Rhodium catalyzed

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

From Synthesis Gas. A rhodium-catalyzed process capable of converting synthesis gas directly into acetaldehyde in a single step has been reported (83,84). 

The stringency of the conditions employed in the unmodified cobalt 0x0 process leads to formation of heavy trimer esters and acetals (2). Although largely supplanted by low pressure ligand-modified rhodium-catalyzed processes, the unmodified cobalt 0x0 process is stiU employed in some instances for propylene to give a low, eg, - 3.3-3.5 1 isomer ratio product mix, and for low reactivity mixed and/or branched-olefin feedstocks, eg, propylene trimers from the polygas reaction, to produce isodecanol plasticizer alcohol. 

Fig. 4. Mechanism for the TPP-modified rhodium-catalyzed oxo reaction of propylene to -butyraldehyde.

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Ca.ta.lysis, The readily accessible +1 and +3 oxidation states of rhodium make it a useful catalyst. There are several reviews of the catalytic properties of rhodium available (130—132). Rhodium-catalyzed methanol carbonylation (Monsanto process) accounted for 81% of worldwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°0 and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likely reduced in a water... 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

Conventional triorganophosphite ligands, such as triphenylphosphite, form highly active hydroformylation catalysts (95—99) however, they suffer from poor durabiUty because of decomposition. Diorganophosphite-modified rhodium catalysts (94,100,101), have overcome this stabiUty deficiency and provide a low pressure, rhodium catalyzed process for the hydroformylation of low reactivity olefins, thus making lower cost amyl alcohols from butenes readily accessible. The new diorganophosphite-modified rhodium catalysts increase hydroformylation rates by more than 100 times and provide selectivities not available with standard phosphine catalysts. For example, hydroformylation of 2-butene with l,l -biphenyl-2,2 -diyl... 

Rhodium-catalyzed hydroformylation has been studied extensively (16—29). The most active catalyst source is hydridocarbonyltris(triphenylphosphine)rhodium, HRhCO[P(CgH )2]3 (30). However, a molecule of triphenylphosphine is presumed to dissociate to form the active species (21,28). Eurther dissociation could occur as shown ia equation 3. 

In the mid-1980s, Ruhrchemie (now Hoechst) converted its oxo capacity to a proprietary water soluble rhodium catalyzed process (27,28), a technology developed jointly with Rhc ne-Poulenc. Product separation in this process is by decantation. Isomer ratios of n- to isobutyraldehyde of about 20 1 are obtained. 

Mitsubishi Chemical uses a proprietary medium pressure rhodium-catalyzed process (29) in some plants which operate at 90—120°C and 5—10 MPa (725—1450 psi), and gives isomer ratios of about 4 1. 

Allylamines have been used as nitrogen protective groups. They can be removed by isomerization to the enamine (t-BuOK, DMSO) or by rhodium-catalyzed isomerization. ... 

Trifluoromethyl-substituted diazonium betaines [176]. Synthetic routes to trifluoromethyl-substituted diazo alkanes, such as 2,2,2-trifluorodiazoethane [ 177, 7 78, 179] and alkyl 3,3,3-trifluoro-2-diazopropionates [24], have been developed Rhodium-catalyzed decomposition of 3,3,3-tnfluoro-2-diazopropionates offers a simple preparative route to highly reactive carbene complexes, which have an enormous synthetic potential [24] [3-1-2] Cycloaddition reactions were observed on reaction with nitnles to give 5-alkoxy-4-tnfluoromethyloxazoles [750] (equation 41)... 

The rhodium-catalyzed isomerization can also be carried out with the chiral catalyst, Ru2Cl4(diop)3 (H2, 20-80°, 1-6 h, 47-90% yield). In this case, optically enriched enol ethers are obtained. ... 

The Paloc group was developed as an amino acid protective group that is introduced with the p-nitrophenyl carbonate (H2O, dioxane, 68-89% yield). It is exceptionally stable to TFA and to rhodium-catalyzed allyl isomerization, but it is conveniently cleaved with Pd(Ph3P)4 (methylaniline, THF, 20°, 10 h, 74-89% yield). ... 

Rhodium-catalyzed isomerization. Ru(cod)(cot) has been used to convert an allylamine into an enamine."... 

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 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. 

In this context, the use of ionic liquids with halogen-free anions may become more and more popular. In 1998, Andersen et al. published a paper describing the use of some phosphonium tosylates (all with melting points >70 °C) in the rhodium-catalyzed hydroformylation of 1-hexene [13]. More recently, in our laboratories, we found that ionic liquids with halogen-free anions and with much lower melting points could be synthesized and used as solvents in transition metal catalysis. [BMIM][n-CgHi7S04] (mp = 35 °C), for example, could be used as catalyst solvent in the rhodium-catalyzed hydroformylation of 1-octene [14]. 

In the rhodium-catalyzed hydroformylation of 1-hexene, it has been demonstrated that there is a correlation between the solubility of 1-hexene in ionic liquids and reaction rates (Figure 5.3-4) [28]. 

J. Herwig, R. Eischer in Rhodium-catalyzed Hydroformylation in Catalysis by Metal Complexes (P. W. N. M. van Leewen, C. Claver eds.), Kluwer Academic Publisher, The Netherlands,... 

Approximately 2.5 million tons of acetic acid is produced each year in the United States for a variety of purposes, including preparation of the vinyl acetate polymer used in paints and adhesives. About 20% of the acetic acid synthesized industrially is obtained by oxidation of acetaldehyde. Much of the remaining 80% is prepared by the rhodium-catalyzed reaction of methanol with carbon monoxide. 

A rhodium catalyzed reduction of pyrrole 2,5-bis(propanoate) 62 afforded the cis-pyrrolidine 63 en route to the unusual heterocycle, azatriquinane (64) <96TL131>. 

In 1968,Horner et al. [22] and Knowles and Sabacky [23] independently demonstrated that low but definite enantiomeric excesses (up to 15% ee) were produced in the rhodium-catalyzed asymmetric hydrogenation of simple alkenes using methylpropylphenylphosphine 7 as chiral ligand (Scheme 1). 

In conclusion, many chiral pyridine-based ligands have been prepared from the chiral pool and have been successfully tested as ligands for the copper- or rhodium-catalyzed cyclopropanation of olefins. Alfhough efficient systems have been described, sometimes leading interestingly to the major cis isomer, the enantioselectivities usually remained lower than those obtained with the copper-bis(oxazoline) system. 

Graening, T, Friedrichsen, W., Lex, J., Schmalz, H.G. (2002) FacUe Construction of the Colchicine Skeleton by a Rhodium-Catalyzed Cyclization/Cycloaddition Cascade. Angewandte Chemie International Edition, 41, 1524-1526. 

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