Novel decomplexation method for alkyne-Co2(CO)6 complexes

Article abstract of DOI:10.1016/S0022-328X(97)00679-7

A novel and general decomplexation method for alkyne-Co₂(CO)₆ complexes has been established, which treats the complexes with ethylenediamine in THF.

Full text of DOI:10.1016/S0022-328X(97)00679-7

Journal of Organometallic Chemistry 554 (1998) 163–166
Novel decomplexation method for alkyne–Co2(CO)6 complexes
Takumichi Sugihara *, Hitoshi Ban, Masahiko Yamaguchi
Faculty of Pharmaceutical Sciences, Tohoku Uni6ersity, Aobayama, Sendai 980-8578, Japan
Received 1 September 1997; received in revised form 13 October 1997
Abstract
A novel and general decomplexation method for alkyne–Co2(CO)6 complexes has been established, which treats the complexes
with ethylenediamine in THF. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Decomplexation; Alkyne complexes; Carbonyl complexes; Cobalt; Ethylenediamine
Alkyne–Co2(CO)6 complexes are often used not only
for protection of alkynes [1] but also as mediator for
construction of new carbon–carbon bonds in the well-
known Nicholas [2] and Pauson–Khand reaction [3]. In
the Nicholas reaction, even if the carbon–carbon bond
formation is achieved in an excellent yield, the next step,
i.e. the decomplexation of alkyne–Co2(CO)6 complexes,
can be cumbersome. Indeed, oxidants, such as Fe3+ ion
[4], ceric ammonium nitrate (CAN) [5], or tertiary amine
oxides [6] are usually used to carry out decomplexation,
which can lead to unacceptable yields due to the propen-
sity of the resulting alkynes to oxidation in some cases.
We recently found that the Pauson–Khand reaction was
carried out in cyclohexylamine at 35°C with a dramatic
rate-enhancement [7]. In the course of our study on the
Pauson–Khand reaction, when 1a was treated with
2-aminoethanol, not only the Pauson–Khand reaction
but also decomplexation took place to afford 3a pre-
dominantly (Scheme 1). This result led us to investigate
a novel decomplexation method for alkyne–Co2(CO)6
complexes without using oxidants [8,9].
At first, we focused on the duality of the reaction
course as shown in Scheme 1 and tried to find conditions
in which the decomplexation could proceed exclusively.
Since we convinced that primary amines can easily
react with alkyne–Co2(CO)6 complexes from our pre-
liminary results [7], we first studied the reaction of 1a
with excess amount of chelatable diamines. The results
under various conditions are summarized in Table 1
(Scheme 2).
Scheme 1.
* Corresponding author. Tel.: +81 22 2176814; fax: +81 22
2176811; e-mail: taku@mail.pharm.tohoku.ac.jp
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00679-7

