A simple, one-pot oxidative esterification of aryl aldehydes through dialkyl acetal using hydrogen peroxide

Article abstract of DOI:10.1007/s11164-015-2408-4

A simple and an efficient one-pot procedure has been developed to synthesize various aryl carboxylic esters directly from aryl aldehydes using hydrogen peroxide without any catalyst. The reaction proceeds smoothly at room temperature. A preliminary investigation suggests the formation of dialkyl acetal as an intermediate during the reaction sequence.

Full text of DOI:10.1007/s11164-015-2408-4

Res Chem Intermed
DOI 10.1007/s11164-015-2408-4
A simple, one-pot oxidative esterification of aryl
aldehydes through dialkyl acetal using hydrogen
peroxide
C. Devarajan1,2 V. Vijayakumar3 C. Ramalingam1
R. Vijayaraghavan4
•
•
•
Received: 18 September 2015 / Accepted: 23 December 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract A simple and an efficient one-pot procedure has been developed to
synthesize various aryl carboxylic esters directly from aryl aldehydes using
hydrogen peroxide without any catalyst. The reaction proceeds smoothly at room
temperature. A preliminary investigation suggests the formation of dialkyl acetal as
an intermediate during the reaction sequence.
Keywords Aryl aldehyde Á Oxidative esterification Á Hydrogen peroxide Á Dialkyl
acetal
Introduction
Aryl carboxylic esters are very important and useful synthetic intermediates or
structural elements in the manufacture of pharmaceuticals and agrochemicals [1, 2].
Generally, aryl carboxylic esters are prepared by the condensation of aryl carboxylic
acids and alcohols, using the Fischer esterification [3–5]. Carboxylic acids are
activated by converting them into acyl chlorides or anhydrides which in turn are
employed in the synthesis of corresponding esters [6]. One-pot oxidative esterification
reaction is receiving increasing attention over classical methods due to its economic
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2408-4)
contains supplementary material, which is available to authorized users.
&
R. Vijayaraghavan
rvijayaraghavan@vit.ac.in
1
2
3
4
School of Bio-Science and Technology, VIT University, Vellore 632 014, India
FMC India Private Limited, India Innovation Centre, Bangalore 560022, India
Centre for Organic and Medicinal Chemistry, VIT University, Vellore 632 014, India
Materials Chemistry Division, Centre for Excellence in Nano Materials, School of Advanced
Sciences, VIT University, Vellore 632 014, India
123

C. Devarajan et al.
viability [7] (Scheme 1). For oxidative esterification reactions, various metal catalysts
such as palladium [8–12], copper [13], manganese [14], iridium [15], tungsten [16],
zinc [17], vanadium [18], calcium and magnesium [19], nanoparticle catalysts such as
gold–nickel oxide nanoparticles [20], gold nanoparticles [21, 22] and iron oxide
nanoparticles [23], N-heterocyclic carbene (NHC) catalysts [24–26], ionic liquids [27,
28], oxoammonium salts [29], and reagents such as iodine [30] and TsNBr2 [31] have
been used. Heavy-metal oxidants such as manganese(IV) oxide [32] and expensive
reagents such as tert-butyl hydroperoxide (TBHP) have also been reported [33].
Catalyst recovery methods and reactivation is critical in these protocols. An
ultrasound-assisted condition is also reported for this reaction [34]. Hydrogen
peroxide is a versatile green oxidant used to effect various organic transformations
[35], and it is also used in oxidative esterification of secondary alcohols into ketones,
primary alcohols into carboxylic acids [36, 37], and aldehydes into carboxylic acids
[38] in the presence of a phase transfer catalyst (PTC).
The synthesis of aryl carboxylic esters by oxidative esterification under mild
conditions with simple reagents would be an attractive approach. We made an
earnest attempt at direct oxidative esterification of alcohols and aryl aldehydes using
hydrogen peroxide under mild condition (Scheme 2) without any catalyst, and it
was found to be successful with appreciable conversion. Hence, we are reporting
this hitherto unreported reaction.
In this newly developed simple and efficient protocol, hydrogen peroxide being a
green oxidant generates water as the only by-product. With 26 equi. of alcohol as a
reactant/solvent and 8 equi. of H2O2 (46 % solution), various aryl aldehydes were
converted into their corresponding aryl esters with up to 73 % yields.
Experimental
Laboratory-grade solvents and reagents were used as received. Boiling points and
melting points were determined on an Elchem Microprocessor-based DT apparatus
and are uncorrected. For thin-layer chromatography (TLC), Merck silica gel 60
F254 pre-coated plates were used, visualized by exposure to UV light (254 nm). 1H
NMR and 13C NMR were determined on a Bruker 400-MHz spectrometer using
Hydrolysis
O
O
O
R'OH
-H2O
[O]
Ar
Ar
Ar
OR'
H
OH
Aryl
Aryl Carboxylic
Aryl Carboxylic
Aldehyde
1
Acid
Ester
3
2
Oxidative esterification
Scheme 1 Synthesis of Aryl carboxylic esters
123

