Synthesis and properties of tetrathiafulvalenes bearing 6-aryl-1,4-dithiafulvenes

Article abstract of DOI:10.3762/bjoc.16.86

Novel multistage redox tetrathiafulvalenes (TTFs) bearing 6-aryl-1,4-dithiafulvene moieties were synthesized by palladium-catalyzed direct C-H arylation. In the presence of a catalytic amount of Pd(OAc)₂, P(t-Bu₃)·HBF₄, and an excess of Cs₂CO₃, the C-H arylation of TTF with several aryl bromides bearing 1,3-dithiol-2-ylidenes took place efficiently to produce the corresponding p-conjugated molecules. We also succeeded in the estimation of the oxidation potentials and number of electrons involved in each oxidation step of the obtained compounds by digital simulations.

Full text of DOI:10.3762/bjoc.16.86

Synthesis and properties of tetrathiafulvalenes
bearing 6-aryl-1,4-dithiafulvenes
Aya Yoshimura1,2, Hitoshi Kimura1, Kohei Kagawa1, Mayuka Yoshioka1, Toshiki Itou1,
Dhananjayan Vasu3, Takashi Shirahata1,2, Hideki Yorimitsu3 and Yohji Misaki*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 974–981.
1Department of Materials Science and Biotechnology, Graduate
School of Science and Engineering, Ehime University, 3 Bunkyo-cho,
Matsuyama, Ehime 790-8577, Japan, 2Research Unit for Power
Generation and Storage Materials, and Research Unit for
Development of Organic Superconductors, Ehime University, 3
Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan and 3Department
of Chemistry, Graduate School of Science, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan
doi:10.3762/bjoc.16.86
Received: 11 February 2020
Accepted: 04 April 2020
Published: 12 May 2020
This article is part of the thematic issue "C–H functionalization for
materials science".
Email:
Guest Editor: K. Itami
Yohji Misaki* - misaki.yohji.mx@ehime-u.ac.jp
© 2020 Yoshimura et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
cross-conjugated systems; electrochemical properties; extended
π-conjugation; digital simulation analysis; tetrathiafulvalene
Abstract
Novel multistage redox tetrathiafulvalenes (TTFs) bearing 6-aryl-1,4-dithiafulvene moieties were synthesized by palladium-cata-
lyzed direct C–H arylation. In the presence of a catalytic amount of Pd(OAc)2, P(t-Bu3)·HBF4, and an excess of Cs2CO3, the C–H
arylation of TTF with several aryl bromides bearing 1,3-dithiol-2-ylidenes took place efficiently to produce the corresponding
π-conjugated molecules. We also succeeded in the estimation of the oxidation potentials and number of electrons involved in each
oxidation step of the obtained compounds by digital simulations.
Introduction
Tetrathiafulvalenes (TTFs) with extended π-conjugation have oxidation [1-16]. Considerable efforts have been devoted to the
attracted attention as possible components of functional materi- development of peripherally benzene- or thiophene-substituted
als, such as molecular conductors, field-effect transistors TTFs. As for peripherally benzene-functionalized TTFs, some
(FETs), and positive electrode materials for rechargeable synthetic approaches, crystal and electronic structures, electro-
batteries because the TTF moiety has strong electron-donating chemical and optical properties, and the nature of radical ion
properties attributed to the formation of stable aromatic 1,3- complexes with DDQ or iodine were reported [17-24]. Periph-
dithiol-2-ylidenes (1,3-dithiole rings) by one- and two-electron erally thiophene-functionalized TTFs, as potential precursors to
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Beilstein J. Org. Chem. 2020, 16, 974–981.
