A modern and practical laccase-catalysed route suitable for the synthesis of 2-arylbenzimidazoles and 2-arylbenzothiazoles

Article abstract of DOI:10.1039/c8ra07377e

Heterocyclic aromatic compounds containing an imine (C═N) bond such as benzimidazoles and benzothiazoles are important active pharmaceutical ingredients. The synthesis of 2-aryl-1H-benzimidazoles and 2-arylbenzothiazoles in good to excellent yields was ac

Full text of DOI:10.1039/c8ra07377e

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A modern and practical laccase-catalysed route
suitable for the synthesis of 2-arylbenzimidazoles
and 2-arylbenzothiazoles
Cite this: RSC Adv., 2018, 8, 39496
*
Mudzuli Maphupha,
Wanyama P. Juma, Charles B. de Koning and Dean Brady
Heterocyclic aromatic compounds containing an imine (C]N) bond such as benzimidazoles and
benzothiazoles are important active pharmaceutical ingredients. The synthesis of 2-aryl-1H-
benzimidazoles and 2-arylbenzothiazoles in good to excellent yields was achieved by reacting 2-
aminoaromatics with various benzaldehyde derivatives catalysed by the commercial laccases Novoprime
and Suberase® at room temperature and in the presence of atmospheric oxygen.
Received 4th September 2018
Accepted 19th November 2018
DOI: 10.1039/c8ra07377e
rsc.li/rsc-advances
well as neuropeptide Y Y1 receptor antagonists.7,8 Furthermore,
through blocking the secretion of gastric acid in the stomach,
Introduction
substituted benzimidazoles such as omeprazole can act as
gastric H+/K+ ATPase inhibitors.9–11 Some of the well-known
available drugs bearing a benzimidazole scaffold include
albendazole, mebendazole, omeprazole, and bendamustine
(Fig. 1).12 Furthermore, compounds with benzimidazole moie-
ties have been utilized in material science applications; they are
oen used as membranes for fuel cells and in organic light-
emitting diodes.13
A related class of compounds, the benzothiazoles, are also an
important class of aromatic heterocyclic compounds composed
of a benzene ring fused to a thiazole ring where a thiazole ring is
a ve membered ring made up of sulphur and nitrogen
atoms.14,15 The benzothiazole moiety substituted with various
functional groups has attracted attention due to their phar-
macological and therapeutic activities (Fig. 2).14 These
compounds have displayed antitumor,16 anticancer,14,15,17 anti-
HIV,18 antiviral,19 antimicrobial,20 antibacterial,21 anthel-
mintic,22 anti-diabetic,23 anti-allergic24 and anti-inammatory
activities.25 Other reported applications include use in
industry as oxidants, industrial dyes and functional
materials.16,26–33
Applying enzymes to address modern challenges experienced in
complex synthetic organic chemistry has proven valuable.
Enzymes provide alternative and sustainable processes and
have helped to minimize the release of hazardous substances
into the environment.1 Laccases (benzenediol : oxygen oxido-
reductases, EC1.10.3.2) are well recognized oxidoreductase
enzymes belonging to the family of blue multi-copper-
containing oxidases.1 They are capable of catalysing oxidation
reactions of several low molecular weight organic compounds
such as polyphenols, aminophenols, methoxyphenols, amino-
phenols, and lignin-related molecules.1–4 The catalytic process
of these enzymes occurs through a one-electron oxidation and
water is released as the by-product.1 Because of their broad
catalytic activity, laccases have been utilised in a variety of
industrial applications ranging from textile to the pulp and
paper industries and from food industries to bioremediation
processes.5
Benzimidazoles are
a class of heterocyclic aromatic
compounds composed of an imidazole ring fused to a benzene
ring. Because of their remarkable biological activities, benz-
imidazoles have been studied for more than a century and are
a common moiety in many active pharmaceuticals (Fig. 1). One
of the prominent natural occurring derivatives of benzimid-
azoles is N-ribosyl-dimethyl benzimidazole; a component of
inhibitors such as factor Xa (Fxa) inhibitors, poly (ADP-ribose)
polymerase (PARP) inhibitors, 5-HT3 antagonists, and 5-lip-
oxygenase inhibitors, and also serves as one of the cobalt
ligands in vitamin B12.6 Substituted benzimidazoles are asso-
ciated with a wide range of biological and pharmacological
activities such as anti-tumour, anti-ulcer, anti-fungal, anti-
hypersensitive, antiviral agents, anti-allergic properties, as
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand,
Johannesburg, South Africa. E-mail: dean.brady@wits.ac.za; Tel: +27-11-7176745
Fig. 1 Pharmaceuticals a benzimidazole nucleus.
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aldehydes 2 with using molecular oxygen as an oxidant.
However, this results not only in the formation of 2-
substituted benzimidazole (3) but also 2,3-disubstituted
benzimidazole (4) as a by-product or even the major product
(Scheme 1).7 Hence, although this protocol was effective, poor
chemoselectivity was a disadvantage. Recently, Leutbecher
et al. discovered that the same reaction using the laccase cat-
alysed enzyme domino reaction between o-phenylenediamine
1 with various aromatic aldehydes 2 in aerobic conditions
using a phosphate buffer and obtained 2-aryl-1H-benzimid-
azoles 3 in good to excellent yields but with some di-
substituted by-product 4.7
It has been shown that in the presence of an oxidizing agent
the reaction of 1 with 2 favours the formation of product 3 over
product 4.34 Following this, our aim was to rst identify how and
why the 1-N-benzylated product 4 was formed and if its forma-
tion could be controlled and possibly eliminated. Our initial
approach was to investigate the selectivity of these enzyme-
facilitated reactions by varying the pH, types of buffers and
co-solvents used in the synthesis of these compounds. The co-
solvents used in this study were methanol, ethanol, DMF, and
acetonitrile and the buffers were acetate and phosphate at
various pH values.
As a starting point, we investigated the reaction between o-
phenylenediamine (1a) with benzaldehydes 2a–d (Fig. 3) (1 : 2
ratio) in acetate buffer (pH 4.5) using the laccase Suberase®
from Novozymes at room temperature (Scheme 1). Initially, the
reaction times for these reactions varied from 2 to 24 h; it was
observed that for the majority of the substrates, the mono-
substituted product (3) forms rst and then followed by the
undesired N-benzylated product (4). Therefore, a reaction time
of 24 h was selected for the model studies to detect the forma-
tion of by-product compound 4.
Fig. 2 Benzothiazoles in biologically active compounds.
Due to their numerous biological activities, development of
efficient synthesis methods for benzimidazoles and benzothia-
zoles compounds has received
a signicant amount of
interest.7,12,14 The conventional methods include condensation–
dehydration reaction of ortho-aminoaromatics with carboxylic
acids and their derivatives under strong acid and high
temperature conditions or condensation with aldehydes under
oxidative conditions.26,34 A number of oxidants have been used
to catalyse these reactions, including iodine, peroxides, 1,4-
benzoquinone, nitrobenzene, cupric acetate, and (bromodi-
methyl)sulfonium bromide, and recent advances for benz-
imidazoles include procedures such as microwave assisted
synthesis and solid phase synthesis.10 The use of transition-
metal catalysed cross-couplings reactions of benzothiazoles
and aromatic halides using aromatic boronic and carboxylic
acids is also known.27
Secondly, the relative ratios of products 3 and 4 obtained by
laccase-catalysed oxidation were profoundly inuenced by the
type of organic solvent used. As has been observed previously
inferior results were obtained while using methanol and
ethanol as co-solvents, as they both showed a great deal of
competitive formation of products 3 and 4.7,12 Improved che-
moselectivity was observed with DMF and acetonitrile; wherein
we were able to minimize the formation of the 1-N-benzylated
product 4 (Table 1).
Most reported methods are disadvantageous because they
involve use of toxic oxidants, abrasive reaction conditions (high
temperatures and pressure, as well as long reaction times).
Furthermore, the transition metals are expensive while solvents
such as NMP, DCM, and DMF are environmentally
unfriendly.26,27
Greener approaches have been reported recently; this
includes the laccase catalysed reaction between o-phenyl-
enediamine and benzaldehydes as well as the two-step laccase-
mediator aerobic oxidation condensation of in situ-produced
salicylaldehyde derivatives with aromatic amines for the
synthesis of benzimidazole derivatives.7
Therefore, alternative and more environmental friendly
preparation
of
2-aryl-1H-benzimidazoles
and
2-aryl-
benzothiazoles involves enzymes.1,7 Herein, we report laccase
catalysed one-pot syntheses of 2-aryl-1H-benzimidazoles and 2-
arylbenzothiazoles from a condensation–dehydration reaction
of 2-aminoaromatics with aryl-aldehydes.
Results and discussion
The synthesis of benzimidazole derivatives can be achieved
Scheme 1 Laccase catalysed reaction of 1 and 2 in acetate buffer (pH
through reactions of derivatives of o-phenylenediamine 1 with 4.5) using various co-solvents.
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each enzyme. Novoprime Base 268 presented the best oxidation
results compared to the two laccases preparations from Myce-
liophthora thermophile (Suberase® and Denilite® II Base) for the
formation of 2-(3,4-dimethoxyphenyl)-1H-benzimidazole (3d).
