Synthesis and pesticidal properties of thio and seleno analogs of some common urea herbicides

Article abstract of DOI:10.1080/10426500802391692

Thio and seleno analogs of fenuron, isoproturon, chlorotoluron, metoxuron, monuron, and diuron were synthesized from the corresponding aryl amines. Their reaction with thiophosgene leads to isothiocyanates. Aryl amines were also converted (via isocyanides) to isoselenocyanates. The reaction of both isothio- and isoselenocyanates with dimethylamine affords the corresponding thio and seleno analogs of the above-mentioned urea herbicides. Herbicidal activity of the synthesized compounds was slightly lower than the activity of the parent urea herbicides. The thio and seleno analogs as well as the parent ureas showed good fungicidal activity at a concentration of 200 ppm against selected fungi.

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Synthesis and Pesticidal
Properties of Thio and Seleno
Analogs of Some Common Urea
Herbicides
Jerzy Zakrzewski a & Maria Krawczyk a
a Institute of Industrial Organic Chemistry, Warsaw,
Poland
Available online: 22 Jun 2009
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Pesticidal Properties of Thio and Seleno Analogs of Some Common Urea Herbicides,
Phosphorus, Sulfur, and Silicon and the Related Elements, 184:7, 1880-1903
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Phosphorus, Sulfur, and Silicon, 184:1880–1903, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802391692
Synthesis and Pesticidal Properties of Thio and Seleno
Analogs of Some Common Urea Herbicides
Jerzy Zakrzewski and Maria Krawczyk
Institute of Industrial Organic Chemistry, Warsaw, Poland
Thio and seleno analogs of fenuron, isoproturon, chlorotoluron, metoxuron,
monuron, and diuron were synthesized from the corresponding aryl amines. Their
reaction with thiophosgene leads to isothiocyanates. Aryl amines were also con-
verted (via isocyanides) to isoselenocyanates. The reaction of both isothio- and
isoselenocyanates with dimethylamine affords the corresponding thio and seleno
analogs of the above-mentioned urea herbicides. Herbicidal activity of the synthe-
sized compounds was slightly lower than the activity of the parent urea herbicides.
The thio and seleno analogs as well as the parent ureas showed good fungicidal
activity at a concentration of 200 ppm against selected fungi.
Keywords Isocyanides; isoselenocyanates; isothiocyanates; selenoureas; thioureas;
urea herbicides
INTRODUCTION
Thio- and selenoureas exhibit a wide scope of biological activity.
The importance of thioureas as biologically active compounds (pes-
ticides, bactericides) has been known for a long time, as summa-
rized in a recent review.1 Pesticidal activities of thioureas have
been described,2–6 and their fungicidal properties have been recently
reviewed.7 Both thioureas and selenoureas are effective superoxide rad-
ical scavengers.8 Selenoureas act as effective antioxidants and show low
toxicity for some human cells.9 N, N-Unsubstituted selenourea deriva-
tives are tyrosinase inhibitors.10 The relationship of thione/thiol and
selone/selenol tautomerism in N, N-disubstituted cyclic thio- and sele-
noureas to their inhibition capacity of peroxidase-catalyzed reactions
was demonstrated.[11]
Received 18 March 2008; accepted 6 August 2008.
We thank the Polish State Commitee for Scientific Research for financial support
(429/E-142/S/2007).
Address correspondence to Jerzy Zakrzewski, Institute of Industrial Organic Chem-
istry, Annopol 6, 03-236 Warsaw, Poland. E-mail: zakrzewski@ipo.waw.pl
1880

Thio- and Selenourea Herbicides
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X
CH3
Rn
H
Urea herbicide, X = O
Fenuron
HN
N
CH3
4-i-C3H7
3-Cl, 4-CH3
3-Cl, 4-CH3O
4-Cl
Isoproturon
Chlorotoluron
Metoxuron
Monuron
Rn
3,4-Cl2
Diuron
FIGURE 1 1,1-Dimethyl-3-aryl ureas—parent urea herbicides described in
this article.
Isothio- and isoselenocyanates are the most useful intermediates
for the synthesis of thio- and selenoureas, respectively. The chem-
istry of isothio- and isoselenocyanates has been recently reviewed.12
The application of isoselenocyanates as useful precursors for selenium-
containing heterocycles has been recently reviewed independently by
Heimgartner et al.[13] and Garud et al.14 In our previous articles, we pro-
posed a 4-isoselenocyanate-2,2,6,6-tetramethylpiperidin-1-oxyl as an
isoselenocyanate bearing a nitroxyl moiety.15,16
1,1-Dimethyl-3-aryl ureas such as fenuron, isoproturon, chloro-
toluron, metoxuron, monuron, and diuron (Figure 1) are known as urea
herbicides.17–19
With the exception of 3d (Rn = 3-Cl, 4-CH3O, X = S), the thio analogs
of the urea herbicides in Figure 1 are known compounds: 3a (Rn =
H, X = S),20,21 3b (Rn = 4-i-C3H7, X = S),22 3c (Rn = 3-Cl, 4-CH3,
X = S),6 3e (Rn = 4-Cl, X = S),20,21 3f (Rn = 3,4-Cl2, X = S).2,22 In
particular, 3a and 3e have been widely described and are commercially
available (Aldrich, Rare Chemicals). To the best of our knowledge, the
corresponding seleno analogs have not been described to date, except
6a (Rn = H, X = Se).23–25 As far as we know, a direct comparison of the
pesticidal properties of ArNHC(X)N(CH3)2 (X = O, S, Se) has not been
reported to date.
Herein we present the synthesis and pesticidal activity of thio and
seleno analogs of the above-mentioned herbicides (Figure 1, X = S,
Se).

