Aerobic oxidation of primary alcohols catalyzed by copper complexes of 1,10-phenanthroline-derived ligands

Article abstract of DOI:10.1039/c2dt31134h

Five copper complexes [(L¹)₂Cu(H₂O)] (ClO₄)₂ (1), [(L¹)Cu(H₂O) ₃](ClO₄)₂ (1a), [(L³) ₂Cu(H₂O)](ClO₄)₂ (2), [(L ⁵)₂Cu(H₂O)](ClO₄)₂ (3) and [(L⁶)₂Cu](ClO₄) (4) (where L¹ = 1,10-phenanthroline, L³ = 1,10-phenanthroline-5,6-dione, L ⁵ = 1,10-phenanthrolinefuroxan and L⁶ = 2,9-dimethyl-1,10-phenanthrolinefuroxan), and in situ prepared copper complexes of 2,9-dimethyl-1,10-phenanthroline (L²) or 2,9-dimethyl-1,10- phenanthrolinedione (L⁴) were used for aerial oxidation of primary alcohols to the corresponding aldehydes under ambient conditions. The copper catalysts have been found to catalyze a series of primary alcohols including one secondary alcohol with moderate turnover numbers and selectivity towards primary alcohols. Copper(ii) complexes 1 (or 1a) and 2 were found to be the better catalysts among all other systems explored in this study. A copper(ii)-superoxo species is implicated to initiate the oxidation reaction. Structural and electronic factors of 1,10-phenanthroline-based ligands affecting the catalytic results for aerial oxidation of alcohols are discussed.

Full text of DOI:10.1039/c2dt31134h

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PAPER
Aerobic oxidation of primary alcohols catalyzed by copper complexes of
1,10-phenanthroline-derived ligands
Oindrila Das and Tapan Kanti Paine*
Received 25th May 2012, Accepted 31st July 2012
DOI: 10.1039/c2dt31134h
Five copper complexes [(L1)2Cu(H2O)](ClO4)2 (1), [(L1)Cu(H2O)3](ClO4)2 (1a), [(L3)2Cu(H2O)](ClO4)2
(2), [(L5)2Cu(H2O)](ClO4)2 (3) and [(L6)2Cu](ClO4) (4) (where L1 = 1,10-phenanthroline,
L3 = 1,10-phenanthroline-5,6-dione, L5 = 1,10-phenanthrolinefuroxan and L6 = 2,9-dimethyl-1,10-
phenanthrolinefuroxan), and in situ prepared copper complexes of 2,9-dimethyl-1,10-phenanthroline (L2)
or 2,9-dimethyl-1,10-phenanthrolinedione (L4) were used for aerial oxidation of primary alcohols to the
corresponding aldehydes under ambient conditions. The copper catalysts have been found to catalyze a
series of primary alcohols including one secondary alcohol with moderate turnover numbers and
selectivity towards primary alcohols. Copper(II) complexes 1 (or 1a) and 2 were found to be the better
catalysts among all other systems explored in this study. A copper(II)-superoxo species is implicated to
initiate the oxidation reaction. Structural and electronic factors of 1,10-phenanthroline-based ligands
affecting the catalytic results for aerial oxidation of alcohols are discussed.
mechanism of alcohol oxidation is controlled by the Cu2+/Cu+
potential of the catalyst. This potential can be tuned in copper(II)
Introduction
Controlled oxidation of primary alcohol to aldehyde is con-
sidered as one of the important reactions in organic chemistry.
