Rhodium catalysts, also

Certain sterically hindered rhodium catalysts also lead to improved selectivity. For... 

Multiatomic [6] as well as cationic [7] rhodium catalysts also display a high preference for linear hydroformylation products. However, a catalyst system which generally yields branched hydroformylation products has not yet been found. Vinylarenes, such as styrene (16), form preferentially the (.vo-aldehyde 20 and not the n-aldehydes. The possibility to form a relatively stable Rh- -allyl complex 18 is most likely the decisive factor for this result [8]. Subsequent oxidation of 20 leads to 2-arylpropionic acids 21, of which some derivatives like 22-24 are of great importance as non-steroidal inflammatory drugs (NSID) (Scheme 3) [9]. For their synthesis by the hydroformylation of styrenes, not only a regioselective but also an enantioselective reaction process is... 

Rhodium catalysts, also important in asymmetric hydrogenation (see Section D.2.5.1.), usually show greater versatility and selectivity in asymmetric hydroformylation than cobalt catalysts. Various rhodium precursors are available, such as Rh203, Rh2(OAc)4, Rh2(C2H4)4Cl2, [Rh(nor-bornadiene)Cl]2 , [Rh(l,5-hexadiene)Cl]2118. [Rh(l,5-cyclooctadiene)Cl]238, [Rh4(CO)12]39, [Rh(CO)2Cl]2106, RhH(CO)(PPh3)33, Rh(allyl)3 139. and [Rh(nbd)2]BF467. 

The rhodium catalyst also efficiently promotes the cyclopropenation of alkyncs with high diastereoselectivities or with enantioselectivities up to 98 %. Compared with the related cyclo-propanations this process seems to be more dependent on the size and the configuration of the alkyl group of the diazoester114. If the 5-catalyst is employed the configuration at the cyclo-propene is predominantly S, w hile the 7 -catalyst induces the opposite configuration. 

Catalytic hydrogenation of oximes to amines requires conditions resembling those for catalytic hydrogenation of nitro compounds and nitriles.20d The catalyst should be as active as possible, e.g., Raney nickel101 (if necessary, platinized), platinum oxide,102 palladium-charcoal,103 palladium-barium sulfate,104 or rhodium-alumina.105 This rhodium catalyst also serves for reduction of an amidoxime to the amidine.106 Hydrogenation may be effected under pressure, but the temperature should be kept as low as possible to avoid formation of secondary amines. 

Rhodium catalysts also show quite peculiar behaviour. - The value of Ti/rh is very small at and below 273 K, but between 273 and 423 K it increases very rapidly, so that the products at low temperature resemble those given by platinum, while those at high temperature are like those that palladium gives hydrogen exchange is however much more marked. Increasing temperature therefore favours alkene desorption over addition. 

Recent research in packed beds [38], ceramic monoliths [20] and microreactors [30] also revealed an excellent performance of a Ce-Zr support mixture, which was explained by an increase in surface area by the addition of zirconium oxide [38]. A zirconium oxide-supported rhodium catalyst also revealed similar ignition performance to a mixed Ce-Zr oxide supported rhodium catalyst on the surface of Fecralloy monoliths with trapezoidal channels [9]. A novel route to a support, which might be useful for CPO and OSR, is the synthesis of silicon carbide foam, recently used for steam reforming [44] (Figure 25.2). This support would also be less acidic and is suitable for building a compact foam catalyst... 

Certain sterically hindered rhodium catalysts also lead to improved selectivity. For example, rhodium triphenylacetate improves the selectivity for 11 over 12 from 5 1 to 99 1. ... 

Decarbonylation of aldehydes is frequently used in synthetic organic chemistry [19]. Aromatic aldehydes and enals, but also saturated aldehydes, have been shortened by one C atom with this transformation. In most cases, rhodium complexes were used in a stoichiometric reaction, but also catalytic transformations have been described [20]. It was found that RhCl3-3H20 modified with dppp was less air-sensitive than [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) modified with dppp or the Wilkinson complex and therefore better suited for lab-scale experiments [21]. While using the Wilkinson complex, strictly oxygen-free conditions were essential for the success. Besides homogeneous rhodium catalysts, also supported complexes were suggested recently [22]. The reaction in ionic liquids is a possibility to recycle the precious rhodium complex [23]. 

