Pd/C-Al-water facilitated selective reduction of a broad variety of functional groups

Article abstract of DOI:10.1039/c6gc03032g

The chemoselective reduction of a broad variety of functional groups by a Pd/C-Al-H₂O system is described. The reduction is based on the reaction of Al with the solvent water that in situ produces hydrogen that is utilized by the supported Pd catalyst toward the hydrogenation of the target functional groups. The hydrogenations are carried out under mild conditions and provided the products in high yields and selectivity. The reduction system appeared to be effective for a broad range of functional groups, including C-C, C-N, C-O and N-O multiple bonds, aromatic rings, hydrogenolysis of C-O, C-N and C-Halogen bonds. The appropriate selection of the reaction conditions allowed the selective preparation of different products from the same substrate. The simplicity, cost, tunability and the environmentally benign character of the catalytic system offer numerous advantages over the currently available methods.

Full text of DOI:10.1039/c6gc03032g

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Pd/C-Al-Water Facilitated Selective Reduction of a Broad Variety
of Functional Groups
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a,
C. Schäfer, C. J. Ellstrom, H. Cho, B. Török *
DOI: 10.1039/x0xx00000x
www.rsc.org/
The chemoselective reduction of a broad variety of functional several key factors are often overlooked. Hydrogen is an
groups by Pd/C-Al-H O system is described. The reduction is based explosive gas and its production through steam reforming
on the reaction of Al with the solvent water that in situ produces requires extensive energy (700-1000 ˚C) and yields
2
a
hydrogen that is utilized by the supported Pd catalyst toward the considerable amount of carbon dioxide, a well-known green-
4
hydrogenation of the target functional groups. The house gas. Therefore, the overall environmental impact of
hydrogenations are carried out under mild conditions and catalytic hydrogenation is clearly not negligible. As a potential
provided the products in high yields and selectivity. The reduction alternative, the applications of metal-Al alloys in aqueous
5
system appeared to be effective for a broad range of substituents, reductions have been explored. The major advantage of the
including C-C, C-N, C-O and N-O multiple bonds, aromatic rings, system is that the dissolution of the Al in the aqueous solvent
hydrogenolysis of C-O, C-N and C-Halogen bonds. The appropriate results in the controlled formation of the hydrogen and the
selection of the reaction conditions allowed the selective additional metal component serves as
6
a
potential
preparation of different products from the same substrate. The hydrogenation catalysts. The method, which was pioneered
simplicity, cost, tunability and the environmentally benign by Tashiro and co-workers allowed the reduction of a broad
character of the catalytic system offer numerous advantages over variety of compounds using a simple setup, mostly using NiAl
7
the currently available methods.
and other metal-Al alloys. In general, two experimental
variables, temperature and the pH of the solution, were used
to control the system. The application of dilute sodium or
potassium hydroxide solutions made this reduction system to
display powerful properties, resulting in deoxygenations,
hydrogenolysis and the partial or complete hydrogenation of
Introduction
The reduction of functional groups is one of the most
1
fundamental transformations in organic synthesis. Over the
8
aromatic rings. While the high reaction rates and conversion
past century a number of different methods were developed,
most of which would fit into the traditional categories of
2
catalytic hydrogenation, metal hydride reductions, dissolving
values are welcome characteristics, when coupled with low
selectivity, the formation of multiple byproducts and
essentially uncontrollable process, the overall value of the
method is questionable.
metal reductions, and other methods including reductions
3
with organic and inorganic reagents. While effective, these
In our previous works using the Ni-Al system we have shown
that the application of strong inorganic bases is not only
unnecessary but by eliminating the base much more
manageable processes can be developed. Examples include
methods often employ harmful reagents, generate toxic
waste, possess low atom economy, use flammable or explosive
solvents, in short do not conform to the current standards of
sustainable chemistry. Therefore, the development of
methods that use safe and convenient reagents and solvents
and prevent or minimize waste formation to create
environmentally sustainable processes is highly desirable.
