Graphene oxide as a metal-free catalyst for oxidation of primary amines to nitriles by hypochlorite

Article abstract of DOI:10.1039/c5cc09463a

Graphene oxide catalyzes oxidation by NaClO of primary benzyl and aliphatic amines to a product distribution comprising nitriles and imines. Nitriles are the sole product for long chain aliphatic amines. Spectroscopic characterization suggests that percarboxylic and perlactone groups could be the active sites of the process.

Full text of DOI:10.1039/c5cc09463a

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DOI: 10.1039/C5CC09463A
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Graphene oxide as metal-free catalyst for hypochlorite oxidation
of primary amines to nitriles
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
b
b
Ana Primo, Marta Puche, Octavian D. Pavel, Bogdan Cojocaru, Alina Jurca, Vasile Parvulescu
a
and Hermenegildo García *
DOI: 10.1039/x0xx00000x
www.rsc.org
Graphene oxide catalyzes NaClO oxidation of primary benzyl and that deeper oxidation by NaClO using G as metal-free catalyst
aliphatic amines to a product distribution comprising nitriles and can result in the conversion of primary amines into the
imines. Nitriles are the sole product for long chain aliphatic corresponding imines or nitriles. Particularly useful from the
amines. Spectroscopic characterization suggests that percarboxylic synthetic point of view is the conversion of long chain aliphatic
and perlactone groups could be the active sites of the process.
amines into the corresponding aliphatic nitriles. Nitriles have
large interest in their own as solvents and also as synthetic
intermediates in the preparation of carboxylic acids, ketones
The use of graphene (G) and related 2D materials as metal-free
1-4
1
0 2
catalysts has emerged as a hot topic in catalysis.
The long
and N-containing heterocycles, including phthalocyanines.
The catalyst is a GO sample (1 to 5 m lateral size, 51, 8 and 44
of C, H and O content, respectively) obtained by Hummers
term goal is to determine the potential of Gs as alternative
catalysts to conventional metal and metal oxides for certain
reaction types. In this context, one process that has attracted
considerable attention recently as benchmark reaction is the

%
oxidation of graphite and subsequent exfoliation, as
reported.{Stankovich, 2006 #60} Using GO as catalyst,
preliminary tests were carried out to determine the most
adequate reaction conditions to oxidize benzyl amine (1a) by
NaClO as oxidizing reagent. Table 1 summarizes conversion
and selectivity data for this reaction. Controls using NaClO as
oxidant in the absence of any catalyst shows that 1a is
5-7
oxidation of primary benzylamines.
In the most common
process, benzylamines undergo oxidative condensation to N-
benzylidene benzylamines promoted by a large variety of
metal oxides of various compositions as well as Au NPs
5, 7, 8
supported on a variety of metal oxides.
More closely
related to the present study, Loh and co-workers have found
that graphene oxide (GO) conveniently purified from
amorphous acidic carbon debris by basic washings and
subsequent neutralization by acids, is an efficient catalyst for
the aerobic oxidation of benzyl amines into the corresponding
N-benzylidene benzyl amines without the presumed assistance
converted around 40
N-benzylidene benzylamine (2a) and benzylidene imine (3a
Table 1, entries 1 and 2). Benzylidene imine 3a is highly
% into two products, namely
)
(
reactive against nucleophiles, including H O and amines,
2
leading to benzaldehyde or benzylidene benzyl amine 2a. For
this reason, imine 3a is not a common reaction product in
benzyl amine oxidations. In the present case, formation of 3a
9
of any metal. It has been proposed that base washings and
13
removal of acid amorphous carbon residues generated in the
Hummers oxidation of graphite results on the release of free9
holes on the GO sheet that are the active sites of the process.
From the synthetic point of view, oxidation of amines into
nitriles is an important functional group transformation.
was safely identified by the absence in C NMR spectra of
peaks at about 190 ppm corresponding to benzaldehyde and
the presence of a peak at 150 ppm (see supplementary
+
information). Also in GC-MS, the M and its main fragment
appeared at 105 and 104 Dalton, respectively. No benzonitrile
was detected in the solution. An increase in the excess of
NaClO in the absence of catalyst did not result in a significant
Continuing with the oxidation of benzylamines by
carbocatalysts, in the present manuscript it will be reported
increase in the conversion of benzylamine 1a
.
