Microwave-assisted, ligand-free, direct C-H arylation of thiophenes in biomass-derived γ-valerolactone

Article abstract of DOI:10.1039/c7nj01540b

The Pd-catalyzed C-H arylation of heterocycles with aryl halides is a straightforward and more environmentally-friendly route to the synthesis of well-defined, pi-conjugated polymers for challenging applications in electronic devices. Although this type of transformation is more atom efficient than cross-couplings, it still poses environmental issues in the form of reaction media and the use of phosphine ligands. A procedure for the C-H arylation of thiophenes, with a substantially improved environmental impact, and a promising application of this protocol to the synthesis of regioregular polythiophenes are reported in this work. γ-Valerolactone (GVL), a non-toxic biomass-derived solvent, has been used in this phosphine-free microwave-assisted process and replaces commonly used dipolar aprotic media. Activated aryl bromides gave arylthiophenes in good yields, while iodides were used and pivalic acid was added when electron-withdrawing groups were present. The reaction was also extended to C-H (hetero)arylation polycondensation and a high molecular weight poly(3-hexyl)thiophene was obtained, using low Pd loading, in high yields and good regioregularity in short reaction times. Computational studies have shed light on GVL's role as a ligand in the catalytic system.

Full text of DOI:10.1039/c7nj01540b

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Microwave-assisted, ligand-free, direct C-H arylation of
thiophenes in biomass-derived γ-valerolactone
Silvia Tabasso,a Emanuela Calcio Gaudinob Laura Rinaldib, Audrey Ledouxc, Paolo Larini*c and
Giancarlo Cravotto*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The Pd-catalyzed, C-H arylation of heterocycles with aryl halides is a straightforward and more environmentally-friendly
route to the synthesis of well-defined, pi-conjugated polymers for challenging applications in electronic devices. Although
this type of transformation is more atom efficient than cross-couplings, it still poses environmental issues in the form
reaction media and the use of phosphine ligands. A procedure for the C-H arylation of thiophenes, with substantially
www.rsc.org/
improved environmental impact, and
a promising application of this protocol to the synthesis of regioregular
polythiophenes are reported in this work. γ-Valerolactone (GVL), a non-toxic biomass-derived solvent, has been used in
this phosphine-free microwave-assisted process and replaces commonly used dipolar aprotic media. Activated aryl
bromides gave arylthiophenes in good yields, while iodides were used and pivalic acid added when electron-withdrawing
groups were present. The reaction has been also extended to C-H (hetero)arylation polycondensation and a high molecular
weight poly(3-hexyl)thiophene was obtained, using low Pd loading, in high yields and good regioregularity in short reaction
times. Computational studies have shed light on GVL’s role as a ligand in the catalytic system. …………………………………………
properties.5
Thiophenes have been used to build materials for organic
electronics and considerable effort has recently been dedicated to
Introduction
The replacement of conventional methods with microwave (MW)
irradiation-based environmentally friendly procedures has gained
increasing interest in recent decades. This interest is certainly well-
deserved as the use of MWs in synthetic organic chemistry offers
many advantages which are in line with green chemistry
recommendations; it promotes rate acceleration, enhanced yields
and selectivity,1 leading to potentially lower energy and metal
consumption, while also avoiding undesirable reactions and by-
products. Moreover, the particular properties of MW irradiation
mean that polar non-toxic solvents, such as water and alcohols, can
be coupled with reduced catalyst amounts. MW irradiation is
therefore a suitable tool with which to overcome the limitations
encountered in challenging organic reactions, such as direct C-H
arylation.2
the synthesis of conjugated polymers via C-H activation-based,
direct (hetero)arylation polymerization (DHAP).6,7 Direct C-H
(hetero)arylation is more environmentally-friendly than traditional
cross-coupling reactions, as it does not lead to the formation of
metallic salts as by-products. Furthermore, no additional synthetic
steps are required to produce the organometallic intermediates.
