A R T I C L E S
Wang et al.
devices (OLEDs).11 Low molecular weight OXDs,12 dendritic,13
and polymeric derivatives14 have been studied in this context.
Recently, electrochromism has been reported in OXD-TTF
hybrids.15
The diyne analogue of HiPA, i.e., 2-methyl-3,5-hexadiyn-2-
ol,19 has not been exploited previously in the synthesis of
terminal butadiynes. It proved to be an efficient reagent for this
purpose, affording 8a,b (Scheme 2) as crystalline solids by
analogy with the synthesis of 4a,b. 2-Methyl-3,5-hexadiyn-2-
ol was less reactive than HiPA in cross-coupling reactions with
aryl bromides 2a,b. Therefore, the iodide derivatives 6a,b were
used to avoid high temperatures. It is notable that 8a,b are stable
to routine purification and can be stored under ambient
conditions for months without any detectable decomposition.
The unusual high stability of 8a can be explained by the packing
mode in the solid state (as revealed by X-ray analysis: see
below).
With 4 and 8 in hand, we proceeded to functionalize the
terminal positions with redox-active moieties. The butadiynes
8a,b are particularly attractive building blocks for the synthesis
of unsymmetrical 1,4-diarylbutadiyne derivatives, as their cross-
coupling reactions preclude any possibility of symmetrical
byproducts being formed.20 Our initial attempt to prepare 10a
by the cross-coupling of 4a with 2,7-dibromofluorenone under
Sonogashira conditions led to self-coupling of 4a to give 9 (90%
yield, Scheme 3). It is well-known that self-coupling of terminal
alkynes can occur as a side-reaction alongside cross-coupling,21
and it is favored when arylbromides are used for the reaction.22
When the more reactive 2,7-diiofluorenone was used, both 10a,b
were isolated in high yields (Scheme 3). Due to the extremely
low solubility of 2,7-dibromo-9-(dicyanomethylene)fluorene in
organic solvents (requiring boiling DMF), it was not possible
to carry out its coupling with 4 or 8. 10c was, however,
synthesized by a condensation of the fluorenone analogue 10b
with malononitrile.
The aims of the present work were 2-fold. (1) To develop
the synthetic chemistry of OXD derivatives possessing terminal
-ethynyl (4a,b) and -butadiynyl (8a,b) substituents, with em-
phasis on the attachment of redox-active chromophores. (2) To
probe the extent of π-conjugation through these molecular wires
as revealed in their structural, redox, and optoelectronic proper-
ties. Our specific targets were a series of linearly conjugated
molecules comprising one or two OXD units linked through
-ethynyl or -butadiynyl bridges to moieties of different π-elec-
tron donating strength, viz., TTF16 (10d, 11, 13), thienylene
(10e), bithienylene (10f), 9-[2-(4,5-dimethyl-1,3-dithiolylene)]-
2,7-fluorenylene (10g), and N,N-dimethyl-/diphenyl-aminophe-
nyl (12, 14), and for comparison, two electron acceptors, viz.
9-fluorenone (10a and 10b) and 9-dicyanomethylenefluorene
(10c).
Results and Discussion
Synthesis. The OXD-containing terminal ethynes 4a and 4b
were synthesized in high yields from the OXD bromides
2 (obtained, in turn, by dehydrative cyclization of 1a and 1b)
first via Sonogashira couplings with 2-methyl-3-butyn-2-ol17
(HiPA) followed by deprotection of the precursor compounds
3 in the presence of a catalytic amount of NaOH in toluene18
(Scheme1). Apart from its low cost, there are two major
attractions in using HiPA to synthesize these terminal alkynes.
(1) The high polarity of the protected derivatives 3 leads to
easy chromatographic separation of the deprotected products 4
from any unreacted 3. (2) The higher boiling point of HiPA
(compared to that of trimethylsilylacetylene) allows higher
reaction temperatures, which are required for some unreactive
aryl bromides.
10d-g were similarly obtained in 67-91% yields from the
corresponding diiodo compound and reagent 4a or b. Cross-
(19) Gusev, I.; Kucherov, V. F. Bull. Acad. Sci. USSR DiV. Chem. Sci. (Engl.
Transl.) 1962, 995.
(20) Unsymmetrically substituted 1,3-butadiynes have been efficiently synthe-
sized via Pd-catalyzed cross-coupling of 1,3-diynylzincs (Negishi, E.; Hata,
M.; Xu, C. Org. Lett. 2000, 2, 3687) or via alkylidine carbenoid
rearrangements (Shi Shun, A. L. K.; Chernick, E. T.; Eisler, S.; Tykwinski,
R. R. J. Org. Chem. 2003, 68, 1339).
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Chem., Int. Ed. 2001, 40, 1372. (b) Bryce, M. R. J. Mater. Chem. 2000,
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(22) Chow, H.-F.; Wan, C.-W.; Low, K.-H.; Yeung, Y.-Y. J. Org. Chem. 2001,
66, 1910. A self-coupling method has been developed on the basis of
Sonogashira conditions without using aromatic halides. (Batsanov, A. S.;
Collings, J. C.; Fairlamb, I. J. S.; Holland, J. P.; Howard, J. A. K.; Lin, Z.;
Marder, T. B.; Parsons, A. C.; Ward, R. M.; Zhu, J. J. Org. Chem. 2005,
70, 703.) The presence of adventitious oxygen due to incomplete air
exclusion from the reaction vessel is likely to assist the formation of 9.
However, to the best of our knowledge, self-coupling as a major reaction
under Sonogashira conditions in the presence of an arylhalide partner is
unusual. For 4a, this could arise due to the increased acidity of the ethynyl
hydrogen caused by the electron-withdrawing OXD unit. It is known that
less acidic terminal ethynes are less reactive to oxidative self-couplings,
and the reaction yields could be improved by the addition of a small amount
of strong base (ref 1). In other words, the higher the acidity of a terminal
arylacetylene, the higher the yield of self-coupling. Semiempirical calcula-
tions (PM3, HyperChem, Version 6.03, Hypercube, Inc., 2000: all
calculations terminated at the RMS gradient of 0.01 kcal Å-1 mol-1
)
indicated that 4a had the smallest net charge (-0.148) on the terminal sp
carbon atom, compared with those of 4-fluorophenylacetylene (-0.157),
TMSA (-0.198), and 4-(phenylethynyl)phenylacetylene (-0.163); i.e., 4a
was the most acidic of these four compounds. We make this comparison
because the less acidic TMSA (Lewis, J.; Raithby, P. R.; Wong, W.-Y. J.
Organomet. Chem. 1998, 556, 219), 4-fluorophenylacetylene, and 4-(phe-
nylethynyl)phenylacetylene have been cross-coupled with 2,7-dibromof-
luorenone under similar conditions. (Ipaktschi, J.; Hosseinzadeh, R.; Schlaf,
P.; Eckert, T. HelV. Chim. Acta 2000, 83, 1224). In addition, the acetylides
of 4 and the diacetylides of 8 were less reactive to nucleophilic reactions
with ketones and aldehydes than were standard alkyl and aryl acetylenes,
which may also be because of the higher acidity of these OXD acetylenes
(hence lower nucleophilicity of their derived acetylides). (Kreher, D.;
Batsanov, A. S.; Wang, C.; Bryce, M. R. Org. Biomol. Chem. 2004, 2,
858). Self-coupling of 8a, in the absence of an aromatic halide, gave the
tetrayne analogue of 9 in 40% yield: details will be published separately.
(17) (a) Havens, S. H.; Hergenrother, P. M. J. Org. Chem. 1985, 50, 1763. (b)
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3790 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006