Selective fluorination of an aryl triazolinone herbicide intermediate

Fluoroaryl 1,2,4-triazolin-5-ones are an important class of herbicidal compounds useful in a variety of crop protection applications, primarily for the control of grassy and broad-leaf plant species. While a number of different synthetic strategies can be used for the step-wise preparation of these complex molecules, it is generally preferred to introduce fluorine later in the synthetic pathway, since the presence of fluorine and other halogens has a dominant effect on all subsequent synthetic steps. We have investigated the reactions that occur between aryl triazolinones and a variety of fluorination agents, including F₂/N₂, XeF₂, (CF₃SO₂)₂NF, Selectfluor^®, CF₂(OF)₂, CF₃OF, CH₃COOF, and CF₃COOF, and have used this knowledge to develop several alternative high-yielding routes to fluoroaryl 1,2,4-triazolin-5-ones. The fluorine introduction strategy and experimental results for a representative example of this important class of compounds are discussed herein.