Current Organic Chemistry, 2011, 15, 3961-3985
3961
Zirconia-Based Solid Acids: Green and Heterogeneous Catalysts for Organic
Synthesis
Meghshyam K. Patil,a Avvari N. Prasad,b and Benjaram M. Reddyb*
a
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Sub-Campus Osmanabad 413 501,
Maharashtra, India
b
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad - 500 607, India
Abstract: This review highlights the application of sulfated, molybdated and tungstated zirconia solid acid catalysts, and their modified
forms for variety of organic synthesis and transformation reactions in the liquid phase. Most of these catalysts offer significant
improvements in various organic reactions with regard to the yield of products, simplicity in the operation, reusability of the catalysts and
green features by avoiding toxic conventional catalysts. Preparation of various zirconia-based solid acid catalysts has been briefly
described. Characterization of these catalysts by different techniques has also been presented. Most of these catalysts are highly
promising for numerous organic reactions in the liquid phase which include condensation, isomerization, esterification and
transesterification, muticomponent reactions and so on.
Keywords: Superacid, sulfated zirconia, molybdated zirconia, tungstated zirconia, tetragonal phase, organic synthesis, solvent-free.
1. INTRODUCTION
Solid acids and superacids have been the subjects of everlasting
interest owing to their numerous applications in many areas of the
chemical industry. The application of acid catalysts is very rampant
in the chemical and refinery industries, and those technologies
employing highly corrosive, hazardous and polluting conventional
liquid acids and Lewis acids such as H2SO4, HCl, HF, HClO4,
H3PO4, AlCl3, BF3, ZnCl2 and SbF5 are being replaced with solid
acids such as clays, zeolites, heteropolyacids, ion exchange resins
(Amberlyst and Nafion-H) and metal oxides. Some of these solid
acids and superacids are characterized by various advantages which
include easy handling, simplicity and versatility of process
engineering, catalyst regeneration, decreasing reactor and plant
corrosion problems and environmental safe disposal [1-15]. Till
date, a number of organic syntheses and transformation reactions
have been conducted with solid acids leading to better regio- and
stereo-selectivity/specificity which depend on the strength of the
acid and type of acidity (Brønsted or Lewis). Over the past few
decades, zirconia (ZrO2) based solid acids have received much
attention, among other solid acids, due to their superior catalytic
activity for hydrocarbon conversions at mild conditions [8,14-18].
These catalysts are finding numerous applications in oil refinery
and petrochemical industries. Among the promoted zirconia-based
solid acid catalysts, the SO42/ZrO2 (SZ) catalyst become more
popular from 1979 when Arata and co-investigators [16,19]
reported that zirconia, upon proper treatment with sulfuric acid or
ammonium sulfate exhibits extremely strong acidity and is capable
of catalyzing the isomerization of n-butane to isobutane at room
temperature. Over the period of time the SZ catalyst has also been
reported to be very active for various organic syntheses and
transformation reactions including multicomponent reactions,
isomerization, alkylation, acetylation, esterification, glycosidation
and some other commercially useful reactions [20-26]. However, a
major disadvantage associated with SZ catalyst is its rapid
*
Address correspondence to this author at the Inorganic and Physical Chemistry
Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad–500 607,
India; Fax: +9140 2716 0921; E-mail: [email protected], [email protected]
1385-2728/11 $58.00+.00
deactivation under both high temperature and in reducing
atmosphere owing to the formation of SOx and H2S, respectively.
Also it forms sulfuric acid if there is water in the reaction medium.
Many efforts were made to improve the activity and stability of the
SZ catalyst which include promotion of the catalyst with transition
metals such as Fe, Mn and Cr, and with noble metal Pt, as well as
with carbon molecular sieves [27]. In 1988 Arata and Hino reported
that solid superacids could be synthesized by incorporating WOx or
MOx into the Zr- or Ti-hydroxides under certain preparation
conditions [28,29]. Our extensive investigations also confirmed
their findings, and we have explored some of these catalysts for
various organic reactions during the last fifteen years.
Using Hammett indicators, Hino and Arata [16] observed that
SZ is an acid 104 times stronger than 100% sulfuric acid. Acids
stronger than 100% sulfuric acid are generally referred to as
superacids [2,17,18]. The strength of an acid can be characterized
by the so-called Hammett acidity function, H0. The greater the
value of the function, the stronger is its acidity. The value of H 0
for 100% sulfuric acid is 12. Therefore, SZ catalyst with H0 = 16
is considered as the strongest halide-free solid superacid [4,30-32].
Subsequent investigations revealed that sulfate-free ZrO2-based
solid superacids could be synthesized by incorporating molybdate
or tungstate promoters under certain preparation conditions [4,3038]. The typical H0 values reported for SZ (650 °C), WOx/ZrO2
(800 °C) and MoOx/ZrO2 (800 °C) catalysts calcined at different
temperatures are 16.1, 14.6 and 13.3, respectively, which reveal
the superacidic character [17,18,38,39]. There were some doubts in
the literature whether SZ is ‘‘just” as strong acid as H-zeolites, or a
true strong superacid [40-46]. More recently, a concerted study
from Japan confirmed the superacidity of the SZ catalyst beyond
doubt [47].
The primary objective of this review was to summarize recent
studies on organic synthesis and transformation reactions catalyzed
by various zirconia-based solid acid catalysts, namely, SZ,
molybdated zirconia (MZ), tungstated zirconia (WZ) and other
modified-ZrO2 catalysts. Preparation and physicochemical
characterization of these catalysts have also been briefly discussed
in this review.
© 2011 Bentham Science Publishers
3962 Current Organic Chemistry, 2011, Vol. 15, No. 23
2. PREPARATION OF CATALYSTS
Promoted zirconia solid acid catalysts could be prepared by
different methods. The catalytic properties generally depend on the
method of preparation, nature of precursors, precipitating agents,
promoting agents, method of impregnation, calcination temperature
and so on. Also, the activity of catalysts depends primarily on the
activation temperature. In this section, methods of preparation of
zirconia-based catalysts, namely, sulfate, molybdate, tungstate ion
promoted zirconia catalysts and other modified catalysts are
presented in brief.
2.1. Preparation of Sulfated, Molybdated and Tungstated
Zirconia
SZ catalysts are prepared mostly by adopting a two-step or a
single step method. Both the preparative methods are equally
exploited for the synthesis of this catalyst. In the two-step method,
zirconium hydroxide is prepared first followed by impregnation
with a sulfating agent [16,19]. To prepare zirconium hydroxide,
different zirconium compounds such as zirconium nitrate,
zirconium chloride, zirconium oxychloride and zirconium
isopropoxide were hydrolyzed with aqueous ammonia or urea [4850]. Sulfate impregnation is carried out by using various sulfating
agents such as H2SO4, (NH4)2SO4 [16,19,48-51], SO3 [52] and
ClSO3H [53]. The resultant sulfated zirconium hydroxide is then
calcined in air at 550-650 °C to generate acidity. Molybdate and
tungstate ion promoted zirconia catalysts are also prepared by using
the same procedure. Molybdate and tungstate ion impregnation is
carried out by using ammonium heptamolybdate and ammonium
metatungstate, respectively. Resultant impregnated zirconium
hydroxide is then calcined in air at 650-800 °C in order to generate
the acidity. SZ and WZ catalysts are also commercially available;
Ramos et al. [54] used SZ and WZ catalysts purchased from MEL
Chemicals.
SZ catalysts are also prepared by a simple one-step sol-gel
method, in which organo-zirconium precursors such as zirconium
n-propoxide and zirconium isopropoxide were used. In this
procedure, to the alcoholic solution of zirconia-precursor, certain
amount of H2SO4 solution was added slowly under vigorous stirring
until a viscous solution was obtained. The gel was heated at 80 °C
to evaporate excess alcohol and calcined at 600 °C for 7 h in air to
get the white SZ solid. SZ is also prepared by thermal
decomposition of Zr(SO4)2 [55,56]. However, this method did not
attract much attention because it does not allow the control of
sulfate content.
Many groups working in this area have carried out
modifications to the above-described methods in order to get better
surface area, mesoporous structure and more tetragonal zirconia
phase [57-66]. Morterra et al. [60] prepared SZ catalyst by
hydrothermal precipitation route, and Rubio et al. [67] synthesized
SZ using ammonium zirconium carbonate. Also, various surfactants
such as triblock copolymer and cetyl trimethyl ammonium bromide
(CTAB) were also used in order to generate high surface area.
2.2. Preparation of Other Zirconia-Based Solid Acids
As mentioned earlier, zirconia-based catalysts were modified by
using appropriate salts of Fe, Mn, Cr, and Pt, as well as with carbon
molecular sieves in order to improve the physicochemical and
catalytic properties [27]. Occelli et al. [68] prepared Cu, Fe, Mn
and Fe-Mn promoted SZ catalysts by adopting a two-step
precipitation-impregnation method by employing CuSO4,
Fe2(SO4)3, or MnSO4 precursors. Many research groups are
Patil et al.
working in this area owing to the commercial significance, and
reported the preparation of Al, Fe, Mn, Cr, and Pt modified SZ
catalysts [69-73].
Interestingly, some mixed oxides also exhibited strong surface
acidity (Brønsted or Lewis) due to the generation of excess negative
or positive charge in the model structure of the binary oxides.
Mixed oxides such as SiO2-ZrO2 [74] and Al2O3-ZrO2 (AZ) [75]
lead to very strong acidic properties, whereas TiO2-ZrO2 (TZ) was
not only a strong acid but also had a distinct basicity [76,77]. Reddy
and co-workers synthesized sulfate promoted CeO2-ZrO2 and
sulfate, molybdate and tungstate ion promoted TZ and AZ catalysts
using two-step procedure i.e., co-precipitation followed by
impregnation method [75,78,79]. Various research groups working
in this area also reported preparation of sulfated AZ, sulfated TZ
and sulfated ceria-zirconia (Ce-ZrO2/SO42) catalysts [80-85]. Also,
Negrón-Silva et al. [86] prepared MCM-41(Mobil Composition of
Matter No.41) supported SZ catalyst from siliceous MCM-41 and
zirconium sulfate. Guo et al. [87] reported SZ catalyst supported
on multi-walled carbon nanotubes. Zhao et al. [88] reported
synthesis of SZ supported on mesostructured -Al2O3 and other
combinations.
3. CATALYST CHARACTERIZATION
Modified or promoted zirconia solid acid catalysts were
extensively characterized by various spectroscopic and
nonspectroscopic techniques such as X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), Fourier transform infrared
(FTIR), Raman spectroscopy (RS), differential thermal
analysis/thermogravimetric analysis (DTA/TGA), scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and
other methods in order to understand the bulk and surface
properties. All characterization results revealed that the
incorporated promoter ions show a strong influence on the surface
and bulk properties of the ZrO2. Especially, XRD and Raman
results suggested that impregnated sulfate, molybdate and tungstate
ions stabilize the metastable tetragonal phase of ZrO2 at ambient
conditions [49,89]. NH3-Temperature programmed desorption
(TPD) and Brunauer-Emmett-Teller (BET) surface area results
indicated that the sulfated catalyst exhibits enhanced acid strength
and higher specific surface area than other promoted and
unpromoted samples [49]. Recently, Park et al. [90] reported a
highest acid strength for 20 wt.% tungstate ion promoted
zirconia catalyst among other catalysts having different
loadings of tungstate-ion from NH3-TPD study. Also, there are
various other methods, which are used to determine the surface
acidity of the catalysts such as Hammett indicator method, TPD of
various base molecules, model test reactions and so on [4,30,31].