This challenge has fuelled the interests of chemists in developing
catalytic systems that can selectively oxidize alcohols to the cor-
responding carbonyl compounds using aerial oxygen as the
terminal oxidant. Copper has drawn the attention as metal of
choice for the catalytic oxidation of alcohols because of its pres-
ence in the active site of galactose oxidase (GO), an enzyme that
couples the oxidation of primary alcohols to aldehydes with con-
complexes of ligands derived from 1,10-phenanthroline by elec-
tronic and structural tuning on the ligand backbone. Solvents
also play an important role in the redox reactions. A varying
degree of catalytic reactivity of copper(II) complexes of functio-
nalized 1,10-phenanthrolines is expected that would provide the
mechanistic insight about the oxidation reaction. We report in
this paper, a series of mononuclear copper complexes of phenan-
throline-derived ligands (Chart 1) that serve as catalysts in aerial
oxidation of alcohols under ambient conditions. Four copper(II)
complexes [(L1)2Cu(H2O)](ClO4)2 (1), [(L1)Cu(H2O)3](ClO4)2
(1a), [(L3)2Cu(H2O)](ClO4)2 (2), [(L5)2Cu(H2O)](ClO4)2 (3) and
one Cu(I) complex [(L6)2Cu](ClO4) (4) have been used for
alcohol oxidation. The effect of different functional groups on
1,10-phenanthroline backbone controlling the catalytic reactivity
is discussed. Complexes 1 and 2 oxidize primary alcohols
including aromatic, heterocyclic and unsaturated alcohols to the
comitant reduction of dioxygen to hydrogen peroxide.1–4
A
number of biomimetic copper complexes of polydentate ligands
have been developed as functional models of GO.5–14 Copper
complexes of pyridine-based ligands like 2,2′-bipyridine and
1,10-phenanthroline have been used for oxidation of primary
alcohols.15–19 Merko et al. reported very active catalytic systems
comprising of CuCl, 1,10-phenanthroline and hydrazine deriva-
tives for aerobic oxidation of allylic and benzylic alcohols.20–23
Copper complexes of various nitrogen donor ligands in combi-
nation with TEMPO have been developed as catalysts for
alcohol oxidation.24–31 Efficiency of TEMPO based catalytic
systems leads to design a ligand system containing TEMPO on
its backbone, which exhibits remarkable catalytic activity.32 It
has been proposed that TEMPO radical takes part in the H-atom
abstraction process,33 similar to that by a copper(II)-superoxide
species proposed in the reaction mechanism of GO. The
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur,
Kolkata-700032, India. E-mail: ictkp@iacs.res.in;
Fax: +91-33-2473-2805; Tel: +91-33-2473-4971
Chart 1 Ligands used in this study.
11476 | Dalton Trans., 2012, 41, 11476–11481
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corresponding aldehydes including one secondary alcohol to the
corresponding ketone with moderate yield.
room temperature. A green solid was precipitated from the sol-
ution after stirring for 12 h. The solid was isolated by filtration
and dried. The filtrate was kept for slow evaporation of solvent
to isolate another portion of green solid. Yield: 0.25 g (71%).
Anal Calcd for C24H14Cl2CuN4O13 (700.83 g mol−1): C, 41.13;
H, 2.01; N, 7.99. Found: C, 41.20; H, 2.03; N, 8.10%. FTIR
(KBr): 3421(br), 3068(w), 1699(s), 1575(s), 1483(m), 1454(m),
1427(s), 1371(m), 1301(w), 1259(w), 1109–1041(vs), 816(m),
725(m), 625(m) cm−1. UV-Vis (in MeCN) [λmax, nm (ε, M−1
cm−1)]: 682 (48), 485 (53), 360 (sh), 313 (13 706), 302
(14 795). ESI-MS (positive ion in MeCN): m/z = 500.85 (20%,
[(L3)2Cu(H2O)]+), 482.84 (100%, [(L3)2Cu]+), 232.96 (30%,
[L3 + Na]+), 210.97 (15%, [L3 + H]+).
Experimental
General
All chemicals used for the synthetic purposes were purchased
from commercial sources. Solvents were distilled and dried prior
to use unless otherwise stated. Ligands L1, L2 and L3 are com-
mercially available. Ligand L4 was prepared according to a
modification of the procedure described in the literature.34,35
Syntheses and characterizations of complexes [(L5)2Cu(H2O)]-
(ClO4)2 (3) and [(L6)2Cu](ClO4) (4), and ligands L5 and L6 have
been reported from our group.36,37 Fourier transform infrared
spectra were recorded on a Shimadzu FT-IR 8400S instrument.
Elemental analyses were performed on a Perkin Elmer 2400
series II CHN analyzer. Solution electronic spectra were
measured at room temperature on an Agilent 8453 diode array
spectrophotometer. Electro-spray Ionization Mass spectra were
recorded with Waters QTOF Micro YA263. Gas chromatographic
data were recorded in an Agilent 6890N Network GC system.
X-band EPR measurements were performed on a JEOL JES-FA
200 instrument.
Catalytic oxidation of alcohols. Copper catalyst (0.01 mmol)
was dissolved in 5 mL acetonitrile. To that, 1 mmol alcohol and
0.5 mmol tetrabutylammonium methoxide solution (20% solu-
tion in methanol) were added. The reaction mixture was
allowed to stir under oxygen at room temperature. After the reac-
tion, the solution was passed through a silica gel column of
60–120 mesh and washed with 15 mL acetonitrile. To the eluent
0.5 mL standard solution of naphthalene was added and the pro-
ducts were quantified by GC.
