Dodecyloxy-substituted 2,4,6-Tris(styryl)pyridines

Article abstract of DOI:10.1002/jhet.5570450346

(Chemical Equation Presented) (E,E,E)-2,4,6-Tris(styryl)pyridines 2a-c with 3, 6 or 9 dodecyloxy substituents were prepared by the highly stereoselective condensation reaction of collidine (8) and the corresponding phenyl-(1-phenyl-methylidene)amines 7a-c

Full text of DOI:10.1002/jhet.5570450346

May-Jun 2008
935
Dodecyloxy-Substituted 2,4,6-Tris(styryl)pyridines
Thorsten Lifka, Annette Oehlhof, and Herbert Meier*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, 55099 Mainz, Germany.
Fax: (+49)6131-39-25396 E-mail: hmeier@mail.uni-mainz.de
Received August 28, 2007
R2
R3
R1
2
a
b
c
R1
H
H
OC12H25
OC12H25
R2 OC12H25
OC12H25
R3
H
OC12H25 OC12H25
R1
R2
R3
R2
N
R3
R1
(E,E,E)-2,4,6-Tris(styryl)pyridines 2a-c with 3, 6 or 9 dodecyloxy substituents were prepared by the
highly stereoselective condensation reaction of collidine (8) and the corresponding phenyl-(1-phenyl-
methylidene)amines 7a-c (Siegrist reaction). In contrast to the corresponding compounds with a benzene or
a 1,3,5-triazine core, 2a-c do not show any hints for the formation of thermotropic liquid crystals. The
major application of such star-shaped systems is in the field of nonlinear optics.
J. Heterocyclic Chem., 45, 935 (2008).
Conjugated star-shaped compounds have interesting
electrical, optical and optoelectronic properties, which
permit various applications in materials science (nonlinear
optics NLO, field effect transistors FET, light emitting
diodes LED, etc.) [1]. This structure unit can also serve as
mesogen for the formation of liquid crystals [LC]
{discotic nematic (ND), columnar nematic (NC) or
columnar (Colh, Colob, Colr, etc.) thermotropic phases}
[2]. Scheme 1 shows 1,3,5-tris(styryl)benzenes 1 and the
Scheme 1
R3
R2
R1
R1
R2
R3
Y
X
X
1
2
3
4
X
CH CH
CH
N
N
N
corresponding pyridines 2, pyrimidines
1,3,5-triazines 4.
3
and
Y
N
CH
Ri: H, Alkoxy (i = 1,2,3)
R1
R3
Although the parent systems (R = H) have been known
for a long time (1 [3], 2 [4], 3 [5], 4 [6]), only very few
derivatives with long flexible side chains R have been
prepared. Such side chains are a precondition for the
formation of LC phases. (E,E,E)-1,3,5-Tris(styryl)-
benzenes 1 with nine OC6H13 or OC12H25 groups (R1 =
R2 = R3) form Colh phases [7-9]. (E,E,E)-2,4,6-Tris-
(styryl)pyridine 2 with six decyloxy groups (R1 = H, R2 =
R2
R3 = OC10H21) and two additional CN groups on the
pyridine ring forms a Colh phase as well [10]. Until now –
to our best knowledge – pyrimidines 3 with side chains
were not studied. Most examples of LC phases exist in the
series of 1,3,5-triazines 4 with six octyloxy chains (R1 =
Table 1
1H NMR data of 2a-c (δ values in CDCl3, TMS as internal standard).
Comp.
