Strategy to isolate ionic gold sites on silica surface: Increasing their efficiency as catalyst for the formation of 1,3-diynes

Article abstract of DOI:10.1016/j.apcata.2020.117444

A new strategy is presented to obtain an efficient heterogeneous gold catalyst constituted by isolated ionic gold sites, which is known to be effective in alkyne coupling reaction. The procedure is based on a significant difference between offered gold amount and available adsorbent sites on the support, ensuring the formation of very active isolated gold ion sites. In order to achieve this purpose, mesoporous silica xerogel was grafted with an ionic silsesquioxane containing charged ammonium quaternary group. The modified silica showed 0.25 mmol of cationic sites per gram of material and presented thermal stability up to 200 °C. This material was applied as support for immobilization of Au(III) ions as square planar AuCl₄^? complex. The gold amount offered was just 12 % of the exchangeable capacity. The catalyst was efficiently applied in the cross coupling reactions, in which only 0.22 mol% was applied to obtain symmetric and non-symmetric 1,3-diynes.

Full text of DOI:10.1016/j.apcata.2020.117444

Applied Catalysis A, General 594 (2020) 117444
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Strategy to isolate ionic gold sites on silica surface: Increasing their
efficiency as catalyst for the formation of 1,3-diynes
a
a
a
a
b
Cezar A. Didó , Felipe L. Coelho , Maurício B. Closs , Monique Deon , Flavio Horowitz ,
b
a,
a,
Fabiano Bernardi , Paulo H. Schneider *, Edilson V. Benvenutti *
Institute of Chemistry, UFRGS, CP 15003, CEP 91501-970, Porto Alegre, RS, Brazil
a
b
Institute of Physics, UFRGS, CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Organic-inorganic hybrid composites
Sol-gel process
Gold
Alkynes
A new strategy is presented to obtain an efficient heterogeneous gold catalyst constituted by isolated ionic gold
sites, which is known to be effective in alkyne coupling reaction. The procedure is based on a significant dif-
ference between offered gold amount and available adsorbent sites on the support, ensuring the formation of
very active isolated gold ion sites. In order to achieve this purpose, mesoporous silica xerogel was grafted with
an ionic silsesquioxane containing charged ammonium quaternary group. The modified silica showed 0.25 mmol
Cross-coupling
C-C coupling
of cationic sites per gram of material and presented thermal stability up to 200 °C. This material was applied as
−
4
support for immobilization of Au(III) ions as square planar AuCl
complex. The gold amount offered was just
12 % of the exchangeable capacity. The catalyst was efficiently applied in the cross coupling reactions, in which
only 0.22 mol% was applied to obtain symmetric and non-symmetric 1,3-diynes.
−
1. Introduction
A very common gold complex is the AuCl
4
anion [31,32], which
can be easily obtained by dissolving gold metal in aqua regia and, as far
as we know, this complex has not been applied in coupling reactions of
alkynes. This complex can be covalently immobilized on the surface of
inorganic supports containing chelating groups, such as amine or sulfur
[33–35]. Another way to immobilize this complex is by using the ion-
exchange process, in which the inorganic matrix is previously modified
with cationic organic sites, such as quaternary ammonium groups
[12,36]. However, the immobilized gold complexes have a thermo-
dynamic trend to aggregate and convert themselves into gold clusters or
nanoparticles [12,31,34,37,38].
Aiming to avoid such aggregation, in the present work we have
designed a catalyst containing a very small amount of isolated ionic
gold sites on the silica surface. This aim was achieved by using a silica
xerogel matrix that was previously functionalized with an ionic silses-
quioxane. This charged compound is a hybrid oligomeric material that
presents organic chain with quaternary ammonium group and it is
covalently bonded to a silica moiety. This charged chain provides water
solubility and ion exchange capacity. Furthermore, the silica part has
silanol groups that undergo crosslink reactions and graft the silses-
Although gold is considered inert noble metal in the bulk form, it
has emerged in the last decade as a powerful catalyst in reactions for
organic transformations when used at the nanoscale or even lower
[
1–7]. Considering that heterogeneous catalysis allows the recovering
of the catalyst, the development of gold catalyst dispersed in several
porous inorganic supports has become important [8–12]. Among the
inorganic supports, silica has been extensively employed because it can
be easily synthesized by sol-gel process, with designed characteristics
such as texture and morphology [12–15]. Additionally, silica presents
intrinsic desired properties such as chemical, thermal and mechanical
stability, even though, it enables surface functionalization by using
grafting reactions [12,14,16].
