Ketalization of ketones to 1,3-dioxolanes and concurring self-aldolization catalyzed by an amorphous, hydrophilic SiO2-SO3H catalyst under microwave irradiation

Article abstract of DOI:10.21577/0103-5053.20180039

The amorphous, mesoporous SiO₂-SO₃H catalyst with a surface area of 115 m² g^-1 and 1.32 mmol H⁺ per g was very efficient for the protonation of ketones on a 10percent (m/m) basis, and the catalyst-bound intermediates can be trapped by polyalcohols to produce ketals in high yields or suffer aldol condensations within minutes under low-power microwave irradiation. The same catalyst can easily reverse the ketalization reaction. Printed in Brazil-

Full text of DOI:10.21577/0103-5053.20180039

http://dx.doi.org/10.21577/0103-5053.20180039
J. Braz. Chem. Soc., Vol. 29, No. 8, 1663-1671, 2018
Printed in Brazil - ©2018 Sociedade Brasileira de Química
Article
Ketalization of Ketones to 1,3-Dioxolanes and Concurring Self-Aldolization
Catalyzed by an Amorphous, Hydrophilic SiO2-SO3H Catalyst under
Microwave Irradiation
Sandro L. Barbosa,*,a Myrlene Ottone,a Mainara T. de Almeida,a Guilherme L. C. Lage,a
Melina A. R. Almeida,a David Lee Nelson,a Wallans T. P. dos Santos,a Giuliano C. Clososki,b
Norberto P. Lopes,b Stanlei I. Kleinc and Lucas D. Zanattad
aDepartamento de Farmácia, Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM),
R. da Glória, 187, Centro, 39100-000 Diamantina-MG, Brazil
bDepartamento de Física e Química, Faculdade de Ciências Farmacêuticas de Ribeirão Preto,
Universidade de São Paulo (USP), Av. do Café s/n, 14040-903 Ribeirão Preto-SP, Brazil
cDepartamento de Química Geral e Inorgânica, Instituto de Química,
Universidade Estadual de São Paulo (Unesp), R. Prof. Francisco Degni, 55, Quitandinha,
14800-900 Araraquara-SP, Brazil
dLaboratório de Química Bioinorgânica, Departamento de Química,
Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo,
Av. Bandeirantes, 3900, 14040-901 Ribeirão Preto-SP, Brazil
The amorphous, mesoporous SiO2-SO3H catalyst with a surface area of 115 m2 g-1 and
1.32 mmol H+ per g was very efficient for the protonation of ketones on a 10% (m/m) basis, and
the catalyst-bound intermediates can be trapped by polyalcohols to produce ketals in high yields
or suffer aldol condensations within minutes under low-power microwave irradiation. The same
catalyst can easily reverse the ketalization reaction.
Keywords: protonated ketones, self-aldolization, ketalization, ketal hydrolysis, sulfonated silica
Introduction
a mesoporous, sulfonated silica as a Brønsted acid, whose
high hydrophilicity made it ideal for the catalysis of water-
forming, proton-assisted organic transformations, such as
the formation of ethers from alcohols and the esterification
of acids with alcohols, equations 1 and 2.7 SiO2-SO3H8 and
SiO2-(CH2)3-SO3H6 have been reported to catalyze ketal
formation from ketones and ethylene glycol, which is also
a water-forming reaction involving the equilibrium shown
in equation 3.
Sulfuric acid can be considered to be the standard acid for
all organic reactions that are catalyzed by strong Brønsted-
Lowry acids, but its anti-Green-Chemistry nature impairs
its widespread use: its residues are highly acidic and its
corrosiveness is widely avoided. On the other hand, solid acids
such as sulfonated silicas have been shown to bear a sufficient
number of acidic sites, with little tendency for lixiviation,1
to make them versatile candidates for the replacement of
the homogeneous sulfuric acid. The SiO2-SO3H catalysts
can be prepared in several ways. Examples range from the
simple addition of H2SO4 to slurries of silica and ether2,3
or the treatment of silica with chlorosulfonic acid4 to more
complex synthesis of mesoporous materials where the
sulfonic groups are separated from the surface of the silica
by carbon chains.5 This method is also a form of diminishing
the hydrophilicity of the catalyst.6 We recently developed
(1)
(2)
The carbonyl groups of aldehydes and ketones are
quite reactive and must be protected if the compounds
bearing such groups are to be subjected to chemical
transformations where the aldehyde or ketone functions
*e-mail: sandro.barbosa@ufvjm.edu.br

