Reduction of diphenylacetylene using Al powder in the presence of noble metal catalysts in water

Article abstract of DOI:10.1016/j.tet.2016.09.021

Diphenylacetylenes can be reduced to the corresponding diphenylethanes (2) in water in excellent yield using Al powder and Pd/C at 60?°C for 3?h in a sealed tube. In addition, the complete reduction of both aromatic rings required 80?°C for 15?h with Al powder in the presence of Pt/C. However, the nature of hydrogenated product formed was found to be strongly influenced by the reaction temperature, time, volume of water and the amount of catalyst being employed.

Full text of DOI:10.1016/j.tet.2016.09.021

Tetrahedron 72 (2016) 6943e6947
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Reduction of diphenylacetylene using Al powder in the presence of
noble metal catalysts in water
,
,
Ummey Rayhan a b, Zannatul Kowser a, Carl Redshaw c, Takehiko Yamato a
*
a Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga 840-8502, Japan
b Department of Chemistry, Dhaka University of Engineering and Technology, Gazipur 1700, Bangladesh
c Department of Chemistry, The University of Hull, Cottingham Road, Hull, Yorkshire HU6 7RX, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2016
Received in revised form 6 September 2016
Accepted 11 September 2016
Available online 14 September 2016
Diphenylacetylenes can be reduced to the corresponding diphenylethanes (2) in water in excellent yield
using Al powder and Pd/C at 60 ꢀC for 3 h in a sealed tube. In addition, the complete reduction of both
aromatic rings required 80 ꢀC for 15 h with Al powder in the presence of Pt/C. However, the nature of
hydrogenated product formed was found to be strongly influenced by the reaction temperature, time,
volume of water and the amount of catalyst being employed.
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Reduction of diphenylacetylene
Noble metal catalyst
Al powder
Water
Cyclohexane ring
1. Introduction
Pd,18e20 Rh,21,22 Ru,23e25 Ni,26 Pt,27,28 Ir,29 Os,30 V,31 Fe,32 and Nb.33
There is particular interest in the use of Pd given its excellent
Hydrogenation of aromatic alkynes to the corresponding al-
kenes and alkanes is of great interest in synthetic organic chem-
istry. This is one of the most widely used chemical reactions with
many industrial applications in the petro, food and pharmaceutical
industries. Styrene is an industrially and commercially valuable
material. Phenylacetylene reduced by semi-hydrogenation is
a process of great industrial importance1,2 because phenylacetylene
is a poisoning impurity in styrene feedstocks, and leads to de-
activation of the styrene polymerization catalyst. The hydrogena-
tion of phenylacetylene proceeds under relatively mild conditions
and the desired product is the intermediate (styrene). Thus, this
methodology is a very convenient tool for the evaluation of process
design2,3 and testing the efficiency of hydrogenation catalysts.4e7
This is why a substantial amount of literature is available featur-
ing both homogeneous8e10 and heterogeneous11e17 selective pro-
cesses for the semi-hydrogenation of alkynes.
performance in the selective hydrogenation of alkynes.34 Recently,
transition-metal nanoparticles in catalysis have drawn much at-
tention due to their high efficiency and unique properties. Palla-
dium nanoparticles possessing high catalytic activity and
controllable particle size have also attracted attention.35 Moreover,
the use of ruthenium nanoparticles for the semi-hydrogenation of
alkyne and platinum-ruthenium nanoparticles for the selective
hydrogenation of phenylacetylene has been reported.36,37 The se-
lective reduction of phenylacetylene was also investigated with
AlMgO particles which were used as an alternative to water-
reactive generator of hydrogen.38 The partial hydrogenation of 3-
hexyne was observed using low-loadings of palladium mono- and
bimetallic catalysts.39 A Hantzsch amido dihydropyridine has been
used as a transfer hydrogenation reagent for
a,b-unsaturated
ketones.40
Catalytic hydrogenation is widely considered to be an environ-
mentally benign process and both heterogeneous and homoge-
neous alternatives are popular in industry.41e47 Nowadays,
heterogeneous catalysts are being used for the selective reduction
of condensed N-heterocycles using water as both a solvent and
a hydrogen source.48 Based on the application of Raney-type NieAl
alloy in aqueous medium, the selective reduction of ketones and
reductive amination of carbonyl compounds has been reported.49,50
More recently, the application of a heterogeneous catalyst in
Catalytic hydrogenation using hydrogen gas or hydride transfer
agents is commonly employed for this transformation. Various
hydrogenation reactions have been studied, for which the active
catalysts commonly employed include systems based on the metals
* Corresponding author. Fax: þ81 952 28 8548; e-mail address: yamatot@cc.
saga-u.ac.jp (T. Yamato).
http://dx.doi.org/10.1016/j.tet.2016.09.021
0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

