Layered Double Hydroxides as Bifunctional Catalysts for the Aryl Borylation under Ligand-Free Conditions
, Alexsandro A. da Silva 4, Luiz F. B. Malta 1,* and Jaqueline D. Senra 1,4,*Round 1
Reviewer 1 Report
Senra and co-workers reported Cu/Al LDH and Pd salt combined catalyst system for arylborylation of haloarenes. The combined catalyst system showed better results. I think this manuscript should be accepted after revisions of following points:
1. LDH is usually synthesized as Mg/Al form. The catalytic activitiy of Cu/Al LDH and Pd should be compared with Mg/Al LDH with Pd to clarify the effect of Cu in LDH structure. The data should be shown in Table 1.
2. Na2PdCl4 easily adsorbed onto LDH surface. Catalyst reuse experiment of the recovered solid should be conducted.
3. To clarify the role of Cu, please show the proposed reaction mechanism as a Scheme.
4. In Table 2, simple bromobenzene showed lower reactivity compared to both electron donating (methoxy) and withdrawing (acetyl) substitutents. Why? The aouthers should mention the reason.
Author Response
-As suggested by the reviewer, result concerning the use of Mg/Al LDH in the presence of Cu(II) and Pd(II) was added to the main text and included in Table 1, entry 8:
Results section, page 5, lines 202-204: “Since the positive effect could be related to the LDH structural and compositional properties, we also tested the most common Mg/Al LDH in the presence of CuSO4 and Na2PdCl4 which rendered a good yield (Table 1, entry 8).”
Discussion section, page 7, lines 297-299: “This fact could be reinforced when testing Mg/Al LDH in the presence of CuSO4 and Na2PdCl4 that resulted in an inferior product yield - compared to the same catalytic conditions by using the Cu/Al matrix.”
-Attempts to reuse the catalyst were performed following the reviewer’ suggestion. However, we presume that the failure can be related to the partial material exfoliation. Concerning this, we added in the main text:
Results section, page 6, lines 237-240: “Encouraged by the preliminary results, we next investigated whether this catalyst could have recycling potential. In such case, it could shed a light on the behavior of Cu/Al LDH as a reservoir or as real catalytically active species. However, attempts to recover the material after reaction work up failed”.
Discussion section, page 8, lines 251-252: “According to the frustrated catalyst recycling attempts, it is also possible that acetonitrile act as a moderate LDH exfoliation agent.”
-As suggested by the reviewer, a mechanistic scheme was proposed in the Discussion section, page 8, Scheme 1.
-The low conversion of bromobenzene was intriguing for us. Since the initial screening pointed - in part - to the positive influence of both EDG and EWG, we would expect a moderate substrate conversion in the absence of electronic effects. Concerning possible reasons for this result, we presume effects of solubility (since there was no preliminary step for acetonitrile drying) as well as alternative catalytic pathways (e.g. Ullmann-type reaction, hydrodehalogenation). This last possibility could be suggested by assuming some acetonitrile hydrolysis in basic medium and that aryl bromide hydrodehalogenation is a possible catalytic pathway in Pd-catalyzed systems. This fact is under investigation in our lab.
Following the reviewer advice, we added in Discussion section, page 8, lines304-307:
“However, the low reactivity of bromobenzene was intriguing. We presume the occurrence of possible alternative pathways (e.g. Ullmann-type reaction, hydrodehalogenation) in minor extension but the solubility factor cannot be ruled out.”
Reviewer 2 Report
In this work, the borylation reaction over Cu/Al layered double hydroxides was reported. The materials were prepared with the presence of Na2PdCl4. The manuscript fits well with the scope of the Catalysts journal. The authors presented condition strategies for catalysts preparation, basing on the results obtained through XRD. The work is focused on the evaluation of contaminations of the samples with malachite and others. The authors suggested that both Cu and Pd contributed to the enhanced catalytic performance when LDHs are free of the contaminations. In my opinion, the manuscript presents very interesting findings. The text should be published after minor corrections:
- Page 2, Section 2.1. – the description of Figure 1a should appear as first, not of Figure 1b. Perhaps the presented diffractograms may be exchanged, as “a” without, and “b” with sodium carbonate.
- In the Y axis of Figure 1 and Figure 2 should not be Counts, I would suggest using Intensity with [a.u.]. If, according to authors, counts are necessary, please unify the Y axis in both Figs. 1 and 2.
