Highly regioselective oxidative polymerization of 4-phenoxyphenol to poly(1,4-phenylene oxide) catalyzed by tyrosinase model complexes [7]

Full text of DOI:10.1021/ja981283m

J. Am. Chem. Soc. 1998, 120, 8529-8530
8529
Scheme 1
Highly Regioselective Oxidative Polymerization of
4-Phenoxyphenol to Poly(1,4-phenylene oxide)
Catalyzed by Tyrosinase Model Complexes
Hideyuki Higashimura,*,†,‡ Kiyoshi Fujisawa,§
Yoshihiko Moro-oka,*,§ Masaaki Kubota,†,‡ Akinobu Shiga,†,‡
Atsushi Terahara,‡ Hiroshi Uyama,| and Shiro Kobayashi*,|,
Joint Research Center for Precision Polymerization
Japan Chemical InnoVation Institute, NIMC
1-1 Higashi, Tsukuba 305-8565, Japan
Tsukuba Laboratory, Sumitomo Chemical Co. Ltd.
6 Kitahara, Tsukuba 300-3294, Japan
Scheme 2
Research Laboratory of Resources Utilization
Tokyo Institute of Technology, 4259 Nagatsuta
Midori-ku, Yokohama 226-8503, Japan
Department of Materials Chemistry
Graduate School of Engineering, Kyoto UniVersity
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
National Institute of Materials and Chemical Research
1-1 Higashi, Tsukuba 305-8565, Japan
ReceiVed April 16, 1998
Regioselective oxidative polymerization of phenols leading to
2,6-unsubstituted poly(1,4-phenylene oxide) (PPO) has long been
a target of research activities in the field of polymer synthesis.
Despite many attempts, no successful regioselective polymeri-
zation of 2,6-unsubstituted phenols has been reported so far,1
although that of 2,6-disubstituted phenols, where the o-positions
are protected, produces useful linear polymers.2 In this paper,
we report highly regioselective oxidative coupling of 4-phenoxy-
phenol (PPL) catalyzed by tyrosinase model complexes. This is
the first example to synthesize PPO showing crystallinity with a
melting point by oxidative polymerization (Scheme 1).
A CuCl/N,N,N′,N′-tetraethylethylenediamine (teed) catalyst
under dioxygen was the only example reported as catalyst for
oxidative coupling of PPL.3 However, this catalyst could not
control regioselectivity of the phenoxy radical coupling. The main
reason might be that the CuCl/teed catalyst generates “electro-
philic” bis(µ-oxo) dicopper(III) complex 4,4,5 which abstracts
hydrogen atoms from PPL to give “free” phenoxy radical 5
(Scheme 2). On the basis of this view, we have paid much
attention to “nucleophilic” µ-η2:η2-peroxo dicopper(II) complex
1,6 a copper-dioxygen model complex for tyrosinase, and reached
a working hypothesis as follows: complex 1 will abstract protons
(not hydrogen atoms) from PPL to give phenoxo-copper(II)
complex 2, equivalent to phenoxy radical-copper(I) complex 3.
Intermediates 2 and/or 3 are not “free” radicals but “controlled”
radicals, and hence, regioselectivity of the subsequent coupling
may be regulated.7 In this study, two copper(II) chloride
complexes, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate)copper(II)
chloride (Cu(tpzb)Cl)8a,c and (1,4,7-triisopropyl-1,4,7-triazacy-
clononane)copper(II) dichloride (Cu(tacn)Cl2)8b,c were selected as
catalysts for the oxidative polymerization of PPL.
The oxidative polymerization of PPL was performed in the
presence of the catalyst (5 mol % based on PPL) in toluene or
THF under dioxygen (1 atm) at 40 °C. As a model system of
free phenoxy radical coupling, polymerization of PPL oxidized
by an equimolar of 2,2′-azobisisobutyronitrile (AIBN) was
examined in toluene under nitrogen at 40 °C. During these
polymerization reactions, small portions of the reaction mixtures
were taken out and dimers of the initial polymerization stages
were analyzed. With the CuCl/teed catalyzed polymerization,
major products detected were four dimers and identified as p-4,
o-4, oo-22, and oo-13 (Table 1) by using liquid chromatography-
† Japan Chemical Innovation Institute.
‡ Sumitomo Chemical Co. Ltd.
§ Tokyo Institute of Technology.
| Kyoto University.
National Institute of Materials and Chemical Research.
(1) (a) Hay, A. S. J. Polym. Sci. 1962, 58, 581-591. (b) Uyama, H.;
Kurioka, H.; Kaneko, I.; Kobayashi, S. Chem. Lett. 1994, 423-426.
(2) (a) Hay, A. S.; Blanchard, H. S.; Endres, G. F.; Eustance, J. W. J. Am.
Chem. Soc. 1959, 81, 6335-6336. (b) Hay, A. S. Macromolecules 1969, 2,
107-108. (c) Ikeda, R.; Sugihara, J.; Uyama, H.; Kobayashi, S. Macromol-
ecules 1996, 29, 8702-8705.
