Oxidative and reductive cyclization in stiff dithienylethenes

Article abstract of DOI:10.3762/bjoc.14.259

The electrochemical behavior of stiff dithienylethenes, undergoing double bond isomerization in addition to ring-closure, has been investigated. Electrochromism was observed in almost all cases, with the major pathway being the oxidatively induced cyclization of the open isomers. The influence of the ring size (to lock the reactive antiparallel conformation) as well as substituents (to modulate the redox potential) on the electrocyclization was examined. In the series of derivatives with 6-membered rings, both the E- and the Z-isomer convert to the closed isomer, whereas for the 7-membered rings no cyclization from the E-isomer was observed. For both stiff and normal dithienylethenes bearing benzonitrile substituents an additional and rare reductive electrocyclization was observed. The mechanism underlying both observed electrocyclization pathways has been elucidated.

Full text of DOI:10.3762/bjoc.14.259

Oxidative and reductive cyclization in stiff dithienylethenes
Michael Kleinwächter, Ellen Teichmann, Lutz Grubert, Martin Herder and Stefan Hecht*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2812–2821.
Department of Chemistry & IRIS Adlershof, Humboldt-Universität zu
Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
doi:10.3762/bjoc.14.259
Received: 26 July 2018
Email:
Accepted: 30 October 2018
Published: 09 November 2018
Stefan Hecht* - sh@chemie.hu-berlin.de
* Corresponding author
This article is part of the thematic issue "Photoredox catalysis for novel
organic reactions".
Keywords:
diarylethenes; electrochromism; molecular switches;
(spectro)electrochemistry
Guest Editor: P. H. Seeberger
© 2018 Kleinwächter et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The electrochemical behavior of stiff dithienylethenes, undergoing double bond isomerization in addition to ring-closure, has been
investigated. Electrochromism was observed in almost all cases, with the major pathway being the oxidatively induced cyclization
of the open isomers. The influence of the ring size (to lock the reactive antiparallel conformation) as well as substituents (to modu-
late the redox potential) on the electrocyclization was examined. In the series of derivatives with 6-membered rings, both the E- and
the Z-isomer convert to the closed isomer, whereas for the 7-membered rings no cyclization from the E-isomer was observed. For
both stiff and normal dithienylethenes bearing benzonitrile substituents an additional and rare reductive electrocyclization was ob-
served. The mechanism underlying both observed electrocyclization pathways has been elucidated.
Introduction
Diarylethenes (DAEs) are a rich family of organic photo- both reaction modes to the molecular structure is still under
switches formally derived from stilbene [1,2]. Upon irradiation discussion [22-26]. There are only few reports about reductive
they are able to undergo reversible photoisomerization based on isomerization, each involving the ionic methylpyridinium group
6π-electrocyclization and -cycloreversion, respectively, be- as a substituent on the photochromic unit [27-29]. However, by
tween two thermally stable states, which make them interesting a combination of suitable substituents, a bidirectional system
components for optical memories [3,4]. In addition to photo- able to operate in both switching directions either electrochemi-
chemistry, the isomerization of DAEs can also be triggered by cally or photochemically has been reported [28].
electrochemical means, therefore providing a stimulus orthogo-
nal to light [5].
We have recently developed a new subclass of DAEs without
the geometric constraint of the central endocyclic olefin bridge
Electrochemically induced isomerization of DAEs is almost ex- yet with two rings each involving one of the bridge’s carbon
clusively based on oxidation. Either cyclization [5-13] or atoms to lock the photoreactive antiparallel conformation and
cycloreversion [14-21] can be observed, while correlation of thus provide stiff dithienylethenes (sDTEs), in analogy to stiff
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stilbenes [30-32]. Due to enabled isomerization of the central subsequently relate to other members of this new family of
exocyclic double bond, sDTEs form a three-state system under- compounds, possessing either different ring sizes or aromatic
going interconversion between ring-open E- and Z-isomers and substituents with different electronic properties (Scheme 2).
a ring-closed C-isomer (Scheme 1).
Interestingly, we found that oxidative cyclization can occur
from both double bond isomers. In addition, a reductive cycliza-
Here we describe the oxidatively and reductively induced isom- tion was discovered in bis(benzonitrile)-substituted DTEs and
erization behavior of sDTEs as investigated by cyclovoltam- also in a DTE with an extended π-system.
metry (CV) and spectro-electrochemistry (SEC) [33].
