A comparative study of different metal acetylacetonates covalently anchored onto amine functionalized silica: A study of the oxidation of aldehydes and alcohols to corresponding acids in water

Article abstract of DOI:10.1039/c2gc35289c

A series of metal acetylacetonates covalently anchored onto amine functionalized silica were prepared by the complexation of metal acetylacetonates [Co(acac)₂, Cu(acac)₂, Pd(acac) ₂, Ru(acac)₃, Mn(acac)₃, Co(acac)₃] with organically modified 3-aminopropyl silica and their catalytic activities were tested for the oxidation of aromatic aldehydes, α,β-unsaturated aldehydes and benzyl alcohols to the corresponding carboxylic acids in aqueous medium. Different metal acetylacetonates have been chosen with a view to select the most active heterogeneous catalyst for oxidations. The characterization of the catalysts was done on the basis of FTIR, TGA and AAS analysis. SiO ₂-Co(acac)₂ catalyzes the oxidation of aromatic aldehydes under air atmosphere without using any additional oxidant; however, t-BuOOH was found to be a highly efficient oxidant for the oxidation of α,β- unsaturated aldehydes and heterocyclic aldehydes, as well as the direct oxidation of benzyl alcohols to the corresponding carboxylic acids. The most active catalyst was found to be highly stable and recyclable under the reaction conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2gc35289c

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PAPER
A comparative study of different metal acetylacetonates covalently anchored
onto amine functionalized silica: a study of the oxidation of aldehydes and
alcohols to corresponding acids in water†
Ravinderpal Kour Sodhi,a Satya Paul*a and J. H. Clarkb
Received 27th February 2012, Accepted 10th April 2012
DOI: 10.1039/c2gc35289c
A series of metal acetylacetonates covalently anchored onto amine functionalized silica were prepared
by the complexation of metal acetylacetonates [Co(acac)2, Cu(acac)2, Pd(acac)2, Ru(acac)3, Mn(acac)3,
Co(acac)3] with organically modified 3-aminopropyl silica and their catalytic activities were tested for
the oxidation of aromatic aldehydes, α,β-unsaturated aldehydes and benzyl alcohols to the corresponding
carboxylic acids in aqueous medium. Different metal acetylacetonates have been chosen with a view to
select the most active heterogeneous catalyst for oxidations. The characterization of the catalysts was
done on the basis of FTIR, TGA and AAS analysis. SiO2–Co(acac)2 catalyzes the oxidation of aromatic
aldehydes under air atmosphere without using any additional oxidant; however, t-BuOOH was found to
be a highly efficient oxidant for the oxidation of α,β-unsaturated aldehydes and heterocyclic aldehydes,
as well as the direct oxidation of benzyl alcohols to the corresponding carboxylic acids. The most active
catalyst was found to be highly stable and recyclable under the reaction conditions.
reported Rh(acac)(CO)2 in combination with polymer supported
Introduction
triphenylphosphine for the hydroformylation of 1-hexene;4a
In recent years, covalently anchored metal complexes onto solid
supports1 have been extensively exploited in organic syntheses,
since the link with the support is stable towards most solvents
and additives, which makes them highly selective and recover-
able under the reaction conditions. Use of such catalysts in water
leads to highly efficient and environmentally-friendly catalytic
systems.2 Metal-catalyzed oxidation of organic compounds is
increasingly becoming an important transformation in modern
day chemistry because of mild conditions and greater selectivity.
