Photochemical transformation of aqueous para-halogenophenylureas: Evidence for the intermediary formation of carbenes

Article abstract of DOI:10.1039/b005646o

N-Substituted 4-iminocyclohexa-2,5-dienylidenes (λ(max) = 300 and 405 nm) are detected at pulse end upon laser flash photolysis of aqueous monuron and metobromuron. In oxygenated medium these carbenes are converted into N-substituted iminoquinone-O-oxides

Full text of DOI:10.1039/b005646o

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Photochemical transformation of aqueous para-halogenophenylureas:
evidence for the intermediary formation of carbenes
Abdelaziz Boukhram and Claire Richard*
L aboratoire de Photochimie Moleculaire et Macromoleculaire (CNRS UMR 6505),
Ensemble Universitaire des Cezeaux, 63177 Aubiere cedex, France.
E-mail: Claire.Richard=univ-bpclermont.fr
L e t t e r
Received (in Montpellier, France) 12th July 2000, Accepted 1st September 2000
First published as an Advance Article on the web 2nd October 2000
N-Substituted 4-iminocyclohexa-2,5-dienylidenes (k
= 300
384 nm. The carbene 4-iminocyclohexa-2,5-dienylidene, pro-
duced by photolysis of aqueous para-halogenoanilines,
absorbs at 390 and 407 nm.12 Both carbenes exhibit the char-
acteristic reactivity of triplet carbenes, adding oxygen to give
the benzoquinone oxide16,17 or the iminoquinone-O-oxide12
and abstracting an H atom from aliphatic alcohols to yield
the phenoxyl radical17 or the anilino radical cation.12 The
addition of the carbene onto a starting molecule yields dimers
max
and 405 nm) are detected at pulse end upon laser Ñash photoly-
sis of aqueous monuron and metobromuron. In oxygenated
medium these carbenes are converted into N-substituted
iminoquinone-O-oxides (k
= 300 nm and 460 nm) and in the
max
presence of H-donor molecules into N-substituted anilino rad-
icals (k = 430 nm).
max
and the reaction with H O gives rise to hydroquinone or
aminophenol.
Substituted phenylureas exhibit herbicidal activity and are
widely used as plant products. After application they are
subject to photolysis and it is therefore of great interest to
determine their mechanisms of phototransformation.1 The
photochemical transformations of para-chlorophenyl-N,N-
dimethylurea (monuron) and para-chloro- or para-bromo-
2
To determine the mechanism of phototransformation of
monuron (M) and metobromuron (MB) in water and detect
eventual carbenes, we investigated the reactions by means of
laser Ñash photolysis and transient absorption spectroscopy.
The photolysis of deoxygenated and neutral aqueous solu-
tions of MB (2 ] 10~4 M) produces the transient absorption
spectra shown in Fig. 1. The spectrum observed at pulse end
exhibits two bands with maxima at 300 and 405 nm. Another
species grows in during the Ðrst microseconds following the
pulse end (Fig. 1, insert). It absorbs at longer wavelength
phenyl-N,N-methoxymethylureas
(monolinuron
and
metobromuron) were investigated in the past.2h6 Many
photoproducts were found, depending on the experimental
conditions. Dehalogenation with hydroxylation was observed
at low substrate concentration (2 ] 10~5 M),2 whereas forma-
tion of numerous biphenyls was reported at higher concentra-
tion (10~3 M).3,4 Demethylation or demethoxylation of the
substituted ureas were observed in all cases. Reductive dechlo-
rination was shown to occur in place of photohydrolysis in
water containing surfactants or in methanol.5,6
These analytical results resemble those obtained by pho-
tolysis of para-halogenophenols and para-halogenoanilines in
water7h12 and explained by the formation of carbenes as
primary intermediates10h12 (Scheme 1). These species, produc-
ed after heterolytic loss of HX or X~, were successfully
