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increases dramatically with increasing number of the constituent carbon
atoms [55]. Therefore, it would be extremely interesting to see if geometry-
selective synthesis is possible under kinetically controlled conditions based
on an organic synthesis method. As an initial step toward this goal, in order
to produce C
78
fullerene in a size-selective manner, three-dimensional cyclo-
phanes 32a and 32b having the [4.3.2]propellane units were prepared as
precursors of polyynes 33a and 33b, respectively. In the negative mode LD
TOF mass spectrum of 32a, an intense peak due to C
78
H
18
(33a
) formed
by expulsion of all indane units was observed. However, only partial
dehydrogenation down to C
78
H
14
took place, just as in the case of C
60
H
18
(28). On the other hand, in the case of hexachloro derivative 32b, frag-
mentation of C
78
H
12
Cl
6
(33b
) formed by the loss of all indane fragments
from 32b took place to release hydrogen and chlorine atoms simultaneously,
leading to the formation of C
78
together with some C
78
H
2
and C
78
H
4
,in
which a few hydrogen atoms remained intact. Although the structure of the
C
78
ion remains to be established, the observed C
2
losses from C
78
down to
C
70
are the strong indication of its fullerene structure.
6.5 CONCLUSIONS
A new class of strain and bent polyynes has become accessible by the
generation of molecular cyclic polyynes through decomposition of structur-
ally well-defined organic precursors, as well as by the laser vaporization of
graphite. This has opened new aspects in relation to the hybrid sp-sp
2
-sp
3
network systems or polymeric carbynoid structures. The molecules of inter-
est spanning from two-dimensional monocyclic, bicyclic, tricyclic, polycyclic
rings, to three-dimensional macrocyclic polyynes, provide a chance of new
challenges for further research on generation, characterization, and reac-
tions of the relatively large polyynes.
ACKNOWLEDGMENT
This work was supported by CREST, Japan Science and Technology
Agency (JST).
REFERENCES
[1] For reviews: (a) Diederich, F. Oligoacetylenes. In: Modern Acetylene Chem-
istry, Stang, P.J.; Diederich, F., Eds. VCH: Weinheim, 1995; 443-471.
(b) Diederich, F.; Rubin, Y. Synthetic approaches toward molecular and poly-
meric carbon allotropes. Angew. Chem., Int. Ed. Engl. 1992, 31(9), 1101-1123.
(c) Diederich, F.; Gobbi, L. Cyclic and linear acetylenic molecular scaffolding.
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