Ligand-free copper-catalyzed arylation of imidazole and N,N′-carbonyldiimidazole, and microwave-assisted synthesis of N-Aryl-1H-imidazoles

Article abstract of DOI:10.1002/hlca.200890156

Microwave-assisted arylation of 1H-imidazoles and N,N′- carbonyldiimidazole under ligand-free copper-mediated conditions in tetraethyl orthosilicate is reported. Valuable evidence for understanding of the Cu-catalyzed mechanism of the Ullmann reaction is

Full text of DOI:10.1002/hlca.200890156

Helvetica Chimica Acta – Vol. 91 (2008)
1435
Ligand-Free Copper-Catalyzed Arylation of Imidazole and N,N’-
Carbonyldiimidazole, and Microwave-Assisted Synthesis of N-Aryl-1H-
imidazoles
by Xiao-Dong Yang, Liang Li, and Hong-Bin Zhang*
Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of
Education, School of Chemical Science and Technology, Yunnan University, Kunming, Yunnan 650091,
P. R. China
(phone: þ 86-871-5031119; fax: þ 86-871-5035538; e-mail: zhanghb@yun.edu.cn or
zhanghbyd@gmail.com)
Microwave-assisted arylation of 1H-imidazoles and N,N’-carbonyldiimidazole under ligand-free
copper-mediated conditions in tetraethyl orthosilicate is reported. Valuable evidence for understanding
of the Cu-catalyzed mechanism of the Ullmann reaction is also presented.
Introduction. – N-Arylimidazoles are important substructures present in natural
products, synthetic materials, and pharmaceuticals [1]. They are also frequently used as
building blocks for a broad array of imidazolium salts used as precursors to
imidazolylidenes, a new family of ligands for homogeneous catalysis1). Traditionally,
N-arylimidazoles have been obtained by nucleophilic aromatic substitution of a
strongly electron-withdrawing activated aryl halide [3] or by conventional stoichio-
metric copper-promoted coupling of imidazole with an aryl iodide [4]. High temper-
atures and long periods of time are generally required for a better yield. Because
catalyzed arylation of imidazoles with transition metals is an attractive way to access N-
arylimidazoles, many new methods featured by addition of a promoting ligand, such as
4,7-dimethoxy-1,10-phenanthroline [5c] or l-proline [5i][5l], which is thought to
increase catalyst solubility and stability, and to prevent aggregation of the metal [5q],
have been developed, and the synthesis of N-arylimidazoles remains a focus of recent
interests [5]. In most cases, however, considerable amount of ligands and long periods
of time were required. In our previous work, we had developed a method for the
synthesis of N-arylimidazoles [6]. Although this method provides a practical access to
the highly sterically hindered N-arylimidazoles, unfortunately, it failed to afford
acceptable yields for the highly electron-rich N-arylimidazoles, which are believed to be
important building blocks for organic functional materials such as liquid crystals and
fluorescent materials [7].
Results and Discussion. – In order to obtain highly electron-rich N-arylimidazoles,
we recently initiated a research project towards the arylation of imidazoles. After a
careful survey of the literature, microwave-assisted arylation of imidazoles has
1
)
For reviews of NHC-carbene, see [2].
ꢀ 2008 Verlag Helvetica Chimica Acta AG, Zürich

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Helvetica Chimica Acta – Vol. 91 (2008)
attracted our attention, although only electron-deficient [3][8] or electron-neutral [9]
arylbromides and aryliodides being used in the literature. We assumed that the
microwave heating should reduce the reaction time and provide good access to the
highly electron-rich N-arylimidazoles. The reaction conditions reported by Wu et al. [9]
were thus tested for the synthesis of electron-rich 1-(3,4,5-trimethoxyphenyl)-1H-
imidazole, as indicated in Scheme 1. Unfortunately, the reaction in N-methylpyrrolidine
(NMP) under microwave irradiation provided an unacceptable low yield of the desired
product, even in the presence of 20% of CuI (see Scheme 1, Methods A and B). Having
failed to copy the literature procedure, we decided to carry out a copper(I)iodide
catalyzed arylation of imidazole in tetraethyl orthosilicate, a solvent which has recently
been identified by us as a beneficial medium for the high-temperature Ullmann
aminations [10].
