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Y. Zhang et al. / Catalysis Communications 11 (2010) 951–954
Table 1
Aerobic oxidation of benzylamine to benzonitrile and benzyamide in water.
Scheme 1. Ru catalyzed aerobic oxidation primary amines to nitrils in water or in PhCF3.
Entry
Catalyst
Conv. [%]
1a [%]
1b [%]
material from particles with diameter of average 40 nm, BET surface
area: 60 m2/g) were purchased from Nanjing Emperor Material Co.
Ltd. The other reagents were purchased from Aladdin Reagent, Sigma-
Aldrich Company and Alfa-Aesar Company without further
purification.
1
Ru(OH)x/anatase
Ru(OH)x/anatase
Ru(OH)x/anatase
Ru(OH)x/anatase
Ru(OH)x/ceria
Ru(OH)x/Al2O3
Ru(OH)x/anatase
No
100
100
98
93
80
78
76
57
83
90
0
7
19
6
5
6
13
10
n. d
2a
3b
4c
5d
6
94
100
100
100
0
7f
8
2.2. General procedure for preparation of Ru(OH)x/anatase and
Ru(OH)x/ceria
Reaction conditions: benzylamine, 0.25 mmol; Ru catalyst (Ru: 5 mol%); water, 1.0 mL,
100 °C; under 1 atm O2; reaction time: 10 h. Conversion and selectivity were
determined by GC using an internal standard.
The anatase powder (0.5 g) was added to a 10 mL of aqueous
solution of RuCl3 (10.9 mM) with vigorously stirring at room
temperature. After 0.5 h, the pH value of the solution was adjusted
to 13 by addition of an aqueous solution of NaOH (3.0 M) and the
resulting slurry was stirred for 24 h at room temperature. The solid
was then filtered off, washed with a large amount of water, and dried
in vacuum to afford 0.35 g of Ru(OH)x/anatase as a pale green powder
(Ru content: 2.19 wt.%). The ceria supported ruthenium hydroxide
was prepared in the same way and 0.30 g of Ru(OH)x/ceria was
obtained as a brown powder (Ru content: 2.09 wt.%). The content of
Ru was detected by ICP-OES (inductively coupled plasma optical
emission spectroscopy). The ICP-OES analyses were performed with
Perkin Elmer OPTIMA 2000DV.
a
At 120 °C, 1 atm O2 in a sealed pressure tube.
At 80 °C, N-benzylbenzamide (10%).
1.0 atm air, N-benzylbenzamide (12%).
Benzaldehyde (15%) and N-benzylbenzamide (10%).
b
c
d
f
The reaction scaled up to 1.0 mmol benzylamine, isolated yield.
(Table 1, Entry 5). As reported by Mizuno, the oxidation gave mainly
benzamide by Ru(OH)x/Al2O3 at 140 °C in water, [32] but under our
condition (100 °C), the oxidation by Ru(OH)x/Al2O3 afforded mainly
benzonitrile in 83% yield with benzamide in 13% yield (Table 1, Entry
6). The results indicated that higher temperature enhanced the
hydrolysis of benzonitrile to benzamide for both Ru(OH)x/Al2O3 and
Ru(OH)x/anatase (Table 1, Entry 1, 2, 6). But Ru(OH)x/Al2O3 was more
active for the hydrolysis of benzonitrile than Ru(OH)x/anatase
(Table 1, Entry 1, 6), thus the Ru(OH)x/anatase was more selective
for benzonitrile. In fact, Mizuno reported the hydrolysis of benzoni-
trile to benzamide at 140 °C with Ru(OH)x/Al2O3 [3]. Although this
issue is still to be solved, the experimental fact is that at high reaction
temperature Ru(OH)x/Al2O3 was more active for hydrolysis of nitriles
to amides than Ru(OH)x/anatase. Through screening of the catalyst
systems, the best catalyst system was found to be the Ru(OH)x/anatase,
although the role of the supports was not investigated thoroughly. The
scaled-up oxidation of benzylamine (1.0mmol) by Ru(OH)x/anatase
afforded benzonitrile in 90% yield (Table 1, Entry 7), which showed the
practicality of the oxidation method. Benzonitrile was easily extracted
by diethyl ether from the water solution with N90% purity, and it can be
further purified by column chromatography on silica gel. Benzonitrile
was not formed in the absence of Ru catalyst or in the presence of
supports only.
