Synthesis of π-extended dibenzophospholes by intramolecular radical cyclization and their properties

Article abstract of DOI:10.1021/ol501189u

Intramolecular radical cyclization of phosphine oxides (R¹R ²P(O)H) induced by radical generators affords the corresponding dibenzophosphole oxides in excellent yields. By applying this method, linearly π-extended ladder-type dibenzo

Full text of DOI:10.1021/ol501189u

Letter
pubs.acs.org/OrgLett
Synthesis of π‑Extended Dibenzophospholes by Intramolecular
Radical Cyclization and Their Properties
Shunsuke Furukawa, Shunsuke Haga, Junji Kobayashi,*,† and Takayuki Kawashima*,‡
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: Intramolecular radical cyclization of phosphine
oxides (R1R2P(O)H) induced by radical generators affords the
corresponding dibenzophosphole oxides in excellent yields. By
applying this method, linearly π-extended ladder-type
dibenzophosphole oxides were successfully synthesized.
he phosphorus-containing π-conjugated five-membered
ring, phosphole, has attracted attention because of its
Scheme 1. Strategy To Construct a Dibenzophosphole
Framework by Intramolecular Radical Cyclization
T
relatively small HOMO−LUMO energy gap and because its
properties can be tuned through diverse functionalization of the
phosphorus atom.1 Recently, π-extended phospholes have been
applied in organic electronic devices such as light-emitting
diodes2 and organic photovoltaic devices,3 and the potential
utility of this class of compounds has led to the development of
unique synthetic methods.4 For simple dibenzophospholes,
several synthetic methods have been reported including
intramolecular Friedel−Crafts type cyclization of 2-biphenyl-
phenylphosphinic acid or aryldichlorophosphines,5 the reaction
of tetraphenylphosphonium bromide with lithium diethyl-
amide,6 and the anionic intramolecular cyclization of triphenyl-
phosphine oxide with 2 equiv of phenyllithium.7 However,
reports on the synthesis of π-extended dibenzophospholes are
limited; available methods include palladium-catalyzed intra-
molecular dehydrogenative cyclization of hydrophosphine
oxides8 and a 4-fold free-radical phosphonylation reaction of
a tetrabromo-p-terphenylene or biphenylthiophene.9 These
methods avoid the use of highly reactive polylithiated species,
which can lead to the formation of oligomeric side products
during the construction of further π-extended phosphole
skeletons.9 In a similarly directed approach, we have developed
an efficient method that avoids the requirement for a rare metal
catalyst. In this paper, we report on intramolecular radical
cyclization of secondary phosphine oxides that can effectively
yield π-extended dibenzophosphole oxides.
dibenzophosphole oxides 2 (Table 1). The reaction of [4,4′-
di-tert-butyl-(1,1′-biphenyl)-2-yl](phenyl)phosphine oxide (1a)
Table 1. Reaction Conditions for the Intramolecular Radical
Cyclization
conditions
a
radical generator
(equiv)
temp
(°C)
conv
(%)
entry
R
solvent
1
2
3
4
5
6
Ph
Ph
Ph
Ph
AIBN (1.1)
BPO (1.1)
benzene
benzene
MeOH
MeOH
MeOH
MeOH
90
90
rt
48
41
73
87
96
85
b
Et3B (1.0)/O2
b
Et3B (2.1)/O2
rt
b
p-anisyl
t-Bu
Et3B (2.1)/O2
rt
b
Et3B (12)/O2
rt
We hypothesized that hydrogen abstraction from the P−H
bond would efficiently form a P-centered radical intermediate
because of the small bond dissociation energy of such bonds
(322 kJ/mol).10 Subsequent addition of the radical species to
unsaturated bonds in a Pudovik-type reaction11 followed by
hydrogen elimination would then give cyclized dibenzo-
phosphole oxides (Scheme 1).
