Efficient synthesis of mono- and bis-arylated ethylenedioxy thiophene-based functional π-conjugated molecules via clay-supported palladium-catalysed direct arylation reactions

Article abstract of DOI:10.1007/s11164-020-04219-3

An active Pd catalyst was prepared by the modification of montmorillonite clay. The catalyst was found to be effective for the synthesis of ethylenedioxy thiophene-based π-conjugated molecules. The reaction proceeded through clay-supported Pd-catalysed di

Full text of DOI:10.1007/s11164-020-04219-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04219-3
Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy
thiophene‑based functional π‑conjugated molecules
via clay‑supported palladium‑catalysed direct arylation
reactions
Sneha Sreekumar1 · Soumya Xavier2 · Avudaiappan Govindan2 ·
Raihamol Erattammottil Thampikannu3 · Kannan Vellayan3 · Beatriz González4
Received: 11 May 2020 / Accepted: 20 July 2020
© Springer Nature B.V. 2020
Abstract
An active Pd catalyst was prepared by the modifcation of montmorillonite clay. The
catalyst was found to be efective for the synthesis of ethylenedioxy thiophene-based
π-conjugated molecules. The reaction proceeded through clay-supported Pd-cata-
lysed direct C–H bond activation (arylation) mechanism. Preparation, characteriza-
tion and application of the catalysts are reported. The products were analysed with
1
LCMS, and comparison with reported spectral data (FT-IR, H NMR). Reusability
of clay-supported palladium catalyst towards direct arylation reaction was tested for
three reaction cycles without a signifcant loss of catalytic activity.
Keywords Clay-supported catalyst · Direct arylation · EDOT
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-020-04219-3) contains supplementary material, which is available to authorized users.
*
Kannan Vellayan
kannanpvl@gmail.com
1
2
College of Integrated Studies, University of Hyderabad, Hyderabad, Telangana 500046, India
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi,
Kerala 682022, India
3
4
Department of Chemistry, Government College, Kattappana, Kattappana, Kerala 685508, India
GIR-QUESCAT, Departamento de Química Inorgánica, Universidad de Salamanca,
37008 Salamanca, Spain
Vol.:(0123456789)
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S. Sreekumar et al.
Introduction
Clays are the most abundant inexpensive natural material with high mechanical
strength and high chemical resistance. They possess a layered structure, which
can be exfoliated to yield an appreciable surface area which can be used for the
adsorption of molecules. Montmorillonite, a type of clay mineral, is chemically
described as a 2:1 clay. The structure contains an interlayer space between the
layers, where mobile-hydrated cations or water can be incorporated [1–3]. A new
class of polymer–clay composites is of great interest as they may be applied as
electroactive components in diferent electrochemical devices, like solid-state
batteries, fuel cells or electrochemical sensors [4–6].
Since conjugated polymers (CPs) have applications in the future generation
electronics such as solar cells [7, 8], light-emitting diodes [9], electronic displays,
electrochromic devices [10] and biosensors, they are one of the most popular top-
ics over the past few decades. These applications depend on the properties of CPs
which can be tuned via modifcations of the molecular structure [11]. Poly(3,4-
ethylenedioxythiophene), PEDOT, one of the derivatives of polythiophene, is
considered as an excellent conjugated conducting polymer because of its rela-
tively high conductivity and stability with temperature, commercial availability,
good biocompatibility, small intrinsic band gap, transparency in the visible range,
suitable morphology and fast doping–undoping mechanism [12–18]. EDOT-
based π-conjugated polymers have been extensively investigated, because they
exhibit unique properties such as electrochromic as well as electrooptic behaviour
[19–22].
The EDOT-based thienyl oligomers are mostly synthesized using in situ pre-
pared 3,4-ethylenedioxythiophenyl-2-magnesium bromide [18, 23–25]. Addition-
ally, the synthesis of EDOT-based thienyl oligomers has also been achieved using
stable 2-tributylstannyl-3,4-ethylenedioxythiophene and the 2-pinacolboronate
ester of 3,4-ethylenedioxythiophene [26]. The EDOT-based oligomers are most
widely prepared by Kumada, Negishi, Stille and Suzuki coupling methods [27].
