Synthesis of cyclopentadienones catalyzed by methylidynetricobalt nonacarbonyl

Article abstract of DOI:10.1039/b107985a

Easily prepared and air-stable methylidynetricobalt nonacarbonyl could be used as a catalyst for the intramolecular [2+2+1]-cocyclization of diynes and carbon monoxide producing cyclopentadienones.

Full text of DOI:10.1039/b107985a

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Synthesis of cyclopentadienones catalyzed by methylidynetricobalt
nonacarbonyl
Takumichi Sugihara,* Akihito Wakabayashi, Hiroko Takao, Hiroshi Imagawa and Mugio
Nishizawa
Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514,
Japan. E-mail: taku@ph.bunri-u.ac.jp; Fax: +81-88-655-3051; Tel: +81-88-622-9611
Received (in Cambridge, UK) 4th September 2001, Accepted 23rd October 2001
First published as an Advance Article on the web 7th November 2001
Easily prepared and air-stable methylidynetricobalt nona-
carbonyl could be used as a catalyst for the intramolecular
[2+2+1]-cocyclization of diynes and carbon monoxide pro-
ducing cyclopentadienones.
Cyclopentadienones are anticipated versatile building blocks in
synthetic organic chemistry due to the formal ease in introduc-
tion of a variety of functionalities at all positions on the ring.1,2
Since the compounds have anti-aromatic nature, availability
would seem to be troublesome. The assistance of transition
metal complexes in the [2+2+1]-cocyclization of alkynes and
carbon monoxide appears as a promising method to prepare
cyclopentadienones.3 In most cases, however, a stoichiometric
amount of complex is required, since the newly formed
cyclopentadienone tends to remain coordinated onto the
metal.4,5 Unexpectedly we were led to discover that the
cocyclization of alkynes and carbon monoxide can be catalysed
by certain polynuclear cobalt carbonyl complexes.
gated (such as THF, DME, CH2Cl2, MeCN, DMF, and EtOH)
toluene seemed to be the best for this reaction in terms of the
yield of 2 and the recovery of 4a. When the reaction was carried
out at 120 °C for 48 h, only 3 was obtained and 4a was not
recovered at all (Entry 4). The prolonged reaction time at higher
temperature brought about the decomposition of the cluster,
which might have promoted isomerization of one of the alkenes
to produce 3. A variety of clusters was also investigated.
Whereas the thermally stable clusters, such as 4b–e, did not
catalyze the cyclization (Entries 5–8), ones having hydrogen
and bromine brought about fruitful results (Entries 3 and 9).
While it is known that treatment of 4f in refluxing toluene gave
4g, 4g itself also catalyzed the cyclization but the yield was
lower than the case of 4f (Entry 10). Finally, dicobalt
octacarbonyl 5 and tetracobalt dodecacarbonyl 6 also catalyzed
the cyclization (Entries 11 and 12), although the catalytic
activity was less than 4a. The difference of reactivity between
4a, 5, and 6 was also observed in the cyclization of 7 (Scheme
1). When 4a was used as a catalyst, the desired 8 was produced
in 48% yield along with the recovery of 7, whereas the reaction
with 5 or 6 was not fruitful. Since the cluster is more stable
against auto-oxidation than the binary cobalt carbonyl com-
plexes such as 5 and 6, placing one carbon unit in the core brings
stability and reactivity. Methylidynetricobalt nonacarbonyl
showed the best catalytic activity in the cyclization.
At first, we chose 1 as a substrate and investigated the
cyclization catalyzed by polynuclear cobalt carbonyl complexes
under various conditions. The results are shown in Table 1.
Since some alkylidynetricobalt nonacarbonyls6 4 were
known to catalyze the Pauson-Khand reaction without any
additive,7,8 we first chose these clusters as catalysts for the
cyclization. When the structurally simplest methylidynetrico-
balt nonacarbonyl (4a) was used, the desired 29 was produced in
24% yield along with 3 even under an argon atmosphere (Entry
1). More than half of the carbon monoxide in the cluster was
incorporated into the products. Whereas the catalytic activity
was not increased dramatically under 1 atm pressure of carbon
monoxide (Entry 2), the reaction under 20 atm of carbon
monoxide proceeded in a good yield (Entry 3). It should be
mentioned that 4a was also recovered in 70% yield by silica gel
column chromatography (Entry 3). Since the reaction of 1 in the
presence of 0.01 equiv. of 4a produced 2 in 32% yield, a certain
amount of 4a was required to complete the reaction and 4a was
recyclable under these conditions. Among the solvents investi-
Table 1 Cyclization of 1 catalyzed by cobalt complexesa
The conditions for the catalytic synthesis of cyclopentadie-
nones were fixed and were applied for various substrates. The
intermolecular cyclization of diphenylacetylene, 1-phenyl-
2-triethylsilylacetylene, bis(trimethylsilyl)acetylene, and bis-
Atmosphere Temp. Time
Catalyst (atm) (°C) (h)
Yield (%)
3
Entry
2
1
1
2
3
4
5
6
7
8
4ab
4ab
4ac
4a
4b
4c
4d
4e
4f
Ar(1)
CO(1)
reflux 10
reflux 10
24
32
96
—
—
—
—
—
80
53
73
55
5
4
51
45
—
—
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
CO(20)
95
120
95
10
48
10
10
10
10
10
10
10
10
—
89
—
—
—
—
2
4
5
8
99
95
95
100
98
98
11
42
15
36
95
95
9
10
11
12
4g
5
6
95
95
95
a A 0.125 M solution of 1 and 0.015 equiv. of the catalyst was stirred.b A
0.05 equiv. of the catalyst was used.c 4a was recovered in 70% yield.
Scheme 1
2456
Chem. Commun., 2001, 2456–2457
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107985a

