Langmuir 1997, 13, 41-50
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Interaction of Optically Active Tartaric Acid with a Nickel-Silica Catalyst: Role of Both the Modification and Reaction Media in Determining Enantioselectivity Mark A. Keane* Department of Chemistry & Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received September 25, 1995. In Final Form: October 11, 1996X An enantioselective Ni/SiO2 catalyst has been prepared by modification with aqueous, methanolic, ethanolic, and butanolic solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral β-keto ester (methyl acetoacetate) to a β-hydroxy ester ((R)-(-)-methyl 3-hydroxybutyrate). The effects of systematically varying the modification time and temperature, modifier concentration, and the incorporation of NaBr as a comodifier on the uptake of TA and on the ultimate activity and enantioselectivity are discussed. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching of nickel from the catalyst surface are presented. The degree of TA adsorption was found to increase and the extent of nickel dissolution to decrease with the following sequence of modifier solvents: water, methanol, ethanol, 1-butanol. Changes in hydrogenation activity and enantioselectivity are reported as a function of the reaction time, temperature, and the reactant to catalyst molar ratio. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation where the alcoholic treatments yielded the highest activities and selectivities. The reaction exhibits typical Langmuir-Hinshelwood behavior and reaction orders with respect to methylacetoacetate and hydrogen concentrations and apparent activation energies are provided. The effect of the reaction solvent on product composition is discussed, and a linear relationship between the apparent rate and dielectric constant of the reaction medium, in the case of alcoholic solvents, is presented. The action of thiophene, acetic acid, and water as additives is identified, and catalyst durability is addressed.
Introduction It is now well documented that nickel, modified by immersion in a solution of optically active material, catalyses the enantioselective hydrogenation of molecules with a prochiral center.1-5 In the case of tartaric acid (TA) modification, the effects of such variables as modification time (tmod),6-9 temperature(Tmod),2,6-8,10 modifier concentration ([TA]),7,8,11-13 and pH2,7-9,12,14-17 have been considered. The adsorption of TA on supported nickel metal is corrosive,6,7,10,12,13,17-19 and the amount of nickel leached into solution has been shown to depend on the modification conditions.7,17 The ultimate catalytic asym* Address for correspondence: Department of Chemical Engineering, The University of Leeds, Leeds LS2 9JT, United Kingdom. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Bosnich, B. Asymmetric Catalysis; NATO ASI Series; Martinus Nijhoff Publishers: Dordrecht, 1986. (2) Izumi, Y. Adv. Catal. 1983, 12, 215. (3) Tai, A.; Harada, Y. Taylored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel: Dordrecht, 1986; p 265. (4) Blaser, H. U.; Muller, M. Stud. Surf. Sci. Catal. 1991, 59, 73. (5) Blaser, H. U. Tetrahedron: Asymmetry 1991, 2, 843. (6) Fu, L.; Kung, H. H.; Sachtler, W. M. H. J. Mol. Catal. 1987, 42, 29. (7) Keane, M. A.; Webb, G. J. Catal. 1992, 136, 1. (8) Tatsumi, S. Bull. Chem. Soc. Jpn. 1968, 41, 408. (9) Fish, M. J.; Ollis, D. F. J. Catal. 1977, 50, 353. (10) Hoek, A.; Sachtler, W. M. H. J. Catal. 1979, 58, 276. (11) Richards, D. R.; Kung, H. H.; Sachtler, W. M. H. J. Mol. Catal. 1986, 36, 329. (12) Bennett, A.; Christie, S.; Keane, M. A; Peacock, R. D.; Webb, G. Catal. Today 1991, 10, 1619. (13) Keane, M. A.; Webb, G. J. Chem. Soc., Chem. Commun. 1991, 1619. (14) Wittmann, G.; Bartok, G. B.; Bartok, M.; Smith, G. V. J. Mol. Catal. 1990, 60, 1. (15) Vedenyapin, A. A.; Klabunovskii, E. I.; Telanov, Yu. M.; Sokolova, N. P. Kinet. Catal. 1975, 16, 436. (16) Klabunovskii, E. I.; Vedenyapin, A. A.; Chankvetadze, B. G.; Areshidze, G. C. Proceedings of the 8th International Congress on Catalysis, Berlin, 1984; Dechema: Frankfurt, 1984; p. 543. (17) Keane, M. A. Catal. Lett. 1993, 19, 197.