EDOT synthesis by Mitsunobu reaction is reported by Quintero et al. [28], and
most common industrially applied route by double Williamsons etherifcation
of 3,4-dihydroxy-2,5-thiophenedicarboxylic acid ester with 1,2-dihaloethanes
involves a number of tedious synthetic steps [14]. The synthesis of stable and
soluble oligo(3,4-ethylenedioxythiophenes) and the synthesis of thienyl vinylenes
incorporating EDOT units were also reported [29, 30].
As a practical method for the production of conjugated polymeric materials,
polycondensation using C–H direct arylation [31] is a promising alternative. It
can be defned as the direct coupling of a nonactivated aryl C–H bond with an
activated arene [32]. This method does not require the use of organometallic
monomers that are essential in polycondensation methods that use conventional
cross-coupling reactions. The elimination of organometallic monomers reduces
the number of synthetic steps for monomer preparation and undesirable metal
containing waste after polymerization. Having fewer synthetic steps, the process
is less expensive and the reduction in waste reduces the environmental burden
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
and increases the purity of the product [33]. Direct C–H arylation of heteroarenes
with aryl halides was reported using a rhodium catalyst [34] and a palladium cat-
alyst [35–38]. Diphenyliodonium trifate is used as coupling partner in C–H bond
activation in the presence of Pd(OAc)2. The reaction seems interesting from the
perspective of atom economy and nontoxic waste [39]. The direct arylation of
thiophene-bearing electron-withdrawing groups was reported by Lemaire and co-
workers [40]. Under ‘Heck-type’ experimental conditions, direct regioselective
C–H arylation of 3-methoxythiophene and 3,4-ethylenedioxythiophene (EDOT)
was performed successfully [41]. Direct C2 arylation of quinoline N-oxides
using boronic esters a metal-free approach was reported by Nazanin et al. [42]
and metal-free arylation by using various sources such as diazonium salt, organo
catalysts, aryliodonium salts and energy sources like microwave, photochemical
reactor, etc. [43].
To the best of our knowledge, no reports were found on direct arylation of EDOT
using supported clay catalysts. In this article, the regioselective arylation of 3,4-eth-
ylenedioxythiophene by an aryl bromide, catalysed by clay-supported Pd catalyst,
is described. This catalyst is considered environmentally benign and cost efective
because a natural clay was used for its preparation, purifed by dispersion–decanta-
tion, which only used a small amount of electric power for stirring and centrifuging
the clay–water dispersions. Although Pd precursors are expensive, considering the
employed amount of Pd acetate, the catalyst is competitive. No other chemical rea-
gents have been used, and no leaching was observed during the catalytic reactions
(vide infra). Mono- and bis-arylation was carried out in the presence of activated
clay, and the products were obtained in moderate yields. The catalyst was reusable
for three reaction cycles without substantial loss of catalytic activity.
Experimental
Materials and methods
NMR spectra were recorded on a Bruker 400 MHz spectrometer with TMS as inter-
nal standard using CDCl3 solvent (STIC, SAIF, CUSAT). Molecular mass of the
EDOT analogues was recorded by GC–MS. FT-IR spectra were recorded by the KBr
pellet method using a JASCO FT-IR 4100 spectrometer. X-ray difraction (XRD)
patterns were recorded between 2 and 65° (2θ) over non-oriented powder samples,
at a scanning speed of 2°/min, by using a Siemens D-500 difractometer, operat-
ing at 40 kV and 30 mA, and employing fltered Cu Kα radiation (λ=1.5418 Å).
