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catalyzed by ionic liquids alone in the absence of metal catalysts [26].
Some of these oxidation processes were carried out using NaClO
and hypervalent iodine as oxidants [27]. Next, the 2, 2, 6, 6-
tetramethylpiperidinyloxy (TEMPO) coupled ionic liquids were
also used as catalysts [28]. Despite the notable achievements in
the metal catalyzed and metal free oxidations of alcohols, the
search for facile, cost-effective and environmentally benign proce-
dures that avoid the use of expensive and toxic metal catalysts and
hazardous stoichiometric agents is of continuing interest and high-
ly desired.
reduced pressure in high vacuum (5 mm Hg) until the BIL remained
constant (Fig. 1). 1H-NMR (500 MHz, DMSO-d6):δ 3.8(s, 6 H),
7.6(d, 2 H), 7.3(d, 2 H), 9.0(s, 2 H), 4.6(t, 4 H), 3.2(t, 2 H), 2.3(t, 2 H),
8.1(d, 1 H), 13C-NMR (125 MHz, DMSO-d6):δ 41.96, 132.90, 133.01,
70.68, 56.56, 26.91, 24.55, 168.21. Micro analytical data: Cal (C: 36.90;
N: 16.55; H: 5.00), found: (C: 36.86; N: 16.49; H: 4.95). FT-IR (KBr,
cm−1): ν 1658, 1433, 1021, 778. HR-MS (EI): cal (421.01), found
(421.66).
2.3. Typical experimental procedure for oxidation reaction
However, G. Sekar et al [29] have reported direct oxidation of pri-
mary alcohols to acids using a catalytic amount of CuCl and anhy-
drous tert-BuOOH (5 M in decane) in acetonitrile/70% tert-BuOOH
(in water) [30] under very mild conditions. Mo Hunsen et al [31]
have shown oxidation of alcohols to aldehydes and ketones, and alco-
hols and aldehydes directly to carboxylic acids using periodic acid as
the terminal oxidant in moderate to excellent yields in a short
amount of time using fluoro chromates as a catalyst. Michele Besson
et al [32] successfully carried out air oxidation of primary alcohols
to the corresponding aldehydes or acid using Pt/C catalysts. Hiromichi
Ohta et al [33] developed a novel approach to the oxidation of
alcohols to acids using biocatalyst. Akichika Itoh et al [34] studied
photo-oxidation of alcohol to corresponding carboxylic acid with mo-
lecular oxygen in the presence of a catalytic amount of an inorganic
reagent, lithium bromide, bromine, and hydrogen bromide. Recently
Jinyu Han et al [35] reported an efficient protocol for oxidation of al-
cohols to corresponding carbonyl compounds with excellent conver-
sions using an amino acid Schiff base copper ligand as the catalyst in
the IL [bmim] BF4.
To a mixture of CuCl2/BIL (0.01 mmol) and benzyl alcohol (5 mmol)
was slowly added H2O2 (10 ml, 30%). The resulting reaction mixture was
stirred at 25 °C until the disappearance of starting material (TLC). After
completion of the reaction, ethyl acetate was added (3×5 mL) to
separate product from catalyst. The organic layer was concentrated
and purified by silica gel column chromatography (n-hexane/diethyl
ether=4.5:0.5 ml) to give the carboxylic acid. Subsequently, the recy-
clability of the used CuCl2/BIL catalyst was demonstrated for the oxida-
tion of benzyl alcohol to benzoic acid. The catalytic system could be
reused directly for a new cycle, after full extraction of the product
three times with 5 mL ethyl ether per extraction. The results shown in
Table 3 demonstrated that this oxidative system was readily recyclable
for five runs without any significant loss of catalytic activity.
3. Results and discussion
The initial study was carried out using benzyl alcohol as the
substrate to optimize the reaction condition, and the results are
summarized in Table 1. At first, the oxidation of alcohol was carried
out in absence of catalyst for 24 h using H2O2 oxidant. The reaction
provided only the corresponding aldehyde with no trace of carboxylic
acid (Table 1, entry 1). Then we tested different ionic liquid such as
3-(3-carboxyethyl)-1-methyl-imidazolium bromide [Cemim]Br, 3-
(3-aminoethyl)-1-methyl-imidazolium bromide [Aemim] Br cata-
lyst for the oxidation with 30% H2O2 the reaction produced 66
and 60% of aldehyde with trace of acid (Table 1, entries 2 and 3).
Next, the reaction was carried out in the presence of CuCl2. It was
found that both the catalytic systems gave aldehyde with comparable
conversion (Table 1, entries 4 and 5). Subsequently, the reaction was
screened with novel BIL and CuCl2 separately with 30% H2O2, the re-
action produced an 80% isolated yield of aldehyde and 60% yield of
acid in 1 h respectively (Table 1, entries 6 and 7).
Yet, other solvents were examined, the use of CH3NO2, CH3CN
(Table 1, entries 8 and 9) as solvent yield comparable results. Other
reaction solvents such as THF, DCM, EtOAc, and benzene (Table 1, en-
tries 10–13) were not useful for this oxidation reaction. Moreover, the
system consisting CuCl2/BIL and H2O2, without solvent led to the effi-
cient oxidation of alcohol (99%) in 30 min (Table 1, entry 14).
Next, various types of oxidant were tested as oxygen resource in
CuCl2/BIL. It is seen that tert-BuOOH, H5IO6 as the oxidants showed
moderate conversion and selectivity, only H2O2 gave good result
(Table 1, entries 15 and 16). Next, the influences of the amount of
H2O2 and ionic liquid were also investigated. When the amount of
H2O2 increased from 30 to 50%, and CuCl2/BIL from 0.01 to 0.1 mmol,
the alcohol could still be oxidized with lesser time duration (Table 1,
From the literature survey, there was no report of using CuCl2/BIL
as catalyst for the direct oxidation. Herein, we report a very facile ox-
idation of aliphatic/aromatic primary alcohols to the carboxylic acids
using CuCl2/BIL catalyst by H2O2 oxidant at 25 °C room temperature
under solvent free condition (Scheme 1). This procedure is very sim-
ple, mild, and clean and works efficiently without any additives.
2. Experimental
2.1. General
Unless specified, all chemicals are commercially available. 1H NMR
spectra were recorded on Bruker (500 MHz) and Mass Spectra were
recorded on JEOL GC MATE II HRMS (EI) spectrometer. FT-IR was
recorded on AVATRA 330 Spectrometer with DTGS detector. Quanti-
tative and qualitative analyses were made in Perkin Elmer Series-
200 HPLC, Brownlee Analytical C-18 150×4.6 mm, 5 micrometer
110 Angstron.
2.2. Preparation of catalysts (BIL)
To a solution of [Cemim] Br (1.0 mmol) in DMSO, diphenylphosphoryl
azide (1.0 mmol) and triethylamine (1.0 mmol) were added successively
at 0 °C and the reaction was stirred for 30 min. To this, [Aemim] Br
(1.0 mmol) in DMSO was added and the reaction mixture refluxed for
about 6 h until completion of the reaction (as monitored by TLC). To
this solution ether was added to separate product and base from the
reaction mixture. Then the traces of solvent were removed under
Fig. 1. 1-methyl-3-(3-(2-(1-methyl-1 H-imidazol-3-ium-3-yl) ethylamino)-3-oxopropyl)-
Scheme 1. CuCl2/BIL catalyzed oxidation of alcohols to acids.
1 H-imidazol-3-ium bromide (BIL).
Karthikeyan, Parasuraman
