View PDF Version
 
DOI: 10.1039/D4GC02415J (Paper) Green Chem., 2024, Advance Article

Copper catalyzed direct C–H double methylation of aromatic aldehydes employing methanol as an alkylating agent†

Sereena Sunny and Ramasamy Karvembu*
Department of Chemistry, National Institute of Technology, Tiruchirappalli-620015, India. E-mail: kar@nitt.edu

Received 16th May 2024 , Accepted 9th August 2024

First published on 15th August 2024

Abstract

A mild sustainable protocol for the direct C–H double methylation of aromatic aldehydes has been accomplished by using un-activated methanol as a C1 source. Readily available, inexpensive copper sulfate has been utilized as a greener catalyst at room temperature. This methodology represents the first example of direct synthesis of tertiary alcohols from aromatic aldehydes operating via a concerted radical mediated pathway with the elimination of H2O2. The successful gram-scale synthesis of the product indicates the feasibility of this protocol in industrial applications.

Introduction

Copper is one of the most important trace metals that exist in all living organisms. It catalyzes various enzymatic activities that include electron transfer (blue copper protein type I), oxygen activation (amine oxidase and galactose oxidase), oxygen reduction to water (multicopper oxidases and heme-copper respiratory oxidases), denitrification (copper nitrite reductase and nitrous oxide reductase) and substrate activation (copper amine oxidases). Most of the biological activities of these enzymes involve electron transfer processes due to the redox property of copper sites present in these systems. The active sites in these enzymes have one or more copper ions with different geometrical and electronic environments to facilitate both single and multielectron transfer processes.1 Due to the high demand for developing sustainable protocols for C–H activation reactions which are pivotal in medicinal and industrial applications, many researchers have attempted to mimic biological systems for the development of greener alternatives.2 One such attempt was to study the action of the methane monooxygenase enzyme and the conversion of methane to methanol for the efficient storage of carbon-derived energy for industrial applications.3 Even though there is significant progress in this regard, the exact mechanism remains elusive. Inspired by the unique properties of copper ions in living systems such as redox behavior and hydrogen atom abstraction, we thought of investigating the potential of simple copper salts in activating the un-activated C–H bonds using greener reaction partners under mild conditions.4

C–H methylation is one of the major techniques to introduce methyl groups directly into organic molecules to improve their biological activities and physical properties.5 Conventional transition metal-catalyzed C–H methylation reactions demand harsh conditions and the use of environmentally toxic solvents and toxic methylating agents such as methyl triflate,6 dimethyl sulfate,7 diazomethane,8 methyl iodide,9 trimethyl oxonium tetrafluoroborate10 and corrosive formaldehyde.11 Considering the green chemistry principles, researchers have discovered greener methylating agents such as dialkylcarbonate,12 formic acid,13 carbon dioxide,14 and methanol.15 Among these, methanol has garnered substantial attention as a sustainable C1 building block.16 Based on the mechanistic investigations, the methylation reactions using methanol occurred mainly through the following routes: (a) borrowing hydrogen strategy17 and (b) photo-redox and hydrogen atom transfer catalysis18 (Fig. 1). Since most of these reactions require either harsh conditions or the presence of toxic noble metal-based photocatalysts, there is still room for further improvement. The recent reports on copper catalyzed oxidative C–H functionalization via a radical redox relay process19 and organocopper(III) intermediates20 have aroused our curiosity to investigate the possibility of employing a simple copper salt as a catalyst for C–H methylation with a greener solvent methanol. Considering the growing attention to the development of sustainable methodologies for the direct C–H functionalization of aldehydes,21 we assumed that aromatic aldehydes would be a suitable substrate for methylation. To our surprise, our initial experimental trials using simple copper salts resulted in the formation of an unexpected tertiary alcohol as a product. This was a fascinating result because the direct synthesis of tertiary alcohols from aldehydes was not known, and the existing protocols with other substrates have a lot of drawbacks. The well-established method for the preparation of tertiary alcohols is the nucleophilic addition of Grignard reagent to ketones.22 Despite its industrial significance, this reaction faces numerous challenges from a sustainability perspective, including tedious prefunctionalization of reagents, handling of air- and moisture-sensitive reaction partners, and unwanted waste emission.23 In this regard, here, we report a sustainable, mild, atom-economical, and novel protocol for the synthesis of tertiary alcohols from aromatic aldehydes and methanol using an inexpensive and green copper catalyst.


image file: d4gc02415j-f1.tif
Fig. 1 Different mechanistic pathways for C–H methylation using methanol.

