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A novel planar chiral N-heterocyclic carbene–oxazoline ligand for the asymmetric hydrosilylation of ketones
Catalysis Communications 10 (2009) 1493–1496
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
A novel planar chiral N-heterocyclic carbene–oxazoline ligand for the asymmetric hydrosilylation of ketones Yongqing Kuang a, Xiaoli Sun b, Hui Chen b, Peng Liu b, Ru Jiang b,* a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China Department of Chemistry, School of Pharmacy, Fourth Military Medical University, 17 Changlexi Street, Xi’an, Shannxi 710032, China
a r t i c l e
i n f o
Article history: Received 8 December 2008 Received in revised form 23 March 2009 Accepted 28 March 2009 Available online 5 April 2009
a b s t r a c t A novel ferrocene-based planar chiral N-heterocyclic carbene–oxazoline ligand was synthesized and applied to the rhodium(I)-catalyzed asymmetric hydrosilylation of ketones. Moderate catalytic activity and enantioselectivity were obtained. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Chiral N-heterocyclic carbene Oxazoline Ferrocene Asymmetric hydrosilylation Ketone
1. Introduction Metal-catalyzed asymmetric hydrosilylation of prochiral ketones followed by hydrolysis provides one of the most efficient methods for the preparation of chiral secondary alcohols. Dating from the pioneering work of Ojima, Kagan and Brunner [1–4], various metals, chiral ligands as well as hydride sources have been screened [5–7]. As for rhodium-catalyzed asymmetric hydrosilylation, bidentate chiral P, N-ligands, particularly phosphine–oxazoline ligands, have played a dominant role and some of them have achieved excellent activities and enantioselectivities [8–16]. Chiral N-Heterocyclic carbenes (NHCs) as effective alternatives to chiral phosphine ligands in asymmetric catalytic processes are attracting increasing attention due to their strong r-donating and weak p-accepting properties [17,18]. Both mono-and bisNHC ligands, of which only the carbene carbon atom(s) coordinate(s) to the metal center, have been applied to rhodiumcatalyzed asymmetric hydrosilylation of ketones with excellent catalytic activities, but poor to moderate enantioselectivities [19–21]. Inspired by the good results obtained with phosphine– oxazoline ligands, several research groups have in recent years synthesized carbene–oxazoline chelating ligands and applied some of them in the asymmetric hydrosilylation of ketones [22]. In 1998, Herrmann reported the synthesis of the first chiral carbene–oxazoline ligand (Fig. 1a). In this bidentate ligand, the oxazoline ring is * Corresponding author. Tel./fax: +86 29 84776945. E-mail addresses: [email protected], [email protected] (R. Jiang). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.03.027
linked in its 2-position to the imidazole ring via a methylene bridge [23]. In 2004, Gade and co-workers reported the synthesis of a family of carbene–oxazoline ligands which were obtained by direct linkage of the two heterocycles (Fig. 1b). Their complexes with rhodium(I) achieved enantioselectivities up to 95% ee in the asymmetric hydrosilylation of prochiral dialkylketones [24,25]. In 2005, Bolm reported a series of ferrocene-based carbene–oxazoline ligands in which the imidazole ring was linked to ferrocene via a methylene bridge (Fig. 1c). All of their complexes with rhodium(I) were active giving the secondary alcohol in high yields but with very low enantioselectivities (<6% ee) [26]. With the goal of improving the catalyst performance, we designed and synthesized a novel ferrocene-based chiral carbene–oxazoline ligand precursor 1 possessing a rigid backbone by direct linkage of the imidazole ring to ferrocene (Fig. 2), and examined its efficiency in the asymmetric hydrosilylation of prochiral ketones. 2. Experimental 2.1. General remarks Solvents were purified by standard procedures. 1H NMR and 13C NMR spectra were recorded on a Bruker AV-400 spectrometer. High performance liquid chromatography (HPLC) was performed by an Agilent 1100 interfaced to a HP 71 series computer workstation with a Daicel Chiralcel OD–H chiral column. Gas chromatography analyses were performed on a chiral CP-Cyclodex-b-236 M-19 column (25 m 0.32 mm) on Varian CP-3800. Optical rotations
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(100 MHz, CDCl3): d 18.2, 18.9, 32.4, 53.6, 64.4, 65.7, 68.4, 68.5, 69.5, 69.6, 72.2, 72.5, 72.7, 92.2, 121.5, 125.9, 129.1, 129.4, 129.5, 132.6, 137.8, 162.5 ppm. HRMS (ESI): calcd. for [MPF6]+: 454.1582, found 454.1539. 2.4. Preparation of 7
Fig. 1. Reported carbene–oxazoline ligands (imidazolium procursors).
