Direct hydrogenation of carboxylic acids to corresponding aldehydes catalyzed by palladium complexes

Article abstract of DOI:10.1246/bcsj.74.1803

A variety of carboxylic acids can be directly hydrogenated into the corresponding aldehydes in high yields by using homogeneous catalysts such as [Pd(PPh₃)₄] or combination of Pd(OAc)₂ with tertiary phosphines in the presence of an excess amount of 2,2-dimethylpropionic anhydride (pivalic anhydride). As a typical example, octanoic acid can be convened into octanal in 99% yield in 3 h at 80 °C under 3.0 MPa of H₂ in acetone in the presence of pivalic anhydride and a catalyst system composed of Pd(OAc)₂ + 5P(p-tol)₃. Such hydrogenation is widely applicable to various aliphatic, aromatic, and heterocyclic carboxylic acids as well as to di- and tribasic carboxylic acids. The process allows the presence of other functional groups such as ketonic carbonyl, nitrile, and ester groups and even internal C=C bonds. Heterogeneous palladium catalyst such as palladium on carbon also showed some catalytic activities.

Full text of DOI:10.1246/bcsj.74.1803

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1803–1815 (2001) 1803
e Chemical
ciety of Ja-
n
e Chemical
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Headline Articles
Direct Hydrogenation of Carboxylic Acids to Corresponding Aldehydes
Catalyzed by Palladium Complexes
01
03
15
SJA8
09-2673
124
Catalytic Direct Hydrogenation of Carboxylic Acids
K. Nagayama et al.
Kazuhiro Nagayama, Isao Shimizu, and Akio Yamamoto*
Department of Applied Chemistry, Graduate School of Science and Engineering, and Advanced Research Institute for
Science and Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169-8555
01
01
(Received April 12, 2001)
A variety of carboxylic acids can be directly hydrogenated into the corresponding aldehydes in high yields by using
homogeneous catalysts such as [Pd(PPh3)4] or combination of Pd(OAc)2 with tertiary phosphines in the presence of an
excess amount of 2,2-dimethylpropionic anhydride (pivalic anhydride). As a typical example, octanoic acid can be con-
verted into octanal in 99% yield in 3 h at 80 °C under 3.0 MPa of H2 in acetone in the presence of pivalic anhydride and
a catalyst system composed of Pd(OAc)2 + 5P(p-tol)3. Such hydrogenation is widely applicable to various aliphatic, ar-
omatic, and heterocyclic carboxylic acids as well as to di- and tribasic carboxylic acids. The process allows the presence
of other functional groups such as ketonic carbonyl, nitrile, and ester groups and even internal CwC bonds. Heteroge-
neous palladium catalyst such as palladium on carbon also showed some catalytic activities.
Because of the pivotal importance of aldehydes as reagents
and intermediates in organic synthesis and as final products for
pharmaceutical, cosmetic, and agricultural uses, quite a variety
of methods have been developed for preparation of various al-
dehydes. Hydroformylation of olefins, partial oxidation of pri-
mary alcohols, ozonolysis of olefins, and partial reduction of
carboxylic acid derivatives can be named among other meth-
ods.1 However, most of the currently available methods entail
some problems and limitations in the choice of starting materi-
als and/or in selectivity of the products. In industrial hydro-
formylation process, for example, applicable olefins are limit-
ed and regioselection for obtaining a desired aldehyde some-
times involves problems. Many useful reactions are known in
partial oxidation of primary alcohols to aldehydes such as the
Swern oxidation, the Dess–Martin oxidation, and oxidation by
PCC or MnO2. However, in these processes stoichiometric
amounts of oxidizing agents are required with parallel forma-
tion of undesirable by-product(s) which must be discarded.
Reduction of carboxylic acid derivatives such as acid chlo-
rides, anhydrides, and esters with metal hydrides provides a
convenient laboratory method, but the reactions require sto-
ichiometric amounts of metal hydrides and have to be carried
out at low temperature to avoid reduction of other functional
groups. Particularly, over-reduction of the produced aldehydes
further to corresponding alcohols presents a considerable prob-
lem, since the produced aldehydes are more reactive than the
starting compounds.
lyst with atmospheric pressure of hydrogen, is widely used be-
cause of high yields and convenient experimental conditions.2
The process can be carried out at room temperature and under
atmospheric pressure of H2.
However, the Rosenmund reaction involves the necessity of
preparing acid chlorides in the first place and is not applicable
to unstable acid chlorides. It requires also addition of a sto-
ichiometric amount of a base to neutralize the hydrogen chlo-
ride. As another type of convenient synthesis of aldehydes,
Fukuyama developed a catalytic hydrogenation of thioesters in
combination with triethylsilane catalyzed by Pd/C. The pro-
cess provides a convenient means of aldehyde synthesis, but
the prior synthesis of thioesters is required.3
Efficient preparative methods of aldehydes directly from
carboxylic acids are rare.4 Reaction of carboxylic acids with
metal hydrides gives inert salts of carboxylic acids with libera-
tion of H2 and does not yield aldehydes. Catalytic hydrogena-
tion of carboxylic acids by metal catalysts such as ruthenium
usually yields the corresponding alcohols because of higher re-
activity of aldehydes toward hydrogenation than that of car-
boxylic acids.5
Recently, a direct, vapor phase hydrogenation of carboxylic
acids into aldehydes catalyzed by chromia catalyst has been
successfully commercialized by Mitsubishi Chemical.6 The
hydrogenation reaction can convert various carboxylic acids
into aldehydes in high yields and selectivities with formation
of only water. A disadvantage of the process, however, is the
requirement of high reaction temperature (350 °C) to vaporize
the starting carboxylic acids, excluding applications of the pro-
The catalytic Rosenmund reduction of various acid chlo-
rides into aldehydes, performed by palladium on carbon cata-