Yu et al. [91] investigated the acidity of modified zirconia catalysts
using solid state NMR technique. Sommer and co-workers [92,93]
exploited another method where the rate of H/D exchange for
methane has been used to measure the relative acidity and reactivity
of SZ catalysts. However, all these methods are not versatile for
different types of solid acids.
Various spectroscopic techniques were extensively used to find
out the exact structure of the SZ and other solid acid catalysts.
Various structures are proposed for the SZ catalyst as shown in Fig.
1. The structures (a) and (b) were proposed by Jin et al. [94] and
Ward and Ko [95], respectively, using various characterization
techniques namely, XPS, IR spectroscopy, in situ and ex situ diffuse
reflectance infrared Fourier transform (DRIFT) and XRD. Kustov
et al. [96] suggested the structures (c), (d) and (e) based on diffuse
reflectance IR spectroscopy; structures (c) and (d) are responsible
Current Organic Chemistry, 2011, Vol. 15, No. 23 3963
Green and Heterogeneous Catalysts for Organic Synthesis
(a)
O
O
(b)
O
*
OH
S
O
Zr
O
Zr
O
O
Zr
(e)
Zr
O
O H
S
O
Zr
O
O
O
Zr
O
O
H
OH HO
S
S
O
Zr
Zr
S
O
Zr
S
O
O
O
#
Zr
O
(f) O
Zr
O O
O
Zr Zr
Zr
O
(j)
O
O
O
O
(i)
(h)
O
O H
(c)
Zr
O
O
O
Zr
O
O
Zr
O
Zr
O
O
O
O
O
(g)
O
Zr
O
S
S
O
O
O
O
Zr
Zr Zr
Zr
O
(k)
S
O
S
O
S
Zr
Zr
O
O
O
O
* Brönsted acid site; # Lewis acid site
(d)
O
O
S
O
O
O
S
Zr
O
O
H
H
O
O
Zr
O
Fig. (1). Structures suggested for sulfated zirconia by various groups.
for Brønsted acidity where as structure (e) is responsible for Lewis
acidity. Structural models (f) and (g) were suggested by Saur et al.
[97]; (h) and (i) were proposed by Rosenberg and Anderson [98].
On the other hand, structural model (j) was suggested by Vedrine et
al. [99] and (k) by White et al. [100]. Likewise, based on several
physicochemical characterization results the surface structure of
WZ has also been proposed as shown in Fig. 2 [101]. It is normally
believed that tungsten oxide could exist on the zirconia surface in
the form of polyoxotungstate clusters as presented in Fig. 2. Sohn et
al. [102] prepared solid superacid catalyst Ce-ZrO2/SO42 and
suggested the structural model presented in Fig. 3 based on IR
spectroscopy.
O
Zr
O
O
O
W
O
Zr
O
ZrO2 Support
O
O
W
W
O
O
O
W
ZrO2 Support
Fig. (2). Proposed mono-tungstate and poly-tungstate on ZrO2 support in
WOx/ZrO2 catalysts.
O
O
O
O
S
S
*
HO
O
O
O
O #
Zr
Zr
Ce
Zr
Zr+
O
O
O
O
O
* Brönsted acid site; # Lewis acid site
Fig. (3). Proposed structural model for Ce-ZrO2/SO42 catalyst.
4. CATALYTIC ACTIVITY OF SULFATED ZIRCONIA
Many industrially as well as scientifically important reactions
have been investigated employing SZ catalyst because of its strong
or superacidic character. Several advantages associated with it
include easy handling, non-corrosive nature, water tolerance, easy
preparation, and easy recovery and reusability, which make this
catalyst highly versatile for numerous applications. After its
discovery, it was exploited mainly for vapour phase reactions such
as tert-butylation of phenol [103], Beckmann rearrangement of
cyclohexanone oxime, and isomerization of butane, pentane and
other hydrocarbons [38,104-109]. Recently, Alemán-Vázquez et al.
[110] studied heptane isomerization with SZ catalyst having
different concentrations of sulfate. Interestingly, the SZ catalyst
was found to be very active for various organic synthesis and
transformation reactions in the liquid phase, thus facilitating
synthesis of several fine and specialty chemicals. Recently,
Thirupathi et al. carried out cyanosilylation of aldehydes [111],
Tyagi et al. [112] studied synthesis of acetyl salicylic acid and
Sasiambarrena et al. [113] investigated sulfonylamidomethylation
of benzylsulfonamides and 2-phenylethanesulfonamides using SZ
catalyst in the liquid phase. In this section application of SZ catalyst
for several liquid phase reactions is discussed briefly.
4.1. Multicomponent Reactions
Multicomponent reactions (MCRs) play a significant role in
combinatorial chemistry because of their ability to yield small druglike molecules with several degrees of structural diversity. A MCR
is a chemical reaction where three or more compounds react to form
a single product [114]. MCRs have been known for over 150 years.
The first reported MCR was the Strecker synthesis of aminocyanides in 1850 from which -aminoacids could be derived.
As mentioned by I. Marek (the Guest Editor for a special issue of
Tetrahedron Symposium-in-Print on MCRs) [115], “the practical
construction techniques available to prepare elaborate products are
still woefully inadequate. A seemingly trivial but rather serious
limitation in practice is set by the mere number of steps
accumulating in linear sequences”. MCRs are thus becoming an
increasingly significant class of reactions as they allow a number of
starting materials to be combined to form a single compound and in
a one-pot operation [116]. They, therefore, exhibit an economy of
steps and often atom economy, most of the incoming atoms being
linked together in a single product.
Several MCRs were attempted by using SZ catalyst. Combining
the advantages associated with MCRs with heterogeneity and
catalysis of SZ reinforces the “greenness” for such reactions. MCRs
catalyzed by SZ include Strecker, Biginelli and Hantzsch reaction,
and synthesis of acetamido carbonyl compounds. Also other
zirconia-based catalysts, namely sulfated ceria-zirconia and WZ,
are also used for multicomponent Mannich reaction between
aldehyde, ketone and amine. Some of these reactions as reported in
the literature are summarized in the following sub-sections.
4.1.1. Strecker Synthesis
As discussed above, Strecker reaction is the first reported MCR,
discovered in 1850. -Aminonitriles, often synthesized by Strecker
reaction, are highly useful synthons for the synthesis of aminoacids [117-122], nitrogen containing heterocycles such as
3964 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
an enolizable ketone or -ketoester, and acetyl chloride. Acetamido carbonyl compounds are useful building blocks for a
number of biologically and pharmaceutically valuable compounds
[133]. Variety of catalysts are used for this MCR such as CoCl2
[134], Cu(OTf)3/Sc(OTf)3 [135] and BiCl3 generated in situ from
BiOCl and acetyl chloride [136], CeCl3·7H2O [137], ZrOCl2·8H2O
[138], I2 [139], a few heterogeneous catalysts [140-143] and so on.
All these methods while contributing towards some advantages also
suffer from different negative aspects such as the use of expensive
catalysts, longer reaction times, high temperature and low yields.
By considering all these aspects, Das et al. [144] studied this
MCR by using SZ catalyst. SZ is found to be most efficient in
offering good yields (81-94%) of various -acetamido carbonyls
within short period of time (1-3 h) (Schemes 3 and 4). In addition,
1,3-diketones (ethyl acetoacetate and methyl acetoacetate) were
found to afford the corresponding -acetamido ketoesters with high
diastereoselectivity (Scheme 4) and in most of the cases, anti
isomer was the major product.
imidazoles and thiadiazoles [123,124], and other biologically useful
molecules such as saframycin A [125]. The efficiency of the
reaction has been increased by the use of catalysts, and reactive
cyanide ion sources. Trimethylsilyl cyanide (TMSCN) is a safer,
more effective and more easily handled anion source compared to
other sources. The reaction can be catalyzed by indium (III) iodide
[126], rhodium (III) iodide hydrate [127], 2-iodoxybenzoic acid
(IBX) and tetrabutylammonium bromide (TBAB) [128], and also
few supported versions or versions based on heterogeneous
catalysts [129-132] that could be found in the literature.
Recently, Reddy et al. [20] utilized various zirconia-based solid
acid catalysts, namely SZ, MZ and WZ for the MCR. Among them
the SZ catalyst was found to be most efficient for this reaction.
These reactions proceeded efficiently and various -aminonitriles
were produced from the corresponding aldehydes and amines in
good to excellent yields within short reaction times (20-360 min)
using dry THF as the solvent under N2 atmosphere (Scheme 1). In
order to evaluate the possibility of 1,3-asymmetric induction,
Reddy et al. extended this MCR to the preparation of optically
active -aminonitriles from (R)-(+)-methylbenzylamine, aldehydes
and the TMSCN. The reaction was found to produce a mixture of
diastereoisomers with both aryl as well as alkyl aldehydes, with one
diastereoisomer predominating (Scheme 2).
R
O+
H2N
R1
H
R and R1 = Alkyl, Aryl
H
TMSCN, THF
SZ, N2, r.t.
20-360 min
R
C
H
N
4.1.3. Biginelli and Hantzsch Reaction
The Biginelli reaction is another MCR that creates 3,4dihydropyrimidinones from an aldehyde, -ketoester and urea
[145,146]. Dihydropyrimidinones are widely used in the
pharmaceutical industry as calcium channel blockers,
antihypertensive agents and 1a-antagonists [147]. This reaction
was reported for the first time by Pietro Biginelli in 1893. The
reaction can be catalyzed by Brønsted acids and/or Lewis acids, and
many reagents such as lanthanum chloride [148], Montmorillonite
KSF [149], heteropolyacids [150], metal triflates [151], silica
supported sulfuric acid [152] and so on were used.
In order to find better alternatives and to overcome the
drawbacks associated with these catalysts, lot of work has been
R1
CN
Yield = 52-93%
Scheme 1.
4.1.2. Synthesis of -Acetamido Carbonyl Compounds
Another important MCR involves synthesis of -acetamido
carbonyl compounds by reaction of aromatic aldehyde, acetonitrile,
O
R
CH3
+
H
NH2
Ph
CH3
TMSCN, THF
Ph
SZ, N2, r.t.
CH3
CN
N
R
H
(R, R)
+
Ph
CN
N
H
(R,S)
R
(R, R):(R, S) Yield (%)
R
________________________________
Phenyl
4-MeO-Ph
i-propyl
n-butyl
24:76
08:92
80
78
36:64
37:63
90
85
Scheme 2.
NHAc O
CHO
R
O
R1
SZ, r.t. CH3CN
R
CH3COCl
X
R1
1-3 h
R = 4-Br, 3-NO2, H etc., X = Me, Ar etc.
Yield = 72-95 %
R1 = Ph, 4-NO2C6H4, 4-BrC6H4, C6H10O, Me
+
X
Scheme 3.
NHAc O
CHO
R
O
R = 4-Cl, 3-NO2, 4-Br,
4-Me
Scheme 4.
O
+
R2 = Me, Et
SZ, r.t. CH3CN
CH3COCl
2
OR
2-3 h
NHAc O
OR2
R
+
OR2
R
O
Anti
Yield = 80-88%
O
Syn
Current Organic Chemistry, 2011, Vol. 15, No. 23 3965
Green and Heterogeneous Catalysts for Organic Synthesis
R1
O
H
O
+
O
OR2
R1
O
X
+
SZ, solvent-free
H2N
NH2
R2
100 0C
NH
N
X
H
Yield = 80-92 %
X = O, S
R1 = H, 4-NO2, 4-OH R2 = Me, Et
Scheme 5.
