A Calix[8]arene-Based Catalyst for Suzuki–Miyaura Couplings with Reduced Pd_Leaching

Round 1

Reviewer 1 Report

This paper describes a calix[8]arene-based palladium catalyst for Suzuki-Miyaura coupling. The newly synthesized catalyst exhibited relatively low reactivity toward the Suzuki-Miyaura coupling reaction, but palladium leaching in the coupling product was strongly reduced by the calix[8]arene core. The results would be valuable for catalytic chemists. Therefore, this reviewer recommends this paper for publication in Catalysts. The following issues are to be addressed in the revision of the manuscript.

1) The yields of compounds 7 and 8 need to be shown in the Experimental Section.

2) Information on the product selectivity in the coupling reaction should be added. 

3) The elemental analysis data or high-resolution mass spectrum for complex 8 should be added.

Minor items

1) p.1, line 48: activites -> activities

2) p.2, line 80: It -> it

3) p.3, line 98: beeing -> being

4) p.4, line 99: that -> than

Author Response

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Reviewer 2 Report

Comments:

In this paper, the authors showed a quinoline decorated calix[8]arene-based carbenic catalyst for Suzuki-Couplings, based on the chelating effect of the coordinating, electron-deficient quinoline ligand, the Pd species can be stabilized and decreasing the Pd leaching during the catalysis. This is an interesting perspective for synthesis new catalysts with good performance. However, there are some major issues need to be resolved.

1.      The synthesis process of the final target Cat. 8 was showed reasonable, but the characterizations for the catalyst is insubstantial with only ¹H NMR and XPS, the ¹³C NMR spectra didn’t show any clear signal to ensure the structure of 8. We couldn’t make sure the precursor 7 is accurately obtained and the Pd species is indeed anchored based on the characterizations at present. Maybe MS or X-ray single crystal measurements or other alternative methods would be considered necessary to support this interpretation.

2.      The Pd loading in all the Tables are denoted as 0,001; 0,5; 0,05 etc. should revised as 0.001; 0.5, 0.05…

3.      The Pd leaching was evaluated by compare with Cat. 2 on one times, the result may be by chance. The author should compare the leaching with more than three catalyst, and test more than three times. The Line 116-121 at Page 4 show that the reduced Pd species leaching in Cat 8 probable due to the coordination, electron-deficient, pi-extended of the quinoline moiety, do you have any evidence for this conclusion?

4.      Why the calix[8]arene employed? What role did it played in the reactions? Is it possible that the leached Pd species be caught by calix[8]arene?

Author Response

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Reviewer 3 Report

This work (communication) continues a series of articles by the authors devoted to the development of new catalysts for cross-coupling reactions based on calix[8]arene and NHC palladium complexes. In this work, the authors introduced a chelating quinoline fragment into the ligand structure, which should make it possible to reduce the leakage of palladium from the catalyst. The work is a logical development of the previous studies of the authors and can be published, but there are several comments.

1) The work is somewhat carelessly designed (references format, fonts, line thickness on schemes, different schemes formats, experimental part format, etc.), all this should be corrected. It seems that the text was written in a great hurry.

2) The resulting catalyst must be additionally characterized using HRMS, this is necessary, especially considering the broadened lines in the NMR spectra.

3) HSQC spectrum cannot be analyzed, authors should at least label cross peaks with color, if any.

4) line 88, "broad scope" is an incorrect statement, the authors used only 6 substrates. It is more logical to speak of a representative scope.

5) Table 1 is very inconvenient to read - combined wells with solvent make analysis very difficult. In addition to the conversion, the output should be indicated in the table. One should also compare TON and TOF.

6) The key achievement - the significance of the results obtained is reflected in Table 2 (in fact, these are only two experiments). The results obtained should be compared with other catalysts described in the literature, and not only with representatives of the series of the authors of the work. And the experiment with the separation of the catalyst itself should be described  more carefully. Can the catalyst be used again? In general, the work obviously lacks references to the literature, now they are limited to general reviews and previous works of the authors. In this case, the comparison is carried out with only one representative of calixarene catalysts.

6) Conclusions should be reduced in volume - they are too wordy.

Author Response

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Reviewer 4 Report

The authors report a really interesting synthesis of a new NHC/quinoline calixarene ligand for palladium catalysis, demonstrating its utility in Suzuky-Miyaura couplings, and as main part of the paper, analysing the palladium-leaching. In spite of this the manuscript needs still to be perfectionate, indeed it contains some typos (i.e. plateforms, éq).

Moreover, the bibliography needs to be expanded, in particular in the context of organometallic chemistry and calixares catalysis.

 

Considering the experimental results, there are also few criticisms:

 

1.       The authors performed a substrate scope almost restricted to para-substituted aryl bromides in combination with phenyl boronic acids. No results about aryl triflates and iodides are given, moreover in order to show the applicability of the new unreported calixarene synthesis more challenging ortho-substituted substrates should be discussed in the paper. I recommend the author to provide those results on the revision.

2.       The authors reports the ICP-MS analyses without error bars. At least a basic error evaluation is required (standard deviation for example) if it possible.

 

In light of my considerations, I strongly recommend the publication of this paper in Catalyst after a major revision (point 1 is mandatory, point 2 is optional).

 

Given all these considerations, I strongly reccomend the pubblication of this paper on this journal after a major revision.

Author Response

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Round 2

Reviewer 2 Report

In this paper, the authors showed a quinoline decorated calix[8]arene-based carbenic catalyst for Suzuki-Couplings, based on the chelating effect of the coordinating, electron-deficient quinoline ligand, the Pd species can be stabilized and decreasing the Pd leaching during the catalysis. This is an interesting perspective for synthesis new catalysts with good performance.  I hope it would be accept now.

Author Response

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Reviewer 3 Report

The authors took into account all comments and improved the manuscript. It seems strange that all attempts to use the methods of MS analysis were unsuccessful, but in this case we have to trust the authors. A brief explanation (based on authors response) should be added to the part of the article devoted to the synthesis of the catalysts. The article can be published.

Author Response

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Reviewer 4 Report

Dear authors, I would reccomend to perform the experiments I suggest on the point 1 of my previous referee report.

At least aryl triflates and aryl iodides must be shown on  simple substrates.

I am confident that those few experiments will be included on the final version of the manuscript.

Author Response

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