Results and Discussion
conducting polymers, and organic metals were also prepared
and characterized [25-29]. To design more tempting molecules, Synthesis
the attachment of 1,3-dithiole rings to aromatic rings appears First, we tried to synthesize compounds 1 and 2 in one step
very appealing since these allow to produce novel multistage from pristine TTF and 5, respectively, through palladium-cata-
redox systems. However, such molecules could formerly not be lyzed C–H arylation (Table 1). When the aryl bromides 6a,b
synthesized by conventional approaches. In 2011, a break- were allowed to react with TTF under the conditions A, the
through synthesis of arylated TTF derivatives by a palladium- products 1a,b were produced in 46 and 48% yields, respective-
catalyzed direct C–H arylation was reported, and the structural ly (Table 1, entries 1 and 2). Attempted isolations of products
and electrochemical properties of the products were clarified 1c and 1d failed, despite complete conversions of TTF, because
[30]. This motivated us to synthesize novel multistage redox- the polarities of the mono-, di-, and triarylated TTFs were
TTFs bearing 1,3-dithiole rings on aromatic rings, 1–3 extremely close to that of the tetraarylated TTF 1c and the solu-
(Figure 1). In addition, we focused on cross-conjugated systems bility of these compounds were low and almost beyond isola-
with 1,3-dithiole rings, which are of interest as novel multi- tion for 1d (Table 1, entries 3 and 4). The palladium-catalyzed
stage redox systems as well as donor components for organic C–H arylation of 5 with 6a,b proceeded to give products 2a,b in
conductors [1,31-41]. The palladium-catalyzed C–H arylation 75 and 86% yields, respectively (Table 1, entries 5 and 6). On
might offer access to new cross-conjugated molecules bearing the other hand, it was difficult to produce 3 in the same manner
vinyl-extended TTF moieties (EBDTs), such as 4 (Figure 1), because 2-bromothiophenes 7 bearing a 1,3-dithiole ring at the
and the electrochemical properties of these representatives 5-position were unstable, for example, 7a decomposed at
should be brought to light. Herein, we report the synthesis around 41–42 °C (Scheme 1a). Therefore, we achieved the syn-
and electrochemical properties of tetrathiafulvalene derivatives thesis of 3a by Pd-catalyzed thienylation of TTF using acetal-
1–4.
protected 8, followed by deprotection using PTSA·H2O and
Figure 1: Target compounds.
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Beilstein J. Org. Chem. 2020, 16, 974–981.
Table 1: Synthesis of compounds 1 and 2.
entry
TTF or 5
6 (equiv)
conditions
yield of 1 or 2 (%)
1
2
3
4
5
6
TTF
TTF
TTF
TTF
5
6a (5)
A
1a: 46
6b (5)
6c (5)
A
1b: 48
A or B
A or B
Aa
1c: 0 (mixture)
1d: 0 (mixture)
2a: 75
6d (5)
6a (2.5)
6b (2.5)
5
Aa
2b: 86
aReaction time 24 h.
Scheme 1: Synthesis of compounds 3.
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Beilstein J. Org. Chem. 2020, 16, 974–981.
P(OEt)3-mediated cross coupling with 11 (Scheme 1b). of redox waves, respectively (around +0.03, +0.10, +0.17, and
The cross-conjugated molecule 4 was prepared in two proce- +0.42 V vs Fc/Fc+ for 1a and −0.05, +0.10, and +0.46 V vs
dures; one was the palladium-catalyzed C–H arylation of TTF Fc/Fc+ for 1b, Figure 4). The redox potentials of 1a,b are sum-
with bromide 12 (Scheme 2a) and the other was the marized in Table 2 together with the related compound TTF.
Vilsmeier–Haack reaction of 1a, followed by triethyl phosphite- The redox waves observed at +0.42 V for 1a and +0.46 V for
mediated cross coupling with 11 (Scheme 2b).
1b were likely related to the second redox of the central TTF
moiety because they were close to the E2 of TTF (+0.37 V).
