Montmorillonite K10 clay-microwave assisted isomerisation of acetates of the Baylis-Hillman adducts: A facile method of stereoselective synthesis of (e)-trisubstituted alkenes

Article abstract of DOI:10.1055/s-2001-16040

Montmorillonite K10-microwave assisted stereoselective isomerisation of acetates of the Baylis-Hillman adducts furnished (E)-trisubstituted alkenes in high yields. A comparative study of this reaction with Mont-K10, ion-exchanged clay (Fe³⁺-Mont-K10) and acid treated regional natural Kaolinite clay and the efficiency of these clays in this reaction is described.

Full text of DOI:10.1055/s-2001-16040

1314
LETTER
Montmorillonite K10 Clay-Microwave Assisted Isomerisation of Acetates of
the Baylis-Hillman Adducts: A Facile Method of Stereoselective Synthesis of
(E)-Trisubstituted Alkenes
Ponnusamy Shanmugam,* Paramasivan Raja Singh
Organic Chemistry Division, Regional Research Laboratory (CSIR), Trivandrum-695 019, Kerala, India
Fax +0091-471-491712; E-mail: shan@csrrltrd.ren.nic.in
Received 14 June 2001
Dedicated to Professor K. Rajagopalan on the occasion of his 60th Birthday.
Montmorillonite K10-microwave combination has been
Abstract: Montmorillonite K10-microwave assisted stereoselec-
utilized for effecting many organic transformations as a
tive isomerisation of acetates of the Baylis-Hillman adducts fur-
catalyst.15-17 In continuation of our research work on clay
nished (E)-trisubstituted alkenes in high yields. A comparative
catalysis, 18-20 we herein report the mont- K10-microwave
study of this reaction with Mont-K10, ion-exchanged clay (Fe3+-
Mont-K10) and acid treated regional natural Kaolinite clay and the assisted stereoselective isomerisation of acetates of Bayl-
efficiency of these clays in this reaction is described.
is-Hillman adducts into its (E)- trisubstituted olefins.
Key words: montmorillonite K10 clay, microwave, stereoselective
isomerisation, Baylis-Hillman reaction, E-alkenes, ion-exchanged
sented in Scheme 1. The Baylis-Hillman acetate adducts 2
clay, natural kaolinite clay
The general isomerisation studies are schematically repre-
(a-p) were prepared according to the literature procedure.8
Stirring the acetate 2a with 50% w/w montmorillonite
K10 clay in CH2Cl2 at r.t. for 48 h furnished the starting
The usefulness of montmorillonite K10 clay in organic
material with 20% deacylated product. Heating the same
synthesis and its application as a catalyst for a number of
reaction mixture at reflux for 24 h again furnished only the
starting material. However, when a slurry of the acetate 2a
organic reactions are well documented.1-4 The montmoril-
lonite K10 and its structurally modified clays are known
with 50% w/w mont-K10 clay without any solvent was ir-
to act as both Bronsted and Lewis acid catalysts for a va-
radiated in a microwave oven for 6 min., a clean isomer-
riety of industrially important organic reactions.1 The clay
ised product 3a was obtained in 60% yield (5-10%
1
catalysts are known as eco-friendly acid catalysts which
decomposition) as determined by H NMR. Among the
several variations tested to optimize the condition,21 the
following conditions involving acetate 2a with 30% w/w
of mont-K10 clay, 70% microwave power level (PL) and
13 min. irradiation time was found to be best and yielded
the clean isomerised product 3a (in 9:1, E:Z isomer)22 in
74% after column purification. The microwave irradiation
of the acetate 2a under similar conditions without any clay
furnished the starting material quantitatively confirming
the necessity of clay catalyst for this reaction. Adduct 2a
(deacetylated) under similar conditions furnished 20% of
the isomerised product with remaining decomposed prod-
ucts. Hence acetate protection of the Baylis-Hillman ad-
ducts is necessary for good yields. The results are
summarised in Table 1.
have potential for replacing the conventional mineral ac-
ids and are non-pollutant. The advantages of the clay-cat-
