Unprecedented Demonstration of Regioselective SEAr Reaction giving Unsymmetrical Regioregular Oligothiophenes

Article abstract of DOI:10.1002/chem.201600159

Aromatization of 4-cyano-3-oxotetrahydrothiophene by sulfuryl chloride gives the new building block 4-cyano-3-pyrrolidylthiophene, which forms unsymmetrical regioregular oligothiophenes with a strict alternation of the donor and acceptor groups along the conjugated system. The self-coupling reactions that form the oligomers are shown to proceed by a regioselective electrophilic aromatic substitution mechanism involving a stabilized Wheland intermediate. First alternate: Regioregular oligothiophenes based on the 3-pyrrolidyl-4-cyanothiophene building block, with a strict alternation of the donor and acceptor groups along the conjugated backbone, have been prepared in one step. The mechanism of formation of the oligomers corresponds to a regioselective self-electrophilic aromatic substitution (self-S_EAr) involving a stabilized Wheland intermediate.

Full text of DOI:10.1002/chem.201600159

DOI: 10.1002/chem.201600159
Communication
&
Oligomerization
Unprecedented Demonstration of Regioselective SEAr Reaction
giving Unsymmetrical Regioregular Oligothiophenes
Chady Moussallem, Simon Olivier, JØrØmie Grolleau, Magali Allain, Charlotte Mallet,
Gurunathan Savitha, FrØdØric Gohier,* and Pierre Fr›re*[a]
Abstract: Aromatization of 4-cyano-3-oxotetrahydrothio-
phene by sulfuryl chloride gives the new building block 4-
cyano-3-pyrrolidylthiophene, which forms unsymmetrical
regioregular oligothiophenes with a strict alternation of
the donor and acceptor groups along the conjugated
system. The self-coupling reactions that form the oligo-
mers are shown to proceed by a regioselective electrophil-
ic aromatic substitution mechanism involving a stabilized
Wheland intermediate.
Thiophene-based oligomers are among the most widely inves-
tigated classes of conjugated materials for the development of
electronic plastics.[1] Careful modifications on the oligothio-
phene backbone by the insertion of substituents on the b and
Figure 1. Various D–A structures in 3-donor-4-cyanothiophene series.
b’ positions of the thiophene units with electron-donating (D)
or electron-acceptor (A) character enable a control of the
energy levels of the HOMO and LUMO frontier orbitals.[2] Fine-
photovoltaic devices.[6] The unsymmetrical structure (Figure 1)
tuning the band gap and energy levels of the conjugated ma-
synthesized (Figure 1) and used as donor materials in organic
terials are crucial to control the electronic properties, such as
absorption and emission properties, ionization potential, and
electronic affinity. Within the class of conjugated materials,
donor–acceptor (D–A) materials, mixing both electron donor
and electron acceptor blocks along the conjugated backbone,
are currently among the most widely investigated materials
that present moderate or low band gap and broad absorption
bands.[3] In this context, we have developed new D–A building
blocks in which the D and A groups are grafted onto the 3-
and 4-positions of a thiophene ring.[4] Due to the asymmetric
electronic distribution, the reactivity at the 2- and 5-positions
of the thiophene are different, thus allowing the regioselective
building of designed D–A structures. The electronic properties
of the D–A derivatives strongly depend on the distribution of
the D and A group along the conjugated backbone.[5] Thus
from 3-alkoxy-4-cyanothiophene, various symmetrical D–A–D
(for small molecules) or (D–A)n structures (for polymers) were
built with a strict alternation of the D and A groups grafted at
the 3- and 4-positions of thiophene represents a new D–A
structure, for which different electronic properties are expecte-
d.[4a] Nevertheless, the synthesis of such D–A structures has
been less developed. Indeed, the regioselective syntheses of
unsymmetrical oligomers presenting an alternating D–A struc-
ture require several steps with similar difficulties encountered
for the regioregular head-to-tail (H–T) structures for the cou-
pling of b-substituted thiophenes.[7] Unsymmetrical bithio-
phenes with H–T structures are generally obtained in three
steps by organometallic coupling of the corresponding halo-
genated and metallated thiophenes.[8] Some examples of regio-
selective metal-free coupling reactions to produce regioregular
3-alkylthiophenes through CÀH activation or regioregular bi-
thiophenes by hypervalent iodine reagents have also been de-
scribed.[9] The synthesis of longer oligomers, built step by step,
requires a high number of steps (at least 5 steps for the trimer
and 6 or 7 steps for the tetramer), necessitating laborious pu-
rification by chromatography after each coupling reaction.[7b,10]
As a further step in our approach to these alternating D–A
structures, we have investigated the influence of electron-do-
nating ability on the reactivity of the D–A monomer by graft-
ing a pyrrolidino group that exhibits a strong donor charac-
ter.[11] We have explored the synthesis of oligomers based on
the new 3-pyrrolidyl-4-cyanothiophene building block and
found that chlorination induces an unprecedented regioselec-
[a] Dr. C. Moussallem, Dr. S. Olivier, J. Grolleau, M. Allain, Dr. C. Mallet,
Dr. G. Savitha, Dr. F. Gohier, Prof. Dr. P. Fr›re
MOLTECH-Anjou, UMR CNRS 6200, University of Angers
2 Bd. Lavoisier 49045 Angers (France)
E-mail: Frederic.gohier@univ-angers.fr
pierre.frere@univ-angers.fr
Supporting information, including full details of experimental procedures,
characterization data, and NMR spectra, and the ORCID identification num-
ber(s) for the author(s) of this article can be found under http://dx.doi.org/
10.1002/chem.201600159.
