Rhodium and Ruthenium Catalysts

Rh(acac)(C0)2 as the catalyst in acetic acid/acetylacetone under an overall pressure of 21 bar, styrene was obtained in high selectivity, albeit in low yield (TON = 23, turnover frequency TOP = 1.88 x 10 s ). The presence of both acetic acid and acetylacetone is crucial the nature of the rhodium(I) precursor is less critical, almost the same results having been obtained with Wilkinson s catalyst RhClfPPhsfs. 

The mechanism is proposed to be analogous to that of the Fujiwara process, in that the initial step consists of an electrophilic attack of the aromatic C—H bond by rhodium(III) under formation of a a-arylrhodium intermediate followed by insertion of ethylene into the C—Rh bond, -hydride ehminahon to release the product and, hnally, proton release/reoxidahon of rhodium(I) to rhodium(in). 

Hydroquinone was beneficial to the reaction outcome, its effect being attributed to the suppression of radical side-reactions. The proposed mechanism consists of iniHal electrophilic attack of the metal onto the C—H bond of the arene to give an arylruthenium species with concomitant proton release, followed by alkene insertion into the Ru—C bond, -hydride elimination to liberate the cinnamate and a ruthenium hydride, and finally regeneration of the active catalyst either by insertion of another alkene into the Ru-H bond protonation of the alkylruthenium complex and elimination of methyl propionate (under an 

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Both the rhodium and ruthenium catalysts have been used to successively hydrogenate the terpene geraniol (3) to citronellol (4) and 3,7-dimethyl-octanol (J08) ... 

Adsorption is commonly used for catalyst removal/recovery. The process involves treating the polymer solution with suitable materials which adsorb the catalyst residue and are then removed by filtration. Panster et al. [105] proposed a method involving adsorbers made from organosiloxane copolycondensates to recover rhodium and ruthenium catalysts from solutions of HNBR. These authors claimed that the residual rhodium could be reduced to less than 5 ppm, based on the HNBR content which had a hydrogenation conversion of over... 

Enantioselective hydrogenation of functionalized alkenes is a well-developed field. A wide variety of rhodium and ruthenium catalysts and substrates are available for this purpose (see Chapters 23 to 28), and the reaction is widely used as a common synthetic tool in both academia and industry. 

With most rhodium and ruthenium catalysts 4 and 5 (Fig. 30.2), only low en-antioselectivities were obtained (Table 30.1, entries 1-6) [2-6]. However, good results were reported by Noyori and coworkers, who used DuPHOS with potassium tert-buloxide activation to hydrogenate substrate 1 in 86% ee (Table 30.1, entry 6) [6], as well as hydrogenating a range of other 1,1-disubstituted alkenes (see Section 30.2.2). 

The application of various ruthenium compounds in acetonitrile330 or rhodium and ruthenium catalysts or their mixtures331 was found to show significant improvements (high product selectivities, high linearities) in the aminomethylation of terminal alkenes to produce tertiary amines. 

The homogeneous catalytic asymmetric hydrogenations of 2-arylacrylic acids have been studied. Both rhodium and ruthenium catalysts have been examined. The reaction temperatures and hydrogen pressures have profound effects on the optical yields of the the products. The presence of a tertiary amine such as triethylamine also significantly increases the product enantiomer excess. Commercially feasible processes for the production of naproxen and S-ibuprofen have been developed based on these reactions. 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

Rylander et al. studied the effect of carriers and water on the stereochemistry of hydrogenation of o-, m-, and / -xylenes over rhodium and ruthenium catalysts at room temperature and an initial hydrogen pressure of 0.44 MPa.66 As seen from the results shown in Table 11.6, carbon-supported catalysts give less trans isomers than do the other supported catalysts. With a few exceptions, rhodium catalysts tend to produce the trans isomers more than do ruthenium catalysts. It is noted that the presence of water greatly reduced the proportion of trans isomer in the hydrogenations of o- and m-xylenes with Ru-C and of p-xylene with Rh-C. 

TABLE 11.6 Effects of Carriers and Water on the Stereochemistry of Hydrogenation of o-, in-, andp-Xylenes with Rhodium and Ruthenium Catalysts 2 ... 

Nakahara and Nishimura studied the selectivities of copper-chromium oxides, nickel, palladium, rhodium, and ruthenium catalysts in the hydrogenation of phenan-threne, 9,10-dihydrophenanthrene (DHP), and 1,2,3,4-tetrahydrophenanthrene (THP), usually in cyclohexane at 80°C (150°C for copper-chromium oxide) and an initial hydrogen pressure of 11 MPa (5 MPa for platinum metals). The hydrogenations over Os-C, Ir-C, and Pt-C were very slow and not investigated further. The varying compositions of the reaction mixture versus reaction time have been analyzed on the basis of the reaction sequences shown in Scheme 11.20 by means of a computer simulation, assuming the Langmuir-Hinshelwood mechanism.262 The results are summarized in Table 11.23. 