1804 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Table 1. Competitive Hydrogenation of Benzoic Anhydride and Pivalic Anhydride
(tBuCO)2O
/mmol
Recovereda)/mmol
Yielda)/mmol
Entry
Time/h
2
3
4
5
0.72
0.44
0
7
1
2
2
6
48
48
120
0.26
0.60
0.60
0.62
0.82
3.96
2.52
1.80
2.80
2.30
1.68
1.62
2b)
3
a) GLC yields. b) [Pd(dba)2]/DPPB 0.02 mmol was used as a catalyst.
cess to thermally unstable compounds and making the process
inconvenient in laboratory synthesis of aldehydes.
anhydride.9 We summarize in the present paper the results of
catalytic hydrogenation of a variety of carboxylic acids and
discuss the scope and limitations of the process.
We have been interested in activation of carbon-oxygen
bonds in oxygen-containing organic compounds by transition
metal complexes7 with an objective of developing a new pro-
cess where use of organic halides can be avoided. As we have
reported previously, carboxylic anhydrides were found to oxi-
datively add to Pd(0) complexes to give acyl(carboxyla-
to)bis(tertiary phosphine)palladium(Ⅱ) complexes (1) with
cleavage of the C–O bond in the carboxylic anhydrides. The
acyl(carboxylato)bis(tertiary phosphine)palladium complexes
thus formed react with molecular hydrogen to liberate alde-
hydes and carboxylic acids accompanied by some alcohols
produced by hydrogenation of the aldehydes formed (Eq. 1).8
Results
Study of Competitive Hydrogenation of a Pair of Car-
boxylic Anhydrides into Aldehydes Catalyzed by a Pd(0)
Complex. In a previous study of oxidative addition of vari-
ous carboxylic anhydrides with Pd(0) complexes, we observed
that the carboxylic anhydrides having more bulky carboxyl en-
tities such as isobutyric anhydride (pivalic anhydride) and dim-
ethylacetic anhydride, were hydrogenated less rapidly with
[Pd(PPh3)4] than the less bulky anhydrides. The result of com-
petitive hydrogenation of benzoic anhydride and pivalic anhy-
dride is shown in Table 1.
When 2 mmol each of benzoic anhydride (2) and pivalic an-
hydride (3) were subjected to hydrogenation under hydrogen
pressure of 3.0 MPa in THF at 80 °C, benzaldehyde (4) and
benzoic acid (5) were produced, whereas only a negligible
amount of pivalaldehyde (6) was formed, accompanied by piv-
alic acid (7) that was produced in an amount comparable to
that of benzaldehyde. When 6 mmol of pivalic anhydride and
2 mmol of benzoyl anhydride were used, formation of benzoic
acid (5) became negligible, with benzaldehyde formed as the
only aldehyde. Use of a bidentate ditertiary bisphosphine, 1,4-
bis(diphenylphosphino)butane (dppb), gave similar results
with somewhat less effectiveness (entry 2). The results sug-
gest that the bulkier entity in the anhydride is less reactive to
the hydrogenation.
(1)
Based on the results a new catalytic process was developed
to hydrogenate carboxylic anhydrides into aldehydes and car-
boxylic acids under mild conditions (Eq. 2).
For comparing the reactivities of the acyl entities in the car-
boxylic anhydrides, we prepared a mixed anhydride, benzoic
pivalic anhydride, having bulkier and less bulky acyl groups as
shown below, and we examined its behavior toward palladium-
catalyzed hydrogenation (Eq. 3).
(2)
The catalytic process provides a convenient means to pro-
duce aldehydes from carboxylic anhydrides without using acyl
halides under neutral mild conditions, making the process ha-
lide-free without liberation of a salt to be discarded. Remain-
ing problems of the process are that preparation of the carbox-
ylic anhydrides is required first and the half of the anhydride
molecule is converted into carboxylic acid which must be re-
leased.
The present paper describes our effort to surmount the dis-
advantages through fundamental studies on the reactivities of
carboxylic anhydrides with Pd(0) complexes. The attempt led
to a novel synthetic method to convert carboxylic acids direct-
ly into aldehydes in the presence of a less reactive carboxylic
(3)
The mixed anhydride containing the benzoyl and pivaloyl enti-

