Solvent-free copper-catalyzed trisilylation of alkynes: a practical and atom-economical approach for accessing 1,1,1-trisilylalkanes

Abstract

Organosilicon compounds are versatile reagents in chemical synthesis and materials sciences. As an important class of organosilanes, 1,1,1-trisilylalkanes can undergo various organic transformations and serve as core units for silicon-containing hyperbranched polymers. The existing catalytic approaches for accessing 1,1,1-trisilylalkanes via alkyne trisilylation not only requires pre-synthesized moisture- and air-sensitive organocalcium and organolanthanum catalysts but also suffers from limited substrate scope for both alkyne and hydrosilane reagents. For example, only alkyl-substituted alkynes can undergo organocalcium-catalyzed trisilylation with alkyl hydrosilanes to provide the desired 1,1,1-trisilylalkane products. Herein, we report a selective copper-catalyzed trisilylation reaction of both alkyl- and aryl-substituted alkynes with a readily accessible copper catalyst that is generated in situ from Cu(OAc)₂ and tributylphosphine PⁿBu₃. This copper-catalyzed trisilylation reaction features easy catalyst preparation, broad substrate scope, and mild solvent-free reaction conditions. Mechanistic studies reveal that this trisilylation reaction occurs through copper-catalyzed deprotosilylation of alkynes to form alkynylsilanes followed by double hydrosilylation of alkynylsilane.

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Introduction

Organosilanes are useful building blocks in organic synthesis and materials sciences because of their diverse reactivity, non-toxicity, high stability, and ease of handling.^1–3 As an important family of organosilicon compounds, 1,1,1-trisilylalkanes, particularly those containing Si–H bonds which allow their further functionalization, are widely used in the synthesis of silicon polymers.^4,5 In addition, 1,1,1-trisilylalkanes can readily undergo base-induced desilylation to generate gem-disilyl-substituted carbanions.⁶ These carbanions are stabilized by the attached silyl groups and can react with various electrophiles.^6–9 However, general approaches for preparing structurally diverse 1,1,1-trisilylalkanes from readily accessible starting materials are rather limited in scope and functional group compatibility, which in turn limits the exploration of their new reactivity. The classic synthesis of 1,1,1-trisilylalkanes has largely been based on stoichiometric reactions of trisilylmethyllithium or trisilylmethyl Grignard reagents with activated alkyl halides.^10,11 However, these reactions require stoichiometric amounts of pyrophoric reagents and generate large quantities of waste when conducted on a large scale.

Metal-catalyzed hydrosilylation of unsaturated hydrocarbons is a straightforward and atom-economical approach for preparing various families of organosilicon compounds,^12–16 such as alkylsilanes, vinylsilanes, allylsilanes, and gem-disilylalkanes.^17–24 Synthetic protocols based on lanthanum-catalyzed dihydrosilylation of silyl-substituted internal alkynes (Scheme 1A) and calcium-catalyzed trisilylation of terminal alkynes (Scheme 1B) to prepare 1,1,1-trisilylalkanes have also been developed but suffer from several significant limitations.^25,26 For example, the scope of alkynes for these metal-catalyzed trisilylation reactions is limited to alkyl-substituted alkynes, and aryl-substituted alkynes only undergo dehydrogenative silylation to provide alkynylsilane products.²⁶ In addition, the scope of hydrosilanes for these reactions is limited to alkylsilanes RSiH₃ because arylsilanes ArSiH₃ can readily undergo silane redistribution reactions to produce SiH₄, Ar₂SiH₂, or Ar₃SiH in the presence of alkaline earth metal or lanthanide catalysts.^27–30 Furthermore, the preparation of these well-defined lanthanum and calcium pre-catalysts is rather challenging because they are highly oxophilic and moisture-sensitive.³¹ Lastly, the highly polar nature of metal–carbon bonds in organolanthanum and organocalcium intermediates renders these trisilylation reactions less compatible towards reactive functional groups.³² Therefore, it remains desirable to identify metal catalysts for selective alkyne trisilylation that can combine broad substrate scope, high functional group tolerance, and convenient catalyst generation.

