Aerobic oxidation of primary alcohols catalyzed by copper salts and catalytically active μ-hydroxyl-bridged trinuclear copper intermediate

Article abstract of DOI:10.1002/adsc.201000456

We have developed a new protocol involving the "copper(II) chloride-cesium carbonate" system for the aerobic oxidation of primary alcohols. Cesium carbonates and the solvents such as toluene and 1,2-dichloroethane play important roles to form a catalytica

Full text of DOI:10.1002/adsc.201000456

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DOI: 10.1002/adsc.201000456
Aerobic Oxidation of Primary Alcohols Catalyzed by Copper
Salts and Catalytically Active m-Hydroxyl-Bridged Trinuclear
Copper Intermediate
a
a
b
a,
Lei Liang, Guodong Rao, Hao-Ling Sun, and Jun-Long Zhang *
a
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, Peopleꢀs Republic of
China
Fax : (+86)-10-6276-7034; e-mail: zhangjunlong@pku.edu.cn
Department of Chemistry, Beijing Normal University, Beijing 100875, Peopleꢀs Republic of China
b
Received: June 11, 2010; Revised: September 9, 2010; Published online: October 12, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000456.
Abstract: We have developed a new protocol in-
volving the “copper(II) chloride-cesium carbonate”
system for the aerobic oxidation of primary alco-
hols. Cesium carbonates and the solvents such as
toluene and 1,2-dichloroethane play important roles
to form a catalytically active intermediate contain-
ing a m-hydroxyl-bridged trinuclear copper moiety
for use in the aerobic oxidation of alcohols. In con-
trast to the tremendous progress on biomimetic
copper catalysts supported by organic ligands, much
less has been reported regarding the “ligand-free”
copper-catalyzed aerobic oxidation of alcohols, which
is equally important to be developed from a simple
bench-top procedure and to further apply it for indus-
[
9]
(
complex 1). From the X-ray crystal structure of
trial-scale applications.
A good example is the
complex 1, an electrostatic interaction between
chloride anions and cesium cations was observed,
which stimulated us to understand the roles of
cesium carbonate in this reaction as a base, stabiliz-
ing and “solvating” intermediates in toluene and
“CuCl-TEMPO” system for the aerobic oxidation of
[
10]
alcohols developed by Semmelhack and, important-
ly, the efficiency of this system has been demonstrated
[
11]
in natural product synthesis.
However, other than
to enhance the efficiency for “ligand-free” copper cat-
alysis, how to clarify the possible intermediates and
even for their catalytic properties is another challenge
for “ligand-free” catalysis.
1,2-dichloroethane.
Keywords: aerobic oxidation; copper catalysis; in-
termediates; primary alcohols
To meet this challenge, from the view of bioinor-
ganic chemistry, the questions such as whether active
metal sites in natural metalloenzymes could be repro-
duced in a“ligand-free” metal catalytic system arise to
understand the reaction mechanisms and further
Creating functional models containing similar active apply insights gained from metalloenzymes to organic
metal sites to metalloenzymes not only allows the bio- catalysis. Indeed, although there is no ligand coordi-
logical reactivity to be deciphered but also fosters the nated to the metal ion, solvents, substrates and other
construction of complexes with distinguished catalytic additives might form the primary and/or secondary
[1]
properties. Copper oxidases provide elegant exam- coordination spheres to the metal ion. For example,
ples of using environmentally benign oxidants such as Sun and co-workers isolated oxotetracuprate
(m -O)Cl ] cluster as an active intermediate in
a
[
2]
4À
10
O to perform organic transformations. Inspired by
ACTHUNGTRENNUNG[ Cu ACHTUNGTRENNUNG
4
2
4
the excellent performance of galactose oxidases the aerobic oxidation of 2,3,6-trimethylphenol in ionic
[
3]
(
GOs), copper catalysts supported by organic li- liquid and four disordered 1-n-butyl-3-methyl-imida-
[12]
gands have exhibited good reactivity in the aerobic zolium cations as counterions of the tetrahedron.
oxidation of alcohols. Following Markꢁꢀs pioneer Encouraged by this, we herein report a “CuCl +
work with the CuCl(Phen-DEADH ) (Phen=1,10- Cs CO ” protocol for the aerobic oxidation of alco-
[
4]
2
[5]
2
2
3
phenanthroline;
DEADH =diethylhydrazinodicar- hols under mild condition and found that Cs CO and
2 2 3
[6] [7]
boxylate) catalytic system, Stack, Wieghardt and the solvents play important roles for efficient cataly-
[8]
the others had developed efficient copper catalysts sis. More importantly, the structure and reactivity of a
Adv. Synth. Catal. 2010, 352, 2371 – 2377
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Lei Liang et al.