Results and Discussion
Our present study is aiming to: 1) elucidate the influence of
possible double bond isomerization on the electrochromism of Cyclization by anodic oxidation
sDTEs; 2) explore the structural effect of varying ring size In initial experiments, the electrochemical behavior of the
as well as electronic modification; and 3) contribute to the methyl-substituted derivative bearing six-membered rings
mechanistic understanding of electrochromism in DAEs in (sDTE66-Me) was investigated. For both configurational
general.
isomers, i.e., E- and Z-sDTE66-Me, an irreversible oxidation
wave corresponding to the transfer of two electrons was ob-
First, we discuss the simple methyl-substituted sDTE deriva- served in the cyclic voltammograms (Figure 1a), with the
tive sDTE66-Me, consisting of two 6-membered rings, and Z-isomer (blue dashed line) being slightly easier to oxidize than
Scheme 1: Combining double bond isomerization (E/Z) and cyclization/cycloreversion (Z/C) in three-state switching sDTEs. Photochemically, both
ring-open isomers are converted to the closed isomer C in high yield when irradiated with near UV light. Upon irradiation with visible light the C-isomer
undergoes quantitative cycloreversion exclusively to the Z-isomer [33].
Scheme 2: Overview of all sDTE and reference DTE compounds investigated in this study. The compound names indicate the molecular frame
(e.g., sDTE66) and the substituents attached to the 5-position of the thiophenes (e.g., -PhOMe). For all compounds, the major isomer obtained in the
synthesis [33] is shown.
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Figure 1: Cyclic voltammograms of sDTE66-Me. a) Both E- (black line) and Z-isomer (blue dashed line) display one irreversible oxidation wave, which
is assigned to the formation of the closed isomer. b) The closed isomer (red line), generated in situ from Z-sDTE66-Me upon irradiation with 313 nm
light prior to the CV measurement, displays two distinct oxidation waves. The first one is reversible whereas the second is quasi-reversible. Experi-
ments were carried out in MeCN with 0.1 M Bu4NPF6, c = 1∙10−3 M, dE/dt = 1 V s−1.
E-sDTE66-Me (black line). In contrast to both open isomers, both E- and Z-sDTE66-Me convert from their colorless initial
two separate one-electron oxidation waves were observed for charge-neutral state into the same oxidized species which
the closed isomer C-sDTE66-Me (Figure 1b), generated either displays characteristic absorption bands centered at 493 nm and
in the irreversible oxidation process or photochemically. Both 621 nm (Figure 2a and b; for an overlay of spectra see Figure
of these oxidation events occur at significantly lower potentials S29, Supporting Information File 1).
compared to the open isomers, reflecting the increased energy
of the HOMO due to the extended π-system generated upon To identify the nature of this product, SEC was performed on
ring-closure.
the photochemically generated closed isomer (Figure 2c,d).
Herein, the reversible first oxidation step yields the radical
Irreversible oxidation of the open isomer of DAEs has already cation C+• (Figure 2c, light blue), identified by its characteristic
been observed and was ascribed to cyclization [6-13]. Indeed, red-shifted absorption at 731 nm and 912 nm due to an unpaired
the two separate one-electron reduction waves arising during electron. The radical cation C+• is stable even at the slow scan
the back-sweep after oxidation of the E- and Z-isomer match rates of SEC and builds up continuously as evidenced by clean
exactly those of the independently photochemically prepared isosbestic points. In a subsequent, second oxidation step, the
closed isomer (Figure 1b). Furthermore, in a second consecu- radical cation C+• is converted to the dication C2+ (Figure 2d,
tive redox cycle (Figure S11, Supporting Information File 1) green), as indicated by the hypsochromic shift due to the
also two oxidation waves for C-sDTE66-Me are observed. absence of an open-shell system. The formed dication exhibits
Interestingly, oxidatively induced cyclization seems to occur characteristic absorption bands centered at 493 nm and 621 nm,
similarly from both Z- and E-configured open isomers, i.e., identical to the species formed upon oxidation of the open
regardless of the double bond geometry. Since ring-closure isomers.
