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Copper Catalysts Anchored on Cysteine-Functionalized Polydopamine-Coated Magnetite Nanoparticles: A Versatile Platform for Enhanced Coupling Reactions

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Abstract
Cysteine plays a crucial role in the development of an efficient copper-catalyst system, where its thiol group serves as a strong anchoring site for metal coordination. By immobilizing copper onto cysteine-modified, polydopamine-coated magnetite nanoparticles, this advanced catalytic platform exhibits exceptional stability and catalytic activity. Chemical modification of the polydopamine (PDA) surface with cysteine enhances copper salt immobilization, leading to the formation of the Fe₃O₄@PDA-Cys@Cu nanostructure. This system was evaluated in palladium-free, copper-catalyzed Sonogashira coupling reactions, effectively catalyzing the coupling of terminal acetylenes with aryl halides. Additionally, the Fe₃O₄@PDA-Cys@Cu platform was employed in click reactions, confirming the enhanced catalytic efficiency due to increased copper content. The reusability of the platform was further investigated, demonstrating improved performance, especially in recyclability tests in click reaction, making it a promising candidate for sustainable heterogeneous catalysis.
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Subject: Chemistry and Materials Science  -   Organic Chemistry

1. Introduction

Transition metal-catalyzed cross-coupling reactions have become increasingly prevalent in contemporary synthetic organic chemistry, facilitating the creation of vital carbon-carbon (C–C) bonds. [1,2] Within the spectrum of strategies for synthesizing biologically or industrially significant organic functional molecules, the Sonogashira coupling reaction emerges as a pivotal technique, notably facilitating the formation of carbon-carbon bonds, particularly in the synthesis of alkynes [3,4].
The coupling reaction between aromatic acetylenes and aryl halides was independently reported in 1975 by Cassar [5], Heck [6], and Sonogashira [7]. To perform those coupling reactions, a wide range of palladium-catalysts along with copper co-catalyst have been frequently employed.
Although the reaction can be performed under mild conditions, applying the Sonogashira reaction to industrial or pharmaceutical synthesis became challenging due to the toxicity and air sensitivity of phosphine ligands, as well as the high price of palladium [8]. To overcome the shortcomings of the aforementioned reaction conditions, various approaches have focused on the development of more sustainable and cost-effective catalytic systems, such as palladium- and phosphine-free Sonogashira-type coupling reactions. In this context, copper has emerged as a promising candidate due to its earth abundance and low toxicity [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. Along with copper metal, several transition metal complexes, including Fe [24,25,26], Zn [27], Co [28,29], Ag [30], Ni [31], Ru [32,33], and Au [34], have been efficiently utilized for the execution of Sonogashira coupling reactions.
Interestingly, recent scientific artworks have highlighted the growing interest in Pd-free copper-catalyzed Sonogashira coupling reactions, with the aim of overcoming the limitations associated with palladium-based catalysis [35,36,37]. More importantly, despite the impressive advancements in Csp–Csp2 bond construction utilizing various transition metal catalytic systems over the past few decades, the development of heterogeneous Cu-based catalyst systems remains an ongoing challenge. To date, several heterogeneous copper-catalytic protocols, including an appropriate rigid support, have been successfully developed and applied to Sonogashira coupling [37].
Of the aforementioned outcomes, utilizing the magnetic support had attracted our attention [38,39,40]. In our previous study, we demonstrated the preparation and application of a heterogeneous copper catalytic platform, where copper was directly immobilized on the surface of PDA-coated magnetite (Fe3O4@PDA@Cu) [41]. Although this platform demonstrated satisfactory performance as a heterogeneous copper catalyst in click reactions, we envisioned a higher copper content in this type of magnetite-based platform. To achieve this goal, we designed further modifications to the PDA surface of magnetite by introducing a biocompatible substance. Cysteine was chosen as the optimal compound for this decoration.
According to previous studies, cysteine, which contains amine, carboxy, and thiol groups, has been widely utilized for effective anchoring with PDA by covalently linking to PDA-modified core-shell nanoparticles for secondary modification [42,43,44]. As a result, the cysteine-modified PDA-magnetite nanoparticles played a significant role for removing Pb from wastewater [45]. Interestingly, a recent study demonstrated an example of cysteine-modified layered double hydroxides (LDHs)-coated magnetite nanoparticles for copper anchoring and its application in click reactions [46]. These outcomes strongly supported our anticipation of enhancing copper immobilization through the action of cysteine moiety on the surface of the Fe3O4@PDA-Cys platform.
In continuation of our efforts in developing efficient protocols for organic synthesis, we herein report the copper-cysteine complex immobilized on the surface of PDA-coated magnetite nanoparticles, (denoted as Fe3O4@PDA-Cys@Cu), as an efficient and recoverable nanocatalyst for Pd-free, Cu-catalyzed Sonogashira coupling and click reaction.