Scheme 1 Catalytic synthesis of 1,1,1-trisilylalkanes.

Deprotosilylation of terminal alkynes to form alkynylsilanes with metal acetylide species as intermediates is a key step in transition metal-catalyzed alkyne trisilylation reactions. Terminal alkynes can readily react with various copper salts under mild conditions to form stable monomeric or high-nuclearity copper acetylides.^33–35 Accordingly, copper acetylide species have been proposed as reactive intermediates in a variety of alkyne functionalization reactions, such as Sonogashira coupling reactions and multi-borylation of terminal alkynes.^36–43 Recently, copper complexes have been employed to catalyze hydrosilylation and deprotosilylation of terminal alkynes to generate vinylsilanes and alkynylsilanes, respectively.^44,45 In these copper-catalyzed reactions between terminal alkynes and hydrosilanes, copper hydride and copper acetylide species have been proposed as key intermediates. Nevertheless, suitable conditions and copper catalysts have not been identified to integrate copper-catalyzed deprotosilylation and double hydrosilylation of alkynes into one process to produce 1,1,1-trisilylalkanes.

In continuation of our efforts in developing selective base-metal catalyzed synthesis of multi-organometallic compounds from readily accessible unsaturated hydrocarbons,^46–53 we became interested in identifying selective base metal catalysts for trisilylation of alkynes to access 1,1,1-trisilylalkane compounds. We envisioned that copper complexes would be potential catalysts to promote 1,1,1-trisilylation reactions of terminal alkynes because copper acetylide and copper hydride species could be formed in the reactions of terminal alkynes with hydrosilanes. Herein, we report a copper-catalyzed 1,1,1-trisilylation reaction of terminal alkynes under mild conditions with commercially available Cu(OAc)₂ and monophosphine ligand PⁿBu₃ (Scheme 1C). Mechanistic studies suggest that alkynylcopper, alkynylsilane, and gem-disilylalkene species are key intermediates for this copper-catalyzed trisilylation reaction.

Results and discussion

Evaluation of reaction conditions

To initiate our studies on the copper-catalyzed trisilylation of alkynes, we evaluated the reaction between phenylacetylene 1a and PhSiH₃ to identify selective copper catalysts and reliable conditions that promote the formation of 1,1,1-trisilylalkane 4a (Table 1). The major possible by-products of this reaction are (E)-vinylsilane 2a and gem-disilylalkane 3a from hydrosilylation and double hydrosilylation of 1a, respectively.¹⁰ The copper catalysts for this study were generated in situ by combining Cu(OAc)₂ and phosphine ligands and activated by their reaction with PhSiH₃. In general, the experiments were performed with alkyne 1a as a limiting reagent in the presence of 4 equivalents of PhSiH₃ and 10 mol% copper catalyst at 40 °C. The results of the selected examples of these experiments are summarized in Table 1.

Table 1 Evaluation of reaction conditions for the copper-catalyzed 1,1,1-trisilylation of terminal alkyne 1a^a

