V. S. Kadam et al. / Tetrahedron Letters 57 (2016) 2608–2611
2609
increases in solubility while the rigidity enhances the favorable
solid state arrangement.15
6 with an excess of sodium hydride under refluxing condition in
dry THF for 15 h in 89% yield. The plausible mechanism of the for-
mation of compound 7 may include the abstraction of a-H of com-
pound 6 to form carbanion which reacts with carboxylate group of
another molecule of compound 6 to cyclize into the p-benzo-
quinone moiety. Most common route to synthesize p-benzo-
quinonedithiophene comprises 3-thiophene carboxylic acid as
starting material.22 After functionalization of thiophene carboxylic
acid into amide, p-benzoquinonedithiophene was obtained by
treating resulting amide with n-BuLi at low temperature. In the
synthesis of compound 7, NaH effectively works as base due to
presence of two electron-withdrawing carboxylate groups in com-
There are only few methods reported in the literature to synthe-
sized BDT and TPD. BDT have been synthesized from p-benzo-
quinonedithiophene, whereas TPD has been synthesized from
thiophene-3,4-dicarboxylic acid. Synthetic methods for BDT and
TPD are multistep. Here we report the synthesis of BDT (donor
unit) and TPD (acceptor unit), two important building blocks for
the conjugated polymers, from a single precursor, dimethyl thio-
phene-3,4-dicarboxylate (6). This compound (6), in turn, was syn-
thesized from maleic anhydride.
pound 6, which, in turn, increase the acidity of a-H of compound 6.
Compound 7 can be used as precursor to obtain many benzodithio-
phene derivatives. Additionally, the carboxylate groups may play
an important role in controlling conformation by nonbonding
interactions in the resulting conjugated systems.23 The electron-
withdrawing carboxylate substituents as the side chains are
known to lower the HOMO level of the conjugated systems with
minor effect on the optical bandgaps.24
The diester 7 was hydrolyzed with aqueous NaOH to afford
dicarboxylic acid 8, which subsequently aromatize and decarboxy-
lated by heating with Cu/quinolone to afford dihydroxy ben-
zodithiophene 9. When compound 7 was treated with aqueous
NaOH solution in presence of Zn powder, it afforded dihydroxy
benzodithiophene dicarboxylic acid (10). Interestingly, when com-
pound 7 was stirred with excess of NaH in ethyl acetate, aromati-
zation with trans-esterification occurred to afford dicarboxylate
(11) (Scheme 2).
Results and discussion
Synthesis of compound 6 was accomplished in six steps from
maleic anhydride as shown in Scheme 1. The maleic anhydride
was converted to 2,3-dimethyl maleic anhydride (1) following
the reported procedure.16 Compound 1 was further converted into
2,3-dimethyl dimethyl maleate (2).17 The bromination of
compound 2 was carried out by NBS and benzoyl peroxide as ini-
tiator.18 Dibromo compound 3 was cyclized to dimethyl 2,5-dihy-
drothiophene-3,4-dicarboxylate (4) using sodium sulfide in THF
as solvent at room temperature. Aromatization of compound 4
was accomplished by oxidation and followed by dehydration.
Compound 4 was oxidized using m-CPBA to afford sulfoxide 5
and further dehydrated using acetic anhydride to afford compound
6. This synthetic pathway to synthesize compound 6 is simple with
an overall good yield of 32%.
Absorption spectra of 7, 8, 10, and 11 showed three distinct
absorption peaks. Compound 7, 8, 10, and 11 showed p–
p⁄ absorp-
tion peaks at 332, 368, 403, 407 nm, respectively (Figs. S8–S12).
Electrochemical experiments were conducted on compound 8
and compound 11 by cyclic voltammetry (CV). Both the com-
pounds 8 and 11 showed oxidation peaks at 0.87 and 0.91 V versus
Ag/Ag+, respectively (Figs. S13 and S14). Compound 8 showed
reduction peak at 0.5 V versus Ag/Ag+.
The traditional route to synthesize thiophene-3,4-dicarboxylic
acid includes the Rosenmund–von Braun Reaction.19 The overall
five step reaction strategy of the synthesis of thiophene-3,4-dicar-
boxylic acid mainly involves reaction of 3,4-dibromothiophene
with toxic cuprous cyanide affording expected compound in mod-
erate yields. The Gewald reaction was used as the another strategy
to synthesize 3,4-thiophene dicarboxylate derivatives.20 Recently,
Leclerc and coworkers have synthesized 3,4-thiophene dicarboxy-
late using the Gewald reaction followed by the Sandmeyer reaction
to extrude the amino group.21 This procedure has been regarded as
TPD was synthesized from compound 6 in three steps, which
mainly includes hydrolysis of dimethyl thiophene-3,4-dicarboxy-
late in aqueous NaOH solution followed by its reaction with oxalyl
chloride to form diacid dichloride and its subsequent reaction with
alkyl amine (Scheme S1, ESI). Similar methodology can be
employed to synthesis of 5-alkyl-thieno[3,4-f]-isoindole-5,7-dione
(TID).25 Thieno[3,4-c]pyrrole-4-6-dione (TPD) has been synthe-
sized in literature from 3,4-thiophenedicarboxylic acid and amine
condensation reaction.26
Crystals of compound 7 and 11 suitable for single crystal X-ray
diffraction were obtained by slow evaporation of saturated solu-
tion in hexane. The compound 7 was crystallizing with triclinic
crystal packing system with space group P-1. Molecular structure
of compound 7 possess center of symmetry. The unit cell of
a
cheap and efficient method to synthesize 3,4-thiophene
dicarboxylate.
Synthesis
of
3,7-dimethyl-4,8-dihydrobenzo[1,2-b:4,5-b0]
dithiophen-4,8-dione-dicarboxylate (7), a precursor to dihydoxy
benzodithiophene 9, was accomplished by new and unconven-
tional approach. Compound 7 was obtained by treating compound
Scheme 1. Synthesis of thiophene-3,4-dicarboxylate.
Scheme 2. Synthesis of benzodithiophene derivatives.