Copper(I)-catalyzed intramolecular direct C-arylation of azoles with aryl bromides

Article abstract of DOI:10.1002/cjoc.201300399

A concise route to access 5H-imidazo[2,1-a]isoindole heterofused compounds by copper(I)-catalyzed intramolecular coupling of non-activated aryl bromides with azoles is reported. With CuI as catalyst, 1,10-phenanthroline as ligand, and K₃PO₄ as base, the reactions of 1-(2-bromobenzyl)-1H- imidazoles in DMF/o-xylene (1:1, V:V) at 145°C afford the corresponding substituted 5H-imidazo[2,1-a]isoindoles in high yields via intramolecular C-arylation. A concise route to access 5H-imidazo[2,1-a]isoindole heterofused compounds by copper(I)-catalyzed intramolecular coupling of non-activated aryl bromides with azoles is reported for the first time. With CuI as catalyst, 1,10-phenanthroline as ligand, and K₃PO₄ as base, the reactions of 1-(2-bromobenzyl)-1H-imidazoles in DMF/o-xylene (1:1, V:V) at 145°C afford the corresponding substituted 5H-imidazo[2,1-a]isoindoles in high yields via intramolecular C-arylation. Copyright

Full text of DOI:10.1002/cjoc.201300399

FULL PAPER
DOI: 10.1002/cjoc.201300399
Copper(I)-Catalyzed Intramolecular Direct C-Arylation of
Azoles with Aryl Bromides
Yuan Huang,a,1 Wei Chen,a,1 Dan Zhao,a Chen Chen,a Huiqing Yin,a Likang Zheng,a Ming Jin,a
and Shiqing Han,*,a,b
a College of Biotechnology and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing, Jiangsu
211816, China
bKey Laboratory of Synthetic Chemistry of Natural Products, Shanghai Institute of Organic Chemistry, CAS,
Shanghai 200032, China
A concise route to access 5H-imidazo[2,1-a]isoindole heterofused compounds by copper(I)-catalyzed intra-
molecular coupling of non-activated aryl bromides with azoles is reported. With CuI as catalyst, 1,10-phenanth-
roline as ligand, and K3PO4 as base, the reactions of 1-(2-bromobenzyl)-1H-imidazoles in DMF/o-xylene (1∶1,
V∶V) at 145 ℃ afford the corresponding substituted 5H-imidazo[2,1-a]isoindoles in high yields via intramolecu-
lar C-arylation.
Keywords copper, intramolecular C-arylation, azole, aryl bromides
Introduction
iodides, under the same conditions it was totally un-
reactive with aryl bromides; secondly, the regioselectiv-
ity and functional group tolerance were significantly
limited owing to the use of strong bases like LiOBu-t.
Herein, we report the synthesis of a series of heterocy-
cle-fused azole derivatives by copper(I)-catalyzed intra-
molecular C－H arylation of azoles with aryl bromides
under mild base and compatible ligand (Scheme 1).
The transition-metal-catalyzed C－H activation for
regioselective formation of aryl-aryl bond is a powerful
method which has attracted substantial interest over the
past century.[1] When we talk about the formation of
aryl-aryl bonds, we often regard as inseparable with
expensive transition metals such as Rh,[2] Ru[3] and Pd.[4]
However, in recent years impressive progress has been
made using much cheaper and less toxic copper salts as
convenient alternative catalysts for the formation of
aryl-aryl bonds.[5] Daugulis and his co-workers[6] de-
veloped a general method for copper-catalyzed arylation
of sp2 C－H bonds with pKa’s below 35 in heterocycles
as well as polyfluoro-benzenes.[6b,6c] Moreover, a vari-
ety of electron-rich and electron-poor heterocycles such
as pyrimidine,[6] azoles, [7] thiophenes, benzofuran, pyri-
dine oxides, pyridazine, and caffeine[8] could be arylated.
Recently, intramolecular direct arylation of several
electron-rich azoles with aryl iodides for the synthesis
of a wide range of valuable heterofused molecules was
performed by SanMartin et al.[9a] Different from forma-
tion of azoles ring species by multicomponent reac-
tions[10] or palladium-catalyzed arylation of carbon-hy-
drogen bonds,[11] this protocol provided a simple and
economical way to synthesize biologically relevant tri-
and tetra-cyclic fused azoles.
