CHIRALITY 26:780–783 (2014)

Short Communication Synthesis of Novel Chiral Tridentate Schiff-Base Ligands and Their Applications in Catalytic Asymmetric Henry Reaction GEN-RONG QIANG, TIAN-HUA SHEN, XIAO-CONG ZHOU, XIAO-XIA AN, AND QING-BAO SONG* State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, P.R. China

ABSTRACT A series of chiral tridentate Schiff-bases were prepared and used as ligands in the catalytic asymmetric Henry reaction. Under the optimal conditions, a variety of arylaldehydes were smoothly converted into corresponding adducts with high yields (up to 98%) and excellent enantioselectivities (up to 97% ee). Chirality 26: 780–783, 2014. © 2014 Wiley Periodicals, Inc. KEY WORDS: asymmetric catalysis; Henry reaction; copper(II) complexes; chiral tridentate Schiff base; enantioselectivities INTRODUCTION

The Henry (nitroaldol) reaction is one of the classical C–C bond forming reactions in synthetic chemistry. The resulting adducts of β-nitro alcohols can be conveniently transformed into various valuable building blocks, such as 1,2-amino alcohols and α-hydroxy carboxylic acids. Since the pioneering work of Shibasaki in 1992,1 the catalytic asymmetric Henry reaction has received considerable attention,2,3 and various types of metal-based catalysts have been developed, such as copper-bis(oxazoline) by Evans et al.4 and Jørgensen and colleagues,5 zinc-aminoalcohol by Trost and Yeh6 and Palomo et al.,7 and cobalt-salen complexes by Yamada and colleagues.8,9 However, some of these catalytic systems suffer from certain limitations, such as low temperature and anhydrous conditions. Therefore, the design and development of novel chiral ligands is still in demand. The chiral tridentate Schiff-base ligand, which is derived from the chiral amino alcohol and salicylaldehyde derivative, is one of the privileged ligands in asymmetric catalysis. In the past few decades, chiral tridentate Schiff-base–metal complexes have been widely applied in many asymmetric reactions.10–16 To the best of our knowledge, only a few studies used tridentate Schiff-base ligands in the catalytic asymmetric Henry reaction.17–20 Wang and colleagues first reported that the substituents on the salicylidenyl aromatic ring of a tridentate Schiff-base have a great influence on the yield and the enantioselectivity of the Henry reaction.19 When a chlorine atom was introduced to the 3- or 5-position of the salicylidenyl aromatic ring, the enantioselectivity could be improved from 78% to 83% or 88% respectively.19 We anticipate that chiral ligands bearing other halogen atoms could also show high catalytic activities in the Henry reaction. In order to explore the effect of halogen, we herein report the preparation of a series of chiral tridentate Schiff-base ligands 1a-f and their application in the catalytic asymmetric Henry reaction. MATERIALS AND METHODS General Procedure for the Preparation of Chiral Schiff-Base Ligands To a solution of an amino alcohol (1 mmol) in 10 mL ethanol, salylaldehyde or its derivative (1 mmol) was added. The reaction mixture © 2014 Wiley Periodicals, Inc.

was stirred at room temperature for 24 h. The solvent was removed under reduced pressure and the residue was purified by column chromatography to get pure ligand.

