Reaction control in heterogeneous catalysis using montmorillonite: Switching between acid-catalysed and red-ox processes

Article abstract of DOI:10.1039/c6ra05293b

The use of montmorillonite, modified with a super-acid (CF₃SO₃H), in the presence of hydroquinone as a radical scavenger and under a nitrogen atmosphere, induced the formation of tetrasubstituted furans as the major product from benzoins. In the absence of a radical scavenger, the only products obtained were 1,2-diketones.

Full text of DOI:10.1039/c6ra05293b

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Reaction control in heterogeneous catalysis using
montmorillonite: Switching between acid-catalysed and red-ox
processes
Received 00th January 20xx,
Accepted 00th January 20xx
José Antonio Morales-Serna,*a Bernardo A. Frontana-Uribe,b,c Rosario Olguín,c Virginia Gómez-
Vidales,c Leticia Lomas-Romero,a Erendira Garcia-Ríos,c Ruben Gaviño,c Jorge Cárdenas*c
DOI: 10.1039/x0xx00000x
www.rsc.org/
The use of montmorillonite, modified with a super-acid (CF3SO3H), in the presence of hydroquinone as a radical scavenger
and under a nitrogen atmosphere, induced the formation of tetrasubstituted furans as the major product from benzoins.
In the absence of a radical scavenger, the only products obtained were 1,2-diketones.
.
Usually, the condensation of
HCl,10
α
presence of p-toluensulfonic
-hydroxyl ketones in the
acid11
or
1. Introduction
iodotrimethylsilane12 gives a mixture of products, where
tetraphenylfuran is typically the principal product (Scheme 1).
Inspired by biocatalytic systems, artificial switchable1
catalysts2 have recently received increased attention because
of their ability to perform specific and complex tasks as a
consequence of external stimuli such as light irradiation,3 pH
variation,4 the introduction of metal ions,5 or modification of
the reaction conditions (solvents, temperature, use of
additives or agitation conditions).6 Despite astonishing
developments in the field of switching founded on changing
reaction conditions, only one example has been reported on
Meanwhile, the oxidation of
α-hydroxyl ketones into the
corresponding 1,2-diketones can be carried out in the
presence of a wide variety of metals,13 including Fe14 (Scheme
1). In this work, the general idea is based on the ability of
montmorillonite to participate as a Lewis or Brønsted acid
catalyst in organic reactions15-16 and as a generator of free
radical species in redox processes.17 This dual functionality
introduces an attractive possibility for the development of a
switching process in presence of super-acid montmorillonite,
that results from the amount and type of metals contained in
this type of materials and makes studying these materials an
interesting research topic.
the switching of
a
heterogeneous catalyst such as
montmorillonite (clay) achieved by the use of different
solvents.7 In that report, solvent induced selectivity was
observed in the acid catalysed allylsilylation, arylsilylation and
silylation of alkynes. To the best of our knowledge, the use of
different reaction conditions to stimulate different non-
interfering metals within a clay such as montmorillonite and to
give rise to a switching process has not been explored.
Oxidation
process
Ar
Condensation
process
O
O
Ar
Ar
Ar
Ar
Ar
Metals
Acid conditions
Ar
Ar
O
OH
O
Herein, we report our recent observation of the ability to
tune the highly chemoselective condensation and oxidation of
benzoins by altering the reaction conditions, which provides
new protocols for the synthesis of tetrasubstituted furans8 or
1,2-diketones,9 depending on the conditions employed.
Scheme 1. Synthesis of tetrasubstituted furans and 1,2-diketones
2. Results and discution
2.1. Material catalytic
Montmorillonite is
a representative 2:1 clay mineral
a.Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av.
San Rafael Atlixco No. 186, Ciudad deMéxico 09340, México; Fax: +52-55-5804-
4666; Tel.: +52-55-5804-491; E-Mails: joseantonio.moralesserna@xanum.uam.mx
(JAMS).
composed of units that consist of two tetrahedral silica sheets
and an octahedral alumina sheet. It exhibits isomorphous
substitution, which produces an excess negative charge. This
excess negative charge is compensated for by the adsorption
of exchangeable cations on the surface layer.18 The treatment
of these clays with acids and super-acids maximizes the
number of Brønsted catalytic sites by replacing exchangeable
cations with protons.19 In the same way, the structural
b.
