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Chemistry Letters Vol.38, No.5 (2009)
Palladium(II)-catalyzed Selective Oxidation of ꢀ,ꢁ-Unsaturated Aldehydes
to ꢀ,ꢁ-Unsaturated Carboxylic Acids with Hydrogen Peroxide
Yoshihiro Kon, Daisuke Imao, Takuya Nakashima, and Kazuhiko Satoꢀ
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
(Received February 5, 2009; CL-090129; E-mail: k.sato@aist.go.jp)
Palladium(II)-catalyzed chemoselective oxidation of ꢀ,ꢁ-
unsaturated aldehydes with hydrogen peroxide to give ꢀ,ꢁ-
unsaturated carboxylic acids was performed. Cinnamaldehyde
was effectively catalyzed by palladium(II) trifluoroacetate to
generate cinnamic acid in 92% yield under organic solvent-free
conditions. The reaction appears to be applicable to various ꢀ,ꢁ-
unsaturated aldehydes.
Table 1. Various catalysts tested for the oxidation of cinnamal-
dehyde with hydrogen peroxidea
Entry
1d
Catalyst
Pd(tfa)2
Yield/%b
Selectivity/%c
73
92
92
70
65
13
6
97
96
95
86
86
43
67
0
2
3e
4
Pd(OAc)2
Pd(tfa)2, NaClf
Pd(PPh3)4
PdCl2
5
6
7
ꢀ,ꢁ-Unsaturated carboxylic acids are important com-
pounds, known to be some of the most valuable intermediates
and precursors for chemical production and pharmaceuticals.1
Despite the oxidation of aldehydes into carboxylic acids being
generally considered to be a simple and downhill reaction,2 sev-
eral methods that have been developed for the preparation of
ꢀ,ꢁ-unsaturated carboxylic acids from their aldehyde deriva-
tives require severe and complex reaction conditions.3 In addi-
tion, they use more than equimolar amounts of oxidants, leading
to the formation of an equimolar co-product as waste.3 As a part
of the green concept, such toxic oxidants are being replaced by
alternative less-toxic reagents.4 Hydrogen peroxide (H2O2) is an
ideal oxidant because water is the only side product and the atom
efficiency is excellent.5,6 To our knowledge, however, few syn-
thetic approaches using H2O2 have been reported that are cata-
lyzed by Mo and/or W complexes,7 phase-transfer catalysts,8
SeO2,9 or benzeneseleninic acid10 for the oxidation of alde-
hydes,11 and there is no example dealing with ꢀ,ꢁ-unsaturated
aldehydes as substrates with good chemo-selectivity.12–14 There
is no such example because these compounds undergo many
side-reactions (e.g. epoxidation of aldehydes,10,15 of acids16
and the Michael addition15,17) due to their reactive conjugated
double bond. They can also give alcohols and formic acid via
the Baeyer–Villiger type fragmentation through hydrolysis.18
We herein report a practical and simple process for the selective
oxidation of ꢀ,ꢁ-unsaturated aldehydes to their carboxylic acids
using H2O2 (30% in water) as a terminal oxidant that is effec-
tively catalyzed by Pd(tfa)2 (tfa = trifluoroacetate) under or-
ganic solvent-free conditions.
8
None
0
aReaction conditions: (E)-cinnamaldehyde (2.0 mmol), 30% H2O2
(4.0 mmol), catalyst (0.040 mmol), 10 ꢁC, 1500 rpm, 2 h, unless other-
wise stated. bYield and conversion on the basis of (E)-cinnamaldehyde,
determined by GC analysis with biphenyl as internal standard. cYield/
conversion (%). d30% H2O2 (2.0 mmol) was used. e30% H2O2
(6.0 mmol) was used. fNaCl (0.080 mmol) was added.
activity for the oxidation of (E)-cinnamaldehyde (Entry 2). The
addition of 1-octene retarded this oxidation of (E)-cinnamal-
dehyde (12% yield) due to the strong coordination of olefin
moiety of 1-octene to Pd(tfa)2. PdCl2 was not effective for the
catalytic synthesis of cinnamic acid because of lower solubility
toward the organic (substrate) phase (6% yield, Entry 7).19 The
addition of NaCl to the H2O2–Pd(tfa)2 catalytic system appa-
rently retarded the oxidation, probably due to the generation of
PdCl2 in situ (Entry 5). In contrast, Pd0 complex, Pd(PPh3)4,
was found less effective for the oxidation of (E)-cinnamaldehyde
(13% yield, Entry 6). The amount of H2O2 is also important
for the catalysis, and we found that using 2 molar equivalents
of H2O2 results in optimal performance (Entries 1–3). It is note-
worthy that no reaction takes place without the catalyst in spite
of the presence of the oxidant with an aldehyde, showing that
cationic palladium does accelerate the reaction with excellent
chemoselectivity.
CHO
+
30% H2O2 (2.0 mol equiv)
ð1Þ
A mixture of (E)-cinnamaldehyde (265 mg, 2.0 mmol) and
Pd(tfa)2 (13.2 mg, 0.040 mmol) placed in a test tube was stirred
at 1500 rpm with a magnetic stirrer at 10 ꢁC, followed by the ad-
dition of 30% H2O2 (453 mg, 4.0 mmol). After vigorous stirring
for 2 h, (E)-cinnamic acid was produced in 92% yield (96%
selectivity, GC analysis) (eq 1 and Table 1, Entry 2). The reac-
tion proceeded highly chemoselectively to give (E)-cinnamic
acid, and benzaldehyde (2% yield) and benzoic acid (1% yield)
were the only side products, in spite of the organic solvent-free
conditions.15 Table 1 shows the activities of various catalysts.
Pd(OAc)2 showed better catalytic activity (70% yield, Entry 4)
than those of copper(II) and nickel(II) acetate complexes (0%
yields). Among the PdII complexes, Pd(tfa)2 exhibited excellent
COOH
Pd(tfa)2 (0.02 mol equiv)
10 °C, 2 h
Yield 92%
This oxidation system was well applicable to the various
ꢀ,ꢁ-unsaturated aldehydes to give the corresponding carboxylic
acids (Table 2).20 Aliphatic ꢀ,ꢁ-unsaturated aldehydes were oxi-
dized to the corresponding carboxylic acids in good yields (71,
80, and 73% yields for C6, C8, and C9 ꢀ,ꢁ-unsaturated alde-
hydes, respectively, Entries 1, 2, and 4). The reaction with
(E)-2-methyl-2-butenal proceeded to generate tiglic acid (47%
yield, Entry 6). Acrolein was effectively oxidized to generate
acrylic acid in 64% yield (Entry 7). Solid ꢀ,ꢁ-unsaturated alde-
Copyright Ó 2009 The Chemical Society of Japan