A new laccase-catalyzed domino process and its application to the efficient synthesis of 2-aryl-1H-benzimidazoles

Article abstract of DOI:10.1016/j.tetlet.2010.11.145

The laccase-catalyzed domino reaction of o-phenylenediamine and benzaldehydes with aerial oxygen as the oxidant exclusively yields 2-aryl-1H-benzimidazoles in good to very good yields. It is easy to perform under very mild reaction conditions.

Full text of DOI:10.1016/j.tetlet.2010.11.145

Tetrahedron Letters 52 (2011) 604–607
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new laccase-catalyzed domino process and its application
to the efficient synthesis of 2-aryl-1H-benzimidazoles
⇑
Heiko Leutbecher, Mihaela-Anca Constantin, Sabine Mika, Jürgen Conrad, Uwe Beifuss
Bioorganische Chemie, Institut für Chemie, Universität Hohenheim, Garbenstraße 30, D-70599 Stuttgart, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The laccase-catalyzed domino reaction of o-phenylenediamine and benzaldehydes with aerial oxygen as
the oxidant exclusively yields 2-aryl-1H-benzimidazoles in good to very good yields. It is easy to perform
under very mild reaction conditions.
Received 28 October 2010
Accepted 26 November 2010
Available online 4 December 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Laccase
Aerobic oxidation
Domino reactions
N-Heterocycles
Laccases are multicopper oxidases which are able to catalyze
the selective oxidation of a number of substrates using oxygen as
an oxidant under mild reaction conditions. The oxidation of the
substrate is accompanied by the reduction of oxygen to give water
as the only side product.1 Over the last few years the interest in
laccase-catalyzed transformations has increased.2 It has been dem-
onstrated that laccases can be used for the oxidation of aromatic
methyl groups,3a benzylic, allylic, and aliphatic alcohols,3b ethers,3c
benzyl amines and hydroxylamines.3d Laccases have also been
employed for the oxidation of catechols and hydroquinones to
the corresponding benzoquinones and related transformations.4
Also, there is ample evidence that laccases catalyze the oxidative
coupling of several phenolics using oxygen as the oxidant.5 In a
number of contributions we have reported that laccase initiated
domino reactions between catechols and several 1,3-dicarbonyls
intramolecular 1,4-addition to yield a tetrahydrophenazine 4. The
last step is the oxidation of this intermediate to afford the 2,3-dia-
minophenazine (2). On an analytical scale this reaction has been
employed for determining the activity of laccases.7c Alternatively,
this transformation can also be performed on a preparative scale
7a
using either FeCl3 or hydrogen peroxide in the presence of cata-
lytical amounts of peroxidases.7b
The oxidative dimerization of 1a has caught our attention for
two reasons: (a) it allows the preparation of 2 with very high
yields, and (b) during the course of the reaction a laccase-catalyzed
aerobic oxidation of a C–N-single bond to a C@N-double bond
takes place. Since transformations of this type play a central
role in heterocyclic chemistry we have studied whether the
air, laccase (A. bisporus)
can be employed for the synthesis of a number of heterocyclic
6b,c
buffer , pH = 6.0
systems, including 1H-pyrano[4,3-b]benzofuran-1-ones,
3,4-dihy-
NH2
NH2
N
NH2
NH2
r.t., 6 h
drodibenzofuran-1(2H)-ones,6b and benzofuro[3,2-c]pyridin-1(2H)-
2
ones.6a
90%
N
As part of our studies of the laccase-catalyzed domino processes
we have observed that the oxidative transformation of o-phenyl-
enediamine (1a) with air as the oxidant in the presence of catalytic
amounts of laccase from Agaricus bisporus exclusively delivers 2,3-
diaminophenazine (2) in 90% yield (Scheme 1).
It is assumed that the oxidative dimerization of 1a starts with
the laccase-catalyzed oxidation of one molecule of the substrate
to the corresponding diimine 3, which reacts with a second
molecule o-phenylenediamine (1a) by means of an inter- and an
1a
2
[O]
[O]
H
N
H
H
NH
NH2
NH2
1,4
1a
+
N
H
NH
3
4
⇑
Corresponding author. Tel.: +49 711 459 22171; fax: +49 711 459 22951.
E-mail address: ubeifuss@uni-hohenheim.de (U. Beifuss).
Scheme 1. Preparation of 2,3-diaminophenazine (2) by laccase-catalyzed aerobic
dimerization of 1a.
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doi:10.1016/j.tetlet.2010.11.145

