Note
DOI: 10.1002/bkcs.10851
BULLETIN OF THE
S.-A. Lee et al.
KOREAN CHEMICAL SOCIETY
Oxidative Esterification of Aldehydes with Urea Hydrogen Peroxide
Catalyzed by Aluminum Chloride Hexahydrate
*
Sin-Ae Lee, Yoon Mi Kim, and Jong Chan Lee
Department of Chemistry, Chung-Ang University, Seoul 06974, Korea. *E-mail: jclee@cau.ac.kr
Received May 2, 2016, Accepted May 28, 2016
Keywords: Alcohol, Esters, Oxidation, Urea Hydrogen Peroxide
Esters are some of the most important functional groups in
organic chemistry and have been found in the sub-structure
of a variety of natural products, industrial chemicals, and
pharmaceuticals.1 Numerous methods have been reported
for the preparation of various esters.2 One of the most com-
mon methods is the reaction of activated carboxylic acid
derivatives with alcohols. In this method, prior to the reac-
tion with alcohols, carboxylic acids must be converted to
reactive intermediates such as acid chlorides, anhydrides, or
1-hydroxybenzotriazoles.3 Other more efficient and eco-
nomically advantageous approaches include the direct ester-
ification of aldehydes with alcohols in the presence of
various oxidants.4 A variety of transition-metal-based cata-
lysts like vanadium,5 iridium,6 palladium,7 copper,8
rhodium,9 and gold10 were utilized for oxidative esterifica-
tion of aldehydes. In addition, a variety of oxidative sys-
In search of a new oxidative esterification method, our
attention was directed to the utilization of readily available
aluminum(III) chloride hexahydrate as the catalyst since
it possesses high Lewis acidity and low toxicity.
Aluminum(III) chloride hexahydrate, well-known as a
promising environmentally benign reagent, has found wide
utility for a variety of organic transformations.24 Initial
attempts at the esterification of benzaldehyde using aqueous
30 wt % hydrogen peroxide with aluminum(III) chloride
hexahydrate in methanol at 60ꢀC for 12 h gave only 58%
yield for the desired methyl benzoate. However, we found
that replacement of aqueous hydrogen peroxide with urea
hydrogen peroxide (UHP) gave methyl benzoate with a sig-
nificantly better yield of 90%. Although UHP has found
applications in various organic transformations,25 to the
best of our knowledge there is no precedent for the applica-
tion of UHP for oxidative esterification. Further attempts at
the esterification of benzaldehydes in methanol with various
catalysts like ZnCl2, ZnBr2, MgCl2, MgBr2, CaCl2, or
CaBr2 in the presence of UHP gave only moderate yield
lower than 70%. Therefore use of UHP in combination with
aluminum(III) chloride hexahydrate is distinctively more
reactive for the direct transformation of benzaldehyde to
methyl benzoate. Replacement of methanol by other larger
alcohols in the present reaction conditions led to somewhat
lower yields as shown in the Table 1. With highly hindered
t-butanol or less nucleophilic phenol, no reaction occurs
under the same reaction conditions. Various aldehydes
undergo oxidative esterification to the corresponding
methyl esters in high yields upon treatment with urea
hydrogen peroxide and catalytic amounts of aluminum(III)
chloride hexahydrate in methanol as shown in Table 2. In a
typical experiment 1.0 mmol of aryl/alkyl aldehyde was
reacted with 5.0 mmol of UHP and 10 mol % of aluminum
chloride hexahydrate in 4.0 mL of methanol at 60ꢀC to
give the desired methyl esters. All of the reactions tested in
this study were completed within 8–12 h. The present pro-
tocol was sufficiently reactive for various types of aromatic,
aliphatic, and heterocyclic aldehydes. Aromatic aldehydes
with electron donating or electron withdrawing substituents
gave equally high yields. Sterically demanding aldehydes
tems were developed for direct esterification, which include
11
Br2,
I2/PhI(OAc)2,12 N-halosuccinimide,13 pyridinium
hydrobromide perbromide,14 NaOCl/NaI,15 potassium
peroxymono-sulfate,16 B(C6F5)3/TBHP,17 and trichloroiso-
cyanuric acid.18 However, all of the above methods usually
require the use of stoichiometric amounts of reagents, toxic
transition metals, expensive catalysts, or harsh reaction con-
ditions. Recently, aqueous hydrogen peroxide mediated
oxidative esterification has become more popular for the
direct esterification of aldehydes. A few transition metal
complexes such as methyltrioxorhenium,19 V2O5,
20
titanosilicate,21 and Ni(II) complex,22 were reported in the
literature to effect the hydrogen peroxide mediated oxida-
tive esterification of aldehydes. Very recently, Wu et al.
reported an environmentally friendly method for the oxida-
tive methyl esterification of aromatic aldehydes utilizing
aqueous hydrogen peroxide with readily available calcium
chloride or magnesium chloride.23 However, even by virtue
of this method in the preparation of aromatic methyl esters,
one might occasionally encounter inconveniences in terms
of low isolated yields, long reaction times, and a narrow
range for the applicable aromatic aldehydes. In particular,
this method gives low yields for both aldehydes containing
electron donating substituents in aromatic rings and hetero-
cyclic aldehydes. Therefore, development of a more gen-
eral, efficient, and greener protocol for the esterification of
aldehydes with readily available catalyst is still desirable.
such
as
2,4-dimethoxybenzaldehydes
and
2,4-
dichlorobenzaldehyde were smoothly converted into the
Bull. Korean Chem. Soc. 2016
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
1
Lee, Sin-Ae
