1992
Organometallics 2010, 29, 1992–1995 DOI: 10.1021/om100127f
New Dipyridylamine Ruthenium Complexes for Transfer Hydrogenation of Aryl Ketones in Water Charles Romain,† Sylvain Gaillard,† Mohammed K. Elmkaddem,† Loı¨ c Toupet,† Cedric Fischmeister,*,† Christophe M. Thomas,*,‡ and Jean-Luc Renaud*,§ †
University of Rennes 1, UMR CNRS 6226, 35042 Rennes Cedex, France, ‡Chimie ParisTech, UMR CNRS 7223, 75005 Paris, France, and §Laboratoire de Chimie Mol eculaire et Thioorganique, UMR CNRS 6507, INC3M, FR 3038, ENSICAEN, University of Caen, 14050 Caen, France Received February 16, 2010
Summary: A new family of cationic organometallic chloro compounds of the type [(arene)Ru(N,N)(Cl)]þ containing N, N-chelating dipyridylamine ligands has been synthesized and isolated as the chloride salts, which are water soluble and stable to hydrolysis. The resulting mononuclear ruthenium complexes catalyze the transfer hydrogenation of aryl ketones in aqueous solution to give the corresponding alcohols with good conversion and interesting recyclability.
Introduction Environmental concerns in chemistry increase the demand for more selective chemical processes with a minimum of waste. For that reason, water-soluble organometallic complexes attract continuously growing interest for applications in catalysis, because of environmentally friendly processing, simple product separation, and pH-dependent selectivity in aqueous media.1,2 Therefore, metal-catalyzed synthesis in two-phase or even purely aqueous media has gained increasing attention during the last decades with remarkable industrial success in CdC and CdO bond hydrogenation, alkene hydroformylation, and carbonylation reactions.1-3 The reduction of carbonyl compounds to alcohols is an industrially relevant reaction for the preparation of fine chemicals, perfumes, agrochemicals, and pharmaceuticals.3b Metal-catalyzed pathways to these kinds of alcohols include H2 hydrogenation, hydrosilylation, or transfer hydrogenation and use catalysts based on palladium, rhodium, iridium, and ruthenium.3b In particular, transfer hydrogenation has emerged as a powerful, practical, and versatile tool for the reduction of carbonyl compounds in which a substrateselective catalyst transfers hydrogen between the substrate and a hydrogen donor or acceptor.4 The method is attractive as an alternative to hydrogenation: the increasing success of *Corresponding authors. (C.F.) E-mail: cedric.fischmeister@ univ-rennes1.fr. Fax: þ33(0)223236939. Tel: þ33(0)223235998. (C.M.T.) E-mail:
[email protected]. Fax: þ33(0)143260061. Tel: þ33(0)144276721. (J.-L.R.) E-mail:
[email protected]. Fax: þ33(0)231452877. Tel: þ33(0)144276721. (1) Cornils, B.; Hermann, W. A. Aqueous-Phase Organometallic Catalysis, 2nd ed.; Wiley-VCH, 2002. (2) Lindstrom, U. M. Chem. Rev. 2002, 102, 2751. (3) (a) Sinou, D. Adv. Synth. Catal. 2002, 344, 221. (b) Meyer, N.; Lough, A. J.; Morris, R. H. Chem.;Eur. J. 2009, 15, 5605. (4) For recent reviews, see: (a) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393. (b) Samec, J. S. M.; B€ackvall, J. E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (c) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. (d) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201. pubs.acs.org/Organometallics
Published on Web 03/29/2010