2. Results

2.1. Characterization

Characterization of the obtained platform was performed using several techniques, including FT-IR, TGA, EDX, and SEM. According to the ICP-OES analysis, the cysteine-modified magnetite particles (Fe3O4@PDA-Cys@Cu) contained a higher copper content (18.96%) compared to the unmodified platform (Fe3O4@PDA@Cu), which had 5.02%.
The thermal stability of the Fe₃O₄@PDA-Cys@Cu platform and related complexes was investigated using TGA (Figure 1). The negligible weight loss below 150 °C is attributed to the physically adsorbed volatile substances. Upon comparing the TGA analysis results, the difference between the Fe₃O₄@PDA@Cu and the cysteine-modified platform (Fe₃O₄@PDA-Cys@Cu) becomes even more pronounced. Interestingly, a very similar pattern was observed in the cysteine-modified platforms (Fe₃O₄@PDA-Cys and Fe₃O₄@PDA-Cys@Cu).
Morphological feature of the Fe3O4@PDA-Cys@Cu was investigated using SEM–EDX and given in Figure 3. The EDX analyses of each platform clearly showed an increase in copper content in the Fe3O4@PDA-Cys platform compared to the Fe3O4@PDA platform.

2.2. Application to Organic Reactions

To verify the catalytic activity of the prepared Fe3O4@PDA-Cys@Cu platform, it was initially employed in the Sonogashira coupling reaction, a cross-coupling reaction involving aryl acetylenes and aryl halides.
Before the general application of the Fe3O4@PDA-Cys@Cu platform, we conducted a preliminary search for optimal parameters for the Sonogashira coupling reactions using phenylacetylene (1a) and 4-iodoanisole (2a) as standard substrates (Table 1).
At first, the model reaction was carried out using excess equivalent (1.2 eq) of phenyl acetylene (1a) with 60 mg of Fe3O4@PDA-Cys@Cu platform in DMSO at 100 ℃, affording the desired coupling product (3a) in 84% isolated yield (Table 1, Entry 1). Significantly, a remarkable improvement in isolated yield (up to 99%) was achieved by increasing the amount of the catalytic platform (up to 80 mg) employed under the same conditions (Table 1, Entry 2). Subsequent trials were conducted to identify a more versatile solvent system, which demonstrated the high efficiency of both ethanol and water as reaction media. For practical convenience, we chose to employ EtOH as the reaction solvent for further investigation. Decreasing the amount of the platform to 60 mg did not significantly affect the isolated yield (Table 1, Entries 5 and 6). Further attempts with 50 mg of Fe3O4@PDA-Cys@Cu platform demonstrated relatively lower catalytic efficiency under similar conditions (Table 1, Entry 7). The use of equimolar amounts of 1a and 2a resulted in a slightly disappointing outcome (Table 1, Entry 8). Interestingly, reaction temperature turned out to be a critical factor for the successful completion of the reaction (Table 1, Entry 9).