S0743-7463(95)00796-7 CCC: $14.00
metric yield reflects the relative contribution of the modified “selective” and bare nickel “nonselective” sites.2,11,12,20,21 Addition of sodium bromide7,22 and carboxylic acids23 to the modifying solution has been reported to enhance selectivity, but the precise source of the catalytic enantiocontrol has yet to be conclusively established. Attempts made to date to measure surface coverage by the modifier have been largely unsuccessful.9,11,20,24,25 With the exception of a report by Sachtler et al.,18 wherein an enantioselectivity of 94% is quoted as resulting from a methanolic TA treatment, all the existing data concern modifications in aqueous media. Tartaric acid modified Ni/SiO2 catalysts have been successfully used in the enantioselective hydrogenation of methylacetoacetate (MAA) to methyl 3-hydroxybutyrate (MHB).6,7,10-13,20,26,27 The hydrogenation of MAA on a bare or unmodified nickel surface generates a racemic product.7,12,17,19,28 Izumi et al., using TA modified Raney nickel, (18) Hoek, A.; Woerde, H. M.; Sachtler, W. M. H. Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980; Tanabe, K., Sieyama, T., Eds.; Kodansha/Elsevier: Tokyo/Amsterdam, 1981; p 376. (19) Keane, M. A. Can. J. Chem. 1994, 72, 372. (20) Nitta, Y.; Utsumi, T.; Imanaka, T.; Teranishi, S. J. Catal. 1986, 101, 376. (21) Harada, T.; Tai, A.; Yamamoto, M.; Ozaki, H.; Izumi, Y. Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980; Tanabe, K., Sieyama, T., Eds.; Kodansha/Elsevier: Tokyo/Amsterdam, 1981; p 364. (22) Harada, T.; Izumi, Y. Chem. Lett. 1978, 1195. (23) Harada, T.; Kawamura, T.; Haikawa, S.; Osawa, T. J. Mol. Catal. 1994, 93, 211. (24) Harada, T.; Haraki, Y.; Izumi, Y.; Muraoka, J.; Ozaki, H.; Tai, A. Proceedings of the 6th International Congress on Catalysis, London, 1976; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; The Chemical Society: London, 1977; p 1024. (25) Yasumori, I.; Inoue, Y.; Okabe, K. Proceedings of the International Symposium on the Relations Between Heterogeneous and Homogeneous Phenomena, Brussels, 1974; Elsevier: Amsterdam, 1975; p 41. (26) Bostelaar, L. J.; Sachtler, W. M. H. J. Mol. Catal. 1984, 27, 387. (27) Nitta, Y.; Sekine, F.; Sasaki, J.; Imanaka, T.; Teranishi, S. J. Catal. 1983, 79, 211. (28) Keane, M. A. Zeolites 1993, 13, 14, 22, 330.
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have reported steady progress in improving enantioselectivity from 3-5% in 195629 to > 80% in 1981.30 The reported dependence of selectivity on reaction variables is conflicting in that, for instance, (a) maxima have been quoted at reaction temperatures of 323 K,24,31 328-333 K,32 and 343-353 K,33,34 (b) temperature independence has been observed in the range 333-393 K,35 and (c) selectivity was shown to decrease as the temperature was raised from 323 to 353 K.36 Transesterification28,37 and decomposition31 of the β-keto ester have also been observed at higher reaction temperatures. The asymmetric process has been reported to be both dependent27,38 and independent33,34 on and of hydrogen pressure, a range of optimum substrate to catalyst ratios14,31,34,38-40 have been given, and a variety of selectivity/conversion relationships27,33,34,39,41 are illustrated in the literature. The available data are further complicated by a reported difficulty in attaining reproducible measurements.10,14,18,24,26,31 The reaction solvent has been shown both to influence31,35 and to have no effect33,34 on reaction rate and selectivity. Whereas a number of workers41-43 have carried out the hydrogenation using an undiluted reactant, a range of solvents including ethyl acetate,20,37,38,43 methyl propionate,35,37 THF,23 methanol,18 ethanol,14 and butanol44 have been used. Thiophene and pyridine,44 n-butylamine, and a number of alkenes37 have been shown to enhance selectivity while acetic acid has been reported to act as a promoter2 and, alternatively, to have no effect on the ultimate product composition.35 Catalyst deactivation has been noted20,45,46 and has been counteracted in two instances.45,47 Taking an overview of the published data, it is evident that one can only, at best, make qualitative comparisons between results originating from different laboratories. Moreover, the range of experimental conditions that have been considered in each study is too narrow to afford a complete understanding of the underlying principles. Physical measurements of TA treatment of Ni/SiO2 in solvents other than water are reported for the first time in this paper, and a direct correlation between enantioselectivity and the dielectric constant of the reaction medium is presented. The response of the modification process to wide variations in treatment time, temperature, and modifier concentration is illustrated and the dependence of product composition on selected reaction variables and prolonged catalyst use is addressed. The data generated in this study are related, where feasible, to (29) Akabori, S.; Sakurai, S.; Izumi, Y.; Fujii, Y. Nature (London) 1956, 178, 323. (30) Harada, T.; Yamamoto, M.; Onaka, S.; Imaida, M.; Ozaki, H.; Tai, A.; Izumi, Y. Bull. Chem. Soc. Jpn. 1981, 54, 2323. (31) Smith, G. V.; Musoiu, M. J. Catal. 