Among the above methods catalytic hydrogenation is
considered to be the most environmentally benign, however,
9
the highly selective reduction of ketones, indoles and
1
quinolines, or the selective reductive amination of ketones
0
11
and aldehydes with NH OH and amines, all carried out with
4
NiAl alloy in an aqueous medium. While the use of the Ni
catalyst allowed the reduction of sulfur-containing substrates,
the tenability of the system is limited and hindered by natural
characteristics of nickel. As a result, it was decided to upgrade
the system by replacing the metal-aluminum alloy with simple
Al powder that would also allow the application of traditional
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noble metal hydrogenation catalysts that are commercially Aesar) catalysts. After the overall analysis of the data, the 5%
available in a great variety.
Pd/C sample obtained from Alfa-Aesar emerged as the catalyst
of choice for further studies based on its performance and
chemically inert support as the BaCO
known to chemically modify certain hydrogenations.
Since it was our goal to establish the Pd-Al-H O system as a
3
supported catalyst is
1
2
Results
Continuing our interest in developing environmentally benign
methodologies, in the present work we describe the
application of a commercially available supported Pd catalyst
2
versatile broad-scope reduction method, compounds with a
wide variety of functional groups have been selected to assess
its activity and selectivity. Due to the variety of the functional
groups four major sets have been created. In the first set the
reduction of C-C multiple bond containing compounds,
including, alkenes, alkynes, aromatic and heteroaromatic
molecules, were studied. The aim was to optimize conditions
for selective reductions, thus most compounds are aromatic
with an unsaturated side chain. The conditions were optimized
in order to identify different experimental parameters when
the reaction proceeds with high yields and selectivity, and the
appropriate selection of the conditions will provide different
products using the same substrate. The results are tabulated in
Table 2.
2
in conjunction with the Al-H O hydrogen generating system for
the highly chemoselective reduction of a broad variety of
functional groups. All major functional groups that can
undergo reduction have been assessed in terms of
applicability, yields and chemoselectivity and the individual
reactions have been optimized toward a desired product.
As heterogeneous Pd catalysts have widespread applications, a
number of such catalysts are available with different support
materials from different suppliers. Thus, as a first step several
commercially available supported Pd catalysts were tested to
select the one with the best performance in terms of reaction
rate or yield and selectivity. Catalysts with three different
2 3 3
supports (C, Al O and BaCO ) were selected and in addition,
the most common Pd/C catalyst was obtained from three
suppliers for testing purposes. Three test reactions were
investigated, the hydrogenation of phenylacetylene,
Table 2. Reductions of C-C multiple bonds including aromatic rings by the 5% Pd/C (Alfa
a
Aesar)-Al/H
2
O catalyst system
‡
Entry
Substrate
Product
T
Time
[h]
Yield
acetophenone and 2-nitrotoluene. The data are summarized
b
[
˚C]
[%]
in Table 1.
1
2
3
4
25
50
25
50
16
1
100
100
100
100
Table 1. Screening of commercial Pd catalysts in the selective reduction of
21
1
phenylacetylene, 2-nitrotoluene and acetophenone using Al-H
2
O hydrogen generating
a
system
Entry
Substrate
Catalyst
Product
Yield
5
6
15
25
240
25
63
b
%]
[
100
1
2
3
4
5
6
7
8
9
Pd/C (Aldrich)
100
100
100
Pd/Al
2
O
3
(Engelhard)
(Alfa-Aesar)
7
8
25
50
16
3
100
100
Pd/BaCO
3
c
Pd/C (Engelhard)
Pd/C (Alfa Aesar)
Pd/C (Aldrich)
6(59)
100
4
9
75
75
24
24
100
100
Pd/Al
2
O
3
(Engelhard)
(Alfa-Aesar)
100
100
1
1
0
1
Pd/BaCO
3
c
Pd/C (Engelhard)
Pd/C (Alfa Aesar)
Pd/C (Aldrich)
7 (6)
25
50
25
50
23
1
100
100
76
1
1
1
1
1
1
0
1
2
3
4
5
100
d
d
12
13
32 (48)
68 (32)
100
Pd/Al
2
O
3
(Engelhard)
(Alfa-Aesar)
19
2
Pd/BaCO
3
1
4
74
d
Pd/C (Engelhard)
Pd/C (Alfa Aesar)
71 (29)
d
98 (1)
1
5
90
16
100
a
Reactions were carried out with 10 mol% catalyst, 2.5 mmol Al, 0.25 mmol
b
c
substrate in 1.5 mL H
2
O, all reactions were carried out at RT for 24h; GC yields;
a
Reactions were carried out with 10 mol% catalyst, 2.5 mmol Al, 0.25 mmol
d
yields in parenthesis correlates to semi-reduced product; yields in parenthesis
correlates to over-reduced product.
