Optimized Synthesis of Dinitrochalcones via Ultrasonic Bath in a Cyclohexane–Methanol Solvent System

Abstract

This study describes the efficient synthesis of five dinitrochalcones (DNCHs) using an ultrasonic bath as an unconventional method to improve reaction yields and reduce reaction times. The Claisen–Schmidt condensation of nitroacetophenones and nitrobenzaldehydes was carried out in a cyclohexane–methanol solvent system under ultrasonic irradiation, achieving yields between 56% and 92%. The application of ultrasound not only accelerated the reaction but also improved the overall efficiency compared to conventional methods such as magnetic stirring. The synthesized compounds were characterized by NMR spectroscopy, which corroborated their structures. Therefore, it is confirmed that obtaining DNCHs with a nitro group in ortho by ultrasonic irradiation is an energetically efficient and environmentally friendly alternative.

1. Introduction

The synthesis of organic compounds with relevant biological and pharmacological properties is an area of great interest in synthetic chemistry [1,2]. Chalcones—a type of α,β-unsaturated compound—have been shown to have a wide range of biological activities [3,4]. The type of substituent determines the biological activities these molecules possess [5]. Among the substituents, the nitro group (NO₂) stands out, which has the ability to attract electrons in aromatic rings due to resonance and, in some cases, facilitates the interaction with nucleophilic sites of proteins, such as enzymes, leading to their inhibition [6]. As the nitro group—being conjugated to the sp² carbon atom—enhances various manifestations of biological activity, this stimulates the therapeutic confirmation of this functional group [7,8]. In this context, the synthesis of nitro-group-containing chalcones on both aromatic rings is of particular interest since these compounds may exhibit improved biological properties compared to their analogs that do not possess substituents [9]. Specific examples of the pharmacological properties of nitro-group-containing chalcones include anti-inflammatory [9], vasorelaxant [10], cytotoxic [11], anti-hyperglycemic [12], anti-leishmanial [13], and antimicrobial [14] activities (Figure 1).

However, synthesizing these compounds often requires conventional reaction conditions, such as strong acids or bases, which can limit their applicability [15]. This has led researchers to try alternative routes of synthesis, among which are methods that apply green chemistry paradigms, an example of which is ultrasound [16,17].

Ultrasound is a technique that has gained popularity in organic synthesis due to its ability to accelerate and improve the efficiency of chemical reactions [18]. Ultrasound is a form of energy produced when a pressure wave propagates through a medium, such as a liquid or gas. When ultrasound energy is applied to a chemical system, physical and chemical phenomena are generated that can influence the reaction. These phenomena include the formation of gas bubbles, the generation of free radicals, and the creation of more favorable reaction conditions—such as the dissolution of insoluble substances and a reduction in the reaction temperature [19,20]. In this context, chemical reactions induced by ultrasonic waves are carried out by a phenomenon called chemical cavitation [21]. Cavitation involves the formation of micrometer-sized voids or bubbles in a liquid when a sufficient pressure drop alters its cohesive forces. The violent collapse of these bubbles to re-establish intermolecular interactions releases a large amount of energy, which explains the effects mentioned above [22,23]. Rapid nucleation, growth, and collapse are the main characteristics of cavitation, a phenomenon that was discovered accidentally in the 19th century by Thorneycroft and Barnaby [24].

The application of ultrasound in organic synthesis has proven effective in improving the efficiency and selectivity of reactions, reducing reaction times, and eliminating toxic residues. In addition, ultrasound can be used to synthesize compounds that cannot be obtained by conventional methods, such as synthesizing compounds that require extreme reaction conditions or synthesizing compounds that are unstable under normal conditions [25,26,27].

In previous work, we reported the synthesis of three chalcone isomers using conventional mechanical stirring conditions, as well as the use of a strong base such as sodium hydroxide in methanol at 0 °C (Figure 2) [28].