H. Leutbecher et al. / Tetrahedron Letters 52 (2011) 604–607
605
R2
R2
N
N
NH2
NH2
N
Cl
+
OHC
N
H
N
H
N
H
R1
R1
5a
5b
1
6
5
OMe
OMe
OMe
N
N
-H2O
[O]
OMe
N
H
N
H
H
N
R2
R2
5c
5d
N
NH2
N
H
O2C
O2C
OMe
OMe
R1
R1
H
N
H
N
8
7
N
H
N
H
Scheme 2. Reaction of o-phenylenediamines 1 with aromatic aldehydes 6 to yield
benzimidazoles 5.
5e
5f
O3S
H
N
laccase-catalyzed imine formation can be applied to the synthesis
of other heterocycles as well.
N
H
No doubt, substituted benzimidazoles and structurally related
compounds occupy a pivotal position in medicinal chemistry.8
These compounds have been studied in great detail because many
of them exhibit remarkable biological activities, resulting in their
use as anti-ulcerative, antitumor, antihypertensive, antifungal,
and antihistaminic agents in therapy. This is why the efficient syn-
thesis of 2-aryl-1H-benzimidazoles 5 and their derivatives is a
highly important and rewarding target for synthetic organic chem-
ists.9
5g
Figure 1. Products 5a–g of the laccase-catalyzed domino reaction between 1a and
6a–g.
air, laccase
(A. bisporus)
buffer, pH = 6.0
NH2
R
R
N
r.t., 3-18 h
R2
N
OHC
+
50-99%
N
H
NH2
N
R1
H
1a
6a-g
5a-g
5
Scheme 3. Laccase-catalyzed aerobic synthesis of 2-aryl-1H-benzimidazoles 5.
One of the standard methods for the preparation of 2-aryl-1H-
benzimidazoles 5 starts with the condensation of an o-phenylene-
diamine 1 with an aromatic aldehyde 6 to yield a Schiff base 7.
Cyclization of 7 yields a benzimidazoline derivative 8, which is then
oxidized to give the corresponding benzimidazole 5 (Scheme 2).
Numerous oxidants have been used for this transformation,
including nitrobenzene,10a 1,4-benzoquinone,10b H2O2/ceric ammo-
nium nitrate,10c (bromodimethyl)sulfonium bromide,10d oxone,10e
iodine,10f and cupric acetate.10g Meanwhile a number of methods
have been developed by making use of the highly attractive oxidant
oxygen.11 However, most of these methods require organic sol-
vents, high reaction temperatures, or the presence of additional
reagents, considerably limiting their attractiveness. It has also been
established that the synthesis of 2-aryl-1H-benzimidazoles 5 by
reaction of an o-phenylenediamine 1 and an aromatic aldehyde 6
can also be accomplished in aqueous solvent systems.10f,g,11b,e
With a number of these methods the formation of the required
2-aryl-1H-benzimidazole 5 is accompanied by the occurrence of
reaction of o-phenylenediamine (1a) with aromatic aldehydes 6
using air as the oxidant. The model reaction chosen was the trans-
formation of o-phenylenediamine (1a) and benzaldehyde (6a). We
found that the laccase-catalyzed reaction of 1a and 6a affords the
2-phenyl-1H-benzimidazole (5a) (Fig. 1) in 74% yield when
performed under aerobic conditions at pH 6.0 (0.2 M phosphate
buffer) and at room temperature (Scheme 3, Table 1, entry 1). A
commercially available laccase from Agaricus bisporus was used
as the catalyst.13 The selectivity of this reaction is quite remarkable
for its exclusive formation of the 1H-benzimidazole ring system.
The dimerization of o-phenylenediamine (1a) to the 2,3-diamino-
phenazine (2) does not take place under the reaction conditions
chosen.
These encouraging findings prompted us to conduct a new
benzimidazole synthesis with a number of substituted benzalde-
hydes (6b–g). In all cases the laccase-catalyzed domino process
was achieved using equimolar amounts of o-phenylenediamine
(1a) and benzaldehydes 6a–g at room temperature.14 The 2-aryl-
1H-benzimidazoles 5b–g (Fig. 1) were isolated with yields ranging
from 50% to 99% (Scheme 3, Table 1, entries 2–7).15
1-benzylated 2-aryl-1H-benzimidazoles
9
as side products,11g
and sometimes this type of compounds has been isolated as the
main product.12
R2
N
Due to their limited solubility in water the reactions with the
aldehydes 6b–d required the addition of some methanol to the
phosphate buffer. All reactions could be performed in a most sim-
ple manner by shaking the reaction mixture using a liquid phase
synthesis system ‘Synthesis 1’. Using this experimental setup suffi-
cient aeration was ensured. The reactions could also be performed
using a magnetic stirrer at high stirring speed. The workup of the
N
R1
9
R2
Therefore, we felt that it was worth to study whether 2-aryl-1H-
benzimidazoles 5 can be obtained by a laccase-catalyzed domino

