Cyclotrimerization of alkynes vs. ketone formation in aqueous microemulsion

Article abstract of DOI:10.1016/j.molcata.2013.11.009

In the course of our attempts to develop sustainable conditions for the catalytic organic reactions by replacement of the traditional but environmentally disfavored organic solvents by water, we studied the cyclotrimerizations of alkynes, in aqueous microemulsions. The catalyst for these reactions was the rhodium-trichloride encaged within silica sol-gel. In acidic aqueous media, alkynes can undergo addition of water to form their corresponding ketones. In order to eliminate completely the formation of ketones, relatively low reaction temperatures are required. At higher reaction temperatures, however, buffered microemulsion media is preferable. Cyclotrimerization of alkynes proved to be dependent on the reaction temperatures, the electronic nature of the substrates, the electronic nature of the surfactant and on the hydrophobicity of the sol-gel support. During the cyclotrimerization reaction, the rhodium complex was turned to Rh(0) nanoparticles characterized by TEM measurements. These investigations may be regarded as model studies for the conversion of alkynes into substituted benzenes in water.

Full text of DOI:10.1016/j.molcata.2013.11.009

Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cyclotrimerization of alkynes vs. ketone formation
in aqueous microemulsion
a
b
c
Zackaria Nairoukh , Monzer Fanun , Michael Schwarze ,
c
a,∗
Reinhard Schomäcker , Jochanan Blum
Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
Center for Colloid and Surface Research, Al-Quds University, East Jerusalem 51000, Palestinian Authority
Institut für Chemie, Technische Universität Berlin, Strasse des 17, Juni 124, D-10623 Berlin, Germany
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
In the course of our attempts to develop sustainable conditions for the catalytic organic reactions by
replacement of the traditional but environmentally disfavored organic solvents by water, we studied
the cyclotrimerizations of alkynes, in aqueous microemulsions. The catalyst for these reactions was the
rhodium-trichloride encaged within silica sol–gel. In acidic aqueous media, alkynes can undergo addition
of water to form their corresponding ketones. In order to eliminate completely the formation of ketones,
relatively low reaction temperatures are required. At higher reaction temperatures, however, buffered
microemulsion media is preferable.
Cyclotrimerization of alkynes proved to be dependent on the reaction temperatures, the electronic
nature of the substrates, the electronic nature of the surfactant and on the hydrophobicity of the sol–gel
support. During the cyclotrimerization reaction, the rhodium complex was turned to Rh(0) nanoparticles
characterized by TEM measurements.
Received 13 September 2013
Received in revised form 8 November 2013
Accepted 10 November 2013
Available online 20 November 2013
Keywords:
Alkynes cyclotrimerization
Alkynes hydration
Heterogeneous catalysis
Microemulsion
Sustainable chemistry
These investigations may be regarded as model studies for the conversion of alkynes into substituted
benzenes in water.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
and sol–gel materials support the catalysts. So far, we found that
our system can be applied to a variety of hydrogenations processes
[20], transfer hydrogenation [21], regioselective hydroformylation
[22], double bond migration in allylic compounds [23], dispropor-
tionation of dihydroarenes [24], and carbon–carbon bond coupling
processes [25,26] (Heck, Suzuki, Stille and multi-component addi-
tion reactions). We now find that the combination of aqueous
emulsions of the substrates and sol–gel entrapped catalysts – the
so-called Emulsion/Solid-heterogenization method for Transport
and catalysis (EST) – is applicable to facile alkynes cyclotrimeriza-
tion processes.
In aqueous microemulsion, however, the cyclotrimerization of
alkynes is competed with the ketones formation process in the
presence of RhCl3 entrapped within silica sol–gel. In order to add
water to alkynes to form ketones, it is essential to stay at low pH. In
aqueous media, RhCl3 can be easily hydrolyzed and provides low
pH solutions. Furthermore, we already demonstrated that efficient
metal-free hydration of alkynes can be performed easily in aqueous
microemulsion at low pH [27].
