Selective oxidation of alcohols using ferrocene-labeled Merrifield resin-supported ionic liquid phase catalysts

Article abstract of DOI:10.1007/s11164-016-2563-2

Abstract: A novel ferrocene-labeled Merrifield resin-supported ionic liquid phase catalysts has been synthesized and successfully utilized for the selective oxidation of primary alcohols into corresponding aldehydes. The catalysts were recovered by simple filtration which greatly simplified the purification step and allowed successive reuse for multiple times without significant loss in activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2563-2

Res Chem Intermed
DOI 10.1007/s11164-016-2563-2
Selective oxidation of alcohols using ferrocene-labeled
Merrifield resin-supported ionic liquid phase catalysts
1
1
•
Rajanikant Kurane
Sharanabasappa Khanapure
•
Prakash Bansode
1
1
•
•
Rajashri Salunkhe
1
Gajanan Rashinkar
Received: 30 January 2016 / Accepted: 26 April 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A novel ferrocene-labeled Merrifield resin-supported ionic liquid phase cat-
alysts has been synthesized and successfully utilized for the selective oxidation of pri-
mary alcohols into corresponding aldehydes. The catalysts were recovered by simple
filtration which greatly simplified the purification step and allowed successive reuse for
multiple times without significant loss in activity.
Graphical Abstract
&
Gajanan Rashinkar
gsr_chem@unishivaji.ac.in
1
Department of Chemistry, Shivaji University, Kolhapur, M.S. 416004, India
123

R. Kurane et al.
Keywords Merrifield resin Á Selective oxidation Á Alcohols Á Ferrocene Á Reusability
Introduction
Selective oxidation of alcohols into aldehydes is a ubiquitous and pivotal process in
organic chemistry [1–4]. Traditionally, high-valent metal oxides of chromium,
manganese and ruthenium are widely employed for this purpose [5–8]. Insights into
these homogeneous catalytic systems have revealed some serious problems, such as
over-oxidation of aldehydes to carboxylic acid, the use of hazardous terminal
oxidants, drastic reaction conditions, tedious work-up procedures and generation of
significant amounts of toxic waste products. The covalent immobilization of these
reagents on solid supports has been carried out to alleviate aforementioned problems
and provide an attractive alternative in organic synthesis in view of the selectivity
and associated ease of manipulation [9–14]. A variety of other reagents, such as
oxone [15], calcium hypochlorite [16], 2-hydroperoxyhexafluoro-2-propanol [17],
vanadium [18], Cu-NHC-TEMPO [19], nano structured rutile TiO [20], palladium
2
[
[
21], choline peroxydisulphate [22], nickel phosphine complexes [23], In(NO ) in
3 3
C mim][FeBr ] [24], derivatives of 1-acetoxy-1,2-benziodoxole-3(1H)-one [25],
4
12
sym-collidinium chlorochromate (S-COCC) [26], Ag-NHC complex [27], etc. have
been used for selective oxidation of alcohols to aldehydes. However, despite of
impressive progress, a mild protocol using a highly robust heterogeneous catalyst
still represents a considerable synthetic challenge.
Environmental and economic concerns have recently raised much attention in
redesigning synthetic processes using green chemistry principles so that the use of
hazardous reagents and the use of toxic waste can be minimized. In this regard,
considerable attention has been focused on the contemporary concept of supported
ionic liquid phase (SILP) catalysis involving covalent immobilization of ionic
liquid-like units onto a surface of a porous high area support material, as it provides
an elegant approach for designing advanced materials for sustainable synthesis [28].
The noteworthy features of this novel class of environmentally benign reagents
include high activity and selectivity along with the ease of separation from the
reaction mixture by simple filtration. The development of novel SILP catalysts for
various organic transformations has dramatically advanced in recent years [29–35].
In continuation of our work related to SILP catalysis [36–38], we report here the
synthesis of ferrocene-labeled Merrifield resin-supported ionic liquid phase catalysts
containing perruthenate, permanganate and perchromate anions for the selective
oxidation of primary alcohols into corresponding aldehydes. Our interest in
incorporating the ferrocene moiety into the backbone of SILP catalysts stem from
the bulk of the ferrocenyl group which causes substantial improvements in the
permeability of the substrate into the catalyst matrix, thereby improving the
accessibility of reactants to the active sites which ultimately facilitates the catalyst
performance of such catalysts [39–44].
1
23

