The first intramolecular pauson-khand reaction in water using aqueous colloidal cobalt nanoparticles as catalysts

Article abstract of DOI:10.1021/ol017043k

Chemical equation presented The first intramolecular Pauson-Khand reaction in water was successfully carried out by using aqueous colloidal cobalt nanoparticles as catalysts.

Full text of DOI:10.1021/ol017043k

ORGANIC
LETTERS
2002
Vol. 4, No. 2
277-279
The First Intramolecular Pauson−Khand
Reaction in Water Using Aqueous
Colloidal Cobalt Nanoparticles as
Catalysts
,†
Seung Uk Son,† Sang Ick Lee,† Young Keun Chung,* Sang-Wook Kim,‡ and
,‡
Taeghwan Hyeon*
School of Chemistry and Center for Molecular Catalysis, Seoul National UniVersity,
Seoul 151-747, Korea, and School of Chemical Engineering and Institute of Chemical
Processes, Seoul National UniVersity, Seoul 151-742, Korea
ykchung@plaza.snu.ac.kr
Received November 13, 2001
ABSTRACT
The first intramolecular Pauson−Khand reaction in water was successfully carried out by using aqueous colloidal cobalt nanoparticles as
catalysts.
Extensive research on new and effective catalytic Pauson-
Khand reactions has continued to find ever more efficient
methods for the preparation of cyclopentenone compounds.1
Several excellent homogeneous catalysts have been devel-
oped for the reactions. Nevertheless, one serious problem in
homogeneous catalysis is separation of the reaction products
from the catalyst. To solve this problem, we recently
developed2 novel heterogeneous Pauson-Khand catalysts
based on metallic cobalt supported on either mesoporous
silicas or charcoal.
conventional organic solvents by the supercritical fluids CO2
or ethylene was demonstrated by Jeong and co-workers.3
However, reaction conditions have not yet seen much
improvement and are still quite harsh, requiring a long
reaction time. Recently, as a consequence of the increasing
demand for efficient, safe, and environmentally friendly
processes, the application of homogeneous catalysts in
aqueous solution and on liquid supports has attracted
tremendous attention for many catalytic reactions.4 These
aqueous catalytic systems can offer a great opportunity for
practicing green chemistry. However, no catalytic systems
that enable the use of water as a solvent in the Pauson-
Khand reaction have been developed yet. Furthermore, it has
Although heterogenized catalysts can solve the separation
problem, they still require an organic solvent as a reaction
medium. Recently, the possibility of the substitution of
† School of Chemistry and Center for Molecular Catalysis.
‡ School of Chemical Engineering and Institute of Chemical Processes.
(1) For recent reviews, see: (a) Brummond, K. M.; Kent, J. L.
Tetrahedron 2000, 56, 3263. (b) Fletcher, A. J.; Christie, S. D. R. J. Chem.
Soc., Perkin Trans. 1 2000, 1657. (c) Chung, Y. K. Coord. Chem. ReV.
1999, 188, 297. (d) Jeong, N. In Transition Metals for Organic Synthesis;
Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, 1998; p 560. (e) Geis,
O.; Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1998, 37, 911.
(2) (a) Kim, S.-W.; Son, S. U.; Lee, S.-I.; Hyeon, T.; Chung, Y. K. J.
Am. Chem. Soc. 2000, 122, 1550. (b) Son, S. U.; Lee, S.-I.; Chung, Y. K.
Angew. Chem., Int. Ed. 2000, 39, 4158.
(3) (a) Jeong, N.; Hwang, S. H.; Lee, Y. W.; Lim, J. S. J. Am. Chem.
Soc. 1997, 119, 10549. (b) Jeong, N.; Hwang, S. H. Angew. Chem., Int.
Ed. 2000, 39, 636.
(4) (a) Aqueous-Phase Organometallic Catalysis; Cornils, B., Herrman,
W. A., Eds.; Wiley-VCH: Weinheim, 1998. (b) Katti, K. V.; Gali, H.; Smith,
C. J.; Berning, D. E. Acc. Chem. Res. 1999, 32, 9. (c) Herrman, W. A.;
Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1524. (d)
Fringuelli, F.; Piermatti, O.; Pizzo, F.; Vaccaro, L. Eur. J. Chem. 2001,
439. (e) Genet, J. P.; Savignac, M. J. Organomet. Chem. 1999, 576, 305.
(f) Hanson, B. E. Coord. Chem. ReV. 1999, 185-186, 795.
10.1021/ol017043k CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/20/2001