With the optimization tests, the scope and applicability of the novel catalytic platform was further explored using various substrates (Table 2). Firstly, the investigation was conducted using 1a and aryl halides containing C–I bond in the presence of Fe3O4@PDA-Cys@Cu platform under standard conditions. Initially, 2-iodotoluene (2b) was reacted with 1a, resulting in the formation of 1-methyl-2-(phenylethynyl)benzene (3b) with an isolated yield of 91% (Table 2, Entry 1). Additionally, the reaction of 1a with 3-fluoro-iodobenzene proceeded well, yielding the corresponding coupled product (3c) in excellent isolated yields (Table 2, Entry 2). In contrast to these positive outcomes, relatively lower performance was observed in the coupling reactions with 4-chloroiodobenzene (2d) and 3-cyano-iodobenzene (2e). Interestingly, a polar functional group (OH) on aryl iodide (2f) was well tolerated, yielding the desired product (3f) in excellent yield (Table 2, Entry 5).
The substrate scope of acetylene was also investigated using various arylacetylenes (1b – 1f) under the standard conditions. Overall, the reaction outcomes were consistent with those observed for 1a, confirming the catalytic efficiency of the developed reaction platform. Entries 6 and 7 in Table 2 demonstrate that 3-iodoaniline (2g) and 3-fluoro-iodobenzene (2c) were effective substrates for the synthesis of their respective asymmetric acetylene derivatives (3g and 3h) from 1-ethynyl-4-methylbenzene (1b) using the optimized Sonogashira coupling (Table 2, Entries 6 and 7). Similar catalytic performance was observed when various arylacetylenes, such as 4-ethynylbenzonitrile (1c) and 1-ethynyl-4-methoxybenzene (1d), were used. The corresponding products (3i and 3j, respectively) were effectively obtained with excellent yields (Table 2, Entries 8 and 9). Furthermore, the presence of bromine (1e) and hydroxyl (1f) groups on the arylacetylene rings did not affect the reaction outcome (Table 2, Entries 10 and 11). The reaction conditions used in this study were well tolerated to provide the final coupling products (3k and 3l) in an excellent manner.
To further extend the scope of this green protocol, we studied the coupling of 1a with various aryl halides bearing a C–Br bond, including 4-bromobenzonitrile, 2-bromothiophene, and 2-bromopyridine. Unfortunately, the reactions proceeded sluggishly or resulted in unseparable mixtures (Table 2, Entries 12–14).
Next, the recyclability of the novel catalytic platform was explored to ensure its advantage as a heterogeneous catalytic system (Table 3). Following standard procedures, a recycling test was carried out using 1a and 2a. After each cycle, the catalyst was effortlessly retrieved using an external magnet, washed consecutively with fresh water and acetone, and subsequently dried in air. The results from the recycling test indicate that the present platform demonstrates high potential as a recoverable and reusable heterogeneous catalytic system for Sonogashira coupling under mild conditions.
As mentioned before, we previously disclosed a novel heterogeneous copper catalyst immonbilized on polydopamine-coated magnetite for click reaction [49]. In our previous study, the catalyst was prepared by immobilizing Cu(OAc)₂ salt onto polydopamine-coated magnetite (denoted as Fe3O4@PDA@Cu). The catalytic activity of the resulting Fe₃O₄@PDA@Cu platform was investigated in a click reaction, utilizing three-component reactions of azide, alkyne, and benzyl surrogates in water, which provided the corresponding 1,2,3-triazoles in high yields. Despite the positive outcomes using the Fe₃O₄@PDA@Cu platform, unsatisfactory results were observed in recycling tests (see Table 5).
