Tetrahedron Letters
Friedel–Crafts acylation using bismuth triflate in [BMI][PF6]
Phuong Hoang Tran a,b, Fritz Duus a, Thach Ngoc Le b,
⇑
a Department of Science, Systems and Models, University of Roskilde, POB 260, Roskilde DK-4000, Denmark
b Department of Organic Chemistry, Faculty of Chemistry, University of Science, Vietnam National University-Hochiminh city 70000, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Bismuth trifluoromethanesulfonate was found to be a good catalyst for the Friedel–Crafts acylation. Bis-
muth triflate immobilized in an ionic liquid was the most efficient catalytic system. Bismuth triflate in
[BMI][PF6] catalyzes this reaction under microwave irradiation allowing the rapid synthesis of aryl
ketones in excellent yields. The catalyst system was easily recovered and reused several times.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 September 2011
Revised 26 October 2011
Accepted 4 November 2011
Available online 13 November 2011
Keywords:
Bismuth triflate
Ionic liquid
Microwave
Friedel–Crafts acylation
Friedel–Crafts acylation is an important reaction for the synthe-
sis of aromatic ketones, which are useful precursors for the synthe-
sis of various valuable pharmaceutical compounds.1 However, the
use of Lewis acids such as AlCl3, BF3, FeCl3, TiCl4, SnCl2, etc., leads
to a number of issues: complexation of the Lewis acid with the ke-
tone product; a stoichiometric amount of acid is needed; hydroly-
sis of excess starting material is always required leading to the
generation of a large amount of corrosive and toxic waste.2 Besides,
the reaction is extremely sensitive to moisture because most Lewis
acids immediately react with water and then lose their reactivity.
The disadvantage of anhydrous conditions is that substrates con-
taining water or aqueous solutions can not be used directly.3
Usually, Lewis acid catalysts are not recovered or recycled.2 Conse-
quently, conducting such a reaction on an industrial scale gener-
ates considerable amounts of hazardous waste.4–7
The use of metal triflates as water-compatible Lewis acids was
first reported in 1991. These triflates possess strong Lewis acidity
and exhibit high tolerance toward water.8 Metal triflates act as
strong Lewis acids due to their hard character and strong affinity
toward carbonyl oxygens. In addition, their hydrolysis was postu-
lated to be slow. Immobilized triflate salts can be recovered after
the reaction and reused.8 Recently, triflates in an ionic liquid9
and triflates in supercritical carbon dioxide10 have been success-
fully employed as environmentally benign Lewis acid catalysts.
Ionic liquids consist of positively and negatively charged ions,
whereas water and organic solvents exist as neutral molecules.
While common salts do not melt below 800 °C, most ionic liquids
remain liquid at room temperature. In general, ionic liquids are
useful for reactions that involve reactive ionic intermediates.11,12
The use of ionic liquids instead of conventional solvents as reaction
media enhances both the selectivity and rates of various reactions.
In addition, the catalyst and solvent are easily recycled.12 Friedel–
Crafts acylation in ionic liquids has been studied extensively.13
Gmouh has shown the acylation of arenes in an ionic liquid.14 More
recently, Alleti applied gadolinium triflate immobilized in imidazo-
lium based ionic liquids for the acetylation of alcohols and
amines,15 and Hardacre reported the Friedel–Crafts benzoylation
of anisole in ionic liquids.16
Microwave irradiation provides a powerful tool for organic syn-
thesis, with its direct reaction mixture heating, thus time and
energy are saved.17 Many organic reactions proceed much faster
with higher yields under microwave irradiation compared to con-
ventional heating.18 The microwave energy can be transferred to
the reaction medium via two mechanisms: dipole rotation and
ionic conduction. Therefore, ionic liquids should be highly suitable
media for microwave-assisted reactions.
As part of our ongoing research on the application of green
chemistry in organic synthesis, we report here the Friedel–Crafts
acylation using bismuth triflate in an ionic liquid under microwave
irradiation.
The acetylation of anisole using acetic anhydride was selected
as a model reaction and Bi(OTf)3 was chosen as the catalyst for
the Friedel–Crafts acylation in an ionic liquid because this catalyst
is water stable and its catalytic activity is much higher than other
metallic triflates [M(OTf)3].19 In our procedure, 5 mol % of Bi(OTf)3
was used to catalyze the acetylation of anisole in [BMI][PF6] (0.5 g).
If the amount of catalyst was increased to 10 mol %, the yield was
slightly improved. All attempts to reduce the amount of catalyst
loading led to diminished yields. After intensively screening the
⇑
Corresponding author.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.