Switchable Mesomeric Betaines Derived from Pyridinium-Phenolates and Bis(thienyl)ethane

Article abstract of DOI:10.1002/ejoc.202100279

Syntheses of push–pull substituted non-symmetric bis(thienyl)ethenes (BTEs) possessing a central perfluorocyclopentene core are described. The substituent effects of anisole, phenole, and phenolate as well as pyridine, pyridinium, and N-methylpyridinium substituents, joined through their 3- or 4-positions to the central BTE core, respectively, cover the range from very strongly electron-donating [σ(4-phenolate)=?1.00] to extremely strongly electron-withdrawing [σ(pyridinium-4-yl)=+2.57] in the title mesomeric betaines. The different isomers possessing 4-yl/4-yl, 4-yl/3-yl and 3-yl/3-yl substituents represent different combinations of conjugated and cross-conjugated partial structures and cause different spectroscopic properties. In addition, through-space conjugation between the 2- and 2′-position of the thiophenes can be observed which circumvents the charge-separation of through-bond cross-conjugation. The BTE possessing the push–pull chromophore consisting of 3-anisole and 4-pyridinium substituents (24) displays the best extinction coefficients within the series of compounds described here (?=33.8/15.7 L/mol ? cm), while the mesomeric betaine possessing an N-methylpyridinium-4-yl and a 4-phenolate substituent (29) displays considerable bathochromic shifts to λ_max=724 nm in its closed form.

Full text of DOI:10.1002/ejoc.202100279

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Switchable mesomeric betaines derived from pyridinium-
phenolates and bis(thienyl)ethane.
Authors: Sven Nagorny, Felix Lederle, Viktor Udachin, Thea
Weingartz, Eike G. Hübner, Sebastian Dahle, Wolfgang
Maus-Friedrichs, Jörg Adams, and Andreas Schmidt
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100279
Link to VoR: https://doi.org/10.1002/ejoc.202100279
01/2020

10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
Switchable mesomeric betaines derived from pyridinium-
phenolates and bis(thienyl)ethane.
Sven Nagorny,1 Felix Lederle,2 Viktor Udachin,3,4 Thea Weingartz,1 Eike G. Hübner,1 Sebastian
Dahle,3,4 Wolfgang Maus-Friedrichs,3,4 Jörg Adams,5 and Andreas Schmidt1*
Abstract: Syntheses of push-pull substituted non-symmetric
bis(thienyl)ethenes (BTEs) possessing central
catalysts.[7] The chemical and physical properties of mesomeric
betaines are dependent on their type of conjugation which can be
identified in different ways. Thus, frontier orbital profiles,
resonance forms, dipole increments, and isoconjugated
equivalents[1] of alternant and nonalternant hydrocarbon anions
and dianions, respectively, are characteristic of the different types
of conjugation.[8] Recent connectivity matrices lead to five distinct
classes of mesomeric betaines as a basis for a deeper
understanding of structure - properties relationships.[9] Among
those five distinct classes, conjugated, cross-conjugated, and
pseudo-cross-conjugated mesomeric betaines are the most
important.[10] Here, pyridinium-phenolate 1 is given as an example
of conjugated mesomeric betaines, and these are the
characteristics (Scheme 1): i) common atoms for either charges
in the resonance forms exist (A), ii) cation and anion are joined
through a starred position of the isoconjugate equivalent (B), iii)
the starred positions are active sites of the highest occupied
molecular orbital (HOMO, C), and iv) 1,3-dipoles can be dissected
from the resonance forms (D). A prominent derivative of 1 is
Reichardt´s betaine which has been used to define the ET(30) and
ETN values of solvent polarities.[11]
a
perfluorocyclopentene core are described. The substituent effects of
anisole, phenole, and phenolate as well as pyridine, pyridinium, and
N-methylpyridinium substituents, joined via their 3- or 4-positions to
the central BTE core, respectively, cover the range from very strongly
electron-donating [(4-phenolate) = -1.00] to extremely strongly
electron-withdrawing [(pyridinium-4-yl)
=
+2.57] in the title
mesomeric betaines. The different isomers possessing 4-yl/4-yl, 4-
yl/3-yl and 3-yl/3-yl substituents represent different combinations of
conjugated and cross-conjugated partial structures and cause
different spectroscopic properties. In addition, through-space
conjugation between the 2- and 2´-position of the thiophenes can be
observed which circumvents the charge-separation of through-bond
cross-conjugation. The BTE possessing the push-pull chromophore
consisting of 3-anisole and 4-pyridinium substituents (24) displays the
best extinction coefficients within the series of compounds described
here ( = 33.8 / 15.7 L/mol•cm), while the mesomeric betaine
possessing an N-methylpyridinium-4-yl and a 4-phenolate substituent
(29) displays considerable bathochromic shifts to max = 724 nm in its
closed form.
Introduction
Mesomeric betaines are conjugated molecules which can
exclusively be represented by dipolar canonical formulae in which
the positive and negative charges are delocalized within a
common -electron system.[1] They play major roles as masked
1,3-dipoles in heterocyclic chemistry (sydnones,[2] münchnones[3]),
as natural products,[4] biologically active molecules (e.g.
Molsidomine[5]), crypto-N-heterocyclic carbenes[6] and as
1
M.Sc. Sven Nagorny, M.Sc. Thea Weingartz, Prof. Dr. Eike G. Hübner,
Prof. Dr. A. Schmidt, Clausthal University of Technology, Institute of
Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld
(Germany)
Fax:(+ 49)-5323-722834
E-mail: schmidt@ioc.tu-clausthal.de
https: www.ioc.tu-clausthal.de
Scheme 1. Characteristics of conjugated mesomeric betaines.
2
3
M. Sc. Felix Lederle, Clausthal University of Technology, Institute of
Energy Research and Physical Technologies, Am Stollen 19 B, D-
38640 Goslar, Germany
M. Sc. Viktor Udachin, Assist. Prof. Dr. Sebastian Dahle, Prof. Dr.
Wolfgang Maus-Friedrichs, Clausthal University of Technology, Institute
of Energy Research and Physical Technologies, Leibnizstrasse 4, D-
38678 Clausthal-Zellerfeld, Germany
Clausthal University of Technology, Clausthal Centre for Material
Technology, Agricolastrasse 2, D-38678 Clausthal-Zellerfeld, Germany
Prof. Dr. Jörg Adams, Clausthal University of Technology, Institute of
Physical Chemistry, Arnold-Sommerfeld-Strasse 4, D-38678 Clausthal-
Zellerfeld, Germany; adams@pc.tu-clausthal.de
By contrast, betaine 2 is cross-conjugated and these are the
characteristics (Scheme 2): i) positive and negative charges are
strictly delocalized in separated parts of the common -electron
system according to the rules of resonance (E), ii) cation and
anion are connected via an unstarred position of the isoconjugate
equivalent (F), iii) the unstarred positions are nodal positions of
the HOMO (G),[12] and iv) 1,4-dipoles can be dissected from the
resonance forms (H). The alkaloid Punicine from Punica
granatum is a derivative of 2 from nature and can exist as cross-
conjugated mesomeric betaine as shown, or as its tautomer.[13]
4
5
*
Correspondence: schmidt@ioc.tu-clausthal.de; Tel.: +49-5323-723861
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
bis(thienyl)ethenes have been published so far,[17],[18] although the
BTE core is of great ongoing interest. BTE 12o is a typical
example with a broad variety of applications,[19] among those
optical nanopattering[20] and ultra-compact nanophotonic optical
modulators[21] as well as electrochemical switches.[22] Therefore,
its properties has been examined experimentally[23] and
theoretically.[24] In general, BTEs reversibly undergo
photochemical 6-electrocyclizations of their ring-opened form
(12o) via their 1,3,5-hexatriene partial structure to give ring-closed
forms such as 12c. BTEs with a 4-hydroxyphenyl group at the 5-
position of a thiophene ring and a 4-pyridyl moiety at the other
have been studied in terms of photochemical pKa-modulation,[25]
as red fluorescent molecular switches,[26] for polarization optical
recording,[27] and for metal complexations (Re, Ru, W) to modulate
the spectroscopic properties.[28] These applications took
advantage of the fact that the -conjugated chain length can be
controlled by the reversible hybridization from sp2 to sp3 of the C2-
position of the thiophenes[29] and reversible changes from
through-bond cross-conjugated systems to through-bond
conjugated push-pull-chromophors. Phenolates and pyridinium
substituents and their precursors cover the entire range from
strongly electron-donating to strongly electron-withdrawing;
Hammett -constants are shown in Table 1. In view of the ongoing
interest in controlling molecular properties of BTEs we report here
on variations of the connectivity via the - and -positions of the
pyridinium ring and the para- and meta-positions of the phenolate
to realize different types of conjugation in the target molecules.
Scheme 2. Characteristics of cross-conjugated mesomeric betaines.
