Rhodium on Alumina1

Rh/Al2O3

[7440-16-6]  · Rh  · Rhodium on Alumina  · (MW 102.91)

(active catalyst under mild conditions for hydrogenation of unsaturated compounds including aldehydes, ketones,1h,7 and aromatic carbocyclic1 and heterocyclic structures;1a,g,k high stereospecificity and low hydrogenolytic activity)

Form Supplied in: rhodium supported on alumina, carbon, or silica containing 0.5-5.0% of the metal are available commercially.

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

Introduction.

The catalytic properties of rhodium, like the other platinum metals, depend secondarily on the support. Alumina is commonly used but other supports may furnish advantages in particular circumstances. Rhodium black avoids possible influences of the support.2 The high activity of rhodium often permits its use at room temperature and atmospheric pressure but higher temperatures and pressures may be used to advantage.

Alkenes and Alkynes.

Although rhodium catalyzes the hydrogenation of alkenes, dienes, and alkynes under mild conditions, it is not the most selective platinum catalyst if double bond migration or isomerization can adversely affect the desired stereochemistry or if a selective hydrogenation of a diene or alkyne is sought.1h The tendency of rhodium for alkene isomerization relative to hydrogenation is only a little greater than that of ruthenium (see Ruthenium Catalysts), which results in the formation of a greater fraction of cis-1,2-dimethylcyclohexane using Ru (93.5%) than using Rh (87.6%) in hydrogenating 1,2-dimethylcyclohexene at 25 °C/1 atm H2.3

The hydrogenation of the double bond in vinyl or allylic compounds is achieved with some success with Rh but generally Pd or Ru is more effective, particularly with vinyl ethers.1h However, the high activity of Rh allows the reaction to be conducted at lower temperatures than with other metals. For example, the asperuloside tetraacetate, which contains both a vinyl and an allyl ether structural unit, was hydrogenated over Rh/C starting at -30 °C and raising the temperature slowly during 3 h to 0 °C to give a virtually quantitative yield of the tetrahydro product (eq 1).4

The activity of Rh/C also is shown by the hydrogenation of the double bond in the b-acyloxy-a,b-unsaturated esters and ketones without causing hydrogenolysis as given by Platinum(IV) Oxide (eq 2).5

Rh/Al2O3 in combination with Palladium on Carbon effected the saturation of an isomer of dodecamethyl[6]radialene (1) to all-trans-hexaisopropylcyclohexane (2) when separately the same catalysts failed (eq 3).6 Apparently the reduction required high reactivity for both hydrogenation (Rh) and alkene isomerization (Pd).

Aldehydes and Ketones.

Rhodium is an excellent catalyst for the hydrogenation of aldehydes and ketones under mild conditions.1a,e,h,7 The reduction of aliphatic carbonyl compounds can be accomplished without the hydrogenolysis of susceptible groups attached elsewhere in the molecule.

In the hydrogenation of cyclohexanones, rhodium excels in the formation of the axial hydroxyl group from monosubstituted compounds; the effect is especially notable for alkyl substituents.1d,8 With methoxy groups in place of alkyl groups at the 2- and 4-positions the fraction of axial isomer increases markedly for Pd, Ir (see Iridium), and Pt but not Rh.9a Hydrogenation of unhindered cyclohexanones in isopropyl alcohol or THF in the presence of small amounts of Hydrogen Chloride gives excellent yields of axial alcohols (eq 4).9b Steroidal 3,17- and 3,20-diones are selectively hydrogenated at C-3 to yield the corresponding 3-axial-hydroxy ketones.

Imines.

Imines are thought to be important intermediates in the hydrogenation of nitriles, anilines, and oximes.1 Ammonia and primary amines condense with aldehydes to give imines which may be hydrogenated. Freifelder describes the conversion of 3,4-dimethoxybenzaldehyde to the benzylamine over Rh/C in 95% ethanol containing ammonia (aq) and NH4OAc, (rt, 3 atm, 64% yield).1k

Nitriles.

