Cis/trans-isomerizations in 1,2-bis(2-thienyl)ethene derivatives: A joint experimental and computational approach

Article abstract of DOI:10.1016/S0022-2860(99)00180-5

Two substituted 1,2-bis(2-thienyl)ethenes were synthesized and their Z/E-isomerization was studied in solution. Despite their structural similarities, they behaved very differently concerning the position of the equilibrium and the rate with which it was established: 1,2-bis(3-methoxy-2- thienyl)ethene (BMTE) was found to exist completely in the E-form and the transformation from Z to E was extremely fast, while 1-cyano-1-(2-thienyl)-2- (3-methoxy-2-thienyl)ethene (CTMTE) displayed an equilibrium mixture composed of 97% Z- and 3% E-CTMTE, which was reached far slower. In an attempt to rationalize these differences ab initio calculations were performed which enabled us to correlate relative energy differences of starting and end conformations and of estimated intermediary states on the one hand with the experimentally observed isomerizational behaviour on the other. Furthermore, the data present insight into the effects of the methoxy and nitrile group on E/Z stability and conjugation length.

Full text of DOI:10.1016/S0022-2860(99)00180-5

Journal of Molecular Structure 485–486 (1999) 87–96
Cis/trans-isomerizations in 1,2-bis(2-thienyl)ethene derivatives: a
joint experimental and computational approach
*
F. Blockhuys, R. Hoefnagels, C. Peten, C. Van Alsenoy, H.J. Geise
Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1; B-2610 Antwerp (Wilrijk), Belgium
Dedicated to Prof. Dr. Larry S. Bartell on the occasion of his 75th birthday
Received 18 January 1999; accepted 4 February 1999
Abstract
Two substituted 1,2-bis(2-thienyl)ethenes were synthesized and their Z/E-isomerization was studied in solution. Despite their
structural similarities, they behaved very differently concerning the position of the equilibrium and the rate with which it was
established: 1,2-bis(3-methoxy-2-thienyl)ethene (BMTE) was found to exist completely in the E-form and the transformation
from Z to E was extremely fast, while 1-cyano-1-(2-thienyl)-2-(3-methoxy-2-thienyl)ethene (CTMTE) displayed an equili-
brium mixture composed of 97% Z- and 3% E-CTMTE, which was reached far slower. In an attempt to rationalize these
differences ab initio calculations were performed which enabled us to correlate relative energy differences of starting and end
conformations and of estimated intermediary states on the one hand with the experimentally observed isomerizational beha-
viour on the other. Furthermore, the data present insight into the effects of the methoxy and nitrile group on E/Z stability and
conjugation length. ᭧ 1999 Elsevier Science B.V. All rights reserved.
Keywords: Quantum-chemical calculations; Cis/trans-isomerism; Conformational stability; Synthesis; Spectroscopy
1. Introduction
the HOMO and LUMO energies and increases the
electron affinity and thus improves the efficiency of
electron injection into the compound. During the
synthesis of CTMTE we noted that its behaviour
towards E/Z isomerization greatly differed from that
of BMTE, which prompted us to investigate the matter
further.
We report here on the half-life and equilibrium
times of their E/Z isomerization reactions. Further-
more, ab initio calculations were performed to esti-
mate their lowest energy conformers and to correlate
their experimentally observed isomerization behavior
with the calculated energy differences of the Z- and E-
forms. The resulting model will be used to predict the
isomerization behaviour of 1-(3-methoxy-2-thienyl)-
2-(2-thienyl)ethene (MTE, see Fig. 1), although this
compound has not yet been synthesized.
In our ongoing research into the semiconductor appli-
cations of arylene vinylene oligomers [1–5], we synthe-
sized 1,2-bis(3-methoxy-2-thienyl)ethene (BMTE) and
1-cyano-1-(2-thienyl)-2-(3-methoxy-2-thienyl)ethene
(CTMTE) shown in Fig. 1. Methoxy groups tend to
increase the solubility and hence the processibility of
such compounds. Unfortunately, the electron donating
properties of alkoxy groups also enhance their sensi-
tivity towards photo-oxidative degradation [6–8]. To
mitigate this, an electron withdrawing nitrile group
was introduced on the ethenediyl spacer–the site of
the photo-oxidation; in addition, the CN group lowers
* Corresponding author. Tel.: ϩ 32-3-820-2349; fax: ϩ 32-3-
820-2310.
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00180-5