The reaction conducted using Suberase® as our laccase affor-
ded a yield of 59% of the 2-substituted benzimidazole 3d and
36% was the 3,4-disubstituted benzimidazole 4d. When using
Denilite® II Base as our laccase, the ratio was 57% compound
3d and 30% compound 4d. Finally, when the enzyme Novo-
prime Base 268 was used, compound 3d was formed in a 78%
yield while compound 4d was not detected under these reaction
conditions.
Finally, in order to identify the optimal conditions for these
reactions we also considered the effects of using different
buffers and pH towards the reaction selectivity. Leutbecher et al.
investigated the effect of different buffers using acetate and
phosphate buffers while varying buffer to co-solvent ratios. It
was shown that the ratios of buffer to co-solvent affect chemo-
selectivity and consequently the yield of the products formed.7
In this study the buffer and pH studies were conducted using
phosphate buffer (0.1 M, pH 6.0–7.5) and acetate buffer (0.1 M,
pH 3.0–5.0) with DMF or acetonitrile as co-solvents at room
temperature. Reactions at lower pH values (3.0–5.0) occurred at
faster rates than reactions carried out at higher pH values
(>7.15), but no signicant effect on selectivity was observed.
Using the discovered optimal conditions (General method),
derivatives of o-phenylenediamine were treated with a series of
aromatic aldehydes using commercial laccase, Novoprime Base
268, from Novozymes at room temperature and the obtained
results are summarised in Table 2 below.
As shown in Table 2, 2-aryl-1H-benzimidazoles were ob-
tained in good to excellent yields (60–94%) from a variety of
aromatic aldehydes. It is clear the method applies for a variety
of aryl aldehydes containing both electron-donating and
electron-withdrawing substituents around the ring. No by-
product (4) was formed in any of the reactions. For
compounds 3e, 5c and 5d by-product was previously observed
by Leutbecher et al. in ratios of 20 : 3, 2 : 1 and 10 : 1 respec-
tively in the absence of laccase.7
Fig. 3 o-Phenylenediamine (1a) and aromatic aldehydes (2a–d) used
in this study.
Thirdly, we explored the possible effect of the type of laccase
used on selectivity of these reactions. When Fan et al. developed
a highly chemoselective scandium(II) triate based method for
synthesis of either 2-benzimidazoles or 2,3-benzimidazoles it
was demonstrated that the presence of a strong oxidizing agent
such as hydrogen peroxide inuences the production of the 2-
substituted benzimidazoles over the 2,3-disubstituted benz-
imidazoles. This implies that the redox potential of the laccase
used could inuence the chemoselectivity of the reactions.35,36
Therefore, using laccases (Suberase®, Denilite® II Base, and
Novoprime Base 268) from Novozymes, we investigated the role
of enzyme type and preparation on these reactions. The reaction
between o-phenylenediamine (1a) with benzaldehyde 2d was
chosen as a model reaction and it was performed using an
acetate buffer (pH 4.5) and acetonitrile at room temperature
(Scheme 1).
The ndings shown in Fig. 4, clearly indicate that the specic
laccase inuences the chemoselectivity of benzimidazole
derivatives to a great extent, based on the redox potential of
Table 1 Suberase®-catalysed formation of benzimidazoles through
reaction of o-phenylenediamine (1a) and benzaldehydes 2a–d in
acetate buffer (0.1 M, pH 4.0) using various co-solventsa
As shown in Scheme 2 below; aer the condensation of the
diamine and the aromatic aldehyde, the subsequent interme-
diate benzimidazoline i may condense further with another
Entry
Aldehyde
Co-solvent
Time
3 Yield %
4 Yield %
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
2d
2d
2d
2d
Methanol
Ethanol
DMF
Acetonitrile
Methanol
Ethanol
DMF
Acetonitrile
Methanol
Ethanol
DMF
Acetonitrile
Methanol
Ethanol
DMF
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
59
—
72
62
—
—
—
—
10
25
—
—
34
—
67
59
20
13
—
—
—
97
98
82
90
70
74
56
66
—
23
36
9
10
11
12
13
14
15
16
Acetonitrile
Fig.
4 Comparison of commercial laccase preparations in the
a
chemo-selective synthesis of 2-benzimidazole (blue) and the by-
product 2,3-benzimidazole (red).
Reaction conditions: Myceliophthora thermophilia laccase (2.0 mL
Suberase®) 50 : 50 solvent/acetate buffer at 25 ꢀC.
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benzothiazoles have been investigated previously.37,38 Sayyahi
et al. (2015) identied ethanol as the best solvent for preparing
2-arylbenzothiazoles under reux conditions while Gao et al.
(2014) described water as the optimal solvent using KI cata-
lyst.38,39 Therefore, in order to identify optimum reaction
conditions we explored the effect of using various co-solvents. 2-
Aminothiophenol (5) and benzaldehyde (6a) were reacted in the
presence of a laccase (Suberase®, from Novozymes) in acetate
buffer (pH 4.0) using various solvents at room temperature
(Table 3).
Acetonitrile gave the best yield under the described reaction
conditions. While using methanol and ethanol as co-solvents,
the reaction did not go to completion aer 24 h of reaction
and the product (7a) (yellow oil) was difficult to isolate from the
starting material (2-aminothiophenol (5)) as they both have
almost indistinguishable Rf values (40% EtOAc/hexane).
Conversely, Sayyahi et al. (2015) when using the ionic liquid
[bmim][FeCl4] as a catalyst obtained excellent yields using
ethanol but not acetonitrile.38
In order to purify compound 7a from the starting material,
the product was dried overnight under high-vacuum to
completely dry-out the oily product to an almost solid form, and
cold acetonitrile was added to precipitate the product. An
additional wash of cold acetonitrile (3 ꢁ 20.0 mL) was used to
remove residual starting material from the product.
To elucidate the role of the enzyme as a catalyst, control
reactions were conducted using benzaldehydes 6a–l under air in
acetate buffer (0.1 M, pH 4.0)/acetonitrile (50 : 50) in the
absence of laccase (Scheme 4). However only two substrates
resulted in the formation of a fused product in very low yield
(<10%) and no activity was observed for the remaining
substrates (Table 4). Conversely, quantitative yields of the same
substrates were obtained in the presence of an enzyme.
Numbers in brackets (7b and 7e) – for these two compounds
non-enzymatic limited background product formation that
occurs in the absence of laccase was observed.
Table 2 Novoprime-catalysed formation of benzimidazoles through
reaction of o-phenylenediamines (1 equiv.) and benzaldehyde deriv-
atives (1 equiv.) in acetonitrile and acetate buffer (pH 4.0) at room
temperaturea
Entry Amine
Benzaldehyde
Time Product Yield %
1
2
3
4
5
6
7
8
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R ¼ H
R1 ¼ 2-OMe
R1 ¼ 3-OH
24 h 3a
24 h 4b
24 h 3c
24 h 3d
69
82
76
78
92
94
88
56
60
95
64
94
89
68
94
93
79
90
R1 ¼ 2,5-diOMe
R1 ¼ 3,4-diOMe
R1 ¼ H
2 h
2 h
4 h
8 h
2 h
4 h
3e
3f
3g
3h
3i
R1 ¼ 2-Cl
R1 ¼ 4-Cl
R1 ¼ 2-NO2
R1 ¼ 2-(Pyridin-4-yl)
R1 ¼ 3-NO2
9
10
11
12
13
14
15
16
17
18
3j
R1 ¼ 4-(Dimethylamino) 8 h
3k
3l
R1 ¼ 4-NO2
2 h
R1 ¼ 3,4,5-TriOMe
R1 ¼ 4-OMe
24 h 3m
24 h 3n
R ¼ 4-Br R1 ¼ 2-(Pyridin-4-yl)
R ¼ 4-Cl R1 ¼ 3,4-diOMe
R ¼ 4-Me R1 ¼ H
4 h
2 h
8 h
3 h
3o
3p
3q
3r
R ¼ H
R1 ¼ 2-Br, 5-OMe
a
Reaction conditions: laccase (0.105 g Novoprime Base 268) 50 : 50
acetonitrile/acetate buffer at room temperature.
molecule of benzaldehyde to afford another intermediate imi-
nium ion ii. Successively, this intermediate may tautomerize to
the most stable form iii to give the resultant disubstituted
benzimidazole product 4.12
In summary, we demonstrated that using acetonitrile in the
presence of a catalytic amount of laccase (Novoprime Base 268)
promotes the exclusive formation of the 2-phenyl-substituted
benzimidazole for a broad range of derivatives. Our improved
method is simple, selective and can afford 2-aryl-1H-benz-
imidazoles in good to excellent yields.
For the synthesis of 2-arylbenzothiazoles, we began our
investigation by exploring the oxidative condensation reaction
between 2-aminothiophenol (5) and benzaldehyde (6a) under
aerobic conditions at room temperature. The laccase-catalysed
cross-coupling reaction between 5 and 6a to afford 2-phenyl-
benzothiazole (7a) (Fig. 5) in 85% yield was performed in an
acetate buffer (0.1 M, pH 4.0) using acetonitrile (50%) as a co-
solvent (Scheme 3 and Table 3, entry 4).