The products of hydrogenating a nitrile depend on the catalyst and whether the nitrile is aliphatic or aromatic.1a,h The reaction conditions including the solvent, the temperature, and the H2 pressure, are important variables. Rhodium catalysts are most useful in the preparation of either primary or secondary amines.

The hydrogenation of an aliphatic nitrile leads to the formation of an imine which in turn may be hydrogenated to a primary amine.1a,g,h However, secondary amines are often the principal product, which apparently follows the condensation of the imine and the primary amine.1g Whether the condensations occur in the homogeneous medium, as first proposed by von Braun et al.,10 or on the surface of the catalyst, is unsettled.11 Recently, the analysis of a kinetic study of the hydrogenation of propionitrile on Rh/C and Rh/Al2O3, to form propylamine and dipropylamine, led to the claim that the results do not support the proposal that the condensations occur in the liquid phase.11 A mechanism involving only surface reactions is proposed to explain the authors' results as well as the literature record.

The catalyst's support influences the effect of increasing pressure on the distribution of the products of hydrogenating pentanenitrile in methanol at rt.12 At 4 atm, Rh/C, Rh/Al2O3, and Rh2O3 mainly yield dipentylamine (93%, 87%, and 77% respectively). Raising the pressure increases the yield of the primary amine. At 90 atm it is the sole product from unsupported Rh (Rh2O3), 54% yield is obtained from Rh/Al2O3, and 29% yield from Rh/C, the remainder being the secondary amine. At an intermediate pressure (50 atm), Rh/C yields only the secondary amine.1g Using Rh/C in the presence of a large excess of butylamine (mole ratio 1.7-6.6) the yield of butylpentylamine is 95-100%.1g

Aromatic nitriles are hydrogenated over Rh/C under mild conditions to dibenzylamines in excellent yields (25 °C, 4 atm).1g

Most recently, acetic acid was shown to be an excellent solvent for the Rh/Al2O3 or Rh/C catalyzed hydrogenation of aliphatic, aromatic carbocyclic, and aromatic heterocyclic nitriles to secondary amines (eq 5).13 3-Hydroxypropanenitrile formed a tertiary amine as a single isomer (eq 6) and the reduction of the tosylate of b-cyanoalanine formed a lactam (eq 7).

Oximes.

Although few examples of its use for the reduction of oximes have been recorded, rhodium has given excellent results, being especially useful in place of catalysts which yield excessive amounts of secondary amines.1h Freifelder et al. found the 5% Rh/Al2O3-catalyzed hydrogenation of cycloheptanone oxime in methanol (60 °C, 0.75-1.0 atm) gave an 80% yield of cycloheptylamine.1a,14 3-Amino-2-methyl-2-butanone and 1-(1-aminoethyl)cyclohexanol were prepared by the 5% Rh/Al2O3-catalyzed hydrogenation of the oximes of 3-hydroxy-3-methyl-2-butanone and 1-acetylcyclohexanol (eq 8) in 90-92% yields.15

Carbocyclic Aromatic Compounds.

Rhodium provides highly active catalysts for the hydrogenation of the aromatic carbocycle under mild conditions (25-80 °C and 1-3 atm).1 It is particularly useful when hydrogenolysis of attached hydroxy or alkoxy groups or benzylic alcohols or ethers is to be avoided.16 The selectivity of rhodium under mild conditions is shown by the Rh/Al2O3-catalyzed hydrogenation in acetic acid at room temperature of (2-methoxyphenyl)propan-2-one to cis-1-(2-methoxycyclohexyl)propan-2-one (eq 9).17

The consecutive addition of hydrogen atoms to the adsorbed cycle presumably proceeds through adsorbed cyclohexadienes and cyclohexenes, but only the desorption of the latter has been observed.18 Of the platinum metals, ruthenium yields the largest amount of cyclohexenic intermediates when hydrogenated in the presence of water, which is also an unusual promoter of ruthenium's catalytic activity (see Ruthenium Catalysts). In nonaqueous media, Rh yields the largest concentrations of cyclohexenic intermediates, as in the hydrogenation of t-butylbenzenes and t-butylbenzoic acids or esters in EtOH, HOAc, or saturated hydrocarbon solvents.1f,19

