Rhodium catalyzed hydrogenation

Catalysts show remarkable product variation in hydrogenation of simple nitriles. Propionitrile, in neutral, nonreactive media, gives on hydrogenation over rhodium-on-carbon high yields of dipropylamine, whereas high yields of tripropylamine arise from palladium or platinum-catalyzed reductions (71). Parallel results were later found for butyronitrile (2S) and valeronitrile (74) but not for long-chain nitriles. Good yields of primary aliphatic amines can be obtained by use of cobalt, nickel, nickel boride, rhodium, or ruthenium in the presence of ammonia (4J 1,67,68,69). 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Ester formation catalyzed by lipase (Mucor miehei) in conjunction with hydrogenation catalyzed by a rhodium complex Sol-gel immobilization of both catalysts... 

The occurrence of multinuclear catalysts in hydrogenations catalyzed by rhodium-DIOP systems seems unlikely, although the trans-RhCl(CO)(DIOP) complex 43 is dimeric (276), and in basic methanolic solution the 1 1 diphos complex exists as Rh3(diphos)3(OMe)2 + (138a, Section II, B, 1). 

Fig. 3.1 G raphical illustration of numbers of reports per year versus date of publication. Data were obtained by searching the Chemical Abstracts Database using the term hydrogenation catalyzed by ruthenium complexes or osmium complexes or rhodium complexes. These are not comprehensive searches but are still representative of the activity in the field.

Asymmetric hydrogenation of a,fi-unsa turated acids.1 2- Aryl-3-methyl-2-bu-tenoic acids (2) undergo highly stereoselective hydrogenation catalyzed by a complex of rhodium with the chiral (aminoalkyl)ferrocenylphosphine (1), but not with... 

The automatic procedure for time-series reference spectra generation was first demonstrated for the homogeneous catalyzed rhodium hydroformylation of cyclo-octene using Rh4(CO)i2 as precursor, n-hexane as solvent and FTIR as the in situ spectroscopy at 298 K [64]. Upon addition of hydrogen to the system, hydroformylation is initiated. A typical reaction spectrum (k=7) and the pre-conditioned... 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Dehydropeptides (21) were employed for the asymmetric hydrogenation, catalyzed by chiral rhodium complexes of the hydroxyproline derivative (13). It was reported that the stereoselectivity is satisfying (ds = 90-95 %)50). 

The reaction of an alkere with carbon monoxide and hydrogen, catalyzed by cobalt or rhodium salts, to form an aldehyde is called hxdmfonnylaiion (or sometimes the oxo process) ... 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

Hydrogen addition to multiple bonds is catalyzed by certain complex metal salts in solution. This may be described as homogeneous catalysis and, compared to heterogeneous catalysis, is a relatively new development in the area of hydrogenation reactions. Rhodium and ruthenium salts appear to be generally useful catalysts ... 

Homogeneous hydrogenation catalyzed by the four-coordinated rhodium complex, Rh[(C6H5)3P]3Cl, has been particularly well investigated. With this catalyst, the first step is formation of the six-coordinated rhodium hydride of known configuration, 16, in which we abbreviate the ligand, triphenylphosphine, (C6H5)3P, as L ... 

Table 2 Some Other Hydrogenations Catalyzed by Cationic Rhodium-Phosphine Complexes...

This preparation illustrates the alkylation of malononitrile under acid-catalyzed conditions, and the use of diborane for the reduction of a dinitrile to a diamine. The procedure for the preparation of tert-butylmalononitrile has been outlined briefly by Boldt and co-workers.2 The generation of diborane in situ and the general method for nitrile reduction is that described by Brown and co-workers.3 Attempts to reduce the dinitrile to the diamine by other methods including catalytic hydrogenation (5% rhodium on alumina, 5 atm.), lithium aluminum hydride, and lithium aluminum hydride-aluminum chloride were singularly unsuccessful. 

The hydrogenations catalyzed by rhodium/24a are not fast at 1 bar pressure. However, at 5 bar in a well-stirred autoclave TOFs of 200-600 h 1 were reached, which is quite satisfactory even for industrial applications. Even faster reactions can be induced by further increasing the pressure. At elevated pressures the amount of catalyst can be substantially reduced to 0.01-0.1mol%. In all these hydrogenations, the enantioselectivity was not affected in any way by pressures up to 60 bar, in contrast to the deterioration of ee that occurs with increase of pressure when using bisphoshines.31... 

Abley and McQuillin (44) have reported asymmetric homogeneous hydrogenations catalyzed by rhodium complexes of chiral amides. In initial experiments the catalyst was generated in situ by treating trichlorotripyridylrhodium(III) with sodium borohydride in an optically active amide solvent (Fig. 20). In later work a 5% solution of the amide in diethylene glycol monoethyl ether was used and products with the same optical purities were obtained. This evidence indi-... 

Ruthenium and rhodium are more susceptible to inhibition by hydrogen halides than are platinum and palladium. Under mild conditions ruthenium is inhibited even by acetic acid, which is generally a good solvent for hydrogenations over rhodium, palladium, and platinum. Hydrogen chloride may become an inhibitor for rhodium-catalyzed hydrogenations. Freifelder has shown that hydrochloric acid is a strong in-... 

Valence tautomerizfttion. Tris(triphenylphosphine)chlororhodium (in the absence of hydrogen) catalyzes valence tautomerization.6 Thus, when exo-tricyclo[3.2.1.02-4]octene (1) is warmed at 90° with 1.3 moles % of the catalyst it is converted into a mixture of (2, 62%), (3, 32%), and (4, 6%). Rhodium on carbon... 

Similarly, there are examples of rhodium(I) and iridium(I) tertiary phosphine complexes that form isolable dihydrides, which with separate treatment with external base yield monohydrides, equation (k) . Hydrogenations catalyzed by rran5-RhCl(CO)(PPh3)2 " may involve rrani-RhH(CO)(PPh3)2 formed according to equation (1) via an undetected dihydride intermediate. In some aminophosphine analogues, a coordinated N atom may act as proton acceptor s. 

It was shown by Buriak and Osborn [80] that non-micelle-forming anions improved the enantioselectivity of an imine hydrogenation catalyzed by rhodium complexes in the same way as reverse micelles. Complexation of the sulfate or sulfonate anion with the catalyst appears to be responsible for the enhancement of the enantioselectivity. The very strong dependence of the product chirality on the structure of the anion is discussed. Finally, a long-chain ephedrinium salt 17 as surfactant, should be mentioned. 

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