Palladium- and copper-mediated direct C-2 arylation of azoles - Including free (NH)-imidazole, -benzimidazole and -indole - Under base-free and ligandless conditions

Article abstract of DOI:10.1002/ejoc.200500957

The first palladium- and copper-mediated C-2 arylations of thiazole, oxazole, N-methylimidazole and N-arylimidazoles, as well as of free (NH)-imidazole, -benzimidazole and -indole, with aryl iodides under ligandless and base-free conditions are described. Complete selectivity has been achieved under these unprecedented conditions, which allow the use of substrates containing base-sensitive groups, such the NH groups of imidazole, benzimidazole or indole, without their prior protection. No N-arylation products were detected in the arylation of free (NH)-imidazole, -benzimidazole and -indole. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200500957

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200500957
Palladium- and Copper-Mediated Direct C-2 Arylation of Azoles − Including
Free (NH)-Imidazole, -Benzimidazole and -Indole − Under Base-Free and
Ligandless Conditions
[a]
[a]
[a]
Fabio Bellina,* Silvia Cauteruccio, and Renzo Rossi*
Keywords: C–C coupling / Synthetic methods / Regioselectivity / Palladium / Copper
The first palladium- and copper-mediated C-2 arylations of
thiazole, oxazole, N-methylimidazole and N-arylimidazoles,
as well as of free (NH)-imidazole, -benzimidazole and -in-
dole, with aryl iodides under ligandless and base-free condi-
tions are described. Complete selectivity has been achieved
under these unprecedented conditions, which allow the use
of substrates containing base-sensitive groups, such the NH
groups of imidazole, benzimidazole or indole, without their
prior protection. No N-arylation products were detected in
the arylation of free (NH)-imidazole, -benzimidazole and -
indole.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
DMF at 140 °C under ligandless conditions.[9b] This work
encouraged us to investigate more deeply the most suitable
reaction conditions under which to perform direct Pd- and
Cu-mediated C-2 arylation of 1-aryl-1H-imidazole and
other azoles, together with their mechanisms. Here we re-
port that π-electron-sufficient heteroarenes such as oxazole,
thiazole, 1-aryl-1H-imidazoles and 1-methyl-1H-imidazole,
as well as free (NH)-imidazole, -benzimidazole and -indole,
can be effectively transformed in highly selective Pd- and
Introduction
The π-electron-sufficient heteroarenes oxazoles, thi-
azoles, imidazoles and benzimidazoles are important struc-
[
1]
tural units frequently found in natural products, pharma-
cologically active substances,[ agrochemicals,
2]
[3]
com-
pounds used to treat endoparasitic diseases of domestic ani-
[
4]
mals and organic functional materials such as liquid crys-
[
5]
tals and fluorescent dyes. The complicated structures and
biological importance of some of these heterocyclic deriva-
tives have provided a challenge to modern synthetic meth-
odology, and in the last fifteen years the direct functionaliz-
ation of C–H bonds of five-membered heterocycles through
selective transition metal-catalysed C-arylation with aryl
Cu-mediated C-2 arylation under base-free and ligandless
[12]
conditions.
These experimental conditions are very dif-
ferent from those employed so far for performing the transi-
tion metal-catalysed arylation of π-excessive or π-sufficient
[6–10]
heteroarenes, which invariably involve the use of a base.
halides has been developed as a very attractive and practical
Moreover, these unprecedented conditions allow the use of
substrates containing base-sensitive groups – such as the
NH groups in imidazole, benzimidazole and indole – with-
out their prior protection.
synthetic methodology.[
6–10]
In fact, it appears to have a
synthetically significant advantage, being able to effect C–
C bond formation without the need for preactivation of the
heterocycles by halogenation or stoichiometric metallation
reactions.
In the course of our investigations into the synthesis of
biologically active vicinal diaryl-substituted five-membered
heterocyclic derivatives[ we recently developed a selective
Results and Discussion
11]
We first examined the reaction between 1-phenyl-1H-imi-
and efficient procedure for the preparation of a large variety dazole (1a) and 4-iodoanisole (2a) in DMF at 140 °C in the
of 1,2-diaryl-1H-imidazoles through direct coupling be- presence of 5 mol-% Pd(OAc) and 2 equiv. of CuI and
2
tween 1-aryl-1H-imidazoles and aryl halides in the presence found that after 26 h the reaction was complete and had
of CuI and of CsF and a catalytic amount of Pd(OAc) in cleanly and selectively given the required imidazole 3a in
2
7
6% isolated yield [Equation (1)].
