Full Papers
doi.org/10.1002/cctc.202100644
ChemCatChem
Highly Ordered Mesoporous Cobalt Oxide as
Heterogeneous Catalyst for Aerobic Oxidative
Aromatization of N-Heterocycles
Yue Cao,[a] Yong Wu,[a] Yuanteng Zhang,[a] Jing Zhou,[a] Wei Xiao,[b] and Dong Gu*[a]
N-heterocycles are key structures for many pharmaceutical
intermediates. The synthesis of such units normally is con-
ducted under homogeneous catalytic conditions. Among all
methods, aerobic oxidative aromatization is one of the most
effective. However, in homogeneous conditions, catalysts are
difficult to be recycled. Herein, we report a heterogeneous
catalytic strategy with a mesoporous cobalt oxide as catalyst.
The developed protocol shows a broad applicability for the
synthesis of N-heterocycles (32 examples, up to 99% yield), and
the catalyst presents high turnover numbers (7.41) in the
absence of any additives. Such a heterogenous approach can
be easily scaled up. Furthermore, the catalyst can be recycled
by simply filtration and be reused for at least six times without
obvious deactivation. Comparative studies reveal that the high
surface area of mesoporous cobalt oxide plays an important
role on the catalytic reactivity. The outstanding recycling
capacity makes the catalyst industrially practical and sustainable
for the synthesis of diverse N-heterocycles.
Introduction
heterogeneous catalysts have caused extensive research due to
their reusability and simple preparation process.[4] Recently,
reported heterogeneous Pt, Pd3Pb, Rh, Au and Ru catalysts were
well-documented for this reaction.[5] However, the majority of
the examples rely on precious metals and display a narrow
substrate scope. The synthetic potentials of the heterogeneous
catalysis employing earth-abundant metals remain to be further
explored.
Catalytic dehydrogenation of saturated organic molecules, such
as N-heterocycles, is of great importance for generating
valuable aromatic N-heterocycles. Because aromatic N-hetero-
cycle is a key skeleton structure in a range of pharmaceuticals,
bioactive heterocycles, and natural products. Therefore, tremen-
dous efforts have been paid to develop efficient catalytic
methods for constructing such structures.[1]
Since the advent of mesoporous materials, they play a key
role in catalysis due to their unique structural features, such as
high surface area, uniform and tunable pore size and shape,
large pore volume, and easy functionalization.[6] Especially
mesoporous metal oxide materials, which possess intrinsic
features such as high concentration of active sites and large
pore size can expedite adsorption and mass transfer, are rarely
investigated compared to the corresponding bulk metal oxide
catalysts.[6l] In the continuous interest to explore mesoporous
metal oxide materials for earth-abundant transition metals
catalysts, we have focused our attention on developing efficient
Catalytic acceptorless dehydrogenation has been proved to
be
a powerful process to access the corresponding N-
heterocycles.[2] Despite the satisfactory performance acquired,
compared with the catalytic acceptorless dehydrogenation
route, the oxidative dehydrogenation protocol could lower the
thermodynamic energy barriers required with the assistance of
active oxygen species. Thus, relative mild reaction conditions
could be achieved in the oxidative dehydrogenation trans-
formation. Aerobic oxidative aromatization of N-heterocycles is
another fundamental approach which needs metal catalysts
and stoichiometric oxidant such as O2. In spite of excellent
catalytic activity at laboratory,[3] homogeneous catalysts are
often limited by the difficulty to recover and scale up, complex
steps of the synthesis of the ligand and high cost. As a contrast,
metal oxide catalysis by the nanocasting route.[7] As
a
representative earth-abundant transition metal oxide, mesopo-
rous cobalt oxides have a novel spinel structure and mix
valence of Co2+ and Co3+ [8]
which exhibit excellent catalytic
,
performances for CO oxidation,[7a] methanol oxidation[9] and
selective oxidation of benzyl alcohol.[10] In this work, we
synthesized a series of mesoporous Co3O4 materials by a surface
casting process method, which could be easily scaled up to
gram level. The specifically designed mesoporous Co3O4
possesses high surface area and highly ordered
mesostructure,[11] which exhibits excellent catalytic activity for
dehydrogenation of N-heterocycles and have considerable
catalyst recovery efficiency. Air is employed as the terminal
oxidant, and no additional additive, high turnover numbers
(7.41) and high substrate scope (32 examples) make the catalyst
superior to the present catalytic systems.
[a] Y. Cao, Dr. Y. Wu, Y. Zhang, J. Zhou, Prof. D. Gu
The Institute for Advanced Studies
Wuhan University
No. 299, Bayi Road
Wuhan 430072, (P. R. China)
E-mail: DGu@whu.edu.cn
[b] Prof. W. Xiao
Hubei Key Laboratory of Electrochemical Power Sources
College of Chemistry and Molecular Sciences
Wuhan University
No. 299, Bayi Road
Wuhan 430072, (P. R. China)
Supporting information for this article is available on the WWW under
ChemCatChem 2021, 13, 1–9
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