Gold complex is a very efficient catalyst in several organic reactions
[
4,17] including the cross-coupling reactions of terminal alkynes to
synthesize symmetric and non-symmetric products of 1,3-diynes
17–23]. These compounds present great relevance as building blocks
and provide a large number of compounds in the chemical, pharma-
ceutical and materials industry. The success of gold complex as catalyst
has been explained based on its redox potential Au(I)/Au(III)
[
quioxane as films onto inorganic surfaces [39]. By using the ion-ex-
−
4
[
6,20,24–27], however the precise nature of the redox process for gold
change process, AuCl
complex was immobilized and the offered gold
chloride complex is not well established. [28–30].
amount was much lower than the available cationic sites. The catalyst
⁎
Corresponding authors.
E-mail addresses: paulos@iq.ufrgs.br (P.H. Schneider), benvenutti@ufrgs.br (E.V. Benvenutti).
https://doi.org/10.1016/j.apcata.2020.117444
Received 14 November 2019; Received in revised form 30 January 2020; Accepted 31 January 2020
Available online 01 February 2020
0926-860X/ © 2020 Elsevier B.V. All rights reserved.

C.A. Didó, et al.
Applied Catalysis A, General 594 (2020) 117444
was efficiently applied in cross coupling reactions to obtain symmetric
purified by washing with distilled water and ethylic alcohol, and va-
and non-symmetric 1,3-diynes.
cuum dried at ambient temperature for additional 3 h. This material
was named SiO
.5. Characterization
UV-Vis spectrum of HAuCl
160 1PC Shimadzu spectrophotometer, at room temperature. For solid
materials, Agilent CARY 5000 equipment was used to acquire UV–vis
diffuse reflectance spectra by applying Kubelka-Munk approach [12].
Shimadzu Instrument model TGA-50 was used to perform the thermo-
gravimetric analysis (TGA), using a heating rate of 20 °C min
argon flow. Chloride potentiometric analysis was performed using
AgNO3 standard solution and Digimed DM-20 potentiometer
equipped with chloride selective electrode. The nitrogen isotherms
were determined at 77 K, using a Tristar II Krypton 3020 Micromeritics
instrument. Samples were previously degassing at 125 °C, under va-
cuum for 14 h. BET multipoint method was applied to estimate the
surface area. BJH and DFT methods were used to obtain the pore size
distributions [41]. Agilent 500 MHz instrument was used to acquire
room-temperature 29Si NMR spectra, using CPMAS probe (4 mm) with
2
/SSQ/Au.
2
. Experimental section
2
2.1. Synthesis of the silsesquioxane (SSQ)
4
aqueous solution was recorded in a UV-
The procedure to obtain the ionic silsesquioxane was based on a
previous report [40]. The 1,4-diazabicyclo[2.2.2]octane (Sigma-Al-
drich, 6.0 g) and 12.2 mL of 3-chloropropyltrimethoxysilane (Aldrich)
were dissolved in dimethylformamide (Sigma-Aldrich, 45 mL). The
mixture was heated up to 76 °C under stirring and nitrogen atmosphere
until a white solid precipitates. Then, this solid was washed with methyl
alcohol and dried for 4 h at 50 °C. The solid was dissolved in 45 mL of
formamide (Vetec), at 75 °C, under stirring. Afterwards, water (0.8 mL)
and HF 40 % (Aldrich, 0.3 mL) were added. The gelation was carried
out at 45 °C. The final white solid, the cationic silsesquioxane, was
named as SSQ.
