PEG-supported imidazolium chloride: A highly efficient and reusable reaction medium for the Heck reaction

Article abstract of DOI:10.1055/s-2005-871566

Abstract: Pd(OAc)² in [PEGmim][Cl] constituted a highly efficient and recyclable catalytic system for the Heck reaction of aryl bromides and activated aryl chlorides in the absence of ligands. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-871566

LETTER
1861
PEG-Supported Imidazolium Chloride: A Highly Efficient and Reusable
Reaction Medium for the Heck Reaction
P
L
EG-Supported
i
Imidaz
a
oliumChlo
n
ride g Wang, Yuhong Zhang,* Chunsong Xie, Yanguang Wang*
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
E-mail: yhzhang@zjuem.zju.edu.cn; E-mail: orgwyg@zju.edu.cn
Received 28 February 2005
recyclable medium for the Pd-catalyzed Heck reaction of
aryl bromides and aryl chlorides.
Abstract: Pd(OAc) in [PEGmim][Cl] constituted a highly effi-
cient and recyclable catalytic system for the Heck reaction of aryl
bromides and activated aryl chlorides in the absence of ligands.
2
As shown in Scheme 1, the preparation of [PEGmim][Cl]
(
3) was accomplished by esterification of 1-(2-hydroxyl-
Key words: Heck reaction, ionic liquids, PEG
ethyl)-3-methyl-imidazolium chloride (1) with PEG-suc-
cinic acid (2), which was derived from PEG 4000 and
1
4
succinic anhydride according to our previous procedure.
The Heck reaction has been extensively utilized in the
preparation of a wide variety of olefins. The traditional
1
1
H NMR spectrum analysis showed that PEG-succinic
acid was completely converted to [PEGmim][Cl]. The
melting point of the resulting [PEGmim][Cl] was around
47 °C, which is lower than PEG 4000 (around 55 °C).
PEGmim][Cl] was readily soluble in water, acetonitrile,
acetone, dichloromethane, and methanol, but it can be
quantitatively precipitated by the addition of diethyl ether.
Heck reaction is performed with a palladium catalyst with
phosphine ligands in the presence of a base under an inert
2
atmosphere. However, the expensive palladium complex
[
is often lost at the end of the reaction, which limited the
large-scale application of Heck reactions. On the other
hand, phosphine ligands, especially the electron-rich
phosphine ligands, are often toxic, water-, and air-sensi-
tive. Consequently, several phosphine-free and/or recy-
clable palladium catalytic systems have been developed,
Cl
ClCH2CH2OH
OH
N
N
N
N
+
70 °C , 24 h
3
which include the heterogeneous palladium catalysts,
4
5
nanopalladium catalysts, sulfur-based palladacyles,
1
6
palladium complexes of nitrogen ligands, and carbene
O
7
O
O
complexes of palladium.
OH
PEG 4000
OCOCH2CH2CO2H
Ionic liquids (ILs) have been adopted as a reaction media
for the recycling of catalysts in a broad range of catalytic
DMAP
2
8
9
reactions, including the Heck reaction. It was reported
that ILs as media could promote the Pd-catalyzed Heck
1
0
Cl
reaction. However, the currently available ILs are
moderately to enormously expensive. For this reason, the
immobilization of catalysts in ILs on silica or polymers
1
, DCC
OCOCH2CH2CO2CH2CH2
[PEGmim][Cl]
N
N
+
DMAP, 12 h, N2
3
1
1
has been reported.
Scheme 1
More recently, poly(ethylene glycol) (PEG) has been suc-
cessfully used as an alternative reagent to replace ILs in
1
2
The combination of [PEGmim][Cl] with Pd(OAc) pro-
2
biphasic catalytic systems. PEGs are non-volatile liq-
uids and are orders of magnitude less expensive. We and
vides a ligandless catalytic system for the Heck reaction.
