Rapid and Simple Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic Acid]

Round 1

Reviewer 1 Report

In this manuscript entitled “Rapid and Efficient Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic acid]”, Phuong Hoang Tran and co-workers described the preparation of a deep eutectic solvent (DES), that is [CholineCl][Oxalic acid], and its application as a catalyst in the synthesis of 2-aryl benzoxazoles by condensation of 2-aminophenols and benzaldehydes. After the easy preparation and characterization of the [CholineCl][Oxalic acid], authors performed an extended optimization of the experimental conditions, using the condensation of 2-amino-4-chlorophenol (1b) and benzaldehyde (2a) as the model reaction: several parameters were tested, including temperature, time, catalyst loading, as well as a comparison with other typical DESs. The optimized conditions were then extended to the reaction of several 2-aminophenols and benzaldehydes, and in the final part of the manuscript a recyclability study of the catalyst was also performed. In general, the present work is nice and interesting, and in my opinion it has the potential to be published. However, I think there are some aspects that need to be addressed before considering its possible acceptance on MDPI Catalysts.

1. First of all, the role of microwave (MW) irradiation in this cyclization reaction is quite unclear. Why did the authors not conduct tests with conventional thermal heating? Could it happen that the same reaction with the use of the traditional oil bath heating is better? In general, I think that authors should better emphasize the role played by MW irradiation in this class of reactions. For this purpose, in my opinion in the introduction a broader discussion should be made on the use of non-conventional energy source in organic synthesis: not only MW irradiation-assisted protocols, but also IR irradiation-assisted (DOI: 10.1002/cssc.202101070), ultrasound-assisted (DOI: 10.1016/j.tetlet.2016.10.060) and mechanical milling-assisted (DOI: 10.3762/bjoc.13.186). This point should be well specified in the introduction, even with the support of the above mentioned literature.

2. A further critical point of the work is the impact and the novelty of the work compared to the previous literature. In fact, the use of DESs as a catalysts for the synthesis of heterocyclic compounds, and in particular of 2-substituted benzoxazoles, has been already reported in literature (that is, 10.1016/j.molliq.2020.115011). I believe that the authors should better emphasize the advantages in the use of a DES as a catalyst in this class of reactions.

3. An important consideration is related to the recyclability test of the [CholineCl][Oxalic acid] catalyst. A slight loss of catalytic activity was observed after each cycle, probably due to the loss of the catalyst in the recovery process. Actually, if only a slight decrease in the reaction conversion was found, the 3c/4c ratio was significantly changed. Could the catalyst loss in the recovery process enough to explain this change in the 3c/4c ratio?

4. Other minor issues. A) The notes [a] and [b] below Table 4 are missing. B) The superscript reference numbers from 24 to 50 (see lines from 41 to 50) are written in bold type: please remove the bold.

Author Response

Reviewer #1

In this manuscript entitled “Rapid and Efficient Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic acid]”, Phuong Hoang Tran and co-workers described the preparation of a deep eutectic solvent (DES), that is [CholineCl][Oxalic acid], and its application as a catalyst in the synthesis of 2-aryl benzoxazoles by condensation of 2-aminophenols and benzaldehydes. After the easy preparation and characterization of the [CholineCl][Oxalic acid], authors performed an extended optimization of the experimental conditions, using the condensation of 2-amino-4-chlorophenol (1b) and benzaldehyde (2a) as the model reaction: several parameters were tested, including temperature, time, catalyst loading, as well as a comparison with other typical DESs. The optimized conditions were then extended to the reaction of several 2-aminophenols and benzaldehydes, and in the final part of the manuscript a recyclability study of the catalyst was also performed. In general, the present work is nice and interesting, and in my opinion it has the potential to be published. However, I think there are some aspects that need to be addressed before considering its possible acceptance on MDPI Catalysts.

1. First of all, the role of microwave (MW) irradiation in this cyclization reaction is quite unclear. Why did the authors not conduct tests with conventional thermal heating? Could it happen that the same reaction with the use of the traditional oil bath heating is better? In general, I think that authors should better emphasize the role played by MW irradiation in this class of reactions. For this purpose, in my opinion in the introduction a broader discussion should be made on the use of non-conventional energy source in organic synthesis: not only MW irradiation-assisted protocols, but also IR irradiation-assisted (DOI: 10.1002/cssc.202101070), ultrasound-assisted (DOI: 10.1016/j.tetlet.2016.10.060) and mechanical milling-assisted (DOI: 10.3762/bjoc.13.186). This point should be well specified in the introduction, even with the support of the above mentioned literature.

