N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aromatic Aldehydes into Carboxylic Acids: A Critical Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

From my point of veiw the suggested manuscript is far from new data or conclusions is if compared with other reviews in NHC oxidative catalysis. See f.e. one of the last https://doi.org/10.1002/chem.202202467, where most of the suggested topics are also reviewed. So I think that the manuscript is out of novelty and should be rejected. In case of other decision I would like to recommend to check all the figures. In presented manuscript most of them are of very low quality. 

Author Response

Comment 1: From my point of veiw the suggested manuscript is far from new data or conclusions is if compared with other reviews in NHC oxidative catalysis. See f.e. one of the last https://doi.org/10.1002/chem.202202467, where most of the suggested topics are also reviewed. So I think that the manuscript is out of novelty and should be rejected.

Response 1: We agree with the reviewer about the high interest of the review by C. De Risi et al. (Chem. Eur. J. 2023, 29, e202202467; doi.org/10.1002/chem.202202467). This review was already cited in the manuscript, i.e., reference 23, lines 436-437.

With all due respect, we believe the reviewer overlooked the main point of our review. Many reviews have already been published on NHC oxidative catalysis, including the review by C. De Risi et al. However, the specific transformation of aldehydes into the corresponding carboxylic acids has not been covered. For example, in C. De Risi et al.'s review, despite citing up to 200 references, there are only four examples of NHC-catalyzed oxidation of aldehydes to carboxylic acids.

• Oxidation of benzaldehyde promoted by PIDA, reference 35 in Chem. Eur. J. that becomes reference 32 cited in the manuscript
• Aerobic oxidation of benzoic acid, references 6 and 7 in Chem. Eur. J. that become references 25 and 26 cited in the manuscript
• Aerobic oxidation of furfural, reference 125 in Chem. Eur. J. that becomes reference 52cited in the manuscript
• Aerobic oxidation of benzoic acid, reference 129 in Chem. Eur. J. that becomes reference 51cited in the manuscript

The formation of C-O, C-S, C-N, and C-C bonds from aldehydes is an oxidation process. However, the important specific transformation of aldehydes into their corresponding carboxylic acids is scattered throughout the large amount of aldehyde transformations into a wide range of valuable derivatives. The goal of this review was to highlight the scope and limitations of this particular transformation.

Comment 2: In case of other decision I would like to recommend to check all the figures. In presented manuscript most of them are of very low quality.

Response 2: With all due respect, all figures have been created using ChemDraw, and the catalytic cycles have been carefully adapted from the literature. Each time, the original citation was mentioned in the caption.

Reviewer 2 Report

Comments and Suggestions for Authors

Favre-Réguillon presents a review entitled “N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aromatic Aldehydes into Carboxylic Acids: A Critical Review”, in which the versatility of NHCs as catalysts for the activation and transformation of aromatic aldehydes, particularly in the formation of C–C and C–heteroatom bonds are generally well demonstrated. The work is well documented, and the use of references is appropriate.

However, given that the title includes the term “critical review”, it is recommended that the conclusion be strengthened to better reflect the author’s critical perspective on the use of NHCs.

Overall, the manuscript is of good quality and may be accepted after addressing a few specific points outlined below:

• I recommend including an index to clarify the organization and classification of the reactions discussed in the review.
• It is necessary to specify all substituents (e.g., R, Ar, X, Y, etc.) in the reaction schemes for clarity and completeness.
• All schemes should be homogenized in terms of formatting and style, as multiple styles are currently presented throughout the manuscript.
• Please assign and include a corresponding number for each compound in the Schemes and the text.
• The yields shown in Figure 1 are inconsistent with the accompanying explanation and should be revised for accuracy.
• In line 113, the author states: “The reactivity of the Breslow intermediate with oxygen or with atmospheric oxygen has been noticed several times in the literature”; however, no specific references are provided. It is recommended to include relevant citations to support this statement.
• The notation for the methyl group should be standardized throughout the manuscript (e.g., consistently using either Me or CH₃).
• I suggest presenting the mechanisms as Figures and the chemical reactions as Schemes, to clearly distinguish between them.

