Encapsulated AlCl3: A convenient catalyst for the alkylation of benzene with dodecene

Article abstract of DOI:10.1016/S0040-4039(02)00702-5

A novel method for the encapsulation of AlCl₃ was successfully carried out using an insoluble polymer wall. The polymer wall was formed by the reaction of poly(styrene-co-dimethylaminoethyl methacrylate) and hydrogenated telechelic polybutadiene containing -COOH groups. The encapsulated AlCl₃ was used to catalyze the Friedel-Crafts alkylation of benzene with dodecene. The alkylbenzenes were obtained in excellent yields and the encapsulated AlCl₃ catalyst was separated by simple filtration.

Full text of DOI:10.1016/S0040-4039(02)00702-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4555–4557
Encapsulated AlCl3: a convenient catalyst for the alkylation of
benzene with dodecene
Kusoomjin Srirattnai,a Somsak Damronglerd,a,* Shinzo Omi,b Sophon Roengsumran,c
Amorn Petsomc and Guang-Hui Mab
aDepartment of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
bGraduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
Tokyo 184-8588, Japan
cDepartment of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Received 20 September 2001; revised 4 December 2001; accepted 10 April 2002
Abstract—A novel method for the encapsulation of AlCl3 was successfully carried out using an insoluble polymer wall. The
polymer wall was formed by the reaction of poly(styrene-co-dimethylaminoethyl methacrylate) and hydrogenated telechelic
polybutadiene containing -COOH groups. The encapsulated AlCl3 was used to catalyze the Friedel–Crafts alkylation of benzene
with dodecene. The alkylbenzenes were obtained in excellent yields and the encapsulated AlCl3 catalyst was separated by simple
filtration. © 2002 Elsevier Science Ltd. All rights reserved.
Friedel–Crafts reactions are amongst the most impor-
tant reactions in organic synthesis. In the course of
more than 100 years of Friedel–Crafts chemistry, only
two catalysts, AlCl3 and BF3 have gained wide recogni-
tion.1 Especially, anhydrous AlCl3 has maintained its
use ever since it was introduced by Friedel and Crafts,2
despite some unfavorable properties such as decomposi-
tion or deactivation in water, safety and lack of recov-
erability. Lewis acid-catalyzed reactions are still of
great interest because of unique reactivities and selectiv-
ities that can be achieved as well as the mild conditions
used.3
that of the monomeric Lewis acid. For polymer-sup-
ported aluminum chloride,4 AlCl3 was immobilized
onto polymer or inorganic support materials, but suc-
cessful applications have been limited.3
The emphasis in this study was on the establishment of
an encapsulation procedure to preserve highly reactive
AlCl3 which easily loses activity in the air or a highly
moist atmosphere.
The utilization of polymer–supported catalysts offers
several advantages in preparative procedures. The sim-
plification of the workup and separation of products
and catalysts are useful for industrial processes. There
are a few examples of polymer-supported Lewis acids.4–6
They comprise a polymer carrier attached by weak
chemical or physical interactions to a Lewis acid. The
Lewis acid can be partially washed out of the polymer
support during reaction.7 For the polymer-supported
scandium based Lewis acid using Nafion8 and a poly-
acryronitrile derivative,9 reactivities were lower than
Keywords: encapsulated AlCl3; Friedel–Crafts alkylation; alkylben-
zene; hydrogenated telechelic polybutadiene.
* Corresponding author.
Figure 1. Formation of the insoluble salt of the polymer wall
from the P(St-co-DMAEMA) and Sat. PB.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00702-5

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K. Srirattnai et al. / Tetrahedron Letters 43 (2002) 4555–4557
The capsules were prepared by a solvent evaporation
technique.10 The polymer wall was formed by the reac-
tion of the dimethylamino groups from poly(styrene-co-
dimethylaminoethyl
methacrylate),11
P(St-co-
DMAEMA) and -COOH groups of hydrogenated
telechelic polybutadiene (Sat. PB)12 to form the insolu-
ble salt of the polymer13 (Fig. 1) by using liquid polybu-
tadiene (PBD) as a stabilizer. In this study, the size
distribution of all the capsules was not wide because the
AlCl3 capsules have a tendency to agglomerate as
shown by optical microscopy (Fig. 2) and scanning
electron microscopy (Fig. 3). At 10 wt% of Sat. PB, 10
wt% of P(St-co-DMAEMA) and 2 wt% of PBD, the
capsules had a smoother surface and achieved more
complete encapsulation than at other ratios.
Alkylation of benzene with 1-dodecene was chosen as a
model reaction to study the catalytic activity of the
encapsulated AlCl3.14 The product was analyzed by GC
and GC–MS. The capsules at various polymer wall and
stabilizer concentrations were effective in alkylation of
benzene with 1-dodecene to give the phenyldodecanes
(2-p, 3-p, 4-p, 5-p and 6-phenyldodecane) except for the
1-phenyl isomer. This is consistent with a carbonium
ion mechanism. The absence of 1-phenyldodecane is
Figure 3. Scanning electron micrograph of capsules at 10 wt%
Sat. PB, 10 wt% P(St-co-DMAEMA) and 2 wt% of PBD.
due to the instability of the primary carbonium ion. It
has been shown that the capsules can be an effective
catalyst for the Friedel–Crafts alkylation of benzene
with 1-dodecene, and the capsules can be easily recov-
ered by a simple filtration. The reaction in the presence
of unencapsulated AlCl3 (free AlCl3) under the same
conditions gave a similar yield to those of encapsulated
AlCl3 (Table 1). The catalytic activity of the encapsu-
lated AlCl3 was affected by the amounts of stabilizer
(runs 2–4). This indicated that 2 wt% of PBD was
enough to stabilize the capsules. The effectiveness of
the capsules at different amounts of Sat. PB (runs 3, 5
and 6) was not so different when compared with the
capsules with different amounts of P(St-co-DMAEMA)
(runs 3, 7 and 8). It shows that at 10 wt% of Sat. PB
and 10 wt% of P(St-co-DMAEMA), the capsules had
high conversion and complete encapsulation.15
Most of the conventional Friedel–Crafts catalysts gen-
erally give complex reaction mixtures which can be
worked up and separated only with difficulty16 and with
the loss of catalysts producing environmentally
unfriendly, toxic waste.17 In the case of this encapsu-
lated AlCl3, the capsules could be dispersed well in
benzene and remained in the solid state, and thus could
be conveniently separated quantitatively at the end of
the reaction by simple filtration. The capsules showed
Figure 2. Photograph of capsules taken by an optical micro-
scope at 10 wt% Sat. PB, 10 wt% P(St-co-DMAEMA) and 2
wt% of PBD.
Table 1. The encapsulation recipe for AlCl3 and its catalytic activity in the reaction of benzene and 1-dodecenea
Run No.
Sat. PB (Wt.%)
PS-DMAEMA (Wt.%)
PBD (Wt.%)
Conversionc (%)
1b
2
3
4
5
6
7
8
–
10
10
10
5
2.5
10
10
–
10
10
10
10
10
15
5
–
1
2
5
2
2
2
2
97
74
97
97
97
94
76
13
a All experiment were carried out at 36°C for 1 h.
b Unencapsulated AlCl3 (free AlCl3).
c Total conversion (2-p, 3-p, 4-p, 5-p and 6-p) by GC analysis using a capillary column (DB-PSWAX).