Centro Conjunto de Investigación en Química Sustentable, UAEM-UNAM,
Carretera Toluca-Ixtlahuaca Km 14.5, C.P. 50200 Toluca, Estado de México,
México.
c. Instituto de Química, Universidad Nacional Autónoma de México, Circuito
Exterior, Ciudad Universitaria, Ciudad de México 04510, México. Fax: +52 55 5616
2217; Tel: +52 55 56224413; E-mail: rjcp@unam.mx (JC).
† Electronic Supplementary Informaꢀon (ESI) available: Experimental details and
characterization data of all compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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transformation brought about by super-acid treatment allows respectively, after three days of reaction. In absence of
free aluminium ions to migrate from the alumina octahedral catalyst, the yield is 15% after the same time.
sheets into the interlayer region, where they displace sodium
With these results in hand, we next investigated the scope
ions, maximizing the number of Lewis catalytic sites present.20- of the condensation and oxidation processes with aromatic
α-
21
hydroxy-ketones 1b-1f to demonstrate whether it is possible
to selectively tune the reaction by controlling the reaction
conditions. To do this, we first studied the reaction in the
presence of hydroquinone under a nitrogen atmosphere. As
Table 1. Elements present in the clay samples was determined by EDS
Elemental concentration weight (%)
Element
shown in Table 4, tetrasubstituted furans 2b-2f were obtained
Natural clay
Clay/CF3SO3H
as major products in excellent yield. On the other hand, when
the reaction was carried out in the absence of hydroquinone
Na2O
K2O
2.60
0.33
3.52
21.30
65.48
1.05
-
0.13
0.30
2.06
13.36
74.60
1.65
2.30
0.27
0.03
5.30
and under an open atmosphere, phenyl α-hydroxy-ketones 1b-
1f were easily converted to desired 1,2-diketones 5b-5f with
MgO
Al2O3
SiO2
CaO
SO3
excellent yields (Table 4).
Table 3. Oxidation and condensation of benzoin
TiO2
MnO
FeO
0.33
0.31
5.08
O
Ph
Ph
Ph
Ph
Ph
Ph
O
OH
Ph
Ph
O
2a
Montmorillonite/CF3SO3H
20%
O
3a
Ph
OH
Ph
O
O
Benzene, reflux
The chemical compositions of montmorillonite and of
montmorillonite modified with a super-acid (CF3SO3H) were
determined by energy-dispersive X-ray spectroscopy (EDS).
The montmorillonite was found to contain Na, K, Mg, Al, Si, Ca,
Ti, Mn, Fe and S, the last of which was provided by the acid
used while the other elements were found in the natural clays
(Table 1). The use of CF3SO3H substantially enhanced the
surface area, pore volume and pore diameter of the modified
Ph
Ph
1a
Ph
Ph
Ph
Ph
4a
O
O
5a
Yield %a
Entry
Product
With hydroquinoneb
Without hydroquinonec
1
2
3
4
2a
3a
4a
5a
60
5
0
0
montmorillonite
compared
with
the
unmodified
5
0
montmorillonite (Table 2). Ample discussion about the
characterisation of this material has been provided in previous
work.17,20
25
95
a
b
Yield of isolated product after chromatographic purification. Reagents and
conditions for the reaction: benzoin 1 (1 mmol), clay (20%w), hydroquinone (50%
mmol), benzene or toluene (50 mL), N2 atmosphere, reflux, 12 h. cBenzoin 1 (1
mmol), clay (20%), benzene or toluene (50 mL), open atmosphere, reflux, 6.
