Synthesis and structure of bis-crown-containing stilbenes

Article abstract of DOI:10.1007/s11178-005-0255-2

An improved procedure was proposed for the synthesis of stilbenes fused to two crown ether fragments at both benzene rings. The structure of new homologous symmetric bis-crown-containing stilbenes was determined by X-ray analysis. Relations were revealed

Full text of DOI:10.1007/s11178-005-0255-2

Russian Journal of Organic Chemistry, Vol. 41, No. 6, 2005, pp. 843–854. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 6, 2005,
pp. 864–875.
Original Russian Text Copyright © 2005 by Vedernikov, Basok, Gromov, Kuz’mina, Avakyan, Lobova, Kulygina, Titkov, Strelenko, Ivanov, Howard, Alfimov.
Synthesis and Structure of Bis-crown-Containing Stilbenes
A. I. Vedernikov1, S. S. Basok2, S. P. Gromov1, L. G. Kuz’mina3, V. G. Avakyan1,
N. A. Lobova1, E. Yu. Kulygina2, T. V. Titkov1, Yu. A. Strelenko4, E. I. Ivanov2,
J. A. K. Howard5, and M. V. Alfimov1
1 Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7A, Moscow, 119421 Russia
e-mail: gromov@photonics.ru
2 Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine, Odessa, Ukraine
3 Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
4 Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia
5 Chemistry Department, Durham University, South Road, Durham DH1 3LE, United Kingdom
Received June 15, 2004
Abstract—An improved procedure was proposed for the synthesis of stilbenes fused to two crown ether
fragments at both benzene rings. The structure of new homologous symmetric bis-crown-containing stilbenes
was determined by X-ray analysis. Relations were revealed between the size of the crown ether moiety and
stilbene conformation in crystal and the mode of crystal packing. Conformational analysis of the prepared
1
stilbenes in solution and in the solid state was performed by H and 13C NMR spectroscopy and by DFT
quantum-chemical calculations.
In the recent years, supramolecular chemistry has
become one of the most extensively developing fields
of organic chemistry [1]. Components of supramolec-
ular systems are often complex and difficultly acces-
sible organic compounds, and their synthesis in many
cases constitutes the subject of a separate study. In
addition, an important problem in supramolecular
chemistry is prediction of properties of compounds in
supramolecular systems and development of selective
procedures for their self-assembly.
A promising sort of molecules for studying supra-
molecular organization in solution, monomolecular
layers, Langmuir–Blodgett films, and polymeric
matrices includes crown ether-fused azobenzenes, azo-
methines, styryl dyes, and related compounds [2–5]
possessing double N=N, C=N, and C=C bonds. In-
teresting properties of these compounds are the ability
to form supramolecules via host–guest-like complex
formation with metal and ammonium ions, consider-
able variations in spectral parameters due to complex
formation, and the possibility of varying their structure
and properties by irradiation. The most typical photo-
chemical processes occurring in the above self-
organized systems and accompanied by considerable
variation of their properties are trans–cis isomerization
of the double bonds, electrocyclic reactions, and
[2+2]-cycloaddition. Crown ether-fused stilbenes were
not studied in much detail [6–8], though these com-
pounds might be expected to exhibit equally interest-
ing photochemical behavior and pronounced ability to
form complexes.
We recently showed that bis(18-crown-6)–stilbene
Id and a number of diammonioalkyl derivatives of
viologen analogs in dilute solutions give rise to
unusual bi- and trimolecular complexes with effective
intermolecular charge transfer due to spatial pre-
organization of the donor and acceptor parts of the
initial molecules via hydrogen bonding [9–11]. Such
supramolecular systems were found to be very stable;
therefore, they can be regarded as convenient models
for studying photoinduced intermolecular electron
transfer [12] and promising species with versatile
electrochemical properties [13]. However, the high
potential of bis-crown-containing stilbenes as building
blocks for assembly of complex supramolecular
systems has not been explored to a sufficient extent.
One of the main reasons is relatively difficult prepara-
tion of these compounds. Therefore, the goal of the
present work was to develop a convenient procedure
for the synthesis of symmetric stilbenes containing
two 12(15, 18)-crown-4(5, 6) fragments and study their
structure.
1070-4280/05/4106-0843 © 2005 Pleiades Publishing, Inc.

VEDERNIKOV et al.
844
Scheme 1.
4
7
MeO
MeO
CH2Br
OMe
5
3
BrCH2CH(OEt)2
H2SO4, AcOH
OMe
OMe
MeO
MeO
1A
6
C5H11OH, Δ
OMe
OMe
2
1
OMe
IIa
III
(E)-Ia, 50%
The first representatives of bis-crown-fused stil-
benes, bis(15-crown-5)–stilbene Ic and bis(18-crown-
6)–stilbene Id were synthesized by reductive dimer-
ization of the corresponding 4-formylbenzocrown
ethers with TiCl4 and zinc in the presence of 1,8-bis-
(dimethylamino)naphthalene in anhydrous tetrahydro-
furan (yield 26 and 31%, respectively) [7]. A disad-
vantage of this procedure is the necessity of carrying
out the reaction at low temperature (–70°C). We re-
produced the synthesis of stilbene Id as described in
[7] and found that cooling with ice is sufficient to
ensure effective reaction. The target product was thus
obtained in 40% yield. Other disadvantages of the
reported method include the use of aggressive or
expensive reagents and thoroughly dehydrated solvent
and difficulties in the isolation of products from the
reaction mixture. Taking into account the above stated,
we analyzed published data [14] with a view to devel-
op a more convenient procedure for the synthesis of
bis-crown-containing stilbenes.
