Chemistry Reference
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Fig. 18 Cycloparaphenylene
envisioned as the unit-cycle
of an armchair nanotube
(double bonds omitted for
clarity)
S
S
Cu
heat
S
S
S
S
pyrolysis
38
39
[2]CPP
Fig. 19 Parekh's attempted synthesis of [2]CPP [
3
]
by Parekh, V
¨
gtle accessed hexaphenyl pentasulfate macrocycle 40 in 65% yield
(Fig.
20a
).
However, thermal cleavage of the sulfide bonds again proved difficult. It is
assumed that, while efficient in many cases for the buildup of strain [
60
], this
motif is inadequate to access the highly rigid, bent aromatic backbone of the
CPPs. V¨gtle et al. then revised their synthesis to target the less energetic [8]CCP
and [10]CPP. To do so, the oligomeric phenyleneethyleneacetylene macrocycles 41
were synthesized from a tetra-Wittig reaction in dilute conditions meant to quell
polymerization (Fig.
20b
). V¨gtle's vision for these macrocycles was that [4+2]
cycloadditions to the ene-yne moieties would build in the final four phenyl rings and
yield the corresponding carbon nanohoops. However, these attempts were never
successful. Lastly, V¨gtle prepared substituted L-shaped cyclohexanes 42 and 43
with the intent to oxidize these to phenyl rings after cyclic oligomerization, but
failed to arrive at the corresponding macrocycles (Fig.
20c
). This cyclohexane
approach would later be employed successfully by Itami et al. [
35
].
4.2 First Synthesis of Cycloparaphenylene
After eluding synthesis for over 70 years, the pioneering synthesis of [9]CPP, [12]
CPP, and [18]CPP was reported by Jasti and Bertozzi [
32
]. Using cyclohexadienes as
masked aryl rings, Jasti envisioned a final reductive aromatization from macrocycles
of the type 46-48. These aromatization reactions were expected to be highly exother-
mic and appropriate for building in considerable strain (Fig.
21
).
To employ this method, diiodide 44 was easily synthesized on a large scale without
chromatography via the addition of benzoquinone into a molar excess of lithiated
diiodobenzene followed by methylation with sodium hydride and iodomethane.
A portion of this diiodide was then converted to the bisboronate 45 and “shotgun”
Suzuki-Miyaura cross-coupling at high dilution offered a mixture of macrocycles.
These macrocycles were easily separated by standard chromatographic procedures.
