Synthesis of a Sidewall Fragment of a (12,0) Carbon Nanotube

Abstract

Synthesis of a carbon nanobelt (CNB) is a very challenging task in organic chemistry. Herein, we report the successful synthesis of an octabenzo[12]cyclacene based CNB (6), which can be regarded as a sidewall fragment of a (12,0) carbon nanotube. The key intermediate compound, a tetraepoxy nanobelt (5), was first synthesized by Diels–Alder reaction, and subsequent reductive aromatization gave the fully conjugated CNB 6. X‐ray crystallographic analysis unambiguously confirmed the belt‐shaped structure of 6. 1H NMR spectrum and theoretical calculations (ACID, NICS, and 2D/3D ICSS) revealed localized aromaticity and stronger shielding chemical environment in the inner region of the belt. The optical properties (absorption and emission) of 6 were studied and correlated to its electronic structure. Strain analysis indicates that the phenyl substituents at the zigzag edges are crucial to the successful synthesis of 6. This report presents a new strategy towards highly strained CNBs.

Synthesis of Zigzag Carbon Nanobelts through Scholl Reactions

“…Because carbon nanobelts are double-stranded macrocycles having two conjugated and non-coincident edges, as uccessful synthesis of carbon nanobelts needs to achieve both macrocyclization and ladderization, the latter of which is ap rocess to form ad ouble stranded structure with the connectivity similar to aladder. In the synthesis of [4n]benzo-[6n]cyclacene (n = 2, 3) by Itami [20] and Chi, [21] macrocyclization and ladderization are achieved simultaneously through repetitive Diels-Alder cycloadditions.T he synthesis of nanobelts 1-3 herein relies on an alternative strategy,w here macrocyclization is followed by ladderization through regioselective Scholl reactions.The synthetic precursors of 1-3 are hybrid cyclic arylene oligomers 4-6,r espectively.F igure 3 shows the structures of 4-6 illustrating two key concepts of designing.F irst, unlike the nanoring precursors for the reported synthesis of armchair and chiral carbon nanobelts, [9] 4-6 contain meta-phenylene units (highlighted in yellow), which not only provide azigzag edge in the resulting nanobelt but also alleviate the strain of nanorings [22] to facilitate macrocyclization. As ar esult, 4-6 are less strained than the corresponding cycloparaphenylenes,a nd in particular, 6 is as train-free macrocycle.S econd, the formation of 1-3 relies on selective formation of C À Cb onds to the a-positions (highlighted with red balls) in the naphthalene units in the course of Scholl reactions.S uch regioselectivity has been preliminarily demonstrated in the reported synthesis of curved polycyclica romatics.…”

Synthesis of Zigzag Carbon Nanobelts through Scholl Reactions

“…[10] Attempts to synthesize cyclacenes were reported by the groups led by Stoddart, [11] Cory, [12] Schlüter, [13] PeÇa [14] and Wang, [15] who successfully constructed belt-shaped molecular frameworks mainly by Diels-Alder reactions,b ut failed to achieve the fully conjugated structures.T he first successful synthesis of zigzag carbon nanobelts has not been achieved until very recently on the basis of am odified design, [m]benzo-[n]cyclacene (m n), [16][17][18] which is more stable than [n]cyclacene because benzannulation increases the number of Clarsa romatic sextets.I tami and co-workers reported, in ap reprint format in May 2020 [19] and later in af ormal publication, [20] the synthesis of [12]benzo [18]cyclacene (Figure 1a)t hrough repetitive Diels-Alder cycloadditions followed by reductive aromatization;a nd af ew months later, Chi and co-workers published the synthesis of [8]benzo- [12]cyclacene (Figure 1a)u sing essentially the same strategy. [21] Herein we report new zigzag carbon nanobelts 1-3 (Figure 1b)o nt he basis of ad esign different from [n]cyclacene or [m]benzo[n]cyclacene.Carbon nanobelts 1 and 2 are apair of constitutional isomers,whose polycyclicframeworks represent two different cutouts from zigzag (16,0) SWCNT, and 3 is alarger analogue of 2 representing asidewall segment of (24,0) SWCNT.F igure 2c ompares the polycyclic frameworks of 1 and 2 with [16]cyclacene by mapping them on an unrolled honeycomb lattice of (16,0)SWCNT,s howing the alkoxylated benzenoid rings in 1 and 2 as grey hexagons and other six-membered rings as light blue hexagons.T he light blue hexagons in 1 and 2 compose the essential belt structures, while the grey hexagons are not necessary to keep the belt structure but allow expansion along the direction of the nanotube axis. [ 16]cyclacene represents ac omplete row of linearly arranged hexagons along the equator in the unrolled lattice of (16,0)SWCNT.I nc ontrast, the essential belt structures of 1 and 2 present aw ave-like arrangement of light blue hexagons across five and three rows,respectively,in the unrolled lattice of (16,0)SWCNT.Such wave-like arrangement of hexagons involves alarger number of Clarsaromatic sextets than the linear arrangement in [n]cyclacene.S pecifically,t he essential belt structures (shown in light blue) of 1 and 2 contain eight Clarsaromatic sextets,while the closedshell form of [16]cyclacene does not contain any Clars aromatic sextet.…”

“…Because carbon nanobelts are double-stranded macrocycles having two conjugated and non-coincident edges, as uccessful synthesis of carbon nanobelts needs to achieve both macrocyclization and ladderization, the latter of which is ap rocess to form ad ouble stranded structure with the connectivity similar to aladder. In the synthesis of [4n]benzo-[6n]cyclacene (n = 2, 3) by Itami [20] and Chi, [21] macrocyclization and ladderization are achieved simultaneously through repetitive Diels-Alder cycloadditions.T he synthesis of nanobelts 1-3 herein relies on an alternative strategy,w here macrocyclization is followed by ladderization through regioselective Scholl reactions.The synthetic precursors of 1-3 are hybrid cyclic arylene oligomers 4-6,r espectively.F igure 3 shows the structures of 4-6 illustrating two key concepts of designing.F irst, unlike the nanoring precursors for the reported synthesis of armchair and chiral carbon nanobelts, [9] 4-6 contain meta-phenylene units (highlighted in yellow), which not only provide azigzag edge in the resulting nanobelt but also alleviate the strain of nanorings [22] to facilitate macrocyclization. As ar esult, 4-6 are less strained than the corresponding cycloparaphenylenes,a nd in particular, 6 is as train-free macrocycle.S econd, the formation of 1-3 relies on selective formation of C À Cb onds to the a-positions (highlighted with red balls) in the naphthalene units in the course of Scholl reactions.S uch regioselectivity has been preliminarily demonstrated in the reported synthesis of…”

Pyrene-Based Diarynes as Precursors for Twisted Fused Polycyclic Aromatic Hydrocarbons: A Comparison of Two Routes

“…There are a few examples where the in situ generation of pyrene-based bis-arynes has been described in the cycloaddition to furans, 9,10 benzofurans, 11 or arylacetonitrils. 12 In all these cases, either pyrene dibromides 9,12 or tetrabromides 10, 11,13 have been used as molecular precursors, which were transformed into the arynes with non-nucleophilic bases, or n-BuLi. Similar to aryl bromides, aryl triflates 14 can be transformed into arynes by non-nucleophilic bases, or, more elegant, ortho-TMS triflates 15 as bench stable precursors that are in situ transformed to arynes by fluoride anions.…”