Gold-catalysed homocoupling of alkynes

Article abstract of DOI:10.3184/174751913X13605921421864

The homocoupling reaction of alkynes was carried out in the presence of 10 mol% AuCl3 using I2 as oxidant to generate 1,3-diynes products in high yield. The method is simple, safe and efficient. Both aryl and alkyl substituted alkynyl substrates reacted smoothly. AuClPPh3 also showed comparable catalytic activity to this transformation under a nitrogen atmosphere.

Full text of DOI:10.3184/174751913X13605921421864

174 RESEARCH PAPER
MARCH, 174–176
JOURNAL OF CHEMICAL RESEARCH 2013
Gold-catalysed homocoupling of alkynes
Peng Yao
Department of Chemistry and Chemical Engineering; Xinxiang University; Xinxiang 453003, P. R. China
The homocoupling reaction of alkynes was carried out in the presence of 10 mol% AuCl3 using I2 as oxidant to gener-
ate 1,3-diynes products in high yield. The method is simple, safe and efficient. Both aryl and alkyl substituted alkynyl
substrates reacted smoothly. AuClPPh3 also showed comparable catalytic activity to this transformation under a
nitrogen atmosphere.
Keywords: alkynes, homocoupling, gold-catalysis, Au(I) /Au(III) catalytic cycle
The formation of C–C bonds by the use of transition metal
catalysts is one of the most interesting and intriguing subjects
in organic chemistry. Various transition metals such as Pd, Ru,
Rh, Ni have been extensively investigated.1–4 The catalysis of
organic reactions by gold compounds has emerged as a power-
ful tool and recently received increased attention.5–13 A variety
of classical organic transformations, especially the formation
of C–C bonds,14–20 have been shown to be mediated by Au(I) or
Au(III) complexes.21–26
While the relationship between Au(I/III) and Pd(0/II) has
been noted,27–29 gold-catalysed “palladium-like” cross-
coupling, such as Sonogashira30–31 and Suzuki32–33 couplings
have been reported. The Au(I/III) system can be accessed
through the use of oxidant, such as tert-butylhydroperoxide,
PhI(OAc)2, or Selectfluor. We report here an atom-economical
process to form alkyne–alkyne bonds by the homocoupling
of alkynes catalysed by AuCl3 using iodine as the oxidant to
facilitate this oxidative transformations.
When we subjected a mixture of phenylacetylene and K2CO3
in DMSO to 10 mol% AuCl3, 1,4-diphenylbuta-1,3-diyne
could be detected in a yield of 25%. When the catalyst loading
was increased to 50%, we obtained the product in a yield of
61%. We speculated that Au(III) would revert to Au(I) during
the catalytic reaction and lose catalytic activity. In light of the
mechanism of Pd-catalysed homocoupling reactions,34,35 we
tried to use additives in order to decrease the catalyst loading
and increase the catalytic activity.
Then we studied the effect of the solvent under these
optimum conditions. The homocoupling did not proceed in
toluene and THF (Table 2, entries 1 and 2). Other solvents such
as CH2Cl2, hexane, CH3OH were inefficient (Table 2, entries
3–5). The reaction proceeded smoothly in DME (1,2-dime-
thoxyethane) under these conditions (Table 2, entry 6). DMSO
and DMF gave the best yields of 94% and 91% to afford the
expected products (Table 2, entries 7 and 8).
The base is also an important factor in this reaction. Organic
bases like Et3N hindered the formation of 1,4-diphenylbuta-
1,3-diyne (Table 3, entry 1). Na2CO3 performed better than
NaOH, K3PO4, KOH or CsF (Table 3, entries 2–6). K2CO3 and
Cs2CO3 showed the best catalytic activity (Table 3, entries 7
and 8).
Having identified the optimum conditions (Table 3, entry 8),
we examined the scope of the reaction, with regards to each of
the alkynes bearing both functionalised aromatic rings and
simple alkyl substituents. These proved to be suitable sub-
strates for homocoupling in good to excellent yield (Table 4,
condition A). Variation of the arylacetylene indicated that
both electron-deficient and electron-rich arenes could be
employed in the C–C bond forming step. (Table 4, entries
1–6). Heterocyclic compounds like pyridylacetylene also gave
the corresponding products smoothly in respectable yield.
Gold chelating with nitrogen atoms might reduce its catalytic
activity. (Table 4, entry 7) The use of aliphatic alkynes resulted
in reduced yields (Table 4, entries 8–9), and cyclohexenylacet-
ylene could also give the diyne in moderate yield (Table 4,
entry 10).
Results and Discussion
We chose tert-butylhydroperoxide, PhI(OAc)2, Selectfluor,
t-BuOOt-Bu and I2 as the oxidants (Table 1, entries 1–6),
1 equiv. of iodine gave the 1,3-diyne in a yield of 81% in the
presence of 10 mol% AuCl3. This was better than the other
oxidants. We examined the amount of I2, and 1.5 equiv. showed
the best yield of 94% (Table 1, entries 6–8).
Since gold catalysed the homocoupling reaction, the activity
of gold should be high enough to insert between C-H bonds
and if conditions permit, alkynes should be able to couple with
aryl halides. In our system, we found that cross-coupling reac-
tion of 4-methoxyiodobenzene and phenylacetylene without
iodine took place in 52% yield of the cross-coupling product,
Table 1 Homocoupling reaction in the presence of various
oxidantsa
Table 2 The screening of various solventsa
Entry
Oxidant/equiv.
Yield /% b
Entry
Solvent
Yield/%b
1
2
3
4
5
6
7
8
-
tert-Butylhydroperoxide
PhI(OAc)2
10
60
68
63
69
81
94
93
1
2
3
4
5
6
7
8
Toluene
THF
CH2Cl2
Hexane
CH3OH
DME
0
5
Selectfluor
t-BuOOt-Bu
I2 (1.0)
16
13
21
60
91
94
I2 (1.5)
I2 (2.0)
DMF
DMSO
a Reaction conditions: phenylacetylene (1.0 mmol), AuCl3
(0.1 mmol), oxidant, K2CO3 (2 mmol), DMSO (5 mL), 100 °C, 12 h.
b GC yields.
a Reaction conditions: phenylacetylene (1.0 mmol), AuCl3
(0.1 mmol), I2 (1.5 mmol), K2CO3 (2 mmol), solvent (5 mL),
100 °C, 12 h.
* Correspondent. E-mail: xinxiangyaopeng@126.com
b GC yields.

