Indian Journal of Chemistry
Vol. 54B, June 2015, pp 779-790
SiO2-H3PO4 catalyzed condensation of amines and aldehydes: Solvent-free
synthesis of some E-imines, spectral correlations of (E)-N-(substituted
benzylidene)-1-benzylpiperidin-4-amines and XRD structure of
(E)-N-(4-nitrobenzylidene)-1-benzylpiperidin-4-amine
P Mayavel, K Thirumurthy, S Dineshkumar & G Thirunarayanan*
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
E-mail: [email protected]
Received 7 July 2014; accepted (revised) 12 March 2015
A series of Schiff bases (aryl E-imines) including (E)-N-(substituted benzylidene)-1-benzylpiperidin-4-amines have
been derived from the SiO2-H3PO4 catalyzed solvent-free condensation of aryl amines including 1-benzylpiperidin amines
and substituted benzaldehydes under microwave irradiation. The yields of the imines are more than 85%. The synthesized
imines are characterized by their physical constants, analytical and spectroscopic data earlier reported in literature. The
group frequencies of imines such as infrared (ν, cm-1), NMR both 1H and 13C chemical shifts (δ, ppm) have been correlated
with Hammett substituent constants, F and R parameters. The XRD structure of the (E)-N-(4-nitrobenzylidene)-1benzylpiperidin-4-amine have been established.
Keywords: (E)-Imines, SiO2-H3PO4, Hammett correlations, X-ray crystal structure
The (E)-imines also called Schiff bases were first
synthesized by Schiff in 1864. Schiff bases named
after Hugoschiff’s formed by the bimolecular condensation products of primary amine with carbonyl
compounds. These (E)-imines are characterized by the
–N=C– imine group which find important in elucidating
the mechanism of transamination and racemization
reaction of biological system1,2. Numerous reagents
have been used for the synthesis of optically active
imines like Lewis acids3, molecular sieves in ionic
liquids4, solid super acid K-10 montmorillonite5, Tandam
catalyst6, MnO2 (Ref 7), CaO (Ref 8), ZnCl2 (Ref 9),
MgSO4-PPTS10, alumina11, P2O5-SiO2 (Ref 12), infrared13,
ultrasound radiation14 and fly-ash:H2SO4 with
microwave irradiation. These catalysts were used for
the synthesis of chiral Schiff bases by condensation of
amines15,16, with carbonyl compounds5,6,17, alcohols18
and acid chlorides3,19. The (E)-imines starting materials
and important intermediates were used for the
synthesis of pharmacologically active triazoles and
trizolones20,21, chiral amines9, pyrimidine derivatives22,
phenylhydrazones10, indoles23, quinoxalines24, imidazoles25,
through hydrogenation26, nucleophilic addition with
organometallics27, and cyclo-addition reaction28. Optically
active Schiff base derivatives possess biological activities
such as antimi-crobial29, anticancer30, antiplasmodicantihypoxic31, antitubularcular32, nematicidal insecticidal17,
anti-inflammatory and lipoxygenase33. Spectroscopic
data were useful for prediction of ground state conformations of organic substrates. Infrared frequencies are
used for prediction of s-cis and s-trans conformation
of ketones, chalcones, acid chloride, esters34-39.
Physical organic chemists and research have been
studies the spectral correlations of these frequencies with
Hammett substituent constants F and R parameters4044
. Recently Suresh et al.35 have synthesized some aryl
imines by solvent-free synthesis method, studied the
effects of substituents on the function spectral
frequencies and the antimicrobial activities of the
amines. There is no report available for solvent-free
synthesis of some imines including substituted
benzylidene-1-benzyl-piperidin-4-amines,
spectral
correlation and X-ray crystal structure of nitro substituted
benzylidene-1-benzylpiperidin-4-amines. Therefore the
authors have taken efforts to synthesis of some (E)imines by SiO2-H3PO4 catalyzed condensation of aryl amines
including benzylidene-1-benzylpiperidin-4-amines with
substituted benzaldehydes. These synthesized Schiff
bases have been characterized by their analytical, physical
constants and spectroscopic data earlier published in
literature. The spectroscopic data of these imines have
been utilized for studying the spectral correlations and
establish the structure of nitro substituted imine using
single crystal X-ray diffraction spectroscopic data.
780
INDIAN J. CHEM., SEC B, JUNE 2015
Results and Discussion
In organic chemistry research laboratory, works
have been carried out to synthesize aryl imine derivatives
by condensation of aryl amines and various benzaldehydes containing electron withdrawing as well as
electron donating substituents. The condensation reaction
is feasible with aryl amines and benzaldehydes in the
presence of vigorous acidic catalyst like SiO2-H3PO4
under microwave irradiation in atmospheric temperature
and pressure. Hence the authors have synthesized the
imine derivatives by the condensation between 2 mmol
of aryl amines, 2 mmol substituted benzaldehydes and
the solid SiO2-H3PO4 (0.5 g) catalyst under microwave
irradiation for 4-6 min at room temperature (Scheme I).
