Subnanometer gold clusters on amino-functionalized silica: An efficient catalyst for the synthesis of 1,3-diynes by oxidative alkyne coupling

Article abstract of DOI:10.1021/acscatal.7b00691

Subnanometer (d = 0.8 ± 0.2 nm) gold particles homogeneously dispersed on amino-functionalized silica catalyze Glaser-type alkyne coupling, providing corresponding 1,3-diynes under mild conditions. Readily available λ³-iodane PhI(OAc)₂ is used as an oxidant and 1,10-phenanthroline is used as an additive. Ten symmetrical 1,3-diynes and three products of heterocoupling containing various functionalities are isolated in high yields. The catalyst can be recycled at least five times, giving consistently high isolated yields and maintaining the size and distribution of gold clusters. This unique combination of stable subnanometer gold clusters and hypervalent iodine thus paves a hitherto unexplored avenue in organic synthesis employing heterogeneous gold catalysis. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.7b00691

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Letter
Sub-Nanometer Gold Clusters on Amino-Functionalized Silica: An Efficient
Catalyst for the Synthesis of 1,3-Diynes by Oxidative Alkyne Coupling
Beáta Vilhanová, Ji#í Václavík, Luca Artiglia, Marco
Ranocchiari, Antonio Togni, and Jeroen Anton van Bokhoven
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00691 • Publication Date (Web): 07 Apr 2017
Downloaded from http://pubs.acs.org on April 7, 2017
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Sub-Nanometer Gold Clusters on Amino-Functionalized Silica: An Ef-
ficient Catalyst for the Synthesis of 1,3-Diynes by Oxidative Alkyne
Coupling
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Beáta Vilhanová,†,‡,¶ Jiří Václavík,†,§,¶ Luca Artiglia,‖ Marco Ranocchiari,*,‖ Antonio Togni,*,† Jeroen
A. van Bokhoven*†,‖
† Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zürich, Vladimir-Prelog-
Weg 1-2, 8093 Zürich, Switzerland
‡ Department of Organic Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague, Czech
Republic
§ Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo nám. 2, 166 10 Prague,
Czech Republic
‖ Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland
ABSTRACT: Sub-nanometer (d = 0.8 ± 0.2 nm) gold particles homogeneously dispersed on amino-functionalized silica catalyze
Glaser-type alkyne coupling, providing corresponding 1,3-diynes under mild conditions. Readily available λ3-iodane PhI(OAc)2 is
used as the oxidant and 1,10-phenanthroline as additive. Ten symmetrical 1,3-diynes and three products of heterocoupling contain-
ing various functionalities are isolated in high yields. The catalyst can be recycled at least five times giving consistently high isolat-
ed yields and maintaining the size and distribution of gold clusters. This unique combination of stable sub-nanometer gold clusters
and hypervalent iodine thus paves a hitherto unexplored avenue in organic synthesis employing heterogeneous gold catalysis.
alkyne coupling, gold, heterogeneous catalysis, hypervalent iodine, subnanometer
The chemistry of 1,3-diynes has been flourishing in the re-
cent years1 thanks to their growing importance as building
blocks for the synthesis of pharmaceuticals,2 optoelectronic
materials,3 and polymers.4 Such compounds are usually pre-
pared by Glaser-type oxidative coupling of the corresponding
alkynes.1a,1b,5 Frequently, the coupling is based on the use of
CuII salts, typically Cu(OAc)2 (Glaser-Eglinton coupling),6 or
CuI salts with ligands such as N,N,N′,N′-tetramethylethane-
1,2-diamine (TMEDA) and oxygen (Glaser-Hay coupling).7
Since heterogeneous catalysis is often preferred with regard to
facile catalyst separation and recycling,8 efficient heterogene-
ous catalysts, mainly based on Cu,9 but also on Pd9a and Ni,9a
have been developed for alkyne homocoupling. Typical draw-
backs of such catalysts include their cumbersome or expensive
preparation, high metal loadings, high amounts of base, and
elevated reaction temperature. Notably, only some of these
heterogeneous catalysts have been investigated in the coupling
of two different alkynes (heterocoupling),9b,h–j which is still a
challenging topic.9a
(Boronat et al. focused on the alkyne coupling itself12c) and
13
Au/La2O3 (but not Au/SiO214) or in surface reactions per-
formed under vacuum without isolating the products.15 There
is thus clearly room for the development of novel heterogene-
ous gold catalysts for alkyne coupling, which could overcome
the drawbacks described above and provide the valuable un-
symmetrical products of heterocoupling.
