A R T I C L E S
Takayama et al.
important.2,6 The synthetic methods developed thus far, however,
have some drawbacks, more or less, such as lack of generality
or low yield, or are not applicable to prepare in quantity.
Polymers and oligomers having an enyne scaffold can be
prepared by topochemical solid-state polymerization of suitably
prearranged and substituted buta-1,3-diynes,7 a requirement,
however, which severely limits their accessibility. Wudl and
Biter reported a synthetic method for preparing a systematic
series of trans-oligoenynes using coupling reaction of an
alkynylmetal compound with trans-1,2-dihalo ethylene as the
key reaction.8 However, extension of this coupling method for
synthesizing oligoenynes having olefin substituent(s) failed,9 and
thus far, a model compound of polymers obtained by polym-
erization of substituted buta-1,3-diynes has not been prepared.
An iterative approach to cis-oligoenynes using the Sonogashira
coupling as the key reaction was reported by Hirsch and co-
workers which allowed preparation of some kinds of oligoenyne
with up to four triple and three double bonds, albeit the overall
yield was rather low.10
Figure 2. Building blocks 1 and 2 for the synthesis of oligoenynes.
Scheme 1
Preparation of monodisperse oligomers having an enediyne
scaffold with trans-olefin configuration was reported by Dieder-
ich et al. They prepared the oligomers with extended size by a
statistical deprotection-oxidative Hay oligomerization protocol
which enabled them to carry out comprehensive structure-
property studies.11 On the basis of their study, the effective
conjugation length could be revealed to comprise 9-11
monomer units, which corresponds to a total of 27-33 double
and triple bonds.11a The multinanometer-long π-conjugated
oligomers having an enediyne scaffold thus prepared have been
shown to work as potential components of future molecular-
scale electronic devices. The synthetic method, however, was
not selective, and the yield of the oligomers decreased notably
in proportion to the increase of the number of monomer units.
Moreover, isolation of the resulting oligomers required the use
of preparative size-exclusion chromatography and/or gel-
permeation chromatography. It should also be noted that no
general entry to oligoenediynes having cis-olefin configuration
has been developed thus far.
Results and Discussion
Synthesis of Oligoenynes. Our synthetic method for prepar-
ing oligoenynes includes efficient and practical synthesis of a
variety of 1-iodo-4-(trimethylsilyl)but-1-en-3-yne derivative with
trans- and cis-olefin configuration, i.e., 1 and 2 shown in Figure
2, and repeated use of 1 and 2 as building blocks, for the
synthesis of trans- and cis-oligoenynes, respectively, using the
Sonogashira coupling reaction13 as the key carbon-elongation
reaction.
Recently, we have developed a one-pot method for synthesiz-
ing 1-trimethylsilyl-1,4-diiodo-1,3-alkadienes 3 via regioselec-
tive coupling of internal acetylenes and ethynyltrimethylsilane
mediated by a divalent titanium reagent Ti(O-i-Pr)4/2 i-PrMgCl,
and the following reaction of the resulting titanacyclopentadienes
with I2.14,15 We have now found that treatment of crude 3 with
pyrrolidine afforded 2 in high overall yield as shown in Scheme
1. Meanwhile, enynes 1 can be synthesized from 2 obtained
using 1-trimethylsilyl-1-alkynes as the internal acetylene by the
conventional reaction sequence, as exemplified by the produc-
tion of 1a from 2c as shown in Scheme 2. As also shown in
Schemes 1 and 2, the Sonogashira coupling of 2 or 1 thus
obtained with 3-methyl-1-butyn-3-ol provided enediynes 4 and
Herein we report a practical synthetic method to prepare
monodisperse π-conjugated oligomers containing an enyne- or
enediyne scaffold with either trans- or cis-olefin configuration,
including oligoenediynes having the effective conjugation
length.12
(6) Diederich, F.; Gobbi, L. Top. Curr. Chem. 1999, 201, 43-79. Diederich,
F. Chem. Commun. 2001, 219-227. Nielsen, M. B.; Diederich, F. Synlett
2002, 544-552. Tykwinski, R. R.; Zhao, Y. Synlett 2002, 1939-1953.
