Novel functional group transformation from acetylene cobalt complex to ketone via ligand exchange

Article abstract of DOI:10.1246/cl.2006.464

A new transformation of acetylene dicobalt tetracarbonyl dppm complexes was found to convert to ketones with high regioselectivity under ligand exchange condition. Copyright

Full text of DOI:10.1246/cl.2006.464

4
64
Chemistry Letters Vol.35, No.4 (2006)
Novel Functional Group Transformation from Acetylene Cobalt Complex
to Ketone via Ligand Exchange
Ã
Akinari Hamajima, Hiyoku Nakata, Masako Goto, and Minoru Isobe
Laboratory of Organic Chemistry, School of Bioagricultural Sciences, Nagoya University,
Chikusa-ku, Nagoya 464-8601
(Received January 23, 2006; CL-060097; E-mail: isobem@agr.nagoya-u.ac.jp)
A new transformation of acetylene dicobalt tetracarbonyl
dppm complexes was found to convert to ketones with high
regioselectivity under ligand exchange condition.
We expected that some ligand-exchange complexes11 might
satisfy these requirements directed to the ketone synthesis.
Table 1 summarizes the effects of some ligands. We clearly
observed the ligand-exchange compound on the TLC, when
9
-menbered ring acetylene cobalt complex 4 was treated with
Acetylene dicobalt hexacarbonyl complexes are used in the
1
triphenylphosphine. After 1 or 2 h, the ligand exchange cobalt
complex was converted to the corresponding acetylene 5 (Entry
1). In this case, no 9-membered ring-ketone 6 was obtained, but
the result suggested the potentiality to stabilize coordinatively
unsaturated complex via ligand exchange by a phosphine group.
Then, we attempted to employ a bidentate ligand. Treatment of
organic synthesis of natural products largely as a protecting
2
group, substitution reaction at its adjacent position (Nicholas re-
3
4
action) and the Pauson–Khand reaction. As for the complexes
situated at the acyclic positions, there exist various regeneration
2
,5,6
methods back into acetylenes with some oxidants.
cyclic complexes, however, only little has been known for the
As for the
ꢀ
the acetylene cobalt complex with dppm at 60 C gave acetylene
7
8
conversion into the corresponding acetylenes or olefins due
to the strain of the acetylene moiety. During the course of syn-
thetic studies toward ciguatoxin known as a marine neurotoxin
causing ciguatera poisoning, we reported the reductive decom-
plexation of dicobalt hexacarbonyl acetylenes into cis-olefins
dppm dicobalt tetracarbonyl complex. After stirring for 1 or 2
days at the same temperature, dppm complex was converted to
the desired ketone 6 in moderate yield (Entry 2). Treatment with
two-methylene diphosphine ligand, dppe or bis(diphenylphos-
phino)benzene, gave a mixture of ligand exchanged complex
and simply decomplexed acetylene 5. After stirring for several
hours, both of the desired ketone 6 and the acetylene 5 were iso-
lated (Entries 3 and 4). Treatment with three or four-methylene
diphosphine ligand gave only decomplexed acetylene 5 (Entries
5 and 6). These results imply that bidentate ligands longer than
dppe similarly behave like triphenylphosphine. Thus, ligand-ex-
change with dppm gave the best result among others as shown in
Table 1.
Table 2 summarizes the result of ketone-formation with
dppm under various ways of oxygen introduction. In Entry 1,
treatment of the cobalt complex 4 with 1.5 equiv. of dppm under
open air gave 6 in lower yield. In Entry 2, treatment of 4 under
controlled air condition gave 6 in moderate yield, but not repro-
ducible. Entries 1 and 2 show that it is difficult to obtain ketone
in a steady yield only by a controlled amount of air. In Entry 3,
addition of excess dppm gave the ketone 6 reproducibly in
moderate yield. In Entry 4, using of excess dppm under pure
9
or vinylsilanes. In the synthetic studies on the central part of
