Jakub Szyling and Jędrzej Walkowiak

Effective one-pot synthesis of (E)-poly(vinyl arylenes) via trans-borylation/Suzuki coupling protocol

De Gruyter | Published online: March 11, 2017

Abstract

A new stereoselective method for the synthesis of (E)-poly(vinylarenes) on the basis of the Ru-H catalyzed trans-borylation reaction and sequential Suzuki coupling is described. Two different strategies of Pd-catalyzed polycondensation leading to unsaturated organic macromolecular compounds are discussed. Development of a one-pot procedure reduces costs and time, needed for the separation of intermediates, the amount of organic solvents used for purification and therefore has a positive influence on the process sustainability.

1 Introduction

Modern industry and academia are intensively striving to design and produce new electronic devices which could be applied in solar cells [1], [2], electroluminescence devices [3], novel transistors [4], [5], sensors [6], [7], [8] or photovoltaics [9], [10], [11]. It is still a challenge to effectively replace silicon-based conducting and optoelectronic materials with their organic, macromolecular counterparts, which are generally cheaper, flexible and easier for preparation. Poly(phenylene vinylenes) (PPVs) are often used as a part (light emitting polymer) of polymer light emitting diodes [12], [13], [14], [15], [16]. Organic light emitting diodes and polymer light emitting diodes have become an alternative to traditional light emitters and their role in electronic industry is still increasing [17], [18]. The wide applications of poly(arylene vinylenes) (PAVs) have stimulated the development of new synthetic protocols for the synthesis of these conjugated conducting macromolecular compounds. The most common methods used for the formation of PAVs via stoichiometric as well as catalytic reactions are summarized in Figure 1.

Figure 1: Methods used for the synthesis of poly(arylene vinylene) polymers.

Figure 1:

Methods used for the synthesis of poly(arylene vinylene) polymers.

Many research groups are engaged in the search for new methods leading to linear and branched PAVs. The conventional stoichiometric approaches providing PAVs include, e.g. the Wittig-Horner polycondensation of α,α′-p-xylenediylbis(triphenylphosphonium chloride) with terephthalaldehyde [19], dehydrogenative Gilch polycondensation of bis(halomethylbenzenes) [20], thermolysis of sulfonium polymer precursors (Wessling route) [21], or Knoevenagel condensation of aldehydes with phenylmethylenes with electron accepting nitrile group in terminal positions [22]. PPVs and other PAVs can be also obtained by chemical vapor deposition using dichloropara[2.2]cyclophane or α,α′-dichloro-p-xylene as reagents [23], [24]. High temperature of the process is responsible for a low selectivity towards the products. The problem with halogen removal is the additional drawback of this process, making it useless in the synthesis of PAVs. These stoichiometric methods are often non-selective and lead to the macromolecular compounds with both possible geometries of double C=C bonds, which is undesirable for the final products applications; procedures are often synthetically complicated and demand application of reagents, which are not commercially available.

Synthesis of PAVs is also possible by catalytic methods as acyclic diene metathesis of 1,4-divinylsubstituted arenes [25] or McMurry condensation of dialdehydes or diketones catalyzed by titanium compounds at a low oxidation state [26]. Pd-catalyzed coupling reactions leading to formation of the new carbon-carbon bonds are by far the most popular catalytic methods applied for the synthesis of PAVs. Heck coupling of ethylene or divinylarenes with dihaloarenes leads to PAV with interesting luminescence properties [27], [28], [29]. This method was also used for the preparation of copolymer of phenyl vinylene with fluorene, which is a rare example of the polymer emitting red color light [30].

Marciniec et al. [31] described highly effective and selective Hiyama coupling between bissilylethenes and dihaloarenes (mainly dibromoarenes) as the efficient method for the formation of PAVs. The availability of substrates, as well as simplicity of the method, are favorable features of this protocol. Another example of Hiyama coupling leading to the synthesis of (E) or (Z)-oligo(arylene vinylenes) is the Pd-coupling reaction of 1,4-diiodobenzenes with 1,4-bis(silylethenyl)benzenes [32].

The Suzuki coupling, which is the most commonly used method for the formation of new carbon-carbon bonds, is used also in the formation of PAVs. Oligo(arylene vinylenes) or PAVs obtained according to the Suzuki coupling, due to the retention of the configuration during the reaction, have a high stereoregularity and the type of the reagents determines the formation of macromolecular compounds with E or Z geometry of the double bond. Wakioka et al. have obtained (Z)-PPV by the Suzuki coupling catalyzed by Pd(PPh3)4 of 2,5-dioctaoxy-1,4-phenyldiboronic acid with (Z,Z)-bis(2-bromoethenyl)benzene, which was further transformed to (E)-PPV by photoisomerization [33].

