Chemoselective reduction of thioureas to N-methylamines and N,N-dimethylamines using a Cu-based catalyst

Abstract

We report a general method for the copper-catalyzed hydrogenative reduction of thioureas to N-methylamines and N,N-dimethylamines (10 examples, 42–99% carbon utilization rate) using PhSiH₃ as the hydrogenation reagent, realizing the highly efficient indirect utilization of carbon in carbon disulfide (CS₂). This strategy provides a mild and environmentally friendly platform for CS₂ pollutant valorization, holding potential for sustainable chemical manufacturing.

---

Sustainability spotlight

A two-step method for the utilization of carbon disulfide (CS₂) to form N-methylanilines or N,N-dimethylanilines via a thiourea intermediate is a promising way to use CS₂ as a feedstock, lowering its emission from rubber, viscose-fiber and agrochemical industries. We report herein a convenient and highly efficient method for the hydrogenation of thioureas with PhSiH₃, using earth-abundant metal catalysis, leading to the formation of N-methylamines and N,N-dimethylamines based on a well-studied first step, proceeding under atmospheric pressure with high carbon utilization. Notable functional group tolerance and environmentally friendly chemoselective reduction highlight the potential of this reaction for chemical transformations of renewable feedstocks. This work aligns with industry, innovation, and infrastructure (UN SDG 9) and responsible consumption and production (UN SDG 12).

Introduction

Realizing the high-value-added directional conversion of industrial waste gases is an important research direction that urgently needs in-depth exploration in the pertinent field of sustainable development. Carbon disulfide (CS₂) is primarily found in natural gas, petroleum gas, water gas, and industrial exhaust gases from the automotive and chemical industries.^1–4 In the environment, it readily transforms into acid rain, thereby causing serious ecological threats and environmental damage.^2,3 Therefore, it is essential to curb its emissions into the atmosphere.^3,5 Various techniques have been developed to remove CS₂, such as physical and chemical adsorption,^6–8 hydrolysis,^9,10 and hydrolysis-oxidation coupling.¹¹ The production of thiourea from CS₂ and amines is an effective method for fixing it, and this has been extensively studied.^1,12–18 Furthermore, reducing thiourea into high-value-added products is an indirect way of utilizing CS₂ waste.

Recently, important progress has been made in the conversion of thioamides into amines or/and thiols by Milstein and other groups^19–26 (Fig. 1a and b); however, studies of the hydrogenation reduction of thioureas remain scarce.²⁷ N-methylamines and N,N-dimethylamines are widely used as solvents and are significant in the synthesis of fine chemicals such as dyes, surfactants, pesticides, petrochemicals, rubber, and pharmaceuticals.^28–33 The catalytic reduction of thioureas to produce them is thus an effective method for recycling the useful carbon resources in CS₂ to obtain high-value-added products (Fig. 1). To achieve this goal, there are challenges including (a) catalyst poisoning due to the sulfur compounds generated during the reaction; and (b) site-selectivity upon cleavage of the C=S and/or C–N bond of thioureas.²⁵ Moreover, because of n_N → π* C=S resonance from thiourea C–N and C=S bonds, and the large radius of the sulfur atom, the thiourea C–N bond has a high rotational barrier, hampering its chemical transformation and application.³⁴ Khurana pioneered the catalytic reduction of 1,3-diphenylthiourea into N-methylanilines using NaBH₄ as a reducing agent, in the presence of NiCl₂ as the catalyst.²⁷

Fig. 1 (a) Previous work on the hydrogenation reduction of thioamides. (b) Previous work on the hydrogenation reduction of thioureas. (c) This work on the the reduction of thioureas with PhSiH₃ using a Cu catalyst.

