Cobalt Nanoparticles on Charcoal: A Versatile Catalyst in the Pauson-Khand Reaction, Hydrogenation, and the Reductive Pauson-Khand Reaction

Article abstract of DOI:10.1021/ol0268889

(Matrix Presented) Dispersions of nanometer-sized cobalt particles with very high stability were prepared in charcoal and analyzed by electron microscopy and X-ray analysis. The resulting cobalt nanoparticles on charcoal (CNC) were successfully used as a catalyst for the carbonylative cycloaddition of alkyne, alkene, and carbon monoxide (Pauson-Khand reaction), hydrogenation, and the reductive Pauson-Khand reaction.

Full text of DOI:10.1021/ol0268889

ORGANIC
LETTERS
2002
Vol. 4, No. 22
3983-3986
Cobalt Nanoparticles on Charcoal: A
Versatile Catalyst in the Pauson−Khand
Reaction, Hydrogenation, and the
Reductive Pauson−Khand Reaction
Seung Uk Son, Kang Hyun Park, and Young Keun Chung*
School of Chemistry and Center for Molecular Catalysis, Seoul National UniVersity,
Seoul 151-747, Korea
ykchung@plaza.snu.ac.kr
Received September 12, 2002
ABSTRACT
Dispersions of nanometer-sized cobalt particles with very high stability were prepared in charcoal and analyzed by electron microscopy and
X-ray analysis. The resulting cobalt nanoparticles on charcoal (CNC) were successfully used as a catalyst for the carbonylative cycloaddition
of alkyne, alkene, and carbon monoxide (Pauson−Khand reaction), hydrogenation, and the reductive Pauson−Khand reaction.
The preparation and characterization of metal nanoparticles
is currently of considerable practical and theoretical interest
because such materials fall into an intermediate state of
matter between the molecular and the bulk and frequently
display unusual physical and chemical properties.1 For noble
metal colloids, catalytic applications are considered,2 since
a unique combination of reactivity, stability, and selectivity
is expected. During the past decade, transition metal colloids
were revealed as very efficient catalysts for various reactions,
such as hydrogenation,3 oxidation,4 hydrosilyation,5 coupling
reaction (Heck reaction and Suzuki reaction),6 and some
photocatalytic reactions.7 Recently, we reported8 the catalytic
use of colloidal cobalt nanoparticles for the Pauson-Khand
reactions. Although they are quite effective, they suffer from
relatively lower stability and are more inconvenient to use
than conventional heterogeneous catalysts such as Co on
charcoal and Co on silica.9 To overcome the disadvantages
of nanoparticles in catalysis, we researched how to combine
the merits of conventional heterogeneous catalysts with the
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Fichtner, P. F. T.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228. Schulz,
J.; Roucoux, A.; Patin, H. Chem. Eur. J. 2000, 6, 618. (c) Johnson, B. F.
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182-183, 89.
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Galow, T. H.; Drechsler, U.; Hanson, J. A.; Rotello, V. M. Chem. Commun.
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EnViron. 2002, 37, 331. (b) Maira, A. J.; Coronado, J. M.; Augugliaro, V.;
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10.1021/ol0268889 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/10/2002

