Organometallics 2010, 29, 3661–3664 DOI: 10.1021/om100601r
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Sulfonate-Functionalized NHC-Based Ruthenium Catalysts for the Isomerization of Allylic Alcohols in Water. Recyclability Studies Arturo Azua, Sergio Sanz, and Eduardo Peris* Departamento de Quı´mica Inorg anica y Org anica, Universitat Jaume I. Avenida Vicente Sos Baynat s/n, Castell on, E-12071 Spain Received June 21, 2010
Two new complexes of the type [Ru(CO3)(η6-arene)(NHC)]Cs, in which the NHC ligand incorporates a sulfonate functionality, have been prepared and fully characterized. The two water-soluble complexes have been tested in the catalytic isomerization of allylic alcohols in water. The p-cymene compound (labeled as 1) was shown to be an excellent catalyst when compared to other related Ru catalysts. This catalyst can be recycled by simple liquid-liquid extractions several times without detected loss of activity.
Introduction During the last two decades, there has been an increasing interest in the use of water as solvent for many homogeneously catalyzed reactions.1,2 The potential advantages of water over usual organic solvents may include cost, safety, and environmental concerns.2 Water-soluble versions of almost any kind of ligand are nowadays known, although phosphines are by far the ones that have been more widely studied,1,3 probably because they have been extensively used in the preparation of homogeneous catalysts. Despite the increasing interest in the use of N-heterocyclic carbenes (NHCs) in the design of homogeneous catalysts during the last two decades, it is rather surprising that these types of ligands have rarely been prepared in their water-soluble versions for homogeneously catalyzed reactions in water.4 NHCs may introduce a series of advantages over phosphines in the design of water-soluble catalysts. For example, (i) NHCs are considered as stabilizing ligands for transition metal complexes; thus they may improve the stability of the catalyst in water; (ii) phosphines are easily oxidized and may generate undesirable residues; and (iii) NHCs are known to be less toxic than phosphines. A few examples of NHC ligands containing hydrophilic groups (mainly sulfonates) *To whom correspondence should be addressed. E-mail: eperis@ qio.uji.es. (1) Herrmann, W. A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1524. (2) Lamblin, M.; Nassar-Hardy, L.; Hierso, J. C.; Fouquet, E.; Felpin, F. X. Adv. Synth. Catal. 2010, 352, 33. Lindstrom, U. M. Chem. Rev. 2002, 102, 2751. (3) Shaughnessy, K. H. Chem. Rev. 2009, 109, 643. Pinault, N.; Bruce, D. W. Coord. Chem. Rev. 2003, 241, 1. (4) A full and detailed review on the synthesis of hydrophilic carbene ligands can be found in: Shaughnessy, K. H. Chem. Rev. 2009, 109, 643. (5) Turkmen, H.; Can, R.; Cetinkaya, B. Dalton Trans. 2009, 7039. Fleckenstein, C.; Roy, S.; Leuthausser, S.; Plenio, H. Chem. Commun. 2007, 2870. Papini, G.; Pellei, M.; Lobbia, G. G.; Burini, A.; Santini, C. Dalton Trans. 2009, 6985. (6) Virboul, M. A. N.; Lutz, M.; Siegler, M. A.; Spek, A. L.; van Koten, G.; Gebbink, R. Chem.;Eur. J. 2009, 15, 9981. Nagai, Y.; Kochi, T.; Nozaki, K. Organometallics 2009, 28, 6131. r 2010 American Chemical Society
have been described in the literature,5,6 but we found only three recent examples in which the complexes obtained were used as water-soluble catalysts.5 Aiming to obtain suitable complexes that can facilitate homogeneously catalyzed reactions in water, we now report the preparation of two new Ru(η6-arene)(NHC) compounds in which the NHC ligand has a sulfonate group. The two new compounds have been studied in the water-catalyzed isomerization of allylic alcohols.
