Benzo-fused Tri[8]annulenes as Molecular Models of Cubic Graphite

Article abstract of DOI:10.1002/anie.202106233

Cyclotrimerization of 9,10-dibromo-9,10-dihydrodibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrene with potassium tert-butoxide in the presence of a transition-metal catalyst afforded two polycyclic aromatic hydrocarbon stereoisomers consisting of three cyclooct

Full text of DOI:10.1002/anie.202106233

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How to cite:
International Edition: doi.org/10.1002/anie.202106233
German Edition: doi.org/10.1002/ange.202106233
Cubic Graphite
Benzo-fused Tri[8]annulenes as Molecular Models of Cubic Graphite
Barbara Ejlli, Pascal Nußbaum, Frank Rominger, Jan Freudenberg,* Uwe H. F. Bunz, and
Klaus Mꢀllen*
In memory of Klaus Hafner
Abstract:
Cyclotrimerization
of
9,10-dibromo-9,10-
with
stability.[10] Attempted synthesis by high-temperature carbon-
ization of hexahalobenzenes with sodium amalgam only
furnished amorphous carbon, illustrating the difficulty in
accessing the three-dimensional, crowded arrangement of
hexaphenylbenzene units.[9] Cubic graphite can be regarded
as a 3D carbon structure[10,11] consisting of hexagons and
octagons: Each benzene ring is part of three equivalent poly-
para-phenylene (PPP) chains while two biphenyl subunits of
neighboring PPP strands form eight-membered[6k,12] tetraphe-
nylene rings, rendering all of the sp2 hybridized carbon atoms
equivalent. If ever prepared, the existence of diffusion
channels, for example, for lithium ions, should qualify cubic
graphite as active material for rechargeable batteries.[10,11]
Subunits of cubic graphite were prepared via bottom-up
approaches to serve as model compounds.[11b,13] Three-dimen-
sional dendritic oligophenylenes[13b] underline the crowded
oligophenylenic nature while the tetraphenylene substruc-
tures[14] are lacking entirely. The latter are present in the
doubly helical ortho-oligophenylenes[14c,15] although the 3D
feature is missing in the linear arrangements.[16] Synthetic
strategies towards these models involve Suzuki couplings[13a]
and Diels–Alder- cycloadditions.[11b,13b] Tetraphenylene sub-
structures[10] were synthesized via thermal ring opening of
biphenylenes[17] or via Suzuki[18] or Ullmann[15c] reactions of
biphenyls, albeit in low yields.
dihydrodibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrene
potassium tert-butoxide in the presence of a transition-metal
catalyst afforded two polycyclic aromatic hydrocarbon stereo-
isomers consisting of three cyclooctatetraene (COT) moieties
connected via a central benzene ring. Both isomeric tri-
[8]annulenes were obtained selectively through the choice of
the catalyst: The a,a,a-form (Ru catalyst) displayed a threefold
symmetrywith the COT subunits forming the side walls of
a (chiral) molecular cup. In the thermodynamically more
stable a,a,b-isomer (Pd catalyst), one of the three boat-shaped
COTs was flipped over and faced the opposite molecular
hemisphere with respect to the central benzene ring as
evidenced by crystal structure analysis. Both title compounds
are small segments of “cubic graphite”, an elusive carbon
allotrope.
T
he discovery of carbon allotropes[1] such as fullerenes,[2]
carbon nanotubes,[3] graphenes[4] and, most recently, other
carbon nanostructures synthesized via on-surface chemistry[5]
has attracted great attention.[6] While the well-known forms of
carbon include diamond[7] and graphite,[8] there is another, yet
elusive 3D carbon allotrope, the so-called “cubic graphite”,
deriving its name from its symmetry and relationship to
conventional graphite. It was first proposed by Gibson et al.[9]
and predicted to possess high thermal and mechanical
Hypothetically, cubic graphite (Figure 1) would be avail-
able by multiple cyclotetramerizations of hexadehydroben-
zene or cyclotrimerizations of octadehydrocyclooctatetraene.
Therefore, we envisage a suitable model compound to result
from cyclotrimerization of a cyclooctatrienyne with sterically
congested benzene rings.
In this contribution, we report two routes towards hydro-
carbon 7, a subunit of cubic graphite, which rely on cyclo-
trimerization reactions (Scheme 1). We expected the less
crowded Cs-symmetric (a,a,b)-conformer 7a to be favored
[*] M. Sc. B. Ejlli, P. Nußbaum, Dr. F. Rominger, Dr. J. Freudenberg,
Prof. U. H. F. Bunz
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢀt Heidel-
berg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: freudenberg@oci.uni-heidelberg.de
M. Sc. B. Ejlli, Dr. J. Freudenberg
InnovationLab
Speyerer Str. 4, 69115 Heidelberg (Germany)
M. Sc. B. Ejlli, Prof. K. Mꢁllen
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202106233.