this technique follows from its operational simplicity and reduction of the risks associated with the use of high pressures of an easily flammable gas of high diffusibility. Furthermore, the donor and the acceptor are environmentally friendly and are also easy to handle. There have been several attempts to develop active catalytic systems for the transfer hydrogenation of ketones in aqueous uss-Fink8a,11,12 have media.5-12 In this regard, Ogo10 and S€ recently reported transfer hydrogenation reactions of ketones with sodium formate or formic acid, catalyzed in aqueous solution by [(arene)Ru(bipy)(OH2)]2þ and related iridium(III) complex or [(arene)Ru(phen)(OH2)]2þ (bipy=2,20 -bipyridine, phen = chelating 1,10-phenanthroline ligands). We have recently investigated the synthesis and characterization of new bidentate 2,20 -dipyridylamine ligands,13 which have shown interesting coordination abilities for the synthesis of welldefined Mg(II) and Zn(II) complexes.14 We envisaged that some of these ligands (i.e., the less sterically encumbered) would (5) (a) Wu, X. F.; Li, X. G.; Hems, W.; King, F.; Xiao, J. Org. Biomol. Chem. 2004, 2, 1818. (b) Li, X. G.; Wu, X. F.; Chen, W. P.; Hancock, F. E.; King, F; Xiao, J. Org. Lett. 2004, 6, 3321. (c) Wu, X. F.; Li, X. G.; King, F.; Xiao, J. Angew. Chem., Int. Ed. 2005, 44, 3407. (d) Wu, X. F.; Vinci, D.; Ikariya, T.; Xiao, J. Chem. Commun. 2005, 4447. (e) Xiao, J.; Wu, X. F.; Zanotti- Gerosa, A.; Hancock, F. Chim. Oggi-Chem. Today 2005, 23, 50. (f) Wu, X. F.; Li, X. H.; McConville, M.; Saidi, O.; Xiao, J. J. Mol. Catal. A: Chem. 2006, 247, 153. (g) Wu, X. F.; Liu, J. K.; Li, X. H.; Zanotti-Gerosa, A.; Hancock, F.; Vinci, D.; Ruan, J. W.; Xiao, J. Angew. Chem., Int. Ed. 2006, 45, 6718. (h) Li, X. H.; Blacker, J.; Houson, I.; Wu, X. F.; Xiao, J. Synlett 2006, 1155. (6) (a) Cortez, N. A.; Rodrı´ guez-Apodaca, R.; Aguirre, G.; Parra-Hake, M.; Cole, T.; Somanathan, R. Tetrahedron Lett. 2006, 47, 8515. (b) Cortez, N. A.; Aguirre, G.; Parra-Hake, M.; Somanathan, R. Tetrahedron Lett. 2007, 48, 4335. (7) (a) Li, L.; Wu, J.; Wang, F.; Liao, J.; Zhang, H.; Lian, C.; Zhu, J.; Deng, J. Green Chem. 2007, 9, 23. (b) Wu, J.; Wang, F.; Ma, Y.; Cui, X; Cun, L.; Zhu, J.; Deng, J.; Yu, B. Chem. Commun. 2006, 1766. (8) (a) Canivet, J.; S€ uss-Fink, G. Green Chem. 2007, 9, 391. (b) Zeror, S.; Collin, J.; Fiaud, J.-C.; Aribi Zouioueche, L. Adv. Synth. Catal. 2008, 350, 197. (9) Zeror, S.; Collin, J.; Fiaud, J.-C.; Zouioueche, L. A. J. Mol. Catal. 2006, 256, 85. (10) (a) Ogo, S.; Makihara, N.; Watanabe, Y. Organometallics 1999, 18, 5470. (b) Ogo, S.; Abura, T.; Watanabe, Y. Organometallics 2002, 21, 2964. (c) Ogo, S.; Uehara, K.; Abura, T.; Watanabe, Y.; Fukuzumi, S. Organometallics 2004, 23, 3047. (11) Canivet, J.; Labat, G.; Stoeckli-Evans, H.; S€ uss-Fink, G. Eur. J. Inorg. Chem. 2005, 4493. (12) Canivet, J.; S€ uss-Fink, G.; Stepnicka, P. Eur. J. Inorg. Chem. 2007, 4736. (13) Elmkaddem, M. K.; Fischmeister, C.; Thomas, C. M.; Renaud, J.-L. Chem. Commun. 2010, 46, 925. (14) Zheng, Z.; Elmkaddem, M. K.; Fischmeister, C.; Roisnel, T.; Thomas, C. M.; Carpentier, J.-F.; Renaud, J.-L. New J. Chem. 2008, 32, 2150. r 2010 American Chemical Society
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Scheme 1. Catalytic Reduction of Ketones
Table 1. Catalytic Activities of the Ru Compounds in the Reduction of Acetophenonea Figure 1. Synthesis of ruthenium compounds 2-5.
entry
compound
conversion (%)b
1 2 3 4
2 3 4 5
89 51 100 65
a [S]/[cat.] = 20, 65 °C, H2O, 24 h, HCO2H/HCO2Na (5 equiv/ 5 equiv). b Determined by 1H NMR.
