Synthesis and properties of benzophospholo[3,2-b]benzofuran derivatives

Article abstract of DOI:10.1021/jo502889r

Benzophospholo[3,2-b]benzofuran (BPBF) derivatives were studied as a new family of phosphole-containing π-conjugated organic materials. The versatile reactivity of the trivalent phosphorus center of the parent BPBF offered access to a variety of derivatives by simple chemical modifications. The diversity-oriented synthetic route was also developed to allow facile access to BPBFs with a different substituent on the phosphorus center. Photophysical properties were revealed based on UV-vis and photoluminescence spectroscopies and theoretical calculation. The characteristic property of BPBF derivatives is highly efficient photoluminescence. In particular, phosphole oxide derivatives demonstrated intense blue fluorescence with high quantum yield over 85% even in the solid state.

Full text of DOI:10.1021/jo502889r

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Article
Synthesis and Properties of Benzophospholo[3,2-b]benzofuran Derivatives
Motonobu Takahashi, Koji Nakano, and Kyoko Nozaki
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo502889r • Publication Date (Web): 18 Mar 2015
Downloaded from http://pubs.acs.org on March 25, 2015
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Synthesis and Properties of
Benzophospholo[3,2ꢀb]benzofuran Derivatives
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†
*‡
*†
Motonobu Takahashi, Koji Nakano, Kyoko Nozaki
†
Department of Chemistry and Biotechnology, Graduate School of Engineering, The
University of Tokyo, 7ꢀ3ꢀ1 Hongo, Bunkyoꢀku, Tokyo 113ꢀ8656, Japan
‡
Department of Organic and Polymer Materials Chemistry Tokyo University of
Agriculture and Technology 2ꢀ24ꢀ16 Nakaꢀcho, Koganei, Tokyo 184ꢀ8588, Japan
Corresponding Authors
*
Eꢀmail: k_nakano@cc.tuat.ac.jp
* Eꢀmail: nozaki@chembio.t.uꢀtokyo.ac.jp
ABSTRACT: Benzophospholo[3,2ꢀb]benzofuran (BPBF) derivatives were studied as a
new family of phospholeꢀcontaining πꢀconjugated organic materials. The versatile
reactivity of the trivalent phosphorus center of the parent BPBF offered an access to a
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variety of derivatives by simple chemical modifications. The diversityꢀoriented
synthetic route was also developed to allow a facile access to BPBFs with a different
substituent on the phosphorus center. Photophysical properties were revealed based on
UVꢀvis and photoluminescence spectroscopies and theoretical calculation. The
characteristic property of BPBF derivatives is highlyꢀefficient photoluminescence. In
particular, phosphole oxide derivatives demonstrated intense blue fluorescence with
high quantum yield over 85% even in the solid state.
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TABLE OF CONTENTS
O
O
O
PhPCl
2
Li
Li
P
P
E
Ph
Ph
P=O, P=S, +PMe –OTf
O
O
Highly-
fluorescent
Cl
2
PNEt
2
MeOH
1) ArLi
) H
P
P
2
2
O
2
O
MeO
Ar
INTRODUCTION
Incorporation of phosphole moiety(s) into the πꢀconjugated system is a promising
1
strategy for new organic functional materials with novel properties. In contrast to the
nitrogen center in pyrrole, the trivalent phosphorus center in a phosphole ring adopts a
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tetrahedral geometry.
Accordingly, the lone pair on the phosphorus center cannot
interact with butadiene πꢀelectrons efficiently, which causes the very low aromaticity of
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2
phosphole. On the other hand, such a tetrahedral geometry allows the effective
interaction of the exocyclic P−C
σ* orbital with the endocyclic butadiene π* orbital,
3
resulting in a lowꢀlying LUMO energy level of phosphole. The tetrahedral geometry
of the phosphorus center also gives phosphole an opportunity for easy modification.
Oxidation, alkylation, and coordination to a Lewis acid give the corresponding
phosphole derivatives with different electronic and steric structures. Based on these
attractive features of phosphole moiety, a variety of phospholeꢀcontaining πꢀconjugated
4
molecules have been developed, such as oligophospholes, phospholeꢀcontaining
5
6
7
polymers,
dithieno[3,2ꢀ
zwitterionic benzoborolo[3,2ꢀ
2,5ꢀdiarylphospholes,
(di)benzophospholes,
8
9
b
;2′,3′ꢀ
d
]phospholes,
benzophospholo[3,2ꢀ
b
]benzophospholes,
and
1
0
b
]benzophospholes,
and found to be promising
candidates for highꢀperformance organic functional materials. In particular, high
emission property is characteristic in phospholeꢀcontaining πꢀconjugated compounds.
For example, 2,5ꢀdiarylphospholes have been systematically studied to be applied as
6
d, 6e
emissive layer for organic lightꢀemitting diodes (OLEDs).
Extended πꢀconjugated
8a, 8b, 8d
9
systems with a dithienophosphole
or phospholo[3,2ꢀ
b
]phosphole moiety with
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extended πꢀconjugated systems have been reported to demonstrate intense fluorescence
with moderate to high quantum yields.
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Benzeneꢀ, thiopheneꢀ, phospholeꢀ, and boroleꢀfused phosphole derivatives have
been reported so far. In particular, thiopheneꢀfused derivatives were intensely studied
(Chart 1). On the other hand, the furanꢀfused phosphole derivatives, such as
dibenzo[
d,
d
']phospholo[3,2ꢀ
b
:4,5ꢀb
']difuran 2 (DBPDF) (Chart 1), have been
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synthesized only in the last few years, and there is a limited information about their
11b
structure‒property relationship.
Recently, a variety of furanꢀfused πꢀconjugated
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molecules have been synthesized and been applied as organic functional materials.
