Carboxylation of styrene in the N(C4H9) 4Br-heptane system

Article abstract of DOI:10.1007/s11172-005-0155-1

The carboxylation of styrene into carboxylic acids in the N(C ₄H₉)₄Br-heptane system in the presence of phosphine complexes and palladium acetate was studied. In the absence of phosphine, the Pd catalyst seems to be stabil

Full text of DOI:10.1007/s11172-005-0155-1

2564
Russian Chemical Bulletin, International Edition, Vol. 53, No. 11, pp. 2564—2567, November, 2004
Carboxylation of styrene in the N(C4H9)4Br—heptane system
ꢀ
A. L. Lapidus, O. L. Eliseev, N. N. Stepin, and T. N. Bondarenko
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5303. Eꢀmail: oleg@ioc.ac.ru
The carboxylation of styrene into carboxylic acids in the N(C4H9)4Br—heptane system in
the presence of phosphine complexes and palladium acetate was studied. In the absence of
phosphine, the Pd catalyst seems to be stabilized in solution by forming anionic complexes with
NBu4Br; the stabilization depends on the acidity of the reaction medium. The catalytic system
can be used repeatedly, its activity being reduced only slightly.
Key words: styrene, palladium catalysts, carboxylation; ionic liquids.
Carbonylation of organic substrates with carbon monꢀ
oxide is an important domain in metal complex catalyꢀ
sis.1—4 In particular, Pdꢀcatalyzed carboxylation of styꢀ
renes affords derivatives of hydratropic acid, which is the
structural fragment of nonsteroidal antipyretics such as
ibuprofen and its analogs.5,6
i. [Pd]/H+, CO, H2O, TBAB/nꢀC7H16
The essential problem in homogeneous catalysis by
metal complexes is their recycling. A catalyst is dissolved
in one (usually, polar) phase, while both reagents and
products are dissolved in another phase. In this case, the
products and the catalyst are separated because the reacꢀ
tion mixture forms two immiscible layers. Most often,
water serves as a polar solvent and waterꢀsoluble ligands
are used.7,8 Recently,9,10 ionic liquids were proposed as
"liquid carriers" for homogeneous catalysts; such liquids
require no special waterꢀsoluble ligands and thus comꢀ
mon usual metal complexes are suitable.
Twoꢀphase carboxylation of olefins is poorly investiꢀ
gated. In the only study dealing with ionic liquids as reacꢀ
tion media for styrene carboxylation,11 the formation of
αꢀarylpropionates was found to be highly selective; howꢀ
ever, a low activity of the catalyst prevented it from being
used repeatedly.
Let us consider the effects of the pressure, the temꢀ
perature, and the concentration of the promoter (HCl)
on the main reaction parameters in the presence of
PdCl2(PPh3)2 as a catalyst. With an increase in the presꢀ
sure of CO from 1 to 7 MPa, the conversion of styrene
increased from 46 to 98%, while the yield of the acids
increased from 37 to 91%. Both the conversion and the
yield depend on the pressure in a nearly linear fashion. An
insignificant amount of ethylbenzene was detected among
the reaction products. At a pressure below 1.5 MPa, the
major product was unbranched acid 2; with an increase in
the pressure, the ratio of 1 to 2 grows (Table 1). This
result agrees with the reported11,12 data on higher yields of
isomeric carboxylation products at elevated pressures. Apꢀ
parently, the effect of the pressure on the regioselectivity
can be associated with reduced steric hindrances. As the
concentration of CO in solution increases, the bulky PPh3
molecules in the Pd catalyst are replaced by CO through
ligand exchange. Accordingly, the resulting catalytic comꢀ
plex is more compact and favors the formation of isoꢀ
meric product 1.
The present work was aimed at developing a stable
Pdꢀbased catalyst for styrene hydrocarboxylation. Use of
a mixture of tetrabutylammonium bromide (TBAB) and
heptane as a reaction medium allows the reaction prodꢀ
ucts to be easily separated from the homogeneous catalyst.
Results and Discussion
With an increase in the reaction temperature, the conꢀ
version of styrene increased. Up to a temperature of
120 °C, the yield of the acids and the styrene conversion
increased in a parallel manner, styrene being insignifiꢀ
cantly hydrogenated into ethylbenzene. With a further
The hydrocarboxylation of styrene in the presence of a
Pd catalyst and a strong inorganic acid as a promoter
yields hydratropic (1) and hydrocinnamic acids (2).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2458—2461, November, 2004.
1066ꢀ5285/04/5311ꢀ2564 © 2004 Springer Science+Business Media, Inc.

