Friedel-Crafts acylation reaction using carboxylic acids as acylating agents

Article abstract of DOI:10.1016/j.tet.2006.07.031

Dehydrative Friedel-Crafts acylation reaction of aromatic compounds with carboxylic acids as acylating agents was investigated in the presence of Lewis acid- or Br?nsted acid-catalyst. Various metal triflates and bis(trifluoromethanesulfonyl)amides showed catalytic activity at high temperature, among which Eu(NTf₂)₃ proved to be the most effective and efficiently catalyzed the acylation reaction of alkyl- and alkoxybenzenes with aliphatic and aromatic carboxylic acids at 250 °C. Bi(NTf₂)₃ was more effective than Eu(NTf₂)₃ at lower temperature, but proved to be hydrolyzed in the presence of a small amount of water to give HNTf₂ and [Bi₆O₄(OH)₄(H₂O)₆](NTf₂)₆. The structure of the latter compound was confirmed by a single crystal X-ray analysis. Among five Br?nsted acids, HOTf, HNTf₂, HCTf₃, TsOH, and Nafion SAC-13, HNTf₂ has proved to be the most efficient catalyst and more effective than Eu(NTf₂)₃ for the acylation of p-xylene with heptanoic acid at 220 °C or lower temperature. HNTf₂ catalyzed the acylation of anisole with carboxylic acids in high yields in refluxing toluene with azeotropic removal of water.

Full text of DOI:10.1016/j.tet.2006.07.031

Tetrahedron 62 (2006) 9201–9209
Friedel–Crafts acylation reaction using carboxylic
acids as acylating agents
y
*
Masato Kawamura, Dong-Mei Cui and Shigeru Shimada
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5,
Tsukuba, Ibaraki 305-8565, Japan
Received 2 June 2006; revised 2 July 2006; accepted 11 July 2006
Available online 4 August 2006
Abstract—Dehydrative Friedel–Crafts acylation reaction of aromatic compounds with carboxylic acids as acylating agents was investigated
in the presence of Lewis acid- or Brønsted acid-catalyst. Various metal triflates and bis(trifluoromethanesulfonyl)amides showed catalytic
activity at high temperature, among which Eu(NTf2)3 proved to be the most effective and efficiently catalyzed the acylation reaction of alkyl-
and alkoxybenzenes with aliphatic and aromatic carboxylic acids at 250 ꢀC. Bi(NTf2)3 was more effective than Eu(NTf2)3 at lower temper-
ature, but proved to be hydrolyzed in the presence of a small amount of water to give HNTf2 and [Bi6O4(OH)4(H2O)6](NTf2)6. The structure of
the latter compound was confirmed by a single crystal X-ray analysis. Among five Brønsted acids, HOTf, HNTf2, HCTf3, TsOH, and NafionÒ
SAC-13, HNTf2 has proved to be the most efficient catalyst and more effective than Eu(NTf2)3 for the acylation of p-xylene with heptanoic
acid at 220 ꢀC or lower temperature. HNTf2 catalyzed the acylation of anisole with carboxylic acids in high yields in refluxing toluene with
azeotropic removal of water.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
reports have described catalytic Friedel–Crafts acylation re-
action using acid anhydrides as acylating agents.2 However,
the use of acid anhydrides necessarily yields 1 equiv of car-
boxylic acids as by-products. Methyl esters, which form
methanol as a by-product, could be employed in the presence
of excess amount of trifluoromethanesulfonic acid.3 Mel-
drum’s acids are also used as acylating agents in Lewis
acid-catalyzed acylations.4 However, the most desirable
acylating agents are probably carboxylic acids, which are
common precursors of acid chlorides and anhydrides, be-
cause the reaction produces water as the only by-product.
Recently there are some reports on catalytic Friedel–Crafts
acylation reactions using carboxylic acids as acylating
agents; zeolites,5,6 heteropoly acids and their salts,5,7
clay,5,8 Lewis acids,9 and graphite/TsOH10 have been used
as catalysts.
The Friedel–Crafts acylation reaction is a useful method for
the preparation of aromatic ketones and is widely used not
only in laboratory scale but also in large scale in industry.1
The most common procedure uses acid chlorides, which
are usually prepared from the corresponding carboxylic
acids and thionyl chloride, as acylating agents and a
stoichiometric or an excess amount of AlCl3 as a reaction
promoter. This procedure suffers from severe corrosion
and waste problems (stoichiometric amounts of SO2 and
HCl are formed as by-products and AlCl3 is hydrolyzed
to form a large amount of waste). Therefore, there are
strong demands for a cleaner alternative that meets
recent requirement for environmentally benign chemical
processes.
Several new methods have been explored to overcome these
disadvantages. One important direction is to substitute the
stoichiometric reaction promoter by efficient catalysts, and
another is to replace acid chlorides by cleaner alternatives
such as acid anhydrides and esters. Recently a number of
Our approach to the catalytic Friedel–Crafts acylation reac-
tion using carboxylic acids as acylating agents was started by
using simple Lewis acid- and Brønsted acid-catalysts, since
the potential of these catalysts for this reaction was not well
explored at the time we started. We have reported that some
Lewis acids are effective for the intramolecular Friedel–
Crafts acylation of carboxylic acids to form cyclic aromatic
ketones.11 In this paper, we describe the details on the inter-
molecular Friedel–Crafts acylation reaction catalyzed by
Lewis acid- and Brønsted acid-catalysts.12
* Corresponding author. Tel.: +81 29 861 6257; fax: +81 29 861 4511;
e-mail: s-shimada@aist.go.jp
Present address: College of Pharmaceutical Sciences, Zhejiang University
y
of Technology, Hangzhou 310014, PR China.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.07.031

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M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
2. Results and discussion
utilized as a catalyst for Friedel–Crafts acylation using
acid chlorides and anhydrides as acylating agents in recent
years,2d,14 also showed relatively good results under these
conditions (Table 1, entry 32). As it can be seen from Table
1, considerable side reaction took place at 180 ꢀC in many
cases; for example, Sc(OTf)3 afforded 1a in 39% yield al-
though the carboxylic acid was completely consumed (Table
1, entry 1). However, the reaction at higher temperature,
250 ꢀC, unexpectedly proved to give the desired product in
higher yields. In most cases, rare earth metal triflates gave
ketone 1b in moderate to high yields at 250 ꢀC, among
which Yb(OTf)3 showed the highest performance (83%
yield, Table 1, entry 24). Although Hf(OTf)4 has high activ-
ity for the conversion of heptanoic acid, secondary reaction
of ketone 1b easily takes place under the reaction conditions
to give uncharacterized side products and longer reaction
time causes the disappearance of 1b (Table 1, entries 28
and 29).