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Ketalization of Ketones to 1,3-Dioxolanes and Concurring Self-Aldolization Catalyzed
J. Braz. Chem. Soc.
might be destroyed. Usually this protection is accomplished
by the reaction of the carbonyl compound with ethylene
glycol in the presence of an acid catalyst.9 The products
are usually referred to as an acetal if prepared from an
aldehyde, or ketal, if formed from a ketone. They contain
the backbone of the 5-membered 1-3-dioxolane ring.
This ring must be hydrolyzed in the presence of an acid
catalyst, to liberate the ethylene glycol and the carbonyl
compound after the desired transformation. Apart from
the great importance of such protection and deprotection
reactions, some ketals are useful themselves, such as those
from glycerol; these ketals can be used as precursors for the
synthesis of monoglycerides, as food emulsifiers,10 as chiral
building blocks in organic synthesis11 or as fuel additives
for biodiesel.12 1,3-Dioxolanes have also been shown to
exhibit antimicrobial activity against Gram-positive and
Gram-negative bacteria and can be used as antiseptics
for sterilization of working surfaces and instruments.13
The six-membered ring ketals, or m-dioxanes, are also of
interest for the pharmacy and fuel industries.14 We now
report that reactions such as equation 3 can be greatly
enhanced by microwave irradiation using our hydrophilic
catalyst and that the procedure can be extended to other
polyols such as trimethylol propane, neopentyl glycol and
crude glycerol. In the absence of alcohols, the ketones
tested were rapidly protonated to produce aldol condensates
(equation 4), except for benzophenone.
as KBr pellets in the 4000-400 cm-1 range on a Varian 640
spectrophotometer operating in the FT mode. X-ray
diffraction (XRD) patterns were collected on a Rigaku
diffractometer at 30 kV and 20 mA using CuKα radiation.
Differential thermal analysis (DTA) was carried out using a
PerkinElmer 1700 analyzer. Ketal contents and yields were
determined with a gas chromatography mass spectrometer
GC-MS-QP 2010 Shimadzu equipped with an AOC 5000
Auto Injector and a 30 m Agilent J&W GC DB-5 MS
column. Direct insertion spectra were measured at 70 eV.
Quantitative analyses were performed on a Shimadzu
GC-2010 gas chromatograph equipped with a flame
1
ionization detector (FID). H and 13C nuclear magnetic
resonance (NMR) spectra were recorded on BrukerAvance
400 and Avance 500 spectrometers. All the reactions were
performed under atmospheric pressure, using microwave
irradiation and monitored by thin layer chromatography
(TLC) with Silica Gel 60 F254 on aluminum. The
chromatograms were visualized by UV or by using an
ethanolic vanillin developing agent. Silica gel (Merck
230-400 mesh) was used for purification of products by
flash column chromatography using hexane/ethyl acetate
(8:2) as eluent.
Preparation of the silica gel and the sulfonated silica,
SiO2‑SO3H7
A mixture of 300.0 g of sand and 600.0 g of sodium
carbonate were homogenized and transferred to porcelain
crucibles, which were heated at 850 °C for 4 h. The hot
solid mixtures were transferred to a glass filter frit and
washed with 600-900 mL of boiling water. The filtered
solution was acidified to pH = 1 with hydrochloric acid,
the white precipitate was filtered, and dried at 400 ºC. The
resulting silica was passed through a 24 mesh sieve for
standardization. 10.0 g of the prepared silica was mixed
with 10.0 mL of H2SO4 and stirred at room temperature
for 12 h, filtered and dried at 150 °C for 4 h, cooled and
stored in a desiccator. The acid strength of 1.32 mmol of
H+ per gram of catalyst was determined by potentiometric
titration.
(3)
(4)
Experimental
Raw materials
The alcohols ethylene glycol, 2-ethyl-2-hydroxy-
methylpropane-1,2-diol (trimethylol propane), and
2,2-dimethylpropane-1,3-diol (neopentyl glycol) and
the ketones propanone, cyclohexanone, acetophenone,
4-methylacetophenone and benzophenone were used
as purchased; crude glycerol was obtained from the
transesterification of waste cooking oil with methanol.
Typical procedures
All the reactions were irradiated in an unmodified
microwave (MW) oven (900 GHz)/360 W using an
open 125 mL two-necked round bottom flask and were
accompanied by TLC or GC-MS. Typically, the amount
of catalyst was weighed directly into the flask, and
the desired quantity of the ketone and alcohol, where
applicable, were added, the mixture quickly mixed by
Instrumentation
The infrared spectra of solid samples were recorded

