Efficient palladium-catalyzed homocoupling reaction and sonogashira cross-coupling reaction of terminal alkynes under aerobic conditions

Article abstract of DOI:10.1021/jo0503310

An efficient method for palladium-catalyzed homocoupling reaction of terminal alkynes in the synthesis of symmetric diynes is presented. The results showed that both Pd(OAc)₂ and CuI played crucial roles in the reaction. In the presence of 2 mo

Full text of DOI:10.1021/jo0503310

Efficient Palladium-Catalyzed Homocoupling Reaction and
Sonogashira Cross-Coupling Reaction of Terminal Alkynes under
Aerobic Conditions
Jin-Heng Li,* Yun Liang, and Ye-Xiang Xie
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, College of Chemistry
and Chemical Engineering, Hunan Normal University, Changsha 410081, China
jhli@hunnu.edu.cn
Received February 21, 2005
An efficient method for palladium-catalyzed homocoupling reaction of terminal alkynes in the
synthesis of symmetric diynes is presented. The results showed that both Pd(OAc)2 and CuI played
crucial roles in the reaction. In the presence of 2 mol % Pd(OAc)2, 2 mol % CuI, 3 equiv of Dabco,
and air, homocoupling of various terminal alkynes afforded the corresponding symmetrical diynes
in moderate to excellent yields, whereas low yields were obtained without either Pd(OAc)2 or CuI.
Moreover, high TONs (turnover numbers; up to 940 000 for the reaction of phenylacetylene) for
the homocoupling reaction were observed. Under similar reaction conditions, cross-coupling of 1-iodo-
4-nitrobenzene with phenylacetylene was also carried out smoothly in quantitative yield. However,
the presence of CuI disfavored the palladium-catalyzed Sonogashira cross-coupling reactions of
the less active aryl iodides and bromides. In the presence of 0.01-2 mol % Pd(OAc)2, a number of
aryl iodides and bromides were coupled with terminal alkynes in good to excellent yields. It is
noteworthy that this protocol employs mild, efficient, aerobic, copper-free, and ligand-free conditions.
Introduction
nogashira cross-coupling reaction of aryl halides with
terminal alkynes2,11-15 represent two particularly effec-
tive approaches to the synthesis of the functionalized
alkynes. For the Pd(II)-catalyzed homocoupling reactions
of terminal alkynes,16 several mild and efficient catalytic
systems have been reported, including the use of
(1) Pd(PPh3)4, CuI, Et3N, and chloroacetone in C6H6;4
(2) Pd(dba)2, n-Bu4NBr, NaOH, and allyl bromide in
CH2Cl2;5 (3) PdCl2(PPh3)2, CuI, and I2 in i-Pr3N;6
(4) PdCl2(PPh3)2, CuI, amines (Et3N or Dabco), and
bromoacetate in THF;7 and (5) PdCl2(PPh3)2, CuI, PPh3,
and O2 (or I2) in Et3N/MeCN.8,9 Besides additives such
PPh3 and TBAB, a stoichiometric amount of a reoxidant
is necessary for successful homocoupling in all cases,
Alkynes are useful building blocks in organic synthesis
and a basic functional group in many natural products
and bioactive compounds.1 For these reasons, much
attention has been given to the development of efficient
and selective methods for the synthesis of alkynes.2-15
Of these transformations, both the palladium-catalyzed
homocoupling reaction of terminal alkynes2-10 and So-
(1) (a) Viehe, H. G. Chemistry of Acetylene; Marcel Dekker: New
York, 1969; p 597. (b) Bohlmann, F.; Burkhart, F. T.; Zero, C. Naturally
Occurring Acetylenes; Academic Press: New York, 1973. (c) Traha-
novsky, W. S. Oxidation in Organic Chemistry; Academic Press: New
York, 1973; Vol. 5-B. (d) Hansen L.; Boll, P. M. Phytochemistry 1986,
25, 285. (e) Kim, Y. S.; Jin, S. H.; Kim, S. L.; Hahn, D. R. Arch. Pharm.
Res. 1989, 12, 207. (f) Matsunaga, H.; Katano, M.; Yamamoto, H.;
Fujito, H.; Mori, M.; Tukata, K. Chem. Pharm. Bull. 1990, 38, 3480.
