Pd(Phen)Cl2 stabilized by ionic liquid: an efficient and reusable catalyst for biphasic oxidative cyclocarbonylation of β-aminoalcohols and 2-aminophenol

Article abstract of DOI:10.1016/j.tetlet.2007.05.048

Biphasic oxidative cyclocarbonylation of β-aminoalcohols and 2-aminophenol to synthesize corresponding 2-oxazolidinones were investigated in the presence of ionic liquid stabilized Pd(phen)Cl₂ complex. Catalytic comparison results showed that, 1-butyl-3-methyl-imidazolium iodide salts (BMImI) can serve simultaneously as a specific stabilizer to protect the transition metal complex against deactivation, a promoter to increase the catalytic performance and a reaction medium to recycle the catalyst with unprecedented TOF value.

Full text of DOI:10.1016/j.tetlet.2007.05.048

Tetrahedron Letters 48 (2007) 4845–4848
Pd(Phen)Cl stabilized by ionic liquid: an efficient and reusable
2
catalyst for biphasic oxidative cyclocarbonylation
of b-aminoalcohols and 2-aminophenol
*
*
Fuwei Li and Chungu Xia
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 73000, China
Received 23 April 2007; revised 8 May 2007; accepted 10 May 2007
Available online 16 May 2007
Abstract—Biphasic oxidative cyclocarbonylation of b-aminoalcohols and 2-aminophenol to synthesize corresponding 2-oxazolidi-
nones were investigated in the presence of ionic liquid stabilized Pd(phen)Cl
2
complex. Catalytic comparison results showed that,
1
-butyl-3-methyl-imidazolium iodide salts (BMImI) can serve simultaneously as a specific stabilizer to protect the transition metal
complex against deactivation, a promoter to increase the catalytic performance and a reaction medium to recycle the catalyst with
unprecedented TOF value.
Ó 2007 Elsevier Ltd. All rights reserved.
2
-Oxazolidinones are very important intermediates in
another efficient, stable and reusable catalyst for the
production of 2-oxazolidinones.
the manufacture of pharmaceuticals, cosmetics, pesti-
cides and other fine chemicals.
1
,2
They are mainly
produced by phosgenation of the corresponding 1,2-
aminoalcohols or 2-aminophenols, which has resulted
The three important factors with respect to transition
metal catalyzed reactions are the stabilization of cata-
lysts, finding effective promoter cooperating with the
expensive metal catalyst to enhance its catalytic effi-
ciency and the reuse of catalysts. Therefore, how to
design and search for an additive that may serve simul-
taneously in the reaction as a stabilizer to protect the
noble metal catalyst against deactivation, a promoter
to increase activity of the main metal catalyst and a reac-
tion medium to recycle catalyst deserve to be pursued in
order to search for an efficient catalytic system.
3
in serious social and environmental problems. With
the increasing environmental concerns, it is worthy to
develop efficient and environmentally benign non-phos-
gene methodologies for the production of 2-oxazolidi-
nones. As one of the approaches adopted recently,
oxidative cyclocarbonylation of 1,2-aminoalcohols or
2
-aminophenols using carbon monoxide as the carbonyl
source catalyzed by transition metal catalysts represents
a valid one-step alternative for the synthesis of 2-oxazo-
lidinones. In 2000, Gabriele and his co-workers reported
a homogeneous palladium catalytic system for the syn-
thesis of 2-oxazolidinones using oxidative carbonylation
Recently, the combination of task specific ionic liquids
(TSILs) as versatile and novel reaction media with tran-
sition metal complex has resulted in many diverse and
flexible ‘platforms’ to establish a highly effective and eas-
ily separable catalytic system. More latest reported stud-
ies and our previous investigation have shown that the
stabilization of metal complexes may be achieved by
the use of imidazolium ILs that may provide electronic
4
process, and we developed a first heterogeneous
5
a
catalyst with good catalytic efficiency. Although the
recycle of expensive transition metal catalyst was
achieved, its catalytic performance was still not so
efficient. Consequently, it is necessary to investigate
6
,7
and/or steric protections.
With our continuous
5
research in oxidative cyclocarbonylation, in this work,
we reported here an effective and reusable process for
the preparation of 2-oxazolidinones by oxidative cyclo-
carbonylation of b-aminoalcohols and 2-aminophenol
Keywords: Biphasic oxidative cyclocarbonylation; Reuse of catalyst;
Ionic liquid; 2-Oxazolidinone.
*
Corresponding authors. Tel.: +86 931 4968089; fax: +86 931
277088; e-mail addresses: chmlifw@nus.edu.sg; cgxia@lzb.ac.cn
8
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.05.048