At this point, the most efficient method presenting the
highest catalytic activity for preparing 2-arylbenzothiazoles was
using acetonitrile and acetate buffer (pH 4.0), similar to the
method described for 2-aryl-benzimidazole. Therefore to study
The effect of organic solvents such as ethanol, acetonitrile,
hexane, chloroform, DMSO and water when preparing
Scheme
2
Mechanism for the synthesis of the 1,2-disubstituted Fig.
5
Products 7a–l of the laccase-catalysed domino reaction
benzimidazoles.
between 5 and 6a–l.
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In conclusion, we have successfully developed a simple and
efficient method for the synthesis of 2-arylbenzothiazole deriv-
atives using an inexpensive commercial laccase as a catalyst at
room temperature. The optimal conditions for conducting
these reactions are using acetate buffer (0.1 M, pH 4.5) and
acetonitrile as a co-solvent.
Scheme 3 Laccase-catalysed synthesis of 2-phenyl-benzothiazole.
Table 3 Suberase®-catalysed synthesis of 2-phenyl-benzothiazole
(7a) by reaction of 2-aminothiophenol (5) and benzaldehydes 6a in
acetate buffer (0.1 M, pH 4.0) using various co-solventsa
Experimental
General method used to synthesize 2-aryl-benzothiazole
derivatives
Entry
Aldehyde
Co-solvent
Time
Product
Yield %
A mixture of 2-aminoaromatic (10.0 mmol) and benzaldehyde
derivative (10.0 mmol) in acetonitrile (10.0 mL) and acetate buffer
(10.0 mL, pH 4.0) was stirred at room for 5 minutes. Laccase was
then added into the mixture and the contents were stirred for 2–
24 h. When the reaction completed the product precipitated from
the solution and was extracted with ethyl acetate (30.0 mL) and
water (3 ꢁ 20.0 mL) and concentrated on a rotary evaporator. The
product was washed several times with cold acetonitrile (3 ꢁ 20.0
mL) to remove any excess starting material.
1
2
3
4
5
6a
6a
6a
6a
6a
Methanol
Ethanol
DMF
Acetonitrile
DCM
24 h
24 h
24 h
24 h
24 h
7a
7a
7a
7a
7a
58
55
78
85
65
a
Reaction conditions: Myceliophthora thermophilia laccase (2.0 mL
Suberase®) 50 : 50 solvent/acetate buffer at 25 ꢀC.
General
All reagents and solvents were purchased from Sigma-Aldrich
(South Africa) or Merck KGaA (South Africa). All solvents used
for chromatographic separation were distilled before use to
remove any impurities. All the chemical reagents were used as
received without any further purication. Reactions were
monitored by TLC carried out on Merck aluminium foil backed
plates coated with silica gel (60 F254) and visualization was
done under UV light. Purication of some of the compounds
was done using Macherey-Nagel silica gel 60 (particle size
0.063 mm to 0.20 mm) purchased from Merck. All the melting
point recordings of the compounds were performed on a Stuart
SMP10 instrument. Bruker 300 and 500 MHz spectrometers
were used to record both the 1H and 13C Nuclear Magnetic
Resonance data using a suitable solvent at room temperature.
Data processing of the spectra was done using MestreNova
Soware under license from Mestrelab Research, CA, USA. The
resonant frequency for all spectra obtained are reported in parts
per million relative to an internal standard, TMS, which appears
at zero parts per million. Coupling constants are reported in
Hertz. A Bruker Tensor-27 Fourier Transform spectrometer was
used to perform infrared spectroscopy.
Scheme 4 Laccase-free synthesis of benzothiazoles.
the range and limitations of the optimised procedure 2-ami-
nothiophenol was treated with a series of aryl-aldehydes in the
presence of a catalytic amount of laccase to yield 2-arylbenzo-
thiazole derivatives in good to excellent yield at room temper-
ature (General method) and the results obtained are
summarized in Table 4. Similar reactivity was observed for
benzaldehydes with a variety of substituent groups; both elec-
tron withdrawing and electron donating at various positions
showed comparable yields and reaction rates.
Table 4 Novoprime-catalysed synthesis of benzothiazoles through
reaction of 1 equiv. of 2-aminothiophenol (5) with 1 equiv. of benz-
aldehydes 6a–1 in acetate buffer (0.1 M, pH 4.0)/acetonitrile (50 : 50)
for 24 h via Scheme 3a
Entry
Benzaldehyde
Time
Product
Yield %
1
2
R1 ¼ H
24 h
24 h
7a
7b
85
56(8)
R1 ¼ 4-
LC-MS: a 10 mL of the sample was injected into the Dionex
Ultimate 3000 UHPLC system (Thermo Scientic, Dionex, Sun-
nyvale, California, USA) and run through a loop for one minute
at 50% solvent A consisting of 0.1% formic acid in H2O (v/v) and
50% solvent B consisting of 0.1% formic acid in acetonitrile (v/v)
at a owrate of 0.3 mL minꢂ1. High resolution mass spectra
were recorded on a inked Bruker Compact Q-TOF mass spec-
trometer (Bruker Daltonics, Bremen, Germany) using an ESI
positive source.
(Dimethylamino)
R1 ¼ 4-OMe
3
4
5
6
7
8
9
10
11
12
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
68
73
65(7)
56
88
87
76
48
74
R1 ¼ 3,4-diOMe
R1 ¼ 3,4,5-TriOMe
R1 ¼ 4-Cl
R1 ¼ 2-(Pyridin-4-yl)
R1 ¼ 4-Acetyl
R1 ¼ 2-Br-5-Ome
R1 ¼ 3-NO2
R1 ¼ 2-OMe
Myceliophthora thermophilia laccases (Suberase® 400 U gꢂ1
and Denilite® II Base; 800 U gꢂ1) and Novoprime Base 268
(origin and units not dened) were obtained from Novozymes.
R1 ¼ 4-NO2
84
a
Reaction conditions: laccase (0.105 g Novoprime Base 268) 50 : 50
acetonitrile/acetate buffer at room temperature.
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(C-9), 135.19 (C-4), 131.70 (C-60), 130.23 (C-40), 122.51 (C-50),
121.96 (C-10), 121.34 (C-30), 118.89 (C-7), 118.63 (C-6), 112.58 (C-
8), 112.38 (C-5), 56.25 (OMe). IR (vmax/cmꢂ1): 3043 (ArC–H); 1619
(C]N); 1584, 1537 (ArC]C); 1306 (C–O).
The following methods were used for the synthesis of
benzimidazoles
Method A. A mixture of o-phenylenediamine (10.0 mmol, 1
equiv.) and benzaldehyde derivative (20.0 mmol, 2 equiv.) in
acetonitrile (10.0 mL) and acetate buffer (10.0 mL, pH 4.5) was
stirred at room temperature for 5 minutes. Suberase® (2.0 mL)
was added to the mixture and the contents were stirred until
reaction completes (monitored by TLC). The product precipitates
from the solution as the reaction proceeds and aer completion
the product was extracted with ethyl acetate (30.0 mL) and water
(3 ꢁ 20.0 mL) and concentrated on a rotary evaporator. The
product was washed several times with cold acetonitrile (3 ꢁ 20.0
mL) to remove any excess starting material.
Synthesis of 2-(2,5-dimethoxyphenyl)-1H-benzimidazole (3c)
Method B. The same as Method A, except that methanol (2.0
mL) was used instead of acetonitrile.
Method C. The same as Method A, except that ethanol (4.0
mL) was used instead of acetonitrile.
Method D. The same as Method A, except that DMF (10.0
mL) was used instead of acetonitrile.
Method E. A repetition of Method A using Denilite® II Base
(0.085 g) and Novoprime Base 268 (0.105 g) laccases instead of
Suberase® (2.0 mL).
Method B. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (0.253 g,
10%). Rf (40% EtOAc/hexane) 0.38.
Method C. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (0.677 g,
25%). Rf (40% EtOAc/hexane) 0.38.
Method E. Novoprime Base 268 (0.105 g), stirring time ¼
24 h. Purication by washing with cold acetonitrile (3 ꢁ 20.0
mL) to remove any excess starting material (conrmed by TLC)
to afford a white solid (1.93 g, 76%). Rf (40% EtOAc/hexane)
Synthesis of 2-(2-methoxyphenyl)-1H-benzimidazole (3a)
+
ꢀ
0.38. Mp ¼ 224–226 C. ([M + H] found: 255.1126 C15H14N2O2
requires [M + H]+, 255.1129). 1H NMR (500 MHz, DMSO-d6):
d 12.09 (1H, s, NH), 7.87 (1H, d, J ¼ 2.8 Hz, H30), 7.63 (2H, dd, J ¼
19.0, 7.2 Hz, H5, H8), 7.19 (3H, m, H6, H7, H60), 7.05 (1H, dd, J ¼
9.0, 2.9 Hz, H40), 3.98 (3H, s, OMe), 3.81 (3H, s, OMe). 13C NMR
(126 MHz, DMSO-d6): d 153.69 (C-2), 151.56 (C-50), 149.21 (C-20),
143.10 (C-9), 135.23 (C-4), 122.61 (C-7), 122.04 (C-6), 118.92 (C-
10), 117.60 (C-40), 114.10 (C-5, C-8), 113.96 (C-20), 112.48 (C-60),
56.65 (OMe-50), 56.03 (OMe-20). IR (vmax/cmꢂ1): 2937 (ArC–H);
1619 (C]N); 1523, 1492 (ArC]C); 1301 (C–O).
