Lewis acid catalyst free synthesis of benzimidazoles and formamidines in 1,1,1,3,3,3-hexafluoro-2-propanol

Article abstract of DOI:10.1016/j.jfluchem.2010.10.002

A simple, inexpensive, environmentally friendly and efficient route for the synthesis of benzimidazole and formamidine derivatives by the reaction of O-phenylenediamines or amines with orthoesters using hexafluoroisopropanol as a solvent/catalyst is described.

Full text of DOI:10.1016/j.jfluchem.2010.10.002

Journal of Fluorine Chemistry 131 (2010) 1377–1381
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Lewis acid catalyst free synthesis of benzimidazoles and formamidines
in 1,1,1,3,3,3-hexafluoro-2-propanol
a,
b
c
a
Samad Khaksar *, Akbar Heydari , Mahmood Tajbakhsh , Seyed Mohammad Vahdat
a
Chemistry Department, Islamic Azad University, Ayatollah Amoli Branch, Amol, Iran
b
Chemistry Department, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
c
Chemistry Department, Mazandaran University, Babolsar, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple, inexpensive, environmentally friendly and efficient route for the synthesis of benzimidazole
and formamidine derivatives by the reaction of O-phenylenediamines or amines with orthoesters using
hexafluoroisopropanol as a solvent/catalyst is described.
Received 1 August 2010
Received in revised form 13 October 2010
Accepted 15 October 2010
Available online 11 November 2010
ß 2010 Elsevier B.V. All rights reserved.
Keywords:
Benzimidazole
Formamidine
Hexafluoroisopropanol
Hydrogen bonding
1
. Introduction
Over the last years, fluorinated alcohols have, due to their
antiviral, antihypertension, anticancer properties [5,6] and express
significant activity against several viruses such as HIV [7], Herpes
(HSV-1) [8], human cytomegalovirus (HCMV) [9] and influenza
[10]. They have also been used as ligands for asymmetric
transformations [11]. According to the literature, benzimidazoles
were typically synthesized by the reaction of an appropriate
substituted 1,2-phenylenediamine with carboxylic acid or its
derivatives [12], nitriles [13], and orthoesters [14] in the presence
of a strong acid at elevated temperature. Recently, several methods
have been introduced, where aldehydes [15], acid chlorides [16],
O-dinitrobenzene [17], Gold’s reagent [18] and 2-nitroanilines [19]
were used as starting materials for this synthesis. However, some
of these methods suffer from one or more of the following
drawbacks such as strong acidic conditions, long reaction times,
low yields of the products, tedious work-up, need to excess
amounts of reagent and the use of toxic reagents, catalysts and/or
solvents. Therefore, there is a strong demand for a highly efficient
and environmentally benign method to prepare these hetero-
cycles. The present investigation describes a facile preparation of
benzimidazole derivatives from O-phenylenediamines and orthoe-
sters in HFIP (Scheme 1).
unique chemical and physical properties, attracted an increasing
interest in the context of green reaction media, as co-solvent or
additive [1]. Highly fluorinated alcohols exhibit high hydrogen
bonding donor ability, low nucleophilicity, high ionizing power
and the ability to solvate water. Reactions in fluorinated alcohols
are generally selective and without effluents, allowing thus a facile
isolation of the product and can be easily recovered by distillation.
Among several of fluorinated alcohols, the most commonly used
and cheapest are trifluoroethanol (TFE) and hexafluoroisopropanol
(HFIP), which are available on a commercial scale. Due to their use
in solvolysis reaction, where the generated cationic intermediates,
can be trapped by nucleophiles [2] and their interaction with
acetals to form the electrophilic species [3], it could be anticipated
that these solvents could be useful as a medium for activation of
orthoesters. In continuation of our interest using of fluorinated
solvents as efficient medium in various organic transformations
[
4], we reasoned that it could assist certain condensation reactions.
In this report we describe our results for the synthesis of
Benzimidazoles. These heterocycles are very important com-
pounds which show a broad range of biological activities such as
2. Results and discussion
In preliminary experiments, O-phenylenediamine (1 mmol) in
1
mL TFE was allowed to stir at room temperature with
*
Corresponding author. Fax: +98 121 2552136.
E-mail addresses: S.khaksar@iauamol.ac.ir, samadkhaksar@yahoo.com
trimethylorthoformate. After 10 h, only 30% of expected benz-
(S. Khaksar).
imidazole 1a was obtained. Our efforts were then focused on HFIP.