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Table 1. Optimization of reaction conditions for the aerobic
oxidation of benzyl alcohol.
Table 2. Base effect of the copper(II)-catalyzed aerobic oxi-
dation of benzyl alcohol.
[a]
[a]
[b]
[c]
[b]
[c]
Entry Cat Base
Conv. [%]
Yield [%]
Entry
Cat.
Base
Conv. [%]
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
CuCl2
–
Cs CO 94
99
0
0
1
2
3
4
5
6
7
8
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2[
CuCl2
Cs CO
94
16
15
3
14
17
15
15
7
99
92
92
95
86
94
99
93
82
97[
2
3
2
3
Cs CO
–
0
0
K CO
2
2
3
3
CuCl2
CuCl2
CuCl2
CuCl
CuI
Na CO
2 3
Cs CO 74
99
99
94
96
92
95
97
94
95
96
82
95
97
Rb CO
2
3
2
3
Cs CO 83
NaOH
KOH
CsOH
2
3
Cs CO 77
2
3
Cs CO 71
2
3
CuSO4
Cs CO 23
t-BuOK
2
3
Cu O
Cs CO 21
9
K PO
3
2
2
3
4
0
1
2
3
4
5
6
Cu
Cu
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf)
Cs CO 49
10
11
12
DMAP
CsHCO3
Cs CO
1
80
20
2
2
3
d]
A( OAc)
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO 90
88
2
2
3
e]
Cu (OH) CO Cs CO 12
95
2
2
3
2
3
2
3
Mn
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OAc)
Cs CO 79
2
2
3
3
3
3
[a]
Reaction conditions: benzyl alcohol (1 mmol), copper
salts (0.01 mmol), base (1.2 mmol) in toluene (5 mL), the
mixture was stirred in oxygen at 408C for 12 h.
Conversions were determined by GC with chlorobenzene
as internal standard.
c]
FeCl3
CoCl2
Cs CO
4
6
9
2
Cs CO
2
NiCl2
Cs CO
2
[b]
[
a]
Reaction conditions: benzyl alcohol (1 mmol), copper
salts (0.01 mmol), base (1.2 mmol) in toluene (5 mL), the
mixture was stirred in oxygen at 408C for 12 h.
Conversions were determined by GC with chlorobenzene
as internal standard.
[
Yields were calculated based on conversions, mass
balances >90%.
[d]
[
[
b]
The other product was benzyl benzoate.
10% TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl free
radical) was used.
[e]
c]
Yields were calculated based on conversions, mass
balances >90%.
catalytically active intermediate containing the Employment of other bases such as t-BuOK, K PO ,
3
4
4+
ACHTUNGTRENNUNG[ Cu ACHTUNGTRENNUNG( m -OH) ACHTUNGTRENNUNG( m -Cl)] moiety with cesium cations as and DMAP also afforded low conversions (Table 2,
3 3 3
counterions have been determined.
entries 8–10). However, 80% conversion was achieved
Through optimization of reaction conditions as when 2 equivalents of CsHCO were used instead of
3
shown in Table 1, when 1.2 equivalents of Cs CO
Cs CO (Table 2, entry 11), although the over-oxi-
2 3
2
3
were employed as base, benzyl alcohol could be oxi- dized product benzyl benzoate was detected. In this
dized to benzaldehyde (conversion: 94%, selectivity> work, an attempt to increase the efficiency by adding
9
9%) by molecular oxygen catalyzed by CuCl2 a catalytic amount of TEMPO (10%) was unsuccess-
1 mol%) at 408C for 12 h (Table 1, entry 1). The con- ful and, surprisingly, the conversion dropped drastical-
trol experiments in the absence of copper catalyst or ly (20%, Table 2, entry 12), different from the previ-
(
[
13]
cesium carbonate showed no oxidized product forma- ously reported “CuCl-TEMPO” system. These re-
tion (Table 1, entries 2 and 3). Using air to replace sults suggested that Cs CO plays an important role to
2
3
molecular O resulted in lower yield (74%, Table 1, increase the reactivity of copper salts in the aerobic
2
entry 4). The effect of 4 ꢃ molecular sieves (4 ꢃ MS) oxidation of alcohols.