Selective oxidation of alcohols using ferrocene-labeled…
Experimental
General
All reactions were carried out under air atmosphere in dried glassware. Infrared
spectra were measured with a Perkin-Elmer one FTIR spectrophotometer. The
1
13
samples were examined as KBr discs *5 % w/w. H NMR and C NMR spectra
were recorded on a Bruker AC (300 MHz for H NMR and 75 MHz for C NMR)
1
13
spectrometer using CDCl as solvent and tetramethylsilane (TMS) as an internal
3
standard. Chemical shifts are expressed in d parts per million (ppm) values with
tetramethylsilane (TMS) as the internal reference, and coupling constants are
expressed in hertz (Hz). Mass spectra were recorded on a Shimadzu QP2010
GCMS. The materials were analyzed by SEM using a JEOL model JSM with 5 and
2
0 kV accelerating voltage. Elemental analyses were performed on the EURO
EA3000 vector model. Melting points were determined on a MEL-TEMP capillary
melting point apparatus and are uncorrected. N,N-Dimethylaminomethyl fer-
rocene was synthesized using a previously reported procedure [70]. The various
aryl alcohols (Spectrochem) and Merrifield resin (2 % cross linked, 200–400 mesh,
ca 2–3 mmol/g; Alfa Aesar) were used as received.
Preparation of [FemDMMerA]Cl (3)
A mixture of N,N-dimethylaminomethyl ferrocene (1) (3.03 gm, 10 mmol)
Merrifield resin (2) (3.0 g) and in 25 mL of DMF was heated at 80 °C in an oil
bath. After 72 h, the polymer was filtered, washed with DMF (3 9 50 mL), MeOH
(
afford [FemDMMerA]Cl (3). The loading was found to be 1.40 mmol/g of matrix.
3 9 50 mL), CH Cl (3 9 50 mL), and dried under vacuum at 50 °C for 24 h to
2
2
FT-IR (KBr, thin film): t = 3408, 3053, 3019, 2918, 2851, 2689, 1602, 1510, 1448,
-
420, 1317, 1219, 1160, 1104, 939, 830, 805, 758, 696, 556, 464 cm ; FT-
1
1
RAMAN (KBr, thin film): t = 2253, 2155, 2088, 2043, 1552, 1399, 1251, 1110,
-
04, 675, 580 cm ; Elemental analysis observed: %C 70.12, %H 5.62, %N 3.02.
1
9
Preparation of [FemDMMerA]RuO (4a)
4
The suspension of 3 (3.0 g) and KRuO (1.84 g, 9 mmol) in a distilled water (25
4
mL) was stirred for 24 h. Afterward the polymer was filtered and washed with
distilled water (3 9 20 ml), as well as washed under Soxhlet and dried under
vacuum at 50 °C for 48 h to afford [FemDMMerA]RuO (4a). The loading was
4
found to be 1.7 mmol/g of matrix. FT-IR (KBr, thin film): t = 3409, 2922, 2854,
1
4
2
5
615, 1507, 1404, 1320, 1258, 1160, 1099, 1017, 840, 808, 699, 668, 558,
-
1
64 cm ; FT-RAMAN (KBr, thin film): t = 2900, 2472, 2303, 2243, 2162, 2102,
053, 2021, 1912, 1754, 1699, 1596, 1216, 1145, 1101, 1031, 922, 841, 737, 694,
-
80, 531 cm ; Elemental analysis observed: %C 51.73, %Fe 11.66, %Ru 16.78.
1
123

