Co-catalytic oxidative coupling of primary amines to imines using an organic nanotube-gold nanohybrid

Article abstract of DOI:10.1039/c4cc07951e

A novel nanohybrid structure was synthesized by assembling gold nanoparticles on polymerized polydiacetylene nanotubes. Combination of the nanohybrid with gallacetophenone afforded an efficient cooperative co-catalytic system for the oxidative coupling of primary amines into imines. The system is highly efficient and sustainable as it operates in high yields using minimal amounts of the metal and the quinone, under ambient atmosphere, at room temperature, in water, and is easily recycled.

Full text of DOI:10.1039/c4cc07951e

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Co-catalytic oxidative coupling of primary amines
to imines using an organic nanotube–gold
nanohybrid†
Cite this: Chem. Commun., 2014,
50, 15251
Received 18th September 2014,
Accepted 20th October 2014
Dhanaji V. Jawale,a Edmond Gravel,*a Elise Villemin,a Nimesh Shah,b
Valerie Geertsen,c Irishi N. N. Namboothiri*b and Eric Doris*a
´
DOI: 10.1039/c4cc07951e
www.rsc.org/chemcomm
A
novel nanohybrid structure was synthesized by assembling on organic nanotubes (AuONT). The latter was used as co-catalyst
gold nanoparticles on polymerized polydiacetylene nanotubes. in the aerobic oxidative coupling of aliphatic and aromatic amines.
Combination of the nanohybrid with gallacetophenone afforded Since gold nanoparticles have previously been shown to catalyze
an efficient cooperative co-catalytic system for the oxidative coupling the oxidation of hydroquinones,13 we conceived that the metallic
of primary amines into imines. The system is highly efficient and species could be applied to the catalytic oxidation of amines to
sustainable as it operates in high yields using minimal amounts of imines in the presence of a redox-active quinone co-catalyst. This
the metal and the quinone, under ambient atmosphere, at room sequence mimics the process that takes place in metallo-enzymes:
temperature, in water, and is easily recycled.
the quinone unit would catalyze the imine formation per se and
gold particles would promote the in situ re-oxidation of the released
Imines are versatile intermediates for the synthesis of fine chemicals hydroquinone, thus completing the catalytic cycle (Scheme 1). In
and are classically produced by condensation of carbonyl com- addition, the use of a supra-molecular organic platform for the
pounds with amines under dehydrating conditions. Other strategies stabilizing and recycling of the noble metallic species is expected to
include the oxidative coupling of amines with alcohols, the direct lead to an overall improvement of the cooperative catalytic process.
oxidation of secondary amines, and the in situ oxidation/condensa-
The AuONT assembly was prepared by a layer-by-layer approach.
tion of primary amines.1 The latter process has recently emerged as Preformed gold nanoparticles (AuNPs) were anchored on the surface
a promising alternative to conventional methods,2 and some metal- of organic nanotubes (ONT) via the interfacing of a polycationic
based3–6 and metal-free7,8 approaches have been devised. Further layer. First, the organic nanotube platform was produced from
developments have also led to the recent discovery of co-catalytic the assembly and photo-polymerization of diacetylene (DA)
systems based on metalloenzyme mimics made of a redox-active nitrilotriacetate (NTA) amphiphiles DANTA (Fig. 1). Based on
co-factor and a metal centre.9 These cooperative biomimetic the protonation state of the NTA polar head, different aggregates
schemes are based on the use of a quinone-type co-catalyst together can be obtained such as spherical micelles (under alkaline
with Cu salts10 or Pt/Ir nanoclusters.11 Although these approaches conditions), or bilayer assemblies (at neutral/acidic pH).15 Here,
are quite efficient for the conversion of primary (and secondary) a concentrated solution of DANTA in MeOH/EtOH 1 : 1 was
amines to imines, some challenges still remain to be tackled such added dropwise to cold water (pH 6.5). After stirring for a few
as, for example, the development of a bioinspired co-catalytic minutes, the clear mixture became turbid, indicating the
system that would operate at room temperature, under ambient formation of supra-molecular nanostructures.
air (no added O2), and that could be easily recycled.
