Redox responsive molecular tweezers with tetrathiafulvalene units: synthesis, electrochemistry, and binding properties

Article abstract of DOI:10.1016/j.tet.2009.10.052

Several new molecular tweezers with tetrathiafulvalene (TTF) arms as well as mono-TTF derivatives bearing 3,5-di-tert-butylbenzylthio groups to provide enhanced solubility were prepared starting from a bis-cyanoethyl-protected tetrathiafulvalene derivativ

Full text of DOI:10.1016/j.tet.2009.10.052

Tetrahedron 65 (2009) 10348–10354
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Redox responsive molecular tweezers with tetrathiafulvalene units: synthesis,
electrochemistry, and binding properties
a
b
a
a,
*
´
´
Maciej Skibinski , Rafael Gomez , Enno Lork , Vladimir A. Azov
a University of Bremen, Department of Chemistry, Leobener Str. NW 2C, D-28359 Bremen, Germany
b
´
Universidad Complutense, Departamento de Qu´ımica Organica, Avda. Complutense s/n, E-28040 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new molecular tweezers with tetrathiafulvalene (TTF) arms as well as mono-TTF derivatives
bearing 3,5-di-tert-butylbenzylthio groups to provide enhanced solubility were prepared starting from
a bis-cyanoethyl-protected tetrathiafulvalene derivative. The X-ray crystallographic analysis of 3 and 7a
showed highly distorted TTF groups and absence of close TTF–TTF contacts in the crystalline state.
Comparative cyclic voltammetry (CV) measurements demonstrated that through space distance-de-
pendent TTF–TTF interactions take place in the TTF-containing molecular tweezers, leading to electronic
Received 17 September 2009
Received in revised form 14 October 2009
Accepted 14 October 2009
Available online 31 October 2009
Keywords:
ꢀ
pairing with formation of mixed valence [TTF]þ2 species and splitting of the first oxidation wave. TTF-
Tetrathiafulvalenes
Molecular tweezers
Cyclic voltammetry
X-ray
containing molecular tweezers were successfully tested as receptors for several electron-deficient
substances.
Ó 2009 Elsevier Ltd. All rights reserved.
Molecular recognition
1. Introduction
only a few TTF-containing rigid molecular tweezers13 or structures
preferring to adopt a tweezers-like conformation in solid state
The discovery of organic conductors1 and charge–transfer (CT)
complexes2 based on tetrathiafulvalenes (TTFs) triggered intensive
studies of this heterocyclic system. Initial research efforts focused
mainly on applications in the field of materials chemistry, making
tetrathiafulvalene one of the most important building blocks for
organic molecular electronics.3 Later, due to their electron-donat-
ing properties and possibility of reversible stepwise oxidation
forming stable radical cation and dication species, tetrathiafulva-
lenes found use in macromolecular,4 supramolecular,5 and other
areas of chemistry. The versatile chemistry of tetrathiafulvalenes6
allows the construction of various functional architectures, such as
TTF-based chiral dimers,7 dendrimers,8 cyclophanes, and cages.9 In
interlocked supramolecular systems, such as catenanes and rotax-
anes, tetrathiafulvalenes efficiently play a role as molecular motors,
triggering positional displacement of supramolecular components
upon oxidation/reduction.10
(either neutral14a or oxidized14b,c) have been reported so far. Very
recently, several TTF-containing calixpyrroles15 were shown to be
efficient guest binders for electron-deficient guests, such as tetra-
cyanoquinodimethane and derivatives of 2,4,7-trinitro-9-fluo-
renylidenemalononitrile, due to the electron-donating properties
of tetrathiafulvalene units.
2. Results and discussion
2.1. Molecular design
In this paper we describe the preparation and characterization of
a series of highly soluble tetrathiafulvalene derivatives bearing 3,5-
di-tert-butylbenzylthio groups. Several of these compounds have
structural features characteristic of molecular tweezers, with tetra-
thiafulvalene arms attached to benzene and naphthalene scaffolds.
This design is similar to the one used before for the construction of
molecular tweezers built on the basis of the dioxa[2.2]orthocyclo-
phane12 or dithia[2.2]orthocyclophane skeletons.16 Interestingly in
these systems, the introduction of redox active TTF groups in mo-
lecular tweezers opens the possibility to control their conforma-
tional and binding preferences by redox stimuli. Previously we have
shown16 that molecular tweezers with two TTF arms covalently
linked to a benzene backbone via a dithia[2.2]orthocyclophane
spacer (Fig. 1) prefer the closed conformation in neutral and mono-
Non-cyclic compounds with cavities, commonly called molec-
ular tweezers or clips,11,12 have been already successfully employed
as molecular hosts for binding of flat molecules in solution as well
as in solid state, typically forming sandwich-like host–guest com-
plexes with them. Despite the broad development of TTF chemistry,
* Corresponding author. Tel.: þ49 421 218 3667; fax: þ49 421 218 3720.
