Nanostructured oxytyramine catalyst for the facile one-pot synthesis of cyclohexanecarbonitrile derivatives

Article abstract of DOI:10.1039/c5ra20150k

The magnetic, recyclable heterogeneous organocatalyst OT@Si@SPIONs has been developed in this report, with the aim of synthesizing cyclohexanecarbonitriles. The prepared nanocatalyst was fully characterized by various techniques and its catalytic activity

Full text of DOI:10.1039/c5ra20150k

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Nanostructured Oxytyramine Catalyst for Peerless One-Pot Synthesis of
Cyclohexanecarbonitrile Derivatives
Priya Arora, Jaspreet Kaur Rajput#, Harminder Singh
Department of Chemistry, Dr. B R Ambedkar National Institute of Technology, Jalandhar ꢀ144011,
Punjab, India
Corresponding Author: #rajputj@nitj.ac.in, Tel.: +91-181-2690301 02x2208; fax: +91 181 2690320
Abstract
Magnetic recyclable heterogeneous organocatalyst OT@Si@SPIONs, has been developed in this
report, with the aim to synthesize cyclohexanecarbonitriles. The prepared nanocatalyst was fully
characterized with various techniques and its catalytic activity has been checked by one pot reaction
which involves in situ formation of imine from cyclohexanone and amines which further undergo
nucleophilic addition with TMSCN. Moreover, although this Strecker reaction is century old reaction,
its applicability with ketones is still a less explored subject, and our investigation provided an insight
into the efficient synthesis of cyclohexanecarbonitrile derivatives using a ketone i.e. cyclohexanone.
As compared with other reported catalytic systems in literature our showed superior catalytic activity
at a remarkably low catalyst loading i.e. 5 mg.
Keywords: Oxytyramine anchored SiꢀSPIONs, magnetically recoverable heterogeneous catalyst,
cyclohexanecarbonitrile derivatives, green, recyclable.
1. Introduction
1
Combination of nanotechnology with green chemistry had proven to be a boon for industries .
Chemical and pharmaceutical industries are keenly looking forward for economical and
environmentally benign measures of recycling catalyst 2 as homogeneous catalysis though endowed
with chemo, regio and enantioselectivity of the catalyst imposes difficulty of catalyst separation from
3
the final product . In such respect, nanoꢀmagnetic catalysis plays a major role in heterogeneous
catalysis as it provides easy magnetic recovery of catalyst from the reaction mixture with the help of a
4
permanent magnet
hence vanishing the use of conventional techniques like filtration and
centrifugation etc. Nanocatalysts either metallic nanoparticles or supported magnetic nanoparticles
have high catalytic activity, high degree of chemical stability, act as simple, robust, readily available
5
high surface area heterogeneous catalysts . Among the different magnetic nanocatalysts known to
date, Fe3O4 nanoparticles have greatest impact in this field as they can efficaciously reinforce the
6
functioning of different moieties by acting as a support to them and result in heterogeneous and
magnetically separable catalysts due to their superparamagnetic character. Albeit bare iron oxide
nanoparticles are hard to handle due to their extreme reactive nature towards oxidation but this
extreme reactivity can be mitigated by tailoring the surface of nanoparticles. Along with high
1

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reactivity, many other pitfalls of bare nanoparticles like formation of large aggregates to minimize
surface energies, loss of dispersibility, low thermal stability and high chemical activity can also be
lessened by applying some protection strategies which include grafting or coating of an inert material
7. The most studied inert material for protecting iron oxide nanoparticles is silica, which can be further
8
used to anchor the catalytic molecules into its pores to generate catalytic centres . Many organic
ligands have also been reported to stabilize the magnetic nanoparticles and also for their particular
9
application . Magnetic iron nanoparticles and their functionalized counterparts have numerous
applications in various fields such as data storage 10, catalysis 6,11, drugꢀtargeting 12, cancer therapy 13
lymph node imaging or hyperthermia 14, magnetic resonance imaging 15, sensors 16 and many more.
,
Ample attention on bifunctional entities has been attracted in organic synthesis because of their vast
applications in drug designing. Amino nitriles are potent representators of bifunctional compounds.
