Silica aerogel-iron oxide nanocomposites: Recoverable catalysts in conjugate additions and in the Biginelli reaction

Article abstract of DOI:10.1016/S0040-4020(03)00051-6

Ferrihydrite silica aerogels containing 11-13% iron and a surface area of 500-600 m²/g are catalysts for the title reactions. They are recovered and reused without loss of activity.

Full text of DOI:10.1016/S0040-4020(03)00051-6

Tetrahedron 59 (2003) 1553–1556
Silica aerogel-iron oxide nanocomposites: recoverable catalysts in
conjugate additions and in the Biginelli reaction
a
a
b
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Sandra Martınez, Miriam Meseguer, Lluıs Casas, Elisenda Rodrıguez, Elies Molins,
a
Marcial Moreno-Manas, Anna Roig, Rosa M. Sebastian and Adelina Vallribera
b
a
a
,
*
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Department of Chemistry, Universitat Autonoma de Barcelona, Edifici C. Unitat de Qumica Organica,
08193 Cerdanyola (Barcelona), Spain
Institut de Ciencia de Materials de Barcelona, Campus de la UAB. 08193 Cerdanyola (Barcelona), Spain
b
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Received 15 October 2002; revised 12 December 2002; accepted 3 January 2003
Abstract—Ferrihydrite silica aerogels containing 11–13% iron and a surface area of 500–600 m2/g are catalysts for the title reactions. They
are recovered and reused without loss of activity. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
knowledge ferrihydrite silica aerogels have not been used in
heterogeneous solid–liquid catalysis.
Heterogeneous catalysis dominates the industrial scenery
mainly due to the facility of recovery and reuse of solid
insoluble catalysts.1 The comparison between hetero-
geneous and homogeneous catalysis has been deeply
discussed.2 Transition-metal containing catalytic active
sites should be anchored to both chemically stable and
physically robust frameworks in order to secure recovery
without detriment of catalytic properties. Silica gel and
silicate-based supports fulfil both conditions.3–5 Another
crucial point is the accessibility of reagents to the active
sites. This requires either the active sites to be at the surface
of the support or materials of high specific surface and
porosity, and aerogels offer these advantages.6 Some of us
have prepared and studied silica aerogels containing iron
oxide nanoparticles.7a,b Nanoparticles of iron oxide were
grown inside the silica aerogel pores by wet chemistry
during sol-gel step. They are accessible because of the high
open porosity and the aerogel acts as supporter of the active
catalyst. The iron oxide particles are round shaped and the
average particle size diameter is of 3 nm. Depending on the
synthesis conditions as well as on further thermal treatment
of the composite aerogels, different phases for the iron oxide
have been obtained: maghemite, magnetite, hematite or
ferryhydrite.7
We present our results12 on the use of nanoparticles of
ferrihydrite anchored in a silica aerogel, 1, having a high
specific surface and high porosity, in the conjugate addition
of pentane-2,4-dione 2 to diethyl diazenedicarboxylate 3
and in the Biginelli reaction.
2. Results
The conjugate addition of b-dicarbonyl compounds to
azodiester 3 is an example of electrophilic amination
which has been performed before under nickel(II) and
ruthenium(II) catalysis.13 Others have used iron(III)
chloride as a catalyst in these reactions.14 In all cases but
one13c catalysts were not recovered. We have now
performed the reaction of diketone 2 and 3 five consecutive
times with the same batch of aerogel 1a in refluxing 1,2-
dichloroethane (Scheme 1). The yields of pure 4 after
recrystallization were 70, 74, 76, 71, and 80%. Analysis of
the reaction crude indicates a leaching of 3% of the iron
present in the aerogel. In the case of ethyl acetoacetate, a
complete consumption of the nucleophile was observed but
the adduct could not be properly isolated.
The Biginelli reaction15 is an old three component
condensation reaction which recently has drawn the
attention of the chemical community because the resulting
4-aryl-3,4-dihydropyrimidin-2(1H)-ones are antihyperten-
sive agents.16 The Biginelli reaction is catalysed by different
Lewis acids, including iron(III) chloride.17 Now we have
found that the three component reaction can be carried out
in refluxing ethanol in the presence of iron-containing
In order to analyse the catalytic activity of iron-containing
aerogels, the ferrihydrite phase was selected as the guest
particles due to the known catalytic activity of ferrihydrite.8
Few references deal with the use of iron aerogels in catalysis
in gas phase9 or in supercritical CO2.10,11 To the best of our
*
Corresponding authors. Tel.: þ34-935813045; fax: þ34-935811265;
e-mail: elies.molins@icmab.es; adelina.vallribera@uab.es
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00051-6

