Rhodium-palladium hydrogenation catalyst

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... 

Alkenes react with hydrogen in the presence of a platinum palladium rhodium or nickel catalyst to form the corresponding alkane... 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

The above generalities apply particularly to palladium. Hydrogenation over platinum or rhodium are far less sensitive to the influence of steric crowding. Reduction of 1-t-butylnaphthalene over platinum, rhodium, and palladium resulted in values of /ci//c2 of 0.42, 0.71, and 0.024, respectively. Also, unlike mononuclear aromatics, palladium reduces substituted naphthalenes at substantially higher rates than does either platinum or rhodium. For example, the rate constants, k x 10 in mol sec" g catalyst", in acetic acid at 50 C and 1 atm, were (for 1,8-diisopropylnaphthalene) Pd (142), Pt(l8.4), and Rh(7.1)(25). 

Styrene, a-ethyl-asymmetric hydroformylation catalysts, platinum complexes, 6, 266 asymmetric hydrogenation catalysts, rhodium complexes, 6, 250 Styrene, a-methyl-asymmetric carbonylation catalysis by palladium complexes, 6, 293 carbonylation... 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

The choice of the metals is strictly related to the catalytic application. As we shall show later, the catal54ic reaction most commonly investigated with polymer supported M / CFP catalysts is hydrogenation (Table 3). The overwhelming majority of catalytic studies concerns the hydrogenation of alkenes and by far the most commonly employed metal is palladium, followed by platinum. Examples of rhodium and ruthenium hydrogenation catalysts supported on pol5uneric supports are very rare. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

This type of hydrodehalogenation has been performed generally in the presence of organic or inorganic bases to neutralize the hydrogen halides formed. Among published results, the use of rhodium complexes as catalysts dominates, but palladium and ruthenium complexes have also been applied on a frequent basis. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

In this chapter, we focus on the rhodium-catalyzed hydrogenation and the development of chiral phosphorous ligands for this process. Although there are other chiral phosphorous ligands, which are effective for ruthenium-, iridium-, platinum-, titanium-, zirconium-, and palladium-catalyzed hydrogenation, they are not discussed in this account. However, this does not preclude complexes of other transition metals as effective catalysts for asymmetric hydrogenation. Fortunately, there are numerous reviews and books that discuss this particular aspect of asymmetric hydrogenation [3]. 

Table II summarizes the systems that have been studied as hydrogenation catalysts. The majority of these involve ds metal complexes mainly rhodium(l), palladium(II), and platinum(II), which have been found to be the most successful of the homogeneous hydrogenation catalysts.

Yields of cis-isomer in the hydrogenation of 1,4-dialkylcyclohexenes over rhodium, palladium and platinum catalysts at atmospheric pressure and 25°C in acetic acid solution... 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

Heterogeneous hydrogenation of the C=N bond is a very widely used synthetic process with application to small and large-scale reactions. Many of the catalysts described in other sections may also be employed, for example those based on supported rhodium, palladium etc, and Raney Nickel. This area has been reviewed extensively recently192. Hydrogenation of oximes and hydrazones results in formation of amines. Milder conditions can be used for oxime reduction if the ethylaminocarbonyl derivative is prepared in situ prior to reduction276. 

HYDROGENATION, CATALYSTS Nickel on alumina. Nickel-Graphite. Palladium-Poly(ethylenimine). Palladium catalysts. Raney nickel. Rhodium catalysts. 

Riesz and Weber compared the selectivities of commercial platinum, palladium, rhodium, and nickel catalysts for hydrogenation of linolenic components in soybean oil.110 Representative results are summarized in Table 3.8. Certain platinum metal catalysts showed higher selectivities than nickel catalysts, as indicated by the values of SL (A in Scheme 3.13) = 2.4-2.7. Generally, nickel catalysts showed selectivities... 

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