Highly Regio- and Stereoselective Heterogeneous Hydrosilylation of

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

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Highly Regio- and Stereoselective Heterogeneous Hydrosilylation of Terminal Alkynes over Cobalt-Metalated Porous Organic Polymer Ren-Hao Li, Xiao-Ming An, Ying Yang, Ding-Chang Li, Zi-Lin Hu, and Zhuang-Ping Zhan* Department of Chemistry, Fujian Provincial Key Laboratory of Chemical Biology, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, P. R. China

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S Supporting Information *

ABSTRACT: A cobalt/POL-PPh3 catalyzed (E)-selective hydrosilylation of alkynes with PhSiH3 has been developed for the synthesis of (E)-β-vinylsilanes with high regio- and stereoselectivity and wide functional group tolerance. It is the first report of using porous organic polymer as a recyclable regio- and stereoselective and efficient ligand in hydrosilylation reactions in which the polymer could be recycled numerous times in a continuous flow system without loss of activity and selectivity. The earth-abundant base-metal catalyst, coordinated by heterogeneous recyclable ligand, shows promise for industrial application.

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inylsilanes are versatile synthetic building blocks owing to their stability, convenient operation, nontoxicity, and propensity to undergo a variety of transformations.1 To date, alkyne hydrosilylation is the most atom-economic and environmental-friendly approach toward such compounds. Although transition-metal-catalyzed hydrosilylation of alkynes is well established for the preparation of vinylsilanes, a serious issue hampering the widespread use of this method is the difficulty in regiocontrol and stereocontrol. For example, the hydrosilylation of a terminal alkyne can generate three possible products, (E)-β-, (Z)-β-, and α-vinylsilanes (Figure 1). In addition, the side reactions from oligomerization, hydrogenation, and the multiaddition of alkyne also complicate the catalytic system.

Figure 1. Different products of the alkyne hydrosilylation.

Figure 2. Hydrosilylation reaction of alkyne and PhSiH3 to form (E)β-vinylsilanes.

During the past decades, a series of noble metals, such as platinum,2 rhodium,3 palladium,4 iridium,5 ruthenium,6 thorium,7 and gold,8 have been successfully developed for the (E)-β-selective hydrosilylation of alkynes. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. For example, noble metal catalyzed hydrosilylation reactions with monosubstituted silanes (RSiH3) often generate multiaddition products because these catalysts are too active to stabilize a monoaddition product (Figure 2a).2c,d,4b The combination of chemical, economic, and environmental challenges has spurred © XXXX American Chemical Society

the exploration of inexpensive, low-toxic, and earth-abundant catalysts such as cobalt, iron, and nickel. Recently, continuing efforts on the development of inexpensive metal catalysts for hydrosilylation have led to progress in this field.9−13 The non-noble metal (such as cobalt Received: July 11, 2018

