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Energy & Fuels 1998, 12, 644-648
Hydrogenation of Aromatic Compounds with a Rhodium Catalyst in Biphasic Systems Shiyong Yang and Leon M. Stock* Department of Chemistry, The University of Chicago, Chicago, Illinois 60616 Received December 2, 1997. Revised Manuscript Received February 12, 1998
The reaction conditions for the catalytic reduction of single-ring aromatic compounds such as tetralin with chloro(1,5-hexadiene)rhodium(I) dimer in a biphasic mixture of hexane and an aqueous buffer containing a surface active agent have been investigated. The reaction proceeds most effectively in a dilute alkaline buffer with a low concentration of a surfactant. Under these conditions, the hydrogenation of tetralin provided cis- and trans-decalin in high yield with no apparent decrease in the catalytic activity after eight catalyst cycles. The reaction rate increases as the pressure and the temperature are increased. The reaction system is effective for the reduction of tetralin in the presence of coal liquids.
Introduction The challenges presented by the reduction of singlering aromatic compounds in petroleum and coal liquids prompted us to explore the chemistry of organorhodium catalysts that have proven to be very effective for the reduction of single-ring aromatic compounds in twophase reaction systems. It is well established that pure single-ring aromatic compounds can be hydrogenated at ambient temperature and one atmosphere of dihydrogen with reactive organometallic reagents,1-13 and the advantages offered by biphasic and phase transfer catalysis have received significant attention in recent years.14-18 To illustrate, the catalyst generated from rhodium(III) trichloride and tricaprylmethylammonium chloride (Aliquat 336) in a biphasic medium catalyzes homogeneous hydrogenation reactions,4,6,10-12 and chloro(1,5-hexadiene)rhodium(I) provides an active, apparently, heterogeneous catalyst for the hydrogenation of these substances.5,9 (1) Maitlis, P. Acc. Chem. Res. 1978, 11, 301. (2) Muetterties, E. L.; Bleeke, J. Acc. Chem. Res. 1979, 12, 324. (3) Bleeke, J. R.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 556. (4) Sasson, Y.; Zoran, A.; Blum, J. J. Mol. Catal. 1981, 11, 293. (5) Januszkiewicz, K. R.; Alper, H. Organometallics 1983, 2, 1055. (6) Blum, J.; Amer, I.; Zoran, A.; Sasson, Y. Tetrahedron Lett. 1983, 24, 4139. (7) Borowski, A. F. Transition Met. Chem. (Weinheim, Ger.) 1983, 8, 226. (8) Borowski, A. F. Transition Met. Chem. (Weinheim, Ger.) 1984, 9, 109. (9) Januszkiewicz K. R.; Alper, H. Can. J. Chem. 1984, 62, 1031. (10) Amer, I.; Amer, H.; Blum, J. J. Mol. Catal. 1986, 34, 221. (11) Blum, J.; Amer, I.; Vollhardt, K. P. C.; Schwarz, H.; Hohne, G. J. Org. Chem. 1987, 52, 2804. (12) Amer, I.; Amer, H.; Ascher, R.; Blum, J.; Sasson, Y.; Vollhardt, K. P. C. J. Mol. Catal. 1987, 39, 185. (13) Nasar, K.; Fache, F.; Lemaire, M.; Beziat, J.; Besson, M.; Gallezot, P. J. Mol. Catal. 1994, 87, 107. (14) Kalck, P.; Monteil, F. Adv. Organomet. Chem. 1992, 34, 219. (15) Joo, F.; Csiba, P.; Benyei, A. J. Chem. Soc., Chem. Commun. 1993, 1602. (16) Wiebus, E.; Cornils, B. Chem. Ing. Technol. 1994, 66, 916. (17) Herrmann, W. A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1524. (18) Lubineau, A.; Auge, J.; Queneau, Y. Synthesis 1994, 8, 741.
