Novel Meta-Diamide Compounds Containing Sulfide Derivatives Were Designed and Synthesized as Potential Pesticides

The meta-diamide (m-diamide) insecticide, Broflanilide, was characterized by its high efficiency, low toxicity and lack of cross-resistance with traditional GABA receptors. In accordance with the principles of drug molecular design, easily derivable sulfur with diverse bioactivities was introduced while leading with the parent Broflanilide. Twelve novel m-diamide target compounds containing sulfide derivatives were synthesized through exploration guided by the literature. Their structures were confirmed by melting points, 1H NMR, 13C NMR and HRMS. Insecticidal activity assessments revealed that most target compounds A–D exhibited 100% lethality against Plutella xylostella (P. xylostella) and Aphis craccivora Koch (A. craccivora) at 500 mg·L−1. Notably, for P. xylostella, compounds C-2, C-3, C-4 and D-2 demonstrated 60.00–100.00% insecticidal activity even at a concentration as low as 0.625 mg·L−1. As determined by structure–activity relationship (SAR) analysis, compounds with R1 = CH3 and R2 = Br (B-1, C-2 and D-2) and sulfoxide compound C-3 contained 100.00% lethality against A. craccivora at 500 mg·L−1, surpassing the lethality when leading with the parent Broflanilide in terms of efficacy. Consequently, it can be inferred that the sulfoxide compound (C-3) requires further investigation as a potential active molecule for new insecticides. These explorations provide valuable references for future research on the synthesis and insecticidal activities of sulfide-containing m-diamide compounds.


Introduction
The continual discovery and development of novel pesticides with unique structures and superior biological activities remain paramount objectives for researchers in the field of agrochemicals [1,2].Broflanilide [3] (BASF (Ludwigshafen, Germany) and Mitsui Chemicals, Inc. (Tokyo, Japan)) and Cyproflanilide [4] (Tahoe Group) (Figure 1) acting on γ-aminobutyric acid (GABA) receptors, which regulate the transmission of chloride ions into cells, causing pests to vomit and excite they are killed, are typical representatives of m-diamide insecticide.Their outstanding insecticidal activities and novel structures have quickly garnered significant interest among pesticide researchers.Recently, the Wu [5] group reported that compounds with sulfides introduced in the place of the trifluoromethyl group when leading with the parent Cyproflanilide exhibited certain insecticidal activities against Nilaparvata lugens (N.lugens).The thioethyl derivative (Figure 1) displayed the most potent insecticidal activity (98.92%, 100 mg•L −1 ), as determined through structureactivity relationship (SAR) studies.Regrettably, the activities of the title compound towards P. xylostella, Tetranychus cinnabarinus (T.cinnabarinus) and A. craccivora were negligible at the same test concentration, indicating significant room for further exploration in the development of novel m-diamide compounds containing sulfur derivatives as potential insecticides.
Molecules 2024, 29, x FOR PEER REVIEW 2 of 13 towards P. xylostella, Tetranychus cinnabarinus (T.cinnabarinus) and A. craccivora were negligible at the same test concentration, indicating significant room for further exploration in the development of novel m-diamide compounds containing sulfur derivatives as potential insecticides.Due to the diverse biological activities and easily derivable characteristics of sulfidecontaining structures, which are often applied in various pesticides, these structures are considered pivotal in the field [6,7].For example, the first commercialized diamide insecticide, Flubendiamide [8], marking the advent of the era of green diamide insecticides, contained an (O)2S-CH3 structural unit.Commonly derived sulfur structures such as (O)nS-CF3 and (O)nS-CH2CF3 have been instrumental in the development of compounds like Fipronil [9], Flupentiofenox [10] and Bisulflufen [11], all of which have demonstrated excellent biological activities in agriculture (Figure 1).
Considering the significance of thioethers, sulfoxides and sulfones as common sulfides, a series of sulfide-containing m-diamide insecticides were designed based on the principle of active fragment transition (Figure 1).Common sulfide active groups in pesticides-(O)nS-CH3/CF3/CH2CF3-were introduced into the leading with parent-Broflanilide-to replace the CF3 group.The exploration of the target synthesis route and insecticidal activities, along with the summarization of SAR, may provide valuable references for future research on sulfur-containing m-diamide compounds.

