Temperature-controlled highly selective dimerization of α-methylstyrene catalyzed by Br?nsted acidic ionic liquid under solvent-free conditions

Article abstract of DOI:10.1016/j.tet.2006.02.025

A temperature-controlled highly selective dimerization of α-methylstyrene to produce 2,4-diphenyl-4-methyl-1-pentene and 1,1,3-trimethyl-3-phenylindan was catalyzed by Bro?nsted acidic ionic liquid [Hmim]⁺BF₄^-. At 60 °C, 2,4-diphenyl-4-methyl-1-pentene was formed in 93% selectivity with >92% conversion under a solvent-free condition while 1,1,3-trimethyl-3-phenylindan could be obtained in 100% selectivity when the reaction temperature was increased to 170 °C. The ionic liquid [Hmim]⁺BF₄^- could be reused with almost no loss of activity.

Full text of DOI:10.1016/j.tet.2006.02.025

Tetrahedron 62 (2006) 3985–3988
Temperature-controlled highly selective dimerization
of a-methylstyrene catalyzed by Br o¨ nsted acidic ionic
liquid under solvent-free conditions
Haiming Wang, Peng Cui, Gang Zou, Fan Yang and Jie Tang
*
Department of Chemistry, East China Normal University, 3663 North Zhongshan Rd., Shanghai 200062, China
Received 21 November 2005; revised 10 February 2006; accepted 10 February 2006
Available online 7 March 2006
Abstract—A temperature-controlled highly selective dimerization of a-methylstyrene to produce 2,4-diphenyl-4-methyl-1-pentene and
1
C
,1,3-trimethyl-3-phenylindan was catalyzed by Br o¨ nsted acidic ionic liquid [Hmim] BF
K
4
. At 60 8C, 2,4-diphenyl-4-methyl-1-pentene was
formed in 93% selectivity with O92% conversion under a solvent-free condition while 1,1,3-trimethyl-3-phenylindan could be obtained in
C
K
1
of activity.
00% selectivity when the reaction temperature was increased to 170 8C. The ionic liquid [Hmim] BF could be reused with almost no loss
4
q 2006 Elsevier Ltd. All rights reserved.
1
producing polystyrene, ABS resin and the likes. The
1. Introduction
saturated cyclic isomer, 1,1,3-trimethyl-3-phenylindan
(Indan), also has wide applications in polymers, for example,
as a photo and thermo stabilizer, plasticizer as well as
Br o¨ nsted acid-catalyzed dimerization of a-methylstyrene
AMS) generally produces a mixture of three isomers: 2,4-
diphenyl-4-methyl-1-pentene (1-PT), 2,4-diphenyl-4-
2
3
(
4
mobility and viscosity modifier in the production of
methyl-2-pentene (2-PT) and the saturated cyclic isomer,
1
polystyrene or ABS resin. The Br o¨ nsted acid-catalyzed
dimerization of AMS is the straightest way to produce these
useful chemicals. However, due to the interconvertion and
1
,1,3-trimethyl-3-phenylindan (Indan) (Scheme 1).
1
the difficulties of separation of these isomers, highly
selective dimerization of AMS is the precondition for
practical process for production of the desired isomers,
especially for the productionof 1-PT. Theacidity of Br o¨ nsted
acid catalysts and reaction temperature have proved to be
crucial to the selectivity in the dimerization of AMS.
1
-PT
Ph
Ph
Ph
Ph
H+
H+
2-PT
Ionic liquids, featuring low volatility as well as high thermal
and chemical stability, have attracted much attention as
environmentally friendly replacement for traditional
volatile organic solvents in catalysis and organic transform-
Indan
Ph
5
ations. Br o¨ nsted acidic ionic liquids have been recently
reported to serve as both reaction medium and catalysts in
Scheme 1. Br o¨ nsted acid-catalyzed dimerization of a-methylstyrene.
6
many organic transformations. A sulfonic acid based
0
imidazolium triflate, was reported to catalyze dimerization
Br o¨ nsted acidic ionic liquid, N-alkyl-N -(4-sulfonic butyl)
Among the dimers, 2,4-diphenyl-4-methyl-1-pentene
1-PT) is a versatile molecular weight regulator that is
7
of AMS under solvent-free conditions. However, a mixture
(
odorless and has few effects on the color and stability of
resulting polymers, thus widely used in polymerization
of 1-PT, 2-PT and Indan was generated with poor
selectivity. We have recently described a highly selective
dimerization of a-methylstyrene for 1-PT in a Br o¨ nsted
C
K
C
K
acidic ionic liquid, [Hmim] BF , where [Hmim] BF
4
4
Keywords: Br o¨ nsted acidic ionic liquid; a-Methylstyrene; Dimerization.
8
*
Corresponding author. Tel.: C86 21 62232764; fax: C86 21 62232100;
e-mail: jtang@chem.ecnu.edu.cn
severed as both catalyst and reaction medium. Herein, we
further report a temperature-controlled dimerization of
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.02.025