Molecular switches can take advantage of the different types of
conjugation of mesomeric betaines. Thus, the 1,4-dipole 3 causes
intramolecular cycloadditions on irradiation to yield a bis-lactam
4[14] (Scheme 3). Under charge neutralization, the alkaloid
Shihunine 5 can be switched into spiro compound 6.[4] Conjugated
pyridinium-ylides 8, formed from pyridinium salts
7
by
deprotonation, undergo reversible disproportionations to form
radical anions 9.- and radical cations 9.+.[15] Similarly, the
aforementioned alkaloid Punicine 10, especially its viologen
derivative,[16] is switchable in terms of photo-induced reversible
redox reactions to form 11.- and 11.+ by disproportionation. The
switching process is accompanied by color changes from yellow
to deep blue.
Scheme 4. Cross-conjugation ↔ conjugation interconversions on switching
bis(thienyl)ethenes.
Table 1. Hammett  values of substituents[34]
substituent
-
position
meta / 3-yl
para / 4-yl
OMe
+0.12
-0.27
OH
O
pyridine
+0.55
-0.94
pyridinium
+2.10
Scheme 3. Switchable mesomeric betaines.
+0.12
-0.37
-0.71
-1.00
+2.57
As part of an ongoing project we aimed at mesomeric betaines of
the pyridinium-phenolate type such as Reichardt´s betaine (c.f.
Scheme 1) and Punicine (c.f. Scheme 2) possessing
bis(thienyl)ethene (BTE) moieties as switchable spacer between
cationic and anionic partial structures (Scheme 4). To the best of
our knowledge, only two mesomeric betaines derived from
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
gaseous HCl to form the corresponding pyridinium salts 26o – 28o
as chlorides, whereupon all pyridine H resonance frequencies
1
shifted to lower field. The shift has the same magnitude as of the
corresponding O-methylated species 23o – 25o, which cannot
form tautomers, under identical conditions. The -protons of the
pyridine ring typically appear at approximately 8.60 ppm (17o, 18o)
and 8.82 ppm (singlet) (19o) in the 1H NMR spectra taken in
DMSO-[D6], whereas the corresponding signals shift by
approximately 0.18 ppm (17o), 0.24 ppm (18o), and 0.50 ppm
(19o) to lower field on formation of the corresponding pyridinium
salts. In addition, bathochromic shifts of the UV absorption
maxima up to 30 nm were observable on protonation to the
pyridinium salts. The protonation was fully reversible in all cases.
Thus, the predominant tautomer in solution is pyridine-BTE-
phenol, as shown in Scheme 5, rather than the zwitterionic
pyridinium-BTE-phenolate.
Results and Discussion
The
target
betaines
were
prepared
starting
from
dibromomethylthiophene 13 which gave the boronic acid 14 in
good yields according to modified literature procedures.[30] Best
yields were achieved when the lowest amount of 1 M NaOH was
used for the work-up procedure. 4-Bromopyridine, 3-
bromopyridine, 4-bromoanisole, and 3-bromoanisole were then
used to prepare the building blocks 15a-d by Suzuki-Miyaura
reactions.[31] We found that the order in which the chemicals were
added had a considerable impact on the outcome of the reaction.
Yields decreased considerably (to 23 - 45%) when the halide was
added after the boronic acid and before the addition of the catalyst
and the base. Vice versa, best yields were achieved (62 - 78%)
when the reagents were added in the order (1.) boronic acid, (2.)
solvent, (3.) base (as aq. solution), (4.) catalyst, (5.) halide. The
subsequent synthesis to yield the non-symmetric BTEs were
carried out in analogy to modified literature procedures.[32] The
best compromise between yield and monosubstitution of
perfluorocyclopentene to give 16a,d was found when 1,5 eq. were
used and added at once. The non-symmetric BTEs were then
prepared starting from 16a,d and 15b-d and best yields were
obtained when the reaction partners were cooled to -80°C prior to
reaction. We prepared the 4,4-, 4,3- and 3,3-isomers 17o, 18o and
19o which readily undergo 6-electrocyclizations on irradiation to
yield their ring-closed forms 17c, 18c, and 19c, respectively. On
irradiation of the open forms at 300 nm in CDCl3 over a period of
120 min. at 0,5 mW applying a Xenarc lamb, 82% of the 4,4-
isomer, 60% of the 4,3-isomer, and 56% of the 3,3-isomer were
converted into their corresponding closed forms (c.f. Table 3), as
determined by 1H NMR spectroscopy. O-Demethylation was
performed applying BBr3 followed by hydrolysis which yielded the
phenols 20o, 21o, and 22o, respectively. The phenol´s proton can
be detected at  = 9.7 ppm (DMSO-[D6]) in the 1H NMR spectra.
A series of methylation conditions [iodomethane, methyl triflate,
Meerwein´s reagent (Me3OBF4)] was tested to prepare the N-
methylpyridinium moieties. Best results were achieved when
dimethylsulfate in the presence of catalytic amounts of
nitrobenzene were used, and the successful reaction was
unambiguously proved by shifts of the pyridine´s protons by
approximately  = 0.25 ppm (-H) and  = 0.85 ppm (-H) to
lower field. The target molecules 29o, 30o, and 31o were obtained
as hygroscopic oils which are exclusively soluble in polar protic
solvents such as aqueous alcohols and water. Under these
conditions, they are in equilibrium with their phenols [29o+H+ OH-],
[30o+H+ OH-], and [31o+H+ OH-], as evidenced by NMR and
UV/Vis spectroscopic examinations. In agreement with
observations concerning the aforementioned pKa-modulators[29]
and also phenol-group substituted diarylethenes which are
stabilized under acidic conditions,[33] the betaines are destabilized
in the absence of water and decompose within some hours.
Therefore, conversion rates of the betaines could not be
determined. Of all target betaines, 29o has been prepared before
following another protocol.[25] To gain information about the
predominant tautomer of 20o – 22o, i.e. [pyridine-BTE-phenol] vs.
[pyridinium+-BTE-phenolate-], we treated these phenols with
Scheme 5. Synthesis of the target molecules 29o – 31o.
In view of the theory of mesomeric betaines[1],[9] and its application
to BTEs, the target betaines can be represented by several
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
dipolar canonical forms, four of which are shown (29o / 29o´ / 29c
/ 29c´) (Scheme 6). According to the rules of resonance, through-
bond cross-conjugation of the open form 29o causes a -
electronic charge separation between the two wings of the
molecule as indicated by strictly separated sites of positive and
negative charges in the canonical formulae. In contrast to
literature-known mesomeric betaines, however, additional
aspects have be taken into account, as 29o is not planar according
to a DFT calculation (N31G6**/PBE0-d3) (Figure 1). As a
consequence,
the
through-bond
cross-conjugation
is
circumvented by through-space conjugation. Thus, whereas the
pyridinium and phenolate rings are almost coplanar with their
adjacent thiophene rings in vacuo (C1-C5-C6-C7= -3.337°; C1´-
C5´-C6´-C7´= -3.326°), the thiophene-pyridinium and the
thiophene-phenolate moieties are twisted by -45.6° and +46.5°
from the plane of the double bond of the cyclopentene (C2-C3-
C1´´-C2´´ & C2´-C3´-C2´´-C1´´). Due to steric reasons the 1,3,5-
hexatriene partial structure is twisted as well. Thus, dihedral
angles C2-C3-C1´´-C2´´ = 11.580° and C2´-C3´-C2´´-C1´´=
15.191° were calculated. As a consequence, the distance
between the 2-positions of the thiophenes in the most stable
conformation of 29o was calculated to be 328.1 pm which is
smaller than the sum of the van-der-Waals radii (340 pm). The 3-
positions have a distance of 309.0 pm. This close proximity
enables a spatial electron communication between the cationic
and anionic parts of the betaine by through-space conjugation. -
Electronic short-circuits have only been documented for through-
bond conjugated bridges in mesomeric betaines so far, which
largely influence their physical and chemical properties,[1] but to
the best of our knowledge through-space conjugations in
mesomeric betaines have not yet been observed. Much interest
has recently been directed to through-space conjugation in face-
Scheme 6. Characteristics of the 4,4-connectivity before (29o) and after the
photochemical ring-closure (29c).
to-face
aromatics[35]
including
cyclophanes,[36]
hexaarylbenzenes,[37] helical foldamers[38] and others, due to their
capacities to transport charges and energies via a spatial
channel.[39] Interplane distances of -stacked aromatic systems of
280 pm in cyclophanes[40] up to 349.4 pm[35] in -stacked
aromatics have been measured. In contrast to the
aforementioned systems, the through-space conjugation in
mesomeric betaine 29o is enabled by the proximity of the 2-
positions of the thiophenes and not by -stacking between the
pyridinium and the phenolate which required the formation of
another conformer. Calculated partial charges of the pyridinium
and phenolate segments (DFT, N31G6**/PBE0-d3) were
determined to be +0.390 and -0.378, respectively, indicating a
high degree of charge-separation in the ground state. On ring-
closure these values decrease. However, their magnitude prove
the considerable contribution of dipolar resonance forms such as
29c´ at the expense of the neutral conjugated form 29c.
Additionally, we re-orientated the y-axis with a different ZMAT, so
that the y-axis is parallel to the line connecting the nitrogen and
the oxygen atom. Along this axis the dipole moment was
calculated to be -26.496 D for the open form 31o and -18.324 D
for the closed form 29c. The change of the type of conjugation on
irradiation is thus well reflected in these values. As summarized
in Table 2, the bond lengths C1”-C3, C5-C6 and C5´-C6´ are
shortened after the ring-closure due to increased -contributions.