Entry	Variation from the standard conditions	Conversion of 1a (%)	Yield of 4a (%)	2a : 3a : 4a
a Reaction conditions: phenylacetylene 1a (0.300 mmol), PhSiH3 (1.20 mmol), Cu(OAc)2 (30.0 μmol), ligand (60.0 μmol for monophosphines and 30.0 μmol for bisphosphines), neat or solvent (0.3 mL) at 40 °C for 12 h; the conversion of 1a, the yield of 4a, and the ratios of 2a : 3a : 4a were determined by GC analysis with tridecane as the internal standard. b CuTC = copper(I) thiophene-2-carboxylate. c NaOtBu (20 mol%) was used.
1	None	>99	76	— : 12 : 88
2	P^nBu3 (30 mol%)	>99	53	— : 16 : 84
3	P^nBu3 (10 mol%)	88	11	31 : 38 : 31
4	PCy3 as the ligand	86	8	65 : 23 : 12
5	P^tBu3 as the ligand	90	—	86 : 14 : —
6	PPh3 as the ligand	85	—	66 : 34 : —
7	Ruphos as the ligand	89	—	84 : 16 : —
8	Johnphos as the ligand	90	—	80 : 20 : —
9	xantphos as the ligand	90	<5	48 : 45 : 7
10	binap as the ligand	88	—	90 : 10 : —
11	dppf as the ligand	55	—	82 : 18 : —
12	toluene as solvent	70	5	21 : 66 : 13
13	CH3CN as solvent	>99	44	8 : 36 : 56
14	THF as solvent	92	36	10 : 30 : 60
15	DMA as solvent	>99	68	— : 12 : 88
16	CuOAc as the precursor	>99	72	— : 24 : 76
17	CuTC^b as the precursor	>99	74	— : 15 : 85
18	(iPr)CuCl as the catalyst^c	30	—	20 : 80 : —

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After evaluating various phosphine ligands and solvents, we found that the neat reaction between 1a and 4 equivalents of PhSiH₃ proceeded smoothly in the presence of 10 mol% Cu(OAc)₂ and 20 mol% PⁿBu₃ and afforded 1,1,1-trisilylalkane 4a in 76% GC yield with 88% selectivity (entry 1 in Table 1). The reaction with 30 mol% PⁿBu₃ showed similar selectivity (entry 2 in Table 1). However, the reaction with 10 mol% PⁿBu₃ proceeded with much lower selectivity (entry 3 in Table 1). The steric properties of trialkylphosphine ligands had profound influence on selectivity. For example, the reaction conducted with 20 mol% PCy₃ showed only 12% selectivity toward 1,1,1-trisilylalkane 4a and the reaction with 20 mol% P^tBu₃ did not generate any detectable amounts of 4a (entries 4 and 5 in Table 1). When copper catalysts were generated from Cu(OAc)₂ and triphenylphosphine PPh₃ or bulky dialkylbiaryl phosphines, such as Johnphos and Ruphos, the reactions proceeded with high conversions of alkyne 1a, but provided (E)-vinylsilane 2a as the major product (entries 6–8 in Table 1). Similar results were obtained for the reactions conducted with copper catalysts containing bisphosphine ligands, such as xantphos, binap, and dppf (entries 9–11 in Table 1). In addition, we also tested various solvents for this reaction and found that the solvent effect on this trisilylation was noticeable (entries 12–15 in Table 1). The reactions conducted in toluene, acetonitrile, and THF proceeded with decreased chemoselectivity, and the reaction in N,N-dimethylacetamide (DMA) occurred with a similar selectivity compared to the neat reaction. Furthermore, we also found that copper catalysts generated in situ from copper(I) salts, such as CuOAc or CuTC, and PⁿBu₃, were similarly active and selective for the copper-catalyzed alkyne trisilylation (entries 16 and 17 in Table 1). However, when the copper(I) complex (iPr)CuCl (10 mol%) together with NaO^tBu (20 mol%) was used as the catalyst for the trisilylation reaction, the reaction proceeded with a low conversion and did not form a detectable amount of 1,1,1-trisilylalkane product 4a (entry 18 in Table 1).

Substrate scope of terminal alkynes

With an active copper catalyst in hand and reliable conditions identified for this Cu-catalyzed 1,1,1-trisilylation (entry 1 in Table 1), we explored the scope of terminal alkynes that undergo this trisilylation reaction, and the results are gathered in Table 2. In general, a wide range of aryl- (1a–1u), alkenyl- (1v), and alkyl-substituted alkynes (1u–1ai) reacted smoothly with PhSiH₃ in the presence of 10 mol% Cu(OAc)₂ and 20 mol% PⁿBu₃ to afford the corresponding 1,1,1-trisilylalkanes (4a–4ai) in moderate to high isolated yields (up to 78%). Furthermore, several alkynes (1aj–1an) derived from commonly used drugs and bioactive molecules also underwent this Cu-catalyzed trisilylation reaction to form 1,1,1-trisilylalkane products (4aj–4an) in good yields (56–76%). The structure of 1,1,1-trisilylalkane 4r was confirmed by single-crystal X-ray diffraction analysis.

