2
M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
Table 1
Various catalysts and conditions for esterification of aldehydes and alcohols.
Substrate
Catalyst and conditions
(1 mmol), NaNO (1 mmol), 70 °C
Cu(ClO (5 mol%)-InBr (5 mol%)-TBHP (1.1 mmol), 100 °C
CuF (0.05 mmol)-TBHP (6 mmol), DMSO/H
NHC (15 mol%), DBU (110 mol%), THF, 25 °C
Oxone (1 mmol), 50 °C
Time (h)
Yield (%)
Refs.
Aromatic aldehydes
Aliphatic and aromatic aldehydes
Aromatic aldehydes
I
2
2
14–36
16
12
0.5–92
18
2–24
4
24
24
24
24–48
24
7–97
[5]
[9]
)
4 2
3
42–91
44–85
15–94
9–98
17–86
76–100
45–71
58–95
10–92
70–76
23–92
71–91
2
2
O, 120 °C
[10]
[12]
[14]
[16]
[19]
[13]
[17]
[18]
[19]
[20]
[22]
Aromatic aldehydes
Aliphatic and aromatic aldehydes
Aliphatic and aromatic aldehydes
Aliphatic and aromatic aldehydes
Benzyl alcohols
Pd(OAc)
Ru(PPh
KI (0.2 mmol), TBHP (3 mmol), CH
Pd(OAc) (5 mol%), Na CO (2 mmol), PMHS (0.1 mmol), CH
PI–Au (1 mol%), K CO (0.5 mmol), O (1 atm.), CH OH/H O, 25 °C
(5 mol%), Xantphos (5 mol%), CH OH/Toluene, reflux
(2 mol%), MAE (6 mol%), Cs CO (10 mol%), acetone, 25 °C
2
(2.5 mol%), XPhos (5 mol%), K
2
CO
3
(0.2 mmol), CH
3
OH/acetone, N
2
, 50 °C
)
3 3
(CO)H
2
3
(5 mol%), Xantphos (5 mol%), CH OH/Toluene, reflux
3
OH, 60 °C
Benzyl alcohols
2
2
3
3
OH, 40 °C
Aliphatic and aromatic alcohols
Aliphatic and aromatic alcohols
Aliphatic and aromatic alcohols
Benzyl alcohols
2
3
2
3
2
Ru(PPh
[Cp IrCl
3
)
3
(CO)H
2
3
⁄
2
]
2
2
3
CuI (20 mol%), DTBP (4 mmol), 1,3-diketimine (nacac) ligand, DMF, 90 °C
4
GO/Oxone/)))
GO/)))
2.2. Typical procedure for the oxidative esterification of benzyl alcohols
ArCHO
ArCOOR
ArCH OH
2
ROH
Oxone/ROH
To a solution of alcohol (1 mmol) in 2 mL of toluene was added
Scheme 1. Sonochemical direct oxidative esterification of aldehydes and benzyl
alcohols using GO/Oxone in alcoholic solvent.
GO (0.3 g). The resulting mixture was sonicated in an Elmasonic P
ultrasonic cleaning unit (ultrasonic bath) with a frequency of
3
7 kHz and 100% output power at 80 °C for the time indicated in
The effect of ultrasound in chemical reactions is known [31,32].
Moreover, the application and efficiency of ultrasound in oxidation
of alcohols have been reported [33–36]. For example, Mills and co-
workers showed that the initial rate of oxidation of alcohols to
Table 4. Then Oxone (1 mmol) and 2 mL of an alcoholic solvent
was added in the reaction medium and the resulting mixture
was irradiated for the time indicated in Table 4. The mixture was
filtered through a sintered funnel and evaporated under reduced
pressure, and extracted with ethyl acetate. The organic layer was
their corresponding ketones with NaBrO
an aqueous CCl , was greater with ultrasonic irradiation than with
stirring alone [33]. The oxidation of alcohols into respective alde-
hydes and ketones by Ni(NO O/I /water system under ultra-
3 4
, mediated by RuO in
4
2 4
dried over Na SO , filtered and evaporated under reduced pressure.
Purification was achieved by column chromatography using n-hex-
ane/EtOAc: 100/3 as eluent. The spectroscopic data of the obtained
esters were compared with authentic samples [5,40,42,43]. Spec-
troscopic data for methyl 3,4-dichlorobenzoate (entry 9, Table 4):
3
)
2
Á6H
2
2
sonic irradiation has been demonstrated [34].
