Synthesis and Toxicological Effect of Some New Pyrrole Derivatives as Prospective Insecticidal Agents against the Cotton Leafworm, Spodoptera littoralis (Boisduval).

Herein, a series of biologically active pyrrole derivatives, namely 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetic acids 2a-c, 2-[(2-hydroxyethyl)-thio]-5-aryl-1H-pyrrole-3-carbonitriles 3a-c, and 2-[(2-amino-ethyl)thio]-5-aryl-1H-pyrrole-3-carbonitriles 4a-c, 2,2'-disulfanediylbis(5-aryl-1H-pyrrole-3-carbonitriles) 5a-c, 2-((3-cyano-5-aryl-1H-pyrrol-2-yl)thio)acetates 6a-c, 2-[(3-cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetohydrazides 7a-c, and 2-{2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetyl}-N-phenyl-hydrazinecarbothioamides 8a-c, as insecticidal agents, were synthesized via adaptable, smoothly accessible 2-(2-oxo-2-arylylethyl)malononitriles 1a-c. The structures were proved using infrared (IR), nuclear magnetic resonance (NMR), and mass spectrum (MS) techniques. Under laboratory conditions, the toxicological characteristics were tested towards Spodoptera littoralis, cotton leafworm insect type. In respect to the LC50 values, compounds 6a, 7a, 8c, and 3c possess the highest insecticidal bioefficacy, with values of 0.5707, 0.1306, 0.9442, and 5.883 ppm, respectively. The study paves the way towards discovering new materials for potential use as insecticidal active agents.

Introduction

A significant threat to agricultural production security is caused by insect pests, which not only may result in large financial losses but could also lead to jeopardizing food security for both people and the country as a whole.[1,2]Spodoptera littoralis (Boisduval) is an example of dangerous polyphagous moth that may feed on more than 100 host plants from 40 economically important plant families, including cotton, potatoes, maize, vegetables, and ornamental plants, generating production losses of up to 50% due to its larval leaf chewing activities.[3−5] This bug is native to Africa; however, it is distributed worldwide,[6] and its control is becoming more difficult due to resistance, and cross-resistance to chemical insecticides and the bioinsecticide Bacillus thuringiensis Berliner.[7,8]

Pyrrole derivatives are a noteworthy family of bioactive molecules that have been used in many sectors, including natural product chemistry, pharmaceutical chemistry, and agrochemical chemistry.[9−12] Many natural products containing the pyrrole moiety exhibit a broad range of agrochemical activity, such as ryanodine, pyrrolomycin, and dioxapyrrolomycin.[13,14] Additionally, some saleable pesticides also contain the pyrrolyl groups within their structures, such as fungicide fludioxonil, fungicide fenpiclonil, and acaricide chlorfenapyr (Figure ). Therefore, pyrrole analogues have a momentous effect on pest control. In addition, pyrrole derivatives have significant biological properties such as antibacterial,[15] COX-1/COX-2 inhibition activities,[16] antitumor activity,[17] antifungal,[18] cyclooxygenase inhibition activities,[19] antianxiety,[20] and antidiabetic.[21] Several pyrrole derivatives are cholesterol-lowering agents,[22] HIV fusion inhibitors,[23] and anticancer.[24] Moreover, pyrrole derivatives are applied for the preparation of semiconducting and fluorescence materials.[25]

Figure 1

Some commercial pesticides containing the pyrrolyl groups within their structures.

Although there is extensive literature that focuses on synthesizing and studying the properties of pyrroles, the development of simple and environmentally acceptable synthesis methods for avoiding pollution and reducing the use of nonrenewable resources in both pharmaceutical and academic fields is the focus of attention.[26,27]

Phase transfer catalysts (PTCs) were frequently employed for the synthesis of organic derivatives in both liquid–liquid and solid–liquid heterogeneous reaction mixtures,[28,29] allowing for the transfer of inter-phase species in two interspecies phases. As a consequence, reaction rates have been accelerated.[30,31] Unlike traditional methods, PTCs deliver economic and environmental benefits for commercial organic synthesis operations. Many studies have shown that PTC is the most efficient approach to produce a large number of active intermediates.[32]

We aim in this work to synthesize a series of pyrrole compounds that possess insecticidal activity using a simple and environmentally acceptable method. Also, we aim to test the potential insecticidal activity of the prepared compounds against the cotton leafworm, S. littoralis.

Results and Discussion

Synthesis

Herein, target products, namely 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetic acids 2a-c, 2-[(2-hydroxyethyl)-thio]-5-aryl-1H-pyrrole-3-carbonitriles 3a-c, and 2-[(2-amino-ethyl)thio]-5-aryl-1H-pyrrole-3-carbo-nitriles 4a-c, were successfully prepared. The obtained yield is 61–90% through cycloaddition reaction of 2-mercaptoacetic acid, 2-mercaptoethylamine, or 2-mercaptoethanol, respectively, on 2-(2-oxo-2-arylethyl)malononitriles 1a-c under phase transfer catalysis (PTC) conditions using tetrabutylammonium bromide as phase transfer catalyst, dioxane as an organic liquid phase, and potassium carbonate as a solid phase (Scheme ).

