Ionic tagged amine supported on magnetic nanoparticles: synthesis and application for versatile catalytic Knoevenagel condensation in water

Abstract

Propylamine modified with imidazolium ionic moiety grafted onto magnetic nanoparticles (MNPs) was prepared and evaluated as a catalyst for Knoevenagel condensation in water at room temperature. The catalyst was efficient in the reaction to give the condensation products in good yields. It is worth noting that the ionic-tagged catalyst performed significantly better than its ionic tag-free counterpart. Finally, the catalyst could be reused for 8 times with a slight loss in its catalytic activity.

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1. Introduction

The Knoevenagel condensation of aldehydes with acidic active methylene compounds represents one of the most fundamental carbon–carbon formation reactions in the field of organic transformations.^1–6 The corresponding condensation products are electrophilic trisubstituted alkenes, which have been widely used not only as starting materials for Michael addition⁷ and domino or one-pot reactions,⁸ but also as important intermediates for the preparation of functional polymers, fine chemicals and pharmaceutical drugs.⁹ Traditionally, this type of condensation reactions are performed in homogeneous solution in the presence of organic bases as well as their salts as catalysts, including dimethylamino pyridine, piperidine, guanidine and ethylenediamine.¹⁰ These methods are obviously limited by the difficulties of catalyst separation and recyclability. Therefore, several methodologies for the Knoevenagel condensation catalyzed by heterogeneous catalysts were developed, in which the catalysts are readily recoverable and are recycled. For example, MgO/ZnO,^11a amine-functionalized polyacrylonitrile fiber,^11b,c La₂O₃/MgO,^11d Ni–SiO₂,^11e Si-MCM-41 supported basic materials^11f and IRMOF-3 (ref. 11g) have been tested to catalyze the condensation reactions. Each of the abovementioned methods has its own merits with at least one of the limitations of low yields, narrow substrate scope, harsh reaction conditions and the involvement of large amounts of harmful and toxic organic solvents. Hence, improved methods for the Knoevenagel condensation using inexpensive, readily available catalysts and less scavenger reagents coupled with easier work-up procedures are highly desirable.

Magnetic nanoparticles (MNPs) are extensively studied as efficient supports for immobilizing homogeneous catalysts for various organic transformations because of their interesting properties such as high surface area and paramagnetic properties, facilitating their separation from reaction media after magnetization with an external permanent magnetic field.¹² MNPs-supported guanidine was developed and used to catalyze the cyanosilylation of carbonyl compounds to achieve good to high yields of the desired products.¹³ In addition, the nanocatalyst can be reused up to 20 times. Gawande and coworkers reported a sulfonic acid catalyst grafted on magnetite and subsequently served as a highly efficient magnetic catalyst for the Ritter and multicomponent reactions.¹⁴ Meanwhile, a new concept of ion-tag strategy that can be achieved by installing an ionic substituent on the catalyst has been presented.¹⁵ Catalysts optimized and designed according to the ion-tag strategy have three advantages, including water compatibility, readily recyclable and higher catalytic activities compared to the parent tag-free catalysts, which, to some extent, is a working hypothesis. Thus, a series of novel ionic tagged catalysts have emerged and have been exploited in versatile organic transformations such as aza-Michael addition,¹⁶ selective hydroformylation,¹⁷ olefin reduction,¹⁸ Suzuki–Miyaura reaction,¹⁹ asymmetric Henry reaction,²⁰ enantioselective α-aminoxylations of ketones and aldehydes,²¹ asymmetric allylic etherification,²² and asymmetric addition.²³ Based on the unique properties of magnetic nanoparticle support and ion tag of ionic liquid, a new model for the design of efficient and recoverable catalysts has been developed via the combination of magnetite and ionic liquid (Scheme 1). Consequently, with the principle for the construction of catalysts in hand and in continuation of our research endeavors on the development of environmentally benign and sustainable chemical processes,²⁴ herein, we report a novel MNPs supported amine with ionic tag and its application as a highly efficient and magnetically recoverable catalyst for the versatile Knoevenagel condensations.

Scheme 1 A model for the design of novel catalysts.

