A simple one-pot procedure for the direct homocoupling of terminal alkynes promoted by copper nanoparticles

Article abstract of DOI:10.1055/S-0029-1217051

The reaction of different terminal alkynes with readily prepared copper(O) nanoparticles led to the formation of the corresponding symmetrical 1,4-disubstituted 1,3-diynes in good yield. No palladium, ligands, or oxidant additives were needed. For aliphatic terminal alkynes, the use of sodium carbonate as base markedly improved the reaction rate and allowed the use of a substoichiometric amount of the copper nanoparticles.

Full text of DOI:10.1055/S-0029-1217051

PAPER
4027
A Simple One-Pot Procedure for the Direct Homocoupling of Terminal
Alkynes Promoted by Copper Nanoparticles
H
omocoupling
a
of
T
ermin
b
al Alkynes
P
i
romote
a
d by Coppe
n
r
N
anopartic
a
les Nador, Leandro Fortunato, Yanina Moglie, Cristian Vitale, Gabriel Radivoy*
Instituto de Química del Sur (INQUISUR-CONICET), Departamento de Química, Universidad Nacional del Sur,
Avenida Alem 1253, 8000 Bahía Blanca, Argentina
Fax +54(291)4595187; E-mail: gradivoy@criba.edu.ar
Received 28 May 2009; revised 4 August 2009
gies with respect to environmental and economic consid-
erations has attracted the attention of diverse research
groups. For example, J.-H. Li and co-workers have report-
ed an efficient amine- and phosphine-free protocol for the
Abstract: The reaction of different terminal alkynes with readily
prepared copper(0) nanoparticles led to the formation of the corre-
sponding symmetrical 1,4-disubstituted 1,3-diynes in good yield.
No palladium, ligands, or oxidant additives were needed. For ali-
phatic terminal alkynes, the use of sodium carbonate as base mark- homocoupling of terminal alkynes in the presence of pal-
edly improved the reaction rate and allowed the use of a
substoichiometric amount of the copper nanoparticles.
ladium(II) chloride, copper(I) iodide, trimethylamine N-
oxide, and sodium acetate.18 It is worthy to note that the
Key words: copper nanoparticles, alkynes, dimerizations, cou-
pling, 1,3-diynes
copper-mediated (palladium-free) self-coupling of termi-
nal alkynes has received limited attention. Furthermore,
there are few reports in the literature of copper-promoted
homocoupling reactions in the absence of any other addi-
Conjugated diynes are very important building blocks in tive or palladium catalyst.19 Regarding the palladium-free
organic synthesis.1 C–C triple bonds, which can be easily synthesis of 1,3-diynes, very recently D. Li and co-work-
functionalized through multiple addition reactions, are ers have reported a copper(I) iodide/iodine mediated path-
key structural moieties in the preparation of intermediates way for the synthesis of symmetrical 1,3-diynes using
for the synthesis of natural products2 and pharmaceuticals sodium carbonate as base instead of amines.20 Taking into
such as antifungal,3 antitumor,4 antibacterial,5 and anti- account environmental considerations, the recently re-
inflammatory agents.6 On the other hand, and mainly due ported copper salt mediated homocoupling of
to the solid-state properties of their homopolymers, 1,3- alkynylboronates21 or alkynyltrifluoroborates22 appears to
diynes have found important application in the construc- be particularly attractive, although both methodologies re-
tion of new molecular materials such as p-conjugated quire the pre-transformation of the corresponding alkynes
acetylenic oligomers and polymers,7 macrocycles,8 and into the starting organoboron compounds.
supramolecular structures.9 Consequently, research into
the coupling of terminal alkynes continues to expand.
nm in diameter) in C–C coupling reactions has attracted
On the other hand, the use of metal nanoparticles (2–10
Traditional methods for the synthesis of 1,4-disubstituted considerable attention in recent years.23 Although the spe-
1,3-butadiynes include copper(I)- or copper(II)-mediated cific role of these nanoparticles in the reaction mecha-
Glaser,10 Eglinton,11 or Hay12 dimerization of terminal nisms is not always easy to ascertain, such nanoparticles
alkynes. In these reactions, the formation of alkynylcop- have been demonstrated to exhibit unique catalytic prop-
per species is proposed, leading to the corresponding 1,3- erties due to their high surface area and the possibility of
diynes by a subsequent oxidative dimerization reaction.13 charge distribution in electron-transfer reactions.24
Palladium-mediated transformations such as the homo-
In the last years, we have been actively working in the
coupling of terminal alkynes catalyzed by palladium(0)–
area of reduction and coupling of a variety of organic
copper(I) systems,14 or the palladium-catalyzed dimeriza-
compounds with new reactive systems based on the use of
tion of terminal alkynyl halides with terminal alkynes,15
a mixture of readily available iron(II) or copper(II) chlo-
have been shown to be the most efficient and selective for
rides (hydrated or anhydrous), an excess of lithium pow-
the synthesis of 1,3-diynes; however, palladium reagents
der, and a catalytic amount of an arene as electron carrier.