During the course of this reaction the SiO2-H3PO4
catalyzes coupling between aryl amines and aldehydes
with elimination of water followed by loss of proton
forms the imines. The yields of the imines in this
condensation are more than 85%. The benzaldehydes
containing electron donating substituents like methoxy
gave higher yields than the electro withdrawing
substituents like nitro and halogens. The physical
constants yield and mass spectral data are presented in
Table I. The proposed general mechanism of this
reaction is given in Scheme II. Further we have also
investigated effect of catalyst loading on this
condensation with equimolar quantities of 4-amino-1benzylpiperidine and benzaldehyde (entry 30). In this
condensation the yield of imine is 94%. The reusability
of the catalyst for this condensation was studied with
the obtained product percentage of further runs. The
first two runs gave 94% yield. The second, third runs
gave 93.5 and 93.5% of imine and fifth run gave 93%
yield. The reusability of the catalyst with the obtained
yield percentage of the runs are presented in Table II.
The amount of catalyst was increased from 0.1-1 g,
the yield also increases from 85-94%. After adding
0.4 g of the catalyst, there is no significant increasing
in the yield was observed. The effect of catalyst loading
was shown in Figure 1. The optimum quantity of
catalyst for this condensation of amine and aldehyde
was found to be 0.4 g.
Scheme I — Synthesis of imines by condensation of aldehydes and amines in presence of SiO2-H3PO4 under microwave irradiation
Table I — Analytical, physical constants and mass fragments (m/z) of the imines synthesized by aryl amines and
substituted benzaldehydes reaction of the type R–NH2 + R′–CHO → R–N=CH–R′
Entry R
R′
Product
M.W. Yield
(%)
1
C6H5
C6H5
C6H5N=CHC6H5
181
86
2
C6H5
4-ClC6H4
C6H5N=CHC6H4Cl (4)
215
87
3
C6H5
4-CH3OC6H4
C6H5N=CHC6H4OCH3 (4)
211
87
4
C6H5
4-CH3C6H4
C6H5N=CHC6H4CH3 (4)
195
88
5
C6H5
4-NO2C6H4
C6H5N=CHC6H4NO2 (4)
226
86
6
C5H4N
C6H5
C5H4NN=CHC6H5
182
85
7
C5H4N
4-ClC6H4
C5H4NN=CHC6H4Cl (4)
216
86
8
C5H4N
4-CH3OC6H4
C5H4NN=CHC6H4OCH3 (4)
212
90
9
C5H4N
4-CH3C6H4
C5H4NN=CHC6H4CH3 (4)
196
89
10 C5H4N
4-NO2C6H4
C5H4NN=CHC6H4NO2 (4)
227
85
m.p.
(°C)
Mass
(m/z)
52-53
(50-52) (Ref 35)
61-62
(61) (Ref 35)
56-57
(54-56) (Ref 35)
36-37
(35-37) (Ref 35)
88-89
(87-89) (Ref 35)
94-96
(92-95) (Ref 35)
85-86
(82-85) (Ref 35)
158-59
(155-57) (Ref 35)
122-24
(119-22) (Ref 35)
123-24
(120-23) (Ref 35)
–
–
–
–
–
–
–
–
–
–
Contd —
781
MAYAVEL et al.: SOLVENT-FREE SYNTHESIS OF E-IMINES
Table I — Analytical, physical constants and mass fragments (m/z) of the imines synthesized by aryl amines and
substituted benzaldehydes reaction of the type R–NH2 + R′–CHO → R–N=CH–R′ — Contd
Entry R
R′
Product
M.W. Yield
(%)
11 4-CH3C6H4
C6H5
4-CH3C6H4N=CHC6H5
195
89
12 4-CH3C6H4
4-OHC6H4
4-CH3C6H4N=CHC6H4OH (4)
212
85
13 4-CH3C6H4
2-OHC6H4
4-CH3C6H4N=CHC6H4OH (2)
212
86
14 4-CH3C6H4
4-CH3OC6H4
4-CH3C6H4N=CHC6H4OCH3 (4)
226
92
15 4-CH3C6H4
4-N(CH3)2C6H4 4-CH3C6H4N=CHC6H4N(CH3)2 (4) 239
85
16 4-CH3C6H4
4-NO2C6H4
4-CH3C6H4N=CHC6H4NO2 (4)
241
85
17 4-BrC6H4
C6H5
4-BrC6H4N=CHC6H5
260
86
18 4-BrC6H4
4-OHC6H4
4-BrC6H4N=CHC6H4OH (4)
277
87
19 4-BrC6H4
2-OHC6H4
4-BrC6H4N=CHC6H4OH (2)
277
88
20 4-BrC6H4
4-N(CH3)2C6H4 4-BrC6H4N=CHC6H4N(CH3)2 (4) 304
88
21 4-BrC6H4
4-NO2C6H4
4-BrC6H4N=CHC6H4NO2 (4)
306
86
22 C2H2N3
C6H5
C2H2N3N=CHC6H5
172
85
23 C2H2N3
4-CH3C6H4
C2H2N3N=CHC6H4CH3 (4)
187
86
24 C2H2N3
4-CH3OC6H4
C2H2N3N=CHC6H4OCH3 (4)
203
90
25 C2H2N3
2-OHC6H4
C2H2N3N=CHC6H4OH (2)
189
86
26 C2H2N3
4-OHC6H4
C2H2N3N=CHC6H4OH (4)
189
88
27 C2H2N3
4-N(CH3)2C6H4 C2H2N3N=CHC6H4N(CH3)2 (4)
216
86
28 C2H2N3
4-NO2C6H4
C2H2N3N=CHC6H4NO2 (4)
218
85
29 C2H2N3
4-ClC6H4
C2H2N3N=CHC6H4Cl (4)
207
85
30 C12H16N
C6H5
C12H16NN=CHC6H5
278
94
31 C12H16N
3-BrC6H4
C12H16NN=CHC6H4Br (3)
357
87
32 C12H16N
3-ClC6H4
C12H16NN=CHC6H4Cl (3)
312
86
33 C12H16N
4-ClC6H4
C12H16NN=CHC6H4Cl (4)
312
87
34 C12H16N
4-N(CH3)2C6H4 C12H16NN=CHC6H4N(CH3)2 (4)
321
88
m.p.
