Substituent effects on the 13C NMR chemical shifts of the imine carbon in N-(4-X-benzylidene)-4-(4-Y-styryl) anilines

Article abstract of DOI:10.1002/poc.3032

Long-range electronic substituent effects were targeted using the substituent dependence of δ_C(C=N), and specific cross-interactions were explored extendedly. A wide set of N-(4-X-benzylidene)- 4-(4-Y-styryl) anilines, p-X-C₆H_4<

Full text of DOI:10.1002/poc.3032

Research Article
Received: 4 May 2012,
Revised: 13 August 2012,
Accepted: 16 August 2012,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3032
13
Substituent effects on the C NMR chemical
shifts of the imine carbon in N-(4-X–
benzylidene)-4-(4-Y–styryl) anilines
Zhengjun Fanga,e, Chenzhong Caob,c,d* and Guanfan Chenb,c,d
Long-range electronic substituent effects were targeted using the substituent dependence of dC(C═N), and specific
cross-interactions were explored extendedly.
A
wide set of N-(4-X–benzylidene)-4-(4-Y–styryl) anilines,
p-X–C6H4CH═NC6H4CH═CHC6H4–p-Y (X= NMe2, OMe, Me, H, Cl, F, CN, or NO2; Y = NMe2, OMe, Me, H, Cl, or CN) were
prepared for this study, and their 13C NMR chemical shifts dC(C═N) of C═N bonds were measured. The results show that
both the inductive and resonance effects of the substituents Y on the dC(C═N) of p-X–C6H4CH═NC6H4CH═CHC6H4–p-Y
are less than those of the substituents Y in p-X–C6H4CH═NC6H4–p-Y. Moreover, the sensitivity of the electronic character
of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the
conjugated chain is elongated. It was confirmed that the substituent cross-interaction is an important factor influencing
dC(C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long-range transmission of
the specific cross-interaction effects on dC(C═N) decreases with increasing conjugated distance between X and Y.
The results of this study suggest that there is a long-range transmission of the substituent effects in
p-X–C6H4CH═NC6H4CH═CHC6H4–p-Y. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper
Keywords: 13C NMR chemical shift; benzylidene aniline; long-range transmission; substituent effect
effects on the 13C NMR chemical shifts dC(C═N) of C═N in
M1 (Scheme 1) are not quite the same as those in M2. Thus, it is
worthwhile to provide more information on the substituent effects
in M1.
In the present work, compounds M1 are synthesized and chosen
as the model compounds, and the 13C NMR chemical shifts of their
imine carbons are used to analyze the substituent effects. Our
primary interest is to study the long-range substituent effects in
benzylidene anilines with elongated chains.
INTRODUCTION
To use new liquid crystals and nonlinear optical materials optimally
and design new candidates efficiently, it is necessary to understand
the charge distribution of the molecules of mesogenic compounds,
which is central to their optical and electronic characters.[1–3] It
has been reported that when the electron density of the carbon
nucleus increases, the field of resonance also increases.[4,5]
Substituent effects on 13C NMR have been investigated.[6–8] Several
13C NMR studies have revealed that the overall electron distribution
can be finely tuned through the electronic effects of remote
substituents.[9–11]
Substituted benzylidene anilines are widely used structural units
in liquid crystals and nonlinear optical materials.[12–15] Neuvonen
and co-workers[16] demonstrated that changes of the substituents
X and Y could finely control the electronic character, and the
presence of the specific cross-interaction between X and Y in
M3 (Scheme 1) was verified. Cao et al.[17] further explored the
substituent-specific cross-interaction effects in the same system
and successfully proposed the new parameter Δs2 (Δs2 = (sX ꢀ sY)2)
to scale the effects, where the sX and sY are Hammett constants of
the substituents X and Y, respectively. It is known that the length
of the conjugated chain can affect the inductive and resonance
effects and thus might lead to changes in the electron distribution.
Cao et al.[18] also investigated the substituent effects in M2
(Scheme 1). Our observations of the substituent effects in M2
encouraged us to prepare other type of benzylidene anilines, in
which the conjugated chain in the aniline unit is elongated. The
higher electronegativity of nitrogen, compared with carbon,
and the presence of a lone pair of electrons in the nitrogen atom
influence the electron distribution. Consequently, the substituent
* Correspondence to: Chenzhong Cao, School of Chemistry and Chemical
Engineering, Hunan University of Science and Technology, Xiangtan 411201,
China. E-mail: czcao@hunst.edu.cn
a Z. Fang
School of Chemistry and Chemical Engineering, Central South University,
Changsha 410083, China
b C. Cao, G. Chen
School of Chemistry and Chemical Engineering, Hunan University of Science
and Technology, Xiangtan 411201, China
c C. Cao, G. Chen
Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry
of Education, Hunan University of Science and Technology, Xiangtan 411104,
China
d C. Cao, G. Chen
Hunan Provincial University Key Laboratory of QSAR/QSPR, Xiangtan 411104,
China
e Z. Fang
School of Chemistry and Chemical Engineering, Hunan Institute of Engineering,
Xiangtan 411104, China
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

Z. FANG, C. CAO AND G. CHEN
Scheme 1. Structures of compounds M1, M2, and M3
a more accurate description of the substituent effects of X in M1.
This is because the dual substituent parameter approach allows
for the different contributions of inductive and conjugative effects.
Thus, this approach is suitable for the analysis of the effects of
substituents X on dC(C═N), in which the susceptibility of dC(C═N)
is dissimilar greatly to the inductive and conjugative effects of X.
Both the signs in front of rF(X) and rR(X) are negative, indicating
that electron-donating (ED) benzylidene substituents X cause
deshielding, whileelectron-withdrawing(EW)ones causeshielding.
This behavior can be understood by considering the resonance
structures of M1 shown in Scheme 2. The nitrogen atom is more
electronegative than the carbon atom. Consequently, the
nitrogen and carbon atoms of the C═N alternate with negative
and positive charges, as the resonance form M1-2 (Scheme 2).