Vol. 29, No. 8, 2018
Barbosa et al.
1665
shaking and, immediately, irradiated. The silica catalyst
does not allow the final temperature of the slurries to
exceed 73 ºC (see also reference 7). To each of the cooled
vessels from the reactions, 30.0 mL of diethyl ether was
added, and the mixture was filtered. The organic extracts
were washed with 10.0 mL of saturated NaHCO3, dried
over anhydrous MgSO4 and concentrated under reduced
pressure. The residues were purified on chromatographic
columns using hexane/ethyl acetate 8:2 as eluent (with the
exception of solketal, which was distilled) to furnish the
pure products as colorless oils. For the synthesis of ketals,
the amounts of ketone (1.0 mmol) and alcohol (5.0 mmol)
were maintained throughout; the amount of catalyst used
in each run was adjusted to maintain a constant 10%
mass-to-mass ratio to the ketone, and the reactions were
irradiated for 2 to 7 min. The ketals were identified by
GC-MS, electrospray ionization time-of-flight (ESI-TOF)
mass spectrometry and 1H and 13C NMR spectroscopy. For
the synthesis of α,β-unsaturated ketones, the amount of
catalyst was adjusted to 10% (m/m) relative to 1.0 mmol of
the ketones (acetone, methyl ethyl ketone, cyclohexanone,
acetophenone and p-methylacetophenone). All the
reaction media were irradiated for 6 min, except for
acetone, whose reaction was performed in a conventional
heating mantle (12 h of reflux). The α,β-unsaturated
ketones were identified by GC-MS and 1H and 13C NMR
spectroscopy.
Results and Discussion
Silica gel and catalyst, SiO2‑SO3H, characterization
The non-crystalline natures of the silica gel and of
the SiO2-SO3H catalyst were confirmed by XRD, which
presented broad peaks centered at 2θ = 23º,7 Figure 1.
All the reactions were performed under atmospheric
pressure, using a heating mantle and, except for acetone,
microwave irradiation, and monitored by TLC with
Silica Gel 60 F254 on aluminum. The chromatograms
were visualized by UV or by using an ethanolic vanillin
developing agent. Silica gel (Merck 230-400 mesh)
was used for purification of products by flash column
chromatography using hexane/ethyl acetate (9:1) as
eluent.
For the hydrolysis of the ketals, the amount of catalyst
was adjusted to 20% (m/m) in relation to the ketal
(1.00 mmol), and an excess of water (10.00 mL) was added
to minimize the auto condensation reactions of the ketone
products.After irradiating 2 min at 360 W, the mixture was
cooled, mixed with 20.00 mL of ethyl ether and filtered.
The organic mixture was extracted with an aqueous NaCl
solution, dried with MgSO4 and concentrated; GC-MS
analyses of these extracts confirmed 94-98% yields. The
alcohols and ketones were finally separated by column
chromatography using a mixture of hexane/ethyl acetate
8:2 as eluent. Ethylene glycol and 20% (m/m) SiO2-SO3H
was irradiated for 6 min to produce diethylene glycol in
96% yield after the same chromatographic purification
procedure.
Figure 1. X-ray powder pattern of (a) precipitated silica gel and
(b) SiO2-SO3H catalyst.
The infrared spectrum of the silica (Figure 2a) is also
representative of an amorphous silica gel.7 The characteristic
absorption bands around 3400 and 1630 cm-1 are associated
with the stretching and bending modes of molecular water,
whereas the shoulder at 3200 cm-1 and the weak absorption
at 960 cm-1 are related to the –OH and Si–OH vibrations of
the silica silanol groups.7 The bands at 1090 cm-1 and the
associated shoulder at 1190 cm-1 are characteristic of the
asymmetric stretching modes of the Si–O–Si bonds.7 The
absorption at 800 cm-1 of the ring-structured tetrahedral
SiO4 (Si–O–Si symmetric stretchings), the Si–O–Si bending
vibrations at 470 cm-1 and the overtones typical of the

