Enhancing the potential of the Michael addition reaction using Betti base-thiourea-derived organocatalysts for the one-pot synthesis of 2,4-disubstituted pyrroles

Abstract

A novel class of Betti base-thiourea-derived bifunctional organocatalysts have been synthesized. The hydroxy group of the naphthyl ring (Betti base) and substituted thiourea moiety effectively enhances the reactivity through hydrogen bonding and facilitates the Michael addition reaction of methyl ketones to nitroolefins, affording 2,4-disubstituted pyrroles, which are core structural motifs in pharmaceuticals and optoelectronic and agrochemicals materials. The comprehensive reaction optimization and broad substrate scope evaluation with moderate to high yields (75–93%) highlight the versatility and efficiency of this catalytic system under mild reaction conditions.

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Introduction

Organocatalysis is one of the most thriving research domains in contemporary organic synthesis approaches after enzymatic and metal-based catalysis.¹ With the development of organocatalysis, substantial progress has been achieved in recent years in solving established synthetic difficulties by uncovering novel catalytic modes and beneficial integrations with various reaction types.² The approach of developing novel organocatalysts is growing into a dynamic activity, and the emergence of Brønsted acid catalysts that enabled increased activity for organic reactions is a prime example. A distinctive single key-activation mechanism recently paved the way to a more enzyme-like use of numerous interactions. The integration of different non-covalent interactions improved the activation methodologies of organocatalysts to obtain high selectivities.³ In these organocatalysts, the activation approaches might lead to enhanced reactivities via H-donor acidity that could not be easily achieved by other means. The concept was found useful in many organic transformations.⁴

Among the different classes of organocatalysts, thiourea-based derivatives have attracted researchers; thus, the high hydrogen bonding activity of urea and thiourea derivatives has led to numerous studies in the field of molecular recognition. Through multi-hydrogen bonding, they have been utilised for detecting nitrate, sulfonic acid, carboxylic acid, and other substances.^5–7 According to recent reports from many groups, urea and thiourea can be used as excellent organocatalysts for synthesising various heterocyclic compounds^8–11 due to their ability to activate electrophiles via hydrogen bonding and offer a compelling alternative by stabilizing the transition state, thus facilitating a range of transformations enhancing both the yield and selectivity.

Many natural products and pharmaceutical molecules contain pyrrole moieties as the core structure (Fig. 1)^12,13 due to their vital role as key intermediates in the synthesis of various anticancer drugs, agrochemicals, and optoelectronic materials. Conventional pyrrole synthesis relies on classical condensation or multicomponent reactions, such as the Hantzsch, Huisgen, and Paal–Knorr syntheses, which require harsh conditions and extended reaction times and involve multistep processes with low atom economy, use of toxic reagents and reactions utilising gaseous reagents such as H₂ or CO at high pressures.¹⁴

Fig. 1 Bio-active molecules containing a pyrrole moiety.

To overcome these challenges, various catalytic systems, including metal catalysts (e.g., Pd, Cu, and Fe) and organocatalysts, have been reported for pyrrole synthesis, offering improved selectivity, use of milder conditions, and higher yields. Organocatalysis has developed into a compelling strategy in modern synthetic chemistry, such as for the Michael addition reaction of methyl ketones to nitroolefins, and the nitro functionality can be transformed into various functional groups, such as ketones, nitrile oxide, amines and carboxylic acids, which play vital roles in the synthesis of complex organic molecules, particularly in pharmaceuticals and natural product synthesis.¹⁵

In particular, the synthetic routes for preparing 2,4-diaryl-substituted pyrrole are underexplored. These methods include the reductive dimerization of phenacyl azides using samarium(II) iodide,¹⁶ coupling of unactivated alkynes and amines with carbon monoxide using zirconocene-derived complexes,¹⁷ hydrogenation of α-phenyl-β-benzoyl propionitrile followed by oxidation using selenium,¹⁸ the condensation reactions like acyclic ketone with aziridine,¹⁹ cyclo-condensation of aminoacetonitrile with different enones followed by dehydrocyanation,²⁰ copper-catalyzed reaction of aryl acetaldehydes with vinyl azides (Scheme 1b),²¹ and formylation of 1,3-diphenyl prop-2-en-1-one using an N-heterocyclic carbene (NHC) catalyst followed by ammonium acetate under microwave irradiation (Scheme 1b).²²

Scheme 1 Previous reports (a and b) and this work (c).