R
R
EtO2C
O
H
+
O
O
+
H2N
EtO2C
SZ
NH2
solvent-free
CO2Et
Me
4h
N
H
Me
Scheme 6.
carried out using SZ catalyst. In 2005, Reddy et al. [48] reported
synthesis of dihydropyrimidinones by condensation reaction
between an aldehyde, -ketoester and urea or thiourea under
solvent-free conditions employing SZ catalyst at 100 °C (Scheme
5). These reactions were found to proceed efficiently and various
dihydropyrimidinones were produced in excellent yields (80-92%)
in short reaction times (40-60 min). Thereafter, Gopalakrishnan et
al. [153] and Kumar et al. [154-156] studied Biginelli reaction
using SZ catalyst under microwave irradiation.
Further, Angeles-Beltrán et al. [157] made a competitive study
of Biginelli versus Hantzsch reaction using SZ catalyst and urea or
thiourea as amine under solvent-free conditions. Reaction was
studied at different temperatures (60, 80, 100 and 150 °C) for 4 h. It
was found that formation of Hantzsch product (Scheme 6) increases
and Biginelli product decreases with increase in temperature from
60-150 °C, and at 60 °C no Hantzsch products were obtained; at 80
°C, traces of Hantzsch products were noted.
4.2. Protection of Alcohols, Phenols and Aromatic Aldehydes
Normally, a protecting group is introduced into a molecule by
chemical modification of a functional group, and that protecting
group allows overcoming the problems of chemoselectivity in the
subsequent chemical reaction. It plays a vital role in multistep
organic synthesis [158]. In many preparations of delicate organic
compounds, some specific parts of their molecules cannot survive
the required reagents or chemical environments. Therefore, these
parts, or groups, must be protected [158]. The SZ has been used as
a promising catalyst for protection and deprotection of various
groups in organic synthesis. Some of these reported examples are
described in the following paragraphs.
4.2.1. Synthesis of 1,1-Diacetates from Aromatic Aldehydes
Acetic anhydride is one of the most commonly used reagents
for formation of 1,1-diacetates from aldehydes. This reaction is
catalyzed by Brønsted acids and/or Lewis acids as well as
heterogeneous catalysts [159-165]. Negrón et al. [166] utilized SZ
catalyst for this reaction, and various 1,1-diacetates were
synthesized from aromatic and heteroaromatic aldehydes in
excellent yields within 5-8 h at 0 °C (Scheme 7). The deprotection
of the resulting acylals was achieved using the same catalyst at 60
°C and acetonitrile as the solvent (Scheme 8). Also, in another
work, S. V. N. Raju [167] reported that SZ is an efficient catalyst
for the synthesis of variety of 1,1-diacetates from the corresponding
aldehydes and ketones.
O
OAc
H
+
R
SZ, 0 0C
Ac2O
OAc
R
5-8 h
R = H, 2-CH3, 4-CH3, 2-CH3, 4-OCH3
2-NO2, 4-NO2, etc.
Scheme 7.
O
OAc
OAc
R
SZ, 60 0C
R
CH3CN
H
Scheme 8.
4.2.2. Methoxymethylation of Alcohols
Methoxymethyl ethers are commonly used in protecting
alcohols
and
phenols
in
natural
product
synthesis.
Methoxymethylation was carried out by using chloromethyl methyl
ether [168,169], which is no longer suitable due to its extreme
carcinogenicity. Therefore, chloromethyl methyl ether is replaced
with dimethoxymethane. This reaction is reported with various
catalysts such as phosphorus pentoxide [170], expansive graphite
[171], FeCl3/3 Å molecular sieves (MS) [172], Nafion-H resin
[173] and so on. Nevertheless, these catalysts having advantages
also associated with some drawbacks such as low product yields,
requiring high reaction temperatures, tedious work-up procedures,
etc. Lin et al. [174] studied this reaction by using sulfated metal
oxides (SMO) namely, ZrO2, TiO2, Fe2O3, SnO2, HfO2, Al2O3 and
SiO2. To identify the optimal SMO catalyst among various
catalysts,
they
investigated
methoxymethylation
of
cyclohexylmethanol with
dimethoxymethane at ambient
temperature. Among all SMO catalysts, SZ was found to be most
promising. Thereafter, methoxymethylation of various alcohols was
carried out with dimethoxymethane at ambient temperature using
SZ catalyst (Scheme 9). It was also found that product yield
followed a preferential order of primary > seconday >> tertiary
alcohol due to steric effect.
OCH3
R
OH
+
OCH3
SZ
r.t.
R = primary, secondary, tertiary
Scheme 9.
OCH3
OR
Yield = 23-99%
3966 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
compounds such as triazolo-, oxadiazolo-, oxazino- and furanobenzodiazepines. These represent an important class of bioactive
compounds. In general, benzodiazepines are synthesized by the
condensation of o-phenylenediamines with ,-unsaturated
carbonyl compounds, -haloketones or ketones. Many reagents
have been reported for this condensation reactions which include
BF3-etherate [192], polyphosphoric acid [193], Yb(OTf)3 [194] and
acetic acid under microwave conditions [195]. Also this
condensation is reported in an ionic liquid medium [196,197].
Considering the advantages of heterogeneous SZ catalysts, such as
non-toxicity and low cost, Reddy et al. [48,198] studied the
synthesis of 2,3-dihydro-1H-1,5-benzodiazepines (Scheme 12) by
the condensation of o-phenylenediamine with ketones under
solvent-free conditions catalyzed by SZ. The advantages associated
with this methodology are easy work-up procedure, excellent
product yields and recyclable catalyst.
4.2.3. Tetrahydropyranylation of Alcohols and Phenols
The tetrahydropyranylation is one of the most commonly used
processes to protect hydroxyl groups in organic synthesis [175177]. The OH-group can be protected under various conditions such
as oxidation, oxidative alkylation, reduction, using Grignard
reagents, acylation etc. Tetrahydropyranylation of alcohols and
phenols was carried out by reaction with 3,4-dihydro-2H-pyran.
This reaction is catalyzed by a variety of reagents such as protic
acids (HCl and p-toluenesulfonic acid (p-TSA)), Lewis acids (BF3OEt2, aluminium sulfate on silica gel and sulfuric acid on silica
gel), clay materials (Montmorillonite K-10 and HY-zeolite), ionexchange resins and so on [178-181]. However, many of these
methods are characterized by several disadvantages such as
expensive reagents, tedious workup procedure, rapid catalyst
deactivation, low product yields, high reaction times, elevated
reaction conditions, and disposal problems. In view of these
reasons, Reddy et al. [182] studied this reaction by using SZ
catalyst. The SZ catalyst was found to be extremely active for
tetrahydropyranylation of various alcohols (primary, secondary, and
tertiary) and phenols providing excellent product yields (82-96%)
within short reaction times (Scheme 10).
O
OH +
R
O
SZ, r.t.
O
4.4. Transesterification
In organic chemistry, transesterification is a process in which
alcohol reacts with ester to provide different alcohol and different
ester through interchange of alkoxy moiety. Transesterification
reactions are important synthetic organic transformations in
industrial as well as in academic laboratories. Transesterification is
utilized in the synthesis of polyester, in which transesterification of
diesters with diols forms macromolecules. For example,
polyethylene terephthalate is synthesized by reaction between
dimethyl terephthalate and ethylene. Methanol is produced as byproduct, which is evaporated to accelerate the rate of reaction. The
reverse reaction i.e., between polyethylene terephthalate and
methanol is also an example of transesterification and has been
used to recycle polyesters into individual monomers. There are
many catalysts available for transesterification reaction which
include acid catalysts such as H2SO4, H3PO4, HCl, p-TSA,
BuSn(OH)3 and Al(OR)3, and base catalysts that include NaOH,
KOH and NaOCH3 and so on. However, these catalysts are toxic,
corrosive, produce large amount of by-products which are difficult
to separate from the reaction medium. In view of environmental
concern, there is a constant search for catalysts which are easily
separable from the products, less toxic and reusable. In this regard,
SZ is a highly promising catalyst which has been successfully used
for
few
transesterification
reactions.
For
example,
transesterification of triacetin with methanol and methyl salicylate
with phenol gives salol that has been studied by employing SZ
catalyst.
Lopez et al. [199] compared the catalytic activity of a number
of solid and liquid catalysts in the transesterification of triacetin
with methanol (Scheme 13) at 60 °C. The order of reactivity among
various acid catalysts investigated is as follows: H2SO4 >
Amberlyst-15 > SZ > Nafion NR50 > WZ > supported phosphoric
acid (SPA) > Zeolite H > ETS-10 (H). Only H2SO4 and
Amberlyst-15 were found to show higher activity than SZ catalyst.
The solid acids SZ, Amberlyst-15, Nafion NR50 and WZ exhibited
57%, 79%, 33% and 10% conversion; and H2SO4 exhibited 99%
R
solvent-free
Yield = 82-96%
R = Alkyl, Aryl
Scheme 10.
4.2.4. Synthesis of Silyl Ethers
Silyl ethers have been developed from alcohols using variety of
silylating agents namely, chlorotrimethyl silane [183],
trimethylsilyl azide [184], triethylsilyl chloride [185], allylsilane
[186], triethylsilyl hydride [187] and hexamethyldisilazane
(HMDS) [188]. The protection of hydroxyl groups by silylation is
imperative in peptide synthesis, and as lipophilicity modifiers for
the peptides [189]. Several groups synthesized an anticancer drug
NCX 4040 by selective silylation of 4-hydroxy benzyl alcohol as
the starting material [190]. Recently, Thirupathi et al. [191]
synthesized silyl ethers using oximes and alcohols employing
TMSCN as the silylating agent and SZ catalyst. These silyl ethers
were generated in short reaction times with excellent yields (8596%) (Scheme 11).
10 mol% SZ
R OTMS + R1CN
solvent-free,
r.t.
R = allylic, cyclic, acyclic, aliphatic, aromatic, hetero aromatic, oxime
R
OR1 + TMSCN
Scheme 11.
4.3. Synthesis of Benzodiazepines
Benzodiazepines and their polycyclic derivatives are finding
numerous new applications; widely used as anticonvulsant, antiinflammatory, analgesic, hypnotic, sedative, and antidepressive
agents; valuable intermediates for the synthesis of fused ring
R
O
NH2
+
R
NH2
R = H, Me
Scheme 12.
R1
R
R2
SZ, r.t.
2-3 h
R1 = CH3, C2H5, Ph
R2 = H, CH3
H
N
R1
R2
R2
R
N
R1
Yield = 84-96%
Current Organic Chemistry, 2011, Vol. 15, No. 23 3967
Green and Heterogeneous Catalysts for Organic Synthesis
OCOCH3
H3COCO
OCOCH3
Acid or Base
Catalyst
+ 3 CH3OH
60 0C
O
OH
+ 3
HO
OH
H3C
OCH3
Scheme 13.
conversion respectively for transesterification of triacetin and
methanol (methanol:triacetin = 6:1) at 60 °C. At 50% conversion of
triacetin, SZ showed 55.1, 31.2 and 13.8% selectivity for diacetin,
monoacetin and glycerol respectively. In another study, Lopez et al.
