Photodegradation of Azole Fungicide Triadimefon

Article abstract of DOI:10.1021/jf960074n

To examine the photostability of the fungicide triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-1H-(1,2,4-triazol-1-yl)butan-2-one] in the field, model experiments with organic solvents were performed. Photolysis in methanol, hexane, and acetone resulted in considerable formation of 1-(4-chlorophenoxy)-3,3-dimethylbutan-2-one, 1-[(4-chlorophenoxy)methyl]-1H-1,2,4-triazole, 1H-(1,2,4-triazol-1-yl)-3,3-dimethylbutan-2-one, and 1-phenoxy-3,3-dimethyl-1H-(1,2,4-triazol-1-yl)butan-2-one. The rate of photodegradation in different solvents followed first-order rate kinetics with a significant correlation coefficient.

Full text of DOI:10.1021/jf960074n

294
J. Agric. Food Chem. 1997, 45, 294−298
P h otod egr a d a tion of Azole F u n gicid e Tr ia d im efon
Subir K. Nag and Prem Dureja*
Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi-110012, India
To examine the photostability of the fungicide triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-1H-
(1,2,4-triazol-1-yl)butan-2-one] in the field, model experiments with organic solvents were performed.
Photolysis in methanol, hexane, and acetone resulted in considerable formation of 1-(4-chlorophe-
noxy)-3,3-dimethylbutan-2-one, 1-[(4-chlorophenoxy)methyl]-1H-1,2,4-triazole, 1H-(1,2,4-triazol-1-
yl)-3,3-dimethylbutan-2-one, and 1-phenoxy-3,3-dimethyl-1H-(1,2,4-triazol-1-yl)butan-2-one. The
rate of photodegradation in different solvents followed first-order rate kinetics with a significant
correlation coefficient.
Keyw or d s: Triadimefon; photodegradation; photoproducts; solvents; rate kinetics
INTRODUCTION
IR spectrophotometer. 1H-NMR spectra were recorded on a
Varian EM 360L (60 MHz) instrument. Deuteriochloroform
(CDCl3) or carbon tetrachloride (CCl4) was used as solvent and
tetramethylsilane (TMS) as the internal standard for NMR.
Gas chromatography-mass spectroscopy was done on a HRDC-
MEGA 2 series, coupled with a Fisons-TRIO 1000 ion trap
mass spectrometer. The ionization potential was 70 eV. The
GC was equipped with a SE-54 capillary column (15 m × 0.25
mm i.d., film thickness, 0.1-0.15 µm). The conditions were
as follows: initial temperature was 60 °C for 1 min and then
Triadimefon, 1-(4-chlorophenoxy)-3,3-dimethyl-1H-
(1,2,4-triazol-1-yl)butan-2-one, first reported in 1973 by
Frohberger, is reported to be a very good protectant and
eradicant fungicide against powdery mildew and rust
fungi (Buchenauer, 1973). It has been claimed to move
systematically after application to roots or leaves of
cereals (Kasper et al., 1975) and basipetally in cucumber
and barley (Scheinpflug and Paul, 1977). Its activity
is probably enhanced as a result of considerable redis-
tribution in the vapor phase. It is also used as a foliage
fungicide (Wright et al., 1983) and as a soil fungicide
(Hardison, 1975, 1976). Clark et al. (1978) studied the
photolysis of triadimefon in methanol using a medium-
pressure mercury vapor lamp and identified only 1H-
1,2,4-triazole, methyl 4-chlorophenyl carbonate, and
4-chlorophenol.
To study systematically the photochemical reactivity
of pesticides on plant surfaces, model photoreactions are
undertaken. Hartmann and Schwack (1994) used or-
ganic solvents with selected functionalities as substi-
tutes for components of plant waxes for photodegrada-
tion of triadimefon. In the present paper, the results
of photochemical degradation of triadimefon in the
presence of hexane, methanol, and acetone as repre-
sentative model substances for the plant cuticle con-
stituents are presented.
heated up to 250 °C (10 min) at 15 °C min-1
used as the carrier gas.
. Helium was
TLC was performed on 20 × 20 cm2 glass plates coated with
0.5 mm silica gel G, using iodine as chromogenic reagent.
Photoproducts were separated by column chromatography
using a glass column (75 cm × 2 cm i.d.) containing 500 g of
60-100 mesh preactivated silica gel in n-hexane and eluting
with hexane and hexane and acetone in different ratios.
