Palladium-catalyzed efficient and one-pot synthesis of diarylacetylenes from the reaction of aryl chlorides with 2-methyl-3-butyn-2-ol

Article abstract of DOI:10.1002/adsc.200600498

An efficient and practical synthetic method has been developed for the preparation of symmetrical diarylacetylenes from the direct reaction of aryl chlorides with 2-methyl-3-butyn-2-ol catalyzed by palladium(II) chloride-bis(tricyclohexylphosphine) [PdCl₂(PCy₃) ₂] under mild reaction conditions. Unsymmetrical diarylated acetylenes could be also obtained by using two different aryl chlorides simultaneously. The catalytic procedure includes a novel one-pot palladium-catalyzed, double Sonogashira coupling of inactivated aryl chlorides without use of copper(1) as co-catalyst.

Full text of DOI:10.1002/adsc.200600498

FULL PAPERS
DOI: 10.1002/adsc.200600498
Palladium-Catalyzed Efficient and One-Pot Synthesis
of Diarylacetylenes from the Reaction of Aryl Chlorides with
2-Methyl-3-butyn-2-ol
Chenyi Yi,a Ruimao Hua,a,* Hanxiang Zeng,a and Qiufeng Huanga
a
Department of Chemistry, Tsinghua University, Innovative Catalysis Program, Key Laboratory of Organic
Optoelectronics & Molecular Engineering of Ministry of Education, Beijing 100084, Peopleꢀs Republic of China
Fax : (+86)-10-6279-2596; e-mail: ruimao@mail.tsinghua.edu.cn
Received: September 30, 2006; Revised: March 8, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient and practical synthetic simultaneously. The catalytic procedure includes a
method has been developed for the preparation of novel one-pot palladium-catalyzed, double Sonoga-
symmetrical diarylacetylenes from the direct reaction shira coupling of inactivated aryl chlorides without
of aryl chlorides with 2-methyl-3-butyn-2-ol cata- use of copper(I) as co-catalyst.
lyzed by palladium(II) chloride-bis(tricyclohexyl-
phosphine) [PdCl2(PCy3)2] under mild reaction con-
G
ditions. Unsymmetrical diarylated acetylenes could Keywords: aryl chlorides; diarylacetylenes; 2-methyl-
be also obtained by using two different aryl chlorides 3-butyn-2-ol; palladium
Introduction
chlorides with terminal alkynes in the presence of
Cs2CO3 in DMSO to afford arylated internal acety-
Transition metal-catalyzed activation of the C X (X= lenes in good to high yields (Scheme 2).[4] In continua-
À
I, Br, Cl) bond of aryl halides and its application to tion of our investigation to extend the generality of
À
C C bond formation have become one of the most this catalytic procedure, we have further examined
important topics of current synthetic research.[1] In the reactions of aryl chlorides with a variety of func-
the past two decades, the palladium-catalyzed cross- tional group-bearing terminal alkynes, and found that,
coupling reaction of aryl halides with terminal alkynes in the reaction of chlorobenzene with 2-methyl-3-
(Sonogashira reaction) has been proven to be a most butyn-2-ol, a low yield (ca. 12%) of diphenylacety-
efficient method for the synthesis of arylated internal lene was obtained. These results encourage us to de-
acetylenes, and a variety of palladium/copper(I), or velop an efficient copper-free PdCl2(PCy3)2-catalyzed
N
copper-free palladium catalyst systems has been de- one-pot synthesis of diarylacetylenes from the reac-
veloped (Scheme 1).[2]
tion of aryl chlorides with 2-methyl-3-butyn-2-ol.[5]
It is well known that 2-methyl-3-butyn-2-ol can be
Aryl chlorides are less expensive, more easily avail-
able, but less reactive compared to aryl bromides and used as an acetylene synthon by its coupling with aryl
iodides. Therefore, the development of efficient cata- bromides or iodides catalyzed by palladium com-
lyst systems for Sonogashira couplings of aryl chlor- plexes with Cu(I) as co-catalyst, and then elimination
ides is a promising and challenging objective.[2a,3] Re- of acetone in the presence of alkalis to give monoary-
cently, we have developed an efficient copper-free lated alkynes.[6] For synthesis of diarylated internal
PdCl2(PCy3)2-catalyzed Sonogashira coupling of aryl acetylene, a separated additional Sonogashira cou-
N
Scheme 2. PdCl2
with terminal alkynes.