Response: We carried out the reactions in conventional heating (120 ^oC, 15 min and 120 ^oC, 60 min) and in ultrasound irradiation (80 ^oC, 15 min) and the results were added to Table 1, entries 15-17.

2. A further critical point of the work is the impact and the novelty of the work compared to the previous literature. In fact, the use of DESs as a catalysts for the synthesis of heterocyclic compounds, and in particular of 2-substituted benzoxazoles, has been already reported in literature (that is, 10.1016/j.molliq.2020.115011). I believe that the authors should better emphasize the advantages in the use of a DES as a catalyst in this class of reactions.

Response: Thank you for the comment. We added the reference [37] in the revised manuscript and emphasized the advantages of the use of DES in the synthesis of benzoxazoles.

In the revised manuscript:

As part of our ongoing research to develop a green and efficient approach for benzoxazole synthesis [42-44], we report herein an improved method for synthesizing benzoxazole compounds via the reaction of different 2-aminophenols and benzaldehydes using deep eutectic solvents under solvent-free MW irradiation. The catalyst was easily prepared from simple and cheap materials such as choline chloride and oxalic acid. Under microwave irradiation, DES provided the 2-arylbenzexazoles in good to excellent yields. Intertingly, the DES could be recovered and reused without significant loss of the catalytic activity.

3. An important consideration is related to the recyclability test of the [CholineCl][Oxalic acid] catalyst. A slight loss of catalytic activity was observed after each cycle, probably due to the loss of the catalyst in the recovery process. Actually, if only a slight decrease in the reaction conversion was found, the 3c/4c ratio was significantly changed. Could the catalyst loss in the recovery process enough to explain this change in the 3c/4c ratio?

Response: Thank you for the comment. We believe that the loss of catalyst could affect the 3c/4c ratio because the decrease of catalytic amount led to the increase of imine intermediate (3c). The corresponding product (4c) was obtained in low yield because the concentration of the catalyst was not enough to convert completely imine intermediate to the final product.

In the revised manuscript:

We believed that the increase of 3c/4c ratios could be explained by the decrease in catalytic concentration. When the catalyst concentration was decreased in the reaction mixture, the imine intermediate could not convert completely to the final product.

4. Other minor issues. A) The notes [a] and [b] below Table 4 are missing. B) The superscript reference numbers from 24 to 50 (see lines from 41 to 50) are written in bold type: please remove the bold.

Response: Thank you for the comment. We corrected these issues in the revised manuscript.

Reviewer 2 Report

In the article “Rapid and Efficient Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic acid]”, deep eutectic solvent ([CholineCl][Oxalic acid]) was synthesized and characterized which was then used as a catalyst for the formation of benzoxazoles with high conversion and selectivity obtained within 15 to 30 min as an environment-benign solvent-free reaction. Although the reaction is well-known and has been thoroughly studied, it is still well complementary as parts of the Tran group’s efforts on DES catalyst.

However, there are some issues that need to be addressed.

First of all, further manuscript polishing is highly recommended. 

The author underlined “microwave-assisted synthesis” in the title. Thus, a control experiment with regular heating should be included. Meanwhile, please keep all compound names in bold including the table legends.

The mechanism provided is arguably not favored. Especially, the last step is shown as a “hydride” leaving. The author claimed there is air oxidation. Please provide the control experiment without air. An additional control experiment is by applying the synthesized 3c as starting material. In principle, the way through intermediate c (Scheme 3) directly to product 4c is more favored. Line 167, HRMS [M+2H]+, another interpretation is the amine from intermediate c + H.

Results and discussion, 1H NMR of [CholineCl][Oxalic acid] has been provided in supporting information. If possible, please include the 13C NMR as well.

Figure 2, the title should be “TGA analysis…”.

In table 1, as mentioned above, a control experiment with normal heating at 120 ^oC should be added. Please clarify how the author gets the conversion through internal standards or otherwise. If possible, please try to interpret the data not just dictate what was obtained.

The description of entries 1-4 is very confusing (lines 99-101). Please rewrite it.