2. Results and discussion

2.1 Preparation and characterization of catalysts

The magnetic nanoparticle supported catalyst 4 (MNPs-IT-PA) was prepared in multiple steps from several commercially available chemicals (Scheme 2). SiO₂@Fe₃O₄ 1 (MNPs) was synthesized by a chemical co-precipitating method reported in the literature,²⁵ followed by silica coating.²⁶ The outer silica shell can not only prevents the aggregation of nanoparticles but also provides suitable surface Si–OH groups for further functionalization.²⁷ N-(3-propyltriethoxysilane) imidazole 2 was prepared via the condensation of imidazole and 3-chloropropyltriethoxysilane, and was then grafted on the MNPs to give the supported imidazole 3. Subsequently, 3 was refluxed with 3-bromine propylamine in dry toluene under nitrogen atmosphere to afford the final catalyst MNPs-IT-PA 4. For comparison, ion-free counterpart 6 (MNPs-PA), in which 3-aminopropyltriethoxysilane 5 was directly immobilized onto the magnetic nanoparticles, was prepared, as outlined in Scheme 2.

Scheme 2 Preparation of magnetic nanoparticle supported catalyst MNPs-IT-PA 4 and MNPs-PA 6.

The transmission electron microscopy (TEM) images in Fig. 1 show that MNPs (a), MNPs-PA (b) and MNPs-IT-PA (c) are uniformly sized magnetic nanoparticles with an almost spherical morphology. The diameter of the all the three kinds of particles is approximately 16 nm, which is significantly larger than naked Fe₃O₄ particles. The similar size and morphology of blank support (a) and amine anchored (b and c) implies that the immobilization process did change the magnetic ferrite core, and the modification reaction for catalyst loading only occurred on the silica shell surface.

Fig. 1 TEM images of MNPs (a), MNPs-PA (b) and MNPs-IT-PA (c).

The X-ray diffraction (XRD) spectra of anchored catalysts MNPs-IT-PA, MNPs-PA and MNPs have the same typical peaks with respect to their positions and relative intensities (Fig. 2). A broad peak from 2θ = 18° to 28° is attributed to the amorphous silica shell formed surrounding the Fe₃O₄ core.²⁸ The peaks at 30.07°, 35.41°, 43.04°, 53.39°, 56.91°, and 62.49° correspond to the (220), (311), (400), (422), (511), and (440) reflections of the crystalline cubic spinel structure of standard naked Fe₃O₄ sample (JCPDS 19-0629), respectively. As shown in Fig. 2, the phase composition of MNPs remained intact after surface modification on nanoparticles.

Fig. 2 XRD spectra of MNPs (a), MNPs-PA (b) and MNPs-IT-PA (c).

Successful functionalization of MNPs can be verified from the FT-IR spectra shown in Fig. 3.

Fig. 3 FT-IR spectra of MNPs (a), MNPs-PA (b) and MNPs-IT-PA (c).

In Fig. 3, the band at 579 cm⁻¹ was assigned to the characteristic Fe–O vibrations and confirmed the existence of Fe₃O₄ component in all the three samples. A broad peak at around 3400 cm⁻¹ is attributed to the Si–OH group. Moreover, the presence of Si–O bond in three samples is evident from two characteristic vibrations at 953 cm⁻¹ and 1086 cm⁻¹. The samples b and c exhibit adsorptions at 2965 cm⁻¹, 2874 cm⁻¹, and 1465 cm⁻¹, as attributed to the C–H stretching vibration of linker 3-chloropropyltriethoxysilane anchored on the surface of SiO₂@Fe₃O₄.²⁹ The imidazole tag was responsible for the covalent anchoring of propylamine by forming an imidazolium-based ionic liquid moiety, which was evident from characteristic bands at 1507 cm⁻¹ of C=N vibration and at 3143 cm⁻¹ of C–H stretching on the imidazole ring,³⁰ as shown in Fig. 3c. Two new bands at 1211 cm⁻¹ and 1153 cm⁻¹ corresponding to the N–H stretching vibration of primary amine indicate that the MNPs are functionalized with propylamine modified by imidazolium-based ionic liquid.