K. Nagayama et al.
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1805
ties was prepared by treating silver benzoate with pivaloyl
chloride. The mixed anhydrides produced were found to con-
tain minor amounts of benzoic acid, together with benzoic and
pivalic anhydrides as revealed by NMR, but the mixture was
subjected to the catalytic hydrogenation without distillation to
avoid the disproportionation process. The catalytic hydroge-
nation of the benzoic pivalic anhydride sample gave benzalde-
hyde as the major product on hydrogenation at 80 °C for 1 day
(Eq. 3). Pivalic acid was formed preferentially, with a minor
amount of benzoic acid. The reaction system examined after
the hydrogenation contained pivalic anhydride and benzoic an-
hydride generated on disproportionation of the mixed anhy-
dride.
formation of a minor amount of pivalaldehyde. Not shown in
Scheme 1 for simplicity is the hydrogenation of benzoic pival-
ic anhydride that gives a mixture of the two aldehydes and the
two carboxylic acids. The carboxylic acids produced there
would be likewise converted into the mixture of carboxylic an-
hydrides in the exchange step shown in the lower part of
Scheme 1 so as to be involved in the catalytic hydrogenation.
In fact, various types of carboxylic acids were successfully
converted into the corresponding aldehydes under H2 pressure
at the cost of pivalic anhydride as shown in Tables 2–7. In
these Tables, the experiments are standardized for the sake of
comparison by a protocol using [Pd(PPh3)4] in THF at 80 °C
with hydrogen pressure of 3.0 MPa. We later found that em-
ployment of more polar solvents such as acetone and DMF
was more favorable to increase the yield, but Tables 2, 3, and
5–8 show the results performed in THF.
The result indicates that the less bulky acyl part in the ben-
zoic pivalic anhydride is preferentially hydrogenated.
Design of a Catalytic Process to Directly Hydrogenate a
Carboxylic Acid. Examination of the result shown in Eq. 3
led us to the idea of employing a mixture of a carboxylic acid
and a less reactive, bulky, carboxylic anhydride such as pivalic
anhydride as an additive for realizing a direct catalytic hydro-
genation of the carboxylic acid into aldehydes. Scheme 1 il-
lustrates the idea.
In the upper part of the Scheme, the competitive hydrogena-
tion of benzoic anhydride and pivalic anhydride is shown. The
palladium-catalyzed hydrogenation of benzoic anhydride pro-
ceeds much faster than the hydrogenation of pivalic anhydride,
yielding corresponding aldehydes and carboxylic acids, re-
spectively. The carboxylic acids produced would then react
with the carboxylic anhydrides, as shown in the lower part of
Scheme 1, generating a mixture of the mixed anhydride and
the two kinds of symmetric anhydrides. The carboxylic anhy-
drides would then react with molecular hydrogen in the pres-
ence of a palladium catalyst, as shown in the upper part of the
scheme, with the more reactive benzoic anhydride reacting
faster with H2. It is thus expected that addition of the less reac-
tive bulky pivalic anhydride would lead to the preferential hy-
drogenation of benzoic acid into benzaldehyde at the cost of
pivalic anhydride, which is converted into pivalic acid with
Table 2 summarizes the results of hydrogenation of various
aliphatic carboxylic acids in the presence of pivalic anhydride
and [Pd(PPh3)4].
Most carboxylic acids without a substituent at the α-carbon
were converted into the corresponding aldehydes in high yields
(Table 2, entries 1–6). In the case of more hindered carboxylic
acids, the yields of the aldehydes were decreased (entries 7–
13). Hydrogenation of cyclohexanecarboxylic acid required a
longer reaction time for obtaining a good yield (entries 7, 8).
Hydrogenation of α-phenylpropionic acid (entry 11) gave sty-
rene as the major product, indicating that decarbonylation of
the acylpalladium intermediate occurred to give α-phenyleth-
ylpalladium intermediate that subsequently underwent β-hy-
drogen elimination to produce styrene. On the other hand,
diphenylacetic acid (entry 12) yielded diphenylmethane. Its
formation may be accounted for by hydrogenation of the puta-
tive intermediate, (diphenylmethyl)palladium species, pro-
duced by oxidative addition of the carboxylic anhydride fol-
lowed by decarbonylation.10 Highly hindered 1-adamantane-
carboxylic acid showed a poor reactivity toward the hydroge-
nation and the recovery of the starting material was confirmed
(entry 13).
Scheme 1. Proposed reaction mechanism of hydrogenation of two anhydrides catalyzed by a palladium complex.

1806 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Table 2. Catalytic Hydrogenation of Aliphatic Carboxylic Acidsa)
Yieldb)/%
Yield/mmol
Recovery/mmol
Entry
1
Carboxylic Acids
Other Products
(nC7H15CO)2Of
9c)
6c)
7d)
3d)
98
0.46
5.20
2.96
2e)
90
0.14
4.04
0.78
3
4
99
70
0.50
0.40
5.64
4.64
3.08
3.32
5
6
92
89
0.54
0.42
4.88
4.62
3.22
3.48
7
63
82
0.52
0.70
5.84
5.58
3.34
3.04
8f)
9
64
23
0.70
0.68
4.08
3.22
3.10
3.82
10
11
12
< 3
0.40 < 4.80
3.58
3.12
13
0.42
5.38
13
8
0.64
3.00
3.22
a) Reaction conditions are shown in the text. b) Yields are based on the starting carboxylic acids. c)
Determined by 1H NMR using (CHCl2)2 as an internal standard. d) Determined by GLC using nC17H36 as
an internal standard. e) (tBuCO)2O, 3 mmol. f) Reaction time, 48 h.
The results of the catalytic hydrogenation of aliphatic acids
having functional groups are summarized in Table 3.
counted for by assuming pre-coordination of a bis(triphe-
nylphosphine)palladium(0) complex to the CwC bonds11 in the
reaction media, followed by the activation of C–O bond of
trans-cinnamic acid.
In the case of hydrogenation of 10-undecenoic acid having a
terminal CwC bond, isomerization of the terminal CwC bond
to an internal bond and partial hydrogenation of the terminal
CwC bond also occurred (entry 22). Attempts to hydrogenate
10-undecenoic acid to unsaturated aldehyde without causing
terminal-to-internal isomerization and the hydrogenation of
the olefinic double bond are summarized in Table 4.
The CwO bonds in esters (entries 14, 15) and ketones (en-
tries 16, 17), and internal CwC bonds (entries 18 and 19) re-
mained unchanged throughout the hydrogenation reactions. In
the case of hydrogenation of trans-cinnamic acid which has an
α,β-unsaturated CwC bond, the yield of trans-cinnamaldehyde
was low when [Pd(PPh3)4] was used as the catalyst (Pd/P ratio
= 1/4, entry 20), while a higher yield was achieved by usage
of [Pd(OAc)2] + 2PPh3 catalyst system (Pd/P = 1/2, entry 21).
The results of the high yields and quite high selectivities (for-
mation of pivalaldehyde was < 1%.) in this case may be ac-
Occurrence of isomerization was found to be suppressed by

