Reaction of 4H-cyclopenta[2,1-b:3,4-b′]dithiophenes with NBS - A route toward 2H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6(4H)-diones

Article abstract of DOI:10.1016/j.tet.2013.01.026

In this paper we present a detailed study of the bromination reaction of cyclopentadithiophene (CPDT) derivatives, by means of NBS, giving access to either 2,6-dibromo-4H-cyclopenta[2,1-b:3,4-b′]dithiophenes or 2H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6(4H)-diones (CPDT-2,6-diones). The CPDT-2,6-diones are fully characterized, including an X-ray single crystal structure of one of the representative materials. A mechanism leading to the formation of the latter, which arise as a new class of compounds within the CPDT family, is proposed and is supported by Gibbs free energy calculations. The influence of the solvent, reaction time, and the number of equivalents of NBS on the selectivity of the reaction is discussed.

Full text of DOI:10.1016/j.tet.2013.01.026

Tetrahedron 69 (2013) 2260e2267
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Reaction of 4H-cyclopenta[2,1-b:3,4-b0]dithiophenes with
NBSda route toward 2H-cyclopenta[2,1-b:3,4-b0]dithiophene-
2,6(4H)-diones
Lidia Marin a, Sarah Van Mierloo a, Yuexing Zhang b, Koen Robeyns c, Benoît Champagne b,
,
,
*
Peter Adriaensens a, Laurence Lutsen d, Dirk Vanderzande a d, Wouter Maes a
a Design & Synthesis of Organic Semiconductors (DSOS), Hasselt University, Institute for Materials Research (IMO-IMOMEC), Agoralaan 1,
Building D, 3590 Diepenbeek, Belgium
b
ꢀ
Laboratoire de Chimie Theorique, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium
c
ꢀ
Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity (MOST), Universite Catholique de Louvain,
Place L. Pasteur 1, bte L4.01.03, 1348 Louvain-la-Neuve, Belgium
d IMEC, IMOMEC Ass. Lab., Universitaire Campus, Wetenschapspark 1, 3590 Diepenbeek, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we present a detailed study of the bromination reaction of cyclopentadithiophene (CPDT)
derivatives, by means of NBS, giving access to either 2,6-dibromo-4H-cyclopenta[2,1-b:3,4-b0]dithio-
phenes or 2H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6(4H)-diones (CPDT-2,6-diones). The CPDT-2,6-
diones are fully characterized, including an X-ray single crystal structure of one of the representative
materials. A mechanism leading to the formation of the latter, which arise as a new class of compounds
within the CPDT family, is proposed and is supported by Gibbs free energy calculations. The influence of
the solvent, reaction time, and the number of equivalents of NBS on the selectivity of the reaction is
discussed.
Received 7 December 2012
Received in revised form 7 January 2013
Accepted 9 January 2013
Available online 18 January 2013
Keywords:
4H-Cyclopenta[2,1-b:3,4-b0]dithiophene
(CPDT)
Bromination
2H-Cyclopenta[2,1-b:3,4-b0]dithiophene-
2,6(4H)-dione
Ó 2013 Elsevier Ltd. All rights reserved.
Gibbs free energy calculations
Solvent effects
1. Introduction
(PCPDTBT) (Fig. 1) was among the first low band gap copolymers to
be explored for OPV applications, affording power conversion effi-
Organic photovoltaics (OPV) has emerged as a powerful and
environmentally friendly technology that converts solar light into
energy.1 An appealing feature of organic solar cells is the relatively
low production cost for large area modules by application of roll to
roll printing techniques. Moreover, due to the high extinction co-
efficients of organic semiconductors, the thicknesses of the active
layers can be in the order of hundreds of nanometers only, which
may ultimately lead to light-weight, flexible and semi-transparent
modules, particularly interesting for portable or wearable electron-
ics and building integrated photovoltaics. With the solar photon flux
peaking around 700 nm, various donoreacceptor materi-
alsddisplaying a smaller band gap and broader absorption win-
dowdhave recently been designed and used in bulk heterojunction
organic solar cells.2,3 Poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-
b:3,4-b0]dithiophene-2,6-diyl)-alt-(2,1,3-benzothiadiazole-4,7-diyl)]
ciencies above 2.8% in combination with a fullerene acceptor.4 In the
presence of a suitable processing additive, the efficiency was almost
doubled, surpassing the 5% threshold.5 As a result, PCPDTBT has
become one of the workhorse low band gap materials in the field.6
The copolymer, in which electron rich 4H-cyclopenta[2,1-b:3,4-b0]
dithiophene (CPDT) and electron poor 2,1,3-benzothiadiazole units
(BT) alternate along the backbone, is generally synthesized by Suzuki
or Stille polycondensations,7 occurring via a step-wise growth of the
* Corresponding author. Tel.: þ32 (0)11 268312; fax: þ32 (0)11 268299; e-mail
Fig. 1. Chemical structure of a PCPDTBT (4,4-bis(2-ethylhexyl)-substituted) low band
gap copolymer.
address: wouter.maes@uhasselt.be (W. Maes).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.01.026

L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
2261
polymer chain and therefore requiring a precise 1:1 ratio of func-
tionalities (halide and boronic ester or trialkyl tin, respectively). Not
fulfilling this precise monomer feed ratio leads to the formation of
low molecular weight species, which show poor optical properties
and the resulting materials can, in consequence, not be used in the
fabrication of highly efficient solar cell devices.