A simple, one-pot oxidative esterification of aryl…
O
OH
O
O
O
H
25-30oC
+
CH3OH + H2O2
+ 2H2O
2
1
3
Scheme 2 Synthesis of Aryl carboxylic esters using one-pot oxidative esterification
DMSO-d6 and CDCl3 solvents. Chemical shifts (d) are reported in ppm and
coupling constants (J) are in Hertz (Hz). GC-MS data were obtained at 70 eV using
a Thermo-fisher Trace Ultra GC and PolarisQ MS using HP-5 (column 30 m
length 9 0.32 mm inner diameter 9 0.25 lm film thickness).
General procedure for the synthesis of methyl benzoate
A mixture of benzaldehyde (5 g, 47.2 mmol) and methanol (50 mL, 26 equi.) was
stirred at 30 °C and 25 mL of 46 % H2O2 (8 equi.) was added dropwise over 10 min.
The reaction mixture was stirred at 30 °C for 3 h and the progress of the reaction was
monitored through TLC. After the completion of the reaction, 250 mL of water was
added to the reaction mixture and the obtained products were extracted using ethyl
acetate (2 9 100 mL). The combined ethyl acetate layer was treated with 500 mL of
20 % solution of sodium bisulfite solution, followed by 100 mL of saturated solution
of sodium bicarbonate and then washing with 100 mL of saturated sodium sulfate
solution. Finally, ethyl acetate was distilled off and the product was collected as a
residue. Colourless liquid; bp 196–198 °C; 1H NMR (400 Hz, CDCl3) d 3.90 (s, 3H,
CH3), 7.43 (m, 2H, ArH), 7.53 (m, 1H, ArH), 8.02 (m, 2H, ArH); 13C NMR (100 Hz,
CDCl3) d 52.12, 115.43, 128.40, 129.62, 130.20, 132.97, 167.20; MS (EI): m/z
(%) = 136 (29) [M?], 105 (100), 92 (29), 77 (73), 51 (28).
Caution: The mixture of hydrogen peroxide and organic solvents forms organic
peroxides which can ignite easily and burn rapidly and intensely. Proper care must
be taken while handling this mixture. It is essential to treat the entire reaction mass
with excess sodium bisulfite to ensure complete destruction of organic peroxides.
Ethyl benzoate (Product of Entry 2, Table 3)
Colourless liquid; bp 210–211 °C; 1H NMR (400 Hz, CDCl3) d 1.40 (t, J = 7.2 Hz, 3H,
CH3), 4.39 (q, J = 7.2 Hz, 2H, –CH2), 7.44 (m, 2H, ArH), 7.54 (m, 1H, ArH), 8.04 (m,
2H, ArH); 13C NMR (100 Hz, CDCl3) d 14.37, 61.02, 128.37, 129.59, 130.56, 132.87,
166.74; MS (EI): m/z (%) = 150 (59) [M?], 122 (23), 105 (100), 77 (29), 51 (10).
Isopropyl benzoate (Product of Entry 3, Table 3)
Colurless liquid; bp 264–266 °C; 1H NMR (400 Hz, CDCl3) d 1.28 (d, J = 6.0 Hz,
6H, CH3), 5.16 (m, 1H, CH), 7.34 (m, 2H, ArH), 7.45 (m, 1H, ArH), 7.95 (m, 2H,
123