Procedure for the Asymmetric Henry Reaction Chiral ligand 1e (13.2 mg, 0.02 mmol) and Cu(OAc)2 · H2O (4.0 mg, 0.02 mmol) were mixed in ethanol (2.0 mL) at room temperature and stirred for 2 h. To the resulting blue solution, p-nitrobenzaldehyde (0.2 mmol) and nitromethane (0.6 mL) were added. The reaction mixture was stirred for a certain time at room temperature. Then the solvent was removed under reduced pressure and the residue was purified by column chromatography to afford the nitroaldol product. The enantiomeric excess (ee) was determined by highperformance liquid chromatography (HPLC) analysis with a Chiralcel AD-H or OD-H column. 1 1b: Yellow solid. mp 145–147 °C; H NMR (500 MHz, CDCl3) δ (ppm) 2.85 (dd, J = 10.3, 13.7 Hz, 1H), 2.87 (s, 1H), 3.08 (dd, J = 1.2, 13.7 Hz, 1H), 4.36 (dd, J = 1.8, 10.3 Hz, 1H), 6.79 (d, J = 8.8 Hz, 1H), 6.97-6.99 (m, 3H), 7.16-7.22 (m, 4H), 7.27-7.33 (m, 4H), 7.42 (t, J = 7.8 Hz, 2H), 7.48-7.50 13 (m, 3H), 7.65 (d, J = 7.5 Hz, 2H), 12.79 (s, 1H); C NMR (125 MHz, CDCl3) δ (ppm) 37.4, 78.8, 79.8, 110.1, 118.8, 119.7, 126.1, 126.2, 126.5, 127.1, 127.3, 128.3, 128.4, 128.5, 129.6, 133.5, 135.1, 138.6, 143.9, 145.1, 159.7, 165.2. MS (m/z): 485 (M + 1). Anal. Calcd. for C28H24BrNO2 : C, 69.14; H, 4.97; N, 2.88. Found: C, 69.05; H, 4.89; N, 2.83. -1 1c: Yellow solid. mp 162–164 °C; IR (KBr, cm ): 3042, 3025, 2924, 1 1630, 1450, 1384, 747, 699; H NMR (500 MHz, CDCl3): δ (ppm) 2.862.90 (m, 2H), 3.08 (dd, J = 1.6, 13.8 Hz, 1H), 4.39 (dd, J = 1.9, 10.4 Hz, 1H), 6.76 (d, J = 2.5 Hz, 1H), 6.98 (d, J = 6.6 Hz, 2H), 7.15-7.21 (m, 4H), 7.26-7.32 (m, 3H), 7.34 (d, J = 2.5 Hz, 1H), 7.41-7.47 (m, 5H), 7.65 (d, 13 J = 7.3 Hz, 2H), 13.92 (s, 1H); C NMR (125 MHz, CDCl3): δ (ppm) 37.3, 78.3, 79.7, 118.9, 122.5, 122.8, 125.9, 126.0, 126.7, 127.3, 127.4, 128.4, 128.5, 128.6, 129.0, 129.6, 132.4, 138.4, 143.7, 144.8, 156.6, 164.6. MS (m/z): 475 (M + 1). Anal. Calcd. for C28H23Cl2NO2 : C, 70.59; H, 4.87; N, 2.94. Found: C,70.54; H, 4.81; N, 2.87. -1 1d: Yellow solid. mp 83–85 °C; IR (KBr, cm ): 3441, 3059, 3025, 2955, 1 1630, 1446, 1384, 747, 700. H NMR (500 MHz, CDCl3) δ (ppm) 2.85-2.90 (m, 2H), 3.07 (dd, J = 1.5, 13.8 Hz, 1H), 4.39 (dd, J = 1.8, 10.4 Hz, 1H), 6.93 *Correspondence to: Qing-bao Song, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Chaowang road 18, Hangzhou 310032, P. R. China. E-mail: [email protected] Received for publication 7 April 2014; Accepted 21 July 2014 DOI: 10.1002/chir.22369 Published online 16 October 2014 in Wiley Online Library (wileyonlinelibrary.com).

