Aerobic oxidation of hydroxylamines with nanoporous gold catalyst as an efficient synthetic method of nitrones Dedicated to Professor Jiro Tsuji on the occasion of his receipt of the 2014 Tetrahedron Prize

Article abstract of DOI:10.1016/j.tet.2015.05.094

Abstract A facile preparation of nitrones has been achieved by use of unsupported nanoporous gold (AuNPore) as a heterogeneous catalyst through aerobic oxidation of N,N-disubstituted hydroxylamines with molecular oxygen as an oxidizing agent under mild conditions. A variety of amines were oxidized to the corresponding nitrones in high chemical yields. The catalyst is robust enough to be reused without leaching.

Full text of DOI:10.1016/j.tet.2015.05.094

Tetrahedron xxx (2015) 1e4
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Aerobic oxidation of hydroxylamines with nanoporous gold catalyst
as an efficient synthetic method of nitrones
a
b
b,
*
Salprima Yudha S , Indra Kusuma , Naoki Asao
a
Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Bengkulu, W.R Supratman, Kandang Limun, 38371 A Bengkulu,
Indonesia
WPI-Advanced Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile preparation of nitrones has been achieved by use of unsupported nanoporous gold (AuNPore) as
a heterogeneous catalyst through aerobic oxidation of N,N-disubstituted hydroxylamines with molecular
oxygen as an oxidizing agent under mild conditions. A variety of amines were oxidized to the corre-
sponding nitrones in high chemical yields. The catalyst is robust enough to be reused without leaching.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 26 March 2015
Received in revised form 19 May 2015
Accepted 25 May 2015
Available online xxx
Dedicated to Professor Jiro Tsuji on the oc-
casion of his receipt of the 2014 Tetrahedron
Prize
Keywords:
Nitrones
Nanoporous gold
Aerobic oxidation
Heterogeneous catalyst
Reusability
1. Introduction
times without leaching as well as any significant loss of the catalytic
activity. In comparison with supported gold nanoparticle catalysts,
Selective oxidation of organic molecules, such as alcohols and
amines, is one of the most fundamental and important trans-
formations in organic synthesis. For this purpose, it is highly de-
sirable to develop an effective and reusable heterogeneous catalyst
the nanoporous gold catalyst can be fabricated easily by selective
leaching of Ag from commercially available or handmade gold-
esilver alloys by just immersion in nitric acid. Particularly, due to
4
the robust 3D network structure composed of ligaments, it shows
1
5,6
together with cheap and environmentally friendly oxidizing agent.
high durability and reusability. These results encouraged us to
During the last decade, gold nanoparticles supported on suitable
explore the catalytic performance of AuNPore in other reactions,
and we focused on the oxidation of N,N-disubstituted hydroxyl-
amines, which are nitrogen-analogues of secondary alcohols. The
expected products are nitrone compounds, which are valuable
oxides have been studied well as efficient heterogeneous catalysts
2
for such oxidation reactions. However, they have drawbacks on the
stability because they have tendency to agglomerate to become
larger particles by repeating the reactions, resulting in the de-
activation. Recently, we were successful in using a monolithic
nanoporous gold material (AuNPore) as an efficient catalyst for
7
building blocks in organic synthesis. They can be used as elec-
8
trophiles in reactions of organometallic reagents as well as in 1,3-
9
dipolar cycloaddition reactions. Furthermore, they are useful as
3
10
11
aerobic oxidation of alcohols with molecular oxygen. The reaction
spin-trap reagents and therapeutic agents. Although a number
of preparation methods have been reported so far,1
methods with recyclable heterogeneous catalysts are still limited.
2e16
effective
proceeded smoothly with a variety of secondary alcohols under
mild conditions, and the corresponding ketone products were ob-
tained in good to high yields. The catalyst is reusable at least four
1
7
In this article, we report that AuNPore exhibits a remarkable cata-
lytic activity in the aerobic oxidation of hydroxylamines with mo-
lecular oxygen at atmospheric pressure, leading to nitrone
18
compounds in good to high yields. The catalyst is recoverable and
reusable many times with no leaching of gold.
