Highly selective aromatic alkylation of phenol and anisole by using recyclable bronsted acidic ionic liquid systems

Article abstract of DOI:10.1002/ejoc.201300579

A highly efficient ionic liquid catalyst system for selective alkylation of phenol and anisole with alkenes is described. By using Bronsted acidic triflate ionic liquids containing the SO₃H group attached to the cation, it was possible to recycle the catalyst and reuse it after a simple workup procedure. Moreover, selectivity towards the monoalkylated products was improved to 93 % by using a biphasic system. A sulfonic acid functionalized ionic liquid is used as catalyst in the hexylation of phenol and anisol, enabling very high product selectivities, simple product isolation and effective catalyst recycling. The observed selectivity boost is mainly due to differential solubility effects of the liquid-liquid biphasic reaction system established by the ionic liquid catalyst. Copyright

Full text of DOI:10.1002/ejoc.201300579

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FULL PAPER
DOI: 10.1002/ejoc.201300579
Highly Selective Aromatic Alkylation of Phenol and Anisole by Using
Recyclable Brønsted Acidic Ionic Liquid Systems
Karin Titze-Frech,[a] Nikolai Ignatiev,[b] Marc Uerdingen,[b] Peter Steffen Schulz,*[a] and
Peter Wasserscheid[a]
Keywords: Homogeneous catalysis / Ionic liquids / Solvent effects / Waste prevention / Alkylation
A highly efficient ionic liquid catalyst system for selective
alkylation of phenol and anisole with alkenes is described.
By using Brønsted acidic triflate ionic liquids containing the
SO3H group attached to the cation, it was possible to recycle
the catalyst and reuse it after a simple workup procedure.
Moreover, selectivity towards the monoalkylated products
was improved to 93% by using a biphasic system.
ample HF,[8] are volatile and toxic. To realize the alkylation
reaction in the desired rate and selectivity, these acids are
often applied in quantities well above stoichiometry. As a
consequence, neutralization of the excess of acid after reac-
tion results in large amounts of waste.[9]
Introduction
In 1887, Charles Friedel and James Mason Crafts in-
vented the conversion of aromatic compounds into their
alkylated or acylated derivatives by using anhydrous alumi-
num trichloride as Lewis acid catalyst.[1] Over the past 125
years, interest in substrate variation, improved selectivity
and catalyst activity, along with environmental and econ-
omic aspects, has led to dynamic research in the catalysis
of aromatic alkylation and related process optimization.
Both bulk and fine chemicals are produced, underlining
the commercial value and the versatility of the reaction.
The selective synthesis of monoalkylated, oxygen-function-
alized arenes, such as alkylated phenols or anisoles, is a sub-
ject of high industrial interest because these products play
a prominent role in the production of fine chemicals.[2] Im-
portant fields of application of these products are lubri-
cants,[3] UV-light absorbers, fragrances,[4] and resins.[5]
Furthermore, phenols with long alkyl substituents have
been used as building blocks for anionic detergents. These
alkylated phenols, with linear or branched side chains, are
formed by alkylation of phenols with Cn-olefins (n = 6–20).