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F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
Fig. 1. Observed Z/E-isomerization behaviour of BMTE (upper row) and CTMTE (middle row) in solution, and predicted Z/E isomerization
behaviour of MTE.
2. Synthesis of the compounds
described by Elsenbaumer et al. [9,10] using trans-
dichloro[1,3-bis(diphenylphosphino)propane]nick-
el(II), NiCl2(dppp), as the catalytic complex. Fig. 2
shows the path followed. Commercially available
E-1,2-bis(3-methoxy-2-thienyl)ethene (3; BMTE)
was synthesized via the Grignard cross-coupling as
Fig. 2. Synthesis of E-1,2-bis(3-methoxy-2-thienyl)ethene (3), BMTE.

F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
89
Table 1
exactly 0.5 eq. of E-1,2-dichloroethene removes side
product (4), run 5. Switching to a one-pot reaction and
reverse addition of the reagents to avoid poisening of
NiCl2(dppp) by E-1,2-dichloroethene (run 6), leads to
a further increase in yield to 61% (lit. 45% [10]).
Z-BMTE was synthesized analogously by adding
commercially available Z-1,2-dichloroethene in the
catalytic step. Due to its extremely fast transformation
into E-BMTE neither physical nor spectroscopic data
could be obtained.
Composition of the reaction products of the Grignard coupling (%)
according to GC; amount of catalyst ncat (mol%), overall yield (%)
and reaction temperature T (ЊC) are given. For experimental details,
see text
Run ncat (mol%) Yield (%) %(2) %(3) %(4) T (ЊC)
1
2
3
4
5
6
1.5
1.7
1.9
1.9
1.9
1.9
27
23
32
36
45
61
13
41
5
0
0
72
52
89
91
100
100
15
7
5
9
0
20
20
20
30
30
30
CTMTE (7) is most conveniently synthesized by
a Knoevenagel condensation, as depicted in Fig. 3.
Starting from commercially available 3-methoxy-
thiophene, the required 3-methoxy-2-thiopheneal-
dehyde (5) was prepared by a Feringa reaction
[12]. Using BuLi, the 2-position of 3-methoxythio-
phene was selectively lithiated and subsequently
formylated using DMF. The light-yellow crystals
were analysed with GC-MS and seemed to be
pure; Normal Phase HPLC (NP-HPLC), however,
demonstrated the presence of non-volatile impuri-
ties which could not be removed by recrystalliza-
tion, but sublimation of the aldehyde yielded
extremely pure white crystals. The step to
CTMTE (7) was carried out by condensation of
3-methoxy-2-thiophenealdehyde (5) and commer-
cially available 2-thienylacetonitrile (6) using
ethanolate as base in ethanol [13]. The product
was recrystallized, but NP-HPLC demonstrated
the presence of starting-products; again (7) could
easily be isolated by sublimation resulting in
bright yellow, needle shaped crystals. Attempts
to perform the condensation using tert-butoxide
in tert-butanol [14] or piperidine in pyridine [15]
both failed; UV–Vis analysis of the reaction
products did not show any trace of the desired
condensation product.
0
0
3-methoxythiophene (1) was coupled to E-1,2-
dichloroethene using the Grignard coupling,
yielding E-BMTE (3). We modified Elsenbaumer’s
procedure slightly to increase yield and purity of
the products.
Initial experiments indicated that the yield and
composition of the reaction products are extremely
dependent on the amount of catalyst used: the amount
proposed in Ref. [10] ( ^ 0.2 mol%) did not produce
solid reaction products, whereas larger amounts did.
At the same time, we identified two important side
products namely 3,30-dimethoxy-2,20-bithiophene
(2), formed by the catalyst, and E-1-chloro-2-(3-
methoxy-2-thienyl)ethene (4), formed after one
cycle. A study of the catalyst’s mechanism [11] indi-
cated that the amount of side product (2) is equimolar
to the amount of the added catalyst, which precludes
the use of a large amount of catalytic complex.
The optimal amount of catalyst was determined by
assessing the composition of the reaction products by
gas chromatography. From the results given in Table
1, it can be seen that the amount of catalyst as well as
the temperature have an influence on the reaction.
Raising the temperature by 10ЊC results (run 4) in
the disappearance of side product (2). The large
amounts of side products in runs 1, 2 and 3 are due
to a slight excess of dichloroethene. Addition of
RP-HPLC (Reversed Phase HPLC) analysis of the
purified CTMTE indicated the presence of two
components in a mixture which were identified by
Fig. 3. Synthesis of 1-cyano-1-(2-thienyl)-2-(3-methoxy-2-thienyl)ethene (7), CTMTE.