was not significant; the yield of alcohol (83%,
A significant solvent effect was also observed and
Table 1, entry 5) without 4 ꢃ MS was slightly lower. the results are summarized in Table 3. In non-polar
Different Cu(I) or Cu(II) sources were employed as solvents such as CH Cl , n-hexane, toluene and 1,2-di-
2
2
catalysts and the product yields varied from 11–85% chloroethane, the conversions of benzyl alcohol (>
Table 1, entries 6–12). In addition, other metal salts 50%, Table 3, entries 1–4) were much higher than that
such as Mn(OAc) , FeCl , CoCl and NiCl (Table 1, in polar solvents such as acetonitrile, THF, DMF,
entries 13–16) were examined and, except for Mn- methanol, acetone and H O (<40%, Table 3, en-
(
ACHTUNGTRENNUNG
2
3
2
2
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OAc) which gave 76% yield, others showed product tries 5–10), opposite to the solvent effect of the
2
[
13]
yields less than 10%.
“CuCl-TEMPO” system
which exhibited much
Interestingly, this reaction was highly dependent on higher reactivity in polar solvents such as DMF, ace-
the type of bases. When Cs CO was replaced by tonitrile and H O. More importantly, we fould that
2
3
2
other metal carbonates such as Na CO , K CO and the polarity of the solvents is not the only factor for
2
3
2
3
Rb CO , the conversions of benzyl alcohol were the solvent effect in the “CuCl -Cs CO ” system. As
2
3
2
2
3
lower than 20% (Table 2, entries 2–4). Similar poor shown in Table 3, higher conversions were obtained
reactivity was observed using metal hydroxides such (93% and 94%, entries 1 and 2, respectively) in 1,2-di-
as NaOH, KOH and CsOH (Table 2, entries 5–7). chloroethane and toluene than in the other solvents.
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Aerobic Oxidation of Primary Alcohols Catalyzed by Copper Salts
Table 3. Solvent effect of the copper(II)-catalyzed aerobic The logarithm of the relative rates plotted against the
[a]
+
oxidation of benzyl alcohol.
s
value of the para-substituent resulted in a linear
relationship. Hammett 1 values of À0.55 in toluene
and À0.31 in 1,2-dichloroethane were obtained, which
are larger than the 1 value (À0.16) observed in
[
b]
[c]
Entry
Solvent
Conv. [%]
Yield [%]
1
2
3
4
5
6
7
8
9
1,2-dichloroethane
toluene
93
94
70
53
6
38
15
31
6
99
99
79
84
99
98
99
89
89
96
[
13]
“CuCl-TEMPO”, indicating a mechanism involving
the formation of more charge character in the transi-
tion state. The primary kinetic isotope effect (k /k )
2
CH Cl2
n-hexane
H
D
CH CN
3
for the CuCl /Cs CO -catalyzed aerobic oxidation of
2
2
3
THF
DMF
this alcohol at 258C was determined to be 1.45. This
value indicates CÀH bond cleavage on progressing to
CH OH
3
the transition state. Thus, a possible mechanism is
proposed. According to previous reports, the catalytic
reaction may be started from the deprotonated alco-
hol reacted with Cs CO . Through coordinating to the
acetone
H O
2
10
8
[
a]
Benzyl alcohol (1 mmol), CuCl ·2H O (0.01 mmol),
2
2
2
3
Cs CO (1.2 mmol) in solvent (5 mL), the mixture was
2
3
copper (II) center, alcoholsꢀ a-H could be abstracted
with 1e transfer and a new C=O bond formed. An
stirred in oxygen at 408C for 12 h.