The ED substituents X inductively stabilize the form M1-2,
increasing its relative contribution, and lead to the deshielding
of the imine carbon. On the other hand, the EW substituents X
inductively destabilize M1-2.
RESULTS
The 13C NMR chemical spectra of the compounds M1 in
Scheme 1 were recorded in CDCl3 at 293 K. The dC(C═N) of M1
are reported in Table 1, and the chemical shifts are expressed
in ppm relative to CDCl3 (77.0 ppm).
DISCUSSION
To give an insight into the changing regularity of the substituent
effects of X and Y on dC(C═N) in M1, we focused on two aspects.
One is the corresponding effect of the benzylidene substituent X
and aniline substituent Y on dC(C═N) in M1, and the other is the
specific cross-interaction effect between X and Y. Also, a distinction
of the substituent effects between M1 and M2, and M1 and M3
was investigated.
Effect of substituents X on dC(C═N)
As seen in Table 2 (columns 5 to 6), rF(X) or rR(X) changes as the
aniline substituent Y changes, suggesting that the contribution of
the inductive and conjugative effects of the substituent X can be
affected by the remote substituent Y. These phenomena are
illustrated in Figs 1 and 2. Good linear correlations revealed that
the electronic effects of X on dC(C═N) can be modified by Y. The
values of rF(X) and rR(X) for M2 and M3 reported by Cao et al.
and Neuvonen et al.[16,18] which were correlated with the dual
substituent parameter approach, are also shown in Figs 1 and 2.
In the comparison of the slopes of the lines in Figs 1 and 2, in each
figure, the line for M1 has the smallest slope, while the slope of the
line corresponding to M2 takes second place. It can be inferred that
the sensitivity of the electronic character of the C═N function
to electron donation/electron withdrawal by the substituent X
attenuates as the length of the conjugated chain is elongated.
In our previous reports,[17,18] the parameter Δs2 (Δs2 = (sX ꢀ
Y)2 = {[sF(X) + sR(X)]ꢀ [sF(Y) + sR(Y)]}2) was used to evaluate the
substituent cross-interaction effect when both X and Y were varied,
and excellent correlations had been observed with Eqn (3). If Y is
fixed, the second and the fourth terms in Eqn (3) are constant; thus,
Y is fixed, and Eqn (3) can be simplified to Eqn (4).
For each substituent Y in M1, when the substituent X was varied,
the dC(C═N) values in Table 1 were fitted using Eqns (1) and (2).
The correlation results are listed in Table 2 (columns 2 to 8).
dCðC ¼ NÞ ¼ rs þ constant
(1)
(2)
dCðC ¼ NÞ ¼ rFsF þ rRsR þ constant
where s, sF, and sR are the Hammett parameters of substituents
(s = sF + sR), sF and sR are the inductive parameter and resonance
parameter, respectively, and r, rF, and rR are the corresponding
coefficients.
The 13C NMR chemical shifts of the C═N carbon were first
correlated with the s parameter according to Eqn (1), and
their results were poor (Table 2; columns 2 to 4). Significant
improvements were obtained with Eqn (2) (Table 2; columns 5 to
8), suggesting that a dual substituent parameter approach provides
Table 1. 13C NMR shifts of the C═N carbons in M1
X
Y
dCðC ¼ NÞ ¼ rFsFðXÞ þ rFsFðYÞ þ rRsRðXÞ þ rRsRðYÞ
NMe2
OMe
Me
H
Cl
CN
þ rðΔs2ÞΔs2 þ constant
(3)
NMe2
OMe
Me
H
Cl
F
159.44 159.66 159.73 159.84 159.69 160.22
158.88 158.87 159.15 159.28 159.38 159.68
159.40 159.69 159.77 159.90 159.86 160.33
159.42 159.72 159.82 159.94 160.03 160.40
157.55 157.89 158.00 158.31 158.25 158.75
157.83 158.13 158.23 158.36 158.43 158.77
156.41 156.93 157.07 157.17 157.32 157.76
155.87 156.27 156.53 156.67 156.81 157.25
dCðC ¼ NÞ ¼ rFsFðXÞ þ rRsRðXÞ þ rðΔs2ÞΔs2 þ constant (4)
These preceding observations suggest that Δs2 may be used
to scale the interaction between X and Y in M1 when Y was fixed.