2,6-dibromo-CPDT 2a in isolated yields as low as 40% have drawn
our attention. The 1H NMR spectra of the crude reaction mixtures
(see Supplementary data, Fig. S1) showed, besides the expected
triplet-like resonance pattern of the dibrominated CPDT 2a (at
d
¼6.93 ppm) and the aliphatic protons of the alkyl side chains (in
this case two 2-ethylhexyl groups), the presence of an additional
triplet at
When Suzuki polycondensation (SPC) is chosen for the synthesis
of PCPDTBT,7c the toxic tin side products generated in the Stille
polycondensation are avoided. The SPC is usually carried out be-
tween the dibrominated CPDTand the bis(boronic acid pinacol ester)
derivative of 2,1,3-benzothiadiazole. The latter can be synthesized in
two steps starting from BT by direct bromination, using a mixture of
bromine in hydrobromic acid,8 followed by palladium catalyzed
halogen-metal exchange in the presence of bis(pinacolato)diboron.7b
On the other hand, dibrominated CPDT derivatives are obtained
upon bromination using N-bromosuccinimide (NBS).7d,9b In order to
obtain the precise 1:1 stoichiometry required in the SPC, high purity
of the difunctional monomers is required. Coupling units bearing
only one of these functionalities, i.e., boronic ester or bromine, act as
end-cappers or ‘dead end chains’ and lead to the formation of lower
molecular weight materials. Such monofunctional compounds must
therefore carefully be removed by tedious purification or, ideally, not
be formed during the transformations.
d
¼6.01 ppm. In the particular case shown in Fig. S1, both
triplets were located in the low field region with the same in-
tegrated value, pointing to a 1:1 ratio of two products. The presence
of a secondary product formed during the bromination reaction
was also confirmed by GCeMS. It should be emphasized here that
the resonance of the unique aromatic proton belonging to the
dibrominated CPDT derivative 2a (at
d
¼6.93 ppm) shows up as an
‘apparent’ triplet (there is no J-coupling) due to the presence of
different stereoisomers. In the case where there is one linear and
one branched (2-ethylhexyl) side chain (2c and 2d), an apparent
doublet was observed (Fig. S9 and 15), whereas in the case of two
linear substituents (2b) only a sharp singlet was noticed (Fig. S4).
In this paper, we present a detailed study of the bromination
reaction of CPDT derivatives, by means of NBS, giving access to both
2,6-dibromo-4,4-bis(alkyl)-4H-cyclopenta[2,1-b:3,4-b0]dithio-
phenes and 2H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6(4H)-dio-
nes (CPDT-2,6-diones), depending on the applied conditions.
Scheme 1. Bromination of CPDTs 1aee with NBS, affording 2,6-dibromo-CPDTs 2aee
and CPDT-2,6-diones 3aee.
2. Results and discussion
2.1. Synthetic exploration
Both reaction products were separated and purified by means of
column chromatography (silica) to recover the dibrominated CPDT
2a (clear oil, 40% yield) and the secondary product 3a (yellow oil,
40% yield), respectively. Complete 1H and 13C NMR analysis, cor-
roborated with GCeMS analysis and IR spectroscopy, allowed to
identify the isolated secondary product as 4,4-bis(2-ethylhexyl)-
The synthesis of CPDT derivatives has been studied intensively
and has been improved steadily over the years. The groups of
Gronowitz10 and Wynberg11 have dedicated long years of research
to this class of compounds. Among the six isomers that can be
obtained depending on the annulation of the two thiophene rings,
4H-cyclopenta[2,1-b:3,4-b0]dithiophene has gained much attention
in the past years and is, to the best of our knowledge, the sole
isomer that has been used in the synthesis of low band gap poly-
mers for solar cell applications. The presence of solubilizing alkyl
side chains on the bridging carbon atom is mandatory to obtain
solution processable materials. Most synthetic protocols toward the
CPDT scaffold involve multistep routes,12 many of which use harsh
reaction conditions and/or controversial reagents (such as the
WolffeKishner reduction and the oxidation of secondary alcohols
using pyridinium chlorochromate, respectively). More recently,
CPDT derivatives bearing various functionalities in the side chains
have been synthesized using a three-step approach.13 This route
enables the synthesis of both symmetrically and asymmetrically
alkyl-substituted (linear or branched) CPDT derivatives and also
allows introduction of ester-functionalized side chains by variation
of the keto(ester) precursor. Regardless of the synthetic approach
used for the synthesis of the CPDT scaffold, bromination in posi-
tions 2 and 6 of the fused aromatic unit is required to afford the
CPDT monomer for SPC.3c,14
2H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6(4H)-dione
(3a)
(Scheme 1), a new compound within the CPDT family. 1H NMR
analysis revealed that the triplet-like signal located around
d
¼6.01 ppm (Fig. S2) results from the protons in
a position of the
carbonyl groups, the latter being confirmed by the presence of
a characteristic C]O resonance around
d
¼194 ppm in the 13C NMR
spectrum (Fig. S3) and by an absorption peak located at
n
¼1700 cmꢀ1 in the IR spectrum. HRMS analysis provided the exact
molar mass of the isolated product, in accordance with the pro-
posed structure. The formation of the CPDT-2,6-dione was not
limited to the bis(2-ethylhexyl) derivative, but was also encoun-
tered for a few analogous CPDT compounds 1bed (Scheme 1). The
experiments that were conducted using 3.5 equiv of NBS in DMF
allowed to isolate both the initially targeted dibrominated CPDTs
2aed and CPDT-2,6-diones 3aed. All novel materials were fully
characterized by 1H and 13C NMR, (HR)MS and IR.