Accordingly, it should make a significant contribution to the field of organic functional
materials to investigate πꢀconjugated furanꢀfused phosphole derivatives. Here we
report the synthesis and properties of benzophospholo[3,2ꢀb]benzofuran (BPBF)
derivatives 3−7, πꢀconjugated molecules with a phospholo[3,2ꢀ ]furan moiety (Chart 1).
b
The desired BPBFs were successfully synthesized via a conventional synthetic approach
for dibenzophospholes. In addition, the diversityꢀoriented synthetic strategy was also
developed in this study, which allows a facile access to BPBFs with a different
substituent on the phosphorus center. Furan moiety can be considered as an electron
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8a, 8b
donor, while phosphole can serve as an electron acceptor.
Accordingly, the
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BPBF framework is expected to be an intramolecular donorꢀacceptorꢀtype πꢀelectron
system, inducing a large Stokes shift and high photoluminescence property. Indeed,
the present BPBFs demonstrated high fluorescence with high quantum yield even in the
solid state.
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Chart 1. Structures of thiopheneꢀ and furanꢀfused phospholes
RESULTS AND DISCUSSION
Synthesis of BPBF Derivatives.
10ꢀphenylꢀbenzophospholo[3,2ꢀ ]benzofuran derivatives (10ꢀPhꢀBPBFs) is described in
Scheme 1. One of the most common synthetic strategies for dibenzophosphole moiety
Synthesis of the target molecules,
b
1
f
is the reaction of dilithiated biaryls with dichlorophosphines. In addition, the sulfur
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analog 1, benzophospholo[3,2ꢀ
b
]benzothiophenes (BPBT) (Chart 1) were synthesized
]thiophene and the subsequent
Accordingly, we designed
]furan (9) as a starting material for 10ꢀPhꢀBPBF 3
via the lithiation of 2ꢀ(2ꢀbromophenyl)ꢀ3ꢀbromobenzo[
b
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reaction
with
dichlorophosphines.
2ꢀ(2ꢀbromophenyl)ꢀ3ꢀiodobenzo[
b
which possesses a trivalent phosphorus center and should be a key intermediate for a
series of BPBF derivatives 4−6 (Scheme 1). Compound 9 was synthesized efficiently
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by iodineꢀmediated electrophilic cyclization of diarylethyne 8 which was obtained in
high yield from the commerciallyꢀavailable compounds via Sonogashira−Hagihara
n
coupling reaction. The obtained compound 9 was then dilithiated with BuLi in the
presence of
N,N,N′,N′ꢀtetramethylethylenediamine (TMEDA), and treated with
dichlorophenylphosphine. The crude product of 3 after column chromatography
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(neutralꢀalumina) under argon atmosphere showed the singlet signal at −29.5 ppm in P
NMR spectroscopy. The chemical shift was highꢀfield shifted compared to that of the
1
4
thiophene analog, BPBT 1 (
P
= P, Chart 1) (‒16.5 ppm), while it was lowꢀfield shifted
= P,
The complete isolation of 3 could not be achieved partially
compared to that of the related furanꢀfused phosphole compound, DBPDF 2 (
P
11b
Chart 1) (‒44.0 ppm).
because of its instability under air. Accordingly, the product was isolated as airꢀstable
phosphole oxide derivative 4 by treatment with aqueous H O (58% yield from 9).
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The lowꢀfieldꢀshifted P NMR resonance (21.5 ppm) clearly supported the formation of
phosphole oxide derivative 4. The chemical shift was again highꢀfield shifted
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compared to that of BPBT oxide 1 (
P = P=O, Chart 1) (26.1 ppm), while it was
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1b
lowꢀfield shifted compared to that of DBPDF oxide 2 (
P
= P=O, Chart 1) (11.9 ppm).
Scheme 1. Synthesis of 10ꢀPhꢀBPBF Derivatives 3‒6
MeO
MeO
PdCl (PPh )
2
3 2
CuI
I
+
Et N
3
2
5 °C, 23 h
Br
Br
8
9
4%
nBuLi
I2
toluene
O
TMEDA
PhPCl2
THF
–78 °C to rt
Br
reflux, 24 h
–78 °C, 15 min 5 min
I
9
9
5%
O
O
aqueous H O
2
2
CH Cl
25 °C, 20 min
8%
2
2
P
P
Ph
O
Ph
5
3
4
31P NMR:
Lawesson's reagent
toluene, reflux, 5 h
= –29.5 ppm
S8
9
5%
MeOTf, CH Cl
CH2Cl2
25 °C, 3 h
40%
2
2
O
2
5 °C, 6 h
5%
4
P
O
Ph
S
5
P
Ph Me
OTf
6
(TMEDA = N,N,N',N'ꢀtetramethylethylenediamine)
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Trivalent phosphole compound 3 was a common precursor for other derivatives
8d
(Scheme 1).
As summarized in Scheme 1, oxidation of 3 with S gave phosphole
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sulfide derivative 5 in 40% yield (from 9). Phosphole sulfide derivative 5 can be
synthesized by treatment of phosphole oxide derivative 4 with Lawesson’s reagent
(
95%) as well. Phosphonium derivative 6 was obtained via the reaction of 3 with
methyl triflate in 45% yield (from 9). These two compounds are also airꢀstable and
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can be easily isolated under ambient condition. The P NMR spectra of compounds 5
and 6 demonstrated lowꢀfield shift (27.8 ppm for 5, 16.3 ppm for 6) relative to the
parent compound 3.
We also developed a facile synthetic route to phospholeꢀfused πꢀconjugated
compounds with a desired substituent on the phosphorus center. The substituent on
the phosphorus center would have significant influence on physical properties of
phospholeꢀfused πꢀconjugated compounds.