Fig. 2. Chiral carbene precursor 1.
were measured on a Perkin–Elmer 343 Polarimeter. Commercial reagents were used as received, unless otherwise stated. THF, Et2O and toluene were dried over sodium and freshly distilled before use. Compounds 3–5 were prepared according to published procedures [27–29]. 2.2. Preparation of 6 To a well-dried Schlenk tube, CuI (48 mg, 0.25 mmol), imidazole (0.51 g, 6 mmol) and Cs2CO3 (3.42 g, 10.5 mmol) were added, evacuated twice and back-filled with nitrogen. Dioxane (5 mL), 5 (2.12 g, 5 mmol) and trans-1,2-cyclohexane diamine (0.11 g, 1 mmol) were then successively added under nitrogen. The Schlenk tube was sealed and the reaction mixture was stirred with heating at 110 °C for 24 h. The reaction mixture was cooled to ambient temperature, diluted with 250 mL of ethyl acetate, and filtered through a plug of silica gel followed by eluting with 500 mL of ethyl acetate. The filtrate was concentrated and the resulting residue was purified by column chromatography (hexane/ethyl acetate 1:5) to provide 0.95 g of 6 (51% yield). [a]D = +26.3 (c 0.135, CHCl3); 1H NMR (400 MHz, CDCl3): d 0.84 (d, J = 6.68 Hz, 3H), 0.91 (d, J = 6.74 Hz, 3H), 1.70 (m, 1H), 3.90 (m, 2H), 4.17– 4.30 (m, 7H), 4.53 (m, 1H), 4.70 (m, 1H), 6.99 (s, 1H), 7.21 (s, 1H), 7.83 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3): d 18.3, 18.8, 32.6, 61.3, 65.7, 66.9, 67.9, 68.5, 69.6, 70.4, 71.2, 72.0, 72.6, 93.9, 122.6, 128.3, 139.8, 163.0 ppm. HRMS (ESI): calcd. for [M + H]+: 364.1112, found 364.1091. 2.3. Preparation of 1 Benzyl bromide (0.85 g, 5 mmol) in dry acetonitrile (3 mL) was added dropwise into a solution of 6 (1.82 g, 5 mmol) in dry acetonitrile (5 mL) under reflux. The mixture was further refluxed for 1.5 h. TLC showed the consumption of 6. TlPF6 (1.75 g, 5 mmol) in acetonitrile (10 mL) was added dropwise into the reaction solution under vigorous stirring. A precipitate formed promptly, and the mixture was further stirred for 20 min. After filtration, the solvent was removed in vacuo. The residue was purified by column chromatography (CH2Cl2) to give 2.13 g of 1 as a brown oil (70% yield). [a]D = 43.2 (c 0.09, CHCl3), 1H NMR (400 MHz, CDCl3): d 0.70 (d, J = 6.63 Hz, 3H), 0.76 (d, J = 6.69 Hz, 3H), 1.53 (m, 1H), 3.69 (m, 1H), 3.83 (t, J = 8.13 Hz, 1H), 4.09 (t, J = 9.41 Hz, 1H), 4.22 (s, 5H), 4.33 (s, 1H), 4.65 (s, 1H), 4.79 (s, 1H), 5.26 (m, 2H), 7.11 (s, 1H), 7.27 (s, 5H), 7.51 (s, 1H), 8.92 (s, 1H) ppm. 13C NMR
t-BuOK (0.11 g, 0.95 mmol), [Rh(COD)Cl]2 (0.22 g, 0.45 mmol) and 1 (0.54 g, 0.90 mmol) were added into dry THF (8 mL) under nitrogen. The mixture was refluxed for 12 h. After the solvent was removed in vacuo, the residue was purified by column chromatography (CH2Cl2) to afforded 0.60 g of 7 as an orange air stable solid (75% yield). [a]D = 541.0 (c 0.083, Acetone); 1H NMR (400 MHz, CDCl3): d 0.87 (t, J = 6.38 Hz, 6H), 1.69–1.74 (m, 3H), 1.94 (s, 3H), 2.09–2.30 (m, 2H), 2.40–2.59 (m, 1H), 3.35 (s, 1H), 3.75–3.78 (m, 2H), 4.15 (m, 1H), 4.37–4.42 (m, 6H), 4.53 (t, J = 9.62 Hz, 1H), 4.68 (m, 1H), 4.88–4.91 (m, 3H), 5.54 (d, J = 15.16 Hz, 1H), 5.67 (d, J = 15.15 Hz, 1H), 7.02 (s, 1H), 7.23– 7.27 (m, 2H), 7.39–7.43 (m, 3H), 7.68 (s, 1H) ppm. 13C NMR (100 MHz, (CD3)2CO): d 17.4, 17.6, 27.9, 32.3, 34.3, 55.1, 67.3, 69.3, 69.4, 70.3, 70.4, 71.0, 71.3, 71.7, 73.0, 80.0, 97.6, 97.7, 98.9, 123.7, 126.7, 128.4, 129.2, 129.3, 129.8, 137.5, 166.9, 167.0, 181.2 (d, 1JC–Rh = 51.1 Hz, C-Rh) ppm. HRMS (ESI): calcd. for [MPF6]+: 664.1498, found 664.1400. 2.5. General procedure for the asymmetric hydrosilylation reaction Under nitrogen, catalyst 7 (16.0 mg, 0.02 mmol) and ketone (1 mmol) were added into THF (1 mL). Silane (1.5 mmol) in dry THF (1 mL) was added by syringe within 20 min at 25 °C. Then the mixture was stirred for 24 h at this temperature. p-Tolylsulfonic acid in methanol (1%, 1 mL) was added to hydrolyze the silyl ether at 0 °C, and the mixture was further stirred for 30 min at ambient temperature. After the solvent was evaporated, the product was purified by column chromatography with petroleum ether/diethyl ether (6:1–2:1). All products had S configuration, which were determined by comparing the optical rotations with the reported values [30]. Enantiomeric excesses were determined by HPLC with a Chiralcel OD–H column or by GC using a chiral CP-Cyclodex-b-236 M-19 column. 3. Results and discussion The planar chiral carbene precursor 1 was synthesized from ferrocene–carboxylic acid 2 (Scheme 1). Treatment of 2 with oxalyl chloride and (S)-valinol successively led to the formation of amide 3 in 86% yield, which was transformed into 4 in 84% yield by cyclization using Appel’s method [27]. After a highly diastereoselective ortho-lithiation followed by treatment with 1,2-diiodoethane, 5 was obtained in 79% yield [28,29]. Then 5 was allowed to react with imidazole in the presence of Cs2CO3, CuI and trans-1,2-cyclohexane diamine in dioxane at 110 °C. The resulting ferrocene imidazole 6 was treated with benzyl bromide followed by anion exchange with TlPF6 to give ferrocene imidazolium salt 1, which is hygroscopic upon exposure to the ambient atmosphere. This behavior, however, did not influence the transformation of imidazolium salt to its corresponding metal complex. The ferrocene imidazolium salt 1 was treated with [Rh(COD)Cl]2 in THF in the presence of t-BuOK to give the carbene rhodium complex 7 as an orange air stable crystal in 75% yield (Scheme 2). The analytical and spectroscopic data for 7 are consistent with its proposed structure. The 13C NMR spectra showed the expected signals for the carbene carbon linking with Rh at around 181.2 ppm. The mass spectra showed strong characteristic signals for the [M+PF6] fragment.
