Applied Clay Science 53 (2011) 106–138
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Applied Clay Science
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Review Article
Organic synthesis using clay and clay-supported catalysts
Gopalpur Nagendrappa ⁎,1
Department of Chemistry, Bangalore University, Bangalore 560 001, India
a r t i c l e
i n f o
Article history:
Received 20 May 2010
Received in revised form 17 October 2010
Accepted 19 October 2010
Available online 6 October 2010
Keywords:
Clay mineral
Activated bentonite
Montmorillonite
Saponite
Organic synthesis
Heterogeneous catalyst
a b s t r a c t
Clays and modified clays are used to catalyze various types of organic reactions such as addition, Michael
addition, carbene addition and insertion, hydrogenation, allylation, alkylation, acylation, pericyclic reactions,
condensation reactions, aldol formation, imine synthesis, diazotization reactions, synthesis of heterocycles,
esterification reactions, rearrangement/isomerization reactions, cyclization reactions, oxidation of alcohols,
dehydrogenation, epoxidation and several more. Clays function as Brønsted and/or Lewis acids, or as bases.
Clays with combined acidic and basic properties have been developed by simple procedures of modification.
Such clays are employed to catalyze a sequence of acid and base-catalyzed reactions in one pot. Good
enantioselectivity and stereoselectivity are achieved using chiral organic compounds and chiral complexes
intercalated between clay layers. Examples from recent literature are described here.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Clays are widespread, easily available and low-cost chemical
substances. Both in their native state and in numerous modified
forms, clays are versatile materials that catalyze a variety of chemical
reactions. Just as they can be molded into any shape, their micro
structure can be changed to suit chemists' needs to promote diverse
chemical reactions. It is convincingly argued that clays initiated,
supported and sustained the process of formation of small molecules
on the earth millions of years ago, which gradually developed into
more complex molecules. In the course of time, there emerged from
the latter the self replicating assemblies that evolved into simple life
forms and progressed to the present elaborate living world of plants
and animals (Saladino et al., 2004; Stern and Jedrzejas, 2008; Ciciriello
et al., 2009).
Clays are nanoparticles with layered structures. The layers possess
net negative charge that is neutralized by cations such as Na+, K+, Ca2+,
etc., which occupy the interlamellar space. The amazing amenability of
clays for modification lies in the fact that these interlamellar cations can
be very easily replaced by other cations or other molecules. Molecules
can be covalently anchored to layer atoms. All this can be done by very
simple procedures. This provides tremendous scope for altering the
properties of clays like acidity, pore size, surface area, polarity and other
characteristics that govern their performance as catalysts. Because of
these wide ranging possibilities, in addition to their environmental
⁎ Permanent address: #13, Basappa Layout, Gavipuram Extension, Bangalore-560019,
India. Tel.: +91 80 26670899.
E-mail address: [email protected].
1
Retired from the organization.
0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.clay.2010.09.016
compatibility and cheapness, much effort is expended in discovering
newer methods of using clays in their native and modified forms as
catalysts for diverse organic reactions.
Clays have a long history of use as catalysts and as supports in organic
reactions (Vogels et al., 2005). Several excellent reviews on clay
catalyzed organic reactions have appeared in the recent past (Varma,
2002; Dasgupta and Török, 2008; Ranu and Chattopadhyay, 2009). Zhou
(2010) has briefly summarized the emerging trends in synthetic claybased materials. The present review attempts to report some of the
developments that have taken place in the area of organic synthesis
using clays and clay-supported catalysts during the past decade.
Much of the work on clays focus on the use of “normal” smectites,
mostly the commercially available K10 and KSF or native varieties with
Brønsted or Lewis acid sites and enhancing their catalytic performance
by pillaring techniques to manipulate the pore size, surface area and
stability or replace interlayer cations to alter acid-base properties (Singh
et al., 2007; Moronta et al., 2008). Clays have been intercalated with a
variety of inorganic and organic ions, metal complexes, and organic
compounds. These have brought about radical changes in the
performance of clays in terms of increasing the rates of reactions, yields,
product selectivity, and stereoselectivity including enantioselectivity.
Clays have been modified to act as acid-base combination catalysts
which have been employed to carry out acid and base-catalyzed
reactions in a sequence in one pot (Motokura et al., 2005, 2009). The
possibilities seem to be limited only to the power of our imagination to
modify clays for any reaction.
The review describes seven types of organic reactions in as many
sections—Addition reactions, Condensation reactions, Diels–Alder and
related reactions, Esterification reactions, Friedel–Crafts and related
reactions, Isomerization reactions, and Oxidation reactions. It should be
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
107
Scheme 1. Addition of allylsilanes to aromatic and aliphatic alkenes.
noted that the literature covered is essentially from articles in
mainstream journals published between 2000 and 2010, with a few
exceptions; the patent literature is completely omitted. As a result the
review is not exhaustive; some aspects and several types of reactions are
left out for various reasons.
2. Addition reactions
In this section examples of addition reactions leading to carbon–
carbon and carbon–heteroatom bonds are considered. In the past ten
years several groups of workers have reported a variety of addition
reactions efficiently facilitated by montmorillonite clays. They include
addition of allylsilanes to C=C and C=O bonds, carbene addition,
epoxidation, Michael addition, etc. Some of them are considered here.
Motokura et al. (2010) have found excellent catalytic performance
by proton-exchanged montmorillonite in the addition of allylsilanes
to aromatic and aliphatic alkenes (Scheme 1).
The mechanism has been studied in detail, a summary of which is
presented in Scheme 2.
Activated montmorillonite K10 clay was found to catalyze the
reaction of allyl trimethylsilane with aromatic aldehydes to give
homoallylic silyl ethers (Dintzner et al., 2009). The authors suggest
that the reaction proceeds through a cyclic transition state in the
interlamellar space of the catalyst and is helped by its Lewis acid
character. This modified Hosomi–Sakurai reaction is environment
friendly and delivers protected homoallylic ethers due to six membered
pericyclic transition state of ene reaction (Scheme 3).
The reaction with aliphatic aldehydes and ketones was successful
in some cases, which are given below, with yields of the homoallylic
ether products mentioned in the parentheses.
O
CHO
O
CHO
(70%)
(90%)
(47%)
CHO
(61%)
CHO
(16%)
(25%)
Allylation of ketones and aldehydes has been carried out using
potassium salts of allyl- and crotyltrifluoroborates using borontrifluoride
etherate or montmorillonite K10 catalyst (Nowrouzi et al., 2009)
(Scheme 4). The authors find that K10 clay catalyzed reactions are robust,
straightforward and easy to work up, and scalable, which means the K10
catalyzed reaction is far superior to the Lewis acid-catalyzed one.
Other supports like alumina, silica gel and charcoal proved to be inferior.
The yields are generally excellent. In each case one stereoisomer is
Scheme 2. Mechanism of addition of allylsilanes to aromatic and aliphatic alkenes.
Scheme 3. Reaction of allyl trimethylsilane with aromatic aldehydes.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 4. Allylation of ketones and aldehydes using potassium salts of allyl- and crotyltrifluoroborates.
Scheme 5. Addition of aniline derivatives to cinnamaldehyde.
predominantly more than the other. The K10 catalyzed reactions are
better stereoregulated with the diastereomeric ratio being greater than in
the case of BF3.OEt2 catalyzed reactions.
Montmorillonite K10 clay catalyzes the addition of aniline derivatives to cinnamaldehyde in a Michael fashion as the first step of a
domino process involving cyclization in the second step followed by
dehydration and oxidation in the final step to deliver quinolines in good
to excellent yields (De Paolis et al., 2009) (Scheme 5). The reaction is
carried out under solvent-free condition and with the assistance of
microwave radiation.
A three-component reaction of enaminones, β-ketoesters/1,3diketones and ammonium acetate takes place under the catalytic
Scheme 6. Reaction of enaminones, β-ketoesters and ammonium acetate to form pyridines.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
109
Scheme 10. Addition of carbenes to C=N double bonds to produce aziridines.
Scheme 7. Synthesis of ethers by the addition of alcohols to olefins.
influence of montmorillonite K10 clay in refluxing isopropyl alcohol to
produce tri-substituted pyridines in good yields (Reddy et al., 2005c)
(Scheme 6). The reaction starts with the initial Michael addition of
NH3 (from NH4OAc) to enaminone, followed by two condensation
reactions, and elimination of NMe2 to the final product.
Synthesis of ethers by the addition of alcohols to olefins is an
important reaction catalyzed by Brønsted acids. The problem, however,
in such reactions is the possibility of extensive isomerization of the
double bond through the intermediate carbocation. Wang and Guin
(2002) have found that sulphuric acid-treated montmorillonite clay is a
highly active catalyst for bringing about addition of methanol to 2,3dimethyl-1-butene, and that it is far more selective to addition than
sulphated zirconia, Nafion, Amberlyst-15 or bentonite clay, which cause
isomerization (Scheme 7). The activity of the catalyst depended on the
sulphuric acid content in the clay.
Dintzner et al. (2006) have suggested an undergraduate green
experiment on the synthesis of a natural insecticide, methylenedioxyprecocene (MDP), with anti-juvenile hormone activity. It is a reaction
of sesamol with 3-methyl-2-butenol catalyzed by basic montmorillonite
K10–K+ (prepared by washing K10 with K2CO3 solution), and assisted
by microwave irradiation under solvent-free condition. The mechanism
involves initial addition of the deprotonated sesamol to enal, the
dehydration of the intermediate and finally an intramolecular heteroDiels–Alder reaction (Scheme 8).
