Carboxylate-Assisted Carboxylation of Thiophene with CO2 in the Solvent-Free Carbonate Medium
Round 1
Reviewer 1 Report
In this manuscript the authors described a solvent-free carboxylation of thiophene using a mixture of salts as reagents. The yields obtained are vey low, being the best only 5% using 1.5 equivalents of the salts (60 mol% Cs2CO3 and 40 mol% potassium pivalate) at high temperature (280 oC) and high CO2 pressure (8 bar). Then, a mechanism pathway was proposed based on theoretical studies.
The experimental details should be furnished in the support information.
The authors called this reaction as a C-H activation; however, this hydrogen is already activated since the pKa≈32.5, this would be a rather a C-H functionalization.
This protocol should be optimized in order to increase the yield and the scope and limitations with substituted thiophene derivatives should also be investigated.
Recently, Lee et al. reported the Ag(I)-catalyzed C-H carboxylation of thiophene derivatives:
ORGANOMETALLICS 40 (18), 3136-3144, 2021
Author Response
Thank the reviewer for the constructive comments. According to the Reviewer’s comments and suggestions, we have made the corresponding revisions and discussions. The corresponding experimental details are shown in the Supporting information. To make the work more complete and the results more reliable, we made the following improvements, that is, we perform the relevant experimental studies to determine the optimal reaction conditions (as shown in Table 2 in main document). The purpose of this paper is to realize the direct carboxylation of thiophene with CO2 to produce the thiophene carboxylic acid products due to its difficulty in the C-H deprotonation, because most of thiophene are broken as organic sulfur impurities in the coking pro-cess and have not been reasonably utilized. The properties of thiophene and benzene are very similar, so it is a substitute for benzene in many fields. Therefore, we only performed the experimental investigation on the substrate thiophene. And to validate the feasibility of this route,, we also, respectively, calculate the energy barrier in deprotonation step for 2-methylthiophene, 3-methylthiophene (Table 1). Results indicate that the height of energy barrier is consistent with the reactivity of the substrate. Special thanks again to you for your good comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work of a team of scientists from China is devoted to studying the reaction of direct carboxylation of thiophene with carbon dioxide in the absence of a solvent. The paper shows how the reaction proceeds depending on the changing parameters of its implementation - such as temperature, a mixture of initial salts, etc. A clear description of the carboxylation mechanism is given, indicating all intermediate stages and their energy barriers calculated using chemical apparatus. Also, the most important geometric characteristics of all the described structures are calculated in the work. In addition to the calculation part, the authors present data from experimental work on the direct conduct of decarboxylation reactions. In general, the work is interesting, but has a number of shortcomings that need to be corrected. Thus, schematic representations of the energy profiles of the reactions should be given in good quality, some signatures are difficult to read, or not readable at all. Also, authors should rethink the presentation of experimental data, making their perception more accessible to readers. Finally, it is worth noting that when reading the work, the significance of the results of the work is very weakly traced, especially in the conclusions formulated to it, which creates the impression of a routine fundamental research. It is worth designating a clear practical result, which, of course, is in the work. Lastly, the authors need to issue a list of references in accordance with the requirements of the publisher, which can be found on the official website.
The work can be accepted after making changes.
Author Response
Thank reviewer for the constructive comments. We have studied your comments carefully and tried our best to make corresponding revised and improved the manuscript content. Special thanks again to you for your good comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have included the table 2 where the yields for obtaining the desired product was improved. Now the best yield is 42%, however there is no information on how the yields where calculated. Thus, the authors should include in both manuscript (as footnote of Table 2) and support information the method used to calculate the yields.
The references should be correct following this journal rules. We also strongly recommend an update of the literature, including the reference that we have cited in the first report.
Author Response
Thank reviewer for the constructive comments. We have studied your comments carefully and tried our best to make corresponding revised and improved the manuscript content. Special thanks again to you for your good comments.