−1
and
a
2
2.2. Synthesis of silica xerogel (SiO )
∼
10 kHz of spinning rate. TMS was employed as reference for che-
2
The SiO was synthesized by sol-gel method. Tetraethylorthosilicate
mical shift. A Nikon D100 camera was used to material pictures. The
XPS analysis was performed at LNLS Synchrotron Laboratory, using an
InSb(111) double-crystal monochromator. The photon energy calibra-
(
Aldrich, 5.0 mL) was added to ethanol (Merck, 5.0 mL). After, 2.0 mL
of distilled water containing 3 drops of hydrofluoric acid 40 % (Aldrich)
were added. The system was stored for 14 days, at room temperature,
for gelation and solvent evaporation. Subsequently, the resulting xer-
ogel was washed with distilled water, ethanol, and dried under vacuum
for 2 h.
tion of monochromator was done at Mo L
calibrations were made using gold, 4f7/2 peak at 84.0 eV, carbon 1 s
peak at 284.6 eV and Si 2p peak of SiO at 103.3 eV. The XPS mea-
1
edge 2866 eV. Additional
2
surements were obtained at a 45° takeoff angle at room temperature.
The data were analyzed using the XPSPeak 4.1 software. The peaks
were adjusted using a Shirley-type background and an asymmetric
Gaussian-Lorentzian sum function (20 % Lorentzian contribution).
2
2 2
.3. Organofunctionalization of SiO with ionic silsesquioxane (SiO /SSQ)
The SiO
The SiO (2.40 g) was added to an aqueous SSQ solution (0.96 g in
0 mL) and the mixture was stirred for 22 h at room temperature.
2
material was organofunctionalized by grafting reaction.
2
3
2
2.6. Catalytic performance of SiO /SSQ/Au
Afterwards, the solid was separated from the supernatant and vacuum
dried for 2 h at 80 °C. Then, the modified xerogel was washed using
distilled water and ethylic alcohol, and vacuum dried at 75 °C for ad-
The alkyne coupling reactions was performed in a sealed reaction
tube charged with phenylacetylene (29 μL, 0.25 mmol), 1,10-phenan-
throline (18 mg, 0.1 mmol), iodobenzene diacetate (80 mg, 0.25 mmol),
gold catalyst (18 mg, 0.22 mol%) in dichloromethane (3 mL). The re-
action mixture was stirred at 70 °C for 24 h. As dichloromethane was
used as solvent and the reaction was performed above its boiling point
in a sealed flask, hypervalent iodine was chosen instead of oxygen from
the open atmosphere. The crude reaction mixture was purified by
column chromatography using hexane as eluent. All characterization
ditional 2 h. The resulting modified xerogel was named as SiO
2
/SSQ.
−
2.4. Immobilization of AuCl
4
in organofunctionalized silica (SiO
2
/SSQ/
Au)
2
The SiO /SSQ material was submitted to ion-exchange process with
−
-3
AuCl
4 2 4
. The SiO /SSQ (2.41 g) was added to HAuCl solution (4.10
1
13
-
1
data ( H NMR and C NMR spectra) of the organic compounds are
presented in Supplementary material (Figures S1-S17).
mol⋅L , 20 mL) previously prepared by dissolving 79 mg of gold
powder (Vetec) in 4 mL of aqua regia (nitric acid 65 % (Aldrich) and
hydrochloric acid 37 % (Merck), v/v 1:3) that were diluted to 100 mL
with distilled water. The system was stirred for 2 h, and then the solid
was separated from the supernatant, and vacuum dried for 3 h at am-
bient temperature. Finally, the resulting yellowish material was
3. Results and discussion
The SSQ ionic silsesquioxane was grafted onto mesoporous silica
2 2
Fig. 1. Representation of silica surface grafted with the ionic silsesquioxane (SiO /SSQ) and the ion-exchange process (SiO /SSQ/Au).
2

C.A. Didó, et al.
Applied Catalysis A, General 594 (2020) 117444
Fig. 2. Solid state MAS 29Si NMR spectra of SSQ ionic silsesquioxane and SiO
/
2
SSQ material.
2
Fig. 5. XPS spectra at the Cl 2p region of the SSQ ionic silsesquioxane and SiO /
SSQ/Au material. The black points represent the experimental data, the dot line
the Shirley background.
xerogel (SiO
to anchor the AuCl
surface and the ion-exchange process are illustrated in Fig. 1.