Accordingly, elucidation of the
substituent effects is important to design new phospholeꢀfused πꢀconjugated
compounds. However, the substituent on the phosphorus center is indeed a phenyl
group in most of phospholeꢀfused πꢀconjugated compounds, and the reports on the
substituent effects have been very limited. Recently, Baumgartner and coworkers
synthesized dithieno[3,2ꢀ
b
;2′,3′ꢀ
d
]phospholes with a variety of substituents on the
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phosphorus center.
Their synthetic approach was the reaction of dilithiobiaryls with
aryldichlorophosphines which should be prepared on demand. In our synthetic route,
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10ꢀMeOꢀBPBF 11 is designed as a common intermediate which afford the desired
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0ꢀarylꢀBPBFs 7 after the reaction with organilithium reagent (Scheme 2). In this
route, the substituent on phosphorus center can be introduced in the last step. In
addition, variety of organolithium can be easily prepared from the
a
commerciallyꢀavailable organohalogen compounds. Accordingly, the library of
BPBFs with a different substituent on the phosphorus center can be readily prepared.
The synthesis of 10ꢀArꢀBPBFs is described in Scheme 2. Compound 9 was
dilithiated and treated with Cl PNEt , affording 10ꢀNEt ꢀBPBF 10 (85% yield). The
2
2
2
1
7
compound 10 is not so airꢀsensitive and can be purified under air.
10 was completely converted to 10ꢀMeOꢀBPBF 11 by the reaction with methanol.
Finally, 10ꢀMeOꢀBPBF 11 was reacted with organolithium reagent and subsequently
Then, the obtained
18
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oxidized with H O to give the desired 10ꢀArꢀBPBF oxides 7.
This procedure gave a
2
2
library of BPBF oxides with electronꢀdonating (4ꢀMeOꢀC H , 4ꢀMe NꢀC H , thienyl)
6
4
2
6
4
and electronꢀwithdrawing (4ꢀCF ꢀC H ) aryl groups on the phosphorus center.
3
6
4
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Scheme 2. Synthesis of 10ꢀArꢀBPBFs 7 by using 10ꢀMeOꢀBPBF 11 as a common
intermediate
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Photophysical Properties of 10-Phenyl-BPBF Derivatives.
properties of compounds 4−6 were evaluated by UV−vis absorption spectroscopy and
photolumenescence spectroscopy. Their spectra are shown in Figure 1, and their
Photophysical
photophysical data are summarized in Table 1. Phosphole oxide and sulfide
derivatives 4 and 5 gave the almost identical longest absorption maxima around at 340
4
nm with each other, while the molar absorption coefficient of 4 (ε = 0.77 × 10
−
1
−1
4
−1
−1
M ·cm at
λ
abs = 341 nm) was larger than that of 5 (
ε
= 0.66 × 10 M ·cm at
λ
abs
= 342 nm). Absorption spectrum of phosphonium derivative 6 showed the longest
absorption maximum at 351 nm, which was redꢀshifted compared to those of 4 and 5.
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Figure 1. UV−Vis absorption and photoluminescence spectra of BPBFs 4−6.
Table 1. Photophysical Data of BPBFs
solution
Stokes
shift
powder
λ
abs
λ
em
Φ
λ
em
Φ
a
b
c
d
c
(
nm)
(nm)
(%)
(nm)
(%)
‒
1
4
5
6
341
342
351
428
426
439
5960
5770
5710
84
<1
98
421
87
<1
50
e
ꢀꢀꢀ
469
a
b
c
In CH
2
Cl
2
.
2 2
In CH Cl . Excitation at 340 nm. Absolute
quantum yield determined by a calibrated integrating sphere
d
e
system. Excitation at 340 nm. Emission maximum cannot
be detected because of low emission intensity.
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The present BPBF derivatives were found to possess high photoluminescence
properties. All of the BPBF derivatives showed broad photoluminescence spectra in
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
—
1
2 2
CH Cl with large Stokes shifts of 5700~6000 cm . Compounds 4 and 6 exhibited
intense blue emission with high quantum yields, which are higher than those of the
8
a, 8b, 8d
previouslyꢀreported dithienophosphole derivatives
and 10ꢀPhꢀBPBT derivatives
1
, and are competitive with highlyꢀemissive bisꢀphosphorylꢀbridged stilbene
9
derivatives. In contrast, the low fluorescence intensity with a very low quantum yield
was observed for the phosphole sulfide derivative 5. It is noteworthy that high
quantum yields of compounds 4 and 6 were maintained in their solid states. In
particular, the quantum yield of the phosphole oxide derivative 4 was 0.87. High
emission property of phosphole oxide derivatives 4 in the solid state may be due to the
8d
stericallyꢀhindered structure around the tetrahedral phosphorus center.
Theoretical Study of Photophysical Properties of 10-Phenyl-BPBF Derivatives.
Phosphole oxide 4 possess the calculated longest wavelength absorption at 344 nm with
large oscillator strength (Table S4 for the selected absorption peaks), which are almost
identical to the experimental values (341 nm, Table 1). The calculated longest
wavelength absorption of phosphonium derivative 6 (363 nm) was longer than the
experimental value, while the redꢀshifted absorption relative to that of 4 was well
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demonstrated. These S →S electron transitions of 4 and 6 are dominated by
0
1
HOMO−LUMO (π−π*) transition. On the other hand, the obvious transition for
10
11
12
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14
15
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59
60
phosphole sulfide 5 is S →S3 electron transition (329 nm) dominated by
0
[
HOMO−2]−LUMO transition, which would be observed as the experimental longest
wavelength absorption maximum (342 nm, Table 1). The lowerꢀenergy transitions at
370 nm (HOMO−LUMO) and 350 nm ([HOMO−1]−LUMO) possess much lower
20
oscillator strength, which were not detected as absorption maxima in UV−vis
absorption spectroscopy. As shown in Figure 2, the HOMO orbital of compound 4
spreads over the BPBF framework, and the slightly larger distribution is observed in the
furan side. In contrast, the LUMO orbital is largely dominated by phosphole oxide
moiety. Such distribution in HOMO and LUMO orbitals may be attributed to that
furan and phosphole moieties work as electron donor and acceptor, respectively,
1
3
resulting in intramolecular donorꢀacceptorꢀtype πꢀelectron systems.