JOURNAL OF CHEMICAL RESEARCH 2013 175
Table 3 Base effect of homocoupling reactiona
Entry
Base
Yield/%b
1
2
3
4
5
6
7
8
Et3N
NaOH
K3PO4
KOH
0
27
29
34
37
55
90
94
CsF
Na2CO3
Cs2CO3
K2CO3
Scheme 2
a Reaction conditions: phenylacetylene (1.0 mmol), AuCl3
(0.1 mmol), I2 (1.5 mmol), base (2 mmol), DMSO (5 mL), 100 °C,
D, which reacts with the terminal alkyne in the formation
of complex E. Finally, the reductive elimination of cross-
coupling products takes place and releases Au(I) to complete
the catalytic cycle. In this catalytic process, the relationship
between Au(III) and Au(I) is very similar to Pd(0) and Pd(II).
Furthermore, we speculated that the Au(I) complexes may
show catalytic activity in homocoupling reactions according to
the mechanism we proposed. hence we used AuClPPh3 as the
catalyst to replace AuCl3 for promoting this reaction. However,
the introduction of air was not successful as in Au(III) cataly-
sis. Similar activity can be obtained when the system was
protected by inert gas and the results were shown in Table 4
(condition B). Notably, heterocyclic or alkyl alkynes gave
increased yields of the corresponding 1,3- diyne. (Table 4,
entries 7–10).
12 h.
b GC yields.
Table 4 Gold-catalysed homocoupling reaction
Entry
R
Yield/%a
Yield/%b
1
2
3
4
5
6
7
8
9
4-MeOC6H4–
4-MeC6H4–
4-EtC6H4–
4-n-PrC6H4–
C6H5–
81
91
86
87
90
81
49
46
43
88
86
89
86
90
83
68
74
77
4-FC6H4–
2-pyridinyl–
n-C4H9–
n-C6H13–
Conclusions
10
65
70
In conclusion, we report our results concerning the synthesis
of 1,3-diynes by gold catalysis. We studied Au(III)-catalysed
homocoupling reactions of terminal alkynes and the key point
of the reaction is the use of the readily available iodine as
oxidant. Aromatic alkynes are more reactive than aliphatic
alkynes. The homocoupling reactions take place using
AuClPPh3 as a catalyst without the presence of iodine under
N2. Heterocyclic or alkyl alkynes gave increased yields of the
corresponding 1,3- diyne.
a Reaction condition A: alkynes (1.0 mmol), AuCl3 (0.1 mmol), I2
(1.5 mmol), K2CO3 (2 mmol), DMSO (5 mL), 100 °C, 12 h,
isolated yields. b Reaction condition B: alkynes (1.0 mmol),
AuClPPh3 (0.1 mmol), K2CO3 (2 mmol), DMSO (5 mL), under N2,
100 °C, 12 h, isolated yields.
while the product of homocoupling was 28% (Scheme 1).
According to our experimental results, gold salts not only cata-
lysed the self-coupling reaction of alkyne, but also brought
about the cross-coupling of aryl halides with alkyne.
Experimental
All solvents were purified and dried according to standard methods
prior to use. The quantitative analysis of the reaction products was
performed on an Agilent 6890/5975 GC/MS instrument with a pro-
grammable split/splitless injector. The injector-port temperature was
set at 270 °C. The oven-temperature program was initially set at 80 °C
and ramped to 220 °C at 5.0 °C min−1, and maintained at 220 °C for
2.0 min at each step. Proton NMR spectra were recorded in CDCl3 on
a Bruker Avance III 400 MHz spectrometer. Proton chemical shifts are
reported in parts per million (ppm) relative to tetramethylsilane (TMS)
with the residual solvent peak as the internal reference. Multiplicities
are reported as: singlet (s), doublet (d), triplet (t) and multiplet (m).
HRMS (EI) data were collected on High Resolution mass
spectrometer(MAT 900 XL, Thermo Finnigan, USA). All materials
were purchased from common commercial sources and used without
additional purification.
With regard to our results and literature reports,36 we pro-
pose the following reaction mechanism (Scheme 2). The triva-
lent gold catalyst reacts with the terminal alkyne to generate
the alkynyl gold complexes A. The homocoupling product is
obtained after the reductive elimination and generates B
(Au(I)), which under the action of iodine is re-oxidised to
trivalent gold C. Then the reaction of the terminal alkyne with
acetylene gave complex A to complete the catalytic cycle.
Without the iodine in the system as oxidant, Au(III) cannot be
reduced to Au(I) after the catalytic reaction. In the cross-cou-
pling reaction between acetylene and aryl halides without I2,
the oxidative addition of Au(I) to iodobenzene forms complex
Scheme 1