Carboxylation of styrene
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 11, November, 2004 2565
Table 1. Effect of the pressure on the styrene carboxylation
catalyzed by 0.5% PdCl2(PPh3)2—40% HCl (110 °C)
Ratio of 1 to 2
1.2
P/MPa Conversion
of styrene
Yield (%)
0.8
0.4
1
2
the
acids
ethylbenzene
(%)
1.0
1.5
2.0
5.0
7.0
45.7
52.2
56.5
91.3
97.8
9.2
16.3
26.2
53.1
66.1
28.1 37.3
31.2 47.5
23.1 49.3
25.0 78.1
24.6 90.7
1.3
2.2
0.9
2.2
0.0
100
120
140
T/°C
Fig. 2. Effect of the temperature on the ratio between acids 1
and 2 (0.5% PdCl2(PPh3)2—40% HCl, 2.0 MPa).
increase in the temperature, the yield of ethylbenzene
increased dramatically, with a respective decrease in the
acid yields. At 150 °C, the conversion of the substrate was
complete and the yields of the acids and ethylbenzene
were 43 and 36%, respectively (Fig. 1).
The ratio between acids 1 and 2 is also temperatureꢀ
dependent. At 130 °C, the selectivity to hydratropic acid 1
was reduced, which correlates with the literature data.12
However, with further increase in the temperature,
the concentration of hydrocinnamic acid 2 became
higher (Fig. 2).
Y (%)
100
80
60
40
20
20
40
Content of HCl (mol. %)
These results suggest that the nature of the catalytic
system changes at t > 120—130 °C. Apparently, the cataꢀ
lytic complex decomposes to give metallic palladium. This
is evident from the sharply enhanced hydrogenating abilꢀ
ity of the catalyst. In addition, the heated reaction mixꢀ
ture turned from yellowꢀorange (this color is characterisꢀ
tic of a dissolved palladium phosphine complex) to greenꢀ
ish black, which also indicates the formation of finely
divided palladium.
Fig. 3. Effect of the concentration of HCl on the styrene converꢀ
sion (0.5% PdCl2(PPh3)2—HCl, 5.0 MPa, 110 °C).
of styrene was only 6.5% and the yield of the acids was 6%.
With an increase in the concentration of HCl, the catalyst
became much more effective (Fig. 3).
In weakly acidic media, products 1 and 2 were formed
in nearly equal amounts. With an increase in the concenꢀ
tration of HCl, the ratio 1 : 2 increases (Fig. 4). The total
selectivity to both acids was reduced from 99 to 80% as
[HCl] increased from 5 to 80%. For [HCl] >40%, the
yield of ethylbenzene was 2%.
Palladium complexes with such bidentate ligands as
1,2ꢀdiphenylphosphinoethane (dppe) and 1,4ꢀdiphenylꢀ
phosphinobutane (dppb) proved to be less effective in
styrene carboxylation. Under the same conditions, the
yields of the acids were 78, 20, and 18% for PdCl2(PPh3)2,
An acid promoter is a necessary component of this
catalytic system. In the absence of an acid, the conversion
Y (%)
100
1
80
2
60
Ratio of 1 to 2
40
3
4
2
20
100
120
140
T/°C
Fig. 1. Effect of the temperature on the styrene carboxylation
catalyzed by 0.5% PdCl2(PPh3)2—40% HCl (P = 2.0 MPa):
(1) the conversion of styrene, (2) the yield of the acids, and
(3) the yield of ethylbenzene.
20
40
Content of HCl (mol. %)
Fig. 4. Effect of the concentration of HCl on the ratio between
acids 1 : 2 (0.5% PdCl2(PPh3)2—HCl, 5.0 MPa, 110 °C).

