Direct and Co-catalytic Oxidation of Hydroxylamines to Nitrones Promoted by Rhodium Nanoparticles Supported on Carbon Nanotubes

Article abstract of DOI:10.1002/cctc.201601155

Rhodium nanoparticles were assembled on carbon nanotubes, and the resulting nanohybrid was studied in the aerobic oxidation of hydroxylamines to nitrones. Two catalytic systems were developed (i.e., direct or co-catalytic) and both operated to provide high yields of the products, under mild conditions, but with their own specificity as regards the kinetics and regioselectivity of the transformation. In addition, the in situ cycloaddition of the produced nitrones with different alkynes was investigated.

Full text of DOI:10.1002/cctc.201601155

Accepted Article
Title: Direct and co-catalytic oxidation of hydroxylamines to nitrones
promoted by rhodium nanoparticles supported on carbon
nanotubes
Authors: Eric Doris, Praveen Prakash, Edmond Gravel, Dinh-Vu
Nguyen, and Irishi Namboothiri
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To be cited as: ChemCatChem 10.1002/cctc.201601155
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10.1002/cctc.201601155
ChemCatChem
COMMUNICATION
Direct and co-catalytic oxidation of hydroxylamines to nitrones
promoted by rhodium nanoparticles supported on carbon
nanotubes
Praveen Prakash,[a] Edmond Gravel,[a] Dinh-Vu Nguyen,[a] Irishi N. N. Namboothiri* [b] and Eric Doris*[a]
Abstract: Rhodium nanoparticles were assembled on carbon
nanotubes and the resulting nanohybrid was studied in the aerobic
oxidation of hydroxylamines to nitrones. Two catalytic systems were
developed (i.e. direct or co-catalytic) and both operated in high
yields, under mild conditions, but with their own specificity as
regards the kinetics and regioselectivity of the transformation. In
addition, the in situ cycloaddition of the produced nitrones with
different alkynes was investigated.
The RhCNT nanohybrid was synthesized according to a
procedure that we previously reported.[9a,12a] In short, carbon
nanotubes (CNTs) were first decorated with lipids incorporating
a nitriloacetic polar head and a diacetylene lipophilic chain. The
lipids self-assembled spontaneously on the CNT surface to form
a negatively charged layer composed of nanoring-like structures
that were polymerized (diyne motif) by UV-irradiation. In the
second step, a cationic polymer (poly(diallyl dimethylammonium
chloride)) was immobilized through electrostatic interactions and
served as
a
stabilizing group for pre-formed rhodium
Nitrones[1] are valuable chemicals[2] that are active, for example,
in cycloaddition reactions such as the 1,3-dipolar cycloaddition
with alkenes and alkynes to form 5-membered rings. The most
common route to nitrones is the oxidation of hydroxylamines
which classically requires strong oxidizing conditions. Various
oxidants used for this purpose include methyltrioxorhenium,[3]
nanoparticles[14] (diameter ca. 2 nm) that were finally added to
the multi-layered assembly. This layer-by-layer approach
allowed dense and homogeneous distribution of the RhNPs at
the surface of the CNTs. The final aqueous suspension was
analyzed by inductively coupled plasma mass spectrometry
(ICP-MS) and a Rh concentration of 6.4 mM was measured.
sodium
hypochlorite,[4]
manganese
dioxide,[5]
N-
bromosuccinimide, mercuric oxide, iodine, ceric ammonium
nitrate, and tert-butyl hydroperoxide.[6] In recent reports,
hypervalent iodine reagents[7] and catalytic systems (based for
example on nano-gold chemistry[8]) have been described to
perform hydroxylamine oxidation. However, the use of harsh
conditions (especially heating) is often required with the latter
systems thus narrowing the scope of substrates and lowering
the yields of isolated products.
We have recently reported the assembly and use of
nanotube-supported metal nanoparticles (Au,[9] Pd,[10] Ru[11] or
Rh[12]) as catalysts for energy-relevant reactions (oxygen or
carbon dioxide reduction[13]
)
and for various organic
transformations (oxido-reductive processes, coupling reactions,
etc.). Among our different nanohybrid catalysts, rhodium
particles supported on carbon nanotubes (RhCNT) showed
excellent performances in the mild oxidation of nitrogen-
containing substrates.[12] With these features in mind, we sought
to expand the scope of application of the rhodium-based
nanhybrid catalyst. In the present article, we thus report the use
of RhCNT for the oxidation of secondary hydroxylamines into the
corresponding nitrones, either directly (Scheme 1, path A) or in
the presence of a quinone co-catalyst (Scheme 1, path B). In
addition, further application to “one-pot” oxidation/cycloaddition
with alkynes is also investigated.
