Catalytic Pauson-Khand reaction in super critical fluids

Full text of DOI:10.1021/ja972099o

J. Am. Chem. Soc. 1997, 119, 10549-10550
10549
the homogeneous reaction media after completion of the
reaction. The reaction with water soluble ligand bound catalysts
in organic/aqueous phase process8 and with perfluorinated
ligands bound catalysts in the conventional organic solvent and
perfluorinated solvent system are devised for the operational
simplicity.9
Catalytic Pauson-Khand Reaction in Super Critical
Fluids
Nakcheol Jeong,*,† Sung Hee Hwang,† Youn Woo Lee,‡ and
Jong Sung Lim‡
Meantime, the use of supercritical fluids (Scfs) as reaction
media is becoming an alternative for the reactions in which the
previously described options are not suitable. The projected
advantages of the reactions in supercritical fluids are the
increased reaction rates and selectivities resulting from the high
solubility of reactant gases, rapid diffusion of solutes, weakening
of the solvation around reacting species, and the local clustering
of reactants or solvents.10 It is also interesting to note, in a
practical sense, that those fluids are easily recycled and allow
the separation of dissolved compounds by a gradual release of
pressure. Sequential and selective precipitations of the catalyst
and product would be possible.
Several recent reports have shown that supercritical CO2 (sc
CO2) can replace the conventional organic solvents in various
transformations such as radical reactions,11 Diels-Alder reac-
tion,12 polymerization,13 homogeneous catalytic hydrocarboxy-
lation,14 and asymmetric hydrogenations.15
Herein, we would like to report our preliminary study of the
first catalytic Pauson-Khand reaction in supercritical fluids.
The catalytic process by dicobalt octacarbonyl had been well
conceived since the discovery of the reaction, but it was in quite
recent years before it was realized.16 To our experiences in
this field, the control of aggregation status of the catalytic active
species played a critical role. We hoped that the catalytic metals
should be well dispersed in Scfs and the chances of the
aggregation of metals would be reduced substantially.
Our initial studies using dicobalt octacarbonyl as a catalyst
were mainly focused in sc CO2 since there was a report dealing
with a hydroformylation of olefin with a catalytic amount of
dicobalt octacarbonyl.16
Catalytic intramolecular Pauson-Khand reactions were per-
formed first in sc CO2 by charging a cylindrical stainless steel
reactor (80 mL capacity) with a catalyst and enynes followed
by pressurization properly with carbon monoxide and carbon
dioxide. A red homogeneous supercritical phase was obtained
upon warming the mixture to 40 °C, and the reaction mixture
was further heated up to an appropriate temperature, and the
reaction was allowed to proceed.
Hanhyo Institutes of Technology 461-6 Chonmingdong
Yusongku, Taejon, 305-390, Korea
Korea Institute of Science and Technology
P.O. Box 131, Cheongryangri, Seoul, 130-650, Korea
ReceiVed June 25, 1997
Cocyclization of alkynes with alkenes and carbon monoxide
by cobalt leading to cyclopentenones (known as Pauson-Khand
reaction) has been accepted as one of the most powerful tools
in the synthesis of cyclopentenones.1 Recent developments in
Pauson-Khand reaction, especially in the 1990s, have been
quite impressive. These include findings of promoters such as
silica gel,2 tertiary amine N-oxide,3 and DMSO4 for the
stoichiometric reaction, enantioselective reactions,5 and catalytic
versions of the reaction.6 Many variations employing other
metals are also reported.7 Despite the successful progress and
potential implications as an industrial process of this reaction,
the use of this remarkable reaction has been limited only to the
laboratory application.
This limitation is mainly attributed to the rather low turnover
number and turnover frequency of the catalytic reaction. More
severe limitations are associated with the practical operational
difficulties such as removal of the catalyst and solvents from
† Hanhyo Institutes of Technology.
‡ Korea Institute of Science and Technology.
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I. U. Ann. N. Y. Acad. Sci. 1977, 295, 2. (c) Pauson, P. L. Tetrahedron
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After much trial experimentations, the range of the required
minimal amount of the catalyst and various parameters of the
reaction were roughly defined. Table 1 lists the trials to
optimize the reaction condition. The required amount of catalyst
is about 2-5 mol %6 but it has not been fully optimized. The
reactions under rather low carbon monoxide pressure (1-5 atm)
frequently led to incompletion of the reaction or low chemical
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S0002-7863(97)02099-4 CCC: $14.00 © 1997 American Chemical Society

10550 J. Am. Chem. Soc., Vol. 119, No. 43, 1997
Communications to the Editor
Table 1. Trials To Optimize the PKR Condition in Supercritical CO2
condition
yield
entry
Co2(CO)8 (mol %)
PCO (atm)
PCO2 (atm)
T (°C)
t (h)
final P (atm)
1 (%)
2 (%)
1
2
3
4
5
0.2
1
2.5
2.5
2.5
6
15
15
1
86 at 34 °C
100 at 36 °C
105 at 34 °C
86 at 34 °C
112 at 37 °C
48
84
69
60
90
70
24
62
24
24
96
177
180
112
198
77
17
0
0
0
0.5
66
85
64
82
30
Table 2. Intramolecular Catalytic PKR in Supercritical CO2
Table 3. Intermolecular Catalytic PKR in Supercritical CO2
also served as a good substrate albeit an eliminated product was
obtained together with the desired one in the presence of a
leaving group (entry 3, Table 2).6
Allyl propargyl ether also produced the corresponding product
in excellent yield (entry 4, Table 2), but tosyl and cbz protected
allyl propargyl amine, which were among the best substrates
in the conventional catalytic reaction,6b,c remained unreacted
under this condition since they are not reasonably soluble in sc
CO2 (entry 5, Table 2).
An intermolecular reaction also worked well under this
condition. Phenyl acetylene can couple with norbonadiene
(excess of) to give the bicyclic compound in 87% (entry 1, Table
3). Biscocyclization of diyne proceeded nicely to furnish the
bis(bicyclicpentenone) in high yield (entry 2, Table 3).
These preliminary studies demonstrate the feasibility of
conducting transition metal mediated transformations with high
efficiency in Scfs. Especially reactions employing metal
carbonyl under carbon monoxide pressure will be perfectly fit
in this special phase due to their favorable solubility profiles.
We are further optimizing the related reactions in SCFs to define
the potential of these novel media and devise environmentally
friendly processes.
yield (entries 1, 2, and 4, Table 1). Higher reaction temperature
(90-100 °C) beyond critical temperature (31 °C at 72.9 atm)
is necessary for the completion in a reasonable reaction time
(<24 h). This reaction mixture requires higher carbon monoxide
pressure (15-30 atm) to make the catalytic metal species as
intact as possible. Otherwise, the metal catalyst was deactivated
by that forming of an unidentified white precipitate.
The scope of the reaction in terms of the substrates was
determined by employing a standard condition as follows: 2.5
mol % dicobalt octacarbonyl along with 30 atm of CO (at 23
°C) in CO2 (final pressure is 110-120 atm at 36-9 °C) was
heated at 90-95 °C for 24 h, and the results are summarized in
Tables 2 and 3.
Supporting Information Available: Representative procedures and
schematic diagram of equipment (3 pages). See any current masthead
page for ordering and Internet acess instructions.
The reaction proceeded pretty well regardless of the substitu-
tion pattern of acetylene. Disubstituted acetylene gave a little
better chemical yield (entry 2, Table 2). 1,1′-Disubstituted olefin
JA972099O