Addition of carbenes to carbon–carbon double bonds to produce
cyclopropanes is a well known reaction. Fraile et al. (2004) have
studied the scope and limitations of the stereochemical course of the
addition of alkoxycarbonyl carbene generated from the corresponding
diazoacetic esters. The reactions were carried out in the presence of
chiral bis(oxazaline)-copper (Box–Cu(II)) complexes supported on
laponite clay-like solid and Nafion-like solid (Scheme 9). The
suitability of the supported catalyst system is influenced by a variety
of factors. Better stereochemical control was possible on the laponitesupported catalyst, compared to other supported catalysts studied.
However, supported catalysts were inferior to the free chiral catalyst
in terms of yields and stereochemical control, though they could be
reused.
The enantiomeric excess depended on the alkyl group of the
diazoester, and it was 83% in the case of menthyl ester, while in the
case of n-butyl ester it was 74%. The phenyl substituted (R = Ph) Box
was a better catalyst than the t-butyl substituted one.
Borkin et al. (2010) have prepared cis-aziridines in high diastereoselectivity (N99%) and excellent yields (82–91%) by reacting Schiff
bases with ethyl diazoacetate in the presence of montmorillonite K10
as catalyst at room temperature for 2 h (Scheme 10). The K10 was the
best catalyst among the several other acid catalysts, such as
H4W12SiO40, Nafion-H, Amberlist-15, and Nafion-H on silicon, in
achieving the highest diastereoselectivity. However, reactions conducted using Nafion-H gave better yields of products (mixtures of cisand trans-aziridines). The efficiency of the catalyst remained the same
even after it was reused three times.
Box–Cu complexes (and Cu-complexes of three other ligands)
immobilized in laponite clay are able to efficiently catalyze the
insertion of carbene formed from methyl phenyldiazoacetate into C–H
bond of THF at the 2-position with high enantioselectivity (up to 88%
ee). The immobilization not only allows recovery and reuse of the
chiral catalyst, but also provides an improvement in selectivity over
the results obtained in solution, probably due to a confinement effect
of the bidimentional support (Fraile et al., 2007) (Scheme 11).
Scheme 8. Synthesis of methylenedioxyprecocene (MDP), a natural insecticide.
Scheme 9. Addition of carbenes to C=C double bonds to produce cyclopropanes.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 11. Insertion of carbene into C–H bond.
An Iranian natural bentonite modified by N-alkylated 1,4diazabicyclo[2,2,2]octane quaternary salt was found to act as a good
catalyst in triphase reactions consisting of the solid catalyst and the
aqueous and organic phases (Ghiaci et al., 2005). When a mixture of
cyclooctene, the catalyst, chloroform and an aqueous solution of
sodium hydroxide was refluxed, dichlorocarbene was generated,
which added to the cyclooctene present in the reaction mixture,
forming dichlorobicyclo[6.1.0]nonane as the product in almost
quantitative yield (Scheme 12).
The triphase systems (water-petroleum ether-catalyst) with water
soluble nucleophiles work well for nucleophilic substitution reactions
also (Scheme 13).
Triazenes have been synthesized (Scheme 14) by adding paminobenzene-1-sulfonyl azide or amide to a cold mixture of sodium
nitrite and acid-treated clay (K10, bentonite, or kaolin), followed by a
cyclic secondary amine, which adds to the diazo intermediate formed
in the previous step (Dabbagh et al., 2007). The yields are moderate in
all the cases and are similar to the yields obtained using HZSM-5 and
sulfated zirconia. The mechanism does not involve formation of the
conventional diazonium salt intermediate. Instead it consists of initial
interaction of nitrite with protonated silicate of clay followed by a
series of nucleophilic addition and elimination processes. (The
Scheme 12. Addition of dichlorocarbene in triphase system.
Scheme 13. Nucleophilic substitution in triphase system.
proposed mechanism is a bit elaborate; the interested reader may
refer to the original paper).
Dithiocarbamic acid A adds to arylideneoxazalones B to form the
Michael adducts C which undergo cyclization to 1,3-thiazinan
derivatives D. The reactions are carried out by microwave irradiation
of the reactants adsorbed on montmorillonite K10, basic and neutral
alumina, and silica gel. The best yields (76–91%) were obtained in the
shortest reaction times from the reactions performed on K10 clay.
Reactions carried out on other solid catalysts were inferior. Microwave irradiation without using the adsorbent K10 clay was found to
be ineffective, demonstrating the significant role of clay for the
success of the reactions (Siddiqui et al., 2010) (Scheme 15).
3. Condensation reactions
Carbon–carbon bond forming reactions are of primary importance in
organic synthesis. Among the numerous procedures developed for this
purpose aldolization/aldol condensation occupies an important position. The aldol reaction is catalyzed by acids as well as bases. A base is the
preferred catalyst to obtain aldol. Because of the importance of these
reactions attention is being paid to develop environment friendly
procedures using heterogeneous clay catalysts.
Hydrotalcites (HT) as solid base catalysts have been successfully
used in bringing about the aldol reactions. For example, Roelofs et al.
(2000) have reported the self condensation of acetone (1) to give
aldol 2 and cross condensation of acetone with citral (3) on modified
hydrotalcite catalysts at 0 °C (Scheme 16). The catalyst shows high
activity and a small amount (5%) of it is enough to effect the aldol
formation. The cross aldol (4) is formed with very high selectivity.
These reactions are 100% atom economic and are examples for good
“green” procedures.
Acetaldehyde (5) condenses with heptanal (6) in the presence of
hydrotalcite-type catalysts to give cross condensation product nonenal (7) in ethanol at 100 °C (Tichit et al., 2003) (Scheme 17). If the
Brønsted base strength is increased due to a higher proportion of MgO
or hydrated sites, the reaction takes a different course through the
intermediate α-anion of heptanal (10) to give 8 and 9.
In an interesting study an acid-layered clay was combined with a
basic layered clay to bring about sequential acid and base-catalyzed
reactions in one pot. Motokura et al. (2005) mixed Ti4+ intercalated
montmorillonite with surface tunable basic hydrotalcites. The acid-
Scheme 14. Triazenes from p-aminobenzene-1-sulfonyl azide.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
111
Scheme 15. 1,3-Thiazinans from dithiocarbamic acid and arylideneoxazalones.
Scheme 16. Aldol reaction of acetone with itself and citral.
base catalyst combination acts sequentially in multistep reactions
that require both acid and base catalysis. The authors report the
following multistep reactions, (i) deacetalization–aldol condensation,
Michael addition–acetalization, (ii) esterification–aldol condensation–
epoxidation, and (iii) esterification–aldol condensation–Michael addition. Each one of these combined three-reaction sequences has been
performed in one pot resulting in excellent yields. The aldol condensation reactions are initiated by deacetalization effected by Ti4+-mont,
followed by aldol condensation catalyzed by basic hydrotalcite. The
reaction sequences are depicted in Scheme 18.
The next four reactions start with Michael addition catalyzed by
HT, followed by acetal formation catalyzed by Ti4+-montmorillonite.
In the last reaction sequence, the first reaction is acid-catalyzed
esterification, the second is acid-catalyzed deacetalization, followed
immediately by base-catalyzed aldol condensation. The olefinic
product 11 is then epoxidized to 12 by hydrogen peroxide under
basic catalysis. Or the olefin 11 can be hydrogenated to 13, which
undergoes Michael addition to acrylonitrile in the same pot to give 14.
None of the intermediate products in any reaction depicted in
Scheme 18 was isolated; each step was performed one after the other
in one pot.
Aldolization creates a chiral centre in aldol product. This provides an
opportunity to design enantio-selective synthetic procedures to deliver
the desired enantiomers by employing suitable chiral catalysts. Such
catalysts immobilized in clays through intercalation have shown
excellent results. Srivastava et al. (2009) report using montmorillonite
clay in which the sodium ions have been exchanged with prolium
(protonated proline) by treating the clay with prolium chloride in
methanol. The immobilized prolium acts as a chiral molecular catalyst to
induce chirality in the newly formed asymmetric carbon of the aldol
product 15 (Scheme 19). They found that intercalated hydroxyproline
behaves in a similar manner. They further report that pillaring the
prolium embedded (Pro-Mont) clay with trimethylbutylammonium
bistriflimide leads to significantly higher yields and enantiomeric
excesses, and that the solvents too have influence on the yields and
enantiomeric excesses.
Montmorillonite K10 has proved to be an effective catalyst in bringing
about Mukaiyama crossed aldol condensation of silyl enol ethers with
various aldehydes (Loh and Li, 1999). If the clay is intercalated with a
chiral catalyst an excess of one enantiomer of the aldol is obtained. For
example, Fabra et al. (2008) describe the use of the chiral diphenylbis
(oxazoline)-Cu2+ complex 16 immobilized on laponite clay to bring
about the Mukaiyama aldol reaction in a stereo-controlled manner
(Scheme 20). They found that 2-(trimethylsilyloxy)furan (17) adds to
the α-ketoesters 18 to give the aldols 19a,b with the anti-isomers as
major products (up to 90% ee; dr, 86:14).
However, after surveying several such chiral Cu2+-complex-clay
catalyzed reactions, Fraile et al. (2009) offer a note of caution that the
application of these catalysts is limited despite excellent results in
some cases.
Knoevenagel condensation of malononitrile with carbonyl compounds has been found to be activated by ultrasound and catalyzed by
alkaline-doped saponites. The dopant ions were Li+ and Cs+. The Cs+
doped clay was far superior to the Li+ doped one in providing higher
yields of products (Martin-Aranda et al., 2005) (Scheme 21). Thermal
reactions with the same basic saponites as catalysts but without
ultrasound were inferior.
