Bromophenyl substituted dithiafulvenes and tetrathiafulvalene vinylogues: Synthesis, structure, and electronic properties

Article abstract of DOI:10.1016/j.tetlet.2013.06.077

Bis(bromophenyl) substituted tetrathiafulvalene vinylogues (TTFVs) were prepared via oxidative dimerization reactions of corresponding bromophenyl substituted dithiafulvene precursors. The synthesis of ortho-bromophenyl TTFVs led to the formation of an un

Full text of DOI:10.1016/j.tetlet.2013.06.077

Tetrahedron Letters 54 (2013) 4666–4669
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bromophenyl substituted dithiafulvenes and tetrathiafulvalene
vinylogues: synthesis, structure, and electronic properties
⇑
Stephen Bouzan, Louise N. Dawe, Yuming Zhao
Department of Chemistry, Memorial University, St. John’s, NL, A1B 3X7, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(bromophenyl) substituted tetrathiafulvalene vinylogues (TTFVs) were prepared via oxidative dimer-
ization reactions of corresponding bromophenyl substituted dithiafulvene precursors. The synthesis of
ortho-bromophenyl TTFVs led to the formation of an unexpected bis-spiro product, the structure of which
was clearly elucidated by single crystal X-ray crystallography. Electronic and redox properties of the
bromophenyl substituted dithiafulvenes, TTFV derivatives, and the related bis-spiro compound were
investigated by UV–vis spectroscopic and cyclic voltammetric analyses. Detailed structure–property rela-
tionships have been discussed.
Received 23 May 2013
Revised 12 June 2013
Accepted 16 June 2013
Available online 27 June 2013
Keywords:
Tetrathiafulvalene
Dithiafulvene
Substitution effect
Redox
Ó 2013 Elsevier Ltd. All rights reserved.
Tetrathiafulvalene vinylogues (TTFVs) belong to an attractive
branch in the field of widely studied tetrathiafulvalene (TTF) chem-
istry, owing to their excellent electron-donating properties and un-
ique conformational switching behavior associated with the redox
processes taking place in TTFV.1 Taking advantage of their unique
redox and structural properties, TTFV derivatives have been re-
cently employed as functional building blocks in the preparation
of various intriguing molecular materials and devices, ranging
from molecular rods,2 conjugated polymers,2,3 shape-persistent
macrocycles,2 redox-active ligands,4 and chemical sensors.5 Aryl
substituted dithiafulvenes (DTF) are the commonly used precur-
sors for the synthesis of TTFV scaffolds, following an oxidative
dimerization mechanism6 as illustrated in Scheme 1.
In the neutral state, the steric crowding around the butadienyl-
ene moiety of a diaryl substituted TTFV tends to drive the molecule
into a pseudo cisoid conformation.1g,5,7 Upon a two-electron oxida-
tion process, the diaryl-TTFV is converted into a dication, in which
significant Coulombic repulsion between the two dithiolium rings
rotates the molecular skeleton into a transoid structure.1g,7,8 Dia-
ryl-TTFVs that prefer a transoid structure in the neutral state are,
however, scarcely known. Previously, Yamashita and co-workers7a
hypothesized that diphenyl-TTFVs would favor the transoid con-
formation when ortho-substituents were introduced to increase
steric interactions. This argument is legitimate and sound, but
has never been convincingly validated by concrete experimental
evidence. Very recently, we reported the first example of a fully
planar transoid bisnapthyl-TTFV, wherein the allylic strain in the
Scheme 1. General mechanism for the oxidative dimerization of an aryl-DFT to
form a diaryl-TTFV product.
1-naphthyl group was enlisted to direct the conformation.9 This re-
sult hence led us to revisit the ortho-substitution effect on the
properties of diphenyl-TTFV derivatives, since gaining better con-
trollability over the conformation of TTFV offers an effective ap-
proach for rational design and fine tuning of new TTFV-based
functional materials.
In this work, a series of bis(bromophenyl) substituted TTFV iso-
mers 1a–c was targeted (Fig. 1). These compounds were chosen as
models in our study because of the relatively large atomic radius
and high electronegativity of bromine, which are expected to
impose significant steric and electronic (inductive) effects when
⇑
Corresponding author. Tel.: +1 (709) 864 8747.
E-mail address: yuming@mun.ca (Y. Zhao).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.077

