1 Introduction

Palladium-based catalysts have become a ubiquitous part of synthetic chemistry because of their widespread application in the synthesis of important natural and medicinal substrates [1,2,3,4,5,6,7,8]. They are successfully employed to promote a wide variety of reactions such as arylation [9,10,11,12,13,14,15,16], alkylation [17,18,19,20,21,22,23,24,25,26,27], hydrogenation [17, 28,29,30,31,32,33,34,35], oxidation [5, 36,37,38], cyclization [39,40,41,42,43,44,45,46], cross-coupling [47,48,49,50,51,52], isomerization [53,54,55,56,57,58,59], and so on. The exceptional catalytic activity of palladium-based catalysts/precatalysts motivates researchers to find new and exciting discoveries every day. Palladium catalysts have also made their impact in the hydrogen borrowing catalysis [60,61,62,63,64,65,66,67,68,69,70,71].

The hydrogen borrowing catalysis has become a green and powerful methodology to construct carbon–carbon and carbon-heteroatom bonds that uses alcohols as alkylating agents [60,61,62,63,64,65,66,67,68,69,70,71]. As this technique has the advantage to use readily available, easy-to-handle and sustainable alcohols as alkylating agents over traditional methods that uses mutagenic alkyl halides. Although, the hydrogen borrowing methodology provides high atom economy and green by-product (water), use of organic solvents limits its sustainability. In the context of developing greener and sustainable synthetic approach; reactions which uses water as the reaction medium, has gained tremendous interest [72,73,74,75,76,77,78]. Water is a cheap, abundant, environmentally friendly, non-toxic and non-flammable unlike organic solvents [79, 80]. In addition, water has a large temperature window, high heat capacity and inherently low oxygen solubility which favors the aerobic reaction with sensitive transition metal catalysts. Furthermore, the high polarity, low viscosity and immiscibility of water makes the reaction workup and product purification easy [81, 82]. Additionally, hydrophobic effect accelerates the organic reactions in water which results in unusual reactivity and selectivity [83,84,85,86].

Immobilization of catalyst makes the separation simpler and offers reusability of the expensive metal catalyst [87]. Among the several supports, polymer anchored palladium catalysts [88,89,90,91,92], especially polystyrene [89] based palladium catalysts have been studied to a greater extent. Polystyrene (PS) anchored palladium catalysts are borderline catalyst which offers the advantages of reusability and simpler purification of the heterogeneous system while retaining selectivity of the homogeneous system. However, in many instances upon longer stirring with organic solvents, polystyrene supported palladium have shown leaching of metal. This problem can be avoided with the use of water as a solvent.

There have been numerous reports for the synthesis of α-alkylated ketones starting from ketones and primary alcohols [93,94,95]. Recently, more abundant secondary alcohols are being used for the β-alkylation of secondary alcohols leading to α-alkylated ketones. However, most of these reports are based on ruthenium- or iridium-based catalysts [96,97,98,99,100,101,102,103,104,105] and are heavily relied on the use of phosphine based ligands and therefore the reaction is carried out under anaerobic and anhydrous condition. Moreover, the application of noble metal catalyst in solvent free or in aqueous condition is also very limited. Realizing the aforementioned needs from a sustainability perspective to apply in hydrogen borrowing catalysis, herein, we report the synthesis and catalytic activity of new polystyrene supported pyrazole-based palladium catalysts/precatalysts with the experience that we gained to utilize pyrazole-based palladacycles for C–C [68, 69] and C–N [67, 69] bond forming reactions via hydrogen borrowing methodology. These catalysts were used in the synthesis of quinoline and α-alkylated ketones from secondary alcohols using double dehydrogenative approach in water. The catalytic recyclability was also studied.