Figure 1. Most stable conformation of 29o and 29c.
Due to the through-space conjugation the frontier orbital profile of
29o is not typical of cross-conjugated mesomeric betaines[12] to
which it formally belongs (Figure 2).[1][4][9][10] Thus, the highest
occupied molecular orbital (HOMO) of 29o is essentially located
in the phenolate ring and in the adjacent thiophene ring in
accordance with the sites of negative charges in the resonance
forms. In analogy, the lowest unoccupied molecular orbital
(LUMO) of 29o is essentially located in the pyridinium ring and in
the adjacent C3 and C4 positions of the adjacent thiophene ring.
However, the through-space conjugation causes active positions
of the HOMO in the thiophen-2-yl pyridinium moiety, and, vice
versa, of the LUMO in the thiophen-2-yl phenolate ring which sets
the betaines clearly apart from typical examples.[12]
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
The HOMO and LUMO of 30o are essentially located in the
phenolate and the pyridinium rings, respectively, with small
contributions in the opposite wings of the molecule (Figure 3). The
HOMO of 30c is in well accord with a substituent in cross-
conjugation, whereas the LUMO is diagnostic for a delocalized
positive charge over the entire -system. These characteristics
sets the 4,3-isomer 30o clearly apart from the 4,4-isomer 29o.
Figure 2. Frontier orbital profiles of 29o and 29c.
Marked differences exist between the 4,4-isomer 29 and the 4,3-
isomer 30 (Scheme 7). According to the rules of resonance, the
negative charge of 30o is restricted to the phenolate ring which
translates into increased group partial charges and a larger
calculated permanent dipole moment in comparison to its isomer
29o. Obviously, the connectivity of the phenolate influences the
group partial charge of the pyridinium, which itself remained
unchanged in comparison to 29. In contrast to the 4,4-isomer, a
conjugated mesomeric betaine 30c is formed after the ring-closure
reaction. This can easily be recognized by common atoms for
either charges in the canonical formulae[1][4] and the group partial
charges. As expected, the values of the latter are between those
of cross-conjugated mesomeric betaines (c.f. 31c) and conjugated
systems (c.f. 29c).
Figure 3. Frontier orbital profiles of 30o and 30c.
In contrast to the 4,4- and 4,3-isomers, the 3,3-isomer 31o
remains a formal cross-conjugated mesomeric betaine after the
photochemical transformation to 31c. As a consequence, the
calculated permanent dipole moment of 31c is the highest of the
entire series (Scheme 8), and group partial charges indicate an
effective -electronic charge separation with respect to the frontier
orbitals by cross-conjugation[1][4][12] before and after the ring-
closure.
Scheme 8. Characteristics of the 3,3-isomers 31o and 31c.
Scheme 7. Characteristics of the 4,3-isomers 30o and 30c.
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
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As expected, calculations predict that the 5,6- and 5´-6´ bond
lengths are shortened to a much lesser extent due to much
weaker -interactions in comparison to the 4,4-connectivity. Table
2 summarizes selected calculated bond lengths of all isomers.
As observed for the other isomers, the HOMO and LUMO of the
open-form betaine 31o are not strictly localized in separated parts
of the -electron system. In the ring-closed form 31c the HOMO
is almost exclusively located in the phenolate ring, whereas the
LUMO is distributed over the entire molecule (Figure 4).
Thus, a phenolate in conjugation as in 29o causes a resonance
frequency at  = 7.23 ppm of the thiophene´s proton, whereas
cross-conjugation gives signals at approximately  = 7.35 ppm
(30o, 31o) in accordance with the restricted delocalization of the
negative charge. In general, the thiophen´s resonance
frequencies of all compounds of the series described here display
considerable up-field shifts on ring-closure (c.f. Table S1,
Supporting Information). The following conclusions can be drawn
from results of UV/Vis measurements of the mesomeric betaine
precursors 17 – 28 (Table 3 and Supporting Information) which
are soluble in chloroform: Exchange of the methoxy group
towards the OH group has the largest influence on the absorption
maxima of the 4,4-isomer (17c→20c; max = 14 nm;  = -0.2
L/mol•cm), but no effect on the corresponding ring-opened forms
(17o→20o; max = 0 nm;  = -10.4 L/mol•cm). A small effect is
observed on the absorption maximum of the cross-conjugated
3,3-isomer accompanied by a considerable effect on the
extinction coefficient (19c→22c, max = 3 nm;  = -8.3 L/mol•cm).
N-Protonation of the ring-closed forms results in considerable
bathochromic shifts, the magnitude of which is dependent on the
connectivity, as it is 72 - 61 nm for the 4,4- (17c→23c, 20c→26c),
59 – 51 nm for the 4,3- (18c→24c, 21c→27c), and 25 – 23 nm for
the cross-conjugated 3,3-isomers (22c → 28c, 19c → 25c),
respectively. The BTE 27o has the highest conversion rate of the
series of compounds. Thus, after 2 h of irradiation with a Xenarc
lamb at  = 300 nm in CDCl3 85% were converted into the closed
form 27c, as determined by 1H NMR spectroscopy.
Figure 4. Frontier orbital profiles of 31o and 31c.
Table 3. UV/Vis spectroscopic properties of the target betaine precursors as
well as of reference BTE 12.
BTE
17o
20o
23o
26o
18o
21o
24o
27o
19o
22o
25o
28o
12o
max
300
300
293
291
300
300
285
353
300
300
305
305
325
/103
31.1
20.7
18.8
10.4
37.0
9.8
BTE
17c
20c
23c
26c
18c
21c
24c
27c
19c
22c
25c
28c
12c
max
604
618
676
679
604
613
663
664
590
593
615
616
613
/103
8.9

conversion
0.82
Table 2. Selected calculated bond lengths before and after the ring-closure
reaction
4,4
4,3
8.7
0.69
Connectivity
Compound
Ring-opened form 29o
Ring-closed form 29c
Difference
Bond 1”-3
145.9 pm
141.1 pm
4.8 pm
Bond 5-6
141.6 pm
138.8 pm
2.8 pm
Bond 5´-6´
142.5 pm
138.4 pm
4.1 pm
12.6
6.2
0.77
0.66
4,4
13.7
2.7
0.60
0.56
Ring-opened form 30o
Ring-closed form 30c
Difference
146.2 pm
140.1 pm
6.1 pm
142.3 pm
139.5 pm
2,8 pm
146.3 pm
143.4 pm
2,9 pm
33.8
8.3
15.7
3.2
0.58
4,3
3,3
0.85
21.2
14.3
17.1
8.4
12.8
4.5
0.56
3,3
Ring-opened form 31o
Ring-closed form 31c
Difference
146.2 pm
138.8 pm
7.4 pm
145.1 pm
143.6 pm
1.5 pm
146.4 pm
143.6 pm
2.8 pm
0.75
10.6
2.6
0.56
0.71
Ref
29.7
17.5
0.70
All spectra were measured in chloroform with a concentration of 0.05 mg/mL.
max is given in [nm], /103 in [L / mol·cm]. Conversion refers to formation of the
closed form starting from the opened form in CDCl3 after irradiation time of 120
min. under the measurement conditions described, determined by 1H NMR
spectroscopy.
The aforementioned discussed characteristics translate into
spectroscopic properties. The 1H NMR resonance frequencies of
the thiophene´s unsubstituted positions are diagnostic of the type
of conjugation. Thus, the pyridinium-4-yl substitution pattern
which is in conjugation with the thiophene ring as realized in 29o
and 30o, causes resonance frequencies at  = 8.23 ppm - 8.24
ppm of the thiophene´s proton (4-H). Cross-conjugation between
pyridinium and the thiophene, realized by the pyridinium-3-yl
substitution pattern of 31o, causes signals at considerably higher
field, i.e.  = 7.93 ppm in accordance with the distribution of the
positive charge in the canonical formulae. The thiophene ring
attached to the phenolate moiety (4´-H) behaves analogously.

As the target betaines are hygroscopic and have a rather limited
solubility in chloroform, UV/Vis measurements were performed in
methanol (Table 4). As mentioned before, the mesomeric
betaines predominantly exist in their protonated form under these
conditions. To obtain the UV-Vis spectra of the betaines, the
protonated open-form compounds (29o+H+), (30o+H+), and
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
(31o+H+) were dissolved in methanol, switched to the closed form
and then treated with aq. 1M NaOH aq. solution. As evidenced by
19F NMR spectroscopy, no substitution of the fluorine atoms takes
place under these conditions. On betaine formation, the 3,3-
connectivity results in a hypsochromic shift [(31c+H+)→31c, max
= 24 nm), whereas the other isomers display bathochromic shifts
on betaine formation [(29c+H+) →29c, max = 61 nm, (30c+H+) →
30c, max = 33 nm]. Betaine synthesis starting from the 4-pyridyl-
BTE-4-anisol precursor 17c thus results in a bathochromic shift of
more than approximately 120 nm to max = 724 nm. The target
betaines, however, proved to be too sensitive to determine
accurate conversion rates to calculate extinction coefficients.
