Palladium on carbon-catalyzed aqueous transformation of primary alcohols to carboxylic acids based on dehydrogenation under mildly reduced pressure

Article abstract of DOI:10.1002/adsc.201401123

The catalytic dehydrogenation of alcohols to carbonyl products is a green sustainable oxidation with no production of waste except for hydrogen, which can be an energy source. Additionally, a reusable heterogeneous catalyst is valuable from the viewpoint of process chemistry and water is a green solvent. We have accomplished the palladium on carbon (Pd/C)-catalyzed dehydrogenation of primary alcohols to carboxylic acids in water under a mildly reduced pressure (800 hPa). The reduced pressure can be easily controlled by the vacuum controller of the rotary evaporator to remove the excess of generated hydrogen, which causes the reduction (reverse reaction) of aldehydes to alcohols (starting materials) and other undesirable side reactions. The present method is applicable to the reaction of various aliphatic and benzylic alcohols to the corresponding carboxylic acids, and the Pd/C could be reused at least 5 times.

Full text of DOI:10.1002/adsc.201401123

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
DOI: 10.1002/adsc.201401123
Palladium on Carbon-Catalyzed Aqueous Transformation of
Primary Alcohols to Carboxylic Acids Based on Dehydrogenation
under Mildly Reduced Pressure
Yoshinari Sawama,a,* Kosuke Morita,a Shota Asai,a Masami Kozawa,b
Shinsuke Tadokoro,b Junichi Nakajima,b Yasunari Monguchi,a
and Hironao Sajikia,*
a
Gifu Pharmaceutical University, 1-25-4 Daigaku-nishi, Gifu 501-1196, Japan
Fax : (+81)-58-230-8109; e-mail: sawama@gifu-pu.ac.jp or sajiki@gifu-pu.ac.jp
Chemical Research Laboratories, Nissan Chemical Industries, Ltd., 2-10-1 Tsuboi-Nishi, Funabashi, Chiba 274-8507,
b
Japan
Received: December 3, 2014; Revised: February 2, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401123.
Abstract: The catalytic dehydrogenation of alcohols the rotary evaporator to remove the excess of gener-
to carbonyl products is a green sustainable oxidation ated hydrogen, which causes the reduction (reverse
with no production of waste except for hydrogen, reaction) of aldehydes to alcohols (starting materi-
which can be an energy source. Additionally, a reusa- als) and other undesirable side reactions. The present
ble heterogeneous catalyst is valuable from the view- method is applicable to the reaction of various ali-
point of process chemistry and water is a green sol- phatic and benzylic alcohols to the corresponding
vent. We have accomplished the palladium on carboxylic acids, and the Pd/C could be reused at
carbon (Pd/C)-catalyzed dehydrogenation of primary least 5 times.
alcohols to carboxylic acids in water under a mildly
reduced pressure (800 hPa). The reduced pressure Keywords: carboxylic acids; dehydrogenation; pri-
can be easily controlled by the vacuum controller of mary alcohols; reduced pressure; water
Introduction
benign solvent (Scheme 1).[6] While the secondary al-
cohols were efficiently transformed into ketones
The oxidation of alcohols to carbonyl products is im- under the stated reaction conditions (Rh/C–base–
portant in organic chemistry.[1] Especially, the transi- H2O; Figure 1, style I), the reaction of primary alco-
tion metal-catalyzed dehydrogenation of hydroxy hols (1) produced lower yields of carboxylic acids (2)
groups is an attractive method to provide the corre- accompanying by the decarbonylation[7] of the inter-
sponding carbonyl products with less waste except for mediate aldehydes (3) to the corresponding alkanes
hydrogen gas,[2–6] and various homogeneous[3] and het-
erogeneous catalysts[4] have been used for the dehy-
drogenation of secondary and primary alcohols to the
corresponding ketones and aldehydes. The direct
transformation of a primary alcohol to the carboxylic
acid via the double dehydrogenation process requires
additional water to form the corresponding hydrate as
an intermediate (A, Scheme 1) from the aldehyde
generated by the first dehydrogenation of a primary
alcohol. The pioneer reaction was developed by Mil-
stein et al. using the homogeneous Ru catalyst in
Scheme 1. Previous results: Rh/C-catalyzed double dehydro-
genation of the primary alcohol (1) into the carboxylic acid
water,[5] and we also recently reported the heteroge-
neous and reusable Rh/C-catalyzed method in water
as a non-flammable, clean, cheap and environmentally I).
(2) in a test tube under atmospheric pressure of argon (Style
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
ÞÞ
These are not the final page numbers!

