Synthesis and Properties of Dithieno-Fused 1,4-Azaborine Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b03316

The first synthesis of dithieno[3,2-b:2′,3′-e][1,4]azaborinine (DTAB) derivatives has been achieved by Buchwald-Hartwig coupling and subsequent Friedel-Crafts-type C-H borylation. A facile method for further π-extension of DTAB was also developed via stannylation and subsequent Kosugi-Migita-Stille cross-coupling reaction. The fundamental properties of DTAB derivatives were also investigated.

Full text of DOI:10.1021/acs.orglett.8b03316

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis and Properties of Dithieno-Fused 1,4-Azaborine
Derivatives
Koichi Mitsudo,*,† Keisuke Shigemori,† Hiroki Mandai,† Atsushi Wakamiya,‡ and Seiji Suga*,†
†Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka,
Kita-ku, Okayama 700-8530, Japan
‡Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: The first synthesis of dithieno[3,2-b:2′,3′-e][1,4]azaborinine (DTAB) derivatives has been achieved by
Buchwald−Hartwig coupling and subsequent Friedel−Crafts-type C−H borylation. A facile method for further π-extension of
DTAB was also developed via stannylation and subsequent Kosugi−Migita−Stille cross-coupling reaction. The fundamental
properties of DTAB derivatives were also investigated.
zaborines, which are benzene ring alternatives that
Ainclude boron and nitrogen atoms, have received
considerable attention since the pioneering work reported by
Dewar,1,2 and several new azaborine derivatives have been
studied extensively over the past decade due to their unique
properties.3 Meanwhile, thienoacenes, acene derivatives that
contain thiophene moieties, have also been the focus of
research as promising compounds for the synthesis of organic
materials.4 The incorporation of main-group atoms into
thienoacenes is also a hot topic these days, and several
heterothienoacenes have been reported.5 Thienoazaborines,
thiophene-fused azaborines, have also been studied.6 For
instance, Perepichka and co-workers reported integrated
compounds that contained thiophene and 1,2-azaborine
units. They synthesized terthiophenes fused by two 1,2-
azaborine rings and found that these compounds exhibited
fluorescence (Figure 1a).6d In 2013, Wang, Yuan, and Pei
reported the synthesis of BN-substituted tetrathienonaphtha-
lene derivatives and their properties as semiconductors.6g In
contrast to thienoacenes, including 1,2-azaborines, there has
been no report on the synthesis of thienoacenes including 1,4-
azaborines as far as we can tell, although they should be
potential candidates for the synthesis of interesting organic
materials. We have been interested in the synthesis and
properties of heterothienoacenes,7 and focus here has been on
dithieno[3,2-b:2′,3′-e][1,4]azaborinine (DTAB) (Figure 1 b).
The syntheses of dibenzo[b,e][1,4]azaborine (DBAB)8 and
dithieno[3,2-b:2′,3′-e][1,4]thiaborine (DTTB), analogues of
DTAB, have already been reported by Clark8a and Liu.9 At the
beginning of our study, we calculated the NICS(1) values of
DBAB, DTTB, and DTAB, which have a methyl group on the
nitrogen atom and a phenyl group on the boron atom (Figure
Figure 1. (a) Representative examples of previously reported
thiophene-fused 1,2-azaborines. (b) Structure of dibenzo-1,4-
azaborine (DBAB), dithieno-1,4-thiaborine (DTTB), and dithieno-
1,4-azaborine (DTAB) with NICS(1) calculated at the GIAO-
B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level. NICS values were
calculated for the structures with R = Me, Ar = Ph.
1(b)). The NICS(1) value of DTAB (−6.3) was the most
negative among them, suggesting that it is highly aromatic.