Interestingly, the crude reaction mixture did not contain
[
a] Dipartimento di Chimica e Chimica Industriale, University of
Pisa,
the biaryl derivative that would have resulted from homo-
coupling of 2a or the triaryl derivative that would have de-
rived from the C-2 and C-5 arylation of 1a. In contrast,
these substances were found in the crude mixture obtained
from the reaction between 1a and 2a in DMF at 140 °C in
Via Risorgimento 35, 56126 Pisa, Italy
Fax: +39-050-2219260
E-mail: bellina@dcci.unipi.it,
rossi@dcci.unipi.it
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2006, 1379–1382
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1379

F. Bellina, S. Cauteruccio, R. Rossi
SHORT COMMUNICATION
(
1)
(
2)
Table 1. Highly selective Pd- and Cu-mediated C-2 arylation of some π-sufficient heteroarenes including free (NH)-imidazole.
Entry[a]
Reagents
Reaction time
(h)
Product
Isolated yield
(%)
1
2
3
1
2
3
4
5
6
7
8
9
1b (X = 4-MeOC
6
H
4
H
N)
N)
2a
48
25
48
48
48
48
48
48
48
3b
3c
3d
3e
3f
66
84
99
53
89
84
47
84
23
1c (X = 4-MeO
1d (X = NMe)
1e (X = NH)
1e
2
SC
6
4
2a
2a
2a
2b (Ar = C
2c (Ar = 4-CF
2d (Ar = 2-MeC H )
6 4
2a
2a
6
H
5
)
1e
3
C
6
H
4
)
3g
3h
3i
1e
1f (X = S)
1 g (X = O)
3j
[a] Conditions: 2 equiv. of ArI were used.
the presence of 2 equiv. of CuI, 2 equiv. of CsF and 5 mol- sulted from scrambling of the aryl moieties of iodides 2
[
9b]
%
Pd(OAc)2.
tempted to reduce the amount of CuI from 2 equiv. to found in the crude reaction mixtures obtained from the Pd-
.5 equiv. we found that the reaction time was much longer catalysed arylation of 1-aryl-1H-imidazoles in the presence
It should also be noted that when we at- with the organic groups of the ligands, which had been
0
(
(
[
9a]
126 h) and that compound 3a was obtained in a lower yield of triarylphosphanes.
49%). Our method thus also allowed the selective targeting of
The good result obtained in the preparation of 3a from C–H bonds in the presence of free N–H functionalities
a and 2a under the experimental conditions reported in [Eq. (2) and Entries 4–7, Table 1] and furnished 2-aryl-1H-
1
Equation (1) prompted us to extend this new procedure to imidazoles and 2-(4-methoxyphenyl)benzimidazole (5) in
the selective preparation of other C-2 aryl-substituted good to excellent yields without the need for the use of a
azoles, including free (NH)-imidazoles and -benzimidazole. base such as MgO, which, according to Sezen and Sa-
Table 1 and Equation (2) summarize the results obtained mes,[ appeared to be necessary in order to perform the
in these highly chemo- and regioselective reactions, which selective C-2 arylation of free (NH)-imidazole and -benzim-
generally gave the required 2-arylheteroarenes in high idazole. Interestingly, compound 3f was obtained in a yield
yields. Notably, the reactions were very clean; in fact, the higher than that reported by Sezen and Sames for the aryl-
8d]
[
8d]
crude reaction mixtures contained no regioisomers of these ation of 1e in the presence of MgO.
On the other hand,
heterocycle derivatives[13] and, in the cases of free (NH)- we were unable to reproduce the results of these authors
imidazole and -benzimidazole, were also free of N-aryl de- and we did not obtain any trace of 3f under the exact reac-
rivatives. Moreover, they were also uncontaminated with tion conditions reported for its preparation [i.e., by treat-
the biaryls that would have derived from Pd-catalysed ment of 1e with 1.2 equiv. of iodobenzene (2b) in dioxane
Ullmann-type reductive couplings of aryl iodides 2, which, at 150 °C for 14 h in the presence of 5 mol-% Pd(OAc)2,
on the contrary, were present in the reaction mixtures ob- 20 mol-% PPh , 1.2 equiv. of MgO and 2 equiv. of
3
[
8d,14]
tained from the Pd- and Cu-mediated reactions between 1- CuI].
