A mononuclear copper(ii) complex immobilized in mesoporous silica: An efficient heterogeneous catalyst for the aerobic oxidation of benzylic alcohols

Article abstract of DOI:10.1039/c3ra41417e

The diamide ligand 2,6-bis[(N-phenyl)amido]-4-methylphenol (HL) has been used to synthesize a mononuclear copper(ii) complex, [CuL₂(DMF) ₂] (1). The X-ray crystal structure of this complex reveals that the copper(ii) center is coordi

Full text of DOI:10.1039/c3ra41417e

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A mononuclear copper(II) complex immobilized in
mesoporous silica: an efficient heterogeneous catalyst
for the aerobic oxidation of benzylic alcohols3
Cite this: RSC Advances, 2013, 3,
1
9455
a
a
a
b
Suvendu Samanta, Sudipto Das, Partha Kumar Samanta, Supriya Dutta*
and Papu Biswas*
a
The diamide ligand 2,6-bis[(N-phenyl)amido]-4-methylphenol (HL) has been used to synthesize a
mononuclear copper(II) complex, [CuL (DMF) ] (1). The X-ray crystal structure of this complex reveals
2
2
that the copper(II) center is coordinated to the two phenoxides and two amide carbonyl oxygens of the HL
ligand and additionally to the amide carbonyl oxygens of two dimethylformamide molecules. This complex
was also characterized using UV-Vis and EPR spectroscopy, along with thermal and electrochemical
analysis. Complex 1 was then immobilized in a 2-D mesoporous silica matrix. This immobilized complex, 1a,
Received 22nd March 2013,
Accepted 30th July 2013
has been characterized using UV-Vis and FT-IR spectroscopy, as well as small angle X-ray diffraction, N
2
sorption and TEM. The immobilized complex was found to heterogeneously catalyze the aerobic oxidation
of benzylic alcohols to the corresponding aldehydes in the presence of TEMPO, with high turnover
numbers and good recyclability for up to four cycles.
DOI: 10.1039/c3ra41417e
www.rsc.org/advances
molecular oxygen as the oxidant. Recently, copper complexes
have attracted significant attention for the catalytic oxidation
Introduction
Oxidation reactions play a significant role in organic synthesis
and presently there is a demand for more selective and
efficient oxidation methods. Among oxidation reactions, the
oxidation of alcohols to the corresponding carbonyl com-
pounds is one of the most important and fundamental
reactions in organic synthesis. Numerous oxidizing reagents
have been exploited for this purpose. However, most of these
reagents are required in stoichiometric quantities, which is
not only expensive, but sometimes also toxic in nature.
Therefore, from an environmental and economic perspective,
catalytic oxidation processes are promising methods. In recent
years there has been a significant growth in the field of aerobic
of alcohols, since they are present in the active site of galactose
11
oxidase (GO) and several copper enzymes are also known to
activate dioxygen for highly selective transformations under
11
mild reaction conditions.
Copper–phenanthroline complexes were observed to react
12
with methanol as early as 1966. Since then, the copper
complexes of 1,10-phenanthroline (phen) and 2,29-bipyridine
(
bipy) have been thoroughly investigated as catalysts in the
13
oxidation of different alcohols. Mark ´o et al. reported the
efficient aerobic oxidation of alcohols to aldehydes and
ketones in the presence of a copper catalyst comprised of
CuCl, 1,10-phenanthroline and di-tert-butylhydrazine-1,2-
1
oxidation catalysis. The use of molecular oxygen as the
14
2
dicarboxylate (DBADH ). Semmelhack et al. introduced a
primary oxidant has several benefits, such as lower costs,
improved safety, higher abundance and ‘‘green’’ methods with
water as the sole by-product. Many transition metals such as
combined CuCl–2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)–
system for the oxidation of benzylic and allylic alcohols to
O
2
the corresponding carbonyl compounds at room tempera-
2
3
4
5
6
vanadium, cobalt, molybdenum, ruthenium, rhodium,
15
ture. More recently, several other catalytic systems based on
7
8
9
10
palladium, nickel, osmium and polyoxometalates have
been employed as catalysts for the oxidation of alcohols with
TEMPO and a copper complex for the aerobic oxidation of
16
alcohols to aldehydes or ketones have also been reported.
However, most of these catalytic processes are homoge-
neous and suffer from a serious limitation, namely, the
separation and recovery of the catalyst. A catalyst with a high
recycling capacity should require minimum effort for its
separation and is the most desirable for industrial processes.
Therefore, heterogeneous alternatives have attracted a lot of
attention because heterogeneous catalysis permits the easy
separation and reuse of the catalyst. This methodology
a
Department of Chemistry, Bengal Engineering and Science University, Shibpur,
Howrah - 711 103, India
b
Department of Chemistry, Budge Budge Institute of Technology, Nischintapur, Budge
Budge, Kolkata - 700 137, India. E-mail: papubiswas_besus@yahoo.com;
Fax: +0091-33-2668 4564; Tel: +0091-33-2668 4561
3
Electronic supplementary information (ESI) available: X-ray crystallographic
file for compound 1 and Fig. S1–S6. CCDC reference number 912042. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ra41417e
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represents a powerful tool in chemistry, allowing extremely
complex chemical transformations to take place in a cleaner,
was performed using a Perkin Elmer Diamond TG thermal
analyzer under a flow of nitrogen at a heating rate of 10 uC
1
7–19
21
more efficient, one-pot process.
The immobilization of
min . Electrochemical measurements were carried out in
transition metal-based catalysts on different solid supports
N,N-dimethylformamide (DMF) at 25 uC under a nitrogen
atmosphere using a Bioanalytical Systems BAS 100B electro-
chemical analyzer. The concentration of the supporting
electrolyte, tetramethylammonium perchlorate (TEAP), was
2
0,21
22
23–25
e.g., silica,
vated carbons,
polymers, mesoporous materials,
acti-
2
6,27
28
29
pillared clays and zeolites, has been
achieved effectively. Recently, a Cu nanoparticle–polyacryla-
mide/SBA-15 (3-D hexagonal mesoporous silica) hybrid has
been reported as a recyclable catalyst for the aerobic oxidation
0.1 M, while that of the complex was 1 mM. Cyclic
voltammetric (CV) and square wave voltammetric (SWV)
measurements were carried out using a three-electrode
assembly comprising either a glassy carbon or platinum
working electrode, a platinum auxiliary electrode, and an
aqueous Ag/AgCl reference electrode. Under the given experi-
mental conditions, the potential of the external standard
3
0
of alcohols. Previously, the recovery of the catalysts in the
above reactions have been achieved by using non-traditional
3
1a
31b
perfluorinated solvents
and ionic liquids,
while copper–
salen based catalysts have been recovered from aqueous
3
2
33
media and heterogeneous Cu–Mn oxides.
+
ferrocene/ferrocenium (Fc/Fc ) couple was measured at +0.390
We report here the synthesis and characterization of a
mononuclear copper(II) complex (1) derived from a new amide
based ligand 2,6-bis[(N-phenyl)amido]-4-methylphenol (HL).
This mononuclear copper(II) complex was immobilized in
MCM-41 type 2D-hexagonal mesoporous silica (HMS).
Absorption spectroscopy (UV-Vis-NIR), Fourier transform
infrared spectroscopy (FTIR), powder X-ray diffraction
V vs. Ag/AgCl. X-band EPR measurements were carried out
using a JEOL JES-FA 200 instrument. Powder X-ray diffraction
(
XRD) patterns were obtained using a Philips PW 1140 parallel
beam X-ray diffractometer with a Bragg–Bretano focusing
geometry and monochromatic Cu-Ka radiation (l
.540598 Å). N -sorption isotherms were obtained using a
=
1
2
Quantachrome Instruments analyzer at 77 K. Gas chromato-
graphic analyses were conducted using an Agilent
Technologies 6890 N network GC system equipped with a
fused silica capillary HP–5 column (30 m 6 0.32 mm) and a
FID detector. The EDS spectra were obtained using a Horiba
EDS analyzer attached to Hitachi S-3400 N scanning electron
microscope (SEM). Transmission electron microscopy (TEM)
images and selected area diffraction (SAED) patterns were
collected using a JEOL JEM-2100 microscope working at 200
kV.