Author Response

• Comment 1 : However, given that the title includes the term “critical review”, it is recommended that the conclusion be strengthened to better reflect the author’s critical perspective on the use of NHCs.

Response 1 : With all due respect, I believe I have sufficiently critiqued and commented on the results described in the literature, especially regarding the use of CO2 as an oxidant. Additionally, I introduced the concept of productivity (Tables 1 and 2), which enables me to compare catalytic systems with the data provided by the authors. Consequently, I have identified catalytic systems that are less efficient than the reference publication I consider, i.e. stirring aldehyde in water (A. Vigalok et al. reference 57).

In section 4, I compare these results with different catalytic systems (photocatalysis, transition metals, etc.). In conclusion, I identify a few catalytic systems that show interesting productivity (references 38,50,53). As suggested by the reviewer (see response 3), I also added a comment on the structure of the carbene.

• Comment 2 : I recommend including an index to clarify the organization and classification of the reactions discussed in the review.

Response 2 : I'm not sure if Catalyst requires an index in these reviews. I can certainly do it if the editor agrees.

• Comment 3 : It is necessary to specify all substituents (e.g., R, Ar, X, Y, etc.) in the reaction schemes for clarity and completeness.

Response 3 : In the first version of this review, I aimed to be exhaustive regarding the structure of carbenes and carbene precursors. However, this made it quite hard to read, with a complexity that hindered understanding. I decided to simplify the structures of carbenes as much as possible. Additionally, the publications never discuss the nature of the substituents R, Ar, X, Y. Each author chose a carbene without explaining how it relates to those already studied. I thought it would be more reader-friendly to present simpler structures. Readers can see the complete structures in the publications.

Two sentences were added in the conclusion, i.e., “(1) The structure of the NHC precursor needs justification. In this review, the structure of the NHC precursor has been simplified because, in the literature, structure–performance relationships have never been studied or structure of the NHC precursor was never justified from previous studies.” (see also response 1).

• Comment 4: All schemes should be homogenized in terms of formatting and style, as multiple styles are currently presented throughout the manuscript.

Response 4: I don't understand what's going on with some of the figures. In my document, all the figures are of good quality (see pdf attached). They were all created with the same software (ChemDraw 23) using the same style, and all reduced to 75%. I hope the revised version will fix the problem.

• Comment 5: Please assign and include a corresponding number for each compound in the Schemes and the text.

Response 5: As suggested by the reviewer, the mechanism will be shown in figures and chemical reactions in schemes (see also response 9). The number of compounds in all figures referenced in the text has been carefully verified.

• Comment 6: The yields shown in Figure 1 are inconsistent with the accompanying explanation and should be revised for accuracy.

Response 6: Various aromatic aldehydes have been evaluated by the authors and discussed in the text. An example was shown in scheme 1. A sentence was added to comment on the yields shown in figure 1, i.e., “For p-bromobenzaldehyde, 63% of ketone was obtained with 5% of ester (Scheme 1).”

• Comment 7: In line 113, the author states: “The reactivity of the Breslow intermediate with oxygen or with atmospheric oxygen has been noticed several times in the literature”; however, no specific references are provided. It is recommended to include relevant citations to support this statement.

Response 7: The citations related to this statement have been comment in the text following the sentence. References were added in the corrected version.

• Comment 8: The notation for the methyl group should be standardized throughout the manuscript (e.g., consistently using either Me or CH₃).

Response 8: I agree with the reviewer that the notation of the methyl group was a mess, see figure 2 a)! All figures and schemes with a methyl group were standardized.

• Comment 9: I suggest presenting the mechanisms as Figures and the chemical reactions as Schemes, to clearly distinguish between them.

Response 9: I agree with the reviewer and the mechanism will be presented in figures and chemical reactions in schemes.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Alain Favre-Réguillon reports in their manuscript entitled “N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aromatic Aldehydes into Carboxylic Acids: A Critical Review” an up date review on the use of different N-Heterocyclic Carbenes (NHCs) as   promising organo-catalysts for the aerobic oxidation of aldehydes to carboxylic acids. The paper seems good, well organized and of interest for the readers of catalysts journal. Nevertheless, there are some corrections to be made before publication:

1- Some figures and schemes need improvement in terms of quality.