- What about the crystallite size of obtained phases? After washing with organic solvents, the reflections of hydrotalcite seem to be wider than the ones registered in Fig. 3.
- The structural parameters typical of hydrotalcites i.e., “a” and “c”, should be calculated.
- What is the reason of shift of the peaks at 2Θ ca. 11° towards lower Bragg angles (Figs. 3 a and b)? A comment should appear in the manuscript.
Author Response
Reviewer 2
-Page 2, section 2.1: we changed the order the figures are cited in text by accordingly assigning Figure 1a the diffractogram of LDH synthesized without sodium carbonate;
-To all diffractograms the Y label was renamed “Intensity (a.u.)" ;
- Mean crystal size decreased from 21 nm to 14 nm upon washing the LDH with organic solvents. We included this information in the text with the following fragment (page 5, first paragraph, lines 181-185):
Besides, the XRD peaks shown in Figure 5 appear wider and less intense than those in Figure 4b, for instance. According to the Full Width Half the Maximum (FWHM) criterium used in the Scherrer formula, the mean crystal size evolved from 21 nm (for samples of Figures 3 and 4) to 14 nm, signaling a decrease in crystallinity upon post synthesis treatment such as washing with organic solvents.
In addition the following fragment was also added to the discussion section (Page 7, lines 245-249) :
By the way the unit cell dimensions obtained for this phase (Table S1, supporting material) evidenced an expanded network in comparison to the carbonate containing phases. This can be understood in terms of attraction electrostatic forces acting between layer and interlayer parts of the material that permit to pack more efficiently when CO32-instead of NO3- is used as the intercalated ion.
- The unit cell parameters for all LDHs synthesized were summarized in Tables in the supporting information file. Intending the incorporation of these data to the work, we included the following text fragments in the manuscript:
Page 3, lines 113-117
Regarding the LDH unit cell parameters(Table S1, supporting information), both XRD were refined as having a rhombohedral crystal system and belonging to the R-3m spatial group. However the sample synthesized without carbonate presented the biggest unit cell dimensions (a=3.039 Åand c=26.921 Åagainst a=2.979 Åand c=22.519 Åfor the carbonate Cu/Al LDH) which means an expanded structural network was obtained.
Page 3, lines 127-128
This is also supported by strong similarities of unit cell dimensions between these two samples (Table S2, supporting information)
Page 4, lines 149-151
Concerning the unit cell dimensions, no significant differences between these two samples can be perceived (Table S3, supporting information)
Page 4, lines 167-168
The calculated unit cell parameters show no significant differences from those previously shown (Table S4, supporting information)
-The referee refers to the XRD patterns in Figure 4 in the revised manuscript, former Figure 3. These XRD are relative to the comparison between LDHs submitted to centrifugation and filtration. Their unit cell parameters were calculated and are presented in supporting information (Table S3). By inspection of these values no significant differences are observed between any of these samples and those presented in Figure 2 and 3 of the revised manuscript (Tables S1 and S2 in supporting information). Even by visual inspection we were not able to detect such shift for the peak at 2theta ca 11o. Besides the referee did not detail in relation to what sample XRD this shift was perceived.
Reviewer 3 Report
Senra et al. discribe in their manuscript the preparation Cu/Al based layered double hydroxides (LDH) and test these materials as catalysts for borylation reactions. Unfortunately, I found that the manuscript was poorly written and unclear (for example the following parts: rows 151-153 and rows 179-182). It is very hard to see that this work would receive any general interest due poor results of borylation experiments. The problem is that there are available good borylation protocols for 1-iodo-4-nitrobenzene such as methods published in European Journal of Organic Chemistry, 2014(7), 1381-1385; 2014 and Journal of Organic Chemistry, 65(1), 164-168; 2000. The current work does not actually offer anything new important. The “LDH method” still needs Pd source together with relatively big loading of Cu. Moreover, the LDH method applies problematic (atom economy) pinB-Bpin as a source of Bpin. For example, papers in J. Org. Chem. 2011, 76, 9602 and J. Org. Chem. 2016, 81, 1535 report the use of highly efficient Barbier type method which applies more convenient HBpin as a source of Bpin. And, that method does not need Pd at all. The reported LDH method seems to work well only for few very common substrates (1-iodo-4-nitrobenzene and entry 4 table 2). For most of the reported substrates LDH method does not seem to work at all (entries 1, 2, 5 and 6, table 2). By the way, is it really that Br was substituted by the OH during the borylation (see entry 6, table 2)?
