Gram-Scale Synthesis and Supramolecular Complex of Precursors of Clar's Hydrocarbon Triangulene

Article abstract of DOI:10.1021/acs.orglett.9b02683

We present to date the most efficient gram-scale synthesis of triangulene-4,8-dione and 12-hydroxytriangulene-4,8-dione, the precursors of Clar's hydrocarbon, in overall yields >50%. The direct dihydroprecursors of triangulene, obtained upon reduction of

Full text of DOI:10.1021/acs.orglett.9b02683

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Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Gram-Scale Synthesis and Supramolecular Complex of Precursors of
Clar’s Hydrocarbon Triangulene
†
,†
,†,‡
̌
́
̌
̌
Peter Ribar, Tomas Solomek,* and Michal Jurícek*
†Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
‡Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: We present to date the most efficient gram-
scale synthesis of triangulene-4,8-dione and 12-hydroxytrian-
gulene-4,8-dione, the precursors of Clar’s hydrocarbon, in
overall yields >50%. The direct dihydroprecursors of
triangulene, obtained upon reduction of triangulene-4,8-
dione, were stabilized in a supramolecular complex with a
tetracationic cyclophane ExBox4+ and characterized by single-
crystal X-ray crystallography. This result represents the first step in an endeavor to stabilize the fragile core of triangulene in an
inclusion complex in solution and solid state.
Scheme 1. Previous Strategies to Triangulene Precursors
he “odd” nature of triangulene has provoked the minds of
1
Tchemists ever since the proposal of Eric Clar in 1950s.
On account of high reactivity, the synthesis of this compound
known as Clar’s hydrocarbon was not successful until 2001,
when Kubo et al. confirmed for the first time the triplet ground
state of its trisubstituted derivative by EPR spectroscopy in
solution.2 After 10 years of (EPR) silence, the chemistry of
triangular graphene fragments started to flourish again, and this
year, triangulene is featured in the logo of the Symposium on
Novel Aromatic Compounds (ISNA) held in Sapporo, Japan.
The most recent breakthrough is the imaging of the naked
triangulene (2017) and its π-extended homologues (2019) on
various surfaces by using STM/AFM techniques (Figure 1).3
5
3a
̌
Bushby et al. (1995) and Pavlicek et al. (2017). None of
them, however, surpassed the synthesis of Clar in terms of
overall chemical yields.
The oxo-substitution of the triangulene core endows the
neutral trioxotriangulene (TOT) radical derived from 2 with
chemical stability. This stabilization allowed the exploration of
the rich redox properties of TOTs,6 which can store up to four
electrons reversibly, and the use of this system as a molecular
component of cathode-active materials in organic batteries.
The tri-tert-butyl TOT exhibited a high discharge capacity of
more than 300 A h kg−1, exceeding those delivered by Li-ion
Figure 1. Recent breakthroughs in the chemistry of triangulene.