The remaining redox processes observed at around +0.03,
Theoretical calculations
The DFT calculations of compounds 1a, 3a, and 4 have been +0.10, and +0.17 V for 1a, and −0.05 and +0.10 V for 1b might
carried out by using the B3LYP/6-31G(d,p) method [42]. have been derived from the first redox of the central TTF
Figure 2 shows an optimized geometry of 1a, and the 1,3- moiety and the redox of the four outer 1,3-dithiole rings. To
dithiole rings are labeled as A–E and A'–E'. This molecule elaborate the exact oxidation potentials and the number of elec-
adopted a nonplanar structure. The angle between the two 1,3- trons involved in each oxidation step, a digital simulation tech-
dithiole rings in the center (A–A') was 158.0°. The dihedral nique was applied [44]. As a result, the observed redox waves
angles between A and B, A and B', A' and C, and A' and C' of 1a matched the simulated waves (Table 2). It was indicated
were 49.8°, 137.7°, 137.7°, and 49.9°, respectively. The that the redox wave at +0.10 V was due to an overlap of the
HOMO, HOMO−1, and LUMO of 1a are shown in Figure 3. sequential two stages of the one- and two-electron transfer
The HOMO of 1a was mainly located on the TTF moiety, waves at +0.07 and +0.12 V, while the other waves corre-
whereas the HOMO−1 was located on the benzene ring and the sponded to one-electron transfer processes. The simulation
outer 1,3-dithiole rings at the peripheral part of TTF. The results of 1a also showed that the redox wave simulated at
LUMO of 1a spread over the whole molecule except the outer +0.020 V might have been derived from the central TTF moiety
1,3-dithiole rings. The orbital energy of the HOMO of 1a because of the close ΔE values (+0.40 V for 1a and +0.46 V for
(−4.41 eV) was slightly higher than that of TTF (−4.57 eV). As TTF). The same discussion was applied to 1b. In addition, the
such, the first oxidation of 1a might occur at a lower potential potentials related to the outer 1,3-dithiole rings of 1a,b were
than TTF [43].
influenced by the substituents, that is, 1b bearing electron-do-
nating methyl groups had more negative redox potentials than
1a. As a consequence, the one-electron redox process of the
Cyclic voltammetry analysis
The redox behavior of 1–4 was investigated by cyclic voltam- TTF moiety and the multi-electron redox processes of the outer
metry. The compounds 1a and 1b exhibited four and three pairs 1,3-dithiole rings might have overlapped in 1b.
Scheme 2: Synthesis of compound 4.
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Beilstein J. Org. Chem. 2020, 16, 974–981.
Figure 2: An optimized structure of 1a. a) Top view, b) side view, and
c) labeling of the 1,3-dithiole rings.
Compound 2a exhibited three pairs of redox waves (−0.05,
+0.09, and +0.49 V vs Fc/Fc+). The redox waves observed at
−0.05 and +0.49 V were likely related to the TTF derivative (5)
moiety, because they were close to the E1 and E2 of 5, respec-
tively (Table 2). The comparison of the peak currents of each
wave indicated that the redox wave observed at +0.09 V
involved a two-electron transfer, while the redox waves ob-
served at −0.05 and +0.49 V corresponded to one-electron
transfer processes (see the differential pulse voltammetry
Figure 3: Molecular orbitals of 1a.
(DPV) in Supporting Information File 1, Figure S2). These and the simulation results are also summarized in Table 2,
results supported the above-mentioned oxidation potentials and together with those of the related compounds TTF and 14 (see
the number of electrons involved in each oxidation step of 1a,b. Figure 5). The redox process observed at +0.53 V was likely
related to the second redox of the central TTF moiety because
Compound 4 exhibited three pairs of redox waves (around this was the closest to the value of the E2 of TTF (+0.37 V) out
−0.09, +0.09, and +0.53 V vs Fc/Fc+). The redox potentials of 4 of all potentials of the related compounds TTF and 14. The
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Beilstein J. Org. Chem. 2020, 16, 974–981.
Figure 4: Cyclic voltammograms of 1a,b, 2a, and 4 in PhCN/CS2 1:1 (v/v) solution.