Scheme 1

Reaction Route to the Synthesis of the Compounds 2a-c–5a-c

A suitable mechanism for the preparation of the molecules 3a-c–5a-c is proposed in Scheme , where the reacting anions are continuously introduced into the organic phase in the form of lipophilic ion pairs with lipophilic cations supplied by the catalyst, i.e., tetrabutylammonium bromide.[33,34]

Scheme 2

Suggested Mechanism for Synthesis of the Molecules

The synthesis of [(3-cyano-5-phenyl-1H-pyrrol-2-yl)sulfanyl]acetic acid 3a was reported by Ikemoto et al.,[35] from 2-(2-oxo-2-phenylethyl)malononitrile 1a and mercaptoacetic acid under reflux in methanol with 60% yield. Here we managed to synthesize [(3-cyano-5-aryl-1H-pyrrol-2-yl)sulfanyl]acetic acids 3a,b, in 85–90% yield, by the treatment of compounds 1a,b with 2-mercaptoacetic acid (2) under PTC conditions at room temperature (Scheme ).

Elemental analyses using infrared (IR), 1H NMR, and 13C NMR spectra are in agreement with the proposed structures (see Experimental Section).

The IR spectrum of product 2a showed bands at 1703, 2228, and 2830–3447 cm–1 corresponding to (C=O)carboxylic, (CN), and (OH)carboxylic groups, respectively. In addition, the 1H NMR spectrum showed the occurrence of methylene protons (S-CH2) as a singlet signal at δ 3.6 ppm; the aromatic protons appeared as multiplet signals at 6.93–7.68 ppm, and as broad D2O-exchangeable signals at 12.51 and 13.74 ppm corresponding to (NH) and (OH) groups, respectively. The 13C NMR spectrum showed signals at 39.9 ppm for the methylene group (S-CH2), at 97.4, 109.22, 124.5, 127.6, 129.3, 131.1, 131.4, and 135.1 ppm due to the sp2-carbons, at 116.7 ppm due to the (CN) group, and at 171.26 ppm, which can assigned to the (CO) group.

The IR spectrum of compound 3a showed the disappearance of the characteristic absorption band for the ketonic (C=O) group and exhibited a new band at 3355 and 3277 cm–1 corresponding to the (NH2) group, and at 2196 cm–1 due to the (CN) group. Its 1H NMR spectrum showed the appearance of triplet signals at 2.94 and 3.55 ppm, assigned to (−CHNH2) and (−SCH−), respectively; the (NH2) group appeared at 4.3 ppm, six aromatic proton signals appeared at 6.92–7.52 ppm, and the (NH) group appeared at 12.51 ppm. The 13C NMR spectrum showed signals at 59.8 and 60.70 for (S-CH2) and (CH2-NH2), respectively. The signals at 99.9, 109.6, 124.7, 128.1, 129.5, 130.2, 130.6, and 135.6 can be ascribed to the sp2-carbons. The signal at 116.83 ppm is due to the (CN) group.

The IR spectrum of compound 4a showed the disappearance of the characteristic absorption band of the ketonic (C=O) group. However, it exhibited new bands at 3418 and 3182 cm–1 for (OH) and (NH) groups, respectively, and at 2224 cm–1 band for the (CN) group. The 1H NMR spectrum showed the appearance of triplet signals at 2.99 and 3.58 ppm, assigned to the (−SCH−) and (−CHOH) groups, respectively, a signal at 5.02 ppm, attributed to the hydroxyl (OH) group, and six aromatic proton signals at 6.98–7.72 ppm, in addition to the signal at 12.48 ppm attributed to the (NH) group.

2,2′-Disulfanediylbis(5-aryl-1H-pyrrole-3-carbonitriles) 5a-c were formed with a yield in the range of 88–92% by a nucleophilic addition reaction of hydrogen sulfide on 2-(2-oxo-2-arylethyl)malononitriles 1a-c in the presence of triethylamine (TEA) as a catalyst (Scheme ). The suggested mechanism for the synthesis of compounds 5a-c is depicted in Scheme .

Scheme 3

Suggested Mechanism for the Synthesis of the Molecules 5a-c

The IR spectrum of compound 5a showed the disappearance of the characteristic absorption band for the ketonic (C=O) group. New sharp bands at 2222 and 3262 cm–1 are assigned to the (CN) and (NH) groups, respectively. The 1H NMR spectrum shows the disappearance of aliphatic protons signals; however, the appearance of aromatic protons as broad doublet and multiplet signals at 7.13–7.79 ppm and the appearance of the (NH) group as an exchangeable singlet signal at 12.82 ppm were also observed. The 13C NMR spectrum shows the appearance of eight signals in the aromatic region at 103.9, 111.7, 125.4, 127.5, 128.5, 129.36, 130.68, and 138.4 ppm, which are characteristic for sp2-carbons. The signal at 115.3 ppm is characteristic of the (CN) group. The mass spectrum (MS) showed a molecular ion peak m/z (rel. intensity %): 398 (11) [M]+, and it also showed the base peak at m/z 199 (100).

