ruthenium catalyst system

Bioorganic & Medicinal Chemistry Letters 25 (2015) 1961–1964

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Asymmetric hydrogenation of aromatic ketones by new recyclable ionic tagged ferrocene–ruthenium catalyst system Di Xu a, Zhi-Ming Zhou a,b, Li Dai a, Li-Wei Tang a, Jun Zhang a,⇑ a b

R&D Center for Pharmaceuticals, School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, PR China State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, PR China

a r t i c l e

i n f o

Article history: Received 7 December 2014 Revised 27 February 2015 Accepted 10 March 2015 Available online 14 March 2015 Keywords: Enantioselectivity Hydrogenation Ferrocene Ionic liquid Recovery Ruthenium

a b s t r a c t Newly developed ferrocene–oxazoline–phosphine ligands containing quaternary ammonium ionic groups exhibited excellent catalytic performance for the ruthenium-catalyzed hydrogenation of aromatic ketonic substrates to give chiral secondary alcohols with high levels of conversions and enantioselectivities. Simple manipulation process, water tolerance, high activity and good recyclable property make this catalysis practical and appealing. Ó 2015 Elsevier Ltd. All rights reserved.

Asymmetric hydrogenation (AH) of inexpensive prochiral ketones using chiral transition-metal catalysts represents the most efficient and ‘green’ way to access enantiomerically enriched secondary alcohols, which are important pharmaceutical intermediates and building blocks in organic synthesis.1 Due to the ease of preparation, modularity and favorable coordinating properties towards a variety of metals, chiral oxazoline derivatives have been employed as ligands in asymmetric catalysis in the past decades.2 Of particular interest to us were several rutheniumphosphine–oxazoline complexes described by the group of Blaser.2d,3 Complexes prepared in situ from RuCl2(PPh3)3 and chiral phosphine–oxazoline ligands were effective catalysts for the hydrogenation of various aryl ketones with ees up to 99%. However, catalyst recycling has not yet been achieved. As the catalysts for AH are usually expensive, development of recyclable phosphine–oxazoline ligands is therefore of practical importance. For several reasons, ionic liquids are interesting reaction media.4 They allow reactions in a two-phase mode and their high affinity to ionic tagged catalysts facilitates efficient catalyst separation and product isolation. Therefore, the use of ionic tags becomes a useful way to minimize catalyst leaching from ionic liquid and at the same time to improve catalyst stability and reusability.5 In spite of the several catalytic methods for the synthesis of

⇑ Corresponding author. Tel.: +86 010 68918982; fax: +86 010 68912664. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.bmcl.2015.03.023 0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

enantioselective benzyl alchols,1–3 the development of efficient and recyclable catalysts for the asymmetric hydrogenation of aryl ketones is still an important goal. Herein, we report novel ferrocene–oxazoline–phosphine ligands containing a quaternary ammonium ion bonded to the C4 of the oxazoline ring through a methylene group (FOPQ) (Scheme 1). These chiral ligands are easy to prepare, efficient, moisture-stable, and recyclable by means of ionic liquids. This is a preliminary report of ruthenium/FOPQ complexes that catalyze the AH of aromatic ketones with excellent enantioselectivity. (S,Rp)-4-(tert-Butyldimethylsilyloxy)methyl-2-[(2-diphenylphosphino)ferrocenyl]oxazoline, L1 (Scheme 1) was synthesized in two steps starting from (R)-4-hydroxymethyl-2-ferrocenyloxazoline,6 and afterwards, (R,Rp)-1-[2-[2-(diphenylphosphino)ferrocenyl]oxazolinyl]methyl-quaternary ammonium salts L2a–f were prepared. The X-ray crystal structure (CCDC 1037749) of L2d is shown in Figure 1. The hydrogenation of acetophenone was selected as model reaction for the initial screening of conditions. Several metal complexes and salts such as RuCl2(p-Cymene), RuCl2(cod), RuCl2(PPh3)3, RhOTf(cod)2, [IrCl(cod)]2, Cu(OAc)2H2O, and Fe3(CO)12 were examined in combination with the ligand L2a which proved to be very effective in Cu(OAc)2H2O catalysed asymmetric 1,3-dipolar cycloaddition carried out in our lab.6f We found that RuCl2(PPh3)3/L2a gave the best enantioselectivity in MeOH at 45 °C in the presence of KOH as a base at 2 MPa of H2 for the standard time of 24 h (Table 1, entry 1). [Ir(cod)Cl]2 gave the

