Zirconacyclopentadiene-Annulated Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1002/anie.201700818

Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach

Full text of DOI:10.1002/anie.201700818

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International Edition: DOI: 10.1002/anie.201700818
German Edition: DOI: 10.1002/ange.201700818
Quinodimethanes Very Important Paper
Zirconacyclopentadiene-Annulated Polycyclic Aromatic
Hydrocarbons
Gavin R. Kiel, Micah S. Ziegler, and T. Don Tilley*
Abstract: Syntheses of large polycyclic aromatic hydrocarbons
(PAHs) and graphene nanostructures demand methods that
are capable of selectively and efficiently fusing large numbers
of aromatic rings, yet such methods remain scarce. Herein, we
report a new approach that is based on the quantitative
intramolecular reductive cyclization of an oligo(diyne) with
a low-valent zirconocene reagent, which gives a PAH with one
or more annulated zirconacyclopentadienes (ZrPAHs). The
efficiency of this process is demonstrated by a high-yielding
fivefold intramolecular coupling to form a helical ZrPAH with
16 fused rings (from a precursor with no fused rings). Several
other PAH topologies are also reported. Protodemetalation of
the ZrPAHs allowed full characterization (including by X-ray
crystallography) of PAHs containing one or more appended
dienes with the ortho-quinodimethane (o-QDM) structure,
which are usually too reactive for isolation and are potentially
valuable for the fusion of additional rings by Diels–Alder
reactions.
to regioselectivity and functional-group tolerance.[4,13] Given
the ability of transition metals to facilitate selective and high-
yielding transformations, they provide a promising platform
for the challenging ring-fusion step. Although the develop-
ment of organometallic methods (e.g., ring-closing alkene
metathesis[14] and [2+2+2] cycloadditions[15,16]) has received
much attention,[17] few are efficient enough for application to
very large PAHs.
We have previously reported the synthesis of p-conju-
gated oligomers,[18] polymers,[19] and macrocycles[20] through
the reductive coupling of alkynes with a low-valent zircono-
cene reagent. Given its ability to form new rings in high yield,
this reaction should be well-suited for PAH syntheses.
Takahashi and co-workers demonstrated its application to
the iterative elongation of an acene (Scheme 1a); however,
T
he isolation of graphene and the elucidation of its extra-
ordinary properties (e.g., ballistic charge transport, mechan-
ical strength, transparency, and flexibility) have motivated
a surge of research on large polycyclic aromatic hydrocarbons
(PAHs), which can be considered to be basic building blocks
of graphene and other carbon-rich nanostructures.[1,2] Thus
significant attention has focused on PAHs as molecular
models for, or synthetic precursors of, this promising class
of materials.[3–5] Large PAHs are also attractive as compo-
nents of electronic and optoelectronic devices.[6–9] Their
suitability for this purpose is due not only to their unique
electronic and photophysical properties,[10] which result from
their extended conjugation and rigidity, but also to their
tendency to assemble into highly ordered structures in the
solid state.[11]
A central challenge for investigations of large PAHs is the
difficulty associated with the synthesis of pure, well-defined
structures, which requires the selective fusion of many
aromatic rings.[12] Near-quantitative yields are necessary for
each ring-fusion event to avoid difficult separations and/or
defects. To date, the most successful approach is based on the
cyclodehydrogenation reaction popularized by Mꢀllen and
co-workers, but there are significant limitations with respect
Scheme 1. Previously reported approaches for the synthesis of PAHs
with zirconacyclopentadiene and rhodacyclopentadiene synthons.
Cp=cyclopentadienyl.
the strategy is lengthy and requires a post-ring-fusion
oxidation step.[21] Wang and co-workers employed zirconacy-
clopentadienes in the annulation of naphthalene diimides, but
alkyne coupling was not involved in the ring-fusion event
[*] G. R. Kiel, M. S. Ziegler, Prof. T. D. Tilley
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
(Scheme 1b).[22]
A related and potentially far-reaching
approach was indicated more than 40 years ago by
Mꢀller,[23] who used diynes with p-conjugated tethers to
access several fully unsaturated fused-ring systems via
a rhodacyclopentadiene synthon (e.g., A; Scheme 1c). Two
promising features of this chemistry are that 1) a PAH is
E-mail: tdtilley@berkeley.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700818.