Despite numerous approaches utilizing magnetite-based copper catalytic systems for click reaction [50], there remains a need for versatile routes that focus on catalyst recyclability and reusability. Here, to expand the library of click reactions catalyzed by magnetic nanoparticle-supported copper catalysts and conduct a comparative study, we further carried out click reactions using various substrates in the presence of our newly designed Fe₃O₄@PDA-Cys@Cu platform. For simple comparison, the results were disclosed in parallel with those obtained using our previous Fe₃O₄@PDA@Cu platform, as shown in Table 4.
In general, no significant differences were observed in the overall reactivity when using Fe₃O₄@PDA@Cu and Fe₃O₄@PDA-Cys@Cu as catalysts in the click reaction, respectively. In both cases, cyclization proceeded smoothly, providing 1,4-disubstituted-1,2,3-triazoles in excellent yields regardless of the functionality on acetylenes or benzyl halides. As evidenced by the yield of the target product, each catalyst system exhibited highly effective catalytic activity in the click reaction.
Next, the recyclability of the novel catalytic platform in click reaction was explored to ensure its advantage over our previous non-modified Fe3O4@PDA@Cu platform. Following standard procedures, a recycling test was carried out as depicted in Table 5. The isolated yields of the coupling products obtained from both recycling tests are comparatively displayed, clearly indicating the effectiveness of the Fe3O4@PDA-Cys@Cu catalyst over the Fe3O4@PDA@Cu catalyst used in our previous study.
Table 5. Reusability test using the Fe3O4@PDA@Cu and Fe3O4@PDA-Cys@Cu platforms.
Table 5. Reusability test using the Fe3O4@PDA@Cu and Fe3O4@PDA-Cys@Cu platforms.
Preprints 120155 i031
Recycling 0 1st 2nd 3rd 4th 5th 6th 7th
Yielda 98% (5 h) 97% (7 h) 72% (7 h) 51% (7 h) - - - -
Yieldb 99% 99% 96% 96% 95% 95% 95% 90%
a Isolated yield of 4a (based on 2a) using Fe3O4@PDA@Cu platform. The number in parentheses indicates the reaction time. b Isolated yield of 4a (based on 2a) using Fe3O4@PDA-Cys@Cu platform. Each run was conducted in 5 h.
As shown in Table 5, the catalytic activity of the two platforms (Fe₃O₄@PDA@Cu and Fe₃O₄@PDA-Cys@Cu) in recycling tests for the click reaction exhibited significant differences. We hypothesized that these differences could be attributed to the variation in copper content retained within the recycled platforms after use. To verify our assumption, we performed ICP analysis to quantify the decrease in copper content before and after use. In our previous study with Fe₃O₄@PDA@Cu, the copper content dropped from 5.01% to 0.04%. Interestingly, in the current study using Fe₃O₄@PDA-Cys@Cu, the reduction in copper content was notably less, decreasing from 18.96% to 17.41% after seven recycling cycles.
Furthermore, a similarly modest reduction in copper content was also observed during the Sonogashira coupling reaction using Fe₃O₄@PDA-Cys@Cu, and confirmed by EDX analysis (Figure 4).