A

DOI: 10.1021/acs.orglett.8b02176 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters and iron)-catalyzed hydrosilylation of alkynes with RSiH3 has emerged to selectively produce (Z)-β-vinylsilanes.10 Nevertheless, the highly selective formation of (E)-β-vinylsilanes still remains tough for the non-noble-metal-catalyzed hydrosilylation of alkyne with RSiH3 (Figure 2b). In 2014, Deng developed a carbine-coordinate cobalt(I) complex and applied it to the hydrosilylation of alkynes and PhSiH3 with poor (E)β-selectivities.11 Thomas described a complex of the MesBIPCoCl2 for stereoselective hydrosilylation of 1-octyne and PhSiH3 to give the (E)-β-vinylsilanes with moderate (E)selectivity.12 Huang reported cobalt complexes of phosphine− iminopyridine ligand for the low (E)-β-selective hydrosilylation of alkyne with PhSiH3.13 Consequently, it is still a challenge to find a ligand that can coordinate with cobalt to facilitate the highly selective formation of (E)-β-vinylsilanes in the alkyne hydrosilylation with RSiH3. Over the past decades, porous organic polymers (POPs) have received special attention due to their high surface areas, excellent stabilities, tunable pore size distribution, and flexible synthetic strategies.14 These insoluble porous polymers can be swelling by a lot of organic solvents, showing behavior similar to those of soluble analogues. Porous polymers with excellent mobility of catalytic sites adopted onto the framework can enhance the catalytic performance. As noted above, all hydrosilylations of alkynes have been widely reported using homogeneous catalysts. However, homogeneous ligand separation and recyclable use are two major concerns in industrial hydrosilylation processes. In the ideal scenario, easily available heterogeneous ligand would coordinate with inexpensive metal and recycle numerous times without loss of activity and selectivity in hydrosilylation. With this in mind, we questioned whether porous organic polymers could serve as a heterogeneous ligand in coordinate with nonnoble metal to promote the alkyne hydrosilylation catalyzation. Herein, we demonstrate the utilization of these P-doped POPs coordinated by Co(acac)2 in the hydrosilylation of alkynes with high selectivity and recyclability. It is the first time using porous organic polymer as a regio- and stereoselective and efficient ligand in hydrosilylation, which could be recycled numerous times with little loss of activity and selectivity. This earth-abundant base-metal catalyst, coordinated by heterogeneous recyclable ligand, shows promise for industrial application. Our studies commenced with the hydrosilylation of 1decyne as a representative substrate. We examined a variety of ligands, additives and cobalt catalysts, for these model reactions. The results are summarized in Table 1. We first tested catalysts generated from Co(acac)2 and bidentate phosphine ligands such as Xantphos, dppf, and dppe (Table 1, entries 1−3), and the reaction afforded a mixture of both the monoaddition product 2a and double-addition product 3a which may be due to the high catalytic activity of Co(acac)2− bidentate phosphine ligands complex for alkyne hydrosilylation. In an attempt to control the selectivity of monoaddition product formation, different kinds of monodentate phosphine ligands were screened. When we carried out the reaction in the presence of PPh3, the highly monoaddition selective product 2a was isolated in 88% yield (Table 1, entry 4). Introduction of P(p-Tol)3 and P(p-FC6H4)3 as the ligand obviously decreased the yield (Table 1, entries 5 and 6). P(oTol)3 and P(tBu)3 were not effective ligands for this reaction (Table 1, entries 7 and 8). Additives were indispensable for promoting the efficacy. Without NaBHEt3, the yield of 2a was

Table 1. Optimization of the Reaction Conditions for Hydrosilylation of 1-Decyne and PhSiH3a

entry

cat.

ligand

additive

yield (%)b (yield (%)c)

1 2 3 4 5 6 7 8 9 10 11 12d

Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2

Xantphos dppf dppe PPh3 P(p-Tol)3 P(p-FC6H4)3 P(o-Tol)3 P(tBu)3 PPh3 PPh3 PPh3 POL−PPh3

NaBHEt3 NaBHEt3 NaBHEt3 NaBHEt3 NaBHEt3 NaBHEt3 NaBHEt3 NaBHEt3

39 (40) 42 (44) 40 (38) 88 65 63 0 0 21 84 28 93

MeMgBr NaOtBu NaBHEt3

a Reaction conditions: 1-decyne (1.0 mmol), phenylsilane (1.1 mmol), cat. (2 mol %), ligand (4 mol %), additive (4 mol %), THF (2 mL), room temperature, 4 h, N2 atmosphere. bThe yields of 2a were determined by 1H NMR spectroscopy. cThe yields of 3a were determined by 1H NMR spectroscopy. dPOL−PPh3 (15 mg/mmol).