*
Corresponding author. Tel./fax: þ81 22 217 6165; e-mail address: asao@m.to-
hoku.ac.jp (N. Asao).
http://dx.doi.org/10.1016/j.tet.2015.05.094
0
040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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2
S. Yudha S et al. / Tetrahedron xxx (2015) 1e4
2
. Results and discussion
In order to verify whether this catalytic performance in this
reaction comes from the solid phase of gold or the dissolved ones,
leaching experiment was conducted as described in Scheme 1.
Initially, AuNPore catalysed aerobic oxidation under standard
conditions was carried out using substrate 1a along with 1,3,5-
trimethoxybenzene as an internal standard, and allowed to react
for 30 min. At this stage, nitrone 2a was produced in 14% yield.
Then, a half volume of supernatant (without solid AuNPore cata-
lyst) was transferred into different reaction vial using syringe
equipped with filter unit, and both reaction solutions were allowed
to react under the same conditions for 1.5 h. In the reaction without
the catalyst, the chemical yield of 2a was unchanged from 14%. On
the other hand, 2a was produced nearly quantitatively in the re-
action with the catalyst. These results clearly showed that hetero-
geneous phase of gold catalyst became the active species for
catalytic performance, not the leached ones. This observation was
also supported by induced coupled plasmaemass spectrometry
The fabrication of AuNPore catalysts was performed through
a selective dealloying of silver as the less noble metal from
Au30Ag70 alloy with a thickness of 40
for 18 h at room temperature. A scanning electron microscopy
m
m using 70 wt % of nitric acid
4
(
SEM) image in Fig. 1a showed that the ligaments with average size
of 30 nm as well as nanoporous channels are formed uniformly
across the entire AuNPore. The result of energy dispersive X-ray
(
(
EDX) analysis showed that the composition of AuNPore is Au98Ag
in atomic %), in which the residual silver usually gives an effect in
2
the catalytic activity of AuNPore.
(ICP-MS) analysis that showed the presence of leached species of
gold in reaction solution could be negligible (50 ppb only).
Fig. 1. The Scanning electronic microscopy (SEM) images. (a) The freshly prepared
AuNPore. (b) The reused AuNpore in the reaction of 1a.
We examined the catalytic aerobic oxidation of N,N-dibenzyl-
hydroxylamines 1a with AuNPore catalyst, and the results were
summarized in Table 1. Treatment of 1a in CH
AuNPore at 60 C for 15 h under oxygen balloon resulted in the
3
CN with 10 mol % of
ꢀ
formation of N-benzyl-1-phenylmethanimine oxide 2a in 63% yield
(
entry 1). The chemical yield was increased to 98e99 % by using
nitromethane or toluene as solvents (entries 2e3). Furthermore,
the reaction rate was dramatically improved by use of MeOH as
a solvent and 2a was obtained in 99% yield after only 2 h (entry 4).
From these results, MeOH was confirmed as the best choice for
solvent in this reaction. Since no reaction occurred with AueAg
mother alloy, it is obvious that the nanostructure of AuNPore is
necessary to create the catalytic activity (entry 5). The reaction
proceeded even under air although the reaction became sluggish
(
7
entry 6). On the other hand, 2a was not produced under Ar (entry
). These results clearly showed that molecular oxygen behaved as
an oxidizing agent in this reaction, and ordinary stoichiometric
Scheme 1. The leaching experiment.