2.3. Procedure for Ru(OH)x catalyzed aerobic oxidation of amines
An amine (0.25 mmol), Ru(OH)x/anatase (Ru: 5 mol%), and water
(1.0 mL) were placed in a Schlenk tube with a magnetic stir bar, and
the reaction was carried out at 100 °C under 1 atm of O2 or air (with a
balloon). After the reaction was completed, the Ru(OH)x/anatase was
separated by filtration, and the products were extracted by 3×1.0 mL
diethyl ether. The conversion and yield were determined by GC with
an internal standard (C16H34). Ru(OH)x/ceria (Ru: 3 mol%) catalyzed
aerobic oxidation of amines in PhCF3 was performed in a similar way.
GC analyses were performed on a Hewlett Packard 5890 instrument
with a FID detector and Hewlett Packard 24 m×0.2 mm i.d. HP-5
capillary column.
3. Results and discussion
3.1. Aerobic oxidation of amines to nitriles in water
With optimized reaction conditions in hand, we investigated the
application scope of the aerobic oxidation reaction. Several amines
were examined for oxygenation with Ru(OH)x/anatase in water
(Table 2). The transformation of benzylamines, which contain
methoxy, chloro and alkyl groups, proceeded efficiently to give the
corresponding benzonitriles in high yields (Table 2, Entry 2–5).
Oxidation of aliphatic amine requires longer time, and octan-1-amine
was converted into octanenitrile smoothly in 86% yield within 16 h
(Table 2, Entry 6).
Oxidation of benzylamine was performed as a model reaction with
O2 in water. Ru(OH)x/anatase catalyst showed the highest catalytic
activity and selectivity for the transformation of benzylamine to
benzonitrile. Ru(OH)x/anatase catalyzed oxidation of benzylamine
gave benzonitrile in 93% yield with benzamide (7%) by-product under
1 atm O2 at 100 °C in water (Table 1, Entry 1). And at a higher
temperature, 120 °C, the oxidation gave benzonitrile in 80% yield and
benzamide in 19% yield under 1 atm O2 (Table 1, Entry 2), which
implied some benzonitrile was hydrolyzed to benzamide. At a lower
temperature, 80 °C, besides benzonitrile (78%) and benzamide (6%),
N-benzylbenzamide was obtained in 10% yield, which indicated that
benzonitrile reacted with benzylamine before completion of the
oxidation (Table 1, Entry 3). Air used as oxidant instead of O2, the
reaction proceeded with a slight loss of the product selectivity (76%)
(Table 1, Entry 4), and some N-benzylbenzamide (12%) formed also.
Compared to Ru(OH)x/anatase, Ru(OH)x/ceria exhibited lower selec-
tivity, and the oxidation afforded benzonitrile in 57% yield with
benzamide (6%), benaldehyde (15%) and N-benzylbenzamide (10%)
3.2. Aerobic oxidation of amines to nitriles in benzotrifluoride
Afterwards, we tested the aerobic oxidation of amines also in
organic solvents. Ru(OH)x/ceria was found the best catalyst for the
transformation of benzylamine to benzonitrile among several cata-
lysts examined in benzotrifluoride. Ru(OH)x/ceria catalyzed oxidation
of benzylamine afforded benzonitrile in 94%, 93% and 85% yields at
120, 100 and 80 °C respectively in 10 h (Table 3, Entry 1, 2, 7). Instead
of Ru(OH)x/ceria, Ru(OH)x/anatase and Ru/Al2O3 [30] exhibited
slightly lower selectivity in benzotrifluoride (Table 3, Entry 3, 8).