Visible-Light-Initiated Manganese-Catalyzed E-Selective

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Visible-Light-Initiated Manganese-Catalyzed E‑Selective Hydrosilylation and Hydrogermylation of Alkynes Hao Liang, Yun-Xing Ji, Rui-Han Wang, Zhi-Hao Zhang,* and Bo Zhang* State Key Laboratory of Natural Medicines, China Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009, China

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S Supporting Information *

ABSTRACT: Manganese-photocatalyzed activation of the Si− H bond in silanes for the hydrosilylation of alkynes has been developed. The mild protocol operates efficiently with high regioselectivity (anti-Markovnikov) and stereoselectivity (Z/E ratio ranges from 92:8 to >99:1), providing a wide range of Zvinylsilanes in high yields. Moreover, visible-light-induced manganese-catalyzed activation of the Ge−H bond for Eselective alkyne hydrogermylation is reported for the first time.

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oxidative potential, this procedure failed to convert simple silanes possessing phenyl or alkyl substituents to the hydrosilylation products. Furthermore, the stereochemical control of this transformation is also an issue, since conducting reactions with aromatic alkynes as substrates result in poor stereoselectivities. These shortcomings limited the application of such a method in synthesis. Therefore, it would be very fascinating and yet challenging to develop new approaches for highly selective hydrosilylation of alkynes with broad generality. Herein, we present a visible-light-driven manganesecatalyzed E-selective anti-Markovnikov hydrosilylation of alkynes. Salient features of our method include (1) an earthabundant 3d-non-noble transition metal as the photocatalyst for alkyne hydrosilylation, (2) excellent levels of control on regio- and stereoselectivity, (3) wide substrate scope, good functional-group tolerance, and simple and mild reaction conditions. Dinuclear metal complexes are proficient catalysts for a range of photomediated radical reactions.10 Owing to the weak bond dissociation energy (BDE) of metal−metal bonds, dinuclear metal complexes undergo light-induced metal− metal homolysis via an energy transfer process to generate metal-centered radicals. For instance, a commercially available and inexpensive dinuclear manganese complex, Mn2(CO)10, can produce a manganese-centered radical [·Mn(CO)5] through photochemical homolysis of its Mn−Mn bond (BDE of Mn−Mn bond = 15 kcal mol−1).10,11 Previous studies have documented that ·Mn(CO)5 has high affinity for halogen atom abstraction from an activated or unactivated alkyl halide to form a carbon-centered radical that further engages in a series of radical transformations (Scheme 1a).10 Despite these precedents, its reactivity toward Si−H activation has hardly been investigated.12 We envisioned that by using · Mn(CO)5 generated from Mn2(CO)10 upon visible-light

wing to their high stability, low toxicity, and ease of handling, vinylsilanes are widely utilized as versatile synthetic building blocks in organic synthesis and material chemistry.1 Accordingly, methods for the preparation of vinylsilanes have gained increased attention from synthetic chemists. Among various methods known, hydrosilylation of alkynes, involving the addition of a Si−H bond across a C−C triple bond, represents the most favorable and straightforward route to access vinylsilanes with 100% atom efficiency.2 The major challenge in this research field is the control of the regioand stereochemistry, as the strategy potentially leads to three products, α-, (Z)-β-, and (E)-β-vinylsilanes. To address this selectivity issue, a range of precious and scarce metal-catalyst systems such as Pt, Ru, Rh, Ir, and Pd have been successfully established to prepare vinylsilanes with high regio- and stereoselectivity.3 However, from an economical and sustainable point of view, the development of new catalytic systems based on inexpensive and earth-abundant metals that can promote efficient hydrosilylation of alkynes is more desirable. Along this line, several well-defined base-metal complexes such as Fe, Ni, and Co have been extensively exploited as catalysts for highly selective hydrosilylation of alkynes.4 In sharp contrast, despite being the third most abundant transition metal, hydrosilylation of alkynes facilitated by manganese catalysis is still in its infancy.5 In this context, Wang and coworkers have recently reported the first manganese-catalyzed hydrosilylation of aromatic alkynes.6 Despite this significant contribution, continuous efforts are still required, especially for the effective hydrosilylation of both aromatic and aliphatic alkynes under mild conditions. Visible-light photocatalysis has been demonstrated to be a powerful synthetic tool for mild and environmentally friendly organic transformations.7 Although numerous advances in this area have been made, photomediated protocols for alkyne hydrosilylation have hardly been reported.8 More recently, Wang and Yao described a photoredox-mediated hydrosilylation of alkynes with (Me3Si)3SiH catalyzed by eosin Y and thiol.9 Although mild and elegant, because of the higher © XXXX American Chemical Society

Received: February 24, 2019

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DOI: 10.1021/acs.orglett.9b00701 Org. Lett. XXXX, XXX, XXX−XXX