These Cu/Al LDH materials may found interest among some inorganic chemistry researchers. I strongly recommend to continue the experimental work in some other field. This manuscript cannot be published in Catalysts – and, unfortunately, I cannot recommend it to any other journal.
Author Response
-In response to the referee, we clarified the sentences in rows 202-204. Indeed, we found that the referred borylation protocols (European Journal of Organic Chemistry, 2014(7), 1381-1385; 2014 and Journal of Organic Chemistry, 65(1), 164-168; 2000) involve relatively harsh and expensive conditions since phosphines are required. Concerning the novelty/importance parameters, the present communication did not intended to disclose an extensive product scope and not even to elucidate the borylation mechanism. Mostly, our preliminary work revealed that preferentially pure Cu/Al LDH can have a novel catalytic application in combination with a Pd(II) salt. Given the importance of borylation reactions, we believe that this simple ligand-free catalyst system can interest not only inorganic chemists but mainly the catalytic community engaged with Pd-catalyzed cross-couplings. Asrecommended by the referee, our group are investigating this layered system but keeping borylation reaction as target. With respect to the use of (Bpin)2, it is in fact a problem from the atom economy perspective. The Barbier-type conditions cited by the referee is an alternative protocol to avoid the (Bpin)2 and Pd catalyst but are strongly limited to dry conditions, inert atmosphere and the use of acidic conditions in the work-up to generate potential dangerous hydrogen gas. In our system, (Bpin)2 seems to have a dual role (and not a “half-waste”): reaction with an aryl halide and reduction of Pd(II) ions to Pd(0). This last role was visually evidenced by the darkening of the Na2PdCl4 solution.Therefore, the following text was inserted in the manuscript
Discussion section, page 8, lines 311-312:
“Lastly, (Bpin)2 seems to be involved in the reduction of Pd(II) species as suggested by the solution darkening according to visual inspection”.
In relation to the reaction with 4-bromobenzyl bromide, we believe that the unexpected alcohol product could be formed from the LDH surface hydroxyl groups.
Reviewer 4 Report
This manuscript presents an interesting and intriguing new twist on the use of LDH as catalytic systems. It is demonstrated that there is a synergistic effect between the Na2PdCl4 added to the solution and the Cu/Al LDH in the synthesis of boronic esters. The manuscript is clearly written and the results seems to be convincing. However, I have a couple of suggestions to improve the manuscript:
1) Formal: I would be appreciated some graphical material to illustrate the structure of materials and/or the expected mechanisms.
2) Content: The authors assume that the (PdCl4)2- ion is always in solution, but it is possible that the incorporation of such species into the material could clarify the synergistic effects. I recommend to investigate the the presence of Pd in LDH materials after they have been treated with Na2PdCl4 solutions.
Author Response
1) We inserted one representation of LDH structure in the Introduction section (Figure 1, page 2). A scheme depicting a proposal of the reaction mechanism was added to the Discussion section (scheme 1, page 8);
2) We have been investigating the formation of composites of LDH with Pd. We have previously published one manuscript in which we describe the reduction of Pd in N,N-dimethylformamide (DMF) medium mediated by Mg/Al LDH (The Sci World J. vol2013, Article ID 456789, 8 pages). The association of these Pd(0) particles to LDH still is a matter to be better studied; nonetheless there is evidence these particles are being deposited on the LDH surface, as described by our research group in a former manuscript applying Au instead of Pd (J.Nanomaterials, vol.2013, Article ID 357069, 6pages). We have some SEM visualizations that are not yet published that evidence Pd deposition or intercalation and Pd oxidation state are strongly influenced by the solvent medium employed, H2O or DMF. However this is a work in progress and it will be the subject of a future manuscript that is yet to come.
Round 2
Reviewer 3 Report
Authors have carried out minor changes to the manuscript. My comments stay and I stick by my previous decision.I cannot see that this work would receive any general interest because of the poor results of the borylation experiments. Moreover, the new finding that the catalyst cannot be recycled decreases the interest of this manuscript. Authors must continue the experimental work and try to find some other more suitable application for their materials. Or perhaps, the material synthesis and characterizations can be published in some inorganic chemistry journal. As a such, it is not very interesting as a catalyst material.
Reviewer 4 Report
After the comments and changes made by the authors I recommend the publication of the manuscript