Results and discussion

We started our investigation with 4-nitrobenzaldehyde and methanol as model substrates in the presence of various commercially available inexpensive Cu(II) salts at room temperature. Interestingly, except for Cu(OAc)2·4H2O, all other copper(II) salts delivered 3aa as a di-methylated product with good yield (Table 1, entries 1–4, 70–94% yield). Among these, CuSO4·5H2O was found to be the most effective. This reiterates the role of counter-ions in determining the efficiency of the reaction. To check the compatibility of other metal sulfates, we also tested some of the M(II) sulfates (M = Co, Ni, and Zn). It has been observed that both nickel sulfate and cobalt sulfate afforded inferior results and only 8% yield was obtained in the case of zinc sulfate, indicating the importance of copper in this protocol (Table 1, entries 5–7). Furthermore, conducting the reaction without copper under identical conditions resulted in a significant reduction in yield, emphasizing the indispensable role of copper in the reaction (Table 1, entry 18). There was no significant enhancement of yield noticed on elevating the temperature (Table 1, entries 15–17) and varying the catalyst amount (Table 1, entries 8–13). Doubling the catalytic amount from 5 to 10 mol% and extending the reaction time from 24 to 36 h resulted in only a 4% increase in the yield. Similarly, increasing the temperature from room temperature to 80 °C also resulted in a modest 2% increase in the yield. Therefore, considering the cost-effectiveness and environmental concerns, we decided to use a 5 mol% catalyst at room temperature for 24 h in the reaction for optimal performance.
Table 1 Optimization of the aldehyde C–H alkylation

image file: d4gc02415j-u1.tif
Entry Metal salt Temperature (°C) mol% Time (h) Yielda (%)
a Isolated yield.
1 CuSO4·5H2O rt 10 36 94
2 CuCl2·2H2O rt 10 36 84
3 Cu(NO3)2·3H2O rt 10 36 70
4 Cu(OAc)2·4H2O rt 10 36 0
5 ZnSO4·7H2O rt 10 36 8
6 CoSO4·7H2O rt 10 36 Trace
7 NiSO4·6H2O rt 10 36 Trace
8 CuSO4·5H2O rt 1 36 12
9 CuSO4·5H2O rt 2 36 54
10 CuSO4·5H2O rt 5 36 92
12 CuSO4·5H2O rt 20 36 93
13 CuSO4·5H2O rt 50 36 90
14 CuSO4·5H2O rt 5 24 90
15 CuSO4·5H2O 60 5 24 89
16 CuSO4·5H2O 80 5 24 92
17 CuSO4·5H2O 100 5 24 88
18 — rt — 24 12

With the optimal conditions in hand, we were curious to evaluate the applicability of this strategy for synthesizing corresponding methylated products. We initiated our process with electron-withdrawing benzaldehydes like 3-nitrobenzaldehyde and 2-nitrobenzaldehyde. To our delight, both the substrates showed good reactivity and produced tertiary alcohols (3ab and 3ac) with excellent yield (Table 2, 94 and 83% respectively). We also tested conjugated systems such as 2-nitrocinnamadehyde. Strikingly, it also showed a very good reactivity and exclusively produced a good yield of tertiary alcohol 3ad as the product (Table 2, 72% yield). This marks the synthesis of a novel conjugated tertiary alcohol derived from 2-nitrocinnamaldehyde.

Table 2 Substrate scope of the aldehyde C–H methylationa,b
a Aldehyde 1 (0.5 mmol), CuSO4·5H2O (5 mol%), MeOH 2a (1 ml), rt, and 24 h.b Isolated yields are given in parentheses.
image file: d4gc02415j-u2.tif