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. 

Rhodium catalysts also give cyclization products via vinylidene complexes generated in situ (Scheme 3.25) [256, 257]. So enediyne 3.505... 

The uncatalyzed addition of hydrogen to an alkene although exothermic is very slow The rate of hydrogenation increases dramatically however m the presence of cer tain finely divided metal catalysts Platinum is the hydrogenation catalyst most often used although palladium nickel and rhodium are also effective Metal catalyzed addi tion of hydrogen is normally rapid at room temperature and the alkane is produced m high yield usually as the only product... 

This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

Meth5l-l,3-propanediol is produced as a by-product. The hydroformylation reaction employs a rhodium catalyst having a large excess of TPP (1) and an equimolar (to rhodium) amount of 1,4-diphenylphosphinobutane (DPPB) (4). Aqueous extraction/decantation is also used in this reaction as an alternative means of product/catalyst separation. 

A synthesis of optically active citroneUal uses myrcene (7), which is produced from P-piaene. Reaction of diethylamine with myrcene gives A/,A/-diethylgeranyl- and nerylamines. Treatment of the aHyUc amines with a homogeneous chiral rhodium catalyst causes isomerization and also induces asymmetry to give the chiral enamines, which can be readily hydrolyzed to (+)-citroneUal (151). 

With Unsaturated Compounds. The reaction of unsaturated organic compounds with carbon monoxide and molecules containing an active hydrogen atom leads to a variety of interesting organic products. The hydroformylation reaction is the most important member of this class of reactions. When the hydroformylation reaction of ethylene takes place in an aqueous medium, diethyl ketone [96-22-0] is obtained as the principal product instead of propionaldehyde [123-38-6] (59). Ethylene, carbon monoxide, and water also yield propionic acid [79-09-4] under mild conditions (448—468 K and 3—7 MPa or 30—70 atm) using cobalt or rhodium catalysts containing bromide or iodide (60,61). 

Rhodium catalyst is used to convert linear alpha-olefins to heptanoic and pelargonic acids (see Carboxylic acids, manufacture). These acids can also be made from the ozonolysis of oleic acid, as done by the Henkel Corp. Emery Group, or by steam cracking methyl ricinoleate, a by-product of the manufacture of nylon-11, an Atochem process in France (4). Neoacids are derived from isobutylene and nonene (4) (see Carboxylic acids, trialkylacetic acids). 

The dimethyl ketal function (51) is one of the most suitable base stable protecting groups for saturated 5a- and 5/i-3-ketones. It is formed by reaction of the ketone (50) with methanol in the presence of a suitable catalyst. Good selectivity can also be achieved with this group since 2-, 6-, 11-, 12-, 17- and 20-ketones do not form dimethyl ketals under these conditions. The 2-ketone is converted in part to the dimethyl ketal in the presence of homogeneous rhodium catalyst. "" y -Toluenesulfonic acid is the catalyst of... 

The activity of homogeneous catalysts also has been demonstrated Wilkin son s catalyst trisftriphenylphosphme rhodium chloride induces perfluoroalkyl iodides to add to olefins at 80 [70] (equation 10) Tetrakis(triphenylphosphine)-... 

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... 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

Rhodium catalysts have also been used. Benzylic halides were converted to carboxylic esters with CO in the presence of a rhodium complex. In this case, the R could come from an ether R20, a borate ester B(OR )3, or an Al, Ti, or Zr alkoxide. Reaction with an a,co-diiodide, BU4NF and Mo(CO)e gave the corresponding lactone. ... 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

Vinyl boranes add to conjugated ketones in the presence of a rhodium catalyst (with high asymmetric induction in the presence of BINAP) 7 Alkynyl-boranes also add to conjugated ketones, in the presence of... 

In the presence of metal catalysts such as rhodium compounds, aldehydes can add directly to alkenes to form ketones. The reaction of co-alkenyl aldehydes with rhodium catalyst leads to cyclic ketones, with high enantioselectivity if chiral ligands are employed. Aldehydes also add to vinyl esters in the presence of hyponitrites and thioglycolates. ° ... 

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