However, it implies the use of formyl derivatives of
benzocrown ethers and preparation of phosphorylated
crown ethers, which make the procedure expensive and
laborious. Among simpler procedures for the synthesis
of symmetric stilbenes having donor substituents in the
aromatic rings, we selected reaction of phenol deriva-
tives with α-haloacetaldehyde diethyl acetal in acid
medium, followed by thermal rearrangement of 1,1-di-
aryl-2-haloethane thus formed into stilbene derivative
[15, 16]. We examined the possibility of extending
this procedure to the preparation of biscrown-fused
stilbenes.
First of all, we studied the synthesis of tetra-
methoxystilbene Ia as model compound in which the
four methoxy groups may be regarded as analogs of
electron-donor crown-ether moieties. The reaction of
1,2-dimethoxybenzene (IIa) with bromoacetaldehyde
diethyl acetal in a mixture of acetic and sulfuric acids
afforded 1,1-bis(3,4-dimethoxyphenyl)-2-bromoethane
1
(III) with a high purity (>90%, according to the H
NMR data). Compound III was converted in a good
yield into stilbene (E)-Ia by heating in 1-pentanol
(Scheme 1).
One of the most popular methods of synthesis of
stilbenes, including crown-fused compounds [8], is
based on the Wittig condensation and its modifications.
The optimized procedure was then applied to the
synthesis of benzocrown ethers IIb–IId. The corre-
sponding bromoethanes (analogous to III) were ob-
tained as oily substances and were subjected (without
identification) to thermal rearrangement to give stil-
benes Ib–Id (Scheme 2). Thermal rearrangement of
1,1-diaryl-2-haloethanes is usually effected by heating
in high-boiling solvents [14]. We examined the effect
of the solvent nature and temperature at the rearrange-
ment stage in the synthesis of bis(15-crown-5)–stilbene
Ic as an example (Table 1). The yield of compound Ic
was the maximal when the corresponding 1,1-diaryl-
2-bromoethane was heated in alcohols. Under these
conditions, the released hydrogen bromide is likely to
react with the alcohol to give alkyl bromide which is
inactive toward crown-fused stilbene. The reaction in
acetic acid was accompanied by formation of a con-
siderable amount of tars, and the low yield of stilbene
Ic can also be explained by sensitivity of the polyether
ring to HBr. The rearrangements in high-boiling sol-
Table 1. Conditions of rearrangement of 1,1-diaryl-2-bromo-
ethane IIIc and yields of bis(15-crown-5)–stilbene Ic
Solvent
Ethanol
Temperature, °C
Yield, %
078
098
29a
51a
1-Propanol
1-Butanol
118
63a
1-Pentanol
1-Hexanol
136–138
156
73a
70a
1-Heptanol
1-Octanol
176
69 (62)a
65 (60)a
60a
195
Cyclohexanol
Ethylene glycol
Nitrobenzene
Acetic acid
161
196–198
210–211
117–118
135–140
47a
39a
17a
No solvent (reduced
pressure)
12a
a
After recrystallization from benzene.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

SYNTHESIS AND STRUCTURE OF BIS-CROWN-CONTAINING STILBENES
845
Scheme 2.
O
β'
(1) BrCH2CH(OEt)2
H2SO4, AcOH
(2) C5H11OH, Δ
O
O
O
O
α'
γ'
O
O
O
(
1
7
O
O
δ'
ε
2
6
5
O
O
1A
O
O
3
)
)
n
n
δ
O
(
4
)
n
γ
α
(
β
IIb–IId
(E)-Ib, (E)-Ic, (E)-Id
(E)-Ib, n = 0, yield 72%; (E)-Ic, n = 1, 73%; (E)-Id, n = 2, 70%;
vents were also accompanied by partial tarring, and
the product yield was lower (for alcohols, starting from
1-heptanol). The optimal temperature for the rear-
rangement of crown-containing 2-bromoethanes into
stilbenes was found to range from 120 to 160°C.
Analysis of the 1H NMR spectra and melting points
of the products and comparison with published data
showed that stilbenes Ic and Id are trans isomers. The
1H NMR spectrum of newly synthesized bis(12-crown-
4)–stilbene Ib also contained a single set of signals.
The presence of weak signals from protons in the
ethylene fragment (J1,1A = 16.2 Hz) indicated E con-
figuration of compound Ib.
presumably as a result of specific packing of molecules
in crystal. The dihedral angle between the mean-square
plane passing through the oxygen atoms in the macro-
ring and the stilbene system is 46.6° for Ib, 24.4°
for Ic, and 39.7° for Id. This specificity gives rise to
some new effects which are not typical of benzocrown
ethers. In fact, all benzocrown ether systems studied so
far, as well as compounds Ia and Id, are characterized
by distortion of the bond angles at the C5 and C6 atoms
of the benzene ring (i.e., atoms included in the macro-
ring; for atom numbering, see Fig. 1). The endocyclic
angles O1C5C6 and O2C6C5 are usually appreciably
smaller (~115°) while the exocyclic angles O1C5C4 and
O2C6C7 are greater (~125°) than the corresponding
Thus the use as starting compounds of accessible
benzocrown ethers rather than their formyl derivatives,
simple experimental procedure, and high overall yield
of the product make the method proposed by us for
the synthesis of compounds Ib–Id more advantageous
than that reported previously.