[200] carried out transesterification of triglycerides and the
esterification of carboxylic acids with ethanol using modified
zirconias, namely SZ, WZ and TZ. Among all these catalysts, SZ is
found to be the most active for both transesterification and
esterification reactions, which however exhibited significant sulfur
loss and was greatly reduced its long term activity. Again, there is
another report on transesterification of triglycerides at 120 °C
[201]. Recently, Petchmala et al. [202] investigated
transesterification of palm oil and esterification of palm fatty acid
in near- and super-critical methanol with SZ catalyst. The SZ
catalyst was prepared with three different sulfur loadings (0.75%,
1.8% and 2.5%) and subjected to two calcination temperatures (500
°C and 700 °C). The most appropriate sulfur loading was found to
be 1.8% and the optimal calcination temperature was 500 °C.
Shamshuddin and Nagaraju [203] synthesized zirconia, SO42
and Mo(VI) ions modified zirconia and studied their catalytic
performance in the synthesis of phenyl salicylate (Salol) via
transesterification of methyl salicylate with phenol (Scheme 14) in
the liquid phase with or without molecular sieves. They studied this
transesterification reaction employing ZrO2, MoO3, SZ and MZ
catalysts at ~189 °C. SZ and MZ were found to be effective
catalysts for synthesis of salol. In case of ZrO2, MoO3 and MZ
catalysts, phenyl salicylate (salol) was the only product; in case of
SZ, salol was formed as a major product along with diphenyl ether.
However, the formation of diphenyl ether with SZ may be
attributed to the presence of ‘very strong’ acid sites [204] on its
surface.
O
OCH3
OH
OH
Ph
O
Acid catalyst
4.5.1. Regioselective Ring-Opening of Aziridines
SZ is found to be effective catalyst for regioselective ringopening of aziridines with potassium thiocyanate and thiols [205].
Aziridines ring can be opened smoothly with KSCN in the presence
of SZ to give the corresponding -aminothiocyanates in high yields.
A series of -aminothiocyanates were prepared using various Ntosyl-2-aryl aziridines or N-tosyl-2-alkyl aziridines with KSCN
(Scheme 15). The conversion required only 2 h at room temperature
using CH3CN as the solvent and the ring-opening of the aziridines
took place regioselectively in high yields. With N-tosyl-2arylaziridines, product 1 resulting from cleavage at the benzylic
position, and with N-tosyl-2-alkylaziridines, product 2 resulting
from the cleavage at the terminal position were formed
predominantly along with minor amounts of the other regioisomers.
In case of symmetrical bicyclic aziridines only product 3 is formed
with trans stereochemistry (scheme 16). Also, a series of aminosulfides were prepared from different N-tosyl-2-aryl
aziridines or N-tosyl-2-alkyl aziridines by treatment with various
thiophenols (products 4 and 5 formed (scheme 17) same as above)
in the presence of SZ. In case of symmetrical bicyclic aziridines
only product 6 is formed with trans stereochemistry (Scheme 18).
NHTs
NTs + KSCN
SZ, CH3CN
R
r.t., 2 h
R = Aryl, Alkyl
SCN
+
SCN R
NHTs
1
2
minor
major
when R = Aryl when R = Alkyl
R
Scheme 15.
NHTs
+ KSCN SZ, CH3CN
r.t., 2 h
NTs
n
n
SCN
3
_______________
n Yield (%)
_______________
1
89
87
2
_______________
O
OH
+
Scheme 16.
Scheme 14.
NTs + R1SH
4.5. Ring-Opening of Aziridines and Epoxides
Aziridines are important precursors for the synthesis of various
nitrogen-containing bioactive molecules such as heterocycles,
alkaloids and amino acids. Regioselective ring-opening of
aziridines contributes mainly to their synthetic utility. Ring-opening
of aziridines with KSCN provides -aminothiocyanates, which are
the precursors of thiazoles or benzothiazoles having pesticidal
properties; with thiols providing -aminosulfides, which are the
precursors of various bioactive compounds. Ring-opening of
epoxides with amines gives -aminoalcohols, which are the key
intermediates to many organic compounds including biologically
active natural and synthetic products and are chiral auxiliaries for
asymmetric synthesis. The SZ catalyst has been utilized for the
ring-opening of N-tosyl aziridines with amines, thiols and
potassium thiocyanate, and ring-opening of epoxides with amines
and N-heterocycles.
R
R = Aryl, Alkyl
r.t., 2 h
R1 = Aryl
SR1
NHTs
SZ, CH3CN
+
NHTs
SR1 R
5
4
major
major
when R = Aryl when R = Alkyl
R
Scheme 17.
n
NTs
Scheme 18.
+
R1SH
SZ, CH3CN
r.t., 2 h
NHTs
n
SR1
6
_______________________
n
Yield (%)
R1
_______________________
1
91
C6H5
2
4-Cl-C6H4 88
_______________________
3968 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
pharmaceuticals [209,210]. Sulfoxides have attracted much
attention as important chiral auxiliaries in asymmetric synthesis and
in carbon-carbon bond forming reactions [211]. Usually, sulfoxides
are prepared by various methods. However, these methods have
some limitations which include use of hazardous, corrosive and
expensive reagents and formation of a mixture of products along
with the desired sulfoxides. Reddy et al. [48] reported the synthesis
of various diaryl sulfoxides from the reaction of diverse activated
and non-activated arenes with thionyl chloride catalyzed by SZ
under solvent-free conditions (Scheme 22). All the diaryl sulfoxides
were formed in excellent yields in short reaction times.
4.5.2. Regioselective Ring-Opening of Epoxides
Reddy et al. [206] carried out the regioselective ring-opening of
epoxides with various aromatic amines catalyzed by SZ under
solvent-free conditions to provide the corresponding aminoalcohols in high yields (70-97%). Ring-opening of styrene
oxide with aromatic amines afforded the major regioisomer 7, and
aliphatic oxirane (propylene oxide) with aromatic amines gave
major regioisomer 8 (Scheme 19). Symmetrical oxirane
(cyclohexeneoxide) with various aryl amines provided the product
of racemic 2-aryl amino cyclohexanol 9, which was identified as
the trans-diastereoisomer (Scheme 20). In another study, NegrónSilva et al. [86] employed SZ and SZ/MCM-41 catalysts for the
regioselective synthesis of -aminoalcohols using various epoxides
with aniline or benzyl amine at 60 °C (conventional heating) or
microwave exposure under solvent-free conditions. Both catalysts
exhibited good catalytic activity, and the catalysts were recovered
and reused without any appreciable loss of activity. Also, Das et al.
[207,208] exploited SZ catalyst for ring-opening of various
epoxides with nitrogen heterocycles such as indoles, pyrroles and
immidazoles in CH2Cl2 (Scheme 21). Reaction took place within 34.5 h offering good yields of products (42-83%).
O
HN
SZ, r.t.
R
O
NH2
NH2
OH
R1
2
OH
R + R1 CHO
NHR
NR R N
H
H
Yield = 73-85%
Scheme 23.
Scheme 20.
aldehydes/ketones in the presence of an acid catalyst to produce
azafulvenium salts. These azafulvenium salts can undergo further
addition reaction with another molecule of indole to produce
bis(indolyl)methanes [213]. Protic acids as well as Lewis acids, and
various other reagents such as lanthanide triflates, clays, ion
4.6. Synthesis of Diaryl Sulfoxides
Sulfoxides and sulfones are important compounds for synthesis
of various organosulfur compounds in the field of drugs and
R3
N
R3
R1
R2
N
CH2Cl2, SZ, r.t., 3-4.5 h
Ar
Ar
Ar
N
R
CH2Cl2, SZ, r.t., 4 h
Ar = Ph
N
Yield = 42-75%
R1
N
R
O
OH
Ar
R2
OH
HO
Ar
N
R
OH
N
R
R=H
45%
16%
6%
R = CH3
42%
12%
8%
NH
CH2Cl2, SZ, r.t., 4 h
Scheme 21.
H
SZ, r.t.
solventfree
R1 = Aromatic,
Heteroaromatic
N
H
R = H, Me
9
Ar
S
Ar
Ar
Yield = 80-92%
solvent-free
Large number of natural products which contain
bis(indolyl)methanes and bis(indolyl)ethanes have been isolated
from both marine and terrestrial sources, and some of them were
found to exhibit interesting biological activity [212]. In general,
bis(indolyl)methanes are synthesized by the analogous reactions of
Ehrlich test, where indoles react with aliphatic or aromatic
OH + 1
R
SZ, r.t.
O
SZ, r.t.
4.7. Synthesis of Bis(indolyl)methanes
Scheme 19.
O + R
Ar
Scheme 22.
H
N
R1
R
45-300 min R1
8
7
R1 = Aryl, Alkyl R = Aryl
major
major
when R1= Aryl when R1 = Alkyl
+R
+H
S
H +
Cl
Cl
Ar
OH
Ar
N
N
Yield = 68%
OH
Current Organic Chemistry, 2011, Vol. 15, No. 23 3969
Green and Heterogeneous Catalysts for Organic Synthesis
exchange resins and zeolites have also been studied for this
reaction. Reddy et al. [48] synthesized bis(indolyl)methanes by
electrophilic substitution reaction of indole with various aldehydes
in the presence of SZ catalyst (Scheme 23). This reaction was
carried out for an appropriate time at room temperature and the
resulting bis(indolyl)methanes were produced in excellent yields.
4.8. Friedel-Crafts Alkylation Reactions
The Friedel-Crafts reactions were developed by Charles Friedel
and James Crafts in 1877. Now, these reactions are used
extensively in the synthesis of fine chemicals and various
intermediates in pharmaceutical as well as in petrochemical
industries. Friedel-Crafts alkylation reactions involve the alkylation
of an aromatic compound with an alkyl halide using a strong Lewis
acid catalyst. The SZ catalyst has been utilized for various FriedelCrafts alkylation reactions such as that of diphenyl oxide, guaiacol,
p-cresol and methoxyphenol.
Yadav and Sengupta [22] carried out the alkylation of diphenyl
oxide with benzyl chloride and synthesized the corresponding
isomeric benzyl chloride in excellent yields using SZ catalyst
(Scheme 24). In another study, Yadav and Rahuman [214] tested
various solid acid catalysts such as Filtrol-24, DTP/K-10, Deloxane
ASP resin, K-10 Montmorillonite clay, and SZ for alkylation of 4methoxyphenol with methyl tert-butyl ether (MTBE) (Scheme 25).
Activity of SZ catalyst was found to be the least among the tested
catalysts. Though the SZ catalyst exhibited the lowest activity as
compared to other solid acid catalysts, it showed maximum
selectivity to monoalkylated products. Thus with 1:3 molar ratio of
4-methoxyphenol and MTBE, the SZ catalyst provided 22%
conversion and 85% selectivity to monoalkylated product. Yadav et
al. [215,216] investigated the O- versus C-alkylation of p-cresol
and guaiacol with cyclohexene (Schemes 26 and 27) by using
several solid acid catalysts including SZ. For example, alkylation of
p-cresol with cyclohexene (1:1 molar ratio) provided 47%
conversion and 82% O-alkylated product selectivity. On the other
hand, the alkylation of guaiacol with cyclohexene (0.226:0.045
molar ratio) provided 74% conversion and 68% O-alkylated
product selectivity.
Apart from these studies, there are some other reports in which
SZ catalyst has been used for alkylation of benzene with benzyl
chloride [217], alkylation of p-cresol with isobutylene [218], and
alkylation of diphenyl oxide with 1-decene [219].