oxidants, such as H
2
O
2
and N-methylmorpholine-N-oxide (NMO),
We next examined the reusability of the catalyst. After com-
pletion of the reaction of 1a, the catalyst was recovered by simple
filtration of the reaction mixture, followed by washing the catalyst
with acetone several times. After dryness under vacuum condi-
tions, the catalyst was reused. The reaction proceeded with the
recovered catalyst, and the corresponding product 2a was obtained
in high yields even after repeating the reaction 8 times (Fig. 2). This
are not necessary.1
2e16
Table 1
a
Aerobic oxidation of N,N-dibenzylhydroxylamine 1a with AuNPore catalyst
Entry
Catalyst
Solvent
Time
Yield of 2a (%)b
1
2
3
4
5
AuNPore
AuNPore
AuNPore
AuNPore
AueAg alloy
AuNPore
AuNPore
CH
CH
3
CN
NO
15 h
15 h
15 h
2 h
2 h
2 h
63
98
99
99
0
3
2
Toluene
MeOH
MeOH
MeOH
MeOH
c
6
7
34
Trace
d
2 h
a
ꢀ
C
Reaction conditions: 1a (0.15 mmol), catalyst (10 mol %), solvent (2 mL), at 60
2
under O .
b
2 2
Determined by NMR using CH Br as an internal standard.
The reaction was conducted under air.
The reaction was conducted under Ar.
c
d
Fig. 2. Reusability of AuNPore in the oxidation of 1a.
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S. Yudha S et al. / Tetrahedron xxx (2015) 1e4
3
observation clearly showed that AuNPore was a robust catalyst
with high durability in this reaction. The SEM image of the re-
covered catalyst also showed that the nanostructure of AuNPore
was unchanged even after 8 times reuse (Fig. 1b).
gold has been demonstrated for the first time. The oxidation re-
action proceeded smoothly in high chemical yields with tolerance
to different functional groups under mild conditions. Furthermore,
the gold catalyst was recoverable and reusable many times with no
leaching of gold.
With the optimized reaction condition established in entry 4 of
Table 1, we investigated the scope of this catalytic reaction with
other hydroxylamines, and the results are summarized in Table 2.
The reaction of N-benzyl-N-butylhydroxylamine 1b proceeded
smoothly, and N-butyl-1-phenylmethanimine oxide 2b was ob-
tained selectively in high yield, and the regioisomer, N-benzylbu-
tan-1-imine oxide, was not produced (entry 1). A bulky t-butyl
group of 1c has no influence on this reaction, and the corre-
sponding 2c was obtained in high yield (entry 2). Not only aliphatic
but also aromatic group is tolerant in this reaction, and benzyl
phenyl hydroxylamine 1d was oxidized in high yield (entry 3). N,N-
dialkyl substituted symmetric substrate 1e gave 2e in good yield
4
4
. Experimental
.1. General
Scanning electron microscopy (SEM) observation was carried
out using a JEOL JSM-6500F instrument operated at an accelerating
voltage of 30 kV. TEM characterization was performed using a JEM-
2100 TEM JEOL (JEOL, 200 kV) equipped with double spherical
aberration (Cs) correctors for both the probe-forming and the
image-forming lenses. The XPS measurements were carried-out
using a VG ESCALAB 250 spectrometer (Thermo Fisher Scientific
K. K.) employing monochromatic Al K X-ray radiation. The base
(
entry 4). The 6-membered cyclic hydroxylamine 1f gave the cor-
responding nitrone 2f in good yield (entry 5). In the reaction of 3,4-
dihydroisoquinolin-2(1H)-ol 1g, 3,4-dihydroisoquinoline 2-oxide
ꢁ
8
1
pressure of the analysis chamber was less than 10 Pa. H NMR and
2
g was obtained in 91% yield, and the unconjugated regioisomer
13
C NMR spectra were recorded on JEOL JNM AL 400 (400 MHz)
spectrometers. Column chromatography was carried out employ-
ing neutral silica gel 60 N (spherical, 40e100 m, KANTO Chemical
was not produced (entry 6).
m
Table 2
Co.) and basic silica gel (DM2035, Fuji Silysia Chemical Ltd.). Ana-
lytical thin-layer chromatography (TLC) was performed on glass
TLC silica gel 60 F254 (neutral, Merck KGaA) and basic TLC plates
a
Aerobic oxidation of a variety of hydroxylamines with AuNPore catalyst
(
(
Chromatorex, Fuji Silysia Chemical Ltd.). Au (99.99%) and Ag
99.99%) were purchased from Tanaka Kikinzoku Kogyo K.K. and
Mitsuwa’s Pure Chemicals, respectively. Structures of known
1
13
products were identified by H and C NMR and HRMS (or
GCeMS).
