Letter Cite This: Org. Lett. 2017, 19, 5776-5779
pubs.acs.org/OrgLett
“Click Chemistry” Mediated Functional Microporous Organic Nanotube Networks for Heterogeneous Catalysis Wei Yu, Minghong Zhou, Tianqi Wang, Zidong He, Buyin Shi, Yang Xu, and Kun Huang* School of Chemistry and Molecular Engineering, East China Normal University, 500 N, Dongchuan Road, Shanghai, 200241, P. R. China S Supporting Information *
ABSTRACT: The synthesis of azide functional microporous organic nanotube networks (N3-MONNs) via a Friedel−Crafts hyper-crosslinking reaction is reported. Subsequently, a general method for obtaining heterogeneous catalysts through a Cu-catalyzed alkyne−azide reaction is presented. The small-molecule catalysts such as 2,2,6,6,tetramethylpiperidine-1-oyl and 4-(N,N-dimethylamino)pyridine can be anchored into the MONNs. Owing to the hierarchically porous structure and high surface area, these catalysts show high activity in selective oxidation of alcohols and acylation reaction, respectively.
O
Up to now, many strategies have been adapted to immobilize small-molecule catalysts on POPs through covalent14 and noncovalent15 attachment,16 in which the copper-catalyzed azide−alkyne cycloaddition (CuAAC) reaction is undoubtedly one of the most extensively used reactions because of its high efficiency and universality in scope for reactive or sensitive molecules within the “click ” chemistry toolbox which was devised in 2001 by Sharpless.17 In particular, there is a growing awareness of using the CuAAC reaction for loading all sorts of functional molecules on POPs.18 Herein, a class of azide-functionalized microporous organic nanotube networks (N3-MONNs) was successfully synthesized by the combination of molecule templating of core−shell bottlebrush copolymers and hyper-cross-linking reaction. Then, small molecule catalysts such as TEMPO and DMAP were further anchored into the N3-MONNs by “click” chemistry as shown in Scheme 1. Owing to the hierarchical-porosity structure and high surface area, both of the resultant heterogeneous catalysts show high catalytic activity in alcohol oxidation and acylation reaction, respectively. We believe this approach can be developed to be a general method to fabricate various functional microporous organic nanotube networks for various kinds of applications in the future. As shown in Scheme S1, the chlorine contained core−shell bottlebrush copolymer precursors were synthesized by the combination of controlled radical and ring-opening polymerizations based on our previous reported method.12b From the 1 H NMR spectrum of PGM-g-(PLA-b-PS/PVBC) (Figure S1), every branch of the precursor is composed of a PLA block with 56 repeat units and a PS/PVBC block with 101 PS units and 21 PVBC units. The molecular weight distributions were characterized by gel permeation chromatography (GPC) and
ver the past decades, porous organic polymers (POPs), including covalent organic frameworks (COFs),1 conjugated microporous polymers (CMPs),2 polymers of intrinsic microporosity (PIMs),3 porous aromatic frameworks (PAFs),4 and hyper-cross-linked polymers (HCPs)5 have garnered considerable attention because of their potential applications in gas separation,6 drug delivery,7 catalysis,8 sensing/detecting,9 etc. Because of their high surface area, robust organic framework, and excellent chemical modifiability, POPs have been extensively investigated as a versatile platform for heterogeneous catalysts.10 Among all kinds of POPs, hypercross-linked polymers (HCPs) with their advantages of tunable pore size, low-cost, monomer diversity, and facile preparation have been recently explored as a type of catalytic supporter for immobilizing small molecule catalysts. For example, the Tan groups have reported a series of cost-effective HCPs-type catalysts for some organic transformations, which showed that the HCPs material could act as a promising porous supporter for organic catalysis.5b,11 Recently, our group developed a novel method to synthesize the microporous organic nanotube networks (MONNs) as a kind of POPs supporter via a Friedel−Crafts hyper-crosslinking reaction.12 The convenience of functionalization and tunable structures give them