2- There are some spaces to be removed.

3- I suggest to the author to remove the red color from the figures and schemes.

Author Response

• Comment 1 : Some figures and schemes need improvement in terms of quality.

Response 1 : I don't understand what's going on with some of the figures. In my document, all the figures are of good quality. They were all created with the same software (ChemDraw 23) using the same style. I hope these issues will be fixed in the new version.

• Comment 2 : There are some spaces to be removed.

Response 2 : I agree with reviewer 2 and added a space between “(yield)” and “%”. See also the response to reviewer 4.

• Comment 3 : I suggest to the author to remove the red color from the figures and schemes.

Response 3 : As suggested by the reviewer, color has been removed from the figure and schemes.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript submitted by Favre-Réguillon presents a comprehensive review of the N-heterocyclic carbenes (NHCs)-catalyzed aerobic oxidation of aromatic aldehydes to the corresponding carbolxylic acids. NHCs are promising organocatalysts not only for oxidation to carboxylic acids but also for conversion to esters and the Stetter reaction. In addition, the manuscript includes extensive literature on mechanistic considerations and catalytic cycles. Therefore, this review is highly beneficial for the wide readership of this journal, and I recommend that the manuscript be published in Catalysts after addressing the following minor remarks.

 

• Adding the key word “organocatalyst” to the Keywords section will increase the visibility of the paper in search engines.
• According to the instructions for authors, reference numbers should be placed before punctuation. Therefore, all reference numbers should be moved before punctuation, and not after.
• L44, L46, title of Scheme 1: Since “benzoin” is a compound name, it should be written in lowercase, and not initial-capitalized.
• Scheme 1: In the structures of compounds 11 and 12, the allylic methylene carbon is missing. Please confirm.
• Figure 3: Insert a space between “2” and “h”.
• L119: Remove the space between “(yield)” and “%”. Also in 131, 136, 209, 217, 240, 248, 254, 270.
• Figure 5: Figures 5, 13 and Table 1 entry 5 show “TBD”, while only “TDB” is described in the Abbreviations section. Which one is correct?
• L159 and L168: The structures of “bromo-benzaldehyde” nor “bromomethyl benzoate (meaning methyl bromobenzoate?)” appeared in Scheme 4, which could confuse readers. Therefore it is better to remove “bromo”.
• Figure 7: Compound 24 is a peroxyacid anion, whereas hydrogen is shown instead of an anion. Please confirm.
• L191: Insert a period after “J” of “J Castells”.
• L205: Remove the under line from “2”. What is “2” in Figure 9? Insert compound number 2 in Figure 9.
• L210: Insert a hyphen between “NHC” and “catalyzed”.
• L252: Does chemical rate mean chemical reaction rate? Please confirm.
• L260: The sign “…” is better replaced by “etc.”.
• L267: Insert a space between “12” and “h”.
• L271: Figure 16 is a reaction of benzaldehyde, while its legend mentioned “2-chlorobenzaldehyde”. Please correct.
• Table 1, footnote b): The sentence “see supporting information for details” is not appropriate, since no supporting information can be found for this manuscript.
• Table 1, footnote e): Insert a period at the end of the sentence.
• Table 1, footnote f): Initial-capitalized for “carboxylic”.
• Table 2: The unit “h” and “%” are better to move under “Time” and “Yield”, like Table 1.
• Table 2, entry 9: The catalyst amount is missing.
• L364: Correct “characterisation” to “characterization”.
• L371 for abbreviation of DPQ: Italicize “tert”.

Author Response

• Comment 1 : Adding the key word “organocatalyst” to the Keywords section will increase the visibility of the paper in search engines.

Response 1: I agree with the reviewer, and the keyword “organocatalyst” has been added.

• Comment 2 : According to the instructions for authors, reference numbers should be placed before punctuation. Therefore, all reference numbers should be moved before punctuation, and not after.

Response 2: The placement of punctuation has been changed throughout the manuscript.

• Comment 3 : L44, L46, title of Scheme 1: Since “benzoin” is a compound name, it should be written in lowercase, and not initial-capitalized.