Results and Discussion Compounds 1 and 2 were prepared from the reaction of [RuCl2(η6-arene)]2 (arene = hexamethylbenzene or p-cymene) and N,N0 -(methyl)(propanesulfonate)imidazolium in the presence of cesium carbonate in refluxing acetonitrile (Scheme 1). Both complexes were insoluble in most organic solvents, although they were soluble in MeOH and water (water solubility >300 g/L). Compounds 1 and 2 were characterized by NMR spectroscopy, electrospray mass spectrometry (ESIMS), and elemental analysis. The most representative NMR data arise from the 13CNMR spectra, which show the signals due to the Ru-Ccarbene carbons at δ 176.4 (1) and 179.4 (2). The 13C NMR signals due to the carbon at the carbonate ligand appear at δ 167.3 (1) and 167.1 (2). The presence of the carbonate ligand is also confirmed by the presence of a broad band at 1595.8 (1) and 1586.2 (2) cm-1, in accordance with the frequencies shown for other related Ru(CO3)(η6-arene)(NHC) complexes previously reported.7 The presence of the carbonate ligand in the Ru complex offered us a good opportunity to study the reaction of this complex with H2, in connection with our previous studies on the reduction of CO2 with Ir and Ru NHC-based complexes.8 The reaction of 1 in the presence of KOH (1 M) and 50 atm of H2 affords the formation of KCHOO, so the (7) Demerseman, B.; Mbaye, M. D.; Semeril, D.; Toupet, L.; Bruneau, C.; Dixneuf, P. H. Eur. J. Inorg. Chem. 2006, 1174. (8) Sanz, S.; Azua, A.; Peris, E. Dalton Trans. 2010, 39, 6339. Sanz, S.; Benitez, M.; Peris, E. Organometallics 2010, 29, 275. Published on Web 07/19/2010
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Azua et al. Scheme 1
formation of carbonate-Ru intermediates may be considered as possible reaction intermediates in the reduction of CO2 under basic reaction conditions. Unfortunately, we did not obtain any clear evidence about the nature of the resulting Ru species after this process. We tested the catalytic activity of complexes 1 and 2 in the isomerization of allylic alcohols. This reaction proceeds with total atom economy and allows the direct preparation of the corresponding carbonyl compounds, thus simplifying the conventional two-step organic procedures.9 Several ruthenium complexes have been reported to show high activities in this catalytic reaction,10-14 and some of them have provided excellent outcomes using water as solvent,10,11,13,15 although there are just a few studies reporting the recycling of the catalyst for its reuse in further catalytic cycles.10,11,13,16 The reactions were carried out in water with a 0.2 mol % catalyst loading at 100 °C. For comparative purposes we also tested RuCl3 3 3H2O, [RuCl2(p-cymene)]2, and [RuCl2(pcymene)(IMe)] (IMe = N,N-dimethylimidazolylidene). As can be seen from the data shown in Table 1, compound 1 is far more