(g) Hudlicky, M. Oxidation in Organic Chemistry; ACS Monograph 186;
American Chemical Society: Washington, DC, 1990; p 58. (h) Sono-
gashira, K. In Comprehensive Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon: Oxford, UK, 1991; Vol. 3, p 551. (i) Alonso, F.;
Beletskaya, I. P.; Yus, M. Chem. Rev. 2004, 104, 3079.
(3) For representative reviews on the palladium-catalyzed homo-
coupling reactions of alkynes, see: (a) Siemsen, P.; Livingston, R. C.;
Diederich, F. Angew. Chem., Int Ed. 2000, 39, 2632. (b) Negishi E.;
Alimardanov, A. In Handbook of Organopalladium Chemistry for
Organic Synthesis; Negishi, E., Ed.; Wiley-Interscience: New York,
2002; Vol. 1, p 989.
(4) Rossi, R.; Carpita, A.; Bigelli, C. Tetrahedron Lett. 1985, 26, 523.
(5) Vlassa, M.; Ciocan-Tarta, I.; Margineanu, F.; Oprean, I. Tetra-
hedron 1996, 52, 1337.
(2) (a) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH: Weinheim, 1998. (b) Miyaura, N. Cross-
Coupling Reaction; Springer: Berlin, 2002.
10.1021/jo0503310 CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/19/2005
J. Org. Chem. 2005, 70, 4393-4396
4393

Li et al.
TABLE 1. Palladium-Catalyzed Homocoupling Reaction
of Phenylacetylene (1a)a
which is consistent with the results of Marder and co-
workers.9,11 Very recently, we also reported a phosphine-
and amine-free, PdCl2/CuI-catalyzed method for homo-
coupling of alkynes in the synthesis of diynes.10 However,
a stoichiometric amount of harmful reoxidant (Me3NO)
was still required. To satisfy environmental concerns, O2
is an attractive alternative to the reported reoxidants.
It is noteworthy that the use of O2 as the reoxidant by
Fairlamb, Marder, 14and co-workers often results in
incomplete reactions to afford moderate to good yields
in many cases.9,11 Therefore, the use of air as an envi-
ronmentally benign reoxidant for the phosphine-free
palladium(II)-catalyzed homocoupling reaction is still
significant.
entry
base (equiv)
time (h)
isolated yield (%)
1b
2
0
7
3.5
2.5
2
8
74
83
90
100
88
81
15
58
65
DABCO (0.5)
DABCO (1.0)
DABCO (2.0)
DABCO (3.0)
DABCO (3.0)
Et3N (3.0)
3
4
5
2
6c
7
12
7
20
2
2
8d
9e
10f
NaOAc (3.0)
DABCO (3.0)
DABCO (3.0)
For the Sonogashira cross-coupling reaction, the com-
bination of palladium, phosphines, and CuI is generally
employed as the catalytic system under degassed condi-
tions.2,11,12 However, many phosphine ligands are air-
sensitive and expensive, resulting in significant limits
on their synthetic applications.12,13 Furthermore, the
presence of CuI can result in the formation of some Cu-
(I) acetylides in situ that can readily undergo oxidative
homocoupling reaction of alkynes.2-11 To overcome these
drawbacks, many phosphine- and copper-free palladium-
catalyzed Sonogashira cross-coupling protocols have been
developed.14,15 However, only one report by Yang and co-
workers on the palladium-catalyzed Sonogashira cross-
coupling reaction employed under aerobic, ligand-free,
and copper-free conditions.15e In the presence of 1 mol %
PdCl2, various aryl iodides were treated with terminal
alkynes to afford moderate to good yields of the corre-
a Unless otherwise indicated, the reaction conditions were as
follows: 1a (1 mmol), Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %), base,
and MeCN (5 mL) at room temperature under air. b Conversion
of 1a was 10% as determined by GC analysis. c Under argon
(bubbled). d Conversion of 1a was 25% as determined by GC
analysis. e Without CuI. Conversion of 1a was 60% as determined
by GC analysis. f Without Pd(OAc)2. Conversion of 1a was 100%
as determined by GC analysis.
sponding cross-coupled products in aqueous media. How-
ever, the scope of this method is limited to aromatic
iodides, and rather high catalyst loading is required. For
these reasons, the development of efficient, ligand-free,
and copper-free palladium-catalyzed Sonogashira cross-
coupling reaction under aerobic conditions still remains
an area of current interest. Here, we report our findings
of those reactions in detail (eqs 1 and 2).
