Synthesis and study of the anodic behavior of 1,3,4,6-tetraaryl-2λ4δ2-thieno[3,4-c] thiophenes

Article abstract of DOI:10.1080/10426500307926

Two heterocyclic compounds based on the thieno[3,4-c]thiophene structure with four aryl substituents were prepared and their behavior in electrooxidation studied. These tetraarylthieno[3,4-c]thiophenes were synthesized in three steps starting from 1,3-dibenzoylmethane in the case of 1,3,4,6-tetraphenyl-2λ⁴δ²-thieno[3,4-c] thiophene 1a and from 1,3-bis(4′-methoxyphenyl)propane-1,3-dione in the case of 1,3,4,6-tetrakis(4′-methoxyphenyl)-2λ⁴δ ²-thieno[3,4-c]thiophene 1b, a new compound. Both cyclic and hydrodynamic voltamperometric analyses indicate two reversible one-electron oxidation stages for compound 1b, while for compound 1a only the first stage is reversible. The preparative electrooxidation of the two compounds results in the opening of one thiophene ring giving rise to γ-keto-thioketones.

Full text of DOI:10.1080/10426500307926

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Phosphorus, Sulfur, and Silicon and the Related
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Synthesis and Study of the Anodic Behavior of 1,3,4,6-
Tetraaryl-2λ 4 δ 2 -thieno[3,4-c]thiophenes
Tahar Douadi a & Michel Cariou b
a Université F. Abbas , Sétif, Algérie
b Université d′ Angers , Angers, France
Published online: 27 Oct 2010.
To cite this article: Tahar Douadi & Michel Cariou (2003) Synthesis and Study of the Anodic Behavior of 1,3,4,6-Tetraaryl-2λ 4
δ 2 -thieno[3,4-c]thiophenes, Phosphorus, Sulfur, and Silicon and the Related Elements, 178:3, 509-520
To link to this article: http://dx.doi.org/10.1080/10426500307926
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Phosphorus, Sulfur and Silicon, 2003, Vol. 178:509–520
C
Copyright 2003 Taylor & Francis
1042-6507/03 $12.00 + .00
DOI: 10.1080/10426500390170660
SYNTHESIS AND STUDY OF THE ANODIC
BEHAVIOR OF 1,3,4,6-TETRAARYL-
2–THIENO[3,4-c]THIOPHENES
4
2
Tahar Douadia and Michel Carioub
Universite´ F. Abbas, Se´tif, Alge´riea and Universite´ d0 Angers,
Angers, Franceb
(Received June 26, 2002)
Two heterocyclic compounds based on the thieno[3,4-c]thiophene struc-
ture with four aryl substituents were prepared and their behav-
ior in electrooxidation studied. These tetraarylthieno[3,4-c]thiophenes
were synthesized in three steps starting from 1,3-dibenzoylmethane
4
in the case of 1,3,4,6-tetraphenyl-2
2–thieno[3,4-c]thiophene 1a
and from 1,3-bis(40-methoxyphenyl)propane-1,3-dione in the case of
1,3,4,6-tetrakis(40-methoxyphenyl)-2
2–thieno[3,4-c]thiophene 1b, a
4
new compound. Both cyclic and hydrodynamic voltamperometric anal-
yses indicate two reversible one-electron oxidation stages for compound
1b, while for compound 1a only the first stage is reversible. The prepar-
ative electrooxidation of the two compounds results in the opening of
one thiophene ring giving rise to -keto-thioketones.
Keywords: Cation radical; electrooxidation; thieno[3,4-c]thiophene;
thioketone; thiophene
INTRODUCTION
It is known that the biphenyl linkage has a stabilizing effect on cation
radicals derived from methoxylated aromatics, especially when in-
cluded in a cyclic system such as fluorene, dibenzofuran, and even
more dibenzothiophene.1a This stabilizing effect is even greater with
two sulfur bridges as in dibenzo[bc,fg][1,4]dithiapentalene. Actually
the preparation, by anodic oxidation, of the mixed valence salt,
bis(2,3,4,6,7,8-hexamethoxydibenzo[bc,fg][1,4]dithiapentalenium) per-
chlorate (Scheme 1) was successful.1b
Address correspondence to Michel Cariou, Laboratoire d’Inge´nierie Mole´culaire et
Mate´riaux Organiques, UMR CNRS 6501, 2, Boulevard Lavoisier, 49045 Angers Cedex,
France. E-mail: michel.cariou@univ-angers.fr
509