Esterification of 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetic acids 2a-c in methyl alcohol in the presence of sulfuric acid as a catalyst under reflux conditions gave the corresponding methyl-2-((3-cyano-5-aryl-1H-pyrrol-2-yl)thio)acetates 6a-c, which were subjected to reaction with hydrazine hydrate at room temperature to give the corresponding 2-[(3-cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetohydrazides 7a-c, which were then reacted with the phenyl isothiocyanate of 2-{2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetyl}-N-phenylhydrazinecarbothioamides 8a-c (Scheme ).

Scheme 4

Reaction Route to the Synthesis of the Compounds 6a-c–8a-c

The IR spectrum of compound 6c showed the removal of the hydroxyl group band and occurrence of a new band at 1708 cm–1, characteristic of the (CO)ester group. Its 1H NMR spectrum showed the removal of the hydroxyl group signal and occurrence of a new signal at 3.63 ppm, characteristic of the (OCH)ester group. Its 13C NMR spectrum showed a new signal at δ 37.94 ppm, characteristic of the (OCH3)ester group.

The IR spectrum of compound 7a showed the disappearance of the band of (CO)ester and appearance of a new band at 1655 cm–1 for the (CO)amidic group, and occurrence of bands at 3327 and 3279 cm–1 for the (NH2) group. Its 1H NMR spectrum shows the removal of a methyl signal and occurrence of new signals at 3.56 and 9.23 ppm for (NH2) and (NH)hydrazide groups, respectively.

The IR spectrum of compound 8c showed the removal of the amino group. 1H NMR shows clearly broad D2O-exchangeable signals at 9.63 and 10.3 ppm, and a signal at 12.9 ppm for (NH) groups.

Insecticidal Bioefficacy Screening

To assess the potential application of the prepared compounds as insecticidal agents, their toxic activities towards S. littoralis (Boisd.) were tested. Accordingly, the LC50 values were determined (see Table and Figures , 3) and used as a measure for comparisons. The results show that compound 7a is more active than the other pyrrole-synthesized derivatives.

Table 1

Insecticidal Activity of Compounds 2–8 against the 2nd and 4th Larvae of S. littoralis (Boisd.) after 72 h of Treatment Compared to Dimilin as the Standard Insecticide

	2nd instar larvae			4th instar larvae
comp.	LC50 (ppm)	slope	toxic ratio	LC50 (ppm)	slope	toxic ratio
2a	8.4025	0.2971 ± 0.0815	0.0155	145.369	0.301 ± 0.0991	0.4586
3c	5.8838	0.1889 ± 0.0756	0.0221	81.406	0.231 ± 0.0880	0.8189
4c	11.0533	0.1716 ± 0.0754	0.0118	154.471	0.225 ± 0.0820	0.4316
5c	8.859	0.1707 ± 0.0752	0.0147	128.376	0.297 ± 0.0978	0.5193
6a	0.5707	0.3167 ± 0.0779	0.2288	67.908	0.297 ± 0.0893	0.9817
7a	0.1306	0.1306 ± 0.0770	1	66.670	0.231 ± 0.0880	1
8c	0.9442	0.2798 ± 0.0771	0.1383	71.670	0.266 ± 0.0912	0.9302
dimilin	0.1021	0.1326 ± 0.0760	1	62.890	0.2103 ± 0.0819	1

Figure 2

Insecticidal activity of Compounds 2–8 against the 2nd and 4th larvae of S. littoralis (Boisd.) after 72 h of treatment compared to Dimilin as the standard insecticide.

Figure 3

Insecticidal activities of compounds 2–8 against the 2nd (black line) and 4th (red line) instar larvae of S. littoralis (Boisd.) after 72 h of treatment.

The target synthesized compounds were screened for their insecticidal bioefficacy as explained below.

Insecticidal Bioefficacy Test for the 2nd Larvae of S. littoralis (Boisd.) after 72 h of Treatment

The results of compounds 2–8 tested against the larvae after 72 h are shown in Figures and 3 and summarized in Table . The results confirm that all compounds have high to low insecticidal activity against the 2nd larvae of S. littoralis (Boisd.), when compared to the commercial Dimilin insecticide, with LC50 values ranging from 0.1306 to 11.053 ppm. The results show that compounds 6a, 7a, and 8c have promising insecticidal activity against the 2nd larvae of S. littoralis (Boisd.). Compounds 6a, 7a, 8c, and 3c possess a high insecticidal bioefficacy, and their LC50 values are 0.5707, 0.1306, 0.9442, and 5.883 ppm, respectively.

Insecticidal Bioefficacy Test for the 4th Larvae of S. littoralis (Boisd.) after 72 h of Treatment

The results of compounds 2–8 after 72 h of treatment on the 4th larvae are illustrated in Figures and 3, and the LC50 values are summarized in Table . The results show that all of the target compounds exhibit strong to weak insecticidal activity compared to the commercial Dimilin insecticide material, which showed assorted values from 66.670 to 154.471 ppm.

By using a computerized regression analysis program,[36] the median lethal concentration (LC50) and slope values of the target compounds were computed and reported as parts per million (ppm) (Figure ). The insecticidal activity of the synthesized compounds (2a, 3c, 4c, 5c, 6a, 7a, and 8c) were compared with Dimilin against S. littoralis (Boisd.), in which 2nd instar larvae are represented by black lines and 4th instar larvae are represented by red lines after 72 h of treatment. The analysis shows that compound 7a is more active towards S. littoralis (Boisd.) than the other synthesized compounds, and much closer to the activity of the Dimilin insecticide.