are expensive and they often require the use of amines as
We used the commercially available hydrated chlorides
well as phosphine ligands which are generally noxious,
(FeCl2·4H2O or CuCl2·2H2O) for the reduction of differ-
sensitive to air, and/or expensive. Moreover, E,Z-enynes
ent organic functionalities (i.e., carbonyl compounds,
as side products are often formed,16 and the presence of a
imines, alkyl and aryl halides, sulfonates),25a–e the crystal-
stoichiometric quantity of an added oxidant (air, O2, I2) is
lization water being the source of hydrogen in these re-
essential for alkyne dimerization in most cases.17 In recent
ducing systems, whereas for the coupling reactions (i.e.,
years, the development of more convenient methodolo-
dimerization of Grignard reagents, reductive coupling of
nitroaromatics, cycloaddition of azides and terminal
alkynes),25f–h we used the corresponding anhydrous chlo-
rides. We have discovered that the above-mentioned reac-
tive systems, under mild reaction conditions (THF as
SYNTHESIS 2009, No. 23, pp 4027–4031
Advanced online publication: 19.10.2009
DOI: 10.1055/s-0029-1217051; Art ID: M02909SS
© Georg Thieme Verlag Stuttgart · New York
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x.
x
x
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F. Nador et al.
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solvent, at room temperature), render very reactive, mono- dergoing Glaser dimerization, probably due to the weaker
disperse (3.0 1.5 nm), spherical iron(0) or copper(0) acidity of the acetylenic proton.26 Under the above-
nanoparticles.25a–c We wish to present herein a simple and outlined reaction conditions, 1-hexyne (1e), 1-dodecyne
economical methodology for a convenient access to sym- (1f), 5-methyl-1-hexyne (1g), and 4-phenyl-1-butyne (1h)
metrical 1,3-diynes via the homocoupling of terminal were converted into the corresponding 1,4-dialkyl-1,3-
alkynes promoted by readily generated copper(0) nano- diynes in good yields (Table 1, entries 6, 8, 10, and 12, re-
particles (CuNPs), in the absence of palladium, ligands, or spectively), although longer reaction times were needed
any oxidant additive. To the best of our knowledge, this is compared to their aryl counterparts. In addition to the in-
the first report describing the direct homocoupling of ter- trinsic lower reactivity of the alkylacetylenes, we suspect
minal alkynes promoted by copper(0) nanoparticles.
that the increased reaction times could also be attributed
to the partial agglomeration of the CuNPs that occurs on
prolonged heating. In fact, we have recently observed by
TEM analysis that these copper nanoparticles agglomer-
ate to a size of 15–30 nm after heat treatment for more
than six hours at the reflux temperature of tetrahydrofuran
(Figure 1).27
The reaction of a series of aryl and alkyl terminal alkynes
(1.0 mmol) with CuNPs, generated in situ from anhydrous
copper(II) chloride (4.0 mmol), lithium sand (4.0 mmol),
and a catalytic amount of 4,4¢-di-tert-butylbiphenyl
(DTBB; 2.5 mol%, relative to the copper salt) as electron
carrier, in tetrahydrofuran at reflux gave the correspond-
ing symmetrical 1,4-disubstituted 1,3-diynes in good
yield (Scheme 1). Control experiments, using phenylacet-
ylene as the test compound, confirmed that no conversion
occurs in the absence of the CuNPs.
CuNPs
R
H
R
R
oxidant-, ligand-
and palladium-free
2a–k
1a–k
R = aryl, alkyl
Scheme 1 CuNPs-promoted homocoupling of terminal alkynes
Figure 1 Left: CuNPs after stirring at room temperature for 6 h;
right: CuNPs after refluxing in tetrahydrofuran for 6 h.
