Study on Design, Synthesis and Herbicidal Activity of Novel 4-Amino-6-(5-Aryl-Substituted-1-Pyrazolyl)-3-Chloro-5-Fluoro-2-Picolinic Acids

6-Aryl-2-picolinic acid herbicides are an important subclass of auxin herbicides, characterized by their good absorption and conductivity, broad weed control spectrum, and excellent herbicidal activity against some resistant weeds. Based on previous studies from our group and the distinct characteristics of physico-chemical properties and biological activities of active skeleton structure containing fluorine atoms, this paper introduces the design and synthesis of 41 novel 4-amino-6-(5-aryl-substituted-1-pyrazolyl)-3-chloro-5-fluoro-2-picolinic acid compounds. The test of inhibiting <i>A. thaliana</i> roots growth showed that most of the <b>S</b>-series compounds exhibited superior inhibitory effects compared to picloram, with six compounds demonstrated even better inhibitory capability than the new herbicidal molecule florpyrauxifen. For example, compound <b>S202</b>, at a concentration of 0.5 µmol/L, exhibited a 78.4% inhibition of <i>A. thaliana</i> root growth, whereas florpyrauxifen showed only a 33.8% inhibition. Root growth inhibition tests on weeds showed that 28 compounds, at a concentration of 250 µM, demonstrated a greater than 80% inhibition of <i>Brassica napus</i> (BN) root growth. Post-emergence herbicidal activity tests showed that most compounds exhibited good inhibitory effects on broadleaf weeds, with 10 compounds achieving a 100% inhibition of the growth of <i>Amaranthus retroflexus L</i> (AL). These results demonstrate that some of the 4-amino-6-(5-aryl-substituted-1-pyrazolyl)-3-chloro-5-fluoro-2-picolinic acid compounds could be used as potential lead structures in the discovery of novel synthetic auxin herbicides.