significant potential application in various heterogeneous catalysts. For example, we reported the synthesis of organic ligand incorporated hyper-cross-linked microporous organic nanotube frameworks (O-HMONFs) as platforms for heterogeneous catalysts through direct copolymerization or cohyper-cross-linking catalytic monomer into the shell layer of bottlebrush copolymers.13 However, for the above methods, special ligand molecules with aromatic rings or expensive ligand monomers are required, such as benzyl amine, 2,2′-bipyridine, or 4-diphenylphosphinostyrene, which make these methods limited. Therefore, to explore a relatively simple, convenient, and universal approach to immobilize functional small molecules into the supporter is quite crucial. © 2017 American Chemical Society
Received: August 31, 2017 Published: October 26, 2017 5776
DOI: 10.1021/acs.orglett.7b02682 Org. Lett. 2017, 19, 5776−5779
Letter
Organic Letters
disappearance of the IR absorption of the azide group (2100 cm−1) (Figure S5C). However, the signals of TEMPO in solid state 13C NMR spectra were covered by the large amount of −CH2− signal produced during the hyper-cross-linking procedure (Figure S4C).22 The morphology of the assynthesized N3-MONNs and TEMPO-MONNs were characterized by transmission electron microscopy. Figure 1A shows a
Scheme 1. Synthetic Route to TEMPO/DMAP-MONNs from Well-Defined Core-Shell Bottlebrush Copolymer Precursors via Hyper-Crosslinking Reaction and “Click Chemistry”
the results show that all polymers exhibit monomodal molecular weight distributions and low polydispersities, which indicates efficient reinitiation and the formation of well-defined copolymer precursors (Figure S2). To obtain the N3-MONNs, the chlorine contained core− shell bottlebrush copolymers were first hyper-cross-linked via a Friedel−Crafts alkylation reaction, in which the pendant phenyl groups from PS/PVBC shell block of bottlebrush copolymers are used as cross-linkable components. After hyper-crosslinking, the PLA core is completely degraded in the presence of FeCl3 as a Lewis acid and HCl (byproduct of Friedel−Crafts reaction). The complete removal of PLA can be confirmed by the disappearance of the characteristic PLA carbonyl stretch peak (1758 cm−1) in the FTIR spectra (Figure S3). Although the chloromethyl group of vinylbenzyl chloride in the bottlebrush precursor can also participate in the Friedel−Crafts alkylation during hyper-cross-linking, a certain amount of unreacted chloromethyl groups would remain.19 The disappearance of the characteristic signals (COO at ∼170 ppm, CH at ∼70 ppm for the PLA) and the decrease of signal at ∼47 ppm corresponding to CH2Cl in 13C solid state NMR spectroscopy also further confirmed the completely hydrolysis of the PLA block and partial alkylation of chloromethyl groups, respectively (Figure S4B). Consequently, the MONNs with remaining chloromethyl group was further reacted with sodium azide to obtain azide functionalized MONNs (N3-MONNs). The successful introduction of azide group can be verified by the appearance of the bands at 2100 cm−1 (Figure S5B). Calculated by the nitrogen content (3.20 wt %) in N3-MONNs, the amount of chloromethyl groups in MONNs participating in the azidation procedure is about 76.7%. It proved that a large proportion of chloromethyl groups of the bottlebrush precursor would remain after hyper-cross-linking and could be further modified. 2,2,6,6,-Tetramethylpiperidine-1-oyl (TEMPO) as a stable organic radical can work as a redox reagent under various reactions.20 Recently, TEMPO-containing solid catalysts have attracted great interest because of their convenience of separation of catalyst and products, which provides the possibility of reusing the catalyst and improves the production proficiency.21 Herein, propargyl ether TEMPO (a) was synthesized, which can react with the N3-MONNs to achieve TEMPO-MONNs through the subsequent “click reaction”. The successful synthesis can be judged by the complete
Figure 1. TEM images of N3-MONNs (A) and TEMPO-MONNs (B); N2 absorption−desorption isotherms and pore distribution of pore size distributions (inset) calculated using DFT methods of N3MONNs (C); and TEMPO-MONNs (D).