DANTA self-assembled into bilayers with polyacid head-groups
With these prerequisites in mind and as part of our long oriented towards the aqueous medium and diacetylene-containing
standing interest in heterogeneous gold-catalyzed reactions,12–14 tails densely packed into an inner hydrophobic domain.16
we developed a novel nanohybrid made of gold nanoparticles
a CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage,
91191 Gif-sur-Yvette, France. E-mail: edmond.gravel@cea.fr, eric.doris@cea.fr
b Department of Chemistry, Indian Institute of Technology Bombay,
Mumbai 400 076, India. E-mail: irishi@chem.iitb.ac.in
c
´
CEA, IRAMIS, Nanosciences et Innovation pour les Materiaux,
´
la Biomedecine et l’Energie, UMR3299, 91191 Gif-sur-Yvette, France
† Electronic supplementary information (ESI) available: Experimental details and
Scheme 1 Overview of the catalytic process.
spectral data for all compounds. See DOI: 10.1039/c4cc07951e
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Fig. 1 (a) Structure of DANTA amphiphile; (b) structure of PDADMAC;
(c) proposed mechanism for the polymerization of diacetylenes upon
UV irradiation.
These bilayers slowly rearranged into twisted ribbons (Fig. 2a
and b) and helices (Fig. 2c and d), and further torsion eventually
led to well-defined nanotubular structures (Fig. 2e) of very narrow
diameter distribution centred on ca. 45 nm (Fig. 2f). The organic
nanotubes were subsequently photo-polymerized at 254 nm to
reinforce cohesion of the supra-molecular assemblies. UV irradia-
tion produced an ene–yne conjugated network (Fig. 1c) that
connected the amphiphilic units together. At this point, stable
ONT displaying carboxylate groups at their surface were obtained.
In a second step, the nanotubes were coated with a cationic polymer,
namely poly(diallyldimethylammonium chloride) (PDADMAC,
Fig. 1b), which was immobilized on the anionic surface by electro-
static interactions. Preformed gold nanoparticles17 were finally
anchored to the tubular platform by interaction with the poly-
ammonium network to deliver the nanohybrid co-catalyst (Fig. 2g).
Transmission electron microscopy (TEM) analysis indicated dense
and homogeneous covering of the organic nanotube surface by
AuNPs of regular shape and size (ca. 3 nm, Fig. 2h). Metal
content of the nanohybrid aqueous suspension was assessed
by ICP-MS which indicated a gold concentration of 0.5 mM. The
Au0-oxidation state of gold was confirmed by X-ray photoelectron
spectrometry (XPS).
Fig. 2 Illustration (left) of the different shapes of DANTA bilayers and corre-
sponding TEM pictures (right): twisted ribbons (a and b), helix (c and d), organic
nanotube (e and f), and AuONT hybrid (g and h).
With the AuONT nanohybrid in hand, we next investigated
its potential in the aerobic oxidative coupling of primary imine 2a (Table S1, entry 5, ESI†). Superiority of the THAP
amines in the presence of a hydroquinone co-catalyst. Benzyl- system can be rationalized by hydrogen-bond stabilization of
amine (1a) was selected as a model substrate and the reaction the Schiff base transition state (Scheme S1, ESI†).
conditions were set working with 3 mol% of the hydroquinone
Kinetic parameters were calculated by running the same
derivative and 0.1 mol% of the AuONT nanohybrid in water, reaction in the presence of 0.01 mol% of gold to give a turn-over
under air, and at room temperature for 24 h (see ESI,† Table S1). number (TON) of 2900 and a turn-over frequency (TOF) of
Among the different quinone precursors that were tested as 121 hꢀ1, taking the total content of gold into account (see ESI†).
co-catalysts, dihydroxybenzenes like pyrocatechol (Table S1, When only gold surface atoms (those in contact with the medium
entry 1, ESI†) or acetylhydroquinone (Table S1, entry 2, ESI†) and active for catalysis) were considered,18 TON and TOF reached
only provided low conversions of 15 and 17%, respectively. The values of 9667 and 402 hꢀ1, respectively.
efficiency of the process could be improved with the use of
Gallacetophenone (THAP) was thus selected as co-catalyst, and
pyrogallol derivatives such as gallic acid (38%, Table S1, entry 3, the scope and limitations of the newly developed process were
ESI†), propyl gallate (40%, Table S1, entry 4, ESI†), and finally studied on a variety of amines (Table 1). In addition to 1a (entry 1),
gallacetophenone (20,30,40-trihydroxyacetophenone, THAP) which variously substituted benzylamines (1b–f) with either electron-
promoted nearly full conversion of 1a into the corresponding donating or withdrawing groups, were tested (entries 2–6).