Preliminary work in our laboratory suggested that the catalyst prepared from the dimer of chloro(1,5-hexadiene)rhodium(I) was especially reactive. However, initial attempts to reduce tetralin in the presence of coal liquids under the conditions described by the previous investigators were unsuccessful, and we investigated the influences of different surfactants, buffers, solvents, and related factors on the hydrogenation of monocyclic aromatic compounds to establish more suitable reaction conditions. Experimental Section Materials. All the reagents and catalyst precursors were acquired from conventional sources. Tetralin was distilled and stored under nitrogen. Water was distilled twice under nitrogen before use. The buffers were prepared by dissolving the commercially available solids free of dyes and preservatives in distilled water. Chloro(1,5-hexadiene)rhodium(I) dimer the other rhodium compounds were obtained from commercial sources and were not purified. Tetrabutylammonium hydrogen sulfate (THS), cetyltrimethylammonium bromide (CTAB), tricaprylmethylammonium chloride (Aliquat 336), sodium dodecyl sulfate (SDS), and Tween 20 were used as received. All the air-sensitive and moisture-sensitive chemicals were handled in a glovebox filled with nitrogen or argon. Hexane was purified by refluxing commercial anhydrous hexane over sodium hydride prior to distillation. The coal liquid, sample 05SP19 from run 260-05-378, was supplied by the US Department of Energy. Hydrogenation. The following general procedure was used for hydrogenation. An oven-dried 250 mL round flask was equipped with a mechanical stirrer guided by a vacuumwithstanding Teflon stirrer bearing, a gas inlet linked to a pressure cylinder, and an outlet connected to a mineral oil bubbler. Tetralin (25 mmol) was added to a mechanically stirred solution of chloro(1,5-hexadiene)rhodium(I) dimer (0.125 mmol, 55 mg) in hexane (30 mL) protected by dihydrogen. Then a buffer solution (10 mL) containing enough surfactant to provide a [surfactant]/[Rh] ratio between 0.5 and 2 was added. An emulsion, which turned black after a few of minutes, was formed immediately. The reaction was allowed to proceed for 20 h at room temperature. The organic phase
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Hydrogenation of Aromatic Compounds was separated and the aqueous phase was washed with hexane several times. The organic phases were combined, washed with water, and dried before analysis. The products were identified by gas chromatographic analysis with a Perkin-Elmer Sigma 3B instrument with a 10% OV101 capillary column. The magnetic resonance data were acquired with a Varian XL 500 MHz spectrometer. The mass spectroscopic data were obtained with an HP Model 5890 Series II chromatograph interfaced with an HP Model 5970 mass spectrometer. Procedure for Determination of Catalyst Stability. These experiments were performed in the same apparatus. Tetralin (25 mmol, 3.4 mL) was added to a mechanically stirred solution of chloro(1,5-hexadiene)rhodium(I) dimer (0.125 mmol, 55 mg) in hexane (30 mL) protected by dihydrogen at atmospheric pressure. The buffer solution (10 mL) containing the surfactant was added. The conversion of tetralin was monitored until the tetralin conversion reached 80-85% and then the stirrer was stopped and the organic phase was withdrawn. Another portion of tetralin (3.4 mL) in hexane (30 mL) was added to the system, and then the reaction started again. In this way, the catalyst could be reused and the effectiveness of the catalyst was assessed from the accumulated moles of tetralin converted per mole of rhodium. Experiments with Tetralin-d12 and Toluene-d8. In a typical experiment, chloro(1,5-hexadiene)rhodium(I) dimer (28 mg, 0.065 mmol) was dissolved in deuterated cyclohexane-d12 (10 mL) under dihydrogen, and toluene-d8 (1.1 mL, 0.01 mol) was added with magnetic stirring. The two-phase reaction system was created by the addition of an aqueous buffer (3.0 mL, pH ) 7.4) containing CTAB (28 mg). Aliquots of the mixture (2 mL) were withdrawn, and the organic phase was separated and investigated by magnetic resonance or mass spectrometry.
Energy & Fuels, Vol. 12, No. 3, 1998 645 Table 1. Effect of Surfactants on the Catalytic Hydrogenation of Tetralin at 25 °C and 1 atm of Dihydrogena surfactant none THS CTAB Aliquat 336 Tween 20 SDS
trans
cis
total
0.0 0.5 4.0 8.0 1.0 4.0 8.0 1.0 1.0 1.0
18 12 7 4 21 13 7 12 8 17
74 76 38 26 79 61 37 66 38 83
92 88 45 30 100 74 44 78 46 100
a The reaction conditions for these experiments were tetralin (3.3 g, 25.0 mmol), chloro(1,5-hexadiene)rhodium(I) dimer (58 mg, 0.126 mmol), hexane (30 mL), pH 7.4 Hydrion buffer (20 mL).
