Microwave-assisted nafion-H catalyzed friedel-crafts type reaction of aromatic aldehydes with arenes: Synthesis of triarylmethanes

Article abstract of DOI:10.1007/s10562-010-0406-0

A new solid acid Nafion-H, a perfluorinated sulfonic acid resin, catalyzed microwave-assisted synthesis of triarylmethanes is described. Various benzaldehydes react readily with arenes to provide the corresponding triarylmethanes in good to excellent yields. The reactions were carried out under solvent free conditions under microwave irradiation in a pressure vessel. The solvent free microwave irradiation methods appears to be an environmentally friendly synthetic protocol providing products in significantly shorter reaction times over traditional heating methods carried out in a pressure tube.

Full text of DOI:10.1007/s10562-010-0406-0

Catal Lett (2010) 138:155–159
DOI 10.1007/s10562-010-0406-0
Microwave-Assisted Nafion-H Catalyzed Friedel–Crafts Type
Reaction of Aromatic Aldehydes with Arenes: Synthesis
of Triarylmethanes
•
Surya G. K. Prakash Gabriella Fogassy
•
George A. Olah
Received: 12 May 2010 / Accepted: 20 June 2010 / Published online: 2 July 2010
Ó Springer Science+Business Media, LLC 2010
Abstract A new solid acid Nafion-H, a perfluorinated
sulfonic acid resin, catalyzed microwave-assisted synthesis
of triarylmethanes is described. Various benzaldehydes
react readily with arenes to provide the corresponding tri-
arylmethanes in good to excellent yields. The reactions
were carried out under solvent free conditions under
microwave irradiation in a pressure vessel. The solvent free
microwave irradiation methods appears to be an environ-
mentally friendly synthetic protocol providing products in
significantly shorter reaction times over traditional heating
methods carried out in a pressure tube.
readily occurs under highly acidic conditions for example
over 2 equivalent of AlCl3 at 60 °C [8] to give a number of
products such as triphenylmethane, diphenylmethane, tri-
phenylmethanol and anthracene in yields varying from 30
to 60% [9]. Further work showed that substituted trip-
henylmethanes and anthracenes were formed by the
reactions of other aromatic aldehydes with aromatic
hydrocarbons such as toluene and xylenes in presence of
excess AlCl3 [10–13]. The initial step in these reactions
was considered to be the decarbonylation of the aldehydes,
the reverse of a Gattermann–Koch reaction [14–17], rather
than a transformylation reaction. The aldehyde derived
from a transformylation reaction was not detected. The
carbon monoxide produced by the decarbonylation was
presumed to react with the aromatic hydrocarbons in some
unspecified way to produce the triarylmethanes and
anthracenes. To ascertain the mechanism, a number of
aromatic aldehydes with a variety of arenes and Lewis and
Brønsted acid catalysts were examined [18]. In many
reactions, triarylmethanes were observed that did not
contain the aromatic moiety corresponding to the original
aldehydes. This might be interpreted as evidence for initial
formation of the ‘‘other aldehydes’’ by transformylation,
followed by subsequent addition reactions to produce the
triarylmethanes.
Keywords Nafion-H ꢀ Microwave irradiation ꢀ
Arylation ꢀ Triarylmethanes ꢀ Benzaldehyde
1 Introduction
Over the past two decades, microwave-assisted organic
synthesis has emerged as an important area that has
attracted significant attention as indicated by extensive
number of publications [1–3]. It is a convenient and time
saving method, which promotes the application of envi-
ronmentally benign approaches such as solvent free and
heterogeneous catalytic reaction conditions [4, 5].Triar-
ylmethanes constitute an important class of compounds
that are ubiquitous in materials, medical, and dyestuff
fields [6]. Acid-catalyzed condensation of benzaldehyde
with benzene is known since 1886 [7]. This classic reaction
To establish the mechanism, different acids were studied
at room temperature [19]. For example in 100% H2SO4
(H0 = -12) [20] benzaldehyde did not react with benzene
at room temperature. In trifluoromethanesulfonic acid
(H0 = -14) however, the reaction is complete in 30 h,
giving a 90% yield of triphenylmethane at room tempera-
ture. Addition of 1% SbF5 to trifluoromethanesulfonic acid
(H0 = -16) enhanced the reaction rate, and the reaction
was completed in 18 h at room temperature. In stronger
acid, CF3SO3H2?B(OSO2CF3)4- (H0 = -18), the reaction
S. G. K. Prakash (&) ꢀ G. Fogassy ꢀ G. A. Olah
Donald P. and Katherine B. Loker Hydrocarbon Research
Institute and Department of Chemistry, University of Southern
California, University Park, Los Angeles, CA 90089-1661, USA
e-mail: gprakash@usc.edu
123

