Rhodium system

There appears to be concentration of rhodium in the surface of the iridium-rhodium clusters, on the basis that the total number of nearest neighbor atoms about rhodium atoms was found to be smaller than the nunber about iridium atoms in both catalysts investigated. This conclusion agrees with that of other workers (35) based on different types of measurements. The results on the average compositions of the first coordination shells of atoms about iridium and rhodium atoms in either catalyst Indicate that rhodium atoms are also incorporated extensively in the interiors of the clusters. In this respect the iridium-rhodium system differs markedly from a system such as ruthenium-copper (8), in which the copper appears to be present exclusively at the surface. 

The Rh and Ir complexes 85-88 (Fig. 2.14) have been tested for the intramolecular hydroamination/cyclisation of 4-pentyn-l-amine to 2-methyl-1-pyrroline (n = 1). The reactions were carried out at 60°C (1-1.5 mol%) in THF or CDCI3 The analogous rhodium systems were more active. Furthermore, the activity of 87 is higher than 85 under the same conditions, which was attributed to the hemilabihty of the P donor in the former complex, or to differences in the trans-eSects of the phosphine and NHC ligands, which may increase the lability of the coordinated CO in the pre-catalyst [75,76]. 

Modified rhodium systems show considerable activity in the hydroformylation of styrene to the branched aldehydes. Chiral diphosphines, diphosphites, and phosphine-phosphites have been the ligands most studied. Hydroformylation experiments have often been performed in situ but the characterization of intermediates has provided an interesting contribution to coordination chemistry.179... 

The hydroformylation of styrene using rhodium systems containing the four structurally related diphosphines dppe, dppp, (86), and (87) has been studied. A systematic analysis of the effect of the pressure, temperature, and the ligand metal molar ratio shows that the five- and six-membered ring chelating diphosphines behave differently from one another.347 An analysis of the effect of pressure, temperature, and ligand metal molar ratio on the selectivity of styrene hydroformylation catalyzed... 

PMMA and Model Reactions. Reactions between PMMA and red phosphorus were carried out with the ratio of PMMA to red phosphorus varying from 10 1 to 1 1, i.e., 1.0g PMMA and 0.1 g phosphorus to 1.0g PMMA and 1.0g phosphorus, at temperatures ranging from 300°C to 375°C. The PMMA-rhodium system used a 1.0g sample of PMMA and 0.5g of CIRh(PPh3)3 at 260°C for 2 hours. Reactions with the model compound, dimethyiglutarate, DMG, utilized a 1 1 stoichiometry, 0.17g DMG and 1.0g CIRh(PPh3)3 (4-7). 

It is unlikely that CIRh(PPh3)3 will ever be useful as a flame retardant due to its red color, expense, and the potential toxicity associated with a heavy metal. An additional disadvantage of the rhodium system is the fact that char formation occurs at a temperature of 250°C, since this is near the processing temperature of PMMA char formation may occur during processing rather than under fire conditions. This discovery is nonetheless... 

Similar results were obtained by Heil and Marko (48) for an unmodified rhodium system (see Table IV). 

One of the most interesting observations is that the a-isomer (9) contains a quaternary carbon atom attached to the formyl group, which is a violation of the rule of Keulemans (49). It must be that electronic effects are dominant for this type of substrate, particularly when the effects of phosphine-modified rhodium systems are considered (vide infra). 

Both modified cobalt and modified rhodium systems have been successfully employed. In general, both have produced good yields of linear aldehyde. Results are tabulated in Table XXVIII. 

In view of rapidly increasing raw material prices and plant construction costs, as well as more stringent environmental standards, it is likely that future practices will favor high-efficiency processes which operate under mild reaction conditions with few by-products. Modified rhodium is advantageous in these respects, as shown in Tables XXXIII and XXXIV. In view of the recent successes with commercial rhodium systems (103, 104, 130, 131), it is likely that these will find more extensive use in the next few years. 

The system is not limited to the use of synthesis gas as feed. Mixtures of carbon dioxide and hydrogen also give rise to the formation of polyhydric alcohols, and it is also claimed that the reaction mixture can consist of steam and carbon monoxide (62). This latter claim is consistent with the presence of C02 in the reaction mixture when CO/H2 is used as feed [infrared data (62)], and suggests that these ionic rhodium systems are also active catalysts for the water gas-shift reaction (vide infra). 

With reference to the homogeneous catalyst systems thus far reported for the synthesis of hydrocarbons/chemicals from carbon monoxide and hydrogen, only the anionic rhodium systems of Union Carbide show any appreciable shift activity. With neutral species of the type M3(CO)12 (M = Ru or Os), only small quantities of carbon dioxide are produced under the synthesis conditions (57). 

