Chemistry Reference
In-Depth Information
6 Cyclacenes . .................................................................................. 280
6.1 Towards the Synthesis of [6]
12
Cyclacene . ........................................... 281
6.2 Towards the Synthesis of [6]
8
Cyclacene . ............................................ 283
6.3 [6.8]Cyclacene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
6.4 Buckybelts ............................................................................. 285
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
1 Radially Oriented
π
-Systems
Molecules with nonplanar p-orbitals have attracted great interest in recent years due
to their fascinating electronics, challenging synthesis, and favorable supramolecu-
lar properties [
1
]. Many of these, such as the calixarenes, cyclotriveratriline, and
corranulenes maintain their electron-rich cavities only weakly and can planarize or
otherwise deform through rotation or bowl inversion [
2
]. In contrast, a perplexing
and synthetically challenging arrangement, theorized more than 80 years ago,
comprises shape-persistent hydrocarbon hoops consisting of all sp or sp
2
hybridized
carbon atoms [
3
]. These rigid hoops possess unique
-systems that are geometri-
cally forced into the center of the molecule (carbon nanotube-like) rather than
above and below the plane of the molecule (graphene-like, Fig.
1
).
The strain associated with these belt systems from nonplanarity renders them
challenging synthetic targets. The simplest of this class are the cyclo[
n
]carbons,
allotropes of carbon consisting of a macrocycle of all sp-hybridized atoms (Fig.
2
).
Cyclo[18]carbon was observed by Diederich and coworkers under mass spec-
troscopic conditions by the threefold
retro
-Diels-Alder liberation of anthracene
from the cleverly designed and stable annulene 1. The all-carbon product, however,
proved to be transient [
4
-
6
]. In spite of the reactivity of cyclocarbon, several
complex hydrocarbon belts of this type have succumbed to organic synthesis and
many more remain in the sights of synthetic chemists (Fig.
3
). Herein we provide a
review of the synthetic challenges, successful syntheses, and fascinating properties
of this novel class of rigid hoops.
π
2 Hydrocarbon “Picotube”
In 1996, Herges reported the groundbreaking first bottom-up organic synthesis of a
fully conjugated, aromatic hydrocarbon with a radial
-system [
7
]. The so-called
“picotube” is a hydrocarbon with the formula C
56
H
32
formed easily by the photo-
induced ring expansion of tetradehydrodianthracene 2 [
7
]. Proceeding through
tandem [2+2] and retro-[2+2] cycloadditions, tetraanthraceneylidene (TDDA) 3
can be obtained in 16% yield after purification (Fig.
4
).
The resulting hydrocarbon “picotube” was found to be extremely stable to
oxidation and methodology to close the remaining fjord regions to form a [4,4]
carbon nanotube 4 has yet to be developed. Herges attempted this challenging
π
