Aerobic oxidation of 2,3,6-trimethylphenol to trimethyl-1,4-benzoquinone with copper(II) chloride as catalyst in ionic liquid and structure of the active species

Article abstract of DOI:10.1021/ja0391964

TMQ is an important precursor in industrial vitamin E synthesis. We report a "green chemistry approach" with respect to the highly selective and environmentally friendly oxidation of 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (TMQ) with molecular oxygen as oxidant and a copper catalyst immobilized in a molten salt. n-Butanol as co-solvent has a positive effect on the activity and selectivity. The structurally characterized catalyst, a 1-n-butyl-3-methylimidazolium oxotetracuprat, is formed in situ via hydrolysis of CuCl₂ in the presence of imidazolium chloride. We propose a mechanism of oxidative phenolate activation at a [Cu₄(μ⁴-O)]⁶⁺ core as electronically coupled electron acceptor, formation of a copper-bound phenolate radical anion, spin delocalization into the aromatic ring, and attack by triplet oxygen at the para position. Attack of Cu(I) as reduction equivalent at the peroxy radical, proton-mediated elimination of a copper(II)-hydroxo species, will either substitute a copper(I) site in the reduced oxo cluster or take up an electron from the reduced mixed valent cluster [Cu₄(μ₄-O)]₆₊ to regenerate the oxidized cluster as the active electron acceptor. Copyright

