Effective oxidation of alcohols with H5IO6 catalyzed by nickel(II) schiff base complexes

Article abstract of DOI:10.1080/15533174.2010.503180

Nickel(II)-Schiff base-triphenylphosphine complexes catalyze oxidation of alcohols to carbonyls in presence of minimum amount of periodic acid. The catalytic oxidation was developed in mild conditions and showed good yields. The effects of temperature, time, and concentrations of catalyst and co-oxidant were studied. Higher catalytic activity has been observed for NiL1 compared to the other complexes.Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2010.503180

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Effective Oxidation of Alcohols with H5IO6 Catalyzed
by Nickel(II) Schiff Base Complexes
Dileep Ramakrishna a & Badekai Ramachandra Bhat a
a Catalysis and Materials Chemistry Division, Department of Chemistry , National
Institute of Technology Karnataka , Surathkal, Srinivasanagar, India
Published online: 31 Aug 2010.
To cite this article: Dileep Ramakrishna & Badekai Ramachandra Bhat (2010) Effective Oxidation of Alcohols with H5IO6
Catalyzed by Nickel(II) Schiff Base Complexes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 40:8, 516-520
To link to this article: http://dx.doi.org/10.1080/15533174.2010.503180
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:516–520, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.503180
Effective Oxidation of Alcohols with H5IO6 Catalyzed
by Nickel(II) Schiff Base Complexes
Dileep Ramakrishna and Badekai Ramachandra Bhat
Catalysis and Materials Chemistry Division, Department of Chemistry, National Institute of Technology
Karnataka, Surathkal, Srinivasanagar, India
Very recently, we developed an efficient method for the se-
lective oxidation of alcohols to the corresponding aldehydes and
ketones using nickel(II) complex containing triphenylphosphine
and a Schiff base as a catalyst in ionic liquid media.[27] As a
part of our ongoing research towards nickel catalyzed oxidation
chemistry, herein, we report the catalytic oxidation of benzylic
and allylic alcohols to the corresponding carbonyl compounds
using periodic acid at ambient temperature. This procedure is
very simple, mild and clean and works efficiently without any
additives.
Nickel(II)-Schiff base-triphenylphosphine complexes catalyze
oxidation of alcohols to carbonyls in presence of minimum amount
of periodic acid. The catalytic oxidation was developed in mild
conditions and showed good yields. The effects of temperature,
time, and concentrations of catalyst and co-oxidant were studied.
Higher catalytic activity has been observed for NiL1 compared to
the other complexes.
Keywords Ni(II) complex, oxidation, periodic acid, Schiff base
INTRODUCTION
EXPERIMENTAL
The oxidation of alcohols to the corresponding aldehydes
and ketones is a key reaction in synthetic organic chemistry,
and numerous oxidizing reagents are available to affect this
important reaction.[1−4] Many of these oxidation systems re-
quire stoichiometric amounts of metal compounds with con-
comitant environmental problems. For example, the chromium
oxidants are used in vast amounts both in the laboratory
and industry.[5] Recent efforts have focused on systems that
employ metal compounds only in catalytic amounts together
with a stoichiometric oxidant.[6−9] Several reaction conditions
have been developed for the oxidation of alcohols to car-
bonyls, such as RuCl3/H5IO6,[10] RuO4,[11] RuCl3/K2S2O8,[12]
Ru–Co bimetallic catalyzed oxidation,[13,14] Cu(MnO4)2 cat-
alyzed oxidation,[15] TEMPO (2,2,6,6-tetramethylpiperidinyl-
1-oxy)-catalyzed oxidation with sodium hypochlorite,[16−21]
Na2WO4/H2O2,[22] and Cu(II)–salen complex/H2O2.[23] The
use of transition-metal complexes as catalysts for oxidation re-
actions has received good attention during the last decades,
particularly by the interest in understanding reactions where the
metal ion plays a central role in the catalytic reaction. However,
despite the extensive studies carried out with cobalt, iron and
manganese complexes, only a few investigations have focused
on nickel catalysts.[24−26]
The complexes were synthesized according to a reported
literature procedure,[27] as shown in Scheme 1.
Catalytic Experiment
A typical experimental procedure was as follows: A solution
of nickel complex (11 mg, 0.02 mmol) in 20 cm3 CH3CN was
added to the solution of substrate (1 mmol) and H5IO6 (230 mg,
1 mmol). The reaction mixture was stirred at 70◦C for 30 min
and the mixture was extracted with ether. The ether solution was
analyzed by GC. Products were isolated by distillation.
Product Analysis
The reaction product analysis was carried out using gas chro-
matography (GC) (Shimadzu 2014, Japan); the instrument has
a 5% diphenyl and 95% dimethyl siloxane Restek capillary
column (30 m length and 0.25 mm diameter) and a flame ion-
ization detector (FID). Nitrogen gas was used as the carrier gas.
The retention times for different compounds were determined
by injecting commercially available compounds under identi-
cal gas chromatography conditions. The oxidation products are
commercially available, and were identified by GC co-injection
with authentic samples.
RESULTS AND DISCUSSION
Received 20 March 2010; accepted 19 May 2010.
Address correspondence to Badekai Ramachandra Bhat, Catalysis
and Materials Chemistry Division, Department of Chemistry, National
Institute of Technology Karnataka, Surathkal, Srinivasanagar 575025,
India. E-mail: chandpoorna@yahoo.com
Catalytic Oxidation
Benzylalcohol was selected as a model substrate for opti-
mization of the process. In order to study the effect of time
516

