13 Photochemical Reactions of Pyrethroid Insecticides R O Y L. H O L M S T E A D , J O H N E . C A S I D A , and L U I S O. R U Z O
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Pesticide Chemistry and Toxicology Laboratory, Department of Entomological Sciences, University of California, Berkeley, Calif. 94720
The natural pyrethrins and many highly insecticidal synthetic chrysanthemates are not suitable for control of agricultural insect pests because of insufficient s t a b i l i t y in l i g h t and a i r ( l ) . Considerable progress has been made in improving the photostability of pyrethroids by suitable formulation ( e . g . , microencapsulation and inclusion complexes) and by adding antioxidants or UV screens. However, the most effective stabilization is achieved by replacing the photol a b i l e groups by others that give enhanced s t a b i l i t y to the overall molecule and equal or increased insecticidal activity (2,3). Knowledge of the photochemical reactions of the earlier pyrethroids contributed to the development of this new generation of photostabilized pyrethroids. It is now necessary to define the photochemistry of these newer compounds and the significance of their photoproducts as residues and environmental contaminants. This review considers the types of photolytic reactions of pyrethroids with emphasis on permethrin (3), NRDC l 6 l (4) and S 5602 (5). Isornerization of the Cyclopropane Ring and of Alkenyl Substituents Epimerization of [lR]-cyclopropanecarboxylate insecticides greatly reduces or destroys their insecticidal activity. As a corollary, epimerization of suitable [IS]-compounds yields the insecticidal conformation. The trans- and cis-isomers also differ in potency and persistence. Photoisomerization of the cyclopropane ring therefore has important consequences. Irradiation (\ > 200 nm) of [ l R ] - or [1RS]-trans- or - c i s chrysanthemic acid or its simple a l k y l esters in hexane with isobutyrophenone or related sensitizers yields equilibrium mixtures of the corresponding [ l R , t r a n s ] - , [IS,trans]-, [ l R , c i s ] and [lS,cis]-compounds in the approximate ratio 32:32:18:18 (6-9) (Figure l ) . The postulated mechanism involves cleavage 137
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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138
SYNTHETIC
PYRETHROIDS
Figure 1 of
t h e C - l t o C-3 bond
diradical
w h i c h may
k possible
the
fragment it
may
rearrange I,
isomerized
on glass
cyclopropane s i l i c a
themates
studies
isomerization isobutenyl
cyclopropane solution nm)
mixture or
in
as deposits sunlight
tion
phase
with
the resulting
NRDC l 6 l s o l u t i o n s ,
in
methanol
o f thin after
a
films
are also
leads
in
thin
in
exposed
photoisomerizaoccurs
undergoing
further
(\ >
290 n m )
On i r r a d i a t i o n
o f isomerization in
2-propanol;
to sunlight, (l4).
on exposure
methanol
Photo-
films
or cis-permethrin
more
( λ >290
occurring
than
(12).
solutions,
degree
both
t o an equilibrium
on glass
mixture
irradiation detected
in
Irradiation
surfaces
occurs
trans-
on glass
by replacing
the isomerization
isomer
small
in
over
undergo
extent
(12-14).
films
andconsiderably
6 days
the acid moiety
aqueous
(see below).
of
obtained
preference
significant
also
I n dilute
chrysan-
a n da i r so that
maytake
g e l or s o i l
o f permethrin
exposure
irradiated
These
ring.
or as thin
on s i l i c a
(13)
at the
light
in
andc i s - e s t e r s ,
(12).
o f isomerization
or cis-permethrin
hexane
photodecomposition
derivatives
in
acid nor
trans-resmethrin
sunlamp X L L ) «
reactions
(\ > 2 9 0 n m ) o f e i t h e r
rapidly
to a
d i d not
o f the
by a dihalovinyl substituent
trans-
o f trans-
rapidly
or a
reactions
isomerization
isomerization to
group
on hydrolysis
with
unstable
other
or
irradia
cis-chrysanthemic
Theabsence
photostabilized
ands o l i d
o f either
more
neither
o f the cyclopropane
Pyrethroids
the senecioate
2h h r s u n l a m p
after
was c o n f i r m e d
are relatively
t h e d i r a d i c a lmay
t o give
TlR,trans]-chrysanthemates
even
sunlight
a a n yo f
(7,9).
was r e c o v e r e d
esters.
ring
g e lwith
short-term
the
Thus,
acid
t o form to y i e l d
dimethrin a n dtetramethrin)
products
(10).
photodecomposed
cleavage
insecticidal
a l l e t h r i n ,
meso-cis-caronic
on
Alternatively,
to a lactone
on k
Studies
tion
isomers.
ring
t h e C - l t o C-3 bond
b y C-2 t o C-3 bond
(pyrethrin reveal
o f the cyclopropane
reform
occurs
on
30$ t r a n s - i s o m e r
is
The senecioate o f permethrin
a n d NRDC
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
13.