Table 2. Selected bond lengths d and bond angles ω in
molecules Ia–Id
d, Å
Bond
Ia
Ib
Ic
Id
C1–C1A
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C7
C7–C2
C5–O1
C6–O2
O1···O2
1.338(3) 1.341(2) 1.334(8) 1.328(4)
1.467(2) 1.464(2) 1.480(5) 1.475(2)
1.386(2) 1.396(2) 1.402(6) 1.396(2)
1.396(2) 1.393(2) 1.391(6) 1.395(2)
1.383(2) 1.387(2) 1.400(6) 1.383(3)
1.414(2) 1.407(2) 1.411(5) 1.417(2)
1.382(2) 1.381(2) 1.381(6) 1.392(2)
1.415(2) 1.411(2) 1.416(6) 1.410(3)
1.370(2) 1.369(1) 1.358(5) 1.372(2)
1.373(2) 1.373(1) 1.379(5) 1.369(2)
2.591(0) 2.606(0) 2.685(0) 2.591(0)
The structures of molecules Ia–Id (frontal and side
views), determined by the X-ray diffraction method,
are shown in Fig. 1, and selected bond lengths and
bond angles therein are given in Table 2. All molecules
occupy a particular position in the crystal symmetry
center and are characterized by trans configuration
of the ethylene fragment and planar structure of the
conjugated bond system. On the other hand, molecules
Ia–Id exist in two different conformations, which are
retained in going to solution. Molecules Ia and Ib
in crystal adopt a more stable syn,syn conformation,
while Ic and Id exist in less favorable anti,anti con-
formation (see below discussion of the NMR data and
quantum-chemical calculations).
Angle
ω, grad
C1AC1C2 127.8(2) 126.2(1) 126.2(5) 127.2(2)
O1C5C4
O2C6C7
O1C5C6
O2C6C5
124.9(1) 124.7(1) 125.6(3) 125.2(2)
125.1(1) 123.5(1) 120.0(4) 125.2(2)
As seen from Fig. 1 (side views), only molecule Ia
is planar as a whole; the methoxy groups therein are
located almost in the same plane as the benzene rings.
The crown ether fragments in molecules Ib–Id decline
to an appreciable extent toward the stilbene plane,
115.9(1) 115.7(1) 115.9(3)
114.9(1) 116.2(1) 119.5(3)
115.1(1)
115.6(1)
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

VEDERNIKOV et al.
846
C9
C12
O3
C11
O2
C13
O2
C10
O4
C6
C7
C2
C6
C7
O1
C5
C4
C1
C1A
O1A
O4A
O1A
O2A
C1A C1
C2
C5
O1
C8
C9
C8
O2A
O3A
C4
C3
C3
O4
O3
C13
C12
Ib
Ia
C9
O3A
C10
C9
C11
O3A
C8
C4
C3
C4
C10
C11
C5
O2A
C8
O6
C5
C6
O2
O1
C3
C2
O5
C12
C13
C2
O5
O4
O4A
O4A
O5A
O1
C6
C1A
C1
C1
O2A
O1A
O4
C1A
O2
O1A
C7
O5A
O3
C16
C15
C7
C14
C14
C13
O6A
C12
C17
O3
C15
Id
Ic
Fig. 1. Structure of molecules Ia–Id according to the X-ray diffraction data (frontal and side views).
standard values. These data contradict the concept
involving steric repulsion of the O1 and O2 atoms: the
distance between O1 and O2 in molecules Ia and Id
(2.59 Å) is shorter than the sum of their van der Waals
radii (~2.8 Å), which is usually interpreted in terms of
conjugation between lone electron pairs (LEP) on the
O1 and O2 atoms (p orbitals) and the benzene ring. The
conjugation is favored by the conformation of the
CAlkOCArCAr fragment where the corresponding torsion
angle approaches either 0 or 180°. In fact, the torsion
angles CAlkOCArCAr in molecules Ia and Id are fairly
small: 9.5, –11.7° and 3.5, –15.3°, respectively.
A different pattern is observed for compounds Ib
and Ic. Molecule Ib is also characterized by appropri-
ate distortion of the bond angles at C5 and C6 (Table 2);
however, this distortion is caused by a different factor.
The torsion angles CAlkOCArCAr in molecule Ib are too
large (–33.3 and 46.8°) to ensure effective conjugation
between the oxygen LEPs and the benzene ring. There
is an empirical rule, according to which the energy of
conjugation is proportional to the squared cosine of
the angle between the corresponding p and π orbitals,
i.e., of the torsion angles given above. This means that
molecule Ib retains conjugation by only 70 and 47%,
respectively. Undoubtedly, the reason for the observed
angular distortion is joint effect of two factors: (1) in-
sufficient flexibility of the relatively small macroring
and (2) crystalline field effects leading to declination
of the mean-square plane of the macroring from the
plane of the stilbene system.