This
intramolecular donorꢀacceptorꢀtype interaction is partially supported by
2
1
solvatochromism of 4.
Increase in solvent polarity caused redꢀshift in
em = 413 nm in hexane [ (30) = 30.9], 423 nm in
photoluminescence spectra (
λ
E
T
benzene [ET(30) = 34.5], 423 nm in CH CN [ET(30) = 46.7], 428 nm in CHCl3 [ET(30)
3
22
=
39.1], 438 nm in MeOH [ET(30) = 55.5], ET(30) = solvent polarity parameter, Figure
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1
1
1
1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
λ
abs
), while significant solvatochromism in the absoption spectra was not observed ( =
3 3
336 nm in hexane, 341 nm in benzene, 341 nm in CH CN, 343 nm in CHCl , 343 nm in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
14
MeOH, Figure 4).
Such an electronic structure may induce large Stokes shifts,
which can diminish the selfꢀquenching of photoluminescence to afford high quantum
1
3, 23
yields.
Figure 2. Molecular orbitals of (a) HOMO and (b) LUMO of 4 by the DFT
calculation at the B3LYP/6ꢀ31G(d) level.
Figure 3. Photoluminescence spectra of BPBF 4 in different solvents.
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49
50
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53
54
55
56
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58
59
60
Figure 4. Absorption spectra of BPBF 4 in different solvents.
Effects of the Substituents on Phosphorus Center on Photophysical Properties.
Absorption and fluorescence properties of BPBFs 7 are summarized in Table 2 and
Figures 5 and 6. As shown in Figure 5, compound 7a, 7c, and 7d show similar
absorption features to compound 4 with absorption maxima at around 300, 310, and 340
nm, while the molar absorption coefficients are depend on the substituents. On the
other hand, compound 7b with 4ꢀ(dimethylamino)phenyl group on the phosphorus
center shows less resolved absorption spectrum. In the photoluminescence spectra,
compound 7a, 7c, and 7d again show similar features with emission at around 430 nm
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1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(Figures 6). In contrast, 4ꢀ(dimethylamino)phenyl group induces redꢀshifted emission.
‒
1
The Stokes shift of 7770 cm is largest in the series of BPBF derivatives in this study.
Such large Stokes shift should be attributed to intramoleclar charge transfer. The DFT
calculation showed that the HOMO of compound 7b is dominated by
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4ꢀ(dimethylamino)phenyl moiety (donor) and the LUMO is localized at BPBF moiety
(Figure S35). This trend is similar to the reported dithienophospholes with a
24
4ꢀ(diphenylamino)phenyl substituent on the phosphorus center.
Table 2. Photophysical Data of BPBFs 7
Stokes shift
a
b
c
λ
abs (nm)
λ
em (nm)
Φ (%)
‒
1
(
cm )
7
7
a
342
341
346
428
464
433
428
5880
7770
5810
5450
88
90
ꢀꢀꢀ
ꢀꢀꢀ
b
7
c
7d
347
a
b
c
In CHCl
3
.
3
In CHCl . Excitation at 315 nm. Absolute
quantum yield determined by a calibrated integrating sphere
system.
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48
49
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52
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59
60
Figure 5. Absorption spectra of BPBFs 4 and 7.
Figure 6. Photoluminescence spectra of BPBFs 4 and 7.
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4
4
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4
4
4
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5
6
0
1
2
3
4
5
6
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9
0
1
2
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5
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8
9
0
1
2
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7
8
9
0
1
2
3
4
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7
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9
0
1
2
3
4
5
6
7
8
9
0
CONCLUSION
In conclusion, a series of BPBF derivatives have been synthesized via simple reaction
sequences. The trivalent phosphole derivative, 10ꢀPhꢀBPBF was obtained by using the
conventional synthetic route and transformed to
Pꢀfunctionalized derivatives. In
addition, the diversityꢀoriented synthetic route was developed by using 10ꢀMeOꢀBPBF
as an easilyꢀtoꢀuse common intermediate. This synthetic approach would be applied to
the synthesis of other dibenzophosphole derivatives and open up the chemistry of
phospholeꢀcontaining πꢀconjugated compounds. Photophysical properties of BPBFs
were revealed based on UVꢀvis and photoluminescence spectroscopies and theoretical
calculation. The highly electronꢀdonating aryl substituent on phosphorus center were
found to induce large Stokes shift, which should be caused by intramolecular charge
transfer. The chracteristics of the present BPBF derivatives are large Stokes shift and
high emission property even in the solid state, which would be derived from the
incorporation of the phosphole moiety into the fusedꢀconjugated system.
EXPERIMENTAL SECTION
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General Procedures. NMR spectra were recorded in deuteriochloroform or C D on
6
6
1
13
31
a 500 MHz spectrometer ( H 500 MHz, C 126 MHz, P 202 MHz) or a 400 MHz
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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29
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41
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50
51
52
53
54
55
56
57
58
59
60
1
13
19
31
spectrometer ( H 400 MHz, C 101 MHz, F 376 MHz, P 162 MHz). Chemical
shifts are reported in ppm relative to the internal standard peak (0 ppm for Me Si) for
4
1
13
H, the deuterated solvent peak (77.16 ppm for CDCl ) for C, and the external
3
31
standard peak (85% H PO ) for P. Data are presented in the following space:
3
4
chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet and/or
multiplet resonances, br = broad), coupling constant in hertz (Hz), and signal area
integration in natural numbers. Melting points were determined on a melting point
apparatus.
