2
14
N. Mameda et al. / Applied Catalysis A: General 505 (2015) 213–216
Table 1
exchanged silicotungstic acid (AgSTA) [26]. However, the prepara-
Optimization of reaction conditions for the hydration of phenylacetylene.a
tion of Au–HS/SO H-PMO(Et) catalyst is very expensive [25] and
3
the catalytic activity of AgSTA is low towards internal alkynes
[
26]. Therefore, the development of an inexpensive, more efficient,
solvent-free and eco-friendly heterogeneous hydration systems
without any additives is highly desirable.
Entry Water (mmol) Catalyst
Temperature ( C) Yieldb (%)
◦
Catalysts based on zeolites possess major importance both in
petroleum and fine chemical industries [27,28]. This is mainly due
to the fact that zeolites have uniform channel size and unique
molecular shape selectivity, as well as strong acidity and good ther-
mal/ hydrothermal stability. The BEA-type of zeolite consists of
an intergrowth of two or more polymorphs comprised of a three
dimensional system of 12-membered ring channels [29]. The BEA
framework topology attracts much attention because of the large
available micro-pore volume, large-pore channel system and the
presence of active sites (Bronsted acid sites in the micropores and
on the external surface and Lewis acid sites predominantly at the
internal surface due to the local defects) in different concentrations.
In continuation of our efforts toward the development of novel and
eco-friendly synthetic protocols using zeolites [30–32], herein we
report a simple, efficient and environmentally benign approach for
the hydration of alkynes using zeolite under solvent-free condi-
tions.
1
2
3
4
5
6
7
8
9
1
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
N/A
4
6
H
100
100
100
98
60
58
27
50
00
00
8
H-mordenite
HZSM-5 (40)
Montmorillonate K10 100
HY
NaY
MCM-41
Amberlyst-15
Amberlite
100
100
100
100
100
100
100
100
100
100
100
RT
60
80
100
100
100
12
33
31
00
25
56
00
00
07
58
30
57
85
c
0
H2SO4
HClc
11
12
Nb2O5.nH2O
H
H
d
e
13
14
15
16
f
N/A
H
H
H
H
H
H
17
18
f
1
2
2
9
0
1
2
. Experimental
a
Reaction conditions: phenylacetylene (2 mmol), H2O (8 mmol), Catalyst
◦
(
100 mg), 100 C, 3 h.
2.1. General information
b
Isolated yields.
.25 mmol of catalyst.
Catalyst (50 mg).
Catalyst (75 mg).
c
0
Alkynes were purchased from Sigma–Aldrich. Hˇ (Si/Al = 15)
d
e
f
zeolite was obtained from Alfa Aesar, England. All chemicals used
were reagent grade and used as received without further purifica-
tion. All the samples were systematically characterized by different
spectroscopic techniques. The XRD patterns of the samples were
obtained on a Regaku miniflux X-ray Diffractometer using Ni fil-
N/A refers to not applicable.
◦
◦
−1
tered CuK␣ radiation at 2 = 2 80 with a scanning rate of 2 min
conditions (Table 1). In order to choose the best catalyst first, the
reaction was carried out over various zeolites and other catalysts
at 100 C for 3 h and the results are presented in Table 1 (entries
and the beam voltage and currents of 30 kV and 15 mA, respectively.
1
H NMR spectra were recorded by using Bruker VX NMR FT-300
◦
1
3
or Varian Unity 500 and C NMR spectra were recorded by using
Bruker VX NMR FT-75 MHz spectrometers instrument in CDCl3. The
chemical shifts (ı) are reported in ppm units relative to TMS as an
internal standard for H NMR and CDCl3 for C NMR spectra. Cou-
pling constants (J) are reported in hertz (Hz) and multiplicities are
indicated as follows: s (singlet), d (doublet), dd (doublet–doublet),
t (triplet), q (quartet), m (multiplet). Column chromatography was
carried out using silica gel (100–200 mesh).
1
–12). Among the catalysts examined, H zeolite showed a higher
catalytic activity and furnished the corresponding methyl ketone
in excellent yield (Table 1, entry 1). Under the similar reaction
conditions, H-mordenite, HZSM-5 (40) and montmorillonite K10
provided the corresponding methyl ketones in < 60% yield (Table
1
13
1
, entries 2–5). NaY and MCM-41delivered virtually no conver-
sion of starting material (Table 1, entries 6–7). When the acidic
resins, homogeneous Bronsted acids and heterogeneous Lewis acid
catalysts were investigated under similar reaction conditions, the
hydration product was obtained in up to 33% yield (Table 1, entries
2.2. General procedure for hydration of alkynes
8
–12). Once H catalyst was found as the best catalyst for hydra-
Reactions were performed in a magnetically stirred round bot-
tomed flask fitted with a condenser and placed in a temperature
controlled oil bath. Zeolite (H) (100 mg) was added to the well
tion of phenylacetylene, the influence of a catalyst amount was
studied. By varying the catalyst amount from 50 to 100 mg, a grad-
ual improvement in yield (25–98%) was observed and any further
increase of amount did not have accountable effect on the yield
stirred solution of alkyne (2 mmol) and H O (8 mmol) and the reac-
2
◦
tion mixture was allowed to stir at 100 C. After disappearance of
(Table 1, entries 1 and 13–14). In the absence of catalyst no hydra-
the alkyne (monitored by TLC) or after an appropriate time, the
reaction mixture was cooled to room temperature, diluted with
ethyl acetate. The catalyst was separated by filtration and the
removal of solvent in vacuo yielded residue. and it was further puri-
fied by column chromatography using silica gel (100–200 mesh) to
afford pure products. All the products were identified on the basis
tion of phenylacetylene was observed, thus confirming the role of
the catalyst in the reaction (Table 1, entry 15).
The reaction was remarkably accelerated by varying the reaction
◦
temperature from room temperature to 100 C (Table 1, entries 1
and 16–18) and yield improved progressively from 0 to 98%. The
mole ratio of phenyl acetylene to H O also had a significant influ-
ence on the product yield. When increasing the mole ratio of phenyl
1
13
2
of H and C NMR spectral data.
acetylene to H O (1:0–1:4), the yield of the desired product was
increased from 30 to 98% (Table 1, entries 1 and 19–21). As can be
seen from the above obtained results, the optimized reaction con-
2
3
. Results and discussion
3.1. Catalyst screening
ditions to get the highest yield for this hydration reaction are 1:4
◦
mole ratio of phenylacetylene to H O at 100 C over H catalyst
In the initial investigation, phenylacetylene was selected as a
2
(100 mg).
model substrate for alkyne hydration to find out the best reaction