Copper-catalyzed oxidative coupling of 2,4,6-Trimethylphenol with oxygen

Article abstract of DOI:10.1246/cl.2000.1318

2,4,6-Trimethylphenol was oxidized with oxygen in the presence of (1,4,7-tribenzyl-1,4,7-triazacyclononane)copper(II) chloride as a catalyst to produce exclusively a coupled product stilbenequinone, without the formation of oxygenated products.

Full text of DOI:10.1246/cl.2000.1318

1
318
Chemistry Letters 2000
Copper-Catalyzed Oxidative Coupling of 2,4,6-Trimethylphenol with Oxygen
Kenichi Oyaizu, Kei Saito, and Eishun Tsuchida*
Advanced Research Institute for Science and Engineering, Department of Polymer Chemistry, Waseda University,
Tokyo 169-8555
(Received August 11, 2000; CL-000768)
2
,4,6-Trimethylphenol was oxidized with oxygen in the
presence of (1,4,7-tribenzyl-1,4,7-triazacyclononane)copper(II)
chloride as a catalyst to produce exclusively a coupled product
stilbenequinone, without the formation of oxygenated products.
The outcome of the copper-catalyzed oxidation of 2,4,6-
trimethylphenol with oxygen has been dominated by oxygenat-
ed products such as 4-hydroxy-3,5-dimethylbenzaldehyde, 2,6-
dimethyl-p-benzoquinone, and 4-alkoxy-2,6-dimethylphe-
nols.1 This reaction, effectively catalyzed by copper(II) chlo-
rides coupled with diethylamine, hydroxylamine, or oximes,
has attracted much attention not only as a convenient synthetic
method for p-hydroxybenzaldehyde that is an important inter-
mediate in the industrial synthesis of dyes, polymers, pharma-
ceutical and agrochemicals, but also as the relevant model for
,2
metric stilbenequinone coupled at the o- or p-position. X-ray
analysis revealed the p-coupled structure having the crystallo-
3
12
the biosynthesis of vitamin E. Another important oxidation
chemistry of phenols is the coupling of o- and/or p-unsubstitut-
ed phenols to give diphenoquinones and phenylene oxide poly-
mers.4 On the other hand, the oxidative coupling of 2,4,6-tri-
alkylated phenols at the benzylic carbon is less frequent, and
has so far required a large amount of strong oxidants in the
absence of oxygen. For example, the oxidation of 2,6-di-tert-
butyl-4-methylphenol with iodine gives a mixture of 1,2-
bis(3,5-di-tert-butyl-4-hydroxyphenyl)ethane and 3,3',5,5'-tetra-
graphic center of symmetry located at the center of the mole-
cule (Figure 1). The product, 3,3',5,5'-tetramethylstilbene-4,4'-
quinone, is a fully oxidized dimer. The partially oxidized struc-
–7
8
tert-butylstilbene-4,4'-quinone, and 2,4,6-trimethylphenol is
9
oxidized by silver carbonate into stilbenequinone, though the
product is not well characterized. Herein we report our suc-
cessful attempts to use oxygen as an oxidant for the oxidative
coupling of 2,4,6-trimethylphenol. Combination of X-ray crys-
tallographic and spectroscopic methods revealed that the prod-
uct is solely the corresponding stilbenequinone 4.
