Electron-poor bowl-shaped polycyclic aromatic dicarboximides: Synthesis, crystal structures, and optical and redox properties

Article abstract of DOI:10.1021/acs.orglett.7b02618

Two new bowl-shaped polycyclic aromatic hydrocarbons, based on corannulene and naphthalene dicarboximide, are synthesized by an improved Suzuki-Miyaura cross-coupling and C-H arylation cascade reaction. Crystallographic analyses confirm structural assignments and provide insight into molecular interactions in the solid state. The new bowl-shaped molecules show reversible oxidation and reduction, intense visible range absorption, and high fluorescence quantum yields. These molecules can be considered bowl-shaped congeners of planar perylene dicarboximides.

Full text of DOI:10.1021/acs.orglett.7b02618

Letter
pubs.acs.org/OrgLett
Electron-Poor Bowl-Shaped Polycyclic Aromatic Dicarboximides:
Synthesis, Crystal Structures, and Optical and Redox Properties
Kazutaka Shoyama,† David Schmidt,†,‡ Magnus Mahl,† and Frank Wurthner*,†,‡
̈
†Institut fur Organische Chemie, Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
‡Center for Nanosystems Chemistry (CNC), Universitat Wurzburg, Theodor-Boveri-Weg, 97074 Wurzburg, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Two new bowl-shaped polycyclic aromatic hydro-
carbons, based on corannulene and naphthalene dicarboximide,
are synthesized by an improved Suzuki−Miyaura cross-coupling
and C−H arylation cascade reaction. Crystallographic analyses
confirm structural assignments and provide insight into molecular
interactions in the solid state. The new bowl-shaped molecules
show reversible oxidation and reduction, intense visible range
absorption, and high fluorescence quantum yields. These
molecules can be considered bowl-shaped congeners of planar
perylene dicarboximides.
he discovery of fullerenes and carbon nanotubes has
Tmotivated the pursuit of nonplanar polycyclic aromatic
hydrocarbons (PAHs).1−5 In particular, bowl-shaped PAHs are
attractive synthetic targets. Interest in curved aromatic scaffolds
continues to increase because such systems offer high prospects
for packing control as desired in organic materials and
supramolecular chemistry (e.g., in bulk heterojunction solar
cells,6,7 liquid crystals,8,9 supramolecular polymerization,10 and
host−guest complexation).11−13 Being commercially available
and mass-producible,14 corannulene has become the most
utilized precursor for bowl-shaped PAHs and has been
functionalized with various moieties.15,16 Simple groups are
introduced by nucleophilic substitution,17,18 metal-catalyzed
C−C bond formation,18−21 or Rieche formylation.22 As well,
substituents may be installed prior to constructing the
corannulene scaffold.21,23,24 Extension of the corannulene π-
scaffold is achieved by introduction of electron-rich aromatic
substituents followed by direct arylation25 or oxidative
condensation.26−28 However, it is challenging to expand the
corannulene π-scaffold with electron-deficient substituents by
the above-mentioned synthetic methods.29 We reasoned that
our recently introduced Pd-catalyzed Suzuki−Miyaura cross-
coupling/C−H arylation cascade protocol for 4,5-dibromo-1,8-
naphthalene dicarboximides and aromatic boronic esters could
be applied to corannulene π-scaffold extension.30,31 This would
afford hitherto unknown, but highly desirable, bowl-shaped
electron-deficient PAHs with potential applications in electric-
field responsive dipolar materials9,32 or as n-type semi-
conductors.27
Figure 1. Structures of corannulene and the first examples of bowl-
shaped polycyclic aromatic dicarboximides 1 and 2.
reaction conditions that favorably effect Heck-type reactions.33
The molecular structures of the newly synthesized bowl-shaped
PAHs were unequivocally assigned by X-ray crystallographic
analysis. Interestingly, while monoannulated shovel-like bowl 1
exhibits one-dimensional columnar bowl-in-bowl packing
commonly seen for bowl-shaped PAHs,12,13,20,24 diannulated
bowl 2 forms a solvent-incorporated sandwich motif.
We first targeted the synthesis of monoannulated bowl 1 by
optimizing the reaction conditions for Pd-catalyzed cascade
Suzuki−Miyaura cross-coupling and direct arylation between
literature-known dibromo-naphthalene dicarboximide 434 and
corannulene monoboronic ester 3 (Scheme 1). Boronic ester 3
was prepared according to a literature-reported iridium-
catalyzed C−H borylation procedure.15,16 To our disappoint-
ment, only traces of the desired product 1 could be observed
when our previously successful annulation method30,31 was
applied (Table 1, entry 1). Fortunately, after optimization of
the phosphine ligand, reaction of boronic ester 3 and
dibromonaphthalene dicarboximide 4 using [Pd2(dba)3]·
CHCl3 as a Pd(0) source, P(o-tolyl)3 as a ligand, and
Cs2CO3 as a base at 90 °C gave bowl 1 in 36% isolated yield
(Table 1, entries 2−4).