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directly obtained from the precursor (e.g., no subsequent
oxidation is required) and 2) functionality can be divergently
introduced using metal-transfer reactions. Surprisingly, this
approach has been employed for only a limited number of
PAHs, which were synthesized via organometallic intermedi-
Figure 1. Resonance structures of o-QDM and a benzannulated
o-QDM.
ates containing
a single annulated metallacyclopenta-
diene.[24,25]
Herein, we report a generalization of Mꢀllerꢁs approach,
enabled by the high efficiency of zirconocene coupling.
Specifically, the quantitative intramolecular reductive cycli-
zation of an oligo(diyne) (B; Scheme 2a) with a low-valent
examples (2a–2c and 2g–2k). These isolable o-QDMs are not
only interesting from a fundamental perspective but they can
also be selectively hydrogenated using a simple procedure to
produce highly alkylated PAHs.
Investigations began with the model system shown in
Scheme 3, which involves the reductive coupling of a diyne
(3–7) to form a phenanthrene annulated with a single
zirconacyclopentadiene (ZrPAHs 1a–1e). There were two
initial goals, namely 1) to establish efficient conditions for the
zirconocene coupling, which needs to be very high-yielding to
be applied in multifold coupling reactions, and 2) to deter-
Scheme 3. Investigations on a model system. * Yields of isolated
products. ZrPAHs 1a and 1c–1e were formed quantitatively according
to 1H NMR spectroscopy (the formation of 1b was not monitored).
Scheme 2. Synthetic strategy explored in this work and accessible
PAHs.
zirconocene reagent gives a zirconacyclopentadiene-annu-
lated PAH (“ZrPAH”, 1). Eleven ZrPAHs (1a–1k; Sche-
me 2b), representing seven different PAH topologies, are
presented, including one with 16 fused rings and five
appended zirconacyclopentadiene rings (1k). Importantly,
these ZrPAHs result from precursors that have no fused rings
mine the suitability of these ZrPAHs as precursors to
o-QDMs, which might then be employed in Diels–Alder
reactions for fusion of additional rings. Towards this end,
ZrPAH 1a was isolated in 92% yield after treatment of diyne
3 with Cp2Zr(pyr)(Me3SiC CSiMe3) (1.1 equiv),[27] a conven-
ꢀ
ient, well-defined source of “Cp2Zr”.[28] Monitoring by
1H NMR spectroscopy revealed that this reaction is rapid
(complete in < 5 min) and quantitative.
À
and could thus be easily assembled using established C C
bond-forming reactions. Furthermore, high-yielding and ste-
reoselective protodemetalation of 1a–1k was exploited as
a novel entry to several PAHs with exocyclic diene functional
groups (2a–2k). Such compounds might be expected to
exhibit high reactivity, as is typically the case for dienes with
the ortho-quinodimethane (o-QDM; Figure 1) structure.[26]
However, they are remarkably stable when incorporated
into a PAH framework (with certain R substituents), which
allowed the isolation and full characterization of eight rare
Treatment of 1a with excess HCl (5 equiv) produced
a surprisingly persistent species consistent with 2a (as judged
by a characteristic quartet at d = 5.7 ppm and the correspond-
ing doublet at d = 1.8 ppm for the vinyl and methyl protons,
respectively) in nearly quantitative yield. Observation of this
species despite the presence of excess strong acid is remark-
able; however, 70% of 2a decomposed to an unidentified
product within 6 h. Replacement of HCl with benzoic acid
2
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produced 2a in similarly high yield, and there were no signs of
its decomposition after storage for one week in [D6]benzene
solution.