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Yoshinari Sawama et al.
Results and Discussion
The dehydrogenation of 6-phenyl-1-hexanol (1a) as
a primary alcohol was initially investigated in the
presence of 10% Rh/C (20 mol%) and NaOH
(2.2 equiv.) in H2O at 1008C under a mildly reduced
pressure (Table 1). The reaction, carried out in a flask
connected to a reflux condenser or test tube under
ambient pressure (1013 hPa), gave the corresponding
carboxylic acid (2a) in 38% yield within 6 h or 50%
yield after 24 h accompanied by the undesirable 1-
phenylpentane (4a)[8] resulting from the decarbonyla-
tion of 6-phenyl-1-hexanal (3a), respectively (entries 1
and 2). The use of Pd/C in a test tube (style I) also
provided a similar result in comparison with that of
Rh/C (entries 3 vs. 2). Hydrogen generated during the
reaction progress could be removed under reduced
pressure at 900 and 800 hPa and 2a was obtained in
Figure 1. Reaction styles in a test tube under argon (Style I)
and a flask connected to a reflux condenser and vacuum
controller (Style II).
(4) as a side reaction. The undesirable side reaction good yields (entries 4 and 5).[9] Further reduction of
can be promoted by Rh/C in the presence of excess the pressure to 700 and 300 hPa decreased the reac-
hydrogen. Additionally, hydrogen stored in the reac- tion efficiency of the dehydrogenation and 1a was not
tion vessel could facilitate the hydrogenation (reverse completely consumed (entries 6 and 7). The generated
reaction) of the intermediate aldehyde to the corre- hydrogen also probably plays a role in activating the
sponding alcohol (starting material), which delays the catalyst metal,[10,11] thus significant removal of hydro-
consumption of the aldehyde to the carboxylic acid gen from the reaction system delays the Rh/C-cata-
and promotes the undesired decarbonylation of the lyzed dehydrogenation step. The dehydrogenation
aldehyde. Therefore, we next investigated the contin- also smoothly proceeded at 808C under 800 hPa to
uous removal of hydrogen generated by the dehydro- provide 2a in 81% yield (entry 9). Meanwhile, the de-
genation of the alcohol (1) and hydrate (A) under re- hydrogenations at 608C, room temperature and
duced pressure by the vacuum controller of a rotary 1208C resulted in dramatically decreased reaction ef-
evaporator (Figure 1, Style II).
ficiency (entries 10–12).
Table 1. Optimization under reduced pressure (Style II).
Entry
Pressure [hPa]
Temperature [oC]
Time [h]
Yield [%][a]
1a
2a
3a
4a[b]
1
1013
–
–
100
100
100
100
100
100
100
80
80
60
r.t.
120
6
20
0
0
0
0
43
41
24
0
69
87
42
38
50
51
80
76
33
10
58
81
11
0
4
0
0
0
0
0
0
0
0
0
0
0
7
10
2
11
13
12
0
6
0
1
0
2[c]
3[c,d]
4
24
24
6
6
6
6
6
6
6
900
800
700
300
900
800
800
800
800
5
6
7
8
9
10
11
12
6
6
16
13
[a]
1
Yields were determined by H NMR using 1,4-dioxane as the internal standard.
[b]
Alkane 4a was comparatively easily vaporized under reduced pressure. Therefore, the yield of 4a may be incorrect due to
its loss during the reaction process.
The reaction was carried out in a test tube (Style I).
[c]
[d]
10% Pd/C was used instead of 10% Rh/C.
2
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
ÝÝ
These are not the final page numbers!