Reactions were conducted under inert atmosphere (N2),
thus avoiding the common side reactions of diynes (or
sensitive starting alkynes) associated with the Glaser cou-
pling reaction and various improved Glaser oxidative
dimerization reactions, which require the use of oxidant
additives.
Under the same reaction conditions, alkynes bearing an
oxy functionality, such as 2-(2-propynyloxy)tetrahydro-
2H-pyran (1i) and 2-propyn-1-ol (1j), were converted into
the corresponding conjugated diynes 2i and 2j in good
yield (Table 1, entries 13 and 14, respectively). Finally,
taking into account the important significance of the ho-
mocoupling of conjugated enynes for the construction of
new molecular materials, we applied our methodology to
1-ethynyl-1-cyclohexene (1k), which was smoothly con-
verted into the corresponding 1,4-di(1-cyclohexenyl)-1,3-
butadiyne (2k) as the only reaction product in good yield
(Table 1, entry 15).
Table 1 summarizes the results obtained for various aryl-
and alkylacetylenes. First, we chose phenylacetylene to
optimize the reaction conditions with the goal of high
yields in minimum reaction times. Thus, we observed that
the use of an excess of CuNPs (4.0 equiv, relative to the
starting alkyne), and heating to 66 °C (reflux temperature
of THF), was essential for the total conversion of the start-
ing phenylacetylene into the desired homocoupled prod-
uct. Under these optimized reaction conditions,
phenylacetylene (1a) was quantitatively converted into
1,4-diphenyl-1,3-butadiyne (2a) (Table 1, entry 1). Low-
ering the reaction temperature from 66 °C to room tem-
perature also resulted in total conversion, but in much
longer reaction times (Table 1, entry 2). With regard to the
effect of electronic properties of the substituents attached
to the aromatic ring in arylacetylenes, we observed that
substrates bearing electron-releasing groups (Table 1, en-
tries 3 and 4) were more reactive than those bearing elec-
tron-withdrawing groups (Table 1, entry 5). The same
reactivity pattern has been previously observed by others
in the homocoupling of terminal arylacetylenes.14a,18,19
In an attempt to reduce the reaction times of the dimeriza-
tion of aliphatic alkynes, we studied the effect of the addi-
tion of a base in order to promote a base-assisted
deprotonation of the starting alkyne. Interestingly, we ob-
served that the addition of sodium carbonate (1.0 equiv,
relative to the starting alkyne) to the reaction medium not
only markedly improved the reaction rate, but also al-
lowed the use of a substoichiometric amount of the
CuNPs (0.5 equiv, relative to the starting alkyne). Thus,
under these reaction conditions, aliphatic alkynes 1e, 1f,
and 1g were converted into the corresponding 1,3-diynes
in 6–10 hours and in good to excellent yield (Table 1, en-
tries 7, 9, and 11). In contrast, the reactivity of the terminal
arylacetylenes was not affected by the presence of sodium
carbonate, both with substrates bearing electron-with-
drawing or electron-releasing groups attached to the aro-
matic ring. Other bases tested, such as triethylamine,
pyridine, sodium acetate, cesium carbonate, or potassium
We then examined the application of the present protocol
to the homocoupling of various alkylacetylenes. It is
known that aliphatic terminal alkynes are sluggish in un-
Synthesis 2009, No. 23, 4027–4031 © Thieme Stuttgart · New York

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Homocoupling of Terminal Alkynes Promoted by Copper Nanoparticles
4029
Table 1 CuNPs-Promoted Homocoupling of Terminal Alkynesa
Entry
1
Starting alkyne
Time (h)
8
Product
Yieldb (%)
90
(2a)
(2a)
(1a)
(1a)
2
3
30c
10
88
75
Me2N
NMe2
(2b)
(1b)
Me2N
4
5
12
18
72
65
MeO
F3C
MeO
F3C
OMe
(1c)
(2c)
CF3
(1d)
(2d)
F3C
F3C
CF3
6
7
24
6d
74
n-Bu
n-Bu
n-Bu
n-Bu
(1e)
(1e)
(1f)
(1f)
(2e)
(2e)
78d
66
n-Bu
n-Bu
8
23
10d
20
n-C10H21
n-C10H21
n-C10H21
n-C10H21
n-C10H21
n-C10H21
(2f)
(2f)
9
69d
65
10
(2g)
(1g)
11
12
6d
80d
71
(2g)
(1g)
20
(1h)
(2h)
13
17
85
(1i)
(2i)
O
O
O
O
O
O
14
15
16
20
68
73
(1j)
(2j)
HO
HO
OH
(2k)
(1k)
a Reaction conditions: 1 (1.0 mmol), CuNPs (4.0 mmol), and THF (7 mL) at the reflux temperature of THF (66 °C), unless otherwise stated.
b Isolated yield after column chromatography (hexane–EtOAc), based on the starting alkyne.
c Reaction performed at room temperature.
d Reaction conditions: 1 (1.0 mmol), CuNPs (0.5 mmol), Na2CO3 (1.0 mmol), and THF (7 mL) at the reflux temperature of THF (66 °C).