6944
U. Rayhan et al. / Tetrahedron 72 (2016) 6943e6947
combination with microwave irradiation has been employed as an
environmentally benign tool for some contemporary organic
synthesis.51,52
(3a) was found in the highest yield (97%) (Table 2, entry 2). On
increasing the reaction time, the amount of the reduction product
(3a) was not found to increase, rather over a longer reaction time of
more than 4 h, the product (3a) started to transform into 4a (Table
2, entry 4).
On the other hand, the utilization of water as a chemical reagent is
an essential aspect of Green Chemistry.53 Water as a solvent for or-
ganic reactions has many advantages over the use of other organic
solvents including cost, safety, simplicity of operation and most
importantly its benign environmental character.54e56 More recently,
water has been used as a stoichiometric H or D atom donor for
tetrahydroxydiboron-mediated palladium-catalyzed transfer hy-
drogenation and deuteriation of alkenes and alkynes.57 In addition,
hydrogenations and deuterium labeling has been carried out with Al-
based metal alloys under aqueous conditions.58 Here in this study,
we illustrate a simple reduction method for diphenylacetylene using
commercially available Al powder in the presence of noble metal
catalysts (Pt/C, Pd/C, Ru/C or Rh/C) in water in a sealed tube.
Table 2
Reduction of diphenylacetylene (1a) using Al powder and Pd/C in H2Oa,b
Entry
Time (h)
2a
3a
Yieldc,d (%) 4a
5a
Recovery 1a
1
2
3
4
2
3
4
5
0
0
0
0
52
0
0
0
3
0
0
0
0
48
3
0
97[84]
100
97
0
a
Substrate: 20 mg (0.11 mmol), Al powder: 100 mg (500 wt %), Catalyst: 4.5 mol %
(metal), H2O: 0.5 mL.
b
Conditions: temp: 60 ꢀC.
The yields were determined by GLC.
The isolated yields are shown in a square bracket.
c
d
2. Results and discussion
It was observed that on increasing the reaction time, the yield of
the desired compound (3a) was found to peak (97%) at 3 h. On
increasing the reaction time further, the amount of product (3a)
increased only very slightly. On the other hand, using Raney NieAl
in dilute alkaline aqueous solution, benzophenone required
a higher amount of catalyst compared to that used in this work,59
Thus, the catalytic system developed here is more economical.
From Table 2, it was found that 60 ꢀC for 4 h was the best con-
ditions for the reduction of diphenylacetylene (1a) to diphenyl-
ethane (3a) when using Al powder and Pd/C. To explore the
corresponding activity of other catalytic systems, the reduction was
carried out under the same condition for 3h. In the case of Rh/C, we
obtained a 27% yield, whilst for Ru/C, no reaction occurred. In Fig. 1,
the pink colour indicates the starting compound diphenylacety-
lenes (DPA), the green colour represents our desired product 1,2-
diphenylethane (DPE) and the blue colour for stilbene product.
In our previous study, a Pt/C catalyst with Al powder in water was
found to be a stronger reducing agent for the reduction of aromatic
rings.60 Basedonthis information, thereductionofdiphenylacetylene
(1a) was examined with this catalyst in a sealed tube.
In order to achieve a more environmentally friendly chemical
process, the reduction of diphenylacetylene (Scheme 1, 1a) was
carried out using Al powder in the presence of a noble metal cat-
alyst in water in a sealed tube. Besides the expected product 1,2-
diphenylethane (Scheme 1, 3a), a mixture of stilbene (Scheme 1,
2a), cyclohexylphenylethane (Scheme 1, 4a) and 1,2-
dicyclohexylethane (Scheme 1, 5a) was obtained upon reduction.
The relative distribution of the products formed was found to de-
pend on the reaction conditions employed. Consequently, the ef-
fects of the reaction temperature, time, amount of catalyst and
required volume of water for the reduction of diphenylacetylene
(1a) were investigated.
Scheme 1. Reduction of diphenylacetylene (1a) by using Al powder and noble metal
catalyst in H2O.
An initial attempt to reduce diphenylacetylene (1a) using only
Al powder in water in a sealed tube failed. However, when the
reduction was carried out using Al powder and Pd/C at 60 ꢀC for 3 h,
diphenylethane (3a) was isolated in good yield (Table 1, entry 2). On
increasing the reaction time, diphenylacetylene (1a) gradually
underwent further reduction and as a result, the intermediate
product (3a) was transformed into 4a over a longer reaction time
(Table 1, entry 3).
The reduction of diphenylacetylene (1a) was conducted at 60 ꢀC
to evaluate the effect of the reaction time at 60 ꢀC using Al powder
and Pd/C. When 1a was subjected to reduction over 3 h, the product
Fig. 1. Reduction of diphenylacetylene (1a) using Al powder and noble metal catalyst.
Table 1
Reduction of diphenylacetylene (1a) using Al powder and noble metal catalyst in
H2Oa,b
Entry
Catalyst
Time (h)
Yieldc (%) 3a
4a
Recovery 1a
When the reduction of diphenylacetylene (1a) was carried out
at 80 ꢀC for 12 h using a Pt/C catalyst with Al powder in water, 37%
of 5a was observed along with 63% recovery of the starting com-
pound (1a) (Table 3, entry 1). The reduction increased on increasing
the reaction temperature up to 100 ꢀC, but this temperature is not
suitable when water is used as the solvent (Table 3, entry 2). Con-
sequently, when using this catalyst, the reaction temperature was
gradually decreased and a 91% yield of compound 5a was obtained
at 80 ꢀC over 15 h. Thus Pt/C turned out to be the best catalyst and it
1
2
3
4
5
Al PowderþPt/C
Al PowderþPd/C
Al PowderþPd/C
Al Powder
3
3
6
6
3
20
95
94
0
0
0
6
0
0
80
5
0
100
9
NieAl
91
a
Substrate: 0.11 mmol, NieAl: 100 mg (500 wt %) (Wako), Al powder: 100 mg
(500 wt %) (Wako), Catalyst: 4.5 mol % (metal) (Wako), H2O: 0.5 mL (Wako).
b
Conditions: temp: 60 ꢀC.
The yields were determined by GLC.
c

