Rare earth(III) perfluorooctane sulfonates and perfluorooctanesulfonic acid in fluorous solvents: Novel and recyclable catalytic systems for Friedel-Crafts acylation of unactivated benzenes

Article abstract of DOI:10.1016/j.jfluchem.2005.05.008

Catalytic Friedel-Crafts acylation of benzene and unactivated benzenes, such as chlorobenzene and fluorobenzene, was successfully accomplished using rare earth(III) perfluorooctane sulfonates (RE(OPf)₃), RE = Sc, Y, La ～ Lu) and perfluorooctanesulfonic acid (PfOH) as catalysts in fluorous solvents. Solutions of Yb(OPf)₃ and PfOH in perfluorodecalin (C ₁₀F₁₈, cis and trans-mixture) are the most suitable catalytic system, with catalyst loading as low as 0.4%mol leading to clean, high-yielding benzoylation of a variety of unactivated benzenes. By simple separation of the fluorous phase containing only catalyst, acylation can be repeated several times.

Full text of DOI:10.1016/j.jfluchem.2005.05.008

Journal of Fluorine Chemistry 126 (2005) 1191–1195
www.elsevier.com/locate/fluor
Rare earth(III) perfluorooctane sulfonates and perfluorooctanesulfonic
acid in fluorous solvents: Novel and recyclable catalytic systems for
Friedel-Crafts acylation of unactivated benzenes
*
Wen-Bin Yi, Chun Cai
Chemical Engineering College, Nanjing University of Science & Technology, Nanjing 210094, China
Received 4 April 2005; received in revised form 19 May 2005; accepted 20 May 2005
Available online 7 July 2005
Abstract
Catalytic Friedel-Crafts acylation of benzene and unactivated benzenes, such as chlorobenzene and fluorobenzene, was successfully
accomplished using rare earth(III) perfluorooctane sulfonates (RE(OPf)3), RE = Sc, Y, La ꢀ Lu) and perfluorooctanesulfonic acid (PfOH) as
catalysts in fluorous solvents. Solutions of Yb(OPf)3 and PfOH in perfluorodecalin (C10F18, cis and trans-mixture) are the most suitable
catalytic system, with catalyst loading as low as 0.4%mol leading to clean, high-yielding benzoylation of a variety of unactivated benzenes.
By simple separation of the fluorous phase containing only catalyst, acylation can be repeated several times.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Friedel-Crafts acylation; Fluorous biphasic catalysis; Rare earth(III) perfluorooctanesulfonates; Perfluorooctanesulfonic acid; Perfluorocarbon
1. Introduction
solvents, especially perfluoroalkanes have some unique
properties which make them attractive alternatives for
conventional organic solvents [6]. The compounds functio-
nalized with perfluorinated groups often dissolve preferen-
tially in fluorous solvents and this property can be used to
extract fluorous components from reaction mixtures [7].
Friedel-Crafts acylation catalyzed by the catalytic system
including hafnium triflate and trifluoromethanesulfonic acid
was reported [8]. However, reusing of these catalysts
required tedious workup procedures, such as filtration,
purification and drying. Shi reported in 2002 that RE(OPf)3
can catalyzed Friedel-Crafts acylation of anisole with acetic
anhydride in fluorous phase effectively [5]. Thus, We were
intrigued to apply the catalytic system including rare
earth(III) perfluorooctane sulfonates (RE(OPf)3) and per-
fluorooctanesulfonic acid (PfOH) in fluorous solvents to
catalyzed Friedel-Crafts acylation of benzene and unac-
tivated benzenes, such as chlorobenzene and fluorobenzene.
As expected that, rather high yields of the corresponding
diarylketones and the robustness of the catalytic system for
recycling using by simple phase-separation were obtained.
We would like to report herein the work on this new
application of the catalytic system.
Friedel-Crafts acylations are widely used in the
manufacture of aryl ketones, which are of interest in the
synthesis of a large number of fine chemicals, such as drugs,
fragrances, dyes and pesticides [1]. However, the large-scale
synthesis, involving more than stoichiometrical amount of
Lewis acids, such as aluminum trichloride (AlCl3), makes
acylation one of the most environmentally harmful
processes. Although some catalysts or catalytic systems
which complete the acylation by catalytic use have been
reported [2], substrates are limited to activated benzenes or
aroyl derivatives in most cases. Thus, development of more
efficient and powerful catalysts is strongly demanded.
Recently, a new kind of Lewis acids of rare earth(III)
perfluorooctane sulfonates (RE(OSO2C8F17)3, RE(OPf)3)
was of special interest [3–5]. The characteristic features of
the catalyst include low hygroscopicity, ease of handling,
robustness for the recycling using and high solubility in
fluorous solvent [4]. On the other hand, perfluorocarbon
* Corresponding author. Fax: +86 25 84315030.
E-mail address: yiwenbin5@163.com (C. Cai).
0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.05.008