2.2. Single crystal X-ray diffraction
Single crystals suitable for X-ray diffraction analysis could be
obtained forone of the CPDT-2,6-diones (3c) (Fig. 2), unambiguously
proving the structure assignment. The structure is highly disor-
dered, at least for the long alkyl chains. The CPDT system is quite
rigid and planar, and the (first atoms of the) alkyl chains are posi-
tioned perpendicular with respect to the CPDT plane.
To brominate 4,4-bis(2-ethylhexyl)-substituted CPDT derivative
1a (Scheme 1) and exclude the presence of the monobrominated
derivative in the reaction mixture, we have conducted the bromi-
nation reaction (for 8e24 h in the dark and under inert atmo-
sphere) by adding an excess of NBS (2.5 up to 3.2 equiv) to a 2 mM
solution of the CPDT derivative in N,N-dimethylformamide (DMF).
Under these conditions, no monobrominated product was detected
and the yields of the isolated dibrominated CPDT 2a (Scheme 1)
were varying between 40 and 80%. Reactions which afforded
2.3. Reaction mechanism
Several processes can account for the formation of CPDT-2,6-
diones 3aed: (1) DielseAlder addition between singlet oxygen

2262
L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
were carried out under an oxygen-free atmosphere, using
degassed solvents and chemicals. Additionally, only 2.1 equiv of
NBS were used for a 1 mM solution of 1a in DMF. Even when
taking all these precautions, GCeMS and NMR analysis showed
the formation of 3a already after 2 h. At this stage of the re-
action, also monobrominated CPDT was still present in low
concentrations. After 3 h, when the monobrominated CPDT
was not detected anymore, the reaction was stopped. These
experiments showed that the exclusion of oxygen from the
reaction medium does not completely avoid the formation of
the CPDT-2,6-dione. Hence, the mechanism leading to the
formation of compound 3a probably does not involve 1O2.
(2) In an attempt to brominate 2-bromo-3,5-di-tert-butylth-
iophene with bromine in an acetic acidenitric acid mixture in
the presence of silver nitrate, Gronowitz et al.17 isolated the
corresponding hydroxyfuranone in 70% yield. The formation of
the latter was supposed to occur by hydrolytic opening of the
thiophene ring followed by dehydrogenation via bromine. Also
this mechanism does not seem to be responsible for the for-
mation of compound 3a, as the replacement of the sulfur atom
in the CPDT ring was not observed.
(3) Upon bromination of naphtho[1,2-c]thiophene using a mixture
of NBS in acetic acid, Lin et al.18 have recovered the expected
dibrominated compound together with two other secondary
products, i.e., a thiolactone and a thioanhydride. Even though
the bromination reaction was conducted for only 20 min, with
just 2.0 equiv of NBS (an equimolar amount), the formation of
the dibrominated compound was not exclusive, as three
products were formed in a 1:0.6:1 ratio. The proposed mech-
anism involves further electrophilic attack of the dibrominated
derivative leading to a tribrominated species, which affords the
thiolactone upon aqueous work-up. Exposure of the latter to air
leads to the formation of the thioanhydride.
Fig. 2. Ortep representation of 3c (R1¼2-ethylhexyl, R2¼octyl), showing thermal dis-
placement ellipsoids drawn at the 50% probability level.
(1O2) (presumably generated in situ) and the non-brominated CPDT
derivative,15 (2) hydrolytic opening of the thiophene ring followed
by dehydrogenation via bromine and (3) overbromination of the
dibrominated compound leading, upon aqueous work-up, to a thi-
olactone, followed by its fast oxidation (in air). To get more insight
into the reaction path leading to the formation of the CPDT-2,6-
diones, these possible mechanisms were analyzed.
(1) It is well known that thiophene derivatives, and especially
photoactive materials, such as poly(3-alkylthiophenes), have
a rather low photochemical stability, which, in the presence of
air, leads to their degradation. The photodegradation products
are believed to be formed via an adduct between the thiophene
ring (part of the p-conjugated system in the case of polymers)
and oxygen, generating carbonyl and sulfonic derivatives or
that, alternatively, the pathway involves the side chains.16 The
fact that such degradations are considerably reduced or even
absent when the thiophene derivatives are irradiated under
inert atmosphere highlights the key role of oxygen in the for-
mation of the photodegradation products. Since the bromina-
tion reaction of CPDT 1a has been conducted in the dark and
under inert atmosphere, this mechanism is not likely to take
place. To verify the involvement of 1O2, all further experiments
By analogy, the same sequence of reactions, i.e., electrophilic at-
tack followed by hydrolysis and fast oxidation, can account for the
formation of CPDT-2,6-dione 3a (Scheme 2). Here, overbromination
at the 2-position of the CPDT ring affords the positively charged
intermediate 2a0, which is stabilized by several resonance forms. The
resonance form bearing the positive charge at the 6-position leads,
upon aqueous work-up, to thiolactone 2a00. Surprisingly, this
Scheme 2. Proposed mechanism for the formation of CPDT-2,6-dione 3a.