2a
Pyridine
3-H, 5-H
7.24 (2H)
2-Styryl, 6-Styryl
4-Styryl
OC12H25
OCH2
4.00 (6H)
olefin. H [a]
7.07 (2H)
7.66 (2H)
7.07 (2H)
7.59 (2H)
aromat. H olefin. H [a]
aromat. H
6.89 (2H)
7.47 (2H)
6.87 (1H)
7.06 (1H)
7.09 (1H)
6.76 (2H)
CH2
1.20-1.85 (60H)
CH3
0.89 (9H)
6.90 (4H)
7.53 (4H)
6.86 (2H)
7.11 (2H)
7.18 (2H)
6.81 (4H)
6.89 (1H)
7.28 (1H)
6.89 (1H)
7.27 (1H)
2b
7.31 (2H)
7.36 (2H)
4.05 (12H)
4.03 (18H)
1.20-1.88 (120H)
1.20-1.85 (180H)
0.89 (18H)
0.89 (27H)
2c
7.10 (2H)
7.56 (2H)
6.91 (1H)
7.24 (1H)
[a] 3J = 16.1 ± 0.1 Hz

936
T. Lifka, A. Oehlhof, and H. Meier
Vol 45
H, R2 = R3 = OC8H17) or nine OC6H13, OC8H17, OC10H21,
OC12H25, OC16H33 or O(CH2)2-CH(CH3)-(CH2)3-CH(CH3)2
chains [11-13]. We studied now pyridines 2 with 3, 6, and
9 OC12H25 groups.
2a and 2b at ambient temperatures with sharp melting
points at 43 and 69 °C, respectively, 2c is a viscous oil,
which solidifies below 0 °C to a glass [19].
Scheme 2
The synthetic approach (Scheme 2) started with the
formation of the corresponding aldimines 7a-c from the
aldehydes 5a-c and aniline 6: 7a [14], 7b [15], 7c [15,16].
Commercially available 2,4,6-trimethylpyridine (collidine)
8 was then reacted with 7a-c. The first two condensation
processes in 2- and 4-position are fast. The third
condensation step (in 6-position) is much slower as 1H nmr
reaction spectra revealed. The yield of the Siegrist reaction
[17] 7 + 8 → 2 is generally lower than that of comparable
processes as for example the Knoevenagel condensation or
the Wittig-Horner reaction, but the stereoselectivity in favor
of the (E)-configurations is extremely high. The kinetically
controlled Siegrist reaction [18] leads to Z/E ratios which
are below 1:99, which means below the thermodynamic
equilibrium, which is in the case of 1,2-di(het)arylethenes
in the range of 5:95.
H
O
+
NH2
R1
R3
R2
5a-c
6
CH3
R1
CH3
N
CH3
N
R2
8
R3
KOH / DMF
7a-c
R2
R1
R3
The 1H nmr characterization of the target compounds 2a-
c is summarized in Table 1. Typical is the splitting Δδ of
the olefinic AB spin systems with coupling constants of
16 ± 0.1 Hz. It indicates the polarization of the olefinic
double bonds which is for 2 higher than for the benzene
derivatives 1, but lower than for the corresponding
1,3,5-triazines 4 [12,14]. According to the push-pull effect,
the resonance of the α and α' protons in 2a-c (Scheme 2,
Table 1) is always at higher field than the resonance of the
β and β' protons. Table 2 shows the 13C nmr data of 2a-c.
The polarization of the olefinic CC bonds is demonstrated
by Δδ values between 4.9 and 8.5 ppm.
"'
!'
R3
R2
!
N
R1
R2
"
R1
2a-c
R3
2
a
b
c
R1
R2
R3
H
H
OC12H25
OC12H25 OC12H25 OC12H25
OC12H25 OC12H25
Thermotropic liquid crystalline phases could not be
detected by DSC for 2a-c. Compound 2c would fulfill the
best prerequisites for a columnar aggregation because the
disc, formed by sp2 carbon atoms, has a diameter of about
190 pm [10] (suitable for the π stacking) and nine
dodecyloxy chains should provide sufficient Van der
Waals interactions. In contrast to the crystalline states of
H
mp [°C] 43
Yield [%] 66
69
45
< 0
19
Star-shaped compounds with polar end groups as 2a-c
have high hyperpolarizabilities. Therefore nonlinear optics
are the preferred area of application in materials science [2].