2 Results and Discussion

Our studies to use homogeneous palladacycles as catalysts/precatalysts reveal that -CF3 functionalized triaryl pyrazoles act as better catalysts/precatalysts. Keeping this in mind we designed and synthesized two styryl-substituted monomers M1 and M2 following the literature reported procedure [106]. Reaction of 1,3-diketone with phenylhydrazine (or) (2- (trifluoromethyl)phenylhydrazine in methanol and acetic acid produced the desired triaryl pyrazoles in good yield. The triaryl pyrazoles were brominated using N-bromosuccinimide. The bromopyrazoles thus formed were reacted with 4-vinylphenyl boronic acid under Suzuki reaction condition to afford the styryl pyrazole monomers M1 and M2. The monomers were then subjected to free radical polymerisation using azobisisobutyronitrile (AIBN) as an initiator in 1,2-dichloroethane (DCE) at 80 °C to produce the desired polymers P1 and P2 (Scheme 1a). The monomers M1 & M2 and the polymers P1 & P2 were characterized by 1H & 13C NMR spectroscopy. The 1H NMR of P1 & P2 showed complete disappearance of the AMX multiplet observed in M1 & M2 which indicates the formation of the polymers (Fig. S12 and S15). Gel permeation chromatography (GPC) (with respect to polystyrene standards) gave a molecular weight (Mn) of 23,500 with a PDI = 2.3 for P1 and 62,300 with a PDI of 1.8 for P2. (see SI, Fig. S79 and S80). Insoluble resin IP1 was prepared by reacting monomer M2 with divinylbenzene in 1:2 ratio under free radical polymerisation condition using AIBN as an initiator in DCE at 80 °C (Scheme 1b). All three polymers P1, P2 and IP1 were subjected to palladation in acetic acid at 100 °C for 1 h using palladium acetate resulting in Pd@P1, Pd@P2 and Pd@IP1 respectively. Although, we expected that Pd@P1 and Pd@P2 acts as soluble polymers, to our surprise they turned out to be insoluble in common organic solvents. Infrared spectroscopic studies reveal the presence of carbonyl functional groups at 1411 and 1564 cm−1 for Pd@P1 and at 1411 and 1568 cm−1 for Pd@P2 which are absent in case of P1 and P2 respectively (Fig. S81 and S82). Similar peaks at 1411 and 1589 cm−1 for Pd@IP1 implies the presence of carbonyl group (Fig. 1). Elemental analyses of Pd@P1, Pd@P2 and Pd@IP1 indicated a palladium loading of 1.12, 1.60 and 0.972 mmol/g respectively. These materials were further characterized using 13C cross-polarization magic-angle spinning (CPMAS) (Fig. S17-20). To further support the presence of palladium, the fresh catalyst Pd@IP1 was analyzed using powder XRD. The XRD pattern showed four peaks at 2θ = 40.2°, 46.7°, 68.3°, and 82.3° which can be assigned to (111), (200), (220), and (311) planes of the cubic crystal system (PDF No. 01–087-0639) for the metallic palladium [107,108,109,110]. The broad peaks at 2θ = 10 o and 20 o can be attributed to the C and N atom of the polymeric pyrazole unit (Fig. 1). Similar observations were made for the catalyst Pd@P1 (PDF No. 01–087-0645) and Pd@P2 (PDF No. 03–065-2867) in powder XRD (Fig. S81 and S82). Since Pd@IP1 showed better catalytic activity among the supported catalysts (see-discussion in next section), it was further characterized by transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). The TEM image of the fresh catalyst showed well dispersed nanoparticle with an average particle size in the range of 2–3 nm which indicates that during the metalation process the polymer stabilizes the formation of Pd-nanoparticles (Fig. 2). Further insight into the structural integrity of the nanoparticle were studied using EDX elemental mapping analysis. As seen in the EDX images, Pd atoms are well dispersed over a cloudy substrate of C atoms which is the polymeric pyrazoles (Fig. 2). The chemical state of the Pd atom were investigated using X-ray photoelectron spectroscopy. The Pd 3d spectrum (Fig. 3) shows two different chemical states of the Pd atom. In the fresh catalyst, the lower binding energies (335 eV and 340.2 eV) can be attributed to metallic palladium (Pd(0)) [111,112,113,114]. The higher components of the binding energy (336.5 and 341.5) can be assigned to Pd(II). Further the molar ratio of Pd(0) and Pd(II) present in both fresh & reused catalyst has been found to be 1.081 and 1.005 respectively.

