Methylthio-TFMS derivatives

Methylthio TFMS Derivatives. Data for the meta- and para-substituted methylthio TFMS derivatives (3-SCH3, 4-SCH3) were not included in Hansch structure—activity correlations for the several weed types. This omission was a result of our noting that when herbicidal data for the methylthio derivatives were included in fits, much poorer Hansch correlations were obtained. This was true whether or not the TFMS compounds were separated into meta- and para-derivatives for fitting purposes. It was also true for all weed types examined, in the presence and absence of surfactant. A typical example of the improvement in statistical parameters effected by omitting methylthio points from the data pool is illustrated by the following Hansch correlation of TFMS activity on Foxtail grass in the presence of surfactant. 

An alternative (and probably more likely) explanation for the anomalous herbicidal activity of the methylthio-TFMS derivatives exists [sug-... 

S02CH3-TFMS and 3-S02CH3-TFMS derivatives as they do in the Foxtail surfactant case (see Table XV). Although methylthio-TFMS derivatives might possibly be protected from in vivo oxidation by Tween 80 absorbed by the Wild Mustard seeds (micelle formation), a process of this type would not be observable experimentally since slower ratedetermining partitioning events control the overall herbicidal activity as previously explained. 

Thus, it seems that the anomalous methylthio-TFMS herbicidal activity we have observed in our structure-activity studies on grass and broadleaf weeds is attributable to in vivo conversion of the -SCH3 side chain to more highly oxidized forms (-SOCH3, -S02CH3). Alternatively, methylthio-TFMS derivatives may function herbicidally via a completely different mode of action than the other TFMS derivatives examined. In light of the arguments presented, however, this seems to be a much less likely possibility than the in vivo oxidation proposal. 

Methylthio-TFMS derivatives (3-SCH3-TFMS, 4-SCH3-TFMS) exhibit anomalous behavior and may be oxidized in vivo to methylsulfinyl (-SOCH3) or methylsulfonyl (-S02CH3) before reacting at the receptor site within the plant or seed. Alternatively, they may function by a mode of action different from the other TFMS pre-emergence herbicides tested. The parent TFMS (H-substituted) appears to function as a 4-substituted series member rather than as a 3-substituted derivative. The predictive capabilities of the derived Hansch relationships were demonstrated for several specific cases. 

Table XV. Calculated and Experimental LD9o Values for Methylthio- and Methylsulfonyl-TFMS Derivatives...

Experimental LD9o values are those listed in Table VII for the methylthio-TFMS and methylsulfonyl-TFMS derivatives. 

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