A Simple and Effective Protocol for the Pechmann Reaction to Obtain 4-Methylcoumarin Derivatives Using a High-Speed Mixer Ball Mill Process

: We hereby report a simple and efﬁcient method for the preparation of 4-methylcoumarins series, including Coumarin 120 (7-amino-4-methylcoumarin) from phenols (or naphthols) and ethyl acetoacetate in the presence of 3 mol% InCl 3. Coumarins were obtained in good yields (52–92%) through Pechmann condensation, under a rapid and environmentally friendly protocol using a high-speed ball mill mixer at room temperature, with short reaction times

These methods usually require harsh conditions (e.g., stoichiometric amounts of strong acids or bases, no reusability of catalyst, production of large acidic wastes, elevated temperature, and prolonged reaction time) with poor regioselectivity, limiting the substrate scope, or protocols that lead into a difficult purification process. Therefore, each developed protocol for coumarins preparation comes with these disadvantages and advantages. However, according to the significance of green and sustainable protocols, there is a perceptible need to develop new, eco-friendly, and simple procedures through a keen investigation of the existing strategies. These methods usually require harsh conditions (e.g., stoichiometric amounts of strong acids or bases, no reusability of catalyst, production of large acidic wastes, elevated temperature, and prolonged reaction time) with poor regioselectivity, limiting the substrate scope, or protocols that lead into a difficult purification process. Therefore, each developed protocol for coumarins preparation comes with these disadvantages and advantages. However, according to the significance of green and sustainable protocols, there is a perceptible need to develop new, eco-friendly, and simple procedures through a keen investigation of the existing strategies.
In this context, Pechmann condensation still attracts a lot of attention from synthetic chemists. Indeed, recently, various interesting protocols for coumarin ring construction based on green chemistry principles have been reported [36][37][38][39][40][41][42][43][44]. These procedures do not only focus on the effectiveness of a catalyst or its reusability but also its safety for both mankind and the environment using several alternative physical methods. Methodologies for the Pechmann reaction include microwave, sonication, photocatalysis, and mechanochemical approaches. The last one, i.e., mechanochemical activation, is the main current approach in free-solvent synthetic chemistry and relies on the application of mechanical energy produced from grinding or milling processes to achieve chemical transformations [45][46][47][48][49][50][51].
Despite the significance of the Pechmann condensation for the preparation of bioactive substituted 4-methylcoumarins (4-methyl-2H-chromen-2-ones) and the valuableness of a mechanochemical approach for chemical transformations as a greener way, there are very little synthetic methods that apply to the mechanochemical methodology to promote this reaction, in which only grinding ("in a mortar by a pestle" by a hand) and ball-milling (BM "in mill apparatus" by grinding balls) techniques were used ( Figure 2). Using "porcelain mortar-pestle" as the manual grinding tool, Indian researchers described for the first time the synthesis of coumarins via Pechmann condensation under solvent-free conditions by applying a grinding technique. They reported higher yields of the obtained 4methylcoumarin derivatives; however, two equivalents of catalysts (p-TsOH and P2O5) to the starting phenols were used [41][42][43]. It is known that manual grinding, carried out by a person, is a labor-intensive and non-automatic process, which often raises some concerns, including the kinetic parameters of the reaction and the reproducibility [51]. Although the hand-grinding technique is an easy and inexpensive approach, mechanical milling using a mixer mill or planetary mill by grinding balls (ball-milling) is becoming a more promising green tool for the synthesis of organic molecules, including heterocyclic In this context, Pechmann condensation still attracts a lot of attention from synthetic chemists. Indeed, recently, various interesting protocols for coumarin ring construction based on green chemistry principles have been reported [36][37][38][39][40][41][42][43][44]. These procedures do not only focus on the effectiveness of a catalyst or its reusability but also its safety for both mankind and the environment using several alternative physical methods. Methodologies for the Pechmann reaction include microwave, sonication, photocatalysis, and mechanochemical approaches. The last one, i.e., mechanochemical activation, is the main current approach in free-solvent synthetic chemistry and relies on the application of mechanical energy produced from grinding or milling processes to achieve chemical transformations [45][46][47][48][49][50][51].
