Lewis acid catalysis by lanthanide complexes with tris(perfluorooctanesulfonyl)methide ponytails in fluorous recyclable phase

Article abstract of DOI:10.1016/S0040-4039(00)01925-0

Scandium and ytterbium tris(perfluorooctanesulfonyl)methide complexes are shown to be immobilized in fluorous recyclable phase and extremely efficient Lewis acid catalysts for alcohol acylation, Friedel-Crafts acylation, Diels-Alder reaction, and Mukaiyam

Full text of DOI:10.1016/S0040-4039(00)01925-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 289–292
Lewis acid catalysis by lanthanide complexes with
tris(perfluorooctanesulfonyl)methide ponytails in fluorous
recyclable phase†
Koichi Mikami,a,* Yutaka Mikami,a Yousuke Matsumoto,a Joji Nishikido,b
Fumihiko Yamamotob and Hitoshi Nakajimab
aDepartment of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan
bThe Noguchi Institute, Tokyo 173-0003, Japan
Received 2 October 2000; revised 23 October 2000; accepted 27 October 2000
Abstract—Scandium and ytterbium tris(perfluorooctanesulfonyl)methide complexes are shown to be immobilized in fluorous
recyclable phase and extremely efficient Lewis acid catalysts for alcohol acylation, Friedel–Crafts acylation, Diels–Alder reaction,
and Mukaiyama aldol reaction by virtue of the highly electron-withdrawing effect of tris(perfluorooctanesulfonyl)methide
ponytails without any hydrocarbon spacer. © 2000 Elsevier Science Ltd. All rights reserved.
lization of lanthanide catalysts (Fig. 1). The key to the
success is the powerful electron-withdrawing effect of
the perfluoroalkanesulfonylmethide group without any
hydrocarbon spacer.5 We report the ‘super’ Lewis acid-
ities and the complete recycle of lanthanide(III) tris-
(perfluorooctanesulfonyl)methide complexes in the
fluorous phase.
Enzymatic reactions based on hydrogen bonding of
gigantic protein enzymes have been recognized as mod-
els for catalytic organic reactions. A wide variety of
Lewis acid complexes in minimal sizes, by contrast,
have thus been developed by mimicking hydrogen
bonding with the Lewis acid–base complexation in
aprotic polar solvents.1 However, the Lewis acid com-
plexes have often been wasted in more than a stoichio-
metric amount. Therefore, it is desirable to decrease the
amount of Lewis acid complex by developing a stronger
Lewis acid catalyst and the recycle process thereof.
Quite recently, the concept of fluorous bi-phasic cataly-
sis (FBC) was introduced as an environmentally benign
recycle process.2 Phosphine or phosphite ligands with
fluorous ponytails and hydrocarbon spacers have been
developed to immobilize late transition metal catalysts
for hydroformylation, hydride reduction, hydrogena-
tion, alkene epoxidation and hydroboration in non-po-
lar media.3 The design and immobilization of strong
Lewis acid catalysts are challenging in this unorthodox
non-polar media for Lewis acid catalysis.4 Numerous
(nine) and long-enough (perfluorooctyl, C8F17) fluorous
ponytails can be employed for fluorous phase immobi-
First, we examined the solubility of the scandium and
ytterbium complexes in fluorous and/or non-fluorous
solvents. The lanthanide complexes were soluble in
Keywords: acylation; environmental benignity; fluorous phase; lan-
thanide; Lewis acid catalysis.
Figure 1. ScL3%, L%=C(SO2C8F17)3. The core structure of
ScL3, L=C(SO2CF3)3 is optimized at the Hartree Fock level
with 6-31 G basis set for C, F, S, O and with CEP-31G
pseudo potential and basis set for Sc. Then the CF3 group in
the core structure is replaced with C8F17 and those chains are
optimized using UFF force field (Ref. 6).
* Corresponding author.
† A part of this work has been presented at the Ann. Meeting of
Chem. Soc. of Jpn., Chiba, March 28, 2000, Mikami, K.; Mikami,
Y.; Matsumoto, Y.; Yamamoto, F.; Nishikido, J.; Nakajima, H.
Abstr. No. 1F415.
0040-4039/01/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01925-0

