A Transition-Metal-Free Synthesis of Multisubstituted Imidazoles

Article abstract of DOI:10.1002/cjoc.201500759

A novel and simple t-BuOLi/I₂-mediated synthesis of 1,2,4-trisubstituted imidazoles was developed without transition-metal added. The transition-metal-free strategy tolerated a range of substrates and provided products in moderate to good yield

Full text of DOI:10.1002/cjoc.201500759

COMMUNICATION
DOI: 10.1002/cjoc.201500759
A Transition-Metal-Free Synthesis of Multisubstituted
Imidazoles
Ping Wu,a Lutao Zhang,a Xueguo Zhang,a Xin Guo,a and Baohua Chen*,a,b
a State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China
b Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou,
Gansu 730000, China
A novel and simple t-BuOLi/I2-mediated synthesis of 1,2,4-trisubstituted imidazoles was developed without
transition-metal added. The transition-metal-free strategy tolerated a range of substrates and provided products in
moderate to good yields with 100% regioselectivity.
Keywords multisubstituted imidazoles, cyclization, benzamidine, N-tosyl-hydrazone, heterocycles
Introduction
Scheme 1 Works in our group to gain imidazole derivatives
Imidazole ring, a key structure among the valuable
nitrogen-containing heterocycles,[1-12] is frequently
found in numerous natural products, bioactive com-
pounds and agrochemical industries.[13-17] The pharma-
cological properties of imidazole derivatives, such as
antimicrobial,[18,19] antiplasmodium[20] and anti-inflam-
matory,[21,22] are increasingly remarkable and widely
exploited. Besides, its good prospects as the raw materi-
als and ligands in organic synthesis have attracted more
and more scientific workers to dedicate to develop
complementary strategies and viable alternatives for
obtaining multisubstituted imidazoles.[23-27]
Given the importance of imidazole derivatives, a
number of transition metals-catalyzed formations of
substituted imidazoles have been reported in the litera-
ture.[28-31] Although these presented methods are gener-
ally efficient, their applications are of great limitation,
such as uneasy operation, poor functional group toler-
ance and high cost of metal catalysts.[32,33] Therefore,
there is still a challenge to develop novel synthetic
methods for preparation of potentially pharmacologi-
cally active imidazole derivatives in terms of opera-
tional simplicity and readily available starting materials.
Our group[34-37] has been dedicated to investigating the
synthesis of imidazole derivatives (Scheme 1). High
regioselectivity as it is, but they request a significant
amount of expensive metal catalyst. In our continued
interest in the synthesis of imidazoles, we herein report
a simple and transition-metal-free alternative that can
enable the construction of the C－N bond through oxi-
dative transformation[38-42] of readily available ben-
zamidines[43-46] with N-tosyl-hydrazones.[47-50]
Previous work in our group:
Ar3
CuI (10 mol% )
H
Ar1
N
R
O2N
bipy ( 20 mol%)
O2, DMF, 90 oC
Ar3
N
Ar2
+
N
R
NH
Ar1
Ar2
R
H
FeCl3 (30 mol%)
Ar1
N
Ar2
+
N
HC
CHO
CH3NO2/DMF, 80 oC
R
N
NH
Ar1
Ar2
I2 (2.5 mol%)
R
O
H
.
O
FeCl3 6H2O (10 mol%)
1,2-DCB, 110 oC
Ar1
N
Ar2
Ar3
+
Ar3
N
R
NH
N
Ar1
Ar2
This work:
H
R
NNHTs
Ar1
N
I2, t-BuOLi
N
Ar2
+
N
Toluene, 70 oC, 5 h
R
Ar1
NH
Ar2
Experimental
All the reactions were carried out in a round-bottom
side arm flask (10 mL). All substrates are easily avail-
able. Flash chromatography was performed with Merck
silica gel 60 F254 plates, and the products were visual-
ized by UV detection. 1H NMR spectra were recorded at
300 MHz and 400 MHz in CDCl3, and 13C NMR spectra
were recorded at 75 MHz and 100 MHz in CDCl3 with
using TMS as internal standard. Melting points were
determined on a microscopic apparatus without correc-
tion. HRMS was performed on an FT-ICRMS mass in-
strument and measured with electrospray ionization
*
E-mail: chbh@lzu.edu.cn; Fax: 0086-0931-8912582
Received November 1, 2015; accepted December 15, 2015; published online February 3, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500759 or from the author.
Chin. J. Chem. 2016, 34, 363—367
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
363