S. Bouzan et al. / Tetrahedron Letters 54 (2013) 4666–4669
4667
Figure 1. Molecular structures and calculated torsion angles for bis(bromophenyl)-
substituted TTFVs 1a–c.
bromine is placed at different positions of the phenyl group.
Especially, our theoretical modeling study using the density func-
tional theory (DFT) method has revealed that the torsion angle
(h) about the butadienylene segment in ortho-bromo isomer 1c is
considerably smaller than the other two isomers 1a and 1b.10
The conformational differences should in turn have a direct impact
on the electronic and redox properties, and therefore would be
ideal for systematic examination of the structure–property correla-
tions in diphenyl substituted TTFVs.
The synthesis of bis(bromophenyl)-TTFVs was undertaken via
the approaches described in Scheme 2. p-Bromobenzaldehyde
(2a) was first subjected to an olefination reaction11 with thione 3
in the presence of trimethylphosphite under heating, affording
dithiafulvene 4a in a yield of 60%. Compound 4a was a yellow-col-
ored crystalline solid. By careful recrystallization, good-quality
crystals of 4a suitable for single-crystal X-ray diffraction analysis
were obtained (see Fig. 2). Compound 4a then underwent an oxida-
tive dimerization reaction at room temperature using iodine as the
oxidant. After a reductive workup with aqueous Na2S2O3, neutral
TTFV 1a was acquired in 54% yield. In the same way, meta-bromo
Figure 2. ORTEP plots (50% ellipsoid probability) of compounds 4a and 5: (A) front
view of 4a, (B) side view of 4a, (C) front view of 5, and (D) side view of 5. Atomic
color code: Gray = C, Green = S, Blue = Br, H represented by spheres.
isomer 1b was synthesized using m-bromobenzaldehyde (2b) as
the starting material. In the synthesis of ortho-bromo isomer 1c,
however, some unexpected result was attained. As shown in
Scheme 2, dithiafulvene intermediate 4c was successfully prepared
from o-bromobenzaldehyde (2c) via the trimethylphosphite-pro-
moted olefination. Oxidative dimerization of 4c in the presence
of iodine followed by treatment with Na2S2O3 resulted in the for-
mation of several products as manifested by thin-layer chromato-
graphic (TLC) analysis. After flash column chromatographic
separation and recrystallization, a yellow crystalline solid was iso-
lated as the major product in yield of 50%. To our great surprise, the
structure of this compound as characterized by NMR and MS anal-
yses (see the Supplementary data) turned out to be the bis-spiro
compound 5 (see Scheme 2) instead of the expected TTFV 1c. 1H
NMR analysis of the crude reaction mixture suggested 1c was also
formed, but only as a minor product. Pure 1c could not be isolated
due to low yield and tremendous difficulty in separation.
Besides NMR and MS characterizations, single-crystal X-ray
crystallography also offered conclusive evidence for molecular
structural elucidation. Figure 2 shows the single-crystal struc-
tures of 4a and 5.12,13 The molecular structure of dithiafulvene
precursor 4a assumes a slightly twisted conformation along the
p-conjugated framework, with the torsion angle between the
phenyl and vinylene units being 19.4°. It is noteworthy that
the C–S–C bond angles in the dithiole ring are around 95°, which
is consistent with the known fact that sulfur atom prefers to
adopt a 90° bond angle.14 The relatively small C–S–C bond angle
hence drives the dithiole five-membered ring to take a non-pla-
nar structure as can be clearly seen in Figure 2B. Of great inter-
est is the unusual bis-spiro motif in compound 5. Compound 5
crystallized with two chemically identical molecules in the
asymmetric unit, but for simplicity only one is shown in Figure 2.
As a result of significant steric crowding in 5, the two dithiole
and two phenyl rings surrounding the central tetrahydrothio-
phene unit are in a nearly perpendicular orientation. The two
bromo groups are positioned in a trans configuration so as to re-
duce the net molecular dipole moment.
Compound 5 was at first speculated to result from the dicationic
intermediate interacting with some kind of sulfur nucleophiles
Scheme 2. Synthesis of bis(bromophenyl) substituted TTFVs 1a, 1b, and spiro-
derivative 5.

