Synthesis and insecticidal activity of diacylhydrazine derivatives containing a 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole scaffold

Background The diacylhydrazine derivatives have attracted considerable attention in recently years due to their simple structure, low toxicity, and high insecticidal selectivity. As well as 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole is an important scaffold in many insecticidal molecules. In an effort to discover new molecules with good insecticidal activity, a series of diacylhydrazine derivatives containing a 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole scaffold was synthesized and bio-assayed. Results Bioassays demonstrated that some of the title compounds exhibited favorable insecticidal activities against Helicoverpa armigera and Plutella xylostella. The insecticidal activity of compounds 10g, 10h, and 10w against H. armigera were 70.8, 87.5, and 79.2%, respectively. Compounds 10c, 10e, 10g, 10h, 10i, 10j and 10w showed good larvicidal activity against P. xylostella. In particular, the LC50 values of compounds 10g, 10h, and 10w were 27.49, 23.67, and 28.90 mg L−1, respectively. Conclusions A series of diacylhydrazine derivatives containing a 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole scaffold was synthesized and bio-assayed. The results of insecticidal tests revealed that the synthesized diacylhydrazine derivatives possessed weak to good insecticidal activities against H. armigera and P. xylostella. Compounds 10g, 10h, and 10x showed much higher insecticidal activity than tebufenozide, and exhibited considerable prospects for further optimization. Primary structure–activity relationship revealed that phenyl, 4-fluoro phenyl and four fluorophenyl showed positive influence on their insecticidal activities, and introduction of a heterocyclic ring (pyridine and pyrazole) showed negative impacts on their insecticidal effects. Electronic supplementary material The online version of this article (doi:10.1186/s13065-017-0279-z) contains supplementary material, which is available to authorized users.


Chemistry
The synthesis of the 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carbohydrazide derivatives are depicted in Scheme 1. Firstly, the key intermediate 3-bromo-1-(3chloropyridin-2-yl)-1H-pyrazole-5-carboxylic acid (5) was obtained in good yield via reactions of hydrazinolysis, cyclization, bromination, oxydehydrogenation, and acidolysis by employing 2,3-dichloropyridine (1), hydrazine hydrate and diethyl maleate as starting materials [24,33,34]. Then compound 5 was allowed to further react with thionyl chloride under reflux to afford 3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carbonyl chloride (7) [35]. Subsequent treatment of intermediate 7, with tert-butyl hydrazine hydrochloride (8) in the presence of triethylamine in trichloromethane at ambient temperature afforded 3-bromo-N′-(tert-butyl)-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carbohydrazide (9) in 80% yield. Finally, the title compounds (10a-10x) were conveniently obtained in an >70% yield by treating of  Structures of the title compounds (10a-10x) were established on basis of their spectroscopic data. In the 1 H NMR spectra, the N-H proton appeared as a broad singlet near δ 11.10 ppm. The proton at position 5 of pyridine appeared as a doublet of doublets near δ 8.45 due to the coupling coefficients from the protons at 3 and 4 positions of the pyridine ring; the coupling constants were 3 J = 4.7 Hz and 4 J = 1.5 Hz respectively. As well as the protons at positions 3 and 4 showed as doublet of doublets near δ 8.2 and 7.7 ppm, respectively, because of the coupling coefficients from both 5 positions and the each other from 4 and 3 positions of the pyridine ring, respectively. 4-pyrazole-H exhibited a singlet near δ 6.90 ppm. The rest of the aromatic protons appeared range from 7.0 to 8.0 ppm, the nine protons (-CH 3 ) 3 appeared as a singlet near δ 1.45 ppm; In 13 C NMR spectra for the fluorine contained compounds, the carbons were split into multiplet due to the coupling coefficients from "F", take compound 10m as example, the carbon near "F" resonance frequency is near δ C 158.27 ppm as a doublet and with the coupling constant ( 1 J C-F ) was 249.5 Hz; and the carbons at ortho-position of F were also split into doublets with coupling constant ( 2 J C-F ) ranged from 18.1 Hz to 21.4 Hz. The properties, 1 H NMR, 13 C NMR, 19 F NMR, and HR-MS data of the synthesized compounds 10a to 10x are summarized in more detail in the "Experimental section".
The results listed in Table 1 indicated that the synthesized compounds displayed weak to good larvicidal activity against Helicoverpa armigera at the test concentration. For example, the larvicidal activity of compounds 10c to 10j, 10l, 10o-10q, 10v, and 10w showed >50% mortality on H. armigera at 500 mg L −1 , and the larvicidal activity of 10g, 10h, and 10w were 70.8, 87.5, and 79.2%, respectively, whereas the concentration was 100 mg L −1 , the mortalities of H. armigera for compounds 10h and 10w were still >50%.
As shown in Table 2, the synthesized compounds shown larvicidal activity against Plutella xylostella, with mortality range from 6.7 to 100%. And it can be seen that most of the synthesized compounds show over 60% activity at 500 mg L −1 , and compounds 10e, 10g to 10j and 10w displayed >90% activities. In particular, compounds 10g, 10h and 10w showed good larvicidal activity, both 10h and 10w showed 100% activities against Plutella xylostella at 200 mg L −1 , and the activity of compound 10g was up to 96.7%. When the concentration was 50 mg L −1 , the activities of compounds 10g, 10h and 10w were 66.7, 76.7 and 70% at 50 mg L −1 , respectively, whereas these three compounds showed moderate activity at 25 mg L −1 .
The median lethal concentrations (LC 50 ) of compounds 10c, 10e, 10g, 10h, 10i, 10j and 10w were further determined. For comparison, the LC 50 value of tebufenozide (a commonly used insecticide) were also evaluated. The results are given in Table 3. The LC 50 values of compounds 10e, 10g, 10h, 10j and 10w were less than 100 mg L −1 (Table 3). In particular, the compounds 10g, 10h, and 10w exhibited excellent insecticidal activities, with LC 50 values of 27.49, 23.67, and 28.90 mg L −1 , respectively. These compounds showed slightly higher insecticidal activity than commercial tebufenozide (LC 50 = 37.77 mg L −1 ). As revealed by data in Tables 1  and 2, the insecticidal activity of the title compound was effected by R group. When R was a benzene ring (10w), the compound showed excellent insecticidal activity (compare with tebufenozide), and the activity could be slightly enhanced by introduction of a fluorine at 4 position of benzene (compound 10g) or four fluorines on benzene (10h). However, the activity decreased when benzene was substituted by tri-fluorine at 3, 4, 5 positions, as well as decreased by introducing other substituents, such as nitro, 2-trifluoromethyl, 3-trifluoromethyl, 3,4-di-chloro, and 4-iodine. In addition, when R was a heterocyclic ring (i.e., pyridine, pyrazole, furan), the corresponding compounds showed much weaker activities than the compounds with a benzene ring. Moreover, a compound containing the benzyl show no larvicidal activity. But interestingly, a compound containing the 2-thiophen-2-yl (10j) was found to show good insecticidal activity.