156
G. K. Surya Prakash et al.
Scheme 1 Mechanism for the
superacid-catalyzed
condensation reaction of
benzaldehyde with benzene
+OH2
H
+OH
H
+OH
H
H
O
H
C
C
C
C
H+
H
+
+
+
H
H
OH
+
2.2 General Method for Thermal and Microwave
Induced Condensation Reactions
was instantaneous with 83% yield of the isolated product.
Similar studies are reported in liquid Brønsted superacids
using isomeric pyridine and quinoline carboxaldehydes as
well as aromatic dialdehydes [21, 22]. This study suggests
that the monoprotonated aromatic aldehydes are not strong
enough to react with aromatics. In O-protonated benzal-
dehyde the carboxonium ion is stabilized both by the
oxygen and by the phenyl group, resulting in a weak carbon
electrophile (Scheme 1). To decrease the neighbouring
group participation, further protonation or protosolvation is
required that will increase the electrophilic reactivity of the
carbonyl carbon. Further protonation of protonated benz-
aldehyde can take place either on oxygen or on carbon on
the phenyl ring.
The thermally heated reactions were carried out in a 15 mL
pressure tube loaded with Nafion-H (200 mg, *0.2 mmol
protonic site), benzaldehyde derivative (50 mg, *0.5 mmol)
and benzene (4 mL). The resulting suspension was magneti-
cally stirred at a controlled temperature. After cooling, the
solid resin sulfonic acid was filtered off, water was introduced
to the reaction mixture followed by extraction using dichlo-
romethane. The combined organic phase was dried over
anhydrous Na2SO4 and the solvent evaporated to obtain the
product/products.
The microwave-induced reactions were carried out in a
7 mL pressure resistant Pyrex glass tube equipped with a
rubber septum and an aluminium cap. The capped reaction
tube with the above mentioned suspension was introduced
into the cavity of the microwave apparatus (CEM Discover,
2.45 GHz, CEM Corporation, NC, USA) and magnetically
stirred at a constant temperature (150 °C) controlled by an
IR sensor. The pressure of the reaction vessel with various
reactants used hovered between 2 and 7 bars. The product
isolation was identical to that of the thermal method.
In this article, we describe a novel, environmentally
benign approach for the synthesis of a wide variety of triar-
ylmethanes using the heterogeneous strong solid acid, Naf-
ion-H under microwave irradiation. The combination of
microwave irradiation and solid superacid catalyst, Nafion-
H (estimated H0 C -12 [23]), decreases the reaction time
significantly. In order to evaluate the effect of the microwave
on the reaction rates, yields and conditions, the reaction was
carried out under classical heating conditions as well.
2.3 Analysis
2 Experimental
The products are all well known [and were identified on the
1
2.1 Materials
basis of their H and 13C NMR (Varian 400 MHz) and
mass spectra (Thermo Finigan GC coupled with a mass
spectrometer) as well as with comparison with the litera-
ture data [22]. The isomer distribution of the product was
The Nafion-H catalysts were prepared from commercial
(DuPont) Nafion-K resin according to the procedure
described in [23]. The different aromatic aldehydes and
arenes were supplied by Aldrich and used as received.
1
determined by H NMR and gas chromatographic analysis
Scheme 2 Reaction of
R'
H
O
benzaldehyde derivatives with
aromatics over Nafion-H
catalyst
C
R
Nafion-H
+
+
°
150 C
R
R'
R'
R
R'
123

Microwave-Assisted Synthesis of Triarylmethanes Over Nafion-H
157
Table 1 Microwave and conventionally heated reaction of benzaldehyde derivatives with benzene
Entry
Aldehyde
Product
Microwavea
Classical
Heating
Time (min)
Yield (%)
92
Time (min)
720
Yield (%)
97
CHO
1
2
3
4
70
O2N
CHO
CHO
30
100
100
79
600
1920
480
100
100
74
O2N
NC
F
NC
190
70
F
CHO
Cl
CHO
CHO
CHO
CHO
5
6
7
8
30
25
30
90
94
94
94
21
300
240
92
94
94
26
Cl
Br
Br
F3CO
600
F3CO
H3C
1080
H3C
CHO
9
70
26
720
45
Conditions: 150 °C, 0.5 mmol aldehyde, 2 mL benzene, 0.4 g Nafion-H
a
50–150 W, 2–7 bar pressure
Thermo Finigan GC (DB-5 capillary column, with helium
as the carrier gas).
microwave irradiation. For comparison, the reaction has
been carried out using traditional heating methods using an
oil bath. The reactions were carried out under solvent free
conditions using excess of aromatics. Different aromatics
and benzaldehydes were used for the arylation reactions.
The Friedel–Crafts arylation of arenes can produce several
products and in some cases isomeric products (Scheme 2).
Wide ranges of triarylmethanes were prepared using
benzene as the reactant with different benzaldehydes
(Table 1). All reactions were driven to 100% conversion.
3 Results and Discussion
Our goal was to develop an efficient and at the same time,
environmentally friendly method for the synthesis of tri-
arylmethanes. The condensation of benzaldehydes with
arenes over Nafion-H catalyst was developed under
123

