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72
Chemistry Letters Vol.33, No.4 (2004)
Novel Acidic Ionic Liquids Catalytic Systems for Friedel–Crafts Alkylation of Aromatic
Compounds with Alkenes
Kun Qiao and Chiaki Yokoyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577
(Received January 7, 2004; CL-040025)
Novel acidic ionic liquids catalytic systems are developed
for the Friedel–Crafts alkylation of aromatic compounds with al-
kenes.
(CH2)n
SO2X
-
CF SO
3
3
1
2
a, n = 3, X = Cl, 1b, n = 4, X = Cl
a, n = 3, X = OH, 2b, n = 4, X = OH
The Friedel–Crafts alkylation of aromatic compounds with
alkenes is among the most important processes in chemical in-
dustry. Traditionally, this type of reaction is carried out in the
presence of AlCl3, HF, or sulfuric acid, which inevitably leads
to a series of environment problems as well as operational diffi-
culties in product separation and purification.
Scheme 1. Acidic ionic liquids.
1
acidic ionic liquids, we first employed them as catalysts for alky-
lation of p-xylene with styrene, a typical example of the Friedel–
Crafts alkylation reactions of aromatic compounds with alkenes
8
that have important practical application in industry. Two dif-
Recently, a few of examples of the Friedel–Crafts alkylation
of aromatic compounds with alkenes have been reported to be
proceeded in room temperature ionic liquids (RTILs), a kind
ferent products, i.e. the monostyrenated and the distyrenated,
are detected and both are the desired products in industry. It
could be seen when p-xylene and styrene are treated with the
Brꢀnsted acidic ionic liquid 2a or 2b (Runs 1and 2), a satisfac-
2
of novel and promising solvents for synthetic chemistry. It is
ꢀ
known that these alkylation reactions could be carried out either
directly using chloroaluminate ionic liquids, especially in their
Lewis acidic form, as catalysts or employing an ionic liquid cat-
alytic system that containing scandium triflate and hydrophobic
ionic liquid such as [BMIm][PF6] (1-butyl-3-methylimidazoli-
tory conversion of styrene is achieved after 5 h reaction at 70 C.
The Lewis acidic ionic liquid 1a is also an effective catalyst for
this reaction (Run 3), but with the increasing the length of the
side chain, the activity of the Lewis ionic liquid 1b notably de-
creased (Run 4), suggesting the difference in the type of acidity,
the Brꢀnsted or Lewis acidity, may affect the reaction in some
degree.
The reaction is carried out in a biphasic mode, so the prod-
ucts in the upper layer could be easily separated from the ionic
liquid by decantation after reaction and the remaining ionic liq-
uid is ready for next run. The Brꢀnsted ionic liquid 2b has shown
good reusability for the alkylation reaction of p-xylene with sty-
rene, for example, even after 5 times cycle under the same reac-
tion condition, the conversion of styrene and selectivity to mon-
oalkylated products still kept at 91.6 and 85.3%, respectively
(Run 5).
1
,3
um hexafluorophosphate).
Unfortunately, from the point of environmental view, both
of these catalytic systems have an intrinsic shortage because of
their instability. The sensitivity of chloroaluminate ionic liquids
toward air and water is already well known. Hydrophobic ionic
liquid such as [BMIm][PF6] is also found to be prone of hydrol-
4
ysis to emit acidic fume containing HF and other species. In
consideration of the possibility of the hydrolysis of hydrophobic
ionic liquid, particularly in the presence of acidic metal triflate, it
becomes difficult to tell which one, metal triflate or acidic impur-
ity species formed from the hydrolysis of ionic liquid, act as the
actual catalysts in the case when catalytic system containing
scandium triflate and hydrophobic ionic liquid is utilized for al-
kylation reactions.
Ionic liquid 2b could also smoothly catalyze the alkylation
reaction of styrene with other aromatic compounds such as ben-
zene and toluene (Runs 6 and 7). Very interesting, however, no
reaction is observed when ionic liquid 1a or 2b is applied as cat-
alyst for alkylation of benzene with 1-hexene (Runs 8 and 9).9
There is also no reaction occurred when only scandium tri-
flate is used as catalyst for alkylation of benzene with 1-hexene
In this paper, we report the research of utilizing a kind of
novel acidic ionic liquids catalytic systems for the Friedel–Crafts
alkylation of aromatic compounds with alkenes. It includes: (i) a
5
kind of novel Lewis acidic ionic liquids (1) is synthesized deriv-
6
10
ing from Cole’s Brꢀnsted acidic ionic liquids (2) (Scheme 1),
(
(Run 10), which is of in accord with the result observed by
1
ii) it is demonstrated that those acidic ionic liquids are effective
Song. A synergistic effect is, however, discovered when the re-
and reusable catalysts for alkylation of aromatic compounds
with styrene, but less active for alkylation of aromatic com-
pounds with other alkenes, (iii) a surprising synergistic effect
has been found between those acidic ionic liquids and metal tri-
flate, leading to active catalytic systems for alkylation reactions
of aromatic compounds with alkenes that only ionic liquids or
metal triflates are ineffective.
action of alkylation of benzene with 1-hexene is carried out in
the presence of the mixture of ionic liquid 2b or 1b with scandi-
um triflate (Runs 11 and 12). High conversions of 1-hexene are
achieved in both cases, but selectivities to monoalkylated prod-
ucts are remarkably varied. Catalytic system containing the
Lewis acidic ionic liquid had exhibited higher selectivity for
monoalkylated products than that of the Brꢀnsted acidic one.
Then the scandium triflate-Lewis acidic ionic liquid 1b cat-
alytic system is applied for the alkylation reactions of p-xylene
or benzene with other different alkenes, including 1-dodecene,
Results of the Friedel–Crafts alkylation of several aromatic
7
compounds with alkenes are summarized in Table 1. To exam-
ine the catalytic performance of both the Brꢀnsted and Lewis
Copyright Ó 2004 The Chemical Society of Japan