Synthesis
The bilinear synthesis routes for the target thioether-containing m-diamine compound (A and B) are shown in Scheme 1 depending on the references and exploration [5,[12][13][14].The key intermediate, 2-fluoro-3-(N-methylbenzamido)benzoic acid (4), can be smoothly prepared from methyl 2-fluoro-3-nitrobenzoate as the starting material.On the basis of the referenced literature, the synthesis of intermediate 6a involved extensive explorations (Table 1).The reaction could not proceed when N,N-Dimethylformamide (DMF) was used as a solvent with 5, NaOH, FeSO4•7H2O and (CF3)2FCI under one-pot reaction conditions [15] (Table 1, entries 1-4).However, employing a two-phase system consisting of ethyl acetate (EA) and water (H2O) as solvents, and tetrabutylammonium hydrogen sulfate (IPC-TBA-HS) as a phase transfer catalyst, with the equivalent ratio of compounds 5:Na2S2O4:NaHCO3:IPC-TBA-HS:(CF3)2FCI being 1:1.1:1.1:0.3:2, the reaction Due to the diverse biological activities and easily derivable characteristics of sulfidecontaining structures, which are often applied in various pesticides, these structures are considered pivotal in the field [6,7].For example, the first commercialized diamide insecticide, Flubendiamide [8], marking the advent of the era of green diamide insecticides, contained an (O) 2 S-CH 3 structural unit.Commonly derived sulfur structures such as (O) n S-CF 3 and (O) n S-CH 2 CF 3 have been instrumental in the development of compounds like Fipronil [9], Flupentiofenox [10] and Bisulflufen [11], all of which have demonstrated excellent biological activities in agriculture (Figure 1).
Considering the significance of thioethers, sulfoxides and sulfones as common sulfides, a series of sulfide-containing m-diamide insecticides were designed based on the principle of active fragment transition (Figure 1).Common sulfide active groups in pesticides-(O) n S-CH 3 /CF 3 /CH 2 CF 3 -were introduced into the leading with parent-Broflanilide-to replace the CF 3 group.The exploration of the target synthesis route and insecticidal activities, along with the summarization of SAR, may provide valuable references for future research on sulfur-containing m-diamide compounds.

