58
Chemistry Letters Vol.32, No.1 (2003)
Clean Synthesis of 3,30,5,50-Tetra-tert-butyl-4,40-diphenoquinone from the Oxidative Coupling
of 2,6-Di-tert-butylphenol Catalyzed by Alkali-promoted Cu–Mg–Al Hydrotalcites
in the Presence of Molecular Oxygen
Keisuke Iwai, Takayoshi Yamauchi, Keiji Hashimoto,y Tomoo Mizugaki,yy Kohki Ebitani,yy and Kiyotomi Kanedayy
K. K. Nisseikagaku kogyosyo, 2-18-110, Yodogawa-ku, Osaka 532-0001
yOsaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536-8553
yyDepartment of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received October7, 2002; CL-020859)
2,6-Di-tert-butylphenol is selectively transformed to
MgCl2Á6H2O (129.6 mmol), and AlCl3ÁH2O (43.2 mmol) was
dissolved in 120 mL of distilled water. A 120 mL of an aqueous
solution of Na2CO3 (0.224 mol) and NaOH (0.39 mol) was slowly
added to the above solution, and then the resultant mixture was
heated at 65 ꢀC for 18 h with vigorous stirring. The obtained slurry
was filtered, and washed with distilled water and drying at 100 ꢀC
3,30,5,50-tetra-tert-butyl-4,40-diphenoquinone in the presence of
molecular oxygen using alkali-promoted Cu–Mg–Al hydrotal-
cites as recyclable heterogeneous catalysts.
3,30,5,50-Tetra-tert-butyl-4,40-diphenoquinone (TBDPQ) is a
valuable raw material for highly functionalized photo-
sensitizers.1 TBDPQ can be conventionally synthesized by the
liquid-phase oxidative coupling of 2,6-di-tert-butylphenol (DBP)
using stoichiometric inorganic oxidants,2 that produces copious
amounts of wastes. With ever-growing environmental issues of
chemical processes,3 much attention has been paid to the use of
molecular oxygen as a ‘‘green’’ oxidant to achieve transformation
of DBP to TBDPQ in the presence of metal catalysts.4;5 Typical
examples are copper(II)-amine complexes coupled with a strong
base such as KOH. Tsuruya et al. tried to heterogenize the Cu
complex catalysts on the surface of poly(4-vinylpyridine)6 and
mesoporous silicate MCM-41 for the liquid-phase oxidation of
DBP in the presence of potassium compounds.7 However, the
oxidation by both catalysts needs a chlorinated solvent of CHCl3.
Moreover, the TBDPQ yield resulted in only 40%, where the Cu/
MCM-41 catalyst could not be reused.7
for15 h to give 12.0 g of Cu–Mg–Al–CO . A 2.0 g of the Cu-
3
hydrotalcite was soaked in 1 M aqueous KOH solution (20 mL)
for 2 h, followed by filtration, washing, and drying afforded a gray
powderof K/Cu–Mg–Al–CO 3 sample. The hydrotalcite structure
of the gray powder was confirmed by its XRD pattern,
ꢀ
and the basal spacing was 7.9 A. [Anal. Calcd for
K
0:14Mg4:89Al1:0Cu0:5(OH)12:90.49CO3ÁnH2O (n = 5): K, 1.1;
Mg, 22.8; Al, 5.2; Cu, 6.1. Found: K, 1.08; Mg, 22.7; Al, 5.2; Cu,
6.2 wt%.] A typical example forthe oxidation of DBP is as
follows. Into a reaction vessel were placed DBP (1.24 g, 6 mmol),
K/Cu–Mg–Al–CO3 (1.20 g, Cu: 1.2 mmol), and o-xylene
(15 mL). The heterogeneous mixture was then stirred at 130 ꢀC
underan O
atmosphere. After 10 h, the hydrotalcite was
2
separated by filtration. LC analysis of the filtrate11 showed a
quantitative yield of TBDPQ. Column chromatography of the
filtrate on silica, followed by recrystallization from ethanol gave
1.18 g of TBDPQ (96% yield). The isolated hydrotalcite was
washed with methanol, and soaked in 20 mL of 1 M aqueous KOH
solution. Afterfiltartion, the solid was washed with waterand
dried at 100 ꢀC, which could be reused keeping its activity and
selectivity forthe above oxidative coupling reaction; the yields of
TBDPQ in the first and second recycle experiments were over
94%, respectively.
Our approach to developing a highly efficient and recyclable
heterogeneous catalyst in the TBDPQ synthesis is incorporation
of CuII species into inorganic base crystals as macroligands.
Hydrotalcites, consisted of a positively charged Brucite-layer and
2À
interlayer of CO3 species,8 have surface base sites.9 Further,
Mg and/or Al cations, composing the Brucite-layer, can be
replaced with various metal cations as the catalytically active
centers.10 We report here a clean and simple synthesis of TBDPQ
from the oxidative coupling of DBP under an O2 atmosphere
using an alkali-promoted Cu-exchanged hydrotalcite, which acts
as a recyclable heterogeneous catalyst (eq. 1).
The oxidation of DBP was carried out using various
hydrotalcites under an O2 atmosphere, which is summarized in
Table 1. Generally, major products under the above conditions
were TBDPQ and 3,30,5,50-tetra-tert-butyl-4,40-biphenyldiol
(TBDPD) (Scheme 1). Notably, the Cu–Mg–Al–CO3 hydrotal-
cites combined with K, Cs, and Rb cations were found to be highly
active catalysts forthe oxidative coupling of DBP to TBDPQ
(entries 1, 4, and 5). These catalysts were more effective than the
corresponding Cu-free Mg–Al–CO3 hydrotalcites (entries 10–
12). In the case of the alkali-free Cu–Mg–Al–CO3 hydrotalcite,
the yield of TBDPQ was quite low (entry 9). Further, the
oxidations in the second and third runs using the spent K/Cu–Mg–
12
Al–CO3 catalyst gave similarTBDPQ yields (entries 2 and 3).
In the oxidation of DBP by the Cu catalyst, the role of KOH is
generally considered to be deprotonation of DBP into a K-
phenolate species, which is subsequently oxidized by CuII to
afford radical intermediates of carbon–carbon coupling.4a,7
A
The hydrotalcite was prepared according to the modified
method in the literature.8 A representative procedure is for K/Cu–
Mg–Al–CO3.
A
mixture of CuCl2Á2H2O (12.9 mmol),
Copyright Ó 2003 The Chemical Society of Japan