Recyclable Lewis acid catalysts by tuning supercritical vs liquid carbon dioxide phases: Lanthanide catalysts with tris(perfluorooctanesulfonyl)methide and bis(perfluorooctanesulfonyl)amide

Article abstract of DOI:10.1016/S0040-4020(02)00986-9

Lanthanide tris(perfluorooctanesulfonyl)methide and bis(perfluorooctanesulfonyl)amide catalysts are shown to be continuously employed in supercritical carbon dioxide by changing supercritical vs. liquid carbon dioxide phases. The lanthanide complexes are

Full text of DOI:10.1016/S0040-4020(02)00986-9

Tetrahedron 58 (2002) 8345–8349
Recyclable Lewis acid catalysts by tuning supercritical vs liquid
carbon dioxide phases: lanthanide catalysts with
tris(perfluorooctanesulfonyl)methide and
bis(perfluorooctanesulfonyl)amide
Joji Nishikido,a Mayumi Kamishima,a Hiroshi Matsuzawab and Koichi Mikamib
,
*
aThe Noguchi Institute, Tokyo 173-0003, Japan
bDepartment of Applied Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received 7 June 2002; accepted 10 June 2002
Abstract—Lanthanide tris(perfluorooctanesulfonyl)methide and bis(perfluorooctanesulfonyl)amide catalysts are shown to be continuously
employed in supercritical carbon dioxide by changing supercritical vs. liquid carbon dioxide phases. The lanthanide complexes are thus
extremely efficient Lewis acid catalysts even in supercritical carbon dioxide phase by virtue of the highly electron-withdrawing and
solubilizing effect of tris(perfluorooctanesulfonyl)methide and bis(perfluorooctanesulfonyl)amide without any hydrocarbon spacer. q 2002
Elsevier Science Ltd. All rights reserved.
1. Introduction
sulfonyl)methide and bis(perfluorooctanesulfonyl)amide as
a new Lewis acid catalyst. Numerous and long-enough
(perfluorooctyl, C8F17) perfluoroalkyl ligands6 can be
attached directly without any hydrocarbon spacer for
increasing the Lewis acidity. These lanthanide catalysts
can be employed for fluorous biphase catalysis (FBC) to be
soluble in fluorous solvent.7,8 FBC has been of concern as an
environmentally benign reaction system to have the
advantage that fluorous catalysts are immobilized in
fluorous solvent and recycled. However this FBC method
is not yet completely green reaction system in the use of an
organic solvent such as 1,2-dichloroethane and toluene. To
solve this drawback, we investigated the use of supercritical
carbon dioxide (scCO2) in place of an organic solvent
(Fig. 1(a)).9 Since fluorous compounds are soluble in
scCO2,3a,f,4c,e,5a,e homogeneous phase with fluorous solvent
immobilizes fluorous lanthanide catalysts. After the reac-
tion, fluorous solvent remains in the reaction vessel by
releasing liquid carbon dioxide with product. In addition, we
tried to reuse the lanthanide catalysts by changing from
scCO2 to liquid carbon dioxide (Fig. 1(b)). Lanthanide
catalysts are insoluble in liquid carbon dioxide even in the
presence of reaction substrate or product. We report here the
completely recyclable use of lanthanide(III) tris(perfluoro-
octanesulfonyl)methide and bis(perfluorooctanesulfonyl)
amide complexes in scCO2.
A wide variety of Lewis acid catalysts have been developed
on the basis of the Lewis acid–base complexation in
organic polar solvents.1 However, the Lewis acid complexes
have often been employed and then wasted after the
reactions in more than a stoichiometric amount. Therefore,
it is desirable to decrease the amount of a Lewis acid
complex in catalytic by developing a stronger Lewis acid
catalyst and the recycle process thereof. The replacement of
conventional liquid solvents by supercritical fluids as
reaction media for homogeneous catalysis has been known
to provide the opportunity to control the reaction in terms of
the reactivity and selectivity because of high gas misci-
bilities, greater diffusivities, clustering effects, and tunable
solvent power by changing their densities along with the
pressure.2 Supercritical fluids have been employed as an
environmentally benign reaction media in late transition
metal catalysis for hydroformylation3 and hydrogenation.4
By contrast, the design and immobilization of strong Lewis
acid catalysts are essentially remained as a challenging
problem in this unorthodox non-polar media for Lewis acid
catalysis.5
We have developed lanthanide(III) tris(perfluorooctane-
Keywords: supercritical carbon dioxide; liquid carbon dioxide;
environmental benignity; lanthanide; Lewis acid catalysis;
tris(perfluorooctanesulfonyl)methide; bis(perfluorooctanesulfonyl)amide.
2. Results and discussion
*
Corresponding author. Tel.: þ81-3-5734-2776; fax: þ81-3-5734-2142;
e-mail: kmikami@o.cc.titech.ac.jp
First, we examined the catalytic activities of the lanthanide
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00986-9

