Copper-Catalyzed Alkylative Deoxygenation of O-Substituted Hydroxamic Acid Derivatives with Grignard Reagents: A Combined Experimental and Computational Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper describes a novel "alkylative deoxygenation" reaction, wherein CuCN and Grignard reagents act synergistically to convert N–O bonds into N–C bonds. By expanding the substrate scope of copper-catalyzed electrophilic amidation strategies and elaborating on the reaction mechanism via detailed DFT calculations, the work demonstrates considerable application potential. Nevertheless, the current manuscript exhibits notable deficiencies in substrate scope, synthetic practicality, mechanistic completeness, and textual standardization. Substantial revisions are therefore prerequisite for the manuscript to be considered for acceptance. Specific revision requirements are outlined below:

1. In the Introduction, the authors present a systematic overview of copper-catalyzed "N–O electrophilic amidation" reactions as well as the utility of Grignard reagents in introducing alkyl and aryl moieties. A critical gap, however, lies in the lack of pioneering commentary on the uniqueness of the "Grignard reagent + Cu catalyst" system for C–N bond construction. To enhance the manuscript’s logical coherence, the authors are advised to supplement this section.
2. The Introduction would also benefit from a supplementation of the advantages and distinctiveness of the present study, as this will help underscore the work’s novelty. Additionally, the authors may wish to compare CuCN with other metals (e.g., Pd, Ni, Co) in analogous transformations, thereby emphasizing the cost-effectiveness, uniqueness, and sustainability of the copper catalytic system.
3. In the condition optimization section, while the authors have evaluated various copper catalysts, they have not elaborated on the rationale behind the selection of the optimal reaction temperature, copper catalyst loading, or Grignard reagent equivalents. Supplementing these details is recommended to improve the rigor of the experimental work.
4. To confirm the pivotal role of the copper catalyst in the reaction system, the authors are encouraged to add a set of control experiments without the copper catalyst, which can be included either in the condition optimization or mechanistic validation section.
5. The inclusion of "scale-up experiment" data is also recommended. The authors may select a model reaction with high yield, increase the substrate dosage, and conduct the reaction under standard conditions to verify the scalability of the proposed methodology.
6. Currently, the mechanistic validation section relies solely on DFT calculations. To corroborate the reaction mechanism, the authors should supplement direct detection and verification experiments (such as NMR and HRMS) aimed at capturing signals of reaction intermediates.
7. In Section 2 ("Results and Discussion"), the preparation and formatting of all figures and tables require further refinement. For instance, the correlation between structural diagrams and table entries in Table 2 is unclear. The authors are therefore advised to optimize all figures and tables in this section to facilitate reader comprehension.
8. The manuscript contains several textual and formatting inconsistencies that demand revision. Examples include inconsistent explanations of "ND" and "NR" in the footnotes of Tables 1 and 2, as well as disorganized formatting of Reference 22.
9. The Supporting Information requires further enhancement and supplementation. For example, the current version only provides spectral data for selected compounds. To strengthen the authenticity of the experimental results, the authors should supplement complete spectral information for all compounds reported in Tables 1 and 2 of the main text.

Author Response

This paper describes a novel "alkylative deoxygenation" reaction, wherein CuCN and Grignard reagents act synergistically to convert N–O bonds into N–C bonds. By expanding the substrate scope of copper-catalyzed electrophilic amidation strategies and elaborating on the reaction mechanism via detailed DFT calculations, the work demonstrates considerable application potential. Nevertheless, the current manuscript exhibits notable deficiencies in substrate scope, synthetic practicality, mechanistic completeness, and textual standardization. Substantial revisions are therefore prerequisite for the manuscript to be considered for acceptance. Specific revision requirements are outlined below:

 

1. In the Introduction, the authors present a systematic overview of copper-catalyzed "N–O electrophilic amidation" reactions as well as the utility of Grignard reagents in introducing alkyl and aryl moieties. A critical gap, however, lies in the lack of pioneering commentary on the uniqueness of the "Grignard reagent + Cu catalyst" system for C–N bond construction. To enhance the manuscript’s logical coherence, the authors are advised to supplement this section. 

