Y.-D. Huang et al. / Tetrahedron 70 (2014) 1274e1282
1281
Fig. 14. A molecular mode of 7d showing different structural motifs.
ꢀ
parallel assignment with a vertical distance about 2.8 A. The co-
plane of pyrazine ring with two OCNH groups is attributed to the
two intra-hydrogen bondings forming by the two protons of amide
groups with the two nitrogen atoms of pyrazine ring. Both the two
spectrometer operated at 100 MHz. Accurate mass data were ob-
tained with an Agilent Technologies 6520 Accurate-Mass Q-TOF LC/
MS instrument and a LCQ Advantage MAX instrument, respectively,
under ESI model. Fourier transform infrared spectroscopy (FTIR)
measurements were performed on a Bio-Rad FTS 6000 spectrom-
eter. Ultra-violetevisible spectroscopy (UV) absorption spectra
were recorded with a Lambda35 spectrometer. X-ray diffraction
ꢀ
intra-hydrogen bondings have the same length of 2.33 A. In-
terestingly, the plane of aryl ring diverges from the pyrazine ring
with a dihedron angle of ca. 56.5ꢀ. Under fully extended confor-
ꢀ
mation of its alkyl chains, the molecular length of 7d is 37.7 A
(Fig. 14). The value is much bigger than the lattice parameter of the
Colh phase of 7d, where a¼26.7 A. This difference stems from the
(XRD) was checked on a Bruker diffractometer (Cu Ka radiation
ꢀ
k¼1.54056 A). Scanning electron microscopy (SEM) images were
taken by Hitachi S-4800 microscope. Polarization optical micros-
copy (POM) was carried out using an Olympus BX51 microscope
equipped with a Linkam LTS 350 platinum heating plate connected
to a Linkam TMS 94 processor and the photographs were taken
with a Fujix Digital camera HC-300Z. Differential scanning calo-
rimetry (DSC) traces were recorded using a Mettler-Toledo DSC1/
500 and the samples were placed in an aluminum crucible and
analyzed, under a stream of helium and nitrogen, with a heating/
cooling rate of 10 ꢀC/min.
ꢀ
shrinkage of alkyl chains in the Colh LC phase rather than the
conformation changing of the core structure, i.e., the pyrazine ring
and two aromatic rings adopting a trans-conformation with no
intramolecular hydrogen bonding. The intramolecular hydrogen
bonding in the compound is strong and it helps to maintain the co-
plane of pyrazine ring with two OCNH groups. The adoption of such
trans-conformation will result in the disappearance of intra-
molecular hydrogen bonding and the twist of the molecule, which
lead to higher energy and less stability. Moreover, a twist structure
will destroy the
pep stacking, an important driving force for the
4.2. Synthesis of the gelators 7aef and control compound 8
molecules self-assembling into long-range ordered columnar
structure and finally showing mesophase. That is to say, the exis-
tence of the mesophase indicates the core structure of compound
does not adopt trans-conformation. As to the shrinkage of alkyl
chains in the Colh LC phase, it is also reported in literature.20
The synthetic route and description for the compounds of 7aef
and 8 was presented in Section 2.1.
Compound 7a: Yellow crystalline solid; yield: 38%. 1H NMR
(300 MHz, CDCl3, TMS):
d
9.85 (s, 2H, eCONH); 7.65 (d, J¼8.8 Hz,
4H, ArH); 6.93 (d, J¼8.8 Hz, 4H, ArH0); 3.98 (t, J¼6.5 Hz, 4H, eOCH2);
3. Conclusion
3.08 (t, J¼6.6 Hz, 6H, pyrazineeCH3); 1.88e1.28 (m, 16H, CH2); 0.91
(m, 6H, eO(CH2)5CH3). 13C NMR (100 MHz, CDCl3):
d 161.16, 156.27,
In summary, a new series of pyrazine carboxamides have been
prepared and their gelation ability, supramolecular architectures,
and liquid crystal properties have been investigated. All the com-
pounds are efficient organogelators and exhibit thermotropic
mesophase. The organogels show multistimuli responsive behav-
iors and the driving forces of gelation are intra-hydrogen bondings,
150.75, 142.81, 130.42, 121.47, 114.84, 68.29, 31.61, 29.26, 26.11,
23.37, 22.62, 14.05. HRMS: calculated for [MþH]þ C32H43N4O4
547.3284, found 547.3285.
