Synthesis of 2-Bromoimidazoles from Alkynes, N-Sulfonylazides, and Bromocyanides

Article abstract of DOI:10.1021/acs.orglett.5b00977

(Chemical Equation Presented) A synthetic method for 2-bromoimidazoles is developed from Rh-catalyzed cyclization of N-sulfonyl-1,2,3-triazoles with bromocyanides. Cu-catalyzed [3 + 2] cycloaddition followed by Rh-catalyzed cyclization starting from alkyn

Full text of DOI:10.1021/acs.orglett.5b00977

Letter
pubs.acs.org/OrgLett
Synthesis of 2‑Bromoimidazoles from Alkynes, N‑Sulfonylazides, and
Bromocyanides
Eunsook Lee, Taekyu Ryu, Eunji Shin, Jeong-Yu Son, Wonseok Choi, and Phil Ho Lee*
†
†
National Creative Research Initiative Center for Catalytic Organic Reactions, Department of Chemistry, Kangwon National
University, Chuncheon 200-701, Republic of Korea
*
S Supporting Information
ABSTRACT: A synthetic method for 2-bromoimidazoles is developed from Rh-catalyzed cyclization of N-sulfonyl-1,2,3-
triazoles with bromocyanides. Cu-catalyzed [3 + 2] cycloaddition followed by Rh-catalyzed cyclization starting from alkynes, N-
sulfonylazides, and bromocyanides is also demonstrated for de novo synthesis of 2-bromoimidazoles in one pot. Moreover, this
work was successfully employed to introduce diverse functional groups to the 2-position of imidazoles via cross-coupling
reaction.
midazoles are a significant scaffold of azaheterocyclic
Scheme 1. Approaches for the Synthesis of 2-
Bromoimidazoles
I
compounds, which are extensively used in biologically active
1
2
compounds, in ionic liquids, and as precursors of stable carbene
3
ligands. Consequently, development of synthetic routes of
highly functionalized imidazoles from easily accessible starting
materials has been a continuing challenge in modern organic
4
synthesis, regardless of the many reported methods. In
particular, because 2-bromoimidazoles can be applied in the
introduction of diverse functional groups to imidazole ring via
cross-coupling reactions, the discovery of novel synthetic
approaches to such heterocycles remains a formidable challenge.
To date, a myriad of 2-bromoimidazoles could be synthesized by
functionalization of a preformed imidazole nucleus or 1,3-
diazacyclopentane derivatives possessing two nitrogen atoms at
the 1,3-position (Scheme 1): reaction of tetrabromomethane
with 2-lithioimidazole derivatives obtained from 1-alkylimida-
zoles or 1-alkylbenzimidazoles with a strong base such as n-BuL5i
at low temperature and t-BuOLi at high temperature (eq 1),
reaction of bromocyanide with N-alkylimidazoles in acetonitrile
6
(
eq 2), and treatment of imidazolones with phosphorus
7
oxybromide (eq 3). However, some of these synthetic methods
are limited by their low yields, severe conditions (low or high
temperature), formation of polybromide, and use of strong bases
under anhydrous conditions. In addition, we are not aware of any
reports describing the synthesis of 2-bromoimidazoles a de novo
procedure.
9
produced imidazoles. In continuation of our ongoing program
related to the synthesis of azaheterocyclic compounds using N-
1
0
sulfonyl-1,2,3-triazoles, we envisioned that the use of
bromocyanide would give 2-bromoimidazoles, and additional
valuable transformation is possible despite the risk of
Recently, a synthetic application of N-sulfonyl-1,2,3-triazoles
as precursors of α-imino Rh carbenoid has been intensively
8
investigated. In particular, Fokin and co-workers reported that
Received: April 4, 2015
Rh-catalyzed transannulation of 1,2,3-triazoles with nitriles
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
debromination. Herein, we report a synthetic method of 2-
bromoimidazoles from Rh-catalyzed cyclization of N-sulfonyl-
Scheme 2. Synthesis of 2-Bromoimidazoles Using N-Sulfonyl-
1,2,3-Triazoles and Bromocyanides
a
1,2,3-triazoles with bromocyanides (eq 4). Tandem Cu-catalyzed
[
3 + 2] cycloaddition and Rh-catalyzed cyclization from alkynes,
N-sulfonylazides, and bromocyanides is also described for de
novo synthesis of 2-bromoimidazoles in one pot (eq 5).
Moreover, this work was extended to introduction of various
functional groups to the 2-position of imidazole derivatives via a
coupling reaction (eq 6).