The first attempts to synthesize triangulene were carried out
by Clar. His efforts using the old-days aromatic hydrocarbon
chemistry gave rise to two oxo-functionalized precursors 1 and
2 (Figure 1) accessible in eight and seven steps with 29% and
36% yields, respectively (Scheme 1).4 Despite the incredible
synthetic methodology advancements in the past 60 years, only
two shorter pathways have been reported since then by
Received: July 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02683
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Organic Letters
Letter
a
batteries. The unpaired electron in TOT radicals is responsible
for the formation of one-dimensional assemblies in the solid
state on account of weak π-stacking. Such assemblies show
semiconducting properties with relatively high conductivities
of ∼10−3 S cm−1. Mixing of a sample of TOT radical with a
sample of its reduced form results in a purely organic mixed-
valence salt, exhibiting conductivity that can reach up to 125 S
cm−1. It has also been demonstrated that other TOT
derivatives can act as promising metal-free electrocatalysts in
the oxygen-reduction reaction. We showed that the tri-tert-
butyl derivative of 2 can easily be converted into donor−
acceptor dyads with rare layered solid-state packing motifs.7
Finally, because of the extra stabilization provided by the
peripheral oxygen atoms, the doubly reduced conjugate base of
2 was the first TOT derivative with a triplet ground state
detected by EPR spectroscopy.8
Scheme 2. Synthesis of Triangulene Precursors 1, 2, and 3
Considering the importance of these achievements, both
fundamentally and from the standpoint of applications, there is
a need for a high-yielding few-step synthesis of 1 and 2 that can
be performed on a gram scale. The common feature of all
previously published synthetic strategies (Scheme 1 and
Schemes S1−S3) is the preparation of 3,3-di-o-tolylisobenzo-
furan-1(3H)-one. This precursor represents the point in
̌
synthesis where the approaches of Bushby and Pavlicek
shorten the original sequence of Clar, although they
significantly forfeit the yield. The subsequent oxidation of
the two methyl groups in this precursor to carboxylic acids is
required to assemble the triangulenedione core in a final acid-
catalyzed cyclization. Each strategy uses rather harsh
̌
(KMnO4/reflux; Bushby, Pavlicek) or even dangerous
(HNO3/200 °C; Clar; precursor on the right in Scheme 1)
oxidation conditions. In fact, when we were reproducing the
synthetic sequence of Clar, we encountered an explosion
during this step in our attempt to oxidize the methyl groups
using a pressure flask (Figure S1). A similar dangerous
outcome was previously reported in the literature by Bushby et
al.9 For this reason, we strongly discourage even skilled
synthetic organic chemists to use this method.
a
Reaction conditions: (a, 1) n-BuLi, THF, −78 °C, 2 h; (a, 2)
(EtO)2CO, rt, 7 days, 74%; (b) CrO3, H2SO4, acetone, reflux, 12 h,
77% (5), 21% (6), 81% (10); (c) H2O2, NaH2PO4, HCl, NaClO2,
MeCN, 10 °C, 6 h, 99%; (d) Cu, H2SO4, 120 °C, 93% (1), 98% (2);
(e) LiAlH4, AlCl3, Et2O/THF, 3.5 h, 11% (3 cycles); (f) Me2SiCl2,
MeCN, rt, 2 h, 99%; (g) Li, C10H10, THF, rt, 14 h, 94%; (h) H2SO4,
120 °C, 2 h, 82%.
To avoid these issues, we designed a new synthesis of 1 and
2 (Scheme 2), which employs an intermediate that is accessible
in a single high-yielding step and which can be oxidized using a
milder oxidant. This intermediate, a C3-symmetric tetraalcohol
4, could be successfully obtained in a very good 74% yield
through (1) a lithium−halogen exchange in commercially
available 2-bromobenzylalcohol with two equiv of n-BuLi,
followed by (2) a 3-fold nucleophilic addition of the resulting
aryl lithium salt to diethylcarbonate. This reaction can be easily
upscaled to ∼50 g of the starting 2-bromobenzylalcohol
without decreasing the yield. Compound 4 does not require
purification by column chromatography and can be obtained
pure by simple recrystallization from ethyl acetate.
The structure of 4 allows for using a milder oxidant to
convert the three hydroxymethyl groups into the correspond-
ing carboxylic acids. Our initial attempts with Jones oxidation
provided a mixture of 5 and 6. While the latter is a byproduct
that cannot be oxidized to the desired compound 7, the former
possesses a formyl group in place of the expected carboxylic
acid. This unexpected result was, however, a fortunate reaction
outcome because 5 can be converted directly to 1, a reduced
form of 2 without the hydroxyl group in position 12. The
optimized conditions of the Jones oxidation provided 5 in 77%
(3.7 g) and 6 in 21% yield. This oxidation is the only step in
our sequence that requires purification by column chromatog-
raphy. Subsequent Friedel−Crafts acylation of 5, employing
concentrated sulfuric acid as the solvent and copper as a
reducing agent,4 resulted in the formation of 1 in excellent 93%
yield (1.9 g). This three-step procedure with an overall 53%
yield is the shortest and best-yielding synthesis of 1 to date.