Compound 7b: Yellow crystalline solid; yield: 39%. 1H NMR
(300 MHz, CDCl3, TMS):
d
9.85 (s, 2H, eCONH); 7.64 (d, J¼8.8 Hz,
4H, ArH); 6.94 (d, J¼8.8 Hz, 4H, ArH0); 3.98 (t, J¼6.5 Hz, 4H, eOCH2);
pep stacking interactions and van der Waals interactions. The
3.10 (t, J¼6.6 Hz, 6H, pyrazineeCH3); 1.88e1.28 (m, 24H, CH2); 0.91
length of the aliphatic chains has great influence not only on gel-
ating ability but also on thermal behavior. Longer alkyl chains fa-
cilitate to promote the gelation efficiency of the gelator while
exhibit irregular influence on their mesomorphic temperature
(m, 6H, eO(CH2)5CH3). 13C NMR (100 MHz, CDCl3):
d 161.19, 156.25,
150.95, 142.17, 130.63, 121.59, 114.97, 68.42, 31.95, 29.39, 26.18,
23.49, 22.80, 14.25. HRMS: calculated for [MþH]þ C36H51N4O4
603.3910, found 603.3907.
domain. The strong
pep interactions result in the molecules of
Compound 7c: Yellow crystalline solid; yield: 41%. 1H NMR
gelators self-assembling into long-range ordered columnar struc-
ture and finally showing mesophase under the cooperation of van
der Waals interactions.
(300 MHz, CDCl3, TMS):
d
9.88 (s, 2H, eCONH); 7.74 (d, J¼8.8 Hz, 4H,
ArH); 6.95 (d, J¼8.8 Hz, 4H, ArH0); 3.98 (t, J¼6.5 Hz, 4H, eOCH2);
3.10 (t, J¼6.6 Hz, 6H, pyrazineeCH3); 1.88e1.28 (m, 32H, CH2); 0.91
(m, 6H, eO(CH2)5CH3). 13C NMR (100 MHz, CDCl3):
d 161.17, 156.30,
4. Experimental section
150.78, 142.76, 130.42, 121.52, 114.88, 68.31, 31.91, 29.57, 29.34,
26.05, 23.37, 22.70,14.14. HRMS: calculated for [MþH]þ C40H59N4O4
659.4536, found 659.4531.
4.1. Materials and measurements
Compound 7d: Yellow crystalline solid; yield: 42%. 1H NMR
All the chemicals were purchased from commercial chemical
suppliers and used without further purification. 1H nuclear mag-
netic resonance (1H NMR) spectra were recorded with a Bruker 400
spectrometer operated at 300 MHz and 13C nuclear magnetic res-
onance (13C NMR) spectra were recorded with a Bruker 400
(300 MHz, CDCl3, TMS):
d
9.84 (s, 2H, eCONH); 7.64 (d, J¼8.8 Hz,
4H, ArH); 6.93 (d, J¼8.8 Hz, 4H, ArH0); 3.97 (t, J¼6.5 Hz, 4H, eOCH2);
3.08 (t, J¼6.6 Hz, 6H, pyrazineeCH3); 1.88e1.28 (m, 40H, CH2); 0.91
(m, 6H, eO(CH2)5CH3). 13C NMR (100 MHz, CDCl3):
d 161.18, 156.23,
150.79, 142.85, 130.42, 121.52, 114.88, 68.31, 31.93, 29.60, 29.43,