Scheme 1. Attempts Towards the Synthesis of 1-(3,4,5-Trimethoxyphenyl)-1H-imidazole
Initially, the reaction was conducted at a temperature of 1458 under microwave
irradiation conditions, but no desired product was detected. However, when the
temperature was increased to 1808, the arylation of 1H-imidazole was observed. A
good yield was obtained when the temperature was switched to 2008 (Scheme 1,
Method C). The same coupling could also be carried out in a high-pressure autoclave in
the presence of CuI (Scheme 1, Method D). These results indicated that the
temperature still plays a crucial role in this arylation, and the microwave heating can
accelerate the coupling reaction. Next, we carried out a number of reactions in order to
get further insight towards this arylation reaction (Table 1).
We found that the copper catalyzed arylation of 1H-imidazole with 4-bromoanisole
under microwave conditions leads to the desired product in an excellent yield, even in
the presence of only 5% CuI. Both, Cu(OAc)2 and copper(II)acetylacetonate could
also be used as active catalysts, however, they were less efficient than CuI. Tetraethyl
orthosilicate remained the solvent of choice, not only for better yields, but also for its
easy handling in product purifications. Although our method was the first reported
procedure for ligand-free arylation of imidazole with aryl chlorides in the presence of
CuI under microwave irradiation, the yields were lower than those with aryl bromides
and aryl iodides (see Table 1). A number of arylimidazoles were then synthesized, and
the results are shown in Table 2.

Helvetica Chimica Acta – Vol. 91 (2008)
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Table 1. Studies on Copper Catalyzed Arylation of Imidazolea)
Entry
Copper
Solvents
Aryl halides
R
X
Yield [%]
1
2
3
4
5
6
7
8
9
CuI
CuI
CuI
CuI
(EtO)4Si
(EtO)4Si
(EtO)4Si
(EtO)4Si
(EtO)4Si
(EtO)4Si
(EtO)4Si
p-xylene
DMF
4-iodoanisole
MeO
MeO
MeO
MeCO
MeCO
MeO
MeO
MeO
MeO
MeO
I
89
80
15
78
4-bromoanisole
4-chloroanisole
4-bromoacetophenone
4-chloroacetophenone
4-bromoanisole
4-bromoanisole
4-bromoanisole
4-bromoanisole
4-bromoanisole
Br
Cl
Br
Cl
Br
Br
Br
Br
Br
CuI
28
Cu(OAc)2
Cu(Acac)2
CuI
CuI
CuI
65b)
72b)
70
37
35
10
DMSO
a) Reaction conditions: aryl halide (1.2 mmol), CuI (9.5 mg, 5 mol-%), imidazole (1 mmol), and K2CO3
(276 mg, 2 mmol) in commercially available organic solvent (2 ml) were subjected to microwave
irradiation at 2008 (CEM Discover) for 40 min. b) Reactions were carried out under identical conditions,
but with Cu(OAc)2 or Cu(Acac)2 (5 mol-%).
Although the Ullmann reaction has been studied for more than one century, its
mechanism has not been well-established [11]. To get some insight into the mechanism,
we decided to use N,N’-carbonyldiimidazole (CDI; 0.5 mmol, 0.5 equiv.), which is not
nucleophilic, as a source of imidazole to see whether an arylation could take place. To
our surprise, the arylation with 4-bromoanisole (1.2 mmol, 1.2 equiv.) under the
conditions described above occurred and led to arylimidazole in excellent yield (85%
yield after isolation, see Table 3). This coupling, to the best of our knowledge,
represents the first example of copper catalyzed arylation of N,N’-carbonyldiimidazole
with both imidazole subunits in CDI (0.5 mmol amount of CDI equals to 1.0 mmol
amount of imidazole) being utilized.