Ir r a d ia tion a n d P h otop r od u cts. Solutions of triadime-
fon (100 ppm) in hexane, methanol, and acetone, respectively,
were irradiated for 24 h with a medium-pressure Hg lamp (125
W, Phillips) and a water-cooled quartz filter. After irradiation,
the solvent was distilled off under reduced pressure. To
produce enough of photoproducts for structural analysis,
triadimefon was irradiated in five different batches.
P r ep a r a tion of Com p a r ison Com p ou n d s. Pinacol hy-
drate and pinacolone were synthesized according to the
procedure described by Vogel (1989).
1-Br om o-3,3-d im et h yl-2-b u t a n on e (1-Br om op in a col-
on e). Bromine (5 mL; Ca u tion : experiment should be carried
out in a fume cupboard, using gloves and facemask), dissolved
in glacial acetic acid (10 mL), was added dropwise to a solution
of pinacolone (10 g) in acetic acid (10 mL) with constant
stirring and maintaining the temperature (25 °C). After
addition of bromine, stirring was continued for another 2 h.
The solution was extracted with n-hexane and dried over
sodium sulfate. Hexane was distilled off, and product was
distilled at 71 °C at 10 mmHg. The IR spectrum of the product
MATERIALS AND METHODS
Ch em ica ls. A technical sample of triadimefon (95%) was
supplied by M/s Bayer Ltd. (New Delhi, India) and was purified
further by repeated crystallization from n-hexane to constant
melting point (80-81 °C) and shown to be chromatographically
pure (TLC, GLC, and HPLC). The solvents used were of
analytical grade.
showed the presence of CdO at 1719 cm-1
δ 4.1 (s, 2H, CH2), 1.3 (s, 9H, C(CH3)3).
.
1H-NMR (CCl4):
Ap p a r a tu s a n d Ch r om a togr a p h y. Triadimefon and its
degradates in rate kinetics studies were analyzed with a GLC
instrument (Hewlett Packard Model 5890, series II gas liquid
chromatograph, equipped with an electron capture detector
and a capillary column [HP-1, methyl silicon gum, 10 m × 0.53
mm i.d. × 2.63 µm film thickness], coupled with a Hewlett
Packard 3390A integrator). The operating conditions were as
follows: oven, injector, and detector temperatures at 160, 250,
and 300 °C, respectively. Nitrogen was used as carrier gas
1-(4-Ch lor op h en oxy)-3,3-d im eth ylbu ta n -2-on e. A solu-
tion of 1-bromopinacolone (100 mg) and 4-chlorophenol (300
mg) was refluxed in dry benzene (10 mL, experiment should
be conducted in fume cupboard) in the presence of anhydrous
potassium carbonate (1 g) for 4 h. The solution was filtered
to remove potassium carbonate. The benzene layer was
separated and treated with sodium hydroxide solution (10%)
to remove excess phenol. It was washed several times with
water. The benzene layer was dried over anhydrous sodium
sulfate. On distillation, a yellow oil was obtained which
solidified on standing; mp 63-65 °C. It was purified by column
chromatography. The IR spectrum of the product showed the
with a flow rate of 20 mL min-1
. The ultraviolet-visible
spectrum of triadimefon was recorded on a Hitachi Model
V-2000 double-beam UV-vis spectrophotometer in methanol
using a quartz cuvette (1 cm path length). The infrared
spectra of products were recorded on a Nicolet Impact-700 FT-
presence of a carbonyl group at 1728 cm-1
.
1H-NMR (CDCl3):
S0021-8561(96)00074-X CCC: $14.00
© 1997 American Chemical Society

Photodegradation of Azole Fungicide Triadimefon
J. Agric. Food Chem., Vol. 45, No. 1, 1997 295
F igu r e 1. Possible photodegradation products of triadimefon (I) in methanol (M), n-hexane (H), and acetone (A).
δ 7.2 (d, 2H, AA′BB, aromatic), 6.8 (d, 2H, AA′BB, aromatic),
4.6 (s, 2H, CH2), 1.1 (s, 9H, C(CH3)3).
1-Phenoxy-3,3-dimethylbutan-2-one (V). MS: m/z 192
(M+).