Molecule Ic is the first example of structures lack-
ing distortion of bond angles at one benzene carbon
atom linked to oxygen atom of the macroring. The
bond angles at C5 are characterized by typical devia-
tions [125.6(3) and 115.9(3)°; Table 2], while the
corresponding angles at C6 are almost similar [120.0(4)
and 119.5(3)°]. The torsion angle C8O1C5C4 is 3.3°,
which suggests conjugation between O1 and the ben-
zene ring, whereas the torsion angle C15O2C6C7 equal
to 110.3° rules out conjugation between O2 and the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

SYNTHESIS AND STRUCTURE OF BIS-CROWN-CONTAINING STILBENES
847
benzene ring. Undoubtedly, this is caused by the crys-
talline field effect which fixes the molecule in a spe-
cific structure lacking for conjugation between the
benzene ring and one of the oxygen atoms. No such
effect was observed previously for other benzocrown
ethers with the same size of the polyether ring.
b
0
c
The presence of a macrocyclic fragment in mole-
cules Ib–Id induces considerable changes in bond
length distribution in the stilbene conjugation system
relative to that found in molecule Ia. The latter is
characterized by a distinct alternation of bonds in the
benzene ring, though this effect is more pronounced in
the C5C6C7C2 fragment. Bond alternation in molecules
Ib–Id is observed only in the C5C6C7C2 fragment
where the central C6–C7 bond is appreciably shorter
than the contiguous bonds. The bonds in the C2C3C4C5
fragment have almost similar lengths. The bridging
ethylene double bond in all the examined molecules is
essentially localized (Table 2). It should be noted that
analogous bond distribution was observed in the ben-
zene rings of molecule Ic where conjugation between
O2 and the benzene ring is absent. This means that
oxygen atom in the meta position with respect to the
ethylene bridge only slightly affects conjugation in
the stilbene system.
a
Fig. 2. Two layers of crystal packing of stilbene Ia.
the crystal structure of stilbene Ib are arranged close to
each other, thus forming extended hydrophilic areas.
As far as crystal packing was presumed to affect the
structure of stilbene molecules Ia–Id and electronic
effects therein, it is reasonable to consider the modes
of crystal packing in more detail. Figure 2 shows
two layers of the crystal packing of compound Ia. No
stacks are formed by molecules in the neighboring
layers, and the layers are superimposed in such a way
that a stair-like motif is obtained.
The tendency to form hydrophilic domains reaches
a limit in the crystal structures of stilbenes Ic and Id,
which are characterized by generally similar packing
motifs (Figs. 4, 5). It becomes obvious that the princi-
pal packing requirement of crown ether fragments is
formation of double hydrophilic layers.
In no case conjugated fragments are packed in
stacks, for stack-like motif (a) rules out contact be-
tween the stilbene fragments. The only way of ensuring
contact between the stilbene fragments is packing
according to stair-like motif (b) as shown below:
The crystal structure of compound Ia is charac-
terized by the existence of hydrophilic domains formed
by methoxy groups of the neighboring molecules.
Presumably, just the formation of large hydrophilic
domains prevents stack-like packing which is more
typical of conjugated organic systems than stair motif.
(a)
(b)
d1
d2
Packing of molecules Ib in crystal is shown in
Fig. 3. Here, conjugated parts of molecules give rise
to a stair–parquet motif. The crown ether moieties in
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

VEDERNIKOV et al.
848
tional behavior in solution is very important for struc-
ture determination of supramolecular systems based
thereon [9–13]. Therefore, we examined structural
parameters of stilbenes Ia–Id with the aid of various
NMR techniques.
a
0
c
Stilbenes Ia–Id in crystal exist as symmetric syn,syn
or anti,anti conformers (Fig. 1), presumably due to
requirements imposed on their structure to ensure the
tightest packing of their molecules. In going to solu-
tion, fast equilibrium may establish between the
anti,anti, intermediate unsymmetrical syn,anti, and
syn,syn conformers (Scheme 3) by analogy with
crown-containing styryl and butadienyl dyes studied
previously [5, 18]. Unfortunately, the NOESY spectra
did not allow us to estimate the fraction of each
particular conformer of compounds I because of strong
interaction between the ethylene protons and both 3-H
and 7-H in all conformers (for atom numbering, see
Schemes 1 and 2 and Fig. 1). An indirect evidence in
favor of syn,syn conformers can be derived from the
positions of signals of the 3-H and 7-H protons in
the benzene ring, which occupy ortho positions with
respect to the ethylene group. The 7-H proton is also
located in the ortho position with respect to the strong
electron-donor alkoxy group; therefore, its signal
should appear in a stronger field relative to the 3-H
signal. However, the opposite pattern was observed
b
Fig. 3. Crystal packing of stilbene Ib.
Here, the dumbbells denoting crown-containing
stilbene molecules are shown in a different projection
(butt-end view), and the line inside the dumbbell is the
projection of the planar stilbene fragment. Obviously,
the distance d2 is shorter than d1, i.e., only stair-like
arrangement of the conjugated fragments ensures
formation of tightly packed areas in crystal, taking into
account weak contacts between the polyether moieties
typical of crown ethers.
Fine structure of organic molecules and their as-
semblies in solution and in crystal can be established
by NMR spectroscopy [17]. Comparison of the NMR
data with the structure of a series of homologous com-
pounds, determined by crystallographic analysis, was
rarely performed, for it was impossible to obtain
suitable crystals for the whole series. As concerns bis-
crown-containing stilbenes, study of their conforma-
1
really; i.e., the 7-H signals in the H NMR spectra of
stilbenes Ia–Id were located in a weaker field (by
0.02–0.06 ppm) relative to those belonging to 3-H.
This situation is possible when the 7-H proton suffers
from deshielding effect by the ethylene fragment, i.e.,
more compact syn,syn conformer of Ia–Id predomi-
nates in the equilibrium mixture. Signals from the
CH2O protons in the crown ether moieties of com-
pounds Ib–Id are successively displaced upfield as the
distance to the benzene ring increases. According to
the NOESY spectra, the most downfield CH2OAr
(CH3OAr) signals are those from the CH2O2 (CH3O2 in
Ia) group which (as well as aromatic 7-H proton) fall
into the area deshielded by the C=C bond in syn,syn
conformers.
a
Compounds Ia–Id in solution in CDCl3 and in the
crystalline state were studied by 13C NMR spectros-
copy. Table 3 contains the carbon chemical shifts and
the differences ΔδC in going from crystal to solution.