R. Kurane et al.
Preparation of [FemDMMerA]MnO (4b)
4
The suspension of 3 (3.0 g) and KMnO (1.42 g, 9 mmol) in a distilled water (25
4
mL) was stirred for 24 h. Afterward the polymer was filtered and washed with
distilled water (3 9 20 ml) as well as washed under Soxhlet and dried under
vacuum at 50 °C for 48 h to afford [FemDMMerA]MnO 4b. The loading was
4
found to be 8.0 mmol/g of matrix. FT-IR (KBr, thin film): t = 3365, 2924, 2851,
-
601, 1508, 1485, 1396, 1253, 1158, 1104, 1018, 853, 702, 567, 464 cm ; FT-
1
1
RAMAN (KBr, thin film): t = 2298, 2162, 2059, 1829, 1792, 1401, 1210, 1009,
-
1
9
17, 807, 574 cm ; Elemental analysis observed: %C 30.80, %Fe 11.67, %Mn
4.10.
4
Preparation of [FemDMMerA]CrO (4c)
4
The suspension of 3 (3.0 g) and CrO (0.9 g, 9 mmol) in a distilled water (25 mL)
3
was stirred for 24 h. Afterward the polymer was filtered and washed with distilled
water (3 9 20 ml) as well as washed under Soxhlet and dried under vacuum at
5
4
1
0 °C for 48 h to afford [FemDMMerA]CrO 4c. The loading was found to be
4
.2 mmol/g of matrix. FT-IR (KBr, thin film): t = 3425, 2924, 2851, 1620, 1507,
454, 1398, 1261, 1099, 1019, 942, 802, 702, 537, 464 cm ; FT-RAMAN (KBr,
-
1
thin film): t = 2591, 2287, 2156, 2086, 2048, 2004, 1917, 1868, 1765, 1640, 1531,
-
1
1
428, 1199, 1096, 955, 754, 585 cm ; Elemental analysis observed: %C 52.94,
Fe 17.91, %Cr 21.93.
%
General procedure for the oxidation of alcohols
A mixture of aryl alcohol (1 mmol) and [FemDMMerA]Y (100 mg) in solvent
(
5 mL) was refluxed in oil bath. After completion of the reaction as monitored by
TLC, the reaction mixture was filtered to remove insoluble SILP catalyst.
Evaporation of solvent in vacuuo followed by column chromatography over silica
gel using petroleum ether/ethyl acetate (95:5 v/v) afforded pure aldehydes.
Spectral data of representative compounds
Table 2, entry d: White solid, M.p.: 103 °C, FT-IR (neat, thin film): t = 2970,
2
1
–
850, 2229, 1701, 1680, 1608, 1571, 1530, 1460, 1430, 1410, 1383, 1296, 1201,
1 1
-
171, 1011, 980, 827, 780, 765 cm ; H NMR (300 MHz, CDCl ) d: 10.11 (s, 1H,
3
1
CHO), 8.00-8.03 (q, 2H, Ar–H), 7.86 (d, 2H, J = 10.2 Hz, Ar–H) ppm; C NMR
3
(
75 Hz, CDCl ) d: 191.3, 138.7, 132.9, 129.9, 117.7, 117.6 ppm; MS (EI): m/z
3
?
31[M] ; Elemental analysis observed: %C: 73.13, %H: 3.82, %N: 10.65.
1
Table 2, entry o: Lusterous red solid, M.p.: 122 °C, FT-IR (neat, thin film):
t = 3093, 2827, 2761, 1682, 1662, 1586, 1510, 1452, 1409, 1371, 1268, 1243,
-
1 1
1
201, 1136, 1106, 1023, 1001, 821, 762 cm ; H NMR (300 MHz, CDCl ) d: 9.98
3
(
1
s, 1H, –CHO), 4.81 (s, 2H, Cp–H), 4.63 (s, 2H, Cp–H), 4.29 (s, 5H, Cp–H) ppm;
3
?
C NMR (75 Hz, CDCl ) d: 193.4, 73.2, 69.7 ppm; MS (EI): m/z 214 [M] ;
3
Elemental analysis observed: %C: 61.48, %H: 4.57, %Fe: 26.05.
1
23

Selective oxidation of alcohols using ferrocene-labeled…
Results and discussion
Our initial studies were focused on the synthesis of SILP catalysts. They were
readily prepared through a two-step procedure (Scheme 1). First, N,N-dimethy-
laminomethyl ferrocene (1) was quaternized with Merrifield resin (2) to afford
[
FemDMMerA]Cl (3). The anion metathesis reaction of 3 with KRuO , KMnO and
4
4
Me
Me
N
+
Cl
Fe
Merrifield Resin (2)
N,N-Dimethylaminomethyl Ferrocene (1)
Me
N+
Cl-
Fe
Me
[FemDMMerA]Cl (3)
KRuO , KMnO and CrO
3
4
4
Me
Y-
N+
Fe
Me
-
[
FemDMMerA]Y (4)
-
-
4
4
4
a Y = RuO
4
-
-
b Y = MnO
4
-
-
c Y = CrO
4
Scheme 1 Synthesis of ferrocene labeled Merrifield resin supported ionic liquid phase catalysts
123

