Stable 1H-Benzo[c]thio- and 1H-Benzo[c]selenophen-2-ium Tetrafluoroborates: Insight into Electronic Structures, Electrochemical Behavior, and Reactivity

Article abstract of DOI:10.1021/acs.orglett.7b00716

The synthesis of 1H-benzo[c]thio- and 1H-benzo[c]selenophen-2-ium tetrafluoroborates by the reaction of triphenylcarbenium tetrafluoroborate with 1-ethylidene-1,3-dihydrobenzo[c]thiophene or 1-ethylidene-1,3-dihydrobenzo[c]selenophene, respectively, is reported. The electronic structure of these novel 1H-benzo[c]chalcogenophenium salts was examined on the basis of their electronic spectra, whereby transitions were assigned in agreement with theoretical calculations. Electrochemical measurements revealed irreversible one-electron reduction waves for these 1H-benzo[c]chalcogenophenium salts.

Full text of DOI:10.1021/acs.orglett.7b00716

Letter
pubs.acs.org/OrgLett
Stable 1H‑Benzo[c]thio- and 1H‑Benzo[c]selenophen-2-ium
Tetrafluoroborates: Insight into Electronic Structures,
Electrochemical Behavior, and Reactivity
Noriyoshi Nagahora,* Ikue Takemoto, Motomi Fujii, Kosei Shioji, and Kentaro Okuma
Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan
S
* Supporting Information
ABSTRACT: The synthesis of 1H-benzo[c]thio- and 1H-benzo[c]selenophen-2-ium
tetrafluoroborates by the reaction of triphenylcarbenium tetrafluoroborate with 1-
ethylidene-1,3-dihydrobenzo[c]thiophene or 1-ethylidene-1,3-dihydrobenzo[c]-
selenophene, respectively, is reported. The electronic structure of these novel 1H-
benzo[c]chalcogenophenium salts was examined on the basis of their electronic spectra,
whereby transitions were assigned in agreement with theoretical calculations.
Electrochemical measurements revealed irreversible one-electron reduction waves for
these 1H-benzo[c]chalcogenophenium salts.
enzothiophene and its selenium congener, benzoseleno-
workers,24,25 which is outlined in Scheme 1. A Friedel−Crafts
Bphene, represent one of the most fundamental classes of acylation of anisole with 2-iodobenzoyl chloride furnished 2-
molecules in the field of organic chemistry. The chemistry of
benzothiophenes in particular has received much attention, as
they not only represent fascinating synthetic building blocks for
new organosulfur compounds, but potentially also offer
promise for functional electronic devices such as organic
field-effect transistors or light-emitting devices.1−7
Scheme 1. Synthesis of 1H-Benzo[c]thio- and 1H-
Benzo[c]selenophen-2-ium Tetrafluoroborates 1a−2b
On the other hand, several papers have been published on
the synthesis of S-alkyl- or S-arylthiophenium salts,8−23 and
some of these compounds have known to be present among
electrophilic alkylation or arylation reagents toward aromatics
and heteroaromatics. However, in contrast to the chemistry of
benzothiophenes, synthetic investigations of thiophenium as
well as selenophenium still remain limited.
As far as superior properties of organic electronic devices
based on benzochalcogenophenes are concerned,1,2,4−7 1H-
benzochalcogenophenium salts are promising candidates for
molecular electronics. Disclosing their properties in photo-
physics and electrochemistry is extremely fascinating from the
viewpoint of not only fundamental chemistry but also
developing molecular electronics.
Herein, we present studies on (i) the synthesis of the novel
1-alkylidene-1H-benzo[c]thio- and 1-alkylidene-1H-benzo[c]-
selenophen-2-ium tetrafluoroborates 1a−2b, (ii) their elec-
tronic structures probed by various experimental methods and
theoretical calculations, (iii) their electrochemical properties,
and (iv) reactivity toward 1,3-butadiene.
The synthetic strategy toward these novel 1-alkylidene-1H-
benzo[c]chalcogenophen-2-ium salts was based on the syn-
thesis of 1,3-dihydroisobenzochalcogephenes by intramolecular
5-exo cyclizations of 2-alkynylphenyl-(4-methoxyphenyl)-
methanechalcogenolate, followed by hydride abstraction
reactions with triphenylcarbenium salt. In order to gain
synthetic access to 1a−2b, we modified the synthetic procedure
for 2-benzoselenopyrylium salts reported by Sashida and co-
iodobenzophenone 3 in 80% yield. A subsequent Sonogashira
coupling of 3 with phenylacetylene or 2,2-dimethylbut-1-yne in
the presence of copper iodide and a palladium catalyst afforded
4a or 4b in 94% or quantitative yield, respectively. Reduction of
4a and 4b with LiAlH4 provided both 5a and 5b in 95% yields,
Received: March 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00716
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Organic Letters
Letter
which reacted smoothly with PBr3 at room temperature to
afford the corresponding benzyl bromides 6a and 6b in 95%
and 85% yields, respectively. Treatment of 6a with NaSH or
NaSeH followed by sequential cyclization reactions yielded the
corresponding 1-benzylidene-3-(4-methoxyphenyl)-1,3-dihy-
droisobenzothiophene 7a (37%) and selenophene 8a (40%),
respectively. The molecular structure of 8a was confirmed by X-
ray crystallographic analysis (Figure 1).26 The corresponding 6-
mostly likely be attributed to the character of the hetero-
butadiene subunit containing a sulfonium or selenonium cation.