O
O
+
O
Cl
SZ
+
90 0C
Scheme 24.
OCH3
OCH3
OCH3
OCH3
+
H3C
CH3
CH3
SZ, 1,4-dioxane
+
150 0C, 3 h
OH
OH
Conversion 28%
OH
BHA (mono)
BHA (di)
Scheme 25.
OH
OH
O
SZ, Toluene
+
+
80 0C, 3 h
Conversion 47%
CH3
CH3
CH3
O-alkylation
(82%)
C-alkylation
Scheme 26.
OCH3
OCH3
OH
+
SZ, Toluene
80 0C
Conversion 74%
Scheme 27.
OCH3
OH
O
+
O-alkylation
(68%)
C-alkylation
3970 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
chlorobenzophenone (Scheme 28). Deutsch et al. [221] studied the
benzoylation of anisole with benzoic anhydride in presence of
various solid acid catalysts including SZ, Nafion-H, Amberlyst-15,
K-10, H-BEA and H-mordenite at 50 °C, and the SZ catalyst
exhibited the best performance for benzoylation (71% yield in 3 h).
Furthermore, benzoylation of various aromatic compounds such as
anisole 10, m-xylene 11 and mesitylene 12 with benzoic anhydride
and benzoyl chloride were studied at 100 °C (Scheme 29), the
reactivity of aromatics was in the order: anisole > mesitylene > mxylene with both the acylating agents. On raising the temperature
from 100 to 136 °C, there was no effect on the order of reactivity of
aromatics. In another study, Deutsch et al. [222] used SZ catalyst
for acylation of anisole and chlorobenzene with various acylating
agents which include aliphatic carboxylic anhydrides, aliphatic acid
chlorides and substituted benzoyl chlorides. But only benzoyl
chlorides reacted with chlorobenzene in the presence of SZ. Later
on, the same group reported the acylation of methoxynaphthalenes
13, dimethylnaphthalenes (14, 15), methylnaphthalenes 16,
naphthalene, and anthracene with benzoic anhydride 17, benzoyl
chloride 18, and acetic anhydride 19 to synthesize aromatic ketones
by employing SZ catalyst (Schemes 30 and 31) [223]. In this study,
it was found that the rate of product formation on SZ was
dependent on the respective aromatic compound, solvent used, and
the ratio of substrate to the acylating agent.
Apart from these, various groups working in this fascinating
area also reported interesting results. Recently, El-Sharkawy and
Al-Shihry, [224] studied Friedel-Crafts acylation of toluene with
acetic anhydride using various sulfated metal oxides such as
4.9. Friedel-Crafts Acylation Reactions
Acylation of aromatic compounds is another class of FriedelCrafts reactions. Acylation reaction has several advantages over
alkylation; the product (aromatic ketone) is less reactive than
original substrate so that the possibility of multiple acylation is
reduced. Aromatic ketones are intermediates or end-products
employed in the formation of pharmaceuticals, cosmetics,
agrochemicals, dyes and speciality chemicals. Friedel-Crafts
acylation is carried out by using various Lewis/Brønsted acids such
as AlCl3, BF3-HF, FeCl3, ZnCl2, SnCl4, InCl4, SbCl5, H2SO4, HCl
and so on. The SZ catalyst has been exploited for various FriedelCrafts acylation reactions such as acylation of benzene, substituted
benzenes, naphthalenes, anthracenes and other.
Cl
O
O
SZ
+
70 0C
Cl
Cl
Scheme 28.
Yadav and Pujari [220] studied acylation of benzene with 4chlorobenzoyl chloride using different solid acid catalysts such as
K-10 clay, Filtrol-24 clay, dodecatungstophosphoric acid (DTPA),
DTPA supported on K-10, Amberlyst-15, Amberlite IR 120,
Indion-130 and SZ. Among them SZ exhibited the best activity.
The acylation reaction was 100% selective to 4O
R3
X
R3
SZ, -HX
R1
R2
O
R1
O
R2
+
isomers
+
isomers
R4
R3
O
O
R2
R4
O
SZ, -CH3CO2H
R1
10 R1 = R2= R4 = H, R3 = OCH3
11 R1 = R3 = CH3, R2 = R4 = H
12 R3 = R1 = R4 = CH3, R2 = H
R4
Scheme 29.
R2
R2
O
+
R1
R3
X
SZ
-HX
17, 18 R3 = C6H5,
13
= H,
= OCH3
X = OCOC6H5/Cl
14 R1, R2 = CH3
19
R3 = CH3,
15 R1 = H, R2 = CH3
X = OCOCH3
16 R1 = CH3, R2 = H
R1
R2
+ isomers(s)
R1
O
R3
Scheme 30.
O
O
+
R
SZ
X
-HX
R = C6H5, CH3;
X = Cl, OCOC6H5,
OCOCH3
Scheme 31.
R
9-Acylanthracene
Current Organic Chemistry, 2011, Vol. 15, No. 23 3971
Green and Heterogeneous Catalysts for Organic Synthesis
O
R
H3C
X
+
H
SZ, r.t.
O
R
solvent-free
10-15 min
H3C
O
O
X
+
CH3
Yield = 85-97%
O
R = 2-naphthyl, 4-acylphenyl, benzyl, cyclopentane etc.
X = O, NH
H3C
OH
Scheme 32.
glycosidation methods have been one of the major concerns in
synthetic organic chemistry due to the structural complexity and the
biological significance of glycosubstances. Toshima et al. working
in this area for several years carried out several stereoselective
glycosidation of glycosyl phosphites; manno- and 2deoxyglucopyranosyl
-fluorides;
2-deoxyand
2,6dideoxyglycosyl diethyl phosphites; mannopyranosyl sulfoxides;
3,4-di-O-protected olivoses; 2-deoxy--D-glucopyranosyl fluoride;
O-benzylated 1-hydroxy sugars with various alcohols using solid
acid catalysts including SZ [25,228-235].
In an interesting investigation, Toshima et al. [25] reported
stereo-controlled glycosidation of totally benzylated 2-deoxy--Dglucopyranosyl fluoride 20 with various alcohols (21-26) using SZ
catalyst to synthesize both the 2-deoxy--and -Dglucopyranosides. They examined the glycosidation reaction of 20
with cyclohexyl methanol 21 using SZ catalyst with or without 5 sulfated tin oxide, SZ containing different amounts of sulfate and
Al2O3-SZ. Yadav and Pimparkar [225] carried out the synthesis of
2,5-dimethoxyacetophenone by acylation of 1,4-dimethoxybenzene
with acetic anhydride over various solid acid catalysts including
SZ.
4.10. Acylation of Phenols, Alcohols and Amines
Acylation of phenols, alcohols and amines is of synthetic
importance. Acylation of phenol and alcohol gives an ester which
corresponds to an important family of intermediates widely
employed in the synthesis of fine chemicals, drugs, plasticizers,
perfumes, food preservatives, pharmaceuticals and chiral auxillaries
[226,227]. SZ was found to be a promising catalyst for acylation of
phenols, alcohols and amines with acetic anhydride as an acylating
agent under solvent-free conditions at room temperature (Scheme
32). In this study, acetylation of diverse alcohols, phenols and
OBn
OBn
O
BnO
BnO
SZ, MS 5A
OR
Et2O, 00C
OBn
O
BnO
BnO
+ R OH
F
20
SZ, 250C
CH3CN
OH
HO
22
23
25
24
OBn
O
O
MeO
MeO
n-C8H17-OH
21
OR
O
21-26
OH
HO
O
BnO
BnO
Me
O OMe
OH
26
N3
Scheme 33.
molecular sieves under various conditions. They observed that
glycosidation using 5 wt.% SZ in CH3CN at 25 °C for 1 h gives 2deoxyglucopyranoside in high yield with high -stereoselectivity.
On the other hand, by using a 100 wt.% SZ in the presence of 5 Å
molecular sieves (500 wt.%) in Et2O at 0 °C, the corresponding 2deoxyglucopyranoside was obtained with -stereoselectivity
(Scheme 33).
In another study, Toshima et al. [235] studied the stereocontrolled glycosidation of manno- and 2-deoxyglucopyranosyl -
amines was carried out with acetic anhydride; products were
formed in excellent yields (85-97%) within short period of time
(10-15 min).
4.11. Stereo-Controlled Glycosidation
Glycosubstances including glycoconjugates, glycolipids,
glycoproteins, oligosaccharides and many antibiotics continue to be
the central focus of research both in chemistry and biology.
Therefore, the development of efficient and stereoselective
BnO
BnO
BnO
X
O
SZ (100wt%)
OR
Et2O, 25 0C
MS 5 A (100wt%)
X
OBn
28
BnO
O
+
R
20 X = H F
27 X = OBn
OH
SZ (5wt%)
CH3CN, 40 0C
OBn
O
BnO
HO
BnO
O
OBn
OMe
OH
29
N3
BnO
BnO
X
O
OR
21-24,
28-30
O
OH
O
BnO
BnO
Scheme 34.
BnO
BnO
BnO
30
OMe
3972 Current Organic Chemistry, 2011, Vol. 15, No. 23
BnO
BnO
BnO
OBn
O
Patil et al.
Y +
BnO
BnO
BnO
solvent
25 0C, 3 h
HO
X
31 X = S(O)Ph, Y = H
O
33
21
32 X = H, Y = S(O)Ph
OBn
O
OH
n-C8H17-OHHO
OH
22
OBn
O
SZ
23
24
O
BnO
BnO
OH
O
OBn
OMe
N3
34
29
Scheme 35.
fluorides using several heterogeneous solid acids, namely
Montmorillonite K-10, Nafion-H and SZ. Their study revealed that
SZ gives better yields for this reaction. Also, SZ provides
maximum -selectivity when CH3CN is employed as the solvent at
40 °C, and -selectivity in Et2O as solvent at 25 °C when used
along with molecular sieves 5 Å (100 wt.%). By taking these
conditions in hand, they carried out manno- and 2deoxyglucopyranosyl -fluorides (20, 27) with several alcohols
(21-24, 28-30) and synthesized both - and -manno- and 2deoxyglucopyranosides selectively (Scheme 34).
In another study, Nagai et al. [232] initially performed
glycosidation of mannopyranosyl sulfoxides 31 and 32 with
cyclohexylmethanol by employing several solid acids such as
Montmorillonite K-10, Nafion-H and SZ in the presence of 5 Å
molecular sieves and different solvents at 25 °C for 3 h. From the
initial study, the glycosidation of -mannopyranosyl sulfoxide 31
and cyclohexylmethanol with 100 wt% Nafion-H and 100 wt.% 5 Å
MS in CH3CN at 25 °C for 3 h exclusively gave the mannopyranoside 33 in high yield (97%) with high
stereoselectivity (/ = 97/3). On the other hand, the glycosidation
of -mannopyranosyl sulfoxide 32 with 300 wt.% SZ and 300 wt.%
5 Å MS in Et2O proceeded smoothly and selectively give the mannopyranoside 33 in high yield (99%) with high
stereoselectivity (/ = 19/81). By taking these conditions in hand,
glycosidation of 31 and 32 with several alcohols 21-24, 29, and 34
(Scheme 35) was carried out resulting in high yields and selectivity
of products.
literature showing a high activity of SZ catalyst for isomerization
reactions in the liquid phase.