The transition-metal catalyzed [2 + 2 + 2] cycloaddition of
alkynes is a very powerful method for the construction of arenes in
a single operational step. It continues to attract great attention by
virtue of its intrinsic atom economy and convergent nature, as well
as the importance of substituted benzenes as synthetic intermedi-
ates [1–4]. Since the first discovery by Reppe and co-workers [5],
the cyclotrimerization takes place in the presence of either transi-
tion metal-derived catalysts [6,7], in aqueous media and biphasic
conditions [8–10], by heterogeneous catalysts [11–14], in super-
critical carbon dioxide [15], or under photochemical conditions
[
16–18].
In the course of our attempts to replace the conventional but
harmful solvents in organic processes by benign aqueous media,
we have already developed a general system in which the sub-
strates are solubilized in aqueous emulsions or microemulsions
and the catalysts can be recycled [19,20]. In these systems suit-
able surfactants promote the solubilization in the aqueous media
In this report we describe an efficient transformation of alkynes
to substituted benzenes by RhCl entrapped within sol–gel support
3
in a regioselective manner under relatively mild conditions within
environmentally favored media.
∗ Corresponding author. Tel.: +972 2 6585329; fax: +972 2 6513832.
E-mail address: jochanan.blum@mail.huji.ac.il (J. Blum).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.11.009

9
4
Z. Nairoukh et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
2
. Experimental
(2.1 ml, 6.68 mmol) (or the corresponding ethyltriethoxysi-
lane, n-propyltrimethoxysilane or phenyltrimethoxysilane), TDW
(0.38 ml) and EtOH (5.6 ml) was stirred for 24 h. Then, to a solu-
tion of tetramethoxysilane (3.6 ml, 24.2 mmol), TDW (2.4 ml) and
MeOH (2 ml) was added RhCl ·3H O (30 mg, 0.114 mmol) and N-
2
.1. Instruments
NMR spectra were recorded with either Bruker DRX-400 or
3
2
Bruker Avance II-500 instrument. Infrared spectra were recorded
with a Perkin-Elmer 65 FTIR spectrometer. Mass spectral mea-
surements were performed with a Q-TOF-II spectrometer. Gas
chromatographic analyses were carried out with a Hewlett-Packard
model Agilent 4890D equipped with either a 30 m long column
packed with Carbowax 20 M-poly(ethylene glycol) in fused silica
trimethoxysilylpropyl-N,N,N-trimethylammonium chloride [50%
in methanol] (60 mg, 0.114 mmol). The two mixtures were com-
bined and the stirring was continued as long as possible (2–7 days).
The resulting gel was aged for 2 days at room temperature, dried at
◦
80 C and 0.1 Torr for 12 h. The ceramic material was washed with
boiling CH Cl2 (2× 15 ml) to ensure the removal of any metallic
2
(
Supelco 25301-U) or with a 15 m long column packed with bonded
compounds that was not entrapped within the sol–gel matrix, and
◦
crosslinked (5% phenyl) methyl polysiloxane [HP-5]. ICP-MS anal-
yses were performed with a Perkin-Elmer model ELAN DRC II
instrument. XPS measurements were performed with a Kratos Axis
Ultra X-ray photoelectron spectrometer. Spectra were acquired
with monochromated Al K␣ (1486.7 eV) X-ray source with 0 take
off angle. The pressure in the test chamber was maintained at 1.5
redried at 80 C and 0.1 Torr to give a constant weight.
2.5. General procedure for the emulsification of the substrates
◦
Typically, a mixture of TDW (11.4–17.6 ml, 89.3 wt.%), and a
suitable surfactant (0.42–0.65 g, 3.3 wt.%) was stirred at room tem-
perature. Then, the substrate (0.102–0.158 g, 1 mmol, 0.8 wt.%) was
added dropwise under vigorous stirring. The emulsion, so formed,
was titrated with n-propanol until a clear transparent mixture was
obtained (usually 1.05–1.63 ml, 6.6 wt.%).
