Sulfonated Tetraphenylethylene-Based Hypercrosslinked Polymer as a Heterogeneous Catalyst for the Synthesis of Symmetrical Triarylmethanes via a Dual C–C Bond-Cleaving Path

Abstract

A sulfonic acid functionalized tetraphenylethylene-based hypercrosslinked polymer (THP-SO₃H) with a well-developed porous network and accessible sulfonic acid sites was synthesized and characterized by different analytical techniques. The catalytic prowess of the synthesized material THP-SO₃H was investigated in a challenging dual C–C bond-breaking reaction for the synthesis of symmetrical triarylmethanes (TRAMs) in high yield. The scope of the developed metal-free method was also explored with a wide variety of substrates. The organocatalyst can be easily recovered by filtration and reused up to five consecutive cycles without substantial loss in its catalytic efficacy.

Key words

hypercrosslinked polymer - heterogeneous catalyst - reusable - C–C bond breaking - triarylmethane

Functionalization of porous organic polymers has provided an exciting platform for catering to a varied array of applications such as adsorption, energy applications, catalysis, etc.[1] The high thermal and chemical stability, high specific surface areas, and hierarchical pore networks of porous organic polymers have attracted significant attention over recent decades.[2] Moreover, they have an added advantage of the ease of preparation and functionalization using mild synthetic conditions. The proper selection of monomer units further aids in designing polymer materials with tailor-made properties for different applications. Based on the reactions involved in the synthetic process, porous organic polymers may be further classified into the categories such as hypercrosslinked polymers, microporous organic polymers, covalent organic framework, etc.[3] Hypercrosslinked polymers possessing high surface areas, inherent porosity, good chemical and thermal stability, and a rigid skeleton for easy incorporation of catalytic sites have been proven to be a promising candidate for their use as heterogeneous catalysts.[4] Heterogeneous catalysts are being gradually preferred over classical homogeneous catalysts owing to their noncorrosiveness, ease of recovery, and reusability.[5] However, common limitations of heterogeneous catalysts include high cost, low yields, and catalyst poisoning in a hydrophilic environment. In this regard, the effective design of hypercrosslinked polymer-based catalysts may help to combine the advantages of both heterogeneous and homogeneous catalysts. Acid-functionalized hypercrosslinked polymers have been recently utilized as heterogeneous catalysts, mostly for the conversion of biomass into biofuel,[6] biofuel additives,[7] and hydroxymethylfurfural (HMF).[8] Bhaumik and his group have recently employed a hypercrosslinked supermicroporous polymer as a heterogeneous catalyst for synthesizing biodiesel.[6] However, the scope of such functionalized polymer-based catalysts has not been investigated much in other organic synthetic applications.

For example, organic reactions proceeding through unusual yet challenging C–C bond-breaking reactions still await the same level of interest as C–C bond-forming counterparts. Over the past years, several catalytic routes for the cleavage of the C–C bonds have been developed by researchers.[9] Our interest on exploring new catalytic approaches for the activation of C–C bonds lead us to develop a facile FeCl₃-catalyzed dual C–C bond-breaking reaction in homogeneous medium for the synthesis of symmetrical and unsymmetrical triarylmethanes (TRAMs).[10] Inspite of tremendous reactivity and selectivity of homogeneous catalysts, it has limited applications in industrial processes due to the difficulties associated with the removal, recovery, and recycling of the active catalysts. Till date, several methods are reported in the literature on the synthesis of TRAMs.[11] To the best of our knowledge, the synthesis of TRAMs via a dual C–C bond-breaking reaction of diarylmethyl-substituted 1,3-dicarbonyl derivatives 1 has not been attempted with heterogeneous catalysts. In this regard, acid-functionalized hypercrosslinked polymers can be used as reusable heterogeneous catalyst for the synthesis of TRAMs. Here, we have synthesized a sulfonic acid functionalized hypercrosslinked polymer derived from tetraphenylethylene (TPE). Hypercrosslinked polymers have been previously used by researchers as polymeric supports due to their porous natures and high thermal stabilities.[12] However, the choice of monomer units heavily influences the physico-chemical characteristics of the polymer. The tetra­phenylethylene (TPE) moiety consists of peripheral phenyl rings, which prevents the π–π stacking of its polymerized form. Hence, the surface area of TPE-based hypercrosslinked polymers is usually very high and provides a suitable platform for the incorporation of numerous catalytic sites. Moreover, increasing the crosslinking between the monomer units improves the thermal stability of the polymeric catalyst. The synthesized TPE-based hypercrosslinked polymer (THP) has a high surface area and optimum pore dimensions, which make it a suitable framework for introducing sulfonic acid sites. The sulfonated polymer (THP-SO₃H) possesses high sulfonic acid content and good thermal stability. Also, THP-SO₃H shows high potential as a heterogeneous catalyst for the synthesis of symmetrical TRAMs via a dual C–C bond-breaking reaction of diarylmethyl-substituted 1,3-dicarbonyl derivatives 1. It is noteworthy to mention that TRAM skeleton is found in several natural products, pharmaceuticals, dyes, etc.[13] Moreover, the application of THP-SO₃H as a reusable heterogeneous catalyst in the dual C–C bond-breaking reaction enhances the practical utility of the present work.