Continuing our interest in designing new synthetic methods²³ for biologically significant compounds such as imidazo[1,2-a]pyridines, 1,4-dihydro quinolines, 4-hydroxy coumarins, and tetrazoles, we present herein the synthesis of 2,4 disubstituted pyrroles via the Michael addition of nitrostyrene to the methyl ketone using Betti base-thiourea-derived bifunctional organocatalysts under mild reaction conditions (Scheme 1c).

Results and discussion

Considering the significance of organocatalysts, we tried to design and synthesize Betti base-thiourea-derived bifunctional organocatalysts following the methods reported in the literature.²⁴ We obtained low to poor yields. Hence, various Lewis acids and reaction conditions were attempted to optimize the reaction, such as p-toluene sulfonic acid, ferric chloride, aluminium chloride, and titanium chloride, which afforded poor yield (Table 1, entries 2, 4, 5 and 7). While phosphorus pentoxide (Table 1, entry 3) failed to give the desired product, the best result was obtained with zinc chloride (Table 1, entry 6). Then, different solvents were optimized. Among the solvents, ethanol at room temperature gave the highest product yield. Under the optimized condition, different organocatalysts were synthesized (Scheme 2).

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst	Solvent	Temp. (°C)	Time (h)	% yield^b
a Reaction conditions: (i) A (1.0 mmol), B (1.0 mmol), C (1.0 mmol), Catalyst (0.2 mmol) in 5.0 mL EtOH. b Isolated yield. c NR = no result.
1.	—	EtOH	25	24	NR^c
2.	p-TSA	EtOH	25	24	45
3.	P2O5	EtOH	25	24	NR
4.	FeCl3	EtOH	25	24	8
5.	AlCl3	EtOH	25	24	18
6.	ZnCl 2	EtOH	25	24	91
7.	TiCl3	EtOH	25	24	9
8.	BBr3	EtOH	25	24	Trace
9.	H2SO4	EtOH	25	24	5
10.	ZnCl2	DCM	25	24	85
11.	ZnCl2	Toluene	25	24	79
12.	ZnCl2	DMSO	25	24	79
13.	ZnCl2	MeCN	25	24	79
14.	ZnCl2	EtOH	40	24	79
15.	ZnCl2	EtOH	78	24	85

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Scheme 2 Substrate scopes for the as-synthesized organocatalysts.

To seek an efficient and optimal reaction condition for the one-pot synthesis of 2,4-disubstituted pyrroles under non-toxic and environmentally friendly conditions, we performed the reaction using acetophenone (0.5 mmol), nitrostyrene (0.5 mmol), and organocatalyst (0.05 mmol), as given in Table 2. We used L-proline, glycine, and boric acid, which afforded poor yields (Table 2, entries 2–4), while dextrose, urea, thiourea and phenylthiourea failed to give any products. Hence, different Betti base-thiourea-derived bifunctional organocatalysts were synthesized (Scheme 2) and used to optimize the proposed scheme (Table 2). Catalysts 1 to 6 were used, out of which catalyst 5 (Cat-5) gave the best results. For the solvent optimization, different solvents were used out of which ethyl acetate, dichloromethane, tetrahydrofuran and acetonitrile failed to give any products, while toluene, dichloroethane, methanol and DMSO gave poor yield (Table 2, entries 12–15).

Table 2 Optimization of the reaction conditions^a,b

Entry	Catalyst	Solvent	Temp. (°C)	Time (h)	% yield^c
a Reaction conditions: (i) 1 (0.5 mmol), 2 (0.5 mmol), Catalyst (0.05 mmol) in 1.0 ml EtOH. (ii) Zn dust (1.0 mmol), HCl (0.5 ml). (iii) I2 (0.5 mmol). b Catalysts dextrose, urea, thiourea and phenylthiourea did not give any reaction. Similarly, no reaction occurred in EtOAc, DCM, THF and CH3CN solvents. c Isolated yield. d NR = no result. e Di-PTU = diphenylthiourea.
1.	—	EtOH	0	24	NR^d
2.	L-Proline	EtOH	0	24	12
3.	Glycine	EtOH	0	24	8
4.	Boric acid	EtOH	0	24	14
5.	Di-PTU^e	EtOH	0	24	Trace
6.	Cat-1	EtOH	0	24	67
7.	Cat-2	EtOH	0	24	75
8.	Cat-3	EtOH	0	24	81
9.	Cat-4	EtOH	0	24	74
10.	Cat-5	EtOH	0	24	93
11.	Cat-6	EtOH	0	24	79
12.	Cat-5	Toluene	0	24	21
13.	Cat-5	DCE	0	24	5
14.	Cat-5	MeOH	0	24	61
15.	Cat-5	DMSO	0	24	24
16.	Cat-5	EtOH	0	10	93
17.	Cat-5	EtOH	0	6	93
18.	Cat-5	EtOH	10	4	91
19.	Cat-5	EtOH	25	4	85
20.	Cat-5	EtOH	50	4	28
21.	Cat-5	EtOH	78	4	93
22.	Cat-5 (5 mol%)	EtOH	0	6	57
23.	Cat-5 (10 mol%)	EtOH	0	6	93
24.	Cat-5 (20 mol%)	EtOH	0	6	93

