P. Karthikeyan et al. / Journal of Molecular Liquids 173 (2012) 180–183
181
Scheme 1. Chemoselectivity: oxidation of benzyl alcohol and 1-octanol with L-AAIL.
139.91, 148.90, 149.22 and 155.44. FT-IR (KBr, cm−1): 3427.12,
3294.10, 2926.10, 1735.36, 1477.33, 1225.03 and 624.96. HR-MS (EI):
calculated mass (349.03), found (349.43).
Fig. 1. 3-(3-(1,2-dicarboxyethylamino)-3-oxopropyl)-1-methyl-1H-imidazol-3-ium
bromide (L-AAIL).
2.3. Typical oxidation procedure
To a mixture of L-AAIL (0.0025 mmol) and benzyl alcohol (2 mmol),
30% H2O2 (5 mmol) was slowly added. The resulting reaction mixture
was stirred at room (25 °C) temperature for 30 min. After completion
of the reaction, ether was added (3×5 mL) to separate the product
from the catalyst. The organic layer was concentrated and purified by
column chromatography (ether: n-Hexane 0.5: 4.5 mL) to give the
benzaldehyde.
processes using H2O2 are an important and challenging goal in mod-
ern oxidation chemistry [34,35].
2. Experimental
2.1. General
1
Unless specified, all chemicals are commercially available. H NMR
3. Result and discussion
spectra were recorded on a Bruker (500 MHz) spectrometer and mass
spectra were recorded on a JEOL GC MATE II HRMS (EI) spectrometer.
FT-IR was recorded on an AVATRA 330 spectrometer with DTGS detec-
tor. Quantitative and qualitative analysis were made with Perkin Elmer
Series-200 HPLC with a Brownlee Analytical C-18 150×4.6 mm column,
5 μm 110 Å.
The explorative experiments have been carried out with benzyl al-
cohol as the model substrate using hydrogen peroxide at 25 °C under
solvent free conditions. Obviously, there is no appreciable reaction in
absence of catalyst (Table 1, entry 1). At first we applied the simple
ionic liquid, [Cemim]Br and L-aspartic acid and observed slight oxida-
tion of benzyl alcohol producing benzaldehyde (18–35%; Table 1,
entries 2 and 3). However conversion and yields could not be im-
proved further with either [Cemim]Br or L-aspartic acid. Next our
interest was turned towards the preparation of L-aspartic acid
coupled imidazolium ionic liquid for alcohol oxidation. To our delight,
the L-aspartic acid coupled imidazolium ionic liquids (L-AAIL;
Scheme 1) were found to be the active system for the selective oxida-
tive conversion of benzyl alcohol to benzaldehyde. Here, benzyl alco-
hol was completely oxidized to benzaldehyde (96%; Table 1, entry 4).
Surprisingly, the oxidation process in organic solvents is not so facile
(Table 1, entries 5–8).
After inventing the active ionic liquid for the oxidation of benzyl al-
cohol, we started to examine the scope of the developed catalyst for a
series of structurally diverse alcohols. As seen from the table, aromatic,
aliphatic and heterocyclic alcohols can be oxidized to carbonyl com-
pounds. The substituted primary benzylic alcohols are selectively oxi-
dized to aldehydes with 88–96% yield (Table 2, entries 1–6). Similarly,
the secondary alcohols have also been converted to ketones with 76–
84% yield (Table 2, entries 7–10).
After having demonstrated the selective oxidation of aromatic al-
cohols, our interest was focused towards the oxidation of aliphatic al-
cohols. This is because many oxidation protocols have significantly
failed in the oxidation of secondary aliphatic alcohols. However, in-
terestingly, the oxidation process we have developed works well in
the oxidation of aliphatic secondary alcohols to corresponding car-
bonyl compounds with 85–86% yield (Table 2, entries 11–12). As
the last example, the oxidation of heterocyclic alcohols was per-
formed. Notably, these alcohols also underwent oxidation and pro-
duced corresponding aldehydes (88–90%; Table 2, entries 13–14).
Apparently, the green oxidation methodology developed worked
well for the selective conversion of structurally diverse aromatic, ali-
phatic, cyclic and heterocyclic alcohols to corresponding carbonyl
compounds.
Further, the chemoselectivity of the alcohol oxidation has been per-
formed using benzyl alcohol and 1-octanol. The mixture of benzyl alcohol
and 1-octanol in equal amounts (2 mmol) was subjected to oxidation by
hydrogen peroxide with L-AAIL under optimized conditions. Interesting-
ly, the oxidation of benzyl alcohol proceeds predominantly over the oxi-
dation of 1-octanol producing 80% of benzaldehyde and 20% of octanal
2.2. Preparation of catalysts (L-AAIL)
A mixture of Boc-aspartic acid (11.0 mmol) and triethyl amine
(22.0 mmol) in DMF was cooled in an ice-bath. The 1-carboxy ethyl-3-
methyl imidazolium bromide [Cemim]Br (10.0 mmol) was added and ag-
itation continued at ambient temperature for 24 h. After completion of
the reaction, the mixture was extracted with ether and poured to water.
The unreacted Boc-aspartic acid was removed by centrifugation. The
aqueous layer was concentrated using a rotary evaporator and the ionic
liquid was dried under vacuum at 70 °C for 6 h. The Boc-3-(3-(1,2-
dicarboxyethylamino)-3-oxopropyl)-1-methyl-1 H-imidazol-3-ium bro-
mide was deprotected using 50% TFA in dichloromethane (5 mL) for 1 h
at 25 °C. After evaporation of the solvent from the mixture, TFA was re-
moved by triturating the residue with 5 mL of methanol (saturated
with ammonia). The resulting mixture was concentrated using rotary
evaporator and the ionic liquid 33-(3-(1,2-dicarboxyethylamino)-3-
oxopropyl)-1-methyl-1 H-imidazol-3-ium bromide (L-AAIL) was dried
under vacuum 80 °C for 3 h. 1H-NMR (500 MHz, DMSO-d6) δ: 2.2
(m, 2H), 2.8 (t, 2H), 3.7 (s, 3H), 4.6 (t, 2H), 5. 2 (m, 2H), 6.9 (s, 2H),
7.6–7.8 (d, 1H), 8.2 (d, 1H), 9 (s 1H), 10.1 (s, 2H). 13C-NMR (125 MHz,
DMSO-d6) δ: 22.16, 23.72, 38.15, 50.05, 66.11, 80.96, 128.21, 128.37,
Table 1
Oxidation of benzyl alcohol with different ionic liquids and amino acid in different solvents at
25 °Ca.
Entry
Solvent
Catalyst
Time (h)
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
–
–
24
24
24
0.5
24
24
24
24
Trace
35
15
100
54
69
Trace
35
8
96
52
63
55
50
–
[Cemim]Br
L-aspartic acid
L-AAIL
L-AAIL
L-AAIL
–
–
CH2Cl2
DMF
CH3OH
CH3CN
L-AAIL
L-AAIL
62
56
a
Reaction condition: 2 mmol benzyl alcohol, 5 mmol H2O2, 0.0025 mmol catalyst.