A Facile FeCl3/I2-Catalyzed Aerobic Oxidative Coupling Reaction: Synthesis of Tetrasubstituted Imidazoles from Amidines and Chalcones

Article abstract of DOI:10.1021/acs.orglett.5b01854

A facile and efficient route for the synthesis of tetrasubstituted imidazoles from amidines and chalcones via FeCl₃/I₂-catalyzed aerobic oxidative coupling has been developed. This new strategy is featured by high regioselectivity an

Full text of DOI:10.1021/acs.orglett.5b01854

Letter
pubs.acs.org/OrgLett
A Facile FeCl /I ‑Catalyzed Aerobic Oxidative Coupling Reaction:
3
2
Synthesis of Tetrasubstituted Imidazoles from Amidines and
Chalcones
†
†
†
†
†
‡
†
†
Yuelu Zhu, Cheng Li, Jidong Zhang, Mengyao She, Wei Sun, Kerou Wan, Yaqi Wang, Bin Yin,
†
,†
Ping Liu, and Jianli Li*
†Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education and College of Chemistry &
Materials Science, Northwest University, Xi’an Shaanxi 710127, P. R. China
‡Xi’an Catalyst Chemical Co., Ltd., Xi’an Shaanxi 710016, P. R. China
*
S Supporting Information
ABSTRACT: A facile and efficient route for the synthesis of
tetrasubstituted imidazoles from amidines and chalcones via FeCl3/
I2-catalyzed aerobic oxidative coupling has been developed. This new
strategy is featured by high regioselectivity and yields, good
functional group tolerance, and mild reaction conditions.
midazoles, one of the most valuable heterocyclic com-
functionalization of amidines with chalcones. On the basis of
the above hypothesis, we present here the first successful
attempt on a FeCl /I -catalyzed aerobic oxidative coupling of
amidines with chalcones for synthesizing tetrasubstituted
imidazoles.
We began our study by taking N-phenylbenzamidine (1a)
and chalcone (2a) as the model substrates. The results are
summarized in Table 1. Delightedly, when the reaction was
1
I
pounds, have been found in many natural products and
2
widely applied in functional materials. In particular, they also
3
2
3
4
have good pharmacological activities, such as antifungal,
5
6
7
antitumor, antibacterial, antiplasmodium, and anti-inflamma-
tory. Current synthesis methods for imidazole derivatives are
mainly restricted to less-substituted imidazoles, while only a
8
9
handful of them provided access to tetrasubstituted imida-
1
0
zoles. Particularly during the last decades, several new
methodologies have emerged, such as cross-coupling of
carried out employing a combination of FeCl
(10 mol %) in toluene at 110 °C under air atmosphere, the
desired product was obtained in 56% (Table 1, entry1).
Control experiments showed the necessities of both FeCl and
for this reaction, without any resulting in the lack of reactivity
(Table 1, entries 2−3). Lower yields were obtained by use of
Fe(NO ·9H O, FeBr , CuCl , and Cu(OAc) instead of
FeCl
3
(20 mol %) and
I
2
1
1
aldimines, the coupling reaction of 2-azido acrylates and
1
2
nitrones, Cu-catalyzed cycloaddition of amidines and nitro-
3
1
3
14
olefins, the three-component reaction,1 Ni-catalyzed dehy-
I
2
5
drogenation of benzylic-type imines, and Zn-catalyzed
cyclization of 2-(tetrazol-5-yl)-2H-azirines and imines.
