Synthesis of α,α′-linked oligophospholes and polyphospholes by using Pd-CuI-promoted stille-type coupling

Article abstract of DOI:10.1021/ol100926q

Palladium-copper-promoted Stille-type cross-coupling reactions between α-stannylphospholes and α-iodophospholes afforded α,α′-linked oligophospholes and polyphosphole efficiently. It has been revealed that the polyphosphole P-oxide possesses an extremely narrow band gap and a high electron-accepting ability as compared to polythiophene.

Full text of DOI:10.1021/ol100926q

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2675-2677
Synthesis of r,r′-Linked Oligophospholes
and Polyphospholes by Using
Pd-CuI-Promoted Stille-Type Coupling
Arihiro Saito,† Yoshihiro Matano,*,† and Hiroshi Imahori†,‡,§
Department of Molecular Engineering, Graduate School of Engineering, Kyoto
UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Institute for Integrated Cell-Material
Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Fukui
Institute for Fundamental Chemistry, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8103, Japan
matano@scl.kyoto-u.ac.jp
Received April 22, 2010
ABSTRACT
Palladium-copper-promoted Stille-type cross-coupling reactions between r-stannylphospholes and r-iodophospholes afforded r,r′-linked
oligophospholes and polyphosphole efficiently. It has been revealed that the polyphosphole P-oxide possesses an extremely narrow band gap
and a high electron-accepting ability as compared to polythiophene.
Polyheteroles (R,R′-linked polymers of heterocyclopentadi-
ene) constitute an important group of conjugated polymers
with applications in organic devices such as organic photo-
voltaic cells, organic field-effect transistors, and organic light-
emitting diodes.1 It is well-known that the inherent properties
of polyheteroles depend heavily on the nature of each
heterole monomer. Phosphole possesses a low-lying LUMO
owing to the effective σ*-π* orbital interaction and an active
lone electron pair at phosphorus.2 In this regard, polyphos-
pholes are expected to show distinguished optical and
electrochemical properties that cannot be attained by other
classes of polyheteroles. Indeed, some theoretical studies
predicted, for example, a considerably narrow band gap for
polyphosphole compared with polypyrrole and poly-
thiophene.3 Experimentally, however, the chemistry of
polyphospholes has been untouched due to the lack of
promising synthetic methods as well as appropriate monomer
precursors.4 Although oligophospholes up to tetramer were
synthesized by using oxidative coupling of R-lithiophosp-
(2) For recent reviews, see: (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) Quin, L. D. Curr. Org. Chem. 2006, 10, 43. (d) Baumgartner,
T.; Re´au, R. Chem. ReV. 2006, 106, 4681; Correction: 2007, 107, 303. (e)
Re´au, R.; Dyer, P. W. In ComprehensiVe Heterocyclic Chemistry III;
Ramsden, C. A., Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsvier: Oxford,
2008; Chapter 3.15, pp 1029-1048. (f) Matano, Y.; Imahori, H. Org.
Biomol. Chem. 2009, 7, 1258. (g) Fukazawa, A.; Yamaguchi, S.
Chem.sAsian J. 2009, 4, 1386.
† Graduate School of Engineering.
‡ Institute for Integrated Cell-Material Sciences.
§ Fukui Institute for Fundamental Chemistry.
(1) For example, see: (a) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S.
Chem. ReV. 2007, 107, 1324. (b) Thompson, B. C.; Fre´chet, J. M. J. Angew.
Chem., Int. Ed. 2008, 47, 58. (c) Allard, S.; Forster, M.; Souharce, B.; Thiem,
H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (d) Grimsdale, A. C.;
Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. ReV. 2009,
109, 897. (e) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. ReV. 2009, 109,
5868, and references cited therein.
(3) (a) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth.
Met. 1998, 96, 177. (b) Ma, J.; Li, S.; Jiang, Y. Macromolecules 2002, 35,
1109. (c) Casanovas, J.; Alema´n, C. J. Phys. Chem. C 2007, 111, 4823. (d)
Zhang, G.; Ma, J.; Wen, J. J. Phys. Chem. B 2007, 111, 11670.