Scheme 1
scheme 1

(a) Synthesis of soluble polymers P1 and P2 and insoluble precatalysts Pd@P1 and Pd@P2, (b) Synthesis of insoluble resin IP1 and pre-catalyst Pd@IP1

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Fig. 1
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IR spectra for IP1 and Pd@IP1 (left). Powder XRD pattern of Pd@IP1 (right; PDF No. 01–087-0639)

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Fig. 2
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TEM images of (a) fresh Pd@IP1; (c) after 7th run of alkylation (Pd@IP1); and (e) after 7th run of quinoline reaction (Pd@IP1). EDX elemental mapping of (b) fresh Pd@IP1; (d) after 7th run of alkylation (Pd@IP1); and (f) after 7th run of quinoline reaction (Pd@IP1

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Fig. 3
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XPS spectra for the Pd 3d of the (left) fresh Pd@IP1 and (right) Pd@IP1 after 7th run of alkylation

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Having synthesized and characterized the catalysts, their catalytic efficiency for C–C bond formation was studied. The reaction of 1-phenylethanol and benzyl alcohol was chosen as a model reaction. Initial screening of the three PS-supported catalysts (Table 1, entry 1–3) revealed that the activity follows Pd@IP1 > Pd@P2 > Pd@P1, which can be attributed to the electronic as well as steric factors as we observed in the homogeneous system [67,68,69]. Among the different bases screened, KOtBu resulted in 59% yield at 100 °C (Table 1, entry 4–7). Further elevation in temperature to 120 °C caused an increase in yield to 76% (Table 1, entry 8). Finally, by using 2 mL of water as the solvent resulted 92% of the α-alkylated ketone (Table 1, entry 9). Decrease of metal loading or base loading resulted in substantial decrease in the product yield (Table 1, entry 10 and 11). The reaction does not proceed without the catalyst (Table 1, entry 12). With the optimised conditions in hand, the scope and limitations of this protocol were screened. Electron donating as well as electron withdrawing groups were well tolerated and resulted in good yields. 4-Methylbenzyl alcohol, 2-methylbenzyl alcohol, 4-methoxybenzyl alcohol and 4-fluorobenzyl alcohol afforded the α-alkylated products in good to excellent yields (81 to 91%) (Table 2, 2b–2e). Varying the secondary alcohol also had little impact on the reactivity of this protocol. 1-(2-Methylphenyl)ethanol and 1-(4-methylphenyl)ethanol afforded the alkylated product in 81 and 79% yield respectively (Table 2, 2f, 2h). 4-Fluoro-α-methylbenzyl alcohol produced the ketone product in 87% yield (Table 2, 2g). Increase of aliphatic chain in the secondary alcohol did not impact the reactivity as 1-phenylpropan-1-ol afforded the desired ketone product in good yield (Table 2, 2i). Cyclic system such as 1,2,3,4-tetrahydronaphthalen-1-ol gave the corresponding α-alkylated product in 75% yield (Table 2, 2k). This protocol has good tolerance towards heteroaryl substituted secondary alcohol such as 1-(thiophen-2-yl)ethan-1-ol which resulted in 71% yield of the ketone product (Table 2, 2j).