(6) Liu, Q.; Burton, D. J. Tetrahedron Lett. 1997, 38, 4371.
(7) Lei, A.; Srivastava, M.; Zhang, X. J. Org. Chem. 2002, 67, 1969.
(8) Fairlamb, I. J. S.; Ba¨uerlein, P. S.; Marrison, L. R.; Dickinson,
J. M. Chem. Commun. 2003, 632.
(9) Batsanov, A. S.; Collings, J. C.; Fairlamb, I. J. S.; Holland, J.
P.; Howard, J. A. K.; Lin, Z.; Marder, T. B.; Parsons, A. C.; Ward, R.
C.; Zhu, J. J. Org. Chem. 2005, 70, 703.
(10) Li, J.-H.; Liang, Y.; Zhang, X.-D. Tetrahedron 2005, 61, 1903.
(11) Nguyen, P.; Yuan, Z.; Agocs, L.; Lesley, G.; Marder, T. B. Inorg.
Chim. Acta 1994, 220, 289.
(12) For recent representative papers on phosphine-palladium
catalysts, see: (a) Bo¨hm, V. P. W.; Herrmann, W. A. Eur. J. Org. Chem.
2000, 3679. (b) Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295.
(c) Me´ry, D.; Heuze´, K.; Astrc, D. Chem. Commun. 2003, 1934.
(d) Gelman, D.; Buthwald, S. L. Angew. Chem., Int. Ed. 2003, 42, 5993.
(e) Elangovan, A.; Wang, Y.-H.; Ho, T.-I. Org. Lett. 2003, 5, 1841.
(f) Feuerstein, M.; Berthiol, F.; Doucet, H.; Santelli, M. Synthesis 2004,
1281 and references therein. (g) Wolf, C.; Lerebours, R. Org. Biomol.
Chem. 2004, 2, 2161. (h) Hierso, J.-C.; Fihri, A.; Amardeil, R.; Meunier,
P.; Doucet, H.; Santelli, M.; Ivanov, V. V. Org. Lett. 2004, 6, 3473.
(i) Heuze´, K.; Me´ry, D.; Gauss, D.; Blais, J.-C.; Astruc, D. Chem. Eur.
J. 2004, 10, 3936. (j) Cheng, J.; Sun, Y.; Wang, F.; Guo, M.; Xu, J.-H.;
Pan, Y.; Zhang, Z. J. Org. Chem. 2004, 69, 5428. (k) Feuerstein, M.;
Doucet, H.; Santelli, M. Tetrahedron Lett. 2004, 45, 8443. (l) Adjabeng,
G.; Brenstrum, T.; Frampton, C. S.; Robertson, A. J.; Hillhouse, J.;
McNulty, J.; Capretta, A. J. Org. Chem. 2004, 69, 5082. (m) Arques,
A.; Aunon, D.; Molina, P. Tetrahedron Lett. 2004, 45, 4337.
(n) Leadbeater, N. E.; Tominack, B. J. Tetrahedron Lett. 2003, 44, 8653.
(13) (a) Pignolet, L. H., Ed. Homogeneous Catalysis with Metal
Phosphine Complexes; Plenum: New York, 1983. (b) Parshall, G. W.;
Ittel, S. Homogeneous Catalysis; J. Wiley and Sons: New York, 1992.
(14) For recent representative papers on phosphine-free ligand-
palladium catalysts, see: (a) Loch, J. A.; Albrecht, M.; Peris, E.; Mata,
J.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 700.