Hydrosilanes are mild and inexpensive reducing agents commonly used in catalytic transfer hydrogenation reactions for reducing various organic functionalities, including esters, amides, ureas, and thioamides.^35–40 Despite their significant advantages with respect to environmental friendliness and cost as compared to borohydrides, a widely applied, operationally simple, mild, and cheap method for the catalytic reduction of thioureas using silanes has yet to be developed. Furthermore, many catalytic reactions rely on noble metals, whose use should be limited due to their low abundance, high price, and high toxicity.^41,42 Regarding green and sustainable chemistry, earth-abundant transition metals can be excellent alternatives to rare precious metals.^43,44 Among them, copper is abundant on Earth and has low cost and toxicity, forming a wide variety of coordination modes with ligands.^41,45–48 Efforts to explore more sustainable and environmentally friendly copper catalysts for the hydrogenation reduction of thioureas and their derivatives are therefore quite valuable.

Herein, we disclose the reduction of thioureas with hydrosilanes as the reduction reagent into N-methylamines and N,N-dimethylamines using an earth-abundant-metal copper-based catalyst, which can indirectly realize the high carbon utilization of CS₂ waste.

Results and discussion

We initially screened various metals and found that Cu metal exhibited good catalytic performance in the hydrosilylation of 1,3-diphenylthiourea (1a) during the experimental exploration stage (Table S1). Among various Cu metal precursors,^49–51 CuCl₂ showed the most prominent catalytic effect (Table S2). Having identified CuCl₂ as the metal precursor, we investigated the influence of ligands on the catalytic performance of Cu-based catalyst systems (Table 1, entries 1–6). We started our investigation by studying the hydrosilylation reaction with PhSiH₃ (5 equivalents) using CuCl₂ (20 mol%) and a bidentate phosphine ligand (40 mol%) at 130 °C for 24 h in THF (3 mL). By evaluating ligands L1–L5, we found that ligands were crucial for controlling the product distribution of the reaction (Table 1, entries 1–5). As the carbon chain structure in ligands L1–L5 became more complex, the proportion of dimethylaniline in the reaction product gradually increased from 20% to 60%, while the proportion of methylaniline also gradually decreased from 62% to 30%. Among them, the catalyst system composed of CuCl₂ and ligand L3 achieved a high carbon utilization rate of 92%, with the yields of monomethylaniline (1b) and dimethylaniline (1c) being 42% and 50%, respectively. It should be noted that CuCl₂ has relatively low catalytic performance in the absence of a ligand (Table 1, entry 6). In addition, appropriately increasing the reaction temperature and time can effectively enhance the reaction rate (Table 1, entries 3 and 8–14), and the carbon utilization rate reaches 96% at 140 °C after 24 h (Table 1, entry 8). However, prolonged reaction times will lead to a decline in carbon utilization.

Table 1 Optimization of the desulfurization reduction of thiourea with silanes^a

Entry	Ligand	T (h)	Temp. (°C)	PhSiH3 (equiv.)	Yield of 1b (%)	Yield of 1c (%)	Carbon utilization^b (%)	Conv. (%)
a Reaction conditions: substrate (0.8 mmol), CuCl2 (20 mol%), ligand (40 mol%), hydrosilane (1.0–7.0 equiv.), THF (3.0 mL), 120–150 °C (bath temperature). Determined by GC using biphenyl as an internal standard unless otherwise noted. Isolated yields are given in parentheses. Identification of the products was also carried out by GC-MS.b Carbon utilization was calculated by adding the yield of 1b and the yield of 1c.c No ligands were used.d Ph2SiH2 was used instead of PhSiH3.e Ph3SiH was used instead of PhSiH3.
1	L1	24	130	5.0	62	20	82	95
2	L2	24	130	5.0	57	27	84	96
3	L3	24	130	5.0	42	50	92	98
4	L4	24	130	5.0	34	52	86	97
5	L5	24	130	5.0	30	60	90	97
6^c	—	24	130	5.0	5	11	16	16
7	L3	24	120	5.0	28	45	71	86
8	L3	24	140	5.0	46	50	96	>99
9	L3	24	150	5.0	39	49	88	>99
10	L3	6	140	5.0	27	27	54	70
11	L3	12	140	5.0	40	47	87	93
12	L3	18	140	5.0	44	47	91	>99
13	L3	30	140	5.0	39	52	91	>99
14	L3	36	140	5.0	33	50	83	>99
15^d	L3	24	140	5.0	0	7	7	33
16^e	L3	24	140	5.0	—	—	—	NR
17	L3	24	140	3.0	44 (41)	49 (48)	93 (89)	>99
18	L3	24	140	1.0	21	20	41	58
19	L3	24	140	7.0	45	45	90	>99

