H3PW12O40/SBA-15 for the Solventless Synthesis of 3-Substituted Indoles
Round 1
Reviewer 1 Report
This manuscript deals with the the solventless synthesis of 2,3-substituted indoles over mesoporous SBA-15 physisorbed with H3PW12O40 catalysts under mild conditions emphasising the effect of tungsten content on the physico-chemical and catalytic properties. Since the selective catalytic reactions and applying heterogeneous catalysis have high impact on the production of chemicals, this investigation deserves attention.
In this form, however, this manuscript cannot be published for the following reasons:
1) p. 1 lines 32–35 The authors state that “Herein, we wish to study the effect of HPA content dispersed over the mesoporous SBA-15 and try to correlate physicochemical properties of HPA/SBA15 catalysts with their catalytic activities in the three-component solventless condensation of ethyl acetoacetate, urea and benzaldehyde under mild conditions.”, but later (p. 2) they also write that “… benzaldehyde, indole, and malononitrile (Scheme 1) …” were used in the studied reaction. Moreover, the titles of Tables 2 and 3 are also misleading. Please, clarify this contradiction.
2) p. 1 lines 37 The authors also assert about this reaction that it is a “… cyclocondensation …”, but it is obviously not (see Scheme 1)! The authors should revise this statement.
3) p. 3 lines 68–69 In the Figure 1 the yield of substituted indoles is shown in the function of the amount of HPA acid (not HPW!) and also depicted the desorption temperature of ammonia observed in the NH3-TPD measurements. Whereas, the latter information is not too informative, moreover, it is very difficult to understand and follow the authors’ explanation about it. Please, extend the description of this phenomenon.
4) p. 3 lines 76–77 The authors state that “Table 3 demonstrated high yields for most of the aromatic aldehydes; however, electron-withdrawing groups afforded slightly better yields than the electron-donating substituents [14-15].” but no further information is reported about the reasons of these differences. You should give a more detailed explanation.
5) The English also needs improvements. There are some typical grammar or typing mistakes:
p. 1 line 1 „H3PW12O40 …” instead of H3PW12O40
line 10 „A series of … were …” instead of A series … was
p. 2 line 55 and elsewhere „… 2-((1H-indol-3-yl)(phenyl)methyl)malononitrile …” instead of 2-[(1H-indol-3-yl)(phenyl)methyl]malononitrile
p. 3 line 74 and elsewhere „… %wt …” instead of wt%
line 76 „Table 3 demonstrated high yields for most of the aromatic aldehydes …” instead of High yields for most of the aromatic aldehydes were obtained, as demonstrated in Table 3
p. 6 line 162 „… ACS Catalysis …” instead of ACS Catal.
As an overall comment, this manuscript reflects a relatively careless work.
Author Response
Response to Reviewer 1
MS ID: catalysts-482743
MS Title: H3PW12O40/SBA-15 for the solventless synthesis of 3-substituted indoles
Authors: Tayebee et al
We thank the reviewer for the comments and have responded constructively to each point below.
1. p. 1 lines 32–35. The authors state that “Herein, we wish to study the effect of HPA content dispersed over the mesoporous SBA-15 and try to correlate physicochemical properties of HPA/SBA15 catalysts with their catalytic activities in the three-component solventless condensation of ethyl acetoacetate, urea and benzaldehyde under mild conditions.”, but later (p. 2) they also write that “… benzaldehyde, indole, and malononitrile (Scheme 1) …” were used in the studied reaction. Moreover, the titles of Tables 2 and 3 are also misleading. Please, clarify this contradiction.
We apologise for this confusing text. This study addresses the three-component condensation of aromatic aldehydes, indole, and malononitrile and we have corrected the text to reflect this.
Action: Manuscript amended
2. p. 1 lines 37. The authors also assert about this reaction that it is a “… cyclocondensation …”, but it is obviously not (see Scheme 1)! The authors should revise this statement.
The reviewer is correct. We have changed "Cyclocondensation" to "condensation".
