A New Approach to the Preparation of 1,3,4-Triarylpyrroles

De Xing Zeng; Yi Chen

doi:10.1055/s-2006-932452

LETTER

A New Approach to the Preparation of 1,3,4-Triarylpyrroles

De Xing Zeng, Yi Chen*
Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100101, P. R. of China
Fax: +86(10)64879375; e-Mail: yichen@mail.ipc.ac.cn;

Abstract

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Abstract

A class of 1,3,4-triaryl-2,5-dihydropyrroles were synthesized using the McMurry coupling reaction as the key step. The non-catalytic photoconversion of 1,3,4-triaryl-2,5-dihydropyrroles furnished 1,3,4-triarylpyrroles in good yields (63-89%). It was found that the photoconversion was facile and very reliable; the solvent was found to play an important role.

Key words

1,3,4-triaryl-2,5-dihydropyrrole - 1,3,4-triarylpyrrole - photoconversion - McMurry reaction

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References

References and Notes

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1a-f; General Procedure 2-Bromoacetophenone or 2-bromoacetone derivative (20 mmol), Na₂CO₃ (20 mmol), and p-anisidine (10 mmol) or aniline derivatives (10 mmol) were dissolved in EtOH (95 %; 20 mL). The mixture was stirred for 0.5 h at ambient temperature followed by heating to reflux for 4 h. The reaction mixture was cooled and diluted with H₂O. The solid was obtained and purified by recrystallization from EtOH. To a suspension of Zn powder (0.3 mol) in THF (350 mL) under nitrogen flux was added TiCl₄ (10 mL) at 0 °C by syringe. The mixture was then refluxed for 1 h. To the mixture was added the solid (15 mmol) obtained above in THF (250 mL) very slowly at ambient temperature. The reaction mixture was stirred for another 24 h in darkness then quenched with K₂CO₃ (40%; 80 mL). The solid was filtered and washed with Et₂O (50 mL). The combined organic phases were concentrated to 50 mL under reduced pressure. H₂O (50 mL) was then added to the residue. The product was extracted with Et₂O (50 mL) and dried over MgSO₄. After evaporation of the solvent, the crude product was purified by flash column chromatography. For 1f 2-bromoacetonephenone was treated with p-anisidine first, followed by reaction with 3-(2,5-dimethyl)-1′-bromoacetylthiophene. 1a: ¹H NMR: δ = 7.30-7.19 (m, 12 H), 6.66 (d, 3 H, J = 7.6 Hz), 4.57 (s, 4 H). MS (EI): m/z = 297 (M⁺, 100). HRMS: m/z calcd for C₂₂H₁₉N [M⁺]: 297.3991; found: 297.4056. 1b: ¹H NMR: δ = 7.30-7.27 (m, 10 H), 6.88 (d, 2 H, J = 8.8 Hz), 6.63-6.60 (d, 2 H, J = 8.9 Hz), 4.54 (s, 4 H), 3.69 (s, 3 H). MS (EI): m/z = 327 (M⁺, 100). HRMS: m/z calcd for C₂₃H₂₁NO [M⁺]: 327.4259; found: 327.6128. 1c: ¹H NMR: δ = 7.30-7.27 (m, 10 H), 7.22 (d, 2 H, J = 8.2 Hz), 6.67 (d, 2 H, J = 8.8 Hz), 4.57 (s, 4 H). MS (EI): m/z = 330 (M⁺ - 1, 99), 138 (100). HRMS: m/z calcd for C₂₂H₁₈ClN [M⁺]: 331.8542; found: 332.0014. 1d: ¹H NMR: δ = 6.92 (d, 2 H, J = 9.0 Hz), 6.57 (d, 2 H, J = 6.0 Hz), 5.87 (s, 2 H), 4.35 (s, 4 H), 3.79 (s, 3 H), 2.24 (s, 6 H), 2.06 (s, 6 H). MS (EI): m/z = 363 (M⁺, 100). HRMS: m/z calcd for C₂₃H₂₅NO₃ [M⁺]: 363.5225; found: 363.5249. 1e: ¹H NMR: δ = 7.93 (d, 4 H, J = 9.0 Hz), 7.28 (d, 4 H, J = 9.0 Hz), 6.93 (d, 2 H, J = 9.0 Hz), 6.68 (d, 2 H, J = 9.0 Hz), 4.64 (s, 4 H), 3.79 (s, 3 H), 2.41 (s, 6 H), 2.06 (s, 6 H). MS (EI): m/z = 517 (M⁺, 52), 119 (100). HRMS: m/z calcd for C₃₃H₃₁N₃O₃ [M⁺]: 517.6259; found: 517.6164. 1f: ¹H NMR: δ = 7.30-7.25 (m, 5 H), 6.88 (d, 2 H, J = 9.0 Hz), 6.68 (s, 1 H), 6.62 (d, 2 H, J = 9.0 Hz), 4.59 (t, 2 H, J = 4.5 Hz), 4.36 (t, 2 H, J = 4.5 Hz), 3.71 (s, 3 H), 2.41 (s, 3 H), 1.95 (s, 3 H). MS (EI): m/z = 361 (M⁺, 100). HRMS: m/z calcd for C₂₃H₂₃NOS [M⁺]: 361.5067; found: 361.5095.