2.1. Lewis acid-catalyzed Friedel–Crafts acylation re-
action of p-xylene with hexanoic and heptanoic acids
The acylation of p-xylene with hexanoic and heptanoic acids
was selected as a model reaction to examine catalytic perfor-
mance of metal triflates, M(OTf)3 and M(OTf)4.13 The re-
sults are summarized in Table 1. The reaction was carried
out using a sealed glass tube with an excess of p-xylene
(30–65 equiv to carboxylic acids) in an oil bath set at the
temperature shown in tables. The reaction can take place
at 180 ꢀC with various Lewis acids, although the yields of
desired ketone 1a are low. At this temperature, Sc(OTf)3
(Table 1, entry 1) afforded 1a in the highest yield, 39%,
among 14 metal triflates tested. Bi(OTf)3, which has been
Table 1. Reaction of p-xylene with hexanoic and heptanoic acidsa
Then we examined four bis(trifluoromethanesulfonyl)-
amide salts, Sc(NTf2)3,15 Yb(NTf2)3,16 Eu(NTf2)3,17 and
Bi(NTf2)3,18 which are stronger Lewis acids than the corre-
sponding triflates. Sc(NTf2)3 afforded 1b in higher yield
than Sc(OTf)3 (Table 2, entry 1) while Yb(NTf2)3 showed
poorer results than Yb(OTf)3 (Table 2, entry 2). Considering
their strong Lewis acidity, Yb(NTf2)3 and Sc(NTf2)3 were
expected to give more promising results. However, their
strong Lewis acidity seemed to promote undesirable side re-
action at the same time. The best result was obtained by us-
ing Eu(NTf2)3 to give 1b in excellent yields (Table 2, entries
3 and 5). The yield of 1b decreased when the amount of cata-
lyst or p-xylene was reduced (Table 2, entries 6–8). Figure 1
shows the effect of temperature on the yield of 1b in
Eu(NTf2)3-catalyzed reaction. At 240 ꢀC or higher, more
than 80% of 1b is formed after 12 h, while the yield drasti-
cally decreases at 230 ꢀC or lower. On the other hand,
Bi(NTf2)3, which is less selective than Eu(NTf2)3 at
250 ꢀC (Table 2, entry 4), proved to be more effective than
Eu(NTf2)3 at 220 ꢀC (Table 2, entry 9). Only 1 mol % of
O
M(OTf)x
R
+
RCOOH
R = CH3(CH2)n
n = 4 or 5
1a: n = 4
1b: n = 5
Entry Catalyst (mol %)
n
Conditions
Conv. (%)b Yield (%)c
1
Sc(OTf)3 (20)
Sc(OTf)3 (20)
Y(OTf)3 (20)
Y(OTf)3 (15)
La(OTf)3 (15)
Pr(OTf)3 (15)
Nd(OTf)3 (20)
Nd(OTf)3 (15)
Sm(OTf)3 (20)
Sm(OTf)3 (15)
Eu(OTf)3 (20)
Eu(OTf)3 (20)
Gd(OTf)3 (15)
Tb(OTf)3 (20)
Dy(OTf)3 (20)
Dy(OTf)3 (15)
Ho(OTf)3 (20)
Ho(OTf)3 (15)
Er(OTf)3 (20)
Er(OTf)3 (15)
Tm(OTf)3 (20)
Tm(OTf)3 (15)
Yb(OTf)3 (20)
Yb(OTf)3 (20)
Lu(OTf)3 (20)
Lu(OTf)3 (15)
Hf(OTf)4 (20)
Hf(OTf)4 (20)
Hf(OTf)4 (20)
Ga(OTf)3 (20)
In(OTf)3 (20)
Bi(OTf)3 (20)
4
5
4
5
5
5
4
5
4
5
4
5
5
4
4
5
4
5
4
5
4
5
4
5
4
5
4
5
5
5
5
4
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
250 ꢀC, 12 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
250 ꢀC, 12 h
180 ꢀC, 45 h
180 ꢀC, 60 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 45 h
250 ꢀC, 12 h
180 ꢀC, 62 h
180 ꢀC, 1.25 h
250 ꢀC, 12 h
250 ꢀC, 12 h
250 ꢀC, 12 h
180 ꢀC, 45 h
>99
>99
40
>99
52
66
20
72
45
97
>99
83
96
21
3
97
30
99
41
>99
15
>99
76
>99
51
>99
46
89
>99
94
39
49
12
65
0
23
Trace
29
20
64
29
52
62
1
2d
3
4
5
6
7
8
9
10
11
12d
13
14
15
16d
17
18
19
20
21
22
23
24d
25
26
27
28e
29d
30d
31d
32
2
80
3
Table 2. Reaction of p-xylene with heptanoic acida
65
16
66
8
43
14
83
28
63
25
36
0
M(NTf2)3
1b
+
CH3(CH2)5CO2H
Entry M(NTf2)3 (mol %) Temp (ꢀC) Time (h) Conv. (%)b Yield (%)c
1
2
3
Sc(NTf2)3 (15)
Yb(NTf2)3 (15)
Eu(NTf2)3 (15)
Bi(NTf2)3 (15)
Eu(NTf2)3 (20)
Eu(NTf2)3 (20)
Eu(NTf2)3 (10)
Eu(NTf2)3 (5)
Bi(NTf2)3 (20)
250
250
250
250
250
250
250
260
220
220
12
10
12
8
12
12
12
18
9
98
>99
>99
>99
>99
87
>99
51
80
92
75
64
91
80
96
75
73
15
53
66
4
22
62
33
5d
6e
7
>99
75
8d
9
a
The ratio of p-xylene/carboxylic acid is 65/1 for the reaction of hexanoic
acid and 50/1 for the reaction of heptanoic acid, unless otherwise noted.