The tetraphenylethylene-based hypercrosslinked polymer (THP) scaffold was synthesized via a simple Friedel–Crafts-based crosslinking reaction with tetraphenylethylene as the monomer and formaldehyde dimethylacetal as the crosslinker.[14] Sulfonation of the polymer was successfully carried out in chlorosulfonic acid (ClSO₃H) at 25 °C under N₂ atmosphere [Scheme S1 of the Supporting Information (SI)].[6] The details of the structural and morphological characterizations of THP-SO₃H are included in the SI.

After characterizing the THP-SO₃H material, we were interested to explore its catalytic prowess for the synthesis of symmetrical TRAMs via a challenging dual C–C bond-cleaving reaction of diarylmethyl-substituted 1,3-dicarbonyl derivatives 1 in a heterogeneous medium. To investigate the optimized reaction conditions, we chose 1,3-diphenyl-2-[phenyl(2,4,6-trimethoxyphenyl)methyl]propane-1,3-dione (1a) and 2-methylfuran (2a) as the model substrates for the synthesis of symmetrical TRAM 3a by the cleavage of both C_sp3–C_sp3 and C_sp3–C_sp2 bonds in substrate 1a (Table [1]). Initially, we screened different solvents (entries 1–5) for the dual C–C bond cleavage in the presence of THP-SO₃H to find out the suitable solvent for the reaction. Among different solvents, the highest yield of the symmetrical TRAM 3a was obtained in DCE solvent using the synthesized organocatalyst at 80 °C in 30 min (entry 3). The reaction gave comparably lower yield of the desired TRAM 3a in MeNO₂, MeCN, and toluene solvents (entries 1, 2, 4). However, polar protic solvent, such as EtOH, gave poor yield of 3a even after 3 h (entry 5). The temperature also played a significant role in the dual C–C bond-breaking reaction. When we decreased the temperature from 80 °C to 55 °C, only 78% yield of the desired product 3a was obtained after 3 h (entry 6). Further decreasing the temperature to room temperature, product 3a was obtained only in 52% yield after 5 h (entry 7). Unlike our previous report,[10] we did not isolate any unsymmetrical TRAM via the cleavage of C_sp3–C_sp3 bond only,[15] resulting dibenzoylmethane as the carbon-based leaving group. In addition, we varied the catalyst loading to determine the optimum amount of THP-SO₃H for the dual C–C bond cleavage in the reaction. It is noteworthy that the use of 96 mg catalyst at 80 °C produced the maximum yield of the product 3a in 30 min (entry 3). An increase in the amount of catalyst loading (144 mg) did not affect the yield of the reaction significantly (entry 8). But a lower catalyst loading (48 mg) resulted in lesser yield of the product 3a (entry 9). Besides, no dual C–C bond-breaking reaction was noticed in the absence of catalyst, and the starting materials were recovered quantitatively (entry 10). It is to be noted that the leaving 1,3-diphenylpropan-1,3-dione and 1,3,5-trimethoxybenzene were isolated in more than 90% yields (entry 3).