 

(our reply) We thank this Reviewer for his/her useful comment that gives a greater relevance to our paper. We have now the following new statement (lines 43-49) in the introductory which includes three new pertinent references (14, 15 and 18).

“Pioneering research in this field was carried out by Johnson and co-workers using organozinc reagents and copper catalysts and N,N-dialkyl-O-acyl hydroxylamine derivatives [14,15]. Subsequently, R₂N⁺ synthons were used as effective nitrogen source for a variety of organometallic reagents, but this reactivity was limited to O-substituted oxime-derivatives to give primary amines after hydrolysis [16,17]. In this scenario, the use of Grignard reagents especially in combination with copper salts has important consequences for selectivity and reaction pathways [17,18]. In particular, the reactivity across oxime derivatives is postulated to occur by the intermediacy of nitrenoid species ending with a sort of S_N2 displacement of the oxygen [16] (Scheme 1, eq. a).”

 

2. The Introduction would also benefit from a supplementation of the advantages and distinctiveness of the present study, as this will help underscore the work’s novelty. Additionally, the authors may wish to compare CuCN with other metals (e.g., Pd, Ni, Co) in analogous transformations, thereby emphasizing the cost-effectiveness, uniqueness, and sustainability of the copper catalytic system.

 

(our reply) In the Introduction, we believe to have clearly highlighted the distinctive aspects of our work in relation to the state of the art. Accordingly, we did not examine other metals, as they fall outside the scope of our research.” It is amply known that copper salts have unique reactivity in combination with Grignard reagents and they are sustainable and cheap as metal.

 

3. In the condition optimization section, while the authors have evaluated various copper catalysts, they have not elaborated on the rationale behind the selection of the optimal reaction temperature, copper catalyst loading, or Grignard reagent equivalents. Supplementing these details is recommended to improve the rigor of the experimental work.

 

(our reply) We have used for the sake of uniformity the same reaction conditions (THF, 5 hours at 0 °C, 3.0 equiv. of Grignard reagent, and 0.2 equiv. of Cu. This was done in order to compare the reactivity of the different starting material of type 4. As expected, we have observed, in limited experiments that lowering the amount of Grignard reagents and of copper catalyst does not allow complete conversion of the starting material. However, we have not performed a systematic investigation and we believe these data does not give an added value to the present work.

 

4. To confirm the pivotal role of the copper catalyst in the reaction system, the authors are encouraged to add a set of control experiments without the copper catalyst, which can be included either in the condition optimization or mechanistic validation section.

(our reply) We have now added in Table 1 (new entry 6) that the reaction carried out without any copper salt showed no conversion.

 

5. The inclusion of "scale-up experiment" data is also recommended. The authors may select a model reaction with high yield, increase the substrate dosage, and conduct the reaction under standard conditions to verify the scalability of the proposed methodology.

(our reply) An experiment on a 2.0 mmol scale has now been performed, described in the experimental section (lines 478-482)  and this data have been included in Table 1 as a new entry 13.

 

6.  Currently, the mechanistic validation section relies solely on DFT calculations. To corroborate the reaction mechanism, the authors should supplement direct detection and verification experiments (such as NMR and HRMS) aimed at capturing signals of reaction intermediates.

(our reply) In the course of our experimental work, we considered the possibility of detecting reaction intermediates; however, no such detection was successful both using NMR and HRMS.

 

7. In Section 2 ("Results and Discussion"), the preparation and formatting of all figures and tables require further refinement. For instance, the correlation between structural diagrams and table entries in Table 2 is unclear. The authors are therefore advised to optimize all figures and tables in this section to facilitate reader comprehension.

 

(our reply) We warmly thank this Reviewer to bring this aspect to our attention. In fact, there were some inconsistencies in the numbering of compound of type 3 in Table 2 and relative Figure of the Table that now have been corrected.

8. The manuscript contains several textual and formatting inconsistencies that demand revision. Examples include inconsistent explanations of "ND" and "NR" in the footnotes of Tables 1 and 2, as well as disorganized formatting of Reference 22.

