Cobalt-catalyzed homocoupling of terminal alkynes: Synthesis of 1,3-diynes

Article abstract of DOI:10.1016/S0040-4039(01)01701-4

Homocoupling of terminal alkynes proceeds using Co₂(CO)₈ pretreated with phenanthroline to give good yields of 1,3-diynes under mild conditions.

Full text of DOI:10.1016/S0040-4039(01)01701-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7733–7736
Cobalt-catalyzed homocoupling of terminal alkynes:
synthesis of 1,3-diynes
M. E. Krafft,* C. Hirosawa, N. Dalal, C. Ramsey and A. Stiegman
Department of Chemistry, Florida State University, Tallahassee, FL 32306, USA
Received 28 August 2001; accepted 10 September 2001
Abstract—Homocoupling of terminal alkynes proceeds using Co2(CO)8 pretreated with phenanthroline to give good yields of
1,3-diynes under mild conditions. © 2001 Elsevier Science Ltd. All rights reserved.
Formation of substituted 1,3-diynes via coupling between
sp hybridized atoms1 represents one class of coupling
reactions which yields polyfunctionalized molecules. The
most commonly used metal salts for alkyne–alkyne
coupling are Cu(+1) or Cu(+2) salts under either catalytic
or stoichiometric conditions. The Glaser1–3 and
Eglington1,4–7 procedures for direct coupling of terminal
alkynes, or the Chodkiewicz–Cadiot procedure1,8–10 for
coupling of a terminal alkyne with a halo alkyne, and
modifications thereof represent the most widely used
alkyne coupling reactions for the synthesis of 1,3-diynes.
To a lesser extent complexes of palladium,11 nickel,12
cobalt13 and Rieke copper14 have been shown to mediate
homo- or heterocoupling of terminal alkynes. Phase
transfer catalysis has been shown by Alper to give alkyne
homocoupling products.15 Recently, heterogeneous cou-
pling of phenylethyne over CuꢀMgꢀAl mixed oxides has
been demonstrated.16 Alkyne coupling reactions to form
both symmetrical and unsymmetrical 1,3-diynes have
been used to generate polymers and monomers,17,18
functional host molecules19 andintermediates forthetotal
synthesis of natural products.3,5,9,20
reaction is conducted in polar solvents. The reactivity of
these species has been well-studied with respect to
hydroformylation chemistry. Fachinetti reported that
[Py6Co][Co(CO)4]2 reacts with hydrogen at ambient tem-
perature under CO/H2 (1:1) atmosphere to give
[Py2H][Co(CO)4].23 Under the same conditions [Py6Co]-
[Co(CO)4]2 reacts with alkenes to give [RCOCo]-
[Co(CO)4]2.24
During the course of our studies on the role of amines
in the catalytic Pauson–Khand reaction25 we investigated
the in situ formation of different amine-ligated cobalt
carbonyl complexes and their subsequent reactions with
enynes. Much to our surprise we discovered that reaction
of 20% of 2,2%-dipyridyl (dipy) with 10% of Co2(CO)8 for
30 min at ambient temperature followed by the addition
of enyne 1 and warming to 70°C for 18 h gave rise only
to 1,3-diyne 2 in 33% yield. Homocoupling of the terminal
alkyne had apparently occurred (Eq. (1)) and none of the
normal Pauson–Khand product 3 was isolated.26 Conse-
quently, we assumed that Co2(CO)8 had been converted
to a HNIP via disproportionation with dipy and the
resultingHNIPwasthereforeresponsiblefortheobserved
coupling. Herein we describe subsequent modifications of
the reaction conditions and the development of an alkyne
coupling using a cobalt HNIP as the catalyst.
Dicobalt octacarbonyl is known to be an efficient catalyst
for the Pauson–Khand reaction as well as for hydro-
formylations.21 A variety of cobalt complexes can also be
prepared from Co2(CO)8 by ligand exchange or dispro-
portionation reactions. In the presence of Lewis bases,
Co2(CO)8 undergoes disproportionation to homonuclear
ion-pairs (HNIP).22 The disproportionation patterns are
dependent on the bases and polarity of solvents. Gener-
ally, amines or alcohols tend to form Co(II) salts
[LnCo][Co(CO)4]2 (Ln=neutral ligand) and isonitriles or
phosphines form Co(I) salts [LnCo][Co(CO)4] when the
20% dipy
10% Co2(CO)8
)
EtO2C
EtO2C
EtO2C
EtO2C
2
1
2
EtO2C
EtO2C
3
O
(1)
* Corresponding author. E-mail: mek@chem.fsu.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01701-4