Over the past decade, metal acetylacetonate complexes have
emerged as versatile catalysts for a broad range of industrially
and academically interesting reactions3 e.g. radical polymeriza-
microencapsulated Cu(acac)2 has been used successfully for the
aziridination of olefins;4b and Co(acac)3/1-butyl-2,3-dimethyl-
imidazolium chloride (BMMImCl)4c has been reported as a
base-free catalytic system for the clean synthesis of N,N′-disub-
stituted ureas; silica supported In(acac)3 has been employed for
the synthesis of oxindoles in a H2O–CH3CN solvent system.4d
More recently, supported metal acetylacetonate catalysts have
shown excellent activity in oxidation reactions e.g. MoO2(acac)2
immobilized onto MCM-41 has been reported for the selective
epoxidation of alkenes;5a Cicco et al. have reported homo-
geneous and heterogeneous catalytic oxidation of benzylic and
secondary alcohols with polymer supported Co(acac)2;5b poly-
benzimidazole resin-supported metal acetylacetonates have been
reported for the oxidation of cyclohexene with t-BuOOH.5c
Oxidation of aldehydes and alcohols to the corresponding car-
boxylic acids is one of the fundamental transformations due to
its diversified potential in organic syntheses and industrial manu-
facturing.6 Traditionally, this transformation has been carried out
using a variety of conventional oxidants, notably Mn and Cr
based reagents,7 which produce copious amounts of undesirable
wastes. In recent years, various improved methods using calcium
hypochlorite,8a oxone8b or hydrogen peroxide as oxidants under
severe basic conditions,8c or in combination with other reagents,
such as formic acid,8d sodium chlorite,8e benzeneselinic acid,8f
magnesium monoperoxyphthalate,8g have been reported.8h The
metal catalyzed oxidation of aldehydes to the corresponding car-
boxylic acids9 using environmentally-friendly oxidants has
gained popularity over the last few years. More recently,
tion of styrenes3a catalyzed by Co(acac)2, Ni(acac)2, Mn(acac)2 or 3
,
and Al(acac)3; acetylation of phenols, alcohols, and amines
using Ru(acac)3 under neat conditions;3b modified cobalt(II)
acetylacetonate complexes have been employed for Negishi-type
coupling.3c Recently, supported metal acetylacetonate complexes
have been developed and found to be more effective and selec-
tive than their homogeneous counterparts.4 Fujita et al. have
aDepartment of Chemistry, University of Jammu, Jammu-180 006,
India. E-mail: paul7@rediffmail.com; Fax: +91-191-2505086;
Tel: +91-191-2453841
bGreen Chemistry Centre of Excellence, Department of Chemistry,
University of York, Heslington, York, United Kingdom. E-mail:
james.clark@york.ac.uk; Fax: +44 (0)1904 322705; Tel: +44 (0)1904
322567
†Electronic supplementary information (ESI) available. DOI: 10.1039/
c2gc35289c
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covalently anchored metal catalysts have been developed for this
transformation that provided easy work-up, more selectivity and
high turn-over numbers. e.g. Au–CeO2 has been reported for the
selective aerobic oxidation of aliphatic and aromatic aldehydes
to acids;10a ε-Keggin polyoxometalates of Ni, Co, Mn and
Cu have been used for the oxidation of aldehydes to acids;10b
Kholdeeva et al. have reported Co containing polyoxometalate-
based heterogeneous catalysts for the oxidation of aldehydes
under ambient conditions;10c Au–C oxidizes aldehydes to car-
boxylic acids in the liquid phase.10d
The direct oxidation of alcohols to the corresponding
carboxylic acids is more desirable since it avoids one step i.e.
oxidation of alcohols to aldehydes. Heterogeneous transition
metal catalysts have also been developed for this transformation
e.g. Pd–C has been reported for the direct oxidation of alcohols
to carboxylic acids;11a Ley et al. have used immobilized oxidiz-
ing agents for the oxidation of benzyl alcohols to carboxylic
acids.11b Thus, very few heterogeneous metal catalysts have been
developed for the oxidation of aldehydes and alcohols to the cor-
responding carboxylic acids. Hence, there is a great need to
develop novel covalently anchored metal catalysts for the oxi-
dation of aldehydes and alcohols to carboxylic acids.
Results and discussion
Preparation and characterization of covalently anchored
M(acac)n onto amine functionalized silica [SiO2–M(acac)n]
Amino propyl silica (AMPS) supported metal acetylacetonate
complexes were prepared following the earlier reported proce-
dure1b with slight modification (Scheme 1). M(acac)n (where
M = Co, Cu, Pd, n = 2; Co, Mn, Ru, n = 3) was refluxed with
AMPS in dry toluene followed by simple filtration and drying in
the oven at 110 °C. All the six heterogeneous catalysts were
characterized by FTIR, TGA and AAS analysis. Among the
various supported metal acetylacetonates tested, SiO2–Co(acac)2
was found to be most active.
