Quan
Lin
,
Liping
Lin
,
Jingjing
Wang
,
Daohong
Yu
*,
Zhengwang
Chen
* and
Zhong-Xia
Wang
*
Jiangxi Provincial Key Laboratory of Synthetic Pharmaceutical Chemistry, College of Chemistry and Materials Science, Gannan Normal University, Ganzhou 341000, P.R. China. E-mail: yudh@gnnu.edu.cn; chenzwang2021@163.com; zhongxiawang@ncu.edu.cn
First published on 3rd July 2025
We present a manganese-mediated cross-electrophile coupling (XEC) that directly constructs C(sp)–Si bonds between alkynyl halides and chlorosilanes. By leveraging manganese as a reductant, this method enables a mild and operationally simple approach. Mechanistic studies reveal the in situ generation of key intermediates alkynylmanganese in amide media, which undergo SN2 substitution with chlorosilanes to afford diverse alkynylsilanes in high yields (up to 99%) with suppressed diyne byproducts. To our knowledge, this is the first discovery of alkynylmanganese species, derived from readily available alkynyl halides, for C(sp)–Si cross-electrophilic coupling. This work expands the scope of XEC in organosilicon chemistry and provides a robust alternative for C(sp)–Si bond formation.
Three fundamental challenges could persist in C(sp)–Si XEC reactions involving alkynyl halides and chlorosilanes. First, the high reactivity of alkynyl halides often results in undesired homocoupling and addition side reactions.5 Second, the low reactivity and elevated bond dissociation energy (BDE) of R3Si–Cl bonds (ca. 117 kcal mol−1),6 compared to R3C–Cl bonds (ca. 84 kcal mol−1),7 adversely affect the XEC reactivity of alkyne electrophiles. Third, the diminished nucleophilicity of commonly reported C(sp)–Ni/Co intermediates, relative to aryl or alkyl-Ni/Co species, further reduces their reactivity toward chlorosilanes.3a,8 Therefore, developing an efficient cross-electrophilic coupling protocol for the direct synthesis of alkynylsilanes from alkynyl halides is both timely and essential.
Herein, we report a manganese-mediated XEC reaction that enables direct C(sp)–Si bond formation between alkynyl halides and chlorosilanes. This method innovatively employs metal manganese as a reductant, circumventing requirements for sophisticated catalytic systems involving transition-metal catalysts and organic ligands. Notably, mechanistic investigations reveal the in situ generation of intermediate alkynylmanganese species from alkynyl halides as precursors in the presence of manganese metal and amide solvent, followed by SN2 nucleophilic substitution reactions with chlorosilanes.9 This protocol enables the synthesis of a variety of alkynylsilanes in high yields (up to 99%) under mild conditions, effectively suppressing the formation of diyne byproducts (Fig. 1b, bottom). As such, this discovery establishes an alternative synthetic toolkit for organosilicon chemistry.
Entry | Variation | Yield (%) | ||
---|---|---|---|---|
3a | 3a′ | 3a′′ | ||
a Standard conditions: 1a (0.15 mmol), 2a (0.45 mmol), Mn (0.45 mmol), DMA (1 mL), 25 °C, Argon, yields were determined by GC using dodecane as the internal standard. b Isolated yield. c Deuterated product. TDAE = tetrakis(dimethylamino)ethylene. Terpy = 2,2′:6′,2′′-terpyridine. w/ = with. w/o = without. | ||||
1 | None | 86 (80)b | 6 | 0 |
2 | w/ NiBr2(dtbbpy) (10 mol%) | Trace | 23 | 57 |
3 | w/ CoBr2 & Terpy (10 mol%) | 23 | 3 | 29 |
4 | w/o Mn | 0 | 0 | 0 |
5 | TDAE instead of Mn | 0 | 0 | 0 |
6 | Mg instead of Mn | 64 | 12 | 0 |
7 | Zn instead of Mn | 25 | 25 | 0 |
8 | Fe instead of Mn | 0 | 0 | 0 |
9 | DMF instead of DMA | 52 | 15 | 0 |
10 | THF instead of DMA | 0 | Trace | 0 |
11 | Toluene instead of DMA | 0 | 0 | 0 |
12 | DMSO instead of DMA | 0 | Trace | 0 |
13 | CH3CN instead of DMA | 0 | Trace | 0 |
14 | Under air | 22 | 29 | 0 |
15 | 1.0 equiv. D2O was added | 25 | 23c | 0 |
16 | 4 mmol of 1a (gram scale) | 64b | 8 | 0 |