The magnetic properties of neat SiO₂@Fe₃O₄ (a), ionic-free supported amine (b) and ionic tagged amine anchored on MNPs (c) were investigated at room temperature (Fig. 4). The saturation magnetizations were 32.2, 36.4, and 56.4 emu g⁻¹, respectively. It should be noted that the high magnetization and superparamagnetism of the ionic-tagged amine loaded sample exhibits good magnetic character and re-dispersion properties, which enables the catalyst to be well-dispersed in the reaction solution and to be easily recovered by an external magnetic force (Fig. 5).

Fig. 4 Room temperature magnetization curves for MNPs (a), MNPs-PA (b) and MNPs-IT-PA (c).

Fig. 5 Photographs of the dispersion of catalyst MNPs-IT-PA (a) and the separation of the catalyst using an external magnet.

The thermal stabilities of the two immobilized catalysts were determined by thermogravimetric (TG) analysis. The results are shown in Fig. 6. The two TG curves were divided into several regions corresponding to different mass loss ranges. The first weight loss for both the samples, which occurred below 200 °C was probably because of the loss of adsorbed solvent or trapped water on the surface and structural water within amorphous silica outside the Fe₃O₄ core. A mass loss of approximately 4.5% of SiO₂@Fe₃O₄-PA occurring between 198 °C and 510 °C was attributed to the loss of propylamine. Further mass loss of the catalyst MNPs-PA at higher temperature was a result of the decomposition of silica shell (Fig. 6a).³¹ For catalyst MNPs-IT-PA, the second weight loss in the range of 199 °C to 350 °C was accounted by the cleavage of imidazolium ionic liquid-modified amine, which was very obvious in the DTG curve (Fig. 6b). The occurrence of further decomposition beyond 351 °C could be designated as the complete loss of imidazolium-functionalized trialkoxysilane moiety.³² The loading amount of propylamine and amine combined with ionic moiety were determined to be 0.21 mmol g⁻¹ and 0.34 mmol g⁻¹, respectively, by elemental analysis, well in accordance with the TG analysis.

Fig. 6 TG-DTG analysis for SiO₂@Fe₃O₄-PA(a) and MNPs-IT-PA (b).

2.2 Catalytic performances in versatile Knoevenagel condensation

The catalytic activities of MNPs-PA and MNPs-IT-PA were investigated using the Knoevenagel condensation between 2-(cyanomethyl)benzimidazole (5 mmol) and 2-thenaldehyde (5 mmol). According to the results provided in Table 1, the catalyst MNPs-IT-PA performed well to give the desired product within 2 h in 79% yield, while ionic tag-free catalyst MNPs-PA gave the same desired product in 62% yield (Table 1, entries 2 and 3). The results confirmed that MNPs-IT-PA catalyzes the model reaction significantly better than its counterpart ionic tag-free MNPs-PA, which may be attributed to the improved mass transfer originating from the hydrophobic ionic moiety.³³ Only 28% yield of the product was obtained when the reaction was carried out in the presence of a blank support (SiO₂@Fe₃O₄) even if the reaction time was prolonged to 5 h (Table 1, entry 1). The reaction conditions were further optimized at different catalyst loadings, revealing that 5 mol% of the catalyst MNPs-IT-PA afforded the best catalytic efficiency in the terms of reaction rate and yield (Table 1, entries 4–7). For comparison, several organic solvents were also exploited as media for the reaction. The reactions catalyzed by MNPs-IT-PA in methanol, toluene and dichloromethane resulted in the product with lower yields (Table 1, entries 8, 10 and 12). It is worth noting that the ionic tag-free catalyst performed better in organic solvents than the ionic-tagged catalyst MNPs-IT-PA (Table 1, entries 8–13). Considering reaction efficiency as well as the principle of green chemistry, the reaction catalyzed by MNPs-IT-PA in water at room temperature was used for further examinations.