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Scheme 1. Synthesis of 1.
Scheme 2. Synthesis of Rh(I)-NHC 7.
The asymmetric hydrosilylations were carried out by treating acetophenone with a silane in the presence of carbene rhodium complex 7. The chemical yield of chiral 1-phenylethanol and its ee value obtained under various conditions are summarized in Table 1. Entries 1–4 show the effect of the solvent on the reaction catalyzed by 7 at 25 °C. The solvent was observed to play a role in both the yield and enantioselectivity, and THF proved to be superior (Table 1, entry 1). CH2Cl2 lowered the yield but did not significantly affect enantioselectivity (entry 2). When the reaction was carried out at 25 °C, the product yield and ee value were higher in all solvents. When the reaction temperature was lowered to 0 °C, no product formed. However, when the temperature was increased to 40 °C, the ees of product dropped (entries 5 and 6). As for the silane, diphenylsilane gave the highest enantioselectivity,
followed by phenylsilane and polymethylhydrosiloxane (PMHS) (entries 1, 7 and 8). We subsequently looked into the effect of the amount of catalyst on the catalytic activity and enantioselectivity. As shown in Table 1, the yield and enantioselectivity increased with the rise of catalyst loading from 0.5 mol% to 2 mol% (entries 1 and 9). Increase of catalyst amount to 5 mol% gave no benefit to enantioselectivity (entry 10). To extend the scope of the carbene-Rh(I) complex 7 as a chiral catalyst in this reaction, various aryl alkyl ketones were screened. As shown in Table 2, these ketones can be transformed to the corresponding secondary alcohols under the optimized conditions. The chemical yield and enantioselectivity of the reaction were affected by the steric and electronic properties of the ketones. Aromatic ketones possessing an electron withdrawing substituent at the para position afforded the corresponding alcohol products with good levels of enantioselectivity, but a lower yield (Table 2, entry 2). Introduction of an electron-donating group such as p-Me or pMeO gave an increase in yields with a decrease in ee to 47% and 46% (entries 3 and 4). Employing 2-acetonaphthone as substrate gave a yield of 68% and an ee of 50% (entry 5). Finally, increasing the bulkiness of the alkyl group led to a significant drop in the stereoselectivity and chemical yield (entries 6, 7 and 8 vs. 1). These results are superior to those of Bolm’s ferrocenyl ligands (66% ee) [26], but not as good as those of some other carbene–oxazoline ligands [22,24,25].
Table 1 Asymmetric hydrosilylation of acetophenone catalyzed by 7.
O
OH 1)1.5 equiv silane, Cat. 7 2) p-TSA, MeOH
Entry
Solvent
Temperature (°C)
Silane
Cat/substrate (mol%)
Yield (%)
ee (%)a
1 2 3 4 5 6 7 8 9 10
THF CH2Cl2 Toluene Et2O THF CH2Cl2 THF THF THF THF
25 25 25 25 40 40 25 25 25 25
Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 PhSiH3 PMHS Ph2SiH2 Ph2SiH2
2 2 2 2 2 2 2 2 0.5 5
73 60 39 36 83 70 68 65 50 78
53 52 46 41 42 39 50 42 46 53
a
Determined by HPLC using a Chiracel OD–H column [31].
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Table 2 Asymmetric hydrosilylation of aryl alkyl ketones catalyzed by 7. 1) 1.5 equiv Ph2SiH2 Cat.7 (2 mol%), THF, 25oC
O Ar
R
2) p-TSA, MeOH
OH Ar
R
Entry
Substrate
Yield (%)
ee (%)a
1 2 3 4 5 6 7 8
Acetophenone 40 -Chloroacetophenone 40 -Methylacetophenone 40 -Methoxyacetophenone 2-Acetonaphthone Propiophenone 2-Methylpropiophenone Butyrophenone
73 64 80 77 68 63 59 60
53 56 47 46 50 42 44 39
a Determined by HPLC using a Chiracel OD–H column or by GC using a chiral CPCyclodex-b-236 M-19 column [31]. All products had S configuration.
4. Conclusion A novel ferrocene-based planar chiral N-heterocyclic carbene– oxazoline ligand has been synthesized and applied to rhodium-catalyzed asymmetric hydrosilylation of ketones. Although enantioselectivities for the tested substrates remain low to moderate, the results represent an improvement when compared to those of Bolm’s ligand in which the imidazole ring was linked to ferrocene via a methylene bridge, possibly due to the more rigid backbone of the new ligand. Further investigation into reactivities and applications are underway.