1H-(1,2,4-Tr ia zol-1yl)-3,3-d im eth ylbu ta n -2-on e. A solu-
tion of 1,2,4-triazole (200 mg) and 1-bromopinacolone (600 mg)
was refluxed in dry acetone in the presence of anhydrous
potassium carbonate (1 g) for 4 h. The solution was filtered
to remove potassium carbonate. The filtrate was evaporated
to dryness. The residue was subjected to column chromatog-
raphy. A solid thus obtained was crystallized from methanol
as a colorless amorphous solid. The IR spectrum of the product
1-(4-Chlorophenoxy)-3,3-dimethylbutan-2-one (VI).
mp: 63-65 °C. The IR spectrum of the photoproduct
showed the presence of carbonyl group at 1728 cm-1
.
1
MS: m/z 226 (M+) (VI, Table 1). Its H-NMR, IR, and
mass spectra were comparable with those of an authen-
tic sample.
2-(4-Chlorophenoxy)ethanol (VII). MS: m/z 172, 171
(base peak), 27/128 (VII, Table 1, Figure 1).
showed the presence of a carbonyl group at 1731 cm-1
.
1H-
NMR (CDCl3): δ 8.2 (s, 1H), 7.9 (s, 1H), 5.3 (s, 2H, CH2), and
1.2 (s, 9H, C(CH3)3).
1-(Phenoxy)-3,3-dimethyl-1H-(1,2,4-triazol-1-yl)butan-
2-one (VII). The IR spectrum of the product showed a
band at 1720 cm-1, indicating the presence of a carbonyl
P h otolysis Kin etics. A solution of triadimefon (10 ppm,
20 mL) in solvents (methanol, acetone, and hexane) was
irradiated in quartz and Pyrex tubes under UV light. Samples
were withdrawn at intervals of 0, 0.5, 1, 3, 5, and 7 h from
quartz tubes and 1, 3, 5, 8, 12, 16, 20, 24, 30, 36, and 48 h
from Pyrex tubes and analyzed by GLC.
1
group. The H-NMR spectrum showed two one-proton
singlets at δ 8.2 and 8.0, corresponding to two triazole
protons. It also showed a multiplet at δ 7.8, corre-
sponding to five aromatic protons, a singlet at δ 6.9 for
one proton at C-1, and a singlet at δ 1.2 for nine protons
of the tert-butyl group. MS: m/z 259 (M+) (Table 1).
RESULTS AND DISCUSSION
1-(Phenoxymethyl)-1,2,4-triazole (IX). MS: m/z 175
(M+) (Table 1), 69, and 95. The mass spectrum showed
the absence of chlorine atom.
Isola t ion a n d Id en t ifica t ion of P h ot op r od u ct s
(F igu r e 1). (i) P h otolysis in Meth a n ol. A methan-
olic solution of triadimefon (I, 1000 ppm) was irradiated
for 24 h. The solvent was removed by rotary vacuum
evaporation, and the residue was subjected to column
chromatography over activated silica gel. Photoprod-
ucts were separated by successive elution with hexane
and hexane-acetone in different ratios. The separated
photoproducts were further purified either by crystal-
lization from appropriate solvent or by the preparative
TLC. The following photoproducts were identified by
IR, 1H-NMR, MS, GC-MS, and comparison with au-
thentic standards.
1-(1,2,4-Triazol-1-yl)-3,3-dimethylbutan-2-one (X). The
IR spectrum of the product showed the presence of a
1
carbonyl group at 1731 cm-1. The H-NMR spectrum
of the product showed two singlets for one proton each
at δ 8.2 and 8.0 for two triazole protons, a singlet of
two protons at δ 5.3 for CH2, and nine proton singlet at
δ 1.2 for the tert-butyl group (Figure 1). MS: m/z 167
(M+) (Table 1). The product was identified by Co-TLC,
Co-IR, and comparison of the mass and NMR spectra
with those of an authentic sample.
1-(4-Chlorophenoxy)-3,3-dimethyl-1-(1,2,4-triazol-1-
yl)butan-2-ol (XI). The product was identified by com-
parison of its IR, NMR, and mass spectra with those of
an authentic sample.
1-[(4-Chlorophenoxy)methyl]-1,2,4-triazole (XII). The
IR spectrum of the product showed the absence of
carbonyl group. The 1H-NMR spectrum showed the
presence of two one-proton singlets at δ 8.3 and 8.0 for
two triazole ring protons. It also showed four aromatic
protons at δ 7.4-7.2. The NMR spectrum showed the
absence of a singlet at δ 1.2 due to the tert-butyl group.