Entries 5-9, Table 1, please clarify whether they are 5 reactions or just a single reaction with 5 times GC-MS monitoring.

Line 102, “as aspected” should be “as expected”. Line 103, “time reaction” should be “reaction time”. 

Table 2, entry 1: there is a scale up data depicted. Please add a description in the context.

Line 139 to line 153: please double check again. The way was written is very confusing. Line 149, in the parenthesis, please double check. The same as line 150.

Table 4, please add the necessary legends to the table. If possible, please attach the GC-MS spectra that determined the ratio of 3/4 in supporting information.

Scheme 3, please fix the arrow in intermediate c, the arrow should point to O, rather than the bond.

There are too many mistakes in supporting information. Please check very carefully. Please add the solvents used for NMRs. If possible, please show all the spectra from 0 ppm. There are no NMRs of 4b.

Author Response

Reviewer #2

In the article “Rapid and Efficient Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic acid]”, deep eutectic solvent ([CholineCl][Oxalic acid]) was synthesized and characterized which was then used as a catalyst for the formation of benzoxazoles with high conversion and selectivity obtained within 15 to 30 min as an environment-benign solvent-free reaction. Although the reaction is well-known and has been thoroughly studied, it is still well complementary as parts of the Tran group’s efforts on DES catalyst.

However, there are some issues that need to be addressed.

1. First of all, further manuscript polishing is highly recommended.

Response: Thank you for the comment. We checked the manuscript carefully and corrected the English in the revised manuscript.

2. The author underlined “microwave-assisted synthesis” in the title. Thus, a control experiment with regular heating should be included. Meanwhile, please keep all compound names in bold including the table legends.

Response: Thank you for the comment. Control experiments under conventional heating and ultrasound irradiation were carried out in the revised manuscript (please see Table 1, entries 15-17). The compounds and Table legends were bolded in the revised manuscript.

In the revised manuscript:

 Control experiments were performed under conventional heating and ultrasound irradiation which provided a lower conversion than microwave heating (entries 15-17, Table 1).

3. The mechanism provided is arguably not favored. Especially, the last step is shown as a “hydride” leaving. The author claimed there is air oxidation. Please provide the control experiment without air. An additional control experiment is by applying the synthesized 3c as starting material. In principle, the way through intermediate c (Scheme 3) directly to product 4c is more favored. Line 167, HRMS [M+2H]+, another interpretation is the amine from intermediate c + H..

Response: Thank you for the comment. We corrected the proposed mechanism. We are sorry for the mistake, the last step is the oxidation with oxygen in the air to form the desired product and water as a byproduct.

4. Results and discussion, 1H NMR of [CholineCl][Oxalic acid] has been provided in supporting information. If possible, please include the 13C NMR as well.

Response: Thank you for the comment. ¹³C NMR was added in the revised supporting information.

In the revised manuscript

[CholineCl][Oxalic acid][58]

¹H-NMR (500 MHz, DMSO-d₆) d ppm 3.81–3.80 (m, 2H, –CH₂–N), 3.42–3.40 (m, 2H, –CH₂–O), 4.71(s, 3H, –OH), 3.11 (s, 9H, –N(CH₃)₃)

¹³C-NMR (125 MHz, DMSO-d₆) d ppm 161.80, 67.42 (t, J = 2.8 Hz), 55.5, 53.6 (t , J = 3.8 Hz).

5. Figure 2, the title should be “TGA analysis…”.

Response: Thank you for the comment. We corrected this in the revised manuscript.

6. In table 1, as mentioned above, a control experiment with normal heating at 120 oC should be added. Please clarify how the author gets the conversion through internal standards or otherwise. If possible, please try to interpret the data not just dictate what was obtained.

Response: Thank you for the comment. We did not use internal standard and the conversion was calculated as follows

Conversion =(%GC_3c +%GC_4c)/(%GC_3c +%GC_4c+%GC_aldehyde)

As suggested by the reviewer, a control experiment was carried out with conventional heating at 120 ^oC (please see Table 1).

7. The description of entries 1-4 is very confusing (lines 99-101). Please rewrite it.

Response: Thank you for the comment. We corrected the issue in the revised manuscript

8. Entries 5-9, Table 1, please clarify whether they are 5 reactions or just a single reaction with 5 times GC-MS monitoring.