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Palladium on Carbon-Catalyzed Aqueous Transformation of Primary Alcohols
Table 2. Efficiency of catalysts and bases.
Entry
Catalyst
X
Base
Yield [%][a]
1a
2a
3a
4a[b]
1
2
3
4
5
6
7
8
10% Rh/C
10% Pd/C
10% Pt/C
10% Ru/C
10% Rh/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
20
20
20
20
5
5
5
5
5
5
5
5
1
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
KOH
LiOH
Na2CO3
NaHCO3
–
0
0
0
48
42
4
14
32
48
72
94
10
25
–
81
86
74
46
52
88
75
50
40
19
2
0
0
0
0
0
0
1
3
4
7
4
0
0
–
0
5
0
5
4
4
1
0
4
0
0
1
1
–
9
10
11
12[c]
13
14[d]
NaOH
NaOH
NaOH
76
50
95[e]
5
[a]
1
Yields were determined by H NMR using 1,4-dioxane as the internal standard.
[b]
Alkane 4a was comparatively easily vaporized under reduced pressure. Therefore, the yield of 4a may be incorrect due to
its loss during the reaction process.
1.1 equiv. of NaOH was used.
For 9 h.
[c]
[d]
[e]
Isolated yield.
Various platinum metals on carbon (e.g., Pd/C, Rh/ (entries 1–9). During the dehydrogenation process,
C, Ru/C and Pt/C) were found to also possess a cata- the generated hydrogen may cause the Pd/C-catalyzed
lyst activity for the dehydrogenation of alcohols.[6,10] hydrogenation of the coexisting reducible functionali-
The double dehydrogenation of a primary alcohol at ties within the same molecule. While aromatic fluo-
808C under 800 hPa in the presence of NaOH ride and nitro groups were totally tolerated under the
(2.2 equiv.) was efficiently catalyzed by 20 mol% of present reaction conditions (entries 7 and 9), an aro-
10% Rh/C, Pd/C and Pt/C, except for Ru/C (Table 2, matic chloride was reductively dechlorinated to give
entries 1–4), and the lower catalyst loading of 10% benzoic acid (2c; entry 8). The yields were dramatical-
Pd/C (5 mol%) still maintained a high catalyst activity ly improved in comparison to our previous Rh/C-cata-
(entry 6), while the use of 5 mol% of Rh/C signifi- lyzed reactions in a test tube (Style I; e.g., 2b: 53%,
cantly delayed the dehydrogenation (entry 5). KOH, 2c: 54%, 2d: 44%, 2e: 38%, 2g: 23% and 2h: 33%).[13]
LiOH, Na2CO3 and NaHCO3, instead of NaOH, were 10-Acetoxy-1-decanol (1k) underwent the NaOH-
insufficient as a base (entries 7–10) and the reaction mediated hydrolysis, and the diol was dehydrogenated
without a base hardly proceeded (entry 11). The re- to give the corresponding dicarboxylic acids (2k). Fur-
duction of NaOH used to 1.1 equiv. or the further de- thermore, the reaction of aldehydes (3b and 3c) was
crease of Pd/C to 1 mol% resulted in lower yields of also applicable to provide the corresponding carbox-
2a. Therefore, the combination of 10% Pd/C ylic acids (2b and 2c) in quantitative yields, respec-
(5 mol%)–NaOH (2.2 equiv.) under 800 hPa at 808C tively (entries 11 and 12).
was optimum[12] and prolonging the reaction time to
9 h provided 2a in 95% isolated yield (entry 14).
The reuse of the heterogeneous catalysts is impor-
tant from the viewpoint of green sustainable chemis-
The present reaction under reduced pressure was try. The Pd/C could be reused at least 5 times after
applied to various aliphatic and benzylic alcohols simple filtration, washing with H2O and MeOH, then
(Table 3). Decanol (1b) as a simple aliphatic alcohol drying [Eq. (1)]. Under the present dehydrogenation
and various benzyl alcohols (1c–1j) bearing electron- in basic media, the carboxylic acids as products were
donating and electron-withdrawing groups on the aro- isolated after acidic quench with aqueous 1N H2SO4.
matic nuclei underwent the double dehydrogenation The acidic quench before the filtration of Pd/C causes
to the corresponding aliphatic carboxylic acid and a slight leaching of Pd (2%) from the carbon support
benzoic acid derivatives in good to excellent yields as based on an ICP-OES analysis. Meanwhile, the
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3
ÞÞ
These are not the final page numbers!