 

(our reply) The explanation of the meaning of ND and NR has been provided in the footnote of Tables 1 and 2.

We have now adjusted the punctuations of Ref. 22.

 

9. The Supporting Information requires further enhancement and supplementation. For example, the current version only provides spectral data for selected compounds. To strengthen the authenticity of the experimental results, the authors should supplement complete spectral information for all compounds reported in Tables 1 and 2 of the main text.

 

(our reply) We have now displayed the NMR data of all compounds described in the paper obtained by us (even if there were already known in the literature).

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Pineschi, Favero and coworkers discovered the CuCN catalyzed method for nucleophilic alkylative deoxygenation of secondary hydroxamic acids. This manuscript presents a highly compelling and well-executed methodology along with the computational study to understand the mechanistic details of the reaction. I believe it requires only minor revisions before acceptance. The authors convincingly demonstrate the methodology using various secondary hydroxamic acids along with series of Grignard reagents. Overall, the readers of this journal will be highly interested in present strategy either for synthesizing these types of compounds or to mimicking it for other relevant chemistry. With few minor revisions suggested, this work is of high quality and fully suitable for publication in Molecules Journal. One comment: Please correct the compound numbers in sentence on line 76, “we hypothesized that compound 2a could arise alkylation of oxyamide 2a”

Author Response

Pineschi, Favero and coworkers discovered the CuCN catalyzed method for nucleophilic alkylative deoxygenation of secondary hydroxamic acids. This manuscript presents a highly compelling and well-executed methodology along with the computational study to understand the mechanistic details of the reaction. I believe it requires only minor revisions before acceptance. The authors convincingly demonstrate the methodology using various secondary hydroxamic acids along with series of Grignard reagents. Overall, the readers of this journal will be highly interested in present strategy either for synthesizing these types of compounds or to mimicking it for other relevant chemistry. With few minor revisions suggested, this work is of high quality and fully suitable for publication in Molecules Journal. One comment: Please correct the compound numbers in sentence on line 76, “we hypothesized that compound 2a could arise alkylation of oxyamide 2a”

(our reply) We warmly thank this Referee for the positive comment and to bring to our attention this mistake that has now been corrected at line 83 of the revised paper.

Reviewer 3 Report

Comments and Suggestions for Authors

Please find my review report in the attached file.

Comments for author File: Comments.pdf

Author Response

 

“The manuscript is interesting; however, it still requires several improvements before it can be considered for publication.

From the experimental chemistry side, I did not identify any clear issues. The main concerns, in my view, lie in the DFT section.

It would be very beneficial for the manuscript to include a kinetic analysis supported by IRC (intrinsic reaction coordinate) calculations. This would allow the authors to propose fully consistent theoretical reaction pathways and report the corresponding activation barriers.  At present, the discussion relies mainly on ΔG values, but the rate-determining step of a reaction is not necessarily governed by overall free-energy changes; it is often controlled by the activation energy (Ea/ΔG‡). I therefore encourage the authors to add an IRC-based kinetic study to clarify whether the proposed mechanism is primarily favored kinetically or thermodynamically, which would substantially strengthen the mechanistic interpretation”.

(our reply) We believe that the reviewer’s comment may arise from a misunderstanding. In the manuscript, the analysis is centered throughout on the activation barriers. The discussion explicitly and consistently addresses ΔG^‡ values and ΔΔG^‡ differences for both key steps of the mechanism (acyl‑nitrene formation, which is the rate‑determining step, and nitrogen methylation). This focus reflects our goal of highlighting how the substrate structure, in particular the O-R² group, modulates the activation energy.

The doc and pdf versions available on the submission portal display the “‡” symbol correctly, so the issue likely originated during file download or conversion has now been fixed.

In addition, Scheme 3 reports the full energy profile, including all minima and transition states, with their energies relative to the reference point. From this representation, all activation barriers can be directly extracted. To make the comparison even clearer, we also included two tables within Scheme 3 summarizing the forward and reverse ΔG^‡ values.