Table 1 Evaluation of the effects of the catalyst and solvent on the Knoevenagel condensation reaction of 2-(cyanomethyl)benzimidazole and 2-thenaldehyde at room temperature

Entry	Catalyst (mol%)	Solvent (10 mL)	Time (h)	Yield (%)^a
a Isolated yield.
1	MNPs (SiO2@Fe3O4) (73.5 mg)	H2O	5	28
2	MNPs-PA (5.0)	H2O	2	62
3	MNPs-IT-PA (5.0)	H2O	2	79
4	MNPs-IT-PA (10.0)	H2O	2	79
5	MNPs-IT-PA (20.0)	H2O	3	80
6	MNPs-IT-PA (2.0)	H2O	2	63
7	MNPs-IT-PA (0.5)	H2O	3	37
8	MNPs-IT-PA (5.0)	Methanol	3	58
9	MNPs-PA (5.0)	Methanol	2	68
10	MNPs-IT-PA (5.0)	Toluene	3	52
11	MNPs-PA (5.0)	Toluene	3	65
12	MNPs-IT-PA (5.0)	CH2Cl2	3	57
13	MNPs-PA (5.0)	CH2Cl2	3	61

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With optimal reaction conditions in hand, we proceeded to investigate the scope of methyl active compounds and various substituted arylaldehydes, and the results are shown in Table 2. A wide range of aromatic aldehydes reacted with 2-(cyanomethyl)benzimidazole to give products in high yields (Table 2, entries 1–7). The reaction rate of aldehydes with electron-withdrawing groups at the aromatic ring is slightly faster than that with electron-donating groups (Table 2, entries 1–2 vs. 3–6). On the other hand, the reaction of mono-, di-, tri-substituted or 9-anthraldehyde proceeded more slowly than that of benzaldehyde because of the steric hindrance of substitutents at the ring (Table 2, entries 1 vs. 3–5 and 7). Gratifyingly, 2-(cyanomethyl)benzothiazole, phenylacetonitrile and indole-2-acetonitrile were also found to be efficient methylene active ingredients to give the desired condensation products in good yields albeit relatively longer reaction times were required for indole-2-acetonitrile (Table 2, entries 15 and 16). It is worth noting that all the obtained products were exclusively E-isomers.

Table 2 MNPs-IT-PA catalyzed the Knoevenagel condensation of various aromatic aldehydes with 2-(cyanomethyl)benzimidazole, 2-(cyanomethyl)benzothiazole and phenylacetonitrile and indole-2-acetonitrile^a

Entry	Substrate	Aldehyde	Time (h)	Yield (%)^b
a Reaction conditions: methyl active compounds (5 mmol), aldehydes (5 mmol), water (10 mL), MNPs-IT-PA (5 mol%), rt.b Isolated yield.
1			2	83
2			1.5	85
3			1	88
4			2	84
5			4	83
6			2	80
7			3	76
8			1.5	84
9			2	85
10			2	85
11			3	90
12			3	78
13			3	85
14			5	82
15			5	86
16			5	81

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Encouraged by the exciting results of cyano-substituted methylene active compounds in the present catalytic system, we also attempted to conduct the condensation of non-cyano substituted ketone (I) and benzaldehyde to form the product 4-methyl-3-oxo-N-phenyl-2-(phenylmethylene)pentanamide (II), which is a key intermediate for the preparation of Atorvastatin calcium(III), which can inhibit 3-hydroxy-3-methylglutaryl-coenzyme (HMG-CoA) reductase (Scheme 3).³⁴ To our pleasure, the reaction proceeded smoothly in the presence of catalyst MNPs-IT-PA in water, and 84% yield of the product (II) was achieved within 4 h (reaction condition A in Scheme 3). Only 59% product yield was obtained when the reaction was promoted by ionic tag-free catalyst SiO₂@Fe₃O₄-PA (condition B). The traditional method for the preparation of III requires a large amount of organic solvent and acid (Condition C). Compared with B and C, the reaction conditions A using ionic-tagged MNPs-IT-PA as catalyst may provide an environmentally friendly alternative route for the synthesis of Atorvastatin calcium on an industrial scale.

Scheme 3 Aqueous Knoevenagel condensation of non-cyano substituted ketone with benzaldehyde under various reaction conditions.