Figure 2. Molecular structure of the cationic complex in [(η6p-cymene)Ru(1)Cl]PF6. The PF6- anion is omitted for clarity.
have the potential to stabilize ruthenium(II) derivatives in water. Herein we report the initial results of our studies on a series of water-soluble arene ruthenium complexes containing an N,N-chelating 2,20 -dipyridylamine ligand. We also describe the catalytic activity and the recycling of these complexes in the transfer hydrogenation of aromatic ketones.
Results and Discussion Our attention was drawn to the 2,20 -dipyridylamine ligand 1 (Figure 1), which was supposed to be more hydrophilic than the archetypical bypy or phen ligands. The dimeric arene ruthenium complexes [(arene)RuCl2]2 (arene = benzene, mesitylene, p-cymene, hexamethylbenzene) react at 50 °C in methanol with the N,N-donor ligand 1 to give the expected cationic chloro complexes [(arene)Ru(1)Cl]Cl (2-5) as the only product in good yields ranging from 72% to 87%. All the complexes, which were isolated as chloride salts, are water soluble and air stable. They have been characterized by 1H, 13C NMR and HRMS. The NMR spectra reveal symmetrical structures that prove no coordination from the bridging nitrogen atom and only the coordination of the two pyridyl moieties. Suitable crystals for X-ray analysis were obtained for the analogous hexafluorophosphate salt of compound 4. The molecular structure of the cationic complex [(η6-p-cymene)Ru(1)Cl]þ is depicted in Figure 2. The structure of the cation consists of a pseudotetrahedral arrangement of a ruthenium atom coordinated to the p-cymene ligand, the two nitrogen atoms of the N,N-ligand, and a chlorine atom. The bond lengths for this cationic
complex are very similar to those reported for related, three-legged piano-stool ruthenium complexes.15 In particular, Ru-C distances fall within the range 2.169(3)-2.237(3) A˚. As expected, the two amino groups of the N,N-ligand are in equatorial positions, which is the more stable conformation.12 Moreover, the central (secondary amine) N atom is nearly planar. Having isolated these complexes, we then carried out our catalytic investigations in hydride transfer (Scheme 1). On the basis of previous studies,10,11 describing arene ruthenium complexes containing bipyridine or phenantholine ligands for the transfer hydrogenation of ketones with formic acid and/or sodium formate as hydrogen donor source in water, we have evaluated the catalytic potential of our complexes in this reaction using acetophenone as a model substrate. Considering the pH-dependence of the catalytic activity of such complexes, we first studied the influence of the hydrogen donor ratio in the presence of catalyst 5. Best results were obtained at pH = 3.8 (i.e., pKa of formic acid) for an equimolar ratio of HCO2H/HCO2Na (5 equiv of each compared to the substrate).5 Under these conditions, the secondary alcohol was obtained in 65% yield within 24 h (entry 4, Table 1). A decrease of the amount of HCO2H/ HCO2Na to 2.5 equiv of formic acid and 2.5 of formate led to a decrease of the catalytic activity, and the alcohol was obtained in 14% yield. The use of 10 equiv of formic acid or sodium formate led to lower reactivity (11% and 3%, respectively). Consequently, a buffer solution with 5 equiv of formic acid and sodium formate has been prepared to evaluate the catalytic activities of the complexes previously synthesized. As showed in Table 1, all the chloro complexes 2-5 were found to catalyze transfer hydrogenation of acetophenone in aqueous solution; however the reactivity depends on the nature of the ancillary arene ligand. In the presence of 5% of catalyst 4, the reduction of acetophenone in phenylethanol is complete within 24 h, whereas with the other catalysts the yields are lower (entries 1-4, Table 1). As previously described by S€ uss-Fink, the catalytic activity is dependent on the arene.8a However, in contrast to his results, it is worth noticing that the conversion with hexamethylbenzene did not provide the best result. In our case, p-cymene as arene seems to be the best compromise between steric effects and electron density on the ruthenium. Even with only 1% of catalyst 4, the catalytic activity is maintained and the secondary (15) Therrien, B.; Ward, T. R.; Pilkington, M.; Hoffmann, C.; Gilardoni, F.; Weber, J. Organometallics 1998, 17, 330.