Wu et al.
Table 1 Screening for optimized conditionsa
COMMUNICATION
1
(ESI). Copies of H NMR and 13C NMR spectra are
provided in the Supporting Information. Commercially
available reagents were used without further purifica-
tion.
NNHTs
H
N
NH
Additive
Base
+
The reactions were carried out in a round-bottom
flask reactor (10 mL). 1a (0.2 mmol), 2a (0.2 mmol), I2
(0.2 mmol), t-BuOLi (0.4 mmol), and toluene (2.0 mL)
were added to the flask with magnetic stirring bar. The
resulting mixture was stirred at 70 ℃ for 5 h. After
cooling to room temperature, the mixture was filtered
and extracted with ethyl acetate (10 mL×3). Then the
filtrate was concentrated under reduced pressure in or-
der to get the crude product, which was further purified
by silica gel chromatography (petroleum/ethyl acetate,
V∶V＝15∶1 as eluent) to obtain product 3a.
O
Solvent
2a
1a
O
N
N
3a
Entry Base (Dosage/equiv.) Additive Temp./℃Solvent Yieldb/%
1
2
t-BuOLi (2.0)
t-BuOK (2.0)
CsCO3 (2.0)
I2
I2
70 Toluene 72
70
70
70
70
70
70
70
70
70
Toluene 48
Toluene Trace
Toluene Trace
Toluene 39
Toluene 40
Toluene 57
Toluene 68
Toluene 26
Toluene 38
Toluene Nr
Toluene 34
Results and Discussion
3
I2
In our initial studies, the reaction of N-phenyl-ben-
zamidine (1a) with 4-methoxyacetophenone N-tosyl-
hydrazone (2a) was chosen as a model reaction for opti-
mizing the reaction conditions. Gratifyingly, the desired
product 3a was obtained in 72% yield in the presence of
I2 (1.0 equiv.) and t-BuOLi (2.0 equiv.) in toluene (2.0
mL) at 70 ℃ for 5 h, which indicated that the usage of
iodine sources is essential to make the cyclization pro-
ceed and to obtain the corresponding products with good
yields, as shown in Table 1. Subsequently, we screened
the iodine sources and bases. The use of other bases in-
cluding t-BuOK, CsCO3, K2CO3 and NaOAc, gave infe-
rior results (Table 1, Entries 2－5), and either increasing
or decreasing the amount of t-BuOLi did not give a better
yield than the use of t-BuOLi (2.0 equiv.) (Table 1, En-
tries 6－8). Furthermore, when other iodine sources in-
cluding tetrabutylammonium iodide (TBAI), 2-iodoxy-
benzoic acid (IBX), PhI(OAc)2 or N-iodosuccinimide
(NIS) were applied, the yield of 3a dramatically de-
creased (Table 1, Entries 9－12). With regard to the sol-
vents, CH3CN, 1,2-dichloroethane (DCE), N,N-dimethyl-
formamide (DMF), dimethyl sulfoxide (DMSO) and di-
oxane failed to afford a better result (Table 1, Entries 13
－17). Employing the different reaction temperature and
reaction time did not provide significant improvements to
reaction yields (Table 1, Entries 18－20).
With the optimized reaction conditions in hand, a
number of amidine substrates were tested and the results
are illustrated in Table 2. Satisfactorily, the transforma-
tion was amenable to a series of N-substituted amidines
affording the corresponding products 3 in moderate to
good yields ranging from 52% to 79%. Generally, lower
yields were obtained when R1 or R2 was electron-with-
drawing substituted group (Table 2, Entries 2－4). On
the other hand, the cyclization reaction afforded the de-
sired products in higher yields with disubstituted ami-
dines (Table 2, Entries 7 and 10－12) and with elec-
tron-donating group at the R2 position (Table 2, Entries
5 or 9).
4
K2CO3 (2.0)
I2
5
NaOAc (2.0)
t-BuOLi (1.0)
t-BuOLi (1.5)
t-BuOLi (2.5)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
t-BuOLi (2.0)
I2
6
I2
7
I2
8
I2
9
TBAI
IBX
10
11
12
13
14
15
16
17
18
19
PhI(OAc)2 70
NIS
I2
70
70
70
70
70
CH3CN
DCE
56
50
Nr
Nr
I2
I2
DMF
I2
DMSO
I2
70 Dioxane Nr
I2
60
80
Toluene 69
I2
Toluene 71
39c
Toluene
72d
64e
Toluene
43f
20c,d
t-BuOLi (2.0)
I2
70
21e,f
22
t-BuOLi (2.0)
t-BuOLi (2.0)
I2
70
70
－
Toluene Nr
a Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), base, addi-
tive (1.0 equiv.), toluene (2.0 mL), 70 ℃ in an oil bath for 5 h,
unless otherwise stated. Isolated yields. c Reacting for 2.5 h.
b
d Reacting for 7 h. Under O2 atmosphere. Adding 4 Å MS (100
e
f
mg).
clization, and either electron-donating groups (CH3 and
OCH3) or electron-withdrawing groups (F, Cl and Br)
were well compatible with this reaction with yields 54%
－72%. Good yields were obtained with electron-with-
drawing group at the para-position. Moreover, the low-
er yield of the corresponding product was obtained
while the stronger electron-withdrawing group was
Furthermore, N-tosyl-hydrazones with different
function groups on the aryl ring were tested for this cy-
364
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© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, 34, 363—367