In contrast, under the same conditions but using 10 wt % of
GO as catalyst, a substantial percentage of nitrile 4a with
selectivity above 50 % was observed at similar conversions of
1a (Table 1, entry 3). It is interesting to note that formation of
benzonitrile 4a is accompanied by a concomitant decrease in
the selectivity of benzylidene imine (3a) from 55 to 6 %, while
a.
Instituto Universitario de Tecnología Química CSIC-UPV, Univ. Politécnica de
Valencia, Avda/ de los Naranjos s/n, 46022, Valencia, Spain. E-mail:
hgarcia
@qim.upv.es
b.
Department of Chemical Technology and Catalysis, University of Bucharest,
Bucharest 030016, Romania
Footnotes relating to the title and/or authors should appear here.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See the selectivity of benzylidene benzylamine (2a) was mostly
DOI: 10.1039/x0xx00000x
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unaffected (compare entries 1 and 3 in Table 1). This change in additional amounts of 2a. The presence of H O should also
2
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DOI: 10.1039/C5CC09463A
product selectivity suggests that benzyl imine 3a is the affect to all the pathways indicated in Scheme 1.
precursor of nitrile 4a and that the presence of GO does not Another point to be addressed is the influence of oxygen on
promote the conversion of into 3a, since conversion of the the benzylamine oxidation by NaClO. It is very common in
1
starting material 1a was similar in the absence and presence of oxidation reactions that the presence of oxygen influences
GO catalyst. Thus, under the present experimental conditions, conversion and selectivity, commonly by favoring radical chain
GO is promoting oxidation of imine 3a to nitrile 4a without mechanisms involving the intermediacy of peroxyl radicals. In
promoting the oxidation of the starting material 1a that is the reaction under study, it has also been reported that GO
spontaneously oxidized in a certain extent by NaClO (Table 1, can be catalyst for the oxidation of benzylamine 1a under
entries 1 and 2). This would explain the similar conversion of certain conditions. For this reason the influence of oxygen
1a and selectivity to 2a in the presence and absence of GO and using NaClO as oxidizing reagent was also checked. It was
the exclusive change in the selectivity between 3a and 4a observed that the presence of oxygen has not influence on
comparing the results with and without GO. A plausible benzylamine (1a) conversion (Table 1, compare entries 1, 2, 3
reaction mechanism compatible with the results obtained is and 4). In contrast, the presence of oxygen has a remarkable
shown in Scheme 1.
influence on product selectivity when using GO as catalyst,
decreasing the percentage of imine (2a in favor of the
),
Table 1. Results of the oxidation of benzyl amine (1a) catalyzed formation of benzonitrile (4a) and benzyl imine (3a) (Table 1,
by GO. Reaction conditions: 0.27 mmol of 1a, 5 mL CH CN as compare selectivity of 2a and 3a in entries 1, 2, 3 and 4). This
3
o
solvent, 0.59 mmol NaClO, 5 h reaction time, 75 C reaction catalytic data suggests that in Scheme 1, while the initial steps
temperature, catalyst: 10 wt % GO respect to 1a
.
starting from benzyl amine (1a) as substrate are not promoted
by oxygen, subsequent secondary processes can be boosted by
the presence of oxygen. Accordingly, using NaClO as oxidizing
reagent and GO as catalyst, the best conditions in terms of
benzonitrile (4a) selectivity were achieved in the presence of
oxygen (Table 1, entry 3).