1
6
)
3
3
2
3
2
2
However, most approaches encounter some drawbacks,
including high catalyst loading, substrates unavailable, limited
applications, long reaction time or low yields under harsh
reaction conditions. Moreover, most substrates are limited to
3
(Table 1, entries 4−7). Other iodide ion sources, such as
NaI, KI, and ZnI were also examined, resulting in a sharp
2
decrease of yield (Table 1, entries 8−10). Other common
solvents, such as DMSO, DMF, DMA, chlorobenzene, 1,2-
dichloroethane (1,2-DCE) and dioxane, were also screened,
and the results showed that chlorobenzene was the most
suitable solvent for this reaction (Table 1, entries 11−16). After
further elaboration of the reaction conditions (Table 1, entries
1
7
1
,2-diketone, aldehydes and primary amines, so those
procedures restrict the synthesis of tetrasubstituted imidazoles
and usually involve poor regioselectivity. Therefore, a novel,
practical and efficient protocol for a straightforward con-
struction of tetrasubstituted imidazoles remains highly desir-
able.
1
7−20), including varying temperature and catalyst loading, the
yield of desired product was improved to 85% when oxygen gas
was used as a oxidant (Table 1, entry 17).
Recently, the direct oxidative C−H bond functionalization by
iron and iodine has gained considerable attention, and is an
excellent way to form various heterocycles from the readily
With the optimized conditions in hand, the scope of various
N-arylbenzamidines with different substituents on one or both
phenyl rings were tested (Scheme 1). With regard to the
electronic effects, it was found that electron-donating
substituents arylamidines (3ba, 3ea, 3ia and 3ja) showed
better reactivities and gave higher yields than electron-
1
8
available reactants. This research shows that, compared to
rare metal catalyst, the iron and iodine not only are available,
inexpensive, and environmentally benign catalysts, but also can
offer complementary selectivity and impressive reactivity.
Therefore, these results encouraged us to hypothesize that it
may be a new protocol for the synthesis of tetrasubstituted
imidazoles through iron and iodine cocatalyzed oxidative C−H
Received: June 27, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01854
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 1. Substrate Scope of Amidines
b
entry
catalyst (mol %)
solvent
yield (%)
1
2
3
4
5
6
7
8
9
FeCl (20), I (10)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMSO
DMF
56
0
3
2
I2 (10)
FeCl (20)
0
3
Fe(NO ) ·9H O (20), I (10)
51
48
37
53
32
43
39
21
23
27
72
37
33
85
53
81
58
3
3
2
2
FeBr (20), I (10)
3
2
CuCl (20), I (10)
2
2
Cu(OAc) (20), I (10)
2
2
FeCl (20), NaI (20)
3
FeCl (20), KI (20)
3
10
11
12
13
14
15
16
17
18
19
20
FeCl (20), ZnI (10)
3
2
FeCl (20), I (10)
3
2
FeCl (20), I (10)
3
2
FeCl (20), I (10)
DMA
3
2
FeCl (20), I (10)
PhCl
3
2
FeCl (20), I (10)
1,2-DCE
dioxane
PhCl
3
2
FeCl (20), I (10)
3
2
c
c
c
c
FeCl (20), I (10)
3
2
FeCl (10), I (5)
PhCl
3
2
,
,
d
e
FeCl (20), I (10)
PhCl
3
2
FeCl (20), I (10)
PhCl
3
2
a
Reaction conditions: 0.6 mmol of 1a and 0.5 mmol of 2a in the
b
presence of catalyst in solvent (2 mL) for 10 h in air. Isolated yields.
c
d
Reaction performed under O balloon. The reaction was carried out
2
a
e
Reaction conditions: 1 (0.6 mmol), 2a (0.5 mmol), FeCl (20 mol
3
at 120 °C. The reaction was carried out at 90 °C.
%
yields.
), I (10 mol %), PhCl (2 mL), O balloon, 110 °C, 10 h; isolated
2
2
withdrawing ones (3ga, 3ha, 3la and 3ma). It should benoted
that the steric hindrance of the substrates almostly have no
effect on the yields (3ca, 3da, 3pa and 3ra). Notably, N-
phenylthiophene-2-carboximidamide could also be well tol-
erated, and the yield of the corresponding product (3na) is
a
Scheme 2. Substrate Scope of Chalcones
7
4%. And substrate scope not limited to arylated benzamidines,
C-alkylimidazole 3ta was isolated in reasonable yields by using
this new methodology. To our delight, the structure of 3na and
3
sa were unambiguously confirmed by X-ray diffraction
analysis. On the whole, for N-arylbenzamidines, these reactions
displayed high functional group tolerance and gave the desired
products in moderate to high yields.
