Palladium-Catalyzed Cross-Coupling Polymerization: A New Access

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Palladium-Catalyzed Cross-Coupling Polymerization: A New Access to Cross-Conjugated Polymers with Modifiable Structure and Tunable Optical/Conductive Properties Kunming Jiang,† Lu Zhang,†,‡ Yucheng Zhao,† Jun Lin,‡ and Mao Chen*,†

Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 11/21/18. For personal use only.

†

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, China S Supporting Information *

ABSTRACT: While the synthesis of conjugated polymers has received significant attention, the preparation of cross-conjugated polymers, where the π-electron delocalization cannot extend along the backbone, has received much less success. We have exploited a new catalytic polymerization process and developed the Pd-catalyzed cross-coupling enabled synthesis of crossconjugated polymers with readily accessible N-tosylhydrazones and aryl halides. A broad scope of cross-conjugated polymers with good processability and thermal stability has been prepared in high yields using dialkylbiarylphospine−Pd precatalysts. Owing to their unique vinyl structures, these polymers could be easily modified in postpolymerization fashions to impart high degrees of structural complexities. Moreover, these materials were shown to have interesting and tunable optical and conductive properties, featuring their great potentials in a variety of applications.

T

Scheme 1. Synthesis of Conjugated and Cross-Conjugated Polymers

he development of polymerization methods based on transition-metal-catalyzed reactions has greatly enriched the field of polymer science, enabling efficient accesses to complex, diverse, and functional materials.1−3 Cross-conjugated polymers (CCPs) provide new opportunities for material engineering because of the ability to mediate π-electron communication.4−6 Related materials have been explored in different areas, such as quantum interference,7 nonlinear optics,8 energies,9 and others.10 However, in contrast to widely investigated conjugated polymers,11,12 the synthetic methods for cross-conjugated polymers, where the π-electron delocalization cannot extend along their backbones in accord with the classical resonance theory,13 have been less reported (Scheme 1).10,14−18 Swager et al.10 and the Koizumi group14 synthesized CCPs with vicinal vinylidenes via Ti- and Pd-catalyzed processes, respectively. The Tilley 15 group prepared polyphenylenevinylidenes via the Zr-mediated process. The Weber16 groups and Itami et al.17 applied Ru- and Pd-catalyzed processes, respectively, to prepared polyphenylenevinylidenes. Existing synthetic approaches to such CCPs require the use of stoichiometric amounts of organometallic agents.14−17 The limited stability and availability of starting organometallic agents have confined the utility of these methods and restricted the scope of CCP materials. In the past decade, the Van Vranken,19,20 Barluenga,21,22 and Wang23,24 groups pioneered the development of the Pdcatalzyed cross-coupling of aryl derivatives with diazo © XXXX American Chemical Society

compounds or tosylhydrazones via a migratory insertion mechanism. This innovative strategy provides a useful alternative to previous C−C couplings, allowing for the efficient synthesis of small molecules with substituted Received: October 9, 2018 Revised: November 12, 2018