aryl-1H-imidazoles 1a–c and aryl halides in the presence of
Our results, besides being important from a synthetic
point of view, have an important implication in the context
CsF under ligandless conditions.[
9b]
On the other hand, the use of ligandless conditions pre- of the mechanism of the Pd- and Cu-mediated arylation of
vented the formation of byproducts that might have re- π-sufficient heteroarenes. In fact, although this mechanism
1380
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 1379–1382

Direct C-2 Arylation of Azoles
SHORT COMMUNICATION
still has to be clarified, the results reported in Table 1 indi- Experimental Section
cate that it might require the presence and participation of
General Procedure for the Pd- and Cu-Mediated Synthesis of 2-Aryl-
H-imidazoles 3 and the 2-Arylbenzimidazole 5 from Aryl Iodides 2
and Compounds 1 and 4, respectively: Compound 1 or 4 (1.0 mmol),
Pd(OAc) (11.2 mg, 0.05 mmol), CuI (0.38 g, 2.0 mmol), and (if a
solid) the aryl iodide 2 (2.0 mmol) were placed in the reaction vessel
a basic pyridine-like nitrogen atom in these heterocycles
and/or of a solvent such as DMF with mildly basic charac-
1
[
15]
teristics, since the Pd- and Cu-mediated C-2 arylation of
2
compounds 1 did not occur when toluene was used in place
of DMF. On the other hand, the presence of a basic pyri- under a stream of argon. The reaction vessel was fitted with a sili-
dine-like nitrogen atom in the heterocyclic substrates was con septum, evacuated and back-filled with argon, and this se-
unnecessary. In fact, in a preliminary experiment we found quence was repeated twice. Deaerated DMF (5 mL) and (if a li-
quid) the aryl iodide 2 (2.0 mmol) were then added successively by
syringe under a stream of argon at room temperature. The resulting
mixture was stirred at 140 °C under argon for the period of time
reported in Table 1 and Equations 1–2. The degree of completion
of the reaction was established by GLC and GLC-MS analysis of
a sample of the crude reaction mixture after treatment with a satu-
that indole (6) is able to react with iodide 2a in DMF at
1
40 °C over 48 h in the presence of 5 mol-% Pd(OAc) and
2
2
equiv. of CuI to furnish 2-(4-methoxyphenyl)-1H-indole
[16]
in 35% isolated yield [Equation (3)].
rated aqueous NH
being cooled to room temperature, the reaction mixture was diluted
with AcOEt and poured into a saturated aqueous NH Cl solution
4
Cl solution and extraction with AcOEt. After
4
and the resulting mixture was stirred in the open air for 0.5 h and
then extracted with AcOEt. The organic extract was washed with
water, dried and concentrated under reduced pressure, and the resi-
due was purified by MPLC on silica gel. The chromatographic frac-
tions containing the required compound were collected and con-
centrated. This procedure was employed to prepare 2-aryl-1H-imi-
dazoles 3a–j [Eq. (1) and Table 1, Entries 1–9] and 2-(4-meth-
oxyphenyl)benzimidazole (5) [Eq. (2)].
It should be noted that the structural identities of compounds 3a–c
[
Eq. (1) and Entries 1–2 of Table 1], which each had chemical
(
3)
purity higher than 99%, were confirmed by comparison of their
GLC and GLC-MS data with those of authentic samples of these
compounds obtained by Pd- and Cu-mediated C-2 arylation of the
In our opinion, the mechanism of the C-2 arylation of
azoles 1, 4 and 6, as we recently reported,[ might involve
the formation of an organocopper() derivative followed by
a transmetallation reaction with a heteroarylpalladium()
halide species and a reductive elimination. On the other
hand, coordination of heterocycles 1 to CuI to form π com-
9b]
[9b]
corresponding 1-aryl-1H-imidazoles 1a–c with aryl iodide 2a.