3. Conclusion

we developed and thoroughly characterized a novel, recoverable catalytic platform composed of magnetic particles (Fe3O4), copper salts, polydopamine, and cysteine (Fe3O4@PDA-Cys@Cu). Using a range of spectroscopic methods, including FTIR, EDX, TGA, SEM, and ICP-OES, we confirmed the enriched copper content in this platform. The enhanced immobilization of copper salt is attributed to the cysteine modification of the PDA surface on the magnetite. The increased copper content proved highly effective in organic reactions. For example, the Fe3O4@PDA-Cys@Cu platform successfully facilitated the Sonogashira coupling reaction, yielding the desired Csp – Csp2 coupling products under mild conditions. Furthermore, the platform demonstrated efficacy in click reactions through three-component reactions involving terminal alkynes, benzyl surrogates, and sodium azide in an aqueous environment, producing the desired 1,4-disubstituted-1,2,3-triazoles in good to excellent isolated yields. A comparative study of the click reaction clearly highlighted the advantage of cysteine modification in the PDA-coated magnetite-based nanoparticle platform for copper immobilization

4. Materials and Methods

4.1. Procedure for Fe3O4@PDA

Fe3O4 nanoparticles were prepared by co-precipitation method following the literatures [47,48].
An iron salt solution was obtained by mixing FeCl3·6H2O (34.6 g) and FeCl2·4H2O (12.7 g), in the molar ratio of 2:1, in 1.5 L of deionized water under nitrogen at room temperature. By dropwise addition of NH3 solution, the pH was adjusted to 12. A black precipitate was formed after continuously stirring for 1 hour. The precipitate was magnetically separated and washed four times with deionized water until the solution reached pH 8 and two times with ethanol and was dried under vacuum at 50 °C overnight. Next, for the preparation of PDA-coated magnetite (Fe3O4@PDA), a mixture of Fe3O4 MNPs (3.0 g) and tris buffer (1.0M, pH 8.5, 20 mL) was sonicated for 30 min. Subsequently, dopamine hydrochloride (6.0 g) were added to the mixed solution, and the mixture was stirred at room temperature for 24 hours. Then, the Fe3O4@PDA particles were collected using a magnet and washed several times with H2O and ethanol. The obtained platform was dried in vacuum at 50 °C for 12 hours.

4.2. Procedure for Fe3O4@PDA-Cys

A solution of Fe3O4@PDA particles in H2O was sonicated for 3 hours at room temperature. In a separate flask, L-cysteine and H2O were combined and sonicated for 1 hour at room temperature. The cysteine solution was then added to the Fe3O4@PDA solution, and the resulting mixture was stirred at 50 °C for 24 hours. The black precipitates were filtered, washed sequentially with H2O and EtOH, and then dried under vacuum at 50 °C for 24 hours.

4.3. Procedure for Fe3O4@PDA-Cys@Cu

A solution of Fe3O4@PDA-Cys particles in EtOH/H2O was sonicated for 60 minutes at room temperature. Subsequently, copper acetate was added to the solution, and the resulting mixture was stirred at room temperature for 24 hours. The Fe3O4@PDA-Cys@Cu particles were then collected using a magnet and washed several times with H2O and ethanol. The obtained products were dried under vacuum at 50 °C for 12 hours.

4.4. General Procedure for Sonogashira Coupling

To a round-bottom flask equipped with a magnetic bar, phenylacetylene (3.6 mmol), aryl halide (3.0 mmol), Fe3O4@PDA-Cys@Cu and EtOH (4.0 mL) were added. Then, the mixture was stirred at a refluxing temperature. After completion, the reaction mixture was cooled down to room temperature. The mixture was filtered and the filtrate was acidified with 3M HCl aqueous solution. The aqueous layer was extracted with diethyl ether (3 X 10 mL). The combined organic layers were washed with brine, dried with anhydrous Na2SO4, and evaporated under reduced pressure. The crude mixture was purified by column chromatography on silica gel (hexanes/ ethyl acetate).

4.5. General Procedure for Click Reaction

Briefly, the mixture of benzyl halide (3.0 mmol), terminal alkyne (3.6 mmol) and sodium azide (3.6 mmol) was stirred at 90 °C in the presence of Fe3O4@PDA-Cys@Cu platform (70 mg) in H2O. After completion of the reaction, the mixture was decanted with the aid of an external magnet. The aqueous layer was then extracted with ethyl acetate, followed by a typical workup and purification on silica gel using eluent (ethyl acetate/hexane).

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org, 1H and 13C NMR data and copies of spectra.