high catalytic activity of cobalt nanoparticles. We chose
charcoal as a support and prepared cobalt nanoparticles on
charcoal (CNC). Herein we report the first synthesis and
characterization of CNC, the CNC-catalyzed Pauson-Khand
reaction and hydrogenation, and the first use of CNC as a
bifunctional catalyst in a sequential reaction of the Pauson-
Khand reaction and hydrogenation in a one-pot reaction.
Synthesis of Cobalt Nanoparticles on Charcoal (CNC).
Surfactant-stabilized nanoparticles of cobalt were prepared
cobalt nanoparticles did not retard the catalytic activity of
cobalt nanoparticles.
Study on the Use of CNC as Catalyst in the Pauson-
Khand Reaction. Using CNC as a catalyst, we investigated
the intramolecular Pauson-Khand reaction as a test reaction
(Scheme 1 and Table 1).
Scheme 1
by the thermal reduction of dicobalt octacarbonyl.10
A
transmission electron microscopic (TEM) image11 confirms
that the particles are well separated and that they are nearly
monodisperse, having a mean diameter of 12 nm. Refluxing
the synthesized cobalt nanoparticles with dried charcoal
yielded cobalt nanoparticles on charcoal (CNC). A charac-
terization of CNC was carried out by using XRD and
HRTEM. The HRTEM study11 shows that the mean size of
nanopartciles is 12 nm, ranging from 7 to 18 nm. The crystal
packing of CNC is an epsilon mode.12 CNC is quite stable
even in air for several months.
As Table 1 shows, the optimum conditions were 5 atm of
CO at 130 °C. When either the reaction temperature was
decreased to 120 °C or the CO pressure was lowered, the
catalytic activity was found to decrease significantly (entries
5 and 7). Thus, to preserve a high catalytic activity, the
reaction temperature and CO pressure have to be maintained
at a minimum of 130 °C and 5 atm of CO.
To check the recyclability, CNC was separated and reused
several times. The results shown in Table 1 confirm that the
catalyst maintained its high activity even after being recycled
five times.
Comparative Study of Cobalt Species in the Pauson-
Khand Reaction. First, we examined the Pauson-Khand
reaction using CNC and other Co catalysts (Table 1).
Table 1. Pauson-Khand Reactions with CNC and Other
Cobalt Catalystsa
Table 2. Pauson-Khand Reaction with Various Substratesa
CO
yield
(%)b
entry
catalyst
(atm)
1
2
3
4
5
6
7e
8
9
Co2(CO)8c
5
5
5
1
3
5
5
5
5
5
5
79
97
nr
0
25
98
62
95
96
98
98
colloidal cobalt nanoparticlesd
Co/Cd
CNC
CNC
CNC
CNC
recovered form #6
recovered form #8
recovered form #9
recovered form #10
10
11
a Reaction conditions: CNC (0.1 g, 12 wt %), enyne (0.48 nmol), 130
°C, 18 h. b Isolated yield. c Amount used ) 0.10 mmol. d Amount of cobalt
used ) 12 mg. e Temperature ) 120 °C.
The comparison of different Co catalysts revealed con-
siderably different reactivities. No reaction was observed with
cobalt on charcoal under 5 atm of CO. Thus, high pressures
were necessary with cobalt on charcoal. Under 5 atm of CO
with Co2(CO)8, a high yield (79%) was observed. Under the
same conditions, cobalt nanoparticles also gave product in
an almost quantitative yield (97%). When the same Pauson-
Khand reaction was carried out using CNC as a catalyst,
98% of the product was isolated. Thus, immobilization of
a Reaction conditions: CNC (0.1 g, 12 wt %), 0.48 mmol of enyne,THF,
130 °C, 18 h. b Isolated yield.
(10) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291,
2115.
To demonstrate the versatility of CNC, we screened
various substrates for the Pauson-Khand reaction and the
results are shown in Table 2. While the normal intramolecular
(11) See Supporting Information.
(12) Dinega, D. P.; Bawendi, M. G. Angew. Chem., Int. Ed. 1999, 38,
1788.
3984
Org. Lett., Vol. 4, No. 22, 2002