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F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
Fig. 4. Atomic numbering of BMTE (left), CTMTE (right) and MTE (bottom). The dotted lines indicate the intramolecular hydrogen
interaction (see text).
LC-MS as the E- and Z-isomers of (7). Interest-
ingly, the NMR spectrum of CTMTE in deuterated
methanol showed the typical doubling of peaks
due to the presence of two isomers, but the
solid state FT-IR spectrum of (7) (KBr matrix)
displayed a simplicity usually associated with
pure compounds and so suggested the presence
of only one isomer. We assumed that CTMTE
crystallizes in only one conformation—most prob-
ably the E-form—and that in solution, a dynamic
equilibrium is established between the two
isomers; this was later corroborated by our
isomerization experiments. The IR spectrum
shows a relatively intense signal at 1580 cmϪ1
for the CyC bond, indicating conjugation between
the thiophene rings and the nitrile group and thus
the planarity of the molecule. The signal of the
nitrile group itself corroborates this, since it is
found at a lower wavenumber (2208 cmϪ1) than
usual. The high lmax (381 nm) value of the UV-
absorption band further reinforces the planarity
statement.
3. Computational methods and conformational
study of the compounds
Ab initio calculations were performed using
Pulay’s gradient method [16,17] incorporated in the
program BRABO for large molecules [18,19] with the
6-31Gء basis set [20–22]. Geometries are calculated
with complete relaxation until convergence was
reached; for convergence criteria see [23]. The atomic
numbering of the compounds is given in Fig. 4. To
keep the calculations within manageable bounds the
results describe conformations characterized by
methoxy groups eclipsing the C(3)–C(4) bond of the
thiophene ring on which they are substituted, i.e. the
torsion angle w(C(4)–C(3)–O–CH3) ˆ 0Њ. To further
reduce computer time, the calculations were
performed initially with planar heavy atom skeletons.
These properties are in line with previous X-ray single
crystal analyses of E-BMTE and similar compounds
[24,25]. Then, in conformity with the Cahn–Ingold–
Prelog priority rules and IUPAC recommendations
[26] the torsion angles wA(S(1A)–C(2A)–

F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
91
Fig. 5. Conformations of BMTE. Top row presents numbering, IUPAC names and relative energies (kcal/mol) of the planar conformations of
the E-isomer, bottom row those of the Z-isomer.
C(6)yC(7)) and wB(S(1B)–C(2B)–C(7)yC(6)) char-
acterize the remaining configurations. They are shown
in Figs. 5 and 6 for BMTE and CTMTE, respectively,
together with their IUPAC names and relative ener-
gies. It should be noted that the methoxy substituted
side (wA) is named first, and the other side (wB) named
second. In case of the symmetrically substituted
BMTE the [ap,sp] conformers equal the [sp,ap]
conformers.
To check the assumption of skeleton planarity the
torsion potentials around the C(2A)–C(6) and C(2B)–
C(7) bond were calculated. Going from [sp,sp] to
[ap,sp] in E-BMTE we found a barrier height of
5.6 kcal/mol near the orthogonal form (wA ˆ 90Њ,
Fig. 6. Conformations of CTMTE. Top row presents numbering, IUPAC names and relative energies (kcal/mol) of the planar conformations of
the E-isomer, bottom row those of the Z-isomer.