The objective of this work was to perform the synthesis of five dinitrosubstituted chalcones in both aromatic rings A and B (DNCHs, compounds 3a–e) (Figure 2) through Claisen–Schmidt condensation between o-, m-, and p-nitroacetophenones and the corresponding o-, m-, and p-nitrobenzaldehydes using a non-conventional method by irradiation with an ultrasound bath. This approach seeks to decrease the drastic reaction conditions and to be aligned with the green chemistry paradigm.

2. Materials and Methods

2.1. General Approach

The reagents utilized in the synthesis and spectroscopic characterization of the DNCHs were of analytical and spectroscopic grade Sigma-Aldrich (Toluca, México) and used without further purification. The reaction progress was monitored using thin-layer chromatography (TLC) on 0.2-mm silica-gel-coated aluminum plates with a fluorescent indicator, F254 Merck (Toluca, México). The mobile phase used for the TLC analysis was a 7:3 mixture of hexane and ethyl acetate for all the examples presented herein. The reported melting points were determined in an SMP 10 (STUART) melting point apparatus (Bibby Scientific, Staffordshire, UK) and not corrected. NMR spectra were obtained using a Bruker Ascend^® 600 MHz NMR spectrometer (Billerica, MA, USA). Chemical shifts are expressed in parts per million (ppm) and referenced to TMS as an internal standard, while coupling constants (J) are expressed in Hertz. The ultrasonic irradiation generator, with a power of 100 W, was a Cole Parmer ultrasonic cleaner model 08891-21 (Vernon Hills, IL, USA). Infrared spectra were obtained using an Agilent Cary 630 FTIR Diamond ATR (Santa Clara, CA, USA).

2.2. Procedure for the Synthesis of Dinitrochalcones (DNCHs)

The procedure for synthesis of the DNCHs was carried out by Claisen–Schmidt condensation between the respective o-nitroacetophenone (1a), m-nitroacetophenone (1b), p-nitroacetophenone (1c), and the respective o-nitrobenzaldehyde (2a), m-nitrobenzaldehyde (2b), p-nitrobenzaldehyde (2c). All nitroacetophenones and nitrobelzaldehydes were equimolar in all reactions.

The technique employed included adding 10 mmol of the corresponding nitroacetophenones and their carbonate equivalents. The mixture was then solubilized with 10 mL of methanol in a 50 mL flask until the nitroacetophenone was completely dissolved. Next, 10 mmol of the corresponding benzaldehyde was added. Subsequently, 1 mL of cyclohexane was added and subjected to ultrasound irradiation for the time and at the temperature specified in the results table. Once the reaction was finished, the solid obtained was filtered and washed with cold water. Finally, the product was purified following a methodology previously reported [28] for the conventional method of dinitrochalcone synthesis through solvent-pair (dichloromethane/n-hexane) recrystallization.

3a (E)-1,3-bis(2-nitrophenyl)prop-2-en-1-one. A total of 56% was obtained as a white solid; m.p.: 140–142 °C [in the lit., 136–137 °C] [29]; ¹H NMR (600 MHz, DMSO-d₆) δ = 8.25 (d, J = 8.2 Hz, 1H), 8.08 (d, J = 8.1 Hz, 1H), 8.01 (d, J = 7.8 Hz, 1H), 7.95 (t, J = 7.5 Hz, 1H), 7.83 (m, 2H), 7.76 (d, J = 7.5 Hz, 1H), 7.69 (t, J = 7.9 Hz, 1H), 7.66 (d, J = 16.1 Hz, 1H), 7.24 (d, J = 16.1 Hz, 1H); DEPTQ NMR (150 MHz, DMSO-d₆) δ = 193.3, 149.3, 147.4, 142.1, 135.8, 135.5, 134.9, 132.7, 132.4, 130.5, 130.1, 125.8, 125.5, 124.3, 124.1. FTIR λ_max/cm⁻¹: 1655 (C=O), 1515 (C=C), 1330 (N—O), 970 (C=C trans).

