Acid-ionic polymer as recyclable catalyst for one-pot three-component Mannich reaction

Abstract

A recyclable solid-catalyst, acid-ionic polymer bearing imidazolium trifluoromethanesulfonate, [PS-Im][OTf] was presented as an efficient catalyst in a one-pot, three-component Mannich reaction of aldehydes, amines and ketones. The protocol was conducted successfully under mild, convenient and metal-free conditions to afford the corresponding products in moderate to excellent yields. In addition, the catalyst was reused four times with no significant loss in activity.

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Introduction

The Mannich reaction is one of the most powerful tools for carbon–carbon bond-forming.¹ It produces β-amino carbonyl compounds, which are crucial building blocks for the synthesis of a wide variety of nitrogen-containing drugs and natural products.² Some of these compounds exhibit biological significance,³ for examples anticancer,⁴ antimicrobial⁵ anti-HIV⁶ and antimalarial⁷ properties (Fig. 1). As a consequence, new synthetic methods to prepare these compounds have been continually developed, especially direct-Mannich reaction in a one-pot three-component process of aldehydes, amines and various nucleophiles.

Fig. 1 Examples of bioactive compound of Mannich bases.

Generally, one-pot, three-component Mannich reactions have been reported with various catalyst systems. Most catalysts are Lewis acids,⁸ Bronsted acids,⁹ organocatalyst¹⁰ and recently nanoparticles.^11,16 However, most of these catalysts have serious disadvantages such as their high cost, toxicity and hazardous nature, in addition to being difficult to separate from the product. Some are unstable, moisture sensitive, environmental contaminants and not reusable. Therefore, high performance non-toxic, inexpensive, reusable and efficient catalysts are desirable. Additionally, practical and metal-free methods with an environmentally benign synthetic process under green context merits more recent attention for one-pot Mannich reactions.

Solid-supported catalysts are currently of great interest as green and reusable catalysts, providing high yields of desirable products for many useful organic reactions.¹² The advantages of these catalysts are moisture resistance, air tolerance, low-toxicity, easily separated post reaction by simple filtration and recyclable. In particular, there are practical advantages in medicinal and pharmacological production because of their non-toxicity and high-throughput synthesis. Mannich reactions that promoted by solid-supported Lewis and Bronsted acid-catalysts have been rare.^13–16 Kobayashi's group in 2003 used a hydrophobic polymer-supported sulfonic acid as a catalyst for three-component Mannich reaction.¹⁴

Subsequently, in 2007, Wu's group employed SiO₂–OSO₃H as the catalyst for the direct-Mannich reaction.¹⁵ Recently, in 2014, Zareyee' group described the use of a hydrophobic sulfonic acid based nanosilica as the catalyst for one-pot three-component Mannich reaction.¹⁶

Bronsted acidic ionic liquids (BAILs) have received attention in past decades as efficient acidic catalysts under a wide range of reaction conditions.^17,18 Recently, BAILs has proven to be effective catalysts in the Mannich reactions.¹⁸ Solid-supported acidic ionic liquid catalysts^19–26 have been reported for acetalization reaction,²⁰ condensation,²¹ Hantzsch reaction,²² multicomponent reaction,²³ esterification,²⁴ hydrolysis of cellulose²⁵ and biodiesel synthesis.²⁶ Herein, we would like to present the utilization of polystyrene bearing acidic ionic liquid (acid-IP) as a reusable heterogeneous catalyst for direct-Mannich reaction of aldehydes, amines and ketones in one-pot process under mild, convenient and metal-free conditions at ambient temperature (Scheme 1).

Scheme 1 Three-component Mannich reaction catalyzed by acid-IP.

Results and discussion

A series of acid-ionic polymers (acid-IP) bearing imidazolium cation with two different acid-anions (pTSA and OTf) was investigated as a recyclable heterogeneous catalyst for one-pot, three-component Mannich reaction of aldehydes, amines, and ketones. Preparation of these acid-ionic polymers used commercially available polystyrene beads (PS) with 1.80 mmol g⁻¹ of chloride loading (Scheme 2, see ESI†).