510
T. Douadi and M. Cariou
SCHEME 1
That is why we thought that compounds with the same heterocyclic moi-
ety, that is, thieno[3,4-c]thiophene with four substituents instead of the
two condensed benzene rings as represented on the chemical scheme 2
below, could also lead to stable cation radicals by electrooxidation.
SCHEME 2
In fact, the chemical oxidation of 1,3,4,6,-tetraphenylthieno[3,4-c]-
thiophene 1a, by action of silver tetrafluoroborate or aluminium chlo-
ride in dichloromethane at 73 C leads to a cation radical.2 Also, the
reaction of potassium with this same compound in dimethoxyethane at
30 C gives the anion radical.2
The chemical oxidation of 1,3,4,6-tetrakis(isopropylsulfanyl)-
thieno[3,4-c]thiophene3 by iodine in hexane at 30 C affords as
well a cation radical. On the other hand its oxidation using nitroso-
nium tetrafluoroborate (NOBF4) in acetonitrile at 30 C gives a
dication.3
Only
a
few substituted thieno[3,4-c]thiophenes have been
synthesized in a stable form. We mention 1,3,4,6-tetraphenylthieno[3,4-
c]thiophene,4 1,3,4,6-tetra(2-thienyl)thieno[3,4-c]thiophene,5 1,3,4,6-
tetrakis(alkylsulfanyl)thieno[3,4-c]thiophene,6 and 1,3-dibromo-4,6-
dicyanothieno[3,4-c]thiophene.7 For the present study, a new one was
synthesized: 1,3,4,6-tetrakis(40-methoxyphenyl)thieno[3,4-c]thiophene
1b.

Synthesis and Study of the Anodic Behavior
511
Handling and using 1,3,4,6-tetraarylthieno[3,4-c]thiophenes 1 is dif-
ficult because they easily undergo oxidation in solution, with opening
of one thiophene ring, under the effect of light and aerial oxygen;4,8 this
requires manipulation away from light, in degassed solution, and under
nitrogen atmosphere.
SYNTHESIS OF 1,3,4,6-TETRAARYL-
4
2
2-THIENO[3,4-c]THIOPHENES
4
1,3,4,6-Tetraphenyl-2
2-thieno[3,4-c]thiophene 1a
The first step is the coupling reaction in the presence of sodium iodide of
brominated dibenzoylmethane9 into 1,4-diphenyl-2,3-dibenzoylbutane-
1,4-dione.10 The reaction of phosphorus pentasulfide with this tetra-
benzoylethane in refluxing pyridine gives directly 1,3,4,6-tetraphenyl-
4
2
2-thieno[3,4-c]thiophene 1a.
This method is faster than the one described in the literature,4 which
needs two additional steps.
1,3,4,6-Tetrakis(40-methoxyphenyl)-
4
2
2-thieno[3,4-c]thiophene 1b
The method used is the same as the one described above for the syn-
thesis of compound 1a, but needs a previous preparation of 1,3-bis(40-
methoxyphenyl)propane-1,3-dione (or dianisoylmethane) by condensa-
tion of 4-methoxyacetophenone with methyl 4-methoxybenzoate in the
presence of sodium hydride11 (An = p.MeO C6H4):
NaH,DMSO/ether
An CO CH3 + MeO CO An
An CO CH2 CO An (56%)
NaI,acetone
→
r . t .,12h
+Br2,CH2Cl2
→
Pyridine, 5 C
(AnCO)2CHBr
→ (AnCO)2CH CH(COAn)2 (53%)
r . t .,12 h
P4S10,pyridine
→ 1b (22%)
Reflux,2 h
Anodic Behavior
1) Voltammetry.
a) Cyclic voltammetry. Cyclic voltammetry measurements were carried
out from solutions of tetraarylthieno[3,4-c]thiophenes 1, 2.5 10 3 M in

