Structure-activity relationships of herbicidal aryltriazolinones

A series of substituted aryltriazolinones, known to inhibit protoporphyrinogen oxidase, were prepared and their structure-activity requirements at positions 4 and 5 of the aromatic ring investigated. A QSAR equation obtained for substituents at the 5 position identified the hydrophobicity term pi and the Sterimol minimum width B-1 as the two parameters affecting in-vitro biological activity. Greenhouse pre-emergence activity correlated with in-vitro activity and the hydrophobicity term pi of the substituent at that position. It was found that the phenoxy-4-oxyacetate group at aromatic position 5 was an outlier and had to be considered separately. SAR analysis of substituents at aromatic position 4 revealed that two different models were required to explain all observed substituent effects. In the first model, where the 5 position was occupied by hydrogen, the 4-chlorobenzyloxy group at aromatic position 4 gave the best compound. The second model, where the 5 position of the aromatic ring was occupied by a group other than hydrogen, resulted in a OSAR equation, previously derived, which links substituent effects at position 4 with pi and with the electronic para inductive term F-p. In this model the chloro group provides optimum biological activity. The need to separate the aryltriazolinone herbicides into several different classes in order to explain their substituent effects at aromatic positions 4 and 5 could be rationalized if more than one binding conformation, within the same binding site, is possible.