Scheme 2 Preparation of acid-ionic polymers.

The formation of β-amino carbonyl compound 4a through one-pot three-component Mannich reaction catalyzed by acid-ionic polymer-based imidazolium salts was investigated using 4-nitrobenzaldehyde 1a, aniline 2 and cyclohexanone 3 as model substrates (Table 1). Initial experiments, 10 mol% of catalysts were tested using 1.0 mmol of each substrate in acetonitrile (MeCN) at room temperature for 24 h (entries 1–4). The highest yield of the desired product 4a was obtained with catalyst III in 82% (entry 2). Subsequently, with the best catalyst III, various solvents were screened in the same reaction condition. Results demonstrated the high yield of desired product 4a was achieved with polar solvents (MeCN and DMF). This was probably due to the generous swelling of acid-ionic polymer in polar solvents that lead to reaction rate enhancement. However, MeCN was selected as the best solvent for this protocol due to more efficient recyclability.

Table 1 Optimization of one-pot three-component Mannich reaction with various catalysts and solvents^a

Entry	Catalyst	Solvent	Yield 4a^b (%)
a All reactions were conducted with 1.0 mmol of 1, 2 and 3 using 10 mol% of catalyst in 3.0 mL of solvent, shaking 800 rpm at room temperature for 24 h.b Isolated yields.c Second run.d Third run.e Fourth run.
1	II, [PS-Im][pTSA]	MeCN	72
2	III, [PS-Im][OTf]	MeCN	82(83)^c(81)^d(80)^e
3	V, [PS-BsIm][pTSA]	MeCN	41
4	VI, [PS-BsIm][OTf]	MeCN	36
5	III, [PS-Im][OTf]	DMF	81(74)^c(54)^d(48)^e
6	III, [PS-Im][OTf]	DCM	38
7	III, [PS-Im][OTf]	DCE	48
8	III, [PS-Im][OTf]	Toluene	27
9	III, [PS-Im][OTf]	THF	60
10	III, [PS-Im][OTf]	1,4-Dioxane	38
11	III, [PS-Im][OTf]	H2O	49
12	III, [PS-Im][OTf]	MeOH	56

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To improve yield of desired product 4a, ratio of substrates was adjusted (Table 2). Slight increase of cyclohexanone 3 to 1.2 equivalents, desired product 4a was produced in higher yield, 89% for 24 h and 93% for 48 h (entry 1). However, reaction time was investigated also from 3–48 h and the result indicated that the reaction was almost completed in 48 h.

Table 2 Optimization of one-pot three-component Mannich reaction with various amount of substrates^a

Entry	SM (1a : 2 : 3) (mmol)	Catalyst	Yield 4a^b (%)
a All reactions were conducted with 1.0 mmol of 1, 1–5 mmol of 2 and 1–5 mmol of 3 using 10 mol% of catalyst in 3.0 mL of MeCN, shaking at 800 rpm at room temperature for 24 h.b Isolated yields.c 48 h.d 1.0 mL MeCN.e 20 mol% of III.f 5 mol% of III.
1	1 : 1 : 1.2	III, [PS-Im][OTf]	89(93)^c
2	1 : 1 : 2.0	III, [PS-Im][OTf]	86
3	1 : 1 : 5.0	III, [PS-Im][OTf]	86
4	1 : 1.2 : 1.2	III, [PS-Im][OTf]	75
5	1 : 2.0 : 1.2	III, [PS-Im][OTf]	71
6	1 : 5.0 : 1.2	III, [PS-Im][OTf]	50
7^d	1 : 1 : 1.2	III, [PS-Im][OTf]	81
8^e	1 : 1 : 1.2	III, [PS-Im][OTf]	77
9^f	1 : 1 : 1.2	III, [PS-Im][OTf]	80
10	1 : 1 : 1.2	pTSA	73
11	1 : 1 : 1.2	TfOH	77
12	1 : 1 : 1.2	—	0

