TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1051–1055
Tandem reactions of Friedel–Crafts/aldehyde cyclotrimerization
catalyzed by an organotungsten Lewis acid
Hsing-Shiun Wang and Shuchun Joyce Yu*
Department of Chemistry, National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan
Received 17 November 2001; revised 5 December 2001; accepted 7 December 2001
Abstract—The tris(2-pyridyl)phosphine complex [P(2-py) W(CO)(NO) ](BF ) acts as a Lewis acid catalyst precursor for the
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tandem reactions of Friedel–Crafts/aldehyde cyclotrimerization, which lead to the formation of a series of hyper-branched star
polymers. © 2002 Elsevier Science Ltd. All rights reserved.
There has been considerable recent research interest
into the chemistry of low-valent organometallic Lewis
the five-membered heterocyclic arenes such as furan
and pyrrole with acrolin and/or crotonaldehyde at both
C-2 and C-5 positions during the course of the tandem
reactions. Subsequently, two aldehydic end groups at
both sides of the F–C adducts undergo further catalytic
cyclotrimerization to afford the hyper-branched star
polymers consisting of five- and six-membered hetero-
cyclic sub-units.
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acids. We communicate herein our development of a
new process for the catalytic tandem reactions of
Friedel–Crafts (F–C)/aldehyde cyclotrimerization by an
organotungsten Lewis acid. The process provides a
notably efficient method for the one-pot synthesis of a
series of hyper-branched star polymers bearing five-
and six-membered heterocyclic sub-units from the reac-
tions of five-membered heterocyclic arenes and a,b-
unsaturated aldehydes.
To verify the initiation and propagation steps for the
formation of these hyper-branched star polymers, two
separate controlled experiments are conducted. In the
The zero-valent tungsten complex [P(2-py) W(CO)-
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first experiment, methyl vinyl ketone (MVK) is treated
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(
NO) ](BF ) 1 possesses strong Lewis acidity upon
loss of the CO ligand, and the relative Lewis acid
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with the reactant arenes such as furan, pyrrol and
thiophene in the presence of 1 mol% of 1. As expected,
the reactions undergo double F–C alkylation to give the
corresponding 2,5-disubstituted arenes in quantitative
yields (Scheme 2A). Besides, there are no other
detectable by-products such as mono- and 2,4-disubsti-
tuted adducts formed. In the second experiment, a
series of aldehydes RCOH (R=Me, Et, n-Pr, i-Pr,
n-Bu) are used straight without solvents in the presence
of 0.3 mol% of 1. As a result, the corresponding cyclic
strength of 1 is found to be comparable to those of
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AlCl and BF . In addition, 1 can be easily prepared in
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two steps with high yield from W(CO) , and can be
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stored as a crystalline solid in air for months without
significant decomposition. The organotungsten Lewis
acid 1 acts as a catalyst precursor for the synthesis of a
series of hyper-branched star polymers. These star poly-
mers are generated from a one-pot reaction process of
double F–C alkylation of five-membered heterocyclic
arenes with a,b-unsaturated aldehydes followed by
cyclotrimerization of the F–C adducts (Scheme 1).
These tandem reactions are carried out in neat sub-
strates at ambient temperature with a catalyst loading
of 0.05–1 mol% to afford quantitative insoluble poly-
mers almost instantaneously (Table 1). It is most likely
that the complex 1 catalyzes double F–C alkylation of
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trimers are obtained without any detectable by-prod-
ucts such as dimers and teramers (Scheme 2B).
In order to obtain smaller sizes of the hyper-branched
star-shaped oligomers for structural analysis, subse-
quent reduction of the aldehydic end-groups with
NaBH in ethanol was performed, and this generated
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the corresponding alcohols to prevent further polymer-
ization via aldehyde cyclotrimerization. The oligomers
*
0
Corresponding author.
thus obtained were readily soluble in CHCl . In a
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040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
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