0
c
b
13
The positions of signals from analogous C nuclei
in stilbenes Ic and Id in solution almost are almost
similar, indicating their similar conformational com-
positions and, probably, the absence of appreciable
Fig. 4. Crystal packing of stilbene Ic.
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SYNTHESIS AND STRUCTURE OF BIS-CROWN-CONTAINING STILBENES
849
Scheme 3.
R
O
R
O
R
O
O
R
O
R
R
O
O
R
R
R
O
O
O
R
O
O
R
R
anti,anti
syn,anti
syn,syn
strain in the macrocyclic fragments. In the 13C NMR
spectrum of homologous bis(12-crown-4)–stilbene Ib,
signals from all sp2-hybridized carbon atoms are dis-
placed downfield relative to the corresponding signals
of compounds Ic and Id, the greatest shifts being
observed for C4 and C7 (up to 4.0 ppm). Obviously,
this pattern results from considerable steric strains in
the 12-membered polyether rings of Ib, which force
LEPs of the O1 and O2 atoms out of conjugation with
the aromatic system. This is consistent with the X-ray
diffraction data (Fig. 1). The positions of most carbon
signals in the spectrum of tetramethoxystilbene Ia are
almost the same as in the spectra of Ic and Id. Excep-
tions are the C4 and C7 signals which are displaced
upfield by 2.7–3.2 ppm. Presumably, methoxy groups
are stronger electron donors than alkoxyethoxy, and
this effect is reflected in δC of the carbon atoms in the
ortho positions with respect to those groups.
overall variations |ΔδC| for C3 and C7 in Ia, Ib, and Id
amount to ~8 ppm, in keeping with the corresponding
δC values for anti,anti and syn,syn conformers in
crystal. In going from crystal to solution, the C7 signals
of Ia and Ib shift downfield while the C3 signals are
displaced upfield; the opposite pattern is observed for
stilbenes Ic and Id. The behavior of δC
3
and δC is con-
7
sistent with the existence of conformational equilib-
rium in solution; however, the predominant conformer
cannot be determined with certainty on the basis of
these data. Presumably, 13C chemical shifts of stilbenes
Ia–Id strongly depend on the mode of crystal packing
whose contribution to ΔδC is difficult to estimate.
In order to determine the most favorable conformer
of the chromophore fragment in (E)-stilbenes Ia–Id
we performed quantum-chemical calculations of the
a
13
The C NMR spectra of all stilbenes Ia–Id in the
solid state contain a single set of relatively narrow
signals (Table 3). Signals from the methylene carbon
atoms of crown-containing stilbenes Ib–Id give rise to
a broad band in the region of δC 71 ppm with several
poorly resolved maxima. The strongest differences in
13
the solid-state C NMR spectra of stilbenes Ia–Id are
observed in the aromatic region (Fig. 6). As follows
from the X-ray diffraction data, the carbon signals
belong to syn,syn conformers of Ia and Ib and anti,anti
conformers of Ic and Id. Taking into account that
13
rigorous assignment of signals in C NMR spectra of
solid substances is impossible, approximate assign-
ment was made on the basis of the X-ray diffraction
data and comparison with the spectra of their solutions.
In going from the solid state to solution, the |ΔδC|
values for most carbon nuclei do not exceed 2.6 ppm.
However, the C3 and C7 signals are displaced by 6 ppm
(except for stilbene Ic possessing an unusual con-
formation of the crown ether fragments in crystal). The
b
0
c
Fig. 5. Crystal packing of stilbene Id.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

VEDERNIKOV et al.
850
Table 3. 13C NMR spectra of stilbenes Ia–Id in solution and in the solid state,a δC, ppm
Comp.
C7
C4
C3
C2
C6
C5 CH=CH Cα (Me) Cα' (Me)
Cβ, Cβ'
69.94
Cγ, Cδ, Cε
no.
Iab
Iac
Ia (Δδ)d 04.9
Ibb
Ibc
108.78 111.38 119.56 130.74 149.20 148.77 126.68
103.90 110.70 123.20 128.10 149.00 149.00 124.70
55.97
56.20
–0.20
71.70
72.20
55.88
53.40
2.4
00.7
–3.6
02.6
00.2
–0.2
02.0
115.81 117.96 121.30 132.43 150.71 150.29 126.99
71.97
71.17, 71.27
113.10 114.40 126.20 132.20 151.50 150.70 126.20
Ib (Δδ)d 02.8
Icb
03.6
–4.9
00.2
–0.8
–0.4
00.8 –0.3 to –2.6
111.80 114.12 120.10 131.23 149.31 148.91 126.72
69.13
69.24 69.63, 69.67 70.56, 70.59,
71.12
Icc
123.80 114.40 118.50 128.60 151.10 149.90 124.80
71.20
Ic (Δδ)d –12.00 –0.3
01.6
02.6
–1.8
–1.0
01.9 –0.1 to –2.1
Idb
Idc
111.96 114.30 120.10 131.25 149.15 148.76 126.70
114.10 114.10 114.10 128.70 149.70 147.70 125.30
69.24
70.60
69.29 69.69, 69.70 70.78, 70.85
Id (Δδ)d –2.1
00.2
06.0
02.6
–0.5
01.1
01.4 –1.4 to 0.30
a
b
c
d
Approximate assignment for the solid state.
Solution in CDCl3, c = 0.05 M, 30°C.
