Effect of substituents on the 13C chemical shifts of the azomethine carbon atom of N-(phenyl substituted)pyridine-3- and -2-aldimines

Article abstract of DOI:10.1016/S0022-2860(02)00403-9

¹³C chemical shifts of the azomethine carbon atom for N-(phenyl substituted)pyridine-3-aldimines, 3-Py-CH=N-C₆H₄-X, and N-(phenyl substituted)pyridine-2-aldimines, 2-Py-CH=N-C₆H₄-X, having a wide rang

Full text of DOI:10.1016/S0022-2860(02)00403-9

Journal of Molecular Structure 642 (2002) 113–118
www.elsevier.com/locate/molstruc
Effect of substituents on the 13C chemical shifts of the azomethine
carbon atom of N-(phenyl substituted)pyridine-3- and -2-aldimines
a,
B.Z. Jovanovic , M. Misic-Vukovic , A.D. Marinkovic , V. Vajs
a
a
b
ˇ
*
´
ˇ ´
´
´
aFaculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P.O. Box 3503, 11120 Belgrade, Yugoslavia
bFaculty of Chemistry, University of Belgrade, P.O. Box 158, 11001 Belgrade, Yugoslavia
Received 5 March 2002; revised 1 August 2002; accepted 1 August 2002
Abstract
13C chemical shifts of the azomethine carbon atom for N-(phenyl substituted)pyridine-3-aldimines, 3-Py–CHyN–C6H4–X,
and N-(phenyl substituted)pyridine-2-aldimines, 2-Py–CHyN–C6H4–X, having a wide range of substituent effects, were deter-
mined in CDCl3 solution. Exceptionally good Hammett correlation of the 13C NMR chemical shifts of azomethine carbons with
electrophilic substituent constants s þ indicate strong resonance interaction of the substituents on the aniline ring with the azo-
methine carbon atom of the investigated series of imines. The demand for electrons in the investigated systems may be compared
to that of the N-benzylidenanilines and N-(phenyl substituted)pyridine-4-aldimines. The mode of transmission of the substituent
effects, both inductive and resonance, in all four systems, was discussed. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: 13C NMR spectroscopy; Spectra-structure correlations; N-(phenyl substituted)pyridine-3-aldimines; N-(phenyl substituted)pyridine-
2-aldimines
1. Introduction
that the substituents in the aniline nucleus largely
influence the deflection of the angle u by their
electronic and/or steric effects (see the general
formula which follow) and determine the confor-
mation of the corresponding molecules.
It has been reported that N-(phenyl substituted)-
pyridine-2- and -3- and -4-aldimines are important
intermediates for the synthesis of aminoalkylpyr-
idines [1] and pharmacologically active triazoles and
triazolines [2,3]. In this latter context, it has also
been established that substituents both from the
aniline ring and from the aryl group on
the azomethine carbon influence the yields of the
triazole products [2].
The intention of the present investigation was to
study the effects of the substituents at the aniline
ring and also of the 3- and 2-pyridine group present
in the corresponding molecule on the 13C NMR
chemical shifts of the azomethine carbon, as a
continuation of our previous study of N-(phenyl
substituted)pyridine-4-aldimines [7]. It should be
possible, on the basis of 13C NMR data, to conclude
about the conformations of the investigated mol-
ecules. The general formula of the investigated
N-(phenyl substituted)pyridine-3-aldimines is:
Numerous previous investigations of the molecular
structures of N-benzylidenanilines [4–6] have shown
*
Corresponding author. Tel.: þ381-11-3370-460; fax: þ381-11-
3370-387.
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00403-9