A
Scheme 1. The Sonogashira coupling reaction.
1738
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1738 – 1742

Palladium-Catalyzed Efficient and One-Pot Synthesis of Diarylacetylenes
FULL PAPERS
Scheme 3. Synthesis of diarylated internal alkynes from the reaction of aryl halides with 2-methyl-3-butyn-2-ol.
pling was usually required (Scheme 3). To the best of
our knowledge, there is only one example in the liter-
ature for the synthesis of diarylacetylenes from
tandem Sonogashira cross-coupling reactions of aryl
bromides or iodides with 2-methyl-3-butyn-2-ol cata-
Table 1. Reaction of chlorobenzene (1a) with 2-methyl-3-
butyn-2-ol.[a]
lyzed by PdCl
2(PPh3)2/CuI,[7] although symmetrical
A
and unsymmetrical diarylacetylenes could be obtained
from the reactions of aryl iodides with trimethylsilyl-
acetylenes,[8,9] or aryl bromides with 4-aryl-2-methyl-3-
butyn-2-ols[8c] in the presence of palladium complexes.
In this paper, we report our results on the one-pot
synthesis of symmetrical and unsymmetrical diaryl-
acetylenes from the reaction of aryl chlorides with 2-
methyl-3-butyn-2-ol in the presence of PdCl2(PCy3)2,
E
in which a sequential double palladium-catalyzed So-
nogashira couplings of aryl chlorides is involved
(Scheme 4).
Results and Discussion
The reaction of chlorobenzene (1a) with 2-methyl-3-
butyn-2-ol was first investigated to optimize the reac-
tion conditions, and a variety of palladium catalysts,
solvents and additives was used (Table 1). When a
mixture of 1a (4.0 mmol), 2-methyl-3-butyn-2-ol
[a]
Reactions were carried out using 4.0 mmol of 1a,
1.0 mmol of 2-methyl-3-butyn-2-ol, 3.0 mmol of Cs2CO3,
0.1 mmol of additive (if used) and 0.05 mmol of catalyst
in 3.0 mL of solvent in a sealed tube at 1208C for 12 h.
GC yield (isolated yield) based on the amount of alkyne
used.
10.0 mmol of 1a was used.
TBAB=tetrabutylammonium bromide.
dppp=1, 3-bis(diphenylphosphino)propane.
[b]
(1.0 mmol), Cs2CO3 (3.0 mmol) and PdCl2(PCy3)2
G
[c]
(0.05 mmol) in DMSO (3.0 mL) was heated with stir-
ring at 1208C for 12 h, diphenylacetylene (2a) was
formed in 35% GC yield (Table 1, entry 1). The use
of other solvents such as NMP( N-methylpyrroli-
[d]
[e]
done), p-xylene, or an excess of 1a as solvent resulted highest yield of 2a was obtained (90% GC yield,
in the formation of 2a in somewhat lower yields Table 1, entry 6). Under the reaction conditions of
(Table 1, entries 2–4). The reaction was found to be entry 6, PdCl
N
N
accelerated significantly by addition of tetrabutylam- moderate catalytic activity affording 2a in 42% and
monium bromide or piperidine as additives. In partic- 50% GC yields, respectively (Table 1, entries 7 and
ular, when piperidine was used as the additive, the 8). In addition, neither PdCl
N
N
Scheme 4. One-pot synthesis of symmetrical diarylated acetylenes from the reaction of aryl chlorides with 2-methyl-3-butyn-
2-ol.
Adv. Synth. Catal. 2007, 349, 1738 – 1742
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1739

Chenyi Yi et al.