Full text of DOI:10.1021/ja0391964

Published on Web 07/20/2004
Aerobic Oxidation of 2,3,6-Trimethylphenol to Trimethyl-1,4-benzoquinone
with Copper(II) Chloride as Catalyst in Ionic Liquid and Structure of the
Active Species
Hongjian Sun,*,† Klaus Harms, and Joerg Sundermeyer
Fachberech Chemie der Philipps-UniVersitaet Marburg, 35032 Marburg, Germany
Received October 23, 2003; E-mail: hjsun@sdu.edu.cn
Table 1. Influence of the Amount of [BMIm]CuCl3 on the
Selectivity of TMQ
In the industrial production of vitamin E, trimethyl-1,4-benzo-
quinone (TMQ) is a key intermediate. The current method for its
production is para-sulfonation of 2,3,6-trimethylphenol (TMP)
followed by oxidation of MnO2. One-step oxidation of TMP to
TMQ has been investigated using different oxidizing agents such
as hydrogen peroxide,1 and peroxy acids.2 In this process, several
catalysts such as iron complex3 and cobalt complex4 were attempted
to increase the selectivity and yield and to decrease the amount of
byproducts. Molecular oxygen as oxidant would be the most
important method because it is environmentally friendly.
In a patent, the oxidation of alkyl-substituted phenols with a
stoichiometric amount of copper(II) salt was carried out in an
oxygen atmosphere (30 bar) in dimethylformamide.5 The corre-
sponding p-benzolquinone was the major product. The oxidation
of TMP afforded TMQ in acetone/water in yield of 80-87% at
full conversion. BASF AG further developed this process over a
catalyst containing copper(II) halide and in an alcohol/water two-
phase medium.6 The yield of TMQ reached 98.5%. Abe’s group
proposed copper(II) as catalyst and lithium chloride as cocatalyst
for the oxidation. In hexanol/water two-phase medium a yield of
97% at full conversion was obtained.7 A serious drawback of these
methods is that almost or even more than a stoichiometric amount
of catalyst is required for a complete and selective oxidation of
TMP.
A great development of this catalytic oxidation was made by
Takehira.8 He reduced the amount of catalyst to 10 mol % by
applying nitrogen-containing organic compounds as cocatalysts.
With the best cocatalyst, hydroxyamine hydrochloride, the yield
of 92% was attained. But during the oxidation the cocatalyst,
hydroxyamine hydrochloride was partly consumed. With 1 mol of
hydroxyamine hydrochloride, about 20 mol of TMQ was produced.
Baiker and co-workers made a progress in this direction.9 TMQ
was obtained in up to 80% yield using only 1.5 wt % CuCl2 catalyst
under ambient conditions. Takaki succeeded in an efficient oxidation
of TMP with polymer-supported copper catalysts.10 But it took a
long time to complete the reaction. Xiao’s group studied catalytic
hydroxylation of TMP with hydrogen peroxide over copper
hydroxyphosphate.11 The selectivity for trimethylhydroquinone was
reached when the reaction was carried out under N2.
a
entry
1
2
3
4
5
[BMIm]CuCl3/mol %
selectivityb
2.5
86
5
89
10
94
20
96
100
98
a 2 mmol TMP, 2 mL of n-BuOH, 10 Bar O2, 60 °C, 5 h. b Conversion
is 100% in all of the experiments.
Table 2. Influence of the Proportion of CuCl2 to [BMIm]Cl on the
Selectivity of TMQ
a
entry
1
2
3
4
5
6
CuCl2/[BMIm]Cl
selectivityb
1:0.5
71
1:1
94
1:2
84
1:4
80
1:8
74
1:20
55
a 2 mmol TMP, 2 mL of n-BuOH, 10 Bar O2, 60 °C, 5 h. b Conversion
100% for entry 1-5; 90% for entry 6.
We herein describe a “green chemistry approach” with respect
to the highly selective and environmentally friendly oxidation of
TMP to TMQ with molecular oxygen as oxidant and a copper(II)
chloride as catalyst in the ionic liquid medium of 1-butyl-3-
methylimidazolium chloride ([BMIm]Cl). n-Butanol as cosolvent
has a positive effect on the selectivity and activity. We found that
the amount of copper(II) chloride catalyst can be substantially
reduced in ionic liquid without a large decrease in the yield and
selectivity. With the amount of copper(II) chloride in 2.5 mol %,
a yield of 86% TMQ could still be attained.
The first results (Table 1) show that with a stoichiometric amount
of copper(II) chloride a yield of 98% could be reached. With the
decrease of the amount of copper(II) chloride dihydrate the yield
gradually declines. When 2.5 mol % copper(II) chloride dihydrate
was used in the experiment, a yield of 86% could still be reached.
If the used amount of copper(II) chloride is constant at 10 mol %
and we only change the amount of [BMIm]Cl, it can be found that
the largest yield could be attained in the proportion (1:1) of copper-
(II) chloride dihydrate to [BMIm]Cl (Table 2). If we increase the
amount of the ionic liquid, we get the lower yield and selectivity.
In the proportion of 1:20 a selectivity of 55% could only be reached.
In almost all of the experiments the conversion is 100%. Besides
TMQ the other product is 2,2′,3,3′,5,5′-hexamethyl-4,-4′-dihydroxy-
biphenyl. TMP is soluble in the ionic liquid in all of the
experiments.
Since the first application of ionic liquid as a reaction medium,12
there have been some reports about application of ionic liquids in
the catalytic process.13 Song’s work proved that oxidation can
advantageously be realized in ionic liquid.14 The epoxidation of
2,2-dimethylchromen could clearly be accelerated with the addition
of ionic liquid. Recently, Gree developed an aerobic oxidation of
alcohols to the corresponding aldehydes and ketones in ionic
liquid.15 However, the oxidation of phenols to quinones in ionic
liquid is not known.
Red cubic crystals were obtained from the catalytic solution.
X-ray structure shows that it is a 1-n-butyl-3-methylimidazolium
oxotetracuprate (Figure 1). Four copper(II) ions form a tetrahedron
core in the cluster compound and have an oxygen atom center.
Moreover, there are six bridging Cl atoms and four end Cl atoms.
Four disordered 1-n-butyl-3-methylimidazolium cations complete
the asymmetric unit of the crystal structure as counterions of the
tetrahedron. The similar structures were found in the literature.16
But this is the first time to isolate the catalytically active species
† School of Chemistry and Chemical Engineering, Shandong University, Shanda
Nanlu 27, Jinan 250100, P.R. China.
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9550
J. AM. CHEM. SOC. 2004, 126, 9550-9551
10.1021/ja0391964 CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
as cosolvent affording 86% yield provides a new alternative to the
copper(II) chloride-catalyzed aerobic oxidation. The advantage in
this catalytic system is that only a catalytic amount of copper(II)
chloride is necessary. A oxotetracuprate [Cu4(µ4-O)Cl10]-4 was
isolated as an active species. We are currently investigating the
scope of this system and the influence of some other reaction
conditions on the selectivity and yield. The preliminary tests verified
this catalytical system is also applicable for oxidation of 2-meth-
ylnaphthol. We also want to test the other ionic liquids as oxidation
reaction medium.
In a typical procedure, a 60-mL thick glass autoclave was charged
with the catalyst solution of copper(II) chloride in mixed medium
of [BMIm]Cl and n-butanol. The autoclave was pressurized with
oxygen (10 bar) and heated to 60 °C. After 5 h, the autoclave was
cooled to the ambient temperature and depressurized. The conver-
sions and yields were determined by GC using an AHRGC-5300
Capillary GC with column DB-5 without removal of the ionic
liquid/Cu complex from the catalytic solution.
To isolate the oxotetracuprate, a mixture of TMP (2 mmol),
CuCl2‚2H2O (0.2 mmol), and 1-butyl-3-methylimidazolium chloride
(0.2 mmol) in 2 mL of 1-butanol was stirred at 60 °C under 10 bar
of O2 for 5 h. The reaction solution was left to stand at room
temperature for several days. Red cubic crystals were formed and
separated by decantation and were washed with pentane. Yield:
51%. Elemental analysis (%) Calcd for C32H60Cl10Cu4N8O: C,
25.53; H, 4.02; N, 7.44; Cl, 25.33; Cu, 37.68. Found: C, 25.95; H,
4.26; N, 7.34; Cl, 25.23; Cu, 37.28. Decomposition temperature
116 °C.
Figure 1. ORTEP view. Selected bond lengths [Å] and angles [deg]: Cu1-
O1 1.919(6), Cu2-O1 1.917(6), Cu3-O1 1.937(6), Cu4-O1 1.913(6);
Cu4-O1-Cu2 109.0(3), Cu4-O1-Cu1 111.7(3), Cu2-O1-Cu1
107.7(3), Cu4-O1-Cu3 110.4(3), Cu2-O1-Cu3 109.8(3), Cu1-O1-Cu3
108.2(3).
Acknowledgment. We thank the Deutsche Forschungsgemein-
schaft.
Scheme 1. Proposed Mechnism of the Aerobic Oxidation
Supporting Information Available: Cryatallographic data collec-
tion and refinement parameters, position and thermal parameters, and
bond distances and angles for the oxotetracuprate (CIF, PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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as an active species plays a key role. At first the bridging Cl atom
is substituted by phenol. Then, through spin delocalization a copper-
bound phenolate radical ion is formed. After the attack by triplet
oxygen at the para position, the peroxy radical is obtained. In the
next step through the attack of Cu(I) species on the peroxy function
group and the proton-mediated elimination the copper cluster can
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In conclusion, the oxidation of TMP to TMQ with 2.5 mol %
copper(II) chloride catalyst in ionic liquid [BMIm]Cl with n-butanol
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