The 29Si NMR spectra of both SiO
/SSQ and silsesquioxane mate-
rials are presented in Fig. 2. NMR spectrum for silsesquioxane presents
2
/SSQ) aiming to prepare a support with high surface area
−
4
2
complex (SiO /SSQ/Au). The modified silica
2
three peaks identified as T , T and T1 silicon species, at -67.5, -58.1
3
2
2
and -49.2 ppm, respectively [12,40]. The spectrum of SiO /SSQ enables
to observe the occurrence of T3 and T2 species coming from the sil-
sesquioxane, as well as, the presence of Q and Q3 species, at 110 and
4
100 ppm, respectively, assigned to the silica bulk. These results are
clear evidences that silsesquioxane grafting reaction took place on silica
surface.
The organofunctionalized silica was submitted to thermogravi-
metric analysis, and the thermogram is presented in Fig. 3, along with
unmodified SiO . The weight loss up to 150 °C is due to water deso-
2
Fig. 3. Thermogravimetric curves for SiO
2
and SiO
2
/SSQ materials.
rption. In the range from 150 to 650 °C, two different processes were
observed, silica dehydroxylation reactions and desorption of organic
2 2 2 4
Fig. 4. Absorption profile, k/s calculated from UV–Vis diffuse reflectance spectra of SiO , SiO /SSQ and SiO /SSQ/Au, along with UV–vis spectrum of HAuCl
aqueous solution. Inset: spectra of assigned region for plasmon resonance of gold nanoparticle. Aside: photographs of SiO /SSQ and SiO /SSQ/Au materials.
2 2
3

C.A. Didó, et al.
Applied Catalysis A, General 594 (2020) 117444
Fig. 6. Textural analysis: (a) N
2
adsorption-desorption isotherms; (b) BJH pore size distribution; Inset Fig. 5b the DFT micropore size distribution.
Table 1
2
group per gram of SiO /SSQ material, considering two chloride ions per
Textural analysis.
charged SSQ group, in good agreement with the TGA analysis discussed
above. The charged sites come from the quaternary ammonium groups,
which should be available to anchor the AuCl - complex.
4
Surface area ( ± 5 m²·g−1)
Pore volume ( ± 0.001 cm3·g−1)
Sample
SiO
SiO
SiO
2
2
2
495
313
310
0.805
0.523
0.568
In a previous work, it was observed that the use of ionic silses-
quioxanes keeps the gold species well dispersed due to multidirectional
and long range attractive field of the ionic interaction [12]. In the
present work, aiming to maintain the gold in the cationic form and to
avoid the nanoparticle formation, a gold complex was highly dispersed
on the silica surface. The catalyst with isolated ionic gold sites was
/SSQ
/SSQ/Au
groups. In the case of SiO
due to silanol condensation of silica surface. On the other hand, for
SiO /SSQ material this value was higher (9.68 %) which corresponds
2
, the weight loss in this range (3.17 %) is only
designed by offering much lower gold amount than available charged
2
−
sites (SiO
2
/SSQ/Au). 0.03 mmol of AuCl
4
2
per gram of SiO /SSQ ma-
not only to dehydroxylation reactions, but also, to decomposition and
desorption of SSQ organic groups. Based on these values, the SSQ
content was estimated as 0.24 mmol of SSQ groups per gram of SiO /
2
SSQ material. It is important to highlight that the SSQ groups grafted on
silica surface present good thermal stability, at least until 200 °C. This
2
characteristic of the organofunctionalized support (SiO /SSQ) is at-
tractive not only for gold species but for other anionic metal complex as
well, in which the catalysis is usually performed below this temperature
42].
The amount of SSQ groups was also estimated by potentiometric
analysis through the evaluation of the exchangeable chloride ions. The
−
terial was offered so as to consider a full incorporation of AuCl
complex, because it corresponds to ca. 12 % of available sites.
4
−
−
Even with such a low gold content in SiO
complex was identified by UV–vis spectroscopy. The spectra of AuCl
solution and SiO
are identified in the spectrum of AuCl
13 nm, due to charge transfer (CT) transition and the other small band
around 400 nm assigned to local (d-d) transition of square planar
complex [43,44]. In the SiO /SSQ/Au spectrum, the CT band can be
2
/SSQ/Au, the AuCl
4
4
2
/SSQ/Au material are depicted in Fig. 4. Two bands
−
4
solution, one of them at
3
[
2
identified as a shoulder, and, although the d-d band cannot be clearly
seen, the picture evidences this transition, because the absorption in the
−1
obtained value was 0.54 mmol·g , corresponding to 0.27 mmol of SSQ
Table 2
Optimization of reaction conditions.