In order to optimize the reaction conditions, the deactivat-
ed bromoanisole was chosen as the initial coupling partner
in combination with styrene. [PEGmim][Cl] (3 g) and
Pd(OAc) (0.5 mol%) were applied as the catalytic system
in the absence of any ligand. As shown in Table 1, no
conversion was observed using Et N as the base (Table 1,
entry 1), while the inorganic bases Na CO (Table 1, entry
) and NaOAc (Table 1, entry 3) gave poor conversions.
However, K PO (Table 1, entry 4) and K CO (Table 1,
entry 5) afforded excellent or quantitative conversion.
Decreasing the reaction temperature led to a drop in the
conversion (Table 1, entry 6). Using PEG 4000 instead of
PEGmim][Cl] as the medium could also result in a sig-
nificant decrease in conversion (Table 1, entry 7). It is
13
others have investigated the Heck reaction in PEG. We
found that Pd(OAc) could be readily reduced to Pd nano-
2
particles by the terminal hydroxy group of PEG under the
1
3b
2
Heck reaction conditions. The resulting nano-Pd/PEG
was an excellent recycling system for the Heck reaction of
aryliodides, but it was ineffective for aryl bromides and
aryl chlorides. In this letter, we report our new research
effort in the development of PEG-supported 3-methyl-
imidazolium chloride ([PEGmim][Cl]) as a more efficient
3
2
3
2
3
4
2
3
SYNLETT 2005, No. 12, pp 1861–1864
0
1
.0
8
.2
0
0
5
[
Advanced online publication: 22.06.2005
DOI: 10.1055/s-2005-871566; Art ID: U05205ST
©
Georg Thieme Verlag Stuttgart · New York

1
862
L. Wang et al.
LETTER
noteworthy that 1-butyl-3-methylimidazolium chloride trans-stilbene. As shown in Table 3, excellent yields were
[BMIm][Cl]), a typical IL, only gave a trace amount of achieved for the electron-deficient aryl chlorides under
(
the product under the same reaction conditions (Table 1, these reaction conditions (Table 3, entries 1–3 and 5–7),
entry 8). These facts demonstrate that the binding of an IL whereas the electron-neutral chlorobenzene gave low to
molecule to PEG via the covalent bond provides a unique modest yields (Table 3, entries 4 and 8).
reaction medium for the Heck reaction. The results of GC
1
Table 2 Heck Reactions of Bromoarenes with Olefins Catalyzed by
and H NMR spectrocopy indicated that the coupling
Pd(OAc) (0.5 mol%) in [PEGmim][Cl]
2
reaction in [PEGmim][Cl] only gave the trans-stilbene.
Similar stereoselectivity was observed when PEG was
0.5 mol%,
Pd(OAc)2
1
3
used as the reaction medium.
In an attempt to extend the application scope of Pd(OAc)2/
PEGmim][Cl] system, several representative aryl bro-
mides were tested. As shown in Table 2, the coupling of Entry
electron-deficient aryl bromides with both styrene and
[
PEGmim][Cl]
R2
R1
Br +
R1
R2
K2CO3, 140 °C
[
R1
R2
Time (h)
Yield (%)a,b
1
NO2
Cl
CO Et
8
8
95
93
97
94
95
96
95
91
97
95
92
93
2
ethyl acrylate gave almost quantitative yields (Table 2,
entries 1–3 and 7–9), while the deactivated aryl bromides
bearing electron-donating substituents also afforded the
coupled products in excellent yields after a prolonged
reaction time (10 h; Table 2, entries 5, 6, 11, and 12). In
all cases, the trans-products were obtained; this was
2
3
CO Et
2
COCH3
H
CO Et
8
2
4
CO Et
10
10
10
8
2
1
5
CH3
OMe
NO2
Cl
CO Et
2
confirmed by GC and H NMR analysis.