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J. Nishikido et al. / Tetrahedron 58 (2002) 8345–8349
Figure 1. The recycle methods of fluorous lanthanide catalysts using scCO2 as a reaction media. (a) fluorous/scCO2 biphase reaction, (b) without fluorous solvent.
complexes for alcohol acylation as a probe reaction in
scCO2/fluorous solvent (perfluorooctane). The ester for-
mation of cyclohexanol (2 mmol) with acetic anhydride
(2.2 mmol) underwent in homogeneous phase involving
soluble fluorous phase immobilized lanthanide catalyst
above critical point. After the reaction, lanthanide catalyst
was soluble in fluorous phase and the product separated
from fluorous phase was obtained by extracting with organic
solvent. The isolated yield of the product was 98% (Table
1). By contrast, the yield of esterification in scCO2 without
lanthanide catalyst was 3%. Therefore lanthanide catalyst is
very effective Lewis acid in scCO2. Fluorous phase
immobilized lanthanide catalyst was recycled in five times
and no loss of activity was observed for the recovered
catalyst.
anhydride (2.2 mmol) underwent in homogeneous phase
involving soluble fluorous lanthanide catalyst above critical
point (Photo 1). However heterogeneous phase appeared
with solid catalyst below critical temperature even in the
presence of products. Taking this advantage of fluorous
lanthanide catalyst with respect to temperature-dependent
solubility,10 we examined to reuse these complexes. The
liquid carbon dioxide was evacuated with products, after the
reaction completed within 10 min in the homogeneous
phase at 408C with 1 mol% of scandium and ytterbium
complexes (Fig. 1(b)). The lanthanide tris(perfluorooctane-
sulfonyl)methide and bis(perfluorooctanesulfonyl)amide
complexes were insoluble in carbon dioxide liquid phase
and the residual lanthanide complexes remained in the
autoclave and were reused without isolation. The reaction
product could be isolated into the carbon dioxide liquid
phase after completing the reaction and lowering the
temperature (2208C). Cyclohexyl acetate was thus obtained
in quantitative yield as calculated by GC analysis (Table 2).
Scandium and ytterbium complexes were completely
remained in the reaction vessel as determined by atomic
emission spectrometry. No loss of activity was observed for
We examined the esterification of cyclohexanol catalyzed
lanthanide complexes without fluorous solvent in scCO2.
The ester formation of cyclohexanol (2 mmol) with acetic
Table 1. Esterification in scCO2/C8F18 catalyzed by Yb[N(SO2C8F17)2]3
Cyclea
%Yieldb
1
2
3
4
5
99 (98)c
99
99
99
99
a
The catalyst in the fluorous phase was recycled.
Calculated by GC analysis using n-nonane as an internal standard.
Value in parenthesis refers to the isolated yields.
b
c
Photo 1. Esterification in (a) homogeneous phase under supercritical
carbon dioxide (b) heterogeneous phase under liquid carbon dioxide.