E-mail address: vazov@uni-bremen.de (V.A. Azov).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.10.052

M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
10349
2,3-bis(2-cyanoethylthio)-tetrathiafulvalene 3 in a phosphite-medi-
ated coupling with the oxo-derivative 4. As expected, compound 3
showed good solubility in various non-polar organic solvents (al-
kanes, chloroalkanes, and aromatics). 2-Cyanoethylthio-protected
tetrathiafulvalene 3 can serve as a versatile building block for the
preparation of various highly soluble TTF derivatives: by treatment
with one or two equivalents of caesium hydroxide mono- or dithio-
lates can be generated and subsequently alkylated.19
Symmetric tetrakis(3,5-di-tert-butylbenzylthio)-tetrathiafulva-
lene 5 was prepared in two steps from 1 by transchalcogenation
with mercuric acetate into the corresponding oxo-derivative 6 fol-
lowed by phosphite-mediated homo-coupling to yield 5. Compound
5 displayed almost unlimited solubility in alkanes, aromatic, and
chlorinated solvents, but crystallized in a fridge from concentrated
(60–80%) hexane solution. NMR experiments in CDCl3 did not imply
any significant self-aggregation of 5 up to ca. 10% concentration.
Tetrathiafulvalene derivatives 7a,b and TTF-containing molecu-
lar tweezers 8a–c (Scheme 2) were prepared by reaction of TTF
dithiolates generated in situ from 2,3-bis(2-cyanoethylthio)te-
trathiafulvalenes 3 and aromatic methylene bromides 9a, 9b,20
10a,21 10b,20 and 10c,22 following the previously reported synthetic
protocols.16,23 Purification by column chromatography in dichloro-
methane/petroleum ether mixtures afforded pure products in 63–
81% yields. The new TTF derivatives are bright yellow (sometimes
with orange hint) solids with high solubility in non-polar organic
solvents, such as dichloromethane, toluene, and even alkanes.
All new tetrathiafulvalene derivatives were characterized by
means of NMR (1H and 13C), MS (high resolution EI, ESI, or MALDI),
and UV/Vis spectroscopy. In the 1H NMR spectra of compounds 7a,b
and 8a–c the methylene protons of the eight-membered dithia
rings appeared as a singlet, suggesting fast conformational equili-
bration at room temperature. The UV/Vis spectra (CH2Cl2, 293 K) of
the new tetrathiafulvalenes featured the typical absorption pattern
for tetrathio-substituted TTF derivatives23,24 with absorption bands
at lmax ca. 300 and 330 nm, as well as a shoulder at ca. 400 nm.
Figure 1. Selected low potential energy conformations with approximate dimensions
of molecular tweezers with benzene scaffold. Dimensions are based on molecular
modeling results.
oxidized states, but undergo conformational transition to the open
conformation upon complete oxidation due to the Coulombic re-
pulsion between the two doubly charged TTF moieties. This obser-
vation has paved the way to the employment of such architectures as
appealing systems for host–guest binding: an affinity to electron-
deficient guests, which is expected in the non-oxidized state, can be
turned off by oxidation of the TTF-containing host.
In this work, we have optimized the solubility and geometry of
TTF-containing molecular tweezers for binding electron-deficient
flat molecular guests as a step further in our research on TTF-
containing systems. Thus, receptors with benzene and naphthalene
backbones and accordingly different TTF–TTF distances and prop-
erties have been prepared and investigated.
2.3. Crystallographic study
2.2. Synthesis and properties
X-ray structures of the TTF derivatives 3 and 7a showed signifi-
cantly non-planar TTF moieties (Figs. 2 and 3) featuring the boat-like
distortions. Their central C2S4 groups are planar, whereas both five-
membered rings are folded along their S/S vectors and tilted
inwards toward each other. In compound 3, the dihedral angles be-
tween the least-squared planes A (S1, S2, C1, C2, S3, S4) and B (S1, S2,
C3, C4) and A and C (S3, S4, C5, C6) are ca. 30.83ꢁ and 12.58ꢁ, re-
spectively. The distortion of the TTF moiety is even more remarkable
in 7a, where the dihedral angles between the least-squared planes A
(S1, S1i, C1, C2, S2, S2i) and B (S1, S1i, C3, C3i) and A and C (S2, S2i, C4,
C4i) reach ca. 27.57ꢁ and 30.44ꢁ, respectively. Such a significant dis-
tortion is caused only by weak intermolecular interactions with
other molecules in the crystal lattice and not bycovalent bonding, for
The initial goal of our study was to render high solubility to
thioalkyl TTF derivatives in non-polar organic solvents, yet avoiding
the use of long alkyl chains as solubilizing groups due to their po-
tential interference with binding substrates. Thus, we have chosen
3,5-di-tert-butylbenzylthio moiety as solubilizing group, being in-
spired by the remarkable application records of analogous 3,5-di-
tert-butylphenyl groups for the solubilization of otherwise low
soluble planar systems, such as porphyrin derivatives.17
The synthesis of TTF-containing tweezers started with the prep-
aration of thione 1 from 3,5-di-tert-butylbenzylbromide17a and
bis(tetraethylammonium)-bis(1,3-dithiole-2-thione-4,5-dithiol)zin-
cate 218 (Scheme 1). Thione 1 was then used for the synthesis of
Scheme 1. Synthesis of 3,5-di(tert-butylbenzylthio)tetrathiafulvalene derivatives 3 and 5.