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S. Bouzan et al. / Tetrahedron Letters 54 (2013) 4666–4669
immediately after the radical dimerization step as shown in
Scheme 1. To test this hypothesis, control experiments were per-
formed, where a large excess of dimethylsulfide (Me2S) was added
as a nucleophile to the dimerization reactions of 2a–c. The results
of these reactions were, however, no different from the normal
dimerization reactions done previously. It was therefore reason-
able to believe that the central sulfur bridge of 5 was introduced
at the workup stage, where the reducing agent, Na2S2O3, severed
as the nucleophile. A tentative mechanism as outlined in Scheme 3
was proposed to account for this transformation. The fact that
there were no cyclized products observed in the dimerization reac-
tions of 2a and 2b indicates that this unique reactivity must orig-
inate from the ortho-bromo substitution effect. In the general
oxidative dimerization mechanism (see Scheme 1), rapid deproto-
nation (elimination) would occur right after the dicationic inter-
mediate is formed. In the case of ortho-bromophenyl substituted
dithiafulvene, this dication can form double intramolecular BrÁ Á ÁH
hydrogen bonds, given the close distance between the acidic ben-
zylic hydrogen and ortho-bromine. In the crystal structure of 5, the
BrÁ Á ÁH distance is observed at a close distance of 2.68 Å (Fig. 2D),
which substantiates our hypothesis. The intramolecular hydrogen
bonding thus provides stabilization to the dication and hinders
the benzylic protons from deprotonation. In the following reduc-
tive workup process, nucleophilic addition of [S2O3]2À competes
against the deprotonation pathway, leading to the formation of 5
as the major product.
Figure 3. Normalized UV–vis absorption spectra of compounds 4a–c, 1a,b, and 5
measured in CH2Cl2 at room temperature.
Electrochemical redox properties of these compounds were
studied by cyclic voltammetry (CV) and their cyclic voltammo-
grams are shown in Figure 4. In Figure 4A, compound 4a exhibits
a pair of redox peaks at +0.78 V and +0.46 V, which are attributed
to a single-electron oxidation process on the dithiafulvene unit and
reduction of the resulting TTFV dication respectively.2 The voltam-
mogram of 4b shows very similar patterns with the redox peaks
slightly shifted to the positive potential direction. The voltammo-
gram of ortho-bromo isomer 4c shows a similar anodic peak at
The electronic properties of dithiafulvene precursors 4a–c,
TTFVs 1a,b, and spiro-compound 5 were investigated by UV–vis
spectroscopic analysis. Figure 3 compares the normalized UV–vis
spectra of these compounds. It can be clearly seen that the absorp-
tion spectra of dithiafulvenes 4a–c are nearly superimposable, and
so are the spectra of TTFVs 1a versus 1b, suggesting that changing
the bromo-substitution position has little effect on the electronic
absorption properties.
The UV–vis spectrum of 5 shows a maximum absorption peak at
311 nm, which is considerably blueshifted in comparison with
those of other compounds at ca. 357 nm. It is also notable that
there is a significantly long absorption tail extending to ca.
470 nm in the spectrum of 5.
Scheme 3. Proposed mechanism for
leading to product 5 after oxidative dimerization.
a
Na2S2O3 involved cyclization pathway
Figure 4. Cyclic voltammograms for (A) bromophenyl dithiafulvene 4a–c, and (B)
TTFVs 1a,b and bis-spiro compound 5.

