CL-151024
Received: November 4, 2015 | Accepted: December 1, 2015 | Web Released: December 5, 2015
Iron-catalyzed Selective Oxidation of α,β-Unsaturated Aldehydes
to α,β-Unsaturated Carboxylic Acids by Molecular Oxygen
Shinji Tanaka, Yoshihiro Kon, Yumiko Uesaka, Ryo Morioka, Masanori Tamura, and Kazuhiko Sato*
Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST),
Central 5, Higashi 1-1-1, Tsukuba
(E-mail: k.sato@aist.go.jp)
Selective oxidation of α,β-unsaturated aldehydes to α,β-
unsaturated carboxylic acids was performed using O2 as the
oxidant in the presence of a simple iron catalyst. The addition
of an alkali metal carboxylate as a cocatalyst enhanced the
selectivity for the desired product. Redox tuning of the iron
catalyst via association with the alkali metal led to a controlled
radical generation during the catalytic O2 oxidation.
non-redox-active base metals might act as simpler catalysts
exhibiting enhanced activity in oxidation reactions. Herein, we
report the preparation of α,β-unsaturated carboxylic acids from
α,β-unsaturated aldehydes using a combination of O2 and a Fe
catalyst whose catalytic activity was induced by the simple
addition of alkali metal carboxylate.
During our study on oxidation of allylic alcohols by Fe
catalyst,14 we found that some product aldehydes oxidized into
carboxylic acid under air after standing a long time, without the
use of β-diketonate metal complex, which is a highly reactive
catalyst.11 Stimulated by this unexpected finding, we carried out
catalytic O2 oxidation of α,β-unsaturated aldehyde by simple Fe
salt, using trans-2-decenal (1a) as a model substrate (Table 1).
1a was treated with 0.01 equiv of Fe(NO3)3¢9H2O in EtOAc
solution at 25 °C under atmospheric pressure of O2, resulting in
the full conversion of the substrate and the formation of trans-
Molecular oxygen (O2) is a readily available, inexpensive
oxidant and is regarded as a promising natural resource for
oxygen-containing chemical products.1,2 Conventional chemical
processes including oxidations often use stoichiometric amounts
of hazardous oxidants, leading to the formation of equimolar
amounts of a by-product as waste. Although oxidation by O2
would proceed with 100% atom efficiency under ideal condi-
tions and thus generate no waste, such dioxygenase-type reac-
tions have been reported less frequently than monooxygenase-
and oxidase-type oxidation reactions, in which only one O atom
of O2 incorporates into substrate and O2 is consumed by re-
oxidation of catalyst, respectively.3 In particular, the synthesis
of value-added chemical products should be replaced by
dioxygenase-type reactions to reduce waste generated during
multistep reactions.
α,β-Unsaturated carboxylic acids are among the most
valuable intermediates and precursors for chemical production
and pharmaceuticals.4 Although the oxidation of aliphatic
aldehyde into carboxylic acids proceeds by O2 even without
a catalyst,5,6 several methods have been developed for the
preparation of α,β-unsaturated carboxylic acids from their
aldehyde derivatives, requiring severe and complex reaction
conditions.7 Recently, N-heterocyclic carbene (NHC)-catalyzed
O2 oxidation reactions of α,β-unsaturated aldehyde were
developed by several groups, yet more than equimolar amounts
of base were essential for good reaction yields.8 From the
standpoint of green-sustainable chemistry, catalytic oxidation
using O2 without large amounts of base is promising even if
an organocatalyst could be employed.9 Nobile et al. reported a
Fe catalyst bearing 2-(acetoacetoxy)ethyl methacrylate ligand-
catalyzed O2 oxidation of trans-2-hexenal; however, substrates
were limited and a halogenated solvent was required.10
Mukaiyama et al. proposed the effectiveness of β-diketonate
ligand to activate metal-catalyzed oxidation.11 More useful
methods and systems to activate a base-metal complex must be
developed to realize practical O2 oxidation for the production of
valuable fine chemicals.
1
2-decenoic acid (2a) in 49% yield (Entry 2). H NMR of the
product mixture indicated the presence of trans-2-perdecenoic
acid (25%), and octanoic acid (12%) as by-products, which
have been generated by undesired oxidation of the olefin part.6c
No oxidation occurred in the absence of the catalyst (Entry 1).
When 0.2 equiv of CH3COONa was added in this catalysis,
product yield and selectivity were improved to 76% and 77%,
respectively (Entry 3), while other sodium salts, NaNO3 and
NaOTf, were less effective (Entries 4 and 5).15 For other Fe salts,
[Fe(acac)3] catalyzed the oxidation to yield 2a in 55% yield with
56% selectivity (Entry 6), and Fe(OAc)2 did not show catalytic
activity due to low solubility (Entry 7). CF3COONa, a more
electronegative carboxylate salt, increased product yield and
selectivity (Entry 8), and thus trifluoroacetic acid and its salts
were screened as additives. Interestingly, higher yields and
selectivity were achieved when salts containing larger mono-
cations were added (Entries 8, 10, 11, and 12), whereas the
addition of acid (Entry 9) or Zn salt (Entry 13) was less
effective. A higher substrate concentration (2 M) induced higher
yield and selectivity of 2a in the presence of Na and K salts
(Entries 14 and 15). Selective oxidation proceeded even under
air, giving 2a in 72% yield with 86% selectivity (Entry 17).
Moreover, 2a was not generated under Ar (Entry 18).
After optimization of catalysis conditions, we screened the
substrates (Table 2). α,β-Unsaturated aldehydes having linear
alkyl groups, such as crotonaldehyde (1b), trans-2-pentenal
(1c), and trans-2-hexenal (1d), were oxidized to corresponding
carboxylic acids in good yields with high selectivity (Entries 1-
3). 3-Methyl-2-butenoic acid (2e), which includes reactive
trisubstituted olefin, was obtained at lower yield (Entry 4). For
aromatic aldehydes, benzaldehyde (1f) and 2-thiophenecarbox-
aldehyde (1g) were oxidized into corresponding acids 2f and
2g in moderate-to-high yields though oxidation of 1g required
longer reaction time (Entry 6). Aldehydes having an alkyne
It is widely accepted that non-redox metal ions have
considerable effect on the redox state of redox-active metal,
promoting electron-transfer reactions12 as well as catalytic
reactions.13 An appropriate combination of redox-active and
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