Response 3 : I agree with the reviewer, and the capitalization of benzoin has been corrected throughout the manuscript.

• Comment 4: Scheme 1: In the structures of compounds 11 and 12, the allylic methylene carbon is missing. Please confirm.

Response 4: We sincerely thank the reviewer for correcting these errors. The structure of compounds 11 and 12 has been corrected.

• Comment 5: Figure 3: Insert a space between “2” and “h”.

Response 5: A space was added.

• Comment 6: L119: Remove the space between “(yield)” and “%”. Also in 131, 136, 209, 217, 240, 248, 254, 270.

Response 6: Spaces were removed between yield and % throughout the manuscript, text, figures, and tables.

• Comment 7: Figure 5: Figures 5, 13 and Table 1 entry 5 show “TBD”, while only “TDB” is described in the Abbreviations section. Which one is correct?

Response 7: TBD for triazabicyclodecene is correct. The misprint was corrected in the abbreviation section.

• Comment 8: L159 and L168: The structures of “bromo-benzaldehyde” nor “bromomethyl benzoate (meaning methyl bromobenzoate?)” appeared in Scheme 4, which could confuse readers. Therefore it is better to remove “bromo”.

Response 8: I agree with the reviewer, bromo was removed.

• Comment 9: Figure 7: Compound 24 is a peroxyacid anion, whereas hydrogen is shown instead of an anion. Please confirm.

Response 9: We would like to sincerely thank the reviewer for the correction of this error. The structure of the compound 24 has been corrected.

• Comment 10: L191: Insert a period after “J” of “J Castells”.

Response 10: A period was added after J.

• Comment 11: L205: Remove the under line from “2”. What is “2” in Figure 9? Insert compound number 2 in Figure 9.

Response 11: In the first draft of this review, I planned to number each carbene precursor and then create a general figure. However, I later realized that this made the text considerably more difficult to read. This is a remnant of that numbering system. The reference to this number has been removed.

• Comment 12: L210: Insert a hyphen between “NHC” and “catalyzed”.

Response 12: A hyphen was added.

• Comment 13: L252: Does chemical rate mean chemical reaction rate? Please confirm.

Response 13: “chemical rate “was corrected to “chemical reaction rate”.

• Comment 14: L260: The sign “…” is better replaced by “etc.”.

Response 14: As suggested by the reviewer, “…” was replaced by “etc.”.

• Comment 15: L267: Insert a space between “12” and “h”.

Response 15: A space was added between numbers and h throughout the manuscript

• Comment 16: L271: Figure 16 is a reaction of benzaldehyde, while its legend mentioned “2-chlorobenzaldehyde”. Please correct.

Response 16: The legend was corrected.

• Comment 17: Table 1, footnote b): The sentence “see supporting information for details” is not appropriate, since no supporting information can be found for this manuscript.

Response 17: Thank you again for your careful review. This is another remnant from the first version of this review, where the equations used to calculate productivity were included in the supplementary material. The reference to the supplementary material has been removed.

• Comment 18: Table 1, footnote e): Insert a period at the end of the sentence.

Response 18: A period was added at the end of footnote e.

• Comment 19: Table 1, footnote f): Initial-capitalized for “carboxylic”.

Response 19: The first letter of Carboxylic has been capitalized.

• Comment 20: Table 2: The unit “h” and “%” are better to move under “Time” and “Yield”, like Table 1.

Response 20: The position of the unit in Tables 1 and 2 has been standardised.

• Comment 21: Table 2, entry 9: The catalyst amount is missing.

Response 21: The catalyst amount was added in Table 2, entry 9.

• Comment 22: L364: Correct “characterisation” to “characterization”.

Response 22: The misprint has been corrected.

• Comment 23: L371 for abbreviation of DPQ: Italicize “tert”.

Response 23: The prefix “tert” has been italicized.

I want to sincerely thank the reviewer for their careful proofreading and especially for correcting the structure of products 11 and 12 in Scheme 1.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors improved the paper so I think it could be considered for the publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The revised version is now suitable to be published in  "Catalysts"