Montmorillonite K10 catalyzes the condensation of primary mono(20) and diamines (21) with β-hydroxy-β-bis(trifluoromethyl) ketones
(22), obtained by aldol reaction of methyl ketones with hexafluoro
Scheme 17. Aldol condensation of heptanal.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 18. Sequential multiple reactions using combined acid and basic layered clay.
Scheme 19. Enantio-selective synthesis of aldols.
acetone hexahydrate, to give mono- (23) and di-imines (24). The
reaction is conducted in chloroform at 70 °C for 4–5 days. A number of
di-imines have been synthesized which find use in the preparation of
fluorinated chelate complexes for vapour deposition in microelectronics
(Marquet et al., 2008) (Scheme 22).
Motokura et al. (2009) have succeeded in preparing catalysts with
coexisting acidic and basic sites in the montmorillonite clay interlayer
by treating the clay first with hydrochloric acid and then with
triethoxysilylalkyl amines. Silicon of –Si(OEt)3 covalently binds with –
SiO– in clay by displacing an EtO group, and thus the alkyl amine is
immobilized. The acid sites and basic sites act independently to bring
about both the acid-catalyzed deacetalyzation and the base-catalyzed
Knoevenagal condensation reactions in tandem in one pot (Scheme 23).
A three-step reaction of addition of phenylhydrazines to dihydro4-H-pyranone derivatives, followed by ring opening and intramolecular condensation to deliver pyrazines in good yields is promoted by
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
113
Scheme 20. Mukaiyama reaction of silyl enol ethers with aldehydes.
Scheme 21. Knoevenagel condensation of malononitrile with carbonyl compounds.
montmorillonite KSF clay in ethanol solvent. The stereochemistry of
the substituents is retained in the products (Yadav et al., 2004b)
(Scheme 24).
A microwave-assisted montmorillonite KSF catalyzed, solventless
aldol condensation of aromatic aldehydes with aryl methyl ketones
has been developed by Chtourou et al. (2010) (Scheme 25). The
reaction takes just about 1 h for completion. It is highly selective (87–
98% trans chalcones) and the yields are good to excellent (80–95%).
The authors maintain that their method is far more efficient than
other known methods which use a number of acid-treated supports,
alkalies and other catalysts in various solvent media.
In a series of reactions leading to the synthesis of conjugationexpanded carba- and azuliporphyrins (28 and 29) an intermediate
step of condensation of bicyclo[2.2.2]octadiene-fused pyrrole (25)
with bicyclo[2.2.2]octadiene-fused 2-acetoxymethyl-4-t-butoxycarbonylpyrrole (26) was carried out using montmorillonite K10 as
catalyst to give tripyrrane (27) in N90% yield (Okujima et al., 2004)
(Scheme 26).
The Biginelli reaction, which involves a one-pot condensation of βketoesters (30)/β-dicarbonyl compounds (31) with aldehydes (32)
and ureas (33) to give dihydropyrimidones (34), is conventionally
conducted in ethanol with strong protic acid or Lewis acid as catalyst.
Salmon et al. (2001) have shown that the reaction takes place on
commercially available bentonite clay TAFF as catalyst under infrared
irradiation and in the absence of solvent (Scheme 27).
Earlier Li and Bao (2003) had demonstrated that the threecomponent Biginelli reaction can be performed efficiently using
samarium trichloride supported on montmorillonite clay (clay:
SmCl3 = 10:1) as catalyst with microwave irradiation under solventfree condition (Scheme 28). The catalyst was found to be reusable.
Hydrotalcite-like materials have been used by Zhu et al. (2009a) as
catalysts for the condensation of aromatic as well as aliphatic primary
amines with aromatic and aliphatic aldehydes, and cyclohexanone to
obtain Schiff bases (35). The reaction is carried out under solvent-free
condition by stirring together the reactants and the catalyst at room
temperature (Scheme 29). The catalyst is recyclable. The yields in
most cases are excellent. Aldehydes reacted faster than ketones.
Malonic acid undergoes Knoevenagel condensation with salicylic
aldehydes in the presence of montmorillonite KSF clay on refluxing in
water for 24 h. The condensation products initially formed cyclize to
give coumarin-3-carboxylic acids, which are isolated by filtering off
the catalyst, evaporation of the solvent and finally recrystallization.
The selectivity was 95% and the yields were N90% in most cases.
However, if diethyl malonate was used in place of malonic acid the
yields of the coumarin-3-carboxylate esters were only 30–48% (Bigi
Scheme 22. Condensation of primary mono- and diamines with ketones.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 23. Reaction using coexisting acidic and basic sites in tandem.
Scheme 24. Pyrazines from dihydropyranones and phenylhydrazines.
et al., 1999) (Scheme 30). Montmorillonite K10 clay was much less
effective than the KSF as catalyst. Since the Knoevenagel reaction
requires basic conditions and the subsequent cyclization needs
protons for its initiation, the authors presume that the clay catalyst
must be ditopic and hence should contain both acid and basic sites.
The other important feature of the catalyst was that it retained its
activity even after five reaction cycles.
Quinoxalines, which find applications in the area of medicines, dyes,
electron luminescent materials, organic semiconductors and others, are
commonly prepared by the condensation of 1,2-diketones with ophenylene diamines. A variety of catalysts are used, particularly acidic
or oxidizing reagents. In most cases an organic solvent is also used. Huang
et al. (2008) have overcome the eco-disadvantages associated with using
such catalysts and organic solvents by making use of montmorillonite K10
as catalyst in water at room temperature. The reaction takes about 3 h and
gives excellent yields (90–100%; in one case, it was 70%, because the
diamine has an electron withdrawing NO2 substituent) (Scheme 31). The
catalyst can be reused without much loss in its activity.
The authors propose a mechanism suggesting an initial protonation of the diketone by the Brønsted acidic K10 clay. The nucleophilic
diamine then adds to the protonated diketone intermediate followed
by dehydration to give the quinoxaline products.
Isobezofuran-1(3H)-one derivatives have been prepared by
microwave irradiation of a mixture of phthalaldehydic acid and
ketones well dispersed in montmorillonite K10 catalyst under
solventless conditions (Landge et al., 2008) (Scheme 32). The best
results were obtained when the irradiation is carried out for 10–
Scheme 25. Microwave-assisted solventless aldol condensation.
Scheme 26. Synthesis of conjugation-expanded carba- and azuliporphyrins.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 27. Solventless method for Biginelli reaction under IR irradiation.
115
reported by Kulkarni and Török (2010). The three-component system
consists of an aromatic aldehyde, aniline or its derivative and phenyl
acetylene or its derivative. Microwave irradiation of the reaction
mixture combined with K10 at 100 °C for about 10 min was found to
be the optimum condition for obtaining the best yields (Scheme 33).
The reaction is a multistep domino process that starts with the
formation of a Schiff base in the first step. In the next step, phenyl
acetylene adds to the base, then the adduct cyclizes followed by
deprotonation to give the observed quinoline product.
Microwave irradiation of a mixture of tryptamines and aromatic
aldehydes dispersed in combined 10% Pd/C/K10 catalyst delivered
β-carbolines in good to excellent yields. The optimum reaction
temperature was 130 °C. The selectivity for β-carbolines was
100% when 10% Pd/C with K10 was used (Kulkarni et al., 2009a)
(Scheme 34). If Pd/C was less than 10%, the intermediate tetrahydroβ-carbolines were isolated along with β-carbolines, and if only
K10 was used as catalyst, only the former compounds were formed
as products. These results indicate that the cyclization process
is catalyzed by K10, while the dehydrogenation is brought about by
Pd/C. Use of other acid catalysts, such as Al2O3 and acetic acid, gave
very low yields. Tryptaphan as substrate in place of triptamine or Pt/C
instead of Pd/C gave the same β-carboline products.
4. Diels–Alder and related reactions
Scheme 28. Microwave-assisted Biginelli reaction without solvent.
Scheme 29. Condensation of primary amines with aldehydes and ketones.
30 min at 170 °C. The catalyst is reusable and its performance in the
fifth trial was as good as in the first one.
A microwave-assisted, montmorillonite K10 catalyzed threecomponent reaction for the preparation of quinolines has been
Pericyclic reactions are one of the most important ways of constructing
more complex organic molecules from the simpler ones. Among these the
most celebrated are those that involve the 6-electron cyclic transition
state, which include electrocyclic and cycloaddition reactions. Since they
are thermally allowed processes, their energy demand can be easily met.
Many of them, in fact, occur at room temperature and even below that in a
number of cases. The Diels–Alder reaction is a cycloaddition process
between a 4-electron and a 2-electron component which could be starting
compounds or intermediates formed in a multistep reaction sequence.
Many such reactions are assisted or initiated by acid catalysts. This has
provided an opportunity to exploit clays, particularly montmorillonites, as
Brønsted or Lewis acid catalysts for Diels–Alder reactions. In this section
some recent examples of such catalyzed reactions are presented.
A major problem in developing a suitable chiral organocatalyst
entrapped in the montmorillonite interlayer is its instability due to
leaching. This problem has been overcome by Mitsudome et al. (2008)
by entrapping the cationic chiral organocatalyst (5S)-2,2,3-trimethyl5-phenylethyl-4-imidazolinone hydrochloride by replacing the interlayer cations in montmorillonite clay. The catalyst is very effective in
carrying out asymmetric Diels–Alder reactions (Scheme 35). Several
other inorganic solids, such as silica, titania, zeolite, MCM-41,
hydroxyapatite and γ-ZrP failed to function as support. The
montmorillonite supported catalyst is stable and could be reused a
few times.