The obvious catalytic role of MNPs-IT-PA in Knoevenagel condensation could be attributed to two factors. As depicted in Fig. 7, nitrogen with a lone pair electron at the primary amine of MNPs-IT-PA (Function domain) could act as an acceptor to form a hydrogen bond with a methylene active ingredient. The resulting carbon anion would attack the aldehyde, followed by the removal of a molecule of H₂O to provide the final condensation product. Second, the hydrophilic domain of ionic moiety in the catalyst could play the role of a phase transfer catalyst (PTC), which would increase the solubility of the reagent in water. It is clear that the poor water solubility of a methylene active compound decreases the catalytic activity of the primary amine, which was verified by ionic-free SiO₂@Fe₃O₄-PA promoted processes (Table 1, entry 2 vs. entry 3 and Scheme 3, condition A vs. condition B). Moreover, the “ionic environment” present electrosteric activation to stablize the transition state of the reaction.¹⁵ Both the hydrophic property and the presence of a good acceptor on the nitrogen of MNPs-IT-PA efficiently facilitate the reaction.

Fig. 7 Proposed explanation of MNPs-IT-PA as a catalyst for the Knoevenagel condensation.

One of the main advantages of the supported catalyst is its recyclability. Thus, we turned our attention to the reusability of the ionic-tagged catalyst MNPs-IT-PA. Upon the completion of the reaction, the magnetic catalyst was readily recovered by an external magnet (Fig. 5b), and was reused in the next run before it was successively washed with ethanol and ethyl acetate. As shown in Fig. 8, the catalyst MNPs-IT-PA could be reused for 8 times and no significant decrease in the product yield was observed. In addition, the amine loading of the catalyst subjected to 8 runs was 0.31 mmol g⁻¹, demonstrating the covalent connection between the ionic-modified amine and MNPs.

Fig. 8 Recyclability study of the catalyst MNPs-IT-PA in an aqueous reaction of 2-(cyanomethyl)benzimidazole (5 mmol) and 2-thenaldehyde (5 mmol) in water (10 mL) within 2 h.

3. Conclusions

In conclusion, the first magnetic nanoparticles supported propylamine with an ionic liquid modification was designed and prepared. The ionic-tagged supported catalyst MNPs-IT-PA was used as highly efficient promoter for the Knoevenagel condensation to afford products in good yields. Moreover, MNPs-IT-PA could also be used as a catalyst to prepare 4-methyl-3-oxo-N-phenyl-2-(phenylmethylene)pentanamide, which is a key intermediate for the synthesis of Atorvastatin calcium. The comparison between MNPs-IT-PA and MNPs-PA shows that the ionic moiety efficiently facilitates the reaction. The catalyst MNPs-IT-PA can be reused 8 times with a slight loss of catalytic activity.

4. Experimental

4.1 Materials and instruments

All the chemical reagents were used without further purification. The reaction monitoring was accomplished by thin layer chromatography (TLC) on gel F254 plates. Powder X-ray diffraction data were obtained using Cu Kα radiation. Fourier transform infrared spectroscopy (FT-IR) was performed on a spectrometer using KBr pellets. Transmission electron microscopy (TEM) was performed with an instrument operating at 40–100 kV. Magnetic measurements were carried out in a vibrating sample magnetometer (VSM) at room temperature. ¹H and ¹³C NMR were recorded on a spectrometer at 400 MHz and 100 MHz in CDCl₃, respectively. Chemical shifts were reported in parts per million (δ), relative to the internal standard of tetramethylsilane (TMS).

