Hydrolysis of prosulfuron at pH 5: evidence for a resonance-stabilized triazine cleavage product
1997
Bray, L.D. | Heard, N.E. | Overman, M.C. | Vargo, J.D. | King, D.L. | Lawrence, L.J. | Phelps, A.W.
Prosulfuron is a herbicide for the selective control of broadleaf weeds in corn. In order to examine the effect of pH on the stability of prosulfuron in aqueous solution, a hydrolysis experiment was conducted in buffer at pH 5, 7 and 9 utilizing [phenyl-14C] and [triazine-14C]prosulfuron. Prosulfuron was found to be stable under neutral and basic conditions and hydrolytically unstable under acidic conditions (T1/2 approximately 10 days). One of the major routes of degradation at pH 5 involved hydrolysis of the sulfonylurea bridge to yield the corresponding phenyl- and triazine-cleavage products and minor amounts of desmethyl-prosulfuron and dihydroxymethyltriazine. In addition to these hydrolysis products, an unknown species was observed at significant levels after 30 days (approximately 20% of the applied dose). A large-scale experiment 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 resonance-stabilized derivative of the parent molecule which stemmed from a hydrolytic opening of the triazine ring of desmethyl-prosulfuron. Although this triazine ring-opening phenomenon was first described for chlorsulfuron, a structurally related sulfonylurea herbicide, the structure of the product was only postulated. Recently, Cambon et al. described the hydrolysis kinetics of thifensulfuron and thifensulfuron-methyl which resulted in cleavage of the sulfonylurea bridge and triazine ring-opening. Our results support these findings and we herein describe the characterization and identification of this resonance-stabilized species by comparison with the recently synthesized authentic reference standard.
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