Action: Manuscript amended
3. p. 3 lines 68–69. In the Figure 1 the yield of substituted indoles is shown in the function of the amount of HPA acid (not HPW!) and also depicted the desorption temperature of ammonia observed in the NH3-TPD measurements. Whereas, the latter information is not too informative, moreover, it is very difficult to understand and follow the authors’ explanation about it. Please, extend the description of this phenomenon.
Phosphotungstic acid is commonly abbreviated as ‘HPW’ in the literature, and we have therefore adopted this convention throughout our manuscript (e.g. Appl. Catal. B 2017, 200, 10; J. Memb. Sci. 2011, 368, 241; Int. J. Hydrogen Energy 2013, 38, 12830; Green Chem. 2014, 16, 1202); HPW is the active component of our catalysts. ‘HPA’ is the abbreviation for generic heteropolyacids, and not for phosphotungstic acid (e.g. Appl. Catal. A 2003, 256, 77). To avoid confusion, we have removed use of the abbreviation HPA throughout the manuscript. Further information on the NH3-TPD analysis protocol is now included in the Experimental section 3.2. The ammonia desorption temperature is directly proportional to the solid acid strength, and hence is analogous to a pKa scale; the strong correlation between indole product yield and ammonia desorption temperature in Fig. 1 indicates that aldehyde activation or intramolecular cyclisation is rate-limiting. A brief discussion of this phenomenon is now included in the manuscript.
Action: Manuscript+ESI amended
4. p. 3 lines 76–77. The authors state that “Table 3 demonstrated high yields for most of the aromatic aldehydes; however, electron-withdrawing groups afforded slightly better yields than the electron-donating substituents [14-15].” but no further information is reported about the reasons of these differences. You should give a more detailed explanation.
A plausible reaction mechanism is now described in the manuscript for the superior performance of substituted aldehydes featuring electron-withdrawing versus -donating groups. According to the proposed mechanism, electron-donating substituents on the phenyl ring of aldehyde deactivate it toward the nucleophilic attack of malononitrile, whereas electron-withdrawing groups activate the carbonyl group of the aldehyde. Fig. S7 is now added to the SI.
Action: Manuscript+ESI amended
5. The English also needs improvements. There are some typical grammar or typing mistakes:
The manuscript has been comprehensively edited and proofread.
Action: Manuscript amended
Reviewer 2 Report
In this work, the authors studied their previously reported H3PW12O40/SBA-15 catalysts with various W loadings for the solventless synthesis of 3-substituted indoles. The supported H3PW12O40/SBA-15 catalysts showed enhanced activity compared with pure H3PW12O40. The effects of HPA loading and acid strength on product yield were investigated. The catalysts were characterized by physisorption, XRD, NH3-TPD, HRTEM, etc. This is an interesting work. However, the current manuscript need to address the following concerns before publication.
1) “H3PW12O40/SBA-15” in title should be “H3PW12O40/SBA-15”
2) The “%” can be deleted from “yield%” in abstract.
3) Reference 10 is incomplete.
4) HPA is not defined before its use.
5) The resolution of scheme 1 can be improved.
6) It would be helpful to provide the original NH3 TPD result in the supporting information.
7) Is the maximum desorption peak temperature used as an index for the acid strength in this work? This should be clarified in the manuscript. Why does the acid strength of HPA/SBA15 increase with the loading of HPA?
8) The “Yield (%)” in the first row of Table 3 is not centered vertically.
9) “Good catalyst stability was attained and a decrease of ~15% over 5 consecutive re-uses was observed for HPA/SBA-15.” Could you provide the tests results of 5 consecutive re-uses in supporting information?
10) It is helpful to provide the mentioned FT-IR of the spent catalyst after 5 re-uses in the supporting information.
Author Response
Response to Reviewer 2
MS ID: catalysts-482743
MS Title: H3PW12O40/SBA-15 for the solventless synthesis of 3-substituted indoles
Authors: Tayebee et al
We thank the reviewer for the comments and have responded constructively to each point below.
1. H3PW12O40/SBA-15” in title should be “H3PW12O40/SBA-15”.
We apologise for these typographical errors and have corrected the subscripts.