reduced to 21% (Table 1, entry 9). MeMgBr or NaOtBu as the activator led to lower yields in comparison with NaBHEt3 (Table 1, entries 10 and 11). On the basis of the above results, an easily available P-doped porous organic polymer POL-PPh3 was synthesized through a free-radical polymerization of vinylfunctionalized PPh3 monomer (Supporting Information (SI), Figure S1). Excitedly, the use of POL-PPh3 as a heterogeneous ligand gave the product in 93% yield (Table 1, entry 12). Hence, the optimum conditions were determined to be conducting the reaction in THF at room temperature with Co(acac)2 as catalyst, POL-PPh3 as ligand, and NaBHEt3 as additive. In addition, we explored the catalytic performance of this Co catalyst for hydrosilylations with hydrosilanes. Secondary hydrosilane (Ph2SiH2), tertiary hydrosilanes (Ph3SiH, Ph2MeSiH, PhMe2SiH, and (MeO)2MeSiH) were unreactive under the reaction conditions (SI, Table S1). In an effort to explore the structure−activity relationship, the POL-PPh3 was characterized by thermo-gravimetry (TG), N2 adsorption−desorption analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The TG shows that POL-PPh3 remains intact at temperatures up to 430 °C, indicating its superior thermal stability (Figure 3a). Nitrogen adsorption−desorption analysis (Figure 3b) shows that POL-PPh3 possesses very favorable hierarchical pore size distributions, which is further confirmed by SEM (Figure 3c) and TEM (Figure 3d) images. The pore sizes were mainly distributed at 0.40−0.80 nm, which was calculated by the method of nonlocal density functional theory (NLDFT) (SI, Figure S2). The BET surface areas and total pore volumes of POL−PPh3 are up to 1150 m2/g and 2.24 cm3/g, respectively. The catalytic efficiency of the Co(acac)2/POL-PPh3 system was evaluated using a variety of alkynes (Scheme 1). All reactions were excellently selective for the formation of the B

DOI: 10.1021/acs.orglett.8b02176 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

yields. The scalability of (E)-dec-1-en-1- yl(phenyl)silane 2a synthesis was evaluated by performing the hydrosilylation of 1decyne on a gram scale. The reaction furnished the desired products in 88% yield, similar to that obtained on smaller scales. Ethynylcyclopropane and 1-ethynylcyclohex-1-ene also proceeded smoothly (Scheme 1, products 2h, 2i). The catalyst is compatible with aliphatic alkynes bearing various functional groups, including hydroxyl (products 2j, 2k), nitrile (product 2l), and chloride (product 2m). Aliphatic alkynes containing protecting groups, such as silyl ether, tosylate, and acetylate (products 2n−p), afforded the expected products in moderate to high yields. The scope of aromatic alkynes that undergo this Co-catalyzed E-selective hydrosilylation is also summarized. A variety of aromatic alkynes bearing electron-donating substituents afforded the corresponding hydrosilylation products (products 2q−v) in good yields and excellent regioselectivities. When dialkyl-substituted alkyne was subjected to this reaction, we obtained an unsymmetrical product 2w smoothly. For broadly synthetic interests, kinds of electron-withdrawing functionalized groups, such as halides, trifluoromethyl (products 2x−ac) were tolerated, and the results show that all reactions were excellently selective for this transformation (rr ≥ 20:1) but reactions of ethynylbenzene containing 4-F, 3Cl, and 4-CF3 moieties led to slightly lower selectivity (products 2z, 2aa, 2ac). The yields of some products (2w− 2ac) were moderate because there exist side products including hydrogenation products from both the substrates and the products and so on. In addition, sulfur-containing heteroaromatic alkynes reacted in good yields and excellent selectivities (products 2ad−ae). Internal alkynes will be reduced to alkene or alkane and cannot afford hydrosilylation products. Continuous flow processes are attractive by current trends in industrial application as they can be performed most advantageously by using immobilized catalysts.15 For example, reaction and filtration operations are carried out simultaneously so that the use of the catalysts can be repeated, whereas in corresponding batch reactions both processes have to be performed separately. As noted above, the ligands of (E)selective hydrosilylation are complex and unrecyclable compounds. Herein, the reusability of POL-PPh3 as a stationary phase in flow system was examined for the hydrosilylation of 1-decyne and phenylacetylene to obtain 2a and 2q. Our flow system was set up as shown in Scheme 2. POL-PPh3 mixed with silica sands was placed into a column16 and coordinated with Co(acac)2, then activated by NaBHEt3,

Figure 3. (a) TG curve of POL−PPh3. (b) N2 sorption isotherms of POL−PPh3. (c) Scanning electron microscopy (SEM) of POL−PPh3. (d) Transmission electron microscopy (TEM) of POL−PPh3.

