Bidirectional iterative synthesis of alternating benzene-furan oligomers towards molecular wires

Article abstract of DOI:10.1039/b207881c

Reaction of propargylic dithioacetal 2a with BuLi gives the sulfur-substituted allenyllithium 3a which is allowed to react with a dialdehyde to yield the corresponding alternating benzene-furan oligoaryls 6. Functional group transformation converts the es

Full text of DOI:10.1039/b207881c

Bidirectional iterative synthesis of alternating benzene–furan oligomers
towards molecular wires†
Chin-Fa Lee,ab Ching-Yuan Liu,b Hua-Can Song,ab Shr-Jie Luo,a Jui-Chang Tseng,ab Hsi-Hua Tsoa
and Tien-Yau Luh*ab
a Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115
b Department of Chemistry, National Taiwan University, Taipei, Taiwan 106.
E-mail: tyluh@chem.sinica.edu.tw; Fax: +886-2-2651-1488; Tel: +886-2-2789-8500
Received (in Cambridge, UK) 12th August 2002, Accepted 10th October 2002
First published as an Advance Article on the web 28th October 2002
Reaction of propargylic dithioacetal 2a with BuLi gives the
sulfur-substituted allenyllithium 3a which is allowed to react
with a dialdehyde to yield the corresponding alternating
benzene–furan oligoaryls 6. Functional group transforma-
tion converts the ester groups in 6 to dialdehyde 8 which can
be used for the synthesis of higher homologues towards
molecular wires. A combination of this furan annulation,
Heck reaction and Sonogashira coupling leads to a variety of
(2)
4a (278 °C, 0.5 h, then rt, 0.5 h) followed by treatment with
TFA (rt, 12 h). After usual work-up, furan-containing diester 5
(mp 110–111 °C) was obtained in 67% yield.‡ It is striking to
learn that the dithioacetal functionality is more reactive than the
ester group towards BuLi under these conditions. This reaction
provides a useful route for the synthesis of functionalized
allenyllithium reagents by means of a lithium/sulfur exchange
reaction. This promising result apparently lays a foundation for
the bidirectional iterative synthesis of furan-containing oligoar-
yls leading to molecular wires.
benzene–furan–alkene/alkyne conjugated oligomers of pre-
cise length.
There has been ever burgeoning study on the design and
synthesis of conjugated oligomers of precise length and
constitution because of their potential optoelectronic applica-
tions.1 Oligoaryls and their vinylene or acetylene homologues
have been widely investigated. Incorporation of heteroaromatic
rings into these systems will tune the optoelectronic properties
of the oligomers, and studies on oligothiophenes or pyrroles and
related compounds has been extensive.1,2 Relatively speaking,
investigations on furan-containing oligoaryls have been only
sporadically explored.3–5 We recently reported a new annula-
tion procedure for the synthesis of 2,3,5-trisubstituted furans
from the corresponding dithioacetals (eqn. (1)).6 Thermally and
(1)
In a similar manner, treatment of 2 equiv. of 3a with
terephthaldehyde (278 °C, 0.5 h, then rt 1 h) followed by
reaction with TFA (rt, 12 h) afforded diester 6a (mp 169–170
°C) in 45% yield. Reaction of 6a with DIBAH (4 equiv., 0 °C.
0.5 h, then rt 5 h) followed by oxidation with 4 equiv. of MnO2
(rt, 20 min) afforded the corresponding dialdehyde 8a (mp
189–190 °C) in 78% yield. Dialdehyde 8a was employed for the
next annulation reaction with 2.4 equiv. of 3a to afford the
corresponding nonaaryl 6b (mp 209–211 °C) in 38% yield. In a
similar manner, 6b was converted into 8b (mp 220–222 °C) in
78% yield by sequential treatment with DIBAH and MnO2. By
employing the same strategy, 13-mer 6c (mp 221–222 °C) was
obtained in 32% yield from 8b and 3a. Because of the presence
of the butyl groups, the solubility of 6 in organic solvents was
good. They can also easily be precipitated by adding methanol
to the organic solutions. Since 6c contains two ester groups at
the terminal phenyl rings, further transformation by repeating
the same procedures just mentioned would lead to higher
homologues of alternating benzene–furan molecular wires.
photochemically stable pentaaryls 1 containing alternating
benzene and furan moieties are conveniently obtained. The use
of these materials as hole transporting materials in electro-
luminescent devices has been disclosed.7 Furthermore, the
presence of an alkyl substituent at the C-3 position of the furan
heterocycle may increase the solubility of 1 in organic solvents.
We felt that this protocol can be applied for the bidirectional
iterative synthesis of furan-containing oligoaryls of different
conjugation lengths leading to molecular wires. Our strategy
therefore involves the synthesis of furan-containing oligoaryls
functionalized at both terminal aryl moieties.
Furan-containing oligoaryls 6 were both thermally (Td
384, 410, 457 °C for 6a, b and c, respectively) and photo-
chemically (Sunlamp 200 W, 160 °C, 24 h) stable under
=
Both organocopper and alkyllithium reagents are known to
react with cyclic dithioacetals yielding the corresponding
alkylated sulfur-stabilized anions.6,8 In order to synthesize the
furan-containing oligoaryls functionalized at terminal aryl
moieties, it is necessary to test the relative reactivity of different
functional groups towards these organometallic reagents. It is
known that an ester group can be stable in certain organolithium
reagents.9 Thus, treatment of 2a with BuLi (278 °C, 50 min)
yielded 3a (eqn. (2)) which was allowed to react with aldehyde
† Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b2/b207881c/
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CHEM. COMMUN., 2002, 2824–2825
This journal is © The Royal Society of Chemistry 2002

