Helicenes from diarylmaleimides

Article abstract of DOI:10.1021/ol500154k

Perkin condensations of arylglyoxylic acids with arylacetic acids, followed by the addition of alkylamine, yield diarylmaleimides in a one-pot procedure. The arylglyoxylic acids are obtained by arene acylation with ClCOCO ₂Et and reduced with NaI and hypophosphorous acid to the arylacetic acids. With 2,7-di-tert-butyl-pyren-4-yl or chrysen-6-yl as the aryl, photocyclodehydrogenation of the diarylmaleimides yields substituted helicenes which can be reduced to stable anions. The helicenes combine bathochromically shifted absorption with hypsochromically shifted fluorescence with respect to their precursors.

Full text of DOI:10.1021/ol500154k

Letter
pubs.acs.org/OrgLett
Helicenes from Diarylmaleimides
Harald Bock,* Daniel Subervie, Pierre Mathey, Anirban Pradhan, Parantap Sarkar, Pierre Dechambenoit,
Elizabeth A. Hillard, and Fabien Durola*
́
Centre de Recherche Paul Pascal, CNRS & Universite de Bordeaux, 115 avenue Schweitzer, 33600 Pessac, France
S
* Supporting Information
ABSTRACT: Perkin condensations of arylglyoxylic acids with arylacetic acids, followed by the addition of alkylamine, yield
diarylmaleimides in a one-pot procedure. The arylglyoxylic acids are obtained by arene acylation with ClCOCO2Et and reduced
with NaI and hypophosphorous acid to the arylacetic acids. With 2,7-di-tert-butyl-pyren-4-yl or chrysen-6-yl as the aryl,
photocyclodehydrogenation of the diarylmaleimides yields substituted helicenes which can be reduced to stable anions. The
helicenes combine bathochromically shifted absorption with hypsochromically shifted fluorescence with respect to their
precursors.
dilemma and toward very large yet highly processable arenes is
the avoidance of planar, well stacking, and thus poorly soluble
systems by the introduction of twist: indeed, we observed that
nonplanar polycyclic arene 3 shows good solubility even in cold
toluene despite its size and the presence of only three tert-butyl
substituents.3
Highly congested triply helicenic structures such as 4 are
efficiently accessible by intramolecular Scholl reactions,4 but
this chemistry precludes the presence of many electronically
interesting substituents. On the other hand, we established that
imides such as 1 are straightforwardly accessible from
unsubstituted polycyclic arene precursors via Friedel−Crafts
acylation followed by glyoxylic Perkin condensation with a
bromoarylacetic acid and ring closure by Pd-assisted dehydro-
bromination.2
The photocyclization of stilbene-type compounds including
diarylmaleimides is an established and efficient approach to
helicenes,5 and therefore we wondered whether the glyoxylic
Perkin reaction6 followed by photocyclization could offer a fast
and straightforward approach to highly nonplanar structures
such as 4 but with electronically desirable imide substituents
such as in 1. We therefore set out to synthesize symmetrical
helicenedicarboximides from accessible polycylic arenes via the
corresponding arylglyoxylic acids.
xtended polycyclic arenes attract ever growing interest due
Eto their optoelectronic properties, which with increasing
size approach those of graphene.1 In contrast to graphene, large
discrete-size arenes allow the tuning of their electronic
properties by the incorporation of electron-donating or
-withdrawing substituents at their rim. But the synthetic
accessibility and the technological utility of large polycylic
arenes are limited by their insolubility that quickly increases
with size. For example, we recently found that already
moderate-sized diperylenoanthracene-tetracarboxdiimide 1
(Figure 1), albeit bearing four long n-undecyl alkyl chains, is
so poorly soluble even in chlorinated solvents that a satisfactory
1H NMR spectrum could only be obtained at 130 °C,2 whereas
in perylene-tetracarboxdiimide 2 four ethyl substituents are
sufficient to impart good solubility in chlorinated solvents at
room temperature. An evident way out of this solubility
Because of inherent regioselectivity challenges addressed ever
since the first helicene photosyntheses,5 polycyclic aromatic
substrates for the Friedel−Crafts acylation with ClCOCO2Et
should meet two requirements in order to provide an
unambiguous access to helicenes: The most reactive sub-
stitution position should have only one neighboring hydrogen-
Figure 1. Planar aromatic diimides with limited solubility (left) and
nonplanar aromatic hydrocarbons with excellent solubility (right). R =
n-undecyl.