(
(
PXRD), high resolution transmission electron microscopy
HRTEM) and X-ray photoelectron spectroscopy studies have
been used to characterize the material. The immobilized
catalyst was employed for the catalytic aerobic oxidation of
primary alcohols to the corresponding aldehydes using
TEMPO as co-catalyst under heterogeneous conditions. The
results show that the conversions of the primary alcohols
occur in high yield and are totally selective towards the
aldehydes. Our catalyst also offers an extra advantage as it
does not require the use of any additives, which are necessary
in most of the reported homogeneous methods. In addition,
the separation and recovery of the catalyst in this hetero-
geneous catalytic system can be performed very easily by either
centrifugation or filtration.
Synthesis of 2,6-bis[(N-phenyl)amido]-4-methylphenol (HL)
2,6-Dicarboxylic-4-methyl phenol (0.98 g,
5 mmol) was
3
dissolved in 25 cm dry dichloromethane in a three-necked
3
round-bottom flask. Oxalyl chloride (1.1 cm , 12.5 mmol) was
added via syringe through a septum followed by the addition
of 2 drops of dimethylformamide (DMF) under a flow of
nitrogen. The resulting mixture was stirred at room tempera-
ture for 3 h and then concentrated under vacuum. The
resulting crude, pale yellow acid chloride was redissolved in 25
Experimental
Materials
3
cm dry dichloromethane for further reaction without any
Reagent grade chemicals were obtained from commercial
sources and were used as received. The solvents were purified
purification. In a separate three-necked round-bottom flask,
3
3
4
20 cm dry dichloromethane, triethylamine (1.26 g, 12.5
and dried according to standard protocols. 2,6-Diformyl-4-
3
5
36
mmol) and aniline (1.03 g, 11.0 mmol) were mixed under a
flow of nitrogen. The crude acid chloride dissolved in dry
dichloromethane was then added slowly to this mixture using
a pressure-equalizing addition funnel. The resulting mixture
was then stirred for 4 h during which time a white product
separated out. The solid product was filtered and then purified
by column chromatography using an 8 : 2 hexane–ethyl
acetate mixture. Yield: 1.38 g (80%). Mp 222 uC. Found: C,
methyl phenol,
2,6-dicarboxylic-4-methyl phenol
and
3
7
4-hydroxymethylbenzaldehyde (HMB) were prepared accord-
ing to methods previously reported in the literature.
Physical measurements
Elemental (C, H, and N) analyses were performed using a
Perkin-Elmer 2400 II elemental analyzer. FT-IR spectra were
recorded using KBr disks with Shimadzu FT-IR 8400S and
JASCO FTIR-460 Plus spectrophotometers. The electronic
absorption spectra were measured at room temperature using
7
5
2.84; H, 5.26; N, 8.12%. C H N O requires: C, 72.82; H,
21 18 2 3
.24; N, 8.09%. IR (KBr, n, cm ) 3354 (w), 3281 (m), 1653 (s),
2
1
1
1597 (s), 1549 (s), 1491 (s), 1445 (s), 1326 (m), 1281 (m), 1254
m), 1230 (w), 1173 (w), 1125 (w), 1080 (w), 948 (m), 793 (m),
an Agilent 8453 diode array spectrophotometer. H NMR
(
7
spectra were obtained using a Bruker Avance DPX spectro-
meter at 300 MHz with chemical shifts (d, ppm) quoted relative
to tetramethylsilane (TMS). Thermogravimetric analysis (TGA)
1
3 H
59 (s), 725 (m), 685 (m), 570 (w), 507 (m). H NMR (CDCl , d ,
ppm) 14.02 (s, 1H, PhOH), 9.37 (s, 2H, NHCLO), 7.86 (s, 2H,
1
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Ar), 7.67 (d, 4H, J = 4.8 Hz, Ar), 7.38 (t, 4H, J = 4.6 Hz, Ar), 7.18
Preparation of HMB–Si–HMS. The anchoring of (3-amino-
propyl)triethoxysilane (3-APTES) into 2-D hexagonal mesopor-
ous silica (HMS) was achieved by stirring 0.1 g HMS with 0.18 g
(t, 2H, J = 4.3 Hz, Ar), 2.30 (s, 3H, CH
3
).
Preparation of the metal complexes
(0.81 mmol) 3-APTES in dry chloroform at room temperature
Caution! The perchlorate salts used in this study are
potentially explosive and therefore should be handled in
small quantities with care.
for 12 h under a N2 atmosphere. The white, solid HMS–
[Si(CH ) NH ) produced was filtered and washed with chloro-
2
3
2 x
form and dichloromethane. This solid was then refluxed with
2.73 g 4-hydroxymethylbenzaldehyde (HMB) (20 mmol) in
[
CuL (DMF) ] (1). The ligand, HL, (0.35 g, 1.0 mmol) was
2 2
3
3
3
dissolved in 25 cm methanol and a 10 cm solution of
Cu(ClO ?6H O (0.19 g, 0.5 mmol) in methanol was added to
methanol (10 cm ) for 3 h at 60 uC. The resulting pale yellow
)
2
2
solid HMB–Si–HMS was then collected by filtration and was
dried in a desiccator (Scheme 1).
4
3
it with stirring. Triethylamine (0.1 g, 1.0 mmol) in 5 cm
methanol was then added drop wise. The resulting solution
was refluxed for 1 h and after some time a greenish-brown
crystalline compound started to appear. This was filtered and
washed, first with chilled dry methanol, and then with dry
The catalytic oxidation of primary alcohols. The immobi-
3
lized copper catalyst 1a (0.02 gm) was dispersed in 5 cm
toluene. A primary alcohol (1 mmol, 0.11 g) and TEMPO (1.6
mg, 0.01 mmol) were then added to this solution. The reaction
mixture was allowed to stir under oxygen at a temperature of
80 uC. After completion of the reaction, the catalyst was
separated from the reaction mixture by centrifugation. The
solution was then passed through a 60–120 mesh silica gel
3
diethyl ether. The compound obtained was dissolved in 25 cm
dimethylformamide (DMF) and heated at 80 uC for 0.5 h. It was
then left undisturbed overnight. Brown single crystals suitable
for X-ray crystallography appeared. The crystals were filtered
through a sintered glass crucible and washed with a 2 : 1
mixture of diethyl ether and DMF. Yield: 0.56 g (70%). Found:
3
column and washed with 10 cm toluene. Chlorobenzene was
added to the eluent as an internal standard and the products
were quantified using GC.
C, 63.92; H, 5.45; N, 9.40. C48
H
48CuN
6
O
8
requires: C, 64.02; H,
2
1
5
1
1
.37; N, 9.33%. FT-IR (KBr, n, cm ) 3253(w), 3039(w, br),
662(s), 1639(s), 1595(s), 1523(s), 1496(m), 1446(s), 1323(m),
X-ray crystallography
295(s), 752(m), 692(m), 576(m). UV-Vis [in DMF, lmax, nm (e,
A single crystal of compound 1 was mounted on a glass fiber
and coated with perfluoropolyether oil. Intensity data were
collected at 150(2) K on a Bruker-AXS SMART APEX II
diffractometer equipped with a CCD detector using graphite-
monochromated Mo-Ka radiation (l = 0.71073 Å). The data
3
21
21
dm mol cm )]: 285 (37 000), 385 (25 000), 450 (550), 750
(
80).