(°C)
Mass
(m/z)
114-15
–
(112-15) (Ref 35)
212-13
–
(210-12) (Ref 35)
105-106
–
(102-105) (Ref 35)
95-96
–
(93-95) (Ref 35)
96
–
(92–95) (Ref 35)
125-26
–
(121-25) (Ref 35)
62-63
–
(61-62) (Ref 35)
170-71
–
(169-70) (Ref 35)
177-78
–
(175-77) (Ref 35)
192-93
–
(189-92) (Ref 35)
175-76
–
(172-75) (Ref 35)
194-95
–
(194) (Ref 35)
191-92
–
(190) (Ref 35)
195-96
–
(195) (Ref 35)
178-79
–
(178) (Ref 35)
194-95
–
(193) (Ref 35)
235-36
–
(235) (Ref 35)
238-39
–
(238) (Ref 35)
205-206
–
(205) (Ref 35)
64-65
278 [M+], 279 [M1+], 265, 212,
201, 187, 152, 113, 102, 94, 81,
69, 52
59-60
357 [M+], 359 [M2+], 317, 304,
279, 265, 237, 198, 155, 128,
124, 119, 110, 103, 97, 65, 52
53-54
312 [M+], 313 [M1+], 314 [M2+],
299, 282, 260, 235, 179, 138,
111, 105, 86, 65, 52
102-103
312 [M+], 313 [M1+], 314 [M2+],
299, 282, 260, 35, 179, 137, 113,
111, 105, 88, 76, 64, 52
57-58
321 [M+], 302, 289, 264, 240,
225, 217, 186, 144, 130, 121,
102, 88, 82, 64, 53
Contd —
782
INDIAN J. CHEM., SEC B, JUNE 2015
Table I — Analytical, physical constants and mass fragments (m/z) of the imines synthesized by aryl amines and
substituted benzaldehydes reaction of the type R–NH2 + R′–CHO → R–N=CH–R′ — Contd
m.p.
(°C)
Mass
(m/z)
87
92-93
308
93
63-64
C12H16NN=CHC6H4CH3O (4)
308
94
49-50
4-CH3C6H4
C12H16NN=CHC6H4CH3 (4)
292
90
51-52
39 C12H16N
3-NO2C6H4
C12H16NN=CHC6H4NO2 (3)
323
87
60-61
40 C12H16N
4–NO2C6H4
C12H16NN=CHC6H4NO2 (4)
323
86
82-83
296 [M+], 298 [M2+], 282, 270,
244, 226, 219, 205, 179, 163,
135, 122, 108, 105, 95, 69, 53
308 [M+], 293, 282, 269, 256,
189, 175, 161, 147, 102, 95, 82,
69, 64, 52.
308 [M+], 293, 282, 252, 231,
226, 217, 188, 162, 134, 120,
107, 82, 56, 53
292 [M+], 293 [M1+], 263, 241,
211, 203, 166, 152, 129, 109, 91,
52
323 [M+], 324 [M1+], 295, 285,
267, 253, 247, 222, 119, 110, 85,
63, 53
323 [M+], 324 [M1+], 295, 285,
267, 253, 247, 222, 119, 110, 85,
63, 53
Entry R
R′
Product
M.W. Yield
(%)
35 C12H16N
4-FC6H4
C12H16NN=CHC6H4F (4)
296
36 C12H16N
3-CH3OC6H4
C12H16NN=CHC6H4CH3O (3)
37 C12H16N
4-CH3OC6H4
38 C12H16N
Scheme II — The proposed mechanism of formation of imines by condensation of aldehydes and amines in presence of SiO2-H3PO4
under microwave irradiation
Table II — Reusability of catalyst on condensation of imine
(2 mmol) and benzaldehyde (2 mmol) under microwave
irradiation (entry 30)
Run
1
2
3
4
5
Yield (%)
94.0
94.0
93.5
93.5
93.0
Further we have studied the effect of solvents on
the condensation of equimolar quantities of 4-amino1-benzylpiperidine and benzaldehyde (entry 30) in
conventional heating with each component of catalyst.
The solvents like methanol, ethanol, dichloromethane
and tetrahydrofuran (THF) have been used for this
condensation. The obtained percentage of products of
these imines with various solvents is shown in
Table III. Carrying out this condensation with above
solvents and solvent-free method, the authors have
observed higher yield of imines by SiO2-H3PO4
catalyzed condensation under microwave irradiation.
Spectral correlations
The spectral linearity of synthesized imines has
been studied by evaluating the substituent effects on
the group frequencies such as νC=N (cm–1), the proton
chemical shifts δ (ppm) of C–H and carbon chemical
783
MAYAVEL et al.: SOLVENT-FREE SYNTHESIS OF E-IMINES
shifts of C=N have been correlated with Hammett
substituent constants F and R parameters using single
and multi-linear regression analysis.