Lopez et al. (2007) have made an interesting observation of
montmorillonite K10 exerting considerable influence in terms of
Scheme 30. Coumarin-3-carboxylic acids from malonic acid and salicylic aldehydes.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 31. Quinoxalines by condensing 1,2-diketones with o-phenylene diamines.
stereochemical outcome and product yields on Diels–Alder reaction
carried out in ionic liquid, 1-hexyl-3-methylimidazolium (HMI)
tetrafluoroborate. The results with other supports like silica or
alumina or without support in the same medium were not good.
Microwave irradiation was also not as good as K10 clay. With K10 clay
as catalyst the yields were as high as 99% and endo:exo ratio was 93:7.
The reaction took just 30 min at room temperature (Scheme 36). The
same medium after work up was reused four times without much loss
in yields.
Several clay catalyzed hetero-Diels Alder reactions have been reported
in the last few years. Dintzner et al. (2007) found that the carbonyl group
of benzaldehyde and its derivatives adds to 2,3-dimethyl-1,3-butadiene
under the influence of montmorillonite K10 clay in carbon tetrachloride at
25 °C to form dihydropyrans. The clay was preheated to 250 °C which
enabled the collapse of the interior structure of clay by extrusion of water
leading to a decrease in Brønsted acidity but an increase in Lewis acidity
that is responsible for the catalytic activity of the clay, suggest the authors.
Based on the fact that benzaldehydes with ortho-substituents having lone
pair electrons give far better yields of dihydropyran products, the authors
propose a clay metal ion coordinated transition state that favours the
reaction (Scheme 37). The less substituted dienes (e.g., isoprene)
produced oligomeric and polymeric products rather than [4+2] addition
products. p-Chloro and p-nitrobenzaldehyde gave very poor yields of the
corresponding dihydropyrans as compared to o-chloro- and onitrobenzaldehyde.
Chiba et al. (1999) have generated in situ the highly reactive oquinomethanes from o-hydroxybenzyl alcohols at room temperature
using wet monmorillonite K10 and lithium perchlorate in nitromethane. The presence of air is necessary. The quinomethanes formed add
instantly to olefins to give benzodihydropyrans (Scheme 38).
Methylene cyclopropanes function as dienophiles in their azaDiels–Alder addition reaction with ethyl (arylimino)acetates under
the catalytic influence of montmorillonite K10 in dichloroethane at
room temperature to produce tetrahydroquinolines. The K10 clay
performs as good as triflic acid in catalyzing these reactions (Zhu et al.,
2009b) (Scheme 39).
Two of the authors of this group, Shao and Shi (2003) had reported
similar aza-Diels–Alder reactions of methylene cyclopropanes with
Schiff bases. In this case they had employed monmorillonite KSF clay
or scandium triflate as catalyst (Scheme 40). The Schiff bases were
produced in situ by the reaction of aryl aldehydes and aryl amines
employed directly in the 3-phase reaction.
Scheme 32. Isobezofuranones from phthaldehydic acid and ketones without solvent.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
117
Scheme 33. Three-component reaction for the preparation of quinolines.
Aryl amines add to endocyclic enecarbonates resulting in the latter
undergoing ring opening to give aralkyl imines under the influence of
montmorillonite KSF clay in THF at room temperature. The imines so
formed are highly reactive and undergo rapid aza-Diels–Alder
reaction with a second molecule of the endocyclic enecarbonate
present to finally give hexahydro-1H-pyrrolo(3,2-c) quinoline derivatives (Yadav et al., 2004a) (Scheme 41).
The same group of workers had previously reported (Yadav et al.,
2002) similar reaction of aryl amines with dihydrofuran and
dihydropyran to produce furano- and pyrano-quinolines (Scheme 42).
A similar mechanistic pathway is proposed for this reaction also.
Guchhait et al. (2009) have reported a multicomponent Povarov
reaction involving aromatic amine, aromatic aldehyde and terminal
acetylene. The three compounds were allowed to react in the
presence of perchloric acid-treated montmorillonite clay at 70 °C in
open air. Substituted quinolines were obtained as products in
moderate to good yields varying from 42–81% depending on the
substituents (Scheme 43). The hetero-Diels–Alder reaction proceeds
by a multistep process in which the acetylene adds to the Schiff base
formed by the condensation of the amine and the aldehyde. Slightly
better yields were obtained in oxygen atmosphere instead of open air.
The research group also found that, among some twenty different
solid supports, the HClO4 treated montmorillonite worked the best in
terms of yields, reaction time, work up procedure, etc.
Cyclopentadiene and furan undergo Diels–Alder addition at room
temperature with trans-2-methylene-1,3-dithiolane-1,3-dioxide in
the presence of Fe3+-doped montmorillonite K10 combined with
2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) to
give the product in a overall yield of 64% (Gültekin, 2004)
(Scheme 44).
Aldimines generated in situ from aliphatic aldehydes and panisidine add to Danishefsky diene in the presence of montmorillonite
Scheme 34. Condensation of tryptamines and aldehydes to give β-carbolines.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 35. Entrapped cationic chiral organocatalyst in asymmetric Diels–Alder reactions.
Esters, which are normally understood to be alkyl or aryl carboxylates, are an important class of naturally occurring compounds,
particularly as oils, fats and waxes. They are a major component of all
living organisms. They have numerous applications in industries and as
foods and fuels, for which esters obtained from both natural as well as
synthetic sources are employed. There are several ester forming
reactions available, such as (i) reaction of alcohols or phenols with
carboxylic acids, carboxylic acid anhydrides or acyl halides, (ii) addition
of carboxylic acids to olefins, (iii) addition of alcohols to ketenes,
(iv) substitution of alkyl halides/tosylates with carboxylates, (v) Baeyer–
Villiger oxidation of ketones, etc. The most common and expedient
method is the acid-catalyzed reaction of a carboxylic acid with an
alcohol, called esterification, which is usually carried out in homogeneous media. Though several Brønsted acids are used as catalysts for this,
the most convenient and commonly used one is concentrated sulfuric
acid. The disadvantages of using this acid, like corrosion problems,
handling difficulties, waste disposal hassles, environmental hazards etc.
are well known. This opens up a good opportunity for acid clays to
replace the hazardous mineral acid in ester forming processes, and a
substantial amount of research activity is going on in this area.
Clays available commercially or occurring naturally, including
those found in the geographical region of the researchers, have been
Scheme 36. Diels–Alder reaction in ionic liquid. K10 is the best catalyst.
Scheme 37. Hetero-Diels–Alder reaction of benzaldehyde with dimethylbutadiene.
K10 in aqueous or aqueous acetonitrile medium to produce 2-alkyl2,3-dihydro-4-pyridones in excellent yields (Akiyama et al., 2002)
(Scheme 45).
Montmorillonite K10, filtrol-24, bentonite and pyrophillite clays
were used as catalysts to bring about Diels–Alder addition of 1,4naphthoquinone and N-phenylmaleimide to 4,6-bis(4-methoxyphenyl)and 4-(4-methoxyphenyl)-6-methylpyran-2(H)-ones under a dry state
adsorbed condition. Filtrol-24 performed the best under this condition.
Further, when montmorillonite K10 and bentonite were modified
by impregnating with AlCl3, ZnCl2 and FeCl3 using their aqueous or
nonaqueous solution (PhNO2 for AlCl3, MeCN for ZnCl2 and FeCl3),
followed by washing with water and drying, the resulting modified K10
and bentonite catalysts performed as well as filtrol-24. Among the doped
catalysts FeCl3-containing ones worked the best (Kamath et al., 2000)
(Scheme 46).
5. Esterification reactions
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
119
Scheme 38. o-Quinomethanes from o-hydroxybenzyl alcohols and their D–A addition.
found to work well with or without suitable modifications. The most
successful esterification catalysts have been the montmorillonites,
normally those that possess high Brønsted acidity. Some selected
examples from recent literature are described here.
Kaolin, montmorillonite K10 and KSF supported with transition
metal chlorides, InCl3, GaCl3, FeCl3, and ZnCl2 were employed to
esterify tert-butanol with acetic anhydride to tert-butyl acetate with
more than 98% selectivity. The K10 clay was found to be the best
support and K10 supported InCl3was the best catalyst followed by
GaCl3/K10, FeCl3/K10 and ZnCl2/K10, in that order. A noteworthy
feature was the low activity of the catalysts for the dehydration of tertbutanol below 50 °C (Choudhary et al., 2001a) (Scheme 47).
In a later study employing the InCl3/K10 catalyst Choudhary et al.
(2004) reported the preparation of thirteen esters by the reaction of
benzyl alcohol, phenol, 4-nitrophenol, 1-naphthol and 2-naphthol with
benzoyl chloride, acetyl chloride and n-butyryl chloride. The reaction
gave high yields of esters (up to 98%) under mild conditions (50 °C, 0.2–
2 h reaction time). They also discovered that the supporting K10 clay is
essential for the reaction, since InCl3 alone was not effective as catalyst.
The yield of benzyl benzoate was only 11% after 1 h of reaction at 50 °C
between benzyl alcohol and benzoyl chloride with InCl3 as catalyst,
while with equivalent amount of InCl3/K10 as catalyst the ester yield
was 96%. The authors further found that the InCl3/K10 catalyst was
almost as effective in its fifth time reuse as in the beginning.
Srinivas and Das (2003) have demonstrated that ferric chloride
supported on montmorillonite K10 clay (Fe3+/K10) is an efficient
esterification catalyst. The catalyst is highly selective in esterifying
both saturated and unsaturated aliphatic carboxylic acids, while the
aromatic acids are unreactive, as demonstrated by using mixtures of
aromatic and aliphatic acids. The yields of esters are high and the
catalyst is reusable. The catalyst is shown to be useful also in the
preparation of amides of aliphatic carboxylic acids with aliphatic as
well as aromatic amines, but ineffective in the preparation of the
amides of aromatic acids with aromatic amines (Scheme 48).
