N-Alkylation of amines with alcohols over nanosized zeolite beta

Article abstract of DOI:10.1039/c3gc41345d

Direct N-alkylation of amines with alcohols was successfully performed by using nanosized zeolite beta, which showed the highest catalytic activity among other conventional zeolites. This method has several advantages, such as eco-friendliness, moderate to high yields, and simple work-up procedure. The catalyst was successfully recovered and reused without significant loss of activity and only water is produced as co-product. In addition, imines were also efficiently prepared from the tandem reactions of amines with 2-, 3- and 4-nitrobenzyl alcohols using nanosized zeolite beta.

Full text of DOI:10.1039/c3gc41345d

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DOI: 10.1039/C3GC41345D
N-Alkylation of amines with alcohols over nanosized zeolite Beta
Marri Mahender Reddy, Macharla Arun Kumar, Peraka Swamy, Mameda Naresh, Kodumuri
Srujana, Lanka Satyanarayana, Akula Venugopal and Nama Narender*
Nanosized zeolite Beta is highly active and selective catalyst for the synthesis of higher amines
from amines and alcohols is reported.
R1
R2
R1
H
N
3 or
OH + H2N R3
or
R
N
R2
N
N
H
R1 = Alkyl, Aryl
Nanosized zeolite Beta
135 °C
R1
R2
R2 = H, Alkyl R3 = Aryl
NH2
OH
+
R'
O2N
R'
O2N
R' = H, Me, OMe, NH2, Cl

Green Chemistry
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DOI: 10.1039/C3GC41345D
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
N-Alkylation of amines with alcohols over nanosized zeolite Beta
Marri Mahender Reddy,a Macharla Arun Kumar,a Peraka Swamy,a Mameda Naresh,a Kodumuri
Srujana,a Lanka Satyanarayana,b Akula Venugopal a and Nama Narender*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Direct Nꢀalkylation of amines with alcohols was successfully performed by using nanosized zeolite Beta,
which showed the highest catalytic activity among other conventional zeolites. This method has several
advantages such as, ecoꢀfriendly, moderate to high yields, simple workꢀup procedure, catalyst was
successfully recovered and reused without significant loss of activity and only water is produced as coꢀ
10 product. In addition, imines also efficiently prepared from the tandem reactions of amines with 2, 3 and 4ꢀ
nitrobenzyl alcohols using nanosized zeolite Beta.
Introduction
50 catalytical systems have disadvantages of the recovery and reuse
Carbonꢀnitrogen bondꢀforming reaction is one of the most
important transformation in organic chemistry and which plays a
15 vital role in the elaboration and composition of biological and
chemical systems.1 NꢀAlkylation of amines is of fundamental
importance in organic synthesis because resulting higher amines
have widely been used as synthetic intermediates for
pharmaceuticals, agrochemicals, fine chemicals, dyes, surfactants
20 and functionalized materials.2
The most frequently used method for the preparation of Nꢀ
alkyl amines is the coupling of amines with alkyl halides3 in the
presence of stoichiometric amounts of inorganic bases, but this
method is often associated with environmental problems. Toxic
25 nature of alkyl halides and related alkylating agents, the
selectivity to the desired secondary amines is generally low. In
addition, this reaction produces large amounts of (in)organic
salts. Reductive amination of carbonyl compounds with amines4
is the another known route to higherꢀorder amines, which requires
30 the use of strong reducing agents or hydrogen gas.
An alternative environmentallyꢀbenign approach to these
methods is the Nꢀalkylation with alcohols as alkylating agents.
The advantage of the use of alcohols instead of alkyl
halides/carbonyl compounds are, that it produces only water as
35 byꢀproduct, high atom efficiency, avoiding the use of toxic
reagents and stoichiometric reducing agents. Until now, many
homogeneous transition metal complexes and salts such as,
ruthenium,5 rhodium,6 iridium,7 palladium,8 gold,9 nickel,10
copper11 and iron12 catalysts, have been reported for the Nꢀ
40 alkylation of amines with alcohols. However, these homogeneous
of expensive catalysts and / or the indispensable use of coꢀ
catalysts such as bases and stabilizing agents. Transition metalꢀ
free protocols have also been reported, although they normally
require harsh reaction conditions (high temperatures and
55 pressures) to achieve reasonable yields of products.13
However, in these environmentally conscious and
economically pressured days, homogeneous catalysts are no
longer acceptable because of inherent problems, such as
corrosion, toxicity, difficulty in catalyst handling, separation from
60 the reaction system, high cost and the creation of (in)organic
waste. Consequently, it has long been appreciated that the use of
alternative solid (heterogeneous) catalysts to replace
homogeneous catalysts is the ultimate goal in catalysis science
and engineering. Heterogeneous catalysts have many advantages
65 over homogeneous ones, such as low cost, tolerance to a wide
range of temperatures and pressures, easy and inexpensive
removal from the reaction mixture by simple filtration or
centrifugation, easy and safe disposal, safe storage, long lifetime,
ecoꢀfriendly nature, regenerability and reuse.