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Russ.Chem.Bull., Int.Ed., Vol. 53, No. 11, November, 2004
Lapidus et al.
Y (%)
%
60
1
2
80
60
40
20
40
1
2
20
3
1
2
3
4
5
6
7
8
N
Fig.
6.
Recycling
of
the
catalytic
system
0.5% PdCl2(PPh3)2—40% TsOH in the twoꢀphase TBAB—hepꢀ
tane medium (2.0 MPa, 110 °C): (1) the conversion of styrene
and (2) the yields of the acids; N is the number of cycles.
20
40
60 Content of HCl (mol. %)
Fig. 5. Effect of the concentration of HCl on the styrene carboxyꢀ
lation catalyzed by 0.5% Pd(OAc)2—HCl (5.0 MPa, 110 °C):
(1) the conversion of styrene, (2) the yield of the acids, and
(3) the yield of ethylbenzene.
Thus, the carboxylation of styrene into carboxylic
acids can be successfully conducted in a twoꢀphase
TBAB—heptane system. Either palladium phosphine
complexes or palladium acetate can be used as effective
catalysts. In the absence of phosphine, palladium seems
to be stabilized in solution through the formation of anꢀ
ionic complexes with TBAB. This catalytic system can be
recycled.
PdCl2(dppe), and PdCl2(dppb), respectively. A similar
reduction in the catalyst efficiency was noted for diꢀ
phosphine ligands.13
The phosphine ligand is not a necessary component of
the catalytic system. With palladium acetate as a catalytic
precursor, carboxylation products were also obtained in
high yields. The activity and selectivity of such a catalytic
system strongly depend on the concentration of an acid
promoter (Fig. 5).
Experimental
Carboxylation of styrene was carried out in a pressurized
50ꢀmL Hastelloyꢀlined steel reactor fitted with a magnetic stirꢀ
ring bar. The reactor was heated with an electric furnace. The
reactor was charged with styrene (0.5 mL, 4.35 mmol), the
catalyst (0.022 mmol, 0.5% with respect to styrene), heptane
(5 mL), NBu4Br (1 g), 37% aqueous HCl, and water. Water was
added in such an amount that its total volume (added waꢀ
ter + water in the solution of HCl) was 0.39 mL (21.8 mmol,
500% with respect to styrene). The reactor was purged with CO
several times and closed and the temperature was controlled.
Then CO was introduced to a required pressure and the stirrer
was switched on. After 2 h, the reactor was cooled to ambient
temperature and the reaction mixture composed of two liquid
layers was withdrawn and analyzed by GLC. In catalyst recyꢀ
cling experiments, the organic material was extracted from the
reaction mixture with ether (20 mL). New portions of styrene
and heptane and a required amount of water were added to the
residue and the reaction was carried out again. Ethereal extracts
were concentrated, treated with diazomethane, and analyzed
by GLC.
In conventional solvents such as ethanol or acetone, a
palladium catalyst should include ligands (most ofꢀ
ten, phosphines) to prevent precipitation of palladium
metal.1—4 However, when a phosphineꢀfree catalyst
is used, palladium black is not detected up to high conꢀ
centrations of HCl, with the catalyst remaining highly
effective. Apparently, such stability of the catalyst is due
to a TBAB melt used as a reaction medium. Based on
available literature data, one can assume that palladium is
stabilized by forming 16ꢀelectron complexes of the type
[NBu4]+[L2PdBr]–, which are effective in carbonylation.14
The stability of such complexes seems to be dependent on
the acidity of the medium. An increase in the concentraꢀ
tion of HCl above 40% makes the catalyst less effective.
With further increase in acidity, the complex decomposes
completely to produce metallic palladium. Under these
conditions, the increase in the conversion of styrene is
due to its hydrogenation to ethylbenzene (see Fig. 5).
Ionic liquids are used in homogeneous catalysis mainly
to ensure catalyst recycling. The catalyst can be used reꢀ
peatedly, with nearly no loss in its activity or selectivity to
acids (Fig. 6). After each cycle, organic material (carboxyꢀ
lation products + unreacted styrene) was extracted from
the reaction mixture with ether. When new portions of
styrene, water, and heptane were added to the system, the
catalyst proved to be still effective (see Fig. 6).
GLC was performed with an Avtokhrom UE5 PID instruꢀ
ment (quartz capillary column 30 m × 0.25 mm, stationary
phase SEꢀ30, helium as a carrier gas, nꢀoctane as the internal
standard).
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Received March 25, 2004;
in revised form October 11, 2004