512
T. Douadi and M. Cariou
FIGURE 1 Cyclic voltammogrammes of 1,3,4,6-tetraarylthieno[3,4-c]-
thiophenes 1a and 1b (c = 2.5 mM), Pt anode (reference Ag/0.01 M Ag+, v =
100 mVs 1) in CH2Cl2 0.1 M nBu4NPF6.
CH2Cl2 0.1 M nBu4NPF6 (CH2Cl2 was used because of the insolubility
of compounds 1 in acetonitrile); a platinum anode (A = 0.785 mm2) was
used as a working electrode, and Ag/0.01 M Ag+ as a reference electrode.
Figure 1a represents the voltammogramme of 1,3,4,6-tetraphenyl-
thieno[3,4-c]thiophene 1a at a scan rate of 100 m Vs 1; the curve shows
clearly the full reversibility of the first peak at 0.27 V; it was found
to be a purely reversible one-electron process (Epa-Epc = 60 mV). The
second oxidation peak, which appears at 1.10 V, is irreversible.
The cyclic voltammogramme relative to the oxidation of 1,3,4,6-
tetrakis(40-methoxyphenyl)thieno[3,4-c]thiophene 1b (Figure 1b)
shows the presence of two reversible one-electron redox couples
respectively at 0.05 V and 0.72 V, Epa-Epc being equal to about 60 mV
and the ratio of peak currents (ipc/ipa) being close to 1. The presence of
a methoxy substituent in the para position of the benzene ring makes
compound 1b much more easily electrooxidizable than compound 1a.
3
b) Hydrodynamic voltammetry. At a concentration of 1.25
10 M,
in CH2Cl2 0.1 M nBu4NPF6 as well, on a polished platinum rotating
disc electrode, compound 1a presents two waves of oxidation having
different heights. However compound 1b shows two oxidation waves of
equal heights (Figure 2). The half-wave potentials are 0.26, 1.12 V for
1a and 0.035, 0.72 V for 1b, versus Ag/0.01 M Ag+ reference electrode.
The current intensity is proportional to the square root of the rotation
rate of the electrode (Figure 3). This confirms that the electrooxidation

Synthesis and Study of the Anodic Behavior
513
(a)
(b)
FIGURE 2 Intensity versus potential plots of 1a (a) and 1b (b) (c = 1.25 mM),
at a rotated-disk platinum electrode (diameter: 2 mm), in CH2Cl2 0.1 M
nBu4NPF6 (reference Ag/0.01 M Ag+) at different rotation rates (in revolutions
1
per second) 1) 500, 2) 750, 3) 1000, 4) 1500, 5) 2000, 6) 2500. Scan rate: 5 mVs
.
of compounds 1a and 1b is a fully diffusion controlled process.12 Besides,
potential variations of the two waves of compound 1b as a function of
the decadic logarithm of the ratio ilim-i/i are parallel lines of 80 mV
slope.

514
T. Douadi and M. Cariou
(a)
(b)
FIGURE 3 Intensity of the limiting current (ilim) as a function of the square
root of the angular velocity of the disc (in radians per second) ω1/2 of the rotated-
disc platinum electrode (diameter: 2 mm), at the potentials of 0.50 V for a
solution 1.25 mM of 1a (a) and 0.25 (for i1): first wave (solid line), and 1.00 V
(for i2): second wave (dashed line), of a solution 1.25 mM of 1b (b).

Synthesis and Study of the Anodic Behavior
515
FIGURE 4 EPR spectra recorded: a) during the anodic oxidation of 1a, at a
platinum microanode, in CH2Cl2 0.2 M nBu4NBF4 (i = 30 A); and b) just
after the end of the electrolysis of 1a.
2) Macroscale electrolyses. The preparative electrolyses were con-
ducted in a three-compartment H-shaped cell, on a platinum foil anode
(total area: 16 cm2) in CH2Cl2 0.1 M nBu4NPF6 with Ag/0.01 M Ag+
as a reference electrode.
The preparative electrolysis at the first oxidation step of 1,3,4,6-
tetraphenylthieno[3,4—c]thiophene 1a (E = 0.25 V) in the presence of a
soluble base, such as 2,6-lutidine, leads exclusively to -keto-thioketone
1
2 by opening of the ring, after consumption of 2 F mol (despite
anolyte degassing and protection of the electrolysis cell from light).
An ESR spectrum was recorded during the electrooxidation of 1a, in
order to observe its variation under our experimental conditions, such
as microcell, equipped of a platinum microanode, in CH2Cl2 0.2 M
nBu4NBF4 with a current intensity of 30 A. The signal assigned to the
cation radical species appeared first alone and progressively decreased
(Figure 4) up to a colorless solution.

