Rhodium catalysts hydrocarbonylation

Rhodium Ca.ta.lysts. Rhodium carbonyl catalysts for olefin hydroformylation are more active than cobalt carbonyls and can be appHed at lower temperatures and pressures (14). Rhodium hydrocarbonyl [75506-18-2] HRh(CO)4, results in lower -butyraldehyde [123-72-8] to isobutyraldehyde [78-84-2] ratios from propylene [115-07-17, C H, than does cobalt hydrocarbonyl, ie, 50/50 vs 80/20. Ligand-modified rhodium catalysts, HRh(CO)2L2 or HRh(CO)L2, afford /iso-ratios as high as 92/8 the ligand is generally a tertiary phosphine. The rhodium catalyst process was developed joindy by Union Carbide Chemicals, Johnson-Matthey, and Davy Powergas and has been Hcensed to several companies. It is particulady suited to propylene conversion to -butyraldehyde for 2-ethylhexanol production in that by-product isobutyraldehyde is minimized. 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

The hydroformylation of alkenes generally has been considered to be an industrial reaction unavailable to a laboratory scale process. Usually bench chemists are neither willing nor able to carry out such a reaction, particularly at the high pressures (200 bar) necessary for the hydrocarbonylation reactions utilizing a cobalt catalyst. (Most of the previous literature reports pressures in atmospheres or pounds per square inch. All pressures in this chapter are reported in bars (SI) the relationship is 14.696 p.s.i. = 1 atm = 101 325 Pa = 1.013 25 bar.) However, hydroformylation reactions with rhodium require much lower pressures and related carbonylation reactions can be carried out at 1-10 bar. Furthermore, pressure equipment is available from a variety of suppliers and costs less than a routine IR instrument. Provided a suitable pressure room is available, even the high pressure reactions can be carried out safely and easily. The hydroformylation of cyclohexene to cyclohexanecarbaldehyde using a rhodium catalyst is an Organic Syntheses preparation (see Section 4.5.2.5). 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Nearly quantitative yields are obtained when 1,4-dienes containing a quaternary center at carbon-3 (R = alkyl, aryl, not H) are converted in the presence of rhodium catalysts 13 14. Here double-bond isomerization in the substrate to form conjugated dienes is suppressed. This hydrocarbonylative cyclization of 1,4-dienes with carbon monoxide gives cyclopentanone products with medium to high regio- and stereoselectivities14-17. Thus, the hydrocarbonylative cyclization of 2.3,3-trimethyl-1.4-pentadiene and similar substrates give predominantly the m-producls [turns cis 1 6). 

Various other rhodium catalysts can initiate hydroacylation reactions. Thus, the indenyl complex [075-C9H7)Rh(J72-C2H4)2] is used in intermolecular hydroacylation44. Rhodium zeolites (RhNaX and RhNaY type zeolites) act as bifunctional catalysts for the synthesis of 2-methyl-3-hexanone and 4-heptanone (1 2 ratio) from propene, carbon monoxide and hydrogen53. In this case, the ketones may be formed via hydrocarbonylation (vide supra), however, according to control experiments, rhodium-free zeolites alone catalyze ketone formation from propene and butyraldehyde53. 

Cobaltcarbonyls are used as catalysts for carbonylations such as hydroformylation (0x0 reaction), hydrocarbonylation and amidocarbonylation [70a]. Hydroformylation is the reaction of preparing aliphatic aldehydes in which carbon number is increased by one [46]. Especially, butylaldehyde has been industrially produced largely from propylene butylaldehyde is used as a raw material for butanol and 2-ethylhexanol, etc. Cobalt and rhodium compounds are used for their catalysts. The reactivity of cobalt catalysts is lower than that of rhodium catalysts. However, more linear products of the reaction shown in eq. (17.27) are obtained. The ratio of... 

The carbonyl group in a molecule has an important directing effect on the location of the hydrocarbonyl addition. Formation of 1,3-dialdehydes is favored. Especially high yields, with a higher proportion of 1,4-dialdehyde or subsequent diol, are obtained with rhodium catalyst [174]. 

In this manner, for instance, the Shell hydroformylation process has been carried to commercial operation (see p. 22). The Shell catalyst (trialkyl phosphine/cobalt hydrocarbonyl), because of its high stability, is especially useful for working at lower CO partial pressures. Rhodium catalysts are also suitable for one-step alcohol syntheses they also catalyze the homogeneous hydrogenation of aldehyde groups [174, 253, 326, 327]. Rhodium catalysts also allow much higher reaction rates than cobalt catalysts. The reaction goes especially smoothly when the rhodium is added as carbonyl or in the form of its oxide. 

Hydroformylation, or the 0X0 process, is the reaction of olefins with CO and H9 to make aldehydes, which may subsequently be converted to higher alcohols. The catalyst base is cobalt naph-thenate, which transforms to cobalt hydrocarbonyl in place. A rhodium complex that is more stable and mnctions at a lower temperature is also used. 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

The reaction of alkenes (and alkynes) with synthesis gas (CO + H2) to produce aldehydes, catalyzed by a number of transition metal complexes, is most often referred to as a hydroformylation reaction or the oxo process. The discovery was made using a cobalt catalyst, and although rhodium-based catalysts have received increased attention because of their increased selectivity under mild reaction conditions, cobalt is still the most used catalyst on an industrial basis. The most industrially important hydrocarbonylation reaction is the synthesis of n-butanal from propene (equation 3). Some of the butanal is hydrogenated to butanol, but most is converted to 2-ethylhexanol via aldol and hydrogenation sequences. 