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D. Xu et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1961–1964

PPh2 O

OTBS N

PPh 2 O N

TBAF

Fe

Table 1 Initial screening of reaction conditionsa

OH

Fe

1) Tf2 O, i-Pr2 NEt

O

2) tertiary amine

L1

OH

RuCl2 (PPh 3) 3/ L2a base, solvent H 2 2 MPa, 24 h

PPh2 O

R X N

Fe L2

L2a: R = 1,2-dimethylimidazole, X = OTf L2b: R = 1,2-dimethylimidazole, X = PF6 L2c: R = 1,2-dimethylimidazole, X = BF4 L2d: R = 1-methylpyrrolidine, X = OTf L2e: R = triethylamine, X = OTf L2f: R = tetramethylethane-1,2-diamine, X=OTf

Scheme 1. Synthesis of ligands L2.

highest yield, but no optical rotation was observed (Table 1, entry 2). For results with other metal complexes and salts see Supporting information. Subsequently, the effect of the base was investigated. As reported in the literature,7 the base has usually a striking effect in asymmetric catalytic hydrogenation. Base dependency was observed in the reaction. As shown in Table 1 RuCl2(PPh3)3/L2a performed well with KOH, while reactions with K2CO3, tBuOK, and MeONa gave lower yield in all cases and comparable or lower enantioselectivity (entries 3–5). A good combination of temperature and pressure was crucial. High enantioselectivity and yield were obtained at 30 °C under 2 MPa of hydrogen in MeOH/H2O (entry 8). A higher temperature (45 °C) gave a much better yield but a lower ee (entry 6). While at 15 °C, ee value increased, but the conversion remained low even when the pressure was increased up to 5 MPa (entry 7). Furthermore, better results were obtained under 2 MPa than under atmospheric pressure (entry 9). Under biphasic conditions satisfactory results were obtained, and a mixture of toluene and water in 10:1 molar ratio proved to be the most appropriate biphasic reaction medium. When toluene/H2O was replaced by iPrOH, the reaction process transferred from AH to ATH (asymmetric transfer hydrogenation),8 and the product was obtained with opposite enantioselectivity (entry 11). An effective catalyst is essential to achieve high conversions and ees.9 Subtle changes in geometric, steric, and/or electronic properties of chiral ligands can lead to dramatic variations of reactivity and selectivity.10 L2b–L2f were tested, and the results are summarized in Table 2. The catalyst was formed in situ from

Figure 1. X-ray crystal structure of L2d.

Entry

Temp. (°C)

Solvent

Base

Yieldb (%)

eec (%)

1 2d 3 4 5 6 7e 8 9f 10 11

45 45 45 45 45 45 15 30 30 30 30

MeOH MeOH MeOH MeOH MeOH MeOH/H2O (10:1) MeOH/H2O (10:1) MeOH/H2O (10:1) MeOH/H2O (10:1) Toluene/H2O (10:1) iPrOH

KOH KOH K2CO3 t-BuOK MeONa KOH KOH KOH KOH KOH KOH

33 75 29 21 4 98 51 70 42 54 55

66.2 — 69.3 65.6 61.8 43.2 55.4 68.4 69.5 87.0 82.4

(S) (S) (S) (S) (S) (S) (S) (S) (S) (R)

a Unless otherwise stated, all reactions were carried out with acetophenone (1 mmol) concentration of 0.5 M in solvent and a base (0.1 mmol) under 2 MPa of H2 for 24 h; acetophenone/RuCl2(PPh3)3/L2a = 200:1:1.1. b Isolated yield. c Determined by chiral HPLC analysis (Daicel OD-H, hexane/i-propane = 98/2). d [Ir(cod)Cl]2 as metal precursor. e Pressure of H2 was 5 MPa. f Pressure of H2 was 0.1 MPa.