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M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
Scheme 2. Synthesis of 3,5-di(tert-butylbenzylthio)tetrathiafulvalene derivatives 7 and 8.
Figure 2. ORTEP plot of 3 (50% probability of thermal ellipsoids). Minor positions of
the disordered atoms are omitted.
Figure 3. ORTEP plot of 7a (50% probability of thermal ellipsoids).
example, due to incorporation of the TTF moiety into a cyclophane
backbone,25 demonstrating remarkable flexibility of the TTF group.
The presence of the bulky 3,5-di-tert-butylbenzyl groups in both
structures did not give a possibility for the normally observed TTF–
TTFstacking, inwhichplanarTTFmolecules are separated byca. 3.6 Å
from each other. The long-range lattice order is governed by the
weak dispersion interactions involving tert-butyl groups and aro-
matic rings, which separate TTF units from each other. (Figs. 4 and 5)
No close contacts between the sulfur atoms could be observed.
Thus, analysis of the X-ray structures evidenced notable flexi-
bility of the TTF moiety, which allows adopting their conformation
to make it suitable for tight packing in the solid state. The same
flexibility can be also advantageous for molecular recognition
processes, allowing the receptor to adopt the best conformation for
the guest binding, as well as close TTF–TTF contacts in compounds
with the benzene scaffold.
electrochemical processes on the cathodic scan, the first one at
E11/2¼0.30–0.40 V (vs Ag/AgCl, Table 1) leading to the TTF radical
cation, and the second one at E21/2¼0.60–0.70 V giving the dica-
tion, as it is known from literature.3 On the other hand, the cyclic
voltammograms of molecular tweezers 8a and 8b, which are
constructed on the benzene scaffold, feature a splitting of the
first oxidation wave into two relatively well-defined waves with
D
E11/2z0.14 V (Table 1 and Fig. 6), indicating the one-electron
sequential formation of the mono-(radical cation) and the bis-
(radical cation).26
This fact suggests the electronic stabilization of the first-formed
radical cation by the
p electrons of the neighboring TTF moiety, thus
leading to a reduction of the potential with formation of mixed
ꢀ
valence [TTF]þ2 species, and to an increase of the second mono-
ꢀ
electronic oxidation step to [TTF]22(þ ) in molecular tweezers 8a and
2.4. Electrochemistry
8b, as a result of the proximity of the first-formed radical cation. In
contrast, molecular tweezer 8c, built onto the naphthalene scaffold,
does not show any splitting of the first oxidation wave (Fig. 6) but an
electrochemical behavior similar to tetraarylthiotetrathiafulvalene
The cyclic voltammograms of mono-TTF derivatives show the
classical redox behavior of TTF, displaying two quasi-reversible

M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
10351
Figure 6. Cyclic voltammograms of molecular tweezers 8a, 8b and 8c. (CH2Cl2/0.1 M
Bu4NPF6, scan rate 200 mV/s).
Figure 4. Crystal packing of 3, projection along the a axis. Hydrogen atoms are omitted
for clarity.
The reversible two-electron process for 8a–c leading to the
formation of the tetracations can be observed at around 0.66 V,
a similar value recorded for TTF 5. No splitting of this wave could be
detected for 8a–c, indicating that the two TTF chromophores are at
large distance and no more in electronic communication with each
other.27 Finally, the cyclic voltammograms of these derivatives are
completed in most cases with one additional oxidation wave at
around 1.90 V, probably arising from the 3,5-di-tert-butylbenzyl
substituents (Table 1).
2.5. Binding studies
Unlike previously reported molecular tweezers, which either
had limited solubility or sterically hindering groups near the
binding site,16 the new compounds lacked these disadvantages and
were suitable for molecular recognition studies. Thus, molecular
recognition properties were tested for molecular tweezers 8a and
8c, built onto a benzene and naphthalene backbone and thus with
different TTF–TTF spans, as well as for the reference compound 7a.
Binding studies were performed by means of NMR binding titra-
tions in chloroform using tetracyanoquinodimethane (TCNQ),
2,4,7-trinitro-9-fluorenylidenemalononitrile (TNF),28 and tetra-
cyanobenzene (TCNB) (Fig. 7) as guest compounds. As expected,
NMR spectra showed fast host–guest exchange. Both molecular
tweezers 8a,c showed binding with TNF (Ka¼16 Mꢂ1 and
Ka¼22 Mꢂ1 for 8a and 8c, respectively), whereas only 8c showed
weak binding with TCNB (Ka¼6 Mꢂ1).29 In binding experiments
with TCNQ, the broadening and disappearance of the NMR reso-
nance of the TCNQ protons were observed for quite low host con-
centrations (<1 mM) with both 8a and 8c, implying generation of
paramagnetic species due to formation of a charge–transfer (CT)
complex. In the IR spectra of TCNQ with 8a and 8c weak CT ab-
sorption bands could be detected at ca. 760 and 850 nm, re-
spectively, although at 5–10 times higher concentrations of a host.