ν = ρσ + ν0
(1)
where ν0 is the frequency for the parent member of
the series.
The observed νC=N stretching frequencies (cm–1) of
the imine derivatives were correlated with various
Hammett substituent constants, F and R parameters
through single and multi-regression analyses including
Swain-Lupton’s parameters. The statistical analysis
results of single parameter correlation were shown in
Table V. The correlation of νC=N (cm–1) frequencies of
imines with Hammett σR substituent constants and R
parameters is found to be fail, with positive ρ value.
This implies that the normal substituent effect
operates in all imine derivatives. This is due to the
absence of polar, field and inductive effects of the
substituents and hence they are unable to predict the
reactivity on C=N stretches. This is associated with
the conjugative structure shown in Figure 2. Some of
the single parameter correlations of νC=N (cm–1)
frequencies with Hammett substituent constants fail in
correlation. So, the authors think that it is worthwhile
to seek the multi-regression analysis which may
produce a satisfactory correlation with Resonance,
Field and Swain-Lupton’s46 constants. This is shown
in the following Eqs 2 and 3.
IR spectral study
The recorded infrared νC=N stretching frequencies
(cm–1) of the synthesized imines (entries 30-40) have
been presented in Table IV. These stretches were
correlated2a,34–45 with Hammett substituent constants,
F, R and Swain-Lupton’s46 parameters. In this regression
analysis the structure parameter correlation Hammett
equation have been employed as shown in Eq. 1.
νC=N (cm–1) 1564.12(±15.384) + 18.721(±3.032)σI
– 32.58(±13.33)σR (R = 0.955,
n = 11, P > 95%)
Figure 1 — The effect of catalyst loading
…(2)
Table III — Effect of solvents in conventional heating and with solvent and microwave irradiation on yield of imine (entry 30)
SiO2
73
MeOH
PA SiO2:PA
77
79
SiO2
74
Solvents
EtOH
PA SiO2:PA
SiO2
DCM
PA SiO2:PA
75
80
80
73
Microwave irradiation
82
SiO2
75
THF
PA SiO2:PA
81
SiO2
PA
SiO2:PA
82
80
94
85
MeOH = Methanol; EtOH = Ethanol; DCM = Dichloromethane; THF = Tetrahydrofuran; PA = Phosphoric acid.
Table IV — Infrared and NMR spectral data of synthesized imines (entries 30-40)
Entry
30
31
32
33
34
35
36
37
38
39
40
Substituent
H
3-Br
3-Cl
4-Cl
4-N(CH3)2
4-F
3-OCH3
4-OCH3
4-CH3
3-NO2
4-NO2
IR
νC=N (cm–1)
1567.12
1565.44
1595.71
1592.24
1560.70
1597.18
1593.45
1575.79
1573.95
1526.95
1598.42
1
δCH
2
H NMR
(ppm)
δCH=N (ppm)
3.550
3.510
3.538
3.543
3.540
3.547
3.552
3.534
3.547
3.580
3.559
8.327
8.172
8.236
8.271
8.19
8.282
8.294
8.235
8.285
8.556
8.400
13
δC=N (ppm)
C NMR
δCH (ppm)
δ Cipso (ppm)
159.16
157.54
157.69
157.8
159.14
157.68
159.88
161.52
159.09
156.51
156.81
67.71
67.57
67.66
67.7
67.64
67.56
67.63
67.67
67.70
67.45
67.89
130.50
122.90
134.78
136.39
152.00
165.45
159.01
164.65
140.71
148.60
148.93
2
784
INDIAN J. CHEM., SEC B, JUNE 2015
Table V — Results of statistical analysis of IR (ν, cm–1) of C=N, NMR chemical shifts (δ, ppm) of CH2, CH, C=N and Cipso of imines
with Hammett substituent constants σ, σ+, σI, σR, F and R parameters (entries 30-40)
Frequency
Constants
r
I
ρ
s
n
σ
0.815
1576.88
0.764
22.92
11
σ+
0.731
1576.97
3.911
22.73
11
σI
0.692
1574.25
8.036
22.82
11
σR
0.791
1571.81
–26.03
21.93
11
F
0.834
1572.70
11.60
22.72
11
R
0.815
1574.10
–9.86
22.64
11
σ
0.855
3.543
0.014
0.01
11
σ+
0.812
3.545
0.007
0.01
11
σI
0.706
3.540
0.014
0.01
11
σR
0.842
3.551
0.031
0.01
11
F
0.738
3.539
0.015
0.01
11
R
0.886
3.551
0.019
0.01
11
σ
0.862
8.273
0.153
0.08
11
σ+
0.854
8.294
0.080
0.09
11
σI
0.841
8.235
0.176
0.10
11
σR
0.889
8.355
0.302
0.08
11
F
0.884
8.234
0.163
0.10
11
R
0.874
8.353
0.199
0.08
11
σ
0.861
158.92
–1.511
1.32
11
σ+
0.899
158.71
–0.778
1.37
11
σI
0.868
159.61
–2.643
1.32
11
σR
0.716
158.34
–1.844
1.41
11
F
0.846
159.64
–2.521
1.33
11
R
0.829
158.31
–1.356
1.41
11
νC=N
δCH
2
δCH
δC=N
δCH
2
σ
0.701
67.65
0.004
0.11
11
σ+
0.700
67.65
0.006
0.11
11
σI
0.816
67.67
–0.070
0.11
11
σR
0.820
67.67
0.094
0.11
11
Correlated derivatives
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
Contd —
MAYAVEL et al.: SOLVENT-FREE SYNTHESIS OF E-IMINES
785
Table V — Results of statistical analysis of IR (ν, cm–1) of C=N, NMR chemical shifts (δ, ppm) of CH2, CH, C=N and Cipso of imines
with Hammett substituent constants σ, σ+, σI, σR, F and R parameters (entries 30-40) — Contd
Frequency
δCipso
Constants
r
I
ρ
s
n
F
0.822
67.68
–0.100
0.11
11
R
0.819
67.67
0.062
0.11
11
σ
0.721
147.04
–8.788
14.22
11
σ+
0.824
145.83
–5.464
14.17
11
σI
0.804
144.21
4.685
14.72
11
σR
0.851
139.87
–29.829
12.63
11
F
0.723
140.17
15.237
14.21
11
R
0.750
139.60
–21.125
12.62
11
Figure 2 — The resonance conjugative structure
νC=N (cm–1) 1567(±15.835) + 15.154(±3.042)F
– 12.212(±2.173)R (R = 0.932,
n = 11, P > 90%)
…(3)
1
H NMR spectral study
In this present study, the 1H NMR spectra of the
synthesized Schiff base derivatives under investigation
have been recorded in deuteriochloroform solution
employing tetramethylsilane (TMS) as internal standard.