active than 2, in agreement with previously reported results by Crochet and co-workers, where the catalytic activity of Ru(p-cymene) complexes is shown to be higher than the Ru(hexamethylbenzene) analogues.13 This result may indicate that the decoordination of the η6-arene ligand may also be playing a role in the activation of the catalyst. [RuCl2(p-cymene)]2 and [RuCl2(p-cymene)(IMe)] also afforded very good catalytic outcomes (entries 7 and 8), although slightly lower than that shown by 1 (entry 6). The differences in catalytic activities between 1, [RuCl2(p-cymene)]2, and (9) Cadierno, V.; Crochet, P.; Gimeno, J. Synlett 2008, 1105. Cadierno, V.; Crochet, P.; Garcia-Garrido, S. E.; Gimeno, J. Dalton Trans. 2010, 39, 4015. Uma, R.; Crevisy, C.; Gree, R. Chem. Rev. 2003, 103, 27. (10) Crochet, P.; Diez, J.; Fernandez-Zumel, M. A.; Gimeno, J. Adv. Synth. Catal. 2006, 348, 93. (11) Cadierno, V.; Garcia-Garrido, S. E.; Gimeno, J. Chem. Commun. 2004, 232. Cadierno, V.; Crochet, P.; Garcia-Garrido, S. E.; Gimeno, J. Dalton Trans. 2004, 3635. (12) Ito, M.; Kitahara, S.; Ikariya, T. J. Am. Chem. Soc. 2005, 127, 6172. Crochet, P.; Fernandez-Zumel, M. A.; Gimeno, J.; Scheele, M. Organometallics 2006, 25, 4846. Doppiu, A.; Salzer, A. Eur. J. Inorg. Chem. 2004, 2244. Fekete, M.; Joo, F. Catal. Commun. 2006, 7, 783. Martin-Matute, B.; Bogar, K.; Edin, M.; Kaynak, F. B.; Backvall, J. E. Chem.;Eur. J. 2005, 11, 5832. Campos-Malpartida, T.; Fekete, M.; Joo, F.; Katho, A.; Romerosa, A.; Saoud, M.; Wojtkow, W. J. Organomet. Chem. 2008, 693, 468. Cadierno, V.; Crochet, P.; Francos, J.; Garcia-Garrido, S. E.; Gimeno, J.; Nebra, N. Green Chem. 2009, 11, 1992. Takai, Y.; Kitaura, R.; Nakatani, E.; Onishi, T.; Kurosawa, H. Organometallics 2005, 24, 4729. Lastra-Barreira, B.; Diez, J.; Crochet, P. Green Chem. 2009, 11, 1681. (13) Diaz-Alvarez, A. E.; Crochet, P.; Zablocka, M.; Duhayon, C.; Cadierno, V.; Gimeno, J.; Majoral, J. P. Adv. Synth. Catal. 2006, 348, 1671. (14) da Costa, A. P.; Mata, J. A.; Royo, B.; Peris, E. Organometallics 2010, 29, 1832. (15) Cadierno, V.; Garcia-Garrido, S. E.; Gimeno, J.; Varela-Alvarez, A.; Sordo, J. A. J. Am. Chem. Soc. 2006, 128, 1360. (16) Knight, D. A.; Schull, T. L. Synth. Commun. 2003, 33, 827. Bianchini, C.; Meli, A.; Oberhauser, W. New J. Chem. 2001, 25, 11.
Table 1. Isomerization of Allylic Alcohols in H2Oa
entry
R
catalyst
time (h)
yieldb
1 2 3 4 5 6 7 8 9 10 11
H H Et Et nPr nPr nPr nPr nPr nBu nBu
2 1 2 1 2 1 [RuCl2(p-cymene)(IMe)] [RuCl2(p-cymene)]2 RuCl3 3 3H2O 2 1
1 1 4 2 4 4 4 4 4 4 4
99 99 6 98 2 98 90 92 0 20 99
a Reactions performed in a sealed tube under N2 atmosphere at 100 °C, allylic alcohol (0.5 mmol), catalyst (0.2 mol %), using 1 mL of H2O as solvent. b Yields and conversions determined by GC and 1H NMR spectroscopy, using anisole as internal standard.