Conversions were determined by GC with chlorobenzene
as internal standard.
Yields were calculated based on conversions, mass
balances >90%.
[
[
b]
active Cu(II) species would be regenerated from O2
c]
oxidizing the Cu(I) intermediate(s).
To extend the scope of the substrate, a series of al-
cohols was oxidized to the corresponding carbonyl
compounds under the optimized conditions and the
the higher reac- results are summarized in Table 4. All the primary al-
[
5,14]
According to the previous studies,
tivity in toluene was hypothesized for the interaction cohols could be selectively oxidized to aldehydes and
between copper ions and aromatic solvent. Similarly, the formation of over-oxidized carboxylic acids was
interactions between 1,2-dichloroethane and cesium not observed. Allylic and benzyl alcohols can be oxi-
cations had also been demonstrated by theoretical dized to the corresponding aldehydes in good conver-
[
15]
and crystallographic studies. Thus, we assumed that sions (>80%, Table 4, entries 1–7). An aliphatic pri-
the possible interactions between the solvents and mary alcohol showed much a lower conversion (50%,
copper and/or cesium cations might be another factor Table 4, entry 8). However, the secondary alcohols
for the solvent effect. Since CuCl and Cs CO are such as 2-phenylethanol (Table 4, entry 9) could not
2
2
3
not soluble in solvents such as 1,2-dichloroethane, tol- be oxidized to acetophenone. Interestingly, when both
uene and n-hexane, according to Markꢁꢀs catalytic the primary and secondary benzyl alcohols are pres-
[
5]
system
and the recently reported “NaAuCl4/ ent in one molecule such as 4-(1-hydroxyethyl)benzyl
[16]
Cs CO ” system, another role of Cs CO may be as alcohol (Table 4, entry 10), only the primary alcohol
2
3
2
3
a heterogeneous support.
was selectively oxidized and the secondary alcohol
Hammett plots were constructed by measuring the was left intact. An exception was observed for 2-hy-
rate of oxidation of various para-substituted benzyl droxy-1,2-diphenylethanone which was oxidized to
alcohols in toluene and 1,2-dichloroethane (Figure 1). the diketone in 72% yield together with 20% benzal-
dehyde (Table 4, entry 11). It is probably due to the
more acidic a-H which could be much more easily ab-
stracted than that of other secondary alcohols. We as-
sumed that the lack of observed reactivity of primary
alkyl alcohols and secondary alcohols may be due to
their greater aÀH bond strengths or/and steric effect
of the alkyl group for secondary alcohols compared
[
4,5,6e,8e,10]
with benzylic or allylic alcohols.
To further investigate the possible intermediate in
the “CuCl +Cs CO ” protocol in toluene or 1,2-di-
2
2
3
chloroethane, we attempted to study the reaction be-
tween CuCl (0.01 mmol), Cs CO (0.01 mmol) with
2
2
3
benzyl alcohol (0.5 mL) in toluene or 1,2-dichloro-
ethane (2 mL). Under these conditions, we isolated a
red crystalline solid (ca. 5%). Nevertheless, similar
treatments afforded only blue precipitates when tolu-
Figure 1. Hammett plots for the reaction of para-substituted ene or 1,2-dichloroethane was replaced by other sol-
benzyl alcohols in toluene and 1,2-dichloroethane.
vents. The same phenomenon was also observed when
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Table 4. Copper(II)-catalyzed aerobic oxidation of different
[a]
alcohols.
Entry Substrate
Product
Cat. Yield
%]
[
1
2
CuCl2 87
CuCl2 89
3
4
5
CuCl2 88
CuCl2 90
CuCl2 87
6
7
CuCl2 86
CuCl2 82
CuCl2 28
8
9
n-C H OH
n-C H CHO
1
0
21
9
19
CuCl2
0
1
1
0
1
CuCl2 80
CuCl2 72
Figure 2. (a) ORTEP structure of complex 1. (b) Crystal
parking structure complex 1. The triangles represent the tri-
copper clusters linked by Cl anions (black ball). Gray balls
represent Cs cations
[
[
a]
Alcohol (1 mmol), catalyst (0.01 mmol), Cs CO
2
3
(
1.2 mmol), toluene (5 mL), 408C, 12 h.