1979, 60, 184. (32) Ozaki, T. Bull. Chem. Soc. Jpn. 1978, 51, 257. (33) Klabunovskii, E. I. Russ. J. Phys. Chem. 1973, 47, 765. (34) Lipgart, E. N.; Petrov, Yu. I.; Klabunovskii, E. I. Kinet. Katal. 1971, 12, 1491. (35) Osawa, T.; Harada, T.; Tai, A. J. Catal. 1990, 121, 7. (36) Yasumori, I.; Yokozeki, M.; Inoue, Y. Faraday Discuss. 1981, 71, 385. (37) Brunner, H.; Muschiol, M.; Wishert, T.; Wiehl, J. Tetrahedron: Asymmetry 1990, 1, 159. (38) Nitta, Y.; Sekine, F.; Sasaki, J.; Imanaka, T.; Teranishi, S. Chem. Lett. 1981, 541. (39) Hubbell, D. O.; Rys, P. Chimia 1970, 24, 442. (40) Petrov, Yu.; Klabunovskii, E. I.; Balandin, A. A. Kinet. Katal. 1967, 8, 814. (41) Gross, L. H.; Rys, P. J. Org. Chem. 1974, 39, 2429. (42) Ozaki, H.; Tai, A.; Kobatake, S.; Watanabe, H.; Izumi, Y. Bull. Chem. Soc. Jpn. 1978, 51, 3559. (43) Nitta, Y.; Kawabe, M.; Imanaka, T. Appl. Catal. 1987, 30, 141. (44) Chernysheva, V. V.; Murina, I. P.; Klabunovskii, E. I. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1983, 689. (45) Tai, A.; Imachi, Y.; Harada, T.; Izumi, Y. Chem. Lett. 1981, 1651. (46) Osawa, T.; Harada, T. Bull. Chem. Soc. Jpn. 1987, 60, 1277. (47) Tai, A.; Tsukioka, K.; Ozaki, H.; Harada, T.; Izumi, Y. Chem. Lett. 1984, 2083.
Keane
results available in the literature, recording both agreement and divergence and identifying relationships that have not been reported previously. Experimental Section A 15.2% (w/w) Ni/SiO2 catalyst was prepared by the homogeneous precipitation/deposition of nickel onto a nonporous microspheroidal Cab-O-Sil 5M silica as described in detail elsewhere.7 The silica support was first washed with triply deionized water and dried in air for 20 h at 373-383 K before use. The precipitation was carried out in a 2 dm3 three-necked round-bottomed flask fitted with a Citenco motor-driven stirrer. A sample of urea (Aldrich Chem. Co., 99+%) was added to a 1.5 dm3 aqueous suspension of Cab-O-Sil 5M in nickel nitrate (Aldrich Chem. Co., 99.999%) at 290 ( 2 K, where the molar ratio Ni(NO3)2‚6H2O/H2NCONH2 equals 0.36. The suspension was slowly heated under constant agitation to 361 ( 3 K and held at this temperature for 6 h. The pH of the suspension was preadjusted (with nitric acid) to 2.8 to prevent premature hydrolysis and the pH was observed to increase to 5.3 on completion of the precipitation step. The suspension was filtered and washed with 4 × 400 cm3 hot deionized water and air-dried in an oven at 373-383 K for 20 h. The nickel loading was determined by atomic absorption (Perkin-Elmer 360 AA spectrophotometer) using the experimental procedure described elsewhere,7 and the water content of the catalyst precursor was measured by thermogravimetry.48 The hydrated supported precursor (sieved in the mesh range 150-125 µm) was reduced, without a precalcination step, by heating in a 150 cm3 min-1 stream of hydrogen at a fixed rate of 5 K min-1 to a final temperature of 723 ( 2 K which was maintained for 18 h. The hydrogen gas (CANOX, 99.9%) was purified by passage through water (activated Molecular Sieve type 5A) and oxygen (1% Pd on WO3) traps which were connected in series. The reduced catalyst was then flushed in a purified stream of nitrogen at 200 cm3 min-1, cooled to room temperature, and contacted with 100 cm3 aqueous or alcoholic solutions of TA. In certain cases 2 g of NaBr was added to the modifier or the modified catalyst was agitated at room temperature in a 0.02 mol dm-3 solution of NaBr. It should be noted that, at the same modifier concentration, the pH of the TA solutions increased in the sequence of solvents, water < methanol < ethanol < 1-butanol, which corresponds to the dissociation equilibrium of TA in these media. The pH of the aqueous modifier was preadjusted to the isoelectric point for TA (5.1) with 1 mol dm-3 NaOH solutions whereas the pH of the alcohol based TA solutions was not adjusted. The resultant solution was thoroughly purged with helium to remove any entrapped air bubbles. The modification was performed in air with constant agitation for 2 h at the desired temperature ((2 K). The tartaric acid was of AnalaR grade and the prepared solutions