R. Kurane et al.
CrO resulted in the formation of the desired ferrocene-labeled Merrifield resin-
3
supported ionic liquid phase catalysts containing perruthenate, permanganese and
perchromate anion acronymed as [FemDMMerA]RuO (4a), [FemDMMerA]MnO4
4
(
4b) and [FemDMMerA]CrO (4c).
4
Due to insolubility of 4a–c in common organic solvents, their structure
elucidation was limited to FT-IR and FT-Raman spectroscopy. The quaternization
reaction was examined by using FT-IR spectroscopy. The negative bands at
-
1
-1
6
almost disappeared while the peaks at 464 cm (Fe–Cp stretching band), 1105
69 cm (C–Cl stretching band) and 1260 cm (wagging bands of CH –Cl) [45]
2
-
1
-
1
and 1160 (stretching modes of C–N), 3019, 2918 and 2851 cm (C–H stretching of
Cp rings) increased in intensity after 72 h, reflecting the substantial degree of
quaternization. Anion metathesis reaction was monitored using FT-IR as well as FT-
-
1
Raman spectroscopy. In the FT-IR spectra, the appearance of a peak at 840 cm
1
-
-1
(
for [FemDMMerA]RuO ), 853 cm (for [FemDMMerA]MnO ), 942 cm (for
4 4
-
FemDMMerA]CrO ), which are characteristic peaks of RuO , MnO4 , and
-
[
CrO4 , confirmed the formation of 4a–c. In addition, the FT-Raman spectra of 4a–c
4
4
-
-
-
-
displayed characteristic peaks of RuO , MnO4 and CrO4 at 1031, 1009 and
4
-
1
1
096 cm , respectively, confirming the proposed structures of the desired SILP
catalysts.
The Ru, Mn, and Cr contents were mapped by EDX analysis ( Fig. 1). The results
that indicated about 1.7, 8.0, and 4.2 mmol of ruthenium, manganese and chromium
-
1
g
were present in 4a, 4b and 4c, respectively.
Scanning electron micrographs (SEM) at various stages of preparation of 4a–c
were recorded to understand the morphological changes on the surface of the
Merrifield resin. Scanning was done across the entire length of the polymeric resin
4 4 4
Fig. 1 EDX of a [FemDMMerA]RuO , b [FemDMMerA]MnO , c [FemDMMerA]CrO
1
23

Selective oxidation of alcohols using ferrocene-labeled…
3
beads. Comparison of images taken at a magnification of *3 9 10 indicated that
the spherical beads of Merrifield resin with smooth surfaces (Fig. 2A) were tainted
upon functionalization. After functionalization, the resin beads in 4a–c were not
spherical like the original Merrifield resin beads (Fig. 2A). In fact, bead degradation
was observed which may be plausibly due to stress generated during quaternization
and the subsequent anion metathesis reaction (Fig. 2B–D) [46]. High ratios of
covalently anchored quaternary ammonium group mass to bead volume leads to a
situation where most of the beads experience enormous tension. These points of
Fig. 2 Scanning electron micrographs of
C [FemDMMerA]MnO and D [FemDMMerA]CrO
A
Merrifield resin,
B
4
[FemDMMerA]RuO ,
4
4
123