Despite the significance of the Pechmann condensation for the preparation of bioactive substituted 4-methylcoumarins (4-methyl-2H-chromen-2-ones) and the valuableness of a mechanochemical approach for chemical transformations as a greener way, there are very little synthetic methods that apply to the mechanochemical methodology to promote this reaction, in which only grinding ("in a mortar by a pestle" by a hand) and ballmilling (BM "in mill apparatus" by grinding balls) techniques were used ( Figure 2). Using "porcelain mortar-pestle" as the manual grinding tool, Indian researchers described for the first time the synthesis of coumarins via Pechmann condensation under solvent-free conditions by applying a grinding technique. They reported higher yields of the obtained 4-methylcoumarin derivatives; however, two equivalents of catalysts (p-TsOH and P 2 O 5 ) to the starting phenols were used [41][42][43]. It is known that manual grinding, carried out by a person, is a labor-intensive and non-automatic process, which often raises some concerns, including the kinetic parameters of the reaction and the reproducibility [51]. Although the hand-grinding technique is an easy and inexpensive approach, mechanical milling using a mixer mill or planetary mill by grinding balls (ball-milling) is becoming a more promising green tool for the synthesis of organic molecules, including heterocyclic compounds and condensation reactions [46]. Recently, a new protocol for the synthesis of simple coumarins under ball milling at an ambient temperature in the presence of methanesulfonic acid (MsOH) was developed by Sharapov et al. [44]. To obtain 4-methylcoumarin molecules with good yields, a Retsch PM 100 swing mill including 10 mL stainless steel ball mill jars and ten stainless steel balls 5 mm in diameter were used, but Pechmann condensations were carried out at 2 h, which is quite slow for this type of technique. Therefore, this topic is still an interesting research open window to investigate in the organic synthesis field. simple coumarins under ball milling at an ambient temperature in the presence of methanesulfonic acid (MsOH) was developed by Sharapov et al. [44]. To obtain 4-methylcoumarin molecules with good yields, a Retsch PM 100 swing mill including 10 mL stainless steel ball mill jars and ten stainless steel balls 5 mm in diameter were used, but Pechmann condensations were carried out at 2 h, which is quite slow for this type of technique. Therefore, this topic is still an interesting research open window to investigate in the organic synthesis field. Considering the above-stated facts and our constant efforts to study the synthesis of new bioactive small heterocyclic molecules under sustainable reaction conditions [52][53][54][55], we developed a simple green procedure for the synthesis of 4-methylcoumarin derivatives, including 7-amino-4-methylcoumarin This molecule has been considered an important molecule and building block in biophysical research as a common fluorescent probe [56,57].
Thus, in this work, we described a practical method for the synthesis of the title compounds via an InCl3-catalyzed Pechmann condensation. As an important novelty of our research, we carried out coumarins synthesis under solvent-free conditions, using a highspeed mixer ball mill process with a milling frequency of 60 Hz, Teflon ball mill jars, and single stainless-steel balls 8 mm in diameter. Considering the above-stated facts and our constant efforts to study the synthesis of new bioactive small heterocyclic molecules under sustainable reaction conditions [52][53][54][55], we developed a simple green procedure for the synthesis of 4-methylcoumarin derivatives, including 7-amino-4-methylcoumarin This molecule has been considered an important molecule and building block in biophysical research as a common fluorescent probe [56,57].
Thus, in this work, we described a practical method for the synthesis of the title compounds via an InCl 3 -catalyzed Pechmann condensation. As an important novelty of our research, we carried out coumarins synthesis under solvent-free conditions, using a high-speed mixer ball mill process with a milling frequency of 60 Hz, Teflon ball mill jars, and single stainless-steel balls 8 mm in diameter.

