exTTF 1 represents the first example of a peculiar type of
radia[n]annulenes,5 the structure and properties of which have
invoked a series of questions both fundamentally and
practically important; for instance: (1) Will the central
radiaannulene π-bridge show the gaining of aromaticity
similar to that of TTFAQ upon oxidation?6 (2) Will the
incorporation of π-extension add new dimensions for control
over solid-state ordering, multistage-redox behavior, and
supramolecular stacking in solution or the solid state?1 (3)
Will enhanced electronic and photonic performances be
achievable through this new design concept? To address these
issues, synthetic access to macrocyclic enyne bridged
TTFAQ analogues is prerequisite.
Scheme 2. Synthesis of Acetylene-Expanded TTFAQs 8a and 8b
Strictly speaking, exTTF 1 does not serve as an adequate
model for the study of π-expanded TTFAQ systems, since
unlike TTFAQs and other exTTFs the two dithiole rings of
1 have no electronic communication through the long
π-spacer as a result of its nonplanar, S-shaped π-framework.4
The structural nonplanarity of 1 arises mainly from the
significant steric crowdedness among the anthraquinone and
macrocyclic enyne moieties. To circumvent this setback, we
then revised our design to a simpler motif, exTTF 2 (Scheme
1), which can be viewed as an expanded version of TTFAQ
derived from directly inserting acetylenic units into the
quinoid spacer of TTFAQ. In principle, motif 2 should afford
a better model system than exTTF 1 because its π-framework
is absent of any steric hindrance and hence capable of taking
a planar ground-state structure in favor of maximal π-con-
jugation.
The key step in the synthesis of exTTF 2 is to construct
the central macrocyclic enyne moiety. This can be ac-
complished through a one-pot, 4-fold Sonogashira cyclization
protocol developed by our group in the synthesis of exTTF
1.4 Following this strategy, two acetylene-expanded TTFAQs
8a and 8b were prepared as outlined in Scheme 2. The
synthesis began with a Wittig-Horner reaction between
ketone 4 and the ylide in situ generated by treatment of
phosphonate 3 with t-BuLi. Dithiole precursor 57 was
obtained in a modest yield and was desilylated with K2CO3
to give corresponding terminal alkyne. This intermediate
showed relatively low stability and was immediately sub-
jected to Sonogashira coupling with diiodobenzene 7a or 7b
under dilute conditions (ca. 4-6 mM). To our delight, the
two macrocyclization reactions successfully afforded the
desired products. After column chromatographic separation,
expanded TTFAQs 8a and 8b were obtained in 20% and
7% yield, respectively. Given the fact that the macrocycliza-
tion underwent four consecutive steps of cross-coupling
reactions in one pot, the yields of 8a,b are actually very
reasonable.8
(3) (a) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Ort´ı, E.; Viruela, P. M.;
Viruela, R. J. Org. Chem. 1998, 63, 1268. (b) Moore, A. J.; Bryce, M. R.
J. Chem. Soc., Perkin Trans. 1 1991, 157.
(4) Chen, G.; Wang, L.; Thompson, D. W.; Zhao, Y. Org. Lett. 2008,
10, 657.
Parallel to the one-pot approach, the synthesis of 8a and
8b through a stepwise route was also explored. As outlined
in Scheme 2, desilylation of dithiole precursor 5 followed
up by cross coupling with excess diiodobenzene 7a or 7b
afforded precursors 9a and 9b. With 9a and 9b in hand,
another iteration of intramolecular 2-fold Sonogashira cou-
pling would furnish the desired products 8a and 8b. This
synthetic route, however, was proven inefficient due to the
sluggish yield of the first coupling step. Further attempts at
macrocyclization through this approach were therefore aban-
doned.
(5) (a) Tykwinski, R. R.; Gholami, M.; Eisler, S.; Zhao, Y.; Melin, F.;
Echegoyen, L. Pure Appl. Chem. 2008, 80, 621. (b) Gholami, M.; Melin,
F.; McDonald, R.; Ferguson, M. J.; Echegoyen, L.; Tykwinski, R. R. Angew.
Chem., Int. Ed. 2007, 46, 9081. (c) Mitzel, F.; Boudon, C.; Gisselbrecht,
J.-P.; Seiler, P.; Gross, M.; Diederich, F. HelV. Chim. Acta 2004, 87, 1130.
(d) Mitzel, F.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.;
Diederich, F. Chem. Commun. 2003, 1634. (e) Gholami, M.; Chaur, M. N.;
Wilde, M.; Ferguson, M. J.; McDonald, R.; Echegoyen, L.; Tykwinski, R. R.
Chem. Commun. 2009, 3038.
(6) The macrocycle enyne framework in 1 and 2 presents a new addition
to a class of carbo-meric molecules that have attracted great interest in
terms of π-delocalization and aromaticity in neutral and ionized states. See:
(a) Lepetit, C.; Nielsen, M. B.; Diederich, F.; Chauvin, R. Chem.sEur. J.
2003, 9, 5056. (b) Lepetit, C.; Godard, C.; Chauvin, R. New. J. Chem. 2001,
25, 572. (c) Godard, C.; Lepetit, C.; Chauvin, R. Chem Commun. 2000,
1833. (d) Chauvin, R. Tetrahedron Lett. 1995, 36, 397.
The molecular structures and purity of expanded TTFAQ
analogues 8a,b and relevant precursors were unambiguously
(7) Analogues of 5 with different substituents were previously reported;
see: (a) Nielsen, M. B.; Moonen, N. N. P.; Boudon, C.; Gisselbrecht, J.-P.;
Seiler, P.; Gross, M.; Diederich, F. Chem. Commun. 2001, 1848. (b) Nielsen,
M. B.; Petersen, J. C.; Thorup, N.; Jessing, M.; Andersson, A. S.; Jepsen,
A. S.; Gisselbrecht, J.-P.; Boudon, C.; Gross, M. J. Mater. Chem. 2005,
15, 2599.
(8) The yield of 8a (20%) corresponds to an average yield of 67% for
each individual Sonogashira coupling, while the yield of 8b (7%) corre-
sponds to 51% yield for each individual coupling.
Org. Lett., Vol. 11, No. 13, 2009
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