DOI: 10.1002/cctc.201600362
Full Papers
Mixed Picolinate and Quinaldinate Iron(III) Complexes for
the Catalytic Oxidation of Alcohols with Hydrogen
Peroxide
Shinji Tanaka, Yoshihiro Kon, Atsuko Ogawa, Yumiko Uesaka, Masanori Tamura, and
Kazuhiko Sato*[a]
A series of Fe mixed picolinate and quinaldinate complexes
that catalyze the H2O2 oxidation of alcohols are presented. The
Fe catalysts are generated in situ by simple mixing of iron ace-
tate with 6-methylpicolinic acid (6-MepicH) and 4-chloropico-
linic acid (4-ClpicH); they showed high catalytic activity for
H2O2 oxidation of 1-phenylethanol to acetophenone. Fe com-
plexes generated in situ as precatalysts were successfully iso-
lated and structurally characterized. Based on the single-crystal
X-ray analysis and electrochemical measurements of the Fe
complexes, the lability of picolinate and/or quinaldinate ligand
and the redox potential of FeIII were found to be important
factors for the catalytic reaction.
Introduction
Iron is an ideal transition-metal catalyst because of its abun-
dance, low expense, and low toxicity, and hence, catalysts for
a variety of catalytic transformations of organic compounds
have been replaced by iron catalysts.[1] In particular, significant
attention has been paid to iron in the field of oxidation
chemistry owing to its remarkable role as a constituent of en-
zymes that catalyze extremely mild and selective oxidation re-
actions.[2] Recently, a variety of iron complexes have been re-
ported as effective oxidation catalysts.[3] Low-valent iron com-
plexes bearing well-designed ligands have been shown to ex-
hibit high catalytic activity for CÀH oxidation, olefin epoxida-
tion, dihydroxylation and so on, yet the difficulty of handling
these catalysts as well as the necessity of multistep syntheses
of ligand/catalyst have hampered their practicality, even
though iron itself is a promising transition metal. There is thus
need for a simpler iron catalyst to develop truly practical or-
ganic transformation processes. In fact, iron-catalyzed oxida-
tion reactions have been investigated for over a century and
are known as Fenton[4] and GIF-type oxidation reactions;[5]
these reactions are easily realized by the mixing of readily
available salts and oxidants. The combination of an iron salt,
picolinic acid (picH), and H2O2, known as the GoAggIII system,
was developed by Barton et al.[6] and Stavropoulos et al.[7] Be-
cause of its potential practicality, this system is still being in-
vestigated to determine its reaction mechanism, but so far it
has only been applied to the oxidation of several alkane sub-
strates. In relation to GoAggIII-type complexes, Fe complexes
associated with quinaldinic acid (qnH) have also been report-
ed.[8] Although Suzuki et al. observed some interesting behav-
ior of these peroxide species, their application to the catalysis
of oxidation reactions has rarely been explored.[8a]
Oxidation of alcohols is one of the most important transfor-
mations of organic compounds, and a variety of methodolo-
gies have been developed for its catalysis.[9] Recently, a combi-
nation of copper salts and organic radicals was found to be an
efficient catalyst for oxidation of alcohols by O2,[10] and some
substrates have even been catalytically oxidized by using or-
ganic radicals alone, without the need of metal salts.[11] Consid-
ering the relatively high cost of organic radical compounds
and the toxicity of copper, however, iron complexes with H2O2
would be a desirable oxidation catalyst in practical terms, and
catalytic systems exhibiting higher activity as well as higher se-
lectivity are still desired.[12]
In general, for transition-metal-based molecular catalysis, the
catalysts are optimized by the repeated design and synthesis
of suitable ligands for specific transformations. In contrast, in
situ generated catalysts comprising more than three com-
pounds can be optimized simply by changing the type, combi-
nation, and ratio of the components.[13] Moreover, such cata-
lysts can be easily handled and reproduced, which are advan-
tages for practical application. In our recent communication,
iron-catalyzed selective oxidations of styrenes and allylic alco-
hols under simple and mild reaction conditions by using
a small excess amount of H2O2 as an oxidant were achieved.[14]
Slight modification of the GoAggIII system, namely, the use of
a combination of two kinds of picolinic acid as ligands for the
iron catalyst, was essential for achieving high reactivity and se-
lectivity. In this study, we further optimized the catalytic per-
formance of Fe catalysts by simply employing a variety of pico-
linic/quinaldinic acids as ligands (Figure 1), and determined the
best catalyst combination for the Fe-catalyzed H2O2 oxidation
of alcohols. In addition, several mixed-ligand iron complexes
[a] Dr. S. Tanaka, Dr. Y. Kon, A. Ogawa, Y. Uesaka, Dr. M. Tamura, Dr. K. Sato
Interdisciplinary Research Center for Catalytic Chemistry
National Institute of Advanced Industrial Science and Technology (AIST)
Central 5, Higashi 1-1-1, Tsukuba (Japan)
Supporting information for this article can be found under http://
ChemCatChem 2016, 8, 1 – 10
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