Results and discussion
Synthesis
All the ligands and their copper complexes were purified and
characterized prior to use. In the oxidation of alcohols using O2
as the oxidant, either copper(II) complexes of phenathroline-
based ligands or combinations of copper(II) salt and ligands were
used (Fig. 1). All six ligands, in the presence of copper(II) per-
chlorate and a base in acetonitrile, were found to oxidize primary
alcohols to the corresponding aldehydes under ambient con-
ditions. The oxidized products were isolated by filtration through
silica gel column and were analysed by gas chromatography
using an internal standard. It is to note that in the absence of any
ligand, only copper(II) in acetonitrile is able to oxidize benzyl
alcohol to benzaldehyde in a stoichiometric amount, whereas
benzyl alcohol remains unreacted in the absence of copper(II)
salt in the system (Table 1). The binding of alcoholate to the
copper(II) centre does not take place in the absence of a base
which is confirmed from the stoichiometric conversion in the
[(L1)2Cu(H2O)](ClO4)2 (1). 1,10-Phenanthroline monohydrate
(0.198 g, 1 mmol) was reacted with Cu(ClO4)2·6H2O (0.185 g,
0.5 mmol) in dichloromethane at room temperature. The result-
ing green solution was stirred at room temperature for 12 h. The
solution was then filtered and kept for slow evaporation of
solvent to isolate a light blue solid. Yield: 0.18 g (56%). Anal
Calcd for C24H18Cl2CuN4O9 (640.87 g mol−1): C, 44.98; H,
2.83; N, 8.74. Found: C, 45.09; H, 2.92; N, 9.02%. FTIR (KBr):
3439(br), 3068(w), 2924(w), 1627–1587(w), 1521(m), 1431(s),
1342(w), 1147–1093(vs), 848(s), 721(s), 623(s) cm−1. UV-Vis
(in MeCN) [λmax, nm (ε, M−1 cm−1)]: 910, 725 (52), 345 (sh),
325 (sh). ESI-MS (positive ion in MeCN): m/z = 422.96 (60%,
[(L1)2Cu]+), 181.02 (75%, [L1 + H]+).
[(L1)Cu(H2O)3](ClO4)2 (1a). 1,10-Phenanthroline monohy-
drate (0.198 g, 1 mmol) was reacted with Cu(ClO4)2·6H2O
(0.37 g, 1 mmol) in acetonitrile at room temperature. After 12 h
the blue solution was filtered and the filtrate was kept for slow
evaporation of solvent to isolate a blue solid. Yield: 0.29 g
(58%). Anal Calcd for C12H14Cl2CuN2O11 (496.70 g mol−1): C,
29.02; H, 2.84; N, 5.64. Found: C, 28.54; H, 2.75; N, 5.39%.
FTIR (KBr): 3437(br), 3051(w), 2924(w), 1628–1583(w), 1516
(m), 1423(s), 1348(w), 1144–1088(vs), 852(s), 719(s), 630(s)
cm−1. UV-Vis (in MeOH) [λmax, nm (ε, M−1 cm−1)]: 702 (36),
345 (sh), 330 (sh). ESI-MS (positive ion in MeCN): m/z =
521.87 (22%, {[(L1)2Cu](ClO4)}+), 482.50 (5%, [(L1)2Cu(H2O)-
(CH3CN)]+), 341.85 (100%, {[(L1)Cu](ClO4)}+), 242.93 (98%,
[(L1)Cu]+).
[(L3)2Cu(H2O)](ClO4)2 (2). To a solution of 1,10-phenanthro-
line-5,6-dione (0.21 g, 1 mmol) in dichloromethane, Cu-
(ClO4)2·6H2O (0.185 g, 0.5 mmol) was added with stirring at
Fig. 1 Different copper complexes used for the oxidation of alcohols.
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reaction. Therefore, a combination of ligand, copper(II) salt and a
base is needed to carry out the catalytic aerobic oxidation of
alcohols.
under nitrogen atmosphere where sub-stoichiometric amount of
benzaldehyde was observed.
