Heterogeneous Gold Catalyst: Synthesis, Characterization, and Application in 1,4-Addition of Boronic Acids to Enones

Article abstract of DOI:10.1021/acscatal.5b01207

The new 1 wt % Au/TiO₂-UVM-7 catalyst was prepared and fully characterized. This heterogeneous catalyst proved to be active, selective and recyclable for the unprecedented gold-catalyzed 1,4-addition of various functionalized arylboronic acids to 2-cyclohexen-1-one and other selected enones using toluene as a solvent. The gold-based catalyst was recycled two times and played an active role in this reaction, and the nature of the solvent determined a remarkable change in the products' selectivities.

Full text of DOI:10.1021/acscatal.5b01207

Research Article
pubs.acs.org/acscatalysis
Heterogeneous Gold Catalyst: Synthesis, Characterization, and
Application in 1,4-Addition of Boronic Acids to Enones
Alaina Moragues,† Florentina Neatu
̧
,*,‡,§ Vasile I. Parvulescu,*,‡ Maria Dolores Marcos,∥
̂
Pedro Amoros,*,† and Veronique Michelet*,⊥
́
́
†Instituto de Ciencia de los Materiales, Universitat de Valen
̀
cia, P.O. Box 22085, 46071 Valencia, Spain
‡Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, 4-12 Regina Elisabeta Boulevard, 030016
Bucharest, Romania
§National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials, 105Bis Atomistilor Street, P.O.
Box MG7 Magurele, Bucharest, Romania
∥Centro de Reconocimiento Molecular y DesarrolloTecnolog
Valencia, Camino de Vera s/n, 46022 Valencia, Spain
́ ́
ico (IDM), Departamento de Química, Universidad Politecnica de
⊥PSL Research University, Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, 11 rue P. et M. Curie, 75005 Paris,
France
ABSTRACT: The new 1 wt % Au/TiO2−UVM-7 catalyst was prepared and
fully characterized. This heterogeneous catalyst proved to be active, selective
and recyclable for the unprecedented gold-catalyzed 1,4-addition of various
functionalized arylboronic acids to 2-cyclohexen-1-one and other selected
enones using toluene as a solvent. The gold-based catalyst was recycled two
times and played an active role in this reaction, and the nature of the solvent
determined a remarkable change in the products’ selectivities.
KEYWORDS: heterogeneous catalysis, gold, 1,4-addition, boronic acids, selectivity, solvent effect
1,4-Addition of arylboronic acids to α,β-unsaturated ketones
is another C−C bond coupling of broad interest.16 Few reports
about the addition of arylboronic acids to 2-cyclohexenone in
the presence of heterogeneous catalysts have been described so
far in the literature,17 and all of them refer to rhodium
complexes. We have selected UVM-7, a nanoparticulated
material with a bimodal pore system,18 as the heterogeneous
support. The most outstanding feature concerning this
nanostructured material is its peculiar (very open) architecture,
which is based on a continuous network constructed from
covalently bonded mesoporous nanoparticles, whose aggrega-
tion defines a non-ordered secondary system of large pores.
The intraparticle mesopore micelle template system has
similarities to the one present in MCM-41-like solids19 and
could therefore be modified to include TiO2 domains on the
silica surface and later gold nanoparticles. The existence of
hierarchic porosity, combined with short-length mesopores,
favors impregnation and subsequent TiO2 dispersion in a highly
homogeneous way. These TiO2 nanodomains are able to act as
1. INTRODUCTION
Nano and subnano gold particles have shown very interesting
applications in catalysis with very high selectivities and TONs
in multiple reactions.1 Most of these examples refer to
supported gold nanoparticles, in which the support has either
the role of stabilizing gold species in different oxidation states
(Auδ+, Au+, or Au3+)1n,2 or the role of ensuring a buffer electron
transfer to these nanoparticles. This interaction differentiates
among the reactions in which these catalysts have been
investigated.3 Among these, gold nanoparticles were reported
to exhibit activity in carbon−carbon (C−C) coupling reactions.