4.2 Preparation of silica coated magnetic nanoparticle-SiO₂@ Fe₃O₄ 1 (MNPs)

Magnetic (Fe₃O₄) nanoparticles were prepared by co-precipitation.²⁵ FeCl₃·6H₂O (8.1 g, 0.03 mmol) and FeCl₂·4H₂O (4.97 g, 0.025 mmol) were added to deionized water (100 mL) and sonicated until the salts dissolved completely. Then, the transparent solution was heated at 85 °C with vigorous mechanically stirring under N₂ atmosphere for 1 h. pH was then adjusted to 9 using concentrated aqueous ammonia (25 wt%). The black precipitate was washed several times with deionized water until the pH of the eluent decreased to 7. The coating of a layer of silica on the surface of the naked Fe₃O₄ was achieved by the sol–gel method.²⁶ The naked Fe₃O₄ (1.0 g) was diluted with ethanol (200 mL) by ultrasonic irradiation. 6 mL concentrated NH₃·H₂O and 2 mL of tetraethoxysilane (TEOS) were added to the well-dispersed magnetic nanoparticles solution and stirred for 24 h at room temperature. The resulting MNPs 1 was collected by an external magnet and washed with water until the solution was neutral. Then, it was washed with ethanol and diethyl ether successively and dried under vacuum.

4.3 Synthesis of the catalysts MNPs-IT-PA and MNPs-PA

Synthesis of MNPs-IT-PA 4. To a solution of imidazole (6.8 g, 100 mmol) in dry toluene (100 mL), 3-chloropropyltriethoxysilane (24 mL, 100 mmol) was added in one portion. The reaction solution was refluxed for 24 h under nitrogen protection, And the mixture was evaporated under reduced pressure to remove the organic solvent. The residue was then purified with neutral alumina column chromatography (eluent: ethyl acetate) to give the intermediate 2. 1.0 g of SiO₂@ Fe₃O₄ 1 (MNPs) was dispersed in 20 mL dry toluene by sonication for 1 h, and then the intermediate 2 (0.5 g) was added to the solution. After mechanical stirring under nitrogen atmosphere at reflux conditions for 48 h, the solid was magnetically separated and washed with ethanol, followed by drying under vacuum to afford Im-MNPs 3. The resultant 3 (1.0 g) was dispersed in 50 mL dry ethanol by sonification for 1 h. 3-Bromopropylamine (0.27 g, 2.0 mmol) in 20 mL dry ethanol was added dropwise to the mixture and was further refluxed for 48 h under nitrogen protection. The solid catalyst MNPs-IT-PA 4 was collected by an external magnetic force, rinsed with ethanol, and then dried under vacuum for 6 h. Finally, 0.96 g grey nanoparticles (MNPs-IT-PA, 4) were obtained with a loading of 0.34 mmol g⁻¹, as determined by elemental analysis.

Synthesis of MNPs-PA 6. 0.5 g of silica coated magnetic nanoparticles 1 (MNPs) was dispersed in dry toluene (30 mL) by ultrasonication for 5 min. (3-Aminopropyl)triethoxy silane (1.8 g) were added and the reaction mixture was refluxed for 12 h under nitrogen atmosphere. The supported catalyst MNPs-PA 6 was obtained by magnetic separation and washed with ethanol, and then dried in a vacuum oven at 60 °C for 6 h. The loading of the catalyst was determined to be 0.21 mmol g⁻¹ by elemental analysis.

NMR data for intermediate 2. ¹H NMR (400 MHz, CDCl₃): δ 0.46–0.50 (m, 2H), 1.13–1.15 (m, 9H), 1.81 (t, J = 8 Hz, 2H), 3.74 (t, J = 7.2 Hz, 6H), 3.86 (t, J = 7.2 Hz, 2H), 6.84 (d, J = 1.2 Hz, 1H), 6.96 (s, 1H), 7.40 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 7.3, 18.2, 24.8, 49.0, 58.4, 118.7, 129.2, 137.1 ppm.

4.4 General procedure for the MNPs-IT-PA catalyzed ambient Knoevenagel condensation in water

Benzaldehyde (5 mmol), methylene active compound (5 mmol), and MNPs-IT-PA (735 mg) were mixed in water (10 mL). The mixture was allowed to react at room temperature for a suitable length of time, which was decided by TLC. Separated from the reaction solution by a magnet, the catalyst was washed with ethanol and ethyl acetate, followed by drying under vacuum and reusing for subsequent runs. The decanted solution was directly filtered and the remaining residue was purified with recrystallization using ethanol or column chromatography using petroleum ether/ethyl acetate as the eluent. The products were characterized by ¹H NMR and ¹³C NMR.