Similar cyclization to form 2 requires that the formyl group
in 5 is first oxidized to the carboxylic acid. This step was
performed with a NaClO2/H2O2 mixture,10 yielding 7 in a
nearly quantitative yield (99%, 3.1 g) without the need for
purification. Finally, cyclization using Cu/H2SO4 afforded 2,
which was obtained pure upon filtration also almost
quantitatively (98%, 2.6 g). This four-step synthetic route
gives 2 in an overall 55% yield, which is ∼1.5 times higher than
that of the best available seven-step synthesis of Clar.4 Because
of low solubility, the structures of both 1 and 2 were confirmed
by 1D/2D NMR spectroscopy in D2SO4, including the full
assignment of 1H and 13C resonances reported here for the first
time (see the Supporting Information).
Since our attempts to prevent the formation of 6 in the
Jones oxidation of the key tetraalcohol 4 were not successful,
we explored an alternative path (Scheme 1, bottom). The key
was to avoid the formation of the acetal moiety in 6 by
reducing the central hydroxy group in 4. First, ether
intermediate 8 was synthesized upon treatment of 4 with
B
DOI: 10.1021/acs.orglett.9b02683
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Letter
dimethylsilyl dichloride11 in almost a quantitative yield. Then,
8 was reduced by lithium naphthalenide12 in 94% yield to triol
9. Subsequent Jones oxidation provided the tricarboxylic acid
10 in a considerably better yield (81%) than the method of
Bushby et al. (54%),5 which required oxidation of two methyl
groups by potassium permanganate to form 10. The final 3-
fold Friedel−Crafts acylation afforded 2 in 82% yield. Although
this alternative route added an extra synthetic step, its overall
yield of 46% is superior to any of the previously reported
syntheses.
phane ExBox4+ as a promising candidate.16 ExBox4+ is known
to scavenge electron-rich polyaromatic hydrocarbons from
their solutions on account of a favorable donor−acceptor
interaction in the formed host−guest complexes, the strength
of which correlates with the number of π-electrons.
Dihydroprecursors 3 possess 20 π-electrons each and are
expected to form a stable host−guest complex with ExBox4+.
Indeed, formation of the 3⊂ExBox·4PF6 complex was
1
confirmed by H NMR spectroscopy in solution (Figure 2)
and X-ray crystallography in the solid state (Figure 3).17
To obtain the direct precursor of pristine triangulene,
ketotriangulenes 1 or 2 must be further reduced. Reduction of
1 to a mixture of 3a and 3b was reported previously twice.3a,13
We felt, however, that both the method and the available
spectroscopic data to characterize 3 were not sufficiently
described, or the samples of 3 were produced in an
unsatisfactory purity because of the sensitivity of 3 to oxygen.
Our initial attempts to reduce 1 using original conditions
AlCl3/LiAlH4 revealed that this protocol leads only to small
quantities of the desired product. It appeared to us that the
reason is the low solubility of 1. As the reaction progressed, the
surface of undissolved solid 1 would be covered by aluminum
oxide, a reaction byproduct, which suppressed the reduction to
proceed further. To partially overcome this problem, 1 was
added into the mixture of AlCl3/LiAlH4 in small portions. The
next important part of the protocol was to isolate soluble red
byproducts, presumably various hydroxyl triangulenes formed
upon contact with oxygen, during the column chromatography.
Even though the presence of oxygen was minimized during the
purification steps, formation of these byproducts could not be
avoided. They were subjected again to the same reduction
conditions, which afforded additional amounts of the desired
product. After three cycles, the combined product samples
were purified once again by oxygen-free column chromatog-
raphy, to afford a pure sample of 3 in 11% overall yield.
