Cross-coupling reactions of gold(I) alkynyl and polyyndiyl complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.11.026

Gold(I) alkynyl complexes are shown to efficiently couple with aryl iodides under mild conditions in the presence of both Pd(II) and Cu(I) co-catalysts. The reaction is not gold catalysed, but rather the Au(I) centre serves to transfer the alkynyl moiety to Cu(I), which then enters the conventional Sonogashira cycles. Using this method, a small range of 1,4-disubstituted diynes, including examples of differentially substituted compounds ArCCCCAr′, have been prepared directly from [(Ph₃P) AuCCCCAu(PPh₃)] and aryl iodides ArI.

Full text of DOI:10.1016/j.jorganchem.2010.11.026

Journal of Organometallic Chemistry 696 (2011) 2172e2176
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cross-coupling reactions of gold(I) alkynyl and polyyndiyl complexes
,
,
Wing Y. Man a, Sören Bock a b, Natasha N. Zaitseva c, Michael I. Bruce c, Paul J. Lowa
*
a Department of Chemistry, Durham University, South Rd, Durham, DH1 3LE, UK
b Technische Universität Chemnitz, Fakultät für Naturwissenschaften, Institut für Chemie, Lehrstuhl für Anorganische Chemie, Straße der Nationen 62, 09111 Chemnitz, Germany
c Department of Chemistry, University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold(I) alkynyl complexes are shown to efficiently couple with aryl iodides under mild conditions in the
presence of both Pd(II) and Cu(I) co-catalysts. The reaction is not gold catalysed, but rather the Au(I)
centre serves to transfer the alkynyl moiety to Cu(I), which then enters the conventional Sonogashira
cycles. Using this method, a small range of 1,4-disubstituted diynes, including examples of differentially
substituted compounds ArC^CC^CAr0, have been prepared directly from [(Ph3P)AuC^CC^CAu(PPh3)]
and aryl iodides ArI.
Received 12 September 2010
Received in revised form
9 November 2010
Accepted 17 November 2010
Keywords:
Gold(I)
Ó 2010 Elsevier B.V. All rights reserved.
Sonogashira
Cross-coupling
CadioteChodiewicz
Diyne
1. Introduction
envisioned. Of particular relevance to the present study it must be
noted that in addition to the readily formed acetylide derivatives
Gold salts and complexes have found an important, rapidly
increasing and varied role in stoichiometric and catalytic trans-
formations of organic substrates, which often takes advantage of
[Au(C^CR)(PPh3)] [28], the bulky {Au(PPh3)} fragment is also able
to stabilise terminal alkynes and more highly reactive carbon-
containing compounds, including polyynes H(C^C)nR [29e31] and
H(C^C)nH [32,33] making these linear carbon fragments available
for use as cross-coupling partners through their gold synthons.
The use of gold polyynes as reagents in Pd/Cu catalysed
CadioteChodkiewicz style cross-coupling reactions with halo-
alkynes has been shown to be of considerable synthetic utility in
the preparation of linear polycarbon chains, including some of
substantial length, supported on a variety of organometallic
supports (Scheme 2) [34e43]. These reactions generally proceed in
moderate to excellent yield in ether-based solvents at ambient
temperatures. In this report we describe the use of gold(I)
complexes of alkynes and diynes as reagents in the Songashira
cross-coupling reaction that afford 1,2-di(aryl) acetylenes (tolans)
and 1,4-di(aryl)-buta-1,3-diynes. Under the conditions described
herein, both Pd(0) and Cu(I) species are necessary to promote the
cross-coupling reaction, which is initiated by a transmetallation
reaction that transfers the acetylide ligand from Au(I) to Cu(I) and
then Pd(II).
the strong
p-acidity of Au(I) and Au(III) species [1e7]. Whilst the
use of gold redox couples within catalytic transformations is not as
prevalent nor as well established as for other metals [8e12], recent
work has established the viability of cross-coupling reactions of
gold(III/I) couples in cross-coupling reactions, with an oxidative
agent not involved in the cross-coupling process used to return the
gold centre to the þIII oxidation state [11e18]. When coupled with
the propensity of Au complexes to activate different CeH bonds
some quite remarkable cross-coupling reactions of otherwise
‘unactivated’ substrates can be achieved (Scheme 1) [19,20].
However, as noted by Hashmi and colleagues [21], the potential
for organogold complexes to enter into transmetallation reactions
with other metals [22e27] coupled with the rather high redox
potential of Au(III) and stability (in the thermodynamic sense) of
many Au(I) species opens an alternative use for Au(I) species in
synthetic chemistry in which the orthogonal reactivity of the two
metals can be utilised to complete otherwise challenging synthetic
processes. The well-established isolobal relationship between [Au
(PPh3)]þ and Hþ allows further CeH activation chemistry to be
2. Results and discussion
The notion of ‘palladium-free’, gold-catalysed Sonogashira
cross-coupling reactions has attracted some attention [44], but very
* Corresponding author. Tel.: þ44 (0)191 334 2114; fax: þ44 (0)191 384 4737.
E-mail address: p.j.low@durham.ac.uk (P.J. Low).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.026