Table 1 Optimization for the synthesis of α-alkylated ketone from 2°-alcohol
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Table 2 Substrate screening for the synthesis of α-alkylated ketones from secondary alcohols
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With this success, we focused our attention for the synthesis of quinoline using the optimized conditions and catalyst mentioned above. 2-Aminobenzyl alcohol is made to react with 1.5 equivalent of 1-phenylethanol in aqueous condition (1 mL H2O) which afforded the desired quinoline product in 93% yield. Substitutions such as –CH3 and –OCH3 on the aryl group of secondary alcohol afforded excellent yield of the quinoline product (Table 3, 3b (87%) and 3c (83%). In general, halo substituted compounds at elevated temperature undergoes dehalogenation and thereby making them incompatible in most of the protocols involving palladium catalysis [67,68,69,70,71]. It is worth mentioning that use of aqueous medium helped in this protocol tolerate fluoro and chloro substituted secondary alcohols underthe experimental conditions and produced the quinoline compounds in 72–89% (Table 3, 3d–3g).

Table 3 Substrate screening for the synthesis of quinoline
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Heteroaryl substrate such as 1-thiophenylethanol afforded the quinoline product in 77% isolated yield (Table 3, 3i). 2-Phenyl-1,8-naphthyridine was also prepared using this protocol in 78% yield (Table 3, 3j). It should be noted that 2-methylquinoline was produced in 63% yield (Table 3, 3k) using an excess amount of isopropyl alcohol (0.5 mL). To our delight 3-aminopropan-1-ol was converted to 2-phenylpyridine in 65% yield (Table 3, 3l). 1-Phenylpropan-1-ol which has one more carbon and cyclic secondary alcohol viz., 1,2,3,4-tetrahydronaphthalen-1-ol generated a good yield (Table 3, 81% (3m) and 74%(3n)) of the quinoline products.

The heterogeneous nature of the catalyst was tested by hot filtration. Two reactions were performed separately, in the first reaction, alkylation of alcohol was tested under the optimized conditions after 5 and 24 h. While, in the second case after 5 h the reaction mixture was filtered at hot condition, and the filtrate was further heated for 24 h after adding the required base. The first reaction gave a 93% of the product yield after 24 h, whereas in the second case 43% of product was observed after 5 h which remained same till 24 h (Fig. 4). This result suggest that the reaction does not proceed through the dissolved palladium and thus implies the heterogeneous nature of the catalyst.

Fig. 4
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Recycling experiment (Top). Hot filtration test (Bottom)

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Recycling experiments were performed for both alkylation and quinoline synthesis. After the first run, the catalyst was filtered using glass filter and was washed with water, methanol and acetone three times prior to the next run. The catalyst showed good activity for both the reactions till the fourth run (Fig. 4; yield ranges from 92–89% in case of α-alkylation and 91–88% in case of quinoline synthesis). After the fourth run, a decrease in the yields were observed in both the cases which eventually dropped down to 77 and 81% in case of alkylation reaction and quinoline synthesis respectively.

To understand the reason behind the decrease in the yield, we analysed the spent catalyst Pd@IP1 using different techniques. The TEM reveal a marginal increase in the average particle size after the seventh run of alkylation (4.5–5 nm) and quinoline (3–4 nm) reactions. (Fig. 2). A similar binding energy for Pd@IP1 was observed after 7th run of alkylation using X-ray photoelectron spectroscopy. Elemental analysis reveal that there was considerable palladium loss observed in both the reactions after the seventh cycle (0.972 mmol/g fresh catalyst; 0.834 mmol/g after 7th cycle-alkylation reaction; 0.85 mmol/g after 7th cycle-quinoline reaction). These observations suggest that palladium leaching is responsible for the slight decrease in activity. Presence of oxygen in the EDX elemental analysis further supports the presence of carbonyl group in the catalyst (See Fig. S74, S76, and S78).

3 Conclusion

In conclusion, we have designed and synthesized polystyrene supported palladium catalysts. Among the three catalysts studied, Pd@IP1 showed better activity. The catalysts were characterized using 13C CPMAS NMR, TEM, XPS, XRD and ICP techniques. Effective use of Pd@IP1 in dehydrogenative cross coupling of secondary alcohols which involves synthesis of quinoline and α-alkylated ketones from secondary alcohols using water as a sustainable medium. This protocol tolerates different substrates including halogen substituted aryl compounds. Interestingly, the recycling experiments reveal that the catalyst is stable under the experimental conditions with slight decrease in yield which is due to the metal leaching. Further studies to use this catalytic system for different organic transformation are currently in progress.