The Rh/C-catalyzed hydrogenation of 2-t-butylbenzoic acid initially yields cis-2-t-butylcyclohexanecarboxylic acid (75%), 6-t-butyl-1-cyclohexenecarboxylic acid (22%), and 2-t-butyl-1-cyclohexenecarboxylic acid (2%); trans-2-t-butylcyclohexanecarboxylic acid is formed only after the unsaturated intermediates begin to be reduced, which occurs when about 80% of the benzoic acid has been converted.19 The result supports the earlier proposal that the formation of trans isomers in the hydrogenation of the xylenes arises from the saturation of desorbed cyclohexenes, even though the latter are undetected.18

The rate of the rhodium-catalyzed hydrogenation of the aromatic ring diminishes with alkyl substitution but with multiple substituents the arrangement of the groups affect the reactivity. The selective hydrogenation of the unsubstituted ring in o-anisylmethylphenylphosphine oxide illustrates that alkoxy groups also lower the reactivity of the benzene ring (eq 10).20

Although rhodium is generally effective in saturating polycarbocyclic aromatic compounds, it is not useful if the selective reduction of a particular cycle is sought.1g-i

Phenols and Phenyl Ethers.

A useful application of rhodium catalysts is the hydrogenation of phenols and phenyl ethers which are sensitive to hydrogenolysis, a particular problem in hydrogenating di- and polyhydroxybenzenes.1a-c,21 Smith and Stump furnish an informative study, both kinetic and preparative, of the hydrogenation of hydroxybenzenes catalyzed by Pt and Rh/Al2O3.22 The kinetic evidence indicates that the mechanism of formation of the cyclohexanols from the phenols does not require ketone intermediates, although some are formed. The authors propose that cyclohexenols are intermediates which may be reduced to cyclohexanols, hydrogenolyzed to cyclohexanes, or isomerized to cyclohexanones. The conversion of resorcinol to dihydroresorcinol (1,3-cyclohexandione) in high yield is described (eq 11).

The hydrogenation of the cresols at 80 °C and 80-100 atm forms intermediate ketones in amounts which permit a quantitative analysis of the kinetics of the process to give a good estimate of the fraction of the cresol which is converted via the ketone to the cyclohexanol.23 The stereochemistry of the final product is determined by the fraction formed via the cyclohexanone (knowing the fraction of cis and trans isomers it forms upon reduction), with the remainder of the product alcohol being the cis isomer. In principle, if cyclohexenol intermediates desorb from the catalyst, readsorption followed by the suprafacial addition of hydrogen could yield some trans isomers.18

The hydrogenation of 1-naphthol over Rh/Al2O3 in ethanol or methanol (25 °C, 4 atm) gave good yields of the isomeric decalols in the ratio of 13.3:3.2:1, the largest component being the cis,cis isomer; the configurations of the others were not specified; hydrogenolysis to decalin was low (3%).24 Of the unsupported platinum metals used to catalyze the hydrogenation of 2-naphthol and the intermediate tetrahydro-2-naphthol (t-BuOH, 80 °C, 50 atm), Rh and Ru are highly stereospecific in forming the cis,cis-2-decalols and afford only small amounts of hydrogenolysis products.25

Anilines.

Ruthenium is generally the preferred catalyst for the hydrogenation of anilines to cyclohexyl amines (see Ruthenium Catalysts), but rhodium catalysts have been used successfully.1g,h To diminish the formation of dicyclohexylamine, the best solvents were found to be t-butyl or isopropyl alcohol; the addition of a small amount of a strong base, preferably Lithium Hydroxide, to Rh hydroxide furnished a catalyst that almost eliminated the diamine's formation.26 Rhodium oxide prepared by LiNO3 fusion with Rhodium(III) Chloride, and used in isopropyl or t-butyl alcohol, leads to excellent yields of the isomeric methoxy- or ethoxycyclohexylamines from the corresponding anilines (eq 12).27 Aniline has been converted to cyclohexylamine (96%, dicyclohexylamine 4%) catalyzed by Rh/Al2O3 in i-PrOH (23-26 °C, 55-70 atm).28

Heterocyclic Aromatic Compounds.

