Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions

Article abstract of DOI:10.1016/j.molcata.2005.04.039

The efficacy of sulfated zirconia catalyst was investigated towards various acid-catalyzed organic syntheses and transformation reactions in the liquid phase. The SO₄^2-/ZrO₂ efficiently catalyzes synthesis of 1,5-benzodiazepine derivatives, electrophilic substitution of indoles with aldehydes to afford the corresponding bis(indolyl)methanes, synthesis of 3,4-dihydropyrimidinones, synthesis of diaryl sulfoxides, and tetrahydropyranylation of alcohols and phenols. Various advantages associated with these protocols include, simple work-up procedure, solvent-free conditions, short reaction times, high product yields and easy recovery and reusability of the catalyst. The SO₄ ^2-/ZrO₂ catalyst was obtained by immersing a finely powdered hydrous Zr(OH)₄ into 1 M H₂SO₄ solution and subsequent drying and calcination at 923 K. The Zr(OH)₄ was prepared from aqueous ZrOCl₂·8H₂O solution by hydrolysis with dilute ammonium hydroxide. The bulk and surface properties of the prepared catalysts were examined by X-ray powder diffraction, BET surface area, ammonia-TPD and Raman spectroscopy techniques. All characterization results revealed that the incorporated sulfate ions show a significant influence on the surface and bulk properties of the ZrO₂. In particular, XRD and Raman results suggest that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO₂ at ambient conditions. Ammonia-TPD and BET surface area results indicate that sulfated catalyst exhibits enhanced acid strength and specific surface area than that of unprompted ZrO₂.

Full text of DOI:10.1016/j.molcata.2005.04.039

Journal of Molecular Catalysis A: Chemical 237 (2005) 93–100
Sulfated zirconia as an efficient catalyst for organic
synthesis and transformation reactions
Benjaram M. Reddy∗, Pavani M. Sreekanth, Pandian Lakshmanan
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
Received 21 February 2005; received in revised form 20 April 2005; accepted 20 April 2005
Available online 4 June 2005
Abstract
The efficacy of sulfated zirconia catalyst was investigated towards various acid-catalyzed organic syntheses and transformation reactions
in the liquid phase. The SO42−/ZrO2 efficiently catalyzes synthesis of 1,5-benzodiazepine derivatives, electrophilic substitution of indoles
with aldehydes to afford the corresponding bis(indolyl)methanes, synthesis of 3,4-dihydropyrimidinones, synthesis of diaryl sulfoxides, and
tetrahydropyranylation of alcohols and phenols. Various advantages associated with these protocols include, simple work-up procedure,
solvent-free conditions, short reaction times, high product yields and easy recovery and reusability of the catalyst. The SO42−/ZrO2 catalyst
was obtained by immersing a finely powdered hydrous Zr(OH)4 into 1 M H2SO4 solution and subsequent drying and calcination at 923 K. The
Zr(OH)4 was prepared from aqueous ZrOCl2·8H2O solution by hydrolysis with dilute ammonium hydroxide. The bulk and surface properties
of the prepared catalysts were examined by X-ray powder diffraction, BET surface area, ammonia-TPD and Raman spectroscopy techniques.
All characterization results revealed that the incorporated sulfate ions show a significant influence on the surface and bulk properties of the
ZrO2. In particular, XRD and Raman results suggest that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO2 at ambient
conditions. Ammonia-TPD and BET surface area results indicate that sulfated catalyst exhibits enhanced acid strength and specific surface
area than that of unprompted ZrO2.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Solid superacid; Sulfated zirconia; Benzodiazepines; Bis(indolyl)methanes; 3,4-Dihydropyrimidinones; Diaryl sulfoxides; Tetrahydropyranylation
1. Introduction
BF3, ZnCl2 and SbF5 pose significant risks in handling, con-
tainment, disposal and regeneration due to their toxic and
corrosive nature. There is an exigent need to eliminate the
aggressive and ecologically harmful mineral acids to carry
out a large number of acid-catalyzed industrial processes.
This can be achieved by the development of strong solid acid
catalysts that are stable, regenerable and active at moderate
temperatures.
