Microwave accelerated selective and facile deprotection of allyl esters catalyzed by montmorillonite K-10

Article abstract of DOI:10.1039/a909265j

Carboxylic acids are regenerated from their corresponding substituted allyl esters by Montmorillonite K-10 using microwave irradiation under solvent free conditions to afford enhanced yields and reduced reaction times compared to thermal conditions. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a909265j

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Microwave accelerated selective and facile deprotection of allyl
esters catalyzed by Montmorillonite K-10
Anil S. Gajare,* Nadim S. Shaikh, Bhushan K. Bonde and Vishnu H. Deshpande*
1
Division of Organic Chemistry: Technology, National Chemical Laboratory, Pune 411 008,
India
Received (in Cambridge, UK) 23rd November 1999, Accepted 5th January 2000
Carboxylic acids are regenerated from their corresponding
substituted allyl esters by Montmorillonite K-10 using
Results and discussion
Table 1 lists a variety of allyl esters which were converted to
their parent acid in the presence of nucleophile, catalyst Mont-
morillonite K-10 by both thermal and microwave accelerated
solvent free conditions. The time required for completion of the
reaction by the microwave method is appreciably shorter than
when using thermal conditions with mild reaction conditions.
It is noted that during the course of reaction, the allyl
cation formed reacts with the aromatic nucleus of toluene, 1,4-
dimethoxybenzene or anisole via alkylation. When toluene is
used as the nucleophile the product is monoalkylated at the
ortho position and is obtained in moderate yield (Scheme 1). It
microwave irradiation under solvent free conditions to
afford enhanced yields and reduced reaction times com-
pared to thermal conditions.
Introduction
When a chemical reaction is to be carried out selectively at one
reactive site in a multifunctional molecule, the other reactive
sites must be temporarily blocked. Many protective groups have
been, and are being, developed for this purpose.1
Functional group protection and deprotection strategies are
essential to target-oriented synthesis in organic chemistry.
Carboxylic acids can be protected as anhydrides,2 amides3 or
esters.4 Unsaturated esters are particularly useful as protect-
ing groups because of their stability, the ease with which
they can be obtained by reaction of the corresponding
alcohol with acid chloride or alkylation of the acid with the
corresponding allyl halide under base catalyzed conditions.5
Deprotection of allyl ester can be done by using methodologies
that employ Pd(OAc)2,6a PdCl2(Ph3P)2,6b (Ph3P)3RhCl,6c
Me2CuLi,6d formic acid7 and sulfated SnO2.8 Reports also show
only a few carboxylic acids which were protected as a but-3-en-
1-yl ester and have been deprotected via ozonolysis9a and
β-elimination.9b Recently we have reported the use of
natural kaolinitic clay and EPZG for selective allyl ester
deprotection.10
As there is a need for ‘clean technology revolution’, the use of
solid, inorganic catalysts promises to go a long way to improve
the current technology which is very inefficient or leads to
unacceptable levels of waste.11 Clays can be used as an efficient
and versatile catalyst for various organic reactions. In mont-
morillonite clay one octahedral aluminate layer is sandwiched
between two tetrahedral silicate layers.12 This clay is non-toxic,
non-corrosive, cheap and recyclable. Montmorillonite clay has
both Brønsted and Lewis acidic catalytic sites available, hence
its natural occurrence, as well as its ion-exchange properties
allows it to function efficiently as a catalyst. Recent reports on
Montmorillonite K-10 focus on some important reactions13–18
and various transformations.19
Scheme 1
is evident from Table 1 that entries 2, 4, 5, 6 and 18 are easily
deprotected whereas the cinnamyl ester requires the presence of
more nucleophilic aromatic species such as anisole. The pres-
ence of a methyl ester in mixed ester selectively deprotects in
good yield (entry 18). It is also important to note that aryl or
alkyl esters (entries 19 and 20) remain unaffected under these
reaction conditions.
Experimental
IR spectra were recorded in chloroform on a Perkin-Elmer
137-E spectrometer. The H NMR spectra were recorded on a
1
Bruker 200 MHz instrument and the chemical shifts were
reported with Me4Si as an internal standard. The mass spectra
were recorded on Finnigan MAT-1020-B 70 eV mass spec-
trometer. Microwave reactions were carried out in an IBF made
domestic microwave oven operating at its full power at 1000
MHz. Catalyst Montmorillonite K-10 and all chemicals were
obtained from Aldrich Chemicals.
Microwave assisted heterogeneous reactions20 with various
solid inorganic support have attracted research interest because
of the simplicity, greater stability and rapid synthesis of a
variety of organic compounds.21 The salient features of the
microwave approach along with the use of mineral supported
reagents or catalysts are the enhanced reaction rates, formation
of pure products in high yields and the ease of manipulation.
Recently, solvent-free microwave assisted reactions22 have
gained more popularity as they provide an opportunity to work
with open vessels. This avoids the risk of development of high
pressure and provides a possibility of upscaling the reaction on
a preparative scale and helps the induction of the reaction
under ‘dry conditions’.22
General procedure
To a mixture of the allyl ester (10 mmol), 1,4-dimethoxy-
benzene or anisole (15 mmol) or toluene (15 ml, in the case of
thermal reactions) and catalyst Montmorillonite K-10 clay
(10% w/w) were added in a glass test tube and mixed thoroughly
followed by irradiation in a microwave oven or heating with
stirring under reflux in an oil bath (toluene 15 ml as solvent).
DOI: 10.1039/a909265j
J. Chem. Soc., Perkin Trans. 1, 2000, 639–640
This journal is © The Royal Society of Chemistry 2000
639

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Table 1 Deprotection of allyl esters catalysed by Montmorillonite
reaction mixture was diluted with chloroform and washed with
K-10
1 M NaOH (3–4 times). The aqueous layer was separated and
acidified with dilute HCl. The acidified aqueous layer was
extracted with organic solvent to obtain the corresponding acid
in pure form. The chloroform layer was washed with brine,
dried and concentrated to get crude alkylated product, which
was purified by column chromatography over silica gel. Pure
monoalkylated product was obtained in moderate yield.
Thermal
Microwave
Yield Time/ Yield
Entry Allyl esters
Time/h (%)a
min
(%)a
In conclusion, the present method provides a useful altern-
ative for deprotection of allyl esters. The notable advantages of
this methodology are mild and solvent free conditions, short
reaction time (10 to 20 min), chemoselectivity and its environ-
mental friendly conditions. We believe this will serve as a useful
addition to modern organic synthetic methodologies.
1
6
94
20
96
2
3
4
5
94b
80
20
20
20
98
85
97
8.5
8.5
90b
Acknowledgements
5
6
14
60b,c
62b,c
20
10
70c
70c
The authors B. K. B. and A. S. G. would like to express their
sincere thanks to the Director, NCL for providing research
facilities and to the trustees of Lady Tata Memorial Trust,
Mumbai for a research fellowship to NSS.
14
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The reaction was monitored by TLC. After the completion of
the reaction, this mixture was cooled to room temperature and
the catalyst was separated by filtration. The reaction mixture
was concentrated to remove excess toluene under vacuum. The
Communication a909265j
640
J. Chem. Soc., Perkin Trans. 1, 2000, 639–640