Scheme 1 General strategy
R
N
R
X
N
X
20 mol% CuI
N
Br
N
1,10-phen, K3PO4
DMF/o-xylene
R'
R'
Results and Discussion
As it could be seen in Table 1, our preliminary stud-
ies focused on the conversion of 1-(2-bromobenzyl)-
1H-imidazole (1) into 5H-imidazo[2,1-a]isoindole (2) as
the model reaction. Initially, we tested CuI/LiOBu-t/
o-xylene to catalyze this reaction with aryl bromides,
however, same to San Martin’s description,[9a] there was
no desired product observed (Table 1, Entry 1). Curi-
ously, combination of 1,10-phenanthroline as ligand and
organic base, such as LiOBu-t or KoBu-t reported by
However, this method still suffered from some limi-
tations: Firstly, the system was only efficient with aryl
*
E-mail: hanshiqing@njut.edu.cn; Tel.: 0086-025-58139970; Fax: 0086-025-58139369
Received May 13, 2013; accepted June 26, 2013; published online July 19, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300399 or from the author.
1 The two authors contributed equally to this work.
Chin. J. Chem. 2013, 31, 1007—1010
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Huang et al.
Table 2 Optimization of the reaction conditions
FULL PAPER
Table 1 Screening reaction conditions for catalytic arylation
N
N
N
N
CuI (Cat.)
N
Br
N
Br
20 mol% copper source (Cat.)
N
N
1,10-Phen,
2 equiv. K3PO4,
Solvent, 24 h
20 mol% ligand,
2 equiv. base,
DMF, 135 oC, 24 h
1
2
2
1
Entrya Copper source
Ligand
Base
LiOBu-t
Yieldb/%
Catalyst
loading/mol%
Entrya
Solvent
DMF
Temp/℃ Yieldb/%
1
2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
—
—
0
0
1
2
3
4
5
6
7
8
9
20
20
135
135
135
135
135
145
155
125
145
145
145
145
56
34
60
49
72
87
76
37
24
54
62
87
1,10-Phen LiOBu-t
1,10-Phen KOBu-t
o-xylene
3
0
DMF/o-xylene (1∶2)
DMF/o-xylene (2∶1)
DMF/o-xylene (1∶1)
DMF/o-xylene (1∶1)
DMF/o-xylene (1∶1)
DMF/o-xylene (1∶1)
DMF/o-xylene (1∶1)
20
20
20
20
20
20
5
4
1,10-Phen
K2CO3
36
47
56
34
12
0
5
1,10-Phen Cs2CO3
6
1,10-Phen
Hacac
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
7
8
TMEDA
L-Proline
—
9
10
11
12
13
14
0
10 DMF/o-xylene (1∶1)
11 DMF/o-xylene (1∶1)
12 DMF/o-xylene (1∶1)
10
15
25
1,10-Phen
1,10-Phen
1,10-Phen
1,10-Phen
0
CuBr
CuCl
Cu2O
34
27
11
a
Conditions: the Schlenk test tube was charged with CuI, 20
mol% phen and 2 equiv. K3PO4. All reactions were run in
non-anhydrous solvent in N2 for 24 h. b Yield of isolated product.
a The Schlenk test tube was charged with 20 mol% of Cu salt, 20
mol% of ligand (when it is added) and 2 equiv. of base. All reac-
tions were run in non-anhydrous solvent at 135 ℃ in N2 for 24 h.
b Yield of isolated product.
and 2). Further studies showed that utilization of mixed
solvent (Table 2, Entries 3 and 4) could afford 2b in
higher yield. Finally, the best result in terms of yield
was achieved using DMF∶o-xylene (1∶1, V∶V) and
0.2 equivalent of CuI at 145 ℃ (Table 2, Entry 6). An
attempt to lower and enhance temperature or the amount
of CuI to 10 mol% (Table 2, Entries 5－12) resulted in
less efficiency.
On the basis of the above results, we concluded that
the optimized conditions for intramolecular coupling
were combination of DMF/o-xylene (1∶1) at 145 ℃
for 24 h using two equivalents of K3PO4 as the base in
the presence of a catalyst that was generated in situ from
CuI (20 mol%) and 1,10-phenanthroline (Phen, 20
mol%).
