Preparation and properties of conjugated polymers containing 1,2-diaryl-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutene units

Article abstract of DOI:10.1246/cl.2005.724

Conjugated polymers containing sterically protected 1,2-di(2-thienyl)-3,4- diphosphinidenecyclobutene units were prepared and their properties were investigated. Red shifts of the polymers were observed, compared to the corresponding monomeric species, wh

Full text of DOI:10.1246/cl.2005.724

724
Chemistry Letters Vol.34, No.5 (2005)
Preparation and Properties of Conjugated Polymers Containing
1,2-Diaryl-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutene Units
Subaru Kawasaki, Junichi Ujita, Kozo Toyota, and Masaaki YoshifujiÃ
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
(Received February 17, 2005; CL-050210)
Conjugated polymers containing sterically protected 1,2-
polymers by a similar method reported previously8 were studied.
Polymers 6a and 6b were successfully obtained from 5a and 5b
in 32 and 38% yield, respectively, after reprecipitation from
Et2O-MeOH. These polymers turned out to be stable in air.
The yields for 6a and 6b were better than that for alkylene-linked
DPCB polymer 8.8 On the other hand, attempted preparation of
polymers from 5c and 5d resulted in the formation of comlplex
mixtures of products including oligomeric products (Mn <
3000), probably because propagation was prevented by steric re-
pulsion between the DPCB units.
6a: Red solid; 31P NMR (162 MHz, CDCl3) ꢁ 198.6 [br,
(E,Z)-DPCB moiety], 177.1 [br, (E,Z)-DPCB moiety], 171.3
[br, (E,E)-DPCB moiety], and À71:8 (s, terminal moiety); IR
(KBr) 1591, 1460 (br), 1392, 1362, 1238, 1203 (br), 1120, and
833 cmÀ1; Mn 59600, Mw=Mn 72.4.16
6b: Red resin; 31P NMR (CDCl3) ꢁ 171.1 [br, (E,E)-DPCB
moiety] and À71:6 (s, terminal moiety); IR (KBr) 1593 (br),
1460 (br), and 800 cmÀ1; Mn 4500, Mw=Mn 4.4.
It should be mentioned that several problems for the alky-
lene-linked DPCB polymer 8 have been overcome in the case
of 6: (i) The solubility of 6 for common organic solvents was im-
proved; (ii) in the preparation of 6, intramolecular cyclization
di(2-thienyl)-3,4-diphosphinidenecyclobutene units were pre-
pared and their properties were investigated. Red shifts of the
polymers were observed, compared to the corresponding mono-
meric species, which indicates extension of the ꢀ system on
polymerization.
Heteroatom-containing conjugated polymers are of current
interest, because of their various functionalities.1 Conjugated
polymers containing ꢀ-bonded heavier main group elements
are also attractive. Some polymers containing multiply bonded
phosphorus atoms2 have recently been prepared and their prop-
erties were investigated: Gates3 and Protasiewicz4 prepared ꢀ-
conjugated polymers containing P=C bond in the main chain.
However the polymers are either sensitive to oxygen or mois-
ture,3 and the extension of the conjugation is supposed to be lim-
ited,4 because the P=C ꢀ-plane and the aromatic ꢀ-plane of the
sterically protecting group are nearly perpendicular in energeti-
cally favored conformation.5 Thus, properties of other types of
conjugated system containing multiply bonded phosphorus
atoms are of interest.
We have reported preparation of the first polymer containing
3,4-diphosphinidenecyclobutene (abbreviated to DPCB) moie-
ties and its polymer complex,6,7 as well as catalytic activity of
the related DPCB-polymer.8 However the DPCB units are elec-
tronically isolated each other by methylene chains. We now re-
port here preparations and properties of ꢀ-conjugated polymers
containing the DPCB units. In these polymers, the phosphorus-
carbon ꢀ-bonds are incorporated in the ꢀ-system of the polymer.
The synthetic route for the polymer is shown in Scheme 1.
The DPCB derivatives are normally prepared from ethynylphos-
phines via bis(phosphaallene) intermediates.8,9 Thus, we pre-
pared compounds 5a and 5b, as monomeric precursors. Com-
pound 1a,10 bearing an octyl group to increase polymer solubil-
ity, was prepared as follows. A Kumada coupling of 1,4-dibro-
mobenzene with 3-octyl-2-thienylmagnesium bromide in the
presence of Pd2(dba)3 and N-heterocyclic carbene ligand
[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene chloride]11
afforded 1a in 89% yield,12 while 1b13 was prepared by the
Suzuki coupling of 2,7-diiodo-9,9-dioctylfluorene and 2-thio-
pheneboronic acid in 91% yield. Reaction of 1a and 1b with
NIS, followed by the Sonogashira coupling and desilylation,
afforded 4a and 4b in 63 and 77% yield, respectively (in 3 steps).
