Ruthenium Catalysts1

Ru

[7440-18-8]  · Ru  · Ruthenium Catalysts  · (MW 101.07)

(selective hydrogenation catalysts,1 high regio- and stereoselective addition to alkenes,2 low activity for alkene isomerization,3 actively hydrogenates carbonyls,4 low hydrogenolytic activity,5 high stereoselective perhydrogenation of aromatic carbocyclic and many aromatic heterocyclic compounds6,7)

Form Supplied in: ruthenium supported on alumina or carbon and RuO2 are available commercially.

Preparative Methods: Ruthenium(III) Chloride is converted by aq. Sodium Hydroxide or Lithium Hydroxide to Ru(OH)3, which may be dried in a vacuum at room temperature for later use.6 Ruthenium black can be prepared by the reduction of RuCl3.3H2O with Formaldehyde, washed repeatedly with distilled water, and stored in ethanol.8

Handling, Storage, and Precautions: the unused supported catalysts are not pyrophoric but ruthenium on finely divided carbon, like carbon itself, can undergo a dust explosion.1d After use, the catalysts are likely to contain adsorbed hydrogen which may ignite when dried.1c,1d Use in a fume hood.

Introduction.

The catalytic characteristics of ruthenium, like other platinum metals, depend secondarily on the support.1c,d The discovery of RuO2 as a selective hydrogenation catalyst encouraged its use.5,7,9 Later the supported catalysts Ru/C and Ru/Al2O3 became readily available. Supported catalysts are the more efficient users of the metal but may not furnish the maximum yield of a desired product if, for example, intraparticle mass transport affects the product distribution.1d

Ruthenium-catalyzed hydrogenations often show a variable and sometimes lengthy induction period which can be avoided by agitating the catalyst and solvent mixture in hydrogen (1 atm) for 1-2 h before introducing the substrate.2,10 The common solvents are water, water-acetic acid, alcohols, and dioxane. Water is a powerful promoter of the activity of ruthenium hydrogenation catalysts.1d,2,6

Alkenes and Alkynes.

Ruthenium exhibits a lower tendency than rhodium (see Rhodium on Alumina) to isomerize alkenes (relative to hydrogenation), particularly at high pressures which are often used with ruthenium catalysts.1c,1d Under mild conditions, a monoalkyl substituted ethylene is hydrogenated selectively in the presence of di- and trisubstituted ethylenes; hydrogenation of alkynes gives the corresponding alkane.2 Ruthenium catalysts add hydrogen suprafacially (cis) to a carbon-carbon double bond, as in the hydrogenation of alkyl substituted cyclohexenes (eq 1),3,11 bicyclo[2.2.0]hexenes or bicyclo[2.2.0]hexadienes (eq 2),12 and bicyclo[4.4.0]decene.13

Carbonyl Compounds.

Ruthenium is an excellent catalyst for the hydrogenation under mild conditions of aliphatic carbonyl compounds and aldehydes such as furfural (eq 3).4 Industrially, it is used for the conversion of glucose to sorbitol.1d

Elevated temperatures and pressures have been used successfully, as in the 5% Ru/C catalyzed hydrogenation of tetramethyl-1,3-cyclobutanedione which gave a mixture of stereoisomers (98%), a far better result than that given by Pd, Pt, or Rh (eq 4).14

Felföldi has given a broad review of the stereochemistry of the hydrogenation of substituted cyclic ketones which includes comparisons of ruthenium with other metals as catalysts.15

Carboxylic Acids to Alcohols.

Either RuO2 or Ru/C catalyzes the hydrogenation of carboxylic acids, both mono- and dicarboxylic, in water to the corresponding alcohols at around 150 °C and 500-700 atm (eq 5).16 Rhenium oxides, particularly ReO, effect this reduction under less vigorous conditions.17

Hydrogenation of Nitro Alkynes.

Ru/C or Ru/Al2O3 favor the reduction of the aromatic nitro group in (3-nitrophenyl)acetylene.18 To avoid the observed deactivation of the catalyst, the alkyne was converted to the acetone adduct, 2-methyl-4-(3-nitrophenyl)-3-butyn-2-ol, which was hydrogenated to 2-methyl-4-(3-aminophenyl)-3-butyn-2-ol (eq 6). The base-catalyzed removal of acetone released the amino alkyne quantitatively.