With the optimized conditions in hand, the scope of
the procedure with respect to other substrates was stud-
ied. As shown in Table 3, azoles derivatives without
substituting group or with electron donating group (i.e.
methyl-, phenyl-) exhibited greater reactivity, affording
the desired arylation products in good to excellent yields
(Table 3, Entries 1－7). This protocol was also applied
to less acidic substrates (DMSO pKa’s above 35) such as
1-(2-bromobenzyl)-4-nitro-1H-imidazole (8a), 1-(2-bro-
mobenzyl)-1H-pyrazole (9a), but both gave negligible
results (Table 3, Entries 8 and 9). It could be explained
according to the mechanism disclosed by Daugulis and
co-workers that the reactions proceed by initial depro-
tonation of a relatively acidic sp2 C－H. C－H bond
DMSO pKa’s below 35 is necessary for copper-cata-
Daugulis and co-workers for the Cu-catalyzed arylation
of aromatic C－H bond,[6a] completely failed to lead to
the formation of product under the reaction conditions
(Table 1, Entries 2 and 3). The success of C－H aryla-
tion reactions might be related not only to the strength
of the base but also to the solubility of both the base
itself[12] and the metal halides byproducts which are
formed during the processes.
Fortunately, the reaction occurred when we tried
mild inorganic base, and the reactivity was better with
K3PO4 as the base (Table 1, Entry 6) than with K2CO3
and Cs2CO3 (Table 1, Entries 4 and 5). In addition to
base, some ligands extensively employed in the cop-
per-catalyzed C-arylation were screened.[13] Notably,
the reaction using 1,10-phenanthroline was more effec-
tive in comparision with Hacac (34%), TMEDA (12%),
L-proline (0%) and without ligand (0%) (Table 1, En-
tries 7－10). As expected, no coupling reaction took
place when the reaction was carried out without the
metal catalyst (Table 1, Entry 11). Among the readily
available copper(I) compounds screened, including
Cu2O, CuCl, CuBr and CuI, the air-stable CuI had the
best performance (Table 1, Entries 6 and 12－14).
To increase product yield, the influences of the sol-
vent, catalyst loading and temperature were further in-
vestigated. As displayed in Table 2, the moderate results
were obtained in DMF or o-xylene with 20 mol% CuI,
20 mol% Phen and 2 equiv. K3PO4 (Table 2, Entries 1
1008
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Chin. J. Chem. 2013, 31, 1007—1010

Copper(I)-Catalyzed Intramolecular Direct C-Arylation of Azoles with Aryl Bromides
Table 3 Intramolecular copper-catalysed C－H arylation of
lyzed direct arylation. We then applied our protocol to
azoles with aryl bromides
9-(2-bromo-benzyl)-1,3-dimethyl-1H-purine-2,6(3H,9H)-
dione (10a), unfortunately, only substrate was recovered
(Table 3, Entry 10). When 1-(2-Bromo-4-chloro-ben-
zyl)-1H-imidazole (11a) and 1-(2-Bromo-4-chloro-ben-
zyl)-1H-benzoimidazole (12a) were used as substrates,
the yields of 11b and 12b were got in 62% and 60%
(Table 3, Entries 11 and 12).
R
R
N
N
CuI (20 mol%)
1,10-Phen (20 mol%)
X
R''
X
N
Br
R''
N
K3PO4
DMF/o-xylene (1:1, V:V)
145 o
C
R'
R'
a
b
Conclusions
Entrya
1
Product
Isolated yieldb/%
87
In conclusion, we have shown an effective protocol
for the intramolecular direct C－H arylation of azoles
with aryl bromides in the presence of less expensive
copper salt catalyst and mild inorganic bases. Mean-
while, the reactions are efficient affording the coupled
products with good to excellent yields. To the best of
our knowledge, it is the first example of copper-medi-
ated C－H functionalization with aryl bromides to build
fused azoles derivatives. The method also provides an
easy access to the synthesis of 5H-imidazo[2,1-a]-iso-
indoles heterofused compounds which are not readily
available by conventional methods. This synthetic ap-
proach may find applications in straightforward prepa-
ration of other novel fused heterocycles. Further studies
to extend the application of this method to other func-
tionalized compounds construction are currently in pro-
gress in our laboratories.