requires the two reactive α-thienyl carbon atoms to approach
each other, presumably an additional electrochemically induced Notably, while the first oxidation step of the closed isomer
E → Z isomerization occurs prior to cyclization. Indeed, there (C → C+•) is fully reversible, the second oxidation (C+• → C2+)
are scattered reports about configurational isomerism in stil- is only quasi-reversible. Formation of an unknown follow-up
bene radical cations [34,35] and simple dithienylethenes product (FP) occurs at the low scan rates of the SEC experi-
[36-38]. As such, we postulate an equilibrium between both ment, indicated by the appearance of a reduction wave at a sig-
E- and Z-radical cations with the latter rapidly reacting to the nificantly lower potential (Epc = −0.530 V) during the return
closed isomer.
scan (Figure 2d). At the same time the reduction waves corre-
sponding to the closed isomer, as observed in the CV experi-
To gain a deeper mechanistic understanding of the oxidative ment with high scan rates (Figure 1), disappear. Furthermore,
cyclization, the evolution of the UV–vis absorption spectra the UV–vis spectrum recorded in the SEC after reduction is an
during a CV cycle was recorded in a SEC cell. It was found that overlay of that of the closed isomer and the new species FP
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Figure 2: Spectroelectrochemistry of sDTE66-Me. Absorption changes during CV, insets showing the corresponding cyclovoltammograms with red
dots marking when UV–vis–NIR spectra were measured. A stable intermediate is formed upon oxidation of: a) E-sDTE66-Me and b) Z-sDTE66-Me.
c) Oxidation of photogenerated C-sDTE66-Me to the stable radical cation C+•. d) Further oxidation of C+• to the dication FP2+, measured in a separate
experiment. At the low sweep rates of SEC an irreversible reduction wave at Epc = −0.530 V occurs in a), b), and d). Experiments were carried out in
MeCN with 0.1 M Bu4NPF6, c = 5∙10−4 M, dE/dt = 10 mV s−1.
possessing an absorbance maximum at 358 nm (Figure S29, tive for sDTE66-Me, it can still be observed at the fast scan
Supporting Information File 1). This kind of irreversible rates (1 V s−1). Notably, for phenyl-substituted sDTEs the
process from an oxidized closed isomer has already been ob- extent of FP2+-formation is lower, and for the donor-substi-
served for DAEs [7], but its nature has not been further dis- tuted C-sDTE66-PhOMe the initial absorption spectrum after a
cussed except that it is different from the typical photochemical complete redox cycle is fully recovered, even at low scan rates
byproduct of DAEs [39,40].
(for a comparison of full redox cycles of C-sDTE66-Me and
C-sDTE66-PhOMe see Figure S30, Supporting Information
Although the exact identity of FP/FP2+ remains elusive, several File 1). This observation confirms the enhanced stabilization of
characteristics can be summarized: 1) FP2+ is formed from both the C2+ cation by electron-donating groups [8]. 3) Upon reduc-
the open and the closed isomers upon oxidation (Figure S41 and tion of FP2+, both FP as well as C are formed. UPLC analysis
Figure S42, Supporting Information File 1) and its reduction of a preparative scale oxidation and subsequent reduction of
wave is shifted to lower potentials compared to C2+ for all ob- Z-sDTE66-Me as well as C-sDTE66-Me (Figure S41 and
served cases by up to >600 mV (see Table S1, Supporting Infor- Figure S42, Supporting Information File 1) showed mainly the
mation File 1). 2) The amount of FP2+ formed depends on both closed isomer after the reduction step. However, in these exper-
the electronic structure of the photoswitch and the scan rate. iments an insoluble off-red film deposited on the platinum elec-
While at slow scan rates (10 mV s−1) the conversion is quantita- trode that resisted further analysis (for a photograph, see Figure
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Beilstein J. Org. Chem. 2018, 14, 2812–2821.
S41d, Supporting Information File 1). 4) The absorption spec-
trum of FP is red-shifted compared to the two open isomers yet
blue-shifted compared to the closed isomer (Figure S29b, Sup-
porting Information File 1). 5) Upon reduction, a species analo-
gous to FP was not observed (vide infra and Figure S43, Sup-
porting Information File 1).