290
K. Mikami et al. / Tetrahedron Letters 42 (2001) 289–292
Scheme 2.
Scheme 1.
aromatic fluorocarbons but not so soluble in aliphatic
fluorocarbons. However, fluoroaromatic solvents were
miscible in non-fluorous solvents. Therefore, we exam-
ined the catalytic activities of the lanthanide complexes
for alcohol acylation as a probe reaction in aliphatic
fluorocarbon/non-fluorous solvents with or without
fluoroaromatics; in the bi-phases or a homogeneous
phase with fluoroaromatics at higher temperature.
The ester formation of cyclohexanol (2 mmol) with
acetic anhydride (2 mmol) in perfluoromethylcyclohex-
ane (5 ml), toluene (5 ml) and perfluorobenzene (3 ml)
was completed within 15 min in the homogeneous
phase at 40°C, with 1 mol% of scandium and ytterbium
complexes (Scheme 1). Then, the reaction mixture was
allowed to stand at 15°C for 3 min, so that the reaction
mixture could separate into the upper phase (19.9%
CF3C6F11, 52.5% toluene, 27.6% C6F6) and the lower
phase (69.9% CF3C6F11, 13.8% toluene, 16.3% C6F6)
(Fig. 2); cyclohexyl acetate was obtained in quantitative
yield as calculated by GC analysis (Table 1). Scandium
and ytterbium complexes were completely (>99%) re-
covered in the lower phase as determined by atomic
emission spectrometry.
Figure 3. Before (25°C) and after (10 sec, 25°C) the reaction.
acetic anhydride (2 mmol) in perfluoromethylcyclohex-
ane (5 ml) and toluene (5 ml) was carried out at 30°C
for 20 min in the presence of 1 mol% of lanthanide
complexes (Scheme 2). Then, the heterogeneous two
phases were separated within 10 seconds into toluene
and the lower fluorous layers (Fig. 3). Cyclohexyl ace-
tate was obtained in good isolated yield. Scandium and
ytterbium complexes were completely (>99%) recov-
ered and reused in the fluorous phase without isolation
(Table 2).
Then, the catalytic activities and recyclable use of the
lanthanide complexes were examined for CꢀC bond
forming (CCF) reactions. The Diels–Alder (D-A) reac-
tion constitutes one of the most efficient construction
processes of six-membered rings.7 The D-A reaction of
2,3-dimethylbutadiene (2 mmol) with methyl vinyl ke-
tone (2 mmol) in perfluoromethylcyclohexane (5 ml)
and 1,2-dichloroethane (5 ml) was carried out at 35°C
for 8 h in the presence of a catalytic amount (5 mol%)
of scandium complex. Then, the heterogeneous two
phases were separated to give acetylcyclohexene in
good isolated yield (Scheme 3). Scandium methide com-
plex was completely (>99.9%) recovered and reused in
the recyclable fluorous immobilized phase (Table 3).
We then conducted the reaction and separation in the
bi-phase (FBC) mode of perfluoromethylcyclohexane/
toluene. The reaction of cyclohexanol (2 mmol) with
Table 2. Esterification in two phases catalyzed by
Yb[C(SO2C8F17)3]3 or Sc[C(SO2C8F17)3]3
Figure 2. Before (15°C), during (40°C) and after (3 min,
15°C) the reaction.
Cyclea
% Yieldb
Sc[C(SO2C8F17)3]3
Table 1. Esterification in homogeneous phase catalyzed by
Yb[C(SO2C8F17)3]3 or Sc[C(SO2C8F17)3]3
Yb[C(SO2C8F17)3]3
1
2
3
4
5
99
99
% Yielda
99 (96)c
98
100 (98)c
99
Yb[C(SO2C8F17)3]3
100
Sc[C(SO2C8F17)3]3
100 (95)b
99
99
99 (96)c
100 (98)c
a The catalyst in the lower phase was recycled.
a Calculated by GC analysis using n-nonane as an internal standard.
b Calculated by GC analysis using n-nonane as an internal standard.
c Values in parentheses refer to the isolated yields.
b Values in parentheses refer to the isolated yields.

