Synthesis of unsymmetrical phenylurea derivatives via oxidative cross coupling of aryl formamides with amines under metal-free conditions

Article abstract of DOI:10.1039/c4nj01668h

A new synthetic approach for phenylurea derivatives involving the cross coupling of N-aryl formamides with amines through the formation of isocyanate intermediates in the presence of hypervalent iodine reagents is described.

Full text of DOI:10.1039/c4nj01668h

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DOI: 10.1039/C4NJ01668H
Graphical abstract
Synthesis of unsymmetrical phenylurea derivatives via oxidative
cross coupling of aryl formamides with amines under metal free
conditions
Nagireddy Veera Reddy, Pailla Santhosh Kumar, Peddi Sudhir Reddy, Mannepalli Lakshmi
Kantam and Kallu Rajender Reddy *
A direct transformation of N-aryl formamides to the corresponding phenylurea derivatives via the formation of isocyanate
intermediates is achieved in good yields using hypervalent iodine as an external oxidant.

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DOI: 10.1039/C4NJ01668H
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LETTER
Synthesis
of
unsymmetrical
phenylurea
derivatives via oxidative cross coupling of aryl
formamides with amines under metal free
conditions†
Cite this: DOI: 10.1039/c3nj00000x
Received 00th XXXXX 2013,
Accepted 00th XXXXX 2013
Nagireddy Veera Reddy, Pailla Santhosh Kumar, Peddi Sudhir Reddy,
Mannepalli Lakshmi Kantam and Kallu Rajender Reddy*
DOI: 10.1039/c3nj00000x
www.rsc.org/njc
A new synthetic approach for phenylurea derivatives
involving the cross coupling of N-aryl formamides with
amines through the formation of isocyanate intermediates
in the presence of hypervalent iodine reagent was
described.
The unsymmetrical urea derivatives have significant importance in
the field of medicine, biology and material science.1–3 Several of
these derivatives are found in biologically active compounds4 and
often serves as an important intermediates5 and bi-functional
organocatalysts6 in organic synthesis. The prevalence of urea
functional group in medicinal chemistry is attributed to metabolic
stability of the N–C(O)–N linkage and to the large number of
variations possible. Among them, phenylureas have been attracted a
special attention because of their herbicidal properties as well as in
Fig. 1 Examples of herbicides with unsymmetrical phenylurea derivatives.
However, with the growing concern of environments and
stringent legislations by the governments, replacements of toxic
phosgene routes are in great demand. Although alternative routes
such as Curtius/Hoffman rearrangements14 as well as oxidative
carbonylation of amines with CO and direct carbonylation of amines
have been reported,15 additional synthetic steps involved in
preparing the starting precursors, expensive metal catalysts, use of
CO gas and high pressure conditions are some of the issues to be
handled.16 Moreover these methods have limitations particularly in
the synthesis of unsymmetrical urea derivatives. Therefore,
development of new synthetic protocols to prepare unsymmetrical
urea derivatives through non-toxic routes under mild conditions has
become an important area of research.
In recent years, functional group transformations involving
oxidative cross couplings have emerged as an important synthetic
strategy in organic chemistry.17 Notable among them are the cross
couplings involving formamide functionalities.18 In continuation of
our ongoing research work on functional group transformations
involving oxidations,19 we have been exploring the application of
formamide functionality in cross-coupling chemistry. During these
investigations formamides have been successfully transformed into
carbamates (I,II,III),20a-c amides (IV)20d and ureas (V).20e However,
use of formamide in large excess (as self solvent) made these
reactions applicable for only a handful of substrates, in particular for
N,N’-dialkyl formamides.
other agricultural applications (Fig. 1).
Revelation of 3-(4-
chlorophenyl)-1,1 -dimethylurea (monuron) possessing herbicidal
activity led to studies of a large number of unsymmetrical
phenylurea derivatives.7 Others such as fenuron, diuron and siduron
were widely used as herbicide chemicals. Furthermore, the
isoproturon has shown to play an effective role in herbicide
chemistry that inhibits photosynthesis of plants.8 The other
derivative, N-butyl-N-methyl-N’-(3,4-dichlorophenyl)urea (neburon)
has been commercially exploited as an efficient herbicide on the
fields of wheat and strawberries.9,10 Most of the commercially
accessible procedure for the synthesis of ureas and their derivatives
are derived by reacting suitable amines with phosgene,11
isocyanates,12 or dimethyl carbonate.13
aInorganic and Physical Chemsitry Division, CSIR-Indian Institute
of Chemical Technology, Tarnaka, Hyderabad–500 007, India.
Fax: (+ 91) 040-27160921. E-mail: rajender@iict.res.in;
rajenderkallu@yahoo.cm
†Electronic Supplementary Information (ESI) available: Experimental
procedure, characterization data, 1H, 13C, IR and mass spectra. See
DOI: 00.0000/b000000x
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 1-3 | 1