Textural properties were determined from nitrogen (Air Liquide, 99.999%) adsorp-
tion data, obtained at −196 °C using a Micrometrics Gemini VII 2390 t, surface
area and porosity apparatus. Specifc surface area was obtained by the BET method,
external surface area and micropore volume by means of the t method and the total
pore volume from the nitrogen adsorbed at a relative pressure of 0.95 [44, 45]. Ele-
ment chemical analyses were carried out at Activation Laboratories Ltd. (Ancaster,
Ontario, Canada), using inductively coupled plasma-atomic emission spectrometry
(ICP-AES).
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S. Sreekumar et al.
General procedure for the synthesis of clay‑supported palladium catalyst
The clay mineral used in this work was raw montmorillonite from Cheto, Arizona,
USA. The natural clay mineral was purifed before its use by dispersion–decanta-
tion, separating the ≤2 μm fraction.
This is a very usual procedure for purifcation of clay materials, avoiding the
presence of admixtures (quartz, feldspar) always present in natural samples. It sim-
ply consists in the dispersion of the clay in water, then the most dense particles will
descent in the column, and the lowest dense (clay particles) may remain in the upper
part of the column. By application of Stokes law, it is easy to determine the length of
the column enriched in≤2 µm fraction for a given decantation time, extracting pure
clay material. Its cation exchange capacity was 0.67 meq/g, its basal spacing 13.60
Å and its BET specifc surface area 89 m2/g. The purifed solid clay is designated
as “M”. Natural montmorillonite was impregnated with Pd(II) acetate by means of
the incipient wetness impregnation method. This is very well-known method in the
preparation of supported catalysts. A precursor of an active phase, usually a salt, is
dissolved in the smallest volume of a solvent, and this solution is used to impregnate
by wetting a given mass of the support. The wet solid is then dried and calcined
under adequate conditions. Thus, the amount of Pd(II) acetate needed for a Pd con-
tent of 5% in the fnal solid was dissolved in the minimum amount of acetone, and
the resulting solution was added to the montmorillonite, all the solid being homoge-
neously wet. The solid was dried and calcined at 500 °C under air atmosphere. This
sample was denoted MPd500. As this catalyst was prepared by a method that did not
involve fltration steps, it was assumed that 5% Pd was incorporated. For compara-
tive purposes, the natural montmorillonite was also calcined at this temperature, and
this sample was denoted M500.
General procedure for the synthesis of bis‑arylated 3, 4‑ethylenedioxythiophene
EDOT and halobenzene derivatives were taken in the ratio 2:1 in 10 ml of dry
dimethylacetamide/dimethylformamide (DMAc/DMF) in three-necked round-bot-
tomed fask under nitrogen atmosphere. To this, sodium acetate (2 mmol), tetrabu-
tylammonium bromide (8 mmol) and clay-supported palladium catalyst (MPd500,
25 mg) were added. The reaction mixture was stirred at 80 °C for 12 h. The mixture
was poured into water and extracted with dichloromethane. The organic layer was
washed several times with brine, water and dried using MgSO4. The product was
recrystallized using chloroform.
Synthesis of 5‑(2‑(2,3‑dihydrothieno[3,4‑b]
[1,4]‑dioxin‑7‑yl)‑benzene‑5‑yl)‑2,3‑dihydrothieno[3,4‑b][1,4]dioxine
Following the general procedure, this compound (Compound 3 according to
Scheme 1) was obtained (57%). The main characterization data are:
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
Scheme 1 Synthesis of mono- and bis-arylated 3, 4-ethylenedioxythiophene
1H NMR (400 MHz, CDCl3): δ 4.26–4.37 (8H, m), 6.43 (2H, s), 7.54 (4H,d).
13C NMR (100.6 MHz, CDCl3): δ 76.2, 74.8, 98.24, 120.81, 125.82, 139.54,
146.73, 152.15.
Mass m/z: 358 (M+).
Synthesis of (10‑(2,3‑dihydrothieno[3,4‑b]
[1,4]‑dioxin‑7‑yl)‑5‑Anthracen‑9‑yl)‑2,3‑dihydrothieno[3,4‑b][1,4]dioxine
Following the general procedure, this compound (Compound 5 according to
Scheme 1) was obtained (52%). The main characterization data are:
1H NMR (400 MHz, CDCl3): δ 4.2.6–4.37 (8H, m), 6.43 (2H, s), 7.2–8 (8H,d).