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For comparison, the reaction was investigated with 24 h and showed yield was not improved when substrate 3 was increased further to 2.0–5.0 equivalents (entries 2 and 3). Unfortunately, when amount of amine substrate 2 was increased yield of the desired product 4a declined (entries 4–6). This was attributed to excess amine base quenching the acidity of catalytic system. Highly concentrated reaction volume produced the slight decrease in yield (entry 7). Amount of catalyst III with 20 and 5 mol% were examined, and resulted in lower yields of desired product 4a in 77 and 80% respectively (entries 8 and 9). Due to greater acidity with 20 mol% of catalyst amine substrate was reduced by reacting with acid catalyst, while reaction was incomplete with 5 mol% of catalyst. In addition, confirmation of catalytic activity of catalyst III, the control reactions were tested with general acids such as pTSA and TfOH and found to give lower yields of the desired product 4a (entries 10 and 11) and no product in the absence of catalyst (entry 12).

Recycle performance of catalyst III was also examined with the optimum reaction over 48 h (Table 2, entry 1). After the first run, the resin catalyst III was recovered by simple filtration and washed with ethyl acetate and dried under vacuum pump. Recovered resin catalyst III was reused, giving similar yields even after four cycles with 93, 91, 92 and 90% of the desired product 4a, respectively.

Under optimum reaction condition (Table 2, entry 1), a variety of both aldehydes and anilines were investigated for their performance (Table 3). Corresponding product yields depended on steric hindrance and electronic effect of substrates. Reaction of aromatic aldehyde substrates with electron-withdrawing group (–NO₂, –F, –Cl, including –H) produced good to excellent yields (73–96% for products 4a–e and 4j–l). Notably, electron-withdrawing group at para-position gave a superior yield than at meta-position (4a compared to 4c; –NO₂). While aromatic aldehyde substrates with an electron-donating group (–OMe) provided products in moderate and good yields (61 and 76% of 4f and 4m, respectively). Moreover, heteoaromatic aldehydes gave good yields with furan-2-carbaldehyde (80%; 4g), low yield with 2-pyridinecarboxaldehyde (26%; 4h), and no desirable product (4i) with pyrrole-2-carbaldehyde. This result is likely due to basicity of nitrogen atom on substrates that could easily attach to an acid-catalyst and reduce catalytic activity. Aniline substrates with an electron-withdrawing group (–Br, –F) could improve product yields (4j–m), by providing a higher electrophilicily of an iminium ion intermediate. Unfortunately, yields were lower in the case of an electron-donating group (–OMe) on aniline substrate (29%; 4n) because the instability of an iminium ion intermediate and product led to generating undesirable products. Moreover, 2-pentanone was investigated as a nucleophile, resulting in low yields of desired products with 24% (4o).

Table 3 Synthesis of β-amino carbonyl derivatives^a

a All reactions were conducted with aldehyde (1, 1.0 mmol), amine (2, 1.0 mmol) and ketone (3, 1.2 mmol) using 10 mol% of catalyst III in MeCN (3.0 mL), shaking 800 rpm at room temperature; % yields were reported as isolated yields; the ratio of syn : anti was determined by NMR.

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Conclusions

In summary, we have demonstrated a metal-free and convenient method for the preparation of β-amino carbonyl compounds via one-pot three-component Mannich reaction of aldehydes, amines and cyclohexanone at ambient temperature catalyzed by recyclable heterogeneous acid-ionic polymer. The acid-ionic polymer-based imidazolium cation with OTf anion, [PS-Im][OTf] (III) exhibited the most efficient catalyst to produce desirable products in moderate to excellent yields with a wide range of aldehyde and amine substrates. In addition, the catalysts are easily separated from the reaction mixture by simple filtration and reusability of catalyst was possible for subsequent reactions.

Acknowledgements

This work was financially supported by Thailand Research Fund (Grant no. TRG5780298), the Center of Excellence for Innovation in Chemistry (PERCH-CIC) and the Research Grant of Burapha University through National Research Council of Thailand (Grant no. 28/2555 and 16/2556). Special thanks to Prof. Dr Frederick W. H. Beamish, Faculty of Science, Burapha University, for his comments and English correction and Dr Byoung Se Lee (FutureChem Co. Ltd., Korea) for helpful discussion.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra04834c

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