Received: October 1, 2013
Published: February 28, 2014
© 2014 American Chemical Society
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bearing carbon in order to offer an unequivocal final
photocyclization geometry, and this carbon to be photo-
dehydrogenated should be the α-position of a naphthalene
subunit to ensure that at least a [5]helicene is formed on
cyclization.
such as 7 and 6 from their arylcarboxylic analogs that might
have formed by carbonyl cleavage, as the second carbonyl signal
at ca. 190 ppm is very characteristic.
Although the Wolff−Kishner reduction of arylgloxylic acids
such as 5-acenaphthylglyoxylic acid to the corresponding acetic
acid has been reported,9 the recent reports by Milne et al. of the
reduction of phenylglyoxylic acid to phenylacetic acid with
phosphorous acid and catalytic sodium iodide,10 and by Hicks
et al. of the reduction of alkyl aryl ketones to alkylarenes with
hypophosphorous acid and catalytic iodine,11 led us to search
for a gentler approach than the Wolff−Kishner reaction by
combining the two latter procedures. Indeed, we found that
treatment of the glyoxylic ester 6 with hypophosphorous acid
and sodium iodide in refluxing acetic acid, with methane-
sulphonic acid added to ensure complete hydrolysis, leads
directly to the corresponding acetic acid 8, which crystallizes in
nearly pure form from the reaction mixture upon cooling to
room temperature. Acidic esterification leads to the methyl
ester 9.
We previously reported that the glyoxylic Perkin reaction can
be coupled in a one-pot procedure with basic esterification of
the formed diarylmaleic anhydride, yielding the corresponding
dialkyl ester directly.2 We now find that imides can likewise be
directly obtained, if excess alkylamine is added to the reaction
mixture after the completion of the Perkin condensation.
Dipyrenylmaleimide 10 is thus obtained in 75% yield from 7
and 8 in a one-pot procedure. To our delight, the cyclization of
yellow 10 proceeded efficiently in refluxing toluene in the
presence of iodine and air, yielding the red [5]helicene 11
overnight in 60% yield, simply using a conventional 200 W light
bulb externally.12 This photooxidation leads to the formation of
an additional aromatic sextet in the Clar structure13 of 11
compared to 10 (Figure 2), which may be a helpful driving
force in the reaction.
Pyrene and perylene are among the most ubiquitous
polycylic arenes in optics and electronics and satisfy the first
of these two requirements: Both the reactive 1-position in
pyrene and the reactive 3-position in perylene are adjacent to
neighboring rings. However, they fail the second criterion, as no
further benzene ring is present beyond the neighboring CH.
But pyrene undergoes tert-butylation in positions 2 and 7,
redirecting Friedel−Crafts acylation onto position 4,7 which
satisfies both criteria, while the tert-butyl group in position 7
ensures that photocyclization cannot go beyond the targeted
[5]helicene to give a planar benzo[ghi]perylene subunit.
Indeed, 2,7-di-tert-butylpyrene 5 reacts with ClCOCO2Et to
yield the 4-glyoxylic ester 6 in the presence of ZrCl4, a Lewis
acid which we found previously to give cleaner Friedel−Crafts
acylations than AlCl3 and no concomitant glyoxylic ester
hydrolysis.2 Saponification with NaHCO3 yields the glyoxylic
acid 7 (Figure 2). Basic hydrolysis is to be preferred over acidic
hydrolysis, as we found that prolonged heating with sulphuric
acid in acetic acid leads to decarbonylation to the carboxylic
acid, in agreement with the reported conversion of phenyl-
glyoxylic acid to benzoic acid.8 13C NMR spectroscopy is
crucial for the differentiation of arylglyoxylic acids and esters
Compared to other synthetic approaches to imide-function-
alized helicenes, such as the Diels−Alder addition of a
maleimide to 3,4,3′,4′-tetrahydro-1,1′-binaphthyl followed by
oxidative aromatization (3% overall yield),14 and compared to
the four-step synthesis of photocyclizable nonalkylated
maleimides from 5-amino-1,4-diaryl-1,2,3-triazoles,15 the Per-
kin/photocyclization approach appears not only more general
but also more efficient. Compared to recently reported
photocyclizations of pyrenyl-naphthyl-ethylenes to [4] and
[5]helicenes without imide substituents,16 the photocyclization
to 11 proceeds with greater yield and speed (and no need for a
Cu-based sensitizer), which may be attributed to the greater
stability and electron-withdrawing effect of the maleimide
bridge.