Method A. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (1.38 g,
62%). Rf (40% EtOAc/hexane) 0.38.
Method B. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (1.32 g,
59%). Rf (40% EtOAc/hexane) 0.38.
Synthesis of 2-(3,4-dimethoxyphenyl)-1H-benzimidazole (3d)
Method D. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (1.61 g,
72%). Rf (40% EtOAc/hexane) 0.38.
Method E. Novoprime Base 268 (0.105 g), stirring time ¼
24 h. Purication by washing with cold acetonitrile (3 ꢁ 20.0
Method A. Stirring time ¼ 24 h. Purication by washing with
mL) to remove any excess starting material (conrmed by TLC) cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
to afford a brown solid (1.55 g, 69%). Rf (40% EtOAc/hexane) material (conrmed by TLC) to afford a brown solid (1.48 g,
0.38. Mp ¼ 178–180 ꢀC (lit. 40 179–180 ꢀC). ([M + H]+ found: 59%). Rf (60% EtOAc/hexane) 0.43. Mp ¼ 230–232 ꢀC (lit. 41
225.1023 C14H12N2O requires [M + H]+, 225.1024). 1H NMR (500 231–232 ꢀC). ([M + H]+ found: 255.1127 C15H14N2O2 requires [M
MHz, DMSO-d6): d 12.11 (1H, s, NH), 8.33 (1H, d, J ¼ 7.6 Hz, + H]+, 255.1129). 1H NMR (300 MHz, chloroform-d): d 8.28–7.99
H60), 7.63 (2H, dd, J ¼ 16.6, 7.5 Hz, H5, H8), 7.48 (1H, t, J ¼ (1H, m, H8), 7.74–7.43 (3H, m, H5, H20, H60), 7.22 (1H, d, J ¼
7.8 Hz, H40), 7.24 (1H, d, J ¼ 8.3 Hz, H7), 7.19 (2H, t, J ¼ 6.5 Hz, 8.2 Hz, H7), 7.14 (1H, dd, J ¼ 8.3, 3.5 Hz, H6), 7.08–6.89 (1H, m,
H30, H50), 7.12 (1H, t, J ¼ 7.5 Hz, H6), 4.03 (3H, s, OMe). 13C H50), 5.75 (1H, s, NH), 4.85–3.50 (6H, m, 2 ꢁ OMe). 13C NMR
NMR (126 MHz, DMSO-d6): d 157.25 (C-2), 149.43 (C-20), 143.20 (126 MHz, DMSO-d6): d 153.70 (C-2), 150.56 (C-30), 149.39 (C-40),
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143.09 (C-9), 136.51 (C-4), 129.86 (C-10), 122.97 (C-7), 122.85 (C- remove any excess starting material (conrmed by TLC) to
6), 122.51 (C-60), 118.95 (C-8), 118.53 (C-5), 112.32 (C-20), 111.38 afford a brown solid (2.14 g, 94%). Rf (40% EtOAc/hexane) 0.58.
(C-50), 56.10 (2 ꢁ OMe): IR (vmax/cmꢂ1): 2940 (ArC–H); 1606 (C] Mp ¼ 231–234 ꢀC (lit. 40 234–235 ꢀC). ([M + H]+ found: 229.0527
N); 1588, 1501 (ArC]C); 1320 (C–O).
C
13H9ClN2 requires [M + H]+, 229.0528). 1H NMR (500 MHz,
Method B. Stirring time ¼ 24 h. Purication by washing with DMSO-d6): d 12.71 (1H, s, NH), 7.92 (1H, d, J ¼ 7.1 Hz, H60),
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting 7.78–7.46 (5H, m, H5, H8, H30, H40, H50), 7.25 (2H, d, J ¼ 6.8 Hz,
material (conrmed by TLC) to afford a brown solid (0.864 g, H6, H7). 13C NMR (126 MHz, DMSO-d6): d 149.56 (C-2), 143.69
34%). Rf (60% EtOAc/hexane) 0.42.
(C-10), 135.13 (C-9), 132.54 (C-4), 132.11 (C-20), 131.65 (C-40),
Method D. Stirring time ¼ 24 h. Purication by washing with 130.81 (C-30), 130.46 (C-60), 127.88 (C-50), 123.18 (C-7), 122.17 (C-
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting 6), 119.56 (C-8), 112.17 (C-5). IR (vmax/cmꢂ1): 3045 (ArC–H); 1622
material (conrmed by TLC) to afford a brown solid (1.71 g, (C]N); 1569, 1539 (ArC]C); 700 (C–Cl).
67%). Rf (60% EtOAc/hexane) 0.42.
Method E. Denilite® II Base (0.085 g), stirring time ¼ 24 h.
Synthesis of 2-(4-chlorophenyl)-1H-benzimidazole (3g)
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a yellow oil (1.45 g, 57%). Rf (60% EtOAc/hexane) 0.42.
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 8 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a brown solid (1.97 g, 78%). Rf (60% EtOAc/hexane) 0.42.
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 4 h.
Synthesis of 2-phenyl-1H-benzimidazole (3e)
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a brown solid (2.02 g, 88%). Rf (40% EtOAc/hexane) 0.42.
Mp ¼ 286–289 ꢀC (lit. 42 291–294 ꢀC). ([M + H]+ found: 229.0527
C
13H9ClN2 requires [M + H]+, 229.0528). 1H NMR (500 MHz,
DMSO-d6): d 12.98 (1H, s, NH), 8.20 (2H, d, J ¼ 8.4 Hz, H20, H60),
7.64 (4H, m, H5, H8, H30, H50), 7.22 (2H, d, J ¼ 6.7 Hz, H6, H7).
13C NMR (126 MHz, DMSO-d6): d 149.56 (C-2), 143.69 (C-9),
135.13 (C-4), 132.54 (C-40), 132.11 (C-10), 131.65 (C-30), 130.81
(C-50), 130.46 (C-20), 127.88 (C-60), 123.18 (C-7), 122.17 (C-6),
119.56 (C-8), 112.17 (C-5). IR (vmax/cmꢂ1): 3052 (ArC–H); 1622
(C]N); 1588, 1540 (ArC]C); 727 (C–Cl).
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 2 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to afford
a brown solid (1.77 g, 92%). Rf (40% EtOAc/hexane) 0.42. Mp ¼
290–293 ꢀC (lit. 40 292–293 ꢀC). [M + H]+ found: 195.0916
C13H10N2 requires [M + H]+, 195.0918). 1H NMR (500 MHz,
DMSO-d6): d 12.89 (1H, s, NH), 8.19 (2H, d, J ¼ 7.5 Hz, H20, H60),
7.68 (1H, d, J ¼ 7.6 Hz, H8), 7.53 (4H, m, H5, H7, H30, H50), 7.21
(2H, dt, J ¼ 15.6, 6.6 Hz, H6, H40). 13C NMR (126 MHz, DMSO-d6):
d 151.69 (C-2), 144.29 (C-9, C-4), 135.47 (C-10), 130.65 (C-40),
130.29 (C-30, C-50), 129.40 (C-20), 126.90 (C-60), 122.99 (C-7), 122.12
(C-6), 119.34 (C-8), 111.78 (C-5). IR (vmax/cmꢂ1): 3047 (ArC–H);
2921 (N–H); 1622 (C]N); 1559, 1542 (ArC]C).
Synthesis of 2-(2-nitrophenyl)-1H-benzimidazole (3h)
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 8 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford an orange-brown solid (1.33 g, 56%). Rf (40% EtOAc/
hexane) 0.22. Mp ¼ 191–193 C (lit. 43 190–193 ꢀC). H NMR
(500 MHz, DMSO-d6): d 12.21 (1H, s, NH), 7.21 (1H, d, J ¼ 8.0 Hz,
H60), 7.15 (1H, d, J ¼ 7.7 Hz, H30), 7.04 (1H, t, J ¼ 7.6 Hz, H50),
6.94 (1H, t, J ¼ 7.7 Hz, H40), 6.79 (2H, s, H5, H8), 6.43 (2H, s, H6,
H7). 13C NMR (126 MHz, DMSO-d6): d 149.44 (C-2), 147.77 (C-20),
133.11 (C-4, C-9), 131.39 (C-50, C-60), 131.36 (C-10, C-40), 124.76
Synthesis of 2-(2-chlorophenyl)-1H-benzimidazole (3f)
1
ꢀ
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 2 h. (C-6, C-7, C-30), 124.71 (C-5, C-8). IR (vmax/cmꢂ1): 3064 (ArC–H);
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to 1625 (C]N); 1573, 1524 (ArC]C).