K. Srirattnai et al. / Tetrahedron Letters 43 (2002) 4555–4557
4557
very good stability to organic solvents and to high
temperature. They also exhibited good catalytic activ-
ity.18 Therefore, the encapsulated AlCl3 made AlCl3
safer and more convenient to handle.
10. A typical experimental procedure, run no. 2, was as
follows: hydrogenated telechelic polybutadiene (Sat. PB)
1.5 g was dissolved in 15 g of cyclohexane, AlCl3 0.8 g
was added into the polymer solution and stirred in the
reactor at 40°C. Then the solution of P(St-co-
DMAEMA) in cyclohexane (1.5 g/15 g) was added drop-
wise at 40°C with constant stirring for 30 min; a
crosslinking polymer was formed around the AlCl3, and
to this solution, a solution of stabilizer (PBD 0.9 g in 90
g of dodecane) was added and stirred. After evaporation
of the solvent at 35–40°C under vacuum, the supernatant
was separated by centrifugation. The capsules were
washed with hexane, and dried under N2 atmosphere.
11. Styrene (St, Kishida Chemical Co., Japan) and dimethyl-
aminoethyl methacrylate (DMAEMA, Wako Pure
Chemical Industries, Ltd, Japan) were commercial grade.
They were distilled under reduced pressure. 2,2%-Azo-
bis(2,4-dimethylvaleronitrile) (V-65, ADVN) was reagent
grade (Wako Pure Chemical Industries, Ltd, Japan), and
was used as an initiator. A typical procedure was as
follows: A mixture containing 90 g of St, 13 g of
DMAEMA, 1 g of ADVN and 20 ml of distilled toluene
was added to the reactor with a stirrer. After the mixture
was bubbled with nitrogen gas for 1 h, the nozzle was
lifted up above the surface of the mixture and the temper-
ature was elevated to 50°C gradually for the polymeriza-
tion. The polymerization was carried out for 24 h under
a nitrogen atmosphere. The polymer was precipitated by
methyl alcohol, separated by centrifugation and dried in
a vacuum.
In conclusion, we have successfully developed a new
method for the encapsulation of AlCl3 using an insolu-
ble polymer wall. The capsules have effectiveness and
stability in Friedel–Crafts alkylation reactions. More-
over, they are strong enough for long-term use. We are
currently studying the activity of these capsules for
other applications. The present technique should be
useful for the encapsulation of other sensitive catalysts
or substances.
Acknowledgements
The authors sincerely thank the Agency of Interna-
tional Exchange of Japan (AIEJ) short-term student
exchange program for providing a one-year scholarship
to K. Srirattnai to carry out this research at Graduate
School of Bio-Applications and Systems Engineering
(BASE) at the Tokyo University of Agriculture and
Technology.
References
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New York, 1973.
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12. Sat. PB is
a hydrogenated telechelic polybutadiene
obtained from Nippon Soda Co., Japan. Trade name: PG
CI-1000 hydrogenated.
13. IR (KBr, cm−1): 3300 (OH stretching), 1600 (CꢀO stretch-
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stretching), along with characteristic absorption bands of
aromatic compounds at 3050 and 1600 cm−1
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14. The reaction was carried out as follows: 1-Dodecene
(0.015 g. 0.089 mmol) was added gradually to a mixture
of encapsulated AlCl3 (30 mg) and benzene (0.885 g. 11.3
mmol) at 36°C for 1 h. The capsules were separated by
simple filtration. The benzene was evaporated and the
product was analyzed by gas chromatography and gas
chromatography/mass spectrometry.
15. Observed from photograph of capsule taken by optical
microscope (Olimpus DP-10, Tokyo).
16. Burgess, D. J.; Singh, O. N. Spontaneous formation of
small sized albumin/acacia coacervate particles. J. Pharm.
Pharmacol. 1993, 45, 586.
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Chemistry; Marcel Dekker: New York, 1984.
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18. Encapsulated AlCl3 can be stored at room temperature
for more than a few months without losing its activity.
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118, 8977.