Table 2 Ligand screeninga

O

R

OH

RuCl2 (PPh 3) 3 Ligand KOH, Toluene/H 2O H2 2 MPa, 30 o C

R

Entry

R

Ligand

Time (h)

Yieldb (%)

eec (%)

1 2 3 4 5 6 7 8 9

3,5-DiCF3 3,5-DiCF3 3,5-DiCF3 3,5-DiCF3 3,5-DiCF3 3,5-DiCF3 2-OMe 2-OMe 2-OMe

L2a L2b L2c L2d L2e L2f L2a L2f L2d

24 24 24 24 24 24 24 24 24

97 90 89 97 91 94 99 99 99

58.4 47.8 44.4 77.6 79.4 73.7 96.0 96.0 99.7

a Unless otherwise stated, all reactions were carried out with a substrate (1 mmol) concentration of 0.5 M and KOH (0.1 mmol) as base, in toluene/H2O (10:1) under 2 MPa of H2, substrate/catalyst = 200:1. b Isolated yield. c Determined by chiral HPLC analysis (Daicel OD-H, hexane/i-propane).

RuCl2(PPh3)3 and the chiral-phosphine–oxazoline ligand (L) in 1:1.1 molar ratio (see Supporting information). As shown in Table 2, the catalytic system RuCl2(PPh3)3/L2d was the most efficient one (entries 4 and 9) in the AH of aryl methyl ketones with both electron-withdrawing (3,5-diCF3) and electron-donating (2-OMe) substituents. The replacement of the anionic fragment OTf with PF6 or BF4 in ligands L2a, L2b, L2c (R = 1,2-dimethylimidazole) did not give better results (entries 1–3). Ligands L2d–f were designed bearing saturated quaternary ammonium cations, which were expected to affect the reduction process positively. The quaternary ammonium cation plays an important role in steric hindrance and functional group effect. In our attempts, L2d gave the best outcomes in AH of ketones probably because of its proper cation size. L2e and L2f possess flexible chain structure cations, and their catalytic activities are lower (entries 5 and 6). The results indicate: (1) the large bite angle created by the ferrocenyl backbone and the


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Table 3 Substrate scope of the ketone-hydrogenation reactiona

O R R1

1

OH

RuCl2(PPh3 )3 L2d

2

KOH, Toluene/H 2O H2 2 MPa, 30 o C, 12 h

R2 R1 2

Entry

R1

R2

Prod.

Yieldb (%)

eec (%)

1 2 3d 4 5 6e 7 8 9 10 11 12 13 14 15 16 17 18

H 4-Me 4-Me 4-OMe 2-OMe 2-OMe 4-NMe2 3,5-DiCF3 4-CF3 4-NO2 2-Br 3-Br 4-Br 4-OMe 4-Me 4-Br H H

Me Me Me Me Me Me Me Me Me Me Me Me Me CH2Br CH2Br CH2Br Et o-MeC6H4

2a 2b 2b 2c 2d 2d 2g 2h 2f 2e 2i 2j 2k 2l 2m 2n 2o 2p

61 99 96 98 99 66 92 98 76 67 55 99 99 47 14 67 7 6

89.6 97.2 86.8 97.0 99.7 90.1 74.6 82.4 68.8 26.2 35.0 82.0 95.4 97.6 94.0 97.6 n.d.f n.d.

a Unless otherwise stated, all reactions were carried out with a substrate (1 mmol) concentration of 0.5 M in solvent and KOH (0.1 mmol) as base under 2 MPa of H2, substrate/catalyst = 200:1. b Isolated yield. c Determined by chiral HPLC analysis (Daicel OD-H, hexane/i-propane). d Substrate/catalyst = 1000:1. e Substrate/catalyst = 10,000:1. f Not determined.

electronic properties of the coordinating atoms plays an important role in the catalytic performance of the hybrid phosphine-N systems L2 as supporting ligands for the Ru-catalyzed asymmetric hydrogenation;11 (2) proper steric demand of the system L2d benefits hydrogenation of various aromatic ketones (Table 3, see below). Thus, L2d was the ligand of choice. The better performance of ionic tagged ligand L2d compared with neutral ligand L1 is illustrated in Schemes 2 and 3. To our delight, higher reactivity of L2d was detected within shorter reaction time for substrates 1-(2-methoxyphenyl)ethanone and 1-(ptolyl)ethanone. Results were equal after long reaction time (24 h). The reasons could be the p-stacking, and hydrogen-bonds owing to the presence of ion-tagged ligand.12

Scheme 2. AH of 1-(2-methoxyphenyl)ethanone.