70
Although there are several reports on the Nꢀalkylation using
heterogeneous catalysts,14 most of them suffer from harsh
reaction conditions, low turnover numbers (TONs) and
frequencies (TOFs), limited substrate scope and catalysts having
poor recyclabilities. Therefore, the development of more active
75 and selective heterogeneous catalysts for the Nꢀalkylation of
amines with alcohols is still challenging.
Catalysts based on various zeolites posses major importance
both in petroleum and fine chemical industries.15 This is mainly
due to the zeolites have uniform channel size, unique molecular
80 shape selectivity, as well as strong acidity and good
thermal/hydrothermal stability. The BEAꢀtype of zeolite consists
of an intergrowth of two or more polymorphs comprising of a
three dimensional system of 12ꢀmembered ring channels.16
Zeolite Beta has pore diameters of 0.76×0.64 nm and 0.55×0.55
85 nm. The BEA framework topology attracts much attention
a I&PC Division, CSIRꢀIndian Institute of Chemical Technology,
Hyderabad 500 007, India. Eꢀmail: narendern33@yahoo.co.in; Fax:
+91ꢀ40ꢀ27160387/27160757; Tel: +91–40ꢀ27191703
45 b Centre for NMR & Structural Chemistry, CSIRꢀIndian Institute of
Chemical Technology, Hyderabad 500 007, India
† Electronic Supplementaery Information (ESI) available: 1H and 13C
NMR spectra. See DOI: 10.1039/b000000x/
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because of the large available micropore volume, largeꢀpore
channel system and the presence of active sites in different
concentrations that are useful in a number of acidꢀcatalyzed
reactions.17
5
However, zeolites often exhibit insufficient activity and/or fast
deactivation, mainly due to poor diffusion efficiency.18 The slow
transport in the zeolite micropores leads to slow reaction rates or
unwanted secondary side reactions resulting from long residence
times. In order to benefit fully from the unique sorption and
10 shapeꢀselectivity effects in the micropores without suffering from
diffusional limitations, the diffusional path length in the
micropores should be very short.19 Nanosized zeolite crystals
with narrow particle size distributions and sizes less than 100 nm
have received much attention because of their great potential
15 applications in catalysis and adsorption. The reduction of particle
size from the micrometer to the nanometer scale leads to
substantial changes in the properties of the material. The decrease
in the crystal sizes results in high external surface areas, reduced
diffusion path lengths and more exposed active sites.20
20
40
Fig. 2 FTꢀIR spectrum of calcined nanosized zeolite Beta.
Synthesis of nanosized zeolite Beta 22
Tetrethylammonium hydroxide (15.5 mL (37 mmol), TEAOH 35
wt % solution in water) was slowly added to 1.2 g (30 mmol) of
NaOH in 4 ml of distilled water. The resulting mixture was
45 stirred for 10 min and added to a solution of (22.3 mL (100
mmol)) tetraethyl orthosilicate in 21 mL of distilled water at
room temperature, then the mixture was allowed to stir for 30
min. 2.1 g (3.33 mmol) of aluminum sulphate (Al2(SO4)3.16H2O)
dissolved in 10.2 mL of warm water was added to the above
50 mixture and stirred for 1 h. Then 1.4 g (3.8 mmol) of
cetyltrimethylammonium bromide (CTAB) dissolved in 40 mL of
ethanol was added slowly to the above reaction mixture and the
final gel was stirred for 2 h. The resulted viscous gel was heated
at 90 °C with continuous stirring to complete dryness. The dry
55 precursor lumps were coarsely crushed and transferred in to a
Teflon cup placed in Teflon lined autoclave. The amount of water
(ca. 0.2 mL per 1 g (dry gel)) was added into the bottom of
autoclave without contacting the dry gel in the Teflon cup. The
crystallization of the dry gel was carried out at 175 ºC for 24 h.
