Synthesis and Characterization of 3-Methyl-1-(4-(trifluoromethyl)phenyl)indeno [1,2-c]pyrazol-4(1H)-one

Abstract

Pyrazoles have potential applications in the agrochemical and medicinal chemistry industries as pesticides, anti-inflammatory medications, and antitumor drugs. Fluorinated fused-ring pyrazoles may also possess medicinally useful properties. Herein, we report the acid-catalyzed synthesis of a new tricyclic, trifluoromethylated indenopyrazole, 3-methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one, from 2-acetyl-1,3-indanedione and 4-trifluoromethylphenylhydrazine. This isomeric pyrazole was obtained in yields ranging from 4–24%. NMR spectroscopic characterization and elemental analysis support the structural assignment, identity, and purity of the product.

1. Introduction

While the preparation of heteroaromatic species such as pyrazoles is well documented, the synthesis and bioactivity studies of fused-ring pyrazoles remain a vibrant area of research with many compounds of this type finding use in the treatment of diseases [1,2,3,4,5,6,7,8]. In addition, fluorine substitution has been extensively investigated in drug research for its ability to enhance biological activity and increase chemical or metabolic stability [9,10,11].

Triketones also possess useful medicinal properties, such as antibiotic activity [12]. Fused-ring triketones, such as 2-acetyl-1,3-indanedione, serve as a useful scaffold upon which highly functionalized, tricyclic N-aryl pyrazoles may be built [3,13]. Earlier efforts showed that the acid-catalyzed condensation of 2-trifluoroacetyl-1,3-indanedione (A) with phenylhydrazine provided the 3-trifluoromethyl fused-ring pyrazole B as the sole regioisomer in 70% yield. See Figure 1 [3].

The product regiochemistry of the reaction depicted above is noteworthy, given that many investigations of pyrazole product formation have shown that conventional and microwave-mediated condensations between 1,3-diketones and hydrazines typically yield two regioisomers as shown in Figure 2, often in unequal proportions [13,14].

It is surmised that a combination of steric and electronic factors conspires to affect the pyrazole product distribution [15]. In the case of compound A, the greater electrophilic character of the trifluoroacetyl group relative to the topologically constrained indanedione ring system carbonyl likely contributes to the initial nucleophilic attack by the hydrazine, which ultimately leads to the observed regiochemistry of compound B.

Correspondingly, a non-fluorinated analog of compound A, 2-acetyl-1,3-indanedione (compound 1), was selected to examine whether the pyrazole isomeric product selectivity would be similar to that observed in the 3-trifluoromethyl indenopyrazole synthesis. Compound 1 contains the 2-COCH₃ group, which is sterically slightly smaller than the -COCF₃ group, but because the methyl group is an inductive electron donor, the carbonyl would have a smaller partial positive charge and perhaps not be as prone to nucleophilic attack by the arylhydrazine. It was thus anticipated that the condensation of 2-acetyl-1,3-indanedione with 4-trifluoromethylphenylhydrazine would yield a mixture of pyrazole isomeric products.

2. Results

The regioisomeric pyrazoles 3-methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one (1a) and 3-methyl-2-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(2H)-one (1b) were prepared according to Scheme 1. Three runs were conducted to compare conventional reflux, microwave, and microwave-sonication methods for effectiveness.

Compound 1a was satisfactorily characterized by ¹H and ¹³C nuclear magnetic resonance, mass spectrometry, and elemental analysis. All preparative methods and characterization data are reported in the experimental section. ¹H and ¹³C NMR spectra, as well as the EI mass spectrum, are available as Supplemental Materials. The preparation, isolation, and characterization of 1b were reported previously, but the characterization data for 1b are provided in ref. [16] and in the experimental section [16].

3. Discussion

3.1. Synthesis and Purification

As indicated earlier, three runs of the reaction in Scheme 1 were conducted. The details for each run are provided in the experimental section. Run 1, a conventional reflux conducted in ethanol, was analyzed by thin-layer chromatography to assess reaction completion after 48 h.

