Indium triflate in 1-isobutyl-3-methylimidazolium dihydrogenphosphate: an efficient and green catalytic system for Friedel–Crafts acylation Thach Ngoc Lea,⇑ a Department of Organic Chemist
Trang 1Indium triflate in 1-isobutyl-3-methylimidazolium dihydrogen
phosphate: an efficient and green catalytic system for Friedel–Crafts
acylation
Thach Ngoc Lea,⇑
a Department of Organic Chemistry, Faculty of Chemistry, University of Science, Vietnam National University, Hochiminh City 70000, Viet Nam
b
Department of Science, Systems and Models, Roskilde University, POB 260, Roskilde DK-4000, Denmark
a r t i c l e i n f o
Article history:
Received 30 December 2014
Revised 5 March 2015
Accepted 12 March 2015
Available online 18 March 2015
Keywords:
Indium triflate
Aryl ketone
Microwave irradiation
Friedel–Crafts acylation
Ionic liquids
a b s t r a c t
Indium triflate in the ionic liquid, 1-isobutyl-3-methylimidazolium dihydrogen phosphate ([i-BMIM]H2PO4), was found to show enhanced catalytic activity in the Friedel–Crafts acylation of various aromatic compounds with acid anhydrides The catalytic system was easily recovered and reused without
a significant loss of activity
Ó 2015 Elsevier Ltd All rights reserved
The Friedel–Crafts acylation is a fundamental reaction for the
preparation of aromatic ketones, which are used among other
things as precursors in the synthesis of pharmaceuticals and
agrochemicals.1–7Challenges still remain in Friedel–Crafts
acyla-tions as the process involves more than a stoichiometric amount
of a metal chloride which is lost during the typical aqueous
tri-flate catalyzed Friedel–Crafts acylation as this method only uses
a catalytic amount of the metal, and is stable in many organic
sol-vents and also in aqueous medium, hence the catalyst can easily be
recovered and reused without a significant loss of its activity.8–21
Recently, metal triflates in ionic liquids have been shown to be
efficient and green catalytic systems for Friedel–Crafts acylations.6
The first report of a green Friedel–Crafts acylation reaction was
described by Ross and Xiao.22Copper triflate was an effective
unfortunately limited to highly activated examples In addition,
toward acid anhydrides, and gave only 10% conversion after one
hour Gmouh et al reported Friedel–Crafts acylations using
Bi(OTf)3/[EMIM]NTf2with a catalyst loading as low as 1 mol %.23
The catalytic system could be easily recycled without loss of activity However, the reaction was limited to benzoyl chloride There are also a few reports describing the use of metal triflates
in ionic liquids for Friedel–Crafts acylations with substrates such
as anisole24,25 and ferrocene.26,27 We have reported Bi(OTf)3/ [BMIM]PF6as a good catalytic system for Friedel–Crafts acylations
of highly activated substrates under both microwave irradiation
activity of bismuth triflate in triflate anion containing ionic liquids
scope was restricted to benzoyl chloride
In this Letter, we report a method for the Friedel–Crafts acylation of various aromatic compounds using four different acid anhydrides as acylating reagents Firstly, we investigated the catalytic activity of different metal triflates in the Friedel–Crafts acetylation of mesitylene under conventional heating in the
Among these metal triflates, the four metal triflates which gave the highest yields (Table 1, entries 11–14) were chosen to be tested for catalytic activity in the Brønsted acidic ionic liquid
acidic counterion because Brønsted acids are known to be good catalysts for Friedel–Crafts acylations.30–41Surprisingly, copper tri-flate, which showed the best catalytic activity under solvent-free
http://dx.doi.org/10.1016/j.tetlet.2015.03.051
⇑ Corresponding author.
E-mail address: lenthach@yahoo.com (T.N Le).
