Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds

Round 1

Reviewer 1 Report

Manuscript ID: inorganics-2193456

In this work the authors have studied a set of three manganese complexes as catalyst in promoting the oxidation of saturated hydrocarbons (cyclohexane, methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The reviewer believes the performed analyzes and the discussion of results are enough to make this work accept after minor revision. Therefore, the reviewer suggests a minor revision of this work.

 

1.     The number of articles used in the introduction section are very few and old and should be updated.

2.     the quality of the Figures should be improved.

3.     Some sentences are not completely clear.

4.     there are few typos and spaces missing.

5.     The used references should be updated with newer references.

6.     English text is weak and should be edited

 

Author Response

Manuscript ID: inorganics-2193456

In this work the authors have studied a set of three manganese complexes as catalyst in promoting the oxidation of saturated hydrocarbons (cyclohexane, methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The reviewer believes the performed analyzes and the discussion of results are enough to make this work accept after minor revision. Therefore, the reviewer suggests a minor revision of this work.

We thank the reviewer's opinion and suggestions, which were taken into account in the revised version as follows:

1. The number of articles used in the introduction section are very few and old and should be updated.

The number of articles cited in the introduction increased, and more recent articles replaced the old ones.

2. the quality of the Figures should be improved.

Done

3. Some sentences are not completely clear.

The text was revised

4. there are few typos and spaces missing.

Done

5. The used references should be updated with newer references.

Done

6. English text is weak and should be edited

The English has been revised and improved.

Reviewer 2 Report

A. Horn Jr. devoted an article to the study of the catalytic activity of manganese complexes for the functionalization of cycloalkanes. The idea of C-H functionalization is of great interest from a scientific and practical point of view, which makes this area of research relevant.

This work lacks novelty for publication in Inorganics. In the first place,

the complexes themselves are not new; their synthesis has long been known. In terms of structural analysis, this work also does not present new data (no XRD). This leaves only the catalytic part of the research. At the same time, similar catalytic reactions with other complexes have been published earlier by the authors themselves. There are also comments on the data presented. The data based on GC-MS on the structures of the resulting products are illusorily presented. For example, 1,2-dichlorocyclohexane, or 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene are simply declared, but the evidence for these particular substances with specific substituent positions or double bonds is not adequately established. On the other hand, radical processes with alkanes are rarely regioselective. How can the selectivity in the processes under study be explained? It is necessary to present the supposed mechanism of the considered reactions with explanations and literature references. In general, such a presentation of these studies raises many questions and doubts. Another issue is the lack of literature data on the oxidation of cycloalkanes in the presence of hydrogen peroxide. Several scientific groups are working in this area.

Author Response

 Horn Jr. devoted an article to studying the catalytic activity of manganese complexes for the functionalization of cycloalkanes. The idea of C-H functionalization is of great interest from a scientific and practical point of view, which makes this area of research relevant.

This work lacks novelty for publication in Inorganics. In the first place, the complexes themselves are not new; their synthesis has long been known. In terms of structural analysis, this work also does not present new data (no XRD). This leaves only the catalytic part of the research. At the same time, similar catalytic reactions with other complexes have been published earlier by the authors themselves. There are also comments on the data presented. The data based on GC-MS on the structures of the resulting products are illusorily presented. For example, 1,2-dichlorocyclohexane, or 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene are simply declared, but the evidence for these particular substances with specific substituent positions or double bonds is not adequately established. On the other hand, radical processes with alkanes are rarely regioselective. How can the selectivity in the processes under study be explained? It is necessary to present the supposed mechanism of the considered reactions with explanations and literature references. In general, such a presentation of these studies raises many questions and doubts. Another issue is the lack of literature data on the oxidation of cycloalkanes in the presence of hydrogen peroxide. Several scientific groups are working in this area.

We want to thank the reviewer for highlighting some points that can be improved. Concerning the compounds employed in the study, the reviewer is right when saying that they are not new, as we have also indicated in the manuscript. However, this does not invalidate the publication of the data related to the catalytic activity, mainly in a special issue of the Inorganics, devoted to “Manganese chemistry: from fundamentals to applications.” Clearly, the data show a new application of a set of manganese compounds; therefore, we believe that the work is suitable for publication in this particular issue. Since we present compounds whose molecular structures were previously reported in the literature, and our analysis indicates that our synthesized compounds are the same, presenting any XRD data does not make sense.