Furthermore, chalcones with different substituents were also
investigated, and satisfactorily, imidazoles could be obtained
smoothly with good results (Scheme 2). When the similar
groups are located on the phenyl rings whatever adjacent to
carbonyl groups or connected to C−C double bond, they
groups had no significant impact on the yields which are high
(
from 72 to 85%). But the substrates with electron-donating
group substituted on phenyl rings (3ae, 3af, 3ag, and 3ak)
show better reactivities and give higher yields than those with
electron-withdrawing group ones (3ah, 3ai, 3aj, and 3al).
Besides, chalcones bearing aromatic heterocycles are also
suitable for the oxidative coupling reactions, such as 3am in
a
Reaction conditions: 1a (0.6 mmol), 2 (0.5 mmol), FeCl (20 mol
3
%
yields.
), I (10 mol %), PhCl (2 mL), O balloon, 110 °C,10 h; isolated
2
2
7
6% yield.
Following our synthetic studies, some controlled experiments
were carried out to gain insight into the reaction mechanism
Scheme 3). No significant decrease in yield was observed
when a radical scavenger TEMPO (1.5 equiv) was added to the
reaction, thus a radical process is probably unlikely to be
involved (Scheme 3, eq A). The reaction under argon
atmosphere delivered imidazolidine 4aa in 14% yield as the
only product, which indicates that the oxygen is critical to the
formation of imidazole and the turnover of the catalytic cycle
(Scheme 3, eq B). Furthermore, when the reactions were
quenched at 2 h, imidazoline 4aa was isolated as the major
(
B
DOI: 10.1021/acs.orglett.5b01854
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Controlled Experiments
Figure 1. (a) The condensed Fukui function f− of nitrogen and
(r)
other surrounding atoms in compound B. (b) The 3D representation
product (43% yield, eq C), along with 23% of imidazole 3aa.
Extending the reaction time to 10 h, imidazoline 4aa was
almost disappeared and the desired imidazole 3aa was obtained
in 85% yield. These results suggest that the reaction proceeds
through the cyclization of the amidines and chalcones to form
the imidazoline first, followed by dehydrogenative oxidation of
the tetrasubstituted imidazoline to the imidazole.
−
of the Fukui function f ( of the iso-value of 0.003 a.u. for compound
r)
B (positive for red color and negative for green color).
and chalcones as the starting materials and tolerates a wide
range of functional groups. This operationally practical protocol
might be a useful and widely applicable method in medicinal,
organic, and material chemistry.
On the basis of the aforementioned results and the literature
1
9
reports, a plausible mechanism concerning the iron and
iodine cocatalyzed coupling between 1a and 2a is proposed in
Scheme 4. The reaction is initiated by the activation of chalcone
ASSOCIATED CONTENT
Supporting Information
■
*
S
General experimental procedure, characterization data of the
compounds, and CIF data for 3na and 3sa. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.orglett.5b01854.
Scheme 4. Plausible Reaction Mechanism
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: lijianli@nwu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
NSFC 21272184; 20972124; J1210057 and 21143010), the
■
(
(
2a) by FeCl , in the next step, the addition of N-
3
Shaanxi Provincial Natural Science Fund Project (No.
015JZ003) and the Xi’an City Science and Technology
Project (No. CXY1429(6); CXY1434(7) for financial support.
arylbenzamidines (1a) to A forming the Michael adduct B,
which was observed by HRMS. The Michael adduct B is in
tautomerization with the enol form C. Then, C reacts with I to
2
2
produce intermediate D, followed by a subsequently intra-
molecular cyclization to afford 4aa, which is oxidated to the
desired product 3aa under aerobic conditions. In the end,
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DOI: 10.1021/acs.orglett.5b01854
Org. Lett. XXXX, XXX, XXX−XXX