606
H. Leutbecher et al. / Tetrahedron Letters 52 (2011) 604–607
Table 1
methanol mixture. When the model reaction was run in the ab-
Synthesis of 2-aryl-1H-benzimidazoles
o-phenylenediamine (1a) and benzaldehydes 6a–g under aerobic conditions using
laccase from A. bisporus as a catalyst
5 by reaction of equimolar amounts of
sence of the laccase from Agaricus bisporus but under otherwise
identical reaction conditions an inseparable mixture of a number
of compounds was formed. The composition of this mixture was
not studied in more detail.
Entry
6
Buffer/MeOH (v:v)
Time (h)
5
Yield (%)
1
2
3
4
5
6
7
a
b
c
d
e
f
1:0
5:2
5:2
8:2
1:0
1:0
1:0
18
5
5
4
3
a
b
c
d
e
f
74
73
50
62
99
82
80
When the phosphate buffer was replaced by a 9:1 mixture of
acetate buffer (0.2 M, pH 4.4) and methanol and the reaction was
again performed in the absence of laccase only 18% of a 9:1 mixture
of 2-phenyl-1H-benzimidazole (5a) and the N-benzylated product
9a (R = H) was isolated (Scheme 4). In the presence of 2 equiv of
benzaldehyde (6a) the formation of 34% of a 20:3 mixture of 5a
and 9a was observed (Scheme 4, Table 2, entry 1). This clearly indi-
cates that the laccase-catalyzed reaction between 1a and 6a pro-
ceeds with much better selectivity and higher yield of 5a (Table 1,
entry 1) than the reaction in the absence of the enzyme.
Despite these findings, the reactions of 1a with the substituted
benzaldehydes 6b–g were also run in the absence of laccase
(Scheme 4, Table 2, entries 2–7).
5
3
g
g
air
buffer, pH = 4.4
r.t., 20 h
NH2
R
OHC
+
NH2
In all cases 1 equiv of 1a was reacted with 2 equiv of the respec-
tive benzaldehyde under air in either acetate buffer or a mixture of
acetate buffer/methanol and in the absence of laccase. It was found
that the reaction with the aldehydes 6c und 6d delivered mixtures
of 5 and 9 (Scheme 4, Table 2, entries 3 and 4). The crude product of
the reaction with 6g could not be analyzed in detail, but 6g was
clearly present among the other compounds. The benzimidazole
5g, though, could not be detected (NMR). The transformations with
6b, 6e, and 6f were the only ones where the benzimidazoles 5b, 5e,
and 5f were obtained as the sole products (Table 2, entries 2, 5 and
6)—but in yields considerably lower than those obtained in the lac-
case-catalyzed reactions (Table 1, entries 2, 5 and 6) and less pure
crude products.
These results unambiguously demonstrate that the laccase-
catalyzed oxidative transformation of 1a and 6 is superior to the
non-enzymatic version with respect to yield and purity of the
2-aryl-1H-benzimidazoles.
To summarize, a new application for the laccase-catalyzed aer-
obic oxidation of a C–N-single bond to a C@N-double bond has
been discovered, namely the efficient synthesis of 2-aryl-1H-benz-
imidazoles 5 by the reaction of o-phenylenediamine (1a) with
benzaldehydes 6 under mild conditions.
1a
6
R
R
R
N
N
N
+
N
H
9
5
Scheme 4. Reaction of 1a and 6 under aerobic conditions in acetate buffer (pH 4.4)
in the absence of laccase.
Table 2
Reaction of 1 equiv o-phenylenediamine (1a) with 2 equiv of benzaldehydes 6a–g
under air in acetate buffer (pH 4.4) and in the absence of laccase
Entry
6
Buffer/MeOH (v:v)
Product(s)a
Total yield (%)
1
2
3
4
5
6
7
a
b
c
d
e
f
9:1
5:2
5:2
8:2
1:0
1:0
1:0
5a/9a (20:3)
5b
5c/9c (2:1)
5d/9d (10:3)
5e
5f
—
34
31
77b
99
82
15
c
g
—
a
All reactions were vigorously stirred using a magnetic stirrer for 20 h in acetate
buffer (0.2 M, pH 4.4) at rt under air.
References and notes
b
In addition, the product also contains a side product of unknown structure.
c
The crude product is a mixture of 6g and products of unknown structure.
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reaction mixtures proved to be extremely simple as well. The prod-
ucts were precipitated by simple salting out using NaCl. After fil-
tration followed by washing with 15% aqueous sodium chloride
solution and then water, heterocycles 5a–g were obtained. In none
of the reactions the formation of the 2,3-diaminophenazine (2) or
any other side product was observed.
Concerning the mechanism it is assumed that the reaction
starts with the formation of the Schiff base 7 followed by an intra-
molecular ring closure to produce the N,N-acetal 8. The laccase-
catalyzed oxidation of 8 finally yields the benzimidazole 5 (Scheme
2).
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In order to elucidate the role of the laccase in the domino reac-
tion a number of additional experiments were conducted. When
the laccase-catalyzed transformation of o-phenylenediamine (1a)
and benzaldehyde (6a) was run in a 9:1 mixture of phosphate buf-
fer (0.2 M, pH 6.0) and methanol 5a was isolated in 64% yield. This
means that the yield obtained in the phosphate buffer/methanol
mixture is only slightly lower (10%) than the one achieved with
the pure phosphate buffer (Table 1, entry 1). Under these condi-
tions, however, the reaction can be more easily performed since
the benzaldehyde completely dissolves in the phosphate buffer/