Over the past few years, the preparation and characteri-
zation of zirconia based solid acids has been receiving much
attention, among other solid acids such as clays, zeolites, het-
eropolyacids and ion exchange resins, due to their superior
catalytic activity for hydrocarbon conversions [3,4]. These
catalysts are finding numerous applications in oil refinery and
petrochemical industries. Among the promoted ZrO2 solid
acid catalysts, the SO42−/ZrO2 become more popular due to
its high activity for light alkane isomerizations even at low
Acid-catalyzed organic reactions are numerous and the
usage of solid acid catalysts is very rampant in several indus-
trial and environmental processes [1]. It can be said that
solid acids are the most important heterogeneous catalysts
used today, considering in terms of both the total amounts
used and the final economical impact. According to a recent
review of industrial acid–base catalysis, of the 127 processes
identified, over 115 are solid acid-catalyzed [2]. It clearly
indicates the significance of these materials and the scope of
their commercial exploitation. The use of conventional liq-
uid acids and Lewis acids such as, H2SO4, HCl, HF, AlCl3,
∗
Corresponding author. Tel.: +91 40 2716 0123; fax: +91 40 2716 0921.
E-mail addresses: mreddyb@yahoo.com, bmreddy@iict.res.in
(B.M. Reddy).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.04.039

94
B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
recorded in the range of 100–4000 cm−1 and a spectral reso-
lution of 2 cm−1 using the 514.5 nm exciting line from an
argon ion laser (Spectra Physics, USA). The temperature
programmed desorption (TPD) measurements using ammo-
nia probe molecule were carried out on an AutoChem 2910
instrument (Micromeritics, USA). A thermal conductivity
detector was used for continuous monitoring of the desorbed
ammonia and the areas under the peaks were integrated using
GRAMS/32 software. Prior to TPD studies, samples were
pre-treated at 473 K for 1 h in a flow of ultra pure helium
gas (40 ml min−1). After the pre-treatment, the sample was
saturated with 10% ultra pure anhydrous ammonia gas (bal-
ance He, 60 ml min−1) at 373 K for 2 h and subsequently
flushed with He (60 ml min−1) at 373 K for 2 h to remove the
physisorbed ammonia. The heating rate for the TPD mea-
surements, from ambient to 1073 K, was 10 K min−1. All
flow rates mentioned are at normal temperature and pressure
(NTP).
temperatures. Many large volume applications based on sul-
fated zirconia are reported in the literature, especially in the
petroleum industry for alkylation, isomerization and cracking
reactions [5–8].
In recent times, inorganic solid acid-catalyzed organic
transformations are gaining much attention due to the proven
advantage of heterogeneous catalysts, like simplified product
isolation, mild reaction conditions, high selectivities, ease
in recovery and reuse of the catalysts and reduction in the
generation of wasteful byproducts [9–11]. In that connec-
tion we were interested in investigating various industrially
important organic reactions aimed at replacing toxic and
corrosive reagents, noxious or expensive solvents and mul-
tistep process, with single step solvent-free ones by using
environmentally benign solid acid catalysts. Interestingly,
sulfated zirconia exhibits excellent activity for a wide range
of organic synthesis and transformation reactions. In this
paper we report the SO42−/ZrO2 catalyzed various organic
synthesis and transformation reactions and the advantages
associated with these catalyst systems.
2.3. Activity studies
All chemicals used in this study were commercially avail-
able and used without further purification. All the reactions
were carried out in the liquid phase batch mode by taking a
mixture of reactants and catalyst in a round bottom flask and
stirred/refluxedforappropriatetimes. Completionofthereac-
tion was monitored by TLC. After completion of the reaction,
catalyst was recovered by simple filtration and reused. The
products were recovered from the filtrate, concentrated on a
rotatory evaporator and chromatographed on a silica gel col-
umn to afford pure products (isolated yields). NMR and mass
spectroscopy techniques were used to analyze the products
and compared with the authentic samples.
2. Experimental
2.1. Catalyst preparation
About 25 g of ZrOCl2·8H2O (Fluka, GR grade) was dis-
solved in doubly distilled water. To this clear solution, dilute
aqueous ammonia was added drop-wise from a burette with
vigorous stirring until the pH of the solution reached 8. The
obtained precipitate was washed with distilled water until
free from chloride ions and dried at 393 K for 24 h. To pre-
pare sulfated ZrO2 catalyst, a portion of the obtained hydrous
zirconia sample was ground to fine powder and immersed in
1 M H2SO4 solution (30 ml) for 30 min. Excess water was
evaporated on a water-bath and the resulting sample was
oven-dried at 393 K for 12 h and calcined at 923 K for 4 h
in air atmosphere and stored in vacuum desiccator. For the
purpose of comparison an unpromoted ZrO2 was also pre-
pared by calcining the hydrous zirconia at 923 K for 4 h in
air atmosphere.