−
9
×
10 Torr during the acquisition process. High resolution XPS
scans were collected for Rh 3d and C 1s peaks with pass energy
0 eV. The XPS binding energy was calibrated with respect to the
2
peak position of C 1s at 285.0 eV. Data analysis was performed
with Vision processing data reduction software (Kratos Analyti-
cal Ltd.) and Casa XPS (Casa Software, Ltd.). Transmission electron
microscopy was done with Scanning Transmission Electron Micro-
scope (STEM) Tecnai G2 F20 (FEI Company, USA) operated at 200 kV
and equipped with EDAX-EDS for identification of the elemental
composition.Transmission electron microscopy was done with a
scanning transmission electron microscope (STEM) Tecnai G2F20
2.6. Emulsification of the substrate when SDBS was used as
surfactant
A mixture of TDW (11.4–17.6 ml, 89.3 wt.%), SDBS (0.42–0.65 g,
◦
3
.3 wt.%) was heated to 50–55 C on a water bath in order to dis-
solve the surfactant. Then, the substrate (0.102–0.158 g, 1 mmol,
.8 wt.% of the desired microemulsion) was added dropwise under
(
FET company) operated at 200 kV and equipped with EDAX-EDS
0
for identification of elemental compositions.
vigorous stirring. The emulsion, so formed, was titrated with n-
propanol until a clear transparent mixture was obtained (usually
2.2. Chemicals
1
.05–1.63 ml, 6.6 wt.%) that disperses a beam of laser light.
Ethynylbenzene, 1-(1,1-dimethylethyl)-4-ethynylbenzene, 3-
ethynylthiophene, dodecyltrimethylammonium bromide (DTAB),
cetyltrimethylammonium bromide (CTAB), sodium benzene
-dodecylsulfonate (SDBS), tetramethoxysilane (TMOS), tetrae-
thoxysilane (TEOS), ethyltriethoxysilane, n-propyltrime-
2
.7. General procedure for the cyclotrimerization of alkynes
The above microemulsion of the substrate, together with immo-
4
bilized rhodium catalyst (15 mg RhCl ), were placed in either an
3
autoclave or in a pressure vessel and heated with stirring to the
desired temperature for the required length of time. The reaction
vessel was cooled to room temperature and the mixture was fil-
tered. The filtrate was treated with NaCl (2 g), which caused the
mixture to separate into two phases. The sol–gel material, as well
thoxysilane and phenyltrimethoxysilane were purchased from the
Sigma–Aldrich Chemical Company. 1-Ethynyl-4-methoxybenzene,
1
-ethynyl-4-methylbenzene,
1-ethynyl-4-chlorobenzene,
1-
ethynyl-4-fluorobenzene and
n-octyltriethoxysilane
were
purchased from the Alfa Aesar Chemical Company. Buffer solution
pH 4 [Phthalate], buffer solution pH 7 [Phosphate] and buffer
solution pH 9 [Borate] were purchased from the J. T. Baker.
as the aqueous layer, were extracted with CH Cl2 (2× 10 ml) to
2
ensure complete removal of the products. The combined organic
solutions were dried (MgSO ), concentrated and analyzed both by
4
GC and 1H NMR and compared with authentic samples. The het-
2
.3. Preparation of hydrophilic immobilized catalyst
◦
erogenized catalyst was dried at 80 C and 0.1 Torr for 5 h in order
to be ready for use in the next run.
Entrapment of RhCl3 within hydrophilic silica sol–gel was
carried out as follows: to a solution of tetraethoxysilane (3 ml,
3.4 mmol), triply distilled water (TDW, 1.33 ml) and EtOH
2.8. Spectral data of selected products
1
(
3.5 ml) was added RhCl ·3H O (30 mg, 0.114 mmol) and N-
3
2
(
(
(
1) 1,2,4-Triphenylbenzene 1H NMR (400 MHz, CDCl ):
ı
ı
trimethoxysilylpropyl-N,N,N-trimethylammonium chloride [50%
in methanol] (60 mg, 0.114 mmol). Stirring was continued as long
as possible (24 h) and the resulting gel was aged for 2 days at
3
7
.17–7.25 (m, 10 H), 7.35–7.72 (m, 8 H) [10].