L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
2263
intermediate has never been detected by GCeMS or NMR analysis,
suggesting that this compound is very efficiently converted to
compound 3a. The loss of aromaticity in compound 3a is reflected in
the chemical shift of the two protons belonging to the fused het-
rings of 1e are
well the experimental values of
2e, the corresponding chemical shift for the
d
¼7.19 and 7.05 ppm, respectively, matching rather
d
¼7.17 and 6.94 ppm for 1a.13a For
b hydrogen atom de-
creased slightly to
d
¼6.90 ppm, close to the experimental value of
erocycle located at
d¼6.01 ppm, well below the normal chemical
d
¼6.93 ppm for 2a. The calculated 1H NMR chemical shift of the
shift range of aromatic (CPDT) protons. The driving force for this
transformation can be found in the formation of a stable quinoid
derivative.19 The CPDT-2,6-dione formation was enhanced by the use
of an excess of NBS (vide infra), which suggests that, under these
conditions, the CPDT-2,6-dione is thermodynamically favored,
whereas the dibrominated CPDT derivative appears under more ki-
netic conditions. The crucial role of a bromonium intermediate in the
NBS promoted rearrangement of 1,1-diarylmethylenecyclopentanes
has recently been pointed out and supports the proposed reaction
pathway.20 Additionally, palladium mediated reactions between
a series of variously substituted CPDT derivatives and sodium
dicyanomethanide, followed by oxidation with aqueous bromine,
have led to the formation of quinoid 2,6-dihydro-4H-cyclopenta[2,1-
b:3,4-b0]dithiophene derivatives.21
b
hydrogen atoms in 3e appeared at 5.97 ppm, in line with the
experimental shift at
on 2e00 showed that the
on the carbonyl sides have chemical shifts of
d
¼6.01 ppm for 3a. In addition, calculations
hydrogen atoms on the dibrominated and
¼6.45 and 5.97 ppm,
b
d
respectively. The resonance at
d
¼6.45 ppm can hence be applied as
a fingerprint to detect the possible presence of 2e00.
2.5. Study of solvent effects
Knowing that in DMF the use of 4.0 equiv of NBS afforded the
CPDT-2,6-dione 3c (R1¼2-ethylhexyl, R2¼octyl) as the major re-
action product (w73% yield; vide infra), we have investigated the
influence of other solvents and/or solvent mixtures generally used
for brominations using NBS. The polarity and protic character of the
solvent were varied. As displayed in Table 1, the use of less polar
solvents, such as chloroform (CHCl3) or carbon tetrachloride (CCl4),
led exclusively to the formation of the dibrominated compound 2c.
The use of polar protic solvents or a 1:1 mixture with a less polar
solvent, such as acetic acid (AcOH) or an acetic acid/chloroform
mixture, respectively, still afforded the dibrominated compound 2c
as the main reaction product, together with small amounts of CPDT-
2,6-dione 3c (the formation of 3c being slightly enhanced in pure
AcOH). These findings suggest that the formation of 3c is favored in
polar solvents.1,2-Dimethoxyethane (DME) and DMF are both polar
aprotic solvents, perfectly miscible with water. Moreover, the sys-
tem NBS-DME is known as a powerful oxidizing medium for sec-
ondary alcohols.21c,24 Surprisingly enough, bromination of 1c
appeared to proceed differently in these two solvents. In DME, the
formation of 3c did not occur at all, whereas in DMF the latter stands
for the major reaction product. In acetone, 2c was also the sole re-
action product, the presence of 3c not being detected. These results
show that there is no correlation between the protic/aprotic char-
acter or the polarity of the solvent (mixture) and the product dis-
tribution (dibrominated CPDT vs CPDT-2,6-dione).
2.4. Theoretical calculations
To support our assumption, theoretical calculations were carried
out for the reactions involving the model compounds 1e, 2e, 2e00, and
3e, together with NBS, succinimide (NS), HBr, and water. The results,
summarized in Fig. 3, showed that the formation of dibromo-CPDT
2e from 4,4-diethyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene (1e)
stabilizes the system by about 46.0 kcal/mol according to B3LYP/6-
311þG(d,p) calculations, and by 50.1 kcal/mol when employing the
MP2/6-311þG(d,p) method, indicating that the formation of the
dibrominated compound is a thermodynamically favored process.
Moreover, the formation of CPDT-2,6-dione 3e further stabilizes the
system by 62.6 [32.2] kcal/mol according to calculations carried out
at the B3LYP/6-311þG(d,p) [MP2/6-311þG(d,p)] level. During the
formation of dibromo-CPDT 2e, part of the product reacts further
with NBS to form 3e, provided NBS is in excess, which is in agree-
ment with the experimental findings. The calculations also indicated
that the formation of the CPDT-2,6-dione derivative 3e via in-
termediate 2e00 is thermodynamically favored, which substantiates
the experimental hypothesis. The formation of 2e00 leads to an ad-
ditional DG stabilization by 33.7 [19.3] kcal/mol with respect to 2e
Table 1
and the transformation of the former to 3e involves an additional
stabilization by 28.9 [12.9] kcal/mol according to B3LYP/6-
311þG(d,p) [MP2/6-311þG(d,p)] calculations.