Under optimized reaction conditions, we undertook an initial exploration of the scope of alkynyl bromides and representative alkynyl iodides in this manganese-mediated XEC utilizing TMSCl, VinylMe2SiCl, or VinylPhMeSiCl. The unactivated alkynyl halides exhibited significant reactivity with chlorosilanes, producing target products in yields ranging from 31% to 87%. Both silicon- and alkyl-substituted alkyne halides demonstrated compatibility within this reaction system (Fig. 2, products 3b–3f). Notably, tertiary alkyl-substituted alkynyl bromides (3f) exhibited significantly higher reactivity than secondary alkyl counterparts (3e), whereas primary alkyl-substituted alkynyl bromides (3c, 3d) required elevated amounts of chlorosilane and manganese, along with prolonged reaction times, to achieve complete conversion. We postulate that primary alkyl-substituted alkynyl bromides exhibit relatively low steric hindrance at the propargylic position may drive substrate isomerization to the allenic form, thereby inhibiting the formation of the alkynylmanganese species and enabling substrate recovery.11 Subsequently, we examined the scope of aromatic alkynyl halides (3g–3y). Remarkably, alkynylferrocene (3g) and alkynylbiphenyls (3h–3i) were found to be compatible with the standard conditions, yielding moderate results. The products 3j–3l indicate that aromatic alkynyl halides containing electron-donating groups (EDGs), such as methoxy and alkyl substituents on the benzene ring, are still able to yield the desired products in moderate to good yields. Similarly, the presence of electron-withdrawing groups (EWGs) such as –CO2Me (3m), –CO2Ph (3n), and –CF3 (3o) on the benzene ring produced comparably favorable outcomes. The reaction also exhibits excellent chemo-selectivity; we performed the XEC with alkynyl bromides containing various active functional groups, synthesizing products such as bromobenzene (3p), benzonitrile (3q), and benzaldehyde (3r) in yields ranging from 42% to 91%, while maintaining the integrity of their functional groups. Reactions involving aromatic alkynyl bromides with heteroaromatic rings proceeded smoothly, yielding alkynylsilanes (3s–3v) in yields of 48% to 95%. Furthermore, by increasing the amount of chlorosilanes, the bisilylation product 3w was obtained with a yield of 73%. In certain cases, silica-based drug analogs exhibited superior pharmacokinetic properties.12 We also explored the incorporation of alkynylsilane fragments into pharmaceutical and natural product scaffolds, including DL-menthol (3x), glycosides (3y), steroids (3z–3aa), icaridine (3ab), podophyllotoxin (3ac), ezetimibe (3ad), and ospemifene (3ae), achieving promising results. Additionally, when utilizing both unactivated and activated alkynyl iodides (1ea, 1ja, 1ka), as well as steroidal alkyne iodides (1za, 1aaa), corresponding silylation products were obtained with yields ranging from 49% to 99%.
We subsequently examined the scope of chlorosilanes by coupling them with alkynyl bromide 1k under standard conditions (Fig. 3). Depending on the varying chain lengths, steric properties, and functional groups of the chlorosilanes, we achieved yields ranging from good to excellent. For instance, TMSCl (3af), TESCl (3ag), chlorodimethyl(butyl)silane (3ah), and chlorodimethyl(isobutyl)silane (3ai) furnished the desired products in yields between 69% and 95%. Additionally, we assessed the compatibility of corresponding alkynyl halides under these conditions. Notably, alkynyl chloride (1kb) did not perform as anticipated, whereas alkynyl iodide (1ka) yielded product 3ab with a 90% yield. Chlorosilanes bearing sterically hindered or conjugated functional groups, such as aryl and alkenyl substituents (3aj–3al), resulted in target products with yields ranging from 60% to 78%. The incorporation of chloromethyl and chloropropyl functionalities onto the chlorosilanes was well tolerated, thus offering enhanced opportunities for late-stage modifications of the resultant products (3am–3an). Chlorohydrosilanes represent a fundamental class of materials in organosilicon chemistry.8c,13 Previous studies have shown that the bond dissociation energy of Si–H (ca. 77 kcal mol−1) is significantly lower than that of the Si–Cl bond.14 Products 3ao–3aq further illustrate that chlorohydrosilanes exhibit remarkable chemical selectivity under standard conditions. Moreover, the present method is also applicable to chlorogermanes, as demonstrated by examples 3ar and 3as.