176 JOURNAL OF CHEMICAL RESEARCH 2013
Typical procedure
(28), 91 (15). HRMS (EI) Calcd for C16H18 (M+) 210.1409; found
210.1408.
Au(III)-catalysed homocoupling: I2 (1.5 mmol)was added to a mixture
of alkynes (1 mmol), AuCl3 (0.1 mmol), K2CO3 (2 mol) and DMSO
(5 mL). The solution was stirred at 100 °C under air for 12 h. After
being cooled to room temperature and filtered through a filter paper,
water (5 mL) was added to the solution which was extracted with Et2O
(3 × 10 mL). The combined organic phases were evaporated under
reduced pressure, and the residue was subjected to flash column
chromatography to obtain the desired product.
Au(I)-catalysed homocoupling: A mixture of the alkynes (1 mmol),
AuClPPh3 (0.1 mmol), K2CO3 (2 mol) and DMSO (5 mL) was stirred
at 100 °C under N2 for 12 h. After being cooled to room temperature
and filtered through a filter paper, water (5 mL) was added to the
solution which was extracted with Et2O (3 × 10 mL). The combined
organic phases were evaporated under reduced pressure, and the
residue was subjected to flash column chromatography to obtain the
desired product.
1-Methyl-4-(phenylethynyl)benzene[Scheme 1]: White solid, m.p.
1
59–60 °C (lit.30 60 °C); H NMR (400 MHz, CDCl3, TMS) δ 7.45–
7.51 (m, 4H), 7.26–7.33 (m, 3H), 6.85 (d, J = 8.0 Hz, 2H), 2.38 (s,
3H). MS (EI): m/z (%): 192 (100) [M+], 177 (36), 149 (30). HRMS
(EI) Calcd for C15H11O (M+) 208.0888; found 208.0889.
This work is accomplished with the help of Gai-rong Chen,
Shao-hong Xu and Shun-mei Wang and I am particularly grate-
ful to them for their help.
Received 4 December 2012; accepted 16 January 2013
Paper 1201657 doi: 10.3184/174751913X13605921421864
Published online: 12 March 2013
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