Although we have studied other compounds in the same catalytic reaction, our previous works were developed with compounds totally different from the manganese compounds reported here, including both ligands and metal ions.  Therefore, we think the catalytic data are relevant since they show a new application for this family of manganese compounds.

Concerning the GC-MS data, we added the fragmentograms of the chemical species formed as supporting information to confirm the identity of the observed species.

This question has also called our attention to the selectivity shown by these systems. The reason for this needs to be clarified for us too. Further than GC-MS, we have also used GC-FID, aiming to identify any other different species. However, only one product (chlorocyclohexane) was observed in the chromatographic analyses using cyclohexane as substrate. However, other products were observed when using methylcyclohexane. In our previous works, we have also identified a  mixture of products when using adamantane and n-hexane, but all are monochloride products. So, it seems that the formation of the chloride product decreases the reactivity of the molecule concerning the starting molecule.

Concerning citations of works related to cyclohexane oxidation in the presence of H2O2, some of them were cited in the introduction. However, since the manganese compounds didn´t work in the presence of H2O2, references were not mentioned in results and discussion.

In the discussion section (paragraphs 6-8), we have discussed that two possible mechanisms can operate together. One is based on forming an intermediate species Mn-OCl, and the other is related to the homolytic cleavage of the N-Cl bond present in the TCCA molecule. Unfortunately, we could not identify, using spectroscopic methods (EPR, ESI-MS), any intermediate that could clarify this point. This is why we prefer to avoid proposing a possible reaction mechanism at the moment. On the other hand, our discussion brings some possibilities based on the literature. We are currently synthesizing other manganese compounds and hope to identify intermediate species for this reaction in the near future.

Reviewer 3 Report

The authors present their research results about the chlorination of cyclohexane and methlycyclohexane with TCCA catalyzed by several Mn(III) complexes or compounds. The results are reasonable and interest to readers. However, the manuscript was poorly organized and written. It is necessary to improve the manuscript then the manuscript can be considered for publication.

1. The oxidation with H2O2 was inactive, so the descriptions related to this item in the manuscript (ether in title or text) should be removed.

2. There are some grammatical  and typing errors.

3. In the discussion section, the second paragraph to the seventh paragraph (The oxidation of ......[Mn(salen)Cl ) should be moved to introduction section or removed directly.

 

Author Response

The authors present their research results about the chlorination of cyclohexane and methlycyclohexane with TCCA catalyzed by several Mn(III) complexes or compounds. The results are reasonable and interest to readers. However, the manuscript was poorly organized and written. It is necessary to improve the manuscript then the manuscript can be considered for publication.

We want to thank the reviewer for the positive evaluation of our work and her/his suggestions.

1. The oxidation with H2O2 was inactive, so the descriptions related to this item in the manuscript (ether in title or text) should be removed.

We changed the title of the article. However, we believe it is important to show that the system was inactive when using H2O2 but active when using TCCA. This will alert the readers that TCCA is an attractive oxidant agent to be studied with other catalysts that do not show—activity in the presence of H2O2.

2. There are some grammatical  and typing errors.

We have carried out a careful evaluation of the text and fixed many grammatical and typing errors.

3. In the discussion section, the second paragraph to the seventh paragraph (The oxidation of ......[Mn(salen)Cl ) should be moved to introduction section or removed directly.

Thanks for the suggestion. We have moved part of the indicated paragraphs to the introduction and removed some redundant ones.

Round 2

Reviewer 2 Report

Unfortunately, the authors made no attempt to improve the quality of the work through a more detailed characterization of the products resulting from the chlorination of cyclohexane and methylcyclohexane. In my opinion, the work leaves a feeling of incompleteness. In the absence of motivation to search for correlations between the structure of complexes and their catalytic activity, this work contains factual material with the absence of any regularities. A similar observation applies to most works devoted to the synthesis of new complexes and further applications in C-H functionalization.
I remain in the opinion (in terms of organic chemistry) that in the absence of NMR spectra of the products obtained using different imaging modes, it is impossible to say that the authors obtained exactly 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohex-ene. If the authors agree on a simple route, without strong confirmation and only on the basis of GC-MS they can claim that monochlorinated methylcyclohexane, methylcyclohexenes (without specifying the double bond) are obtained.

Reviewer 3 Report

All my concerns have been addressed. So I recommend to accept this manuscript for publication in the present form.