Despite these advantages, this reaction still suffers from a number
of drawbacks, which include the frequent need for high catalyst
loadings (1-10 mol%),8 and monophosphine or diphosphine
ligands.9 Wu et al. have recently developed a phosphine free, direct
C−H arylaꢀon of thiophenes at low catalyst amounts,10 which,
however, required prolonged reaction times (24 h) in order to
achieve reasonable yields. Other ligandless protocols with low
catalyst loadings have been developed by Doucet and co-workers,
but the reaction still took 20 h to complete.11,12 Additional impetus
for change can be found in the fact that C-H arylations usually occur
in dipolar, aprotic solvents, which, despite being very useful in
promoting such reactions, are to be banned in the near future
because of their toxicity and environmental impact.13 For example,
DMA is the most widespread solvent used in C-H sp2 arylation of
heterocycles, being classified as toxic to reproduction by the ECHA14
(European Chemical Agency). This has obviously led to effort being
shifted to the search for a green substitute for this solvent class. A
significant amount of recent interest has focused on the use of γ-
valerolactone (GVL), in particular, as a bio-based, dipolar, aprotic
solvent.15
Direct C-H arylation holds significant synthetic potential for the
synthesis of various biologically active compounds,3 and organic
functional materials,4 that bear biaryl and heteroaryl skeletons. A
great deal of attention has been devoted, in particular, to the
synthesis of arylthiophenes because of their biological and physical
GVL is commonly synthesized from the hydrogenation of biomass-
derived levulinic acid.16 The MW-assisted, cascade production of
GVL directly from lignocellulosic biomass has recently been
reported,17 and is worth interest as GVL is both the process solvent
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and product. Despite GVL’s noteworthy chemical and physical
properties, which make it a valuable, green alternative to polar
aprotic solvents,18 only a few papers have so far dealt with its
application to organic synthesis.19 While a number of works on its
use as a green solvent in cross-coupling reactions do already
exist,20-23 only one report has been, to the best of our knowledge,
made in the C-H activation of heteroarenes (1,2,3 triazoles) by
Vaccaro and Ackermann.24 This work describes a ligand-free, MW-
assisted coupling of aryl halides with thiophenes using low catalyst
loadings and GVL as the solvent. The synthesis of a polythiophene
has been carried out under optimized conditions and MW
irradiation, via direct, (hetero)arylation polymerization (DHAP).
Moreover, computational methods have granted useful insights
into the mechanism of this phosphine-free reaction.
mol%) was added. The mixture was heated to the required
temperature under MW irradiation for 2 hours (average power
960 W) in a N2 (1 MPa) atmosphere and under magnetic
stirring (450 rpm), the product was then precipitated after the
addition of water, filtered off and purified by column
chromatography (hexane) after solvent evaporation.
Alternatively, crude reactions were filtered on a silica pad,
washed with 3 mL of EtOAc and, after solvent evaporation,
purified by flash column chromatography (hexane).
Procedure for MW-assisted coupling polymerization of 2-
bromo-3-hexylthiophene
2
-bromo-3-hexylthiophene (2 mmol, 490 mg), K2CO3 (6 mmol,
800 mg), and PivOH (2 mmol, 204 mg) were placed into a
quartz vial equipped with a magnetic stirrer and suspended in
GVL (2 mL). The vial was purged with N2, and Pd(OAc)2 (1 mg,
0.004 mmol, 0.2 mol%) was added.
Experimental
Materials and instruments
All starting organic reagents and solvents were purchased from
Aldrich and used without further purification. NMR spectra
were recorded on a Bruker Avance 300 (300 MHz and 75 MHz
The mixture was heated to 100 °C for 3 hours under MW
irradiation (average power 960 W) and magnetic stirring (450
rpm) in a N2 (1 MPa) atmosphere. After cooling to room
temperature, the crude reaction was poured into vigorously
stirred MeOH (10 mL). Centrifugation of the resulting mixture
at 4000 rpm for 15 minutes led to the precipitation of the
polymer. The supernatant was removed by decantation, 10 mL
of distilled water were added and the mixture was centrifuged
under the same conditions to remove the water soluble salts.
The deep purple residue obtained after the decantation of
water was then dried under vacuum to give a solid of poly(3-
hexyl)thiophene.
1
for H and 13C, respectively) at 25°C in CDCl3. Chemical shifts
were calibrated to the residual solvent proton and carbon
resonances. GC-MS analyses were performed on a GC Agilent
6890 (Agilent Technologies - USA), fitted with a Agilent
Network 5973 mass detector, using a HP-5 column (30 m long
capillary column, i.d of 0.25 mm and film thickness 0.25 μm).
MW-assisted reactions were carried out in a SynthWAVE (MLS
GmbH, Milestone Srl) reactor which houses a closed MW-
cavity. SEC THF polymer analyses were performed on a
Viskotek GPCmax VE 2001.
