Chemistry Reference
In-Depth Information
Fig. 37 Formation of [10]CPP·C
60
observed by
1
H NMR [
37
]
Isobe started with hexyl-functionalized dibromo crysene 76, and accessed the
metallocycle 78 via the diboronate 77 using platinum chloride and cesium fluoride.
Reductive elimination with triphenylphosphine offered the hydrocarbon 79 cleanly.
Given that the rotation of the aromatic domains of this macrocycle through the
cavity is highly disfavored at room temperature (even in the absence of the
solubilizing hexyl chains), they represent side wall portions of (10,10), (11,9),
and (12,8) carbon nanotubes depending on isomerization (Fig.
40
)[
44
].
Isobe observed that above 100
C a single atropisomer, isolated by HPLC, will
scramble to yield all six isomers. Models such as [4]CC with extended
π
-systems
bring us closer to the synthesis of an ultrashort CNT. One can envision, with an
appropriate extension strategy, employing Isobe's synthesis to produce selectively
armchair (10,10) and chiral (11,9) and (12,8) nanotubes from a single precursor [
44
].
Along this route to carbon nanohoop elongation, the first
-extended cyclopar-
aphenylene was synthesized by the Itami lab in 2012 [
47
]. Using the quinone
addition and reductive aromatization motif developed by Jasti, dibromide 82 was
synthesized by addition to naphthoquinone and macrocyclized using a nickel-
catalyzed “shotgun” approach. This macrocyclization gave a poor yield (2%) of
the desired nine-membered cyclic oligomer 83 which was painstakingly isolated
from the reaction mixture. Single-electron reductive aromatization with granular
lithium metal offered the first benzannulated CPP, [9]CN (Fig.
41
).
Itami reported the fascinating observation that the proton NMR spectrum of this
compound is highly convoluted, indicating restricted rotation of the naphthalene
rings through the center of the molecule [
47
]. Progress towards the closure of the
fjord regions in [9]CN to offer the first example of a short CNT obtained by organic
synthesis is no doubt underway.
π