Solid state, room temperature.
Δδ = δ(solution) – δ(solid).
C5, C6
CH=CH
C3
Ia
C2
C4
C7
C5, C6
C5, C6
C5, C6
CH=CH,
C3
C2
Ib
Ic
Id
C4
C7
C2
CH=CH
C7
C3
C4
C2
C3, C4, C7
CH=CH
Fig. 6. Solid-state 13C NMR spectra of stilbenes Ia–Id in the aromatic region.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

SYNTHESIS AND STRUCTURE OF BIS-CROWN-CONTAINING STILBENES
851
Table 4. Parameters of X-ray analysis and crystallographic data for compounds Ia–Id
Parameter
Ia
C18H20O4
300.34
Ib
C26H32O8
472.53
Ic
C30H40O10
560.62
Id
Molecular formula
C34H48O12
648.72
Molecular weight
Crystal system
Space group
a, Å
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
C2/c
08.9484(5)
06.7152(3)
12.4323(7)
92.500(3)
746.35(7)
2
10.0733(6)
07.2364(4)
16.1510(8)
97.464(2)
1167.34(11)
2
19.4172(8)
08.3901(4)
08.5481(4)
92.262(2)
1391.51(11)
2
42.778(2)0
09.0073(3)
08.6096(3)
95.988(2)
3299.3(2)
4
b, Å
c, Å
β, deg
V, Å3
Z
ρ
calc, g/cm3
1.336
1.344
1.338
1.306
F(000)
320
504
600
1392
μ(MoKα), mm–1
Crystal habit, mm
Temperature, K
Scan range θ, deg
Reflection index range
0.094
0.099
0.100
0.098
0.36×0.24×0.10
120.0(2)
2.28–23.26
0.36×0.26×0.22
120/0(2)
2.04–30.00
0.38×0.32×0.06
120.0(2)
1.05–27.50
0.56×0.40×0.02
100.0(2)
2.31–26.00
0–8 ≤ h ≤ 9,
0–7 ≤ k ≤ 7,
–13≤ l ≤ 12
0–8 ≤ h ≤ 14,
–10 ≤ k ≤ 10,
–22 ≤ l ≤ 14
–25 ≤ h ≤ 25,
–10 ≤ k ≤ 8,
–10 ≤ l ≤ 11
–43 ≤ h ≤ 52,
–11 ≤ k ≤ 11,
–10 ≤ l ≤ 10
Total number of reflections
2775
8279
8143
9016
Number of independent
reflections
1064 (Rint = 0.0278)
3344 (Rint = 0.0221)
3172 (Rint = 0.0534)
3184 (Rint = 0.0679)
Number of reflections
with I > 2σ(I)
1064
141
2553
218
2071
245
2135
305
Number of refined
parameters
Divergence factors R
[I > 2σ(I)]
R1 = 0.0339,
wR2 = 0.0844
R1 = 0.0425,
wR2 = 0.1163
R1 = 0.1025,
wR2 = 0.2409
R1 = 0.0435,
wR2 = 0.0839
Divergence factors (all
reflections)
R1 = 0.0448,
wR2 = 0.0892
R1 = 0.0579,
wR2 = 0.1287
R1 = 0.1545,
wR2 = 0.2637
R1 = 0.0829,
wR2 = 0.0949
Reliability with respect to
1.008
1.029
1.096
1.015
F2
Residual electron density
–0.149/0.168
–0.188/0.364
–0.505/1.829
–0.202/0.210
(min/max), e/Å3
syn,syn and anti,anti conformers of the simplest
representative, tetramethoxystilbene Ia, in terms of the
density functional theory (DFT). Molecule Ia contains
no flexible macrocyclic fragments which could give
rise to numerous local minima on the potential energy
surface; calculations of conformations of such struc-
tures often give inaccurate results, for these local
minima may be close to global. The calculated bond
lengths and bond angles in the syn,syn conformer of Ia
were in a good agreement with the corresponding
parameters determined by X-ray analysis: the maximal
differences between the theoretical and experimental
values were 0.013 Å and 0.6°, respectively. A slight
difference was observed between the calculated and
experimental dihedral angles between the ethylene
bond and benzene ring planes (7.8°), as well as for the
torsion angles COCArCAr (9.5 and 11.7°). This differ-
ence is likely to result from the effect of crystalline
field on the structure of stilbene Ia. The calculations
showed that the syn,syn conformer of Ia is thermo-
dynamically more stable than the anti,anti conformer
by 0.65 kcal/mol. According to the Boltzmann formula,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

VEDERNIKOV et al.
852
the ratio of the syn,syn and anti,anti conformers in
equilibrium mixture at room temperature should be
75:25 (with no account taken of solvation effects).
a rate of 5.2 kHz with a stream of air. The 13C chemical
shifts were determined by substitution relative to the
high-frequency line of solid glycine (δC 176.03 ppm)
and were measured with an accuracy of 0.1 ppm. The
mass spectra (electron impact, 70 eV) were recorded
on a Varian MAT-311A instrument with direct sample
admission into the ion source. The elemental composi-
tions were determined at the Microanalysis Laboratory
(Nesmeyanov Institute of Organometallic Compounds,
Russian Academy of Sciences, Moscow). Benzocrown
ethers IIb–IId were synthesized by the procedure
described in [19]. Bromoacetaldehyde diethyl acetal
was purchased from Aldrich.