3986
H. Wang et al. / Tetrahedron 62 (2006) 3985–3988
AMS for highly selective production of 1-PT and Indan
catalyzed by [Hmim] BF under solvent-free conditions.
results were collected in Table 2. As revealed in Table 2, the
dimerization showed almost no change with respect to
conversion and selectivity.
C
K
4
2
. Results and discussion
C K
4
Table 2. Recycling [Hmim] BF in the dimerization of AMS
a
b
D
.1. Effects of the loading of [Hmim] BF4
L
Recycle
Conv. (%)
Indan
1-PT
2-PT
2
on the dimerization
1
2
3
4
5
6
95
96
95
93
96
95
3.5
4.5
4.4
3.9
4.1
4.3
90.1
89.5
90.3
90.2
90.5
89.6
6.4
6.1
5.3
5.9
5.4
6.1
C
Although [Hmim] BF represents the simplest and the
K
4
most readily prepared ionic liquid, it would be attractive,
especially in practical respect, to use the ammonium in
minimum amount in the dimerization of AMS, for example,
as catalyst, if the high selectivity for 1-PT could be
C
a
K
4
Reaction was carried out at 60 8C for 24 h with [Hmim] BF /AMSZ1:2
(mol/mol).
Determined by GC.
C
K
b
maintained. To our delight, when [Hmim] BF4 previously
used as solvent was reduced to 50 mol% of AMS the
selectivity for 1-PT and rate of the dimerization of AMS
just slightly decreased within experimental errors at 60 8C.
Further investigations showed the selectivity in the
dimerization of AMS was not sensitive to the loading of
the Br o¨ nsted acidic ionic liquids (Table 1). For example,
2
.3. Effects of temperature on the dimerization
The reaction temperature showed a great effect on both the
selectivity and the rate of dimerization of AMS catalyzed by
5 mol% [Hmim] BF (Table 3). The conversion of AMS
4
C
K
C
K
4
2
when the molar ratio of [Hmim] BF to AMS was reduced
from 4/1, 1/1 to 1/2, the selectivity for 1-PT decreased from
decreased from 92.5 to 78.4% for 48 h as reaction
temperature being lowered from 60 to 40 8C (Table 3,
entries 1 and 2). Further elongating the reaction time did not
increase the AMS conversion remarkably at 60 8C, implying
that the reaction could be very slow at low concentration of
AMS. However, when the reaction was conducted at 80 8C
the conversion reached 92.7% in 24 h, just half of that at
60 8C (Table 1, entry 10), but the 1-PT selectivity decreased
to 86.9% (Table 3, entry 6). No AMS was detected by GC
after 10 and 5 h when the reaction was carried out at 120 and
9
9
1
3.5, 91.2 to 90.3% and with conversion of AMS reaching
6.4, 93.1 and 93.5%, respectively, in 10 h (Table 1, entries
, 3 and 6). In fact, further reducing the loading of
C
K
[
Hmim] BF to 25 mol% of AMS slightly increased the
4
