Lewis acid catalysis in supercritical carbon dioxide. Use of scandium tris(heptadecafluorooctanesulfonate) as a Lewis acid catalyst in Diels-Alder and aza Diels-Alder reactions

Article abstract of DOI:10.1246/cl.2000.178

Diels-Alder reactions of carbonyl dienophiles with dienes and aza Diels-Alder reactions of imines with a diene have been successfully carried out using scandium tris(heptadecafluorooctanesulfonate) (Sc(OSO₂C₈F₁₇)₃) as a Lewis acid catalyst in supercritical carbon dioxide (scCO₂). It was revealed that the length of perfluorocarbon chains of the scandium catalyst was an essential factor for the catalytic activity in scCO₂.

Full text of DOI:10.1246/cl.2000.178

178
Chemistry Letters 2000
Lewis Acid Catalysis in Supercritical Carbon Dioxide.
Use of Scandium Tris(heptadecafluorooctanesulfonate) as a Lewis Acid Catalyst
in Diels-Alder and Aza Diels-Alder Reactions
−
Jun-ichi Matsuo, Takehiro Tsuchiya, Kazunori Odashima, and Shu Kobayashi*
Graduate School of Pharmaceutical Sciences, The University of Tokyo, CREST, Japan Science and Technology Corporation (JST),
Hongo, Bunkyo-ku, Tokyo 113-0033
(Received November 10, 1999; CL-990967)
Diels-Alder reactions of carbonyl dienophiles with dienes
and aza Diels-Alder reactions of imines with a diene have been
successfully carried out using scandium tris(heptadecafluo-
rooctanesulfonate) (Sc(OSO2C8F17)3) as a Lewis acid catalyst
in supercritical carbon dioxide (scCO2). It was revealed that
the length of perfluorocarbon chains of the scandium catalyst
was an essential factor for the catalytic activity in scCO2.
Lewis acid-catalyzed reactions are now of great interest
because they proceed under mild conditions to achieve unique
reactivity and selectivity.1 While various types of Lewis acids
have been employed in many synthetic processes, the reactions
are usually carried out in anhydrous aprotic organic solvents
such as dichloromethane. On the other hand, we have focused
on use of Lewis acid catalysts in water.2 Water is no doubt a
cheap, safe, and environmentally friendly solvent. While most
Lewis acids are believed to be decomposed or deactivated in
the presence of even a small amount of water, we have recently
found that rare earth compounds3 and some other metal salts4
are stable and work as Lewis acid catalysts in water. In these
reactions, although water works as a Lewis base to coordinate
to the Lewis acid, the coordination occurs under equilibrium
conditions and Lewis acid catalysis has been performed effi-
ciently in such media. Similarly, it was expected that such
Lewis acids would work well in supercritical carbon dioxide
(scCO2), which has also been regarded as a desirable substitute
for some toxic organic solvents to accomplish benign chemical
reactions.5 In this paper, we report our preliminary results on
Lewis acid catalysis in scCO2, especially focused on Diels-
Alder reactions and aza Diels-Alder reactions using scandium
tris(heptadecafluorooctanesulfonate) as a Lewis acid catalyst.6
Diels-Alder reactions provide one of the most useful meth-
ods for the preparation of cyclic compounds with high stereose-
lectivities.7 Recently, although these reactions in scCO2 have
been investigated, major interest has been directed to the stereos-
electivity, and synthetic applications are limited because the
reactions proceed sluggishly in scCO2 in many cases.8 In order
to overcome the low reactivity in scCO2, we decided to examine
use of Lewis acids in Diels-Alder reactions in scCO2. It was
also anticipated that stereoselectivities would be improved by
using Lewis acids. We first examined the Diels-Alder reaction
between methylvinylketone (MVK) and isoprene as a model and
using rare earth salts as Lewis acids. One problem incurred in
using Lewis acids in scCO2 was the low solubility of most
Lewis acids. Since the use of perfluoroalkylated groups was
known to increase solubility, we decided to employ scandium
perfluoroalkanesulfonates. In our first experiments, MVK and
isoprene were introduced by the Feed Line method described by
Danheiser et al.8a The effect of perfluoroalkyl substituents of
