Chemistry and Action of N-Phenylmaleamic Acids and Their Progenitors as Selective Herbicide Antidotes

Several N-substituted maleimides and related compounds were prepared and tested for their effects on plant growth. N-Alkylmaleimides were inactive but N-(4-chlorophenyl)maleimide (CPMI) protected sorghum from injury caused by alachlor without reducing its herbicidal activity. The highest protection was obtained on simultaneous application of CPMI and alachlor immediately after sowing. CPMI possessed both high botanical and chemical specificity, protecting only sorghum among the six crops evaluated and only against alachlor; it was ineffective as an antidote for EPTC or chlorsulfuron. Structural features favorable for high antidotal activity are the unsubstituted maleimide ring for N-(4-chlorophenyl)maleimides and a 4-chloro, 4-fluoro, or 4-methyl substituent on the phenyl ring for the almost equally active N-phenylmaleimides, -isomaleimides, and -maleamic acids. CPMI and N-(4-chlorophenyl)isomaleimide undergo rapid hydrolysis to N-(4-chlorophenyl)maleamic acid (CPMA). N-Phenylmaleimides and -isomaleimides thus appear to be proantidotes and N-phenylmaleamic acids are the actual antidotes. CPMI and CPMA, as the earlier antidotes R-25788 and flurazole, increase the glutathione level in sorghum roots.