In this study we would like to report an efficient catalytic method for the synthesis of PPV with trans stereoregularity from commercially available reagents: vinylboronates, styrenes and dihaloarenes via one-pot strategies based on the trans-borylation (borylative coupling) reaction and Suzuki polycondensation. Two different strategies will be discussed. The first one is based on borylative coupling of vinylboronate with bromosubstituted styrenes and further Suzuki polycondensation of obtained borylethenylbromoarene to PPVs. The homocoupling of vinylboronates followed by the Suzuki coupling with dihaloarenes is the second discussed method. Application of a one-pot procedure has great impact on the processes sustainability, since the separation of the intermediates obtained in trans-borylation reaction is not necessary. Therefore, such a protocol reduces the amount of organic solvents used for the intermediate separation and saves time for their isolation. The whole process is less time-consuming which has a positive influence on its economy.

2 Materials and methods

2.1 Materials

Unless mentioned otherwise, all operations were performed under argon atmosphere (5.0 class, Messer Polska Sp. z o.o., Chorzow, Poland) using the standard Schlenk line technique. Solvents were purified by an mBraun MB SPS 800 purification system and stored over molecular sieves type 4 Å. All dihalogenoarenes, 3-bromostyrene, 4-bormostyrene, 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaborolane, [Pd(PPh3)4], [Pd(PCy3)2], acetone, hexane, THF and toluene were purchased from Sigma-Aldrich, Poznan, Poland. Deuterated solvents were obtained from Dr. Glaser AG Basel, Basel, Switzerland. Potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), sodium hydroxide (NaOH) and potassium hydroxide (KOH) were purchased from P.O.Ch., Gliwice, Poland, Gliwice. Reagents were used as supplied without further purification. Vinylboronates: 2-Vinyl-1,3-dioxaborolane [34] and 2-vinyl-1,3-dioxaborinane [35] were synthesized according to literature methods with some modifications. The ruthenium – [Ru(CO)Cl(H)(PCy3)2 [36] – and palladium catalyst – [PdBr(PCy3)2Ph] [37] [Pd(dba)(PCy3)2] [38] were synthesized according to the references.

2.2 Analytical methods

1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on Varian XL 300 MHz and Varian Gemini 300 MHz spectrometers in chloroform-d1, benzene-d6, DMSO-d6 and THF-d8 solution. Chemical shifts are reported (in ppm) with reference to the residue solvent. FT-IR spectra were measured on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a built-in ATR accessory with ATR diamond unit. In all experiments, 16 scans at a resolution of 2 cm−1 were used to record the spectra. Gas chromatographic (GC) analyses were performed on a Varian Star 3400CX with a DB-5 fused silica capillary column (30 m×0.15 mm) and thermal conductivity detector (TCD). Mass spectra of the substrates and products were obtained by GC-MS analysis on a Bruker Scion 436-GC with a 30 m Varian DB-5 0.25 mm capillary column and a Scion SQ-MS mass spectrometry detector. Thermogravimetric (TG) analysis and differential TG analysis (DTG) were carried out using Netzsch TG 209 F1 Libra-Thermo-Microbalance apparatus. All experiments were performed in nitrogen atmosphere at temperatures from the range 30–850°C at the heating rate 10°C/min. Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) using an Alliance 135 GPCV 2000 apparatus.

2.3 Borylative coupling procedures

The [Ru(CO)Cl(H)(PCy3)2] catalyst (2% mmol) was evacuated in the Schlenk vessel for 20 min, then refilled with argon. Subsequently, toluene, vinylboronate (1.0 mmol) and olefin (1.0 mmol) were added. The concentration of reagents in toluene was approximately 0.5 m. The reaction was carried out for 18 h at 80°C under argon atmosphere. The progress of the reaction was monitored by GC-MS analyses. The solvent was evaporated and the crude product was isolated from the residue of catalyst on silica gel using column chromatography with hexane/ethyl acetate (8/1) as eluent. The product was analyzed on the basis of GC-MS, 1H NMR and 13C NMR. The E or Z isomer was only formed during the process.

(E)-1,2-di(1,3,2-dioxaborinan-2-yl)ethene (1a): Isolated yield: 94%; 1H NMR (300 MHz, CDCl3, 25°C): δ=1.26 (4H, m, CH2CH2O), 3.42 (8H, br, OCH2), 6.42 (2H, br, BCH=CHB) ppm. 13C NMR (75 MHz, CDCl3, 25°C): δ=28.2 (OCH2CH2CH2O), 68.9 (OCH2CH2CH2O) ppm. Cα to boron atom is not observed. MS (EI) (m/z [%]): 196(M+, 23), 138(94), 129(100), 111(7), 67(23), 53(31).

(E)-1,2-di(1,3,2-dioxaborolan-2-yl)ethene (1b): Isolated yield: 80%; 1H NMR (300 MHz, CDCl3, 25°C): δ=3.42 (8H, m, OCH2CH2O ), 6.37 (2H, br, BCH=CHB) ppm. 13C NMR (75 MHz, CDCl3, 25°C): δ=66.6 (OCH2CH2O) ppm. Cα to boron atom is not observed. MS (EI) (m/z [%]): 168(M+, 31), 139(87), 111(100), 97(9), 68(41), 53(18).

