Chemistry Reference
In-Depth Information
Fig. 48 Crystal structure of [6.8]
3
cyclacene showing boat-like cyclooctatetraene moieties [
74
]
It was estimated that this bending disrupts the conjugation around the cyclacene,
diminishing it to about 31% [
74
]. Even though this structure is electronically
distinct from the highly-conjugated [6]cyclacenes,
it possesses inward-facing
π
-orbitals and represents an important step forward in the synthesis of hydrocarbon
belts as models for short carbon nanotubes.
6.4 Buckybelts
Over the years, the Schl¨ ter lab has synthesized several saturated belts, including
the [6]
18
cyclacene framework [
75
]. When it became clear that [6]cyclacenes were
too reactive to isolate, Schl¨ ter proposed and found evidence for the formation
of a buckybelt 110 having the structure of the equator region of fullerene C
84
(Fig.
49
)[
76
-
78
].
Reaction of hydrocarbon dienophile 103 with diene synthon 104 gave a greater
than 90% yield of
endo
and
exo
adducts which were separated by benchtop chroma-
tography. Generation of benzyne followed by trapping with furan and reaction with
tetracyclone offered
syn
and
anti
products 106 and 107 from both
endo
and
exo
105
respectively in 30% yield over three steps. Heating of both pure
exo
and pure
endo
106 and 107 separately both led cleanly to the belt 108 having the framework of the
target aromatic hydrocarbon. Treatment with
para
-toluenesulfonic acid installed
four alkenes to offer 109. Subsequent conversion of 109 to the desired belt, however,
was not observed under acidic conditions, and is yet to be realized [
76
,
78
]. In 2008
it was shown that, upon fragmentation of tetraacetate derivative 111 in a mass
spectrometer, the parent ion for 110,
m
/
z
932 can be observed, confirming the
production, if transient, of the desired buckybelt (Fig.
50
).
As such, the synthesis of a fully aromatic, two-stranded hydrocarbon belt
remains a fascinating challenge in the realm of synthetic organic chemistry.
¼