Table 2 Scope of terminal alkynes for Cu(OAc)₂/PⁿBu₃-catalyzed trisilylation^a

a Reaction conditions: terminal alkyne 1 (0.300 mmol), PhSiH₃ (1.20 mmol), Cu(OAc)₂ (30.0 μmol), PⁿBu₃ (60.0 μmol), 40 °C, 12 h, and yields of the isolated products. b Alkyne 1an (0.100 mmol) was used.

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This Cu-catalyzed 1,1,1-trisilylation reaction tolerates various reactive groups. For example, alkynes containing sulfide (4f and 4ad), carboxylic ester (4h and 4ak–4an), fluoro (4i), chloro (4j, 4ae, and 4al), bromo (4k), silyl (4l), pinacol boronic ester (4m), cyano (4n), siloxy (4ag), carboxylic amide (4ah), acetal (4aj), and sulfonamide (4ak) moieties are compatible with the identified reaction conditions. In addition, alkynes containing heterocyclic aromatic groups also reacted with PhSiH₃ to provide the desired 1,1,1-trisilylalkanes containing carbazole (4s), thiophene (4t), pyridine (4u), and indole (4af) in good yields.

We also conducted the copper-catalyzed 1,1,1-trisilylation of terminal alkyne 1a and 1w with ⁿC₆H₁₃SiH₃, an alkyl-substituted hydrosilane. These two reactions proceeded to form the desired 1,1,1-trisilylalkanes 4a′ and 4aw′, respectively, albeit in low isolated yields (eqn (1)).

Synthetic utilities

After establishing the scope of this 1,1,1-trisilylation reaction, we subsequently showed the synthetic utility of this protocol (Scheme 2). A gram-scale reaction of phenylacetylene 1a with PhSiH₃ was performed, and this reaction proceeded smoothly under standard conditions to afford 1,1,1-trisilylalkane 4a (2.46 g) in 59% isolated yield (Scheme 2A). Trisilylalkane products from these trisilylation reactions contain three primary silyl groups and they can be readily converted to other organosilicon compounds. For example, CH₂ carbene formed from CH₂I₂ and Et₂Zn could readily be inserted into all six Si–H bonds of 4a to produce trisilylalkane 5, which contains three tertiary silyl groups, in 78% isolated yield (Scheme 2B).⁵⁴ Sequential alkoxylation/protodesilylation of 4a with methanol-D₄ in the presence of KHMDS as a catalyst generated gem-disilylalkane 6-D₁₃ in 80% isolated yield (Scheme 2C).⁵⁵ Compound 4a could undergo arylation/desilylation and diarylation/desilylation reactions with a phenyl Grignard reagent to form gem-disilylalkanes 7 and 8 in good yields, respectively (Scheme 2D and 2E).⁵⁶ The corresponding sequential alkylation/desilylation reaction with n-propylmagnesium chloride afforded gem-disilylalkane 9 in 65% isolated yield (Scheme 2F).

Scheme 2 Gram-scale synthesis of and derivatization of 4a.