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unique reactivity of manganese in this reaction. An attempt to reduce the catalyst loading to 5 mol % led to a decrease in yield (Table 1, entry 11). Control experiments were conducted and confirmed that both Mn2(CO)10 and visible light are indispensable for the hydrosilylation to proceed (Table 1, entries 12 and 13). When this reaction was performed at 120 °C, 3 was obtained in 40% yield (Table 1, entry 14). With optimized experimental conditions in hand, we next sought to explore the scope of this reaction (Scheme 2). As

Scheme 1. C−Br/I and Si−H Activation by Light-Initiated Mn Catalysis

Scheme 2. Scope of Alkynesa

irradiation as a hydrogen atom transfer (HAT) catalyst, the Si−H bond might be efficiently activated, thus enabling alkyne hydrosilylation in a simple and mild manner. To evaluate the hypothesis, we first investigated the hydrosilylation of phenylacetylene 1 with dimethyl(phenyl)silane 2 in the presence of Mn2(CO)10 as the catalyst in CH2Cl2 at room temperature under the irradiation of 6 W blue LEDs. Gratifyingly, the reaction proceeded with excellent regio- and stereoselectivity to deliver hydrosilylation product 3 in 84% yield as a single Z-isomer (Table 1, entry 1). The influence of Table 1. Optimization of Reaction Conditionsa

entry

catalyst (mol %)

solvent

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13c 14c,d

Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn2(CO)10 (10) Mn(CO)5Br (10) Fe2Cp2(CO)4 (10) Mn2(CO)10 (5) none Mn2(CO)10 (10) Mn2(CO)10 (10)

CH2Cl2 CH3CN DMF MeOH EtOAc n-hexane cyclohexane decalin decalin decalin decalin decalin decalin decalin

84 0 0 trace trace 30 50 95 40 trace 65 0 0 40

a

Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), and catalyst in solvent (2.0 mL) were irradiated with 6 W blue LEDs at room temperature under N2 for 12 h. bIsolated yields. cThe reaction was conducted in darkness. dThe reaction was conducted at 120 °C.

Isolated yields. bThe Z/E ratios were determined by 1H NMR spectroscopy. cThe Z/E ratios were determined by GC-MS.

various solvents on this transformation was then examined (Table 1, entries 2−8). It turns out that decalin was the best candidate, providing 3 in 95% yield with consistent Eselectivity. Notably, a mononuclear manganese complex, Mn(CO)5Br, also triggers this transformation but with a lower yield (Table 1, entry 9). The stronger Mn−Br BDE (75 kcal mol−1)11 is most likely the reason behind the lack of reactivity with Mn(CO)5Br. We also surveyed a dinuclear iron complex, Fe2Cp2(CO)4, as the catalyst for the hydrosilylation (Table 1, entry 10). However, this reaction did not work well under the same conditions, albeit with a relatively low BDE of the Fe−Fe bond (28 kcal mol−1),11 thereby implying the

shown in 4−18, a broad range of phenylacetylene derivatives bearing different sterically and electronically varied aryl substituents all proceeded smoothly, affording the corresponding Z-vinylsilanes in high yields with excellent stereoselectivities. A diverse array of functional groups such as alkyl (4, 5, 13, 15), alkoxyl (6, 7, 18), fluoro (8), chloro (9, 14, 16), bromo (10, 17), trifluoromethoxy (11), and phenyl (12) were well tolerated. Both 2-ethynyl-naphthalene and sulfurcontaining heteroaromatic alkyne underwent successful hydrosilylation in good yields and excellent stereoselectivities (19, 20). Apart from aromatic alkynes, aliphatic alkynes could also participate in this transformation and reacted with high Z/E

a

B


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Scheme 4. First Visible-Light-Initiated ManganeseCatalyzed E-Selective Hydrogermylation of Alkynesa,b

a

Isolated yields. bThe Z/E ratios were determined by 1H NMR spectroscopy.

To gain insight into the mechanism of this reaction, we carried out some experiments (Scheme 5). Initially, a Scheme 5. Mechanistic Studies

Scheme 3. Scope of Silanesa,b

a Isolated yields. bThe Z/E ratios were determined by 1H NMR spectroscopy.