Scheme 39. Aza-Diels–Alder reaction of arylimino esters with methylene cyclopropanes.
Scheme 40. Aza-Diels–Alder reaction of Schiff bases with methylene cyclopropanes.
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Scheme 41. Hexahydropyrroloquinolines from aryl amines and enecarbonates.
Scheme 42. Furano-/pyrano-quinolines from aryl amines and dihydrofuran/dihydropyran.
Some Brazilian natural clays (smectite, atapulgite and vermiculite), without pretreatment or activation, have been demonstrated by
Silva et al. (2004) to act as good catalysts for transesterification of
ethyl acetoacetate and ethyl bezoylacetate by six carbohydrateacetonides. Refluxing a mixture of the reactants and the catalyst in
toluene for ~48 h produced the acetonide esters in good yields by
replacing the ethyl group of the β-keto esters. The reaction mixture on
cooling to room temperature was stirred (~24 h) with bezylamine to
produce β-benzyl enamino esters (Scheme 49).
Vijayakumar et al. (2004, 2005a,b) have shown that acid activated
Indian bentonite is a good catalyst, in some cases better than zeolites,
for the preparation of aryl and alkyl esters of fourteen different
aromatic and aliphatic carboxylic acids. Particularly, the yields of aryl
esters of long chain fatty acids are very impressive, which were more
than 90%, are very impressive. Results of a part of their work are
depicted in Scheme 50.
Earlier Kantam et al. (2002) observed that Fe3+-impregnated
montmorillonite clay catalyzed the esterification of various aliphatic
Scheme 43. Multicomponent Povarov reaction—synthesis of quinolines.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
121
Scheme 44. Diels–Alder reaction of 2-methylene-1,3-dithiolane-1,3-dioxide.
Scheme 47. Esterification of tert-butanol with acetic anhydride.
acids, including the long chain fatty acids, aromatic acids, and α,βunsaturated mono- and dicarboxylic acids with alcohols, under mild
reaction conditions. The yields were obtained in the range of good to
excellent.
In three reports Reddy et al. (2004, 2005a,b) describe their study
on the esterification of monocarboxylic and dicarboxylic acids with
phenols and alcohols including ethylene glycol. They found that Al3+
exchanged montmorillonite GK-129 was the best catalyst compared
with the same clay exchanged with divalent and other trivalent metal
cations. It is better than the similarly treated montmorillonite K10 or
Indian bentonite. Its performance is comparable to the results
obtained using zeolite H-β or p-toluene sulfonic acid catalyst. They
discuss various experimental conditions such as temperature,
reaction time, solvents, catalyst preparation, etc., that give the best
results, as well as the catalyst's performance in its reuse. Their results
are consolidated in Scheme 51. Reddy et al. (2007) have carried out
esterification of succinic acid with iso-butyl alcohol to di-iso-butyl
succinate in connection with the evaluation of surface activity of Mn+montmorillonite clay catalysts, where Mn+ = Al3+, Fe3+, Cr3+, Zn2+,
Ni2+, Cu2+ and H+. The reactions performed on Al3+-mont and H+mont gave the best yields (96% and 97%), Fe3+-mont and Cr3+-mont
gave good yields (74% and 51%), while the other Mn+-mont catalysts
gave poor yields (23–27%). The results were rationalized based on
various properties of catalysts due to exchanged ions.
As noted in the section on condensation reactions, Motokura et al.
(2005) have esterified cyanoacetic acid with methanol to methyl
cyanoacetate over the combined Ti4+-mont, HT catalyst (Section 3,
Scheme 18).
Scheme 48. Ferric chloride supported on K10 for esterification.
Methyl mandelate, used in flavouring and perfumery, has been
prepared by esterification of mandelic acid with methyl alcohool in
the presence of montmorillonite K10 supported dodecatungstophosphoric acid (DTP/K10) and its cesium salt (Cs-DTP/K10) (Yadav and
Bhagat, 2005) (Scheme 52). The Cs-DTP/K10 catalyst was found to be
better than K10 and other solid catalysts like S-ZrO2. The catalyst was
shown to be recyclable. Using the same catalyst (Cs-DTP/K10 clay)
Yadav and George (2008) have carried out the esterification of
benzoic acid with phenol to phenyl benzoate which then undergoes
Fries rearrangement to give 2- and 4-hydroxybenzophenones. The
selectivity of the products depended on a number of experimental
parameters.
The esterification strategy was used by Mittal (2007) to increase
the basal plane spacing in clays in order to achieve shear induced
exfoliation. This was accomplished by modifying the clay platelets with
Scheme 45. Alkyldihydropyridones from Danishefsky diene and aldimines formed in situ.
Scheme 46. Diels–Alder addition of 1,4-naphthoquinone to N-phenylmaleimide.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 49. Transesterification of esters by carbohydrate-acetonides.
quaternary ammonium ions carrying hydroxyalkyl groups, which were
then reacted with long chain fatty acids (Scheme 53). The paper reports
various aspects of catalyst modification and resulting properties.
Wallis et al. (2007) have employed the esterification of maleic
anhydride with p-cresol, along with a transacetalization reaction,
diacetylation of bezaldehyde with acetic anhydride and tetrahydropyranylation of ethanol (Scheme 54), to assess and improve the
catalytic activity of different batches of commercially available K10
montmorillonite clays. They have found that acid treatment improves
the activity of the clay and suggest that esterification is suitable for
determining the degree of clay delamination. They suggest that loss of
layer stacking and increase in available exchange sites for protonation
are responsible for clay activity enhancement. They compare this with
an earlier observation by Reddy et al. (2005a) who had observed
Scheme 50. Esterification using Indian bentonite.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
123
Scheme 51. Preparation of diesters using Al3+ exchanged montmorillonite GK-129.
Scheme 52. K10 supported dodecatungstophosphoric acid-Cs salt for esterification.
dramatic improvement in the catalytic activity of K10 montmorillonite clay with much reduced specific surface area on acid treatment.
Thermally activated Nigerian Ukpor kaolinite clay and Udi clay
were shown to be good catalysts for the preparation of n-propyl
acetate (Igbokwe et al., 2008). Various experimental parameters were
measured to obtain the best yield of the ester.
Esterification of long chain fatty acids, stearic, oleic and palmitic
acid, with short chain alcohols, methanol, ethanol, 1-propanol, 1butanol and 2-butanol has been carried out using a series of
montmorillonite based clay catalysts, KSF/0, KP10, K10 (Neji et al.,
2009) (Scheme 55). The product esters are meant to be used as
biodiesel. A variety of reaction conditions were investigated. KSF/0,
which had the lowest pH value, was found to be the best catalyst. The
yields in the case of primary alcohols were almost quantitative, but in
the case of 2-butanol the ester was obtained in only 40% yield. The
catalyst was recycled twice without significant loss in its activity.
p-Hydroxybenzoic acid esters, with the sobriquet parabens, find
application in cosmetic, pharmaceutical and food industries. Hazarika
et al. (2007) have prepared the methyl, ethyl and n-propyl p-hydroxy
benzoates by refluxing the mixtures of the aromatic acid and
appropriate alcohol on montmorillonite K10 clay for 10–15 h, and
obtained the esters in 81–90% yields (Scheme 56). The acid-treated
clay gave slightly better yields (~2% more).
Acetic acid reacts efficiently with 2-methoxyethanol in the
presence of clay catalyst treated with sulfuric acid and aluminium
salts and calcined at 313–633 K, to give methoxyethyl acetate. Both
Lewis acid and Brønsted acid sites are active in catalyzing the
esterification process (Wang and Li, 2000) (Scheme 57).
The waxy stearyl stearate ester was synthesized by reacting stearic
acid with stearyl alcohol on montmorillonite clay under solvent-free
condition. The temperature was strictly maintained at 170 °C
throughout the bulk of the reaction mixture in the pilot scale reactor
by using microwave irradiation. The pure ester is obtained in 95%
yield on filtering off the solid catalyst. The use of microwave radiation
reduces the reaction time by a factor of 20–30 times compared to the
time required by the reaction done in conventional reactor (Esveld
et al., 2000) (Scheme 58).
6. Friedel–Crafts and related reactions
Scheme 53. Strategy to increase the basal plane spacing in clays by esterification.
The Friedel–Crafts reaction is an important carbon–carbon bond
forming process. It enjoys numerous applications in the synthesis of
bulk chemicals, fine chemicals, pharmaceutical and perfumery
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 54. Esterification for determining the degree of clay delamination.
chemicals, and several others. It is an electrophilic substitution reaction
of usually aromatic and heteroaromatic compounds by alkyl or acyl
groups catalyzed by a number of Lewis and Brønsted acids under
homogeneous or heterogeneous conditions. Alkyl halides, alcohols,
sulfonates and olefins are used as alkylating agents, and acyl halides or
acid anhydrides are generally used for acylation. The most commonly
used catalysts are AlCl3, ZnCl2, SnCl4, BF3, FeCl3, SbCl5, H2SO4, H3PO4, etc.,
mostly under homogeneous conditions. Though these Lewis acids are
Scheme 55. Esters for using as biodiesel.
Scheme 56. Preparation of p-hydroxybenzoic acid esters.
Scheme 57. Preparation of methoxyethyl acetate.
indeed catalysts, they are not really used in catalytic quantities, but in
much larger amounts. This obviously poses serious problems of
handling, recovery and disposal of the waste products. In this respect,
Friedel–Crafts reactions are an antithesis of Green chemistry principles.