Conversion of carboxylic acids was determined by GC analysis using
naphthalene (for hexanoic acid) or docosane (for heptanoic acid) as an
internal standard.
Yields are based on carboxylic acids and are determined by GC analysis
using naphthalene (for hexanoic acid) or docosane (for heptanoic acid) as
an internal standard.
10d Bi(NTf2)3 (1)
48
b
c
a
The ratio of p-xylene/carboxylic acid is 50/1 unless otherwise noted.
Conversion of the carboxylic acid was determined by GC analysis using
docosane as an internal standard.
Yields are based on the carboxylic acid and are determined by GC analysis
using docosane as an internal standard.
b
c
d
e
d
e
p-Xylene/carboxylic acid¼65/1.
p-Xylene/carboxylic acid¼65/1.
p-Xylene/carboxylic acid¼60/1.
p-Xylene/carboxylic acid¼30/1.

M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
9203
Table 3. Eu(NTf2)3-catalyzed Friedel–Crafts acylation of p-xylene and
anisole using various carboxylic acids
O
Eu(NTf2)3
(15 mol %)
R
+
RCOOH
250 °C
50 equiv
1c–1u
Time (h) Conv. (%)a Product Yield (%)b
Entry
R
1
2
3
4
5
6
7
8
CH3
C2H5
8
8
9
16
9
11
7
10
17
12
12
13
13
14
14
94
>99
>99
91
98
>99
96
>99
>99
>99
ndd
>99
>99
>99
>99
>99
94
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
1s
1t
48
56
61
34
70
55
n-C3H7
i-C3H7
n-C4H9
i-C4H9
n-C5H11
i-C5H11
c-C6H11
n-C7H15
n-C8H17
n-C9H19
n-C10H21
n-C11H23
Ph
65 (78)c
61
21
9
10
11
12
13
14
15
16
17
18
19
80
86
82
76
72
66
59
76
Figure 1. Temperature and time dependence of the yield of 1b in the
Eu(NTf2)3-catalyzed reaction of p-xylene with n-heptanoic acid.
2-MeC6H4 14
4-MeC6H4 14
2-FC6H4
4-FC6H4
Bi(NTf2)3 afforded 1b in 66% yield after 48 h (Table 2, entry
10).
13
16
>99
60
76
60
1u
a
As mentioned above, considerable side reactions take place
depending on the catalysts and reaction conditions. We tried
to identify the structures of side products and succeeded to
elucidate the structure of one of the major by-products, 2.
In the reaction of p-xylene with heptanoic acid using
20 mol % of Yb(NTf2)3 at 250 ꢀC, 2 was isolated in 19%
yield in addition to 51% of 1b. The structure of 2 was con-
firmed by 1H and 13C NMR, IR, and HRMS spectroscopies.
Compound 2 is probably derived from the dehydrative con-
densation of 1b and p-xylene. Although other by-products
were detected by TLC analysis, their structures could not
be identified.
Conversion of the carboxylic acid was determined by GC analysis using
docosane as an internal standard.
Isolated yield unless otherwise noted.
The value in the parenthesis is GC yield.
Not determined.
b
c
d
lower than the reaction temperature and probably caused
the decreased yields. The use of branched carboxylic acids,
especially at the a-position, decreased the yields consider-
ably (Table 3, entries 4, 6, 8, and 9), suggesting the high
sensitivity of p-xylene to the steric hindrance around the
carboxyl group. Acylation with aromatic carboxylic acids
also successfully proceeded to give benzophenones (Table
3, entries 15–19).