(b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (c) Batey,
R. A.; Shen, M.; Lough, A. J. Org. Lett. 2002, 4, 1411. (d) Eckhardt,
M.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 13642. (e) Yong, B. S.;
Nolan, S. P. Chemtracts: Org. Chem. 2003, 205. (f) Rau, S.; Lamm,
K.; Goerls, H.; Schoeffel, J.; Walther, D. J. Organomet. Chem. 2004,
689, 3582. (g) Gossage, R. A.; Jenkins, H. A.; Yadav, P. N. Tetrahedron
Lett. 2004, 45, 7689. (h) Alonso, D. A.; Botella, L.; Najera, C.; Pacheco,
M. C. Synthesis 2004, 10, 1713. (i) Thakur, V. V.; Kumar, N. S. C. R.;
Sudalai, A. Tetrahedron Lett. 2004, 45, 2915.
Results and Discussion
Pd(OAc)2 and CuI-Catalyzed Homocoupling of
Terminal Alkynes. As shown in Table 1, our initial goal
was to evaluate the effect of bases on the Pd(II)-catalyzed
homocoupling of phenylacetylene (1a). The results showed
that use of DABCO as the base gave the best results,
and the amount used also affected both rates and yields
of the reaction (entries 1-8). Without base, only 8% yield
of the corresponding diyne 2a was isolated after 7 h in
(15) For recent representative papers on ligand-free palladium
catalysts, see: (a) Alami, M.; Ferri, F.; Linstrumelle, G. Tetrahedron
Lett. 1993, 34, 6403. (b) Heidenreich, R. G.; Kohler, K.; Krauter, J. G.
E.; Pietsch, J. Synlett 2002, 1118. (c) Urgaonkar, S.; Verkade, J. G. J.
Org. Chem. 2004, 69, 5752 and references therein. (d) Son, S. U.; Jang,
Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Lee, J.; Hyeon, T. J.
Am. Chem. Soc. 2004, 126, 5026. (e) Liang, B.; Dai, M.; Chen, J.; Yang,
Z. J. Org. Chem. 2005, 70, 391.
(16) Compared with the copper-mediated homocoupling reactions
of terminal alkynes, the palladium-catalyzed homocoupling transfor-
mations are arguably more mild, efficient, and chemoselective. More-
over, the palladium-catalyzed homocoupling reactions of aliphatic
alkynes as well as aromatic alkynes afford good to excellent yields,
whereas low to moderate yields are often obtained when homocoupling
of aliphatic alkynes were employed under the Hay reaction conditions.
Reference 3a has reviewed the scope and limitations of both the copper-
catalyzed homocoupling reactions and palladium-catalyzed homo-
coupling reactions in detail.
4394 J. Org. Chem., Vol. 70, No. 11, 2005

Homocoupling and Cross-Coupling Reactions of Terminal Alkynes
TABLE 2. Palladium-Catalyzed Homocoupling
Reactions of Alkynesa
TABLE 3. Screening the Catalytic Efficiency of the
Palladium-Catalyzed Homocoupling Reaction in Aira
entry alkyne Pd(OAc)2 (mol %) time (h) yield (%)b
TON
1
2
3
4
5
6
1a
1a
1f
1f
1l
0.001
0.0001
0.001
0.0001
0.001
0.0001
10
12
12
12
20
24
100
94
100
90
60
41
100 000
940 000
100 000
900 000
60 000
1l
410 000
a Unless otherwise indicated, the reaction conditions were as
follows: 1 (1 mmol), Pd(OAc)2, CuI (2.0 mol %), DABCO (3 equiv),
and MeCN (5 mL) at room temperature in air. b Isolated yield.
palladium-catalyzed homocoupling reaction tolerated a
variety of functional groups, and the yields and rates are
based upon the substrates. For homocoupling of aromatic
alkynes 1b-e, the corresponding diynes 2b-e were
obtained in 98, 70, 75, and 91% yields, respectively, in
the presence of Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %),
and DABCO (3.0 equiv) (entries 1-4). It is noteworthy
that aliphatic alkynes 1f-h give the corresponding
coupled products 2f-h in quantitative yields (entries 5,
7, and 8). Homocoupling of alkynes bearing different
functional groups such as 1-ethynyl cyclohexene (1i),
1-ethynyl cyclohexanol (1j), 3,3,5-trimethyl hex-1-yn-3-
ol (1k), and propargyl acetate (1l), was also carried out
smoothly to afford the corresponding diynes 2i-l in 97,
95, 93, and 71% yields, respectively (entries 9-12).