typical TEM image of the N3-MONNs, in which a batch of unique organic nanotube networks is developed from the bottlebrush copolymers. It can be seen that most of the bottlebrush precursors preserved the cylindrical shape, and the hollow tubular structure could also be observed after etching of the PLA layer. The average length of the cylindrical units is ∼67.1 nm with a channel of ∼3.3 nm in diameter and a shell thicknesses of ∼6.3 nm. It is noteworthy that the hollow tubular morphology with similar sizes still remained (Figure 1B). Measured by elemental analysis, the resultant TEMPOMONNs contains ∼0.34 mmol g−1 of TEMPO groups, which will provide enough active sites for further reactions (Table S1). To obtain the textural information, the N2 absorption/ desorption isotherms and pore size distributions of the N3MONNs and TEMPO-MONNs were characterized. Figure 1 panels C and D exhibit characteristic type IV curves with a distinct hysteresis loop at a high pressure region (P/P0 = 0.7− 1.0 and 0.8−1.0), indicating the presence of a hierarchically (micro/mesopore) porous structure. An absorption uptake at low relative pressure confirms the formation of the micropore in the polymeric matrices. The micropore is produced from the hyper cross-linking PS shell layer, while the mesopore comes from the selectively removed PLA core layer. According to the Brunauer−Emmett−Teller (BET) model, the N3-MONNs is proven to have high specific surface area of 940.2 m2/g and a pore volume of 1.02 cm3/g, meanwhile the counterparts of TEMPO-MONNs are 617.9 m2/g and 1.01 cm3/g. The decreased surface areas may be ascribed to the successful introduction of the TEMPO groups in the nanotube networks, and might block part of micropore. Besides, the distribution of pore sizes (average sizes from 1.0 to 7.4 nm, respectively) calculated by the density functional theory (DFT model) indicates that both of N3-MONNs and TEMPO-MONNs possess a hierarchically porous structure. These results also agree with the TEM characterization. 5777
DOI: 10.1021/acs.orglett.7b02682 Org. Lett. 2017, 19, 5776−5779
Letter
Organic Letters As an evaluation of the catalytic performance of TEMPOMONNs, the selective oxidation of alcohols under Anelli’s conditions was examined (Table 1, Scheme 2). The results Table 1. Oxidation of Alcohols with Different Substratesa
Figure 2. Catalytic performance of the recycled TEMPO-MONNs.
its recovery and reuse procedure will limit its applications.25 To verify our method can be widely generalized, propargyl ether DMAP (b) is selected as another example to react with the N3MONNs. Similarly, the disappearance of the IR absorption at 2100 cm−1 suggests the successful introduction of the DMAP group on the MONNs (Figure S5D).The resultant DMAPMONNs contains ∼0.21 mmol g−1 of DMAP group based on elemental analysis (Table S1). However, the signals of DMAP in the solid state 13C NMR spectra were also covered by quaternary aromatic carbon adjacent to the methylene in the cross-linked shell (Figure S4D).26 The TEM (Figure S8) and BET (Figure S7B) measurements of the obtained DMAPMONNs further confirm that the hierarchical porosity morphology can be preserved well after modifications. The acylation of alcohols with Ac2O is chosen as model reaction to evaluate its catalytic activity (Scheme 3). The generality of the
a
Reaction condition: alcohol (0.5 mmol), TEMPO-MONNs (0.83 mol %) in CH2Cl2 (0.75 mL), aq NaClO (1.5 mol %), KBr (2 mol %) at 0 °C, pH = 9.1. bGC yield.