Scheme 1. Overview of the direct (RhCNT, path A) or co-catalytic oxidation
(RhCNT/quinone, path B) of hydroxylamines to nitrones.
The above-prepared catalyst was subsequently investigated in
the direct oxidation of secondary hydroxylamines into nitrones.
N,N-dibenzylhydroxylamine (1a) was chosen as
a model
substrate and was subjected to the aerobic action (open flask) of
RhCNT (1 mol%). Under these reaction conditions, a high
dependency on the solvent system was observed. In fact, only
trace amounts of the product were detected after 12 h in a
CHCl3/H2O mixture (Table 1, Entry 1), however full conversion
was observed after 8 h in MeOH (Entry 2). Relying on previous
works that we carried out on bio-inspired oxidative
systems,[12a,15] we conceived that the addition of a “quinone” co-
catalyst could impact the overall efficiency of the reaction. Using
10 mol% of hydroquinone as co-catalyst, the reaction was
complete in the biphasic CHCl3/H2O mixture after 3.5 h (Entry 3)
and the process was even faster in MeOH, reaching full
[a]
[b]
Dr. P. Prakash, Dr. E. Gravel, D.-V. Nguyen, Dr. E. Doris
Service de Chimie Bioorganique et de Marquage (SCBM), CEA,
Université Paris-Saclay, 91191 Gif-sur-Yvette, France.
E-mail: eric.doris@cea.fr
Prof. Dr. I. N. N. Namboothiri
Department of Chemistry, Indian Institute of Technology Bombay,
Mumbai 400 076, India.
E-mail: irishi@chem.iitb.ac.in
Supporting information for this article is given via a link at the end of
the document.
conversion in only 1.2
transformation, the redox-active quinone catalyzed the oxidation
h (Entry 4). In the co-catalytic
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10.1002/cctc.201601155
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COMMUNICATION
reaction per se and RhCNT promoted the in situ reoxidation of
the released hydroquinone, thus completing the catalytic cycle
(cf. Scheme 1). Two blank experiments conducted with either 10
mol% of hydroquinone only (no RhCNT, Entry 5) or 10 mol% of
1,4-benzoquinone only (no RhCNT, Entry 6), provided no
conversion in the former case, and 10% of 2a in the latter case.
These results highlight the cooperative contribution of the two
co-catalysts that synergistically catalyzed the oxidation reaction.
Table 2. Scope of the RhCNT catalyzed oxidation of N,N disubstituted
hydroxylamines 1 to nitrones 2.[a], [b]
2/2’
ratio
Yield
Entry Substrate 1
Product(s) 2
Path t (h)
(%)[c]
A
B
8
1.2
-
-
99
98
1
2
3
A
B
8
0.5
-
-
96
95
Table 1. Optimization of the aerobic oxidation of N,N-dibenzylhydroxylamine 1a
to nitrone 2a.[a]
A
B
14
8
-
-
89
85
A
B
20
9
-
-
96
92
4
5
[c]
Entry Co-catalyst
Solvent
t (h)
Yield (%)[b] TOF (h−1
)
A
B
17
10
-
-
94
88
1
-
CHCl3/H2O 3:1
MeOH
12
8
trace
99
-
2
-
12.4
27.4
65.3
N/A
N/A
A
B
2
2
-
-
99
94
6
7
3
10 mol%
10 mol%
10 mol%
10 mol%
CHCl3/H2O 3:1
MeOH
3.5
1.2
24
12
96
4
98
A
B
26
15
1:0
1:0.4 89
93
5[d]
6[f]
MeOH
0[e]
MeOH
10[e]
A
B
19
12
1:1
1:1
98
97
8
9
[a] Conditions: 1a (0.1 mmol), RhCNT (1 mol%), solvent (2 mL), room temp.,
open flask. [b] Yield of isolated product. [c] TOF = (mmolproduct/mmolcatalyst)/time.
[d] Experiment conducted in the absence of RhCNT. [e] measured by 1H-NMR.
[f] Experiment conducted in the absence of RhCNT using 1,4-benzoquinone
instead of hydroquinone.
