Rhodium-on-carbon catalyzed hydrogen scavenger- and oxidant-free dehydrogenation of alcohols in aqueous media

Article abstract of DOI:10.1039/c4gc00434e

The efficient and catalytic dehydrogenation of alcohols is a clean approach for preparing carbonyl compounds accompanied only by the generation of hydrogen gas. We have accomplished the heterogeneous rhodium-on-carbon catalyzed dehydrogenation of secondary, as well as primary, alcohols to the corresponding ketones and carboxylic acids in water under basic conditions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00434e

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Rhodium-on-carbon catalyzed hydrogen
scavenger- and oxidant-free dehydrogenation
of alcohols in aqueous media†
Cite this: Green Chem., 2014, 16,
3439
Received 12th March 2014,
Accepted 12th April 2014
Yoshinari Sawama,* Kosuke Morita, Tsuyoshi Yamada, Saori Nagata, Yuki Yabe,
Yasunari Monguchi and Hironao Sajiki*
DOI: 10.1039/c4gc00434e
www.rsc.org/greenchem
The efficient and catalytic dehydrogenation of alcohols is a clean lyzed dehydrogenation of secondary, as well as primary, alco-
approach for preparing carbonyl compounds accompanied only by hols in water as a clean, nontoxic, non-flammable, cheap and
the generation of hydrogen gas. We have accomplished the environmentally benign solvent, into the corresponding
heterogeneous rhodium-on-carbon catalyzed dehydrogenation of ketones and carboxylic acids.
secondary, as well as primary, alcohols to the corresponding
ketones and carboxylic acids in water under basic conditions.
The oxidation of alcohols is one of the most fundamental
organic reactions used to prepare carbonyl compounds.1
Among the various methods for the oxidation of alcohols, the
transition metal-catalyzed dehydrogenation2 of hydroxyl
groups without an organic hydrogen scavenger (compounds
bearing easily reducible functionalities are used to scavenge
the hydrogen gas (H2)), or an oxidant (e.g., molecular oxygen3),
is a clean approach to provide the corresponding carbonyl pro-
ducts, due to its safety and the production of less waste, except
for hydrogen gas. Additionally, the hydrogen gas generated
during the dehydrogenation process as a side-product can be
utilized as a valuable energy source. While a variety of homo-
geneous4 and heterogeneous catalysts using Ag,5 Au,6 Cu,7
Co,8 Pt9 and Ni10 as a transition metal on a support have been
developed for the dehydrogenation of secondary and primary
alcohols into the corresponding ketones and aldehydes, only
Milstein et al. have pioneered the direct transformation of
primary alcohols into carboxylic acids, using a homogeneous
Ru catalyst in water.11 Since the reuse of catalysts is eagerly
desired from the viewpoint of green chemistry, dehydrogena-
tion reactions using a reusable water soluble Ir catalyst,4i and
heterogeneous catalysts,5–10 are considered to be the most
sustainable processes without any waste, with the exception of
H2. However, all of the previous reactions using heterogeneous
catalysts were carried out in organic solvents (e.g. toluene and
xylene). Herein, we demonstrate the heterogeneous Rh/C cata-
ð1Þ
We have already reported the redox reactions of secondary
alcohols under Pd/C catalyzed hydrogenation conditions.12
A platinum group metal, supported on carbon, generally dis-
plays a catalytic activity for hydrogenation.13 We have also uti-
lized not only Pd/C, but also Rh/C, Ru/C and Pt/C for the
hydrogenation of various substrates, such as arene nuclei,
alkyl chlorides and fluorinated arenes.14 Dehydrogenation
using a heterogeneous platinum group metal-on-carbon poten-
tially includes the occurrence of a problematic issue, with the
undesirable and inverse reduction of carbonyl products into
the corresponding alcohols (substrates) by the in situ-gene-
rated H2 gas (eqn (1)). Nevertheless, we have developed de-
hydrogenation reactions of alcohols when combined with
some platinum group metals on carbon analogs, along with an
inorganic base and water as the solvent, during the course of
our studies.