Mechanistic considerations

To get a preliminary understanding of this copper-catalyzed trisilylation process, we monitored the reaction of 4-ethynylanisole 1e with PhSiH₃ with an attempt to identify potential intermediates. The GC-MS analysis of the reaction mixture showed that a significant amount (up to 25% GC yield) of gem-disilylalkene 10e was formed in the early stage of the reaction and then 10e was fully consumed in the late stage of the reaction (Scheme 3A). To verify the intermediacy of gem-disilylalkenes in this trisilylation reaction, we prepared gem-disilylalkene 10e and subjected it to this copper-catalyzed trisilylation reaction. As expected, 10e was converted to 1,1,1-trisilylalkane 4e in 94% GC yield (Scheme 3B). In addition, we found that alkynylsilane 11e reacted with PhSiH₃ to afford 1,1,1-trisilylalkane 4e in 85% yield under standard conditions (Scheme 3C), suggesting that alkynylsilane 11e is a potential intermediate for the trisilylation reaction of alkyne 1e. Indeed, the reaction of phenylacetylene 1a with PhMe₂SiH, a bulky hydrosilane, in the presence of Cu(OAc)₂/PⁿBu₃ stopped at the dehydrogenative silylation stage and afforded alkynylsilane 12 in 60% GC yield (Scheme 3D). Furthermore, we also carried out the stoichiometric reaction between copper(I) phenylacetylide 13 and 4 equivalents of PhSiH₃ in the presence of 2 equivalents of PⁿBu₃, and this reaction provided trisilylalkane 4a in 80% GC yield (Scheme 3E). However, the corresponding reaction in the absence of PⁿBu₃ did not produce any detectable amount of 4a.

Scheme 3 Control experiments and monitoring of 1,1,1-trisilylation of alkyne 1e.

Deuterium-labelling experiments were also carried out on this trisilylation reaction. For example, 4-ethynylanisole 1e-D₁ reacted with PhSiH₃ under standard conditions to afford trisilylalkane 4e in 61% isolated yield and no deuterium incorporation was detected by ²H NMR spectroscopic analysis (Scheme 4A). The corresponding reaction between alkyne 1e and PhSiD₃ produced 4e-D₈ with deuterium atoms located at the benzylic carbon and silicon atoms (Scheme 4B).

Scheme 4 Deuterium-labelling experiments and proposed catalytic cycles for this copper-catalyzed 1,1,1-trisilylation of terminal alkynes.

Based on the results of the above mechanistic experiments and the precedent for copper-catalyzed hydrosilylation/silylation reactions of terminal alkynes,^44,45 we proposed a plausible catalytic pathway for this copper-catalyzed 1,1,1-trisilylation reaction, as depicted in Scheme 4C. The activation of Cu(OAc)₂ with PhSiH₃ in the presence of PⁿBu₃ (L) forms a copper hydride species L_nCuH, which then reacts with an alkyne to form an alkynylcopper intermediate (I) with the concomitant release of hydrogen gas, which was detected by GC analysis. σ-Bond metathesis between copper acetylide I and PhSiH₃ produces alkynylsilane 11 and regenerates L_nCuH. Hydrocupration of alkynylsilane 11 with L_nCuH forms an alkenylcopper species (II), which then reacts with PhSiH₃ to give gem-disilylalkene intermediate 10. Subsequently, hydrocupration of gem-disilylalkene 10 with L_nCuH generates an alkylcopper intermediate (III), which then reacts with PhSiH₃ to afford 1,1,1-trisilylalkanes 4. Based on the reaction profile of the trisilylation of terminal alkyne 1e (Scheme 3A), accumulation of gem-disilylalkene 10e was observed, which suggests that the hydrosilylation of gem-disilylalkene 10 to generate 1,1,1-trisilylalkane 4 is the slowest reaction compared to dehydrogenative silylation of alkynes 1 and hydrosilylation of alkynylsilane 11 as shown in Scheme 4C.

Conclusions

In summary, we have developed an effective and practical protocol to access 1,1,1-trisilylalkanes by copper-catalyzed 1,1,1-trisilylation of terminal alkynes with PhSiH₃. A series of alkyl- and aryl-substituted alkynes undergo this trisilylation reaction in the presence of Cu(OAc)₂ and PⁿBu₃. Mechanistic studies reveal that this trisilylation reaction proceeds through a reaction sequence combining copper-catalyzed dehydrogenative hydrosilylation and double hydrosilylation of alkynylsilane intermediates. These 1,1,1-trisilylalkane products can be readily converted to other multi-silylated compounds by manipulating their Si–H bonds. Further development of copper-catalyzed multi-functionalization reactions of unsaturated hydrocarbons and the synthetic application of 1,1,1-trisilylalkanes will be the subject of future studies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Education of Singapore (A-8000984-00-00). J. L. thanks the China Scholarship Council (CSC) for providing him a PhD scholarship.