W.Y. Man et al. / Journal of Organometallic Chemistry 696 (2011) 2172e2176
2173
Scheme 1. A gold catalysed cross-coupling of unactivated substrates [19].
Scheme 2. The preparation of a C19 chain from a gold-modified CadioteChodkiewicz coupling reaction [43].
recent re-examination of the conditions suggests that the role of
adventitious palladium contamination of the gold salts used in
these processes must be considered carefully. Indeed, as little as
0.1 mol % Pd in the form of [Pd2(dba)3] and 2 mol% AuI/dppe in the
presence of K2CO3 can catalyse the cross-coupling reaction of 4-
iodoacetophenone with 1-ethynyl-4-methylbenzene (tolylacety-
lene) in refluxing toluene (Table 1) [45].
Similar results have been obtained from mixed Pd/Au catalysed
cross-coupling reactions of aryl halides [46,47] and arene diazo-
nium salts [48] with terminal alkynes in amine or amine containing
solvent mixtures, at temperatures ranging from ambient to the
reflux point of the solvent. In each case, no effective coupling was
observed in the absence of palladium, and the reactions can be
considered as analogues of the conventional Pd/Cu co-catalysed
Sonogashira reaction, with the gold reagent taking the place of the
Cu(I) salts more commonly employed. Very recently, pre-formed
gold-alkynyl complexes [Au(C^CAr)(L)] (L ¼ phosphine, Ar ¼ aryl
group) have also been cross-coupled with aryl iodides Ar’-I under
‘copper-free’ palladium-catalysed conditions to give the corre-
sponding tolan, AreC^CeAr0, in quantitative yield, albeit under
rather forcing conditions (Scheme 3) [45].
Given the considerably milder conditions under which organ-
ometallic halides and gold-alkynyl derivatives enter into cross-
coupling reactions in the presence of both Pd(II) and Cu(I)
complexes and salts (ether-based solvents, room temperature),
a range of simple cross-coupling reactions of [Au(C^CPh)(PPh3)]
(1) were explored. Treatment of 1 with iodo benzene in thf in the
presence of [PdCl2(PPh3)2] (1 mol %) and CuI (1 mol %) resulted in
complete conversion to diphenylacetylene (tolan) within 2 h at
60 ꢀC. There was no significant conversion in the absence of either
of the co-catalysts. The reaction mechanism under these mild
conditions therefore likely involves transmetallation of the alkynyl
moiety from Au(I) to Cu(I), which then enters the normal Sonoga-
shira cycle. The more activated aryl iodides 4-iodo nitrobenzene
Table 1
The Au(I)/Pd(0) catalysed cross-coupling of 4-iodoacetophenone and tolylacetylene [45].
Pd/mol%
conversion/%
yield/%
e
<2
6
16
<2
6
1.2 ꢁ 10ꢂ4
1.2 ꢁ 10ꢂ3
1.2 ꢁ 10ꢂ2
0.12
16
24
82
78
<2a
24
100
100
<2
1.2
1.2
a
Room temperature.

