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-SO3 H) possesses high sulfonic acid content and good thermal stability. Also, THP-SO3 H 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-SO3 H 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 (ClSO3 H) at 25 °C under N2 atmosphere [Scheme S1 of the Supporting Information (SI)].[6 ] The details of the structural and morphological characterizations of THP-SO3 H are included in the SI.
After characterizing the THP-SO3 H 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 Csp3 –Csp3 and Csp3 –Csp2 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-SO3 H 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 Csp3 –Csp3 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-SO3 H 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 Conditionsa
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-SO3 H. 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-SO3 H (96 mg), DCE (2 mL), 80 °C. (b) 3i –q and 4a –c : 1 (1.0 mmol), 2 (2.0 mmol), THP-SO3 H (96 mg), DCE (2 mL), 80 °C.
Hence, to study the catalytic efficacy of THP-SO3 H 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 Csp3 –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 Csp3 –Csp3 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 Csp3 –Csp3 bond is relatively easier to cleave than Csp3 –Csp2 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-SO3 H 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-SO3 H
The FT-IR spectrum of the reused catalyst after the fifth cycle shows the same pattern as the synthesized THP-SO3 H 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-SO3 H 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-SO3 H has abundant acidic sites due to the presence of a large number of –SO3 H 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 Csp3 –Csp3 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 Csp3 –Csp2 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-SO3 H) 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-SO3 H showed promising catalytic activity in the synthesis of symmetrical TRAMs via the cleavage of both Csp3 –Csp3 and Csp3 –Csp2 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
