Singlet molecular oxygen-mediated photooxidation of 2-substituted 3-hydroxypyridines

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

“…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). While processes [5] and [8] can be merely physical events, process [6] can involve the consumption of both Rf and AHMPD.…”

“…The competitive pathways represented by processes [3] (and, subsequently, [5] and [6]) and [8] are generated from 3 Rf* (37,38). While processes [5] and [8] can be merely physical events, process [6] can involve the consumption of both Rf and AHMPD. The predominance of a given process will depend on the competition between O 2 and AHMPD for the quenching of 3 Rf*.…”

“…In particular, in many cases the photo-induced elimination from the aqueous environment of said N-heterocycles has shown to be a potentially applicable procedure, and in some cases the reactions involved have been studied (4). 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).…”

“…The reaction sequence, which has been already sufficiently described (4,11), is depicted in processes [1]- [6], where S is a dye sensitizer responsible for the visible light absorption (RB in the case of ref. (9)), 1 S* and 3 S* are the respective singlet and triplet excited states of S, O 2 ( 3 Σ g − ) is the ground-state molecular oxygen, and O 2 ( 1 ∆ g ) is the singlet molecular oxygen generated by energy transfer from 3 S* to O 2 ( 3 Σ g − ).…”

“…(9)), 1 S* and 3 S* are the respective singlet and triplet excited states of S, O 2 ( 3 Σ g − ) is the ground-state molecular oxygen, and O 2 ( 1 ∆ g ) is the singlet molecular oxygen generated by energy transfer from 3 S* to O 2 ( 3 Σ g − ). O 2 ( 1 ∆ g ) can be physically deactivated by collision with solvent molecules (process [4]), or with AHMPD and (or) the sensitizer (process [5]), or can react with the latter compounds giving rise to oxidation products (process [6]). Our results indicated that the degradation of AHMPD by sensitized photooxidation is efficient only in highly alkaline solutions -a very improbable case in natural waters -and that this procedure could be exclusively applicable in programmed experiments under controlled conditions.…”