FULL PAPERS
displayed any catalytic activity (Table 1, entries 9 and with 2-methyl-3-butyn-2-ol to afford bis(2-thiophenyl)-
10). acetylene (2g) in 82% yield (Table 2, entry 6). Under
To demonstrate the efficiency and generality of this the same reaction conditions, electron-poor aryl chlor-
process, we have examined this transformation with ides could undergo the present transformation
both electron-poor and electron-rich aryl chlorides smoothly. Indeed, in the case of methyl 3-chloroben-
under the optimized reaction conditions as indicated zoate (1h), the corresponding product 2h was ob-
in entry 6 of Table 1, and the obtained results are tained in 77% isolated yield (Table 2, entry 7).
summarized in Table 2. 4-Chlorotoluene (1b) showed
However, it should be noted that in the cases of
similar reactivity as 1a to afford di-p-tolylacetylene much more electron-deficient chlorobenzene deriva-
(2b) in 88% yield (Table 2, entry 1). para- and ortho- tives such as 1-chloro-4-nitrobenzene and chloropen-
vinyl-substituted chlorobenzenes 1c–e also furnished tafluorobenzene, the desired diarylacetylene could
the corresponding diarylacetylenes in high yields not be obtained in satisfactory yields, because the for-
(Table 2, entries 2–4). These results indicated that the mation of some unidentified compounds with molecu-
vinyl group was tolerated under the reaction condi- lar weights more than that of the corresponding diary-
tions. When 1-chloronaphthalene (1f) was used, 2f lated acetylene were found (confirmed by GC-MS).
was isolated in 95% yield (Table 2, entry 5). A heter-
As shown in Scheme 3, the reported procedure for
ocyclic compound, 2-chlorothiophene (1g) reacted the synthesis of unsymmetrical diarylated acetylenes
from the reaction of aryl bromides or aryl iodides
with 2-methyl-3-butyn-2-ol usually includes two isolat-
ed steps. To extend the application of the present
methodology, we have also briefly examined the reac-
tion of two different aryl chlorides with 2-methyl-3-
butyn-2-ol to try to establish a one-pot synthetic
method for unsymmetrical diarylated acetylenes.
Table 3 shows the results of the reactions of 1a, 1b or
1f with 1h and 2-methyl-3-butyn-2-ol. The desired un-
symmetrical diarylated acetylenes 3 could be isolated
in fair yields. As expected, the symmetrical diarylated
acetylene 2 was also formed. In these cases, 2h could
be obtained in 30–40% yield, but 2a, 2b, or 2f was
formed in only a small amount. However, although
the reaction 1a with 1g and 2-methyl-3-butyn-2-ol also
gave a mixture of arylated acetylene products, at-
tempts to isolate and purify the corresponding unsym-
metrical diarylated acetylenes were not successful.
Table 2. Palladium-catalyzed reaction of aryl chlorides with
2-methyl-3-butyn-2-ol.[a]
Conclusions
In conclusion, we have developed an efficient, practi-
cal one-pot synthesis of symmetrical diarylacetylenes
in good to high yields by the direct cross-coupling of
aryl chlorides with 2-methyl-3-butyn-2-ol catalyzed by
PdCl2(PCy3)2. The present catalytic procedure in-
G
cludes two sequential Sonogashira couplings of inacti-
vated aryl chlorides, and has the advantages of ready
availability of the starting materials and easy handling
of the catalyst. Although the unsymmetrical diarylat-
ed acetylenes could be also obtained by the simulta-
neous coupling reaction of two different aryl chlorides
with 2-methyl-3-butyn-2-ol, the yields need to be im-
proved.
[a]
Reactions were carried out using 4.0 mmol of 1,
1.0 mmol of 2-methyl-3-butyn-2-ol, 3.0 mmol of Cs2CO3,
0.1 mmol of piperidine and 0.05 mmol of catalyst in
3.0 mL of DMSO in a sealed tube at 1208C for 12 h.
[b]
Isolated yield based on the amount of alkyne used.
1740
asc.wiley-vch.de
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1738 – 1742

Palladium-Catalyzed Efficient and One-Pot Synthesis of Diarylacetylenes
FULL PAPERS
Table 3. Synthesis of unsymmetrical diarylated acetylenes.[a]
36.8 mg,) in DMSO (3.0 mL) was charged under nitrogen in
a sealed tube and heated with stirring at 1208C for 12 h.