C. Devarajan et al.
ArH); 13C NMR (100 Hz, CDCl3) d 22.05, 68.47, 128.37, 129.61, 131.02, 132.80,
166.27; MS (EI): m/z (%) = 164 (16) [M?], 123 (34), 105 (100), 77 (29), 51 (9).
Methyl 2-chlorobenzoate (Product of Entry 4, Table 3)
1
Colurless liquid; bp 232–234 °C; H NMR (400 Hz, CDCl3) d 3.90 (s, 3H, CH3),
7.27 (m, 1H, ArH), 7.39 (m, 2H, ArH), 7.78 (m, 1H, ArH); 13C NMR (100 Hz,
CDCl3) d 52.40, 126.56, 130.09, 131.05, 131.38, 132.54, 133.67, 166.15; MS (EI):
m/z (%) = 170 (37) [M?], 172 (13), 139 (100), 111 (29), 75 (23).
Methyl 3-chlorobenzoate (Product of Entry 5, Table 3)
1
Colurless liquid; bp 230–232 °C; H NMR (400 Hz, CDCl3) d 3.80 (s, 3H, CH3),
7.23 (m, 1H, ArH), 7.39 (m, 1H, ArH), 7.79 (m, 1H, ArH), 7.89 (s, 1H, ArH); 13C
NMR (100 Hz, CDCl3) d 52.34, 127.68, 129.66, 131.88, 132.91, 134.51, 165.81;
MS (EI): m/z (%) = 170 (59) [M?], 172 (20), 139 (100), 111 (31), 75 (21).
Methyl 4-chlorobenzoate (Product of Entry 6, Table 3)
1
Colurless liquid; bp 245–247 °C; H NMR (400 Hz, CDCl3) d 3.88 (s, 3H, CH3),
7.37 (d, J = 8.4 Hz, 2H, ArH), 7.93 (d, J = 8.4 Hz, 2H, ArH); 13C NMR (100 Hz,
CDCl3) d 52.20, 128.67, 130.95, 139.33, 166.14; MS (EI): m/z (%) = 170 (48)
[M?], 172 (17), 139 (100), 111 (28), 75 (20).
Methyl 3-hydroxybenzoate (Product of Entry 9, Table 3)
Pale brown solid; bp 70–71 °C; 1H NMR (400 Hz, DMSO-d6) d 3.81 (s, 3H, CH3),
7.03 (m, 1H, ArH), 7.29 (m, 1H, ArH), 7.37 (m, 2H, ArH), 9.83 (s, 1H, OH); 13C
NMR (100 Hz, DMSO-d6) d 52.00, 115.55, 119.77, 120.26, 129.77, 130.81,157.46,
166.22; MS (EI): m/z (%) = 152 (61) [M?], 121 (100), 93 (39), 65 (30), 63 (12).
(Dimethoxymethyl) benzene (4)
Colurless liquid; bp 86–88 °C/18 mmHg; 1H NMR (400 Hz, CDCl3) d 3.34 (s, 6H,
CH3), 5.40 (s, 1H, CH), 7.47 (m, 3H, ArH), 7.52 (m, 2H, ArH); 13C NMR (100 Hz,
CDCl3) d 52.70, 103.21, 125.70, 126.71, 128.21, 128.88, 129.01, 129. 82; MS (EI):
m/z (%) = 152 (1) [M?], 121 (100), 91 (25), 77 (14).
Result and discussion
The conversion of benzaldehyde 1 into the methyl benzoate 3 was considered as a
representative reaction and hence initially the reaction was attempted with 13 equi.
of methanol and 8 equi. of H2O2 (46 % solution) at RT which led to the formation
of 48 % methyl benzoate 3 with 49 % conversion of benzaldehyde after 3 h (Entry
1, Table 1). We attempted to derive the optimal conditions for the synthesis of
123

A simple, one-pot oxidative esterification of aryl…
methyl benzoate by varying the reactant quantity, reaction duration and temper-
ature. Accordingly, when the reaction was allowed to continue for 24 h, it led to the
formation of methyl benzoate 3 (with 21 % yield) and benzoic acid 2 (with 70 %
yield) with conversion of 91 % benzaldehyde. When the reaction was carried out at
65 °C, the complete conversion of benzaldehyde was observed with the formation
of 38 % methyl benzoate and 62 % benzoic acid (Entries 2 and 3, Table 1).
Reaction with 26 equi. of methanol at RT substantially favoured the formation of
methyl benzoate (the desired product) up to a maximum of 71 % with a total
conversion of 73 % (Table 1, Entry 4).
The formation of benzoic acid 2 was attributed to the hydrolysis of the produced
ester 3. Initially, we thought that the conversion of 1 into 3 was through benzoic
acid 2 as the intermediate, and hence we employed benzoic acid directly under the
same reaction conditions as mentioned in Entry 4 of Table 1, which failed to
produce methyl benzoate (Scheme 3). This conclusively revealed that the reaction
did not proceed through benzoic acid as the intermediate.
When we increased the methanol to 39 and 52 equi (Entries 5 and 6, Table 1) a
by-product of (dimethoxymethyl) benzene 4 (dialkyl acetal) was obtained with
yields of 13 and 20 %, respectively, with concomitant decrease of the methyl
benzoate yield. This could possibly indicate that the reaction proceeds through an
intermediate of dialkyl acetal type (acetal intermediate) (Scheme 4). In order to
capture the dialkyl acetal intermediate 4, the reaction was carried out under dilute
H2O2 concentration (Table 2).
As per Entry 1, Table 2 substantial formation of acetal intermediate 4 (67 %) was
observed after 2 h along with 8 % of ester 3 when just 1.6 equi. of H2O2 solution
(46 % solution) was used. However, on continuing the reaction to 3 h, the dialkyl
acetal intermediate 4 was decreased to 59 % (desired ester was increased to 18 %)
(Entry 2, Table 2), which could be due to a portion of the acetal intermediate 4
Table 1 Synthesis of methyl benzoate
Entry
MeOH equi.
Time (h)
Conversiona (%)
Yielda (%)
2
3
1
13
13
13
26
39
52
3
24
3
49
91
0
70
62
1
48
21
38
71
60
56
2
3b
100
73
4
3
5c
6d
3
73
0
3
76
0
Hydrogen peroxide (46 % solution, 8 equi.) was added to the solution of benzaldehyde in MeOH for
10 min and the mixture was stirred at 30 °C for the specified duration
a
GC results
b
Reaction temperature is 65 °C
(Dimethoxymethyl) benzene was detected as by product (13 %)
c
d
(Dimethoxymethyl) benzene was detected as by product (20 %)
123

