Novel phenoxyacetic herbicides synthesized from longifolene-derived primary amine for sustainable weed management

Because of low water solubility, herbicides containing a phenoxy acid group, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), are applied with an amine, like dimethylamine (DMA) and isopropylamine (IPA), to form ammonium salts. However, the use of amine poses substantial health and environmental risks during manufacturing and utilization. The development of non-toxic high-performance herbicidal formulations using natural compounds is therefore highly desired but remains limited. In this work, three longifolene-derived ammonium phenoxyacetates and one glyphosate were synthesized and characterized. Their herbicidal activities were evaluated against Lolium multiflorum Lam. and Brassica campestris. The results showed that almost all target compounds exhibited higher herbicidal activity than DMA or IPA formulations prepared by their corresponding commercial herbicides. Particularly, compounds 6b and 6c containing the Cl atom tended to be the most active candidates, especially with notable half maximal inhibitory concentrations (IC₅₀) values of around 0.0002 mmol L^-1 against the root and shoot growth of Brassica campestris, which both showed complete inhibition for Lolium multiflorum Lam. root growth and Brassica campestris shoot growth at concentrations of 0.039 and 0.156 mmol L^-1, respectively. In addition, compound 6c showed a good broad-spectrum herbicidal effect on the root growth of 6 different weeds, especially on rice, with an IC₅₀ of 0.000085 mmol L^-1. It is suggested that compounds 6b and 6c could be considered as promising botanical herbicides for sustainable weed management.