selectivity of 1-PT (92.8%) while the rate of dimerization
decreased sharply and only 23.9% AMS was converted after
1
0 h. Interestingly, the selectivity of 1-PT, 2-PT and Indan
remained almost unchanged at low conversion of AMS
Table 1, entries 8–10).
(
1
50 8C, respectively (Table 3, entries 8 and 9). However, the
C
Table 1. [Hmim] BF
K
4
promoted dimerization of AMS at 60 8C
selectivity of the dimerization was poor and cyclic isomer,
Indan, became the major product in the later case (Table 3,
entries 9 and 10).
a
Entry IL/AMS Time (h) Conv. (%) Distribution of dimers (%)
Indan
1-PT
2-PT
1
2
3
4
5
6
7
8
9
1
4/1
1/1
1/1
1/1
1/2
1/2
1/2
1/4
1/4
1/4
10
5
10
24
5
10
24
5
10
24
96.4
90.0
93.1
98.1
69.5
93.5
94.7
13.8
23.9
49.2
0.5
2.7
2.9
4.0
3.0
3.4
3.5
1.6
1.8
2.1
93.5
91.5
91.2
89.1
91.5
90.3
90.1
93.0
92.8
92.2
6.0
5.8
5.9
6.9
5.5
6.3
6.4
5.4
5.4
5.7
C
Table 3. Temperature effects on the [Hmim] BF
of AMS
K
4
catalyzed dimerization
a
b
Entry T (8C)
Time (h) Conv. (%) Distribution of dimers (%)
Indan
1-PT
2-PT
1
2
3
4
5
6
7
8
40
60
60
80
80
80
120
120
150
150
48
48
90
5
10
24
5
10
5
10
78.4
92.5
92.7
44.8
64.4
92.7
98.0
100
100
100
1.8
2.2
2.5
4.2
4.4
5.8
37.0
43.5
62.0
76.5
93.4
92.6
92.1
89.1
88.4
86.9
48.5
39.5
24.5
9.5
4.8
5.2
5.4
6.7
7.2
7.3
14.5
17.0
13.5
13.0
0
a
Determined by GC.
D
.2. Recycling of the ionic liquid [Hmim] BF4
L
2
9
10
C
Separation of [Hmim] BF from the organics is very
simple. Organic layer was readily separated from the ionic
liquid by decanting and the residue was washed by hexane
and dried in vacuum to recycle the ionic liquid
K
4
C
a
K
Reaction was carried out with [Hmim] BF4 /AMSZ1:4 (mol/mol).
Determined by GC.
b
C
K
4
[
weight of [Hmim] BF was lost for each cycle. Thus for
Hmim] BF . Due to the gel-like property, about 5–10%
According to the mechanism of acid-catalyzed dimerization
of AMS, Indan was formed from ionic dimeric intermediate,
which could be formed not only from dimerization of AMS
but also from 1-PT and 2-PT under acidic conditions
(Scheme 2).
C
K
4
C
each reuse, some (5–10% weight) fresh [Hmim] BF was
K
4
added to keep the constant weight of the ionic liquid for
each experiment. Since the dimerization was very slow at
low loading of [Hmim] BF it reasonable to omit the
C
K
4
influence of the newly added ionic liquid on the
dimerization. The reuse of catalytic system was investigated
Therefore, we anticipated, using the same catalyst system, it
should be possible to convert the chain isomers 1-PT and 2-
PT into cyclic isomer Indan, for which most acid-catalyzed
C
K
with [Hmim] BF /AMSZ1:2 (mol/mol) at 60 8C and
4

