Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Porous Organic Polymer as a Heterogeneous Ligand for Highly Regio- and Stereoselective Nickel-Catalyzed Hydrosilylation of Alkyne Yun-Bing Zhou,† Zhi-Kai Liu,† Xin-Yang Fan,† Ren-Hao Li,† Guo-Liang Zhang,† Li Chen,† Ying-Ming Pan,‡ Hai-Tao Tang,*,‡ Jia-Hao Zeng,† and Zhuang-Ping Zhan*,†
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†
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China ‡ State Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and Pharmaceutical Sciences of Guangxi Normal University, Guilin 541004, People’s Republic of China S Supporting Information *
ABSTRACT: A porous organic polymer (POL-Xantphos) was synthesized and employed as a heterogeneous ligand for selective hydrosilylation of alkynes. It exhibits high selectivity and catalytic efficiency toward a broad range of alkynes. Owing to the confinement effect of the micropore structure, POL-Xantphos was far superior to the monomeric Xantphos ligands in controlling the selectivity. By performing hydrosilylation in a flow reactor system, separation and regeneration of the Ni/POL-Xantphos catalyst are easily achieved without any loss in selectivity or activity.
H
ydrosilylation of alkynes1 has been of high interest, as an efficient and atom-economic method for vinylsilanes2 that are versatile intermediates in organic synthesis. A key issue that needs to be solved in the hydrosilylation of alkynes is the control of stereo- and regioselectivities. In general, at least three isomers such as (E)-β-, (Z)-β-, and α-vinylsilanes can be obtained in this transformation. At the same time, other isomers arising from alkene isomerization,3 alkyne oligmerization,4 and hydrogenation5 accompany the Si−H addition process. Over the past several decades, a series of precious metals, such as Pt,6 Rh,7 Ru,8 Pd,9 and Th,10 have been used almost exclusively as catalysts for selective alkyne hydrosilylation. From an economical viewpoint, the use of nonprecious metal catalysts for hydrosilylation is more desirable and attractive. Recently, several groups have focused on the development of nonprecious metal-catalyzed highly selective hydrosilylation of alkynes11 in the presence of welldesigned homogeneous ligands (Scheme 1). For example, Cocatalyzed highly (E)-β-selective hydrosilylation of alkynes with Ph2SiH2 was achieved by using IAd or didentate phosphine as ligands.12 Note that the utility of Xantphos led to mainly double-addition products when using PhSiH3 as a silicon source.12c With oxazoline iminopyridine (OIP), Huang’s and Lu’s groups successfully developed Co-catalyzed highly αselective hydrosilylation.13 In addition, the catalytic systems such as FeCl2/PDI, Co(acac)2/PDI, and CoCl2/PCNN provided selective access to (Z)-β-vinylsilanes.14 Ni complexes, as an inexpensive and abundant alternative to platinum, have © XXXX American Chemical Society
Scheme 1. Catalysts Used in Hydrosilylation of Alkynes
also proven to have excellent selectivity for alkene hydrosilylation15 but poor selectivity for alkyne hydrosilylation.16 Consequently, hydrosilylation of unsaturated hydrocarbon with homogeneous catalysts finds industrial applications including fluids, molding products, silicone-based surfactants, and release coatings.17 However, problems associated with competing side reactions, catalyst activation, catalyst separation, and regeneration have still persisted. Therefore, the Received: September 25, 2018
A
DOI: 10.1021/acs.orglett.8b03064 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters development of heterogeneous catalysts capable of promoting highly selective Si−H additions to alkynes remains both significant and challenging. Porous organic polymers (POPs), a class of highly crosslinked amorphous polymers, consist mainly of carbon, oxygen, nitrogen, and phosphorus that are connected through strong covalent bonds.18 Porous organic polymers with various functionalities can be synthesized successfully with the bottom-up approach. By taking advantage of their high surface areas, permanent porosity, and excellent stabilities, POPs have been applied in many areas such as gas separation,19 postcombustion carbon capture,20 drug delivery,21 and energy storage.22 More importantly, porous organic polymers could serve as a promising platform for incorporating homogeneous catalytic moieties into the framework. Therefore, POPs could play the roles as both ligand and support simultaneously to prepare the efficient heterogeneous catalyst.23 As far as we know, only one example involving heterogeneous cobaltcatalyzed hydrosilylation of alkynes was reported