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Ketalization of Ketones to 1,3-Dioxolanes and Concurring Self-Aldolization Catalyzed
J. Braz. Chem. Soc.
features at 2900-2100 cm-1 and the concomitant increase
of the H2O bending at 1637 cm-1. These changes certainly
reflect the accommodation of additional water molecules
via hydrogen bonds to the –SiOH and –O–SO3H groups of
the SiO2-SO3H. The splitting of the absorptions from 1210
to 1040 cm-1 in the spectrum of the catalyst, which indicates
a profound modification of the overall surface structure
of the Si–O–Si original arrangement, are also reflected in
the further weakening of the Si–OH band at 960 cm-1. The
contributions of the –O–SO3H groups to the absorptions of
the IR spectrum of the catalyst are ambiguous because of the
close proximity of the atomic weights of tetrahedral silicon
and sulfur. However, one might assume that the enhancement
of the Si–O–Si longitudinal-transverse optical vibration (LO)
absorption at 1210 cm-1 and the transversal optical (TO)
rocking of the Si–O–Si bonds at 592 cm-1 may be due to the
presence of the –OSO3H groups of the catalyst.7
The striking differences between the original silica
and the SiO2-SO3H catalyst is also evident from the field
emission scanning electron microscopy (FE-SEM) images
shown in Figures 3 (silica) and 4 (catalyst) at different
magnifications. It is apparent that the even distribution of
the particles of the silica, which are responsible for its large
surface area, tend to form aggregates upon sulfonation. The
aggregates formed upon sulfonation are well separated by
macropores several nanometers in diameter.
Textural parameters of materials were studied by N2
adsorption/desorption isotherms, Figure 5, that showed a
type IV behavior with H1 hysteresis, typical of mesoporous
materials, which are related to pores with a constant
transversal section.7 After silica surface modification with
–SO3H groups, there was a decrease in surface area of 507
to 115 m2 g-1, and a volume of pores decrease of 0.78 to
0.38 cm3 g-1.
Figure 2. FTIR spectra of the (a) silica gel and (b) SiO2-SO3H.
amorphous silicas in the region of 1800-1900 cm-1 complete
the spectrum of the gel.7 The infrared (IR) spectrum of
the SiO2-SO3H catalyst (Figure 2b) presented remarkable
differences. For instance, the H2O bands at 3400 cm-1 are
much broader, with additional intramolecular hydrogen bond
The thermal behaviors of the new silica gel and
catalyst were evaluated. The thermogravimetric analysis
Figure 3. FE-SEM images of the amorphous silica gel.