Scheme 3. AH of 1-(p-tolyl)ethanone.

On the basis of the results reported above, the optimized reaction conditions were set as following: 0.5 mol % of RuCl2(PPh3)3 and L2d as the catalyst, KOH as the base, toluene/H2O as the solvent with a substrate concentration of 0.5 M, and 2 MPa of H2 at 30 °C. To demonstrate the flexibility of this methodology, the hydrogenation of a series of aromatic ketonic substrates was investigated with Ru–L2d complex under the optimized conditions (Table 3). Conversions and enantioselectivities were dependent on the substituents: electron-deficient substrates gave low conversions and enantioselectivities (entries 8–10); with bromide as substituent, the position in the ring of substrate 1 affects the outcomes (entries 11–13). Bulky substituents in a to CO (R2), especially ethyl and omethylphenyl groups, impact the conversions negatively (entries 14–18). Yields and enantioselectivities for the hydrogenation turned out to be very high when using electron-rich substrates (entries 2–6). The highest yield and enantioselectivity were achieved with 1-(2-methoxyphenyl)ethanone (entry 5, yield 99%, ee = 99.7%). The results showed that the electron density of the aromatic ring has a strong effect on the reactivities and enantioselectivities. Overall, products with electron-donating substituents were formed quantitatively or nearly so. The reaction performs well when scaling up and using commercial ketones as received. Upon increasing the S/C ratio to 104:1, Ru– L2d gave (S)-1-(2-methoxyphenyl)ethanol (2d) in 66% yield with slight decrease of the ee (Table 3, entry 6). We have demonstrated that the catalyst can be recycled and reused several times. At the end of each run, the toluene phase with the hydrogenation product was readily removed under nitrogen. The IL phase was simply recharged with substrate and toluene and subjected to hydrogenation. After screening numerous ionic liquids, [Bmim]PF6 was chosen for the immobilization of the catalysts as it forms a well separated phase from toluene and appears to be stable under the reaction conditions. In addition, the ionic liquid seems to promote the reaction (yields and ees of the first and second runs of Table 4 are slightly higher than those of entries 2 and 5 of Table 3). As shown in Table 4, the Ru–L2d catalytic system was successfully recycled and reused in the AH of two substrates. Deterioration of the catalytic activity was detected after the fourth run, but the asymmetric induction of the catalyst was retained for at least six runs. The MS analysis of the anion of the catalyst after the sixth run showed that the anion group of the ligand was stable. In summary, new ionic tagged ferrocene–Ru catalysts were designed and prepared, and a family of aromatic alcohols was obtained through a simple and versatile synthetic method with

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Table 4 Recycling of the catalyst Ru–L2da Run

1 2 3 4 5 6

1-(2-Methoxyphenyl)ethanone (2d)

1-(p-Tolyl)ethanone (2b)

Yieldb (%)

eec (%)

Yieldb (%)

eec (%)

99 99 97 84 73 67

>99.9 99.9 97.8 93.4 92.5 92.2

98 92 89 86 67 59

97.1 99.8 99.7 98.5 95.6 95.2

a All reactions were carried out with a substrate (1 mmol) concentration of 0.5 M in solvent and KOH (0.1 mmol) as base under 2 MPa of H2, 30 °C, 12 h, substrate/catalyst = 200:1. b Isolated yield. c Determined by chiral HPLC analysis (Daicel OD-H, hexane/i-propane).

high yields (up to 99%) and excellent enantioselectivities (up to 99.7%). The chiral aromatic alcohols are important pharmaceutical intermediates. Compared to the neutral catalyst L1, the new catalytic system reveals higher activity in AH reactions. Change of the solvent from methanol or toluene/H2O to iPrOH caused reversed enantioselectivity. Moreover, the use of ionic liquid facilitates the recyclability of the catalyst which remains in the lower phase. Therefore, the catalyst was recycled by simple removal of the upper organic phase and reused after re-charge of toluene and substrate. Further applications of the catalyst are currently underway. Acknowledgments

3.

4.

5.

We appreciated the financial support by the National Technological Project of the Manufacture and Innovation of Key New Drugs (2009ZX09103-143). We also appreciate the experimental suggestions and language revisions from Professor Marta Catellani. Supplementary data Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre. CCDC number: 1037749. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.ca.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.03. 023.

6.

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