This intramolecular cyclization strategy was considered to be
a promising method with which to construct π-extended
dibenzoheteroles; indeed, we have previously demonstrated
such an approach for silicon analogues.12
a
b
The conversion was determined by 1H NMR spectroscopy. Air was
added by a syringe to an argon-filled vessel.
with a typical radical generator such as 2,2′-azobis-
(isobutyronitrile) (AIBN) or benzoyl peroxide (BPO) under
heating in benzene gave the corresponding cyclized product 2a
in moderate yields (41−48%) (entries 1−2). Performing the
reaction under milder conditions by using triethylborane with
oxygen as a radical generator gave the product in higher yield
It was found that treatment of 2-biphenylarylphosphine
oxides 1 with radical generators affords the desired
Received: May 1, 2014
© XXXX American Chemical Society
A
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Organic Letters
Letter
(73%) at room temperature (entry 3). After optimization of the
reaction conditions, we found that the use of 2.1 equiv of
triethylborane in the presence of oxygen gave the best result
(87%) (entry 4). These reaction conditions were applicable to
other substrates bearing a p-anisyl group 1b (entry 5) or a tert-
butyl group 1c (entry 6) on the phosphorus atom, and they
gave the desired products 2b and 2c in good yields (96% and
85%, respectively). For the bulky substrate 1c, an excess of
triethylborane was required for high conversion, which may be
because of the kinetically stabilized P−H bond in 1c and the
short lifetime of the ethyl radical generated in the
triethylborane/O2 system.
This intramolecular cyclization was also applicable to the
construction of π-extended ladder-type dibenzophospholes. By
adding a small excess of triethylborane to bis(phenylphos-
phinyl)-p-terphenyl 3 in the presence of oxygen, doubly
cyclized phosphole oxides anti-4 and syn-4 were obtained as
a diastereomeric mixture (Scheme 2). These diastereomers
Scheme 2. Double Intramolecular Radical Cyclization
Figure 1. ORTEP drawings of dibenzophosphole sulfides anti-5 (a)
and syn-5 (b) (50% probability).
The photophysical properties of the ladder-type dibenzo-
phosphole oxides 4 and sulfides 5 were evaluated by UV−vis
absorption and fluorescence spectroscopy, and the obtained
data are summarized in Table 2, together with those of simple
were successfully separated by silica gel column chromatog-
raphy to give anti-4 and syn-4 in 16% and 28% isolated yields,
respectively. In an attempt to confirm the anti/syn con-
formation of the products 4, single crystals of the products were
not suitable for single-crystal X-ray crystallographic analysis.
The conformation was, however, determined from X-ray
crystallographic analysis of the corresponding phosphole sulfide
5, which were obtained from 4 with retention of configuration
by using Lawesson’s reagent (Scheme 3).13 As expected,
phosphole sulfides anti-5 and syn-5, prepared from phosphole
oxides anti-4 and syn-4, respectively, afforded single crystals
that were suitable for X-ray analysis. The X-ray structures of
anti-5 and syn-5 revealed that both compounds have almost
flat, ladder-type p-terphenyl units (Figure 1).
Table 2. Photophysical Data for Ladder-Type Phosphole
Oxide and Sulfide Derivatives
fluorescence
a
a
absorption
solution
solid
λem [nm]
b
b
compd λabs [nm] log ε λem [nm]
ΦF
ΦF
2a
340
391
391
385
3.19
3.68
3.61
3.73
3.72
382
0.67
−
454
−
anti-4
syn-4
anti-5
syn-5
426, 438
426, 438
430
0.89
0.46
0.63
0.12
0.04
0.79
476
0.004
0.004
437, 471, 501
476, 500
386
431
a
b
In CH2Cl2. Absolute quantum yield determined by an integrating
sphere system.
dibenzophosphole oxide 2a. Dibenzophosphole oxide 2a
showed an absorption maximum at λabs = 340 nm (log ε =
3.19) that was derived from the HOMO−LUMO transition
(Figure S1). In the ladder-type phosphole oxide anti-4, the
absorption maximum was significantly red-shifted to 391 nm
(log ε = 3.68) because of the π-expansion through the σ*−π*
conjugation of the main π-framework to the σ* orbital of the
exocyclic P−C bonds. This result is consistent with a previous
report for the ladder-type dibenzophosphole oxide.9 The
diastereomer syn-4 showed a similar absorption profile to
that of anti-4. Fluorescence spectra showed that the emission
wavelengths of the ladder-type derivatives 4 are also red-shifted
(λem = 426, 438 nm for both anti-4 and syn-4) compared with
that of 2a (λem = 382 nm). The ladder-type dibenzophosphole
oxides 4 showed high fluorescence quantum yields of 0.89 and
Scheme 3. Conversion of Phosphole Oxides to Phosphole
Sulfides with Lawesson’s Reagent
B
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Organic Letters
Letter
0.79 for anti- and syn-4, respectively, which result from their
rigid main π-framework. Interestingly, a slight difference in the
fluorescence spectra was found between anti-4 and syn-4 in the
solid state. The emission maximum of the latter in the solid
state (λem = 476 nm) was slightly red-shifted compared with
that of the former (λem = 454 nm) (Figure S2), and the
fluorescence quantum yield of syn-4 (ΦF = 0.63) was larger
than that of anti-4 (ΦF = 0.46). These results suggest that
intermolecular interactions in the solid state are different in the
anti and syn forms of 4 because of their conformational
difference.
