Indium(III)-catalyzed synthesis of N-substituted pyrroles under solvent-free conditions

Abstracts

A variety of N-substituted pyrroles have been synthesized by reacting γ-diketones (R¹C(O)CH2CH2C(O)R²: R¹, R² = Me, Ph) with amines (RNH2: R=Alkyl, Aryl, TsNH) or diamines (1,6-diaminohexane and 1,2-diaminoethane) in the presence of indium tribromide, indium trichloride or indium trifluoromethanesulfonate at room temperature under solvent-free conditions. The experiment protocol features simple operations, and the products are isolated in high to excellent yields (81-98%).

indium tribromide; indium trichloride; indium trifluoromethanesulfonate; pyrroles; solvent-free conditions

Vários pirróis N-substituídos foram sintetizados pela reação de γ-dicetonas (R¹C(O)CH2CH2C(O)R²: R¹, R² = Me, Ph) com aminas (RNH2: R = Alkyl, Aryl, TsNH) ou diaminas (1,6-diaminohexano and 1,2-diaminoetano) na presença de tribrometo de índio, tricloreto de índio ou trifluorometanossulfonato de índio a temperatura ambiente e sem solventes. O protocolo envolve operações simples e os produtos são isolados em excelentes rendimentos (81-98%).

ARTICLE

Indium(III)-catalyzed synthesis of N-substituted pyrroles under solvent-free conditions

Jiu-Xi Chen; Miao-Chang Liu; Xiao-Liang Yang; Jin-Chang Ding; Hua-Yue Wu^*

College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325027, P. R. China

ABSTRACT

A variety of N-substituted pyrroles have been synthesized by reacting γ-diketones (R¹C(O)CH₂CH₂C(O)R²: R¹, R² = Me, Ph) with amines (RNH₂: R=Alkyl, Aryl, TsNH) or diamines (1,6-diaminohexane and 1,2-diaminoethane) in the presence of indium tribromide, indium trichloride or indium trifluoromethanesulfonate at room temperature under solvent-free conditions. The experiment protocol features simple operations, and the products are isolated in high to excellent yields (81-98%).

Keywords: indium tribromide, indium trichloride, indium trifluoromethanesulfonate, pyrroles, solvent-free conditions

RESUMO

Vários pirróis N-substituídos foram sintetizados pela reação de γ-dicetonas (R¹C(O)CH₂CH₂C(O)R²: R¹, R² = Me, Ph) com aminas (RNH₂: R = Alkyl, Aryl, TsNH) ou diaminas (1,6-diaminohexano and 1,2-diaminoetano) na presença de tribrometo de índio, tricloreto de índio ou trifluorometanossulfonato de índio a temperatura ambiente e sem solventes. O protocolo envolve operações simples e os produtos são isolados em excelentes rendimentos (81-98%).

Introduction

Heterocyclic small molecules play an important role in the search for new therapeutic and drug candidates.¹ Pyrroles are an important class of heterocyclic compounds and are structural units found in a vast array of natural products, synthetic materials, and bioactive molecules, such as heme, vitamin B12, and cytochromes.² Pyrroles also play crucial roles in nonlinear optical materials as well as in supramolecular chemistry.³ Therefore, the enormous number of procedures have been developed for the construction of pyrroles. Classical methods for their preparation include the Knorr,⁴ Hantzsch,⁵ and Paal-Knorr condensation reactions.^6-16 Among these methods, the most commonly used and straightforward approach for the preparation of pyrroles is the direct condensation of γ-diketones and amines (Paal-Knorr reaction) in the presence of protonic acids such as AcOH,⁶p-TsOH.⁷ Some improved procedures were reported to utilize Lewis acids such as Bi(NO₃)₃.5H₂O,⁸ Sc(OTf)₃,⁹ SnCl₂¹⁰ and heterogeneous catalyst like montmorillonite (KSF,¹¹ K10¹² and Fe(III)¹³), zirconium compounds.¹⁴ Recently, this condensation reaction has also been performed under microwave technology¹⁵ and in ionic liquid medium.¹⁶

However, there are some limitations with these methodologies such as prolonged reaction times,^11,14,16 elevated temperatures,⁷ use of volatile organic solvents,^8,11 the requirement of excess reagents or catalysts,¹⁰ moisture sensitive/hazardous catalysts,¹⁰ use of an additional microwave oven,¹⁵ use of costly solvent (ionic liquids),¹⁶ unsatisfactory and yields¹⁴. Thus, the development of new reagents with great efficiency, environmentally friendly, convenient procedure, and delivery of better yields methods for the synthesis of pyrroles is of great interest.