Carbocyclic Aromatic Compounds.

Ruthenium, as well as rhodium, is noted for its effective catalysis of the hydrogenation of aromatic carbocycles.1e,6 Other reducible functional groups usually remain intact. With more than one substituent attached, the configuration of the saturated product corresponds mainly to addition of hydrogen to one face of the aromatic cycle. As with other platinum metals, the selectivity is affected by the extent to which unsaturated intermediates desorb from the catalytic sites and whether the intermediates are readsorbed and reduced with selectivities differing from that of their precursor, or transformed to products other than those of simple hydrogen addition.

Aromatic Hydrocarbons.

Of all the platinum metals studied, ruthenium forms the largest amount of an observable cyclohexene in the hydrogenation of benzene or the xylenes at near atmospheric pressures.19 Don and Scholten showed the importance of water in maximizing the yield of cyclohexene when ruthenium is the catalyst.20 The formation of trans saturated isomers from substituted benzenes has been attributed to the hydrogenation of the desorbed intermediates.21

The Ru(OH)3-catalyzed hydrogenation of o-xylene at 85 °C and 80-100 atm yields a cis/trans ratio of 1,2-dimethylcyclohexanes of 12.3 compared to 9.5 for Rh (eq 7).6 Only Os and Ir (see Iridium) yield larger fractions of the cis isomer.

Ruthenium is the most stereoselective catalyst for the perhydrogenation of naphthalene or methylnaphthalenes (95% cis).13 High stereoselectivity is shown in the hydrogenation of triptycene to the cis-anti-perhydrotriptycene (eq 8).22

Fullerenes C60 and C70 in a ca. 85:15 ratio, dissolved in toluene and with an equal volume of water added, were hydrogenated over 5% Ru/C to yield a mixture of hydrogenated fullerenes from C60H2 to C60H40.23 The hydrides retain the fullerene carbon skeleton.

Phenols and Phenyl Ethers.

Among the first reported uses of Ru was the hydrogenation of a phenolic ring contained in a polycyclic compound.9 The reductions were performed with RuO2 at 50 °C and 100 atm H2, which avoided hydrogenolysis of the hydroxyl group and was stereoselective.

The hydrogenation of phenols produces cyclohexanone intermediates which probably arise from the isomerization of a cyclohexenic intermediate, a vinyl alcohol. Palladium gives the highest yields of cyclohexanone, approaching 100%; ruthenium yields are smaller but more than is obtained with other platinum metals.24 The maximum diminishes with an increase of pressure while the fraction of the cis saturated product increases. This result indicates that the hydrogenation of the ketone is not as cis stereoselective as is the direct hydrogenation of the phenol.

That enols are among the initial desorbed intermediates in the hydrogenation of phenols is supported by the study of the hydrogenation of ethyl p-tolyl ether in ethanol over unsupported platinum metals (25 °C, 1 atm).25 Ruthenium affords the simplest result in that the enol ether is the principal intermediate; ethyl 4-methyl-3-cyclohexenyl ether is formed in smaller amounts (eq 9). With platinum metals other than Ru, the hydrogenation is accompanied by the formation of 4-methyl-1,1-diethoxycyclohexane and loss of the ethoxy group, leading to 4-methylcyclohexanone and 4-methylcyclohexanol. For Ru, the cis/trans ratio diminishes with conversion as the fraction of the saturated ether formed via the direct path diminishes.

To avoid the hydrogenolysis of the ether linkages, 5% Ru/Al2O3 was used to hydrogenate Dibenzo-18-crown-6 in 1-butanol to Dicyclohexano-18-crown-6 (eq 10).26 Other crown ethers were treated similarly using RuO2 or the supported catalyst and the conversions were thought to be nearly quantitative.