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Organometallics, Vol. 29, No. 8, 2010
Table 2. Reduction of Several Ketones Catalyzed by 4 via Hydride Transfera
Romain et al. Table 3. Recycling of Catalyst 4a entry
run
HCO2H/HCO2Na
conversion (%)b
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 1 2 3 4 5 6
5/5 (starting conditions) 0.5/0.5 0.5/0.5 0.5/0.5 0.5/0.5 5/5 5/5 (starting conditions) 0/0 0/0 0/0 0/0 5/5
97 90 43 43 36 86 97 45 15 7 5 5
a
[S]/[cat.] = 20, 65 °C, H2O, 24 h. b Determined by GC.
a [S]/[cat.] = 20, 65 °C, H2O, 24 h, HCO2H/HCO2Na (5 equiv/ 5 equiv). b Determined by 1H NMR.
alcohol is produced in quantitative yield within 24 h (TON = 100). In order to determine the potential applicability of this catalytic system, we then extended the reaction to a wider range of aromatic ketones. Whatever the substituent on the aromatic ring (electron-donating or -withdrawing on ortho and/or para position), the conversion and the isolated yields were good (entries 1-6, Table 2). However, it is worth mentioning that with 4-bromobenzophenone some trace of reduction of the C-Br bond was also observed in the reaction conditions. Having demonstrated the potential of these new complexes in the reduction of acetophenone, we envisioned to prepare the aqua complexes [(arene)Ru(1)(H2O)]2þ (isolated as tetrafluoroborate or chloride salts) in order to increase the activity of our ruthenium catalysts and to compare their catalytic activity to the corresponding chloro complexes [(arene)Ru(1)(Cl)]þ in the transfer hydrogenation of acetophenone in water. Both complexes were soluble in water in our previous conditions and found to have catalytic activities for this reaction. However, no noticeable difference was observed between the chloro complexes and the corresponding aqua complexes, which seems to indicate that the same active catalytic species is involved in this reaction (i.e., presumably a hydride complex). An important goal of this work was to study the recyclability of our ruthenium catalysts. Indeed, due to the high solubility and stability of the precatalytic species in water, separation and recycling should be easy to perform. Thus, at the end of the reduction, diethyl ether could be added to extract the organic compounds and the ruthenium complex immobilized in the water phase reused directly. For this purpose, two different sequences for recycling have been carried out: a first set of experiments (entries 1-6, Table 3) for which donor hydrogen sources were added at the end of each run, and a second one (entries 7-12, Table 3) without addition of donor hydrogen source. In both cases, recycling is possible. However, the catalytic activity decreased run after run, even if the drop is less important by adding 0.5 equiv of formic acid and formate at the end of the reaction (Figure 3). However, in the two series of experiments, the reason for this decrease is different. In the first series, the decrease could be explained by a diminution of the quantity of the hydrogen donor source. Indeed, after the fifth run, the addition of a donor hydrogen source (an equimolar ratio of 5 equiv
Figure 3. Recycling of catalyst 4. [S]/[cat.] = 20, 65 °C, H2O, 24 h. Conversion determined by GC.