Conventionally, gold-catalyzed C−C bond formations were
merely investigated using homogeneous catalytic systems,
which exhibit very high TONs but suffer from the isolation/
recycling point of view.4 Heterogeneous gold-supported
catalysts were, however, reported in these reactions as well:
homocoupling reactions,5 cross-coupling,6 Suzuki−Miyaura
cross-coupling,7 Sonogashira cross-coupling,8 Ullmann cou-
pling,9 sequential oxidation addition reactions,10 benzylation of
aromatics,11 oxidative C−C coupling reactions,12 amination
reactions,13 cycloisomerization reactions,14 and nucleophilic
addition.15
Received: June 9, 2015
Revised: July 16, 2015
© XXXX American Chemical Society
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preferred inorganic anchors for the gold nanoparticles and, due
to the open framework of UVM-7, will present an enhanced
accessibility. Having these specific characteristics in mind, we
therefore embarked on the synthesis of a Au/TiO2−UVM-7
catalyst, anticipating superior activity compared with other gold
heterogeneous systems. As part of our ongoing program toward
applications of supported gold nanoparticles in catalysis,14d,20
we wish therefore to report herein our preliminary results on
the preparation of gold supported on mesoporous UVM-7 silica
and its unprecedented use in 1,4-addition of various arylboronic
acid (2) to 2-cyclohexen-1-one (1) (Scheme 1) and other
selected enones.
equipped with EPMA (electron probe microanalysis). Powder
X-ray diffraction patterns were recorded with a Seifert 3000TT
θ−θ diffractometer using Cu Kα radiation. The TEM and
HRTEM microstructural characterizations were carried out
using a JEOL JEM-1010 instrument operating at 100 kV and
equipped with a CCD camera and a Tecnai G2F20(FI)
instrument, respectively. STEM−HAADF (scanning trans-
mission electron microscopy−high-angle annular dark-field)
images were acquired on a JEOL-2100F microscope. The N2
adsorption−desorption isotherms were recorded at −196 °C
using a Micromeritics ASAP2020 automated instrument.
Calcined samples were degassed for 15 h at 130 °C and 10−6
Torr before analysis. Surface areas were estimated according to
the BET model, and pore size dimensions were calculated using
the BJH method. Raman spectroscopy was performed using a
Horiba spectrometer equipped with a He−Ne (λ = 633 nm)
laser. The spectra were recorded in the 200−4000 cm−1 range.
2.3. Catalytic Tests. To a mixture of arylboronic acid (2.30
mmol), Na2CO3 (0.92 mmol), 2-cyclohexen-1-one (0.46
mmol), and toluene (4 mL) was added 30 mg of catalyst.
The reaction mixture was heated at 80 °C until completion of
the reaction (TLC) and then cooled to room temperature. The
reaction mixture was filtered on a short pad of silica gel with
cyclohexane/ethyl acetate (7:3) and evaporated under reduced
pressure. No further purification was necessary. The purity of
Scheme 1. 1,4-Addition of Arylboronic Acids to 2-
Cyclohexen-1-one
2. EXPERIMENTAL PART
2.1. Catalysts Preparation. The synthesis of UVM-7 is
based on the atrane route, a general preparative strategy
previously described in detail.18 Once the UVM-7 mesostruc-
tured solid was obtained, to prepare the final porous material,
the template was removed by calcination at 550 °C for 6 h
(heating ramp = 5 °C·min−1) under static air atmosphere.
Once the silica support was prepared, it was loaded with 3.7
wt % of Ti/(Ti + SiO2) in a way similar to that described
elsewhere.20 The corresponding amount of titanylacetylaceto-
nate (TiO(acac)2) (0.19 g) was mixed with 100 mL of ethanol
and vigorously stirred. Subsequently, deionized water (100 mL)
was slowly added. After that, the UVM-7 silica support (0.6 g)
was stirred for 2 h with the Ti solution. The mixture was
filtered, washed, and allowed to dry overnight at room
temperature. The TiO2−UVM-7 was calcined at 300 °C for 5
h (heating ramp = 1 °C·min−1) under static air atmosphere.