4846
F. Li, C. Xia / Tetrahedron Letters 48 (2007) 4845–4848
catalyzed by specific ILs (BMImI) promoted Pd(Phen)-
Cl2 system under biphasic conditions, in which the
stabilization and recycle of noble metal catalyst were
accomplished, simultaneously.
the ILs had a strong impact on the catalytic activity
and BMImI was maybe the best co-catalyst for
Pd(phen)Cl2 complex in the oxidative cyclocarbonyl-
ation of ethanolamine. In order to gain insight of the
BMImI-promoted Pd(phen)Cl system, other two com-
2
Initially, ethanolamine (1a) was selected as a probe
substrate to investigate the possibility of a certain ILs
acting simultaneously as a stabilizer, promoter and reac-
tion medium in the reaction, the results are listed in
Table 1 . As shown, with BMIm as cation, the effect
of different types of anions on the reaction was studied
first under the same reaction conditions. Only about
parison experiments were carried out to investigate the
difference between BMIm and other conventional iodide
salts, in which inorganic iodide salt KI and organic tet-
raethylammonium iodide salt [Et] I utilized as additives,
4
8
+
respectively, instead of BMImI, the catalyst black pre-
cipitate was observed after the completion of catalytic
reaction, only 35% conversion of 1a obtained (Table 1,
entries 7 and 8). The results of these three comparison
experiments suggested that imidazolium cation involved
IL, BMImI showed more efficient ability in increasing
catalytic activity than that of other conventional inor-
ganic or organic ammonium iodide salts. It is probably
because that some special interaction between the imi-
dazolium-based iodide IL and palladium complex hap-
pened and therefore BMImI can act not only as a
promoter to enhance the catalytic performance but also
as a novel stabilizer to protect the palladium complex
1
5% of conversions were achieved, respectively, using
BMImBF , BMImPF , BMImCl or BMImBr as an
4
6
additive for the palladium complex catalyst while high
selectivity was obtained (Table 1, entries 1–4). Subse-
quently, the comparison experiment was carried out
using Pd(phen)Cl as a catalyst in the absence of ionic
2
liquid, only 6% conversion of 1a and 90% selectivity of
2
-oxazolidinone (2a) were observed (Table 1, entry 5);
furthermore, the deactivation of palladium catalyst
was founded in the reaction while this phenomenon
was not observed in the presence of ILs. All the above
6
a
against deactivation.
ꢀ
ꢀ
ꢀ
ꢀ
mentioned results show that BF4 , PF6 , Br , Cl
+
involved BMIm type ILs can stabilize and protect the
palladium catalyst particle free from deactivation, and
then improve the activity and selectivity of the imidazo-
lium ILs-promoted palladium complex system to a
certain content in comparison with that of Pd(phen)Cl2
alone. However, their abilities in promoting catalytic
performances were quite unsatisfactory. When BMImI
was introduced into the catalytic system as a promoter
or stabilizer instead of other anion type ILs, interest-
ingly, the catalytic activity was greatly improved and
In order to investigate the capability of ILs as a reaction
medium under a two-phase condition, the recycling of
the Pd(phen)Cl /BMImI catalyst system was also exam-
2
ined. Since BMImI is insoluble with DME, after reac-
tion, the resulting product and unreacted substrate
dissolved in the DME phase while the Pd(phen)Cl was
2
1
1
immobilized in the ILs phase; therefore, the reaction
mixture containing 2-oxazolidinones can be separated
from the system by simple decantation and the IL phase
including the palladium complex catalyst can be reused
after evaporation to remove the water liberated in the
reaction. When catalyst was used three times in the oxi-
dative cyclocarbonylation of 1,2-aminoalcohol under the
same conditions shown in this typical reaction, a minor
9
4% conversion of 1a with high selectivity was obtained
at mild reaction conditions, the corresponding TOF
ꢀ
1
exceeded 3200 h , which is 3.0 times higher than our
ꢀ
1
previous reported record of 1000 h (Table 1, entry
). This experiment as result showed that the anions of
ꢀ1
6
loss in activity and 2913 h of TOF were obtained (Ta-
ble 1, entry 9). This result indicated, in addition to high
activity and selectivity, that the Pd(phen)Cl /BMImI
showed good stability in recycle process.
Table 1. Oxidative cyclocarbonylation of ethanolamine (1a) catalyzed
by imidazolium ILs-promoted palladium complex system
2
a
O
The highly efficient BMImI/Pd(phen)Cl2 catalytic
system was also found to be applicable to a variety of
b-aminoalcohols (1b–e) under the optimal reaction con-
ditions. The results are listed in Table 2. As shown,
H
H
Pd(phen)Cl2/ILs
HN
H
O
+
CO/O2
110 ºC, 5.2 MPa
H2N
OH
H
ꢀ1 b
Entry
Ionic liquid
BMImBF
Conv. (%)
Select. (%)
TOF (h )
Pd(phen)Cl /BMImI catalytic system exhibited excellent
2
1
2
3
4
4
15
13
16
13
6
94
36
35
85
95
90
95
95
90
98
97
95
96
508
417
540
440
190
3288
1246
1187
2913
catalytic activity to almost all the employed b-amino-
alcohols under the same reaction conditions, providing
the corresponding 2-oxazolidinones with high turnover
frequency and selectivity. Furthermore, the high effi-
ciency of fulfilling the reaction under considerably mild
oxidizing conditions has also permitted the application
of this methodology to 2-aminophenol (1f), and 89%
BMImPF
BMImCl
BMImBr
—
6
c
5
6
BMImI
KI
d
7
e
8
f
4
[Et] I
ꢀ
1
of conversion and 2955 h
of TOF were obtained.
9
BMImI
These above mentioned results suggested that Pd(phen)-
a
Reaction conditions: the mol ratio of substrate/Pd catalyst = 3570,
BMImI 1 ml; temperature 110 °C, PCO=O2 ¼ 5:0=0:2, time 1 h.
Moles of 2-oxazolidinones produced per mole of catalyst per hour.
Deactivation of palladium catalyst was observed.
Cl /BMImI was a robust and efficient catalytic system
2
b
c
d
e
f
for the oxidative cyclocarbonylation of b-aminoalcohols
and 2-aminophenol.
KI 10 mg and 20 equiv of Pd complex, were used.
[
4
Et] I 14.5 mg and 20 equiv of Pd complex, were used.
As to the reaction mechanism of this process, it is not
well understood at this stage. Since a poor result was ob-
The catalyst system used for the third time.

