Synthesis, Structure, and Reactivity of a Cylinder-Shaped Cyclo[12]orthophenylene[6]ethynylene: Toward the Synthesis of Zigzag Carbon Nanobelts

Article abstract of DOI:10.1021/acs.orglett.6b02702

Benzannulated cyclacenes (BCs) have been proposed as stable zigzag carbon nanobelts. Density functional theory (DFT) calculations revealed a closed-shell ground state for [12]BC, whereas an open-shell ground state was suggested for [12]cyclacene. The calc

Full text of DOI:10.1021/acs.orglett.6b02702

Letter
pubs.acs.org/OrgLett
Synthesis, Structure, and Reactivity of a Cylinder-Shaped
Cyclo[12]orthophenylene[6]ethynylene: Toward the Synthesis of
Zigzag Carbon Nanobelts
Katsuma Matsui, Masako Fushimi, Yasutomo Segawa,* and Kenichiro Itami*,†,‡,§
†
†
,†,‡
†
Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡
JST, ERATO, Itami Molecular Nanocarbon Project, Chikusa, Nagoya 464-8602, Japan
§
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan
*
S Supporting Information
ABSTRACT: Benzannulated cyclacenes (BCs) have been proposed
as stable zigzag carbon nanobelts. Density functional theory (DFT)
calculations revealed a closed-shell ground state for [12]BC, whereas
an open-shell ground state was suggested for [12]cyclacene. The
calculated strain energy and frontier molecular orbital energies of
[
12]BC also implied high stability. An unstrained macrocycle 1,
consisting of orthophenylene and ethynylene moieties, was designed
as a potential precursor for [12]BC and synthesized by sequential
Suzuki−Miyaura cross-coupling of diphenylacetylene derivatives. While the conversion of 1 into [12]BC is still under
investigation, an unexpected rearrangement of the triene moieties in 1, affording a tribenzo[f,k,m]tetraphene structure, was
discovered during the screening of reaction conditions. An attempt was made to rationalize this result by proposing a plausible
reaction mechanism that proceeds via intermediates containing cyclobutadiene or Dewar benzene moieties. The proposed
mechanism is partially supported by DFT calculations.
olecules that represent constituent segments of carbon
M
nanotubes (CNTs) are fascinating compounds due to
their unique cyclic π-conjugation systems and their application as
buildingblocks inthe bottom-up synthesis of structurally uniform
1
CNTs. CNTs can be subdivided into three classes, according to
2
their chiral indices (n,m). Accordingly, (n,m)CNTs with n = m, n
>
m > 0, and m = 0 are called “armchair”, “chiral”, and “zigzag”,
respectively, and the corresponding segment molecules for each
1c
type of CNT have been targeted by synthetic organic chemists.
The shortest segments for armchair CNTs are para-connected
cyclic oligophenylenes, the so-called cycloparaphenylenes
1
,3
(
CPPs). Although the CPP structure was proposed for the
4
Figure 1. (a) Zigzag CNTs. (b) [n]Cyclacenes and [n]BCs. (c)
Resonance structures of [n]cyclacenes (left) and [n]BCs (right). Clar
aromatic sextets are drawn as circles.
first time in the 1930s, it took almost 80 years to accomplish their
synthesis due to a lack of efficient synthetic methods to construct
such highly distorted structures. In 2008, Bertozzi and Jasti
achieved the synthesis of [n]CPPs (n = 9, 12, and 18), and since
then, [n]CPPs (n = 5−16, and 18) have been reported based on
synthetic methods developed by Jasti, Itami, and Yamago. As a
matter of fact, [5]-, [9]-, [12]-, and [15]CPPs are now
commercially available. After the successful synthesis of these
CPPs, their size-dependent photophysical properties, guest-
encapsulating behavior, adsorption properties, and reactivity
with metal complexes were investigated.
5a
16
state was predicted to be favored in the ground state. Given that
this feature endows cyclacenes with high reactivity, it is not
5
6
7
15
surprising that their synthesis has not yet been accomplished.
8
We designed the new cyclacene derivatives [n]benzo[n]-
cyclacenes ([n]BCs) in which two benzene rings are annulated to
each naphthalene moiety of the corresponding [n]cyclacenes.
Because of this structural modification, these [n]BCs include Clar
aromatic sextets, and thus, they may accordingly exhibit increased
thermal and chemical stability. As[n]BCs stillexhibit the segment
9
10
11
12
Since the discovery of suitable synthetic routes to CPPs, the
13
14
constituent segments of chiral and zigzag CNTs have also
15
beenstudied.ThesimplestzigzagCNTsegmentsarecyclacenes
Figure 1a,b). As the closed-shell structure of cyclacenes does not
contain a Clar aromatic sextet (Figure 1c), the open-shell singlet
(
Received: September 8, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02702
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
structure of (n,0)zigzag CNTs, [n]BCs can also be categorized as
zigzag carbon nanobelts. In this study, we conducted theoretical
calculations on the electronic properties and strain energies of
quantitatively, and a subsequent Suzuki−Miyaura coupling of 3
withanexcessof2proceededina1:2mannertofurnishtrimer4in
52% yield. Borylation of 4 by a dilithiation and treatment with i-
PrOBpin (Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
afforded 5 as a mixture containing small amount of impurities.