60 After hydrothermal treatment, the solid product was filtered and
washed with water until neutral and dried in air. The surfactant
was removed from the asꢀsynthesized nanosized zeolite Beta
through calcination for 8 h at 550 ºC.
Fig. 1 XRD patterns of nanosized zeolite Beta samples: (a) uncalcined;
(b) calcined; (c) reused catalyst.
As a part of our ongoing research programme to develop
25 environmentally friendlier synthetic procedures using zeolites.21
Herein, we report a simple and efficient catalytic method for the
Nꢀalkylation of amines with alcohols using nanosized Beta
zeolite.
65
Fig. 3 27Al NMR spectrum of calcined nanosized zeolite Beta.
General procedure for the N-alkylation of amines with
alcohols / tandem reactions of amines with nitrobenzyl
alcohols
Experimental
Reactions were performed in a magnetically stirred round
30 Reagents and materials
70 bottomed flask fitted with
a condenser and placed in a
Alcohols and amines were purchased from Aldrich. The
following chemicals were employed for the preparation of
nanosized zeolite Beta. Tetraethyl orthosilicate ((C2H5O)4Si, 99%
Lancaster), Tetraethylammonium hydroxide (TEAOH, 35%
temperature controlled oil bath. Nanosized zeolite Beta (100 mg)
was added to the well stirred solution of amine (2 mmol) in
alcohol (6 mmol) and the reaction mixture was allowed to stir at
135 °C in an open (air) atmosphere. After disappearance of the
35 Fluka),
Sodium
hydroxide
(NaOH,
98%
Loba), 75 amine (reaction was monitored by TLC) or after the appropriate
Cetyltrimethylammonium bromide (CTAB, 98% Lancaster) and
Aluminum sulphate (Al2(SO4)3.16H2O, Universal). All the
chemicals used in this study were in asꢀreceived form.
time, the reaction mixture was cooled to room temperature. The
catalyst was removed by filtration, rinsed with ethyl acetate and
removal of solvent in vacuo yielded a crude residue. The crude
residue was further purified by column chromatography using
2
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DOI: 10.1039/C3GC41345D
silica gel (100ꢀ200 mesh) to afford pure products. All the
products were identified on the basis of NMR and mass spectral
data.
frame work vibration region is shown in Fig. 2. A band at 462
cmꢀ1 characteristic to the pore opening, absorption band at 520ꢀ
45 570 cmꢀ1 which is characteristic of highly crystalline Beta
zeolite.23 The band at 900ꢀ950 cmꢀ1 is due to terminal SiꢀOꢀH+
groups. The large and broad peak appearing in the range of 1060ꢀ
1090 cmꢀ1 is due to asymmetric stretching vibration (υa(oTo))24.
Characterization and analysis
5
All the samples were systematically characterized by various
analytical and spectroscopic techniques. The XRD patterns of the
Table 1 Optimization for Nꢀbenzylation of aniline with benzyl alcohola
samples were obtained on
a
Regaku miniflux Xꢀray
Diffractometer using Ni filtered CuKα radiation at 2θ = 2–80º
with a scanning rate of 2º minꢀ1 and the beam voltage and
Catalyst
Ph
H N Ph
Ph
OH
+
Ph
N
H
2
10 currents of 30 kV and 15 mA, respectively. The FTꢀIR spectrum
of the sample recorded on a Nicolet 740 FTꢀIR spectrometer at
ambient conditions using KBr as the diluents. The solid state
MASꢀNMR analysis has been done by employing Avanceꢀ500
WB (Varian) spectrometer with a frequency of 130.31 MHz and
15 [Al(H2O)6]3+ used as a standard for 27Al. Transmission electron
microscopy (TEM) images of the samples were recorded using a
50
Entry
Catalyst
Temperature
Time
(h)
Yieldb (%)
(°C)
135
135
135
135
135
135
135
135
135
135
135
135
1
2
HZSMꢀ5 (40)
6.3
6.3
6.3
6.3
6.3
6.3
6.3
6.3
6.3
6.3
6.3
6.3
ꢀ
ꢀ
Mordenite
Tecnai
F 12 TEM instrument. Temperature programmed
desorption (TPD) of ammonia was carried out on a laboratoryꢀ
built apparatus. In a typical experiment about 0.1 g of the sample
20 was pretreated at 300 °C for 1 h by passing helium (99.9%, 50
mL min−1). After pretreatment, the sample was saturated with
anhydrous ammonia (10% NH3 balance He) at 100 °C for 1 h and
subsequently flushed with He at the same temperature to remove
physisorbed ammonia. Then the TPD analysis was carried out
25 from ambient temperature to 800 °C at a heating rate of 10 °C
min−1 and the desorbed NH3 was measured using a gas
chromatography equipped with thermal conductivity detector.