Synthesis of the 2-D hexagonal mesoporous silica matrix
HMS). The mesoporous material was prepared using a slightly
(
3
9
3
8
were processed with SAINT, and absorption corrections were
modified procedure described previously.
The cationic
3
9
made with SADABS. The structures were resolved using
direct and Fourier methods and refined with a full-matrix
surfactant cetyl trimethylammonium bromide (CTAB, 1.48 g)
was added to an aqueous solution of sodium dodecyl sulfate
2
40
least-squares method based on F using SHELXTL and
(
SDS, 0.24 g) with vigorous stirring at room temperature.
Stirring was continued for another 30 min. Gel formation
occurred immediately leading to viscous suspension.
4
1
SHELX-97 software packages. The non-hydrogen atoms were
refined anisotropically, while the hydrogen atoms were placed
at geometrically calculated positions with fixed isotropic
thermal parameters. The crystallographic data and selected
details of the structural determination are given in Table 1.
a
Tetraethyl orthosilicate (1.75 g) was added to this suspension
with continuous stirring over 30 min. After 2 h continuous
stirring, tetraethylammonium hydroxide was added dropwise
until the color of the solution turned white. The pH value of
the final mixture was about 11. The resultant mixture was aged
overnight while stirring at room temperature and then
transferred into a polypropylene bottle and heated at 80 uC
for 72 h without stirring. The solid produced was recovered by
filtration, then washed several times with water, and dried at
room temperature. The resulting powder was calcined at 700 K
to remove the organic surfactants. The molar ratio of the
various components was 1 : 0.5 : 0.1 : 90 (TEOS/CTAB/SDS/
Result and discussion
Synthesis and characterization of the metal complex
[
CuL (DMF) ] (1)
2 2
The synthesis of 2,6-dicarboxylic-4-methyl phenol was
3
6
achieved by following the method reported by Okawa et al.
The reaction of 2,6-dicarboxylic-4-methyl phenol with oxalyl
chloride in the presence of catalytic amount of DMF affords
the di-acid chloride, which reacts with aniline in presence of
triethlylamine to give the ligand, HL. The synthesis of HL is
outlined in Scheme 2. The mononuclear copper(II) complex is
obtained from the reaction between one equiv. of both HL and
Cu(ClO ) ?6H O in presence of one equiv. triethylamine with
4 2 2
methanol as a solvent. At first we obtained a greenish-brown
complex, which was then recrystallized from DMF to obtain
H
2
O).
Preparation of immobilized catalyst 1a. Immobilization of
the complex [CuL (DMF) ] (1) was carried out by suspending
2
2
3
1.0 g of mesoporous silica in a 20 cm DMF solution
containing 0.2 g of the complex, followed by vigorous stirring
at room temperature for 4 h. The solid was then filtered,
washed several times with DMF and dried under vacuum at
ambient temperature. Found: C, 5.57; H, 0.39; N, 0.62%. 8.34
wt% immobilization of C42
.38; N, 0.62%.
H
34CuN
4
O
4
requires: C, 5.58; H,
[CuL
2 2
(DMF) ] (1).
1
0
The H NMR spectrum of the ligand, HL, was recorded in
CDCl
3
and the chemical shift values and their related spectral
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Scheme 1 Organic modification of a MCM-41 type 2-D HMS using 3-APTES in CHCl
3
and anchoring 4-hydroxymethylbenzaldehyde (HMB) onto 2-D HMS (in
methanol).
2
1
assignments are given in the experimental section. Scheme S1
3255 and 3190 cm are due to the N–H stretching vibration,
which is hydrogen-bonded to both coordinated phenolate
oxygens and uncoordinated amide carbonyl oxygens. The CLO
1
(
ESI
3
) and Fig. S1 (ESI
3
) show the proton labelling and H NMR
spectrum of the HL ligand, respectively.
2
1
The FT-IR spectrum of the HL ligand exhibits two peaks at
vibration in the ligand is observed at about 1640 cm . On the
2
1
3281 and 1599 cm
due to the stretching and bending
other hand, the CLO vibration of the coordinated DMF
2
1
frequencies of NH, respectively (Fig. 1a). The stretching
vibration relating to the amide carbonyl groups appears at
appears at 1664 cm
.
The absorption spectroscopic behaviour of compound 1 was
studied in dimethylformamide (DMF). The UV-Vis-NIR spec-
tral data for this compound are given in the experimental
section. Complex 1, exhibits three very strong absorption
bands at 286 nm (e = 73 600 dm mol cm ), 330 nm (e =
1 340 dm mol cm ), and 385 nm (e = 49 990 dm mol
2
1
1
650 cm . The IR spectrum of complex 1 exhibits several
diagnostic features. Two weak broad bands observed at about
3
21
21
3
21
21
3
21
2
2
1
2 2
Table 1 Crystallographic data of [CuL (DMF) ] (1)
cm ) due to ligand centred p–p* transitions. The six-
coordinated octahedral copper complex exhibits two bands
Empirical formula
C
48
H
48
N
6
O
8
Cu
3
21
21
in the visible range at 440 nm (e = 650 dm mol cm ),
3
21
which appears as a shoulder, and 750 nm (e = 78 dm mol
M
T/K
900.46
150(2)
Monoclinic
P2
9.950(4)
13.482(5)
17.253(6)
105.438(9)
2230.9(14)
2
1.340
0.550
942
0.36 6 0.28 6 0.15
30 207
9046
574
8450
2
1
cm ). The shoulder at 440 nm is most likely due to a
Crystal system
Space group
II
phenolate oxygen A Cu ligand-to-metal charge transfer
1
(LMCT) transition, while the other band observed is due to
a/Å
b/Å
c/Å
b/u
d–d transitions. Fig. 2 shows the Vis-NIR spectrum of
compound 1.
3
The thermal stability of [CuL
over the temperature range 30–700 uC using thermogravi-
metric (TG) measurements. Fig. S2 (ESI ) shows the TG data of
2 2
(DMF) ] (1) has been studied
U/Å
Z
2
3
D/g cm
m/mm
F(000)
3
21
compound 1, which indicate that the solvated compound
remains stable up to 140 uC and desolvation then occurs
between 140 and 200 uC. The desolvated product is stable up to
Crystal size/mm
No. of measured reflections
No. of observed reflections
Parameters refined
315 uC. The observed weight loss (16.75%) is in agreement
with the value (16.23%) expected for the loss of two DMF
molecules. With a further increase in temperature from 315 to
700 uC, compound 1 eventually loses all of its organic
backbone, producing CuO.
The electrochemical properties of complex 1 have been
studied in dimethylformamide (DMF) solution. Complex 1 is
No. of reflections [I . 2s(I)]
a
Goodness of fit, S
1.027
0.0303, 0.0742
0.0344, 0.0770
b
c
c
Final R
1
, wR
2
[I . 2s(I)]
(all data)
b
R
1
, wR
2
a
2
o
2
K
)/(N 2 P)] where N is the number of data values
S = [gw(F
2 F
c
b
and P the total number of parameters refined.
1 o
R (F) = S||F | 2
c
2
2
o
2 2
2 2 1/2
|F
c
||S|F
o
|. wR
2
(F ) = [Sw(F
2 F
c
) /Sw(F
o
) ]
.
found to undergo one quasi-reversible oxidation at about E
K
=
1
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Scheme 2 The preparation of 2,6-bis[(N-phenyl)amido]-4-methylphenol (HL).
Fig. 1 FT-IR spectra of complex 1, the mesoporous silica matrix and immobilized catalyst 1a (a) and the % transmittance of (–CLO)amide vs. the weight of the
immobilized catalyst obtained from FTIR (b).
+
0.800 V (DEp = 0.085 V at 0 to +1.0 V) and two successive
irreversible reductions at about E = 20.45 and 20.73 V (Fig.