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S. Martınez et al. / Tetrahedron 59 (2003) 1553–1556
1554
X-Ray diffractogram of 1b shows a broad peak centred at
228 of 2u, which is typical of silica aerogels (Fig. 1). The
remaining peaks are also broad and their position cannot be
established with high precision. They could correspond to
different iron oxide phases, except the peak located at 45.58.
This one is characteristic of the ferrihydrite phase. X-Ray
diffractograms of ferrihydrites largely depend on the
sample’s crystallinity, but those ferrihydrites showing the
whole set of peaks are usually known as ‘6-line ferrihy-
drite’.18 The crystalline structure can be described as
hexagonal close-packed layers of O22, OH2 and H2O
with the Fe(III) occupying octahedral positions, the
structure is identical to that of hematite but in the case of
hematite, the hexagonal layers are only of O22 and there are
no-vacancies.
Scheme 1.
aerogel 1b. We performed three consecutive albeit not
identical reactions with the same batch of 1b. Indeed,
aldehyde 7 was varied in each reaction to afford three
different 4-aryl-3,4-dihydropyrimidin-2(1H)-ones, 8a–c, in
this order in 65, 34, and 61% unoptimised yields (Scheme 2).
Scheme 2.
Figure 1.
¨
Mossbauer spectrum (Fig. 2) at 300 K shows a single
3. Conclusion
doublet characteristic of paramagnetic iron sites. At 70 K
the magnetic ordering appears. At 4.2 K the spectrum is
totally split in six broad lines. The hyperfine parameters
In summary, iron(III)-containing silica aerogels are pre-
pared and used as efficient catalysts for condensation
reactions. The aerogels are recovered and reused without
decrease of activity.
correspond to those of ferrihydrite.19 Mossbauer measure-
¨
ments were also performed at 10 K and under 6 T magnetic
field,7b hyperfine field distribution agrees with the sper-
omagnetic behaviour of ferrihydrite.
4. Experimental
4.1. Preparation and characterization of ferrihydrite
silica aerogel (1)
Fe(NO3)2·9H2O (7.3 g, 0.018 mol) was dissolved in ethanol
(12 mL). Then, tetramethoxysilane (20 mL, 0.09 mol) was
added drop-wise while stirring. After 20 min of continuous
stirring the solution was kept at 408C. The resulting sol took
a darker colour and gelified after 9 days (gel precursor of
1a). Another batch was prepared with triple amount of
reactants and gelified after 11 days (gel precursor of 1b).
Supercritical drying of the gels was performed at super-
critical conditions of ethanol (T¼516 K, pressure¼100–
140 bar). The complete process taking 8 h. They presented
the following analyses. Compound 1a: C, 8.99 and 8.92%;
H, 1.29 and 1.25%; Fe, 11.8 and 11.2%. Compound 1b: C,
6.91 and 6.97; H, 0.95 and 0.93; Fe, 12.9 and 13.0 (carbon
comes from non-hydrolysed ethers). Iron has been deter-
mined by atomic absorption spectrophotometry. Bulk
densities: 0.65 g/mL (for 1a) and 0.64 g/mL (for 1b); BET
surface area: 603 m2/g (for 1a) and 511 m2/g (for 1b).
Figure 2.