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Synthesis of 2-(pyridin-4-yl)-1H-benzimidazole (3i)
Method E
Novoprime Base 268 (0.105 g), stirring time ¼ 8 h. Purication
by washing with cold acetonitrile (3 ꢁ 20.0 mL) to remove any
excess starting material (conrmed by TLC) to afford an orange
solid (1.52 g, 64%). Rf (40% EtOAc/hexane) 0.17. Mp ¼ 201–
202 ꢀC (lit. 44 202–204 ꢀC). ([M + H]+ found: 238.1339 C15H15N3
requires [M + H]+, 238.1340). 1H NMR (500 MHz, DMSO-d6):
d 8.14 (1H, s, NH), 7.99 (2H, d, J ¼ 8.7 Hz, H20, H60), 7.48 (2H, d, J
¼ 9.2 Hz, H5, H8), 7.14 (2H, d, J ¼ 9.2 Hz, H6, H7), 6.83 (2H, d, J
¼ 8.7 Hz, H30, H50), 3.00 (6H, s, 2 ꢁ N–CH3). 13C NMR (126 MHz,
DMSO-d6): d 163.47 (C-2), 152.72 (C-40), 151.74 (C-9, C-4), 128.02
(C-20, C-60), 121.80 (C-6, C-7), 117.83 (C-5, C-8, C-10), 112.32 (C-30,
C-50). IR (vmax/cmꢂ1): 2887 (ArC–H); 2773 (N–H); 1637 (C]N);
1593, 1504 (ArC]C).
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 2 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a brown solid (1.69 g, 60%). Rf (40% EtOAc/hexane) 0.13.
Mp ¼ 211–214 ꢀC (lit. 42 214 ꢀC). ([M + H]+ found: 196.0872
C
12H9N3 requires [M + H]+, 196.0870). 1H NMR (500 MHz,
DMSO-d6): d 13.23 (1H, s, NH), 8.76 (2H, d, J ¼ 4.9 Hz, H30, H50),
8.10 (2H, d, J ¼ 4.8 Hz, H20, H60), 7.66 (2H, d, J ¼ 68.5 Hz, H5,
H8), 7.50–7.02 (2H, m, H6, H7). 13C NMR (126 MHz, DMSO-d6):
d 150.97 (C-2), 149.24 (C-30, C-50), 144.13 (C-10), 137.60 (C-9),
135.49 (C-4), 124.03 (C-6, C-7), 122.73 (C-20), 120.79 (C-60),
119.93 (C-8), 112.27 (C-5). IR (vmax/cmꢂ1): 3051 (ArC–H); 2877
(N–H); 1665 (C]N); 1562, 1537 (ArC]C).
Synthesis of 2-(4-nitrophenyl)-1H-benzimidazole (3m)
Synthesis of 2-(3-nitrophenyl)-1H-benzimidazole (3j)
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 2 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a brown solid (2.27 g, 95%). Rf (40% EtOAc/hexane) 0.52.
Mp ¼ 320 ꢀC (lit. 40 >300 ꢀC). ([M + H]+ found: 240.0767
C
13H9N3O2 requires [M + H]+, 240.0769). 1H NMR (500 MHz,
DMSO-d6): d 13.28 (1H, s, NH), 8.42 (4H, s, H20, H30, H50, H60),
7.74 (1H, d, J ¼ 7.6 Hz, H8), 7.59 (1H, d, J ¼ 7.6 Hz, H5), 7.28
(2H, m, H6, H7). 13C NMR (126 MHz, DMSO-d6): d 149.48 (C-2),
148.30 (C-40), 144.32 (C-10), 136.53 (C-9), 135.71 (C-4), 127.88 (C-
20, C-60), 124.79 (C-30, C-50), 124.07 (C-7), 122.80 (C-6), 119.94 (C-
8), 112.28 (C-5). IR (vmax/cmꢂ1): 2748 (ArC–H); 1604 (C]N);
1513, 1433 (ArC]C).
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 4 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a brown solid (2.26 g, 95%). Rf (40% EtOAc/hexane) 0.43.
Mp ¼ 202–205 ꢀC (lit. 43 201–203 ꢀC). ([M + H]+ found: 196.0872
C
13H9ClN2 requires [M + H]+, 196.0870). 1H NMR (500 MHz,
Synthesis of 2-(3,4,5-trimethoxyphenyl)-1H-benzoimidazole (3n)
DMSO-d6): d 13.27 (1H, s, NH), 9.02 (1H, s, H20), 8.62 (1H, d, J ¼
7.7 Hz, H60), 8.33 (1H, d, J ¼ 8.2 Hz, H40), 7.86 (1H, t, J ¼ 8.0 Hz,
H50), 7.66 (2H, d, J ¼ 50.7 Hz, H5, H8), 7.26 (2H, d, J ¼ 6.7 Hz,
H6, H7). 13C NMR (126 MHz, DMSO-d6): d 147.43 (C-2), 146.75
(C-30), 130.84 (C-4, C-9), 130.12 (C-60), 129.04 (C-10, C-50), 122.55
(C-40), 119.20 (C-6, C-7, C-20), 116.36 (C-5, C-8). IR (vmax/cmꢂ1):
3098 (ArC–H); 1623 (C]N); 1588, 1517 (ArC]C).
Method E. Novoprime Base 268 (0.105 g), stirring time ¼
24 h. Purication by washing with cold acetonitrile (3 ꢁ 20.0
mL) to remove any excess starting material (conrmed by TLC)
to afford a white solid (2.53 g, 89%). Rf (40% EtOAc/hexane)
0.19. Mp ¼ 258–262 ꢀC (lit. 40 258.7–259.7 ꢀC). ([M + H]+
Synthesis of 4-(1H-benzimidazol-2-yl)-N,N-dimethylaniline (3k)
1
found: 285.1230 C16H16N2O3 requires [M + H]+, 285.1224). H
NMR (500 MHz, DMSO-d6): d 12.82 (1H, s, NH), 7.66 (1H, d, J ¼
7.8 Hz, H8), 7.55–7.53 (3H, m, H5, H6, H7), 7.20 (2H, p, J ¼
7.0 Hz, H20, H60), 3.91 (6H, s, 2 ꢁ OMe), 3.74 (3H, s, OMe). 13C
NMR (126 MHz, DMSO-d6): d 153.69 (C-2), 151.68 (C-30, C-50),
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144.22 (C-9), 139.38 (C-4), 135.44 (C-40), 125.95 (C-10), 122.89 (C- d6): d 157.53 (C-300), 156.83 (C-20), 152.31 (C-2), 143.35 (C-4),
7), 122.09 (C-6), 119.17 (C-5), 111.61 (C-8), 104.30 (C-20, C-60), 135.68 (C-9), 132.39 (C-60), 132.02 (C-40), 129.17 (C-700), 127.94
60.62 (OMe), 56.53 (2 ꢁ OMe). IR (vmax/cmꢂ1): 2952 (ArC–H); (C-500), 124.64 (C-200), 122.66 (C-7), 122.03 (C-6), 120.98 (C-50),
1589 (C]N); 1498, 1482 (ArC]C); 1311 (C–O).
120.57 (C-600), 120.00 (C-10), 119.54 (C-8), 111.96 (C-5), 111.33 (C-
30), 111.16 (C-400), 55.75 (OMe), 55.73 (OMe), 43.25 (C-100). IR
(vmax/cmꢂ1): 3063 (ArC–H); 1603 (C]N); 1582, 1518 (ArC]C);
1327 (C–O).
Synthesis of 2-(4-methoxyphenyl)-1H-benzimidazole (3o)
Method C. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (0.447 g,
13%). Rf (40% EtOAc/hexane) 0.34.
Synthesis of 3-(1-(3-hydroxybenzyl)-1H-benzoimidazol-2-yl)
phenol (4b)
Method E. Novoprime Base 268 (0.105 g), stirring time ¼
24 h. Purication by washing with cold acetonitrile (3 ꢁ 20.0
mL) to remove any excess starting material (conrmed by
TLC) to afford a light brown solid (1.53 g, 68%). Rf (40%
EtOAc/hexane) 0.31. Mp ¼ 226–228 ꢀC (lit. 42 225–226 ꢀC). ([M
+ H]+ found: 285.1230 C16H16N2O3 requires [M + H]+,
1
285.1024) H NMR (500 MHz, DMSO-d6): d 12.73 (1H, s, NH),
8.12 (2H, d, J ¼ 8.6 Hz, H20, H60), 7.56 (2H, d, J ¼ 52.8 Hz, H5,
H8), 7.20–7.14 (2H, m, H6, H7), 7.11 (2H, d, J ¼ 8.6 Hz, H30,
H50), 3.84 (3H, s, OMe). 13C NMR (126 MHz, DMSO-d6):
d 161.07 (C-2), 151.82 (C-40), 144.37 (C-9), 135.44 (C-4), 128.48
(C-20, C-60), 123.19 (C-7), 122.49 (C-6), 121.98 (C-8), 121.91 (C-
5), 118.96, 114.84 (C-30, C-50), 111.48 (C-10), 55.80 (OMe). IR
(vmax/cmꢂ1): 3044 (ArC–H); 1621 (C]N); 1583, 1500 (ArC]C);
1317 (C–O).