(E)-1,2-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethene (1c): Isolated yield: 97%; 1H NMR (300 MHz, CDCl3, 25°C): δ=1.22 (24H, s, CH3), 6.54 (2H, br, BHC=CHB) ppm. 13C NMR (75 MHz, CDCl3, 25°C): δ=23.7 (CH3), 84.0 B(OCCH3) ppm. Cα to boron atom is not observed. MS (EI) (m/z [%]): 280(M+, 4), 265(100), 193(20), 152(85), 137(37), 83(95), 55(73).

(E)-2-(4-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1d): Isolated yield: 92%; 1H NMR (300 MHz, CDCl3, 25°C): δ=1.11 (12H, s, CH3), 6.06 (1H, d, =CHB, JH-H=19.1 Hz), 7.00 (1H, d, =CH-Ph, JH-H=18.8 Hz), 7.36–7.42 (2H, C6H4Br), 7.49–7.53 (2H, m, C6H4Br) ppm. 13C NMR (75 MHz, CDCl3, 25°C): δ=24.0 (CH3), 83.4 B(OCCH3), 123.0 128.9, 131.6, 135.4 (C6H4Br) 146.7 (=CHC6H4Br) ppm. Cα to boron atom is not observed. MS (EI) (m/z [%]): 308(M+, 58), 293(24), 264(11), 251(13), 224(80), 209(7), 143(100), 77(5).

(E)-2-(4-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1e): Isolated yield: 90%; 1H NMR (300 MHz, CDCl3, 25°C): δ=1.16 (12H, s, CH3), 6.11 (1H, d, =CHB, JH-H=19.0 Hz), 6.98 (1H, d, =CH-Ph, JH-H=19.0 Hz), 7.27 (1H, m, C6H4Br), 7.44–7.57 (3H, m,C6H4Br) ppm. 13C NMR (75 MHz, CDCl3, 25°C): δ=24.2 (CH3), 84.1 B(OCCH3), 122.6 127.2, 130.1, 130.9, 131.6. 134.9 (C6H4Br) 148.4 (=CHC6H4Br) ppm. Cα to boron atom is not observed. MS (EI) (m/z [%]): 308(M+, 4), 293(15), 264(11), 251(9), 223(37), 209(51), 143(100), 77(32).

2.4 Suzuki polycondensation

2.4.1 Homopolycondensation of boryl(boromostyryl)ethenes:

A 50 ml Schlenk vessel was charged with [PdBr(PCy3)2Ph] (5% mmol), evacuated for 20 min and refilled with argon. Subsequently (E)-2-(4-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.0 mmol) or (E)-2-(4-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.0 mmol), 2 m aqueous solution of K2CO3 (20 mmol) and THF (0.1 m) were added. The reaction was carried out for 24 h at 50°C. The yellow-orange product was filtered, washed with hexane and acetone, dried under vacuum and analyzed by 1H NMR, FT-IR and TG/DTG analysis.

Poly(p-phenylene vinylene) (2a): Isolated yield: 86%; 1H NMR (300 MHz, DMSO-d6, 25°C): δ=5.77–6.01 (br, =CH), 6.09–6.33 (br =CH), 7.44–7.63 (br m, Ar) ppm; FT-IR (ATR, cm−1): 783.0, 837.0, 965.6, 1010.4, 1109.0. 1173.2, 1521.2, 1587.2, 1706.2, 2961.4, 3022.0; TG: td5=160.2, td10=178.1; char yield: 21.05%.

Poly(m-phenylene vinylene) (2b): Isolated yield: 79%; 1H NMR (300 MHz, DMSO-d6, 25°C): δ=5.88 (d, =CH, JH-H=15.2 Hz), 6.16 (d =CH, JH_H=15.4 Hz), 7.39–7.51 (br m, Ar) ppm.; FT-IR (ATR, cm−1): 792.0, 963.8, 1011.2, 1109.2, 1177.0, 1210.0, 1336.0, 1562.0, 2964.0, 3022.9; TG: td5=235.2, td10=252.1; char yield: 49.19%.

2.4.2 Polycondensation of bisborylethenes with dihalogenoarenes:

A 50 ml Schlenk vessel was charged with [Pd(Br)(Ph)(PCy3)2] (5% mmol), evacuated for 20 min and refilled with argon. Subsequently, bisborylethene (1.0 mmol), dihalogenoarene (1.0 mmol), 2 m aqueous solution of Cs2CO3 (20 mmol) and THF (0.1 m) were added. The reaction was carried out for 24 h at 50°C. The product was filtered, washed with hexane and acetone, dried under vacuum and analyzed by 1H NMR, FT-IR and TG/DTG analysis.

Poly(p-phenylene vinylene) (3a): Isolated yield: 83%; 1H NMR (300 MHz, DMSO-d6, 25°C): δ=5.96 (d, =CH, JH-H=19.1 Hz), 6.12 (d, =CH, JH-H=19.1 Hz), 7.31–7.56 (br m, Ar) ppm; FT-IR (ATR, cm−1): 781.0, 835.0, 966.6, 1013.4, 1106.0. 1175.2, 1519.2, 1594.2, 1703.2, 2961.8, 3023.6.; TG: td5=167.0, td10=184.7, char yield: 31.03%.