4 Experimental Section

4.1 General Information

Reagents and starting materials were purchased from Alfa-Aesar, Sigma-Aldrich and Spectrochem chemical companies and used as received unless otherwise noted. Chlorinated solvents were distilled from CaH2. THF was distilled from Na/benzophenone prior to use. 1,3,5-Triphenyl-1H-pyrazole and 3,5-diphenyl-1-(2-(trifluoromethyl)phenyl)-1H-pyrazole were prepared according to literature procedure15. All 400 or 700 MHz 1H and 100 or 176 MHz 13C NMR, and 377 MHz 19F spectra were recorded on a spectrometer operating at 400 or 700 MHz referenced internally to solvent signals. 19F NMR spectra were externally referenced to α,α,α- trifluorotoluene in CDCl3 (δ = -63.73 ppm). High-resolution mass spectra (HRMS) were recorded on a Bruker microTOF-QII mass spectrometer. Gel permeation chromatography (GPC) analyses were performed on a Shimadzu-LC20AD system referenced to poly(styrene) standards. THF was used as the mobile phase with a flow rate of 1.0 mL min−1. Morphological study and elemental mapping analyses of the samples were performed using a transmission electron microscope (equipped with HRTEM, JEOL 2100F, operated at 200 kV). Particle size was measured using ImageJ software. The elemental composition of the synthesized catalysts were verified by using an inductively coupled plasma-optical emission spectrophotometer (iCAP 7000 ICP-OES). The powder X-ray diffraction data were collected on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) as the X-ray source. XPS measurements were carried out using Thermo Kalpha + spectrometer using micro focused and monochromated AlKα radiation with energy 1488.6 eV.

Synthetic procedure for tetraarylpyrazole M1: Monomer M1 was prepared by following the literature reported method [1,2,3,4,5,6,7,8]. The quantities involved are as follows. Bromopyrazole 3 (1.00 g, 2.67 mmol), Pd(PPh3)4 (0.09 g,0.08 mmol) Na2CO3 ( 0.56 g, 5.34 mmol) and 4-vinylbenzeneboronic acid (0.47 g, 3.20 mmol) were taken in a 100 mL two neck round bottom flask. The whole set up was evacuated and nitrogen was purged into it. A degassed dimethoxyethane and water in 20:8 was added to it and was refluxed for 24 h. Aqueous layer was separated and the organic layer was collected using dichloromethane (3 × 20 mL). The organic phase was dried with Na2SO4 and concentrated under vacuum. After purification by column chromatography, the compound was isolated as white solid (0.76 g, 72%). 1H NMR (400 MHz, CDCl3) δ 7.55–7.47 (m, 2H, Ar–H), 7.32–7.26 (m, 7H, Ar–H), 7.25–7.21 (m, 4H, Ar–H), 7.21–7.16 (m, 2H, Ar–H), 7.04 (t, J = 8.0 Hz, 4H, Ar–H), 6.65 (dd, J = 17.6, 10.9 Hz, 1H, Ph-CH = CH2), 5.69 (d, J = 17.6 Hz, 1H, Ph-CH = CH2), 5.19 (d, J = 10.9 Hz, 1H, Ph-CH = CH2) ppm. 13C NMR (101 MHz, CDCl3) δ 150.37(CPz), 141.54(CPz), 140.01(CPz), 136.75(Ph-CH = CH2), 135.86(CHAr), 133.19 (CAr), 132.76 (CAr), 130.90(CHAr), 130.59(CHAr), 130.17(CAr), 128.91(CHAr), 128.62(CHAr), 128.47(CHAr), 128.37(CHAr), 127.84 (CAr), 127.41 (CAr), 126.24(CHAr), 125.49(CHAr), 120.45(CHAr), 113.67 (Ph-CH = CH2) ppm.