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.10.002

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S. Khaksar et al. / Journal of Fluorine Chemistry 131 (2010) 1377–1381
R1
R1
NH2
NH2
The syntheses of amidines have in recent years received
N
HFIP
R3
significant attention. This is because of their wide range of biological
[20] and pharmaceutical activities [21] such as histamine-receptor
antagonists [22], serine-protease inhibitors [23] and nitric oxide
synthaseinhibitors[24]. Furthermore, thesecompoundscanbeused
as pesticides and acaricides [25], analgesic [24], antibacterial and
antipyretic [26]. Moreover, substituted amidines are valuable
synthon in the synthesis of many azacyclic compounds [27]. The
amidine group was also found to be an effective protecting group for
primary amines [28], electrophiles [29], nitrogen-based nucleophile
[30], auxiliaries [31] and a linker in solid-phase synthesis [32].
Classical method for the synthesis of amidine derivatives is the
condensation of amines with orthoesters using organic and mineral
acids under rather forcing conditions [33]. Numerous variations of
procedures have been reported in the literature. These include the
nucleophilic addition of amines to nitriles (the most convenient and
atom-economic method) [34], condensation reactions of amines
with activated amide intermediates, e.g. imino esters [35], imidoyl
chlorides [36] or O-triflated imidates [37], thioamides [38],
carboxylic acids [39], carboxylic esters [40], orthoesters [41] as a
solvent and a reagent in the presence of condensation agents and
Beckman-type rearrangement of ketoximes [42]. In addition,
formamidines are usually formed by the condensation of primary
+
3
4
R C(OR )
3
r.t, 2 h
R2
R2
N
H
1
a-o
Scheme 1.
As a strong H-bond donor (
power (YOTs = 3.79), and polarity (P
a
= 1.96, pK
a
= 9.3), with high ionizing
s
= 11.08), it could activate the
orthoesters towards the nucleophilic attack of amine groups. The
reaction was then investigated in HFIP: where a solution of O-
phenylenediamine (1 mmol), trimethylorthoformate (1 mmol) in
HFIP (Table 1, entry 1) was stirred at room temperature. The
reaction was remarkably fast (2 h) and, after distilling off the HFIP,
the benzimidazole 1a was isolated in 95% yield. Further experi-
ments revealed that a similar procedure is applicable for the
preparation of a wide range of compounds analogous to adduct 1
(
Table 1). The study was extended to aromatic (entries 1–11),
aliphatic (entries 12 and 13), and heterocyclic diamines (entries 14
and 15) furnishing the products by a smooth cyclocondensation
within 2 h. The yields in general are high, regardless of the
structural variations of the diamines.
Table 1
Synthesis of benzoimidazole derivatives in HFIP.
Entry
Diamine
Ortho-ester
Product
Yield (%)
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
1
CH(OMe)
3
1a
1b
1c
1d
1e
1f
95
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
2
3
4
5
6
7
CH(OEt)
3
92
92
92
90
90
90
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
3
CH CH(OMe)
3
Me
N
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
Me
Me
Me
NH2
Me
Me
CH(OMe)
3
NH2
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
CH(OEt)
3
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
Me
N
3
CH CH(OMe)
3
Me
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
Me
Me
NH2
N
CH(OMe)
3
1g
NH2
N
H

S. Khaksar et al. / Journal of Fluorine Chemistry 131 (2010) 1377–1381
1379
Table 1 (Continued )
Entry
Diamine
Ortho-ester
Product
Yield (%)
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
Me
Me
NH2
N
8
CH
3
CH(OMe)
3
1h
90
Me
NH2
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
Me
Me
NH2
Me
Me
N
9
CH
3
CH(OMe)
3
Me
1i
92
85
88
90
90
85
88
NH2
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
Cl
Cl
NH2
NH2
Cl
N
1
1
1
1
1
1
0
1
2
3
4
5
CH(OMe)
3
1j
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
Cl
N
CH
3
CH(OMe)
3
3
3
Me
1k
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
N
CH(OMe)
3
1l
NH2
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
Me
CH
3
CH(OMe)
1m
1n
1o
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
CH(OMe)
3
N
N
N
H
[
T
D
$
I
N
L
I
N
E
]
[TD$INLE]
NH2
NH2
N
CH
3
CH(OMe)
Me
N
N
N
H
amines and N,N-dialkylformamide dimethylacetals under neutral
conditions [43]. Recently, Lin et al. demonstrated that sulfated
zirconia catalyzed the reaction between aniline and trimethylortho-
formate to produce formamidine at 40 8C [44]. In fact, formamidines
are known [45] to be particularly unstable under many of the
conditionsencounteredduringtheirpurificationsteps. Thesefactors
make the trans-amidination of N,N-dimethyl formamidines a very
attractive synthetic route for the preparation of N ,N -unsymme-
trical formamidines, but are not suitable for a combinatorial
approach or for high throughput synthesis.