Despite the fact that alkylation of aromatic substances
can, in principle, be carried out under noncatalyzed thermal
conditions,[6] all commercial processes apply catalysts to
promote reaction rate and to increase the selectivity to the
desired monoalkylated products.[7] Most industrial pro-
cesses use strong acids or even superacidic (more acidic
than 100% H2SO4) systems.[2] Some of these acids, for ex-
In recent years, significant effort has been devoted to im-
prove alkylation reactions of functionalized aromatic sub-
strates, in particular of phenol alkylation.[10] Approaches to
improve phenol alkylation by advanced solvent concepts in-
clude the use of supercritical water[11] and acidic ionic li-
quids.[12] In contrast to reactions in supercritical water,
which require corrosion-resistant, high-pressure equipment,
reactions in ionic liquids can be performed in standard re-
actors. Further advantages of ionic liquids as reaction me-
dia in alkylation reactions include – in addition to their
extremely low vapor pressures – the tuneability of their
acidity and miscibility with reagents and products.[13] By
using acidic ionic liquids as catalyst phase, it is possible to
separate the products from the catalyst phase simply by
either liquid–liquid phase separation (requiring a miscibility
gap between the IL and the product phase) or by product
evaporation (requiring an appreciable vapor pressure of the
product). In both cases the ionic liquid catalyst remains in
the reaction system and can be reused. Compared with
traditional homogeneous alkylation reactions, no neutral-
ization of the ionic liquid’s catalyst is required, which avoids
significant amounts of salt formation, waste water treat-
ment and release of acidic gases. Thus, alkylation reactions
performed by using acidic ionic liquids are typically charac-
terized by a much better E-factor[14] compared with the
same reaction performed with traditional systems, such as
AlCl3 or sulfuric acid.
[a] Lehrstuhl Chemische Reaktionstechnik, Friedrich-Alexander-
Universität Erlangen-Nürnberg,
Egerlandstrasse 3, 91058 Erlangen, Germany
E-mail: peter.schulz@fau.de
http://www.ltc1.uni-erlangen.de/
However, it has to be clearly stated that the most com-
mon Lewis acidic ionic liquids, chloroaluminate ionic li-
quids of the type [cation]Cl/AlCl3 with n([cation]Cl)/
n(AlCl3) Ͻ 1, are not suitable for the alkylation of oxygen-
[b] Ionic Liquid Research Laboratory, Merck KGaA,
Frankfurter Str. 250, 64293 Darmstadt, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300579.
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P. S. Schulz et al.
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containing aromatic substrates such as phenol or anisole. tached to the core to improve the acidity of the ionic liquid
Whereas such chloroaluminate melts have been widely ap- further.[29] Gu et al.[13b] used 1-(4-sulfobutyl)-3-methylimid-
plied to catalyze the alkylation of heteroatom-free aromatic azolium triflate ([MIMBS][OTf]) as solvent and catalyst for
compounds,[12a,12c] they form stable phenoxy compounds the oligomerization of olefins. Qureshi et al.[30] performed
with most phenolic substrates. From these stable com- the alkylation of phenol with styrene by using 1-(4-sulfo-
pounds the alkylated product can only be released by hy- butyl)-3-methyl-imidazolium hydrogen-sulfate/sulfuric acid
drolysis of the melt, a procedure that again inevitably leads as catalyst and solvent, albeit with rather low conversions.
to loss of the ionic liquid catalyst.
Elavarasan et al.[31] and Gui et al.[32] reported the selective
Published attempts to overcome this major limitation in- alkylation of phenol with tert-butyl alcohol by using vari-
clude the application of less oxophilic, acidic chloroindate ous SO3-functionalized imidazolium salts with hydro-
ionic liquids[15] and the use of Brønsted acidic ionic li- gensulfate counterions.
quids.[16] The utility of chloroindate melts has been demon-
strated by Seddon et al. for the alkylation of phenol and
catechol with alkenes[15,17] and by Leitner et al. for the reac-
tion of anisole and acetic anhydride.[18] The use of Brønsted
acidic ionic liquids has been realized by Sesing et al. for
the Fries rearrangement[19] and by Ignatiev et al. for the
dehydration of cyclohexanol.[20] In the work by Sesing et
al., a weakly Brønsted acidic ionic liquid, such as 1-butyl-
3-methylimidazolium hydrogensulfate ([BMIM][HSO4]), is
mixed with a strong mineral acid, such as sulfuric acid, to
obtain an ionic catalyst phase of suitable acidity.[19] In this
case, a mineral acid that dissolves in the product phase still
needs to be neutralized as part of the product isolation pro-
cedure.