detected by nanosecond laser Ñash photolysis in aqueous
medium and at ambient temperature.10h12
(j \ 440 nm) and is long-lived. In air-saturated medium,
max
the 300/405 nm transient is detected again at pulse end. A
di†erent species, absorbing strongly with maxima at 300 and
460 nm (Fig. 2), appears in the 4 ls following the pulse end
(Fig. 2, curve A of insert). The growing in is made faster by an
increase of the oxygen concentration: the apparent Ðrst-order
rate constant of formation is equal to 6.6 ] 105 s~1 in air-
saturated medium and to 2.9 ] 106 s~1 in oxygen-saturated
solution. This species decays slowly (k \ 1.0 ] 104 s~1).
d
Lastly, in nitrogen-saturated solution containing 2-propanol
(0.17 M), a species with an absorption maximum at 430 nm is
Scheme 1
The carbene 4-oxocyclohexa-2,5-dienylidene, which was gen-
erated for the Ðrst time at cryogenic temperatures by photoly-
sis of the corresponding quinone diazide and characterized by
matrix isolation spectroscopy,13h15 has an absorption spec-
trum showing a two-band structure with maximum at 370 and
Fig. 1 Transient spectra measured from
a nitrogen-saturated
aqueous solution of MB (2 ] 10~4 M); A(266) \ 0.60; P \ 1.6 mJ
pulse~1: (…) at pulse end, (L) 8 ls after pulse end. The insert shows
the absorbance time course at 440 nm.
DOI: 10.1039/b005646o
New J. Chem., 2000, 24, 849È851
849
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Fig. 2 Transient spectra measured from an aqueous solution of MB
(2 ] 10~4 M); A(266) \ 0.60; P \ 1.6 mJ pulse~1: (>) 6 ls after pulse
Fig. 3 Dependence on pulse energy of the absorbances of species
obtained from photolysis of MB (2 ] 10~4 M): (…) at pulse end at
end in an air-saturated solution, (K) 2 ls after pulse end in an N -
2
saturated solution containing 2-propanol (0.17 M). The insert shows
410 nm in an N -saturated solution; (>) 6 ls after pulse end at 450
2
the absorbances time courses (A) at 450 nm in an air-saturated solu-
nm in an air-saturated solution; (K) 2 ls after pulse end at 430 nm in
tion and (B) at 430 nm in an N -saturated solution containing 2-
an N -saturated solution containing 2-propanol (0.17 M).
2
2
propanol (0.17 M).
medium exhibits the same spectral characteristics (broad
unstructured band) and the same kinetics of formation
(k \ 2.3 ^ 0.2 ] 109 M~1 s~1) as the iminoquinone-O-oxide12
produced by addition of oxygen onto the carbene and should
be the N-substituted iminoquinone-O-oxide. Lastly, the 430
nm species formed in solutions containing 2-propanol should
be the N-substituted anilino radical. To prove this assignment,
we produced this radical independently by photolyzing solu-
tions containing 1,1-dimethyl-3-phenylurea and K S O . In
this way, sulfate radicals are photogenerated; they oxidize the
phenylurea in a one-electron process20 and the N-substituted
anilino radical is expected to be produced by further depro-
tonation of the radical cation. The absorption spectrum mea-
sured from this reaction mixture 30 ls after pulse end (Fig. 4)
is similar to that observed in solutions containing MB and
2-propanol. It conÐrms that the 430 nm species is the N-
substituted anilino radical.
The carbene 4-iminocyclohexa-2,5-dienylidene was shown
to absorb at shorter wavelengths (ca. 15 nm) than the anilino
radical cation, both species exhibit a two-band absorption
spectrum.12 The carbenes derived from M and MB also have
an absorption band blue-shifted by 25 nm compared to the
N-substituted anilino radical, but contrary to their
unsubstituted counterparts, they show an unstructured
absorption band.
To determine the quantum yields of carbene formation we
photolyzed M and MB at 266 nm using low intensity contin-