HCO2H/5 equiv HCO2Na) restored the conversion obtained in the second run. This result demonstrated that the catalytic system remains active after several runs, but its activity depends on the amount of hydrogen donor source in solution and needs to be adjusted before a new reaction. The lower hydrogen concentration after each run might be explained by the consumption of more than 1 equiv of hydrogen donor during the reaction. Indeed, in aqueous phase, it is known that such a catalytic system can transform formic acid or formate into CO2 and H2.10,11 The dramatically decreasing catalytic activity observed in the second series (entries 7-12, Table 3) shows again the crucial importance of the amount of hydrogen donor. During the third run (entry 9) the solution turned from yellow to green, indicating an evolution of the catalytic species such as a decoordination of at least one of the ligands and then formation of a nonreactive intermediate. The last entry of the table provided evidence of the decomposition of the catalyst, as the addition of hydrogen donor does not lead to any conversion (entry 12, Table 3). In conclusion, we have synthesized several (arene)Ru(dipyridylamine) complexes that are interesting catalysts for the transfer hydrogenation of aromatic ketones in water. Moreover, we also demonstrated that such complexes could be reused several times. We will then proceed to the extension of this methodology to chiral auxiliaries, in order to develop a library of ruthenium-based complexes for enantioselective catalysis.
Experimental Section General Procedures. All experiments were carried out in an inert atmosphere using standard Schlenk techniques and freshly distilled solvents. The starting materials [(arene)RuCl2]2 and the
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Organometallics, Vol. 29, No. 8, 2010
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ligand 1 were prepared according to the published methods.16,17 All other reagents were commercially available and were used without further purification. NMR spectra were recorded on an ARX Bruker 400 MHz spectrometer using solvent residual peak as locking agent. Electro-spray mass spectra were obtained on a Micromass ZABSpecTOF spectrometer. Preparation of the Arene Ruthenium Compounds [(arene)Ru(1)Cl]Cl (arene = C6H6, mesitylene, p-cymene, C6Me6). Ligand 1 (2 equiv, 0.10 mmol) was added to a suspension of [(arene)RuCl2]2 (0.05 mmol) in methanol (5 mL). The mixture was stirred overnight at 50 °C. After evaporation to dryness, the residue was washed with ether (3 2 mL) to give the expected product in 72-87% yields. [(C6H6)Ru(1)Cl]Cl (2). Yield: 72%. 1H NMR (400 MHz, δ, CD3OD): 5.85 (s, 6H), 7.18-7.22 (m, 2H), 7.25 (dq, J = 0.6 Hz, 8.4 Hz, 2H), 7.91-7.95 (m, 2H), 8.72 (ddd, J = 5.9, 1.7, 0.5 Hz, 2H), NH not observed. 13C NMR (100 MHz, δ, CD3OD): 88.1 (d), 115.0 (d), 120.7 (d), 141.9 (d), 154.9 (s), 155.8 (d). HRMS (ESI): m/z 386.0001 [C16H15ClN3Ru]þ. [(mesitylene)Ru(1)Cl]Cl (3). Yield: 80%. 1H NMR (400 MHz, δ, CD3OD): 1.82 (s, 9H), 5.26 (s, 3H), 7.22-7.28 (m, 4H), 7.94 (ddd, J = 8.4, 7.3, 1.7 Hz, 2H), 8.60 (ddd, J = 5.9, 1.7, 0.6 Hz, 2H), NH not observed. 