Green Dynamic Kinetic Resolution—Stereoselective Acylation of Secondary Alcohols by Enzyme-Assisted Ruthenium Complexes

Round 1

Reviewer 1 Report (Previous Reviewer 3)

The authors revised the manuscript according my previous comments. In my opinion, the manuscript looks much improved after revision. Some minor comment:

1) No HPLC traces for the racemic ester of 1-S-phenylethanol and for the racemic and enantiopure 1-(1-naphthyl)ethanol.

2) There is mistake in ref. 8.

Author Response

The response to the remarks of Reviewer 1.

We appreciate your work of reviewing our work. We introduced changes according to your remarks as listed below. We are convinced that the revision, according to your suggestions has risen the quality of our work.

The authors revised the manuscript according my previous comments. In my opinion, the manuscript looks much improved after revision. Some minor comment:

1) No HPLC traces for the racemic ester of 1-S-phenylethanol and for the racemic and enantiopure 1-(1-naphthyl)ethanol.

The processes have been monitored by GC. Appropriate chromatograms - the results of racemization and DKR has been padded to the Supplementary Materials.

2) There is mistake in ref. 8.

Thank you. The mistake has been corrected.

The work has been corrected by an English proofreading office with a Native speaker who holds PhD in chemistry.

Reviewer 2 Report (New Reviewer)

Reviewer’s Comments:

The manuscript “Green dynamic kinetic resolution-stereoselective acylation of secondary alcohols by enzyme-assisted ruthenium complexes” is very interesting work. In this study, The resulting catalytic system of the ruthenium racemization catalysts and enzymatic acylation led to chiral esters obtained by dynamic kinetic resolution. The immobilized catalytic system in the ionic liquid expresses the same activity and selectivity as the homogeneous system while allowing for convenient separation of the desired products. Additionally, the efficient reuse of the catalytic system has been demon strated fulfilling the paradigm of green processes. I believe this topic is of great interest to our reader, I think it needs minor revision before it is ready for publishing. So, I strongly recommend this manuscript for publication in the Journal of Catalysts with major revisions.

1. In this manuscript, the authors did not explain the importance of ruthenium complexes in the introduction part. The authors should explain the importance of ruthenium complexes.

2. The author should provide reason about this statement with recent references “The immobilized catalytic system in the ionic liquid expresses the same activity and selectivity as the homogeneous system while allowing for convenient separation of the desired products”.

3. Introduction part is not impressive and systematic. Cite the following articles in the introduction part. (i) 10.3390/molecules27196580 (ii) 10.3389/fchem.2022.995820

4. The authors should justify the following statement “The possibility of creating an efficient heterogeneous catalyst in which the ruthenium complex is non-covalently attached to support was also sought due to the simplicity of the system and the least influence of the catalytic properties”.

5. The authors should explain regarding the recent literature why “The studies showed that neither the MWCNT support nor the IL affects the activity of the ruthenium catalyst”.

6. The authors should explain the statement with reference to the literature. “This is because the catalyst is confined in a small volume of the IL as opposed to being dispersed throughout the entire volume in the absence of the addition of an IL”.

7. Comparison of the present results with other similar findings in the literature should be discussed in more detail. This is necessary in order to place this work together with other work in the field and to give more credibility to the present results.

8. The conclusion part is very weak. Improve by adding the results of your studies.

9. The authors should pay more attention to the English grammar, and the abbreviation of journal names in Ref.

Author Response

Reviewer 2

We appreciate your work of reviewing our work. We introduced changes according to your remarks as listed below. We are convinced that the revision, according to your suggestions has risen the quality of our work.

The manuscript “Green dynamic kinetic resolution-stereoselective acylation of secondary alcohols by enzyme-assisted ruthenium complexes” is very interesting work. In this study, The resulting catalytic system of the ruthenium racemization catalysts and enzymatic acylation led to chiral esters obtained by dynamic kinetic resolution. The immobilized catalytic system in the ionic liquid expresses the same activity and selectivity as the homogeneous system while allowing for convenient separation of the desired products. Additionally, the efficient reuse of the catalytic system has been demonstrated fulfilling the paradigm of green processes. I believe this topic is of great interest to our reader, I think it needs minor revision before it is ready for publishing. So, I strongly recommend this manuscript for publication in the Journal of Catalysts with major revisions.

1. In this manuscript, the authors did not explain the importance of ruthenium complexes in the introduction part. The authors should explain the importance of ruthenium complexes.

Thank you for the suggestion. The importance of ruthenium catalysts has been emphasized in the introduction (lines 42-46) and several works have been additionally cited.

2. The author should provide reason about this statement with recent references “The immobilized catalytic system in the ionic liquid expresses the same activity and selectivity as the homogeneous system while allowing for convenient separation of the desired products”.