Acknowledgments We thank the National Natural Science Foundation of China (No: 20302014, 20672141) and Shannxi Province (2007K01-35) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2009.03.027. References [1] I. Ojima, M. Nihonyanagi, Y. Nagai, J. Chem. Soc. Chem. Commun. (1972) 938a. [2] W. Dumont, J.C. Poulin, T.P. Dang, H.B. Kagan, J. Am. Chem. Soc. 95 (1973) 8295–8299.
[3] H. Brunner, R. Becker, G. Riepl, Organometallics 3 (1984) 1354–1359. [4] H. Nishiyama, K. Itoh, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, second ed., Wiley-VCH, 2000, pp. 111–143. [5] E.N. Jacobsen, A. Pfaltz, H. Yamamoto, Comprehensive Asymmetric Catalysis, vol. 1, Springer, Berlin, 1999. [6] O. Riant, N. Mostefai, J. Courmarcel Synth. (2004) 2943–2958. [7] S. Díez-González, S.P. Nolan, Org. Proc. Pre. Int. 39 (2007) 523–561. [8] S. Yao, J.C. Meng, G. Siuzdak, M.G. Finn, J. Org. Chem. 68 (2003) 2540–2546. [9] X. Hu, H. Chen, H. Dai, Z. Zheng, Tetrahedron Asymmetry 14 (2003) 3415– 3421. [10] T. Hayashi, C. Hayashi, Y. Uozumi, Tetrahedron Asymmetry 6 (1995) 2503– 2506. [11] B. Tao, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 3892–3894. [12] Y. Nishibayashi, K. Segawa, K. Ohe, S. Uemura, Organometallics 14 (1995) 5486–5487. [13] L.M. Newman, J.M.J. Williams, Tetrahedron Asymmetry 7 (1996) 1597–1598. [14] T. Langer, J. Janssen, G. Helmchen, Tetrahedron Asymmetry 7 (1996) 1599– 1602. [15] A. Sudo, H. Yoshida, K. Saigo, Tetrahedron Asymmetry 8 (1997) 3205–3208. [16] A.G. Coyne, P.J. Guiry, Tetrahedron Lett. 48 (2007) 747–750. [17] W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290–1309. [18] V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619–636. [19] W.A. Herrmann, L.J. Goossen, C. Köcher, G.R.J. Artus, Angew. Chem. Int. Ed. 35 (1996) 2805–2807. [20] T. Chen, X.G. Liu, M. Shi, Tetrahedron 63 (2007) 4874–4880. [21] J.W. Faller, P.P. Fontaine, Organometallics 25 (2006) 5887–5893. [22] L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718–725. [23] W.A. Herrmann, L.J. Goossen, M. Spiegler, Organometallics 17 (1998) 2162– 2168. [24] L.H. Gade, V. César, S. Bellemin-Laponnaz, Angew. Chem. Int. Ed. 43 (2004) 1014. [25] V. Cesar, S. Bellemin-Laponnaz, H. Wadepohl, L.H. Gade, Chem. Eur. J. 11 (2005) 2862–2873. [26] Y. Yuan, G. Raabe, C. Bolm, J. Organomet. Chem. 690 (2005) 5747–5752. [27] C.J. Richard, A.W. Mulvaney, Tetrahedron Asymmetry 7 (1996) 1419–1430. [28] R. Kuwano, T. Uemura, M. Saitohb, Y. Itob, Tetrahedron Asymmetry 15 (2004) 2263–2271. [29] C. Bolm, K. Fernandez, A. Seger, G. Raabe, K. Gunther, J. Org. Chem. 63 (1998) 7860–7867. [30] W.A. Herrmann, D. Baskakov, E. Herdtweck, S.D. Hoffmann, T. Bunlaksananusorn, F. Rampf, L. Rodefeld, Organometallics 25 (2006) 2449– 2456. [31] Entry 1 (1-phenylethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 17.1 (minor), tS (min) = 19.9 (major); entry 2 (1(40 - chlorophenyl)ethanol): chiral CP-cyclodex-b-236 M-19 column, 0.25 mm 50 m, T = 130 °C, p = 15 psi, tR (min) = 25.2 (minor), tS (min) = 27.6 (major); entry 3 (1-(40 -methylphenyl)ethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 99:1, flow rate 0.5 mL min1, tR (min) = 16.3 (minor), tS (min) = 17.5 (major); entry 4 (1-(4’-methoxyphenyl)ethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 98:2, flow rate 0.5 mL min1, tR (min) = 34.3 (minor), tS (min) = 36.6 (major); entry 5 (1-(2’-naphthyl)ethanol): Daicel Chiralcel OD–H, hexane: iPrOH = 98:2, flow rate 1.0 mL min1, tS (min) = 21.7 (major), tR (min) = 23.6 (minor); entry 6 (1-phenyl-1-propanol): Daicel Chiralcel OD–H, hexane: iPrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 17.4 (minor), tS (min) = 20.4 (major); entry 7 (2-methyl-1-phenyl -1-propanol): chiral CP-cyclodex-b-236 M19 comuln, 0.25 mm 50 m, T = 115 °C, p = 15 psi, tR (min) = 28.2 (minor), tS (min) = 28.6 (major); entry 8 (1-phenyl-1-butanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 16.3 (minor), tS (min) = 17.5 (major).