Contents lists available atScienceDirect
Tetrahedron Letters
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / t e t l e t
Trang 2conditions, afforded only a 15% yield when dissolved in the
[i-BMIM]H2PO4ionic liquid Unlike the reactions of other metal
tri-flates, the acetylation of mesitylene catalyzed by copper triflate in
Previously, Ross and Xiao observed the formation of this
precipi-tate and identified it as Cu(OAc)2H2O by NMR spectroscopy.22
Interestingly, indium triflate improved the yield of the
correspond-ing ketone when dissolved in [i-BMIM]H2PO4(Table 1, entry 13)
Indium triflate has been demonstrated to be a good catalyst for
Herein, we report the Friedel–Crafts acylations of aromatic
com-pounds using In(OTf)3in [i-BMIM]H2PO4ionic liquid This is the
first time [i-BMIM]H2PO4has been synthesized and used as a
cat-alytic system with In(OTf)3in Friedel–Crafts acylations of aromatic
compounds with acid anhydrides
The general synthesis of ionic liquids involves an alkylation–
metathesis procedure.47–50In our synthesis, the first step generates
a bromide-containing ionic liquid via the alkylation of
1-methylim-idazole with isobutyl bromide The second step involves anion
exchange in this precursor with KH2PO4 Both steps are carried
out under solvent-free conditions at 80 °C in a few minutes [see
Supporting information (SI), Section S2] Indium triflate in
catalytic activity to afford better yields in comparison with only
indium triflate as the catalyst Previously, the Friedel–Crafts
acyla-tion using acid anhydrides with challenging substrates such as
alkylbenzenes gave low yields (48–53%) in long reaction times
(overnight) in the presence of metal triflates and ionic liquids.22
It is noteworthy that our method has been applied extensively to
slightly activated substrates The method gave good yields within
only 0.5–3 h (conventional heating) and in 30 min (microwave
irradiation) (see below)
The Friedel–Crafts acetylation of anisole was chosen as the
model reaction and indium triflate (Table 1, entry 13) was selected
obtained in the absence of indium triflate and the yield of the
product decreased significantly on reducing the catalyst loading
to 1 mol % The conditions for the acetylation of anisole were
opti-mized and the highest yield of 81% was obtained within only
30 min at 100 °C (SI, Section S3) These conditions were then
applied to other substrates and acylating reagents Interestingly, arenes containing slightly activated substituents such as alkylben-zenes and polycyclic benzenoid aromatic compounds such as naphthalene, fluorene, and anthracene were also acylated in good yields
Under conventional heating, the Friedel–Crafts benzoylation of various substrates was investigated in the presence or absence of
presented inTable 2 In all cases, the yields of the corresponding ketones were significantly improved when using the ionic liquid reaction medium The results of the benzoylation of aromatic com-pounds show that electron-rich (Table 2, entries 1–7) as well as some slightly activated arenes (Table 2, entries 11 and 14) were reactive Alkylbenzenes were benzoylated in moderate yields because of their low electrophilicity (Table 2, entries 8–10) Surprisingly, the benzoylation of mesitylene gave a 58% yield of a monobenzoylated product and 32% yield of a dibenzoylated by-product under conventional heating, whilst the yield of this by-product was only 10% under microwave irradiation (Table 2, entry 12) Aromatic rings without an electron-donating sub-stituent, such as naphthalene and anthracene, were also reactive (Table 2, entries 13 and 15) In general, the benzoylation of several
of the aromatic substrates proceeded smoothly to give the corresponding ketones in good to excellent yields within short
1,3-Dimethoxybenzene also gave a good yield of the expected product (Table 2, entry 4) However, in 1,4-dimethoxybenzene, in which the aromatic ring has a counteracting orientation of the two meth-oxy substituents, was only acylated in moderate yields (Table 2, entry 5)
Reactions with aliphatic anhydrides under conventional heating and microwave irradiation are presented inTable 3 The aliphatic anhydrides appeared to be less reactive than benzoic anhydride, therefore slightly activated substrates such as alkylbenzenes were not reactive, with the exception of mesitylene (Table 3, entry 6) Strongly activated substrates were acylated in good yields (Table 3, entries 1–4) Unfortunately, naphthalene and anthracene were found to be unreactive toward aliphatic anhydrides The Friedel–Crafts acylation was also studied under microwave irradiation Although microwave irradiation has been applied to a wide range of organic reactions, its use in metal triflate catalyzed
Table 1 Effect of different metal triflates on the Friedel–Crafts acetylation of mesitylene using acetic acid anhydride under conventional heating
O
CH 3 COOH metal triflate (5 mol%)
120 o C, 90 min
2 equiv
(%)
a Conversions in parentheses are those obtained when the reactions were carried out in [i-BMIM]H 2 PO 4 Only monoacetylated product was obtained.