In this study, we compare the performance of acidic ionic
liquids of the general type [MIMBS]X and [MIMBS]X/HX
with the reactivity of the pure corresponding mineral acids
in the alkylation of phenol and anisole. Pure [MIMBS][OTf]
(see also Scheme 1) has been reported to be more acidic
than [BMIM][HSO4] but less acidic than pure triflic acid.[33]
Phenol and anisole are activated arenes, therefore mul-
tiple alkylation at the aromatic ring causes serious selectiv-
ity problems. In addition, the carbocation generated from
the olefin can attack the oxygen atom to form the corre-
sponding ether compound as another byproduct. Scheme 1
shows the alkylation of phenol and anisole with 1-hexene
as well as the products and byproducts formed in the reac-
tion. The desired mono-ring-alkylated products appear as
different regioisomers depending on the position of the alk-
ylation in the aromatic ring versus the O-substitution.
First, we present our experimental results in phenol hex-
Results and Discussion
In this paper we describe the results of our investigation ylation. In all cases, the reaction mixtures in phenol hex-
to identify new and more efficient ionic liquid catalyst sys- ylation were homogeneous (monophasic) and the organic
tems for selective alkylation of phenol and anisole with hex- products were isolated by extraction with cyclohexane after
ene. In our study we focused on Brønsted acidic triflate reaction. Figure 1 shows the GC-spectra of the isolated or-
ionic liquids containing the SO3H group attached to the ganic product mixtures after 15 min reaction time at 80 °C.
cation (Scheme 1). This class of task-specific ionic liquids Three different catalyst systems were compared: (a) pure
(TSIL) was introduced by Cole et al.[21] and has already triflic acid, (b) an equimolar mixture of [MIMBS][OTf]
been tested in several synthetic applications such as the syn- with triflic acid, and (c) pure [MIMBS][OTf]. Triflic acid
thesis of ethyl acetate,[21] diketals,[13a] quinoline,[22] pyruval- (HOTf) proved to be by far the most active catalyst, but
dehyde dimethylacetals,[23] 1-(benzothiazolylamino)methyl- many side-products were formed (Figure 1, a). Apart from
2-naphthols,[24] levulinic acid,[25] nitriles,[26] and aryl hal- mono-alkylated products (P1), many different multiply
ides.[27] These TSILs have also been applied in biphasic re- alkylated arenes (P2) were detected in the GC analysis. By
actions with water as a second phase,[22] for which recycling using the [MIMBS][OTf]/HOTf equimolar mixture as cata-
of the IL catalyst phase has been demonstrated.[13a,21,22,24] lyst, the selectivity in the hexylation of phenol increased
For heterogeneous, catalyzed reactions these TSIL were significantly (Figure 1, b). The best selectivity, with almost
used either as heteropolyanion-based ionic liquids or were exclusive formation of mono-alkylated hexylphenol, was
immobilized on silica.[28] More acidic groups can be at- obtained with pure [MIMBS][OTf] catalyst (Figure 1, c). It
Scheme 1. Alkylation of phenol (R = H) and anisole (R = methyl) by using 1-hexene as alkylating agent. The desired monoalkylated
product is shown together with O-alkylated and multiply alkylated by-products.
2
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Aromatic Alkylation of Phenol and Anisole
is noteworthy that the selectivity in the latter case was found even with a 1:1 mixture of phenol and alkene, whereas with
to be almost independent of 1-hexene conversion (the stoi- the same feedstock mixture the selectivities were only 40%
chiometrically limiting substrate). Four main mono-alkyl- in the case of [MIMBS][OTf]/HOTf (1:1) and only 20%
ated products were detected: o- and p-hexylphenol, each with pure HOTf (Table 1, entries 5–7). By addition of an
with 2-/3-hexyl connectivity. At the beginning of the reac- organic solvent, which forms a second phase with the ionic
tion, mainly p-2- and o-2-hexylphenols were produced, pre- liquid, the selectivity for the mono-alkylated product could
sumably due to the slower isomerization rate of 1-hexene be further improved (Table 1 entry 9).