Materials and Instruments
The reagents and solvents used in the synthesis of intermediate and final compounds were of purity grade for synthesis. All reagents were purchased from Merck, J.T. Baker, Sigma, and Aldrich Chemical Co. and used without further purification. Reactions monitoring the purity of the final product were followed by thin layer chromatography (TLC) on Silufol UV254 plates of a 0.25 mm thickness. TLC plates were revealed in a UV light chamber of 254 nm or in an ethanolic solution of phosphomolybdic-sulfuric acids. Uncorrected melting points were measured in a Fisher-Johns melting point apparatus.
Reactions were carried out in a mixer mill apparatus (amalgamator Zenith, Z-1A) at a frequency of 60 Hz at room temperature using grinding jars of different materials (Nylon, Teflon, or stainless steel of 1.5-2 mL volume) and a single stainless-steel ball with different sizes: d = 5 mm (0.6401 g) and 8 mm (2.1031 g) (See, ESI: Table S1). All vessels were applied for 1-3 mmol runs.
The acquisition of nuclear magnetic resonance spectra 1 H and 13 C was performed in a Bruker Avance-400 spectrometer (400 MHz for 1 H and 101 MHz for 13 C). Chemical shifts were reported in ppm with solvent resonance as the internal standard (DMSO-d6: δ 2.50 ppm). Coupling constants (J) are given in Hz; a multiplicity of signals was expressed by the following abbreviations: (s) singlet, (d) doublet, (dd) doublet of doublets, and (m) multiplet.
Infrared spectra were recorded on an FTIR Bruker Tensor 27 spectrophotometer coupled to a Bruker platinum with an attenuated reflectance (ATR) cell at 31 scans and a 2 cm −1 resolution. Elemental analyses were performed on a Thermo Scientific CHNS-O analyzer (Model. Flash 2000) and were within ±0.4 of theoretical values.

General Procedure for the Synthesis of 4-Methylcoumarin Derivatives 3a-g
In a cylindrical Teflon vessel (8.1573 g, 2 mL), the respective phenol 1a-g (2.85 mmol) ethyl acetoacetate 2 (2.84 mmol) and InCl 3 (3 mol%) were added. Subsequently, a stainlesssteel ball of 8 mm in diameter with a weight of 2.1031 g was incorporated into the reaction mixture. Once the reactor was carefully sealed, it was subjected to a vibration of 60 Hz at a determined time in a mixer mill apparatus (amalgamator Zenith, Z-1A). This reaction was monitored by TLC, and once completed, the crude was poured onto crushed ice, obtaining the solids corresponding to products 3a-g, which were subsequently recrystallized in ethanol. All yields refer to the isolated products (Table 1). A total of 2 mL Teflon ball mill vessels were applied for 1-3 mmol runs.  13

Results and Discussion
The wide application of the Pechmann reaction arises from exceptionally simple procedures and reaction conditions using easily available simple reactants, substituted phenols and β-ketoesters (mainly ethyl acetoacetate) in the presence of acid catalysts [25]. Since, in our research, we needed 7-amino-4-methylcoumarin (coumarin 120), a rather expensive reagent, we resorted to the Pechmann condensation. Its preparation according to modified protocols based on this reaction is available in the chemical literature [58][59][60][61][62]. However, although we were able to reproduce a protocol synthesizing the desired coumarin via a three-step method proposed by Pozdnev [62,63] (See ESI, Scheme S1), we were not satisfied with the obtained results. Therefore, we decided to study the reaction of 3-aminophenol 1 and ethyl acetoacetate 2 to obtain straightforward 7-amino-4-methylcoumarin 3 (Scheme 1). Then, looking for the best green reaction conditions under solvent-free methodologies, we used a high-speed mixer ball mill (HSMBM) process at room temperature.

