Journal of the Chinese Chemical Society, 2005, 52, 1101-1104
1101
Facile and Selective Transesterification of b-Keto Esters using NaIO4,
KIO4, and Anhydrous CaCl2 as Inexpensive Catalysts under Neutral
Conditions
Babasaheb P. Bandgar,a* Arshad M. Hashmia and Shivaji S. Panditb
Organic Chemistry Research Laboratory, School of Chemical Sciences, Swami Ramanand Teerth
Marathwada University, Nanded-431606, India
b
P. G. and Research Centre, Department of Chemistry, Padmashri Vikhe Patil College Pravaranagar,
A/P. Loni. Kd. Tal. Rahata. Dist. Ahmednagar-413713, India
a
NaIO4, KIO4 and anhydrous CaCl2 catalyzed selective transesterification of b-keto esters is described.
Neutral reaction conditions and inexpensive catalysts are important features of this method.
Keywords: NaIO4; KIO4; Anhydrous CaCl2; Transesterification; b-Keto esters.
INTRODUCTION
Transesterification is one of the classic organic reactions that has enjoyed numerous laboratory uses and industrial applications.1 The b-keto esters, an important class of
versatile intermediates, are extensively used in agrochemical, pharmaceutical, and dyestuff industries.2 b-Keto esters
are also useful organic building blocks for the synthesis of
complex natural products.2 In general, protic acids carry out
transesterification of b-keto esters, which causes environmental problems.1 Most of the reported methods are not general enough to accelerate the reaction in a forward direction,
which requires one of the reactants in an excess amount to obtain good yields. 1 Dimethylaminopyridine (DMAP) catalyzed transesterification of b-keto esters gave good yields;
however, the catalyst is toxic, expensive and requires a high
temperature.3,4 Otera5,6 et al. reported the transesterification
of b-keto esters using tetrabutyldistannoxanes as a catalyst
under mild conditions; however, the preparation of catalysts
is difficult. Sulfated SnO2,7a amberlysts-15,7b and transition
metal oxide super acids8 fail with aromatic b-keto esters.
The transesterification of b-keto esters is catalyzed by
transition metal complexes9 like Mn(III) salen complex and
vanadyl(IV) acetate, which are difficult to prepare. Zeolite,10
kaolinic clay,11 and yttria-zirconia,12 have been reported for
effective transesterification of b-keto esters with some limitations. Disubstituted b-keto esters enolates underwent
1,3-ester shift under basic conditions.13 Very few examples
are reported on transesterification of b-keto esters with propargylic alcohols.14,15 The transesterification of simple esters
with a stoichiomertric amount of iron(III) perchlorate, 16
metal alkoxides of titanium(IV), 17 magnesium, 18 have also
been reported in the literature. Thus, there is need of a general
and practical method for b-keto esters involving easily available and inexpensive catalysts. We report here the use of
NaIO4, KIO4 and anhydrous CaCl2 as inexpensive and easily
available catalysts for transesterification of b-keto esters
(Scheme I).
RESULTS AND DISCUSSION
A wide range of b-keto esters with various primary,
secondary, tertiary, allylic, benzilic and propargylic alcohols
in the presence of a catalytic amount of NaIO4, KIO4, and anhydrous CaCl 2 in toluene at 100-110 °C underwent trans-
Scheme I
O
O
1
OR
R
1
O
O
2
+ R
3
OH
2
* Corresponding author. E-mail: [email protected]
Catalysts
0
100-110 C
3
1
OR
R
3
2
+ R
OH
4
1102
J. Chin. Chem. Soc., Vol. 52, No. 6, 2005
Bandgar et al.
Facile and Selective Transesterification of b-Keto Esters
esterification in good to excellent yields (entries, a-m). The
present procedure of transesterification is quite general as a
wide range of structurally varied b-keto esters such as open
chain, cyclic and aromatic ones underwent transesterification
with a variety of alcohols. The reaction with bulky and less
reactive tertiary alcohols (entries, d, e, j) which is often problematic in acid catalyzed reactions or fails to occur with
Ti(OEt)4 is also realized by these catalysts though in moder-
J. Chin. Chem. Soc., Vol. 52, No. 6, 2005
1103
ate yields.
The transesterification of b-keto esters with unsaturated alcohols is difficult5 because of facile decarboxylative
rearrangements.19 Another noteworthy feature of this methodology is that unsaturated alcohols like allylic and propargylic underwent smooth transesterification furnishing the
corresponding b-keto esters in high yields (entries b, d, f, l).
The present procedure is superior because it worked equally
1104
J. Chin. Chem. Soc., Vol. 52, No. 6, 2005
well with aromatic b-keto esters in good to excellent yields
(entries k-m). It is important to mention that this methodology is highly specific only for the b-keto esters. Other esters
like simple esters, a-halo esters and unsaturated esters (n-q)
failed to undergo transesterification. An attempt to carry out
the transesterification of b-keto esters with other nucleophiles like phenols, thiophenols and amines (entries r-t) under the same reaction conditions failed. It is also important to
note that NaIO4, KIO4 worked better than CaCl2 in terms of
yields and reaction time.
The catalysts NaIO4, KIO4 and CaCl2 are weak Lewis
acids and therefore, they might be accepting a loan pair of
electrons from both the carbonyl groups. As a result of this,
carbonyl ester becomes more reactive due to the good leaving
group -OR2 and therefore nucleophilic attack with R3OH is
accelerated and results in the formation of transesterified
products.
In conclusion, we have demonstrated that NaIO4, KIO4
and CaCl2 are efficient and selective catalysts for transesterification of b-keto esters. The ready availability of these neutral inexpensive commercial catalysts is an important advantage. The superiority and flexibility of this method over the
existing methods lies in ease of operation and the simplicity
in the workup involving filtration of the catalysts. The short
reaction time and good to excellent yields using inexpensive
catalysts are attractive features of this protocol.
EXPERIMENTAL SECTION
All chemicals are of analytical grade. IR spectra were
recorded on a Bomen FT-IRMB-104 Spectrophotometer with
zinc selenide optics. 1 H NMR were recorded on a Brucker
AC-300 spectrometer (300 MHz) in CDCl3 using TMS as an
internal standard. CHN analyses were recorded on a VarioEL analyzer. TLC was monitored on 0.25 mm E. Merck precoated silica gel plates (60F-254).
General Procedure
A mixture of b-keto esters (5 mmol), alcohol (5 mmol)
and catalysts (1 mmol) in toluene (20 mL) was heated at 100110 °C in a round bottom flask provided with a distillation
condenser to remove methanol or ethanol. After completion
of reaction (TLC), the catalysts were filtered, and the filtrate
was concentrated to get crude product which was purified by
column chromatography on silica gel (petroleum ether:ethyl
acetate) to afford the ester as a viscous colorless liquid in excellent yields.
Bandgar et al.
ACKNOWLEDGEMENT
SSP thanks UGC, New Delhi, for teacher fellowship
under FIP scheme.
Received March 17, 2005.
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