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
A novel planar chiral N-heterocyclic carbene–oxazoline ligand for the asymmetric hydrosilylation of ketones Yongqing Kuang a, Xiaoli Sun b, Hui Chen b, Peng Liu b, Ru Jiang b,* a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China Department of Chemistry, School of Pharmacy, Fourth Military Medical University, 17 Changlexi Street, Xi’an, Shannxi 710032, China
a r t i c l e
i n f o
Article history: Received 8 December 2008 Received in revised form 23 March 2009 Accepted 28 March 2009 Available online 5 April 2009
a b s t r a c t A novel ferrocene-based planar chiral N-heterocyclic carbene–oxazoline ligand was synthesized and applied to the rhodium(I)-catalyzed asymmetric hydrosilylation of ketones. Moderate catalytic activity and enantioselectivity were obtained. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Chiral N-heterocyclic carbene Oxazoline Ferrocene Asymmetric hydrosilylation Ketone
1. Introduction Metal-catalyzed asymmetric hydrosilylation of prochiral ketones followed by hydrolysis provides one of the most efficient methods for the preparation of chiral secondary alcohols. Dating from the pioneering work of Ojima, Kagan and Brunner [1–4], various metals, chiral ligands as well as hydride sources have been screened [5–7]. As for rhodium-catalyzed asymmetric hydrosilylation, bidentate chiral P, N-ligands, particularly phosphine–oxazoline ligands, have played a dominant role and some of them have achieved excellent activities and enantioselectivities [8–16]. Chiral N-Heterocyclic carbenes (NHCs) as effective alternatives to chiral phosphine ligands in asymmetric catalytic processes are attracting increasing attention due to their strong r-donating and weak p-accepting properties [17,18]. Both mono-and bisNHC ligands, of which only the carbene carbon atom(s) coordinate(s) to the metal center, have been applied to rhodiumcatalyzed asymmetric hydrosilylation of ketones with excellent catalytic activities, but poor to moderate enantioselectivities [19–21]. Inspired by the good results obtained with phosphine– oxazoline ligands, several research groups have in recent years synthesized carbene–oxazoline chelating ligands and applied some of them in the asymmetric hydrosilylation of ketones [22]. In 1998, Herrmann reported the synthesis of the first chiral carbene–oxazoline ligand (Fig. 1a). In this bidentate ligand, the oxazoline ring is * Corresponding author. Tel./fax: +86 29 84776945. E-mail addresses: [email protected], [email protected] (R. Jiang). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.03.027
linked in its 2-position to the imidazole ring via a methylene bridge [23]. In 2004, Gade and co-workers reported the synthesis of a family of carbene–oxazoline ligands which were obtained by direct linkage of the two heterocycles (Fig. 1b). Their complexes with rhodium(I) achieved enantioselectivities up to 95% ee in the asymmetric hydrosilylation of prochiral dialkylketones [24,25]. In 2005, Bolm reported a series of ferrocene-based carbene–oxazoline ligands in which the imidazole ring was linked to ferrocene via a methylene bridge (Fig. 1c). All of their complexes with rhodium(I) were active giving the secondary alcohol in high yields but with very low enantioselectivities (<6% ee) [26]. With the goal of improving the catalyst performance, we designed and synthesized a novel ferrocene-based chiral carbene–oxazoline ligand precursor 1 possessing a rigid backbone by direct linkage of the imidazole ring to ferrocene (Fig. 2), and examined its efficiency in the asymmetric hydrosilylation of prochiral ketones. 2. Experimental 2.1. General remarks Solvents were purified by standard procedures. 1H NMR and 13C NMR spectra were recorded on a Bruker AV-400 spectrometer. High performance liquid chromatography (HPLC) was performed by an Agilent 1100 interfaced to a HP 71 series computer workstation with a Daicel Chiralcel OD–H chiral column. Gas chromatography analyses were performed on a chiral CP-Cyclodex-b-236 M-19 column (25 m 0.32 mm) on Varian CP-3800. Optical rotations
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(100 MHz, CDCl3): d 18.2, 18.9, 32.4, 53.6, 64.4, 65.7, 68.4, 68.5, 69.5, 69.6, 72.2, 72.5, 72.7, 92.2, 121.5, 125.9, 129.1, 129.4, 129.5, 132.6, 137.8, 162.5 ppm. HRMS (ESI): calcd. for [MPF6]+: 454.1582, found 454.1539. 2.4. Preparation of 7
Fig. 1. Reported carbene–oxazoline ligands (imidazolium procursors).
Fig. 2. Chiral carbene precursor 1.