Method A. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a brown solid (2.58 g,
82%). Rf (40% EtOAc/hexane) 0.23. Mp ¼ 246–249 ꢀC (lit. 45
243–245 ꢀC). ([M + H]+ found: 316 C20H16N2O2 requires [M + H]+,
316.1286). 1H NMR (500 MHz, DMSO-d6): d 9.77 (1H, d, J ¼
30.8 Hz, H60), 9.43 (1H, s, H20), 7.73 (1H, d, J ¼ 7.7 Hz, H8), 7.39
(1H, d, J ¼ 7.5 Hz, H5), 7.33 (1H, q, J ¼ 7.7 Hz, H50), 7.23 (3H, td,
J ¼ 17.7, 16.0, 8.7 Hz, H6, H7, H600), 7.17–7.06 (2H, m, OH), 6.95
(1H, d, J ¼ 8.1 Hz, H700), 6.65 (1H, d, J ¼ 8.0 Hz, H500), 6.50 (1H, d,
J ¼ 7.5 Hz, H40), 6.40 (1H, s, H300), 5.49 (2H, s, H100). 13C NMR
(126 MHz, DMSO-d6): d 156.09 (C-30), 155.91 (C-400), 151.69 (C-2),
140.98 (C-4), 136.71 (C-9, C-200), 134.25 (C-10), 129.58 (C-50),
128.19 (C-600), 120.96 (C-7), 120.52 (C-6), 117.87 (C-60), 117.55 (C-
700), 115.28 (C-8), 114.99 (C-5), 114.38 (C-40), 112.82 (C-300), 111.07
(C-20), 109.45 (C-500), 45.81 (C-100).
Synthesis of 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-
benzimidazole (4a)
Method C. Stirring time ¼ 24 h. Purication by washing with
Method B. Stirring time ¼ 24 h. Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting material (conrmed by TLC) to afford a brown solid (3.06 g,
material (conrmed by TLC) to afford an orange solid (0.689 g, 97%). Rf (40% EtOAc/hexane) 0.23.
20%). Rf (40% EtOAc/hexane) 0.34. Mp ¼ 148–152 ꢀC (lit. 44
Method D. Stirring time ¼ 24 h. Purication by washing with
151–153 ꢀC). ([M + H]+ found: 345.1596 C22H20N2O2 requires [M cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
+ H]+, 334.1599). 1H NMR (500 MHz, DMSO-d6): d 7.67 (1H, d, J material (conrmed by TLC) to afford a brown solid (3.10 g,
¼ 7.7 Hz, H60), 7.52 (1H, t, J ¼ 7.8 Hz, H50), 7.42 (1H, d, J ¼ 98%). Rf (40% EtOAc/hexane) 0.23.
7.4 Hz, H700), 7.36 (1H, d, J ¼ 7.1 Hz, H30), 7.27–7.12 (4H, m, H6,
Method E. Novoprime Base 268 (0.105 g), stirring time ¼ 4 h.
H7, H8, H40), 7.07 (1H, t, J ¼ 7.4 Hz, H600), 6.92 (1H, d, J ¼ 8.2 Hz, Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
H5), 6.75 (1H, t, J ¼ 7.4 Hz, H500), 6.58 (1H, d, J ¼ 7.4 Hz, H400), remove any excess starting material (conrmed by TLC) to
5.21 (2H, s, H100), 3.67 (6H, s, OMe). 13C NMR (126 MHz, DMSO- afford a brown solid (2.72 g, 86%). Rf (40% EtOAc/hexane) 0.23.
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Rf (60% EtOAc/hexane) 0.21. ([M + H]+ found: 405.1805
C
Synthesis of 1-(2,5-dimethoxybenzyl)-2-(2,5-dimethoxyphenyl)-
1H-benzimidazole (4c)
1
24H24N2O4 requires [M + H]+, 405.1810). H NMR (500 MHz,
DMSO-d6): d 7.88 (1H, s, H20), 7.71 (1H, d, J ¼ 7.9 Hz, H60), 7.53
(1H, d, J ¼ 7.8 Hz, H8), 7.34–7.16 (3H, m, H5, H6, H7), 6.91–6.79
(2H, m, H50, H600), 6.74 (1H, s, H300), 6.62 (1H, d, J ¼ 16.3 Hz,
H700), 5.90 (2H, m, H100), 3.88–3.57 (12H, m, 4 ꢁ OMe). 13C NMR
(126 MHz, DMSO-d6): d 153.71 (C-2), 150.59 (C-30), 149.41 (C-40),
149.36 (C-400), 149.18 (C-500), 148.60 (C-4), 143.10 (C-9), 129.87 (C-
10), 122.99 (C-7), 122.85 (C-6), 122.16 (C-200), 119.76 (C-60), 119.45
(C-700), 118.95 (C-8), 118.56 (C-5), 112.36 (C-20), 112.49 (C-60),
111.37 (C-50), 110.78 (C-300), 56.11 (4 ꢁ OMe), 47.74 (C-100). IR
(vmax/cmꢂ1): 2970 (ArC–H); 1651 (C]N); 1454 (ArC]C); 1228
(C–O).
Method B. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow oil (2.66 g, 66%).
Rf (60% EtOAc/hexane) 0.21.
Method D. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a white solid (0.927 g,
23%). Rf (60% EtOAc/hexane) 0.21.
Method A. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford an oil (2.25 g, 56%). Rf
(40% EtOAc/hexane) 0.36. ([M + H]+ found: 405.1820 C24H24N2O4
requires [M + H]+, 405.1810). 1H NMR (500 MHz, DMSO-d6):
d 7.73–7.57 (1H, m, H60), 7.48–7.32 (1H, m, H8), 7.28–7.14 (2H, m,
H30, H40), 7.12 (2H, d, J ¼ 9.3 Hz, H400, H500), 7.02–6.85 (1H, m,
H7), 6.85 (1H, d, J ¼ 8.9 Hz, H5), 6.75 (1H, dd, J ¼ 8.9, 2.7 Hz, H6),
6.15 (1H, s, H700), 5.19 (2H, s, H100), 3.74–3.45 (12H, m, 4 ꢁ OMe).
13C NMR (126 MHz, DMSO-d6): d 153.42 (C-2), 153.32 (C-50),
151.90 (C-600), 151.58 (C-300), 151.01 (C-20), 143.22 (C-4), 135.63 (C-
9), 125.80 (C-200), 122.76 (C-7), 122.10 (C-6), 120.61 (C-8), 119.56
(C-5), 117.48 (C-10), 117.10 (C-40), 114.80 (C-700), 113.23 (C-30),
113.00 (C-60), 112.19 (C-500), 111.37 (C-400), 56.22 (OMe-50), 56.04
(OMe-600), 55.68 (OMe-300), 55.37 (OMe-20), 43.26 (C-100).
Method B. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford an oil (3.64 g, 90%). Rf
(40% EtOAc/hexane) 0.36.
Method C. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford an oil (2.82 g, 70%). Rf
(40% EtOAc/hexane) 0.36.
Method D. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford an oil (3.79 g, 94%). Rf
(40% EtOAc/hexane) 0.36.
Method E. Denilite® II Base (0.085 g), stirring time ¼ 24 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a yellow oil (1.21 g, 30%). Rf (60% EtOAc/hexane) 0.21.
The following methods were used to synthesize
benzothiazoles
Method F. A mixture of 2-aminothiophenol (1.63 g, 15.0
mmol) and benzaldehyde derivative (1.06 g, 10.0 mmol) in
acetonitrile (10.0 mL) and acetate buffer (10.0 mL, pH 4.0) was
stirred at room for 5 minutes. Suberase® (2.0 mL) was then
added into the mixture and the contents were stirred for 1 h.
When the reaction completes, the product precipitates from the
solution and was extracted with ethyl acetate (30.0 mL) and
water (3 ꢁ 20.0 mL) and concentrated on a rotary evaporator.
The product was washed several times with cold acetonitrile (3
ꢁ 20.0 mL) to remove any excess starting material.
Method G. The same as Method A, except that methanol (2.0
mL) was used instead of acetonitrile.
Method H. The same as Method A, except that ethanol (4.0
mL) was used instead of acetonitrile.
Method I. The same as Method A, except that DMF (10.0 mL)
was used instead of acetonitrile.
Synthesis of 1-(3,4-dimethoxybenzyl)-2-(3,4-dimethoxyphenyl)-
1H-benzimidazole (4d)
Method J. The same as Method A, except that DCM (10.0 mL)
was used instead of acetonitrile.
Synthesis of 2-phenylbenzothiazole (7a)
Method A. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow oil (1.45 g, 36%).
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Method F. Stirring time ¼ 2 h. Purication by washing with Synthesis of 2-(4-methoxyphenyl)benzothiazole (7c)
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a bright yellow solid
(1.79 g, 85%). Rf (40% EtOAc/hexane) 0.93. Mp ¼ 96–98 ꢀC (lit.
46 102–104 ꢀC). ([M + H]+ found: 212.0528 C13H9NS requires [M
+ H]+, 212.0530). 1H NMR (500 MHz, chloroform-d): d 8.15–
8.04 (3H, m, H8, H20, H60), 7.91 (1H, d, J ¼ 8.0 Hz, H5), 7.50
(4H, m, H6, H7, H30, H50), 7.39 (1H, t, J ¼ 7.6 Hz, H40). 13C NMR
(126 MHz, chloroform-d): d 154.17 (C-2), 135.08 (C-4), 133.65
Method F. Stirring time ¼ 8 h. Purication by washing with
(C-10), 130.97 (C-9), 129.03 (C-20, C-60), 127.57 (C-50), 126.32 (C-
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
30), 125.19 (C-40), 123.26 (C-6, C-7), 121.62 (C-5, C-8). IR (vmax
/
material (conrmed by TLC) to afford white solid (1.64 g, 68%). Rf
cmꢂ1): 3075 (ArC–H); 1588, 1558 (ArC]C); 1644 (C]N); 727
(C–S).