The stability of 2a in solution motivated its isolation as
examples of isolable o-QDMs are very rare and invariably
involve substitution patterns that preclude their use as
synthetic intermediates.[29,30] Treatment of 1a with excess
benzoic acid afforded pure 2a as a colorless crystalline solid
after a simple filtration of the reaction mixture diluted with
hexanes through a plug of silica gel. The stability of 2a
permitted its full characterization, including by X-ray crys-
tallography, which revealed its “twisted” nature in the solid
state (Figure 2a). This twist can be quantified by two angles:
1) the torsion angle of the exocyclic diene unit (51.38) and
2) the twist angle of the biphenyl backbone (19.88). Com-
pound 2a can be handled for a few hours in air (in solution or
as a solid) without detectable decomposition, but prolonged
exposure to air at ambient temperature (ca. 218C) leads to
slow decomposition, probably by O2-catalyzed oligomeriza-
tion. The solid can be stored indefinitely under a nitrogen
atmosphere at ambient temperature. In the absence of O2,
ring closure to cyclobutarene 8 (Scheme 3) is the major
decomposition pathway, but heating to 1008C for 24 h was
required for complete conversion. In contrast, the unsubsti-
tuted analogue 9 could not be isolated and has been reported
to rapidly decompose by [4+2] dimerization.[31]
Figure 2. Crystal structures of 2a, 2g, 2h, and 2i. Thermal ellipsoids
set at 50% probability.
The analogous n-propyl-substituted o-QDM 2b was
prepared in 95% yield by in situ generation of 1b using
Negishiꢁs Cp2ZrCl2/nBuLi reagent,[32] followed by treatment
with aqueous HCl. This high yield was possible despite the use
of excess strong acid, but short reaction times (< 10 min) were
required. Successful isolation of SiMe3-substituted o-QDM 2c
required stoichiometric HCl as this compound rapidly reacted
with excess HCl to form an unidentified product. Employ-
ment of benzoic acid gave a mixture containing only a small
amount of 2c (ca. 10%).
An attempt to isolate diaryl-substituted o-QDM 2d
instead led to cyclobutarene 10 as the sole product upon
protodemetalation of 1d with HCl, likely through electro-
cyclic ring closure of 2d. Treatment with benzoic acid gave
a complex mixture. An intermediate case was mono-aryl-
substituted o-QDM 2e, which cyclized
lation led to rapid (< 5 min) 1,4-addition of HCl to yield 13 as
the major product. The intermediacy of 2 f was supported by
the isolation of Diels–Alder adduct 14 in 76% yield (in one
pot, from 12) upon protodemetalation in the presence of
N-ethylmaleimide at 218C. In contrast, the analogous Diels–
Alder reaction with 2a (Scheme 4) required elevated temper-
ature to proceed at a reasonable rate (708C for 15 h). The low
reactivity of 2a in the presence of a good dienophile is
surprising, but the adduct 15 forms quantitatively and was
isolated in high yield (90%). Compound 13 is not a precursor
to 14 as addition of N-ethylmaleimide after generation of 2 f
resulted only in isolation of 13. Importantly, the efficient one-
pot reaction to form 14 from 12 suggests that this method
to 11 with a half-life of about two days
at 198C upon protodemetalation of 1e
with HCl. The intermediate 2e was
1
identified by H NMR spectroscopy in
93% yield. It is well known that aryl
groups lower the barrier for this rever-
sible ring-closing reaction.[33]
Thiophene-annulated o-QDM 2 f
(Scheme 4) was predicted to be more
reactive than its benzannulated ana-
logue 2a for two reasons, namely 1) the
diminished steric interaction between
the methyl groups and annulated rings
and 2) the lower aromaticity of the
annulated thiophene rings. Indeed,
generation of ZrPAH 1 f from diyne
Scheme 4. Diels–Alder reactivity of the thiophene-annulated o-QDM 2 f and comparison with its
12 followed by its in situ protodemeta- benzannulated analogue 2a.
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should also be of value for more traditional, reactive o-QDM
chemistry.
The extension of this approach to larger polyaromatic
systems by multifold coupling reactions began with precursor
bis(diyne)s 16–18 (Scheme 5). These compounds were easily
accessible by Suzuki couplings (see the Supporting Informa-
greater interest. Diels–Alder reactions are the obvious
choice; however, initial difficulties with the aromatization of
15 prompted the establishment of an alternative route to
valuable functionality. A 1,4-addition of H2 across the diene is
potentially the simplest transformation and would give
unique PAHs decorated with alkyl groups. Such functionality,
which is currently difficult to install, improves solubility and
often promotes liquid crystallinity, with minimal perturbation
of electronic properties. A selective 1,4-addition was observed
upon exposure of 2a, 2b, and 2g–2i to 1 atm of H2 over Pd/C
to furnish the alkylated PAHs 19–23 in good to very high
yields (Scheme 6). A wide range of alkylated PAHs are now
within reach using this approach.