C. Devarajan et al.
COOCH3
Scheme 3 Selective formation
of methyl benzoate from
benzaldehyde
COOH
CHO
H2O2
X
CH3OH
+
+
0%
COOCH3
H2O2
CH3OH
71%
O
O
OR'
O
2R'OH
H2O2
H2O
Ar
H
Ar
Ar
OH
OR'
Acetal
Ar
OR'
- R'OH
- H2O
Aryl
- R'OH
Aryl
Aryl
carboxylic
aldehyde
carboxylic
intermediate
ester
3
acid
2
1
4
H2O
H2O2
- H2O
OH
Ar
OH
Aldehyde
Hydrate
Scheme 4 Proposed course of oxidative esterification reaction and by products formation
being converted into the ester 3. A further 1.6 equi. addition of H2O2 to the above
reaction mixture led to a significance decrease of acetal intermediate 4 to 34 %
(along with a significant increase of ester 3 to 38 %) in 1 h (Entry 3, Table 2). The
reaction eventually showed 23 % acetal intermediate 4 with 56 % product 3 after
24 h (Entry 4, Table 2).
Table 2 Synthesis of methyl benzoate with dilute peroxide concentration
Entry
H2O2 equi.
Time (h)
Conversiona (%)
Yielda (%)
4
3
1
1.6
1.6
3.2
3.2
2
3
75
78
73
79
67
59
34
23
8
2b
3b
4
18
38
56
4
24
H2O2 was added to the solution of benzaldehyde (5 g) in 100 mL MeOH for 10 min and the mixture was
stirred at 30 °C for specified duration
a
GC results
b
Entries 2 and 3 are the same reactions distinguished by different amounts of H2O2 addition
123