In early 2016, we disclosed a material featuring exceptional-
ly small sub-nanometer (d = 0.8 ± 0.2 nm) gold particles sup-
ported on 3-aminopropyl-functionalized silicate SBA-15.16
Literature comparison reveals that the particles are as small as
17
17a,18
Au13 or even Au11
clusters. This material (further re-
ferred to as catalyst A) was soon found to be highly active and
selective in dehydrogenation of formic acid16 and transfer hy-
drogenation of various N-heterocyclic compounds.19 Conse-
quently, we wish to report here the use of the stable sub-
nanometer gold particles of A in a practical and efficient
method for oxidative alkyne coupling with good to excellent
yields even for the heterocoupled products.
Interestingly, despite the profound interest in heterogeneous
gold catalysis and its indisputable role in fine chemical syn-
thesis,10 Au-catalyzed alkyne coupling studies are scarce, with
only a handful of homogeneously catalyzed examples report-
ed.11 In addition, 1,3-diynes have been observed as side-
We started our investigations by mixing phenylacetylene
(1a) with catalyst A in reagent-grade dichloromethane (DCM)
– no reactivity was detected during 18 h at ambient tempera-
ture (Table 1, entry 1). Adding hypervalent iodine-based oxi-
dant PhI(OAc)211b,c,20 to the reaction mixture did not promote
12
products in Sonogashira coupling catalyzed by Au/CeO2
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Table 1. Screening the reactivity of A with alkyne 1a, oxi-
Table 2. Screening of alkynes 1a–j in oxidative coupling
catalyzed by A.a
dant PhI(OAc)2, and various basic additives.a
1
2
3
4
5
6
7
8
9
A
A
+
R
R'
R
R'
PhI(OAc)2, additive,
DCM, rt
PhI(OAc)2, 1,10-phen,
DCM, rt
1
1
2
1a
2a
yield (%)b
96
entry
additive
--
conversion (%)b
entry
1c
1
2
1c
0
0
1a
2a
2b
2c
2d
2c
3
1b
1c
1d
98
94
89
2
--
3
1,10-phen
pyridine
99
0
10
11
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4d
5d
6d
7d
8
4
benzo[h]quinoline
triethylamine
TMEDA
0
0
0
5c
6
1e
1f
2e
2f
92
97
MeO
OMe
48
68
2,2¢-bipyridine
2,2¢-bipyridinee
9
O
aReaction conditions: substrate 1a (0.03 mmol), catalyst A (1
mol% Au vs. substrate), PhI(OAc)2 (0.045 mmol, 1.5 equiv), ad-
ditive (0.012 mmol, 0.4 equiv), DCM (0.40 mL), rt, 18 h. Deter-
mined by GC-MS. Reaction without PhI(OAc)2. Reaction time
of 6 h; 0.4 and 1.0 equiv tested, giving the same result. e1 equiv.