Scheme 6. Hydrogenation of 2a, 2b, and 2g–2j.
As a test for the applicability of the method to more
demanding multifold couplings, PAHs containing three and
five o-QDM units were targeted (2j and 2k; Scheme 7).
Remarkably, treatment of tris(diyne) 24 and pentakis(diyne)
Scheme 5. Twofold couplings for the synthesis of compounds 2g–2i.
ꢀ
tion). Methyl substituents were chosen to simplify structural
analysis and promote the crystallinity of the target bis(o-
25 with Cp2Zr(pyr)(Me3SiC CSiMe3 (3.3 and 5.5 equiv),
followed by excess benzoic acid, furnished 2j and 2k in
excellent yields (89 and 83%) after isolation. Structural
evidence was provided by the characteristic 1H NMR spectra
of these isolable o-QDMs (see above). The synthesis of the
helical PAH 2k not only demonstrates the high efficiency of
the method, but this compound is also the first member of
a new class of helicenes with alternating angular and linear
fusion. Hydrogenation of 2j furnished the angular PAH 26
(Scheme 6) in 66% yield.
ꢀ
QDM)s. Upon treatment of 16–18 with Cp2Zr(pyr)(Me3SiC
CSiMe3) (2.2 equiv) followed by excess benzoic acid, bis(o-
QDM)s 2g–2i were isolated in high yields. The intermediate
ZrPAHs 1g–1i were isolated as crude solids, resuspended in
benzene or toluene, and then treated with benzoic acid, but
isolation of the ZrPAH is optional. While Cp2Zr-
ꢀ
(pyr)(Me3SiC CSiMe3) is an extremely convenient reagent
(especially on a small scale), the Negishi protocol (see above)
is often preferred. Thus, to demonstrate both the scalability
and applicability of this protocol, it was employed to isolate
1.5 g of 2g (89% yield). The structures of 2g–2i were
elucidated by X-ray crystallography (Figure 2b–d). Each
diene unit was found to possess a similar amount of distortion
as 2a, as determined by their diene torsion and biphenyl twist
angles. They also have qualitatively similar stabilities. To the
best of our knowledge, these are the first examples of isolable
compounds with more than one o-QDM subunit.
In conclusion, the method presented herein is a highly
efficient means to fuse multiple rings into PAHs that contain
valuable zirconacyclopentadiene functional groups (ZrPAH).
Protodemetalation of these ZrPAHs gives a new entry to
PAHs containing embedded o-QDMs. The high yields of
these reactions suggest their suitability for the synthesis of
even larger PAHs and graphene nanostructures. The discov-
ery of a general class of isolable o-QDMs, which are usually
generated as fleeting intermediates, opens new possibilities,
including reactions with other reactive intermediates (e.g.,
arynes) or precise control of stoichiometry (e.g., for a Diels–
The isolable o-QDMs are structurally fascinating, but
their promise for post-coupling functionalization is perhaps of
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Acknowledgements
Initial phases of the synthetic work were funded by the
National Science Foundation (CHE-0314709). Other aspects
of the work were supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
Chemical Sciences, Geosciences, and Biosciences Division
(DE-AC02-05CH11231) and by the University of California,
Berkeley. We also acknowledge the National Institutes of
Health for funding of the ChexRay X-ray crystallographic
facility (College of Chemistry, University of California,
Berkeley; S10-RR027172). We thank Daniel S. Levine for
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Conflict of interest
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The authors declare no conflict of interest.
Keywords: graphene nanostructures · metallacycles ·
polycyclic aromatic hydrocarbons · quinodimethanes ·
zirconocene
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Manuscript received: January 23, 2017
Final Article published: && &&, &&&&
6
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Communications
Quinodimethanes
G. R. Kiel, M. S. Ziegler,
T. D. Tilley*
&&&& — &&&&
Zirconacyclopentadiene-Annulated
Polycyclic Aromatic Hydrocarbons
Zirconocene zipper: An efficient fivefold
reductive cyclization demonstrates the
promise of a new strategy for the con-
struction of graphene nanostructures.
Protodemetalation of the resulting zirco-
nacyclopentadiene-annulated polycyclic
aromatic hydrocarbons provides a con-
ceptually unique means for the genera-
tion of valuable ortho-quinodimethane
structures.
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