General procedure for MW-assisted coupling of thiophenes
and electron-deficient aryl halides
Results and discussion
The aryl bromide (0.5 mmol), thiophene derivative (1 mmol)
and KOAc (1 mmol) were placed into a quartz vial equipped
The reaction between 2-methylthiophene and 4-
with a magnetic stirrer and suspended in GVL (3 mL). The vial bromonitrobenzene was studied, using a variety of bases that
are commonly used for C-H activation under conventional
conditions (Table 1), in order to set up the protocol for C-H
activation in GVL.
was purged with N2 and Pd(OAc)2 (0.2 mol%) was added. The
mixture was heated to the required temperature under MW
irradiation for 2 hours (average power 960 W) in a N2 (1 MPa)
atmosphere and under magnetic stirring (450 rpm). The
reactor was cooled after 2 hours and solid products were
recovered via precipitation after the addition of water (4 mL),
filtered off and washed with 1 mL of cold water. The solid was
then dried under vacuum and purified by flash column
chromatography (hexane). Alternatively, liquid products were
filtered on a silica pad, washed with 3 mL EtOAc and purified
by flash column chromatography (hexane) after solvent
evaporation.
Table 1. Screening of bases for arylation between 4-bromonitrobenzene and 2-
methylthiophene.
Entry
Base (eq.)
KOAc (0.25)
TEA (2)
Cs2CO3 (2)
K2CO3 (0.25)
DBU (2)
Conversion (%)
Yield (%)
1
2
3
4
5
100
-
4
75
40
99
-
General procedure for MW-assisted coupling of thiophenes
and non-activated aryl halides
2a
61b
11
The aryl iodide (0.5 mmol), thiophene derivative (1 mmol),
K2CO3 (1.5 mmol) and PivOH (0.5 mmol) were placed into a
quartz vial equipped with a magnetic stirrer and suspended in
GVL (3 mL). The vial was purged with N2 and Pd(OAc)2 (0.2
aBiphenyl was also obtained. b5,5′-dimethyl-2,2′-bithiophene was also
obtained.
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irradiation in DMAc, afforded the product in a 38% yield,
The organic bases, triethylamine and DBU, proved to be whereas the reaction did not proceed in GVL. Wu et al. have
unsatisfactory (Table 1, entries 2 and 5), whereas inorganic reported on the addition of pivalic acid (PivOH) to the coupling
bases gave good results. Organic bases probably did not give of 4-bromoanisole with methylthiophene in DMAc, using
good results due to their lack in establishing the CMD K2CO3 as the base.10 In fact, enhanced product yields have
(Concerted Metalation Deprotonation) process (see the herein been achieved under these conditions in DMAc (64%),
mechanistic part below). Both carbonate bases (Table 1, while MW irradiation led to shorter reaction times than in the
entries 3 and 4), gave evidence of the cation effect. However, previous work (2h instead of 24h). This result caused us to
caesium furnished very low conversion (Table 1, entry 3), change the reaction conditions in order to test the coupling of
probably due to the low solubility of this base in the reaction electron-rich substrates with thiophenes in GVL (Table 3).
media, while potassium provided improved conversion, but, While, for electron-deficient substrates, aryl bromides were
unfortunately, also a bis-thiophene by-product (Table 1, entry more promising than other aryl halides, only aryl iodides led to
4). Finally, the best results were obtained using potassium the desired products in the presence of the methoxy group
acetate as the base (Table1, entry 1), although an excess of (Table 3, entries 4 and 5). Not even acetyl thiophene reacted
methylthiophene (2 eq.) was necessary to avoid the formation with 3-bromoanisole, probably because of coordination
of the homo-coupling product.
between the acetyl function and palladium (Table 3, entry 3).
Substrate scope
These preliminary reactions occurred in very promising
reaction times, which were far lower than those previously
reported under conventional heating using DMAc as the
solvent and the same catalyst amounts.25 It was therefore
decided to assess the scope and limitations of this reaction by
studying the coupling of electron-deficient aryl halides with
thiophenes under the optimized conditions (Table 2).