With this standardized condition the catalytic oxidations of
alcohols were carried out with other phenanthroline-derived
ligands. Table 2 shows the catalytic conversion of benzyl alcohol
in the reaction with different 1,10-phenanthroline ligands. A
good correlation of catalytic yield with the structural and elec-
tronic properties of the ligands can be made. In general, ligands
derived from 2,9-dimethyl-1,10-phenanthroline (L2, L4 and L6)
show relatively poor yield of aldehyde compared to other three
systems. Additionally, no appreciable change in TON was
observed by changing the ratio of Cu2+ salt and ligand.
To standardize the reaction conditions, a set of catalytic exper-
iments were carried out using Cu(ClO4)2·6H2O and L3 in a 1 : 1
ratio (catalyst), 100 equivalents of benzyl alcohol and 50 equiva-
lents of tetrabutylammonium methoxide (TBAM). The resulting
solution was allowed to stir under air (O2) and the formation of
benzaldehyde was monitored with time (Fig. 2a). The conversion
of alcohol to aldehyde increases with time up to 7 h; thereafter
no further oxidation was observed. To see the effect of base,
catalytic reactions were carried out with different concentration
of TBAM. The highest conversion (turnover number, TON) of
alcohol was observed in the presence of 50 mol% of base
(Fig. 2b).
The effectiveness of the catalytic system was checked with
increasing concentration of the substrate under similar reaction
conditions. From the plot of concentration of benzyl alcohol vs.
TON, (Fig. 3) a sharp increment in TON was observed with
increasing alcohol concentration until it reaches a value of 236
in the presence of 1250 equivalents of benzyl alcohol. Upon
further increment of alcohol concentration, the catalytic TON
does not increase. The involvement of aerial oxygen in the cata-
lytic oxidation was established by carrying out the reaction
The catalytic alcohol oxidations were performed with isolated
copper(II) complexes (1, 1a and 2) of L1 and L3. Attempts were
made to isolate the copper(II) complexes with the metal/ligand
ratio of 1 : 1 and 1 : 2 in acetonitrile. Except for L1, all other
Table 1 Effect of different components on catalytic oxidation of
benzyl alcohol
Benzyl
alcohol
(mmol)
L3
(mmol)
Cu(ClO4)2·6H2O
(mmol)
TBAM
(mmol)
% Yield of
benzaldehydea
0.01
—
0.01
0.01
—
1
1
1
1
1
0.5
—
—
0.01
0.01
0.01
4.5
1.9
55
0.5
a Yield of benzaldehyde was calculated from GC by comparing the peak
area with respect to that for internal standard naphthalene.
Fig. 3 Dependence of catalytic TON on the amount of benzyl alcohol.
Reaction conditions: 0.01 mmol catalyst in the presence of 0.5 mmol
TBAM in acetonitrile for 7 h at 25 °C.
Fig. 2 (a) Rate of catalytic oxidation of benzyl alcohol. Reaction conditions: 0.01 mmol catalyst, 1 mmol benzyl alcohol and 0.5 mmol TBAM in
acetonitrile for 7 h at 25 °C. (b) Dependence of catalytic TON on the amount of base. Reaction conditions: 0.01 mmol catalyst, 1 mmol benzyl alcohol
in acetonitrile for 7 h at 25 °C.
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Table 2 Catalytic oxidative conversion of benzyl alcohol to
Table 3 Catalytic oxidation of different alcohols by 1 and 2
benzaldehyde using different phenanthroline-based copper catalysts
TON (Time)
1
Catalyst
TON
Entry
1
Substrate
Product
2
Cu2+ : L1 (1 : 1)
67
65
48
55
54
24
62
62
67
47
11
Cu2+ : L1 (1 : 2)
62 (7 h)
67 (7 h)
Cu2+ : L2 (1 : 2)
Cu2+ : L3 (1 : 1)
Cu2+ : L3 (1 : 2)
Cu2+ : L4 (1 : 2)
2
38 (72 h)
30 (48 h)
[(L1)2CuII(H2O)](ClO4)2 (1)
[(L1)CuII(H2O)3](ClO4)2 (1a)
[(L3)2CuII(H2O)](ClO4)2 (2)
[(L5)2CuII(H2O)](ClO4)2 (3)
[(L6)2CuI](ClO4) (4)
3
4
5
33 (7 h)
58 (27 h)
72 (7 h)
39 (7 h)
47 (27 h)
60 (7 h)
ligands form 1 : 2 complexes irrespective of the metal/ligand
ratio and reaction conditions. The copper(II) complex of L1 (1a)
with the metal/ligand ratio of 1 : 1 could be isolated from aceto-
nitrile. Copper(II)-1,10-phenanthroline complexes with metal/