Rough estimation based on donor and acceptor oxidation/reduction
potentials30 indicated that the degree of CT for complexes of TCNQ
Figure 5. Crystal packing of 7a, projection along the c axis. Hydrogen atoms are
omitted for clarity.
Table 1
Electrochemical data of TTF derivativesa
ox1
ox10
1/2
ox2
Compound
E
(V)
E
(V)
E
(V)
1/2
Eox3 (V)
1/2
3
5
0.36
0.33
0.33
0.33
0.26
0.27
0.34
0.68
0.66
0.69
0.68
0.66
0.67
0.66
–
1.90
1.86
1.31, 1.94
1.89
1.90
1.86
7a
7b
8a
8b
8c
0.40
0.43
a
Data were obtained using a one-compartment cell in CH2Cl2/0.1 M Bu4NPF6, Pt
as the working and counter electrodes. Values given at room temperature vs Ag/Agþ
scan rate 200 mV/s.
,
derivative 5. This fact implies that in molecular tweezers 8a,b TTF–
TTF interaction takes place exclusively through space, and in 8c this
interaction does not seem to be possible as a consequence of the
long TTF–TTF distance, independently of the tweezers’ conforma-
tion. Intermolecular nature of the splitting due to tweezers’
dimerization13c seems quite unlikely due to the following reasons:
(a) splitting was observed only for the tweezers with the benzene
scaffold, in which close TTF–TTF contacts upon conformational
distortion are possible; (b) the splitting shows a well-defined dou-
blet, whereas for a dimer more complex pattern can be expected.13c
(c) Measurements with different scan rates did not lead to signifi-
cant changes of the voltammograms.
Figure 7. Structures of guests used for molecular recognition studies.

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M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
with hosts 8 should be rather small. Thus, other reasons for the
signal broadening, such as dynamic host–guest exchange with
a timescale similar to the one of NMR, cannot be fully ruled out.
Dilution studies with 7a and 8a,b did not reveal any self-association
of the molecular host at the concentrations used for binding titra-
tions. Additional host–guest binding studies using other solvents
and guest compounds are currently under way.
Binding studies have thus showed the feasibility of the molec-
ular design used in our molecular tweezers systems, with two TTF
moieties aligned in parallel fashion, although relatively low binding
constants imply that significant tuning of the receptor structure
is necessary to improve the binding efficiency. Thus, molecular
tweezers bearing pyrrolo-31 and alkyl-substituted tetrathiafulva-
lenes, which are know for their better electron-donating3 and
binding32 properties than thioalkyl-substituted TTFs, are planned
to be tested as hosts for electron-deficient guests.
mixture turned brown and then gradually changed its color to
yellow-green. The reaction was stirred overnight, and then evap-
orated to dryness. The residue was dissolved in CH2Cl2/petroleum
ether (1:3, 40 mL), and the undissolved material was filtered off.
After evaporation of the solvent, the product was purified by flash
chromatography (CH2Cl2/petroleum ether, 1:4–1:3) to give the
product as a viscous yellow syrup, which crystallized upon cooling
yielding a bright yellow solid (2.086 g, 3.46 mmol, 90%). Mp: 76.5–
78.5 ꢁC. Rf¼0.28 (CH2Cl2/petroleum ether, 1:4). 1H NMR (200 MHz,
CDCl3):
d
¼1.30 (36H, s), 3.96 (4H, s), 7.05 (4H, d, J¼2.0 Hz), 7.33 (2H,
t, J¼2.0 Hz) ppm. 13C NMR (50 MHz, CDCl3):
¼31.55; 34.86, 41.77,
d
121.98, 123.40, 134.78, 137.98, 151.27, 211.45 ppm. HRMS (EI): m/z
calcd for C33H46S5 (Mþ) 602.22031, found 602.22177.
4.2.2. 2,3-Bis(2-cyanoethylthio)-6,7-bis(3,5-di-tert-butylbenzylthio)-
tetrathiafulvalene (3). A suspension of 1 (1.66 g, 2.76 mmol) and 4
(0.795 g, 2.9 mmol) in P(OEt)3 (5 mL) was degassed by a freeze–
pump–thaw cycle, and then heated with stirring at 120 ꢁC for 2 h.
The reaction mixture gradually turned from yellow to bright yel-
low-orange, and a precipitate appeared. The solvent was removed
under high vacuum; the residue was suspended in MeOH (20 mL)
and the undissolved material, mostly consisting of 2,3,6,7-tetra-
kis(2-cyanoethylthio)tetrathiafulvalene, was filtered off. Methanol
was then removed under vacuum, and the residue was first sub-
jected to crude chromatography (CH2Cl2) using a short SiO2 plug,
and then the product was additionally purified by flash chroma-
tography (petroleum ether/EtOAc, 7:3). Compound 3 crystallized
upon concentration of the solvent affording bright yellow crystal-
line powder (1.08 g, 1.26 mmol, 46%). Mp: 119–120 ꢁC. Rf¼0.34
3. Conclusions
In summary, the attachment of 3,5-di-tert-butylbenzylthio
groups to TTF has been shown to be an efficient strategy to obtain
soluble tetrathiafulvalene derivatives. It was used to render solu-
bility to several novel TTF derivatives; some of those with archi-
tectures of molecular tweezers. Two new tetrathiafulvalene
derivatives were characterized using X-ray, showing significant
distortion of the TTF moieties due to the hindering influence of the
bulky 3,5-di-tert-butylbenzyl groups, which prevent efficient TTF–
TTF stacking. Cyclic voltammetry measurements evidenced that
only distance-dependent through space TTF–TTF interactions are
possible in the TTF-containing molecular tweezers. Finally, TTF-
containing molecular tweezers were successfully tested as re-
ceptors for several electron-deficient substances, and, although the
binding constants were relatively low, this result is promising.