Influence of the solvent (properties) on the bromination of 1c (using 4.0 equiv of
NBS)
Solvent
(mixture)
2c:3c (%)
Dipole
Dielectric
Miscibility
with water
moment (D)a
constant (ε)a
Theoretically calculated NMR chemical shifts were in good
agreement with the experimental ones. The B3LYP/6-311þG(2d,p)
DMF
CHCl3
CCl4
AcOH
CHCl3/AcOH (1:1)
DME
Acetone
27:73
100:0
100:0
83:17
96:4
3.82
1.01
0
1.74
d
36.70
4.81
2.24
6.15
d
Yes
No
No
Yes
d
calculated 1H NMR chemical shifts (corrected using the
a/b linear
regressions)22 for the
a
and
b
hydrogen atoms on the thiophene
100:0
100:0
1.71
2.88
7.20
21.0
Yes
Yes
a
Ref. 25.
2.5.1. In situ 1H NMR experiments. To be able to follow the evolu-
tion of the reaction mixture in time, we have then performed
a series of in situ 1H NMR experiments. For this purpose, the bro-
mination reaction was conducted directly in an NMR tube, in either
DMF-d7, CDCl3, CH3COOD or CH3COOD/CDCl3 (1:1 v/v).
2.5.1.1. DMF-d7. The first spectrum (after 5 min only) revealed
the presence of the dibrominated CPDT 2c together with traces of
the monobrominated derivative. The last recorded spectrum (after
3 days) displayed almost no changes compared to the one recorded
after 1 h, showing a main peak corresponding to the dibrominated
Fig. 3. Variation of the Gibbs free energy (kcal/mol) along the reaction coordinate.

2264
L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
product and two additional small peaks. The spectrum of the hy-
drolyzed sample, however, showed the resonance at
corresponding to the CPDT-2,6-dione 3c, as the major signal, in an
almost 9:1 ratio with respect to the dibrominated CPDT 2c
CPDT-2,6-dione as the unique reaction product. The distribution of
the mol fractions of 2c and 3c in the crude reaction mixture, as
d
¼6.01 ppm,
deduced from the integration of the protons located at
d
¼6.93 and
6.01 ppm in the 1H NMR spectrum, respectively, showed a sigmoid
variation with the number of equivalents of NBS used, which could
be an indication of an autocatalytic reaction (Fig. 4).23
(
at
d
¼6.93 ppm), together with two other minor doublet-like signals
d
¼6.30 and 7.10 ppm. GCeMS analysis of an aliquot collected
after 1 h and the hydrolyzed sample (after 3 days) showed the same
composition. These facts strongly suggest that the formation of the
CPDT-2,6-dione is significantly accelerated upon hydrolysis and/or
exposure to air. Under these conditions (4.0 equiv of NBS, DMF as
a solvent) it became very clear that the reaction time has little or no
influence on the formation of the CPDT-2,6-dione.
2.5.1.2. CDCl3. The first spectrum (after 5 min) showed that the
starting product was fully converted to the dibromo-CPDT 2c. No
traces of the monobrominated derivative or other secondary
products could be detected at this stage of the reaction. The system
looked stable and did not evolve in the next 5 h. No major changes,
except for two minor doublet-like signals at
d
¼6.30 and 7.10 ppm,
were noticed until the end of the experiment. After work-up, the 1H
NMR spectrum of the fully hydrolyzed mixture showed the pres-
ence of the dibrominated CPDT 2c and small amounts of CPDT-2,6-
dione 3c. As in the DMF case, the two other minor doublet-like
signals were equally present.
Fig. 4. Influence of the number of equivalents of NBS on the reaction product ratio
2c:3c (experiments conducted in DMF).
2.5.1.3. CH3COOD and CH3COOD/CDCl3 (1:1 v/v). For the re-
actions performed in acetic acid or a mixture acetic acid/chloro-
form, in situ 1H NMR experiments pointed again toward an increase
in the percentage of the CPDT-2,6-dione 3c upon aqueous work-up,
this trend being enhanced in pure acetic acid.
3. Conclusions
In this study we have carefully investigated the bromination of
a series of CPDT derivatives, with NBS as the bromine source. Among
the parameters influencing the reaction, e.g., concentration of the
starting product, solvent, reaction time and the presence/absence of
oxygen, the solvent and the number of equivalents of NBS seemed to
have a great impact on the yields of the dibrominated products and
the 2H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6(4H)-dione second-
ary products. In DMF, full control over the reaction could be achieved
by tuning the number of equivalents of NBS. The use of 2.1 equiv
afforded the dibrominated CPDT derivatives as the unique reaction
products. On the other hand, the use of 5.0 equiv led to exclusive
formation of the CPDT-2,6-diones. The formation of the CPDT-2,6-
diones is supposed to occur via a bromination-hydrolysis se-
quence, followed by fast conversion to the dione. The obtained in-
sights are of relevance for all those involved in the synthesis and
application of CPDT-based materials (notably in organic electronics).