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Figure 1. A hypothetical segment of cubic graphite (eight-membered-
Part of the “Special Collection in Memory of Klaus Hafner”
rings are hightlighted in blue).
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Scheme 1. Possible synthetic approaches towards cyclotrimer 7a,b and strained cycloalkynes discussed herein. Reagents and conditions:
a) KOtBu, Pd(dba)2, PPh3, toluene, 1008C, 3 d, 42% yield 2, 50% yield 7a; b) (bromomethyl)triphenylphosphoniumbromide, NaHMDS, Et2O,
À788C—rt, 16 h, 80% yield; c) 2-biphenylboronic acid, Pd(PPh3)4, K2CO3, THF/H2O, 758C, 16 h, 64% yield; d) DDQ, Cu(OTf)2, chloroform, up to
1508C, microwave, up to 48 h; e) Br2, CHCl3, 758C, 16 h, 73% yield; f) [(C5H5)Ru(CH3CN)3]PF6, KOtBu, DCM, rt, 2 d, 29% yield 7b. Relative
stereochemistry depicted.
over the (a,a,a)-rotamer 7b, where the COT rings all point
toward the same side of the central benzene ring.
temperature, Nuckolls et al.[24] crystallized the respective
(a,a,b)-cyclotrimer with 12 benzene rings less than 7a,b
(Scheme 1) as a discrete by-product, starting from B. By
contrast, we only obtained the (a,a,b)-isomer 7a upon Pd
catalysis at 1008C. These opposing selectivities may be
a consequence of the different steric demands of the
intermediately generated cycloalkynes A (boat-shaped) and
B (approximately planar) at the catalytic center. At 1008C
under Ru-catalysis, selectivity is inverted and the more stable
(a,a,b)-cyclotrimer 7a is formed, albeit in only trace amounts
among several decomposition products. Note that both new
cyclotrimers 7a and 7b formed selectively through catalyst
and temperature control.
In a first attempt, brominated dibenzosuberenone 1 was
subjected to Pd catalyzed cyclotrimerization in the presence
of triphenylphosphine under basic conditions to furnish 2 in
42% yield. Benzannulated “tris-tropone” 2, previously pre-
pared in Ar matrices[19] or via demetallation of a platinum-
alkyne complex,[20] thus became available on the gram scale.
As its ring expansion[21] failed, 2 was transformed into
bromovinyl derivative 3 after Wittig reaction in 80% yield.
Subsequent Suzuki coupling with 2-biphenylboronic acid
furnished the hydrocarbon 4 (64% yield), which was then
subjected to an established tandem oxidative ring expansion
in the presence of Cu(OTf)2.[22] Even reactions at 1508C in
a pressurized microwave reactor for 48 h only resulted in
traces of compound 7a among its hard-to-separate inter-
mediates (see Supporting Information, section 2.1).
Both isomers failed to show thermal or photochemical
1
interconversion: Temperature-dependent H NMR spectros-
copy in tetrachloroethane from À358C to 708C exhibited
slight downfield shifts of the resonances with increasing
temperature, but did not indicate interconversion (see
Compound 7a was, however, accessed via an alternative
route: Bromination of dibenzo[3,4:7,8]cycloocta[1,2-l]phen-
anthrene (5)[22] led to dibromo derivative 6. Double HBr
elimination with KOtBu generated the corresponding cyclo-
octatrienyne A, which cyclotrimerized in situ at 1008C in the
presence of Pd(dba)2 as catalyst and triphenylphosphine as
ligand to furnish 7a in a yield of 50% (at room temperature,
only debromination was observed). The proton NMR spec-
trum of 7a (Figure 2, top) exhibited three characteristic
downfield-shifted resonances of the three different phenan-
threnylene bay protons (1, 2 and 3) at chemical shifts
exceeding 8.5 ppm (phenanthrene: 8.75 ppm in CDCl3).[23]
Strikingly, treatment of 6 under basic conditions in the
presence of [(C5H5)Ru(CH3CN)3]PF6 at room temperature
furnished a different cyclotrimer in 29% yield, albeit with
a NMR spectrum suggesting a C3 symmetry with only one
resonance for the phenanthrene bay protons (relative inten-
sity of 6 for protons a; Figure 2, bottom)—we identified the
second cyclotrimer as the (a,a,a)-isomer 7b. When attempt-
ing olefin metathesis under similar conditions at room
1
Figure 2. Aromatic region of the H NMR spectra of 7a (top,
600 MHz) and 7b (bottom, 600 MHz) in CD2Cl2.
2
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Supporting Information for details). Thermal isomerization
clooctatetraene (84.4–85.18),[30] which is also observed in case
was also not observed after keeping neat 7a and 7b at 4008C
for 10 minutes. Photoirradiation of solutions of 7a and 7b in
cyclohexane in a Rayonet photoreactor (300 nm) under inert
atmosphere for 24 h provided only starting materials.