We initially examined the catalyst efficiencies for the de-
hydrogenation of benzhydrol (1a) in water at 100 °C for 6 h
under an argon atmosphere (Table 1). The 10% Pd/C catalyzed
reaction gave a mixture of the desirable ketone (2a) and di-
phenylmethane (3a) as a hydrogenolysis product of the in situ-
generated H2 (entry 1, without base). Furthermore, the reac-
tions using 10% Pt/C, Ru/C and Rh/C produced mixtures of an
unreacted or inversely-hydrogenated15 substrate (1a), a ketone
product (2a), diphenylmethane (3a), and bis(diphenylmethyl)-
ether (4a) (entries 3, 5, 7 and 8, without base). The generation
ratio of 2a could be improved by the addition of Na2CO3 (1.1
equiv.) (entries 1, 3, 5, and 7, with Na2CO3). The incremental
increase of Rh/C, from 10 mol% to 20 mol%, gave 2a in nearly
Laboratory of Organic Chemistry, Gifu Pharmaceutical University, 1-25-4 Daigaku-
nishi, Gifu 501-1196, Japan. E-mail: sajiki@gifu-pu.ac.jp; Fax: +81-58-230-8109
†Electronic supplementary information (ESI) available: The typical procedure,
the reusability tests of the catalyst, the ICP-OES results of metal leaching and the
spectroscopic data of the products are depicted. See DOI: 10.1039/c4gc00434e
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Table 1 The catalyst efficiencies towards the dehydrogenation of benz-
hydrol (1a) as a sec-alcohol
Table 3 Solvent effects
Entry
Solvent
Ratio of 1a/2a/3a/4a a
Ratio of 1a/2a/3a/4a a
1
2
3
4
5
H2O
MeOH
Toluene
CPME
1,4-Dioxane
0/98/2/0
94/6/0/0
68/30/0/2
72/24/2/2
96/4/0/0
b
Entry
Catalyst
Without base
With Na2CO3
1
2
3
4
5
6
7
8
10% Pd/C (10 mol%)
10% Pd/C (20 mol%)
10% Pt/C (10 mol%)
10% Pt/C (20 mol%)
10% Ru/C (10 mol%)
10% Ru/C (20 mol%)
10% Rh/C (10 mol%)
10% Rh/C (20 mol%)
0/68/32/0
—
29/46/1/24
—
67/28/1/4
—
32/56/11/1
20/66/4/10
0/70/30/0
29/71/0/0
9/82/9/0
a The ratios were determined by 1H NMR spectroscopy.
1/87/7/0
22/77/1/0
59/49/0/0
16/78/6/0
0/98(92)c/2/0
Table 4 Double dehydrogenations of pri-alcohol (6a) into the corres-
ponding carboxylic acid (8a)
a The ratios were determined by 1H NMR spectroscopy. b 1.1 equiv. of
Na2CO3 were used. c Isolated yield.
Table 2 Base efficiencies
Entry
Solvent
Ratio of 6a/7a/8a a
Entry
Base
Time (h)
Ratio of 1a/2a/3a/4a a
1
2
3
4
5
6
Na2CO3
K2CO3
Li2CO3
NaHCO3
NaOH
Et3N
0/0/100 (32)b
12/44/44 (22)c
8/39/53 (24)c
45/3/52 (30)c
0/0/100 (53)b
0/0/100 (37)c
1
2
3
4
5
6
7
8
Na2CO3
Na2CO3
NaOH
NaHCO3
NaHCO3
Et3N
3
6
3
3
6
3
3
6
35/63/2/0
0/98(92)b/2/0
45/53/2/0
35/65/0/0
5/92/2/1
74/26/0/0
21/78/1/0
2/97(91)b/1/0
a The ratios were determined by 1H NMR spectroscopy. Other
products, which could not be identified, were also obtained. b Isolated
yields of 8a. c The yields of 8a were determined by 1H NMR
spectroscopy using 1,2-methylenedioxybenzene as the internal
standard.