The FTIR spectra of silica and AMPS exhibited strong bands,
which are characteristic of the support matrix. The bands in the
range of 3765–3308 cm−1 were assigned to surface –OH groups
and two strong bands at about 1103 and 799 cm−1 were assigned
to νas(Si–O–Si) and νs(Si–O–Si), respectively. After surface
modification, 3-aminopropylsilica (AMPS) displayed character-
istic –CH2 stretching bands at 2933 and 2974 cm−1. After com-
plexation, the –CH2 stretching band got shifted to 2925 cm−1
and the characteristic band due to CvN bond appeared at
1637 cm−1 in the case of most effective catalyst i.e. [SiO2–
Co(acac)2]. Another band at 1572 cm−1 also appeared, which
was assigned to the acetylacetonate ring. The characteristic
CvN bands for silica supported metal acetylacetonates are pre-
sented in Table 1.
Due to our continued interest in the development of hetero-
geneous catalysts,1b–g we have prepared different metal acetylaceto-
nates covalently anchored onto amine functionalized silica [SiO2–
M(acac)n, n = 2 or 3; M = Co, Cu, Mn, Pd, Ru] and their catalytic
activities were tested for the selective oxidation of aromatic and
α,β-unsaturated aldehydes to the corresponding carboxylic acids;
and alcohols to carboxylic acids, with a view to select the most
effective recyclable and stable heterogeneous catalyst.
The stability of the catalysts was determined by thermogravi-
metric analysis. The TGA was recorded by heating the sample at
Scheme 1 General procedure for the preparation of silica functionalized metal acetylacetonates and their proposed structures.
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Table 1 Characterization of catalysts by FTIR, AAS and thermal
analysis
Table 2 Comparison of the catalytic activities of different metal
acetylacetonates anchored onto AMPS for the oxidation of aldehydes to
acidsa
AAS
TGAc (°C)
FTIRa
(CvN,
νmax in
analysisb
(Metal
per g
Solvent-free
Yieldb
Water
Loss of
residual
solvent
Loss of
organic
functionality
Yieldb
Time (h) (%)
Entry Catalyst
cm−1
1637
1639
)
catalyst)
Entry Catalyst
Time (h) (%)
1
2
3
4
5
6
7
SiO2–Co
(acac)2
0.0145
0.0140
0.013
0.011
0.023
0.011
0.015
100
103
110
95
260–450
250–456
195–500
190–450
260–500
250–350
180–450
1
2
3
4
5
6
SiO2–Co(acac)2
SiO2–Mn(acac)3 3.5
SiO2–Co(acac)3
SiO2–Pd(acac)2 4.5
SiO2–Cu(acac)2
SiO2–Ru(acac)3
2
90
85
82
70
60
50
4.5
5
5
5
5
80
75
73
75
55
50
SiO2–Co
4
d
(acac)2
SiO2–Mn 1630
5
6
(acac)3
SiO2–Co
(acac)3
SiO2–Pd
(acac)2
SiO2–Cu
(acac)2
5
1635
1639
1633
1600
a Reaction
conditions:
4-methoxybenzaldehyde
(1
mmol),
SiO2–M(acac)n [M = Cu, Ru, Pd, Co, Mn and n = 2 or 3 (5 mol% Metal)]
at 50 °C under solvent-free conditions or 70 °C in water (5 mL).
b Isolated yields.
110
99
SiO2–Ru
(acac)3
88
a FTIR was recorded on Perkin-Elmer FTIR spectrophotometer using
KBr discs. b AAS analysis was carried on GBC Avanta-M atomic
absorption spectrometer. The catalyst was stirred in dil. HCl for 10 h and
then subjected to AAS analysis. c Thermal analysis was carried out on
DTG-60 Shimadzu make thermal analyzer with heating rate of 10 °C
min−1 d FT-IR, AAS and TGA analysis of the catalyst after the 5th run.
.
the rate of 10 °C min−1. The TGA curve of SiO2–Co(acac)2
showed an initial weight loss up to 120 °C, which was attributed
to loss of the residual solvent and water trapped onto the surface
of silica. Further, decomposition of organic functionalities starts
from 260 to 450 °C. The major weight loss at high temperature
is characteristic of chemisorbed material and confirms that the
aminopropyl group is chemically bound onto the surface of
silica. Thus, the catalyst is stable up to 260 °C and it is safe to
carry out the reaction at 70 °C under heterogeneous conditions.
The major weight losses of all the six catalysts are presented in
Table 1.
