HYDROLYSIS OF PROSULFURON AT PH 5 - EVIDENCE FOR A RESONANCE-STABILIZED TRIAZINE CLEAVAGE PRODUCT

Prosulfuron is a herbicide for the selective control of broadleaf weed s in corn. In order to examine the effect of pH on the stability of pr osulfuron in aqueous solution, a hydrolysis experiment was conducted i n buffer at pH 5, 7 and 9 utilizing [phenyl-C-14] and [triazine-C-14]p rosulfuron. Prosulfuron was found to be stable under neutral and basic conditions and hydrolytically unstable under acidic conditions (T(1/2 )similar to 10 days). One of the major routes of degradation at pH 5 i nvolved hydrolysis of the sulfonylurea bridge to yield the correspondi ng phenyl- and triazine-cleavage products and minor amounts of desmeth yl-prosulfuron and dihydroxymethyltriazine. In addition to these hydro lysis products, an unknown species was observed at significant levels after 30 days (similar to 20% of the applied dose). A large-scale expe riment was subsequently performed to generate additional material for the spectroscopic characterization and identification of this unknown degradate. A thorough spectral evaluation of this unknown revealed a r esonance-stabilized derivative of the parent molecule which stemmed fr om a hydrolytic opening of the triazine ring of desmethyl-prosulfuron. Although this triazine ring-opening phenomenon was first described fo r chlorsulfuron, a structurally related sulfonylurea herbicide, the st ructure of the product was only postulated. Recently, Cambon et al. de scribed the hydrolysis kinetics of thifensulfuron and thifensulfuron-m ethyl which resulted in cleavage of the sulfonylurea bridge and triazi ne ring-opening. Our results support these findings and we herein desc ribe the characterization and identification of this resonance-stabili zed species by comparison with the recently synthesized authentic refe rence standard.