158
G. K. Surya Prakash et al.
Table 2 Microwave and conventionally heated reaction of benzaldehyde with aromatics
Entry
Aromatic
Product
Microwavea
Classical
Heating
Time (min)
Yield (%)
Time (min)
240
Yield (%)
1
2
Toluene
25
70 (oo:op:pp = 15:17:68)
82 (oo:op:pp = 3:27:70)
CH3
CH3
CH3
H3C
H3C
p-Xylene
10
30
120
68
CH3
Cl
Cl
3
Chlorobenzene
160
62 (oo:op:pp = 19:32:49)
2700
46 (oo:op:pp = 8:28:64)
Conditions: 150 °C, 0.5 mmol benzaldehyde, 2 mL aromatics, 0.4 g Nafion-H
a
50–150 W, 5–7 bar
Table 3 Microwave and conventionally heated reaction of p-nitro-benzaldehyde with aromatics
Entry
Aromatic
Product
Microwavea
Classical
Heating
Time (min)
Yield (%)
Time (min)
270
Yield (%)
1
2
3
Toluene
30
95 (oo:op:pp = 15:17:68)
90 (oo:op:pp = 3:21:76)
CH3
CH3
O2
N
CH3
H3
C
O2
N
p-Xylene
15
66
120
82
CH3
H3
C
Cl
Cl
O2
N
Chlorobenzene
155
100 (oo:op:pp = 4:35:61)
2520
100 (oo:op:pp = 5:35:60)
Conditions: 150 °C, 0.5 mmol p-nitro-benzaldehyde, 2 mL aromatics, 0.4 g Nafion-H
a
50–150 W, 5–7 bar
In most cases the formation of triarylmethanes was pre-
ferred, with benzene, however, diarylmethane was also
formed (Entries 4, 8 and 9).
Arylation of benzaldehyde has been performed with
toluene, p-xylene and chlorobenzene (Table 2) in order to
evaluate effect of the nature of the reactants.
In any case, microwave or classical heating result in
comparable yields for the respective triarylmethanes
derivatives. However, benzaldehyde containing weakly
electron donating groups such as a phenyl or a methyl
group (Entry 8 and 9, Table 1) result in lower product
yields under microwave irradiation 21%/26% and 26%/
45%, respectively. Using benzaldehydes with strongly
electron-withdrawing groups such as nitro- or cyano-,
quantitative yields can be achieved. However, the longest
reaction time was observed with cyanobenzaldehyde.
Among halobenzaldehydes, the chloro- and bromo-deriv-
atives gave higher yields than the fluoro-derivatives.
Microwave mediated heating results in much faster reac-
tion times compared to regular heating.
The different aromatics have different effects on the
product yields: alkylbenzene derivatives gave higher yields
for microwave-irradiated reactions and halogen ones gave
higher yields with classical heating. An isomeric mixture of
three triarylmethanes was obtained (Table 2) in case of
mono-substituted aromatics, corresponding to the ortho–
ortho0 (oo), ortho–para0 (op) and para–para0 (pp) posi-
tional isomers as expected. With the conventional heating
the para–para0 products were the most preferred. With the
microwave irradiation, the isomeric distribution moved
toward the formation of ortho–ortho0 products indicating
higher reactivity.
The study was also extended to p-nitro-benzaldehyde
(Table 3).
123

Microwave-Assisted Synthesis of Triarylmethanes Over Nafion-H
159
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A novel effective solid superacid-catalyzed, economic and
environmentally benign method for triarylmethane syn-
thesis has been developed. The combination of microwave
irradiation and Nafion-H catalyst greatly reduce the reac-
tion time compared to the classical heating methods. The
short reaction time, solvent free condition, ease of product
isolation, and the safe nature of the catalyst make the
process an attractive alternative for the synthesis of triar-
ylmethanes in a practical way.
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Acknowledgments Financial support provided by the Loker
Hydrocarbon Research Institute is gratefully acknowledged.
´
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