Response: Thank you for the comment. They are the 5 individual reactions.

9. Line 102, “as aspected” should be “as expected”. Line 103, “time reaction” should be “reaction time”.

 Response: Thank you for the comment. We corrected these issues in the revised manuscript.

10. Table 2, entry 1: there is a scale up data depicted. Please add a description in the context.

Response: Thank you for the comment. We added a description in the revised manuscript.

11. Line 139 to line 153: please double check again. The way was written is very confusing. Line 149, in the parenthesis, please double check. The same as line 150.

Response: Thank you for the comment. We checked and corrected the paragraph in the revised manuscript.

12. Table 4, please add the necessary legends to the table. If possible, please attach the GC-MS spectra that determined the ratio of 3/4 in supporting information.

Response: Thank you for the comment. We added legends in the revised manuscript. We added the MS spectra in the revised supporting information.

13. Scheme 3, please fix the arrow in intermediate c, the arrow should point to O, rather than the bond.

Response: Thank you for the comment. We checked and corrected the mechanism in the revised manuscript.

14. There are too many mistakes in supporting information. Please check very carefully. Please add the solvents used for NMRs. If possible, please show all the spectra from 0 ppm. There are no NMRs of 4b.

Response: Thank you for the comment. We checked and corrected the revised supporting information.

Reviewer 3 Report

This manuscript describes a convenient preparation of 2-arylbenzooxazoles catalyzed by an accessible [CholineCl][Oxalic acid] catalyst.  Along with the authors’ series of studies (Refs. 50, 62-66), the present work is a piece of extension.  Because [CholineCl][Oxalic acid] catalyst is reported (Refs. 72 and 73), the authors have better address this issue in early stage of the introduction section. Although [CholineCl][Oxalic acid] is a reported catalyst, the present method looks like to exhibit high performance in this condensation reaction as demonstrated in Table 2. 

The reviewer basically recommends the publication in Catalysts after the major revision commented below: There are a number of improper descriptions and the authors should carefully revise this issue.       

<Comments and suggestions>

1.        Title; “Efficient” should be deleted especially in the title as a recent reliable journal style. 

2.        Abstract; The first two sentences are repetitive. These should be unified in one sentence.

3.        Abstract; line 12: microwave irradiation → microwave irradiation (MW). Accordingly, “(MW)” in line 17 should be moved.

4.        Introduction; Too many citations are listed. The reviewer suggests that more relevant ones should be selected.

5.        Line 25; biological → pharmaceutical and agrochemical

6.        Line 29; was … → is conventional and carried out …

7.        Line 31; mild → milder   et al. → group or coworkers (as a recent style)

8.        Line 39; in the presence of …… → mediated by …… catalysis

9.        Line 46; no chemical reaction → without any special covalent-bond forming reaction

10.     Line 51; reports → applications          

11.     Line 64; [CholineCl][Oxalic acid] → [CholineCl][Oxalic acid] DES catalyst  previous studies → our previously reported studies?

12.     Line 66; for 30 min at 100 °C → at 100 °C for 30 min

13.     Line 67; demonstrated → determined

14.     Line 72; as-synthesized ?

15.     Line 85; Full name of TGA should be addressed.

16.     Fig. 2; Why is rapid decomposition of oxalic acid?  Decarboxylation? Please mention this issue briefly.

17.     Line 97; irradiation. → irradiation (Table 1).

18.     Line 106; the reaction was investigated …… → Without the catalyst, the reaction was completely retarded to afford intermediate 3c.

19.     Table 1; “Catalyst loading (mol%)” should be moved before “Reaction condition”

20.     Line 115; deep eutectic solvent → DES

21.     Lines 115-119; Relevant citations for each catalyst should be addressed.

22.     Line 121; “recognized with” should be deleted.

23.     Line 124; 1/99 → 1/99 (5 mmol scale)

24.     Line 131; demonstrated the comparison of → shows the comparison data of

25.     Lines 132 and 135; two “other reported catalysts” appeared. (repetition)

26.     Table 3; citations in Table 3 should be avoided.  The other catalysis should be briefly described in the text.  The reviewer considers that the catalysis (entries 2 and 4) is considerably milder with regard to the temperature.  The comment on this issue should be provided.