516
T. Douadi and M. Cariou
SCHEME 3
The following mechanism is proposed for the formation of -keto-
thioketone 2.
SCHEME 4
Anodic oxidations of 1,3,4,6-tetrakis(40-methoxyphenyl)thieno[3,4-
c]thiophene 1b carried out, both at the potential of the first peak and
at the potential of the second peak, in the same conditions as 1a, gave
the same kind of -keto-thioketone 3 (characterized by IR and mass
spectrometry):
SCHEME 5

Synthesis and Study of the Anodic Behavior
517
CONCLUSION
In spite of the similarities of the structure between dibenzodithiapen-
talene and tetraarylthieno[3,4-c]thiophene 1, it is not possible to obtain
stable cation radical salts by electrooxidation, because of the high ten-
dency to the opening of one of the cycles.
EXPERIMENTAL
General Methods
The reaction progress and purity of samples were checked by T.L.C. on
silica gel plates (Merck, 5714). Purification of starting materials and
electrolysis products was carried out by silica gel column chromatogra-
phy (Merck, 7736). The proton NMR spectra were recorded at 60 MHz,
with TMS as an internal reference, in CDCl3.
Starting Materials
4
1,3,4,6-Tetraphenyl-2
2-thieno[3,4-c]thiophene 1a
1
1,3-Diaryl-2-bromopropane-1,3-dione. This compound was syn-
thesized according to the procedure of Garg G. H.:9 bromination of an
ice-cold mixture of dibenzoylmethane and pyridine in dichloromethane:
m.p. 94 C (methanol) (lit.:9 92–93 C).
2 1,4-Diphenyl-2,3-dibenzoylbutane-1,4-dione. It was prepared by
the catalytic coupling,10 at room temperature for 12 h, of 1,3-diary1-2-
bromopropane-1,3-dione in the presence of potassium iodide in acetone:
m.p. 210 C (lit.:10 204–208 C).
4
3 1,3,4,6-Tetraphenyl-2 2-thieno[3,4-c]thiophene 1a. Under a ni-
trogen atmosphere, a mixture of 1,4-diphenyl-2,3-dibenzoylbutane-1,4-
dione (3.0 g, 6.7 mmol) in pyridine (30 ml) with phosphorus pentasulfide
(3.0 g, 13.5 mmol) was refluxed for 2 h. A red violet color appeared. After
cooling, the reaction mixture was poured onto ice. The filtration and re-
crystallization in acetic anhydride of the precipitate gave red crystals
4
of 1,3,4,6-tetraphenyl-2 2-thieno[3,4-c]thiophene 1a (700 mg, 23%):
m.p. 245 C (lit.:4 245–247 C).
1,3,4,6-Tetrakis(40-methoxyphenyl)-
4
2
2- thieno[3,4-c]thiophene 1b
1
1,3-Bis(40-methoxyphenyl)-2-bromopropane-1,3-dione. This com-
pound was prepared according to reference:9 bromination of an