S3a, ESI ). To assign these redox processes, we also carried out
cyclic voltammetry experiments on the HL ligand in dimethyl-
formamide (Fig. S3b, ESI ). The ligand demonstrates a quasi-
reversible oxidation at about EK = + 0.745 V (DE = 0.090 V)
processes that occur in complex 1 are due to ligand-centered
processes. The first reduction process observed can be
K
II
I
3
assigned as the Cu /Cu reduction potential.
The X-band EPR spectrum of complex 1 has been captured
in 4 : 1 ethanol/methanol solution at 77 K (Fig. S4, ESI ). The
3
EPR spectrum shows an axially symmetric copper(II) center
3
p
and a quasi-reversible reduction at about E
K
= 20.715 V (DE
p
=
with g) = 2.05 and g|| = 2.26. These values indicate that the
2
2
0.20 V). Therefore, the oxidation and the second reduction
ground state of Cu(II) is predominantly d
.
x 2y
The crystal structure of [CuL
2
(DMF)
2
] (1)
An ORTEP representation of [CuL
2
(DMF) ] (1) along with atom
2
labels is shown in Fig. 3 and selected bond distances and
angles are given in Table 2.
In the asymmetric unit, the copper(II) centre is coordinated
with the two phenoxides and two carbonyl oxygens of the
ligand, HL. The metal center is additionally coordinated with
two carbonyl oxygens from DMF molecules. The coordination
6
environment around the six-coordinated metal centre [CuO ]
may be considered to be a slightly distorted octahedron. It
should be noted that, in terms of an octahedral description,
the metal centre is located in the basal plane formed by the
atoms O1, O2, O4, and O5. However, these atoms are displaced
from the least-squares plane by ¡ 0.015(3) Å and the metal ion
is situated 0.0067(4) Å below this plane. Oxygen atoms O7 and
O8 from the two coordinated DMF molecules are trans-axially
disposed to the metal centre. The two Cu–O(phenolate)
distances [1.9044(13) and 1.9061(13) Å] are identical, as are
the two Cu–O(carbonyl) distances [2.0316(15) and 2.0387(15)
Å]. On the other hand, the axial Cu–O(DMF) distances
[2.3670(20) and 2.3400(20) Å] are significantly longer relative
Fig. 2 UV-Vis-NIR spectrum of complex 1 in dimethylformamide and the
immobilized catalyst 1a (dispersed in dimethylformamide).
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a
Table 3 Catalytic oxidation of benzyl alcohol alcohols under various conditions
Entry
Catalyst
Co-catalyst/base
Time (h)
Yield (%)
1
2
3
1a
1a
1a
1
Triethylamine
Piperidine
TEMPO
TEMPO
TEMPO
24
24
16
16
24
35
50
80
45
10
b
4
c
5
a
Reaction conditions: benzyl alcohol (1 mmol), base (1 mmol),
TEMPO (0.01 mmol), 20 mg catalyst (1a or 1) in toluene.
b
c
Homogeneous reaction in toluene. Blank reaction in the presence
of the mesoporous silica matrix and TEMPO only.
The packing diagram of compound 1 reveals several short
interactions between aromatic hydrogen atoms and the
centroid of the p cloud of another aromatic ring. Compound
…
1
exhibits one intermolecular C–H p interaction in addition
2 2
Fig. 3 An ORTEP representation of the complex [CuL (DMF) ] (1) showing 50%
to two intramolecular interactions. Generally, the upper
acceptable limit for the distance of a C–H hydrogen atom
probability displacement ellipsoids.
…
from the centroid of an aromatic ring in a C–H p interaction
…
is 3.5 Å. The metrical parameters for the C–H p interactions
Table 2 Selected bond distances and bond angles of [CuL
2
(DMF)
2
] (1)
…
…
in compound 1 (Table S1, ESI
3
) indicate that the C–H p
distances range from 2.80 to 3.02 Å, indicating that the C–H p
interactions are strong. Another interesting structural feature
of this compound is the presence of an intermolecular p–p
interaction between the aromatic ring composed of C2, C3, C4,
C5, C6, and C7 and the aromatic ring composed of C23, C24,
C25, C26, C27, and C28 (the distance between the two
centroids is 3.53 Å). All these short interactions and the
intermolecular hydrogen-bonding give rise to a two dimen-
sional sheet structure in the compound, as shown in Fig. S5,
Bond distances (Å)
Bond angles (u)
Cu(1)–(O1)
Cu(1)–(O2)
Cu(1)–(O4)
Cu(1)–(O5)
Cu(1)–(O7)
Cu(1)–(O8)
1.9044(13)
2.0316(15)
1.9061(13)
2.0387(15)
2.3670(20)
2.3400(20)
O(1)Cu(1)O(2)
O(1)Cu(1)O(4)
O(1)Cu(1)O(5)
O(1)Cu(1)O(7)
O(1)Cu(1)O(8)
O(2)Cu(1)O(5)
O(2)Cu(1)O(4)
O(2)Cu(1)O(7)
O(2)Cu(1)O(8)
O(4)Cu(1)O(5)
O(4)Cu(1)O(7)
O(4)Cu(1)O(8)
O(5)Cu(1)O(7)
O(5)Cu(1)O(8)
O(7)Cu(1)O(8)
92.12(6)
179.03(8)
88.54(6)
89.53(8)
91.35(8)
178.74(10)
87.01(6)
85.50(8)
96.09(8)
92.34(6)
90.82(8)
88.33(8)
93.44(7)
84.96(8)
178.15(9)
ESI.
3
Characterization of the immobilized catalyst
The FT-IR spectra of the mesoporous silica host and
immobilized catalyst 1a are shown in Fig. 1a. 1a exhibits
2
1
several bands in the 1700–1400 cm region, which can be
attributed to the immobilization of compound 1 in the
mesoporous silica host. These bands are not found in the
spectrum of the mesoporous host. This result indicates that
the immobilization of compound 1 in the mesoporous host
was successful. In order to determine if the distribution of the
copper complex in catalyst 1a was homogenous, FTIR spectra
were carried out in a quantitative fashion. The percentage
transmittance (the –CLO group of the amide ligand was used
as a probe) was plotted against the weight of sample used and
a linear curve fit (Fig. 1b) was obtained, signifying a near
to the Cu–O(axial) distances. The trans-axial O7–Cu–O8 angle
is 178.15(9)u, while the trans-equatorial angles are 179.03(8)u
for O1–Cu–O4 and 178.74(10)u for O2–Cu–O5 . The trigonal
planes comprising O1, O2, and O7, and O4, O5, and O8 are
inclined with respect to one another by 1.14u, which again
indicates a slightly distorted octahedral geometry.
A noteworthy feature of the molecular structure of 1 is the
…
occurrence of both intramolecular and intermolecular N–H O
bonds involving amide nitrogen and both coordinated
phenolate oxygens and uncoordinated carbonyl oxygens, as
listed in Table 3. The hydrogen atoms of amide nitrogens N2
and N4, are intramolecularly hydrogen-bonded to phenolate
oxygens O1 and O4. On the other hand, N1 and N3 are
involved in intermolecular hydrogen-bonding with uncoordi-
nated amide carbonyl oxygens, O3 and O6. It should be noted
that the amide nitrogen atoms act as donors while both types
42
homogeneous distribution of copper complex 1.
The absorption spectroscopic behaviour of immobilized
catalyst 1a (dispersed in DMF) is shown in Fig. 2. Catalyst 1a
exhibits two bands in the visible range at 440 nm, which
appears as a shoulder, and 730 nm due to phenolate oxygen A
II
Cu ligand-to-metal charge transfer (LMCT) and d–d transi-
tions, respectively. This again demonstrates the immobiliza-
tion of compound 1 in the mesoporous host. In addition, we
have obtained UV-Vis-NIR spectra of complex 1 and immobi-
…
of oxygen act as acceptors. The donor–acceptor N O distances
…
lie between 2.601(3) and 2.793(3) Å and the N–H O angles
range from 139.00u to 153.00u (Table 3), indicating that the
hydrogen bonds are very strong.