Both amino and nitrile moieties can easily undergo diversity of modifications allowing rapid advance
towards synthetically important organic products. Transformation of nitrile group in αꢀaminonitriles
to carboxylic acid by hydrolysis leads to formation of αꢀamino acid and this transformation was first
carried out by Adolph Strecker in 185017, thereafter termed as Strecker synthesis, since then vast
research has been carried out on this reaction with various catalysts like Yb(OTf)3ꢀpybox18,
lanthanum(III)ꢀbinaphthyl disulfonate19, Jacobsen’s thiourea catalyst20, mesoporous aluminosilicate
(AlꢀMCMꢀ41)21, Fe(Cp)2PF622, Nꢀheterocyclic carbene (NHC)ꢀamidate palladium (II) complex23,
25
BINOLꢀphosphoric acid24, K2PdCl4
,
gallium (III) triflate26, IBX/TBAB27, nanocrystalline
magnesium oxide28, superparamagnetic iron oxide29 and Lewis base30 e.g. N,Nꢀ
dimethylcyclohexylamine etc., various sources of amino and cyano groups like HCN, KCN,
(EtO)2P(O)CN, Et2AlCN, Bu3SnCN and TMSCN31,32. Strecker reaction is the simplest and
economical method for the synthesis of αꢀaminonitriles which are versatile synthons of various amino
acids and many other bioactive compounds including natural products. Recently various
pharmacologically useful compounds like phthalascidin 62233, hepatitis C virus NS3 serine protease
inhibitors34 and boron containing retinoids35 have been synthesized following this strategy. Strecker
reaction with ketones has been rarely studied due to very slow reaction rates as ketones are
challenging substrates due to their steric demands during the formation of CꢀC bond36. Often ketone
Strecker reaction is carried out stepwise by using premade imines or by using high pressure
conditions. Although some of one pot procedures have been reported but most of them involve the use
of expensive reagents, harsh conditions, tedious work up and long reaction times. So we herein report
the Strecker reaction involving ketones i.e. cyclohexanone as substrate with good results at ambient
temperature and solvent free conditions. The advantage of this methodology is the simplicity of the
procedure, which avoided the use of tedious chromatographic purification of products and no use of
harsh organic solvents thus supporting the central issue of today’s research i.e. green chemistry.
So in continuation of our previous work 37,38, we describe here environmentally sound protocol for
facile synthesis of cyclohexanecarbonitrile derivatives using cyclohexanone. Herein, we also report
2

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the preparation and structural determination of a new organocatalyst oxytyramine immobilized
silica@Fe3O4 nanocatalyst. Oxytyramine is a naturally occurring catecholamine which acts as an
inotropic agent. It is an essential part of human body as it has many important functions like it
controls the motor abilities, motivation, concentration, sleep, mood, memory and learning etc 39. It
can improve the functioning of the heart and is also used for the treatment of Parkinson’s disease. So
the oxytyramine anchored SiꢀSPIONs can be used in variety of applications. The utilization of
magnetic nanoparticles as catalysts for this reaction is one approach i.e. green, inexpensive, facile and
widely applicable.
2. Result and Discussion
2.1 Synthesis of OTꢀSiꢀSPIONs
The core of the oxytyramine graftedꢀsilica coated superparamagnetic iron oxide nanoparticles (OTꢀSiꢀ
40
SPIONs) has been synthesized by coꢀprecipitation method combined with low power sonication.
41
Modified Stöber method in addition to ultrasonication, has been used to prepare OTꢀSiꢀSPIONs,
which has an advantageous effect that this method does not require any high temperature treatment to
get crystalline nanoparticles as ultrasonic waves generate some localized hot spots which induce in
situ calcinations 42. Oxytyramine moiety has been coated on SiꢀSPIONs as former is high affinity
binding molecule and thus stabilize the SPIONs. The –NH2 group on the moiety could be used as the
reaction site. The synthesis of magnetite nanoparticles and its functionalization have been represented
in Scheme I.
OC2H5
O
O
C2H5O
Si
Si
OC
HO
O
O
OH
Fe2+
O
NH4OH
Fe3O4
Fe3O4
2Fe3+
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Scheme I: Schematic representation of coating of silica and oxytyramine over magnetite nanoparticles.
2.2 Characterization of prepared OTꢀSiꢀSPIONs
FT-IR:-
In order to depict the surface modification of SPIONs with TEOS and oxytyramine, the FTꢀIR spectra
of SPIONs, SiꢀSPIONs and OTꢀSiꢀSPIONs are illustrated in Fig. 1A in which Fig. 1(a) displays FTꢀIR
spectrum of SPIONs. The absorbance bands at 590 and 633 cmꢀ1 are ascribed to FeꢀO vibration and
are consistent with the reported IR spectrum for spinel Fe3O4 43. The IR bands at 3393 and 1625 cmꢀ1
are due to stretching and deformation vibration of –OH groups which are due to adsorbed water on the
surface of magnetite nanoparticles. FTꢀIR spectrum of SiꢀSPIONs is shown in Fig. 1(b). The
absorption band at 1097 cmꢀ1 characteristic of SiꢀO group, demonstrate covalent bond between
nanoparticles surface and silane. Bands at 957 cmꢀ1 and 801 cmꢀ1 are due to vibration of SiꢀOꢀSi
groups. These results indicate successful immobilization of SiO2 on the surface of Fe3O4 nanoparticles.
Fig. 1(c) depicts the FTꢀIR spectrum of prepared OTꢀSiꢀSPIONs. Several new peaks at 1281, 1488
and 1614 cmꢀ1, characteristic of CꢀO vibrations, benzene ring CꢀC vibrations and NꢀH stretching
respectively, confirms the successful coating of oxytyramine over silica coated magnetite
nanoparticles.
XRD:-
As presented in Fig. 1B, SPIONs show characteristic diffraction peaks at different values of 2θ i.e.