1 3

S. Sreekumar et al.
13C NMR (100.6 MHz, CDCl3): δ 76.2, 74.8, 98.24, 124, 129. 2, 130, 139.5,
146.73, 152.15.
Mass m/z: 459 (M+).
General procedure for the synthesis of mono‑arylated 3,
4‑ethylenedioxythiophene
EDOT and halobenzene derivatives were taken in the ratio 1:1 in 10 ml of dry
DMAc/DMF in three-necked round-bottom fask under nitrogen atmosphere. To
this, sodium acetate (2 mmol), tetrabutylammonium bromide (8 mmol), and clay-
supported palladium catalyst (MPd500, 25 mg) were added. The reaction mixture
was stirred at 80 °C for 12 h. The mixture was poured into water and extracted with
dichloromethane. The organic layer was washed several times with brine, water and
dried using MgSO4. The product was recrystallized using chloroform.
Synthesis of 4‑(2,3‑dihydrothieno[3,4‑b][1,4]dioxin‑5‑yl)‑benzaldehyde
Following the general procedure, this compound (Compound 7 according to
Scheme 1) was obtained (62%). The main characterization data are:
1H NMR (400 MHz, CDCl3): δ 4.26–4.37 (8H, m), 6.43 (2H, s), 7.6–7.9
(4H,d), 9.8 (1H,s).
13C NMR (100.6 MHz, CDCl3): δ 76.2, 74.8, 98.24, 128.5, 139.54, 146.73,
152.15.192.
Mass m/z: 246 (M+).
Synthesis of 5‑(4‑nitrophenyl)‑2,3‑dihydrothieno[3,4‑b][1,4]dioxin
Following the general procedure, this compound (Compound 9 according to
Scheme 1) was obtained (48%). The main characterization data are:
1H NMR (400 MHz, CDCl3): δ 4.26–4.37 (8H, m), 6.43 (2H, s), 7.72–8.3
(4H,d), 9.8 (1H,s).
13C NMR (100.6 MHz, CDCl3): δ 76.2, 74.8, 98.24, 127.3, 146.73,149.2,
152.15.
Mass m/z: 263.27 (M+).
Synthesis of 5‑(4‑methoxyphenyl)‑2,3‑dihydrothieno[3,4‑b][1,4]dioxine
Following the general procedure, this compound (Compound 11 according to
Scheme 1) was obtained (60%). The main characterization data are:
1H NMR (400 MHz, CDCl3): δ 4.26–4.37 (8H, m), 6.43 (2H, s), 6.7–7.4
(4H,d), 3.8 (1H,s).
13C NMR (100.6 MHz, CDCl3): δ 57.2, 76.2, 74.8, 98.24, 115.5, 129, 146.73,
149.2, 163.7.
Mass m/z: 248 (M+).
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
Results and discussion
Synthesis and characterization
Scheme 1 shows the direct C–H arylation of 3,4-ethylenedioxythiophene (1) with
substituted aryl halides catalysed by clay-supported Pd catalyst. All the mono-
and bis-arylated oligomers were obtained in good yield. The resulting mono- and
1
bis-arylated products were characterized by H-NMR, 13C-NMR and GC–MS
analysis.
The clay-supported Pd catalyst (MPd500) was found to be active towards the
direct arylation reaction. Bis arylation reported earlier required Pd(PPh3)4 and
other conventional catalysts were costlier, not reusable and required more time
for the completion of the reaction as compared to the present catalyst. The usual
mechanism for the C–H activation pathway is expected to be operating here also.