Methyl Phenyl Carbonate (II). TLC: Rf 0.93 (solvent
system, n-hexane-acetone, 95:5 v/v). MS: m/ z 152
(fragments recorded in Table 1).
Methyl 4-Chlorophenyl Carbonate (III). TLC, Rf 0.85
(solvent system, n-hexane-acetone, 9:1 v/v). MS: m/ z
186 (M+) (III, Figure 1, Table 1), base peak at m/ z 127/
128 (M+ - 59).
4-Chlorophenol (IV). mp: 43 °C. bp: 217 °C. 1H-
NMR (CDCl3): δ 7.8 (s, 4H, aromatic). MS: m/z 128/
130 (M+). Identified by Co-TLC, Co-GLC, and mmp
comparison with an authentic sample.

296 J. Agric. Food Chem., Vol. 45, No. 1, 1997
Nag and Dureja
Ta ble 1. Ma ss Sp ectr a l Da ta of P h otod egr a d a tion
P r od u cts of Tr ia d im efon
quartz tubes for 48 h. NMR and mass spectra of
isolated products were in good agreement with photo-
products IV, V, VIII, X, XII, XIII, and XIV identified
earlier (Table 1, Figure 1).
mass
abundance
(%)
yield
(%)
product found (m/z)
structure
I
293
258
236
208
166
127/128
152
121
93
186
155
127
128
192
177
135
57
226
169
141
127
172
171
155
141
127
259
202
174
93
18
10
12
100
15
80
80
100
80
70
20
100
100
15
100
35
70
35
28
60
65
90
100
80
30
40
8
10
100
45
100
95
40
10
15
80
15
15
10
M+
15.0
The UV spectrum of triadimefon (I) in methanol and
water exhibited a band at 225 nm for allowed π-π*
transition of the phenyl ring and a band at 274 nm,
which is essentially n-π* in character, resulting from
the combined transitions of the carbonyl system and
lower energy band of the aromatic rings. These π-π*
and n-π* transitions can lead to the production of either
singlet or triplet excited states. Thus, no unique excited
state can be invoked to explain the variety of photore-
actions undergone by the various functional groups of
triadimefon.
Figure 1 shows the possible degradation products of
triadimefon. The photoproducts identified can be ra-
tionalized as originating from any one of the following
photochemical processes (A-E) (Figure 2).
The photochemical cleavage via pathway A (Figure
2) involves bond breaking between phenolic oxygen and
the C-1 carbon atom (Ia ), leading to the formation of
two radicals. These two radicals abstract protons from
the medium and yield 4-chlorophenol (IV) and 1-(1,2,4-
triazol-1-yl)-3,3-dimethylbutan-2-one (X).
Pathway B (Figure 2) involves bond breaking between
C-1 and C-2 yielding Ib and is evidently the preferred
pathway in solution phase photolysis based on the yield
of the products observed. The radical Ib abstracts a
proton from the solution and yields XII. A similar
product was identified by Hartmann and Schwack
(1994) when triadimefon was irradiated in 2-propanol.
Pathway C (Figure 2) involves breaking of the bond
between C-1 and N-1 (Ic). The radical so formed gives
products VI and XIV by abstraction of protons from the
medium. These photoproducts were earlier identified
by Clark et al. (1978).
The carbonyl group of triadimefon undergoes reduc-
tion to give photoproduct XI (triadimenol), which also
shows fungicidal activity.
Photoproduct XV, which was identified only when
triadimefon was irradiated in hexane solution, may be
formed by breaking the bond between C-1 and N-1 of
photoproduct XII and subsequently abstracting a proton
from the solvent. Photoproduct III was also detected
by Clark et al. (1978) on irradiation of triadimefon in
methanolic solution under UV light.