During the last decade particularly after the review by Cintas,¹⁷ indium(III) based carbon-carbon bond formation and other organic transformations have gained much attention. In accordance with the recent surge of interest in indium(III) salts has emerged as a promising catalyst for various types of organic transformations. Particularly, indium tribromide,¹⁸ indium trichloride¹⁹ and indium trifluoromethanesulfonate²⁰ as a novel type of water-tolerant green Lewis acid catalyst have received considerable attention for various transformations, because they have advantages of water stability, operational simplicity, recyclability, strong tolerance to oxygen and nitrogen-containing reaction substrates and functional groups, and it can often be used in just catalytic amounts. Recently, Zhang²¹ reported a few examples of InBr₃-catalyzed organic reactions. Ghosh and coworkers²² reviewed systematically In(OTf)₃-catalyzed organic syntheses.

In continuation of our interest in Lewis acid-catalyzed and green chemistry organic reactions,²³ we herein wish to report a simple, practical and efficient method for the synthesis of pyrroles from γ-diketones and amines using indium(III) salt as catalyst at room temperature under solvent-free conditions (Scheme 1).

Results and Discussion

Initially, the efficacy of various catalysts was tested in the model reaction under solvent-free condition at room temperature (Table 1). The results showed that InCl₃, InBr₃ and In(OTf)₃ were superior to other Lewis acids with respect to reaction times and product yields. Entry 1 showed the blank reaction without addition of any catalyst, in this case only 29% 2,5-dimethyl-1-phenyl-1H-pyrrole (3a) was obtained after 24 h. For the sake of comparison, the reaction was carried out using other anhydrous chloride, such as ZnCl₂, NiCl₂, SrCl₂ and CuCl₂ under solvent free conditions (Entries 2-8). However, these chlorides showed lower activity than InCl₃ under same reaction conditions. We examined influence of solvents on the reaction yields. It was found that the reactions gave lower yields of the desired product in solvents such as CH₂Cl₂, H₂O, CH₃NO₂ after prolonged reaction time. (Entries 10-13, 15-18, 20-23). We also checked the reusability of the catalyst by separation and reloading in a new run and found that the catalyst could be reused three times without any decrease in the yield (Entry 19). In light of these good results, subsequent studies were carried out under the following optimized conditions: with 5 mol% InBr₃, InCl₃ or In(OTf)₃ at room temperature under solvent-free conditions.

To evaluate the scope of catalyst's application, various γ-diketones were treated with amines under the above conditions and the results presented in Table 2. The desired products were characterized by NMR, IR and mass spectrometry and also by comparison with authentic samples.

The Indium(III)-catalyzed reactions were found to be dependent on electronic and steric factors of amines. Anilines with electron-donating groups gave a high yield (up to 98%) due to the increased electron density of the aromatic system (entries 2 and 6). Whereas anilines with strongly electron-withdrawing group (NO₂) afforded in moderate yield (71%) after 40 min (entry 7), which showed electronic effect. ortho-substituted anilines whatever the character of the substituted groups required longer reaction times. It was revealed that yields were significantly decreased when the sizes of the ortho-substituent groups were large. For example, 2,6-dimethylaniline or 2,6-diisopropylaniline (entries 4-5) were found to be less active and gave mild yields after 1 h. This may be due to the steric hindrance exhibited by the 2,6-diisopropyl and 2,6-dimethyl groups of the aniline towards the approaching acetonylacetone. Moreover, we also examined the condensation reaction of aliphatic amines with acetonylacetone (entries 8-18). The corresponding products (3h-3r) were obtained with excellent yields. In the case of 1,6-diaminohexane or 1,2-diaminoethane, two equivalents of acetonylacetone were used giving products with dipyrryl groups 3n or 3o (entries 14-15).

On the other hand, it was found that heterocyclic amines (2-aminopyridine) and less nucleophilic aromatic amine (α-naphthalenamine) did not make any difference in this reaction (entries 19 and 20). It is noteworthy that optically active amines were converted into the products (3p-3q) without any racemization or inversion (entries 16 and 17). We also investigated the reaction of various γ-diketones such as 1,4-diphenylbutane-1,4-dione and 1-phenylpentane-1,4-dione with amines. Similarly, the corresponding pyrroles (3u, 3v) were obtained in high yields in the presence of InCl₃, InBr₃ or In(OTf)₃ at 50 °C.

Finally, we examined the condensation reactions of γ-diketones with p-toluenesulfonylhydrazide (Scheme 2). N-(2,5-Dimethyl-1H-pyrrol-1-yl)-4-methylbenzenesulfonamide (4a) were obtained with good yields (87-91%) when the reaction was performed at 70 °C in the presence of Indium(III) salts. It is found that this condensation reaction gave 4a in moderate yield (53%) under solvent-free conditions due to poor solubility of p-toluenesulfonylhydrazide in γ-diketones and the yields showed significant improvement in CH₃NO₂. Nevertheless, trace product was obtained in the absence of catalysts, which also further proved that Indium(III) salts do play an important role in this reaction.