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DOI: 10.1039/C4NJ01668H
For the initial optimization studies N-phenyl formamide and
dibutylamine were chosen as reacting partners and reactions were
performed by varying solvents and oxidants under metal and metal
free conditions and results are summarized in Table 1. Reactions
with [bis(acetoxy)iodo]benzene [PhI(OAc)2] as an oxidant in
dichloromethane (DCM) at room temperature does not provide any
product formation (Table 1, entry 1). A similar observation was also
o
made with THF solvent at 60 C (Table 1, entry 3). Whereas, a
marginal improvement in product formation was observed by raising
the temperature of DCM solvent (Table 1, entry 2). However, a
sharp rise with maximum yield (85%) was noticed when
[bis(trifluoroacetoxy)iodo]benzene [PhI(OCOCF3)2] was used as an
oxidant (Table 1, entry 4). No further improvement in product
formation was observed with copper catalysts in presence of TBHP
or H2O2 as the terminal oxidants (Table 1, entries 5-7).
Scheme 1 Organic transformations involving the formamide functional
Under the optimized reactions conditions, the general
applicability of this method was further evaluated with structurally
diverse aryl formamides and results are shown in the Table 2. In the
presence of [PhI(OCOCF3)2] oxidant, different formamides reacted
with dibutylamine to afford the corresponding urea products in
excellent yields (Table 2, 3a-3g ). A slight drop in yields were
observed when aryl ring system is substituted with electron
withdrawing groups (Table 2, 3d, 3e ) compared to the electron
donating groups (Table 2, 3b, 3c ).
group.
Interestingly our recent work revealed that aryl formamides could
be utilized in stoichiometric quantities and transformed them into
carbamates using hypervalent iodine reagents in the presence of
alcohols(VI).20f We have postulated here the formation of
carbamates via an isocyanate intermediate, which is a direct
transformation rather than the rearrangements involved Curtius and
Hofmann procedure (Scheme 2).
Looking at the synthetic
importance of phenylureas in agrochemical applications, we have
extended the above concept towards the synthesis of unsymmetrical
phenylureas and the results are presented in this work.
Table 2. Direct synthesis of N-phenylurea (3) derived from N-aryl
formamides (1) and amines (2).a,b
O
O
H
R2
H
N
H
N
N
PhI(OCOCF3)2
R3
H
R2
N
+
R3
rt, 2 h, DCE,
4Ao MS, N2
R1
1(1 mmol)
R1
2 (2 mmol)
3
O
O
O
N
O
H
N
H
H
H
N
N
N
N
N
N
Scheme 2 Synthesis of phenyl urea derivatives via isocyanate intermediates.
3c (88%)
3a
3d (73%)
(85%)
O
3b (83%)
Cl
O
N
O
N
O
N
H
H
Table 1 Optimization studies of phenylurea synthesis for the reaction
between N-phenylformamide and dibutylamine.a
H
H
N
N
N
N
N
Ph
3h
3e
(82%)
(77%)
3f
3g
(78%)
(80%)
Br
O
O
O
O
H
H
H
H
N
N
N
N
N
N
N
N
Ph
Ph
3j
3i
(76%)
O
3k
(72%)
(82%)
O
3l (
70%)
Cl
O
O
H
H
H
H
N
N
Ph
N
N
N
N
Ph
N
N
Ph
Ph
Ph
3m
(68%)
3p
(83%)
3n (65%)
3o (
70%)
Cl
N
O
O
O
O
H
H
H
H
N
N
N
N
N
N
N
O
O
3r
(76%)
Br
3q (80%)
Cl
3t
(80%)
3s (84%)
aReaction conditions: N-arylformamide (1 mmol), amine (2 mmol),
dichloroethane (3 mL), [bis(trifluoroacetoxy)iodo]benzene (1 mmol), MS 4
Å, N2, 2 h. b Isolated yields.
aReaction conditions: N - phenylformamide (1 mmol), dibutylamine (2
mmol), solvent (3 mL,), oxidant (1 mmol), MS 4Å, N2, 2 h.
2 | J. Name., 2012, 00, 1-3
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Substitution at ortho-position of aryl formamide with ethyl- and
In summary, we have reported a mild and efficient procedure for