present the distorted octahedral geometry with two
b]
Isolated yield of the product after column chromatogra-
phy.
long CuÀCl bonds (2.773 ꢃ–2.852 ꢃ) for each copper
ion defining the Jahn–Teller distortion axis. The tri-
metric cluster is created by m -Cl bridges and by the
2
Cs CO was replaced by other metal carbonates. X- capping m -OH and m -Cl groups. The oxygen of the
2
3
3
3
ray diffraction quality crystals of the red species hydroxy group is located 0.846 ꢃ above the [Cu3]
complex 1) were obtained by slow diffusion of dilut- plane and the m -Cl (Cl1) is 1.993 ꢃ below the plane.
(
3
ed benzyl alcohol containing CuCl to toluene con- Crystal packing of 1 shows a that 2D coordination
2
4+
taining Cs CO . The molecular structure of complex 1 polymer is built from trinuclear [Cu ACHTUNGTRENUNNG( m -OH) ACHTNUTRGNEG(NU m -Cl)]
2
3
3 3 3
(
Figure 2, a) revealed that 1 consists of a trinuclear core units. The Cl2 atoms acts as m bridges and join
2
4+
[
Cu
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(m -OH)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(m -Cl)] core in which the copper ions, neighboring clusters by the Cu1ÀCl2ÀCu2A bond and
3
3
3
consisting of three independent copper ions linked by form a zig-zag chain. The double bridges Cu2ÀCl6À
m -OH, m -Cl and m -Cl bridges to a scalene triangle Cu3A and Cu3ÀCl6AÀCu2A hold the chain together
3
3
2
with the distances of 3.086 ꢃ (Cu1···Cu2), 3.102 ꢃ into a 2D framework. To the best of our knowledge,
[
17]
(
Cu2···Cu3), and 3.072 ꢃ (Cu3···Cu1).
The lengths structurally characterized trinuclear copper oxo or hy-
(m -OH)] moiety in 1,
of CuÀCu bonds are shorter than those supported by droxide systems, like the [Cu
AHCTUNGTRENNUNG
3
3
oxime-/oximate, N,N-pyrazole-, or N,N-triazole type were not unprecedented but are extremely rare with
[
18]
peripheral ligands (3.165–3.322 ꢃ) but much longer m2-Cl anion bridging. Moreover, complex 1 is hitherto
than those in the [Cu (m -O)] moiety (2.812– the only example of the hydroxide complex of a [Cu3
AHCTUNGTRENNUNG
3
3
[19]
2
.862 ꢃ) . All copper ions in the trimetric unit
ACHTUNGTNERNU(GN m -OH)] unit isolated from the “ligand-free” copper-
3
adopt an (OH)Cl5 coordination environment and catalyzed aerobic oxidation of alcohols.
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Aerobic Oxidation of Primary Alcohols Catalyzed by Copper Salts
Table 5. Complex 1 catalyzed aerobic oxidation of different
[a]
alcohols.
Entry Substrate
Product
Cat. Yield
%]
[
1
2
1
94
1
75
3
4
n-C H OH
n-C H CHO
1
1
15
0
1
0
21
9
19
5
1
86
Figure 3. Time-dependent course of the copper-catalyzed
aerobic oxidation of benzyl alcohol in toluene.
[a]
Alcohol (1 mmol), catalyst (0.01 mmol), Cs CO3
2
(
1.2 mmol), toluene (5 mL), 408C, 12 h.
[b]
Isolated yield of the product after column chromatogra-
phy.
From Figure 2, (b), we observed that Cs cations act
as counterions dispersed in the channels and between
layers of the 2D network, which shows that the dis- 5). Thus, we speculated such trinuclear [Cu ACHTUNGNRTEUNG( m -OH)-
3 3
4+
tances of the inter-ion contacts between Cs and Cl
ACHTUNGTNERNGNU( m -Cl)] intermediate was generated through cop-
3
varied from 3.422 to 3.996 ꢃ. The interaction between per(II) chloride with molecular oxygen and, more im-
Cl anions and Cs cations had been shown in the crys- portantly, acts as a entire catalytic center or dissoci-
tal of Cs CuCl and the distances between Cs and Cl ates to mononuclear copper active species to perform
2
4
[
20]
varied from 3.460 to 3.630 ꢃ. Although such an inter- aerobic oxidation of alcohols.