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tive coupling leading to strictly D–A-alternated regioregular
oligomers (Figure 1).
Table 1. Aromatization of 2 with various amounts of SO2Cl2.[a]
The synthesis of 3-pyrrolidyl-4-cyanothiophene 3 and oligo-
mers was realized in two steps from the readily accessible 4-
cyano-3-oxotetrahydrothiophene 1[12] (Scheme 1). For the first
step, the addition of pyrrolidine to 1 was realized by using
Entry
SO2Cl2 [equiv]
T
Products (conversion[b] or yield[c] [%])
1
2
3
4
5
6
0.5
0.9
1.1
1.1
1.5
2
RT
RT
08C
reflux
reflux
reflux
3 (50)[b]
3 (90)[b]
3 (78), 4 (8), 5 (4) 6 (2)[c]
3 (58), 4 (8-12), 5 (4-6), 6 (4)[c]
oligomers
oligomers
[a] Solvent=CHCl3; [b] conversion based on NMR signals of non-isolated
compounds 2 and 3; [b] yields of isolated compounds.
the yields of isolated oligomers were not well reproducible
and could vary even without modification of the parameters of
the reaction (e.g., time of reflux or initial concentration of 2).
Increasing the amount of SO2Cl2 to 1.5 or 2 equivalents led to
a complete disappearance of the monomer 3, alongside an
increase in the yield of oligomers (not quantified; Table 1, en-
tries 5 and 6). MALDI-TOF mass spectrometry indicated the for-
mation of the tetramer, pentamer, hexamer, and heptamer (see
the Supporting Information, Figure S14). These results clearly
show that the oligomerization process takes place after the ar-
omatization and is brought about by the excess of SO2Cl2. Al-
though SO2Cl2 is a well-known halogenating agent in electro-
philic aromatic substitution (SEAr) reactions, the reaction per-
formed with 0.2 equivalents of SO2Cl2 directly on the monomer
3 at 08C did not allow isolation of the chlorinated derivative 3-
Cl. TLC showed the disappearance of 3, but the reactional
medium became dark quickly and we were unable to isolate
3-Cl, while the formation of oligomers was also observed. No-
tably, the yields of isolated dimer and trimer were inferior to
those obtained when using 1.1 equivalents of SO2Cl2 on dihy-
drothiophene 2. To better understand how the excess of
SO2Cl2 works in the oligomerization, the reaction was moni-
tored directly in an NMR tube at 208C by adding 0.5, 1, and
1.5 equivalents of SO2Cl2 solution in CDCl3 to the dihydro-
thiophene 2 in CDCl3 (Figure 2). The solutions in each NMR
tube were neutralized with sodium bicarbonate to facilitate
NMR spectroscopy, because the pyrrolidyl group is partially
protonated due to HCl emission in the course of the reaction.