1
2) 1,3,5-Triphenylbenzene
H
NMR (400 MHz, CDCl ):
3
◦
7.39–7.54 (m, 9H), 7.70–7.75 (m, 6H), 7.82 (s, 3H) [10].
room temperature, dried at 80 C and 0.1 Torr for 12 h. The ceramic
1
3) 1,2,4-Tris(4-fluorophenyl)benzene
H
NMR (400 MHz,
material was washed with boiling CH Cl2 (2× 15 ml) to ensure the
2
CDCl ): ı 6.89–6.97 (m, 4H), 7.07–7.20 (m, 6H), 7.46 (d,
removal of any metallic compound that was not entrapped within
3
◦
J = 7.6 Hz, 1H), 7.54–7.67 (m, 4H) [10].
the sol–gel matrix, and redried at 80 C and 0.1 Torr to give a con-
stant weight.
(
4) 1,3,5-Tris(4-fluorophenyl)benzene 1H NMR (400 MHz,
CDCl ): ı 7.15–7.25 (m, 6H), 7.63–7.68 (m, 6H), 7.69 (s, 3H)
3
[
10].
5) 1,2,4-Tris(4-chlorophenyl)benzene 1H NMR (400 MHz,
CDCl ): ı 7.05–7.12 (m, 4H), 7.21–7.24 (m, 4H), 7.42–7.49 (m,
2.4. Preparation of hydrophobic immobilized catalyst
(
Entrapment of RhCl3 within octylated silica sol–gel was
3
3
H), 7.55–7.62 (m, 4H) [10].
carried out as follows:
a solution of n-octyltriethoxysilane

Z. Nairoukh et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
95
Scheme 1. Cyclotrimerization vs. hydration of alkynes in aqueous microemulsion.
◦
(
(
6) 1,3,5-Tris(4-chlorophenyl)benzene 1H NMR (400 MHz,
of acetophenone (compare entries 1–4, Table 1). We chose 60 C as
our standard temperature for reasonable reaction rates and yields.
The immobilized catalyst proved to be an efficient cyclotrimer-
ization catalyst for a variety of alkynes. Table 2 shows the electronic
CDCl ): ı 7.43–7.47 (m, 6H), 7.56–7.60 (m, 6H), 7.69 (s, 3H)
3
[
10].
7) 1,2,4-Tris(4-methylphenyl)benzene 1H NMR (400 MHz,
◦
CDCl ): ı 2.35 (s, 3H), 2.36 (s, 3H), 2.42 (s, 3H), 7.05–7.15 (m,
effect of the substrates on the reaction rate at 60 C after 1 h.
3
8
4
H), 7.28–7.32 (m, 2H), 7.49 (d, J = 8 Hz, 1H), 7.58–7.66 (m,
Electron withdrawing groups accelerate the reaction rate while
electron donating groups decelerate the reaction rate (for exam-
ple, compare entries 2 and 4, Table 2). Steric effect can be
noticed (by comparison of entries 2 and 3 in Table 1). It should
be mentioned that both 1-ethynyl-4-methoxybenzene and 1-
(1,1-dimethylethyl)-4-ethynylbenzene give only the asymmetric
substituted benzene (entries 5 and 6, Table 2). The immobilized
RhCl3 proved also to be an efficient cyclotrimerization catalyst for
heteroaromatic substrates (entry 7, Table 1). Because of the low
reaction rate of alkynes with electron donating groups, we per-
H) [10].
(
8) 1,3,5-Tris(4-methylphenyl)benzene 1H NMR (400 MHz,
CDCl ): ı 2.45 (s, 9H), 7.10–7.15 (m, 6H), 7.57–7.65 (m, 6H),
3
7
.76 (s, 3H) [10].
