Three-component Au—Chitosan—SiO2 systems as heterogeneous catalysts for intramolecular cyclization of 2-(2-phenylethynyl)aniline

Article abstract of DOI:10.1007/s11172-015-1232-8

A series of three-component heterogeneous catalysts Au—Ch—SiO₂ with Ch for chitosan was prepared. The size of gold nanoparticles immobilized on the chitosan matrix depends on the method of sample preparation. The particles with the size <3 nm are formed when the preliminary prepared homogeneous Au—Ch complex is supported on SiO₂. The catalysts were tested in the intramolecular cyclization reaction of 2-(2-phenylethynyl)aniline to for 2-phenylindole. The maximum conversion of 2-(2-phenylethynyl)aniline (25% within 35 h) was obtained on the Au(0.4%)—Ch(2.9%)/SiO₂ catalyst.

Full text of DOI:10.1007/s11172-015-1232-8

2
816
Russian Chemical Bulletin, International Edition, Vol. 64, No. 12, pp. 2816—2820, December, 2015
Threeꢀcomponent Au—Chitosan—SiO systems
2
as heterogeneous catalysts for intramolecular cyclization
of 2ꢀ(2ꢀphenylethynyl)aniline
E. D. Finashina,a O. P. Tkachenko, A. Yu. Startseva, V. G. Krasovsky, L. M. Kustov, and I. P. Beletskayab
a
a
a
a,b
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Eꢀmail: lmk@ioc.ca.ru
b
Chemical Department, M. V. Lomonosov Moscow State University,
/3 Leninskie Gory, 119992 Moscow, Russian Federation
1
A series of threeꢀcomponent heterogeneous catalysts Au—Ch—SiO with Ch for chitosan
2
was prepared. The size of gold nanoparticles immobilized on the chitosan matrix depends on
the method of sample preparation. The particles with the size <3 nm are formed when the
preliminary prepared homogeneous Au—Ch complex is supported on SiO . The catalysts were
2
tested in the intramolecular cyclization reaction of 2ꢀ(2ꢀphenylethynyl)aniline to for 2ꢀphenylꢀ
indole. The maximum conversion of 2ꢀ(2ꢀphenylethynyl)aniline (25% within 35 h) was obꢀ
tained on the Au(0.4%)—Ch(2.9%)/SiO catalyst.
2
Key words: catalytic intramolecular hydroamination of 2ꢀ(2ꢀphenylethynyl)aniline, cataꢀ
lysts based on gold nanoparticles, chitosan.
Reactions of intramolecular cyclization of 2ꢀalkynylꢀ
anilines are among the most promising methods for the
preparation of indoles, which are components of many
prevent enlargement of small particles in these catalytic
systems. One of the approaches frequently used to prevent
gold nanoparticle agglomeration is their immobilization
1
10
natural and synthetic substances with biological activity.
into matrices of various polymers.
II
II
These reactions are usually catalyzed by the Pd and Cu
Chitosan (poly(1—4)ꢀNꢀacetylꢀβꢀDꢀglucosamine) is
linear polysaccharide with polymer chain built of βꢀ1,4ꢀ
bonded glucosamine residues and minor amounts of
Nꢀacetylglucosamine (chitin) units. The primary and secꢀ
ondary structures of this polysaccharide resemble the celꢀ
lulose structure. Chitosan is the product of the deacetylaꢀ
tion of natural biopolymer chitin, which is the major comꢀ
ponent of shells of various Crustacea. Therefore, chitosan
is the second in abundance in nature after cellulose.
2
,3
compounds. The possibility that this reaction occurs
in the presence of the AuIII salts or Au compounds has
recently been shown (Scheme 1). However, only homoꢀ
geneous catalytic systems were used in the published
I
4
—7
works.
Scheme 1
Since a monomeric unit of chitosan contains nitroꢀ
genꢀ and oxygenꢀcontaining functional groups, chitosan
exhibits a unique sorption ability toward many metal ions
and even metal atoms. The polymer matrix of chitosan
can irreversibly bind various ions and for this reason it
attracts attention mainly as an efficient system of removal
of heavy metal cations from dilute aqueous solutions. The
studies of the sorption properties of the gold—chitosan
i. CH Cl /HCl , EtOH, EtOH/H O, ~20 °C.
2
2
aq
2
R = Ar, Alk, vinyl, H
1
1,12
complexes have been started fairly long ago,
and the
Data on the application of gold nanoparticles as heteroꢀ
geneous catalysts of intramolecular cyclization of various
ethynylanilines are very restricted.
possibilities of applying the chitosan polymer matrix for
the stabilization of gold nanoparticles and using these sysꢀ
tems in heterogeneous catalysis were considered.9
8
,9
It is known that the heterogeneous goldꢀcontaining
catalysts are characterized by size effects, since their acꢀ
tivity depends on the particle size. It is very important to
The use of chitosan as a support in catalysis is fraught
with problems. The first problem is a small specific surface
area of chitosan, and the second problem is insufficient
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2816—2820, December, 2015.
066ꢀ5285/15/6412ꢀ2816 © 2015 Springer Science+Business Media, Inc.
1

