Synthesis and Herbicidal Evaluation of Novel Ammonium Phenoxyacetates Based on Camphene-Derived Primary Amine

To increase solubility, amines, including dimethylamine (DMA) and isopropylamine, are included in commercial formulations of phenoxyacetic acid herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), but amine volatilization during production and use poses challenges for the social environment and living organisms. To mitigate the problem by replacing currently applied volatilized amines and developing high-efficacy and environment-friendly herbicides, three camphene-derived ammonium phenoxyacetates and one glyphosate were synthesized and characterized. The preliminary herbicidal activity tests showed that several compounds displayed higher herbicidal performance against Lolium multiflorum Lam. and Brassica campestris than their corresponding herbicide-amine salts. Compared to DMA salts of 2,4-D and MCPA, compounds 5b-5c containing one or two chlorine atoms presented similar or higher herbicidal activity against B. campestris even at a lower concentration (0.0006 mmol/L). Besides, compound 5b with the half maximal inhibitory concentration (IC₅₀) value of 0.000281 mmol/L against B. campestris shoot growth showed 611.7% higher herbicidal activity than that of DMA salt of 2,4-D, whereas compound 5c with an IC₅₀ value of 0.026 mmol/L against L. multiflorum Lam. shoot growth presented 38.5% higher herbicidal activity than DMA salt of MCPA. This study indicated that compounds 5b-5c could be promising herbicidal candidates.