As anticipated, electron-withdrawing aldehydes 3ae, 3af, and 3ag yielded the desired product with good to moderate yields (76, 80, and 68%, respectively) (Table 2). In contrast, the strong electron-donating aldehyde anisaldehyde yielded only a trace amount of the product 3ah (Table 2). However, 4-methylthio benzaldehyde provided a good yield of the corresponding tertiary alcohol 3ai (78%, Table 2). Benzaldehyde, a neutral one, did not yield the product 3aj, while naphthaldehyde achieved a 60% yield of the desired product 3ak (Table 2). Furthermore, the heterocyclic compound 3al exhibited promising results with a 54% yield (Table 2). Conversely, furfural, a naturally occurring and bio-based aldehyde, yielded an inferior result (Table 2). Notably, 3-carboxybenzaldehyde, an aldehyde with an unsaturated substituent, also demonstrated a favourable outcome (3an, 65%, Table 2). These results underscore the significant influence of the electronic and binding properties of substituents on substrates in shaping the outcome of the reaction. It is worth mentioning that the tertiary alcohols 3aa, 3ab, 3ac, 3ae, 3af, 3ag, 3ak, 3al, and 3an produced via this protocol are prohibitively expensive when synthesized using alternative methods. Nevertheless, this approach offers a cost-effective and environmentally friendly means of producing these compounds.

To check the applicability of this protocol to other alcohols, various alcohols, including ethanol, isopropanol, butanol, and allyl alcohol, were evaluated as alkylating agents under identical conditions. Interestingly, ethanol resulted in good to moderate yields of tertiary alcohols (Table 3, 4aa–4ag, 90–40% yield), whereas isopropanol and butanol delivered negligible amounts of the products, indicating the influence of steric hindrance in the reaction. Notably, the more reactive allyl alcohol produced allylated products with yields up to 92% (Table 4, entries 5aa–5ag, yields ranging from 92 to 40%), highlighting a pathway for green and cost-effective synthesis of these products. It is worth mentioning that this method of obtaining alkylated and allylated products is more environment friendly and cost-effective than the existing protocol suggested in the literature.24

Table 3 Substrate scope of the aldehyde C–H ethylationa,b
a Aldehyde 1 (0.5 mmol), CuSO4·5H2O (5 mol%), EtOH 2b (1 ml), rt, and 24 h.b Isolated yields are given in parentheses.
image file: d4gc02415j-u3.tif

Table 4 Substrate scope of the aldehyde C–H allylationa,b
a Aldehyde 1 (0.5 mmol), CuSO4·5H2O (5 mol%), allyl alcohol 2c (1 ml), rt, and 24 h.b Isolated yields are given in parentheses.
image file: d4gc02415j-u4.tif

The synthetic utility of this method was further investigated in the gram scale synthesis of 3aa (Table 5). The product was successfully obtained with excellent yield (88%) even under operationally milder conditions with cheap starting materials and catalyst. The result further indicates that this protocol has high potential for industrial-level applications.

Table 5 Gram scale synthesis
image file: d4gc02415j-u5.tif

To evaluate the environmental impact of our protocol, we assessed key green chemistry metrics, including atom economy (AE) and process mass intensity (PMI). We examined how effectively our catalytic system utilizes reactants and incorporates them into the final products by calculating the AE (ESI, Table S1†) and comparing these values with the recent literature on tertiary alcohol synthesis. Our AE values, ranging from 83 to 89%, demonstrate that our process is highly efficient and comparable to other methods reported in the literature (ESI, Table S3†).24 However, while some previous methods report higher atom economy values, their use of heavy metals, harsh reaction conditions, and strong bases undermines other green chemistry aspects, reducing their overall sustainability. Additionally, to assess the overall sustainability of our catalytic process, we calculated the PMI values. The relatively low values (194 to 464 kg kg−1) further indicate the greenness of our protocol (ESI, Table S2†).

To gain insight into the mechanism, we also performed certain control experiments. The radical trap experiment was successfully executed by using the radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) (Table 6a). It is observed that the formation of product 3aa was almost completely inhibited by TEMPO and the adduct 5 could not be detected by HR-MS. However, the addition of butylated hydroxytoluene (BHT) to the reaction mixture under standard conditions did not affect the progress of the reaction and the resultant product 3aa was isolated with a yield of 90% (Table 6b). The suppression of the reaction in the presence of TEMPO may arise from its interaction with the metal center, thereby hindering the essential electron transfer process required for the reaction to proceed effectively. The ineffectiveness of BHT in hindering the reaction indicates the concerted addition rather than a stepwise radical mechanism.