In nuclear magnetic resonance spectra, the 1H or the
13
C chemical shifts (δ, ppm) depend on the electronic
environment of the nuclei concerned. The signals of
the Schiff base proton chemical shifts (δ, ppm)
CH=N, have been assigned and are presented in
Table IV. These chemical shifts (δ, ppm) have been
correlated with reactivity parameters. Thus the Hammett
equation has been used in the form as shown in Eq. 4.
logδ = logδ0 + ρσ
(4)
where δ0 is the chemical shifts of the corresponding
parent compound.
The assigned proton chemical shifts (δ, ppm) of
Schiff bases have been correlated2a,34–45 with Hammett σ
constants, F and R parameters. The results of statistical
Correlated derivatives
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
H, 3-Br, 3-Cl, 4-Cl, 4-N(CH3)2, 4-F, 3-OCH3, 4-OCH3,
4-CH3, 3-NO2, 4-NO2
analysis are presented in Table V. These proton
chemical shifts (ppm) fail in correlation with Hammett
substituent constants and F and R parameters. All
correlations give positive ρ values. This shows that
the normal substituent effect operates in all systems.
The failure in correlation is attributed to the conjugative
structure shown in Figure 2.
In view of the inability of the Hammett σ constants
to produce individually satisfactory correlations with
the imine proton chemical shifts (δ, ppm), the authors
think that, it is worthwhile to seek multiple correlations
involving either σI and σR constants or SwainLupton’s46 F and R parameters. This is shown in the
following Eqs 5-8 for CH=N proton chemical shifts
(δ, ppm).
δCH2 (ppm) 3.549(±0.011) + 0.004(±0.022)σ I
+ 0.029(±0.023)σR (R = 0.944,
n = 11, P > 90%)
δCH2 (ppm) 3.546(±0.01) + 0.010(±0.022)F
+ 0.017(±0.016)R (R = 0.949,
n = 11, P > 90%)
δCH (ppm) 8.319(±0.056) + 0.086(±0.011)σ I
+ 0.271(±0.115)σR (R = 0.972,
n = 11, P > 95%)
δCH (ppm) 8.307(±0.057) + 0.110(±0.010)F
+ 0.182(±0.080)R (R = 0.969,
n = 11, P > 95%)
13
…(5)
…(6)
…(7)
…(8)
C NMR spectral study
Physical organic chemists and researchers2a,34–45
have made extensive study of 13C NMR spectra for a
786
INDIAN J. CHEM., SEC B, JUNE 2015
large number of ketones, styrenes, styryl ketones and
keto-epoxides. They have studied linear correlation of
the chemical shifts (δ, ppm) of Cα, Cβ and CO carbons
with Hammett σ constants in alkenes, alkynes, acid
chlorides and styrenes. In the present study, the
chemical shifts (δ, ppm) of Schiff base C=N and ipso
carbons have been assigned and are presented in
Table IV. Attempts have been made to correlate these
chemical shifts (δ, ppm) with Hammett substituent
constants, field and resonance parameters, with the
help of single and multi-regression analyses to study
the reactivity through the effect of substituents.
The chemical shifts (δ, ppm) observed for the δC=N
have been correlated with Hammett constants and the
results of statistical analysis2a,34–45 are presented in
Table V. The δC=N, δCH2 and ipso carbon chemical shifts
(δ, ppm) gave poor correlation with Hammett σ constants,
F and R parameters. This is due to the reason stated
earlier with resonance conjugative structure as shown
in Figure 2.
In view of inability of some of the σ constants to
produce individually satisfactory correlation, the
authors think that it is worthwhile to seek multiple
correlation involving all σI, σR, F and R parameters (Swain
and Lupton, 1968). This is given in the following
correlation Eqs 9-14.