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W.-B. Yi, C. Cai / Journal of Fluorine Chemistry 126 (2005) 1191–1195
Table 1
Effect of the catalytic systems on the benzoylationa
Entry
Catalystic system
Yield (%)b
1
2
Sc(OPf)3 + PfOH
Y(OPf)3 + PfOH
La(OPf)3 + PfOH
Ce(OPf)3 + PfOH
Pr(OPf)3 + PfOH
Nd(OPf)3 + PfOH
Sm(OPf)3 + PfOH
Eu(OPf)3 + PfOH
Gd(OPf)3 + PfOH
Tb(OPf)3 + PfOH
Dy(OPf)3 + PfOH
Ho(OPf)3 + PfOH
Er(OPf)3 + PfOH
Tm(OPf)3 + PfOH
Yb(OPf)3 + PfOH
Lu(OPf)3 + PfOH
86
62
69
71
53
81
65
59
68
67
74
72
80
79
85
69
3
Scheme 1.
4
5
6
7
2. Results and discussion
8
9
The benzoylation of benzene with benzoyl chloride was
used as a test reaction to compare the catalytic activity of
these catalytic system under standardized conditions
(Scheme 1). Table 1 shows the effect of sixteen catalytic
systems on the benzoylation in perfluorodecalin (C10F18,
cis and trans-mixture). Among these catalytic acylations,
the systems containing Sc(OPf)3 and Yb(OPf)3 are most
efficient. This would be ascribed to the higher Lewis
acidity of Sc(OPf)3 and Yb(OPf)3 than those of other
RE(OPf)3 [9]. Much lower yields were obtained in the
presence of PfOH or Yb(OPf)3 (13 and 20% yields,
respectively). We found that, upon heating at 80 8C, the
organic phase is miscible with fluorous phase and the
acylation became a homogenous reaction. The control
experiment elucidates that only 3% product of acylation
could be obtained in the absence of catalyst and fluorous
solvent. In addition, we found that a catalyst loading of
only 0.4 mol% was required when using fluorous phase
technology, which is more effective than the 10 mol% of
hafnium triflate and trifluoromethanesulfonic acid required
to catalyze the acylation.
10
11
12
13
14
15
16
a
All reactions were carried out in perfluorodecalin (C10F18, cis and
trans-mixture) at 80 8C for 12 h.
Isolated yields based on the acylating agent.
b
perfluorodecalin (C10F18, cis and trans-mixture) is the best
fluorous solvent for benzoylation.
We decided to use the relatively cheap and similarly
active catalytic system including Yb(OPf)3 and PfOH and
perfluorodecalin (C10F18, cis and trans-mixture) as a
fluorous solvent for benzoylation. The benzoylation of
various aromatics with benzoylating agents has been
examined (Scheme 2). The results are summarized in
Table 3. Not only benzoyl chloride but also benzoic
anhydride was efficient acylating agents for this reaction. It
was very exciting to find that unactivated benzenes, such as
chlorobenzene and fluorobenzene, reacted smoothly to
afford the corresponding aromatic ketones in the presence of
Yb(OPf)3 and PfOH. In all cases, the reactions proceeded
very cleanly (checked by TLC) and no side reaction products
were observed. In addition, it is noted that the reaction
proceeded in a highly regioselective manner to give para-
regioisomer. When the reaction was finished, the reaction
mixture was cooled to room temperature. The fluorous phase
with RE(OPf)3 and PfOH catalysts can separate from the
organic layer return to the bottom layer. Based on GC–MS
data, no loss of perfluorodecalin (C10F18, cis and trans-
mixture) to the organic phase can be detected.
Next, perfluorohexane (C6F14), perfluoromethylcyclo-
hexane (C7F14), perfluorotoluene (C7F8), perfluorooctane
(C8F18) and perfluorooctyl bromide (C8F17Br) were also
selected as fluorous solvents and the effect of such solvents
was examined for the Friedel-Crafts acylation (Scheme 1).
Table 2 shows the influence of fluorous solvents on the
reaction under the controlled conditions. The results showed
that yields of the desired ketones in perfluorooctane (C8F18)
and perfluorooctyl bromide (C8F17Br) are lower than other
solvents. The fluorous solvents perfluorohexane (C6F14) and
perfluorotoluene (C7F8) are in fact miscible with aromatic
substrates, such as benzene at room temperature. Thus, it is
impossible to recover fluorous phase by phase-separation. At
the same time, we found that during repeated benzoylation
reactions the loss of fluorous solvent is very serious when
using perfluoromethylcyclohexane (C7F14) as a fluorous
solvent because it is very volatile (bp 76 8C). Therefore,
Attempts were made to recycle these catalytic systems.
The benzoylation of benzene with benzoyl chloride under
the conditions described in Table 1 with RE(OPf)3 and PfOH
as catalysts was run for five consecutive cycles, furnishing
Scheme 2.