3b (E)-1-(2-nitrophenyl)-3-(3-nitrophenyl)prop-2-en-1-one. A total of 92% was obtained as a white solid; m.p.: 145–147 °C [in the lit., 143–145 °C] [29]; ¹H NMR (600 MHz, DMSO-d₆) δ = 8.64 (s, 1H), 8.29 (m, 3H), 7.97 (t, J = 7.5 Hz, 1H), 7.88 (t, J = 7.8 Hz, 1H), 7.79 (d, J = 7.5 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.63 (d, J = 16.3 Hz, 1H), 7.56 (d, J = 16.3 Hz, 1H); DEPTQ NMR (150 MHz, DMSO-d₆) δ = 193.1, 149.3, 147.5, 144.1, 136.9, 136.1, 135.4, 132.5, 131.3, 130.0, 129.1, 125.9, 125.5, 124.5, 123.9. FTIR λ_max/cm⁻¹: 1651 (C=O), 1522 (C=C), 1345 (N—O), 983 (C=C trans).

3c (E)-1-(2-nitrophenyl)-3-(4-nitrophenyl)prop-2-en-1-one. A total of 86% was obtained as a yellow solid; m.p.: 175–177 °C [in the lit., 168–169 °C] [29]; ¹H NMR (600 MHz, DMSO-d₆) δ = 8.23 (m, 3H), 8.03 (d, J = 8.8 Hz, 2H), 7.93 (t, J = 7.5 Hz, 1H), 7.84 (t, J = 7.7 Hz, 1H), 7.76 (d, J = 7.5 Hz, 1H), 7.53 (d, J = 16.2 Hz, 1H), 7.48 (d, J = 16.2 Hz, 1H); DEPTQ NMR (150 MHz, DMSO-d₆) δ = 193.0, 149.3, 147.5, 143.6, 141.3, 135.9, 135.5, 132.6, 130.8, 130.2, 130.0, 125.6, 124.9. FTIR λ_max/cm⁻¹: 1664 (C=O), 1508 (C=C), 1333 (N—O), 980 (C=C trans).

3d (E)-3-(2-nitrophenyl)-1-(3-nitrophenyl)prop-2-en-1-one. A total of 65% was obtained as a white solid; m.p.: 160–162 °C [in the lit., 159–161 °C] [29]; ¹H NMR (600 MHz, DMSO-d⁶) δ 9.35 (t, J = 1.9 Hz, 1H), 9.09 (m, 1H), 9.02 (m, 1H), 8.73 (dd, J = 7.9, 1.3 Hz, 1H), 8.62 (dd, J = 8.2, 1.1 Hz, 1H), 8.57 (d, J = 15.5 Hz, 1H), 8.46 (d, J = 15.5 Hz, 1H), 8.38 (m, 2H), 8.24 (m, 1H); RMN DEPTQ (150 MHz, DMSO-d₆) δ = 188.6, 149.7, 149.2, 140.9, 139.2, 135.8, 134.7, 132.2, 131.6, 130.6, 130.4, 128.6, 126.8, 125.7, 124.6. FTIR λ_max/cm⁻¹: 1653 (C=O), 1522 (C=C), 1336 (N—O), 970 (C=C trans).

3e (E)-1,3-bis(3-nitrophenyl)prop-2-en-1-one. A total of 88% was obtained as a yellow solid; p.f.: 214–216 °C [in the lit., 210–211 °C] [29,30]; ¹H NMR (600 MHz, DMSO-d₆): δ 8.87 (s, 1H), 8.81 (s, 1H), 8.64 (d, J = 7.7 Hz, 1H), 8.52 (m, 1H), 8.38 (d, J = 7.7 Hz, 1H), 8.30 (m, 1H), 8.22 (d, J = 15.7 Hz, 1H), 7.94 (d, J = 15.7 Hz, 1H), 7.90 (t, J = 6 Hz, 1H), 7.77 (t, J = 8.0 Hz, 1H); RMN DEPTQ (150 MHz, DMSO-d₆) δ = 188.6, 149.4, 149.2, 143.8, 139.4, 137.3, 136.2, 135.8, 131.6, 131.3, 128.5, 125.9, 125.2, 124.2, 123.9. FTIR λ_max/cm⁻¹: 1660 (C=O), 1524 (C=C), 1329 (N—O), 981 (C=C trans).

