Reaction between 4-Nitro-1,3-diarylbutan-1-ones and Ammonium Acetate in the Presence of Morpholine and Sulfur: An Efficient Synthesis of 2,4-Diarylpyrroles

Abstract

An efficient synthesis of 2,4-diarylpyrroles is described. Heating a mixture of a 4-nitro-1,3-diarylbutan-1-one and ammonium acetate in the presence of morpholine and sulfur afforded the corresponding 2,4-diarylpyrroles in excellent yields.

Key words

2,4-diarylpyrroles - 4-nitro-1,3-diarylbutan-1-ones - morpholine - sulfur - heterocycles - cyclization

• References and Notes

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• 20 General Procedure for the Preparation of 2,4-Diarylpyrroles 3a–o, Exemplified with 3a A mixture of 4-nitro-1,3-diphenylbutan-1-one (1a, 1 mmol), sulfur (3 mmol), morpholine (3 mmol), and NH₄OAc (6 mmol) within a 5 mL round-bottomed flask was magnetically stirred in a silicone oil bath at 80 °C for 30 min. Progress of the reaction was followed by TLC monitoring. Then the reaction mixture was cooled to ambient temperature, H₂O (2.5 mL) was added, and the product was extracted into CH₂Cl₂ (2 × 2.5 mL). The organic layer was dried over Na₂SO₄, filtered, the solvent was removed, and the residue was purified by crystallization from EtOAc. 2,4-Diphenyl-1H-pyrrole (3a) Colorless crystals, mp 176–177 °C, yield 0.214 g, 98%. IR (KBr): 3435 (NH), 1598, 1481, 1451, 1411, 1258, 1185, 1157, 1129, 1075, 1030, 903, 807, 746, 688 cm^–1. ¹H NMR (300.1 MHz, DMSO-d ₆): δ = 6.96 (1 H, s, CH), 7.13 (1 H, t, J = 7.5 Hz, CH), 7.18 (1 H, t, J = 7.5 Hz, CH), 7.32 (2 H, t, J = 7.5 Hz, 2 × CH), 7.35 (1 H, s, CH), 7.37 (2 H, t, J = 7.6 Hz, 2 × CH), 7.61 (2 H, d, J = 7.5 Hz, 2 × CH), 7.69 (2 H, d, J = 7.5 Hz, 2 × CH), 11.45 (1 H, br s, NH). ¹³C NMR (75.5 MHz, DMSO-d ₆): δ = 103.7 (C³H), 117.1 (C⁵H), 123.9 (2 × CH), 124.9 (2 × CH), 125.2 (C⁴), 125.5 and 126.2 (2 × CH), 129.0 (2 × CH), 129.2 (2 × CH), 132.7 and 133.1 (2 × C) 136.2 (C²). MS: m/z (%) = 219 (100) [M⁺], 191 (37), 165 (15), 115 (38), 110 (13), 102 (10), 95 (6), 89 (11), 77 (10), 63 (10), 51 (11). HRMS (ES⁺): m/z calcd for C₁₆H₁₄N [M + H]⁺: 220.1126; found: 220.1124. 