Results and Discussion
The wide application of the Pechmann reaction arises from exceptionally simple procedures and reaction conditions using easily available simple reactants, substituted phenols and β-ketoesters (mainly ethyl acetoacetate) in the presence of acid catalysts [25]. Since, in our research, we needed 7-amino-4-methylcoumarin (coumarin 120), a rather expensive reagent, we resorted to the Pechmann condensation. Its preparation according to modified protocols based on this reaction is available in the chemical literature [58][59][60][61][62]. However, although we were able to reproduce a protocol synthesizing the desired coumarin via a three-step method proposed by Pozdnev [62,63] (See ESI, Scheme S1), we were not satisfied with the obtained results. Therefore, we decided to study the reaction of 3aminophenol 1 and ethyl acetoacetate 2 to obtain straightforward 7-amino-4-methylcoumarin 3 (Scheme 1). Then, looking for the best green reaction conditions under solventfree methodologies, we used a high-speed mixer ball mill (HSMBM) process at room temperature. First, following the tutorial review of Stolle et al. on ball mills as chemical reactors [45], we studied several important technical variables of the ball mill process (frequency of vibration process, type of milling material, size of milling balls and milling time), in addition to the most important chemical parameter (nature of catalyst) for this model reaction ( Figure 3). Our selection of the ball milling apparatus used fell to the Zenith amalgamator because it was simple to operate, cheap and affordable, and had three rotational speed ranges (4800, 4200, and 3600 rpm). We decided to carry out all planned reactions at a milling frequency of 60 Hz (i.e., working at 3600 rpm) to avoid secondary or consecutive reactions which could occur at higher frequencies (higher energies) [45]. In the initial stage, we used Duralon jars and a single stainless-steel ball with a diameter of 5 mm Scheme 1. The model reaction of 1a and 2 to obtain 7-amino-4-methylcoumarin 3a and evaluated reaction parameters of the HSMBM process.
First, following the tutorial review of Stolle et al. on ball mills as chemical reactors [45], we studied several important technical variables of the ball mill process (frequency of vibration process, type of milling material, size of milling balls and milling time), in addition to the most important chemical parameter (nature of catalyst) for this model reaction ( Figure 3). Our selection of the ball milling apparatus used fell to the Zenith amalgamator because it was simple to operate, cheap and affordable, and had three rotational speed ranges (4800, 4200, and 3600 rpm). We decided to carry out all planned reactions at a milling frequency of 60 Hz (i.e., working at 3600 rpm) to avoid secondary or consecutive reactions which could occur at higher frequencies (higher energies) [45]. In the initial stage, we used Duralon jars and a single stainless-steel ball with a diameter of 5 mm (0.6401 g). Based on our own experience with the solvent-less synthesis of heterocycles, which was catalyzed by amorphous milled cellulose sulfonic acid (AMCell-SO 3 H) [64], this promoter was the first choice. However, even the milling times (45 min) in the presence of this acid catalyst did not promote the formation of the desired product 3a ( Figure 3A,  entries 1-3). Similarly, it should be noted that when this catalyst was used in the traditional methodology of the Pechmann reaction, it did not lead to the synthesis of 3a. Therefore, we presumed that this unsuccessful result could be due to a lack of H + ions in the media. This kind of solid support promoter can be used in Pechmann condensation under free solvent conditions; however, high temperatures (80-160 • C) or another additional energy source, such as an ultrasound, is necessary [26]. On the other hand, Lewis acids have been widely used as efficient catalysts in the conventional synthesis of 4-methylcoumarins, BiCl 3, and InCl 3 [65,66]. Therefore, we decided to use these compounds where bismuth(III) chloride (10 mol%) afforded the coumarin 3a in a poor yield (29%) (Entry 4), while indium(III) chloride with the same load gave 3a at a considerable yield (70%) highlighting that the milling time was only 20 min (Entry 5). Likewise, it was decided to reduce the InCl 3 load (from 10 to 5 mol%) in the reaction, noting only a small decrease