PPh3-Assisted Esterification of Acyl Fluorides with Ethers via C(sp3)–O Bond Cleavage Accelerated by TBAT
Round 1
Reviewer 1 Report
Nishihara and co-workers developed PPh3-catalyzed TBAT-accelerated esterification of acyl fluorides with cyclopentyl methyl ether. The reaction showed good functional-group tolerance, and broad substrate scope for the synthesis of (hetero)aromatic esters. Moreover, the proposed mechanism is reliable, and the manuscript is well presented. In this viewpoint, this reviewer recommends the publication in Catalysts after minor revision.
One example of the efficient esterification reaction using CO could be cited:
C. Zhu, B. Yang, Y. Qiu, J.-E. Bäckvall, Angew. Chem. Int. Ed. 2016, 55, 14405–14408.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this manuscript, the authors reported the PPh3-catalyzed methoxylation of acyl fluorides with cyclopentyl methyl ether (CPME) mediated by tetrabutylammonium difluorotriphenysilicate (TBAT) via regiospecific C–OMe bond cleavage. The scope of methodology covers a wide range of substrate with a good functional group tolerance and works under metal free conditions. The content of this paper is within the scope of the Catalysts. However, I think some modifications and further clarifications are needed before publication in the journal. I would recommend it for publication after addressing the following queries.
1. Authors should explain optimization study in more details.
- Effect of catalyst (PPh3) load on reaction yield. Why 30 % of PPh3 have selected as optimum catalyst load?
- Effect of additive load on reaction yield.
2. Authors should explain the reason why the yield of reaction in Entry 15 (no catalyst) is higher than the yield of reactions in Entry 2, 3, and 4 (with other phosphine catalysts). (Table 1)
3. In optimization study mainly phosphine ligands with electron donating groups have investigated. Authors should investigate the effect of phosphine ligands with electron withdrawing groups.
4. The method is also feasible with different other ethers (2b, 2c, and 2d) however the title is very specific for CPME. It might be more useful to modify title with more general version.
5. The authors claim that the reaction mechanism does not proceed via radical path and they proved this by using radical scavengers. They should give more detail about the method (EPR or ?) and analysis of the outcomes of the study.
6. To understand the reaction mechanism authors designed an experiment in which Ph3SiOMe (possible intermediate) was used in the absence and presence of TBAT (scheme 4). Based on the outcomes of this study, they obtained 91% of yield in the presence of both reagents. This is higher than the yield of a reaction in the optimized conditions. Authors should explain the role of Ph3SiOMe in the progress of the reaction.
7. The authors claim that the reaction of acyl fluoride with PPh3 produces Ph3SiF which was confirmed by 19F{1H} NMR spectrum. Authors should provide this spectrum in manuscript or ESI.
8. The authors claim that all compounds have characterized by (1H, 13C{1H}, 31P{1H}, and 19F{1H}) NMR however, they only provided the 1H NMR results. Authors should provide other characterization data as well.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Nishihara and co-workers describe the methoxylation of acyl fluorides with cyclopentyl methyl ether as a methoxylating reagent in the presence of tetrabutylammonium difluorotriphenylsilicate. With this protocol, various aroyl fluorides were converted to the corresponding methyl esters in good yields with high functional-group compatibility. References and Data are good. The authors mentioned that PPh3 catalyzed this transformation. However, this reviewer does not agree this (see comment 1). This reviewer recommends a new submission (or transfer) to other journals. Major comments are shown below.
1) In the absence of PPh3, the reaction proceeded to give a methyl ester in 53% yield (Table 1, entry 15). After addition of “30 mol%” of PPh3, the yield was increased to 74% (entry 1). The difference is only “21%”. This is not a “catalytic” reaction (stoichiometric effect). Thus, the title word “PPh3-catalyzed” should be changed to “PPh3-assited” or “PPh3-enhanced”.
2) How about applicability of aliphatic acid fluorides?
3) In Table 2 for 3i; Did the author observe any byproducts?
4) In Scheme 5; All of components (Bu4N+, carbocation from A, F on the silicate etc.) should be drawn exactly. Electrical charge is also incorrect in the chemical formula.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
In this manuscript, the authors reported the PPh3-Assisted Esterification of Acyl Fluorides with Ethers via C(sp3 )-O Bond Cleavage Accelerated by TBAT. Authors have addressed the queries and added missing experiments in the revised manuscript. The content of this paper is within the scope of the Catalysts and I would recommend it for publishing.
Reviewer 3 Report
The revised manuscript by Nishihara was significantly improved. However, this paper describes “a stoichiometric reaction” between acyl fluorides, TBAT and CPME with PPh3 as an additive (NOT catalysts and catalytic reactions). Thus, this paper is not in line with the journal policy of “Catalysts”. [Catalysts (ISSN 2073-4344; CODEN: CATACJ) is a peer-reviewed open access journal of catalysts and catalyzed reactions published monthly online by MDPI.] This reviewer recommends a new submission (or transfer) to other journals such as “Molecules”. This reviewer believes that this paper is suitable for publication in such journals.
In Scheme 5; Several components are still missing. Where is the cyclopentyl cation from A to go? Where is the F anion from B to go? Any comments on this are welcome.