A

DOI: 10.1021/acs.macromol.8b02163 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules olefins.25,26 However, to our knowledge, this catalytic process has not been exploited in the context of polymer chemistry for challenges that lie in the restraint of possible side reactions, such as thermal degradation of hydrazones,27 cyclopropanation,28 dehalogenation, and others26 (Scheme S1), which would have dramatic influences on the polymerization, resulting in low molecular weight, broad polydispersity, and poor structural fidelity. We envisioned that the Pd−carbene migratory insertion chemistry could serve as a novel motif to build CCPs of poly(hetero)arylenevinylidenes (iso-PAVs) through judicious selection of conditions. Different repeating units Ar1/Ar2 (Scheme 1) could be easily assembled into the polymer backbone in an alternative sequence with appropriate monomers, allowing the generation of a library of CCPs with tunable/enhanced properties and thereby presenting a versatile platform for material engineering. Furthermore, the introduction of vinylidenes into the main chain could facilitate postprocessing with a variety of methods, leading to various functional materials.10,14 Herein, we report the first synthesis of a new series of CCPs via the Pd-catalyzed cross-coupling polymerization of N-tosylhydrazones and aryl halides, which provides polymers with well-defined structures, good processability, and thermal stability at good to excellent yields. Additionally, investigations on three different aspects including postmodification as well as optical and conductive properties have been conducted to demonstrate the interesting/unique characteristics of these CCP materials. We began our investigations with model reactions between N-substituted-dibromocarbazole (A1) and bis(N-tosylhydrazone) (B1) using 1 mol % Pd2(dba)3 (dba = dibenzylideneacetone) and 3 mol % XPhos (2-dicyclohexylphosphino2′,4′,6′-triisopropylbiphenyl) ligand in the presence of LiOtBu and dioxane. After initial optimization of the amount of base and concentration of monomers (Table S1), up to 98% conversion of A1 was obtained. The crude mixture was precipitated with methanol for two times to afford P1 in 83% yield (Scheme 2, entry 1, Mn = 3.8 kDa). To improve the molecular weight, we turned our attention to Buchwald precatalysts as these complexes are highly active for a range of Pd-catalyzed reactions and are readily available with a library of variants.29−31 However, when the third-generation precatalyst29 (2 mol % XPhos-Pd G3) with extra XPhos (1 mol %) was used, P1 was afforded with Mn = 3.2 kDa (entry 2). The decreased Mn was probably attributed to the undesired C−N bond formation between A1 and carbazole byproduct generated through the activation of precatalyst,30 leading to dead chain ends during the polymerization (Figure S1).32 With this concern, the fourth-generation precatalyst (G4),30 which can eliminate side reactions of A1, was employed in further studies. Gratifyingly, adding 2 mol % XPhos-Pd G4 and 1 mol % XPhos to the reaction improved the Mn to 6.1 kDa with 85% yield (entry 3). Unfortunately, the integration ratio of Ha/Hb was less than 2/1 in the 1H NMR spectrum of isolated polymer (Figure 1a), suggesting the terminal vinyl group was partially consumed by undesired reactions, such as the Pd-catalyzed Heck reaction of the vinyl group and aryl bromide (Figure S2). The integration of Hc, which is attributed to carbazole, is higher than its theoretical value, also supporting the hypothesized side reaction. When other dialkylbiaryl phosphine ligands were used instead of XPhos, polymers with much lower Mn values were afforded.

Scheme 2. Optimization for a Model CCP

Conditions: A1 (0.5 mmol), Pd (2 mol %, Pd/XPhos = 1/1.5), LiOtBu (2.2 mmol), dioxane (5 mL), 110 °C, 4 h. Conversion was determined by gas chromatography. aMn = number-average molar mass. Đ = molecular weight distribution. Mn and Đ were measured by THF SEC. bIsolated yield. cAbsolute molecular weight was determined by THF SEC using a MALLS detector.

We next decreased the ratio of A1/B1 to suppress the arylation of the vinyl group. When 1.10 equiv of B1 was used, the Mn increased to 6.2 kDa with a full conversion of A1 (entry 4). However, the integration ratio of Ha/Hb was 3.7/2.0 (Figure 1b), and new peaks located at 1.8 ppm (Hd) and 2.3 ppm (He) were observed in a nearly 1/1 ratio. We speculated that an increase in B1 accelerated the generation of diazo species through the Bamford−Stevens reaction,33 leading to cyclopropanation (Figure S3),28 with vinyl groups (gave He), and protonation of diazo intermediates (gave Hd) in the growing chain.27 Then, with 1.05 equiv of B1, the amount of water was optimized to suppress those side reactions.22 When 5 vol % of water in dioxane was used, P1 was obtained with 97% isolated yield and a highest Mn (entry 6, Mn = 15.7 kDa, Đ = 2.01). The absolute molecular weight determined by multiangle laser light scattering (MALLS) was much higher (33.7 kDa), indicating that the conventional calibration underestimated its Mn. The well-defined structure was confirmed by the 1H and 13C NMR spectra (Figure 1c,d). In the 1H NMR spectrum, protons attributed to (hetero)aryl, vinyl, and alkyl groups were all observed, and their integrations were in a good agreement with the targeted structure of P1. With the optimal polymerization conditions in hand, various monomers were examined to expand the polymer scope (Scheme 3). Using 2 mol % XPhos-Pd G4 precatalyst and 1 mol % XPhos, all substrates gave full conversions in 4 h, affording P1 to P15 in good to excellent yields (82−98%) upon simple precipitation with methanol for two times.34 This method is not only compatible with aromatic groups but also shows high reactivity in the presence of N-, S-, or O-atomcontaining heterocycles, which are important and versatile units that pervasively appeared in applications.11 Polymers with both terminal and internal vinyl groups were obtained (e.g., P9 vs P10) without apparent influences on the molar masses. Moreover, this new series of polymers possesses good B