Here we report the first examples for this unknown class of
bowl-shaped electron-poor PAHs that comprise corannulene
and one or two naphthalene dicarboximide moieties (1 and 2,
Figure 1). These compounds were obtained by a modification
of our previously reported cascade annulation reaction using
Received: August 23, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02618
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Organic Letters
Letter
solubility of lithium salts in organic solvents. Pivalate, which can
also coordinate to Pd(II) and is known to positively assist
various direct arylation reactions by concerted metalation−
deprotonation,36 did not improve the yield of 1. This implies
that concerted metalation−deprotonation is not the likely
mechanism for this direct arylation reaction.37
Scheme 1. Synthesis of Bowl-Shaped Polycyclic Aromatic
Dicarboximide 1
With the optimized reaction conditions for 1 we were also
able to synthesize diannulated bowl-shaped PAH 2. Pd-
catalyzed reaction of chloro-bromo-naphthalene dicarboximide
5 and diboronic acid diester 7 gave bowl 2 in 31% isolated yield
(Scheme 2). Effects of additives were similar to those for the
Table 1. Optimization of Reaction Conditions for Scheme
1
a
Scheme 2. Synthesis of Bowl-Shaped Polycyclic Aromatic
Bis(dicarboximide) 2
a
b
yield (%)
entry
condition
ligand
SPhos
additive
1
6
1
2
3
4
A
A
A
A
A
A
A
A
A
B
B
B
B
−
−
−
−
−
−
−
traces
19
traces
20
HBF4·PCy3
P(mesityl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
P(o-tolyl)3
11
18
c
36
12
d
5
24
14
6
12
e
6
f
19
c
7
29
a
Reaction conditions: 7 (1.0 equiv), 5 (3.0 equiv), [Pd2(dba)3]·CHCl3
8
Bu4NCl
Bu4NBr
−
Bu4NCl
LiCl
CsOPiv
15
11
29
28
(20 mol %), P(o-tolyl)3 (80 mol %), Cs2CO3 (4.5 equiv), water (100
equiv), o-DCB, 90 °C, 2 h then added an additive (1.0 equiv) and 160
°C, 24 h. Yields were determined by 1H NMR using 1,1,2,2-
9
26
10
11
12
13
n.d.
n.d.
n.d.
n.d.
b
c
41
tetrachloroethane as an internal standard. Reaction carried out at 160
c
°C from the beginning (24 h). Isolated yield.
17
18
a
onefold annulation. Diboronic diester 7 was prepared similarly
to 3 by adjusting the stoichiometry of bis(pinacol)ester and was
isolated as a regioisomeric mixture after column chromatog-
raphy. The composition of 7 was confirmed by mass
spectrometry (MALDI-TOF) and elemental analysis (see SI).
We note here that the above-mentioned iridium catalysis is
sensitive to steric effects38 and therefore the two boronic ester
moieties in 7 are likely to be introduced at distal positions. In
accordance with this, we did not isolate the regioisomer of 2
where the second naphthalene dicarboximide substituent was
annulated at the [2,3] or [8,9] positions. Therefore, we deduce
that the second boronic ester of 7 is introduced at the 4, 5, 6, or
7 position which in all cases leads to 2 as the sole product.
