One-pot synthesis of substituted furans and pyrroles from propargylic dithioacetals. New annulation route to highly photoluminescent oligoaryls [7]

Full text of DOI:10.1021/ja0004736

4992
J. Am. Chem. Soc. 2000, 122, 4992-4993
One-Pot Synthesis of Substituted Furans and
Pyrroles from Propargylic Dithioacetals. New
Annulation Route to Highly Photoluminescent
Oligoaryls
Chin-Fa Lee, Lian-Ming Yang, Tsyr-Yuan Hwu,
An-Shuan Feng, Jui-Chang Tseng, and Tien-Yau Luh*
Department of Chemistry, National Taiwan UniVersity
Taipei, Taiwan 106
ReceiVed February 8, 2000
Oligoaryl is an important class of compounds which exhibit a
variety of fascinating properties for optoelectronic interests.1
Incorporation of five-membered heteroaromatic moieties into these
conjugated molecules will occasionally increase fluorescence
quantum yields and the optoelectronic properties of the oligomers
can be tuned.1 Most syntheses of these heteroaromatic containing
oligomers involve the transition-metal catalyzed cross coupling
reactions of the corresponding aryl components.2 In general, the
presence of a long chain aliphatic substituent in these heteroaro-
matic rings will increase the solubility in organic solvent and
hence enhance the processibility of these materials.1 However,
introduction of such alkyl substituent at C3 and/or C4 positions
in these heteroaromatic rings for further cross coupling reactions
is not trivial.3 Cyclization of the 1,4-dicarbonyl moiety with
heteroatom-containing reagents provides an alternative procedure
for the construction of these five-membered heterocycles.3-5 It
is known that annulation of allenylmethanols can afford the
corresponding five-membered oxygen heterocycles.6 In addition,
annulation of allenyl carbonyl compounds,7 propargylic acetals,8
or oxiranes9,10 furnishes a powerful arsenal for the synthesis of
substituted furans. The applications of propargylic metallic species
have paved a useful path for the construction of furan skeletons.11
We recently reported that propargylic dithioacetal 1 can serve as
an allene 1,3-zwitterion synthon (eq 1).12 The interesting feature
for this reaction involves an umpolung of one of the two carbon-
sulfur bonds in the dithioacetal functionality. The organocopper
intermediate 2 can react with a number of electrophiles leading
to either allenyl or alkynyl product 3 or 4. The chemoselectivity
of this reaction depends on the nature of the electrophile. It is
envisaged that the reaction of 2 with an aldehyde 6 or an aldimine
8 may yield the intermediate alcohol 5a, or amine 5b, respectively.
Since the thioether moiety in 5 could be a good leaving group,
cyclization to eliminate the sulfur moiety may lead to the
substituted furan 7 or pyrrole 9 (eq 2). In this communication,
(1) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner,
G., Eds.; Wiley-VCH: Weinheim, 1998.
(2) For examples: Pelter, A.; Jenkins, I.; Jones, D. E. Tetrahedron 1997,
53, 10357. Pelter, A.; Rowlands, M.; Jenkins, I. H. Tetrahedron lett. 1987,
28, 5213. Pelter, A.; Rowlands, M.; Clements, G. Synthesis 1987, 51. Fanta,
P. E. Synthesis 1974, 9. Crisp, G. T. Synth. Commun. 1989, 19, 307. Gronowitz,
S.; Bobosik, V.; Lawitz, K. Chem. Scr. 1984, 24, 5. Yassar, A.; Garnier, F.;
Deloffre, F.; Horowitz, G.; Ricard, L. AdV. Mater. 1994, 6, 660. Gronowitz,
S.; Bobosik, V.; Lawitz, K. Chem. Scr. 1984, 23, 120.
(3) For reveiews, see: Lipshutz, B. H. Chem. ReV. 1986, 86, 795. Hou, X.
L.; Cheung, H. Y.; Hon, T. Y.; Kwan, P. L.; Lo, T. H.; Tong, S. Y.; Wong,
H. N. C. Tetrahedron 1998, 54, 1955.
(4) Bean, G. P. In Pyrroles; Jones, R. A., Ed.; Wiley: New York, 1990; p
194. Jackson, A. H. In ComprehensiVe Organic Chemisty; Sammes, P. G.,
Ed.; Pergamon Press: Oxford, 1979; Vol. 4, p 296.
(5) Wynberg, H.; Metselaar, J. Synth. Commun. 1984, 14, 1. Moriarty, R.
M.; Prakash, O.; Duncan, M. P. Synth. Commun. 1985, 15, 789. Nakayama,
J.; Murabayashi, S.; Hoshino, M. Heterocycles 1987, 26, 2599. Asano, T.;
Ito, S.; Saito, N.; Hatakeda, K. Heterocycles 1977, 6, 317. Kooreman, H. J.;
Wynberg, H. Recl. TraV. Chim. Pays-Bas 1967, 86, 37.
we report a convenient one-pot annulation reaction leading to
furan or pyrrole moiety as a part of the oligoaryls.