11 shows good solubility in chlorinated solvents, and single
crystals suitable for structure analysis could be obtained by
diffusion of methanol into a DCM solution. The crystals are
racemic; i.e., no separation of enantiomers is observed upon
crystallization, albeit this is reported for other helicenes.5 While
the outer phenanthrene moieties in the two pyrene blades are
nearly undistorted and planar, the bulk of the helical distortion
is localized in the three inner benzene rings, with a central
torsion angle along the three inner bonds of ca. 35° (Figure 3).
To test whether higher helicenes can be accessed by our
glyoxylic approach with similar ease, we looked for a substrate
prone to yield a [7]helicene. Although phenanthrene reacts in
Friedel−Crafts acylations mainly in the 3-position suitable for
[7]helicene construction,17 we preferred chrysene 12, because
its regioselectivity in acylations is reported to be superior to
Figure 2. Synthesis of helicene-dicarboximides via glyoxylic Perkin
reactions. (a) ClCOCO2Et, ZrCl4, DCM, 25 °C, 16 h, 61%/81%; (b)
NaHCO3, H2O, EtOH, reflux, 2 h, 100%/97%; (c) NaI, H3PO2, H2O,
AcOH, reflux, 6 h, then addition of MeSO3H, reflux, 64 h, 98%/94%;
(d) MeOH, H2SO4, reflux, 16 h, 68%/91%; (e) NEt3, Ac2O, dioxane,
reflux, 3 h, then addition of Et2CHNH2, reflux, 16 h, 75%/68%; (f) I2,
O2, PhMe, reflux, 16 h/64 h, 60%/68%.
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Figure 4. Absorption (continuous lines, left) and normalized
fluorescence (dashed, right) spectra of 10 (orange, excitation at 460
nm), 11 (red, 510 nm), 17 (turquoise, 420 nm), and 18 (blue, 470
nm) in chloroform (20 μM).
Figure 3. Crystallographic structures of pyrene-derived [5]helicene 11
and chrysene-derived [7]helicene 18 in stick (top, hydrogen atoms
omitted) and calotte (bottom) representations.
The anodic and cathodic electrochemistry of 11 and 18 were
evaluated by differential pulse voltammetry (DPV) and cyclic
voltammetry (CV) in DCM. 11 has a reversible reduction at
−1.67 V (all potentials are given vs ferrocene/ferrocenium (Fc/
Fc+)) and a reversible oxidation process at +0.85 V. 18 likewise
shows a reversible reduction at −1.62 V. However, an
irreversible oxidation at 1.15 V suggests that the radical cation
of 18 is unstable. Both compounds thus exhibit their first
reduction and oxidation potentials within the electrochemical
window of DCM, which allows direct comparison of the
electrochemical band gap with the optical band gap derived
from the absorption spectra. With the energy level of Fc/Fc+ at
4.8 eV below vacuum, voltammetry yields highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels of EHOMO(11) = −5.65 eV,
phenanthrene,17 and because no isomeric alternative to
helicene formation is possible in the photocyclization.
The acylation of chrysene with ClCOCO2Et and ZrCl4 is
indeed highly regioselective and yields the expected 6-glyoxylic
ester 13, which is saponified to the glyoxylic acid 14 or reduced
to the acetic acid 15 with the same ease as in the case of the
pyrene series. 15 with sulphuric acid in refluxing methanol gives
the methyl ester 16. One-pot Perkin coupling of 14 and 15 plus
imidification yields the dichrysenylmaleimide 17 in 68% yield.
Under similar conditions, the photoreaction of yellow 17 to
the orange [7]helicene 18 proceeds more slowly than the
cyclization of 10 to 11 in the pyrene series: after 3 days of
reaction, besides a yield of 68% of 18, traces of starting material
17 are still present and have to be separated by column
chromatography. As in the case of 11, the formation of 18 from
17 leads to a Clar structure13 with an additional aromatic sextet
in the newly formed six-membered ring (Figure 2).