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Yoshinari Sawama et al.
Table 3. Scope of the substrates.
transition metal-catalyzed oxidation of alcohols using
O2 is also a green process to obtain carbonyl products
accompanied with H2O as the only byproduct.[14–16]
The reaction of 1-decanol (1b) without Pd/C never
proceeded [Eq. (3)] and the reaction efficiency of Pd/
C under argon was even better in comparison to that
under O2 conditions in a test tube (Style I) [Eq. (4)].
Therefore, the first conversion step of the alcohol to
the aldehyde is considered to mainly proceed by the
Pd/C-catalyzed dehydrogenation. Furthermore, 1-dec-
anal (3b) was completely transformed to decanoic
acid (2b) in H2O without Pd/C and NaOH under
800 hPa at 808C,[17] while the addition of NaOH
rather delayed the transformation of 3b to 2b [Eq.
(5)]. Meanwhile, the reaction in the presence of Pd/C
efficiently proceeded in aqueous basic media as
shown in Table 3, entry 11. Since the di-18O-labeled
product was efficiently obtained by using H218O in-
stead of H216O, the present transformation of 3b to 2b
mainly proceeded by the dehydrogenation of the in-
termediate hydrate (A’) derived from 3b [Eq. (6)].
While the formation of the hydrate (A’) is an equilib-
rium reaction, the reaction demonstrates a tendency
to converge on the 18O-labeled intermediate (A’) due
to the isotopic effect. Under the O2 oxidation of the
aldehyde, the non-labeled 3b can be transformed to
18O-labeled 3b’ via the hydrate (A’), and 3b’ is theo-
retically converted to the mono-18O-labeled 2b ac-
cording to a previous paper.[17a] Nevertheless, the
di-18O-labeled 2b was actually obtained as the main
product under the Pd/C–NaOH–H218O conditions
[Eq. (6)]. The 18O-labeling reaction using 2b as a sub-
strate under the same reaction conditions never pro-
ceeded [Eq. (7)]. Therefore, in the present reaction,
the two continuous transformations of alcohol to alde-
hyde and aldehyde to carboxylic acid mainly pro-
gressed based on the Pd/C-catalyzed dehydrogena-
[a]
The Rh/C-catalyzed dehydrogenation of 1b–1e and 1g,
1h in a test tube (Style I; at 1008C for 24 h) gave low
yields (2b: 53%, 2c: 54%, 2d: 44%, 2e: 38%, 2g: 23%
and 2h: 33%), respectively.
[b]
4 equiv. of NaOH were used.
acidic quench of the resulting filtrate after filtration tion.[18]
could eliminate the problem and the Pd/C could be
reused without leaching [Eq. (2)].
It is noteworthy that the present dehydrogenation
could be accomplished using a normal evaporator
During the dehydrogenation of primary alcohols equipment [Eq. (8), a photograph is also shown in the
under reduced pressure, oxygen (O2) as a component Supporting Information]. Namely, benzyl alcohol (1c;
of the air could be an oxidant for the alcohols. The 5 mmol), 10% Pd/C (5 mol%), NaOH (2.2 equiv.) and
4
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
ÝÝ
These are not the final page numbers!