Scheme 1. Scope of Alkynes for the Co(acac)2/POL-PPh3Catalyzed Anti-Markovnikov Hydrosilylation with PhSiH3a−c

Scheme 2. Synthesis in Flow System and Reuse of Co/ POL−PPh3 toward Hydrosilylation a Reaction conditions: alkynes (1.0 mmol), PhSiH3 (1.1 mmol), Co(acac)2 (2 mol %), POL−PPh3 (15 mg/mmol), NaBHEt3 (4 mol %), THF (2 mL), room temperature, N2 atmosphere, 4 h. bYields of isolated products. cThe selectivity for product (r.r. = E-β-vinylsilane product/all other isomers) was ≥20:1 unless otherwise noted, determined by 1H NMR spectroscopy. dGram scale: 1-decyne (10.0 mmol, 1.38 g). eThe rr ≥ 10:1.

(E)-β-vinylsilane products (rr ≥ 20:1, rr = E-β-vinylsilane product/all other isomers) unless otherwise noted. The straight-chain aliphatic alkynes and branched-chain aliphatic alkynes gave the corresponding products 2a−g in high isolated C

DOI: 10.1021/acs.orglett.8b02176 Org. Lett. XXXX, XXX, XXX−XXX

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Chen, J.; Walter, M. W.; Cook, M. J. Org. Biomol. Chem. 2013, 11, 4488. (3) (a) Morales-Cerón, J. P.; Lara, P.; López-Serrano, J.; Santos, L. L.; Salazar, V.; Á lvarez, E.; Suárez, A. Organometallics 2017, 36, 2460. (b) Huckaba, A. J.; Hollis, T. K.; Howell, T. O.; Valle, H. U.; Wu, Y. Organometallics 2013, 32, 63. (4) (a) Zhang, J. W.; Lu, G. P.; Cai, C. Green Chem. 2017, 19, 2535. (b) Yamashita, H.; Uchimaru, Y. Chem. Commun. 1999, 1763. (5) Field, L. D.; Ward, A. J. J. Organomet. Chem. 2003, 681, 91. (6) (a) Gao, R.; Pahls, D. R.; Cundari, T. R.; Yi, C. S. Organometallics 2014, 33, 6937. (b) Zaranek, M.; Marciniec, B.; Pawluć, P. Org. Chem. Front. 2016, 3, 1337. (7) (a) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550. (b) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.; Eisen, M. S. Organometallics 2001, 20, 5084. (8) (a) Corma, A.; González-Arellano, C.; Iglesias, M.; Sanchez, F. Angew. Chem., Int. Ed. 2007, 46, 7820. (b) Ishikawa, Y.; Yamamoto, Y.; Asao, N. Catal. Sci. Technol. 2013, 3, 2902. (9) (a) Zuo, Z. Q.; Yang, J.; Huang, Z. Angew. Chem., Int. Ed. 2016, 55, 10839. (b) Guo, J.; Lu, Z. Angew. Chem., Int. Ed. 2016, 55, 10835. (c) Sun, J.; Deng, L. ACS Catal. 2016, 6, 290. (d) Porter, T. M.; Hall, G. B.; Groy, T. L.; Trovitch, R. J. Dalton Trans. 2013, 42, 14689. (e) Wu, C.; Teo, W. J.; Ge, S. ACS Catal. 2018, 8, 5896. (f) Guo, J.; Shen, X.; Lu, Z. Angew. Chem., Int. Ed. 2017, 56, 615. (10) (a) Teo, W. J.; Wang, C.; Tan, Y. W.; Ge, S. Angew. Chem., Int. Ed. 2017, 56, 4328. (b) Greenhalgh, M. D.; Frank, D. J.; Thomas, S. P. Adv. Synth. Catal. 2014, 356, 584. (c) Challinor, A. J.; Calin, M.; Nichol, G. S.; Carter, N. B.; Thomas, S. P. Adv. Synth. Catal. 2016, 358, 2404. (11) Mo, Z. B.; Xiao, J.; Gao, Y. F.; Deng, L. J. Am. Chem. Soc. 2014, 136, 17414. (12) Docherty, J. H.; Peng, J. Y.; Dominey, A. P.; Thomas, S. P. Nat. Chem. 2017, 9, 595. (13) Du, X. Y.; Hou, W. J.; Zhang, Y. L.; Huang, Z. Org. Chem. Front. 2017, 4, 1517. (14) (a) Zhou, Y. B.; Wang, Y. Q.; Ning, L. C.; Ding, Z. C.; Wang, W. L.; Ding, C. K.; Li, R. H.; Chen, J. J.; Lu, X.; Ding, Y. J.; Zhan, Z. P. J. Am. Chem. Soc. 2017, 139, 3966. (b) Li, R. H.; Ding, Z. C.; Li, C. Y.; Chen, J. J.; Zhou, Y. B.; An, X. M.; Ding, Y. J.; Zhan, Z. P. Org. Lett. 2017, 19, 4432. (c) Jiang, J.; Wang, C.; Laybourn, A.; Hasell, T.; Clowes, R.; Khimyak, Y. Z.; Xiao, J.; Higgins, S. J.; Adams, D. J.; Cooper, A. I. Angew. Chem., Int. Ed. 2011, 50, 1072. (15) (a) Kirschning, A.; Monenschein, H.; Wittenberg, R. Angew. Chem., Int. Ed. 2001, 40, 650. (b) Kreituss, I.; Bode, J. W. Nat. Chem. 2017, 9, 446. (c) Yamamoto, H.; Sasaki, I.; Hirai, Y.; Namba, K.; Imagawa, H.; Nishizawa, M. Angew. Chem., Int. Ed. 2009, 48, 1244− 1247. (16) Silica sands can increase the gap of stationary phase in column to enhance fluid flow. (17) For a detailed procedure, see the Supporting Information. (18) For a discussion about the proposed mechanism, see the Supporting Information.