HOLMSTEAD ET
l6l
as
thin
sunlight
tion
films
to
rate
is
via
the
markedly (6-8).
of
NRDC l 6 l
on
irradiation
where
sensitizers
(\
but
sensitizer
permethrin
is
in
water
290
nm)
chrysanthemic
with
also
isomerization >
for
excited
increased
in
is
state
since
to
acid the
case
the
hexane
only in
is
with
and
acid
reaction
the
the
the
its
other
moiety
observed
presence
major
and
isomeriza
isobutyrophenone
the
photodebromination
substituents
to
group
(Figure
t r i p l e t
This
photoisornerization, converted
of
of
triplet
reaction
without
(13).
Alkenyl
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and
trans7cis-isomerization
occurs
sensitizers
enyl
sunlight
139
Reactions
lk).
(12,
The esters
Photochemical
AL.
a
of
in
i . e . ,
cyclopropyl jasmolin
the
the
I
rethronyl moiety
a l l y l
group
substituent
gives
the
of
and
also
undergo
allethrin
the
is
Z(cis)-pent-2-
Ε(trans)-isomer
(15)
2).
hv
Oxidation
of
F u n c t i o n a l Groups
Photooxidation themates
and
of
various
the
earlier
moieties
of
residual
persistence.
allethrin, yields acids, tion
of of
at
products ester
which
the
possible Figure
alcohol I
and
the
in
moiety
were
oxidation furylmethyl on
glass
major
thin
films
I,
on
glass
Saponification
of
pyrethroid liberates
compounds
originate
double
bond
pathways
to
account
3.
Although products
not
examined, the
their
pyrethrin
or
to
chrysan-
alcohol
limits
of
(10 ) .
each
in
Moieties
the
other
and a l l y l
groups
are
and
pyrethroids
for
of
compounds very
compounds
from
l a b i l i t y
oxida
isobutenyl
these
derived
high
from
the
the
12-16
of
the pyrethrin
indicates
susceptible
to
that photo-
(10).
Resmethrin or
case
from
as
the
group
allethrin relative
pentadienyl
light
each
identified
substituent;
in
greatly
tetramethrin
in
and A l c o h o l
substituent
groups
pyrethroids
trans-methyl
are
shown
isobutenyl
Acid
Photodecomposition
products the
the
functional
dimethrin and
11-15
mixture
the
of
in
group
sunlamp or
as
other
undergo
irradiation
deposits
on
photodecomposition
isobutenyl
substituent
to
with
rapid oxidation in
aqueous
s i l i c a
route give
gel R-
medium
5-benzyl-3exposed or
as
k).
epoxidation
at
and
to
sun
thin
(Figure
(ll)
involves the
the when
S-epoxides.
films One
the Formation
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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140
of
SYNTHETIC
the
the
poses by
other
furan by
the
to
a
as
photoproducts a
cyclic
following
rearrangement
detected or
major
ring
its
hydrogen
benzyl
group
benzyl
cation
to
pathways:
the
give
the
the
(ll);
position
hydroxy
r a d i c a l to
give
peroxide
reduction
derivative
r a d i c a l from or
i n i t i a t e d by
cyclopentenolone
epoxide
to
is
ozonide-type
lactone the
to
(i)
a
oxidation which d i o l is
migration
of
symmetrical ( i l l ) ;
of
decom-
followed
which
benzyloxy
PYRETHROIDS
also a
to
proton the
migration lactone
of
the
(IV)
(11). No p h o t o o x i d i z e d identified
to
(Ik)
5602
the
and
S
date
a c i d moiety
resistant
to
in
derivatives
r e t a i n i n g the
ester
group
the
on p e r m e t h r i n
(12),
NRDC
l6l
bond
of
(12). and
The the
studies
halogen-substituted
3-phenoxybenzyl
group
double appear
to
photooxidation.
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
be
are
quite
13.
HOLMSTEAD
Reductive The tion
Photochemical
E T A L .
Dehalogenation
the acid
duces t h e p o s s i b i l i t y halide.
permethrin halogen
Less
of reductive extensive
irradiated
(λ
monodechlorinated
derivative
moiety
b u t always
in
larger
amounts
lengths
minor
o f these
o n NRDC 1 6 1 .
Thus,
preference
steric
accounts l 6 l
of
with
debromination
products
from
l6l
been
is
The
light
in
is
with
l 6 l
in
l i k e l y
This
hexane
proceeds
o fd i -
unity
is
i . e . ,
(13).
not
the
The
f o r photodebromination
is
or thin
(12-14),
amount
preference
o f NRDC l 6 l ,
material
product
monodebrominated
steric
necessary
to
the findings
the trans-debrominated
approximates is
solutions
n o t formed
films
(l4).
no secondary
monodechloro-permethrin
hexane
o f Carbon
with
on irradia Inthe
oxidation
o r monodebromo-NRDC
sources
o f photodecarboxylation
on the structure
Photoelimination
linkage
light
group o f
containing
short
Dioxide
or other (l6).