The chemical reactivity of this new class of compounds toward
various transformations, e.g., reductions, Knoevenagel reactions,
condensations, dimerizations, thionylations, etc., specific to carbonyl
functionalities, is currently under investigation within our group, as
is the exploration of the possible applications of the novel thieno-
quinoidal derivatives in organic electronics.26
Summarizing, in situ 1H NMR experiments allowed to confirm the
strong dependence of the reaction outcome on the solvent employed.
No clear intermediates could be identified and/or isolated. The find-
ings also point to a crucial change in composition upon aqueous
work-up and support the suggested mechanism, i.e., over-
bromination followed by hydrolysis and fast conversion to the dione.
2.6. Influence of the number of equivalents of NBS on the
product distribution
Having in mind that, provided NBS is in excess, the bromination
reaction leads to the formation of two distinct classes of products,
we were interested to obtain full control/selectivity toward the
formation of either product. For this purpose we have decided to
perform the bromination reaction of 1c in DMF using an excess of
NBS ranging from 2.1 to 5.0 equiv, the reaction time being limited to
3 h. This series of experiments (Table 2) pointed out that by using
only a slight excess of NBS (2.1 or 2.5 equiv), the CPDT-2,6-dione
formation could virtually be suppressed. The use of 3.0, 3.5, and
4.0 equiv of NBS afforded the CPDT-2,6-dione in 15, 50, and 73%
yield, respectively, whereas the use of 5.0 equiv of NBS gave the
4. Experimental section
Table 2
Influence of the number of equivalents of NBS on the
reaction product distribution 2c:3c
4.1. Materials and methods
Equiv NBS
2c:3ca,b
NMR chemical shifts (d, in parts per million) were determined
2.1
2.5
3.0
3.5
4.0
5.0
98:2
100:0
85:15
50:50
27:73
0:100
relative to the residual CHCl3 absorption (7.26 ppm) or the 13C
resonance shift of CDCl3 (77.16 ppm). Gas chromatographyemass
spectrometry (GCeMS) analyses were carried out applying
Chrompack Cpsil5CB or Cpsil8CB capillary columns. Exact mass
measurements were performed in the EI mode at a resolution of
10,000. The structures of the model compounds were optimized
using density functional theory (DFT) with the B3LYP hybrid
exchange-correlation functional and the 6-311þG(d,p) basis set. All
a
Deduced from the integration of the protons situated
at
d
¼6.93 and 6.01 ppm, belonging to compounds 2c and
3c, respectively; 3 h reaction at rt in DMF (c¼0.02 mM).
b
Within the 1H NMR detection limit.

L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
2265
vibrational frequencies are real, confirming that these structures
are minima. G values were evaluated at the same level of ap-
NMR (300 MHz, CDCl3)
d
¼6.01 (s, 2H), 1.86e1.82 (m, 4H), 1.29e1.20
D
(m, 40H), 0.87 (t, J¼6.0 Hz, 6H); 13C NMR (75 MHz, CDCl3)
¼194.3
d
proximation for T¼298.15 K and P¼1.0 atm. Based on the B3LYP/6-
311þG(d,p) optimized structures, the chemical shifts of all systems
were calculated with the same functional, the 6-311þG(2d,p) basis
set, and the GIAO method to ensure origin-independence. The
MøllerePlesset second-order perturbation theory (MP2) method
was also employed to perform single point calculations on these
(C]O), 176.1, 145.3, 115.1 (CH), 52.6, 39.4 (CH2), 32.0 (CH2), 29.8
(CH2), 29.77 (CH2), 29.75 (CH2), 29.70 (CH2), 29.6 (CH2), 29.4 (CH2),
29.4 (CH2), 22.8 (CH2), 14.2 (CH3); IR (NaCl, cmꢀ1
)
nmax¼3076 (w,
unsaturated CeH), 2958/2923/2852 (s, saturated CeH), 1703 (s, CO);
UVevis (CHCl3, nm) lmax (log ε)¼380 (4.497).
geometries. For both B3LYP and MP2 electronic energies, the
D
G
4.2.5. Methyl-9-(2,6-dibromo-4-(2-ethylhexyl)-4H-cyclopenta[2,1-
b:3,4-b0]dithiophene-4-yl)nonanoate (2d). According to general pro-
cedure 1: methyl 9-(4-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]
dithiophene-4-yl)nonanoate (1d) (0.315 g, 0.685 mmol), NBS
(0.260 g, 1.46 mmol, 2.1 equiv), DMF (17.5 mL), eluent hexane/ethyl
acetate 9:1; yellow oil (0.266 g, 63%); GCeMS (EI) m/z¼616/618/620
values were then evaluated by including the temperature-
dependent contributions obtained at the B3LYP level. For both the
structure optimizations and property calculations, the Polarizable
Continuum Model (PCM) using the integral equation formalism
(IEFPCM) was employed to account for solvent effects (DMF). All
calculations were performed using Gaussian09.27
[Mþ]; 1H NMR (300 MHz, CDCl3)
d¼6.92/6.91 (2ꢂ s, intensity ratio 1/
1, 2H), 3.64 (s, 3H), 2.26 (t, J¼7.5 Hz, 2H), 1.81 (t, J¼4.8 Hz, 2H),
1.76e1.70 (m, 2H), 1.61e1.51 (m, 2H), 1.30e0.88 (m, 19H), 0.77 (t,
J¼6.9 Hz, 3H), 0.62 (t, J¼7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3)
4.2. General procedure 1 (allowing to obtain both the di-
brominated and the CPDT-2,6-dione products)
d
¼174.5 (COeO), 155.9/155.8, 136.6/136.5, 125.0/124.9, 111.1/111.0
4,4-Bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene
(1a) (0.797 g, 1.98 mmol) was dissolved in DMF (100 mL) and the
mixture was cooled to 0 ꢁC by means of an ice bath (the reaction
was performed in the dark and under N2 balloon pressure). Sub-
sequently, a solution of NBS (0.178 g, 5.97 mmol, 3.0 equiv) in DMF
(100 mL) was added drop-wise during 30 min and the mixture was
allowed to react at rt overnight. The mixture was again cooled
down to 0 ꢁC, water (100 mL) was added and the aqueous layer was
extracted with diethyl ether. The combined organic layers were
washed with water and brine, dried over anhydrous MgSO4 and
filtered. Evaporation of the solvent under reduced pressure affor-
ded a dark yellow viscous oil. The crude mixture was purified by
means of column chromatography (silica) using a gradient of hex-
ane/ethyl acetate from 0% (isolation of the dibrominated CPDT 2a)
to 10% (isolation of the CPDT-2,6-dione 3a).