were found to be better than 99% pure by HPLC; the concentrations of the TA solutions used in the modifications were reproducible to within (1%. After modification, the catalyst was decanted and washed with the particular modifier solvent (1 × 25 cm3), methanol (2 × 25 cm3) and the reaction solvent (2 × 25 cm3) before being stored in the latter prior to use; the reagents used in the washing step were thoroughly degassed in helium. The activated catalysts were also contacted, before and after modification, with thiophene and acetic acid to poison the bare metal sites. In each case, the catalyst was immersed in 40 cm3 of methanol and stirred at room temperature. A 10 cm3 aliquot of thiophene or acetic acid was then added and the resultant suspension was agitated for 30 min. The poisoned catalyst was decanted and washed with methanol (2 × 25 cm3) and 1-butanol (2 × 25 cm3). The nickel content of the postmodifier, resulting from an acid leaching of the catalyst, was determined by AA spectrophotometry.7 Nickel metal dispersions, before and after modification, were determined by carbon monoxide chemisorption7 at 273 K, assuming a nickel surface atom to adsorbed carbon monoxide stoichiometry of 2:1.49 The amount of TA adsorbed on selected samples was determined by analyzing the pre- and postmodification solutions by HPLC (Spectra-Physics). A Spherisorb 5µ C8 packed column (Alltech Assoc., 250 × 4.6 mm) was (48) Coughlan, B.; Keane, M. A. J. Catal. 1990, 123, 364. (49) Romanowski, W. Highly Dispersed Metals as Adsorbents; Halsted Press, John Wiley & Sons: New York, 1987; p 171.
Determining Enantioselective Conditions used with 0.2 mol dm-3 phosphoric acid (Aldrich, >99%) as the mobile phase. Detection was by UV (λ ) 187 nm) in conjunction with a Pye Unicam PU4810 integrator. As nickel tartrate (NiTA) has been tentatively identified as the stereoselective active site formed by the corrosive adsorption of TA on nickel,6,10 a correction for the presence of NiTA in the postmodifier was made by preparing a range of TA/NiTA solutions (of varying NiTA concentration) which were then used to construct the calibration plot. The amount of TA adsorbed was calculated (to within (3%) as the difference between the values obtained for the solutions before and after modification. Nickel tartrate was prepared according to the method of Sachtler et al.10 The liquid phase hydrogenation of MAA in a range of organic solvents was carried out in the temperature ((2 K) range 288 K e T e 373 K in a 250 cm3 glass vessel fitted with a condenser, hydrogen inlet, and thermocouple well. The MAA:solvent molar ratio was varied but the total volume was kept at 50 cm3, and a 10-240 cm3 min-1 stream of purified hydrogen was bubbled through the suspension which was either stagnant or kept under constant agitation (600-800 rpm). The hydrogen partial pressure was varied by dilution in nitrogen, keeping the overall flow rate constant at 60 cm3 min-1. The ultimate degree of conversion was achieved after 15-80 h, at which point the catalyst was removed from the reaction mixture by filtration. The extent of hydrogenation was determined by HPLC using a Pirkle type 1A 5µ reversible column (250 × 4.6 mm) with a 10% IPA/90% hexane mixture as the mobile phase. The overall level of hydrogenation was converted to mol % MHB using a 21 point calibration plot; a quadratic equation was used to fit these data to better than (2%. Optical yields (O.Y.) were determined from measurements of optical rotation (AA-10 Automatic Digital Polarimeter) using the equation
O.Y. ) [R]TD/[R]T0 ) 100 R/[R]T0lc [R]T
where D is the specific rotation of the product solution measured at the sodium D-line and temperature T (293 ( 3 K), [R]T0 is the specific rotation of the pure enantiomer under the same conditions (-22.95° for R-(-)-MHB), R is the measured optical rotation, l is the path length (20 cm) of the cell, and c is the solute concentration. Optical rotation vs R-(-)-MHB concentration data were fitted to a quadratic equation to better than (2%, which was then used to determine the optical yield of the product. In this paper, optical selectivity is expressed in terms of enantiomeric excess (ee) which is defined as
% ee ) 100{[(R)-(-)-MHB] - [(S)-(+)-MHB]}/ {[(R)-(-)MHB] + [(S)-(+)-MHB]}
Results and Discussion The results reported in this paper are related to the treatment of a 15.2% (w/w) Ni/SiO2 catalyst with (R)-(+)TA which yielded (R)-(-)-MHB as the predominant product; replacement of the modifier by its antipode generated identical values of enantiomeric excess but with a reversal of the sign. Moreover, modification with optically inactive meso-TA yielded a virtually racemic (ee
