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Synlett 2008(9): 1341-1344  
DOI: 10.1055/s-2008-1072766
LETTER
© Georg Thieme Verlag Stuttgart · New York

Ytterbium(III) Perfluorooctanoate Catalyzed One-Pot, Three-Component Synthesis of Fully Substituted Pyrazoles under Solvent-Free Conditions

Li Shen, Song Cao*, Nianjin Liu, Jingjing Wu, Longjie Zhu, Xuhong Qian*
Shanghai Key Laboratory of Chemical Biology, Center of Fluorine Chemical Technology, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, P. R. of China
Fax: +86(21)64252603; e-Mail: scao@ecust.edu.cn; e-Mail: xhqian@ecust.edu.cn;
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Publication History

Received 20 February 2008
Publication Date:
07 May 2008 (online)

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Ytterbium(III) Perfluorooctanoate Catalyzed One-Pot, Three-Component Synthesis of Fully Substituted Pyrazoles under Solvent-Free ConditionsSynlett 2008; 2008(11): 1746-1746
DOI: 10.1055/s-2008-1067137

Abstract

A convenient one-pot synthesis of fully substituted pyrazoles via three-component condensations of phenylhydrazine, aldehydes, and 1,3-dicarbonyl compounds using ytterbium perfluorooctanoate [Yb(PFO)3] as catalyst under solvent-free conditions is described. The reusable catalyst can be easily prepared and was efficient for the reaction. A possible mechanism is suggested.

Key words

pyrazole - solvent-free synthesis - three-component synthesis - Yb(PFO)3 - catalyst

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Experimental Benzaldehyde and phenylhydrazine were freshly distilled before used. The catalyst Yb(PFO)3 was prepared according to ref. 15; other chemicals were purchased commercially and used without further purification. Melting points were recorded in open capillary using Büchi melting point B540 apparatus and were uncorrected. The 1H NMR and 13C NMR spectra were recorded on a Bruker AM-400 spectrometer (400 MHz and 100 MHz, respectively) using TMS as internal standard. High-resolution mass spectra (HRMS) were recorded under electron-impact conditions using a MicroMass GCT CA 055 instrument.

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General Procedure for the One-Pot Syntheses of Pyrazoles To the appropriate aldehyde 1 (1 mmol), phenylhydrazine (1 mmol) was added dropwise. The mixture was stirred for 0.5 h, 1,3-dicarbonyl compound 2 (1.2 mmol) and Yb(PFO)3 (0.1 mmol) were added, the mixture was heated at 120 °C for another 1-1.5 h. After the reaction was complete (monitored by TLC), the mixture was cooled to r.t., CH2Cl2 (3 mL) was added, the catalyst recovery was accomplished simply by filtration and drying in air or under vacuum. The filtrate was washed with sat. aq NaCl solution and dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to leave the crude product which was recrystallized by EtOAc and hexane to give the pure compound. If necessary, the product was purified by chromatography over SiO2.

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Typical Data for a Representative Compound: Ethyl 1-Phenyl-3-(4-bromophenyl)-5-methylpyrazole-4-carboxylate (3b) White solid, mp 60.8-61.3 °C. 1H NMR (400 MHz, CDCl3): δ = 7.58 (d, J = 8.4 Hz, 2 H), 7.52-7.51 (m, 4 H), 7.48-7.46 (m, 3 H), 4.27 (q, J = 7.2 Hz, 2 H), 2.59 (s, 3 H), 1.26 (t, J = 7.2 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 163.9, 152.4, 145.0, 138.6, 132.1, 131.1, 130.8, 129.3, 128.8, 125.8, 122.5, 110.5, 60.1, 14.1, 12.8. HRMS: m/z calcd for C19H17BrN2O2 [M+]: 384.0473; found: 384.0473.