δC=N (ppm) 159.28(±0.931) – 2.302(±1.837)σI
– 1.039(±0.897)σR (R = 0.949,
n=11, P > 95%)
…(9)
δC=N (ppm) 159.23(±0.915) – 2.226(±1.758)F
– 1.012(±0.128)R (R = 0.956,
n = 11, P > 95%)
…(10)
δCH2 (ppm) 67.718(±0.078) – 0.114(±0.055)σI
+ 0.135(±0.016)σR (R = 0.923,
n = 11, P > 90%)
…(11)
67.72(±0.077)
–
0.124(±0.049)F
δCH2 (ppm)
+ 0.082(±0.008)R (R = 0.941,
n = 11, P > 90%)
…(12)
δCipso (ppm) 133.16(±8.589)+ 6.341(±0.694)σI
– 35.54(±17.490)σR (R = 0.987,
n = 11, P > 95%)
…(13)
δCipso (ppm) 130.32(±7.961)+22.393(±15.298)F
– 24.58(±11.14)R (R = 0.965,
n = 11, P > 95%)
…(14)
Single crystal XRD structure of (E)-1-(4-nitrobenzylidene)-1-benzylpiperidin-4-amine
The named imine (entry 40) (Figure 3) having the
methylene protons in one plane and the azomethine
protons (N=C–H) also occupied one plane. The single
lattice unit, lattice packing diagram of (E)-1-(4nitrobenzylidene)-1-benzylpiperidin-4-amine (entry 40)
are shown in Figures 4 and 5. Unit cell parameters
and intensity data (Table VI) were obtained at 298 K
on a Bruker-Nonius SMART APEX CCD single crystal
diffractometer equipped with a graphite monochromator
and a MoKα fine-focus sealed tube (λ = 0.71073 Å)
operated at 2.0 kW. In each case, the detector was
placed at a distance of 6.0 cm from the crystal. Data
Figure 3 — Single crystal diffraction refinement structure of (E)1-(4-nitrobenzylidene)-1-benzylpiperidin-4-amine (entry 40)
Figure 4 — A single lattice unit of (E)-1-(4-nitrobenzylidene)-1benzylpiperidin-4-amine (entry 40)
Figure 5 ― Lattice packing diagram of (E)-1-(4nitrobenzylidene)-1-benzylpiperidin-4-amine (entry 40)
787
MAYAVEL et al.: SOLVENT-FREE SYNTHESIS OF E-IMINES
were collected with a scan width of 0.3° in ω and an
exposure time of 15 s/frame. Data acquisition and
data extraction were performed with SMART and the
SAINT-Plus software was used for absorption
correction. Both structures were solved by direct
methods and refined on F2 by full-matrix least-squares
procedures. All non-hydrogen atoms were refined
anisotropically. Hydrogens were included in the
structure factor calculation at idealized positions by
using a riding model. The methylene protons of C–13
carbons exist in one plane. The SHELX-97 programs
were used for structure solution and refinement. The
atomic coordinates and equivalent isotropic displacement
parameters, bond lengths and angles and anisotropic
displacement parameters for entry 40 are presented in
Tables VII–IX.
Table VI — Crystal data, data collection and refinement
parameters for (E)-1-(4-nitrobenzylidene)-1-benzylpiperidin-4amine (entry 40)
CCDC No.
Empirical formula
Formula weight
Crystal size (mm)
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Calculated density (g cm–3)
Absorption coefficient (mm–1)
F(000)
θ Range for data collection (°)
Index ranges
Reflections collected / unique
Completeness to θ
Absorption correction
λ (Å)
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I ≥ 2σ(I)]
R indices (all data)
Largest diff. peak / hole (e Å–3)
926151
C19H21N3O2
323.39
0.19 × 0.18 × 0.16
273
Monoclinic
P21/c
13.906(5)
11.650(4)
11.406(4)
90.00
107.485(5)
90.00
1762.4(11)
4
1.219
0.081
688
1.54–24.97
−16 ≤ h ≤ 16, −13 ≤ k ≤ 13,
−13 ≤ l ≤ 13
16415 / 3098 [Rint = 0.0416]
24.97 (99.8%)
None
MoKα (0.71073)
Full-matrix least square on F2
3095 / 0 / 220
1.079
R1 = 0.0578, wR2 = 0.1268
R1 = 0.0846, wR2 = 0.1394
0.135 / –0.204
Experimental Section
Materials and methods
All chemicals used in this work were purchased
from Sigma-Aldrich Chemicals Private Limited. Melting
points of all imines have been determined in open
glass capillaries on Biom melting point apparatus
(Universal Bio Chemicals Enzyme House, Madurai-3) and
are uncorrected. Infrared spectra (KBr, 4000-400 cm–1)
have been recorded on Avatar-300 Fourier transform
spectrophotometer (Thermo Nicolet, USA). The NMR
spectra of all imines were recorded in Bruker AV400
spectrometer (Bruker AXS GMBH, Karlsruhe,
Germany), operated 400 MHz frequency for recording
1
H and 100 MHz for 13C NMR spectra in CDCl3
solvent using TMS as internal standard. Electron
impact (70 eV) and chemical ionization mode FAB +
mass spectra have been recorded in Varian-Saturn
2200 GC-MS spectrometer (Varian 92 Medical Systems,
Palo Alto, CA, USA). The single crystal XRD pattern
was recorded in Bruker-Nonius SMART APEX CCD
single crystal diffractometer. These data (CCDC No.