NMR data for representative products.
2-(1H-Benzoimidazol-2-yl)-3-(thiophen-2-yl)acrylonitrile (Table 1, model reaction). ¹H NMR (400 MHz, CDCl₃): δ 7.15–7.22 (m, 1H), 7.30–7.37 (m, 2H), 7.50–7.52 (m, 1H), 7.66–7.70 (m, 2H), 7.76–7.77 (m, 1H), 7.87–7.89 (m, 1H), 8.63 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 98.1, 115.6, 115.7, 116.7, 123.6, 129.1, 134.5, 136.8, 137.0, 139.3, 147.6 ppm.
2-(1H-Benzoimidazol-2-yl)-3-phenylacrylonitrile (Table 2, entry 1). ¹H NMR (400 MHz, CDCl₃): δ 7.31–7.34 (m, 2H), 7.50–7.52 (m, 3H), 7.65 (s, 2H), 7.98–7.80 (m, 2H), 8.51 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 111.2, 116.8, 119.7, 123.6, 124.2, 129.3, 130.0, 132.1, 132.7, 146.3, 146.8 ppm.
2-(1H-Benzoimidazol-2-yl)-3-(3-methoxyphenyl)acrylonitrile (Table 2, entry 4). ¹H NMR (400 MHz, CDCl₃): δ 3.89 (s, 3H), 7.07–7.09 (m, 1H), 7.33–7.44 (m, 3H), 7.55–7.59 (m, 2H), 7.66 (s, 2H), 8.53 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 55.5, 105.8, 114.1, 116.5, 119.0, 121.7, 123.4, 123.7, 126.0, 127.0, 130.2, 133.6, 135.0, 146.9, 153.6, 160.0, 162.7 ppm.
2-(1H-Indol-2-yl)-3-phenylacrylonitrile (Table 2, entry 15). ¹H NMR (400 MHz, CDCl₃): δ 7.28–7.33 (m, 2H), 7.41–7.49 (m, 4H), 7.60–7.63 (m, 2H), 7.88 (d, 2H, J = 5.6 Hz), 8.00 (d, 1H, J = 6.0 Hz) ppm, 8.58 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 106.5, 112.4, 113.1, 118.9, 120.0, 121.5, 123.6, 124.4, 125.8, 128.9, 129.1, 129.8, 134.9, 137.3, 138.2 ppm.
2-(1H-Indol-2-yl)-3-p-tolylacrylonitrile (Table 2, entry 16). ¹H NMR (400 MHz, CDCl₃): δ 2.42 (s, 3H), 7.26–7.33 (m, 4H), 7.45(d, 1H, J = 6.4 Hz), 7.57–7.60 (m, 2H), 7.79 (d, 2H, J = 6.4 Hz), 8.00 (d, 1H, J = 6.0 Hz), 8.46 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 21.8, 105.3, 112.3, 113.3, 119.1, 120.0, 121.4, 123.5, 124.4, 125.4, 128.9, 129.8, 132.1, 137.2, 138.5, 140.2 ppm.
4-Methyl-3-oxo-N-phenyl-2-(phenylmethylene)pentanamide (Scheme 3, II). ¹H NMR (400 MHz, CDCl₃): δ 1.21 (d, 6H, J = 6.4 Hz), 3.36 (m, 1H), 7.14–7.18 (m, 1H), 7.32–7.38 (m, 5H), 7.47 (d, 2H, J = 7.6 Hz), 7.56–7.59 (m, 3H), 7.64 (s, 1H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ 19.1, 36.7, 120.3, 125.0, 129.0, 129.1, 130.0, 130.1, 133.0, 136.2, 137.4, 140.7, 165.5 ppm.

Acknowledgements

We are grateful for the financial supports for this research by the National Natural Science Foundation of China (Grant 21106090 and 21272169), Foundation of Low Carbon Fatty Amine Engineering Research Center of Zhejiang Province (2012E10033), and Zhejiang Provincial Natural Science Foundation of China (no LY12B02004 and LQ13B070002).

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