Clark and Liu reported that the boron-containing rings of
DBAB and DTTB were almost planar, indicating their
aromaticity, and the introduction of a bulky substituent such
as a mesityl group conferred stability upon them. We assumed
Received: October 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03316
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
that DTAB would be stable on account of its high aromaticity
even without a bulky substituent on the boron atom. Based on
this hypothesis, we started to synthesize DTAB derivatives that
had a phenyl group on the boron atom.
Our synthetic strategy is illustrated in Scheme 1. We
considered that DTAB (1) could be synthesized by a Friedel−
dithienylamine (2a and 2b) could be synthesized by a similar
method reported by Rasmussen and co-workers.11 P(t-Bu)3
was an effective ligand, and 2a and 2b were obtained in
respective yields of 75% and 72%. For the synthesis of N-
methyl-3-benzo[b]thiophenyl-3-thienylamine (2c), the use of
P(t-Bu)3 was not effective. Optimization of the reaction
conditions revealed that RuPhos, a commonly used ligand for
Buchwald−Hartwig coupling reactions, was effective. With
RuPhos, the reaction of 3a with 3-bromobenzo[b]thiophene
afforded the desired product 2c in 78% yield. A tandem
coupling reaction using 3,6-dibromothieno[3,2-b]thiophene
was also carried out; the use of Pd[P(t-Bu)3]2 gave both the
dehalogenated monocoupling product 2d and the double-
coupled diamine 2e in respective yields of 21% and 13%. When
RuPhos was used as a ligand, the yield of 2e increased to
58%.12
Scheme 1. Our Synthetic Strategy for Dithienoazaborines
Next, we chose 2a as a model compound, and C−H
borylation was carried out (Table 1). In chlorobenzene,
Table 1. Synthesis of Dithienoazaborine 1a by C−H
a
Crafts-type C−H borylation10 of 3,3′-dithienylamine 2 derived
from 3-thienylamine 3 and 3-bromothiophene via a Buch-
wald−Hartwig cross-coupling reaction. Furthermore, the thus-
obtained 1 could be transformed to more π-extended
derivatives by cross-coupling reactions.
Borylation under Various Conditions
While 3,3′-dithienylamines have been known to be useful
precursors for dithienopyrroles, there has been only one report
on the synthesis of more π-extended 3,3′-dithienylamine
derivatives.11 Therefore, we first developed a synthetic method
for the construction of 2 by a Buchwald−Hartwig cross-
coupling reaction (eqs 1−5). N-Methyl- and N-phenyl-3,3′-
b
entry
Et3N (equiv)
time (h)
yield (%)
1
2
3
4
5
0
48
48
24
48
48
40
61
88
1.0
2.0
2.0
2.0
c
>99 (96)
c d
,
82
a
Reaction conditions: 2a (0.20 mmol), PhBCl2 (1.5 equiv), Et3N (0−
b
1
2 equiv), PhCl (0.29 M), 135 °C. Determined by H NMR with
c
1,1,2,2-tetrachloroethane as an internal standard. Isolated yield.
d
Performed with 1.0 mmol of 2a.
treatment of 2a with PhBCl2 (1.5 equiv) at 135 °C for 48 h
gave the desired azaborine 1a regioselectively in 40% yield
(entry 1). Further optimization revealed that the addition of
Et3N increased the yield of 1a. With 1 equiv of Et3N, the NMR
yield of 1a increased to 61% (entry 2). The reaction proceeded
quite efficiently with 2 equiv of Et3N. Compound 1a was
obtained in 88% yield even within 24 h (entry 3), and the
reaction proceeded quantitatively within 48 h (entry 4, > 99%
NMR yield, 96% isolated yield). When the reaction was
performed on a 1 mmol scale, 1a was also obtained in good
yield (entry 5).
The structure of 1a was confirmed by X-ray analysis (Figure
2). The sum of the bond angles around the nitrogen, boron,
and carbon atoms that comprised the azaborine ring was 360°,
and the sum of the interior angles of the azaborine ring was
720°, indicating that the azaborine ring is completely planar.
The packed structure of 1a is illustrated in Figure 2b.