Table 2. Influence of Buffer Composition on the Hydrogenation of Tetralin at 25 °C and 1 atm of Dihydrogena expt no 1 2 3 4 5
Results and Discussion Reaction Conditions. Preliminary work indicated that chloro(1,5-hexadiene)rhodium(I) dimer provided a very reactive aromatic hydrogenation catalyst, and we systematically investigated the reduction of tetralin as a representative single-ring aromatic compound in the fossil fuels to establish the most favorable reaction conditions. The reactions were intentionally carried out under mild conditions, 25 °C and 1 atm of dihydrogen, that required about 10 and 20 h for 50% and 100% conversion, respectively, so that the differences in the reaction parameters could be evaluated. The reduction of tetralin does not occur with the chloro(1,5-hexadiene)rhodium(I) dimer alone in pure hexane under anhydrous conditions. However, tetralin is reduced to decalin in 92% yield in a biphasic system containing hexane and a buffer at pH 7.4. The influence of several phase transfer agents on this reaction was examined. The results, Table 1, indicate that high yields of decalin can be obtained in the absence of any added surfactant molecule but that more complete conversions can be obtained with modest concentrations of CTAB and SDS. At equivalent low relative concentrations, these two substances appear to be more effective than THS, Aliquat 336, or Tween 20. However, high relative concentrations of the surfactant molecules are detrimental. For example, when the surfactant to rhodium ratio was increased to 8 for THS and CTAB, the conversion of tetralin to decalin decreased to less than 50%. The influences of several different buffer solutions on the hydrogenation in the biphasic system in the absence
yield of decalin, %
[surfactant]/[Rh] (mol/mol)
6 7b
yield (%) buffer composition pH ) 7.4 Hydrion pH ) 7.0 Hydrion pH ) 8.0 Hydrion pH ) 7.4 phosphate (0.1 M) pH ) 7.4 phosphate (0.001 M), NaCl (0.138 M), KCl (0.027 M), Tween 20 (0.05% w/v) pH ) 7.4 Trizma saline (0.05 M), NaCl (0.56%) water
trans-decalin cis-decalin total 18
74
92
9
71
80
5
33
38
6
29
34
4
36
41
3
25
28
6
38
44
a
The conditions were the same as the conditions presented in Table 1. b Water (20 mL) replaced the buffer in this experiment.
of surfactants are shown in Table 2. All the reaction systems were effective for the reduction, but a complex pattern of reactivity was observed. Both the pH and the buffer composition affect the conversion of substrate. The buffers with the same composition but different pH values (entries 1, 2, 3) and those with same pH but different compositions (entries 1, 4, 5, 6) all provide different conversions of tetralin. The highest conversion (92%) was obtained by using the Hydrion buffer at pH 7.4, and this buffer was used in most subsequent experiments. The relative volumes of the aqueous and organic phases influence the hydrogenation. The information shown in Figure 1 indicates that the conversion of tetralin increases steeply as the volume of buffer phase increases until the [aqueous phase]/[organic phase] ratio is between 0.3 and 0.6, then shows no further significant change. The hydrogenation proceeds in many organic solvents, but chlorocarbons, dimethylethylene oxide, and alcohols provide unsatisfactory results relative to aliphatic hydrocarbons. Hexane, cyclohexane, and heptane are good solvents for the reaction. However, large paraffinic molecules such as hexadecane provide decreased conversions of tetralin.
646 Energy & Fuels, Vol. 12, No. 3, 1998
Yang and Stock
Figure 1. Relationship between the conversion of tetralin and the relative volume of the aqueous phase (Hydrion buffer at pH 7.4) to the organic phase (hexane). Table 3. Comparison of Rhodium Catalysts for the Hydrogenation of Tetralin at 25 °C and 1 atm of Dihydrogena Rh compound
conversion (%)
chloro(1,5-hexadiene)rhodium(I) dimer chloro(1,5-cyclooctadiene)rhodium(I) dimer rhodium(III) trichloride rhodium(III) acetylacetonate
100 79 24 0
a The same reaction conditions were used in all experiments with tetralin (25.0 mmol), Rh compound (0.125 mmol), hexane (30 mL), Hydrion buffer (pH ) 7.4, 20 mL), and CTAB (0.125 mmol).