H. Leutbecher et al. / Tetrahedron Letters 52 (2011) 604–607
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13. For all experiments commercially available laccase (EC 1.10.3.2) from A.
bisporus (Fluka) was used. Laccase activity was determined according to (a)
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14. General procedure for synthesis of 5a–g: 1.0 equiv 6 (0.2 mmol) [6b (0.13 mmol),
6c (0.18 mmol), 6d (0.11 mmol), respectively] and 1.0 equiv o-phenylenedi-
amine (1a) were dissolved in phosphate buffer (14 mL, 0.2 M, pH 6.0) or a
phosphate buffer/methanol mixture (see Table 1). 31 U laccase from Agaricus
bisporus were added. The reaction mixture was shaken with
a shaking
frequency of 850 rpm using a liquid phase synthesis system ‘Synthesis 1’
(Heidolph) for 3–18 h (see Table 1) at room temperature under air until the
substrates had been fully consumed. Alternatively, the reaction could be
performed using a magnetic stirrer at high stirring speed. The reaction mixture
was saturated with sodium chloride and filtered through a Büchner funnel. For
isolation of products 5a–d 2 M KOH (1 mL) was added before filtration. The
filter cake was washed with 15% sodium chloride solution (20 mL) and water
(2 mL) and dried at room temperature to yield pure heterocycles 5a–g (NMR).
Analytically pure compounds could be obtained via recrystallization.
~
m 2652 (NH), 1626, 1563 and 1454 (C@C), 1196
15. Selected data for 5g: IR (ATR)
(SO3), 753 cmꢀ1 (@C–H); UV–vis (MeOH) kmax (log
e) 202 (4.47), 238 (3.96),
278 nm (4.07); 1H NMR (300 MHz, CD3OD) d 7.52–7.57 (m, 2H, 5-H and 6-H),
7.63–7.73 (m, 1H, 50-H), 7.71–7.75 (m, 1H, 40-H), 7.74–7.78 (m, 2H, 4-H and 7-
3
H), 7.77–7.82 (m, 1H, 60-H), 8.13 (d, J3 -H, 4 -H = 7.6 Hz, 1H, 30-H); 13C NMR
(75 MHz, CD3OD) d 115.47 (C-4 and C-7), 122.38 (C-10), 127.94 (C-5 and C-6),
129.75 (C-30), 132.14 (C-50), 132.75 (C-60), 133.04 (C-3a and C-7a), 134.28 (C-40),
146.88 (C-20), 150.48 (C-2); MS (EI, 70 eV) m/z 274 [M+] (15%), 256 [M+ꢀH2O]
(3), 224 (2), 210 (8), 194 [M+ꢀSO3] (100), 166 (4), 91 (4), 44 (17); HRMS (EI,
70 eV) m/z calculated for C13H10N2O3S [M+] 274.0412; found 274.0416.
0
0
12. (a) Jacob, R. G.; Dutra, L. G.; Radatz, C. S.; Mendes, S. R.; Perin, G.; Lenardão, E. J.
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Baghbanzadeh, M. Tetrahedron Lett. 2006, 47, 2557.