---

Subsequently, Ph₂SiH₂ and Ph₃SiH were used as reducing reagents⁵² (Table 1, entries 15 and 16). With a decrease in Si–H, the reduction efficiency gradually declined (Table 1, entries 8, 15 and 16). Finally, we attempted to carry out the catalytic reaction under conditions with a lower amount of PhSiH₃ (Table 1, entries 17 and 18). When PhSiH₃ is reduced to 3 equivalents, it has a reduction efficiency similar to that of 5 equivalents⁵³ (Table 1, entry 17). However, further reduction in the amount of PhSiH₃ will lead to a significant decrease in reduction efficiency (Table 1, entry 18).

With the optimal reaction conditions for the reduction of substrate 1a in hand, we explored the substrate scope for the catalytic hydrosilylation of thiourea derivatives to N-methylaniline (b) and N,N-dimethylaniline (c) using CuCl₂ and the L3 ligand as the in situ catalyst system (Table 2). Initially, we selected substrates with electron-donating functional groups at the para positions of the benzene rings (Table 2, entries 2 and 4), which exhibited carbon utilization rates exceeding 90%, and the yield of N,N-dimethylamine was higher than that of N-methylamine. In addition, we checked electron-withdrawing functional groups at the para positions of the benzene rings, such as halides or trifluoromethyl; the carbon utilization rates ranged between 73% and 90% (Table 2, entries 5–8), and the yield of N-methylamine was higher than that of N,N-dimethylamine in the product in some cases (Table 2, entry 8). Additionally, we investigated a case where the benzene rings were substituted at the ortho-positions. Surprisingly, the carbon utilization rate reached 73%. All the product in this reaction was the substituted N-methylamine, and no substituted N,N-dimethylamine (c) was formed, possibly because of the steric hindrance effect brought about by the functional groups at the ortho position (Table 2, entry 3). As expected, compared with the symmetric thioureas above, the asymmetric thiourea 1-phenyl-3-butyl-2-thiourea exhibited a 43% carbon utilization rate, lower than the carbon utilization rates of the symmetric thioureas, with 29% yield of N-methylaniline and 14% yield of N,N-dimethylaniline in the product (Table 2, entry 9). Finally, we found that when the two benzene rings on N,N-diphenylthiourea were completely replaced with aliphatic substituents like 1,3-dicyclohexyl and 1,3-dibenzyl groups, the carbon utilization rates decreased significantly, with carbon utilization rates of 63% and 32%, respectively (Table 2, entries 10–11). In summary, the exploration of the substrate scope of the catalyst system implied that the catalyst system demonstrated good tolerance toward para-substituents on the benzene ring, such as methyl, methoxy, trifluoromethyl, and halogens, as well as moderate tolerance toward benzyl and cyclohexyl groups, and the nature of substituents on the thiourea benzene ring was able to significantly influence the reaction outcome.

Table 2 The catalytic hydrogenation of organic thiourea derivatives to N-methylanilines and N,N-dimethylanilines by the copper catalyst^a