Table 1 Optimization of Reaction Conditions^a

Entry	Solvent	Temp. (°C)	Time (min)	Yield (%)b
1	MeNO2	80	45	89
2	MeCN	80	60	81
3	DCE	80	30	94
4	toluene	80	90	64
5	EtOH	80	180	56
6	DCE	55	180	78
7	DCE	RT	300	52
8c	DCE	80	30	96
9d	DCE	80	30	78
10e	DCE	80	60	nil

^a Reaction conditions: 1a (480 mg,1.0 mmol), 2a (246 mg, 3.0 mmol), catalyst (96 mg), and solvent (2 mL).

^b Isolated yields.

^c Catalyst (144 mg).

^d Catalyst (48 mg).

^e No catalyst.

After optimizing the reaction conditions, we explored the substrate scope of the developed method with our in-house synthesized organocatalyst THP-SO₃H. From our previous work, we experienced that the combination of 1,3-diphenylpropan-1,3-dione (as the 1,3,-dicarbonyl substituent) and 2,4,6-trimetheoxyphenyl unit (as the electron-rich arene substituent) in the starting substrates 1 showed the best results during dual C–C bond-breaking reaction.[10]

Scheme 1 Substrate scope of the C–C bond-breaking reaction for the synthesis of TRAMs. Reagents and conditions: (a) 3a–h: 1 (1.0 mmol), 2 (3.0 mmol), THP-SO₃H (96 mg), DCE (2 mL), 80 °C. (b) 3i–q and 4a–c: 1 (1.0 mmol), 2 (2.0 mmol), THP-SO₃H (96 mg), DCE (2 mL), 80 °C.

Hence, to study the catalytic efficacy of THP-SO₃H for the synthesis of symmetrical TRAMs, we varied only the R group in 1, keeping 1,3-diphenylpropan-1,3-dione and 2,4,6-trimetheoxyphenyl units intact in the precursor 1 (Scheme [1]). We found that the aromatic ring bearing EWG in substrate 1 generated slightly higher yields of the symmetrical TRAMs 3b–d than the aromatic ring bearing EDG 3e,f. A heteroaryl substituent in substrate 1g provided the corresponding product 3g in 71% yield. Furthermore, the reaction performed well when 2,5-dimethylfuran (2b, Figure [1]) was used as nucleophile producing the desired TRAM 3h in good yield. Then, we examined other nucleophiles based on their performance in the dual C–C bond-cleaving reaction. In the presence of indole derivatives 2c–i, the corresponding bisindolylmethanes 3i–o were obtained in good yield. Surprisingly, 5-methoxyindole (2g) gave a lower yield of the desired bisindolylmethane derivative 3m after prolonged reaction time. The N-substituted indoles 2h,i were effective in the dual C–C bond-cleaving reaction generating the symmetrical TRAMs 3n,o in excellent yields. 2-Methylthiophene (2j) also reacted well with substrate 1a to give the symmetrical TRAM 3p in 79% yield. It is important to note that 4-methoxythiophenol (2k) took part in the reaction producing 55% yield of the product 3q with two new C_sp3–S bonds at the cost of two C–C bonds. While examining the scope of nucleophiles, we noticed that the symmetrical TRAM did not form during the reactions between substrate 1a and nucleophiles 2l–n via the dual C–C bond-breaking reaction. Instead, we isolated unsymmetrical TRAMs 4 due to the exclusive C_sp3–C_sp3 bond-cleaving reaction of 1a in the presence of the above-mentioned nucleophiles. We previously noted the similar observations in our FeCl₃-catalyzed dual C–C bond-breaking work.[10] These results indicate that the C_sp3–C_sp3 bond is relatively easier to cleave than C_sp3–C_sp2 bond in substrate 1 in our developed reaction conditions.[16]

Figure 1 List of nucleophiles used in the dual C–C bond cleavage reaction

In view of commercial applications, we also explored the potential for reusability of the synthesized organocatalyst THP-SO₃H in the dual C–C bond-breaking reaction of substrate 1a and 2a. After 30 min of the reaction time in each cycle, the catalyst was recovered by filtration, washed with DCE, and dried at 100 °C under vacuum oven for 3 h (the detailed procedure is given in the SI). The recovered catalyst was reused for four consecutive cycles without significant loss in its catalytic efficiency (Figure [2]).