2174
W.Y. Man et al. / Journal of Organometallic Chemistry 696 (2011) 2172e2176
Scheme 3. The palladium-catalysed cross-coupling of 4-iodoacetophenone with a gold(I) alkynyl complex [45].
Table 2
coupling protocol, the conditions described in Table 2 may be useful
when attempting to cross-couple aryl halides bearing particularly
base-sensitive substituents. Despite the cost differential of gold-
and silyl-substituted alkynes, there are also some identifiable
advantages in the conditions employed in Table 2 over Nishihara’s
sila-Sonogashira protocol in terms of the quantity of copper salt
necessary to effect the cross-coupling and the use of the lower-
boiling thf solvent [49].
Effect of catalyst and halide on the Sonogashira cross-coupling of 1 with aryl halides.
With these results in hand, attention was turned to the cross-
coupling reactions of the bis(gold) compound [(Ph3P)AuC^CC^-
CAu(PPh3)] (2). Preparative routes to symmetric 1,4- substituted
diynes are commonly based on oxidative homo-coupling reactions
of terminal alkynes [50e53], and as such end-group trans-
formations of a pre-formed 1,3-diyne such as 2 offer an interesting
alternative reaction sequence.
AreX
[Pd]a/mol%
CuI/mol%
T/ꢀC
t/h
%conversion
C6H5I
C6H5I
C6H5I
C6H5Br
e
5
1
5
5
1
1
1
1
1
1
5
e
1
5
5
1
1
1
1
1
1
60
60
60
60
60
25
25
25
60
60
25
17
17
2
22
22
1.5
1.5
16
2
e
e
100
Trace
e
C6H5Cl
The bis(gold) reagent 2, which is readily available from [AuCl
(PPh3)] and Me3SiC^CC^CSiMe3, was only sparingly soluble in thf,
and consequently all reactions with this reagent were carried out
in CH2Cl2 to ensure homogenous reaction conditions. At room
temperature, 2 reacted smoothly and essentially completely with
2 M equivalents of 4-iodonitrobenzene, 4-iodo methyl benzoate
and 4-iodo methyl benzene in the presence of 0.5 mol% (per gold
equivalent) [PdCl2(PPh3)2] and CuI (Table 3). An entirely analogous
reaction with the bis(halide) 4-iodo bromobenzene also proceeded
smoothly and with excellent chemoselectivity to afford 1,4-bis(40-
bromophenyl)-buta-1,3-diyne as the only detectable product. Aryl
iodides bearing more electron donating groups were also quanti-
tatively converted in [PdCl2(PPh3)2]/CuI catalysed reactions with 2
to give the 1,4-disubstituted buta-1,3-diynes in reactions that
proceeded slowly at room temperature, but much more efficiently
at slightly elevated temperatures (Table 3). These results are
complementary to preparative routes involving Glaser, Hay or other
oxidative dimerisation reactions of 1-alkynes [54]. Clearly the
conditions set out in Table 3 will not be competitive with known
strategies in every case, and certainly not for the preparation of
IC6H4NO2
IC6H4CO2Me
IC6H4NH2
IC6H4NH2
IC6H4OMe
BrC6H4NO2
100
100
e
100
100
e
2
20
a
[Pd] ¼ [PdCl2(PPh3)2]
and 4-iodo methylbenzoate smoothly cross-coupled with 1 at
25 ꢀC, and on a preparative scale 1-phenyl-2-(40-nitrophenyl)
acetylene was isolated in 87% yield from 1 and 4-iodo nitrobenzene.
Bromobenzene coupled significantly more slowly, at 60 ꢀC, whilst
chlorobenzene was essentially an inactive coupling partner: such
results fall completely in line with expectation based on the ease of
oxidative addition to the palladium catalyst (Table 2). When
compared with the conditions summarised in Scheme 3, it can be
seen that the introduction of CuI permits the cross-coupling of
a gold acetylide with, preferably, and aryl iodide under very mild
conditions, in ether-based solvent and without an additional base.
Although not generally superior to the ‘normal’ Sonogashira cross-
Table 3
Cross-coupling reactions of 2 with aryl halides.
AreX
[Pd]a/mol%
CuI/mol%
T/ꢀC
t/h
%conversion
IC6H4NO2
IC6H4CO2Me
IC6H4Br
IC6H4CH3
IC6H4NH2
IC6H4OMe
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
25
25
25
25
60
60
6
100
97
90
100
100
100
24
24
24
2
2
a
[Pd] ¼ [PdCl2(PPh3)2]