were measured on a Perkin–Elmer 343 Polarimeter. Commercial reagents were used as received, unless otherwise stated. THF, Et2O and toluene were dried over sodium and freshly distilled before use. Compounds 3–5 were prepared according to published procedures [27–29]. 2.2. Preparation of 6 To a well-dried Schlenk tube, CuI (48 mg, 0.25 mmol), imidazole (0.51 g, 6 mmol) and Cs2CO3 (3.42 g, 10.5 mmol) were added, evacuated twice and back-filled with nitrogen. Dioxane (5 mL), 5 (2.12 g, 5 mmol) and trans-1,2-cyclohexane diamine (0.11 g, 1 mmol) were then successively added under nitrogen. The Schlenk tube was sealed and the reaction mixture was stirred with heating at 110 °C for 24 h. The reaction mixture was cooled to ambient temperature, diluted with 250 mL of ethyl acetate, and filtered through a plug of silica gel followed by eluting with 500 mL of ethyl acetate. The filtrate was concentrated and the resulting residue was purified by column chromatography (hexane/ethyl acetate 1:5) to provide 0.95 g of 6 (51% yield). [a]D = +26.3 (c 0.135, CHCl3); 1H NMR (400 MHz, CDCl3): d 0.84 (d, J = 6.68 Hz, 3H), 0.91 (d, J = 6.74 Hz, 3H), 1.70 (m, 1H), 3.90 (m, 2H), 4.17– 4.30 (m, 7H), 4.53 (m, 1H), 4.70 (m, 1H), 6.99 (s, 1H), 7.21 (s, 1H), 7.83 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3): d 18.3, 18.8, 32.6, 61.3, 65.7, 66.9, 67.9, 68.5, 69.6, 70.4, 71.2, 72.0, 72.6, 93.9, 122.6, 128.3, 139.8, 163.0 ppm. HRMS (ESI): calcd. for [M + H]+: 364.1112, found 364.1091. 2.3. Preparation of 1 Benzyl bromide (0.85 g, 5 mmol) in dry acetonitrile (3 mL) was added dropwise into a solution of 6 (1.82 g, 5 mmol) in dry acetonitrile (5 mL) under reflux. The mixture was further refluxed for 1.5 h. TLC showed the consumption of 6. TlPF6 (1.75 g, 5 mmol) in acetonitrile (10 mL) was added dropwise into the reaction solution under vigorous stirring. A precipitate formed promptly, and the mixture was further stirred for 20 min. After filtration, the solvent was removed in vacuo. The residue was purified by column chromatography (CH2Cl2) to give 2.13 g of 1 as a brown oil (70% yield). [a]D = 43.2 (c 0.09, CHCl3), 1H NMR (400 MHz, CDCl3): d 0.70 (d, J = 6.63 Hz, 3H), 0.76 (d, J = 6.69 Hz, 3H), 1.53 (m, 1H), 3.69 (m, 1H), 3.83 (t, J = 8.13 Hz, 1H), 4.09 (t, J = 9.41 Hz, 1H), 4.22 (s, 5H), 4.33 (s, 1H), 4.65 (s, 1H), 4.79 (s, 1H), 5.26 (m, 2H), 7.11 (s, 1H), 7.27 (s, 5H), 7.51 (s, 1H), 8.92 (s, 1H) ppm. 13C NMR
t-BuOK (0.11 g, 0.95 mmol), [Rh(COD)Cl]2 (0.22 g, 0.45 mmol) and 1 (0.54 g, 0.90 mmol) were added into dry THF (8 mL) under nitrogen. The mixture was refluxed for 12 h. After the solvent was removed in vacuo, the residue was purified by column chromatography (CH2Cl2) to afforded 0.60 g of 7 as an orange air stable solid (75% yield). [a]D = 541.0 (c 0.083, Acetone); 1H NMR (400 MHz, CDCl3): d 0.87 (t, J = 6.38 Hz, 6H), 1.69–1.74 (m, 3H), 1.94 (s, 3H), 2.09–2.30 (m, 2H), 2.40–2.59 (m, 1H), 3.35 (s, 1H), 3.75–3.78 (m, 2H), 4.15 (m, 1H), 4.37–4.42 (m, 6H), 4.53 (t, J = 9.62 Hz, 1H), 4.68 (m, 1H), 4.88–4.91 (m, 3H), 5.54 (d, J = 15.16 Hz, 1H), 5.67 (d, J = 15.15 Hz, 1H), 7.02 (s, 1H), 7.23– 7.27 (m, 2H), 7.39–7.43 (m, 3H), 7.68 (s, 1H) ppm. 13C NMR (100 MHz, (CD3)2CO): d 17.4, 17.6, 27.9, 32.3, 34.3, 55.1, 67.3, 69.3, 69.4, 70.3, 70.4, 71.0, 71.3, 71.7, 73.0, 80.0, 97.6, 97.7, 98.9, 123.7, 126.7, 128.4, 129.2, 129.3, 129.8, 137.5, 166.9, 167.0, 181.2 (d, 1JC–Rh = 51.1 Hz, C-Rh) ppm. HRMS (ESI): calcd. for [MPF6]+: 664.1498, found 664.1400. 2.5. General procedure for the asymmetric hydrosilylation reaction Under nitrogen, catalyst 7 (16.0 mg, 0.02 mmol) and ketone (1 mmol) were added into THF (1 mL). Silane (1.5 mmol) in dry THF (1 mL) was added by syringe within 20 min at 25 °C. Then the mixture was stirred for 24 h at this temperature. p-Tolylsulfonic acid in methanol (1%, 1 mL) was added to hydrolyze the silyl ether at 0 °C, and the mixture was further stirred for 30 min at ambient temperature. After the solvent was evaporated, the product was purified by column chromatography with petroleum ether/diethyl ether (6:1–2:1). All products had S configuration, which were determined by comparing the optical rotations with the reported values [30]. Enantiomeric excesses were determined by HPLC with a Chiralcel OD–H column or by GC using a chiral CP-Cyclodex-b-236 M-19 column. 3. Results and discussion The planar chiral carbene precursor 1 was synthesized from ferrocene–carboxylic acid 2 (Scheme 1). Treatment of 2 with oxalyl chloride and (S)-valinol successively led to the formation of amide 3 in 86% yield, which was transformed into 4 in 84% yield by cyclization using Appel’s method [27]. After a highly diastereoselective ortho-lithiation followed by treatment with 1,2-diiodoethane, 5 was obtained in 79% yield [28,29]. Then 5 was allowed to react with imidazole in the presence of Cs2CO3, CuI and trans-1,2-cyclohexane diamine in dioxane at 110 °C. The resulting ferrocene imidazole 6 was treated with benzyl bromide followed by anion exchange with TlPF6 to give ferrocene imidazolium salt 1, which is hygroscopic upon exposure to the ambient atmosphere. This behavior, however, did not influence the transformation of imidazolium salt to its corresponding metal complex. The ferrocene imidazolium salt 1 was treated with [Rh(COD)Cl]2 in THF in the presence of t-BuOK to give the carbene rhodium complex 7 as an orange air stable crystal in 75% yield (Scheme 2). The analytical and spectroscopic data for 7 are consistent with its proposed structure. The 13C NMR spectra showed the expected signals for the carbene carbon linking with Rh at around 181.2 ppm. The mass spectra showed strong characteristic signals for the [M+PF6] fragment.