References

1. M. A. Brook, Silicon in Organic, Organometallic, and Polymer Chemistry, J. Wiley, New York, 2000.
2. I. Fleming, A. Barbero and D. Walter, Chem. Rev., 1997, 97, 2063–2192.
3. P. Gaspar and R. West, in The Chemistry of Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig, John Wiley and Sons, Chester, UK, 1998, vol. 2.
4. R. J. P. Corriu, M. Granier and G. F. Lanneau, J. Organomet. Chem., 1998, 562, 79–88.
5. K. D. Safa, S. Tofangdarzadeh and H. H. Ayenadeh, Heteroat. Chem., 2008, 19, 365–376.
6. H. Sakurai, K.-i. Nishiwaki and M. Kira, Tetrahedron Lett., 1973, 14, 4193–4196.
7. B.-T. Gröbel and D. Seebach, Angew. Chem., Int. Ed. Engl., 1974, 13, 83–84.
8. S. Inoue and Y. Sato, Organometallics, 1986, 5, 1197–1201.
9. K. Itami, T. Nokami and J.-i. Yoshida, Org. Lett., 2000, 2, 1299–1302.
10. C. L. Smith, L. M. James and K. L. Sibley, Organometallics, 1992, 11, 2938–2940.
11. K. D. Safa and M. Babazadeh, J. Organomet. Chem., 2005, 690, 79–83.
12. Hydrosilylation: A Comprehensive Review on Recent Advances, ed. B. Marciniec, Springer, Berlin, 2009.
13. Y. Nakajima and S. Shimada, RSC Adv., 2015, 5, 20603–20616.
14. X. Du and Z. Huang, ACS Catal., 2017, 7, 1227–1243.
15. J. Sun and L. Deng, ACS Catal., 2016, 6, 290–300.
16. J.-W. Park, Chem. Commun., 2022, 58, 491–504.
17. W. J. Teo, C. Wang, Y. W. Tan and S. Ge, Angew. Chem., Int. Ed., 2017, 56, 4328–4332.
18. C. Wang, W. J. Teo and S. Ge, Nat. Commun., 2017, 8, 2258.
19. M.-Y. Hu, J. Lian, W. Sun, T.-Z. Qiao and S.-F. Zhu, J. Am. Chem. Soc., 2019, 141, 4579–4583.
20. X. Du, Y. Zhang, D. Peng and Z. Huang, Angew. Chem., Int. Ed., 2016, 55, 6671–6675.
21. J. Guo, H. Wang, S. Xing, X. Hong and Z. Lu, Chem, 2019, 5, 881–895.
22. H. L. Sang, Y. Hu and S. Ge, Org. Lett., 2019, 21, 5234–5237.
23. S. Nishino, K. Hirano and M. Miura, Chem. – Eur. J., 2020, 26, 8725–8728.
24. J. Guo, X. Shen and Z. Lu, Angew. Chem., Int. Ed., 2017, 56, 615–618.
25. W. Chen, H. Song, J. Li and C. Cui, Angew. Chem., Int. Ed., 2020, 59, 2365–2369.
26. T. Li, R. Liu, X. Liu and Y. Chen, Org. Lett., 2023, 25, 761–765.
27. X. Liu, L. Xiang, E. Louyriac, L. Maron, X. Leng and Y. Chen, J. Am. Chem. Soc., 2019, 141, 138–142.
28. C. Guo, M. Li, J. Chen and Y. Luo, Chem. Commun., 2020, 56, 117–120.
29. M. Itoh, K. Inoue, J.-i. Ishikawa and K. Iwata, J. Organomet. Chem., 2001, 629, 1–6.
30. T. Li, K. N. McCabe, L. Maron, X. Leng and Y. Chen, ACS Catal., 2021, 11, 6348–6356.