In the absence of Cu salts, however, identical microwave irradiation provided
mainly 4-bromoanisole and CDI, no significant hydrolysis of CDI was observed by TLC
analysis. When the reaction was carried out with stoichiometric amounts of CuI
(1 mmol), CDI (0.5 mmol), and K2CO3 (2 mmol) in the absence of 4-bromoanisole, a
grey-blue colored mixture was obtained. This mixture was subjected to TLC analysis,
and only a trace of CDI was detected. Addition of 4-bromoanisole to this blue-colored
mixture followed by heating under microwave irradiation (2008) led to 1-(4-
methoxyphenyl)-1H-imidazole, although in relatively low yield (30 – 45%). We also
carried out a control reaction with a stoichiometric amount of CuI (1 mmol), imidazole
(1 mmol), and K2CO3 (2 mmol) in the absence of 4-bromoanisole. A grey-blue colored
mixture was also formed after heating under microwave irradiation (2008) for 20 min,
and only a trace of imidazole was detected by TLC analysis. Addition of 4-

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Helvetica Chimica Acta – Vol. 91 (2008)
Table 2. CuI-Catalyzed Synthesis of Arylimidazoles in Tetraethyl Orthosilicate Assisted by Microwave
Irradiationa)
Entry
Ar
R
Yield [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
3,4,5-trimethoxyphenyl
4-phenylphenyl
4-tert-butylphenyl
pyridin-2-yl
H
H
H
H
H
H
H
H
H
H
H
Ph
H
53b)
94
36; 71b)
83
4-nitrophenyl
85
2,4-dimethoxyphenyl
4-methoxy-2-methylphenyl
4-(N,N-dimethylamino)phenyl
phenanthren-9-yl
anthracen-9-yl
phenyl
4-phenylphenyl
4-acetylphenyl
57b)c)
48b)c)
90
40b)c)
18b)c)
88
91
78b)
a) Reagents and conditions: Aryl bromides (1.2 mmol), CuI (9.5 mg, 0.05 mmol), imidazole (1 mmol),
and K2CO3 (276 mg, 2 mmol) in commercially available tetraethyl orthosilicate (2 ml) were subjected to
microwave irradiation at 2008 (CEM Discover) for 40 min. b) Reactions were carried out under identical
conditions, but with 20% of CuI (38 mg, 0.2 mmol). The indicated yields represent the yields of isolated
products. c) The low yields were due to steric hindrance.
Table 3. Arylation of N,N’-Carbonyldiimidazole (CDI)a)
Entry
Ar
Yield [%]
1
2
3
4
5
6
7
4-methoxyphenyl
3,4,5-trimethoxyphenyl
4-phenylphenyl
85
51b)
90
pyridin-2-yl
87
59
41b)c)
18b)c)
2,4-dimethoxyphenyl
phenanthren-9-yl
anthracen-9-yl
a) Reagents and conditions: Aryl bromides (1.2 mmol), CuI (9.5 mg, 0.05 mmol), CDI (0.5 mmol), and
K2CO3 (276 mg, 2 mmol) in commercially available tetraethyl orthosilicate (2 ml) were subjected to
microwave irradiation at 2008 (CEM Discover) for 40 min. b) Reactions were conducted under identical
reaction conditions, but with 20% of CuI (38 mg, 0.2 mmol). The indicated yields represent the yields of
isolated products. c) The low yields were due to steric hindrance.

Helvetica Chimica Acta – Vol. 91 (2008)
1439
bromoanisole to this blue-colored mixture followed by heating under microwave
irradiation (2008) also led to 1-(4-methoxyphenyl)-1H-imidazole in fair yield (65%).
To date, four main mechanisms for Ullmann-type coupling reactions have been
described in the literature: 1) oxidative addition/reductive elimination of ArX on
copper(I) proposed by Cohen in 1974 [12]; 2) p-complexation of ArX on copper(I)
proposed by Paine in 1987 [13]; 3) Lewis-type complexation of ArX on copper(I) [11];
and 4) aryl-radical intermediates [4][14]. The results observed in this work are in favor
of Paineꢁs proposal. The initial step in the catalytic cycle might be the formation of a
copper-imidazole complex (a grey-blue colored mixture). It would be less likely that an
oxidative addition of an arylbromide towards the copper(I)iodide occurred first. The
possible pathway is shown in Scheme 2.
Scheme 2. Proposed Pathwayfor the Arylation of Imidazole and CDI
Conclusions. – In summary, a microwave-assisted synthesis of N-arylimidazoles by a
ligand-free copper-catalyzed Ullmann reaction was disclosed. Although trends have
been directed toward the discovery of efficient ligands (in most cases, a 10 – 20%

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Helvetica Chimica Acta – Vol. 91 (2008)
amount of ligand is required [5], in an extreme case, a 60% amount of ligand is used to
ensure a good yield [5i]) for palladium or copper catalyzed arylation of imidazoles, the
ligand-free copper catalyzed Ullmann process is still attractive in view of industrial
application. The arylation results obtained with CDI, as well as imidazole in this
research also reveal some interesting evidence towards the mechanism of the copper
catalyzed Ullmann reaction.