Structure-Action Relationship

Generally, our results demonstrate that the synthesized moiety-containing pyrrole derivatives of 3c, 6a, 7a, and 8c compounds are more active against the 2nd larvae of S. littoralis (Boisd.) than the rest of the other tested compounds. Also, the results show that compounds containing acetohydrazide and the cyano group as a part of their structure show insecticidal activity against the 2nd and 4th larvae of S. littoralis (Boisd.), in comparison to the commercial Dimilin insecticide.

Conclusions

We have successfully developed and prepared a novel and ecologically acceptable pathway to synthesize a series of some new bioactive pyrrole derivatives that possess insecticidal activity using a simple and environmental acceptable method. The structures of the newly synthesized compounds were proved using IR, MS, and 1H, 13C NMR spectroscopic techniques. Furthermore, the toxicological parameters of the prepared products under laboratory circumstances were investigated towards the cotton leafworm, S. littoralis (Boisduval), and estimated LC50 values were used for comparisons. The results show that compounds 3c, 6a, 7a, and 8c possess a high and promising insecticidal bioefficacy, with LC50 values of 5.883, 0.5707, 0.1306, and 0.9442 ppm, respectively. Compound 7a has comparative values to the commercial Dimilin drug against the cotton leafworm, S. littoralis (Boisduval).

Materials and Methods

General Experimental Procedures

All starting materials are from Merck, Fluka, and Aldrich suppliers, and were used as received without any further purification. Using precoated plate silica G/UV-254 of 0.25 mm thickness (Merck 60F254) to display UV light (254/365 nm), all reactions were monitored by chromatography of thin layers. Melting points were detected with a Kofler melting point device and were uncorrected. IFRED spectra were collected using an FT-IR-ALPHBROKER-Platinum-ATR spectrophotometer with the ATR technique. The DMSO-d6 for the Bruker Bio Spin AG spectrometer at 400 and 100 MHz was used to record 1H NMR and 13C NMR spectra, respectively. The chemical shifts (δ) for 1H NMR, as an internal standard (δ = 0), are in parts per million (ppm) with reference to the tetramethylsilane (TMS). Coupling constants (J) in hertz (Hz) and other information are reported as follows: chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, m = multiple, dd = doublet). For 13C NMR, TMS (δ = 0), the internal standard was employed, and spectra with full proton disassembly were produced. In the Perkin-Elmer model, CHN analyzers were used for elemental analyses.

General Procedure for the Synthesis of 2-(2-Oxo-2-arylethyl)malononitriles 1a-c[37]

A mixture of malononitrile (6.6 g, 0.1 mol) and an appropriate phenacyl bromide derivative (0.1 mol) in ethanol (150 mL) was cooled to 0 °C. A solution of potassium hydroxide (8.4 g, 0.15 mol) in ethanol (50 mL) was added dropwise and the reaction mixture was stirred for 2 h (TLC). Then, the reaction, with caution, was quenched with ice-H2O and 1 M HCl solution. Finally, the formed solid was collected by filtration, dried, and crystallized from ethanol to give a white product; yield 85–87%.

General Procedure for the Synthesis of 2-[(3-Cyano-5-aryl-1H-pyrrol-2-yl)thio]acetic Acids 2a-c

A mixture of 2-mercaptoacetic acid (9.2 g, 0.1 mol), anhydrous potassium carbonate (42 g, 0.3 mol), and tetrabutylammonium bromide (TBAB) (2.58 g, 0.008 mol) in dioxane (150 mL) was stirred for 30 min at room temperature; then, the appropriate 2-(2-oxo-2-arylethyl)malononitrile derivative (0.1 mol) was added. The reaction mixture was stirred for 2.30 h (TLC). The reaction mixture was filtered off, the precipitate was treated with distilled water, and acidified with diluted HCl. The formed precipitate was filtered off, left to dry, and crystallized from ethanol.

2-[(3-Cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetic Acid (2a)

White powder, yield: (23.3 g, 90%), mp: 135–137 °C, IR (cm–1): 1703(CO), 2228 (CN) 2830–3447 (OHcarboxylic). 1H NMR (DMSO-d6) δ: 3.6 (s, 2H, CH2), 6.93–7.68 (m, 6H, Haromatic), 12.51 (s, 1H, NH), 13.74 (s, 1H, OH). 13C NMR (DMSO-d6) δ: 39.9, 109.2, 97.4, 116.7, 124.5, 127.6, 129.3, 131.1, 131.4, 135.1, and 171.26. Elemental Analysis Calcd for C13H10N2O2S (258.05): C, 60.45; H, 3.90; N, 10.85; S,12.41. Found: C, 60.35; H, 3.95; N, 10.90; S,12.46.