Table 6 Mechanistic studies
(a) 4-Nitrobenzaldehyde 1a (0.5 mmol), CuSO4·5H2O (5 mol%), methanol 2a (1 ml), 2,2,6,6-tetramethyl-1-piperidinyloxy (0.75 mmol), rt, and 24 h. (b) 4-Nitrobenzaldehyde 1a (0.5 mmol), CuSO4·5H2O (5 mol%), methanol 2a (1 ml), butylated hydroxytoluene (0.75 mmol), rt, and 24 h. (c) 3-Nitrobenzaldehyde 1b (0.5 mmol), CuSO4·5H2O (5 mol%), CD3OD 2c (1 ml), rt, and 24 h. (d) 3-Nitrobenzaldehyde 1b (0.5 mmol), anhydrous CuSO4 (5 mol%), MeOH 2a (1 ml), rt, 24 h, and argon atmosphere.
image file: d4gc02415j-u6.tif

The deuterium labeling experiments were also conducted using CD3OD as a reaction partner to gain deeper insight into the mechanism (Table 6c). The disappearance of peaks corresponding to the methyl protons in the 1H NMR spectrum indicates that the C1 source in this process is exclusively methanol. However, the retention of the alcoholic proton peak eliminates the possibility of hydrogen atom transfer from methanol during the reaction (ESI Fig. 27 and 28†). To investigate the potential involvement of crystallization water in the catalyst as a source of hydrogen during hydrogen atom transfer (HAT), we conducted the reaction using anhydrous copper sulfate under strictly anhydrous conditions (Table 6d). The decrease in the yield of 3ab suggests the necessity of a hydrated catalyst for the efficient progression of the reaction, likely due to its role in facilitating HAT. Further detailed studies are required to validate this hypothesis.

Based on the preliminary studies, we have proposed a plausible mechanism for the synthesis of tertiary alcohols from aromatic aldehydes (Fig. 2). Initially, the metal center interacts with the substrate through both the substituent group and the carbonyl group, resulting in the weakening of the C[double bond, length as m-dash]O bond. Subsequently, the addition of the methyl radical to the carbonyl carbon, followed by single electron transfer (SET) from Cu(II) to methanol, generates B. Another addition of the methyl radical, along with the breaking of the C–H bond and hydrogen atom transfer (HAT), yields the product C. Finally, the regeneration of Cu(II) occurs via elimination of H2O2. The successful detection of H2O2 in the reaction medium with an XploSens PS™ peroxide detection strip further supports the proposed mechanism (ESI, Fig. S1†). It is evident that both the counter ion and crystallized water in the catalyst play crucial roles in the reaction. This could be attributed to their potential involvement in facilitating hydrogen atom transfer (HAT) and stabilizing high oxidation state intermediates during the reaction. However, further detailed studies are necessary to precisely define their contributions and confirm this hypothesis. The detailed mechanism is still undergoing evolution, and ongoing studies are being conducted in our lab.


image file: d4gc02415j-f2.tif
Fig. 2 Plausible mechanistic pathway.

Conclusions

In summary, a novel and sustainable method has been devised for C–H double methylation of aromatic aldehydes, facilitating the synthesis of tertiary alcohols with high atom efficiency. The activation of un-activated methanol was achieved under ambient conditions by using cost-effective copper sulfate as the catalyst. The successful synthesis of the product at a gram scale demonstrates the practicality of employing this protocol in industrial contexts. Control experiments have confirmed that the reaction proceeds through concerted radical addition, with the catalyst regenerating via the elimination of H2O2. Key aspects of this work include the activation of methanol under ambient conditions without any thermal or photoinduction, the usage of environmental-friendly, readily available, and inexpensive catalysts, and the utilization of green solvents like methanol and ethanol, along with relatively simple reaction conditions. This research could pave the way for the development of methods to activate small molecules using simple metal salts under mild conditions in the future.

Author contributions

Sereena Sunny conducted the chemical experiments, characterized the products, analyzed the data, formulated the methodology, and drafted the paper. Ramasamy Karvembu supervised the project and edited the paper.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Sereena Sunny thanks the Department of Science and Technology, India, for the INSPIRE doctoral fellowship (IF180052) and Praveen D Raj for helpful discussions during the preparation of the manuscript.