J. Nishikido et al. / Tetrahedron 58 (2002) 8345–8349
Table 2. Esterification in scCO2 catalyzed by Yb and Sc complexes
8347
dioxide. The lanthanide complexes are thus extremely
efficient Lewis acid catalysts even in scCO2 by virtue of the
highly electron-withdrawing effect of tris(perfluorooctane-
sulfonyl)methide and bis(perfluorooctanesulfonyl)amide
ligands without any hydrocarbon spacer.
Cyclea
%Yieldb
3. Experimental
3.1. General
Yb[N(SO2C8F17)2]3
Sc[C(SO2C8F17)3]3
1
2
3
99 (98)c
99
98
99 (98)c
100 (98)c
99
1H and 13C NMR spectra were measured on a JEOL JNM-
EX400 (400 MHz) spectrometers. Chemical shifts of H
1
a
The residual catalyst was recycled.
Calculated by GC analysis using n-nonane as an internal standard.
Values is parenthesis refer to the isolated yields.
b
c
NMR were expressed in parts per million relative to
chloroform (d 7.26) or tetramethylsilane (d 0.00) as an
internal standard in chloroform-d. Chemical shifts of 13C
NMR were expressed in parts per million relative to
chloroform-d (d 77.0) as an internal standard. GC analysis
was carried out on SHIMADZU GC-1700AF and GC–MS
analysis was taken by Hewlett–Packard G1800 A GLS.
Atomic emission spectrometer is of IRIS/AP Nippon Jarrell
Ash Co. 1,2-Dichoroethane was distilled from phosphorous
pentaoxide.
the catalyst recovered. The isolated yield of the product was
98%. Thus, the reaction of cyclohexanol (2 mmol) with
acetic anhydride (2.2 mmol) was carried out at 408C for
10 min, in the presence of 1 mol% of lanthanide
complexes. Then, the liquid carbon dioxide was evacuated.
Cyclohexyl acetate was obtained in good isolated yield. The
remaining scandium and ytterbium complexes were com-
pletely recovered and reused in scCO2 without isolation
(Table 2).
3.2. Ester formation of cyclohexanol with acetic
anhydride by using fluorous solvent
Then, the catalytic activities and recyclable use of the
lanthanide complexes were examined for C–C bond
forming (CCF) reactions. Friedel–Crafts(F–C) reaction
constitutes one of the most useful processes in organic
synthesis.11 The F–C acylation reaction of anisol (2 mmol)
with acetic anhydride (4 mmol) was also carried out at 808C
for 2 h, in the presence of a catalytic amount (3 mol%) of
scandium and ytterbium complexes. The remaining scan-
dium and ytterbium complexes were completely (.99%)
recovered and reused in the supercritical carbon dioxide
phase without isolation (Table 3).
A 20 ml stainless steel autoclave equipped with magnetic
stirring bar was charged with cyclohexanol (0.20 g,
2 mmol), acetic anhydride (0.22 g, 2.2 mmol), and ytterbium
(III) tris[bis(perfluoroocatanesulfonyl)amide] (1 mol%
based on cyclohexanol) as a Lewis acid catalyst in
perfluorooctane (5 ml). After adding carbon dioxide to
apply pressure of 20 MPa to the autoclave, the reaction
mixture was stirred for 15 min at 408C. After cooling below
08C, the pressure in the autoclave was slowly released. The
autoclave was opened at room temperature, and dichloro-
ethane (5 ml) was added to the autoclave and stirred for
15 min to extact the product. Then, the reaction mixture was
allowed to stand for 5 min, so that the reaction mixture
could separate into the upper organic phase and the lower
fluorous phase. As a result of the gas chromatographic
analysis of the dichloroethane layer dissolving the product,
the yield of cyclohexyl acetate was 99% using n-nonane as
an internal standard. Cyclohexyl acetate was obtained from
dichloroethane layer after evaporation under reduced
pressure and silica gel chromatography (0.280 g, 98%
isolated yield).
In summary, we have disclosed lanthanide tris(perfluoro-
octanesulfonyl)methide and bis(perfluorooctanesulfonyl)
amide complexes as efficient and recyclable Lewis acid
catalysts by tuning supercritical (scCO2) vs liquid carbon
Table 3. Friedel–Crafts reaction in scCO2 catalyzed by Yb and Sc
complexes
After removing the dichloroethane layer, to which cyclo-
hexanol (0.2 g, 2 mmol) and acetic anhydride (0.22 g,
2.2 mmol) were added. After adding carbon dioxide to
apply pressure of 20 MPa to the autoclave, the reaction
mixture was stirred for 15 min at 408C. After cooling below
08C, the pressure in the autoclave was slowly released. The
autoclave was opened at room temperature, and dichloro-
ethane (5 ml) was added to the autoclave and stirred for
15 min to extract the product. As a result of the gas
chromatographic analysis of the dichloroethane layer
dissolving the product, the yield of cyclohexyl acetate was
99% using n-nonane as an internal standard. The similar
manner was repeated further three times, and the yields of
cyclohexyl acetate were 99, 99 and 99%, respectively.
Cyclea
% Yieldb
Yb[N(SO2C8F17)2]3
Sc[C(SO2C8F17)3]3
1
2
3
79 (77)c
77
77
82 (79)c
80
79
a
The residual catalyst was recycled.
Calculated by GC analysis using n-decane as an internal standard.
Values is parenthesis refer to the isolated yields.
b
c