K. Nagayama et al.
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1807
Table 3. Catalytic Hydrogenation of Aliphatic Carboxylic Acid Having Functional Groupsa)
Yieldb)/%
Yield/mmol
Recovery/mmol
Entry
Carboxylic Acids
Other Products
9c)
6c)
7d)
3d)
14
15
99
0.60
5.12
3.30
99
97
0.60
0.38
4.76
—
3.28
3.40
16
17
85
0.14
3.98
3.54
18
19
oleic acide)
erucic acidf)
99g)
96g)
0.52
0.42
5.80
5.52
2.84
3.08
20
30
84
0.02
0.02
3.96
3.68
3.82
3.80
21g)
olefinc)
external
internal
48%
38%
22
87h)
0.26
4.38
3.50
hydrogenated (14%)
a) Typical conditions are given in the text. b) Yields are based on the starting carboxylic acids. c) Determined by
1H NMR using (CHCl2)2 as an internal standard. d) Determined by GLC using nC17H36 as an internal standard.
e) Oleic acid = cis-9-octadecenoic acid.
f) Erucic acid = cis-13-docosenoic acid.
g) Pd(OAc)2/2PPh3 was used as a catalyst.
h) Total aldehyde yield.
using polar solvents such as N,N-dimethylformamide and 1,2-
dimethoxyethane.
Most aromatic acids are reduced in high yields into the cor-
responding aldehydes as shown in Table 5.
try 33), whereas chloro-substituted benzoic acids at para- and
ortho-positions gave complicated mixtures. 1- and 2-Naphtoic
acids were hydrogenated into the corresponding aldehydes in
moderate yields with formation of naphthalene as the decar-
boxylation product (entries 34–35).
The present catalytic hydrogenation process can be applied
to heterocyclic carboxylic acids, as summarized in Table 6.
Among pyridinecarboxylic acids, only nicotinic acid having
the carboxyl group at the β-position was hydrogenated to pro-
duce the corresponding aldehyde quantitatively. 2-Pyridine-
carboxylic acid was not hydrogenated, whereas only a small
amount of 4-pyridinecarboxylic acid was hydrogenated. The
reason for the poor reactivity of 4-pyridinecarboxylic acid is
not clear. Furancarboxylic acids and thiophenecarboxylic ac-
ids were hydrogenated in moderate to good yields into the cor-
responding aldehydes (entries 39–42), although the hydroge-
nation proceeded slowly.
The meta- and para-substituted benzoic acid derivatives
having electron-withdrawing as well as electron-releasing
groups were successfully hydrogenated into aldehydes in ex-
cellent yields (entries 23–27). In the case of 4-methoxybenzo-
ic acid bearing the electron-donating group at the para-posi-
tion, a longer reaction time was required to complete the reac-
tion (entry 26). On the other hand, o-methylbenzoic acid was
resistant to reduction to o-tolualdehyde owing to the steric hin-
drance of the methyl group around the carbonyl carbon in the
carboxyl group. In hydrogenation of monofluorobenzoic acids
(entries 30–32), conversion of the starting carboxylic acids
into the corresponding aldehydes occurred in excellent yields
for meta- and para-substituted fluorobenzoic acids, whereas o-
fluorobenzaldehyde was obtained in a moderate yield. m-
Chlorobenzoic acid was hydrogenated selectively into m-chlo-
robenzaldehyde without causing the C–Cl bond cleavage (en-
Aliphatic and aromatic dicarboxylic acids were also con-
verted into corresponding dialdehydes in excellent yields, as
shown in Table 7 (entries 43, 44, and 47, 48), exceptions being

1808 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Table 4. Attempts of Chemoselective Hydrogenation of 10-Undecenoic Acid
Total aldehyde
Olefin isomerizationa)
Yield of
Conditions
acetone,
yield/%
68
terminal:internal
86:14
10-Undecenal/%
58
80 °C, 5 h
MeO(CH2)2OMe,
80 °C, 3.5 h
MeCN,
80 °C, 3 h
DMF
90
45
69
91:9
80:20
98:2
82
36
68
80 °C, 5 h
DMF 2.5 cm3
H2 6.0 MPa
83
96:4
80
(tBuCO)2O 3 mmol
80 °C, 5 h
a) The amount of hydrogenated product was negligible as determined by 1H NMR.
those which easily form cyclic carboxylic anhydrides. lar solvents such as acetone and DMF than in THF, dioxane,
Glutaric acid (entry 45), and succinic acid (entry 46) were
converted into six- and five-membered cyclic anhydrides,
whereas phthalic acid (entry 49) was quantitatively converted
into phthalic anhydride. Aromatic tribasic acid, 1,3,5-benzene-
tricarboxylic acid, was hydrogenated to tricarbaldehyde in a
high yield, accompanied by formation of a small amount of
isophthalaldehyde as a byproduct formed by removal of one
carboxyl group (entry 50).
Table 8 shows the effects of variation of experimental condi-
tions on the yield of octanal by hydrogenation of octanoic acid
in the presence of 0.02 mmol of [Pd(PPh3)4] under 3.0 MPa of
H2 in THF at 80 °C. When the amount of pivalic anhydride
was increased from 2 mmol/2 mmol of octanoic acid to 3, 4,
and 6 mmol, the yield of octanal produced increased from 78%
to 90, 92, and 98%.
Increase in the yield of pivalaldehyde as by-product was
also noted when the higher ratio of the pivalic anhydride to oc-
tanoic acid was employed.
Lowering the hydrogen pressure to 0.5 MPa caused a de-
crease in the yield of octanal down to 47%. Lowering the reac-
tion temperature to 50 °C from 80 °C also caused the decrease
in the aldehyde yield under otherwise similar conditions.
Since pivalic anhydride can be considered in a sense as a de-
hydrating agent, the effects of various dehydrating agents were
also examined (Table 9).
and toluene.
In Table 11 are summarized the effects of catalysts and cata-
lyst precursors on the yield of octanal by hydrogenation of oc-
tanoic acid.
The hydrogenation of octanoic acid with [Pd(PPh3)4] in ace-
tone under 3.0 MPa of H2 at 80 °C for 5 h in the presence of
pivalic anhydride gives octanal in 91% yield. Addition of an
extra molar amount of PPh3 to [Pd(PPh3)4] suppressed the al-
dehyde formation to less than half. The effect of addition of
PPh3 ligand in varying molar amounts to a Pd(0) complex was
examined next using [Pd(dba)2]. The result showed that use of
[Pd(dba)2] in combination with 4 molar amounts of PPh3
showed much less activity than the case when [Pd(PPh3)4] was
used. The results imply that the dba ligand attached to Pd(0)
may hinder the reaction of the Pd(0) complex in reaction with
the substrate. Further increase in the extra amount of PPh3
caused further suppression in the catalytic activity.
Although a Pd(0) species is believed to be involved in the
catalytic system, it is not necessary to use a Pd(0) complex it-
self as the catalyst. Use of palladium(Ⅱ) acetate in combina-
tion with a phosphine ligand12 was found to be effective as
well in the hydrogenation. Addition of 4 molar amounts of
PPh3 ligand was effective to give the octanal in a high yield,
whereas addition of more PPh3 had an inhibition effect. Addi-
tion of 5 molar amounts of P(o-tol)3, which is known as a very
bulky ligand, severely hindered the hydrogenation, whereas
addition of PPh2(p-tol) had a similar effect to that of PPh3. The
highest aldehyde yield was obtained with use of Pd(OAc)2
with five molar amounts of P(p-tol)3.
Adamantanecarboxylic anhydride, another bulky carboxylic
anhydride similar to pivalic anhydride, was effective as an ad-
ditive in the hydrogenation reaction of octanoic acid. Other
dehydrating agents such as acetic anhydride used in excess,
dibutyl dicarbonate, and dicyclohexylcarbodiimide showed
some effects to enhance the hydrogenation activity, whereas o-
sulfobenzoic anhydride and diphenylphosphinic anhydride
were ineffective in promoting the hydrogenation.
Effects of other tertiary phosphine and phosphite ligands
which showed poor promotion effects are also included in Ta-
ble 11. Among the catalyst precursors examined, the system
containing Pd(OAc)2 and PMePh2 showed a different behavior,
giving more pivalaldehyde than octanal. The reason for such
abnormal behavior was not pursued further.
Table 10 shows the effect of solvents in the hydrogenation
reaction. In general, the hydrogenation proceeds faster in po-