Further synthetic efforts are aimed to the construction of analogous
receptors with improved binding properties using more electron
rich pyrrolo-TTF and alkyl-TTF arms instead of thioalkyl-TTFs.
(CH2Cl2). 1H NMR (200 MHz, CDCl3):
d
¼1.31 (36H, s), 2.75 (4H, t,
J¼7.0 Hz), 3.10 (4H, t, J¼7.0 Hz) 3.85 (4H, s), 7.09 (4H, d, J¼2.0 Hz),
7.31 (2H, t, J¼2.0 Hz) ppm. 13C NMR (50 MHz, CDCl3):
¼18.74,
d
31.15, 31.39, 34.66, 41.06, 106.91, 114.27, 117.42, 121.43, 123.08,
127.71, 129.51, 135.34, 150.75 ppm. IR (KBr): nmax¼2251 (C^N)
cmꢂ1. UV–vis (CH2Cl2): lmax
(3
)¼310 (16,300), 331 (16,900), 393
(2950 dm3 molꢂ1 cmꢂ1) nm. HRMS (EI): m/z calcd for C42H54N2S8
(Mþ) 842.20527, found 842.20639.
4. Experimental section
4.1. General
4.2.3. 4,5-Bis(3,5-di-tert-butylbenzylthio)-1,3-dithiole-2-one
(6). Compound 1 (388 mg, 0643 mmol) was dissolved in CHCl3
(7.5 mL) and Hg(OAc)2 (410 mg, 1.29 mmol) was added to it, fol-
lowed by AcOH (2.5 mL). The reaction mixture was stirred over-
night, and then the white precipitate was filtered off using Celite.
The resulting organic phase was washed with saturated NaHCO3
solution, the aqueous phase was extracted two times with CH2Cl2.
The combined organic phase was dried (MgSO4) and evaporated to
dryness affording an almost colorless syrup that crystallized upon
standing in a fridge. The product was additionally purified by flash
chromatography (CH2Cl2/petroleum ether, 1:2) yielding colorless
crystalline material (357 mg, 0.608 mmol, 95%). Mp: 87.5–89 ꢁC.
Rf¼0.37 (CH2Cl2/petroleum ether, 1:2). 1H NMR (200 MHz, CDCl3):
Reagent grade chemicals and solvents were used without fur-
ther purification unless otherwise stated. All reactions were carried
out under the atmosphere of dry N2. Chemical shifts (
d) are
reported in parts per million (ppm) downfield from tetrame-
thylsilane using the residual solvent peak as internal reference:
CDCl3 (7.26 ppm for 1H, 77.23 ppm for 13C). 1H and 13C NMR spectra
were recorded with a Bruker Avance DPX-200 spectrometer. EIMS
spectra were measured on a Finnigan MAT 8200 spectrometer, ESI-
MS spectra were measured on a BrukermicrOTOF spectrometer us-
ing direct injection method, MALDI-MS spectra were measured on
a Perseptive (Applied Bioystems) Voyager DE Pro spectrometer. UV/
Vis measurements were performed on a Varian Cary 50 Conc
spectrophotometer. IR spectra were recorded on a Perkin–Elmer
Paragon 500 FTIR spectrometer. Analytical thin layer chromatog-
raphy (TLC) was performed on 0.2 mm silica gel aluminum cards
with F-254 fluorescent indicator. Flash chromatography (FC) was
d¼1.28 (36H, s), 3.92 (4H, s), 7.05 (4H, d, J¼2.0 Hz), 7.31 (2H, t,
J¼2.0 Hz) ppm. 13C NMR (50 MHz, CDCl3):
¼31.62; 34.98, 41.76,
d
121.92, 123.43, 129.12, 135.14, 151.29, 190.33 ppm. HRMS (EI): m/z
calcd for C33H46OS4 (Mþ) 586.24315, found 586.24240.
4.2.4. 2,3,6,7-Tetrakis(3,5-di-tert-butylbenzylthio)tetrathiafulvalene
(5). A suspension of compound 6 (333 mg, 0.567 mmol) in P(OEt)3
(3 mL) was degassed by a freeze–pump–thaw cycle, then heated
with stirring at 120 ꢁC for 2 h. The reaction mixture gradually
turned from colorless to yellow-orange. The solvent was removed
under high vacuum, the oily residue was subjected to flash chro-
matography (CH2Cl2/petroleum ether, 1:4) affording compound 5
as a yellow-orange oil, which crystallizes upon standing (232 mg,
0.203 mmol, 72%). Mp: 63–65 ꢁC. Rf¼0.44 (CH2Cl2/petroleum ether,
carried out using 230–440 mesh (particle size 36–70 mm) silica gel.
