Cobalt oxide supported gold nanoparticles as a stable and readily-prepared precursor for the in situ generation of cobalt carbonyl like species

Article abstract of DOI:10.1016/j.tetlet.2011.09.067

A treatment of cobalt oxide supported gold nanoparticles (Au/Co ₃O₄) under syngas atmosphere effectively generated a cobalt carbonyl-like active species in the reaction vessel. The preparation of Au/Co₃O₄ was quite simple and the in situ generated cobalt species could be used as a stable and easy handling alternative for dicobalt octacarbonyl without bothersome purification prior to use. The reactions, which are sensitive to the purity of the dicobalt octacarbonyl, such as the alkoxycarbonylation of epoxides and the Pauson-Khand reaction, smoothly progressed with Au/Co₃O₄.

Full text of DOI:10.1016/j.tetlet.2011.09.067

Tetrahedron Letters 52 (2011) 6869–6872
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Tetrahedron Letters
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Cobalt oxide supported gold nanoparticles as a stable and readily-prepared
precursor for the in situ generation of cobalt carbonyl like species
Akiyuki Hamasaki a,b, Akiko Muto a,b, Shingo Haraguchi a,b, Xiaohao Liu a,b, Takanori Sakakibara a,b
,
Takushi Yokoyama a,b, Makoto Tokunaga a,b,c,
⇑
a Department of Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
b JST (Japan Science and Technology Corporation), CREST, Japan
c International Research Center for Molecular Systems (IRCMS), Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2011
Revised 10 September 2011
Accepted 15 September 2011
Available online 21 September 2011
A treatment of cobalt oxide supported gold nanoparticles (Au/Co3O4) under syngas atmosphere effec-
tively generated a cobalt carbonyl-like active species in the reaction vessel. The preparation of Au/
Co3O4 was quite simple and the in situ generated cobalt species could be used as a stable and easy han-
dling alternative for dicobalt octacarbonyl without bothersome purification prior to use. The reactions,
which are sensitive to the purity of the dicobalt octacarbonyl, such as the alkoxycarbonylation of epox-
ides and the Pauson–Khand reaction, smoothly progressed with Au/Co3O4.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Cobalt carbonyls
Pauson–Khand reaction
Alkoxycarbonylation
Alternative catalyst
Cobalt carbonyls are one of the most versatile reagents among
metal carbonyls. Dicobalt octacarbonyl1 (Co2(CO)8) in particular
is broadly used as a catalyst for hydroformylation,2 carbonylation
reactions,3 and amidocarbonylation.4 It is also employed as a stoi-
chiometric reagent for the Nicholas reaction5 and the protection of
alkynes.6 However, Co2(CO)8 often requires sublimation and/or
recrystallization prior to use to obtain good catalytic activity and
repeatability because of decomposition during storage even below
0 °C under an inert gas atmosphere due to the dissociation of CO.
Furthermore, a less obvious problem is that a preparation of
Co2(CO)8 from Co(II) salt requires a harsh condition, for example
CO/H2 pressure of 3500 psi (ca. 24 MPa), and a temperature of
150 °C.7,8
tal solution and a base solution at rt (co-precipitation method).9b,c
Since the obtained solid consisted of gold(0) and cobalt oxide, it
was fairly stable to air and moisture at least for several months.
Au/Co3O4 worked as a recyclable heterogeneous catalyst in nonpo-
lar solvents such as heptane.9a,c However, our initial investigations
proved that this heterogeneous system showed poor activity for
the alkoxycarbonylation of epoxides and the Pauson–Khand reac-
tions (PKR). Therefore, we have attempted to use Au/Co3O4 as a
precursor of homogeneous Co2(CO)8-like species by employing
resoluble polar solvents. This catalyst system was able to generate
fresh active species continuously during the reaction, thus it was
consummately ideal for the reactions sensitive to the purity of
Co2(CO)8.
Recently, we have reported a novel function of cobalt oxide sup-
ported gold nanoparticles (Au/Co3O4) to provide Co2(CO)8-like spe-
cies under a CO/H2 atmosphere.9a–c The active species were formed
by the reduction of the support metal and by subsequent binding
to CO. The existence of Co(0) after the treatment of Au/Co3O4 under
H2 was clearly observed using an XRD and a XANES spectra.9d Au
nanoparticles played an important role for the first reduction step
to generate spillover hydrogen, therefore Co3O4 without Au did not
show any catalytic activity for the hydroformylation of olefins.9a,c
Au/Co3O4 can be readily prepared from commercial HAuCl4ꢀ4H2O
and Co(NO3)2ꢀ6H2O even on a large scale by simple mixing of a me-
Co2(CO)8 (5 mol%)
3-hydroxypyridine
(10 mol%)
OH
O
CO (4 MPa)
O
ð1Þ
OMe
MeOH, THF
3
1a
3
2a
fresh Co2(CO)8: 91% conv., 81% yield
aged Co2(CO)8: 70% conv., 37% yield
Co2(CO)8 catalyzes the alkoxycarbonylation of epoxides to give
b-hydroxyesters. This was first reported in 196110a and several ex-
tended works were found in recent years.10b–h,11 Although specific
mention about sensitivity to the purity of Co2(CO)8 did not appear
⇑
Corresponding author. Tel./fax: +81 92 642 7528.
E-mail address: mtok@chem.kyushu-univ.jp (M. Tokunaga).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.067