References

  1. E. I. Solomon, D. E. Heppner, E. M. Johnston, J. W. Ginsbach, J. Cirera, M. Qayyum, M. T. Kieber-emmons, C. H. Kjaergaard, R. G. Hadt and L. Tian, Chem. Rev., 2014, 114, 3659–3853 CrossRef CAS PubMed .
  2. (a) L. Que Jr and W. B. Tolman, Nature, 2008, 455, 333–340 CrossRef PubMed ; (b) P. Gamez, P. G. Aubel, W. L. Driessen, J. Reedijk and W. L. Driessen, Chem. Soc. Rev., 2001, 30, 376–385 RSC ; (c) H. M. L. Davies, J. Du Bois, J. Yu, H. Lu and X. P. Zhang, Chem. Soc. Rev., 2011, 40, 1899–1909 RSC ; (d) L. Ciano, G. J. Davies, W. B. Tolman and P. H. Walton, Nat. Catal., 2018, 1, 571–577 CrossRef CAS ; (e) W. Sun and Q. Sun, Acc. Chem. Res., 2019, 52, 2370–2381 CrossRef CAS PubMed ; (f) L. Vicens, G. Olivo, M. Costas, L. Vicens, G. Olivo and M. Costas, ACS Catal., 2020, 10, 8611–8631 CrossRef CAS ; (g) R. M. Bullock, J. G. Che, L. Gagliardi, P. J. Chiri, O. K. Farh, C. H. Hendo, C. W. Jone, J. A. Keit, J. Klosin, S. D. Mintee, R. H. Morri, A. T. Radosevic, T. B. Rauchfus, N. A. Strotma, A. Vojvodic, T. R. War, J. Y. Yan and Y. Surendranath, Science, 2020, 369, eabc3183 CrossRef CAS PubMed .
  3. (a) S. C. Bete, L. K. May, P. Woite, M. Roemelt and M. Otte, Angew. Chem., Int. Ed., 2022, 61, e202206120 CrossRef CAS PubMed ; (b) Y. Zhu, C. W. Koo, C. K. Cassidy, M. C. Spink, T. Ni, L. C. Zanetti-Domingues, B. Bateman, M. L. Martin-Fernandez, J. Shen, Y. Sheng, Y. Song, Z. Yang, A. C. Rosenzweig and P. Zhang, Nat. Commun., 2022, 13, 5221 CrossRef CAS PubMed ; (c) C. W. Koo, F. J. Tucci, Y. He and A. C. Rosenzweig, Science, 2022, 375, 1287–1291 CrossRef CAS PubMed ; (d) R. J. Jodts, M. O. Ross, C. W. Koo, P. E. Doan, A. C. Rosenzweig and B. M. Hoffman, J. Am. Chem. Soc., 2021, 143, 15358–15368 CrossRef CAS PubMed .
  4. (a) J.-L. Pierre, Chem. Soc. Rev., 2000, 29, 251–257 RSC ; (b) E. I. Solomon, P. Chen, M. Metz, S. K. Lee and A. E. Palmer, Angew. Chem., Int. Ed., 2001, 40, 4570–4590 CrossRef CAS ; (c) J. W. Whittaker, Chem. Rev., 2003, 103, 2347–2363 CrossRef CAS PubMed ; (d) J. Liu, S. Chakraborty, P. Hosseinzadeh, Y. Yu, S. Tian, I. Petrik, A. Bhagi and Y. Lu, Chem. Rev., 2014, 114, 4366–4369 CrossRef CAS PubMed .
  5. D. Aynetdinova, M. C. Callens, H. B. Hicks, C. Y. X. Poh, B. D. A. Shennan, A. M. Boyd, Z. H. Lim, J. A. Leitch and D. J. Dixon, Chem. Soc. Rev., 2021, 50, 5517–5563 RSC .
  6. X. Zheng, J. Zeng, M. Xiong, J. Huang, C. Li, R. Zhou and D. Xiao, Asian J. Org. Chem., 2019, 8, 1325–1331 CrossRef CAS .
  7. J. S. Chang, Y. Da Lee, L. C. S. Chou, T. R. Ling and T. C. Chou, Ind. Eng. Chem. Res., 2012, 51, 655–661 CrossRef .
  8. K. Maruoka, A. B. Concepcion and H. Yamamoto, Synthesis, 1994, 1283–1290 CrossRef CAS .
  9. M. Selva and A. Perosa, Green Chem., 2008, 10, 457–464 RSC .
  10. R. De Marco, M. L. Di Gioia, A. Liguori, F. Perri, C. Siciliano and M. Spinella, Tetrahedron, 2011, 67, 9708–9714 CrossRef CAS .
  11. J. Liu, Y. Song, X. Wu and L. Ma, ACS Omega, 2021, 6, 22504–22513 CrossRef CAS PubMed .