b
2
substrate in 1.5 mL H O; GC yields.
The data show that the hydrogenation of a variety of C-C
multiple bonds occur readily with the Pd/C-Al-H O system. It
Table 1 shows that the different Pd catalysts exhibit fairly
similar characteristics. The highest activity and selectivity were
2
appears that C-C double and triple bonds undergo the
hydrogenation even at room temperature, although the
3
demonstrated by the Pd/C (Alfa-Aesar) and Pd/BaCO (Alfa-
2
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completion of the reaction required extended time (16-21h). benzaldehyde at higher temperature; however, the reaction
The same results can be achieved with the same substrates at appears to be quantitative and highly selective for
50
˚
C in much shorter reactions (1-3h). It is interesting to note benzylalcohol (entry 2). The epoxide ring is also susceptible to
that at 15 C phenylacetylene (entry 5) yielded styrene in good the reaction conditions; styrene oxide readily undergoes
yield and high selectivity, indicating that the hydrogenation of partial hydrogenolysis to the expected primary alcohol.
the C C triple bond likely occurs in a step-by-step manner. Further increase in reaction temperatures resulted in the
˚
≡
Similar characteristics were observed in the hydrogenation of hydrogenation and deoxygenation of the two carbonyl
aromatic compounds. Naphthalene was hydrogenated to compounds to yield methyl and ethylbenzene respectively
tetraline, while indole yielded indoline (entries 12-15). The (entries 4, 5). The catalytic system appears to tolerate the
complete reduction of the aromatic ring in styrene and presence of F-substituents even at 90 ˚C and provides the
phenylacetylene, while required higher temperature, occurred expected 4-F-benzylalcohol in quantitative yield (entry 6).
in quantitative yields. The reduction of the benzene ring of However, the reduction of 4-bromobenzaldehyde at room
benzoic acid also occurred selectively to yield cyclohexanoic temperature resulted in debromination, while at 50 ˚C the
acid, although under harsher conditions (entry 16). quantitative formation of 4,4’-dimethylbiphenyl was observed
The second set of compounds includes C-O multiple bond (Scheme 1, see later). Similar conditions resulted in complete
containing substrates such as carbonyl compounds and dechlorination in the case of 4-chlorobenzaldehyde. The
epoxides. Similarly to Table 2, compounds with aromatic side reduction of 2-bromoacetophenone also occurred with low
chains were selected in order to investigate the step-by-step selectivity providing multiple debrominated products with
nature of the process, i.e. whether it is possible to reduce the ethylbenzene (66%) as the major product. Finally, the
functional group first and then the aromatic ring in a reduction of acetophenone at 75
successive reaction. The potential of the C-O hydrogenolysis, complete hydrogenation and deoxygenation of the substrate
thus the synthesis of de-oxygenated products, was also to ethylcyclohexane in highly selective and nearly
˚C for 24 h, resulted in a
a
studied. The data are collected in Table 3.
quantitative reaction (entry 7).
As the catalytic system appears to work efficiently in C-C and
C-O bond reductions a similar set of reactions have been
carried out with C-N or N-O multiple bond containing
compounds. As above, typical substituents/functional groups
have been selected, including nitro, nitril, imino, oxime and
aromatic imino. The optimized results are collected in Table 4.