6870
A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872
Table 2
in the literatures, this reaction seemed to be greatly dependent on
the fineness of Co2(CO)8. We investigated the difference of catalytic
activity between a freshly sublimed Co2(CO)8 and an aged one by
following Jacobsen’s procedure10d (Eq. 1). A fresh catalyst gave
the result of 91% conversion and 81% yield. On the other hand,
the catalytic activity decreased to 70% conversion and 37% yield
after storage for several weeks at 4 °C under a N2 atmosphere.
The result of the initial survey of the reaction conditions including
solvent, additive, syngas ratio, temperature, and reaction time is
shown in Table 1. Under the conditions of Table 1, entry 1, only
2% of the desired b-hydroxyester was obtained along with a large
amount of nucleophilic ring opening ether products. Whereas an
addition of 3-hydroxypyridine was reported in the literature to im-
prove the reaction efficiency,10c it was found to be ineffective for
the Au/Co3O4 catalyst system with a negligible improvement of
the product yield (Table 1, entry 2). Recently, pyrazole was also re-
ported as an additive,10f and it was employed for the reaction be-
cause it worked better than 3-hydroxypyridine (Table 1, entry 3).
A low yield of the desired product was caused by the formation
of several by-products including ketones, acetals, and ethers.10f
While hydroformylation of epoxides could take place under a CO/
H2 atmosphere,12 b-hydroxyaldehyde or reduced 1,3-diol products
were not detected in the reaction mixture. Since the most signifi-
cant side reaction among them was an ether formation by a nucle-
ophilic attack of MeOH, the amount of MeOH was restricted. As a
result, the decrease in MeOH suppressed the formation of byprod-
ucts and gave a better yield of b-hydroxyesters (Table 1, entries 4
and 5). Additionally, increasing the reaction time at a milder reac-
tion temperature (65 °C) and a higher CO partial pressure effi-
ciently improved the results (Table 1, entries 6–8). Pretreating
the catalyst prior to each reaction was important for the efficient
generation of the active species. The CO/H2 atmosphere seemed
to be preferable to H2 since the active species exist at the initial
stage of the reaction which avoids the side reactions irrespective
of the catalysis (Table 1, entries 1–4 vs 5–8).
Au/Co3O4 catalyzed alkoxycarbonylation using various epoxides and alcoholsa,b
11.4 wt% Au/Co3O4
(0.6 mol% Au, 11 mol% Co)
Pyrazole (3 mol%)
CO (4 MPa)
OH
O
O
R
R
OR'
R'OH/THF (1:4, 5 mL)
65 °C, 40 h
2
1
Entry
1
Substrate
R0OH
Product
Yieldc (%)
O
MeOH
2b
58
79
O
2
MeOH
2c
5
9
O
3
4
MeOH
MeOH
2d
2e
45
86
O
O
5
MeOH
2f
5
O
O
6
7
EtOH
2g
2h
82
33
3
3
2-PrOH
a
The catalyst was pretreated under CO/H2 (3:1, 2 MPa) at 120 °C for 3 h in the
reaction solvent prior to the reaction.
b
The reaction was conducted on a 5 mmol scale.
GC yield.
c
reactivity similar to MeOH (Table 2, entry 6). However, 2-PrOH
gave a lower yield probably due to the steric problem (Table 2,
entry 7).
Next, we turned our attention to the substrate scope of the reac-
tion. Although a slight difference of reactivity according to the
chain length was observed, alkyl epoxides were smoothly reacted
to give the desired b-hydroxyesters in moderate to good yields
(Table 2, entries 1–4). Styrene oxide showed a low reactivity
(Table 2, entry 5). As for alcohol nucleophiles, EtOH showed a
The PKR is a formal [2+2+1] cyclization for providing cyclo-
pentenone derivatives. Since this reaction is quite useful for con-
structing complex molecular skeletons, it is incorporated in many
natural product syntheses.13 This reaction was originally reported
as a stoichiometric reaction using Co2(CO)8,14 and catalytic ver-
sions were developed by employing various additives15 or highly
Table 1
Optimization of reaction conditions for Au/Co3O4 catalyzed alkoxycarbonylation of epoxides
11.4 wt% Au/Co3O4
(0.6 mol % Au, 11 mol% Co)
additive (3 mol%)
CO/H2 (4 MPa)
OH
O
O
OMe
65 °C, 40 h
3
3
2a
1a
Entry
Pretreatmentb
Additive
None
3-Hydroxypyridine (10 mol%)
Pyrazole
Pyrazole
Pyrazole
Pyrazole
Pyrazole
Pyrazole
Solvent
CO:H2
Yielda (%)
1c,d
2c,d
3c,d
4c,d
5c,e
6e
A
A
A
A
B
B
B
B
MeOH
MeOH
MeOH
MeOH/THF (1:1)
MeOH/THF (1:4)
MeOH/THF (1:4)
MeOH/THF (1:4)
MeOH/THF (1:4)
3:1
3:1
3:1
3:1
3:1
3:1
7:1
CO only
2
3
17
26
33
48
59
68
7e
8e
a
GC yield.
b
Pretreatment method: (A) The catalyst was pretreated under H2 (2 MPa) at 120 °C for 2 h in the reaction solvent prior to the reaction; (B) The catalyst was pretreated
under CO/H2 (3:1, 2 MPa) at 120 °C for 3 h in the reaction solvent prior to the reaction.
c
The reaction was carried out at 80 °C for 20 h.
The reaction was conducted on a 2 mmol scale in 2 mL of the solvent.
The reaction was conducted on a 5 mmol scale in 5 mL of the solvent.
d
e