Author Contributions

Synthesis and Characterization, Y.J. Jo and S.W. Park; writing—original draft preparation, S.H. Kim; writing—review and editing, S.H. Kim and W.S. Shin. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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  50. For references on the Fe₃O₄-based copper catalyst in click reactions, refer to our previous study (reference 49 above) and the references cited therein.
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Figure 1. Comparing TGA analyses of Fe3O4@PDA-Cys@Cu platform and related substrates.
Figure 1. Comparing TGA analyses of Fe3O4@PDA-Cys@Cu platform and related substrates.
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Figure 2. IR spectra.
Figure 2. IR spectra.
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Figure 3. EDX analyses of Fe3O4@PDA@Cu (up) and Fe3O4@PDA-Cys@Cu (down).
Figure 3. EDX analyses of Fe3O4@PDA@Cu (up) and Fe3O4@PDA-Cys@Cu (down).
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Figure 4. EDX analysis of recovered Fe3O4@PDA-Cys@Cu.
Figure 4. EDX analysis of recovered Fe3O4@PDA-Cys@Cu.
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Table 1. Screening optimal parameters for the Sonogashira coupling.
Table 1. Screening optimal parameters for the Sonogashira coupling.
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Entry 1a 2a Cu-Platform Solvent Temp. Time Yielda
1 3.6 mmol 3.0 mmol 60 mg DMSO 100 ℃ 4 h 84%
2 3.6 mmol 3.0 mmol 80 mg DMSO 100 ℃ 4 h 99%
3 3.6 mmol 3.0 mmol 80 mg EtOH reflux 5 h 99%
4 3.6 mmol 3.0 mmol 80 mg H2O reflux 5 h 99%
5 3.6 mmol 3.0 mmol 70 mg EtOH reflux 5 h 99%
6 3.6 mmol 3.0 mmol 60 mg EtOH reflux 5 h 97%
7 3.6 mmol 3.0 mmol 50 mg EtOH reflux 5 h 93%
8 3.0 mmol 3.0 mmol 80 mg EtOH reflux 4 h 87%
9 3.6 mmol 3.0 mmol 70 mg EtOH rt 24 h trace
a Isolated yield of 3a based on 2a.
Table 2. Sonogashira coupling.
Table 2. Sonogashira coupling.
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Entry Ar1 Ar2 Product Yielda
1 Preprints 120155 i003
(1a) 		Preprints 120155 i004
(2b) 		Preprints 120155 i005
(3b) 		91%
2 (1a) Preprints 120155 i006
(2c) 		Preprints 120155 i007
(3c) 		93%
3 (1a) Preprints 120155 i008
(2d) 		Preprints 120155 i009
(3d) 		86%
4 (1a) Preprints 120155 i010
(2e) 		Preprints 120155 i011
(3e) 		88%
5 (1a) Preprints 120155 i012(2f) Preprints 120155 i013
(3f) 		98%
6 Preprints 120155 i014 (1b) Preprints 120155 i015(2g) Preprints 120155 i016
(3g) 		98%
7 (1b) (2c) Preprints 120155 i017
(3h) 		92%
8 Preprints 120155 i018(1c) (2g) Preprints 120155 i019
(3i) 		94%
9 Preprints 120155 i020(1d) (2f) Preprints 120155 i021
(3j) 		91%
10 Preprints 120155 i022(1e) (2b) Preprints 120155 i023
(3k) 		95%
11 Preprints 120155 i024
(1f) 		(2g) Preprints 120155 i025
(3l) 		96%
12 (1a) Preprints 120155 i026 No reaction -
13 (1a) Preprints 120155 i027 No reaction -
14 (1a) Preprints 120155 i028 No reaction -
a Isolated yield based on aryl halide.
Table 3. Recyclability test in Sonogashira coupling.
Table 3. Recyclability test in Sonogashira coupling.
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Recycle 1st 2nd 3rd 4th 5th 6th 7th
Yielda 98% 98% 96% 97% 95% 94% 90%
a Isolated yield based on 2a.
Table 4. Click reaction using Fe3O4@PDA-Cys@Cu.
Table 4. Click reaction using Fe3O4@PDA-Cys@Cu.
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Entry X Y Z Product Resulta Resultb
1 H 4-Br Br 4b 80% 95%
2 H 4-t-Bu Br 4c 88% 98%
3 H 4-F Cl 4d 90% 90%
4 H 4-CN Cl 4e 85% 90%
5 H 2,4-Cl2 Cl 4f 95% 96%
6 4-F H Br 4g 90% 96%
7 4-C4H9 H Br 4h 86% 97%
8 4-OCH3 H Br 4i 96% 98%
9 4-NO2 H Br 4j 90% 91%
10 3-OH H Br 4k 88% 95%
11 4-NO2 4-F Cl 4l 88% 92%
12 4-F 2,4-Cl2 Cl 4m 80% 92%
13 3-CH3 4-t-Bu Br 4n 96% 97%
a Isolated yield based on benzyl halide using Fe3O4@PDA@Cu platform (results cited from reference [49]) b Isolated yield based on benzyl halide using Fe3O4@PDA-Cys@Cu platform [this work].
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