Compounds 4a and 4b were then converted to 5a (87%) and 5b
(70%) by metallation of 4a and 4b with ethylmagnesium bro-
mide in THF followed by reaction with chloro(2,4,6-tri-t-butyl-
phenyl)phosphine.14 Compounds 5c and 5d (Chart 1) were also
prepared from 4c and 4d15 in a similar manner (5c: 40% yield;
5d: 48% yield).
a: R2 = C8H17,
R2 R2
Ar
–Ar– =
R1
R1
S
S
b: R2 = H,
1a,b: R1 = H
2a,b: R1 = I
3a,b: R1 = C≡CSiMe3
4a,b: R1 = C≡CH
5a,b: R1 = C≡CP(H)Mes*
C8H17 C8H17
a
b
c
–Ar– =
d
M
Mes* P
P Mes*
R2
R2
Ar
e
5a,b
S
S
n
f
6a,b: M = none
7a,b: M = PdCl2
Mes* = 2,4,6-t-Bu3C6H2
Scheme 1. (a) NIS, CHCl3, AcOH; (b) Me3SiCꢀCH, NEt3,
CuI, Pd(OAc)2, PPh3, DMF; (c) NaOH–MeOH, THF; (d) (i)
EtMgBr, THF; (ii) MesÃP(H)Cl; (e) (i) n-BuLi, DME; (ii)
BrCH2CH2Br, then reflux in toluene; (f) PdCl2(MeCN)2, THF.
As the precursors 5a–5d were obtained, preparation of the
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
725
gated polymer containing double bonded phosphorus. These
polymer metal complexes can be regarded as an example of ‘as-
sembled complexes,’ which are of current interest.19
This work was supported in part by the Grants-in-Aid for
Scientific Research (Nos. 13304049, 14044012, and 16033207)
from the Ministry of Education, Culture, Sports, Science and
Technology.
References and Notes
1
2
For recent reviews, see: A. Kraft, A. C. Grimsdale, and A. B.
Holmes, Angew. Chem., Int. Ed., 37, 402 (1998); S. Yamaguchi
and K. Tamao, Chem. Lett., 34, 2 (2005).
‘‘Multiple Bonds and Low Coordination in Phosphorus Chemistry,’’
ed. by M. Regitz and O. J. Scherer, Georg Thieme Verlag, Stuttgart
(1990); M. Yoshifuji, J. Organomet. Chem., 611, 210 (2000).
V. A. Wright and D. P. Gates, Angew. Chem., Int. Ed., 41, 2389
(2002).
3
4
R. C. Smith, X. Chen, and J. D. Protasiewicz, Inorg. Chem., 42, 5468
(2003); R. C. Smith and J. D. Protasiewicz, J. Am. Chem. Soc., 126,
2268 (2004).
Figure 1. UV–vis spectra of 6a, 6b and 9 in CH2Cl2. Molar ab-
sorption coefficients for 6a and 6b are calculated based on the
formal repeating unit.
5
S. Shah, T. Concolino, A. L. Rheingold, and J. D. Protasiewicz,
Inorg. Chem., 39, 3860 (2000); S. Shah, B. E. Eichler, R. C. Smith,
P. P. Power, and J. D. Protasiewicz, New J. Chem., 27, 442 (2003).
N. Yamada, K. Toyota, and M. Yoshifuji, Chem. Lett., 2001, 248.
For recent reviews on polymer complexes, see: ‘‘Metal Complexes
OC8H17
R
C8H17 C8H17
6
7
and Metals in Macromolecules,’’ ed. by D. Wohrle and A. D.
¨
Pomogailo, Wiley-VCH, Weinheim (2003).
R
R
R
8
9
a) K. Toyota, J. Ujita, S. Kawasaki, K. Abe, N. Yamada, and
M. Yoshifuji, Tetrahedron Lett., 45, 7609 (2004). For reviews of
monomeric DPCB-metal catalysts, see: b) M. Yoshifuji, J. Synth.