Au—Ch—SiO as heterogeneous catalysts
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 12, December, 2015 2817
2
resistance to attrition. One of the methods for surmountꢀ
ing these problems is the preparation of threeꢀcomponent
systems metal—chitosan—oxide support (the latter should
was refluxed for 6 h in an ethanolic solution of NaAuCl4 with
a reflux condenser. The progress of the reaction was followed
by the decolorization of an ethanolic solution of the goldꢀconꢀ
taining precursor. After reflux, the precipitate was filtered,
washed with EtOH, and dried in air for 24 h.
To obtain samples IV and V, the homogeneous Au—Ch comꢀ
plex was preꢀsynthesized. The calculated amount of the goldꢀ
containing precursor corresponding to the ionꢀexchange capacꢀ
ity of chitosan17 was introduced into a chitosan solution in 1.5%
AcOH. The suspension was stirred to the complete dissolution of
sodium chloroaurate. The obtained Au—Ch complex was immoꢀ
bilized in silica gel by the incipient wetness impregnation method.
After impregnation, the catalysts were kept for 16 h in a 0.5 М
NaOH solution. The catalyst was filtered and multiply washed
with distilled water to a neutral pH. The prepared catalysts were
dried in air for 24 h.
Intramolecular hydroamination was carried out in a roundꢀ
bottom flask with a reflux condenser. In a standard experiment, the
reaction mixture was stirred at ~20 °C in argon. The molar ratio subꢀ
strate : catalyst was 20 : 1, and ethanol (3—6 mL) served as a solvent.
The progress of the reaction was monitored using GLC
have a sufficient surface area and relatively high mechaniꢀ
cal strength1
3—16
).
The purpose of this work is the development of active
threeꢀcomponent catalysts Au—Ch—SiO (Ch is chitoꢀ
2
san) for the intramolecular cyclization of 2ꢀ(2ꢀphenylꢀ
ethynyl)aniline to 2ꢀphenylindole.
Experimental
Commercial reagents 2ꢀ(2ꢀphenylethynyl)aniline (Oakꢀ
woodChemical, 99+%), chloroauric acid, sodium chloroaurate,
2
ꢀphenylindole (95%) (Sigma—Aldrich), ethanol, and glacial
acetic acid (reagent grade, Reakhim) were used as received.
Chitosan was obtained from crab shells (South Korea, moꢀ
lecular weight 100 000—150 000, degree of deacetylation of amino
groups 70%, moisture content 3 wt.%) and was used without
additional purification.
(
KristaLyuks 4000 M chromatograph, capillary column OV 1
2
–1
Silica gel КСК (SBET = 330 m g (GOST 3956ꢀ76, Karpov
with a length of 30 m, flameꢀionization detector, helium as
a carrier gas). The temperatures of the evaporator, detector, and
column (isotherm) were 240, 240, and 210 °C, respectively. The
total analysis time was 11 min. The sensitivity coefficients of the
detector to the components of the reaction mixture determined
by calibration mixtures were 1.1 and 0.9 for 2ꢀ(2ꢀphenylꢀ
ethynyl)aniline and 2ꢀphenylindole, respectively.
Chemical Plant, Mendeleevsk, Russia) and silica gel КСС No. 3
2
–1
(
SBET = 450 m g (GOST 3956ꢀ76, Reakhim) were used as
supports. The silica gels were multiply washed with dilute HCl to
remove iron impurities.
The threeꢀcomponent catalysts were obtained by three difꢀ
ferent methods: samples I and II were prepared by depositing
gold and chitoses on silica gel according to the published proceꢀ
Diffuse reflectance infrared Fourier transform spectra
9
dure, sample III was obtained by depositing gold on silica gel
(
DRIFTS) were recorded at ~20 °C on a Nicolet 460 Protégé
that was impregnated several times with chitosan, and samples
IV and V were prepared by depositing the preꢀsynthesized homoꢀ
geneous Au—Ch complex on silica gel. The calculated composiꢀ
tion of the samples and their designations are presented in Table 1.
To obtain sample III, silica gel КСК (fraction 0.85—0.1 mm,
humidity ratio 0.4 mL g–1) was three times impregnated with
a chitosan solution in 1.5% AcOH (2.2 g of chitosan in 150 mL of
the acid) using the incipient wetness impregnation method.
After each impregnation, silica gel with supported chitosan was
placed in a 0.5 М NaOH solution and kept for 3 h, after which
the impregnated silica gel was washed to a neutral pH with disꢀ
tilled water and filtered. The obtained white powder was dried
for 1 h at 80 °C. The amount of chitosan supported on silica gel
was determined from an increase in the weight of the initial
spectrometer with the diffuse reflectance attachment. Catalyst
samples were placed in ampules with a СаF window. A CaF
2
2
powder was used as a reference sample. To obtained a satisfactory
signal to noise ratio, 500 spectra were accumulated. IR spectra
–
1
were measured in a frequency range of 400—6000 cm with
a resolution of 4 cm– . Carbon monoxide served as a molecular
test, which was adsorbed at ~20 °C and at an equilibrium presꢀ
sure of 15 Torr.
1
The phase composition of the catalyst samples was deterꢀ
mined by Xꢀray diffraction analysis (XRD). The XRD patterns
were detected on a DRONꢀ2 instrument in Niꢀfiltered CuꢀKα
radiation (λ = 0.1542 nm) in the step scan mode (with an increꢀ
ment of 0.02°) in the range 2θ = 10—80°. The crystalline phases
were identified by comparing of the position and intensity of
peaks in the Xꢀray pattern with ICDD references.
silica gel. To deposite gold on the Ch—SiO support, the latter
2
The photoelectron (XPS) spectra were recorded on a SES
2
002 photoelectron spectrometer with a hemispherical analyzer
Table 1. Composition of the threeꢀcomponent Au—Ch—SiO2
catalysts
(
GammadataꢀScienta, monochromatic AlꢀKα radiation, hν =
–
10
=
1486.6 eV). The main chamber was evacuated to 4•10
Torr.
The measurements were carried out at the energy of the analyzer
fixed at a value of 200 eV that gives the resulting absolute resoluꢀ
tion equal to 0.25 eV. The charge of the sample surface was
compensated by a lowꢀenergy electron beam. In the further proꢀ
cessing, the Si2p binding energy (102.7 eV) was used as a referꢀ
ence to additionally correct the XPS peak positions. The binding
energy (BE) scale was preꢀcalibrated by the position of peaks of
the framework levels Au4f7/2 (84.0 eV), Cu2p3/2 (932.7 eV), and
Ag3d5/2 (368.3 eV). The surface atomic ratios of elements were
calculated from the line intensities and corresponding photoionꢀ
Catalyst
Trade mark of
silica gel
Composition of
threeꢀcomponent catalyst
I*
II
III
IV
V
КСС No. 3
КСС No. 3
КСК
КСК
КСС No. 3
Au(3.5%)—Ch(18%)—SiO2
Au(4%)/Ch(29%)—SiO2
Au(0.8%)/Ch(5.9%)—SiO2
Au(0.4%)—Ch(2.9%)/SiO2
Au(0.6%)—Ch(4.3%)/SiO2
*
According to the ICPꢀOES data, the gold content in the cataꢀ
1
8
lyst is 2.78 wt.%.
ization cross sections.

