Base-mediated cyclization reaction of 2-alkynylphenylphosphine oxides: Synthesis and photophysical properties of benzo[b]phosphole oxides

Article abstract of DOI:10.1021/ol800841v

(Chemical Equation Presented) The base-mediated intramolecular cyclization reaction of 2-alkynylphenylphosphine oxides affords benzo[b]phosphole oxides, which show intense blue-green fluorescence. Benzo[b]phospholes are also prepared by the reduction of b

Full text of DOI:10.1021/ol800841v

ORGANIC
LETTERS
2008
Vol. 10, No. 13
2689-2692
Base-Mediated Cyclization Reaction of
2-Alkynylphenylphosphine Oxides:
Synthesis and Photophysical Properties
of Benzo[b]phosphole Oxides
Takanobu Sanji,* Kentaro Shiraishi, Taigo Kashiwabara, and Masato Tanaka*
Chemical Resources Laboratory, Tokyo Institute of Technology,
4259-R1-13, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
sanji@res.titech.ac.jp; m.tanaka@res.titech.ac.jp
Received April 12, 2008
ABSTRACT
The base-mediated intramolecular cyclization reaction of 2-alkynylphenylphosphine oxides affords benzo[b]phosphole oxides, which show
intense blue-green fluorescence. Benzo[b]phospholes are also prepared by the reduction of benzo[b]phosphole oxides.
Among the variety of π-conjugated systems with potential
applications as electronic materials, such as organic thin film
transistors and organic electroluminescent (EL) devices, main
group element-containing π-conjugated molecules have at-
tracted considerable interest recently.1 This is because the
orbital interaction of the main group element moiety plays
an intriguing role in the π-conjugated framework, and the
resultant π-systems reduce the HOMO-LUMO gaps com-
pared with the parent π-system. In this context, phosphacy-
clopentadienes (phospholes) form interesting building blocks
for the construction of conjugated systems, where an
interaction between the butadiene π* orbital and the low-
lying σ*(P-R) orbital takes place in the ring.2 In addition,
phosphorus-containing π-systems retain a versatile reactivity
of a central phosphorus atom, and this makes it possible to
tune the electronic and optical properties of the materials
using simple chemical modifications, such as oxidation.
Several reports have addressed the synthesis of phospholes,3,4
dibenzophospholes,5,6dithienophospholes,7andtheirderivatives.8
Indoles are basic skeletons in organic chemistry and are
found in natural products and biologically active sub-
stances.9,10 On the other hand, synthesis and properties of
benzo[b]phosphole, phosphorus analogues of indoles, have
not been explored to date.11,12 Here, we report the synthesis
and photophysical properties of benzo[b]phosphole oxides
and benzo[b]phospholes.13–15
Because the metal- and base-mediated cyclization reaction
of alkynylamines, which proceeds through intramolecular
addition of the N-H bond to the triple bond, is one of the
(1) (a) Mathey, F. Angew. Chem., Int. Ed. 2003, 42, 1578. (b) Hissler,
M.; Dyer, P. W.; Re´au, R. Coord. Chem. ReV. 2003, 244, 1. (c) Mathey, F.
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L.; Re´au, R. Angew. Chem., Int. Ed. 2000, 39, 1812. (c) Hay, C.; Hissler,
M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet, L.; Nyula´szi, L.; Re´au,
R. Chem. Eur. J. 2001, 7, 4222. (d) Hay, C.; Fave, C.; Hissler, M.; Rault-
Berthelot, J.; Re´au, R. Org. Lett. 2003, 5, 3467. (e) Fave, C.; Cho, T.-Y.;
Hissler, M.; Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Re´au, R. J. Am. Chem.
Soc. 2003, 125, 9254. (f) Fave, C.; Hissler, M.; Ka´rpa´ti, T.; Rault-Berthelot,
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10.1021/ol800841v CCC: $40.75
Published on Web 05/29/2008
2008 American Chemical Society