Rhodium has been used effectively to catalyze the hydrogenation of furans, pyrroles, pyridines, and other structures containing aromatic heterocycles.1a,h

Furans.

Furans are saturated under mild conditions with Rh/C. 2,5-Diferrocenylfuran is hydrogenated quantitatively to cis-2,5-diferrocenyltetrahydrofuran (5% Rh/C, rt, 1 atm, 2 h) (eq 13); after a time (6 h) at higher pressures (60 atm) the ether linkages are hydrogenolized to form 1,4-diferrocenylbutane.29

Pyrrole.

Rhodium is the preferred catalyst for the hydrogenation of pyrroles.30 Pyrrole and 1-methylpyrrole, without solvent, are reduced to the pyrrolidines in high yield over Rh/Al2O3 (rt, 2-3 atm).1g In compounds containing an aromatic carbocycle which is not fused to the pyrrole, the carbocycle tends to be reduced preferentially,1g but the reverse selectivity has also been observed (eq 14)31 and attributed to the trisubstitution in the carbocyclic ring.1g

The high cis selectivity in the Rh/Al2O3-catalyzed hydrogenation of a 2,5-disubstituted pyrrole is illustrated in (eq 15).32

Pyridine.

For the hydrogenation of pyridines, rhodium is the most effective catalyst under mild conditions and maintains its advantage at higher pressures.1h 2,6-Dimethylpyridine in glacial AcOH is hydrogenated to cis-2,6-dimethylpiperidine over 5% Rh/Al2O3 (eq 16).33

Although a variety of catalysts can affect satisfactorily the reduction of the 2- and 4-pyridinecarboxylic acids, the decarboxylation observed with the other catalysts in the hydrogenation of nicotinic acid (pyridine-3-carboxylic acid) is avoided by 5% Rh/Al2O3 in aqueous ammonia (eq 17).34

Generally, catalytic hydrogenation of pyridines with attached or fused aromatic carbocycles occurs preferentially in the pyridyl ring.1h 4-Benzylpyridine is converted quantitatively to 4-benzylpiperidine (5% Rh/C, EtOH, 60 °C, 4 atm).35 The pyridine ring in (3-methoxy-2-pyridyl)-2-propanone is reduced selectively with Rh/Al2O3 in water containing one equiv of Hydrogen Bromide, the selectivity is achieved only when the pyridylpropanone has been carefully purified by distillation (eq 18).36

A variety of other nitrogen heterocycles have been hydrogenated successfully over rhodium catalysts.1g,1k


