Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 11819–11823
Iron Catalysis Very Important Paper
Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of Amines using
a Hybrid Hydroquinone/Cobalt Catalyst as Electron Transfer Mediator
Abstract: Herein we report the first FeII-catalyzed aerobic
biomimetic oxidation of amines. This oxidation reaction
involves several electron transfer steps and is inspired by
biological oxidation in the respiratory chain. The electron
transfer from the amine to molecular oxygen is aided by two
coupled catalytic redox systems, which lower the energy barrier
and improve the selectivity of the oxidation reaction. An iron
hydrogen transfer complex was utilized as the substrate-
selective dehydrogenation catalyst along with a bifunctional
hydroquinone/cobalt Schiff base complex as a hybrid electron
transfer mediator. Various primary and secondary amines were
oxidized in air to their corresponding aldimines or ketimines in
good to excellent yield.
literature of direct reoxidation of an SSRCred by O2 or
H2O2.[2,3] In Nature this oxidation problem is solved by
enlisting multiple enzymes and co-enzymes as electron trans-
fer mediators (ETMs), which lower the overall barrier for
electron transfer from the SSRCred to H2O2 or O2 as shown in
Scheme 1. In natural aerobic systems, these ETMs are part of
the respiratory chain, which is responsible for producing ATP
in many organisms. The overall process of the respiratory
chain is analogous to the process shown in Scheme 1 and ends
with reduction of O2 to H2O.[4] Over the years we have
developed a number of biomimetic oxidations that work
according to the principle shown in Scheme 1, where ruthe-
nium,[5] palladium,[6] and osmium,[7] have been used as
substrate-selective redox catalysts.[8]
O
xidation processes constitute an important fundamental
class of transformations in organic chemistry. Although
numerous oxidation reactions have been developed over the
years, the demand for milder, more efficient, and sustainable
methods has increased in recent times with the growing
interest in green chemical procedures.[1] With regard to green
methods, of particular interest are those inspired by biological
processes,[2,3] where environmentally friendly and inexpensive
oxidants such as molecular oxygen (O2) or hydrogen peroxide
(H2O2) are often used. However, direct selective oxidation of
an organic substrate by H2O2 or O2 remains an unmet
challenge because of the large energy barriers and low
selectivity of such direct oxidations. The use of a substrate-
selective redox catalyst (SSRC) may solve this problem,
where the reduced form of the SSRC (i.e. SSRCred) is re-
oxidized by H2O2 or O2. However, direct re-oxidation of the
SSRCred to SSRC by O2 or H2O2 may still be too slow and
there are only a limited number of examples known in the
Scheme 1. Principle for oxidation with O2 or H2O2 using ETMs
(ETM=electron transfer mediator; SSRC=substrate selective redox
catalyst).
Recently the groups of Beller and Bolm declared that the
age of iron had begun, and it is certainly true that the field has
advanced rapidly since then.[9] Over the past two decades,
inexpensive iron catalysts have been employed in many
elegant synthetic transformations which have traditionally
been dominated by noble transition metal catalysts, such as
cross-coupling and transfer hydrogenation, among many
others.[10–12] We have recently developed iron-catalyzed reac-
tions including DKR of sec-alcohols, cycloisomerization of
functionalized allenes, and biomimetic aerobic oxidation of
alcohols.[13,14] In the present work we have developed a novel
iron-catalyzed aerobic oxidation of amines via the biomimetic
approach in Scheme 1, where ETM1 and ETM2 are merged
into the bifunctional ETM I, which acts as a hybrid catalyst
(Scheme 2).
[*] Dr. A. Guꢀmundsson,[+] Dr. S. Manna,[+] Prof. Dr. J.-E. Bꢁckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
10691 Stockholm (Sweden)
E-mail: jeb@organ.su.se
Prof. Dr. J.-E. Bꢁckvall
Department of Natural Sciences
Mid Sweden University
85170 Sundsvall (Sweden)
A prominent class of iron catalysts that have been used for
transfer hydrogenation are the (cyclopentadienone)iron tri-
carbonyl complexes II, originally synthesized by Reppe and
Vetter in the 1950’s (Scheme 3).[15] Iron hydride complex III
was first prepared and isolated by the group of Knçlker.[16,17]
Knçlkerꢀs complex III and its related iron tricarbonyl
complexes II have found extensive use in transfer hydro-
genation reactions.[18,19] The first use of complex III in
catalysis was reported by the group of Casey in 2007 for the
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Angew. Chem. Int. Ed. 2021, 60, 11819 –11823
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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