to 2- and 3-hexene compared to alkylation rate. 1-Hexene,
The possibility for recycling is another very relevant ad-
produced solely 2-hexylphenols. After its isomerization to vantage of the ionic liquid system reported here. Recycl-
2- and 3-hexene, all four main products were generated. At ability of the ionic liquid catalyst is a crucial feature from
full conversion, the o/p ratio was 0.9 and the 2-/3-hex- an economic and ecological point of view because the syn-
ylphenol ratio was 1.6. Detailed reaction conditions, con- thesis of the ionic liquid catalyst is certainly more expensive
versions, and selectivity are given in Table 1.
and resource-demanding than the synthesis of pure mineral
acids. We therefore assessed the ionic liquid recovery and
its reuse in phenol alkylation. To this end, the reaction mix-
ture was extracted with cyclohexane, the remaining ionic
liquid was dried under vacuum, and fresh reactants were
added. Six recycle runs of [MIMBS][OTf] were performed
in this way without loss of activity or selectivity (Figure 2).
No thermal decomposition of the ionic liquid was detected
by 1H or 13C NMR spectroscopic analysis after the six recy-
cle runs.
Figure 1. GC spectra of phenol hexylation experiments with
(a) HOTf, (b) [MIMBS][OTf]/HOTf (1:1), and (c) [MIMBS][OTf]
as catalyst after 15 min at 80 °C. Molar ratio 1-hexene/phenol/cata-
lyst (1:5:0.71); denoted time intervals in the spectra: P = phenol,
P1 = mono-alkylated phenols, P2 = multiply alkylated phenols.
Table 1. Acid-catalyzed alkylation of phenol with 1-hexene at 80 °C
after 15 min. The IL/hexene ratio was 0.71.
Entry Catalyst
Phenol/
Conv. Select.
[%][b] [%][c]
Figure 2. Recycling experiments of the [MIMBS][OTf] catalytic
phase in the hexylation of phenol. T = 80 °C, t = 15 min; phenol/
1-hexene/IL, 5:1:0.71; conversion (black bars) and selectivity (grey
bars).
1-hexene[a]
1
2
3
4
5
6
7
8
9
HOTf[d]
5:1
100
100
100
100
100
100
100
100
99
41
78
88
73
20
40
65
34
93
HOTf/[MIMBS][OTf], 1:1[e] 5:1
[MIMBS][OTf][f]
5:1
5:1
1:1
[g]
Encouraged by these results of phenol hexylation, we ap-
plied the same ionic liquid system to anisole hexylation. In
contrast to alkylation of phenol, the reaction system for
anisole hexylation with pure [MIMBS][OTf] showed a li-
quid–liquid miscibility gap. The reaction systems contain-
ing HOTf formed a single-phase reaction mixture. Selectiv-
ity-wise, anisole alkylation is more demanding than phenol
alkylation because the methoxy group offers additional pos-
sibilities for side-product formation. Figure 3 shows the GC
spectra of anisole hexylation with (a) pure HOTf, (b) an
equimolar [MIMBS][OTf]/HOTf mixture, and (c) pure
[MIMBS][OTf]. The displayed product mixtures where ob-
H2SO4
HOTf[d]
[MIMBS][OTf]/HOTf, 1:1[e] 1:1
[MIMBS][OTf][f]
1:1
1:1
5:1
[g]
H2SO4
[MIMBS][OTf][h]
[a] Molar ratio. [b] Conversion of hexene. [c] Selectivity to monoalk-
ylated products; Reaction conditions: 4.35 g (46.2 mmol) phenol,
0.778 g (9.2 mmol) 1-hexene. [d] Triflic acid (0.97 g, 6.5 mmol).
