Iron(III) catalysed synthesis of unsymmetrical di and trisubstituted ureas - A variation of classical Ritter reaction

Article abstract of DOI:10.1039/c2ob06916d

An application of the classical Ritter reaction for the synthesis of unsymmetrical di and trisubstituted ureas catalyzed by FeCl₃ is described. The protocol is of significant interest in view of the easy availability of precursors, mild reaction conditions employed and interestingly its applicability for the alkylation of alcohols capable of forming stable carbocationic intermediates even to the sterically hindered moieties. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06916d

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COMMUNICATION
Iron(III) catalysed synthesis of unsymmetrical di and trisubstituted
ureas – a variation of classical Ritter reaction†
Hosamani Basavaprabhu and Vommina V. Sureshbabu*
Received 13th November 2011, Accepted 9th January 2012
DOI: 10.1039/c2ob06916d
the vast diversity of synthetic applications of ureas it is desirable
to augment a simple, safer and an alternative protocol for their
synthesis.
An application of the classical Ritter reaction for the syn-
thesis of unsymmetrical di and trisubstituted ureas catalyzed
by FeCl3 is described. The protocol is of significant interest
in view of the easy availability of precursors, mild reaction
conditions employed and interestingly its applicability for the
alkylation of alcohols capable of forming stable carbocationic
intermediates even to the sterically hindered moieties.
In the ongoing studies,18 we were employing glycosyl cyana-
mides for the preparation of guanidinoglycosides. During the
course of the work, we realized that cyanamides19 resemble
nitriles in reactivity. In addition, J. Anatol et al.20 described the
synthesis of acyl and sulfonyl ureas from the corresponding acyl
and sulfonylcyanamides. However, their attempt to synthesize
substituted ureas from the corresponding cyanamides and t-butyl
alcohol under reflux condition, in the presence of strong acids
such as conc. H2SO4 and HCl resulted in very low yields of
desired ureas. Thus we started to investigate a simple, mild and
an alternative protocol for the synthesis of unsymmetrical di and
trisubstituted ureas through a variation of Ritter’s reaction.
Ritter’s reaction21 is the classical reaction for the C–N bond
formation i.e., amidation of alcohols or alkenes with nitrile in
the presence of a Lewis acid.22,23 Recently, iron catalysis has
been considered as an alternative not only because of its lower
toxicity and cost compared to other metals but also because it
possesses some useful properties which have been utilized in
many transformations.24 Iron catalysed C–C, C–N and C–O
bond forming reactions have recently been developed.25 In the
Urea functional groups have received considerable diligence due
to its wide range of applications in agriculture as agrochemicals,
pharmaceutical drugs, petrochemicals, in biology as well as in
materials science.1 They also serve as important intermediates
and organocatalysts in organic synthesis.2 Recent literature cited
a few examples of ureas as potent HIV-1 protease inhibitors, p38
MAP kinase inhibitors for the treatment of inflammatory dis-
eases and peptidomimetics with increased metabolic stability.3,4
Several methods have been developed for the synthesis of
unsymmetrically substituted ureas (Fig. 1). Curtius rearrange-
ment of acyl azides into isocyanates and their further reaction
with amine is a well known route for the synthesis of urea.5
Diphenylphosphoryl azide (DPPA) is the reagent of choice as it
allows the direct conversion of carboxylic acids into ureas
through Curtius rearrangement in one pot.6 Besides this, Hoff-
mann7 and Lossen8 rearrangements have also been explored for
the synthesis of urea starting from amide and hydroxamic acid
respectively. A number of carbonylating reagents9 were devel-
oped including phosgene,10 triphosgene11 for the synthesis of
title molecules but their preparation suffers inherent limitations.
Many carbamates,12 carbonates,13 formamides14 were also devel-
oped to serve as a source for the generation of isocyanate which
were the active intermediates for the preparation of urea. In
addition, carbodiimides15 on hydrolysis exclusively yields sub-
stituted ureas. Other approaches include the oxidative carbonyla-
tion of amine with CO in presence of transition metal catalysts16
or the direct carbonylation of amine by CO2.17 Thus, owing to
Peptide Research Laboratory, Department of Studies in Chemistry
Central College Campus, Dr. B. R. Ambedkar Veedhi, Bangalore
University, #109, Bangalore 560 001, India. E-mail: hariccb@
hotmail.com, sureshbabuvommina@rediffmail.com, hariccb@gmail.com
†Electronic supplementary information (ESI) available: 1H spectra of
2h, 2j, 1H and 13C NMR spectra of 4a–d, 4f–k, 4m, 4o, 4r, 4s and
HRMS spectra of 4e, 4l, 4n, 4p and 4q. See DOI: 10.1039/c2ob06916d
Fig. 1 Schematic representation of various routes reported in the litera-
ture for the synthesis of unsymmetrical N,N’-disubstituted ureas.
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Table 1 Screening of catalysts and solvents for optimizing the reaction
condition
Entry
Catalyst
Solvent
Time (h)
Yielda (%)
Fig. 2 Retro-synthetic route for the synthesis of unsymmetrical substi-
1
2
3
4
5
6
BF3·Et2O
AlCl3
FeCl3
FeCl3
FeCl3
DCM
DCM
DCM
DCE
THF
CH3CN
6
6
6
4.5
6
33
24
68
86
43
39
tuted urea.