Table 2
13C NMR data of 2a-c (δ values in CDCl3, TMS as internal standard).
Vinylene Bridges Benzene Rings [a]
Comp. Pyridine Ring
C-2,6 HC-3,5
OC12H25 Chains [a]
C-4
145.7
α-CH2
126.2
β-CH2
132.2
α'-CH2
123.9
β'-CH2
132.0
CH
Cq
CqO
OCH2
67.9
CH2
22.7
26.0
29.2
29.3
22.7
26.0
29.3
CH3
14.1
2a
2b
2c
155.9 117.0
156.0 117.7
156.0 116.4
114.6
114.7
128.3
128.4
111.6
113.3
129.0 159.3
29.4
29.6
31.9
145.7
145.6
126.4
128.1
132.6
133.0
124.1
126.1
132.6
133.0
129.3 149.1
129.8 149.2
149.5
69.1
69.3
29.4
29.6
31.9
14.1
14.1
149.9
105.7
131.5 138.3
131.9 139.3
153.3
69.1
73.5
22.7
26.1
29.4
29.7
31.9
153.4
[a] Partly overlapping signals

May-Jun 2008
Dodecyloxy-Substituted 2,4,6-Tris(styryl)pyridines
937
Acknowledgement. We are grateful to the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for financial support.
EXPERIMENTAL
The melting points were determined on a Büchi melting point
apparatus and are uncorrected. Their control was performed with
1
a Perkin-Elmer DSC-7. The H and 13C nmr data were obtained
REFERENCES AND NOTES
on Bruker AM 400 and AC 200 spectrometers using CDCl3 as
solvent and TMS as internal standard. The FD mass spectra were
recorded on a Finnigan M95 spectrometer. A Beckman Acculab
spectrometer served for the measurements of the ir spectra in
KBr or as neat films. The elemental analyses were performed in
the microanalytical department of the institute.
[1] Meier, H. in Carbon-Rich Compounds, From Molecules to
Materials, Haley, M. M.; Tykwinski, R. R., eds, Wiley-VCH,
Weinheim, 2006, pp 476-528 and references therein.
[2] See for example: Laschat, S.; Baro, A.; Steinke, N.;
Giesselmann, F.; Hägele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer,
S.; Schreivogel, A.; Tosoni, M. Angew. Chem. 2007, 199, 4916; Angew.
Chem. Int. Ed. 2007, 46, 4832.
[3] Malkes, L. Y.; Kovalenko, N. P. J. Org. Chem. USSR (Engl.
Transl.) 1966, 2, 288.
[4] Koenigs, W.; von Bentheim, A. Ber. Dtsch. Chem. Ges.
1905, 38, 3907.
General Procedure for the Preparation of the
(E,E,E)-2,4,6-Tris(styryl)pyridines 2a-c. A slow oxygen-free
stream of N2 was purged for 30 min through a solution of
2,4,6-trimethylpyridine (collidine) (8) (606 mg, 5.0 mmol) and
16.5 mmol aldimine 7a-c [14,15] in DMF (150 mL). To the
vigorously stirred solution powdered KOH (3.70 g, 66 mmol)
was added and the temperature was raised to 90 °C. After 1 h the
reaction mixture was cooled to 0-5 °C and CH3OH (200 mL)
was added dropwise. The products 2a and 2b precipitated and
were washed with aqueous CH3OH. Oily product 2c was
extracted with 2 x 100 mL C2H5OC2H5 from the CH3OH phase
to which 200 mL H2O was added. The dried (MgSO4), crude
products were purified by column chromatography (6 x 50 cm
SiO2, toluene or toluene/ethyl acetate 100:2). The compounds 2a
and 2b can be recrystallized from acetone.
[5] Bowman, A. J. Chem. Soc. 1937, 494.