Table 1. Measured TLC R_f values for conventional run 1 crude reaction mixture.

Entry	Possible Intermediate & Pyrazole Structures	Rf (CH2Cl2)	Rf (EtOAc/Hexane)	Relative Spot Size/ Color (1 = Largest; 6 = Smallest)
1/2	or	0.94	0.97	2/Orange
1/2		0.81	0.85	4/Yellow-orange
3		0.46	0.75	1/Pale brown
4		0.36	0.58	3/Pale brown
5/6	or	0.16	0.46	5/Pale brown
5/6		0.08	0.38	6/Pale brown
	Reactants
7		0.06	0.15	Not present
8		0.28	0.66	Not present

provides R_f values for the crude reaction mixture eluted with CH₂Cl₂ and 30:70 v:v EtOAc:Hexane.

As Table 1 shows, the crude reaction mixture contains multiple intermediate and product species formed during the reaction while the starting materials were fully depleted. Based on the spot color on the developed TLC plate, the medium-intensity orange and intense yellow-orange compounds (entries 1/2) were assessed to be regioisomeric arylhydrazone intermediates. This color range and intensity are consistent with the arylhydrazone intermediates reported previously. [15] Entry 3, which we assessed to be the major pyrazole isomer (1b), was by far the largest diameter spot but more brownish in appearance. Entry 4, the minor pyrazole (1a) was much less intense but had a similar brown appearance. Entries 5/6, assessed to be the regioisomeric pyrazol-ols, were the smallest spots and the least intense with a slight brown coloration.

Following the TLC analysis of Run 1, the solvent was removed under reduced pressure. After workup, the crude product mixture (0.484 g, 71% mass balance) was subjected to the first chromatographic separation, which provided three major fractions containing mixtures of compounds 1/2, 3/4, and 5/6, respectively. Each major fraction was then combined, concentrated to dryness, and weighed: compounds 1/2 (0.122g, 18% yield), the pyrazole isomers (compounds 3/4) (0.320 g, 47% yield), and compounds 5/6 (0.020 g, 3% yield). No further attempts were made to isolate the intermediates 1, 2, 5, and 6. The fraction containing the pyrazole isomers was then analyzed by gas chromatography (Figure 3). Based on the areas of the two eluent peaks, 1b constituted 85.4% of the pyrazole isomeric mixture (0.273 g), while 1a comprised 14.6% of the pyrazole mixture (0.047 g). From these data, a 1b:1a ratio of 5.9:1 was determined.

To isolate the individual pyrazoles, a second column chromatographic separation was undertaken using a 10–30% gradient elution of EtOAc/hexane. The second column afforded the pyrazole isomers in two separate fractions: 1b—0.270 g; 1a—0.046 g. Each pyrazole isomer was then recrystallized twice from hot ethanol to give pyrazole 1b (0.236 g, 36%) and pyrazole 1a (0.026 g, 4%) for an overall moderate yield of 40% with a final 1b:1a ratio of 9:1. The unsatisfactory yield of 1a was thus attributed, in part, to the necessity of performing two chromatographic separations and mechanical loss from two recrystallizations to achieve isomeric purity.

To examine whether a microwave-mediated process would change the product distribution, run 2 was conducted in ethanol at 80 °C for a period of 1 h on the variable power mode. The overall yield was only 30%, possibly a result of the lowered reactant concentration compared to run 1 and the brief reaction time (1 h). However, the 1b:1a ratio was determined to be 4:1, a slight decrease from the initial 5.9:1 ratio found for run 1. At present, it is unclear whether the microwave-mediated process contributed to the change in product distribution. The use of tandem radial chromatography afforded satisfactory separation of the hydrazone intermediates (compounds 1 and 2) as the major fractions (0.205 g combined mass), pyrazole isomers 1a (0.020 g), and 1b (0.078 g) as well as the pyrazol-ol intermediates (0.033 g). The pyrazole isomers did not require further purification by recrystallization, thus minimizing mechanical product loss.