FeCl3
6
a Isolated yield under laboratory conditions.
Scheme 1 Synthesis of unsymmetrical substituted urea through a vari-
ation of Ritter reaction.
present context the high Lewis acidity and catalytic activity of
FeCl3 towards benzyl and allyl alcohols26 render us to investi-
gate its use for urea preparation.
The following mechanistic analysis revealed to us the suit-
ability of Ritter’s reaction for the synthesis of unsymmetrical
ureas from cyanamides and alcohols (Fig. 2).
The utility of Ritter’s reaction was not much explored other
than for amide bond formation. In view of the above mechanistic
analysis, we sought to synthesize ureas from the cyanamides and
readily available alcohols which form stable carbocations as
starting materials through a variation of the Ritter reaction,
wherein the cyanamide serves as the nitrile source (Scheme 1).
The cyanamide (2) precursors for the present protocol were pre-
pared through a simple route using CNBr in diethyl ether/tetra-
hydrofuran (THF) as a solvent under mild conditions in the
presence of 1.0 eq. of triethylamine (TEA) to scavenge HBr
released in the reaction.27 The reaction is simple, mild and
straight forward in yielding desired cyanamides in excellent
yields.
Initially, benzyl alcohol (3a) and phenylcyanamide (2a) were
chosen as substrates to screen the optimized conditions. The
results are summarized in Table 1.
Various Lewis acids and solvents were examined, among
which FeCl3 in dichloroethane (DCE) proved advantageous in
affording good yield (86%) of desired urea (4a).
The reaction yield mainly depended on the solvent used
(Table 1). DCE and dichloromethane (DCM) were found to be
the most effective solvents in terms of reaction duration and
yield. The yield was lower with coordinating solvents such as
THF and acetonitrile. Thus, we had chosen DCE as the solvent
for further work. It has been previously observed that in the pres-
ence of a catalytic amount of Lewis acid (in this case FeCl3),
benzyl alcohols were rapidly converted into dibenzyl ether (A)
by eliminating water.28
Presumably in the presence of nucleophile i.e., cyanamide, the
ether is polarized by FeCl3 and generates an incipient benzylic
carbocation. Further the reaction proceeds by the electrophilic
addition of thus formed carbocation to the cyanamide. This
results in the formation of nitrilium ion, which is then
Fig. 3 Plausible reaction pathway for the benzyl substituted urea via
dibenzyl ether formation.
hydrolysed by H2O (which is being generated during ether for-
mation) and affords the final product (Fig. 3). In one of the
experiments, dibenzyl ether was isolated and confirmed by
NMR.
Also the synthesis of 4a was undertaken employing dibenzyl
ether as precursor under optimized Ritter condition. The reaction
proceeded well in affording 4a in quantitative yield. The efficacy
of the protocol was further exemplified using benzyl alcohol and
variety of amines i.e., precursors for cyanamides (Table 2).
Next we turned our attention to the synthesis of t-butyl ureas
(Scheme 2). Unfortunately, when t-butyl alcohol (3b) and phe-
nylcyanamide (2a) were employed under the optimized reaction
conditions poor yield was observed. In order to increase the
yield, a 0.5 eq. of acetic acid was used which assist by forming
t-butylacetate, a better source of carbocation which further par-
ticipates in the reaction yielding urea (4i) with much ease com-
pared to its alcohol counterpart.29 Thus in the presence of FeCl3
and AcOH, t-butyl ureas were obtained in good to excellent
yields as summarized in Table 3.
In the next part of the work, other alcohols such as diphenyl-
methanol (3c), 1-phenylethanol (3d) and allyl alcohol (3e) furn-
ished the corresponding ureas in good to excellent yields
(Table 4).
The mechanism for the synthesis of t-butyl and allyl urea pre-
sumably involves the formation of stable carbocation (t-butyl
and allylic cation respectively), as in case of a typical Ritter’s
reaction mechanism.
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Table 2 List of benzyl substituted ureas synthesized through benzyl alcohol
Yieldb
(%),
Reaction
time (h)
Yielda
(%),
Rean.
time (h)
M.P. (°C),
Obsd
(Lit.)
M.P. (°C),
Obsd
(Lit.)
Sl.
no.
Sl.
no.
Cyanamide
(2)
Cyanamide (2) Urea (4)a
Urea (4)a
1
86 (4.5)
80 (4)
176–178
(175–176)
5
79 (5)
71 (5)
103–104
(102)
2
187–189
6
227–230
3
4
74 (6)
63 (6)
197–198
168–169
7
8
82 (5)
78 (5)
139–141
(137–139)
89–93
a Cyanamide (1.0 eq.), benzyl alcohol (2.0 eq.), FeCl3 (30 mol%), DCE (10 mL) as solvent, reflux. b Isolated yield.
thermal conditions, t-butyl urea dissociates into the correspond-
ing isocyanate and furnishes the desired urea as reported by
Shudo.30 Also, trisubstituted ureas were also accessed by the
reaction of cyanamide derived from secondary amines with the
benzyl alcohol under the optimized Ritter conditions (4e–h).