NH2
O
H
CN
N
NH
+
+
+
1a
2a
3a
4a
5a
Entry
Conversion (%)
Selectivity (%)
In order to determine the scope of the reaction, primary
benzylamines (Table 2) having substituents on the aromatic
ring as well as aliphatic amines (Table 3) were also tested as
substrates. Overall, Table 2 shows that for substituted
benzylamines the major product of the NaClO oxidation
promoted by GO are the corresponding benzyl imines 3b-d or
related compounds (6e) and aryl nitriles 4b-e are obtained in
1
a
2a
3a
4a
5a
-
a
1
43
42
48
47
54
42
45
29
40
26
60
88
55
0
a,b
2
71
6
0
-
-
3
54
41
16
10
1
very minor percentages (see supporting information for MS, H
13
b
and C NMR spectroscopic data of the reaction products).
Thus, the presence of methyl substituent at the para position
4
33
24
-
-
b,c
(Table 2) disfavors the selectivity towards nitrile (4b) and the
5
-
major product was the corresponding benzylidene imine (3b)
accompanied by some 4-methylbenzaldehyde (5b). 3b exhibits
d
6
1
13
in C NMR spectroscopy no peak at 190 ppm corresponding to
a) no catalyst; b) no oxygen; c) double amount NaClO; d) 1 mL aldehyde and a peak at 150 ppm attributable to the imine
water; e) dried NaClO; f) reaction time :1 h; g) reaction time: 3 carbon (see supporting information). Similarly, 2- and 4-
h.
methoxybenzyl amine (Table 2) undergo oxidation in the
presence of NaClO resulting in the predominant formation of
The presence of water was found to be an important factor the corresponding benzylidene imines (3c and 3d) with very
controlling conversion of 1a
selectivity and product high selectivity particularly for the 2-methoxy derivative. In the
,
distribution (Table 1, entry 6). If water is purposely added, the case of the 4-methoxy derivative, the percentage of 4-
conversion of 1a decreases somewhat from 48 to 42 % with methoxybenzaldehyde (5d), probably derived from the
the complete absence of imine 3a, a high selectivity towards hydrolysis of the imine (3d), was significantly higher in the final
benzylidene benzylamine (2a) and only a marginal formation reaction mixture. The case of 4-chlorobenzylamine is
of nitrile (4a) about 10 %. Importantly, in the presence of remarkable, since for this compound, particularly under dry
water, instead of imine (3a), benzaldehyde (5a) was detected conditions, the major final product observed was the
in a very small percentage (Table 1, entry 6), in agreement corresponding N,N-bis(4-chlorobenzyl)hydroxylamine
with the expected reactivity of imine 3a
accompanied by small percentages of 4-chlorobenzaldehyde
A reasonable explanation for the influence of H O in the
5e) and 4-chlorobenzonitrile (4e). Compound 6e was
(6e)
.
(
2
1
process is also included in Scheme 1. According to this, H O identified by the presence in H NMR spectroscopy of the
2
will promote hydrolysis of imine 3a to benzaldehyde 5a that methylene groups as singlets at 3.85 ppm and their
13
would react quickly with starting benzylamine 1a affording corresponding aliphatic C in C NMR spectrum appearing at 52
2
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ppm (see supporting information). It seems that the presence remarkable since generally aliphatic amines undergo oxidation
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DOI: 10.1039/C5CC09463A
of moderate electron withdrawing Cl substituent changes the giving rise to a wide range of products that make this process
1
1 12
reactivity pattern from the formation of benzyl imines 3a to extremely unselective and lacking of synthetic interest.
In
the hydroxylamine 6e. In the case of the chloro derivative, the contrast, using NaClO as oxidant and GO as catalyst either the
presence of water in the reaction mixture does not alter oxidative coupling product or the nitrile were selectively
substantially product distribution, although a slight increase in obtained, depending on the chain length. The change in the
the percentage of 4-chlorobenzonitrile (4e) was measured, the reactivity pattern was observed for heptamine. A salient
most remarkable catalytic data being the conversion decrease observation was that for the long alkyl chain, the
of the starting material (1e) in the presence of water (Table 2 corresponding nitriles
4 were formed as the only products
compare entries 7 and 8 with 9 and 10). It is, however, evident observed at any substrate conversion (Table 3, entries 5-8).
that bis(4-chlorobenzyl)hydroxylamine 6e is structurally Data of Table 3 contrasts with the current state of the art and
related to benzylidine benzylamine 2e by water addition.
opens the possibility to develop a general process for aliphatic
nitrile formation from amines.