The reaction proceeds very cleanly without the formation of any by-products except water. Because the reaction can be performed using a solvent-free procedure, at the end of the reaction, the crude mixture can be directly charged on a chromatographic column to obtain the pure product, avoiding any tedious work up.

In summary, we have developed a facile and efficient method for the synthesis of N-substituted pyrroles using Indium(III) salts. The advantages of the method include (i) absence of a solvent, (ii) short reaction times, (iii) high yields, (iv) no work-up, and (v) use of trace amounts of a catalyst that could be recovered and re-used.

Experimental

Melting points were recorded on Digital Melting Point Apparatus WRS-1B and are uncorrected. ¹H NMR and ¹³C NMR spectra were taken with a VARIAN Mercury plus-400 instrument using CDCl₃ as the solvent with tetramethylsilane (TMS) as an internal standard at room temperature. Chemical shifts are given in δ relative to TMS, the coupling constants J are given in Hz. IR spectra was recorded on a AVATAR 370 FI-Infrared Spectrophotometer. Mass spectra were recorded on Finnigan Trace DSQ (EI, CI) or Thermo Finnigan LCQ-Advantage (ESI).

Typical experimental procedure for the synthesis of 2,5-dimethyl-1-phenyl-1H-pyrrole (3a)

A mixture of acetonylacetone (570 mg, 5 mmol) and aniline (465 mg, 5 mmol) was stirred in the presence of InCl₃ (55 mg, 5a mol%), InBr₃ (89 mg, 5 mol%) or In(OTf)₃ (141 mg, 5 mol%) at room temperature for 30 min. After completion of the reaction (TLC), the crude products were separated by column chromatography on Et₃N pre-treated silica gel using petroleum ether/EtOAc (12:1 to 5:1) as eluent to afford a pure product 3a in 90%, 93% or 94% yields, respectively. The spectral and analytical data of some representative compounds are given below.

2,5-dimethyl-1-phenyl-1H-pyrrole (3a)¹³

A white solid, mp 50.1-50.3 ºC (46-47 ºC); IR ν_max/cm^-1 3060, 2922, 1599, 1520, 1498, 1403, 1321 (film); ¹H NMR (400 MHz, CDCl₃) δ 7.49-7.41 (m, 3H, ArH), 7.27-7.22 (m, 2H, ArH), 5.92 (s, 2H, pyrrole), 2.05 (s, 6H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 139.9, 129.0, 128.8, 127.6, 105.6, 13.0; MS (EI): m/z (%) 171 (100) [M⁺], 156 (8). Compounds 3b-3m and 3p-3v were obtained following a similar procedure.

2,5-dimethyl-1-((R)-1-phenylethyl)-1H-pyrrole (3p)²⁴

A colorless oil, [α]_D²⁰: +42.95(c 0.098, CH₃OH); IR ν_max /cm^-1 3088, 2975, 2926, 1519, 1496, 1448, 1395, 1293 (neat); ¹H NMR (400 MHz, CDCl₃) δ 7.27 (t, J 7.2 Hz, 2H, ArH), 7.20 (t, J 7.2 Hz, 2H, ArH), 7.02 (d, J 7.2 Hz, 2H, ArH), 5.80 (s, 2 H, pyrrole), 5.45 (q, J 7.2 Hz, 1H, CHCH₃), 2.06 (s, 6H, CH₃), 1.83 (d, J 7.2 Hz, 3H, CHCH₃). ¹³C NMR (100 MHz, CDCl₃) δ 142.3, 128.3, 128.1, 126.7, 125.9, 106.0, 52.3, 19.2, 13.7; MS (EI): m/z (%) 200 (52) [M+1]⁺, 199 (100), 124 (10), 96 (35).

1-(furan-2-ylmethyl)-2,5-dimethyl-1H-pyrrole (3r)⁹

A colorless oil, IR ν_max/cm^-1 3109, 2975, 2931, 1577, 1521, 1505, 1442, 1406, 1343, 1296, 1216, 1147 (neat); ¹H NMR (400 MHz, CDCl₃) δ 7.34 (s, 1H, furan), 6.31 (s, 1H, furan), 6.05 (s, 1H, furan), 5.83 (s, 2H, pyrrole), 4.93 (s, 2H, CH₂), 2.30 (s, 6H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.3, 142.1, 127.9, 110.2, 106.9, 105.3, 40.5, 12.4; MS (EI): m/z (%) 176 (43) [M+1] ⁺, 175 (100) [M⁺], 81 (13).