30.12, 35.54, 43.13, 53.56, 57.17, 62.78 with corresponding diffraction planes 220, 311, 400, 422, 511
44
and 440 which are totally consistent with spinel cubic Fe3O4 diffraction peaks (JCPDS 19ꢀ0629)
.
Fig. 1B(b) shows the XRD pattern of SiꢀSPIONs, showing exactly the same peaks as that of Fe3O4
nanoparticles in addition to it a broad hump at 2θ= 20ꢀ25 is observed which is characteristic of
º
amorphous silica. The repetition of the same peaks depict that the crystallinity of the nanoparticles has
been restored even after its functionalization. The XRD pattern of OTꢀSiꢀSPIONs is shown in Fig. 1B
(c). The broad peak at 2θ= 20ꢀ25 in SiꢀSPIONs has been shifted to lower values due to synergistic
º
effect of amorphous silica. Also the intensity of the characteristic peaks of SPIONs further decreases
because of surface coating of shell layers, which confirms the coverꢀup of subsequent layers. The
average size of SPIONs, as calculated by Debye Scherrer formula is found to be 13 nm.
~
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1A
1B
OT-Si-SPIONs
SPIONs
Si-SPIONs
c
b
a
Fig. 1: A. FTꢀIR spectra of : (a) SPIONs; (b) SiꢀSPIONs; (c) OTꢀSiꢀSPIONs, B. XRD Spectra of: (a) SPIONs; (b) Siꢀ
SPIONs; (c) OTꢀSiꢀSPIONs.
Morphology:-
Fig. 2 displays the TEM (aꢀc) images of SPIONs, SiꢀSPIONs and OTꢀSiꢀSPIONs which reveal that
iron oxide nanoparticles are spherical in nature with size ranging from 12ꢀ15 nm which is consistent
with the size calculated from XRD data and are present in nonꢀaggregate form. Fig. 2b shows the
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coreꢀshell structure of silica coated iron oxide nanoparticles and Fig. 2c portray the mosaic type
structure of OTꢀSiꢀSPIONs in which organic moiety, oxytyramine has been coated over multiple Siꢀ
SPIONs. Fig. 2e shows the SEM image of OTꢀSiꢀSPIONs confirming its spherical shape.
Fig. 2 TEM images of: (a) SPIONs; (b) SiꢀSPIONs; inset shows its coreꢀshell structure (c) OTꢀSiꢀSPIONs; (d) Recovered
OTꢀSiꢀSPIONs after 5th cycle; (e) SEM image of OTꢀSiꢀSPIONs.
Magnetic Properties:-
Vibrating sample magnetometer was used to examine the magnetic properties of synthesized
nanoparticles i.e. OTꢀSiꢀSPIONs at room temperature. Fig. 3 describes the VSM plot of oxytyramine
anchored SiꢀSPIONs, which depicts superparamagnetic nature of nanoparticles in which the saturation
magnetization value comes out to be 2.413 emu/g which is very small as compared to its bulk value of
uncoated SPIONs i.e. approximately 90 emu/g 45. This decrease in saturation magnetization is because
of very small size of the synthesized nanoparticles as there is a linear correlation between particle size
and saturation magnetization and also due to nonꢀmagnetic layer on the surface of nanoparticles the
value further decreases. The zero coercivity and small remanence value also proves the
superparamagnetic properties of synthesized functionalized nanoparticles.
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Fig. 3 VSM image of OTꢀSiꢀSPIONs.
Surface properties:-
Fig. 4 shows the N2 adsorptionꢀdesorption isotherm of the OTꢀSiꢀSPIONs. The BET surface area of
the particles was found to be 5.9115m2/g, as calculated by linear part of the BET plot. The total pore
volume at P/P0 = 0.98 is 0.0106 cm3/g and average pore diameter is 26.00626 nm. This pore size
depicts the particles are mesoporous in nature 46. The BET isotherm is of Type IV characteristic of
porous material with pore size in the range of 1.5ꢀ100 nm 47.
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(a)
(b)
Fig. 4: (a) BET plot of OTꢀSiꢀSPIONs; (b) Type IV isotherm plot of OTꢀSiꢀSPIONs.
UV-Vis :-
UVꢀVis spectral analysis (Fig. 5) has been used to confirm the formation and stability of magnetic
nanoparticles in an ethanolic colloidal solution. Pure Fe3O4 nanoparticles absorbed light with
wavelengths mainly below 750 nm (Fig. 5a), in comparison to Fe3O4 nanoparticles shift of absorption
band towards lower wavelength i.e. blue shift was noticed on silica as well as oxytyramine coating
over Fe3O4 nanoparticles. The UVꢀVis spectra of the synthesized nanoparticles were also observed
after 10 days, showing no significant change in the absorption peaks which confirms the stability of
nanocatalysts.
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(c)
(b)
(a)
Fig. 5 UVꢀVis spectra of: (a) SPIONs; (b) SiꢀSPIONs; (c) OTꢀSiꢀSPIONs.