ligand ratio of 1 : 1 and coordinated solvents are known in the lit-
erature.38,39 Catalytic reactions with 1 (metal : L1 = 1 : 2) and 1a
(metal : L1 = 1 : 2) exhibit almost similar turnover numbers. The
experimental results indicate a 1 : 1 copper(II) complex as the
active complex that reacts with alcohol to affect catalytic oxi-
dation. One of the ligands in complex 1 most likely gets disso-
ciated in solution and acetonitrile or water (in solvent)
coordinates to the copper(II) center. However, reaction with 2
shows a higher value of TON than that obtained in the reaction
when L3 and copper(II) perchlorate are used separately. Here
again a combination of 1 : 1 metal : ligand ratio and 1 : 2 afford a
similar TON. Therefore 1 and 2 were used for the catalytic oxi-
dation of a few other primary alcohols including the secondary
alcohol (Table 3). The catalytic TONs obtained in the oxidation
of different alcohols were found to be moderate. The reaction of
2 with a mixture (1 : 1) of benzyl alcohol and 2-phenyl ethanol
shows the formation of 62% benzaldehyde and 10% acetophe-
none. The catalyst is therefore selective towards the oxidation of
primary alcohols. Of note, 6-methyl-2-pyridine methanol (entry
3 in Table 3) is observed to afford poor yield of the oxidized
product. A blue shift of the d–d band in the optical spectrum of
[(L1)CuII(H2O)3](ClO4)2 (1a) from 702 nm to 595 nm is
observed upon addition of a basic solution of 6-methyl-2-pyri-
dine methanol. This supports the binding of 6-methyl-2-pyridine
methanol to the copper(II) center. The binding of the substrate
leads to a thermodynamically stable copper(II) complex which
remains inert towards oxygen activation.
The electronic effects of different substituent on the catalytic
yields were tested with substituted benzyl alcohols (Table 3,
entries 5 to 8). It is found that 3-methoxy benzyl alcohol exhibits
excellent yield of the corresponding aldehyde in the presence of
both 1 and 2. The yield of aldehyde is decreased with substrate
having electron withdrawing substituents (Table 3, entries 6 and
7). This result further supports the electronic effect on the
efficiency of the catalyst. In case of 4-hydroxy benzyl alcohol,
very poor yield of the corresponding aldehyde may be explained
on the basis of the coordination of phenolate oxygen to the
copper(II) center. It is worth mentioning here that aliphatic
primary and secondary alcohols are not oxidized by the copper(II)
complexes under similar experimental conditions.
6
7
40 (7 h)
42 (7 h)
16 (24 h)
42 (24 h)
8
8 (24 h)
9 (24 h)
Fig. 4 Optical spectra of (a) 2 (0.5 mM solution in MeCN–THF (1 : 1)
solvent mixture), (b) after addition of a basic solution of benzyl alcohol
into 2 under nitrogen and (c) solution after 7 h under inert atmosphere
at 25 °C.
Mechanistic studies
To get an insight into the mechanism of catalytic oxidation of
alcohols, the reaction of 2 (0.5 mM in acetonitrile–tetrahydro-
furan (1 : 1) solvent mixture) with 10 equivalents of benzyl
alcohol and TBAM was monitored spectrophotometrically under
nitrogen atmosphere at room temperature (Fig. 4). Strong metal-
to-ligand charge transfer bands at 430 nm and 550 nm are gener-
ated in the visible region. The charge-transfer bands are
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Fig. 5 X-band EPR spectra of (a) complex 2, (b) after addition of a
basic solution of benzyl alcohol into 2, (c) solution after 7 h under nitro-
gen in a frozen MeCN–THF (1 : 1) solvent mixture at 123 K.
characteristic of a copper(I) complex supported by phenanthro-
line-based ligand.19 The solution is stable under nitrogen but
changes to green upon exposure to air. This result clearly
suggests the reduction of copper(II) to copper(I) in the presence
of alcohol under an inert atmosphere.
Scheme 1 Proposed mechanism for catalytic aerobic oxidation of alco-
hols by copper complexes of phenanthroline-based ligands.