Scheme 2 SiO2–Co(acac)2 catalyzed oxidation of aldehydes or benzyl
alcohols to carboxylic acids in air atmosphere or using t-BuOOH as an
oxidant.
active and SiO2–Ru(acac)3 is the least active. To make the devel-
oped protocol general, the reaction conditions were then
extended for the oxidation of aromatic aldehydes possessing
both electron-withdrawing and electron-donating groups, α,β-un-
saturated aldehydes and heterocyclic aldehydes and found that
aromatic aldehydes possessing one methoxy or one halogen
atom, cinnamaldehyde and furfural undergo oxidation efficiently
without requiring additional oxidant (Scheme 2, Table 3) in
solvent-free as well as under aqueous conditions. However, in
the case of β-chlorocinnamaldehydes, aromatic aldehydes pos-
sessing nitro- and hydroxy groups, two chloro- or three methoxy
groups showed poor results. So in order to make SiO2–Co(acac)2
a general catalyst for the oxidation of all types of aldehydes, the
reaction conditions were then employed for the oxidation of 2,4-
dichlorobenzaldehyde using various oxidants (molecular O2,
H2O2 and t-BuOOH) since it gave poor results under an air
atmosphere. Among these oxidants, 70% t-BuOOH gave the best
results (Table 4, entry 4). Use of tert-butyl hydroperoxide as an
oxidant corresponds to modern trends in organic syntheses since
it is moderately active toward most organic substrates, easily
available and can be used for large scale synthesis. Further, reac-
tion without catalyst in the presence of t-BuOOH provided a
trace amount of acid, indicating that SiO2–Co(acac)2 is catalyz-
ing the reaction (Table 4, entry 5). The generality of the
The amount of the metal supported onto AMPS was deter-
mined by AAS analysis. SiO2–Co(acac)2 contained 0.0145 g of
cobalt per g of the catalyst. The AAS of all the six catalysts is
presented in Table 1.
Catalyst testing for the oxidation of aldehydes and alcohols to
carboxylic acids
Initial attempts to optimize the reaction conditions for the oxi-
dation of aldehydes to the corresponding carboxylic acids were
done using 4-methoxybenzaldehyde as a test substrate and the
oxidation was carried out in the presence of different silica func-
tionalized metal acetylacetonates, oxidants and solvents.
To select the most appropriate heterogeneous metal acetylace-
tonate catalyst, the oxidation of the test substrate was carried out
using different silica functionalized metal acetylacetonate com-
plexes [SiO2–M(acac)n (M = Co, Cu, Mn, Pd and Ru, n = 2 or 3,
5 mol% metal] at 50 °C under solvent-free conditions or 70 °C
under aqueous conditions in air atmosphere. The results are pre-
sented in Table 2. It was found that SiO2–Co(acac)2 is the most
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Table 3 SiO2–Co(acac)2 catalyzed oxidation of aldehydes to carboxylic acids in air under solvent-free conditions and using water as solventa
Solvent-free
Time (h)
1.5
Water
Time (h)
3
Entry
1.
Substrate
Product
Yieldb (%)
93
Yieldb (%)
82
m.p/lit. m.p (°C)12
120–122/12212a
2.
2
90
4.5
80
180–183/182–18512a
3.
4.
4.5
3
93
92
5
5
81
80
236–238/238–24112a
250–252/252–25412a
5.
6.
5
5
90
85
8
77
80
133–135/132–13512a
128–132/128–13512a
7.5
7.
10
65
15
62
145–148/144–14612b
a Reaction conditions: aldehyde (1 mmol), SiO2–Co(acac)2 (0.2 g, 5 mol% Co) at 50 °C in case of solvent-free conditions or at 70 °C using water
(5 mL) as a solvent.b Isolated yields.
In order to find out the role of SiO2–Co(acac)2 as the hetero-
Table 4 Effect of different oxidants on the oxidation of aldehydes to
carboxylic acidsa
geneous catalyst, the reaction with benzaldehyde (Table 6) was
carried out in the presence of activated silica, 3-aminopropyl
silica (AMPS), homogeneous Co(acac)2 and without using cata-
lyst and oxidant other than air. The results are summarised in
Table 6. Under homogeneous conditions, the reaction was com-
pleted in 0.5 h (91% isolated yield of benzoic acid). However,
with SiO2–Co(acac)2 under heterogeneous conditions, 90% (iso-
lated yield) benzoic acid was obtained in 1.5 h. The use of
heterogeneous systems would be more advantageous as cata-
lyst recovery is easier, process is more efficient and more-
over, the catalyst could be recycled for several runs. Thus,
SiO2–Co(acac)2 was selected as the heterogeneous catalyst for
the oxidation of aldehydes to acids. Once we were able to trans-
form aldehydes into carboxylic acids successfully, then we
thought to extend our protocol for the direct oxidation of benzylic
alcohols to carboxylic acids under similar conditions (Scheme 2).