The products of the coupling reaction were identified by NMR
spectra using a Bruker VX NMR FTꢀ300 or Varian Unity 500
30 spectrometers instrument in CDCl3. Chemical shifts are reported
in parts per million (ppm) downfield from TMS. ESI mass
spectra were obtained by using Micromass Quattro LC mass
spectrometer and Highꢀresolution mass spectra obtained by using
ESIꢀQTOF mass spectrometry.
3
HY
NaY
41
ꢀ
4
5
HX
ꢀ
6
Hβ
74
47
ꢀ
7
MCMꢀ41
Montmorillonite K10
SiO2ꢀAl2O3
SiO2
8
9
49
ꢀ
10
11
12
Absence of catalyst
ꢀ
Nanosized zeolite
Beta
98
13
14
15
16
17
18
19
20
Nanosized zeolite
Beta
145
125
100
135
135
135
135
135
5.3
24
94
63
35 Results and Discussions
Nanosized zeolite
Beta
Catalyst characterization
Fig. 1 displays the XRD patterns of calcined and uncalcined
nanosized zeolite Beta. The Bragg peaks are corresponding to the
typical reflections of BEA structure with high crystallinity.
Nanosized zeolite
Beta
24
21
Nanosized zeolite
Beta
6.3
6.3
6.3
6.3
6.3
87c
32d
20e
88f
70g
Nanosized zeolite
Beta
Nanosized zeolite
Beta
Nanosized zeolite
Beta
Nanosized zeolite
Beta
a
Reaction conditions: Benzyl alcohol (6 mmol), Aniline (2 mmol),
b
Catalyst (100 mg), open atmosphere. Products were characterized by
NMR, Mass spectra and isolated yields calculated based on aniline.
40
Fig. 4 TEM image of nanosized zeolite Beta.
c Catalyst (50 mg), d Catalyst (25 mg). e Catalyst (10 mg). f Benzyl alcohol
g
55 (4 mmol), Aniline (2 mmol). Benzyl alcohol (2 mmol), Aniline (2
The FTꢀIR spectrum of calcined nanosized zeolite Beta in the
mmol).
This journal is © The Royal Society of Chemistry [year]
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The peak appearing in 3200ꢀ3500 cmꢀ1 region is characteristic of
silanol groups attached through strong hydrogen bands with 55 aniline to benzyl alcohol at 135 °C over nanosized zeolite Beta
to get highest yield for this alkylation reaction is 1:3 mole ratio of
neighbouring silanols and hence corresponds essentially to
internal SiꢀOH defect groups25.
(100 mg).
Table 2 NꢀAlkylation of amines with benzyl alcohol over nanosized
zeolite Betaa
5
The location of the Al atoms in the sample was investigated by
27Al NMR spectroscopy (Fig. 3). The 27Al NMR spectrum
contains only one symmetrical peak at 57.87 ppm, which is
characteristic of tetrahedral Al+3 species in the framework.
However, no octahedral aluminum was found in the zeolite.
H
Nanosized zeolite Beta
135 °C
N
Ph
R
NH2
HO
Ph
+
R
10 These results demonstrate that Al+3 species were uniformly
dispersed in the nanosized zeolite Beta.
The TEM image of nanosized zeolite Beta is displayed in Fig.
4. It shows that the nanosized zeolite Beta has uniform particle
size of ca. 30ꢀ40 nm.
Entry
1
Amines
Time
(h)
Product
Yieldb
(%)
H
N
NH2
Ph
6.3
19
98
90
15 Catalyst Screening
NH2
Me
H
N
Ph
At the start of our studies, we investigated the reaction of aniline
with benzyl alcohol (3 equiv.) as a model system to optimize the
different parameters of the reaction (Table 1). In order to choose
the best catalyst, first the reaction was carried out with various
20 zeolites, montmorillonite K10, SiO2 and SiO2ꢀAl2O3 at 135 °C.