K. Mikami et al. / Tetrahedron Letters 42 (2001) 289–292
Table 4. Friedel–Crafts acylation reaction catalyzed by
291
Sc[C(SO2C8F17)3]3
Cyclea
% Yieldb
1
2
3
4
94 (87)c
93
93
92
Scheme 3.
a The catalyst in the lower phase was recycled.
b Calculated by GC analysis using n-decane as an internal standard.
c Values in parentheses refer to the isolated yield.
Table 3. Diels–Alder reaction catalyzed by
Sc[C(SO2C8F17)3]3
Cyclea
% Yieldb
1
2
3
4
95
94 (91)c
95
95 (92)c
a The catalyst in the lower phase was recycled.
b Calculated by GC analysis using n-nonane as an internal standard.
c Values in parentheses refer to the isolated yields.
Scheme 5.
Table 5. Mukaiyama-Aldol reaction catalyzed by
Yb[C(SO2C8F17)3]3 or Sc[C(SO2C8F17)3]3
Cyclea
% Yieldb
Sc[C(SO2C8F17)3]3
Scheme 4.
Yb[C(SO2C8F17)3]3
1
2
3
84
85
83
88 (84)c
88
86
The Friedel–Crafts (F-C) reactions constitute one of
the most useful CCF processes in organic synthesis.8
The F-C acylation reaction of anisole (2 mmol) with
acetic anhydride (4 mmol) was also carried out in the
presence of a catalytic amount (10 mol%) of the lan-
thanide complexes in perfluoromethylcyclohexane (6
ml) and 1,2-dichloroethane (6 ml) at 70°C for 6 h. The
aromatic ketone product was obtained in good isolated
yields (Scheme 4). Scandium complex was completely
(99.8%) recovered and reused in the fluorous phase
(Table 4).
a The catalyst in the lower phase was recycled.
b Calculated by GC analysis using n-nonane as an internal standard.
c Values in parentheses refer to the isolated yield.
sulfonyl)methide ponytails without any hydrocarbon
spacer.11
References
The Mukaiyama-aldol reaction is a synthetically and
biologically important CCF process.9 The aldol reac-
tion of benzaldehyde (2 mmol) with trimethylsilyl enol
ether derived from methyl 2-methylpropanoate (2.1
mmol) was completed within 15 min even in the pres-
ence of only 1 mol% of the lanthanide complexes in
perfluoromethylcyclohexane (6 ml) and toluene (6 ml)
at 40°C (Scheme 5). The aldol product was obtained in
good isolated yields. Lanthanide methide complex was
completely (99.8%) recovered and reused (Table 5).
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In summary, we have disclosed lanthanide tris(per-
fluorooctanesulfonyl)methide complexes10 as extremely
efficient Lewis acid catalysts in the immobilized and
recyclable fluorous phases by virtue of the powerful
electron-withdrawing effect of the tris(perfluorooctane-

292
K. Mikami et al. / Tetrahedron Letters 42 (2001) 289–292
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82% conversion at 110°C for 2 h using ytterbium bis(per-
fluorohexanesulfonyl)perfluorooctanesulfonylmethide (10
mol%), anisole (1 mmol), and acetic anhydride (2 mmol)
in perfluoromethyldecaline (1 ml). The homogeneous mix-
ture was diluted with CH2Cl2 (5 ml) and then the layers
were separated. The organics were extracted three times
with hot (85°C) perfluoromethyldecaline (3×5 ml) to give
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acid by DG=7.7 kcal/mol and tris(trifluoromethanesul-
fonyl)methane is stronger than bis(trifluoromethanesul-
fonyl)amine by DG=2.8 kcal/mol. They have also
reported that bis(perfluorobutanesulfonyl)amine is
stronger than bis(trifluoromethanesulfonyl)amine by
DG=7.7 kcal/mol: J. Am. Chem. Soc. 1994, 116, 3047.
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11. Scandium
and
ytterbium
bis(perfluorooctanesul-
fonyl)amide catalysts can also be used effectively in this
fluorous system. Unfortunately, tris(perfluorobutanesul-
fonyl)methide is too short hair to render the complexes in
fluorous media.
.