With 0.5 equivalents of SO2Cl2, only aromatization took place
and 3 was formed in around 50% yield. When the amount of
SO2Cl2 was increased to1 equivalent, the signals associated to
3 remained predominant, thus conforming the quasi-complete
aromatization reaction. However, a signal at d=3.81 ppm, indi-
cating the presence of a small quantity of 2, is always present,
and a small signal is detected at d=7.57 ppm. When 1.5 equiv-
alents of SO2Cl2 were used, the signal at d=3.81 ppm disap-
peared, whereas that at d=7.57 ppm increased in intensity
and new signals corresponding to the pyrrolidyl protons ap-
peared at d=3.65 and 1.95 ppm. These signals would corre-
spond to chlorinated 3-Cl, obtained by regioselective chlorina-
tion of 3. Signals corresponding to dimer 4 (d=7.85 ppm) and
traces of longer oligomers (small signal at d=7.90 ppm) were
also detected, showing that the oligomers were formed to the
detriment of 3-Cl. When the NMR tube was left for several
Scheme 1. Synthesis of 3-pyrrolidyl-4-cyanothiophene 3 and oligomers.
formic acid in ethanol to give 2 in 87% yield.[13] Aromatization
of 2 was carried out with various oxidants, such as 2,3-di-
chloro-5,6-dicyanobenzoquinone (DDQ) or quinone, to give
thiophene 3 in yields that did not exceed 65%.[14] By using an
excess of the oxidant, a symmetrical bithiophene derivative
was formed with pyrrolidyl groups at the internal positions
(see the Supporting Information, Scheme S3). Aromatization by
using sulfuryl chloride (SO2Cl2) at 08C in methylene chlori-
de[15]afforded 3 in 88% yield. Moreover, when a slight excess
(1.1 equiv) of SO2Cl2 was employed, several oligomers were
also isolated and identified by MALDI-TOF mass spectrometry
and NMR spectroscopy as being the dimer 4, the trimer 5, and
1
the tetramer 6. H NMR spectra of 4, 5, and 6 include a singlet
at d=7.8–7.9 ppm for the proton adjacent to the electron-
withdrawing cyano (A) group and a broad singlet at d=5.8–
5.9 ppm for the proton close to the donor pyrrolidyl (D) group,
indicating the presence of only one isomer and suggesting the
formation of alternating oligomers. For each isolated oligomer,
13C NMR also confirmed the presence of only one unsymmetri-
cal isomer. The strict D–A-alternated structures for the three
oligomers were confirmed by X-Ray diffraction of single crys-
tals (see below). To our knowledge, the direct formation of
regioregular oligomers following the aromatization step has
never been described. To further elucidate this reaction and to
test whether the oligomerization process takes place during or
after the aromatization, the reaction was examined with
various amounts of SO2Cl2 (Table 1).
With less than 1 equivalent of oxidant, conversion of 2 into
3 (Table 1, entries 1 and 2) was not complete but correspond-
ed to the exact amount of SO2Cl2 engaged for aromatization.
When 1.1 equivalents were used at 08C, monomer 3 formed
as the major product in 78% yield and the dimer 4, trimer 5
and tetramer 6 were isolated in 8, 4, and 2% yields, respective-
ly (Table 1, entry 3). When using 1.1 equivalents of SO2Cl2 at
reflux in CHCl3 (Table 1, entry 4), the amount of monomer de-
creased while the quantities of oligomers increased. However,
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Scheme 2. Tautomeric equilibrium between Wh3-HCl+ and Am3-HCl+.
culations of the two forms were performed at the ab initio
density functional level with the Gaussian 09 package (see the
Supporting Information, Figure S1).[18] The aminium form incor-
porates a bond length of the heterocycle that is compatible
with the aromatic thiophene ring, whereas the CÀN bond be-
tween the thiophene and pyrrolidyl rings corresponds to
a single bond. For the iminium form, the sulfur heterocyle has
lost its aromatic character but the CÀN bond length with the
pyrrolidyl group (130 pm) is as expected for a connection with
an iminium bond. Nevertheless, the comparison of the calcu-
lated energy for the two forms shows that iminium form Wh3-
HCl+ is slightly more stable than the aminium Am3-HCl+ with
a difference of energy DE=1.95 kcalmolÀ1. Due to the stabili-
zation of the s-complex Wh3-HCl+, its reaction with a good
nucleophile, such as 3, can become competitive in comparison
to the loss of proton leading to the formation of 3-Cl. Thus,
after aromatization, the action of SO2Cl2 on 3 starts with the
formation of Wh3-HCl+, which acts as electrophile to react
Figure 2. 1H NMR analysis in CDCl3 of dihydrothiophene 2 in the presence of
different amount of SO2Cl2.
hours at room temperature, the orange solution became dark
red and analysis showed complete disappearance of 3-Cl and
formation of numerous compounds, which were difficult to
identify. Moreover, the reactional medium is again acidic,
indicating that the chlorinated compound had eliminated HCl.