Poly(m-phenylene vinylene) (3b): Isolated yield: 74%; 1H NMR (300 MHz, DMSO-d6, 25 oC): δ=5.78 (d, =CH, JH-H=15.2 Hz), 6.06 (d =CH, JH-H=15.4 Hz), 7.30–7.55 (br m, Ar) ppm; FT-IR (ATR, cm−1): 791.0, 965.0, 1009.6, 1111.8, 1179.0, 1215.0, 1332.0, 1565.0, 2968.0, 3024.8; TG: td5=239.0, td10=257.5; char yield: 50.28%.

Poly(anthracene-9,10-vinylene) (3c): Isolated yield: 69%; FT-IR (ATR, cm−1): 602.0, 726.0, 906.0, 964.0, 1011.2, 1125.2, 1131.0, 1167.0, 1272.0, 1620.0, 2954.0, 3020.9, 3048.2; TG: td5=285.2, td10=341.1; char yield: 47.05%.

Poly(fluorene-2,7-vinylene) (3d): Isolated yield: 74%; FT-IR (ATR, cm−1): 723.2, 754.0, 820.6, 888.2, 969.0, 1088.2, 1246.8, 1376.4, 1466.2, 1602.0, 2952.2, 3030.0; TG: td5=240.2, td10=182.0; char yield: 44.82%.

Poly(pyridine vinylene) (3e): Isolated yield: 66%; FT-IR (ATR, cm−1): 743.4, 960.0, 1028.2, 1397.4, 1485.2, 1599.0, 1625.8, 2830.4, 3029.0; TG: td5=260.2, td10=278.1; char yield: 36.18%.

Poly(benzophenone-4,4′-vinylene) (3f): Isolated yield: 58%; FT-IR (ATR, cm−1): 699.2, 705.0, 814.0, 966.2, 1020.0, 1076.6, 1177.2, 1206.4, 1281.4, 1690.8, 2966.2, 3018.0; TG: td5=190.2, td10=221.1; char yield: 42.76%.

Poly(2,5-bis(dodecyloxy)-p-phenylene vinylene) (3g): Isolated yield: 70%; 1H NMR (300 MHz, THF-d8, 25 oC): δ=0.91 (m, CH3), 1.21–1.42 (m, C8H16), 3.92 (s, OCH2) 5.53 (br, =CH), 6.74 (br =CH), 7.12 (s, Ar) ppm; FT-IR (ATR, cm−1): 722.2, 866.0, 966.2, 865.8, 1041.4, 1218.2, 1391.0, 1420.2, 1470.6, 1504.4, 1607.0, 1677.0, 2204.8, 2852.2, 2925.0, 3033.0 TG: td5=263.7, td10=289.5; char yield: 19.08%.

2.5 One-pot procedure for the synthesis of PAV

2.5.1 Synthesis of PPV via trans-borylation of vinylboronate with bromosubstituted styrene and Suzuki coupling:

The [Ru(CO)Cl(H)(PCy3)2] catalyst (2% mmol) was evacuated in the Schlenk vessel for 20 min, then refilled with argon. Subsequently toluene, vinylboronate (1.0 mmol) and bromosubstituted styrene (1.0 mmol) were added. The concentration of reagents in toluene was approximately 0.5 m. The reaction was carried out for 18 h at 80°C under argon atmosphere. The solvent was evaporated and [PdBr(PCy3)2Ph] (5% mmol), 2 m aqueous solution of K2CO3 (20 mmol) and THF (0.1 m) were added. The reaction was carried out from 5 min to 24 h at 50°C. The product was filtered, washed with hexane and acetone, dried under vacuum to give the corresponding compounds.

2.5.2 Synthesis of PAV via homocoupling of vinylboronates and Suzuki coupling:

The [Ru(CO)Cl(H)(PCy3)2] catalyst (2% mmol) was evacuated in the Schlenk vessel for 20 min, then refilled with argon. Subsequently, toluene and vinylboronate (2.0 mmol) were added. The concentration of reagents in toluene was approximately 0.5 m. The reaction was carried out for 18 h at 80°C under argon atmosphere. The solvent was evaporated and the [PdBr(PCy3)2Ph] (5% mmol), dihalogenoarene (1.0 mmol), 2 m aqueous solution of Cs2CO3 (20 mmol) and THF (0.1 m) were added. The reaction was carried out from 18 h to 24 h at 50°C. The product was filtered, washed with hexane and acetone, dried under vacuum to give the corresponding compounds.

3 Results and discussion

Our general idea was to develop a new, simple, highly effective and stereoselective method for the synthesis of PAVs on the basis of commercially available reagents, taking into account the process time, economy and sustainability. For our studies, we chose two catalytic reactions (trans-borylation and Suzuki coupling), which are characterized with really high selectivity towards specific isomers or give the product with the total retention of the configuration.

Developed by Marciniec, borylative coupling (trans-borylation) of olefins with vinylboronates or their homocoupling yielded the products that can be a source of vinylene fragment for PAVs synthesis in Suzuki polycondensation. The reaction occurs via activation of the C-B bond in vinylboronate and the C-H bond in olefin (or the second vinylboronate molecule).