recently, but this method suffered from the use of activating agent, limited silane substrate scope, and the need for constant addition of catalyst during flow reaction.24 Furthermore, the porous structure of polymers as catalyst carriers could affect catalytic activity,25 but its effect on selectivity control has been rarely investigated.26 Herein, we demonstrate for the first time that the beneficial confinement effect of the microporous structure in POPs has a positive effect on controlling selectivities in the hydrosilylation of alkynes. A nickel-metalated POP bearing Xantphos moieties Ni(I)/POL-Xantphos, used as a heterogeneous catalyst with good reusability, promotes the (E)-βselective hydrosilylation of alkynes with the aim of exploiting the porous structure to induce selectivity. In exploratory experiments the model reaction of 1-decyne with Ph2SiH2 was performed in THF at 30 °C, using Ni(acac)2 as the nickel source. First we investigated the efficiency of different homogeneous phosphine ligands in this model reaction. The monodentate ligands such as PPh3 and P(pTol)3 afforded a mixture of E- and Z-selective products in very poor yields (Table 1, entries 1 and 2). Meanwhile, large quantities of structurally undetermined products were also observed, aside from low yields and selectivities. These results are similar to those of the bidentate ligands such as dppb, dppe, and DPEphos (Table 1, entries 3−5). To our delight, switching the phosphine ligand to Xantphos resulted in obviously improved E/Z selectivity and yield for the (E)-βvinylsilane 2a, although moderate β/α selectivity and isomer 5a arising from alkene isomerization were obtained (Table 1, entry 6). Replacement of Ni(acac)2 with other nickel sources such as Ni(cod)2, Ni(OAc)2, and NiCl2 failed to provide the desired products (Table 1, entries 7−9). Inspired by our previous work on the selective oxidative Heck reaction with CMP ligands,26 we envisioned that porous materials as heterogeneous ligands may improve the selectivity for the hydrosilylation. Therefore, based on the observations previously mentioned, Xantphos was selected as a functional unit and further integrated into the skeleton of POPs to serve as a heterogeneous catalytic ligand. POL-Xantphos was successfully prepared from a vinyl-functionalized Xantphos monomer through solvothermal polymerization (Scheme S2, see the supporting information). The resulting polymerbearing Xantphos moieties in the skeleton allow for high loading of catalytic sites. This well-designed POL-Xantphos acts as both ligand and support simultaneously to prepare the
Table 1. Impact of Different Ligands on Selective Hydrosilylation of Alkynesa
entry
ligand
β:αb
E:Zc
yield %d (yield %e)
1 2 3 4 5 6 7f 8g 9h 10
PPh3 P(p-Tol)3 dppb dppe DPEPhos Xantphos POL-Xantphos or Xantphos POL-Xantphos or Xantphos POL-Xantphos or Xantphos POL-Xantphos
>20:1 >20:1 >20:1 >20:1 >20:1 5:1 − − − 22:1
1:2 2:1 1:2 1:3 3:2 >20:1 − − − >20:1
8 20:1. bIsolated yield. cRatio of E-β/α. dStereospecific product. esyn/trans = 14:1.
Scheme 2. Comparison of Heterogeneous System with Homogeneous Systema
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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.8b03064. Experimental procedures, analytical data for compounds, and NMR spectra (PDF) a
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Conditions: alkynes (1 mmol), Ph2SiH2 (1.2 mmol), Ni(acac)2 (0.01 mmol), Xantphos (0.01 mmol), THF (2 mL), 30 °C, 30 h.
AUTHOR INFORMATION
Corresponding Authors
toluene was eluted through the resulting column at room temperature, and then 89% yield and β/α selectivity of 21:1 were obtained, which was comparable with the scenario where both catalyst and substrates were mixed together in one pot. It was found that Ni(acac)2 would be almost washed off from the glass column by the eluent in the absence of silane. We assume that Ni(acac)2 may be reduced into Ni(I) species by silane, and the resulting Ni(I) species will not be washed off easily by the elution during the flow reaction process owing to its good binding ability to POL-Xantphos. The elution was repeated 10 times, and Ni/POL-Xantphos works without any loss in
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Ying-Ming Pan: 0000-0002-3625-7647 Hai-Tao Tang: 0000-0001-7531-0458 Zhuang-Ping Zhan: 0000-0002-6310-8663 Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.orglett.8b03064 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 21772166) and NFFTBS (No. J1310024) is acknowledged.
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DOI: 10.1021/acs.orglett.8b03064 Org. Lett. XXXX, XXX, XXX−XXX
