August 1998
SYNLETT
851
First TaCl -SiO Catalyzed Prins Reaction : Comparative Study of Conventional Heating
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#
vs Microwave Irradiation
S. Chandrasekhar* and B.V. Subba Reddy
Indian Institute of Chemical Technology, Hyderabad-500 007, India
Received 3 March 1998
Abstract : TaCl -SiO catalyzed Prins reaction has been achieved for
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the first time using microwave irradiation.
The condensation of olefins with aldehydes (mostly formaldehyde),
1a,b
generally called the Prins reaction,
is the only organic transformation
wherein a new C-C bond together with 1,3-diol formation is achieved.
2
This reaction is catalyzed by mineral acids, however some literature
3
reports have been documented wherein milder Lewis acids have also
4
5
been utilized. Recent additions include solid catalysts such as resin,
6
7
Zeolite and K clay.
10
Our interest in the development of new methodologies for 1,3-diol
formation en-route to biologically important hydroxyl compounds has
prompted us to study the TaCl -SiO catalyzed Prins reaction of olefins
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2
and formaldehyde (Scheme and Table), the results being presented
herein.
Scheme
The present study has unequivocally confirmed that the conventional
refluxing temperature and longer reaction times required for Prins
reaction are totally avoided using microwave irradiation, which is
becoming an alternate and substitute-heating source.
When the commercially available styrene, 1a (0.01 mol),
7,8
paraformaldehyde (0.01 mol) and TaCl -SiO
(0.001 mol) were
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admixed efficiently and exposed to Microwaves at 600 watts for 3 min,
an excellent yield of 1,3-diol derivative 2a was obtained after filtration
through a small pad of silica gel. This reaction, however, took
approximately 12 h of refluxing in dioxane and a tedious workup before
80% yield of 2a was isolated.
α-Methyl styrene 1b (entry-b), β-methyl styrene, 1c (entry-c), p-methyl
styrene 1d (entry-d), p-chloro styrene 1e (entry-e), p-nitro styrene 1f
(entry-f), p-methyl-β-methyl styrene 1g (entry-g), stilbene 1h (entry-h),
and undecenoate 1i (entry-i) were similarly transformed to
corresponding 1,3-diol derivatives (2a-i) after irradiation with
microwaves for 3-5 minutes. All the reactions were also conducted in
conventional round bottomed flasks with refluxing in dioxane.
Invariably the products obtained by microwave irradiation were purified
with more ease.
ethyl acetate mixture (80/20). Removal of volatiles furnished 1,3-
dioxane derivative (2a, 1.48 g, 90%).
b) Conventional Method : Styrene (1.04 g, 0.01 mol) was added to a
mixture of paraformaldehyde (0.9 g, 0.01 mol) and TaCl -SiO (0.37 g,
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10 mol percent) in dioxane and it was refluxed for 12 hr. at 110°C under
atmosphere. On completion, the reaction mixture was poured into
N
2
In conclusion a thorough and comparative study of Prins reaction
water and extracted with ethyl acetate. Standard workup and removal of
volatiles afforded 1,3-dioxane derivative (2a, 1.32 g, 80%).
catalyzed by TaCl -SiO under both solvent and solvent free conditions
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have been studied for the first time.
Typical Experimental Procedure
Acknowledgement : B.V.S. thanks C.S.I.R. for the award of fellowship.
(a) Microwave Irradiation: Styrene (1.04 g, 0.01 mol) and
paraformaldehyde (0.9 g, 0.01 mol) were absorbed on activated silica
gel (2g, 100-200 mesh, dried overnight at 110°C) and stirred at room
References:
1.
a) Prins, H.J. Chem. Weekbl. 1919, 16, 1072. [C.A. 13, 3155,
1919]. Proc. Acad. Sci. Amsterdam., 1919; 22:51. [C.A. 14, 1662,
1920]. b) Adams, D.R.; Bhatnagar, S.P. Synthesis 1977, 661.
temperature for 1 hr. under N atmosphere. To this TaCl -SiO (0.37 g,
2
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2
10 mole percent) was added and admixed thoroughly and irradiated in a
microwave oven (600 watts) for 3 min. This mixture was cooled to room
temperature, charged on a small silica pad and eluted with n-hexane/
2.
Arundale, E.; Mikeska, L.A. Chem. Rev. 1952, 51, 505.
Chandrasekhar