The 13C NMR spectra of 7a, 8a, and 8b exhibited signals for
the C3 carbon atoms at 55.3 ppm (7a), 52.0 ppm (8a), and
51.4 ppm (8b), the latter two of which exhibited satellite peaks
1
(1JCSe = 54.8 Hz for 8a, JCSe = 54.7 Hz for 8b) arising from
coupling between carbon and selenium. Moreover, the
characteristic signals at 144.4/130.0 ppm (7a) and 138.1/
128.1 ppm (8a) were assigned to the C1 and C4 carbon atoms.
In selenium analogue 8a, these signals were accompanied by
1
2
77Se satellites with JCSe = 112 Hz, JCSe = 11.7 Hz (for 8a).
Conversely, 1-alkylidene-1H-benzo[c]chalcogenophenium tet-
rafluoroborates 1a, 2a, and 2b showed signals at 197.2, 212.6
(1JCSe = 210 Hz), and 212.8 ppm (1JCSe = 257 Hz), respectively.
1
The JCSe coupling constants of 2a and 2b are larger than that
of the corresponding monocyclic selenopyrylium tetrafluor-
oborate (1JCSe = 155.4 Hz) and smaller than those of
selenocarbonyl compounds such as t-Bu2CSe (1JCSe = 213.6
Hz) or selenofenchone (1JCSe = 220.8 Hz).27−29 However, it is
comparable to those of selenoamides30 and selenoesters (1JCSe
= 202−211 Hz),27−29 suggesting a partial double-bond
character for the C3−Se bond in 2a and 2b.
Figure 1. Molecular structure of 8a (left) and 11b (right) with thermal
ellipsoid plots (50% probability). All hydrogen atoms are omitted for
clarity.
The 77Se NMR spectra of 8a and 8b in CDCl3 exhibited a
doublet of doublets at 472.4 ppm (JHSe = 13.7/13.4 Hz) and
531.5 ppm (JHSe = 16.8/14.0 Hz), while 2a and 2b in CD3CN
showed only one doublet at 746.0 ppm (3JHSe = 17.9 Hz) and
739.9 ppm (3JHSe = 21.9 Hz), respectively. The observed
chemical shift of 2a and 2b is characteristic for heterocycles
containing a selenium atom, e.g., selenophene (δSe = 613),31
and slightly upfield shifted relative to the parent selenopyrylium
perchlorate (δSe = 976 in CF3CO2D),32 selenopyrylium
tetrafluoroborate (δSe = 975.7 in CD3CN),25 or 3-tert-butyl-2-
benzoselenopyrylium tetrafluoroborate (δSe = 890 in
CD3CN).25 Moreover, the chemical shift of the selenium
analogues is low-field shift as compared with those of
selenonium salts (δSe = 256 for Me3Se+I−), selenonium ylides,
and selenoamides. The coupling constants of 2a (3JHSe = 17.9
Hz) and 2b (3JHSe = 21.9 Hz) are slightly larger than those of
previously reported heterocycles (2.3−16 Hz) and similar to
those of compounds containing an sp2C−Se bond, e.g.,
PhSeSePh (19 Hz) or benzo[b]selenophene (21 Hz).27
Accordingly, the multinuclear NMR analysis clearly confirmed
their molecular structures of 1H-benzo[c]chalcogenophenium
tetrafluoroborates 1a−2b in solution.
endo-cyclized products, 1H-isothio- and 1H-isoselenochro-
menes, were not detected in the reaction mixture, which is in
sharp contrast to the results reported by Sashida and co-
workers.24,25 In the case of compound 6b, the reaction with
NaSH or NaSeH afforded benzo[c]thiophene 9b in 41% yield
and a 1:1 mixture of 1-alkylidene-1,3-dihydroisobenzoseleno-
phene 8b and benzo[c]selenophene 10b in 50% total yield. In a
final step, the reactions of 7a, 8a, and 8b with [Ph3C][BF4] in
nitromethane furnished 1-benzylidene-1H-benzo[c]-
thiophenium and 1H-benzo[c]selenophenium tetrafluorobo-
rates 1a, 2a, and 2b in 85%, 53%, and 58% isolated yields,
respectively. Hydride abstraction of benzo[c]thiophene 9b
using [Ph3C][BF4] also proceeded in the formation of 1H-
benzo[c]thiophenium tetrafluoroborate 1b as orange crystals in
47% yield. Crystalline 1a−2b do not show any signs of
decomposition under atmospheric conditions and are moreover
thermally stable under an inert atmosphere of argon (their
melting or decomposition temperatures are shown in the SI).