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(d, J = 2.3 Hz, 1H), 6.97 (d, J = 6.6 Hz, 2H), 7.14-7.22 (m, 4H), 7.26-7.32 (m, 3H), 7.40-7.47 (m, 5H), 7.63 (d, J = 2.3 Hz, 1H), 7.65(d, J = 7.4 Hz, 2H), 13 14.06 (s, 1H); C NMR (125 MHz, CDCl3) δ (ppm) 37.3, 78.2, 79.7, 109.3, 112.3, 119.4, 125.9, 126.0, 126.7, 127.3, 127.4, 128.5, 128.6, 128.7, 129.6, 132.8, 137.9, 138.3, 143.7, 144.8, 158.0, 164.4. MS (m/z): 563 (M + 1). Anal. Calcd. for C28H23Br2NO2 : C, 59.49, H, 4.10; N, 2.48. Found: C, 59.42; H, 4.01, N, 2.42. -1 1e: Yellow solid. mp 104–106 °C; IR (KBr, cm ): 3423, 3057, 3024, 2929, 1 1626, 1438, 747, 700. H NMR (500 MHz, CDCl3) δ (ppm) 2.85-2.90 (m, 2H), 3.05 (d, J = 13.2 Hz, 1H), 4.39 (dd, J = 1.4, 10.2 Hz, 1H), 6.96 (d, J = 6.7 Hz, 2H), 7.10 (d, J = 1.9 Hz, 1H), 7.14-7.22 (m, 4H), 7.27 (t, J = 7.8 Hz, 2H),7.31 (d, J = 8.9 Hz, 2H), 7.41-7.46 (m, 4H), 7.65 (d, J = 7.7 Hz, 2H), 7.98 13 (d, J = 2.0 Hz, 1H), 14.19 (s, 1H); C NMR (125 MHz, CDCl3) δ (ppm) 37.3, 78.0, 78.8, 79.7, 88.0, 119.2, 125.9, 126.0, 126.7, 127.3, 127.4, 128.5, 128.6, 128.7, 129.6, 138.3, 139.8, 143.7, 144.8, 148.9, 161.2, 164.2. MS (m/z): 659 (M + 1). Anal. Calcd. for C28H23I2NO2 : C, 51.01; H, 3.52; N,2.12. Found: C, 50.89; H, 3.43; N, 2.05. -1 1f: Yellow solid. mp 163–164 °C; IR (KBr, cm ): 3441, 3058, 1619, 1440, 737, 1 700. H NMR (500 MHz, CDCl3): δ (ppm) 2.88 (s, 1H), 5.49 (s, 1H,), 7.13 (t, J = 7.2 Hz, 1H), 7.12-7.20 (m, 7H), 7.24 (t, J = 7.3 Hz, 1H), 7.31-7.36 (m, 4H), 7.38 (d, J = 2.0 Hz, 1H), 7.57 (d, J = 7.5 Hz, 2H), 8.01 (d, J = 2.0 Hz, 1H), 8.11 13 (s, 1H), 14.17 (s, 1H); C NMR (125 MHz, CDCl3): δ (ppm) 78.8, 79.3, 80.5, 87.4, 120.0, 126.4, 126.7, 127.0, 127.4, 127.8, 127.9, 128.0, 128.4, 129.5, 137.7, 140.1, 143.5, 144.2, 148.9, 160.5, 164.8. MS (m/z): 645 (M + 1). Anal. Calcd. for C27H21I2NO2 : C,50.26; H, 3.28; N, 2.17. Found: 50.19; H, 3.19; N, 2.11.

RESULTS AND DISCUSSION

Initially, the catalytic activities of chiral ligands 1a-1f were examined in the model reaction of p-nitrobenzaldehyde with nitromethane. When a bromine atom was introduced to the para position of the salicylidenyl aromatic ring of the chiral ligand 1a, ligand 1b was obtained. The yield and enantioselectivity both increased slightly (Table 1, entries 1, 2). Furthermore, the ligand 1d with two bromine atoms gave better results than the ligand 1b (Table 1, entry 4 vs. 2). Surprisingly, ligands 1c, 1d, and 1e bearing different halogen atoms on the salicylidenyl aromatic ring gave yield and ee values close to each other. The replacement of the Bn group with a Ph group on the chiral carbon of 1e led to an obvious decrease in both yield and ee value (Table 1, entries 5, 6). Based on these observations, the ligand 1e was chosen for further optimization. Next we examined the effect of solvents in the asymmetric Henry reaction (Table 1, entry 5, 7–13). As was reported, alcoholic solvents, such as methanol, ethanol, or i-propanol, were superior to other solvents (Table 1, entries 5, 7, 8). Ethanol was found to be the best solvent for this reaction, affording the corresponding product in high yield and excellent enantioselectivity (Table 1, entry 5). To further optimize the reaction conditions, the loading of ligand was also tested. When the loading of ligand was increased from 2.5 mol% to 10 mol%, significant improvement occurred in both yields and enantioselectivities (Table 1, entries 14, 15, 5). A further increase in the ligand loading resulted in a reduction in the yield (Table 1, entries 16, 17). Under the optimized conditions, a variety of arylaldehydes were investigated, giving the corresponding nitroaldol products in high yields (up to 98%) and excellent enantioselectivities (89%–97% ee values, see Table 2). As can be seen from Table 2, benzaldehydes bearing strong electron-withdrawing groups gave high yields and enantioselectivities except for the o-nitrobenzaldehyde (Table 2, entries 1–4). Aldehyde with a large sterical hindrance, such as 2-naphthaldehyde, could also give the corresponding adducts with good yield and high enantioselectivity (Table 2, entry 7). When a heteroaromatic aldehyde was tested, high yield and high enantioselectivity were obtained as above (Table 2, entry 8). What’s more,