6
CO Et
2
Table 1 Heck Reaction of 4-Bromoanisole with Styrene Catalyzed
by Pd(OAc) (0.5 mol%) in [PEGmim][Cl], PEG, or [BMIm][Cl]
a
2
7
C H
6
5
5
5
5
5
5
0.5 mol%,
8
C H
8
6
Pd(OAc)2
Ph
MeO
Br +
Ph
MeO
base, solvent
9
COCH3
H
C H
8
6
Entry Temperature Base
°C)
Solvent
Conversionb
(%)
10
C
6
H
10
10
10
(
1
1
1
2
CH3
OMe
C H
6
1
2
3
4
5
6
7
140
140
140
140
140
120
140
140
Et N
[PEGmim][Cl]
[PEGmim][Cl]
[PEGmim][Cl]
[PEGmim][Cl]
[PEGmim][Cl]
[PEGmim][Cl]
PEG 4000
–
11
3
C H
6
Na CO
2
3
a
Isolated yield.
All of the products were isolated as trans-isomers.
b
NaOAc
K PO
8
92
3
4
While this system was less efficient in catalyzing the cou-
pling of non-activated chloroarenes, it is still significantly
more active than most of the results previously reported in
ILs and immobilized ILs.9 For example, immobilized
K CO
100
51
2
3
3
3
3
K CO
2
,10
K CO
52
2
Pd(OAc) in IL on silica was found to be highly efficient
2
9
j
in promoting the Heck reaction of aryl iodides. The poly-
mer-supported carbene complexes of palladium were
active for aryl bromides, but it was not possible to convert
any of the chlorobenzene even under harsh reaction con-
ditions.15 Besides the solvent effects of [PEGMIm]Cl as
the IL, the high efficiency of PEG-supported imidazolium
8
K CO
[BMIm][Cl]
Trace
2
a
The reactions were carried out under the conditions indicated for 10
h.
b
Obtained by GC analysis.
These results encouraged us to investigate the possibility salt is possibly related to the formation of NHCs. The
to couple aryl chlorides and olefins using the Pd(OAc)2/ investigation of NHCs as the ligands for the complex has
[
PEGmim][Cl] system. We employed 4-nitrochloroben- revealed that the active Pd-carbene species can be formed
zene as the Heck coupling partner in combination with in situ from their 1,3-disubstituted imidazolium salts
styrene. Low conversion was obtained under the reaction under basic conditions, and the sterically demanding imi-
conditions described above. However, we surprisingly dazolium salts lead to a significant increase in the reaction
found that using Cs CO instead of K CO in a nitrogen activity.16 In the PEG-supported imidazolium salt, PEG
2
3
2
3
atmosphere, prolonging the reaction time (28–48 h), and not only remarkably diluted the IL, but also constituted a
increasing the reaction temperature (160 °C) ensured the bulky imidazolium salt, which possibly results in the
reaction proceeded smoothly and afforded the desired active NHC-palladium catalyst in situ.
Synlett 2005, No. 12, 1861–1864 © Thieme Stuttgart · New York

LETTER
PEG-Supported Imidazolium Chloride
1863
Table 3 Heck Reactions of Activated Chloroarenes with Olefins
Table 4 Conversion and Isolated Yield from the Recycling Experi-
Catalyzed by Pd(OAc) (5 mol%) in [PEGmim][Cl]
ments Using Pd(OAc) (0.5 mol%) in [PEGmim][Cl] in the Coupling
of Bromobenzene with Styrene
2
2
a
5
mol% Pd(OAc)2
PEGmim][Cl]
[
R2
b
R1
R1
Entry
Cycle
Conversion (%)
Yield (%)
Cl +
R2
Cs2CO3, 160 °C
1
2
3
4
5
1
2
3
4
5
6
100
100
100
100
100
88
95
95
96
94
96
82
Entry
R1
R2
Time (h)
28
Yield (%)a,b
1
2
3
4
5
6
7
NO2
CF3
COCH3
H
C H
96
95
93
62
97
96
78
23
6
5
5
5
5
C H
28
6
C H
48
6
C H
48
6
6
NO2
CF3
COCH3
H
CO Et
48
2
a
All the reactions were performed using K CO as base at 140 °C for
2
3
CO Et
48
10 h.