This statement in the abstract has been made more precise by providing the observed yield and ee selectivity.

3. Introduction part is not impressive and systematic. Cite the following articles in the introduction part. (i) 10.3390/molecules27196580 (ii) 10.3389/fchem.2022.995820

Thank you for the remark. The introduction section has been revised. The citation has been added to the conceptual description (lines 100-104)

4. The authors should justify the following statement “The possibility of creating an efficient heterogeneous catalyst in which the ruthenium complex is non-covalently attached to support was also sought due to the simplicity of the system and the least influence of the catalytic properties”.

Thank you. As suggested, we clarified and justified this idea.

5. The authors should explain regarding the recent literature why “The studies showed that neither the MWCNT support nor the IL affects the activity of the ruthenium catalyst”.

Thank you for pointing us the unclear statement. It is the conclusion of our blank test presented in Table 1. To make it more unequivocal, the sentence has been modified.

6. The authors should explain the statement with reference to the literature. “This is because the catalyst is confined in a small volume of the IL as opposed to being dispersed throughout the entire volume in the absence of the addition of an IL”.

The statement has been justified and referenced to the theory of the IL’s impact on catalysis.

7. Comparison of the present results with other similar findings in the literature should be discussed in more detail. This is necessary in order to place this work together with other work in the field and to give more credibility to the present results.

We agree with the Reviewer’s view. Therefore, we have provided a comparison and references to the results presented in the literature (lines 200-220).

8. The conclusion part is very weak. Improve by adding the results of your studies.

The conclusions have been elaborated on and presented in more detail, which according to your suggestions gives a more precise view of the work’s results. Thank you for helping us increase the level of your work.

9. The authors should pay more attention to the English grammar, and the abbreviation of journal names in Ref.

The work has been corrected by an English proofreading office with a Native speaker who holds PhD in chemistry.

The list of references has been revised and corrected.

Round 2

Reviewer 2 Report (New Reviewer)

Thank you for addressing the comments. The new insertions are relevant. Could be accepted in the present form.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The manuscript entitled “Green dynamic kinetic resolution exemplified on stereoselective acylation of secondary alcohols” authored by Nikodem Kuźnik and co-workers describes the use of non-covalent immobilization of ruthenium complexes on multi-wall carbon nanotubes as heterogeneous catalysis of the dynamic kinetic resolution process in the presence and absence if ionic liquids to enhance the racemization.

The authors have described the non-covalent immobilization of ruthenium complexes on multi-walled carbon nanotubes and their characterization data. They also presented various experiments to show the differences in racemization efficacy of their catalysts and the Recycling ability.

The addition of ionic liquids seems to have some impact on the acceleration of the racemization process.

On the other hand, in the final reaction, there is no significant difference in the rate of the DKR process was noticed. Moreover, with and without ionic liquids, the yield and enantiomeric excess were found same.  The authors themselves agree that “MCF-deposited ruthenium complexes obtained in the past by our research group had good racemization properties”.

Hence, the present manuscript lacks the novelty to be published in catalysis.  

Author Response

Thank you very much for your comments and review.

There is a subtle influence of the addition of an ionic liquid on the DKR process. In addition, the novelty is the study of the influence of the ionic liquid on the work of the catalyst and the creation of a unique heterogeneous system that allows the use of the possibilities of a difficult-to-immobilize ruthenium catalyst.

An appropriate explanation of the novelty has been added to the introduction. The entire manuscript has been thoroughly revised. 

We believe that the revision will meet your expectations.

We appreciate your work in reviewing our manuscript.

Kind regards,

Nikodem KUŹNIK

Reviewer 2 Report

Ionic liquid assisted dynamic kinetic resolution using ruthenium complexes and lipases has been developed and reported as the references 1 and 25 cited. The authors tried to add multi-wall carbon nanotubes (MWCNT) to recycle the catalysts; however, they did not show their results using Ru(1)-MWCNT or Ru(1)-IL-MWCNT catalysts for DKR experiments at all! Thus their progress or improvement along this direction is not known.

Others:

It seems that the interaction between Ru and MWCNT is non-covalent, pi-pi stacking. Therefore, leaking of Ru catalysts during recycling should be an issue. The interaction between IL and MWCNT is not clear, either. The authors should justify the combination of Ru complexes, lipases with IL and MWCNT.

Author Response

Thank you very much for your comments and review.