been expected5 that low-valent cobalt organometallics would
be unstable and become oxidized in the presence of water.
In these circumstances, the development of aqueous colloidal
cobalt nanoparticles6 has attracted our attention. We envision
that the use of aqueous colloidal cobalt nanoparticles should
provide an opportunity to study the Pauson-Khand reaction
in an aqueous solution. Herein we report the first catalytic
intramolecular Pauson-Khand reaction by aqueous colloidal
cobalt nanoparticles in an aqueous solution. This catalytic
system exhibited excellent catalytic performance for intra-
molecular Pauson-Khand reactions and can be recycled.
Aqueous colloidal cobalt nanoparticles were prepared by
reducing an aqueous solution of cobalt acetate containing
sodium dodecyl sulfate (SDS) surfactant.6b,7 The aqueous
colloidal cobalt nanoparticles are stable for a limited time
(2-3 days) in air, quite stable in the absence oxygen, and
easy to handle. The intramolecular Pauson-Khand reaction
of an enyne has been investigated as a test reaction8 (Scheme
1).
Scheme 2
the cobalt nanoparticles. The nonpolar alkyl chains remain
in a nonpolar environment and the hydrocarbon chains can
dissolve the organic substrates. Thus, there was no require-
ment to employ water-soluble substrates. The solubilized CO
molecules might react with the outer surface of cobalt
nanoparticles to generate in situ cobalt carbonyl species,
which can act as catalysts for the carbonylative cycloaddition.
The situation seems to be quite close to that of the Lewis
acid surfactant combined catalysts (LASC) in water.9
Heterogeneous catalysts often suffer extensive leaching
of active metal species during reactions and eventually lose
their catalytic activity. Most organic substances are insoluble
in water, and many reactive substrates and catalysts are
decomposed or deactivated by water. Thus, recovery and
reusability of the catalytic system were tested by carrying
out consecutive cycles (entries 4-7 in Table 1), with the
same catalyst in aqueous solution, carefully separated from
Scheme 1
Table 1 summarizes the results of reactions using cobalt
nanoparticles, cobalt on charcoal, or Co2(CO)8.2 To optimize
the reaction conditions, the pressure of CO and the reaction
temperature were screened. As Table 1 shows, the optimum
reaction conditions were 20 atm of CO at 130 °C. Under
the optimum condition, the yield of the reaction was 92%.
In contrast, the yields for Co2(CO)8 and cobalt on charcoal
were 8% and 24%, respectively. We envision that the
stabilizing SDS surfactant micelles act as nanoreactors to
induce better solubilization of organic reactants and ulti-
mately allow the reactants to interact more intimately with
(5) Sigman, M. S.; Fatland, A. W.; Eaton, B. E. J. Am. Chem. Soc. 1998,
120, 5130.
(6) (a) Ershov, B. G.; Sukhov, N. L.; Janata, E. J. Phys. Chem. B 2000,
104, 6138. (b) Russier, V.; Petit, C.; Legrand, J.; Pileni, M. P. Phys. ReV.
B 2000, 62, 3910.
(7) Cobalt acetate tetrahydrate (1.0 g, 4.0 mmol) and sodium dodecyl
sulfate (SDS) (10.0 g) were dissolved in degassed water at room temperature.
An aqueous solution of NaBH4 (0.4 g, 10 mmol) in 10 mL of water was
added dropwise to the mixture. The color of the solution changed
immediately from pink to black, indicating the formation of colloidal
particles. The resulting solution was used directly in the PK reaction.
(8) Cobalt nanoparticles (23 mg, the amount of calculated Co) in 5 mL
of water and the enyne shown in Scheme 1 (0.10 g, 0.58 mmol) were put
in a high-pressure reactor. The reactor was pressurized with 20 atm of CO
at room temperature and was heated at 130 °C for 12 h. After the reactor
was cooled to room temperature and the excess CO was released, the
reaction mixture was transferred into a separatory funnel. The product was
extracted with diethyl ether (15 mL × 5). The ethereal solution was dried
over anhydrous MgSO4, concentrated, and chromatographed on a silica gel
column, eluting with hexane and diethyl ether (v/v, 2:1). Removal of the
solvent gave the product in 92% yield (0.11 g). When the catalyst was
recycled, excess diethyl ether was added to the reaction mixture after the
reaction. The resulting solution was stirred, and the upper layer was pipetted.
Removal of the solvent gave the product. The remained aqueous solution
was reused for the further catalytic reaction.
Table 1. Pauson-Khand Reaction with Stabilized Cobalt
Nanoparticle in Watera
entry
catalyst
T (°C)
P (atm)
yield (%)b
1
2
3
4
5
6
7
8c
9d
cobalt stabilized
cobalt stabilized
cobalt stabilized
recovered from 2
recovered from 4
recovered from 5
recovered from 6
Co2(CO)8
130
130
110
130
130
130
130
130
130
10
20
20
20
20
20
20
20
20
45
92
10
95
91
96
95
8
Co/C
24
a Reaction conditions: substrate(0.58 mmol), cobalt nanoparticle (23 mg),
130 °C, 20 atm CO, and 12 h. b Isolated yield. c Using 50 mol % Co2(CO)8.
d Using 0.2 g Co/C (13 wt % Co).
(9) Manabe, K.; Mori, Y.; Wakabayashi, T.; Nagayama, S.; Kobayashi,
S. J. Am. Chem. Soc. 2000, 122, 7202.
278
Org. Lett., Vol. 4, No. 2, 2002