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The best result was observed with the ethanol solvent, and then the temperature was optimised as 0 °C in 6 hours (Table 2, entry 23). Taking insights from the literature,^25a–c UV-vis spectroscopy was performed to observe the interaction of starting materials with the organocatalyst Cat-5, for which 1a, 2a and Cat-5 were taken in an equimolar ratio in ethanol, and the UV-vis spectra were recorded for individual samples as well as their cross combinations. Fig. 2 shows that Cat-5 is interacting with both 1a (blue curve) and 2a (orange curve), while the red curve illustrates the bifunctional nature of the organocatalyst Cat-5, as it interacts with 1a and 2a simultaneously. The lower yield of the reaction with Cat-1 as compared to Cat-5 indicates that the hydroxy group on the aldehyde A3 is enhancing the reactivity of the organocatalyst, as hydroxyl groups play a vital role by participating in hydrogen bonding and stabilising the transition state (Scheme 6),^25d–f while Cat-4 and Cat-6, having bulky chlorine attached at the ortho-position, provide hindrance for interaction with the starting materials, and hence, lower yields were observed. Catalyst recycling was then examined in Fig. 3, showcasing that the organocatalyst can be easily reused (recovered from the reaction mixture using column chromatography) up to four times without any noticeable drop in conversion and yield of the product.

Fig. 2 Interaction of the starting materials with Cat-5.

Fig. 3 Catalyst recovery cycles.

Under the optimised reaction conditions (Table 2, entry 23), 2,4-disubstituted pyrroles were successfully synthesised using the dual interaction of the organocatalyst (Cat-5). Different methyl ketones and nitrostyrene derivatives, such as mono- and di-substituted, electron-withdrawing, and electron-donating groups, were investigated, and minor yield differences were observed (Scheme 3). Nitrostyrene having electron-withdrawing moieties, such as p-bromo, p-chloro, p-nitro and o-nitro afforded 3b, 3c, 3e and 3j in 87%, 89%, 69% and 55% yields, respectively, while the electron-donating groups such as p-methoxy, p-methyl, o-methyl and o-methoxy gave 3d, 3q, 3s and 3ae in 93%, 91%, 86% and 81% yields, respectively. Similarly, the aromatic methyl ketone having electron-withdrawing moieties such as p-chloro, p-bromo and p-nitro afforded 3f, 3i and 3l in 88%, 90% and 68% yields, respectively. Aromatic methyl ketone gave 3z, 3aa and 3ad in 90%, 88% and 91% yields, respectively. Di-substituted nitrostyrene and methyl ketones afforded 3m, 3n, 3o, 3t, 3v and 3ag in 87%, 92%, 89%, 90%, 80% and 77% yields, respectively. The biologically important moieties such as tetrazole and coumarin were integrated into the starting materials and afforded 3w, 3x, 3ah and 3ai in 79%, 75%, 81% and 76% yields, respectively. The lower yield of starting materials containing nitro groups confirmed that the interaction of nitrostyrene with the organocatalyst as the nitro group of starting materials hinders the nitrostyrene interaction. Following the optimized conditions, we also tried Scheme 4 but failed to obtain substrates 3aj, 3aj and 3ak probably due to the steric hindrance and cis–trans geometry in the transition state 3 (TS-3) (Scheme 6). Similarly, to increase the substrate scopes, we also performed reactions with cinnamonitrile with methyl ketones. However, it failed to give the desired product, which might be due to less hydrogen bonding interaction with cinnamonitrile and a Betti base-thiourea-derived bifunctional organocatalyst. The scale-up reaction employing the ongoing scheme gave the desired products 3d (1.02 g) and 3n (1.44 g), with good yields of 82% and 81%, respectively (Scheme 5).