This work was supported by a grant (706052) from the Foundation of the Chinese Ministryof
Education for Fostering Important Scientific Program and a grant (2007B0006Z) from the Natural Science
Foundation of Yunnan Province, which are gratefully acknowledged.
Experimental Part
General. Starting materials and reagents used in the reactions were obtained commercially from
Acros, Aldrich, and Fluka, and were used without purification, unless otherwise indicated. M.p.: XT-4
melting-point apparatus, uncorrected. NMR Spectra: Bruker-Avance-300 instrument; TMS as internal
reference, d in ppm, J in Hz. EI-MS: VG Autospec-3000 mass spectrometer; in m/z (rel. %).
General Procedure for the Synthesis of N-Aryl-1H-imidazoles. A mixture of CuI (9.5 – 38 mg, 0.05 –
0.2 mmol), K2CO3 (276 mg, 2 mmol, 2.0 equiv.), N,N’-carbonyldiimidazole or imidazoles (0.5 mmol or
1 mmol), and aryl halides (1.2 mmol) in tetraethyl orthosilicate (2 ml) was put in a vial (CEM Discover).
The sealed vial was then heated at 2008 under microwave irradiation (CEM Discover) for 40 min. After
cooling to r.t., the residue was diluted with AcOEt (3 ml) and 95% EtOH (3 ml). NH4F/H2O on SiO2
(2 g, pre-prepared by addition of NH4F (10 g) in H2O (150 ml) to SiO2 (50 g, 100 – 200 mesh)) was added,
and the resulting mixture was kept at r.t. for 3 – 5 h. The solidified materials were filtered and washed
with AcOEt. After removal of the solvents, the residue was chromatographed on SiO2 (AcOEt/
petroleum ether) to afford the products.
1-(3,4,5-Trimethoxyphenyl)-1H-imidazole [7]. Colorless oil. 1H-NMR (300 MHz, CDCl3): 7.72 (s,
1 H); 7.16 (s, 1 H); 7.10 (s, 1 H); 6.51 (s, 2 H); 3.83 (s, 6 H); 3.79 (s, 3 H). 13C-NMR (75 MHz, CDCl3):
153.52; 137.12; 135.39; 132.88; 129.62; 118.29; 99.42; 60.47; 55.88.
1
1-(Biphenyl-4-yl)-1H-imidazole [5l]. White solid. M.p. 150 – 1528. H-NMR: 7.93 (s, 1 H); 7.71 (d,
J ¼ 8.5, 2 H); 7.63 (dd, J ¼ 7.8, 1.3, 2 H); 7.47 – 7.52 (m, 4 H); 7.42 (d, J ¼ 7.4, 1 H); 7.34 (s, 1 H); 7.26 (s,
1 H). 13C-NMR: 139.62; 138.75; 135.50; 134.63; 129.47; 127.96; 127.51; 126.80; 126.01; 120.77; 117.24.
1-[4-tert-Butylphenyl]-1H-imidazole [15]. White solid. M.p. 78 – 808. 1H-NMR: 7.76 (s, 1 H); 7.42 (d,
J ¼ 8.7, 2 H); 7.24 (d, J ¼ 8.7, 2 H); 7.19 (s, 1 H); 7.12 (s, 1 H); 1.28 (s, 9 H). 13C-NMR: (75 MHz, CDCl3):
150.76; 135.65; 134.94; 130.23; 126.70; 121.24; 118.33; 34.63; 33.28.
2-(1H-Imidazol-1-yl)pyridine [5l]. Colorless solid. M.p. 38 – 408. 1H-NMR: 8.33 (d, J ¼ 4.0, 1 H); 8.25
(s, 1 H); 7.64 – 7.70 (m, 1 H); 7.53 (s, 1 H); 7.22 (d, J ¼ 8.2, 1 H); 7.07 – 7.11 (m, 2 H). 13C-NMR: (75 MHz,
CDCl3): 148.49; 138.43; 134.37; 130.03; 121.42; 115.58; 111.71.