It is interesting that excellent correlations were observed with
Eqn (4) (Table 2; columns 9 to 13), and the standard error was
CN
NO2
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SUBSTITUENT EFFECTS IN BENZYLIDENE ANILINES
Table 2. Correlation of dC(C═N) values for M1 according to Eqns (1), (2), and (4)
Correlation
dC(C═N) versus s(X)
r(X)
dC(C═N) versus sF(X) and sR(X)
rF(X) rR(X)
dC(C═N) versus sF(X), sR(X) and Δs2
Y
R
s
R
s
rF(X)
rR(X)
r
R
s
n
ðΔs2Þ
NMe2 ꢀ2.43 ꢁ 0.52 0.8872 0.71 ꢀ4.90 ꢁ 0.44 ꢀ1.21 ꢁ 0.28 0.9876 0.26 ꢀ3.18 ꢁ 0.50 ꢀ0.11 ꢁ 0.30 ꢀ0.77 ꢁ 0.19 0.9974 0.14 8
OMe ꢀ2.23 ꢁ 0.52 0.8694 0.71 ꢀ4.77 ꢁ 0.31 ꢀ0.96 ꢁ 0.20 0.9930 0.19 ꢀ4.15 ꢁ 0.19 ꢀ0.79 ꢁ 0.09 ꢀ0.56 ꢁ 0.12 0.9990 0.08 8
Me
H
Cl
ꢀ2.17 ꢁ 0.50 0.8718 0.68 ꢀ4.60 ꢁ 0.34 ꢀ0.95 ꢁ 0.21 0.9913 0.20 ꢀ4.09 ꢁ 0.24 ꢀ0.89 ꢁ 0.12 ꢀ0.56 ꢁ 0.16 0.9979 0.11 8
ꢀ2.15 ꢁ 0.49 0.8709 0.68 ꢀ4.55 ꢁ 0.38 ꢀ0.95 ꢁ 0.24 0.9887 0.23 ꢀ4.15 ꢁ 0.19 ꢀ1.11 ꢁ 0.11 ꢀ0.67 ꢁ 0.14 0.9983 0.10 8
ꢀ2.00 ꢁ 0.51 0.8476 0.69 ꢀ4.42 ꢁ 0.44 ꢀ0.79 ꢁ 0.28 0.9832 0.26 ꢀ4.34 ꢁ 0.26 ꢀ1.30 ꢁ 0.22 ꢀ0.74 ꢁ 0.22 0.9954 0.15 8
ꢀ2.01 ꢁ 0.49 0.8577 0.67 ꢀ4.43 ꢁ 0.32 ꢀ0.80 ꢁ 0.20 0.9913 0.19 ꢀ4.84 ꢁ 0.19 ꢀ1.67 ꢁ 0.24 ꢀ0.55 ꢁ 0.14 0.9983 0.09 8
CN
Scheme 2. Resonance structures of M1
not be ignored, even if Y is unchanged. To further demonstrate
this statement, we correlated the dC(C═N) values of M2
and M3 reported in the literature,[16,18] using Eqn (4), and the
results obtained are listed in Table 3 (columns 8 to 13). To make
comparisons easier, the results of Eqn (2) for M2 and M3 are also
given in Table 3 (columns 3 to 7). The results obtained show that
the correlations obtained with Eqn (4) are much better than
those obtained with Eqn (2) (Table 3). Thus, we can conclude that
Δs2 is suitable to evaluate the interaction effect between X and Y
on dC(C═N) in different benzylidene anilines, even if Y is fixed.
As seen in Tables 2 and 3, the signs in front ofr
are the same
ðΔs2Þ
in all cases, while their coefficients rðΔs2Þ are different, indicating
that the relative magnitude of the interactions between X and Y is
different for each substituent Y.
Figure 1. Plots of rF(X) for three series of substituted benzylidene
anilines (M1, M2, and M3) versus s(Y)
Effect of substituents Y on dC(C═N)
A preferable correlation of the dC(C═N) with a dual substituent
parameter approach (Eqn (2)) was achieved in the studying of
the effect of substituents X for M1, M2, or M3 than that with a
single parameter treatment (Eqn (1)). Therefore, when substitu-
ent X was fixed and Y was varied, we correlated the dC(C═N)
and the substituent parameters of Y according to Eqn (2) (Table 4;
columns 2 to 5), and the correlation results were listed in Table 4
(columns 6 to 10).
Positive rF and rR values indicate that EW substituents Y cause
deshielding, while ED ones cause shielding. The dC(C═N) behav-
ior can also be explained by the resonance structure M1-2
shown in Scheme 2. The EW substituents Y inductively stabilize
the form M1-2. Consequently, the electron density of the imine
carbon decreases, and the 13C NMR chemical shift of the C═N
carbon increases. However, ED substituents have an opposite
effect on M1-2, resulting in shielding of the imine carbon.
These effects are similar to the influence of X on dC(C═N), where
the corresponding coefficient rF(Y) or rR(Y) clearly changes with
the variation of X (Table 4; columns 2 to 3). This behavior was
depicted in the lines for M1 in Figs 3 and 4. The plots of rF(Y) or
rR(Y) versus s(X) for M3 (data from literature[16]) and M2 (data from
literature[18]) are also shown in Figs 3 and 4. As can be seen, the
Figure 2. Plots of rR(X) for three series of substituted benzylidene
anilines (M1, M2, and M3) versus s(Y)
reduced to approximately half of that of the corresponding
series obtained with Eqn (2). This confirmed that the effect of