F. Li, C. Xia / Tetrahedron Letters 48 (2007) 4845–4848
Table 2. Oxidative cyclocarbonylation of b-aminoalcohols (1b–e) and
4847
References and notes
2-aminophenol (1f) to synthesize corresponding 2-oxazolidinones
1
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1
996, 96, 835; (b) K o¨ ll, P.; L u¨ tzen, A. Tetrahedron:
R1
H2N
R2
Pd(phen)Cl2/BMImI
H2O
HN
O
Asymmetry 1996, 7, 637; (c) L u¨ tzen, A.; K o¨ ll, P. Tetra-
hedron: Asymmetry 1997, 8, 29; (d) L u¨ tzen, A.; K o¨ ll, P.
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+
CO/O2
CO/O2
-
OH
R1
R2
(
1b-1e)
(
2b-2e)
H
N
NH2
Pd(phen)Cl2/BMImI
H2O
+
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1
999, 64, 8731; (g) Catalytic Asymmetric Synthesis; Ojima,
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OH
I., Ed.; Wiley: New York, 2000.
(
1f)
(2f)
2
. (a) Seneci, P.; Caspani, M.; Ripamonti, F.; Ciabatti, R. J.
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ꢀ1
Sub.
R
1
R
2
Conv. (%) Select. (%) TOF (h )
1
1
1
1
1
b
c
d
H
CH
CH
CH
H
3
94
93
96
92
90
97
97
99
95
94
3250
3220
3390
3120
3020
3
3
CH
2
H
H
b
e
(CH
2-Aminophenol
3
)
2
CH
c
f
3
9, 673; (d) Lohray, B.; Baskaran, S.; Rao, B. S.; Reddy,
a
Reaction conditions: the mol ration of substrate/Pd catalyst = 3570,
BMImI 1 ml; temperature 110 °C, PCO=O2 ¼ 5:0=0:2, time 1 h.
L enantiomer.
B. Y.; Rao, I. N. Tetrahedron Lett. 1999, 40, 4855; (e)
Bager, D. J.; Prakas, I. H.; Schaad, D. R. Aldrichim. Acta
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Nicholson, R. L.; Sanganee, H. J.; Smith, A. D. Tetra-
hedron Lett. 1999, 40, 6677.
b
c
10 ml DME was used.
3
4
. Puschin, N. A.; Mitic, R. V. Justus Leibigs Ann. Chem.
1
937, 532, 300.
tained with Pd(phen)Cl complex alone as the catalyst,
2
. (a) Gabriele, B.; Salerno, G.; Brindisi, D.; Costa, M.;
Chiusoli, G. P. Org. Lett. 2000, 2, 625; (b) Gabriele, B.;
Mancuso, R.; Salerno, G.; Costa, M. J. Org. Chem. 2003,
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M. Chem. Commun. 2003, 486.
and the addition of conventional inorganic or organic
ammonium iodide salts still could not enhance the
catalytic performance markedly, an excellent selectiv-
ity of 2-oxazolidinone and TOF was obtained over
BMImI stabilized Pd(phen)Cl complex system. Based
5. (a) Li, F. W.; Xia, C. G. J. Catal. 2004, 227, 542; (b) Li, F.
W.; Peng, X. G.; Xia, C. G.; Hu, B. Chin. J. Chem. 2005,
23, 643; (c) Peng, X. G.; Li, F. W.; Xia, C. G. Synlett 2006,
2
on the latest published results by Dupont and Migow-
6
a
ski, it can be conjectured that the highly synergistic
1
161; (d) Liu, J. M.; Peng, X. G.; Liu, J. H.; Zheng, S. Z.;
stabilization effect between Pd(phen)Cl and the task
2
Sun, W.; Xia, C. G. Tetrahedron Lett. 2007, 48, 929.
specific ILs leads to the outstanding stability and activ-