4.2. Synthetic procedures
4.2.1. 4,5-Bis(3,5-di-tert-butylbenzylthio)-1,3-dithiole-2-thione
(1). To a solution of bis(tetraethylammonium)bis(1,3-dithiole-2-
thione-4,5-dithiol)zincate 2 (1.38 g, 1.92 mmol) in MeCN (30 mL)
a
solution of 1-bromomethyl-3,5-di-tert-butylbenzene (2.30 g,
8.12 mmol) in MeCN (30 mL) was added dropwise. The reaction
1:3). 1H NMR (200 MHz, CDCl3):
d¼1.30 (72H, s), 3.86 (8H, s), 7.10

M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
10353
(8H, d, J¼2.0 Hz), 7.29 (4H, t, J¼2.0 Hz) ppm. 13C NMR (50 MHz,
132.85, 135.28, 135.71, 151.12 ppm. UV–vis (CH2Cl2): lmax
(
3
)¼292
CDCl3):
d
¼31.67; 35.01, 41.44, 111.01, 121.77, 123.45, 129.36, 135.71,
(35,600), 335 (36,000), 396 sh (5800 dm3 molꢂ1 cmꢂ1) nm. HRMS
(ESI) m/z calcd for C82H102S16 (Mþ) 1598.3507, found 1598.3517; calcd
for (MClꢂ) 1633.2867, found 1633.3255.
151.15 ppm. UV–vis (CH2Cl2): lmax
(3
)¼309 (17,500), 332 (16,800),
396 (3090 dm3 molꢂ1 cmꢂ1) nm. HRMS (EI): m/z calcd for C66H92S8
(Mþ) 1140.49648, found 1140.49887.
4.3.4. 2,10-Bis[4,5-bis(3,5-di-tert-butylbenzylthio)-1,3-dithiol-2-yli-
dene]-5H,7H,13H,15H-6,14-dihexyloxy-bis[1,3]dithiolo[4,5-b:40,50-
b0]benzo[1,2-f:4,5-f0]bis[1,4]dithiocine (8b). Two consecutive flash
chromatographies (CH2Cl2/petroleum ether, 1:2; then cyclohexane/
EtOAc, 12:1), afforded the product as a yellow syrup (113 mg,
0.0627 mmol, 63%), which crystallized upon trituration with
a small volume of hexane giving a yellow-orange crystalline pow-
der. Mp: 133–135 ꢁC. Rf¼0.48 (cyclohexane/EtOAc, 19:1). 1H NMR
4.3. Synthesis of compounds 7 and 8, general procedure
TTF derivative 3 (0.2 mmol) was dissolved in dry DMF (20 mL)
and degassed by
(0.42 mmol, 0.84 mL) was added as a 0.5
a
freeze–pump–thaw cycle; then CsOH
solution in MeOH at
M
0 ꢁC. The mixture was allowed to warm to rt and stirred for 30 min,
turning from orange to dark brown-red in color. Aromatic bromo-
methyl derivative 9 (0.2 mmol) or 10 (0.1 mmol) was dissolved in
dry THF (3 mL), and the solution was degassed by a freeze–pump–
thaw cycle. The THF solution was added in one portion to the
previously prepared DMF solution at ꢂ20 ꢁC, then the mixture was
allowed to warm gradually to rt. The reaction mixture turned or-
ange-yellow, sometimes a yellow precipitate formed. The reaction
was allowed to stir for additional 30 min at rt, and then was
evaporated to dryness. The residue was purified by flash chroma-
tography on silica gel.
(200 MHz, CDCl3):
d
¼0.98 (6H, t, J¼6.8 Hz), 1.25 (72H, s), 1.38–1.61
(12H, m), 1.83–1.97 (4H, m), 3.75 (8H, s), 4.12 (4H, t, J¼6.6 Hz), 4.34
(8H, s), 7.04 (8H, d, J¼1.8 Hz), 7.25 (4H, t, J¼1.8 Hz) ppm. 13C NMR
(50 MHz, CDCl3):
34.98, 41.55, 110.52, 115.62, 121.72, 123.46, 129.45, 129.85, 130.55,
d
¼14.41, 22.91, 25.89, 30.49, 31.63, 32.01, 32.59,
135.71, 151.09, 152.23 ppm. UV–vis (CH2Cl2): lmax
(
3
)¼304 (38,200),
332 (35,800), 396 sh (5800 dm3 molꢂ1 cmꢂ1) nm; HRMS (MALDI,
matrix: DHB): m/z calcd for C94H126O2S16 (Mþ) 1798.5289, found
1798.5431.