alyzed reactions are that they are generally mild, solvent
free and work-up is easy. The Baylis-Hillman reaction is
one of the important Carbon-Carbon bond forming reac-
tions and has been used in organic synthesis for the syn-
thesis of a variety of compounds having diverse functional
groups and has been used as the starting point for a variety
of synthetic organic transformations.5-13 Stereoselective
synthesis of (E)-trisubstituted alkenes is one of the diffi-
cult tasks in organic synthesis and only a few methods are
known in the literature.8, 9,14 Stereoselective isomerisation
of acetates of Baylis-Hillman adducts catalyzed by TM-
SOTf,8,9 trifluoroacetic acid,10 benzyl-trimethylammoni-
um fluoride11 have appeared in the literature.
H
OAc
OH
Z
O
Z
Z
Z
a
b
c
+
R
R
R
R
H
1(a-p)
2-(a-p
OAc
3(a-p)
3a E:Z=9:1
R=Ph, 4-Cl-Ph, 2,4-Cl2-Ph, 4-CH3-Ph, 4-CH3O-Ph, Naphth-1-yl, Naphth-2-yl;
Z= CO2Et, CN, COCH3
Scheme 1 Reagents and Conditions: a) DABCO, Neat; b) Acetyl chloride, Py. 0 °C, 1h; c) 30% w/w mont. K10, MW, 13 min (70% PL),
Neat, 57–80%.
Isomerisation of Baylis-Hillman adducts catalysed by mont.K10-microwave irradiation
Synlett 2001, No. 8, 1314–1316 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Montmorillonite K10 Clay-Microwave Assisted Isomerisation of Acetates
1315
Table 1 Mont K10 claya-Microwaveb combine assisted isomerisation of Baylis-Hillman acetates 2a into 3a under
various conditions.
aMontmorillonite K10 clay was dried at 85 ºC for 2h before each use.
bMicrowave irradiation was carried out on a KenStar (model MWO 9817) microwave oven (800 W/operating frequency
2450 MHz).
cE/Z ratio was assigned based on 1H and 13C NMR.
a
Table 2 Montmorillonite K10 clay -microwave combine assisted isomerisation of Baylis-Hillman acetates 2 (a-p) into
3(a-p)
aMontmorillonite K10 clay was dried at 85 ºC for 2 h before each use.
bMicrowave irradiation was carried out on a KenStar microwave oven with 70% PL.
c30% w/w montmorillonite-K10 clay was used in each case.
dE/Z-ratio was assigned based on 1H and 13C NMR.
eAfter column purification.
In order to exemplify the general nature of this reaction, well as acid treated regional natural kaolinite clay.24,25
we chose a variety of Baylis-Hillman adducts 2(a-p), The use of Fe3+-mont-K10 was found to be as good as
which underwent facile isomerisation with mont-K10-mi- montmorillonite K10 clay while with acid treated regional
crowave combination to give clean isomerised products natural kaolinite (1:1 layer type) clay, the reaction was un-
3(a-p) in good yield. Yields of adduct with nitrile and car- successful and starting material was recovered quantita-
bonyl groups were lower compared to the ester functional tively. The reason for this observation with natural
group at the activated alkene and needed longer irradia- kaolinite clay24,25 may be that the interlayer distance is <
tion time with higher power level. The isomer ratio (E:Z) 7 Å when compared to montmorillonite K10 clay whose
of the isomerised products were as estimated by 1H NMR interlayer gap is 10 Å where the clay reactions actually
(by integrating the peaks at 7.9 and 7.83) (for -CN sub- taken place.1,2 Due to the small interlayer distance in the
stitution the ratio is more comparable to the CO2Et sub- acid treated regional natural kaolinite clay,25 the reacting
stitution at the alkene). The results are summarised in molecules are presumably unable to enter the interlayer
Table 2.
space and hence the reaction failed. The results are sum-
marised in Table 3. The clay recovered from the reaction
mixture by filtration can be recycled three times without
losing its activity by activating the clay at 100 ºC for 3 h.
In conclusion, we have demonstrated the usefulness of
The efficiency of commercial montmorillonite-K10 (2:1
layer type, available from Aldrich Co.,) 1,2 clay was com-
pared with Fe3+-mont-K1023 (an ion exchanged clay) as
Synlett 2001, No. 8, 1314–1316 ISSN 0936-5214 © Thieme Stuttgart · New York