Trang 3Table 2
Comparison of the benzoylation of aromatic compounds in [i-BMIM]H 2 PO 4 and under solvent-free conditions
O
In(OTf) 3 /[i-BMIM]H2 PO 4
O R R
+
Solvent-free [i-BMIM]H 2 PO 4
°C, 30 min
O
OMe
72 (7:0:93) b
88 (6:0:94) b
2 D, 140°C, 120 min
O
OEt
78 (4:0:96) b 90 (0:0:100) b
92 (0:0:100) b
3 D, 100°C, 30 min
O
OMe
4 D, 100 °C, 30 min
O
OMe
OMe
73 (6:94) d 82 (6:94) d
5 D, 140 °C, 30 min
O OMe
OMe
6 D, 140°C, 30 min
O OMe
OMe OMe
90
7 D, 140°C, 120 min
O
SMe
72 (8:0:92) b 86 (4:0:96) b
90 (0:0:100) b
8 D, 180 °C, 120 min
O
Me
52 (25:5:70) b
63 (25:4:71) b
9 D, 160 °C, 180 min
O
Et
55 (18:8:74) b
64 (16:5:79) b
10 D, 160 °C, 180 min
O
n-Pr
56 (10:5:85) b
67 (13:5:82) b
11 D, 160 °C, 180 min
O Me
Me
69 (0:100) e
82 (7:93) e
12 D, 160°C, 180 min
MW, 100 °C, 15 min
O Me Me Me
74
nd c
58 f
65 g
13 D, 160 °C, 180 min
O
52 (77:23) h
65 (74:26) h
Trang 4Table 2 (continued)
Solvent-free [i-BMIM]H 2 PO 4
14 D, 160 °C, 180 min
O
15 D, 160 °C, 180 min
O
42 (68:32) i 70 (82:18) i
a
Isolated yield of pure monobenzoylated product after column chromatography Isomer ratios in parentheses were determined by GC-FID.
b
o-/m-/p-Isomers.
c
nd: not determined.
d
2,6-/2,4-Dimethoxybenzophenone isomers.
e
2,6-/2,4-Dimethylbenzophenone isomers.
f Dibenzoylated product was obtained in 32% yield.
g Dibenzoylated product was obtained in 10% yield.
h
1-/2-Benzoylated naphthalene isomers.
i
9-/1-Benzoylated anthracene isomers.