Using Ru/Al2O3 (110-126 °C, 70 atm) the cresols are converted to the methylcyclohexanols with high selectivities (94-98%) but with moderate cis stereoselectivities, which are larger in ethanol than in water.27

The selectivities of the platinum metals in the hydrogenation of 2-naphthol are comparable in the formation of the two tetrahydro derivatives, 1,2,3,4- and 5,6,7,8-tetrahydronaphthols, for their tendency towards hydrogenolysis, and for their stereochemistry in the formation of the saturated products (eq 11).28

Anilines.

A large number of substituted anilines have been hydrogenated successfully to the cyclohexyl amines over ruthenium catalysts.1c-f Normally, varying amounts of dicyclohexylamines are formed which arise from the reaction of the unsaturated intermediates. At elevated pressures, ruthenium catalysts usually give the least amount of such coupled products.29

The formation of the dicyclohexylamine from aniline as a function of the alcohol used as solvent decreases in the order methyl > ethyl >> isopropyl &AApprox; t-butyl; the addition of a small amount of either NaOH or LiOH further inhibits the formation of the dialkylamine.30

Ru/Al2O3 catalyzes the hydrogenation of alkyl-substituted anilines with high chemoselectivities to the substituted cyclohexylamine (>90% except for o- and m-t-butylaniline) and stereoselectivities, mainly cis, in ethanol on Ru/Al2O3 (110 °C, 70 atm).1e,27 On the same catalyst the hydrogenation of the isomeric toluidines, aminophenols, and alkoxyanilines gives similar results. Although the rate of hydrogenation diminishes by substituting acetyl or alkyl for H in the amino group, the yields and stereoselectivities remain equally good (eq 12).

Substituted Benzoic Acids.

Ruthenium has been used to hydrogenate benzoic acids, or their esters, which are substituted with alkyl, hydroxyl, amino, or additional carboxyl groups.1e,31 Generally, the stereoselectivities are greater than those observed with other metals except possibly Rh, which may be used at lower temperatures.

Ru/C is included amongst the catalysts used in the study of the hydrogenation of o-, m-, and p-t-butylbenzoic acids (20 °C, 1 atm).32 Rh/C gave the largest amount of the unsaturated intermediate while Ru/C resulted in the higher stereoselectivity (cis). The Ru/C-catalyzed hydrogenation of terephthalic acid qualifies as a preparative procedure for cis-cyclohexene-3,6-dicarboxylic acid (70%) (eq 13).33

Heterocyclic Compounds.

Ruthenium is most effective for the perhydrogenation of aromatic heterocycles such as pyrroles, pyridines, and furans. Hydrogenolysis of the cycle or that of attached negative groups is generally small.1d Pyridine is converted quantitatively to Piperidine using RuO2 at 95 °C and 70-100 atm of H2, in less than 0.5 h.7 A large variety of substituted pyridines also were hydrogenated in water, methanol, or ethanol under comparable conditions, with 80-94% yields.

Reaction conditions can affect the chemoselectivity. The RuO2-catalyzed hydrogenation of 3-cyanopyridine to 3-aminomethylpyridine was done in a mixed solvent, methanol and Ammonia, at 95 °C and 120 atm H2 (68%); 3-aminomethylpiperidine was obtained in comparable yield from the cyanopyridine in ammonia at 100-125 °C and 150 atm (eq 14).

A kinetic study of the Ru/C-catalyzed hydrogenation of Quinoline, 2- and 8-methylquinoline, and isoquinoline shows the high selectivity towards 1,2,3,4-tetrahydroquinolines.34 The latter as well as the 5,6,7,8-tetrahydro derivatives are converted more slowly to the decahydroquinolines (eq 15).

Five-membered heterocyclic aromatic compounds which contain nitrogen or oxygen also have been hydrogenated over ruthenium catalysts. 2,5-Bis-(3-oxobutyl)furan in dioxane is converted to 2,5-bis(3-hydroxybutyl)tetrahydrofuran over Ru/C (eq 16).35 RuO2 or Ru/C (70-100 °C, 100 atm) catalyze the perhydrogenation of pyrrole, 2-methylpyrrole, and carbazole.1b

Aromatic heterocyclic diazines such as 2-methylpyrazine and cinnoline (5,6-benzopyridazine) also are hydrogenated on RuO2 or Ru/C.1b


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Samuel Siegel

University of Arkansas, Fayetteville, AR, USA



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