2-{[5-(4-Chlorophenyl)-3-cyano-1H-pyrrol-2-yl]thio}acetic Acid (2b)

White powder, yield: (24.88 g, 85%), mp: 143–145 °C, IR (cm–1): 1713(CO), 2225 (CN) 2850–3430 (OHcarboxylic). 1H NMR (DMSO-d6) δ: 3.72 (s, 2H, CH2), 6.92–7.98 (m, 5H, Haromatic), 12.49 (s, 1H, NH), 13.50 (s, 1H, OH). 13C NMR (DMSO-d6) δ: 39.0, 99.4, 107.9, 117.0, 124.1, 128.9, 129.9, 129.3, 133.9, 134.3, and 171. Elemental Analysis Calcd for C13H9ClN2O2S (292.74): C, 53.34; H, 3.10; Cl, 12.11; N, 9.57; S,10.95. Found: C, 53.04; H, 3.85; N, 10.95; S,12.46.

2-{[3-Cyano-5-(4-nitrophenyl)-1H-pyrrol-2-yl]thio}acetic Acid (2c)

Brown powder, yield: (26.66 g, 88%), mp: 163–165 °C, IR (cm–1): 1713 (CO), 2225 (CN) 2850–3430 (OH). 1H NMR (DMSO-d6) δ: 3.65 (s, 2H, CH2), 6.95–8.32 (m, 5H, Haromatic), 12.50 (s, 1H, NH), 13.50 (s, 1H, OH). 13C NMR (DMSO-d6) δ: 39.0, 99.4, 107.9, 117.0, 124.1, 124.4, 126.2, 129.9, 141.9, 147.9, and 171.1. Elemental Analysis Calcd for C13H9N3O4S (303.29): C, 51.48; H, 2.99; N, 13.85; S,10.57. Found: C, 51.48; H, 2.90; N, 13.80; S,10.62.

General Procedure for the Synthesis of 2-[(2-Aminoethyl)thio]-5-aryl-1H-pyrrole-3-carbonitriles 3a-c

A mixture of 2-aminoethanethiol hydrochloride (1.14 g, 0.01 mol), anhydrous potassium carbonate (4.2 g, 0.03 mol), and a catalytic amount of tetrabutylammonium bromide (TBAB) (0.258 g, 0.0008 mol) in dioxane (30 mL) was stirred for 40 min at room temperature; then, the appropriate phenacyl malononitrile derivative (0.01 mol) was added. The reaction mixture stirring was continued for 8 h (TLC). The mixture was filtered off, the filtrate was evaporated, and the formed precipitate was triturated with petroleum ether several times, then filtered off, and crystallized from ethylacetate–hexane (1:1).

2-[(2-Aminoethyl)thio]-5-phenyl-1H-pyrrole-3-carbonitrile (3a)

Yellow powder, yield: (1.56 g, 64%), mp: 205–208 °C, IR (cm–1): 2196 (CN), 3277, 3355 (NH2). 1H NMR (DMSO-d6) δ: 2.96 (t, J = 6.74 Hz, 2H, CH2NH2), 3.55 (t, J = 6.77 Hz, 2H, SCH2), 4.30 (s, 2H, NH2), 6.92–7.52 (m, 6H, Haromatic), 12.51 (s, 1H, NH). Elemental Analysis Calcd for C13H13N3S (243.33): C, 64.17; H, 5.39; N, 17.27; S, 13.18. Found: C, 64.00; H, 5.46; N, 17.60; S,13.23.

2-[(2-Aminoethyl)thio]-5-(4-chlorophenyl)-1H-pyrrole-3-carbonitrile (3b)

Yellow powder, yield: (1.83 g, 66%), mp: 210–212 °C, IR (cm–1): 2196 (CN), 3275,3352 (NH2). 1H NMR (DMSO-d6) δ: 2.92 (t, J = 6.8 Hz, 2H, CHNH2), 3.58 (t, J = 6.76 Hz, 2H, SCH), 4.32 (s, 2H, NH2), 6.90–7.95 (m, 5H, Haromatic), 12.51 (s, 1H, NH). Elemental Analysis Calcd for C13H12ClN3S (277.77): C, 56.21; H, 4.35; Cl, 12.76; N, 15.13; S, 11.54 Found: C, 56.15; H, 4.41; Cl, 12.75; N, 15.13; S, 11.55.

2-[(2-Aminoethyl)thio]-5-(4-nitrophenyl)-1H-pyrrole-3-carbonitrile (3c)

Yellow powder, yield: (1.9 g, 66%), mp: 222–224 °C, IR (cm–1): 2198 (CN), 3277,3350 (NH2). 1H NMR (DMSO-d6) δ: 2.96 (t, J = 6.77 Hz, 2H, CHNH2), 3.57 (t, J = 6.8 Hz, 2H, SCH), 4.31 (s, 2H, NH2), 6.90–8.33 (m, 5H, Haromatic), 12.51 (s, 1H, NH). Elemental Analysis Calcd for C13H12N4O2S (288.32): C, 54.15; H, 4.20; N, 19.43; S, 11.12 Found: C, 54.15; H, 4.20; N, 19.45; S, 11.10.

