Ld. Bray et al., HYDROLYSIS OF PROSULFURON AT PH 5 - EVIDENCE FOR A RESONANCE-STABILIZED TRIAZINE CLEAVAGE PRODUCT, Pesticide science, 51(1), 1997, pp. 56-64
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.