High resolution mass spectra are taken by electrospray
ionizationꢀtimeꢀofꢀflight (ESI−TOF) method or by atmospheric pressure chemical
ionizationꢀtimeꢀofꢀflight (APCI−TOF) method. UV absorption spectra were recorded
on a UV−VIS scanning spectrophotometer. Photoluminescence spectra were recorded
on a spectrofluorometer. Absolute quantum yields were determined by calibrated
integrating sphere system.
All manipulations involving airꢀ and/or moistureꢀsensitive compounds were carried
out in a glove box under argon atmosphere or with the standard Schlenk technique
under argon. All the solvents used for reactions were distilled under argon after drying
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2
2
2
2
2
2
2
3
3
3
3
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3
3
3
4
4
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4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
over an appropriate drying reagent or passed through solvent purification columns.
Most of reagents were used without further purification unless otherwise specified.
Analytical thinꢀlayer chromatography was performed on a glass plates coated with
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.25ꢀmm 230–400 mesh silica gel containing a fluorescent indicator. Column
chromatography was performed by using silica gel (spherical neutral, particle size
3ꢀ210 m).
6
ꢀ
Computational Studies. The DFT and TD‒DFT calculations were performed by
25
using the Gaussian 03 program (compounds 4 and 5) or Gaussian 09 program
26
(
compounds 6 and 7b) at the B3LYP/6‒31G(d) level.
Synthetic Procedures. The followings are synthetic procedures for BPBFs in this
study.
2
7
1
-Bromo-2-[(2-methoxyphenyl)ethynyl]benzene (8).
A flameꢀdried 250ꢀmL
Schlenk tube containing a magnetic stirring bar was charged with 2ꢀethynylanisole
(
2.78 g, 21 mmol), 1ꢀbromoꢀ2ꢀiodobenzene (5.67 g, 20 mmol), PdCl (PPh ) (282 g,
2 3 2
0
.40 mmol), CuI (38 g, 0.20 mmol), Et N (50 mL) under argon atmosphere. After
3
degassed by freezeꢀpumpꢀthaw cycles (three times), the mixture was stirred under argon
2
atmosphere at 25 °C for 23 h. White precipitate was filtered off with Et O, and the
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filtrated was concentrated under reduced pressure. The resulting residue was purified
by silicaꢀgel column chromatography (Hex/AcOEt = 8/1, Rf = 0.43) to give the title
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
compound as a paleꢀyellow oil (5.39 g, 94% yield): H NMR (500 MHz, CDCl
3
)
δ
7
1
8
1
.62–7.58 (m, 2H), 7.55 (dd,
J
= 7.6, 1.8 Hz, 1H), 7.35–7.31 (m, 1H), 7.28 (td,
= 7.5, 0.8 Hz, 1H), 6.91 (d, J =
J = 7.6,
.1 Hz, 1H), 7.16 (td, = 7.7, 1.8 Hz, 1H), 6.95 (td,
J
J
1
3
1
.5 Hz, 1H), 3.93 (s, 3H); C{ H} NMR (101 MHz, CDCl
3
) δ 160.1, 133.7, 133.3,
32.4, 130.2, 129.3, 127.0, 125.8, 125.5, 120.5, 112.2, 110.9, 92.0, 90.6, 55,9.
12k
3
-Iodo-2-(2-bromophenyl)benzo[
b
]furan (9).
A
flameꢀdried 200ꢀmL
roundꢀbottom flask containing a magnetic stirring bar was charged with 8 (1.0 g, 3.5
mmol), iodine (3.6 g, 14 mmol), and toluene (30 mL). After the mixture was stirred
under reflux for 24 h in the dark, saturated aqueous sodium thiosulfate solution (100
mL) was added to the mixture. The resulting mixture was stirred at 25 °C for 30 min,
and the organic layer was separated. The aqueous layer was extracted with toluene (20
mL × 3), and the combined organic layers were dried over Na SO , filtered, and
2
4
concentrated under reduced pressure. The resulting reside was purified by silicaꢀgel
column chromatography (hexane as an eluent, Rf = 0.30) to give the title compound as a
1
colorless oil (1.32 g, 95%): H NMR (500 MHz, CDCl
3
)
δ
7.74 (dd,
J
= 8.1, 1.3 Hz,
1H), 7.57 (dd,
J
= 7.6, 1.8 Hz, 1H), 7.53–7.51 (m, 1H), 7.50–7.48 (m, 1H), 7.45 (td, J =
2
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1
1
1
1
1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
3
1
7.6, 1.1 Hz, 1H), 7.41 (td,
J = 7.6, 1.4 Hz, 1H), 7.38–7.35 (m, 2H); C{ H} NMR (101
MHz, CDCl
3
) δ 154.9, 154.5, 133.5, 133.1, 131.5, 131.5, 131.2, 127.3, 125.9, 124.2,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
123.7, 121.9, 111.6, 66.4. Anal. Calcd for C H BrIO (%): C, 42.14; H, 2.02. Found
1
4
8
(%): C, 41.98; H, 2.12.