27.     Line 139; a variety of → several

28.     Line 146; between (1a) → between 2-aminophenol (1a)

29.     Line 150; products 2a-2d → products 4a-4d

30.     Line 163; aniline → nitrogen atom of aniline

31.     Scheme 3; (D) should be placed near the imine.  Extrusion of the catalyst should be provided in the array between (D) and (3c), which completes catalytic cycle.

32.     Line 174; (1b) and (2a) should be inserted in the appropriate position.

33.     Fig. 4; The reviewer feels a slight difference of wave numbers between “Fresh” and “Reused, in addition, the latter of which shows more sharp spectra.

34.     Line 191; prominent → advantageous        

35.     Line 219; (a-f, → (2a-f,

36.     Lines 230-233; Recycling procedure (4.4) is ambiguous.  More                          

Author Response

Reviewer #3

This manuscript describes a convenient preparation of 2-arylbenzooxazoles catalyzed by an accessible [CholineCl][Oxalic acid] catalyst.  Along with the authors’ series of studies (Refs. 50, 62-66), the present work is a piece of extension.  Because [CholineCl][Oxalic acid] catalyst is reported (Refs. 72 and 73), the authors have better address this issue in early stage of the introduction section. Although [CholineCl][Oxalic acid] is a reported catalyst, the present method looks like to exhibit high performance in this condensation reaction as demonstrated in Table 2.

The reviewer basically recommends the publication in Catalysts after the major revision commented below: There are a number of improper descriptions and the authors should carefully revise this issue.      

1. Title; “Efficient” should be deleted especially in the title as a recent reliable journal style.

Response: Thank you for the comment. We corrected it in the revised manuscript.

2. Abstract; The first two sentences are repetitive. These should be unified in one sentence.

Response: Thank you for the comment. We checked and corrected the abstract in the revised manuscript.

3. Abstract; line 12: microwave irradiation → microwave irradiation (MW). Accordingly, “(MW)” in line 17 should be moved.

Response: Thank you for the comment. We corrected the abstract in the revised manuscript.

4. Introduction; Too many citations are listed. The reviewer suggests that more relevant ones should be selected.

Response: Thank you for the comment. We cited the references in the revised manuscript.

5. Line 25; biological → pharmaceutical and agrochemical

Response: Thank you for the comment. We corrected it in the revised manuscript.

6. Line 29; was … → is conventional and carried out …

Response: Thank you for the comment. We corrected it in the revised manuscript.

7. Line 31; mild → milder et al. → group or coworkers (as a recent style)

Response: Thank you for the comment. We corrected it in the revised manuscript.

8. Line 39; in the presence of …… → mediated by …… catalysis

Response: Thank you for the comment. We corrected it in the revised manuscript.

9. Line 46; no chemical reaction → without any special covalent-bond forming reaction

Response: Thank you for the comment. We corrected it in the revised manuscript.

10. Line 51; reports → applications

Response: Thank you for the comment. We corrected it in the revised manuscript.

11. Line 64; [CholineCl][Oxalic acid] → [CholineCl][Oxalic acid] DES catalyst previous studies → our previously reported studies?

Response: Thank you for the comment. We corrected it in the revised manuscript.

12. Line 66; for 30 min at 100 °C → at 100 °C for 30 min

Response: Thank you for the comment. We corrected it in the revised manuscript.

13. Line 67; demonstrated → determined

Response: Thank you for the comment. We corrected it in the revised manuscript.

14. Line 72; as-synthesized ?

Response: Thank you for the comment. We corrected it in the revised manuscript.

15. Line 85; Full name of TGA should be addressed.

Response: Thank you for the comment. We corrected it in the revised manuscript.

16. Fig. 2; Why is rapid decomposition of oxalic acid? Decarboxylation? Please mention this issue briefly.

Response: Thank you for the comment. TGA was performed under air condition. The rapid decomposition of organic compounds is oxidation at high temperatures.

17. Line 97; irradiation. → irradiation (Table 1).

Response: Thank you for the comment. We corrected it in the revised manuscript.

18. Line 106; the reaction was investigated …… → Without the catalyst, the reaction was completely retarded to afford intermediate 3c.

Response: Thank you for the comment. We corrected it in the revised manuscript.

19. Table 1; “Catalyst loading (mol%)” should be moved before “Reaction condition”

Response: Thank you for the comment. We corrected it in the revised manuscript.