Interconversion of the conformers is controlled by
the height of the energy barrier to rotation about the
formally single C1–C2. The barrier calculated by the
DFT method as the energy of transition state for
internal rotation was 5.0 kcal/mol in going from the
syn,syn conformer to syn,anti and 5.6 kcal/mol in
going from the anti,anti conformer to syn,anti. These
values are approximately twice as high as the barrier to
rotation about a single C–C bond in alkanes. The
barrier for stilbene Ia is higher due to conjugation
between the double bond and the benzene ring, which
increases π-order of the C1–C2 bond. However, this
barrier is insufficient to considerably hamper intercon-
version of the conformers under standard conditions,
and both conformers are present in the equilibrium
mixture.
Thus we have developed an effective procedure for
the synthesis of symmetric bis-crown-fused stilbenes
from accessible benzocrown ethers. Conformational
analysis of the synthesized stilbenes using the X-ray
diffraction data and NMR spectroscopy has revealed
specific structural features of crown–stilbenes in the
crystalline state and in solution. The results of
quantum-chemical calculations have shown that the
syn,syn conformation of the conjugated fragment in
(E)-stilbenes possessing four alkoxy substituents is
more favorable.
2-Bromo-1,1-bis(3,4-dimethoxyphenyl)ethane
(III). Bromoacetaldehyde diethyl acetal, 3.76 ml
(0.025 mol), was added to a solution of 6.36 ml
(0.05 mol) of 1,2-dimethoxybenzene (IIa) in 8 ml of
glacial acetic acid. The mixture was cooled to 0°C, and
a mixture of 6 ml of glacial acetic acid and 11 ml of
concentrated sulfuric acid was added over a period of
30 min at such a rate that the temperature did not
exceed 10°C. The mixture was then stirred for 15 min
and poured into 100 ml of water. An oily substance
separated and was extracted into chloroform (3×
75 ml), and the extract was washed in succession with
water, a solution of Na2CO3, and water again, dried
over Na2SO4, and evaporated under reduced pressure
to isolate 8.62 g of crude bromoethane III as a green-
ish viscous oily substance (purity >90%, according to
1
the NMR data). H NMR spectrum (DMSO-d6, 30°C),
δ, ppm: 3.71 s (6H, MeO), 3.74 s (6H, MeO), 4.11 d
(2H, CH2Br, J = 7.9 Hz), 4.25 t (1H, CH, J = 7.9 Hz),
6.86 br.s (4H, 5-H, 6-H), 6.99 br.s (2H, 2-H). Mass
spectrum, m/z (Irel, %): 382/380 (15/16) [M]+, 301 (12),
300 (91), 288 (10), 287 (100), 285 (24), 273 (13), 82
(12), 80 (13), 58 (31).
EXPERIMENTAL
The H and 13C NMR spectra were recorded on
1
a Bruker DRX 500 spectrometer operating at 500.13
and 125.76 MHz, respectively, from solutions in
CDCl3 and DMSO-d6 at 30°C; the chemical shifts were
measured relative to the corresponding solvent signals
(δ 7.27 and 2.50 ppm; δC 77.00 ppm); the signals were
(E)-1,2-Bis(3,4-dimethoxyphenyl)ethene (Ia).
A solution of 1.35 g of crude bromoethane III in 12 ml
of 1-pentanol was heated for 6 h under reflux. After
cooling, the precipitate was filtered off, washed with
1-pentanol, and dried in air. Yield 0.53 g (50%, cal-
culated on the initial compound IIa), slightly yel-
lowish crystals, mp 148–150°C; published data [7]:
assigned using homonuclear two-dimensional H–1H
1
COSY and NOESY spectra and heteronuclear H–13C
1
COSY (HSQC and HMBC) spectra. The chemical
shifts were determined with an accuracy of 0.01 ppm,
and the coupling constants, with an accuracy of 0.1 Hz.
High-resolution solid-state 13C NMR spectra were
obtained at room temperature on a Bruker MSL 300
spectrometer (75.47 MHz) with spinning at a magic
angle and using the cross-polarization technique
(cross-polarization period 1 μs) and powerful decoupl-
ing from protons. In all experiments, ZrO2 rotors with
a diameter of 7 mm were used; they were rotated at
1
mp 149–152°C. H NMR spectrum (CDCl3, 30°C, c =
0.05 M), δ, ppm: 3.91 s (6H, 1-OCH3), 3.95 s (6H,
2-OCH3), 6.87 d (2H, 4-H, J = 8.2 Hz), 6.93 s (2H,
CH=CH), 7.05 d.d (2H, 3-H, J = 8.2, 1.8 Hz), 7.07 d
(2H, 7-H, J = 1.8 Hz).
Stilbenes Ib–Id (general procedure). Freshly dis-
tilled bromoacetaldehyde diethyl acetal, 3.76 ml
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

SYNTHESIS AND STRUCTURE OF BIS-CROWN-CONTAINING STILBENES
853
(0.025 mol), was added to a solution of 0.05 mol of
benzocrown ether IIb–IId in 8 ml of glacial acetic
acid, the mixture was cooled to 0°C, and a mixture of
6 ml of glacial acetic acid and 11 ml of concentrated
sulfuric acid was added over a period of 15 min at such
a rate that the temperature did not exceed 10°C. The
mixture was stirred for 2 h at 10°C and poured into
100 ml of water. An oily material separated and was
extracted with CHCl3 (3×75 ml), and the extract was
washed in succession with water, a solution of Na2CO3,
and water again, dried over MgSO4, and evaporated
under reduced pressure to isolate the corresponding
1,1-disubstituted 2-bromoethane as a viscous oily sub-
stance. 1-Pentanol, 75 ml, was added to the product,
and the mixture was heated for 6 h under reflux. After
cooling, the precipitate was filtered off and heated with
100 ml of boiling ethanol. The mixture was cooled,
and the precipitate was filtered off and dried in air.