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M. E. Krafft et al. / Tetrahedron Letters 42 (2001) 7733–7736
We attempted to optimize the yield of the coupling
product by screening a series of ligands which would
form HNIPs in the reaction with Co2(CO)8. Reaction of
enyne 1 with 10 mol% of Co2(CO)8 pre-treated for 30
min with a variety of ligands gave diyne 2 and starting
enyne 1 in varying yields (Table 1, Eq. (2)). Reactions
with phosphines and aliphatic amines led to the forma-
tion of metal deposits upon prolonged heating and
resulted only in the recovery of the starting material
(entries 1–4). The conjugated aromatic diamines were
found to be good ligands, with 1,10-phenanthroline
(phen) giving the best results. Since no Pauson–Khand
product was formed it seemed evident that pre-treat-
ment of Co2(CO)8 with phenanthroline in acetonitrile
was giving rise to a new complex which was inactive as
a catalyst for the Pauson–Khand cycloaddition.
several spectroscopic experiments. The structure of 4,
which was prepared from 2 equiv. of phenanthroline
and 1 equiv. of Co2(CO)8, was predicted to be
[phen3Co][Co(CO)4]2 based on the previous reports.27,28
1
The H NMR spectrum in CD3CN showed four broad
signals of equal intensity (l 17.2, 32.9, 49.5, 106.5
ppm). These strong downfield shifts clearly suggest the
formation of a paramagnetic Co(II) species and the
symmetry of ligands suggest three ligands associated
with the metal center. The 13C NMR spectrum of 4
showed three downfield carbons (200.2, 389.5, 686.6
ppm) and one highly upfield carbon (−182.9 ppm) for
phen (two quaternary carbons were not observed) and
eight CO carbons between 200 and 230 ppm for a pair
−
of [Co(CO)4 ] anions. These spectra are consistent with
the expected structure, [phen3Co][Co(CO)4]2. The IR
spectrum for 4 showed only one CO absorption at 1892
cm−1 which is typical for the isolated [Co(CO)4 ]. The
−
To determine the structure of catalyst 4 generated in
situ from Co2(CO)8 and phenanthroline, we carried out
1H NMR spectrum of the analogous complex 5, which
was prepared from 1 equiv. of Co2(CO)8 and 2 equiv. of
dipy, also showed chemical shifts similar to 4 in the
downfield region (l 14.5, 46.0, 83.7, 87.8 ppm), suggest-
ing the formation of [dipy3Co][Co(CO)4]2. These com-
plexes were easily isolated by removal of the solvent in
vacuo.28 The resulting solid was moderately stable
under air and could be stored in the freezer for a week
with little observable decomposition. The reactivity of
the isolated solid was essentially the same as the cata-
lyst that was generated in situ. Additional support for
the presence of Co(+2) was provided by magnetic sus-
ceptibility measurements over the wide temperature
range −269 to 25°C, using a SQUID magnetometer.
The measured susceptibility yielded a magnetic moment
of 4.8 BM, which is well within the range 4.7–5.2 for
distorted octahedral complexes of Co(+2).29
Table 1.
CH3CN
+
[LnCo][Co(CO)4]2
n=3,6
Co2(CO)8
ligand
reflux
(2)
4: L = phen, n=3
5: L = dipy, n=3
Entry
Ligand
(%)
2 (%)
1 (%)
1
2
3
4
5
6
7
8
Ph3P
DPPE
Triethylamine
TMEDA
Pyridine
Dipyridyl
30
15
40
20
40
20
20
20
0
0
0
100
100
100
100
ꢀ100
59
0
Trace
33
45
0
Phenanthroline
45
87
Dimethyl-phena
Further optimization of the reaction conditions
involved a change in temperature, solvent, atmosphere,
amount of catalyst and concentration (Table 2). Similar
a 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline.
Table 2. Optimization of general reaction conditionsa
Entry
Catalyst (%)
Additive (%)
Solvent
Temp. (°C)
Conc. (mol/l)
Yield (%)
RSM (%)
1
2
3
4
5
6
7
8
Co2(CO)8 (10)
Co2(CO)8 (10)
Co2(CO)8 (10)
Co2(CO)8 (10)
Co2(CO)8 (10)
4 (7)
4 (7)
4 (10)
4 (10)
4 (7)
4 (10)
4 (7)
4 (3)
4 (7)
4 (7)
4 (7)
4 (7)
o-phen (20)
o-phen (20)
o-phen (20)
o-phen (20)
o-phen (14)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
C2H5CN
C2H5CN
CH3NO2
DMF
60
70
80
80
80
80
80
80
80
80
80
80
80
97
80
80
80
0.067
0.067
0.067
0.067
0.067
0.02
0.067
0.067
0.067
0.1
10
45
77
70
72
45
74
82
3
85
86
71
22
0
80
45
5
12
9
54
20
18
87
11
0
10
67
100
100
83
56
9b
10
11
12
13
14
15
16
17
0.1
0.2
0.067
0.067
0.067
0.067
0.067
0
6
33
C2H5OH
a All reactions carried out under a blanket of CO except entry 4 in which argon was used.
b Reaction was carried out in a sealed tube.