This mode of reactivity seems to be general also for other vinylmetalloids as well as alkynes and reagents with the heteroatom-hydrogen bonds (alcohols, silanols, boronic acids and amines). In the case of the borylative coupling reaction, only E isomers are formed in the reaction with olefin. Homocoupling of vinylboronates is less selective, but when 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaborolane is used as a reagent, the E isomer is exclusively formed (see Figure 2).

Figure 2: General protocol for: (A) activation of C-H, N-H and O-H bonds by vinylmetalloids, (B) borylative coupling of vinylboronates with olefins, (C) homocoupling of vinylboronates.

Figure 2:

General protocol for: (A) activation of C-H, N-H and O-H bonds by vinylmetalloids, (B) borylative coupling of vinylboronates with olefins, (C) homocoupling of vinylboronates.

Borylative coupling reactions are catalyzed by ruthenium complexes, which comprise or generate during the process the Ru-H or Ru-B bonds. The most active complex of this transformation is so far [Ru(CO)Cl(H)(PCy3)2] (I). The process occurred with the evolution of ethylene as a by-product, immediately removed from the reaction system.

Our first strategy for the synthesis of vinylene arylene polymers was based on the formation of monomer for Suzuki polycondensation in the borylative coupling of bromosubstituted styrenes with vinylboronates. The products were obtained with quantitative yield. No homocoupling of vinylboronate was observed, even if an equimolar ratio of reagents was used. The product synthesized within the trans-borylation process was a proper monomer for further polymerization, due to the fact that boron as well as halogen atoms were directly attached to Csp2, and both required functions were present in the same molecule. Moreover, the exclusive formation of the (E) isomer in the borylative coupling reaction has a high impact on the stereoregularity of PAVs, because the Suzuki coupling occurs with the retention of configuration. The trans-borylation was carried out under argon atmosphere at 80°C with 2% mol of catalyst (I) in toluene. The conversion of reagents was monitored by GC and GC-MS analyses, while the formation of (E) isomer was evidenced by 1H NMR.

Isolated borylsubstituted bromostyrenes (1d, 1e) were tested as monomers in Suzuki polycondensation using different Pd catalysts, solvents (THF, toluene), reaction temperatures and bases (K2CO3, Cs2CO3 NaOH, KOH).

We tested several Pd(0) and Pd(II) complexes: [Pd(PPh3)4] (II), [PdBr(PCy3)2Ph] (III) [Pd(PCy3)2] (IV) and [Pd(dba)(PCy3)2] (V) in the polymerization of both reagents. We used 5 mol% of the catalyst in all procedures. The best results were obtained when the Pd(II) – complex (III) was used in the Suzuki coupling. Complete consumption of the reagent was observed after 5 min in the case of borylsubstituted 4-bromostyrene. When borylsubstituted 3-bromostyrene was used as a reagent, its total conversion was observed after 24 h under the same process conditions. The structure of reagents has a surprisingly strong influence on the reaction rate, and when bromine is in the meta position to the vinyl substituent, the reaction is much longer. This means that the formation of linear PPVs is much more favorable. The lowest conversion and reaction rates were observer for the most popular catalyst (II) used in the Suzuki transformation. Using this Pd(0) complex deborylation, dehalogenation as well as other coupling by-products were observed in the mixture of products. We also checked the influence of the base on the Suzuki coupling for the processes carried out in the presence of catalyst (II) and the most active one [PdBr(PCy3)2Ph] (III). When Cs2CO3 and K2CO3 were used, comparable results were observed. For 2 m solution of NaOH or KOH, the yields were a bit lower. Because of a lower price of K2CO3, relative to that of the cesium analog, its 2 m solution was employed in the reaction procedure. The reaction was much faster in THF then in toluene because of a better solution of the reagents in a more polar tetrahydrofurane. We also noticed the temperature influence on the reagents conversion and the best results were observed when the reaction took place at elevated temperatures. Table 1 presents the optimization of the Suzuki polycondensation for both substrates which differ in the position of bromine atom in the phenyl ring.

Table 1:

Optimization of Suzuki polycondensation procedure for (E)-2-(4′-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1d) and (E)-2-(3′-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1e).

Table 1: Optimization of Suzuki polycondensation procedure for (E)-2-(4′-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1d) and (E)-2-(3′-bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1e).
No. Reagent [Pd] Base Solvent Temperature (oC) Time Conversion/(isol. yield) (%)b
1a (1d) Pd(PPh3)4 K2CO3 THF 50 24 h 37
2 Cs2CO3 THF 50 24 h 36
3 KOH THF 50 24 h 30
4 NaOH THF 50 24 h 27
5 K2CO3 Toluene 50 24 h 32
6 K2CO2 THF 80 24 h 55
7 PdBr(PCy3)2Ph K2CO3 THF 50 5 min 100 (86)
8 Cs2CO3 THF 50 5 min 100
9 KOH THF 50 5 min 87
10 NaOH THF 50 5 min 83
11 K2CO3 THF 20 4 h 100
12 K2CO3 Toluene 50 45 min 100
13 Pd(PCy3)2 K2CO3 THF 50 3 h 100
14 K2CO3 Toluene 50 3 h 78
15 Pd(dba)(PCy3)2 K2CO3 THF 50 3 h 94
16 K2CO3 Toluene 50 3 h 87
17 (1e) Pd(PPh3)4 K2CO3 THF 50 24 h 27
18 PdBr(PCy3)2Ph K2CO3 THF 50 24 h 100 (79)
19 Pd(PCy3)2 K2CO3 THF 50 24 h 78
20 Pd(dba)(PCy3)2 K2CO3 THF 50 24 h 75

    a[Pd]:[(1d) or (1e)]:[base]=5×10−2:1:20, base (2 m) aq. solution.

    bDetermined by GC-MS.