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2a-f; General Procedure A solution of 1,3,4-trisubstituted 2,5-dihydropyrroles (0.5 mmol) in CH₂Cl₂ (100 mL) was irradiated with UV light (high-pressure Hg lamp, 500 W) until the starting material was no longer detected by TLC. After the solvent was evaporated the crude product were purified by column chromatography (EtOAc-PE). 2a: ¹H NMR: δ = 7.71 (d, 2 H, J = 7.7 Hz), 7.53 (d, 2 H, J = 7.4 Hz), 7.48 (s, 2 H), 7.34-7.21 (m, 11 H). MS (EI): m/z = 295 (M⁺, 100). HRMS: m/z calcd for C₂₂H₁₇N [M⁺]: 295.3833; found: 295.4122. 2b: ¹H NMR: δ = 7.57 (d, 2 H, J = 9.0 Hz), 7.32-7.17 (m, 12 H), 7.05 (d, 2 H, J = 8.8 Hz), 3.81 (s, 3 H). MS (EI): m/z = 325 (M⁺, 100). HRMS: m/z calcd for C₂₃H₁₉NO [M⁺]: 325.4101; found: 325.4708. 2c: ¹H NMR: δ = 7.72 (d, 2 H, J = 8.8 Hz), 7.52 (d, 2 H, J = 8.7 Hz), 7.46 (s, 2 H), 7.31-7.20 (m, 8 H). MS (EI): m/z = 329 (M⁺, 100). HRMS: m/z calcd for C₂₂H₁₆ClN [M⁺]: 329.8384; found: 329.9123. 2d: ¹H NMR: δ = 7.36 (d, 2 H, J = 8.8 Hz), 6.98 (d, 2 H, J = 9.0 Hz), 6.94 (s, 2 H), 5.86 (s, 2 H), 3.85 (s, 3 H), 2.25 (s, 6 H), 2.18 (s, 3 H). MS (EI): m/z = 361 (M⁺, 100). HRMS: m/z calcd for C₂₃H₂₃NO₃ [M⁺]: 361.4387; found: 361.4401. 2e: ¹H NMR: δ = 7.96 (d, 4 H, J = 9.0 Hz), 7.45 (d, 2 H, J = 9.0 Hz), 7.32 (s, 2 H), 7.27 (d, 4 H, J = 8.9 Hz), 7.00 (d, 2 H, J = 8.9 Hz), 3.85 (s, 3 H), 2.41 (s, 6 H), 2.12 (s, 6 H). MS (EI): m/z = 515 (M⁺, 65), 119 (100). HRMS m/z calcd for C₃₃H₂₉N₃O₃ [M⁺]: 515.6106; found: 515.6164. 2f: ¹H NMR: δ = 7.43-7.30 (m, 6 H), 7.25-7.23 (m, 2 H), 7.02-7.00 (m, 3 H), 6.57 (s, 1 H), 3.87 (s, 3 H), 2.45 (s, 3 H), 2.22 (s, 3 H). MS (EI): m/z = 359 (M⁺, 100). HRMS: m/z calcd for C₂₃H₂₁NOS [M⁺]: 359.4909; found: 359.4972.