After cooling, the reaction mixture was diluted with CH2Cl2
to 5.0 mL and octadecane (0.2 mmol, 50.0 mg) was added as
internal standard for GC analysis. After GC and GC-MS
analyses, removal of the solvents and volatiles under
vacuum, the residue was then subjected to preparative TLC
isolation (silica, eluted with cyclohexane). 2a was obtained
as a white solid; yield: 153.0 mg (0.86 mmol, 86%). The re-
sults of GC analysis of the reaction mixture revealed that 2a
was formed in 90% yield.
Typical Procedure for the Reaction of 4-Chloro-
toluene (1b), Methyl 3-Chlorobenzoate (1h) with
2-Methyl-3-butyn-2-ol Affording Methyl 3-Phenyl-
ethynylbenzoate (3b) (Table 3, entry 2)
A
mixture of 1b (2.0 mmol, 253.0 mg), 1h (2.0 mmol,
341.2 mg), 2-methyl-3-butyn-2-ol (1.0 mmol, 84.0 mg),
Cs2CO3 (3.0 mmol, 978.0 mg), piperidine (0.1 mmol), and
PdCl2(PCy3)2 (0.05 mmol, 36.8 mg,) in DMSO (3.0 mL) was
N
charged under nitrogen in a sealed tube and heated with
stirring at 1208C for 12 h. After treatment as described
above and two-fold careful preparative TLC isolation, 3b
was obtained as yellow solid (yield: 80.0 mg, 0.32 mmol,
32%), and 2h was also isolated (yield: 64.6 mg, 0.22 mmol,
22%).
[a]
[b]
Reactions were carried out using 2.0 mmol of 1,
2.0 mmol of 1h, 1.0 mmol of 2-methyl-3-butyn-2-ol,
3.0 mmol of Cs2CO3, 0.1 mmol of piperidine and
0.05 mmol of catalyst in 3.0 mL of DMSO in a sealed
tube at 1208C for 12 h.
Products 2e, 2h, 3b and 3c are new compounds, their
1
structures were characterized by H, 13C NMR, mass spectra,
elemental analyses and/or high resolution MS. 2a is commer-
Isolated yield based on the amount of alkyne used.
cially available, and other products are known compounds
1
which were characterized by H, 13C NMR and mass spectra.
The characterization data for known products and the
copies of H and 13C NMR spectra for all products are given
in the Supporting Information. The spectroscopic data of
the new products are given below.
1
Experimental Section
General Methods
1,2-Bis[4-(2-propenyl)phenyl]acetylene (2e): Pale-yellow
1
solid, mp 140–1428C; H NMR (300 MHz, CDCl3): d=7.49
All organic starting materials were analytically pure and
(d, 4H, J=8.4 Hz), 7.44 (d, 4H, J=8.4 Hz), 5.42 (d, 2H, J=
0.7 Hz), 5.13 (d, 2H, J=0.7 Hz), 2.15 (s, 3H), 2.14 (s, 3H);
13C NMR (75 MHz, CDCl3): d=142.5, 140.9, 131.5, 125.4,
122.4, 113.2, 89.9, 21.6; GC-MS: m/z (% rel. int.)=258 (M+,
100), 243 (18), 202 (37), 189 (8), 115 (16), 91 (6); anal. calcd
for C20H18: C 93.02, H 6.98; found: C 94.02, H, 7.01; HR-
MS: m/z=258.1402 (M+), calcd. for C20H18: 258.1408.
used without further purification. PdCl2(PCy3)2 was pre-
C
1
300 MHz and 75 MHz, respectively. H chemical shifts (d)
were referenced to TMS, and 13C NMR chemical shifts (d)
were referenced to internal solvent resonance. GC analyses
of organic compounds were performed on an Agilent Tech-
nologies 1790 GC (with a TC-WAX capillary 25 m column)
instrument. Mass spectra were obtained on a Hewlett Pack-
ard 5890 Series II GC/MS spectrometer with a PEG-25M
column. High-resolution mass spectra were obtained with a
ZAB-HS mass spectrometer at the Department of Chemis-
try of Beijing University. Elemental analyses were obtained
with a Flash EA 1112 Element Analyzer in the Institute of
Chemistry, Chinese Academy of Sciences.