(
9) 1,2,4-Tris(4-methoxyphenyl)benzene 1H NMR (400 MHz,
CDCl ): ı 3.81 (s, 3H), 3.88 (s, 3H), 3.89 (s, 3H), 6.78–6.85 (m,
3
3
H), 6.99–7.06 (m, 3H), 7.10–7.18 (m, 3H), 7.46 (d, J = 7.6 Hz,
1
H), 7.56–7.69 (m, 5H) [10].
(
(
10) 1,2,4-Tris(4-tert-butylphenyl)benzene 1H NMR (400 MHz,
◦
CDCl ): ı 1.29 (s, 9H), 1.30 (s, 9H), 1.35 (s, 9H), 7.05–7.13 (m,
formed the reactions at 100 C for 3 h (entries 8 and 9, Table 2).
3
4
[
H), 7.21–7.24 (m, 4H), 7.46–7.48 (m, 3H), 7.58–7.62 (m, 4H)
28].
Although the reaction rate was increased, only acetophenone
derivatives were isolated. These results are compatible with our
previous studies which demonstrated that alkynes with electron
donating groups can be hydrated more easily [27]. In order to
eliminate completely the ketone byproduct, we performed the
cyclotrimerization reaction in buffered microemulsions. Although
high temperatures were required, byproducts were completely
eliminated (compare entries 1 and 3, Table 3). The buffer pH also
can affect the reaction rate. A higher reaction rate was obtained at
low pH buffer (entries 3 and 4, Table 3). No conversion was detected
at higher pH buffers (pH 9). We assume that hydrolysis of RhCl3 is
required for the cyclotrimerization reaction, therefore, at high pH
buffer the RhCl3 hydrolysis is slow as is the reaction rate.
The cyclotrimerization reaction proved to depend on the nature
of the surfactant. For example, the cyclotrimerization of ethynyl-
benzene shown in Table 4 indicates that the yields in the presence of
cationic surfactants are higher than those observed in the presence
of anionic additives (compare entries 1–3, Table 4). This observa-
tion can be rationalized by the fact that cationic surfactants are
more suitable to be adhered onto the partially negatively charged
silica sol–gel matrix [30,31]. No significant influence on the reac-
tion yield among the various cationic alkylammonium bromides
was obtained.
1
11) 1,2,4-Tris(3-thiopheneyl)benzene H NMR (400 MHz, CDCl ):
3
ı 6.80 (dd, J = 1.3, J = 5.0 Hz, 1H), 6.83 (dd, J = 1.3, J = 5 Hz,
1
2
1
2
1
H), 7.07 (dd, J = 1.3, J = 3.0 Hz, 1H), 7.12 (dd, J = 1.3,
1
2
1
J2 = 3.0 Hz, 1H), 7.16–7.23 (m, 2H), 7.39–7.44 (m, 2H),
7
.48–7.52 (m, 2H), 7.58 (dd, J = 2.0, J = 8.0 Hz, 1H), 7.67 (d,
1 2
J = 1.9 Hz, 1H) [29].
(
12) 1,3,5-Tris(3-thiopheneyl)benzene 1H NMR (400 MHz, CDCl ):
3
ı 7.41–7.48 (m, 6H), 7.54 (dd, J = 1.4, J = 3 Hz, 3H), 7.72 (s, 3H)
1
2
[
29].
. Results and discussion
Although alkynes do not undergo cyclotrimerization in pure
3
water by the RhCl3 catalyst system, the reaction takes place
upon addition of a suitable surfactant that solubilizes the sub-
strate. Entrapment of RhCl3 within non-hydrophobicized sol–gel
matrix (i.e., prepared from TEOS alone) is not leachable if N-
trimethoxysilylpropyl-N,N,N-trimethylammonium chloride was
introduced. In previous studies [27], we showed that by intro-
ducing mineral acid to the metal-free reaction mixtures, alkynes
◦
were transformed to their corresponding ketones at 80–140 C.
Another factor that influences the cyclotrimerization of alkynes
is the hydrophobicity of the ceramic support of the catalyst.