4.3.1. 2-[4,5-Bis(3,5-di-tert-butylbenzylthio)-1,3-dithiol-2-ylidene]-
5H,10H-1,3-dithiolo[4,5-c][2,5]benzodithiocine (7a). Two consecu-
tive flash chromatographies (CH2Cl2/petroleum ether, 1:3; then
cyclohexane/EtOAc, 20:1) afforded the product as a yellow syrup
(128 mg, 0.152 mmol, 76%), which crystallized upon trituration
with a small volume of hexane giving a bright yellow crystalline
powder. Mp: 219–221 ꢁC. Rf¼0.55 (cyclohexane/EtOAc, 19:1). 1H
4.3.5. 2,11-Bis[4,5-bis(3,5-di-tert-butylbenzylthio)-1,3-dithiol-2-yli-
dene]-5H,8H,14H,17H-bis[1,3]dithiolo[4,5-b:40,50-b0]naphtho[2,3-
f:6,7-f0]bis[1,4]dithiocine (8c). Due to its low solubility, compound
10c was added to the reaction mixture as a powder. Flash chro-
matography (CH2Cl2; then cyclohexane/EtOAc, 9:1) with gradual
addition of CH2Cl2 until the entire fraction containing the product
was collected. After concentration of the solution, the product was
obtained as an amorphous yellow-orange powder (128 mg,
0.0775 mmol, 77%). Mp: 200–210 ꢁC (decomp.). Rf¼0.44 (cyclo-
NMR (200 MHz, CDCl3):
7.04 (4H, d, J¼1.8 Hz), 7.21–7.34 (4H, m), 7.28 (2H, t, J¼1.8 Hz) ppm.
13C NMR (50 MHz, CDCl3):
¼31.64, 35.00, 38.69, 41.48, 111.86,
112.29, 121.73, 123.41, 128.89, 129.64, 130.24, 130.78, 134.82, 135.78,
d
¼1.28 (36H, s), 3.76 (4H, s), 4.30 (4H, s),
d
hexane/EtOAc, 19:1). 1H NMR (200 MHz, CDCl3):
d
¼1.25 (72H, s),
3.73 (8H, s), 4.43 (8H, s), 7.02 (8H, d, J¼1.8 Hz), 7.26 (4H, t, J¼1.8 Hz),
7.67 (4H, s) ppm. 13C NMR (50 MHz, CDCl3):
¼31.62, 34.98, 39.46,
41.47, 111.71, 112.71, 121.72, 123.39, 129.49, 131.01, 132.95, 133.79,
151.15 ppm. UV–vis (CH2Cl2): lmax
(
3
)¼295 (15,200), 335 (18,500),
d
400 sh (2600 dm3 molꢂ1 cmꢂ1) nm. HRMS (EI): m/z calcd for
C44H54S8 (Mþ) 838.19913, found 838.20284.
135.72, 151.12 ppm. UV–vis (CH2Cl2): lmax
(
3
)¼292 sh (43,600), 330
(37,900), 396 sh (6800 dm3 molꢂ1 cmꢂ1) nm. HRMS (ESI): m/z calcd
for C86H104S16 (MþH)þ 1649.3742, found 1649.3730; calcd for
(MþCl)ꢂ 1683.33.63, found 1683.3375.
4.3.2. 2-[4,5-Bis(3,5-di-tert-butylbenzylthio)-1,3-dithiol-2-ylidene]-
5H,10H-6,9-dihexyloxy-1,3-dithiolo[4,5-c][2,5]benzodithiocine
(7b). Two consecutive flash chromatographies (CH2Cl2/petroleum
ether, 1:3; then cyclohexane/EtOAc, 25:1) afforded the product as
a yellow syrup (169 mg, 0.162 mmol, 81%), which crystallized upon
trituration with a small volume of hexane giving a bright yellow
crystalline powder. Mp: 125–126 ꢁC. Rf¼0.67 (cyclohexane/EtOAc,
4.4. X-ray crystallography
Crystallographic measurements were performed on a Siemens-
P4 diffractometer with a graphite monochromated Mo K
a radiation
19:1). 1H NMR (200 MHz, CDCl3):
(36H, s), 1.33–1.58 (12H, m), 1.73–1.87 (4H, m), 3.75 (4H, s), 3.95
d
¼0.94 (6H, t, J¼6.4 Hz), 1.27
(l 71.073 pm) and the low-temperature device LT2. The structure
was solved by direct methods and refined by full matrix least-
squares technique on F2 using SHELX33 program package. All non-
H-atoms were refined anisotropically, the positions of hydrogen
atoms were calculated as a ‘riding’ model.
Crystallographic data (excluding structure factors) for the
structures 3 and 7a in this paper have been deposited with the
Cambridge Crystallographic Data Centre under deposition numbers
747,600 and 747,601, respectively. Copy of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: þ44 (0)1223 336033 or e-mail: depos-
it@ccdc.cam.ac.uk). Abbreviated crystallographic data are given
below.
(4H, t, J¼6.4 Hz), 4.35 (4H, s), 6.79 (2H, s), 7.04 (4H, d, J¼1.8 Hz), 7.27
(2H, t, J¼1.8 Hz) ppm. 13C NMR (50 MHz, CDCl3):
¼14.35, 22.90,
d
26.09, 29.64, 31.63, 31.80, 32.02, 34.98, 41.50, 69.50, 111.90, 112.50,
112.80, 121.72, 123.41, 125.27, 129.60, 130.94, 135.80, 150.74,
151.14 ppm. UV–vis (CH2Cl2): lmax
(
3
)¼318 (17,900), 331 (19,800),
400 sh (2500 dm3 molꢂ1 cmꢂ1) nm. HRMS (EI): m/z calcd for
C56H78O2S8 (Mþ) 1038.37676, found 1038.37727.