To characterize the species generated under the reaction con-
ditions discussed above, X-band EPR spectra were recorded in
frozen solution at 123 K (Fig. 5). EPR spectrum of the mono-
nuclear copper(II) complex (2), exhibiting four-line copper
species (B), which abstracts the α-H atom of coordinated alcoho-
late.5 It is important to mention here that the presence of
TEMPO radical does not affect the yield of benzaldehyde in the
catalytic oxidation, suggesting the involvement of an intramole-
cular H-atom abstraction step. Theoretical study of H-atom
abstraction by TEMPO in alcohol oxidation suggest a pathway
where the overlap of HOMO of alcoholate to π* LUMO of
TEMPO is proposed.33 Similarly, a copper(II)-superoxide orbital
is proposed to interact with an accessible HOMO of alcoholate
in the abstraction of α-H atom. Moreover, the presence of 10
equivalents of TEMPOH, an external superoxide intercepting
reagent, lowers the yield of benzaldehyde from 67% to 51%.
This confirms the abstraction of H-atom from alcoholate by a
copper(II)-superoxide species to form a copper(II)-hydroperoxo
species. An electronic rearrangement of copper(II)-hydroperoxide
(C) species results in the formation of aldehyde and hydrogen
peroxide. The hydrogen peroxide thus produced gets decom-
posed in solution in the presence of copper complex. The result-
ing copper(I) complex then undergoes further oxygen activation
for catalytic cycle.
hyperfine pattern with g⊥ = 2.06, g = 2.28, and A = 142 ×
k
k
10−4 cm−1, changes upon addition of a basic solution of benzyl
alcohol. The new EPR spectrum shows axially symmetric
copper(II) center with g⊥ = 2.03, g = 2.20, A⊥ = 47 × 10−4 cm−1
k
and A = 160 × 10−4 cm−1, suggesting the formation of a new
k
copper(II) complex with dx2−y2 ground state. The g /A value of
k
k
137 predicts a distorted square planar geometry at the copper(II)
center.40 The intensity of the EPR signal attains a maximum
value after 4 h. The EPR parameters of the new copper(II) signal
imply an interaction/binding of alcoholate with the metal centre.
Interestingly, the EPR signal of the copper(II) disappears after
7 h. The EPR spectral changes confirm the formation of a
copper(I) complex upon treatment of the copper(II) complex (2)
with a basic solution of benzyl alcohol and the results are in line
with the optical spectral observation. The copper(I)-alcoholate
species activate molecular oxygen to initiate the oxidation of
alcohol.
The activation of dioxygen by copper(I) complex is further
supported by the lower yield of aldehyde with sterically demand-
ing 2,9-dimethyl-1,10-phenanthroline-based catalytic systems
(Table 2). These ligands prefer to stabilize copper in +1 state
with a tetrahedral geometry and thereby increase the Cu+/Cu2+
potential. Additionally, 1,10-phenanthrolines, being good
π-acceptor, decrease the electron density at the metal center and
thereby make the complexes less susceptible for dioxygen acti-
vation. This effect is more prominent in the oxidation of alcohol
with two-electron oxidized furoxan ligands (L5 and L6) that
show low yield of aldehyde.
Conclusions
We have reported the catalytic oxidation of primary alcohols to
the corresponding aldehydes by copper complexes of phenan-
throline-derived ligands using dioxygen as the terminal oxidant.
A series of primary alcohols including one secondary alcohol
were oxidized where the copper complexes show selectivity
towards primary alcohols in the catalytic oxidation. A copper(II)-
superoxo species has been proposed in the reaction mechanism
to initiate the catalytic reaction. The electronic property of the
supporting ligand has been shown to affect the catalytic
efficiency of the copper catalysts. Copper(II) complexes of
On the basis of the above results, a mechanism for the cataly-
tic oxidation is proposed in Scheme 1. The copper(I)-alcoholate
species (A) activates O2 to form a putative copper(II)-superoxo
11480 | Dalton Trans., 2012, 41, 11476–11481
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16 X. Liu, A. Qiu and D. T. Sawyer, J. Am. Chem. Soc., 1993, 115, 3239–
3243.
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18 H. Korpi, P. Lahtinen, V. Sippola, O. Krause, M. Leskelä and T. Repo,
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electron rich 1,10-phenathroline derivative would probably be
better catalysts for aerial oxidation of alcohols. Further structural
and electronic tuning by functionalization of 1,10-phenanthroline
with electron-donating groups towards the development of a
more efficient transition metal catalyst for aerial oxidation of
alcohols is in progress.
21 I. E. Marko, M. Tsukazaki, P. R. Giles, S. M. Brown and C. J. Urch,
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Acknowledgements
We are grateful to the Department of Science and Technology
(DST), Government of India for the financial support (Project:
SR/SI/IC-51/2010).
24 P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon, Chem.
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