In addition to the polymeric rhodium catalysts previously discussed, monomeric rhodium systems prepared from [Rh(CO)2Cl]2 by addition of strong acid (HC1 or HBF4) and Nal in glacial acetic acid have also been shown to be active homogeneous shift catalysts (80). The active species is thought to be an anionic iodorhodium carbonyl species, dihydrogen being produced by the reduction of protons with concomitant oxidation of Rh(I) to Rh(III) [Eq. (18)], and carbon dioxide by nucleophilic attack of water on a Rh(III)-coordinated carbonyl [Eq. (19)]. 

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. 

The differences between the iridium and rhodium systems were interpreted by considering that Int 1 may open an oxygenation path via intramolecular rearrangement. In this case, the scission of a M-0 bond between the semi-quinone and metal center would be followed by intra-diol insertion of an oxygen and ultimately by the formation of muconic acid and water. The results indicate partial preference of the rhodium complex toward the oxygenation path. 

Josiphos-rhodium systems have been also used to hydrogenate 2- or 3-substi-tuted pyridines and furans, yet both the activities (TOF = 1-2) and enantioselec-tivities were rather low (Scheme 16.22) [86, 87]. Comparable results were obtained with a number of chiral chelating diphosphines of various symmetries. 

Morken and co-worker (58) used a similar approach for the discovery of a selective reductive aldol catalyst. Through screening 192 different sets of reaction conditions Morken settled on a rhodium system shown in Scheme 28. This system is an excellent example of the power of this type of approach. Three parameters were screened simultaneously. It was shown that the selectivity and yield of the reaction are dependent on the hydride source, transition metal and ligand used on that transition metal. In this case, GC was used to evaluate the results. 

A similar system based on rhodium has been studied (123) and was found to be less active than the equivalent iridium catalysts. Selective hydrogenation of acetylenes to olefins and dienes to monoolefins can be performed using the rhodium system, and the authors note that although propan-2-ol is an effective source of hydrogen (via oxidation to acetone), mild pressures of hydrogen gas can also be employed. 

Abstract This chapter presents the latest achievements reported in the asymmetric hydroformylation of olefins. It focuses on rhodium systems containing diphosphites and phosphine-phosphite ligands, because of their significance in the subject. Particular attention is paid to the mechanistic aspects and the characterization of intermediates in the hydroformylation of vinyl arenes because these are the most important breakthroughs in the area. The chapter also presents the application of this catalytic reaction to vinyl acetate, dihydrofurans and unsaturated nitriles because of its industrial relevance. 

After the discovery of the high ee provided by rhodium/diphosphite and rhodium/phosphine-phosphite complexes, with total conversion in aldehydes and high regioselectivities, rhodium systems became the catalysts of choice for asymmetric hydroformylation. Important breakthroughs in this area have been the use of rhodium systems with chiral diphosphites derived from... 

Although aldehydes obtained through the hydroformylation of dihydrofurans are interesting building blocks for organic synthesis, few studies have been reported on the subject. In 1998, previous work on the control of the regio-selectivity in the hydroformylation of dihydrofurans has been reported with rhodium systems modified with different ligands [77,78]. In the hydroformylation of 2,5-dihydrofuran 46 the expected product is the tetrahydrofuran 3-carbaldehyde 49 (Scheme 7). 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

A particularly interesting aspect is the dependence of rate on [H2O] which attains a maximum at ca. 5wt% H2O. The ability to operate at relatively low [H2O] results in a less costly product purification than the high-water rhodium system. 

DR. MORTON HOFFMAN (Boston University) I would like to present some of our results on the rhodium system. I would like to focus on the fate of the rhodium(I) species and, in particular, consider the path of the production of molecular hydrogen from water, which, of course, is the basis of so many of these studies of reduced metal species. 

The reaction of alcohols with CO can also be catalysed by palladium iodides, and various ligands or solvents. Acetic acid is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a palladium compound, an ionic iodide compound, a sulfone solvent at conditions similar to those of the rhodium system (180 °C, 60 bar), and, in some cases, traces of a nickel-bipyridine compound were added. Sulfones or phosphine oxides play a stabilising role in preventing metal precipitation [26], Palladium(II) salts catalyse the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective [27],... 

With the exceptions of a few rhodium systems (see following), the catalytic pyridine-synthesis relies exclusively on cobalt as the active metal. The reaction can be carried out advantageously in a one-pot reaction by generating the cobalt catalysts in situ [Eq.(2)] (74GEP2416295, 74S575 75USP4006149). 

Muller has explored enantioselective C-H insertion using optically active rhodium complexes, NsN=IPh as the oxidant, and indane 7 as a test substrate (Scheme 17.8) [35]. Chiral rhodium catalysts have been described by several groups and enjoy extensive application for asymmetric reactions with diazoalkanes ]46—48]. In C-H amination experiments, Pirrung s binaphthyl phosphate-derived rhodium system was found to afford the highest enantiomeric excess (31%) of the product sulfonamide 8 (20equiv indane 7, 71% yield). 

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