TGA :-
To investigate the stability of the synthesized nanoparticles, thermo gravimetric analysis of SPIONs,
SiꢀSPIONs and OTꢀSiꢀSPIONs has been performed from 40ꢀ750 °C at a heating rate of 10 °C/min
under nitrogen atmosphere as shown in Fig. 6. In case of SPIONs initial weight loss of 1.9 % was
observed in the temperature range of 100ꢀ350 ºC due to loss of moisture and after 350 °C only 0.9 %
weight loss was seen till 700 °C. In case of SiꢀSPIONs, two stages of weight loss are observed in the
temperature range of 40ꢀ100°C and 100ꢀ600°C. The initial weight loss of 3.8 % at first stage
corresponds to loss of adsorbed water and ethanol. The second stage weight loss of 4 % could be
attributed to the removal of residual TEOS as well as structural water. In OTꢀSiꢀSPIONs initially 14.6
% weight loss was observed till 193.8 °C which is due to loss of moisture and volatile components.
Further, from 194ꢀ424 °C weight loss of 11.9 % was there due to loss of structural water or entrapped
hydroxyl groups on the surface of nanoparticles and also may be because of degradation of
oxytyramine. After that, no significant loss was seen till 750 °C which showed better thermal stability
of OTꢀSiꢀSPIONs. Also very less percentage of weight loss was observed at around 100 °C which
confirms the stability of the catalyst in our reaction protocol.
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(a)
(b)
(c)
Fig. 6 TGA plots of: (a) SPIONs; (b) SiꢀSPIONs; (c) OTꢀSiꢀSPIONs.
2.3 Catalytic activity of OTꢀSiꢀSPIONs
The catalytic activity of OTꢀSiꢀSPIONs was tested as a heterogeneous catalyst (Schemeꢀ
II) under various conditions (Table 1) by chosing reaction of cyclohexanone (1), aniline (2a) and
trimethylsilylcyanide (3) as a model reaction. Initially the influence of the catalyst on the model
reaction was explored at room temperature by performing the reaction without catalyst and with
catalyst (OTꢀSiꢀSPIONs) giving 67.59 % yield of the product in 94 minutes and 94.48.30 % yield of
the product in 7 minutes respectively (Table 1, Entry 1, 2). These results encouraged us to optimize
the reaction conditions. Hence further study of the loading of the catalyst was examined. As shown in
Table 1, it was found that the catalyst loading affected the reaction obviously. The use of 2.5 mg of
OTꢀSiꢀSPIONs catalyst gave 4a in only 81.72 % yield (Table 1, Entry 3). With 5 mg of the catalyst
yield has been increased to 94.48 % taking time of 7 minutes to complete the reaction (Table 1, Entry
2). Further on increasing the amount of the catalyst, decrease in yield of 4a upto 86.43 % was
obtained (Table 1, Entry 6). Thus 5 mg of the catalyst was the best choice.
In the subsequent study the reaction was conducted with various solvents to find out the appropriate
solvent. Various solvents like H2O, CH3OH, C2H5OH, THF, Toluene, CH3CN and H2O: C2H5OH
(1:1) mixture have been tried in the model reaction (Table 1, Entries 7ꢀ13). Among all of these
solvents reaction with CH3CN gave the best results with 83.43% yield in 18 minutes (Table 1, Entry
10). Further the model reaction was also carried out in the absence of solvent which gave the best
yield of 4a i.e. 94.48 % in 7 minutes (Table 1, Entry 2).
The effect of temperature was studied by carrying out the model reaction at different reaction
conditions under the optimized conditions (R.T., under refluxing on water bath [≈60°C], under
refluxing on oil bath at 120°C) (Table 1, Entries 2, 14, 15) and the best results were obtained at R.T.
(Table 1, Entry 2).
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Using all the optimized reaction conditions, the model reaction was carried out to investigate the
reaction time which was found to be 7 minutes at room temperature for compound 4a.
Further to examine the significance of supporting the catalyst and efficiency of protocol bare Fe3O4
nanoparticles (Table 1, Entry 16), silica coated Fe3O4 nanoparticles (Table 1, Entry 17), bulk
oxytyramine (Table 1, Entry 18) were also studied. To our delight, the best results were obtained with
our catalyst being reported in this paper. These results confirm that the catalytic activity derives from
the oxytyramine species present on the nanocatalyst surface as well as it is increased significantly due
to increase in the surface area of the catalyst , which is a result of supporting the active phase i.e.
oxytyramine on nanosized material.
Table 1 Optimization of different proportions of OTꢀSiꢀSPIONs nanocatalyst along with solvents and
temperature for the synthesis of 4a.
NH2
C
N
OTꢀSiꢀSPIONs
O
Si
C
N
(CH3)3
Solvent,
Temp.
N
H
1
3
2a
4a
Scheme- II
Entry
Catalyst Amount
Solvent
Temperature
Yield/ Time
(%) / (min.)