The same conditions worked well and excellent results were
obtained (Table 5). Further, the catalytic activity of the developed
protocol was compared with reported catalytic systems under het-
erogeneous conditions (Table 7). The results demonstrate that our
catalytic system is superior to the reported catalytic systems.
Entry
Oxidants
Time (h)
Yieldb (%)
1
2
3
4
5
Atmospheric O2
H2O2
Molecular O2
70% t-BuOOH
70% t-BuOOH
3
3
3
3
6
10
65
40
93
15c
a Reaction was carried out by stirring
a
mixture of 2,4-
dichlorobenzaldehyde (1 mmol), in presence of oxidant and
SiO2–Co(acac)2 (0.2 g, 5 mol% Co) at 70 °C using water (5 mL).
b Isolated yields. In the case of entries 1 and 5, yield refers to column
chromatography yields. c The reaction was carried out in the absence of
SiO2–Co(acac)2.
oxidation catalytic system i.e. combination of SiO2–Co(acac)2
and t-BuOOH has been demonstrated by the oxidation of
various aldehydes (Table 5, Scheme 2).
Furthermore, several solvents like acetonitrile, ethanol,
toluene and water were screened using 3-(4-methoxyphenyl)-3-
chloroacryldehyde as the test substrate. Out of these, water
turned out to be the best solvent, providing the highest yield of
acid in the shortest time with maximum selectivity (Fig. 1). The
reaction with test substrate was also tried under solvent-free con-
ditions using 70% t-BuOOH, but unfortunately poor results were
obtained.
Recyclability
The deactivation and recyclability of the catalysts becomes more
important when performing oxidation reactions. To test this, a
1652 | Green Chem., 2012, 14, 1649–1656
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Table 5 SiO2–Co(acac)2 catalyzed oxidation of aldehydes or benzyl alcohols to carboxylic acids using aqueous 70% t-BuOOH at 70 °C in watera
Entry
1
Substrate
Product
Time (h)
5
Yieldb (%)
90
m.p/lit. m.p (°C)12
145–148/144–14612b
2
3
6
5
92
94
150–152/149–15112b
195–200
4
5
6
7
4
89
94
75
70
205–235/199–24012a
235–237/237–24012a
147–149/146–14812a
213–215/213–21712a
2
5.5
7
8
5.5
3
60
93
90
155–157/158–16112a
155–157/157–16012a
135–138/139–14112a
9
10
7
11
12
3
2
91
92
168–170/168–17112a
Liq./16212a
13
14
3.5
4.5
91
90
97–98/95–9812a
180–183/182–18512a
15
16
5
4
90
92
236–238/238–24112a
250–252/252–25412a
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Table 5 (Contd.)
Entry
17
Substrate
Product
Time (h)
10
Yieldb (%)
92
m.p/lit. m.p (°C)12
235–237/237–24012a
18
5.5
91
98–99/98–10012a
a Reaction conditions: aldehyde or alcohol (1 mmol), 70% t-BuOOH (0.5 mL, 4.5 mmol), SiO2–Co(acac)2 (0.2 g, 5 mol% Co) at 70 °C in water
(5 mL). b Isolated yields.
Fig. 1 Effect of solvents on the oxidation of 3-(4-methoxyphenyl)-3-chloroacryldehyde; Reaction conditions: 3-(4-methoxyphenyl)-3-chloroacrylde-
hyde (1 mmol), aqueous 70% t-BuOOH (0.5 mL, 4.5 mmol), solvent (5 mL) and SiO2–Co(acac)2 (0.2 g, 5 mol% Co).
the fresh catalyst (1637 cm−1). This indicates that the catalyst
Table 6 Comparison of activity of SiO2–Co(acac)2, activated silica,
AMPS and homogeneous Co(acac)2 for the oxidation of benzaldehydea
retained its original structure during the reaction.