A simple, one-pot oxidative esterification of aryl…
In a separate attempt, (dimethoxymethyl) benzene 4 was prepared by the reaction
of benzaldehyde 1 with 1 equi. of benzoyl peroxide in methanol solvent (Scheme 5).
Formation of 90 % (dimethoxymethyl) benzene 4 was observed which was isolated
and then treated with methanol and H2O2 under the conditions mentioned in Entry 4,
Table 1, in which the formation of 65 % methyl benzoate was observed. This
confirms conclusively that the oxidative esterification reaction depicted in this
report proceeds through acetal intermediate 4 mentioned in Scheme 4.
Screening of various aldehydes were carried out (Table 3) under the optimized
conditions mentioned in Entry 4 of Table 1. During the screening experiments, the
holding time of the reaction was increased to 24 h if the reaction did not show
appreciable conversion after 3 h. As mentioned earlier, benzaldehyde afforded
71 % methyl benzoate with 73 % conversion (Entry 1, Table 3 and Entry 4,
Table 1). With ethanol, benzaldehyde afforded 58 % ethyl benzoate with 59 %
conversion (Entry 2, Table 3). When isopropyl alcohol was employed instead of
methanol in the same reaction, benzaldehyde afforded 24 % isopropyl benzoate and
21 % benzoic acid with 45 % conversion; the reaction was found to be slow and it
required 24 h for this conversion (Entry 3, Table 3). This observation can be
attributed to the steric factor possessed by the isopropyl group. Formation of
benzoic acid is due to direct oxidation of benzaldehyde with H2O2. This was
confirmed by doing separate blank reactions between benzaldehyde (1 g) and H2O2
(5 mL) under the same conditions, which afforded 24 % benzoic acid with 25 %
conversion of benzaldehyde after 3 h.
Once the reaction conditions were optimized, the oxidative esterification was
extended to mono-substituted chloro benzaldehydes under the optimized condition.
The 2-chloro, 3-chloro and 4-chloro benzaldehydes found to afford 66, 69 and 71 %
of corresponding methyl benzoates, respectively (Entries 4, 5 and 6, Table 3).
However, these reactions required 24 h for the conversion (71, 73 and 76 %,
respectively) that was realized by benzaldehyde in 3 h.The reaction with 2,
4-dichlorobenzaldehyde was very slow and exhibited only 20 % conversion with
10 % formation of corresponding methyl benzoate even after 24 h (Entry 7,
Table 3). In general, chloro-substituted benzaldehydes exhibited sluggish reaction
rates when compared to benzaldehyde. When hydroxybenzaldehydes were engaged,
3-hydroxybenzaldehyde showed 73 % formation of methyl-3-hydroxybenzoate with
73 % conversion (Entry 9, Table 3). With 2-hydroxybenzaldehyde (Salicylalde-
hyde), the reaction was slow, as 16 % methyl salicylate was observed even after
24 h with 16 % conversion (Entry 8, Table 3). When 4-hydroxybenzaldehyde was
O
H
O
O
O
Ph
O
25-30oC
+ CH3OH +
O
Ph
O
1
4
Scheme 5 Preparation of (dimethoxymethyl) benzene
123

C. Devarajan et al.
Table 3 Synthesis of various aldehydes using oxidative esterification with hydrogen peroxide
Entry
Aldehyde
Ester
Conversionb (%)
Yield (%)b
4
2
3
CHO
CO2Me
CO2Et
CO2iPr
CO2Me
1
2
3
4
73
1
1
71c
CHO
CHO
CHO
59
–
–
3
1
58
24
66
45a
71a
21
–
Cl
Cl
73a
4
–
69
CHO
CO2Me
5
Cl
Cl
76a
20a
16a
73
4
–
–
–
–
71
10
16
73
CHO
CO2Me
6
7
8
9
Cl
Cl
Cl
CHO
Cl
CO2Me
Cl
10
–
Cl
CHO
CO2Me
OH
OH
CHO
CO2Me
–
OH
OH
HO
CHO
CO2Me
10
11
12
70a
53a
0
–
–
–
–
10d
11e
–
HO
CHO
CO2Me
15
–
MeO
MeO
CHO
CHO
CO2Ph
13
14
41
99
–
–
–
38
–
CO2Me
CO2Me
O
O
99
CHO
The mixture of aldehyde (5 g) and alcohol (26 equi.) was stirred at 30 °C and 8 equi. of 46 % H2O2 was
added dropwise over 10 min. Reaction was stirred at 30 °C for 3 h
a
Monitoring after 24 h
b
GC results
c
Yield is 69 % based of wt% assay using acetophenone as internal standard
d
46 % hydroquinone was observed as a by-product
e
26 % mequinol was observed as a by-product
123

A simple, one-pot oxidative esterification of aryl…
engaged, only 10 % methyl-4-hydroxybenzoate was observed with 70 % conversion
after 24 h. The major by-product was identified as hydroquinone which was formed
to the extent of 46 %. A similar phenomenon was observed with 4-methoxyben-
zaldehyde which afforded 11 % methyl-3-methoxybenzoate along with 26 %
mequinol (4-methoxyphenol) (Entries 10 and 11, Table 3).
When benzaldehyde was treated with phenol, no reaction was observed (Entry
12, Table 3). When a heterocyclic aldehyde such as furfuraldehdye was reacted,
38 % methyl 2-furoate was formed with 41 % conversion. However, product
isolation was found to be difficult (Entry 13, Table 3). When an aliphatic aldehyde
such as propionaldehyde was used, formation of propionaldehyde dimethyl acetal
was observed (Entry 14, Table 3).
Conclusion
A simple and efficient generic protocol has been developed for the oxidative
esterification of aryl aldehydes at room temperature with yields up to a maximum
73 %, and the reaction proceeds through dialkyl acetal as intermediate. This
synthetic protocol without any catalyst could be of significant value in organic
synthesis.
Acknowledgments The authors are thankful to FMC Corporation for providing the opportunity and to
Dr. Ravi Datar for providing support to carry out this work. The authors thank VIT University for support
and encouragement.
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