This has provided challenging opportunity to explore alternative
catalytic procedures. Thus reusable, environmentally benign solid acid
catalysts including clays are being explored for these reactions. Clays
and modified clays have been used successfully as catalysts for bringing
about Friedel–Crafts alkylation and acylation.
Cyclopentyl and cyclohexyl derivatives of benzene, toluene, oand p-xylene, mesitylene and anisole have been prepared in 85–95%
yield by refluxing a solution of the respective aromatic substrate
with cyclopentanol or cyclohexanol in 1,2-dichloroethane on Fe3+montmorillonite with 10 mol% of TsOH or MsOH as cocatalyst
(Chaudary et al., 2002) (Scheme 59). The suggested mechanism
involves the formation of sulfonate ester of the alcohol as intermediate
which alkylates the arene selectively. In the absence of the cocatalyst
TsOH or MsOH, the reaction with cyclopentanol produces only a minor
amount of the expected alkylated product (~30%), while the major
product is dicyclopentyl ether (~60%). The reaction does not take place
in the absence of Fe3+–montmorillonite.
An interesting case of Fe–Mg–hydrotalcite anionic clay being used
for the Friedel–Crafts alkylation has been described by Choudhary
et al. (2005a) (Scheme 60). They have benzylated anisole, mesitylene,
p-xylene, toluene and naphthalene with high degree of conversions,
using benzyl choride. They observed that calcining increases the
activity of the catalyst, and that the higher the calcining temperature
the more active the catalyst is. They attribute this to dehydration at
200 °C, formation of metal oxides on calcining at 500 °C and higher
temperatures up to 800 °C. They also found that the used catalyst is
more active than the one used for the initial reaction. This observation
is explained as due to a possible formation of Lewis acid sites resulting
from the reaction of HCl liberated in the benzylation process.
In–Mg hydrotalcite anionic clay is found to function in a similar
manner (Choudhary et al., 2005b). Ga–Mg–hydrotalcite anionic clay
was used earlier for benzylation and benzoylation of benzene
(Choudhary et al., 2001b).
Scheme 58. Microwave-assisted esterification without solvent.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
125
Scheme 59. Mesilates and tosylates in F–C alkylation.
Scheme 60. Anionic clay for the Friedel–Crafts alkylation.
Benzene has been alkylated with propylene to cumene on Al3+exchanged synthetic Zn-saponite with 99% conversion (Scheme 61),
compared to just 0.3% conversion when the same reaction was
performed using commercial solid phosphoric acid (SPA), (Vogels
et al., 2005).
To introduce an isopropyl group on xylenes, isopropyl alcohol is
used by Yadav and Kamble (2009). They have obtained dimethylcumenes by reacting xylenes with isopropyl alcohol in an autoclave in
the presence of cesium substituted dodecatungstophosphoric acid
supported on K10 as catalyst (Scheme 62). The Cs-DTP/K10 was the
most active among the five catalysts studied, including K10, DTP/K10,
sulfated zirconia and filtrol-24. Various aspects of catalytic activity
and kinetics of reactions are considered. Selectivity for monoisopropyl products is very high, with the byproducts, diisopropylated
xylenes, diisopropyl ether and propylene being formed in small
amounts.
Cs-DTP/K10 has been used fruitfully in several other Friedel–Crafts
type reactions. For example, 1,3-dibenzyloxybenzene has been
acetylated to 3,5-dibenzyloxyacetophenone with acetic anhydride as
shown in Scheme 63 (Yadav and Badure, 2008).
Phenol has been benzoylated by benzoic acid to 4-hydroxybenzophenone via the formation of phenyl benzoate as intermediate
which underwent Fries rearrangement, a variant of Friedel–Crafts
acylation (Yadav and George, 2008) (Scheme 64). Cs-DTP/K10 has
been found to be a good catalyst for benzoylation of p-xylene to
2,5-dimethylbezophenone (Yadav et al., 2003). Similar results were
obtained when resorcinol was treated with phenylacetic acid in
the presence of montmorillonite K10 clay-POCl3 under microwave
irradiation. The effect of this catalyst was comparable to the silica
gel-POCl3 catalyzed reaction (Devi, 2006).
A number of other clay-based catalytic systems have been
developed by several groups of researchers for bezylation reactions.
The reactions are conducted in liquid phase or solvent-free conditions
with or without microwave irradiation. Studies were directed to find
the right experimental conditions to obtain the best results by
preparing the most active catalyst.
Ga/AlClx-grafted montmorillonite-K10 was found to be an efficient
catalyst for benzylation as well as benzoylation of benzene, substituted
benzenes and naphthalene, using benzyl chloride and benzoyl chloride
respectively. The catalyst is highly active, and as such benzoylation
occurs even if a strong electron withdrawing group like NO2 is present
on benzene ring (Choudhary and Jha, 2008) (Scheme 65).
Clay catalyists obtained from Pakistani clay minerals were found
to be useful in benzylation of toluene, naphthalene, anthracene,
quinoline, 8-hydroxyquinoline and pyridine (Ehsan et al., 2006).
Scheme 61. Al3+-exchanged synthetic Zn-saponite for alkylation of benzene.
Scheme 62. Alkylation of xylenes using Cs-DTP/K10.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 63. Friedel–Crafts acetylation using Cs-DTP/K10.
Durap et al. (2006) have used Fe3+, Cr3+ and Al3+ pillared bentonite
for benzylation of benzene and toluene. The Fe3+-pillared clay was
found to be the most active catalyst. Kurian and Sugunan (2005) have
carried out benzylation of benzene on alumina pillared transition
metal cation (Cr3+, Fe3+, etc.) exchanged clays. Mechanistic investigation indicated the formation of benzyl cation as intermediate.
Ahmed and Dutta (2005) have prepared acid-treated Zn2+ and Cd2+
ion-exchanged clay composites which act as very good catalysts for
benzylation of benzene. Barton et al. (2003) developed a process for
benzylation of toluene where toluene is first brominated by bromine
either in situ or separately to generate benzyl bromide, which then
benzylates toluene to benzyl toluenes. Both the reactions are
catalyzed by ion-exchanged bentonite and montmorillonite K10
(Scheme 66).
Friedel–Crafts acylation of arenes has been carried out using
chloroacetyl chloride on Fe3+-exchanged montmorillonite K10 in
liquid phase (Paranjape et al., 2008) (Scheme 67). The yields are good
only with polymethylated benzene derivatives, namely, durene,
mesitylene, p-xylene and m-xylene.
Liu et al. (2009) have prepared an efficient montmorillonite K10
supported antimony trichloride catalyst for alkylation of nitrogen
heterocycles, pyrrole, indole and indole derivatives, using epoxides as
alkylating agents. The reaction occurs at room temperature under
solvent-free conditions and takes less than an hour for completion in
most cases (Scheme 68). The yields are good to excellent.
Acid-washed montmorillonite K10 was found to be a good catalyst
for alkylation as well as acylation of aromatic and heterocyclic
Scheme 64. Benzoylation of aromatic compounds.
Scheme 65. Alkylation and benzoylation of aromatic compounds.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
127
Scheme 66. Benzylation of aromatic compounds.
compounds under microwave irradiation and solvent-free condition
(Devi and Ganguly, 2008). The yields are good and comparable to or
better than the yields obtained from reactions using similarly treated
silica gel catalyst (Scheme 69).
Scheme 67. Chloroacetylation of alkylbenzenes.
Singh et al. (2002) synthesized three sesquiterpenes, elvirol,
curcuphenol and sesquichemaenol by Friedel–Crafts alkylation of
appropriate cresols using suitable alkylating agents. The reaction was
brought about by heating the reaction mixtures in presence of
montmorillonite K10 as catalyst. The targeted sesquiterpenes were
formed in major amounts accompanied by minor quantities of
isomeric compounds (Scheme 70).
Zhang et al. (2008) have prepared a number of dihydrocoumarin
derivatives by a microwave-assisted reaction of cinnamoyl chloride
with phenol and its several derivatives using montmorillonite K10 as
catalyst in chlorobenzene as solvent (Scheme 71). The reaction
proceeds by initial formation of phenyl ester of cinnamic acid
followed by alkylation-cyclization of the ester to give the observed
Scheme 68. Alkylation of nitrogen heterocycles by epoxides.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 71. Dihydrocoumarins by acylation–cyclization.
Scheme 69. Alkylation and acylation of aromatic and heterocyclic compounds.
products. The microwave irradiation reduces the reaction time from
several hours to just a few minutes. The yields vary from poor to
excellent.
Methylation of toluene with methanol in the presence of chromiapillared montmorillonite K10 gives xylenes selectively. Mixed pillaring with chromia and titania or zirconia or alumina was found to
produce more efficient catalysts than pillaring with a single metal
oxide. In contrast the natural (unmodified) clay does not exhibit any
shape selectivity and di- and tri-substituted products are also
obtained as well (Binitha and Sugunan, 2008) (Scheme 72). The
catalyst was regenerated by heating and reused four times with little
loss of activity. The authors propose that the nature of porosity in the
pillared clay is responsible for selectivity and that methylation occurs
by an Eley–Rideal type mechanism.
A microwave-assisted montmorillonite K10 catalyzed alkylation of
indoles by tert-butyl alcohol and hexane-2,5-diol has been reported
by Kulkarni et al. (2009b). The alkylation occurs selectively to form 3tert-butylindole. In the case of the diol, the alkylation is followed by
cyclization to give finally carbazole derivatives (Scheme 73). Good
results are obtained when the irradiation is carried out at 130 °C for 8–
15 min. Though the GC yields are reported to be good, the isolated
yields are moderate.