Article

Macromolecules

Figure 1. (a−c) 1H NMR results of polymers obtained in optimization in Scheme 2. (a) is for entry 3, (b) is for entry 4, and (c) is for entry 6. (d) 13 C NMR spectra of P1 in entry 6 of Scheme 2.

solubility at room temperature in many organic solvents, such as tetrahydrofuran (THF), dichloromethane, chloroform, and ethyl acetate, which is important for processing. Molecular weight distributions (Đ) of 1.56−2.15 were observed as measured with size-exclusion chromatography (SEC) in THF since the reaction is presumably a step-growth polymerization. Meanwhile, SEC profiles for all polymers showed good Gaussian distributions (Figure 2) with no discernible shoulder peak, implying minimal side reactions. When measured by MALLS, absolute molecular weights of

13.4−46.3 kDa were obtained. The detailed microstructures were further characterized with 1H NMR, 13C NMR, and IR analyses (Figures S4−S48), confirming the successful preparation of targeted polymers. The thermal properties of P1 to P15 were measured with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Figures S49−S63). DSC analyses showed that polymers were amorphous without detectable melting transition. The glass transition temperatures ranged from 68.2 to 178.5 °C depending on the flexibility of polymer chains. C


Article

Macromolecules Scheme 3. Synthesis of Various CCPs

Conditions: A (0.5 mmol), A/B = 1/1.05, XPhos-Pd G4 (2 mol %), XPhos (1 mol %), LiOtBu (2.2 mmol), 5 vol % H2O in dioxane (2 mL), 110 °C, 4 h. Isolated yield. Mn and Đ were measured by THF SEC. Absolute molecular weights shown in parentheses were determined by THF SEC using a MALLS detector. Tg = glass transition temperature, Td = decomposition temperature. The Tg and Td values were determined by DSC and TGA measurements, respectively. All values were obtained under N2 at a scan rate of 10 °C/min. Temperatures at 5% weight loss (Td) are given.

Importantly, all polymers showed good thermal stability with decomposition temperatures above 300 °C at 5% weight loss. Taking advantages of the versatility of the vinyl group in organic synthesis, P7 was employed as a model substrate in the postsynthetic modification to demonstrate the flexibility/utility of iso-PAVs. Upon three pathways (Scheme 4) including reduction (route a), hydroboration/oxidation (route b), and hydroboration/pinacol protection (route c), polymers P16− P18 were obtained in 82−88% yields. While the NMR and IR results (Figures S64−S72) have confirmed their well-defined chemical structures, the SEC profiles (Figure S73) have further indicated that the polymer backbones remained intact during diverse chemical processes.