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J. Zakrzewski and M. Krawczyk
RESULTS AND DISCUSSION
The synthesis of the thio and seleno derivatives of fenuron, isopro-
turon, chlorotoluron, metoxuron, monuron, and diuron is presented in
Scheme 1. The yields of the synthesized derivatives 2–6, as well as
their physicochemical and spectral data, are presented in Tables I–V.
SCHEME 1 (i) CSCl2, NEt3, benzene; (ii) HN(CH3)2, benzene; (iii)
CHCl3/NaOH, PTC; (iv) Se, CHCl3.
Appropriate aniline derivatives 1a–f were converted to the corre-
sponding isothiocyanates 2a–f using thiophosgene26 in the presence of
triethylamine as a catalyst, as described previously.27 Compounds 2a–f
were isolated by means of vacuum distillation and column chromatog-
raphy, as well as crystallization, and were obtained in 19–73% yield
(Table I).
Amines 1a–f were converted to the corresponding isocyanides 4a–f
in the isonitrile reaction.28–34 Compounds 4a–f were isolated by means
of vacuum distillation as well as column chromatography and were
obtained in 32–66% yields (Table III).
Direct selenation of isocyanides 4a–f with gray selenium35 affords
the corresponding isoselenocyanates 5a–f in analogy to a previously
described procedure.15,16 Compounds 5a–f were isolated by means of
column chromatography and were obtained in 21–91% yield (Table IV).
This is in contrast to a report in the literature,36 where the authors
claim that isoselenocyanates cannot be purified chromatographically.
Some isoselenocyanates were isolated by means of vacuum distillation,
but in these cases, the yields were significantly lower.
Isothio- and isoselenocyanates (2a–f and 5a–f, respectively) were
converted to the thio- and selenoureas 3a–f and 6a–f, respectively,
using dmethylamine in benzene solution as described previously.16,37

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Thio- and Selenourea Herbicides
1895
Compounds 3a–f and 6a–f were purified by means of crystallization
(benzene) and were obtained in 74–92% (3a–f, Table II) and 31–82%
(6a–f, Table V) yields, respectively.
The structure of all synthesized compounds was confirmed by com-
parison of the physicochemical data with those reported in the liter-
1
ature (if available) and by the set of spectroscopic data (MS, H and
13C NMR, IR) (Tables I–V): 2a,38–40 2b,41 2c,42,43 2d,41,43 2e,39,40,43–45
2f,46,47 3a,1,20,21,48 3c,6 3e,20,21,49 3f,2 4a,50–52 4e,52 4f,53 5a,54 5e,54,55
6a.23–25
It should be noted that in the 13C NMR spectra, no signals were
reasonably attributed to an isoselenocyanate group in 5a–f, despite
using a relaxation delay up to 5 sec. The only chemical shifts that
might be attributed to an isothiocyanate group are those for 2d (135.8
ppm) and 2e [136.9 ppm (136.70,40 136.8745)]; however the intensity of
the broadened signals was very weak.
The observed problems with the detection of both isothio- and isose-
lenocyanate signals are consistent with the literature data. For a series
of aryl isoselenocyanates, no signal for the isoselenocyanate group was
detected.54 For 4-methylphenyl isoselenocyanate, no resonance was ob-
served for the isoselenocyanate carbon atom under a variety of ex-
perimental conditions, included extended relaxation delays of 5 sec.56
Measurements of the 13C NMR signals of an isoselenocyanate group
in the series of glycosyl isoselenocyanates indicate that the 13C chem-
ical shift of the isoselenocyanate carbon atom is of low intensity and
appears in the range of 144.8–146.8 ppm.57 The 13C NMR spectra of a
series of 16 4-substituted phenyl isothiocyanates were recorded using a
rather high concentration of 10 mol% and appropriate decoupling con-
ditions to increase the intensity of the isothiocyanate signals. Despite
using the above conditions, the signals of the isothiocyanate carbon
atoms (132.13–139.99 ppm) exhibited very characteristic quadrupolar
broadening and low intensity.45 Analogously, for phenyl isothiocyanate
(2a) enriched with 13C isotope at the isothiocyanate carbon atom, only
a very broad resonance at 135.95 ppm was detected and was attributed
to the isothiocyanate carbon atom.58 However, there are data in the lit-
erature where 13C NMR signals of an isothiocyanate group are detected
without any comments concerning the potential broadening and inten-
sity. Such a situation is described for aliphatic isothiocyanates, 130.4–
134.3 ppm,59 and for aryl isothiocyanates with both electron-donating
and electron-withdrawing substituents, 133.8 (4-methoxyphenyl isoth-
iocyanate) and 140.3 (4-nitrophenyl isothiocyanate), respectively.40