(6) Marshall, J. A.; Wang, X.-J. J. Org. Chem. 1991, 56, 4913. Marshall,
J. A.; Bartly, G. S. J. Org. Chem. 1994, 59, 7169.
(7) Marshall, J. A.; Bennett, C. E. J. Org. Chem. 1994, 59, 6110. Marshall,
J. A.; Robinson, E. D. J. Org. Chem. 1990, 55, 3450. Marshall, J. A.; Wallace,
E. M. J. Org. Chem. 1995, 60, 796.
A THF solution of 1 was allowed to react with 0.6 equiv of
Bu2CuLi13 in THF at -78 °C followed by treatment with an
aldehyde 6. Without workup, the mixture was subsequently treated
with trifluoroacetic acid to give the corresponding furans 7 in
satisfactory yield. A range of 2,3,5-trisubstituted furans can be
conveniently synthesized by this annulation procedure and repre-
(8) Kim, S.; Kim, Y. G. Synlett 1991, 869. Obrecht, D. HelV. Chim. Acta
1989, 72, 447. Ly, N. D.; Schlosser, M. HelV. Chim. Acta 1977, 60, 2085.
(9) Marshall, J. A.; DuBay, W. J. J. Org. Chem. 1991, 56, 1685. Marshall,
J. A.; DuBay, W. J. J. Am. Chem. Soc. 1992, 114, 1450. Sham, H. L.;
Betebenner, D. A. J. Chem. Soc., Chem. Commun. 1991, 1134.
(10) McDonald, F. E.; Schultz, C. C. J. Am. Chem. Soc. 1994, 116, 9363.
(11) Shu, H.-G.; Shiu, L.-H.; Wang, S.-H.; Wang, S.-L.; Lee, G.-H.; Peng,
S.-M.; Liu, R.-S. J. Am. Chem. Soc. 1996, 118, 530. Iwasawa, N.; Maeyama,
K.; Saitou, M. J. Am. Chem. Soc. 1997, 119, 1486.
(13) Less than a stoichiometric amount of the organocopper reagent (e.g.
0.6 equiv) has been used for the Michael addition of acrolein (Cf.: Matsuzawa,
S.; Horiguchi, Y.; Nakamura, E.; Kuwajima, I. Tetrahedron 1989, 45, 349.
See also: Lipshutz, B. H. In Organometallics in Synthesis; Schlosser, M.,
Ed.; Wiley: Chichester, 1994; p 298).
(12) Tseng, H.-R.; Luh, T.-Y. J. Org. Chem. 1997, 62, 4568. Tseng, H.-
R.; Lee, C.-F.; Yang, L.-M.; Luh, T.-Y. J. Org. Chem. 1999, 64, 8582.
10.1021/ja0004736 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/05/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 20, 2000 4993
Table 1. Synthesis and Properties of Trisubstituted Furans 7
sentative examples are summarized in Table 1. Pyrroles 9 were
also obtained in a similar manner when imines 8 were used in
place of aldehydes. It is noteworthy that BF3‚OEt2 appeared to
be a better Lewis catalyst for the cyclization in pyrrole synthesis.
Results are outlined in Table 2. The structures of 7 and 9 were
unambiguously assigned by NOESY, HMQC, and/or HMBC.
As shown in Tables 1 and 2, the reaction is particularly
promising to introduce a substituent at C3. More importantly, this
substituent can be a long chain aliphatic moiety. As mentioned
earlier, it is a common strategy to have such an alkyl group to
increase the solubility for the convenience of processing leading
to devices for optoelectronic investigations. Our synthesis indeed
provides a very convenient entry for the synthesis of such
oligoaryls or polyarylenes with these aliphatic substituents.
Starting from the dialdehydes 10, oligoaryls 11a-d having two
furan moieties were obtained in one pot (eq 3). In a similar
manner, when a diimine 12 was employed, bis-pyrrole 11e was
prepared (eq 3). The results are tabulated in Table 3.
As shown in Table 1, a wide range of substituents having
different functionalities remains intact under these reaction
conditions. Accordingly, further modification of these functional-
ity may lead to a variety of oligoaryls and the lengths of
conjugation can thus be tuned. For example, Heck reaction of
11b with PhI afforded 13 in 83% yield.
Table 2. Synthesis and Photophysical Properties of Substituted
Pyrroles 9 (R1 ) R2 ) Ph)
The λem values measured in EtOAc and the fluorescence
quantum yields (Φf) for these oligomers are also tabulated in
Tables 1-3 for comparison. It is noteworthy that penta- or
hexaaryl oligomers 11 (Table 3) exhibit very bright emission in
the blue light region. As expected, the emission maximum of 13
(λem 460, 479 nm, Φf ) 0.67) shifted bathochromically. Ther-
mogravimetric analyses showed that 11c and 11d remained intact
at 200 °C for at least 24 h and Td values were 412 and 435 °C,
respectively. The potential optoelectronic applications are under
investigation.
Table 3. Synthesis and Properties of Penta- and Hexaaryls 11
In summary, we have uncovered a useful annulation procedure
starting from the propargylic dithioacetals via the corresponding
organocopper intermediates resulting in a convenient synthesis
of furan and pyrrole heterocycles. The reaction is particularly
appealing for the preparation of a range of oligoaryls containing
furan or pyrrole moieties.
Acknowledgment. This work was supported by the National Science
Council and the Ministry of Education of the Republic of China.
Supporting Information Available: Experimental procedures for the
preparation of 1, 7, 9, 11, and 13 and their 1H NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0004736