An efficient synthesis of 5,5′-diaryl-2,2′-bichalcophenes

Article abstract of DOI:10.1016/j.tetlet.2005.11.091

A convenient and high yielding synthesis of 5,5′-diaryl-2,2′- bichalcophenes (furans, thiophenes, and selenophenes) by the hexabutylditin-mediated homocoupling of 5-bromochalcophenes is described. This approach has been applied to the synthesis of the blu

Full text of DOI:10.1016/j.tetlet.2005.11.091

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 795–797
An efficient synthesis of 5,50-diaryl-2,20-bichalcophenes
Mohamed A. Ismail,a David W. Boykina and Chad E. Stephensb,*
aDepartment of Chemistry and Center for Biotechnology and Drug Design, Georgia State University, Atlanta, GA 30303, USA
bDepartment of Chemistry and Physics, Augusta State University, Augusta, GA 30904, USA
Received 8 September 2005; revised 11 November 2005; accepted 18 November 2005
Available online 5 December 2005
Abstract—A convenient and high yielding synthesis of 5,50-diaryl-2,20-bichalcophenes (furans, thiophenes, and selenophenes) by the
hexabutylditin-mediated homocoupling of 5-bromochalcophenes is described. This approach has been applied to the synthesis of the
blue-light-emitting compound 5,50-bis(4-aminophenyl)-2,20-bifuryl (PFDA).
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
been described. Finally, a low yielding benzidine rear-
rangement has been employed to prepare 5,50-bis(4-ami-
nophenyl)-2,20-bifuryl (PFDA, II), a blue-light-emitting
compound utilized in the development of photolumines-
cent polyimides.12
Bi- and oligo-chalcophenes (furans, thiophenes, seleno-
phenes, etc.) continue to receive significant attention as
advanced materials.1 For example, 5,50-diaryl-2,20-
bichalcophenes (I) have been studied as a result of their
luminescent,2 electrochemical,3 and semiconducting4
properties. Oligochalcophenes, particularly oligothio-
phenes,5 have been heavily investigated as organic semi-
conductors, with particular application to thin-film
transistors (TFTs),1a,c light-emitting devices (LEDs),1b
and photovoltaics.1d This includes the unique oligo-
selenophenes, which have recently been investigated as
a TFT material.1e
X
X
O
O
Ar
Ar
NH2
H2N
X = O, S, and Se
PFDA
II
Ar = aryl and heteroaryl
I
The preparation of 2,20-bichalcophenes has historically
involved classical heterocyclic ring-forming methods,
such as the Trofimov and Paal–Knorr syntheses.6 More
recent approaches to 2,20-bichalcophenes that do not in-
volve ring formation include the sequential cross-cou-
pling of metalated chalcophenes with aryl halides,2,3,7
the Suzuki coupling of bromochalcophenes with aryl-
boronic acids,4d,7a,8 and the Heck arylation.4c Copper9 or
iron10 mediated homocoupling of lithiated (or trans-
metalated) chalcophenes has also been heavily utilized;
however, the presence of base sensitive functional
groups (e.g., cyano) can limit this approach. The direct
oxidative homocoupling of neutral chalcophenes at an
unsubstituted site using a Pd/Ag combination11 has also
The homocoupling of halogenated aryls by reaction
with a hexaalkylditin and a palladium catalyst has been
known for some time now.13 Surprisingly, this reaction
has received little attention thus far in the synthesis of
bi- and oligochalcophenes,14 although it represents a
straightforward approach that should tolerate a variety
of functional groups as anion formation is not required.
As a result of our synthetic efforts to prepare biologi-
cally active15 and fluorescent16 diamidines, we have
previously developed a convenient method for the
preparation of 2-bromo-5-(4-cyanophenyl)chalcoph-
enes.17 We thus decided to explore the viability of the
hexaalkylditin-mediated homocoupling of these bromi-
nated precursors for the direct synthesis of our targeted
5,50-diaryl-2,20-bichalcophenes (I). Herein, we describe
our excellent results with this homocoupling approach.
We also describe a new, higher yielding synthesis of
PFDA (II) that employs this homocoupling reaction as
a key step.
Keywords: Bichalcophene; Bifuran; Bithiophene; Biselenophene;
PFDA; Homocoupling.
*
Corresponding author. Tel.: +1 706 667 4995; fax: +1 706 667
4519; e-mail: cstephe7@aug.edu
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.11.091