Table 2. Conversion and selectivity for the NaClO oxidation of
different amines catalyzed by GO. Reaction conditions: 0.27 Table 3. Results of the oxidation of aliphatic amines by NaClO
o
mmol of substrate, 5 mL CH CN, 0.59 mmol NaClO, 5 h, 75 C, catalyzed by GO. Reaction conditions: 0.27 mmol aliphatic
3
o
catalyst: 10 % w/w GO respect to substrate, 5 atm O2
amine, 5h, 75 C, 10% w/w GO respect substrate, 5 ml CH CN,
3
5 atm O
.
2
NH2
NH
O
H
CN
+
+
Entry
Amines
Amine
conver
sion (%)
Selectivity (%)
CH3
4b
CH3
CH3
3b
CH3
5b
1b
Coupling Nitri Aldehy
Entry
1
Conversion (%)
Selectivity (%)
2
le
4
de
5
1
b
3b
93
91
4b
3
5
5b
4
4
98
94
1
2
3
4
pentylamineb
73
100
68
100
100
84
0
0
11
<1
0
0
5
6
a
2
pentylamine
Heptylamine
NH2
NH
OCH3
CN
4c
OCH3
OCH3
+
Heptylamine
68
94
b
1c
3c
5
6
7
8
9
Dodecylamin
e
37
25
43
27
0
0
0
0
100
100
100
100
0
0
0
0
Entry
3
Conversion (%)
Selectivity (%)
1
c
3c
96
99
4c
4
1
Dodecylamin
79
91
b
e
a
4
Hexadecylam
NH2
NH
O
H
CN
a
ine
+
+
Hexadecylam
a,b
ine
OCH3
4d
OCH3
OCH3
3d
OCH3
5d
Stearylamine
58
57
0
0
100
100
0
0
1d
1
0
Stearylamine
Entry
5
Conversion (%)
Selectivity (%)
b
1
d
3d
83
63
4d
2
3
5d
15
34
a
b
solvent is hexane; no oxygen;
100
99
a
6
NH2
O
H
Comparison of the product distribution of Tables 1, 2 and 3
shows that the reaction mechanism should be complex and
that the outcome of the oxidation is influenced by the
structure of the starting material and the reaction conditions.
Focusing on the oxidation of benzylamine 1a, additional
support to the mechanistic proposal shown in Scheme 1 was
sought. Temporal evolution of the products showed that upon
full conversion of 1a, no change in the selectivity of
benzylidene benzylamine 2a occurs upon prolonged reaction
time. In contrast, some decrease in the selectivity of 3a in
favor of 4a is observed. This indicates that compound 2a is not
involved in the formation of benzonitrile 4a that derives from
imine 3a. In this route the role of water seems to be the
CN
Cl
Cl
OH
N
+
+
Cl
Cl
Cl
5e
1
e
2e
4e
Entry
7
Conversion (%)
Selectivity (%)
1
e
4e
4
5e
1
2e
94
89
98
95
64
76
100
a
8
9
5
6
b
1
1
a,b
1
0
100
1
4
a
b
No oxygen; dry conditions;
Aliphatic amines can also be oxidized by NaClO using GO as
metal-free catalyst. The results shown in Table 3 are
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promotion hydrolytic conversion of benzyl imine 3a into
aldehyde 5a
View Article Online
DOI: 10.1039/C5CC09463A
.
To gain at deeper insight into the role of benzaldehyde in the
process, an additional experiment in which an equimolar
mixture of benzaldehyde and ammonium chloride in
acetonitrile was submitted to NaClO oxidation under the
D band(CC)
benzonitrile
G band
conditions employed in the oxidation of
1
was performed. The
b
a
formation of benzonitrile (4a) was observed, although the
yield (1 %) was very low with respect to the amount of
benzaldehyde (5a). Further attempts to increase the formation
of benzonitrile (4a), by adding an excess of ammonium to the
acetonitrile solution of benzaldehyde (5a), only increased the
yield of benzonitrile (4) marginally to 3 %. Therefore, these
experiments indicate that if benzaldehyde (5a) is formed by
hydrolysis of imine 3a, the main product to be expected in the
presence of benzylamine 1a 9is the benzylidene benzylamine
1000
1250
1500
1750
2000
-
1
Raman Shift (cm )
Figure 1. Raman spectra of GO before (a) and after treatment
with NaClO under reaction conditions (b).