2
b
Isolated yields.
CO Et
48
2
8
CO Et
48
2
Synthesis of 1-(2-hydroxyl-ethyl)-3-methylimidazolium chlo-
ride (1): A solution of chlorohydrin (9.8 g, 0.12 mol) was added to
a
Isolated yield.
All of the products were isolated as trans-isomers.
b
1
-methyl imidazole (8.2 g, 0.1 mol). The resulting solution was
stirred at 70 °C for 24 h. After cooling the solution was poured into
anhydrous diethyl ether to precipitate the product as a white solid.
To evaluate the recycling potential of the catalyst, the
coupling of bromobenzene with styrene was used as a
model Heck reaction. The product could be extracted
The white solid was dried under vacuum to yield 15 g (92%) of the
1
product 1. H NMR (500 MHz, D O): d = 8.66 (s, 1 H), 7.41 (d, 2
2
13
H, J = 27.8 Hz), 4.22 (t, 2 H, J = 4.9 Hz), 3.83–3.80 (m, 5 H);
C
conveniently with anhydrous diethyl ether. The residual NMR (125 MHz, D O): d = 136.6, 123.9, 122.7, 60.0, 51.8, 36.0.
2
Pd(OAc) /[PEGmim][Cl] was reused as the reaction MS: The anionic component (chloridion) of 1 is symbolized as B
2
and the cationic component [1-(2-hydroxyethyl)-3-methylimidazo-
medium and catalyst for the next run without regeneration
or addition of Pd(OAc) and [PEGmim][Cl]. The first five
cycles gave 100% conversion (Table 4, entries 1–5).
However, the sixth cycle resulted in a drop in the conver-
sion (88%) (Table 3, entry 6) and the medium become vis-
cous due to the formation of an inorganic salt. In addition,
the stability of the [PEGmim][Cl] was detected by H
NMR spectroscopy. The results showed that the ester product was isolated by filtration and dried under reduced pressure
lium] of 1 as A. MS (ESI): m/z = 126.9 (A), 288.9 (2A + B), 450.8
2
+
(
3A + 2B); MS (EI): m/z (A) = 83.4 [M ], 109.2, 127.1.
Preparation of PEG-supported succinic acid (2): To a solution of
PEG 4000 (20 g, 5 mmol) in CH Cl (100 mL) was added succinic
2
2
anhydride (3 g, 30 mmol) and DMAP (1 g, 8 mmol). The solution
was refluxed for 3 h. CH Cl was removed under reduced pressure
2
2
1
and anhydrous diethyl ether (200 mL) was added to the residue. The
1
linkage between PEG 4000 and succinic acid was stable to yield 20 g of the product 2. H NMR (500 MHz, CDCl ): d = 4.26
under the reaction conditions. However, cleavage of suc-
cinic acid and IL took place during the reaction, and a Preparation of PEG-supported methyl imidazolium chloride
3
(
t, 2 H, J = 4.4 Hz), 3.83–3.46 (m, 187 H), 2.66 (q, 4 H, J = 4.4 Hz).
1
0% loss of the ester was observed in every cycle. The (3): To a solution of 2 (20 g) and 1 (2.43 g) in acetonitrile (100 mL)
was added DMAP (0.6 g, 5 mmol) and DCC (3.1 g, 15 mmol). The
first five cycles gave 100% conversion, the Heck reaction
appeared to be insensitive to the cleavage of the [PEG-
mim][Cl]. The accumulation of the inorganic salt was
probably the main reason for the decrease in reactivity.
mixture was refluxed for 12 h under a nitrogen atmosphere. After
the reaction the solution was concentrated under reduced pressure.