Synthetic procedure for tetra aryl pyrazole M2: Monomer M2 was prepared using the procedure used for the preparation of M1. The quantities involved are as follows: Bromopyrazole 4 (1.5 g, 3.38 mmol), Pd(PPh3)4 (0.12 g, 0.10 mmol), Na2CO3 (0.72 g, 6.76 mmol) and 4-vinylbenzeneboronic acid (0.60 g, 4.05 mmol). After purification by column chromatography, the compound as isolated as white solid (1.02 g, 65%). 1H NMR (700 MHz, CDCl3) δ 7.78–7.72 (m, 1H, Ar–H), 7.59–7.53 (m, 2H, Ar–H), 7.51 (t, J = 6.9 Hz, 2H, Ar–H), 7.42–7.37 (m, 1H, Ar–H), 7.32–7.26 (m, 5H, Ar–H), 7.20–7.04 (m, 7H, Ar–H), 6.69 (dd, J = 17.6, 10.9 Hz, 1H, Ph-CH = CH2), 5.73 (d, J = 17.6 Hz, 1H, Ph-CH = CH2), 5.23 (d, J = 10.9 Hz, 1H, Ph-CH = CH2) ppm. 19F decoupled 13C NMR (176 MHz, CDCl3) δ 150.30(CPz), 143.53(CPz), 137.82(CPz), 136.71(Ph-CH = CH2), 135.94(CHAr), 133.10(CAr), 132.65(CAr), 132.29(CHAr), 131.04(CHAr), 130.91(CHAr), 130.4(CAr), 129.46(CHAr), 129.31(CHAr), 128.59(CHAr), 128.32(CHAr), 128.27(CHAr), 127.85(CHAr), 126.26(CHAr), 124.47(CAr), 121.74(CAr), 119.81(CHAr), 113.70(Ph-CH = CH2) ppm. 19F NMR (376 MHz, CDCl3) δ -59.61 ppm.

Synthetic procedure for soluble polymeric tetra aryl pyrazole P11: A schlenk tube was charged with the monomer M1 (0.500 g, 1.25 mmol) and free radical initiator azobisisobutyronitrile (0.004 g, 0.02 mmol). The entire system was purged with nitrogen followed by the addition of 1 mL of DCE. The entire system was degassed using the freeze–pump–thaw cycle for three times. Then the reaction was stirred for 24 h at 80 °C. Then the reaction mixture slowly added to the distilled hexane in order to precipitate the polymeric material. The precipitate was then dissolved in dichloromethane and reprecipitated from the hexane. This process was reiterated three times. The resulting solid was dried under high vacuum to obtain the white solid. Yield = 0.400 g, (80%). 1H NMR (400 MHz, CDCl3) 7.6–5.9 (aromatic H) and 2.39–0.51 (polymeric backbone) ppm. 13C NMR (101 MHz, CDCl3) δ 150.16, 141.18, 139.90, 133.37, 130.32, 128.81, 128.24, 127.70, 127.24, 125.25, 120.37, 39–41 (polymeric backbone) ppm.

Synthetic procedure for soluble polymeric tetra aryl pyrazole P2: Polymer P2 was prepared following the procedure used for the preparation of P1. The quantities involved are as follows: Monomer M2 (0.5 g, 1.07 mmol) and azobisisobutyronitrile (0.004 g, 0.02 mmol). Yield = 0.38 g (76%). 1H NMR (400 MHz, CDCl3) 7.8–5.9 (Aromatic H) and 2.12–0.57 (b, polymeric backbone) ppm. 13C NMR (176 MHz, CDCl3) δ 150.04, 143.22, 137.63, 133.12, 132.17, 130.80, 130.14, 129.36, 129.05, 128.02, 127.57, 125.36, 123.80, 122.25, 120.70, 119.79 and 39–41 (polymeric backbone) ppm. 19F NMR (376 MHz, CDCl3) δ -59.48 ppm.