S. Bouzan et al. / Tetrahedron Letters 54 (2013) 4666–4669
4669
+0.81 V, but the profile of the reverse scan appears to be very com-
References and notes
plex, with a number of weak anodic peaks observable. Such a fea-
ture is indicative of chemical irreversibility and is likely due to
some electrochemical reaction (EC) mechanisms (e.g., electro-
chemical hydrodehalogenation15); however, the exact mechanisms
still await further investigations to clarify.
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10. See the Supplementary data for detailed computational results.
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12. Crystal data for 4a: C12H11BrS4, M = 363.36, orthorhombic, a = 8.275(4) Å,
The voltammograms of TTFV isomers 1a and 1b feature a simi-
lar pair of reversible redox peaks that can be assigned to the known
simultaneous two-electron transfer on the TTFV unit.2 Unlike the
case of dithiafulvene precursors, the different bromo-substitution
positions in 1a and 1b result in noticeable differences in the redox
potentials. As shown in Figure 4B, the redox peaks of meta-bromo
isomer 1b are shifted to the negative potential direction by ca.
40 mV relative to 1a. The redox properties of 5 appear to be com-
plex. Two anodic peaks are clearly seen at +0.63 and +1.05 V in the
positive scan. On the reverse scan, several cathodic peaks emerge.
Clearly, the ortho-bromo substitution induces some kinds of elec-
trochemical reactions, the exact origins of which await further
investigations to clarify.
In summary, we have prepared a series of bromophenyl substi-
tuted dithiafulvenes and subjected them to oxidative dimerization
to form TTFV isomers for the study of structure–property relation-
ships. In our work, ortho-bromo substitution has been found to
give rise to abnormal redox properties and reactivity. Of particular
note is that an unprecedented bis-spiro compound 5 was obtained
and its formation could be rationalized on the basis of an intramo-
lecular BrÁ Á ÁH hydrogen bonding model. Our findings underscore
the uniqueness of ortho-substitution effect on diphenyl-TTFV syn-
thesis and related property tuning.
Acknowledgments
b = 11.616(5) Å, c = 14.704(7) Å,
a
= 90.00°, b = 90.00°,
c
a
= 90.00°, V = 1413.4(11)
Å3, T = 123(2) K, space group P212121, Z = 4,
l(Mo K
) = 3.482 mmÀ1, 14,131
reflections measured, 3206 independent reflections, 3151 with
(Rint = 0.0444). R1 = 0.0265 (I >2
(I)), wR(F2) = 0.0584 (all data). The goodness
of fit on F2 was 1.069. Flack parameter = 0.006(7). CCDC 936355.
13. Crystal data for 5: C24H22Br2S9, M = 758.79, triclinic, a = 10.990(10) Å,
b = 13.760(11) Å, c = 20.50(2) Å, = 75.00(4)°, b = 88.43(5)°, = 73.35(4)°,
) = 3.509 mmÀ1
23751 reflections measured, 1,714 independent reflections, 9071 with I >2 (I)
(I)), wR(F2) = 0.2086 (all data). The goodness of
I >2r(I)
We thank NSERC, CFI, and Memorial University for funding
support.
r
a
c
3
ꢀ
Supplementary data
V = 2865(4) Å , T = 123(2) K, space group P1, Z = 4,
l(Mo K
a
,
r
(Rint = 0.0773). R1 = 0.0695 (I >2
r
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
06.077.
fit on F2 was 1.073. CCDC 936356.
14. Demiralp, E.; Goddard, W. A., III J. Phys. Chem. A 1997, 101, 8128–8131.
15. Cheng, H.; Scott, K.; Christensen, P. A. Electrochim. Acta 2004, 49, 729–735.