Entry	Substrate	Yield of b (%)	Yield of c (%)	Carbon utilization^b (%)	Conv. (%)
a Reaction conditions: substrate (0.8 mmol), CuCl2 (20 mol%), L3 (40 mol%), PhSiH3 (3.0 equiv.), THF (3.0 mL), 140 °C (bath temperature), 24 h. Determined by GC using biphenyl as an internal standard unless otherwise noted. Identification of the products was also carried out by GC-MS and ^1H NMR. Isolated yields are given in parentheses.b Carbon utilization was calculated by adding the yield of 1b and yield of 1c.c Reaction conditions: substrate (0.4 mmol), CuCl2 (20 mol%), L3 (40 mol%), PhSiH3 (3.0 equiv.), THF (3.0 mL), 140 °C (bath temperature), 24 h.d Determined by GC using 1,3,5-trimethylbenzene as an internal standard.e Reaction conditions: PhSiH3 (5.0 equiv.).
1		44 (41)	49 (48)	93 (89)	>99
2		34	56	90	>99
3		73 (69)	0	73 (69)	93
4		26 (23)	73 (69)	99 (92)	>99
5^c		29	47	73	>99
6		44	43	87	>99
7^c^,^d		41	49	90	95
8		60	22	82	96
9		29	14	43	91
10		37	26	63	85
11^e		19	13	32	71

---

After exploring the substrate suitability of the Cu-based catalyst, the reaction mechanism for catalysing the hydrosilylation of thiourea derivatives to prepare N-methylaniline (1b) and N,N-dimethylaniline (1c) was explored.^54,55 This reaction may proceed through the following two paths: (1) thiourea first forms amine and thioformamide via a single C–N bond cleavage, and then the latter continues to be hydrogenated to form monomethylamine via C–S bond cleavage; or (2) thiourea forms formamidine via initial C=S double bond cleavage, and this then hydrogenates to generate monomethylamine via single C–N bond cleavage. Based on this, we carried out the hydrosilylation reactions of thioformamide (intermediate A) and formamidine (intermediate B), respectively (Fig. 2a and b). Thioformamide was successfully reduced to N-methylamine and N,N-dimethylamine products (Fig. 2a), while it was difficult for formamidine to undergo hydrosilylation reactions (Fig. 2b). Therefore, this reaction may generate the N-methylamine product from the thioformamide intermediate A rather than the formamidine intermediate B.

Fig. 2 (a) Catalytic hydrosilylation reaction of N-phenylthioformamide (intermediate A). (b) Catalytic hydrosilylation reaction of N,N′-diphenylformamidine (intermediate B). (c) Catalytic hydrosilylation reaction of N-phenylthioformamide (intermediate A) and N-methylaniline (1b). (d) Catalytic hydrosilylation reaction of N-methylaniline (1b).

Encouraged by this, we also investigated the source of N,N-dimethylamine. When N-methylamine and thioformamide were used as substrates simultaneously, in addition to the complete conversion of thioformamide, 30% of N-methylamine was also converted, and similar yields of N,N-dimethylamine (68%) and amine (61%) were produced (Fig. 2c). Moreover, N-methylamine as a substrate alone cannot yield N,N-dimethylamine under the same reaction conditions (Fig. 2d). These results suggest that N,N-dimethylamine is formed by the further reaction of the N-methylamine product and the thioformamide intermediate A. Based on the control experiments and our and other groups' previous reports on urea hydrogenation reactions,^56–62 along with reports on the C–N cleavage reaction of thioamide,^63,64 we propose a possible reaction mechanism for the reduction of thiourea by PhSiH₃ to N-methylamine and N,N-dimethylamine (Fig. 3). Thiourea is first hydrogenated to generate intermediate A and an amine via C–N bond cleavage. Intermediate A is then reduced by PhSiH₃ to form N-methylamine. Meanwhile, the Si–H bond of PhSiH₃ dissociates to form a Si–S bond, which is eliminated in the form of PhH₂Si–S–SiH₂Ph ([Si]–S–[Si]). Furthermore, the generated methylamine and intermediate A react to form intermediate C, which subsequently desulfurizes to produce N,N-dimethylamine.

Fig. 3 The proposed mechanism.