K. Nagayama et al.
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1809
Table 5. Catalytic Hydrogenation of Aromatic Carboxylic Acidsa)
Yieldb)/%
11c)
Yield/mmol
Recovery/mmol
Other Products
Entry
Carboxylic Acids
6c)
7d)
3d)
23
24
91
0.62
5.20
2.84
(PhCO)2Of) 5%d)
99
99
95
0.52
0.54
0.76
5.38
5.30
5.38
2.86
25
3.22
4.52
26e)
27
97
84
30
0.42
0.64
0.40
5.02
4.46
4.04
2.86
3.22
3.74
Ph2Of)
28
29e)
30
31
32
ortho
meta
para
78
99
93
0.74
0.68
0.96
4.62
5.08
4.80
3.14
3.14
2.68
33
meta
99
0.52
4.40
3.36
34
1-
2-
50
60
0.32
0.26
3.84
3.16
3.68
3.96
f) 27%d)
f) 10%d)
35g)
a) Typical conditions are shown in the text. b) Yields are based on the starting carboxylic acids. c) Deter-
1
n
mined by H NMR using (CHCl2)2 as an internal standard. d) Determined by GLC using C17H36 as an
internal standard. e) Reaction time, 48 h. f) Detected with GC-MS. g) H2 6.0 MPa.
Other divalent palladium complexes such as trans-
[PdCl2(PPh3)2], [Pd(η3-allyl)Cl]2 without and with phosphines
showed no or poor catalytic activity. Pd/C also catalyzed the
hydrogenation, giving the aldehyde in a yield over 50%,13 but
the yield of pivalaldehyde dominated over that of octanal.
Figure 1 shows the time course of the hydrogenation of oc-
tanoic acid with [Pd(PPh3)4] and Pd(OAc)2 + 5P(p-tol)3 sys-
tems. Usage of Pd(OAc)2 + 5P(p-tol)3 combination as a cata-
lyst precursor afforded almost quantitative conversion of oc-
tanoic acid to octanal within 3 h.
bond. Various aromatic and heterocyclic carboxylic acids can
be smoothly hydrogenated in a reasonable reaction time under
mild experimental conditions. Application of the present hy-
drogenation method to dibasic and tribasic carboxylic acids
may be of considerable potential interest to produce di- and tri-
carbaldehydes. Furthermore, it is possible to protect one of the
carboxylic groups by converting it into an ester that is not sus-
ceptible to the hydrogenation. That part can be later hydro-
lyzed, after the first step of hydrogenation, to regenerate the
carboxylic part.
The present process is regarded as complementary to the
known Mitsubishi Chemical process of direct hydrogenation
of carboxylic acids using chromia. One advantage of the
present process is that the reaction can be performed under
mild conditions, whereas the Mitsubishi Chemical process re-
quires the high temperature of 350 °C and its scope is thus
somewhat limited.
Discussion
Scope and Limitations. The catalytic hydrogenation pro-
cess found in the present study provides a new and convenient
means to prepare quite a variety of aldehydes directly from
free carboxylic acids in one pot. The present process is appli-
cable to various aliphatic acids and is tolerant of the presence
of carbonyl groups and does not hydrogenate the inner CwC
The present method does not need to start from acyl halide,