1
1-(4-Nitrophenyl)-1H-imidazole [7]. Yellow solid. M.p. 177 – 1788. H-NMR: 8.38 (d, J ¼ 9.1, 2 H);
8.00 (s, 1 H); 7.60 (d, J ¼ 9.1, 2 H); 7.40 (s, 1 H); 7.29 (s, 1 H). 13C-NMR: 146.30; 142.00; 135.41; 131.75;
125.75; 121.05; 117.63.
1
1-(2,4-Dimethoxyphenyl)-1H-imidazole [5l]. Pale yellow oil. H-NMR: 7.66 (s, 1 H); 7.17 (s, 1 H);
7.15 (s, 1 H); 7.11 (d, J ¼ 8.5, 1 H); 6.57 (d, J ¼ 2.5, 1 H); 6.51 (dd, J ¼ 8.5, 2.5, 1 H); 3.83 (s, 3 H); 3.79 (s,
3 H). 13C-NMR: 160.39; 153.97; 138.00; 128.64; 126.48; 120.64; 120.06; 104.45; 99.83; 55.79; 55.63.
1-(4-Methoxy-2-methylphenyl)-1H-imidazole. Colorless oil. 1H-NMR: 7.52 (s, 1 H); 7.16 (s, 1 H); 7.13
(d, J ¼ 8.5, 1 H); 6.99 (s, 1 H); 6.82 (d, J ¼ 2.5, 1 H); 6.78 (dd, J ¼ 8.5, 2.5, 1 H); 3.82 (s, 3 H); 2.11 (s, 3 H).
13C-NMR: 159.64; 137.83; 135.55; 129.71; 129.19; 127.71; 120.84; 116.21; 111.83; 55.50; 17.72. ESI-MS: 189
(12, [M þ 1]þ), 188 (100, Mþ), 187 (7), 174 (70), 161 (36), 147 (37), 146 (60), 134 (10), 118 (12), 91 (11),
77 (10). HR-ESI-MS: 189.1032 ([M þ 1]þ, C11H13N2Oþ; calc. 189.1027).
4-(1H-Imidazol-1-yl)-N,N-dimethylaniline [16]. Colorless solid. M.p. 109 – 1108. 1H-NMR: 7.32 (s,
1 H); 7.22 (d, J ¼ 8.9, 2 H); 7.17 (s, 2 H); 6.75 (d, J ¼ 8.9, 2 H); 2.99 (s, 6 H). 13C-NMR: (75 MHz, CDCl3):
149.96; 135.98; 129.69; 127.03; 123.05; 118.92; 112.78; 40.54.

Helvetica Chimica Acta – Vol. 91 (2008)
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1-(Phenanthren-9-yl)-1H-imidazole. Pale yellow solid. M.p. 97 – 998. 1H-NMR: 8.78 (d, J ¼ 8.4, 1 H);
8.73 (d, J ¼ 8.4, 1 H); 7.92 (d, J ¼ 7.8, 1 H); 7.82 (s, 1 H); 7.67 – 7.77 (m, 4 H); 7.59 – 7.62 (m, 2 H); 7.32 (d,
J ¼ 9.6, 2 H). 13C-NMR: 138.54; 132.75; 131.20; 130.71; 130.31; 129.65; 128.93; 128.71; 127.88; 127.75;
127.66; 127.54; 124.59; 123.22; 123.13; 122.81; 121.75. ESI-MS: 245 (19, [M þ 1]þ), 244 (100, Mþ), 243
(37), 217 (38), 204 (20), 189 (26), 176 (29), 165 (5), 151 (8), 135 (5), 122 (12), 107 (8), 94 (13), 88 (9).
HR-ESI-MS: 245.1074 ([M þ 1]þ, C17H13Nþ2 ; calc. 245.1078).
1-(Anthracen-9-yl)-1H-imidazole. Yellow solid. M.p. 154 – 1568. 1H-NMR: 8.60 (s, 1 H); 8.08 (d, J ¼
8.2, 2 H); 7.79 (s, 1 H); 7.47 – 7.54 (m, 7 H); 7.26 (dd, J ¼ 6.2, 1.2, 1 H). 13C-NMR: 139.66; 131.27; 129.73;
128.83; 128.47; 128.36; 127.58; 125.90; 122.76; 122.43. ESI-MS: 245 (19, [M þ 1]þ), 244 (100, Mþ), 243
(44), 216 (48), 204 (15), 189 (17), 176 (25), 163 (3), 149 (6), 122 (14), 107 (18), 94 (12), 88 (9). HR-ESI-
MS: 245.1085 ([M þ 1]þ, C17H13N2þ ; calc. 245.1078).