1
0-Phenylbenzophospholo[3,2-
b]benzofuran (3). A flameꢀdried 20 mL Schlenk
tube containing a magnetic stirring bar was charged with 9 (200 mg, 0.5 mmol), Et O
2
(
10 mL), and N,N,N′,N′ꢀtetramethylethylenediammine (TMEDA, 30
under argon atmosphere. The mixture was stirred for 30 min at −78 °C, and
ꢀbutyllithium (0.67 mL of 1.67 M solution in hexane, 1.1 mmol) was added dropwise
ꢀL, 2.0 mmol)
n
to the mixture at −78 °C. After stirred at the same temperature for 15 min, PPhCl
2
(90
ꢀ
L, 0.66 mmol) was added in one portion to the reaction mixture. The reaction
mixture was allowed to warm quickly to 25 °C (over 5 min) and concentrated under
reduced pressure. The residue was dissolved with CH Cl (50 mL) and passed through
2
2
a short pad of neutral alumina under argon atmosphere. The filtrate was concentrated
under reduced pressure and further purified by neutralꢀalumina column chromatography
under argon atmosphere (CH Cl /hexane = 2/1 as an eluent, Rf = 0.40) to provide 3 as a
2
2
1
pale yellow solid (86 mg, 57 % yield): H NMR (500 MHz, CDCl
3
) δ 7.80 (d, JHH = 7.6
Hz, 1H), 7.69–7.66 (m, 1H), 7.62–7.59 (m, 2H), 7.45–7.42 (m, 3H), 7.33–7.23 (m, 6H);
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5
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7
8
9
1
3
1
C{ H} NMR (101 MHz, CDCl
146.9 (d, JCP = 7.7 Hz), 134.0 (d, JCP = 16.3 Hz), 133.2 (s), 132.5 (d, JCP = 20.1 Hz),
130.8 (d, JCP = 20.1 Hz), 129.6 (s), 128.9 (d, JCP = 7.7 Hz), 128.7 (s), 128.3 (d, JCP
3
) δ 163.5 (d, JCP = 8.6 Hz), 159.1 (d, JCP = 4.8 Hz),
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
=
1
3.4 Hz), 127.3 (d, JCP = 7.7 Hz), 124.6 (s), 123.8 (s), 120.9 (s), 120.1 (s), 118.1 (d, JCP
31
1
=
3
5.8 Hz), 112.4 (s); P{ H} NMR (202 MHz, CDCl ) δ −29.5 (s).
1
0-Phenylbenzophospholo[3,2-b]benzofuran oxide (4). A flameꢀdried 20 mL
Schlenk tube containing a magnetic stirring bar was charged with 3, which was prepared
from 200 mg of 9 (0.50 mmol) according to the procedure described above, and CH Cl
2
2
(10 mL) under argon atomosphere. H O (0.80 mL, 35% aqueous solution) was added
2 2
in one portion. After the resulting mixture was stirred 25 °C for 20 min, saturated
aqueous Na S O (3 mL) was slowly added to the reaction mixture. Organic layer was
2
2
3
separated, and the aqueous layer was extracted with AcOEt (20 mL × 3). The
combined organic layers were dried over MgSO , filtered, and concentrated under
4
reduced pressure.
The resulting residue was purified by silicaꢀgel column
chromatography (AcOEt/hexane = 2/1 as an eluent, Rf = 0.38) to give the title
1
compound as a colorless solid (92 mg, 58% yield from 9): mp 160−162 °C; H NMR
(
400 MHz, CDCl
3
) δ 7.80 (dd, JHP = 13.5, JHH = 7.3 Hz, 2H), 7.72–7.66 (m, 2H), 7.62–
7
.59 (m, 2H), 7.56–7.51 (m, 2H), 7.44–7.38 (m, 3H), 7.36–7.32 (m, 1H), 7.28 (t, JHH
=
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1
1
1
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1
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
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4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
3
1
6
3 CP
.9 Hz, 1H); C{ H} NMR (101 MHz, CDCl ) δ 168.0 (d, JCP = 36.4 Hz), 159.4 (d, J
= 10.5 Hz), 138.7 (d, JCP = 106.4 Hz), 133.0 (d, JCP = 12.5 Hz), 132.8 (d, JCP = 1.9 Hz),
132.7 (d, JCP = 2.9 Hz), 131.1 (d, JCP = 11.5 Hz), 130.2 (d, JCP = 11.5 Hz), 130.02 (d,
JCP = 8.6 Hz), 129.98 (d, JCP = 110.2 Hz), 129.1 (d, JCP = 13.4 Hz), 125.7 (s), 125.4 (d,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
JCP = 7.7 Hz), 124.9 (s), 121.5 (s), 120.1 (d, JCP = 8.6 Hz), 112.5 (s), 112.3 (d, JCP
=
3
1
1
1
21.7 Hz); P{ H} NMR (162 MHz, CDCl ) δ 21.5 (s). Anal. Calcd for C H O P
3 20 13 2
(
%): C, 75.95; H, 4.34. Found (%): C, 75.75; H, 4.34.
1
0-Phenylbenzophospholo[3,2-b]benzofuran sulfide (5). A flameꢀdried 20 mL
Schlenk tube containing a magnetic stirring bar was charged with 3, which was prepared
from 200 mg of 9 (0.50 mmol) according to the procedure described above, and CH Cl
2
2
(10 mL) under argon atmosphere. An excess sulfur S (320 mg, 10 mmol) was added
8
in one portion, and the reaction mixture was stirred 25 °C for 3 h. After the reaction
mixture was concentrated under reduced pressure, the resulting crude residue was
purified by silicaꢀgel column chromatography (CHCl /hexane = 3/2 as an eluent, Rf
=
3
0
1
7
7
.35) to give the title compound as a colorless solid (67 mg, 40% yield from 9): mp
1
68−170 °C; H NMR (500 MHz, CDCl
3
)
δ
7.86 (dd,
J
HP = 15.0,
J
HH = 7.4 Hz, 2H),
.72–7.68 (m, 2H), 7.63–7.60 (m, 2H), 7.56–7.53 (m, 1H), 7.49–7.46 (m, 1H), 7.43–
13
1
3 CP
.34 (m, 4H), 7.29 (t, J = 7.4 Hz, 1H); C{ H} NMR (126 MHz, CDCl ) δ 166.0 (d, J
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= 31.7 Hz), 159.4 (d, JCP = 11.5 Hz), 142.4 (d, JCP = 88.3 Hz), 132.4 (d, JCP = 2.9 Hz),
132.2 (s), 130.9 (d, JCP = 12.5 Hz), 130.23 (d, JCP = 12.5 Hz), 130.22 (d, JCP = 86.4 Hz),
129.6 (d, JCP = 10.6 Hz), 129.0 (d, JCP = 13.4 Hz), 125.8 (s), 125.1 (d, JCP = 8.6 Hz),
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
124.9 (s), 121.0 (s), 120.3 (d, JCP = 7.7 Hz), 114.0 (d, JCP = 105.6 Hz), 112.6 (s);
3
1
1
P{ H} NMR (202 MHz, CDCl
3
)
δ
27.8 (s). Anal. Calcd for C20
H13OPS (%): C,
72.28; H, 3.94. Found (%): C, 72.15; H, 4.05.