the interaction between X and Y is an important factor that could
J. Phys. Org. Chem. (2012)
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Z. FANG, C. CAO AND G. CHEN
Table 3. Correlation of dC(C═N) values for M2 and M3 according to Eqns (2) and (4)
Correlation
dC(C═N) versus sF(X) and sR(X)
rF(X) rR(X)
dC(C═N) versus sF(X), sR(X) and Δs2
Y
R
s
F
rF(X)
rR(X)
r
R
s
F
n
ðΔs2Þ
M3a NMe2 ꢀ3.67 ꢁ 0.48 ꢀ0.65 ꢁ 0.30 0.9939 0.31 133.16 ꢀ3.67 ꢁ 0.48 ꢀ0.65 ꢁ 0.30 ꢀ1.03 ꢁ 0.19 0.9984 0.13 524.94
OMe ꢀ4.19 ꢁ 0.21 ꢀ1.13 ꢁ 0.10 0.9943 0.22 159.04 ꢀ4.19 ꢁ 0.21 ꢀ1.13 ꢁ 0.10 ꢀ0.73 ꢁ 0.13 0.9987 0.09 661.91
9
9
9
9
9
9
9
9
Me
H
ꢀ4.04 ꢁ 0.17 ꢀ0.91 ꢁ 0.08 0.9940 0.18 183.28 ꢀ4.04 ꢁ 0.17 ꢀ0.91 ꢁ 0.08 ꢀ0.58 ꢁ 0.12 0.9987 0.08 620.25
ꢀ4.01 ꢁ 0.14 ꢀ0.87 ꢁ 0.08 0.9948 0.14 232.30 ꢀ4.01 ꢁ 0.14 ꢀ0.87 ꢁ 0.08 ꢀ0.45 ꢁ 0.11 0.9986 0.08 578.29
ꢀ4.28 ꢁ 0.13 ꢀ1.11 ꢁ 0.11 0.9930 0.08 566.36 ꢀ4.28 ꢁ 0.13 ꢀ1.11 ꢁ 0.11 ꢀ0.54 ꢁ 0.11 0.9985 0.08 566.36
ꢀ4.24 ꢁ 0.14 ꢀ1.09 ꢁ 0.08 0.9939 0.17 184.57 ꢀ4.24 ꢁ 0.14 ꢀ1.09 ꢁ 0.08 ꢀ0.57 ꢁ 0.11 0.9987 0.08 658.48
ꢀ4.19 ꢁ 0.15 ꢀ0.95 ꢁ 0.18 0.9893 0.14 166.76 ꢀ4.19 ꢁ 0.15 ꢀ0.95 ꢁ 0.18 ꢀ0.44 ꢁ 0.11 0.9979 0.08 399.33
Cl
F
CN
NO2 ꢀ4.18 ꢁ 0.17 ꢀ0.92 ꢁ 0.22 0.9889 0.14 149.71 ꢀ4.18 ꢁ 0.17 ꢀ0.92 ꢁ 0.22 ꢀ0.41 ꢁ 0.11 0.9980 0.08 334.20
M2b NMe2 ꢀ2.59 ꢁ 0.31 ꢀ1.78 ꢁ 0.19 0.9931 0.17 107.40 ꢀ1.30 ꢁ 0.09 ꢀ1.10 ꢁ 0.05 ꢀ0.54 ꢁ 0.04 0.9999 0.02 5365.64 6
OMe ꢀ2.13 ꢁ 0.27 ꢀ1.37 ꢁ 0.17 0.9916 0.15 87.63 ꢀ1.53 ꢁ 0.07 ꢀ1.30 ꢁ 0.03 ꢀ0.47 ꢁ 0.04 0.9999 0.02 2706.13 6
Me
H
Cl
F
CN
ꢀ1.85 ꢁ 0.22 ꢀ1.21 ꢁ 0.14 0.9926 0.12 99.73 ꢀ1.45 ꢁ 0.12 ꢀ1.23 ꢁ 0.05 ꢀ0.38 ꢁ 0.08 0.9993 0.04 524.50
ꢀ1.73 ꢁ 0.18 ꢀ1.06 ꢁ 0.11 0.9942 0.10 129.09 ꢀ1.51 ꢁ 0.08 ꢀ1.17 ꢁ 0.05 ꢀ0.30 ꢁ 0.06 0.9995 0.03 705.77
6
6
ꢀ1.70 ꢁ 0.22 ꢀ1.14 ꢁ 0.14 0.9915 0.12 87.12 ꢀ1.59 ꢁ 0.06 ꢀ1.35 ꢁ 0.04 ꢀ0.38 ꢁ 0.06 0.9999 0.02 3846.93 6
ꢀ1.81 ꢁ 0.22 ꢀ1.15 ꢁ 0.14 0.9924 0.12 97.50 ꢀ1.59 ꢁ 0.03 ꢀ1.47 ꢁ 0.03 ꢀ0.39 ꢁ 0.03 0.9997 0.03 1010.06 6
ꢀ1.58 ꢁ 0.29 ꢀ0.96 ꢁ 0.17 0.9928 0.12 68.45 ꢀ1.72 ꢁ 0.03 ꢀ1.53 ꢁ 0.05 ꢀ0.32 ꢁ 0.02 1.0000 0.01 4160.96 5
NO2 ꢀ1.51 ꢁ 0.21 ꢀ0.95 ꢁ 0.13 0.9901 0.11 74.63 ꢀ1.79 ꢁ 0.13 ꢀ1.59 ꢁ 0.21 ꢀ0.33 ꢁ 0.10 0.9985 0.05 215.13
6
aData were taken from Cao et al.[18]
bData were taken from Neuvonen et al.[16]
.
.
Table 4. Correlation of dC(C═N) values for M1 according to Eqns (2) and (5)
Correlation
dC(C═N) versus sF(Y) and sR(Y)
dC(C═N) versus sF(Y), sR(Y) and Δs2
X
rF(Y)
rR(Y)
R
s
rF(Y)
rR(Y)
r
ðΔs2Þ
R
s
n
NMe2
OMe
Me
H
Cl
F
0.34 ꢁ 0.30
0.51 ꢁ 0.31
0.51 ꢁ 0.19
0.63 ꢁ 0.09
0.62 ꢁ 0.23
0.57 ꢁ 0.08
0.78 ꢁ 0.08
0.74 ꢁ 0.13
0.51 ꢁ 0.67
0.60 ꢁ 0.31
0.62 ꢁ 0.20
0.65 ꢁ 0.03
0.86 ꢁ 0.17
0.64 ꢁ 0.03
0.92 ꢁ 0.12
0.99 ꢁ 0.12
0.9084
0.9322
0.9721
0.9943
0.9803
0.9957
0.9979
0.9952
0.14 ꢀ0.41 ꢁ 0.67 ꢀ0.07 ꢁ 0.33
0.35 ꢁ 0.24
0.41 ꢁ 0.21
0.24 ꢁ 0.15
0.15 ꢁ 0.02
0.28 ꢁ 0.02
0.13 ꢁ 0.02
0.02 ꢁ 0.09
0.16 ꢁ 0.08
0.9564
0.9772
0.9882
0.9998
0.9922
0.9997
0.9980
0.9985
0.11
0.10
0.06
0.01
0.07
0.01
0.04
0.04
6
6
6
6
6
6
6
6
0.15
0.09
0.04
0.10
0.04
0.04
0.06
0.09 ꢁ 0.31
0.31 ꢁ 0.20
0.56 ꢁ 0.03
0.61 ꢁ 0.17
0.53 ꢁ 0.03
0.80 ꢁ 0.12
0.92 ꢁ 0.12
0.54 ꢁ 0.12
0.63 ꢁ 0.08
0.71 ꢁ 0.02
1.10 ꢁ 0.16
0.71 ꢁ 0.02
0.96 ꢁ 0.16
1.33 ꢁ 0.17
CN
NO2
Figure 3. Plots of rF(Y) for three series of substituted benzylidene
anilines (M1, M2, and M3) versus s(X)
Figure 4. Plots of rR(Y) for three series of substituted benzylidene
anilines (M1, M2, and M3) versus s(X)
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SUBSTITUENT EFFECTS IN BENZYLIDENE ANILINES
slopes of the lines in each figure decrease when the distance
between the substituents X and Y increase, suggesting that the
substituent effect of X on rR(Y) and rF(Y) becomes weaker
with the elongation of the conjugated chain. In other words, the
substituent effect of X on dC(C═N) can be modified by substituent
Y, but this influence decreases with the increasing distance
between X and Y.