3. Results and discussion
The surface and bulk properties of ZrO2 and SO42−/ZrO2
catalysts were examined by various spectroscopic and non-
spectroscopic techniques namely, X-ray powder diffraction,
BET surface area, ammonia-TPD and Raman spectroscopy.
The XRD patterns of ZrO2 and SO42−/ZrO2 samples cal-
cined at 923 K are shown in Fig. 1. The corresponding phase
composition, crystallite size and BET surface area results are
presented in Table 1. As can be seen from Fig. 1, the hydrous
zirconia sample calcined at 923 K is in poorly crystalline
form with a mixture of monoclinic and tetragonal phases.
On the other hand, the sulfated sample exhibits prominent
lines due to tetragonal phase indicating that the impregnated
sulfate ions show a strong influence on the phase modifica-
tion of zirconia from thermodynamically more stable mono-
clinic to the metastable tetragonal phase. It can be observed
from Table 1 that the sulfated zirconia shows smaller crys-
tallite size and more tetragonal phase when compared to
unpromoted ZrO2. It appears from XRD results (Fig. 1 and
Table 1) that the incorporated sulfate ions retard the forma-
2.2. Catalyst characterization
The powder X-ray diffraction patterns of the prepared
samples have been recorded on a Siemens D-5000 diffrac-
tometer by using Cu K␣ radiation source and a scintillation
counter detector. The XRD phases present in the samples
were identified by using JCPDS data files. The BET surface
area of the sample was determined by nitrogen physisorption
at liquid nitrogen temperature on a Micromeritics Gemini
2360 instrument by taking 0.162 nm2 as the area of cross-
section for N2 molecule. Prior to analysis, the samples were
oven-dried at 393 K for 10 h and flushed with argon gas for
1 h. Raman spectra were obtained on a DILOR XY spec-
trometer equipped with a CCD detector. The spectra were

B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
95
lier published papers wherein sulfation inhibited the crystal
growth and increased the specific surface area and stabilized
the tetragonal phase of zirconia [3,4,7].
3.1. Synthesis of 3,4-dihydropyrimidin-2(1H)ones
Dihydropyrimidinones (DHPMs) are important class of
compoundsduetotheirdiversetherapeuticandpharmacolog-
ical applications. Many functionalized derivatives of DHPMs
are known to act as anti-hypertensive agents, alpha-la-
antagonists, nueropeptideY(NPY)antagonistsandalsoserve
as integral backbones of several calcium channel blockers
[18,19]. Several marine alkaloids containing the dihydropy-
rimidinone units are found to exhibit diverse biological
activities as anti-viral, anti-bacterial, anti-tumor and anti-
inflammatory agents [20,21]. Among those, batzelladine
alkaloids have been found to be potential HIV gp-120-CD4
inhibitors [22]. In view of these reasons, this family of
compounds received a great deal of attention towards their
synthesis. The general method for the synthesis of DHPMs
involves three-component condensation reaction between
aldehyde, ␤-keto ester and urea under strong acidic condi-
tions. This reaction was first reported by Biginelli in 1893, but
suffers from low product yields especially with substituted
aromatic aldehydes [23]. Subsequently several modifications
were reported for the Biginelli condensation reaction to
improve the yields. Apart from this classical one-pot Biginelli
reaction many multistep protocols were also reported for the
synthesis of DHPMs [24]. The Biginelli reaction was carried
out employing many reagents such as lanthanum chloride,
manganese acetate, montmorillonite KSF, silica-supported
sulfuric acid and metal triflates [25–27]. Most of these
reported catalysts suffer from various drawbacks such as
stringent reaction conditions, tedious work-up procedures,
long reaction times, use of stoichiometric amounts of
catalysts and some of the catalysts employed are expensive.
In view of these reasons, still many efforts are going on
to develop greener, faster and high yield protocols for this
reaction. Herein, we report a facile method for the synthesis
of 3,4-dihydropyrimidone-2(1H)–ones by a one-pot con-
densation reaction between an aldehyde, ␤-keto ester and
urea or thiourea under solvent-free conditions catalyzed by
sulfated zirconia (Scheme 1).