Intermolecular N−B interaction was not observed, and the
packed structure was mainly effected by π−π interaction.
Next, we performed C−H borylation of 2b−d under the
optimized conditions (Scheme 2). Phenyl(di-3-thienyl)amine
(2b) could also be used for this reaction, and the
corresponding DTAB 1b was obtained in 66% yield with 3
equiv of PhBCl2. Precursors bearing 3-benzo[b]thienyl and 3-
B
DOI: 10.1021/acs.orglett.8b03316
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Stannylation of 1c
Table 2. Synthesis of π-Extended DTAB 5 by Kosugi−
a
Migita−Stille Cross-Coupling
Figure 2. (a) ORTEP drawing of 1a with 50% thermal ellipsoids. (b)
Packed structure of 1a.
Scheme 2. Synthesis of Several Dithienoazaborines via C−H
a
Borylation
b
entry
R
X
5
yield (%)
c
1
H
I
I
I
Br
I
5a
5b
5c
5d
5e
5f
85
92
71
79
91
97
99
2
3
4
5
6
7
CH3
CH3O
Ph2N
F
CF3
CN
I
I
5g
a
Reaction conditions: 4 (0.20 mmol), aryl halide (2 equiv), Pd[P(t-
b
c
Bu)3]2 (5 mol %), toluene (0.1 M). Isolated yield. Performed on a 1
mmol scale.
to give the corresponding π-extended DTABs in good to
excellent yields (entries 2−7).
Next, the fundamental physical properties of DTABs were
investigated. UV−vis absorption spectra of 1a−e are illustrated
in Figure 3. The onset of absorbance (λonset) of 1a was 369 nm,
a
Reaction conditions: 2 (0.20 mmol), PhBCl2 (1.5 equiv), Et3N (2
b
equiv), PhCl (0.29 M). Isolated yield. Performed with 3 equiv of
PhBCl2. Performed with 2e (0.32 mmol), PhBCl2 (3 equiv), Et3N (4
c
equiv) in PhCl (0.07 M).
thieno[3,2-b]thienyl groups were efficient for this reaction, and
the π-expanded DTAB derivatives 1c and 1d were obtained in
respective yields of 86% and 84%. Notably, a more π-expanded
ladder-type DTAB 1e was readily obtained from precursors 2e,
which have two reaction sites (61% yield). As expected, 1a−e
were all stable, could be easily handled in air under ambient
conditions, and could be purified by simple column
chromatography on silica gel. This is likely due to their high
aromaticity.
Figure 3. UV−vis spectra of 1a−e measured in o-C6H4Cl2 (1.0 ×
10−5 M).
For further derivatization of the thus-obtained DTABs, a
stannyl group was introduced to 1c. When 1c was treated with
t-BuLi (2.2 equiv) and TMEDA (2.6 equiv) at −78 °C,
lithiation proceeded smoothly, and a subsequent reaction with
Me3SnCl gave the stannylated DTAB 4 in 84% yield (Scheme
3).13
Thus-obtained 4 was a good precursor of π-extended DTAB
derivatives, and several DTAB derivatives were obtained by
Kosugi−Migita−Stille cross-coupling (Table 2). In the
presence of Pd[P(t-Bu)3]2 (5 mol %), treatment of 4 with
iodobenzene (2 equiv) at 80 °C gave phenyl-substituted
DTAB 5a in 85% yield (entry 1). Aryl halides bearing electron-
donating or -withdrawing groups could be used in the reaction
which is similar to that of 1b (367 nm). The value of λonset
increased with π-extension, and that of 1e was 418 nm, which
is the longest among them. These results suggest that the
HOMO−LUMO gaps of DTABs become smaller by π-
extension and that of 1e is the smallest among them (2.96 eV).