10.1021/ol100926q  2010 American Chemical Society
Published on Web 05/03/2010

holes,5 reductive dimerization of R-free phospholes,6 [1,5]
sigmatropic shift of 1,1′-biphosphole,7 or hydrophosphination
of 1,3-diyne8 as key steps, these methods appeared to have
difficulty in polymer synthesis in terms of availability of the
precursors. Therefore, we decided to develop a new strategy
that would allow a short-step synthesis of polyphospholes
and focused on a cross-coupling methodology to link
phosphole rings at their R and R′ positions.9 Herein, we
report the first synthesis of R,R′-linked oligophospholes and
polyphosphole by using Pd-CuI-promoted Stille-type cou-
pling and their optical/electrochemical properties.
enabled us to introduce tributylstannyl and iodo groups to
R,R′ positions of the phosphole ring in three to four steps
from commercially available reagents. With these precursors
in hand, we first evaluated the efficiency of a Stille coupling
method for the synthesis of R,R′-biphosphole. In the presence
of Pd2(dba)3, tris(2-furyl)phosphine, and CuI,12 Stille-type
cross coupling between 3 and 5 proceeded smoothly at room
temperature to give R,R′-biphosphole 7 in 85% yield
(Scheme 2). It should be noted that both the Pd catalyst and
R-Stannylphosphole 310 and R,R′-distannylphosphole 4a,
which are prerequisite constituents for Stille coupling, were
prepared from the corresponding diynes 1 and 2 by utilizing
a Ti(II)-mediated cyclization method as a key step (Scheme
1).4f,8,11 To prevent undesirable side reactions in the fol-
Scheme 2. Synthesis of Diphosphole 7 and Terphosphole 8
Scheme 1. Synthesis of R-Stannyl- and R-Iodophospholes
CuI are indispensable for producing 7 with high efficiency.
Under the same reaction conditions, homocoupling of 3 also
proceeded, albeit more slowly than Stille-type coupling, to
give 7.13 When 4a was reacted with 5, terphosphole 8 was
obtained as the major product. The relatively low-yield
formation of 8 is attributable to a competitive homocoupling
of 4a, which led to small amounts of quaterphosphole (detected
by MS) and other uncharacterized oligophospholes. Compounds
7 and 8 were characterized by standard spectroscopic techniques.
The 31P NMR spectra indicated the presence of two diastere-
omers for 7 and three diastereomers for 8.14
lowing steps, the phosphorus center was oxygenated at this
stage. The 31P peaks of 3 and 4a appeared at δP 64.9 and
78.2, respectively, with characteristic coupling patterns
arising from R-tin atoms (2JP-Sn ) 116-122 Hz). Treatment
of 3 and 4a with stoichiometric amounts of N-iodosuccin-
imide (NIS) afforded R-iodophosphole 5 (δP 51.5) and R,R′-
diiodophosphole 6a (δP 48.4), respectively. This protocol
(4) Conjugated copolymers containing phosphole units have been
reported by several groups. For example, see: (a) Mao, S. S. H.; Tilley,
T. D. Macromolecules 1997, 30, 5566. (b) Lucht, B. L.; Mao, S. S. H.;
Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 4354. (c) Morisaki, Y.; Aiki,
Y.; Chujo, Y. Macromolecules 2003, 36, 2594. (d) Tomita, I.; Ueda, M.
Macromol. Symp. 2004, 209, 217. (e) Sebastian, M.; Hissler, M.; Fave, C.;
Rault-Berthelot, J.; Odin, C.; Re´au, R. Angew. Chem., Int. Ed. 2006, 45,
6152. (f) de Talance´, V. L.; Hissler, M.; Zhang, L.-Z.; Ka´rpa´ti, T.; Nyula´szi,
L.; Caras-Quintero, D.; Ba¨uerle, P.; Re´au, R. Chem. Commun. 2008, 2200.
(5) (a) Deschamps, E.; Mathey, F. Bull. Soc. Chim. Fr. 1992, 129, 486.
(b) Deschamps, E.; Ricard, L.; Mathey, F. Angew. Chem., Int. Ed. Engl.
1994, 33, 1158.