Synthetic procedure for insoluble resin IP1: A 100 mL two neck round bottom flask was charged with the monomer M2 (0.5 g, 1.07 mmol) and azobisisobutyronitrile (0.004 g, 0.02 mmol). Divinyl benzene (0.28 g, 2.14 mmol) was added to it. The set up was purged with nitrogen and then 1 mL DCE was added to it. The whole set up was degassed using freeze–pump–thaw cycle for three times. The reaction was stirred at 80 °C for 48 h which resulted in the formation insoluble yellow compound. The insoluble compound was transferred to a frit connected to a conical flask and was washed three times with dichloromethane, methanol, and acetone. The yellowish white compound obtained was grounded to a fine powder using mortar and pestle and was dried under high vacuum. Yield = 0.47 g, (94%). 13C CPMAS NMR (101 MHz) δ 153.47–109.49 and 62.77–23.49 (polymeric backbone) ppm.

Synthetic procedure for catalyst Pd@P1: 0.5 g of polymer P1 was suspended in 10 mL acetic acid at 100 °C. Palladium acetate (0.28 g) was added to it and was stirred for 1 h. Then insoluble precipitates were filtered from the reaction mixture under hot condition. The residue was again washed with dichloromethane, methanol, and acetone three times each. Then the brownish black solid obtained was grounded using mortar and pestle and was dried under high vacuum. Palladium content of the catalyst was estimated from ICP-AES analysis using HCl:HNO3:H2O2 (37%) in 3:1:1 ratio as the digesting solution under microwave digestion. Palladium loading = 1.12 mmolg−1. 13C CPMAS NMR (101 MHz) δ 152.35–108.76 and 60.43–20.35 (polymeric backbone) ppm.

Catalyst Pd@P2 was prepared following the similar procedure as used for Pd@P1. Palladium loading = 1.6 mmolg−1. 13C CPMAS NMR (101 MHz) δ 152.60–106.33 and 51.88–20.13 (polymeric backbone) ppm.

Catalyst Pd@IP1 was prepared following the similar procedure as used for Pd@P1. Palladium loading = 0.972 mmolg−1. 13C CPMAS NMR (101 MHz) δ 161.11–106.33 and 57.58–10.42 (polymeric backbone) ppm.

General procedure for alkylation reaction: Pd@IP1 (1 mol% of Pd), KOtBu (0.5 mmol), primary alcohol (0.5 mmol) and secondary alcohol (0.75 mmol) were added to a seal tube. Deionised water (2 mL) was added to it and the tube was sealed. The reaction was stirred at 120 °C. After 24 h, the reaction mixture was filtered using a filter paper. Aqueous layer was separated from the reaction mixture and the organic layer was collected using dichloromethane (3 × 10 mL). The organic layer was dried on Na2SO4 and concentrated under vacuum. Product was purified by column chromatography.

General procedure for quinoline synthesis: Pd@IP1 (1 mol% of Pd), KOtBu (0.5 mmol), 2-aminobenzyl alcohol (0.5 mmol) and secondary alcohol (0.75 mmol) were added to a seal tube. Deionised water (1 mL) was added to it and the tube was sealed. The reaction was stirred at 120 °C. After 24 h, the reaction mixture was filtered using a filter paper. Aqueous layer was separated from the reaction mixture and the organic layer was collected using dichloromethane (3 × 10 mL). The organic layer was dried on Na2SO4 and concentrated under vacuum. Product was purified by column chromatography.

General procedure for recycling experiment: Pd@IP1 (1 mol% Pd), KOtBu (10 mmol), primary alcohol/aminobenzyl alcohol (10 mmol) and secondary alcohol (15 mmol) were added to a seal tube. Deionised water (15 mL) was added to it. Then the tube was sealed and the reaction was stirred at 120 °C. After 24 h, the reaction mixture was filtered using frit apparatus. Then the catalyst was washed with water to remove excess base followed by washing with dichloromethane, methanol and acetone three times each. The catalyst was the dried under high vacuum for 3 h and reused for the next cycle. This process was reiterated 7 times.