Titanosilicate (TS-1) catalyzed oxidation of aromatic aldehydes to esters

Article abstract of DOI:10.1055/s-2002-19744

A facile conversion of aromatic aldehydes to esters using molecular sieves, TS-1, and 30% H₂O₂ is described.

Full text of DOI:10.1055/s-2002-19744

LETTER
267
Titanosilicate (TS-1) Catalyzed Oxidation of Aromatic Aldehydes to Esters
Titanosilicate
u
CatalyzedO
x
b
idation of
A
h
romatic
A
ld
a
ehydes toE
s
sters h P. Chavan,* Shubhada W. Dantale, Chitra A. Govande, Meenakshi S. Venkatraman, Cherukupally Praveen
Division of Organic Chemistry: Technology, National Chemical Laboratory, Pune-411008, India
Fax +91(20)5893614; E-mail: spchavan@dalton.ncl.res.in
Received 23 November 2001
Thus benzaldehyde was smoothly converted to methyl
benzoate using dilute H2O2 as the oxidant in refluxing
methanol employing TS-1 (Si/Ti = 38) as the catalyst. In
order to establish the generality and applicability of this
method, various aromatic aldehydes were subjected to the
same oxidizing conditions to furnish the corresponding
esters in good to excellent yields. A notable point of this
method is that high yields of esters are obtained indepen-
dent of the substrate. Thus, aromatic aldehydes with elec-
tron donating as well as electron withdrawing substituents
are smoothly and efficiently converted to the esters (Ta-
ble). A noteworthy feature of the present protocol is the
efficient oxidation of trimethoxybenzaldehyde (entry 7,
Table) to the corresponding ester. This may be contrasted
with the results obtained from other oxidizing agents,
which lead to formation of aryl formates akin to Dakin ox-
idation. However the reaction with anisaldehyde led to the
formation of the phenol in 82% yield instead of the corre-
sponding ester. This contrasting behaviour is surprising to
us and we do not have any good explanation for this at this
moment. As depicted in the proposed mechanism
(Scheme 1) the loss of proton rather than the aryl migra-
tion leads to the selective formation of esters. In the ab-
sence of the catalyst it was observed that the reaction is
extremely sluggish and too low yielding to be of any prac-
tical utility.
Abstract: A facile conversion of aromatic aldehydes to esters using
molecular sieves, TS-1, and 30% H2O2 is described.
Keywords: oxidation, aldehyde, carboxylic acid
The oxidation of aldehydes to carboxylic acids or esters is
one of the most frequently encountered reactions in organ-
ic chemistry. This transformation has been accomplished
in a variety of ways. Two-step methods include the oxida-
tion of cyanohydrins,1 hemiacetals,2 acetals3,4 and the like.
The one-pot, one-step conventional methods reported re-
6
quire the use of heavy metal oxidants like KMnO4,5 CrO3
or the highly expensive silver,7 ruthenium8 or rhodium9
catalysts. Oxidation using alkaline iodine10 and peroxides
with chlorites11 are more commonly used. NIS-mediat-
ed,12 electrochemical13 as well as very recently using
H2O214 mediated oxidation of aldehydes to esters has also
been reported.
Most of the reported methods employ heavy metal oxi-
dants in stoichiometric amounts. The disposal of reduced
by-products, which are usually toxic metals, becomes a
problem and is also environmentally unacceptable. In this
era of heightened global awareness we thought it neces-
sary to develop a more convenient and efficient oxidation
protocol.
Zeolites are microporous, crystalline aluminosilicates
well endowed with special shape-selective heterogeneous
catalytic properties. The advent of the titanium silicates,
TS-1 (MFI)15,16 and TS-2 (MEL)17 has further given a new
impetus to catalytic oxidation methods. The catalytic ac-
tivity of these zeolites and TS-1 in particular has been re-
viewed in great detail.18–20 TS-1 has been used for the
oxidation of thioethers to sulfoxides,21 aryl amines to
Scheme 1
Table Oxidation of Various Aldehydes
azoxybenzenes22 and for C-C and C-heteroatom double Entry
Substrate
Time (h)
Yield (%)
bond cleavage.23 Unfunctionalized alkanes24,25 have also
been oxidized to alcohols, aldehydes or ketones using TS-
1
C6H5CHO
5
8
65
83
97
97
89
82a
92
99
1.
2
3
4
5
6
7
8
2-Cl-C6H4CHO
4-Cl-C6H4-CHO
3-NO2-C6H4-CHO
4-NO2-C6H4-CHO
4-OMe-C6H4-CHO
3,4,5(OMe)3-C6H2-CHO
4-OHC-C6H4-CHO
As a part of our ongoing program on zeolites as well as de-
velopment of environment-friendly methodologies, we
undertook the study of direct conversion of aldehydes to
esters. Herein we wish to report a novel and extremely ef-
ficient method for the one-pot oxidation of aromatic alde-
hydes to the corresponding esters.
8
8
8
8
12
8
Synlett 2002, No. 2, 01 02 2002. Article Identifier:
1437-2096,E;2002,0,02,0267,0268,ftx,en;D12301ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
a 4-Hydroxyanisole was obtained instead of ester.