This silica−titania support was then loaded with 1 wt % of Au/
(Au + support) (nominal composition) by a deposition−
precipitation method. Working in a 70 °C oil bath, 10 mg of
HAuCl4·3H2O was added to 50 mL of water, and the pH was
adjusted to 7 using 0.1 M NaOH. The impregnation on 0.5 g of
TiO2−UVM-7 was then carried out for an hour. The mixture
was filtered, washed, and dried at 90 °C overnight (Au/TiO2−
UVM-7, Scheme 2). The heterogeneous catalysts 1 wt % Au/
SiO2, 1 wt % Au/UVM-7, and 1 wt % Au/TiO2 were prepared
by the same deposition−precipitation method as used for the
Au/TiO2−UVM-7 catalyst.
1
the product (>98%) was checked by H and 13C NMR and
GC/MS analyses.15,16 To investigate the chemical stability of
the catalysts, the content of the leached metal into the reaction
liquid was determined by ICP−OES (Agilent Technologies,
700 series).
3. RESULTS AND DISCUSSIONS
3.1. Catalyst Characterizations. The UVM-7 silica is a
mesoporous material with a hierarchic porosity, with large
interparticle voids and 3D interconnected mesopores. This
open framework endows the material with enhanced
accessibility, favoring mass diffusion throughout the hierarchical
pore structure.18c These structural features are highly beneficial
for the next stage of synthesis, the addition of Ti. The open
nature of the support favors a regular and homogeneous
impregnation with the titanium complex solution. Then the
material is loaded with TiO2 using a simple wet impregnation
of UVM-7 with titanium(IV) oxyacetylacetonate and calcining
the samples at 300 °C.21
We have used EPMA to verify the chemical homogeneity of
the resulting TiO2−UVM-7 solids. Table 1 gathers the data
corresponding to the bulk chemical composition. These values
have been calculated by averaging EPMA data of ∼20 different
particles. The relatively low standard deviation of the
measurements allows us to reasonably assume that the
TiO2−UVM-7 sample is chemically homogeneous with a
regular dispersion of Ti species on the silica walls. The
incorporation of TiO2 domains does not alter the UVM-7
organization. The TEM image in Figure 1 shows that the
bimodal nanoparticle topology, typical of UVM-7 solids, is
preserved. Both intra- and interparticle pores can be clearly
distinguished. The N2 adsorption−desorption isotherm (Figure
2) is in good accordance with TEM image. The relatively low
proportion of TiO2 domains included in the UVM-7
mesostructure allows preserving high values of the surface
area and pore volume (similar to typical values observed for
pure UVM-7 silica).
2.2. Catalyst Characterizations. The chemical analysis of
the materials was performed using an XL 30 ESEM instrument
Scheme 2. Three-Step Preparation Procedure of Au/TiO2−
UVM-7 Catalyst
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Table 1. Selected Preparative and Physical Data
nominal content M/M+
support (% wt)
EPMA results M/M + support
(% wt)
sample
M = Ti
M = Au
M = Ti
M = Au
BET area (m2/g)
BJH volume (cm3/g)
TiO2−UVM-7
Au/TiO2−UVM-7
3.7
3.8
3.8 0.3
4.6 0.7
1028.7
697.7
1.74
1.56
1.0
1.6 0.3
Figure 1. TEM image of the TiO2−UVM-7 sample.
As expected, the soft experimental conditions used during
the Au deposition (neutral pH and 70 °C) ensure the
conservation of the UVM-7-type organization to a great extent.