4-(3-Methylphenyl)-2-phenyl-1H-pyrrole (3c) Colorless crystals, mp 140–141 °C, yield 0.214 g, 92%. IR (KBr): 3401 (NH), 1597, 1479, 1451, 1413, 1265, 1225, 1188, 1130, 1038, 929, 898, 760, 688 cm^–1. ¹H NMR (300.1 MHz, DMSO-d ₆): δ = 2.34 (3 H, s, CH₃), 6.95 (1 H, d, J = 7.5 Hz, CH), 6.97 (1 H, s, CH), 7.19 (1 H, t, J = 7.8 Hz, CH), 7.21(1 H, dd, J = 8.1, 7.8 Hz, CH), 7.33 (1 H, s, CH), 7.36 (1 H, d, J = 7.5 Hz, CH), 7.40 (2 H, t, J = 7.5 Hz, 2 × CH), 7.46 (1 H, s, CH), 7.69 (2 H, d, J = 7.8 Hz, 2 × CH), 11.45 (1 H, br s, NH). ¹³C NMR (75.5 MHz, DMSO-d ₆): δ = 21.7 (CH₃), 103.7 (C³H), 117.0 (C⁵H), 122.1 (CH), 123.8 (2 × CH), 125.2 (C⁴), 125.5, 126.1, 126.2 and 128.9 (4 × CH), 129.2 (2 × CH), 132.6, 133.1, 136.0 and 137.9 (3 × C and C²). MS: m/z (%) = 233 (100) [M⁺], 217 (17), 202 (13), 189 (15), 165 (7), 129 (11), 115 (25), 102 (10), 89 (8), 77 (13), 63 (7), 51 (8). HRMS (ES⁺): m/z calcd for C₁₇H₁₆N [M + H]⁺: 234.1283; found: 234.1291. 2-(4-Methylphenyl)-4-phenyl-1H-pyrrole (3h) Colorless crystals, mp 207–209 °C, yield 0.214 g, 92%. IR (KBr): 3385 (NH), 1598, 1532, 1491, 1442, 1259, 1212, 1184, 1132, 1078, 1033, 922, 802, 747, 688 cm^–1. ¹H NMR (300.1 MHz, DMSO-d ₆): δ = 2.30 (3 H, s, CH₃), 6.90 (1 H, s, CH), 7.12 (1 H, t, J = 7.5 Hz, CH), 7.19 (2 H, d, J = 7.8 Hz, 2 × CH), 7.31 (1 H, s, CH), 7.32 (2 H, t, J = 7.5 Hz, 2 × CH), 7.58 (2 H, d, J = 7.5 Hz, 2 × CH), 7.61 (2 H, d, J = 7.8 Hz, 2 × CH), 11.39 (1 H, br s, NH). ¹³C NMR (75.5 MHz, DMSO-d ₆): δ = 21.2 (CH₃), 103.1 (C³H), 116.6 (C⁵H), 123.8 (2 × CH), 124.8 (2 × CH), 125.0 (C⁴), 125.4 (CH), 129.0 (2 × CH), 129.7 (2 × CH), 130.4, 132.8, 135.3 and 136.2 (3 × C and C²). MS: m/z (%) = 233 (100) [M⁺], 217 (25), 202 (21), 189 (21), 178 (8), 165 (9), 128 (8), 115 (43), 102 (12), 89 (15), 77 (16), 63 (11), 51 (11). HRMS (ES⁺): m/z calcd for C₁₇H₁₆N [M + H]⁺: 234.1283; found: 234.1278. 4-(4-Methoxyphenyl)-2-(4-methylphenyl)-1H-pyrrole (3j) Colorless crystals, mp 179–181 °C, yield 0.239 g, 91%. IR (KBr): 3437 (NH), 1610, 1581, 1533, 1500, 1439, 1297, 1238, 1177, 1130, 1033, 1032, 923, 829, 793, 716, 631 cm^–1. ¹H NMR (300.1 MHz, DMSO-d ₆): δ = 2.30 (3 H, s, CH₃), 3.75 (3 H, s, OCH₃), 6.81 (1 H, s, CH), 6.90 (2 H, d, J = 8.7 Hz, 2 × CH), 7.18 (2 H, d, J = 8.1 Hz, 2 × CH), 7.19 (1 H, s, CH), 7.51 (2 H, d, J = 8.7 Hz, 2 × CH), 7.56 (2 H, d, J = 8.1 Hz, 2 × CH), 11.27 (1 H, br s, CH). ¹³C NMR (75.5 MHz, DMSO-d ₆): δ = 21.2 (CH₃), 55.5 (OCH₃), 102.9 (C³H), 114.4 (2 × CH), 115.7 (C⁵H), 123.8 (2 × CH), 124.9 (C⁴), 126.0 (2 × CH), 129.0 (C), 129.7 (2 × CH), 130.5 and 132.6 (2 × C), 135.1 (C²), 157.5 (CO). MS: m/z (%) = 263 (100) [M⁺], 248 (90), 220 (12), 204 (12), 178 (14), 165 (7), 152 (6), 132 (10), 119 (23), 102 (6), 91 (17), 77 (10), 65 (7).
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