(CH), 55.1, 51.6 (CH3), 41.7, 39.4 (CH2), 35.4 (CH2), 34.2 (CH), 34.1 (CH2),
29.9 (CH2), 29.31 (CH2), 29.26 (CH2), 29.18 (CH2), 28.6 (CH2), 27.4
(CH2), 25.0 (CH2), 24.3 (CH2), 22.9 (CH2), 14.2 (CH3), 10.8 (CH3); IR
(NaCl, cmꢀ1
)
nmax¼3084 (w, unsaturated CeH), 2928/2855 (s, satu-
rated CeH), 1739 (s, COeO); UVevis (CHCl3, nm) lmax (log ε)¼250
(3.884), 339 (4.297).
4.3. General procedure 2 (allowing to push the reactions to-
ward the formation of the CPDT-2,6-dione products)
4-(2-Ethylhexyl)-4-octyl-4H-cyclopenta[2,1-b:3,4-b0]dithio-
phene (1c) (0.276 g, 0.68 mmol) was dissolved in DMF (10 mL) and
the mixture was cooled down to 0 ꢁC by means of an ice bath (the
reaction was performed in the dark and under N2 balloon pressure).
Subsequently, a solution of NBS (0.484 g, 2.72 mmol, 4.0 equiv) in
DMF (10 mL) was added drop-wise and the mixture was allowed to
react at rt overnight. The mixture was again cooled down to 0 ꢁC,
water (100 mL) was added and the aqueous layer was extracted
with diethyl ether. The combined organic layers were washed with
water and brine, dried over anhydrous MgSO4, and filtered. Evap-
oration of the solvent under reduced pressure afforded a dark
yellow viscous oil. The crude mixture was purified by means of
column chromatography (silica) using a gradient of hexane/ethyl
acetate from 0% (isolation of the dibrominated compound 2c) to
20% (isolation of the CPDT-2,6-dione 3c).
4.2.1. 2,6-Dibromo-4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]
dithiophene (2a).7d Light-yellow oil (0.400 g, 36%). 1H NMR
(300 MHz, CDCl3)
1.85e1.75 (m, 4H), 1.32e1.21 (m, 2H), 0.90e0.85 (m, 16H),
0.80e0.76 (m, 6H), 0.64e0.60 (m, 6H).
d
¼6.94/6.93/6.92 (3ꢂ s, intensity ratio 1/2/1, 2H),
4.2.2. 4,4-Bis(2-ethylhexyl)-2H-cyclopenta[2,1-b:3,4-b0]dithiophene-
2,6(4H)-dione (3a). Yellow oil (0.342 g, 40%). GCeMS (EI) m/z¼432
[Mþ];HRMS (EI) calcd for C25H36O2S2 432.2157; found m/z¼432.2166;
1H NMR (300 MHz, CDCl3)
d
¼6.02/6.01/6.00 (3ꢂ s, intensity ratio 1/2/
1, 2H), 1.92e1.82 (m, 4H), 1.15e0.99 (m, 18H), 0.81 (t, J¼6.0 Hz, 6H),
4.3.1. 2,6-Dibromo-4-(2-ethylhexyl)-4-octyl-4H-cyclopenta-[2,1-b:3,4-
0.71 (t, J¼6.0 Hz, 6H); 13C NMR (75 MHz, CDCl3)
d¼193.7 (C]O),176.1,
b0]dithiophene (2c). Slightly yellow oil (0.060 g, 16%); GCeMS (EI) m/
145.7,115.5 (CH), 52.1, 44.5(CH2), 35.6 (CH), 33.9 (CH2), 28.2(CH2), 27.1
z¼558/560/562 [Mþ]; 1H NMR (300 MHz, CDCl3)
¼6.94/6.93 (2ꢂ s,
d
(CH2), 22.8(CH2),14.1 (CH3),10.3 (CH3);IR (NaCl, cmꢀ1
)
nmax¼3077 (w,
intensity ratio 1/1, 2H), 1.83 (t, J¼4.8 Hz, 2H), 1.78e1.72 (m, 2H),
1.27e0.89 (m, 21H), 0.86 (t, J¼7.1 Hz, 3H), 0.79 (t, J¼6.9 Hz, 3H), 0.64
unsaturated CeH), 2958/2927/2871/2857 (s, saturated CeH), 1700 (s,
CO); UVevis (CHCl3, nm) lmax (log ε)¼278 (4.422), 381 (4.456).