General Procedure for the Synthesis of 2-[(2-hydroxyethyl)thio]-5-aryl-1H-pyrrole-3-carbonitriles 4a-c

A mixture of 2-mercaptoethanol (0.78 g, 0.01 mol), anhydrous potassium carbonate (2.8 g, 0.02 mol), and a catalytic amount of tetrabutylammonium bromide (TBAB) (0.258 g, 0.0008 mol) in dioxane (25 mL) was stirred for 20 min at room temperature; then, the appropriate phenacyl malononitrile derivative (0.01 mol) was added. The reaction was stirred for 6 h (TLC). The reaction mixture was filtered off, the filtrate was evaporated, the dried formed precipitate was triturated with petroleum ether several times, and recrystallized from ethylacetate–hexane (1:1).

2-[(2-Hydroxyethyl)thio]-5-phenyl-1H-pyrrole-3-carbonitrile (4a)

Brown powder, yield: (1.49 g, 61%), mp: 108–110 °C, IR (cm–1): 2224 (CN), 3182 (NH), 3418 (OH). 1H NMR (DMSO-d6) δ: 2.99 (t, J = 6.8 Hz, 2H, SCH), 3.58 (t, J = 6.84 Hz, 2H, CHOH), 5.02 (s, 1H, OH), 6.98–7.72 (m, 6H, Haromatic), 12.48 (s, 1H, NH). Elemental Analysis Calcd for C13H12N2OS (244.31): C, 63.91; H, 4.95; N, 11.47; S, 13.12. Found: C, 63.86; H, 4.90; N, 11.52; S,10.67.

5-(4-Chlorophenyl)-2-[(2-hydroxyethyl)thio]-1H-pyrrole-3-carbonitrile (4b)

Brown powder, yield: (1.72 g, 62%), mp: 114–116 °C, IR (cm–1): 2224 (CN), 3182 (NH), 3418 (OH). 1H NMR (DMSO-d6) δ: 2.96 (t, J = 6.78 Hz, 2H, SCH), 3.6 (t, J = 6.77 Hz, 2H, CHOH), 5.02 (s, 1H, OH), 6.98–7.98 (m, 5H, Haromatic), 12.48 (s, 1H, NH). Elemental Analysis Calcd for C13H11ClN2OS (278.76): C, 56.01; H, 3.98; Cl, 12.72; N, 10.05; S, 11.50. Found: C, 56.01; H, 3.95; Cl, 12.70; N, 10.08; S,11.50.

2-[(2-Hydroxyethyl)thio]-5-(4-nitrophenyl)-1H-pyrrole-3-carbonitrile (4c)

Brown powder, yield: (1.67 g, 60%), mp: 118–120 °C, IR (cm–1): 2224 (CN), 3182 (NH), 3418 (OH). 1H NMR (DMSO-d6) δ: 2.95 (t, J = 6.82 Hz, 2H, SCH), 3.61 (t, J = 6.81 Hz, 2H, CHOH), 5.05 (s, 1H, OH), 6.97–8.32 (m, 5H, Haromatic), 12.48 (s, 1H, NH). Elemental Analysis Calcd for C13H11N3O3S (278.76): C, 53.97; H, 3.83; N, 14.52; S, 11.08. Found: C, 53.93; H, 3.87; N, 14.55; S,11.12.

General Procedure of the Synthesis of 2,2′-disulfanediylbis (5-phenyl-1H-pyrrole-3-carbonitriles) 5a-c

These were prepared as previously reported by Moiseeva et al.[38] as follows:

An appropriate phenacyl malononitrile derivative (0.02 mol) and catalytic amount of TEA were placed in ethanol (50 mL); then, dry hydrogen sulfide gas was passed through this mixture until the saturation. The reaction mixture was refluxed for 1 h and left to cool; the formed precipitate was filtered off, washed with ethanol, dried, and crystallized from dioxane.

2,2′-Disulfanediylbis(5-phenyl-1H-pyrrole-3-carbonitrile) (5a)

Yellow powder, yield: (7.33 g, 92%), mp: 263–265 °C, IR (cm–1): 2222 (CN), 3262 (NH). 1H NMR (DMSO-d6) δ: 7.13–7.79 (m, 12H, Haromatic), 12.82 (s, 1H, NH). 13C NMR (DMSO-d6) δ: 103.9, 111.7, 125.4, 127.5, 128.5, 129.36, 130.68, 115.3, and 138.4. Mass spectrum m/z (rel. intensity %): 398 (11) [M]+. Elemental Analysis Calcd for C22H14N4S2 (398.50): C, 66.31; H, 3.54; N, 14.06; S, 16.09. Found: C, 66.25; H, 3.57; N, 14.09; S, 16.09.

2,2′-Disulfanediylbis (5-(4-chlorophenyl)-1H-pyrrole-3-carbonitrile) (5b)

Yellow powder, yield: (8.22 g, 88%), mp: 277–279 °C, IR (cm–1): 2225 (CN), 3265 (NH). 1H NMR (DMSO-d6) δ: 7.15–8.01 (m, 10H, Haromatic), 12.82 (s, 1H, NH). Elemental Analysis Calcd for C22H12Cl2N4S2 (467.39): C, 56.53; H, 2.59; Cl, 15.17; N, 11.99; S, 13.72. Found: C, 56.49; H, 2.61; Cl, 15.19; N, 11.95; S, 13.74.