The introduction of ethyl, propyl, octyl or phenyl moieties into
the ceramic support and the non-hydrophobicized sol–gel (i.e.,
prepared from TEOS alone) have been compared. ICP analy-
ses indicated that the entrapment of the rhodium complex
within the aforementioned supporters do not leach as long
RhCl3 entrapped within sol–gel matrix can be hydrolyzed in aque-
ous media and provide a suitable pH for such transformation.
Therefore, we performed the cyclotrimerization of alkynes under
relatively low temperatures (Scheme 1). Table 1 shows the effect
of the temperature on the reaction regioselectivity and the reac-
tion yields. Thus, ethynylbenzene can undergo cyclotrimerization
◦
◦
at low temperature (60–100 C) while at 140 C it give almost 93%

9
6
Z. Nairoukh et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
Table 1
Cyclotrimerization of ethynylbenzene at different temperatures.
a
◦
Conversion (%)b
Yield of 1 (%)b
Yield of 2 (%)b
Yield of 3 (%)b
Entry
Reaction Temp. ( C)
Ratio (1:2)
1
2
3
4
60
80
100
140
32
51
95
21
35
71
5
11
16
24
2
–
–
–
2:1
2.2:1
3:1
100
93
2.5:1
a
Reaction conditions: alkyne (1.0 mmol, 0.8 wt.% of the microemulsion), CTAB (3.3 wt.%), 1-PrOH (6.6 wt.%), TDW (89.3%) together with heterogenized rhodium catalyst pre-
pared from RhCl3·H2O (15 mg, 0.057 mmol), N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride [50% in methanol] (30 mg, 0.057 mmol), TEOS (3 ml, 13.4 mmol);
1
h.
b
The percentages were determined by GC, and are the average of at least two experiments that did not differ by more than ± 3%.
Table 2
Cyclotrimerization of some alkynes by heterogenized RhCl3 in an aqueous medium at 60 C.a
◦
Entry
Substrate
Conversion (%)b
Yield of 1 (%)b
Yield of 2 (%)b
Yield of 3 (%)b
Ratio (1:2)
1
40
25
15
–
1.6:1
2
3
28
32
22
21
6
–
–
3.6:1
2:1
11
4
5
10
6
6
4
–
–
–
1.5:1
6
–
6
7
15
32
80
15
21
57
–
–
–
4
–
11
19
1.9:1
3:1
8c
9c
73
37
–
36
–
a
Reaction conditions as shown in Table 1.
b
c
The data are average of at least two experiments that did not differ by more than ± 3%.
◦
The reaction performed at 100 C for 3 h.

Z. Nairoukh et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
97
Table 3
Effect of buffer solutions on the cyclotrimerization of 1-ethynyl-4-methylbenzene by heterogenized RhCl3.
a
◦
Conversion (%)b
Yield of 1 (%)b
Yield of 2 (%)b
Entry
Buffer
Reaction Temp. ( C)
Ratio (1:2)
1
2
3
4
pH 4
pH 7
pH 4
pH 7
100
100
120
120
7
9
56
23
5
6
40
17
2
3
16
6
2.5:1
2:1
2.5:1
2.8:1
a
Reaction conditions as shown in Table 1 except that buffer solution was used instead of TDW and the reaction was continued for 3 h.
The data are average of at least two experiments that did not differ by more than ± 3%.
b
Table 4
Effect of the nature of the surfactant on the cyclotrimerization of ethynylbenzene in aqueous media at 60 C.a
◦
Entry
Surfactant
Conversion (%)b
Yield of 1 (%)b
Yield of 2 (%)b
Ratio (1:2)
1
2
3
DTAB
CTAB
SDBS
43
32
8
26
21
5
17
11
3
1.5:1
2:1
1.6:1
a
Reaction conditions as shown in Table 1 except that a variety of surfactants were used.