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F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
Table 2
˚
Selected calculated distances (A) in the lowest energy conformations of Z- and E-isomers of BMTE, CTMTE and MTE, and in the maximum
energy forms Z[sp,sp] in the rotation around C(2B)–C(7). The last colums gives the energy difference E ˆ E(Z[sp,sp]) Ϫ E(Z[sp,ort]) (kcal/mol)
E-isomer [sp,sp]
Z-isomer [sp,ort]
Top at Z[sp,sp]
E-isomer [sp,sp]
Z-isomer [sp,ort]
Top at Z[sp,sp]
E
r[H3CO(HC(6)]
r[C(2B)–C(7)]
BMTEa
CTMTE
MTE
2.517
2.377
2.512
2.440
2.403
2.427
2.336
2.256
2.329
1.456
1.474
1.460
1.459
1.484
1.460
1.460
1.476
1.464
3.18
6.68
4.00
a C(2B)–C(7) and C(2A)–C(6) are equal in BMTE.
wB ˆ 0Њ): this supports the planarity of the heavy atom
skeleton.
meric stabilization; indeed, releasing one 3-methoxy-
2-thienyl ring of Z-BMTE led to an energy minimum
at wA(S(1A)–C(2A)–C(6)yC(7)) ˆ 109Њ. This ortho-
gonal conformation is 3.18 kcal/mol below the planar
[sp,sp] form, which here represents an energy
maximum.
Similarly, releasing in Z-CTMTE the unsubstituted
2-thienyl ring one energy minimum was found along
the wB-axis at wB(S(1B)–C(2B)–C(7)yC(6)) ˆ 99Њ
with an energy of 6.68 kcal/mol below the planar
[sp,sp] form. Again, the planar [sp,sp] and [sp,ap]
forms are energy maxima, and the lowest energy
form is an orthogonal conformer, which we name
[sp,ort].
Similar results support the planarity of the E-
CTMTE rotamers. The corresponding barrier heights
are 5.3 kcal/mol for rotation around C(2A)–C(6)
([sp,sp] ! [ap,sp]) and 2.4 kcal/mol for rotation
around C(2B)–C(7) ([sp,sp] ! [sp,ap]). The differ-
ence between these two apparently similar torsions
are attributed to the existence of an intramolecular
hydrogen interaction between a H-atom of the ethe-
nediyl spacer and the O-atom of the methoxy group, in
other words an attractive CH–n(O) interaction[27], as
shown by the dotted lines in Fig. 4. Breaking this
interaction to carry out the torsion requires energy,
hence the torsion of the unsubstituted ring B is the
preferred action. The presence of the CH–n(O) inter-
action in a five-membered ring (rather than in a six-
membered ring [27]), together with the presence of an
In addition we observed the unrotated part of Z-
BMTE [sp,ort] and that of Z-CTMTE [sp,ort] to
remain virtually planar. Moreover, for the four
…
isomers involved, the H3CO HC(6) distances of
all
transoid
C(3A)yC(2A)–C(6)yC(7)–
the lowest energy conformations (see Table 2) are
˚
well below the sum (2.60 A) of the Van der Waals
C(2B)yC(3B) system is considered to cause the
preference of [sp,xx] over [ap,xx] confomers in
these compounds.
Turning to the Z-isomers, we noted significant
steric hindrance even in the energetically lowest,
planar [sp,sp] conformers. Twisting one ring out of
the molecular plane might overcome the loss of meso-
˚
˚
radii of H (1.20 A) and O (1.40 A). This corroborates
the presence of a CH–n(O) interaction between a 3-
methoxy-2-thienyl ring and the methylidene unit,
other than conjugation.
Based on the results obtained sofar we calculated
for MTE (see Fig. 4) only the most important forms.
Table 3
Results of the isomerization experiments; t1/2 (min) gives the time needed to reach the 50/50 mixture and teq (min) gives the time needed to reach
equilibrium (97% Z-CTMTE/3% E-CTMTE in methanol)
Solvent
Type of glassware
t1/2
teq
Methanol
Methanol
Hexane/ethyl acetate (95/5; v/v)
Hexane/ethyl acetate (95/5; v/v)
Transparent
Amber
Transparent
Amber
36
82
46
520
135
Ͼ 1440
Ͼ 120
ӷ

