Ionic liquids as reagent and reaction medium: Preparation of alkyl aryl ethers

Article abstract of DOI:10.1007/s00706-006-0562-5

Room temperature ionic liquid, [bmIm]OH, is used as a green recyclable reaction medium and reagent for the alkylation of phenols in excellent yields. The recovered ionic liquid was reused five to six times with consistent activity.

Full text of DOI:10.1007/s00706-006-0562-5

Monatshefte f €u r Chemie 138, 47–49 (2007)
DOI 10.1007/s00706-006-0562-5
Printed in The Netherlands
Short Communication
Ionic Liquids as Reagent and Reaction
Medium: Preparation of Alkyl Aryl Ethers
Farajollah Mohanazadeh1 and Majid Aghvami2
;ꢀ
1
Institute of Industrial Chemistry, Iranian Research Organization for Science
and Technology, Tehran, Iran
2
Institute of Chemistry, Tarbiat Moallem University, Tehran, Iran
Received August 12, 2006; accepted (revised) September 7, 2006
Published online January 2, 2007 # Springer-Verlag 2007
Summary. Room temperature ionic liquid, [bmIm]OH, is used as a green recyclable reaction medium
and reagent for the alkylation of phenols in excellent yields. The recovered ionic liquid was reused five
to six times with consistent activity.
Keywords. Alkylations; Catalysts; Phenols; Ethers; Green Chemistry.
Introduction
Preparation of alkyl aryl ethers is an important synthesis reaction for which a wide
variety of procedures has been developed during the last hundred years [1, 2]. Most
of the commonly used methods involve alkylation of the parent phenol or derived
phenoxide ion, with the latter being by far the more important.
O-Alkylated phenols produced by regioselective synthesis belong to an extreme-
ly attractive domain in organic chemistry. One possible way of accomplishing the
O-alkylation is by using a stoichiometric amount of a strong base. The established
methods include the use of alkali metal hydroxides [3], alkali metal hydrides [4],
and phase-transfer catalytic conditions [5, 6].
Phenoxides undergo O-alkylation in solvents, such as DMSO, DMF, ethers,
and alcohols. In water and trifluoroethanol, however, extensive C-alkylation occurs
[
7, 8]. These latter solvents form particularly strong hydrogen bonds with the
oxygen atom of the phenolate anion. This strong solvation decreases the reactivity
at the oxygen and favors C-alkylation.
Though some of the above-mentioned methods provided good yields of alkyl
aryl ethers, they also involve the use of hazardous and carcinogenic dipolar aprotic
ꢀ Corresponding author. E-mail: mohannazadeh@irost.org

4
8
F. Mohanazadeh and M. Aghvami
organic solvents. Moreover, during the workup these solvents are converted into
waste byproducts, making their recycling impossible. Further, some of the above-
mentioned bases are flammable. Thus, the development of an efficient, safe, and
environmentally friendly method of phenol O-alkylation constitutes an important
challenge.
In recent years, ionic liquids (ILs) [9] have attracted extensive interest as envi-
ronmentally friendly solvents due to their favorable properties, such as non-flamm-
ability, negligible vapor pressure, reusability, and high thermal stability [10]. Over the
past few years, a variety of catalytic reactions has been successfully conducted using
ILs as an alternative for organic solvents [11, 12]. Especially, one of the advantages
of ILs is their behavior as an acid or base catalyst. Recently, Ranu and Banerjee [13]
reported the dramatic influence of a new tailor-made, task-specific, and stable ionic
liquid, 1-butyl-3-methylimidazolium hydroxide [bmIm]OH, in Michael additions.
In our quest to develop a general method for the O-alkylation of phenols we
now discovered that [bmIm]OH efficiently promotes this reaction without any
additional catalyst and solvent.
Results and Discussion
A number of different phenols and alkyl halides were subjected to the alkylation
reaction to test its generality, and the results are summarized in Table 1. The results
Table 1. Preparation of alkyl aryl ethers
Yield
a
%
b
%
b
%
Entry
Y
R
X
1
1
2
1
2
3
4
5
6
7
8
9
H
benzyl
Br
Br
Br
Br
Cl
Br
Br
Br
Br
I
92
95
80
83
80
95
82
70
44
80
65
87
100
100
99
0
0
1
0
3
0
6
4
0
0
4
1
H
4-nitrbenzyl
4-methylbenzyl
4-chlorobenzyl
benzyl
H
H
100
97
H
NO2
CH3
H
benzyl
100
94
benzyl
ethyl
96
H
2-propyl
ethyl
100
100
96
1
1
1
0
1
2
H
H
ethyl
Cl
I
Cl
ethyl
99
a
b
Isolated yield; GC yield

Preparation of Alkyl Aryl Ethers
49
indicate that deactivated phenols are more active and selective for O-alkylation
Entry 6). Introduction of an electron-donating substituent on the aromatic ring
(
substantially decreases the reaction rate (Entry 7). Alkyl iodides react more readily
than bromides, with chlorides being still less reactive.
In conclusion, the present procedure using a basic (pH ¼ 9.3) and easily avail-
able ionic liquid [bmIm]OH provides an efficient and convenient procedure for the
O-alkylation of phenols without requiring any other base and organic solvent. This
method offers marked improvements with regard to operational applicability,
high isolated yields of products, and greenness of procedure, avoiding hazardous
organic solvents and toxic catalysts, and thus, it provides a facile and practical
procedure for the preparation of alkyl aryl ethers. In addition, it turned out that
[bmIm]OH could be reused up to six times after recovery with unchanged activity.
Experimental
Yields refer to pure isolated products. All compounds are known and were characterized by compar-
ison of their physical and spectroscopic data with those of authentic samples. The IL [bmIm]OH was
prepared according to Ref. [13].
Preparation of Alkyl Aryl Ethers
ꢁ
Alkyl halide (10 mmol) was added to a mixture 10 mmol phenol in 10mmol [bmIm]OH at 70 C, and
the reaction mixture was stirred for 2 h until completion of reaction (TLC). The reaction mixture was
3
3
extracted with 15 cm ethyl acetate. The extract was washed with 5 cm H O, dried, and evaporated.
2
The crude product was purified by column chromatography over silica gel providing the pure product
3
Table 1). The ionic liquid left in the reaction vessel was mixed with 10cm ether containing 1% HBr,
(
ꢁ
decanted, and heated at 80 C under vacuum for 30min. The resulting ionic liquid, 1-butyl-3-methy-
limidazolium bromide, was converted to the base according to Ref. [13]. The recovered ionic liquid
was reused six times with consistent activity.
References
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[
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[
[
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