[6] Grundmann, C.; Weisse, G. Chem. Ber. 1951, 84, 684.
[7] Lehmann, M.; Fischbach, I.; Spiess, H. W.; Meier, H. J. Am.
Chem. Soc. 2004, 126, 772.
[8] Meier, H.; Lehmann, M.; Kolb, U. Chem. Eur. J. 2000, 1,
2462.
[9] See also: Meier, H.; Karpouk, E.; Lehmann, M.;
Schollmeyer, D.; Enkelmann, V. Z. Naturforsch. B 2003, 58, 775.
[10] Attias, A.-J.; Cavalli, C.; Donnio, B.; Guillon, D.; Hapiot, P.;
Malthête, J. Mol. Cryst. Liq. Cryst. 2004, 415, 169.
[11] Holst, H. C.; Pakula, T.; Meier, H. Tetrahedron 2004, 60,
6765.
(E,E,E)-2,4,6-Tris[2-(4-dodecyloxyphenyl)vinyl]pyridine (2a)
Yield: 3.097 g (66%). Pale yellow crystals which melted at
43 °C; ir (KBr): 2890, 2820, 1565, 1495, 1450, 1415, 1245,
1120, 950 cm–1; FD ms: m/z (%) 938 (100) [M+•]. Anal. Calcd.
for C65H95NO3 (938.5): C, 83.19; H, 10.20; N, 1.49. Found: C,
83.12; H, 10.21; N, 1.66.
[12] See also: Meier, H.; Holst, H. C.; Oelhof, A. Eur. J. Org.
Chem. 2003, 4173.
[13] Meier, H.; Karpuk, E.; Holst, H. C. Eur. J. Org. Chem. 2006,
2609.
[14] Zerban, G.; Meier, H. Z. Naturforsch. B 1993, 48, 171.
[15] Meier, H.; Prass, E.; Zerban, G.; Kosteyn, F. Z. Naturforsch.
B 1988, 43, 889.
[16] The reaction can be performed without solvent when the
generated water is removed by working at 2.1 kPa. The yields of the
products are then almost quantitative: 7a (95%), 7b (97%), 7c (98%).
[17] Siegrist, E. Helv. Chim. Acta 1967, 50, 906.
[18] See: Meier, H. Angew. Chem. 1992, 104, 1425; Angew.
Chem. Int. Ed. 1992, 31, 1399.
(E,E,E)-2,4,6-Tris[2-(3,4-didodecyloxyphenyl)vinyl]pyridi-
ne (2b) Yield: 3.354 g (45%). Pale yellow crystals which melted
at 69 °C; ir (KBr): 2895, 2825, 1590,. 1570, 1520, 1495, 1455,
1235, 1160, 960, 840 cm–1; FD ms: m/z (%) 1492 (100) [M+H+].
Anal. Calcd. for C101H167NO6 (1491.4): C, 81.34; H, 11.29; N,
0.94. Found: C, 81.35; H, 11.14; N, 1.08.
[19] The phase transition enthalpies ΔH for 2a and 2b amount to
31 and 55 kJ mol–1, respectively. An LC phase of 2c within a small
temperature interval above 0 °C could not be completely excluded by
DSC, but such a possible phase would have no applications in materials
science.
(E,E,E)-2,4,6-Tris[2-(3,4,5-tridodecyloxyphenyl)vinyl]pyr-
idine (2c) Yield: 1.944 g (19%) [20]. Pale yellow oil; ir (neat
film): 2890, 2820, 1565, 1485, 1420, 1365, 1325, 1230, 1105,
955 cm–1; FD ms: m/z (%) 2046 (100) [M+H+]. Anal. Calcd. for
C137H239NO9 (2044.4): C, 80.44; H, 11.78; N, 0.69. Found: C,
80.11; H, 11.95; N, 0.82.
[20] Extension of the reaction time to 6 h enhances somewhat the
yield of the crude product, but makes the purification more difficult.