Among the catalysts examined, Hβ zeolite showed the highest
catalytic activity and furnished the corresponding secondary
amine in 74% yield (Table 1, entry 6). Under the same reaction
conditions, HY, MCMꢀ41, SiO2ꢀAl2O3 provided <50% yield
25 (Table 1, entries 3, 7 and 9). Whereas, HZSMꢀ5, mordenite, NaY,
HX, montmorillonite K10 and SiO2 delivered virtually no
conversion of starting material (Table 1, entries 1,2,4,5,8 and10).
Then we investigated with nanosized zeolite Beta under similar
reaction conditions produced much higher yield (98%) towards
30 corresponding product compared to other catalysts (Table 1, entry
12). The best performance of nanosized zeolite Beta is possibly
due to higher acidity of nanosized zeolite Beta (412 µmol/g) than
the Hβ zeolite ca. 269 µmol/g. Excess amount of alcohol was
used not only as a reactant but also as a solvent.
2
3
4
Me
H
N
NH2
Ph
24
69
Me
Me
Me
H
N
Ph
NH2
Me
24
24
24
43
ꢀ
Me
Me
H
N
Ph
NH2
5
6
H
N
Ph
NH2
45
OMe
OMe
H
N
NH2
Ph
7
8
24
24
14
ꢀ
MeO
MeO
NH2
H
N
Ph
R1
R1
R4
R3
R4
R3
OH
Nanosized zeolite Beta
135 °C
OH
H2O
N
HN
+
NH2
OH
+
H
N
Ph
R2
9
4
53
59
95
67
R2
35
Br
Br
Scheme 1 Zeolite catalyzed Nꢀalkylation of amines with alcohols.
H
N
NH2
Ph
10
24
Optimization of reaction conditions
Once nano Beta was found as the best catalyst for Nꢀalkylation of
aniline with benzyl alcohol, the influence of temperature was
Cl
Cl
NH2
H
N
Ph
40 studied. By varying the reaction temperature from 100 °C to 135
°C, a gradual improvement in yield (21% to 98%) was observed
and further increase of temperature did not have any accountable
effect on the yield (Table 1, entries 12ꢀ15). The mole ratio of
amine to alcohol also have a significant influence on the product
45 yield. While increasing the mole ratio of amine to alcohol (1:1 to
1:3), the yield of the desired product was increased from 70% to
98% (Table 1, entries 12, 19 and 20). The present reaction was
also conducted using different amounts of the catalyst and it was
found that 100 mg of catalyst has shown the best results (Table 1,
50 entries 12 and 16ꢀ18). It is observed that the reaction did not
proceed in the absence of a catalyst (Table 1, entry 11), thus
confirming the role of the catalyst in the reaction. The above
presented results indicate that the optimized reaction conditions
11
6.3
24
Cl
Cl
Cl
Cl
H
N
NH2
Ph
12
13
14
Cl
Cl
H
N
Ph
NH2
24
24
58
65
MeOOC
MeOOC
H
N
Ph
NH2
CN
CN
4
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H
NH2
excellent yields of the corresponding Nꢀbenzylated products.
Aniline produced the corresponding mono Nꢀbenzylated amine
with 98% yield in 6.3 h (Table 2, entry 1). In order to determine
15 the influence of substitution on aromatic ring of aniline on the
reaction path with this reagent system, we studied the reaction
with different substitutions. Moderately activating or deactivating
groups present on aromatic ring of aniline provided moderate to
excellent yields of the respective mono Nꢀbenzylated products
20 (Table 2, entries 2ꢀ4 and 9ꢀ12) independent of the electronꢀ
donating or withꢀdrawing character of the substituents. Highly
activating groups present on aromatic ring of aniline gave less
yields or no conversion was observed (Table 2, entries 6ꢀ8).
Whereas, highly deactivated anilines gave moderate to excellent
25 yields (Table 2, entries 14ꢀ18). However, the reaction with more
sterically hindered 2,6ꢀdiethylaniline was unsuccessful (Table 2,
entry 5).
The position of substitution on the phenyl ring of aniline affect
the reaction yield. Interestingly, moderately activating groups i.e.
30 methyl at ortho position furnished higher yield compared to para
position (Table 2, entries 2 and 3). Whilst, highly deactivating
group i.e. NO2 at ortho position provided lesser yield compared
to meta/para position (Table 2, entries 16ꢀ18). The highly
deactivated aniline i.e. 4ꢀchloroꢀ2ꢀnitroaniline (Table 2, entry 19)
35 failed to react under these reaction conditions even after
prolonged reaction time (24 h). Secondary aromatic amine i,e. Nꢀ
methylaniline provided the corresponding Nꢀbenzylated product
(tertiary amine) in moderate yield (Table 2, entry 20). Various
substituents such as methyl, methoxy, bromo, chloro, ester, cyano
40 and nitro groups can be tolerated in the present reaction
conditions.
