Chloro(η2,η2-cycloocta-1,5-diene){1-benzyl-3-[(S)-2-hydroxy-1-methylethyl]benzimidazol-2-ylidene}rhodium(I)
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper reports the synthesis and characterization of two Rh(I) complexes containing a chiral NHC, 1,5-cyclooctadiene (cod) and a chloro or bromo ligand (compounds 1b and 2b). The synthesis of a new chiral NHC precursor is also included (2a).
The three compounds are new (checked by Scifinder search) and have been characterized by means of their NMR and ESI-MS data. Satisfactory elemental analyses are given for 2a and 2b but not for 1b. The 1H NMR spectrum of 1b shows broad signals at ca. 5.9, 5.3 and 3.9 ppm which were not explained.
The spectroscopic and analytical data agree with the proposed structures. However, the presence of a lower intensity subset of signals in the NMR spectra of 1b and 2b is attributed to the formation of isomers, but nothing is mentioned about the structure of these isomers. A plausible explanation could be that, owing to the high steric hindrance of the NHC ligand, rotation around the Rh-C(NHC) bond could be slow at room temperature. In addition, O-H...Halogen hydrogen bonding could also prevent Rh-C(NHC) rotation. Then, two diastereomeric conformers should be observed in the NMR spectra.
In summary, the article reports the synthesis of three new compounds of potential interest in organic synthesis and catalysis. Overall, the synthetic and characterization work has been properly done, but needs some improvement. The article has been clearly and concisely presented.
I recommend reconsider acceptance after addressing the following issues:
1) The origin and characterization of the minor isomers of 1b and 2b should be discussed.
2) The 1H NMR spectrum of 1b shows broad signals at ca. 5.9, 5.3 and 3.9 ppm which were not explained. It should be purified and a clean 1H-NMR spectrum or a satisfactory (C, H, N)-analysis given.
3) I assume that the starting material [Rh(cod)(OH)]2 has been prepared by the authors. In this case, the work describing its synthesis should be cited.
4) The current title only refers to one of the reported Rh(I) complexes. It should be modified to refer to both Rh(I) complexes.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe study conducted by Skaguchi focuses on the synthesis of hydroxyalkyl-functionalized N-heterocyclic carbene Rh(I)-complexes through a one-pot deprotonation route. The rhodium complexes were characterized using NMR, HRMS measurement, and elemental analysis.
Acceptance is recommended after addressing the following points.
1.) The title is the name of only one compound, it would be advisable to change the title to a general one which includes both compounds (chloro- and bromo- NHC complexes) in general form.
2.) In the Abstract I recommend the use of [RhX(cod)(NHC)] general formula because the present form does’t include the bromo-complex 2b. I would like to mention here that in Line 169 the compound name is inaccurate, 2b is not a chloro-complex; please correct it. Please remove from Keywords ‘ruthenium’.
3.) In Line 62 the authors should replace ‘First’ with ‘The synthesis started with’.
4.) Line 80-82 this is a standard method from literature for preparation of Rh-NHCs, not a new development.
5.) Line 96-99 the authors statement regarding the alkoxide formation should be reconsidered. In my opinion signal of OH group at δ = 3.27 ppm chemical shift is to low compared to the OH resonance in the ligand around 5 ppm. Prove that this functional group is not tethered and please clarify the detailed structure, for characterization use other spectroscopic methods. In the absence of these demonstrations I do not consider this structure acceptable.
6.) What could be the reason for insolubility of 2b in various solvents? Is it not possible that this compound is a polymer? Did the authors examine the solubility of 2b in acetone or dimethylformamide?
7.) It is much surprising to obtain a yield of 100% after purification with column chromatography. What could be the explanation, provided the product is totally pure?
8.) Please indicate the experimental details of the NMR measurments in the Supplementary Material for each spectrum and show the measurement window between 0-12 ppm.
9.) Reference 12 is not a published article ‘Trends in Org. Chem. 2024, in press.’ That is why I couldn't see the structures, but from the title we can conclude the catalysts are copper hydroxyalkyl-N-heterocyclic carbene complexes while on Scheme 1 we can find a Ru-p-cymene complex. This may be a mistake please eliminate this reference or correct the Scheme 1 accordingly.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsIn the new manuscript version, the origin of the previously undetermined minor isomer of of the Rh complexes has been explained by reference to previous work.
The purity of compound 1b has not been improved, although its structure is supported by NMR and HRMS data. The authors recognize in the supporting information the presence of small impurities in the sample.
Point 3) has been addressed.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