Entry
1
Hydroxylamines
Nitrones
Yield of 2 (%)b
4.2. Fabrication of AuNPore catalyst
1b
1c
2b
2c
85
Au (99.99%) and Ag (99.99%) were melted with electric arc-
melting furnace under Ar atmosphere to form Au/Ag alloy (30:70,
2
91
in at. %), which was milled down to thickness of 40
mm. The
ꢀ
resulting foil was annealed at 850 C for 20 h. The foil was cut into
2
small pieces (3ꢂ3 mm ). Treatment of the resulting chips (50 mg)
with 70 wt % nitric acid (5.6 mL) for 18 h at room temperature
resulted in the formation of the nanoporous structure by selective
leaching of silver. The materials were washed successively with
pure water and acetone. Drying of the materials under reduced
pressure gave the nanoporous gold (24.3 mg) and its composition
3
4
1d
1e
2d
2e
93
89
2
was found to be Au98Ag from EDX analysis result.
4.3. General procedure for aerobic oxidation of secondary
hydroxylamines
5
1f
2f
86
91
31.9 mg (0.15 mmol) of N,N-dibenzylhydroxylamines 1a was
placed into a 4 mL vial equipped with a magnetic stirrer bar. 2 mL of
dehydrated methanol was added and then stirred to obtain a solu-
tion. AuNPore (10 mol %, 2.96 mg) was put into the reaction solu-
tion, which was placed in the bottom of vial. Oxygen balloon was
prepared and then attached into the vial through cap equipped
with a septum to make an oxygen atmosphere. Reaction was
6
1g
2g
ꢀ
allowed to proceed for 2 h at 60 C. The formation of product(s) was
a
monitored by TLC. After completion of reaction, the resulting so-
lution was simply taken using a pipette while the vial was washed
several times with methanol. Combined solution was concentrated
under vacuum condition to give a residue. Obtained residue was
purified by passing it through a basic silica column chromatography
with ethyl acetate as the eluent to give 31.1 mg of 2a in 98% yield.
All of products in Tables 1 and 2 are identified with the reported
2
Reaction conditions: 1a (0.15 mmol), solvent (2 mL), under O .
Isolated yield.
b
3
. Conclusion
In conclusion, the aerobic oxidation of secondary hydroxyl-
amines into their corresponding nitrones catalysed by nanoporous
17a
19
12b
20
13j
19
13b
data in the literature: 2a, 2b, 2c,
2d, 2e, 2f, 2g.
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S. Yudha S et al. / Tetrahedron xxx (2015) 1e4
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.4. Procedure for leaching experiment
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before reaction started. The reaction was allowed to proceed for
3
was taken using a syringe filter and transferred into a different vial,
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taken to be analysed by NMR to detect the yield of nitrone 2a. Both
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5
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The aerobic oxidation reaction of secondary hydroxylamines 1a
1
was carried out. After 2 h, reaction solution was taken while the
remaining AuNPore catalyst was recovered by a simple filtration
through cotton, washed several times with acetone and then dried
under a reduced pressure. New reaction solution of 1a was pre-
pared and allowed to react with the used AuNPore and then treated
like the previous one. This treatment was repeated several times
and the yield of 2a in each repetition was analysed by NMR. After
the eighth cycle, the porous structure of reused AuNPore was
analysed using SEM and compared with the fresh one.
4
1
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Please cite this article in press as: Yudha S, S.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.05.094