For run 3, the microwave reaction time was extended to 2 h on the variable power mode and the volume of ethanol lowered to one-third of that used in run 2. Following μ-wave irradiation at 80 °C, the sample was sonicated at 60 °C for an additional hour. Tandem radial chromatography was employed to isolate the isomeric pyrazole products in a pure form (74% yield). Additional fractions collected were the hydrazone intermediates (compounds 1 and 2 (0.032 g combined mass)) and the pyrazol-ol intermediates 5 and 6 (0.015 g combined mass). It is clear from the small quantities of the intermediates remaining that the microwave-sonication method was more effective than either the conventional reflux or the 1 h microwave-only method in the formation of these fused-ring pyrazoles. These modifications led to a 2.47-fold increase in the product yield over that obtained in run 2. Additionally notable was the reduction in the 1b:1a ratio to 2.1:1.

Improvements observed in the overall yield of compound 1a appear to be due in part to modifications in the μ-wave reaction conditions, the continuation of the reaction under sonication conditions, and the radial chromatographic method used to isolate the pyrazole products. These improvements point to the value of combining μ-wave irradiative and sonication methods to mediate the preparation of pyrazoles. Future studies are warranted to expand the scope of arylhydrazine and other bifunctional nucleophile condensations with 2-acetyl-1,3-indanedione.

3.2. Spectral Characterization

With the target compound 1a isolated, key resonances in the ¹H, ¹³C, and ¹⁹F NMR spectra were used to establish product identity. Figure 4 shows the chemical shift assignments for 1a and 1b that support the product structures. See Supplementary Materials.

The ¹⁹F nuclear magnetic resonances observed for 1a and 1b at −60.06 and −61.00 ppm, respectively, are consistent with a -CF₃ group attached to an aromatic ring, which generally varies from −60–−64 ppm [17]. That there is only a modest ¹⁹F chemical shift variation between the pyrazole isomers is also consistent with the fact that the electronic environments the CF₃ groups encounter are very similar. Evidence in the ¹³C NMR of aromatic trifluoromethyl group presence for 1a was confirmed by CF₃ (C₁) resonance centered at 122.8 ppm, which appeared as a quartet with a ¹J_C-F = 285 Hz, as well as a C-CF₃ (C₂) resonance centered at 128.8 ppm, which appeared as a quartet with a ²J_C-F = 32 Hz. For 1b, the CF₃ (C₁) resonance centered at 124.2 ppm appeared as a quartet with a ¹J_C-F = 272 Hz, as well as a C-CF₃ (C₂) resonance centered at 129.2 ppm, which appeared as a quartet with a ²J_C-F = 32 Hz. The carbonyl carbon (C₃) chemical shifts at 183.0 ppm (1a) and 183.5 ppm (1b) are consistent with a ketone conjugated with an aromatic ring. The pyrazolic CH₃ (H₄) singlet at 2.29 ppm for 1a is slightly upfield relative to the CH₃ (H₄) singlet for 1b at 2.48 ppm. The downfield shift of 1b’s H₄ proton signal may be due in part to the extended conjugation and, therefore, deshielding imparted by the α,β unsaturated ketone structural feature circled in green in Figure 4. This extended conjugation may also influence the C₄ carbon resonance of 1b (12.7 ppm), which is downfield relative to 1a’s C₄ carbon resonance at 12.2 ppm. These chemical shift differences, while small, provide a rationale for the regioisomeric pyrazole assignments.