ciguatoxin, high-pressure hydrogenation of the cobalt complex
1
0
gave the desired ketone as a major product. At that time,
high-pressure hydrogenation was the only method for conver-
sion of acetylene cobalt complex to ketone in a single step. Since
this reaction mechanism was unclear, it was difficult to improve
for a practically useful method.
A 7-membered ring cobalt complex 1 was exposed to air
under heating condition to give two ketones of regioisomer
Eq 1). But application of this condition to 9-membered ring
cobalt complex 4 resulted in affording a complex mixture (Eq
). It implied that a carbonyl ligand of acetylene cobalt complex
was removed by heating to result in a formation of coordinative-
ly unsaturated complex and then coordinating with molecular
oxygen to give the ketone. No 7-membered ring acetylene can
exist because of the ring strain, therefore, the coordinatively un-
saturated complex shows longer-life. On the other hand, 9-mem-
bered acetylene can exist, so the coordinatively unsaturated
complex decomposed easily.
(
2
Table 1.
Ph
Ph
Ph
Ph
Ph
O
Ph
H
H
H
H
H
H
H
H
H
H
H
ligand
H
H
O
9
O
O
O
O
O
O
H
O
O
(1)
O
H
7
toluene
C2H4Cl2
or
toluene
Co2(CO)6
H
Co2(CO)6 100 °C
O
O
O
air
H
H
O
2
3
1
4
5
6
3
2% (2:3 = 1:15)
Entry
Ligand
Time/h
Yield/% 5
80
6
Ph
H
H
1
2
3
4
5
6
PPh3
dppm
dppe
dppBz
dppp
dppb
4
22
15
15
14
14
0
O
9
Complex
mixture
trace
18
45
80
46
28
28
0
O
H
toluene
(2)
Co2(CO)6 100 °C
air
4
If a coordinatively unsaturated complex could somehow be
stabilized, ketone formation would apply for 9-membered ring.
80
0
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.4 (2006)
465
Table 2.
Ph
Ph
H
H
H
H
O
O
dppm
conditions
4
O
O
O
H
Co2(CO)4
dppm
toluene
H
1
0 min
100 °C
7
6
dppm
equiv.)
1.5 open air
1.5 air injected by syringe
Entry
Conditions
Time Yield
(
1
2
3
4
5
5 h 21%
15 h 47%
8 h 55%
3 h 45%
15
15
15
open air
O2
sealed (non oxygen condition) 24 h
0%a
adppm complex 7 was recovered in quantitative yield.
Figure 1. Possible 3D structure of 7.
in moderate yields, which would widely provide a new method
for organic synthesis.
Table 3.
dppm
References
1
Entry
1
Substrates
Products
1
00°C, air
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Ph
8
OBz
Ph
9 32%
Ph
H
H
H
H
H
H
H
O
O
H
H
H
O
O
2
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O
O
Co2(CO)4dppm
O
H
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2
3
1
0
4
7% (2 : 3 = 3 : 1)
3
4
5
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8
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OTIPS
Me
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H
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D'
E
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D'
E
H
O
F
O
H
H
AcO
O
F
H
H
AcO
H
H
H
Co2(CO)4dppm
1
4
15 70%
6
7
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yield. Under non-oxygen condition (Entry 5), on the other hand,
no ketone was obtained, instead dppm complex 7 was recovered
in quantitative yield.
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of cyclic or acyclic cobalt complex into the corresponding
ketones under optimized condition. Contrary to the result of
Eq 1, treatment of 1 with dppm gave the regioisomer 2 as the
major product (Entry 2).
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side chain is less hindered, so that dppm was exchanged for
two carbon monoxide in the less hindered side as shown in
Figure 1. Finally, molecular oxygen attack cobalt atom from less
hindered side opposite to dppm, and consequently, the oxygen
atom is shifted toward this carbon to give the ketone.
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exposed to air under heating to give the corresponding ketone
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Published on the web (Advance View) April 5, 2006; DOI 10.1246/cl.2006.464