Acylation of toluene is less sensitive to the steric hindrance
of carboxylic acids than that of p-xylene and afforded
ketones in moderate to good yields with usual p-selectivity
(Table 4, entries 1–3). Other alkylbenzenes such as cumene,
o- and m-xylene, and p-diethylbenzene gave ketones in good
yield (Table 4, entries 4–7), while benzene afforded 8b only
in 4% yield under the same reaction conditions (Table 4, en-
try 8). The acylation of anisole is more efficient than that of
alkylbenzenes as expected and gave ketones in good yields
with excellent p-selectivity (Table 4, entries 9–11). The reac-
tions mentioned above were carried out by using large excess
amount of aromatic compounds as reactants as well as sol-
vents. Since chlorobenzene does not react with carboxylic
acids under the present reaction conditions, it can be used
as a solvent of this reaction. Acetylation of 1-naphthol was
carried out using excess amount of acetic acid in chloroben-
zene to give ketone 10c in good yield (Table 4, entry 12). In
the same way, 2-bromoanisole was acetylated to give 11c as
a single isomer although the yield was low (Table 4, entry
13). The reaction of 1-naphthol may proceed via naphthyl
acetate (O-acylated product), which readily undergoes Fries
rearrangement to form 10c.20
2
It is noteworthy that the acylation using aliphatic carboxylic
acids is successful despite high reaction temperature because
Friedel–Crafts acylation reaction of aliphatic acid chlorides
with a catalytic amount of Lewis acids is often only success-
ful for highly reactive aromatic compounds such as ani-
sole.19 At higher reaction temperature, side reactions of
ketone products are suggested.2d,e
In order to clarify the scope and limitation of Eu(NTf2)3-
catalyzed Friedel–Crafts acylation, reaction of p-xylene
with various carboxylic acids (Table 3) as well as that of
various aromatic compounds with aliphatic carboxylic acids
(Table 4) was examined. In the reaction of p-xylene, straight
chain carboxylic acids afforded ketones in good yields
(Table 3, entries 5, 7, and 10–14) although the yields de-
creased to some extent in the cases of short chain carboxylic
acids (Table 3, entries 1–3), whose boiling points are much

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M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
Table 4. Reaction of various aromatic compounds with aliphatic carboxylic acids catalyzed by Eu(NTf2)3
Eu(NTf2)3
(15 mol %)
O
+
ArH
RCOOH
250 °C
Ar
R
50 equiv
Entry
1
ArH
Product
Time (h)
17
Yield (%)a
78
Isomer ratiob
(CH2)5CH3
o-/m-/p-¼22/5/73
O
3b
i-C3H7
2
24
24
19
12
12
12
58
58
71
74
85
74
(o-+m-)/p-¼14/86
(o-+m-)/p-¼12/88
(o-+m-)/p-¼15/85
96/4c,d
3f
O
c-C6H11
3
O
3k
(CH2)5CH3
4
O
4b
i-C3H7
i-C3H7
(CH2)5CH3
5
O
5b
(CH2)5CH3
6
88/12e,f
O
6b
Et
Et
7g
(CH2)5CH3
Et
Et
O
7b
O
8
24
6
4
87
77
(CH2)5CH3
8b
(CH2)5CH3
9
o-/p-¼<3/>97f
O
MeO
MeO
9b
O
i-C3H7
10
16
MeO
MeO
9f
MeO
MeO
O
c-C6H11
11
16
18
80
83
9k
OH
O
OH
12h
Me
10c
O
Br
Br
MeO
13h
15
11
Me
11c
MeO
a
Isolated yields.
b
c
d
e
f
The ratio of regioisomers was determined by GC analysis unless otherwise noted.
The ratio of regioisomers was determined by GC–MS analysis.
1-(2,4-Dimethylphenyl)-1-heptanone/other regioisomers.
1-(3,4-Dimethylphenyl)-1-heptanone/1-(2,3-dimethylphenyl)-1-heptanone.
The ratio of regioisomers was determined by 1H NMR.
g
h
Eu(NTf2)3 (10 mol %) was used.
Reaction was carried out in chlorobenzene using 4 equiv of AcOH.
2.2. Evaluation of catalytic activity of Bi(NTf2)3 and
HNTf2
(Table 2, entry 10). Since it has been reported that Bi(NTf2)3
is hydrolyzed by water18b and the present reaction produces
water as a by-product, it is possible that HNTf2, which will
be produced by the hydrolysis of Bi(NTf2)3, is working as
a catalyst at least partially in Bi(NTf2)3-catalyzed reactions.
Since complete hydrolysis of Bi(NTf2)3 produces 3 equiv of
We had interest in the exceptional activity of Bi(NTf2)3, only
1 mol % of which catalyzed the reaction of p-xylene with
heptanoic acid to give 1b in 66% yield, as mentioned above

M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
9205
all (Table 6, entry 4). The use of HCTf321 and NafionÒ SAC-
13 afforded a complex mixture (Table 6, entries 6 and 7). In
the latter case, the formation of p-xylene dimer (2,5,40-tri-
methyldiphenylmethane) as a by-product was confirmed
by GC–MS analysis of the reaction mixture.22
Table 5. Reaction of various aromatic compounds with aliphatic carboxylic
acids catalyzed by Eu(NTf2)3
Bi(NTf2)3 or HNTf2
+
CH3(CH2)5COOH
1b
180 °C or 250 °C
50 equiv
Entry Catalyst (mol %) Temp (ꢀC) Time (h) Conv. (%)a Yield (%)a
To evaluate the generality of HNTf2-catalyzed reaction,
acylation of p-xylene and anisole with several carboxylic
acids was examined (Schemes 1 and 2). Acetylation of
p-xylene afforded ketone 1c in a low yield probably because
the boiling point of acetic acid is much lower than the reac-
tion temperature, while dodecanoic acid and benzoic acid
gave ketones in moderate to good yields. The acylation of
anisole can be conducted in refluxing toluene with azeo-
tropic removal of water. By using 20 mol % of HNTf2, acyl-
ation of anisole efficiently took place to give ketones in good
yields. Acylation products of toluene were not observed at
all under these conditions.
1
2
3
4
5
6
7
8
Bi(NTf2)3 (1)
Bi(NTf2)3 (1)
Bi(NTf2)3 (20)
Bi(NTf2)3 (20)
HNTf2 (3)
HNTf2 (3)
HNTf2 (60)
HNTf2 (60)
180
220
180
220
180
220
180
220
50
50
48
9
50
28
48
9
46
85
41
80
57
97
56
97
12
65
27
53
18
84
25
78
a
The yields and the conversions were determined by GC analysis using
docosane as an internal standard.
O
HNTf2, catalytic activity of Bi(NTf2)3 was compared with
that of 3 equiv of HNTf2 (Table 5). At 180 ꢀC, very similar
results were obtained (Table 5, entries 1, 3, 5, and 7), while
3 equiv of HNTf2 showed higher activity than Bi(NTf2)3 at
220 ꢀC (Table 5, entries 2, 4, 6, and 8). Since we have
confirmed partial hydrolysis of Bi(NTf2)3 (vide infra), the
results in Table 5 probably suggest that HNTf2 mainly
catalyzes the reaction in Bi(NTf2)3-catalyzed dehydrative
acylations, while Bi(NTf2)3 acts as a Lewis acid catalyst at
the beginning stage of the reaction.