268
S. P. Chavan et al.
LETTER
A noteworthy feature of the above transformation is that
even the presence of bulky substituents ortho to the alde-
hyde (entry 2) does not have significant detrimental effect
on the yield as well as the efficiency of the reaction.
Acknowledgement
The authors thank Dr. S. Sivasanker and Dr. T. Sen for providing
the catalyst. SWD, CAG, MSV and CHP thank CSIR, New Delhi,
India for award of fellowships. Funding from CSIR, India under
Young Scientist Award Scheme is gratefully acknowledged.
Another noteworthy feature is the almost quantitative
yields obtained in the oxidation of terepthaldehyde (entry
8) to the corresponding diester. This is a commercially im-
portant transformation in view of the fact that the diester
is used for the manufacture of terylene. The diester is at
present commercially prepared26 by the air oxidation of
dialkyl benzenes at elevated temperatures in gas phase
with Cobalt catalyst followed by esterification of the re-
sulting diacid.
References
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A plausible mechanism depicting the formation of esters
from aldehydes involving the catalytic cycle with TS-1 is
depicted in Scheme 2.
(8) Rajendran, S.; Trivedi, D. C. Synthesis 1995, 153.
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Scheme 2
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1990, 345, 240.
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Aliphatic aldehydes viz. hexanaldehyde and dihydrophe-
nylcinnamaldehyde were also converted to methyl hex-
anoate and methyl dihydrophenylcinnamate respectively
in moderate yields.
Thus a mild and highly efficient methodology has been
developed for the conversion of aromatic aldehydes to the
corresponding esters.27 Use of 30% H2O2 as an oxidant
leads to only water as the by-product. Additionally the
catalytic activity and reusability of TS-1 by virtue of its
heterogeneous nature makes the work-up procedure ex-
tremely simple involving a mere filtration of the catalyst
and removal of the solvent to furnish the esters.
(27) Typical Procedure for the Preparation of Dimethyl
Terephthalate: In a typical reaction, 0.56 mL aq solution of
H2O2 (30%, 5 mmol) was added dropwise to a stirred
solution of 0.134 g of terephthaldehyde (1 mmol) and TS-1
(10% by weight) in MeOH and the mixture refluxed until
completion of reaction (8 h). The solvent was removed by
evaporation and the residue washed with water and extracted
with EtOAc. Drying over anhyd Na2SO4 and removal of
solvent furnished 0.19 g essentially of pure diester in 99%
yield.
Thus the present protocol developed by us is operationally
simple and is an important addition to the already existing
methodologies used to effect similar transformations.
IR(nujol): 1710, 1680, 1440, 1370, 1260, 1190, 1100 cm–1
1H NMR (200 MHz): = 3.95 (s, 6 H); 8.10 (s, 4 H)
MS (m/e): 194(24) [M+], 179(5), 164(11), 163(100),
135(27), 120(9), 119(9), 104(17), 103(23), 77(19), 76(34),
75(23).
Synlett 2002, No. 2, 267–268 ISSN 0936-5214 © Thieme Stuttgart · New York