On the basis of optimization experiments, the following conditions were chosen for the polycondensation procedure: 5 mol% of catalyst (III), 2 m aq. K2CO3, THF and 50°C.

Based on the borylative coupling reaction as well as Suzuki polycondensation we proposed a one-pot protocol, which saves time and costs, usually spent for isolation of intermediates. The proposed protocol permits a reduction in the amount of expensive and toxic organic solvents used for separation whereby the whole process has a positive impact on the environment (Figure 3). After the trans-borylation reaction, THF, the catalyst and K2CO3 were added to the vessel and the reaction mixture was heated up to 50°C. The total conversion of reagents, monitored by GC, was observed after a few minutes from the beginning of the process.

Figure 3: One-pot synthesis of poly(phenylene vinylenes) (PPVs) via trans-borylation of vinylboronate with bromosubstituted styrene and Suzuki coupling.

Figure 3:

One-pot synthesis of poly(phenylene vinylenes) (PPVs) via trans-borylation of vinylboronate with bromosubstituted styrene and Suzuki coupling.

The polymer that precipitated during the reaction was filtered off, washed with hexane and acetone to remove low condensed products and the residues of the catalyst and dried under vacuum. The high stereoselectivity as well as linearity of the obtained PPVs polymer are the reasons for the really low solubility of these macromolecular compounds in typical organic solvents. Therefore, their spectroscopic analysis as well as their average molecular weight and polydispersity were difficult to estimate.

The second approach to PAVs, which offers a possibility to incorporate other arenes in the polymer chain, was based on the sequential reaction of vinylboronate homocoupling and Suzuki polycondensation. Homocoupling of vinylboronates leads to bisborylethene, which can be applied as a monomer for the Suzuki polycondensation with dihalogenoarenes.

Table 2 presents the results of homocoupling of different vinylboronates in the presence of catalyst (I). For further investigation, bisborylethene obtained from the pinacolborane vinyl derivative was chosen, as no (Z) isomer was formed in the reaction. A small amount of this undesirable isomer was observed when other vinylbornanes (2-vinyl-1,3,2,-dioxaborolane and 2-vinyl-1,3,2-dioxaborinane) were used as substrates.

Table 2:

Influence of vinylboronate on selectivity and productivity of homocoupling reaction.

Table 2: Influence of vinylboronate on selectivity and productivity of homocoupling reaction.
No. Vinylboronate Product Yielda (%) SelectivitybE/Z
1 (a) (1a) 100 8/1
2 (b) (1b) 88 6/1
3 (c) (1c) 100 1/0

    aReaction yields were calculated on the basis of gas chromatographic (GC) analysis.

    bGC-MS and 1H NMR were used for selectivity determination.

The equimolar ratio of both reagents, bisborylethene and dihalogenoarenes, under the Suzuki polycondensation reaction conditions leads to the formation of macromolecular compounds with vinylene arylene mers. Several dihalogenoarenes, with different functional groups as well heteroatoms in the arene ring, were applied in the polymerization reaction. Two catalysts (III) and (IV) were used for the polymerization, as their use was the most effective in the first method.

According to the above described one-pot procedure for the synthesis of PPVs, a new protocol of polymerization from commercially available reagents was also developed. Bisborylethene – a homocoupling product was directly used, without any isolation, in the Suzuki polycondensation. After borylative homocoupling of vinylboronic acid pinacol ester, THF, dihalogenoarene, the catalyst and base Cs2CO3 were added and the reaction was carried out for 18–24 h, until the total consumption of reagents, which was monitored by GC analysis (see Table 3). We preliminarily checked that Cs2CO3 gave better results than K2CO3 for the coupling of bisborylethenes with dihalogenoarenes.

Table 3:

Synthesis of poly(arylene vinylenes) (PAVs) via homocoupling of vinylboranes and Suzuki coupling.

Table 3: Synthesis of poly(arylene vinylenes) (PAVs) via homocoupling of vinylboranes and Suzuki coupling.

The macromolecular compounds precipitated during the polymerization process due to the fact that PAVs are really poorly soluble in traditional organic solvents. The solubility of PAVs is still the main problem for their potential application in electronic industry, which has been tackled by many research groups. Functionalization of the starting reagents with long alkoxy chains slightly improved the solubility of the synthesized polymers, but still was insufficient to calculate their average molecular weight.

The GPC analyses of the obtained polymers were not representative since only the oligomers with short chains (a few mers) were dissolved in THF used for the analysis. Therefore, we are still thinking about another method for polymers analysis. In the near future, we plan to determine the polymer length (the number of mers in the polymer chain) by the elemental analysis of products, and calculate the residue of halogen (bromine or iodide) in the product.