67.59 / 94
94.48 / 7
( in mg)
ꢀ
(°C)
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
60°C
1
2
ꢀ
5a
ꢀ
3
2.5a
7.5a
10a
15a
5a
5a
5a
5a
5a
ꢀ
ꢀ
81.72 / ꢀ
4
91.62 / 18
87.26 / 4
5
ꢀ
6
ꢀ
86.43 / 10
58.61 / 24
74.31/ 23
51.05 / ꢀ
7
CH3OH
C2H5OH
H2O
8
9
10
11
12
13
14
CH3CN
THF
Toluene
83.43 / 18
77.83 / 21
79.25 / 20
52.59 / ꢀ
5a
5a
5a
1:1 (C2H5OH:H2O)
ꢀ
65.67 / 29
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15
16
17
18
5a
5b
5c
5d
ꢀ
ꢀ
ꢀ
ꢀ
120°C
R.T.
60.58 / 28
79.94 / 1 day
82.19 / 1 day
76.05 / 28
R.T.
R.T.
Reaction conditions a: cyclohexanone (1, 1 mmol), aniline (2a, 1 mmol), Trimethylsilylcyanie (TMSCN, 3, 1.3
mmol), OTꢀSiꢀSPIONs (5 mg), b: Fe3O4 (SPIONs) as catalyst, c: SiꢀFe3O4 (SiꢀSPIONs) as catalyst, d:
Oxytyramine (OT) as bulk catalyst.
Further the versatility of this protocol was examined by the reaction of cyclohexanone and TMSCN
with different amines under the optimized reaction conditions and the results are summarized in Table
2. Reactions were carried out using 1mmol of cyclohexanone, 1mmol amines, 1.3 mmol of TMSCN
and low catalyst loading (5 mg) in solvent less condition. The reaction mixture was stirred using a
magnetic stirring bar at room temperature. Various substituted amines like amines containing both
electron donating as well as electron withdrawing groups (Table 2) have been investigated. Amines
containing both electron withdrawing and electron donating groups showed almost similar trend of
reactivity. The completion of the reaction was indicated by solidification of reaction medium and
TLC. After completion, 5 ml of methanol was added to dissolve the product completely and catalyst
was separated magnetically. Formation of no byproduct was observed. The product was then
1
recrystallized with methanol and was characterized with melting point and HꢀNMR spectroscopic
techniques.
Table 2 OTꢀSiꢀSPIONs catalyzed reaction with different substituted amines to synthesize
cyclohexanecarbonitriles
O
, R.T.
OTꢀSiꢀSPIONs
(5 mg)
CN
NH2
Ar
N
(CH3)3SiꢀC
3
N
H
Solventless
R
1
2(aꢀz)
4 (aꢀz)
Entry
2a
Ar (Amine)
C6H5
Product
Time
Yield (%)
94.48
(min.)
CN
7
N
H
4a
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CN
2b
2ꢀClC6H4
17
48.33
N
H
Cl
4b
4c
CN
2c
2d
4ꢀCl C6H4
4ꢀFC6H4
23
4
97.44
94.60
N
H
Cl
CN
N
H
F
4d
CN
2e
2ꢀBrC6H4
12
45.50
N
H
Br
4e
CN
2f
2ꢀOCH3C6H4
2ꢀCH3C6H4
66
45
52.69
78.96
N
H
H3CO
4f
CN
2g
N
H
H3C
4g
CN
2h
3ꢀBrC6H4
11
89.76
N
H
Br
4h
CN
2i
2j
4ꢀBrC6H4
7
91.60
29.82
N
H
Br
4i
CN
2ꢀOCH2CH3C6H4
62
N
H
H3CH2CO
4j
CN
2k
3ꢀCH3C6H4
42
92.06
N
H
CH3
4k
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CN
2l
3ꢀClC6H4
2ꢀFC6H4
16
19
54.29
68.24
N
H
Cl
4l
CN
2m
N
H
F
4m
Reaction conditions: Cyclohexanone (1, 1 mmol), amine (2aꢀz, 1 mmol), TMSCN (3, 1.3 mmol), OTꢀSiꢀ
SPIONs (5 mg)
The present protocol as compared to reported methods is more environmentally compatible,
economical and green (Table 3).
Table 3 Comparison of synthesis of αꢀaminonitrile derivatives with reported protocols for compound
4a
NH2
C
N
Catalyst
O
Si
C
N
(CH3)3
Solvent,
conditions
N
H
1
3
2a
4a
Entry
Catalyst used
Conditions/ Solvent
Catalyst
loading
50 mg
5 mg
Time (min.)
Yield (%)
1.
2.
BꢀMCMꢀ4148
MCMꢀ41ꢀSO3H49
* R.T./ EtOH
* R.T./ EtOH
404
180
89
91
3.
4.
5.
6.
SBAꢀ15ꢀPhꢀPrꢀSO3H50
^50 °C/ ꢀ
# R.T./ ꢀ
¤ R.T./ ꢀ
ꢁR.T./ ꢀ
5 mol%
0.1 eq.
10 wt%
5 mg
480
25
95
88
88
51
LiBF4
Sulfated Tungstate
OTꢀSiꢀSPIONs
360
7
94.48
(Present
work)
* For 1 mmol of 1, 1 mmol of 2a and 1.2 mmol of 3, ^ For 3 mmol of 1, 3 mmol of 2a and 3.6 mmol of 3, # For
1 eq of 1, 1 eq of 2a and 1.1 eq of 3, ¤ For 1 eq of 1, 1 eq of 2a and 1 eq of 3, ꢁ For 1 mmol of 1, 1 mmol of 2a
and 1.3 mmol of 3.