Table 2: Redox potentials of 1, 2a, 4, and related compoundsa.
compound
observed or
simulated value
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
1a
observed
around +0.10
+0.03b
+0.17
+0.42
simulated
observed
observed
observed
simulated
observed
observed
observed
+0.020 +0.070 +0.120
+0.200 +0.420
+0.10 +0.46b
1b
2a
4
−0.05
−0.05
+0.09
+0.49
+0.09
around −0.09
+0.53b
−0.060 −0.030 +0.010 +0.047 +0.053 +0.098 +0.102 +0.110 +0.180 +0.500
TTFc
5c
−0.09
−0.01
−0.07
+0.37
+0.42
+0.09
14c
aIn PhCN/CS2 1:1 (v/v) containing 0.1 M n-Bu4NPF6. bAnodic peak. cIn PhCN containing 0.1 M n-Bu4NPF6. All potentials were measured against an
Ag/Ag+ reference electrode and converted to vs Fc/Fc+.
remaining redox processes observed at around −0.09 and tively. The observed potentials of 4 were generally consistent
+0.09 V might have been due to the first redox of the central with the simulated ones. The results of a digital simulation also
TTF moiety, and the overall redox of the EBDT sites, respec- showed that the redox wave observed at around −0.09 V and
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Beilstein J. Org. Chem. 2020, 16, 974–981.
+0.09 V corresponded to three stages of one-electron transfer reasons: a) overlap of the central TTF moiety and the redox of
and six stages of one-electron transfer processes, respectively. the EBDT sites, b) the succeeding eight-electron oxidations of
In addition to the overlap of the first redox of the central TTF the non-equivalent EBDT sites, and c) the small ΔE value
moiety and the redox of the EBDT sites, each redox potential of (0.16 V) of the EBDT sites.
the succeeding eight-electron oxidations of the EBDT sites
might have slightly shifted due to the non-equivalence of them.
Also, the small ΔE value (0.16 V) of 14 caused the redox wave
overlap. For these reasons, the first and second redox waves of
4 were broad compared to those of 1a and 1b.
Supporting Information
Supporting Information File 1
Synthetic procedures, theoretical chemical and
electrochemical details, and copies of NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-86-S1.pdf]
Funding
This work was partially supported by a Grant-in-Aid for Scien-
tific Research (JP15H03798), from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT). This work
was also supported by a Grant-in-Aid for Research Promotion,
Ehime University, to The Research Unit for Development of
Figure 5: Related compound 14.
The redox waves of 1a,b and 4 derived from the second redox Organic Superconductors and to The Research Unit for Power
of the central TTF moiety (+0.42 V for 1a, +0.46 V for 1b, and Generation and Storage Materials.
+0.53 V for 4) shifted to higher potentials than the second redox
of TTF because of the instability of the hexacationic state of
ORCID® iDs
1a,b, and the decacationic state of 4 compared to the dicationic
states of TTF caused by on-site Coulomb repulsion between
positive charges in the central TTF moiety and the outer 1,3-
dithiole rings. The same discussion could be applied to com-
pounds 2a. In addition, the observed peak currents of 1a and 4
in the high potential region at +0.4 to +0.5 V were smaller than
those of the simulated waves. This phenomenon might be
understood by considering that electron transfer of this redox
reaction was slow enough to become the rate-determining step
because the crowded structure by which the TTF core is partici-
pating in this redox process is surrounded by extended aromatic
rings bearing 1,3-dithiol rings.
Aya Yoshimura - https://orcid.org/0000-0001-9967-4598
Dhananjayan Vasu - https://orcid.org/0000-0001-7597-3710
Takashi Shirahata - https://orcid.org/0000-0003-0899-3767
Hideki Yorimitsu - https://orcid.org/0000-0002-0153-1888
Yohji Misaki - https://orcid.org/0000-0002-9079-8487
Preprint
A non-peer-reviewed version of this article has been previously published
as a preprint doi:10.3762/bxiv.2020.16.v1
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