CL-160989
Received: November 2, 2016 | Accepted: December 13, 2016 | Web Released: February 3, 2017
Synthesis of Nitriles from Aerobic Oxidation of Amines Catalyzed
by Ruthenium Supported on Activated Carbon
Baoqiang Niu, Fei Lu, Hong-Yu Zhang, Yuecheng Zhang, and Jiquan Zhao*
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China
(E-mail: zhaojq@hebut.edu.cn)
Nitriles were synthesized from the aerobic oxidation of
amines over commercially available catalysts, which were
activated carbon-supported ruthenium catalysts (Ru/AC). The
5%Ru/AC catalyst can tolerate a wide range of substrates,
such as aromatic, aliphatic, and heterocyclic amines, and afford
the target nitriles in good-to-excellent yields. The 5%Ru/AC
catalyst was easily recovered and no ruthenium leaking took
place in the catalytic run.
ruthenium-based heterogeneous catalysts, such as Ru/hydroxy-
apatite,13 Ru/Al2O3,14 Ru(OH)3/Al2O3,15 Ru(OH)3/Fe3O4,4a
RuO2¢xH2O/Co3O4,16 Ru/TiO2,17 and Co3O4/NGr@C18 are
effective in the aerobic oxidation of amines to nitriles without
requiring the presence of any additives. The simple heteroge-
neous catalysts make purification of the products easy. Inspired
by the excellent performance of the ruthenium-based catalysts,
we evaluated some commercially available activated carbon-
supported ruthenium catalysts (Ru/AC) in the aerobic oxidation
of amines to nitriles without the presence of any additives. It was
found the Ru/AC catalysts showed high activity in the reaction,
and can tolerate a wide range of substrates, including aromatic,
aliphatic, and heterocyclic amines.
The Ru/AC catalysts were evaluated for the aerobic
oxidation of amines to nitriles with benzylamine as a model
substrate. Four catalysts with different ruthenium contents
supported on AC were compared under the same reaction
conditions. As shown in Table 1, all the catalysts showed high
activity in view of the conversion of benzylamine (higher than
99.9%), but the selectivity towards benzonitrile changed with
the ruthenium content of the catalysts. In the case of a ruthenium
content of 3% (Table 1, Entry 1), the selectivity towards
benzonitrile was 81.7% with benzaldehyde as the only by-
product. When the content was increased from 3% to 5%, the
selectivity of benzonitrile was maintained constant (Table 1,
Entries 1 and 2). Further increasing the ruthenium content led
to the decrease in the selectivity of benzonitrile. When the
ruthenium content was 7%, the selectivity towards benzonitrile
decreased to 79.7% due to the increase in the selectivity towards
benzaldehyde (Table 1, Entry 3). In the case of the ruthenium
content of 10%, a small amount of N-(benzyl)benzylimine
derived from the homocoupling of benzylamine was detected,
and the selectivity of benzonitrile was 77.6% (Table 1, Entry 4).
The 5%Ru/AC catalyst was then thoroughly investigated in the
subsequent experiments.
Keywords: Aerobic oxidation
| Nitrile | Ruthenium
Nitriles are important intermediates for the synthesis of
dyes, agricultural chemicals, fine chemicals, and polymers.1
Additionally, compounds containing a cyano group are used
as pharmaceuticals and functional materials.2 Nitriles are
generally prepared by nucleophilic reaction of substrates with
appropriate leaving groups such as organic halide, aryl sulfo-
nates, alcohols, diazonium salts, and nitro or amino compounds
with various metal cyanides.3 However, these methods need
stoichiometric or excessive amounts of polluting or toxic
reagents, harsh reaction conditions, and produce large amount
of by-products. Due to the important role of nitriles in organic
synthesis, exploring new environmentally friendly methods to
synthesize nitriles is still an important subject.
In recent reports, many novel protocols have been explored
to prepare nitriles, such as the dehydration of primary amides,4
oxidative dehydrogenation of benzylic alcohols in the presence
of various nitrogen source,5 and using NH3, DMF, or TMSCN
as the CN source for C-H cyanidation of arenes.6 Compared
with these methods, the catalytic oxidation of primary amines by
molecular oxygen is a more desirable route for the synthesis of
nitriles. In this respect, homogeneous catalytic systems, such as
the cyclometalated ruthenium(III) complex,7 has been reported.
However, this catalytic system is only active in the oxidation
of aromatic amines to the corresponding nitriles, meanwhile,
requires the presence of a base in the reaction. Though an amide-
derived NNN-Ru(II) hydride complex,8 a ruthenium-terpyridyl
complex,9 and an iron Schiff base complex10 have been found
efficient in the oxidation of amines to nitriles under mild
conditions, the complicated ligands are hard to synthesize,
which makes them less attractive for practical purposes.
Heterogeneous catalytic systems, mainly copper-, iron-, and
ruthenium-based catalyst systems, have been described. Stahl
et al. reported a Cu/nitroxyl system11 that enabled selective
formation of nitriles from diverse primary amines under room
temperature. However, it needs ligand, nitroxyl, and DMAP as
a base to coexist to realize the catalytic process. Red copper in
the presence of NH4Br as the cocatalyst4c and Fe2O3/NGr@C
catalyst12 are also found effective in the synthesis of nitriles
from amines. However, generally, many additives are required
to facilitate the reaction. It has been reported that several
Table 1. Evaluation of the catalystsa
O
CN
H
N
NH
Ru/AC
2
+
+
O
2
Selectivity/%b
Nitrile Aldehyde Imine
Conversionb
/%
Entry Catalyst
1
2
3
4
3%Ru/AC
5%Ru/AC
7%Ru/AC
10%Ru/AC
99.9
99.9
99.9
99.9
81.7
81.7
79.7
77.6
18.3
18.2
20.2
20.7
®
®
®
1.7
aConditions: benzylamine 1 mmol, Ru/AC 5 mol %, toluene
5 mL, reaction temperature 150 °C, 0.5 MPa O2, reaction time
b
4 h. Determined by GC, anisole as internal standard.
© 2017 The Chemical Society of Japan