Table 3. Reductions of C-O multiple bond containing compounds by the 5% Pd/C (Alfa
a
Aesar)-Al/H
2
O catalyst system
Entry
Substrate
Product
T
Time
[h]
Yield
b
[
˚C]
[%]
O
1
25
24
98
Table 4. Reductions of C-N, N-O multiple bond containing compounds by the 5% Pd/C
a
(
2
Alfa Aesar)-Al/H O catalyst system
2
3
50
75
21
1
99
95
Entry
Substrate
Product
T
Time
h]
Yield
[
b
[
˚C]
[%]
1
2
25
50
15
1
100
100
O
4
75
1
99
CN
3
4
25
50
22
1
100
100
5
6
25
50
22
3
67*
84
5
6
90
90
75
24
1
100
99
7
90
90
1
98
O
7
24
99
8
1.5
100
a
a
Reactions were carried out with 10 mol% catalyst, 2.5 mmol Al, 0.25 mmol
Reactions were carried out with 10 mol% catalyst, 2.5 mmol Al, 0.25 mmol
b
2
b
substrate in 1.5 mL H O; GC yields.
2
substrate in 1.5 mL H O; GC yields.
The data in Table 4 show that a variety of N containing
functional groups can be selectively reduced by the Pd/C-
Table 3 indicates that the reduction of the C-O based
substrates also readily occur, although the conditions are more
diverse and less flexible for the individual substrates.
Acetophenone (entry 1) can be selectively reduced to
methylbenzylalcohol at room temperature in quantitative
Al/H O catalytic system. Similarly to the C-C multiple bond
2
hydrogenations (Table 2), most reductions can be carried out
both at room temperature and 50 ˚C, although the lower
temperature experiments require significantly longer time to
reach completion. However, this feature can be very useful in
the presence of other hydrogenation sensitive groups,
allowing the selective, temperature controlled reduction of
yield. In contrast, at 75 ˚C, as a result of successive C=O
reduction and hydrogenolysis, ethylbenzene formed also
quantitatively (entry 4). Similar reactions occur with
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one or both groups. The reduction of C=N-containing aromatic
ring, such as the one in isoquinoline occurs with quantitative
yield, although at higher temperature (entry 8). The reduction
of cyclohexanone oxime (entry 7) gave a coupled product
dicyclohexyl amine. Its formation is plausible to explain by the
step-by-step reduction of the oxime, first to imine, which can
undergo further reduction to cyclohexyl amine. However, such
imines are highly unstable and in a parallel reaction rapidly
Scheme 2. Reduction of compounds with multiple reducible functional groups
with the 5% Pd/C-Al/H O system
2
O
5% Pd/C-Al-H2O
Ullman Coupling
Br
50 C, 2.5h
100 %
OH
1
3
hydrolyze to carbonyl compounds,
in this case
cyclohexanone, which undergoes reductive amination with the
cyclohexyl amine (Scheme 1).
RT, 25h
,
-unsaturated compounds
O
5
% Pd/C-Al-H O
2
100 %
OH
50 C,1h
Scheme 1. Proposed mechanism for the formation of dicyclohexyl amine from
2
cyclohexanone oxime with the 5% Pd/C-Al/H O system
100 %
OH
RT, 21h
O
5% Pd/C-Al-H2O
98%
50 C,21h
69%
After systematically testing a broad range of functional groups
a fourth set of compounds have been designed and tested. As for every catalytic process, the leaching of the Pd content of the
Most of these reactions were designed to observe potential catalyst could cause problems. Thus, experiments were conducted
chemoselectivity of the Pd/C-Al-H
2
O system using compounds to determine the actual leaching of the Pd to the reaction mixture.
that possess multiple hydrogenation sensitive groups other The test reaction was the hydrogenation of phenylacetylene at
than the aromatic ring. First compounds with halogen room temperature for 24h (Table 2, entry 3). After the reaction was
substituent were selected. Such compounds are commonly completed the catalyst was centrifuged and the supernatant
expected to undergo dehalogenation under the current filtered. The Pd content of the obtained filtrate was determined by
1
5
experimental conditions via the hydrogenolysis of the halogen a UV-Vis spectrophotometric method after complexation with KI.