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2.4 Recyclability of catalyst
The ability of the OTꢀSiꢀSPIONs particles to act as a heterogeneous recyclable catalyst has been
examined by carrying out repeated runs on the same batch of the used 5 mg magnetic catalyst in
reaction of cyclohexanone (1, 1 mmol), aniline (2a, 1 mmol) and TMSCN (3, 1.3 mmol) (Fig. 8). The
catalyst was separated from the reaction mixture magnetically after the completion of the reaction and
was washed with methanol and acetone successively and then was dried at 50º C for 30 minutes. The
catalytic activity of the nanoparticles did not decreased significantly even after five catalytic cycles.
The TEM analysis (Fig. 2d) of the used catalyst revealed that the morphology of the recovered
nanoparticles remained almost unaltered.
100
90
80
70
60
50
40
1
2
3
4
5
Figure 8 Recyclability of OTꢀSiꢀSPIONs upto 5 cycles
Scheme III represents the proposed mechanism for the synthesis of cyclohexanecarbonitriles. The
catalytic NH2 group forms Hꢀbondng with the O of cyclohexanone thus facilitating the rate
determining step of Strecker reaction i.e. formation of imine intermediate, which further reacts with
TMSCN to form the product.
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SiO2
Fe3O4
SiO2
HO
Si
OC2H5
Fe3O4
O
HO
Si
OC2H5
O
O
O
O
O
Si
O
Si
O
HO
i
S
O
O
i O
S
HO
O
O
O
H
N
H
H
N
H
H
N
O
OH
NHR
HO
H
R
NC
NHR
H
OH
(CH3)3Si
RNH2
C
N
(CH3)3Si
SchemeꢀIII Proposed mechanism for synthesis of cyclohexanecarbonitriles
3. Experimental Section
3.1 Materials
All chemicals were purchased from local dealers and were used without any further purification.
3.2 Instrumentation
Fourier transform infrared spectra (FTꢀIR) were recorded using Perkin Elmer spectrum RXꢀI Fourier
transform infrared spectrophotometer using KBr pellets in the scan range of 4000ꢀ 400 cmꢀ1. The
crystalline phase of the nanoparticles was characterized by means of Xꢀray diffraction (XRD)
measurements using Cu Kα radiation (λ=0.154 nm) on Panlytical XPERTPRO (NDP) Xꢀray
diffractometer in the 2θ range of 15ꢀ80°. The morphology, nanostructure and particles size of the
functionalized nanoparticles was studied by transmission electron microscopy (TEM) on Hitachi
S7500 instrument. The sample was prepared by dispersing a small amount of solid nanoparticles in
ethanol and then it was deposited over carbon coated Cu grids by dropcasting and followed by drying.
Magnetic measurements were carried out of dried sample to evaluate magnetic properties of
multifunctional nanoparticles at room temperature. For this Vibrating Sample Magnetometry was
carried out on Princeton applied research model 155. Scanning electron microscope images were
taken on JEOL JSMꢀ6610LV instrument. The thermogravimetric analysis (TGA), differential thermal
analysis (DTA) and differential thermal gravimetry (DTA) curves were recorded using an
EXSTAR6000TG/DTA 6300 instrument at heating rate of 10 °C/min. under nitrogen atmosphere. The
surface area, pore volume and pore diameter of nanoparticles was obtained by micromeritics ASAP
2010 model of accelerated surface area and porosimetry system. The UV spectra have been examined
on Agilent Technologies Cary series UVꢀVis spectrophotometer. The organic products were
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1
characterized by H nuclear magnetic resonance spectroscopy at 400 MHz with the aid of Advance
400 spectrophotometer using tetramethylsilane as the internal standard and DMSOꢀd6 solvent.
3.3 Synthesis of OTꢀSiꢀSPIONs
3.3.1 Synthesis of SPIONs (Fe3O4 nanoparticles)
Superparamagnetic iron oxide nanoparticles were synthesized by modified coꢀprecipitation method
without using any surfactants at room temperature in an ambient atmosphere. Ferrous sulphate
(FeSO4.7H2O) and ferric sulphate (Fe2(SO4)3.nH2O) were used as iron salts in their respective
stoichiometry ratio Fe2+: Fe3+ = 1:2. At first, 10 ml of aqueous solutions of both iron salts [Fe2+(7.19
mmol, 2.0 g)] and Fe3+[(13.0 mmol, 5.2 g)] were made separately and ultrasonicated for 15 minutes at
room temperature. Both the solutions were mixed and again ultrasonicated for 30 minutes. Then
NH4OH (25%) was added dropwise to the above mixed solution to reach pH value of approximately
10 along with sonication and constant stirring to attain homogeneous mixture. After complete
addition, the mixture was stirred at 60 °C for 1 hour. The precipitates were isolated in the magnetic
field and the supernatant was discarded by centrifugation (5000 rpm, 15 min.). Successive washings
with deionized water were done to attain pH ≈ 7. Then the precipitates were dried in an oven at 50 °C
for 24 hours.
