Propeller-Shaped D3h-Symmetric Macrocycles with Three 1,8-Diazaanthracene Blades as Building Blocks for Photochemically Induced Growth Reactions

Article abstract of DOI:10.1002/ejoc.201500496

The efficient synthesis of new D_3h-symmetric propeller-shaped, double-decker compounds with three vertically standing diazaanthracene units arranged so as to allow for photochemically induced intermolecular growth reaction by [4+4] cycloaddition is presented. The propellers bear alkyl chains of four different lengths (ethyl, propyl, hexyl, dodecyl) to have flexibility in creating ordered arrays by packing them into single crystals, onto solid substrates or into mesophases. All propellers are obtained in only six steps and on a 50-200 mg scale. The sequence passes through the ditriflated 1,8-diazaanthracene derivatives 6 which are a very versatile building block for many purposes and were therefore synthesized on a 30 g scale. A new class of propeller-shaped macrocycles is accessible via a six-step procedure with overall yields ranging from 6-16%. These cyclophanes are equipped with 1,8-diazaanthracene units and are designed for photochemical growth reactions in two dimensions.

Full text of DOI:10.1002/ejoc.201500496

FULL PAPER
DOI: 10.1002/ejoc.201500496
Propeller-Shaped D3h-Symmetric Macrocycles with Three 1,8-Diazaanthracene
Blades as Building Blocks for Photochemically Induced Growth Reactions
Marco Servalli,[a] Luzia Gyr,[a] Junji Sakamoto,*[b] and A. Dieter Schlüter[a]
Keywords: Functional materials / Propellers / Cyclophanes / Photochemistry / Cross-coupling / Cycloaddition
The efficient synthesis of new D3h-symmetric propeller-
shaped, double-decker compounds with three vertically
standing diazaanthracene units arranged so as to allow for
photochemically induced intermolecular growth reaction by
[4+4] cycloaddition is presented. The propellers bear alkyl
chains of four different lengths (ethyl, propyl, hexyl, dodecyl)
to have flexibility in creating ordered arrays by packing them
into single crystals, onto solid substrates or into mesophases.
All propellers are obtained in only six steps and on a 50–
200 mg scale. The sequence passes through the ditriflated
1,8-diazaanthracene derivatives 6 which are a very versatile
building block for many purposes and were therefore synthe-
sized on a 30 g scale.
At the same time, we are interested in providing access
not only to building blocks (monomers) that can mediate
growth in confined geometries such as in single crystals, at
the air/water interface or on solid substrates, but also in
liquid crystalline phases and possibly even in homogeneous
solution. We were therefore interested in having a series of
versatile monomers at hand, all sharing the same structural
skeleton but differing in their functionalities to meet the
respective requirements for these different methods of poly-
merization. In the present work, we introduce D3h-symmet-
ric propeller-shaped compounds of type 1 (Figure 1) with
three photoreactive blades, spaced by acetylene moieties
and connected to two central orthogonal benzene cores.
The persistent molecular geometry ensures that the blades,
which are spatially separated from each other by 120°, can-
not react intramolecularly. In addition, intermolecular syn-
alignment of the blades[7] is impossible. Consequently, reac-
tions of compounds 1 are preset for the intermolecular,
anti-selective [4+4] cycloaddition, supposed a face-to-face
Introduction
Shape-persistent compounds with two or three photosen-
sitive anthracene (A) or 1,8-diazaanthracene (DAA) units
in the proper geometric relation have been reported as at-
tractive monomers for photochemically induced poly-
merizations in single crystals as well as in monolayers
spread at the air/water interface. Two-dimensional polymers
(2DP),[1] covalent monolayer sheets[2] and a novel rigid rod
polymer[3] were obtained this way. The growth chemistry in
these cases was mainly [4+4] cycloaddition between either
two A and two DAA units, or one A and one DAA unit.
In a particular case [4+2] cycloaddition between an A unit
and an acetylene also proved successful. Fascinated by the
fact that this growth chemistry can pave the way to two-
dimensionally extended entities, may it be 2DPs[4] or cova-
lent monolayer sheets,[5] and in combination with the pres-
ent interest in 2D materials,[6] we decided to work on a
broad synthetic access to more such building blocks in or-
der to explore the area of rational and mild sheet synthesis
more systematically. This shall in future enable us to investi-
gate the use of covalent sheets with high degrees of internal
order for applications such as ultra-selective membranes, ul-
tra-sensitive pressure sensors, scratch-resistant surface coat-
ings, novel ferroelectric materials, scaffoldings for novel cat-
alyst systems and the like.
[a] Laboratory of Polymer Chemistry, Department of Materials,
ETH Zürich,
Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
E-mail: dieter.schluter@mat.ethz.ch
[b] Department of Applied Chemistry, Graduate School of
Engineering, Osaka University,
2–1 Yamadoaka, Suita, 565-0871 Osaka, Japan
E-mail: sakamoto@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500496.
Figure 1. Chemical structure of the D3h-symmetric cyclophane pro-
pellers 1a–d synthesized in the present study. All compounds carry
six esters with differently long alkyl groups.
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M. Servalli, L. Gyr, J. Sakamoto, A. D. Schlüter
FULL PAPER
(ftf) arrangement can be realized. Compounds 1a–d are for- under azeotropic removal of water in benzene and afforded
mally cyclophanes[8] bearing six ester groups each with dif- the diesters 3a–d. 3a and 3b were conveniently purified by