The results for different rhodium compounds in the catalytic hydrogenation of tetralin are shown in Table 3. These experiments were performed under identical conditions with the same buffer and surfactant. Like chloro(1,5-hexadiene)rhodium(I) dimer, chloro(1,5-cyclooctadiene)rhodium(I) dimer is an active catalyst precursor in the biphasic system. However, rhodium(III) trichloride and acetylacetonate did not form reactive catalysts under the same conditions. The dependence of the reaction rate on the temperature and pressure was examined. These experiments were carried out with chloro(1,5-hexadiene)rhodium(I) dimer in the biphasic buffered reaction system with CTAB. The conversion of tetralin increases as the temperature is increased. The rates at 25, 37, and 44 °C are 4.6, 9.3, and 9.6 mol h-1 Rh-1, respectively. An additional increase in temperature to 53 °C apparently decreased the rate to 6.7 mol h-1 Rh-1, but this observation, which may have be a consequence of a failure of the equipment, was not verified. The influence of dihydrogen pressure on the reduction of tetralin under the same conditions is shown in Figure 2. A linear relationship was observed under these experimental conditions with the rates, 4.6, 8.9, and 11.3 mol h-1 Rh-1 under 1.0, 2.0, and 2.7 atm of dihydrogen, respectively. The hydrogenation reactions of other aromatic compounds were investigated under the same reaction conditions (Table 4). Substituted benzenes including toluene, butylbenzene, tetralin, and 1,3-dimethylbenzene are hydrogenated into the corresponding substituted cyclohexane derivatives in more than 94% yield. Naphthalene is reduced to the mixture of cis- and transdecalin with the cis/trans ratio of 6.7. It was established by gas chromatography that tetralin is an intermediate in the process. 1,3-Dimethoxybenzene was hydrogenated to the 1,3-dimethoxycyclohexane in 98% yield. In
Figure 2. Relationship between the reaction rate and the hydrogen pressure for the biphasic reaction system in the presence of CTAB. Table 4. Hydrogenation of Aromatic Compounds at 25 °C and 1 atm of Dihydrogena substrate
product (%)
conversion (%)
tetralin naphthalene o-xylene toluene m-dimethoxybenzene n-butylbenzene
trans-decalin (21), cis-decalin (79) trans-decalin (13), cis-decalin (87) 1,2-dimethylcyclohexane (94) methylcyclohexane (100) m-dimethoxycyclohexane (98)
100 100 94 100 98
a
n-butylcyclohexane (75)
75
The reactions conditions are shown in Table 3.
this case, the product was an equimolar mixture of the cis and trans isomers. Catalyst Stability. Although the catalyst exhibited high reactivity in the absence of surfactants, it was not stable and could not be effectively reused because it apparently deactivated by the formation of solid deposits on the reactor wall. The tetralin conversion to rhodium ratio rarely exceeded 200, even under otherwise optimum conditions, in the absence of a surfactant. Exploratory work suggested that surfactants prevented the deposition of the solids, and this aspect of the chemistry was investigated to improve the stability of the catalyst while keeping its high activity. As already mentioned, the addition of high concentrations of surfactant molecules reduced the conversion of tetralin, but small amounts of some surfactants provided good yields and stabilized the catalyst enabling high tetralin conversion to rhodium ratios. Figure 3 shows the dependence of decalin yields on the [CTAB]/[Rh] molar ratios. The catalytic activity did not decline until the [CTAB]/[Rh] reached 2. Work on other surfactants indicated that THS and SDS exhibit behavior similar to CTAB. However, Aliquat 336 and Tween 20 reduce the catalytic activity even when [surfactant]/[Rh] was less than 1. The relationship between the effectiveness of the catalyst (moles of tetralin reduced per mole of rhodium) for the reaction with Hydrion buffer and CTAB through eight reaction cycles is shown in Figure 4. There was no decrease in catalyst activity, and the tetralin conversion to rhodium ratio, i.e., the amount of tetralin reduced relative to the amount of catalyst employed in the reaction, increased linearly with the reaction time in this experiment and reached about 1.4 × 103. Reaction Pathway. Previous studies have concluded that a homogeneous catalyst is obtained from
Hydrogenation of Aromatic Compounds
Energy & Fuels, Vol. 12, No. 3, 1998 647 Table 5. Relative Intensity of the Complex Signals Near 0.85 and 2.25 ppma relative intensity time (h)
I2.2/ITMSb
I0.8/ITMSc
0.5 1.0 2.5 4.5 24
1.1 3.2 6.1 4.3 0.0
2.0 3.4 7.0 17 200
a The reaction conditions are described in the Experimental Section. b The relative intensity of the signal located at near 2.25 ppm relative to TMS. c The relative intensity of the signal located near 0.85 ppm of relative to TMS.