A
B
72
36
1:1
1:1
95
91
The scope of the process was further investigated in MeOH with
or without the hydroquinone co-catalyst (Table 2). Various
disubstituted hydroxylamines[16] were thus reacted under the
above-conditions (path A: RhCNT (1 mol%), room temperature,
open flask; or path B: RhCNT (1 mol%), hydroquinone (10
mol%), room temperature, open flask). Apart from N,N-
dibenzylhydroxylamine (1a, Entry 1), other symmetrical
substrates were investigated such as N,N-diethylhydroxylamine
(1b, Entry 2) which was readily oxidized into its nitrone
equivalent 2b in nearly quantitative yields. It is to be noted that
while the direct oxidation took 8 h to reach completion, the co-
catalytic process required only 0.5 h. Cyclic hydroxylamines 1c,
1d, and 1e were also converted to their nitrone counterparts (i.e.
compounds 2c, 2d and 2e, respectively) but the catalytic
process required longer reaction times (Entries 3-5).
Unsymmetrical hydroxylamines were as well efficiently oxidized
[a] Path A: 1 (0.1 mmol), RhCNT (1 mol%), MeOH (2 mL), room temp., open
flask. [b] Path B: 1 (0.1 mmol), hydroquinone (10 mol%), RhCNT (1 mol%),
MeOH (2 mL), room temp., open flask. [c] Isolated yields.
regardless of the path involved. Thus, there seems to be no
major electronic effect of the aryl-substituents on the
regioselectivity of RhCNT-triggered oxidation of benzyl
hydroxylamines. As shown in Table 2, the use of the
hydroquinone co-catalyst significantly increased the rate of the
oxidation in the vast majority of the investigated transformations
with kinetics improved up to a factor of 16 (oxidation of 2b). The
only exception being the oxidation of 1f, for which no difference
was observed. However, in the case of 1g, the use of the co-
catalyst also led to a loss in regioselectivity.
Since our system efficiently promoted the oxidation of
hydroxylamines under mild conditions, we wanted to further
explore the “one-pot” 1,3-dipolar cycloaddition of the produced
nitrones with alkynes as previously investigated by Pezacki and
co-workers.[17] For this study, benzannulated cyclooctyne 3 was
chosen as a model of strained alkyne whereas dimethyl
with
the
RhCNT
hybrid,
(1f, Entry 6)
including
and
N-benzyl-N-
N-benzyl-N-
phenylhydroxylamine
methylhydroxylamine (1g, Entry 7). Noteworthy, while direct
oxidation (path A) of 1g led to the formation of a single isomer
(compound 2g), the co-catalytic process (path B) was less
selective and afforded a 1:0.4 mixture of regioisomers (2g/2g’).
The influence of the electronic properties of the benzyl
groups borne by the hydroxylamines was also scrutinized as
regards the regioselectivity of the oxidation reaction. However,
acetylenedicarboxylate (DMAD, 5) provided
a
model of
electrophilic alkyne. Hydroxylamine 1a was first reacted with
RhCNT in MeOH for 8 h after which the dipolarophile was added
to the mixture. After addition of cyclooctyne 3, full conversion
into cycloadduct 4 was observed in just 1 h (90% yield). The
sequential addition of reagents was needed to avoid some
oxidative degradation of the alkyne. On the other hand, the
cycloaddition of 2a with DMAD resulted in lower yield (48%) of
isoxazoline 6 and required longer reaction time (8 h, Scheme 2).
hydroxylamines substituted with either
a
para-electron
withdrawing (4-fluoro, Entry 8) or donating 4-methoxy group
(Entry 9) both provided a 1:1 mixture of oxidized isomers
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The lower yield observed here was attributed to the slow in situ
degradation of cycloadduct 6 in the presence of the methanolic
solvent. The same type of “one-pot” transformation could be
achieved under path B conditions, as cycloadducts 4 and 6 were
produced in comparable yields, but more rapidly (92%/2.5 h and
44%/9 h, respectively).
In conclusion, we have developed an efficient and
straightforward method for the oxidation of hydroxylamines into
nitrones, using rhodium nanoparticles supported on carbon
nanotubes. The process is operative under mild and very
convenient conditions (room temperature, open flask, easy
recovery of the product) and can even be further accelerated by
using a quinone co-catalyst. The two systems have their own
specificity as regards the kinetics of the reaction (the co-catalytic
system is much faster) and also the regioselectivity (the direct
system is more selective). A variety of hydroxylamines, including
aliphatic and aromatic ones, either symmetrical or
unsymmetrical, were readily converted into their nitrone
counterparts. Furthermore, the methodology was applied to the
one-pot oxidation–cycloaddition of hydroxylamines with alkyne
dipolarophiles.