Org. Chem. Jpn., 61, 1116 (2003). c) F. Ozawa and M. Yoshifuji,
C. R. Chimie, 7, 747 (2004).
a) R. Appel, V. Winkhaus, and F. Knoch, Chem. Ber., 120, 243
(1987). b) M. Yoshifuji, K. Toyota, M. Murayama, H. Yoshimura,
A. Okamoto, K. Hirotsu, and S. Nagase, Chem. Lett., 1990, 2195.
c) K. Toyota, K. Abe, K. Horikawa, and M. Yoshifuji, Bull. Chem.
Soc. Jpn., 77, 1377 (2004).
C8H17O
4c: R = C≡CH
5c: R = C≡CP(H)Mes*
4d: R = C≡CH
5d: R = C≡CP(H)Mes*
Mes*
P
R1
R1
Mes* P
S
P Mes*
S
P
10 S. C. Ng, J. M. Xu, and H. S. O. Chan, Synth. Met., 92, 33 (1998).
11 J. Huang and S. P. Nolan, J. Am. Chem. Soc., 121, 9889 (1999).
12 The carbene ligand-Pd catalyst system improved the yield. See
Supporting Information.
13 M. Belletete, J.-F. Morin, S. Beaupre, M. Ranger, M. Leclerc, and
G. Durocher, Macromolecules, 34, 2288 (2001).
(CH2)8
Mes*
n
8
9
R1 = 2-thienyl
ˆ
´
Chart 1.
14 G. Markl and P. Kreitmeier, Angew. Chem., Int. Ed. Engl., 27, 1360
¨
was suppressed to lead to better yield due to the rigidity of link-
ers; (iii) polymer 6 is expected to show better electronic interac-
tion between the DPCB moieties, due to ꢀ-conjugation: Figure 1
shows UV–vis spectra of 6a and 6b and the related compound
9.9c Apparent red shifts were observed in the ꢀ–ꢀÃ absorption
bands of 6a and 6b, compared with that of 9.17 These facts indi-
cate relatively good ꢀ conjugation in 6a and 6b. The degree of
the red shift is 6a < 6b, probably because the octyl group on the
thienyl ring of 6a hinders coplanarity between the thienyl and
phenylene moieties.
(1988); A. H. Cowley, J. E. Kilduff, N. C. Norman, M. Pakulski,
J. L. Atwood, and W. E. Hunter, J. Am. Chem. Soc., 105, 4845
(1983).
15 C. Weder and M. S. Wrighton, Macromolecules, 29, 5157 (1996);
B. Liu, W.-L. Yu, J. Pei, S.-Y. Liu, Y.-H. Lai, and W. Huang,
Macromolecules, 34, 7932 (2001).
16 Molecular weights were determined by gel-permeation chromatog-
raphy using polystyrene as standard. For the reason for the large
Mw=Mn ratio of 6a, see Ref. 8a.
17 The ꢀÃ orbitals of DPCB correspond to two LUMOs. See, F. Ozawa,
S. Kawagishi, T. Ishiyama, and M. Yoshifuji, Organometallics, 23,
1325 (2004).
18 The 31P NMR data were obtained by monitoring the reaction mix-
tures, indicating complex formation. Signals due to the terminal
moieties were not detected because of low concentration due to poor
solubility of 6. After working up, the complexes were hardly soluble
in common organic solvents.
Next, transition metal complex formation of the polymers
was investigated. When 1 molar amount of PdCl2(MeCN)2
was allowed to react with the polymers in THF at room temper-
ature for 3 h, dark brown polymer complexes 7a and 7b were ob-
tained in ca. 65% and 70% yield, respectively. 7a: 31P NMR
(THF-C6D6) ꢁ 152 (br.); IR (KBr) 1718, 1591, 1462, 1437,
19 For example, T. Ito, T. Hamaguchi, H. Nagino, T. Yamaguchi, H.
Kido, I. S. Zavarine, T. Richmond, J. Washington, and C. P. Kubiak,
J. Am. Chem. Soc., 121, 4625 (1999); Y. M. A. Yamada, K. Takeda,
H. Takahashi, and S. Ikegami, J. Org. Chem., 68, 7733 (2003).
and 1122 cmÀ1; 7b: 31P NMR (THF-C6D6) ꢁ 150 (br.); IR
18
(KBr) 1724, 1595, 1460, 1265, 1122, 1072, and 802 cmÀ1
This is the first example of the complex formation of conju-
.
Published on the web (Advance View) April 16, 2005; DOI 10.1246/cl.2005.724