The behaviors observed in Figs 3 and 4 indicate the existence
of mutual interference between X and Y in M1, when X is fixed.
Here, we also used Δs2 to evaluate the interaction between
X and Y (Eqn (5)), obtaining only a slight improvement on the
correlations for each series (Table 4; columns 6 to 10), although
the improvement becomes greater as the absolute values of
Table 6. Differences in the dC(C═N) between compounds
a
M1 and M3 [ΔdC(M1 ꢀ M3) = dC(C═N)M1 ꢀ dC(C═N)M3
]
X
Y
NMe2 OMe Me
H
Cl
F
CN NO2
NMe2
OMe
Me
H
Cl
CN
2.71 3.19 3.25 3.45 3.48 3.46 4.19 4.36
1.01 0.99 1.23 1.31 1.21 1.29 1.58 1.51
0.16 0.21 0.19 0.23 0.06 0.23 0.22 0.21
ꢀ0.37 ꢀ0.36 ꢀ0.41 ꢀ0.40 ꢀ0.43 ꢀ0.40 ꢀ0.65 ꢀ0.66
ꢀ0.82 ꢀ0.61 ꢀ0.81 ꢀ0.68 ꢀ0.86 ꢀ0.68 ꢀ0.83 ꢀ0.84
ꢀ1.64 ꢀ1.86 ꢀ1.96 ꢀ2.04 ꢀ2.15 ꢀ2.05 ꢀ2.45 ꢀ2.50
aThe values of dC(C═N)M3 were taken from Neuvonen et al.[16]
.
r
ðΔs2Þ
increase.
dCðC ¼ NÞ ¼ rFsFðYÞ þ rRsRðYÞ þ r
Δs2 þ constant (5)
ðΔs2Þ
(M1ꢀM3)| is in line with the increasing ED or EW capability of Y. This
suggests that for each substituent X in M1, when the aniline
substituents Y become more ED, the shielding of the C═N carbon
decreases relative to M3, and the deshielding of the imine carbon
also decreases. In other words, the effect of the substituents Y on
dC(C═N) in M1 decreases relative to that in M3. This is due to
the fact that the distance between the substituents Y and the
azomethine carbon center is different for M1 and M3. For
substituents Y, the number of chemical bonds far from the C═N
carbon is 12 in M1 and 6 in M3.
By comparing Tables 2–4, it can be observed that |rðΔs2Þ | are
smaller in the former, suggesting that the interaction effect
between X and Y in case that X is fixed is smaller than Y is fixed.
To further clarify this point, we correlated the dC(C═N) values of
M3 and M2 reported by Neuvonen et al. and Cao et al.[16,18] by
using Eqn (5), and the results are listed in Table 5 (columns 8 to
13). Likewise, the results of Eqn (2) for M2 and M3, with the use of
the published data,[16,18] are given in Table 5 (columns 3 to 7). From
the data in Tables 3 and 5, we can conclude that in substituted
benzylidene anilines, the interaction effect between X and Y, when
X is fixed, also affects the value of dC(C═N), but the influence is not
as important as when Y is fixed.
Specific cross-interaction effect between substituents X and
Y on dC(C═N)
To investigate the effects of Y in more detail, we evaluated the
differences in dC(C═N) between M1 and M3 (ΔdC(M1ꢀM3)) (Table 6),
as both substituents X and Y in M1 correspond to those in M3.
When Y belongs to an ED group, the value of ΔdC(M1ꢀM3) is positive;
otherwise, the value is negative. The increasing value of |ΔdC
In the preceding discussion concerning the effect of substituents
X or Y on dC(C═N), it was demonstrated that substituent cross-
interaction was an important factor influencing the dC(C═N), even
if either X or Y was fixed. To further testify the relative importance
of the parameter Δs2 when X or Y was fixed, and the other
Table 5. Correlation of dC(C═N) values for M2 and M3 according to Eqns (2) and (5)
Correlation
dC(C═N) versus sF(Y) and sR(Y)
rF(Y) rR(Y)
dC(C═N) versus sF(Y), sR(Y) and Δs2
rR(Y) rðΔs2Þ
X
R
s
F
rF(Y)
R
s
F
n
M2a
NMe2 2.81 ꢁ 0.29 4.66 ꢁ 0.18 0.9976 0.17 528.47 3.59 ꢁ 0.55 5.16 ꢁ 0.35 ꢀ0.35 ꢁ 0.22 0.9986 0.15 462.97
OMe 3.13 ꢁ 1.27 4.71 ꢁ 0.79 0.9973 0.19 463.08 3.42 ꢁ 0.37 5.08 ꢁ 0.18 ꢀ0.44 ꢁ 0.23 0.9986 0.16 469.80
8
8
8
7
8
8
8
8
8
8
8
8
7
8
8
Me
H
2.94 ꢁ 0.32 4.90 ꢁ 0.20 0.9971 0.19 464.30 3.33 ꢁ 0.33 5.06 ꢁ 0.16 ꢀ0.45 ꢁ 0.23 0.9987 0.15 498.44
2.94 ꢁ 0.55 4.96 ꢁ 0.33 0.9925 0.32 265.36 3.23 ꢁ 0.37 5.04 ꢁ 0.24 ꢀ0.51 ꢁ 0.30 0.9981 0.20 256.14