Ring inversion of benzannulated cyclooctatetraenes
occurs via planarization[25] or a twisted, non-planar transition
state[26] (symmetry reduction), which minimizes the ortho,
ortho’ hydrogen interactions—however the barrier remains as
high as 75.8 kcalmolÀ1 for (in comparison to compounds 7a,b
more flexible and less crowded) tetraphenylene.[26c]
A simple molecular model of 7a,b (see Supporting
Information) with one manually planarized eight-membered
ring illustrates the repulsion of neighboring protons (b, g, h
etc.) in the fjord regions and supports the difficulty of
interconverting 7a,b. Density functional calculations (DFT,
B3LYP/6–311G**) show that the (a,a,b)-isomer 7a is 16 kcal
molÀ1 more stable than the (a,a,a)-isomer 7b (see Supporting
Information, section 2.7 for details).
of the single crystals of 7b (87.1–98.58).
Here, however, all three boat-shaped eight-membered
rings face the same side with respect to the central benzene
ring (Figure 3, right). The COT subunits form the sidewalls of
chiral molecular cups—l- and d-enantiomers are present in
the crystal due to the head-to-face alignment of the non-
planar phenanthrylenes. The resulting triangle-shaped cav-
ities should be able to host a suitable alkali cation upon
reduction, as their diameter is about 2.2 ꢀ (Method 1) and
2.7 ꢀ (Method 2) (see Supporting Information, section 2.6 for
details), illustrating a feature of cubic graphite.
Both 7a and 7b are thus valid three-dimensional model
compounds of cubic graphite (cf. Figure 1). They consist of
sterically congested alternating benzene and cyclooctate-
traene rings. Whereas 7a illustrates the expansion in three-
dimensional space slightly better due to the different ori-
entation of the boat-shaped COTs, 7b models the cavities
within cubic graphite.
The absorption spectra of 7a/7b were essentially indis-
tinguishable as expected for these conformers (see Support-
ing Information, Figure S38). The least intense absorption
features at long wavelengths (lmax,abs = 356/358 nm) were
similar to those of phenanthrene,[27] the largest p-conjugated
subunit in both compounds—COT-annulation did not lead to
a pronounced red-shift.[28] 7a and 7b fluoresced violet-blue at
lmax,em = 360/365 nm.[29]
Single crystal analysis of 7a unambiguously confirmed the
formation of the p-extended tri[8]annulene consisting of
three boat-shaped COT moieties. One of them faces the
opposite molecular hemisphere with respect to the central
benzene ring compared to the other two (Figure 3, left). The
phenanthrene moieties are oriented nearly perpendicular
(80.4–91.38) to the benzene core. These angles deviate from
those spanned by the opposing phenylenes in tetrabenzocy-
In conclusion, we synthesized the two non-interconverting
conformers 7a and 7b as model compounds of cubic graphite
in a selective fashion through catalyst control from an in situ
generated strained cycloalkyne A. An alternative strategy via
ring expansion by oxidative rearrangements of seven-mem-
bered rings was not successful. Yet, a simple gram scale
synthetic route to benzannulated “tris-tropone” 2 became
possible, a highly attractive building block for negatively
curved polycyclic hydrocarbons. Future challenges include i)
p-extension of model compounds 7a, b toward higher
homologues, ii) electron transfer reactions to explore the
resulting anion structures and iii) host-guest chemistry of the
“molecular cup” 7b. A quantum-chemical study of the
cyclotrimerization reaction mechanisms involving A (and B)
may provide mechanistic insights into the surprising selectiv-
ities.
Acknowledgements
We thank Steffen Maier and Marvin Nathusius for assistance
with quantum-chemical calculations. We thank Kerstin Brçd-
ner for preliminary experiments on cyclotrimerization to
afford the benzannulated “tris-tropone” 2. Open access
funding enabled and organized by Projekt DEAL.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: carbon allotrope · cubic graphite · cyclooctatetraene ·
cyclotrimerization · molecular cup
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[29] Note that we were unable to compare optical gaps to electro-
tionszentrum Karlsruhe Access Structures service www.ccdc.
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[31] Deposition Numbers 2080525 (for 7a), 2080526 (for 7b), and
2080527 (for 3) contain the supplementary crystallographic data
for this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinforma-
Manuscript received: May 8, 2021
Revised manuscript received: June 11, 2021
Accepted manuscript online: June 22, 2021
Version of record online: && &&, &&&&
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Communications
Cubic Graphite
B. Ejlli, P. Nußbaum, F. Rominger,
J. Freudenberg,* U. H. F. Bunz,
K. Mꢁllen*
&&&& — &&&&
Benzo-fused Tri[8]annulenes as
Molecular Models of Cubic Graphite
Two benzo-fused tri[8]annulenes were
synthesized via cyclotrimerization reac-
tions, introducing two valid model com-
pounds of the yet elusive carbon allotrope
cubic graphite. Both conformers do not
interconvert via temperature- or light-
induced stimuli.
6
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ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!