The two adsorption steps and their respective peaks in the BJH
pore size distribution can be observed for the gold-containing
material (Figure 2). However, although the gold incorporation
implies an appreciable decrease in both the BET area and the
BJH pore volume, the final catalyst yet preserves a high and
accessible porosity. The preservation of the UVM-7 meso-
structure is further supported by TEM images in Figure 3. We
observe the typical aggregates of mesoporous nanoparticles
describing the bimodal pore organization. The HAADF
micrographs display a good gold dispersion. The HRTEM
shows the existence of ordered nanodomains associated with
the gold and the TiO2 particles included on the amorphous
silica surface.
Figure 3. (a)TEM, (b) HAADF, and (c) HRTEM images of the Au/
TiO2−UVM-7 sample (domains highlighted with dashed and
continuous lines correspond to gold and anatase particles,
respectively) and (d) gold particle size distribution histogram.
associated with the amorphous silica support. There are no
peaks that can be attributed to crystalline gold or TiO2. Titania
is found as anatase with a small part of rutile. As can be seen in
the Raman spectra in Figure 4, the sample presents the three
characteristic peaks of anatase with shoulders that can be
associated with the rutile structure.
The small size of these ordered domains prevents their
detection by XRD techniques. In fact, the XRD powder pattern
of the Au/TiO2−UVM-7 sample shows only large signals
Figure 2. (a) N2 adsorption−desorption isotherms and (b) BJH pore size distributions of TiO2−UVM-7 and Au/TiO2−UVM-7 samples.
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Figure 4. (a) High-angle XRD pattern and (b) Raman spectrum of Au/TiO2−UVM-7 sample. The Raman spectra of anatase and rutile have been
included for comparison.
a
Table 2. 1,4- Addition of Arylboronic Acid (2) to 2-Cyclohexen-1-one (1) over Au/TiO2−UVM-7 Heterogeneous Catalyst
b
c
entry
R−C6H4−B(OH)2
2a R: −H
2b R: −OCH3 (para)
2c R: −Br (meta)
product
conversion (%)
yield (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
98
55
66
64
87
88
60
0
95
53
64
61
85
86
57
2d R: −Cl (meta)
2e R: −CF3 (meta)
2f R: −CF3 (para)
2g R: −CH3 (ortho)
2h naphthylboronic acid
3g
3h
0
a
b
1 equiv of 1 (0.46 mmol), 5 equiv of 2, 2 equiv of Na2CO3, toluene (4 mL), 30 mg of catalyst 1% Au/TiO2−UVM-7, 80 °C, 16 h. Determined by
c
GC. Isolated yield.
3.2. Catalytic Tests. Having prepared and characterized the
Au/TiO2−UVM-7 catalyst, we decided to investigate its use in
an original and novel process implying boronic acids. Indeed,
inspired by the seminal work of Corma on heterogeneous gold-
catalyzed homocoupling of boronic acids,5d−f we wondered if
arylgold species could add to 2-cyclohexen-1-one. Table 2
compiles results evidencing the role of the substituent of the
boronic acid in the 1,4-addition of 2 to 1 using the Au/TiO2−
UVM-7 catalyst.
Pleasingly, we observed the desired formation of the arylated
ketone when reacting enone 1 with phenylboronic acid in the
presence of gold catalyst. Depending on the aromatic ring of
the boron derivative, the yields varied from 0% (for
naphthylboronic acid) to 95% for the nonsubstituted one
(Table 2, entries 1−8). It is worth noting that the addition of
phenylboronic acid to 2-cyclohexen-1-one in the presence of
only 0.33 mol % of gold (Table 2, entry 1) occurred with a
TOF (turn over frequency) of 18 (in 16 h), that is, 3−4 times
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a
Table 3. Catalyst and Solvent Influences in the 1,4-Addition of Phenylboronic Acid (2a) to 2-Cyclohexen-1-one (1)
b
c
c
entry
catalyst
solvent
toluene
conversion (%)
3a yield (%)
4 yield (%)
1
2
Au/TiO2−UVM-7
Au/SiO2
98
11
23
70
95
toluene
n.d.
n.d
n.d.