K2CO3
K2CO3
a The ratios were determined by 1H NMR spectroscopy. b Isolated
yields.
quantitative isolated yields (92%, entry 8, with Na2CO3). The
role of the Na2CO3 is unclear and is under investigation.
The dehydrogenation efficiency of 1a, using various bases,
was estimated using 10% Rh/C as the catalyst for 3 h (Table 2,
entries 1, 3, 4, 6 and 7). Na2CO3, NaHCO3, and K2CO3 were
found to be more effective than NaOH and Et3N. An organic
base was less efficient due to the catalytic poison effect by the
coordination ability of Et3N towards Rh metal. For com-
parison, the generation ratios of 2a using Na2CO3, NaHCO3,
and K2CO3 for 6 h (entries 2, 5 and 8) were investigated.
Na2CO3 was chosen as the appropriate base for the Rh/C
catalyzed dehydrogenation.
While the present dehydrogenation in organic solvents
[MeOH, toluene, cyclopentyl methyl ether (CPME) and
1,4-dioxane] led to low conversion yields (Table 3, entries 2–5),
the desired ketone (2a) was nearly quantitatively obtained in
water (entry 1).
Namely, the first dehydrogenation stage of decanol (6a), as a
primary alcohol, can give the corresponding decanal (7a),
which is subsequently transformed into the hydrate intermedi-
ate (A) in water, and the following second dehydrogenation (A)
produces the corresponding carboxylic acid (8a).11,16 NaOH
was found to be the most effective base for the double de-
hydrogenation of decanol (6a) to provide decanoic acid (8a) with
a yield of 53% (entry 5 in comparison with entries 1–4 and 6),
since the hydroxyl ion of NaOH synergistically facilitated the
formation of the hydrate (A). Decanal (7a) as the starting
material was also efficiently converted into decanoic acid (8a)
under the same reaction conditions with a yield of 52%
(eqn (2)), which clearly indicates that the presented double
dehydrogenation of 6a into 8a proceeded via the formation of
7a as an intermediate. The reaction of 6-phenylhexanol (6b)
also gave the desired carboxylic acid (8b) with a yield of 50%,
accompanied by trace amounts of pentylbenzene (9b) and
1-phenyl-1-pentene (10b) as byproducts (eqn (3)). Undesirable
decarbonylations of the terminal aldehyde and the carboxylic
Water as a solvent can also play a crucial role in the trans-
formation of primary alcohols into carboxylic acids (Table 4).
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Table 5 Scope of the substratesa
Table 5 (Contd.)
Time
(h)
Yield
(%)
Time
(h)
Yield
(%)
Entry
10
Substrate
Product
Entry
1b
Substrate
Product
6
82
6
74
81
76
11
12
13
24
38 (42)e
2
3
4
6
6
51
74
48
6
24
76
7
14
12
62 (10)e
15c
16c
24
24
54
44
5
6
6
6
64
89
17c
24
38
7
8
9
6
24
6
89
89
a Na2CO3 (2.2 equiv.) was used for the dehydrogenation of the
secondary alcohols and NaOH (2.2 equiv.) was used for the double
dehydrogenation of the primary alcohols as the bases, unless
otherwise noted. b Na2CO3 (1.1 equiv.) was used as the base. c After the
reaction, the reaction mixture was neutralized with diluted H2SO4, and
then filtered and extracted. d The yields were determined by 1H NMR
spectroscopy because the generated products were inseparable.
e Isolated yields of the starting materials.
acid under the transition metal-catalyzed hydrogenation con-
ditions17 could not be completely suppressed.