Although this yield is lower than the reported ones, our
method is reliable for obtaining very pure material. The NMR
spectroscopy revealed that 3 is formed as a 10:1 mixture of two
inseparable dihydro isomers 3a and 3b. The experimental ratio
of 10:1 is in an excellent agreement with the populations of
89% and 11% for 3a and 3b, respectively, calculated from our
DFT data. Both compounds were fully characterized by 2D
1
Figure 2. Structures of 3a, 3b, and ExBox·4PF6 (top) and H NMR
spectra (CD3CN, 25 °C; bottom) of ExBox·4PF6 and its 1:1 and 1:2
complexes with 3 (3a/3b ∼ 10:1). Intensity of the parts of spectra on
the right from the dashed line was increased with respect to the left
part for clarity.
1
NMR spectroscopy, including the full assignment of H and
13C resonances, for the first time (see the Supporting
Information).
It was reported by Kubo et al. that three tert-butyl groups in
the apexes of triangulene improved its kinetic stability and
permitted recording an EPR spectrum in a frozen toluene
matrix at 123 K.2 An intermediate increase of the temperature
resulted, however, in a rapid oligo-/polymerization of the
diradical in the sample. A better steric protection of triangulene
would require introduction of bulky substituents at positions 4,
8, and 12, which is not a straightforward synthetic task because
of the unexpected chemical reactivity of π-extended aromatic
ketones such as 1 and 2.14 An alternative strategy to prevent
bimolecular reactions of triangulene in solution is an inclusion
of the fragile triangulene core into a large protective molecular
shell or, in other words, stabilizing triangulene in the form of a
supramolecular host−guest complex. Such an approach has
previously enabled kinetically stabilizing a number of well-
known reactive intermediates.15 The first step in this endeavor
is a successful formation of a supramolecular complex of
triangulene precursors 3. We selected a tetracationic cyclo-
Figure 3. Different crystallographic views of the solid-state structure
of 3⊂ExBox·4PF6.17 The top part of ExBox4+ in the structure on the
left is not displayed for clarity. Thermal ellipsoids are shown at a 50%
probability level.
1
In the H NMR spectrum of a 1:1 mixture of 3 and ExBox·
4PF6 in CD3CN, upfield shifts of β and γ protons and a
downfield shift of the C6H4 protons for ExBox·4PF6 as well as
upfield shifts for all signals of 3a and 3b were observed. These
shifts are in agreement with those observed for inclusion of
aromatic guests inside ExBox4+. X-ray diffraction (XRD)
analysis of the single crystal of 3⊂ExBox·4PF6, obtained by a
slow vapor diffusion of diisopropyl ether into the acetonitrile
C
DOI: 10.1021/acs.orglett.9b02683
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Organic Letters
Letter
solution of 3 and ExBox·4PF6, confirmed that 3 resides inside
the cavity of ExBox4+. A comparison of the carbon−carbon
bond lengths of 3a (the major species) in the complex with
those obtained from DFT calculations suggests that the
prevalent of the three possible orientations of 3a inside the
cavity is the one shown in Figure 3 (the longest Csp3−Csp2
bonds are in the range 1.45−1.47 Å by XRD and 1.50−1.52 Å
by DFT). This orientation agrees well with the prevalent
orientation in solution: the largest upfield shift is observed for
protons 5 and 11, smaller shift for protons 7, 8, and 9, and only
a very small shift for protons 1 and 2 (Figure 2, orange lines).
The association constant for the binding of 3 inside ExBox4+
elucidated by NMR titration (Figure S3) is ∼0.8 × 104 M−1, in
between those of 18-π-electron [4]helicene (∼0.6 × 104 M−1)
and 20-π-electron perylene (∼9 × 104 M−1).16 Although the
affinity of 3 toward ExBox4+ is relatively high, so is the rate
constant for the guest-exchange process, judging from the
presence of only one set of averaged signals for the bound and
unbound species (both 3 and ExBox4+) in the NMR spectrum
at room temperature. As a result, our initial attempts to
generate triangulene trapped in the supramolecular complex in
solution at room temperature using oxidants such as chloranil
̌
Michal Jurícek: 0000-0001-5595-431X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (Grant Agreement
716139), the Swiss National Science Foundation (SNSF,
T . S . / P Z 0 0 P 2 _ 1 7 4 1 7 5 , M . J . / P Z 0 0 P 2 _ 1 4 8 0 4 3 ,
PP00P2_170534), and Experientia Foundation (T.S.). We
would liketo thank our colleagues at the University of Basel,
namely, PD Daniel Haussinger (2D NMR), Dr. Markus
Neuburger and Dr. Alessandro Prescimone (XRD), and Prof.
