Metabolic pathway
Phenothrin is the name given to the 1RS-cis-trans isomer mixture
(racemic). The product now used is d-phenothrin which is 95% 1R and
75% trans. It has no field use because the chrysanthemate moiety is very
sensitive to photodegradation. Nevertheless, information on its photochemistry
and its fate in soils and plants has been published. Several
studies in rodents have been reported; this is a reflection of its use in
public health. Phenothrin is degraded mainly by photo-oxidation and by
hydrolysis and oxidation in plants and animals.
Degradation
Phenothrin is stable under normal storage conditions but it is labile to
base, being hydrolysed to trans-2,2-dimethyl-3-(2-methylprop-1-enyl)-
cyclopropanecarboxylic acid (11, trans-chrysanthemic acid) and 3-phenoxybenzyl
alcohol (13,3PBAlc) (Scheme 2). It is sensitive to light and, for
example, as a thin film at midday in the summer at 55 ° N, it was degraded
with a DT50 of 2.5-3.0 hours (Samsonov and Makarov, 1996).
When (lR)-trans-[14C-carboxyl]phenotwhraisn irradiated in degassed
benzene solution, the only product was the cis-isomer. However, in oxygenated
benzene solution, degradation was about 10-fold faster and many
products were detected (Ruzo et al., 1982). A similar array of products was
seen on exposure of thin films to sunlight. Major products (Scheme 1)
were formed by oxidation at the isobutylene substituent giving the
epoxide (2), the alcohol (3), the aldehyde (4) and the carboxylic acid (5).
Caronaldehyde (6) and the caronic acid derivative (7) were formed by
cleavage of ozonolysis products and the hydroperoxide (8) was formed by
ene reactions at the 1’-position. Ester cleavage to 3PBAlc, 3PBAl and 3PBA
(not shown in Scheme 1) was relatively minor.