nitrogen atmosphere. However, they underwent decomposition
upon irradiation in the presence of air.
presence of pyrrole rings resulted in a red shift of the
emission.
A pyrrole moiety can also be introduced into the oligoaryl
system when imine was employed as an electrophile. Thus, the
reaction of diimine 9a with 3d at 278 °C followed by treatment
with BF3·OEt2 afforded 13 in 47% yield.
Incorporation of double and/or triple bonds into this oligoaryl
system is also feasible by combining Heck10 or Sonogashira11
reaction with the furan annulation protocol. Thus, Heck reaction
(10 mol% Pd(OAc)2, 15 mol% Ph3P and K2CO3 in CH3CN, 48
h) of the divinylpentaaryl 12a with excess 4b afforded 14 in
72% yield. In a manner similar to that described above,
annulation of 14 with 3b gave 16 in 62% yield.
Treatment of 11a with 4b under Sonogashira conditions (5
mol% of PdCl2(PPh3)2 10 mol% of CuI and Et3N in acetonitrile)
gave 78% yield of dialdehyde 15. Annulation of 15 with 3c
under our usual conditions yielded 17 (45%). Similarly, 18 was
obtained in 48% yield from the reaction of 15 with 3b. Since
16–18 contain double or triple bonds at the terminal phenyl
rings, further transformation by repeating the same procedures
would lead to higher homologues.
In summary, we have demonstrated a new route for the
synthesis of a variety of alternating benzene–furan oligoaryls up
to 5 nm in length. Further extension by using the same strategy
will be feasible leading to the synthesis of molecular wires of
well-defined conjugation lengths.
We thank the Ministry of Education and the National Science
Council of the Republic of China for financial support.
Notes and references
Electrochemical studies showed that teraryl 5 exhibited a
reversible one-electron redox process whereas pentamer 6a
showed a reversible two-electron redox process. Slight decom-
position was observed when 6b and 6c were subjected to two-
electron oxidation. The first oxidation potentials for oligoaryls
are summarized in Table 1. As expected, the first oxidation
potential of the oligoaryls decreases with increasing conjuga-
tion length. Relatively speaking, substrates containing double or
triple bonds (e.g. 14–18) were less stable towards electro-
chemical oxidation. The absorption, fluorescence data and
fluorescent quantum yields are also provided in Table 1. As
expected, lmax and lem increase with the increasing conjugation
length and reach saturation at the nonamer (6b) stage. The
‡ All new compounds gave satisfactory spectroscopic and analytical data.
The details are described in the ESI.†
1 (a) Electronic Materials: The Oligomer Approach, ed. K. Müllen and G.
Wegner, Wiley-VCH, Weinheim, 1998; (b) J. M. Tour, Acc. Chem.
Res., 2000, 33, 791.
2 For a review see: A. Gandini and M. N. Belgacem, Prog. Polym. Sci.,
1997, 22, 1203 and references therein.
3 H. Saadeh, T. Goodson, III and L. Yu, Macromolecules, 1997, 30,
4608.
4 (a) R. E. Niziurski-Mann and M. P. Cava, Adv. Mater., 1993, 5, 547; (b)
R. E. Niziurski-Mann, C. Scordilis-Kelley, T.-L. Liu, M. P. Cava and R.
T. Carlin, J. Am. Chem. Soc., 1993, 115, 887; (c) A. Hucke and M. P.
Cava, J. Org. Chem., 1998, 63, 7413; (d) J. K. Politis, J. C. Nemes and
M. D. Curtis, J. Am. Chem. Soc., 2001, 123, 2537; (e) S. M. Pyo, S. I.
Kim, T. J. Shim, H. K. Park and M. Ree, Macromolecules, 1998, 31,
4777.
Table 1 Photophysical and electrochemical properties of oligoaryls.
Compd.
E1/2a/eV
lmaxb/nm
lemb/nm
Ff
5 G. Dufresne, J. Bouchard, M. Belletete, G. Durocher and M. Leclerc,
Macromolecules, 2000, 33, 8252.
5
6a
6b
6c
13
14
15
16
17
18
0.92
0.57
0.31
0.20
0.41
0.37
0.40
0.18
0.33
0.28
364
398
418
422
361, 395
344, 407
348, 409
422
397
402
402, 423
454, 482
474, 480
474, 499
451, 485, 523
484
494
497
491
494
0.74
0.74
0.53
0.42
0.79
0.58
0.59
0.73
0.48
0.63
6 C.-F. Lee, L.-M. Yang, T.-Y. Hwu, A.-S. Feng, J.-C. Tseng and T.-Y.
Luh, J. Am. Chem. Soc., 2000, 122, 4992.
7 L.-Z. Zhang, C.-W. Chen, C.-F. Lee, C.-C. Wu and T.-Y. Luh, Chem.
Commun., 2002, 2336.
8 (a) H. Ikehira, S. Tanimoto and T. Oida, J. Chem. Soc., Perkin Trans 1,
1984, 1223; (b) A. Krief, B. Kenda and P. Barbeaux, Tetrahedron Lett.,
1991, 32, 2509.
9 A. Boudier, L. O. Bromm, M. Lotz and P. Knochel, Angew. Chem., Int.
Ed., 2000, 39, 4414.
10 A. de Meijere and S. Bräse, J. Organomet. Chem., 1999, 576, 88.
11 K. Sonogashira, Metal-Catalyzed Cross-coupling Reactions, ed. F.
Diederich and P. J. Stang, Wiley-VCH, Weiheim, Germany, 1998, pp.
203–229.
a The first oxidation potential vs. ferrocene/ferrocenium ion. b Measured in
CHCl3 solution.
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