(3) Mijs, W. J.; van Lohuizen, O. E.; Bussink, J.; Vollbracht, L. Tetrahedron
1967, 23, 2253-2264.
(4) Mahadevan, V.; Hou, Z.; Cole, A. P.; Root, D. E.; Lal, T. K.; Solomon,
E. I.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 119, 11996-11997. In this
paper, the reaction of copper(I)/peralkylated-1,2-cyclohexanediamine com-
plexes with dioxygen generated bis(µ-oxo) dicopper(III) complexes, which
reacted with 2,4,6-tri-tert-butylphenol to yield the phenoxy radical and bis-
(µ-OH) dicopper(II) complexes.
(7) Hay, A. S. Polym. Eng. Sci. 1976, 16, 1-10. In this paper, the concept
of using copper complexes with bulky ligands and “controlled” radicals in an
attempt to avoid coupling at open o-position(s) in phenols was proposed, and
in this direction, the oxidation of o-cresol using copper complexes with
2-alkylpyridines to give high molecular weight polymers was found.
(8) (a) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K., Nakamura, A. J. Am. Chem. Soc.
1992, 114, 1277-1291. (b) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.;
Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1994, 116, 9785-9786. (c)
We determined the structures of Cu(tpzb)Cl and Cu(tacn)Cl2 by using X-ray
crystallography.
(5) (a) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young,
V. G., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996,
271, 1397-1400. (b) Tolman, W. B. Acc. Chem. Res. 1997, 30, 227-237.
(6) (a) Kitajima, N.; Fujisawa, K.; Moro-oka, Y.; Toriumi, K. J. Am. Chem.
Soc. 1989, 111, 8975-8976. (b) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994,
94, 737-757.
S0002-7863(98)01283-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/07/1998

8530 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Table 1. Oxidative Polymerization of 4-Phenoxyphenola
Communications to the Editor
dimers at the initial stage of polymerization
dimer ratio (%)
polymers
g
time
(h)
conv.b yieldc
(%)
time conv.b yielde
Tm
entry
catalyst
oxidant solvent
(%) p-4d o-4d oo-22d oo-13d (h)
(%)
(%)
Mn
Mw
(°C)
f
f
i
1
2
3
4
5
6
Cu(tpzb)Clh O2
toluene
THF
toluene
THF
toluene
0.25
13
11
9
12
17
27
9
7
91
91
93
89
79
82
9
9
7
7
6
4
0
0
0
1
2
2
0
0
14
70
55
91
4
54
89
52
77
8
700
1500
1200
1100 186
2900 194
4700 171
Cu(tpzb)Clh O2
1.7
i
i
i
i
h
Cu(tacn)Cl2 O2
0.20
7.5
0.02
8
0
19
98
Cu(tacn)Cl2 O2
9
12
15
3
13
12
81
86
100
69
3800 13700 ndj
5400 29100 ndj
h
CuCl/teedk
O2
24
AIBNl toluene 120
380
1600
3100 ndj
a Polymerization of PPL (0.60 mmol) in solvent (1.2 g) at 40 °C. b Conversion of PPL. c Total yield of dimers. d Dimers shown below. e Methanol-
f
insoluble part. Determined by size exclusion chromatography. g Temperature of the largest endothermic peak in the second scan of DSC measurement
under nitrogen. h Cu complex (0.030 mmol) and 2,6-diphenylpyridine (0.30 mmol). i Under dioxygen (1 atm). j Not detected. k CuCl (0.030 mmol)
and teed (0.015 mmol). l Oxidized by AIBN (0.60 mmol) under nitrogen.
mass spectroscopy and NMR spectroscopy.9 Products p-4 and
o-4 are formed by the C-O coupling. Formation of oo-22 and
oo-13 is based on the C-C coupling. In the range of PPL
conversion less than 30%, the conversion of PPL was fairly close
to the total yield of the four dimers, and therefore, the ratio of
the dimers can be taken as a strong measure of the regioselectivity
of the coupling.
complex 1 in nonpolar solvents such as toluene.5a It was reported
that complex 1 reacted with HBF4 to yield hydrogen peroxide,8b
and the reaction of another complex 1 with 4-fluorophenol gave
complex 2.6b These previous studies indicate that, for entries 1-3,
µ-η2:η2-peroxo dicopper(II) complexes 1 are formed and react
with phenols to regenerate intermediates 2 or 3 and hydrogen
peroxide,12 and thus, the free radical coupling processes would
be completely excluded.