When 4 and the impure 5 were subjected to the cross-coupling
reaction conditions catalyzed by Pd (dba) /X-Phos (dba =
17
[
n]BCs. Moreover, wereport the synthesis of apossible precursor
for [12]BC and its reactivity including an unexpected rearrange-
ment reaction.
In order to estimate the stability and electronic properties of
2
3
[
n]BCs, density functional theory (DFT) calculations were
dibenzylideneacetone, X-Phos = 2-dicyclohexylphosphino-
2′,4′,6′-triisopropylbiphenyl), 1 was synthesized in 29% yield.
By X-ray crystallography, the dihedral angle of the biphenyl
moiety of 4 was foundto be64.5°, which isalmost identicalto that
of the cyclic hexamer 1 (60°). This preorganized structure of 4
should have contributed to the relatively high yield in the
macrocyclization.
Themolecularstructureof1wasunambiguouslydeterminedby
X-ray crystallography (Figure 3b). A single crystal of 1 was
obtained from vapor diffusion of pentane into a chloroform
solution of 1 at room temperature. In the solid state, 1 forms a
18
carried out (Table 1). Because of the steric repulsion at the fjord
Table 1. Strain Energies and Orbital Energies for
[
12]Cyclacene and [12]BC
orbital energy (eV)
strain energy (kcal·mol−1) HOMO LUMO energy gap
compd
[
12]cyclacene
12]BC
110.4
82.6
−4.48
−5.15
−2.45
−2.01
2.03
3.14
[
19
regions,[n]BCswereslightlydistortedina“waggling” fashion.A
distorted hexagonal structure with C symmetry, which contains
2
closed-shell character was found for the optimized structures of
four chloroform molecules in the cavity. Considering that the
[
(
n]BCs(n=8, 12, 16, 20, 24, and28). Forexample, astrainenergy
SE)of82.6kcal·mol wasestimated for[12]BConthebasisofa
optimized structure of 1 exhibited C symmetry (Figure 3c), the
6
−1
observed slight distortion of 1 in the solid state should be
attributedtopacking interactionsbetween 1andtheincorporated
chloroform molecules. The CH Cl solution of 1 has UV
strain energy calculation method for carbon nanobelts that we
developed (see the Supporting Information for details). As this
valueiscomparabletothatof[12]cyclacene (110.4kcal·mol ),
1
7
2
2
−1
17
absorption with the peak at 296 nm and weak blue fluorescent
the strain energy of [12]BC should be derived predominantly
from the [12]cyclacene framework, and the benzannulation of
(Φ = 0.07) as shown in Figure 3d.
F
Subsequently,weattemptedtofindsuitablereactionconditions
to convert cyclic precursor 1 into [12]BC (Figure 4a), starting
from previously reported conditions for the transformation of
[
12]cyclaceneinto[12]BCshouldhaveastabilizingeffect.TheSE
of [12]BC per carbon atom is thus 1.03 kcal·mol , which is
−1
−
1
6d
20
slightly smaller than that of [9]CPP (1.21 kcal·mol ).
bis(2-biphenylyl)acetylene into dibenzo[g,p]chrysene. Un-
Consequently, the HOMO−LUMO gap increases upon
benzannulation ([12]cyclacene, 2.03 eV; [12]BC, 3.14 eV).
Therefore, [12]BC should exhibit an increased stability relative to
fortunately, a variety of reaction conditions, including treatment
with ICl followed by PdCl (PPh ) /NaOAc, TFA/TfOH
2
3 2
followed by FeCl /MeNO , AuCl/AgOTf, and PtCl /AgOTf
3
2
2
[
12]cyclacene (see the SI for other BCs).
Our strategy for the synthesis of [12]BC is shown in Figure 2.
(Ac = acetyl, Tf = trifluoromethanesulfonyl), afforded complex
mixtures, and the targeted [12]BC could not be detected by
MALDI-TOF MS in any of the obtained reaction mixtures.
During our attempts, we discovered that heating 1 in N,N-
dimethylacetamide (DMAc) to 150 °C for 2 days resulted in the
unexpected formation of 6 via a formal [2 + 1 + 2 + 1]
Based on previously reported reactions, which described the
21
cycloaddition. Compound 6 was isolated in 9% yield, and the
structure was confirmed by X-ray crystallography.
A plausible reaction mechanism for the formation of 6 from 1 is
shown in Figure 4b, in which the structures of 1 and 6 are
22
simplified to A and D, respectively, for the DFT calculations.