F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
93
Fig. 7. Isomerization of CTMTE (top row), MTE (middle row) and BMTE (bottom row): the energies, given in parentheses, are relative to the
starting-structures (kcal/mol), dipole moments m in Debye units.
Those are for the E-configuration the lowest energy
conformation, E[sp,sp], and for the Z-configuration
the lowest energy conformation, Z[sp,ort], and the
maximum energy form at Z[sp,sp]. MTE is a struc-
tural intermediate between BMTE and CTMTE,
which shows (see Table 2) in the energy differences
between Z[sp,sp] and Z[sp,ort], as well as in some key
distances.
the time (teq) needed to reach equilibrium at 97% (Z)
and 3% (E).
A methanol solution of sublimed CTMTE made
and kept in the dark contained only one isomer,
which absorbed in the UV–Vis at lmax ˆ 381 nm
(e ˆ 3.0 × 104). After approximately 3 h of illumina-
tion in daylight, the solution contained 97% of the
other isomer, which absorbed at lmax ˆ 345 nm (e ˆ
1.9 × 104). These observations unequivocally prove
that the solid state contains exclusively the E-isomer
in a conformation with a large mesomeric space, most
likely the planar[sp,sp]. The illuminated solution then
contains predominantly the Z-isomer in conforma-
tion(s) with a smaller mesomeric space, consistent
with the [sp,ort] conformation. Also the experiments
showed the Z-isomer to be about 2 kcal/mol more
stable than the E-isomer; this must be attributed to
solvent effects in the polar methanol. Sublimation,
4. Z/E-isomerization study of CTMTE and BMTE
We monitored by reverse phase HPLC (RP-HPLC)
the isomerization of CTMTE under various conditions
to study the influence of daylight on this transforma-
tion, as well as to investigate if we could gain access
to the separate isomers. Table 3 gives the time (t1/2)
needed to reach the 50/50 mixture of both isomers and

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F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
being performed at relatively low temperature, allows
the molecule to assume in the gas phase its energeti-
cally most stable form, E[sp,sp], which is retained in
the crystal. In amber colored glassware the isomeriza-
tion in methanol slows down considerably, but does
not stop: after 24 h the mixture consists of 77% Z- and
23% E-isomer.
From these experiments and the summary of rele-
vant calculated data given in Fig. 7 and Table 2, a
model was developed from which the E/Z preference
in CTMTE can be envisaged. Starting from the
E[sp,sp], the lowest energy conformation of the E-
isomer, the reaction proceeds to a intermediary
state, which could be approached in shape and energy
by the planar Z[sp,sp] form, and finally ring B rotates
to the stable conformation Z[sp,ort]. The end product
has only a small energetic disadvantage, but an
entropic advantage over the starting-product. This
together with the relatively large energy difference
in dipole moments (Fig. 7) indicates that in polar
solvents only CTMTE may give a significant percen-
tage of Z-isomer in the equilibrium mixture. In BMTE
and MTE stable E-isomers will be preferred.
to an almost orthogonal position destroys the conjuga-
tion that existed in the E-configuration between ring B
and the remainder of the system. In other words the
conjugation length is significantly decreased by
excluding ring B. Then, the small difference between
BMTE and MTE can only be attributed to a small
extra destabilization in the end product of BMTE
…
due to the loss of one CH n(OCH3) interaction.
The large drop in CTMTE suggests that excluding
ring B in this case causes a relatively small loss in
conjugation. The conjugation of ring B to the
remainder of the p-system can be estimated by
comparing the C(2B)–C(7) bond lengths in the three
compounds (see Table 2). It reveals that the consid-
ered bond length in CTMTE is consistently longer
than in the other compounds, which strongly suggests
lower bond order and thus lower conjugation. We
suggest that the effective conjugation length in
CTMTE extends over ring A, the olefinic link and
stops at the nitrile group. This would also explain
the relatively small difference of 36 nm between the
UV–Vis absorption maximum of E-CTMTE (lmax
ˆ
381 nm) and that of Z-CTMTE (lmax ˆ 345 nm) as
compared to a difference of 68 nm between the
absorption maximum of 1,2-bis(2-thienyl)ethene
(lmax ˆ 343 nm [28]) and of 2-vinylthiophene
(lmax ˆ 275 nm [29]).
As might be expected from the calculated dipole
moments, the isomerization rate decreases further in
a solvent (mixure) of low polarity, e.g. hexane/ethyl
acetate (95/5; v/v). Thus in amber coloured glassware
reactions (e.g. lithiations) can be performed selec-
tively on E-CTMTE in apolar solvents.
We conclude that the structural changes when
going from BMTE to CTMTE lead to the following.
The removal of the methoxy group eliminates a stabi-
The calculated energy differences between the
isomers of BMTE and of MTE proposes an explana-
tion for the fact that E-BMTE is extremely stable and
does not isomerize, and suggests similar behaviour in
MTE. Nevertheless, these energy factors do not suffi-
ciently explain the influence of the nitrile group.
Fig. 7 indicates that upon isomerization of the E-
compounds the step from the starting-product to the
intermediary state, Z[sp,sp], demands an energy fairly
similar (about 9 kcal/mol) for the three compounds
considered. In contrast, going from transition state
to end product, the energy drop originating from
twisting ring B out of the molecular plane, puts
CTMTE apart from BMTE and MTE. Since the
drop is mainly due to a decrease in steric hindrance,
counteracted by a decrease in mesomeric stabiliza-
tion, this suggests that the loss of conjugation is less
for CTMTE than for BMTE and MTE. The small drop
in the latter compounds suggests that twisting ring B
…
lizing intramolecular CH n(OCH3) interaction and
the nitrile functionality simplifies torsion around
C(2B)–C(7) by lowering the electron density and
bond order. These factors are powerfull enough to
rationalize why Z-CTMTE can twist easily in polar
solvents into a favourable orthogonal conformation.
BMTE on the other hand is kept in the stable planar E-
…
form by two intramolecular CH n(OCH3) interac-
tions and its transformation into the Z-isomer can
only be accomplished at great cost.
5. Experimental
The syntheses were carried out using Schlenk tech-
niques. RP-HPLC analyses were performed on a
Waters Nova-Pak᭨ C18 column, particle size 4 mm,
i.d. 4.6 mm, length 250 mm with methanol/water (75/