The data are average of at least two experiments that did not differ by more than ± 3%.
b
Table 5
Cyclotrimerization of ethynylbenzene in aqueous microemulsion by RhCl3 entrapped within differently hydrophobicized sol–gel at 60 C.a
◦
Entry
Hydrophobic Moiety
Conversion (%)b
Yield of 1 (%)b
Yield of 2 (%)b
Ratio
(1:2)
1
2
3
4
5
None
Ethyl
Propyl
Octyl
32
6
5
47
12
21
4
3
30
8
11
2
2
17
4
2:1
2:1
1.5:1
1.7:1
2:1
Phenyl
a
Reaction conditions as in Table 1 except for entries 2, 3 and 5 where 6.68 mmol of ethyltriethoxysilane, n-propyltriethoxysilane and phenyltrimethoxysilane, respectively
were applied instead of the octylated precursor. In entry 1, the sol–gel prepared from TEOS only.
b
The data are average of at least two experiments that did not differ by more than ± 3%.
◦
Fig. 1. TEM micrograph of a typical silica sol–gel support entrapped RhCl3 (a) before and (b) after cyclotrimerization of ethynylbenzene at 60 C under EST conditions.
as N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride
was used. Table 5 shows the differences in the reaction rates
among the different hydrophobic ceramic supports studied. These
results indicate that the catalyst within octylated sol–gel (i.e.,
prepared from 80 mol% of tetramethoxysilane and 20 mol% of octyl-
triethoxysilane) and the non-hydrophobicized sol–gel lead to high
conversion (entries 1 and 4, Table 5). Octylated sol–gel undergoes
gelation within 7 days, therefore for practical reasons we used the
non-hydrophobicized sol–gel. The catalyst within ethylated, propy-
lated and phenylated ceramic support give lower conversions.
Finally, after performing the cyclotrimerization reaction in
aqueous microemulsion, the orange sol–gel supported catalyst
turns black. TEM analysis indicated that this black sol–gel contains
Rh(0) nanoparticles having diameters of 2–5 nm (Fig. 1). However,
XPS analysis of the used heterogenized catalyst recovered from the
cyclotrimerization reaction in toluene indicates that no metallic
Rh(0) nanoparticles were formed. Thus, we assume that the aque-
ous surfactant is the main reducing factor. This phenomenon was
also observed and intensively studied in our previous studies [23].
4. Conclusions
Alkynes were transformed to substituted benzenes in aque-
ous microemulsion by sol–gel entrapped RhCl . The reaction was
3
performed at low temperatures in order to completely eliminate
the ketones byproducts. Reaction at higher temperature required
buffer microemulsion media. Cyclotrimerization was also found to
be dependent (i) on the electronic nature of the substrate, (ii) on the
electronic nature of the surfactant, and (iii) on the hydrophobicity
of the sol–gel support.

9
8
Z. Nairoukh et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 93–98
Acknowledgements
[11] H. Imamura, H. Miura, T. Konishi, Chem. Lett. 19 (1990)
495–1496.
12] A.K. Jhingan, W.F. Maier, J. Org. Chem. 52 (1987) 1161–1165.
13] P.W. Kletnieks, A.J. Liang, R. Craciun, J.O. Ehresmann, D.M. Marcus, V.A. Bhirud,
M.M. Klaric, M.J. Hayman, D.R. Guenther, O.P. Bagatchenko, D.A. Dixon, B.C.
Gates, J.F. Haw, Chem. Eur. J. 13 (2007) 7294–7304.
1
[
[
We gratefully acknowledge the financial support of this trilat-
eral study by the Deutsche Forschungsgemainschaft (DFG) through
grant SCHO687/8-2. We also thank Prof. David Avnir for his contin-
uous support and collaboration.
[
[
[
14] R.M. Ormerod, R.M. Lambert, J. Chem. Soc., Chem. Commun. (1990) 1421–1423.
15] J.-S. Cheng, H.-F. Jiang, Eur. J. Org. Chem. (2004) 643–646.
16] D. Dallinger, M. Irfan, A. Suljanovic, C.O. Kappe, J. Org. Chem. 75 (2010)
5
278–5288.