3.03 ꢁ 0.38 5.22 ꢁ 0.22 0.9964 0.23 359.19 3.08 ꢁ 0.28 4.87 ꢁ 0.24 ꢀ0.60 ꢁ 0.24 0.9986 0.16 491.34
3.12 ꢁ 0.46 5.73 ꢁ 0.27 0.9956 0.27 294.60 2.39 ꢁ 0.44 4.52 ꢁ 0.56 ꢀ0.71 ꢁ 0.29 0.9983 0.20 400.62
Cl
NO2
M3b NMe2 2.33 ꢁ 0.28 3.78 ꢁ 0.18 0.9966 0.17 364.95 3.21 ꢁ 0.49 4.33 ꢁ 0.31 ꢀ0.39 ꢁ 0.20 0.9983 0.13 388.54
OMe 2.66 ꢁ 0.31 4.31 ꢁ 0.19 0.9968 0.18 393.72 3.13 ꢁ 0.34 4.44 ꢁ 0.17 ꢀ0.42 ꢁ 0.21 0.9984 0.14 418.08
Me
H
Cl
2.77 ꢁ 0.34 4.53 ꢁ 0.21 0.9967 0.20 373.93 3.20 ꢁ 0.33 4.58 ꢁ 0.16 ꢀ0.47 ꢁ 0.22 0.9984 0.15 413.14
2.88 ꢁ 0.34 4.77 ꢁ 0.21 0.9969 0.20 399.78 3.16 ꢁ 0.29 4.66 ꢁ 0.17 ꢀ0.48 ꢁ 0.23 0.9985 0.16 451.30
2.98 ꢁ 0.38 5.06 ꢁ 0.23 0.9966 0.22 566.36 3.05 ꢁ 0.27 4.67 ꢁ 0.24 ꢀ0.55 ꢁ 0.24 0.9986 0.16 465.34
2.85 ꢁ 0.35 4.77 ꢁ 0.22 0.9966 0.21 369.89 3.08 ꢁ 0.29 4.59 ꢁ 0.19 ꢀ0.50 ꢁ 0.23 0.9984 0.16 423.66
3.32 ꢁ 0.51 5.55 ꢁ 0.28 0.9958 0.26 235.62 2.75 ꢁ 0.49 4.61 ꢁ 0.54 ꢀ0.67 ꢁ 0.35 0.9981 0.20 263.81
3.29 ꢁ 0.48 5.97 ꢁ 0.30 0.9960 0.28 308.01 2.73 ꢁ 0.37 4.81 ꢁ 0.47 ꢀ0.75 ꢁ 0.28 0.9986 0.19 468.69
3.33 ꢁ 0.51 6.17 ꢁ 0.32 0.9956 0.30 284.59 2.54 ꢁ 0.43 4.71 ꢁ 0.55 ꢀ0.81 ꢁ 0.29 0.9986 0.20 460.31
F
CF3
CN
NO2
aData were taken from Cao et al.[18]
bData were taken from Neuvonen et al.[16]
.
.
J. Phys. Org. Chem. (2012)
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Z. FANG, C. CAO AND G. CHEN
one was varied, we investigated the relative contributions (Ψr) or
fraction contributions (Ψf) of the corresponding parameters in Eqns
(4) and (5) to the dC(C═N).[19,20]
evaluate the substituent-specific cross-interaction effects between
X and Y in M1, with the variation of both X and Y. The calculated
results with Eqn (9) or (8) showed that Δs2 was a factor affecting
dC(C═N) of M1 that should not be neglected, especially as the
difference of electronic effect between X and Y increased; for
example, when X was the NO2 group andY was NMe2, thedeviation
between the calculated and experimental dC(C═N) was only
0.07 ppm with Eqn (9), while it was 0.25 ppm with Eqn (8); in
case X was the NMe2 group and Y was CN, the deviation was
0.11 ppm with Eqn (9) and 0.29 ppm with Eqn (8), respectively.
The statistical data for the correlations are shown in the Supporting
Information. To evaluate systematically the long-range effects
of the substituent-specific cross-interaction, Eqns (10) and
(11) reported in our previous works[17,18] are given as follows.
ꢀ
cr ¼ miXi
(6)
R2jcrðiÞj
X
cf ¼
ꢂ 100%
(7)
jcrðiÞj
i
ꢀ
where the mi and Xi are the coefficient and the average value of
the ith parameter in Eqn (4) or (5). The contribution results for
the parameters of Eqns (4) and (5) are shown in Tables 7
and 8, respectively.
As seen in Table 7, for each different substituent Y, as the
substituent X was varied, the relative contributions or fraction
contributions of Δs2 increased in parallel with the increasing ED
or EW ability of the substituent Y. It means that Δs2 plays more
important role as Y become more ED or EW. The behavior can be
understood by consideringthe average value oftheΔs2 parameter.
The value increased as the ED or EW ability of the substituent
Y improves. Thus, increasing the electron effect of substituent Y
increased the contribution of Δs2.
For M1; dCðC ¼ NÞ ¼ 159:86 ꢀ 4:47sFðXÞ þ 0:66sFðYÞ
ꢀ0:96sRðXÞ þ 0:62sRðYÞ ꢀ 0:20Δs2
(9)
R ¼ 0:9913; R2 ¼ 0:9827; s ¼ 0:18; n ¼ 48; F ¼ 476:46
For M2; dCðC ¼ NÞ ¼ 161:56 ꢀ 1:63sFðXÞ þ 3:18sFðYÞ
ꢀ1:40sRðXÞ þ 5:01sRðYÞ ꢀ 0:33Δs2
(10)
R ¼ 0:9974; R2 ¼ 0:9948; s ¼ 0:17; n ¼ 53; F ¼ 1794:53
Similarly, we could see from Table 8 that, when X was fixed,
the relative contributions or fraction contributions of Δs2
increased in parallel with the increasing ED or EW ability of the
substituent X. It also can be explained by the increasing average
value of the Δs2 as X becomes more ED or EW.