3
lized catalyst 1a in the solid state (Fig. S6, ESI ), which also
1
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Fig. 4 Small angle X-ray diffraction patterns of the mesoporous silica host and
immobilized complex catalyst 1a.
Fig. 5 N
2
adsorption–desorption isotherms for (a) mesoporous silica material
and (b) immobilized complex 1a.
indicate the successful immobilization of complex 1 in the
mesoporous silica matrix.
walls are quite thin, which might help the catalytic sites to be
located closer to the surface of the catalyst.
Small-angle X-ray diffraction patterns of the mesoporous
host as well as immobilized catalyst 1a are shown in Fig. 4. In
the low angle region (2h y 1.5–5u), three characteristic Bragg
reflections were observed. These diffraction lines can be
indexed by assuming that the materials have hexagonal
symmetry. The prominent d100 reflection at 2h y 2.35u and
the less intense d110 and d200 peaks at 2h y 3.85u and y 4.80u,
respectively, correspond well to the 2D hexagonally arranged
pore structure of the mesoporous silica framework. This
confirms the long-range structural ordering of these materials.
All of the peaks were well-resolved, which suggests the
retention of the highly ordered structure even after the loading
of the metal complex. The mesoporous host exhibits a
relatively weak additional peak at 2h y 6.05u, which can be
assigned to a d210 reflection. In the case of 1a, the weak
reflection for d210 is suppressed and lost in the background
noise.
High resolution transmission electron microscopy (HRTEM)
images of catalyst 1a are shown in Fig. 6. Catalyst 1a features
hexagonal porous tubules in an open-ended lamellar type
arrangement. When an electron beam falls on the 2-D
hexagonal mesoporous silica perpendicular to the pore axis,
the pores are seen to be arranged in patches composed of
4
4,45
regular rows.
A TEM micrograph of 1a viewed along the
pore axis reveals a hexagonal array with channel dimension of
y4 nm, which is consistent with the XRD results.
The C, H and N elemental analysis results indicate that
2
complex 1 is immobilised in the mesoporous host as [CuL ]
and that the overall loading of the complex is 8.34 wt%. It
seems that the oxygen atoms present in the mesoporous silica
matrix form a weak coordination to the copper atom and thus
2
N sorption isotherms for the mesoporous matrix host and
the immobilized catalyst, exhibit typical type IV isotherms with
a steep increase that is due to capillary condensations, which
4
3
are characteristic of mesoporous materials.
2
N adsorption–
desorption isotherms for the mesoporous silica matrix and
catalyst 1a are shown in Fig. 5. The BET surface area and
average pore diameter of the 2D hexagonal mesoporous host
2
21
were 1056 m
g
and 2.9 nm, respectively. The Barrett–
Joyner–Halenda (BJH) pore-size distribution for this 2D-
hexagonal mesoporous material is centered at 2.9 nm,
suggesting a very narrow range of pore sizes (Fig. S7, ESI ).
3
Adsorption studies were also carried out on immobilized
catalyst 1a. The surface area and pore diameter for immobi-
2
21
lized catalyst 1a were found to be 517 m g and 2.35 nm,
respectively. The surface area and pore diameter for catalyst 1a
were smaller than those of the mesoporous silica host. The
decrease in pore diameter may be attributed to the adherence
of the metal centers to the inner walls of the pores. The pore
Fig. 6 HRTEM image of the catalyst 1a. Inset: Selected area electron diffraction
pattern.
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hold the copper(II) complex to the inner walls of the matrix.
Moreover, complex 1 also contains four uncoordinated amide
–NH groups, which can form strong hydrogen bonding
interactions with oxygen atoms to prevent leaching of the
immobilized complex from HMS. The FT-IR spectrum of
immobilized catalyst 1a exhibits strong broad features around
2
1
3
500–3000 cm , which can be attributed to strong hydrogen
bonding interactions. The metal content of the catalyst 1a was
also investigated using EDS (SEM). EDS spectra obtained from
different points on the material (Fig. S8, ESI ) confirm the
3
presence of Cu in the mesoporous silica matrix.
Catalytic activity
The aerobic heterogeneous oxidation of alcohols was carried
out using the immobilized copper(II) complex (1a) as a catalyst.
In order to standardize the reaction conditions, we carried out
the catalytic reaction with benzyl alcohol in the presence of
various bases and using TEMPO as co-catalyst. Bases such as
triethylamine and piperidine gave poor yields (Table 3, entries
Fig. 7 % Conversion of benzyl alcohol to benzaldehyde over time. (Catalyst
used: 20 mg).
1
and 2), whereas in the presence of TEMPO, the reaction gave
an excellent yield (Table 3, entry 3). We also carried out the
reaction homogeneously, which resulted in a relatively poor
yield (Table 3, entry 4) under the same reaction conditions. A
blank reaction was conducted without any catalyst under the
same experimental conditions and this experiment demon-
strated a remarkable decrease (Table 3, entry 5) in the
conversion of the substrate. Therefore, we decided to carry
out the catalytic reactions heterogeneously using TEMPO as a
co-catalyst. In a typical general heterogeneous experiment, a
mixture of the alcohol (1 mmol) and TEMPO (0.01 mmol) in 5
alcohols possessing both electron withdrawing and donating
groups on the benzene ring, i.e., 4-nitro-, 4-bromo-, 4-methoxy-,
and 4-methyl-benzyl alcohol (Table 4, entries 2–5), were
oxidized, giving the corresponding aldehydes in high yields.
The catalytic TONs obtained from the oxidation of the different
benzylic alcohols were found to be moderate. On the other
hand, the oxidation of allylic alcohols such as cinnamyl alcohol
and 4-bromo cinnamyl alcohol (Table 4, entries 6 and 7) and
aliphatic alcohols such as heptanol (Table 4, entry 8) and
decanol (Table 4, entry 9) gave poor yields and required much
longer reaction times. It can be seen from the proposed
3
cm toluene was stirred at 80 uC. After completion of the
reaction, the product in the reaction mixture was analyzed
using gas chromatography. Products were identified by
comparison with known standards. The effects of reaction
time and the amount of catalyst used were thoroughly
evaluated and benzyl alcohol was chosen as a standard
substrate for this purpose. In order to further standardize
the reaction conditions, a set of catalytic experiments were
carried out using the immobilized catalyst (1a), benzyl alcohol
3
mechanism (Scheme S2, ESI ) that the alcohol coordinates to
the copper(II) centre during the catalytic cycle. As the
coordinating ability of aliphatic alcohols is greatly inferior to
that of aromatic alcohols, the former show very poor yields even
with longer reaction times. We have also carried out the
reaction using secondary alcohols, such as 1-phenylethanol
(1 mmol) and TEMPO (1 mol%). The resulting solution was
(Table 4, entry 10) and cyclohexanol (Table 4, entry 11), but they
allowed to stir at 80 uC under aerobic conditions and the
formation of benzaldehyde was monitored over time (Fig. 7).
The conversion of the alcohol to the aldehyde increased with
time, up to 16 h, after which no further oxidation was
observed. It was also found that the oxidation reaction only
provided benzaldehyde and no over-oxidation to benzoic acid
was observed. To find out the effect that the amount of catalyst
used had on the reaction, a separate set of catalytic reactions
were carried out using different amounts of catalyst. The
highest conversion of the alcohol was observed in the presence
of 20 mg catalyst. Increasing the amount of catalyst further did
not improve the yield of the reaction. Therefore, the optimum
reaction conditions were 16 h reaction time and the use of 20
mg immobilized catalyst in toluene. A control experiment
without either catalyst 1a or TEMPO showed no reaction.