substituents were readily converted to Z-vinylsilanes in good yields and high stereoselectivities (Z/E = 95:5 to >99:1) (38, 40, 41). Sterically encumbered silanes proved to be competent substrates, leading to products 39 and 42 with >99:1 Z/E selectivities. 1,4-Bis(dimethylsilyl)benzene was employed to prepare monohydrosilylation product 43 in 69% yield. Remarkably, we showed that the hydrosilylation could also be applied to siloxanes. For instance, Z-vinylsiloxane 44 was successfully prepared in 75% yield with >99:1 Z/E selectivity from 1,1,1,3,3-pentamethyldisiloxane. The robustness of this transformation turned our attention to examining heavier group 14 hydrides (Scheme 4). Unfortunately, we found that reactions conducted with nBu3SnH failed to afford the expected hydrostannation products. Conversely, the reaction of phenylacetylene 1 with nBu3GeH took place with 97:3 Z/E selectivity to give Zvinylgermanium 45 in 72% yield. Other aromatic alkynes were then tested, and the corresponding products 46−49 were obtained in high yields. Finally, the alkyne derived from cholestanol was found to be eligible for the reaction and afforded product 50 in 82% yield with high selectivity. To the best of our knowledge, these results represent the first activation of the Ge−H bond performed through visiblelight-initiated manganese catalysis.

deuterium-labeling experiment was performed to verify the origin of the C2-proton (Scheme 5a). When hex-3-yne was treated with PhMe2SiD under the standard conditions, a deuteriosilylation product [D]-32 was obtained in 61% yield with the deuterium atom located at the C2 position. Additionally, a competition reaction of hex-3-yne with a 1:1 mixture of 2:[D]-2 was investigated, and the reaction afforded a 3:1 mixture of products (KIE = 3.0, Scheme 5b). This result supports the occurrence of a HAT process. It was reported that isomerization of E-alkenes upon visible light irradiation could produce less-stable Z-isomers,13 which might be a factor for the formation of Z-vinylsilanes. To verify this possibility, Evinylsilane 51 was prepared and subjected to the optimized reaction conditions. No Z-vinylsilane 3 was detected, thereby implying that a photoisomerization process is not involved in the reaction (Scheme 5c). We speculated that a Mn−H species might be generated during the reaction and be part of the catalytic cycle. If so, the addition of a trityl cation, which is known as a hydride abstractor for organometallic compounds,14 should suppress the reaction. As expected, the addition of catalytic amounts of triphenylcarbenium hexachloroantimonate resulted in complete inhibition of this transformation (Scheme 5d). Several radical inhibition experiments were conducted. Performing the reaction in the C


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presence of TEMPO, galvinoxyl, or hydroquinone as a radical scavenger led to no reaction, demonstrating that a radical mechanism is operative (Scheme 5d). Furthermore, we found that the disilane 52 could be generated from dimethyl(phenyl)silane 2 in 30% yield under the standard conditions (Scheme 5e). To probe the role of disilane 52, we ran a reaction using 52 and 1 as substrates under the standard conditions. No product 3 was observed, which reveals that the disilane 52 is not the intermediate on the way to 3. Based on the above observations, a plausible mechanism for the visible-light-promoted hydrosilylation of alkynes is proposed in Scheme 6. First, initiation occurs by visible-

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.Z.). *E-mail: [email protected] (Z.-H.Z.). ORCID

Bo Zhang: 0000-0001-7042-2397 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21702230), the Natural Science Foundation of Jiangsu Province (BK20160743), the Program for Jiangsu Province Innovative Research Team, “Double FirstClass” Project of China Pharmaceutical University (CPU2018GY35, CPU2018GF05), and the 111 Project (B16046).

Scheme 6. Proposed Reaction Mechanism

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REFERENCES

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light-induced Mn−Mn bond homolysis, thus transforming Mn2(CO)10 into ·Mn(CO)5.10 Subsequently, the resulting · Mn(CO)5 abstracts a hydrogen atom from the hydridic Si−H bond to generate a silyl radical along with the formation of HMn(CO)5. Finally, the addition of the silyl radical to the alkyne delivers radical adducts A and B, which undergo another HAT process to yield the desired hydrosilylation product.6,15 This process allows the regeneration of ·Mn(CO)5, thereby sustaining the radical chain. The stereochemistry for the formation of Z-vinylsilanes is likely set for steric reasons in the step that includes the hydrogenolysis of A and B as indicated in Scheme 6.6,16 Moreover, both a light on/ off experiment and quantum yield measurements (Φ = 2.6) support a short radical chain propagation pathway (see the Supporting Information for details).17 In summary, we have documented that earth-abundant and inexpensive manganese complexes act as catalysts for visiblelight-mediated hydrosilylation of alkynes. The mild protocol proceeds with excellent control of regio- and stereoselectivity and provides a range of valuable Z-vinylsilanes with yields up to 98%. We also showed the potential of this chemistry for latestage functionalization of complex compounds. Reactions are easy to conduct and exhibit good functional-group tolerance. Moreover, the first visible-light-induced manganese-catalyzed activation of Ge−H bonds for E-selective hydrogermylation of alkynes was reported. All of these features make this protocol highly practical. Further mechanistic studies and synthetic application of this methodology are ongoing in our laboratory.

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Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00701. Experimental details and characterization data for the products (PDF) D


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Visible-Light-Initiated Manganese-Catalyzed E-Selective

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