3
Au/UVM-7
Au/TiO2
toluene
4
toluene
5
TiO2−UVM-7
toluene
6
toluene
d
e
7
Au/TiO2−UVM-7
Au/TiO2−UVM-7
Au/TiO2−UVM-7
Au/TiO2−UVM-7
toluene/water
toluene/water
water
94
95
80
82
90
61
34
36
1
8
30
44
43
9
10
methanol
a
b
c
1 equiv of 1 (0.46 mmol), 5 equiv of 2, 2 equiv of Na2CO3, solvent (4 mL), 30 mg of catalyst. Determined by GC. Isolated yield reported to 2-
d
e
cyclohexen-1-one. 4.9 mL toluene/0.1 mL water. 2 mL toluene/2 mL water.
higher than those reported on other heterogeneous catalytic
systems.17 The use of substituted phenylboronic acids led to
lower isolated yields (55−86%) of the corresponding ketones
3b−3g (Table 2, entries 2−7). Both steric and electronic effects
could explain this behavior. A lower yield was obtained for
hindered boronic acid such as ortho-methylphenylboronic acid
(Table 2, entry 7). The high steric influence on the reaction
was also demonstrated by the nonreactivity of 2-naphthylbor-
onic acid, which is the organometallic partner bearing the
bulkiest group (Table 2, entry 8). Moderate reactivity was
observed in the case of electron-rich boronic acid, such as the
para-methoxyphenylboronic acid, most plikely as a result of a
slower transmetalation step and, therefore, competitive
protodeboronation reaction. Very good yields were observed
in the case of boronic acids bearing electron-withdrawing
groups, the best results being obtained, as anticipated, for
trifluoromethyl substituents, having the strongest electron-
withdrawing effects in the series (−Br < −Cl ≪ −CF3). The
ketones 2e and 2f were isolated in 85% and 86% isolated yields,
respectively (Table 2, entries 5−6).
influence of water in this reaction. Thus, phenylboronic acid
was allowed to react in the presence of a catalytic amount of the
catalysts, in biphasic reaction media (1:1 vol/vol ratio of
toluene and water), in air at 80 °C. Water addition to the
reaction media resulted in the appearance of a byproduct
(biphenyl) not observed under previous conditions (toluene).
Interestingly, the formation of biphenyl is proportional to the
amount of water added to the system (Table 3, entries 7−8).
This result is in agreement with previous studies on
homocoupling of phenylboronic acid over gold nanoparti-
cles,5d−f which demonstrate the formation of biphenyl in
aqueous media, in contrast with a nonpolar solvent, such as
toluene, in which the formation of biphenyl does not occur.
Performing the reaction only in water (Table 3, entry 9)
afforded a conversion of 80%, with an increased selectivity
(56%, 44% isolated yield) toward the biphenyl byproduct.
Choosing methanol as solvent gave results similar to those for
the reaction performed in water (Table 3, entry 10). Obviously,
carrying out the reaction in a polar medium led to a competitive
reaction between the 1,4-addition of 2 to 1 versus
homogeneous coupling of phenylboronic acid.
In an effort to validate the role of the support, catalytic tests
were performed using Au/TiO2, Au/SiO2, and Au/UVM-7
catalysts (Table 3). The heterogeneous catalysts Au/SiO2 and
Au/UVM-7 (1 wt %) exhibited a low activity for the 1,4-
addition of phenylboronic acid to 2-cyclohexen-1-one (Table 3,
entries 2, 3). The small difference between these catalysts is
most probably attributable to the open architecture of UVM-7,
It is noteworthy that the heterogeneous catalyst Au/TiO2−
UVM-7 (Table 4, entry 1) has a similar excellent activity or
Table 4. Comparison between Au/TiO2−UVM-7 and Other
Rh Heterogeneous Catalytic Systems Able To Promote the
1,4-Addition of Phenylboronic Acid to 2-Cyclohexen-1-one
allowing a better dispersion of gold. The use of 1 wt % Au/
TiO2 allowed the formation of the desired ketone 3a (Table 3,
entry 4), but with a lower conversion compared with the use of
Au/TiO2−UVM-7 complex (Table 3, entry 1). The superior
activity of the Au/TiO2−UVM-7 catalyst may be explained by
the high dispersion of Au combined with a specific interaction
of Au with the dispersed titanium oxide and also by the
existence of a high accessibility to the active sites, thanks to the
hierarchical porosity of UVM-7 combined with the short-length
mesopores. Blank experiments carried out in the absence of
catalyst (Table 3, entry 6) and in the presence of the support
(Table 3, entry 5) did not result in product formation, thus
demonstrating the catalytic activity of Au nanoparticles.