ꢀ
ꢀ
(40% EtOAc/hexane) 0.89. Mp ¼ 120–122 C (lit. 48 120–122 C).
([M + H]+ found: 242.0632 C14H11NOS requires [M + H]+, 242.0635).
1H NMR (500 MHz, chloroform-d): d 8.04 (3H, d, J ¼ 8.7 Hz, H5,
H20, H60), 7.88 (1H, d, J ¼ 8.0 Hz, H8), 7.47 (1H, t, J ¼ 7.6 Hz, H6),
7.35 (1H, t, J ¼ 7.6 Hz, H7), 7.00 (2H, d, J ¼ 8.7 Hz, H30, H50), 3.88
(3H, s, OMe). 13C NMR (126 MHz, chloroform-d): d 167.85 (C-2),
161.94 (C-40), 154.24 (C-4), 134.87 (C-10), 129.12 (C-9), 126.47 (C-
60), 126.19 (C-20), 124.79 (C-6), 122.83 (C-7), 121.50 (C-5, C-8), 114.38
(C-30, C-50), 55.46 (OMe). IR (vmax/cmꢂ1): 3060 (ArC–H); 1574, 1557
(ArC]C); 1603 (C]N); 729 (C–S), 1310 (C–O).
Method G. Stirring time ¼ 24 h. Purication by washing
with cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess
starting material (conrmed by TLC) to afford a white solid
(1.23 g, 58%). Rf (40% EtOAc/hexane) 0.93.
Method H. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a white solid (1.16 g,
55%). Rf (40% EtOAc/hexane) 0.93.
Method I. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a white solid (1.66 g,
78%). Rf (40% EtOAc/hexane) 0.93.
Synthesis of 2-(3,4-dimethoxyphenyl)benzothiazole (7d)
Method J. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a white solid (1.37 g,
65%). Rf (40% EtOAc/hexane) 0.93.
Synthesis of 4-(benzothiazol-2-yl)-N,N-dimethylaniline (7b)
Method F. Stirring time ¼ 2 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a cream solid (1.98 g,
73%). Rf (40% EtOAc/hexane) 0.67. Mp ¼ 132–134 ꢀC (lit. 48
132–134 ꢀC). ([M + H]+ found: 272.0740 C15H13NO2S requires [M
+ H]+, 272.0741). 1H NMR (500 MHz, chloroform-d): d 8.04 (1H,
d, J ¼ 8.2 Hz, H5), 7.88 (1H, d, J ¼ 7.9 Hz, H8), 7.71 (1H, s, H2),
7.60 (1H, d, J ¼ 9.5 Hz, H60), 7.47 (1H, t, J ¼ 7.7 Hz, H6), 7.36
(1H, t, J ¼ 7.6 Hz, H7), 6.95 (1H, d, J ¼ 8.4 Hz, H50), 4.03 (3H, s,
OMe), 3.96 (3H, s, OMe). 13C NMR (126 MHz, chloroform-d):
d 167.93 (C-2), 154.17 (C-4), 151.60 (C-30), 149.37 (C-40), 134.92
(C-9), 126.71 (C-10), 126.23 (C-6), 124.88 (C-7), 122.85 (C-5),
121.51 (C-8), 121.16 (C-60), 111.05 (C-20), 109.84 (C-50), 56.15
(OMe-30), 56.06 (OMe-40). IR (vmax/cmꢂ1): 2964 (ArC–H); 1520,
1479 (ArC]C); 1598 (C]N); 732 (C–S), 1312 (C–O).
Method F. Stirring time ¼ 2 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a cream solid (1.43 g,
ꢀ
56%). Rf (40% EtOAc/hexane) 0.81. Mp ¼ 172 C (lit. 47 170–
171 ꢀC). ([M + H]+ found: 255.0984 C15H14N2S requires [M +
1
H]+, 255.0952). H NMR (500 MHz, chloroform-d): d 8.03–7.91
(2H, m, H20, H60), 7.84 (1H, d, J ¼ 7.9 Hz, H5), 7.44 (1H, t, J ¼
7.4 Hz, H6), 7.30 (1H, t, J ¼ 7.6 Hz, H7), 6.82–6.54 (3H, m, H8,
H30, H50), 3.01 (6H, d, J ¼ 53.2 Hz, N–CH3). 13C NMR (126 MHz,
chloroform-d): d 168.79 (C-2), 154.43 (C-40), 152.20 (C-4),
134.56 (C-9), 128.87 (C-60), 127.71 (C-20), 125.97 (C-6), 125.29
(C-7), 124.18 (C-10), 122.29(C-8), 121.34 (C-5), 120.46 (C-50),
112.29(C-30), 40.48 (N–CH3), 40.18 (N–CH3). IR (vmax/cmꢂ1):
3053 (ArC–H); 2904 (C–N); 2815 (N–C–H), 1556, 1526 (ArC]C);
1604 (C]N); 720 (C–S).
Synthesis of 2-(3,4,5-trimethoxyphenyl)benzothiazole (7e)
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126.82 (C-6), 126.20 (C-7), 123.94 (C-8), 121.87 (C-5), 121.20 (C-
20, C-60). IR (vmax/cmꢂ1): 3035 (ArC–H); 1698 (C]N); 1552, 1503
(ArC]C); 704 (C–S).
Method F. Stirring time ¼ 2 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow solid (1.96 g,
65%). Rf (40% EtOAc/hexane) 0.74. Mp ¼ 148–151 ꢀC (lit. 49
149.5–151.1 ꢀC) ([M + H]+ found: 302.0836 C16H15NO3S requires
1
[M + H]+, 302.0847). H NMR (500 MHz, chloroform-d): d 8.06
Synthesis of 1,4-bis(benzothiazol-2-yl)benzene (7h)
(1H, d, J ¼ 8.2 Hz, H5), 7.89 (1H, d, J ¼ 8.0 Hz, H8), 7.49 (1H, t, J
¼ 7.7 Hz, H6), 7.38 (1H, t, J ¼ 7.6 Hz, H7), 7.34 (2H, s, H20, H60),
3.96 (9H, d, J ¼ 30.7 Hz, OMe). 13C NMR (126 MHz, Chloroform-
d): d 167.80 (C-2), 154.10 (C-4), 153.61 (C-30, C-50), 140.71 (C-40),
135.06 (C-9), 129.09 (C-10), 126.36 (C-6), 125.13 (C-7), 123.10 (C-
8), 121.56 (C-5), 104.84 (C-20, C-60), 61.02 (2 ꢁ OMe), 56.39
(OMe). IR (vmax/cmꢂ1): 2941 (ArC–H); 1581, 1518 (ArC]C); 1698
(C]N); 1331 (C–O); 709 (C–S).
Method F. 2-Aminothiophenol (2 equiv.), stirring time ¼ 2 h.
Purication by washing with cold acetonitrile (3 ꢁ 20.0 mL) to
remove any excess starting material (conrmed by TLC) to
afford a yellow solid (2.98 g, 87%). Rf (40% EtOAc/hexane) 0.81.
Mp ¼ 261–264 ꢀC (lit. 52 263 ꢀC). ([M + H]+ found: 345.0515
Synthesis of 2-(2-chlorophenyl)benzothiazole (7f)
C
20H12N2S2 requires [M + H]+, 345.0516). 1H NMR (500 MHz,
chloroform-d): d 8.23 (4H, s, H20, H30, H50, H60), 8.12 (2H, d, J ¼
8.1 Hz, H5, H700), 7.94 (2H, d, J ¼ 8.0 Hz, H8, H400), 7.53 (2H, t, J
¼ 7.6 Hz, H6, H600), 7.42 (2H, t, J ¼ 7.5 Hz, H7, H500). 13C NMR
(126 MHz, chloroform-d): d 166.81(C-2, C-100), 154.19 (C-4, C-800),
135.70 (C-10, C-400), 135.21 (C-9, C-300), 128.09 (C-20, C-30, C-50, C-
60), 126.56 (C-6, C-600), 125.56 (C-7, C-500), 123.48 (C-8, C-400),
121.70 (C-5, C-700). IR (vmax/cmꢂ1): 3057 (ArC–H); 1697 (C]N);
1558, 1522 (ArC]C); 724 (C–S).
Method F. Stirring time ¼ 18 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a cream solid (1.35 g,
55%). Rf (40% EtOAc/hexane) 0.93. Mp ¼ 110–112 ꢀC (lit. 50
110–112 ꢀC). ([M + H]+ found: 246.0139 C13H8ClNS requires [M +
H]+, 246.0140). 1H NMR (500 MHz, chloroform-d) d 8.07 (1H, d, J
¼ 8.2 Hz, H5), 8.03 (2H, d, J ¼ 8.4 Hz, H20, H60), 7.90 (1H, d, J ¼
8.0 Hz, H8), 7.48 (3H, m, H6, H30, H50), 7.40 (1H, t, J ¼ 7.6 Hz,
H7).13C NMR (126 MHz, chloroform-d) d 166.60 (C-2), 154.08 (C-
5), 137.03 (C-10), 135.06 (C-40), 132.13 (C-9), 129.27 (C-60), 128.71
(C-20), 126.48 (C-30, C-50), 125.41 (C-6), 123.31 (C-7), 121.65 (C-5,
C-8). IR (vmax/cmꢂ1): 3055 (ArC–H); 1646 (C]N); 1588, 1569
(ArC]C); 730 (C–Cl), 709 (C–S).