The substituent with electron donating groups enhanced the reac-
tivity both towards the cyanamide formation as well as the corre-
sponding urea preparation (4b, 4c, 4k, 4l) compared to their
withdrawing counterparts. Substituents with electron withdraw-
ing groups lowered the reactivity of cyanamide to some extent
resulting in comparatively lower yields of urea (4f, 4d). The sub-
stituted benzyl alcohols (3f, 3g) afford corresponding ureas (4t,
4u) in quantitative yield, despite the nature of the substituent
(electron donating or withdrawing). Also, it is worthy to note
that the bulkier and sterically hindered alcohols viz. t-butyl
alcohol (3b), diphenylmethanol (3c) and 1-phenylethanol (3d)
which can form resonance stabilized carbenium ions, afford
desired ureas in good yields (4i–n, 4o–q). The protocol can be
applied efficiently to the substrates possessing other functional
moieties (4c, 4d, 4f). However 4d and 4n were obtained in
lesser yield due to the steric constraint and the electron with-
drawing ability of the substituents respectively. In addition, allyl
alcohol (3e) which forms stable allylic cations stabilized by π
electrons of the olefinic double bond was employed and the reac-
tion worked well in furnishing the corresponding allyl ureas
(4r, 4s) in excellent yields, which were the interesting class of
molecules in polymer science.31 The notable advantages of this
method are the operational simplicity, direct use of cheap and
readily available alcohols as precursor elements and inexpensive,
nontoxic FeCl3 as catalyst which renders the method an impor-
tant alternative to existing methods.
Scheme 2 Synthesis of N,N′-disubstituted ureas employing t-butyl
alcohol.
1
The structures of all the products were confirmed through H
and 13C NMR analyses.
1
In a typical example, in the H NMR spectrum of 2i a signal
appears at δ 4.66 (NHCN) and 13C spectrum exhibited a signal
1
at δ 116.7 (NH–CuN). Whereas the H NMR spectrum of 4j
(as a typical example) revealed two signals at δ 5.64 and 6.02
(–NH–CO–NH–) and 13C spectrum of 4j showed peak at δ
156.4 (–NH–CO–NH–). The disappearance of a peak at δ 116.7
(–NH–CuN) as in the 13C spectrum of 2i and appearance of a
new peak at δ 156.4 corresponding to the carbonyl carbon of
1
urea was observed in the 13C spectrum of 4j. From the H NMR
analyses of both the reactant 2i and product 4j it was inferred
that, during the transformation there is an apparent downward
shift of the –NH proton signal, where NH proton signal in 2i
shifts from δ 4.66 to δ 6.02 in 4j. Not much significant shift was
observed for the benzylic protons, but appearance of a new
signal at δ 5.64 corresponding to another –NH proton of urea
was observed. Thus NMR analyses of 2i and 4j indicated an
efficient transformation of cyanamide to corresponding urea in
one step.
The t-butyl ureas (4i–n) synthesized through this route could
have considerable application as they serve as safe and nonhazar-
dous forms of isocyanate. In the presence of an amine under
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Table 3 List of t-butyl substituted ureas synthesized through t-butyl alcohol
Yieldb (%),
Reactin
time (h)
Yieldb
(%), Rean. M.P. (°C),
Sl.
M.P. (°C),
Obsd (Lit.)
Sl.
no. Cyanamide (2) Urea (4)a
no. Cyanamide (2) Urea (4)a
time (h)
Obsd (Lit.)
9
85 (6)
172–174
(171–172)
12
83 (6)
185–186
(184–186)
10
11
91(6)
111–114
(109–111)
13
14
87 (7)
77 (6)
224–226
(223–224)
81(5.5)
175–177
143–145
(142–143)
a Cyanamide (1.30 eq.), tert-butyl alcohol (2.0 eq.), FeCl3 (mol%), AcOH (0.5 eq.), DCE (10 mL) as solvent, reflux. b Isolated yield.
Table 4 List of N,N′-di and trisubstituted ureas employing other alcohols
Entry
15
Alcohol (3)
Urea (4)a
Yieldb (%), Reaction time (h)
83 (5)
M.P. (°C), Obsd (Lit.)
241–244
16
17
81 (5)
80 (6)
181–182 (180)
84–86 (83–84)
18
19
68 (7)
73 (7)
214–216
191–193
20
21
83 (7)
81 (7)
186–188
196–198
a Cyanamide (1.0 eq.), alcohol (2.0 eq.), FeCl3 (30 mol%), DCE (10 mL) as solvent, reflux. b Isolated yield.
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In summary, herein we describe an application of the classical
Ritter’s reaction for the synthesis of unsymmetrical di and trisub-
stituted ureas catalysed by a safe and eco-friendly reagent
system, FeCl3. This protocol is of significant interest in view of
the easy availability of precursors, mild reaction conditions
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alcohols capable of forming stable carbocationic intermediates.
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Acknowledgements
We thank the Department of Science and Technology (DST)
(grant No. SR/S1/OC-26/2008), New Delhi, India for financial
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