A new perspective seemed to be needed for the efficient
workup. Therefore the use of fluorinated solvent seemed to open
interesting perspectives for the efficient preparation of these
compounds, hoping to greatly simplifying their isolation. Herein,
we report an efficient method for synthesizing formamidines
which relies exclusively on hydrogen bond activation of
orthoesters.
The reaction was remarkably fast (3 h) and, after distilling off the
HFIP, the amidine 3a was isolated in 95% yield. Further experiments
revealed that a similar procedure is also applicable for the
preparation of a wide range of compounds analogous to adduct 3
(Scheme 2).
This method is equally effective with p-bromoaniline and p-
chloroaniline (Scheme 2, entry 4) bearing electron withdrawing
groups on the aromatic ring. After successfully synthesizing a
series of formamidine derivatives in high, to excellent yields, we
turned our attention towards the synthesis of unsymmetrical
formamidine derivatives via unsymmetrical amidination under
similar conditions. Thus, in an experiment, we probed the
condensation of aniline (1 mmol), trimethylorthoformate
(1 mmol) and p-toluidine (1 mmol) in HFIP (Scheme 2, entry 7)
for 3 h at room temperature to afford a clean reaction. We carried
out the three component coupling reaction of different amines and
trimethylorthoformate in HFIP. In all cases, different amines
afforded the desired products in high yields under the same
reaction conditions as shown in Scheme 2. It is known from the
2
3
In a preliminary experiment, aniline (2 mmol) in 1 mL HFIP was
allowed to stir at room temperature with trimethylorthoformate.

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S. Khaksar et al. / Journal of Fluorine Chemistry 131 (2010) 1377–1381
H
H
N
O
HFIP
R
NH2
+
R'
NH2
+
O
O
N
r.t, 3 h
R
R'
1
2
H
3
a-i
Entry
1
Amine 1
Amine 2
NH2
Product (%)
H
NH2
NH2
NH2
N
N
N
3
5%
a
9
H
N
NH2
2
9
0% 3b
H
NH2
NH2
N
N
3
4
90% 3c
Br
Br
Cl
Br
H
N
Br
NH2
N
80% 3d
Cl
Cl
Cl
H
N
N
9
0%
3e
NH2
NH2
5
6
N
N
H
NH2
NH
2
9
0% 3f
H
N
NH2
N
NH2
9
0% 3g
7
H
N
NH2
N
NH2
NH2
9
0% 3h
8
9
Br
Br
H
NH2
N
N
90% 3i
Cl
Br
Scheme 2. Preparation of formamidine in HFIP.
literature that formamidines with an aliphatic residue, compared
to aryl formamidines have a reduced stability and are quickly
decomposed to the corresponding amines when exposed to e.g.,
silica gel [43]. We have also used our method for the synthesis of
aliphatic formamidines. Our approach is well suited to prepare
aliphatic formamidines in high purity (Scheme 2, entries 3, 4). The
notable advantages of this method are the operational simplicity,
direct use of amines and inexpensive, reusable and non-toxic HFIP
medium which render this method an important alternative to
previously reported methods.
high chemoselectivity, (v) no side reaction, and (vi) low costs and
simplicity in process and handling. The recovered HFIP can be
reused. Further studies and efforts to extend the scope of this
method for other useful reactions are currently underway.