The residue was poured into the anhydrous diethyl ether to precipi-
tate the product. The product was isolated by filtration and dried un-
1
der reduced pressure to yield 20 g of 3. H NMR (400 MHz, CDCl ):
In conclusion, the PEG-supported IL [PEGmim][Cl] has
been synthesized and applied to the Pd-catalyzed Heck
coupling reaction as a novel reaction medium. Using the
3
d = 10.70 (s, 1 H), 7.59 (s, 1 H), 7.33 (s, 1 H), 4.74 (s, 1 H), 4.53 (s,
1
2
H), 4.20 (t, 2 H J = 5.5 Hz), 4.07 (s, 2 H), 3.81–3.43 (m, 180 H),
.67–2.60 (m, 4 H).
Pd(OAc) /[PEGmim][Cl] system, the Heck reaction of
2
General procedure for the Heck reaction: A mixture of K CO
2
3
both aryl bromides and activated aryl chlorides with
olefins afforded the coupled products in excellent yields.
(
0.828 g, 6 mmol), Pd(OAc) (4.4 mg, 0.02 mmol), and 3 or PEG
2
4
000 (3 g) was heated at 80 °C for 1 h with stirring. Afterward aryl
Furthermore, the Pd(OAc) /[PEGmim][Cl] catalytic
2
bromide (4 mmol) and olefin (6 mmol) was added and the tempera-
system could be recycled five times without obvious
ture was increased to 140 °C for the indicated time. For aryl chlo-
deactivation. On the basis of even the current efforts, it ride Cs CO was used as base and the reaction solution was
2
3
seems likely that a number of other transition metal-cata- degassed and the reaction was carried out under a nitrogen atmo-
sphere. The amount of Pd(OAc) was increased (44 mg, 0.2 mmol).
After the reaction the solution was cooled to room temperature and
the resulting solid was extracted with anhydrous diethyl ether
lyzed reactions could be performed in [PEGmim][Cl] in-
stead of the conventional [BMIm] series; the investigation
of this is in progress.
2
(
4 × 15 mL). The residue was subjected to a second run by charging
the reaction vessel with the same substrates (bromobenzene styrene
and K CO ) without further addition of Pd(OAc) or [PEG-
2
3
2
Synlett 2005, No. 12, 1861–1864 © Thieme Stuttgart · New York

1
864
L. Wang et al.
LETTER
(6) (a) Consorti, C. S.; Zanini, M. L.; Leal, S.; Ebeling, G.;
mim][Cl]. The combined ethereal phase was analyzed by GC/MS
and then concentrated. Further purification of the product was
achieved by flash chromatography on a silica gel column.
Dupont, J. Org. Lett. 2003, 5, 983. (b) Beletskaya, I. P.;
Kashin, A. N.; Karlstedt, N. B.; Mitin, A. V.; Cheprakov, A.
V.; Kazankov, G. M. J. Organomet. Chem. 2001, 622, 89.
(
7
c) Yao, Q.; Kinney, E. P.; Yang, Z. J. Org. Chem. 2003, 68,
528. (d) Rocaboy, C.; Gladysz, J. A. Org. Lett. 2002, 4,
Acknowledgment
1993.
We thank the Natural Science Foundation of P. R. China (No.
(
7) (a) Bourissou, D.; Guerret, O.; Gabba, F. P.; Bertrand, C. B.
Chem. Rev. 2000, 100, 39. (b) Herrmann, W. A. Angew.
Chem. Int. Ed. 2002, 41, 1290. (c) Nolan, S. P.; Lee, H. M.;
Yang, C. Org. Lett. 2001, 3, 1511. (d) Regitz, M. Angew.
Chem., Int. Ed. Engl. 1996, 35, 725.
2
0373061) for support as well as the Teaching and Research Award
Program for Outstanding Young Teachers in Higher Education
Institutions of MOE, P. R. China
(
8) (a) Hussey, C. L.; Quigley, R.; Seddon, K. R. Inorg. Chem.
References
1995, 34, 370. (b) Welton, T. Chem. Rev. 1999, 99, 2071.