[e] Triflic acid (0.49 g, 3.2 mmol) and [MIMBS][OTf] (1.18 g,
3.2 mmol). [f] [MIMBS][OTf] (2.38 g, 6.5 mmol). [g] 96% H2SO4
(0.63 g, 6.5 mmol). [h] [MIMBS][OTf] (2.38 g, 6.5 mmol) and sol-
vent (cyclohexane) (15.6 g, 185.4 mmol).
In general, the use of an excess of arene versus alkene tained after 60 min reaction time at a reaction temperature
led to better selectivities for the mono-alkylated products of 140 °C. The results follow the same trend to that of
(Table 1, entries 1–3). Remarkably, the use of [MIMBS]- phenol alkylation but the differences are much more pro-
[OTf] resulted in 65% selectivity for mono-hexyl phenol nounced. The catalysis by HOTf was again fastest, but by
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far the least selective. The selectivity for monoalkylated an- Table 2. Acid-catalyzed anisole alkylation (molar ratio anisole/1-
hexene/catalyst, 1:5:0.71.).
isole was only 5% (Figure 3, A2; for details on the reaction
conditions, selectivities and conversion rates, see Table 2).
The phenol (P), methylphenols, and aromatic hexyl ethers
(A1) and multiply alkylated arenes (A3) dominated in the
pattern of product distribution. The picture changed quite
significantly when, instead of the pure triflic acid, an equi-
molar mixture of [MIMBS][OTf] and HOTf was applied as
catalyst phase (Figure 3, b). The selectivity increased drasti-
cally to 89% monoalkylated anisole and anisole conversion
remained high (75%). This positive effect of the acidic ionic
liquid became even more pronounced when pure
[MIMBS][OTf] was applied as the catalyst (Figure 3, c). Al-
though the reaction was slower, excellent selectivity to the
monoalkylated product was achieved. This is an astonishing
result in the light of the numerous possible pathways for
byproduct formation. As seen in the phenol hexylation, 2-
hexylanisoles were produced first. At full conversion the o/p
ratio was 1 and the 2-/3-hexylphenol ratio was 1.5. For as-
signment of the peaks, see the Supporting Information (S4).
Entry Catalyst
t
T
Conv. Select.
[h] [°C] [%][a]
[%][b]
1
2
3
4
5
6
HOTf[c]
2
2
2
2
2
140
140
140
140
120
95
75
59
68
48
99
5
[MIMBS][OTf]/HOTf, 1:1[d]
89
97
40
99
98
[MIMBS][OTf][e]
[f]
H2SO4
[MIMBS][OTf][e]
[MIMBS][OTf][e]
24 120
[a] Conversion of hexene. [b] Selectivity to monoalkylated products;
reaction conditions: anisole (5 g, 46.2 mmol), hexene (0.778 g,
9.2 mmol). [c] Triflic acid (0.97 g, 6.5 mmol). [d] Triflic acid (0.49 g,
3.2 mmol)
and
[MIMBS][OTf]
(1.18 g,
3.2 mmol).
[e] [MIMBS][OTf] (2.38 g, 6.5 mmol). [f] 96% H2SO4 (0.63 g,
6.5 mmol).
activity is caused by a reduction in acidity as a result of
either leaching or water intake during the recycling pro-
cedure.
Figure 4. Recycling experiments of the [MIMBS][OTf] catalytic
phase in the hexylation of anisol. T = 120 °C, t = 4 h; anisol/1-
hexene/IL, 5:1:0.71; conversion (dark grey bars) and selectivity
(grey bars); * After addition of three drops (ca. 30 μL) of triflic
acid.
Figure 3. GC spectra of anisole hexylation experiments with
(a) HOTf (single phase), (b) [MIMBS][OTf]/HOTf 1:1 (single
phase), and (c) [MIMBS][OTf] (biphasic) as catalyst after 60 min
at 140 °C. Molar ratio 1-hexene/anisole/catalyst (1:5:0.71); denoted
intervals in the spectra: A = anisole, P = phenol, A1 = methylphen-
ols and hexyl ethers, A2 = desired products: monoalkylated anisole,
A3 = multiply alkylated arenes and monoalkylated phenols.