(7) Wegner, G. Z. Naturforsch. 1969, 24b, 824-832. Wenz, G.; Mu¨ller, M.
A.; Schmidt, M.; Wegner, G. Macromolecules 1984, 17, 837-850. Coates,
G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs, R. H.
Angew. Chem., Int. Ed. Engl. 1997, 36, 248-251. Sarkar, A.; Okada, S.;
Matsuzawa, H.; Matsuda, H.; Nakanishi, H. J. Mater. Chem. 2000, 10,
819-828 and references therein.
(8) (a) Wudl, F.; Bitler, S. P. J. Am. Chem. Soc. 1986, 108, 4685-4687. (b)
See also: Polhuis, M.; Hendrikx, C. C. J.; Zuilhof, H.; Sudho¨lter, E. J. R.
Tetrahedron Lett. 2003, 44, 899-901. (c) Lindsell, W. E.; Preston, P. N.;
Tomb, P. J. J. Organomet. Chem. 1992, 439, 201-212.
(9) Giesa, R.; Schulz, R. C. Polym. Int. 1994, 33, 43-60. Crousse, B.; Alami,
M.; Linstrumelle, G. Tetrahedron Lett. 1995, 36, 4245-4248.
(10) Kosinski, C.; Hirsch, A.; Heinemann, F. W.; Hampel, F. Eur. J. Org. Chem.
2001, 3879-3890. See also: Bharucha, K. N.; Marsh, R. M.; Minto, R.
E.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 3120-3121.
(11) (a) Edelmann, M. J.; Odermatt, S.; Diederich, F. Chimia 2001, 55, 132-
138. (b) Martine, R. E.; Gubler, U.; Cornil, J.; Balakina, M.; Boudon, C.;
Bosshard, C.; Gisselbrecht, J.-P.; Diederich, F.; Gu¨nter, P.; Gross, M.;
Bre´das, J.-L. Chem. Eur. J. 2000, 6, 3622-3635. (c) Martin, R. E.; Ma¨der,
T.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 817-821. (d) Martin,
R. E.; Gubler, U.; Boudon, C.; Gramlich, V.; Bosshard, C.; Gisselbrecht,
J.-P.; Gu¨nter, P.; Gross, M.; Diederich, F. Chem. Eur. J. 1997, 3, 1505-
1512.
(12) Portions of this work have been communicated. Takayama, Y.; Delas, C.;
Muraoka, K.; Sato, F. Org. Lett. 2003, 5, 365-368.
(13) For general reviews, see: Sonogashira, K. In Handbook of Organopalladium
Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley-Interscience: New
York, 2002; Vol. 1, pp 493-529. Sonogashira, K. In Metal Catalyzed
Cross-Coupling Reactions; Diedrich, F., Stang, P. J., Eds.; Wiley-VCH:
Weinheim, 1998; Chapter 5.
(14) Fukuhara, K.; Takayama, Y.; Sato, F. J. Am. Chem. Soc. 2003, 125, 6884-
6885. Nakajima, R.; Delas, C.; Takayama, Y.; Sato, F. Angew. Chem., Int.
Ed. 2002, 41, 3023-3025.
(15) Recent reviews for synthetic reactions mediated by a Ti(O-i-Pr)4/2 i-PrMgX
reagent: Sato, F.; Urabe, H.; Okamoto, S. Chem. ReV. 2000, 100, 2835-
2886. Sato, F.; Okamoto, S. AdV. Synth. Catal. 2001, 343, 759-784. Sato,
F.; Urabe, H. In Titamiun and Zirconium in Organic Synthesis; Marek, I.,
Ed.; Wiley-VCH: Weinheim, Germany, 2002; pp 319-354.
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