Concerning the photostability, we chose BTE 24 for a comparison
with the reference BTE 12, because it has the highest extinction
coefficient of the entire series which we prepared. We subjected
12 and 24 dissolved in CDCl3 to irradiation with a Jasco FP-8500
spectrofluorometer (Xenarc lamp with 10 nm bandwidth and an
intensity of 0.5 mW) to monitor the behaviour by means of 1H and
13C NMR spectroscopy. After 24 h the NMR spectra did not show
any decomposition products.
to the reference BTE 4,4''-(perfluorocyclopent-1-ene-1,2-
diyl)bis(5,5'-dimethyl-2,2'-bithiophene) 12 from the literature.
Experimental Section
All reactions were carried out under an atmosphere of nitrogen in flame or
oven-dried glassware. All chemicals were purchased and used without
further purification unless otherwise mentioned. Anhydrous solvents were
dried according to standard procedures before usage. Melting points are
uncorrected and were determined in an apparatus according to Dr. Tottoli
(Büchi). The ATR-IR spectra were obtained on a Bruker Alpha in the range
1
of 400 to 4000 cm-1. H NMR spectra were recorded at 400 MHz or 600
MHz. 13C NMR spectra were recorded at 100 MHz or 150 MHz, with the
solvent peak used as the internal reference. Multiplicities are described by
using the following abbreviations: s = singlet, d = doublet, t = triplet, q =
quartet, and m = multiplet. Signal orientations in DEPT experiments were
described as follows: o = no signal; + = up (CH, CH3); - = down (CH2). The
mass spectra (EIMS) were measured with a Varian 320 MS Triple Quad
GC/MS/MS. The HR-MS spectra were obtained with a Bruker Impact II and
Bruker Daltonik Tesla-Fourier transform-ion cyclotron resonance mass
spectrometer. UV measurements were performed using a Jasco FP-8500
spectrofluorometer. Chromatography: The reactions were traced by thin
layer chromatography with silica gel 60 (F254, company MERCK KGAA).
For the detection of substances, quenching was used at either 254 nm or
366 nm with a mercury lamp. The preparative column chromatography was
conducted through silica gel 60 (230 400 mesh) of the company MERCK
KGAA.
Table 4. UV/Vis spectroscopic properties of the target betaines.
Compd.
(29+H+)o
29o

max [nm]
294
Compd.
(29+H+)c
29c

max [nm]
663
4,4
4,3
3,3
316
724
(30+H+)o
30o
342
(30+H+)c
30c
641
Calculations:
300
608
(31+H+)o
307
(31+H+)c
602
Calculations. All density-functional theory (DFT)-calculations were carried
out by using the multithreaded Firefly 8.2.0 QC package,[42] which is
partially based on the GAMESS (US)[43] source code, running on Windows
10 Pro (Version 10.0.17763.914) (x86_64) on an 16 core AMD 2950X
processor workstation. MM2 optimized structures were used as starting
geometries. Complete geometry optimizations were carried out on the
implemented N31G6** basis set and with the PBE0 density functional
including D3 dispersion correction. All calculated structures were proven
to be true minima by the absence of imaginary frequencies. Orbital plots
were obtained using Jmol 14.27.2.[44] Compositions of molecular orbitals
were calculated using the AOMix program.[45],[46] Partial charges were
obtained with NBO 5.9 from the results of the DFT calculations.[47]
31o
300
31c
626
All spectra were measured in methanol with a concentration of 0.05 mg/mL.
Conclusions
Series of push-pull substituted non-symmetric bis(thienyl)ethenes
(BTEs) possessing a central perfluorocyclopentene core under
variation of the connectivity and the type of conjugation have been
prepared, among those the target mesomeric betaines of the
pyridinium-phenolate type. The non-planarity of the BTEs enable
through-space conjugations by a close proximity of the 2/2´-
positions of the thiophene rings which short-circuit the through-
bond cross-conjugation of the ring-opened form. Depending on
the 4,4-, 4,3-, and 3,3-connectivities of anisole, phenole, and
phenolate in combination with pyridine, pyridinium, and N-
methylpyridinium substituents, photochemical conversions
change the type of conjugation which is well reflected in
calculated permanent dipole moments, group partial charges,
frontier orbital profiles as well as spectroscopic properties. Among
the switchable molecules prepared in this project, robust BTEs
possessing push-pull chromophores have been identified, among
those the cationic 4-pyridinium-BTE-3-methoxyphenyl derivative
24 which displays the highest extinction coefficient of the series
of BTEs described here ( = 33.8 / 15.7 L/mol•cm) in its ring-
opened and ring-closed form, respectively. Concerning the
absorption maxima, the mesomeric betaine possessing N-
methylpyridinium-4-yl and 4-phenolate substituents 29c displays
considerable bathochromic shifts to max = 724 nm in comparison
4,4''-(Perfluorocyclopent-1-ene-1,2-diyl)bis(5,5'-dimethyl-2,2'-
bithiophene (12o). The reaction is carried under light protection. A solution
of 2.71 g (9.9 mmol) of 4-bromo-5,5’-dimethyl-2,2’-bithiophene in dry THF
is cooled to -80°C. This mixture is treated with 5.80 mL (10.4 mmol) of n-
BuLi (1.8
M in hexane). After 30 min 0.66 mL (4.9 mmol) of
octafluorocyclopentene are added and the reaction is stirred for 3 hours at
-80°C. After the mixture reached room temperature, the reaction is
quenched with water and extracted with diethyl ether. The organic phases
were collected and dried over MgSO4. The solvent was removed in vacuo.
Column chromatography was used to purify the crude product using
hexanes as eluent to afford the pure product, stored light protected in a
fridge. Yield 1.49 g (54%), a yellow solid. 1H-NMR (400 MHz, CDCl3): δ =
6.95 (s, 2H, 3-H), 6.84 – 6.83 (m, 2H, 3’-H), 6.66 – 6.65 (m, 2H, 4’-H), 2.40
(s, 3H, 6-H), 1.85 (s, 3H, 6’-H) ppm. Spectroscopic data are in agreement
with those reported in the literature.[20a]
3,5-Dibromo-2-methylthiophene (13). To a solution of 10.00 g (101.9
mmol) of 2-methylthiophene in glacial acetic acid 36.3 g (203.7 mmol) of
NBS are added in portions. The reaction is stirred at room temperature for
48 h. The reaction mixture is extracted with DCM and the organic phase is
washed with 1 M aqueous NaOH (2 L). After the organic phase is dried
over MgSO4 the solvent is removed in vacuo to afford the product. Yield
25.92 g (99%), a yellow liquid. 1H-NMR (400 MHz, CDCl3): δ = 6.86 (s, 1H,
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
4-H), 2.33 (s, 3H, 6-H) ppm. Spectroscopic data are in agreement with
those reported in the literature.[19g]
The reaction is carried out under nitrogen atmosphere. 1 eq. halide is
dissolved in dry THF and the mixture is cooled to -80°C. To this solution
1.1 eq of 2.8 M n-BuLi in cyclohexane solution are added. After 30 – 60
min octafluorocyclopentene (1.5 eq.) is added in one portion. After 2 h the
reaction is allowed to warm up to room temperature and is stirred overnight.
Water is added to the solution. Diethyl ether is used for extraction, the
organic phase is washed with water. The combined organic layers are
dried over MgSO4 and the solvent is removed in vacuo followed by column
chromatography to afford the target molecule.
(4-Bromo-5-methylthiophen-2-yl)boronic acid (14). A solution of 17 g
(66.4 mmol) of dibromothiophene 13 in dry diethyl ether is cooled to -80°C
followed by the addition of 24.9 mL (69.7 mmol) of a 2.8 M n-BuLi in
cyclohexane solution. After 10 min of stirring at this low temperature 19.7
mL (73.06) mmol of B(OBu)3 is added. The reaction is allowed to warm to
room temperature. 1 M aqueous HCl is added to the reaction followed by
the extraction with diethyl ether. The organic phase is collected and treated
with 1 M NaOH aqueous solution. The aqueous layer is collected. Under
virgorous stirring 37% of HCl is slowly added to the solution until a
precipitate is formed. The solid is filtered and dried in vacuo. Yield 11.35 g
(77.4%), orange powder. 1H-NMR (400 MHz, DMSO d6): δ = 8.35 (s, 2H,
6-H), 7.49 (s, 1H, 3-H), 2.36 (s, 3H, 7-H) ppm. Spectroscopic data are in
agreement with those reported in the literature.[30]
4-(5-Methyl-4-(perfluorocyclopent-1-en-1-yl)thiophen-2-yl)pyridine
(16a). According to Procedure 2, a solution of 3.00 g (11.8 mmol) of 15a,
4.2 mL (11.8 mmol) n-BuLi and 2.2 mL (16.5 mmol) of
octafluorocyclopentene are reacted. Ethyl acetate is used as eluent. Yield
1.357 g (31.3%), black oil. 1H-NMR (400 MHz, CDCl3): δ = 8.63 – 8.61 (m,
2H, 3’-H), 7.45 – 7.43 (m, 2H, 2’-H), 7.44 (s, 1H, 3-H), 2.52 (s, 3H, 6-H)
ppm. Spectroscopic data are in agreement with those reported in the
literature.[32b]
General procedure of the Suzuki-Miyaura Coupling to prepare 15a-d
(Procedure 1): The reaction is carried out under nitrogen atmosphere. The
boronic acid (13) is dissolved in dry tetrahydrofuran. To this solution was
added a solution of Na2CO3 in water followed by 1 eq. of the halide and 5
mol-% of Pd(PPh3)4. The reaction outcome is monitored by TLC. After the
conversion water is added to the solution, the solution is filtered through
celite and extracted with diethylether. The organic phase is then washed
with water and dried over MgSO4. The solvent is removed in vacuo
followed by column chromatography to afford the desired product.