2,2′-Disulfanediylbis(5-(4-nitrophenyl)-1H-pyrrole-3-carbonitrile) (5c)

Brown powder, yield: (8.4 g, 90%), mp: 285–287 °C, IR (cm–1): 2227 (CN), 3263 (NH). 1H NMR (DMSO-d6) δ: 7.15–8.37 (m, 10H, Haromatic), 12.81 (s, 1H, NH). Elemental Analysis Calcd for C22H12N6O4S2 (467.39): C, 54.09; H, 2.48; N, 17.20; S, 13.13. Found: C, 54.02; H, 2.50; N, 17.24; S, 13.13.

General Procedure for the Synthesis of Methyl-2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetates 6a-c

A mixture of an appropriate amount of 2-[(5-aryl-3-cyano-1H-pyrrol-2-yl)thio]-acetic acid 7a-c (0.1 mol) in methanol (100 mL) and a catalytic amount of sulfuric acid were refluxed for 2 h (TLC). The reaction mixture was left to cool, the precipitate was filtered off, washed with water, dried, and crystallized from ethanol-water.

Methyl-2-[(3-cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetate (6a)

Gray powder, yield: (23.14 g, 85%), mp: 85–87 °C, IR (cm–1): 1712 (CO), 2220 (CN), 3295 (NH). 1H NMR (DMSO-d6) δ: 3.6 (s, 3H, CH3), 4.30 (s, 2H, CH2), 6.95–7.70 (m, 6H, Haromatic), 12.89 (s, 1H, NH). Elemental Analysis Calcd for C14H12N2O2S (272.32): C, 61.75; H, 4.44; N, 10.29; S, 11.77. Found: C, 61.63; H, 4.48; N, 10.32; S, 11.80.

Methyl-2-{[5-(4-chlorophenyl)-3-cyano-1H-pyrrol-2-yl]thio}acetate (6b)

White powder, yield: (24.54 g, 80%), mp: 95–98 °C, IR (cm–1): 1714 (CO), 2222 (CN) 3297 (NH). 1H NMR (DMSO-d6) δ: 3.62 (s, 3H, CH3), 4.29 (s, 2H, CH2), 6.92–7.92 (m, 5H, Haromatic), 12.95 (s, 1H, NH). Elemental Analysis Calcd for C14H11ClN2O2S (306.77): C, 54.81; H, 3.61; Cl, 11.56; N, 9.13; S, 10.45. Found: C, 54.83; H, 3.65; Cl, 11.53; N, 9.11; S, 10.46.

Methyl-2-{[3-cyano-5-(4-nitrophenyl)-1H-pyrrol-2-yl]thio}acetate (6c)

Brown powder, yield: (26.65 g, 84%), mp: 120–122 °C, IR (cm–1): 1282, 1327 (NO2), 1708 (CO), 2219 (CN), 3295 (NH). 1H NMR (DMSO-d6) δ: 3.63 (s, 3H, CH3), 4.26 (s, 2H, CH2), 7.35–8.30 (m, 5H,Haromatic), 12.99 (s, 1H, NH). 13C NMR (DMSO-d6) δ: 37.94, 52.88, 101.6, 113.4, 115.8, 124.85, 125.44, 130.43, 133.77, 136.85, 146.41, and 169.47. Elemental Analysis Calcd for C14H11N3O4S (317.32): C, 52.99; H, 3.49; N, 13.24; S, 10.10. Found: 52.99; H, 3.55; N, 13.26; S, 10.12.

General Procedure for the Synthesis of 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetohydrazides 7a-c

To a solution of the appropriate quantity of 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetate 12a-c (0.1 mol) in methanol (70 mL), hydrazine hydrate (0.2 mol) was added dropwise at room temperature, the reaction mixture was stirred for 2 h (TLC), the formed precipitate was filtered off, washed with methanol, and crystallized from methanol.

2-[(3-Cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetohydrazide (7a)

White powder, yield: (25.05 g, 92%), mp: 176–178 °C, IR (cm–1): 1655 (CO), 2218 (CN), 3248 (NH), 3279, 3327 (NH2). 1H NMR (DMSO-d6) δ: 3.56 (s, 2H, NH2), 4.35 (s, 2H, CH2), 7.04–7.73 (m, 6H, Haromatic), 9.23 (s, 1H, NHhydrazide), 12.65 (s, 1H, NHpyrrole). 13C NMR (DMSO-d6) δ: 38.22, 99.73, 109.90, 124.8, 128.0, 129.41, 130.87, 135.67 and 167.66. Elemental Analysis Calcd for C13H12N4OS (272.33): C, 57.34; H, 4.44; N, 20.57; S, 11.77. Found: C, 57.36; H, 4.47; N, 20.55; S, 11.75.

2-{[5-(4-Chlorophenyl)-3-cyano-1H-pyrrol-2-yl]thio}acetohydrazide (7b)

White powder, yield: (23.9 g, 78%), mp: 196–198 °C, IR (cm–1): 1659 (CO), 2221 (CN), 3248 (NH), 3281, 3329 (NH2). 1H NMR (DMSO-d6) δ: 3.56 (s, 2H, NH2), 4.40 (s, 2H, CH2), 7.1–7.95 (m, 5H, Haromatic), 9.28 (s, 1H, NHhydrazide), 12.63 (s, 1H, NHpyrrole). Elemental Analysis Calcd for C13H11ClN4OS (306.77): C, 50.90; H, 3.61; Cl, 11.56; N, 18.26; S, 10.45. Found: C, 50.88; H, 3.63; Cl, 11.56; N, 18.28; S, 10.47.