Figure 2. SEC traces for polymers from P1 to P15. D


Article

Macromolecules

the λmax in the range 250−325 nm in THF. Different from conjugated polymers, which cover light emissions within blue to red region due to the highly delocalized electronic character,35 CCPs through P1 to P15 all displayed maximum emissions in the high-energy region of visible light, offering an alternative compensation to conjugated polymers. Particularly, several CCPs such as P10 even gives purple light emission at 384 and 400 nm with ΦF = 0.16 (Figure 3d, for optical property of other CCPs; see Table S2), which locates in the highest energy region of visible light. Because the repeating units A/B of CCPs can be easily changed by choosing different coupling partners, this method provides a convenient access to polymeric materials with tunable properties as indicated with the optical property. On the other hand, one advantage of having consecutive cross-conjugation structure along polymer backbone is that its conjugation length can be easily tuned through doping, leading to a dramatic change of its bulk properties. The conductivity was investigated taking P13 as an example. A film made of untreated P13 is insulated. In contrast, after doping with iodine and bromine (Figure 3e, Figures S74−S77), materials show conductivities of 3.2 × 10−2 and 0.83 S/cm, respectively. When the percentages of iodine by weight were decreased from 65% to 20%, the conductivity were declined from 10−2 to 10−5 S/cm, suggesting less delocalized carbocations appeared upon a simple treatment. In conclusion, we have demonstrated the first example of the Pd-catalyzed cross-coupling polymerization of N-tosylhydrazones and aryl halides for the synthesis of CCPs, which are otherwise difficult to prepare with existing polymerization methods. It allows a highly efficient access to a variety of welldefined polymers with high yields from easily accessible starting materials. Moreover, these polymers exhibit good processability and decent thermal stability. Investigations on the postmodification, photophysical property, and conductivity have demonstrated interesting characteristics of iso-PAVs, such

Scheme 4. Postsynthetic Modification of P7

Conditions: (a) p-toluenesulfonyl hydrazide, tri-n-hexylamine, 2,6-ditert-butyl-4-methylphenol (cat.), o-xylene, reflux, 5 h; (b) BH3−THF, rt, 4 h, then NaOH and H2O2 in water, rt, 1.5 h; (c) BH3−THF, rt, 4 h, then pinacol, rt, 3 h.

Next, we examined the photophysical and conductive properties of CCPs. First of all, we synthesized P13a−P13c with different molecular weights by varying the reaction times. As their molar masses changed from 10.3 to 36.3 kDa, the molar absorptivity steadily increased from 0.51 × 106 to 1.60 × 106 L/(mol cm) (Figure 3a). From P13a to P13c, three polymers show similar UV−vis absorption and fluorescent emission profiles (Figure 3b) with only slight increases of the λmax and λem (maximum emission wavelength) values, and these materials at condensed state are colorless (e.g., Figure 3c). We reasoned that the light absorption at nearly constant wavelength was caused by the constrained π-electron communication within the polymer chain, which would dictate related optical properties with limited lengths of linear conjugation paths. Also, when the optical properties of P1−P15 were examined, UV−vis spectra showed that these polymers possess

Figure 3. Investigations on the optical property and conductivity. (a, b) UV−vis and emission spectra respectively for P13 with different Mn in THF (10−6 M) at 25 °C. P13a: Mn = 10.3 kDa; P13b: Mn = 16.4 kDa; P13c: Mn = 36.3 kDa. (c) A film made of P13c. (d) Light emissions of selected polymers in THF (10−6 M) at 25 °C. (e) A film of P13 doped with iodine. E