Target compounds were obtained in good yields when the
various thiophenes were reacted with 4-bromonitrobenzene
(Table 2, entries 1-3). Quite surprisingly, 2-acetylthiophene
was less reactive in these conditions, as 51% of the starting
material was recovered unreacted (Table 2, entry 4). As
expected, the meta-substituted bromonitrobenzene showed
almost the same reactivity as its para-substituted analogue
(Table 2, entry 3), but this behaviour was not reflected by
bromoacetophenone. Indeed, while 4-bromoacetophenone
gave the desired product in good yields (Table 2, entry 5), 3-
bromoacetophenone was less reactive and the ortho-isomer
did not react at all (Table 2, entries 6 and 7). An increase in
temperature did not afford the desired products either. This is
probably a result of coordination between the acetyl function
and palladium. 2-n-butylthiophene reacted with 4-
bromoacetophenone more slowly than in the same reaction
with 2-methylthiophene and the yield was 40 % after 2 hours.
However, it was nevertheless faster than under conventional
heating (Table 2, entries 8 and 9). Disappointing results were
also obtained with 4-iodoacetophenone, as the major product
was biphenyl (Table 2, entry 10). The presence of an additional
halide makes the aryl bromide unreactive (Table 2, entry 11),
even at higher temperatures.
Attempts to couple unsubstituted aryl bromides with
thiophenes led to the reaction not proceeding (harsher
conditions gave biphenyl as the only product), while only 12%
product was formed using aryl iodide (yield as determined by
GC). This result was dramatically different to when the same
reaction was performed using DMAc as the solvent (78% yield
from bromobenzene). Additionally, the reaction between
deactivated 4-bromoanisole and methylthiophene, under MW
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Table 2. Palladium-catalysed coupling of heteroaromatics and electron-deficient aryl
halides.a
Table 3. Palladium-catalysed coupling of heteroaromatics and non-activated aryl
halidesa
Entry
1
Heteroaryl
Aryl
T
Product
Yieldb
(%)
Entry
1
Heteroaryl
Aryl halide
Product
Yieldb
(%)
halide
(°C)
99
140
0
(82)
10
11
1
2
94
0
0
2
2
3
4
140
140
140
(76)
3c
99
11
10
(80)
3
4d
48% (42)
36
(28)
4
5
59% (50)
60% (52)
5
6
12
13
74
5
140
200
200
140
140
(66)
aConditions: Pd(OAc)2 (0.2 mol %), heteroaryl (2 equiv), aryl halide (1 equiv),
K2CO3 (1.5 equiv), PivOH (1 equiv), GVL, 140 °C, 2 h. bYields as determined by GC,
yields in parentheses are isolated. cThe reaction was performed at 180 °C. d3
equiv. of methylthiophene were necessary to avoid the formation of biphenyl.
26
6
(13)
6
7
7
0
Direct (hetero)arylation polymerization: synthesis of poly(3-
hexyl)thiophene (P3HT).
It was decided to assess the versatility of this environmentally-
improved methodology in the synthesis of poly(3-
hexyl)thiophene (P3HT);26 a reference polymer used in the
development of electronic devices, such as flexible solar
cells.27 The properties of this polymer depend on its
regioregularity, which is the ratio of repeating head-to-tail
units. Thompson and co-workers obtained P3HT (Mn = 14.3
kDa), using Pd(OAc)2/PivOH and K2CO3 in DMAc.28 The amount
of Pd(OAc)2 was relatively high (2.0 mol%) and the reaction
lasted for 48h, affording the polymer in modest yields (44 %),
relatively high molecular-weight (Mn = 14.3 kDa) and a good
degree of regioregularity (RR = 88%). This work sees direct,
MW-assisted C-H (hetero)arylation polymerization performed
on 2-bromo-3-hexylthiophene (1M concentration), using low
catalyst loading (Pd(OAc)2, 0.2 mol %), PivOH and K2CO3 as the
base (3 equiv.) in GVL (100°C, 3h) (Figure 1).
40
8
(28)
8
8
20
9c
(14)
14%
(8)
(86)d
10
11
140
200
5
9
0
aConditions: Pd(OAc)2 (0.2 mol %), heteroaryl (2 equiv), aryl halide (1 equiv),
KOAc (2 equiv), GVL, 140°C,
2
h. bYields as determined by GC, yields in
parentheses are isolated. c Conventional heating. d(biphenyl).
On the other hand, good yields were quite surprisingly
achieved even with a non-activated congested substrate, 2-
iodotoluene (Table 3, entry 6).