Figure 3. Relationship between the decalin yields and the [CTAB]/[Rh] ratio for the biphasic reaction of tetralin.
Figure 4. Relationship between the conversion of tetralin and the number of reaction cycles. Each reaction cycle was approximately 1 day. These reactions were conducted under the optimized reaction conditions.
rhodium(III) trichloride and Aliquat 336 and that a heterogeneous catalyst is obtained form chloro(1,5hexadiene)rhodium(I) dimer. Experiments with labeled compounds and reagents have been carried out with the catalyst derived from rhodium(III) trichloride by Blum and his associates.11 For comparison, we performed the same experiments with the catalyst obtained from the rhodium(I) dimer. The reaction products obtained from the reduction of tetralin with deuterium gas under typical conditions were characterized by gas chromatography-mass spectrometry. For trans-decalin, the intensity of the signal at m/z 144 (C10H12D6•+, 20%), which corresponds to the mass of the reduced but unexchanged product is accompanied by many other signals with higher mass: C10H11D7•+ (145, 35%), C10H10D8•+ (146, 55%), C10H9D9•+ (147, 75%), C10H8D10•+ (148, 95%), C10H7D11•+ (149, 100%), C10H6D12•+ (150, 75%), C10H5D13•+ (151, 30%), and C10H4D14•+ (152, 10%). The importance of the exchange reaction is illustrated by the observation that the signal with the greatest intensity appears at m/z 149, indicating that five protons were replaced with deuterons during the hydrogenation. The results for cis-decalin are comparable. This product contains some deuterium-deficient cation radicals: C10H13D5•+ (143, 30%) and C10H14D4•+ (142, 20%), but the most abundant ion appears at m/z 148, C10H8D10•+, with four excess deuterons. In contrast, the treatment of pure cis-decalin with dideuterium in the presence of the catalyst under the same conditions did not result in
exchange, indicating that exchange occurs during the reduction process. It is apparent that the original hydrogen atoms in the aromatic and aliphatic rings exchange during the catalytic reduction. The chloro(1,5-hexadiene)rhodium(I) biphasic catalyst system was also used for the reduction of toluene-d8 in cyclohexane-d12 as the solvent. When a solution of toluene-d8 and the catalyst was stirred in pure cyclohexane-d12 for 2 h under an atmosphere of dihydrogen, the solution remained colorless and only the resonances of the original compounds were detectable. The fact that no new signals appeared in the sensitive proton magnetic resonance spectrum confirmed that no reactions occur in the absence of the aqueous phase and suggest that the organorhodium dimer does not complex with the aromatic compound at the beginning of the reaction. When an aqueous solution of a neutral buffer with or without a surfactant molecule was introduced into the same reaction system, both liquid phases turned black in 10-15 min, implying new rhodium species were being formed. The study of the spectra revealed that the signals of the original catalyst in the cyclohexane phase declined slowly but did not disappear until after the reaction had proceeded for about 2 h. The spectra of samples withdrawn from the reaction system during the same time interval and during the full course of the reaction clearly arose from the starting material, isomers of methyl-1,3-cyclohexadiene, and methylcyclohexane. The definitive resonances of methyl-1,4cyclohexadiene and methylcyclohexene were not detected. More specifically, the proton NMR spectrum of the cyclohexane phase exhibits several new signals including a set of signals at 2.21-2.25 ppm. This resonance increased in intensity as the reaction proceeded, then decreased, and finally disappeared at the end of the reaction when toluene was fully converted to methylcyclohexane. The relationship between the intensity of the new signals, which is summarized in Table 5, shows the changes of the relative intensities of this complex signal to that of tetramethylsilane, the internal standard. The signals between 2.21 and 2.25 ppm were assigned to the protons of methylene groups (-CDH-) in the mixture of isomeric methyl-1,3-cyclohexadienes. It is pertinent that no signals appeared in the region from 2.70 to 2.90 ppm, indicating that the resonance of the methylene group in the corresponding 1,4-cyclohexadiene was absent, and implying that the aromatic ring was first selectively reduced to 1,3-cyclohexadiene derivatives. The signal strength in the region from 0.83 to 0.92 ppm continuously increased. These resonances were assigned to the methyl groups in methylcyclohexane and the isomeric methylcyclohexadienes. The
648 Energy & Fuels, Vol. 12, No. 3, 1998