Article

Macromolecules

(9) Jing, Y.; Liang, Y.; Gheytani, S.; Yao, Y. Cross-Conjugated Oligomeric Quinones for High Performance Organic Batteries. Nano Energy 2017, 37, 46. (10) Swager, T. M.; Grubbs, R. H. Synthesis and Properties of a Novel Cross-Conjugated Conductive Polymer Precursor: Poly(3,4diisopropylidenecyclobutene). J. Am. Chem. Soc. 1987, 109, 894. (11) Muellen, K.; Reynolds, J. R.; Masuda, T. Conjugated Polymers: A Practical Guide to Synthesis; Royal Society of Chemistry: Cambridge, 2014. (12) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting Polymers: Conjugated Polymers, Theory, Synthesis, Properties, and Characterization, 3rd ed.; CRC Press LLC: 2007. (13) Phelan, N. F.; Orchin, M. Cross conjugation. J. Chem. Educ. 1968, 45, 633. (14) Nishioka, N.; Hayashi, S.; Koizumi, T. Palladium(0)-Catalyzed Synthesis of Cross-Conjugated Polymers: Transformation into LinearConjugated Polymers through the Diels−Alder Reaction. Angew. Chem., Int. Ed. 2012, 51, 3682. (15) Mao, S. S. H.; Tilley, T. D. Cross-Conjugated Polymers via Condensation of A Zirconocene Alkynyl(benzyne) Derivative Generated by Thermolysis of Cp2ZrMe(C6H4CCSiMe3). J. Organomet. Chem. 1996, 521, 425. (16) Londergan, T. M.; You, Y.; Thompson, M. E.; Weber, W. P. Ruthenium Catalyzed Synthesis of Cross-Conjugated Polymers and Related Hyperbranched Materials. Copoly(arylene/1,1-vinylene)s. Macromolecules 1998, 31, 2784. (17) Itami, K.; Ohashi, Y.; Yoshida, J.-i. Triarylethene-Based Extended π-Systems: Programmable Synthesis and Photophysical Properties. J. Org. Chem. 2005, 70, 2778. (18) Kayser, L. V.; Hartigan, E. M.; Arndtsen, B. A. Multicomponent Coupling Approach to Cross-Conjugated Polymers from VanillinBased Monomers. ACS Sustainable Chem. Eng. 2016, 4, 6263. (19) Greenman, K. L.; Carter, D. S.; Van Vranken, D. L. Palladiumcatalyzed insertion reactions of trimethylsilyldiazomethane. Tetrahedron 2001, 57, 5219. (20) Kudirka, R.; Devine, S. K. J.; Adams, C. S.; Van Vranken, D. L. Palladium-Catalyzed Insertion of α-Diazoesters into Vinyl Halides To Generate α,β-Unsaturated γ-Amino Esters. Angew. Chem., Int. Ed. 2009, 48, 3677. (21) Barluenga, J.; Moriel, P.; Valdés , C.; Aznar, F. NTosylhydrazones as Reagents for Cross-Coupling Reactions: A Route to Polysubstituted Olefins. Angew. Chem., Int. Ed. 2007, 46, 5587. (22) Barluenga, J.; Escribano, M.; Aznar, F.; Valdés, C. Arylation of α-Chiral Ketones by Palladium-Catalyzed Cross-Coupling Reactions of Tosylhydrazones with Aryl Halides. Angew. Chem., Int. Ed. 2010, 49, 6856. (23) Peng, C.; Wang, Y.; Wang, J. Palladium-Catalyzed CrossCoupling of α-Diazocarbonyl Compounds with Arylboronic Acids. J. Am. Chem. Soc. 2008, 130, 1566. (24) Zhou, L.; Ye, F.; Ma, J.; Zhang, Y.; Wang, J. PalladiumCatalyzed Oxidative Cross-Coupling of N-Tosylhydrazones or Diazoesters with Terminal Alkynes: A Route to Conjugated Enynes. Angew. Chem., Int. Ed. 2011, 50, 3510. (25) Barluenga, J.; Valdés, C. Tosylhydrazones: New Uses for Classic Reagents in Palladium-Catalyzed Cross-Coupling and MetalFree Reactions. Angew. Chem., Int. Ed. 2011, 50, 7486. (26) Xia, Y.; Qiu, D.; Wang, J. Transition-Metal-Catalyzed CrossCouplings through Carbene Migratory Insertion. Chem. Rev. 2017, 117, 13810. (27) Fulton, J. R.; Aggarwal, V. K.; de Vicente, J. The Use of Tosylhydrazone Salts as a Safe Alternative for Handling Diazo Compounds and Their Applications in Organic Synthesis. Eur. J. Org. Chem. 2005, 2005, 1479. (28) Barluenga, J.; Quinones, N.; Tomas-Gamasa, M.; Cabal, M.-P. Intermolecular Metal-Free Cyclopropanation of Alkenes Using Tosylhydrazones. Eur. J. Org. Chem. 2012, 2012, 2312.

as ease of functionalization, molar mass irrelevant absorption wavelength, purple light emission, and tunable electronic performance, further highlighting the diversified potential utilities of this new polymer platform. Given the great potentials of cross-conjugated structures in many applications, we believe the approach we demonstrated here would open new avenues for material engineering.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02163.