The catalytic efficacy was further evaluated, and the
results are summarized in Table 3. For coupling of
alkynes 1a, 1f, and 1l, the catalyst loadings could be
decreased to 0.0001 mol %, and moderate to good yields
could still be obtained after prolonged stirring. For
example, phenylacetylene (1a) was efficiently coupled in
high yield (94%, TON ) 940 000) for 12 h when the
catalyst loading was decreased to 0.0001 mol %. The
results also suggested that Pd(OAc)2 played a criucial role
in the reaction. Without Pd(OAc)2, 1a was consumed
completely in 2 h to afford 65% yield of 2a (entry 10 in
Table 1), whereas 94% yield of 2a was obtained even in
the presence of 0.0001 mol % Pd (entry 2 in Table 3).
Pd(OAc)2-Catalyzed Sonogashira Cross-Coupling
Reaction of Aryl Halides with Terminal Alkynes.
As demonstrated in Table 4, we were gratified to observe
that the Sonogashira cross-coupling reactions of aryl
halides with terminal alkynes were also carried out
smoothly under the above optimized reaction conditions.
In the presence of Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %),
and air, 1-iodo-4-nitrobenzene (3a) was reacted with 1a
to afford the corresponding cross-coupled product 4 in
quantitative yield (entry 1). However, our goal was to
employ palladium catalysts under ligand-free, copper-
free, and aerobic conditions to effect the Sonogashira
cross-coupling reaction. As expected, quantitative yield
of 4 was still obtained in the absence of CuI (entry 2).
Furthermore, the presence of CuI disfavored the cross-
coupling reactions of the less active aryl iodides and
bromides (entries 7, 8, 11, and 12). For example, in the
presence of Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %), and
air, only 11% yield of 7 was isolated after 24 h, whereas
the yield of 7 was increased to 74% without CuI (entries
7 and 8). The results also indicated that Pd(OAc)2 was
highly efficient for the Sonogashira cross-coupling reac-
tions. In the presence of 0.01 mol % Pd(OAc)2, coupling
a Unless otherwise indicated, the reaction conditions were as
follows: 1 (1 mmol), Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %), DABCO
(3 equiv), and MeCN (5 mL) at room temperature under air.
b Without Pd(OAc)2. Conversion of 1f was 55% as determined by
GC analysis. c Without Pd(OAc)2. Conversion of 1l was 30% as
determined by GC analysis.
the presence of Pd(OAc)2 (2.0 mol %), CuI (2.0 mol %),
and air (entry 1), whereas 74% yield of 2a was obtained
after 3.5 h when 0.5 equiv of DABCO was added (entry
2). The yield of 2a was increased sharply to 100% by
further increasing the amount of DABCO to 3 equiv
(entry 5). Other bases such as Et3N and NaOAc were less
effective than DABCO (entries 5, 7, and 8). The previous
results2,9,11 suggested that the presence of O2 could
improve the homocoupling reaction, so we then tested the
effect of O2. Indeed, the reaction is slow, and only 90%
yield was obtained for 12 h when argon was bubbled to
remove air (entry 6). It is noteworthy that both Pd(OAc)2
and CuI play crucial roles in the reaction (entries 5, 9,
and 10). Without either Pd(OAc)2 or CuI, low yields of
2a were isolated after 2 h. The following results in Table
2 also demonstrated that Pd(OAc)2 had a fundamental
influence on the reaction. Without Pd(OAc)2, CuI-
catalyzed homocoupling of 1f and 1l, respectively, af-
forded the corresponding products 2f and 2l in rather low
yields in the presence of DABCO and air (50 and 26%
yields, respectively; entries 6 and 13 in Table 2).
Under the optimized reaction conditions, the homo-
coupling reactions of various other terminal alkynes 1b-l
were carried out smoothly to afford the corresponding
diynes 2b-l in moderate to high yields, and the results
are summarized in Table 2. The results show that the
J. Org. Chem, Vol. 70, No. 11, 2005 4395

Li et al.
15). As earlier reports,2,11,13,14 we expected to shift the
selectivity toward the cross-coupling reaction by increas-
ing the amount of alkynes. Indeed, good yields of the
desired products were obtained at 50 °C when the
amount of alkynes was increased to 1.8 equiv (entries
16 and 17). An attempt to couple bromobenzene (3h) was
not successful (entry 18).