13C NMR (100 MHz, δ, CD3OD): 18.5 (q), 78.3 (d), 109.2 (s), 114.9 (d), 121.3 (d), 141.6 (d), 154.9 (s), 155.4 (d). HRMS (ESI): m/z 428.0455 [C19H21ClN3Ru]þ. [( p-cymene)Ru(1)Cl]Cl (4). Yield: 87%. 1H NMR (400 MHz, δ, CD3OD): 1.23 (d, J = 6.9 Hz, 6H), 2.10 (s, 3H), 2.62 (hept, J = 6.9 Hz, 1H), 5.57 (d, J = 6.3 Hz, 2H), 5.67 (d, J = 6.3 Hz, 2H), 7.19-7.23 (m, 4H), 7.91-7.95 (m, 2H), 8.63 (ddd, J = 5.9, 1.7, 0.8 Hz, 2H), NH not observed. 13C NMR (100 MHz, δ, CDCl3): 18.4 (d), 22.4 (q), 32.2 (q), 85.3 (d), 86.9 (d), 101.4 (s), 108.1 (s), 115.1 (d), 120.7 (d), 141.8 (d), 154.5 (s), 155.8 (d). HRMS (ESI): m/z 442.0621 [C20H23ClN3Ru þ K]þ. [(C6Me6)Ru(1a)Cl]Cl (5). Yield: 81%. 1H NMR (400 MHz, δ, CD3OD): 1.92 (s, 18H), 7.23-7.29 (m, 4H), 7.91-7.96 (m, 2H), 8.33 (ddd, J = 5.9, 1.7, 0.6 Hz, 2H), NH not observed. 13C NMR (100 MHz, δ, CD3OD): 15.7 (q), 95.9 (s), 115.1 (d), 121.6 (d),
141.7 (d), 154.6 (s), 155.4 (d). HRMS (ESI): m/z 470.0935 [C22H27ClN3Ru]þ. Transfer Hydrogenation Catalysis with Acetophenone. Transfer hydrogenation reactions of acetophenone (0.50 mmol) using 2-5 (0.025 mmol) were carried out under inert atmosphere in water (4 mL) using a buffer of HCOOH (2.5 mmol) and HCOONa (2.5 mmol) as hydrogen donor source. The products were extracted by Et2O and identified by gas chromatography. Transfer Hydrogenation Catalysis. Transfer hydrogenation reactions of the appropriate ketones (0.50 mmol) using 4 (0.025 mmol) were carried out under inert atmosphere in water (4 mL) using a buffer of HCOOH (2.5 mmol) and HCOONa (2.5 mmol) as hydrogen donor source. The products were extracted by Et2O, and conversions were calculated by 1H NMR from the crude product. After purification by column chromatography using silica gel and ethyl acetate/cyclohexane (80/20) as eluant, the isolated products were identified by 1H NMR. 1-p-Tolylethanol. 1H NMR (400 MHz, δ, CDCl3): 1.49 (d, J = 6.5 Hz, 3H), 2.35 (s, 3H), 4.88 (dq, J = 6.5, 2.7 Hz, 1H), 7.17 (d, J = 2.7 Hz, 2H), 7.27 (d, J = 2.7 Hz, 2H). 1-(4-Methoxyphenyl)ethanol. 1H NMR (400 MHz, δ, CDCl3): 1.35 (d, J = 6.5 Hz, 3H), 3.69 (s, 3H), 4.71 (q, J = 6.4 Hz, 1H), 6.77 (d, J = 8.4 Hz, 2H), 7.17 (d, J = 8.4 Hz, 2H). 1-Naphthylethanol. 1H NMR (400 MHz, δ, CDCl3): 1.57 (d, J = 6.5 Hz, 3H), 5.02 (q, J = 6.5 Hz, 1H), 7.53-7.46 (m, 3H), 7.77-7.86 (m, 4H). 1-(2-methoxyphenyl)ethanol. 1H NMR (400 MHz, δ, CDCl3): 1.40 (d, J = 6.6 Hz, 3H), 3.74 (s, 3H), 5.00 (q, J = 6.5, 1H), 6.77 (dd, J = 8.2, 0.8 Hz, 1H), 6.86 (dt, J = 7.5, 1.1 Hz, 1H), 7.17-7.12 (m, 1H), 7.25 (dd, J = 7.5, 1.1 Hz, 1H).
(16) Bennet, M. A. Coord. Chem. Rev. 1997, 166, 225. (17) Gaillard, S.; Elmkaddem, M. K.; Fischmeister, C.; Thomas, C. M.; Renaud, J.-L. Tetrahedron Lett. 2008, 49, 3471.
Supporting Information Available: Crystallographic data for [(η6-p-cymene)Ru(1)Cl]PF6 as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful to the Agence Nationale pour la Recherche (ANR) for grant “Jeunes Chercheurs-Jeunes Chercheuses” (ANR-06-JCJC-0013), the ENSCP, the CNRS, and the French Ministry of Higher Education and Research for financial support.