Satoh et al. [236,237] studied skeletal isomerization of
cycloalkanes such as cycloheptane, cyclooctane, cyclodecane and
cyclododecane with SZ in liquid phase at 50 °C. The main product
of methylcyclohexane was formed from cycloheptane along with
small amounts of trans 1,2-dimethylcyclopentane, cis and trans 1,3dimethylcyclopentanes, 1,1-dimethylcyclopentane and ethylcyclopentane. The major product from
cyclooctane was
ethylcyclohexane along with other products namely, cis 1,3dimethylcyclohexane, small amounts of trans 1,2-, 1,3-, 1,4dimethylcyclohexanes, 1,1-dimethylcyclohexane and methylcycloheptane. Cyclodecane was dehydrogenated into cis- or transdecaline with the evolution of dihydrogen, and no isomerization
occurred. Cyclododecane was converted into more than 30 products
resulting from processes of isomerization, dehydrogenation and
cracking.
The isomerization of -pinene produces bicyclic and
monocyclic compounds such as camphene 35, tricyclene 36, fenchene 37, bornylene 38, and monocyclic compounds such as terpinene 39, limonene 40, -terpinene 41, terpinolene 42, pcymene 43, etc (Scheme 36). This isomerization reaction was
carried out in the presence of acid catalysts and acidity of catalyst
was directly related to the activity as well as camphene yield. SZ
catalysts were briefly studied for isomerization of -pinene [238241]. Comelli et al. [238] examined the isomerization of -pinene
with SZ catalyst and compared with ZrO2 and H2SO4. Also to
understand the effect of pre-treatment on the catalytic activity, the
SZ catalyst was treated in a muffle furnace for 2 h at 250, 350 and
550 °C; these catalysts were designated as SZ250, SZ350 and
SZ500. In the case of ZrO2 catalyst that possesses only Lewis acid
sites therefore no activity was observed. On the other hand
Brønsted acid H2SO4 exhibited barely any activity. SZ possesses
4.12. Isomerization Reactions
SZ catalyst came to the focus from isomerization reaction: nbutane to iso-butane. Thereafter, it was used for various reactions
such as the isomerization of butane, pentane and other
hydrocarbons in vapour phase. A few reports were found in the
35
39
Scheme 36.
36
40
38
37
41
42
43
Current Organic Chemistry, 2011, Vol. 15, No. 23 3973
Green and Heterogeneous Catalysts for Organic Synthesis
both Brønsted as well as Lewis acid sites, it showed good activity.
It was also found that the treatment temperature of SZ played a
major role, the SZ250 catalyst exhibited a high activity and the ratio
between bicyclic and monocyclic compounds was maximum.
Various catalysts, namely, SZ, SZ250, SZ350, SZ500 and H2SO4,
after 2 h reaction exhibited a conversion of 85.8, 88.2, 78.4, 56.5
and 1.4% and selectivities for camphene were 58.9, 67.4, 56.8, 59.3
and 12.0%, respectively. In another study, Grzona et al. [241]
prepared SZ catalyst with three different sulfur loadings (5, 10, and
20 wt.% H2SO4) and utilized for -pinene isomerization. All three
SZ catalysts were found to be active for obtaining camphene. With
increasing the amount of catalyst loading the catalytic activity
increased while the selectivity for camphene decreased. A catalyst
concentration of 1 wt.% was found to be the most suitable for high
activity and selectivity.
Flores-Moreno et al. [242] studied the isomerization of pinene oxide 44 to campholenic aldehyde 45 using sulfated
alumina, titania and zirconia in the liquid phase at 0 °C (Scheme
37). They have found that sulfated alumina produces campholenic
aldehyde (76% yield) at full conversion of the reactant where as SZ
produced it with 37% yield again at full conversion.
anhydride with 2-ethylhexanol in the presence of solid acid
catalysts such as natural zeolites, synthetic zeolites (ZEOKAR-2,
ASHNCH-3), heteropolyacids (H4Si(W3O10)4) and SZ. These
reactions were carried out under solvent-free conditions. The SZ
catalyst showed the best activity and efficiency among the
investigated catalysts (Scheme 39). They have also studied the
esterification of trans-(2-hexenyl)succinic anhydride and dibasic
acids such as sebacic acid, adipic acid and caproic acid with various
alcohols such as 2-ethylhexanol, diethyleneglycol and
pentaerythritol using SZ catalyst. In another study, Ardizzone et al.
[248] studied the esterification reaction of benzoic acid to methyl
benzoate with methanol by employing different SZ catalysts.
Excellent product yields under mild reaction conditions were
reported.
O
O
O +2 R
OH
Solid acid
OR
-H2O
OR
O
O
Scheme 39.
O
SZ, 0 0C
CHO
100% conversion
45
37%
44
Scheme 37.
Tyagi et al. [21] reported the isomerization of longifolene 46,
decahydro-4,8,8-trimethyl-9-methylene-1-4-methanoazulene to isolongifolene 47, 2,2,7,7-tetramethyltricycloundec-5-ene, (Scheme
38) employing nano-crystalline SZ catalyst obtained by sol-gel
technique in acidic medium using one-step as well as in basic and
neutral medium using two-step procedure. Almost all catalysts
exhibited excellent selectivity (90-93%) with 100% conversion at
180 °C reaction temperature in solvent-free isomerization of
longifolene to iso-longifolene. In order to obtain the maximum
conversion, the reaction was carried out at different temperatures in
the range of 120-200 °C. It was observed that conversion increases
from 120 to 180 °C then it does not change until 200 °C. Apart
from the above applications, the SZ catalyst was also used for
isomerization of citronellal to isopulegol [243].
4.14. Synthesis of Aromatic ,-Dihalobenzyl Derivatives
,-Dihalo aromatic compounds (gem-dihalides) are important
intermediates in the pharmaceutical, agricultural and dye industries
[249-251]. These are also used as starting materials for several C-C
coupling reactions and for the synthesis of imines as well as parent
raw materials for the preparation of the corresponding amines, acids
and alcohols [252,253]. Wolfson et al. [254] investigated the
synthesis of aromatic gem-dihalides from their corresponding
aromatic aldehydes by using various acid catalysts including both
homogeneous and heterogeneous Lewis and Brønsted acids. They
observed that AlCl3 and SZ are the most active homogeneous and
heterogeneous catalysts, respectively. Benzoyl chloride was more
reactive than acetyl and propionyl chloride. Replacing the chloride
with bromide also resulted in increased activity. Performance of the
reaction in polar solvent and in neat benzaldehyde resulted in
higher product yields. The SZ catalyst provided benzal chloride in
22% yield (Scheme 40), which is highest among the solid acids
used. Also they have carried out the oxidative regeneration of spent
SZ catalyst in air at 550 °C and fully recovered its catalytic activity
that allowed multiple catalysts recycling.
O
O
H
catalyst
+
SZ
46
47
Cl
Scheme 38.
Cl
100 0C, 1 h
Cl
H
+ (C6H5-CO)2O
Yield = 22%
4.13. Esterification Reactions
Esters have a fruity odour and are prepared in large quantities
for various purposes such as artificial fruit essences, flavorings and
components of perfumes. Lot of research has been carried out on
esterification reactions by using SZ catalysts such as esterification
of acetic acid (with ethanol [244] and butanol [245]), palm fatty
acid distillate [246], long-chain fatty acids [202], 4methoxyphenylacetic acid (with dimethyl carbonate) [64],
phenylacetic acid (with p-cresol) [247] and so on.
Sejidov et al. [24] investigated the synthesis of di-2ethylhexylphthalate (DOP) via esterification reaction of phthalic
2
Scheme 40.
4.15. Knoevenagel Condensation
The Knoevenagel condensation of aldehydes with active
methylene compounds is one of the important C-C bond forming
reactions in organic synthesis. Knoevenagel condensation has been
extensively investigated in view of its significance [255] and has
been commonly employed in the synthesis of numerous speciality
chemicals and chemical intermediates. This condensation reaction
is generally catalyzed by bases, acids or catalysts containing both
3974 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
acid-base sites [256], and numerous acid-base reagents or catalysts
are reported. Reddy et al. [257] investigated this reaction by
employing SZ catalyst under reflux and solvent-free conditions.
Various aliphatic, aromatic and heterocyclic aldehydes with
malononitrile produced corresponding Knoevenagel product
(Scheme 41) in short period of time (0.5-6 h) and in excellent yields
(78-98%).
O
H
CN
+
CN
R
CN
SZ
Reflux
30-300 min
SO42/Fe2O3, SO42/HfO2, SO42/SnO2, H-form of mordenite
(CBV21A , CBV90A and CBV10A ), and -zeolite (CP811E-75,
CP811E-150 and CP811E-300). Aniline (45.1 mmol) was mixed
with trimethyl orthoformate (130.2 mmol), 0.3 g of catalyst was
added and the solution was stirred at 40 °C for 1 h. The SZ is found
to be excellent as compared to other studied catalysts in view of
better selectivity (98%) and conversion (35%) (Scheme 44).
Intrestingly, methyl-N-phenylformimidate yield was less than 3 %
and no N-methylformanilide was found in the product mixture.
H3CO
Ar
CN
R
Yield = 78-98%
NH2 + H3CO C
H3CO
SZ
ArN=CH-NHAr + 3 CH3OH
40 0C, 1 h
98%
conversion 35%
H
Scheme 41.
Scheme 44.
4.16. Aza-Michael Reaction
4.18. Cyclodehydration of Some Diols
The aza-Michael addition is one of the most important organic
reactions especially for the synthesis of C-N heterocycles
containing -aminocarbonyl functionality. The product from azaMichael reaction not only constitutes a component of biologically
active natural product but also serves as an essential intermediate in
the synthesis of -aminoketones, -aminoacids and -lactam
antibiotics. Various catalysts are reported in the literature for this
fascinating reaction. Reddy et al. [258] employed SZ for azaMichael reaction of ,-unsaturated carbonyl compounds with
aromatic amines. A variety of ,-unsaturated carbonyl compounds
such as methyl vinyl ketone, methyl acrylate, ethyl acrylate and
cyclohexenone underwent 1,4-addition with a wide range of
aliphatic, aromatic, heterocyclic amines and primary as well as
secondary amines in the presence of SZ catalyst under solvent-free
conditions at room temperature to provide the corresponding aminoketones in high yields (70-95%) in short reaction times
(Scheme 42). To check the selectivity of the reaction, a mixture
(1:1) of morpholine and aniline was treated with an excess methyl
vinyl ketone in the presence of SZ, only the morpholine adduct was
formed as the product (Scheme 43).
Cyclodehydration of diols is highly useful method to obtain
oxygen heterocycles such as tetrahydrofuran, 2,5-dihydrofuran, 1,4dioxane and so on. Oxygen heterocycles are mainly used as
intermediates in the production of drugs, pesticides and other
chemicals on industrial scale [260]. Several catalysts are reported in
the literature for the reactions of diols in the liquid or gaseous
phases. Wali and Pillai [261] carried out a comparative study with
SZ and H-ZMS-5 catalysts for cyclodehydration reaction. Initially,
cyclodehydration of diethylene glycol was studied with SZ and HZMS-5 at 200 °C, and SZ was found to be better than H-ZSM-5
with respect to recyclability, yield and selectivity. They also
compared the results of SZ with other catalysts (Al3+Montmorillonite K10, HMPT and Al2O3) reported in the literature.
Further, SZ has also been successfully employed for
cyclodehydration of various diols such as butane-1,4-diol, pentane1,5-diol, hexane-1,6-diol, cyclohexane-1,4-diol and triethylene
glycol to produce the corresponding oxygen heterocycles with high
conversion, selectivity and yield.