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B. Jovanovic et al. / Journal of Molecular Structure 642 (2002) 113–118
114
Table 2
13C NMR chemical shifts of the azomethine carbon atoms of N-
(phenyl substituted)pyridine-2-aldimines in CDCl3 solution
Compound
X
dC
DdC
s
m or sþm or sþp
sp
(1)
H
160.331
0
0
0
(2)
p-CH3
p-Br
159.640 20.691 20.17 20.31
(3)
160.950
160.823
0.619
0.492
0.23
0.23
0.15
0.11
where X: H (1); p-CH3 (2); p-Br (3); p-Cl (4); p-
OCH3 (5); p-N(CH3)2 (6); m-NO2 (7); p-COCH3
(8); m-CF3 (9) and for the N-(phenyl substituted)-
pyridine-2-aldimines:
(4)
p-Cl
(5)
p-OCH3 158.092 22.239 20.27 20.78
p-N(CH3)2 155.234 25.097 20.83 21.70
(6)
(7)
m-NO2
p-COCH3
m-CF3
163.062
162.188
161.452
163.354
2.731
1.857
1.621
3.023
1.857
0.947
0.71
0.50
0.43
0.78
0.45
0.37
0.67
0.50
0.52
0.79
0.49
0.39
(8)
(9)
(10)
(11)
(12)
p-NO2
p-COOCH3 162.188
m-Cl 161.278
and substituted anilines in appropriate solvents. The
N-( p-nitrophenyl)pyridine-3-aldimine, which was
also synthesized in this work, was not used in the
present study of 13C NMR spectra because it was not
soluble in CDCl3.
where X: H (1); p-CH3 (2); p-Br (3); p-Cl (4); p-
OCH3 (5); p-N(CH3)2 (6); m-NO2 (7); p-COCH3
(8); m-CF3 (9); p-NO2 (10); p-COOCH3 (11); m-Cl
(12).
The corresponding 13C NMR shifts are given in
Tables 1 and 2.
The purity of the obtained compounds was checked
by melting points (mp), elemental analysis and NMR
spectra which were in agreement with literature data.
The new compounds which, to our knowledge, have
not been registered in the literature, were synthesized
in the same manner and are as follows: N-( p-
acetylphenyl)pyridine-3-aldimine, mp 112–113 8C
(found %C 75.29; %H 5.21; %N 12.78; calculated
for C14H12N2O: %C 75.0; %H 5.36; %N 12.5), N-( p-
methoxycarbonylphenyl)pyridine-3-aldimine, mp
114–115 8C: (found %C 69.90; %H 5.07; %N
11.75; calculated for C14H12N2O2: %C 70.0; %H
5.0; %N 11.7), N-( p-bromophenyl)pyridine-2-aldi-
mine mp 62–63 8C (found %C 55.10; %H 3.31; %N
10.71; calculated for C12H9N2Br: %C 55.10; %H
3.45; %N 10.70), N-(p-dimethylaminophenyl)pyri-
dine-2-aldimine mp 92–93 8C (found %C 74.60; %H
6.42; %N 18.49; calculated for C14H15N3: %C 74.70;
%H 6.70; %N 18.6), N-( p-acetylphenyl)pyridine-2-
aldimine mp 124–126 8C (found %C 74.05; %H 5.43;
%N 11.78; calculated for C14H12N2O: %C 75.0; %H
5.4; %N 12.5), N-( p-methoxycarbonylphenyl)pyri-
dine-2-aldimine mp 109–110 8C (found %C 69.05;
%H 5.25; %N 11.09; calculated for C14H12N2O2: %C
2. Experimental
The majority of the N-(phenyl substituted)pyri-
dine-3-aldimines and N-(phenyl substituted)pyridine-
2-aldimines used in this study were synthesized by
literature procedures [2,3,8,9] by heating equimolar
amounts of the corresponding pyridine-carbaldehyde
Table 1
13C NMR chemical shifts of the azomethine carbon atoms of N-
(phenyl substituted)pyridine-3-aldimines in CDCl3 solution
Compound
X
dC
DdC
s
m or sp sþm or sþp
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
H
156.799
0
0
0
p-CH3
p-Br
156.071 20.728 20.17
20.13
0.15
157.400
157.509
0.601
0.701
0.23
0.23
p-Cl
0.11
p-OCH3 154.778 22.021 20.27
p-N(CH3)2 151.884 24.915 20.83
20.78
21.70
0.73
m-NO2
p-COCH3 158.656
m-CF3 158.347
159.640
2.841
1.857
1.548
0.71
0.50
0.43
0.50
0.57

