Catalysis by gold(I) and gold(III): A parallelism between homo- and heterogeneous catalysts for copper-free sonogashira cross-coupling reactions

Article abstract of DOI:10.1002/anie.200604746

(Chemical Equation Presented) Zero, one, or three? The role of Au ⁰, Au^I, and Au^III cations in the heterogeneous gold catalyst Au/CeO₂ was investigated with the copper-free Sonogashira cross-coupling reaction of iodobenzene and phenyl acetylene (see picture). Reactions carried out under both heterogeneous and homogeneous conditions using distinct Au⁰, Au^I, and Au^III species reveal that Au^I is the active species for the cross-coupling reaction.

Full text of DOI:10.1002/anie.200604746

Communications
DOI: 10.1002/anie.200604746
Gold Catalysis
Catalysis by Gold(I) and Gold(III): A Parallelism between Homo- and
Heterogeneous Catalysts for Copper-Free Sonogashira Cross-Coupling
Reactions**
Camino Gonzµlez-Arellano, Alberto Abad, Avelino Corma,* Hermenegildo García,
Marta Iglesias, and FØlix Sµnchez
There is an increasing interest in the chemistry of gold(I) and
gold(III) compounds. A major driving force for this interest
has been the utility of soluble gold compounds and the
observation that gold nanoparticles and supported gold
nanoparticles are active, for instance, for CO oxidation,[1]
ray photoelectron spectroscopy results (see Figure S1 in the
Supporting Information) show the presence of Au0, AuI, and
AuIII. When this solid (5 mol% of gold) was used as a catalyst
for the condensation between iodobenzene and phenylacety-
lene (1:2 molar ratio) in N,N-dimethylformamide (DMF) as
solvent at 1508C using sodium carbonate as base (2 equiv),
full conversion of iodobenzene was obtained in 24 h. The
products observed correspond to 89% of the cross-coupling
product and 11% of biphenyl (the homocoupling product of
iodobenzene). Meanwhile, the phenylacetylene that was
present in twofold excess also gives the corresponding
homocoupling product, 1,4-diphenylbutadiyne, in 30%
yield. We found that when using DMF as solvent, some
leaching of gold occurs. Then, the experiment was repeated
under the same experimental conditions but using o-xylene as
solvent. In this case, the conversion was 50% while the
selectivities of the Sonogashira cross-coupling and the
homocoupling of iodobenzene and phenylacetylene were
nearly the same as before. Au leaching was not observed, and
the Au/CeO2 catalyst could be reused after washing with
acetonitrile and maintained its activity and selectivity.
From these results, we can conclude that the Au/CeO2
catalyst, which contains Au0, AuI, and AuIII species, is active
for performing the copper-free Sonogashira cross-coupling
reaction and also produces, although at a much lower rate, the
homocoupling products of iodobenzene and the alkyne. The
question now is what species are responsible for which
reaction. To answer this question, we used colloidal gold with
a gold particle size distribution (mean diameter 5 nm) very
similar to that observed for gold nanoparticles supported on
nanocrystalline ceria (see the Supporting Information) as
catalyst for the Sonogashira reaction. Under the conditions
described above and using o-xylene as solvent, the conversion
attained with colloidal gold was only 6%, with 46%
selectivity for the desired cross-coupling reaction and 54%
selectivity for the homocoupling product of iodobenzene, and
less than 1% yield of the alkyne homocoupling product. From
the above results, we conclude that Au0 shows much lower
activity for these reactions, probably arising from the
unsaturated gold atoms presents in the small gold nano-
particles. Thus, we were left with AuI and AuIII as potential
active centers for the cross- and homocoupling condensations
observed.
