium catalyzed reaction of A -butenyl-l,3-propanediamines 397 with a mixture of H2 and CO gave usually a mixture of hydroformylated 398 and 399 and carbonylated products 400 and 401 in the presence of a phosphite [PPha, PBu3, PCCgHiOa, P(o-tol)3] (97TL4315, 97T17449). When the hindered biphosphite, BIPEPHOS, and a 9 1 or 1 1 mixture of H2 and... 

The intermolecular version of the above described reaction has also been reported [92]. In the first example the reaction of a rhodium catalyst carbonyl ylide with maleimide was studied. However, only low enantioselectivities of up to 20% ee were obtained [92]. In a more recent report Hashimoto et al. were able to induce high enantioselectivities in the intermolecular carbonyl ylide reaction of the... 

The rhodium-catalyzed tandem carbonyl ylide formation/l,3-dipolar cycloaddition is an exciting new area that has evolved during the past 3 years and high se-lectivities of >90% ee was obtained for both intra- and intermolecular reactions with low loadings of the chiral catalyst. 

The anionic Rh(I) porphyrin [Rh(OEP) induced ring-opening reactions with 4- and 5-membered ring lactones to give organometallic products with the rhodium bonded to the alkoxide carbon rather than the carbonyl carbon. 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

The carbonylation of methanol was developed by Monsanto in the late 1960s. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. An older method involves the same carbonylation reaction carried out with a cobalt catalyst (see Section 9.3.2.4). For many years the Monsanto process has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATIVA process, developed by BP, has come on stream (see Section 9.3.2) ... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

However, when a less active olefin (e.g., diisobutylene or cyclohexene) or a liganded system (Bu3P/Co = 2/1,80 atm CO/H2, 190°C) was used, the hydrido species, e.g., HCo(CO)3PBu3, predominated throughout the reaction. The author concluded that in slower systems, initial interaction of the olefin with the hydrido species HCo(CO)3L could be the ratedetermining step. These results are complementary to those discussed (vide supra) for the rhodium carbonyl catalysis. 

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

Pettit and coworkers—metal hydride intermediates by weak base attack over Fe carbonyl catalysts. Pettit et al.ls approached the use of metal carbonyl catalysts for the homogeneous water-gas shift reaction from the standpoint of hydroformyla-tion by the Reppe modification.7 In the typical hydroformylation reaction, an alkene is converted to the next higher aldehyde or alcohol through reaction of CO and H2 with the use of a cobalt or rhodium carbonyl catalyst. However, in the Reppe modification, the reduction is carried out with CO and H20 in lieu of H2 (Scheme 6) ... 

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