Pymetrozine [97] is a triazinone insecticide [98] that acts on the specific insecticide target protein transient receptor potential vanilloid (TRPV) ion channel, and showed no cross-resistance with other insecticides [99]. Compounds 54–55 (Figure 11) were developed by Nankai University with both phenoxypyridine groups and triazinone groups. The activity against aphids of Compound 54, which was synthesized by Yang et al. [100] by constructing phenoxypyridine structure and introducing a methyl group to the imino group, was significantly improved. At the concentration of 5 mg/kg, the activities against aphids of 54a (80%) and 54b (80%) were both higher than those of pymetrozine (30%). Meanwhile, 54 also exhibited significant insecticidal activity against mosquitoes and lepidopteran pests (cotton bollworm, corn borer, and oriental stick insect). By modifying the linker arm, Wang et al. [101,102] designed and synthesized a series of triazinone derivatives 55 containing an acylhydrazone structure. These compounds had certain activities against aphids, cotton bollworm, corn borer, and armyworm.

Some insecticides and acaricides (flufenerim, purimidifen, tebufenpyrad, and tolfenpyrad [103]) worked by inhibiting the mitochondrial electron transport (MET) at complex I to disrupt respiration, known as complex I inhibitors [104]. Most of the 4-aminopyrimidine [105] derivatives that were synthesized by Wang et al. [106] through intermediate derivatization methods showed good activity against Myzus persicae, among which 56 (Figure 12) had the highest activity and the lowest LC₅₀ value of 0.34 mg/L. The structure-activity relationships suggested that the linker of -CH₂CH₂- was favorable for bioactivity; the halogen substituent at the X position (X = Cl, Br) was more beneficial to the activity; for R¹, the ethyl group with large steric resistance was generally conducive to improve the activity. The substituted thienopyrimidine amines 57 (Figure 12) that were synthesized by Chai et al. [107] had broad-spectrum insecticidal and acaricidal activity, which were very effective against lepidoptera pests, homoptera, and mites even at a very low dose, especially against aphids, Tetranychus cinarcini, Plutella xylodes, and armyworm.

Pyrazole-5-carboxamide insecticides 58 (Figure 13) containing an azo structure were synthesized by Shao et al. [108], many of which had 100% activity against Aphis craccivora Koch and Tetranychus cinnabarinus. Compound 59 [109] showed broad-spectrum insecticidal activity and a 100% mortality rate against Plutella xylostella and Myzus persicae at 600 mg/L. At the same time, several compounds had good activity against Blumeria graminis and southern corn rust. Pyrazole derivatives 60 that were designed and synthesized by Okada et al. [110] had good insecticidal activity against various insect pests (Plutella xylostella, Nilaparvata lugens, and the eggs and adults of Tetranychus urticae).

Phenoxypyridine-containing compounds with insecticidal activity are summarized in Figure 14. Pyridalyl [111] inhibited cellular protein synthesis in insect cell lines but not mammalian cell lines. The novel dihalopropene ether insecticides that were synthesized by Liu et al. [112] exhibited good insecticidal activity. The LC₅₀ of Compound 61, which introduced phenoxypyridine, was 4.05 mg/L and 9.82 mg/L against M. separate and P. litura, respectively, was better than the control pyridalyl (LC₅₀ = 4.81 mg/L and 10.07 mg/L) and better than the compounds with other aromatic ring substitutions. Alkylphenyl sulfide derivatives 62 that was reported by Kumiai Chemical Industry Co., Ltd. [113] had more than 90% control of Tetranychus urticae (Koch) at a concentration of 4 mg/L. Inspired by juvenile hormone, the analogues 63 that were prepared by Li et al. [114] with the introduction of phenoxypyridine were more than 85% effective against Nilaparvata lugens at a concentration of 200 mg/L. Using phenoxypyridine molecular plug-ins, the sulfoximine and oxime ether, Compounds 64 and 65 with insecticidal activity were synthesized by Liu et al. [115] and Du et al. [116]. The neonicotinoids 66 that was designed and synthesized by Tang et al. [117] had certain activities against lepidoptera, homoptera, coleoptera, and the larvae and adults of orthoptera.

In pesticide applications, phenoxypyridine played an important role in the development of lead compounds. Compounds that were derived by linking phenoxypyridine to different active fragments or changing the substituents of phenoxypyridine exhibited a wide range of biological activity, such as herbicidal, fungicidal, bactericidal, and insecticidal activities. In this paper, the derivatives with different activities were classified. The summary of the structure-activity relationship of the derivatives indicated that structural modifications at different positions of phenoxypyridine could improve its activity. Previous studies had focused on compounds that were linked to the phenoxy group at position 2 of pyridine, possibly due to the difficulty of synthesis, so the relationship between the position of the N atom on pyridine and biological activity was unclear. The inhibitory effects of these compounds may be performed by different mechanisms and, therefore, further studies on the mechanism (or targets) are necessary for better evaluations. Still, a lot of activity of phenoxypyridine needs to be prospected in bactericides. In conclusion, phenoxypyridine could be considered as the promising active scaffold for pesticides.