Note that the different activity trends in phenol and anis-
ole alkylation during recycling can be explained by the fact
that phenol alkylation is much faster and requires less
acidic conditions for full conversion (compare also the reac-
tion conditions and conversion levels). Therefore, a slight
loss in acidity does not show up in the form of reduced
As in the case of phenol alkylation, the recyclability of levels of conversion in phenol alkylation under the applied
the [MIMBS][OTf] catalyst was also assessed for anisole conditions, whereas it does in case of the more demanding
alkylation. Figure 4 shows the results of four recycling runs. anisole alkylation.
After each run, the organic phase was decanted from the
The much higher selectivity of the ionic liquid catalyst
ionic liquid catalyst phase and fresh feedstock mixture was systems in anisole alkylation can be attributed to the ionic
added to the catalyst. Whereas the selectivity for the desired liquid/organic biphasic system (with its solubility-driven in-
monoalkylated anisole remained excellent over the four re- crease in aromatic/alkene ratio at the place of reaction, see
cycling runs, there was a clear drop in the catalytic activity. the Supporting Information, S1). In addition, the acidity
The addition of a few drops of triflic acid to the reaction of [MIMBS][OTf] is clearly more suitable for the selective
mixture after recycling run four (ca. 30 μL on 1.5 mL of the alkylation of phenol and anisole compared with triflic acid.
catalyst layer) led to complete recovery of the initial activity
To separate solubility and acidity effects, we determined
(with constant selectivity), suggesting that the decrease in the acidity of [MIMBS][OTf] by a solvent-independent
4
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Aromatic Alkylation of Phenol and Anisole
31P NMR technique according to Gutmann.[34] The method
applies triethylphosphane oxide (TEPO) as the indicator
and derives the dimensionless Acceptor Number (AN) from
the NMR shift of the phosphorus atom in the presence of
an acid. The 31P NMR chemical shifts of the indicator are
recorded for solutions with different indicator concentra-
tions, standardized and extrapolated to infinite dilution of
the indicator. The method has been applied several times in
the literature for the determination of the Lewis acidity of
Phenol and Anisole Hexylation Experiments: The ionic liquid
(8 mmol), was placed in a 50 mL flask with a 15 mm PTFE stirring
bar. After evacuating the flask for 30 min at 80 °C, the ionic liquid
was heated to the reaction temperature. The aromatic compound,
the alkylation reagent, and the internal standard undecane were
added. Most reactions were carried out without the addition of
further solvents. Exceptions in phenol alkylation are noted in the
text, here, cyclohexane was added to generate a biphasic system.
The reaction mixtures were stirred at 80–140 °C for 2–24 h. For
better comparison, an identical molar amount of acidic protons
ionic liquids.[35] Using this method, we found that the acid- was applied in all experiments, leading to the use of 8 mmol tri-
fluoromethanesulfonic acid (HOTf), the use of a mixture of 4 mmol
[MIMBS][OTf] with 4 mmol HOTf, and to the use of 8 mmol
[MIMBS][OTf], respectively. The IL recycling was performed by
product extraction with cyclohexane in the case of phenol alkyl-
ation and by decanting the organic layer in case of anisole alkyl-
ation. In both cases, the ionic liquid phase was washed with cyclo-
hexane until all organic products were extracted. This was checked
by GC and 1H NMR spectroscopic analysis. After two hours evac-
uation at 80 °C, the ionic liquid was ready for reuse.