  12. E. B. Hemming, A. F. Masters, A. Perosa, M. Selva and T. Maschmeyer, Molecules, 2019, 24, 3986 CrossRef CAS PubMed .
  13. C. Qiao, X. F. Liu, X. Liu and L. N. He, Org. Lett., 2017, 19, 1490–1493 CrossRef CAS PubMed .
  14. J. R. Cabrero-Antonino, R. Adam and M. Beller, Angew. Chem., Int. Ed., 2019, 58, 12820–12838 CrossRef CAS PubMed .
  15. Sheetal, P. Mehara and P. Das, Coord. Chem. Rev., 2023, 475, 214851 CrossRef CAS .
  16. (a) K. Natte, H. Neumann, M. Beller and R. V. Jagadeesh, Angew. Chem., Int. Ed., 2017, 56, 6384–6394 CrossRef CAS PubMed ; (b) V. Goyal, N. Sarki, A. Narani, G. Naik, K. Natte and R. V. Jagadeesh, Coord. Chem. Rev., 2023, 474, 214827 CrossRef CAS .
  17. P. Biswal, M. S. Subramani, S. Samser, V. Chandrasekhar and K. Venkatasubbaiah, J. Org. Chem., 2023, 88, 5135–5146 CrossRef CAS PubMed .
  18. J. Jin and D. W. C. MacMillan, Nature, 2015, 525, 87–90 CrossRef CAS PubMed .
  19. (a) B. C. Figula, T.-A. Chen, J. A. Bertke and T. H. Warren, ACS Catal., 2022, 12, 11854–11859 CrossRef CAS ; (b) Z. Zhang, P. Chen and G. Liu, Chem. Soc. Rev., 2022, 51, 1640–1658 RSC .
  20. (a) S. J. Li, Y. Lan and Y. Lan, Chem. Commun., 2020, 56, 6609–6619 RSC ; (b) H. Liu and Q. Shen, Coord. Chem. Rev., 2021, 442, 213923 CrossRef CAS ; (c) Y. Luo, Y. Li, J. Wu, X.-S. Xue, J. F. Hartwig and Q. Shen, Science, 2023, 381, 1072–1079 CrossRef CAS PubMed .
  21. (a) X. Zhang and D. W. C. MacMillan, J. Am. Chem. Soc., 2017, 139, 11353–11356 CrossRef CAS PubMed ; (b) T. Kawasaki, N. Ishida and M. Murakami, J. Am. Chem. Soc., 2020, 142, 3366–3370 CrossRef CAS PubMed ; (c) V. Murugesan, A. Ganguly, A. Karthika and R. Rasappan, Org. Lett., 2021, 23, 5389–5393 CrossRef CAS PubMed ; (d) L. Wang, T. Wang, G. J. Cheng, X. Li, J. J. Wei, B. Guo, C. Zheng, G. Chen, C. Ran and C. Zheng, ACS Catal., 2020, 10, 7543–7551 CrossRef CAS ; (e) P. Fan, C. Zhang, L. Zhang and C. Wang, Org. Lett., 2020, 22, 3875–3878 CrossRef CAS PubMed ; (f) H. Y. Wang, X. H. Wang, B. A. Zhou, C. L. Zhang and S. Ye, Nat. Commun., 2023, 14, 1–11 Search PubMed .
  22. (a) B. Bieszczad and D. G. Gilheany, Angew. Chem., Int. Ed., 2017, 56, 4272–4276 CrossRef CAS PubMed ; (b) Y. L. Liu and X. T. Lin, Adv. Synth. Catal., 2019, 361, 876–918 CrossRef CAS ; (c) M. Shibasaki and M. Kanai, Chem. Rev., 2008, 108, 2853–2873 CrossRef CAS PubMed .
  23. S. E. García-Garrido, A. P. Soto, E. Hevia and J. García-Álvarez, Eur. J. Inorg. Chem., 2021, 2021, 3116–3130 CrossRef .
  24. (a) J. B. C. Mack, J. D. Gipson, J. Du Bois and M. S. Sigman, J. Am. Chem. Soc., 2017, 139, 9503–9506 CrossRef CAS PubMed ; (b) D. Hu and X. Jiang, Green Chem., 2022, 24, 124–129 RSC ; (c) K. Fang, G. Li and Y. She, J. Org. Chem., 2018, 83, 8092–8103 CrossRef CAS PubMed .

Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, and 1H NMR, 13C NMR and HR-MS spectra of compounds. See DOI: https://doi.org/10.1039/d4gc02415j
This journal is © The Royal Society of Chemistry 2024