31. K. P. Kepp, Inorg. Chem., 2016, 55, 9461–9470.
32. Organometallic Chemistry and Catalysis, ed. D. Astruc, Springer Berlin Heidelberg, 2007, pp. 289–311.
33. A. W. Cook, Z. R. Jones, G. Wu, S. L. Scott and T. W. Hayton, J. Am. Chem. Soc., 2018, 140, 394–400.
34. G.-y. Zhao, S. Hu, S.-c. Luo, J. Lei and Y. Wei, J. Alloys Compd., 2023, 949, 169885.
35. H. Lang, A. Jakob and B. Milde, Organometallics, 2012, 31, 7661–7693.
36. C. Ghiazza, T. Billard and A. Tlili, Chem. – Eur. J., 2017, 23, 10013–10016.
37. K. Jouvin, J. Heimburger and G. Evano, Chem. Sci., 2012, 3, 756–760.
38. K. Sonogashira, J. Organomet. Chem., 2002, 653, 46–49.
39. Y.-Q. Ren, C.-J. Yang, Z.-L. Li, Q.-S. Gu, L. Liu and X.-Y. Liu, J. Fluorine Chem., 2023, 267, 110107.
40. F.-L. Wang, C.-J. Yang, J.-R. Liu, N.-Y. Yang, X.-Y. Dong, R.-Q. Jiang, X.-Y. Chang, Z.-L. Li, G.-X. Xu, D.-L. Yuan, Y.-S. Zhang, Q.-S. Gu, X. Hong and X.-Y. Liu, Nat. Chem., 2022, 14, 949–957.
41. L. Liu, K.-X. Guo, Y. Tian, C.-J. Yang, Q.-S. Gu, Z.-L. Li, L. Ye and X.-Y. Liu, Angew. Chem., Int. Ed., 2021, 60, 26710–26717.
42. X. Bai, C. Wu, S. Ge and Y. Lu, Angew. Chem., Int. Ed., 2020, 59, 2764–2768.
43. X. Liu, W. Ming, Y. Zhang, A. Friedrich and T. B. Marder, Angew. Chem., Int. Ed., 2019, 58, 18923–18927.
44. Z.-L. Wang, F.-L. Zhang, J.-L. Xu, C.-C. Shan, M. Zhao and Y.-H. Xu, Org. Lett., 2020, 22, 7735–7742.
45. Q.-C. Gan, Z.-Q. Song, C.-H. Tung and L.-Z. Wu, Org. Lett., 2022, 24, 5192–5196.
46. W. J. Teo and S. Ge, Angew. Chem., Int. Ed., 2018, 57, 12935–12939.
47. W. J. Teo, X. Yang, Y. Y. Poon and S. Ge, Nat. Commun., 2020, 11, 5193.
48. M. Hu and S. Ge, Nat. Commun., 2020, 11, 765.
49. Y. e. You and S. Ge, Angew. Chem., Int. Ed., 2021, 60, 20684–20688.
50. X. Yang and S. Ge, Organometallics, 2022, 41, 1823–1828.
51. Y. Zhao and S. Ge, Angew. Chem., Int. Ed., 2022, 61, e202116133.
52. J. Li and S. Ge, Angew. Chem., Int. Ed., 2022, 61, e202213057.
53. B. B. Tan, M. Hu and S. Ge, Angew. Chem., Int. Ed., 2023, 62, e202307176.
54. H. Wen, X. Wan and Z. Huang, Angew. Chem., Int. Ed., 2018, 57, 6319–6323.
55. J. S. Peake, W. H. Nebergall and Y. T. Chen, J. Am. Chem. Soc., 1952, 74, 1526–1528.
56. N. Hirone, H. Sanjiki, R. Tanaka, T. Hata and H. Urabe, Angew. Chem., Int. Ed., 2010, 49, 7762–7764.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2235641. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4gc00220b

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