[RuCl2(p-cymene)(IMe)] are more important when shorter reaction times are used. For example, for catalyst 1, the reaction is almost complete after 1 h (yield: 85%), while for [RuCl2(p-cymene)]2 and [RuCl2(p-cymene)(IMe)] yields are below 60%. Remarkably, all the reactions proceed without the need of the addition of an external base, which not only simplifies the reaction procedure but also increases the atom economy of the overall process. We also monitored the isomerization of 1-hexen-3-ol using catalyst 1 in H2O and D2O in order to determine if there is a measurable kinetic isotopic effect (KIE) for this process. According to Figure 1, we calculated a KIE value of kH/kD = 1.33 (based on the determination of t1/2 values at 100 °C), but this low value is likely to be due to a solvent isotopic effect rather than an effect attributed to the intrinsic reaction mechanism. The time-course plots show a sigmoidal shape, indicating that the catalyst needs an activation period of ca. 30 min, which is probably the time needed for the loss of the carbonate ligand. In a recent study we observed that the KIE measured for the isomerization of an O-deuterated allylic alcohol was negligible, indicating that the O-H(D) bond activation is not the rate-determining step of the reaction.14 Due the great solubility of 1 in water, the reaction products can be easily separated from the catalyst by simple liquidliquid extraction once the reaction is complete. Also, the high activity of the catalyst in the absence of any external base simplifies the experimental workup for the catalyst reuse. The reactions were carried out using a 1 or 0.5 mol % catalyst loading at 100 °C. The isomerization of two allylic alcohols (1-hepten-3-ol and 1-hexen-3-ol) was studied. After each cycle, the organic products were extracted with CHCl3, and
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Figure 1. Time course of isomerization of 1-hexen-3-ol using catalyst 1 (0.5 mol %) in H2O and D2O at 100 °C. Table 2. Catalyst Recycling in the Isomerization Reactions of Allylic Alcohols Running in H2Oa
entry
R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
nBu
% Ru 1
nPr
1
nPr
0.5
run
yieldb
TONc
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6
99 99 99 99 99 99 99 99 26 10 99 99 99 99 99 99 97 90 34 20 99 99 92 88 86 5
99 198 297 396 495 594 693 792 818 828 99 198 297 396 495 594 691 781 815 835 198 396 580 756 928 938
a Reactions performed in a sealed tube under N2 atmosphere at 100 °C, allylic alcohol (0.5 mmol) and catalyst, using 1 mL of H2O as solvent. Reaction time: 2 h. b Yields and conversions determined by GC and 1H NMR spectroscopy, using anisole as internal standard. c Cumulative TONs.
the aqueous phase containing the catalyst was reused for the successive cycles. In order to compare the activity of each individual cycle in each of the sequences, we decided to perform cycles with a fixed reaction time of 2 h. As can be seen from the data shown in Table 2, catalyst 1 can be recycled up to eight times, without important loss of activity, when an initial 1 mol % catalyst loading was used. For the reactions carried out with a 0.5 mol % catalyst loading, up to five runs could be made before the catalyst showed any important loss of activity. These data compare well with the results reported by Crochet and Gimeno,10 which are considered the best reported so far in terms of recycling capacity, although in their case KOtBu was added to the reaction medium in order
to activate the catalyst. In a more recent example, Caminade and co-workers reported a Ru-based dendrimer terminated with water-soluble phosphines that was also recycled in the catalytic isomerization of allylic alcohols,17 although in this example the reactions took longer (24 h each run) and the loss of activity was detected after four runs.
Conclusions We have prepared two new complexes of the type [Ru(CO3)(η6-arene)(NHC)]Cs, in which the NHC ligand incorporates a sulfonate functionality. Both compounds are soluble in water, but very insoluble in most organic solvents, for which the complexes are a priori good candidates for designing recyclable catalytic systems in which the catalyst is separated with the H2O phase by simple liquid-liquid extraction. The pcymene complex 1 resulted in being a very effective catalyst in the isomerization of allylic alcohols, but the most remarkable result is its capability to be recycled several times without loss of activity. To the best of our knowledge, catalyst 1 is the best catalyst in terms of reuse capacity together with a previous example reported by Crochet, Gimeno, and co-workers,10 but our system has the advantage that the reaction can be carried out in the absence of an external base, which in fact simplifies the reaction procedure and transforms the reaction into a more environmental friendly process. Our work constitutes a new report of the very few examples of the use of NHC-based catalysts incorporating a hydrophilic functionality for the study of a homogeneously catalyzed reaction in water.