Figure 1. Synthesis of poly(3-hexyl)thiophene (P3HT)
These conditions afforded P3HT with improved molecular
weight (Mn = 25 kDa) and yield (75%). P3HT, synthesized in
these conditions, shows
a molecular weight which is
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comparable to Thompson’s, while being in stark contrast with
the P3HT reported by Koizumi and Hayashi,29 which had low
Mn (5.5 kDa) due to the use of Pd/C as a heterogeneous
catalyst. These results show that GVL allows an active, catalytic
species homogeneous phase to form, which occurred thanks
to the coordination of GVL with Pd. To further understand the
role that GVL plays in the internal coordination sphere of the
metal center, it was decided to investigate the reaction
mechanism using computational studies.
Figure 3. Isodesmic reaction between Pd(0)GVL2 and Pd(0)DMAc2.
Reaction mechanism
In order to further understand the impact that GVL has on the
reaction of non-activated aryl halides, our attention was shifted to
the pathways involved in the catalytic cycle (Figure 4). DFT
computational studies of the mechanism behind the C(sp2)-H
activation of thiophenes have previously been reported by
Gorelsky,34 and Ozawa.35 However, to the best of our knowledge, a
reaction pathway involving a ligand-free C(sp2)-H arylation of
thiophene had not yet been computed. We herein present the most
likely reaction pathway, according to DFT computational studies.
After oxidative addition, complex I36 associates with methyl
thiophene giving rise to a more reactive intermediate II (∆G = 2.3
kcal mol-1). The pivalate ligand changes its coordination from ꢀ2 to
ꢀ1 while methyl thiophene establishes a ꢀ2 coordination with Pd(II)
II. TS-III (∆G = 15.8 kcal mol-1) consequently corresponds to the C-H
activation step that takes place via a CMD (Concerted Metalation
Deprotonation), leading to product III (∆G = 4.3 kcal mol-1). From
here, TS-IV (∆G = 21 kcal mol-1) is the reductive elimination step
yielding a C-C bond between the p-OMe phenyl moieties and the
thiophene, which irreversibly leads to final product IV (∆G = -8.2
kcal mol-1). Since the energy difference between TS-III and TS-IV is
∆∆s = 5.2 kcal mol-1, reductive elimination and not C-H activation
can be considered the rate limiting step, in accordance with work
by Ozawa and co-workers.26
Pd(OAc)2 gives rise to insoluble (Pd black) and soluble nanoparticles
at temperatures above 120 °C in the absence of ancillary ligands.30
Experimental results,27 led the authors to believe that the
catalytically active species was to be found in the homogeneous
phase, meaning that Pd is probably solubilized in the reaction
medium. The homogeneous palladium could be either in the form
of monomeric and/or soluble nanoparticles (PdNP). This hypothesis
is corroborated by the complete selectivity towards the C2
(C2:C3>99:1) position observed in the benzothiophene reaction, as
is in accordance with Glorius and co-workers.31 These authors
reported the selective C3 C-H arylation of benzo[b]thiophenes with
aryl chloride (C3:C2>99:1), using a ligand-free dual heterogeneous
catalytic system (Figure 2). Furthermore, they demonstrate that
site-selectivity was reversed when homogeneous Pd(0) was used,
leading to a C2 arylated product.
Figure 2. Selectivity in the arylation of benzo[b]thiophenes.
Thus, it was decided to investigate the effects of GVL on the present
reaction, with the hypothesis of having
a monometallic
homogeneous palladium, as the catalytically active species.
Computational investigations, at the DFT level (Density Functional
Theory), were pursued in order to support our hypotheses of GVL’s
impact and further our interest in the mechanistic studies of
palladium catalysed, C-H activation reactions.32 Since the starting,
catalytically active palladium species possesses an oxidation
number of 0, the coordination strength of GVL was compared with
that of DMAc.33 The maximum number of GVL molecules that can
coordinate a Pd(0) atom was found to be two. Pd(0)GVL2 was found
to be more stable than Pd(0)GVL by ∆G = 9.8 kcal mol-1. The same
result was obtained when DMAc was considered (∆G = 10.2 kcal
mol-1 in favor of Pd(0)DMAc2). An isodesmic reaction between Figure 4. Gibbs free energy change along the reaction pathway of the direct
arylation of 2-methylthiophene and oxidative addition complex
I
Pd(0)GVL2 and Pd(0)DMAc2 showed that the equilibrium is indeed in
favor of the latter by ∆G = 1.1 kcal mol-1 (Figure 3). To summarize,
DMAc was found to be more efficient than GVL in coordinating Pd
atoms and could thus favor the presence of a higher amount of
homogeneous Pd. However, the low ∆ value indicates that DMAc
and GVL have coordination strengths to Pd (0) that are very similar.