Action: Manuscript amended
2. The “%” can be deleted from “yield%” in abstract.
The text has been amended accordingly.
Action: Manuscript amended
3. Reference 10 is incomplete.
Reference 10 is completed.
Action: Manuscript amended
4. HPA is not defined before its use.
‘HPA’ is the abbreviation for generic heteropolyacids (e.g. Appl. Catal. A 2003, 256, 77), however to avoid confusion, we have removed use of the abbreviation HPA throughout the manuscript.
Action: Manuscript amended
5. The resolution of scheme 1 can be improved.
We have re-drawn Scheme 1 to improve the resolution.
Action: Manuscript amended
6. It would be helpful to provide the original NH3 TPD result in the supporting information.
The original ammonia desorption spectra are now included in the Supporting Information as a new Figure S4.
Action: Manuscript+ESI amended
7. Is the maximum desorption peak temperature used as an index for the acid strength in this work? This should be clarified in the manuscript. Why does the acid strength of HPA/SBA15 increase with the loading of HPA?
Further information on the NH3-TPD analysis protocol is now included in the Experimental section 3.2. The ammonia desorption temperature is directly proportional to the solid acid strength, and hence is analogous to a pKa scale. The acid strength of HPW increases with loading due to the nucleation of isolated Keggin clusters (which lack acid sites associated with water held within the secondary heteropolyacid structure) and concomitant formation of two-dimensional HPW aggregates linked by crystalline water (J. Phys. Chem. B 1998, 102, 10817). A comment to this effect is now included in the manuscript.
Action: Manuscript amended
8. The “Yield (%)” in the first row of Table 3 is not centered vertically.
The axis title is now vertically centered.
Action: Manuscript amended
9. “Good catalyst stability was attained and a decrease of ~15% over 5 consecutive re-uses was observed for HPA/SBA-15.” Could you provide the tests results of 5 consecutive re-uses in supporting information?
The results of 5 consecutive reactions are now shown in a new Figure S5.
Action: Manuscript+ESI amended
10. It is helpful to provide the mentioned FT-IR of the spent catalyst after 5 re-uses in the supporting information.
The FTIR spectra of as-prepared and post-reaction catalysts (after 5 consecutive reactions) are shown in a new Figure S6.
Action: Manuscript+ESI amended
Round 2
Reviewer 1 Report
The authors have adapted my suggestions and have corrected the mistakes in their manuscript but, unfortunately, some errors still remained in it.
However, the revised manuscript in this form, after correcting the following inaccuracies, can be published.
p. 1 line 16 and elsewhere „… 2-[(1H-indol-3-yl)(phenyl)methyl]malononitrile …” instead of 2-[(1H-indol-3-yl)(phenyl)methyl]malononitrile (H is italic!)
line 36 „… Zn(salphen) [12], N,N1-dioxide Zn(II) [13], copper(II) sulfonato salen [14] …” instead of Zn(salphen) [12], N,N’-dioxide–Zn(II) [13] or copper(II) sulfonato Salen [14] complexes
p. 3 line 95 „… [18-19] … ” instead of [18,19]
p. 5 line 139 “ … 150 cm3 min−1 …” instead of 150 cm3 min−1
line 147 “… (n-hexane: ethylacetate, 3:2).” instead of (hexane: ethyl acetate, 3:2).
line 152 “… [20-21].” instead of [20,21].
p. 6 line 186 “2. … Chemical Reviews …” instead of Chem. Rev.
line 200 “… 2H-indazolo [2, 1-b] …” instead of 2H-indazolo [2,1-b]
p. 7 line 220 “16. … Appl. Catal. B: Environmen. …” instead of Appl. Catal. B: Environ.
Author Response
Many thanks again for the minor comments. All corrections are done in the revised version.
Reviewer 2 Report
The reviewer thanks the authors for the modification and clarification. The revised version has addressed most of the reviewer's concerns and I have no further comments. I recommend the manuscript for publication.
Author Response
Many thanks again. I hope by consideration of minor comments of Reviewer 1, Introduction is further improved.
