10.1002/cctc.201801058
ChemCatChem
COMMUNICATION
Aerobic activation of C-H bond in amines over a nanorod
manganese oxide catalyst
Hai Wang [a], Liang Wang*[a], Sai Wang [a], Xue Dong [b], Jian Zhang [a,c] and Feng-Shou Xiao *[a,c]
process,[8] but there are still many by-products such as nitriles and
ABSTRACT: The development of heterogeneous catalysts for the
aldehydes. To the best of our knowledge, it has not been
synthesis of pharmaceutically relevant compounds is always
successful for efficient and selective oxidation of amines to
important for chemistry research. Here, we report a selective aerobic
amides over non-noble metallic heterogeneous catalysts yet.
oxidation of aromatic and aliphatic amines to corresponding amides
Herein, we show that the earth-abundant manganese oxide
over a nanorod manganese oxide (NR-MnOx) catalyst. The kinetic
nanorods (NR-MnOx) are highly efficient and selective for aerobic
studies reveal that the NR-MnOx catalyzed amine-to-amide reaction
oxidation of amines to amides with a sole by-product of water,
proceeds the oxidative dehydrogenation of the amines into nitriles,
exhibiting obviously sustainable features compared with
followed by hydrolysis of nitrile into amides. The NR-MnOx exhibits
traditional routes for production of amides in industrial processes
fast kinetics and high selectivities in these steps, as well as hinders
(Scheme 1), and outperforming the state-of-art heterogeneous
the by-product formation. More importantly, the NR-MnOx catalyst is
catalyst containing noble metal.
stable and reusable in the continuous recycle tests with water as a
sole by-product, exhibiting superior sustainability and significant
advancement to outperform the traditional amide production route in
acidic or basic media with toxic by-products.
The activation of C-H bond to form C=O species in organic
molecules has been paid much attention in pharmaceutical and
agrochemical industries.[1,2] However, most of these processes
are still carried out over homogeneous catalysts, which are costly
and environmentally unfriendly because of their difficulty in
separation and regeneration from the reaction system.[3]
Therefore, development of highly efficient heterogeneous
catalysts to replace the homogeneous ones is always an
important topic for the synthesis of pharmaceutically relevant
compounds, but it is still a challenge due to the unsatisfied
efficacy of heterogeneous catalysts.[4]
Scheme 1. Various routes for amide production.
The NR-MnOx was synthesized via co-precipitation of MnSO4
and KMnO4, giving weak and broad peaks in the XRD pattern
assigned to typical α-MnO2 crystals (Figure S1). TEM image of
the sample exhibits nanorod morphology (Figure S2), and N2-
sorption measurement demonstrates that the NR-MnOx has high
surface area (183 m2/g) with narrow pore diameter distribution
centralized at 8.3 nm (Figure S3) originated from the nanorod
aggregation.
As an important class of compounds for production of
peptides and proteins, anti-block reagents, and color pigments for
inks,[5] amides are traditionally prepared from reduction of
activated carboxylic acid derivatives with ammonia and the
rearrangements of ketoximes over liquid acids,[6] which usually
requires stoichiometric reagents with a huge amount of toxic by-
products. Solution of this problem is to develop sustainable routes
for production of amides over heterogeneous catalysts with a
complete avoidance of stoichiometric reagents and liquid acids.
The direct oxidation of amines by activation of C-H bond in
the α-methylene group has been regarded as a sustainable route
to produce amides,[7] but it still has a challenge owing to
occurrence of side-reactions such as over-oxidation,
condensation, and deamination.[8,9] To overcome these issues,
the stoichiometric RuO4 was used as an oxidant.[10] To enhance
the sustainability of this process, it has been employed molecular
oxygen rather than stoichiometric RuO4. Furthermore, it is
developed noble metal (ruthenium hydroxide) as a catalyst for this
We initially explored catalytic performances of various
catalysts in the oxidation of benzylamine to benzamide as a model
reaction at 130 °C, where benzonitrile, benzaldehyde, benzoic
acid, and imine were appeared as by-products (Table 1). In the
blank run without catalyst, amide is completely undetectable but
imine was formed with a yield of 15% from the side reaction of
condensation (entry 1). Notably, a series of manganese-based
catalysts are active for this reaction, but they display quite
different product selectivities. The NR-MnOx catalyst provides a
superior selectivity to benzamide at >99.9% with full conversion
of benzylamine (entry 2, Figure S4). In contrast, the other
manganese oxide catalysts of β-MnO2, commercial activated
MnO2, Mn2O3, Mn3O4, and MnO exhibit relatively low yields to
benzamide (entries 3-7, <66.2%, Figure S5). Even if the
homogeneous catalysts of manganese salts (KMnO4 and MnSO4)
are employed, the benzamide yields are still very low (7.6% over
KMnO4 and undetectable yield over MnSO4, entries 8 and 9).
When metal oxides of CeO2, Co3O4, and Fe3O4 (Figure S5) are
used, the desirable product of benzamide is undetectable (entries
10-12). Supported Au catalysts (Figure S6, Table S1), which have
been considered as one of highly efficient noble-metal catalysts
[a]
H. Wang, Dr. L. Wang, S. Wang, Dr. J. Zhang, Prof. F.-S. Xiao
Key Lab of Applied Chemistry of Zhejiang Province, Department of
Chemistry
Zhejiang University
Hangzhou 310028, China
E-mail: liangwang@zju.edu.cn; fsxiao@zju.edu.cn
X. Dong
Department of Chemistry and Biochemistry, Texas Tech University,
Texas 79409, United States.
[b]
[c]
Dr. J. Zhang, Prof. F.-S. Xiao
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing University of Chemical Technology
Beijing 100029, China
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