Fig. 1. X-ray powder diffraction patterns of pure and sulfate promoted zir-
conia samples calcined at 923 K. (᭹) Characteristic lines due to tetragonal
zirconia; (ꢀ) characteristic lines due to monoclinic zirconia.
tion of larger crystallites of zirconia and stabilize them in the
metastabletetragonalphase[12]. Intheabsenceofpromoters,
calcination of hydrous zirconia normally leads to the for-
mation of thermodynamically more stable monoclinic form
with large crystallite sizes [13]. The specific surface area of
SO42−/ZrO2 sampleisalsohigherthanthatofZrO2 (Table1).
Ammonia-TPD results revealed two temperature maximums
on both the samples indicating the presence of two different
types of acids sites with different acid strength distribution.
The total amount of ammonia desorbed in the case of sulfated
sample is much higher than that of pure ZrO2. It clearly indi-
cates that impregnated sulfate ions show a strong influence
on the acidic properties of the zirconia. The Raman spec-
trum of unprompted ZrO2 exhibited prominent bands due to
a mixture of monoclinic (180, 188, 221, 331, 380, 476 and
637 cm−1) and tetragonal (148, 290, 311, 454 and 647 cm−1
)
phases and the bands due to tetragonal phase were less intense
than the peaks due to monoclinic phase [14,15]. Whereas in
the spectrum of sulfated zirconia catalyst bands represent-
ing the tetragonal phase were more intense. A strong band
at 1032 cm−1 with a shoulder was also observed in the spec-
trum of SO42−/ZrO2 sample. This is due to hydrated sulfate
groups attached to the zirconia support [16]. Raman results
supported the observations made from XRD studies, wherein
sulfation increased the proportion of tetragonal phase [17].
The charaterization results are in agreement with the ear-
The reaction was performed by taking a mixture of
stoichiometric amounts of an aldehyde, ␤-keto ester and
urea/thiourea along with a catalytic amount of sulfated zir-
conia in a round bottom flask and heated at 373 K for an
appropriate time. The reaction proceeds efficiently under
these conditions and the dihydropyrimidinones are produced
Table 1
BET surface area, amount of monoclinic and tetragonal phases of ZrO2 and their crystallite size, and total acidity
Catalyst
BET SA
Monoclinic
Amount (%)
Tetragonal
NH3 desorbed
(m2 g−1
)
Size (nm)
Amount (%)
Size (nm)
(ml g−1
)
ZrO2
SO42−/ZrO2
42
100
76
20
11.2
07.3
24
80
13.0
12.3
5
16

96
B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
Scheme 1.
in excellent yields in short reaction times (40–60 min). Aro-
matic aldehyde bearing either electron withdrawing or elec-
tron donating groups reacted smoothly and produced the
corresponding dihydropyrimidinones in high yields and high
purity. These results are compiled in Table 2. This procedure
offers several advantages including mild reaction conditions,
greater selectivity, high product yields as well as simple
experimental and isolation procedure, which makes it use-
ful and attractive process for large scale synthesis of these
compounds.
3.2. Synthesis of bis(indolyl)methane derivatives
Table 2
Indoles and their derivatives are important intermedi-
ates in organic synthesis and widely featured in variety of
pharmacologically active compounds [28]. During the past
few years a large number of natural products containing
bis(indolyl)methanes and bis(indolyl)ethanes have been iso-
lated from both marine and terrestrial sources and some
of them were found to exhibit interesting biological activ-
ity [29]. Therefore, there is a great deal of interest in the
synthesis of this class of compounds. The acid-catalyzed
reaction of electron rich heterocyclic compounds like indoles
and pyrroles with p-dimethylaminobenzaldehyde is known as
Ehrlich test [30]. Generally, bis(indolyl)methanes are synthe-
sizedbytheanalogousreactionsofEhrlichtest, whereindoles
reactwithaliphaticoraromaticaldehydes/ketonesinthepres-
ence of an acid catalyst to produce azafulvenium salts. These
azafulvenium salts can undergo further addition with a sec-
ond indole molecule to produce bis(indolyl)methanes [31].
Protic-acids as well as Lewis acids are known to promote
these reactions [32,33]. The use of various other reagents
such as lanthanide triflates, clays, ion exchange resins and
zeolites have also been studied for this reaction [34–36].