Entry
1,10-phen (equiv)
PhI(OAc)
2
(equiv)
Temp. (°C)
Time (h)
Solvent
Yield (%)
1
2
3
4
5
0.4
0.4
0.4
–
1.5
1.5
1.5
1.5
–
25
70
70
70
70
70
70
70
70
70
70
70
70
70
6
6
DCM
DCM
DCM
DCM
DCM
DCM
DCM
MeCN
–
37
99
4
18
24
24
18
18
18
18
18
18
18
18
18
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.2
–
a
6
7
–
–
99
99
80
53
9
78
99
54
84
b
8
9
1.5
1.5
1.5
1.5
1.0
0.5
1.0
HCCl
THF
3
1
1
1
1
1
0
1
2
3
4
EtOH
DCM
DCM
DCM
a
diphenyliodonium triflate ((Ph)
bis(trifluroacetoxy)iodobenzene (PhI(OCOCF ) as oxidant.
2
IOTf) as oxidant.
b
3 2
)
4

C.A. Didó, et al.
Applied Catalysis A, General 594 (2020) 117444
Table 3
Substrates scope for the terminal alkyne homocoupling.
Entry
1
Alkyne
Diyne
Yield
2a (99%)
2
3
2b (90%)
2c (92%)
4
5
2d (63%)
2e (74%)
6
2f (70%)
Fig. 7. Recyclability tests of material SiO
2
/SSQ/Au for coupling reaction of
phenylacetylene under optimized conditions.
7
8
2g (30%)
2h (99%)
character may account for its slightly less anionic behaviour, eviden-
−
cing the AuCl
tested by UV–vis spectroscopy discussed above.
Fig. 6 shows the textural analysis. The N
isotherms are presented in Fig. 6a. After the grafting reaction, a de-
crease in the N adsorption amount is clearly seen in two isotherm
regions. The first, in low relative pressures (P/P < 0.05), corresponds
to micropores and the second one in higher P/P values is related to
4
2
species presence in the SiO /SSQ/Au material, as at-
2
adsorption/desorption
9
2i (91%)
2
0
0
mesopores [41]. The BJH mesopore size distribution curves are shown
in Fig. 6b, in which a sharp peak with maximum around 5.7 nm is
clearly observed for SiO2 material. A shift to lower diameter, with
maximum around 5.1 nm, is evident after the grafting reaction with
−
violet region provides the yellowish AuCl
4
colour (aside of Fig. 4). The
nonappearance of band due to plasmon resonance of gold in the na-
noparticles form, which usually appears in the range from 500 to
550 nm [12,32] is evidence of its absence. Even more, the SiO
2
/SSQ/Au
ionic silsesquioxane (SiO /SSQ). This shift of 0.6 nm was interpreted
2
material was annealed at 70 °C, which is the temperature that the
material was further applied as catalyst and, again, no plasmon re-
sonance band of gold nanoparticles was observed (Supplementary
material, Figure S18), therefore confirming the highly dispersed gold
complex.
considering the presence of polymeric silsesquioxane film on the me-
sopore surface that also promotes a reduction in BJH mesopore volume,
as presented on Table 1. The reduction in the BET surface area after the
grafting reaction (Table 1) is mainly produced by trapping of micro-
pores, which are noticed in the DFT micropore distribution curves
(inset Fig. 6b).
XPS analysis was performed for SiO
of Au 4d (bigger photoionization cross section at Eph = 3000 eV) or Au
f regions could be detected, due to the very small gold amount pre-
sented in the Supplementary material Figure S19. However, changes in
the Cl 2p XPS spectrum of the SiO /SSQ/Au material were identified
2
/SSQ/Au material and no peaks
−
After the AuCl4 adsorption by ion-exchange process (SiO /SSQ/
2
4
Au) the isotherms presented very similar profile, although the BJH
mesopore curve shows a slight shift to higher diameter values (Fig. 6b).