R4
R1
SZ, r.t.
NH +
R2
R3
EWG solvent-free
15-120 min
R1
R3
N
R2
EWG
R4
Yield = 70-95%
Scheme 42.
4.17. Synthesis of Formamidine
Lin et al. [259] studied the reaction between aniline and
trimethyl orthoformate to produce formamidine by using various
solid acid catalysts, namely, SZ, SO42/TiO2, SO42/Al2O3,
4.19. Synthesis of Coumarins
Pechmann’s reaction is a simple and easy procedure to
synthesize coumarins by reacting phenols and -ketoesters in the
presence of acid catalysts. Several catalysts are reported in the
literature for this reaction that include Lewis acids, ionic liquids,
zirconium tetrachloride, sulfated ceria-zirconia, H2SO4 promoted
silica gel, SZ and so on [262-267]. Tyagi et al. [26] prepared nanocrystalline SZ catalysts by one-step as well as two-step sol-gel
techniques and exploited for synthesis of 7-substituted 4-methyl
coumarins under different conditions such as increasing phenol to
substrate weight ratio, solvent-free/nitrobenzene or toluene as
solvent, and thermal/microwave irradiation. 7-Amino-4-methyl
coumarin was synthesized from the reaction of m-aminophenol with
O
NH2
O
O
NH +
+
SZ, r.t.
solvent-free
45 min
N
100%
O
O
N
H
0%
(constitution of products)
Scheme 43.
Current Organic Chemistry, 2011, Vol. 15, No. 23 3975
Green and Heterogeneous Catalysts for Organic Synthesis
R
O
+
HO
OH
O
R1
OEt
R2
R1 = CH3, CH2Cl
R2 = H, Cl
R = H, OH
SZ, 80 0C or
60-65 0C
24 h, solvent-free or
small amount of EtOH
HO
O
O
R2
R
R1
Yield = 52-93%
Scheme 45.
ethyl acetoacetate, and 7-hydroxy 4-methyl coumarin from the
reaction of m-hydroxyphenol (resorcinol) with ethyl acetoacetate.
Amino- derivative was formed in excellent yield (~100%) and
selectivity (~100%) in solvent-free condition and also in the
presence of nitrobenzene solvent. Reaction was very fast in solventfree conditions and the complete conversion was attained at
comparatively lower temperature within few minutes than with
nitrobenzene. The solvent-free microwave-assisted synthesis was
found to be the most suitable way to synthesize the hydroxy
derivative giving excellent yields at lower temperatures and in
much lesser times as compared to thermal heating. In another study,
Tyagi et al. [268] used nanocrystalline SZ catalyst for microwaveassisted solvent-free synthesis of hydroxy derivatives of 4-methyl
coumarins by Pechmann reaction. The nanocrystalline catalyst
showed good activity for activated resorcinol substrates, such as
phloroglucinol and pyrrogallol with ethyl acetoacetate for the
synthesis of 5,7-dihydroxy-4-methyl coumarin and 7,8-dihydroxy4-methyl coumarin, respectively, showing significant yields (7885%) within short reaction times (5-20 min) at 130 °C. On the other
hand, phenol and m-methylphenol were found to be inactive for the
synthesis of 4-methylcoumarin and 4,7-dimethylcoumarin
respectively, under the same experimental conditions. Apart from
these, Rodríguez-Domínguez and Kirsch [269] also used small
amounts of SZ catalyst (1wt.%) for the synthesis of
hydroxycoumarins via Pechmann reaction without solvent or in
some cases using a small amount of ethanol with rapid stirring for
24 h at 80 °C or 60-65 °C, and synthesized various coumarins
(Scheme 45) in moderate to good yields.
4.20. Synthesis of -Amino-,-Unsaturated Ketones and
Esters
-Amino-,-unsaturated esters and ketones find synthetic
importance, particularly in the construction of heterocyclic
compounds such as dihydropyridines, pyridines, pyrimidines,
indoles and isothiazoles. Different methods for the synthesis of amino-,-unsaturated esters and ketones have been reported in the
literature [270,271]. The condensation of 1,3-dicarbonyl
compounds with amines is one of the most simple and
straightforward synthetic routes. This transformation has been
catalyzed by variety of catalysts such as HCl, H2SO4, p-TSA,
trimethylsilyl trifluoromethanesulfonate (TMSTf), Montmorillonite
K-10, I2, BF3-OEt2, Al2O3, silica gel, Zn(ClO4)2·6H2O,
CeCl3·7H2O, NaAuCl4, Bi(OTf)3, natural clays and so on. Zhang
and Song [272] exploited SZ catalyst for synthesis of -amino-,unsaturated esters and ketones from 1,3-dicarbonyl compounds and
amines under solvent-free conditions at room temperature (Scheme
O
O
R1
R1
R2
R3
R2
+ H2N
R
SZ, r.t.
10-360 min
= CH3, Ph
R = Alkyl, Aryl
=H
R3 = OMe, OEt, CH3
Scheme 46.
R
R1
NH
46). Initially, they investigated the reaction between 2-bromoaniline
and acetylacetone in order to optimize the reaction conditions. It
was found that without catalyst after 24 h, 90% of 2-bromoaniline
was recovered from the reaction mixture. On the other hand, using
SZ as catalyst -enaminone product has been isolated with 86%
yield under the same reaction conditions. Various amines (primary,
benzylic and aromatic amines) reacted with acetylacetone
effectively to afford the corresponding -enaminone in good to
better yields (78-95%) within short reaction times (10-360 min).
4.21. Mannich-Type Reaction
As discussed earlier, carbon-carbon bond forming reactions
continue to be the central focus of research in synthetic organic
chemistry. One of the important carbon-carbon bond forming
reactions is the Mannich-type reaction of ketene silyl acetals and
aldimines to produce corresponding -aminoesters in a single step
which is of considerable importance for synthesizing biologically
active molecules containing nitrogen atom [272-275]. Wang et al.
[276] extensively studied this reaction by using SZ catalysts. To
optimize the reaction conditions, the Mannich-type reaction
between aldimine 48 and ketene silyl acetal 49 (Scheme 47) was
studied in different solvents and by varying the quantity of catalyst.
It was found that the reaction proceeded smoothly when 50 or 100
wt.% SZ in CH3CN at room temperature (25 °C) was used; with
150 wt.% the reaction time was shortened (from 20 to 5 h) to give
the -aminoester 50 in high yield, and CH3CN was found to be
superior to other solvents (ethanol and toluene). By using these
excellent reaction conditions (i.e., catalyst: 150 wt.%, solvent:
acetonitrile, reaction time: 5 h), they explored the Mannich-type
reaction of various ketene silyl acetals with a range of aldimines to
give the corresponding -aminoesters in good to excellent yields.
OMe
SZ
+
OMe
N
Ph
PMP
OTMS
O
25 0C Ph
49
48
NH
OMe
50
Scheme 47.
4.22. Bamberger Rearrangement
Bamberger rearrangement of phenylhydroxylamine (PHA) to paminophenol (PAP) was investigated at 80 °C, with water as
solvent, employing various solid acid catalysts such as -zeolite,
Montmorillonite K10 clay, sulfonated silica and SZ [277]. Both
activity and selectivity were affected by the choice of the catalyst.
NH2
NHOH
O
R3
R
R2
Yield = 78-95%
SZ
R = H, 3-OH, 3-NH2
Scheme 48.
R
80 0C
OH
3976 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
The selectivity for PAP was found to be 17% for K10 clay,70% for
sulfonated silica, 84% for -zeolite and >90% for SZ respectively.
The SZ catalyst calcined at 650 °C was found to be the promising
catalyst for this reaction. Also, it showed good activity and
selectivity for Bamberger rearrangement of substituted
phenylhydroxylamines (Scheme 48).
of triazenes [284], cyclization of citronellal to isopulegol [285],
rearrangement of allyl-2,4-di-tert-butylphenyl ether to 6-allyl-2,4di-tert-butylphenol [286], acetylation of benzo crown ethers [287],
synthesis of conjugated nitroalkenes [288] and isomerization and
arylation of oleic acid [289]. Interested readers are advised to see
these original publications for more details.
4.23. Synthesis of Dypnone from Acetophenone
5. REACTIONS WITH TUNGSTATED ZIRCONIA (WZ)
Dypnone is a highly useful intermediate for the production of a
large range of compounds such as softening agents, plastisizers and
perfumery bases. Venkatesan and Singh [278] investigated the
synthesis of dypnone from acetophenone by utilizing SZ catalyst.
They initially compared the activity of several acid catalysts,
namely SO42/TiO2, H-, AlCl3 and SZ for dypnone synthesis and it
was found that SZ exhibits highest acetophenone conversion (40
wt.%) and highest selectivity for dypnone (86%) among various
catalysts investigated (Scheme 49). Latter on, they studied the
effect of SZ concentration and reaction temperature. The
conversion of acetophenone was found to increase noticeably up to
2 h of reaction time with an increase of SZ/acetophenone ratio from
0.05 to 0.15. However, for longer reaction times (5 h), the
conversion of acetophenone was same at all catalyst concentrations.
Also, they studied the effect of reaction temperature on the
conversion of acetophenone. The reaction temperature was varied
from 403 to 443 K. With increasing the reaction temperature the
conversion increased.
The WZ is an excellent and environmentally benign solid acid
catalyst. There are many reports in the literature on WZ catalysts
dealing with its preparation, characterization and catalytic acidity
[290-293]. The WZ catalyst has been mostly employed for various
vapour phase reactions, namely, tert-butylation of p-cresol [294],
production of biodiesel [295] and isomerization [38,39]. This
catalyst also exhibits excellent activity for various liquid phase
organic reactions as compiled in this section. A major advantage
associated with this catalyst is that it is highly stable and does not
undergo deactivation unlike SZ where sulfate loss during the course
of reaction is normally expected under severe heat and reducing
conditions.
Acetylation of alcohols, phenols and amines was carried out by
using WZ catalyst and acetic anhydride as the acylating agent to
give O-acylated products in case of alcohols and phenols (Scheme
51), and N-acylated products in case of amines (Scheme 52) [35]. In
a typical reaction procedure a mixture of 1:2 molar amounts of
alcohol and acetic anhydride along with the catalyst (0.4 g) were
taken in a round bottom flask and refluxed for required times.
Reaction took place efficiently providing excellent yields (89-98%)
of products within short reaction times (1-4 h). In another study,
Sakthivel et al. [36] prepared WZ catalysts with 5, 10, 15, 19 and
25 wt.% WO3 and exploited for Friedel-Crafts acetylation of
anisole with acetic anhydride to make 2- and 4methoxyacetophenone. In this study, it was found that catalyst
having 19 wt.% WO3 is the best among studied catalysts with
regard to conversion and selectivity. Bordoloi et al. [37] employed
different WZ catalysts (having different WO3 loading 5-30 wt.%)
for acetylation of veratrole with acetic anhydride to produce 3,4dimethoxy acetophenone and toluene alkylation with 1-dodecene.