In order to gain an insight into the effect of different
starting materials on the yield of aldehyde obtained, various
benzylic, allylic, and aliphatic alcohols were used as the
starting materials. The results are listed in Table 4. Benzylic
also exhibit very poor yields. Next, in order to study the
selectivity of the reaction, the oxidation of a 1 : 1 mixture of
benzyl alcohol and 1-phenylethanol was investigated. The
reaction afforded benzaldehyde and acetophenone in 76%
and 1% yields, respectively. It is worth mentioning that our
catalyst exhibits very good TONs in contrast to most of the
previously reported homogeneous and heterogeneous Cu(II)
based catalysts used for the aerobic oxidation of benzylic
alcohols, which is a very important factor for industrial viability.
Hot filtration test
In order to verify whether catalyst was leaching from the solid
host during the reaction, the liquid phase of the reaction
mixture was collected by centrifugation, at reaction tempera-
ture, after the oxidation reaction was 40% completed. The
corresponding filtrate was then heated again and stirred under
air. The reaction did not proceed any further after the filtering
catalyst 1a from the reaction vessel. This result suggests that
1
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a
Table 4 The catalytic oxidation of different alcohols using catalyst 1a
b
Entry
1
Alcohol
Product
Yield (%)
80
Time (h)
TON
16
20
18
16
16
20
432
2
3
4
5
6
75
79
81
80
40
406
428
438
432
216
7
35
20
189
8
9
20
22
5
24
24
24
108
120
28
1
1
a
0
1
2
24
11
b
Reaction conditions: alcohol (1 mmol), TEMPO (0.01 mmol), catalyst (20 mg) in toleuene. The number of moles of aldehyde formed per
mole of catalyst.
the catalyst was not leaching from the catalyst during
oxidation.
prepared according to the method shown in Scheme 1. A
solution of benzyl alcohol (1 mmol) and TEMPO (0.01 mmol)
was stirred in the presence of immobilized copper catalyst 1a
4
6
Three-phase test
(0.02 mg) and HMB–Si–HMS (1 g) at 80 uC under aerobic
To ascertain if the reactions are truly heterogeneous we
performed a three-phase test on the oxidation of benzylic
alcohols using a specially designed benzylic alcohol under the
usual conditions. During this test, one of the reactants (a
benzylic alcohol, namely, 4-hydroxymethylbenzaldehyde) is
anchored to the modified surface of Si–HMS. The HMS-
anchored 4-hydroxymethylbenzaldehyde, HMB–Si–HMS, was
conditions for 16 h. The reaction mixture was collected by
centrifugation. The solution was then passed through a 60–120
mesh silica gel column and washed with 10 cm toluene.
Chlorobenzene was added to the eluent as an internal
standard and the products were quantified using GC, which
showed that over 75% of the benzyl alcohol had been
converted to benzaldehyde, as expected.
3
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Fig. 9 Absorption spectra of immobilized catalyst 1a with benzyl alcohol (---)
and immobilized catalyst 1a, benzyl alcohol and TEMPO (—).
Fig. 8 The catalytic activity of catalyst 1a for the oxidation of benzyl alcohol. The
activity of the catalyst decreases slightly with the increasing number of catalytic
cycles.
stable under nitrogen, but diminished upon exposure to air.
This clearly indicates that the reduction of copper(II) to
copper(I) was taking place in the presence of the alcohol and
TEMPO, under the inert atmosphere. We also carried out
spectrophotometric measurements on a catalyst–benzyl alco-
hol mixture as well as a catalyst–TEMPO mixture. In both cases
the formation of a MLCT band was not observed.
The residue obtained from the reaction mixture was then
hydrolyzed with 2 N aqueous HCl solution for 3 h under
refluxing conditions. The resulting solution was neutralized,
extracted with ethyl acetate, concentrated, and the analyzed
using GC. The only compound obtained from this process was
4-hydroxymethylbenzaldehyde. The expected oxidation pro-
duct of 4-hydroxymethylbenzaldehyde, terephthalic aldehyde,
was not detected in the extracted liquid. This clearly
demonstrates that 4-hydroxymethylbenzaldehyde does not
participate in a coupling reaction while anchored to HMS.
On the basis of the above observation, a mechanism for the
catalytic oxidation can be proposed, as shown in Scheme S2
1
6a,b
(ESI ), which was originally proposed by Sheldon et al.
3
In
the proposed mechanistic cycle, the alcohol coordinates to the
copper(II) centre. Meanwhile, TEMPO also coordinates to the
Recycling of the catalyst
2
copper(II) ion in a g fashion. The coordination of TEMPO in
A significant factor for the industrial viability of a hetero-
geneous catalytic method is in the ability to easily remove the
catalyst from the reaction mixture and reuse it for subsequent
reactions until the catalyst is sufficiently deactivated. The
recyclability of the immobilized catalyst was examined using
the reaction of benzyl alcohol at 80 uC under air, which was
completed after 16 h. After the completion of reaction, the
catalyst was recovered by centrifugation and then washed
thoroughly with DMF followed by dichloromethane. The
recovered catalyst was dried under vacuum at 90 uC overnight.
The performance of the recycled catalyst in the alcohol
oxidation reaction for to four successive cycles is shown in
Fig. 8. It can be seen that the catalytic activity of recovered
immobilized catalyst 1a decreases marginally in each succes-
sive cycle.
its radical form to a Cu(II) center has previously been
4
7
?
reported.
Abstraction of an a-hydrogen by the O of
TEMPO then takes place resulting in the formation of a Cu(I)
species, TEMPOH and an aldehyde. The copper complex is
then regenerated by the TEMPO-mediated oxidation of Cu(I) to
Cu(II) after the aerobic oxidation of TEMPOH back to TEMPO.
Conclusion
We have synthesized a mononuclear copper(II) complex (1)
using the diamide ligand 2,6-bis[(N-phenyl)amido]-4-methyl-
phenol (HL). This complex was successfully immobilized in a
mesoporous silica matrix. The immobilized copper complex
(
1a) was then exploited as a catalyst for the aerobic oxidation
of alcohols to aldehydes using TEMPO as a co-catalyst. The
conversion of various benzylic alcohols to the corresponding
aldehydes occurred in high yields with moderate TONs. This
method may also be used for the selective oxidation of primary
alcohols in the presence of secondary alcohols. Moreover, the
catalyst can be recovered and reused without significant loss of
the catalytic activity.
Mechanistic study
In order to gain an insight into the mechanism of the catalytic
oxidation of alcohols, the reaction with 1 mmol of benzyl
alcohol and 1 mol% TEMPO was monitored spectrophotome-
terically under a nitrogen atmosphere at room temperature
(Fig. 9). A strong MLCT band at 470 nm was generated in the
visible region. This charge-transfer band is characteristic of
copper(I) complexes. The orange color of the solution was
1
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8
B. M. Choudary, M. L. Kantam, A. Rahman, C. V. Reddy
and K. K. Rao, Angew. Chem., Int. Ed., 2001, 40, 763.
Acknowledgements
S. D. and S. S. are indebted to CSIR, India for their JRF [08/
9 (a) K. S. Coleman, M. Coppe, C. Thomas and J. A. Osborn,
Tetrahedron Lett., 1999, 40, 3723; (b) P. A. Shapley,
N. Zhang, J. L. Allen, D. H. Pool and H.-C. Liang, J. Am.
Chem. Soc., 2000, 122, 1079.
0
03(0072)/2010-EMR-I] and [08/003(0083)/2011-EMR-1], respec-
tively. P.B. acknowledges DST-India for the Fast Track Project
No.SR/FT/CS-022/2009). The authors also acknowledge the
(
facility developed in the Department of Chemistry, BESUS
through MHRD, India and UGC-SAP.
10 A. Haimov and R. Neumann, Chem. Commun., 2002, 876.
11 (a) N. Ito, S. E. V. Phillips, C. Stevens, Z. B. Ogel, M.