518
T. Douadi and M. Cariou
ice-cold solution of 1,3-bis(40-methoxyphenyl)propane-1,3-dione10 in
dichloromethane for 30 mins: m.p. 95–96 C.
2
1,4-Bis(40-methoxyphenyl)-2,3-bis(40-methoxybenzoyl)butane-1,4-
dione. To a solution of 1,3-bis(40-methoxyphenyl)-2-bromopropane-1,3-
dione (8.5 g, 23.5 mmol) in acetone (20 mL), was added dropwise a
solution of sodium iodide (14.1 g, 94 mmol) in acetone (23 mL). Then,
after stirring for 12 h at room temperature, the reaction mixture was
diluted with dichloromethane, washed twice with an aqueous solu-
tion of sodium thiosulfate and several times with water, and dried
over sodium sulfate. After usual work-up and recrystallization in acetic
acid, were isolated 3.5 g (53%) of 1,4-bis(40-methoxyphenyl)-2,3-bis(40-
methoxybenzoyl)butane-1,4-dione: m.p. 222 C (AcOH); NMR (CDCl3)
OMe 3.80 (s, 12H), CH 6.66 (s, 2H), Ar 6.82–6.99–7.95–8.11 (A2X2
system, JAX = 9 Hz, 16H); MS m/z 566 (C34H30O8).
4
3
1,3,4,6-Tetrakis(40-methoxyphenyl)-2 2-thieno[3,4-c]thiophene
1b. A mixture of 1,4-bis(40-methoxyphenyl)-2,3-bis(40-methoxy-
benzoyl)butane-1,4-dione (3.0 g, 5.3 mmol) in pyridine (30 mL) and
phosphorus pentasulfide (3 g) was refluxed for 2 h under a nitrogen
atmosphere. A violet red color appeared. After cooling, the reaction
mixture was poured onto ice. The filtration and recrystallization in
acetic anhydride of the precipitate gave 0.650 g (22%) of red crystals
of thienothiophene 1b: m.p. 263–264 C (Ac2O); NMR (CDCl3) OMe
3.80 (s, 12H), Ar 6.70–6.85–7.16–7.31 (A2X2 system, JAX = 9 Hz,
16H); MS: m/z 564 (C34H28O4S2). Anal: calc: C 70.92; H 4.93; O 10.88;
S 12.28. Found: C 72.31; H 4.99; O 11.33; S 11.35.
Macroscale Electrolyses
Macroscale electrooxidations were carried out at room temperature,
with bubbling nitrogen, in a three-compartment H-shaped cell. The
intermediate compartment (useful volume: 5 mL) was used to mini-
mize the mixing of the catholyte (V: 30 mL) and anolyte (V: 65 mL);
the compartments were separated by two glass-frits (inside diameter:
2 cm). The anode was a platinum foil (total area: 16 cm2), the cath-
ode a graphite rod (diameter: 1 cm) and reference electrode Ag/0.01 M
Ag+ in MeCN. A volume of 100 mL CH2Cl2 0.1 M nBu4NPF6 was di-
vided among the three compartments and salt bridge. Between 0.5 and
1 mmol of tetraarylthienothiophene was added into the anodic com-
partment, in the presence of 2 mmol of 2,6-lutidine, which was closed
by a silicone stopper. The electrolysis was carried on until the intensity
dropped to zero.

Synthesis and Study of the Anodic Behavior
519
Anodic Oxidation of 1,3,4,6-Tetraphenyl-
4
2
2-thieno[3,4-c]thiophene 1a
Compound 1a (444 mg, 1 mmol) was oxidized at a platinum anode,
in CH2Cl2—nBu4NPF6 0.1 M in the presence of 2,6-lutidine (0.23 mL,
2 mmol), at a potential of 0.25 V; a green coloration appeared. After 24 h,
at the end of the electrolysis, the electricity consumption was about
2 Faradays per mole. The anolyte was diluted with dichloromethane
(250 mL) and washed with a 1% aqueous solution of acetic acid (100 mL),
decanted off, washed with water (4 50 mL), and dried over Na2SO4.
After usual work-up, the residue was purified by chromatography on sil-
ica gel (dichloromethane 50/cyclohexane 50) to give a green compound,
2,5-diphenyl-4-benzoyl-3-thiobenzoylthiophene 2 (430 mg, 93%): m.p.
120 C (lit.:3 115 C). IR (KBr)
1650,
1225 cm 1. NMR (CDCl3)
CO
CS
Ar 7.08–7.60 (m, 16H) and 7.62–7.85 (m, 4H). MS m/z 460 (C30H20OS2).
Anodic Oxidation of 1,3,4,6-Tetrakis(40-methoxyphenyl)-
4
2
2-thieno[3,4-c]thiophene 1b
Compound 1b (282 mg, 0.5 mmol) was oxidized, at a platinum an-
ode in CH2Cl2 nBu4NPF6 0.1 M at a potential of 0.05 V (another run,
at a potential of 0.70 V led to the same result) for 24 h; a green col-
oration appeared. After 24 h, at the end of the electrolysis, the elec-
tricity consumption was about 2 Faradays per mole. The anolyte was
diluted with dichloromethane (150 mL), washed with water (4 50 mL),
and dried over Na2SO4. After the usual work-up, the residue was puri-
fied by chromatography on silica gel (dichloromethane 50/cyclohexane
50) to give a green compound, 2,5-bis(40-methoxyphenyl)-4-benzoyl-3-
thiobenzoylthiophene 3 (150 mg, 66%): m.p. 181 C. IR (KBr)
CS
1640,
CO
1160 cm 1. NMR (CDCl3) Ar 7.08–7.60 (m, 16H) and 7.62–7.85
(m, 4H). MS m/z 516 (C34H28OS2).
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