X-ray crystallographic analysis of bowls 1 and 2 gave
unequivocal structural assignments and insight into the effects
of the number of imide moieties on packing motifs. Crystals
suitable for X-ray analysis were obtained by slow diffusion of
hexane into tetrachloroethane solution of 1 and toluene
solution of 2. The bowl depths of the central corannulene
moiety of 1 and 2, defined as the mean distances between the
central five-membered ring and the mean plane of the 10 rim
carbons, are 0.92 and 1.00 Å (Figure 2a, b) while that of
corannulene is 0.87 Å.21 This indicates that an increasing
number of naphthalene imide moieties leads to a deeper
curvature. The crystal structure of 1 consists of columnar stacks
of bowls with the shortest contact between two neighboring
molecules being 3.16 Å (Figure 2c). Neighboring stacks
alternate with respect to the curvature direction of corannulene
moieties. We note that columnar stacks of molecules are often
seen in reported crystal structures of bowl-shaped
PAHs.12,13,20,24 Each column consists of molecules of 1 that
are stacked at the corannulene moiety with naphthalene
dicarboximide moieties of each successively stacked molecule
offset at an angle of ca. 60° (Figure 2e). The crystal structure of
Condition A: 3 (0.04 mmol), 4 (1.5 equiv), [Pd2(dba)3]·CHCl3 (10
mol %), ligand (40 mol %), Cs2CO3 (1.5 equiv), toluene/water (2:1),
90 °C, 12 h. Condition B: 3 (0.04 mmol), 5 (1.0 equiv), [Pd2(dba)3]·
CHCl3 (10 mol %), ligand (40 mol %), Cs2CO3 (3.0 equiv), water
(100 equiv), o-DCB, 90 °C, 2 h, then added additive (0.5 equiv), 160
°C, 24 h. Yield determined by H NMR using 1,1,2,2-tetrachloro-
ethane as an internal standard. Isolated yield. Dry toluene was used
as solvent instead of toluene/water mixture. Corannulene boronic
acid was used instead of ester 3. 2.5 mol % of catalyst and 10 mol % of
ligand were used. o-DCB: o-dichlorobenzene. SPhos: 2-dicyclohexyl-
phosphino-2′,6′-dimethoxybiphenyl. n.d.: not detected.
b
1
c
d
e
f
The main side product of this reaction was di(corannulenyl)-
naphthalene dicarboximide 6, which indicates that intermo-
lecular Suzuki−Miyaura reaction is competing with an
intramolecular direct arylation reaction. In accordance with
this, we also observed increased amounts of 6 when we used
corannulene boronic acid instead of ester 3 or employed a
lower catalyst concentration (Table 1, entries 6, 7). Attempts to
promote the direct arylation reaction by adding quaternary
ammonium salts35 failed, possibly because these salts also
promote competing Suzuki−Miyaura couplings (Table 1,
entries 8, 9).
In order to suppress this side reaction we examined a
stepwise cascade reaction using a newly synthesized bromo-
chloro-naphthalene dicarboximide 5 (see Supporting Informa-
tion (SI) for synthesis). The first-step Suzuki−Miyaura cross-
coupling was performed at 90 °C, and the subsequent arylation
was performed in situ at 160 °C (Table 1, entries 10−13).
Additives that assist the Heck reaction or direct arylation
reaction were also tested. We found that tetrabutylammonium
chloride has a positive effect on this cascade, and bowl 1 was
isolated in an improved yield of 41%. Another chloride salt,
LiCl, did not exert a significant effect, possibly due to the lower
B
DOI: 10.1021/acs.orglett.7b02618
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Letter
Figure 3. UV−vis absorption (solid line) and fluorescence (dashed
line) spectra of 1 (red) and 2 (black) in dichloromethane (c = 1.0 ×
10−5 M; 25 °C). λex = 473 nm for 1 and 503 nm for 2.
3.18 and 5.00 ns, respectively. Albeit lower than those of PMIs
and PBIs, these fluorescence quantum yields are apparently the
highest reported for π-conjugated bowl-shaped chromo-
phores.41 As recently pointed out by Hariharan,42 core-twisted
aromatics suffer from enhanced intersystem crossing and thus
the excellent fluorescence properties of 1 and 2 are indeed
remarkable.
Electrochemical properties were characterized by cyclic and
square wave voltammetry in dichloromethane at room
temperature using 0.1 M tetrabutylammonium hexafluoro-
phosphate as the electrolyte and the Fc+/Fc redox couple as an
internal standard (Figure S1). Bowl 1 showed two reversible
reduction processes at −1.38 and −1.67 V, while bowl 2
showed three at −1.22, −1.34, and −1.72 V. Their oxidation
processes were observed at 1.10 and 1.14 V. The oxidation and
reduction potentials of 1 and 2 are less dependent on the
number of imide substituents than those of imide-substituted
rylenes.43,44 Also, all redox potentials are 0.1−0.3 eV higher
than those of PMI and PBI,40 demonstrating the impact of a
more electron-rich central unit.
In summary, we synthesized bowl-shaped imide-function-
alized PAHs using a recently introduced novel palladium-
catalyzed cascade C−C bond formation reaction that comprises
Suzuki−Miyaura cross-coupling and direct arylation. This
cascade reaction was improved by using a Heck reaction
additive, tetrabutylammonium chloride,35 which is consistent
with our recently reported mechanistic rationale.33 These are
the first examples of corannulene annulation with electron-poor
aromatic moieties via six-membered ring annulation. The
obtained bowl-shaped molecules show exciting functional
properties. Despite possessing non-planar π-scaffolds, they are
highly luminescent and exhibit low-lying frontier orbitals as
desired for acceptors in supramolecular photosystems45 and n-
type semiconductors.40 Further explorations of the self-
assembly behavior of bowl-shaped imide-functionalized PAHs
and extensions of our cascade reaction to various substrates are
currently underway in our laboratory.
