Neuvonen et al.
SCHEME 1
the substituents X and Y affect the overall electron distribution
of X-C6H4-Z-C6H4-Y and therefore among others the
conformation of the molecule. An understanding of the mech-
anisms of charge generation at a molecular level is critical for
an understanding of charge generation, transport, and trapping
in photorefractive liquid crystals and for the design of new
materials for nonlinear optical purposes.
Although NMR shielding is not determined only by the
electron density, linear correlations with positive slopes between
the atomic charges and the 13C NMR chemical shifts for probe
nuclei have been observed in several systems when the
substitution is varied.7-14 Good correlations have also been
observed between the chemical shifts of the unsaturated car-
bons in the side chains of aromatic rings and substituent
parameters.7,8,12,15-18 Although the shift data do not usually fit
the single-parameter correlation (eq 1), good to excellent
correlations have been obtained with eq 2,
qualitative level that the substituent on the benzylidene ring can
influence the sensitivity of the azomethine carbon to the aniline
substituent. Analogously, the aniline substituent seems to affect
the sensitivity of the azomethine carbon to the benzylidene ring
substituent.20 For substituted phenyl benzoates p-Y-C6H4-
CO2C6H4sp-X, we recently observed that the electronic effects
of the remote aromatic ring substituents systematically modify
the sensitivity of the CdO group to the electronic effects of
the phenyl or benzoyl ring substituents.21 Liu et al. recently
reported alike substituent cross-interaction effects in the theo-
retical bond dissociation energies concerning radical chemistry
of anilines and aromatic silanes.22 In the present study, our goal
is to clarify whether the substituted benzylidene anilines, p-Xs
C6H4sCHdNsC6H4sp-Y, display a cross-interaction effect
analogous to that observed for phenyl benzoates.21 We wish
especially to shed light on the substituent effects on the
electronic character of the CdN bridging group in imines, an
important group of mesogenic compounds.
SCS ) Fσ + constant
SCS ) FFσF + FRσR
(1)
(2)
where SCS (substituent-induced change in the chemical shift)
is the 13C NMR shift of the side-chain carbon for a substituted
compound relative to that for the unsubstituted one, and σF and
σR are the inductive and resonance parameters, respectively, for
the aromatic ring substituent in question.
By means of 13C and 15N NMR studies, together with the
computational data for a set of benzaldehyde derivatives
possessing a CdN double bond in the side chain, p-XsC6H4s
CHdNsY, we have recently shown that the sensitivity of the
electronic character of the CdN unit to the benzylidene
substituent X is dependent on the group Y (Y ) Ph, CH2Ph,
C6H4-p-NO2, Me, C(Me)3, OMe, OH, NHPh, or NH2). However,
a detailed analysis of the origin of the phenomenon was not
achieved.8 The early NMR works of Kawasaki19 and Akaba et
al.20 on substituted benzylidene anilines demonstrated on a
Results
The 13C NMR spectra of the benzylidene anilines shown in
Scheme 1 were measured in CDCl3. The measurements were
performed with a low and constant sample concentration (0.1
M) with a view to diminishing intermolecular associations. The
13C NMR chemical shifts for the azomethine carbon in the
benzylidene anilines 1-8 (cf. Scheme 1) are listed in Table 1.
The chemical shifts range from 151.51 to 162.71 ppm.
(4) Miyajima, S.; Nakazato, A.; Sakoda, N. Liq. Cryst. 1995, 18, 651.
(5) Pin˜ol, R.; Ros, M. B.; Serrano, J. L.; Sierra, T.; De La Funte, M. R.
Liq. Cryst. 2004, 31, 1293.
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2003, 58, 735.
(7) Neuvonen, H.; Neuvonen, K.; Koch, A.; Kleinpeter, E.; Pasanen, P.
J. Org. Chem. 2002, 67, 6995.
(8) Neuvonen, K.; Fu¨lo¨p, F.; Neuvonen, H.; Koch, A.; Kleinpeter, E.;
Pihlaja, K. J. Org. Chem. 2003, 68, 2151.
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(12) Neuvonen, K.; Fu¨lo¨p, F.; Neuvonen, H.; Koch, A.; Kleinpeter, E.;
Pihlaja, K. J. Org. Chem. 2001, 66, 4132.
(13) Matsumoto, K.; Katsura, H.; Uschida, T.; Aoyama, K.; Machiguchi,
T. Heterocycles 1997, 45, 2443.
(14) Alvarez-Ibarra, C.; Quiroga-Feijo´o, M. L.; Toledano, E. J. Chem.
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(15) Craik, D. J.; Brownlee, R. T. C. Prog. Phys. Org. Chem. 1983, 14,
1.
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(b) Bonesi, S. M.; Ponce, M. A.; Erra-Balsells, R. J. Heterocycl. Chem.
2004, 41, 161. (c) Mezzina, E.; Spinelli, D.; Lamartina, L.; Buscemi, S.;
Frenna, V.; Macaluso, G. Eur. J. Org. Chem. 2002, 203.
(18) (a) Neuvonen, K.; Fu¨lo¨p, F.; Neuvonen, H.; Pihlaja, K. J. Org. Chem.
1994, 59, 5895-5900. (b) Neuvonen, K.; Fu¨lo¨p, F.; Neuvonen, H.;
Simeonov, M.; Pihlaja, K. J. Phys. Org. Chem. 1997, 10, 55. (c) Neuvonen,
H.; Neuvonen, K. J. Chem. Soc., Perkin Trans. 2 1999, 1497.
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Discussion
Our main goals were to study the effect of the aniline
substituent on the sensitivity of the electronic properties of the
CdN group to the benzylidene substituent and the effect of the
benzylidene substituent on the sensitivity of the electronic
properties of the imine bridging group to the aniline substituent.
We first investigated separately the effects of the benzylidene
and aniline substituents on δC(CdN). Tables 2 and 3 present
the substituent-induced changes in the chemical shifts, SCS [δC-
(CdN)(substituted compound) - δC(CdN)(unsubstituted com-
pound)], of the CdN carbon with respect to the substituents X
and Y, respectively.
Effect of the Benzylidene Substituent X on the CdN
Carbon Resonance δC(CdN). Both electron-withdrawing (EW)
and electron-donating (ED) benzylidene substituents cause
shielding of the CdN carbon as compared with the unsubstituted
derivative X ) H (Tables 1 and 2). This is opposite to the
(20) Akaba, R.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1985,
1186.
(21) Neuvonen, H.; Neuvonen, K.; Pasanen, P. J. Org. Chem. 2004, 69,
3794.
(22) (a) Cheng, Y.-H.; Zhao, X.; Song, K.-S.; Liu, L.; Guo, Q.-X. J.
Org. Chem. 2002, 67, 6638. (b) Song, K.-S.; Liu, L.; Guo, Q.-X. J. Org.
Chem. 2003, 68, 262.
3142 J. Org. Chem., Vol. 71, No. 8, 2006