M+ - Cl
M+ - C(CH3)3
M+ - OdCsC(CH3)3
M+ - p-chlorophenoxy
ClC6H5O*
M+
II
1.0
2.0
M+ - OCH3
C6H5O*
III
M+
M+ - OCH3
ClC6H5O*
M+
IV
V
15.0
4.5
M+
M+ - CH3
M+ - C(CH3)3
+C(CH3)3
VI
M+
9.5
2.5
M+ - C(CH3)3
M+ - OdCsC(CH3)3
ClC6H5O*
M+
VII
M+ - H
M+ - OH
M+ - CH2OH
ClC6H5O*
M+
VIII
4.5
M+ - C(CH3)3
M+ - OdCsC(CH3)3
C6H5O*
IX
X
175
147
93
167
110
82
M+
2.5
M+ - NsCH2
C6H5O*
M+
10.5
M+ - C(CH3)3
M+ - OdCsC(CH3)3
1,2,4-triazole
M+
69
XI
295
260
208
127
209
141
127
82
94
69
42
142
127
11.0
15.0
M+ - Cl
12
20
95
50
M+ - CHOHC(CH3)3
ClC6H5O
XII
M+
M+ - 1,2,4-triazole
ClC6H5O
95
100
100
80
50
100
95
M+ - CH2-C2H2N3
M+
XIII
XIV
1.0
5.5
M+
M+ - NCH
M+
XV
0.5
M+ - 15
MS: m/z 209/211 (M+), indicating the presence of a
chlorine atom in the molecule (Table 1).
Triadimefon (I) and photoproducts III, VI, and XII
undergo dechlorination to give VIII, II, V, and IX,
respectively.
Two minor products were identified by GC-MS of the
irradiated solution. They were identified as phenol
(XIII) and 1,2,4-triazole (XIV) (Figure 1, Table 1).
(ii) P h otolysis in Hexa n e. A solution of triadimefon
(500 mg) in hexane (500 mL) was irradiated in quartz
tubes for 48 h. The solvent was distilled under vacuum,
and the residue showed the formation of a number of
photoproducts on TLC. It showed the formation of IV,
VIII, IX, X, XII, XIII, and XIV by comparison of their
NMR and mass spectra with those of photoproducts
obtained earlier.
Elution of the column with n-hexane-acetone (97:3
v/v, fractions I-V) gave a colorless liquid. Its mass
spectrum showed a molecular ion peak at m/z 142 (Table
1). It was tentatively identified on the basis of mass
spectrum as methyl 4-chlorophenyl ether (XV, Figure
1).
(iv) Ra te Kin etics. The rate of photodegradation of
triadimefon in solvents such as methanol, hexane, and
acetone was studied under UV light from a medium-
pressure Hg lamp using a quartz filter (254-316 nm).
No degradation occurred in the dark, since 98% of
triadimefon was recovered unchanged after 24 h. The
rate of photodegradation in methanol, hexane, and
acetone followed first-order kinetics with a significant
correlation coefficient (r ) 0.99). Half-lives were found
to be 2.83 h, 2.49 h, and 36.70 min in methanol, hexane,
and acetone, respectively (Figure 3). Photodegradation
was more in acetone solution than in the presence of
methanol and hexane. Methanol and hexane can be
considered as a protecting solvents due to their tendency
to shift the UV absorption maxima to the shorter
wavelength, thus reducing molar absorptivity at 270 nm
(iii) P h otolysis in Aceton e. A solution of triadime-
fon (500 mg) in acetone (500 mL) was irradiated in

Photodegradation of Azole Fungicide Triadimefon
J. Agric. Food Chem., Vol. 45, No. 1, 1997 297
F igu r e 2. Proposed photochemical pathways to account for the observed photoproducts.
about photoreactivity, photoproducts, and possible pho-
tochemical pathways in the environment. Buchel et al.
(1975) stressed the importance of the stability of the C-1
of triazole with respect to the biological activity of
triadimefon. But we have observed that, during pho-
tolysis of triadimefon, cleavage of this bond does occur.
Thus, the formation of different products as a result of
irradiation in different solvents simulating environmen-
tal conditions showed the possibility of detoxification of
triadimefon in the field as a result of photolysis. We
can conclude that there will be a rapid breakdown of
triadimefon on the leaf surface of non-fungitoxic com-
pounds.
LITERATURE CITED
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Informations, Section 3, Chemical Control, 8th International
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Frohberger, P. E. Mitt. Biol. Bundesanst. Land-Forstwirtsch.,
Berlin-Dahlem 1973, 151, 61.
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and giving slow degradation, whereas acetone acts as a
triplet sensitizer and thus enhances the rate of degra-
dation.
Con clu sion . Irradiation of triadimefon in different
solvents under UV light has given basic information

298 J. Agric. Food Chem., Vol. 45, No. 1, 1997
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