O
7c
8
1g
1h
2g
2h
98 (92)
84
F
F
b
c
d
F3
C
CF3
F3
C
CF3
the coupling reaction (Table 1, entry 2). Gratifyingly, howev-
er, when using the oxidant together with 1,10-phenanthroline
(1,10-phen) as additive,11c we suddenly observed full conver-
sion to the target 1,3-diyne (diphenyldiacetylene, 2a) (Table 1,
entry 3). Replacing additive 1,10-phen with pyridine, ben-
zo[h]quinoline, triethylamine, or TMEDA7,21 yielded no prod-
uct (Table 1, entries 4–7). On the contrary, reactions using
2,2′-bipyridine proceeded with moderate conversion to 2a
(Table 1, entries 8 and 9). Full conversion was achieved in
1,2-dichloroethane (DCE, Table S4, entries 1 and 2), but sig-
nificantly lower reactivity was observed in DMF (17% con-
version), THF (32%), 1,4-dioxane (63%), acetonitrile (72%),
and chloroform (9%) (Table S4, entries 3–19). Varying the
amount of 1,10-phen (Table S5, entries 1–5) dramatically in-
fluenced the reactivity, with the initially chosen 0.4 molar
equivalents being optimal (Table S5, entry 4). The amount of
PhI(OAc)2 was found to provide best efficiency within 1–1.5
equivalents (Table S5, entries 4 and 6–9).
9
1i
1j
2i
2j
F3CO
OCF3
90
93
O
10
O
11d 1e+1g 2eg
12d 1e+1h 2eh
MeO
F
80
60
CF3
MeO
CF3
13d 1e+1i 2ei
60
MeO
OCF3
aReaction conditions: substrate 1 (0.3 mmol, 1 equiv), catalyst A
(5 mol% Au vs. alkyne 1), 1,10-phenanthroline (0.12 mmol, 0.4
equiv), PhI(OAc)2 (0.45 mmol, 1.5 equiv), DCM (4 mL), rt, 18 h.
bYield of isolated products (2 mmol scale in parentheses). c1
mol% Au vs. substrate. Amount of 1g–i: 2 equiv vs. 1e; amount
of A: 3.3 mol% vs. both alkynes together.
d
our preliminary screening with alkynes 1e, 1f and 1i displayed
slower reaction rates than with 1a at room temperature, the
catalyst loading was increased to ensure full conversion with-
out the necessity of individual subtle optimization. The corre-
sponding symmetrical 1,3-diynes 1a–j were isolated in high
yields and purity (Table 2, entries 1–10) even on a larger scale
(Table 2, entry 7). In addition, products of heterocoupling of
non-polar fluorinated alkynes 1g–i with slightly more polar
methoxy-substituted 2e were obtained in good yields (Table 2,
entries 11–13) under the same reaction conditions. Diyne 2eg
is known and has been isolated before at yields of 44% (20
mol% of CuBr2 + ligand and pyrrolidine in DCM at rt),23 57%
(10 mol% of CuI + pyrrolidine in ethyl lactate, 50 °C),24 75%
(0.2 mol% of Na2PdCl4 + 2 mol% of CuI + ligand + Et3N in
DMF at rt),25 81% (1 mol% of Cu@silica + piperidine in DCM
at rt),9b and 82% (6 mol% of Cu@MnOx in toluene at 105
°C).9j This shows that the yield of 80% (Table 2, entry 11)
achieved with A under mild conditions is fairly competitive in
relation to the literature examples.