In fact, this feature was already observed in a previous report [47], and
it was interpreted as an ionic silsesquioxane leaching process that took
place during the ion exchange process. This leaching can produce an
2
when compared with the Cl 2p XPS spectrum of SSQ ionic silses-
quioxane (Fig. 5). The Cl 2p peak of SSQ shows only a single component
(
blue line), with maximum at 196.7 eV (Fig. 5). This peak corresponds
to the chloride ion of SSQ ionic silsesquioxane, as already reported
45]. On the other hand, the Cl 2p peak of SiO /SSQ/Au material shows
erosive process of SiO pore wall. Additionally, the strong acid medium,
2
coming from the HAuCl solution (aqua regia), should also contribute to
4
[
2
this erosive effect, by converting siloxane to silanol groups, in non-
the presence of a new component (Fig. 5). This new component with
higher binding energy (red line), which appears around 199.4 eV, is
attributed to the chloride from AuCl - complex [46], whose covalent
4
calcined sol-gel silica materials [48]. After the ion exchange process,
the surface area value remains the same, while the pore volume un-
derwent a slight increase (Table 1), in agreement with the leaching
Scheme 1. Non-symmetrical diynes.
5

C.A. Didó, et al.
Applied Catalysis A, General 594 (2020) 117444
process proposed above.
2
Subsequently, the recyclability of the material SiO /SSQ/Au was
Assuming that the ionic gold is uniformly dispersed on the surface,
the density of anchored gold “d” can be estimated as:
investigated by means of six repetitions, since it is one of the most
important issues from the viewpoint of heterogeneous catalysis.
Recycling of the catalyst was performed in a homocoupling reaction
using phenylacetylene as model substrate under the optimized condi-
tions. After each cycle, the crude reaction was diluted and the organic
phase removed. Reagents were reloaded into the reaction vessel and the
next cycle was conducted. The results are shown in Fig. 7.
The catalyst showed an excellent conversion in the first cycle (99 %
yield), whereas a slight decrease was observed in the subsequent re-
cycles. This trend was interpreted considering that the catalyst was
comminuted by the stirrer, leading to a likely loss of mass. Even though,
in the sixth cycle, the reaction yield was still 70 %. Another explanation
could be the contribution of the homogeneous catalysis considering the
possibility of some gold was leached to the solution. In this way, two
d = ([Au] × N)/SBET
−
where “[Au]” is the amount of gold offered (0.03 mmol of AuCl
gram of SiO
specific surface area of SiO
4
per
2
/SSQ support), “N” is Avogadro’s number, and SBET is the
/SSQ material. Thus, the average of inter
” is defined as:
2
complex distance; “
ℓ
ℓ=(1/d)1/2
The found density “d” was 0.058 gold complex per square nan-
ometer and therefore, the average distance between them was 4.2 nm,
revealing that the gold is really highly dispersed and very far from each
other, hindering the gold nanoparticle formation, as shown by UV–vis
analysis.
additional experiments were performed. Instead of use SiO /SSQ/Au
2
material, equal amount of NaAuCl4 or AuCl were applied under opti-
It is important to point out that the SiO
2
/SSQ/Au material gathers
mized reaction conditions. In both cases, the added salts did not provide
the desired 1,3-dyine 2a, after 18 h. Our assumption is that, in organic
media, the gold salt does not present enough solubility to provide the
reactive ions or, as the ions become available they could aggregate to
form nanoparticles. In both suppositions, deactivation would be ex-
desirable characteristics to be used as heterogeneous catalyst, such as: i)
thermal stability until 70 °C; ii) high dispersion of metal ions, in which
every gold atom could be a catalytic site; iii) high surface area, pore
volume and pore size that enable fast diffusion and mobility of the
species during the catalytic process.
pected. In this way, a significant gold leaching from SiO /SSQ material
2
The performance of SiO
2
/SSQ/Au material as catalyst was in-
was discarded. Consequently, in the applied conditions, our design
vestigated in the oxidative alkyne-alkyne coupling reactions in which
the phenylacetylene was selected as the model reaction. The optimal
reaction conditions were determined through a series of experiments
and the results are summarized in Table 2. First, only 0.22 mol% of the
catalyst was used in the dimerization of phenylacetylene (1a), together
heterogeneous catalyst is really the responsible by the reaction.