7. Isomerization reactions
Isomerization or rearrangement reactions are important in
producing a large number of bulk as well as fine chemicals. A majority
of such transformations are brought about by acid-catalyzed processes using conventional Brønsted and Lewis acids. A variety of acid clays
Scheme 70. Synthesis of sesquiterpenes by Friedel–Crafts alkylation.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 72. Methylation of toluene with methanol and its mechanism.
Scheme 73. Microwave-assisted alkylation of indoles by alcohols.
also have served as good catalysts for functional group transformation
and skeletal rearrangements. The clay catalyzed conversion of low
octane hydrocarbon fuels to high octane grade is an old well known
reaction. Due to enormous progress achieved in the use of claysupported catalysts in various areas of organic synthesis, much
attention is being focused on the study of clay catalyzed isomerization/rearrangement reactions, in order to replace the hazardous
conventional Brønsted and Lewis acids. Many times the products
formed from clay catalyzed reactions are quite different from those
that are formed in homogenous mineral acid or Lewis acid-catalyzed
reactions. This aspect gives an opportunity to choose catalysts and
reaction conditions to prepare the desired compounds.
129
Epoxides are good substrates for isomerization under a variety of
acid catalytic conditions. The epoxide ring opens under the influence of
Brønsted as well as Lewis acid catalysis. The incipient cation may
undergo skeletal rearrangement through bond migrations and/or
experience nucleophilic attack. The products have diverse applications.
The diastereomeric (R)-(+)-limonene diepoxides (36a,b) underwent isomerisation at room temperature on synthetic K10 clay
calcined at 100 °C or natural ascanite-bentonite clay to give 37, 38, 39
and 40a,b in a ratio of ~4:3:7:2 (Salomatina et al., 2005) (Scheme 74).
Il'ina et al. (2007) have reported an extensive study of the isomerization of allyl alcohols of the pinene series and their epoxides using
askanite–bentonite clay (calcined at 110 °C) (Scheme 75). Treatment
of (+)-trans-pinocarveol ((+)-41) with askanite–bentonite at
room temperature led to the formation of isomeric allyl alcohol
(−)-myrtenol ((−)-42), and the dimeric ether (+)-43, selectively. In
contrast, the allylic alcohol (+)-41 gives Wagner–Meerwein rearrangement product 44, under homogeneous acidic conditions, due to
protonation of the exocyclic double bond. On the other hand, when
(−)-42 was kept on clay for one hour at room temperature (+)-41
and the rearranged products (+)-45 and 46 were obtained. The
authors suggest the formation of a common intermediate A from (+)41 and (−)-42.
To explain the formation of (+)-45 and 46, the authors propose
that (−)-42 gets protonated at –OH to give the carbocation A as well
as at the double bond to give B, while proposing protonation of only –
OH group in the case of (+)-41.
A similar treatment of the epoxides (+)-47 and (−)-48 of these
two allyl alcohols on askanite–bentonite clay yielded slightly differing
results, though the two epoxides were visualized to give the same
intermediate C Scheme 76. The pinocarpeol epoxide (+)-47 yielded,
among other unidentified products, the α,β-unsaturated ketone (+)49 and the monocyclic keto alcohol (+)-50. Myrtenol epoxide (−)-48
produced the aromatic alcohol 4-isopropylbenzyl alcohol 46 and the
hydroxy aldehyde (+)-50. Interestingly, the epoxide (+)-47 did not
produce 46, and myrtenol epoxide (−)-48 did not give the
unsaturated ketone (+)-49 (Scheme 76).
(+)-trans-Verbinol ((+)-51) and the epoxide (−)-53 of (−)-cisverbinol ((−)-52) were similarly treated with askanite–bentonite
clay. The results obtained are shown in Scheme 77.
Storing (−)-cis-verbenol ((−)-52) and its epoxide on askanite–
bentonite clay in the presence of aldehydes produced heterocyclic
compounds 57–65 as depicted in Scheme 78.
Montmorillonite K10 treated with the heteropolyacid dodecatungstophosphoric acid has been found to be effective in opening 1,2epoxyoctane (66) to octanal (67), octenol (68) and other products
(Yadav, 2005) (Scheme 79).
Isolongifolene (70) is a commercially important tricyclic sesquiterpene used extensively in the perfumery industry. It is obtained by
acid-catalyzed isomerization of longifolene (69). A number of
homogeneous acid conditions are used. However, as isolongifolene
has the tendency to undergo further isomerization the conditions
have to be carefully maintained. Singh et al. (2007) have used alumina
and zirconia pillared clays and Ce3+ and La3+ modified montmorillonite clays for the conversion of longifolene to isolongifolene. They
Scheme 74. Isomerization of diastereomeric (R)-(+)-limonene diepoxides.
130
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 75. Isomerization of allyl alcohols of pinenes.
found that Al3+ pillared montmorillonite clay shows the highest
conversion rate and reasonable selectivity (Scheme 80).
Hydroisomerization of n-heptane to isoheptane was carried out by
Vogels et al. (2005) on synthetic Co-, Mg- and Co/Mg-saponites. They
relate the isomerization to the Lewis acidity of the catalyst (Scheme 81).
Moronta et al. (2008) have studied the isomerization of 1-butene
over natural smectite clay (STx-1, USA) ion-exchanged with Al3+, Fe3+
or pillared with Al and Fe polyoxocations. The results show that Al-
pillared clay is the best catalyst for the conversion of 1-butene to trans2-butene and cis-2-butene accompanied by small amounts of cracking
products (Scheme 82).
Nucleophilic substitution at allylic position (SN1' or SN2' type)
with isomerization of double bond has been observed by Shanmugam
and Vaithiyanathan (2008) on K10 clay under neat conditions in the
case of the derivatives of oxindole 71 (Scheme 83). The products
(72a–c) were further used for construction of spirolactone ring (not
Scheme 76. Isomerization of epoxy alcohols of pinenes.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
131
Scheme 80. Longifolene to isolongifolene.
Scheme 77. Isomerization of trans-verbinol and cis-verbinol epoxide.
Scheme 81. Hydroisomerization of n-heptane to isoheptane.
Scheme 78. Reactions of cis-verbinol and its epoxide with aldehydes.
shown in the scheme) through Morita–Baylis–Hillman reaction
followed by hydrolysis.
Isomerization of enamine 75 to imine 76 has been observed by
Shanbhag and Halligudi (2004). They report hydroamination of
phenylacetylene (73) carried out on Zn2+-exchanged montmorillonite K10 clay by the addition of aniline (74) to the triple bond, followed
by isomerization of the product enamine (75) to imine (76)
(Scheme 84). The authors suggest the activation of the triple bond
by π-complexing with Zn2+ ions in K10, which enables the
nucleophilic addition. The isomerization is due to protonation of the
double bond followed by removal of proton on nitrogen.
Ortega et al. (2006) have achieved success in enriching the cisisomer 77a in an isomeric mixture of cis- and trans-lauthisan (77a and
77b) from an isomer ratio of cis:trans = 1:1.7 to 17:1, on montmorillonite K10. It should be noted with much appreciation that many
other acid catalysts including BF3.OEt2, CF3COOH and TsOH had either
failed to bring about any reaction or had led to decomposition of 77a,
b. This K10 catalyzed enrichment procedure enabled the authors to
develop a short route to the desired isomer (+)-cis-lauthisan in good
yield (Scheme 85).
Tomooka et al. (2000) have observed an interesting case of
stereoregulated substitutive migration of a phenyl group from TBDPS
ether 78 to give 79, thereby achieving an asymmetric synthesis of αaryl and β-hydroxycyclic amines and silanols (Scheme 86).
Two highly branched 25-carbon isoprenoid olefins, a diene (80)
and a triene (81), isolated from Haslea ostrearia were treated with
montmorillonite K10 clay at room temperature. The diene was found
to unergo isomerization of a disubstituted double bond to a trisubstituted double bond, whereas the triene underwent cyclization.
The results were rationalized by assuming that K10 clay acts as
Brønsted acid and hence protonates one of the double bonds to give
tertiary cations 82 and 83, which finally gave the observed products
84, 85 and 86 (Belt et al., 2000) (Scheme 87). The authors claim that
the work has implications in understanding the presence of the
observed organic compounds in sedimentary materials.
Dintzner et al. (2004) have investigated the migration of isoprenyl
group in phenyl isoprenyl ether (87) using montmorillonite K10 and
montmorillonite KSF clays. With K10 clay the isoprenyl group moves
mainly to ortho-position by a [1,3] migration to give ortho-prenylated
phenol 88 (Scheme 88). o-Prenylated phenols show broad range of
pharmacological activity, and therefore the reaction has the potential as
a ‘green chemical’ method of synthesis of o-prenyl phenolic derivatives.
Scheme 79. 1,2-Epoxyoctane ring opening.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 82. Isomerization and cracking of 1-butene.
Scheme 85. cis-Lauthisan to trans-lauthisan isomerization.
Scheme 83. Allylic nucleophilic substitution–isomerization.
The isoprenyl group also migrates to the para-position by a [1,5]
migration to give a small amount of p-prenylated phenol 89. o-Prenyl
phenol 88 undergoes further protonation and cyclization to dihydropyran derivative 90. The authors observed that the mont.KSF clay also
catalyzes the isomerisation but at a very slow rate. For example, with
mont.K10, the reaction is complete in just 30 min, while with mont.KSF
it takes 15 h to complete. The K10 catalyzed reaction was faster in
CH2Cl2, but in CCl4 the selectivity was higher.