5-(3-Methoxyphenyl)-2-methyl-3-(perfluorocyclopent-1-en-1-
yl)thiophene (16d). According to Procedure 2, a solution of 4.6460 g (16.2
mmol) of 15d, 5.8 mL (16.2 mmol) of n-BuLi and 3.0 mL (22.7 mmol) of
octafluorocyclopentene are reacted. Petroleum ether is used as eluent.
Yield 4.10 g (63.7%), yellow oil. 1H-NMR (400 MHz, CDCl3): δ = 7.29 (t,
JH,H = 7.9 Hz, 1H, 5’-H), 7.23 (s, 1H, 4-H), 7.14 – 7.11 (m, 1H, 4’-H), 7.07
– 7.05 (m, 1H, 6’-H), 6.87 – 6.84 (m, 1H, 2’-H) 3.83 (s, 3H, 7’-H), 2.46 (s,
3H, 6-H) ppm.. Spectroscopic data are in agreement with those reported
in the literature.[32b]
4-(4-Bromo-5-methylthiophen-2-yl)pyridine (15a). According to
Procedure 1, a solution of 4.50 g (20.4 mmol) of 13, 8.64 g (81.5 mmol) of
soda, 3.22 g (20.4 mmol) of 4-bromopyridine hydrochloride and 1.18 g of
Pd(PPh3)4 (1.02 mmol) is reacted. Ethyl acetate is used as eluent. Yield
3.06 g (59.2%), a yellowish brown solid. 1H-NMR (400 MHz, CDCl3): δ =
8.59 – 8.57 (m, 2H, 3’-H), 7.38 – 7.36 (m, 2H, 2’-H), 7.31 (s, 1H, 3-H), 2.44
(s, 3H, 6-H) ppm. Spectroscopic data are in agreement with those reported
in the literature.[31b]
General Procedure for the Synthesis of the non-symmetric BTEs 17o-
19o (Procedure 3):
The reaction is carried out under nitrogen atmosphere. 1 eq. of the halide
is dissolved in dry THF and the mixture is cooled to -80°C. To this solution
1.1 eq of 2.8 M n-BuLi in cyclohexane solution is added. After 30 – 60 min
a solution of the cyclopentene unit dissolved in dry THF at -80°C is added.
After 2 h the reaction is allowed to warm up to room temperature and is
stirred overnight. Water is added to the solution. Diethyl ether is used for
extraction, the organic phase is washed with water. The combined organic
layers are dried over MgSO4 and the solvent is removed in vacuo followed
by column chromatography to afford the BTE.
3-(4-Bromo-5-methylthiophen-2-yl)pyridine (15b). According to
Procedure 1, a solution of 3.50 g (15.9 mmol) of 13, 6.72 g (63.4 mmol) of
soda, 2.50 g (15.9 mmol) of 3-bromopyridine and 0.92 g of Pd(PPh3)4 (0.8
mmol) is reacted. Petroleum ether and ethyl acetate is used as eluent (5:1).
Yield 2.88 g (71.5%), brown solid. 1H-NMR (400 MHz, CDCl3): δ = 8.78 (m,
1H, 2’-H), 8.52 – 8.50 (m, 1H, 4’-H), 7.77 – 7.74 (m, 1H, 6’-H), 7.30 – 7.27
(m, 1H, 5’-H), 7.15 (s, 1H, 3-H), 2.43 (s, 3H, 6-H) ppm. Spectroscopic data
are in agreement with those reported in the literature.[31d]
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(4-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridine
(17o). According to Procedure 3, a solution of 1.28 g (4.5 mmol) of 15c,
1.6 mL (4.5 mmol) of n-BuLi and 1.67 g (4.5 mmol) of 16a were reacted.
Petroleum ether and ethyl acetate is used as eluent (1:1). Yield 1.395 g
(50.4%), a beige powder. 1H-NMR (400 MHz, CDCl3): δ = 8.61 – 8.59 (m,
2H, 3’’’-H), 7.48 (s, 1H, 4’-H), 7.47 – 7.45 (m, 2H, 2’’-H), 7.42 – 7.40 (m,
2H, 2’’’-H), 7.14 (s, 1H, 3-H), 6.92 – 6.90 (m, 2H, 3’’-H), 3.83 (s, 3H, 6’’-H),
2.01 (s, 3H, 6-H), 1.95 (s, 3H, 6’-H) ppm. Spectroscopic data are in
agreement with those reported in the literature.[32b]
3-Bromo-5-(4-methoxyphenyl)-2-methylthiophene (15c). According to
Procedure 1, a solution of 3.50 g (15.9 mmol) of 13, 6.72 g (63.4 mmol) of
soda, 2.96 g (15.9 mmol) of 4-bromoanisole and 0.92 g of Pd(PPh3)4 (0.8
mmol) is reacted. Petroleum ether is used as eluent. Yield 1.12 g (25.0%),
a bright yellow solid. 1H-NMR (400 MHz, CDCl3): δ = 7.44 – 7.42 (m, 2H,
2’-H), 6.98 (s, 1H, 4-H), 6.91 – 6.89 (m, 2H, 3’-H), 3.82 (s, 3H, 5’-H), 2.40
(s, 3H, 6-H) ppm. Spectroscopic data are in agreement with those reported
in the literature.[31a]
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridine
(18o). According to Procedure 3, a solution of 1.00 g (3.9 mmol) of 16d,
1.4 mL (3.9 mmol) of n-BuLi and 1.56 g (3.9 mmol) of 15a are reacted.
Petroleum ether and ethyl acetate is used as eluent (1:1). Yield 0.824 g
(38.0%), beige powder. 1H-NMR (600 MHz, CDCl3): δ = 8.61 – 8.60 (m,
2H, 3’’’-H), 7.48 (s, 1H, 4’-H), 7.41 – 7.40 (m, 2H, 2’’’-H), 7.29 (t, JH,H = 8.3
Hz, 1H, 5’’-H), 7.25 (s, 1H, 3-H), 7.13 – 7.11 (m, 2H, 4’’-H), 7.05 – 7.04 (m,
1H, 6’’-H), 6.86 – 6.83 (m, 1H, 2’’-H), 3.83 (s, 3H, 7’’-H), 2.00 (s, 3H, 6-H),
1.97 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz, CDCl3): δ = 160.1 (o, 3’’-C),
150.4 (+, 3’’’-C), 143.8 (o, 2’-C), 142.4 (o, 2-C), 141.3 (o, 5-C), 139.0 (o,
5’-C), 136.9 (o, 7-C), 135.5 (o, 7’-C), 134.5 (o, 1’-C), 130.1 (+, 5’’-C), 126.4
(o, 3’-C), 125.6 (o, 4-C), 125.0 (+, 4’-C), 122.6 (+, 3-C), 119.6 (+, 2’’’-C),
118.2 (+, 6’’-C), 116.2 (o, 8-C, 8’’-C), 113.4 (+, 4’’-C), 111.5 (+, 2’’-C),
111.1 (o, 9-C), 55.4 (+, 7’’-C), 14.7 (+, 6-C), 14.6 (+, 6’-C) ppm. IR (ATR):
3-Bromo-5-(3-methoxyphenyl)-2-methylthiophene (15d). According to
Procedure 1, a solution of 12.00 g (54.3 mmol) of 13, 23.03 g (217.3 mmol)
of soda, 10.16 g (54.3 mmol) of 3-bromoanisole and 3.14 g of Pd(PPh3)4
(2.7 mmol) is reacted. Petroleum ether is used as eluent. Yield 9.76 g
(63.4%), brown solid. 1H-NMR (400 MHz, CDCl3): δ = 7.27 (t, JH,H = 7.9 Hz,
1H, 5’-H), 7.10 – 7.08 (m, 1H, 4’-H), 7.09 (s, 1H, 4-H), 7.03 – 7.02 (m, 1H,
6’-H), 6.84 – 6.81 (m, 1H, 2’-H) 3.83 (s, 3H, 7’-H), 2.41 (s, 3H, 6-H) ppm.