Transformation of 4 into 5 by Lawesson’s reagent. A flameꢀdried 50ꢀmL
roundꢀbottom flask containing a magnetic stirring bar was charged with 4 (158 mg, 0.5
mmol), Lawesson’s reagent (810 mg, 2.0 mmol), and toluene (20 mL). After stirring
under reflux for 5 h, the reaction mixture was concentrated under reduced pressure. The
resulting crude residue was purified by silicaꢀgel column chromatography
(
CHCl /hexane = 2/1 as an eluent, Rf = 0.33) to give 5 as a colorless solid (158 mg, 95%
3
yield).
1
0-Methyl-10-phenylbenzophospholo[3,2-b]benzofuran-10-ium
trifluoromethansulfonate (6). A flameꢀdried 20 mL Schlenk tube containing a
magnetic stirring bar was charged with 3, which was prepared from 100 mg of 9 (0.25
mmol) according to the procedure described above, and CH Cl (8 mL) under argon
2
2
atmosphere. Methyl triflate (110 ꢀL, 1.1 mmol) was added in one portion, and the
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reaction mixture was stirred 25 °C for 6 h. After the reaction mixture was
concentrated under reduced pressure, the title compound was obtained by
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
reprecipitation from acetone as a colorless solid (52 mg, 45% yield from 9): mp
1
1
1
1
79.5−181.0 °C; H NMR (400 MHz, CDCl
3
)
δ
8.50 (dd,
J
HP = 11.7 Hz,
J
HH = 7.7 Hz,
H), 8.08 (dd, JHP = 15.1 Hz, JHH = 7.4 Hz, 2H), 7.92 (dd, JHP = 3.4 Hz, JHH = 7.5 Hz,
1
3
1
H), 7.86–7.62 (m, 7H), 7.55–7.47 (m, 2H), 2.95 (d, JHP = 15.0 Hz, 3H); C{ H} NMR
169.7 (d, CP = 31.7 Hz), 159.8 (d, CP = 11.5 Hz), 135.9 (d,
.9 Hz), 135.5 (d, JCP = 1.9 Hz), 134.0 (d, JCP = 9.6 Hz), 133.0 (d, JCP = 11.5 Hz), 132.6
(
126 MHz, CDCl
3
)
δ
J
J
CP
J =
2
(
d, JCP = 12.5 Hz), 132.3 (d, JCP = 12.5 Hz), 130.8 (d, JCP = 14.4 Hz), 128.6 (d, JCP
=
1.2 Hz), 127.6 (s), 126.6 (s), 123.8 (d, JCP = 9.6 Hz), 122.3 (d, JCP = 8.6 Hz), 121.3 (s),
20.9 (q, JCF = 320.5 Hz, one of the signals of the quartet overlaps with the doublet at
9
1
122.3 ppm), 116.3 (d, JCP = 90.2 Hz), 113.3 (s), 100.9 (d, JCP = 113.2 Hz), 7.7 (d, JCP
=
31
1
+
53.7 Hz); P{ H} NMR (162 MHz, CDCl ) δ 16.3 (s). HRMS–ESI (
m
/z
) calcd for
3
+
C H OP {[M–OSO CF ] } 315.0939, found315.0940.
21 16
2
3
1
0-Diethylaminobenzophospholo[3,2-b]benzofuran (10). A flameꢀdried 80 mL
Schlenk tube containing a magnetic stirring bar was charged with 9 (800 mg, 2.0
mmol), Et O (30 mL), and N,N,N′,N′ꢀtetramethylethylenediammine (TMEDA, 1.20 mL,
2
8.0 mmol) under argon atmosphere. The mixture was cooled to −78 °C, and
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n
ꢀbutyllithium (1.69 mL of 2.6 M solution in hexane, 4.4 mmol) was added dropwise to
2 2
the mixture at −78 °C. After stirred at the same temperature for 1 h, Cl PNEt (0.39
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
mL, 2.6 mmol) was added dropwise to the mixture at −78 °C. The resulting mixture
was stirred for 30 min at the same temperature and allowed to warm quickly to ambient
temperature. After stirring overnight, the reaction mixture was passed through a short
pad of neutral alumina with Et O wash. The filtrate was concentrated, and hexane (20
2
mL) was added. The resulting suspension was filtered and concentrated to give
1
10ꢀNEt
2
ꢀBPBF 10 as a yellow oil (504 mg, 85% yield). H NMR (500 MHz, C
6
D
6
)
δ
,
7.70 (d, JHH = 7.6 Hz, 1H), 7.66–7.61 (m, 2H), 7.42 (d, JHH = 7.9 Hz, 1H), 7.13–7.02
13
1
(
m, 4H), 2.82–2.75 (m, 4H), 0.89 (t, JHH = 7.2 Hz, 6H); C{ H} NMR (126 MHz,
CDCl 163.6 (d, CP = 12.0 Hz), 159.7 (d, CP = 3.6 Hz), 147.5 (d, JCP = 8.4 Hz),
3
)
δ
J
J
133.3 (d, JCP = 3.6 Hz), 130.9 (d, JCP = 20.4 Hz), 129.2 (d, JCP = 13.7 Hz), 129.1, 127.4
(d, JCP = 7.2 Hz), 124.6, 124.0, 121.2, 119.7 (d, JCP = 15.6 Hz), 119.6, 112.5, 44.7 (d,
3
1
1
JCP = 14.4 Hz), 15.3 (d, JCP = 3.67 Hz), P{ H} NMR (162 MHz, C D ) δ 28.6 (s).