When developing catalyst A, we also synthesized materials
B–D consisting of larger gold particles supported on NH2-
SBA-15 or SBA-15 (Tables S1–S3).16 While the catalyst fea-
turing large gold nanoparticles on amino-functionalized silica
(B) gave 35% conversion in 6 h (Table S6, entry 1), those on
SBA-15 (C, D) were catalytically inactive (Table S6, entries 2
and 3). Similarly, test reactions with plain NH2-SBA-15, SBA-
15, and without catalyst gave no conversion (Table S6, entries
4–6). This implies that the alkyne coupling requires gold parti-
cles on the amino-functionalized support. Smaller particles
exhibit considerably higher reactivity than large ones – this
can be related to the number of accessible gold atoms and
numbers of edge and corner sites, which are highest with the
smallest particles.22
Optimized reaction conditions thus comprised catalyst A (1
mol% gold with respect to alkyne), 1,10-phen (0.4 equiv),
oxidant PhI(OAc)2 (1.5 equiv), reagent-grade DCM as solvent,
and room temperature. To examine the scope of the oxidative
homocoupling reaction catalyzed by A, structurally different
alkynes 1a–j were tested on a preparative scale (Table 2). As
Next, we investigated the recyclability of catalyst A in the
homocoupling of 1b on the preparative scale – after each cy-
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cle, the catalyst was filtered off, washed with DCM, dried, and The reaction kinetics featured an induction period within the
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weighed for the subsequent run because each time a small
amount of A was inevitably lost due to manipulation. The
filtrate and solvent from catalyst washing were combined and
the product was isolated by column chromatography. Under
these conditions, catalyst A was reused five times, delivering
product 2b in excellent yields (Figure 1). Z-contrast scanning
electron microscopy (ZC-STEM) revealed that the sub-
nanometer gold particles of A retained their size and homoge-
neous distribution even after five cycles (Figure 2).
first hour, after which the reaction followed a first-order kinet-
ic profile (Figure 3 right). Employing degassed solvent pro-
vided an identical kinetic curve (Figure 3 right), suggesting
that oxygen is not required for the reaction like in the case of
copper-catalyzed Glaser-Hay coupling.1a,1b,5,7 When PhI(OAc)2
was added after 1 h of stirring of all other reaction compo-
nents, the induction period was still present, but adding the
alkyne after stirring the other reactants for 1 h resulted in a
faster onset of the reaction with the induction part being less
pronounced (Figure S3). Also, the initial rate of the second
catalytic cycle was notably higher compared to the first run
(Figure S4).
9
The heterogeneous nature of the catalyst was tested by sepa-
rating A from the reaction mixture after 1 h and 1.5 h and
measuring the conversion of alkyne 1a to product 2a in the
stirred filtrate. In both cases the conversion increased only by
1% within 24 h (Figure 3 left), which can be attributed to
measurement uncertainty rather than catalytic activity of the
liquid phase. An ICP-OES analysis of the filtrate showed gold
concentration of about 1.3 ppm (= µg/g), which accounts for
<1 % of the total gold content in the reaction. At 40 °C the
leached amount of gold increased to 3.6 ppm, and therefore
ambient temperature was preferred in the reactions described
above so as to minimize the undesirable leaching.
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The kinetic experiments, particularly the behavior of the in-
duction period upon changing the reaction conditions, indicate
that the catalyst undergoes some type of activation, presuma-
bly via partial oxidation of gold by reacting with the λ3-iodane
PhI(OAc)2. Toste and co-workers showed that dendrimer-
encapsulated gold clusters on SBA-15 are oxidized by PhICl2
to AuIII species – as detected by X-ray photoelectron spectros-
copy (XPS) measurements – that catalyzes diastereoselective
cyclopropanation.26 By means of XPS, we checked the oxida-
tion state of the gold clusters in A before and after the catalytic
reaction. The spectra (Figure 4) featured large Au 4f peaks,
which could be separated into two doublets. The one at lower
binding energy (red line), displaying the 4f7/2 and 4f5/2 at 85.0
and 88.6 eV, respectively, can be associated to gold nanoclus-
ters (Au).27 As compared to the binding energy of bulk gold
(84.0 eV), this peak is positively shifted by 1.0 eV. This is a
final state effect due to the sub-nanometer dimension of the
gold clusters, which at d < 1.0 nm are no longer metallic28 and
can readily undergo redox processes.29 The second doublet
(blue line), whose peaks are found at 86.4 and 89.7 eV, sug-
gests the presence of gold in a higher oxidation state
(AuIII).26,27c,d,30 Furthermore, the Au/AuIII ratio does not change
after the reaction, in good agreement with the data about the
stability of the catalyst (Figure 1 and 2), and indicates that
exposure to oxygen leads to partial oxidation.29
A
5 times
PhI(OAc)2, 1,10-phen,
DCM, rt
1b
2b
100
80
60
40
20
0
1
2
3
4
5
Cycle
Figure 1. Isolated yields of diyne 2b obtained from 1b using cata-
lyst A (5 mol%) and oxidant PhI(OAc)2 (1.5 equiv) in DCM
overnight in five cycles.