4. Materials and Methods

4.1. Instrumentation

IR spectra were collected on a Thermo Scientific Nicolet iS5 FT-IR spectrometer (iD5 ATR) with a resolution of 2 cm⁻¹. NMR spectra were collected on a Bruker Avance 300 NMR spectrometer operating at 300.13 MHz for ¹H, 282.40 MHz for ¹⁹F, and 75.47 MHz for ¹³C with d⁶-DMSO as solvent. Hexafluorobenzene served as an internal standard for ¹⁹F NMR chemical shifts. Mass spectral data were collected on a Shimadzu QP 2010S GC-MS instrument. Melting points were obtained on an SRS Digimelt MPA160 apparatus and are uncorrected. Reaction temperatures were maintained using a J-Kem^® 410 temperature controller. Microwave reactions were carried out in a CEM Discover^® microwave reactor using the variable power mode. Sonications were performed using a Branson 1800 sonicator operating at 40 Hz. Combustion analysis was conducted by Atlantic Microlab, Inc., Norcross, GA, USA.

4.2. Chemicals

All chemicals were obtained from Sigma-Aldrich and used as purchased unless otherwise indicated. Thin layer chromatography was accomplished on precut SiO₂ plates, using both CH₂Cl₂ and a 30:70 mixture of EtOAc/Hexane (v:v) as the eluting solvent or solvent mixture. Column chromatography was accomplished using a 130–270 mesh silica gel (Sigma-Aldrich, St. Louis, MO, USA) in a 2” diameter Kontes glass column. Radial chromatography was accomplished on a Chromatotron^TM (Harrison Research, Palo Alto, CA, USA) using 4 mm silica gel/gypsum plates (Analtech, Newark, DE, USA). A gradient elution (20–60% EtOAc/Hexane (v:v)) was employed for radial chromatographic separations.

4.3. Methods for the Preparation and Isolation of 3-Methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one (1a)

4.3.1. Run 1

A 50 mL round-bottom flask equipped with a magnetic stirrer was charged with 25 mL ethanol, 2-acetyl-1,3-indanedione (2 mmol), and 4-trifluoromethylphenylhydrazine (2 mmol). Then, 1 drop of concentrated sulfuric acid was added while stirring. A reflux condenser was affixed, and the reaction mixture was refluxed at 78 °C for 48 h. The resulting solution was analyzed by TLC, evaporated to dryness under reduced pressure and the solid residue was neutralized with 15 mL of saturated Na₂CO₃. This solution was extracted with 3 × 5 mL of CH₂Cl₂, and the organic layer was dried over Na₂SO₄. The solvent was evaporated under reduced pressure to obtain 0.484 g (71% mass balance) of solid products and intermediates. The pyrazole mixture was purified by column chromatography (2× columns): 1st column using CH₂Cl₂ as the mobile phase, followed by a 2nd column using a gradient elution of 10–30% ethyl acetate/hexane.

3-methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one (1a) This compound was obtained as golden-brown crystals (ethanol), 0.026 g (4% yield), m.p. 154–156 °C. FT-IR (ATR): 1708 cm⁻¹ (C=O), 1321 cm⁻¹ (Ar-CF₃). NMR: ¹H δ 2.29 (3H, s), 7.26 (1H, d, ³J_H-H =7.50 Hz), 7.35–7.59 (3H, m), 7.91–7.98 (4H, m). ¹³C δ 12.2, 120.6, 122.6, 122.8 (CF₃, q, ¹J_C-F = 285 Hz), 124.1, 124.6, 125.1, 128.8 (C-CF₃, q, ²J_C-F = 32 Hz), 130.5, 131.8, 133.7, 134.4, 135.5, 139.9, 141.3, 146.7, 157.1, 183.0. ¹⁹F δ −60.06 (s, 3F). MS: m/z 328 (100%, M⁺), 327 (40%, M − H⁺), 259 (5%, M − CF₃⁺). Analysis calculated for C₁₈H₁₁F₃N₂O: C, 65.85, H, 3.38, N, 8.53. Found: C, 65.96, H, 3.43, N, 8.46 (See Supplementary Materials).