For M3; dCðC ¼ NÞ ¼ 160:25 ꢀ 4:18sFðXÞ þ 3:24sFðYÞ
ꢀ1:15sRðXÞ þ 4:67sRðYÞ ꢀ 0:59Δs2
We also studied the importance of the item Δs2 in case both
substituents X and Y in M1 were varied. Firstly, the dC(C═N)
values were correlated with sF and sR parameters, and Eqn (8)
was obtained.
(11)
R ¼ 0:9975; R2 ¼ 0:9950; s ¼ 0:17; n ¼ 80; F ¼ 2937:31
It can be seen in Eqns (9), (10), and (11) that the signs in front
of rF and rR are alternated and they are in good agreement with
the signs in front of the rF and rR shown in Tables 2–5 (except
for the rF(Y) and rR(Y) in Table 4 when X = NMe2). The alternated
signs suggest that the maximum interference occurs between
the most ED and EW substituents; for example, the strongest
effect was observed, or X was the NMe2 group and Y was NO2.
In contrast, there is minimum interaction in both X and Y being
the most ED, or both being the most EW substituents; for
dCðC ¼ NÞ ¼ 159:87 ꢀ 4:61sFðXÞ þ 0:59sFðYÞ
(8)
ꢀ0:94sRðXÞ þ 0:73sRðYÞ
R ¼ 0:9876; R2 ¼ 0:9753; s ¼ 0:21; n ¼ 48; F ¼ 424:44
Then, the dC(C═N) values were correlated with sF, sR, and Δs2,
and Eqn (9) was obtained. The much better correlation of Eqn (9)
than that of Eqn (8) demonstrated that Δs2 was appropriate to
Table 7. The relative and fraction contribution (Ψr and Ψf ) of parameters sF(X), sR(X), and Δs2 to the dC(C═N) of M1, M2, and M3
Δs2 Δs2
sF(X)
Ψf (%)
M1 NMe2 ꢀ0.9858 54.49 0.0278 1.54 ꢀ0.7860 43.45 M3 NMe2 ꢀ1.1662 47.45 0.1343 5.46 ꢀ1.1494 46.77
sR(X)
sF(X)
sR(X)
Y
Ψr
Ψr
Ψf (%)
Ψr
Ψf (%)
Y
Ψr
Ψf (%)
Ψr
Ψf (%)
Ψr
Ψf (%)
OMe ꢀ1.2865 76.58 0.1995 11.88 ꢀ0.1906 11.35
OMe ꢀ1.3315 72.21 0.2335 12.66 ꢀ0.2741 14.87
Me
H
ꢀ1.2679 76.42 0.2247 13.54 ꢀ0.1595 9.61
ꢀ1.2865 74.32 0.2803 16.19 ꢀ0.1584 9.15
ꢀ1.3454 71.35 0.3283 17.41 ꢀ0.1945 10.32
ꢀ1.5004 66.46 0.4217 18.68 ꢀ0.3279 14.52
Me
H
ꢀ1.2838 73.03 0.1881 10.70 ꢀ0.2814 16.01
ꢀ1.2743 76.32 0.1798 10.77 ꢀ0.2110 12.64
ꢀ1.3601 72.88 0.2294 12.29 ꢀ0.2712 14.53
ꢀ1.3474 73.58 0.2253 12.30 ꢀ0.2538 13.86
ꢀ1.3315 65.23 0.1963 9.62 ꢀ0.5049 24.73
Cl
CN
Cl
F
CN
M2 NMe2 ꢀ0.3293 29.41 0.3263 29.14 ꢀ0.4638 41.42
OMe ꢀ0.3876 42.57 0.3857 42.36 ꢀ0.1370 15.05
NO2 ꢀ1.2958 58.06 0.2323 10.41 ꢀ0.6947 31.13
Me
H
Cl
F
CN
NO2
ꢀ0.3673 44.22 0.3649 43.93 ꢀ0.0973 11.71
ꢀ0.3825 47.63 0.3471 43.23 ꢀ0.0726 9.04
ꢀ0.4028 44.82 0.4005 44.56 ꢀ0.0953 10.60
ꢀ0.4028 41.86 0.4361 45.32 ꢀ0.1228 12.76
ꢀ0.5229 39.56 0.5447 41.21 ꢀ0.2543 19.24
ꢀ0.4535 36.82 0.4717 38.29 ꢀ0.3029 24.59
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SUBSTITUENT EFFECTS IN BENZYLIDENE ANILINES
Table 8. The relative and fraction contribution (Ψr and Ψf ) of parameters sF(Y), sR(Y), and Δs2 to the dC(C═N) of M1, M2, and M3
sF(Y)
Ψf (%)
sR(Y)
Ψf (%)
Δs2
sF(Y)
Ψf (%)
sR(Y)
Ψf (%)
Δs2
X
Ψr
Ψr
Ψr
Ψf (%)
X
Ψr
Ψr
Ψr
Ψf (%)
M1
M2
NMe2 ꢀ0.0943 21.91 0.0205 4.76 0.2789 64.80 M3 NMe2 0.9951 39.88 ꢀ1.0933 43.82 ꢀ0.3981 15.96
OMe
Me
H
0.0207 7.00 ꢀ0.1584 53.56 0.1033 34.93
0.0713 22.53 ꢀ0.1848 58.40 0.0529 16.72
0.1288 34.88 ꢀ0.2083 56.41 0.0320 8.67
0.1403 25.31 ꢀ0.3227 58.22 0.0827 14.92
0.1219 33.90 ꢀ0.2083 57.92 0.0292 8.12
0.1840 38.16 ꢀ0.2816 58.41 0.0146 3.03
0.2116 28.17 ꢀ0.3901 51.93 0.1473 19.61
OMe 0.9703 43.29 ꢀ1.1211 50.01 ꢀ0.1430 6.38
Me 0.9920 43.32 ꢀ1.1565 50.51 ꢀ0.1339 5.85
H
0.9796 43.03 ꢀ1.1767 51.69 ꢀ0.1135 4.99
Cl
F
CN
NO2
Cl 0.9455 41.55 ꢀ1.1792 51.82 ꢀ0.1446 6.35
F
0.9548 29.46 ꢀ1.1590 35.76 ꢀ1.1166 34.45
CF3 0.7189 28.81 ꢀ1.4159 56.74 ꢀ0.3512 14.07
CN 0.9863 37.14 ꢀ1.2145 45.74 ꢀ0.4471 16.84
NO2 0.7874 30.34 ꢀ1.1893 45.82 ꢀ0.6116 23.56
NMe2 1.1129 40.02 ꢀ1.3029 46.85 ꢀ0.3573 12.85
OMe
Me
H
Cl
CN
1.0602 42.41 ꢀ1.2827 51.31 ꢀ0.1498 5.99
1.0323 42.23 ꢀ1.2777 52.27 ꢀ0.1282 5.24
0.9090 35.13 ꢀ1.5624 60.39 ꢀ0.1061 4.10
0.9548 40.66 ꢀ1.2297 52.37 ꢀ0.1571 6.69
0.7409 30.53 ꢀ1.1413 47.03 ꢀ0.5361 22.09
example, the weakest effect would be observed when X and Y all
were NO2 group, or all were NMe2.