Table 2. Properties of montmorillonite clays17,20
Parameters
Natural clay
Clay/CF3SO3H
Surface area (m2/g)
Pore volume (cc/g)
Pore diameter (Å)
23
185
On the basis of the above results, which are consistent with
previous mechanistic proposals in which an acid catalyses the
condensation reaction,10,11 a putative reaction mechanism is
shown in Scheme 2. Modified montmorillonite clays are known
for their acidic properties and have been used as efficient solid
acid catalysts in a number of organic reactions that require
Brønsted and/or Lewis acid sites.15-16 Here, we hypothesize
that the protons and aluminium ions present in the interlayer
region as a consequence of the treatment with CF3SO3H are
responsible for catalysing the condensation reaction. The first
step of this reaction is the self-condensation of benzoin 1a to
0.140
23.00
0.603
107.79
2.2. Catalytic activity
Once we characterised the material, we turned our focus
to the study of the switching process. We found that the
condensation of benzoin 1a in the presence of modified
montmorillonite, hydroquinone and using benzene or toluene
as solvent afforded a 60% isolated yield of tetraphenylfuran 2a
product (Table 3, entry 1). Additionally, it was possible to
A
, which is a precursor of
H2O, furnishing deoxybenzoin
the past as a key intermediate in this condensation reaction.
B
and 5a. The intermediate
B loses
C
, which has been proposed in
isolate and identify three side products, 3a, 4a and 5a, whose
origin will be explained later (Table 3, entries 2 and 3). The
heterogeneous reaction, when carried out in the absence of
hydroquinone, produces 1,2-diketone 5a as the only product
(95%, Table 3, entry 4). It is noteworthy, that the modified
montmorillonite can be recycled four times without significant
losses in its catalytic activity in both processes, and that when
the reactions were carried out in presence of natural clay the
products 2a and 5a were obtained in low yield, 20 and 30%
Deoxybenzoin
C
condenses with other molecule of benzoin 1a
,
forming a carbon-carbon bond and resulting in intermediate
D
.
After the elimination of two molecules of H2O,
tetrasubstituted furan 2a is obtained from . Finally, the
formation of compounds 3a and 4a can be rationalized from
intermediate (Scheme 2). However, neither of these
products (3a and 4a), nor 1,2-diketone 5a 22
are precursors of
D
A
,
2 | J. Name., 2012, 00, 1-3
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tetrasubstituted furan 2a, because they were subject to
reaction conditions studied in this work, tetrasubstituted furan
2a was not obtained. It is important to remark that the origin
of 5a in condensation process is different than its proposed
origin in the oxidation reaction. In this case 5a is
a
disproportionation product that occurs as a consequence of a
thermal process. Conversely, in the oxidation reaction, 5a is
produced by a free radical process as discussed below.
Table 4. Switching between acid-catalysed and redox processes
Catalyst 20%
Hydroquinone
Benzene, reflux,
N2 atmosphere
Catalyst 20%
Benzene, reflux,
open atmosphere
O
5
Ar
O
Ar
Ar
Ar
Ar
Ar
O
Ar
Ar
O
2
Condensation process
Lewis and Brønsted
acid catalysis
Redox process
Fe (III) catalysis
OH
1
Product (Yield %)a
With hydroquinoneb
Without
hydroquinonec
Entry
Ar
1
2
3
4
5
4-MeC6H4 1b
4-MeOC6H4 1c
4-EtC6H4 1d
4-BrC6H4 1e
3-MeC6H4 1f
2b (62)
5b (33)
2c (65)
5c (33)
2d (65)
5d (31)
2e (60)
5e (28)
2f (60)
5f (32)
-
5b (95)
-
5c (95)
-
5d (95)
-
5e (90)
-
Figure 1. EPR spectra of: a) super-acid montmorillonite without solvent, b) super-acid
montmorillonite in benzene at 70 °C scanned every 5 minutes and c) benzoin 1a in
benzene at 70 °C in the presence of super-acid montmorillonite scanned every 5
minutes.
5f (95)
a
b
Yield of isolated product after chromatographic purification. Reagents and
conditions for the reaction: benzoin 1 (1 mmol), clay (20%w), hydroquinone (50%
mmol), benzene (50 mL), N2 atmosphere, reflux, 12 h. cBenzoin 1 (1 mmol), clay
(20%), benzene (50 mL), open atmosphere, reflux, 6 h.