Recently, it was shown that the oxidative polymerization of
2
,6-difluorophenol with oxygen is accomplished using copper
complexes with 1,4,7-trialkyl-1,4,7-triazacyclononane ligands as
catalysts.10 The strong oxidizing power of the complex demon-
strated by the reaction led us to examine the oxidation of 2,4,6-
trimethylphenol. The overall oxidation from trimethylphenol to
stilbenequinone is a 3-electron, 3-proton transfer process. Under
argon, upon the addition of a CH Cl solution of 3 equiv of
tures such as 1,2-bis(3,5-dimethyl-4-hydroxyphenyl)ethane and
trans-1,2-bis(3,5-dimethyl-4-hydroxyphenyl)ethylene are
excluded by the planar framework of the molecule and the short
C–O distance which is close to the typical bond length of a car-
bonyl group. The short C(8)–C(8)* bond length corresponds to
the partial double bond in stilbenequinone. After the reaction, a
reasonable amount of the copper(I) complex (Figure 2) was iso-
2
2
(1,4,7-tribenzyl-1,4,7-triazacyclononane)copper(II) chloride
[(tacn)CuCl ]) to a solution of 2,4,6-trimethylphenol in CH Cl
2
13
(
lated. It should be noted that the partially oxidized dimers are
2
2
containing 3 equiv of triethylamine at −5 °C, the greenish yel-
low solution slowly turned to a transitory brown color indica-
tive of the formation of the corresponding copper(I) and phe-
not produced even when only 0.3 equiv of [(tacn)CuCl ] is
2
reacted with 2,4,6-trimethylphenol under argon; 4 is obtained as
the sole product in 10% yield, and unreacted trimethylphenol is
recovered from the solution. The absence of the partially oxi-
dized by-products supports the fact that they are more easily oxi-
8
noxyl radical (Scheme 1), which changed to dark red upon
standing overnight. Layering the solution with diethyl ether
1
8
afforded dark red crystals of the product in 90% yield. The H
dized by the copper(II) complex. On the contrary, 4 is obtained
and 13C NMR spectra of the crystallized product revealed a
coupled quinoid structure having only two nonequivalent
methyl groups, which corresponded to those of the centrosym-
11
in 100% yield when the same reaction is carried out in the pres-
ence of oxygen, indicating that the catalytic oxidation with oxy-
gen is accomplished. The amount of oxygen uptake with respect
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1319
References and Notes
1
2
3
4
M. Shimizu, Y. Watanabe, H. Orita, T. Hayakawa, and K.
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A. S. Hay, H. S. Blanchard, G. F. Endres, and J. W.
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5
6
A. S. Hay, J. Polym. Sci., A, 36, 506 (1988).
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Chem., A21, 1081 (1984).
7
8
9
D. G. Hewitt, J. Chem. Soc., C, 1971, 2967.
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1
339 (1971).
K. Oyaizu, K. Saito, and E. Tsuchida, Macromolecules, 33,
766 (2000).
1
0
1
–
to the yield of 4 (150 mol%) was in accordance with the 4e
5
reduction of oxygen.1
4
1
1
Spectroscopic data for 4: H NMR (500 MHz, CDCl , TMS,
3
The question arises as to the origin of the contrast with the
precedented oxygenation of 2,4,6-trimethylphenol at the p-
methyl group that takes place when the conventional copper(II)
ppm) δ: 2.09 (methyl, 6H, s), 2.13 (methyl, 6H, s), 7.05
(
phenyl, 2H, s), 7.23 (phenyl, 2H, s), 7.55 (methide, 2H, s);
13
C NMR (500 MHz, CDCl , TMS, ppm) δ: 16.45 and 17.04
3
1
,2
complexes are used as the catalyst (Scheme 2).
Recently,
(methyl C), 128.1, 133.1, 136.4, 137.5, 138.0, 138.4 and
1
(
–1
87.3 (quinone methide C); IR (KBr, cm ) νmax: 2948, 2915
CH ), 1637 (C=O), 1595, 1502, 1435, 1370, 1323, 1233,
3
1
177, 1027, 941, 907, 781, 610, 456.
1
2
3
Crystal data for 4: C H O , MW = 266.34, monoclinic,
1
8
18
2
space group P2 /c (#14), a = 4.340(3), b = 12.252(4), c =
1
3
1
3.670(4) Å, β = 91.56(6)°, V = 726.6(6) Å , D = 1.217 g
calc
–3 –1
cm , Z = 2, µ (Mo Kα) = 0.78 cm , 1977 reflections meas-
ured, 1758 unique (R = 0.028), 586 observations (I >
int
3
.00σ(I)), 128 variables, final R = Σ||F | – |F ||/Σ|F | = 0.073,
o c o
2 2 1/2
Kobayashi et al. reported that a phenoxyl radical-copper(I) inter-
mediate is generated instead of the free phenoxyl radical during
the oxidative polymerization of phenol catalyzed by (1,4,7-triiso-
R = (Σw(|F | – |F |) /(ΣwF )) = 0.045.