There is also little work on the use of immobilized ruthenium complexes in DKR, especially when it comes to non-covalent immobilization. This work addresses both of these problems by showing the possibilities and disadvantages of the non-covalent immobilization of one of the best-performing ruthenium catalysts. On the other hand, it shows the effective use of the combination of the ruthenium complex, the ionic liquid and the enzyme as a complex heterogeneous system. The system presented in the publication is two-phase. The organic solvent phase contains the substrate first, then the product, while the ionic liquid phase contains the ruthenium complex, the enzyme, and the activating base. A pure product can be obtained by simply separating toluene from the ionic liquid and by evaporating the solvent present in very small amounts. Compared to the work of Kim and Park, where, although the DKR itself did not require the use of a solvent, only an ionic liquid, and the product had to be extracted, here it is possible to easily separate the systems and reuse the mixture with the ionic liquid.

An appropriate explanation of the novelty has been added to the work (introduction and results & discussion).

We believe that the revision will meet your expectations.

We appreciate your work in reviewing our manuscript.

Kind regards,

Nikodem KUŹNIK

Reviewer 3 Report

The authors report the investigation of dynamic kinetic resolution of alcohols in the presence of RuCp catalysts and enzyme. After the reading of the manuscript, I have comments (see below), and I recommend to rewrite the introduction (to explain, what is the aim of this study; what was done in the literature on this topic), more clearly and in details provide all results and perform discussion part. Next comments have to be addressed:

1) From introduction it is not clear what is new in this manuscript in comparison with literature data.

2) According green chemistry: I don’t see any results in paper which proofs the concept (IL used as an additive and not as a solvent, solvent is toluene, catalyst is homogeneous!).

3) Moreover, there is no role of IL in the reaction (see Table 3).

4) I cannot understand why the authors didn’t perform the DKR reaction with Ru(1)-MWCNT and Ru(1)-IL-MWCNT. Otherwise, what was the benefit to investigate the heterogenous system in racemization reaction? Moreover, in abstract is indicated: ”It has been demonstrated that immobilization results in convenient separation of the enantiomeric product and reuse of the catalytic system full-filling the green development approach”, however, I don’t see this result in manuscript, or maybe I’m wrong…

5) For ”The study showed that neither the MWCNT support nor the IL adversely affects the operation of the ruthenium catalyst. Then, a blank test was performed which confirmed that neither MWCNT nor IL solely could racemize secondary alcohols”, the authors have to provide the data in Table 1.

6) What the authors wanted to say about enzyme on Lines 125-133, if they still talking about racemization and influence of IL on it on Lines 134-137.

7) No detailed procedure about immobilization of Ru complexes on carbon nanotube.

8) I’m not familiar with TGA analysis, but how the authors could use the method to determine the Ru loading in the sample? Could them provide the detailed information?

9) Please, indicate more clearly the alcohols in a footnote, otherwise it was difficult to find them in the manuscript.

10) No conclusions about the recycling experiments (Table 2). Why the efficiency is lowering?

11) The Bäckvall group (doi/10.1021/ja051576x) already reported the work about application of RuCp complex and lipase in the DKR. It would be better to explain what is new here in comparison with that work.

12) Table 4. How the catalyst and enzyme was recycled?

13) How was determined the ee of the alcohol and the products?

14) There is a lot or errors in literature references:

-          Ref. 4 and 9 is same; further, ref. 2 and 12; ref. 7 and 23 are also same.

-          Ref. 20, no page numbers; ref. 22 – no publishing data

-          What is it under ref. 18?

Author Response

Dear Reviewer,

Thank you very much for your comments and review.

Please find our answers below.

• From introduction it is not clear what is new in this manuscript in comparison with literature data.

There is also little work on the use of immobilized ruthenium complexes in DKR, especially when it comes to non-covalent immobilization. This work addresses both of these problems by showing the possibilities and disadvantages of the non-covalent immobilization of one of the best-performing ruthenium catalysts. On the other hand, it shows the effective use of the combination of the ruthenium complex, the ionic liquid, and the enzyme as a complex heterogeneous system.

An appropriate explanation of the novelty has been added to the introduction.

• According green chemistry: I don’t see any results in paper which proofs the concept (IL used as an additive and not as a solvent, solvent is toluene, catalyst is homogeneous!).

Although the catalyst is homogeneous, the system presented in the publication is two-phase. The organic solvent phase contains the substrate first, then the product, while the ionic liquid phase contains the ruthenium complex, the enzyme, and the activating base. A pure product can be obtained by simply separating toluene from the ionic liquid and by evaporating the solvent present in very small amounts. Compared to the work of Kim and Park, where, although the DKR itself did not require the use of a solvent, only an ionic liquid, and the product had to be extracted, here it is possible to easily separate the systems and reuse the mixture with the ionic liquid.

• Moreover, there is no role of IL in the reaction (see Table 3).