the coefficient in front of each parameter decreased as the length
of the chain increases and the rðΔs2Þ was inversely proportional
to m2 (m represents the bond number between X and Y). This sug-
gests that there is a long-range transmission effect of substituent
effects as the distance between X and Y increases. Whether the
quantitative relationship between rðΔs2Þ and the bond numbers is
The values of rF(Y) and rR(Y) were smaller in Eqn (9) than in
Eqns (10) and (11), indicating that the inductive and resonance
effects of the substituents Y on dC(C═N) are less than those of
the substituents Y in M3. This observation is in good agreement
with the discussion concerning the influence of Y on dC(C═N).
a general rule is being investigated.
Meanwhile, the coefficient r
decreased as the chemical
ðΔs2Þ
bond numbers between X and Y increased in case that both X
and Y are varied. Here, we try to describe the quantitative relation-
ship between the coefficientrðΔs2Þ and bond numbers (m), in which
EXPERIMENTAL SECTION
The compounds M1 shown in Scheme 3 were synthesized according
to published procedures, with slight modifications.[21–23] The detailed
m is the chemical bond number between X and Y. In M1, M2,
and M3, their values of m are 17, 13 and 11, respectively, and their
coefficients are 0.20, 0.33, and 0.59, respectively. It is not difficult to
find that the r
is nearly inversely proportional to m2.
ðΔs2Þ
CONCLUSION
Considering the respective effects of substituents X and Y in
molecules M1 together with M2 and M3, we found that both X
and Y act systematically on the imine carbon. By a comparison
of the substituent effects on dC(C═N) in M1, M2, and M3, it was
observed that the inductive and resonance effects of substituents
Y on dC(C═N) attenuated with the increasing distance between Y
and the imine carbon center, while as the length of the conjugated
chain between X and the imine carbon center was elongated, the
inductive effects of substituents X on dC(C═N) decreased, and
the change of the resonance effects was nearly ignorable. It is
interestingthat thecorrelations improvedwhenΔs2 was employed
to quantify the effect of X or Y on dC(C═N), and the correlations are
much better in the studying of the effect of X than that of Y on dC
(C═N), and that Δs2 plays a more important role as X or Y become
more ED or EW. This shows that the interaction between X and Y
cannot be neglected in case one of substituents X and Y is
fixed. It was confirmed that Δs2 was also suitable to evaluate the
substituent cross-interaction effect on dC(C═N) of M1 in case
both X and Y were varied, and the calculated dC(C═N) is more close
to the experimental values when the difference between the
electronic effect of X and Y increased. The results showed that
compared with the quantitative equation of M1, M2, and M3,
Scheme 3. Synthetic procedures for the titled compounds M1
J. Phys. Org. Chem. (2012)
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Z. FANG, C. CAO AND G. CHEN
analytical data of the synthesized compounds are available in the
Supporting Information.
Acknowledgements
The project was supported by the National Natural Science
Foundation of China (no. 21072053) and the Scientific Research
Fund of Hunan Provincial Education Department (no. 10K025).
Preparation of compounds III
A mixture of p-nitrobenzyl chloride (4.53 g, 30 mmol) and triethyl phosphate
(5.48g, 33mmol) was heated at 140 ꢃC for 5h before being cooled to room
temperature, and the product was not further purified. Then, p-substituted
benzaldehyde (30mmol) by substituents Y (Y= NMe2, OMe, Me, H, Cl, or
CN) was added, and the mixture was stirred in dry tetrahydrofuran
(100 mL) containing NaH (2.16g, 90 mmol) under a nitrogen atmosphere
at 50ꢃC for30min. Colored solids were obtained after precipitation in water.
The crude products were recrystallized from hot ethanol togive compounds
III. The products were dried in vacuum at 60ꢃC for 18 h.
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Preparation of compounds IV
To a suspension of compounds III (25 mmol) ina mixture ofethanol (150 mL)
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Preparation of compounds M1
Equimolar amounts of compounds IV and p-substituted benzaldehydes
by substituents X (X = NMe2, OMe, Me, H, Cl, F, CN, or NO2) were dissolved
in ethanol, followed by stirring at 80 ꢃC for 1 h. The solvent was removed
by rotary evaporation and recrystallized from dichloromethane and
ethanol to give compounds M1.
SUPPORTING INFORMATION
SUPPORTING INFORMATION
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