Considering all these findings, a mechanism for the oxidative
cyclization of sDTE66-Me (Scheme 3) can be derived: Starting
from either open isomer fast isomerization to the same dication
C2+ takes place upon oxidation. Both, the isomerization of the
double bond of the E-isomer as well as the cyclization reaction
are instantaneous on the timescale of the experiment. The
closed dication C2+ can be reduced stepwise to its neutral form.
In addition, when C2+ is not stabilized by donor substituents it
undergoes a subsequent reaction to the structurally yet unidenti-
fied follow-up product FP2+, which upon reduction, at least
partially, transforms into the charge-neutral closed isomer C.
Note that during oxidative cyclization the characteristic absorp-
tion of the cation radical C+• at 731 nm is not observed
(Figure 2a and b), suggesting a concerted two-electron oxida-
tion of the open isomers and thermal cyclization in the dica-
tionic state (vide infra).
Figure 3: Anodic peak potentials (Epa) of sDTEs and reference com-
pounds in MeCN. Solid circles refer to the first oxidation potential,
hollow circles to the second oxidation potential (if available). For
detailed data, see Table S1, for all respective cyclovoltammograms,
see Figure S10 to Figure S27 in Supporting Information File 1.
For most ring-open compounds investigated, the E-isomer
Influence of ring size and substitution
displayed a higher or at least equal oxidation potential when
To correlate structural parameters with the observed electro- compared to the Z-isomer. This difference for both open
chemical behavior, sDTEs with other ring sizes (sDTE55-Me isomers is rather surprising in view of their very similar
and sDTE77-R) and derivatives bearing peripheral phenyl sub- absorption maxima in first approximation reflecting the
stituents with different electronic properties (-OMe, -Br, -CN, HOMO–LUMO gap. The sole exception to this trend is the only
(-CF3)2) as well as various reference compounds were exam- derivative with an H-substituted double bond ethene-Me, which
ined. The oxidation potentials for the investigated compounds exhibit the expected behavior of a less facile oxidation of the
(see Scheme 2) are summarized in Figure 3 and Table S1 (Sup- Z-isomer due to its somewhat twisted π-system leading to less
porting Information File 1).
pronounced π-conjugation and lowering the HOMO level. Even
Scheme 3: Proposed mechanism for the oxidative cyclization of sDTE66-Me. Upon two-fold oxidation, both open isomers cyclize to the dication C2+.
This dication can reversibly be reduced to the closed isomer C in two consecutive steps. Upon low sweep rates, an unidentified follow-up product FP
forms that at least partially can be reduced to C.
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more surprising is the comparison of methyl-substituted deriva- Cyclization by cathodic reduction
tives of different ring sizes. Instead of the expected ease of oxi- Except for rare examples of methylpyridinium substituted DTEs
dation with an increasing number of carbons due to their donat- [27,28] and dithiazolylethenes [29], ring closure under reduc-
ing inductive (+I) effect, the exact opposite trend was observed tive conditions has not been reported for DAEs. For most struc-
with sDTE77-Me being most difficult to oxidize. In the group tures the reduction potential of the open isomer is too negative
of sDTE66-R derivatives, however, the influence of the substitu- to be reached within the redox window determined by the elec-
ent is in accordance with its electron-donating or electron-with- trolyte. Thus, a strongly electron-deficient substituent such as
drawing ability.
pyridinium is necessary to shift the reduction potential to acces-
sible values. However, not every electron-withdrawing group
The closed isomers are, in general, much easier to oxidize as the appears to be suited to induce cathodic cyclization [40,42].
open isomers, even in cases such as sDTE77-Me, in line with
the largely reduced HOMO–LUMO gap of the colored closed In the case of the electron-deficient benzonitrile derivative
isomers implying an energetically higher and thus more acces- sDTE66-PhCN the reduction potential could be reached and for
sible HOMO level. The first and second oxidation potential are the Z-isomer an irreversible two-electron reduction wave at
shifted depending on the electron-donating or electron-with- Epc = −2.526 V was detected (Figure 4). Upon re-oxidation, a
drawing character of the attached substituents, similarly to what new peak matching that of the photochemically generated
is known for normal DAEs [40,41]. The differences between closed isomer, determined in a separate experiment, was ob-
the first and second oxidation wave seem to depend on the served. Likewise, in a second cycle (Figure S24, Supporting
nature of the substituents with donors reducing the gap, presum- Information File 1), the reduction wave of the closed isomer at
ably by more efficient stabilization of the dication, and on con- Epc = −1.920 V appeared, clearly indicating reductive cycliza-
formational rigidity dictating the extent of π-conjugation be- tion. The reductive formation of C-sDTE66-PhCN was
tween both hemispheres.