1896
J. Zakrzewski and M. Krawczyk
The 13C NMR signal observed for the –HN–(C=X)–N (X = S, Se)
moiety (δ =∼ 180 ppm) is in accordance with those reported in the
literature.56,60
All synthesized compounds have been investigated for herbicidal
and fungicidal activities involving in vitro and in vivo tests. All urea
derivatives exibit herbicidal activity when applied to leaves or soil, but
the activity of the thio (3a–f) and seleno (6a–f) analogs is slightly lower
than the activity of the parent urea herbicides. The parent compounds
as well as their thio and seleno analogs show good fungicidal activity
against most fungi at a concentration of 200 ppm. In some cases, the
seleno derivatives (6a, 6b, 6d) are more active than the thioureas and
the parent compouds (fenuron, isoproturon, metoxuron). The herbicidal
and fungicidal activity of the synthesized derivatives 3a–f and 6a–f is
presented in Tables VI and VII.
EXPERIMENTAL
The general protocols are analogous to those described earlier:
isothiocyanates 2,27 thio- and selenoureas 3 and 6,16,37 respectively,
isocyanides 4,32–34 and isoselenocyanates 5.15,16 All reagents were com-
mercially available and were used as received without further purifica-
tion. All experiments were performed in a multinecked round-bottomed
flask equipped with a magnetic bar, a thermometer, a reflux condenser
protected against humidity, and a dropping funnel. The formation of
the products was monitored by means of TLC. TLC control and col-
umn chromatography were carried out on silica gel Merck Alurolle
5562, Alufolien 5554, and Merck 1.09385.1000 (0.040–0.063 mm, 230–
400 mesh), respectively. The abbreviations for the mobile phases used
throughout the text are as follows: HA9 = hexane:ethyl acetate 9:1 and
BA9 = benzene:ethyl acetate 9:1. MS (EI, 70 eV, m/e, int. [%]) data
were recorded using an AMD 604 mass spectrometer; IR (ν [cm−1])
data were recorded using a FT/IR Jasco 420 spectrophotometer. 1H
NMR (200 MHz, δ [ppm], J [Hz], TMS) and 13C NMR (50 MHz, δ [ppm],
TMS) data were recorded using a Varian UNITYplus 200 spectrometer.
Isothiocyanates 2: General Procedure
A solution of thiophosgene (4.83 g, 0.042 mol, 3.2 mL) in anhydrous
benzene (17 mL) was added dropwise at 10◦C into an efficiently stirred
solution of the amine 1 (0.04 mol) and anhydrous triethylamine (12
mL) in anhydrous benzene (85 mL). The reaction mixture was stirred
for 10 min at r.t. The precipitate of triethylamine hydrochloride was