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B. Jovanovic et al. / Journal of Molecular Structure 642 (2002) 113–118
115
70.0; %H 5.0; %N 11.7), N-(m-nitrophenyl)pyridine-
2-aldimine mp 98–99.5 8C (found %C 62.84; %H
3.50; %N 19.08; calculated for C12H9N3O2: %C
63.43; %H 3.96; %N 18.50).
The 13C NMR spectra of the investigated com-
pounds were recorded in CDCl3 solution with
tetramethylsilane (TMS) as the internal reference
standard using a Varian–Gemini 200 spectropho-
tometer. All the spectra were recorded at ambient
temperature.
shifts were registered for electron acceptor
substituents.
For quantitative assessment of the substituent
effects on the 13C NMR chemical shifts, we postulated
a Hammett equation of the type:
DdC ¼ rs
ð2Þ
where r is the proportionality constant (reaction
constant) reflecting the sensitivity of the 13C NMR
chemical shift to the substituent effects.
The following correlation parameters were
obtained (Fig. 1) using the data from Table 1 for N-
(phenyl substituted)pyridine-3-aldimines:
3. Results and discussion
The 13C NMR chemical shifts of the azomethine
carbon atom (dC) in the solvent CDCl3 and the
differences in the chemical shifts (DdC) together with
the corresponding s [10] and s þ [11] constants for
substituents in the m- and p-position of the aniline ring
of the imine are shown in Tables 1 and 2 (Figs. 1 and 2).
The values of the DdC are the differences between
the 13C NMR chemical shifts of the substituted imines
and the unsubstituted compound of the series: N-
phenylpyridine-3- and -2-aldimine (Eq. (1)).
DdC ¼ 4:93sm=p 2 0:47
ð3Þ
ðr ¼ 0:991; s ¼ 0:338; n ¼ 9Þ
The data from Table 2 for N-(phenyl substituted)pyr-
idine-2-aldimines gave the following correlation
parameters, (Fig. 2):
DdC ¼ 4:92sm=p 2 0:57
ð4Þ
ðr ¼ 0:987; s ¼ 0:337; n ¼ 12Þ
DdC ¼ dCðsubst:Þ 2 dCðunsubst:Þ
ð1Þ
where r is the correlation coefficient; s the standard
deviation and n the number of data included in the
correlation.
The general conclusion derived from the data in
Tables 1 and 2 is that all the substituents on the aniline
ring of the imine, influence, via their electronic
effects, the value of the 13C NMR chemical shifts of
the azomethine carbon atom. For electron donor
substituents these are upfield shifts, while downfield
A much better correlation coefficient was obtained
if s þ, the so called electrophilic substituent constants,
were used. As the s þ constant for the p-COCH3
group was not found in the literature, and keeping in
mind that s and s þ are usually very close for electron
Fig. 1. Plot of the azomethine carbon chemical shifts of N-(phenyl substituted)pyridine-3-aldimines vs. the s þ constants.