3.3.2 Preparation of silica coated magnetite nanoparticles (SiꢀSPIONs)
41
Stöber method with some alteration and combination of sonochemistry was used to prepare silica
layer encapsulated SPIONs. 0.5 g of as prepared Fe3O4 nanoparticles were dispersed in 50 ml of
deionized water by sonication for 30 minutes. Precipitates were isolated magnetically and redispersed
in 25 ml of water and 90 ml of ethanol by sonication for 30 minutes. Solution was turned basic by
adding 5 ml (25%) of ammonia solution. After thorough mixing by mechanical stirrer, 7ml of TEOS
was added dropwise. Then, the mixture was stirred for 5.5 hours at room temperature. The as prepared
nanoparticles were washed several times, magnetically separated, centrifuged (3500 rpm, 15 min.)
and dried in an oven at 50 °C. These nanoparticles obtained were abbreviated as SiꢀSPIONs.
3.3.3 Functionalization of SiꢀSPIONs with oxytyramine (OT)
SiꢀSPIONs (0.1 g) were added in a mixture of 5 ml of water and 1 ml of ethanol and was sonicated for
30 minutes. Side by side 0.1g (mmol) oxytyramine solution in 2 ml of water was also sonicated for 30
minutes. Both the solutions were mixed, sonicated for 15 minutes and stirred overnight at 50 °C. Then
the particles were washed with water, centrifuged (5000 rpm, 15 min.) and dried in an oven at 50 °C.
3.3.4 General procedure for the synthesis of cyclohexanecarbonitriles
To the mixture of cyclohexanone (1 mmol, 103.64ꢂl), amine (1mmol) and TMSCN (1.3 mmol, 162
ꢂl) into a roundꢀbottom flask, OTꢀSiꢀSPIONs (5 mg) was added and stirred at room temperature for
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the desired time. The reaction progress was followed by solidification of the product as well as thin
layer chromatography (TLC) in 30% ethylacetate: hexane solution. After the completion of the
reaction, the solid was dissolved completely in 5 ml of methanol and the catalyst was separated by an
external magnet, washed with CH3OH, CH3COCH3 and dried for further use. The product was
recrystallized with methanol which was further characterized by its melting point and 1HꢀNMR.
3.3.5 Spectral data for all compounds
4a: 1ꢀ(phenylamino)cyclohexanecarbonitrile
M.p. 71ꢀ72°C; 1HꢀNMR (400 MHz, CDCl3) • (ppm): 1.27ꢀ1.33 (m, 1H), 1.57ꢀ1.77 (m, 6H), 1.79ꢀ1.8
(m, 1H), 2.31ꢀ2.34 (t, 2H), 3.62 (s, 1H, NH), 6.88ꢀ6.92 (t, 3H), 7.22ꢀ7.26 (t, 2H).
4b: 1ꢀ(2ꢀchlorophenylamino)cyclohexanecarbonitrile
1
M.p. 89°C; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.37ꢀ1.42 (m, 1H), 1.68ꢀ1.78 (s, 7H), 2.37ꢀ2.39 (t,
2H), 4.38 (s, 1H, NH), 6.75ꢀ6.80 (m,1H), 7.19ꢀ7.21 (d, 2H), 7.29ꢀ7.31 (d, 1H).
4c: 1ꢀ(4ꢀchlorophenylamino)cyclohexanecarbonitrile
1
M.p. 98ꢀ100°C; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.23ꢀ1.36 (m, 1H), 1.68ꢀ1.81 (m, 7H), 2.28ꢀ
2.32 (m, 2H), 3.63 (s, 1H, NH), 6.82ꢀ6.86 (m, 2H), 7.17ꢀ7.21 (m, 2H).
4d: 1ꢀ(4ꢀfluorophenylamino)cyclohexanecarbonitrile
1
M.p. 104ꢀ106 °C; H NMR (400MHz, CDCl3) • (ppm): 1.26ꢀ1.35 (m, 1H), 1.58ꢀ1.74 (m, 6H), 1.78ꢀ
1.81 (m, 1H), 2.21ꢀ2.27 (m, 2H), 3.48 (s, 1H, NH), 6.90ꢀ6.98 (m, 4H).
4e: 1ꢀ(2ꢀbromophenylamino)cyclohexanecarbonitrile
M.p. 82ꢀ84 °C; 1HꢀNMR (400 Hz, CDCl3) • (ppm): 1.65ꢀ1.80 (m, 8H), 2.34ꢀ2.37 (t, 2H), 4.40 (s, 1H,
NH), 6.69ꢀ6.73 (t, 1H), 7.10ꢀ7.26 (q, 2H), 7.46ꢀ7.48 (dd,1H).