benzyl- group does not influence much on the product yields (Table the synthesis of unsymmetrical phenylurea derivatives involving the
2, 3f, 3g ). Moreover, we could not observe the oxidation of benzylic cross coupling of N-aryl formamides with amines through the
carbon in these two cases. Because of the synthetic importance of formation of isocyanate intermediates in the presence of hypervalent
phenylureas bearing N,N’-dimethylamine groups, next we looked at iodine reagent. Although there are practical difficulties with respect
the reactivity of aryl formamides with di-methylamines under the cost and side product formations with hypervalent iodine reagents,
optimized conditions. These reactions provided the products 3h-3j in the direct transformation of N-arylformamides via isocyanate
good yields and among them 3h and 3i are important fungicides and formation may have a synthetic importance over Hofmann and
are known as fenuron and monuron respectively. Reactions with Curtius type rearrangements. Moreover, the easy synthesis of aryl
secondary amines such as N-ethylbutan-1-amine, N-ethylaniline and formamide and their transformation into isocyanate intermediate can
di-benzylamine with aryl formamides resulted in the corresponding be readily utilized for various organic transformations, which are
ureas in good to excellent yields (Table 2, 3k- 3o ). We could also currently under investigation in our laboratory.
extend the present method successfully with cyclic amines viz.,
pyrrolidine, 2-methylpiperidine and morpholine providing the
phenylureas in more than 80% yields (Table 2, 3p- 3t). Our attempts
with primary amines such as aniline and butylamine were not
Experimental
In a reaction vessel 1 mmol of N-aryl formamide was dissolved
mL of dichloroethane solvent, and slowly added the
successful. We could observe formation of small amount of azo-
compound, which could be resulted from aniline or through the
decomposition of formamide. Formation of azo- compounds from
anilines were recently reported by Majee, Hajra and co-workers
with hypervalent iodine reagents.21 Upon the addition of small
amount of water to the standard reaction conditions and performing
the reaction in the absence of external nucleophile at room
temperature, we observed the formation of symmetrical urea
derivatives (Table 3). This could be because of the decomposition of
isocyanate intermediate, which led to the formation of aniline and its
subsequent coupling with N-aryl formamide resulted in symmetrical
urea derivatives in moderate yields (Table 3).
in
3
[bis(trifluoroacetoxy)iodo]benzene (oxidant, 1 mmol) to the solution
with stirring over a period of 2 minutes in the presence of molecular
sieves (4 Å) under a nitrogen atmosphere. Then 2 mmol of amine
was added to the above reaction mixture and the reaction mixture
was stirred for two hours at room temperature. The reaction was
monitored by thin layer chromatography. After completion of the
reaction, the reaction mixture was filtered through a silica gel bed
using ethyl acetate and the filtrate was concentrated under reduced
pressure. The residue was purified by column chromatography on
silica gel using a petroleum ether/ethyl acetate mixture used as an
eluent to afford the corresponding products. The products were
confirmed by 1H, 13C NMR, IR and mass spectroscopic analysis.
Table 3 Formation of symmetrical urea derived from N-aryl formamides.a,b
Notes and references
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Scheme 3 Plausible mechanism for isocyanate formation.
This journal is © The Royal Society of Chemistry 2012
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