Conclusions

In summary, we have developed a general copper-catalyzed system for the reductive desulfurization of thioureas to N-methylamines and N,N-dimethylamines using PhSiH₃ as the reduction reagent. This new catalytic system not only prepared extra N,N-dimethylamine products, but it also showed higher utilization of carbon. This system is applicable to various aliphatic and aromatic thiourea derivatives. In particular, good utilization of carbon is obtained for various aromatic thioureas, including those with halogen, trifluoromethyl, hydroxyl, methyl, methoxy or other groups on the benzene rings. This catalytic system may offer a new perspective and accelerate the further development of the reduction of unsaturated organic sulfides and the chemical recovery of sulfur-containing resins, while also broadening the application of chemo-selective hydrogenation reactions and transfer hydrogenation reactions of unsaturated organic sulfides.

Author contributions

Yuanchen Jie: investigation, formal analysis, visualization and writing – original draft. Jun Zhu: methodology and writing – review and editing. Yongtao Wang: visualization. Jia Yao: methodology and resources. Haoran Li: conception, funding acquisition, project administration, supervision and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional data on experimental procedures, and optimization, GC-MS, ¹H NMR and ¹³C NMR data (Fig. S1 to S39, Tables S1 to S10). See DOI: https://doi.org/10.1039/d6su00242k.

Acknowledgements

This research was supported by the National Key R&D Program of China (2022YFA1503200) and the National Natural Science Foundation of China (No. 22303079 and 22073081). We thank Guochun Lan, Dr Yaqin Liu, and Prof. Dr Qiaohong He (Chemistry Instrumentation Center, Zhejiang University) for their respective technical support with GC-MS, NMR, and ESI-MS.

