is an interesting feature in addition to conformational
flexibility and the transannular π-π interaction. [3.3](2,6)-
Pyridinophane can act as a bidentate nitrogen ligand for
transition metals. Tsuge and co-workers reported the prepa-
ration of Pt(II) and Pd(II) complexes by the reaction of dithia-
[3.3](2,6)pyridinophane with PtCl2(PhCN)2 and PdCl2-
(PhCN)2, respectively.4 An X-ray crystallographic investigation
of these complexes revealed that two pyridine rings were
fixed into the syn geometry with a dihedral angle of about
90°.
Scheme 1 a
a Reagents and conditions: (i) (1) PBr3, Br2, rt, (2) EtOH, 0 °C,
87%; (ii) NaBH4, EtOH, rt to reflux, 70%; (iii) HBr, AcOH, reflux,
87%; (iv) thiourea, EtOH, reflux, 91%; (v) NaBH4, MeOH, rt; (vi)
4, NaOH, EtOH-benzene, reflux, 34%.
On the other hand, the catalytic activity of palladium
complexes with regard to the phosphine-free Heck reaction
in air was affected by the bidentate nitrogen ligands such as
1,2-bis(2-pyridylethynyl)benzene5a and bis-N-methylimida-
zole.5b Therefore, the polymers bearing a pyridinophane-
palladium complex are potentially applicable in the polymer-
supported catalysts in air, which are used for palladium-
catalyzed coupling reactions, as well as recyclable polymer
catalysts. In the present study, we synthesized a novel poly-
(p-phenylene-ethynylene)-type polymer6 with dithia[3.3]-
(2,6)pyridinophane in the main chain as a repeating unit and
investigated the stereochemistry of the resulting polymer.
Moreover, we studied the complexation of this polymer with
the palladium complex and its catalytic performance and the
recycling aspect for the Heck coupling reaction.
The polymerizations of monomer 7 with 8a-c were
carried out in toluene-diisopropylamine solution in the
presence of a catalytic amount of PdCl2(PPh3)2/PCy3/CuI
(Cy ) cyclohexyl) at 80 °C for 48 h (Scheme 2). After the
Scheme 2
The key monomer 6,15-dibromo-2,11-dithia[3.3](2,6)-
pyridinophane 7 was synthesized according to Scheme 1.3c,7
Commercially available chelidamic acid monohydrate 1 was
converted into 2,5-bisbromomethyl-3-bromopyridine 4 with
a good yield.8 Both the bromomethyl groups of 4 were
reacted with thiourea in EtOH to give compound 5 with a
yield of 91%. Finally, the target monomer 7 was synthesized
with a yield of 34% by the reduction of 5 via intermediate
dithiol followed by a treatment with 4.
Comonomers 2,5-dialkoxy-1,4-diethynylbenzenes 8a-c
were prepared according to the established procedures.9
Flexible alkoxy side chains were attached to momomers
8a-c to increase the solubility of the resulting conjugated
polymers.10
reaction was completed, inorganic byproducts were removed
by washing with aqueous NH3, and the filtrate was dried in
vacuo. The THF-soluble portion of the crude polymer was
purified by reprecipitation from a large amount of MeOH
to yield target polymers 9a-c in the form of a yellowish
brown powder. Molecular weight measurements were per-
formed by gel permeation chromatography (GPC) in eluent
CHCl3 using a calibration curve of polystyrene standards.
The structures of monomer 7 and polymers 9a-c were
(4) Moriguchi, T.; Kitamura, S.; Sakata, K.; Tsuge, A. Polyhedron 2001,
20, 2315.
(5) (a) Kawano, T.; Shinomaru, T.; Ueda, I. Org. Lett. 2002, 4, 2545.
(b) Park, S. B.; Alper, H. Org. Lett. 2003, 5, 3209.
(6) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605.
(7) Kawashima, T.; Kurioka, S.; Tohda, Y.; Ariga, M.; Mori, Y.; Misumi,
S. Chem. Lett. 1985, 1289.
1
1
confirmed by the H and 13C NMR spectra. The H NMR
spectra of [3.3]metacyclophane and [3.3]metapyridinophane
(8) (a) Takalo, H.; Kankare, J. Acta Chem. Scand. 1987, B41, 219. (b)
Takalo, H.; Kankare, J. Acta Chem. Scand. 1988, B42, 373.
(9) (a) Moroni, M.; Moigne, J. L. Macromolecules 1994, 27, 562. (b)
Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31,
52.
(10) (a) Le Moignem, J.; Moroni, M.; Coles, H.; Thierry, A.; Kajzar, F.
Mater. Res. Soc. Symp. Proc. 1992, 247, 65. (b) Rutherford, D.; Stille, J.
K.; Elliott, C. K.; Reichert, V. R. Macromolecules 1992, 25, 2294. (c)
Sasabe, H.; Wada, T.; Hosoda, H.; Ohkawa, H.; Yamada, A.; Garito, A. F.
Mol. Cryst. Liq. Cryst. 1990, 189, 155.
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