4. Experimental
4.1. General procedure
To a solution containing trimethyl orthoformate (1 mmol), in
HFIP (1 mL) was added the amine (2 mmol) or O-phenylenedia-
mine (1 mmol) and the mixture was vigorously stirred at r.t. for 2–
3
. Conclusions
3
h. The products were isolated after selective evaporation of the
In summary, we have described an efficient methodology for
HFIP and were purified by a simple silica gel column chromatog-
raphy eluted by an EtOAc and hexane (4:1) mixture, to afford the
corresponding pure disubstituted formamidine or respectively the
benzimidazole in very good yields. The physical data (mp, NMR, IR)
of these known compounds were found to be identical with those
reported in the literature. The spectral data for selected products:
the synthesis of benzimidazole and formamidine derivatives using
various electronically and structurally divergent amines in good to
excellent isolated yields. In contrast to the existing methods using
potentially hazardous catalysts/additives, this new method offers
the following competitive advantages: (i) avoiding the use of any
base, metal or Lewis acid catalyst, (ii) short reaction times, (iii) ease
of product isolation/purification by non-aqueous work-up, (iv)
1H-Benzimidazole (1a): Pale yellow solid, mp 170–171 8C (lit.
1
[14d] 170–172 8C); H NMR (CDCl
3
, 500 MHz):
d
7.10–7.32 (m, 2H),

S. Khaksar et al. / Journal of Fluorine Chemistry 131 (2010) 1377–1381
1381
1
3
[7] (a) A.R. Porcari, R.V. Devivar, L.S. Kucera, J.C. Drach, L.B. Townsend, J. Med. Chem.
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7
1
.44–7.66 (m, 2H), 8.11 (s, 1H), 10.75 (br s, NH); C NMR (CDCl
25 MHz): 115.3, 121.3, 137.1, 140.5.
-Methyl-1H-benzimidazole (1c): Pale yellow solid, mp 176–
3
,
4
(
d
2
Michejda, J. Med. Chem. 40 (1997) 4199–4207.
1
[
[
8] M.T. Migawa, J.L. Girardet, J.A. Walker, G.W. Koszalka, S.D. Chamberlain, J.C. Drach,
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1
3
77 8C (lit. [14d] 175–176 8C); H NMR (CDCl
H), 7.08–7.10 (m, 2H), 7.44–7.45(m, 2H), 12.2 (br s, NH); C NMR
, 125 MHz): 14.5, 114.2, 121.0, 139.1, 151.2.
,5-Dimethyl-1H-benzimidazole (1f): Pale yellow solid, mp 201–
3
, 500 MHz): d 2.48 (s,
1
3
(CDCl
3
d
[10] J. Mann, A. Baron, Y. Opoku-Boahen, E. Johansson, G. Parkinson, L.R. Kelland, S.
Neidle, J. Med. Chem. 44 (2001) 138–144.
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2
1
2
02 8C (lit. [14a] 202–203 8C); H NMR (CDCl
3
, 500 MHz): d 2.31 (s,
(2002) 137–144.
3
H), 2.49 (s, 3H), 7.02 (d, J = 8.0 Hz, 1H), 7.31 (s, 1H), 7.42 (d,
[
12] (a) D.W. Hein, R.S. Alheim, J.J. Leavitt, J. Am. Chem. Soc. 79 (1957) 427–429;
13
J = 8.0 Hz, 1H), 10.76 (br s, NH); C NMR (CDCl
3
, 125 MHz):
9.5, 114.3, 114.6, 123.6, 131.97, 137.3, 138.7, 155.7.
-Chloro-2-methyl-1H-benzimidazole (1k): Pale yellow solid, mp
d 14.1,
(b) W.O. Pool, H.J. Harwood, A.W. Ralston, J. Am. Chem. Soc. 59 (1937) 178–179;
(
2
(
c) M. Raban, H. Chang, L. Craine, E. Hortelano, J. Org. Chem. 50 (1985) 2205–
210;
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1
5
1
1
2
1
1
36–138 8C (lit. [14a,b] 137–140 8C); H NMR (CDCl
3
, 500 MHz): d
[
[
14] (a) Z.H. Zhang, L. Yin, Y.M. Wang, Catal. Commun. 8 (2007) 1126–1131;
b) M.M. Heravi, N. Montazeri, M. Rahmizadeh, M. Bakavoli, M.J. Ghassemzadeh,
.41 (s, 3H), 7.18 (d, J = 8.8 Hz, 1H), 7.43 (d, J = 8.8 Hz, 1H), 7.51 (s,
(
13
3
H), 11.20 (br s, NH); C NMR (CDCl , 125 MHz): d 15.1, 114.69,
J. Chem. Res. (S) (2000) 584–585;
15.48, 122.94, 128.04, 137.27, 139.38, 156.90.