N
1b
2b
N
N
N
2
85
N
3
4
95
77
3b
4b
N
N
N N
N
5
67
5b
N
Experimental
N
6
7
58
65
6b
7b
A Bruker AM-300 MHz instrument (Brucker, Swit-
zerland) was used to acquire 1H NMR spectra with TMS
as internal standard. Chloroform-D was used as solvents.
Low resolution mass spectra (LRMS) were recorded on
an HP-5989A instrument (HP, U.S.A.). An Elementar
Vario ELⅢinstrument (Elementar, Germany) was used
to acquire data of elemental analysis. Melting points
were determined with WRS-2A.
Unless otherwise noted, all reagents were obtained
from commercial suppliers and used without further
purification. Solvents were dried by refluxing for at
least 24 h over CaH2 (DMF), or sodium (o-xylene), and
freshly distilled prior to use. All syntheses and manipu-
lations were carried out under N2 atmosphere.
N
N
N
N
8
9
0
0
8b
9b
O2N
N
N
N
O
N
N
N
N
10
0
10b
O
General procedure for preparation of compounds
(1b－7b, 11b, 12b)
Cl
A flame-dried Schlenk test tube with a magnetic
stirring bar was charged with CuI (38 mg, 0.2 mmol),
1,10-phenanthroline (36 mg, 0.2 mmol), K3PO4 (424 mg,
2.0 mmol), 1a－7a (1.0 mmol) and DMF/o-xylene (1∶
1) (2 mL) under N2. A rubber septum was replaced with
a glass stopper, and the system was then evacuated
twice and back filled with N2. The reaction mixture was
stirred for 10 min at room temperature, and then heated
at 145 ℃ for 24 h. The reaction mixture was then
cooled to ambient temperature, diluted with 3－4 mL of
CH2Cl2, filtered through a plug of silica gel, and washed
N
11
12
62
60
11b
12b
N
N
Cl
N
a
Conditions: the Schlenk test tube was charged with 20 mol%
CuI, 20 mol% phen and 2 equiv. K3PO4 in DMF∶o-xylene (1∶1,
V∶V). All reactions were run in non-anhydrous solvent in N2 at
145 ℃ for 24 h. b Yield of isolated product.
Chin. J. Chem. 2013, 31, 1007—1010
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Huang et al.
FULL PAPER
with 10－20 mL of CH2Cl2. The combined organic ex-
tracts were concentrated and the resulting residue was
purified by column chromatography on silica gel to
provide the desired product 1b－7b, 11b, 12b.
1H, ArH), 7.17 (s, 1H, ArH), 7.21 (dd, J＝8.2, 2.1 Hz,
1H, ArH), 7.29 (d, J＝8.0 Hz, 1H, ArH), 7.73 (d, J＝2.0
Hz, 1H, ArH). MS m/z: 191.0 [M＋H]＋.
3-Chloro-11H-benzo[4,5]imidazo[2,1-a]isoindole
5H-Imidazo[5,1-a]isoindole (1b)[9a] White solid.
M.p. 195－198 ℃ (from hexane); 1H NMR (300 MHz,
CDCl3) δ: 5.00 (s, 2H, CH2), 7.18 (s, 1H, ArH), 7.24 (dd,
J＝7.5, 7.5 Hz, 1H, ArH), 7.37 (dd, J＝8.1, 8.1 Hz, 1H,
1H, ArH), 7.39 (d, J＝7.5 Hz, 1H, ArH), 7.55 (d, J＝7.2
Hz, 1H, ArH), 7.72 (s, 1H, ArH); MS m/z: 156.07 [M＋
H]＋. Anal. calcd for C10H8N2: C 76.90, H 5.16, N 17.94;
found C 76.77, H 5.11, N 17.84.
(12b)[9b] Yellow solid. M.p. 192 －194 ℃ (from
1
hexane); H NMR (300 MHz, CDCl3) δ: 5.12 (s, 2H,
CH2), 7.01 (s, 1H, ArH), 7.05 (s, 2H, ArH), 7.38－7.50
(m, 1H, ArH), 7.52 (s, 1H, ArH), 7.70 (d, J＝7.5 Hz, 2H,
ArH). MS m/z: 241.5 [M＋H]＋.