A similar tendency was observed in the UV−vis absorption
spectra of 5a−g.14 We also measured fluorescence spectra of
DTABs and found that DTAB 1a−e exhibited weak
fluorescence (Φ < 0.1). The fluorescence emission (λem) of
1a−e was observed around 377−417 nm, and the value was
increased with their π-extension. Among 5a−g, the fluo-
C
DOI: 10.1021/acs.orglett.8b03316
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
rescence of 5d was the strongest (Φ = 0.27) probably because
the diphenylamino moiety would work as an efficient donor
unit.15
Hiroki Mandai: 0000-0001-9121-3850
Atsushi Wakamiya: 0000-0003-1430-0947
Seiji Suga: 0000-0003-0635-2077
Cyclic voltammetry (CV) was then carried out for 1a−e and
5a−g.14 In all of the cyclic voltammograms, irreversible
oxidation peaks were observed. In contrast, no clear reduction
peak was observed in the range of the electron window. The
onset values of oxidation (Eonset) were in the range of 0.55−
0.77 V for 1a−e16 and 0.33−0.59 V for 5a−g (vs Fc/Fc+). The
combined electrochemical and optical data led to estimated
HOMO−LUMO levels for 1a−e and 5a−g (Table 3 and
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for
Scientific Research (C) (Nos. 25410042, 16K05695) from
JSPS, Japan, by the Collaborative Research Program of
Institute for Chemical Research, Kyoto University (Grant
No. 2018-42), Okayama Foundation for Science and
Technology, and by JST, ACT-C, Japan. The authors thank
Prof. Yasujiro Murata (Kyoto University) for fruitful
discussions.
a
Table 3. Electrochemical and Optical Data for DTAB 1a−e
λonset/Egopt
(nm/eV)
EHOMO
(eV)
ELUMO
(eV)
1
λmax (nm) log ε
1a
1b
1c
1d
1e
351
350
305
293
303
4.24
4.37
4.41
4.40
4.71
369, 3.36
367, 3.38
395, 3.14
382, 3.24
418, 2.96
−5.52
−5.57
−5.35
−5.44
−5.37
−2.16
−2.19
−2.21
−2.20
−2.19
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Supporting Information). All of the DTABs have HOMO
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In conclusion, we have achieved the first syntheses of DTAB
1a−e by Buchwald−Hartwig coupling and subsequent C−H
borylation. Further π-extension of 1c to afford 5a−g was also
achieved. The fundamental physical properties of the DTAB
derivatives were also studied. Further investigations of these
derivatives are in progress in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03316.
Experimental details, photophysical and electrochemical
properties of 1a−e and 5a−g, spectral data for all new
compounds, and data for theoretical calculations (PDF)
Accession Codes
CCDC 1858599, 1858601−1858602, and 1858671 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
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Mater. 2011, 23, 4347.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: mitsudo@cc.okayama-u.ac.jp.
*E-mail: suga@cc.okayama-u.ac.jp.
ORCID
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Koichi Mitsudo: 0000-0002-6744-7136
D
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Letter
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(12) For details of the Buchwald−Hartwig coupling, see the
Supporting Information.
(13) Prior to the stannylation, optimization of the lithiation
conditions was carried out by a deuteration reaction. For details,
see the Supporting Information.
(14) For the details of physical properties of DTABs, see the
Supporting Information.
(15) Ito, M.; Ito, E.; Hirai, M.; Yamaguchi, S. J. Org. Chem. 2018, 83,
8449.
(16) The low oxidation potential of 5d would be derived from the
highly electron-rich triphenylamine moiety. DFT calculations suggest
that the HOMO of 5d would be delocalized on the triphenylamine.
(17) The LUMO level (−2.16 eV) of DTAB 1a was lower than that
of DTTB (−1.43 eV, see ref 9), suggesting higher electron-accepting
ability of DTABs.
(18) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.;
̈
Bassler, H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551.
E
DOI: 10.1021/acs.orglett.8b03316
Org. Lett. XXXX, XXX, XXX−XXX