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Table 1 Formation of symmetrical iminesa
the superior activity of the AuONT nanohybrid system (95% yield,
Table S2, entry 4, ESI†). In addition, THAP in combination with the
ONT/PDADMAC assembly (without AuNPs) also proved unsuccess-
ful (Table S2, entry 5, ESI†). Recyclability of the heterogeneous
catalyst was also assessed by performing five consecutive benzyl-
amine oxidations. After each cycle, the AuONT co-catalyst was
recovered by centrifugation and reused without any loss of catalytic
activity, as constant yields of 95% were obtained throughout the
sequential runs. In addition, TEM analysis of the recycled catalyst
showed no sintering of the gold nanoparticles. A postulated
mechanism for the co-catalytic AuONT/THAP-mediated oxida-
tive dimerization is illustrated in Scheme 2.
The first step involves the aerobic oxidation of THAP by the
AuONT nanohybrid to afford the active ‘‘quinone’’ co-catalyst a.
In the first catalytic cycle, the initial condensation of amine 1
leads to the formation of imine b with concomitant elimination
of a molecule of water (X = O). Rearomatization through proton
transfer then affords isomeric imine c which, upon addition of a
second equivalent of 1, yields aminal intermediate d. Further
evolution of the transient intermediate d, by elimination of
acetylaminoresorcinol e, finally releases the expected imine 2.
In the second catalytic cycle, the aerobic oxidation of e by AuONT
produces iminoquinone a0 (X = NH) which is likely also active
in catalyzing imine formation. This mechanism involving the
intermediacy of e is supported by the fact that ammonia could be
detected in the course of the reaction using potassium tetra-
iodomercurate (Nessler’s reagent). In addition, aminoacetyl-
resorcinol e could be isolated after completion of the reaction,
whereas no remaining THAP was detected.
Entry Amine 1
Imine 2
Yieldb (%)
95
1
2
3
4
98
98
97
5
95
6
7
98
87
8
33c
9
—
NRd
90
10e
The co-catalytic synthesis of unsymmetrical imines using the
THAP/AuONT system was also studied by reacting benzylamine (1a)
with less readily oxidized amines 3, under the above conditions
a
Conditions: 1 (0.1 mmol), AuONT (200 mL of a 0.5 mM suspension
in H2O, 0.1 mol%), THAP (0.003 mmol, 3 mol%), H2O (1 mL), room
b
d
temp., air, 24 h. Yield of isolated product. c 1H NMR yield. No
1
(Table 2). The ratio of imine products was assessed by H-NMR
e
reaction. Reaction run in MeOH (1 mL).
analysis of the crude mixture after total consumption of starting
1a (ca. 24 h). The use of 1 equivalent of p-anisidine 3a afforded
These substrates were all converted into the corresponding
symmetrical imines within 24 h, in excellent yields (95–98%).
Moreover, under the same conditions, furfurylamine 1g readily
reacted to afford imine 2g in 87% yield (entry 7). In contrast,
a-substituted phenylethylamine 1h could only be partially
converted (33%) after 24 h of reaction (entry 8), and secondary
N-methylbenzylamine 1i failed to react (entry 9). To assess the
reactivity of aliphatic amines with our catalytic system, cyclo-
hexylmethylamine 1j was tested which underwent conversion
into 2j in 90% yield (entry 10). It is to be noted that in the latter
case, the aqueous solvent had to be replaced by methanol
which had a beneficial effect on the isolated yield of product.
The reactivity of the AuONT hybrid on the oxidation of benzyl-
amine was compared to that of other co-catalysts, in association
with THAP. The first control experiment was conducted using
THAP alone (see ESI,† Table S2, entry 1) but no conversion was
detected. Neither the combination of THAP with gold salts
(HAuCl4, Table S2, entry 2, ESI†) nor non-supported gold nano-
particles (Table S2, entry 3, ESI†) provided satisfactory conversions,
as the former was totally unreactive while the latter only led to
15% of the condensed imine, after 24 h. These results highlight
Scheme 2 Proposed mechanism for the AuONT–gallacetophenone
co-catalyzed formation of imines.
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Table 2 Formation of unsymmetrical iminesa
Support from the Indo-French Centre for the Promotion of
Advanced Research (IFCPAR)/Centre Franco-Indien pour la
´
Promotion de la Recherche Avancee (CEFIPRA) is gratefully
acknowledged (Project no. 4705-1). 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).
Entry
1
Amine 3
Imine 4
4/2ab
40 : 60c
55 : 45d
Notes and references
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a
Conditions: 1a (0.1 mmol), 3 (0.2 mmol), AuONT (200 mL of a 0.5 mM
suspension in MeOH, 0.1 mol%), THAP (3 mol%), MeOH (1 mL), room
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`
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`
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