R. Kurane et al.
tension evolve and give rise to stress in the bead which eventually bursts. Although
the morphology of SILP catalysts was not that of the original Merrifield resin, there
was no effect on their catalytic potential.
The thermogravimetric analysis (TGA) of 4a–c was performed in the temperature
range of 25–1000 °C in air to support their stability and robustness (Fig. 3A–C).
The initial weight losses of 5.31, 7.69 and 5.90 % in the respective TGA curves of
4
a, 4b and 4c from room temperature to 100 °C are mainly due to evaporation of
physically absorbed water. The second weight loss in the range 100–290 °C for 4a–
c is attributed primarily due to the decomposition of organics including the
ferrocenyl group. An abrupt weight loss between 300–420 °C suggests the
breakdown of the Merrifield resin backbone. Finally, the residual weights of
around 19.35 % (in 4a), 26.21 % (in 4b) and 34.94 % (in 4c) in the thermograms
correspond to the formation of non-volatile oxides of Ru, Mn and Cr, respectively,
and carbonaceous matter like graphitic carbon and carbonates due to incomplete
combustion of polymers because of the high ramping rate used in TGA analysis.
Our next task was to assess the catalytic activity of the synthesized SILP
catalysts. The oxidation of benzyl alcohol was chosen as a model reaction to
optimize the reaction conditions in terms of the catalyst loading and solvent.
Initially, the effect of various solvents on the model reaction was studied. The
model reaction was performed under reflux conditions in various solvents including
nonpolar toluene, 1,4-dioxane and dichloromethane, polar aprotic acetonitrile, THF,
DMF and DMSO and polar protic methanol and ethanol. The experimental
procedure for the reaction was trouble-free and straightforward. A mixture of benzyl
alcohol (1 mmol) in each solvent (5 mL) was heated to reflux in the presence of 4a–
c (100 mg) until the completion of the reaction as monitored by TLC. The reaction
was sluggish in dichloromethane, methanol and ethanol (Table 1, entries 1–3) while
the use of the toluene, 1,4-dioxane and acetonitrile afforded the anticipated products
in moderate yields (Table 1, entries 10–12). The reaction could not be initiated to a
synthetically useful degree in solvents such as DMF and DMSO (Table 1, entries 4
and 5). It was observed that THF is the most effective in providing the highest
conversion (Table 1, entry 7). The enhanced oxidation rate in THF is accounted for
on the basis of the swelling factor of resin which is a crucial factor in solid phase
organic synthesis. It is well established that a greater degree of swelling leads to a
higher degree of diffusion rate of the reactant onto the surface of the resin, causing
the accessing of the more reactive sites. This results in shorter reaction time and
high chemical conversion [47–49]. Griffith et al. have extensively investigated the
swelling properties of Merrifield resin in various solvents and proved that the resin
has a very high degree of swelling in THF [50]. In our case, measurements of the
swelling degree of SILP catalysts (4a–c) and Merrifield resin were carried out
according to a previously reported procedure [51]: to a known weight of 4a–c and
Merrifield resin in a separate 1 mL syringe, a known amount of THF was added and
allowed to swell to full capacity. The swelling degree was determined by the
measurement of bead diameter using fluorescence microscopy. An approximately
3
0 % increase in the size of 4a–c and Merrifield resin was observed compared with
untreated materials. Overall, this proves that the highest activity of oxidation in
THF is reasonably due to the high degree of swelling of the SILP catalysts in THF.
1
23

Selective oxidation of alcohols using ferrocene-labeled…
4 4 4
Fig. 3 Thermograms of A [FemDMMerA]RuO , B [FemDMMerA]MnO and C [FemDMMerA]CrO
Next, the effect of catalyst loading on the model reaction was investigated. As
shown in Table 1, the yield of benzaldehyde increased by increasing the quantity of
the catalysts from 50 to 100 mg (Table 1, entries 6–7). However, a further increase
in the amount of the catalysts did not have a significant effect on the product yield
and reaction time (Table 1, entries 8–9).
123

