Ruthenium-Catalyzed Tandem C–H Fluoromethylation/Cyclization of N-Alkylhydrazones with CBr3F: Access to 4-Fluoropyrazoles

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Figure

4-Fluoropyrazoles are accessible in a single step from readily available aldehyde-derived N-alkylhydrazones through double C–H fluoroalkylation with tribromofluoromethane (CBr3F). RuCl2(PPh3)3 has been proven to be the most efficient catalyst for this transformation when compared to a range of other Cu-, Pd-, or Fe-based catalyst systems.

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Alexis Prieto

Univ Lyon, Université Claude Bernard Lyon 1, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (ICBMS, CNRS UMR 5246), F-69622 Villeurbanne, France
J. Org. Chem.201782 (6), pp 3311–3316
DOI: 10.1021/acs.joc.7b00085

Ruthenium-Catalyzed Tandem C–H Fluoromethylation/Cyclization of N-Alkylhydrazones with CF3BR: Access to 4-Fluoropyrazoles

http://pubs.acs.org/doi/10.1021/acs.joc.7b00085


The importance of fluorine-containing pyrazoles to the pharmaceutical and agrochemical industries has been steadily increasing in recent years. As a consequence, the development of methods suitable for the incorporation of fluorine or fluoroalkyl groups into the pyrazole ring continues to be the subject of intense research.
Predicated on their previous copper-catalyzed synthesis of 4-substituted pyrazoles, Bouyssi, Monteiro and their co-worker from the Institut de Chemie et Biochemie Moléculaires et Supramoléculaires reported a ruthenium-catalyzed synthesis of substituted-4-fluoropyrazoles ( J. Org. Chem. 2017823311). The requisite starting materials, aldehyde derived N,N-dialkylhydrazones, were readily synthesized. Tribromofluoromethane served as the source of fluorine.
The commercially available and inexpensive ruthenium complex, RuCl2(PPh3)3, was discovered to be a very effective catalyst for this transformation. Diglyme was the preferred solvent for the reaction. The reaction displayed good tolerance for a variety of functional groups, including cyano, ester, formyl, and halide.
In general, higher yields were obtained with electron-withdrawing substituents. This novel methodology affords substituted-4-fluoropyrazoles in good yields in one step from readily available starting materials.

                                                 2K

3-(Benzo[d][1,3]dioxol-5-yl)-4-fluoro-1-methyl-1H-pyrazole (2k)

Chromatography using ethyl acetate/cyclohexane (gradient elution 30:70 to 50:50) gave the title compound as a pale yellow solid (79 mg, 60%).
Mp = 82–85 °C.
1H NMR (400 MHz, CDCl3) δ 7.35–7.31 (m, 2H), 7.27 (d, J = 4.8 Hz, 1H), 6.85 (d, J = 8.5 Hz, 1H), 5.97 (s, 2H), 3.83 (s, 3H).
13C NMR (101 MHz, CDCl3) δ 148.0, 147.2, 146.8 (d, J = 248.0 Hz), 136.7 (d, J = 6.2 Hz), 125.3 (d, J = 4.2 Hz), 119.8 (d, J = 4.7 Hz), 117.5 (d, J = 28.6 Hz), 108.6, 106.6 (d, J = 3.7 Hz), 101.1, 40.0 (s).
19F NMR (282 MHz, CDCl3) δ −178.2 (s). HRMS (ESI): Calcd for C11H10FN2O2 [M + H+]: 221.0721, found 221.0728.

 

Figure

 

alexis prieto

Alexis prieto

Chercheur postdoctoral chez Melchiorre group, ICIQ

Melchiorre group, ICIQ

Didier Bouyssi at Claude Bernard University Lyon 1
Univ Lyon, Université Claude Bernard Lyon 1, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (ICBMS, CNRS UMR 5246), F-69622 Villeurbanne, France

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