Synthesis
The bilinear synthesis routes for the target thioether-containing m-diamine compound (A and B) are shown in Scheme 1 depending on the references and exploration [5,[12][13][14].
The key intermediate, 2-fluoro-3-(N-methylbenzamido)benzoic acid (4), can be smoothly prepared from methyl 2-fluoro-3-nitrobenzoate as the starting material.On the basis of the referenced literature, the synthesis of intermediate 6a involved extensive explorations (Table 1).The reaction could not proceed when N,N-Dimethylformamide (DMF) was used as a solvent with 5, NaOH, FeSO 4 •7H 2 O and (CF 3 ) 2 FCI under one-pot reaction conditions [15] (Table 1, entries 1-4).However, employing a two-phase system consisting of ethyl acetate (EA) and water (H 2 O) as solvents, and tetrabutylammonium hydrogen sulfate (IPC-TBA-HS) as a phase transfer catalyst, with the equivalent ratio of compounds 5:Na 2 S 2 O 4 :NaHCO 3 :IPC-TBA-HS:(CF 3 ) 2 FCI being 1:1.1:1.1:0.3:2, the reaction system was successfully completed after heating and refluxing for about 12 h as monitored by thinlayer chromatography (TLC) [5] (Table 1, entries [5][6][7][8][9][10][11].The intermediates 6b and 6c were prepared following the established synthesis route. system was successfully completed after heating and refluxing for about 12 h as monitored by thin-layer chromatography (TLC) [5] (Table 1, entries [5][6][7][8][9][10][11].The intermediates 6b and 6c were prepared following the established synthesis route.To circumvent the use of harsh conditions, such as the employment of strong bases at low temperatures (e.g., Lithium Diisopropylamide (LDA) or Sodium Hydride (NaH) at −70 °C) [16] or solvents with high boiling points [2], the study aimed to prepare target compounds A under milder conditions.A comprehensive review of the literature and an analysis of the reaction mechanism facilitated the exploitation of differences in the nucleophilicity and leaving abilities of halogen atoms for the synthesis of target compounds A. The exploration ultimately confirmed that compounds A could be efficiently achieved using potassium iodide (KI) [2] as the nucleophilic initiator in an acetonitrile (CH3CN) solvent system.
In the endeavor to synthesize target compounds B, the research initially attempted to employ the method depicted in Scheme 2. This method started from carboxylic acid 4 with 6-bromo-2-(substitutedthio)-4-(perfluoropropan-2-yl)aniline 7, utilizing the mild re- To circumvent the use of harsh conditions, such as the employment of strong bases at low temperatures (e.g., Lithium Diisopropylamide (LDA) or Sodium Hydride (NaH) at −70 • C) [16] or solvents with high boiling points [2], the study aimed to prepare target compounds A under milder conditions.A comprehensive review of the literature and an analysis of the reaction mechanism facilitated the exploitation of differences in the nucleophilicity and leaving abilities of halogen atoms for the synthesis of target compounds A. The exploration ultimately confirmed that compounds A could be efficiently achieved using potassium iodide (KI) [2] as the nucleophilic initiator in an acetonitrile (CH 3 CN) solvent system.
In the endeavor to synthesize target compounds B, the research initially attempted to employ the method depicted in Scheme 2. This method started from carboxylic acid 4 with 6-bromo-2-(substitutedthio)-4-(perfluoropropan-2-yl)aniline 7, utilizing the mild reaction conditions previously successful for compounds A. Unfortunately, despite variations in the amount of the nucleophilic reagent (KI), as well as adjustments to the reaction temperature and duration, the reaction did not proceed.The failure was hypothesized to be due to significant steric hindrance around the amino group in the substituted aniline moiety, which hindered the nucleophilic substitution reaction under the mild conditions.Consequently, the focus of the study shifted to using target compounds A as the starting material.This approach involved an electrophilic substitution reaction at the six-position of the benzene ring to introduce a bromine atom, leading to the successful preparation of target compounds B. Further investigation into the electrophilic substitution reaction on the benzene ring for Br introduction revealed that the target compounds B could be synthesized efficiently by using 1.1 equivalents of N-Bromosuccinimide (NBS) and 2.5 equivalents of potassium carbonate (K 2 CO 3 ) under reflux conditions for 3 h.action conditions previously successful for compounds A. Unfortunately, despite variations in the amount of the nucleophilic reagent (KI), as well as adjustments to the reaction temperature and duration, the reaction did not proceed.The failure was hypothesized to be due to significant steric hindrance around the amino group in the substituted aniline moiety, which hindered the nucleophilic substitution reaction under the mild conditions.Consequently, the focus of the study shifted to using target compounds A as the starting material.This approach involved an electrophilic substitution reaction at the six-position of the benzene ring to introduce a bromine atom, leading to the successful preparation of target compounds B. Further investigation into the electrophilic substitution reaction on the benzene ring for Br introduction revealed that the target compounds B could be synthesized efficiently by using 1.1 equivalents of N-Bromosuccinimide (NBS) and 2.5 equivalents of potassium carbonate (K2CO3) under reflux conditions for 3 h.As illustrated in Scheme 3, the oxidation reactivity of sulfur atoms is closely related to the nature of the R1 group.When R1 is an