W.-B. Yi, C. Cai / Journal of Fluorine Chemistry 126 (2005) 1191–1195
1193
Table 3
Benzoylation of various aromaticsa
Table 2
Effect of fluorous solvents on the benzoylationa
Entry R1 R2
R3 Temperature Time Yield Ratio of
(%)b ortho/parac
Entry
PFC
Yield (%)b
(8C)
(h)
1
2
CF3(CF2)4CF3
78
86
1
2
F
Cl
H
100
120
120
80
24
24
48
16
24
24
12
24
16
24
89
90
62
87
88
81
77
86
79
80
<1/>99
5/95
8/92
–
Cl Cl
Br Cl
H
3
H
3
88
4
H
PhCOO
H
5
Cl PhCOO
H
120
100
80
<1/>99
<1/>99
–
6
F
PhCOO
Cl
H
4
5
6
CF3(CF2)6CF3
76
74
85
7
H
Cl
Cl
Me
CF3(CF2)6CF2Br
8
Cl Cl
Cl
Cl Cl
120
80
2/98
–
9
H
10
Me 120
2/98
a
a
b
c
All reactions were carried out in the presence of Yb(OPf)3 and PfOH at
All reactions were carried out in the presence of Yb(OPf)3 and PfOH.
Isolated yields based on the acylating agent.
80 8C for 12 h.
b
Isolated yields based on the acylating agent.
All ratios of ortho/para were determined by GC.
the corresponding benzophenones, with 85, 84, 85, 84 and
83% isolated yields. The robustness of the catalyst for
recycling using may partly be attributed to thewater-repellent
nature of the perfluoroalkane chain ‘‘(–CF2–CF2–)n’’ of
RE(OSO2C8F17)3 which refuses the approach of water or acid
molecules to the central metal cation, thus maintaining its
high Lewis acidity [2]. The separated fluorous phase
containing only catalyst could be reused for the next acylation
withoutanytreatment, andthisworkupprocedureofrecycling
was accomplished by simple phase-separation.
reuse of this novel catalytic system are expected to
contribute to the development of more benign Friedel-
Crafts acylation.
3. Experimental
3.1. General
MPs were obtained with Shimadzu DSC-50 thermal
analyzer. IR spectra were recorded on a Bumem MB154S
infrared analyzer. 1H NMR spectra were measured on Bruke
Advance RX300. Mass spectra were recorded with a Saturn
2000GC/MS instrument. Inductively coupled plasma (ICP)
spectra were measured on Ultima2C apparatus. Elemental
analyses were performed on a Yanagimoto MT3CHN
corder. The orientation of acylation was determined by
HP4890 GC analyzer with HP-5 silica chromatography
column. Commercially available reagents were used without
further purification.
Finally, the assumed catalytic cycle is shown in Scheme
3. A key acylating agent is proposed to be acyl perflate 19,
which reacts even with unactivated benzenes to afford the
corresponding diarylketones accompanied by regeneration
of PfOH. RE(OPf)3 is assumed to catalyze both generation
of 1 and successive acylation.
In conclusion, the use of RE(OPf)3 and PfOH as catalysts
in properly chosen fluorous solvents for Friedel-Crafts
acylation can be considered as an interesting new alternative
to existing homogeneous catalysts. Unactivated benzenes,
such as chlorobenzene and fluorobenzene, reacted smoothly
to afford the corresponding aromatic ketones in this catalytic
system. By simple separation of the fluorous phase
containing only catalyst, acylation can be repeated several
times. The simple procedures as well as easy recovery and
3.2. Typical procedure for preparation of RE(OPf)3
RE(OPf)3 was prepared according to literature [4].
Method A: the mixture of PfOH solution (aq) and
Scheme 3. Assumed catalytic cycle of acylation.

1194
W.-B. Yi, C. Cai / Journal of Fluorine Chemistry 126 (2005) 1191–1195
YbCl3ꢁ6H2O solution (aq) was stirred at room temperature.
Method B: the mixture of PfOH solution (aq) and Yb2O3
powder was stirred at boiling. In both methods, the resulting
gelatin-like solid was collected, washed and dried at 150 8C
in vacuo to give a white solid, which does not have a clear
melting point up to 500 8C, but shrinks around 380 and
450 8C. IR (KBr) y 1237 (CF3), 1152 (CF2), 1081 (SO2),
1059 (SO2), 747 (S–O) and 652 (C–S) cmꢂ1. ICP: Calcd. for
C24O9F51SYb: Yb, 10.30%. Found: Yb, 9.88%. Anal. Calcd.
for C24O9F51SYbꢁH2O: C, 17.21%; H, 0.10%. Found: C,
17.03%; H, 0.18%.
Acknowledgements
We thank the National Defence Committee of Science
and Technology (40406020103) for financial support. We
also thank the Zhen-Ya Rare Earth Co. Ltd. for providing
several kinds of rare earth oxides.
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