The catalyst with 15 wt.% WO3 calcined at 800 °C was found to be
the most active in acylation and alkylation reactions. This most
active catalyst showed 67% acetic anhydride conversion in
veratrole acylation (veratrole/acetic anhydride molar ratio 2 and 4 h
time) at 70 °C, and 99% dodecene conversion with >99%
monododecyl toluene selectivity at 100 °C (toluene/1-dodecene
molar ratio 10 and time 1 h). The advantages of WZ catalyst are
easy operation and simplicity in the work-up, which involves mere
filtration of the catalyst, and its reusability. Same groups reported
[296] the alkylation of phenol with 1-octene, 1-decene and 1dodecene employing WZ catalyst. As in alkylation and acetylation
reactions, WZ catalyst with 15 wt.% WO3 calcined at 800 °C
exhibited a maximum conversion at 70 °C reaction temperature.
The selectivity to phenyldodecyl ether was high at low reaction
temperatures (51% at 70 °C) and decreased with an increase in
temperature and totally disappeared at 120 °C.
O
O
CH3
2
CH3
SZ
+ H2O
H
140 0C
Scheme 49.
4.24. Cyanosilylation of Aldehydes
Thirupathi et al. [111] employed solid acid catalysts, namely
SZ, MZ and WZ for cyanosilylation of aldehydes with TMSCN.
Initially, to find out better catalyst and suitable conditions, they
studied reaction between benzaldehyde and TMSCN using different
solvents and different amounts of catalysts. The SZ catalyst, under
solvent-free condition at room temperature was found to be best for
this reaction. By using the optimized conditions, they performed the
reaction by using variety of aldehydes with TMSCN to synthesize
corresponding cyanohydrin silyl ethers (Scheme 50). Most of the
reactions proceeded smoothly under these mild conditions and
produced cyanohydrin silyl ethers in good to excellent yields within
short reaction times. Also, they carried out mechanistic study of this
methodology by using XPS and FTIR techniques.
O
OTMS
H
SZ, TMSCN
R
H
sovent-free
R = Aryl, Heteroaromatic etc. N2, r.t.
50-300 min
R
CN
Yield = 75-96%
Scheme 50.
There are some more examples of synthesis and transformation
reactions catalyzed by SZ in the literature exhibiting moderate to
good catalytic activity, which include sulfonylamidomethylation of
benzylsulfonamides and 2-phenylethanesulfonamides [113],
synthesis of N,N’-diphenylenediamines [279], cyclization of 1phenyl-2-propen-1-ones into 1-indanones [280], nitration of
chlorobenzene [281], dehydration of fructose to 5hydroxymethylfurfural [282], Koch carbonylation [283], synthesis
R
OH + Ac2O
WZ
2-4 h
R
OAc + AcOH
Yield = 89-97%
Scheme 51.
R
NH2 + Ac2O
WZ
R = Benzyl, Cyclohexyl
Scheme 52.
1h
R
NHAc + AcOH
Yield = 95-98%
Current Organic Chemistry, 2011, Vol. 15, No. 23 3977
Green and Heterogeneous Catalysts for Organic Synthesis
OH
O
HO
O
+
R
O
O
WZ, Toluene
OEt
OH
110 0C, 5 h
R
R = CH3, OH
CH3
Yield = 50-80%
Scheme 53.
NH2
O
Ar
CHO +
+
WZ
R
CH3
R = Ph or CH3
NH
r.t., N2
solvent-free
O
Ar
R
Scheme 54.
Ar'
O
Ar
CHO + Ar'
Ar'
O
Ar
WZ
NH2 +
NH
+
r.t., N2
solvent-free
NH
O
Ar
anti
syn
Scheme 55.
Ramu et al. [297] reported the esterification of palmitic acid
with methanol using WZ catalysts. They investigated a series of
WZ catalysts containing 2.5 to 25 wt.% WO3 and found that the
catalyst with 5 wt.% loading shows maximum catalytic activity.
This is a good example of the utility of WZ catalyst in the liquid
phase large-volume applications apart from its several vapour phase
catalytic utilities. Reddy et al. [298] reported the synthesis of
substituted coumarins from resorcinol and substituted resorcinol
with ethyl acetoacetate and ethyl -methylacetoacetate by
employing WZ catalyst resulting in good yields (50-80%) at 110 °C
in toluene solvent within 5 h (Scheme 53). In another study, Reddy
et al. [299] exploited WZ catalysts for multicomponent Mannich
reaction between various aromatic aldehydes, amines and ketones at
room temperature under solvent-free conditions (Schemes 54 and
55). Various Mannich adducts were formed in good yields (6690%) in short reaction times (1-8 h).
Apart from all the applications discussed with WZ catalysts in
liquid phase, there are some more examples in the literature on
synthesis and transformation reactions catalyzed by WZ which
include esterification of oleic acid with methanol, Beckmann
rearrangement of cyclohexanone, synthesis of aryl-14Hdibenzo[a.j]xanthenes by one-pot condensation of -naphthol and
aryl aldehydes, etc [300-303].
6. REACTIONS WITH MOLYBDATED ZIRCONIA (MZ)
The MZ is another very promising modified zirconia solid acid
catalyst. Lot of research work is going on over this catalyst as an
alternative to SZ catalyst. Mostly, it is used in the vapour phase
reactions. Very little work is done by using this catalyst in the
liquid phase. Reddy and Reddy [33] reported the synthesis of
substituted diphenylureas in the presence of MZ catalyst (Scheme
56). Various substituted diphenylureas were synthesized in good
yields (60-75%) from substituted aniline and ethyl acetoacetate
under reflux conditions at 180 °C. In another study, Reddy et al.
[304] carried out the esterification of -ketoester with various
alcohols employing the MZ catalyst. This reaction was carried out
by using variety of alcohols (aliphatic, unsaturated, aromatic and
hetero aromatic) at 110 °C under reflux conditions with toluene as
the solvent and excellent product yields were obtained (43-98%).
After completion, the reaction mixture was separated and the wet
catalyst was reused. No appreciable change in the activity was
observed in several cycles. Manohar et al. [34] reported
esterification of mono- and dicarboxylic acids employing the true
eco-friendly MZ catalyst under reflux conditions for 1-4 h (Scheme
57). The MZ catalyst exhibited excellent product yields for various
esterification reactions. Further, Reddy et al. [305] prepared Ptpromoted TZ catalyst and utilized it for selective protection of
NH2
O
O
+
H
N
MZ
OMe
O
180 0C, 6 h
R
H
N
R
Yield = 60-75%
Scheme 56.
O
R1
OH
R1 = CH3, Ph
Scheme 57.
R2-OH
MZ
O
R1
OR2
; HOOC
R2 = Bu, Pr etc.
n
COOH
n = 1,2
R-OH
MZ
n
ROOC
R = CH3
COOR
3978 Current Organic Chemistry, 2011, Vol. 15, No. 23
Patil et al.
carbonyl compounds Variety of carbonyl compounds with ethane1,2-diol in the presence of eco-friendly Pt-Mo/ZrO2 solid acid
catalyst provided corresponding 1,3-dioxolanes in excellent yields
within 6 h.
7. REACTIONS
CATALYSTS
WITH
OTHER
ZIRCONIA-BASED
In order to obtain better catalysts than SZ, to increase the
tetragonal phase of ZrO2 and the stability of catalyst, are lots of
works in the literature and several investigations are still going on.
Various modified zirconia catalysts and promoted SZ catalysts were
prepared, characterized and exploited for various organic synthesis
and transformation reactions. Recently, Chen et al. [306] utilized
SBA-15 supported SZ catalyst for regioselective oxybromination.
Yadav and co-workers reported novel mesoporous superacidic
zirconia-based catalysts, namely, UDCaT-4, UDCaT-5 and
UDCaT-6 and their modified versions which exhibited high
catalytic activity, stability and reusability. They studied alkylation
of m-cresol with tert-butanol, alkylation of mesitylene with tertbutanol and alkylation of o-cresol with iso-propanol using UDCaT4, UDCaT-5 and UDCaT-6 catalysts [307-309], and acylation of
1,4-dimethoxybenzene with acetic anhydride using UDCaT-5 and
other solid acids [225]. Devulapelli and Weng [64] prepared SZ
catalyst by a conversional method and mesoporous-SZ by using
CTAB as template and compared the catalytic activity for
esterification of 4-methoxyphenylacetic acid with dimethyl
carbonate. Zhao et al. [88] synthesized SZ catalyst supported on
mesostructured -Al2O3 and studied Friedel-Crafts benzoylation of
anisole with benzoyl chloride and dealkylation of 1,3,5-tri-tertbutyl-benzene. Also a comparison is made between SZ and MCM41 supported SZ catalysts. Reddy et al. [78,310] prepared sulfated
CexZr1-xO2 solid acid catalyst by a co-precipitation method followed
by sulfate impregnation using H2SO4 and utilized this catalyst for
solvent-free synthesis of coumarins via Pechmann reaction and
multicomponent Mannich reaction between aldehyde, ketone and
amines. Very recently, Reddy et al. [311] reported for the first time
the original use of solid acid catalysts (various zirconia-based
catalysts) in regioselective organic synthesis for the formation of
NC bond (-aminoalcohols) (Schemes 58 and 59) and CC bond
(Friedel-Crafts alkylation) (Scheme 60) by using epoxides and
anilines/indoles, and water as the reaction medium. Among various
solid acid catalysts, the TZ mixed oxide catalyst exhibited better
activity with excellent product yields (7596%) towards the
regioenriched desired product.
In another study, they prepared sulfate, molybdate and tungstate
ion promoted TZ solid acid catalysts by co-precipitation followed
by impregnation with sulfuric acid, ammonium heptamolybdate and
ammonium
metatungstate
precursors.
Physico-chemical
characterization of the prepared catalysts was achieved by using
XRD, BET surface area, FT-Raman and XPS methods [79].
Molybdate ion promoted TZ showed good catalytic activity for the
reaction of variety of aromatic and aliphatic aldehydes with acetic
anhydride to produce corresponding 1,1-diacetates [312]. In the
R1
O
Scheme 60.
R2
O +
R3
R4
N
H
OH
TZ, r.t.
water
N
R2
R1
Scheme 58.
R1
O
R2
R3
R6
+ HN
R4
R5
R1 OH
TZ
R3
+
water, r.t. R2
R5
4
N R
6
R
R1 HO R3
R2
R4
N
R5 R6
Scheme 59.
8. CONCLUDING REMARKS
In spite of the expected drawbacks of the SZ catalyst in terms of
deactivation due to sulfate loss, crystalline-phase transformation
from tetragonal to monoclinic, and coke formation, the SZ catalyst
has received tremendous interest in recent times due to its
simplicity, versatility and superior performance for various organic
synthesis and transformation reactions as elaborated in this review.
Promoted SZ catalysts have shown good catalytic activity or
sometimes better than SZ. Catalytic activity of these catalysts
depends on the method of preparation, precursors used, nature of
promoting agents, calcination temperature etc. All these solid acid
catalysts (mainly SZ) are useful for variety of organic reactions
including MCRs, condensation reactions, isomerization reactions,
esterification, trans-esterification and so on. These solid acid
catalysts having strong incentives to which one can replace the
unfriendly H2SO4 and HF acids in many industrial processes, and in
this direction there is a lot of scope and advantage to work. Many
efforts were undertaken in the last decades on the catalytic activity
of SZ and related catalysts, but still there is tremendous scope to
study and exploit these catalysts for numerous reactions.
ACKNOWLEDGEMENTS
We wish to acknowledge all the researchers whose work is
described in this review for their valuable contributions. A.N.P. is
the recipient of the junior research fellowship of CSIR, New Delhi.
M.K.P thanks to UGC, New Delhi for a research project [No: 39727/2010(SR)].
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Accepted: 27 April, 2011