2a, as reported in other cases.
In summary, in the present study, it has been shown that GO is
a suitable carbocatalyst to promote NaClO oxidation of
aromatic and aliphatic benzyl amines, leading to the formation
of benzonitriles and benzyl imines in variable proportions,
depending on the reaction conditions. Kinetic data show that
benzyl imines are the initial oxidation product that are
converted into nitriles due to the catalytic activity of GO. The
reaction is of synthetic interest for the preparation of the
corresponding nitriles from long alkyl chain aliphatic amines,
for which complete selectivity are attained. Overall, our report
constitutes another remarkable example on the use of GO as
metal-free oxidation catalyst and complements prior studies
on benzylamine oxidation by Gs.
Scheme 1. Mechanistic proposal for benzonitrile formation by
NaClO oxidation of benzylamine catalysed by GO.
Notes and references
See supporting information for experimental procedures and
product characterization data.
To understand the role of GO as catalyst, a suspension of GO in
acetonitrile was treated with NaClO for 3 h at 75 C and the
o
resulting treated GO was characterized by Raman
spectroscopy (see Figure 1). It was observed that under these
oxidizing conditions, new vibration bands appear in the
1
. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J. O. Müller, R. Schlögl
and J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893-4908.
-1
2. Y. Wang, J. Zhang, X. Wang, M. Antonietti and H. Li, Angew. Chem. Int. Ed.,
spectrum at 1750 and 1875 cm . These new peaks are
attributable to the formation of C=O groups on the G sheet,
probably in strained cycles or as esters and anhydrides. Also,
2
3
010, 49, 3356-3359.
. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K.
Domen and M. Antonietti, Nature Mat., 2009, 8, 76-80.
variations on the relative intensity and position of the G and D 4. A. Primo, F. Neatu, M. Florea, V. Parvulescu and H. García, Nature Comm.,
bands upon NaClO treatment were noticed (see Figure 1). 2014, 5, 5291.
5
6
7
8
4
9
. K. Yamaguchi and N. Mizuno, Angew. Chem. Int. Ed., 2003, 42, 1480-1483.
. R. D. Patil and S. Adimurthy, Adv. Synth. Catal., 2011, 353, 1695-1700.
. A. Grirrane, A. Corma and H. García, J. Catal., 2009, 264, 138-144.
. P. Sudarsanam, A. Rangaswamy and B. M. Reddy, RSC Advances, 2014, 4,
6378.
Accordingly, we propose that percarboxylic acids, perlactones
and related oxygenated substructures generated on GO under
oxidative conditions could act as the active sites in this
oxidation. The oxidizing activity of hydroperoxy acids,
perlactones and related hydroperoxy functional groups is well
known in general organic chemistry and due to the presence of
.
C. Su, M. Acik, K. Takai, J. Lu, S. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki, Y.
J. Chabal and K. P. Loh, Nature Comm., 2012, 3, 1298.
abundant carboxylic acid groups on GO, it could be expected 10. G. DeSantis, Z. Zhu, W. A. Greenberg, K. Wong, J. Chaplin, S. R. Hanson,
that similar substructures could be generated upon treatment B. Farwell, L. W. Nicholson, C. L. Rand, D. P. Weiner, D. E. Robertson and M.
-
J. Burk, J. Am. Chem. Soc., 2002, 124, 9024-9025.
with ClO as oxidant.
1
2
1
1. J. S. Reddy and P. A. Jacobs, J. Chem. Soc. Perkin Trans., 1993, 2665-
666.
2. K. E. Gilbert and W. T. Borden, J. Org. Chem., 1979, 44, 659-661.
4
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