Notes and references

1. X. Chen, B. Shen, H. Sun, G. Zhan and Z. Huo, Ind. Eng. Chem. Res., 2017, 56, 6499–6507.
2. C. M. McGuirk, R. L. Siegelman, W. S. Drisdell, T. Runčevski, P. J. Milner, J. Oktawiec, L. F. Wan, G. M. Su, H. Z. H. Jiang, D. A. Reed, M. I. Gonzalez, D. Prendergast and J. R. Long, Nat. Commun., 2018, 9, 5133.
3. F. Wang, H. Chen, X. Sun, C. Wang, Y. Ma, X. Song, K. Li, P. Ning and H. He, Sep. Purif. Technol., 2021, 258, 118086.
4. H. Yoshino, M. Saigo, T. Ehara, K. Miyata, K. Onda, J. Pirillo, Y. Hijikata, S. Takaishi, W. Kosaka, K. Otake, S. Kitagawa and H. Miyasaka, Angew. Chem., Int. Ed., 2025, 64, e202413830.
5. X. Cui, P. Li, B. Hu, T. Yang, H. Fu, S. Chen and X. Zhang, Molecules, 2023, 28, 4627.
6. G. Bo, L. Chang and K. Xie, Fuel Process. Technol., 2006, 87, 873–881.
7. X. Wang, F. Wang, W. Chen, P. Ning, Y. Ma, L. Wang and Z. Bian, Ind. Eng. Chem. Res., 2014, 53, 13626–13634.
8. J. Yang, P. Juan, Z. Shen, R. Guo, J. Jia, H. Fang and Y. Wang, Carbon, 2006, 44, 1367–1375.
9. P. D. Clark, N. I. Dowling and M. Huang, Appl. Catal. B Environ., 2001, 31, 107–112.
10. L. Tang, H. Guo, K. Li, P. Ning, C. Wang and X. Sun, Adv. Mater. Res., 2014, 894, 293–299.
11. L. Wang, D. Wu, S. Wang and Q. Yuan, J. Environ. Sci., 2008, 20, 436–440.
12. F. Liang, J. Tan, C. Piao and Q. Liu, Synthesis, 2008, 2008, 3579–3584.
13. G. M. Kyne, M. E. Light, M. B. Hursthouse, J. d. Mendoza and J. D. Kilburn, J. Chem. Soc., Perkin Trans. 1, 2001 10.1039/B102298A1258-1263.
14. S. D. Bhosle, K. A. Jadhav, S. V. Itage, S. Bandaru, R. S. Bhosale and J. S. Yadav, J. Sulfur Chem., 2023, 44, 186–200.
15. R. Ballini, G. Bosica, D. Fiorini, R. Maggi, P. Righi, G. Sartori and R. Sartorio, Tetrahedron Lett., 2002, 43, 8445–8447.
16. N. Azizi, A. Khajeh-Amiri, H. Ghafuri and M. Bolourtchian, Mol. Div., 2011, 15, 157–161.
17. R. Gutiérrez, J. Vázquez, S. Bernès, Y. Reyes, M. Moya, P. Sharma and C. Alvarez, Synthesis, 2004, 2004, 1955–1958.
18. S. Techapanalai, R. M. Annuur, M. Sukwattanasinitt and S. Wacharasindhu, ChemistrySelect, 2023, 8, e202302045.
19. J. Luo, M. Rauch, L. Avram, Y. Ben-David and D. Milstein, J. Am. Chem. Soc., 2020, 142, 21628–21633.
20. Z. Yang, W. Li, Y. Jing, X. Wei, Y. Li, Y. Liu, P. Su, N. Wang, Z. Chen, J. Su and Z. Ke, Eur. J. Org Chem., 2025, 28, e202500387.
21. R. Sharma, G. Kenguva, R. Dandela and P. Daw, Chem. Commun., 2025, 61, 9460–9463.
22. P. Nad, S. Goswami, H. K. Kisan and A. Mukherjee, Chem. Eur J., 2025, 31, e202500738.
23. Y. Yu, F. Zhang, J. Cheng, J. Hei, W. Deng and Y. Wang, Org. Lett., 2018, 20, 24–27.
24. Q. Zhang and C. Darcel, Eur. J. Org Chem., 2025, 28, e202500048.
25. Z. Wang, S. Chen, C. Chen, Y. Yang and C. Wang, Angew. Chem., Int. Ed., 2023, 62, e202215963.
26. K. Fukumoto, A. Sakai, K. Hayasaka and H. Nakazawa, Organomet., 2013, 32, 2889–2892.
27. J. M. Khurana, G. Kukreja and G. Bansal, J. Chem. Soc., Perkin Trans., 2002, 1(22), 2520–2524.
28. M. Tamura, A. Miura, Y. Gu, Y. Nakagawa and K. Tomishige, Chem. Lett., 2017, 46, 1243–1246.
29. T. Taeufer and J. Pospech, J. Org. Chem., 2020, 85, 7097–7111.
30. Z. Qian, J. Bai, M. Ma, H. Liu, M. Cai, J. Chen and S. Sun, Fuel, 2025, 393, 135006.