Regarding the reviewer’s suggestion about IRC and kinetic characterization, IRC calculations were performed for all transition states, as reported in the section “Material and Methods-Computation part For the methylation step—where several intermediates are involved—we also employed the NEB (Nudged Elastic band) method, allowing a detailed mapping of the potential energy hypersurface (see Materials and Methods-Computational part). These analyses confirm the connectivity of the transition states and support the experimental results.

“A DFT study should not be limited to reporting Gibbs free energies. It is also important to examine additional, straightforward descriptors—such as intra- and intermolecular interactions in key intermediates and transition states. For example, RDG (reduced density gradient) analysis provides a qualitative and visual way to assess noncovalent interactions and can help clarify the nature of the contacts present in the TS. Therefore, I recommend that the authors include RDG isosurfaces/plots for the relevant transition states.

(our reply) The mechanism we propose consists of two steps. In the first one (the rate‑determining step), an heterolytic cleavage of the covalent N–O bond occurs, leading to the formation of a reactive intermediate, the acyl‑nitrene, in the second one, nitrogen methylation takes place through reaction with an organometallic species (the alkycuprate). The bonds involved in these elementary steps are thus covalent or organometallic in nature.

As the reviewer correctly notes, the RDG (Reduced Density Gradient) analysis highlights regions of low electron density and small density gradients, which are characteristic of non‑covalent interactions (e.g., van der Waals contacts, weak hydrogen bonds, etc.), and not of processes dominated by covalent bond breaking and forming. For this reason, RDG analysis is not expected to provide a useful additional mechanistic insight for the present reaction pathway.

“In addition, the dual descriptor can help pinpoint the most nucleophilic and electrophilic regions/atoms, and this interpretation can be further supported by population analyses (e.g., NBO charges and donor–acceptor interactions). Including these complementary descriptors would strengthen the electronic-structure rationale behind the proposed mechanism.

 

(our reply) As suggested by the reviewer, we performed the second–order perturbation analysis of the Fock matrix in the NBO basis. The results were very informative and helped us to clarify the specific aspects we were interested in, namely the precise identification of the electronic factors responsible for the experimentally observed differences in reactivity among the various substrates. In particular, this analysis allowed us to pinpoint the electronic effects stabilizing both the transition state and the acyl‑nitrene intermediate involved in the rate‑limiting step (the heterolytic cleavage of the N–O bond). For this reason, structures reported in Scheme 3 have been updated accordingly, and the corresponding results have been incorporated and discussed in the revised manuscript. A new figure highlighting the strong stabilization of the nitrene intermediate by the alkylcuprate fragment has been included in the text as “Figure 2”. However, in order not to overburden the main text, given the predominantly experimental nature of the article, the detailed NBO data (Tables S1 and S2) have been placed in the Supporting Information section.

Regarding the viewer’s suggestion to employ the “dual descriptor” based on the Fukui function, in our opinion this tool is not particularly informative in the present case. In the first step (the rate‑limiting step), the mechanism involves the heterolytic cleavage of the N–O bond, leading to the formation of a nitrene species whose strong electrophilic character is well known. In our system, this electrophilic center is strongly stabilized by the electron‑donating effect of the Cu–R fragment, which is typically nucleophilic. Moreover, no alternative regiochemical outcomes are possible in this process. Nevertheless, out of curiosity, we computed the f⁻ function for intermediate A‑IV, and, as expected, the nitrogen atom was identified as the most electrophilic site in the molecule. Since this result is straightforward and does not provide additional mechanistic insight, we opted not to include it in the manuscript.

 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I highly appreciate the authors' dedicated efforts and the significant progress achieved during the revision. Upon careful re-evaluation, I find that the authors have effectively addressed most of the questions and suggestions raised in previous review, and the overall quality of the manuscript has been markedly improved. In summary, this study reveals the unique reactivity enabled by the combination of catalytic copper(I) cyanide and Grignard reagents, successfully achieving the deoxygenation of secondary hydroxamic acids bearing weak leaving groups to furnish secondary amides. Although the synthetic utility of this method is still not fully optimized, the work clearly identifies the key factors enabling the observed reactivity through a combination of experimental and theoretical investigations, offering new insights into the reactivity and speciation of organomagnesium-copper complexes. The experimental design is generally sound, the data are reliable, and the discussion is logically coherent. After addressing the following minor issues, the manuscript will be suitable for publication. The specific revision comments are as follows:

1. Correction of numbering in the main text: There is a numbering error in the main text, where the authors mistakenly refer to "Entry 7" in Table 1 as "Entry 6". Please correct this to ensure consistency between the text and the table entries.
2. Clarification in Tables 1 and 2: It is recommended to explicitly label the structures of compound 3 and compound 4 in the reaction schemes provided in Tables 1 and 2. This will help readers quickly identify the corresponding structures.
3. Improvements to Figure 1: Please assign consistent numbering to the transition states leading from Complex I and Complex II to the acyl nitrene intermediate to enhance the readability of the figure. Additionally, ensure uniform formatting for the labels of the R¹ and R² groups throughout the figure to maintain typographical consistency.
4. Differentiation in Scheme 3: The numbering of transition states in Scheme 3 may be easily confused with the numbering of complexes in Figure 1. It is advisable to use distinct numbering systems for the two sets of species to improve clarity and avoid ambiguity, thereby aiding readers in following the correspondence between key intermediates.

Author Response

1.    Correction of numbering in the main text: There is a numbering error in the main text, where the authors mistakenly refer to "Entry 7" in Table 1 as "Entry 6". Please correct this to ensure consistency between the text and the table entries.

(our reply) We thank this referee to bring this mistake to our attention. The correction has accordingly been done.

2.    Clarification in Tables 1 and 2: It is recommended to explicitly label the structures of compound 3 and compound 4 in the reaction schemes provided in Tables 1 and 2. This will help readers quickly identify the corresponding structures.

(our reply) We believe that all structures of compounds of type 3 and 4 are clear in the succinct way we have proposed 

3.    Please assign consistent numbering to the transition states leading from Complex I and Complex II to the acyl nitrene intermediate to enhance the readability of the figure. Additionally, ensure uniform formatting for the labels of the R¹ and R² groups throughout the figure to maintain typographical consistency.

(our reply) In response to the reviewer’s comment, Figure 1 has been modified accordingly. Specifically, we have added the labels for the transition states involved in the rate determining step, as well as the corresponding reference to the energy profile scheme. Additionally, we took this opportunity to standardize the font, style, and character size across the entire manuscript. For example, boldface is now consistently applied to “Scheme”, “Table”, “Figure”, and to the structure labels.

4.    Differentiation in Scheme 3: The numbering of transition states in Scheme 3 may be easily confused with the numbering of complexes in Figure 1. It is advisable to use distinct numbering systems for the two sets of species to improve clarity and avoid ambiguity, thereby aiding readers in following the correspondence between key intermediates.

(our reply) We have decided to maintain our original numbering system in order to avoid excessively high numbering. However, we have implemented several modifications in the main text, in Scheme S2, and in Table S1 to eliminate any possible source of confusion. As clarified in the revised version, the reference point for each scheme (0.0 kJ/mol) is complex I, and the structures appearing along the energy profile follow a consistent progressive numbering.
Since the transition state for acyl nitrene formation in Scheme S2 is associated with complex II rather than complex I, Scheme S2 has been corrected accordingly: complex II is now explicitly included, and the former TSII and Int III have been renamed TSIII and Int IV, respectively. Consistent updates have been applied to Table S1 and to the “Cartesian Coordinates and Thermochemical Data” section.
In the main text, we have also revised the caption of Scheme 3. The expression “key-intermediate structures” (which could be misinterpreted) has been replaced with “structures of key intermediate A, B, and C,” clarifying the type of key intermediates discussed.  Moreover, we have explicitly specified “(a)” for allyl based substrates and “(b)” for benzyl based substrates.
Finally, in lines 229–237, we now refer to structures TSIII with explicit mention of the corresponding scheme (Scheme S2 or Scheme 3) and the relevant table (Table S1), in order to ensure complete clarity and avoid ambiguity.

 

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

 Accept in present form

Author Response

We thank the Referee for his/her comment