23
The [2 + 2] cycloaddition of two alkyne moieties in triyne A
should occur initially to afford cyclobutadiene B. Subsequently,
another alkyne moiety should react with the cyclobutadiene
moiety to form the Dewar benzene structure C via a [2 + 2]
24
Figure 2. Retrosynthesis of [12]BC.
cycloaddition. Finally, this Dewar benzene moiety should
24
isomerize via an electrocyclic ring-opening reaction to furnish
tribenzo[f,k,m]tetraphene D. The structures of A−D and two
transitionstates(TSBC andTSCD)couldbesuccessfullyoptimized
conversion of bis(2-biphenylyl)acetylene into dibenzo[g,p]-
2
0
chrysene, we designed cyclo[12]orthophenylene[6]ethynylene
1)asapossibleprecursorfor[12]BC. Asthebiphenylmoietiesin
can move flexibly, we envisage that macrocycle 1 should be
(
1
by DFT calculations, while the optimization of TS failed,
AB
23
probably due to the presence of a radical mechanism. These
results indicate that this rearrangement should be highly
unstrained. This notion is supported by DFT calculations on a
−
1
hypothetical homodesmotic reaction, which suggest a strain
exothermic (99.0 kcal·mol ) on account of the aromatic
−1
energyof0.2kcal·mol for1.Thus,weenvisionedthat1mightbe
synthesized selectively by sequential Suzuki−Miyaura cross-
coupling reactions of diphenylacetylene derivatives bearing
bromo or boryl groups.
stabilization arising from the formation of a benzene ring, and
−
1
the calculated energy barriers (B → TS : 2.6 kcal·mol , C →
CD
140 °C.
BC
−1
TS :25.1kcal·mol )arelowenoughforareactiontoproceedat
Macrocycle 1 was synthesized on the basis of this synthetic
strategy (Figure 3a). Lithiation, borylation, and hydrolysis of
bis(o-bromophenyl)acetylene (2) afforded boronic acid 3
In summary, we have proposed [n]BCs as stable zigzag carbon
nanobelts. DFT calculations revealed a closed-shell ground state
for [12]BC. The strain and the frontier orbital energy values for
B
DOI: 10.1021/acs.orglett.6b02702
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure3. (a)Synthesisof1.(b)ORTEPdrawingof1(left, sideview;right, topview)with50%probability:allhydrogenatomsareomittedforclarity. Key:
black,carbon;red,chlorine.(c)Structureof1optimizedbyDFTcalculations.(d)UV−visabsorption(solidline)andfluorescence(brokenline)spectraof
the CH Cl solution of 1.
2
2
Figure 4. (a) Formation of 6 by the thermal reaction of 1 and ORTEP drawing of 6 with 50% probability: all hydrogen atoms and solvent molecules are
omitted for clarity. (b) Plausible mechanism for the sequential transformation of triyne A (a simplified model of 1) into 9,18-diphenyltribenzo[f,k,m]-
tetraphene D (a simplified model of 6) with calculated Gibbs free energies relative to that of A.
[
12]BC implied desirably high stability. Unstrained macrocycle 1
AUTHOR INFORMATION
■
*
*
wasdesigned as apotential precursor for[12]BC, and synthesized
via sequential Suzuki−Miyaura cross-coupling reactions of the
corresponding diphenylacetylene derivatives. For the trans-
formation of 1 into [12]BC, a variety of reaction conditions
were tested. Even though these reaction conditions did not afford
Corresponding Authors
E-mail: ysegawa@nagoya-u.jp.
E-mail: itami@chem.nagoya-u.ac.jp.
Notes
[
12]BC, we observed an unexpected rearrangement of 1 into 6,
The authors declare no competing financial interest.
which exhibits a tribenzo[f,k,m]tetraphene structure. A plausible
reaction mechanism via cyclobutadiene and Dewar benzene
intermediates was proposed, which is partially supported by DFT
calculations. Further optimizations of the reaction conditions to
synthesize [n]BCs are currently ongoing on our laboratory.
ACKNOWLEDGMENTS
■
ThisworkwassupportedbytheERATOprogramfromJST(K.I.).
K.M. is a recipient of a JSPS fellowship for young scientists. We
thank Prof. Takanori Suzuki (Hokkaido University) for valuable
discussions. Calculations were performed using the resources of
the Research Center for Computational Science, Okazaki, Japan.
ITbM is supported by the World Premier International Research
Center (WPI) Initiative, Japan.
ASSOCIATED CONTENT
■
*
S
Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b02702.
REFERENCES
Experimentaldetailsandspectraofnewcompounds(PDF)
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DOI: 10.1021/acs.orglett.6b02702
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DOI: 10.1021/acs.orglett.6b02702
Org. Lett. XXXX, XXX, XXX−XXX