TABLE 1. Asymmetric Henry reaction of p-nitrobenzaldehyde a with nitromethane

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

Ligand

Ligand loading (mol%)

Solvent

Time (h)

Yield (%)

Ee (%)

1a 1b 1c 1d 1e 1f 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e 1e

10 10 10 10 10 10 10 10 10 10 10 10 10 2.5 5 15 20 10

EtOH EtOH EtOH EtOH EtOH EtOH MeOH i PrOH THF CH2Cl2 PhMe DMF CH3CN EtOH EtOH EtOH EtOH EtOH

48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 24

88 90 96 95 98 79 76 80 43 32 24 38 20 44 71 89 83 78

82 86 93 94 97 65 92 95 87 29 42 63 31 89 93 95 95 97

b

c

a

All reactions were performed on 0.2 mmol of p-nitrobenzaldehyde and 0.6 mL of nitromethane in 2 mL of EtOH in the present of ligand-Cu(OAc)2 · H2O (1:1) at ambient temperature for 48 h. b Isolated yields. c Enantiomeric excess was determined by HPLC analysis.

TABLE 2. Asymmetric Henry reaction of arylaldehydes with a nitromethane Entry 1 2 3 4 5 6 7 8 9

ArCHO 4-NO2C6H4CHO 3-NO2C6H4CHO 2-NO2C6H4CHO 4-F3CC6H4CHO C6H5CHO 4-CH3OC6H4CHO 2-Naphthaldehyde 4-Bromothiophene-2carboxaldehyde CH3CH2CHO

b

c

Time (h)

Yield (%)

Ee (%)

Configd

48 48 48 48 72 72 48 48

98 93 95 94 92 87 93 94

97 95 90 96 95 92 91 95

S S S S S S S S

48

89

90

S

a

All reactions were performed on 0.2 mmol of arylaldehyde and 0.6 mL of nitromethane in 2 mL of EtOH in the present of 10 mol% 1e and 10 mol% Cu(OAc)2 · H2O at ambient temperature. b Isolated yields. c Enantiomeric excess was determined by HPLC analysis.

aliphatic aldehyde also gave high yields and enantioselectivities in this reaction (Table 2, entry 9). Chirality DOI 10.1002/chir

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Fig. 1. The catalytic cycle of the Henry reaction.

The catalytic cycle of the reaction was proposed based on experiment results. Cu2+ could be used as a strong Lewis acid. Acetic could be used as a Brønsted alkali and the oxygen atom in phenol hydroxyl of salicylic aldehyde could be used as a strong base. Cu2+ could accept the lone pair electrons from the ligands, and form a stable complex compound 1. With the activation of complex 1, nitromethane would seize the protons and afforded copper complex 4. This was the intramolecular proton transfer for the first time. Then the asymmetric Henry reaction occurs between intermediate 4 and aldehyde through transition state 6, 7, and 8 to afford the desired product 9. Two intramolecular proton transfer and one C-C bond formation were included in the whole catalytic cycle. In addition, chiral ligands could strengthen the chiral environment and had a positive impact on stereoselectivity and yield. (Figure 1) CONCLUSION

In summary, we have developed some new effective metalbased catalysts derived from Cu(OAc)2 · 2H2O and chiral tridentate Schiff-base ligands for catalytic the asymmetric Henry reaction of various arylaldehydes with nitromethane. The results suggested that the halogen atoms on the salicylidenyl aromatic ring of Schiff-base ligands played an important role in the enantioselective catalytic process. Further studies on the applications of these ligands in other asymmetric reactions are underway in our group. ACKNOWLEDGMENTS