F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
95
25; v/v) as the mobile phase. NP-HPLC analyses were
performed on an Alltech AdsorboSphere᭨ SI column,
particle size 10 mm, i.d. 4.6 mm, length 250 mm with
different mobile phases depending on the analyte.
Chemical shifts d are in ppm with respect to TMS.
added. The temperature is raised to room temperature
and the mixture is stirred for 30 min. The reaction
mixture is then poured into a mixture of 100 ml of
36% HCl and 850 ml of water and vigorously stirred.
Using a saturated NaHCO3 solution the pH is raised to
6. The water phase is separated and washed 7 times
with ether. The combined organic phases are dried,
the solvent is evaporated and the resulting solids puri-
fied by sublimation at 50ЊC/0.2 mmHg, yielding 3.0 g
5.1. E-1,2-bis(3-methoxy-2-thienyl)ethene (3), BMTE
An amount (27.5 ml) of 1.6 M BuLi solution in
hexane (0.044 mol) was added dropwise to a solution
of 5 g of 3-methoxythiofeen (0.044 mol) in 100 ml
freshly dried diethyl ether and the resulting mixture
is stirred for 2 h. Meanwhile, 1.07 g of Mg metal
(0.044 mol) and 3.8 ml of 1,2-dibromoethane
(0.044 mol) are mixed in 40 ml of ether and this
mixture is refluxed until the metal is completely
dissolved. The second mixture is then added at once
to the first and the new mixture is stirred for 1 h. To
the resulting Grignard reagent are added consecu-
tively 450 mg of NiCl2(dppp) (0.830 mmol), 2 ml of
E-1,2-dichloroethene (0.022 mol) and 50 ml of
diethyl ether. The temperature is raised to 30ЊC and
kept at 30ЊC during the night. The organic phase is
washed thrice with 50 ml of water, the solvent is
evaporated, the residue precipitated in hexane, filtered
and dried; the yield is 3.4 g (61%). M.p. ˆ 121–
123ЊC. UV–Vis (EtOEt): lmax ˆ 361 nm (log e ˆ
4.3). 1H NMR (CDCl3): CH3O–(3.90; s), –CHy
(6.95; s), H(5) (7.00; d;5.5 Hz), H(4) (6.79; d;
5.5 Hz). 13C NMR (CDCl3): CH3O– (58.8), –CHy
(116.1), C(2) (120.9), C(3) (154.7), C(4) (116.8),
C(5) (121.5).
1
(0.021 mol; 48%) of product. M.p. ˆ 84ЊC. H NMR
(CDCl3): CH3O–(3.90; s), –CHO (9.90; d; 1.2 Hz), a-
H (7.64; dd; 1.2 and 5.5 Hz), b-H (6.88; d; 5.5 Hz).
13C NMR (CDCl3): CH3O–(58.9), –CHO (181.1),
ring-C2 (115.9), ring-C3 (165.1), ring-C4 (121.4),
ring-C5 (135.0).
5.4. E-1-cyano-1-(2-thienyl)-2-(3-methoxy-2-
thienyl)ethene (7), CTMTE
One gram of 3-methoxy-2-thiophenealdehyde
(0.007 mol) (5) and 0.9 g of 2-thienylacetonitrile
(0.007 mol) (6) are dissolved in 20 ml of freshly
dried ethanol. In a separate vessel 0.2 g of Na metal
(0.009 mol) is dissolved in 15 ml of dry ethanol and
this solution is added dropwise to the former; a yellow
precipitate is immediately formed. The solvent is
boiled for an instant and then immediately cooled.
The resulting precipitate is filtered off and the filtrate
is kept and yields another precipitate. CTMTE is puri-
fied by sublimation first at 50ЊC/0.2 mmHg, to remove
the aldehyde (5), and then at 110ЊC/0.1 mmHg to
isolate CTMTE (7); the yield is 0.9 g (52%). M.p. ˆ
110–112ЊC. UV–Vis (CH3OH): lmax(E) ˆ 381 nm
(log e ˆ 4.5), lmax(Z) ˆ 345 nm (log e ˆ 4.3). MS
(ES): m/z ˆ 248 [MHϩ], 233[MH ϩ –CH3], 221, 216,
172, 97 [C5H5S ϩ ] (base peak). IR (KBr): n ˆ
1239 cmϪ1[n (COC)], 1435, 1462, 1511, 1527[g(thio-
phene ring)], 1580[n(CyC)], 2208[n(CxN)],
2855[n(CH3)], 3020[n(–CHy)], 3107[n(CH)(thio-
5.2. Z-1,2-bis(3-methoxy-2-thienyl)ethene
The Z-form of BMTE was synthesized by adding
2 ml of Z-1,2-dichloroethene (0.022 mol) in the final
step, but could not be isolated.
5.3. 3-methoxy-2-thiophenealdehyde (5)
1
phene ring)]. H NMR ((CD3)2CO): E: CH3O–(4.04;
At room temperature, 38 ml of a 1.6 M BuLi solu-
tion in hexane is added to a mixture of 6.2 g of
TMEDA
(0.053 mol) and 5 g of 3-methoxythiophene
(0.044 mol) in 30 ml of dry hexane. After refluxing
for 30 min, 60 ml of dry THF are added and the
resulting mixture is cooled to Ϫ 40ЊC. During a
period of 10 min, 6 ml of DMF (0.078 mol) are
s), –CHy (7.71;d;0.5 Hz), H(4A) (7.14; d; 5.6 Hz),
H(5A) (7.76; dd; 5.6 and 0.5 Hz), H(3B) (7.28; dd;
3.7 and 1.2 Hz), H(4B) (7.12; dd; 5.1 and 3.7 Hz),
H(5B) (7.48; dd; 5.1 and 1.2 Hz); Z: CH3O–(4.00;
s), –CHy (7.66; d; Hz), H(4A) (7.02;d;5.6 Hz),
H(5A) (7.74; dd; 5.6 and 0.5 Hz), H(3B) (7.26; dd;
3.7 and 1.2 Hz), H(4B) (7.19; dd; 5.1 and 3.7 Hz),
H(5B) (7.58; dd; 5.1 and 1.2 Hz). 13C NMR
(N,N,N0,N0-tetramethylethylenediamine)