Table 2. Pauson-Khand Reaction with Various Substrates in Watera
a Reaction conditions: substrate (0.58 mmol), cobalt nanoparticle (23 mg, the amount of caculated Co), 130 °C, 20 atm CO, and 12 h. b Isolated yield.
the organic phase at the end of each run. The conversion
yields were quite high even at the fourth run. The relative
invariance of the conversion yields means that the catalyst
system is quite stable during the recycling. The catalytic
system can be recycled at least four times without loss of
performance and without producing any waste.
2). Aqueous colloidal cobalt nanoparticles are quite effective
for intramolecular Pauson-Khand reaction.
In conclusion, we have demonstrated that aqueous colloidal
cobalt nanoparticles are quite effective catalysts for many
intramolecular Pauson-Khand reactions in water. The cobalt
nanoparticles can be reused several times without any
apparent loss of activity. Aqueous colloidal cobalt nanopar-
ticles are easily available and can be applied to various types
of catalytic reactions in water. Thus, their use as catalysts
in aqueous media will contribute to progress in chemical
processes.
Various substrates have been successfully used in the
aqueous colloidal cobalt nanoparticles catalyzed Pauson-
Khand reaction. Table 2 summarizes representative examples
of the reactions. When a terminal enyne (entry 1 in Table 2)
was used as a substrate, the corresponding enone and the
reduced ketone were obtained in the ratio 3:1 with a 95%
overall yield. This catalytic reaction was quite effective with
substituted enynes. The heteroatom tethered substrates
(entries 4 and 5 in Table 2) were tested to generate the
corresponding products in high yields.
Acknowledgment. Y.K.C. thanks the Korea Research
Foundation (Grant KRF-2001-015-DS0025) and T.H. thanks
the Brain Korea 21 Program of the Korean Ministry of
Education.
Reaction of endo-cyclic enynes10 produced the corre-
sponding tricyclic enone compounds in high yields (Scheme
OL017043K
H), 3.74 (s, 3 H), 3.70 (s, 3 H), 3.10 (m, 1 H), 2.78 (s, 2 H), 1.95 (m, 2 H),
(10) Procedure of Synthesis of Substrate and Characterization Data
of New Compounds. Substrate A in Scheme 2 [(η3-C3H5PdCl)2] (3 mg,
8.2 × 10-3 mmol), dppe (7 mg, 0.017 mmol), and 5 mL of THF was put
in a 50 mL Schlenk flask. After the solution was stirred for 10 min,
3-acetoxycyclohexene (0.23 g, 1.6 mmol), dimethyl 2-butynylmalonate (0.36
g, 1.9 mmol), N,O-bis(trimethylsilyl)acetamide (0.2 mL), and KOAc (a trace
amount) were added to the solution. After the resulting solution was stirred
for 18 h, low-boiling chemicals were removed by rotary evaporator. The
high-boiling residue was chromatographed on a silica gel column, eluting
with hexane and diethyl ether (v/v, 10:1). Removal of the solvent gave the
1.82 (m, 2 H), 1.74 (s, 3 H), 1.55 (m 1 H), 1.33 (q, 11.0 Hz, 1 H) ppm. 13
C
NMR (CDCl3, 75 MHz): δ 1705, 170.2, 128.6, 127.6, 78.6, 73.7, 60.5,
52.3, 52.0, 38.6, 24.7, 24.1, 22.8, 22.2, 3.38 ppm, HRMS (M+) calcd
264.1356, obsd 264.1364. Substrate B in Scheme 2. The same procedure
as the synthesis above was applied; yield 94%. 1H NMR (CDCl3, 300
MHz): δ 5.80 (m, 1 H), 5.70 (dd, 4.7, 11.0 Hz, 1 H), 3.73 (s, 6 H), 3.16
(dd, 3.8, 10.0 Hz, 1 H), 2.77 (q, 2.6 Hz, 2 H), 2.15 (m, 2 H), 1.86 (m, 1 H),
1.83 (dt, 4.1, 13.0 Hz, 1 H), 1.74 (t, 2.6 Hz, 3 H), 1.68 (m, 2 H), 1.24 (m,
1 H), 1.20 (m, 1 H) ppm. 13C NMR (CDCl3, 75 MHz): δ 170.2, 132.6,
131.4, 78.2, 73.6, 60.5, 52.0, 42.6, 31.4, 29.3, 27.6, 25.7, 23.5, 3.13 ppm.
HRMS (M+) calcd 278.1518, obsd 278.1517
1
product in 87% yield (0.38 g). H NMR (CDCl3, 300 MHz): δ 5.73 (s, 2
Org. Lett., Vol. 4, No. 2, 2002
279