electron-donating methyl, the electron density on the sulfur atom increases, facilitating oxidation.In the presence of 3-chloroperbenzoic acid (m-CPBA) as the oxidant, this leads to the formation of the corresponding sulfoxides (C-1 and C-2, at room temperature) and sulfones (D-1 and D-2, under reflux conditions).However, when R1 is an electron-withdrawing trifluoromethyl group, it was observed that oxidation did not occur under m-CPBA conditions (Table 2, entries 1-5).Subsequent explorations confirmed that using 5 equivalents of H2O2 as the oxidant in trifluoroacetic acid solvent at −10 °C (Table 2, entries 6-13) yielded the sulfoxide (C-3) at a low yield of 29%.The sulfoxide compound C-4, where R1 is CH2CF3, can be prepared under similar conditions to those used for C-3 using H2O2 as the oxidant.Regrettably, the reaction conditions for the oxidation of corresponding compounds from thioethers to corresponding sulfone compounds were not successfully explored when R1 was trifluoromethyl or trifluoroethyl.As illustrated in Scheme 3, the oxidation reactivity of sulfur atoms is closely related to the nature of the R 1 group.When R 1 is an electron-donating methyl, the electron density on the sulfur atom increases, facilitating oxidation.In the presence of 3-chloroperbenzoic acid (m-CPBA) as the oxidant, this leads to the formation of the corresponding sulfoxides (C-1 and C-2, at room temperature) and sulfones (D-1 and D-2, under reflux conditions).However, when R 1 is an electron-withdrawing trifluoromethyl group, it was observed that oxidation did not occur under m-CPBA conditions (Table 2, entries 1-5).Subsequent explorations confirmed that using 5 equivalents of H 2 O 2 as the oxidant in trifluoroacetic acid solvent at −10 • C (Table 2, entries 6-13) yielded the sulfoxide (C-3) at a low yield of 29%.The sulfoxide compound C-4, where R 1 is CH 2 CF 3 , can be prepared under similar conditions to those used for C-3 using H 2 O 2 as the oxidant.Regrettably, the reaction conditions for the oxidation of corresponding compounds from thioethers to corresponding sulfone compounds were not successfully explored when R 1 was trifluoromethyl or trifluoroethyl.
temperature and duration, the reaction did not proceed.The failure was hypothesized to be due to significant steric hindrance around the amino group in the substituted aniline moiety, which hindered the nucleophilic substitution reaction under the mild conditions.Consequently, the focus of the study shifted to using target compounds A as the starting material.This approach involved an electrophilic substitution reaction at the six-position of the benzene ring to introduce a bromine atom, leading to the successful preparation of target compounds B. Further investigation into the electrophilic substitution reaction on the benzene ring for Br introduction revealed that the target compounds B could be synthesized efficiently by using 1.1 equivalents of N-Bromosuccinimide (NBS) and 2.5 equivalents of potassium carbonate (K2CO3) under reflux conditions for 3 h.As illustrated in Scheme 3, the oxidation reactivity of sulfur atoms is closely related to the nature of the R1 group.When R1 is an electron-donating methyl, the electron density on the sulfur atom increases, facilitating oxidation.In the presence of 3-chloroperbenzoic acid (m-CPBA) as the oxidant, this leads to the formation of the corresponding sulfoxides (C-1 and C-2, at room temperature) and sulfones (D-1 and D-2, under reflux conditions).However, when R1 is an electron-withdrawing trifluoromethyl group, it was observed that oxidation did not occur under m-CPBA conditions (Table 2, entries 1-5).Subsequent explorations confirmed that using 5 equivalents of H2O2 as the oxidant in trifluoroacetic acid solvent at −10 °C (Table 2, entries 6-13) yielded the sulfoxide (C-3) at a low yield of 29%.The sulfoxide compound C-4, where R1 is CH2CF3, can be prepared under similar conditions to those used for C-3 using H2O2 as the oxidant.Regrettably, the reaction conditions for the oxidation of corresponding compounds from thioethers to corresponding sulfone compounds were not successfully explored when R1 was trifluoromethyl or trifluoroethyl.The synthesized target compounds (A-D) underwent comprehensive structural characterization through melting points, 1 H NMR and 13 C NMR to elucidate their molecular structures.Meanwhile, the consistency between the theoretical and measured values of HRMS further proved the correctness.In 1 H NMR spectra, the characteristic proton peak of the amide bond N-H appeared at δ 10.06-10.87ppm for all m-diamide products.The signals of N-H were observed in δ 10.06-10.63among compounds A. Based on chemical structure analysis, it was speculated that this might be due to the electron-withdrawing properties of Br adjacent to the amino group in compounds B, causing the chemical shift to migrate towards a lower field and higher position (δ 10.53-10.71).Notably, the N-CH 3 signal exhibited a chemical shift of δ 3.32-3.44ppm, while the range chemical shift for (O) n =S-CH 3 affected by the different oxidation states of sulfur was broad (δ 2.50-3.43).The typical proton peak of (O) n =S-CH 2 CF 3 in A-3, B-3 and C-4 were presented in δ 4.04-4.37due to the electron-withdrawing effect of CF 3 .
Based on the above, it could be concluded that some sulfide-containing m-diamine compounds as designed in the study contained excellent insecticidal activities against P. xylostella and A. craccivora.Furthermore, the sulfoxide compounds C-2, C-3 and C-4, along with the sulfone compound D-2, demonstrated significantly higher bioactivities compared to the others.