Spectroscopic data are in agreement with those reported in the
literature.[31c]
General procedure for the synthesis of a BTE-precursors 16a,d
(Procedure 2):
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
ν
̃
= 2953, 2922, 2840, 1735, 1592, 1436, 1334, 1268, 1190, 1110, 1047,
(600 MHz, CDCl3): δ = 9.03 – 8.83 (m, 1H, 2’’’-H), 8.67 – 8.50 (m, 1H, 4’’’-
H), 7.58 – 7.50 (m, 1H, 6’’’-H), 7.43 – 7.37 (m, 1H, 5’’’-H), 7.34 (s, 1H, 4’-
H), 7.29 (t, JH,H = 8.3 Hz, 1H, 5’’-H), 7.25 (s, 1H, 3-H), 7.13 – 7.11 (m, 1H,
6’’-H), 7.05 – 7.03 (m, 1H, 2’’-H), 6.85 – 6.83 (m, 1H, 4’’-H), 3.83 (s, 3H,
7’’-H), 2.00 (s, 3H, 6-H), 1.97 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz,
CDCl3): δ = 160.1 (o, 3’’-C), 147.8 (+, 4’’’-C), 145.7 (+, 2’’’-C), 142.7 (o, 2’-
C), 142.2 (o, 2-C), 141.3 (o, 5-C), 137.9 (o, 5’-C), 136.6 (o, 7-C), 135.6 (o,
7’-C), 134.5 (o, 1’-C), 133.3 (+, 6’’’-C), 130.0 (+, 5’’-C),127.0 (+, 5’’’-C),
126.2 (o, 3’-C), 125.5 (o, 4-C), 124.0 (+, 4’-C), 122.5 (+, 3-C), 118.1 (+, 6’’-
C), 116.1 (o, 8-C, 8’’-C), 113.3 (+, 4’’-C), 111.4 (+, 2’’-C), 110.9 (o, 9-C),
984, 892, 811, 776, 686, 535, 477 cm-1. HRMS (ESI): calc.: 552.0885
[M+H+], found: 552.0879 [M+H+]. mp.: 51.1 °C.
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1’-yl)-5’-methylthiophen-2’-yl)pyridine
(19o). According to Procedure 3, a solution of 1.20 g (4.7 mmol) of 16d,
1.7 mL (4.7 mmol) of n-BuLi and 1.87 g (4.7 mmol) of 15b are reacted.
Petroleum ether and ethyl acetate is used as eluent (1:1). Yield 1.612 g
(62.2%), a beige powder.1H-NMR (600 MHz, CDCl3): δ = 8.83 – 8.81 (m,
1H, 2’’’-H), 8.56 – 8.52 (m, 1H, 4’’’-H), 7.82 – 7.79 (m, 1H, 6’’’-H), 7.33 (s,
1H, 4’-H), 7.29 (t, JH,H = 8.3 Hz, 1H, 5’’-H), 7.26 (s, 1H, 3-H), 7.16 – 7.15
(m, 1H, 5’’’-H), 7.13 – 7.11 (m, 1H, 6’’-H), 7.05 – 7.05 (m, 1H, 2’’-H), 6.86
– 6.83 (m, 1H, 4’’-H), 3.84 (s, 3H, 7’’-H), 2.00 (s, 3H, 6-H), 1.98 (s, 3H, 6’-
H) ppm. 13C-NMR (150 MHz, CDCl3): δ = 160.1 (o, 3’’-C), 148.9 (+, 4’’’-C),
146.7 (+, 2’’’-C), 142.6 (o, 2’-C), 142.3 (o, 2-C), 141.3 (o, 5-C), 138.4 (o,
5’-C), 136.7 (o, 7-C), 135.8 (o, 7’-C), 134.5 (o, 1’-C), 132.7 (+, 6’’’-C), 130.1
(+, 5’’-C), 126.2 (o, 3’-C), 125.7 (o, 4-C), 124.2 (+, 5’’’-C), 123.7 (+, 4’-C),
122.6 (+, 3-C), 118.2 (+, 6’’-C), 116.2 (o, 8-C, 8’’-C), 113.4 (+, 4’’-C), 111.5
(+, 2’’-C), 111.1 (o, 9-C), 55.4 (+, 7’’-C), 14.6 (+, 6-C), 14.5 (+, 6’-C) ppm.
55.3 (+, 7’’-C), 14.5 (+, 6-C), 14.5 (+, 6’-C) ppm. IR (ATR): ν̃ = 3151, 2918,
2252, 1600, 1273, 1118, 903, 727, 649 cm-1. HRMS (ESI): calc.: 538.0729
[M], found: 538.0729 [M]. mp.: 102.0 °C.
General Procedure for the Demethylation of the Anisole BTEs to give
20o -22o (Procedure 5):
To a solution of the anisole BTE in dry DCM 1.2 eq. of BBr3 is added at
room temperature. The reaction is controlled via TLC. If necessary more
BBr3 is added till full completion. Water is added to the solution. Diethyl
ether is used for extraction, the organic phase is washed with water. The
combined organic layers are dried over MgSO4 and the solvent is removed
in vacuo followed by column chromatography to afford the deprotected
BTE.
IR (ATR): ν̃ = 2969, 2928, 2882, 1463, 1373, 1306, 1125, 949, 816, 641
cm-1. HRMS (ESI): calc.: 574.0704 [M+Na+], found: 574.0707 [M+Na+].
mp.: 85.4 °C.
General Procedure for the Protonation of the Pyridine of the BTEs to
form 23o-25o (Procedure 4):
The BTE is dissolved in chloroform and treated with gaseous HCl. The
solvent is removed in vacuo to obtain the fully protonated BTE.
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(2-methyl-5-(pyridin-4-yl)thiophen-3-
yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)phenol (20o). According
to Procedure 5 576 mg (1.0 mmol) of BTE 17o is treated with 607 mg (2.4
mmol) of BBr3. Yield 454 mg (81%), light yellow solid. 1H-NMR (400 MHz,
CDCl3): δ = 8.63 – 8.57 (m, 2H, 3’’’-H), 7.50 (s, 1H, 4’-H), 7.48 – 7.45 (m,
2H, 2’’-H), 7.40 – 7.38 (m, 2H, 2’’’-H), 7.10 (s, 1H, 3-H), 6.87 – 6.85 (m,
2H, 3’’-H), 2.03 (s, 3H, 5-H), 1.97 (s, 3H, 2’-H) ppm. Spectroscopic data
are in agreement with those reported in the literature.[32b]
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(4-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium(23o). According to Procedure 4 100 mg (0.2 mmol) of BTE 17o are
treated with gaseous HCl. Yield 102 mg (96%), beige powder. 1H-NMR
(600 MHz, CDCl3): δ = 8.79 – 8.72 (m, 2H, 3’’’-H), 7.87 – 7.83 (m, 2H, 2’’’-
H), 7.73 (s, 1H, 4’-H), 7.45 – 7.42 (m, 2H, 2’’-H), 7.10 (s, 1H, 3-H), 6.91 –
6.89 (m, 2H, 3’’-H), 3.82 (s, 3H, 5’’-H) 2.07 (s, 3H, 6-H), 1.94 (s, 3H, 6’-H)
ppm. 13C-NMR (150 MHz, CDCl3): δ = 159.8 (o, 3’’-C), 148.6 (o, 2’-C),
147.3 (o, 1’’’-C), 143.0 (o, 2-C), 142.9 (+, 3’’’-C), 140.4 (o, 5-C), 138.2 (o,
7-C), 135.8 (o, 5’-C), 134.3 (o, 7’-C), 129.2 (o, 1’’-C), 127.0 (+, 2’’-C), 125.8
(o, 3’-C), 125.2 (o, 4-C), 121.7 (+, 2’’’-C), 120.9 (+, 3-C), 116.0 (o, 8-C, 8’’-
C), 114.5 (+, 3’’-C), 111.5 (o, 9-C), 14.9 (+, 6-C), 14.5 (+, 6’-C) ppm. IR
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(2-methyl-5-(pyridin-4-yl)thiophen-3-
yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)phenol (21o). According
to Procedure 5 800 mg (1.5 mmol) of BTE 18o is treated with 843 mg (3.4
mmol) of BBr3. Yield 584 mg (75%), a light yellow solid1H-NMR (600 MHz,
CDCl3): δ = 8.66 – 8.59 (m, 2H, 3’’’-H), 7.48 (s, 1H, 4’-H), 7.46 – 7.44 (m,
2H, 2’’’-H), 7.23 (t, JH,H = 8.3 Hz, 1H, 5’’-H), 7.23 (s, 1H, 3-H), 7.09 – 7.07
(m, 1H, 5’’’-H), 7.04 – 7.03 (m, 1H, 2’’-H), 6.81 – 6.79 (m, 1H, 4’’-H), 2.00
(s, 3H, 6-H), 1.95 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz, CDCl3): δ = 156.7
(o, 3’’-C), 150.2 (+, 3’’’-C), 144.0 (o, 2’-C), 142.3 (o, 2-C), 141.3 (o, 5-C),
140.8 (o, 1’’’-C), 138.8 (o, 5’-C), 136.9 (o, 7-C), 135.5 (o, 7’-C), 134.7 (o,
1’’-C), 130.4 (+, 5’’-C), 126.5 (o, 3’-C), 125.6 (o, 4-C), 125.2 (+, 4’-C), 122.4
(+, 3-C), 119.9 (+, 2’’’-C), 117.8 (+, 6’’-C), 116.2 (o, 8-C, 8’’-C), 115.2 (+,
4’’-C), 112.7 (+, 2’’-C), 111.1 (o, 9-C), 14.8 (+, 6-C), 14.6 (+, 6’-C) ppm. IR
(ATR): ν̃
= 3275, 2248, 2122, 1655, 1023, 821, 759, 618 cm-1. HRMS
(ESI): calc.: 574.0699 [M+Na+], found: 574.0704 [M+Na+]. mp.: 99.1 °C.