HNTf2 (10 mol %)
R
+
RCOOH
200 °C
50 equiv
1c (R = CH3): 22 h, 21%
1p (R = C11H23): 22 h, 84%
1q (R = Ph): 28 h, 58%
Scheme 1. HNTf2-catalyzed acylation of p-xylene with carboxylic acids.
O
2.3. HNTf2-catalyzed Friedel–Crafts acylation reaction
using carboxylic acids as acylating agents
HNTf2 (20 mol %)
R
+ RCOOH
toluene, reflux
MeO
Dean-Stark trap
Based on the above results, we examined the catalytic activ-
ities of several Brønsted acids for the reaction of p-xylene
with heptanoic acid (Table 6). Among five Brønsted acids
tested, HNTf2 showed the highest performance; 10 mol %
of HNTf2 at 200 or 220 ꢀC afforded 1b in high yields (Table
6, entries 2 and 3). HOTf is less efficient than HNTf2 (Table
6, entry 5), while p-toluenesulfonic acid did not afford 1b at
MeO
50 equiv
9b (R = C6H13): 36 h, 89%
9d (R = C2H5): 24 h, 64%
9p (R = C11H23): 36 h, 84%
Scheme 2. HNTf2-catalyzed acylation of anisole with carboxylic acids.
2.4. Structure of hydrolysis product of Bi(NTf2)3
Table 6. Brønsted acid-catalyzed Friedel–Crafts acylation of p-xylene with
heptanoic acida
Anhydrous Bi(NTf2)3 is highly moisture sensitive and fumes
in air.18b It is reported that Bi(NTf2)3 is hydrolyzed in the
presence of water,18b while no structural information is
available on the bismuth-containing product by the hydro-
lysis. Therefore, we have tried the hydrolysis of Bi(NTf2)3
under various conditions and succeeded to isolate colorless
single crystals of the hydrolysis product under the conditions
similar to those of the catalytic acylation reaction, i.e., in
p-xylene in the presence of n-C6H13CO2H or c-C6H11CO2H
and a small amount of water.23 The hydrolysis was attemp-
ted in the presence of 2.4, 3, and 6 equiv (to Bi) of water
and gave crystals of the same hydrolysis product in all cases.
The structure was identified by a single crystal X-ray
analysis to be [Bi6O4(OH)4(H2O)6](NTf2)6 12 (Fig. 2).
Brønsted acid
+
CH3(CH2)5COOH
1b
200 °C or 220 °C
50 equiv
Entry Brønsted acid
(mol %)
Temp (ꢀC) Time (h) Conv. (%)a Yield (%)a
1
2
3
4
5
6
7
HNTf2 (20)
HNTf2 (10)
HNTf2 (10)
HOTs (10)
HOTf (10)
HCTf3 (10)
220
220
200
220
220
220
15
16
16
16
24
9
>99
98
82
89
89
0
70
20
10
94
38b
98
97
52
NafionÒ SAC-13c 220
50
Similar hexanuclear [Bi6O4(OH)4] structures were reported
24
for [Bi6O4(OH)4](NO3)6$(H2O)n
and [Bi6O4(OH)4]-
a
(ClO4)6$(H2O)n.25 Each oxygen atom of [Bi6O4(OH)4] units
disordered in two positions with nearly equal occupancies
(showed in red and yellow in Fig. 2). This disorder is under-
stood to show the positions of oxide and hydroxide oxygen
Reactions were carried out in a sealed glass tube, and yields and conver-
sions were determined by GC analysis using docosane as an internal
standard unless otherwise noted.
Determined by GC analysis using tetradecane as an internal standard.
20 wt %.
b
c

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M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
connected to six other [Bi6O4(OH)4(H2O)6] units to form
a 3D-network structure.
In order to evaluate the catalytic activity of 12, Friedel–
Crafts acylation reaction of p-xylene with hexanoic acid
was carried out in the presence of 12 (Bi/hexanoic acid/
p-xylene¼0.2/1/65; 180 ꢀC, 45 h). The yield of ketone
(2%; 5% carboxylic acid conversion) was much lower than
that (27%) obtained in the reaction carried out under the sim-
ilar reaction conditions using Bi(NTf2)3 (Table 5, entry 3).
This result supports our assumption that the main catalyti-
cally active species is HNTf2 in the Bi(NTf2)3-catalyzed
dehydrative acylations.
3. Conclusions
Dehydrative Friedel–Crafts acylation reaction of aromatic
compounds with carboxylic acids as acylating agents can
be catalyzed by simple Lewis acid- and Brønsted acid-
catalysts. Lewis acid-catalyzed reaction of p-xylene with
aliphatic carboxylic acids can take place at 180 ꢀC with
various metal triflates and bis(trifluoromethanesulfonyl)-
amides, while higher reaction temperature generally affords
desired ketones with higher selectivities. Eu(NTf2)3 shows
the highest performance at 250 ꢀC and efficiently catalyzes
the acylation reaction of alkyl- and alkoxybenzenes with
aliphatic and aromatic carboxylic acids. Brønsted acid-cata-
lyst HNTf2 has proved to be more efficient than Eu(NTf2)3
for the acylation of p-xylene with carboxylic acids at
200 ꢀC. HNTf2 catalyzes the acylation of anisole with car-
boxylic acids in high yields in refluxing toluene with azeo-
tropic removal of water. These catalytic systems do not
require any additives and can be applied to a wide range of
carboxylic acids and aromatic compounds. Therefore, the
present catalytic systems are superior to the reported coun-
terparts in terms of simplicity and generality. These results
suggest that Lewis acid- and Brønsted acid-catalysts have
a great potential as catalysts for dehydrative Friedel–Crafts
acylation using carboxylic acids as acylating agents.