N
Ph
15
16
17
20
24
24
93
29
NC
NC
NH2
H
N
Ph
NO2
NH2
NO2
H
N
67
74
ꢀ
Ph
NO2
NO2
NH2
H
N
24
24
Ph
18
19
O2N
O2N
Cl
H
N
NH2
Ph
Cl
NO2
NO2
Me
NH
Me
N
Ph
20
21
22
24
24
24
24
62
ꢀ
NH2
Ph
N
H
NH2
H
N
Ph
ꢀ
Ph
H
N
17
N
N
23
24
25
26
27
28
NH
We also examined the alkylation of aliphatic primary amines,
aliphatic secondary cyclic amines and electronꢀpoor hetero
aromatic amine (3ꢀaminopyridine) (Table 2, entries 22, 28 and
45 29). Unfortunately, our attempts to alkylate these amines were
unsuccessful. Surprisingly, 1,2,3,4ꢀtetrahydroquinoline gave the
corresponding tertiary amine in 17% yield, whereas 1,2,3,4ꢀ
tetrahydroisoquinoline yielded 72% yield of the respective
tertiary amine (Table 2, entries 23 and 24). Further, the coupling
50 reactions of benzyl alcohol with pyrazole, 1,2,4ꢀtriazole afforded
the corresponding tertiary amines with 70% and 35% yield,
respectively (Table 2, entries 25 and 27), while the reaction with
imidazole was not quite successful (Table 2, entry 26).
18
48
48
48
24
Ph
72
70
ꢀ
N
N
NH
NH
NH
N
N
N
N
N
Ph
Ph
Ph
N
N
N
35
ꢀ
N
NH
PH
N
NH2
H
ꢀ
N-Alkylation of aniline
N
Ph
29
24
N
55 Structurally diverse alcohols, including benzylic, heteroaromatic,
allylic and aliphatic alcohols reacted with aniline under optimized
reaction conditions and provided the corresponding monoꢀNꢀ
alkylated amines in moderate to excellent yield (Table 3). Benzyl
alcohol with moderately activating groups reacted smoothly and
60 furnished excellent yields of the respective monoꢀNꢀalkylated
anilines (Table 3, entries 1 and 2), while substitution of
moderately deactivating groups on benzene ring decreases the
reactivity, yielded the corresponding monoꢀNꢀalkylated anilines
in moderate yield (Table 3, entries 3ꢀ6). Benzylic secondary
65 alcohols also reacted smoothly with aniline gave the
corresponding monoꢀNꢀalkylated anilines in moderate to good
yields (Table 3, entries 7ꢀ10).
N
a
Reaction conditions: Benzyl alcohol (6 mmol), Amine (2 mmol),
Nanosized zeolite Beta (100 mg), 135 °C, open atmosphere. b Products
were characterized by NMR, Mass spectra and isolated yields calculated
based on amines.
5
With optimized reaction conditions in hand, we explored the
scope of the nano Beta catalyzed Nꢀalkylation with various
combinations of substrates (amines and alcohols) and results are
summarized in Table 2 &3 (Scheme 1).
N-Benzylation of amines
10 As shown in Table 2, various amines successfully reacted with
benzyl alcohol in presence of nano beta afforded moderate to
In the case of hetero aryl methyl alcohols different results were
observed. Furfuryl alcohol and thenyl alcohol were converted to
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H
their respective secondary amines in moderate yield, whereas 3ꢀ
pyridinemethanol did not react under similar reaction conditions
N
OH
OH
Ph
52
11
12
4
4
O
S
O
even after prolonged reaction time (Table 3, entries 11ꢀ13).
Reaction with allylic alcohols also delivered the corresponding
secondary anilines in excellent yields (Table 3, entries 14 and
15). In the case of aliphatic alcohols, reactivity decreased with
increasing the chain length. 1ꢀHexanol and 1ꢀoctanol provided
the respective monoꢀNꢀalkylated products in 41% and 10% yield,
respectively, whilst no reaction was observed with 1ꢀdecanol
H
N
Ph
41
ꢀ
S
5
NH
Ph
24
OH
N
13
N
Ph
HN
OH
10 (Table 3, entries 16ꢀ18). Unfortunately, 2ꢀphenylethyl alcohol
and aliphatic secondary alcohols failed to give any product under
similar conditions (Table 3, entries 19ꢀ21).