3-methyl-2-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(2H)-one (1b) This compound was obtained as brown crystals (ethanol), 0.236 g (36% yield), m.p. 149–152 °C. Physical and spectroscopic data for this compound were previously reported in the literature. [16] FT-IR (ATR): 1705 cm⁻¹ (C=O), 1318 cm⁻¹ (Ar-CF₃). NMR: ¹H δ 2.48 (3H, s), 7.20–7.59 (4H, m), 7.69–7.98 (4H, m). ¹³C δ 12.7, 121.1, 123.1, 124.1, 124.2 (CF₃, q, ¹J_C-F = 272 Hz), 124.6, 125.5, 127.0, 127.5, 127.6, 129.2 (C-CF₃, q, ²J_C-F = 32 Hz), 130.9, 132.3, 134.2, 140.3, 147.2, 157.6, 183.5. ¹⁹F δ −61.00 (s, 3F). MS: m/z 328 (50%, M⁺), 327 (100%, M − H⁺), 259 (15%, M − CF₃⁺). Analysis calculated for C₁₈H₁₁F₃N₂O: C, 65.85, H, 3.38, N, 8.53. Found: C, 66.08, H, 3.42, N, 8.33 (See Supplementary Materials).

4.3.2. Run 2

A 30 mL microwave vial equipped with a magnetic stirrer was charged with 12 mL ethanol, 2-acetyl-1,3-indanedione (1 mmol), and 4-trifluoromethylphenylhydrazine (1 mmol). Then, 1 drop of concentrated sulfuric acid was added while stirring. The vial was capped, placed in the microwave reactor, and irradiated for 1 h at 80 °C using the variable power setting. The resulting solution was evaporated to dryness under reduced pressure and the solid residue (mass 0.340 g, 92% mass balance) was neutralized with 15 mL of saturated Na₂CO₃. This solution was extracted with 3 × 5 mL of CH₂Cl₂, and the organic layer was dried over Na₂SO₄. The solvent was evaporated under reduced pressure, and the pyrazole mixture was purified by tandem radial chromatography (10–30% ethyl acetate/hexane). Fractions collected, in order of elution, were compounds 1/2 (0.205 g), 1b (0.078 g), 1a (0.020 g), and compounds 5/6 (0.033 g).

3-methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one (1a) This compound was obtained as golden-brown crystals, 0.020 g (6% yield). Physical and spectroscopic data for this compound are consistent with that reported in run 1.

3-methyl-2-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(2H)-one (1b) This compound was obtained as brown crystals, 0.078 g (24% yield). Physical and spectroscopic data for this compound are consistent with that reported in the literature [16].

4.3.3. Run 3

A 10 mL microwave vial equipped with a magnetic stirrer was charged with 4 mL of ethanol, 2-acetyl-1,3-indanedione (0.5 mmol), and 4-trifluoromethylphenylhydrazine (0.5 mmol). Then, 1 drop of concentrated sulfuric acid was added while stirring. The vial was capped, placed in the microwave reactor, and irradiated for 2 h at 80 °C using the variable power setting. Following irradiation, the sample was sonicated at 60 °C for an additional hour. The resulting solution was evaporated to dryness under reduced pressure, and the solid residue (mass 0.178 g, 96% mass balance) was neutralized with 15 mL of saturated Na₂CO₃. This solution was extracted with 3 × 5 mL of CH₂Cl₂, and the organic layer was dried over Na₂SO₄. The solvent was evaporated under reduced pressure, and the pyrazole mixture was purified by tandem radial chromatography (10–30% ethyl acetate/hexane). Fractions collected, in order of elution, were compounds 1/2 (0.032 g), 1b (0.087 g), 1a (0.042 g), and compounds 5/6 (0.015 g).

3-methyl-1-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(1H)-one (1a) This compound was obtained as golden-brown crystals, 0.042 g (24% yield). Physical and spectroscopic data for this compound are consistent with that reported in run 1.

3-methyl-2-(4-(trifluoromethyl)phenyl)indeno[1,2-c]pyrazol-4(2H)-one (1b) This compound was obtained as brown crystals, 0.087 g (50% yield). Physical and spectroscopic data for this compound are consistent with that reported in the literature [16].