1
1-Phenyl-1H-imidazole [15]. Colorless solid. M.p. 53 – 558. H-NMR: 7.79 (s, 1 H); 7.40 (t, J ¼ 8.5,
2 H); 7.26 – 7.32 (m, 3 H); 7.21 (s, 1 H); 7.14 (s, 1 H). 13C-NMR: 137.33; 135.52; 130.35; 129.82; 127.41;
121.39; 118.16.
1
1-(Biphenyl-4-yl)-1H-benzimidazole [17]. Colorless solid. M.p. 188 – 1908. H-NMR: 8.16 (s, 1 H);
7.89 – 7.92 (m, 1 H); 7.79 (d, J ¼ 8.5, 2 H); 7.65 (d, J ¼ 7.9, 2 H); 7.58 – 7.61 (m, 3 H); 7.48 – 7.53 (m, 2 H);
7.42 – 7.44 (m, 1 H); 7.35 – 7.38 (m, 2 H). 13C-NMR: 144.18; 14.24; 141.11; 139.82; 129.01; 128.68; 127.89;
127.12; 124.31; 123.74; 122.83; 120.70; 110.50. ESI-MS: 271 (25, [M þ 1]þ), 270 (100, Mþ), 269 (32), 241
(4), 165 (5), 152 (10), 135 (5), 121 (8), 109 (3), 77 (2). ESI-HR-MS: 271.1240 ([M þ 1]þ, C19H15N2þ ; calc.
271.1235).
1-[4-(1H-Imidazol-1-yl)phenyl]ethanone [7]. White solid. M.p. 110 – 1118. 1H-NMR: 8.07 (d, J ¼ 8.7,
2 H); 7.93 (s, 1 H); 7.48 (d, J ¼ 8.7, 2 H); 7.34 (s, 1 H); 7.23 (s, 1 H); 2.62 (s, 3 H). 13C-NMR: 196.43;
140.74; 135.81; 135.37; 131.17; 130.30; 120.70; 117.68; 26.55.
1
1-(4-Methoxyphenyl)-1H-imidazole [7]. White solid. M.p. 60 – 618. H-NMR: 7.68 (s, 1 H); 7.21 (d,
J ¼ 8.8, 2 H); 7.12 (s, 1 H); 7.10 (s, 1 H); 6.90 (d, J ¼ 8.8, 2 H); 3.75 (s, 3 H). 13C-NMR: 158.87; 135.76;
130.65; 130.00; 123.07; 118.66; 114.86; 55.52.
REFERENCES
[1] Z. Jin, Z. Li, R. Huang, Nat. Prod. Rep. 2002, 19, 454; M. Kimura, K. Shi, K. Hashimoto, Z. Z. Hu,
Luminescence 2007, 22, 229, and refs. cit. therein; M. M. Ramla, M. A. Omar, A.-M. M. El-Khamry,
H. I. El-Diwani, Bioorg. Med. Chem. 2006, 14, 7324; N. A. Magnus, W. D. Diseroad, C. R. Nevill Jr.,
J. P. Wepsiec, Org. Process Res. Dev. 2006, 10, 556, and refs. cit. therein.
[2] E. Colacino, J. Martinez, F. Lamaty, Coord. Chem. Rev. 2007, 251, 726; R. E. Douthwaite, Coord.
Chem. Rev. 2007, 251, 702; W. A. Herrmann, Angew. Chem., Int. Ed. 2002, 41, 1290, and refs. cit.
therein.
ˇ
´
ˇ
´
[3] M. Meciarova, J. Podlesna, S. Toma, Monatsh. Chem. 2004, 135, 419.
[4] J. Lindley, Tetrahedron 1984, 40, 1433.
[5] a) L. Zhu, L. Cheng, Y. Zhang, R. Xie, J. You, J. Org. Chem. 2007, 72, 2737; b) A. K. Verma, J. Singh,
V. K. Sankar, R. Chaudhary, R. Chandra, Tetrahedron Lett. 2007, 48, 4207; c) R. A. Altman, S. L.
Buchwald, Org. Lett. 2006, 8, 2779; d) M. L. Kantam, G. T. Venkanna, C. Sridhar, B. Sreedhar, B. M.