Synthesis of 2-(2-bromo-5-methoxyphenyl)benzothiazole (7i)
Synthesis of 2-(pyridin-4-yl)benzothiazole (7g)
Method F. Stirring time ¼ 8 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow solid (2.42 g,
ꢀ
76%). Rf (40% EtOAc/hexane) 0.93. Mp ¼ 123–125 C. ([M +
H]+ found: 319.9739 C14H10BrNOS requires [M
+
H]+,
1
Method F. Stirring time ¼ 2 h. Purication by washing with 319.9740). H NMR (500 MHz, chloroform-d): d 8.68 (1H, d, J
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting ¼ 2.3 Hz, H5), 8.10 (1H, d, J ¼ 8.2 Hz, H8), 7.93 (1H d, J ¼
material (conrmed by TLC) to afford a bright yellow solid 7.9 Hz, H30), 7.57–7.46 (2H, m, H6, H60), 7.39 (1H, t, J ¼
(1.86 g, 88%). Rf (40% EtOAc/hexane) 0.37. Mp ¼ 126–130 ꢀC (lit. 7.5 Hz, H7), 6.94 (1H, d, J ¼ 8.8 Hz, H40), 4.04 (3H, s, OMe).
51 126–130 ꢀC). ([M + H]+ found: 213.0481 C12H8N2S requires [M 13C NMR (126 MHz, chloroform-d): d 161.43 (C-2), 156.22 (C-
+ H]+, 213.0482). 1H NMR (500 MHz, chloroform-d): d 8.78 (2H, 50), 152.00 (C-4), 136.20 (C-10), 134.12 (C-9), 131.88 (C-30),
d, J ¼ 5.7 Hz, H30, H50), 8.13 (1H, d, J ¼ 8.2 Hz, H5), 7.99–7.87 126.11 (C-6), 124.93 (C-7), 124.05 (C-8), 122.99 (C-5), 121.24
(3H, m, H8, H20, H60), 7.55 (1H, t, J ¼ 7.7 Hz, H6), 7.46 (1H, t, J ¼ (C-40), 113.81 (C-60), 113.46 (C-20), 56.03 (OMe). IR (vmax
/
7.6 Hz, H7). 13C NMR (126 MHz, chloroform-d): d 165.10 (C-4), cmꢂ1): 3058 (ArC–H); 1694 (C]N); 1587, 1571 (ArC]C); 1315
153.99 (C-2), 150.77 (C-30, C-50), 140.49 (C-10), 135.23 (C-9), (C–O); 751 (C–Br); 723 (C–S).
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Synthesis of 2-(3-nitrophenyl)benzothiazole (7j)
Synthesis of 2-(4-nitrophenyl)benzothiazole (7l)
Method F. Stirring time ¼ 24 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow solid (2.15 g,
84%). Rf (40% EtOAc/hexane) 0.92. Mp ¼ 243 ꢀC (lit. 54 233 ꢀC).
([M + H]+ found: 257.0379 C13H8N2O2S requires [M + H]+,
257.0380). 1H NMR (500 MHz, chloroform-d): d 8.36 (2H, d, J ¼
8.7 Hz, H30, H50), 8.27 (2H, d, J ¼ 8.7 Hz, H20, H60), 8.13 (1H, d, J
¼ 8.2 Hz, H5), 7.96 (1H, d, J ¼ 8.0 Hz, H8), 7.56 (1H, t, J ¼ 7.7 Hz,
H6), 7.47 (1H, t, J ¼ 7.6 Hz, H7). 13C NMR (126 MHz, chloro-
form-d): d 164.84 (C-2), 154.11 (C-4), 149.05 (C-10), 139.19 (C-40),
135.50 (C-9), 128.24 (C-20, C-60), 126.93 (C-6), 126.23 (C-7), 124.32
(C-30), 123.95 (C-50), 121.85 (C-5, C-8). IR (vmax/cmꢂ1): 3060 (ArC–
H); 1603 (C]N); 1574, 1557 (ArC]C); 1310 (N–O); 729 (C–S).
Method F. Stirring time ¼ 8 h. Purication by washing with
cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess starting
material (conrmed by TLC) to afford a yellow solid (1.23 g,
ꢀ
48%). Rf (40% EtOAc/hexane) 0.81. Mp ¼ 185–187 C (lit. 53
186.8–187.3 ꢀC). ([M + H]+ found: 257.0379 C13H8N2O2S
1
requires [M + H]+, 257.0380). H NMR (500 MHz, chloroform-
d): d 8.94 (1H, s, H20), 8.43 (1H, d, J ¼ 7.8 Hz, H60), 8.34 (1H, d, J
¼ 8.2 Hz, H40), 8.12 (1H, d, J ¼ 8.2 Hz, H5), 7.95 (1H, d, J ¼
8.0 Hz, H8), 7.69 (1H, t, J ¼ 8.0 Hz, H50), 7.55 (1H, t, J ¼ 7.7 Hz,
H6), 7.46 (1H, t, J ¼ 7.6 Hz, H7). 13C NMR (126 MHz, chloro-
form-d): d 164.89 (C-2), 153.97 (C-4), 148.79 (C-30), 135.32
(C-60), 135.20 (C-10), 132.99 (C-9), 130.10 (C-50), 126.84 (C-6),
126.03 (C-7), 125.18 (C-40), 123.77 (C-20), 122.35 (C-8), 121.83
(C-5). IR (vmax/cmꢂ1): 3085 (ArC–H); 1612 (C]N); 1579, 1547
(ArC]C); 1362 (N–O); 715 (C–S).
Conclusions
Laccases are proving themselves to be very useful oxidising
catalysts that enable access to a wide range of chemicals, as was
demonstrated here, synthesising pharmaceutically relevant
compounds such as 2-aryl-1H-benzimidazoles and 2-arylben-
zothiazole derivatives in good to excellent yields. This could be
achieved using readily available commercial laccases, making
this technique accessible to the general chemistry community.
Chemoselectivity was achieved in the synthesis of 2-aryl-1H-
benzimidazoles by changing the solvent and laccase to Novo-
prime Base 268 in an acetate buffer – acetonitrile solvent. The
same protocol was applied to the synthesis of 2-arylbenzothia-
zoles and good to excellent yields were again obtained.
Synthesis of 2-(2-methoxyphenyl)benzothiazole (7k)
The reaction proceed under very mild conditions, with low
environmental impact (green), and the transformations require
a simple work-up routine to obtain pure product with a simple
wash with cold acetonitrile. In addition, this method eliminated
the use of transition metal catalysts (e.g. palladium, ruthenium or
copper); use of toxic oxidants such as DDQ, NiO2 or KI and the
release of iodinated intermediates from oxidants; and the use of
solvents with major environmental concerns such as DMF.
Method F. Stirring time ¼ 24 h. Purication by washing
with cold acetonitrile (3 ꢁ 20.0 mL) to remove any excess
starting material (conrmed by TLC) to afford a brown solid
(1.78 g, 74%). Rf (40% EtOAc/hexane) 0.89. Mp ¼ 121 ꢀC (lit. 54
120–122 ꢀC). ([M + H]+ found: 242.0634 C14H11NOS requires [M
+ H]+, 242.0635). 1H NMR (500 MHz, chloroform-d): d 8.54 (1H,
d, J ¼ 7.8 Hz, H5), 8.10 (1H, d, J ¼ 8.2 Hz, H8), 7.93 (1H, d, J ¼
7.9 Hz, H60), 7.56–7.42 (2H, m, H6, H7), 7.37 (1H, t, J ¼ 7.5 Hz,
H50), 7.14 (1H, t, J ¼ 7.6 Hz, H40), 7.06 (1H, d, J ¼ 8.3 Hz, H30),
4.05 (3H, s, OMe). 13C NMR (126 MHz, chloroform-d): d 153.49
(C-2), 150.51 (C-20), 148.80 (C-4), 137.60 (C-9), 134.06 (C-40),
133.86 (C-60), 130.78 (C-6), 129.93 (C-7), 129.60 (C-10), 125.66
(C-8), 123.29 (C-5), 116.28 (C-50), 115.26 (C-30). IR (vmax/cmꢂ1):
3053 (ArC–H); 1604 (C]N); 1556, 1526 (ArC]C); 1315 (C–O);
720 (C–S).
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We would like to thank Eric Mori, Thapelo Mmbhele and Relwe
Moepya for technical assistance with Mass Spectrometry and Dr
Hendrik Henning for technical assistance with NMR spectrom-
etry: Support from the Department of Science and Technology
Biocatalysis Initiative (Grant 0175/2013) is greatly appreciated.
39508 | RSC Adv., 2018, 8, 39496–39510
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