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Table 2 The cyclization of diynes catalyzed by methylidynetricobalt
tether had an aromatic or silyl group on both termini of the
alkynes (Table 2). In all cases shown in Table 2, the desired
cyclopentadienones9 were produced in good yields.
In conclusion, we have developed a method for catalytic
synthesis of cyclopentadienones by methylidynetricobalt nona-
carbonyl. The cluster combines the advantages of being both
easy to handle and catalytically active.
This work is supported in part by grants from the Asahi Glass
Foundation and the Ministry of Education, Science, Sports, and
Culture, Japan. TS gratefully thanks Professor Masahiko
Yamaguchi at the Graduate School of Pharmaceutical Sciences,
Tohoku University, Japan for his helpful discussion.
nonacarbonyla
4a
Yield
Recovery of
S. M. (%)
Entry
S.M.
(mol%) Product (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
9
1.5
3.0
1.5
1.5
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
3.0
10
10
12
14
16
18
20
22
24
26
28
30
32
70
92
92
90
62
85
45
88
72
70
83
84
35
15
—
—
—
29
8
34
—
—
—
—
—
60
9
11
13
15
17
19
21
23
25
27
29
31
Notes and references
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a All reactions were carried out in a 0.125 M solution of toluene at 95 °C
under 20 atm of carbon monoxide in the presence of 4a.
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4 Cyclization was shown to proceed in a catalytic manner when the
resulting cyclopentadienones were transformed into stable molecules to
reduce their coordinating ability. See: (a) S. H. Hong, J. W. Kim, D. S.
Choi, Y. K. Chung and S.-G. Lee, Chem. Commun., 1999, 2099; (b) S.
U. Son, S.-J. Paik, S. I. Lee and Y. K. Chung, J. Chem. Soc., Perkin
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5 Recently, Shibata et al. reported the synthesis of cyclopentadienones
catalyzed by iridium complexes. See, T. Shibata, K. Yamashita, H.
Ishida and K. Takagi, Org. Lett., 2001, 3, 1217.
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9 The cyclopentadienones shown here were quite stable stored in
dichloromethane, benzene, and toluene solution. In the concentrated
state, Diels–Alder-type dimerization proceeded.
10 The [2+2+2]-cyclotrimerization of alkynes catalyzed by alkylidyne-
tricobalt nonacarbonyl was reported, although the catalytic activity was
low and the reaction conditions were vigorous. See, R. S. Dickson and
G. R. Tailby, Aust. J. Chem., 1970, 23, 229.
(triphenylsily)acetylene did not produce the cyclopentadie-
nones and recovered the starting alkynes and 4a. When
oct-4-yne was treated under the same conditions, hexa(n-
propyl)benzene was produced in 99% yield.10 The cyclization
could proceed only when the diynes with three or four atoms’
Chem. Commun., 2001, 2456–2457
2457