796
M. A. Ismail et al. / Tetrahedron Letters 47 (2006) 795–797
(Bu3Sn)2, Pd(PPh3)4
X
X
Br
X
PhMe, 120 oC
4-6 hr
Y
Y
Y
CN
NC
NC
Scheme 1.
2. Results
2,20-bichalcophenes. Considering the widespread and
continued development of bi- and oligochalcophenes
as advanced materials, including cyano-substituted
derivatives,22 it is expected that this approach will be
of immediate interest to those synthesizing these type
compounds.
As shown in Scheme 1, our conditions for homocou-
pling involved reaction of the 2-bromo-5-(4-cyanophen-
yl)chalcophenes (1 equiv) with hexabutylditin (0.6
equiv) in the presence of catalytic Pd(PPh3)4 (ꢀ1.5–2
mol %) using refluxing toluene as solvent. After a
period of 4–6 h, the insoluble product was filtered off
and recrystallized from DMF to give the bifurans, bithio-
phenes, as well as biselenophenes in good to excellent
yield (78–92%).18 In addition to the phenyl-substituted
bichalcophenes (Table 1, entries 1–3), the 2-pyridyl ana-
logues were also obtained in similar high yield (entries
4–6). It is noteworthy that a good yield (77%) of the phen-
yl-substituted bifuran (entry 1) was obtained in an
experiment employing excess (1.1 equiv) hexabutylditin,
suggesting that the homocoupling product is preferred
over the stannylated monochalcophene product. The
opposite appears to be the case with certain benzene19
and pyridine20 derivatives.
Acknowledgments
This work was supported by the Bill and Melinda Gates
Foundation and NIH (Grants RO1GM61587 and
RO1AI46365).
References and notes
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To further illustrate the use of this homocoupling
approach to bichalcophenes, the fluorescent bifuran
PFDA (II)12 was also prepared by this route. As shown
in Scheme 2, bifuran II was readily obtained in 50%
overall yield21 from 2-bromo-5-(4-nitrophenyl)furan
after homocoupling and nitro reduction by catalytic
hydrogenation. The previously described route to II
involves isolation of a diazonium and a 20% yielding
benzidine rearrangement.12d
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In conclusion, we have described a straightforward and
high-yielding homocoupling approach to 5,50-diaryl-
Table 1.
Entry
X
Y
Yielda
Mp (°C)
1
2
3
4
5
6
O
S
Se
O
S
CH
CH
CH
N
N
N
78
91
92
81
85
85
298–299
298–300
285–286.5
>300
>300
>300
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Se
a Yield of isolated, analytically pure compound.
(Bu3Sn)2, Pd(PPh3)4
O
Br
O
O
PhMe, 120 oC
4 hr
O2N
X
X
X = NO2
H2, Pd/C
X = NH2
(PFDA)
Scheme 2.

M. A. Ismail et al. / Tetrahedron Letters 47 (2006) 795–797
797
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and Pd(PPh3)4 (200 mg, 0.17 mmol) in toluene (40 mL)
was added hexa-n-butylditin (3.48 g, 6 mmol). The reac-
tion mixture was heated under N2 at 120 °C for 6 h, then
cooled, and the precipitate was filtered and washed with
ether. Recrystallization from DMF gave a yellow crystal-
line solid (1.67 g, 91%), mp 298–300 °C. 1H NMR
(DMSO-d6);
d 7.45 (d, J = 4.2 Hz, 2H), 7.68 (d,
J = 4.2 Hz, 2H), 7.85–7.89 (m, 8H). 13C NMR; d 140.3,
137.1, 137.0, 132.5, 126.8, 125.7, 125.4, 118.0, 109.6. MS
(m/z, rel int.); 368 (M+, 100), 334 (5), 222 (8), 184 (20).
High resolution MS calcd for C22H12N2S2: 368.04419.
Observed 368.04476. Analysis for C22H12N2S2: Calcd: C,
71.71; H, 3.28. Found C, 71.48; H, 3.40.
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hexa-n-butylditin (1.22 g, 2.1 mmol), and Pd(PPh3)4
(0.058 g, 0.05 mmol, 1.25 mol %) for 4 h according to the
representative procedure gave 5,50-bis-(4-nitrophenyl)-
2,20-bifuran as a red solid (0.68 g, 90%), mp 280–283 °C.
Recrystallization from DMF/MeOH gave the analytical
1
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sample with the same mp. H NMR (DMSO-d6): 7.18 (d,
J = 3.6, 2H), 7.53 (d, J = 3.9 Hz, 2H), 8.05 (d, J = 9.0 Hz,
4H), 8.31 (d, J = 9.3 Hz, 4H). Analysis for C20H12N2O6:
Calcd: C, 63.83; H, 3.21; N, 7.44. Found: C, 63.49; H,
3.37; N, 7.50. A suspension of the bis-nitro compound
(0.75 g, 2.0 mmol) in EtOAc (100 ml) and EtOH (10 ml)
was hydrogenated with 10% Pd/C (0.25 g, Lancaster) at
60 psi until uptake subsided (3 days). The mixture was
then filtered over Celite and concentrated in vacuo to a
brown solid (0.55 g). Recrystallization from benzene gave
5,50-bis-(4-aminophenyl)-2,20-bifuryl (PFDA, II) as a yel-
low/tan crystalline solid (0.35 g, 56%), mp 218–221 °C. A
second recrystallization from EtOH gave fine golden
scales, mp 221–222 °C (Lit.12d mp 203–205 °C). 1H
NMR (DMSO-d6): 5.35 (br s, 2NH2), 6.61 (d,
J = 8.7 Hz, 4H), 6.66 (d, J = 3.6 Hz, 2H), 6.70 (d,
J = 3.6 Hz, 2H), 7.42 (d, J = 8.7 Hz, 4H). 13C NMR
(DMSO-d6): 103.9, 107.1, 114.0, 118.1, 124.9, 143.9, 148.7,
153.8. MS (ES): m/z 317 (100%, MH+). Analysis for
C20H16N2O2: Calcd: C, 75.92; H, 5.10; N, 8.86. Found: C,
75.95; H, 5.05; N, 8.77.
18. Representative procedure: 5,50-Bis(4-cyanophenyl)-2,20-
bithiophene (Table 1, entry 2). To a solution of 2-
bromo-5-(4-cyanophenyl)thiophene (2.64 g, 10 mmol),
22. Miyata, M.; Matsumi, N.; Chujo, Y. Macromolecules
2001, 34, 7331.