most powerful tools for synthesizing indoles,16 our synthetic
strategy for the benzophosphole oxides was the base-
mediated intramolecular cyclization reaction of 2-alky-
nylphenylphosphine oxides 1.17,18 Scheme 1 shows the
reaction of 2a-dwith trichlorosilane in toluene at 110 °C
afforded benzo[b]phospholes 3a-d in 56-90% yields. The
structures of 2a-d and 3a-d were fully characterized using
1H, 13C, and 31P NMR, MS, and elemental analysis.
The base-mediated intramolecular addition reaction of
H-P(O) compounds to alkynes was employed for π-extend-
ed starting materials with two alkyne parts, as shown in
Scheme 2. For example, the reaction of 4, which has two
H-P(O) groups at the central benzene ring, gave 5 as a
mixture of cis and trans isomers with respect to the directions
of the PdO bonds.21 The isomers were easily separated from
Scheme 1
(5) (a) Aitken, R. A. In Fused FiVe-Membered Hetarenes with One
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synthesis of benzobphosphole oxides 2a-d. For example, the
reaction of 1a (R ) Ph) in the presence of potassium
t-butoxide (20 mol %) as a base in DMSO at 70 °C for 24 h
led to a phosphole oxide 2a in 79% yield.19,20 The reaction
of 1b (R ) 4-MeOC6H4) and 1d (R ) 2-thienyl) gave 2b
and 2d, respectively, in good yields, but the reaction of 1c
(R ) 4-CF3C6H4) gave 2c only in 15% yield. Subsequently,
reduction of the benzophosphole oxides was examined. The
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Scheme 2
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(12) The synthesis of benzo[c]phosphole has also been reported: (a)
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2690
Org. Lett., Vol. 10, No. 13, 2008