Figure 2. ZC-STEM images of material A – a) before performing
catalysis, and b) after five consecutive uses as catalyst in homo-
coupling of 1b. Particle size histograms, average particle size and
standard deviation are included, as obtained from these images.
100
80
60
40
20
0
100
80
60
40
20
0
Standard reaction
Catalyst removed after 1.5 h
Catalyst removed after 1 h
Reagent grade DCM
Degassed DCM
0
4
8
12
16
20
24
0
1
2
3
4
5
6
Time (h)
Time (h)
Figure 3. Time-conversion plots of homocoupling of alkyne 1a
using catalyst A and PhI(OAc)2. Left: Standard run and runs after
removal of A. Right: Comparison of reagent-grade and degassed
DCM.
Figure 4. XPS spectra of material A before and after its use in
catalysis.
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The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures and supplementary data (characteriza-
tion of materials, supplementary results) as addressed in the text
(PDF)
Literature points to three conceivable mechanistic pathways.
The calculations done by Corma et al. on oxidized Au clusters
on CeO2 show that two vicinal gold-activated alkyne mole-
1
2
3
4
5
6
7
8
12c
cules couple together on the gold surface, yielding the homo-
coupling product. The second one is based on homogeneous
gold catalysis works,11c,d suggesting that geminal alkynyl moi-
eties of an AuIII complex reductively eliminate to form the 1,3-
diyne. While Peng and co-workers assume that the required
ACKNOWLEDGMENT
The authors thank Dr. Frank Krumeich from the Electron Micros-
copy Center of ETH Zürich (EMEZ) for the TEM imaging. We
also thank Prof. T. Jung for providing access to the XPS instru-
ment and J. Nowakowski for performing the measurements.
bis(alkynyl)AuIII
compound
originates
from
a
mono(alkynyl)AuIII species by reacting with another alkyne
molecule,11c Corma et al.11d propose that an alkynyl-AuI spe-
cies is oxidized by Selectfluor to form a AuI-AuIII digold com-
plex that gives the bis(alkynyl)AuIII intermediate via
transmetallation. The last scenario, proposed by de Haro and
Nevado31 and adopted by Zhu et al.,11b involves the formation
of a gold-activated alkynyl(phenyl)iodonium salt32 that reacts
with another alkyne molecule to form a vinyl-gold intermedi-
ate that decomposes while producing the 1,3-diyne.
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Our catalytic system requires the presence of 1,10-phen (or
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pling, providing the unsymmetrical 1,3-diynes under the same
conditions. The catalyst was perfectly recyclable owing to
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AUTHOR INFORMATION
Corresponding Author
* E-mail: marco.ranocchiari@psi.ch (M.R.), atogni@ethz.ch
(A.T.), jeroen.vanbokhoven@chem.ethz.ch (J.v.B.).
Author Contributions
¶B.V. and J.V. contributed equally.
Notes
Any additional relevant notes should be placed here.
ASSOCIATED CONTENT
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Isolated yields of diyne 2b obtained from 1b using catalyst A (5 mol%) and oxidant PhI(OAc)2 (1.5 equiv) in
DCM overnight in five cycles.
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ZC-STEM images of material A – a) before performing catalysis, and b) after five consecutive uses as
catalyst in homo- coupling of 1b. Particle size histograms, average particle size and standard deviation are
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Time-conversion plots of homocoupling of alkyne 1a using catalyst A and PhI(OAc)2. Left: Standard run and
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XPS spectra of material A before and after its use in catalysis.
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TOC Graphic
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