Figure 2. Structure of [Bi6O4(OH)4(H2O)6](NTf2)6. Only NTfꢁ2 anions
bound to the top bismuth atom are shown for clarity. Oxygen atoms in
yellow are one set of disordered oxygen atoms (see text).
atoms judging from Bi–O distances, which suggest O2 and
˚
O8 (Bi–O, 2.107(12)–2.204(13) A) are oxide oxygens and
˚
O1 and O9 (Bi–O, 2.297(14)–2.557(16) A) are hydroxide
oxygens.24,25 Each bismuth atom is coordinated by one
water molecule (disordered in two positions; only one of the
disordered water oxygen (O3) is shown in Fig. 2) and three
oxygen atoms of NTfꢁ2 anions. Bi–O distances for water
ꢁ
˚
molecule is 2.59(2) and 2.65(3) A and those for NTf2 anions
˚
ranges from 2.693(6) to 2.909(8) A. The bismuth atoms,
therefore, eight coordinate and their geometric arrangement
can be described as a distorted square antiprism. Among four
oxygen atoms of each NTf2ꢁ anion, three are used for the
coordination to bismuth atoms and one is used for the
hydrogen bonding. As shown in Figure 3, each [Bi6O4-
(OH)4(H2O)6] unit is coordinated by 12 NTfꢁ2 anions, every
two of which are shared between two [Bi6O4(OH)4(H2O)6]
units. Therefore, each [Bi6O4(OH)4(H2O)6] unit is
4. Experimental
4.1. General
Reagents were purchased from Tokyo Kasei Kogyo, Junsei
Chemical, Kanto Chemical, Aldrich, and Wako Pure Chemi-
cal Industries and used as received unless otherwise noted.
Anisole, m-xylene, and cumene were distilled from CaH2
prior to use. Preparative TLC was performed on Wakogel
1
60N (Wako Pure Chemical Industries). H and 13C NMR
1
were recorded on a JEOL JMN-LA500 (499 MHz for H,
125 MHz for 13C) using CDCl3 as a solvent and tetramethyl-
silane as an internal standard. IR spectra were measured
using a JASCO FT/IR-610. GC analyses were performed
on a Shimadzu GC-18A using an Ultra AlloyÒ FFAP capil-
lary column (30 Mꢂ0.25 mm, Frontier Laboratories) or TC-
FFAP capillary column (30 Mꢂ0.25 mm, GL Sciences Inc.).
Low- and high-resolution GC–MS (EI) were measured on
a Shimadzu QP-5000 spectrometer with a Shimadzu GC-
17A and a DB-1 (25 Mꢂ0.32 mm, J&W Scientific), and
JEOL JMS-GC MATE (BU20) spectrometer with a Hewlett
Figure 3. A view showing seven [Bi6O4(OH)4] units.

M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
9207
Packard HP6890 GC system and a NEUTRA BOND-1
capillary column (30 Mꢂ0.25 mm, GL Science).
Characterization data for 2: 1H NMR d 0.85 (t, 3H,
J¼7.5 Hz), 1.21–1.30 (m, 4H), 1.41 (quintet, 2H,
J¼7.0 Hz), 2.00 (q, 2H, J¼7.5 Hz), 2.10 (s, 3H), 2.24 (s,
6H), 2.29 (s, 3H), 5.70 (t, 1H, J¼7.5 Hz), 6.85–6.94 (m,
3H), 6.97 (dd, 1H, J¼7.5, 1.5 Hz), 7.03 (d, 1H, J¼7.5 Hz),
7.05 (d, 1H, J¼7.5 Hz); 13C NMR d 14.04, 19.58, 20.49,
20.96, 20.99, 22.49, 29.26, 29.60, 31.53, 127.17, 127.46,
129.92, 130.32, 130.60, 130.97, 132.26, 133.08, 133.79,
134.40, 134.65, 140.11, 140.16, 142.77; IR (neat) n 2952,
2925, 2862, 1496, 1454, 808 cmꢁ1; HRMS (EI) calcd for
C17H26O [M+] 306.2348, found 306.2351.
4.2. A typical procedure for Friedel–Crafts acylation
A mixture of Eu(NTf2)3 (87.8 mg, 0.0885 mmol), octanoic
acid (85.1 mg, 0.590 mmol), docosane (22.3 mg, an internal
standard for GC analysis), and p-xylene (3.62 mL,
29.5 mmol) was stirred at 250 ꢀC for 12 h with periodical
monitoring of octanoic acid conversion by GC. After cooling
to room temperature, water (3 mL) was added to the mixture
and the organic layer was separated. The aqueous layer was
extracted with ethyl acetate (3ꢂ10 mL). The combined or-
ganic layers were dried over MgSO4, filtered, and concen-
trated under reduced pressure. The residue was purified by
preparative TLC (hexane/EtOAc¼27/1) to give 1-(2,5-di-
methylphenyl)-1-octanone 1l as a pale yellow oil (109 mg,
80% yield).
4.2.4. 1-(4-Isopropylphenyl)-1-heptanone and its isomers
(4b). The title compounds were obtained as a mixture of iso-
mers (p-isomer:(o-+m-isomers)¼85/15) by the reaction of
heptanoic acid (50.8 mg, 0.390 mmol) and cumene
(2.71 mL, 19.5 mmol) as described in the typical procedure
in 71% yield (64.7 mg) as a pale yellow oil after the purifi-
cation by preparative TLC (hexane/EtOAc¼22/1). The
minor isomers were only characterized by GC–MS that
showed M+ ion peaks and the similar fragment patterns as
Characterization data for the new compounds are shown be-
low. All new compounds except for the by-product 2 were
determined to be >95% pure by H NMR spectroscopy.