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J. Nishikido et al. / Tetrahedron 58 (2002) 8345–8349
Ytterbium complexes were completely (.99%) remained in
perfluorooctane layer as determined by atomic emission
spectrometry. Thus, the catalyst can be reused, since the
catalytic activity was not lowered.
pressure and silica gel chromatography (0.231 g, 77%
isolated yield).
After successive introduction of liquid carbon dioxide, the
pressure in the autoclave was released to give the catalyst, to
which anisol (0.22 g, 2 mmol) and acetic anhydride (0.41 g,
4 mmol) were added. After adding carbon dioxide to apply
pressure of 10 MPa to the autoclave, the reaction mixture
was stirred for 2 h at 808C. After cooling to 2208C, liquid
carbon dioxide was successively introduced to the autoclave
at the flow rate of 1 ml/min for 1 h under 6 MPa. The
product was extracted with dichloroethane (5 ml). As a
result of the gas chromatographic analysis of the dichloro-
ethane layer dissolving the product, the yield of p-
methoxyacetophenone was 77% using n-decane as an
internal standard. The similar manner was repeated, and
the yields of at third attempts were 77%. Ytterbium
complexes were completely (.99%) remained in the
reaction vessel as determined by atomic emission spectro-
metry. Thus, the catalyst can be reused, since the catalytic
activity was not lowered.
3.3. Ester formation of cyclohexanol with acetic
anhydride without fluorous solvent
A 20 ml stainless steel autoclave equipped with magnetic
stirring bar was charged with cyclohexanol (0.20 g,
2 mmol), acetic anhydride (0.22 g, 2.2 mmol), and ytterbium
(III) tris[bis(perfluoroocatanesulfonyl)amide] (1 mol%
based on cyclohexanol) as a Lewis acid catalyst. After
adding carbon dioxide to apply pressure of 10 MPa to the
autoclave, the reaction mixture was stirred for 10 min at
408C. After cooling to 2208C, liquid carbon dioxide was
successively introduced to the autoclave at the flow rate of
1 ml/min for 1 h under 6 MPa. The product was extracted
with dichloroethane (5 ml). As a result of the gas
chromatographic analysis of the dichloroethane layer
dissolving the product, the yield of cyclohexyl acetate was
99% using n-nonane as an internal standard. Cyclohexyl
acetate was obtained from dichloroethane layer after
evaporation under reduced pressure and silica gel chromato-
graphy (0.279 g, 98% isolated yield).
3.5. Experimental data
1
3.5.1. Cyclohexyl acetate. H NMR (400 MHz, CDCl3): d
1.20–1.44 (m, 5H), 1.52–1.58 (m, 1H), 1.70–1.74 (m, 2H),
1.84–1.87 (m, 2H), 2.03 (s, 3H), 4.71–4.77 (m, 1H); 13C
NMR (100 MHz, CDCl3): d 21.45, 23.81, 25.37, 31.65,
72.61, 170.40; MS (EI, 70 eV): m/z 127 (C6H11COþ2 ), 99
(C6H11Oþ), 82 (C6H1þ0), 67, 43 (CH3COþ); Elemental
analysis (%) calcd for C8H14O2: C 67.57, H 9.92, found: C
67.40, H 9.94.
After successive introduction of liquid carbon dioxide, the
pressure in the autoclave was released to give the catalyst, to
which cyclohexanol (0.2 g, 2 mmol) and acetic anhydride
(0.22 g, 2.2 mmol) were added. After adding carbon dioxide
to apply pressure of 10 MPa to the autoclave, the reaction
mixture was stirred for 10 min at 408C. After cooling to
2208C, liquid carbon dioxide was successively introduced
to the autoclave at the flow rate of 1 ml/min for 1 h under
6 MPa. The product was extracted with dichloroethane
(5 ml). As a result of the gas chromatographic analysis of
the dichloroethane layer dissolving the product, the yield of
cyclohexyl acetate was 99% using n-nonane as an internal
standard. The similar manner was repeated, and the yields of
cyclohexyl acetate at third attempts were 98%. Ytterbium
complexes were completely (.99%) remained in the
reaction vessel as determined by atomic emission spectro-
metry. Thus, the catalyst can be reused, since the catalytic
activity was not lowered.
3.5.2. p-Methoxyacetophenone. 1H NMR (400 MHz,
CDCl3): d 2.55 (s, 3H), 3.87 (s, 3H), 6.93 (d, J¼9.8 Hz,
2H), 7.94 (d, J¼9.8 Hz, 2H); 13C NMR (100 MHz, CDCl3):
d 26.36, 55.44, 113.57, 130.22, 130.46, 163.31, 196.55; MS
(EI, 70 eV): m/z 150 (Mþ), 135 (MeOPhCOþ), 107
(MeOPhþ), 92, 77, 64, 63, 43 (CH3COþ); Elemental
analysis (%) calcd for C9H10O2: C 71.98, H 6.71, found:
C 72.07, H 6.66.
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