6
6
1
0-Methoxybenzophospholo[3,2-
b
]benzofuran (11).
A flameꢀdried 80 mL
Schlenk tube containing a magnetic stirring bar was charged with 10 (492 mg, 1.67
mmol), toluene (3 mL), and methanol (3 mL) under argon atmosphere. The resulting
mixture was stirred at 65 °C for 4 h. Concentration of the mixture gives
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
1
0ꢀMeOꢀBPBF 11 as a yellow oil (420 mg, 99% yield). H NMR (400 MHz, C
6
D
6
)
δ
,
7.50–7.40 (m, 3H), 7.32–7.29 (m, 1H), 7.00–6.93 (m, 3H), 6.90–6.84 (m, 1H), 2.86 (d,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
1
J
6 6
PH = 8.3 Hz, 3H); C{ H} NMR (126 MHz, C D ) δ 166.0 (d, JCP = 6.0 Hz), 159.6 (d,
JCP = 4.8 Hz), 143.7 (d, JCP = 24.0 Hz), 133.9, 131.8 (d, JCP = 22.8 Hz), 130.4, 128.9 (d,
JCP = 14.4 Hz), 128.4 (d, JCP = 6.0 Hz), 124.9, 124.4, 121.5, 119.6, 117.3 (d, JCP = 30.0
31
1
Hz), 112.4, 53.8 (d, JCP = 14.4 Hz); P{ H} NMR (202 MHz, C D ) δ 80.6 (s).
6
6
General Procedure for the Synthesis of 10-Arylphospholo[3,2-b]benzofuran (7).
The flameꢀdried 20 mL Schlenk tube containing a magnetic stirring bar was charged
with haloarene (1.0 mmol), THF (2.0 mL), and TMEDA (0.30 mL, 2.0 mmol) under
argon atmosphere. The mixture was cooled to −78 °C, and nꢀbutyllithium (0.38 mL of
2.6 M solution in hexane, 1.0 mmol) was added dropwise to the mixture at −78 °C.
After stirred at the same temperature for 1 h, the THF solution of 11 (1.0 mL of 0.50 M)
was added dropwise at −78 °C. After stirred at the same temperature for 30 min, the
reaction mixture was allowed to warm to 50 °C and stirred at 50 °C for overnight. The
reaction mixture was concentrated under reduced pressure, and the residue was
suspended with Et O (10 mL) under argon atmosphere. The resulting suspension was
2
passed through a short pad of neutral alumina with Et O wash, and the filtrate was
2
concentrated. The resulting residue was dissolved in CH Cl (5 mL), and H O (1.0
2
2
2
2
2
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mL, 35% aqueous solution) was added in one portion. After the resulting mixture was
stirred 25 °C for 1 h, water (5 mL) was added. The organic layer was separated,
washed with saturated aqueous Na S O (5 mL), dried over Na SO , filtered, and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
2
3
2
4
concentrated under reduced pressure. The resulting reside was purified by silicaꢀgel
column chromatography.
1
0-(4-Methoxyphenyl)benzophospholo[3,2-b]benzofuran oxide (7a). The crude
residue was obtained by using 1ꢀbromoꢀ4ꢀmethoxybenzene (0.13 mL, 1.0 mmol). The
resulting residue was purified by silicaꢀgel column chromatography (AcOEt/hexane =
5/1 as an eluent, Rf = 0.28) as a pale yellow solid (70 mg, 42% yield). mp 138.0–140.0
1
°C; H NMR (400 MHz, CDCl
3
)
δ
7.75–7.64 (m, 4H), 7.61–7.59 (m, 2H), 7.53 (t,
J
HH
=
7.6 Hz, 1H), 7.41–7.32 (m, 2H), 7.29 (d, JHH = 8.0 Hz, 1H), 6.92 (dd, JHP = 2.3 Hz, JHH
1
3
1
= 8.8 Hz, 2H), 3.80 (s, 3H); C{ H} NMR (101 MHz, CDCl
Hz), 163.3 (d, JCP = 2.9 Hz), 159.4 (d, JCP = 10.5 Hz), 139.21 (d, JCP = 106.4 Hz), 133.0
d, JCP = 13.4 Hz), 132.9 (d, JCP = 12.5 Hz), 132.7 (d, JCP = 1.9 Hz), 130.2 (d, JCP
3
) δ 167.9 (d, JCP = 35.5
(
=
1
1.5 Hz), 129.8 (d, JCP = 8.6 Hz), 125.6 (s), 125.5 (d, JCP = 7.7 Hz), 124.9 (s), 121.6 (s),
20.5 (d, JCP = 116.9 Hz), 120.0 (d, JCP = 8.6 Hz), 114.8 (d, JCP = 13.4 Hz), 112.5 (d,
1
31
+
JCP = 121.7 Hz), 112.5 (s); P{1H} NMR (162 MHz, CDCl ) δ 21.6 (s). HRMS–ESI
3
+
(
m/
z
) calcd for C H O PNa [{M+Na} ] 369.0657, found 369.0650.
21
15
3
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