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Table 1 Dissociation energy
(kcal/mol) for CPPA fullerene
ΔG { with C 60
ΔG { with C 70
CPPA
[6]CPPA a
9.9
0.2
9.6
0.2
[7]CPPA a
9
9
<
<
16 a
10.8
0.3
10.1
0.2
17 a
9
11.9
0.8
<
18 b
14.1
0.3
a
[ 22 ]
b [ 23 ]
Fig. 13 A CPPA-fullerene
onion complex [ 24 ]
19·16·C 60
very reactive cyclocarbons [ 4 - 6 ]. These approaches have hinged upon masking
either the phenyl rings [ 28 - 30 ] or one or more alkynes [ 31 ] as curved moieties that
can be converted to the desired functionality after macrocyclization.
Both Hopf and Tsuji attempted the synthesis of [ n 2 ]CPPAs (having two alkynes
per phenyl ring and n phenyl rings) using masked aromatic rings [ 28 - 30 ]. Hopf
reported attempts towards [2 2 ]CPPA and [3 2 ]CPPA using syn -substituted 1,4-
cyclohexadienes to install curvature (Fig. 14 ). Structure 20 was synthesized by
double addition of lithium trimethylsilylacetylide into benzoquinone followed by
methylation of the resulting diol. After deprotecting the alkynes, Glaser-Ellington
conditions at high dilution (4
10 5 M) offered low yields of macrocycles 21
and 22.
Conversion of these to the corresponding CPPAs has not been reported, presum-
ably due to a lack of appropriate aromatization methods [ 28 , 32 ].
Tsuji had success in the synthesis of [6 2 ]CPPA using Dewar benzenes as
masked aromatic rings which can be irreversibly aromatized by irradiation in the
final synthetic step (Fig. 15 )[ 30 , 33 ]. Tsuji's approach consists of the synthesis of
Dewar benzene derivative 24 by [2+2] cycloaddition of 1,2-dichloroethylene to
dimethylacetylenedicarboxylate. This adduct was then reduced and protected as
the cyclohexyl ketal 26, which is resistant to photoisomerization. Elimination
afforded a mix of dichlorides 27 and 28 which were easily separated by chroma-
tography [ 29 ].
Tsuji proved the usefulness of this building block in the synthesis of less strained
phenylacetylene macrocycles [ 29 , 33 ]. Sonogashira alkynylation, Glaser-Ellington
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