Mechanism of arylation reaction using clay‑supported catalyst
Aryl halide was adsorbed on the metal surface of clay, making it more reactive
followed by the oxidative addition of EDOT with elimination of HX. The prod-
ucts formed were eliminated with reductive elimination regenerating the active
sites of the catalyst. DMF acted as solvent, while the Pd present on the interlayer
and also on the surface of montmorillonite may also participate in the reaction. If
two moles of aryl halide were used, it gave bis-arylated product, and the mecha-
nism is given in Scheme 2.
Characterization of the catalyst
The X-ray difractogram showed that the structure of montmorillonite was main-
tained after impregnation and calcination (Fig. 1). All the characteristic peaks of
montmorillonite were observed after the impregnation. Compared to M500 solid,
the only diference was that the basal spacing was slightly lower for the impreg-
nated solid that could be expected for the additional thermal treatment after the
impregnation process. These difratograms were less intense than that of the orig-
inal sample (see Fig. S1), because these samples were calcined at 500 °C, for
removing the organic part of the Pd precursor, and also for removing structural
water from the clay, and to obtain a more stable catalysts, which caused a certain
deleterious efect on the stacking of the layers.
The nitrogen adsorption–desorption isotherm is typical of layered solids. The
BET specifc surface area of the impregnated solid was 89 m2/g, the external
surface being 69 m2/g (ca. 75% of the total surface), and the micropore volume
0.012 cm3/g [46]. These values are slightly higher than for M500 solid, which
may be due to the development of textural properties by the removal under cal-
cination of the acetate anions from the precursor and eventually some acetone
molecules were retained after the impregnation process. The value of 89 m2/g
1 3

S. Sreekumar et al.
Scheme 2 Mechanism of arylation reaction with MPd500 catalyst
M500
MPd500
200
10
20
30
40
50
60
2θ
Fig. 1 X-ray difractogram of the montmorillonite clay and the impregnated Pd-montmorillonite clay,
both at 500 °C
guaranties the access of the reagent organic molecules to the Pd active sites
(Fig. 2). The palladium content of MPd500 was confrmed by ICP/OES.
Additional characterization by FT-IR spectroscopy showed that both solids
were typical clays [47, 48]. The spectra are very similar, and the only diferences
were the absence of the hydroxyl band near 3600 cm−1 in the impregnated solid,
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
70
M500
MPd500
60
50
40
30
20
10
0,0
0,2
0,4
0,6
0,8
1,0
P/P0
Fig. 2 Nitrogen adsorption–desorption isotherms of M500 and MPd500 solids
due to the interaction with acetone and the further calcination carried out in the
preparation, as well as the small and sharp band in the catalyst near 1400 cm−1,
due to acetate anions, showing the presence of traces of these anions even after
the calcinations (Fig. 3).
To analyse the heterogeneity and Pd leaching from MPd500 catalyst, hot fltra-
tion test was conducted using ICP/OES. For this, the hot reaction mixture after
12 h was fltered and the sample of fltrate was evaporated and digested in HNO3.
No characteristic Pd metal content in the ICP/OES analysis of the prepared
Fig. 3 FT-IR spectra of the montmorillonite clay and the impregnated Pd-montmorillonite clay, both cal-
cined at 500 °C
1 3

S. Sreekumar et al.
sample revealed the poor leaching property and good heterogeneous nature of
MPd500 catalyst.
Catalytic performance
Catalytic screening
Catalyst screening was done by selecting 4-bromobenzaldehyde and EDOT as
substrates. Diferent catalysts such as Pd(PPh3)4, Pd(PPh3)2Cl2, Pd2(dba)3, M500
and MPd500 were screened, and better results were shown by MPd500 catalyst
(Table 1). The results from the literature are also given in the table. It was observed
that the clay-supported catalyst was found to be efective in terms of yield and it was
reusable. Even though Pd(PPh3)4 gives good result, it is not reusable and hence clay-
supported catalyst was found to be superior than other catalysts. Entry 2 in Table 1
showed the efect of montmorillonite catalyst calcined at 500 °C. M500 gave no
product indicating the role of Pd in the direct arylation reaction.