(t, J¼7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3)
¼155.9/155.8, 136.6/
d
136.5, 125.0/124.9, 111.05/111.0 (CH), 55.1, 41.6 (CH), 39.4 (CH2), 35.5
(CH2), 34.1 (CH2), 31.9 (CH2), 30.0 (CH2), 29.43 (CH2), 29.40 (CH2), 28.6
(CH2), 27.4 (CH2), 24.4 (CH2), 22.9 (CH2), 22.8 (CH2), 14.3 (CH3), 14.2
4.2.3. 2,6-Dibromo-4,4-didodecyl-4H-cyclopenta[2,1-b:3,4-b0]dithio-
phene (2b).14 According to general procedure 1: 4,4-didodecyl-4H-
cyclopenta[2,1-b:3,4-b0]dithiophene (1b) (0.370 g, 0.718 mmol), NBS
(0.383 g, 2.16 mmol, 3 equiv), DMF (50 mL), eluent hexane; light-
yellow oil (0.510 g, 72%); GCeMS (EI) m/z¼671/673/675 [Mþ]; 1H
(CH3), 10.8 (CH3); IR (NaCl, cmꢀ1
2957/2926/2855 (w, saturated CeH); UVevis (CHCl3, nm) lmax
(log ε)¼250 (3.984), 339 (4.282).
)
nmax¼3082 (w, unsaturated CeH),
NMR (300 MHz, CDCl3)
d
¼6.92 (s, 2H), 1.77e1.72 (m, 4H), 1.29e1.14
(m, 40H), 0.87 (t, J¼6.0 Hz, 6H); 13C NMR (75 MHz, CDCl3)
d¼156.1,
4.3.2. 4-(2-Ethylhexyl)-4-octyl-2H-cyclopenta[2,1-b:3,4-b0]dithio-
phene-2,6(4H)-dione(3c). Deep-yellow oil (0.205 g, 70%); GCeMS (EI)
m/z¼432 [Mþ]; HRMS (EI) calcd for C25H36O2S2 432.2157; found m/
136.4, 124.7, 111.2 (CH), 55.2, 37.7 (CH2), 32.1 (CH2), 31.1 (CH2), 30.08
(CH2), 29.78 (CH2), 29.74 (CH2), 29.5 (CH2), 25.8 (CH2), 24.6 (CH2),
22.9 (CH2), 14.3 (CH3).
z¼432.2164; 1H NMR (300 MHz, CDCl3)
¼6.01/6.00 (2ꢂ s, intensity
d
ratio 1/1, 2H), 1.88e1.86 (m, 2H), 1.84e1.80 (m, 2H), 1.26e0.98 (m,
21H), 0.83 (t, J¼6.9 Hz, 3H), 0.80 (t, J¼6.9 Hz, 3H), 0.70 (t, J¼7.2 Hz,
4.2.4. 4,4-Didodecyl-2H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6(4H)-
dione (3b). Light-yellow oil (0.078 g, 20%); MS (CI) m/z¼544 [Mþ];
HRMS (EI) calcd for C33H52O2S2 544.3409; found m/z¼544.3370; 1H
3H); 13C NMR (75 MHz, CDCl3)
145.4, 115.3/115.2 (CH), 52.3, 43.1 (CH), 40.9 (CH2), 35.9 (CH2), 33.8
d
¼193.99/193.95 (C]O), 176.2/176.1,

2266
L. Marin et al. / Tetrahedron 69 (2013) 2260e2267
(CH2), 31.8 (CH2), 29.7 (CH2), 29.2 (CH2), 28.3 (CH2), 27.0 (CH2), 24.4
(CH2), 22.9 (CH2), 22.6 (CH2), 14.15 (CH3), 14.10 (CH3), 10.4 (CH3); IR
References and notes
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Acknowledgements
We gratefully thank BELSPO in the frame of the IAP P6/27 and
IAP 7/05 networks and the IWT (Institute for the Promotion of In-
novation through Science and Technology in Flanders) for financial
support by means of the SBO project 060843 ‘PolySpec’. Y.Z. and
B.C. acknowledge the financial support of the Fonds Speciaux de
Recherche of the Academie universitaire Louvain, co-funded by the
Marie Curie Actions of the European Commission (ADi/DB/
986.2011). The calculations were performed on the Plateforme
Technologique de Calcul Intensif of the University of Namur, for
which we acknowledge financial support of the F.R.S.-FRFC (Con-
ventions No. 2.4.617.07.F and 2.5020.11), and of the University of
Namur.
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Supplementary data
Crystallographic data (excluding structure factors) for the
structure in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
912552. A copy of the data can be obtained, free of charge, on ap-
plication to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
þ44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk). Further
supplementary data associated with this article (1H and 13C NMR
spectra of 2bed and 3aed, and more details on the X-ray structure)
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