Angewandte
Communications
Chemie
Catalytic Hydrogenation
A Reusable Co Catalyst for the Selective Hydrogenation of
Functionalized Nitroarenes and the Direct Synthesis of Imines and
Benzimidazoles from Nitroarenes and Aldehydes
Tobias Schwob and Rhett Kempe*
Abstract: The use of abundantly available transition metals in
reactions that have been preferentially mediated by rare noble
metals, for example, hydrogenations, is a desirable aim in
catalysis and an attractive strategy for element conservation.
The observation of novel selectivity patterns with such
inexpensive metal catalysts is especially appealing. Herein,
we report a novel, robust, and reusable cobalt catalyst that
permits the selective hydrogenation of nitroarenes in the
presence of highly hydrogenation-sensitive functional groups,
as well as the direct synthesis of imines from nitroarenes and
aldehydes or ketones in the presence of such substituents.
Furthermore, we introduce the first base-metal-mediated direct
synthesis of benzimidazoles from nitroarenes and aldehydes.
Functional groups that are easy to hydrogenate are again well
tolerated.
highly hydrogenation-sensitive, such as olefins, nitriles and
ketones. Furthermore, we introduce the first base-metal-
mediated direct synthesis of benzimidazoles from nitroarenes
and aldehydes.
Imines and their derivatives are an important class of
compounds. They have been used extensively as ligands[6] and,
because of their diverse reactivity, for the synthesis of
materials,[7] fragrances, fungicides, pharmaceuticals, and agri-
cultural chemicals.[8] Therefore, the development of novel
imine synthesis methods is of great interest.[9,10] Benzimida-
zoles and the development of efficient methods for their
synthesis are similarly important.[11] We have recently intro-
duced a variety of silicon carbonitride (SiCN) metal nano-
composite catalysts,[12] and have very recently introduced
highly active homogenous 3d-metal or base-metal catalysts
for reactions classically mediated by expensive noble
metals.[13]
Our novel cobalt nanocomposite catalyst was synthesized
in a two-step procedure. In the first step, an amidinato
cobalt(II) complex (Figure 1, top left) and a commercially
available polysilazane were dissolved in tetrahydrofuran
(THF), followed by crosslinking using dicumylperoxide
(DCP). After removal of the solvent under vacuum, the
sample was pyrolyzed under a constant nitrogen flow at
7508C. Inductively coupled plasma optical emission spec-
trometry (ICP-OES) measurements revealed 3.8 wt% cobalt
in the pyrolyzed sample. The change from the paramagnetic
behavior of the Co containing crosslinked polysilazane to
superparamagnetic properties in the nanocomposite was
confirmed by magnetic measurements (Figure 1, center
panels). This is in agreement with the presence of isolated
Co2+ ions after crosslinking and their reduction to small
metallic Co nanoparticles (NPs) during pyrolysis. The pres-
ence of small homogenously distributed metal NPs was
additionally verified via transmission electron microscopy
(TEM; Figure 1, top right). The mean Co particle size, as
measured by TEM, is 1.7 nm. The Co particle size distribution
is shown in Figure 1 (top right). After washing the as-
synthesized Co-SiCN nanocomposite material with an aque-
ous basic solution, a specific surface area of 320 m2 gÀ1
(Brunauer–Emmet–Teller) was observed (Figure S1 in the
Supporting Information). Pore-size distribution calculations
revealed a hierarchically structured Co catalyst containing
micro- and mesopores. Temperature-programmed reduction
(TPR) of the Co catalyst was performed next (Figure 1,
bottom left). The samples were heated under a reductive
atmosphere (95% N2, 5% H2, 5 KminÀ1) up to 5508C and
held for 1 h. Comparison to a cobalt oxide reference sample
indicated the presence of reducible cobalt oxide species in the
H
ydrogenation reactions are of significant and continuing
interest for the chemical industry and academic research.[1]
The hydrogenation of aromatic nitro compounds with reus-
able catalysts is the method of choice for the production of
aniline derivatives, an extremely important class of com-
pounds.[2] Corma and co-workers reported a breakthrough
with regard to functional-group tolerance in 2006.[3] They
applied gold catalysts and observed selectivity of over 95%
for hydrogenation of the nitro group in 3-nitrostyrene, 4-
nitrobenzaldehyde, 4-nitrobenzonitrile, and 4-nitrobenza-
mide. Beller and co-workers have recently shown that
heterogeneous catalysts based on abundantly available tran-
sition metals, such as iron[4] and cobalt[5] (3d metals or base
metals), can also mediate the highly selective hydrogenation
of nitroarenes. The replacement of expensive and rare noble
metals with base metals is key to a sustainable future, since it
helps to preserve our element resources. The use of such
abundantly available metals is especially attractive if novel
selectivity patterns are observed.
Herein, we report a novel, reusable, and robust Co
catalyst that permits the selective hydrogenation of nitro-
arenes in the presence of iodo, olefin, aldehyde, ketone, and
nitrile functional groups. More importantly, we describe the
first application of a base-metal catalyst for the direct
synthesis of imines from nitroarenes and aldehydes or ketones
in a method that is tolerant of functional groups regarded as
[*] T. Schwob, Prof. Dr. R. Kempe
Anorganische Chemie II - Katalysatordesign
Universitꢀt Bayreuth, 95440 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
Supporting information for this article can be found under:
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!