1
The H NMR spectra of 7a, 8a, and 8b in CDCl3 showed
singlet signals at 5.93/7.15 ppm (7a), 6.11/7.56 ppm (8a), and
6.61/5.97 ppm (8b) for the H3 and H4 hydrogen atoms
(Figure 2). In contrast, 1-alkylidene-1H-benzo[c]-
chalcogenophenium tetrafluoroborates 1a, 2a, and 2b revealed
the corresponding singlet resonances for the H4 hydrogen
atoms at 8.96, 8.97, and 8.30 ppm, respectively. The deshielded
resonances of 1a, 2a, and 2b relative to 7a, 8a, and 8b can
In order to examine the electronic structure of 1a−2b, their
UV−vis absorption spectra were measured (Figure 3), and the
Figure 3. UV−vis absorption (solid line) and emission (broken line)
spectra of 1-alkylidene-1H-benzo[c]chalcogenophenium tetrafluoro-
borates 1a−2b in CH2Cl2 at room temperature. Inset: photographs of
CH2Cl2 solution of 1a and 2a (concentration: 2.5 × 10−5 mol/L for
1a, and 2.0 × 10−5 mol/L for 2a) upon irradiation with UV light (365
nm).
Figure 2. Chemical structures of compounds 7a−8b as well as 1a−2b.
B
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Letter
photophysical parameters are summarized in Table 1.
Commensurate with the large absorption coefficients, the
1a′−2b′, respectively. Taking these calculations into consid-
eration, we draw the following conclusions for 1-alkylidene-1H-
benzo[c]chalcogenophenium cations 1a′−2b′: (i) structure A
should be the dominant mesomeric resonance structure
(Scheme 2), and (ii) cations 1a′−2b′ have striking features
of s-trans heterodutadiene analogues containing sulfonium or
selenonium.
Table 1. Photophysical and Electrochemical Data of 1-
Alkylidene-1H-benzo[c]chalcogenophenium
Tetrafluoroborates 1a−2b in CH2Cl2
absorption
λ
ε
emission λ
Stokes shift
Scheme 2. Conceivable Resonance Structures for 1-
Alkylidene-1H-benzo[c]chalcogenophenium Cations 1a′−
2b′
compd (nm) (M−1 cm−1
)
(nm)
(cm−1
)
Epc
1a
2a
1b
2b
565
572
501
518
50600
45300
22100
29300
612
631
1390
1570
−0.55
−0.39
−0.53
−0.51
absorption maxima at 565 (1a), 572 (2a), 501 (1b), and 518
nm (2b) were assigned to the π−π* electron transitions of the
1H-benzo[c]chalcogenophenium chromophore. The succes-
sively larger bathochromic shifts (2a > 1a, 2b > 1b) indicate
that the introduction of consecutively heavier group 16 atoms
in the heterobutadiene system results in a decrease of the
HOMO−LUMO energy gap.
The electrochemical behavior of 1a−2b was analyzed by
cyclic and differential pulse voltammetry, and the correspond-
ing voltammograms are shown in Figure 4. In CH2Cl2 at room
Figure 3 shows the emission spectra of 1a and 2a in CH2Cl2,
which exhibited emission maxima at 612 (1a) and 631 nm
(2a).33 The emission energies were found to decrease in the
order 1a > 2a, whereby the Stokes shifts of 1a and 2a were
found to be comparable.
In order to gain further insight into the electronic structures
of 1a−2b, theoretical calculations were carried out on model
compounds 1a′−2b′, in which the tetrafluoroborate anions
were omitted.34,35 In 1a′−2b′, the LUMO is delocalized over
the entire molecular framework, whereas the HOMO is
predominantly localized on the 1-alkylidene-1H-benzo[c]-
chalcogenophenium moieties including the anisyl group
(Figures S3−S6). An assignment of the absorption maxima in
the UV−vis spectra was based on TD-DFT calculations of 1a′−
2b′ at the B3LYP/6-311+G(d,p) level of theory. The lowest
energy absorption maxima of 1a−2b should accordingly be
assigned to symmetry-allowed π−π* electron transitions
(HOMO−LUMO). The HOMO−LUMO transition energies
of 1a′−2b′ are expected to decrease monotonically in the order
1a′ > 2a′, 1b′ > 2b′ (Table 2), which is in good agreement with
the experimentally observed results.