Bis(3-methoxycarbonylphenyl)acetylene (2h): Pale-yellow
1
solid, mp 169–1718C; H NMR (300 MHz, CDCl3): d=8.21
(s, 2H), 8.01 (d, 2H, J=7.7 Hz), 7.70 (d, 2H, J=7.7 Hz),
7.45 (virtual t, 2H, J=7.7 Hz), 3.94 (s, 6H); 13C NMR
(75 MHz, CDCl3): d=166.5, 135.8, 132.9, 130.6, 129.5, 128.6,
123.4, 89.2, 52.3; GC-MS: m/z (% rel. int.)=294 (M+, 100),
263 (75), 235 (15), 189 (17), 176 (48), 150 (16), 102 (8); anal.
calcd for C18H14O4: C 73.45, H 4.76; found: C 72.79, H 4.64;
HR-MS: m/z=294.0889 (M+), calcd. for C18H14O4: 294.0892.
Methyl 3-(p-tolylethynyl)benzoate (3b): Pale-yellow solid,
mp 127–1298C; 1H NMR (300 MHz, CDCl3): d=8.20 (m,
1H), 7.98 (d, 1H, J=7.8 Hz), 7.68 (d, 1H, J=7.8 Hz), 7.41–
7.46 (m, 3H), 7.15–7.17 (d, 2H, J=8.2 Hz), 3.93 (s, 3H),
2.39 (s, 3H); 13C NMR (75 MHz, CDCl3): d=166.5, 138.7,
135.6, 132.7, 131.6, 130.4, 129.2, 129.0, 128.5, 124.0, 119.8,
90.5, 87.7, 52.3, 21.5; GC-MS: m/z (% rel. inten.)=250 (M+,
100), 219 (34), 189 (29), 176 (9), 139 (5), 109 (9), 95 (7), 82
Typical Procedure for the Reaction of Chlorobenzene
(1a) with 2-Methyl-3-butyn-2-ol Affording Diphenyl-
acetylene (2a) (Table 1, entry 6)
A mixture of 1a (4.0 mmol, 450.0 mg), 2-methyl-3-butyn-2-ol
(1.0 mmol, 84.0 mg), Cs2CO3 (3.0 mmol, 978.0 mg), piperi-
dine (0.1 mmol, ca. 10 mL), and PdCl2(PCy3)2 (0.05 mmol,
A
Adv. Synth. Catal. 2007, 349, 1738 – 1742
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1741

Chenyi Yi et al.
FULL PAPERS
(5); HR-MS: m/z=250.0991 (M+), calcd. for C17H14O2:
250.0994.
wald, Angew. Chem. Int. Ed. 2003, 42, 5993–5996;
c) A. Kollhofer, T. Pullmann, H. Plenio, Angew. Chem.
Int. Ed. 2003, 42, 1056–1058; d) Z. Novak, A. Szabo, J.
Repasi, A. Kotschy, J. Org. Chem. 2003, 68, 3327–
3329; e) M. Feuerstein, H. Doucet, M. Santelli, Tetrahe-
dron Lett. 2004, 45, 8443–8446; f) J.-C. Hierso, A.
Fihri, R. Amardeil, P. Meunier, H. Doucet, M. Santelli,
V. Ivanov. Org. Lett. 2004, 6, 3473–3476; g) K. W. An-
derson, S. L. Buchwald, Angew. Chem. 2005, 117,
6329–6333; h) M. Lemhadri, H. Doucet, M. Santelli,
Tetrahedron 2005, 61, 9839–9847; i) Y. Liang, Y.-X.
Xie, J.-H. Li, J. Org. Chem. 2006, 71, 379–381.
Methyl 3-(1-naphthylethynyl)benzoate (3c): Viscous oil;
1H NMR (300 MHz, CDCl3): d=8.45 (d, 1H, J=8.5 Hz),
8.31 (m, 1H), 8.01 (d, 1H, J=7.6 Hz), 7.75–7.85 (m, 4H),
7.41–7.60 (m, 4H), 3.93 (s, 3H); 13C NMR (75 MHz,
CDCl3): d=166.4, 135.7, 133.2, 132.7, 130.6, 130.5, 129.7,
129.3, 129.1, 128.6, 128.4, 126.9, 126.5, 126.1, 125.3, 123.9,
120.5, 93.2, 88.5, 52.3; GC-MS: m/z (% rel. int.)=286 (M+,
100), 255 (12), 226 (49), 200 (5), 127 (8), 113 (20), 100 (5);
HR-MS: m/z=286.0992 (M+), calcd. for C20H14O2: 286.0994.