entry
catalyst
additives
ref
3a yield (%)
a
1
2
3
4
5
Au/TiO2−UVM-7
Rh(III)/hydrotalcite
Rh(I)/hydrotalcite
Rh(III)/NiZn
this work
17a
95
a
1,5-COD
81
a
17b
98
b
1,5-COD
1,5-COD
17c
92
b
RhFAP
17d
90
a
b
1
Isolated yield. Calculated yield from GC or H NMR.
compares favorably with other known Rh heterogeneous
systems, such as Rh(III)/hydrotalcite, Rh(I)/hydrotalcite,
Rh(III)/NiZn, and RhFAP (FAP = fluoroapatite), that require
1,5-COD17a,c,d (Table 4, entries 2, 4, 5) as additives or a higher
catalytic loading17b (Table 4, entry 3).
Considering previous findings in the presence of rhodium
heterogeneous catalysts,17b we also envisaged studying the
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As preliminary results, the activity of our catalytic system was
tested on other enones, such as cyclopenten-2-one 4, (E)-non-
3-en-2-one 6, and (E)-1,3-diphenylprop-2-en-1-one 8 (Scheme
3). Under standard conditions, for example, in the presence of
after each reaction. The 1,4-addition of phenylboronic acid (2a)
to 2-cyclohexen-1-one (1) over Au/TiO2−UVM-7 was selected
as our benchmark reaction. After each step, the catalyst was
separated via centrifugation, washed with toluene, and dried at
60 °C for 2 h. As can be seen from Table 5, the recycled
Scheme 3. 1,4-Addition of Phenylboronic Acid (2) to
Selected Enones
Table 5. Recycling Tests for 1,4-Addition of Phenylboronic
Acid 2a to 2-Cyclohexen-1-one 1
run
conversion (%)
isolated yield (%) of 3a
1
2
3
98
97
97
95
93
91
supported catalysts did not exhibit any significant loss of
activity. Only a slight decrease in the isolated yield was
observed after three runs. Moreover, the ICP−OES analysis of
the filtered solution after the catalytic tests showed that the
gold concentration was below the detection limit of the
equipment.
1 wt % Au/TiO2−UVM-7 at 80 °C in toluene, the reaction of
phenylboronic acid with cyclopenten-2-one 4 cleanly allowed
the formation of the desired ketone 5 in 87% isolated yield
(Scheme 3, eq 1). The reaction conditions were also
compatible with an acyclic linear enone, such as (E)-non-3-
en-2-one 6, and an aryl-substituted ketone, such as (E)-1,3-
diphenylprop-2-en-1-one 8. The enones were transformed to
the corresponding 4-phenylnonan-2-one 7 and 1,3,3-triphenyl-
propan-1-one 9 in excellent isolated yields, 91% and 86%,
respectively (Scheme 3, eqs 2, 3).
Mechanistically, we can consider the first step as a
transmetalation step based on homogeneous22 and hetero-
geneous5d,e Au(III) catalysts’ activity reports from the literature
(Scheme 4). The Lewis acid properties of Au(III) catalyst, as
recently highlighted by Toste’s group,23 would favor the 1,4-
carbometalation, leading to intermediate C. The protodemeta-
lation step followed by dissociation of the arylated ketone
would lead to regeneration of the catalyst A.