Reaction conditions: 4-bromobenzaldehyde (1 mmol), EDOT (1 mmol), sodium
acetate (2 mmol), tetrabutylammonium bromide (8 mmol), MPd500 (25 mg), DMF
(10 ml) temperature: 80 °C, time: 12 h.
Theoretical studies
Attempts were made to predict the transition state of the reaction by theoretical
methods using Gaussian 09 [49]. Theoretical insight into the direct arylation reac-
tion was obtained using Quasi Static Newton method (QST2 and QST3) and Intrin-
sic Reaction Coordinate (IRC) analysis. In the present work we used Beck’s three-
parameter Lee Yang and Parr exchange correlation functional (B3LYP) and Los
Alamos double zeta basis set (LAN L2 dz) for the calculation. Various reaction steps
involved in the mechanism, and their transition states are given below.
Oxidative addition
In the oxidative addition step, the bond between the organic group and the heter-
oatom X is broken, and two new bonds are formed with the metal, which increases
its oxidation state by two units (Scheme 3). A concerted mechanism is proposed
for oxidative addition. Mechanisms consider palladium (0) as a nucleophile and the
Table 1 Screening of the
Entry
Catalyst
Yield (%)
catalyst for arylation of
4-bromobenzaldehyde and
EDOT
1
2
3
4
5
MPd500
M500
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd2(dba)3
62
–
50 [28]
40
33
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
Scheme 3 Mechanism of oxidative addition
organohalide as an electrophile. Concerted pathway entails the simultaneous forma-
tion of the Pd–C and Pd–X bonds in the transition state, which leads to retention of
confguration at a stereogenic carbon atom in the case of chiral electrophiles, R–X.
Ligand substitution
In this step, (Scheme 4) EDOT-Pd-Ar is formed through a fve-coordinated transi-
tion state by the removal of one acetate group.
Ligand association and elimination
Association of one acetate group creates a concerted transition state followed by
elimination of HBr to give the product (Scheme 5).
Reductive elimination
This step consists of coupling of two organic groups in cis position (Scheme 6).
The generally accepted mechanism for this step is concerted and features a cyclic
three-coordinated transition state that results in the fnal C–C coupling and con-
comitant regeneration of the catalytic species. Reductive elimination step is usually
irreversible.
Scheme 4 Ligand substitution
Scheme 5 Mechanism of ligand association and elimination
1 3

S. Sreekumar et al.
Scheme 6 Mechanism of reductive elimination
Recycling studies
After completion of the reaction, catalyst was fltered and washed with dichlo-
romethane repeatedly, followed by drying at 100 °C for an hour and reused. Recy-
cling studies were carried out by taking 4-bromobenzaldehyde (1 mmol), EDOT
(1 mmol), sodium acetate (2 mmol), tetrabutylammonium bromide (8 mmol),
MPd500 (25 mg), DMF (10 ml), temperature: 80 °C, time: 12 h. The catalyst was
found to be active over three consecutive cycles. A small decrease in yield may be
due to leaching of metal ions from the clay interlayer. Results are summarized in
Fig. 4.
Conclusions
Preparation of a new palladium-supported clay catalyst and its characterization
using various physico-chemical techniques were reported. The synthesis of variety
of mono- and bis-arylated EDOT-based oligomers has been achieved in moderate
Fig. 4 Recycling studies of the catalyst
1 3

Efcient synthesis of mono‑ and bis‑arylated ethylenedioxy…
yields, involving direct C–H arylation of EDOT with various substituted heteroaryl
halides using clay-supported palladium catalyst (MPd500). Mechanism for the for-
mation of direct arylation of EDOT-based oligomers in the presence of clay-sup-
ported palladium catalyst was explained. The reusability of the catalyst was also
tested for three reaction cycles without substantial loss of catalytic activity.
Acknowledgements BG thanks a pre-doctoral grant from Universidad de Salamanca. This research did
not receive any specifc grant from funding agencies in the public, commercial or not-for-proft sectors.
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