W.Y. Man et al. / Journal of Organometallic Chemistry 696 (2011) 2172e2176
2175
Table 4
The preparation of differentially substituted diynes from 2.
AreX
[Pd]a/mol%
CuI/mol%
T/ꢀC
t/h
%conversion
IC6H4NO2
IC6H4CO2Me
IC6H4NH2
IC6H4OMe
IC6H4Br
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
35
35
35
35
35
24
24
24
24
24
100
100
100
100
100
a
[Pd] ¼ PdCl2(PPh3)2.
bulk quantities of material. However, there is a potential advantage
for the preparation of compound libraries in that only a single,
readily available diyne precursor is used in conjunction with an aryl
halide, as opposed to requiring the initial pre-synthesis of the
terminal alkyne, which normally entails a coupling and depro-
tection approach.
reagents were purchased and used as received. All reactions were
performed under nitrogen atmosphere in air-tight sealed test-tubes
or GC-vials which were prepared and filled in an Innovative Tech-
nologies glove box. GCeMS analyses were performed on an Agilent
Technologies 6890 N gas chromatograph equipped with a 5973
Inert Mass Selective Detector and a 10 m fused silica capillary
column (5% cross-linked phenylmethylsilicone). UHP helium was
used as the carrier gas. The percentage conversion of the starting
material was monitored with reference to an internal dodecane
To further test the applicability of the reaction conditions, the
sequential reactions of 2 with two different aryl iodides was also
examined. Initial reactions of 2 with one equivalent of either
4-iodo methylbenzoate or 4-iodo nitrobenzene, followed by the
addition of a second aryl iodide were complicated by the facile
reaction of the activated iodide with both gold groups in the
initial stage of the reaction. Cleaner reactions were obtained from
initial reaction of iodobenzene and 2 for 3 h, presumably to
generate [Au(C^CC^CPh)(PPh3)] in situ, followed by treatment
with other aryl iodides (Table 4). Careful control of the stoichi-
ometry of the first reaction step is required if double coupling to
give 1,4-diphenyl-buta1,3-diyne is to be avoided. Nevertheless, this
sequence of cross-coupling reactions neatly illustrates the prepa-
ration of differentially substituted 1,4-diynes from a symmetrically
substituted precursor, and complements existing preparative
methods such as the CadioteChodkiewicz coupling protocol.
calibrant, which was present in all solutions at 100
ml/100 ml
of solvent, and estimated to be accurate to within ꢃ10%.
4.2. General conditions
The gold reagent 1 (25 mmol), arylhalide (25 mmol), [PdCl2(PPh3)2]
(1.0 ml of a 1.24 ꢁ 10ꢂ3 M stock solution in thf, 1.24 ꢁ 10ꢂ6 mol), CuI
(0.5 ml of a 2.47 ꢁ 10ꢂ3 M stock solution in NCMe, 1.24 ꢁ 10ꢂ6 mol)
were added successively to a vial, topped up to 5 ml with THF and
sealed with a rubber septum or crimp-cap. The reaction was stirred,
and heated if indicated, and monitored by GCeMS. For reactions
described in Table 3, 50
reactions described in Table 4, 25
initially. After 3 h of reaction at 35 ꢀC, 25
m
mol of aryl iodide was employed. For the
mol of iodobenzene was added
mol of the second aryl
m
m
iodide was added, and the reaction allowed to proceed overnight
before analysis.
3. Conclusion
Gold(I) alkynyl and diyndiyl complexes can be cross-coupled
with aryl iodides under mild conditions in the presence of both
Pd(II) and Cu(I) co-catalysts, without the need for any additional
base. The reaction likely proceeds via the ready transmetallation
of the alkynyl fragment to Cu(I), followed by a conventional
Sonogashira cross-coupling mechanism. The simple conversion of
the masked butadiyne reagent Me3SiC^CC^CSiMe3, via [(Ph3P)
AuC^CC^CAu(PPh3)] to 1,4-diaryl buta-1,3-diynes has been
achieved, and stepwise functionalisation of each Au moiety has
been demonstrated to afford differentially 1,4-disubstituted
1,3-diynes.
4.3. Preparation of 1,4-bis-triphenylphosphinegold-buta-1,3-diyne
(2)
1,4-Bis-trimethylsilyl-buta-1,3-diyne [0.16 g, 0.8 mmol] and
sodium hydroxide [0.64 g, 16 mmol] were dissolved in methanol
[120 ml]. After stirring for 30 min, [AuCl(PPh3)] [0.75 g, 1.52 mmol]
was added, and the resulting white suspension stirred for 3 h. The
resulting solid was collected by filtration and recrystallised
(CH2Cl2/Et2O) to afford the title compound (0.5 g, 65%). 31P NMR
(CD2Cl2):
d
¼ 43.39 ppm.
4. Experimental
4.4. Preparation of 1-phenyl-2-(40-nitrophenyl) acetylene
4.1. General details
A Schlenk flask was charged with compound 1 (0.150 g,
0.27 mmol) and 4-iodo nitrobenzene (0.067 g, 0.27 mmol) in
thoroughly degassed thf (10 ml) and treated with [PdCl2(PPh3)2]
[9 mg, 0.01 mmol] and CuI [2 mg, 0.01 mmol]. The reaction was
allowed to stir at room temperature for 4 h, after which time the
The compounds [Au(C^CPh)(PPh3)] [28], [(Ph3P)AuC^CC^CAu
(PPh3)] [32] and [PdCl2(PPh3)2] [55] were prepared according to
literature methods, or the minor variations described below. Other

2176
W.Y. Man et al. / Journal of Organometallic Chemistry 696 (2011) 2172e2176
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solvent was removed. The residue was dissolved in the minimum
volume of benzene, and hexane added, causing the precipitation of
[AuI(PPh3)] as a white solid (0.105 g, 67%). The solution was purified
by preparative TLC using hexane as eluant. The first band contained
PhC^CC^CPh (1.7 mg, 6%), the major slightly yellow band con-
tained the title compound (0.052 g, 87%). The last band contained
additional [AuI(PPh3)], together with some unidentified material.
Acknowledgements
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We thank Prof. T.B. Marder for generous access to GCeMS
facilities. S.B. was on placement at Durham University from Tech-
nische Universität Chemnitz. We thank the EPSRC and University of
Durham for funding. W.Y.M. holds a studentship from the Durham
Doctoral Training Account. M.I.B. held an EPSRC Visiting Fellowship.
P.J.L. holds an EPSRC Leadership Fellowship.
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