-5
atom. It was observed, that while the dehalogenation is The Pd content of the reaction mixture was found to be 9.8x10
common for Cl- and Br-containing compounds, the presence of mol/L, which corresponds to 0.81% leaching of the Pd content of
Br on an aromatic ring leads to the exclusive formation of the catalyst. This observation is in line with previous findings
16
biphenyl derivatives, commonly known as the Ullman regarding similar Pd catalysts.
1
4
reaction, under the experimental conditions. It was observed
that the reaction is controlled by steric and electronic effects;
only p-Br-derivatives (Scheme 2) appeared to react, sterically
hindered o-Br substrates did not undergo the Ullman reaction.
Similarly, it appears that the reaction gives quantitative
coupling in the presence of electron withdrawing substituents
Conclusions
In conclusion, applying a broad range of substrates we have
demonstrated that the Pd-Al-H O system represents an
2
effective hydrogenation/reduction method without the use of
hydrogen gas. It appears that the commercial Al powder can
readily serve as a safe, inexpensive hydrogen generator in the
aqueous medium.
(e.g. formyl), moderate yield with bromobenzene, however,
electron donating substituents, such as amino, block the
coupling under the same conditions.
Other reaction of interest included the reduction of
α,β-
The major advantages of the present method as compared to
available alternatives are as follows: (i) the reaction occurs in
water the most environmentally benign and inexpensive
solvent; (ii) the reaction does not produce any harmful by-
unsaturated carbonyl compounds, such as isophorone and
chalcone. It was observed that using a freshly acquired Al
powder isophorone underwent a complete hydrogenation to
the corresponding saturated alcohol. While the reaction can
be carried out at room temperature, it requires 25h. In
3
product. The only byproduct is Al(OH) a nontoxic and easily
recyclable material; (iii) the reaction does not require high
pressure hydrogen, which is an explosive gas and requires the
use of high pressure reaction vessels; (iv) the method can be
applied for nearly all reducible functional groups; and (v) the
contrast, at 50 ˚C the product formation is quantitative in 1 h.
Similarly, the reduction of chalcone also resulted in the
formation of the saturated alcohol at room temperature,
however, at 50 ˚C C-O bond hydrogenolysis occurred and the
appropriate selection of
the reaction conditions
1,3-diphenylpropane formed via deoxygenation.
(temperature/time) allows the selective step-by-step
4
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reduction of compounds with multiple reducible groups, such
as aromatic ketones, styrenes etc. Therefore, this method can
be considered an environmentally benign replacement for
several existing procedures.
S.M. Landge, V. Atanassova, M. Thimmaiah, B. Török,
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Notes and references
4
0
The reactions were carried out in de-ionized water. The
palladium catalyst was a 5% Pd/C sample form Alfa-Aeasar (Type
8
7L, A12623, Lot#: LB0722, LZ0118, particle size 3-25nm). The
aluminium powder was also obtained from Alfa-Aeasar (99.5%
purity, 11067, Lot#:K05Z034, 325 mesh, particle size 7-15 M).
The Pd content of the reaction mixture was determined by a
ꢀ
1
5
spectrophotometric method
based on Pd-KI complex
formation at a 1:400 molar ratio using a Hewlett-Packard Model
8
‡
0
453 Diode-Array Ultraviolet-Visible Spectrometer.
Typical reaction conditions: 5% Pd/C (50 mg, 0.025 mmol,
.1equiv.) and Al powder (68mg, 2.5 mmol, 10 equiv.) are mixed
in 2mL H O in a capped vial and sonicated for 1h (Branson 1510
2
sonicator). Phenyl acetylene (25mg, 0.25 mmol, 1 equiv.) is
added to the mixture and it is stirred at room temperature for
21h. The product is identified by GCMS (Agilent GC6850
MSD5973) in comparison with a commercially available sample.
1
2
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