Oxidative Esterification of Aromatic Aldehydes Using Polymer-Supported Green Bromine

Article abstract of DOI:10.1055/s-0035-1561347

A facile one-pot green chemical conversion of aromatic aldehydes to alkyl esters is described using polymer-supported bromine chloride resin in the presence of potassium carbonate at room temperature. This eco-friendly methodology with simple workup procedure and recovery and recyclability of the resin makes the transformation simple and efficient.

Full text of DOI:10.1055/s-0035-1561347

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©
Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
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letter
Synlett
A. Sridhar et al.
Letter
Oxidative Esterification of Aromatic Aldehydes Using Polymer-
Supported Green Bromine
A. Sridhar
S. Swarnalakshmi
M. Selvaraj*
Department of Chemistry, Guru Nanak College,
Velachery, Chennai 600 042, India
mariselvaraj@yahoo.com
Received: 23.11.2015
Accepted after revision: 30.12.2015
Published online: 08.02.2016
preparation of a polymer-supported bromine chloride resin
and oxidation reactions of aromatic aldehydes carried out
with this reagent.
DOI: 10.1055/s-0035-1561347; Art ID: st-2015-d0909-l
Gopalakrishnan et al.18 have reported the preparation of
polymer-supported bromine chloride resin by treating
commercially available Amberlite IRA-400 chloride resin
with bromine in carbon tetrachloride. Since this method in-
volves the use of elemental bromine and carbon tetrachlo-
ride, we simplified the preparation of the same bromine
chloride resin by treating commercially available, oven-
dried IRA-400 resin with a mixture of potassium bromate
and potassium bromide in the presence of dilute hydro-
chloric acid. Bromine liberated from a bromate–bromide
mixture in acid medium converts IRA-400 chloride resin
into the bromine chloride resin (Scheme 1).
Abstract A facile one-pot green chemical conversion of aromatic alde-
hydes to alkyl esters is described using polymer-supported bromine
chloride resin in the presence of potassium carbonate at room tem-
perature. This eco-friendly methodology with simple workup procedure
and recovery and recyclability of the resin makes the transformation
simple and efficient.
Key words bromine chloride resin, esterification, oxidation, aldehyde,
hypobromite ion
The oxidative one-pot transformation of an aldehyde to
an ester is an extremely useful functional-group intercon-
version in organic synthesis. Several protocols have been
reported for this transformation using different transition-
5KBr + KBO3 + 6H+
3Br2 + 3H O
2
1
2
3
4
metal oxidants such as V O , Fe(ClO ) , ZnBr , palladium,
2
5
4
3
2
5
gold, halogen-derived oxidants such as N-halosuccinim-
ide, hypohalites, pyridinium hydrobromide perbromide,
molecular bromine, molecular iodine, hypervalent(III)io-
dine, N,N-dibromo-p-toluenesulfonamide, and NaIO4/
LiBr. Despite intensive efforts into the oxidative esterifica-
tion of aldehydes, the development of a more effective,
mild, and eco-friendly method still remains a challenge
since many reported methods require heavy-metal oxi-
dants, anhydrous conditions, and extended reaction time or
give poor yields of the products.
3Br2
+
N
N
ClBr2
Cl
6
7
8
9
10
Scheme 1 Preparation of bromine chloride resin using a bromide–bro-
mate mixture
11
12
13
The bromine chloride resin prepared by both methods
possesses the same amount of bromine, as indicated by the
same increase in the weight of the IRA-400 resin and also
by volumetric estimation with Na S O (Table 1). There was
2
2
3
no appreciable change in the bromine content after expo-
sure to microwave irradiation for five minutes or storage for
five days (Table 2 and Table 3).
Bromine can behave as a mild oxidizing agent under ba-
sic conditions due to the formation of hypobromite. This
was confirmed through UV-Vis absorption spectral studies.
UV absorption spectra were recorded for Winkler’s solu-
tion, containing potassium carbonate and an aqueous sus-
Application of polymer-supported reagents in organic
synthesis has grown over the years due to the convenience
in handling, easy workup procedures, and recyclability of
1
4
15
the reagents. Polymer-supported iodate, periodate, bro-
16
17
mate, hypochlorite, and dichloroiodo resins, used as oxi-
dizing agents, have reported in the literature. However,
there is no report on polymer-supported bromine chloride
resin in oxidation reactions. In this paper, we describe the
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A. Sridhar et al.