1810 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Table 6. Catalytic Hydrogenation of Heterocyclic Carboxylic Acidsa)
Yieldb)/%
Yield/mmol
6c) 7d)
Recovery/mmol
Entry
Carboxylic Acids
13c)
3d)
36
37
38
α
β
γ
0
99
4
0
1.06
0.94
1.98
5.38
5.68
4.48
2.52
2.48
39e)
40e)
2-
3-
87
90
0.84
0.72
4.90
4.68
2.80
3.18
41e)
42e)
2-
3-
72
73
1.10
0.78
5.42
4.84
2.90
3.06
a) Typical conditions are shown in the text. b) Yields are based on the starting carboxylic
acids. c) Determined by 1H NMR using (CHCl2)2 as an internal standard. d) Determined by
GLC using nC17H36 as an internal standard. e) Reaction time, 48 h.
nor does it need use of any extra base, and it produces no extra
RCOOH would be converted into a mixture of the carboxylic
anhydrides: (RCOOCOtBu), (RCO)2O, and (tBuCO)2O as
shown in Scheme 2.14 The palladium(0) complex will attack
the less bulky carboxyl carbon preferentially to form an
acyl(carboxylato)palladium intermediate that carries the cata-
lytic cycle. Limited amounts of pivalaldehyde are generated in
all runs due to occurrence of the hydrogenolysis of pivalic an-
hydride by the palladium complex.
Although the main cycle in Scheme 2 accommodates the re-
sults of stoichiometric reactions of the isolated acyl(carboxyla-
to)palladium(Ⅱ) to release the aldehyde and carboxylic acid,
the exact course of catalytic processes requires further valida-
tion.
byproducts that should be discarded. Thus the method can be
regarded as an environmentally benign process and may be
suitable for laboratory synthesis of various aldehydes. As a
limitation, the present process requires addition of pivalic an-
hydride, posing a cost problem and limiting the commercial
applications to production of somewhat expensive products.
Mechanism. In our preliminary communication, we have
proposed the reaction mechanism as shown in Scheme 2 to ac-
count for the conversion of a carboxylic acid in the presence of
pivalic anhydride into an aldehyde catalyzed by a palladium
catalyst under pressurized H2.
Since [PdL2(styrene)] type complexes having tertiary phos-
phine ligands such as PMe3 and PMe2Ph are known to undergo
the oxidative addition of carboxylic anhydride to give the
acyl(carboxylato)bis(tertiary phosphine)palladium(Ⅱ) 1, as
shown in Eq. 1, and complex 1 reacts with H2 to release the al-
dehyde and carboxylic acid (Eq. 2), the reaction mechanism
shown in Scheme 3 accommodates the known experimental re-
sults. The number of ligands L attached to the active Pd spe-
cies in Scheme 2 is tentative and in certain cases the number
may be smaller than 2. The configuration of the acyl(carboxy-
lato)(tertiary phosphine)palladium obtained in the stoichiomet-
ric process was in the trans form, but it may have a cis form in
the catalytic process depending on the tertiary phosphine
ligand employed, since ditertiary phosphine ligand also
showed some catalytic activity.
A theoretical study with a DFT calculation15 suggested that
hydrogenation of trans-[Pd(Ac)(OAc)(PMe3)2] gives acetalde-
hyde and a acetato(hydrido)palladium species, trans-[PdH
(OAc)(PMe3)2], as shown on the left hand cycle in Scheme 3.
The result suggested the possibility of reaction of the carboxy-
lato(hydrido)palladium species with carboxylic anhydride, as
shown on the right hand of Scheme 3 to liberate carboxylic
acid with formation of acyl(carboxylato)palladium complex as
shown at the bottom in Scheme 3. The acyl(carboxylato)palla-
dium complex may carry the catalytic cycle to react with hy-
drogen. The intermediate species are shown in Scheme 3 as
trans forms to be consistent with the experimental results car-
ried out on stoichiometric processes, but the catalytic process
may proceed through cis complexes.
Outside the main catalytic cycle are shown the secondary
cycles to convert the carboxylic acid released in the hydroge-
nation of the acyl(carboxylato)palladium(Ⅱ) into a mixture of
anhydrides on reaction with pivalic anhydride added to the sys-
tem. On interaction with pivalic anhydride, the carboxylic acid
The proposed mechanism is novel and involves intermedia-
cy of only Pd(Ⅱ) species. Scheme 3 stands in contrast to the
well accepted mechanism operating through the catalytic cycle
composed of Pd(0) and Pd(Ⅱ) species or with the recently ar-
gued catalytic cycle involving Pd(Ⅱ) and Pd(Ⅳ).16

K. Nagayama et al.
Table 7. Catalytic Hydrogenation of Di- and Tricarboxylic Acidsa)
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1811
Yieldb)/%
Yield/mmol
Recovery/mmol
Other Products
Entry
Carboxylic Acids
15c)
6c)
7d)
3d)
43
44
99
98
0.48
0.74
5.44
4.62
2.92
3.12
e)
45
46
—
0.70
0.64
3.52
3.40
3.82
4.04
79%d)
99%d)
0
47
48
99
94
0.36
0.26
4.66
4.60
3.22
3.38
49
0
0.72
0.34
3.04
4.56
4.04
3.06
f) quant.d)
50g)
86h)
f) 6%d)
a) Typical conditions are shown in the text. b)Yields are based on the starting carboxylic acids. c) Determined by
1H NMR using (CHCl2)2 as an internal standard. d) Determined by GLC using nC17H36 as an internal standard. e)
The aldehyde signal was observed by 1H NMR at δ 9.35 (CDCl3) in 12% yield. f) Detected with GC-MS. g) Car-
boxylic acid, 0.67 mmol. h) Trialdehyde yield.
Table 8.
Hydrogenation of Octanoic Acid
Effect of Experimental Conditions in Catalytic
on treatment with H2 in agreement with Scheme 3. Currently
we are trying to clarify the mechanism of the catalytic hydro-
genation of carboxylic anhydride, examining the feasibility of
the mechanisms shown in Scheme 2 or Scheme 3 by experi-
mental and theoretical studies. The results will be reported
separately.
Conclusion
Since the useful methodologies to directly convert carboxyl-
ic acids into aldehydes are quite limited, the present method
provides a convenient general route to selective production of
aldehydes under mild and simple conditions from carboxylic
acids. The process is based on the previous studies about re-
lease of aldehyde from acyl(carboxylato)palladium complexes
on treatment with hydrogen. The direct hydrogenation of car-
boxylic acids was realized by combining the catalytic hydroge-
nation process for conversion of carboxylic anhydrides into al-
dehyde and carboxylic acid with auxiliary cycles involving the
exchange processes between the carboxylic acid and bulky car-
boxylic anhydride.
(tBuCO)2O
H2 Pressure
Temperature
C7H15CHO
Yielda)/%
tBuCHOa)
/mmol
/MPa
3.0
3.0
3.0
3.0
/°C
80
80
80
80
/mmol
0.46
0.26
0.14
0.08
6
4
3
2
98
92
90
78
6
6
0.5
3.0
80
50
47
68
0.36
0.14
a) Determined by 1H NMR.
Our preliminary study on the reaction of trans-
[PdH(OCOCF3)(PMe3)2] with propionic anhydride gave evi-
dence supporting the formation of propionic acid and of trans-
[Pd(COEt)(OCOCF3)(PMe3)2] that liberates propionaldehyde
Experimental
All manipulations were carried out under argon. Solvents were
purified by usual methods under argon. Tetrakis(triphenylphos-