Entry
Catalyst
Time (h)
Yieldb (%)
Experimental
1
2
3
4
5
No catalyst
Activated silica
AMPS
Co(acac)2
SiO2–Co(acac)2
8
8
8
0.5
1.5
NRc
30
20
91
90
General
The chemicals used were either prepared in our laboratories or
purchased from Aldrich Chemical Company or Merck. Pet-
roleum ether (60–80 °C) was used in all experiments. The
spectra of the products were compared with those obtained from
a Reaction conditions: benzaldehyde (1 mmol), and catalyst (0.2 g for
entries 1–3; 0.003 g, 5 mol% Co for entry 4; and 0. 2 g, 5 mol% Co for
entry 5) at 50 °C in solvent-free conditions. b Isolated yields. In the case
of entries 2 and 3, yield refers to column chromatography yield. c NR:
No reaction.
1
Aldrich, unless otherwise specified by a reference. The H or
13C NMR data were recorded in CDCl3 or DMSO-d6 on Bruker
DPX 200 spectrometer. The FTIR spectra were recorded on
Perkin-Elmer FTIR spectrophotometer and mass spectral data
were recorded on Bruker Esquires 3000 (ESI). The amount of
metal in catalysts was determined by AAS analysis and thermal
analysis was carried out on a DTG-60 Shimadzu thermal analy-
zer. All yields refer to the isolated yields.
series of 5 consecutive runs for the oxidation of 4-nitrobenzalde-
hyde and 4-bromobenzylalcohol to the corresponding carboxylic
acids were carried out and the results are represented in Fig. 2,
which demonstrate that SiO2–Co(acac)2 is highly active and
recyclable up to the 5th run with very little loss of activity. This
may be due to either microscopic changes in the structure of the
catalyst during drying or leaching of the active species (about
3% determined by AAS analysis of the catalyst after the 5th
run). Further, FTIR of the catalyst after the 5th run showed the
absorption at 1639 cm−1 (CvN), which is slightly higher than
General procedure for the synthesis of aminofunctionalized
M(acac)n
Activated silica (5 g) was added to the solution of 3-amino-
propyl(trimethoxy) silane (0.89 g, 5 mmol) in dry toluene
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Table 7 Comparison of the catalytic activity of SiO2–Co(acac)2 with reported heterogeneous catalytic systems for the oxidation of benzaldehyde and
4-methoxybenzylalcohol to corresponding carboxylic acidsa
Entry
Catalyst/conditions
Oxidant
Solvent/temp
Conversion (%)
Time (h)
1
2
3
4
5
6
7
SiO2–Co(acac)2
Air
O2
O2
O2
t-BuOOH
Air
Air
H2O/70 °C
H2O/90 °C
H2O/90 °C
100
24
20
99
100
92 (% Yield)
92 (% Yield)
3
2
2
16
4.5
15
48
Au–Carbon10d
Pt–Carbon10d
#xX_CYR_HEX_404;-Keggin type10b polyoxometalates
SiO2–Co(acac)2
Toluene/90 °C
H2O/70 °C
Pd–Carbon11a (NaBH4, KOH)
CH3OH–H2O/RTb
CH3CN–H2O/RT
PS-TEMPO,11b PS-Chlorite, Ps-Dihydrogen phosphate, KBr, NaOCl
a Reaction conditions: Oxidation of benzaldehyde to benzoic acid (Entries 1–4); Oxidation of 4-methoxybenzylalcohol to 4-methoxybenzoic acid
(Entries 5–7). b RT: Room temperature.
Fig. 2 Recyclability of SiO2–Co(acac)2; Reaction conditions: aldehyde to carboxylic acid: 4-nitrobenzaldehyde (1 mmol), 70% t-BuOOH (0.5 mL,
4.5 mmol), SiO2–Co(acac)2 (0.2 g, 5 mol% Co) at 70 °C for 2 h using water (5 mL); alcohol to acid: 4-bromobenzylalcohol (1 mmol), 70% t-BuOOH
(0.5 mL, 4.5 mmol), SiO2–Co(acac)2 (0.2 g, 5 mol% Co) at 70 °C for 4 h using water (5 mL).