in its yield, which was not considered significant. Taking into account the fact that the grinding time was not increased and that only half of the catalyst originally used was effective (Entry 6), we used these reaction parameters to follow the optimization. Then, despite the fact that the desired 7-amino-4-methylcoumarin 3a was successfully obtained, it was observed how the precursors reacted with the vessel's material, drastically deteriorating it. Therefore, it was necessary to change the reactors for others to be totally inert to the reagents. under free solvent conditions; however, high temperatures (80-160 °C) or another a tional energy source, such as an ultrasound, is necessary [26]. On the other hand, L acids have been widely used as efficient catalysts in the conventional synthesis methylcoumarins, BiCl3, and InCl3 [65,66]. Therefore, we decided to use these compo where bismuth(III) chloride (10 mol%) afforded the coumarin 3a in a poor yield ( (Entry 4), while indium(III) chloride with the same load gave 3a at a considerable (70%) highlighting that the milling time was only 20 min (Entry 5). Likewise, it wa cided to reduce the InCl3 load (from 10 to 5 mol%) in the reaction, noting only a s decrease in its yield, which was not considered significant. Taking into account the that the grinding time was not increased and that only half of the catalyst originally was effective (Entry 6), we used these reaction parameters to follow the optimiza Then, despite the fact that the desired 7-amino-4-methylcoumarin 3a was successfull tained, it was observed how the precursors reacted with the vessel s material, drasti deteriorating it. Therefore, it was necessary to change the reactors for others to be to inert to the reagents. With these findings, we studied Pechmann condensation and the influence o milling materials nature, changing Duralon vessels with stainless steel and Teflon mi jars ( Figure 3B). When 5 mol% InCl3-catalyzed condensations under the same tech parameters of the ball mill process were realized, we observed a strong difference in reaction effectiveness. Then, operating with stainless steel jars, a large decrease in th action yield and an increase in the milling time were recorded, compared to the Dur vessel ( Figure 3B, entries 1 and 2, respectively). By contrast, in using a Teflon reactor, b With these findings, we studied Pechmann condensation and the influence of the milling materials' nature, changing Duralon vessels with stainless steel and Teflon milling jars ( Figure 3B). When 5 mol% InCl 3 -catalyzed condensations under the same technical parameters of the ball mill process were realized, we observed a strong difference in the reaction effectiveness. Then, operating with stainless steel jars, a large decrease in the reaction yield and an increase in the milling time were recorded, compared to the Duralon vessel ( Figure 3B, entries 1 and 2, respectively). By contrast, in using a Teflon reactor, better results were obtained since the reaction was completed in just 15 min, and its yield increased to 82% (Entry 3). Teflon jars are resistant to reagents and therefore do not react with the precursors used, as well as lightweight, which allows the equipment to remain vibrating steadily. Conversely, using a stainless-steel capsule, a greater effort in the amalgamator to work normally was observed experimentally. This was due to the greater weight of the steel capsule. When using light containers, the activation energy was low, and the reactions were driven toward the formation of thermodynamically stable products, unlike what happens when the vessels are dense, requiring more energy to reach comparable levels of product yield [45].
In the next stage, with the parameters found so far, and a frequency at 60 Hz, the Teflon reactor, and InCl 3 as the catalyst, we proceeded to study the behavior of the reaction, varying the size of the grinding body because it is known that larger balls lead to an increased energy input per collision [45,67] For this, stainless steel balls of 5 and 8 mm in diameter were used ( Figure 3C). As can be seen in this Figure, by reducing the catalyst to 3 mol% and using the smallest ball (d, 5 mm), the reaction took twice as long, and its yield decreased (Entry 2 vs. Entry 1). Unlike the observation when an 8 mm diameter ball was used, product 3a was obtained with a 92% yield in just 8 min (Entry 3 vs. Entry 1).