unequivocally proven by a preparative electrolysis of
Z-sDTE66-PhCN and subsequent product analysis, showing a
Similar to the model compound sDTE66-Me, all available closed to Z-isomer ratio of 69:31 (Figure S43, Supporting Infor-
sDTE66 derivatives as well as both Z-configured sDTE77 deriv- mation File 1). Interestingly and in strong contrast to the
atives undergo electrocyclization upon oxidation. This also Z-isomer, for the E-isomer a reversible reduction wave was ob-
holds true for all cyclopentene-bridged DTE derivatives. How- served and no cyclization product was formed (Figure 4a). Note
ever, in butene-Me the formation of FP2+ is the predominant that, however, both open isomers undergo oxidative cyclization
reaction pathway even at high scan rates, while for E-sDTE55- (Figure S19b and Figure S20b, Supporting Information File 1).
Me as well as both isomers of ethene-Me no characteristic
cathodic waves of the closed isomers were observed. For To investigate the generality of reductive cyclization mediated
E-sDTE77-Me neither oxidative cyclization nor formation of by cyano groups, we subjected the cyclopentene-bridged DTE
FP2+ was found despite its irreversible oxidation wave.
with benzonitrile substituents (DTE-PhCN) to these conditions.
Figure 4: Cyclic voltammograms of sDTE66-PhCN. The reduction of a) E-sDTE66-PhCN (black line) is reversible, whereas the reduction of
b) Z-sDTE66-PhCN (blue line) yields the closed isomer, indicated by the characteristic oxidation wave of C-sDTE66-PhCN (red dashed line), gener-
ated photochemically. Experiments in a) DMF with 0.1 M Bu4NPF6, c = 5∙10−4 M, dE/dt = 1 V s−1 and b) MeCN with 0.1 M Bu4NPF6, c = 5∙10−4 M,
dE/dt = 1 V s−1.
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The electrochemistry of this compound [8] and its hexafluoro- nonsymmetrical DAEs bearing two electronically distinct
cyclopentene derivative [43] have already been investigated, but aryl moieties (CF3- and Me-thiazole) [5] or thiophenes
its behavior under reductive conditions has not been discussed. possessing donor and acceptor substituents (-Ph/-PhOMe,
Indeed, an irreversible reduction wave at Epc = −2.436 V was -Ph(CF3)2/-PhNMe2, and -Ph(CF3)2/-PhOMe) have been inves-
found, giving rise to an oxidation wave corresponding to the tigated [8,40]. However, sufficient separation of the oxidation
closed isomer (Figure S23, Supporting Information File 1). waves in order to assure a fully stepwise oxidation process is
Once formed, the closed isomers of both nitrile-substituted difficult to achieve and could only be realized in the case of
compounds, i.e., sDTE66-PhCN and DTE-PhCN, can be re- modified thiazoles [5] or using the strongly donating -PhNMe2
versibly reduced and oxidized (Figure S20 and Figure S23, Sup- substituent [40]. In these compounds, the first oxidation wave is
porting Information File 1). The two-electron reduction of the fully reversible and anodic cyclization occurs only after the
closed isomer was found to occur in a stepwise manner as indi- second step, forming the open dication (EEC mechanism).
cated by SEC of C-DTE-PhCN clearly showing an intermedi-
ate radical anionic species C−• (Figure S40, Supporting Infor- By combining strongly electron-donating and electron-with-
mation File 1).
drawing substituents within one molecule (Me2NPh-DTE-
PhCN), we could access an open isomer showing two separat-
Moreover, the phenomenon of reductive cyclization appears not ed one-electron waves upon both oxidation and reduction
to be restricted to DTEs with strongly electron-withdrawing (Figure 6). As in the examples above, the first oxidation wave
groups. We found that the electronic effects of an extended of O-Me2NPh-DTE-PhCN is fully reversible and anodic cycli-
π-system lead to similar results. As such, the bis(4-(9,9- zation occurs only from the open dication (Figure 6b). In addi-
dimethyl-9H-fluoren-2-yl)phenyl)-substituted DTE-PhFluo- tion, also the first reduction wave is fully reversible (Figure 6a).