General Experimental Details
Reagents and solvents were purchased from Titan Corporation and used without further purification.Melting points were measured by the SGWX-4B melting point analyzer and uncorrected.NMR spectra were recorded on a Brucker Avance NEO (400, 101 MHz) spectrometer, using DMSO-d6 (TMS as the 0 point internal standard) as the solvent.HRMS data were obtained on Thermo Q Exactive Focus with ESI ionization.The 1 H NMR, 13 C NMR and HRMS spectra for target compounds were provided in Supplementary Materials.

Synthesis and Characterization of the Compounds
A mass of synthesis routes for m-diamide pesticides have been reported in recent years due to their exceptional insecticidal activities [5,[12][13][14].The sulfur-containing m-diamide target compounds were designed based on literature reports and exploration.The use of methyl 2-fluoro-3-nitrobenzoate as the starting material to obtain the target compounds through a bilinear chain reaction is depicted in Schemes 1 and 2.

Drug Preparation
The raw materials were dissolved in DMF to prepare a 1% mother liquor, diluted with 0.1% Tween 80 distilled water to prepare the corresponding concentration and set aside.

Insecticidal Activity Methods
The lethal rate of the target compounds against P. xylostella, N. lugens and A. craccivora were investigated under the contrast of Broflanilide and a blank control without any medication in a greenhouse.
The activity of P. xylostella was determined using the leaf-soaking method.Radish leaves were soaked in moderation to the prepared test fluid for 30 s.Then, they were placed in a plastic culture dish with filter paper and dried naturally in the shade.Each dish was infested with 8 second-instar diamondback moth larvae and placed in an observation room at a temperature of 25 • C. The test results were observed after 48 h.They were considered dead if there was no response or the inability to crawl normally was observed when touching the insect body lightly with a brush.This was repeated three times for each sample.
The activity of N. lugens was determined using the spray method.A rice seedling with two leaves and one core was selected and placed on a 6 cm Petri dish; then, quartz sand was spread on the Petri dish.Each dish was infested with 20 third-instar early brown planthopper nymphs and treated with 2.5 mL of spray with a Potter spray tower, which were then placed in an observation room at a temperature of 25 • C. The test results were observed after 48 h.They were considered dead if there is no response or the inability to crawl normally was observed when touching the insect body lightly with a brush.This was repeated three times for each sample.
The activity of A. craccivora was determined using the spray method.The dishes were infested with 30 alfalfa aphid nymphs and treated with 2.5 mL of spray with a Potter spray tower and then placed in a 25 • C observation room for cultivation.After 48 h of investigation, the insect body was touched with tweezers, and if there was no response, it was regarded as dead.A blank control was set up without adding any medication.

Data Statistics and Analysis
The number of deaths of each processed target was counted, and the lethal rate was calculated.
Lethal rate (%) = number of dead insects total number of insects × 100%

Conclusions
A series of novel sulfide-containing m-diamine target compounds A-D were prepared through exploration by referencing and improving upon the literature.These compounds were characterized by melting points, 1 H NMR, 13 C NMR and HRMS.The mild preparation of the thioether compounds A was achieved by cleverly utilizing I -for its excellent nucleophilicity and leaving properties, thereby avoiding the need for harsh conditions such as strong bases and low temperatures.The insecticidal activity results revealed that most target compounds A-D exhibited 100% lethal rates against P. xylostella and A. craccivora at 500 mg•L −1 .Specifically, for P. xylostella, the sulfoxide-compounds C-2, C-3 and C-4 and sulfone compound D-2 displayed insecticidal activities ranging from 60.00% to 100.00% even at a concentration of 0.625 mg•L −1 .It was observed that compounds with R 1 = CH 3 and R 2 = Br (B-1, C-2 and D-2) and sulfoxide compound C-3 achieved 100.00% lethal rates against A. craccivora at 500 mg•L −1 , outperforming the lethal rates achieved when leading with the parent Broflanilide as per the SAR.Some newly synthesized sulfide-containing m-diamine target compounds had a broader insecticidal spectrum.From this, it can be inferred that the novel sulfoxide compounds C-2, C-3 and D-2 are promising candidates for further study as potential active molecules in new insecticides.

Figure 1 .
Figure 1.The structures of m-diamide, sulfide-containing drugs and the target compounds designed in this paper.

Figure 1 .
Figure 1.The structures of m-diamide, sulfide-containing drugs and the target compounds designed in this paper.

Scheme 2 .
Scheme 2. The exploration synthesis route for novel m-diamide compounds containing thioether B.

Scheme 2 .
Scheme 2. The exploration synthesis route for novel m-diamide compounds containing thioether B.

Scheme 2 .
Scheme 2. The exploration synthesis route for novel m-diamide compounds containing thioether B.

Scheme 1 .
The synthesis route for novel m-diamide compounds containing thioether (A and B).

Table 1 .
Optimization of reaction conditions for the synthesis compound

Scheme 1 .
The synthesis route for novel m-diamide compounds containing thioether (A and B).

Table 1 .
Optimization of reaction conditions for the synthesis compound

Table 2 .
Optimization of reaction conditions for the synthesis compound C-3.

Table 3 .
Insecticidal activity of target compounds against P. xylostella.

Table 4 .
Insecticidal activity of target compounds against N. lugens and A. craccivora.