1812 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Table 9. Effect of Dehydrating Agent in Catalytic Hydroge-
nation of Octanoic Acid
Table 11. Effect of Palladium–Phosphine Catalyst
C7H15CHO
Pd–Phosphine Cat.
tBuCHOa)
/mmol
Yielda)/%
C7H15CHO
Yielda)/%
RCHOa)
/mmol
0.46 (R = tBu)
[Pd(PPh3)4]
[Pd(PPh3)4]
91
41
0.34
0.12
Dehydrating agent
+ PPh3
98
[Pd(dba)2]
+ 4PPh3
+ 5PPh3
+ 6PPh3
13
7
5
0.02
< 0.01
0.02
1.12
99
(R = adamantyl)
[Pd(OAc)2]
+ 4PPh3
+ 5PPh3
+ 6PPh3
79
50
24
0.20
0.14
0.06
0
—
Ac2O (1.1 cm3),b) 48 h
40c)
54
0
—
—
—
+ 2P(o-tol)3
+ 3P(o-tol)3
+ 4P(o-tol)3
+ 5P(o-tol)3
4
7
5
8
0.02
0.04
0.02
0.02
+ 4PPh2(p-tol)
+ 5PPh2(p-tol)
74
62
0.18
0.12
31
—
+ 3P(p-tol)3
+ 4P(p-tol)3
+ 5P(p-tol)3
+ 6P(p-tol)3
67
90
96
33
0.18
0.38
0.48
0.16
1
a) Determined by H NMR unless otherwise noted. b)
The reaction mixture was stirred at 80 °C for 1 h before
introduction of H2. c) GLC yield.
+ 5P(2-furyl)3
0
0
Table 10. Effect of Solvent in Hydrogenation of Octanoic
Acid
+ 4PMePh2
+ 5PMePh2
20
12
1.18
0.70
+ 2P(tBu)3
0
0
+ 4P(OPh)3
+ 5P(OPh)3
2
< 1
0.10
< 0.01
+ 5P(NMe2)3
0
0
C7H15CHO
tBuCHOa)
Run
Solvent
Yielda)/%
/mmol
0.26
0.26
0.14
0.34
0.22
+ 2DPPE
+ 2DPPP
2
15
0.04
0.22
1
2
3
4
5
THF
dioxane
toluene
acetone
DMF
78
69
66
91
93
+ 4PPh3
+ 5PPh3
+ 6PPh3
14
11
8
0.04
0.04
0
a) Deterrmined by 1H NMR.
[PdCl2(PPh3)2]
0
0
15
< 0.01
< 0.01
1.26
phine)palladium(0),17 bis(dibenzilydeneacetone)palladium(0),18
[PdCl2(PPh3)2] + 3PPh3
(η3-allyl)chloropalladium(Ⅱ),19
and
adamantanecarboxylic
10% Pd/C (30.7 mg)b)
anhydride20 were synthesized by the reported methods. All car-
boxylic acids and anhydrides were commercial products and were
used without further purification. NMR data were obtained with
JEOL AL300 in CDCl3 as a solvent at room temperature. Gas
a) Yields were determined with 1H NMR using (CHCl2)2 as
an internal standard. b) THF (5 cm3) was used as a solvent.

K. Nagayama et al.
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1813
Scheme 2. Proposed mechanism of catalytic hydrogenolysis of carboxylic acid with a Pd complex.
less autoclave (100 cm3) was purchased from Taiatsu Glass Co.
General Procedure of Hydrogenation of Carboxylic Acids:
A THF (5 cm3) solution of [Pd(PPh3)4] (0.02 mmol), carboxylic
acid (2 mmol, monobasic), and pivalic anhydride (6 mmol) was
placed in a 100 cm3 stainless autoclave purged with argon. The
autoclave was flushed with dihydrogen (ca. 5 MPa, twice), and
pressurized with dihydrogen (3.0 MPa at room temperature). The
mixture was stirred at 80 °C for 24 h. The products were analyzed
by GC-MS and 1H NMR (CDCl3, r.t.) by comparison with authen-
tic samples. Some aldehydes as shown below were isolated by the
usual silica-gel chromatography (hexane/EtOAc) and were identi-
fied by spectroscopic means and elemental analysis.
cis-9-Octanal: Yield 396 mg (74%). 1H NMR δ 9.73 (1H, t,
J = 1.7 Hz), 5.26–5.36 (2H, m), 2.38 (2H, dt, J = 7.3, 1.7 Hz),
1.92–1.99 (4H, m), 1.55–1.64 (2H, m), 1.22–1.28 (20 H, m), 0.85
(3H, t, J = 6.8 Hz); 13C{1H} NMR δ 202.9, 130.0, 129.7, 43.9,
31.9, 29.75, 29.65, 29.50, 29.31 (overlapping of two carbon sig-
nals), 29.23, 29.12, 29.03, 27.20, 27.13, 22.67, 22.06, 14.09. IR
(neat) 2715 (νC(=O)H), 1729 (νHC=O) cm−1; FAB-MS: Found m/z
267.2704 (M+H), Calcd for C18H34O: 266.26097 (M).
EtOC(=O)(CH2)10CHO: Yield 357 mg (74%). 1H NMR δ
9.68 (1H, t, J = 1.7 Hz), 4.02 (2H, q, J = 7.1 Hz), 2.35 (2H, tq, J
= 7.2, 1.7 Hz), 1.55 (4H, m), 1.15–1.21 (17H, m); 13C{1H} NMR
δ 202.2, 173.4, 59.8, 43.7, 34.1, 29.15, 29.14, 29.12, 29.02, 28.94,
Fig. 1. Time-yield curves in catalytic hydrogenation of oc-
tanoic acid to octanal.
28.90, 24.8, 2.9, 14.1. IR (neat) 2717 (νC(=O)H), 1736 (νHC=O
)
cm−1; Found: C 69.19, H 11.14%. Calcd for C14H26O3: C 69.38,
H 10.81%; FAB-MS: Found m/z 243.1926 (M+H), Calcd for
C14H26O3: 242.18819 (M).
chromatography was carried out on a Hitachi 263-30 equipped
with 5%-SE30. Low-resolution mass spectra combined with gas
chromatograph were obtained with JEOL JMS-SX102A. FAB
analysis and elemental analysis were performed by the Material
Characterization Central Laboratory of Waseda University. Stain-
PhC(=O)(CH2)2CHO: Yield 296 mg (91%). 1H NMR δ
9.75 (1H, t, J = 0.7 Hz), 7.81–7.90 (2H, m), 7.30–7.47 (3H, m),
3.19 (2H, t, J = 6.4 Hz), 2.78 (2H, t, J = 6.4 Hz); 13C{1H} NMR