The difference between our computational system and Ozawa’s lies
in the presence of GVL as the ligand, in place of PH3. This leads to a
slightly larger energy gap between TS-III and TS-IV (∆∆G = 5.2 kcal
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mol-1) in our case than in computations with PH3 as the ligand (∆∆G
= 3 kcal mol-1). This is due to the lowering of the activation barrier
for TS-III (CMD step) when GVL is the ligand. The rate limiting step
of the compute pathway, being TS-IV (∆G = 21 kcal mol-1), means
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Conclusions
In this work, we have successfully developed more
environmentally-friendly conditions for the direct C-H arylation of
thiophenes. This has been accomplished via
a series of
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24 X. Tian, F. Yang, D. Rasina, M. Bauer, S. Warratz, F. Ferlin, L.
Vaccaro, L. Ackermann, Chem. Commun., 2016, 52, 9777.
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26 Q.Wang, R. Takita, Y. Kikuzaki, F. Ozawa, J. Am. Chem. Soc.,
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27 Y. J. Cheng, S. H. Yang, C.S. Hsu, Chemical Reviews, 2009,
109, 5869.
,
enhancements: the use of toxic ligands, such as phosphines, has
been avoided; the amount of palladium has been lowered to 0.2
mol %; a green bio-sourced solvent (GVL) has been used and, finally,
microwave heating has furnished reduced reaction times. The
ligand-free procedure was then successfully applied to the synthesis
of poly(3-hexyl)thiophene (P3HT), although it is limited to relatively
reactive substrates.11
Computational investigations have quantified the coordinating
ability of GVL towards the Pd atom as being slightly weaker than
DMAc’s. This most likely has an effect on the heterogeneous:
homogeneous Pd equilibrium, which is thought to be the rate
limiting step of the entire process.30 Furthermore, the activation
barriers for this C-H arylation are similar to those computed when
PH3 is the ligand,26 suggesting that the role of the ancillary ligand is
not key to the above mentioned reaction steps, although it is more
likely to be in the stability of the active homogeneous palladium
species.
Experimental and theoretical investigations into the impact of green
solvents and additives on the Pd black:Pd nanoparticle:
homogeneous Pd equilibrium will be the focus of our future studies,
the goal of which will be enhancing ligandless C-H arylations in
environmentally-benign conditions
28 A. E. Rudenko, C.A. Wiley, S. M. Stone, J. F. Tannaci, B. C.
,
Thompson, J. Polym. Sci., Part A: Polym. Chem., 2012, 50
3691.
29 S. Hayashi, Y. Kojima, T. Koizumi, Polym. Chem., 2015, 6, 881.
30 M. T. Reetz, J. G. de Vries, Chem. Commun., 2004, 1559; J. G.
de Vries, Dalton Trans., 2006, 421; A. J. Reay, I. J. S. Fairlamb,
Chem. Commun., 2015, 51, 16289; A. H. M. de Vries, J. M. C.
A. Mulders, J. H. M. Mommers, H. J. W. Henderickx, J. G. de
Vries, Org. Lett., 2003, 5, 3285.
31 D-T. D. Tang, K. D. Collins, F. Glorius, J. Am. Chem. Soc., 2013,
135, 7450.
32 P. Larini, C.E. Kefalidis, R. Jazzar, A. Renaudat, E. Clot, O.
Baudoin, O., Chemistry, 2012, 18, 1932; A. Goutierre, H.V.
Trinh, P. Larini, R. Jazzar, O. Baudoin, Organometallics, 2016,
Acknowledgements
The authors thank the University of Turin (ricerca locale 2015),
CNRS, CPE Lyon, UCBL and INSA for funding. CCIR-ICBMS
(UCBL) is gratefully acknowledged for the allocation of their
computational resources.
535
Larini, E. Clot, O. Baudoin, Chem. Sci., 2013,
Holstein, M. Vogler, P. Larini, G. Pilet, E. Clot, O. Baudoin,
ACS Catal. 2015, , 4300; A. Millet, D. Dailler, P. Larini, O.
,
DOI: 10.1021/acs.organomet.6b00535; A. Millet, P.
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ARTICLE
Hitce, J., Larini, P., Jazzar, R., Baudoin, O. Chem. Eur. J. 2014,
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Ligand-free green synthesis of arylthiophenes and poly(3-hexyl)thiophene in GVL under microwave
irradiation
C-H
activation
Low Pd loading
Ligand-free
MW