(
1) (a) Bräse, S.; Meijere, A. In Metal-Catalyzed Cross-
Coupling Reactions; Diederich F., Stang P. J., Wiley-VCH:
Weinheim, 1998, 99–166. (b) Beletskaya, I. P.; Cheprakov,
A. V. Chem. Rev. 2000, 100, 3009. (c) Dounay, A. B.;
Overman, L. E. Chem. Rev. 2003, 103, 2945.
(c) Wasserscheid, P.; Keim, W. Angew. Chem. Int. Ed. 2000,
39, 3773. (d) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z.
Chem. Rev. 2002, 102, 3667.
(9) (a) Jeffery, T. Tetrahedron 1996, 52, 10113. (b) Kaufmann,
D. E.; Nouroozian, M.; Henze, H. Synlett 1996, 1091.
(c) Calo, V.; Nacci, A.; Monopoli, A.; Lopez, L.; Cosmo, A.
Tetrahedron 2001, 57, 6071. (d) Silberg, J.; Schareina, T.;
Kempe, R.; Wurst, K.; Buchmeiser, M. R. J. Organomet.
Chem. 2001, 622, 6. (e) Boehm, V. P. W.; Gstoettmayr, C.
W. K.; Weskamp, T.; Herrmann, W. A. J. Organomet.
Chem. 2000, 595, 186. (f) Bouquillon, S.; Ganchegui, B.;
Estrine, B.; Henin, F.; Muzart, J. J. Organomet. Chem. 2001,
634, 153. (g) Choudary, B. M.; Madhi, S.; Chowdari, N. S.;
Kantam, M. L.; Sreedhar, B. J. Am. Chem. Soc. 2002, 124,
14127. (h) Park, S. B.; Alper, H. Org. Lett. 2003, 5, 3209.
(i) Xiao, J. C.; Twamley, B.; Shreeve, J. M. Org. Lett. 2004,
6, 3845. (j) Hagiwara, H.; Sugawara, Y.; Isobe, K.; Hoshi,
T.; Suzuki, T. Org. Lett. 2004, 6, 2325. (k) Liu, S. F.;
Fukuyama, T.; Sato, M.; Ryu, I. Synlett 2004, 1814.
(10) Boehm, V. P.; Herrmann, W. A. Chem.–Eur. J. 2000, 6,
1017.
(11) (a) Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.;
Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932.
(b) Kim, D. W.; Chi, D. Y. Angew. Chem. Int. Ed. 2004, 43,
483. (c) Mehnert, C. P.; Mozeleski, E. J.; Cook, R. A. Chem.
Commun. 2002, 3010. (d) Handy, S. T.; Okello, M.;
Dickenson, G. Org. Lett. 2003, 5, 2513.
(12) (a) Heldebrant, D. J.; Jessop, P. G. J. Am. Chem. Soc. 2003,
125, 5600. (b) Chandrasekhar, S.; Narsihmulu, C.; Sultana,
S. S.; Reddy, N. R. Chem. Commun. 2003, 1716.
(
2) (a) Littke, A. F.; Fu, G. C. Angew. Chem. Int. Ed. 2002, 41,
4176. (b) Cornils, B.; Herrmann, W. A. Applied
Homogeneous Catalysis with Organometallic Compounds;
VCH: Weinheim, 1996. (c) Beller, M.; Bolm, C. Transition
Metals for Organic Synthesis; Wiley-VCH: Weinheim,
1998.
(
3) (a) Zhao, F.; Bhanage, B. M.; Shirai, M.; Arai, M. Chem.–
Eur. J. 2000, 6, 843. (b) Bhanage, B. M.; Shirai, M.; Arai,
M. J. Mol. Catal. A: Chem. 1999, 145, 69. (c) Varma, R. S.;
Naicker, K. P.; Leisen, P. J. Tetrahedron Lett. 1999, 40,
2075. (d) Biffis, A.; Zecca, M.; Basato, M. Eur. J. Inorg.