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Thio- and Selenourea Herbicides
1899
filtered off and washed thoroughly with benzene (2 × 50 mL). Ben-
zene was evaporated under reduced pressure. The residue was purified
by means of vacuum distillation (2a–c), column chromatography [2c
(hexane), 2f (HA9)], or crystallization [2d (benzene or hexane)] to give
isothiocyanates 2a–f (Table I). If the starting amine was detected (TLC)
in the filtrate (1e → 2e), it was washed with 1N hydrochloric acid (100
mL) and then with a saturated sodium bicarbonate solution (50 mL).
The aqueous layer was extracted with benzene (2 × 50 mL). The com-
bined benzene layers were dried over magnesium sulphate. The drying
agent was filtered off, and the solvent was evaporated under reduced
pressure. The residue was purified by means of crystallization (hexane)
to give 2e (Table I).
Isocyanides 4: General Procedure
A 50% sodium hydroxide solution (15 mL) was added to the vigorously
stirred solution of the amine 1 (0.05 mol), chloroform (0.05 mol, 6.0
g, 4 mL), TEBA chloride (0.125 g), and methylene chloride (15 mL).
The reaction mixture was efficiently stirred and heated at 40◦C (gently
refluxing) for 1–3 h (4h, 16 h). The progress of the reaction was thor-
oughly monitored by means of TLC. The reaction mixture was allowed
to cool to r.t., and cold water (50–100 mL) was added. The aqueous
phase was extracted with methylene chloride. The organic layer was
washed with concentrated sodium bicarbonate solution and dried over
magnesium sulphate. The drying agent was filtered off, and the solvent
was evaporated under reduced pressure. The residue was subjected
to vacuum distillation (4a–c), column chromatography [4a,c (hexane),
4d,e (HA9)] or crystallization [4e,f, (hexane)] to give isocyanides 4a–f
(Table III).
Isoselenocyanates 5: General Procedure
A suspension of the isocyanide 4 (12 mmol) in anhydrous chloroform
(16 mL) and at least a twofold molar excess of powdered gray selenium
were stirred at reflux (60◦C) for about 3 h (5a–d), 40 h (5f), or 60 h
(5e). The time of the reaction was determined by thorough monitoring
of the isoselenocyanate (5) formation and the decay of the amount of
isocyanide by means of TLC (silica gel, HA9, BA9, visualization with
UV 254 and irradiation with UV lamp for 5–10 min). The suspension
was allowed to cool to r.t. and filtered through a Cellite bed. Chloroform
was evaporated under reduced pressure. The residue was subjected to
column chromatography: HA9 (5a–d), hexane (5a,e,f). In the case of

1900
J. Zakrzewski and M. Krawczyk
5b, the compound was also isolated and purified by means of vacuum
distillation (Table IV).
Thio- and Selenoureas (3, 6): General Procedure
A solution of an excess of liquid dimethylamine (about 8 mL) in an
approximately equal volume of anhydrous benzene was slowly added
dropwise at about 0–5◦C to the solution of the appropriate isothio-
or isoselenocyanate (2 and 5, respectively) (6–7 mmol) in anhydrous
benzene (15 mL). The stirred reaction mixture was left overnight at
r.t. in the open flask (except 3e, 6c, 6e, 6f: left at r.t. for 1 h). The
precipitate formed or the crystalline residue obtained after evaporation
of the volatiles (6b: oil solidifying in a refrigerator) was thoroughly
pressed on a filter, washed with benzene (2–3 mL), and crystallized
from benzene (Tables II and V).
Herbicidal Bioassay, Pre- and Post-Emergence Experiments
Herbicidal activity of the thio and seleno compounds was evaluated
using different weed species in pot experiments under controlled con-
ditions. Polyethylene pots, 3.5 L capacity, were filled with 0.75 kg of
soil (physicochemical characteristic: sandy clay, pH/KCl/ 6.7; organic
matter 2.8%) and were wetted with water. Seeds of the 10 weed species
were planted into earth (0.5 cm depth): common chickweed (Stellaria
media), curly dock (Rumex crispus), ribwort (Plantago lanceolata), fat-
hen (Chenopodium album), common poppy (Papaver rhoeas), black
mustard (Brassica nigra), cleavers (Galium aparine), gallant soldier
(Galinsoga parviflora), barnyard grass (Echinochloa crus-gali), and
corn chamomile (Anthemis arvensis). Plants were grown to the two-leaf
stage under normal glasshouse propagation conditions (temperature,
20 5◦C; lighting, 14 h photoperiod of daylight supplemented by lamps,
400 W). The pots were watered overhead. The test compounds were dis-
solved in an appropriate volume of acetone:water solution (1:3) with the
addition of Tween 20 (0.05% v/v) to give the dose of 1 and 0.5 kg/ha. All
treatments were applied as a pre-emergence or post-emergence spray
at a volume rate of 300 L/ha using track laboratory sprayer (nozzle
TeeJet60, pressure 0.2 MPa). There were three replicate pots per treat-
ment arranged in a randomized block design. Pots after spraying were
transferred to the growth chamber (temperature, day/night 20/15◦C;
lighting, 16 h photoperiod, white fluorescent tubes giving 200 µmol
m
−2s−2 PAR). A visual assessment of phytotoxicity separately for each
species was made (18 days or 25 days after treatment for pre- and

Thio- and Selenourea Herbicides
1901
post-emergence experiments) as a percentage compared to the un-
treated plants. The results expressed as mean percent of phytotoxicity
for 10 weed species were calculated and presented in Table VI.
In Vitro Fungicidal Bioassay
The methods for assesing fungicidal activity involving in vitro and in
vivo tests were identical to those previously published.37
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