Several groups have utilized Oxone (potassium hydrogen mon-
opersulfate) as an effective oxidant for numerous organic transfor-
mations [37,38]. Parida and co-workers have reported the
oxidation of primary and secondary alcohols into their correspond-
ing oxidized products using 2–6 equivalents of Oxone [37]. Esteri-
fication of heterocyclic aldehydes has been investigated by Mineno
Pale yellow, M.P. 44.7 °C; IR (KBr)
589, 1435, 1378, 1301, 1110, 757 cm
CDCl ) d = 3.94 (s, 3H, CH ), 7.53 (d, J = 8.3 Hz, 1H, CH Arom),
.87 (dd, J = 8.3, 1.9 Hz, 1H, CH Arom), 8.13 (d, J = 1.9 Hz, 1H, CH
m = 3089, 3022, 2958, 1729,
À1
1
1
;
H NMR (300 MHz,
3
3
7
1
3
Arom);
1
2
3
C NMR (75 MHz, CDCl ) d = 52.54, 128.63, 129.94,
3
and co-workers using Oxone in the presence of In(OTf) [38]. More
30.52, 131.53, 132.92, 137.56, 165.21; MS (EI) (70 eV), m/z (%):
recently, we have reported the use of Oxone/iron(II)sulfate/GO as a
highly efficient system for the oxidation of alcohols to the corre-
sponding carboxylic acid or ketone compounds under ultrasonic
irradiation in water [39]. We have demonstrated that GO most
likely participates in the oxidation process through the generation
of sulfate and hydroxyl radicals or as an oxidant itself. In continu-
ation of our efforts on the use of graphite oxide as an effective oxi-
dant [29,30,39], we report herein a new and simple method for
direct oxidative esterification of various aromatic aldehydes and
alcohols using GO/Oxone in an alcoholic solvent under ultrasonic
irradiation (Scheme 1).
+
+
+
08 (5) [M+4] , 206 (31) [M+2] , 204 (50) [M] , 177 (10), 175
(
62), 173 (100), 145 (30), 109 (20), 74 (18).
3
. Results and discussion
Initially, we screened the oxidative esterification of 4-nitro-
benzaldehyde (1 mmol) in the presence of GO/Oxone in methanol
under different conditions. The results are summarized in Table 2.
First, the oxidative esterification of 4-nitrobenzaldehyde (1 mmol)
in the presence of Oxone (2 mmol) without GO at reflux of metha-
nol afforded the corresponding ester in 80% yield after 7.5 h (entry
1, Table 2). When the oxidation of 4-nitrobenzaldehyde was per-
formed in the presence of 0.2 g of GO and Oxone (2 mmol) in meth-
anol, the corresponding ester methyl 4-nitrobenzoate was
obtained in 70% yield after 90 min using an ultrasonic homoge-
nizer (Bandelin Sonopuls HD 3100) with probe model MS 73 at
100% power (entry 2, Table 2). The oxidative esterification of 4-
nitrobenzaldehyde in the presence of 0.2 g of GO and Oxone
(1 mmol) in reflux of methanol gave methyl 4-nitrobenzoate in
70% yield after 4 h (entry 3, Table 2). Finally, this reaction was car-
ried out in the presence of 0.2 g of GO and 1 mmol of Oxone in an
Elmasonic P ultrasonic cleaning unit (ultrasonic bath) with a fre-
quency of 37 kHz and 100% output power at 60 °C. A rise in the
yield and a decrease in the reaction time were observed; methyl
4-nitrobenzoate was isolated in 95% yield only after 15 min (entry
4, Table 2). The comparison of entries 3 and 4 clearly indicates that
2
. Experimental section
2.1. Typical procedure for the oxidative esterification of aldehydes
To a solution of aldehyde (1 mmol) in 4 mL of methanol was
added Oxone (1 mmol) and GO (0.2 g). The resulting mixture was
sonicated with an Elmasonic P ultrasonic cleaning unit (ultrasonic
bath) with a frequency of 37 kHz and 100% output power at 60 °C
for the time indicated in Table 3. The mixture was filtered through
a sintered funnel and extracted with ethyl acetate (EtOAc). The
2 4
organic layer was dried over Na SO , filtered and evaporated under
reduced pressure. Purification was achieved by column chroma-
tography using n-hexane/EtOAc: 100/3 as eluent. The spectro-
scopic data of the obtained acids were compared with authentic
samples [5,15,40,41].