Chem. 2001, 1131. (e) Djakovitch, L.; Koehler, K. J. Am.
Chem. Soc. 2001, 123, 5990. (f) Djakovitch, L.; Koehler, K.
J. Mol. Catal. A: Chem. 1999, 142, 275. (g) Biffis, A.;
Zecca, M.; Basato, M. J. Mol. Catal. A: Chem. 2001, 173,
2
49. (h) Augustine, R. L.; Buchmeiser, M. R.; Schareina, T.;
Kempe, R.; Wurst, K. J. Organomet. Chem. 2001, 634, 39.
i) Li, J.; Mau, A. W.-H.; Strauss, C. R. Chem. Commun.
997, 1275. (j) Tonks, L.; Anson, M. S.; Hellgardt, K.;
(
1
Mirza, A. R.; Thompson, D. F.; Williams, J. M. J.
Tetrahedron Lett. 1997, 38, 4319.
(4) (a) Reetz, M. T.; de Vries, J. G. Chem. Commun. 2004,
1
559. (b) Klingelhofer, S.; Heitz, W.; Greiner, A.; Oestreich,
S.; Forster, S.; Antonietti, M. J. Am. Chem. Soc. 1997, 119,
0116. (c) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M.
1
A. Org. Lett. 2000, 2, 2385. (d) Pham-Huu, C.; Keller, N.;
Charbonniere, L. J.; Ziessel, R.; Ledoux, M. J. Chem.
Commun. 2000, 1871. (e) Choudary, B. M.; Chowdari, N.
S.; Jyothi, K.; Kumar, N. S.; Kantam, M. L. Chem. Commun.
(13) (a) Chandrasekhar, S.; Narsihmulu, Ch.; Sultana, S. S.;
Reddy, N. R. Org. Lett. 2002, 4, 4399. (b) Luo, C.; Zhang,
Y.; Wang, Y. J. Mol. Catal. A: Chem. 2005, 229, 7.
(14) Lin, X. F.; Zhang, J.; Cui, S. L.; Wang, Y. G. Synthesis 2003,
1569.
2002, 586. (f) Kiraly, Z.; Veisz, B.; Mastalir, A.; Razga, Z.;
Dekany, I. Chem. Commun. 1999, 1925. (g) Reetz, M. T.;
Westermann, E. Angew Chem. Int. Ed. 2000, 39, 165.
5) (a) Gruber, A. S.; Zim, D.; Ebeling, G.; Monteiro, A. L.;
Dupont, J. Org. Lett. 2000, 2, 1287. (b) Bergbreiter, D. E.;
Osburn, P. L.; Wilson, A.; Sink, E. M. J. Am. Chem. Soc.
(15) Schwarz, J.; Boehm, V. P. W.; Gardiner, M. G.; Grosche,
M.; Herrmann, W. A.; Hieringer, W.; Raudaschl-Sieber, G.
Chem.–Eur. J. 2000, 6, 1773.
(
(16) (a) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org.
Chem. 1999, 64, 3804. (b) Lee, H. M.; Nolan, S. P. Org.
Lett. 2000, 2, 2053. (c) Herrmann, W. A.; Boehm, V. P. W.
J. Organomet. Chem. 1999, 572, 141. (d) Huang, J.; Grasa,
G.; Nolan, S. P. Org. Lett. 1999, 1, 1307. (e) Carmichael,
A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon,
K. R. Org. Lett. 1999, 1, 997. (f) Xu, L.; Chen, W.; Xiao, J.
Organometallics 2000, 19, 1123. (g) Yang, C.; Nolan, S. P.
Synlett 2001, 1539.
2000, 122, 9058. (c) Gruber, A. S.; Pozebon, D.; Monteiro,
L.; Dupont, J. Tetrahedron Lett. 2001, 42, 7345.
Synlett 2005, No. 12, 1861–1864 © Thieme Stuttgart · New York