U. Rayhan et al. / Tetrahedron 72 (2016) 6943e6947
6945
Table 3
noteworthy that this is also accomplished without the need of the
presence of any strong base. Moreover, the noble metal catalyst
most likely readily adsorbs hydrogen and thereby ensures the ef-
fective hydrogenation of the substrates. In addition, the reaction
does not produce any harmful waste materials and the only by-
product that forms is non-toxic Al(OH)3/Al2O3 that can be filtered
and used in other hydrogenation processes.
Reduction of diphenylacetylene (1a) using Al powder and Pt/C in H2Oa
Entry
Temp(ꢀC)
Time (h)
3a
Yieldb,c(%) 4a
5a
Recovery 1a
1
2
3
4
80
100
80
12
12
15
18
0
9
6
7
0
0
0
0
37
87
91[78]
92
63
4
3
80
1
a
Substrate: 20 mg, Al powder: 100 mg (500 wt %), catalyst: 4.5 mol % (metal),
H2O: 0.5 mL.
Table 4
b
c
Reduction of substituted diphenylacetylenes (1) using Al powder and Pd/C in H2Oa,b
The yields were determined by GLC.
The isolated yields are shown in a square bracket.
Entry
R
2
Yieldc,d (%) 3
Recovery 1
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
0
0
53
16
0
100
68
37
36
100[90]
41
0
is well-known that Pt/C can act as a more effective catalyst when
used in the presence of Raney NieAl alloy and Al powder.61 The
intermediate product diphenylethane was also investigated under
the same reaction conditions and it led to excellent yields via the
reduction of both aromatic rings.
Subsequently, the reduction of substituted diphenylacetylenes
was examined to afford compounds 3 and 5. Following exposure of
the substituted diphenylacetylenes to reduction, under the estab-
lished standard conditions, 4-methyldiphenylacetylene (1b) gave
the product 3b with a yield of 68% along with recovery of the
starting compound (1b), which is probably due to steric hindrance
(Table 4, entry 2). Interestingly, given in the presence of the bulky
group, 4-methoxydiphenylacetylene (1c) gave a 53% yield of stil-
bene product (2c) whereas 4-tert-butyl-diphenylacetylene (1d) and
4-trifluorodiphenylacetylene (1f) produced a negligible amount of
the stilbene products (2d) and (2f) along with the corresponding
diphenylethane derivatives. In case of an electron-donating group,
32
10
48
0
19
40
a
Substrate: 0.11 mmol, Al powder: 100 mg (500 wt %), H2O: 0.5 mL.
Conditions: temp: 60 ꢀC, time: 3 h.
The yields were determined by GLC.
b
c
d
The isolated yields are shown in a square bracket.
Table 5
Reduction of substituted diphenylacetylenes (1) using Al powder and Pt/C in H2Oa,b
Entry
R
3
Yieldc (%) 4
5
Recovery 1
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
6
0
91
36
3
5
0
0
0
0
25
25
13
100
7
34
4(4a)
77
0
7(5a)þ48
10
0
a
complete reduction of the alkyne group occurred for 4-
15
78
dimethylaminodiphenylacetylene (1e) under the conditions used
(Table 4, entry 5). The outcome of the reduction depends on the
substituents present in the diphenylacetylene (see Scheme 2).
a
Substrate: 20 mg, NieAl: 100 mg (500 wt %), Al powder: 100 mg (500 wt %), Pt/
C: 4.5 mol % (metal), H2O: 0.5 mL.
b
Conditions: temp: 80 ꢀC, time: 18 h.
The yields were determined by GLC.
c
2.1. Proposed reaction mechanism
The reaction mechanism for the reduction is not yet fully clear. It