ity of [MIMBS][OTf] is in the range of 96% H2SO4 (see the
Supporting Information, S2 for details). Therefore, it was
interesting to directly compare the results of catalysis by
[MIMBS][OTf] and 96% H2SO4 in phenol and anisole hex-
ylation. As described earlier, all phenol alkylation systems
of this study form single phase reaction mixtures. The reac-
tion catalyzed by pure [MIMBS][OTf] was found to be sig-
nificantly more selective than the reaction with sulfuric acid
(compare Table 1, entries 3 vs. 4 and 7 vs. 8). Even the
slightly more acidic 1:1 mixture of [MIMBS][OTf] and
triflic acid was more selective than the sulfuric acid cata-
lyzed reaction. These results indicate that, in addition to
providing the appropriate acidity, the ionic liquids provide
additional benefits. However, a much stronger boost in
selectivity is obtained for the more demanding anisole alk-
ylation for which the ionic liquid catalysts form biphasic
reaction systems. Here, the product selectivity of the
[MIMBS][OTf]-catalyzed reaction is 97% compared with
only 40% selectivity in the case of the monophasic reaction
with H2SO4 being the catalyst (at comparable conversions).
The greater selectivity demonstrated in the case of anisole
are attributed to the biphasic reaction system, which leads
All reactions were monitored and analyzed by gas chromatography
(GC) with a Varian 3900 gas chromatography and FID detector.
The GC was equipped with a FS-CS-Pona column (50 °C for
1 min, 5 K/min to 200 °C, 20 K/min to 250 °C; detector and injec-
tor held at 250 °C). The products were identified by GC and GC–
MS. The GC–MS (Varian 450-GC/Varian 220-MS) was equipped
with a VF-5MS column from Varian.
Conversions were determined by GC analysis. 1H and 13C NMR
spectra to check the purity and stability of the applied ionic liquids
were recorded with a JEOL ECX 400 MHz spectrometer.
Supporting Information (see footnote on the first page of this arti-
cle): S1: Determination of the solubility of the substrate in the ap-
plied ionic liquid [MIMBS][OTf]; S2: Determination of IL acidities
with 31P NMR spectroscopy; S3: 1H and 13C NMR spectra of
to a solubility-driven aromatic/olefin ratio that is higher [MIMBS][OTf]; S4: Product distribution of monoalkylated anisoles
(mass spectroscopy).
than nominal in the catalytic phase.
Acknowledgments
Conclusions
The authors gratefully acknowledge financial support by Merck
We have studied in detail a very promising approach to KGaA and by the Cluster of Excellence “Engineering of Advanced
Materials” at the University of Erlangen-Nuremberg, which is
funded by the German Research Foundation (DFG) within the
framework of its “Excellence Initiative”.
the highly selective hexylation of phenol and anisole. The
use of the Brønsted acidic ionic liquid [MIMBS][OTf] en-
abled very high product selectivities, simple product isola-
tion, and reuse of the catalyst phase for at least four repeti-
tive runs. We consider that the selectivity benefits are due
to the biphasic operation with its differential solubility ef-
fects, and due to the specific acidity created by the ionic
liquid environment. We anticipate that the results reported
here are not only of technical interest for companies in the
phenol and anisole alkylation business, but may also pave
the way for the development of new alkylation methods for
heteroaromatic and heteroatom-containing aromatic feed-
stocks in general.
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P. S. Schulz et al.
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Received: April 22, 2013
Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30579
/KAP1
Date: 09-09-13 18:19:13
Pages: 7
Aromatic Alkylation of Phenol and Anisole
Ionic Liquids
A sulfonic acid functionalized ionic liquid
is used as catalyst in the hexylation of
phenol and anisol, enabling very high prod-
uct selectivities, simple product isolation
and effective catalyst recycling. The ob-
served selectivity boost is mainly due to
differential solubility effects of the liquid–
liquid biphasic reaction system established
by the ionic liquid catalyst.
K. Titze-Frech, N. Ignatiev, M. Uerdingen,
P. S. Schulz,* P. Wasserscheid ........... 1–7
Highly Selective Aromatic Alkylation of
Phenol and Anisole by Using Recyclable
Brønsted Acidic Ionic Liquid Systems
Keywords: Homogeneous catalysis / Ionic
liquids / Solvent effects / Waste prevention /
Alkylation
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