hydrochlorination of acetylene,[2] chemoselective reduction
[4]
of substituted nitroarenes by H2,[3] C C bond formation,
ꢀ
and selective oxidation of alcohols.[5] Moreover, the use of Au
complexes in homogeneous catalysis has undergone a renais-
sance and spectacular achievements have recently been
reported.[6] Carbon–carbon coupling reactions are ubiquitous
in organic synthesis. Heck,[7] Suzuki,[8] and Sonogashira
couplings,[9] occupy a special place among those as a result
of the mild reaction conditions required. The coupling
products are mostly used as intermediates for polymers,
natural products, and bioactive compounds. The most com-
monly used catalytic systems for the Sonogashira reaction
include [PdCl2(PPh3)2], PdCl2/PPh3, and [Pd(PPh3)4] together
with CuI as the co-catalyst, and large amounts of amines as
the solvents or co-solvents.[10]
Taking into account that gold has been successful for
performing the same role as Pd during carbon–carbon bond
formation in the Suzuki–Miyaura condensation,[11] while AuI
has the same d10 electronic structure as CuI and can easily and
properly interact with the acetylenic group,[12] it appeared to
us that gold could catalyze the copper-free Sonogashira cross-
coupling reaction. In order to explore this possibility, we
chose a solid catalyst formed by gold supported on nano-
crystalline CeO2 (Au/CeO2). The reason for this choice is that
AuI and AuIII species are stabilized on this catalyst,[13] and X-
[*] A. Abad, Prof. A. Corma, Prof. H. García
Instituto de Tecnología Química, UPV-CSIC
Avda. de los Naranjos s/n, 46022 Valencia (Spain)
Fax: (+34)96-387-7809
E-mail: acorma@itq.upv.es
Dr. C. Gonzµlez-Arellano, Pr. M. Iglesias
Instituto de Ciencia de Materiales de Madrid, CSIC
C/Sor Juana InØs de la Cruz 3
Cantoblanco, 28049 Madrid (Spain)
Pr. F. Sµnchez
Instituto de Química Orgµnica, CSIC
C/Juan de la Cierva 3, 28006 Madrid (Spain)
[**] The authors thank the Ministerio de Educación y Ciencia (projects
MAT2006-14274-C02-01 and -02) for financial support. C. Gonzµlez
thanks the I3P program for financial support.
To study their potential catalytic role, we prepared AuI
and AuIII complexes with Schiff base ligands derived from 1,1-
binaphthyl-2,2’-diamine ([AuI] and [AuIII]; Figure 1). The
corresponding PdII complex ([PdII]) was also prepared for
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
Table 1: Palladium- and gold-catalyzed Sonogashira cross-coupling
reaction.[a]
R
R’
Yield [%]
AuI
Pd0
PdII
[AuCl(PPh3)]
H
H
Ph
n-C10H17
23
20
30
25
54
35
10[b]
(50)[c]
10
97[b]
(80)[c]
40
H
COOEt
CH2CH(CO2Me)2
Ph
25
25
15
14
10[b]
(90)[c]
95[b]
(30)[c]
[a] Conditions: alkyne (15 mmol), aryl halide (10 mmol), catalyst (20%),
and K3PO4 (20 mmol) in o-xylene at 1308C. Yields after 24 h are given
unless otherwise stated. [b] 48 h. [c] 30% catalyst.
reaction times were necessary with K2CO3 to obtain reason-
able conversions.
In the case of the AuI complex [AuI], moderate to
excellent yields with very high selectivities (ꢁ 99%) for the
cross-coupling Sonogashira reaction were obtained (Table 1).
On the other hand, when the AuIII complex was used as the
catalyst for several of the reactions, the product observed
comprised 10% of alkyne homocoupling product with a very
small amount of the iodobenzene homocoupling product.
When the reaction was carried out under the same exper-
imental conditions but in the absence of iodobenzene, only
the alkyne homocoupling product was detected (8% yield). In
contrast, when the reaction was performed in the absence of
the AuIII complex no reaction occurred which confirms that
AuIII catalyzes the homocoupling reaction of 1-phenyl-
acetylene.
Figure 1. Structures of gold and palladium complexes.
comparison reasons. The soluble ligands and their respective
metal complexes were obtained in high yields by using a
similar method to that described previously.[14] In this type of
potentially tridentate ligands, there are two types of nitrogen
donor ligands and one oxygen phenolate. The nitrogen center
on the pyrrolidine ring is expected to act as a hemilabile
ligand, while the Schiff base nitrogen and oxygen centers act
as the principal donors.