To test the formation of chlorinated derivatives suppressed
by the presence of excess SO2Cl2 during the aromatization pro-
cess, the reaction was carried out with 4-cyano-3-hexyloxy-
2,5-dihydrothiophene (see the Supporting information,
Scheme S6). The experiment showed the expected regio-
selective chlorination of the thiophene derivative and the
chlorinated thiophene compound was isolated in 90% yield
from the dihydrothiophene with 2.5 equivalents of SO2Cl2. The
formation of oligomers of 4-cyano-3-hexyloxythiophene was
not observed.
with monomer
3
to give an intermediate dimer DCl+
(Scheme 3). This dimer undergoes elimination of HCl and H+
to re-aromatize and afford dimer 4. Chloride ion is a good leav-
ing group allowing the easy formation of HCl. The reaction
proceeds regioselectively due to the important difference in
site reactivity (nucleophilic vs. electrophilic), thus leading only
to the D–A-alternated dimer. Stepwise reactions with 3 from
DCl+ or after chlorination of the dimer 4 will lead to longer
oligomers. The chlorination of dimer 4 has also been per-
formed by using SO2Cl2 in stoichiometric amount (see the Sup-
The difference in behavior of 4-cyano-3-pyrrolidylthiophene
and 4-cyano-3-hexyloxythiophene towards SEAr with SO2Cl2,
leading to the formation of oligomers to the detriment of the
chlorinated derivative for 3, can be explained by the difference
in stability between the Wheland intermediates (cationic s-
complexes). The study by Forlani and co-workers on 1,3,5-tria-
minobenzene showed that piperidyl or pyrrolidyl groups
strongly stabilized Wheland s-complexes as the mesomeric
iminium form.[16] This stabilization of the amino cationic s-com-
plex intervenes during the chlorination of 3 but also corre-
sponds to the protonated form of 3-Cl. Indeed, as presented
on Scheme 2, there are two possible tautomeric forms for the
protonation of 3-Cl; direct protonation of the nitrogen atom
to give aminium derivative Am3-HCl+ or protonation of the
carbon atom to give Wh3-HCl+, which corresponds to the
Wheland intermediate.
Recently De Rosa and Arnold demonstrated for 3-aminopyr-
role that the iminium tautomer was favored.[17] Theoretical cal-
Scheme 3. Proposed mechanism for the formation of oligomers.
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porting Information, Scheme S5). The chlorinated dimer 4-Cl
was isolated in 46% yield. Tetramer 6 was also isolated in 4%
yield, whereas no trimer was detected by mass spectrometry.
Thus, as in the SEAr reaction of 4 with SO2Cl2, the formation of
4-Cl by loss of H+ from the Wheland complex Wh4-HCl+ com-
petes with the reaction between 4 and Wh4-HCl+ leading to
6. However, both the better stability of chlorinated 4-Cl and
the lower nucleophilic reactivity of dimer 4 disfavor the oligo-
merization process.
of the chlorinated derivative reacts as an electrophile with 3-
cyano-4-pyrrolidylthiophene in an electrophilic aromatic substi-
tution. A synergistic effect of pyrrolidyl and cyano groups indu-
ces the reaction. The electronic dissymmetry at the 2- and 5-
positions of the thiophene ring is enhanced by the strong elec-
tron donor ability of the pyrrolidyl group, which increases the
nucleophilic character of the thiophene derivative. The cyano
group strengthens the electrophilic character of the Wheland
complex stabilized by the pyrrolidyl group in the iminium
form. The monomer and the dimer are useful for the synthesis
of various oligomers with well-defined structures. The influ-
ence of the relative positions of the donor and acceptor
groups along the conjugated chain on the electronic
properties of the oligomers is under investigation and will be
reported in future publications.
Single crystals suitable for X-ray diffraction analysis were ob-
tained for trimer 5 and tetramer 6 by slow evaporation of
a chloroform/ethanol solution. The two structures with strict al-
ternation of the cyano and pyrrolidyl groups present a good
planarity of the conjugated systems, with torsion angles be-
tween the thiophene rings of <128 (Figure 3). The thiophene
Acknowledgements
The authors acknowledge Angers Loire MØtropole for grants to
C.M. and G.S.
Keywords: electrophilic substitution
·
oligomerization
·
organic synthesis · regioselectivity · thiophenes
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Received: January 13, 2016
Published online on March 24, 2016
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