Prenylated phenolic compounds, such as prenylated xanthones
have been prepared by microwave irradiation of hydroxyxanthones
(91a–c) with prenyl bromide (92) in chloroform solution in the
presence of montmorillonite K10 clay. The reaction with 91a takes
just 20 minutes to give about 86% yield of the final dihydropyran
product 93 (Castanheiro et al., 2009) (Scheme 89). The conventional
heating at 100 °C takes 1 h and at room temperature the reaction
takes 5 days to give 63% yield of the product. When the reaction was
carried out at 200 °C with ZnCl2 as catalyst, the yield of the product
was only 22%. The other two hydroxyxanthones 91b and 91c give
mixtures of products because two ortho-positions are available for
cyclization in the prenyl ether intermediates. In the absence of K10
Scheme 86. Stereoregulated substitutive migration of phenyl group.
clay, microwave irradiation of a mixture of 91a and 92 delivers
intermediate prenyl ether (not shown in the scheme) and not 93. The
authors have not suggested any mechanism for the reaction, but it is
conceivable that initially prenyl ethers are formed which then
undergo [1,3] migration, followed by proton catalyzed cyclization of
the intermediates to the observed products 93–98 (Scheme 89).
A [3,3]-sigmatropic shift following the addition of arylhydrazine
hydrochlorides (99) to cyclic enol ethers and enol lactones (100) takes
place, in a manner exactly similar to the one in Fischer indole reaction, to
give a variety of substituted indoles (101) (Scheme 90). The reaction is
catalyzed by several Brønsted acids. However, montmorillonite K10
promoted reaction in 50% aqueous N,N-dimethylacetamide at 80 °C
gave the best yields. Amberlyst-15 and amberlyst-120 gave reasonably
good results, but zeolite-HY and silica performed poorly (Singh et al.,
2008).
Scheme 84. Addition–isomerization reaction.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
133
Scheme 87. Isomerization of isoprenoid olefins.
8. Oxidation reactions
In this section oxidation reactions that are brought about by a few
different types of oxidizing reagents in the presence of clay catalysts are
reviewed. The reactions described include oxidation of aldehydes to
carboxylic acids, alcohols to carbonyl compounds, epoxidation, Baeyer–
Villiger oxidation, oxidation and dehydrogenation of hydrocarbons.
An interesting difference between montmorillonite K10 and
montmorillonite KSF in their interaction with aldehydes has been
observed by Dintzner et al. (2010) (Scheme 91). For example, the K10
clay mainly catalyzes the trimerisation of aldehydes to trialkyl 1,3,5trioxanes, which is a temperature dependent equilibrium reaction
with trimer-to-aldehyde ratio of 3.9:1 attained at −20 °C. However,
when KSF clay is used, oxidation to carboxylic acid takes place under
aerobic conditions. Only aliphatic aldehydes undergo oxidation, but
not the aromatic or the unsaturated aliphatic aldehydes.
The common procedures for the oxidation of alcohols to ketones or
aldehydes make use of compounds of transition metals, such as
chromium, manganese, vanadium, etc., which are toxic. It is desirable
to reduce their use or replace them by oxidants that are less hazardous
Scheme 88. [1,3] Migration of prenyl group-cyclization of product.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
Scheme 89. Preparation of prenylated xanthones and their cyclization.
to health and environment. Many clay-based oxidizing agents that fulfill
this Green chemistry condition have been developed. Eftekhari-Sis et al.
(2007) reported the oxidation of alcohols to aldehydes and ketones
using hydrogen peroxide as oxidizing agent in the presence of lithium
chloride supported on montmorillonite K10. The proposed mechanism
involves the formation of lithium hypochlorite, which is the active
oxidizing agent (Scheme 92).
Hydrogen peroxide or iodosylbenzene epoxidizes cyclooctene to
cyclooctene epoxide and cyclohexane to cyclohexanone in the presence
of modified natural saponite clay (Scheme 93). The modification
procedure involves first, the intercalation of the clay with aluminium
polycation, followed by calcination at 500 °C to get alumina pillared clay
and then impregnation with nickel nitrate or chloride or acetoacetate
(Mata et al., 2009). The amount of nickel loaded may be varied. The
nitrate and acetoacetate added catalysts give higher yields of products.
The oxidation takes place by a free radical mechanism.
Cyclohexanone undergoes Baeyer–Villiger oxidation to ε-caprolactam
by hydrogen peroxide in the presence of Brazilian kaolinite intercalated with a porphyrin derivative, [meso-tetrakis(pentafluorophenyl)
porphyrin]-iron(II), Fe(TPFPP). The porphyrin moiety is introduced
between the clay layers after expanding and functionalizing the
interlayer space by pretreatment of the clay. The authors (Bizaia et al.,
2009) claim that this is the first example of a porphyrin-in-clay
catalyzed Baeyer–Villiger reaction. In the presence of the same catalyst
iodosylbenzene brings about the epoxidation of cyclooctene and
oxidation of cylcohexane to cyclohexanone (Scheme 94). The catalyst
was reused up to five times without significant diminution in the
yields of products.
Scheme 90. Arylhydrazine addition to cyclic enol ethers and enol lactones to form indoles.
G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
135
Scheme 91. Difference between K10 and KSF action on aldehydes.
Scheme 94. Porphyrin-in-clay catalyzed oxidation reactions.
Scheme 95. V2O5-on-clay/H2O2-AcOH for hydroxylation of benzene to phenol.
Scheme 92. Oxidation of alcohols to aldehydes and ketones.
Gao and Xu (2006) have used vanadium oxide catalyst supported
on clay (chlorite, illite, and atapulgite from Inner Mongolia) for the
oxidation of benzene to phenol using hydrogen peroxide as cooxidant. The presence of acetic acid enhances the hydrogen peroxide
reaction as it avoids phase separation problem. A 14% conversion with
94% selectivity was achieved (Scheme 95). Several other oxides of
metals, such as copper, iron, manganese, chromium, molybdenum,
tungsten, etc., were tried in place of vanadium, but were found to be
not as effective. However, the TS-1 was found to be equally good.
Nitric acid and metal nitrate salts are used as both oxidizing and
nitrating agents depending on the nature of the substrate and reaction
conditions. In most cases the reaction is slow, but if vigorous conditions
are used to hasten the reaction the substrate may decompose. However,
reports show that such reactions can be accomplished under very mild
conditions, if the reagents are supported on smectite clays. For example,
Scheme 96. Substrate-based oxidation or nitration using nitric acid or nitrate salts.
dilute nitric acid in the presence of natural bentonite oxidizes benzylic
alcohols to the corresponding carbonyl derivatives. In contrast, the same
reagent-catalyst system nitrates activated aromatic compounds regioselectively (Bahulayan et al., 2002) (Scheme 96).
Ethylbenzene has been dehydrogenated to styrene using a
processed Venezuelan smectite clay intercalated with trinuclear iron
complex, [Fe3O(OAc)6.3H2O]+. Ethylbenzene was heated over the
catalyst for 1 h at 410 °C. There was 50% increase in styrene formation
with the use of this catalyst (Huerta et al., 2003) (Scheme 97).
Scheme 93. Cyclooctene epoxidation and cyclohexane oxidation by H2O2 or C6H5IO.
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G. Nagendrappa / Applied Clay Science 53 (2011) 106–138
References
Scheme 97. Ethylbenzene dehydrogenation to styrene.
Scheme 98. Pt-in-synthetic TS-1 in dehydrogenation of ethylbenzene to styrene.
Dehydrogenation of ethylbenzene has been carried out at 400 °C
using natural or aluminium pillared Venezuelan clay impregnated with
cobalt nitrate or cobalt acetate. The selectivity for styrene varies from 75
to 90% and the conversion is 20% (Gonzalez and Moronta, 2004).
Moran et al. (2007) have dehydrogenated ethylbenzene to styrene
using synthetic clay TS-1 impregnated with 0.5 or 1.0 wt.% of platinum.
A 50% conversion was achieved (Scheme 98). The activity of Pt-TS-1
catalyst is attributed to its high thermal stability and improved surface
area of the support.
9. Conclusions
Natural as well as commercially available clays and their diverse
modified forms are good catalysts for accomplishing a large variety of
organic reactions. Since the procedures employed for their modification
are usually simple chemical operations, there is great scope to prepare
newer clay-supported catalysts that are capable of steering organic
reactions in any desired direction to achieve higher yields and greater
selectivity including diastereo- and enantioselectivity. The present
knowledge about the application of a modified catalyst for a particular
reaction is still empirical; it is difficult to predict accurately whether
newly modified clay can catalyze a specific reaction in a desired manner.
Some kind of structure activity correlation studies could throw some
light on this aspect.
Clays are ubiquitous. Many scientific groups from different parts of the
Globe have shown that the clay varieties available in their geographical
locality are as good as the commercially available ones. This provides a
great opportunity to synthetic chemists to get involved in low budget
research activity, even in undergraduate colleges, and yet achieve
significant results. This is particularly relevant as part of Green chemistry
curriculum.
There is much scope and need to translate laboratory scale
procedures to industrial scale procedures, as there is very little activity
at present in this aspect. This is particularly important because clay
catalysts are environmentally benign, recyclable and economical, and
there is urgent need to replace the not-so-desirable conventional
catalysts. There can probably be no other catalytic system that would
qualify to be called ‘Green’ than the clay-based one.
Acknowledgements
The author is much indebted to the following persons for their
support: Mr. R.K. Rao, Dr. S. Hari Prasad, Dr. M.N. Mallya, Dr. Ravi Kalyan
and Ms. Divya Jyothi in literature search and my wife Dr. Renukarani and
my son Mr. Suchit Gopalpur in preparing the manuscript.
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