926151) can be obtained free of charge from the
Cambridge Crystallographic Data Centre (CCDC),
Table VII — Atomic coordinates (×104) and equivalent isotropic
displacement parameters (Å2 × 103) for (E)-1-(4-nitrobenzylidene)-1-benzylpiperidin-4-amine (entry 40). Ueq is defined as
one third of the trace of the orthogonalized Uij tensor
C(1)
C(5)
C(4)
N(1)
C(6)
C(7)
C(3)
C(2)
N(3)
C(11)
O(2)
N(2)
C(14)
C(12)
C(8)
C(9)
C(15)
C(10)
C(19)
O(1)
C(17)
C(18)
C(13)
C(16)
x
y
Z
Ueq
225(2)
–1142(2)
–1182(2)
1116(1)
–440(2)
999(2)
–552(2)
150(2)
–1900(2)
3468(2)
–2399(2)
2984(1)
4378(2)
2702(2)
1915(2)
1469(2)
4160(2)
2272(2)
5236(2)
–1943(2)
5644(2)
5863(2)
3711(2)
4788(2)
2161(2)
969(2)
491(2)
3547(2)
1802(2)
3036(2)
837(2)
1669(2)
–436(2)
4997(2)
–809(2)
6045(2)
6747(2)
4058(2)
4407(2)
5543(2)
5958(2)
6434(2)
7384(2)
–820(2)
6431(3)
7230(3)
6947(2)
5794(3)
10544(2)
9389(2)
10477(2)
9708(2)
9433(2)
10619(2)
11594(2)
11620(2)
10444(2)
9450(2)
9462(2)
8861(2)
8080(2)
9322(2)
9925(2)
9412(3)
7154(2)
9487(3)
8260(2)
11412(2)
6651(3)
7548(3)
8883(3)
6447(3)
51(1)
57(1)
57(1)
60(1)
56(1)
56(1)
63(1)
59(1)
77(1)
66(1)
96(1)
65(1)
65(1)
62(1)
60(1)
74(1)
82(1)
81(1)
81(1)
136(1)
92(1)
92(1)
85(1)
97(1)
788
INDIAN J. CHEM., SEC B, JUNE 2015
Table VIII — Bond lengths (Å) and angles (°) for (E)-1-(4nitrobenzylidene)-1-benzylpiperidin-4-amine
(entry
40).
Symmetry transformations used to generate equivalent atoms
Table IX — Anisotropic displacement parameters (Å2 × 103) for
(E)-1-(4-nitrobenzylidene)-1-benzylpiperidin-4-amine (entry 40).
The anisotropic displacement factor exponent takes the form:
–2 π2 [h2 a*2 U11 +... + 2 h k a* b* U12]
Bond angles (°)
Bond lengths (Å)
C(1)–C(2)
1.386(3)
C(1)–C(6)
1.390(3)
C(1)–C(7)
1.466(3)
C(5)–C(6)
1.368(3)
C(5)–C(4)
1.377(3)
C(4)–C(3)
1.372(3)
C(4)–N(3)
1.463(3)
N(1)–C(7)
1.250(3)
N(1)–C(8)
1.461(3)
C(3)–C(2)
1.369(3)
N(3)–O(2)
1.207(3)
N(3)–O(1)
1.209(3)
C(11)–N(2)
1.457(3)
C(11)–C(12)
1.504(3)
N(2)–C(13)
1.453(3)
N(2)–C(10)
1.457(3)
C(14)–C(15)
1.364(3)
C(14)–C(19)
1.368(3)
C(14)–C(13)
1.504(3)
C(12)–C(8)
1.512(3)
C(8)–C(9)
1.504(3)
C(9)–C(10)
1.507(3)
C(15)–C(16)
1.368(4)
C(19)–C(18)
1.369(4)
C(17)–C(18)
1.349(4)
C(17)–C(16)
1.362(4)
C(2)–C(1)–C(6)
C(2)–C(1)–C(7)
C(6)–C(1)–C(7)
C(6)–C(5)–C(4)
C(3)–C(4)–C(5)
C(3)–C(4)–N(3)
C(5)–C(4)–N(3)
C(7)–N(1)–C(8)
C(5)–C(6)–C(1)
N(1)–C(7)–C(1)
C(2)–C(3)–C(4)
C(3)–C(2)–C(1)
O(2)–N(3)–O(1)
O(2)–N(3)–C(4)
O(1)–N(3)–C(4)
N(2)–C(11)–C(12)
C(13)–N(2)–C(10)
C(13)–N(2)–C(11)
C(10)–N(2)–C(11)
C(15)–C(14)–C(19)
C(15)–C(14)–C(13)
C(19)–C(14)–C(13)
C(11)–C(12)–C(8)
N(1)–C(8)–C(9)
N(1)–C(8)–C(12)
C(9)–C(8)–C(12)
C(8)–C(9)–C(10)
C(14)–C(15)–C(16)
N(2)–C(10)–C(9)
C(14)–C(19)–C(18)
C(18)–C(17)–C(16)
C(17)–C(18)–C(19)
N(2)–C(13)–C(14)
C(17)–C(16)–C(15)
118.5(2)
119.03(19)
122.50(19)
118.4(2)
122.0(2)
118.8(2)
119.2(2)
117.89(19)
121.2(2)
124.00(19)
118.7(2)
121.1(2)
122.8(2)
119.1(2)
118.0(3)
110.45(18)
109.80(18)
112.05(19)
108.78(19)
117.5(2)
122.8(2)
119.6(2)
110.42(18)
109.48(18)
110.87(18)
109.27(19)
111.8(2)
121.2(2)
111.04(19)
121.4(3)
119.5(3)
120.1(3)
115.5(2)
120.2(3)
12 Union Road, Cambridge CB2 1EZ, UK, Tel: (+44)
1223-336-408, Fax: (+44) 1223-336-033, E-mail:
[email protected].