4.3.3. 2,10-Bis[4,5-bis(3,5-di-tert-butylbenzylthio)-1,3-dithiol-2-yli-
dene]-5H,7H,13H,15H-bis[1,3]dithiolo[4,5-b:40,50-b0]benzo[1,2-f:4,5-
f0]bis[1,4]dithiocine (8a). Two consecutive flash chromatographies
(CH2Cl2/petroleum ether,1:2; then cyclohexane/EtOAc,10:1) afforded
the product as a yellow syrup (107 mg, 0.0668 mmol, 67%), which
crystallized upon trituration with a small volume of hexane giving
a bright yellow crystalline powder. Mp: 260–270 ꢁC (decomp.).
Rf¼0.29 (cyclohexane/EtOAc, 19:1). 1H NMR (200 MHz, CDCl3):
Compound 3. C42H54N2S8, MW¼843.35, triclinic, P-1 (#2),
a¼1027.30 pm, b¼1222.8 pm, c¼1900.2 pm,
a
¼73.520ꢁ,
b
¼80.290ꢁ,
g
¼86.090ꢁ, V¼2.2556 nm3, Z¼2, T¼173 K,
m
¼0.427 mmꢂ1
¼1.242
,
r
Mg mꢂ3, GOF on F2¼1.019, R1¼0.0451, wR2¼0.1205 [I>2
s(I)]. Crystals
were grown by slow evaporation of a cyclohexane solution.
d
¼1.26 (72H, s), 3.75 (8H, s), 4.29 (8H, s), 7.04 (8H, d, J¼1.8 Hz), 7.09
Compound 7a. C44H54S8, MW¼839.35, monoclinic, P21/m (#11),
(2H, s), 7.26 (4H, t, J¼1.8 Hz) ppm. 13C NMR (50 MHz, CDCl3):
d
¼31.64,
a¼929.40 pm, b¼1869.80 pm, c¼1363.8 pm,
a
¼90ꢁ,
b
¼105.860ꢁ,
34.98, 38.19, 41.53, 111.84, 114.24, 121.73, 123.42, 129.48, 129.70,
g
¼105.860ꢁ, V¼2.2798 nm3, Z¼2, T¼173 K,
m
¼0.421 mmꢂ1
,

10354
M. Skibin´ski et al. / Tetrahedron 65 (2009) 10348–10354
¼1.223 Mg mꢂ3
,
GOF on F2¼1.029, R1¼0.0525, wR2¼0.1381
P.-T.; Chiu, S.-H. Chem. Commun. 2006, 2854–2856; (c) Chiang, P.-T.; Chen,
r
N.-C.; Lai, C.-C.; Chiu, S.-H. Chem.dEur. J. 2008, 14, 6546–6552.
14. (a) Tachikawa, T.; Izuoka, A.; Sugawara, T. J. Chem. Soc., Chem. Commun. 1993,
1227–1229; (b) Tachikawa, T.; Izuoka, A.; Sugawara, T. Solid State Commun.
1993, 88, 207–209; (c) Fujiwara, H.; Arai, E.; Kobayashi, H. J. Mater. Chem. 1998,
8, 829–831.
[I>2s(I)]. Crystals were grown by slow evaporation of a CHCl3/hep-
tane solution.
Acknowledgements
15. (a) Poulsen, T.; Nielsen, K. A.; Bond, A. D.; Jeppesen, J. O. Org. Lett. 2007, 9, 5485–
5488; (b) Nielsen, K. A.; Sarova, G. H.; Martı´n-Gomis, L.; Ferna´ndez-La´zaro, F.;
Stein, P. C.; Sanguinet, L.; Levillain, E.; Sessler, J. L.; Guldi, D. M.; Sastre-Santos,
V.A.A. thanks Fonds der Chemischen Industrie for financial
support. Help of Dr. Thomas Du¨lcks and Ms. D. Kemken (MS) and
Mr. J. Stelten (NMR) is gratefully acknowledged.
´
A.; Jeppesen, J. O. J. Am. Chem. Soc. 2008, 130, 460–462; (c) Nielsen, K. A.;
Martı´n-Gomis, L.; Sarova, G. H.; Sanguinet, L.; Gross, D. E.; Ferna´ndez-La´zaro, F.;
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Stein, P. C.; Levillain, E.; Sessler, J. L.; Guldi, D. M.; Sastre-Santos, A.; Jeppesen,
J. O. Tetrahedron 2008, 64, 8449–8463.
´
1909–1917.
16. Azov, V. A.; Gomez, R.; Stelten, J. Tetrahedron 2008, 64,
Supplementary data
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NMR and UV spectra of the new compounds, CV plots with
description of CV measurements, full crystallographic data, and
binding plots with description of binding experiments. Supple-
mentary data associated with this article can be found in the online
version, at doi:10.1016/j.tet.2009.10.052.
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