For this reaction, because we worked in toluene medium, the
mesostructure resists heating for a long period, which has been
corroborated through electron microscopy and N2 adsorption−
desorption isotherms. No significant degradation of the UVM-
7-type architecture, according to TEM images, even after three
catalytic cycles (Figure 5a), was observed. The architecture
based on the aggregation of primary mesoporous nanoparticles
was pleasingly preserved. We detected only a slight loss of
order in the intraparticle mesopore system; however, this minor
structural change may be irrelevant from the catalytic point of
view if accessibility to the gold nanoparticles is preserved to a
large extent. At this point, the N2 adsorption−desorption
isotherm confirmed that the bimodal hierarchic porosity typical
of the UVM-7 support was maintained for the used catalyst.
The two adsorption steps and their respective peaks in the BJH
pore size distribution could be clearly appreciated in Figure
5c,d, respectively. After being used, only a relatively small
decrease in the textural parameters was observed (BET surface
area = 547.3 m2/g and BJH pore volume = 1.27 cm3/g) when
compared with the starting catalyst. On the other hand, dealing
with the active sites, for example, the gold nanoparticles,
3.3. Catalyst Stability and Recyclability. The catalyst
stability and recyclability were established by performing three
successive reaction tests and characterization of the catalyst
Scheme 4. Proposed Mechanism for 1,4-Addition of Arylboronic Acid to 2-Cyclohexen-1-one
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Figure 5. (a) TEM, (b) HAADF, and (c) N2 adsorption−desorption isotherms and (d) BJH pore size distributions of the Au/TiO2-UVM-7 material
after three catalytic cycles.
although a good dispersion of the gold nanoparticles was
preserved according to HAADF (Figure 5b), a small nano-
particle growth (gold particle size from 6.5 to 9.7 nm after three
cycles) has been detected. These small changes in the catalyst
architecture could explain the low decrease in the catalytic
activity during the recycling tests.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: florentina.neatu@chimie.unibuc.ro.
*E-mail: vasile.parvulescu@g.unibuc.ro.
*E-mail: pedro.amoros@uv.es.
*E-mail: veronique.michelet@chimie-paristech.fr.
Notes
■
The authors declare no competing financial interest.
CONCLUSIONS
■
A silica−titania support was successfully prepared and loaded
with 1 wt % gold. The prepared material was shown to be an
efficient catalyst in the unprecedented gold-catalyzed 1,4-
addition of various functionalized arylboronic acids to 2-
cyclohexen-1-one using toluene as a solvent. The reaction
conditions were efficiently applied to other selected enones,
such as cyclic, linear, and aryl-substituted ones. The addition of
phenylboronic acid to 2-cyclohexen-1-one led to a TOF of 18,
which is 3−4 times higher than other heterogeneous catalytic
systems reported in the literature for this reaction in the
presence of rhodium catalysts. The superior activity of the Au/
TiO2−UVM-7 catalyst was demonstrated compared with other
gold-based heterogeneous catalysts and may be explained by
the high dispersion of Au, a specific interaction of Au with the
dispersed titanium oxide, and the existence of a high
accessibility to the active sites, thanks to the hierarchic porosity
of UVM-7. The gold-based catalyst was recycled and therefore
played an active role in this reaction, the nature of the solvent
determining a remarkable change in the products selectivity.
Further studies will be dedicated in the study of the generality
of this catalytic system toward other Michael acceptors.
ACKNOWLEDGMENTS
■
This work was supported by a grant from the Romanian
Ministry of Education, CNCS−UEFISCDI, Project No. PN-II-
RU-PD-2012-3-0295, the Centre National de la Recherche
Scientifique (CNRS), the Ministerio de Economia y Com-
petitividad (MAT2012-38429-C04-03), and the Generalitat
Valenciana (PROMETEO/2009/108). A.M. thanks MEC for a
FPI fellowship.
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