J. McPherson, J. N. Keen, K. D. S. Yadav and P. F. Knowles,
Nature, 1991, 350, 87–90; (b) Enzyme Catalysis in Organic
Synthesis, ed. K. Drauz and H. Waldmann, VCH,
Weinheim, 1995; (c) J. W. Whittacker, Chem. Rev., 2003,
References
1
03, 2347–2663; (d) N. Ito, S. E. V. Phillips, K. D. S. Yadav
1
Julio Piera and Jan-E. B ¨a ckvall, Angew. Chem., Int. Ed., 2008,
7, 3506–3523.
and P. F. Knowles, J. Mol. Biol., 1994, 238, 704–814.
2 (a) V. A. Golubev, E. G. Rozantsev and M. B. Neiman, Izv.
Akad. Nauk. SSSR, Ser. Khim., 1965, 1927; (b) V. A. Golubev,
E. G. Rozantsev and M. B. Neiman, Bull. Acad. Sci. USSR,
Div. Chem. Sci. (Engl. Transl.), 1965, 14, 1898.
3 (a) M. Munakata, S. Nishibayashi and H. Sakamoto, J.
Chem. Soc., Chem. Commun., 1980, 219–220; (b) X. Liu,
A. Qiu and D. T. Sawyer, J. Am. Chem. Soc., 1993, 115,
4
1
1
2
(a) M. Kirihara, Y. Ochiai, S. Takizawa, H. Takahata and
H. Nemoton, Chem. Commun., 1999, 1387; (b) Y. Maeda,
N. Kakiuchi, S. Matsumura, T. Nishimura, T. Kawamura
and S. Uemura, J. Org. Chem., 2002, 67, 6718; (c)
S. Velusamy and T. Punniyamurthy, Org. Lett., 2004, 6, 217.
(a) T. Yamada and T. Mukaiyama, Chem. Lett., 1989, 519;
3
(
b) T. Iwahama, Y. Yoshino, T. Keitoku, S. Sakaguchi and
3
1
239–3243; (c) I. Skibida and A. Sakharov, Catal. Today,
996, 27, 187–193; (d) H. Korpi, P. Lahtinen, V. Sippola,
Y. Ishii, J. Org. Chem., 2000, 65, 6502; (c) T. Iwahama,
S. Sakaguchi, Y. Nishiyama and Y. Ishii, Tetrahedron Lett.,
O. Krause, M. Leskel ¨a and T. Repo, Appl. Catal., A, 2004,
68, 199–206; (e) H. Korpi, P. J. Figiel, E. Lankinen, P. Ryan,
M. Leskel ¨a and T. Repo, Eur. J. Inorg. Chem., 2007,
465–2471.
1995, 36, 6923; (d) V. B. Sharma, S. L. Jain and B. Sain,
2
Tetrahedron Lett., 2003, 44, 383.
S. Velusamy, M. Ahamed and T. Punniyamurthy, Org. Lett.,
4
5
2
2
004, 6, 4821.
(a) H. Egami, S. Onitsuka and T. Katsuki, Tetrahedron Lett.,
005, 46, 6049; (b) H. Egami, H. Shimizu and T. Katsuki,
1
4 (a) I. E. Mark o´ , P. R. Giles, M. Tsukazaki, S. M. Brown and
C. J. Urch, Science, 1996, 274, 2044–2046; (b) I. E. Mark o´ ,
M. Tsukazaki, P. R. Giles, S. M. Brown and C. J. Urch,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2208–2210; (c) I.
E. Mark ´o , P. R. Giles, M. Tsukazaki, I. Chell ´e -Regnaut,
A. Gautier, S. M. Brown and C. J. Urch, J. Org. Chem., 1999,
2
Tetrahedron Lett., 2005, 46, 783; (c) H. Shimizu, S. Onitsuka,
H. Egami and T. Katsuki, J. Am. Chem. Soc., 2005, 127, 5396;
(
d) T. Matsushita, K. Ebitani and K. Kaneda, Chem.
Commun., 1999, 265; (e) B. Hinzen, R. Lenz and S. V. Ley,
Synthesis, 1998, 977; (f) K. Yamaguchi, K. Mori, T. Mizugaki,
K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2000, 122, 7144;
6
4, 2433–2439; (d) I. E. Mark o´ , A. Gautier, R. Dumeunier,
K. Doda, F. Philippart, S. M. Brown and C. J. Urch, Angew.
Chem., Int. Ed., 2004, 43, 1588–1591.
5 (a) M. F. Semmelhack, C. R. Schmid, D. A. CortRs and C.
S. Chou, J. Am. Chem. Soc., 1984, 106, 3374; (b) M.
F. Semmelhack, C. R. Schmid and D. A. CortRs, Tetrahedron
Lett., 1986, 27, 1119.
6 (a) P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon,
Chem. Commun., 2003, 2414–2415; (b) P. Gamez, I. W. C.
E. Arends, R. A. Sheldon and J. Reedijk, Adv. Synth. Catal.,
(
g) I. E. Mark ´o , P. R. Giles, M. Tsukazaki, I. Chelle-Regnaut,
C. J. Urch and S. M. Brown, J. Am. Chem. Soc., 1997, 119,
2661; (h) A. Dijksman, I. W. C. E. Arends and R. A. Sheldon,
1
1
1
Chem. Commun., 1999, 1591; (i) G. Csjernyik, A. H. Ell,
L. Fadini, B. Pugin and J.-E. Backvall, J. Org. Chem., 2002, 67,
1657; (j) K. Yamaguchi and N. Mizuno, Angew. Chem., Int.
Ed., 2002, 41, 4538; (k) B.-Z. Zhan, M. A. White, T.-K. Sham, J.
A. Pincock, R. J. Doucet, Ramama, K. V. Rao, K. N. Robertson
and T. S. Cameron, J. Am. Chem. Soc., 2003, 125, 1195.
J. Martin, C. Martin, M. Faraj and M. Bregeault, New. J.
Chem., 1984, 8, 141.
2004, 346, 805–811; (c) J. S. Uber, Y. Vogels, D. van den Helder,
I. Mutikainen, U. Turpeinen, W. T. Fu, O. Roubeau, P. Gamez
and J. Reedijk, Eur. J. Inorg. Chem., 2007, 4197–4206; (d) E. T.
T. Kumpulainen and A. M. P. Koskinen, Chem.–Eur. J., 2009,
6
7
(a) K. Kaneda, M. Fujii and K. Morioka, J. Org. Chem., 1996,
1
5, 10901–10911; (e) M. F. Semmelhack, C. R. Schmid, D.
A. Cort ´e s and C. S. Chou, J. Am. Chem. Soc., 1984, 106,
374–3376; (f) A. Dijksman, I. W. C. E. Arends and R.
61, 4502; (b) T. Nishimura, T. Onoue, K. Ohe and S. Uemura, J.
Org. Chem., 1999, 64, 6750; (c) K. P. Peterson and R. C. Larock,
J. Org. Chem., 1998, 63, 3185; (d) G.-J. ten Brink, I. W. C.
E. Arends and R. A. Sheldon, Science, 2000, 287, 1636; (e)
Y. Uozumi and R. Nakao, Angew. Chem., Int. Ed., 2003, 42, 194;
3
A. Sheldon, Org. Biomol. Chem., 2003, 1, 3232–3237; (g)
G. Ragagnin, B. Betzemeier, S. Quici and P. Knochel,
Tetrahedron, 2002, 58, 3985–3991; (h) Z. Lu, T. Ladrak,
O. Roubeau, J. van der Toorn, S. J. Teat, C. Massera, P. Gamez
and J. Reedijk, Dalton Trans., 2009, 3559–3570; (i) Z. Lu, J.