Table 3
Acylation using aliphatic acid anhydrides in [i-BMIM]H 2 PO 4
R O
O R
O
In(OTf)3/[i-BMIM]H2PO4
R' R: CH 3 , C 2 H 5 , C 3 H 7
R
O R'
+
Entry Reaction conditions a
(Isomer distribution) b
MW, 80 °C, 30 min
H 3 C O
OMe
82 (4:96) b 83 (4:96) b
C 2 H 5 O OMe
C 3 H 7 O
OMe
2
D, 140 °C, 120 min
MW, 100 °C, 15 min H 3 C
O OEt
83 (5:95) b
90 (6:94) b
D, 140 °C, 180 min
MW, 100 °C, 15 min C2H5
O
OEt
78 (3:97) b
65 c (4:96) b
D, 140 °C, 120 min
MW, 100 °C, 15 min C 3 H 7
O
OEt
72 d (3:97) b 60 e (0:100) b
3
D, 100 °C, 30 min
MW, 80 °C, 10 min H3C
O
OMe
OMe
D, 80 °C, 30 min
MW, 80 °C, 10 min C 2 H 5
O
OMe OMe
Trang 5Friedel–Crafts acylations has been reported in only a few
Letters.27,51–53Under microwave irradiation, mild conditions are
sufficient For example, the yields for the acetylation of phenetol,
the butyrylation of thioanisole and the acetylation of mesitylene
were improved at low temperatures in short reaction times
(Table 3, entries 2, 5, and 6) However, in some cases,
propionyla-tion and butyrylapropionyla-tion of aromatic compounds under microwave
irradiation afforded lower yields in comparison with conventional
heating (Table 3) This can be explained by a decrease in
micro-wave absorption by the acylating reagent with increasing carbon
chain length In general, the Friedel–Crafts acylation usually stops
cleanly after one reaction to give monoacylated products, whereas
diacylated products were also obtained in the propionylation and
butyrylation with acid anhydrides using our method (Table 3,
(Table 3, entry 6) Previously, in traditional Friedel–Crafts acyla-tions of benzene derivatives, triacylated products were not obtained Although diacylated products were also reported in a few Letters, heteroaromatics or polybenzenes, as substrates, were required.54–59
The recyclability of the In(OTf)3/[i-BMIM]H2PO4 system was investigated in the Friedel–Crafts benzoylation of anisole under conventional heating The procedure for the recovery and reuse
of the catalyst is simple and no significant loss in the catalytic activity was apparent after three consecutive runs (SI, Section S4)
In conclusion, a new ionic liquid was synthesized and applied as
a solvent in the Friedel–Crafts acylation of aromatic compounds with acid anhydrides The ionic liquid was found to enhance the catalytic activity of indium triflate under conventional heating
Table 3 (continued)
Entry Reaction conditions a
(Isomer distribution) b
D, 100 °C, 30 min
MW, 80 °C, 10 min C 3 H 7
O
OMe
OMe
4
D, 100 °C, 30 min
MW, 80 °C, 10 min
H 3 C
O OMe
OMe OMe
D, 80 °C, 30 min
MW, 80 °C, 10 min
C2H5
O OMe
OMe OMe
D, 100 °C, 30 min
MW, 80 °C, 10 min
C 3 H 7
O OMe
OMe OMe
79 i (8:92) d 70 j (0:100) d
MW, 120 °C, 15 min
H 3 C O
SMe
64 (0:100) b
52 (5:95) b
C 2 H 5 O
SMe
57 k (0:100) b
55 l (3:97) b
C3H7 O
SMe
60 m (0:100) b
76 n (3:97) b
6
D, 120 °C, 90 min
MW, 100 °C, 15 min H 3 C
O Me
Me Me
D, 120 °C, 30 min
MW, 100 °C, 15 min C2H5
O Me
Me Me
70 o
47 p
D, 120 °C, 90 min
MW, 100 °C, 15 min C 3 H 7
O Me
Me Me
a D: conventional heating, MW: microwave irradiation.
b
Isolated yield of pure monobenzoylated product after column chromatography Isomer ratios in parentheses were determined by GC-FID.
c–n
Diacylated product was also obtained as a by-product: c
20%, d
15%, e
25%, f
5%, g
10%, h
5%, i
10%, j
7%, k
32%, l
20%, m
15%, n
5%.
d
2,4,5-/2,3,6-Trimethoxybutyrophenone isomers.
o–r Di- and tri-acylated products were also obtained as by-products during the acylation of mesitylene: o 10% and 3%, p 5% and 5%, q 15% and 5%, r 5% and 5%.
Trang 6and microwave irradiation The catalytic system can be easily
recovered and reused in three consecutive cycles without any
sig-nificant loss of the activity, which is promising for large scale
appli-cations This research has also provided a new method for the
di-and tri-acylations of aromatic substrates Further research to
develop other catalytic systems for the Friedel–Crafts acylations
of deactivated substrates are now in progress
Acknowledgment
We are grateful to the Vietnam National University—Hochiminh
City (Grant No C2014-18-08) for financial support
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.03
051
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