Scheme 2. Illustration of the TEMPO-MONNs Catalyst for the Oxidation of Alcohols
Scheme 3. Illustration of the DMAP-MONNs Catalyst for Acylation of Alcohols
show that the primary benzylic and aliphatic alcohols (entry 1− 7) can be easily oxidized to the corresponding aldehydes with high yields (97%−99%), and no carboxylic acids are produced. As to secondary alcohol (entry 8), the oxidation is more challenging under the same conditions, and a longer reaction time (40 min) is required to achieve 94% conversion with 99% selectivity. All of the high selectivity may be ascribed to the hydrophobicity of the organic nanotube networks, which can impede the hydrated aldehyde from contact with the entrapped TEMPO radical and further oxidation to corresponding acids.23 Moreover, compared with the various reported TEMPOimmobilized catalysts, the catalytic activity of TEMPOMONNs displays a relatively high value (Table S2). To explore the recyclability of the TEMPO-MONNs, benzyl alcohol was oxidized on a larger scale, and the catalyst could be used for at least 14 times without significant activity loss (Figure 2). A kinetic study was also conducted and the conversion can reach 97% even in 5 min (Figure S6). In addition, the morphology of the TEMPO-MONNs is not apparently destroyed after the recycles and the hollow tubular structure of the network units is still maintained. Furthermore, the N2 absorption measurements of the after-recycles TEMPOMONNs were conducted and the preserved hierarchically porous structure and retained high specific surface area (459.6 m2/g) further confirm the excellent stability of the TEMPOMONNs catalyst (Figure S7A). 4-(N,N-Dimethylamino)pyridine (DMAP) is an efficient nucleophilic organic catalyst used in a large amount of organic transformations.24 It often exhibits highly dermal toxicity, and
acylation using DMAP-MONNs is tested. As shown in Table 2, the acylation reactions of aromatic alcohols (entry 1), aliphatic alcohols (entry 2), and phenols (entries 3−5) with various substituent groups (electron-withdrawing, electron-donating) were successfully converted to the corresponding esters with Table 2. Acylation of Alcohols with Different Substratesa
a
Condition A: alcohol (0.1 mmol), DMAP-MONNs (1.4 mol %), Ac2O (1.5 equiv), and NEt3 (1.5 equiv) in dried CH2Cl2 (0.5 mL) at rt. Condition B: alcohol (0.1 mmol), DMAP-MONNs (5 mol %), and Ac2O (1.5 equiv) in dried CH2Cl2 at rt. bGC yield.
5778
DOI: 10.1021/acs.orglett.7b02682 Org. Lett. 2017, 19, 5776−5779
Letter
Organic Letters
(6) Biswal, B. P.; Chaudhari, H. D.; Banerjee, R.; Kharul, U. K. Chem. - Eur. J. 2016, 22, 4695−9. (7) Fang, Q.; Wang, J.; Gu, S.; Kaspar, R. B.; Zhuang, Z.; Zheng, J.; Guo, H.; Qiu, S.; Yan, Y. J. Am. Chem. Soc. 2015, 137, 8352−5. (8) Kaur, P.; Hupp, J. T.; Nguyen, S. T. ACS Catal. 2011, 1, 819− 835. (9) Yanai, N.; Kitayama, K.; Hijikata, Y.; Sato, H.; Matsuda, R.; Kubota, Y.; Takata, M.; Mizuno, M.; Uemura, T.; Kitagawa, S. Nat. Mater. 2011, 10, 787−93. (10) (a) Ma, L.; Wanderley, M. M.; Lin, W. ACS Catal. 