S. Costa, O. Roubeau, I. Mutikainen, U. Turpeinen, S. J. Teat,
P. Gamez and J. Reedijk, Dalton Trans., 2008, 3567–3573; (j)
S. Velusamy, A. Srinivasan and T. Punniyamurthy,
Tetrahedron Lett., 2006, 47, 923; (k) B. Betzemeier,
M. Cavazzini, S. Quici and P. Knochel, Tetrahedron Lett.,
(f) M. J. Schultz, S. S. Hamilton, D. R. Jensen and M.
S. Sigman, J. Org. Chem., 2005, 70, 3343; (g) T. Nishimura,
N. Kakiuchi, M. Inoue and S. Uemura, Chem. Commun., 2000,
1245; (h) T. Iwasawa, M. Tokunaga, Y. Obora and Y. Tsuji, J.
Am. Chem. Soc., 2004, 126, 6554; (i) D. R. Jensen, M. J. Schultz,
J. A. Mueller and M. S. Sigman, Angew. Chem., Int. Ed., 2003,
42, 3810; (j) B. A. Steinhoff, S. A. Fix and S. S. Stahl, J. Am.
Chem. Soc., 2002, 124, 766.
2000, 41, 4343; (l) G. Ragagnin, B. Betzemeier, S. Quici and
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 19455–19466 | 19465

View Article Online
Paper
P. Knochel, Tetrahedron, 2002, 58, 3985; (m) I. A. Ansari and
RSC Advances
58, 3985; (c) I. Ansari and A. R. Gree, Org. Lett., 2002, 4, 1507;
(d) N. Jiang and A. J. Ragauskas, Org. Lett., 2005, 7, 3689.
32 S. Velusamy, A. Srinivasan and T. Punniyamurthy, Tetrahedron
Lett., 2006, 47, 923.
33 G. Yang, J. Ma, W. Wang, J. Zhao, X. Lin, L. Zhou and
X. Gao, Catal. Lett., 2006, 112, 83–87.
R. Gree, Org. Lett., 2002, 4, 1507; (n) N. Jiang and A.
J. Ragauskas, Org. Lett., 2005, 7, 3689; (o) J. M. Hoover, J.
E. Steves and S. S. Stahl, Nat. Protoc., 2012, 7, 1161–1167; (p)
N. J. Hill, J. M. Hoover and S. S. Stahl, J. Chem. Educ., 2013, 90,
102–105; (q) I. Gamba, I. Mutikainen, E. Bouwman, J. Reedijk
and Sylvestre Bonnet, Eur. J. Inorg. Chem., 2013, 115–123.
7 R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003,
3
4 D. D. Perrin, W. L. Armarego and D. R. Perrin, Purification
of Laboratory Chemicals, 2nd ed. Pergamon: Oxford, U.K.,
1
1
1
1
2
2
2
2
2
1
977–1986.
8 M. Tada and Y. Iwasawa, Coord. Chem. Rev., 2007, 251,
702–2716.
980.
5 (a) F. Ullman and K. Brittner, Ber. Dtsch. Chem. Ges., 1909, 42,
539; (b) R. R. Gagne, C. L. Spiro, T. J. Smith, C. A. Hamman,
W. R. Theis and A. K. Shiemke, J. Am. Chem. Soc., 1981, 103,
073.
3
2
2
9 F. Biginame, A. Corradini, C. Quarantelli and G. Sartori, J.
Catal., 2007, 250, 222–230.
0 M. R. Maurya, S. J. J. Titinchi and S. Chand, J. Mol. Catal. A:
Chem., 2003, 201, 119–130.
1 S. Mukherjee, S. Samanta, A. Bhaumik and B. C. Ray, Appl.
Catal., B, 2006, 68, 12–20.
2 Y. Liu, K. Murata, M. Inaba and N. Mimura, Catal.
Commun., 2003, 4, 281–285.
3 H. U. Blaser, B. Pugin and F. Spinder, J. Mol. Catal. A:
Chem., 2005, 231, 1–20.
4 M. S. Saraiva, N. L. D. Filho, C. D. Nunes, P. D. Vaz, T.
G. Nunes and M. J. Calhorda, Microporous Mesoporous
Mater., 2009, 117, 670–677.
5 S. M. Coman, M. Florea, V. I. Parvulescu, V. David,
A. Medvedovici, D. De Vos, P. A. Jacobs, G. Poncelet and
P. Grange, J. Catal., 2007, 249, 359–369.
4
3
3
6 H. Okawa, M. Honda and S. Kida, Chem. Lett., 1972, 1027.
7 N. M. Loim and E. S. Kelbyscheva, Russ. Chem. Bull., 2004,
5
3, 2080–2085.
8 D. Chandra and A. Bhaumik, Ind. Eng. Chem. Res., 2006, 45,
879.
9 SAINT, version 6.02, SADABS, version 2.03, Bruker AXS, Inc.:
Madison, WI, 2002.
3
3
4
4
4
4
0 SHELXTL, version 6.10, Bruker AXS, Inc.: Madison, WI,
2002.
1 G. M. Sheldrick, SHELXL-97, University of G ¨o ttingen,
G ¨o ttingen, Germany.
2
2 (a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli
and J. S. Beck, Nature, 1992, 359, 710; (b) D. Chandra, B.
K. Jena, C. R. Raj and A. Bhaumik, Chem. Mater., 2007, 19,
2
6 P. McMorn and G. J. Hutching, Chem. Soc. Rev., 2004, 33,
6290.
108–122.
4
4
4
3 K. Sarkar, K. Dhara, M. Nandi, P. Roy, A. Bhaumik and
P. Banerjee, Adv. Funct. Mater., 2009, 19, 223.
4 A. Chenite, Y. Le Page and A. Sayari, Chem. Mater., 1995, 7,
2
7 (a) A. R. Silva, V. Budarin, J. H. Clark, C. Freire and B. de
Castro, Carbon, 2007, 45, 1951–1964; (b) A. R. Silva,
M. Martins, M. M. A. Freitas, J. L. Figueiredo, C. Freireand
and B. de Castro, Eur. J. Inorg. Chem., 2004, 2027–2035.
8 I. Kuzniarska-Biernacka, A. R. Silva, R. Ferreira, A. P. de
Carvalho, J. Pires, M. B. de Carvalho, C. Freire and B. de
Castro, New J. Chem., 2004, 28, 853–858.
1015.
5 C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J. A. Sousa
2
2
Gonc u¨ alves and L. F. Gladden, J. Phys. Chem., 1996, 100,
390.
9 R. Ferreira, M. Silva, C. Freire, B. de Castro and J.
L. Figueiredo, Microporous Mesoporous Mater., 2000, 38,
46 (a) C. Baleizao, A. Corma, H. Garcia and A. Leyva, J. Org.
Chem., 2004, 69, 439; (b) D. D. Das and A. Sayari, J. Catal.,
2007, 246, 60.
391–401.
3
0 R. J. Kalbasi, A. A. Nourbakhsh and M. Zia, J. Inorg.
Organomet. Polym. Mater., 2012, 22, 536–542.
1 (a) B. Betzemeier, M. Cavazzini, S. Quici and P. Knochel,
Tetrahedron Lett., 2000, 41, 4343; (b) G. Ragagnin,
B. Betzemeier, S. Quici and P. Knochel, Tetrahedron, 2002,
47 (a) A. Caneschi, A. Grand, J. Laugier, P. Rey and R. Subra, J.
Am. Chem. Soc., 1988, 110, 2307; (b) J. Laugier, J. M. Latour,
A. Caneschi and P. Rey, Inorg. Chem., 1991, 30, 4474; (c) J.
M. Hoover, B. L. Ryland and S. S. Stahl, J. Am. Chem. Soc.,
2013, 135, 2357–2367.
3
1
9466 | RSC Adv., 2013, 3, 19455–19466
This journal is ß The Royal Society of Chemistry 2013