M. Gruttadauria et al. / Tetrahedron 59 (2003) 4997–5002
5001
in n-propyl alcohol, according to a published procedure.33 The
hydrolysis was carried out by an abrupt addition of aqueous
solution (in excess with respect to the stoichiometric ratio) to
zirconium precursor. The reactants and the relative amount in
a typical preparation are the following: Zr(OCH2CH2CH3)4
(40 g) was added to n-propyl alcohol/H2O (1:1, 100 mL). The
white precipitate was refluxed at 1008C overnight, then was
filtered and washed with bi-distilled water. After drying at
1008C for 12 h, the solid was ground and calcined in air in two
steps: at 4008C for 1 h, then at 6008C for 2 h. According to the
literature,33 the crystallization products ofamorphous zirconia
contain along with the monoclinic also the metastable
tetragonal phase. The sample has surface area of 35 m2/g (as
determined by nitrogen adsorption/desorption measurements
overnight. The resulting solid was filtered, washed with bi-
distilled water and dried at 1008C for 20 h. The Cr(III) in the
washing solution was determined spectrophotometrically as
CrO242. The calculated content of entrapped chromium was
9 wt%. In order to have an oxidizing material, the sol–gel
entrapped Cr(III) was oxidized to Cr(VI) by successive
calcination in air at 4008C for 2 h, then at 6008C for 1.5 h.
During the calcination process, crystallization of zirconia
from amorphous to tetragonal phase occurs. No diffraction
lines of the monoclinic form were detected. The sample
showed a surface area of 40 m2/g with a pore size
˚
distribution centered at 20 A as determined by nitrogen
adsorption/desorption measurements at 77 K.
˚
at 77 K) and a mean pore size distribution of 30 A.
4.1.4. General procedure for oxidation reactions. In a
typical procedure with adCr/SiO2 a round-bottom flask was
filled with Cr(VI)-reagent (1.00 g), dichloromethane or
dichloromethane/diethyl ether 3:1 (0.05 or 0.02 M) and the
alcohol (21.6 mg) under argon. The reactions were carried
out at 258C with magnetic stirring. At the end the reaction
mixture was filtered, washed with 5 mL of ethyl acetate. The
Cr(VI)-reagent was dried in the oven (808C) for 1 h, then
regenerated.
4.1.2. Preparation of sgCr/SiO2. Tetraethoxysilane
(20.83 g, 0.1 mol) was added to an aqueous solution of
HCl (0.01N, 10 mL) and mixed with ethanol (5 mL) as co-
solvent. The mixture was stirred for 30 min to obtain a sol
containing principally hydrolyzed tetraethoxysilane. In
order to complete the hydrolysis step, the ethanol was
removed by distillation under vacuum at 358C. Next, the sol
solution was cooled at 08C to avoid gelation that would be
promoted at room temperature. Chromium(III) chloride
hexahydrate (2.66 g, 0.01 mol) was dissolved in 10 mL of
bi-distilled water and cooled at 08C. Finally, the solution
containing chromium chloride was added slowly to the sol
and the pH was raised from 2 to 4.
4.1.5. General procedure for regeneration by calcina-
tion. Adsorbed and entrapped Cr(VI) samples were
regenerated by calcination in air, in dynamic conditions,
by increasing the temperature from room temperature to
400–6008C (heating rate 108C/min). Usually, 2 g of spent
sample was put in a ceramic dish and calcined up to 4008C
for 1 h, in the case of adCr/SiO2, and up to 6008C 2 h for
adsorbed and sol–gel Cr/ZrO2.
The gelation occurred in about 20 h. The resulting oxide
ground in granules (,425 mm) was washed with water and
sonicated for 30 min in the same solvent in order to remove
any chromium compound that adhered onto the outer
surface of the silica matrix. The resulting material was
heated to 708C until a constant weight was achieved (24 h).
The Cr(III) in the washing solution was determined
spectrophotometrically as CrO242. The calculated content
of the silica entrapped chromium was of 4% wt. Before use
the resulting oxide was dried at 258C at 1 mmHg for 12 h.
4.1.6. General procedure for regeneration with ozone.
Adsorbed or entrapped Cr/SiO2 was regenerated at room
temperature by packing the reagent in a column and letting
the substrate stand in contact with a continuous ozone flow,
usually 50 mg/h within 48 h at 40 mA.
Acknowledgements
The sgCr(III)/SiO2 (3.217 g) was placed in a column and a
continuous ozone flow (10 Nl/h, ca. 50 mg O3/h) was
allowed to pass through for 31 h. At the end of the process
the starting green silica turned to a red-brown material.
The authors thank the University of Palermo for financial
support (funds for selected research topics) and CNR
(Roma).
The sample showed a very high surface area (650 m2/g)
˚
with a pore size distribution centered at 10–20 A as
determined by nitrogen adsorption/desorption measure-
ments at 77 K. The X-ray diffraction pattern showed only
the background of the silica amorphous glass. No reflections
due to any chromium oxide phase were detected.
References
1. Larock, R. C. Comprehensive Organic Transformations: A
Guide to Functional Group Preparations; 2nd ed. Wiley-
VCH: New York, 1999.
4.1.3. Preparation of sgCr/ZrO2. The sol–gel entrapped
Cr/ZrO2 was synthesized by hydrolytic polycondensation of
zirconium n-propoxide Zr(OCH2CH2CH3)4, using CrCl3·6-
H2O as chromium precursor. In a typical preparation,
CrCl3·6H2O (8 g, 0.03 mol) was dissolved in 20 mL of bi-
distilled water, then the resulting solution was added at
room temperature to a well-stirred solution of the zirconium
n-propoxide (40 g, 0.12 mol) in n-propyl alcohol. After the
addition of chromium solution, a fine precipitate immedi-
ately was formed and the suspension was refluxed at 1008C
2. (a) Bowden, K.; Heilbron, I. M.; Jones, E. R. H.; Weedon, B. C.
L. J. Chem. Soc. 1946, 39–45. (b) Djerassi, C.; Engle, R. R.;
Bowers, A. J. Org. Chem. 1956, 21, 1547–1549.
3. Collins, J. C.; Hess, W. W.; Frank, F. J. Tetrahedron Lett.
1968, 10, 3363–3366.
4. (a) Corey, E. J.; Suggs, J. W. Tetrahedron Lett. 1975, 17,
2647–2650. (b) Piancatelli, G.; Scettri, A.; D’Auria, M.
Synthesis 1982, 245–258.
5. Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979, 21,
399–402.
Gruttadauria, Michelangelo