2011, 1, 691−697. (b) Liu, F.; Wang, L.; Sun, Q.; Zhu, L.; Meng, X.; Xiao, F. S. J. Am. Chem. Soc. 2012, 134, 16948−50. (11) (a) Wang, K.; Jia, Z.; Yang, X.; Wang, L.; Gu, Y.; Tan, B. J. Catal. 2017, 348, 168−176. (b) Jia, Z.; Wang, K.; Tan, B.; Gu, Y. ACS Catal. 2017, 7, 3693−3702. (c) Jia, Z.; Wang, K.; Tan, B.; Gu, Y. Adv. Synth. Catal. 2017, 359, 78−88. (12) (a) Zhang, H.; Xiong, L.; He, Z.; Zhong, A.; Wang, T.; Xu, Y.; Huang, K. New J. Chem. 2016, 40, 7282−7285. (b) Zhang, H.; Xiong, L.; He, Z.; Zhong, A.; Wang, T.; Xu, Y.; Zhou, M.; Huang, K. Polym. Chem. 2016, 7, 4975−4982. (c) Zhang, H.; Zhou, M.; Xiong, L.; He, Z.; Wang, T.; Xu, Y.; Huang, K. J. Phys. Chem. C 2017, 121, 12771− 12779. (d) Zhou, M.; Zhang, H.; Xiong, L.; He, Z.; Wang, T.; Xu, Y.; Huang, K. Polym. Chem. 2017, 8, 3721−3730. (13) (a) Xu, Y.; Wang, T.; He, Z.; Zhou, M.; Yu, W.; Shi, B.; Huang, K. Appl. Catal., A 2017, 541, 112−119. (b) Xu, Y.; Wang, T.; He, Z.; Zhong, A.; Yu, W.; Shi, B.; Huang, K. Polym. Chem. 2016, 7, 7408− 7415. (14) (a) Kundu, S. K.; Bhaumik, A. ACS Sustainable Chem. Eng. 2015, 3, 1715−1723. (b) Zhang, Y.; Zhang, Y.; Sun, Y. L.; Du, X.; Shi, J. Y.; Wang, W. D.; Wang, W. Chem. - Eur. J. 2012, 18, 6328−34. (c) Gilhespy, M.; Lok, M.; Baucherel, X. Catal. Today 2006, 117, 114− 119. (15) (a) Wang, J.; Sng, W.; Yi, G.; Zhang, Y. Chem. Commun. 2015, 51, 12076−9. (b) Saha, B.; Gupta, D.; Abu-Omar, M. M.; Modak, A.; Bhaumik, A. J. Catal. 2013, 299, 316−320. (16) Fernandes, A. E.; Jonas, A. M.; Riant, O. Tetrahedron 2014, 70, 1709−1731. (17) Kolb, H. C.; Finn, M.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (18) (a) Mason, B. P.; Bogdan, A. R.; Goswami, A.; McQuade, D. T. Org. Lett. 2007, 9, 3449−51. (b) Taskin, O. S.; Kiskan, B.; Weber, J.; Yagci, Y. Macromol. Mater. Eng. 2015, 300, 1116−1122. (c) Cho, K.; Yoo, J.; Noh, H.-W.; Lee, S. M.; Kim, H. J.; Ko, Y.-J.; Jang, H.-Y.; Son, S. U. J. Mater. Chem. A 2017, 5, 8922−8926. (19) (a) Macintyre, F. S.; Sherrington, D. C.; Tetley, L. Macromolecules 2006, 39, 5381−5384. (b) Pulko, I.; Wall, J.; Krajnc, P.; Cameron, N. R. Chem. - Eur. J. 2010, 16, 2350−4. (20) Ciriminna, R.; Pagliaro, M. Org. Process Res. Dev. 2010, 14, 245− 251. (21) (a) Wang, Y.; Song, X.; Shao, S.; Zhong, H.; Lin, F. RSC Adv. 2012, 2, 7693. (b) Shi, Y.; Nabae, Y.; Hayakawa, T.; Kakimoto, M.-a. RSC Adv. 2015, 5, 1923−1928. (c) Machado, A.; Casimiro, M. H.; Ferreira, L. M.; Castanheiro, J. E.; Ramos, A. M.; Fonseca, I. M.; Vital, J. Microporous Mesoporous Mater. 2015, 203, 63−72. (22) Fey, T.; Fischer, H.; Bachmann, S.; Albert, K.; Bolm, C. J. Org. Chem. 2001, 66, 8154−8159. (23) Michaud, A.; Gingras, G.; Morin, M.; Béland, F.; Ciriminna, R.; Avnir, D.; Pagliaro, M. Org. Process Res. Dev. 2007, 11, 766−768. (24) (a) Hale, K. J.; Grabski, M.; Flasz, J. T. Org. Lett. 2013, 15, 370− 3. (b) Meng, L.-G.; Li, C.-T.; Zhang, J.-F.; Xiao, G.-Y.; Wang, L. RSC Adv. 2014, 4, 7109. (25) Höfle, G.; Steglich, W.; Vorbrüggen, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 569−583. (26) Chen, H. T.; Huh, S.; Wiench, J. W.; Pruski, M.; Lin, V. S. J. Am. Chem. Soc. 2005, 127, 13305−11.