In the oxidative polymerization of PPL catalyzed by CuCl/
teed catalyst (entry 5), considerable amounts of two C-C coupling
dimers of oo-2 and oo-13 were detected, and p-4 selectivity was
consequently low (79%). These values were almost the same as
those oxidized by AIBN (entry 6), in which the formation of the
C-C coupling dimers is characteristic. However, for Cu(tpzb)
catalyst in toluene (entry 1) and in THF (entry 2), and for Cu-
(tacn) catalyst in toluene (entry 3), none of the C-C coupling
dimers were detected and high regioselectivity of p-4 (maximum
93%) was achieved. These data strongly indicate that the
regioselectivity of phenoxy radical coupling can be controlled
by these catalysts. The Cu(tacn) catalyst in THF (entry 4),
however, yielded the C-C coupling dimers to some extent.
In the polymerization without the C-C dimer formation at the
initial reaction stage (entries 1-3), the polymers (methanol-
insoluble part) showed the same IR spectra as crystalline PPO,10a
and their melting points (171-194 °C) were detected by dif-
ferential scanning calorimetric (DSC) analysis.10b In the polym-
erization involving the formation of the C-C dimers (entries
4-6), no clear melting points of the polymers were detected.
On the basis of the above data, the polymerization mechanism
is postulated as follows (Scheme 2). First, copper(II) chloride
complexes 6 of Cu(tpzb)Cl and Cu(tacn)Cl2 react with PPL or
oligomers of PPL to give phenoxo-copper(II) complexes 2,11
equivalent to phenoxy radical-copper(I) complexes 3. Regiose-
lective coupling takes place via 2 and/or 3 to produce copper(I)
complexes 7 of Cu(tpzb) and Cu(tacn)Cl. In case of the Cu-
(tpzb) complex, formation of µ-η2:η2-peroxo dicopper(II) complex
1 from 7 was confirmed under dioxygen8a in both toluene and
THF. For the Cu(tacn) complex, 7 was also known to afford
For the Cu(tacn) complex under dioxygen in THF, it was
reported that 7 gave bis(µ-oxo) dicopper(III) complex 4;5a 4 did
not react with HBF4,13a and a similar complex of 4 abstracted
hydrogen atoms of its substituents to give bis(µ-OH) copper(II)
complex 8.13b These data show that, in case of entry 4, bis(µ-
oxo) dicopper(III) complex 4 is formed, followed by abstraction
of hydrogen atoms from phenols to give free phenoxy radical 5,
in the radical coupling of which the regioselectivity cannot be
regulated, and complex 8 producing intermediates 2 or 3. For
entry 5, the polymerization catalyzed by the CuCl/teed is seemed
to be almost governed via the free radical coupling.14
In conclusion, we have first achieved synthesis of crystalline
PPO showing a melting point by highly regioselective oxidative
polymerization of 4-phenoxyphenol using tyrosinase model
complex catalysts, from which nucleophilic µ-η2:η2-peroxo di-
copper(II) complexes were formed in situ as the active dioxygen
intermediates. The present study is very significant not only in
the fields of polymer chemistry and polymeric materials but also
in that of bioinorganic chemistry, because only few examples have
been reported that enzyme model complexes are really advanta-
geous to functional material productions.
Acknowledgment. We thank Dr. Yoshinobu Nagawa (NIBH) for
NMR analysis. This work was supported by NEDO for the project on
Technology for Novel High-Functional Materials (AIST).
Supporting Information Available: Text describing the characteriza-
tion data for Cu(tpzb)Cl and Cu(tacn)Cl2, the synthetic procedures and
characterization data for p-4, o-4, oo-22, and oo-13, and the typical
procedure for polymerization (5 pages, print/PDF). See any current
masthead pages for ordering information and Web access instructions.
JA981283M
(9) The authentic samples of p-4, o-4, oo-22, and oo-13 were synthesized
and characterized by the method reported in the Supporting Information.
(10) (a) van Dort, H. M.; Hoefs, C. A. M.; Magre´, E. P.; Scho¨pf, A. J.;
Yntema, K. Eur. Polym. J. 1968, 4, 275-287. (b) The melting points of the
resulting polymers were lower than that of linear high molecular weight PPO
(298 °C),10a which may be because the polymers contained a small amount of
ortho C-O linkage and/or had lower molecular weight.
(11) When the copper(II) complexes of Cu(tpzb)Cl and Cu(tacn)Cl2 were
reacted with phenols, giving the phenoxo-copper(II) complexes, hydrogen
chloride was generated. To neutralize generated hydrogen chloride, an excess
of 2,6-diphenylpyridine, having no coordination ability to copper atom, was
added to the complexes.
(12) We proved that a similar copper(I) complex was reacted not only with
dioxygen but also with hydrogen peroxide to form the µ-η2:η2-peroxo dicopper-
(II) complex.
(13) (a) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc.
1996, 118, 11575-11586. (b) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.;
Pan, G.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1995,
117, 8865-8866.
(14) The dimer ratio by the Cu(tacn) in THF was slightly different from
that by the CuCl/teed in toluene. This might be due to the difference in copper
valences of the starting complexes, reactivity of the phenoxo-copper
complexes, or solvents of the reaction.