Figure 4. Cyclic voltammograms of 1a−2b in [(n-Bu)4N][PF6]/
CH2Cl2 at room temperature (scan rate: 100 mV s−1).
temperature, irreversible one-electron reduction waves were
observed for 1a−2b between −0.39 and −0.55 V (all potentials
vs Fc/Fc+, Table 1), whereas no oxidation waves were observed
during anodic sweeps. The observed trends are consistent with
the theoretically calculated LUMO energy levels of 1a′−2b′
(Table 2).
Finally, we have attempted Diels−Alder reactions of 1b and
2b with 2,3-dimethyl-1,3-butadiene in order to disclose their
reactivity (Scheme 3). A solution of 1b with dimethylbutadiene
Scheme 3. Diels−Alder Reactions of 1b and 2b
Table 2. Calculated Molecular Orbital Energy Levels and
First Excited-State Energies for 1-Alkylidene-1H-
benzo[c]chalcogenophenium Cations 1a′−2b′
HOMO LUMO
HOMO−
transition energy
(eV) (λ (nm))
compd
(eV)
(eV)
LUMO (eV)
f
1a′
2a′
1b′
2b′
−9.28
−9.25
−9.56
−9.50
−6.70
−6.70
−6.69
−6.68
2.58
2.55
2.87
2.82
2.41 (514)
2.35 (528)
2.71 (458)
2.63 (472)
0.668
0.617
0.595
0.574
in acetonitrile at room temperature smoothly afforded 11b in
91% isolated yield. The molecular structure of 11b was
determined by X-ray crystallographic analysis,26 as shown in
Figure 1, which clarified that Diels−Alder reaction of the exo-
alkene subunit of 1b proceeded. Compound 2b was subjected
to a Diels−Alder reaction, which generated not only exo-alkene
adduct 12b but also CSe+ adduct 13b in total 78% yield in a
ratio of 1:1. These results revealed novel reactivity of the 1-
alkylidene-1H-benzo[c]chalcogenophenium fragment.
A natural population analysis36 of 1a′−2b′ (Table S2)
revealed charge values between +0.68 and +0.55 at the
chalcogen atoms of 1a′−2b′. The remaining nine carbon
atoms in the 1-alkylidene-1H-benzo[c]chalcogenophenium
framework exhibited merely small negative values (−0.20 to
0.00). The bond order analysis based on the Wiberg bond
index37 (Table S3) suggested bond order values of 1.30−1.35
and 1.63−1.74 for the C3−chalcogen and C1−C4 bonds in
C
DOI: 10.1021/acs.orglett.7b00716
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Organic Letters
Letter
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In conclusion, we have successfully synthesized a series of the
first stable 1-alkylidene-1H-benzo[c]chalcogenophen-2-ium
tetrafluoroborates (1a−2b) and disclosed their photophysical
properties as well as their electrochemical behavior. The
selenium-containing heavy heterocycles 2a and 2b exhibit
absorption and emission maxima at significantly longer
wavelengths relative to sulfur derivatives 1a and 1b. Further
research on the intriguing properties of 1a−2b is currently in
progress in our laboratory and will be reported in due course.
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00716.
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Experimental procedure, characterization data, NMR
spectra for all products, X-ray crystallographic analyses,
and theoretical calculations for the model compounds
(PDF)
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Soc., Perkin Trans. 1 2002, 606−612.
(26) Crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre under the supplementary publication
nos. CCDC 1533077 (for 8a) and 1533078 (for 11b). Copies can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nagahora@fukuoka-u.ac.jp.
ORCID
Noriyoshi Nagahora: 0000-0002-9663-0677
Notes
(30) Murai, T.; Fujishima, A.; Iwamoto, C.; Kato, S. J. Org. Chem.
2003, 68, 7979−7982.
The authors declare no competing financial interest.
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Denoel, J. Org. Magn. Reson. 1976, 8, 354−356.
ACKNOWLEDGMENTS
■
This work was partially supported by the Collaborative
Research Program of the Institute for Chemical Research,
Kyoto University (Grant Nos. 2015-97 and 2016-96), and a
Grant-in-Aid for Scientific Research (C) (No. JP16K05709)
from the Japan Society for the Promotion of Science.
́
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methane solution at room temperature were found to be 12.5% and
3.5%, respectively.
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M06-2X/6-311+G(d,p) level of theory using the Gaussian 09 program.
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Wallingford CT, 2013 (full reference in the Supporting Information).
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