General procedure for the SiO2–Co(acac)2 catalyzed oxidation of
aldehydes to carboxylic acids using air as the oxidant
(100 mL) and refluxed for 15 h. The 3-aminopropylsilica
(AMPS) was filtered off, washed with hot toluene
(100 mL) and dried at 110 °C for 5 h to give the surface
bound amino groups (AMPS). For the preparation of
The mixture of aldehyde (1 mmol) and SiO2–Co(acac)2 (0.2 g,
5 mol% Co) in a round-bottom flask (25 mL) was stirred at
50 °C (solvent-free conditions) or 70 °C in water (5 mL) for an
appropriate time (Table 3) in air atmosphere. On completion
(monitored by TLC), the reaction mixture was treated with
EtOAc (20 mL) and filtered. The filtrate was treated with satu-
rated NaHCO3 solution and extracted with EtOAc. The aqueous
layer was acidified using 2 M HCl and extracted with ethyl
acetate. The organic layer was concentrated and finally the
product was obtained after crystallization with EtOAc–petroleum
ether. The residue (recovered catalyst) was washed with EtOAc
(3 × 5 mL) followed by double distilled water (3 × 10 mL). It
was dried at 100 °C for 2 h and reused for subsequent reactions.
SiO2–M(acac)n,
(AMPS, 3 g) and M(acac)n [0.5 mmol, 0.12 g Co(acac)2,
0.13 Cu(acac)2, 0.15 Pd(acac)2, 0.19 Ru(acac)3,
a mixture of organically modified silica
g
g
g
0.12 g Co(acac)3 and 0.17 g Mn(acac)3] in dry toluene
(100 mL) was stirred at 120 °C for 15 h. The catalyst was
filtered off, washed with toluene (200 mL) till the washings
were colourless and dried in a hot air oven at 110 °C for
5 h. In order to remove any physisorbed M(acac)n, the cata-
lysts were conditioned by refluxing for a total of 12 h in xylene
at 130 °C (2 × 2 h), ethanol at 78 °C (2 × 2 h) and acetonitrile at
80 °C (2 × 2 h). Finally, the catalysts were dried in a hot air oven
at 110 °C for 5 h.
This journal is © The Royal Society of Chemistry 2012
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General procedure for the SiO2–Co(acac)2 catalyzed oxidation of
aldehydes and alcohols to carboxylic acids using t-BuOOH in
water
To a mixture of aldehyde or alcohol (1 mmol), SiO2–Co(acac)2
(0.2 g, 5 mol% Co) and 70% t-BuOOH (0.5 mL, 4.5 mmol),
water (5 mL) was added and the reaction mixture was stirred at
70 °C until the complete conversion of aldehyde or alcohol took
place (monitored by TLC) (Table 5). After completion of the
reaction, it was filtered and washed with EtOAc (10 mL). The
filtrate was treated with saturated NaHCO3 solution and extracted
with EtOAc. The aqueous layer was acidified with 2 M HCl and
extracted with ethyl acetate. The organic layer was concentrated
and the product was obtained after crystallization with EtOAc–
pet. ether. The recovered catalyst was washed with EtOAc (3 ×
5 mL) followed by double distilled water (3 × 10 mL). It was
dried at 100 °C for 2 h and reused for subsequent reactions.
1
The products were confirmed by IR, H and 13C NMR, mass
spectral data (see ESI†) and comparison with authentic samples
obtained commercially or prepared according to the literature
methods.
Conclusions
In conclusion, a series of different metal acetylacetonates cova-
lently anchored onto amine functionalized silica were prepared
and their catalytic activity was evaluated for the oxidation of
aldehydes to carboxylic acids in air, and using t-BuOOH as an
oxidant. SiO2–Co(acac)2 was found to the most active for the
oxidation of most aldehydes using air as an oxidant. Further,
aldehydes bearing dihalogens, nitro groups and three methoxy
groups as well as α,β-unsaturated, heterocyclic and aliphatic
aldehydes undergo oxidation efficiently to the corresponding car-
boxylic acids using t-BuOOH as an oxidant. The developed pro-
tocol was also successful for the direct oxidation of benzyl
alcohols to the corresponding carboxylic acids. The catalyst was
found to be highly active and could be recycled for five consecu-
tive runs without significant loss of activity.
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Acknowledgements
We thank the Director, IIIM Jammu for spectral and library
facilities; We also thank the Department of Chemistry, University
of Jammu for TGA analysis.
Notes and references
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1656 | Green Chem., 2012, 14, 1649–1656
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