Scheme 2 Plausible reaction mechanism for the reaction between 1a and 2a catalyzed by THP-SO₃H

The FT-IR spectrum of the reused catalyst after the fifth cycle shows the same pattern as the synthesized THP-SO₃H material (Figure S7 of the SI), which indicates that the active site of the heterogeneous catalyst remains intact even after the fifth cycle.

Figure 2 Recyclability of THP-SO₃H catalyst in the dual C–C bond-breaking reaction

Based on the above observations and the related literature about the dual C–C bond-breaking reaction, we propose a plausible reaction mechanism for the reaction between substrate 1a and 2a in Scheme [2]. THP-SO₃H has abundant acidic sites due to the presence of a large number of –SO₃H groups at the surface of the hypercrosslinked polymer and provides ample H⁺ ion in the reaction medium. The reaction may be initiated by the activation of carbonyl groups of the starting material 1a in the presence of H⁺ to produce a species A.[10] 2-Methylfuran (2a) combines with the electrophilic species A′ to form unsymmetrical TRAM B and consequently releases 1,3-diphenylpropan-1,3-dione by the cleavage of C_sp3–C_sp3 bond. The electron-donating –OMe group of the species B undergoes conjugation and gets protonated in the acidic medium to generate an ionic species C which may decompose to species D via the elimination of 1,3,5-trimethoxybenzene.[17] Subsequently, a second molecule of 2a reacts with the electrophilic center in species D to produce the desired symmetrical TRAM 3a via a C_sp3–C_sp2 bond-breaking reaction, and a proton is consequently released in the reaction medium.

In conclusion, we have synthesized a novel sulfonic acid functionalized tetraphenylethylene-based hypercrosslinked polymer (THP-SO₃H) with a porous network and accessible sulfonic acid sites. Due to the abundant accessible acidic sites in the material, its catalytic property was examined on a dual C–C bond-breaking reaction in diarylmethyl-substituted 1,3-dicarbonyl derivatives. THP-SO₃H showed promising catalytic activity in the synthesis of symmetrical TRAMs via the cleavage of both C_sp3–C_sp3 and C_sp3–C_sp2 bonds in mild reaction conditions.[18] The generality of the reaction was explored on a diverse range of substrates, and the desired product was obtained in high yield. Due to its heterogeneity in the reaction medium, the catalyst could be recycled for further use. The catalyst was reused up to five reaction cycles without any substantial decrease in its catalytic efficiency. The results described here demonstrate the first-ever synthesis of symmetrical TRAMs via a metal-free, dual C–C bond-breaking strategy using sulfonated tetraphenylethylene-based hypercrosslinked polymer as a heterogeneous catalyst.

Figure 3

Acknowledgment

The NMR Research Centre, IISc Bangalore; SAIC, TU; Centre for Energy, IIT Guwahati; MSE, IIT Kanpur and SAIF, IIT Bombay are also acknowledged for analytical facilities.

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• 18 Typical Procedure for the Synthesis of 3aA 25 mL round-bottomed flask equipped with a magnetic bar and water condenser were charged with 1a (1.0 mmol), 2a (3.0 mmol), DCE (2.0 mL), and THP-SO₃H (96 mg) in an air atmosphere. The flask was placed in a constant temperature oil bath at 80 °C, and the progress of the reaction was monitored by TLC. After 30 min, the mixture was filtered to separate the catalyst and washed twice with DCE (2 × 5 mL). Then the filtrate was removed under reduced pressure, and the crude product was purified by dry column vacuum chromatography (silica gel G, petroleum ether 60–80 °C/EtOAc) to give a yellow oily liquid; yield 94%.¹H NMR (400 MHz, CDCl₃): δ = 2.158 (s, 6 H), 5.256 (s, 1 H), 5.788 (d, J = 3.2 Hz, 4 H), 7.159–7.243 (m, 5 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 13.65, 45.12, 106.08, 108.19, 126.97, 128.40, 128.44, 140.00, 151.46, 152.85 ppm.

Publication History

Received: 12 July 2020

Accepted after revision: 01 October 2020

Accepted Manuscript online:
01 October 2020

Article published online:
27 November 2020