(c) D. Villemin, M. Hammadi, B. Martin, Synth. Commun. 26 (1996) 2895–2898;
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S.F. Hojatia, J. Iran. Chem. Soc. 5 (2008) 65–70.
0
N,N -Diphenyl-formamidine (3a): White solid, mp 137–139 8C
1
(lit. [33a,b] 139–140 8C); H NMR (CDCl
3
, 500 MHz):
d 6.90–7.35
15] (a) A. Ben Alloum, K. Bougrin, M. Soufiaoui, Tetrahedron Lett. 44 (2003) 5935–
5937;
13
(m, 10H), 8.21(s, 1H), 9.22 (br s, NH); C NMR (CDCl , 125 MHz):
3
d
(
(
(
b) P. Gogi, D. Konwar, Tetrahedron Lett. 47 (2006) 79–82;
c) T. Itoh, K. Nagata, H. Ishikawa, A. Ohsawa, Heterocycles 62 (2004) 197–201;
d) R. Trivedi, S.K. De, R.A. Gibbs, J. Mol. Catal. A 245 (2006) 8–11.
1
19.1, 129.3, 132.6, 143.0, 149.5.
N,N -Di-p-tolyl-formamidine (3b): H NMR (CDCl
0
1
3
, 500 MHz):
1
white solid, mp 140–142 8C (lit. [33a] 141–143 8C); H NMR
CDCl , 500 MHz): 2.35(s, 6H), 6.95 (d, J = 8.1 Hz, 4H), 7.12 (d,
J = 8.1 Hz, 4H), 8.19 (s, 1H), 9.32 (br s, NH); C NMR (CDCl
[16] (a) R.N. Nadaf, S.A. Siddiqui, T. Daniel, R.J. Lahoti, K.V. Srinivasan, J. Mol. Catal. A
214 (2004) 155–160;
(
3
d
13
(b) M.M. Heravi, M. Tajbakhsh, A.N. Ahmadi, B. Mohajerani, Monatsh. Chem. 137
3
,
(2006) 175–179.
1
25 MHz):
d
20.7, 119.0, 129.8, 132.6, 143.0, 149.5.
[
17] H. Wang, R.E. Partch, Y. Li, J. Org. Chem. 62 (1997) 5222–5225.
0
N,N -Bis-(4-bromo-phenyl)-formamidine (3c): White solid, mp
69–170 8C (lit. [33a,b] 170–172 8C); H NMR (CDCl
.89 (d, J = 5.95 Hz, 4H), 7.12(d, J = 5.95 Hz, 4H), 8.09 (s, 1H), 9.52
[18] J.T. Gupton, K.F. Gorreia, B.S. Foster, Synth. Commun. 16 (1986) 365–368.
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1
1
6
3
, 500 MHz): d
[
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62–373.
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3
13
(br s, NH); C NMR (CDCl
3
, 125 MHz): d 116.4, 120.7, 132.3, 143.9,
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1
48.8.
0
N,N -Bis-(1-phenyl-ethyl)-formamidine (3f): Viscous colourless
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23] J.W. Liebeschuetz, S.D. Jones, P.J. Morgan, C.W. Marray, A.D. Rimmer, J.M.E.
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L. Brady, K. Wilkinson, J. Med. Chem. 45 (2002) 1221–1232.
1
oil [43]; H NMR (CDCl
3
, 500 MHz):
d 1.55 (d, J = 6.85 Hz, 6H), 4.31–
4
1
1
.36 (m, 2H), 4.47 (q, J = 6.85 Hz, 2H), 7.25–7.41 (m, 11H), 9.89 (s,
13
[24] J.L. Collins, B.G. Shearer, J.A. Oplinger, S. Lee, E.P. Garvey, M. Salter, C. Dufry, T.C.
Burnette, E.S. Furtine, J. Med. Chem. 41 (1998) 2858–2871.
3
H); C NMR (CDCl , 125 MHz): d 22.7, 57.2, 126.0, 126.1, 128.1,
29.1, 141.8.
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N-Phenyl-N -p-tolyl-formamidine (3g): White solid, mp 104–
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1
3
06 8C (lit. [33b] 105–106 8C); H NMR (CDCl
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42.7, 145.3, 149.8.
3
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for language correction and further assistance in the preparation of
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