Moreover, despite the catalyst amount being reduced, coumarin was isolated in the same yield after 10 min of grinding (Entry 4 vs. Entry 3). These results suggest that it is very likely that the increase in the size of the ball could counteract the decrease in the catalyst since its contact surface is greater and facilitates the crushing of the precursors. Considering these observations, we proceeded to decrease the InCl 3 load to 1.5 mol%. However, the grinding time tripled, and its yield dropped to 48% (Entry 5). Likewise, when the reaction was carried out in the absence of a catalyst, it was found that the precursors remained unreacted after 45 min (Entry 6).
Therefore, the study of all these variables on a model reaction led us to develop a high-speed mixer ball mill protocol for Pechmann condensation. To evaluate its scope, other substituted phenols 1b-e (naphthols 1f-g) and ethyl acetoacetate 2, were used as the main precursors to obtain corresponding 4-methylcoumarins 3b-g in good yields under solvent-free conditions and at room temperature ( Figure 4). It is important to note that no reported ball milling or grinding procedure [41][42][43][44] offered the direct preparation of 7-amino-4-methylcoumarin.
In the next stage, with the parameters found so far, and a frequency at 60 Hz, the Teflon reactor, and InCl3 as the catalyst, we proceeded to study the behavior of the reaction, varying the size of the grinding body because it is known that larger balls lead to an increased energy input per collision [45,67] For this, stainless steel balls of 5 and 8 mm in diameter were used ( Figure 3C). As can be seen in this Figure, by reducing the catalyst to 3 mol% and using the smallest ball (d, 5 mm), the reaction took twice as long, and its yield decreased (Entry 2 vs. Entry 1). Unlike the observation when an 8 mm diameter ball was used, product 3a was obtained with a 92% yield in just 8 min (Entry 3 vs. Entry 1).
Moreover, despite the catalyst amount being reduced, coumarin was isolated in the same yield after 10 min of grinding (Entry 4 vs. Entry 3). These results suggest that it is very likely that the increase in the size of the ball could counteract the decrease in the catalyst since its contact surface is greater and facilitates the crushing of the precursors. Considering these observations, we proceeded to decrease the InCl3 load to 1.5 mol%. However, the grinding time tripled, and its yield dropped to 48% (Entry 5). Likewise, when the reaction was carried out in the absence of a catalyst, it was found that the precursors remained unreacted after 45 min (Entry 6).
Therefore, the study of all these variables on a model reaction led us to develop a high-speed mixer ball mill protocol for Pechmann condensation. To evaluate its scope, other substituted phenols 1b-e (naphthols 1f-g) and ethyl acetoacetate 2, were used as the main precursors to obtain corresponding 4-methylcoumarins 3b-g in good yields under solvent-free conditions and at room temperature ( Figure 4). It is important to note that no reported ball milling or grinding procedure [41][42][43][44] offered the direct preparation of 7amino-4-methylcoumarin.  The identity of the products 3a-g (Table 1) was confirmed by their IR, 1 H NMR, 13 C NMR, and mass spectral data and by comparison with authentic samples.

Conclusions
During a detailed study of the reaction conditions, where technical parameters such as the vibration frequency of the equipment, material, and size of the grinding implements, as well as chemical aspects, catalysts, and precursors, were evaluated, it was possible to establish for the first time an appropriate methodology for the von Pechmann reaction via mechanochemical activation and under green chemistry postulates.
The synthesis of a series of 4-methylcoumarins 3a-g was carried out in short reaction times with good and excellent yields (52-95%) through a high-speed mixer ball mill process with a milling frequency of 60 Hz. The developed protocol avoided the use of auxiliary substances such as hazardous solvents or acid catalysts and long heating times. In addition, it allowed the isolation of products with a very high degree of purity, thus evading additional purification processes that lead to the unnecessary expense of more chemical substances. Particularly, 7-amino-4-methylcoumarin 3a, a compound of high commercial value, was synthesized in a short time (10 min) and with an excellent yield (92%) at rt. Due to the HSMBM process, it was possible to keep away from the generation of secondary products observed in traditional methodologies and the use of protective groups that would not only prolong the experimental process but also entail the use of greater quantities of reagents, solvents, and catalysts.