1. Fieser, L.; Fieser, M. (a) FF 1967, 1, 979. (b) FF 1974, 4, 418. (c) FF 1977, 6, 503. (d) FF 1980, 8, 418. (e) FF 1984, 11, 460. (f) Kieboom, A. P. G.; van Rantwijk, F. Hydrogenation and Hydrogenolysis in Synthetic Organic Chemistry; Delft University Press: Delft, 1977. (g) Freifelder, M. Catalytic Hydrogenation in Organic Synthesis: Procedures and Commentary; Wiley: New York, 1978. (h) Rylander, P. N. Catalytic Hydrogenation in Organic Synthesis; Academic: New York, 1979. (i) Rylander, P. N. Hydrogenation Methods; Academic: London, 1985. (j) Bartók, M. Stereochemistry of Heterogeneous Metal Catalysis; Wiley: New York, 1985. (k) COS 1991, 8.
2. Takagi, Y.; Naito, T.; Nishimura, S. BCJ 1965, 38, 2119.
3. Nishimura, S.; Sakamoto, H.; Ozawa, T. CL 1973, 855.
4. Berkowitz, W. F.; Choudhry, S. C.; Hrabie, J. A. JOC 1982, 47, 824.
5. Rozzell, J. D. TL 1982, 23, 1767.
6. Golan, O.; Goren, Z.; Biali, S. E. JACS 1990, 112, 9300.
7. Breitner, E.; Roginski, E.; Rylander, P. N. JOC 1959, 24, 1855.
8. Mitsui, S.; Saito, H.; Yamashita, Y.; Kaminaga, M.; Senda, Y. T 1973, 29, 1531.
9. (a) Nishimura, S.; Katagiri, M.; Kunikata, Y. CL 1975, 1235. (b) Nishimura, S.; Ishige, M.; Shiota, M. CL 1977, 963.
10. von Braun, J.; Blessing, G.; Zobel, F. CB 1923, 56B, 1988.
11. Dallons, J. L.; van Gysel, A.; Jannes, G. In Catalysis of Organic Reactions; Pascoe, W. E., Ed.; Dekker: New York, 1992; pp 93-104.
12. Rylander, P. N.; Hasbrouck, L.; Karpenko, I. ANY 1973, 214, 100.
13. Galán, A.; de Mendoza, J.; Prados, P.; Rojo, J.; Echavarren, A. M. JOC 1991, 56, 452.
14. Freifelder, M.; Smart, W. D.; Stone, G. R. JOC 1962, 27, 2209.
15. Newman, M. S.; Lee, V. JOC 1975, 40, 381.
16. Stocker, J. H. JOC 1962, 27, 2288.
17. Cantor, S. E.; Tarbell, D. S. JACS 1964, 86, 2902.
18. Siegel, S. COS 1991, 8, 417.
19. Van Bekkum, H.; Van de Graaf, B.; Van Minnen-Pathuis, G.; Peters, J. A.; Wepster, B. M. RTC 1970, 89, 521.
20. Vineyard, B. D.; Knowles, W. S.; Sabacky, M. J. J. Mol. Catal. 1983, 19, 159.
21. Burgstahler, A. W.; Bithos, Z. J. JACS 1960, 82, 5466.
22. Smith, H. A.; Stump, B. L. JACS 1961, 83, 2739.
23. (a) Takagi, Y.; Nishimura, S.; Taya, K.; Hirota, K. J. Catal. 1967, 8, 100. (b) Takagi, Y.; Nishimura, S.; Hirota, K. J. Catal. 1968, 12, 214. (c) Takagi, Y.; Nishimura, S.; Hirota, K. BCJ 1970, 43, 1846.
24. Meyers, A. I.; Beverung, W.; Garcia-Munoz, G. JOC 1964, 29, 3427.
25. Nishimura, S.; Ohbuchi, S.; Ikeno, K.; Okada, Y. BCJ 1984, 57, 2557.
26. Nishimura, S.; Shu, T.; Hara, T.; Takagi, Y. BCJ 1966, 39, 329.
27. Nishimura, S.; Uchino, H.; Yoshino, H. BCJ 1968, 41, 2194.
28. Greenfield, H. ANY 1973, 214, 233.
29. Yamakawa, K.; Moroe, M. T 1968, 24, 3615.
30. Gribble, G. W. COS 1991, 8, 603.
31. Dolby, L. J.; Nelson, S. J.; Senkovich, D. JOC 1972, 37, 3691.
32. Turner, W. W. JHC 1986, 23, 327.
33. Overberger, C. G.; Palmer, L. C.; Marks, B. S.; Byrd, N. R. JACS 1955, 77, 4100.
34. Freifelder, M. JOC 1963, 28, 1135.
35. Freifelder, M.; Robinson, R. M.; Stone, G. R. JOC 1962, 27, 284.
36. Barringer, D. F., Jr.; Berkelhammer, G.; Carter, S. D.; Goldman, L.; Lanzilotti, A. E. JOC 1973, 38, 1933.

Samuel Siegel

University of Arkansas, Fayetteville, AR, USA



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.