pared from 64.3 g (0.24 mol) of benzo-15-crown-5
(IIc) according to the procedure described above was
divided into 12 equal portions. Appropriate solvent,
35 ml, was added to each portion, and the resulting
solution was heated for 6 h under reflux or a portion of
IIIc was heated without a solvent for 6 h at 135–140°C
(5 mm). The mixture was cooled, and the precipitate
was filtered off (in the reaction with acetic acid, the
mixture was poured into 120 ml of water) and heated
in 20 ml of boiling ethanol (except for the reaction in
ethanol). The mixture was cooled, and the precipitate
was filtered off and dried in air. The conditions and
yields of compound Ic are given in Table 1.
X-Ray analysis. Single crystals of stilbenes Ia–Id
were obtained by slow evaporation of their solutions in
a mixture of hexane and methylene chloride. Measure-
ments were performed on a Bruker SMART-6 CCD
diffractometer (MoKα irradiation, λ = 0.71073 Å). The
structures were solved by the direct methods and were
refined by the least-squares procedure in full-matrix
anisotropic approximation. The crystallographic
parameters and parameters of the X-ray diffraction
experiments are collected in Table 4. All calculations
were performed using SHELXTL-Plus software
package [20].
(E)-1,2-Bis[3,4-(3,6-dioxaoctamethylenedioxy)-
phenyl]ethene (Ib). Yield 9.3 g (72%), mp 183–
185°C. 1H NMR spectrum (CDCl3, 30°C, c = 0.05 M),
δ, ppm: 3.81 s (8H, γ-CH2O), 3.88 m (8H, β-CH2O,
β'-CH2O), 4.20 m (4H, α-CH2O), 4.23 m (4H,
α'-CH2O), 6.88 s (2H, CH=CH), 6.95 d (2H, 4-H, J =
8.2 Hz), 7.08 d.d (2H, 3-H, J = 8.2, 1.8 Hz), 7.14 d
(2H, 7-H, J = 1.8 Hz). Mass spectrum, m/z (Irel, %):
472 (91) [M]+, 296 (57), 240 (22), 223 (13), 212 (18),
197 (13), 133 (13), 105 (16), 73 (19), 58 (100).
Calculated, %: C 66.09; H 6.83. C26H32O8. Found, %:
C 66.01; H 6.89.
Quantum-chemical calculations of the syn,syn
and anti,anti conformers of stilbene Ia with full
geometry optimization were performed by the DFT
method (B3LYP/6-31G**//B3LYP/6-31G** basis set)
using GAUSSIAN 98 software. The barrier to rotation
about the C1–C2 bond was determined by calculating
(with the same basis set) the structure and geometry of
the corresponding transition state with account taken
of zero-point vibration energy. Calculations of vibra-
tional frequencies for a structure where the torsion
angle between the C=C bond and the benzene ring is
89.3° revealed a single negative frequency, which is
a necessary and sufficient condition for indicating
attainment of a transition state.
(E)-1,2-Bis[3,4-(3,6,9-trioxaundecamethylenedi-
oxy)phenyl]ethene (Ic). Yield 10.3 g (73%), mp 190–
1
192°C [7]. H NMR spectrum (CDCl3, 30°C, c =
0.05 M), δ, ppm: 3.77 br.s (16H, γ-CH2O, δ-CH2O),
3.93 m (8H, β-CH2O, β'-CH2O), 4.16 m (4H, α-CH2O),
4.20 m (4H, α'-CH2O), 6.85 d (2H, 4-H, J = 8.3 Hz),
6.87 s (2H, CH=CH), 7.01 d.d (2H, 3-H, J = 8.3,
1.6 Hz), 7.05 d (2H, 7-H, J = 1.6 Hz).
(E)-1,2-Bis[3,4-(3,6,9,12-tetraoxatetradecameth-
ylenedioxy)phenyl]ethene (Id). Yield 11.3 g (70%),
This study was performed under financial support
by the Russian Foundation for Basic Research (project
nos. 03-03-32178 and 03-03-32929), by President of
the Russian Federation (project no. MK-3666.2004.3),
by the Foundation for Support of Russian Science, by
the INTAS Foundation (grant no. 2001-0267), by the
Ministry of Science and Technology of the Russian
Federation, and by the Russian Academy of Sciences.
The authors are grateful to A.A. Botsmanova for
providing data on the synthesis of stilbene Id by
reductive dimerization.
1
mp 154–156°C [7]. H NMR spectrum (CDCl3, 30°C,
c = 0.05 M), δ, ppm: 3.69 s (8H, ε-CH2O), 3.73 m (8H,
δ-CH2O), 3.78 m (8H, γ-CH2O), 3.93 m (4H, β-CH2O),
3.95 m (4H, β'-CH2O), 4.18 m (4H, α-CH2O), 4.22 m
(4H, α'-CH2O), 6.86 d (2H, 4-H, J = 8.3 Hz), 6.87 s
(2H, CH=CH), 7.01 d.d (2H, 3-H, J = 8.3, 1.7 Hz),
7.06 d (2H, 7-H, J = 1.7 Hz).
Study of the solvent and temperature effects on
the yield of stilbene (Ic). 2-Bromoethane IIIc pre-
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VEDERNIKOV et al.
11. Ushakov, E.N., Gromov, S.P., Vedernikov, A.I.,
854
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