We also found that phosphole oxides and sulfides have
drastically different photophysical properties. Dibenzo-
phosphole sulfides 5 showed absorption maxima at 385 nm
(log ε = 3.73) and 386 nm (log ε = 3.72) for anti- and syn-5,
respectively (Figure 2). DFT calculations suggested that the
used to produce dibenzophosphole oxides 2. This intra-
molecular radical cyclization can be applied to double
cyclization, which affords ladder-type π-extended dibenzo-
phosphole oxides 4. The anti/syn conformations of the
products were determined by X-ray crystallographic analysis
of the corresponding dibenzophosphole sulfides 5. We also
investigated the photophysical properties of the isolated anti/
syn forms of dibenzophosphole oxides and sulfides and found
that the conformational difference between the diastereomers
affects the absorption and emission properties especially in the
solid state. This new synthetic approach involving intra-
molecular radical cyclization will facilitate the creation of
novel phosphorus-containing π-conjugated molecules and will
contribute to further developments in the field of materials
science.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedure, NMR spectra, CIF for anti-5
and syn-5. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: junji@icu.ac.jp.
*E-mail: kawashima.t@gunma-u.ac.jp.
Present Addresses
†Department of Material Science, College of Liberal Arts,
International Christian University, 3-10-2 Osawa, Mitaka,
Tokyo 181-8585, Japan.
Figure 2. Absorption (dashed lines) and emission (solid lines) spectra
of ladder-type phosphole sulfides anti-5 (light blue) and syn-5 (blue)
in CH2Cl2 (triangle) and in the solid state (circle).
‡Graduate School of Science and Technology, Gunma
University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan.
Notes
HOMO of the sulfide 5 is mainly localized on the lone pairs of
the exocyclic sulfur atoms, whereas the LUMO is composed of
the π*-orbital of the main π-skeleton (Figure S3). TD-DFT
calculations revealed that these absorption maxima mainly
correspond to HOMO−4 → LUMO with a relatively large
oscillator strength (f = 0.0994), because HOMO → LUMO
and other transitions involving the orbitals of the lone pairs
were calculated to be forbidden transitions (f < 0.00339)
(Table S1). These electronic properties lead to blue-shifted
absorption maxima of phosphole sulfides compared with those
of phosphole oxides. The fluorescence quantum efficiencies of
the sulfides 5 in the solution state were much lower (ΦF =
0.004 for both anti-5 and syn-5) than those of oxides 4. This
may result from nonradiative decay of the (n,π*) excited state
and an internal heavy atom effect of the sulfur atoms. On the
other hand, fluorescence quantum efficiencies of these sulfides
in the solid state increased to 0.12 and 0.04 for anti-5 and syn-
5, respectively. Crystal packing of these conformers indicated
that rotation of the phenyl rings on the phosphorus atoms is
restricted because of interactions between neighboring
molecules in the solid state (Figure S4), with the result that
thermal decay of the excited state would be partially
suppressed. Such a phenomenon is now commonly referred
to as aggregation-induced emission, and this interesting
property of phosphole derivatives is expected to have
applications in fluorescence sensors.1b,2b,14
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This paper is dedicated to Professor Renji Okazaki (professor
emeritus at The University of Tokyo) on the occasion of his
77th birthday. This work was supported by the Global COE
Program for Chemistry Innovation. Partial financial support
from MEXT (Kakenhi, 20685005, 21108507) is gratefully
acknowledged. We also thank Tosoh Finechem Co., Ltd. for
the generous gifts of alkyllithium reagents. We would also like
to thank Prof. Mao Minoura (Rikkyo University) for his
valuable advice in the crystallographic analysis.
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