However, a major problem associated with the conventional
Lewis acid catalysts is that they deactivate or some times
even decompose due to the nitrogen containing reactants.
Moreover, many of these Lewis acid reagents are required in
stoichiometric amounts.
Sulfated zirconia catalyzed synthesis of 3,4-dihydropyrimidin-2(1H)-ones
Entry Aldehyde Beta-keto ester
Urea/thiourea
Yield
1
90
2
88
3
4
92
90
5
6
80
82
In this study, we successfully synthesized bis(indolyl)
methanes by electrophilic substitution reaction of indole with
various aldehydes in the presence of sulfated zirconia cata-
lyst (Scheme 2). This reaction was carried out by taking a
mixture of aldehyde and indole (1:2.5 mole ratio) in a round
bottomed flask and stirred for an appropriate time at room
temperature along with the catalytic amount of sulfated zir-
Scheme 2.

B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
97
conia. Under these optimized conditions various aldehydes
smoothly reacted with indole and furnished corresponding
bis(indolyl)methanes in excellent yields in short reaction
times. These results are summarized in Table 3. Under sim-
ilar conditions the unpromoted ZrO2 failed to produce the
desired products in good yields. It can be noted from Table 3
that sulfated zirconia is an efficient catalyst for the synthesis
of bis(indolyl)methanes in terms of product yields, reaction
temperature and reaction times.
spectrum of biological activity, these compounds received
a lot of attention towards their synthesis. The general
method for the synthesis of 1,5-benzodiazepines involves
an acid-catalyzed condensation of o-phenylenediamines with
␣,␤-unsaturated carbonyl compounds or ␤-halo ketones or
ketones. Many reagents have been utilized for this reac-
tionincludingpolyphosphoric acid-SiO2, BF3·OEt2, NaBH4,
Yb(OTf)3, MgO-POCl3, and more recently acetic acid under
microwave conditions [40–42]. In this study various 1,5-
benzodiazepine derivatives were synthesized by the conden-
sation reaction of o-phenylenediamine with various ketones
using the sulfated zirconia catalyst under solvent-free condi-
tions (Scheme 3).
The condensation reaction (Scheme 3) was carried out
by taking a mixture of o-phenylenediamine and ketone in
a 1:2.5 mole ratio into a round bottom flask along with
the catalytic amount of sulfated zirconia and the reaction
mixture was stirred at ambient conditions for an appro-
priate time. In the presence of sulfated zirconia catalyst
various ketones were found to react efficiently with sub-
stituted o-phenylenediamines giving high yields of 1,5-
benzodiazepines. Cyclic ketones like cyclohexanone also
reacted smoothly to produce the corresponding fused ring
benzodiazepines. The results of this reaction are summarized
in the Table 4. Under identical conditions unpromoted ZrO2
was found to exhibit very negligible activity. Therefore, the
observed high yields can be attributed to the superacidity of
the sulfated zirconia catalyst.
3.3. Synthesis of 2,3-dihydro-1H-1,5-benzodiazepines
Benzodiazepines and their polycyclic derivatives are an
important class of bioactive compounds. Many functional-
ized benzodiazepines are widely used as anti-convulsant,
anti-anxiety, analgesic, sedative, anti-depressive and hyp-
notic agents [37]. These compound also finding applica-
tions as dyes for acrylic fibers and anti-inflammatory agents
[38]. 1,5-Benzodiazepines are key intermediates for the syn-
thesis of various fused ring compounds such as triazolo-,
oxadiazolo- and oxizino-diazepines [39]. Due to their broad
Table 3
Sulfated zirconia catalyzed synthesis of bis(indolyl)methanes
Entry
Indole
Aldehyde
Yield
85
1
3.4. Synthesis of diaryl sulfoxides
Sulfoxides and sulfones are important intermediates for
the synthesis of large variety of organic sulfur compounds in
the field of drugs and pharmaceuticals [43,44]. Recently, sul-
foxides have received much attention as important chiral aux-
iliaries in asymmetric synthesis and in carbon–carbon bond
forming reactions [45]. Usually, sulfoxides are prepared by
indirect methods, which involve reduction of sulfones [46],
oxidation of sulfides [47], and the reaction of organometal-
lic reagents with sulfinic acid esters, mixed anhydrides or
sulfines [48]. In addition to these indirect processes, diaryl
sulfoxides can also be synthesized by the Friedel–Crafts
2
3
84
78
¨
sulfinylation of arenes using Lewis as well as Bronsted acids
[49,50]. Some other methods like, addition of aryl Grignard
reagents to thionyl chloride and reaction of SO2 with arenes
in the presence of magic acid also produce diaryl sulfoxides
[51]. Veryrecently, thistransformationhasbeenreportedwith
4
82
5
6
73
78
Scheme 3.