Na2CO3 was added to the reaction mixture prior to the addition of the alkyne.
phosphate, did not show any improvement in the reactivi- of the starting alkyne prior to its coordination to the
ty of either alkyl- or arylacetylenes.
CuNPs.
Although the mechanism involved is difficult to ascertain, Finally, although the active species in the reaction medi-
based on previous results published by Rothenberg and um is most probably a copper(I) or copper(II) species, our
co-workers on the copper nanoclusters promoted Sono- in situ generated copper(0) nanoparticles exhibited a dif-
gashira-type cross-coupling reaction,28 we suppose that ferent behavior compared to copper salts, which have
the terminal alkyne could coordinate to the copper nano- been shown to need the presence of a palladium catalyst,
particles forming an {alkynyl–CuNPs} intermediate, an oxidant, or other additives to promote the homocou-
which could then suffer a reductive elimination process pling of terminal alkynes. As for many of the copper-
leading to the corresponding 1,3-diyne product. The en- promoted coupling reactions,29 a clear understanding of
hanced reaction rate achieved in the presence of an added the exact role of additives or ligands, as well as the copper
base (Na2CO3) could be related to the deprotonation step species involved, is still open to study.
Synthesis 2009, No. 23, 4027–4031 © Thieme Stuttgart · New York

4030
F. Nador et al.
PAPER
with those of a commercially available, pure sample. For new com-
pounds, physical and spectroscopic data follow:
In conclusion, we have developed a new and simple one-
pot methodology for the synthesis of 1,3-diynes from ter-
minal alkynes, under mild reaction conditions, by using
readily generated copper(0) nanoparticles, in the absence
of palladium, ligands, amine reagents, or any oxidant ad-
ditive. For aliphatic terminal alkynes, the addition of sodi-
um carbonate to the reaction medium markedly improves
the reaction rate and allows the use of a substoichiometric
amount of the CuNPs. We are actively exploring new ap-
plications of this coupling system to the synthesis of un-
symmetrical 1,4-disubstituted 1,3-diynes, as well as to
Sonogashira-type coupling reactions.
1,4-Bis[3,5-bis(trifluoromethyl)phenyl]-1,3-butadiyne (2d)
Reddish brown semisolid.
1H NMR (300 MHz, CDCl3): d = 7.83–7.91 (m, 4 H, ArH), 7.92–
8.02 (m, 2 H, ArH).
13C NMR (75 MHz, CDCl3): d = 76.2 (2 × C), 79.8 (2 × C), 122.8
(q, 1JC–F = 272.3 Hz, 4 × CF3), 123.0 (2 × ArCH), 123.7 (2 × ArC),
132.4 (4 × ArCH), 132.5 (q, 2JC–F = 34.0 Hz, 4 × ArCCF3).
MS (EI, 70 eV): m/z (%) = 475 (20) [M+ + 1], 474 (100) [M+], 455
(26), 405 (13).
HRMS: m/z calcd for C20H6F12: 474.2468; found: 474.2471.
All moisture-sensitive reactions were carried out under N2 atmo-
sphere. Anhydrous THF was freshly distilled from sodium/ben-
zophenone ketyl. Other solvents were treated prior to use by
standard methods.30 Column chromatography was performed with
Merck silica gel 60 (0.040–0.063 mm, 240–400 mesh). Thin-layer
chromatography was performed on precoated silica gel plates
(Merck 60, F254, 0.25 mm). NMR spectra were recorded on a Bruk-
er ARX-300 spectrometer using CDCl3 as solvent (unless otherwise
stated) and TMS as internal reference. Mass spectra (EI) were ob-
tained at 70 eV on a Hewlett Packard HP-5890 GC/MS instrument
equipped with a HP-5972 mass selective detector. The purity of vol-
atile compounds and gas chromatographic analyses were deter-
mined with a Shimadzu GC-9A instrument equipped with a flame-
ionization detector and a 2-m column (1.5% OV17 9A SUS Chrom
103 80/1000), using N2 as carrier gas. All the starting alkynes 1a–k
(see Table 1) were purchased from commercial sources (Aldrich,
Fluka, Merck) and were used as received.