The AuI complex [AuI] was synthesized according to
Equation (1). Thus, [AuCl(PPh3)] (1 mmol) in THF was
For comparison purposes, [PdII] and [Pd(PPh3)4] com-
plexes were also prepared and tested under the same reaction
conditions (Table 1). The results show that the activity of Pd
catalysts is similar to that of the gold analogues, with also a
very high selectivity for the Sonogashira reaction.
The homogeneous catalytic results presented show that
AuI is able to selectively catalyze the cross-coupling reaction,
while AuIII complexes do not catalyze the cross-coupling but
rather the alkyne homocoupling, although at a much lower
reaction rate than AuI catalyzes the Sonogashira cross-
coupling (see Scheme 1).
ðC32H30N2ÞOꢀKþþ½AuClðPPhÞ3Š !
ð1Þ
½ðC32H28N2ÞOfAuðPPhÞ3g3ŠCl þ KCl
added to a solution of the corresponding phenolate ligand
(1 mmol) in THF (20 mL) at 408C. The resultant mixture was
stirred for 2 h, cooled to room temperature, and filtered, and
the filtrate was concentrated under vacuum. The residue was
extracted with dichloromethane and precipitated with pen-
tane; the precipitate was washed several times, filtered, and
dried to afford the yellow-brown, air-stable solid complex in
high yield.
The homogeneous complexes were studied with the
Sonogashira cross-coupling of R-Ph-I (R = H, COOEt) with
a series of alkynes containing electron-donating and electron-
withdrawing substituents (R’) in the alkyne. The reaction was
carried out in a 25-mL vessel at 1308C during 24 h. In a typical
run, a mixture of aryl halide (10 mmol), alkyne (15 mmol),
aqueous potassium carbonate or phosphate (20 mmol), and
catalyst (0.3 mmol) was stirred in 3 mL of o-xylene. The
standard reaction conditions were applied to a series of
catalysts, including Pd0, [Pd(PPh3)4], and the PdII-, AuIII-, and
AuI-Schiff base complexes ([AuI], [AuIII], and [PdI], respec-
tively). The conversions obtained with those catalysts and the
AuI complex [AuCl(PPh3)] are presented in Table 1. For these
reactions, we chose K3PO4 as the mild base because longer
If we establish a parallelism between the results obtained
with Au/CeO2 and the gold complexes, we conclude the
following:
a) The homogeneous catalysis studies show us that AuI,
which has the same d10 electronic configuration as Pd
metal and CuI, is active and very selective for performing
the Sonogashira reaction.
Scheme 1. Product distribution observed in the Sonogashira reaction
catalyzed by gold complexes.
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Communications
b) AuIII does not catalyze the cross-coupling reaction but
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Experimental Section
Sonogashira reaction: The reaction was carried out in a 25-mL vessel
at 1308C. In a typical run, a mixture of aryl halide (10 mmol), alkyne
(15 mmol), aqueous potassium carbonate or phosphate (20 mmol),
and catalyst (0.3 mmol) in o-xylene (3 mL) was stirred, and the
reaction was followed by GC-MS. Gas chromatography analysis were
performed in a Hewlett-Packard 5890 II or/and in a Hewlett-Packard
G1800 A with a quadrupole mass detector using a cross-linked
methylsilicone column. After the desired time, the reaction mixture
was allowed to cool, and a 1:1 mixture of ether/water (20 mL) was
added. The organic layer was washed and separated, the aqueous
layer was further washed with another 10-mL portion of diethyl ether,
and the combined organic extracts were dried with anhydrous MgSO4
and filtered. The solvent and volatiles were completely removed
under vacuum to give the crude product, which was subjected to
column chromatographic separation to give the pure compounds.
Received: November 22, 2006
Published online: January 17, 2007
ꢀ
Keywords: alkynes · C C coupling · gold ·
homogeneous catalysis
.
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