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Synlett 2005(14): 2147-2150  
DOI: 10.1055/s-2005-872226
LETTER
© Georg Thieme Verlag Stuttgart · New York

Ionic Liquid-Grafted Mn(III)-Schiff Base Complex: A Highly Efficient and Recyclable Catalyst for the Epoxidation of Chalcones

Yanqing Peng, Yueqin Cai, Gonghua Song*, Jing Chen
Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides & Pharmaceuticals, East China University of Science & Technology, Shanghai 200237, P. R. China
Fax: +86(21)64252603; e-Mail: ghsong@ecust.edu.cn;
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Publication History

Received 15 June 2005
Publication Date:
20 July 2005 (online)

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Abstract

A new Mn(III)-Schiff base complex with imidazolium phase tag was synthesized and employed as an efficient and recyclable catalyst for the epoxidation of chalcones with MCPBA/NMO. It can be recovered and reused (at least five times) without any loss of activity.

Key words

Mn(III)-Schiff base complex - imidazolium - catalyst

18

Synthesis of Imidazolium-Tagged Ligand(3).
A mixture of salicylaldehyde (1.22 g, 10 mmol), 2 (2.71 g, 10 mmol) and ionic liquid [bmim][PF6] (2.84 g, 10 mmol) was stirred at r.t. for 1 h. After completion of the reaction, as indicated by TLC, the reaction mixture was diluted with EtOH (20 mL). The precipitate was then filtered and dried to afford 3.01 g (80% yield) of ligand 3 as a pale yellow solid; mp 129-130 °C. FT-IR (KBr): νmax = 3416, 3152, 3093, 2914, 2867, 1637, 1618, 1568, 1505, 1470, 1287, 1163, 1100, 848 cm-1. 1H NMR (500 MHz, CD3COCD3): δ = 4.01 (s, 3 H, CH 3), 4.18 (t, 2 H, J 1 = 5.81 Hz, J 2 = 5.49 Hz, NCH 2), 4.80 (t, 2 H, J 1 = 5.64 Hz, J 2 = 5.83 Hz, NCH 2), 6.83-7.33 (m, 4 H, Ph), 7.70 (s, 1 H, NCH), 7.80 (s, 1 H, NCH), 8.55 [s, 1 H, N(H)CN], 9.10 (s, 1 H, CH), 12.60 (s, 1 H, OH).

19

Synthesis of Ionic Liquid-Grafted Mn(III)-Schiff Base Complex(4).
To a solution of ligand 3 (0.50 g, 1.30 mmol) in EtOH (20 mL) was added Mn(OAc)2·4H2O (0.16 g, 0.65 mmol). After 4 h of refluxing, 0.04 g of LiCl (0.65 mmol) was added and the reaction mixture was further refluxed for 2 h until the starting material had been completely consumed as judged by TLC. On completion of the reaction, the reaction mixture was cooled to r.t. The precipitate was collected by filtration, washed with EtOH (10 mL), and dried to give compound 4 as brown powder (0.43 g, 77% yield). FT-IR (KBr): νmax = 3167, 3110, 2976, 1611, 1545, 1478, 1439, 1136, 835, 754, 634, 472 cm-1. 1H NMR (500 MHz, DMSO-d 6): δ = 4.00 (s, 3 H, CH 3), 4.15 (t, 2 H, J 1 = 5.76 Hz, J 2 = 5.70 Hz, NCH 2), 4.75 (t, 2 H, J 1 = 5.73 Hz, J 2 = 5.86 Hz, NCH 2), 6.85-7.35 (m, 4 H, Ph), 7.65 (s, 1 H, NCH), 7.75 (s, 1 H, NCH), 8.50 [s, 1 H, N(H)CN], 9.01 (s, 1 H, CH). MS (ESI): m/e (%) = 693 (8), 360 (50), 319 (80), 229 (100). Anal. Calcd for C26H30N6O2MnClP2F12: C, 37.20; H, 3.60; N, 10.02. Found: C, 37.13; H, 3.52; N, 9.91.

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General Procedure for the Epoxidation of Chalcones. To a well-stirred solution of chalcone (1 mmol), NMO (0.17 g, 1.5 mmol) and catalyst 4 (8 mg, 0.01 mmol) in MeCN (10 mL) at -25 °C was added MCPBA (0.26 g, 1.5 mmol) in four equal portions over a 2-min period. The homogeneous mixture was stirred at -25 °C until chalcone had been completely consumed (monitored by TLC), followed by removal of MeCN on a rotary evaporator. The residue was then extracted with Et2O (3 × 10 mL). The combined extracts were washed with sat. Na2SO3 (3 × 10 mL), dried over Na2SO4, filtered, and concentrated to afford essentially pure products.
Compound 6f: mp 138-139 °C. FT-IR (KBr): νmax 3112, 3045, 1672, 1600, 1519, 1487, 1408, 1358, 1215, 1089, 1036, 864 cm-1. 1H NMR (500 MHz, CD3COCD3): δ = 7.43-7.60 (m, 4 H, Ph), 7.45 (d, J = 15.7 Hz, 1 H, CH-CH), 7.80 (d, J = 15.7 Hz, 1 H, CH-CH), 8.15-8.35 (m, 4 H, Ph). MS (GC-MS): m/z (%) = 303 (1), 287 (70), 252 (100), 207 (15), 179 (25), 165 (45), 102 (30), 76 (20).
Compound 6g: mp 188-191 °C. FT-IR (KBr): νmax = 3364, 3171, 3045, 2972, 1626, 1540, 1328, 1235, 1149, 844 cm-1. 1H NMR (500 MHz, CD3COCD3): δ = 2.95 [s, 6 H, N(CH3)2], 6.90 (d, J = 15.5 Hz, 1 H, CH-CH), 7.65-7.57 (m, 4 H, Ph), 7.80 (d, J = 15.5 Hz, 1 H, CH-CH), 8.25-8.35 (m, 4 H, Ph). MS (GC-MS): m/z (%) = 312 (1), 296 (100), 250 (25), 207 (3), 179 (25), 165 (20), 105 (10), 76 (10).
Compound 6h: mp 212-213 °C. FT-IR (KBr): νmax = 3112, 2999, 2906, 1646, 1586, 1520, 1493, 1448, 1334, 1202, 1103, 1036, 924 cm-1. 1H NMR (500 MHz, CD3COCD3): δ = 6.01 (s, 2 H, OCH 2O), 6.82-7.25 (m, 3 H, Ph), 7.30 (d, J = 15.5 Hz, 1 H, CH-CH), 7.75 (d, J = 15.5 Hz, 1 H, CH-CH), 8.15-8.35 (m, 4 H, Ph). MS (GC-MS): m/z (%) = 313 (1), 297 (100), 250 (25), 207(5), 175 (25), 145 (30), 105 (8), 76 (10).