References
[
[
[
[
[
17] Y.-Y. Lin, S.-C. Tsai, S.J. Yu, J. Org. Chem. 73 (2008) 4920–4928.
18] D.D. Young, L. Sripada, A. Deiters, J. Comb. Chem. 9 (2007) 735–738.
19] R. Abu-Reziq, D. Avnir, J. Blum, Angew. Chem. Int. Ed. 41 (2002) 4132–4134.
20] R. Abu-Reziq, J. Blum, D. Avnir, Chem. Eur. J. 10 (2004) 958–962.
21] C. Batarseh, Z. Nairoukh, I. Volovych, M. Schwarze, R. Schomäcker, M. Fanun, J.
Blum, J. Mol. Catal. A: Chem. 366 (2013) 210–214.
22] Z. Nairoukh, J. Blum, J. Mol. Catal. A: Chem. 358 (2012) 129–133.
23] D. Meltzer, D. Avnir, M. Fanun, V. Gutkin, I. Popov, R. Schomäcker, M. Schwarze,
J. Blum, J. Mol. Catal. A: Chem. 335 (2011) 8–13.
24] T. Yosef, R. Schomäcker, M. Schwarze, M. Fanun, F. Gelman, J. Blum, J. Mol. Catal.
A: Chem. 351 (2011) 46–51.
25] A.R.-B. Baruch, D. Tsvelikhovsky, M. Schwarze, R. Schomäcker, M. Fanun, J.
Blum, J. Mol. Catal. A: Chem. 323 (2010) 65–69.
26] D. Tsvelikhovsky, J. Blum, Eur. J. Org. Chem. (2008) 2417–2422.
27] Z. Nairoukh, D. Avnir, J. Blum, ChemSusChem 6 (2013) 430–432.
28] L. Xu, R. Yu, Y. Wang, J. Chen, Z. Yang, J. Org. Chem. 78 (2013) 5744–5750.
29] E. Rebourt, B. Pépin-Donat, E. Dinh, Polymer 36 (1995) 399–412.
30] S. Minakata, M. Komatsu, Chem. Rev. 109 (2008) 711–724.
31] E. Tyrode, M.W. Rutland, C.D. Bain, J. Am. Chem. Soc. 130 (2008)
[1] F. Diederich, P.J. Stang, R.R. Tykwinski, Acetylene Chemistry: Chemistry, Biol-
ogy, and Material Science, Wiley-VCH, Weinheim, 2005.
[
[
2] X. Bu, Z. Zhang, X. Zhou, Organometallics 29 (2010) 3530–3534.
3] A. Joosten, M. Soueidan, C. Denhez, D. Harakat, F. Hélion, J.-L. Namy, J.-L. Vasse,
J. Szymoniak, Organometallics 27 (2008) 4152–4157.
[
[
[
[
[
[
[
[
4] N. Morohashi, K. Yokomakura, T. Hattori, S. Miyano, Tetrahedron Lett. 47 (2006)
1
157–1161.
5] W. Reppe, O. Schichting, K. Klager, T. Toepel, Justus Liebigs Ann. Chem. 560
1948) 1–92.
6] Y.-l. Xu, Y.-m. Pan, Q. Wu, H.-s. Wang, P.-z. Liu, J. Org. Chem. 76 (2011)
472–8476.
7] K. Yoshida, I. Morimoto, K. Mitsudo, H. Tanaka, Tetrahedron 64 (2008)
800–5807.
8] I. Amer, T. Bernstein, M. Eisen, J. Blum, K.P.C. Vollhardt, J. Mol. Catal. 60 (1990)
13–321.
9] W. Baidossi, N. Goren, J. Blum, H. Schumann, H. Hemling, J. Mol. Catal. 85 (1993)
53–162.
10] V. Cadierno, S.E. García-Garrido, J. Gimeno, J. Am. Chem. Soc. 128 (2006)
5094–15095.
[
[
(
8
[
[
[
[
[
[
5
3
1
[
17434–17445.
1