1
1
that of the major isomer. H NMR (for p-isomer) d 0.89 (t,
1H NMR spectrum of compound 2 showed some small
unidentified signals in the aliphatic region.
3H, J¼7.2 Hz), 1.27 (d, 6H, J¼7.0 Hz), 1.25–1.42 (m,
6H), 1.72 (quintet, 2H, J¼7.5 Hz), 2.94 (t, 2H, J¼7.5 Hz),
2.96 (septet, 1H, J¼7.0 Hz), 7.31 (d, 2H, J¼8.4 Hz), 7.90
(d, 2H, J¼8.4 Hz); 13C NMR (for p-isomer) d 14.02,
22.52, 23.66, 24.45, 29.07, 31.66, 34.20, 38.52, 126.59,
4.2.1. 1-(2,5-Dimethylphenyl)-3-methyl-1-butanone (1h).
The title compound was obtained by the reaction of isovale-
ric acid (51.1 mg, 0.500 mmol) and p-xylene (3.06 mL,
25.0 mmol) as described in the typical procedure in 55%
yield (52.4 mg) as a pale yellow oil after the purification
by preparative TLC (hexane/EtOAc¼28/1). 1H NMR
d 0.98 (d, 6H, J¼6.8 Hz), 2.25 (nonet, 1H, J¼6.8 Hz),
2.35 (s, 3H), 2.43 (s, 3H), 2.75 (d, 2H, J¼6.8 Hz), 7.09–
7.38 (m, 3H); 13C NMR d 20.51, 20.87, 22.65, 25.07,
50.65, 128.72, 131.58, 131.68, 134.37, 134.99, 138.68,
204.87; IR (neat) n 1686 cmꢁ1; HRMS calcd for C19H30O
[M+] 190.1358, found 190.1366.
128.29, 134.98, 154.27, 200.25; IR (neat) n 1684 cmꢁ1
;
HRMS (EI) calcd for C16H24O [M+] 232.1827, found
232.1820.
4.2.5. 1-(2,4-Dimethylphenyl)-1-heptanone (5b). The title
compound was obtained by the reaction of heptanoic acid
(75.7 mg, 0.582 mmol) and m-xylene (3.56 mL,
29.1 mmol) as described in the typical procedure in 74%
yield (94.6 mg) as a colorless oil after the purification by
preparative TLC (hexane/EtOAc¼27/1). GC–MS analysis
of the product showed the presence of a small amount of
isomer (ca. 4% by the TIC integration). 1H NMR d 0.88 (t,
3H, J¼7.0 Hz), 1.23–1.40 (m, 6H), 1.68 (quintet, 2H,
J¼7.5 Hz), 2.34 (s, 3H), 2.47 (s, 3H), 2.86 (t, 2H,
J¼7.5 Hz), 7.02–7.06 (m, 2H), 7.56 (d, 1H, J¼8.5 Hz);
13C NMR d 13.97, 21.24, 21.35, 22.49, 24.52, 29.00,
31.63, 41.32, 126.16, 128.81, 132.73, 135.27, 138.31,
141.46, 204.13; IR (neat) n 1684 cmꢁ1; HRMS (EI) calcd
for C15H22O [M+] 218.1671, found 218.1664.
4.2.2. 1-(2,5-Dimethylphenyl)-1-undecanone (1o). The
title compound was obtained by the reaction of undecanoic
acid (81.5 mg, 0.437 mmol) and p-xylene (2.68 mL,
21.9 mmol) as described in the typical procedure in 76%
yield (90.9 mg) as a colorless oil after the purification by
1
preparative TLC (hexane/EtOAc¼30/1). H NMR d 0.88
(t, 3H, J¼7.5 Hz), 1.39–1.20 (m, 14H), 1.68 (quintet, 2H,
J¼7.5 Hz), 2.35 (s, 3H), 2.43 (s, 3H), 2.86 (t, 2H,
J¼7.5 Hz), 7.08–7.41 (m, 3H); 13C NMR d 14.08, 20.61,
20.90, 22.65, 24.39, 29.29, 29.32, 29.45, 29.48, 29.55,
31.87, 41.67, 128.73, 131.61, 131.69, 134.46, 135.02,
138.40, 205.17; IR (neat) n 1684 cmꢁ1; HRMS calcd for
C19H30O [M+] 274.2297, found 274.2298.
4.2.6. 1-(3,4-Dimethylphenyl)-1-heptanone and 1-(2,3-
dimethylphenyl)-1-heptanone (6b). The title compounds
were obtained as a mixture (1-(3,4-dimethylphenyl)-1-hep-
tanone/1-(2,3-dimethylphenyl)-1-heptanone¼88/12) by the
reaction of heptanoic acid (73.0 mg, 0.561 mmol) and o-
xylene (3.42 mL, 28.0 mmol) as described in the typical pro-
cedure in 85% yield (104 mg) as a colorless oil after the
4.2.3. 1,1-Bis(2,5-dimethylphenyl)-1-heptene (2). A mix-
ture of Yb(NTf2)3 (101 mg, 0.100 mmol), heptanoic acid
(65.1 mg, 0.500 mmol), and p-xylene (3.06 mL,
25.0 mmol) was stirred at 250 ꢀC for 12 h. After cooling to
room temperature, water (3 mL) was added to the mixture
and the organic layer was separated. The aqueous layer
was extracted with ethyl acetate (3ꢂ5 mL). The combined
organic layers were dried over MgSO4, filtered, and concen-
trated under reduced pressure. The residue was purified
by preparative TLC (hexane/EtOAc¼30/1) to give 1b
(56.2 mg, 51% yield) and 2 (30.5 mg, 19% yield).