Vol. 29, No. 8, 2018
Barbosa et al.
1667
Figure 4. FE-SEM images of the SiO2-SO3H.
Figure 5. N2 adsorption/desorption isotherms and (inset) Barrett-Joyner-
Halenda (BJH) method to pore size distributions for pure silica (SiO2)
(black squares) before modifications and SiO2-SO3H (red squares).
(TGA/DTG/DTA) curves for silica and catalyst, respectively,
are presented in Figure 6.
The decomposition of silica gel in a synthetic air
atmosphere takes place in one step. This step, from room
temperature to 149.71 ºC, is related to the endothermic
elimination of water molecules and the formation of
anhydrous silica. The decomposition of the catalyst takes
place in two stages. The first step, from room temperature
to 169.39 ºC, is also related to the endothermic elimination
of water molecules and the formation of anhydrous
catalyst; the second step, up to 240 °C, corresponds to the
endothermic loss of sulfur trioxide.
Figure 6. TGA/DTG/DTA curves of (a) new silica gel and (b) catalyst in
a synthetic air atmosphere (25 to 900 ºC).
example involves the preparation of the cyclohexanone
ketal in 90% yield using toluene as the solvent and heating
at reflux temperature for 3 h.6 The same reaction was
performed using SiO2-SO3H as the catalyst in the absence
of solvent under low microwave irradiation power (360 W).
The temperature of the slurry did not exceed 73 ºC, and a
99.9% yield was obtained in only 2 min (entry 1 in Table 1).
General remarks
Of particular interest for this work, both SiO2-SO3H8
and SiO2-CH2-CH2-CH2-SO3H6 have been used with
success for ketalization reactions using conventional
heating with reaction times extending up to 12 h. An

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Ketalization of Ketones to 1,3-Dioxolanes and Concurring Self-Aldolization Catalyzed
J. Braz. Chem. Soc.
However, these experimental conditions yielded only about
50% of the acetophenone ketal (entry 5, Table 1), against
93% yield (10 h reflux) using the propyl-modified catalyst.6
The formation of six-membered ketals seems to be
favored over the formation of 1-3-dioxolanes, in particular
those of the seemingly less reactive benzophenone
(entries 14 and 15, Table 1). No formation of the
six-membered isomers of the ketals involving glycerol,
which did fail to react with benzophenone under our
solvent-free condition, was observed. Amongst those
glycerol ketals shown in Table 1, the ciclohexanone
derivative had already been obtained in 90% yield by high
power microwave irradiation (600 W, 140 ºC, 15 min).15
All the ketalization reactions in Table 1 can be reversed by
the addition of water to the ketals in the presence of SiO2-
SO3H, as has already been pointed out by Rajput et al.;2
under microwave irradiation, this reaction takes only 2 min
to complete (see Experimental section).
During our attempts to carry on the ketalization
reactions using conventional heating, it became clear
that concurrent reactions occurred using our SiO2-SO3H
catalyst, namely, etherification and aldol condensation.
Table 1. Ketalization of ketones with polyalcohols using SiO2-SO3H catalyst and MW irradiation. Alcohol to ketone ratio 5:1, 10% (m/m) catalyst
(1)
(2)
(3)
(4)
99.9%
2 min
99.0%
4 min
65.6%
7 min
70.5%
5 min
(5)
(6)
(7)
(8)
50.5%
2 min
93.6%
5 min
66.3%
3 min
58.9%
2 min
(9)
(10)
(11)
(12)
49.7%
2 min
75.1%
5 min
63.6%
3 min
55.6%
5 min
(13)
(14)
(15)
(16)
28.9%
5 min
49.3%
5 min
53.5%
8 min
0%
8 min