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T. Sugihara et al. / Journal of Organometallic Chemistry 554 (1998) 163–166
Table 1
The decomplexation reaction of 1a under various conditionsa
Entry
Solvent
Additive
(Eq.)
Temp (°C)
Time
Yield (%)
3a
1a
1
2
3
4
5
6
7
8
9
H2NCH2CH2NH2
HN(CH2CH2NH2)2
(CH2NHCH2CH2NH2)2
THF
THF
THF
THF
EtOH
CH2Cl2
35
35
35
25
65
65
65
25
25
5 min
5 min
5 min
10 h
5 min
30 min
2 h
97
90
94
99
—
—
—
—
—
—
5
H2NCH2CH2NH2
H2NCH2CH2NH2
H2NCH2CH2NH2
H2NCH2CH2NH2
Fe(NO3)3 ·9H2O
NMOc
10
3.5
2.3
1.2
4.5
1.5
94
89
75b
77b
4 h
10 h
—
—
d
—
a All reactions were carried out in 0.2 M solution. b The undetermined polar byproducts were also produced. c NMO, N-methylmorpholine
N-oxide. d 2a was produced in 97% yield instead of 3a.
When ethylenediamine was used as a solvent, the
desired decomplexation exclusively proceeded in a short
reaction time to give 3a in an excellent yield (entry 1).
Although the use of diethylenetriamine and tri-
ethylenetetramine as solvents also promoted decom-
plexation (entry 2 and 3), the simplest one, i.e.
ethylenediamine, gave the cleanest results. Reducing the
amount of ethylenediamine to ten equivalents still al-
lowed to carry out the reaction in a reasonable time (10
h) at 25°C (entry 4). At more elevated temperature, the
amount of ethylenediamine could be reduced down to
3.5 equivalents with respect to the complex, and the
reaction was completed in a short time (entry 5). En-
tries 6 and 7 showed that the use of smaller amount of
the diamine slowed down the reaction and decreased
the yield of 3a. Among various solvents such as
toluene, 1,4-dioxane, 1,2-dichloroethane, and 1,2-
dimethoxyethane, tetrahydrofuran (THF) has proven to
be the best for both conditions, i.e. with 3.5 equivalents
of the amine at 65°C (Conditions A) and with ten
equivalents of ethylenediamine at 25°C (Conditions B)
[10]. Under previously published conditions [4], polar
byproducts were also formed (entry 8). When 1a was
treated with N-methylmorpholine N-oxide (NMO) in
dichloromethane, the bicyclic enone 2a was produced in
an excellent yield (entry 9) [11]. The present method
should become an important alternative procedure for
the decomplexation of alkyne–Co2(CO)6 complexes
without using oxidants.
With these suitable conditions for decomplexation in
our hand, we investigated its scope (Scheme 3; Table 2).
In most of the cases shown in Table 2 (except entry 12),
the desired decomplexation was finished within 30 min
at 65°C and within 1 day at 25°C to afford the original
alkynes 3 in excellent yields. Protection of alcohol
moieties of the side chain was not necessary for decom-
plexation (entries 3, 5 and 13). Terminal alkynes could
also undergo this reaction (entry 7). It is also worth
pointing out that amides and sulfides did not affect the
reaction course to give the original alkynes in good
yields (entries 8–10). The trimethylsilyl group on the
alkynyl carbon, which is sometimes used as a protective
groups for terminal alkynes, mostly survived under
conditions A, while nearly half of it was cleaved during
decomplexation under conditions B presumably due to
the longer reaction time (entry 11). In contrast, when
1-benzyloxy-5-trimethysilyl-4-pentyne (3l) was treated
under both conditions A and B, 3l was recovered in
quantitative yield. The cobalt in the reaction mixture
might be responsible for the cleavage of the trimethylsi-
lyl group in the present case (entry 11). On the other
hand, the triethylsilyl group survived under both condi-
tions despite a longer reaction time (entry 12). It should
be mentioned that decomplexation of simple propargyl
ether–Co2(CO)6 complexes such as 1-benzyloxy-2-
propyne did not work under both conditions.
Considering the mechanisms, sterically less hindered
amines such as primary ones might come close to the
Scheme 2.