rene undergoes reductive as well as oxidative cyclization However, because the second reduction potential was not acces-
(Figure 5), a phenomenon that we have recently also observed sible, no cyclization was observed. It is noteworthy that the
for reductively and oxidatively induced azobenzene Z→E isom- reduction of O-Me2NPh-DTE-PhCN and O-DTE-PhCN
erization [44,45].
occur at very similar potentials (−2.457 V and −2.436 V, re-
spectively), thus indicating that both hemispheres in open DTEs
are only very weakly conjugated.
Mechanism of the electrochemical
isomerization
In the literature, either mono- [7,18,22,23,46] or bis-oxidized From the fact that the radical anion does not undergo cycliza-
[9-11] open DAEs have been reported and discussed [8,47] as tion we conclude that a concerted EEC mechanism is valid for
the key intermediate undergoing thermal cyclization or cyclore- both oxidative and reductive cyclization in our compounds
version, typically in the context of so-called “ECE” and “EEC” (Scheme 4). There is, however, a marked difference between
mechanisms, respectively [48]. To contribute to this discussion, the oxidative and reductive pathway: In the dicationic state a
Figure 5: Cyclic voltammogram of DTE-PhFluorene. The ring-closed isomer (red dashed line) is formed both under a) reductive and b) oxidative
conditions from O-DTE-PhFluorene (blue line), as shown by the emerging characteristic oxidation and reduction waves, respectively. Experiments in
a) DMF with 0.1 M Bu4NPF6, c = 1∙10−3 M, dE/dt = 1 V s−1 and b) DCM with 0.2 M Bu4NPF6 c = 7∙10−4 M, dE/dt = 100 mV s−1. For two consecutive
oxidation and reduction cycles, see Figure S25 in Supporting Information File 1.
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Figure 6: Cyclic voltammograms of Me2NPh-DTE-PhCN displaying separated one-electron anodic and cathodic waves. a) Reversible first one-elec-
tron reduction (blue line) of O-Me2NPh-DTE-PhCN. The second reduction potential cannot be accessed. First (red dashed line) and second one-elec-
tron reduction (red dotted line) of C-Me2NPh-DTE-PhCN are shown for comparison. b) Oxidative cyclization of O-Me2NPh-DTE-PhCN occurs only at
the second oxidation/reduction step (blue dotted line), whereas the first step (blue line) is reversible. The oxidation waves of the closed isomer (red
dashed line) are shown for comparison. The asterisk marks the oxidation wave of residual open isomer after irradiation. Experiments in MeCN with
0.1 M Bu4NPF6, c = 1∙10−3 M, dE/dt = 1 V s−1.
Scheme 4: Proposed mechanism to explain the observed selectivity of anodic and cathodic cyclization in sDTE66 derivatives: a) Upon two-fold oxida-
tion, an equilibration between the two dications of the E- and Z-isomers is possible due to rotation about the formed central single bond. Once formed,
Z2+ is trapped as C2+. b) In contrast, bond rotation is not possible for the dianion E2− and as a consequence, reductive cyclization is only possible
from Z2−.
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Beilstein J. Org. Chem. 2018, 14, 2812–2821.
ORCID® iDs
resonance structure exists in which the central double bond is
resolved and thus bond rotation is allowed. This is reflected by
the fact that both the E- and the Z-isomers of sDTEs are able to
undergo oxidative cyclization. In contrast, in the dianionic state
formal cross-conjugation between the thiophene moieties and
the central double bond persists and no rotation takes place.
Thus, only the dianion of the Z-isomer undergoes cyclization
while the reduction of the E-isomer is reversible. This behavior
differs strongly from that of stilbene, in which reduction occurs
at the double bond and causes an equilibration of the double
bond isomers in favor of the E-stilbene [49].
Ellen Teichmann - https://orcid.org/0000-0002-4301-1530
Stefan Hecht - https://orcid.org/0000-0002-6124-0222
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