20. Line 115; deep eutectic solvent → DES

Response: Thank you for the comment. We corrected it in the revised manuscript.

21. Lines 115-119; Relevant citations for each catalyst should be addressed.

Response: Thank you for the comment. We added the citation in the revised manuscript.

In the revised manuscript:

The other DESs were prepared according to the previously reported literature [53-57].

22. Line 121; “recognized with” should be deleted.

Response: Thank you for the comment. We corrected it in the revised manuscript.

23. Line 124; 1/99 → 1/99 (5 mmol scale)

Response: Thank you for the comment. We corrected it in the revised manuscript.

24. Line 131; demonstrated the comparison of → shows the comparison data of

Response: Thank you for the comment. We corrected it in the revised manuscript.

25. Lines 132 and 135; two “other reported catalysts” appeared. (repetition)

Response: Thank you for the comment. We corrected it in the revised manuscript.

26. Table 3; citations in Table 3 should be avoided. The other catalysis should be briefly described in the text. The reviewer considers that the catalysis (entries 2 and 4) is considerably milder with regard to the temperature. The comment on this issue should be provided.

Response: Thank you for the comment. We corrected it in the revised manuscript.

In the revised manuscript:

Zhou and co-workers reported [BMIm]₂[WO₄] exhibited the good activity for the synthesis of the desired product in 1,4-dioxane [49]. Next, Gorepatil and co-workers developed a simple, green, and efficient method for the synthesis of benzoxazoles using samarium triflate as a reusable acid catalyst under mild reaction conditions in an aqueous medium [50]. Cho and co-workers reported a cyanide-catalyzed synthesis of 2-substituted benzoxazoles from Schiff bases via aerobic oxidation [51]. Sirgamalla and co-workers developed the synthesis of 2-arylbenzoxazoles by using a copper catalyst at room temperature [7]. Although the previous literature was carried out at a lower temperature than the current method, our work exhibited a higher conversion (99%) and shorter reaction time (15 min) than other reported catalysts.

27. Line 139; a variety of → several

Response: Thank you for the comment. We corrected it in the revised manuscript.

28. Line 146; between (1a) → between 2-aminophenol (1a)

Response: Thank you for the comment. We corrected it in the revised manuscript.

29. Line 150; products 2a-2d → products 4a-4d

Response: Thank you for the comment. We corrected it in the revised manuscript.

30. Line 163; aniline → nitrogen atom of aniline

Response: Thank you for the comment. We corrected it in the revised manuscript.

31. Scheme 3; (D) should be placed near the imine. Extrusion of the catalyst should be provided in the array between (D) and (3c), which completes catalytic cycle.

Response: Thank you for the comment. We corrected it in the revised manuscript.

32. Line 174; (1b) and (2a) should be inserted in the appropriate position.

Response: Thank you for the comment. We corrected it in the revised manuscript.

33. Fig. 4; The reviewer feels a slight difference of wave numbers between “Fresh” and “Reused, in addition, the latter of which shows more sharp spectra.

Response: Thank you for the comment. We agree with the reviewer the reused catalyst demonstrated unsharp signals when compared with the fresh catalyst. We do not know exactly the reason why the signals in low wavenumbers of reused catalyst showed more sharp signals than the fresh catalyst

34. Line 191; prominent → advantageous

Response: Thank you for the comment. We corrected it in the revised manuscript.

35. Line 219; (a-f, → (2a-f,

Response: Thank you for the comment. We corrected it in the revised manuscript.

36. Lines 230-233; Recycling procedure (4.4) is ambiguous. More

Response: Thank you for the comment. We corrected it in the revised manuscript.

In the revised manuscript

To recover the [CholineCl][Oxalic acid] after the completion of the reaction, the reaction mixture was washed times with ethyl to extract both the starting materials and products entirely from [CholineCl][Oxalic acid]. The residue was dried under vacuum at 80 ^oC for 1 h, the [CholineCl][Oxalic acid] was recovered and reused for the consecutive run.

Round 2

Reviewer 2 Report

Dear editor and authors,

Thanks for the authors’ efforts to address the concerns. 

Within the revised version, the author has carefully addressed the raised issues. Therefore, the reviewer recommends the publication on Catalysts.

Reviewer 3 Report

I cannot find "The other DESs were prepared according to reported literature [53-57]," in the text.