1316
P. Shanmugam, P. Raja Singh
LETTER
Table 3 Comparison of mont-K10a, Fe3+-mont.-K10a and acid treated regional natural kaolinite clay
catalysts on isomerisation of acetate of the adduct 2a into 3a.
aAll the clays were dried at 85 ºC for 2 h before each use.
bMicrowave irradiation was carried out on a KenStar microwave oven (70% PL).
cE/Z- ratio was estimated based on 1H and 13C NMR.
dAfter column purification
montmorillonite K10 clay-microwave combination as an (14) Kelly, S. E. In “Comprehensive Organic Synthesis” Vol.1.
Eds: Trost, B.M.; Fleming, I. pp: 797, Pergamon Press, New
York. 1991.
(15) Kad, G. L.; Singh, V.; Kaur, K.P.; Singh, J. Tetrahedron Lett.
1997, 38,1079 and references cited therein.
(16) Verma, R.S. Green Chemistry 1999,1, 43.
(17) Hammadi, M.; Villemin, D. Synth. Commun. 1996, 26, 2901.
(18) Shanmugam, P.; Nair, V. Synth. Commun. 1996, 26, 3007.
(19) Shanmugam, P. Synth. Commun. 1999, 29, 4409.
(20) Shanmugam, P.; Luxmi Varma, R. Indian J. Chem., Sec.(B)
2001, 40B, 0000.
(21) Typical experimental procedure for isomerisation: A
mixture of the acetates of Baylis-Hillman adducts (200 mg,
0.8 mmole) and montmorillonite K-10 (60 mg, 30% w/w of
the adduct) was taken in a stoppered 25 mL conical flask and
irradiated in the microwave oven (70% power mode) for 13
minutes. The mixture was cooled to room temperature and
treated with CH2Cl2 (10 mL). Montmorillonite K-10 clay was
recovered by filtration and washed with CH2Cl2 (2 x 5 mL).
The solvent was removed in vacuo and the crude mixture was
purified by silica gel column chromatography using petroleum
ether-ethyl acetate (92:8) to give pure colourless isomerised
products in 9:1 (E:Z) isomers as estimated by 1H NMR (300
MHz) and 13C NMR (75 MHz).
alternative, useful, speedy and efficient catalyst for the
isomerisation of a variety of acetates of Baylis-Hillman
adducts which provides stereoselective (E)-alkenes. Fur-
ther studies involving the above catalyst system are under-
way.
Acknowledgement
The authors thank Dr. G. Vijay Nair, Director and Dr. Mangalam S.
Nair for their interest in this work and for infrastructure facilities
provided. One of the authors (PR) thanks CSIR (New Delhi) for the
award of Junior Research Fellowship. Thanks are due to Mrs. S. Viji
for recording IR spectra and Mrs. Rugmini Sukumar, Clay and Clay
Minerals Division for natural kaolinite clay used for comparative
studies.
References and Notes
(1) Balogh, M.; Laszlo, P. In Organic Chemistry Using Clays,
Springer-Verlag, New York 1993.
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Mckillop, A.; Clossold, D. W.; Pannavaia, T. J.; Foucaud, A.;
Adams, J. M. In Preparative Chemistry Using Supported
Reagents, Eds. Laszlo, P. Academic Press, New York 1987,
Part 8, pp 453.
(3) Kabalka, G, W.; Pagni, R. M. Tetrahedron 1997, 53, 7999.
(4) Corma, A. Chem. Rev. 1995, 95, 559.
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(6) Cignek, E. In Organic Reactions, Vol. 51; Paquette, L.A., Ed.,
Wiley, New York, 1997, pp 201-350.
(7) Hofmann, H. M.R.; Eggert, U.; Poly, W. Angew. Chem., Int.
Ed. Engl. 1987, 26, 1015.
(8) Basavaiah, D.; Muthukumaran, K.; Sreenivasulu, B. Synthesis
2000, 545.
(9) Basavaiah, D.; Padmaja, K.; Satyanarayana,T. Synthesis 2000,
1662 and references cited therein.
(10) Kim, H.S.; Kim, T.Y.; Ley, K.Y.; Chung, Y.M.; Lee, H.J.;
Kim, J. N. Tetrahedron Lett. 2000, 41, 2613.
(11) Foucaud, A.; El Guemmount, F. Bull. Soc. Chim. Fr. 1989,
403.
(22) The 1H NMR spectra of the isomerised product 3a showed a
peak at 7.83 for the presence of E-vinylic proton and a minor
peak at 7.9 for Z-isomer. In the 13C NMR spectra of
trisubstituted olefins, the allylic carbon cis to aromatic group
appears up field while the same carbon trans to aromatic ring
appears down field. The values we obtained herein are exactly
matching with the values reported8,9 in the literature.
(23) Kwai, M.; Onaka, M.; Izumi, Y. Bull. Chem. Soc., Jpn. 1988,
61, 1237.
(24) The natural Kaolinite clay (layer type 1:1) used in this reaction
was obtained from Clay and Clay Minerals Division, RRL,
Trivandrum. The natural kaolinite clay was treated with 2 N
HCl for 0.5 h and calcined at 400 ºC for 3 h. The interlayer
distance of this acid treated clay was found to be lesser than
that of untreated natural kaolinite clay. See ref. 25.
(25) Sabu, K. R.; Sukumar, R.; Lalithambika, M. Bull. Chem. Soc.,
Jpn. 1993, 66, 3535.
(12) Basavaiah, D.; Dharma Rao, P.; Suguna Hyma, R.
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Article Identifier:
1437-2096,E;2001,0,08,1314,1316,ftx,en;D08801ST.pdf
Synlett 2001, No. 8, 1314–1316 ISSN 0936-5214 © Thieme Stuttgart · New York