E
E
LUMO(11) = −3.13 eV, EHOMO(18) = −5.80 eV, and
LUMO(18) = −3.18 eV and electrochemical band gaps of
ΔELUMO−HOMO(11) = 2.52 eV and ΔELUMO−HOMO(18) = 2.62
eV. As the LUMO energies of 11 and 18 are nearly identical,
the difference in band gap can be attributed primarily to the
difference in HOMO energies.
From the absorption onset wavelengths (Figure 4) of ca. 560
nm for 11 and ca. 520 nm for 18, optical band gaps of
ΔEopt(11) = 2.21 eV and ΔEopt(18) = 2.38 eV are obtained.
The differences between the electrochemical and optical band
gaps of 0.31 eV for 11 and 0.24 eV for 18 can be identified as
the exciton binding energies in the neutral excited species.19
In summary, we have found that our synthetic approach
based on the one-pot reduction and hydrolysis of arylglyoxylic
esters with hypophosphorous acid and iodide to arylacetic acids
and their glyoxylic Perkin reaction coupled with in situ
imidification and subsequent photocyclization can lead quickly
and efficiently from planar polycyclic arenes to alkylimide-
substituted helicenes, which exhibit strongly enhanced
solubility compared to planar arene analogues of similar size
and substitution. They show considerably smaller Stokes shifts
than their maleimide precursors and form stable anions upon
electrochemical reduction.
This approach can be extended to other dicarboxylic
derivatives such as diesters and is also intrinsically suitable for
the buildup of nonsymmetric helicenes because the photo-
cyclized diarylmaleimide is built dissymmetrically from two
complementary building blocks, the arylglyoxylic acid and the
arylacetic acid, whose aryl parts need not be identical. If interest
is vested in enantiomer separation or enantioselective synthesis
of helicenes, the use of homochiral amines in the post-Perkin
Albeit not bearing solubility-enhancing tert-butyl groups like
11, 18 is equally soluble in chlorinated solvents and yielded
racemic crystals suitable for X-ray analysis by methanol
diffusion. The ca. 36° inner torsion angle of 18 is nearly
identical to that of 11, showing that the two inward-pointing
tert-butyl groups of 11 are roughly equivalent to the two
terminal benzene rings of 18 in their effect on the helical
distortion. In both 11 and 18, the chirality of the individual
1
molecule is expressed in both the H and 13C NMR spectra,
where the two ethyl branches of the alkyl substituent are
differentiated.
The absorption spectra are shifted bathochromically by about
50 nm upon cyclization of 10 to 11 and of 17 to 18. Both
maleimide precursors and both final helicenes are characterized
by a broad plateau-like long wavelength absorption band of
moderate intensity (Figure 4). Swallow-tail imides of
condensed arenes are known to show intense fluorescence in
solution,18 and 11 and 18 are no exception in this respect.
Astonishingly, albeit absorbing at higher wavelengths, both
helicenes show their fluorescence maxima at lower wavelengths
than their maleimide precursors. This striking reduction in
Stokes shifts illustrates that conformational relaxation of the
excited state prior to emission is much more restricted in the
rigidified helicenes than in their flexible precursors.
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imidification may be an attractive option. If larger nonplanar
and soluble carboxy-functionalized arenes are desired, suitable
bifunctional building blocks, i.e. arylene-diglyoxylic and
-diacetic acids satisfying our two design criteria for
unambiguous helicene buildup, are to be targeted. A possible
limitation of our approach is the general requirement of dilute
solutions for photocyclizations, limiting the quantities of
obtainable material per solvent volume in the final synthetic
step.
́
(16) Bedard, A.-C.; Vlassova, A.; Hernandez-Perez, A.-C.; Bessette,
A.; Hanan, G. S.; Heuft, M. A.; Collins, S. K. Chem.Eur. J. 2013, 19,
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J. Chem. Res., Synop. 1980, 40.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic structures, electrochemical studies, synthetic
procedures, 1H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(19) Knupfer, M. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 623.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: bock@crpp-bordeaux.cnrs.fr.
*E-mail: durola@crpp-bordeaux.cnrs.fr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financed partially by CNRS and ERASMUS
MUNDUS (external co-operation window with Asia) grants.
We are grateful to Thierry Cardinal and Alain Garcia (Institut
̀ ́
de Chimie de la Matiere Condensee de Bordeaux) for help with
the fluorescence measurements.
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