98
B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
Table 4
Table 5
Sulfated zirconia catalyzed synthesis of 1,5-benzodiazepine derivatives
Sulfated zirconia catalyzed synthesis of diaryl sulfoxides
Entry Diamine
Ketone
Product
Yield
Entry
Arene
Product
Yield
90
1
94
1
2
3
92
85
2
91
3
4
96
84
4
5
6
83
88
80
5
6
94
91
ated with this procedure are cleaner reaction profiles, mild
reaction conditions, shorter reaction times and operational
simplicity.
metal triflates and in the presence of ionic liquids [52]. How-
ever, many of these methods have limitations that include,
use of hazardous, corrosive and expensive reagents, forma-
tion of a mixture of products containing sulfonium salts and
chlorinated byproducts along with the desired sulfoxides.
Accordingly, a simple and high yielding one-pot approach for
the synthesis of diaryl sulfoxides under mild reaction condi-
tions is highly desirable. Our systematic investigations on this
reaction revealed that sulfated zirconia efficiently catalyzes
this reaction (Scheme 4).
This reaction (Scheme 4) was carried out by taking arene
and thionyl chloride (2:1 mole ratio) with catalytic amount
of sulfated zirconia in a round bottom flask and stirred for
an appropriate time under solvent-free conditions. Under
these reaction conditions several substituted arenes reacted
smoothly with thionyl chloride leading to the formation of
corresponding diaryl sulfoxides in good to excellent yields.
The efficiency of sulfated zirconia catalyst for the synthesis
of diaryl sulfoxides with various activated and non-activated
arenes is presented in Table 5. Major advantages associ-
3.5. Tetrahydropyranylation of alcohols and phenols
Tetrahydropyranylation is one of the most frequently used
processes in organic synthesis for the protection of hydroxyl
groups [53]. This is mainly due to high stability of result-
ing THP ethers in variety of reaction conditions, such as
reduction, oxidation, strongly acidic and basic media, as
well as ease in the deprotection of formed THP ethers [54].
Under acidic conditions alcohols and phenols react with 3,4-
dihydropyran (DHP) to give tetrahydropyranyl ethers. Vari-
etiesofreagentshavebeenreportedforthisreactionincluding
protic-acids, Lewis acids, ion exchange resins, clays and
expensive metal triflates [55–57]. Many of these methods
are associated with several drawbacks such as long reaction
times, severe refluxing conditions and some of the reagents
employed are expensive.
Scheme 4.
Scheme 5.

B.M. Reddy et al. / Journal of Molecular Catalysis A:Chemical 237 (2005) 93–100
99
Table 6
Sulfated zirconia catalyzed tetrahydropyranylation of alcohols and phenols
Acknowledgement
Entry
Alcohol
Product
Yield
P.M.S and P.L. thank Council of Scientific and Industrial
Research (CSIR), New Delhi for the award of senior research
fellowships.
1
94
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In this study a variety of alcohols and phenols were
treated with 3,4-dihydro-2-H-pyran in the presence of cat-
alytic amount of sulfated zirconia under solvent-free con-
ditions at room temperature (Scheme 5). Tetrahydropyranyl
ethers were produced in excellent yields at less reaction time
[58]. The catalytic results obtained with sulfated zirconia are
summarized in Table 6. The present procedure for tetrahy-
dropyranylation is quite simple and can be employed for a
wide range of hydroxy compounds.
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transformation reactions leading to the formation of various
pharmacologically/biologically important molecules such
as 1,5-benzodiazepines, bis(indolyl)methane derivatives and
dihydropyrimidinones. Simple work up procedure, milder
reaction conditions and shorter reaction times are some
of the advantages associated with the sulfated zirconia
catalyzed processes. More importantly this catalyst is able to
facilitate an increasing number of solvent-free conversions.
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