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Scheme 1. Synthesis of 1.
Scheme 2. Synthesis of Rh(I)-NHC 7.
The asymmetric hydrosilylations were carried out by treating acetophenone with a silane in the presence of carbene rhodium complex 7. The chemical yield of chiral 1-phenylethanol and its ee value obtained under various conditions are summarized in Table 1. Entries 1–4 show the effect of the solvent on the reaction catalyzed by 7 at 25 °C. The solvent was observed to play a role in both the yield and enantioselectivity, and THF proved to be superior (Table 1, entry 1). CH2Cl2 lowered the yield but did not significantly affect enantioselectivity (entry 2). When the reaction was carried out at 25 °C, the product yield and ee value were higher in all solvents. When the reaction temperature was lowered to 0 °C, no product formed. However, when the temperature was increased to 40 °C, the ees of product dropped (entries 5 and 6). As for the silane, diphenylsilane gave the highest enantioselectivity,
followed by phenylsilane and polymethylhydrosiloxane (PMHS) (entries 1, 7 and 8). We subsequently looked into the effect of the amount of catalyst on the catalytic activity and enantioselectivity. As shown in Table 1, the yield and enantioselectivity increased with the rise of catalyst loading from 0.5 mol% to 2 mol% (entries 1 and 9). Increase of catalyst amount to 5 mol% gave no benefit to enantioselectivity (entry 10). To extend the scope of the carbene-Rh(I) complex 7 as a chiral catalyst in this reaction, various aryl alkyl ketones were screened. As shown in Table 2, these ketones can be transformed to the corresponding secondary alcohols under the optimized conditions. The chemical yield and enantioselectivity of the reaction were affected by the steric and electronic properties of the ketones. Aromatic ketones possessing an electron withdrawing substituent at the para position afforded the corresponding alcohol products with good levels of enantioselectivity, but a lower yield (Table 2, entry 2). Introduction of an electron-donating group such as p-Me or pMeO gave an increase in yields with a decrease in ee to 47% and 46% (entries 3 and 4). Employing 2-acetonaphthone as substrate gave a yield of 68% and an ee of 50% (entry 5). Finally, increasing the bulkiness of the alkyl group led to a significant drop in the stereoselectivity and chemical yield (entries 6, 7 and 8 vs. 1). These results are superior to those of Bolm’s ferrocenyl ligands (66% ee) [26], but not as good as those of some other carbene–oxazoline ligands [22,24,25].
Table 1 Asymmetric hydrosilylation of acetophenone catalyzed by 7.
O
OH 1)1.5 equiv silane, Cat. 7 2) p-TSA, MeOH
Entry
Solvent
Temperature (°C)
Silane
Cat/substrate (mol%)
Yield (%)
ee (%)a
1 2 3 4 5 6 7 8 9 10
THF CH2Cl2 Toluene Et2O THF CH2Cl2 THF THF THF THF
25 25 25 25 40 40 25 25 25 25
Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2 PhSiH3 PMHS Ph2SiH2 Ph2SiH2
2 2 2 2 2 2 2 2 0.5 5
73 60 39 36 83 70 68 65 50 78
53 52 46 41 42 39 50 42 46 53
a
Determined by HPLC using a Chiracel OD–H column [31].
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Table 2 Asymmetric hydrosilylation of aryl alkyl ketones catalyzed by 7. 1) 1.5 equiv Ph2SiH2 Cat.7 (2 mol%), THF, 25oC
O Ar
R
2) p-TSA, MeOH
OH Ar
R
Entry
Substrate
Yield (%)
ee (%)a
1 2 3 4 5 6 7 8
Acetophenone 40 -Chloroacetophenone 40 -Methylacetophenone 40 -Methoxyacetophenone 2-Acetonaphthone Propiophenone 2-Methylpropiophenone Butyrophenone
73 64 80 77 68 63 59 60
53 56 47 46 50 42 44 39
a Determined by HPLC using a Chiracel OD–H column or by GC using a chiral CPCyclodex-b-236 M-19 column [31]. All products had S configuration.
4. Conclusion A novel ferrocene-based planar chiral N-heterocyclic carbene– oxazoline ligand has been synthesized and applied to rhodium-catalyzed asymmetric hydrosilylation of ketones. Although enantioselectivities for the tested substrates remain low to moderate, the results represent an improvement when compared to those of Bolm’s ligand in which the imidazole ring was linked to ferrocene via a methylene bridge, possibly due to the more rigid backbone of the new ligand. Further investigation into reactivities and applications are underway.