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium(24o). According to Procedure 4 100 mg (0.2 mmol) of BTE 18o are
treated with gaseous HCl. Yield 106 mg (99%), beige powder. 1H-NMR
(600 MHz, CDCl3): δ = 8.90 – 8.83 (m, 2H, 3’’’-H), 8.02 – 7.97 (m, 2H, 2’’’-
H), 7.84 (s, 1H, 4’-H), 7.28 (t, JH,H = 8.3 Hz, 1H, 5’’-H), 7.22 (s, 1H, 3-H),
7.10 – 7.08 (m, 1H, 4’’-H), 7.03 – 7.01 (m, 1H, 6’’-H), 6.86 – 6.83 (m, 1H,
2’’-H), 3.83 (s, 3H, 7’’-H), 2.00 (s, 3H, 6-H), 1.97 (s, 3H, 6’-H) ppm. 13C-
NMR (150 MHz, CDCl3): δ = 160.2 (o, 3’’-C), 149.7 (o, 2’-C), 142.9 (o, 2-
C), 141.7 (+, 3’’’-C),141.5 (o, 5-C), 139.0 (o, 5’-C), 138.3 (o, 7-C), 135.2 (o,
7’-C), 134.3 (o, 1’-C), 130.2 (+, 5’’-C), 126.4 (o, 3’-C), 125.6 (o, 4-C), 125.1
(+, 4’-C), 122.6 (+, 3-C), 121.7 (+, 2’’’-C) 118.2 (+, 6’’-C), 116.0 (o, 8-C, 8’’-
C), 113.5 (+, 4’’-C), 111.6 (+, 2’’-C), 111.0 (o, 9-C), 55.4 (+, 7’’-C), 14.8 (+,
(ATR): ν̃ = 2910, 2787, 2671, 2573, 1734, 1600, 1467, 1333, 1270, 1186,
1107, 1051, 984, 895, 850, 814, 779, 736, 689, 562, 530, 475 cm-1. HRMS
(ESI): calc.: 538.0729 [M+Na+], found: 538.0728 [M+Na+]. mp.: 123.9 °C.
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(2-methyl-5-(pyridin-3-yl)thiophen-3-
yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)phenol
(22’o).
According to Procedure 5 950 mg (1.7 mmol) of BTE 19o is treated with
999 mg (4.0 mmol) of BBr3. Yield 870 mg (94%), light yellow solid. 1H-NMR
(600 MHz, DMSO-d6): δ = 9.69 (s, 1H, 7’’-H), 8.83 – 8.81 (m, 1H, 2’’’-H),
8.49 – 8.47 (m, 1H, 4’’’-H), 8.02 – 7.99 (m, 1H, 6’’’-H), 7.61 (s, 1H, 4’-H),
7.43 – 7.41 (m, 1H, 5’’’-H), 7.35 (s, 1H, 3-H), 7.18 (t, JH,H = 7.7 Hz, 1H, 5’’-
H), 7.01 – 6.99 (m, 1H, 6’’-H), 6.96 – 6.95 (m, 1H, 2’’-H), 6.73 – 6.71 (m,
1H, 4’’-H), 1.96 (s, 3H, 6-H), 1.94 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz,
DMSO-d6): δ = 158.5 (o, 3’’-C), 147.8 (+, 4’’’-C), 145.2 (+, 2’’’-C), 142.8 (o,
2’-C), 142.4 (o, 2-C), 140.9 (o, 5-C), 137.9 (o, 5’-C), 137.0 (o, 7-C), 136.4
(o, 7’-C), 134.9 (+, 1C, 6’’’-C), 134.1 (o, 1’’-C), 130.8 (+, 5’’-C), 129.8 (o,
1C, 3’-C),125.7 (o, 1’’’-C), 125.2 (+, 5’’’-C), 125.1 (o, 4-C), 124.5 (+, 4’-C),
122.7 (+, 3-C), 116.6 (+, 6’’-C), 116.2 (o, 8-C, 8’-C), 115.8 (+, 4’’-C), 112.4
6-C), 14.2 (+, 2’-C) ppm. IR (ATR): ν̃ = 3055, 2923, 2838, 2569, 2077,
1629, 1498, 1629, 1498, 1336, 1269, 1193, 1110, 1047, 984, 893, 812,
732, 687, 559 cm-1. HRMS (ESI): calc.: 552.0885 [M], found: 552.0879 [M].
mp.: 67.3 °C.
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-methoxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium(25o). According to Procedure 4 100 mg (0.2 mmol) of BTE 19o are
treated with gaseous HCl. Yield 100 mg (94%), beige powder. 1H-NMR
(+, 2’’-C), 111.2 (o, 9-C), 14.5 (+, 6-C), 14.5 (+, 6’-C) ppm. IR (ATR): ν
̃ =
2972, 2935, 2737, 2674, 2489, 1589, 1441, 1332, 1269, 1185, 1113, 1049,
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10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
985, 896, 848, 797, 679, 465 cm-1. HRMS (ESI): calc.: 560.0548 [M+Na+],
found: 560.0554 [M+Na+]. mp.: 130.1 °C.
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(4-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)-1-
Protonation to give 26o – 28o.
methylpyridin-1-ium (29o + H+). According to Procedure 6 300 mg (0.6
mmol) of BTE 20o are treated with 0.1 mL (1.0 mmol) of DMS. Yield 284
mg (77%), red oil. 1H-NMR (600 MHz, DMSO-d6): δ = 8.82 – 8.80 (m, 2H,
3’’’-H), 8.29 – 8.28 (m, 2H, 2’’’-H), 8.24 (s, 1H, 4’-H), 7.40 – 7.38 (m, 2H,
2’’-H), 7.23 (s, 1H, 3-H), 6.78 – 6.76 (m, 2H, 3’’-H), 4.21 (s, 3H, 5’’’-H), 2.06
(s, 3H, 6-H), 1.91 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz, DMSO-d6): δ =
158.2 (o, 3’’-C), 149.6 (o, 2’-C), 146.9 (o, 1’’’-C), 146.1 (+, 3’’’-C), 143.1 (o,
2-C), 140.3 (o, 5-C), 137.4 (o, 7-C), 135.7 (o, 5’-C), 135.2 (o, 7’-C), 131.6
(o, 1’’-C), 127.3 (+, 2’’-C), 124.9 (o, 3’-C), 124.0 (o, 4-C), 122.5 (+, 2’’’-C),
120.7 (+, 3-C), 116.6 (+, 3’’-C), 116.3 (o, 8-C, 8’’-C), 111.5 (o, 9-C), 47.6
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(4-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium (26o). According to Procedure 4 100 mg (0.2 mmol) of BTE 20o are
treated with gaseous HCl. Yield 99 mg (93%), yellow solid. (600 MHz,
DMSO-d6): δ = 8.83 – 8.77 (m, 2H, 3’’’-H), 8.21 – 8.20 (m, 2H, 2’’’-H), 8.19
(s, 1H, 4’-H), 7.39 – 7.38 (m, 2H, 2’’-H), 7.23 (s, 1H, 3-H), 6.79 – 6.77 (m,
2H, 3’’-H), 2.05 (s, 3H, 6-H), 1.91 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz,
DMSO-d6): δ = 158.3 (o, 3’’-C), 149.7 (o, 2’-C), 147.4 (o, 1’’’-C), 143.5 (+,
3’’’-C), 143.1 (o, 2-C), 140.3 (o, 5-C), 138.1 (o, 7-C), 136.5 (o, 5’-C), 135.5
(o, 7’-C), 130.8 (o, 1’’-C), 127.3 (+, 2’’-C), 124.9 (o, 3’-C), 124.0 (o, 4-C),
122.1 (+, 2’’’-C), 120.7 (+, 3-C), 116.4 (+, 3’’-C), 116.3 (o, 8-C, 8’’-C), 111.5
(+, 5’’’-C), 15.0 (+, 6-C), 14.5 (+, 6’-C) ppm. IR (ATR): ν̃ = 2960, 1638,
1445, 1197, 1055, 996, 764 cm-1. HRMS (ESI): calc.: 552.0885 [M], found:
552.0880 [M+H+].
(o, 9-C), 14.9 (+, 6-C), 14.5 (+, 6’-C) ppm. IR (ATR): ν̃ = 3275, 2248, 2122,
1655, 1023, 821, 759, 618 cm-1. HRMS (ESI): calc.: 538.0729 [M+], found:
538.0733 [M+]. mp.: 130.4 °C.