Letter
Table 1 Determination of Bromine Content in the Bromine Chloride Resin
Weight of the resin (g)
Increase in the weight (g)
2 2 3
Volume of Na S O (0.05 N) (mL) Bromine content (mmol/g)
–
–
Prepared by Br
2
/CCl
4
Prepared by Br /BrO
3
0
1
1
.5
0.129
0.258
0.386
10.4
20.9
31.2
0.8125
1.6187
2.4250
0.8062
1.6125
2.4125
.5
Table 2 Stability of Bromine Chloride Resin under Microwave Irradia-
tion
hypobromite ion. These observations lead to the conclusion
that both Winkler’s solution and the bromine chloride resin
derived from commercially available Amberlite IRA 400
chloride produces the hypobromite ion in basic medium. In
addition, titration with Na S O using starch as an indicator,
Entry
Microwave time (min)
Bromine content (mmol/g)
1
2
3
4
5
6
0
1.6125
1.6063
1.5938
1.5688
1.5438
1.5125
2
2
3
0.5
1
when one equivalent of potassium carbonate was added to
resin containing one equivalent of bromine, also indicated
the conversion of bromine into hypobromite. This observa-
tion prompted for the use of K CO and the resin in equimo-
2
2
3
3
lar amounts with regard to bromine content for oxidation
reactions.
5
Table 3 Stability of Resin during Storage
Entry
Days
Bromine content (mmol/g)
1
2
3
4
5
6
0
1
2
3
4
5
1.6125
1.5750
1.5250
1.4875
1.4652
1.4375
pension of bromine chloride resin, and also for a mixture
containing bromine chloride resin and potassium carbonate
Figure 1 A & B: winkler’s solution and aqueous solution of bromine
chloride resin. C & D: base + winkler’s solution and aqueous solution of
bromine chloride resin
(
Figure 1). The UV spectrum showed absorption bands at λ
389, 265 nm for Winkler’s solution and the aqueous sus-
pension of bromine chloride resin, corresponding to molec-
=
19
ular bromine and hypobromite ion, respectively, due to
the equilibrium established between molecular bromine
and hypobromite ion. Basic potassium carbonate can, how-
ever, shift the equilibrium towards hypobromite ion
For optimizing reaction conditions, oxidation of benzal-
dehyde was carried out using methanol as solvent in the
presence of potassium carbonate. The highest yield of
methyl benzoate was obtained in the presence of K CO at
2
3
(
Scheme 2).
room temperature. A series of aromatic aldehydes was sub-
jected to similar oxidative esterification using methanol as
solvent at room temperature, and the corresponding methyl
esters were obtained in good yields (Table 4). Aromatic al-
dehydes with electron-withdrawing substituents required
shorter reaction times than aromatic aldehydes with elec-
tron-donating substituents. In all the reactions, isolation of
the product was very simple. However, aldehydes with or-
tho substituents gave lower yields, presumably due to steric
interactions. Lower yields were obtained when reactions
Br2
HBr
+
+
H2O
OH–
HOBr
Br–
+
+
HBr
H2O
Scheme 2 Equilibrium of bromine in water
The UV spectrum of Winkler’s solution and bromine
chloride resin containing potassium carbonate showed an
intense absorption band at λ = 265nm corresponding to the
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Synlett
A. Sridhar et al.
Letter
were carried out with ethanol and isopropanol (Table 5),
and this may also be attributed to the increased steric
crowding in the hemiacetal intermediate. These observa-
tions support the probable mechanism of formation of
hemiacetal intermediate followed by oxidation of the hemi-
acetal to the ester (Figure 2).
Table 4 (continued)
Entry Substrate
Product 2 Time Substrate/ Yield
(h)
reagent/base (%)a
CHO
7
2g
4
1:3:3
86
Table 4 Oxidation of Aldehyde to Methyl Ester Using Bromine Chloride
Resin
Br
CHO
CHO
COOCH3
N
ClBr2
8
2h
2i
2
2
4
1:3:3
1:3:3
1 :3:3
89
92
71
N
K2CO3, MeOH, r.t.