We initiated our studies with N-tosyl-4-phenyl-1,2,3-triazole
1a) generated from [3 + 2] cycloaddition of phenylacetylene
(
with tosyl azide in the presence of Cu(I) thiophene-2-carboxylate
1
1
(
Table 1). Although 2.0 mol % of Rh (OAc) , Rh (Oct) , and
2
4
2
4
a
Table 1. Reaction Optimization
a
Reactions were carried out with 1 (0.2 mmol) and 2 (5.0 equiv) in
b
entry
cat. (mol %)
Rh (OAc)
solvent
DCE
time (h)
yield (%)
the presence of Rh (esp) (2.0 mol %) in benzene (2.0 mL, 0.1 M) at
2
2
b
7
0 °C. 1 h.
1
2
3
4
5
6
7
8
9
12
12
12
1
(44)
(16)
0
2
4
Rh (Oct)
2
DCE
4
Rh (S-DOSP)
2
DCE
4
the cyclization of 1g (0.2 mmol, 1.0 equiv) with 2 (5.0 equiv)
using Rh (esp) (2.0 mol %) in benzene (2.0 mL, 0.1 M) at 70 °C
Rh (esp)
DCE
43
2
2
2
2
2
2
2
2
2
2
2
2
2
Rh (esp)
2
CHCl3
THF
12
12
12
12
12
1
18 (8)
(68)
(34)
(16)
0
for 0.5 h, providing 3g in 98% yield. The structure of 3g was
confirmed by X-ray crystallography.
With the optimal reaction conditions, we then explored the
scope and limitation of the present method by investigating a
wide range of substituents on the aryl group of N-[(4-
trifluoromethylbenzene)sulfonyl]-4-aryl-1,2,3-triazoles 1
Rh (esp)
2
Rh (esp)
2
1,4-dioxane
EtOAc
cyclohexane
toluene
benzene
benzene
benzene
Rh (esp)
2
Rh (esp)
2
1
1
1
1
0
1
2
3
Rh (esp)
2
50
Rh (esp)
2
1
57
(
Scheme 3). Electronic variation of substituents on the aryl
c
c
Rh (esp)
2
1
64
ring of 1 did not largely affect the reaction efficiency. For
,
d
e
Rh (esp)
2
0.5
86 (85)
a
Reactions were carried out with 1a (0.2 mmol) and 2 (3.0 equiv) in
the presence of Rh catalyst (2.0 mol %) in solvent (0.8 mL, 0.25 M) at
Scheme 3. Synthesis of 2-Bromoimidazoles Using Triazoles
and Bromocyanides
a
b
7
0 °C. NMR yield using CH Br as an internal standard. Numbers in
2 2
c d
parentheses are NMR yield of 1a. 2 (5.0 equiv) was used. Solvent
e
(
2.0 mL, 0.1 M) was used. Isolated yield of 3a.
Rh (S-DOSP) as a catalyst are totally ineffective for cyclization
2
4
of 1a with bromocyanide 2 (3.0 equiv) (entries 1−3), Rh (esp)
2
2
(
2.0 mol %) gratifyingly gave the desired product, 2-bromo-4-
phenyl-1-tosylimidazole (3a), in 43% yield in DCE (0.25 M) at
0 °C for 1 h (entry 4). Electron-deficient dirhodium complexes
such as Rh (tfa) and Rh (pfb) as catalyst are ineffective. Next, a
7
2
4
2
4
variety of solvents such as DCE, CHCl , THF, dioxane, ethyl
3
acetate, cyclohexane, toluene, and benzene were examined.
Gratifyingly, the reaction in benzene produced 3a in 57% yield
via the cyclization (entry 11). Because of the low boiling point,
excessive use of 2 (5.0 equiv) increased the product yield up to
64% (entry 12). Dilution of concentration of the reaction
solution from 0.25 to 0.1 M gave superior results (entry 13).
Next, triazoles 1 bearing a number of sulfonyl groups at N1
were investigated in the reaction with 2 (Scheme 2). A
modification of the sulfonyl groups at N1 of triazoles 1 did not
largely effect on the efficiency of the cyclization. Methane- and
isopropanesulfonyl triazoles provided the 2-bromoimidazoles 3b
and 3c in 90% and 86% yields, respectively. N-[(4-
Methoxybenzene)sulfonyl]-1,2,3-triazole 1d is slightly less
reactive. In contrast, (benzenesulfonyl)triazoles possessing
electron-withdrawing groups such as chloro, nitro, and
trifluoromethyl afforded the desired 2-bromoimidazoles 3e, 3f,
and 3g in excellent yields. The optimal result was achieved from
a
Reactions were carried out with 1 (0.2 mmol) and 2 (5.0 equiv) in
the presence of Rh (esp) (2.0 mol %) in benzene (2.0 mL, 0.1 M) at
70 °C for 30 min. 2 (10.0 equiv) was used. N-Sulfonylimine of 1-
hexanal.