Materials and instruments
All aromatic acids were purchased from Accela Chem-Bio Co., Ltd (Shanghai, China). Melting points were determined using a XT-4 binocular microscope (Beijing Tech Instrument Co., China) and left uncorrected. The NMR spectra was recorded on a AVANCE III HD 400M NMR (Bruker corporation, Switzerland) or JEOL ECX 500 NMR spectrometer (JEOL Ltd., Japan) operating at room temperature using DMSO as solvent. HR-MS was recorded on an Orbitrap LC-MS instrument (Q-Exative, Thermo Scientific ™ , American). The course of the reactions was monitored by TLC; analytical TLC was performed on silica gel GF254. All reagents were of analytical grade or chemically pure. All anhydrous solvents were dried and purified according to standard techniques just before use.

Synthetic procedures General procedure for intermediates (2-6)
Intermediates 2-6 were prepared by following the known procedures, [24,33,34] and the acyl chloride (7) was synthesized according to reported method [35]. The detailed synthetic procedures and physical properties for these intermediates can be found in Additional file 1.

Synthesis of intermediate (9)
To a well-stirred suspension of tert-butyl hydrazine hydrochloride 8 in dichloromethane, two equivalents of triethylamine was added, the resulted mixture was stirred at room temperature for 10 min, then the solution of acyl chloride 7 in dichloromethane was then added dropwise. After stirring and refluxing for 2 h, dichloromethane was removed in vacuo. The mixture was washed with saturated sodium bicarbonate solution. The solution was  filtered to obtain a crude product, which was recrystallized with ethanol to obtain the 3-bromo-N′-(tert-butyl)-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carbohydrazide (9). Brown solid, yield, 80%, 1

General procedure for the preparation of title compounds (10a-10y)
Different fresh acyl chloride (1 mmol) were added to a well-stirred solution of 9 (1 mmol) in chloroform (5 mL) in present of triethylamine. The resulting mixture was stirred for 50 min at ambient temperature to afford a white solid, and then filtered and recrystallized from ethanol in good yield.

Biological assay
All bioassays were conducted on test organisms reared in the lab and repeated at 25 ± 1 °C according to statistical requirements. Mortalities were corrected using Abbott's formula [37]. Evaluations were based on a percentage scale (0 = no activity and 100 = complete eradication), at intervals of 5%.

Insecticidal activity against H. armigera
The insecticidal activities of some of the synthesised compounds and avermectins against Helicoverpa armigera were evaluated by the diet-incorporated method [33]. A quantity of 3 mL of prepared solutions containing the compounds was added to the forage (27 g), subsequently diluted to different concentrations and then placed in a 24-pore plate. One larva was placed in each of the wells on the plate. Mortalities were determined after 72-96 h.

Insecticidal activity against P. xylostella
The insecticidal activities of compounds 10a-10y against third instar larvae of P. xylostella were evaluated according to a previously reported procedure [33][34][35]. Fresh cabbage discs (diameter: 2 cm) were dipped into the prepared solutions containing compounds 10a-10y for 10 s, air-dried, and then placed in a Petri dish (diameter: 9 cm) lined with filter paper. Then, ten third instar larvae of P. xylostella were carefully transferred to the Petri dish. Each assay was conducted in triplicate. Mortality was calculated 72 h after treatment. The control groups were treated with distilled water containing TW-80 (0.1 mL/L). Commercial insecticides (i.e., chlorantraniliprole, chlorpyrifos, and avermectins) were tested and compared under the same conditions.