scandium catalysts is shown in Table 1. Among the scandium
10
compounds we tested,9 Sc(OSO2C8F17)3 gave the best result.
These results showed that the scandium catalysts having longer
perfluoroalkyl chains gave high activity and catalyzed the Diels-
Alder reaction more efficiently.11 It was observed that
Sc(OSO2C8F17)3 was dissolved in scCO2 to form a homoge-
neous solution, while Sc(OTf)3 was not completely soluble
under the same conditions.12 In addition, it is noted that the
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
179
other Lewis acid-catalyzed reactions in scCO2.
We are grateful to Dr. Toshiyasu Sakakura (National
Institute of Materials and Chemical Research) for his valuable
advice in setting up and designing experimental apparatus.
References and Notes
1
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“Lanthanides: Chemistry and Use in Organic Synthesis,” ed by S.
Kobayashi, Springer, Berlin (1999).
2
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regioselectivity (1/2) has been improved by using Lewis acid
catalysts.13
Several examples of the Diels-Alder reactions using
Sc(OSO2C8F17)3 as a catalyst are shown in Table 2. In all
cases, the reactions proceeded smoothly to afford the desired
adducts in high yields with high selectivities. Even when 1
mol% of Sc(OSO2C8F17)3 was used, the Lewis acid catalysis
was successfully carried out in scCO2 to give the cycloaddition
product in high yield (entry 2), and these results indicate the
synthetic utility of the present Diels-Alder reactions in scCO2
using Sc(OSO2C8F17)3 as a catalyst.
Moreover, it was revealed that aza Diels-Alder reactions
of imines with Danishefsky’s diene (3)14 proceeded cleanly in
scCO2 using Sc(OSO2C8F17)3.12,15 Not only the imine derived
from benzaldehyde but also the imines derived from 2-
pyridinecarboxaldehyde and cyclohexanecarboxaldehyde
reacted with 3 to afford the corresponding piperidone deriva-
tives in good to excellent yields (Table 3). It is noted that 81%
yield of the adduct was obtained even when 0.5 mol% of the
catalyst was employed.
A typical experimental procedure is described for the reac-
tion of MVK with isoprene (Table 1, entry 4): Under an argon
atmosphere, Sc(OSO2C8F17)3 (138 mg, 0.089 mmol) was
placed in a 10-mL reactor along with a magnetic bar, and pre-
cooled low-pressure CO2 was introduced to the sealed reactor
at room temperature. After a mixture of MVK (131 mg, 1.87
mmol) and isoprene (0.56 mL, 5.60 mmol) was added through
the Feed Line, the reactor was pressurized to 150 atm at 50 °C.
After 24 h, the pressure in the reactor was slowly released, the
reactor was opened under 0 °C, and H2O (10 mL) was added to
the residue. The mixture was extracted with Et2O, and the
chemical yield (74%) and the stereoselectivity (1/2 = 94/6)
were determined by GC using an internal standard.
6
7
8
During our studies, Rayner et al. reported Sc(OTf)3-catalyzed Diels-
Alder reactions in scCO2, and discussion on diastereoselectivity was
made. R. S. Oakes, T. J. Heppenstall, N. Shezad, A. A. Clifford,
and C. M. Rayner, Chem. Commun., 1999, 1459.
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9
We used three scandium catalysts, Sc(OTf)3, Sc(OSO2C4F9)3,
Sc(OSO2C8F17)3, which could be prepared easily from Sc2O3 and
the corresponding acids (Sc(OTf)3, Sc(OSO2C4F9)3) or ScCl3 and
the corresponding acid (Sc(OSO2C8F17)3).
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12 It was confirmed that all substrates were dissolved in scCO2 to form
homogeneous solutions.
In summary, we have developed Diels-Alder reactions of
carbonyl dienophiles with dienes and aza Diels-Alder reactions
of imines with a diene using Sc(OSO2C8F17)3 as a Lewis acid
catalyst in scCO2. The synthetic utility of these reactions has
been demonstrated, and effective Lewis acids and catalysis in
scCO2 have been proposed. Use of the water-stable scandium
catalyst as well as scCO2 as a solvent is expected to lead to
benign chemical processes. We are currently investigating
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