high yields (93−99%) at room temperature. In a comparison of the DMAP-MONNs with previous reported DMAP heterogeneous catalysts, this work also showed high values for the esterification reaction (Table S3). The recyclability of DMAPMONNs also has been investigated using a-phenethyl alcohol as the substrate. The result shows that it can be recycled for at least 10 times and still retain a high activity (Figure S9). Similarly, the kinetic study showed that the conversion reached 94% in 5 h (Figure S10). In conclusion, a new method to synthesize an azide-MONNs has been developed by a combination of hyper-cross-linking and molecule templating of a chlorine contained core−shell bottlebrush copolymer. Small molecule catalysts, such as TEMPO and DMAP, can be efficiently introduced into the MONNs by the CuAAC reaction and made the material act as a heterogeneous catalyst. Furthermore, both of the assynthesized TEMPO- and DMAP-MONNs show excellent catalysis activity and recyclability due to their hierarchicalporosity structure and high surface area. In addition, this work might provide a general method to attach any appropriately small molecules to the N3-MONNs for various applications.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02682. Synthesis and characterization of catalysts; general procedures; spectroscopic data (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kun Huang: 0000-0003-2737-1189 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The work is supported by the National Science Foundation of China (Grants 51273066, 21574042). The large instruments were supported by the Open Foundation of East China Normal Universtiy.
■
REFERENCES
(1) (a) Vyas, V. S.; Vishwakarma, M.; Moudrakovski, I.; Haase, F.; Savasci, G.; Ochsenfeld, C.; Spatz, J. P.; Lotsch, B. V. Adv. Mater. 2016, 28, 8749−8754. (b) Ding, S.-Y.; Wang, W. Chem. Soc. Rev. 2013, 42, 548−568. (2) Xu, Y.; Jin, S.; Xu, H.; Nagai, A.; Jiang, D. Chem. Soc. Rev. 2013, 42, 8012. (3) (a) McKeown, N. B.; Budd, P. M. Chem. Soc. Rev. 2006, 35, 675− 83. (b) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E. Chem. Commun. 2004, 230−1. (4) Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M.; Qiu, S.; Zhu, G. Angew. Chem., Int. Ed. 2009, 48, 9457−60. (5) (a) Tan, L.; Li, B.; Yang, X.; Wang, W.; Tan, B. Polymer 2015, 70, 336−342. (b) Jia, Z.; Wang, K.; Li, T.; Tan, B.; Gu, Y. Catal. Sci. Technol. 2016, 6, 4345−4355. (c) Wang, J.; Wei Yang, J. G.; Yi, G.; Zhang, Y. Chem. Commun. 2015, 51, 15708−11. (d) Yang, Y.; Tan, B.; Wood, C. D. J. Mater. Chem. A 2016, 4, 15072−15080. 5779
DOI: 10.1021/acs.orglett.7b02682 Org. Lett. 2017, 19, 5776−5779