Rhodium carbonyl catalysts effect hydrocarbonylation under very mild reaction conditions at higher rates and selectivity. Rhodium introduced as Rh4(CO)i2, for example, probably is converted to RhH(CO) , but there is no direct evidence for such species. These rhodium carbonyls are not particularly attractive since they produce mostly branched aldehydes. 

Table V. Influence of the Structure of the Substrate on the Prevailing Chirality and on the Maximum Optical Yield Obtained in Asymmetric Hydrocarbonylation with Rhodium or Palladium Catalysts in the Presence...

Example 5.2. Hydroformylation of propene [2]. Hydroformylation converts an olefin to an aldehyde of next higher carbon number by addition of carbon monoxide and hydrogen. The reaction is catalyzed by dissolved hydrocarbonyl complexes of transition-metal ions such as cobalt, rhodium, or rhenium. The carbon atom of the carbon monoxide can attach itself to the carbon atom on either side of the olefinic double bond, so that two aldehyde isomers are formed. If the catalyst also has hydrogenation activity, the aldehydes are converted to alcohols and paraffin is formed as by-product. For propene and such a catalyst the (simplified) network is ... 

Besides hydrocarbonylation of olefins with carbon monoxide, hydroacylation can also be achieved by addition of aldehydes to olefins in the absence of carbon monoxide. This reaction is usually induced by rhodium complexes, mainly of the Wilkinson s catalyst type. Other catalysts are also active, e.g., systems derived from ruthenium complexes. Hydroacylation via aldehyde addition reactions has only rarely been surveyed24. 

Catalysts are typically cobalt hydrocarbonyl or either cobalt or rhodium-based complexes modified with phosphines. The original commercial catalyst was cobalt hydrocarbonyl which required temperatures of 110-170°C (230-340°F) and 100-275 bars (1500-4000 psig) pressure. The newer cobalt and rhodium complexes allow the reaction pressures to be reduced to the 20-35 bars (300-500 psig) range. Most producers have switched to the newer catalysts, however, a few plants still operate with the original cobalt 0x0 catalyst. 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

Active catalysts are nickel, cobalt, iron, rhodium, ruthenium and palladium, as well as their salts, carbonyls or hydrocarbonyls. 

Besides nickel and cobalt, almost all of the catalysts discussed in the last chapter which were suited for the formation of free acids can be applied, e. g. rhodium, palladium and, with certain restrictions, iron. Cobalt hydrocarbonyl catalyzes the stoichiometric ester synthesis at mild reaction conditions [35, 121]. The initially formed acylcobalt carbonyls react rapidly with alcohols even at 50 °C and, in the presence of Na-alcoholate, even at 0 °C to give esters [121]. Dienes with isolated double bonds react with carbon monoxide and alcohols at mild reaction conditions in the presence of Pd/HCl to give unsaturated monocarboxylic acid esters and at more severe conditions to give saturated dicarboxylic acid esters [508]. 

The following facts support the assumption that the true catalysts are the transition metal hydrocarbonyls rather than the corresponding metal carbonyls. Only metals which can form hydrocarbonyls, like cobalt, rhodium and iron, can act as catalysts [123, 280, 673, 674], whereas nickel, e.g., is inactive [123, 673] in most cases. Furthermore it is known that cobalt, rhodium and iron carbonyls, under the reaction conditions applied, are able to abstract hydrogen from alcohols, amines and even from the unreactive cycloparaffins to form metal hydrocarbonyls [121-124]. 

More recently, systems based on polypyridine coordination compounds of ruthenium(II) [46-49], rhodium(I) [50a] and iridium(I) [50] have been shown to efficiently catalyse the thermal WGSR. An important effect of the substituent ortho to the nitrogen atom of the ligand has been demonstrated in the case of Ir(I) leading to one of the most efficient catalysts known today [50b]. [Ru(bpy)2(CO)Cl] has also been studied and all of the possible intermediates within the catalytic cycle (hydrocarbonyl complex, metal hydride, aquo species) have been isolated and characterized [48]. 

Petroleum-based production of -butanol is through the oxo process, which involves first the production of -butyraldehyde by the carboxylation of propylene with carbon monoxide and hydrogen over a rhodium hydrocarbonyl catalyst. Further reduction of -butyraldehyde with hydrogen produces -butanol [4, 6, 7]. Isobutanol is co-produced with -butanol in the oxo process or through the... 

Celanese has developed the so-caUed TCX process," in which methanol, formed from synthesis gas from the steam reforming of natural gas, is converted via a hydro-carbonylation reaction into acetic acid. The hydrocarbonylation of methanol is the standard method to make acetic acid via the aid of a rhodium-based catalyst as described in UUmann (Ulhnaim s Encyclopedia of Industrial Chemistry (2), 2005) ... 

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