63d
ð2Þ
29d
ð3Þ
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Table 6 Reusability testsa
heterogeneous Rh/C could be recovered by a simple procedure
and reused up to four times. The presented dehydrogenation
is the first successful case using water as the solvent for this
heterogeneous catalytic system, and carboxylic acids were
obtained from primary alcohols using water as part of the
oxygen source.
Run
Ratio of 1a/2a/3a
Isolated yield of 2a
1st
0/100/0
0/100/0
1/99/0
0/100/0
3/96/1
91
93
95
96
94
84
2nd
3rd
4th
5th
6th
Acknowledgements
We thank the N.E. Chemcat Corporation for the kind gift of
the catalysts, and Nissan Chemical Industries, Ltd for the fruit-
ful suggestion for the application of this method, and the
ICP-OES measurements. This work was partially supported by
the Public Foundation of Chubu Science and Technology
Center.
14/86/0
a 10% Rh/C was reused after simple filtration, washing with H2O and
MeOH, and drying in vacuo. In each run, 91–99% of the Rh/C could be
recovered.
The scope of application of the substrates was next investi-
gated, using Na2CO3 for the secondary alcohols and NaOH for
the primary alcohols as the bases (Table 5). Various secondary
benzylic and aliphatic alcohols, bearing linear and cyclic sub-
structures, efficiently underwent dehydrogenation to give the
corresponding carbonyl products (entries 1–14). As mentioned
in the introduction, a platinum group metal-on-carbon poten-
tially possesses a hydrogenation activity.13 Therefore, the H2
gas generated under the dehydrogenation conditions may
cause the hydrogenation of the coexisting reducible functional-
ities. The alkene was completely hydrogenated (entry 12),
while the cyclopropane, the aromatic fluoride18 and the benzyl
ether functionalities were partially reduced (entries 4, 9, 13
and 14). Meanwhile, the aromatic chloride was perfectly tole-
rated under the Rh/C catalyzed conditions (entry 8).18 Primary
benzylic alcohols also underwent double dehydrogenation in
water to give the corresponding carboxylic acids in moderate
yields (entries 15–17).
Since reuse is one of the key components for heterogeneous
transition metal-catalyzed reactions, from the view point of
green chemistry and cost performance, the reuse of 10% Rh/C
was attempted during the dehydrogenation of 1a.19 While Rh/
C was reusable up to four times without any loss of the cata-
lytic activity, after simple filtration and washes with H2O and
MeOH, a gradual decrease in the catalytic activity was observed
in the fifth and sixth runs. Since analysis using Inductively
Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)
indicated no metal leaching into the reaction mixture,20 the
modest decrease in the catalytic activity might have been due
to the chemical or mechanical damage caused by the basic
conditions and/or the stirring (Table 6).
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Conclusions
We have accomplished the Rh/C catalyzed, hydrogen scaven-
ger- and oxidant-free, dehydrogenation of secondary and
primary alcohols in water, under basic conditions, to provide
the corresponding ketone and carboxylic acid derivatives. The
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777–782; (d) Y. Yabe, Y. Sawama, Y. Monguchi and 19 The reactions in Tables 1–5 were carried out in a test-tube
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15 Rh/C has the hydrogenation activity as shown below. The
reaction of 2a under a hydrogen atmosphere in H2O at
100 °C in the presence of Na2CO3 provided 1a as the main
reduced product.
under argon (balloon), and stirred using the Chemist Plaza
personal organic synthesizer (Shibata Scientific Techno-
logy, Ltd, Tokyo). Meanwhile, the reusability tests listed in
Table 6 were carried out in a two-necked flask attached to a
reflux condenser. Although control of the accumulation of
the generated hydrogen gas can effectively revise the dehy-
drogenation activity, the reproducibility of the reaction can
be easily obtained. Actually, the reaction of 1a in test-tube
(Table 2, entry 2) could be investigated several times to give
similar yields of 2a. See details in the ESI.†
20 The results of metal leaching are described in the ESI.†
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