R. Kurane et al.
It is also worth mentioning that 4a was found to be a very efficient catalyst
compared with 4b and 4c in terms of both reaction time and the yield of product.
The excellent oxidation performance of 4a may be rationalized on the basis of the
-
stabilization of RuO4 anion by alcohol solvation [52]. As better results were
obtained with 4a, it was used in further studies.
With a reliable set of conditions to hand, we probed the generality of the protocol
by oxidizing structurally diverse aryl alcohols with 4a in THF. The results are
summarized in Table 2. In all cases, corresponding aldehydes were obtained as sole
products without any over-oxidation. In general, alcohols with electron-donating
Table 1 Optimization of reaction conditions
CHO
CH OH
-
-
2
(
a)Y = RuO
4
FemDMMerA]Y- (b) Y = MnO
-
-
[
4
-
-
(
c) Y = CrO
4
Solvent, Reflux
6a
5a
-
a
Yield (%) FcMerY
-
Entry Solvent
Amount of catalyst (mg) Time (h) FcMerY
-
-
Yb
-
-
-
Yb
-
Ya
Yc
Ya
Yc
b
b
b
b
b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Dichloromethane 100
24
18
19
29
32
8
24
20
22
32
29
10
7
24
24
26
35
35
11
8
35
45
46
25
35
32
26
30
29
Methanol
Ethanol
DMSO
DMF
100
100
100
100
Trace Trace Trace
Trace Trace Trace
c
50
THF
71
95
96
96
65
68
65
69
–
65
72
73
72
61
58
55
–
61
71
73
72
59
62
52
–
b
d
e
b
b
b
THF
100
150
200
100
100
100
3
THF
3
7
8
THF
3
7
8
0
1
2
3
4
5
Toluene
1,4-Dioxane
Acetonitrile
15
18
13
9
19
22
18
–
22
20
15
–
KRuO
KMnO
CrO
4
20.4 (0.1 mmol)
15.8 (0.1 mmol)
10 (0.1 mmol)
4
–
15
–
–
68
–
–
3
–
18
–
65
Optimal reaction conditions: 5a (1 mmol.), solvent (5 ml), SILP (100 mg), and reflux
a
Isolated yields after column chromatography
b
%
Loading of catalyst per 100 mg: (0.17 mmol of 4a), (0.8 mmol of 4b) and (0.42 mmol of 4c)
c
d
e
%Loading of catalyst per 50 mg (0.085 mmol of 4a), (0.4 mmol of 4b) and (0.21 mmol of 4c)
%Loading of catalyst per 150 mg (0.25 mmol of 4a), (1.2 mmol of 4b) and (0.63 mmol of 4c)
%Loading of catalyst per 200 mg (0.34 mmol of 4a), (1.6 mmol of 4b) and (0.84 mmol of 4c)
1
23

Selective oxidation of alcohols using ferrocene-labeled…
substituents on the aryl ring gave good yields (Table 2, entries b, e, h–k and p) as
compared with those with electron-withdrawing substituents (Table 2, entries d, f
and g). In addition, heterocyclic alcohols such as furfuryl alcohol and thenyl alcohol
could also be oxidized into corresponding aldehydes in excellent yields (Table 2,
entries l and m). Moreover, organometallic alcohol such as ferrocenyl methanol
could also serve as a substrate in this reaction (Table 2, entry o). In addition, the
protocol tolerated the presence of functional groups in the ortho position of the
primary alcohols (Table 2, entries b, c and i). The reaction of the sterically hindered
2
-naphthyl alcohol even gave a yield of 87 %. In addition, the oxidation of
cinnamyl alcohol resulted in selective oxidation, yielding corresponding cin-
namaldehyde in modest yield highlighting the general applicability of the protocol.
1
The identity of all the compounds was ascertained on the basis of IR, H NMR,
13
C
NMR spectroscopy as well as by mass spectrometry. The physical and spectroscopic
data are consistent with the proposed structures and are in harmony with the
literature values [19, 53–60].
4
Table 2 [FemDMMerA]RuO catalyzed oxidation of alcohols
[
FemDMMerA]RuO4
R
CHO
R
CH OH
2
Alcohol
THF, Reflux
Aldehyde
5
6
a
Yield (%)
Entry
Alcohol (5) R
Product (6)
Time (h)
Conversion %
a
b
c
d
e
f
C
6
H
5
–
6a
6b
6c
6d
6e
6f
3
3
4
3
4
3
3
4
5
4
5
4
4
5
4
5
7
95
89
92
96
89
95
94
91
88
89
86
92
89
87
92
86
72
[99
[99
[99
[99
[95
[99
[99
[99
[99
[99
[99
[99
[99
[96
[99
[94
[96
2-OH–C
2-NO –C
4-CN–C
4-MeO–C
6 4
H –
2
6
H
4
–
6
H
4
–
6 4
H –
3-NO
2
2
–C H –
6 4
g
h
i
4-NO
–C
6
H
4
–
6g
6h
6i
6 4
3-MeO–C H –
2-CH
3-CH
4-CH
3
–C
3
–C
3
–C
6
6
6
H
4
H
4
H
4
–
–
–
j
6j
k
l
6k
6l
2-Furyl-
m
n
o
p
q
2-Thenyl-
Naphthyl-
6m
6n
6o
6p
6q
Ferrocenyl-
3,4-Di-MeO–C
Cinnamyl-
6 4
H –
Optimal reaction conditions: 5 (1 mmol.), THF (5 ml), SILP (100 mg.) and reflux
a
Isolated yields after column chromatography
123