2818
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 12, December, 2015
Finashina et al.
The gold content in the catalysts was determined by Inducꢀ
tively coupled plasma/optical emission atomic spectroscopy
ICPꢀOES) on an optical emission spectrometer (Unicam,
PU700).
Au4f7/2
Au4f5/2
(
Results and Discussion
The XRD study of the obtained catalysts shows that
the size of gold nanoparticles immobilized in the chitosan
matrix depends on the preparation method (Table 2). The
size of gold particles in samples IV and V prepared by the
deposition of the preꢀformed homogeneous Au—Ch comꢀ
plex on silica gel does not reach 3 nm. The synthesis acꢀ
94
92
90
88
86
84
82
80 E /eV
b
9
cording to a described procedure gives rather large gold
particles: 14—15 nm in size (samples I and II). The partiꢀ
Fig. 2. XPS Au4f spectrum.
cles 26 nm in size are formed by gold immobilization on
the Ch—SiO support (sample III).
2
The shape of the XPS spectrum of the Au(3.5%)/Ch—
SiO sample (Fig. 1) resembles that of the spectrum charꢀ
2
acteristic of silica, since it contains highꢀintensity peaks
from С1s and framework levels Si2p, Si2s, and О1s, as
well as lowꢀintensity Au4f doublets. The lines of the N1s
framework level are also well seen in the spectra.
The position of the lines of the Au4f7/2 framework levꢀ
els of the Au(3.5%)/Ch—SiO sample (Fig. 2) is insignifiꢀ
2
cantly shifted toward higher BE compared to the BE corꢀ
responding to the metallic state of gold (84.0 eV). In addiꢀ
tion, the components of the Au4f doublet in the spectrum
of this sample have a larger FWHH (full width at halfꢀ
height) compared to the spectra of two other samples,
indicating an additional state of gold. In fact, the deꢀ
composition of the spectrum into components (Fig. 3) in
the form of the Voigt function (deconvolution of the
Gaussian and Lorentzian functions) gives two doublets
with the intensity ratio 8.2 : 1. A more strong Au4f7/2 line
with a BE of 84.0 eV corresponds to the metallic state
of gold, whereas the component with weaker intensity of
the doublet has the BE equal to 85.7 eV, which is close
to the value determined earlier19 for the organic gold
complexes.
9
2
90
88
86
84
82
80 Eb/eV
Fig. 3. Deconvolution of the photoelectron Au4f spectrum of the
Au(3.5%)/Ch—SiO sample.
2
The initial catalysts I, III, IV, and V and the same
catalysts after the catalytic reaction were studied by
the DRIFTS method. Prior to CO feeding, the samꢀ
ples were treated in vacuo at 20 °C for 3 h to remove
physically adsorbed gases and water. The adsorption
of CO was carried out at ~20 °C and at an equilibrium
pressure of 15 Torr. The obtained results are presented
in Table 2.
When using carbon monoxide as a testing molecule to
specify the electronic state of gold, it was found that
CO was not adsorbed on all initial samples even after
prolong (17 h) storage in CO at ~20 °C. Lowꢀintenꢀ
sity bands of CO adsorbed on the Au sites were detectꢀ
ed in the spectra of catalysts III and IV used in catalꢀ
ytic intramolecular cyclization. The intensity of the
spectra is very low. After desorption to vacuum at ~20 °C,
no band of adsorbed CO was observed in the spectra.
O1s
The stretching vibration frequencies in CO molecules
adsorbed on the Au sites are 2188 and 2172 cm–
1
.
O KLL
Si2p
These bands can be assigned to stretching vibrations
in CO molecules in the linear carbonyl complexes
with gold cations (Au ). The difference in the vibraꢀ
C1s Si2s
Au4f
N1s
n+
1
200 1000
800
600
400
200
E /eV
tion frequencies can be related to both different effective
charges of the gold cations and their different environꢀ
ments.
b
Fig. 1. XPS review spectrum of the Au(3.5%)/Ch—SiO sample;
E is the binding energy.
2
2
0
b

