Rates of reaction of singlet oxygen with some systemic pyrimidine fungicides and related compounds

Abstract

Rate constants have been measured for the reaction in chloroform solution of singlet oxygen with the fungicides ethirimol (5‐butyl‐2‐ethylamino‐6‐methylpyrimidin‐4‐ol) and dimethirimol (5‐butyl‐2‐dimethylamino‐6‐methylpyrimidin‐4‐ol), and with the compounds 2‐dimethylamino‐5,6‐dimethylpyrimidin‐4‐ol, 2‐dimethylamino‐6‐methylpyrimidin‐4‐ol, 4‐benzyloxy‐2‐dimethylamino‐5,6‐dimethylpyrimidine and 2‐diethylamino‐6‐methylpyrimidin‐4‐ol. The values obtained show that singlet oxygen reacts readily with these compounds; the differences between the rate constant values have been rationalised in terms of the different structural features of the compounds studied. The possibility that singlet oxygen may react with agricultural chemicals under natural conditions is considered.

Photooxygenation of 5‐aryl‐2,4‐diaminopyrimidines leading to 4‐amino‐1,3,5‐triazin‐2‐yl ketones and, in the presence of sodium borohydride, to 5,6‐dihyro‐4(3H)‐pyrimidinones

“…Discussion. ~ It is known that unstable endoperoxides are formed in aprotic solvents upon reaction of '0, with pyrimidines exhibiting a substitution pattern like Me at C(2), Me or EtO at C(4), H at C(5), and EtO at C(6) [6]. This corresponds with our results concerning the decomposition of photooxygenation products derived from l a in aprotic solvents (Table).…”

Environmental photodegradation of pyrimidine fungicides — Kinetics of the visible-light-promoted interactions between riboflavin and 2-amino-4-hydroxy-6-methylpyrimidine

“…2) indicate the participation in the photoprocesses of reactive oxygen species generated directly or indirectly by 1 Rf* and (or) 3 Rf*. In spite of the high value of the rate constant for the interaction 1 Rf*-AHMPD ( 1 k q , process [7]) -close to the diffusion limit -relatively high concentrations of AHMPD are necessary to prevent triplet state population (75 mM, to quench 50% 1 Rf*, process [7]). The competitive pathways represented by processes [3] (and, subsequently, [5] and [6]) and [8] are generated from 3 Rf* (37,38).…”

“…Results indicate the operation of a complex mechanism that possibly includes specific interactions of AHMPD with singlet ( 1 Rf*) and triplet ( 3 Rf*) excited states of Rf, represented by processes [7] and [8]. , which is equal to the ratio 1 k q / 1 k d , where 1 k d is the rate constant for thermal deactivation of 1 Rf* (process [9]).…”

“…On the other hand, 1 k q values (process [7]) can also be graphically calculated through Stern-Volmer treatment (1/ 1 τ = 1/ 1 τ o + 1 k q [AHMPD]) by evaluation of the respective 1 τ values in the presence of different concentrations of AHMPD (Fig. 3, inset).…”

“…Following this line, in recent papers (5,6) we have reported the kinetics of the dye-sensitized photooxygenation of several hydroxypyridines, hydroxypyrimidines, and hydroxyquinolines with the basic molecular structures of known N-heteroaromatic pesticides (1). The kinetics of the photosensitized oxidation, in aqueous or chloroform solution, of the pyrimidine fungicides ethirimol (5-nbutyl-2-diethylamino-4-hydroxy-6-methylpyrimidine), dimethirimol (5-n-butyl-2-dimethylamino-4-hydroxy-6-methylpyrimidine), and four analogous pyrimidines, all with structures closely related to that of the model compound 2-amino-4-hydroxy-6-methylpyrimidine (AHMPD), has been previously studied (7,8).…”