M. E. Krafft et al. / Tetrahedron Letters 42 (2001) 7733–7736
7735
Table 4.
results were obtained whether the catalytic species was
prepared in situ (entries 1–5) or pre-formed and stored
in a freezer (compare entry 3 with entries 7 and 10).
The catalyst showed a narrow temperature range over
which it was active, 60–80°C, (entries 1–3) with the best
results obtained at 80°C in most cases.
CO
EtO2C
EtO2C
)
2
Catalyst
EtO2C
EtO2C
CH3CN
1
reflux, 18 h
2
Entry
Catalyst (%)
Yield (%)
Acetonitrile proved to be the solvent of choice for the
coupling reaction (entries 11, 14–17). The cobalt salts
are insoluble in ethereal solvents such as Et2O, 1,2-
dimethoxyethane or 1,4-dioxane regardless of whether
they are prepared in situ or pre-made and stored in a
freezer. Reactions performed under an atmosphere of
carbon monoxide gave the highest yields. It seems
plausible that the carbon monoxide atmosphere
enhances the catalyst lifetime although the coupling
proceeded under an argon atmosphere (entry 4). Inter-
estingly, diyne formation was almost completely sup-
pressed when the reaction was performed in a sealed
tube. To demonstrate the generality of the coupling
process, a series of terminal alkynes was subjected to
the optimized conditions (Table 3).
1
2
3
PPNCo(CO)4 (10)
Phen3Co(ClO4)2 (10)
PPNCo(CO)4 (20), Phen3Co(ClO4)2 (10) 56
0
0
PPN, bis(triphenylphosphoranylidene)ammonium.
diyne 2 in 56% yield (entry 3). These results suggest that
both of the ions phen3Co(II) and [Co(CO)4 ] are
involved in the catalytic cycle for the alkyne–alkyne
coupling, however, their specific role is still unclear at
this point. Further work on elucidating the mechanistic
pathway is underway.
−
In summary, we have described a new cobalt-catalyzed
terminal alkyne homocoupling to generate substituted
1,3-diynes under mild conditions.30
In an attempt to understand the mechanism of the
coupling and the role of the two cobalt ions, we exam-
ined the reaction of alkyne 1 with different cobalt salts
which have only one cobalt ion in the complex (Table
4). Complexes [phen3Co][ClO4]2 and PPNCo(CO)4 did
not show any catalytic activity for the coupling reaction
when they were used independently (entries 1 and 2).
Interestingly, the mixture of [phen3Co][ClO4]2 and
[PPN][Co(CO)4] catalyzed the coupling reaction to give
Acknowledgements
This work was supported by the National Science
Foundation and donors to the Krafft Research Fund.
We thank Professor Ken Goldsby (FSU) for perform-
ing electrochemical measurements on complex 4.
Table 3.
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Co2(CO)8 (21 mg, 0.06 mmol) and 1,10-phenanthroline
(16 mg, 0.08 mmol) was added 1 mL of acetonitrile under
a CO atmosphere. After stirring for 30 min at ambient
temperature, a solution of dimethyl 4-butenylpropargyl-
malonate (49 mg, 0.21 mmol) in 1 mL of acetonitrile was
added to the preformed catalyst solution. The solution
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