R. Kurane et al.
On the basis of the literature, a tentative mechanistic rationale for the oxidation
of aryl alcohols using 4a is formulated in Scheme 2. The mechanism is likely to
proceed via a free radical-like transition state, in which the metal alcoholate
complex is formed [61–63] by the attack of alcohol on the oxometallate ion, as
shown in Scheme 2. Finally, the alcoholate species undergoes the usual b-hydride
elimination to afford the respective aldehyde along with metal hydride species
which can be reoxidised by means of filtration and subsequent washing of the spent
SILP catalyst for the next catalytic cycle. The reoxidation step is usually initiated by
the action of electron-proton mediators such as quinone [64–66] and 2,2,6,6-
tetramethylpiperidin-1-yloxy (TEMPO) [67, 68]. It is noteworthy that no such
mediators are required for the oxidation reaction [69].
In order to explore whether the catalyst is truly heterogeneous, we have
performed a number of heterogeneity tests. A hot filtration test was performed with
[FemDMMerA]RuO4
O
-
u
R
R
4
u
O
-
4
-
4
-
4
Merrifield
Resin
-
RuO
4
-
RuO
4
RuO4-
-
4
n
O
io
t
a
r
O
H
H
H
O
e
n
Ru
e
g
e
r
-O
O
r
te
f
A
R
Alcohol
O
O
HO
O
O
HO
Ru
O
Ru
O-
O-
O
H
R
R
H
H
Aldehyde
Metal alcoholate complex
Proton transfer
H O
-
2
Support is removed for simplicity
Scheme 2 Plausible mechanism for the oxidation of alcohols using [FemDMMerA]RuO
4
1
23

Selective oxidation of alcohols using ferrocene-labeled…
Fig. 4 Reusability of [FemDMMerA]RuO
4
the model reaction. After 50 % of completion of the reaction (GC), the catalyst was
removed by filtration. The resulting filtrate was subjected to heating for a further
1
0 h. Gas chromatographic analysis revealed no corresponding enhancement in the
product beyond 50 %, confirming the heterogeneity of 4a. Further, no evidence of
leaching was observed during the catalytic reaction as no ruthenium was detected by
ICP-AES analysis of the filtrate after removal of the catalyst. This indicates that the
catalyst is predominantly covalently attached in the Merrifield resin matrix, thus
making this SILP catalyst leaching-resistant.
The recovery and recyclability of the heterogeneous catalyst must be focused
before any catalytic operation, as it is an obvious parameter for industrial and
commercial scale as well as an essential aspect of green chemistry. The reusability
of 4a was tested for the model reaction. After the reaction, the catalyst was
separated by simple filtration, washed with bountiful amounts of THF, dried in an
oven at 60 °C for 12 h and reused for the next cycle. We were delighted to note that
4
a could be reused for six runs for the model reaction without significant loss in the
product yield, conversion and catalytic activity without prolonged reaction time
Fig. 4). The slight reduction in the conversion observed on successive recycles is
(
thought to be due to attrition during filtration and recovery of the catalyst. Another
phenomenal feature of 4a was its sturdiness. The catalytic activity of 4a did not alter
after 1 week of air exposure at room temperature, indicating its remarkable
constancy.
Conclusion
In conclusion, we have demonstrated a simple and efficient strategy for the
oxidation of primary alcohols into corresponding aldehydes using ferrocene-labeled
Merrifield resin-supported ionic liquid phase catalysts. The protocol offers a number
123

R. Kurane et al.
of significant key features, such as excellent product yields, relatively shorter
reaction times, ease of recovery and reuse of the catalyst, which make this method
financially viable, liberal and a waste-free chemical process for the oxidation of
alcohols.
Acknowledgments The authors thank UGC, New Delhi for financial assistance [F. No. 40-96/2011
(SR)].
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