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)-1-
methylpyridin-1-ium (30o + H+). According to Procedure 6 300 mg (0.6
mmol) of BTE 21o are treated with 0.1 mL (1.0 mmol) of DMS. Yield 319
mg (86%), a green oil. 1H-NMR (600 MHz, DMSO-d6): δ = 8.81 – 8.80 (m,
2H, 3’’’-H), 8.28 – 8.26 (m, 2H, 2’’’-H), 8.23 (s, 1H, 4’-H), 7.34 (s, 1H, 3-H),
7.17 (t, JH,H = 8.3 Hz, 1H, 5’’-H), 7.00 – 6.98 (m, 1H, 5’’’-H), 6.93 – 6.92 (m,
1H, 2’’-H), 6.71 – 6.69 (m, 1H, 4’’-H), 4.22 (s, 3H, 5’’’-H), 2.04 (s, 3H, 6-H),
1.93 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz, DMSO-d6): δ = 158.4 (o, 3’’-
C), 149.5 (o, 2’-C), 146.9 (o, 1’’’-C), 146.1 (+, 3’’’-C), 142.6 (o, 2-C), 141.7
(o, 5-C), 138.1 (o, 7-C), 135.8 (o, 5’-C), 135.6 (o, 7’-C), 134.0 (o, 1’’-C),
131.5 (+, 4’-C), 130.9 (+, 5’’-C), 126.5 (o, 3’-C), 127.0 (o, 4-C), 122.6 (+,
3-C), 122.5 (+, 2’’’-C), 116.6 (+, 6’’-C), 116.2 (o, 8-C, 8’’-C), 115.9 (+, 4’’-
C), 112.4 (+, 2’’-C), 111.3 (o, 9-C), 47.6 (+, 5’’’-C), 14.9 (+, 6-C), 14.5 (+,
4-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium (27o). According to Procedure 4 100 mg (0.2 mmol) of BTE 21o are
treated with gaseous HCl. Yield 104 mg (97%), yellow solid. 1H-NMR (600
MHz, DMSO-d6): δ = 8.85 – 8.78 (m, 2H, 3’’’-H), 8.27 – 8.25 (m, 2H, 2’’’-
H), 8.25 (s, 1H, 4’-H), 7.36 (s, 1H, 3-H), 7.18 (t, JH,H = 8.3 Hz, 1H, 5’’-H),
7.01 – 6.99 (m, 1H, 4’’-H), 6.95 – 6.94 (m, 1H, 6’’-H), 6.73 – 6.71 (m, 1H,
2’’-H), 2.06 (s, 3H, 6-H), 1.95 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz,
DMSO-d6): δ = 158.4 (o, 3’’-C), 149.1 (o, 2’-C), 142.8 (o, 2-C), 142.6 (+,
3’’’-C),141.7 (o, 5-C), 138.0 (o, 7-C), 135.5 (o, 7’-C), 136.3 (o, 5’-C), 134.1
(o, 1’-C), 131.2 (+, 4’-C), 130.9 (+, 5’’-C), 126.9 (o, 3’-C), 124.9 (o, 4-C),
122.3 (+, 3-C), 122.3 (+, 2’’’-C) 116.6 (+, 6’’-C), 116.0 (o, 8-C, 8’’-C), 115.9
(+, 4’’-C), 112.4 (+, 2’’-C), 110.9 (o, 9-C), 14.9 (+, 6-C), 14.6 (+, 6’-C) ppm.
6’-C) ppm. IR (ATR): ν̃ = 3262, 3065, 2956, 1637, 1137, 1032, 877, 572
cm-1. HRMS (ESI): calc.: 552.0885 [M], found: 552.0885 [M].
IR (ATR): ν̃ = 3199, 3065, 2921, 2843, 1628, 1439, 1335, 1269, 1189,
1109, 1052, 984, 893, 781, 638, 540, 466 cm-1. HRMS (ESI): calc.:
574.0699 [M+Na+-H+], found: 574.0704 [M+Na+-H+]. Mp: 139.3 °C
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)-1-
methylpyridin-1-ium (31o + H+). According to Procedure 6 300 mg (0.6
mmol) of BTE 22o are treated with 0.1 mL (1.0 mmol) of DMS. Yield 257
3-(4-(3,3,4,4,5,5-Hexafluoro-2-(5-(3-hydroxyphenyl)-2-methyl-
thiophen-3-yl)cyclopent-1-en-1-yl)-5-methylthiophen-2-yl)pyridin-1-
ium (28o). According to Procedure 4 100 mg (0.2 mmol) of BTE 22o are
treated with gaseous HCl. Yield 106 mg (99%), yellow solid. 1H-NMR (600
MHz, CDCl3): δ = 9.72 – 9.68 (m, 1H, 2’’’-H), 9.48 – 9.44 (m, 1H, 4’’’-H),
8.33 – 8.30 (m, 1H, 6’’’-H), 7.84 – 7.82 (m, 1H, 5’’’-H), 7.48 (s, 1H, 4’-H),
7.24 (t, JH,H = 7.7 Hz, 1H, 5’’-H), 7.23 (s, 1H, 3-H), 7.10 – 7.08 (m, 1H, 6’’-
H), 7.03 – 7.02 (m, 1H, 2’’-H), 6.81 – 6.78 (m, 1H, 4’’-H), 2.08 (s, 3H, 6-H),
2.00 (s, 3H, 6’-H) ppm. 13C-NMR (150 MHz, CDCl3): δ = 156.2 (o, 3’’-C),
144.8 (+, 4’’’-C), 142.5 (+, 2’’’-C), 142.4 (o, 2’-C), 141.4 (o, 2-C), 139.4 (o,
5-C), 139.0 (+, 6’’’-C), 137.7 (o, 7-C), 135.8 (o, 7’-C), 134.5 (o, 5’-C), 133.8
(o, 1’-C), 132.7 (+, 5’’’-C), 130.4 (+, 5’’-C), 129.7 (o, 3’-C), 126.8 (o, 4-C),
125.3 (+, 4’-C), 122.4 (+, 3-C), 118.1 (+, 6’’-C), 116.4 (o, 8-C, 8’’-C), 115.2
(+, 4’’-C), 112.7 (+, 2’’-C), 111.0 (o, 9-C), 14.9 (+, 6-C), 14.7 (+, 6’-C) ppm.
1
mg (70%), green oil. H-NMR (600 MHz, DMSO-d6): δ = 9.39 – 9.38 (m,
1H, 2’’’-H), 8.83 – 8.82 (m, 1H, 4’’’-H), 8.71 – 8.69 (m, 1H, 6’’’-H), 8.08 –
8.05 (m, 1H, 5’’’-H), 7.93 (s, 1H, 4’-H), 7.36 (s, 1H, 3-H), 7.18 (t, JH,H = 7.7
Hz, 1H, 5’’-H), 7.02 – 6.99 (m, 1H, 6’’-H), 6.95 – 6.94 (m, 1H, 2’’-H), 6.72
– 6.70 (m, 1H, 4’’-H), 4.33 (s, 2H, 7’’’-H), 1.99 (s, 3H, 6-H), 1.93 (s, 3H, 6’-
H) ppm. 13C-NMR (150 MHz, DMSO-d6): δ = 158.3 (o, 3’’-C), 144.2 (+, 4’’’-
C), 142.6 (+, 2’’’-C), 142.5 (o, 2’-C), 141.7 (o, 2-C), 140.7 (o, 5-C), 140.4
(+, 6’’’-C), 137.7 (o, 7-C), 135.8 (o, 7’-C), 134.5 (o, 5’-C), 134.0 (o, 1’-C),
132.8 (+, 5’’’-C), 130.9 (+, 5’’-C), 128.4 (o, 3’-C), 126.1 (o, 4-C), 125.1 (+,
4’-C), 122.6 (+, 3-C), 116.6 (+, 6’’-C), 116.4 (o, 8-C, 8’’-C), 115.9 (+, 4’’-C),
112.4 (+, 2’’-C), 111.1 (o, 9-C), 48.6 (+, 7’’’-C), 14.7 (+, 6-C), 14.5 (+, 6’-C)
ppm. IR (ATR): ν̃
= 3206, 3100, 2922, 1630, 1185, 1046, 882, 576 cm-1.
IR (ATR): ν̃ = 3198, 2755, 2263, 1559, 1440, 1269, 1188, 1108, 984, 790,
670, 540, 461 cm-1. HRMS (APCI): calc.: 538.0722 [M], found: 538.0722
[M]. mp.: 145.2 °C.
HRMS (ESI): calc.: 552.0885 [M], found: 552.0895 [M].
Acknowledgments
General Procedure for the N-Methylation of the BTEs to give the
Target Betaines 29o + H+ – 31o + H+ (Procedure 6):
The phenol BTE is dissolved in acetone. To this reaction 1.5 eq. of
dimethylsulfate (DMS) and a few drops of nitrobenzene are added and
then the solution is heated at reflux temperature for 6 h. After cooling down
to room temperature the reaction is treated with diethyl ether and extracted
with bidest. water. The aqueous phase is collected and the water is
removed in vacuo to afford the methylpyridinium salt. The equivalents of
MeSO4- ions in the spectra are 3 instead of 1. Possible are coordinations
of two more ions ore traces of the excess of DMS. Because of the
appearance of an oil there is no crystallization performable.
Prof. Dr. Christian Rembe (Clausthal University of Technology,
Institute for Electrical Information Technology) and Prof. Dr.
Alexander Egner (Georg-August-Universität Göttingen, Göttingen
Graduate Center for Neurosciences, Biophysics, and Molecular
Biosciences) is gratefully acknowledged for helpful discussions.
Keywords: BTE • through-space conjugation • mesoionic
compounds • cross-conjugation • charge-separation
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202100279
European Journal of Organic Chemistry
FULL PAPER
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