Next, we attempted to synthesize polyphosphole from 4a
and 6a by using the Pd-CuI-promoted Stille-type coupling.
Judging from gel permeation chromatography (GPC) analy-
sis, however, only low-molecular-weight polymer was
obtained probably due to its low solubility. Hence, phosphole
2
monomers 4b (δP 78.7, JP-Sn ) 116 Hz) and 6b (δP 49.8)
bearing a p-dodecyloxyphenyl group as a solubilizing func-
tion were newly synthesized according to the titanacycle
protocol (Scheme S1, Supporting Information) and applied
to the Stille-type coupling (Scheme 3).15 As expected, the
(6) (a) Mercier, F.; Mathey, F.; Fischer, J.; Nelson, J. H. J. Am. Chem.
Soc. 1984, 106, 425. (b) Mercier, F.; Mathey, F.; Fischer, J.; Nelson, J. H.
Inorg. Chem. 1985, 24, 4141. (c) Mercier, F.; Holand, S.; Mathey, F. J.
Organomet. Chem. 1986, 316, 271.
(7) (a) Mathey, F.; Mercier, F.; Nief, F.; Fischer, J.; Mitschler, A. J. Am.
Chem. Soc. 1982, 104, 2077. (b) Bevierre, M.-O.; Mercier, F.; Ricard, L.;
Mathey, F. Angew. Chem., Int. Ed. 1990, 29, 655. (c) Laporte, F.; Mercier,
F.; Ricard, L.; Mathey, F. J. Am. Chem. Soc. 1994, 116, 3306.
(8) Matano, Y.; Nakashima, M.; Imahori, H. Angew. Chem., Int. Ed.
2009, 48, 4002.
(12) Devreux, V.; Wiesner, J.; Jomaa, H.; Rozenski, J.; der Eycken,
J. V.; Calenbergh, S. V. J. Org. Chem. 2007, 72, 3783.
(13) Pd-catalyzed homocoupling of stannylarenes was reported previ-
ously. For example, see: (a) Liebeskind, L. S.; Riesinger, S. W. J. Org.
Chem. 1993, 58, 408. (b) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth,
G. P. J. Org. Chem. 1993, 58, 5434. (c) Liebeskind, L. S.; Yu, M. S.; Wang,
J.; Hagen, K. S. J. Am. Chem. Soc. 1993, 115, 9048.
(9) Very few attempts were reported for cross coupling of R-halophos-
pholes. Holand, S.; Gandolfo, F.; Ricard, L.; Mathey, F. Bull. Soc. Chim.
Fr. 1996, 133, 33.
(10) 2-Stannylphosphole was previously reported. Deschamps, B.;
Mathey, F. Bull. Soc. Chim. Fr. 1996, 133, 541.
(14) Density functional theory (DFT) calculations (B3LYP/6-31G*) on
7 showed that there was little difference in free energies between the
respective diastereomers. For details, see the Supporting Information.
(15) In this polymerization, a small amount of 3 (5 mol % per 4b)
was added as an end-cap unit for the purpose of controlling the molecular
weight.
(11) (a) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Imahori, H. J. Org.
Chem. 2006, 71, 5792. (b) Matano, Y.; Miyajima, T.; Imahori, H.; Kimura,
Y. J. Org. Chem. 2007, 72, 6200. (c) Sanji, T.; Shiraishi, K.; Tanaka, M.
Org. Lett. 2007, 9, 3611
.
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Org. Lett., Vol. 12, No. 11, 2010

Scheme 3
.
Synthesis of R,R′-Polyphosphole 9
Table 1. Optical Data and Redox Potentials of 7-10 in CH2Cl2
compd
λab/nm (log ε)
λema/nm (ΦF)b
Eoxc/V
Eredc/V
7
8
9
10
463 (4.35)
511 (4.40)
655 (3.80)d
386 (4.15)
583 (0.02)
605 (<0.01)
e
+0.76
+0.64
+0.30f,g
+1.19
-1.75
-1.62
-1.20f
-2.02
491 (0.19)
a λex ) 440 nm for 7 and 10, 510 nm for 8. b Relative fluorescence
quantum yields. c Determined by DPV (0.1 M nBu4N+PF6-; Ag/Ag+). First
oxidation (Eox) and reduction (Ered) potentials vs Fc/Fc+. d Calculated per
f
repeating unit. e Nonfluorescent. Eonset
.
