Combining Furan Annulation, Heck Reaction, and Sonogashira Coupling for the Synthesis of Oligoaryls

Full text of DOI:10.1021/ol026941t

ORGANIC
LETTERS
2002
Vol. 4, No. 24
4305-4307
Combining Furan Annulation, Heck
Reaction, and Sonogashira Coupling for
the Synthesis of Oligoaryls
,†,‡
Ching-Yuan Liu† and Tien-Yau Luh*
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106, and
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115
tyluh@chem.sinica.edu.tw
Received September 21, 2002
ABSTRACT
A variety of benzene−furan−alkene/alkyne conjugated oligomers of precise length and constitution were synthesized iteratively by combining
furan annulation, Heck reaction, and Sonogashira coupling.
Oligoaryls and their vinylene or acetylene homologues of
desired conjugation length have been widely investigated
because of their potential optoelectronic applications.1 There
has been extensive uses of heteroaromatic rings such as
thiophene in these applications.1 Interestingly, studies on
furan derivatives for these applications have been rare.2-5
We recently reported a convenient one-pot synthesis of
symmetric oligoaryls 2 containing two furan moieties (eq
1).6,7 These oligoaryls are thermally stable and exhibit bright
fluorescence in the blue light region. It is noteworthy that
2a has been shown to be a highly efficient non-amine-based
hole-transporting material in electroluminescent devices.7 The
basic synthetic strategy shown in eq 1 involves the reaction
of a thiolato-substituted allenyl anion 3, generated in situ
from 1 and BuLi, with a dialdehyde followed by the acid-
catalyzed ring closure. Since both vinyl and alkynyl sub-
stituents are stable under the reaction conditions,6 it is
envisaged that substrates containing such functionalities can
undergo Heck8 and Sonogashira9 reactions, respectively, to
extend the chain length while incorporating the aldehyde
functionality at both ends of the oligoaryl chain. Further
annulation leading to furan rings can then proceed to yield
the homologues of oligoaryls. In this paper, we report a
bidirectional iterative synthesis of furan-containing oligoaryls
by combining furan annulation, Heck reaction, and Sono-
gashira coupling.
† National Taiwan University.
‡ Academia Sinica.
(1) (a) Electronic Materials: The Oligomer Approach; Mu¨llen, K.,
Wegner, G., Eds. Wiley-VCH: Weinheim, Germany, 1998. (b) Tour, J.
M. Acc. Chem. Res. 2000, 33, 791 (c) Bunz, U. F. W. Chem. ReV. 2000,
100, 1605.
(2) For a review, see: Gandini, A.; Belgacem, M. N. Prog. Polym. Sci.
1997, 22, 1203 and references therein.
(3) Saadeh, H.; Goodson, T., III; Yu, L. Macromolecules 1997, 30, 4608.
(4) (a) Niziurski-Mann, R. E.; Cava, M. P. AdV. Mater. 1993, 5, 547.
(b) Niziurski-Mann, R. E.; Scordillis-Kelley, C.; Liu, T.-L.; Cava M. P.;
Carlin, R. T. J. Am. Chem. Soc. 1993, 115, 887. (c) Hucke A.; Cava, M. P.
J. Org. Chem. 1998, 63, 7413. (d) Politis, J. K.; Nemes J. C.; Curtis, M. D.
J. Am. Chem. Soc. 2001, 123, 2537. (e) Pyo, S. M.; Kim, S. I.; Shim, T. J.;
Park H. K.; Ree, M. Macromolecules 1998, 31, 4777.
(5) Dufresne, G.; Bouchard, J.; Belletete, M.; Durocher G.; Leclerc, M.
Macromolecules 2000, 33, 8252.
Treatment of 1c with BuLi yielded the corresponding
allenyllithium 3b, which was allowed to react with 5a
followed by acid treatment, affording 4b in 52% yield.
Removal of the TMS group was achieved by treating 4b
with K2CO3 in methanol-THF to give 4c in 98% yield. In
a manner similar to that described in eq 1, reaction of 3a
with 6 gave 7 in 45% yield.
(6) Lee, C.-F.; Yang, L.-M.; Hwu, T.-Y.; Feng, A.-S.; Tseng, J.-C.; Luh,
T.-Y. J. Am. Chem. Soc. 2000, 122, 4992.
(7) Zhang, L.-Z.; Chen, C.-W.; Lee, C.-F.; Wu C.-C.; Luh, T.-Y. Chem.
Commun. 2002, 2336.
(8) de Meijere, A.; Bra¨se, S. J. Organomet. Chem. 1999, 576, 88.
(9) Sonogashira, K. In Metal-Catalyzed Cross-coupling Reactions;
Diederich F., Stang, P. J., Eds. Wiley-VCH: Weiheim, Germany, 1998;
pp 203-229.
10.1021/ol026941t CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/05/2002

Table 1. Photophysical Properties of Oligoaryls
substrate
λmax (nm)b
λem (nm)b
Φf
8
9
10
11
350, 399
340, 400
397
508
486
484
507
0.45
0.81
0.91
0.64
348, 412
reaction. Thus, treatment of 7 with 4 equiv of 5b in the
presence of 5 mol % Pd(OAc)2, Ph3P, and K2CO3 under an
argon atmosphere for 48 h resulted in the formation of 8 in
75% yield. In a similar manner, the Sonogashira reaction
was employed to enable the coupling of 4c with 5b in the
presence of CuI, Ph3P, Et3N, and a catalytic amount of PdCl2-
(PPh3)2 in MeCN. Oligoaryl 9 was isolated in 63% yield.
Annulation of 9 with 3a generated in situ from the reaction
of 1d and BuLi afforded 10 in 38% yield. In a similar
manner, reaction of 8 with 3b gave 11 in 42% yield. Both
oligoaryls 10 and 11 have either double bonds or triple bonds
at both ends of the conjugated system. Accordingly, further
elaboration would be possible by repeating the same
procedures leading to higher homologues of oligoaryls, which
can be used as molecular wires. Although the yields of the
annulation are moderate, compounds 8-11 were easily
precipitated by the addition of methanol to the CH2Cl2
solutions of the crude products.
The photophysical properties of 8-11 were briefly ex-
amined, and the results are summarized in Table 1. These
oligoaryls exhibited a bluish green emission in CHCl3
solution with high quantum efficiency. Compound 11 was
somewhat unstable, with slow decomposition observed when
the CHCl3 solution was exposed to ambient light. As
The presence of double bonds and triple bonds in 7 and
4c, respectively, is essential for the Heck or Sonogashira
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Org. Lett., Vol. 4, No. 24, 2002

illustrated in Table 1, the conjugation lengths in 8-11 already
reached saturation and essentially the λmax values for each
of these oligoaryls remained similar.
Acknowledgment. We thank the Ministry of Education
and the National Science Council of the Republic of China
for financial support.
In summary, we have demonstrated a new convenient route
by combining furan annulation and Heck and Sonogashira
reactions for the synthesis of a variety of benzene-furan-
alkene/alkyne conjugated oligomers of precise length (up to
6 nm) and constitution. Further extension by using this
iterative strategy will be feasible leading to the synthesis of
oligoaryls of well-defined conjugation lengths.
Supporting Information Available: Experimental pro-
cedures for the preparation of 4b,c and 8-11. This material
is available free of charge via the Internet at http://pubs.acs.org.
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