Choudary, J. Org. Chem. 2006, 71, 9522; e) R. Hosseinzadeh, M. Tajbakhsha, M. Alikarami,
Tetrahedron Lett. 2006, 47, 5203; f) M. L. Kantam, G. T. Venkanna, C. Sridhar, K. B. S. Kumar,
Tetrahedron Lett. 2006, 47, 3897; g) M. L. Kantam, B. Neelima, C. V. Reddy, V. Neeraja, J. Mol. Cat.,
A: Chem. 2006, 249, 201; h) Y.-X. Xie, S.-F. Pi, J. Wang, D.-L. Yin, J.-H. Li, J. Org. Chem. 2006, 71,
8324; i) X. Lv, Z. Wang, W. Bao, Tetrahedron 2006, 62, 4756; j) M. Kuil, E. K. Bekedam, G. M.
Visser, A. van den Hoogenband, J. W. Terpstra, P. C. J. Kamer, P. W. N. M. van Leeuwena, G. P. F.
van Strijdonck, Tetrahedron Lett. 2005, 46, 2405; k) T. Jerphagnon, G. P. M. van Klink, J. G. de Vries,
G. van Koten, Org. Lett. 2005, 7, 5241; l) H. Zhang, Q. Cai, D. Ma, J. Org. Chem. 2005, 70, 5164;
m) B. M. Choudary, C. Sridhar, M. L. Kantam, G. T. Venkanna, B. Sreedhar,J. Am. Chem. Soc. 2005,
127, 9948; n) L. Liu, M. Frohn, N. Xi, C. Dominguez, R. Hungate, P. J. Reider, J. Org. Chem. 2005, 70,
`
10135; o) E. Alcalde, I. Dinares, S. Rodrꢂguez, C. G. de Miguel, Eur. J. Org. Chem. 2005, 1637; p) J.-

1442
Helvetica Chimica Acta – Vol. 91 (2008)
B. Lan, L. Chen, X.-Q. Yu, J.-S. You, R.-G. Xie, Chem. Commun. 2004, 188; q) H.-J. Cristau, P. P.
Cellier, J.-F. Spindler, M. Taillefer, Chem. Eur. J. 2004, 10, 5607, and references therein.
[6] J. Liu, J. Chen, J. Zhao, Y. Zhao, L. Li, H. Zhang, Synthesis 2003, 2661.
[7] A. Mori, A. Sekiguchi, K. Masui, T. Shimada, M. Horie, K. Osakada, M. Kawamoto, T. Ikeda,J. Am.
Chem. Soc. 2003, 125, 1700.
[8] Y. Wan, M. Alterman, A. Hallberg, Synthesis 2002, 1597.
[9] Y.-J. Wu, H. He, A. Lꢁ Heureux, Tetrahedron Lett. 2003, 44, 4217; Y.-J. Cheng, Tetrahedron 2002, 58,
887; A. P. Combs, S. Saubern, M. Rafalski, P. Y. S. Lam, Tetrahedron Lett. 1999, 40, 1623.
[10] Y. Zhao, Y. Wang, H. Sun, L. Li, H. Zhang, Chem. Commun. 2007, 3186.
[11] A. Ouali, M. Taillefer, J.-F. Spindler, A. Jutand, Organometallics 2007, 26, 65.
[12] T. Cohen, J. Wood, A. G. Dietz Jr., Tetrahedron Lett. 1974, 15, 3555.
[13] A. J. Paine, J. Am. Chem. Soc. 1987, 109, 1496.
[14] H. L. Aalten, G. van Koten, D. M. Grove, T. Kuilman, O. G. Piekstra, L. A. Hulshof, R. A. Sheldon,
Tetrahedron 1989, 45, 5565.
[15] A. Kiyomori, J.-F. Marcoux, S. L. Buchwald, Tetrahedron Lett. 1999, 40, 2657.
[16] L. Cerrada, J. Cudero, J. Elguero, C. Pardo, J. Chem. Soc., Chem. Commun. 1993, 23, 1713.
[17] L. K. Rasmussen, M. Begtrup, T. Ruhland, J. Org. Chem. 2004, 69, 6890.
Received January29, 2008