intensity of these resonances, which arise from the exchange reactions of the deuterated methyl group, increases considerably as the reaction proceeds (Table 5). In contrast, the resonances of the aromatic protons of toluene systematically declined throughout the entire course of the reaction. Reduction of Tetralin in the Presence of a Coal Liquid. The reactive biphasic catalyst system with chloro(1,5-hexadiene)rhodium(I) dimer, [tetralin]/[Rh] ) 200, and the CTAB surfactant, [CTAB]/[Rh] ) 2.0, was used to hydrogenate tetralin in the presence of a coal liquid provided by the U.S. Department of Energy. From 78 to 95% of the tetralin is reduced when the rhodium to coal liquid ratio is about 0.025. Replicate experiments established that 80-85% of the tetralin (3000 mg) could be converted to decalin in the presence of this coal liquid (1000 mg). However, very high concentrations of the coal liquid interfered with the reduction of tetralin under these mild conditions. Conclusion The investigation of different organic solvents, buffers, surfactants, organorhodium compounds, and related reaction parameters establishes that the catalytic reduction of tetralin proceeds with high efficiency at high substrate to catalyst ratios, 200:1, when the catalyst precursor, chloro(1,5-hexadiene)rhodium(I) dimer, which was originally introduced by Januszkiewicz and Alper,5,9 is used in a biphasic mixture of hexane and an aqueous buffer containing a low concentration of a surfactant at ambient temperature and 1 atm. The surfactant appears to stabilize the catalyst, perhaps by coating the catalyst and modifying its behavior as described by Marques, Silva, and Tundo,21,22 and enables the reaction to be performed yielding more than 1.4 × 103 mol of decalin per mole of rhodium. Under these conditions the catalyst produces both cis- and trans-decalin in contrast to the higher selectivity of the reagent employed by Januszkiewicz and Alper.5,9 Although it was not necessary to exploit the finding in this study, the (19) NATO Advanced Research Workshop. Conference Preprints Aqueous Organometallic Chemistry and Catalysis, Debrecen, Hungary, 1994. (20) Wiebus, E.; Cornils, B. Chemtech 1995, 1, 33. (21) Marques, C. A.; Silva, M.; Tundo, P. J. Org. Chem. 1993, 58, 5256. (22) Marques, C. A.; Silva, M.; Tundo, P. J. Org. Chem. 1994, 59, 3430.
Yang and Stock
rate of the catalytic reaction accelerates as the pressure and the temperature are increased. The initial observations concerning the reduction of tetralin in the presence of a coal liquid indicate that the catalyst system is effective for the reduction of tetralin and presumably for the coal liquids themselves under such conditions, implying that the catalyst system can be operated in the presence of some noxious coal liquid components. Although this study was not directed toward the definition of the specific reaction pathway, our observation that the catalyst precursor remains in the organic phase for about 2 h may be related to the fact that the homogeneous hydrogenation of benzene with the rhodium(III) trichloride Aliquat 336 system exhibits an induction period.11 The work with naphthalene indicates, as expected, that tetralin is an intermediate, and the work with toluene indicates that methyl-1,3-cyclohexadienes are observable intermediates but that the corresponding isomeric methylcyclohexenes are not formed in detectable amounts. The study of the reactions of the labeled toluene and tetralin indicates that neither the starting material nor the fully saturated reaction product undergo extensive exchange under the reaction conditions. However, the products are very substantially exchanged. We infer that the reduction reactions of toluene and methylcyclohexene are essentially irreversible, whereas the reduction reactions of the complexed cyclohexadienes proceed more slowly, enabling these molecules to dissociate from the catalyst. While there are some subtle differences, the significant exchange of the methyl group in the product and the mass spectroscopic exchange data for tetralin that were observed in the reactions of the rhodium(I) dimer are very similar to the data for reactions with the catalyst provided by rhodium(III) trichloride and Aliquat 336.11 Broadly speaking, the available data suggest that the actual catalysts prepared from the rhodium(I) dimer5,9 and from the rhodium(III) compounds11 are very similar and that the conclusions reached in the very thorough study by Blum and his colleagues11 can be employed in the discussion of both reaction systems. All of the data that were obtained in this study are also compatible with the formulation of the catalytic reaction by the pathways that were proposed by Muetterties and Bleeke.2,3 EF9702198