2-{[3-Cyano-5-(4-nitrophenyl)-1H-pyrrol-2-yl]thio}acetohydrazide (7c)

White powder, yield: (27.6 g, 87%), mp: 188–190 °C, IR (cm–1): 1657 (CO), 2215 (CN), 3241 (NH), 3273, 3325 (NH2). 1H NMR (DMSO-d6) δ: 3.56 (s, 2H, NH2), 4.40 (s, 2H, CH2), 7.30–8.31 (m, 5H, Haromatic), 9.28 (s, 1H, NHhydrazide), 12.67 (s, 1H, NHpyrrole). Elemental Analysis Calcd for C13H11N5O3S (317.32): C, 49.21; H, 3.49; N, 22.07; S, 10.10. Found: C, 49.23; H, 3.51; N, 22.05;S, 10.10.

General Procedure for the Synthesis of 2-{2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio]acetyl}-N-phenylhydrazinecarbothioamides 8a-c

A mixture of the appropriate quantity of 2-[(3-cyano-5-aryl-1H-pyrrol-2-yl)thio] acetohydrazide 13a,c (0.02 mol) and phenyl isothiocyanate (2.38 g, 0.02 mol) in ethanol (100 mL) was refluxed for 3 h (TLC). The reaction was left to cool, the formed precipitate was filtered off, washed with ethanol, and crystallized from ethanol.

2-{2-[(3-Cyano-5-phenyl-1H-pyrrol-2-yl)thio]acetyl}-N-phenylhydrazinecarbothioamide (8a)

White powder, yield: (6.92 g, 85%), mp: 195–197 °C, IR (cm–1): 1671 (CO), 2227 (CN), 3311 (NH). 1H NMR (DMSO-d6) δ: 3.81 (s, 2H, CH2), 6.92–7.95 (m, 11H, Haromatic), 9.65 (s, 2H, 2NHthioamidic), 10.29 (s, 1H, NHamidic), 12.9 (s, 1H, NHpyrrole). Elemental Analysis Calcd for C20H17N5OS2 (407.51): C, 58.95; H, 4.20; N, 17.19; S, 15.74. Found: C, 58.92; H, 4.20; N, 17.20; S, 15.74.

2-{2-[(5-(4-Chlorophenyl)-3-cyano-1H-pyrrol-2-yl)thio]acetyl}-N-phenylhydrazinecarbothioamide (8b)

White powder, yield: (7.42 g, 84%), mp: 206–208 °C, IR (cm–1): 1668 (CO), 2220 (CN), 3335 (NH). 1H NMR (DMSO-d6) δ: 4.11 (s, 2H, CH2), 7.12–8.12 (m, 10H, Haromatic), 9.44 (s, 2H, 2NHthioamidic), 10.33 (s, 1H, NHamidic), 12.80 (s, 1H, NHpyrrole). Elemental Analysis Calcd for C20H16ClN5OS2 (441.96): 54.35; H, 3.65; Cl, 8.02; N, 15.85; S, 14.51. Found: C, 54.15; H, 3.75; Cl, 8.52; N, 15.95; S, 14.71.

2-{2-[(3-Cyano-5-(4-nitrophenyl)-1H-pyrrol-2-yl)thio]acetyl}-N-phenylhydrazinecarbothioamide (8c)

White powder, yield: (7.86 g, 87%), mp: 210–212 °C, IR (cm–1): 1671 (CO), 2227 (CN), 3311 (NH). 1H NMR (DMSO-d6) δ: 3.82 (s, 2H, CH2), 7.18–8.31 (m, 10H, Haromatic), 9.63 (s, 2H, 2NHthioamidic), 10.30 (s, 1H, NHamidic), 12.9 (s, 1H, NHpyrrole). Elemental Analysis Calcd for C20H16N6O3S2(452.51): C, 53.08; H, 3.56; N, 18.57; S, 14.17. Found: C, 53.10; H, 3.60; N, 18.55; S, 14.14.

Biology Assay

The insecticidal activity of all synthesized pyrrole derivatives was measured via the leaf dipping bioassay method.[39] The results of laboratory screening are reported here for the most active derivatives to find out the required concentrations that kill 50% (LC50) of the larvae. In this search, five concentrations of each of the synthesized pyrrole derivatives (100, 50, 25, 12.5, and 6.25 ppm) plus 0.1% Tween 80 as surfactant were used. Disks (9 cm diameter) of castor bean leaves were dipped in the tested concentration for 10 s, then left to dry and afforded to 2nd and 4th larvae, nearly of the same size. The larvae were placed in glass jars (5 pounds), and every treatment was replicated three times (10 larvae per each). Control disks were dunked in distilled water and Tween 80, then transferred to the untreated larvae, which were allowed to feed on castor bean for 48 h. Mortality percentage was recovered after 72 h for all insecticides. Mortality was redressed by Abbott’s formula.[40] The measurements of the mortality relapse line were dissected by probit analysis.[36] Harmfulness index was determined by sun equations.[41]