Vol. 29, No. 8, 2018
Barbosa et al.
1669
Because we had already noted the strong tendency of our
catalyst to catalyze the formation of ethers from benzyl
alcohol,7 we decided to perform the detailed study of
the interaction of cyclohexanone and ethylene glycol at
different ketone to alcohol ratios and different catalyst loads
using 360 W MW irradiation. The load of the catalyst, from
7 to 20% (m/m), had only a minor effect on the reaction
product distribution, which was very dependent on the
alcohol to ketone ratio. Figure 7 resumes these findings.
benzyl alcohol, we observed that, at higher concentrations,
the catalyst SiO2-SO3H had an increased affinity for
organic alcohols versus organic acids, and we proposed5
the existence of a catalyst-bound species of the type
[Bz‑OH2]+** (Bz-OH = benzyl alcohol; ** = catalyst
bound species) as important intermediates, so that reactions
such as equations 5 and 6 could result in the exclusive
formation of dibenzyl ether from a mixture of benzyl
alcohol and benzoic acid.7
BzOH + H+** → [BzOH2]+**
(5)
[Bz-OH2]+** + Bz-OH → Bz-O-Bz + H2O** + H+** (6)
We propose that the catalyst is even more selective for
the protonation of ketones over alcohols, and, therefore,
the following sequence of reactions could apply, where **
stands for the catalyst bound species, Scheme 1.
An examination of the yields in Table 1 shows that, as
the stability of the intermediates [R2C+–OH]** increases
from [Cy+–OH] to [Ph2C+–OH], the yields of the ketals
decrease. This observation probably indicates that the
catalyst holds those stable species more tightly. This
effect, along with the difficult diffusion of glycerol through
SiO2-SO3H in solvent-free reactions, also explains the
lack of products in the reaction between benzophenone
and glycerol. The six-membered ring ketals seemed to be
preferred over the 5-membered ethylene glycol derivatives
when the [R2C+–OH]** species are stabilized by aromatic
rings, but this rule was not followed by glycerol, which gave
exclusively the 5-membered ketal, entries 5-12 in Table 1.
This is probably another evidence of the difficulty of the
diffusion of the third CH2–OH group of glycerol through the
hydrophilic pores of the catalyst. It would seem, therefore,
that the mesoporous sulfonated silica used as catalyst in this
and previous works7 showed a higher affinity for substrates
following the order: organic acids < alcohols < ketones. The
results suggest that the outcome of the reactions promoted
by these highly hydrophilic catalysts are dependent on
both the concentration of the catalyst and of the ratio of
the reactants.
Figure 7. Product distribution of the reaction of 1 mol equiv. of
cyclohexanone to 0-3 mol equiv. ethylene glycol, 10% (m/m) of catalyst
to ketone; 2 min MW irradiation at 360 W; final temperature ca. 70 ºC.
Figure 7 shows that cyclohexanone readily self-
condenses in the presence of the sulfonated catalyst, but this
reaction becomes negligible at 2:1 or higher ratios of glycol
to ketone. However, at these higher ratios, the condensation
of the alcohol becomes important. In the absence of a ketone,
ethylene glycol is converted to bis-ethylene glycol (17) in
96% yield (see Experimental section). It must be noted that
the catalyst could be used three times, with very little loss
in activity (under microwave irradiation conditions). Table 2
summarizes the self-condensation of the ketones.
Proposed mechanism
While examining the esterification of benzoic acid with
Table 2. Direct aldol condensation of acetone, cyclohexanone, acetophenone and p-methyl acetophenone with SiO2-SO3H catalyst using MW irradiation
(18)
(19)
(20)
(21)
95.3%
7 min
98.5%
6 min
96.1%
7 min
99%a
aConventional heating, 12 h, reflux.

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Ketalization of Ketones to 1,3-Dioxolanes and Concurring Self-Aldolization Catalyzed
J. Braz. Chem. Soc.
Scheme 1.
Conclusions
Acknowledgments
For the first time it is reported that a mesoporous
sulfonated silica with a small surface area, large pore
diameters and high hydrophilicity presented a higher
affinity for ketones than for alcohols and easily promoted
aldol condensations when used in concentrations from
7 to 24% (m/m) to the concentrations of ketones. In the
presence of the poly-alcohols ethylene glycol, glycerol,
trimethylolpropane and neopentylglycol, the ketones
acetophenone, p-methylacetophenone and benzophenone
were converted to the respective ketals in reasonable to
good yields. However, benzophenone failed to react with
glycerol; pure alkyl ketones such as cyclohexanone were
more easily condensed and ketalized. In the absence
of ketones, ethylene glycol was smoothly converted to
bisethylene glycol. All the reactions occurred within
minutes under solvent-free conditions using microwave
irradiation at 360 W.
The financial support of the Brazilian agencies CNPq,
CAPES and FAPEMIG were greatly appreciated.
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Submitted: November 15, 2017
Published online: March 7, 2018
This is an open-access article distributed under the terms of the Creative Commons Attribution License.