2,11-Dimethyl-5,7-dodecadiyne (2g)
Yellow oil.
1H NMR (300 MHz, CDCl3): d = 0.89 (d, J = 6.9 Hz, 12 H, 4 ×
CH3), 1.36–1.47 (m, 4 H, 2 × CH2), 1.65–1.76 (m, 2 H, 2 × CH),
2.26 (t, J = 7.2 Hz, 4 H, 2 × CH2).
13C NMR (75 MHz, CD3OD): d = 17.8 (2 × CH2), 22.5 (4 × CH3),
28.4 (2 × CH), 38.5 (2 × CH2), 66.3 (2 × C), 77.8 (2 × C).
MS (EI, 70 eV): m/z (%) = 190 (0.15) [M+], 175 (11), 133 (12), 119
(59), 105 (58), 91 (100), 77 (42), 63 (22), 51 (21), 43 (33), 41 (67).
HRMS: m/z calcd for C14H22: 190.3314; found: 190.3306.
1,6-Bis(tetrahydro-2H-pyran-2-yloxy)-2,4-hexadiyne (2i)
Green semisolid.
1H NMR (300 MHz, CDCl3): d = 1.35–1.88 (m, 12 H, 6 × CH2),
3.35–3.84 (m, 4 H, 2 × CH2), 4.26 (s, 4 H, 2 × CH2), 4.73–4.97 (m,
2 H, 2 × CH).
13C NMR (75 MHz, CDCl3): d = 19.0 (2 × CH2), 25.3 (2 × CH2),
30.2 (2 × CH2), 54.4 (2 × CH2), 62.0 (2 × CH2), 70.0 (2 × C), 75.3
(2 × C), 97.0 (2 × CH).
MS (EI, 70 eV): m/z (%) = 278 (0.02) [M+], 85 (100), 77 (14), 76
(17), 75 (11), 67 (17), 57 (20), 55 (21), 43 (19), 41 (30), 39 (15), 29
(15), 28 (12).
Homocoupling of Terminal Alkynes; General Procedure
A mixture of lithium sand (28 mg, 4.0 mmol), DTBB (26 mg, 0.1
mmol), and CuCl2 (538 mg, 4.0 mmol) in THF (5.0 mL) was stirred
at r.t. under N2 atmosphere. The reaction mixture, which was initial-
ly dark green, changed to black, indicating the formation of the cop-
per(0) nanoparticles (CuNPs). The reaction flask was introduced
into a preheated silicon oil bath, at a temperature high enough to en-
sure the reflux of the solvent, and then the alkyne (1.0 mmol) in
THF (2 mL) was slowly added by syringe. For the base-assisted
dimerization of aliphatic alkynes, the CuNPs were generated by
stirring a mixture of lithium sand (14 mg, 2.0 mmol), DTBB (26 mg,
0.1 mmol), and CuCl2 (67 mg, 0.5 mmol) in THF (5.0 mL); after
heating the mixture for 30 min, Na2CO3 (108 mg, 1.0 mmol) was
added. Then, the alkyne (1.0 mmol) in THF (2.0 mL) was slowly
added by syringe. After total conversion of the starting material
(TLC, GC), the resulting suspension was diluted with Et2O (10 mL),
then treated with 10% HCl (10 mL), and the aqueous layer was ex-
tracted with Et2O (3 × 10 mL). The combined organic extracts were
dried (anhyd Na2SO4) and concentrated (20 mbar). The resulting
residue was purified by flash column chromatography (silica gel,
hexane–EtOAc) to give the corresponding 1,3-diyne.
HRMS: m/z calcd for C16H22O4: 278.3513; found: 278.3520.
Acknowledgment
We thank the CONICET (Project PIP-5104) and SGCyT-UNS
(Project 24/Q026), Argentina for financial support. F.N. also thanks
the SGCyT-UNS for a doctoral fellowship.
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Homocoupling of Terminal Alkynes Promoted by Copper Nanoparticles
4031
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Synthesis 2009, No. 23, 4027–4031 © Thieme Stuttgart · New York