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B. Jovanovic et al. / Journal of Molecular Structure 642 (2002) 113–118
116
Fig. 2. Plot of the azomethine carbon chemical shifts of N-(phenyl substituted)pyridine-2-aldimines vs. the s þ constants.
acceptor substituents, the s constant was used for this
substituent.
The correlation for N-(phenyl substituted)pyridine-
2- and 3-pyridine groups directly attached to that atom
than in the N-benzylidenanilines.
Finally, it is very interesting to compare the r þ
values in the series of substituted N-benzylidenani-
lines and isomeric pyridine aldimines investigated in
this and the previous study [7], with electron donating
substituents in the p-position of the aniline ring ( p-H;
p-CH3; p-OCH3 and p-N(CH3)2) where these sub-
stituents probably allow the nearly planar confor-
mation of the investigated compound. The calculated
values of the reaction constants r þ are as follow: 2.81
for N-benzylidenanilines; 3.36 for 4-pyridine aldi-
mines [7]; 3.05 for 2-pyridine aldimines and 2.92 for
3-pyridine aldimines, give the following relationship:
3-aldimines is as follows:
DdC ¼ 3:11sþm=p þ 0:295
ð5Þ
ðr ¼ 0:997; s ¼ 0:192; n ¼ 9Þ
and for N-(phenyl substituted)pyridine-2-aldimines:
DdC ¼ 3:20sþm=p þ 0:24
ð6Þ
ðr ¼ 0:998; s ¼ 0:150; n ¼ 12Þ
The better correlation coefficient with the electro-
philic substituent constants s þ undoubtedly shows
considerable resonance interaction of the electron
donor substituents in the aniline ring of the imines
with the imino group carbon.
phenyl , 3-pyridine , 2-pyridine , 4-pyridine
According to this order a conclusion may be drawn
that the 2I and 2R effects of the nitrogen atom in the
pyridine ring affect the degree of resonance inter-
action through the molecule as a whole. There is an
interesting observation that there exists an important
difference between the effect of the 4- and 2-pyridyl
groups which shows that the nitrogen atom of the
4-pyridyl group effectively delocalizes the electron
density through the rest of the molecule, whereas in
the case of the 2-pyridyl group the explanation for the
lower r þ value requires further investigation of the
stereochemistry of the molecule.
If the literature values [12,13] of the 13C NMR
chemical shifts of N-benzylidenanilines (H, 160.31;
p-CH3, 159.45; p-OCH3, 158.36; m-NO2, 162.37;
p-N(CH3)2, 155.52; p-Cl, 160.69) are correlated with
the 13C NMR chemical shifts from both
the investigated series with the same substituents in
It is of interest to compare the values of
rþ ¼ 3:11 for N-(phenyl substituted)pyridine-3-aldi-
mines and rþ ¼ 3:20 for N-(phenyl substituted)pyr-
idine-2-aldimines with the value of rþ ¼ 2:73 of
substituted N-benzylidenanilines [12]. It may be seen
that in the case of both 2- and 3-pyridine aldimines
there is an increased demand for electrons of the
azomethine carbon atom relative to the demand of the
corresponding carbon in N-benzylidenanilines. This
could most probably be attributed to the effect of the
2- and 3-pyridine groups bonded to the azomethine
carbon atom. It is evident that the electron donating
effect of the substituents in the aniline nucleus towards
the azomethine carbon atom will be more pronounced
in both investigated series with electron-accepting