g A broad and weak peak was
observed for the first oxidation process.
polymerization occurred efficiently at room temperature to
give R,R′-linked polyphosphole 9 as a dark blue powder in
51% yield after filtration through a 0.45-µm membrane filter
and repeated precipitations from CH2Cl2-hexane and
CH2Cl2-MeOH. The number average molecular weight (Mn)
and polydispersity index (PDI) of 9 were determined by GPC
(relative to polystyrene standard) as 13000 and 2.3, respec-
2). Notably, the onset of 9 reaches around 850 nm (Eg )
1.46 eV), indicating that the polymerization at the R,R′
tively.16 Polyphosphole 9 showed a group of rather broad 31
P
peaks at δP 51.6-56.4, which covers the region observed for 7
(δP 54.0-55.1) and 8 (δP 51.9-55.5). Unfortunately, treatment
of 9 with HSiCl3 did not proceed cleanly, and we have not
succeeded in obtaining polyphosphole of the σ3-P type.
The average polymerization degree of 9 (n ) 32) is
sufficiently large for discussing the optical and electrochemi-
cal properties of R,R′-linked polyphosphole by comparison
with those of polythiophenes. The observed data for 7-9
and monomer 1017 are summarized in Figure 1 and Table 1.
Figure 2. Monomer reference 10, oligothioiphenes 11a,b, and
polythiophene 12.
positions expands conjugation of the phosphole π-system
quite effectively. While 7 and 8 are weakly fluorescent, 9 is
nonfluorescent in solution.
Redox potentials of 7-9 were determined by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV).
The polymer 9 showed a broad first reduction process at Eonset
) -1.20 V (vs Fc/Fc+), which is less negative than Ered of
monomer 10 (-2.02 V), dimer 7 (-1.75 V), and trimer 8
(-1.62 V). It is evident that the electron-accepting ability
of the phosphole π-system is largely enhanced by polym-
erization. The redox behavior of 9 is in marked contrast to
that of polythiophene and polypyrrole.
In summary, we have established convenient methods for
the synthesis of R-stannyl- and R-iodophospholes. With these
precursors available, the first example of R,R′-linked polyphos-
phole as well as related oligophospholes was successfully
prepared by using the Stille-type coupling. Notably, polyphos-
phole possesses an extremely narrow band gap and a high
electron-accepting ability. The present monomers are applicable
to the synthesis of phosphole-containing conjugated copolymers,
and further studies on this project are now in progress.
Figure 1. UV/vis absorption spectra of 7-10 in CH2Cl2.
The absorption maxima (λab) shift to longer wavelength with
increasing the number of phosphole units, which is com-
monly seen for R,R′-linked heteroles. The λab values of 7,
8, and 9 are significantly red-shifted (∆λab ) 90-110 nm)
relative to the respective values reported for bithiophene
11a18 (λab ) 376 nm), terthiophene 11b18 (λab ) 404 nm),
and polythiophene (36-mer) 1219 (λab ) 546.3 nm) (Figure
Acknowledgment. This work was partially supported by
Grants-in-Aid (No. 21108511 “π-Space” and No. 22350016)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
(16) Attempts to prepare 9 by the homocoupling of 4b resulted in
failure probably due to its inefficiency compared to the heterocoupling
of 4b/6b.
(17) Saito, A.; Miyajima, T.; Nakashima, M.; Fukushima, T.; Kaji, H.;
Matano, Y.; Imahori, H. Chem.sEur. J. 2009, 15, 10000.
(18) Lee, S. A.; Hotta, S.; Nakanishi, F. J. Phys. Chem. A 2000, 104,
1827.
Supporting Information Available: Experimental details
and DFT calculation results. This material is available free
of charge via the Internet at http://pubs.acs.org.
(19) Izumi, T.; Kobashi, S.; Takimiya, K.; Aso, Y.; Otsubo, T. J. Am.
Chem. Soc. 2003, 125, 5286.
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