4f: 1ꢀ(2ꢀmethoxyphenylamino)cyclohexanecarbonitrile
1
M.p. 82ꢀ84 ºC; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.25ꢀ1.36 (m, 1H), 1.67ꢀ1.79 (m, 7H), 2.37ꢀ
2.39 (t, 2H), 3.84 (s, 3H), 4.35 (bs, 1H, NH), 6.81ꢀ6.83 (q, 2H), 6.87ꢀ6.91 (m, 1H), 7.08ꢀ7.10 (dd,
1H).
4g: 1ꢀ(2ꢀtolylphenylamino)cyclohexanecarbonitrile
1
M.p. 68ꢀ70 ºC; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.33ꢀ1.38 (m, 1H), 1.69ꢀ1.79 (m, 7H), 2.17 (s,
3H), 2.37ꢀ2.40 (t, 2H), 3.49 (s, 1H, NH), 6.78ꢀ6.82 (t, 1H), 7.10ꢀ7.18 (m, 3H).
4h: 1ꢀ(3ꢀbromophenylamino)cyclohexanecarbonitrile
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M.p. 94ꢀ98 °C; 1H NMR (400MHz, CDCl3) • (ppm): 1.25ꢀ1.37 (m, 1H), 1.67ꢀ1.81 (m, 7H), 2.31ꢀ2.35
(t, 2H), 3.71 (s, 1H, NH), 6.82ꢀ6.85 (d, 1H), 6.90ꢀ7.01 (d, 2H), 7.0ꢀ7.11 (t, 1H).
4i: 1ꢀ(4ꢀbromophenylamino)cyclohexanecarbonitrile
1
M.p. 116ꢀ120 °C; H NMR (400MHz, CDCl3) • (ppm): 1.29ꢀ1.35 (m, 1H), 1.64ꢀ1.81 (m, 7H), 2.29ꢀ
2.33 (q, 2H), 3.65 (s, 1H, NH), 6.77ꢀ6.80 (m, 2H), 7.31ꢀ7.34 (m, 2H).
4j: 1ꢀ(2ꢀethoxyphenylamino)cyclohexanecarbonitrile
1
M.p. 92ꢀ94 °C; H NMR (400MHz, CDCl3) • (ppm): 1.34ꢀ1.36 (m, 1H), 1.40ꢀ1.44 (t, 3H), 1.67ꢀ1.77
(m, 7H), 2.35ꢀ2.38 (d, 2H), 4.04ꢀ4.09 (q, 2H), 4.40 (bs, 1H, NH), 6.77ꢀ6.82 (m, 2H), 6.86ꢀ6.9 (m,
1H), 7.09ꢀ7.11 (d, 1H).
4k: 1ꢀ(3ꢀtolylphenylamino)cyclohexanecarbonitrile
1
M.p. 88ꢀ90 ºC; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.28ꢀ1.36 (m, 1H), 1.56ꢀ1.72 (m, 6H), 1.76ꢀ
1.80 (m, 1H), 2.30ꢀ2.34 (q, 5H), 3.56 (s, 1H, NH), 6.71ꢀ6.74 (d, 3H), 7.10ꢀ7.14 (t, 1H).
4l: 1ꢀ(3ꢀchlorophenylamino)cyclohexanecarbonitrile
1
M.p. 80 ºC; HꢀNMR (400 MHz, CDCl3) • (ppm): 1.32 (s, 1H), 1.67 (s, 7H), 2.33 (s, 2H), 3.70 (s,
1H), 6.23ꢀ6.25 (t, 1H), 6.8 (s, 2H), 7.1 (s, 1H).
4m: 1ꢀ(2ꢀfluorophenylamino)cyclohexanecarbonitrile
M.p. ºC; 1HꢀNMR (400 Hz, CDCl3) • (ppm): 1.64ꢀ1.78 (m, 8H), 2.31ꢀ2.34 (t, 2H), 4.19 (s, 1H, NH),
6.45ꢀ6.89 (t, 1H), 7.0ꢀ7.26 (q, 2H), 7.41ꢀ7.48 (dd,1H).
4. Conclusions
In summary, a new type of versatile and efficient heterogeneous magnetic nanocatalyst (OTꢀSiꢀ
SPIONs) has been developed using combined sonochemical and coꢀprecipitation process, further this
recyclable catalyst has been successfully applied in the synthesis of various new
cyclohexanecarbonitriles. The reaction was carried out under stirring at room temperatures which
gave excellent results and render this protocol economical. Synthesis of cyclohexanecarbonitriles is
rarely discussed because of its slow reaction rate but in our protocol these have been synthesized
efficiently. The simple procedure for the catalyst preparation, easy recovery of the catalyst and
recyclability of the catalyst without any significant loss in its activity even after 5 cycles makes it an
ideal catalytic system. The excellent features of this protocol are short reaction time, simple work up
process, green synthesis, no column process and environmentally benign.
Acknowledgements
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We are thankful to SAIF, Panjab University Chandigarh for FT‐ ‐NMR, IIT Ropar for
IR, TEM and 1H
SEM and XRD, CSMCRI Bhavnagar for BET. Two of the authors (P.A. and H.S.) are thankful to
MHRD, UGC and NIT Jalandhar for providing the research fellowship.
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