31. B. Hong, X. Wang, Y. Xie, R. Qin, J. Zuo, X. Wu, L. Ye, W. Da, G. Fu and Y. Yuan, Appl. Catal. B Environ., 2026, 382, 125995.
32. Z. Guo, B. Zhang, X. Wei and C. Xi, ChemSusChem, 2018, 11, 2296–2299.
33. Z. Huang, X. Jiang, S. Zhou, P. Yang, C. X. Du and Y. Li, ChemSusChem, 2019, 12, 3054–3059.
34. Z. He, C. Yan, M. Zhang, M. Irfan, Z. Wang and Z. Zeng, Synthesis, 2021, 54, 705–710.
35. A. Chardon, O. Maubert, J. Rouden and J. Blanchet, ChemCatChem, 2017, 9, 4460–4464.
36. M. D. Greenhalgh and S. P. Thomas, ChemCatChem, 2014, 6, 1520–1522.
37. V. Skrypai, J. J. M. Hurley and M. J. Adler, Eur. J. Org Chem., 2016, 2016, 2207–2211.
38. R. Luo, X. Lin, Y. Chen, W. Zhang, X. Zhou and H. Ji, ChemSusChem, 2017, 10, 1224–1232.
39. U. Prieto-Pascual, I. Bustos, Z. Freixa, A. Kumar and M. A. Huertos, RSC Sustain., 2024, 2, 1052–1057.
40. C. Dewangan, R. Rameshan, S. Perumal, N. V. Kalevaru, S. Wohlrab, R. V. Jagadeesh and K. Natte, RSC Sustain., 2026, 4, 1456–1463.
41. H. Guo, Y. Su, Y. Shen, Y. Long and W. Li, J. Colloid Interface Sci., 2019, 536, 646–654.
42. W. Xu and N. Yoshikai, ChemSusChem, 2019, 12, 3049–3053.
43. C. Masaro, G. Meloni, M. Baron, C. Graiff, C. Tubaro and B. Royo, Chem. Eur J., 2023, 29, e202302273.
44. T. P. L. Cosby and C. B. Caputo, RSC Sustain., 2023, 1, 1547–1553.
45. M. Attarroshan, B. C. Vazquez, C. R. Andrews, S. L. Stokes and J. P. Emerson, Tetrahedron Lett., 2025, 161, 155567.
46. L. Cheng and N. P. Mankad, Chem. Soc. Rev., 2020, 49, 8036–8064.
47. Y. N. Zheng, H. Zheng, T. Li and W. T. Wei, ChemSusChem, 2021, 14, 5340–5358.
48. G. A. Correia, C. H. J. Franco, M. V. Kirillova, A. Pradal, G. Poli, F. Gallou and A. M. Kirillov, RSC Sustain., 2025, 3, 3396–3406.
49. X. Li, S. Xia, K. Chen, X. Liu, H. Li and L. He, Green Chem., 2018, 20, 4853–4858.
50. K. Motokura, N. Takahashi, D. Kashiwame, S. Yamaguchi, A. Miyaji and T. Baba, Catal. Sci. Technol., 2013, 3, 2392.
51. S. Zhang, Q. Mei, H. Liu, H. Liu, Z. Zhang and B. Han, RSC Adv., 2016, 6, 32370–32373.
52. X. Liu, R. Ma, C. Qiao, H. Cao and L. He, Chem. Eur J., 2016, 22, 16489–16493.
53. W. Li, D. Zhu, G. Li, J. Chen and J. Xia, Adv. Synth. Catal., 2019, 361, 5098–5104.
54. M. Avalos, R. Babiano, C. García-Verdugo, J. L. Jiménez and J. C. Palacios, Tetrahedron Lett., 1990, 31, 2467–2470.
55. N. Sarkar, R. K. Sahoo and S. Nembenna, Eur. J. Org Chem., 2022, 2022, e202200941.
56. J. Zhu, Y. Wang, J. Yao and H. Li, Chem. Sci., 2024, 15, 2089–2099.
57. J. Zhu, Y. Wang, J. Yao and H. Li, Chem. Sci., 2024, 15, 20534–20544.
58. J. Zhu, S. Zhang, Y. Wang, J. Yao, X. Hong and H. Li, ACS Catal., 2025, 15, 4911–4920.
59. J. Zhu, Y. Zhang, Z. Wen, Q. Ma, Y. Wang, J. Yao and H. Li, Chem. Eur J., 2023, 29, e202300106.
60. T. Iwasaki, N. Saito, Y. Yamada, S. Ajiro and K. Nozaki, Organomet., 2024, 43, 924–928.
61. B. Sole, J. S. Kolb, R. Marcial-Hernandez, J. Luk, T. Williams, O. V. Magdysyuk, D. Sheppard, G. Walker and A. Kumar, ACS Sustain. Chem. Eng., 2025, 13, 17173–17181.
62. X. Liu and T. Werner, Chem. Sci., 2021, 12, 10590–10597.
63. Y. Liu, X. Mo, I. Majeed, M. Zhang, H. Wang and Z. Zeng, Org. Biomol. Chem., 2022, 20, 1532–1537.
64. Q. Zhang, M. Berro and C. Darcel, Synthesis, 2025, 57, 3394–3402.

---

Footnote

† These authors contributed equally.

---