R = H, CN, NO2, CH3, OCH3, Cl etc
R
R
CHO
9
N
Entry Substrate
Product 2 Time Substrate/ Yield
h)
reagent/base (%)a
(
CHO
CHO
10
2j
1
2a
0.75 1:2:2
94
CHO
CHO
NO2
1
1
2k
2l
2
2
1:3:3
1:3:3
82
85
CHO
OCH3
2
2b
2c
0.5
1:2:2
1:3:3
96
92
NO2
CN
12
CHO
3
1
1
OCH3
CHO
CHO
1
3
4
2m
2n
4
2
1:3:3
1:3:3
72
4
2d
1:3:3
91
CH3
COOCH3
CH CH
CHO
CHO
50:50b
1
5
6
2e
2f
4
4
1:3:3
1:3:3
82
80
CHO
Cl
1
5
6
2o
2p
5
5
1:3:3
1:3:3
54
62
CHO
CHO
1
Cl
a
Isolated yield.
Based on H NMR yield.
b
1
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Synlett
A. Sridhar et al.
Letter
O
Table 5 Oxidation of Aldehyde to Ethyl Ester Using Bromine Chloride
Resin
H
CHO
COOC2H5
O
Br
N
ClBr2
H
OR
OBr
HBr
K CO , EtOH, r.t.
2
3
R
R
R = CN, NO2
O
Entry Substrate
Product 3 Time
Substrate/
Yield
(
h)
reagent/base (%)a
OR
CHO
Figure 2 Plausible mechanism for the oxidative esterification of the
aldehyde
1
3a
5
1:5:5
68
NO2
To study the recyclability of the resin, after the comple-
tion of reaction, the resin was collected by filtration, sub-
jected to bromination, and reused in the oxidation of m-ni-
trobenzaldehyde in the presence of methanol and K CO .
This process was repeated six times, and results indicate
that the reagent can be reloaded and reused at least five
times without significant decrease in the yield as indicated
in Table 6.
CHO
2
3b
5
5
1:5:5
1:5:5
72
65
2
3
NO2
CHO
3
3c
In conclusion, we have achieved a green chemical meth-
od for a facile one-pot conversion of aromatic aldehydes to
their respective methyl esters in good yield at room tem-
perature using polymer-supported bromine chloride in the
CN
a
Isolated yield.
20,21
presence of respective methanol and K CO .
The bro-
2
3
Table 6 Recyclability of the Resin
mine chloride resin is prepared by treating commercially
available IRA-400 chloride resin with a mixture of potassi-
um bromate and potassium bromide solutions in the pres-
ence of dilute hydrochloric acid. The recovery and reusabil-
ity of the reagent makes the methodology economical.
Number of recycles
Recovered resin (g)
Product yield (%)a
1
2
3
4
5
6
0.98
0.96
0.95
0.95
0.93
0.87
94
92
91
91
90
89
Acknowledgment
The authors thank the University Grants Commission, New Delhi, In-
dia for financial support and Prof. K.K. Balasubramanian, IIT Madras,
India for his valuable suggestions.
a
Isolated yield.
Supporting Information
References and Notes
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©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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Synlett
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Letter
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A mixture of bromine chloride resin and K CO in MeOH–H O
2
3
2
(
(8:2) 3 mL was placed in a round-bottom flask fitted with a
magnetic stirrer. To this, the aldehyde (1 mmol, 1 equiv) was
added dropwise, and the reaction mixture stirred at r.t. The
progress of the reaction was monitored by TLC. After comple-
tion of the reaction the mixture was filtered, and the filtrate
was extracted with Et O. The ether layer was washed with H O,
(
2
2
(
(
(
(
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ARKIVOC 2006, (xi), 162.
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brine, dried over Na SO , filtered, and concentrated. The residue
2 4
was purified by column chromatography, if required, over silica
gel (60–120 mesh) using a mixture of PE and EtOAc (9:1) as
eluent.
2
014, 55, 5538.
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(21) Procedure for the Preparation of Bromine Chloride Resin
A solution of 0.1 M KBr–KBrO3 (20 mL) was added to a flask.
Oven-dried Amberlite IRA-400 chloride resin (10 g) was added
2
5
to the flask followed by 5 mL of HCl–H O (1:1). The solution was
2
stirred for 5 min. The resin was filtered and washed with H O
2
and dried.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E