As π-conjugated polymers are really poorly soluble in organic solvents, it was difficult to accomplish their full spectroscopic characterization. The resonance lines characteristic of protons attached to the C=C sp2 bonds with the coupling constants in the range 15–19 Hz were observed in the 1H NMR spectra recorded mostly in THF-d8 or DMSO-d6. Such coupling constants are typical of the compounds with trans-stereoregularity. The presence of (E)-vinylene functionality was also confirmed by the FT-IR analysis. The CH bending of trans-vinylene was visible in all products in the range of 960–969 cm−1, which are characteristic of these groups. The signals assigned to the functional groups attached to arene as well as from heteroarenes were also detected by FT-IR spectroscopy. It is worth emphasizing that Suzuki polycondensation is tolerant to many functional groups attached to arene in monomer and leads to the synthesis of miscellaneous interesting polymers with various electronic and conducting properties. The electronic and electrochemical characteristics of PAVs obtained by the described procedures will be the subject of a separate article in the near future.

The application of arylene vinylene oligomers and polymers in solar and photovoltaic cells, or in optoelectronics, demands determination of their thermal stability because the devices with PAVs work often in high temperatures. The TG analysis and differential thermal analysis were carried out for selected macromolecular compounds (Figure 4). The comparison of thermal properties of PPVs obtained via both one-pot procedures did not show any significant difference in their properties. This means that both methods can be interchangeably used for the synthesis of PAVs. The temperatures after which there is 5% or 10% loss in mass are similar for both samples (Figure 4A and B). Increased values of these parameters were observed when alkoxy substituents were attached to the phenyl ring (Figure 4C). Not only the solubility of polymers is slightly better, but also their thermal resistance is higher. TG analyses were carried out for most of the synthesized macromolecular compounds. The properties are in agreement with the literature data.

Figure 4: Comparison of thermogravimetric (TG)/differential TG (DTG) analysis of polymers: 2a (A), 3a (B) and 3g (C).

Figure 4:

Comparison of thermogravimetric (TG)/differential TG (DTG) analysis of polymers: 2a (A), 3a (B) and 3g (C).

As follows from a comparison of the one-pot procedures developed within this research with the previously described synthesis of PAVs from bissilylethenes and dihalogenoarenes by Hiyama coupling, the Suzuki polycondensation occurs at a higher rate than the analogous Pd-coupling of silicon compounds. Necessity of the activation of the C-Si bond (increase in the bond nucleophilicity) by TBAF is essential for obtaining high conversion of bissilylethene. This initial step affects the reaction rate. Such an activation is not necessary in the Suzuki coupling and in the applied catalytic condition, the polycondensation occurred immediately.

4 Conclusions

Within this study, we developed two highly selective one-pot procedures based on trans-borylation/Suzuki polycondensation for the synthesis of (E)-PPVs and (E)-PAVs. The first method was based on the borylative coupling of vinylboronates with bromosubstituted styrenes and the polymerization of obtained products via Suzuki coupling. Homocoupling of vinylboronates followed by Suzuki polycondensation with dihalogenoarenes are the reactions used in the second synthetic protocol. In both procedures, the reactions occurred at high reaction rates and the products were obtained with high stereoselectivity towards (E)-configuration of all double C=C bonds. The use of commercially available reagents, simplicity of the procedures, and their high effectiveness are the main advantages of these methods. Moreover, the reduction in large amounts of organic solvents used for the separation of intermediate trans-borylation products, as well as shortening of the process time and lowering costs of the processes make the process more sustainable and greener. Several PAVs with different arene cores were synthesized according to the developed protocols. Their electronic and electrochemical properties will be a subject of a separate publication.

Acknowledgments

This work was supported by the Foundation for Polish Science as part of the HOMING PLUS programme (grant no. HOMING PLUS/2012-5/14) and the National Centre for Research and Development (Poland) (grant no. LIDER/26/p527/L-5/13/NCBR/2014).

References

[1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Science 1995, 270, 1789–1791. Search in Google Scholar

[2] Chen J, Cao Y. Acc. Chem. Res. 2009, 42, 1709–1718. Search in Google Scholar

[3] Grimsdale AC, Leok Chan K, Martin RE, Jokisz PG, Holmes AB. Chem. Rev. 2009, 109, 897–1091. Search in Google Scholar

[4] Kano M, Minari T, Tsukagoshi K. Appl. Phys. Express 2010, 3, 051601. Search in Google Scholar

[5] Lei T, Xia X, Wang J-Y, Liu C-J, Pei J. J. Am. Chem. Soc. 2014, 136, 2135–2141. Search in Google Scholar

[6] Gerard M, Chaubey A, Malhotra B. Biosens. Bioelectron. 2002, 17, 345–359. Search in Google Scholar

[7] Guimard NK, Gomez N, Schmidt CE. Prog. Polym. Sci. 2007, 32, 876–921. Search in Google Scholar