Acknowledgement
This work was supported by grants from Natural
Science Foundation of China (Grant Nos. 20942006,
21072095), National High Technology Research and
Development Program of China (863 Program
2011AA02A209) and Program for Changjiang Scholars
and Innovative Research Team in University (Grant No.
IRT1066).
11H-Isoindolo[2,1-a]benzimidazole
(2b)[9a]
White solid. M.p. 190－192 ℃ (from hexane); 1H
NMR (300 MHz, CDCl3) δ: 5.03 (s, 2H, CH2), 7.24－
7.29 (m, 3H, ArH), 7.45－7.54 (m, 3H, ArH), 7.80－
7.90 (m, 1H, ArH), 8.08 (d, J＝7.5 Hz, 1H, ArH); MS
m/z: 207.1 [M＋H]＋. Anal. calcd for C14H10N2: C 81.53,
H 4.89, N 13.58; found C 81.44, H 4.91, N 13.47.
2-Methyl-5H-imidazo[2,1-a]isoindole (3b) White
solid. M.p. 182－185 ℃ (from hexane); 1H NMR (300
MHz, CDCl3) δ: 2.28 (s, 3H, CH3), 4.95 (s, 2H, CH2),
6.82 (s, 1H, ArH), 7.33－7.44 (m, 2H, ArH), 7.58－
7.66 (m, 2H, ArH); MS m/z: 171.8 [M＋H]＋. Anal.
calcd for C11H10N2: C 77.62, H 5.92, N 16.46; found C,
77.54, H 5.99, N 16.40.
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MHz, CDCl3) δ: 5.28 (s, 2H, CH2), 7.53－7.56 (m, 2H,
ArH), 7.69－7.72 (dd, J＝3, 3.9 Hz, 1H, ArH), 7.83－
7.86 (dd, J＝3.9, 3.6 Hz, 1H, ArH), 8.17 (s, 1H, ArH);
MS m/z: 158.1 [M＋H]＋. Anal. calcd for C9H7N3: C
68.78, H 4.49, N 26.74; found C 68.78, H 4.51, N 26.53.
2,3-Diphenyl-5H-imidazo[2,1-a]isoindole
(5b)
1
Yellow solid. M.p. 143－146 ℃ (from hexane); H
NMR (300 MHz, CDCl3) δ: 5.06 (s, 2H, CH2), 6.88－
6.90 (m, 1H, ArH), 7.17－7.25 (m 7H, ArH), 7.36－
7.43 (m, 5H, ArH), 7.95 (s, 1H, ArH); MS m/z: 310.1
[M＋H]＋. Anal. calcd for C22H16N2: C 85.69, H 5.23, N
9.08; found C 85.64, H 5.17, N 9.05.
2-Phenyl-5H-imidazo[2,1-a]isoindole
(6b)
1
Yellow solid. M.p. 172－175 ℃ (from hexane); H
NMR (300 MHz, CDCl3) δ: 5.22 (s, 2H, CH2), 6.05－
6.42 (m, 6H, ArH), 7.67－7.82 (m, 4H, ArH); MS m/z:
234.1 [M＋H]＋. Anal. calcd for C16H12N2: C 82.73, H
5.21, N 12.06; found C 82.47, H 5.27, N 12.01.
7,8-Dimethyl-11H-isoindolo[2,1-a]benzimidazole
(7b)[9a]
White solid. M.p. 221 －224 ℃ (from
1
hexane); H NMR (300 MHz, CDCl3) δ: 2.36 (s, 6H,
2CH3), 5.43 (s, 2H, CH2), 6.46－6.50 (s, 1H, ArH), 6.95
(s, 1H, ArH), 7.15－7.57 (m, 3H, ArH), 7.67 (s, 1H,
ArH); MS m/z: 235.1 [M ＋H] ＋ . Anal. calcd for
C16H14N2: C 82.02, H 6.02, N 11.96; found C 82.04, H
6.03, N 11.98.
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8-Chloro-5H-imidazo[2,1-a]isoindole
(11b)[9b]
White solid. M.p. 197－199 ℃ (from hexane); 1H
NMR (300 MHz, CDCl3) δ: 4.78 (s, 2H, CH2), 7.14 (s,
(Zhao, X.)
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Chin. J. Chem. 2013, 31, 1007—1010