A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872
6871
Table 3
Au/Co3O4 catalyzed intramolecular PKR of various substratesa
11.4 wt% Au/Co3O4
(0.6 mol% Au, 11 mol% Co)
CO/H2 (3:1, 4 MPa)
R1
R1
Y
R2
Y
O
DME (2 mL)
100 °C, 60 h
R2
3
4
(0.5 mmol)
Entry
1
Substrate
EtO2C
Product
Yieldb (%)
Ph
Ph
EtO2C
3a
4a
4b
4c
81
91
81
EtO2C
O
EtO2C
EtO2C
EtO2C
O
O
O
2
3
3b
EtO2C
EtO2C
EtO2C
Me
Ph
Me
Ph
EtO2C
3c
EtO2C
EtO2C
EtO2C
EtO2C
EtO2C
4
5
3d
4d
89
96
EtO2C
Me
Ph
Me
Ph
TsN
O
3e
4e
4f
TsN
O
6c
7e
15d
30
Ph
Ph
3f
O
O
a
b
c
The catalyst was pretreated under H2 (2 MPa) at 100 °C for 3 h in DME prior to each run.
Isolated yield.
The reaction was conducted on a 1 mmol scale at 100 °C for 65 h.
The starting material 3f was recovered at 79%.
d
e
The reaction was conducted at 160 °C for 20 h.
pure Co2(CO)816 from the middle of the 1990s. Other homogeneous
Co sources also provided effective catalyst systems.17 Alongside
the development of homogeneous catalysts, heterogeneous
catalysts including Co on silica,18a Co nanoparticles,18b,c Ru/Co
heterobimetallic nanoparticles,18d Rh/Co heterobimetallic nano-
particles,18e,f and Pd/Co heterobimetallic nanoparticles18g have
been actively investigated by Chung in recent years. Under these
situations, we believed Au/Co3O4 would catalyze the PKR based
on a unique mechanism which was different from the above cata-
lyst systems.
An intermolecular PKR employing a reactive alkene and termi-
nal alkynes was attempted. An equimolar of norbornene (5) and
1-hexyne (6a) were treated with Au/Co3O4 (0.6 mol % Au and
11 mol % Co) and 4 MPa of CO/H2 (3:1) at 100 °C for 61 h to afford
7a in 62% isolated yield (Eq. 2). The reaction using phenylacetylene
(6b) in place of 6a resulted in a steep decrease in the yield.
11.4 wt% Au/Co3O4
(0.6 mol% Au, 11 mol% Co)
CO/H2 (3:1, 4 MPa)
R
O
+
R
As expected, several 1,6-enyne substrates were effectively
transformed into cyclopentenone derivatives under the optimal
condition (Table 3). A substitution of the substrates on alkyne
and alkene moieties generally decreases the reactivity due to a ste-
ric reason.15b A non-substituted substrate 3b indeed gave a slightly
better yield among malonate-tethered substrates (Table 3, entry 2),
but other compounds with methyl and/or phenyl substituents also
showed satisfactory reactivities as well (Table 3, entries 1, 3 and 4).
A tether like malonate or N-tosylate was preferable to locate two
reaction sites closer together (Table 3, entry 5). An ether tether
was also expected to have a similar character, but the reaction
was very sluggish at 100 °C and the desired product 4f was ob-
tained in 15% with 79% of intact 3f (Table 3, entry 6). Under a
harsher reaction temperature, only slight improvement of the yield
was observed while all the substrate was consumed in 20 h (Table
3, entry 7). An explanation for this result is that the in situ forma-
tion of Co-hydride species led to cleavage of the ether moiety by
hydrogenolysis, especially under high temperature.19
DME (2 mL)
5
6a
6b
: R = n-Bu
100 °C
0.5 mmol
: R = Ph
7a
7b
: R = n-Bu 62% (61 h)
27% (60 h)
0.5 mmol
: R = Ph
ð2Þ
The two reactions in this study required dissolution of the ac-
tive species for a smooth progress of the reactions. It was a com-
pletely different concept from a common heterogeneous catalysis
since Au/Co3O4 was used as a precursor of the actual active species.
A treatment of the catalyst in MeOH/THF under H2 at 120 °C for 3 h
gave a colorless suspension (Fig. S1, b). On the other hand, the
same treatment under CO/H2 resulted in a dark brown solution
indicating the formation of a CO-bound active species (Fig. S1, c).
A polar solvent was preferable for these reactions and this solvent
effect might be explained by the solubility of the cobalt species.
Interestingly, Co2(CO)8 is soluble in common organic solvents,