uous light. 2-Propanol (0.17 M) was added to e†ect the
reduction of the carbenes. The quantum yields of substrate
loss are found to be equal to 0.068 ^ 0.008 and 0.12 ^ 0.01,
respectively, whereas those of reduction photoproduct forma-
tion (1,1-dimethyl-3-phenylurea in the case of M) are equal to
detected (Fig. 2). It reaches a maximum concentration 2 ls
after the pulse end (Fig. 2, curve B of insert). The apparent
Ðrst-order rate constant of formation is equal to 3.0 ] 106
s~1.
The photolysis of M in water yields transient species
exhibiting the same absorption spectra and the same reactivity
as those obtained from MB. It proves that dehalogenation has
occurred, considering that the presence of Cl or Br on the
aromatic ring should induce a shift of several nm in the
absorption maxima of the transient species observed. To
detect the eventual formation of chloride radicals, we pho-
tolyzed M in the presence of Cl~ (10~2 M). No absorption
2 2
8
due to the Cl ~~ radical anion was observed within the wave-
2
length range 330È360 nm,19 showing that dehalogenation is
not homolytic.
All the transients are formed by monophotonic processes as
indicated by the linear dependences of the absorbances with
P, the laser pulse energy (Fig. 3). Based on actinometry with
K S O , the e ] / values were evaluated. Data are listed in
2 2
8
Table 1.
The transients observed upon photolysis of M and MB
show strong analogies with those obtained by photolysis of
para-chlorophenol and para-chloroaniline10h12 and the
300/405 nm transient is likely to be a carbene and the precur-
sor of all the other species observed. Unfortunately, the over-
lapping of the absorption spectra prevented us from verifying
that in each case the rate of disappearance of the 300/405 nm
transient corresponds to the rate of formation of the second-
ary transient. The 440 nm transient formed in deoxygenated
solutions may be an intermediate produced by addition of the
carbene onto a starting molecule or water and yielding
biphenyls in the former case and a hydroxylated product in
the latter. The 300/460 nm transient observed in oxygenated
0.051 ^ 0.008 and 0.060 ^ 0.008. Assuming
a complete
reduction of carbenes, we deduce that the quantum yields of
Table 1 Values of e ] / products and estimated e
e ] //M~1 cm~1
Transient (j /nm)
MB
M
e/M~1 cm~1
max
N-substituted 4-iminocyclohexa-2,5-dienylidenes
220 ^ 40
at 410 nm
670 ^ 100
at 450 nm
260 ^ 40
at 430 nm
210 ^ 40
at 410 nm
680 ^ 100
at 450 nm
220 ^ 40
at 430 nm
3800 ^ 800
at 410 nm
11 000 ^ 2000
at 450 nm
4300 ^ 800
at 430 nm
(300/405)
N-substituted iminoquinone-O-oxides
(300/460)
N-substituted anilino radicals
(430)
850
New J. Chem., 2000, 24, 849È851

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In conclusion, carbenes are produced upon irradiation of
monuron and metobromuron in water. The reaction mecha-
nism via the carbene pathway is given in Scheme 2.
Experimental
M and MB were purchased from Aldrich (St Quentin-
Fallavier and used as received. The water was puriÐed using a
Milli-Q device (Millipore). The laser Ñash photolysis was per-
formed using
a
frequency-quadrupled Nd : YAG laser
(Quanta Ray GCR130, pulse duration 9 ns, j \ 266 nm18).
exc
Steady state irradiations were performed using a high pressure
xenon lamp equipped with a Schoe†el monochromator. Sub-
strate consumption was measured by HPLC (UV detection,
conventional reversed phase column).
Fig. 4 Spectrum of the N-substituted anilino radical measured 30 ls
after pulse end in a solution containing 1,1-dimethyl-3-phenylurea
(5 ] 10~4 M) and K S O (0.1 M) at pH 3.
2 2
8
Notes and references
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