The neutral triazaadamantane phosphine PTA (38) has been used by Darensbourg s group to solubilize rhodium and ruthenium catalysts without impairing their selectivity... 

Rhodium-chiraphos cations also hydrogenate ketone and epoxide functionalities, albeit with low optical yields, and are, therefore, not synthetically useful. While this rhodium system seems somewhat limited to the preparation of amino acids, other rhodium and ruthenium catalyst precursors are currently available which show enhanced activity and selectivity for a much broader group of hydrogenation substrates. 

Oxygen-containing Cl and C2 molecules can be efficiently synthesized from CO and H2 (syngas) using cobalt, rhodium, and ruthenium catalysts. Among these catalysts, ruthenium is very efficient and selectively provides products with less than C2 units [8,9] this is in contrast to the Rh and Co catalysts, which produce byproducts with more than C3 units (Eq. 11.2). 

Chiral rhodium and ruthenium catalysts - have been reported to catalyze the Diels-Alder reaction of methacrolein with cyclopcntadiene. Several bis(oxazolidine) and 2-pyridyl-l,3-oxazolidine ligands were used as chiral ligands. The adducts were obtained with only moderate enantioselectivities. 

Hydrogenation of various unsaturated substrates has been carried out in water or in a two-phase system using preformed or in situ rhodium and ruthenium catalysts associated with water-soluble ligands. Complexes such as RhCl(PTA)3... 

The fact that water-soluble sulfonated phosphines may combine the properties of a ligand and a surfactant in the same molecule was first mentioned in 1978 by Wilkinson etal. [11] in their study of the hydroformylation of 1-hexene using rhodium and ruthenium catalysts modified with TPPMS (triphenylphosphine mono-... 

Unlike the tripodal ligand 1, the chelating diphosphine 2 forms rhodium and ruthenium catalysts (by simple reaction with the corresponding hydrated trichlorides) that catalyze the hydrogenation of BT in water/hydrocarbon biphase systems... 

When an enantiomerically pure catalyst is used in the directed hydrogenation of a racemic reactant then the enantiomers will react at intrinsically different rates since the transition states are diastereomeric. With some rhodium and ruthenium catalysts the difference [expressed as a selectivity factor S = k(fast)/k(slow)] is > 10 which makes the process synthetically useful. This is the case for all the x-hydroxyalkylacrylates described in Section 2.5.1.1.1. when complexes based on rhodium(DiPAMP)+ are employed as catalysts. The procedure is operationally simple in that the reaction is run to the point where the enantiomeric excess of recovered reactant is ca. 95% (57-65 % reaction) and the hydrogenation is then stopped. The starting material and product arc separated after removal of the catalyst. Similar results are obtained for a-hydro-xyalkyl-39, x-amidoalkyl-40 and a-carboxyalkylacrylatcs41 (entries 1-3, in the table below). 

Since platinum, rhodium, and ruthenium catalysts operate with similar activation energies, their differences in catalytic activity can be directly traced to differences in the A factor, which may be related to the % d-char-acter of the metal bond in the three metals above. Since the % d-character is 50, 50, and 44 for ruthenium, rhodium, and platinum, respectively (S), it is seen that this sequence is similar to that of the catalytic activity. During catalysis, the palladium surface becomes a chemical compound represented by various stages of interstitial hydride formation, whose d-charac-ter is essentially different from that of the metal. Therefore, the position of palladium in the % d-character sequence is not directly comparable to that of palladium in the catalytic activity sequence. 

Rhodium and ruthenium catalysts may alternatively be used and sometimes show useful selective properties. Rhodium allows hydrogenation of alkenes without concomitant hydrogenolysis of an oxygen function. For example, hydrogenation of the plant toxin, toxol 5 over rhodium-alumina gave the dihydro compound 6 (7.6) with platinum or palladium catalysts, however, extensive hydrogenolysis took place and a mixture of products was formed. 

Asymmetric reduction of prochiral imines, ketoximes, and several dihydroisoquinoline derivatives was successfully performed with chiral rhodium and ruthenium catalysts (Table 7). 

Vazquez-Zavala, A., Fuentes, S., Pedraza, F. (1994). The Influence of Sulfidation on the Crystalline Structure of Palladium, Rhodium and Ruthenium Catalysts Supported on Silica. Applied Surface Science, Vol.78, No.2, (June 1994), pp. 211-218, ISSN 0169-4332... 

In our recent study [13, 14] on stereo- and regioselective coupling reactions of styrene with vinylsilanes, catalyzed by ruthenium complexes, we used p-substituted styrenes as well as new, more efficient rhodium and ruthenium catalysts. In the presence of Grubbs catalyst, a reaction of two initial substances, proceeding via a metallacarbene mechanism and yielding the same products, was recently revealed [11,12]. The two reactions often proceed quantitatively according to Eq. 3. 

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