We gratefully acknowledge financial support from the foundation of Natural Science of Zhejiang Province (No. LY12B02016) and the State Key Laboratory Breeding Base of Green ChemistrySynthesis Technology, Zhejiang University of Technology. Chirality DOI 10.1002/chir

SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article at the publisher’s web-site. LITERATURE CITED 1. Sasai H, Suzuki T, Arai S, Arai T, Shibasaki M. Basic character of rare earth metal alkoxides. Utilization in catalytic carbon-carbon bondforming reactions and catalytic asymmetric nitroaldol reactions. J Am Chem Soc 1992;114:4418–4420. 2. Boruwa J, Gogoi N, Saikia PP, Barua NC. Catalytic asymmetric Henry reaction. Tetrahedron: Asymmetry 2006;17:3315–3326. 3. Palomo C, Oiarbide M, Laso A. Recent advances in the catalytic asymmetric nitroaldol (Henry) reaction. Eur J Org Chem 2007;2561–2574. 4. Evans DA, Seidel D, Rueping M, Lam HW, Shaw JT, Downey CW. A new copper acetate-bis(oxazoline)-catalyzed, enantioselective Henry reaction. J Am Chem Soc 2003;125:12692–12693. 5. Christensen C, Juhl K, Hazell RG, Jørgensen KA. Copper-catalyzed enantioselective Henry reactions of α-keto esters: an easy entry to optically active β-nitro-α-hydroxy esters and β-amino-α-hydroxy esters. J Org Chem 2002;67:4875–4881. 6. Trost BM, Yeh VSC. A dinuclear Zn catalyst for the asymmetric nitroaldol (Henry) reaction. Angew Chem Int Ed 2002;41:861–863. 7. Palomo C, Oiarbide M, Laso A. Enantioselective Henry reactions under dual Lewis acid/amine catalysis using chiral amino alcohol ligands. Angew Chem Int Ed 2005;44:3881–3883. 8. Kogami Y, Nakajima T, Ashizawa T, Kezuka S, Ikeno T, Yamada T. Enantioselective Henry reaction catalyzed by optically active ketoiminatocobalt complexes. Chem Lett 2004;33:614–615. 9. Kogami Y, Nakajima T, Ikeno T, Yamada T. Enantioselective Henry reaction catalyzed by salen-cobalt complexes. Synthesis-Stuttgart 2004;1947–1950. 10. Holmquist M, Blay G, Muñoz MC, Pedro JR. Enantioselective addition of nitromethane to 2-acylpyridine n-oxides. Expanding the generation of quaternary stereocenters with the Henry Reaction. Org Lett 2014;16:1204–1207.

NOVEL CHIRAL TRIDENTATE SCHIFF-BASE LIGANDS 11. Blay G, Hernandez-Olmos V, Pedro JR. The construction of quaternary stereocenters by the Henry reaction: circumventing the usual reactivity of substituted glyoxals. Chem Eur J 2011;17:3768–3773. 12. Blay G, Hernandez-Olmos V, Pedro JR. Enantioselective Henry addition of methyl 4-nitrobutyrate to aldehydes. Chiral building blocks for 2-pyrrolidinones and other derivatives. Org Lett 2010;12:3058–3061. 13. Blay G, Domingo LR, Hernandez-Olmos V, Pedro JR. New highly asymmetric Henry reaction catalyzed by CuII and a C1-symmetric aminopyridine ligand, and its application to the synthesis of miconazole. Chem Eur J 2008;14:4725–4730. 14. Sema HA, Bez G, Karmakar S. Asymmetric Henry reaction catalysed by L-proline derivatives in the presence of Cu(OAc)2: isolation and characterization of an in situ formed Cu(II) complex. Appl Organomet Chem 2014;28:290–297. 15. Ma K, You JS. Rational design of sterically and electronically easily tunable chiral bisimidazolines and their applications in dual Lewis

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Chirality DOI 10.1002/chir

Synthesis of novel chiral tridentate Schiff-base ligands and their applications in catalytic asymmetric Henry reaction.

A series of chiral tridentate Schiff-bases were prepared and used as ligands in the catalytic asymmetric Henry reaction. Under the optimal conditions,...
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