Experimental Section
General procedure for the direct oxidation of hydroxylamines to
nitrones (path A)
The preparation of 2a is given as a representative example. To a solution
of N,N-dibenzyl hydroxylamine 1a (21.3 mg, 0.1 mmol) in methanol (2
mL), was added RhCNT (6.4 mM methanol suspension, 1 mol%). The
reaction mixture was stirred at room temperature for 8 h. After completion
of the reaction (monitored by TLC), the catalyst was removed by filtration
and methanol was evaporated under vacuum to afford the pure nitrone
2a in 99% yield.
Scheme 2. “One-pot” 1,3-dipolar cycloaddition of hydroxylamine 1a with either
strained alkyne 3 or electrophilic alkyne 5.
Recyclability of the RhCNT catalyst under our set of conditions
was evaluated over three consecutive oxidation reactions. After
each cycle, the catalyst was separated from the mixture by
simple benchtop centrifugation, washed with MeOH, and reused
as such for the next run. In the absence of the co-catalyst (path
A) a moderate drop in catalytic activity was observed after each
recycling which was evidenced by a slight increase of the
reaction time needed to reach full conversion. In the presence of
hydroquinone (path B), the process was more efficient but the
loss of catalytic activity over the successive cycles was more
drastic and the reaction time increased from 1.2 to 4 h between
the first and the third run, suggesting slow deactivation of the
catalyst. The origin of this deactivation is not understood yet as
ICP-MS analysis of the mother liquors showed no leaching of Rh,
while TEM and XPS indicated no alteration of recycled RhCNT.
General procedure for the co-catalytic oxidation of hydroxylamines
to nitrones (path B)
The preparation of 2a is given as a representative example. N,N-dibenzyl
hydroxylamine 1a (21.3 mg, 0.1 mmol), hydroquinone (1.1 mg, 0.01
mmol) and RhCNT (6.4 mM methanol suspension, 1 mol%) were stirred
in methanol at room temperature for 1.2 h. The catalyst was removed by
filtration and methanol was removed under vacuum. The residue was
subjected to column chromatography (silica gel, cyclohexane/ethyl
acetate) to afford the desired nitrone 2a in 98% yield.
Recycling experiment (path A)
To a solution of N,N-dibenzyl hydroxylamine 1a (21.3 mg, 0.1 mmol) in
methanol (2 mL), was added RhCNT (6.4 mM methanol suspension, 1
mol%). The reaction mixture was stirred at room temperature and the
progress of the reaction was monitored by TLC. After 8 h of reaction, the
catalyst was recovered by centrifugation and the solvent was evaporated
under vacuum to afford pure product 2a. The recovered catalyst was
washed with methanol and reused in subsequent oxidation reactions.
Table 3. Recycling of the RhCNT catalyst either alone[a] or with hydroquinone
co-catalyst.[b]
Recycling experiment (path B)
path A
path B
N,N-dibenzyl hydroxylamine 1a (21.3 mg, 0.1 mmol), hydroquinone (1.1
mg, 0.01 mmol) and RhCNT (6.4 mM methanol suspension, 1 mol%)
were mixed in methanol. The reaction mixture was stirred at room
temperature and the advancement of the reaction was monitored by TLC.
After 1.2 h of reaction, the catalyst was recovered by centrifugation and
the solvent was evaporated under vacuum. The crude residue was
purified by column chromatography (silica gel, cyclohexane/ethyl acetate)
to afford pure product 2a. The recovered catalyst was washed with
methanol and reused in subsequent oxidation reactions.
Entry
Catalyst
t (h)
yield (%)[c]
t (h)
yield (%)[c]
1
2
3
cycle 1
cycle 2
cycle 3
8
99
98
96
1.2
2.5
4
98
97
98
10
13
[a] Path A: 1 (0.1 mmol.), RhCNT (1 mol%), MeOH (2 mL), room temp., open
flask. [b] Path B: 1 (0.1 mmol.), Hydroquinone (10 mol%), RhCNT (1 mol%),
MeOH (2 mL), room temp., open flask. [c] Isolated yields.
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General
procedure for
the
one-pot
oxidation/1,3-dipolar
Donck, E. Gravel, N. Shah, D. V. Jawale, E. Doris, I. N. N. Namboothiri,
ChemCatChem 2015, 7, 2318.
cycloaddition of nitrones with alkynes
[11] (a) D. V. Jawale, E. Gravel, C. Boudet, N. Shah, V. Geertsen, H. Li, I. N.
N. Namboothiri, E. Doris, Chem. Commun. 2015, 51, 1739; (b) P. Basu,
P. Prakash, E. Gravel, N. Shah, K. Bera, E. Doris, I. N. N. Namboothiri,
ChemCatChem 2016, 8, 1298.