ity of our IL-stabilized palladium complex catalyst
system.
6
. (a) Migowski, P.; Dupont, J. Chem. Eur. J. 2007, 13, 32–39;
(
b) Ott, L. S.; Cline, M. L.; Deetlefs, M.; Seddon, K. R.;
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In conclusion, we have developed a Pd(phen)Cl /BMI-
2
mI catalytic system that gave good catalytic activity in
oxidative cyclocarbonylation reaction under consider-
ably mild conditions (a temperature of 110 °C and a
total pressure of 2.1 MPa), 1-butyl-3-methyl-imidazolium
iodide salts showed better ability in the stabilization
of transition metal active sites and promotion of
catalytic performance than other anion type of imidazo-
lium ionic liquids and conventional iodide salts;
furthermore, the catalyst could be recycled with minor
loss in activity and selectivity. All these results demon-
strate that the combination of a suitable ionic liquid
as stabilizer, promoter and reaction medium with a
metal complex is a pathway towards highly effective
and stable heterogeneous catalysts for the oxidative
cyclocarbonylation.
7. Li, F. W.; Xiao, L. F.; Xia, C. G.; Hu, B. Tetrahedron
Lett. 2004, 45, 8307.
8
. The ionic liquid, BMImI (BMIm = 1-butyl-3-methyl-
imidazolium), BMImBF , BMImPF , BMImCl, BMImBr
4
6
and Pd(phen)Cl
according to the reported literatures.
2
complexes were, respectively, synthesized
9,10
Oxidative cyclo-
carbonylation reactions were performed in a 100-mL
stainless-steel reactor which was equipped with a magnetic
stirrer and an automatic temperature control system. For
each typical reaction, known quantities of b-aminoalco-
hols (1a–e) or 2-aminophenol (1f) (10 mmol), Pd complex
catalyst (1 mg), IL (1 ml) and 6 ml DME were successively
charged into the reactor, the reactor was flushed three
times with CO, and then was pressurized with CO and O2
to a total pressure of 5.2 MPa (CO 5.0 MPa and O2
0
.2 MPa). This condition corresponded to about 96% of
CO in the CO–O mixture and is already beyond the
2
4
c
explosive limit for CO–O2 mixture. Then the reaction
proceeded at 120 °C for 1 h. After the reaction, the DME
phase containing 2-oxazolidinones can be easily separated
from the reaction mixture by simple decantation and the
ILs phase including the palladium complex catalyst can be
reused after evaporation with a vacuum pump to remove
water liberated in the reaction. Qualitative analyses were
conducted with a HP 6890/5973 GC–MS with chemsta-
tion ?tul?> containing a NIST Mass Spectral Database
Acknowledgements
This research was financially supported by the National
Science Foundation of China (20533080) and the
National Science Fund for Distinguished Young Schol-
ars (20625308).

4848
F. Li, C. Xia / Tetrahedron Letters 48 (2007) 4845–4848
ˆ
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9