Pauson-Khand reactions proceeded smoothly under 5 atm
of CO (entries 1-3), internal enyne (entry 4) and heteroatom-
bridged enynes (entries 5 and 6) required a higher CO
pressure of 10 atm. CNC is also effective for intermolecular
Pauson-Khand reaction (entries 7 and 8) under 15 atm of
CO. When the same reactions (entries 7 and 8) were carried
out using cobalt nanoparticles as catalysts, the pressure of
CO was 10 atm. Thus, the catalytic activity of CNC is
slightly lower than that of the colloidal cobalt nanoparticles
for the intermolecular Pauson-Khand reaction.
Cobalt Nanoparticles on Charcoal as a Hydrogenation
Catalyst. Complexes of cobalt are known to be hydrogena-
tion catalysts but are mostly selective for conjugated
systems.13 Co2(CO)8 and derivatives are known as hydro-
genation catalysts for alkenes under specific conditions,
although they are decomposed to metal under a pure H2
atmosphere.
In the previous section, we have demonstrated that CNC
is a quite effective catalyst for carbonylative cycloaddition
reaction. This result suggests that treatment of CNC with
carbon monoxide leads to in situ generation of cobalt
carbonyl species or adsorption of carbon monoxide on the
surface of CNC. Thus, we envision that CNC can be used
as a catalyst for hydrogenation of alkene and other related
compounds as Co2(CO)8 does.
80 °C for N-benzylideneaniline, and >60 °C for cyclopenta-
diene dimer. CNC is not effective for the hydrogenation of
acetophenone and the debenzylation of benzylphenyl ether.
The hydrogenation of R,â-unsaturated enone by CNC implies
the possibility of CNC as a bifunctional catalyst in sequential
reactions of the Pauson-Khand reaction and hydrogenation.
With these observations, we studied the use of CNC as
bifunctional catalysts in the sequential carbonylative cyclo-
addition and hydrogenation of enye or alkyne and alkene in
one pot. Although the sequential reaction of Pauson-Khand
reaction and hydrogenation is a useful methodology to the
synthesis of natural products,16 the known reductive Pauson-
Khand reactions use stoichiometric or substoichiometric
amounts of cobalt complexes.17
Cobalt Nanoparticles on Charcoal as Bifunctional
Catalysts: The Reductive Pauson-Khand Reaction in
One Pot. Recently, the use of one particular catalyst in a
tandem reaction or successive reaction has attracted much
attention due to experimental simplicity and economical
factors. Multifunctional catalysis involves a combination of
two or more reactions requiring different types of catalytic
sites. Recently, the use of the bifunctional or multifunctional
catalysts has been reported in the synthesis of complex
organic molecules.18
To use CNC as a bifunctional catalysts in the Pauson-
Khand reaction and hydrogenation in a one-pot, tandem
reaction, CNC had to satisfy the following conditions: first,
carbon monoxide or hydrogen should not interfere with the
catalytic activity of CNC for each reaction, and second,
hydrogenation of substrates should not take place before a
Pauson-Khand reaction of the substrate occurs. We first
tested the catalytic activity of CNC for the hydrogenation
of a Pauson-Khand reaction product (Table 4). When the
We examined various substrates for hydrogenation reac-
tions, and the results are shown in Table 3. CNC is an
Table 3. Hydrogenation of Olefins with Various Functional
Groupsa
Table 4. Hydrogenation of Enones under Various Condtiona
temp CO pressure H2 pressure time yield
entry catalyst (°C)
(atm)
(atm)
(h)a (%)b
1
2
3
4
5
CNC
CNC
CNC
CNC
CNC
130
130
130
130
130
1
5
5
5
5
18
18
18
18
18
90c
88d
93
a Reaction conditions: CNC (0.1 g, 12 wt %), 0.48 mmol of substrate,
5
10
20
THF, 130 °C, 18 h, 5 atm of H2. b Isolated yield.
95
a Reaction conditions: CNC (0.1 g, 12 wt %), 0.48 mmol of sub-
strate,THF, 130 °C, 18 h. b Isolated yield. c Reactant (4 mol %) was
recovered. d Reactant (3 mol %) was recovered.
effective catalyst for the hydrogenation of R,â-unsaturated
enone, dimethyl itaconate, imine, and olefin. As expected,
carbon-carbon and carbon-nitrogen double bonds were
hydrogenated over carbonyl groups.15 The reaction temper-
ature was slightly dependent upon the substrate itself: 130
°C for R,â-unsaturated enone, 100 °C for dimethyl itaconate,
pressure of hydrogen was 1 atm, no hydrogenation was
observed. However, under 5 atm of H2, 90% of the
hydrogenated product was obtained with 4% recovery of the
reactant. Interestingly, the presence of carbon monoxide did
not interfere with the hydrogenation.
(13) Handbook of Coordination. In Catalysis in Organic Chemistry;
Chaloner, P. A., Ed.; Butterworth: London, 1986; p 38.
Org. Lett., Vol. 4, No. 22, 2002
3985

Pauson-Khand reaction and hydrogenation in a one-pot
reaction (Table 5). Under our reaction conditions, no
hydrogenation of enyne substrates (entries 1-6) was ob-
served before the Pauson-Khand reaction occurred. This
sequential reaction provides a straightforward approach to
the bicyclic and tricyclic ketone frames from enynes. The
structure in entry 2 was previously prepared by reductive
Pauson-Khand cyclization.18 The structures in entries 4 and
5 in Table 5 appear as core skeletons in many natural
syntheses such as those of the linear and angularly fused
triquinane sesquiterpenes. Thus, CNC is a quite effective
catalyst in the reductive Pauson-Khand reaction.
Table 5. Sequential Pauson-Khand Reaction and
Hydrogenationa
We have demonstrated that CNC has quite a high activity
in Pauson-Khand reactions and can be used as a bifunctional
catalyst in the Pauson-Khand reaction and hydrogenation
in a one-pot reaction. CNC is easily recovered and reused
many times without losing catalytic activity and is quite
stable even in the air for several months. The use of CNC
can be extended to other catalytic systems, and further work
in this direction is in progress.
Acknowledgment. Y.K.C. thanks the KOSEF (1999-1-
122-001-5) and the KOSEF through the Center for Molecular
Catalysis at Seoul National University for financial support,
and K.H.P. and S.U.S. acknowledge the receipt of the Brain
Korea 21 fellowship. The authors are indebted to Hyun-mi
Kim in the Research Institute of Advanced Materials.
Supporting Information Available: Experimental details
and characterization of compounds entries 5 and 6 in Table
6 and TEM images. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL0268889
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a Reaction conditions: CNC (0.1 g, 12 wt %), 0.48 mmol of sub-
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Contrary to our expectation, as the pressure of CO
increases, the yield becomes higher. Thus, when the pressures
of H2 and CO were 5 atm and greater than 10 atm,
respectively, the yield was greater than 93%. After much
experimentation, the optimum reaction conditions for the
sequential Pauson-Khand reaction and hydrogenation were
established: 130 °C under 5 atm of CO and 5 atm of H2 for
18 h. We screened various substrates for the sequential
3986
Org. Lett., Vol. 4, No. 22, 2002