can be proposed that in the presence of Pt/C, the Al powder reacts
with water producing a reactive form of hydrogen that gets
adsorbed on the catalyst’s surface. Also, we have noted that when
Al powder and Pt/C were combined with Raney NieAl alloy as a co-
catalyst, the system becomes a much more potent reducing agent in
water. Here, the solvent water may play an important role in the
creation of the catalytic surface (Fig. 2).62
Scheme 2. Reduction of substituted diphenylacetylenes (1) using Al powder and Pt/C
in H2O.
We also explored the substituent effects on diphenylacetylenes
(1) to observe a wider picture of the reduction products obtained
using the above catalysts under the same reaction conditions. In
the case of 4-methyldiphenylacetylene (1b) and 4-tert-butyldi-
phenylacetylene (1d), a completely reduced products (5b) and (5d)
were obtained in low yield, namely 36% and 10%, respectively along
with the other products due to the steric hindrance of the re-
spective methyl and tert-butyl groups (Table 5, entries 2 and 4).
Only 4-N,N-dimethylaminodiphenyl-acetylene (1e) afforded
a quantitative yield for the alkyne group reduction product 3e
which may be due to the electron donating nature of the sub-
stituent. In the case of 4-methoxydiphenylacetylene (1c), a 48%
yield of compounds 5c and 5a (7%) was achieved (Table 5, entry 3).
This result strongly suggests that the present system not only re-
duces the aromatic ring but also cleaves the polar CeO bond. On
the other hand, a 78% reduction occurred with the formation of
compounds 3f (7%) and 4f (15%) in the case of 4-
trifluoromethyldiphenylacetylene (1f) (Table 5, entry 6).
Fig. 2. Proposed reaction pathways for the reduction of diphenylacetylenes.
Based on the GCeMS analysis, a detailed reaction pathway for
the reduction of diphenylacetylenes can be proposed.61 In pathway
1 the triple bond is reduced to single bond directly and in pathway
2 the single bond is afforded through the intermediate product
stilbene (2). The intermediate product 2 only forms when a bulky
One of the most important advantages of this method is that the
reaction can be carried out in water which amongst all its impor-
tant environmental properties, is both readily available and cheap.
In addition, water serves as an economic source of hydrogen and as
a result, no extra hydride or hydrogen gas addition is required. It is

6946
U. Rayhan et al. / Tetrahedron 72 (2016) 6943e6947
substituent is introduced (Table 4), which makes the pathway 2
relatively slower. So, we can assume that pathway 1 is the faster
route compared to pathway 2. It may be concluded here that the
approach of the diphenylacetylenes towards the catalyst’s surface is
very important in terms of achieving a high yield of the product.
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Acknowledgements
This work was conducted under the Cooperative Research Pro-
gram of ‘Network Joint Research Center for Materials and Devices
(Institute for Materials Chemistry and Engineering, Kyushu Uni-
versity)’. The EPSRC (EP/L012804/1) is thanked for financial support
in the form of a travel grant to CR.
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