HOMOGENEOUS CATALYTIC OXIDATION OF ALCOHOLS
517
PPh3
Cl
R
O
N
HO
N
Ni
CH2Cl2
NiCl2(PPh3)2
+
N
NH
R
NH
N
∆
NiL1: R=H, NiL2: R= Cl, NiL3: R=Br, NiL4: R=NO2, NiL5: R= OCH3.
SCH. 1. Synthesis of nickel (II) complexes.
on the activity, the product analysis was done at regular in-
The oxidation of other benzylic and allylic alcohols was
tervals of time under similar reaction conditions (Figure 1). It then examined using the optimized reaction conditions. The
was observed that the total reaction time was only 30 minutes. results for the oxidation of a variety of alcohols are summa-
This implies that Ni(II)-complex/CH3CN-H5IO6 catalytic sys- rized in Table 2. Benzyl alcohols with functional groups were
tem showed good efficiency. Therefore this catalytic system was also oxidized smoothly to the corresponding aldehydes in high
studied in detail (Table 2). The oxidation occurred only in poor yields. As expected, secondary alcohols were oxidized more
yield by simply bubbling molecular oxygen through reaction quickly than primary alcohols. Most oxidations of secondary
mixture under similar reaction conditions (Table 1, entry 2). To alcohols were completed within 45 min and gave the corre-
evaluate the catalytic effect of catalyst, the oxidation of benzy- sponding ketones cleanly. Oxidation of allylic alcohols to α,
lalcohol was carried out under similar reaction conditions in the β-unsaturated carbonyl compounds has long been of interest.
absence of catalyst and no conversion was observed (Table 1, It was found that cinnamaldehyde was obtained from cinnamyl
entry 3). Alternatively, the reaction was carried out at room tem- alcohol in ∼90% conversion after 30 min of stirring whereas
perature under the same conditions in which the reaction was NaIO4/2,2,6,6-tetramethylpiperidine-1-oxyl/NaBr system gives
complete after 210 min (Table 1, entry 4). Further, when the only 18% of aldehyde from the same substrate.[28] Interestingly,
oxidation of benzylalcohol was carried out using less amount catalytic property of nickel(II) complexes is not affected even for
of catalyst, the yield was low (Table 1, entry 5). Also the reac- chelating substrate (o-hydroxy benzylalcohol) as it is in many
tion was carried out using low amount of oxidant under same homogeneous catalytic systems. This present catalytic system
conditions and was observed that the yield was low (Table 1, requires a very short reaction period of about 30–45 minutes
entry 6).
compared to reported catalytic systems.[29]
TABLE 1
Oxidation of benzylalcohol to benzaldehydea
100
80
60
40
20
0
Entry
Reaction time (min)
% Conversion
1
30
300
300
210
30
92.5
<20
NRd
90.6
56.9
64.9
2b
3c
4e
5f
6g
30
aUnless otherwise indicated, all reactions were carried out with
1.08g (1 mmol) of benzylalcohol with 11 mg (0.02 mmol) of Ni(II)
catalyst 230 mg (1 mmol) of H5IO6, stirring at 70◦C.
bThe reaction was carried out under an atmosphere of O2 instead
of H5IO6.
cThe reaction was carried out without Ni(II) catalyst.
d NR = reaction did not occur.
0
10
20
30
40
50
eThe reaction was carried out at room temperature.
f The concentration of Ni(II) catalyst was decreased to 0.01 mmol.
gThe concentration of the oxidant was decreased to 0.5 mmol.
Time (min)
FIG. 1. Effect of time with % conversion.