Experimental Section NMR spectra were recorded on Varian Innova 300 and 500 MHz spectrometers, using CD3OD and D2O as solvents. Elemental analyses were carried out in an EA 1108 CHNS-O Carlo Erba analyzer. Electrospray mass spectra (ESI-MS) were recorded on a Micromass Quatro LC instrument, and nitrogen was employed as drying and nebulizing gas. [RuCl2(p-cymene)]2,18 [RuCl2(C6Me6)]2,19 and [Ru Cl2(p-cymene)(IMe)]20 were prepared (17) Servin, P.; Laurent, R.; Gonsalvi, L.; Tristany, M.; Peruzzini, M.; Majoral, J. P.; Caminade, A. M. Dalton Trans. 2009, 4432. (18) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974, 233. (19) Bennett, M. A.; Huang, T. N.; Matheson, T. W.; Smith, A. K. Inorg. Synth. 1982, 21, 74. (20) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Chem.;Eur. J. 1996, 2, 772.
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according to literature procedures. Solvents and reagents were used as received from commercial suppliers. Synthesis of 1. A solution of [RuCl2(p-cymene)]2 (61 mg, 0.1 mmol), (N,N0 -(methyl)(propanesulfonate)imidazolium) (41 mg, 0.2 mmol), and cesium carbonate (391 mg, 1.2 mmol) was refluxed in CH3CN (30 mL) overnight. The suspension was filtered through Celite, and the solvent was evaporated under reduced pressure. The crude solid was purified by column chromatography. If it is necessary, elution with CH3OH/acetone afforded the separation of a light brown band that contained the compound. The complex was obtained as a light brown solid by precipitation from MeOH/ Et2O solution (yield: 106.1 mg, 84%). 1H NMR (D2O, 500 MHz): δ 7.29 (s, 1H, Himid), 7.22 (s, 1H, Himid), 5.76-5.70 (m, 2H, C6H4), 5.36-5.29 (m, 2H, C6H4), 4.21-4.13 (m, 1H, NCH2), 4.00-3.91 (m, 1H, NCH2), 3.61 (s, 3H, CH3N), 2.92-2.83 (m, 1H, CH2CH2SO3), 2.81-2.69 (m, 2H, CH2CH2SO3, CHCH3), 2.19-2.09 (m, 2H, CH2CH2SO3), 1.92 (s, 3H, CH3(C6H4)), 1.20-1.11 (m, 6H, CH(CH3)2). 13C{1H} NMR (CD3OD, 300 MHz): δ 176.37 (1C, Cimid), 167.31 (1C, CO3), 123.92 (1C, Cimid), 121.42 (1C, Cimid), 108.10 (1C, CiPr, p-cymene), 97.64 (1C, CMe, p-cymene), 83.47 (2C, C6H4), 80.07 (2C, C6H4), 49.47 (1C, CH2SO3), 48.68 (1C, NCH2), 36.94 (1C, NCH3), 31.85 (1C, CHMe2), 26.79 (1C, CH2CH2SO3), 22.05 (2C, CHMe2), 17.81 (1C, MeC6H4). IR (KBr, cm-1): 1595.81 sbr (CdO), 1199.51 sbr (SO3), 1041.37 s (SO3). Electrospray HR MS (15 V, m/z): 499.0478 [M]-. Anal. Calcd for C18N2O6SH25RuCs (mol wt 631.44): C, 34.24; H, 3.99; N, 4.44. Found: C, 34.51; H, 4.23; N, 4.81. Synthesis of 2. A solution of [RuCl2(C6Me6)]2 (69 mg, 0.1 mmol), (N,N0 -(methyl)(propanesulfonate)imidazolium) (41 mg, 0.2 mmol), and cesium carbonate (391 mg, 1.2 mmol) was refluxed in CH3CN (30 mL) overnight. The suspension was filtered through Celite, and the solvent was evaporated under reduced pressure. The crude solid was purified by column chromatography. If it is necessary, elution with CH3OH/acetone afforded the separation of a yellow band that contained the compound. The complex was obtained as a yellow solid by precipitation from MeOH/Et2O solution (yield: 101.6 mg, 77%). 