2
-(4-Methoxyphenyl)-1-methyl-1H-imidazole (3d): The crude reac-
tion product obtained from the Pd- and Cu-mediated reaction be-
tween 1d and 2a (Table 1, Entry 3) was purified by MPLC on silica
gel with a mixture of CH Cl and MeOH (96:4) as eluent to give 3d
2 2
1
plexes might significantly lower the pK of their C–H bonds (186 mg, 99%) as a pale yellow oil. H NMR (300 MHz, CDCl ): δ
a
3
at position 2 and facilitate their conversion into C–Cu = 7.55 (m, 2 H), 7.08 (d, J = 1.0 Hz, 1 H), 6.97 (m, 2 H), 6.92 (d,
[
17]
13
bonds. However, we cannot at present exclude a reaction J = 1.0 Hz, 1 H), 3.83 (s, 3 H), 3.69 (s, 3 H) ppm. C NMR
(
75.5 MHz, CDCl
22.0, 113.9 (2 C), 55.3, 34.4. GLC analysis showed that 3d had
chemical purity higher than 98%. The spectroscopic data of this
3
): δ = 159.9, 147.8, 130.0 (2 C), 128.1, 123.2,
mechanism involving the presence of organopalladium()
and organopalladium() species and organocopper() de-
rivatives.
1
[
18]
[
19]
compound were in agreement with those previously reported.
2-(4-Methoxyphenyl)-1H-indole (7): Indole (6, 1.0 mmol), Pd(OAc)
2
(
(
11.2 mg, 0.05 mmol), CuI (0.38 g, 2.0 mmol) and 4-iodoanisole
2a, 2.0 mmol) were placed in a reaction vessel under a stream of
Conclusion
argon. The reaction vessel was fitted with a silicon septum, evacu-
ated and back-filled with argon, and this sequence was repeated
twice. Deaerated DMF (5 mL) was then added by syringe at room
A reliable new method for the direct and regioselective
C-2 arylation of azoles has been developed. The unprece-
dented reaction conditions for this procedure, which do not temperature under a stream of argon and the mixture was stirred
involve the use of a base, can also be employed for the aryl- under argon at 140 °C. The reaction was monitored by GLC and
ation of substrates containing base-sensitive groups, such GLC-MS analyses of a sample of the crude reaction mixture after
4
treatment with a saturated aqueous NH Cl solution and extraction
as the NH groups in imidazole, benzimidazole and indole,
without their prior protection.
with AcOEt. After 48 h at 140 °C the degree of conversion of the
reaction was 72%. The reaction mixture was then cooled to room
temperature, diluted with AcOEt and poured into a saturated aque-
The results of this study also include the observation that
the Pd- and Cu-mediated arylation, under base-free condi-
tions, of azoles that do not contain a basic pyridine-like
nitrogen atom requires the use of a solvent with mildly basic
characteristics. Applications of this method to the synthesis
4
ous NH Cl solution, and the resulting mixture was stirred in the
open air for 0.5 h and then extracted with AcOEt. The organic
extract was washed with water, dried and concentrated under re-
duced pressure, and the residue was purified by MPLC on silica
of pharmacologically significant compounds are in develop- gel with a mixture of toluene and hexane (60:40) as eluent to give
1
ment.
7 (78 mg, 35%) as a pale yellow solid: m.p. 224–227 °C. H NMR
Eur. J. Org. Chem. 2006, 1379–1382
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1381

F. Bellina, S. Cauteruccio, R. Rossi
SHORT COMMUNICATION
(
300 MHz, [D
6
]DMSO): δ = 11.43 (s, 1 H), 7.80 (m, 2 H), 7.49 (d,
V. Penalva, M. Lemaire, J. Organomet. Chem. 1998, 567, 49–
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5; i) C. Gozzi, L. Lavenot, K. Ilg, V. Penalva, M. Lemaire,
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J = 1.7 Hz, 1 H), 3.80 ppm (s, 3 H). 1 C NMR (75.5 MHz, [D
DMSO): δ = 158.7, 137.7, 136.8, 128.8, 126.3 (2C), 124.8, 121.0,
19.6, 119.1, 114.3 (2 C), 111.0, 97.2, 55.1 ppm. GLC analysis
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study can be found in the Supporting Information.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for and characterization of com-
pounds 3e, 3g, 3h, 3i, 3j and 5. This material is available free of
charge via the Internet at “http://www.eurjoc.org” or from the au-
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Acknowledgments
We thank the Ministero dellЈIstruzione, dellЈUniversità e della
Ricerca (MIUR) and the University of Pisa for financial support.
We are also grateful to Mr. Piergiorgio Vergamini for recording IR
spectra.
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1
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[
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[
I
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Published Online: January 30, 2006
1382
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Eur. J. Org. Chem. 2006, 1379–1382