14
15
16
17
18
19
20
21
5
78
88
41
10
ꢀ
Ph
N
2
24
24
24
24
24
24
24
OH
H
2
Direct synthesis of imines from tandem reactions of amines
with nitrobenzyl alcohols
Ph
N
4
6
8
OH
4
6
8
H
15 Interestingly, when benzyl alcohol bearing a strong electronꢀwith
drawing group i.e. NO2 on phenyl ring reacted with aniline
Ph
Ph
N
H
OH
OH
Table 3 NꢀAlkylation of aniline with alcohols over nanosized zeolite
Betaa
N
H
Nanosized zeolite Beta
H2N Ph
R
NH
Ph
Ph
Ph
R
OH
+
OH
OH
N
135 °C
Ph
H
ꢀ
Ph
HN
ꢀ
Yieldb
(%)
5
5
Entry
1
Alcohol
Time
(h)
Product
OH
NH
Ph
ꢀ
3
3
9
95
OH
HN
Ph
Me
Me
a
20 Reaction conditions: Alcohol (6 mmol), Aniline (2 mmol), Nanosized
b
zeolite Beta (100 mg), 135 °C, open atmosphere. Products were
characterized by NMR, Mass spectra and isolated yields calculated based
on aniline.
OH
7
NH
Ph
90
2
3
Me
Me
OH
NH
Ph
gave the corresponding imines in moderate to excellent yields
25 under similar reaction condition (Table 4). 4ꢀNitrobenzyl alcohol
furnished higher yield compared to 2 and 3ꢀnitrobenzyl alcohols
(Table 4, entries 1ꢀ3). Imines have widespread applications in
laboratory and industrial synthetic processes due to their diverse
reactivity.26
24
24
57
39
Br
Br
OH
NH
Ph
4
5
6
Br
Br
OH
OH
NH
24
24
21
40
30
Encouraged by these results, we next examined a range of
different substituted anilines with 4ꢀnitrobenzyl alcohol under
similar reaction conditions and results are summarized in Table 4.
Introduction of activating groups on para position of aniline
(Table 4, entries 5, 7 and 8) provided excellent yields of the
Ph
Cl
Cl
NH
Ph
Cl
Cl
35 desired products (imines), whereas strongly deactivated anilines
(Table 4, entries 11 and 12) did not react under these conditions
even after prolonged reaction time. orthoꢀSubstituted anilines
(Table 4, entries 4 and 6) gave lesser yields due to ortho effect
(steric hindrance). In the case of 1,4ꢀdiaminobenzene excellent
40 chemoselectivity was observed, in which only one amino group
converted into imine, other amino group remains unaffected
(Table 4, entry 8). When halogenated anilines treated under
present reaction conditions afforded less yields or no conversion
was observed (Table 4, entries 9 and 10).
OH
7
8
8
70
34
NH
Ph
OH
NH
Ph
24
Br
Br
OH
NH
Ph
9
10
67
72
45
The reusability of the catalyst is one of the most important
property for the commercial applications and environmental
proportion. The catalyst was easily separated from the reaction
mixture by simple filtration and the collected catalyst was
calcined at 450 °C to use in next cycle. The reusability of the
OH
10
1.3
NH
Ph
6
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Table 4 Synthesis of imines from anilines and nitrobenzyl alcohols over nanosized zeolite Betaa
Nanosized zeolite Beta
R'
R
OH
H2N R'
+
R
N
135 °C
Entry
1
Alcohols
Amines
Time (h)
Product
Yieldb (%)
35
OH
NO2
H2N
24
N
NO2
OH
H2N
2
3
4
5
24
67
92
34
80
N
O2N
NO2
O2N
O2N
O2N
O2N
O2N
N
OH
H2N
H2N
21
24
24
Me
Me
O2N
OH
OH
OH
N
Me
O2N
H2N
Me
N
MeO
N
MeO
H2N
O2N
6
7
8
9
24
24
15
24
62
83
78
OMe
NH2
Cl
O2N
N
O2N
O2N
O2N
H2N
H2N
H2N
OMe
NH2
OH
OH
OH
O2N
O2N
O2N
N
Cl
Br
N
54
ꢀ
Br
CN
O2N
O2N
O2N
10
24
H2N
OH
OH
OH
N
N
ꢀ
O2N
O2N
11
12
24
24
H2N
H2N
CN
NO2
ꢀ
N
NO2
a Reaction conditions: Alcohol (6 mmol), Amine (2 mmol), Nanosized zeolite Beta (100 mg), 135 °C, open atmosphere.
b Products were characterized by NMR, Mass spectra and isolated yields calculated based on amines.