Scheme 3
Figure 1. ORTEP drawing of 5-cis (major isomer).
each other using silica gel chromatography, the isomers of
5 being obtained in 32% and 8% yields. The structure of
the major isomer was identified to be a cis structure from
single-crystal X-ray analysis (Figure 1). 5-cis showed a nearly
coplanar structure with the dihedral angles between the
benzene ring and the two phosphole oxide rings of 7.2-
7.2 ° along with moderate C-C/CdC bond length alterna-
tions (∆d ) 0.04-0.13 Å). The reduction of 5 with
trichlorosilane gave a diphospholenobenzene-type compound
6 in good yield as a mixture of cis and trans isomers. The
ratio of the two isomers was found to be 1/0.9, estimated
Table 1. Photophysical Properties of Benzo[b]phosphole Oxide
Derivatives and Related Compoundsa
absorption
fluorescence
λFLmax/nmc
ΦFL
d,e
compd
λmaxb/nm
ε /cm-1 M-1
2a
2b
2c
2d
3a
3b
3c
3d
5g
347
362
342
368
321
340
329
350
433
396
387
8800
12600
8400
417
443
414
0.30
0.41
0.65
0.23
-
0.09
-
0.07
0.44
0.61
0.29
12000
13800
16600
13600
15600
23800
28500
32700
444
n.d.f
412
2007, 72, 5119. (j) Matsuda, T.; Kadowaki, S.; Goya, T.; Murakami, M.
Org. Lett. 2007, 9, 133. (k) Miguel, L. S.; Porter III, W. W.; Matzger, A. J.
Org. Lett. 2007, 9, 1005. (l) Agou, T.; Kobayashi, J.; Kawashima, T.
Chem.-Eur. J. 2007, 13, 8051.
n.d.f
417
(14) Very recently bisphosphoryl-bridged stilbenes have been reported:
(a) Fukazawa, A. ; Hara, M.; Okamoto, T.; Son, E.-C.; Xu, C.; Tamao, K.;
Yamaguchi, S. Org. Lett. 2008, 10, 913.
495
438, 463
431, 456
6h
8h
(15) After the submission of this manuscript we have known that
Nakamura et al. have reported the synthesis of benzo[b]phospholes by BuLi-
mediated cyclization of (2-alkynylphenyl)phosphine: (a) Tsuji, H.; Sato,
K.; Ilies, L.; Itoh, Y.; Sato, Y.; Nakamura, E. Org. Lett. 2008, 10, 2263–
2265.
a In THF. b Longest wavelength absorption maximum is shown. c Excited
at the wavelength of the absorption maximum. d Fluorescence quantum yield.
e Determined with quinine sulfate as a standard. f Not detectable. g Major
isomer (cis). h Mixture of trans and cis isomers.
(16) For example, see: (a) Arcadi, A.; Cacchi, S.; Marinelli, F.
Tetrahedron Lett. 1989, 30, 2581. (b) Rudisillt, D. E.; Stille, J. K. J. Org.
Chem. 1989, 54, 5856. (c) Cacchi, S.; Carnicelli, V.; Marinelli, F. J.
Organomet. Chem. 1994, 475, 289. (d) Ezquerra, J.; Pedregal, C.; Lamas,
C.; Barluenga, J.; Pe´rez, M.; Garc´ıa-Mart´ın, M. A.; Gonza´lez, J. M. J. Org.
Chem. 1996, 61, 5804. (e) Hiroya, K.; Itoh, S.; Sakamoto, T. J. Org. Chem.
2004, 69, 1126. (f) Sakai, N.; Annaka, K.; Konakahara, T. Org. Lett. 2004,
6, 1527. (g) Trost, B. M.; McClory, A. Angew. Chem.Int. Ed 2007, 46,
2074. (h) Arcadi, A.; Bianchi, G.; Marinelli, F. Synthesis 2004, 4, 610. (i)
Yasuhara, A.; Kanamori, Y.; Kaneko, M.; Numata, A.; Kondo, Y.;
Sakamoto, T. J. Chem. Soc., Perkin Trans 1 1999, 529. (j) Rodriguez, A. L.;
Koradin, C.; Dohle, W.; Knochel, P. Angew. Chem., Int. Ed. 2000, 39, 2488.
(k) Barluenga, J.; Trincado, M.; Rubio, E.; Gonza´lez, J. M. Angew. Chem.,
Int. Ed 2003, 42, 2406.
(17) (a) Clark, P. W.; Mulraney, B. J. J. Organomet. Chem. 1981, 217,
51. (b) Chatt, J.; Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett, C. J.; Rankin,
D. A. J. Chem. Soc., Dalton Trans. 1985, 1131. (c) Casey, C. P.; Paulsen,
E. L.; Beuttenmueller, E. W.; Proft, B. R.; Petrovich, L. M.; Matter, B. A.;
Powell, D. R. J. Am. Chem. Soc. 1997, 119, 11817. (d) Bunlaksananusorn,
T.; Knochel, P. J. Org. Chem. 2004, 69, 4595.
from the integration ratio of the NMR spectra.22 However,
the separation of these isomers from each other using
chromatography failed at this stage. The reaction of 7, having
two H-P(O) groups at the terminal phenyl rings, also led
to 8 in 42% yield (Scheme 3).23
The photophysical properties of benzophosphole oxides
2a-d, benzophospholes 3a-c, and the related compounds are
of interest. The photophysical data in solution are sum-
marized in Table 1 and the fluorescence spectra of 2a-d, 5,
and 8 are shown in Figure 2. In the absorption spectra, the
benzophosphole oxides 2a-d display an absorption band at
350-370 nm, where the position of the absorption depends
on the substituent on the benzophosphole ring. When
substituted with a 4-methoxyphenyl (2b) or 2-thienyl group
(2d) on the phosphole oxide ring, the absorption maximum
is red-shifted relative to that of 2a (R ) Ph). Introduction
of a 4-trifluoromethylphenyl group causes a blue shift of the
(18) The metal-catalyzed intermolecular cyclization of P-OH to alkynes
has been reported: (a) Peng, A.-Y.; Ding, Y.-X. J. Am. Chem. Soc. 2003,
125, 15006.
(19) We also examined the base-mediated cyclization reaction of phenyl-
(2-phenylethynylphenyl)phosphine. The reaction gave the corresponding
benzo[b]phosphole in 54% isolated yield together with its oxide in 12%
yield.
(20) The reaction conditions were not optimized completely, and based
on the intermolecular addition of H-P(O). to alkenes.16d We found the
reaction time depends on the substrate. For example, the reaction of 2d
proceeded with ca. 50% conversion for 7 h, but the reaction of 4 completes
in 2 h.
(22) The ratio of cis/trans isomers was not changed ranging in
temperature from 0 to 50 °C by measurement with the 31P NMR. See
Supporting Information.
(23) The isomer ratio could not be estimated because of the overlap of
the NMR chemical shifts. An attempt to separate these isomers from each
other using chromatography also failed.
(21) After the reaction, the isomer ratio was 1/0.32, estimated from the
integration ratio of the 31P NMR spectra.
Org. Lett., Vol. 10, No. 13, 2008
2691