1814 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
HEADLINE ARTICLES
Scheme 3. Proposed mechanism of catalytic hydrogenation of carboxylic anhydride involving a palladium hydride intermediate.
δ 200.3, 197.5, 136.0, 132.9, 128.3, 127.7, 37.4, 30.8; IR (neat)
2831 and 2729 (νC(=O)H, separated by the Fermi resonance), 1716
(νHC=O) cm−1; Found: C 73.83, H 6.59%. Calcd for C10H10O2: C
74.06, H 6.21%; FAB-MS: Found m/z 163.0726 (M+H), Calcd
for C10H10O2: 162.06808 (M).
112, 7050 (1990). b) Y. Kanda and T. Fukuyama, J. Am. Chem.
Soc., 115, 8451 (1993).
4
a) L. I. Zakharkin and I. M. Khorlina, Zh. Obshch. Khim.,
34, 1029 (1964). b) H. C. Brown and N. M. Yoon, J. Am. Chem.
Soc., 88, 1464 (1966). c) H. C. Brown and P. M. Weissman, J. Am.
Chem. Soc., 87, 5614 (1965). d) M. Muraki and T. Mukaiyama,
Chem. Lett., 1974, 1447. e) T. D. Hubert, D. P. Eyman, and D. F.
Wiemer, J. Org. Chem., 49, 2279 (1984). f) H. C. Brown, P.
Heim, and N. M. Yoon, J. Am. Chem. Soc., 92, 1637 (1970). g) H.
C. Brown, P. Heim, and N. M. Yoon, J. Org. Chem., 37, 2942
(1972). h) H. C. Brown, C. G. Rao, and S. U. Kulkarni, Synthesis,
1979, 704. i) H. C. Brown, J. S. Cha, N. M. Yoon, and B. Nazer, J.
Org. Chem., 52, 5400 (1987). j) S. Kim and S. S. Kim, Tetrahe-
dron Lett., 1987, 1913. k) S. Kim, S. S. Kim, S. T. Lim, and S. C.
Shim, J. Org. Chem., 52, 2114 (1987). l) S. Kim, Pure Appl.
Chem., 59, 1005 (1987). m) E. Negishi and H. C. Brown, Synthe-
sis, 1974, 77. n) J. Boyer, C. Breliere, R. J. P. Corriu, A. Kpoton,
M. Poirier, and G. Royo, J. Organomet. Chem., 311, C39 (1986).
b) R. J. P. Corriu, G. F. Lanneau, and M. Perrot, Tetrahedron Lett.,
28, 3941 (1987). o) F. Sato, T. Jinbo, and M. Sato, Synthesis,
1981, 871. p) A. O. Bedenbaugh, J. H. Bedenbaugh, W. A.
Bergin, and J. D. Adkins, J. Am. Chem. Soc., 92, 5774 (1970). q)
J. H. Wagenknecht, J. Org. Chem., 37, 1513 (1972).
p-Cyanobenzaldehyde: Yield 241 mg (92%). 1H NMR δ
10.1 (s, 1H), 7.91–7.95 (m, 2H), 7.76–7.80 (m, 2H); 13C{1H}
NMR δ 190.6, 138.7, 132.9, 129.9, 117.67, 117.61; IR (KBr disc)
2754 (νC(=O)H), 1722 (νHC=O) cm−1, Found: C 73.26, H 3.72, N
10.83%, Calcd for C8H5NO: C 73.27, H 3.84, N 10.68%; MS (EI,
70 eV): Found m/z 131.0337 (M), Calcd for C8H5NO: 131.03711
(M).
CHO(CH2)10CHO: Yield 147 mg (75%). 1H NMR δ 9.74
(2H, J = 1.8 Hz), 2.40 (4H, J = 7.3, 1.8 Hz), 1.52–1.66 (4H, m),
1.22–1.34 (12H, m); 13C{1H} NMR δ 202.9, 43.8, 29.19 and
29.18 (overlapping), 29.0, 21.9; IR (neat) 2750 (νC(=O)H), 1713
(νHC=O) cm−1; Found: C 73.08, H 11.39%. Calcd for C12H22O2: C
72.68, H 11.18%; FAB-MS: Found m/z 199.1715 (M+H), Calcd
for C12H22O2: 198.16186 (M).
This study was supported by grants from the Ministry of Ed-
ucation, Science, Sports and Culture and from Nippon Zeon
Co., Ltd. The support of the work by Sankio Co. as Chemical
Award in Synthetic Organic Chemistry, Japan (2000) given to
K. N. is gratefully acknowledged. We also thank the Material
Characterization Central Laboratory of Waseda University for
measurement of FAB-MS and elemental analyses and for tech-
nical guidance.
5
Some examples of catalytic hydrogenation of carboxylic
acids to alcohols: a) D.-H. He, N. Wakasa, and T. Fuchikami, Tet-
rahedron Lett., 36, 1059 (1995). b) M. Bianchi, G. Menchi, F.
Francalanci, F. Piacenti, U. Matteoli, P. Frediami, and C. Botteghi,
J. Organomet. Chem., 188, 109 (1980).
6
a) T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, and
T. Maki, App. Catal. A, 88, 149 (1992). b) N. Ding, J. Kondo, K.
Maruya, K. Domen, T. Yokoyama, N. Fujita, and T. Maki, Catal.
Lett., 17, 309 (1993). c) T. Maki, M. Nakajima, T. Yokoyama, and
T. Setoyama (Mitsubishi Chemical Co.), Jpn. Pat. 62-108832
(1987). d) T. Yokoyama, N. Matsuyama, and T. Maki, Jpn. Pat. 4-
210936 (1992). e) T. Yokoyama and K. Fujii, PETROTECH (in
Japanese), 14, 633 (1991).
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