Figure 2. Crystal structures of 1 and 2. Top and side view of (a) 1 and
(b) 2 (ORTEP drawing in 50% probability for thermal ellipsoids).
Crystal packing of (c) 1 and (d) 2. (e) Two stacked molecules of 1
seen from the top side of a columnar stack. (f) A sandwiching pair of
2. Incorporated solvent molecules are omitted except in (d) where
sandwiched toluene molecules are shown (see text).
2 consists of dimers that are sandwiching two toluene solvate
molecules (Figure 2d). In each dimer the corannulene moiety is
placed over one of the naphthalene dicarboximide arms of the
other molecule (Figure 2f). This clamp-like sandwiched
packing motif is rare for bowl-shaped PAHs23,27 and contrasts
with the tendency of bowl-shaped molecules to pack into
columnar stacks as in the crystal structure of 1.
These new bowl-shaped molecules resemble planar perylene
mono- and bis(dicarboximides) (PMI and PBI). Although PBIs
are valued fluorescence dyes with quantum yields close to
unity39 and are outstanding n-type semiconductors,40 the
impact of the corannulene moiety on optical and redox
properties is hard to predict. Contrary to the commonly seen
insolubility of imide-substituted PAHs, bowls 1 and 2 have high
solubility in chlorinated or aromatic solvents owing to their
curved π-scaffold (see SI for details). Despite larger π-scaffolds,
the solubility of 1 and 2 in chloroform also exceeds that of the
PBI derivative with the same bulky imide substituents (9 and 15
vs 6 mg/mL). The optical properties of bowls 1 and 2 were
characterized by UV−vis and fluorescence spectroscopy in
dichloromethane (Figure 3). The absorption spectra of bowls 1
and 2 reveal S0−S1 transition maxima at 503 and 538 nm,
respectively, which reflect the extended π-conjugation of 2.
These values are indeed similar to those of their planar
congeners PMI (λmax = 510 nm) and PBI (λmax = 526 nm).39
However, the spectrum of 2 shows a nonprogressional vibronic
structure, which stems from overlapping transitions to higher
electronic states as revealed by TDDFT calculations (SI). The
emission maxima of 1 and 2 appeared at 534 and 573 nm,
respectively, from which Stokes shifts were calculated to be
1200 and 1100 cm−1. Such large Stokes shifts are not observed
for PMIs and PBIs and resemble those of other reported bowl-
shaped PAHs.41 More importantly, bowls 1 and 2 have high
fluorescence quantum yields of 0.55 and 0.48 with lifetimes of
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.7b02618.
C
DOI: 10.1021/acs.orglett.7b02618
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Detailed synthetic and experimental procedures, charac-
Letter
(22) Rajeshkumar, V.; Lee, Y. T.; Stuparu, M. C. Eur. J. Org. Chem.
2016, 2016, 36−40.
(23) Fellows, W. B.; Rice, A. M.; Williams, D. E.; Dolgopolova, E. A.;
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N. B. Angew. Chem., Int. Ed. 2016, 55, 1927−1927.
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(27) Lu, R.-Q.; Zhou, Y.-N.; Yan, X.-Y.; Shi, K.; Zheng, Y.-Q.; Luo,
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terization data, NMR spectra, and crystallographic
information (PDF)
Crystal data for 1 (CCDC 1569916); Crystal data for 2
(CCDC 1569917) (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wuerthner@uni-wuerzburg.de.
ORCID
Frank Wurthner: 0000-0001-7245-0471
̈
Notes
(29) Lin, Z.; Li, C.; Meng, D.; Li, Y.; Wang, Z. Chem. - Asian J. 2016,
11, 2695−2699.
The authors declare no competing financial interest.
(30) Seifert, S.; Shoyama, K.; Schmidt, D.; Wurthner, F. Angew.
̈
ACKNOWLEDGMENTS
■
Chem., Int. Ed. 2016, 55, 6390−6395.
We thank the Deutsche Forschungsgemeinschaft (DFG) for
financial support (Grant No. WU 317/20-1).
(31) Seifert, S.; Schmidt, D.; Wurthner, F. Org. Chem. Front. 2016, 3,
̈
1435−1442.
(32) Furukawa, S.; Suda, Y.; Kobayashi, J.; Kawashima, T.; Tada, T.;
Fujii, S.; Kiguchi, M.; Saito, M. J. Am. Chem. Soc. 2017, 139, 5787−
5792.
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