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Synlett 2006(3): 0490-0492  
DOI: 10.1055/s-2006-932452
LETTER
© Georg Thieme Verlag Stuttgart · New York

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;
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Publication History

Received 16 November 2005
Publication Date:
06 February 2006 (online)

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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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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.