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B. Jovanovic et al. / Journal of Molecular Structure 642 (2002) 113–118
117
the aniline nucleus, the following relationships are
obtained:
nilines [7], it may be concluded that the þR
substituent effect in this series, according to the
calculated ratios, has the following order:
dCð2N; XÞ ¼ 1:13dCðH; XÞ 2 20:96
ð7Þ
4-pyridyl . 2-pyridyl . 3-pyridyl . phenyl
ðr ¼ 0:998; s ¼ 0:206; n ¼ 6Þ
The obtained order of the resonance effects, which is
essentially the order of decreasing electron density on
the azomethine carbon atom, is in agreement with the
discussion on the p,p delocalization when electron
donor substituents are present as contrasted by n,p
delocalization for electron acceptor substituents.
Therefore, the previous calculation [7] showing that
the þR resonance effect is the most pronounced in the
series of 4-pyridine aldimines could be attributed to
the effect of nearly direct conjugation of the electron
donor substituents on the aniline part of the molecule
and the ‘aza’ substituent. The observation that the 2-
pyridyl group causes a smaller positive resonance
effect in the series of 2-pyridine aldimines both in
correlation with the s þ values and in the use of the
DSP method, compared to 4-pyridine aldimines, can
be understood as the effect of steric inhibition of the
resonance where the 2-pyridyl group is rotated out-of-
plane to a certain degree. Therefore, it could be
visualized that the less pronounced þR effect of
electron donating substituents in 2-pyridine aldi-
mines, is due to a probably small, but definite
deflexion from planarity for the angle w, caused by
repulsion between the lone pair of the ortho-nitrogen
and p-electrons from the azomethine bond. Deflection
of the pyridine–imine bond by an angle w in 2-
pyridine aldimines has been previously observed [9],
namely the molecule of N-(phenyl substituted)pyr-
idine-2-aldimine would rotate up to 1808 around that
bond to achieve complex formation with dimethyltin
dichloride.
dCð3N; XÞ ¼ 1:12dCðH; XÞ 2 22:27
ð8Þ
ðr ¼ 0:997; s ¼ 0:216; n ¼ 6Þ
The high correlation coefficients obtained here, as
well as the one reported earlier for 4-pyridyl
derivatives [7], prove that the mechanism of trans-
mission of polar substituent effects to the azomethine
carbon atom is the same as in substituted N-
benzylidenanilines. This also implies that the diedral
angle is not significantly different. However, the
above intercorrelation includes both electron accep-
tors and electron donors.
It was of interest to apply the Taft equation (DSP
method) in the form:
DdC ¼ rIsI þ rþR sþR
ð9Þ
in order to separate the inductive and resonance
effects of the substituents in the aniline ring and to
observe their individual influence on the 13C NMR
chemical shifts of the azomethine carbon atom. If the
substituents on the aniline ring with dominantly
electron donor properties are taken into consideration
(H, p-CH3, p-OCH3 and p-N(CH3)2), using literature
values of sI and sþR [14], for N-(phenyl substituted)-
pyridine-2-aldimines the following correlations are
obtained:
DdC ¼ 2:81sI þ 3:05sþR þ 0:088
ð10Þ
ðr ¼ 0:990; s ¼ 0:130; n ¼ 4Þ
Generally, it could be stated that linear free energy
principles could be applied to the study of the
transmission of substituent effects in all three series
of N-(phenyl substituted)pyridine aldimines using the
data from 13C NMR chemical shifts. The Hammett,
Brown–Okamoto and Taft (DSP) methods were
employed and it was concluded that both inductive
and resonance effects, as well as the stereochemistry
of the molecules influence the electron demand of the
azomethine carbon.
The same approach for the N-(phenyl substituted)pyr-
idine-3-aldimines, with the same substituents in the
aniline ring, gives the following result:
DdC ¼ 3:35sI þ 2:95sþR 2 0:069
ð11Þ
ðr ¼ 0:999; s ¼ 0:105; n ¼ 4Þ
On the basis of the obtained ratio rþR =rI; which
is 2:95=3:35 ¼ 0:88 for 3-pyridine aldimines,
3:05=2:81 ¼ 1:08 for 2-pyridine aldimines, 1.12 for
4-pyridine aldimines [7] and 0.8 for N-benzylidena-
The specific behaviour of the 2-pyridine group as a
substituent on the azomethine carbon is yet another

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B. Jovanovic et al. / Journal of Molecular Structure 642 (2002) 113–118
118
example of the well known ortho-effect which causes
a smaller negative resonance effect of the pyridine
nitrogen and, consequently, a lower electron demand
of the azomethine carbon. On the other hand, very
good correlation with the Hammett s constants for 3-
and 4-pyridine aldimines, and excellent correlation
coefficients obtained with s þ and sRþ for predomi-
nantly electron donating substituents in the aniline
ring, indicate extensive delocalization and strong
resonance interaction in these systems. This is further
corroborated by the increased ratio l ¼ rþR =rI in these
systems. Therefore, it is probably true that as long as
electron donating substituents are present in the
aniline ring, there is a substantial decrease of the u
angle and that the corresponding molecules attain
nearly planar conformation. This is not a new notion,
as it has been reported in the literature that in special
cases, strong electron donor substituents present in the
aniline ring and strong electron acceptor substituents
on the azomethine carbon can cause near-planar
conformation of the corresponding molecules due to
the complete through delocalization [12,13]. It was
also stated that although, it has not been proved that an
increase in r þ refers to a decrease of the diedral angle
u, it definitely indicates an increased electron releas-
ing resonance through the phenylimino system,
induced mainly by the strong electron demand of
the azomethine carbon. In this context it should be
noted that the pyridyl group as a substituent is an
exceptionally strong electron-accepting substituent,
which is best exemplified by the values of the s
constants for pyridine for all three positions of
nitrogen in the nucleus [15].
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