3. Results

Initially, we proceeded to optimize the reaction conditions avoiding drastic conditions, taking as a starting point the reaction of m-nitroacetophenone (1b) and m-nitrobenzaldehyde (2b) to obtain m,m-DNCH (3e) reported by Wei and coworkers [30]; for the optimization of the reaction, several variations in the reaction conditions were tested, which are summarized in

Table 1. Optimization of reaction conditions for compound 3e.

Entry	Activation Method	Reaction Conditions	Solvent	Time	Yield
1 *	Magnetic stirring	NaOH (0.6 Equiv.), rt	Methanol	180	33
2 *	Reflux	NaOH (2 Equiv.)	Methanol	240	25
3 *	Reflux	K2CO3 (0.3 Equiv.)	Methanol	240	29
4	Ultrasound	K2CO3 (0.3 Equiv.)	Methanol	60	22
5	Ultrasound	K2CO3 (0.4 Equiv.)	Methanol	60	25
6	Ultrasound	K2CO3 (0.4 Equiv.)	Methanol/ cyclohexane	60	52
7	Ultrasound	K2CO3 (0.6 Equiv.)	Methanol/ cyclohexane	60	49
8	Ultrasound	K2CO3 (0.9 Equiv.)	Methanol/ cyclohexane	60	44
9	Ultrasound	Na2CO3 (0.4 Equiv.)	Methanol/ cyclohexane	30	88
10	Ultrasound	Li2CO3 (0.4 Equiv.)	Methanol/ cyclohexane	30	48
11	Ultrasound	Cs2CO3 (0.4 Equiv.)	Methanol/ cyclohexane	15	80
12	Ultrasound	CaCO3 (0.4 Equiv.)	Methanol/ cyclohexane	60	----

The experiments were carried out at a temperature of 60 °C (experiments were performed in the ultrasonic bath at 60 °C, which remained a constant temperature). The reaction yields of all experiments were “isolated yields”. * Magnetic stirring.

.

By way of comparison, a protocol was established where conventional conditions were established, using reflux (entries 2 and 3). The effect of the base was tested by comparing the effectiveness of the reaction in the presence of NaOH, K₂CO₃, Na₂CO₃, Li₂CO₃, Cs₂CO₃, and CaCO₃.

The same methanol/cyclohexane mixture was employed as the reaction medium for synthesizing the remaining isomeric DNCH. However, experimental temperature modifications were necessary to optimize the yield of the reactions. The optimum conditions for obtaining each product are summarized in Figure 3 and

Table 2. DNCH synthesized by ultrasonication.

DNCH	Time (min)	Temperature (°C)	Yield (%)	m.p. (°C)
3a	60	0	56	140–142
3b	60	0	92	145–147
3c	60	0	86	175–177
3d	60	0	65	160–162
3e	30	60	88	214–216

.

4. Discussion

The synthesis of DNCHs using ultrasound, presented in this work, stands out for avoiding conventional reaction conditions, aligning with principle 6 of green chemistry (energy efficiency) [31]. The optimization of experimental conditions, detailed in Table 1 and Table 2, show several observations regarding reaction efficiency and factors influencing yields, although the experiments initially included magnetic stirring and refluxing (Table 1)—conventional synthesis methods—to establish a point of comparison with ultrasonic activation. Magnetic stirring, performed at room temperature, and refluxing at elevated temperatures, are widely used in organic synthesis due to their ability to promote chemical reactions under controlled conditions efficiently. However, these methods often require long reaction times and can result in low yields, confirmed in this work. For example, magnetic stirring with NaOH did not produce high yields (33%), while refluxing produced slightly decreased yields (25% and 29% with NaOH and K₂CO₃, respectively, in 240 min). These results show the limitations of conventional methods specifically for these molecules.

In contrast, ultrasound irradiation reduced the reaction time to 60 min or less, increasing the yields in most of the experiments carried out by this non-conventional technique. Ultrasonic cavitation and the generation of free radicals may explain this improvement in efficiency [32]. These effects facilitate the cleavage and formation of bonds, promoting Claisen–Schmidt condensation between the nitroacetophenones and nitrobenzaldehydes.