Scheme 3 Substrate scope of Cat-5.

Scheme 4 Failed substrate scope.

Scheme 5 Scale-up synthesis.

Scheme 6 Plausible reaction mechanism.

Plausible mechanism

Following the literature survey,²⁶ the plausible reaction mechanism proposed is the interaction of nitrostyrene (2a) with the organocatalyst (Cat-5) leading to the transition state 1 (TS-1) which further interacted with methyl ketone (2a) to form the transition state 2 (TS-2). The activated methene group in the transition state 3 (TS-3) attacked nitrostyrene, giving the intermediate-1 (Int-1) which was reduced to form the intermediate-2 (Int-2) using zinc and conc. HCl (ref. 27) and further aromatized using iodine²⁸ in one-pot synthesis that led to the desired product 3a upon oxidation with iodine. Moreover, Int-1 and Int-2 were isolated separately and confirmed by ¹H and ¹³C-NMR.

Conclusions

In this study, we have developed a novel Betti base-thiourea-derived bifunctional organocatalyst for promoting the Michael addition reaction of methyl ketones and nitrostyrenes for the synthesis of 2,4-disubstituted pyrroles. The thiourea catalyst-bearing hydrogen bonding sites efficiently facilitated the Michael addition through hydrogen-bonding activation, achieving good to excellent yields under mild reaction conditions, highlighting its versatility in heterocyclic chemistry. The mechanistic insights and a broad range of substrate scope evaluations further adduce the robustness and practical applicability of the catalytic system.

Experimental

General procedure

(a) Synthesis of organocatalysts. First, 0.2 mmol of zinc chloride was added to a round-bottom flask containing a solution of 1.0 mmol of each of aldehyde, phenylthiourea and beta-naphthol in 5.0 ml of ethanol, and the resulting solution was stirred at room temperature for 24 hours. After completion of the reaction, 25.0 ml of cold water was added to afford the precipitate, which was filtered off and recrystallized using ethanol.

(b) Synthesis of 2,4-disubstituted pyrroles. First, 0.5 mmol of methyl ketone was added to a round-bottom flask containing a solution of nitrostyrene (0.5 mmol) and Cat-5 (0.05 mmol) in 1.0 ml of ethanol kept at 0 °C and stirred for 6 h at room temperature. After consumption of the starting material, 1.0 mmol of zinc dust and 0.5 ml of HCl were added at the same temperature and stirred for 2 hours. Afterwards, 0.5 mmol of I₂ was added and stirred at 60 °C for 3 hours to afford the final product, which was purified by column chromatography.
2,4-diphenyl-1H-pyrrole (3a). Yield 93%; brown solid; mp: 174–176 °C. ¹H NMR (500 MHz, CHLOROFORM-D) δ 8.49 (s, 1H), 7.49 (td, J = 8.4, 1.7 Hz, 4H), 7.43–7.21 (m, 6H), 7.11 (dd, J = 2.7, 1.7 Hz, 1H), 6.77 (dd, J = 2.8, 1.7 Hz, 1H). ¹³C NMR (126 MHz, CHLOROFORM-D) δ 135.3, 132.1, 132.0, 131.0, 129.2, 128.8, 126.9, 126.0, 125.2, 125.1, 116.0, 104.4. IR spectrum (KBr), ν_max, cm⁻¹: 3448 (NH), 1549, 1422, 1434, 1173, 1094, 1076, 953, 706. HRMS analysis: (M − H)⁺ = 218.0972. CHNS analysis (C₁₆H₁₃N): calculated: C (87.64%), H (5.98%), N (6.39%) found: C (87.68%), H (5.99%), N (6.32%).

Author contributions

Rajeev Singh – experiments and original writing. Naseem Ahmed – conceptualization, editing and writing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors declare that the data will be available as reported in this paper.

Supporting information for this article is available. See DOI: https://doi.org/10.1039/D5CY00312A.

Acknowledgements

This research was supported by IIT Roorkee and DST-FIST [SR/FST/CS-II/2018/72(C)] (funding to the Chemistry Department, IIT Roorkee, for the provision of 500 MHz NMR and XRD facilities). Rajeev Singh is grateful to the MHRD, IIT Roorkee, for providing a Senior Research fellowship (SRF). The reviewers are highly appreciated for their valuable comments to improve the manuscript.

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