96
F. Blockhuys et al. / Journal of Molecular Structure 485–486 (1999) 87–96
[9] K.Y. Jen, H. Eckhardt, T.R. Jow, L.W. Shacklette, R.L. Else-
nbaumer, J. Chem. Soc. Chem.: Commun. (1988) 215.
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88 00945 A1, 1988, 38.
[11] K. Tamao, K. Sumitani, Y. Kiso, M. Zembayashi, A. Fujioka,
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((CD3)2CO): E: CH3O–(58.7), –CHy (129.1), CCN
(98.4), CN (117.2), C(2A) (115.0), C(3A) (161.6),
C(4A) (115.7), C(5A) (130.0), C(2B) (139.5), C(3B)
(125.4), C(4B) (128.2), C(5B) (125.7); Z: CH3O–
(58.6), –CHy (136.1), CCN (97.6), CN (119.6),
C(2A) (114.1), C(3A) (161.9), C(4A) (115.0), C(5A)
(129.0), C(2B) (133.3), C(3B) (129.8), C(4B) (128.0),
C(5B) (131.8).
Acknowledgements
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C.V.A. thanks the Flemish Science Foundation
(F.W.O.) for his appointment as “onderzoeksdirec-
teur”. The authors gratefully acknowledge support
by the University of Antwerp under Grant GOA-
BOF-UA No. 23.
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