2-(2,5-dimethyl-1H-pyrrol-1-yl)pyridine (3t)¹⁴

A white solid, mp 33.2-33.9 °C; IR ν_max/cm^-1 3055, 2922, 1587, 1525, 1473, 1438, 1397, 1325, 1285, 1234 (film); ¹H NMR (400 MHz, CDCl₃) δ 8.62 (d, J 4.4 Hz, 1H, pyridine), 7.84 (t, J 7.8 Hz, 1H, pyridine), 7.31 (t, J 6.4 Hz, 1H, pyridine), 7.26-7.22 (m, 1H, pyridine), 5.91 (s, 2H, pyrrole), 2.13 (s, 6H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 152.0, 149.2, 138.0, 128.6, 122.3, 122.0, 106.9, 13.1; MS (EI): m/z (%) 173 (25) [M+1]⁺, 172 (100) [M⁺], 171 (47), 157 (25), 94 (13).

1-benzyl-2-methyl-5-phenyl-1H-pyrrole (3u)²⁵

A colorless oil, ¹H NMR (400 MHz, CDCl₃) δ 7.30-7.20 (m, 8H, ArH), 6.92 (d, J 7.2 Hz, 2H, ArH), 6.23 (d, J 2.8 Hz, 1H, pyrrole), 6.05 (s, 1H, pyrrole), 5.11 (s, 2H), 2.12 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 138.9, 134.6, 133.7, 130.4, 128.7, 128.6, 128.3, 126.9, 126.6, 125.6, 107.9, 107.1, 47.6, 12.5; MS (ESI): m/z (%) 248.2 (100) [M+1]⁺.

1,2,5-triphenyl-1H-pyrrole (3v)²⁶

A white solid, mp 230-232 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.26-7.03 (m, 15H, ArH), 6.5 (s, 2H, pyrrole). ¹³C NMR (100 MHz, CDCl₃) δ 138.9, 135.8, 133.2, 128.9, 128.7, 127.8, 127.2 (2C), 126.20, 109.9; MS (ESI): m/z (%) 296.1 (100) [M +1]⁺.

Typical experimental procedure for the synthesis of 1,6-bis(2,5-dimethyl-1H-pyrrol-1-yl)hexane (3n)

A mixture of acetonylacetone (1.14 g, 10 mmol) and 1,6-diaminohexane (580 mg, 5 mmol) was stirred in the presence of InCl₃ (55 mg, 5 mol%), InBr₃ (89 mg, 5 mol%) or In(OTf)₃ (141 mg, 5 mol%) at room temperature for 45 min. After completion of the reaction (TLC), the crude products were separated by column chromatography on Et₃N pre-treated silica gel using petroleum ether/EtOAc (15:1 to 5:1) as eluent to afford a pure product 3n in 88, 87 or 90% yields. The spectral and analytical data of some representative compounds are given below.

1,6-bis(2,5-dimethyl-1H-pyrrol-1-yl)hexane (3n)²⁷

A white solid, mp 102.8-103.0 °C; IR ν_max/cm^-1 3103, 3093, 2969, 2924, 1518, 1475, 1410, 1373, 1301 (film); ¹H NMR (400 MHz, CDCl₃) δ 5.76 (s, 4H, pyrrole), 3.70 (t, J 7.6 Hz, 4H), 2.20 (s, 12H, CH₃), 1.65-1.58 (m, 4H), 1.38-1.34 (m, 4H). ¹³C NMR (100 MHz, CDCl₃) δ 127.2, 105.0, 43.5, 30.9, 26.7, 12.5; MS (EI): m/z (%) 272 (100) [M⁺], 257 (8), 164 (38), 108 (30), 94 (18). Compound 3o was obtained following a similar procedure.

N-(2,5-dimethyl-1H-pyrrol-1-yl)-4-methylbenzenesulfonamide (4a)²⁸

A white solid, mp 143-144 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.68 (d, J 8.0 Hz, 2H, ArH), 7.31 (d, J 8.0 Hz, 2H, ArH), 7.07 (s, 1H, NH), 5.69 (s, 2H, pyrrole), 2.45 (s, 3H, ArCH₃), 1.82 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 145.0, 135.3, 129.9, 128.9, 128.4, 104.9, 21.7, 11.2; MS (ESI): m/z (%) 265.2 (100) [M+1]⁺.

Acknowledgments

We are grateful to the Natural Science Foundation of Zhejiang Province (No. Y405015 and Y405113) for financial support.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br, as PDF file.

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Received: April 11, 2007

Web Release Date: April 18, 2008

Supplementary Information

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Publication Dates

• Publication in this collection
  05 Aug 2008
• Date of issue
  2008

History

• Received
  11 Apr 2007
• Accepted
  18 Apr 2008