TABLE 4. Pd(OAc)2-Catalyzed Sonogashira
Cross-Coupling Reaction of ArX (X ) I, Br) with
Terminal Alkynesa
Conclusion
In summary, both ligand-free palladium-catalyzed
homocoupling reaction and Sonogashira cross-coupling
reaction under aerobic conditions have been developed.
In the presence 2 mol % of Pd(OAc)2, 2 mol % CuI, 3 equiv
of DABCO, and air, homocoupling of various terminal
alkynes was carried out efficiently, providing moderate
to excellent yields and high TONs (maximal TONs up to
940 000). This protocol not only tolerates a range of
functional groups but also does not require any additives
such as phosphine ligands and TBAB. We also found that
the presence of CuI disfavored the palladium-catalyzed
Sonogashira cross-coupling reactions of the less active
aryl iodides and bromides. Without CuI, the cross-
coupling reactions of a number of aryl iodides and
bromides with terminal alkynes afforded good to excellent
yields of the desired products in the presence of 0.01-
2 mol % Pd(OAc)2. It is noteworthy that our protocol
employs a relatively low palladium catalyst loading under
mild, aerobic, copper-free, and ligand-free conditions.
Currently, further efforts to apply these methods in other
transformations are underway in our laboratory.
Experimental Section
(1) Typical Experimental Procedure for the Pal-
ladium-Catalyzed Homocoupling Reactions of Alkynes.
A mixture of alkyne 1 (1 mmol), Pd(OAc)2 (2.0 mol %), CuI
(2.0 mol %), DABCO (3 equiv), and MeCN (5 mL) was stirred
under air at room temperature for the desired time until
complete consumption of starting material as judged by TLC
and GC analysis. After the usual workup, the residue was
purified by flash column chromatography to give 2 (hexane or
hexane/ethyl acetate).
(2) Typical Experimental Procedure for the Pd(OAc)2-
Catalyzed Sonogashira Cross-Coupling Reactions. First,
Pd(OAc)2 (4.5 mg, 2.0 mol %) was dissolved in MeCN (200 mL).
Then, the indicated amount of Pd(OAc)2 acetonitrile solution
was added to a mixture of alkyne 1 (the indicated amount in
Table 4), aryl halide 3 (1 mmol), DABCO (3 equiv), and MeCN
(5 mL). Then, the mixture was stirred under air at the
indicated reaction temperature for the desired time until
complete consumption of starting material as judged by TLC
and GC analysis. After the mixture was filtered and evapo-
rated, the residue was purified by flash column chromatogra-
phy (hexane or hexane/ethyl acetate) to afford the desired
coupled products 4-10.
a Unless otherwise indicated, the reaction conditions were as
follows: 3 (1 mmol), 1 (1.2 mmol), Pd(OAc)2, DABCO (3 equiv),
and MeCN (5 mL) at room temperature under air. b Isolated yield.
c CuI (2 mol %), and ca. 90% yield of 2a was isolated (yield based
on 1a). d At 70 °C. e 1 (1.8 mmol) at 50 °C.
of 3a with 1a still afforded good yield of 4 after prolonged
stirring (92%, TON: 9200; entry 4). It is noteworthy that
coupling of a number of aryl iodides, including the
deactivated 1-iodo-4-methoxybenzene (3f), with alkynes
are carried out smoothly to afford moderate to excellent
yields in the presence of 0.1 mol % Pd(OAc)2 (entries
5-13). For the coupling reactions of aryl bromides;
however, the catalytic efficiency was decreased to some
extent (entries 14-18). For example, treatment of 1-bromo-
4-nitrobenzene (3g) with 1a, Pd(OAc)2 (2 mol %), and
DBCO (3 equiv) afforded a rather low yield even at
70 °C due to the competition between the homocoupling
reaction and the cross-coupling reaction (entries 14 and
Acknowledgment. We thank the National Natural
Science Foundation of China (No. 20202002) for finan-
cial support.
Supporting Information Available: Analytical data and
spectra (1H and 13C NMR) for all products 2 and 4-10. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO0503310
4396 J. Org. Chem., Vol. 70, No. 11, 2005