The influence of the ionic liquid on the DKRu process is subtle, however, it is noticeable in the form of a higher yield of the desired product compared to the lack of addition of the ionic liquid while maintaining high enantioselectivity. Moreover, as stated below in Table 2, there is no very marked change upon the addition of the ionic liquid, however other advantages of using such a system are noted.

• I cannot understand why the authors didn’t perform the DKR reaction with Ru(1)-MWCNT and Ru(1)-IL-MWCNT. Otherwise, what was the benefit to investigate the heterogenous system in racemization reaction? Moreover, in abstract is indicated: ”It has been demonstrated that immobilization results in convenient separation of the enantiomeric product and reuse of the catalytic system full-filling the green development approach”, however, I don’t see this result in manuscript, or maybe I’m wrong…

DKR was not performed with the participation of immobilized ruthenium complexes on carbon nanotubes since significant leaching of the catalyst from this support was observed, which prevents its multiple uses and thus eliminates the possibility of using it as a heterogeneous catalyst. So there was no message to test these catalysts at the DKR. Nevertheless, the following tests were carried out: First, tests were carried out to see whether the system is capable of racemizing secondary alcohols at all. Obtaining very positive effects, the next step was the recycling of the heterogeneous catalyst, which did not give the expected results. Therefore, its further use was abandoned and new catalytic solutions were sought. A brief description of these studies is added below in Table 2.

The assumptions of "green development" described in the abstract are embedded in a convenient procedure for separating the product from the catalyst by separating two liquid phases.

• For ”The study showed that neither the MWCNT support nor the IL adversely affects the operation of the ruthenium catalyst. Then, a blank test was performed which confirmed that neither MWCNT nor IL solely could racemize secondary alcohols”, the authors have to provide the data in Table 1.

Thank you for pointing us to the missing information. Yes, a blind test was performed, and the relevant results confirming the validity of the hypothesis assumed were included in Table 1.

• What the authors wanted to say about enzyme on Lines 125-133, if they still talking about racemization and influence of IL on it on Lines 134-137.

Thank you for showing the inconsistency of the paragraph. We rearranged this section in order to make it more clear.

• No detailed procedure about immobilization of Ru complexes on carbon nanotube.

Thank you for the suggestions. The description of the procedure has been added to the experimental section.

• I’m not familiar with TGA analysis, but how the authors could use the method to determine the Ru loading in the sample? Could them provide the detailed information?

It is a routine analysis that generally assumes that “carbon, nitrogen, …” components could leave the sample as volatile products (mainly oxides), while the metallic components remain in the residue. Therefore, the mass of these components could be determined. This analysis is supported and consistent with SEM-EDS. Both data are presented in the Supporting information accompanying the manuscript.

• Please, indicate more clearly the alcohols in a footnote, otherwise it was difficult to find them in the manuscript.

Corrected according to the remark.

• No conclusions about the recycling experiments (Table 2). Why the efficiency is lowering?

The conclusion and probable explanation have been added beneath the Table 2.

• The Bäckvall group (doi/10.1021/ja051576x) already reported the work about application of RuCp complex and lipase in the DKR. It would be better to explain what is new here in comparison with that work.

Yes, the Bäckvall group described the cooperation of RuCp with lipase in DKR, but in their publication, it was the use of a homogeneous catalyst. In the case of our publication, we are looking for the possibility of using one of the best racemization catalysts in the form of a heterogeneous system.

• Table 4. How the catalyst and enzyme was recycled?

Both the catalyst and the enzyme remained in the ionic liquid phase (this phase was reused), while the toluene phase was separated together with the product. A description of the recycling procedure has been added to the Materials & Methods section.

• How was determined the ee of the alcohol and the products?

ee was determined using chiral stationary gas chromatography (exact data in the Material and methods section).

• There is a lot or errors in literature references:

Ref. 4 and 9 is same; further, ref. 2 and 12; ref. 7 and 23 are also same.

 Ref. 20, no page numbers; ref. 22

What is it under ref. 18?

The literature references have been corrected. Thank you for pointing out the mistakes.

We believe that the revision will meet your expectations.

We appreciate your work in reviewing our manuscript.

Kind regards,

Nikodem KUŹNIK

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have addressed the concerns and modified their introduction accordingly. 

Reviewer 3 Report

The authors revised the manuscript according the reviewers’ comments. Unfortunately, I didn’t find novelty and connection between the racemization part and DKR reaction in the manuscript. The racemization reaction of alcohols with heterogeneous catalysts is well known and not new. The effect of IL is not sufficient in the reaction. Some minor comments:

1) The DKR reaction have to be provided in Table 3.

2) HPLC traces for the racemic and enantiopure compounds have to be given.

3) The references should be in one manner.