w
o
c
o
1
Another curious feature is the first isolation and crystal struc-
ture determination of a copper(I) complex bearing tacn lig-
and, which has been difficult because of its high reactivity
tward O2. Single crystals were grown by carefully layering
the solution with acetonitrile containing tetrabutylammonium
triflate strictly in the absence of oxygen. Crystal data for
[(tacn)Cu(CF SO )·(CH CN)]: C H CuF N O S, MW =
15,16
propyl-1,4,7-triazacyclononane)copper(II) chloride.
In addi-
tion, it was shown that a phenoxocopper(II) complex isolated
from the reaction of 4-fluorophenol and a copper(II) complex
with a bulky ligand could be equibrated with a copper(I) phe-
1
7
noxyl complex in solution due to the resonance. Thus, the
3
3
3
30 36
3
4
3
6
53.24, orthorhombic, space group Pbca (#61), a = 19.17(6),
resulting phenoxyl radical could stay in the coordination sphere
3
b = 20.03(6)), c = 16.14(8) Å, V = 6198(35) Å , D = 1.400
g cm , Z = 8, µ (Mo Kα) = 8.27 cm , 7756 reflections
measured, 7754 unique (R = 0.373), 4114 observations (I >
of copper(I).18 A similar reaction has been reported for the oxi-
calc
–
3
–1
19
dation of catechol to give a copper(I) semiquinone complex.
int
By analogy to these reactions, it is postulated that the phenoxyl
radical is bound to copper(I) (Scheme 1). Although the isolation
of the copper(I) phenoxyl complex 2 is unsuccessful, it seems
reasonable to suppose that the coupling reaction prevails due to
the lower reactivity of 2 toward oxygen (Scheme 1) than that of
the free radical 5 (Scheme 2). This is partly corroborated by a
control experiment using (1,4,7-triazacyclononane)copper(II)
3
.00σ(I)), 520 variables, final R = Σ||F | – |F ||/Σ|F | = 0.064,
o c o
2 2 1/2
R = (Σw(|F | – |F |) /(ΣwF )) = 0.049.
w
o
c
o
14 In the presence of phenols, the copper(I) complex (Figure 2)
reduced O into H O without the formation of partially
2
2
reduced oxygen species.
15 H. Higashimura, K. Fujisawa, Y. Moro-oka, M. Kubota, A.
Shiga, A. Terahara, H. Uyama, and S. Kobayashi, J. Am.
Chem. Soc., 120, 8529 (1998).
chloride as a catalyst, which should less strongly binds
phenolate:20 concurrent side reactions give a significant amount
16
H. Higashimura, M. Kubota, A. Shiga, K. Fujisawa, Y.
Moro-oka, H. Uyama, and S. Kobayashi, Macromolecules,
of the oxygenated products. The control of phenoxyl coupling by
the coordination of a copper complex is reminiscent of the
regioselective oxidative polymerization of 2,6-unsubstituted phe-
3
3, 1986 (2000).
1
7
K. Fujisawa, Y. Iwata, N. Kitajima, H. Higashimura, M.
Kubota, Y. Miyashita, Y. Yamada, K. Okamoto, and Y.
Moro-oka, Chem. Lett., 1999, 739.
15,16
nol.
The high selectivity for the coupling of 2 should allow
kinetic and energetic studies, which is the topic of our continuous
research.
18 F. Harrod, Can. J. Chem., 47, 637 (1969).
19 M. Berreau, S. Mahapatra, J. A. Halfen, R. P. Houser, V. G.
Young, Jr., and W. B. Tolman, Angew. Chem., Int. Ed., 38,
207 (1999).
E. T. is a CREST investigator, JST. This work was partially
supported by a Grant-in-Aid for Scientific Research (No.
20
P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem., 35, 329
(1988).
11650878) from the Ministry of Education, Science, Sports, and
Culture in Japan.