518
D. RAMAKRISHNA AND B. R. BHAT
TABLE 2
Oxidation of alcohols catalyzed by Ni(II) complexesa in CH3CN-H5IO6 system
% Yield of carbonyl compoundb
Alcohols
Product
Time (min)
30
NiL1
NiL2
NiL3
NiL4
NiL5
92.6
89.5
84.6
85.4
81.0
30
30
91.7
96.4
86.2
95.6
86.9
84.6
85.6
84.6
80.1
80.6
30
40
90.1
91.9
86.2
85.4
82.4
80.1
88.6
80.2
78.4
80.0
40
40
95.6
92.1
87.8
87.2
89.6
90.0
85.2
80.1
82.1
76.3
35
30
85.1
87.1
90.0
89.5
88.3
82.6
88.5
86.1
79.8
81.3
(continued on next page)

HOMOGENEOUS CATALYTIC OXIDATION OF ALCOHOLS
519
TABLE 2
Oxidation of alcohols catalyzed by Ni(II) complexesa in CH3CN-H5IO6 system (continued)
% Yield of carbonyl compoundb
Alcohols
Product
Time (min)
30
NiL1
NiL2
NiL3
NiL4
NiL5
95.7
91.3
77.6
71.9
80.1
30
40
76.2
80.2
75.5
76.6
76.0
74.6
73.6
78.1
71.2
76.5
30
30
75.7
72.1
74.9
72.6
72.6
71.2
73.1
76.1
72.5
70.0
a1 mmol alcohol, 1 mmol H5IO6, 0.02 mmol Ni (II) complex, 20 mL CH3CN, stirring at 70◦C.
bGC yield, average of three trials.
Spectral evidence for the complex formation between cata- was observed and a hyperchromicity was observed at 365 nm
lyst and substrate was obtained from UV-Vis spectra of catalyst, (Figure 2). This may be attributed to the formation of a Ni-
benzylalcohol and both in CH3CN. A bathochromic shift of substrate complex.[30] On the basis of above discussion, a plau-
about 15 nm from 350 nm to 365 nm in the spectra of Ni(II) sible reaction mechanism is given (Scheme 2).
CH3CN
H5IO6
NiCl(L)PPh3
O=NiCl(L)PPh3
HIO3
+
2H2O
+
+
R
H
H
HO
O=NiCl(L)PPh3
O
NiCl(L)PPh3
+
R
H
O
H
R
H
O
NiCl(L)PPh3
O
H2O
O
NiCl(L)PPh3
+
+
R
H
SCH. 2. Proposed reaction mechanism for the catalytic oxidation of alcohols.

520
D. RAMAKRISHNA AND B. R. BHAT
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