1H NMR (D2O, 300 MHz): δ 7.42 (s, 1H, Himid), 7.33 (s, 1H, Himid), 4.32-4.23 (m, 1H, NCH2), 3.98-3.89 (m, 1H, NCH2), 3.64 (s, 3H, CH3N), 2.98-2.90 (m, 1H, CH2CH2SO3), 2.81-2.71 (m, 1H, CH2CH2SO3), 2.32-2.18 (m, 2H, CH2CH2SO3), 2.11 (s, 18H, C6Me6). 13 C{1H} NMR (CD3OD, 300 MHz): δ 179.37 (1C, Cimid), 167.10 (1C, CO3), 123.91 (1C, Cimid), 121.34 (1C, Cimid), 92.79 (6C, C6Me6), 49.25 (1C, CH2SO3), 48.24 (1C, NCH2), 36.59 (1C, NCH3), 26.77 (1C, CH2CH2SO3), 14.96 (6C, C6Me6). IR (KBr, cm-1): 1586.16 sbr (CdO), 1185.04, sbr (SO3), 1039.44 s (SO3). Electrospray MS (15 V, m/z): 527.0 [M]-. Anal. Calcd for C20N2-
Azua et al. O6SH29RuCs (mol wt 659.49): C, 36.42; H, 4.43; N, 4.25. Found: C, 36.83; H, 4.73; N, 4.49. Synthesis of (N,N0 -(Methyl)(propanesulfonate)imidazolium). A solution of N-(methyl)imidazole (235 mg, 2.8 mmol) and propanesufate (2.05 g, 16.8 mmol) reacted in 5 mL of CH2Cl2 at 60 °C in a pressure tube for 15 h. The white precipitate was filtered and washed with CH2Cl2, forming a pure white product (yield: 542.64 mg, 95%). 1H NMR (D2O, 300 MHz): δ 8.83 (s, 1H, Himid), 7.61 (s, 1H, Himid), 7.56 (s, 1H, Himid), 4.50-4.39 (m, 2H, NCH2), 3.98 (s, 3H, CH3N), 3.04-2.94 (m, 2H, CH2CH2SO3), 2.47-2.34 (m, 2H, CH2CH2SO3). 13C{1H} NMR (D2O, 300 MHz): δ 136.33 (1C, Cimid), 123.88 (1C, Cimid), 122.31 (1C, Cimid), 47.85 (1C, CH2SO3), 47.32 (1C, NCH2), 35.82 (1C, CH3N), 25.22 (1C, CH2CH2SO3)). Electrospray MS (15 V, m/z): 203.1 [M H]-. Anal. Calcd for C7N2O3SH12 (mol wt 204.25): C, 41.16; H, 5.92; N, 13.72. Found: C, 41.32; H, 6.11 N, 13.97. Catalytic Isomerization of Allylic Alcohols in Water. In a sealed tube, catalyst (0.005 mmol) and the corresponding allylic alcohol (0.5 mmol) were mixed in 1 mL of desoxigenated water under inert atmosphere and stirred at 100 °C for the appropiate time. The products were extracted twice with 2 mL of CHCl3, and anisole (0.5 mmol) was added to the resulting solution. The reaction mixture was analyzed by NMR and gas chromatography. Recycling Procedure. After cooling the reaction mixture, the product was extracted twice with 2 mL of CHCl3, and anisole (0.5 mmol) was added to the resulting solution. The resulting solution was analyzed by NMR and gas chromatography. The traces of organic solvent were eliminated at reduced pressure. Substrate (0.5 mmol) was added and the mixture was heated at 100 °C. Reaction of 1 with Pressurized Hydrogen. Ten mg of compound 1 and 120 mg of KOH were dissolved in 2 mL of D2O. This solution was pressurized up to 50 bar of H2 in a Hastelloy Autoclave Mini-Reactor system for 16 h. After this time the KCHOO signal is observed by 1H NMR.
Acknowledgment. We gratefully acknowledge financial support from the Ministerio de Ciencia e Innovaci on of Spain (CTQ2008-04460 and CTQ2007-31175-E/BQU) and Bancaixa (P1.1B2007-04). The authors are grateful to the Serveis Centrals d’Instrumentaci o Cientı´ fica (SCIC) of the Universitat Jaume I for providing us with spectroscopic facilities. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
