Nanoporous solid acid catalyst for the Diels-Alder reaction of 1,3-dienes with acrylates

Article abstract of DOI:10.1246/cl.2002.166

Nanoporous aluminosilicate, Al-HMS with a high aluminum content, strong acidity, and nanoporosity was found to catalyze the Diels-Alder reaction of 1,3-dienes with methacrylate and acrylate, and turned out to be a green alternative to homogeneous acid catalysts.

Full text of DOI:10.1246/cl.2002.166

166
Chemistry Letters 2002
Nanoporous Solid Acid Catalyst for the Diels-Alder Reaction of 1,3-Dienes with Acrylates
Makoto Onaka,ꢀ Naoki Hashimoto, Ryota Yamasaki, and Yasuyoshi Kitabata
Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902
(Received November 16, 2001; CL-011158)
Nanoporous aluminosilicate, Al-HMS with a high aluminum
1055 m2/g, and pore diameter of 1.9 nm.
content, strong acidity, and nanoporosity was found to catalyze
the Diels-Alder reaction of 1,3-dienes with methacrylate and
acrylate, and turned out to be a green alternative to homogeneous
acid catalysts.
To Al-HMS (0.15 or 0.3 g) predried at 400 ꢁC/0.5 mmHg
(1 mmHg ꢂ 133:3 Pa) for 2 h was added a mixture of acrylate
(1 mmol) and 1,3-diene (3 mmol) in solvent (6 ml) at ꢃ1 ꢁC, and
the mixture was stirred at ꢃ1 ꢁC for a specified time. After
filtration of Al-HMS, the Diels-Alder adducts were purified by
distillation, or analyzed by GC. The stereoselectivity of the
products was determined by NMR.
The Diels-Alder reaction is an important and widely used
reaction in organic synthesis and in the chemical industry. Rate
enhancement of this reaction has been achieved by the use of
Lewis acids such as La(OTf)3,1 5 M LiClO4 in Et2O,2 solvents
such as water,3 supercritical CO2,4 and ionic solvent,5 and under
high pressure,6 or radiation of microwave7 and ultrasound.8 Solid
acids such as zeolite,9 clay,10 and alumina11 also accelerate the
reaction.
From a standpoint of Green Chemistry concerning safe
handling and disposal, solid acids are expected to be alternatives
to mineral acids and Lewis acids which have been conventionally
employed in fine chemicals synthesis in chemical industry. We
previously found that acidic mesoporous aluminosilicate, Hþ-Al-
MCM-41 and Zn2þ-Al-MCM-41 with mesopores of 2.5 nm in
diameter and high specific surface area of over 1000 m2/g were
efficient catalysts for the Diels-Alder reaction of methyl acrylate
with cyclopentadiene.12 However, when the catalysts were
applied to the reaction of less reactive, bulkier dienophile like
methyl methacrylate with acyclic 1,3-diene, poor results
(chemical yield ¼< 50%) were obtained.
The acid catalysis of Al-HMS on the Diels-Alder reaction
was influenced by the reaction media: nonpolar hydrocarbons
such as hexane and cyclohexane are a more desirable solvent than
benzene and dichloromethane.15
Table 1 shows the comparison of the acid catalysis between
representative aluminosilicate solid acids such as Al-HMS,
amorphous silica-alumina (SiO2-Al2O3), commercially available
solid acid K10, dealuminated Y-type zeolite (H-Y), and ꢀ-type
zeolite (H-ꢀ) in the reaction of methacrylate or acrylate with
isoprene or cyclopentadiene. Aluminum sites in the alumino-
silicates are responsible for acid catalysis, and hence the weight of
solid acid catalyst was adjusted so as to have the same or more
amount of aluminum atoms than that of Al atoms contained in Al-
HMS. In this comparison, Al-HMS (Si/Al ¼ 6:8) was the best
solid acid for the Diels-Alder reaction of methacrylate and
acrylate with 1,3-dienes. It is likely that the aluminum atoms are
homogeneously dispersed in the silicate framework and hence
effectively function as acid sites though Al-HMS (6.8) contains a
high content of aluminum.
Al-HMS (6.8) was confirmed to be recyclable as acid catalyst
in the reaction of methyl acrylate with isoprene: 1st run, 94%; 2nd
run, 90%; 3rd run, 87%. A gradual decrease in the catalytic
activity is mainly due to irreversible adsorption of polymeric by-
products from isoprene on Al-HMS which are unable to be
removed with a rinse of organic solvent.
Table 2 summarizes the comparison between Al-HMS and
AlCl3-catalyzed Diels-Alder reactions. The molar quantity of
AlCl3 used is the same as that of aluminum atoms in Al-HMS
used. The acid catalysis of Al-HMS was found to be comparable
to that of homogeneous Lewis acid AlCl3 in the Diels-Alder
reaction of acrylate derivatives. Considering that it is often a
troublesome work to separate AlCl3 from products, Al-HMS
becomes an alternative to homogeneous acid catalysts in fine
chemicals synthesis in terms of safe handling and easy work-up
procedures of the solid acid.
Then, we focused on another type of mesoporous alumino-
silicate, HMS developed by Pinnavaia13 in order to find more
active solid acid catalyst for the reaction. HMS has several
advantages: 1) HMS can be easily formed in the sol-gel reaction
of Si(OR)4 and Al(OR’)3 in the presence of primary alkylamine as
template at room temperature (RT). 2) The acidic properties of
HMS are dependent on the density and distribution of aluminum
sites in the silicate structure. HMS with a high Al content and high
dispersion of Al sites can be prepared via the sol-gel process. 3)
The pore structure of HMS is rather wormhole-like than straight
channel-like, but the pore-size distribution is narrow. 4) HMS
consists of small primary particles forming textural mesoporos-
ity. Pinnavaia urged that as compared with MCM-41, HMS
should be more favorable for a catalyst in liquid-phase reactions
owing to its textural mesoporosity which enables more reaction
substrates to diffuse into the mesopores from a solution.14
To a vigorously stirred solutionof n-C16H33NH2 (3.31 mmol)
in EtOH (153 mmol) and deionized water (257 mmol) was added
at a time a homogeneous mixture of Si(OEt)4 (10 mmol) and
Al(OiPr)3 (1.43 mmol) at RT. The mixture was vigorously stirred
for 48 h at RT. The white gels formed were collected, dried at RT
under a N2 flow, and finally calcined in dry air in an electric
furnace at 500 ꢁC for 5 h. The prepared aluminum-containing
HMS (abbreviated as Al-HMS) was analyzed with ICP and N2 gas
adsorption methods: a Si/Al ratio of 6.8, specific surface area of
In summary, we developed a green solid acid with strong
acidity and nanoporosity which catalyze the Diels-Alder reaction
of 1,3-dienes with relatively poor dienophiles like methacrylate.
We gratefully acknowledge Prof. Masakazu Iwamoto of
Tokyo Institute of Technology for the analysis of Al-HMS with
ICP.
We dedicate this article to Professor Teruaki Mukaiyama on
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
167
Table 1. Comparison of solid acid catalysts in the Diels-Alder reactionsa
the occasion of his 75th birthday.
Table 2. Al-HMS- and AlCl3-Catalyzed Diels-Alder reactions
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