Therefore, the SiO /SSQ/Au catalyst presents excellent activity and
2
high efficiency concerning the synthetic protocol applied and taking
into account the good yields obtained even after six cycles, the possi-
bility of recovering the starting material as well as its simple prepara-
tion method.
with 1,10-phenanthroline, di(acetoxyiodo)benzene (PhI(OAc)
chloromethane (DCM).
2
) in di-
No reaction was observed at room temperature after 24 h. The in-
crease in temperature up to 70 °C, in a sealed Schlenk tube, resulted in
4. Conclusions
3
7 % yield after 6 h and a complete conversion into the desired 1,3-
dyine 2a occurred after 18 h (Table 2, entries 2–3). The homocoupling
did not occur at all if no oxidant (PhI(OAc) ) or ligand (1,10-phen) is
added in the reaction system (Table 2, entries 4–5). Additional ex-
periment in which PhI(OAc) was replaced by diphenyliodonium tri-
3 2
flate ((Ph) IOTf) or bis(trifluroacetoxy)iodobenzene (PhI(OCOCF ) )
Gold catalyst containing isolated ionic complex sites dispersed on
the surface of mesoporous silica was successfully obtained. In order to
achieve this purpose, mesoporous silica with sharp pore size distribu-
tion, 5.7 nm, was synthesized by sol-gel method. This silica was grafted
with ionic silsesquioxane that shows thermal stability up to 200 °C and
provides ionic ammonium quaternary sites. This silica modified with
ionic groups was used as matrix for ion-exchange immobilization of
ionic gold complex. The offered amount of gold was only 12 % of the
available exchangeable sites. This small quantity of gold ions provides
isolated sites on the matrix surface, with estimated average distance
between them of 4.2 nm, hindering the gold aggregation that could lead
to the formation of gold nanoparticle, in this way keeping all offered
gold as available catalytic sites. The catalyst was efficiently applied in
the cross coupling reactions to obtain symmetric and non-symmetric
1,3-diynes, since only 0.22 mol% of the catalyst was sufficient for the
coupling reaction. It is important to mention that the reaction system
could also be recycled, because good results were obtained even after
six reaction runs.
2
2
2
also resulted in full conversion into 2a (Table 2, entries 6–7), revealing
that these other sources of iodonium oxidant are also effective.
The influence of the solvent in the product yield was studied. As can
be seen (Table 2, entries 8–11), chloroform, tetrahydrofuran and
ethanol were detrimental for the reaction. The amount optimization of
both ligand and oxidant (Table 2, entries 12–14) revealed that the best
reaction condition consists of 1,10-phenanthroline (0.4 eq.), di(acet-
oxyiodo)benzene (1.0 eq.) in dichloromethane at 70 °C in closed
system. Therefore, this condition was selected to be applied in a new
series of reactions where the substrate scope for the terminal alkynes
coupling reaction was studied (Table 3). Also, no reaction occurred by
2
using only the SiO /SSQ support, without gold in this condition.
Aryl- and alkyl-substituted alkynes were submitted to homo and
heterocoupling and resulted in yields from good to high. Functionalized
Acknowledgements
phenylacetylene bearing electron-donating groups [o-NH
2
¸ p-OMe or p-
) under-
Me] or electron-withdrawing groups (3,5-(CF ), p-F or p-NO
3
2
We are grateful to CAPES, CNPq, INCT-CMN and FAPERGS for fi-
went oxidative homocoupling and yielded the corresponding 1,3-diynes
in the range from moderate to excellent yields. In case of aliphatic
substituted alkynes, different results were observed. While 2-methyl-3-
butyn-2-ol resulted in full conversion into product 2h, only 30 % yield
was obtained for 2g. The procedure was readily extended to the pre-
paration of non-symmetrical diynes by heterocoupling between phe-
nylacetylene 1a and terminal alkynes 1c and 1d using the same opti-
mized conditions. In this case, the heterodiynes 2j and 2k (Scheme 1),
which are concomitantly formed under these conditions, reaching 61 %
and 77 % yields, respectively, were separated from the homodiyne
products 2c and 2d by column chromatography.
nancial and technical support.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2020.117444.
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