emission maxima (431 and 456 nm) are slightly red-shifted
compared with those of the parent compound 2a. The
quantum yields of 5 and 8 are relatively high. The benzo-
phosphole derivative 6 exhibits an absorption maximum at
396 nm and emission maximum at around 450 nm, which
are blue-shifted relative to those of the benzophosphole
oxides 5. These examples demonstrate that the P ) O
moieties in 5 and 8 cause the significant red-shift of 20-30
nm for the absorption and emission maxima as compared
with the silicon analogues,13e due to the low-lying LUMO
of the phosphole oxide ring.
In summary, we have demonstrated the synthesis of
benzo[b]phosphole oxides by means of the base-mediated
reaction of 2-alkynylphenylphosphine oxides. These benzo-
phosphole oxides display blue-green emission with good
quantum yield. In addition, benzo[b]phospholes are easily
synthesized by reduction of the phosphorus atom of the
phosphole ring. The base-mediated cyclization reaction of
2-alkynylphenylphosphine oxides offers a simple procedure
for obtaining phosphorus-containing extended π-systems.
Benzo[b]phosphole oxides should be more seriously scru-
tinized as new candidates for π-conjugated systems with
potential applications as optoelectronic materials. Further
study is in progress.
Figure 2. Fluorescence spectra of 2a-d, 5-cis, and 8 in THF.
absorption maximum. In the fluorescence spectra, 2a-d
display an emission at 420-440 nm with a quantum yield
of 23-65%. The emission maxima are also shifted to longer
wavelengths with increasing electron-donating ability of the
group on the ring. On the other hand, the benzophospholes
3a-c exhibit absorption maxima at 320-350 nm, which are
blue-shifted relative to those of the benzophosphole oxides.
In the fluorescence spectra, 3b and 3d exhibit only weak
emissions at around 415 nm with quantum yields of 9 and
7%, respectively, and 3a and 3c show practically no
emission.
Acknowledgment. This work was supported by Special
Education and Research Expenses (Post-Silicon Materials
and Devices Research Alliance) and a Grant-in-Aid for
Scientific Research on Priority Areas (No. 18065008) from
the Ministry of Education, Culture, Sports, Science, and
Technology of Japan.
Compound 5 with two benzophosphole oxides has a red-
shifted absorption maximum at 433 nm and also a red-shifted
emission maximum at 495 nm, where the photophysical
properties of the cis/trans isomers are essentially identical
to each other. This indicates that the π-conjugated system is
successfully extended. For 8, the absorption (387 nm) and
Supporting Information Available: Experimental pro-
cedures, spectral data of all new compounds and details of
the X-ray analysis of 5-cis. This material is available free
of charge via the Internet at http://pubs.acs.org.
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