Such polynuclear
action was observed in the crystalline solid state of Cu(II) centers in multicopper oxidases like particulate
complex 1, it suggested that the electrostatic interac- methane monooxygenase (pMMO) and nitrous oxide
tion between Cs cations and the [Cu ACHTUNGRTNEUNG( m -OH)] moiety reductase were proposed to be important to catalyze
3 3
[
1]
or other copper intermediates might exist in solution. the four-electron reduction of molecular dioxygen.
Thus, the Cs cation plays a role stabilizing and “sol- Moreover, synthetic model complexes containing [Cu3
4+
vating” intermediates such as [Cu
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
ACHTUNGTNERNGNU( m -O/OH)] moieties had been demonstrated to be
3
3
3
unit through the interaction of Cl counterions and sol- active for oxidative coupling phenols by molecular
[
6,20]
vents like 1,2-dichloroethane and toluene.
oxygen.
Addition of 10 mol% TEMPO to the re-
To demonstrate the reactivity of trinuclear 1 con- action resulted in decrease of reactivity which resem-
4+
taining [Cu ACHTUNGTRENNUNG( m -OH) ACHTUNGERNNTU(G m -Cl)] moiety, we carried out bled that with CuCl as catalyst. These results indicat-
3 3 3 2
a time-course study of the aerobic oxidation of benzyl ed that complex 1 containing [Cu ACUTHNGTRENNUG(N m -OH) ACHTUNGTRENNUNG( m -Cl)]
3 3 3
alcohol catalyzed by 1 and copper chloride under the moiety is catalytically active towards oxidation of al-
optimized condition (Figure 3). An initial induction cohol and possesses similar reactivity in comparison
period (0–3 h) using copper chloride was observed to that using CuCl as catalyst.
2
whereas 1 exhibited a similar rate without induction
In summary, we have developed a new protocol of
period. Thus, formation of 1 may be the intermediate “CuCl -Cs CO ” system for aerobic oxidation of pri-
2
2
3
to the active specie(s) during the catalysis using CuCl2 mary alcohols. In such catalytic system, cesium car-
as catalyst. bonate and solvents such as toluene and 1,2-dichloro-
From Table 5, we found that 1 exhibited compara- ethane play important roles to form catalytically
4+
ble reactivity to copper chloride. For benzyl alcohol, active intermediate containing [Cu ACHTUNGTRENNNUG( m -OH) ACHTUNRTGNEG(NU m -Cl)]
3 3 3
allylic primary alcohol (R=Ph), 1 could catalytically moiety (complex 1). From the X-ray crystal structure
oxidize them to the corresponding aldehyde in similar of complex 1, the electrostatic interaction between Cl
yields (Table 5, entries 1 and 2) to that achieved with anions and Cs cations was observed, which stimulated
CuCl (Table 1, entry 1 and Table 4, entry 7). Oxida- us to understand the roles of Cs CO in this reaction
2
2
3
tion of aliphatic primary alcohol by 1 showed lower as a base, stabilizing and “solvating” intermediates in
yield (15%, Table 5, entry 3) than that with CuCl2 toluene and 1,2-dichloroethane. To our knowledge, al-
(
28%, Table 4, entry 8).
Chemoselective oxidation of primary alcohols were taining [Cu AHCTUNTGRENNUNG
though synthetic models of multicopper oxidases con-
(m -O/OH)] moieties such as ascorbate
3
3
also observed with 1 as catalyst (Table 5, entries 4 and oxidase, laccase, ceruloplasmin and particulate meth-
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Lei Liang et al.
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ane monooxygenase (pMMO) have been extensively
studied, few of them have been employed as catalysts
in oxidations. The structural information we obtained
may provide a unique avenue toward further design
of polynuclear copper catalysts mimicking the active
metal sites of multicopper oxidases. Detailed mecha-
nism is under investigation and further studies are
aimed at improve this system to become more appli-
cable to different varieties of alcohols.
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