T. Sugihara et al. / Journal of Organometallic Chemistry 554 (1998) 163–166
165
reported previously (i.e. 5“6“7) [7]. In contrast,
when chelatable diamines are used instead of
monoamines, the vacant coordination site could be
filled by the other amino group to form a chelate
complex such as 8. The resulting complex becomes
more electron-rich and might start to release the
alkyne 3. The mechanisms of reactions of alkyne–
Co2(CO)6 complexes with amines, which presented in
the above hypothesis, are currently under further in-
vestigation.
Scheme 3.
Co atom of alkyne–Co2(CO)6 complexes and displace
one of the CO to give 4 (Scheme 4). The driving
force for the substitution reaction is thought to be
strong affinity between cobalt and nitrogen. It is
known that ‘hard’ ligands, which contain a N or O
atom, on metal–carbonyl complexes make the exist-
ing CO ligands labile and therefore facilitate the lig-
and substitution reaction [12]. The amine on the
complex might trigger to open up a vacant coordina-
tion site due to its labilizing effect to form complexes
such as 5. When an existing olefin is accessible to this
site, the Pauson–Khand reaction could proceed as we
Acknowledgements
The authors kindly thank Mamiko Yamada for her
valuable technical assistance. This work was sup-
ported in part by grants from the Japan Society
of Promotion of Science (Research for the Future
Program) and the Ministry of Education, Science,
Sports, and Culture, Japan (no. 08404050
&
09771890). The authors would also like to thank Dr
Christophe Copere´t at the Scripps Research Institute
for his kind advice during the preparation of this
manuscript.
Table 2
The decomplexation reaction of alkyne–Co2(CO)6 complexes 1a
Entry
1
R1
R2
Conditions Ab
Time (min)
Conditions Bc
Yield of 3 (%)
Time (h)
Yield of 3 (%)
1
2
3
4
5
6
7
8
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
Ph
Ph
Ph
n-Bu
Ph
Ph
H
Ph
Ph
Ph
CH2OBn
CH2OH
CH2OBn
10
30
10
20
15
10
10
15
15
15
15
60
20
20
10f
94
98
90
92
97
98
92
93
88
94
85d
93
90
94
99g
10
5
98
97
92
96
100
99
98
90
84
92
47e
94
80
95
98g
24
20
24
10
10
8
C(CH3)2OH
(CH2)2OBn
(CH2)2OBn
(CH2)2NHBz
(CH2)2NHTs
(CH2)2SPh
(CH2)3OBn
(CH2)3OBn
(CH2)3OH
(CH2)3OTBDPS
9
10
8
10
11
12
13
14
15
Ph
Me3Si
Et3Si
Ph
Ph
Ph
20
60
20
20
7h
a All reactions were carried out in 0.2 M solution. b Conditions A: A mixture of the complex and 3.5 molecular equivalents of ethylenediamine
in THF was stirred at 65°C. c Conditions B: A mixture of the complex and ten molecular equivalents of ethylenediamine in THF was stirred at
room temperature. d The desilylated acetylene, 1-benzyloxy-4-pentyne, was also produced in 8% yield. e The desilylated acetylene, 1-benzyloxy-4-
pentyne, was also produced in 38% yield. f Seven molecular equivalents of ethylenediamine were used. g 3p is 1,4-diphenyl-1,3-butadiyne. h Twenty
molecular equivalents of ethylenediamine were used.

166
T. Sugihara et al. / Journal of Organometallic Chemistry 554 (1998) 163–166
Scheme 4.
alkynes. See, (a) M. Isobe, C. Yenjai, S. Tanaka, Synlett (1994)
References
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[9] There is a report that the Co2(CO)6 complex of 1-trimethylsilyl-1-
alkyne derivatives can be decomplexated with concomitant desily-
lation by the treatment with n-Bu4NF. The applicability of this
method, however, is unknown. See ref. [6].
[10] The typical experimental procedures for conditions A: Under an
argon atmosphere, a mixture of 1a (543 mg, 1.19 mmol) and
ethylenediamine (0.28 ml, 4.19 mmol) in THF (5.6 ml) was stirred
at 65°C for 5 min. After cooling, Et2O and water were added to
the reaction mixture. The organic layer was separated, washed
with 3% HCl aq. and sat. NaHCO3 aq. successively, dried over
MgSO4, and concentrated. The resulting residue was passed
through silica gel pad (n-hexane) to give 3a (190 mg, 94%) as a
colorless oil. For conditions B: Under an argon atmosphere,
ethylenediamine (0.82 ml, 12.3 mmol) was added to a solution of
1a (562 mg, 1.23 mmol) in THF (6.2 ml) at 25°C. After 10 h, Et2O
and water were added to the reaction mixture. The organic layer
was separated, washed with 3% HCl aq. and sat. NaHCO3 aq.
successively, dried over MgSO4, and concentrated. The resulting
residue was passed through silica gel pad (n-hexane) to give 3a
(207 mg, 99%) as a colorless oil.
[11] For the tertiary amine oxide-promoted Pauson–Khand reaction,
see, (a) S. Shambayati, W.E. Crowe, S.L. Schreiber, Tetrahedron
Lett. 31 (1990) 5289. (b) N. Jeong, Y.K. Chung, B.Y. Lee, S.H.
Lee, S.-E. Yoo, Synlett (1991) 204.
[12] For instance, see, C.M. Lukehart, in: Fundamental Transition
Metal Organometallic Chemistry, Brooks/Cole, Monterey, 1985,
pp. 63–67.
[8] Reductive decomplexation methods have been reported but all of
them provide the corresponding alkenes instead of the original