Preparation of SiO2-H3PO4 catalyst
The SiO2-H3PO4 catalyst was prepared by literature
method45. In a 50 mL borosil beaker, 2 g of silica
(10-20 µ) 2 mL of ortho phosphoric acid were taken and
mixed thoroughly with glass rod. This mixture was
C(1)
C(5)
C(4)
N(1)
C(6)
C(7)
C(3)
C(2)
N(3)
C(11)
O(2)
N(2)
C(14)
C(12)
C(8)
C(9)
C(15)
C(10)
C(19)
O(1)
C(17)
C(18)
C(13)
C(16)
U11
U22
U33
U23
U13
U12
50(1)
52(1)
50(1)
54(1)
57(1)
53(1)
61(1)
57(1)
62(1)
49(1)
76(1)
55(1)
59(1)
54(1)
53(1)
58(1)
73(2)
71(2)
78(2)
138(2)
83(2)
66(2)
83(2)
114(2)
59(1)
64(1)
59(1)
73(1)
67(1)
67(1)
78(2)
75(2)
79(2)
76(2)
82(1)
55(1)
57(1)
61(1)
72(2)
70(2)
90(2)
66(2)
81(2)
166(2)
115(2)
113(2)
69(2)
100(2)
46(1)
53(1)
64(2)
51(1)
44(1)
47(1)
53(1)
43(1)
90(2)
73(2)
112(2)
89(1)
76(2)
72(2)
54(1)
101(2)
86(2)
116(2)
81(2)
108(2)
88(2)
90(2)
113(2)
91(2)
–2(1)
–1(1)
9(1)
–3(1)
3(1)
–7(1)
16(1)
2(1)
21(1)
–5(1)
17(1)
–5(1)
1(1)
2(1)
–5(1)
–7(1)
–14(2)
–12(2)
–3(1)
39(2)
22(2)
19(2)
–19(2)
–15(2)
17(1)
14(1)
21(1)
15(1)
17(1)
11(1)
22(1)
14(1)
22(1)
17(1)
1(1)
28(1)
17(1)
19(1)
14(1)
35(1)
28(2)
43(2)
21(2)
45(2)
38(2)
16(2)
42(2)
53(2)
9(1)
6(1)
12(1)
–3(1)
6(1)
5(1)
14(1)
12(1)
3(1)
4(1)
–10(1)
–1(1)
–9(1)
6(1)
–2(1)
5(1)
–28(2)
5(1)
–25(2)
–52(2)
0(2)
–26(2)
–16(1)
–26(2)
heated on a hot air oven at 85 °C for 1 h, cooled to RT,
stored in a borosil bottle and tightly capped. This was
characterized by infrared spectra and SEM analysis.
Synthesis of imines
An appropriate equimolar quantity of aryl amines
containing electron withdrawing and electron donating
substituents (2 mmol), substituted benzaldehydes
(2 mmol) and SiO2-H3PO4 (0.5 g) were taken in a borosil
glass tube and closed with lid. This mixture was subjected
to microwave irradiation for 4-6 min in a microwave
oven (Scheme I) at 550 W, 2540 MHz frequency
(Samsung Grill, GW73BD Microwave oven, 230V A/c,
50 Hz, 2450 Hz, 100–750 W (IEC-705)). The completion
of reaction was monitored by TLC. After completion
of the reaction, added 10 mL of dichloromethane and
product was separated by evaporation of dichloromethane extract. Further the compound was purified
by recrystallization with ethanol. The 4-niro substituted
MAYAVEL et al.: SOLVENT-FREE SYNTHESIS OF E-IMINES
imine (entry 40) was growed in ethanol as well refined
brown crystal among the other imines. The solid
catalyst was washed with 10 mL of ethyl acetate and
dried at 110 °C in and hot air oven. The catalyst was used
for further reaction runs.
Conclusions
A series of aryl imines have been synthesized by
condensation of aryl amines and substituted benzaldehydes using microwave irradiation in the presence of
SiO2-H3PO4 under solvent-free conditions. This reaction
protocol offers a simple, eco-friendly, non-hazardous,
easier work-up procedure and high yields. These
imines were characterized by their physical constants,
spectral data. The structure of 4-nitro substituted
imine (entry 40) was established and confirmed by
single crystal XRD data. The IR, NMR spectral data
of these imines has been correlated with Hammett
substituent constants, F and R parameters. From the
results of statistical analyses, multi-regression gave
satisfactory and good correlation coefficients.
Acknowledgement
The authors thank DST NMR Facility Unit, Department of Chemistry, Annamalai University, Annamalainagar 608 002 for recording NMR spectra of compounds
and XRD Facility, School of Chemistry,
University of Hyderabad, 500 046 for recording
single crystal XRD of entry 40.
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