[8] Janata J, Josowicz M. Nat. Mater. 2003, 2, 19–24. Search in Google Scholar

[9] Adam G, Yohannes T, White M, Montaigne A, Ulbricht C, Birckner E, Rathgeber S, Kästner C, Hoppe H, Sariciftci NS. Macromol. Chem. Phys. 2014, 215, 1473–1484. Search in Google Scholar

[10] Cheng Y, Yang S, Hsu C. Chem. Rev. 2009, 109, 5868–5923. Search in Google Scholar

[11] Marks R, Halls J, Bradley D, Friend R, Holmes A. J. Phys. Condens. Matter 1994, 6, 1379–1394. Search in Google Scholar

[12] Burroughes J, Bradley D, Brown A, Marks R, Mackay K, Friend R, Burns P, Holmes A. Nature 1990, 347, 539–541. Search in Google Scholar

[13] Chen J-T, Hsu C-S. Polymer 2013, 54, 4045–4058. Search in Google Scholar

[14] Rodrigues PC, Fontes BD, Torres B, Sousa WS, Faria GC, Balogh DT, Faria RM, Akcelrud L. J. Appl. Polym. Sci 2015, 132, 42579–42586. Search in Google Scholar

[15] Venegoni I, Carniato F, Olivero F, Bisio C, Pira NL, Lambertini VG, Marchese L. Nanotechnology 2012, 23, 435702. Search in Google Scholar

[16] White MS, Kaltenbrunner M, Głowacki ED, Gutnichenko K, Kettlgruber G, Graz I, Aazou S, Ulbricht C, Egbe DA, Miron MC. Nat. Photon. 2013, 7, 811–816. Search in Google Scholar

[17] Becker H, Spreitzer H, Kreuder W, Kluge E, Vestweber H, Schenk H, Treacher K. Synth. Met. 2001, 122, 105–110. Search in Google Scholar

[18] Zheng H, Zheng Y, Liu N, Ai N, Wang Q, Wu S, Zhou J, Hu D, Yu S, Han S. Nat. Commun. 4, 2013, 1–7. Search in Google Scholar

[19] McDonald RN, Campbell TW. J. Am. Chem. Soc. 1960, 82, 4669–4671. Search in Google Scholar

[20] Gilch H, Wheelwright W. J. Polym. Sci. Part A Polym. Chem. 1966, 4, 1337–1349. Search in Google Scholar

[21] Wessling R, Zimmerman R, Patent 3401152, 19661103, Chem. Abstr. 1968, p. 87735. Search in Google Scholar

[22] Lenz RW, Handlovits CE. J. Org. Chem. 1960, 25, 813–817. Search in Google Scholar

[23] Iwatsuki S, Kubo M, Kumeuchi T. Chem. Lett. 1991, 1071–1074. Search in Google Scholar

[24] Staring E, Braun D, Rikken G, Demandt R, Kessener Y, Bouwmans M, Broer D. Synth. Met. 1994, 67, 71–75. Search in Google Scholar

[25] Kumar A, Eichinger B. Macromol. Rapid Commun. 1992, 13, 311–314. Search in Google Scholar

[26] Rehahn M, Schlüter AD. Macromol. Rapid Commun. 1990, 11, 375–379. Search in Google Scholar

[27] Tsukamoto T, Maeyama K, Higashibayashi S, Sakurai H. Chem. Lett. 2015, 44, 1780–1782. Search in Google Scholar

[28] Nojima M, Saito R, Ohta Y, Yokozawa T. J. Polym. Sci. Part A: Polym. Chem. 2015, 53, 543–551. Search in Google Scholar

[29] Greiner A, Heitz W. Macromol. Rapid Commun. 1988, 9, 581–588. Search in Google Scholar

[30] Zou Y, Pan C, Zhang Z, Liu B, He Y, Zhou K, Zhou Y, Li Y. J. Cent. South Univ. Technol. 2010, 17, 269–276. Search in Google Scholar

[31] Prukała W, Pawluć P, Posała K, Marciniec B. Synlett 2008, 2008, 41–44. Search in Google Scholar

[32] Katayama H, Nagao M, Ozawa F, Ikegami M, Arai T. J.Org. Chem. 2006, 71, 2699–2705. Search in Google Scholar

[33] Wakioka M, Ikegami M, Ozawa F. Macromolecules 2010, 43, 6980–6985. Search in Google Scholar

[34] Brinckman F, Stone F. J. Am. Chem. Soc. 1960, 82, 6218–6223. Search in Google Scholar

[35] Brown HC, Basavaiah D, Bhat N. Organometallics 1983, 2, 1309–1311. Search in Google Scholar

[36] Yi CS, Lee DW, Chen Y. Organometallics 1999, 18, 2043–2045. Search in Google Scholar

[37] Barrios-Landeros F, Carrow BP, Hartwig JF. J. Am. Chem. Soc. 2009, 131, 8141–8154. Search in Google Scholar

[38] Majchrzak M, Kostera S, Kubicki M, Kownacki I. Dalton Trans. 2013, 42, 15535–15539. Search in Google Scholar

Received: 2016-11-30
Accepted: 2017-1-23
Published Online: 2017-3-11
Published in Print: 2017-5-24

©2017 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.