Path A conditions: to a solution of N,N-dibenzyl hydroxylamine 1a (10.7
mg, 0.05 mmol) in methanol (2 mL), was added RhCNT (6.4 mM
methanol suspension, 1 mol%) and the reaction mixture was stirred at
room temperature. After the disappearance of hydroxylamine (monitored
by TLC), the alkyne (0.05 mmol) was added and the reaction mixture was
stirred for the corresponding time. The catalyst was removed by filtration
and methanol was removed under vacuum. The residue was subjected to
column chromatography (silica gel, cyclohexane/ethyl acetate) to afford
the cycloadduct.
[12] (a) D. V. Jawale, E. Gravel, N. Shah, V. Dauvois, H. Li, I. N. N.
Namboothiri, E. Doris, Chem. Eur. J. 2015, 21, 7039; (b) S. Donck, E.
Gravel, A. Li, P. Prakash, N. Shah, J. Leroy, H. Li, I. N. N. Namboothiri,
E. Doris, Catal. Sci. Technol. 2015, 5, 4542.
[13] (a) A. Morozan, S. Donck, V. Artero, E. Gravel, E. Doris, Nanoscale
2015, 7, 17274 ; (b) T. N. Huan, P. Prakash, P. Simon, G. Rousse, X.
Xiangzhen, V. Artero, E. Gravel, E. Doris, M. Fontecave,
ChemSusChem 2016, DOI : cssc.201600597
Path B conditions: N,N-dibenzyl hydroxylamine 1a (10.7 mg, 0.05 mmol),
hydroquinone (0.5 mg, 0.005 mmol) and RhCNT (6.4 mM methanol
suspension, 1 mol%) were stirred in methanol at room temperature for
1.2 h. After the disappearance of hydroxylamine (monitored by TLC), the
alkyne (0.05 mmol) was added and the reaction mixture was stirred for
the corresponding time. The catalyst was removed by filtration and
methanol was removed under vacuum. The residue was subjected to
column chromatography (silica gel, cyclohexane/ethyl acetate) to afford
the cycloadduct.
[14] Y. Wang, J. Ren, K. Deng, L. Gui, Y. Tang, Chem. Mater. 2000, 12,
1622.
[15] D. V. Jawale, E. Gravel, E. Villemin, N. Shah, V. Geertsen, I. N. N.
Namboothiri, E. Doris, Chem. Commun. 2014, 50, 15251.
[16] Disubstituted hydroxylamines were synthesized according to previously
reported procedures: (a) R. F. Borch, M. D. Bernstein, H. D. Durst, J.
Am. Chem. Soc. 1971, 93, 2897; (b) I. A. O’Neil, E. Cleator, D. J.
Tapolczay, Tetrahedron Lett. 2001, 42, 8247; (c) C. Gella, E. Ferrer, Ra.
Alibés, F. Busqué, P. de March, M. Figueredo, J. Font, J. Org. Chem.
2009, 74, 6365.
[17] A. R. Sherratt, M. Chigrinova, D. A. MacKenzie, N. K. Rastogi, M. T. M.
Ouattara, A. T. Pezacki, J. P. Pezacki, Bioconjugate Chem. 2016, 27,
1222.
Acknowledgements
Supports from the Indo-French Centre for the Promotion of
Advanced Research (IFCPAR)/Centre Franco-Indien pour la
Promotion de la Recherche Avancée (CEFIPRA) (Project no.
4705-1) and the Enhanced Eurotalent program are gratefully
acknowledged. The TEM-team platform (CEA, iBiTec-S) is
acknowledged for help with TEM images. The “Service de
Chimie Bioorganique et de Marquage” belongs to the Laboratory
of Excellence in Research on Medication and Innovative
Therapeutics (ANR-10-LABX-0033-LERMIT).
Keywords: carbon nanotubes • heterogeneous catalysis •
hydroxylamines • nitrones • rhodium nanoparticles
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Entry for the Table of Contents
COMMUNICATION
Rhodium nanoparticles were
P. Prakash, E. Gravel, D.-V. Nguyen,
I. N. N. Namboothiri* and E. Doris*
assembled on carbon nanotubes and
studied in the aerobic oxidation of
hydroxylamines to nitrones. High
conversion yields were obtained under
two sets of mild conditions (i.e. direct
or co-catalytic). In addition, the in situ
cycloaddition of the produced nitrones
with different alkynes was
Page No. – Page No.
Direct and co-catalytic oxidation of
hydroxylamines to nitrones promoted
by rhodium nanoparticles supported
on carbon nanotubes
investigated.
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