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

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

References

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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): ν_max = 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): ν_max = 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.

20

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): ν_max 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): ν_max = 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): ν_max = 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).