■

Condition optimizations; 1H NMR, 13C NMR, DSC, TGA, and IR analyses of polymers; optical properties of polymer solutions; 1H NMR and 13C NMR analyses of monomers (PDF)

AUTHOR INFORMATION

Corresponding Author

*(M.C.) E-mail: [email protected]. ORCID

Jun Lin: 0000-0002-2087-6013 Mao Chen: 0000-0002-5504-3775 Author Contributions

K.J. and L.Z. contributed equally. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is financially supported by the starting up funding from National Program for Thousand Young Talents of China. We thank Mr. Yuwei Gu (MIT), Dr. Yang Zou (Shenzhen University), Dr. Yang Yang (Caltech), and Dr. Liming Zhang (Fudan University) for discussions.

■

REFERENCES

(1) Matyjaszewski, K.; Tsarevsky, N. V. Macromolecular Engineering by Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2014, 136, 6513. (2) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129. (3) Bielawski, C. W.; Grubbs, R. H. Living Ring-Opening Metathesis Polymerization. Prog. Polym. Sci. 2007, 32, 1. (4) Gholami, M.; Tykwinski, R. R. Oligomeric and Polymeric Systems with a Cross-conjugated π-Framework. Chem. Rev. 2006, 106, 4997. (5) Hopf, H.; Sherburn, M. S. Dendralenes Branch Out: CrossConjugated Oligoenes Allow the Rapid Generation of Molecular Complexity. Angew. Chem., Int. Ed. 2012, 51, 2298. (6) Zou, Y.; Ji, X.; Cai, J.; Yuan, T.; Stanton, D. J.; Lin, Y.-H.; Naraghi, M.; Fang, L. Synthesis and Solution Processing of a Hydrogen-Bonded Ladder Polymer. Chem. 2017, 2, 139. (7) Solomon, G. C. Cross-Conjugation and Quantum Interference. In Cross Conjugation; Hopf, H., Sherburn, M. S., Eds.; Wiley-VCH: Weinheim: Germany, 2016; p 397. (8) Zhao, Y.; Slepkov, A. D.; Akoto, C. O.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Synthesis, Structure, and Nonlinear Optical Properties of Cross-Conjugated Perphenylated iso-Polydiacetylenes. Chem. - Eur. J. 2005, 11, 321. F


Article

Macromolecules (29) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Design and Preparation of New Palladium Precatalysts for C-C and C-N CrossCoupling Reactions. Chem. Sci. 2013, 4, 916. (30) Bruno, N. C.; Niljianskul, N.; Buchwald, S. L. N-Substituted 2Aminobiphenylpalladium Methanesulfonate Precatalysts and Their Use in C-C and C-N Cross-Couplings. J. Org. Chem. 2014, 79, 4161. (31) Biscoe, M. R.; Fors, B. P.; Buchwald, S. L. A New Class of Easily Activated Palladium Precatalysts for Facile C−N CrossCoupling Reactions and the Low Temperature Oxidative Addition of Aryl Chlorides. J. Am. Chem. Soc. 2008, 130, 6686. (32) The presence of side reactions was supported by control experiments under the same conditions (Figures S1−S3). (33) Bamford, W. R.; Stevens, T. S. The Decomposition of Toluenep-sulphonylhydrazones by Alkali. J. Chem. Soc. 1952, 4735. (34) Although full conversions were afforded for all examples, different isolated yields were obtained with the same precipitation procedure as these CCPs have different solubilities. (35) For example, conjugated polymers such as poly(arylenevinylene)s, poly(paraphenylene)s, poly(aryleneethynylene)s, and poly(thiophene)s exhibit blue to red light emissions; see ref 7b.

G


Palladium-Catalyzed Cross-Coupling Polymerization: A New Access

Recommend Documents