5
catalyst was checked by the reaction of aniline with benzyl
alcohol under optimized reaction conditions. The reused catalyst
showed consistent activity even after third reuse (Table 5). The
catalyst was highly crystalline before and after the reaction,
conditions. Reaction of benzyl alcohol with aniline under air
(open) or inert atmosphere gave the corresponding Nꢀalkylated
product without forming imine. Different results were obtained in
case of 4ꢀnitrobenzyl alcohol with aniline depending on reaction
10 which was confirmed by XRD (Fig. 1). After the reaction 20 atmosphere. Imine was obtained under air (open), whilst
quenching of aluminium or silicon from nanosized zeolite Beta
was not observed and confirmed by elemental analysis.
To study mechanism for the formation of Nꢀalkylated product
and imine from alcohols and amines using nanosized zeolite Beta,
corresponding Nꢀalkylated product was formed under inert
atmosphere. In order to find out the intermediate product for
imine formation, the reactions (4ꢀnitrobenzyl alcohol with
aniline) were carried out at different time periods, such as, 10 h,
15 a series of reactions were carried out at standard reaction 25 13 h and 16 h yielded the corresponding imine, along with small
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DOI: 10.1039/C3GC41345D
Table 5 Nꢀbenzylation of aniline with benzyl alcohol ꢀ reusability of the
methodology for organic synthesis.
catalysta
Acknowledgements
Entry
Cycle
Conversion
of aniline
(%)
NꢀBenzylaniline
40 M.M.R, M.A.K and M.N acknowledge the financial support from
CSIR, India in the form of fellowship. P.S and K.S acknowledge
the financial support from UGC, India in the form of fellowship.
Selectivity
Yieldb (%)
(%)
1
2
3
First
Second
Third
97
95
98
99
99
99
96
94
97
Notes and references
1
(a) A. Ricci, Modern Amination Reactions, WileyꢀVCH, Weinheim,
Germany, 2000; (b) J. F. Hartwig, Handbook of Organopalladiam
Chemistry for Organic Synthesis, ed. E. Negishi and A. Meijere,
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Amines : Synthesis properties and application, ed. S. A. Lawrence,
Cambridge University press, Cambridge, 2004.
45
50
55
60
65
70
75
80
a
Reaction conditions: Benzyl alcohol (6 mmol), Aniline (2 mmol),
Nanosized zeolite Beta (100 mg), 135 °C, open atmosphere, reaction time
6.3 h. b Products were characterized by NMR, Mass spectra and isolated
yields calculated based on aniline.
2
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5
amounts of Nꢀalkylated product. Moreover, 4ꢀnitrobenzyl alcohol
in the absence of aniline under inert or air atmosphere exclusively
yielded the corresponding ether and formation of aldehyde was
10 not observed. The same phenomenon was observed with benzyl
R2
H
3
(a) S. L. Buchwald, C. Mauger, G. Mignani and U. Scholz, Adv.
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:
N
H
O
H
H
N
H
O
O
O
R2
H2O
Si
Al
Si
H
O
R1
O
Si
Al
Si
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O2
H
O
N
R2
H2O
Si
Al
Si
R1
Scheme 2 Plausible reaction mechanism for the formation of Nꢀalkylated
product and imine.
alcohol also. Based on above experimental observations (see ESI
15 for other experimental details) and literature reports,27 we
propose a probable mechanism for the formation of Nꢀalkylated
product and imine (Scheme 2).
6
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30 procedure for producing higher amines from amines and alcohols
using nanosized zeolite Beta as a catalyst. Moreover, imines were
also efficiently prepared from the tandem reactions of amines
with 2, 3 and 4ꢀnitrobenzyl alcohols. Notable advantages offered
by this method are broad substrate scope, high atom economy
35 (only water is side product), reusability of catalyst, 100
environmentally benign, higher yields of the desired products and
simple workꢀup procedure, which make it an attractive and useful
95
8
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