2
2
b
c
B
DOI: 10.1021/acs.orglett.5b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
instance, 4-aryl-1,2,3-triazoles having electron-donating methyl
and methoxy groups were subjected to the cyclization, affording
the corresponding 2-bromoimidazoles in good to excellent yields
ranging from 75% to 94%. Also, electron-withdrawing chloro and
bromo groups delivered the desired products. 4-Nitro- and 4-
trifluoromethyl-substituted 4-aryltriazoles are applicable to the
present transformation, leading to 2-bromoimidazoles 3p and 3q
in 80% and 97% yields, respectively. To our delight, Rh-catalyzed
cyclization using thiophene-3-yl substituted triazole 1r took
place to afford 3r in 60% yield even with use of 2 (10 equiv).
However, when n-butyltriazole 1s was subjected to Rh catalyst,
the corresponding bromoimidazole 3s was produced in 44%
yield together with the N-sulfonylimine of 1-hexanal in 48% yield
Scheme 5. Suzuki Reaction of 2-Bromoimidazoles
a
10b,12
Reaction conditions: 5 mol % of Pd(PPh ) , Na CO (3.2 equiv) and
g (0.2 mmol) and 7 (1.06 equiv) in benzene (0.6 mL) and ethanol
60 μL) at 80 °C for 20 h. 3a (R = 4-CH C H SO ) (0.2 mmol) was
through β-hydride migration.
Although 5-phenyltriazole
3
4
2
3
3
(
was not totally ineffective, 4,5-disubstituted triazole 1t turned out
to be compatible with the optimal reaction conditions, affording
b
1
3 6 4 2
used.
13
2-bromoimidazole 3t in 85% yield.
When a wide range of terminal alkynes was subjected to
afforded a variety of 2-arylated imidazoles in yields ranging from
reaction with bromocyanide to demonstrate the practicability of
the one pot process, the corresponding 2-bromoimidazoles were
gratifyingly produced in good yields.
14
7
4% to 93%.
-(p-Methoxyphenyl)- and 2-isopropylsulfenylated imidazoles
0a and 10b were produced in good yields from the reaction of 2-
2
1
15
bromoimidazole (3g) with indium tris(organothiolates) (9).
Imidazoles having a bromo group at the 2-position provide a
chance for further functionalization to access diverse imidazole
derivatives (Scheme 4). When 2-bromoimidazole 3g was reacted
In summary, Rh-catalyzed cyclization of N-sulfonyl-1,2,3-
triazoles with bromocyanides was developed for the synthesis of
a
Scheme 4. Sonogashira Reaction of 2-Bromoimidazoles
2
-bromoimidazoles. Sequential Cu-catalyzed [3 + 2] cyclo-
addition and Rh-catalyzed cyclization starting from alkynes, N-
sulfonylazides, and bromocyanides is also demonstrated for de
novo synthesis of 2-bromoimidazoles in one pot. Moreover, this
work was successfully employed to introduce diverse functional
groups to the 2-position of imidazole derivatives via a cross-
coupling reaction.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Experimental procedures, characterization data, X-ray crystallog-
raphy data (3g), and copies of NMR spectra for all products. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.orglett.5b00977.
a
Reaction conditions: 5 mol % of PdCl (PPh ) , 6 mol % of CuI,
2
3 2
3
g(0.2 mmol), and 4 (1.5 equiv) in Et N (0.67 mL, 0.3 M) at 80 °C
3
b
1
for 12 h. 3a (R = 4-CH C H SO ) (0.2 mmol) was used.
3
6
4
2
AUTHOR INFORMATION
■
with phenylacetylene under Sonogashira reaction conditions
PdCl (PPh ) , CuI, Et N], the alkynylated imidazole 6a was
Corresponding Author
*E-mail: phlee@kangwon.ac.kr.
Author Contributions
[
2
3 2
3
obtained in 90% yield. N-Tosyl-substituted bromoimidazole 3a
was smoothly converted to the coupled product 6b in 84% yield.
The substituents on the aryl group of 4 have no effect on the
efficiency of the reaction. Those arylacetylenes 4 with electron-
donating and -withdrawing substituents on the aryl ring were
treated with 2-bromo-4-phenylimidazoles 3 to furnish the
desired products 6c−h in good to excellent yields ranging from
†
These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
8
4% to 95%.
■
The Suzuki reaction of 2-bromoimidazole 3g with phenyl-
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(2014001403). This study was also supported by a 2014
Research Grant from Kangwon National University (No.
120140199).
boronic acid produced the corresponding N-sulfonyl-2,4-
diphenylimidazole 8a in quantitative yield (Scheme 5).
Electronic variation of substituents on the aryl ring of arylboronic
acid 7 did not significantly affect the reaction efficiency and
C
DOI: 10.1021/acs.orglett.5b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
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DEDICATION
■
Dedicated to Professor Sunggak Kim, Ewha Womans University,
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D
DOI: 10.1021/acs.orglett.5b00977
Org. Lett. XXXX, XXX, XXX−XXX