A Transition-Metal-Free Synthesis of Multisubstituted Imidazoles
Table 2 Substrate scope of amidinea
effect.
H
R1
NNHTs
N
NH
H
N
I2 (1.0 equiv.)
NNHTs
I2 (1.0 equiv.)
t-BuOLi (2.0 equiv.)
Toluene, 70 oC, 5 h
NH
+
t-BuOLi (2.0 equiv.)
+
R2
Toluene, 70 oC, 5 h
O
R3
1a
1m
2a
1f
2a
2l
R1
O
N
N
(1)
R3
N
N
3n
3y
R2
3a
3m
Br
O
Entry
1
Compound 1
R1, R2
H, H
Product Yieldb/%
Ph
72
53
56
52
68
57
74
59
62
79
60
66
55
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
N
2
3-Cl, H
4-Cl, H
H, 2-Cl
H, 3-ethyl
4-CH3, H
N
N
N
N
Ph
N
N
3
N
Ph
Ph
Ph
4
3n, 57%
3o, 65%
3p, 63%
3q, 72%
5
F
Cl
NO2
6
7
4-CH3, 4-CH3
4-OCH3, H
H, 4-CH3
1g
1h
1i
3g
3h
3i
8
N
N
N
N
N
9
N
N
N
Ph
10
11
12
13
4-CH3, 4-OCH3
3-CH3, 4-Cl
4-CH3, 4-Br
2-CH3, H
1j
3j
Ph
Ph
Ph
1k
1l
3k
3l
3u, 67%
3r, 0%
3t, 54%
3s, 60%
1m
3m
O2N
a Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), t-BuOLi
(2.0 equiv.), I2 (1.0 equiv.), toluene (2.0 mL), 70 ℃ in an oil
bath for 5 h. b Isolated yields.
N
N
N
N
N
N
Ph
N
N
Ph
Ph
Ph
employed, which indicated that the reaction was more
electronic effect sensitive when the C－H amination
was undertaken (Eq. 1, 3q, 3t and 3u). The yield was
higher with CH3 at the para-position (Eq. 1, 3p) than
that of the meta-position (Eq. 1, 3s), in addition, the use
of substrate with bulky aromatic group such as 2'-aceto-
naphthone N-tosyl-hydrazone (2y) offered the yield of
47% (Eq. 1, 3y), which indicated the steric hindrance
affected the formation.
3x, 31%
3y, 47%
3v
, 0%
3w, 27%
NNHTs
H
N
NH
I2 (1.0 equiv.)
t-BuOLi (2.0 equiv.)
+
Toluene, 70 oC, 5 h
2m
1a
Encouragingly, the reaction of aliphatic ketones such
as 2-butanone (2j) and methyl isobutyl ketone (MIEK,
2k) proceeded smoothly to produce the desired products
4-ethyl-2-phenyl-1-(p-tolyl)-1H-imidazole (3w) and
4-isobutyl-2-phenyl-1-(p-tolyl)-1H-imidazole (3x) with
yield 27% and 31%, respectively (Eq. 1, 3w and 3x).
Substrate with CH3 at the ortho-position (2m) provided
an acceptable yield of 62% when reacted with
N-phenylbenzimidamide (Eq. 2). However, substrates
with other groups including 4-nitro- and 2-nitro- did not
give satisfying results (Eq. 1, 3r and 3v), even prolong-
ing reaction time or increasing temperature, which was
probably caused by the strong electron-withdrawing
N
(2)
N
3z
On the basis of aforementioned results and those re-
ported in the literature,[51-59] a plausible mechanism is
proposed, as outlined in the Scheme 3. Firstly, the oxi-
dative iodination of the N-tosylhydrazone 2b affords an
iodo-substituted intermediate B, which can be subse-
quently converted to the azoalkene C, with a molecular
HI leaving at the same time.[51] Then, the intermediate A,
which can be generated from the substrate amidine 1a
Chin. J. Chem. 2016, 34, 363—367
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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365

Wu et al.
COMMUNICATION
Scheme 2 Proposed mechanism
H+
Ph
Ba
se
Ph
NH
1a
Ph
Ph
-
N
HN
HN
A
Ph
NNHTs
Ph
Ph
N
I
N
I2
N
Ref. 51
N
Ph
Ph
Ph
3
Ts
C
NNHTs
B
HI
I2
Base
NH
oxidation
Ph
Ph
N
N2
N
N
Ph
I2
Ph
Ph
D
Ph
Ph
HN
H+
N
Ph
I-, N2
Ph
E
releasing a proton on the effect of the base, attatches the
azoalkene C to generate the diazo compound D.[51-56]
Subsequent I2-mediated oxidation of D gives the corre-
sponding radical cation E. Meanwhile, the N2 leaves
upon heating in the presence of the base. An intra-
molecular cyclization can undergo to furnish the dihy-
droimidazole derivative F. Further oxidative aromatiza-
tion of F leads to 1,2,4-triphenyl-1H-imidazole 3 as the
target product.[51,57-58]
21372102).
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