6872
A. Hamasaki et al. / Tetrahedron Letters 52 (2011) 6869–6872
including nonpolar solvents.1 This fact indicated the possibility
that the in situ generated species and Co2(CO)8 were not exactly
identical. An IR analysis of the dark brown solution after the pre-
treatment under CO/H2 showed a characteristic peak at 1965 cm
4. Wakamatsu, H.; Uda, J.; Yamakami, N. J. Chem. Soc. D 1971, 1540; Parnaud, J.-J.;
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20
did not correspond to Co2(CO)8, HCo(CO)4, or Co4(CO)12
.
An
atomic absorption spectrophotometry of the post-reaction mixture
of the PKR revealed that 38% of the Co atoms were leached from
Au/Co3O4. This result implied that it would be difficult to recycle
the catalyst since a spontaneous reassembly of the proper Au/
Co3O4 structure from the solution state is difficult to achieve.
In summary, the alkoxycarbonylation of epoxides and the Pau-
son–Khand reaction effectively proceeded using Au/Co3O4 as a pre-
cursor of the Co active species. Au/Co3O4 can be readily obtained by
simple mixing of the two solutions and is fairly stable to air and
moisture even at ambient temperature. The largest feature of this
catalyst system was the continuous supply of fresh active species
during the reaction. Thus, it is quite effective for the reactions that
are sensitive to the purity of Co2(CO)8. Additional studies to utilize
this catalyst system are currently underway.
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We gratefully acknowledged to the Institute for Materials
Chemistry and Engineering (IMCE), Kyushu University for the MS
spectrum measurements. This work was supported by a Grant-
in-Aid for the Global-COE program, ‘Science for Future Molecular
Systems’, Scientific Research (A) (No. 20245014) (to M.T.), and Re-
search Activity Start-up (No. 21850023) (to A.H.) from the Ministry
of Education, Culture, Science, Sports and Technology of Japan.
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Supplementary data
Supplementary data (experimental procedures and copies of 1H
and 13C NMR spectra for all products) associated with this Letter
can be found, in the online version, at doi:10.1016/j.tetlet.2011.09.
067.
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