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Fig. 3 The scope of alkynyl chlorosilanes. Reaction conditions: 1k (0.15 mmol), 2 (0.45 mmol), Mn (0.45 mmol), DMA (1 mL), 25 °C, Argon, isolated yield. n.d. = not detected. |
The versatility of alkynylsilane functionality enables further structural variations (Fig. 4). The synthesized 3ao derivatives are readily accessible for cross-coupling reactions, exemplified by the formation of compound 5, which was obtained in 78% yield from the reaction of 3ao with methyl 4-vinylbenzoate and 1-(4-iodophenyl)ethan-1-one.15 In the nucleophilic addition of 4-methoxybenzenethiol to 3p, product 6 is preferentially formed, yielding 42% and demonstrating exceptional chemical selectivity.16 Additionally, alkynylgermane 3ar undergoes a Diels–Alder reaction with 2-pyrones to afford biphenyl compound 7 in a yield of 70%, accompanied by the cleavage of the trimethylgermyl group.17 Furthermore, a cobalt-catalyzed regioselective [3 + 2] annulation of 3v with ortho-formylphenylboronic acid produced cyclized product 8 in a remarkable yield of 95%.18 These compounds hold significant promise in biomedical applications and organic synthesis.5a,19
To elucidate the mechanism underlying this process, several control experiments were conducted. Firstly, in the presence of Michael acceptors 4a–4f (3 equivalents), the reactions proceeded effectively, yielding the desired products with results ranging from 56% to 89%. Notably, no radical trapping product 9 was observed (see Scheme S1†). Furthermore, radical capture and inhibition experiments involving the addition of TEMPO and BHT to the silylation reaction demonstrated no significant effect on the formation of the desired product. In contrast, the introduction of 1,4-dinitrobenzene, a known single-electron transfer (SET) inhibitor, completely inhibited the reaction (Fig. 5a), indicating that the SET process may be involved in the oxidative addition of alkynyl bromide with active manganese (Mn*). Secondly, when the reaction was quenched with HCl (1 M, aqueous), no hydrogenation products were detected in the absence of chlorosilane, and nearly all of reactant 1a was remained (Fig. 5b, top). Substituting AcCl (2 equiv.) for chlorosilane yielded the hydrogenated product in 87% yield with Mn as the reductant, while the yield decreased to 6% with Mg, leaving a surplus of 28% of reactant 1a (Fig. 5b, bottom). The decomposition of chlorosilane in the reaction milieu Mn* facilitates the formation of the alkynylmanganese reagent.5b,20 Thirdly, to identify the type of alkynylmanganese reagent involved in this reaction, the 1ki solution generated from 1k and manganese activated with acyl chloride was analyzed by LC-HRMS (Fig. 5c, eqn (1)). The results corresponded with the di(alkynyl)manganese species (HRMS data: exact mass [M + H]+: 318.0447, found: 318.0445). This 1ki solution was subsequently reacted with TMSCl, resulting in the formation of 3ab with a yield of 17% (Fig. 5c, eqn (1)). Alternatively, 1ki was synthesized through a previously established transmetallation of an acetylene lithium reagent with MnBr2, followed by reaction with TMSCl, yielding 3ab with a yield of 30% (Fig. 5c, eqn (2)). To isolate or trap the di(alkynyl)manganese species 1ki, 1,10-phenanthroline (Phen) was added. Upon the addition of Phen to the 1ki solution formed via both pathways, an intermediate (Int-I) precipitated from the solution, with both exhibiting consistent paramagnetic NMR spectra. Unfortunately, the X-ray structure of Int-I was not identified as a di(alkynyl)manganese species; rather, it corresponded to (Phen)2MnCl2 (Fig. 5c). We speculate that the formation of MnX2 occurs concurrently with the generation of the di(alkynyl)manganese species or stems from the activation of manganese with acyl chloride or TMSCl. Furthermore, results from the control experiments show that after the coupling of 1k with TMSCl under standard conditions, the filtrate remains suitable for the synthesis of Int-I (X = Cl), indicating that a significant amount of MnCl2 is produced during the reaction.21
Based on these experimental results and previous reports,3,4,9a,12 a reaction pathway is illustrated in Fig. 6b. Alkyne bromide 1k reacts with Mn* through an oxidative addition mechanism that involves a solvent-facilitated process, resulting in the formation of alkynylMn(II) bromide TS3. This is subsequently followed by transmetallation to yield 1ki, accompanied by the generation of MnX2.22 The nucleophilic substitution of 1ki with chlorosilanes produces the desired product, and the reaction pathway viaTS3 with chlorosilanes is ruled out.23 Density Functional Theory (DFT) computations from 1k to TS3 provide support for this mechanistic proposal (Fig. 6a).21 Our calculations indicate a significant decrease in free energy of 71.6 kcal mol−1 as the Mn(DMA)5 complex24 binds to the bromine atom of 1kvia a SET process, forming the highly stable intermediate IM1, which features a prominent six-membered ring with notable hydrogen bonding between the DMA and Br atoms. This finding underscores the indispensable role of amide solvents in facilitating this reaction (Table 1 and Fig. 6a). The reductive Mn(I) complex interacting with the electron-accepting compound 1k leads to the formation of a three-membered transition state TS1, accompanied by the dissociation of a DMA molecule, requiring a moderate energy barrier of 6.2 kcal mol−1. To achieve the transition state TS3, the synergistic process includes the dissociation of multiple DMA molecules and necessitates overcoming an additional energy barrier of 28.1 kcal mol−1, which represents the rate-determining step of the entire reaction. DFT computations suggest that if the synergistic process fails to facilitate the dissociation of DMA, a less stable transition state TS2 would form, requiring the overcoming of a higher energy barrier, thereby rendering this pathway unfavorable.
Footnote |
† Electronic supplementary information (ESI) available: Synthetic details, spectroscopic characterization and crystallographic data. CCDC 2423603. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc03449c |
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