Samarium(III) Triflate as an Efficient and Reusable Catalyst for Facile Synthesis of Benzoxazoles and Benzothiazoles in Aqueous Medium

Abstract

A simple, green, and efficient method is presented for the synthesis of benzoxazoles and benzothiazoles from reaction of o-aminophenols, o-aminothiophenols, and aliphatic or aromatic aldehydes using samarium triflate as a reusable acid catalyst under mild reaction conditions in aqueous medium.

Key words

benzoxazoles - benzothiazoles - samarium triflate

• References and Notes

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• 24 General Procedure To a mixture of the requisite o-aminophenol/o-amino-thiophenol (1 mmol) and aldehyde (1 mmol) in EtOH–H₂O (2:2 mL), samarium triflate catalyst (10 mol%) was added, and the resulting mixture was stirred at 60 °C. After completion of the reaction, as monitored by TLC, the mixture was diluted with H₂O–EtOAc (1:1, 10 mL) and catalyst recovered by filtration. The filtrate was extracted with Et₂O (2 × 10 mL) and dried with anhydrous Na₂SO₄. After filtration and evaporation of solvent, the crude product was recrystallized from EtOAc or MeOH. All the structures were confirmed by their analytical data and comparison with literature data.^25–30 Spectroscopic Data for Selected Compounds 2-Phenylbenzo[d]oxazole (Table 2 Entry 1) Mp 100–102 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.39 (m, 2 H), 7.57 (m, 3 H), 7.64 (m, 1 H), 7.80 (m, 1 H), 8.29 (m, 2 H). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 110.6, 119.9, 124.5, 125.1, 127.3, 127.5, 128.9, 131.5, 142.2, 150.8, 163.0. IR (KBr): 740, 1171, 1355, 1511, 1640, 3100 cm^–1. MS (EI): m/z = 195.1 [M⁺]. 2-(4-Methylphenyl)benzo[d]oxazole (Table 2 Entry 3) Mp 114–116 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 2.45 (s, 3 H), 7.36 (d, 2 H, J = 8.1 Hz), 7.41 (t, 1 H, J = 8.4 Hz), 7.51 (t, 1 H, J = 8.4 Hz), 7.96 (d, 1 H, J = 8.0 Hz), 8.00–8.06 (m, 3 H). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 21.2, 121.6, 122.9, 125.0, 126.2, 127.3, 129.7, 131.0, 135.0, 141.6, 154.2, 168.0. IR (KBr): 760, 1186, 1409, 1590, 1621, 2968 cm^–1. MS (EI): m/z = 209.07 [M⁺]. 2-(4-Chlorophenyl)benzo[d]oxazole (Table 2 Entry 4) Mp 147–149 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.44 (t, 1 H, J = 7.6 Hz), 7.50–7.56 (m, 3 H), 7.98 (d, 1 H, J = 8.1 Hz), 8.05–8.09 (m, 3 H). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 121.7, 123.2, 125.4, 126.5, 128.7, 129.2, 132.2, 135.1, 136.8, 154.1, 166.5. IR (KBr): 775, 1175, 1371, 1640, 2980 cm^–1. MS (EI): m/z = 229 [M⁺] and 231 [M + 2]. 2-(4-Nitrophenyl)benzo[d]thiazole (Table 2 Entry 11) Mp 225–227 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.51 (t, 1 H, J = 7.5 Hz), 7.60 (t, 1 H, J = 7.6 Hz), 8.01 (d, 1 H, J = 7.8 Hz), 8.16 (d, 1 H, J = 8.1 Hz), 8.36 (q, 4 H, J = 9.4 Hz). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 121.9, 123.8, 124.4, 126.1, 126.8, 128.2, 135.6, 139.2, 149.0, 154.1, 165.0. IR (KBr): 807, 1169, 1370, 1556, 1690, 3056 cm^–1. MS (EI): m/z = 256.0 [M⁺]. 2-(Pyridin-4-yl)benzo[d]thiazole (Table 2 Entry 14) Mp 130–132 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.44–7.50 (m, 2 H), 7.57 (t, 1 H, J = 8.1 Hz), 8.01 (d, 1 H, J = 7.9 Hz), 8.13 (d, 1 H, J = 8.2 Hz), 8.40 (dt, 1 H, J = 8.0, 1.7 Hz), 8.73 (dd, 1 H, J = 4.8z, 1.7 Hz). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 121.8, 123.3, 123.8, 125.6, 129.6, 134.4, 135.1, 148.5, 154.0, 164.7. IR (KBr): 690, 1150, 1400, 1653, 2360, 2905 cm^–1. MS (EI): m/z = 212.03 [M⁺]. 2-(4-Methoxyphenyl)benzo[d]thiazole (Table 2 Entry 15) Mp 122–124 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 3.89 (s, 3 H), 7.02 (d, 2 H, J = 8.4 Hz), 7.39 (t, 1 H, J = 7.2 Hz), 7.51 (t, 1 H, J = 7.2 Hz), 7.94 (d, 1 H, J = 7.8 Hz), 8.06 (m, 3 H). ¹³C NMR (75 MHz, DMSO-d ₆): δ = 55.4, 114.3, 121.6, 122.7, 124.8, 126.2, 126.4, 129.0, 135.0, 154.3, 162.0, 167.7. IR (KBr): 781, 1170, 1450, 1597, 1650, 3010 cm^–1. MS (EI): m/z = 241 [M⁺].
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