Au—Ch—SiO as heterogeneous catalysts
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 12, December, 2015 2819
2
Table 2. Sizes of gold particles. character of CO adsorption, and electronic state of gold in the threeꢀcomponent
Au—Ch—SiO catalysts obtained by the interaction of HAuCl with the NH groups of the support and reduced on
2
4
2
reflux of the polymer matrix in EtOH
Catalyst
Size of Au
particles/nm (XRD)
Character of CO adsorption
according to DRIFTS data
Electronic state
of Au (XPS)
0
+
I
14
No adsorption is observed
Au , Au ,
organic gold complexes
0
II
15
26
<3
<3
No adsorption is observed
No adsorption is observed
Lowꢀintensity bands of adsorbed CO
Lowꢀintensity bands of adsorbed CO
Au
Au
0
III
IV*
V
Was not studied
Was not studied
*
Unreduced sample.
Table 3. Results of the intramolecular cyclization of 2ꢀ(2ꢀphenylethynyl)aniline on the threeꢀcomponent Au—Ch—SiO2
catalysts (~20 °C)
Catalyst
Synthesis time
Conversion
of 2ꢀ(2ꢀphenylethynyl)aniline (%)
Specific productivity•103
/mmole of indole (mmole of Au) h–1
–1
I
II
20
84
3
1.0
+8*
4
10
2.0
6
.0
III
IV
V
85
35.5
44.5
12.6
25.1
13.8
30
140
60
*
60 °С.
The results obtained on investigating the catalysts by
the DRIFTS methods are presented in Table 2 and supꢀ
plemented by the XRD and XPS data.
This work was financially supported by the Russian
Science Foundation (Project No. 14ꢀ50ꢀ00126).
Catalysts I—V were tested in the intramolecular hydroꢀ
amination of 2ꢀ(2ꢀphenylethynyl)aniline. According to the
chromatographic analysis data and the results of prelimiꢀ
nary study of the reaction mixtures by Н and С NMR
spectroscopy, 2ꢀphenylindole is the only product of the
conversion of 2ꢀ(2ꢀphenylethynyl)aniline under the studꢀ
ied conditions on all the threeꢀcomponent Au—Ch—SiO2
catalysts (Scheme 2).
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in revised form October 23, 2015