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S. Martınez et al. / Tetrahedron 59 (2003) 1553–1556
1555
1
2976, 1724, 1695, 1647, 1219, 1085 cm21; H NMR (d6-
DMSO, 250 MHz) d 1.09 (t, J¼7.0 Hz, 3H), 2.24 (s, 3H),
3.99 (q, J¼7.0 Hz, 2H), 5.14 (d, J¼3.2 Hz, 1H), 7.22–7.32
(m, 5H), 7.73 (s, 1H), 9.19 (s, 1H). 13C NMR (d6-DMSO,
62.5 MHz) d 14.2, 17.9, 54.1, 59.3, 99.4, 126.4 (2C), 127.4,
128.5 (2C), 145.0, 148.5, 152.3, 165.5.
4.1.3. Preparation of 5-(ethoxycarbonyl)-6-methyl-4-(4-
chlorophenyl)-3,4-dihydropyrimidin-2(1H)-one
(8b).
The title compound 8b was prepared following the same
procedure as for 8a. Urea (4.2 g, 70 mmol), p-chloro-
benzaldehide (7 g, 50 mmol) and ethyl acetoacetate (8 mL,
62.3 mmol) were added to the recovered aerogel in 15 mL
of EtOH. When the reaction was finished the aerogel was
washed several times with hot ethanol and with hot
tetrahydrofuran. The recovered aerogel was used in the
following Biginelli reaction. The combined extracts were
evaporated and the residue was recrystallized in tetra-
hydrofuran to afford 8b (4.7 g, 34%), mp 2148C (lit.20 mp
210–2128C): IR (KBr) 3234, 3110, 2966, 1714, 1700, 1647,
1219, 1085 cm21; 1H NMR (d6-DMSO, 250 MHz) d 1.10 (t,
J¼7.2 Hz, 3H), 2.24 (s, 3H), 3.98 (q, J¼7.2 Hz, 2H), 5.13
(d, J¼3.2 Hz, 1H), 7.24 (d, J¼8.3 Hz, 2H), 7.39 (d,
J¼8.3 Hz, 2H), 7.78 (s, 1H), 9.25 (s, 1H). 13C NMR (d6-
DMSO, 62.5 MHz) d 14.9, 18.6, 54.3, 60.1, 99.7, 129.02
(2C), 129.2 (2C), 132.6, 144.6, 149.5, 152.7, 166.1.
Figure 3.
Dark field TEM image of 1a (Fig. 3) is the most useful to
visualize the iron oxide particles, since those are more
crystalline than the silica aerogel matrix then resulting in a
brighter contrast. The particles are round shaped, homo-
geneously distributed and with sizes ranging from 1 to 5 nm,
few larger particles having sizes between 6 and 10 nm are
also observed. The particle distribution histogram is also
shown and the average particle size is of 3 nm of diameter.
4.1.4. Preparation of 5-(ethoxycarbonyl)-6-methyl-4-(4-
methoxyphenyl)-3,4-dihydropyrimidin-2(1H)-one (8c).
The title compound 8c was prepared following the same
procedure as for 8a. Urea (4.5 g, 75 mmol), p-methoxy-
benzaldehyde (6 mL, 50 mmol) and ethyl acetoacetate
(6.4 mL, 50 mmol) were added to the recovered aerogel in
15 mL of EtOH. When the reaction was finished the aerogel
was washed several times with hot ethanol and with hot
tetrahydrofuran. The recovered aerogel was used in the
following Biginelli reaction. The combined extracts were
evaporated and the residue was recrystallized in tetra-
hydrofuran to afford 8c (8.9 g, 61%), mp 204–2058C (lit.20
mp 191–1938C): IR (KBr) 3234, 3100, 2947, 1723, 1704,
1647, 1223, 1090 cm21; 1H NMR (d6-DMSO, 250 MHz) d
1.10 (t, J¼7.2 Hz, 3H), 2.24 (s, 3H), 3.72 (s, 3H), 3.98 (q,
J¼7.2 Hz, 2H), 5.09 (d, J¼1.0 Hz, 1H), 6.88 (d, J¼8.6 Hz,
2H), 7.14 (d, J¼8.6 Hz, 2H), 7.67 (s, 1H), 9.16 (s, 1H). 13C
NMR (d6-DMSO, 62.5 MHz) d 14.9, 18.6, 54.2, 55.9, 60.0,
100.4, 114.54, 128.21 (2C), 137.9 (2C), 148.8, 152.9,
159.29, 166.1.
4.1.1. Preparation of 3-[N,N0-bis(ethoxycarbonyl)hydra-
zino]-2,4-pentane-2,4-dione (4). A solution of pentane-2,4-
dione 2 (1.2 g, 11.6 mmol) and diethyl azodicarboxylate 3
(1.0 g, 5.7 mmol) in 1,2-dichloroethane (30 mL) was
refluxed for 72 h under mechanical stirring in the presence
of iron-containing aerogel 1a (0.1 g, 0.21 mmol of iron).
Then the solution was decanted and the aerogel washed
several times with 1,2-dichloroethane never permitting it to
dry. The recovered aerogel was used in the following
preparation of 4. The combined solvent extracts were
evaporated and the residue was recrystallized from toluene
to afford pure 4 (1.10 g, 70%), mp 122–1258C (lit.13c mp
122–1258C): IR (KBr) 3263, 2985, 1760, 1698, 1244 cm21
.
1H NMR (CDCl3, 250 MHz) d major keto tautomer: 1.29 (t,
J¼7.3 Hz, 6H), 2.23 (s, 6H), 4.22 (q, J¼7.3 Hz, 4H), 6.79
(s, 1H); minor enol tautomer: 1.29 (t, J¼7.3 Hz, 6H), 2.17
(s, 6H), 4.22 (q, J¼7.3 Hz, 4H), 16.02 (s, 1H). 13C NMR
(CDCl3, 62.5 MHz) d tautomeric mixture 14.4, 14.5, 22.1,
62.3, 63.5, 63.7, 117.9, 155.7, 155.8, 156.3, 191.9. MS (m/z,
%): 274 (5) [Mþ], 232 (7), 159 (10), 43 (100).
4.1.2. Preparation of 5-(ethoxycarbonyl)-6-methyl-4-
phenyl-3,4-dihydropyrimidin-2(1H)-one (8a): general
procedure. A solution of urea 6 (4.55 g, 75 mmol) and
benzaldehyde 7b (5.1 mL, 50 mmol) in ethanol (15 mL)
was added into a suspension of iron-containing aerogel 1b
(1.77 g, 4.0 mmol of iron) in ethyl acetoacetate 5 (8 mL,
62.3 mmol). The mixture was refluxed for 84 h under
mechanical stirring. After cooling the solution was decanted
and the aerogel was washed with ethanol several times
never permitting it to dry. The recovered aerogel was used
in the following Biginelli reaction. The combined ethanol
extracts were evaporated to afford a residue which was
recrystallized from ethanol to afford 8a (7.92 g, 65%), mp
204–2058C (lit.20 mp 202–2048C): IR (KBr) 3234, 3110,
Acknowledgements
We thank MCYT (projects PB98-0902, BQU2002-04002
and MAT2000-2016) and CIRIT (Projects SGR98-0056,
SGR99-0205, SGR2000-0062 and SGR2001-0181) for
´
financial support. We are indebted to ‘Ramon y Cajal’
programme for a contract to R. M. S.
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