[4] C. Yi, R. Hua, J. Org. Chem. 2006, 71, 2535–2537.
[5] Copper-free means that a copper salt has not been
added as a co-catalyst, however, this does not exclude
the presence of a trace amount of copper in the palladi-
um complex. Indeed, trace amounts of copper (1.36ꢂ
10À4 mg of copper per mg of catalyst) were found in a
palladium catalyst by ICP-MS analyses.
Acknowledgements
This project (20573061) was supported by National Natural
Science Foundation of China. The authors thank Dr. Asish
K. Sharma for his kind English proofreading.
[6] For pioneering work, see: a) D. E. Ames, D. Bull, C.
Takundwa, Synthesis 1981, 364–365; example of recent
reports, see: b) G. Menchi, A. Scrivanti, U. Matteoli, J.
Mol. Chem. A: Chem. 2000, 152, 77–82.
[7] a) Z. Novak, P. Nemes, A. Kotschy, Org. Lett. 2004, 6,
4917–4920; b) H.-F. Chow, C.-W. Wan, K.-H. Low, Y.-
Y. Yeung, J. Org. Chem. 2001, 66, 1910–1913.
[8] a) M. J. Mio, L. C. Kopel, J. B. Braun, T. L. Gadzikwa,
K. L. Hull, R. G. Brisbois, C. J. Markworth, P. A.
Grieco, Org. Lett. 2002, 4, 3199–3202; b) J. Gil-Molto,
C. Najera, Adv. Synth. Catal. 2006, 348, 1874–1882;
c) A. Nagy, Z. Novak, A. Kotschy, J. Organomet.
Chem. 2005, 690, 4453.
[9] The formation of diarylacetylene has also been found
in the reaction of chlorobenzene with trimethylsilylace-
tylene as reported in our recent paper.[4] In addition,
the formation of diarylacetylenes as a considerable by-
product in the reaction of 4-iodotoluene with trimethyl-
silylacetylene under microwave heating has been men-
tioned, see: N. E. Leadbeater, M. Marco; B. J. Tomi-
nack, Org. Lett. 2003, 5, 3919–3922.
References
[1] Examples of reviews, see: a) E.-I. Negishi, Acc. Chem.
Res. 1982, 15, 340–348; b) N. Miyaura, A. Suzuki,
Chem. Rev. 1995, 95, 2457–2483; c) A. Suzuki, J. Orga-
nomet. Chem. 1999, 576, 147–168; d) I. P. Beletskaya,
A. V. Cheprakov, Chem. Rev. 2000, 100, 3009–3066;
e) J. G. de Vries, Can. J. Chem. 2001, 79, 1086–1092;
f) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lem-
aire, Chem. Rev. 2002, 102, 1359–1470; g) C. E. Tucker,
J. G. de Vries, Top. Catal. 2002, 19, 111–118; h) F. Belli-
na, A. Carpita, R. Ross. Synthesis 2004, 2419–2440;
i) L. Bai, J.-X. Wang, Cur. Org. Chem. 2005, 9, 535–
553.
[2] Examples of recent reviews, see: a) A. F. Littke, G. C.
Fu, Angew. Chem. Int. Ed. 2002, 41, 4176–4211; b) K.
Sonogashira, J. Organomet. Chem. 2002, 653, 46–49;
c) E. Negishi, L. Anastasia, Chem. Rev. 2003, 103,
1979–2017; d) K. C. Nicolaou, P. G. Bulger, D. Sarlah,
Angew. Chem. Int. Ed. 2005, 44, 4442–4489.
[3] a) M. R. Eberhard, Z. Wang, C. M. Jensen, Chem.
[10] N. W. Alcock, T. J. Kemp, F. L. Wimmer, J. Chem. Soc.,
Commun. 2002, 818–819; b) D. Gelman, S. L. Buch-
Dalton Trans. 1981, 635–638.
1742
asc.wiley-vch.de
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1738 – 1742