Acknowledgments We thank the National Natural Science Foundation of China (No: 20302014, 20672141) and Shannxi Province (2007K01-35) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2009.03.027. References [1] I. Ojima, M. Nihonyanagi, Y. Nagai, J. Chem. Soc. Chem. Commun. (1972) 938a. [2] W. Dumont, J.C. Poulin, T.P. Dang, H.B. Kagan, J. Am. Chem. Soc. 95 (1973) 8295–8299.
[3] H. Brunner, R. Becker, G. Riepl, Organometallics 3 (1984) 1354–1359. [4] H. Nishiyama, K. Itoh, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, second ed., Wiley-VCH, 2000, pp. 111–143. [5] E.N. Jacobsen, A. Pfaltz, H. Yamamoto, Comprehensive Asymmetric Catalysis, vol. 1, Springer, Berlin, 1999. [6] O. Riant, N. Mostefai, J. Courmarcel Synth. (2004) 2943–2958. [7] S. Díez-González, S.P. Nolan, Org. Proc. Pre. Int. 39 (2007) 523–561. [8] S. Yao, J.C. Meng, G. Siuzdak, M.G. Finn, J. Org. Chem. 68 (2003) 2540–2546. [9] X. Hu, H. Chen, H. Dai, Z. Zheng, Tetrahedron Asymmetry 14 (2003) 3415– 3421. [10] T. Hayashi, C. Hayashi, Y. Uozumi, Tetrahedron Asymmetry 6 (1995) 2503– 2506. [11] B. Tao, G.C. Fu, Angew. Chem. Int. Ed. 41 (2002) 3892–3894. [12] Y. Nishibayashi, K. Segawa, K. Ohe, S. Uemura, Organometallics 14 (1995) 5486–5487. [13] L.M. Newman, J.M.J. Williams, Tetrahedron Asymmetry 7 (1996) 1597–1598. [14] T. Langer, J. Janssen, G. Helmchen, Tetrahedron Asymmetry 7 (1996) 1599– 1602. [15] A. Sudo, H. Yoshida, K. Saigo, Tetrahedron Asymmetry 8 (1997) 3205–3208. [16] A.G. Coyne, P.J. Guiry, Tetrahedron Lett. 48 (2007) 747–750. [17] W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290–1309. [18] V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619–636. [19] W.A. Herrmann, L.J. Goossen, C. Köcher, G.R.J. Artus, Angew. Chem. Int. Ed. 35 (1996) 2805–2807. [20] T. Chen, X.G. Liu, M. Shi, Tetrahedron 63 (2007) 4874–4880. [21] J.W. Faller, P.P. Fontaine, Organometallics 25 (2006) 5887–5893. [22] L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718–725. [23] W.A. Herrmann, L.J. Goossen, M. Spiegler, Organometallics 17 (1998) 2162– 2168. [24] L.H. Gade, V. César, S. Bellemin-Laponnaz, Angew. Chem. Int. Ed. 43 (2004) 1014. [25] V. Cesar, S. Bellemin-Laponnaz, H. Wadepohl, L.H. Gade, Chem. Eur. J. 11 (2005) 2862–2873. [26] Y. Yuan, G. Raabe, C. Bolm, J. Organomet. Chem. 690 (2005) 5747–5752. [27] C.J. Richard, A.W. Mulvaney, Tetrahedron Asymmetry 7 (1996) 1419–1430. [28] R. Kuwano, T. Uemura, M. Saitohb, Y. Itob, Tetrahedron Asymmetry 15 (2004) 2263–2271. [29] C. Bolm, K. Fernandez, A. Seger, G. Raabe, K. Gunther, J. Org. Chem. 63 (1998) 7860–7867. [30] W.A. Herrmann, D. Baskakov, E. Herdtweck, S.D. Hoffmann, T. Bunlaksananusorn, F. Rampf, L. Rodefeld, Organometallics 25 (2006) 2449– 2456. [31] Entry 1 (1-phenylethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 17.1 (minor), tS (min) = 19.9 (major); entry 2 (1(40 - chlorophenyl)ethanol): chiral CP-cyclodex-b-236 M-19 column, 0.25 mm 50 m, T = 130 °C, p = 15 psi, tR (min) = 25.2 (minor), tS (min) = 27.6 (major); entry 3 (1-(40 -methylphenyl)ethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 99:1, flow rate 0.5 mL min1, tR (min) = 16.3 (minor), tS (min) = 17.5 (major); entry 4 (1-(4’-methoxyphenyl)ethanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 98:2, flow rate 0.5 mL min1, tR (min) = 34.3 (minor), tS (min) = 36.6 (major); entry 5 (1-(2’-naphthyl)ethanol): Daicel Chiralcel OD–H, hexane: iPrOH = 98:2, flow rate 1.0 mL min1, tS (min) = 21.7 (major), tR (min) = 23.6 (minor); entry 6 (1-phenyl-1-propanol): Daicel Chiralcel OD–H, hexane: iPrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 17.4 (minor), tS (min) = 20.4 (major); entry 7 (2-methyl-1-phenyl -1-propanol): chiral CP-cyclodex-b-236 M19 comuln, 0.25 mm 50 m, T = 115 °C, p = 15 psi, tR (min) = 28.2 (minor), tS (min) = 28.6 (major); entry 8 (1-phenyl-1-butanol): Daicel Chiralcel OD–H, hexane: i-PrOH = 96:4, flow rate 0.5 mL min1, tR (min) = 16.3 (minor), tS (min) = 17.5 (major).