free
This
dioxide
a n dr e l a t e d radical
type
in
o f the acid
o f carbon
certain pyrethroids
an α-cyano
ester
it
on analogy o f NRDC
o f the
(12).
depends
conditions.
the
in
importance
reaction contain
ratio donor
wave
Figure 5
dechlorination o f the chlorophenyl
Photoelimination
generally
somewhat
observed.
evident
wavelength
ando f the a c i d 5);
at shorter
not assigned,
(13).
debrominated
benzene either
Reductive 5602
"with
carbon-
the stereochemistry
the a c i d moiety
carried out t o date
have
(Figure
irradiation, a small
formed
o f a hydrogen
o f either
studies
S
is
NRDC l 6 l s i n c e
tion
to a
gives the
ester
f o r ~ 80$> o f t h e t o t a l
debromo-NRDC
starting
is
product
so that
on prolonged
trans/cis
func
intro
expected
4%
debromination
derivatives;
presence
it
relative
are obtained
Although
permethrin
with
evident
(12)
12 %
5)
but
290 n m ) in w a t e r
>
amounts
the trans-dehalogenated
(Figure
is
on their
o f the parent
\ > 220 n m ) .
(e.g.,
monodechlorinated
a
isobutenyl
photodehalogenation
dehalogenation
products
N R D C 161
be
f o r the labile
to photooxidation
strengths.
Permethrin
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group
t h a n w i t h NRDC l 6 l b a s e d
bond
141
o f Dihalovinyl Substituent s
d i h a l o v i n y l replacement
stabilizes
vinyl
Reactions
ester is
a
compounds
stabilizing
o f reaction
photolysis
andi r r a d i a t i o n
is
prominent which group
α to
negligible
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
in
142
SYNTHETIC
unsubstituted
esters
major
f o r S 5602
pathway
(Figure
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(12).
6)
such
as permethrin
a n d in h e x a n e ,
o f S 5602
(χ
derivative
on s i l i c a
is
o f ERDC
l 6 l , the decarboxylation
in
methanol
(Figure
6)
with
combination the
acid
Ester
in
a l l e t h r i n , the
with
acid
products
C
,
CI
ether
is
a
amount
r a d i c a l
significant
a n dt e t r a m e t h r i n
t h e newer
pyrethroids
a n da
cyano
the transalcohol
conditions
methyl
bond
formed
from
reactions
gives
in
W ^ V " A r ^
1
is
water
a
esters
yields
W
Xa
o
alcohol
•
h
methanol
o f these (12)
a^XA,O H
26%
11%
29%
13%
atoms
carbon.
o r hexane in
major
halogen
a t the benzyl
photolysis
the methyl
C
It
contain
a n dc i s - d i c h l o r o v i n y l acids
whereas
o f 3-phenoxybenzyl
permethrin
reaction f o r
not f o r pyrethrin
(10).
which
group
o f trans -permethrin
phenoxybenzyl l a r
formed
due t o a
o f other
( l l ) but apparently
dimethrin
moiety
irradiation major
o f the benzylic
(12).
is
smaller
probably
a n dt h e v a r i e t y
o f the ester
a n dc i s - r e s m e t h r i n
reaction
f o r 28 d a y s
on
Cleavage
Photolysis I,
o f S 5602
is
solvents
photoproduct
product
b u t in m u c h
difference
the
t o NRDC l 6 l .
Bond
trans-
This
o f the s t a b i l i t y
portion
available
a n dhexane
S 5602.
is
a n dother
g e l to sunlight
t h e case than
it
t h e major
In
(13)
but
> 2 9 0 n m ) in m e t h a n o l
acetonitrile-water
Thedecarboxylated
exposure
(12)
on irradiation
PYRETHROIDS
and3under
acids
(Figure
Thus, as simi
and t h e 7 ) .
c i s -
+ HO^Ar 28%
hv \>290^
Permethrin l6l
gives
undergoes the trans-
acetonitrile-water the
viscosity
2-propanol), cleavage
with
analagous
NRDC
a n dt h e i r is
reactions
3
53% (12).
methyl
esters
increases
a relative
in
in
in
(13 ) .
As
ethanol and
the extent
phase,
NRDC
hexane a n d
methanol
(methanol,
decrease
l 6 l a n d in t h e s o l i d
in
Figure 7
Similarly,
a n dc i s - d i b r o m o v i n y l acids
o f the solvent there
CH 0^Ar
OCH3
CH3OH
o f
ester
sunlight,
this
Elliott; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
13.
H O L M S T E A D
ceases
E T
t o be t h e major
Dimerization
o f Free
Irradiation S
Photochemical
AL.
5602
in
r e a c t i o n pathway ( l U ) .
Radicals
( λ > 2 9 0 n m ) o f N R D C l 6 l in h e x a n e
several
generated
solvents
(12)
leads
during the photolysis Ihexane " U>290
NC
NRDC 161 S 5602
4%