H. Wang et al. / Tetrahedron 62 (2006) 3985–3988
3987
3. Conclusion
H+
Ph
In conclusion, we described a Br o¨ nsted acidic ionic liquid
catalyzed and temperature-controlled highly selective
dimerization of a-methylstyrene to produce widely used
chemicals in polymers: 2,4-diphenyl-4-methyl-1-pentene
and the saturated cyclic isomer, 1,1,3-trimethyl-3-phenyl-
indan. At low reaction temperature, such as 60 8C,
2,4-diphenyl-4-methyl-1-pentene was formed in 93%
selectivity with O92% conversion using 25 mol%
-
H+
H+
Ph
Ph
Ph
Ph
+
-
H+
C
K
H+
Ph
[Hmim] BF under a solvent-free condition while Indan
could be obtained in 100% selectivity when the reaction
temperature was increased to 170 8C. The ionic liquid
-
4
+
H+
C
Hmim] BF was recycled six times showing no decrease
K
4
[
of activity after the mention of quantity loss of the ionic
liquid during work-up procedure.
Scheme 2. A general mechanism for acid-catalyzed dimerization of AMS.
procedure required use of organic solvents to form a
homogenous catalysis. Recently, Shan et al. reported a
solvent-free procedure for preparation of Indan from AMS.
However, an extremely moisture-sensitive Lewis acidic
4
. Experimental
All commercial chemicals were used without further
purification. Ionic liquid [Hmim] BF was prepared
according to the reported procedure. GC and GC-Mass
analyses were performed on a HP-Agilent 6890 with a 30 m
Hp-5MS column. H NMR spectra were acquired on a
C
K
4
3
ionic liquid Et NHCl–AlCl was used as catalyst. Reaction
3
3
6
c
temperature was screened for conversion of chain isomers
C
K
4
into Indan with [Hmim] BF /AMSZ1:2 (mol/mol). The
1
results were compiled in Table 4. From Table 4, it looked
like that conversion of both 1-PT and 2-PT into Indan was
slow at reaction temperature ranging from 120–150 8C. For
example, at 120 and 150 8C, Indan, among the dimers, just
increased from 41 and 76% to 54 and 88%, respectively,
when the reaction time was prolonged from 8 to 48 h
Bruker Avance 500 spectrometer in CDCl using TMS as
3
internal standard.
4.1. General procedure for the dimerization of
a-methylstyrene (AMS)
(
Table 4, entries 1–4). The similar trend was also observed
even at higher temperature 170 8C. For example, no AMS
remained in the system within 4 h and distribution of dimers
was 89% Indan, 5% 1-PT and 6% 2-PT. Under this
condition, both 1-PT and 2-PT were slowly converted into
Indan and disappeared after 18 and 30 h, respectively,
To a 25 mL flask charged with a magnetic stirrer and a
reflux condenser was added 1.7 g (10 mmol) [Hmim] BF
and 2.4 g (20 mmol) a-methylstyrene. The resulting
biphasic mixture was heated in oil bath at designed
C
K
4
temperature under N . After being cooled to room
2
(
Table 4, entries 5–10). It is clear that, at higher reaction
temperature, the organic layer was separated from the
ionic liquid by decanting. Composition of products was
analyzed by GC and compared with authentic samples.
Mass balances (O95%) were obtained in all cases and only
traces of organics (substrate AMS and products) were found
remained in ionic liquid phase, which was removed by wash
with hexane in the recycling of the ionic liquid.
temperature, the dimerization produces Indan as the major
product and the minor isomers 1-PT and 2-PT could be
converted into Indan ultimately. Compared with the
recently reported dimerization of AMS catalyzed by
Br o¨ nsted and Lewis acid ionic liquids, advantages of our
C
K
4
procedure are obvious: the ionic liquid [Hmim] BF was
readily available and air and moisture stable, and more
importantly, both of two useful products could be obtained
in high selectivity.
1
4
(
.1.1. 2,4-Diphenyl-4-methyl-1-pentene.
H NMR
CDCl , 500 MHz) d: 1.30 (s, 6H, CH ), 2.91 (s, 2H,
3
3
CH ), 4.89 (d, JZ1.8 Hz, 1H, ]CH ), 5.22 (d, JZ1.8 Hz,
2
2
C
Table 4. Cyclolization of AMS catalyzed by [Hmim] BF
Ka
4
1H, ]CH ), 7.19–7.30 (m, 10H, Ph); MS (EI) m/z: 236
2
C C
M] , 221 [MKCH ] , 119, 91, 77.
[
3
b
Entry T (8C) Time (h) Conv. (%) Distribution of dimers (%)
Indan
1-PT
2-PT
1
.1.2. 1,1,3-Trimethyl-3-phenylindan. H NMR (CDCl ,
4
3
1
2
3
4
5
6
7
8
9
1
120
120
150
150
170
170
170
170
170
170
8
48
8
48
4
100
100
100
100
100
100
100
100
100
100
41
54
76
88
89
91
93
97
98
100
43
25
13
2
5
2
16
21
11
10
6
7
6
3
2
—
500 MHz) d: 1.03 (s, 3H, CH ), 1.34 (s, 3H, CH ), 1.68 (s,
3 3
3
1
2
H, CH ), 2.19 (d, JZ13 Hz, 1H, CH ), 2.42 (d, JZ13 Hz,
3 2a
C
H, CH ), 7.10–7.27 (m, 9H, Ph). MS (EI) m/z: 236 [M] ,
2
b
C
21 [MKCH ] , 143, 128, 91, 77.
3
8
12
18
24
30
1
—
—
—
Acknowledgements
0
C
a
K
Project supported by the National Natural Science Foun-
dation of China (No. 20172016), Shanghai Science and
Reaction was carried out with [Hmim] BF
Determined by GC.
4
/AMSZ1:2 (mol/mol).
b

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H. Wang et al. / Tetrahedron 62 (2006) 3985–3988
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