The high selectivity of the oxidation processes in the
heterogeneous conditions studied herein cannot be explained
in terms of a Lewis or Brønsted acid catalysed mechanism, but
rather as a result of performing the reactions in the presence
of super-acid treated montmorillonite. The treatment with
acid could dissolve structural iron in the clay, allowing the now
free iron cations to reach the interlayer or surface of the
clay,17,20 where the oxidation of benzoin 1a to benzil 5a may be
catalysed. To obtain more insight on this point, the EPR
spectrum of modified montmorillonite was recorded without
solvent and is shown in Figure 1a. The six peaks with a
hyperfine coupling resonance signal centred at g=2.01 are
assigned to MnO2 and MnO species, which possess S=5/2 and
S=1/2 electron spin states, respectively. Meanwhile, the singlet
signal at approximately 165 mT and g=4.36 corresponds to
high-spin Fe(III), which is also present in the clay. These signals
disappeared when the spectrum was recorded at 70 °C and
used benzene as solvent (Figure 1b). To our delight, when
benzoin 1a was added to the EPR tube and the experiment was
recorded at 70 °C, the presence of a free radical centred on
carbon was observed at approximately 328 mT g = 2.003, after
a half hour of reacting (Figure 3c). Once the oxidation was
completed, that signal disappeared. Additionally, this signal
was not observed when the same experiment was carried out
in presence of hydroquinone. Based on these results, we
propose that benzoin 1a donates an electron to Fe(III) to form
O
Ar
OH
H
H
Ar
Ar
O
HO OH
O
Ar
Ar
O
O
Ar
Ar
OH
OH
2
Ar
OH
Ar
Ar
1a
B
A
5a
1a
Ar
Ar
Ar
OH
H
Ar
-2 H O
H
Ar
Ar
Ar
Ar
Ar
H
2
Ar
Ar
OH
OH
OH
-
H O
2
Ar
Ar
Ar
O
O
H
O OH
D
2a
C
B
Scheme 2. Proposed mechanism of condensation process
Ar
OH2
H
Ar
OH
H
O
Ar
Ar
O
Ar
Ar
O
H
Ar
Ar
Ar
H
Ar
Ar
Ar
3a
O OH
O
OH
HO OH
A
H
E
[1,3]H
H
O
H
OH
Ar
Ar
Ar
Ar
Ph
4a
s gim atrop ic
s hift
-
H O
2
Ar
Ph
O
Ar
O
O
F
Scheme 3. Proposed mechanism for side products
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benzoin radical cation
G
. Next,
which isomerises to
Figure 1c). The donation of an electron from
furnishes , which loses a proton produce the 1,2-diketone 2.
G
loses a proton to produce Royal Society and Newton Fund (UK) for the International
(detectable in EPR, Alumni Grant.
to Fe(III)
radical structure H 23
,
I
I
J
Notes and references
Finally, Fe(II) may be reoxidised to Fe(III) by oxygen in the
atmosphere (Scheme 4).
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oxidation process, the reactions was carried out in presence of
silica and MCM-41, which are material free of Fe(III). In both
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Fe(II)
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Fe(III)
Scheme 4. Proposed mechanism of oxidation process
3. Conclusions
In conclusion, we have shown that for a montmorillonite
clay modified with a super-acid, two different modes of
behaviour can take place simply by a judicious choice of
reaction conditions. The introduction of free radicals favours
the oxidation of benzoins, while the presence of a free radical
scavenger triggers the condensation of the same benzoins. EPR
experiments provide significant support in determining the
radical mechanism involved in the oxidation reaction. Several
side products were isolated and characterised to shed light on
the acid-base mechanism related to the synthesis of
tetrasubstituted furans through the condensation reaction.
This finding enriches the understanding of the behaviour of
such materials in organic reactions. Further application in
different organic reactions and synthesis studies to further
investigate the material properties reported here are currently
underway in our laboratory.
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4. Acknowledgements
The authors wish to acknowledge the SNI-CONACyT
(Sistema Nacional de Investigadores) for the distinction of
their membership and their stipend. JAMS is grateful to the
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DOI: 10.1039/C6RA05293B
Reaction control in heterogeneous catalysis using montmorillonite: Switching between
acid-catalysed and red-ox processes
José Antonio Morales-Serna, Bernardo A. Frontana-Uribe, Rosario Olguín, Virginia Gómez-
Vidales, Leticia Lomas-Romero, Erendira Garcia-Ríos, Ruben Gaviño, Jorge Cárdenas
For a montmorillonite clay modified with a super-acid (CF3SO3H)., two different modes of behaviour
can take place simply by a judicious choice of reaction conditions.