1
purification by preparative TLC (hexane/EtOAc¼20/1). H
NMR d 0.86–0.92 (m, 3H), 1.27–1.42 (m, 6H), 1.66–1.75
(m, 2H), 2.27–2.34 (m, 6H), 2.83 (t, 0.24H, J¼7.5 Hz, for
the minor isomer), 2.92 (t, 1.76H, J¼7.5 Hz, for the major
isomer), 7.13 (t, 0.12H, J¼7.5 Hz, for the minor isomer),
7.17–7.24 (m, 1H), 7.28 (d, 0.12H, J¼7.5 Hz, for the minor
isomer), 7.69 (dd, 0.88H, J¼8.0, 2.0 Hz, for the major iso-
mer), 7.73 (d, 0.88H, J¼2.0 Hz, for the major isomer); 13C
NMR (for the major isomer) d 14.39, 20.12, 20.32, 22.89,

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M. Kawamura et al. / Tetrahedron 62 (2006) 9201–9209
Table 7. Crystallographic data for [Bi6O4(OH)4(H2O)6](NTf2)6
24.90, 29.44, 32.04, 38.89, 126.17, 129.56, 130.10, 135.45,
137.18, 142.62, 200.93; IR (neat) n 1681 cmꢁ1; HRMS calcd
for C15H22O [M+] 218.1671, found 218.1664.
Formula
C12H16Bi6F36N6O38S12
3174.82
0.14ꢂ0.13ꢂ0.11
Trigonal
R-3 (no. 148)
19.1789(8)
19.1789(8)
15.8377(13)
90
90
120
5045.1(5)
3
3.135
4344
16.18
153(2)
7602
2473
202
0.0319
0.0859
Formula weight
Crystal size (mm)
Crystal system
Space group
4.2.7. 1-(2,5-Diethylphenyl)-1-heptanone (7b). The title
compound was obtained by the reaction of heptanoic acid
(82.6 mg, 0.635 mmol) and p-diethylbenzene (4.94 mL,
31.8 mmol) as described in the typical procedure in 74%
yield (115 mg) as a colorless oil after the purification by pre-
parative TLC (hexane/EtOAc¼20/1). 1H NMR d 0.89 (t, 3H,
J¼7.0 Hz), 1.20 (t, 3H, J¼7.5 Hz), 1.24 (t, 3H, J¼7.5 Hz),
1.28–1.42 (m, 6H), 1.70 (quintet, 2H, J¼7.5 Hz), 2.65 (q,
2H, J¼7.5 Hz), 2.76 (q, 2H, J¼7.5 Hz), 2.87 (t, 2H,
J¼7.5 Hz), 7.18 (d, 1H, J¼8.0 Hz), 7.22 (dd, 1H, J¼2.0,
8.0 Hz), 7.33 (d, 1H, J¼2.0 Hz); 13C NMR d 14.01, 15.51,
16.11, 22.51, 24.31, 26.43, 28.34, 28.97, 31.65, 42.19,
127.18, 130.16, 130.36, 138.85, 140.61, 141.40, 205.86;
IR (neat) n 1691 cmꢁ1; HRMS calcd for C17H26O [M+]
246.1984, found 246.1989.
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
g (ꢀ)
˚
V (A)
Z
Dcalc (g cmꢁ3
F(000)
)
m(Mo Ka) (cmꢁ1
T (K)
)
No. of rflns measd
No. of unique rflns (Rint
No. of variables
R1 (I0>2s(I0))
wR2 (all data)
GOF
)
1.05
1.80, ꢁ1.64
ꢁ3
˚
4.3. A typical procedure for HNTf2-catalyzed Friedel–
Crafts acylation with azeotropic removal of H2O
Diff peak, hole (eA
)
A mixture of HNTf2 (141 mg, 0.500 mmol), heptanoic acid
(325 mg, 2.50 mmol), anisole (13.5 g, 125 mmol), and tolu-
ene (30 mL) was refluxed for 36 h in 100 mL flask equipped
with Dean–Stark apparatus. After cooling to room tempera-
ture, water was added to the mixture and the organic layer
was separated. The aqueous layer was extracted with ethyl
acetate. The combined organic layers were dried over
MgSO4 and concentrated under reduced pressure. The resi-
due was purified by preparative TLC (hexane/EtOAc¼24/1)
to give 1-(4-methoxyphenyl)-1-heptanone 9b as a white
solid (488 mg, 89% yield). Mp: 38–39 ꢀC (lit.26 mp 39–40 ꢀC).
5. Crystallographic data
Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited at the
Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 608158. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: +44 1223
336033 or e-mail: deposit@ccdc.cam.ac.uk].
Acknowledgements
4.4. Hydrolysis of Bi(NTf2)3
D.-M.C. thanks New Energy and Industrial Technology
Development Organization (NEDO) for a postdoctoral
fellowship.
4.4.1. Hydrolysis of Bi(NTf2)3 in p-xylene. Bi(NTf2)3
(105 mg, 0.10 mmol), p-xylene (4 mL), and carboxylic
acid (heptanoic acid, 72 mL, 66 mg, 0.51 mmol or cyclohex-
anecarboxylic acid, 64 mg, 0.50 mmol) were mixed in
a glass tube. Under these conditions, some Bi(NTf2)3 re-
mained undissolved (the amount of Bi(NTf2)3 was too
much for complete dissolution). Then 4.7, 5.4, or 10.4 mL
of water was added. In some cases, the mixture was heated
to reflux for a few minutes, while in other case the mixture
was kept at room temperature. In all cases, colorless crystals
of [Bi6O4(OH)4(H2O)6](NTf2)6 were formed within a few
days.
References and notes
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[Bi6O4(OH)4] was assumed to keep the charge balance as
well as from the similar structures reported.23,24 O3 atom
was judged to be an oxygen atom of a water molecule from
its position as well as the Bi–O distance. Disorder was ob-
served for O1 (O8), O2 (O9), O3, S2, F4–F6, O6, O7, and
C2 atoms (the atomic numberings refer to those in Fig. 2).
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