while a tetrahydrofuran solution of alkyne and PhSiH3 passed through the column. Additionally, Co(acac)2 needed to be supplemented once every five runs, since the combination between Cobalt and PPh3 was unstable and each run would release a small amount of Cobalt.17 As can be seen, POL-PPh3 was recycled 20 times without loss of activity and selectivity (Scheme 2). A mechanism accounting for the formation of (E)-βvinylsilane is presented in Scheme S2. On the basis of the precedents of Co-catalyzed hydrosilylation processes,10a,11,13 we propose that the catalytic cycle involves a low-valent Co(I) silyl intermediate (A). Once formed, migratory insertion of the silyl moiety to the CC triple bond in B will produce the alkenyl metal species C. Complex C could further react with PhSiH3 to yield the hydrosilylation product (E)-β-vinylsilane and regenerate A.18 In summary, we have developed a highly selective Cobalt/ POL-PPh3-catalyzed hydrosilylation of alkynes with PhSiH3 to produce (E)-β-vinylsilanes. It is first reported to use porous organic polymer as a recyclable regio- and stereoselective and efficient ligand in hydrosilylation reaction. The reusability of Co/POL-PPh3 as a stationary phase in flow system was examined for the hydrosilylation and recycled numerous times with little loss of activity and selectivity. This earth-abundant base-metal catalyst is expected to find promising applications in industrial synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02176. Full experimental details and characterization data for all products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhuang-Ping Zhan: 0000-0002-6310-8663 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (No. 21772166) and NFFTBS (No. J1310024). REFERENCES

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DOI: 10.1021/acs.orglett.8b02176 Org. Lett. XXXX, XXX, XXX−XXX