Marcel Mayor for supporting our research. We also acknowl-
edge Leos Valenta (University of Zurich) for help with NMR
̌
̌
̈
̌
measurements.
DEDICATION
■
birthday.
́
Dedicated to Miroslav Kozak on the occasion of his 60th
t
or non-nucleophilic bases (NaH, BuOK) were unsuccessful.
Increasing the activation energy preventing triangulene from
escaping from its inclusion complex is thus inevitable. A
pursuit of such a goal is in progress in our laboratories.
In conclusion, we presented the most efficient synthesis to
date of triangulene-4,8-dione and 12-hydroxytriangulene-4,8-
dione. Both compounds were synthesized on gram scales in
overall yields >50% using a common route with only a single
purification by column chromatography. Our simple approach
promises a rapid access to C3-symmetrically oxo-substituted
triangulenes. We also proposed a new strategy to stabilize
triangulene in a solution or solid state within a supramolecular
complex. By means of NMR spectroscopy and single-crystal X-
ray diffraction, we demonstrated the first step of this strategy,
namely, the complexation of ExBox·4PF6 and the direct
precursor of triangulene, the synthesis of which was here
optimized to produce samples of high purity.
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02683.
(6) (a) Morita, Y.; Nishida, S.; Murata, T.; Moriguchi, M.; Ueda, A.;
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Shiomi, D.; Sato, K.; Takui, T. Bull. Chem. Soc. Jpn. 2018, 91, 922−
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Kariyazono, K.; Moriguchi, M.; Okamoto, H.; Morita, Y.; Nakai, H.
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Synthetic procedures and characterization data for all
new compounds (PDF)
Accession Codes
CCDC 1943899 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
̌
̈
(7) Ribar, P.; Solomek, T.; Le Pleux, L.; Haussinger, D.; Prescimone,
̌
A.; Neuburger, M.; Jurícek, M. Synthesis 2017, 49, 899−909.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: tomas.solomek@unibas.ch.
*E-mail: michal.juricek@chem.uzh.ch.
ORCID
(13) Hara, O.; Tanaka, K.; Yamamoto, K.; Nakazawa, T.; Murata, I.
Tetrahedron Lett. 1977, 18, 2435−2436.
̌
̌
́
Tomas Solomek: 0000-0003-0013-4116
D
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(14) (a) Ribar, P. Doctoral dissertation, University of Basel, 2019.
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Furukawa, K.; Wu, J. Chem. Sci. 2014, 5, 1908−1914.
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̌
(16) (a) Barnes, J. C.; Jurícek, M.; Strutt, N. L.; Frasconi, M.;
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L.; Sarjeant, A. A.; Stoddart, J. F. J. Am. Chem. Soc. 2013, 135, 183−
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Stoddart, J. F. J. Org. Chem. 2013, 78, 11962−11969. (c) Dale, E. J.;
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(17) Crystal parameters for 3⊂ExBox·4PF6: [C22H14⊂C48H40N4·
4(PF6)·2.8(C2H3N)]; orange plate, 0.16 × 0.14 × 0.03 mm; triclinic,
̅
P1 (no. 2); a = 10.1937(7), b = 10.6981(8), and c = 18.2104(15) Å; α
= 96.700(5), β = 106.062(3), and γ = 107.208(3)°; V = 1779.80(14)
Å3; Z = 1; T = 123 K; ρcalc = 1.536 g cm−3; μ = 1.994 mm−1; S =
1.1042. CCDC no. 1943899. Molecules of 3 show rotational
displacement disorder inside the cavity of ExBox4+ (Figure S2).
E
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