The use of different bases also had a crucial impact on the reaction yields. Optimization showed that the most effective base was Na₂CO₃, with a yield of 88% (entry 9), while other bases such as K₂CO₃ and Li₂CO₃ resulted in less efficient yields of 49% and 48%, respectively. This behavior may be related to the strength and solubility of the bases in the reaction system. Thus, Na₂CO₃ appears to provide an optimal balance between the catalysis of the reaction and the formation of undesired side products, a critical aspect to ensure the selectivity of the synthesis.

The solvent played a crucial role in optimizing the yields. The combination of methanol and cyclohexane markedly increased the yields compared to using methanol alone, as observed when contrasting entries 5 and 6 (25% vs. 52%). Cyclohexane probably acted as a co-solvent, enhancing the solubilization of reactants and facilitating better interactions between the reagents and the base, although experiments are still needed to verify this claim. However, something similar has been reported previously, where cyclohexane, in solvent mixtures, is used as a surfactant to improve the efficiency of reactions by increasing the accessibility of the substrate to the catalysts [33,34]. This surface effect is a concrete example of how solvents do not only dilute the reactants but also directly influence the reaction dynamics.

On the other hand, Table 2 presents the yields of DNCHs synthesized under optimal ultrasound conditions. An analysis of these data reveals a remarkable influence of the position of the nitro groups in the aromatic rings on the yields and reaction conditions. Chalcones with at least one nitro group in the ortho position (3a–3d) show yields between 56% and 92%, while the chalcone (3e) with nitro groups in the meta position on both rings reached a yield of 88%, although it required a higher temperature (60 °C) to optimize the reaction in only 30 min. This behavior can be attributed to the inductive effect of the nitro group, which increases the electrophilic character of the carbonyl carbon, favoring condensation, while the higher temperature required for (3e) could be related to a lower reactivity of the target positions.

Compound (3b), with a nitro group in the ortho position on ring A and the meta position on ring B, presented the highest yield (92%). This structural configuration may generate a favorable combination of inductive and steric effects that optimize the reactivity of the reagents. The reactivity also seems to be influenced by the possibility that the nitro groups in the ortho position facilitate the formation of intramolecular interactions, reducing the energetic barriers of the reaction.

Proton Nuclear Magnetic Resonance (¹H NMR) and DEPTQ experiments confirmed the proposed structures of the DNCHs. In the ¹H NMR spectra, two double signals characteristic of the α,β-unsaturated system were observed, with coupling constants ranging between J = 15.5 Hz and J = 16.3 Hz, values consistent with a trans configuration. On the other hand, in the DEPTQ spectrum, the quaternary carbons of the molecules are identified, which appeared in a negative phase, supporting the structural assignment. The ¹H nuclear magnetic resonance (NMR) and Fourier transform infrared spectra are presented in Figures S1–S10 and Figures S11–S15, respectively (see Supplementary Materials).

The ultrasound-mediated synthesis of these DNCHs could represent a promising starting material for the construction of more complex compounds, such as those containing heterocycles. This is because the nitro group can be reduced, thus facilitating its subsequent cyclization or expanding the possibilities of structural functionalization [35].

5. Conclusions

The synthesis of five isomers of DNCHs (3a–e) was achieved under mild reaction conditions, using ultrasound, and adding a co-solvent that optimizes the interaction of the organic phase with the basic catalyst. The spectroscopic characterization of the obtained products agrees with the expected structures. Differences were found in the reaction conditions necessary to optimize the synthesis of each of the compounds, which is attributed, in principle, to the electron-attracting effect exerted by the nitro group on the aromatic systems that depends on the relative position of the nitro group concerning the corresponding carbonyl. The reaction yields were improved for compounds (3a–c) compared to the conventional methodology reported above. At the same time, compounds 3d and 3e also obtained good yields, proving that this ultrasound synthesis methodology is effective for these DNCHs.