ferently long alkyl chains (ethyl to dodecyl). Functional distillation, while the higher esters 3c and 3d were purified
groups that could disturb the desired anthracene π-π inter- by column chromatography. Inspired by the procedure pub-
actions (such as H-bond acceptors/donors) have been lished by Molock and Boykin,[12] the diesters were then sub-
avoided. It is noted that similar cyclophanes have already jected to double aza-Michael additions with 2,6-diamino-
been realized by Moore[9] and Zhang.[10] However either the toluene as nucleophile in mixtures of CHCl3 and a protic
“blades” consist of phenylene-ethynylene units (rather than solvent such as MeOH (for 3a–c) or EtOH (for 3d), to selec-
anthracenes) which were not designed for intermolecular re- tively ensure (Z)-isomer formation.[13] This resulted in fast
actions or the compound was not shape-persistent which and high-yielding formation of compounds 4a–d, which
reduces the probability for an intermolecular ftf packing as were purified by column chromatography (4c–d), while the
required for topochemistry.[1,11] Here we report the efficient lower congeners precipitated out during the reaction and
synthesis of a series of compounds 1a–d following a few- could be used without further treatment. Double intramo-
steps procedure and passing through the novel ditriflates lecular cyclization under dilute conditions in refluxing di-
6a–d which are a particularly potent building block for phenyl ether afforded the pyridoquinolone derivatives 5a–d
cross-coupling reactions and nucleophilic substitutions.
in good yields ranging from 81% to 91%. In particular, the
steric bulk provided by longer alkyl chains such in the case
of 5b–d was beneficial for the scale-up of the reactions. It
prevented intermolecular reactions and allowed using more
concentrated solutions (e.g. 118 mm instead of 43 mm),
though this effect was not fully investigated. Triflation of
5a–d by treatment with Tf2O, 2,6-lutidine as base and
DMAP as catalyst in dry DCM afforded quantitatively the
ditriflates 6a–d on a multi-gram scale without the need of
column chromatography. The ditriflates turned out to be
quite stable in the solid state under ambient conditions, but
due to their photoreactive nature they needed to be stored
in the dark. In solution, the compounds 6a–d are also quite
activated towards nucleophilic aromatic substitution
Results and Discussion
The synthesis of the propeller-shaped compounds 1a–d
is depicted in Scheme 1 (a). 2,7-Disubstituted 1,8-di-
azaanthracene-4,5-ditriflates 6a–d were selected as key
building blocks constituting the blades for monomers 1a–d,
respectively. They were then assembled into the propeller-
shaped cyclophanes by Pd-mediated cross-coupling reac-
tions. The sequence starts off with the commercially avail-
able acetylenedicarboxylic acid 2. The acid was first esteri-
fied with the corresponding alcohol (a: ethanol, b: n-prop-
anol, c: n-hexanol, d: n-dodecanol) using TsOH as catalyst
Scheme 1. (a) The synthetic sequence to the propeller-shaped monomers 1a–d. (b) Synthesis of building block 8 and dibromide 9.
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Building Blocks for Photoinduced Growth Reactions
through loss of the triflate moiety, as evidenced by their could not be obtained even by measuring at 80 °C because
immediate reaction with secondary amines such as piper- of insufficient solubility. It is noted that the poor solubility
idine. 2,6-Lutidine being non-nucleophilic and sterically of these two compounds is a key element of their structure
hindered, proved to be the optimal choice as mild base for design as it will help force these propellers into the crystal-
working with these compounds. In the next step, the direct line state. Thus, their structure proof rests only on HRMS
1
target precursors were assembled. For this purpose the di- and the H-NMR spectra, the latter of which are virtually
triflates 6a–d were coupled in excess with 1,3,5-triethynyl- superimposable to that of their soluble congeners (see Sup-
benzene 8 (Scheme 1, b) under standard Sonogashira condi- porting Information). Compounds 1a–b can be crystallized
tions with Pd(PPh3)4 and CuI as catalyst and co-catalysts, from o-dichlorobenzene to form crystals of up to 100 μm
respectively, and 2,6-lutidine as base to afford the monomer of various shapes, the structures of which is presently under
precursors 7a–d in yields ranging from 50–70%.
investigation. The melting points are above decomposition
It needs to be mentioned here that while compounds 7c temperature. The UV/Vis absorption of all the monomers
and 7d could be easily purified by rGPC (recycling gel per- are virtually superimposable and therefore the propyl series
meation chromatography), their lower molar mass conge- was chosen as representative for the characterization.
ners 7a and 7b were poorly soluble which caused difficulty Cross-talk between the DAA units in the same compound
in purification. Despite washings and recrystallizations is minor as the comparison with model compound 12 shows
these two compounds could not be obtained in pure form [λmax(1b) = 422 nm; λmax(12) = 431 nm]. The fluorescence
and were used as obtained. If strictly necessary, these com- spectra were also measured for the propyl representatives
pounds can be nevertheless purified by rGPC in small by diluting the same solutions used for the UV/Vis study
amounts. The NMR spectra of these two compounds in and do not display any particular feature (Figure S34). Fi-
the highest achieved purity are displayed in the SI. Column nally, compounds 1a–d were found to be stable up to ap-
chromatography was not considered an option due to the proximately 300 °C (heating rate: 10 °C/min) by thermo-
six nitrogen functionalities and the sensitivity of the three gravimetric analysis.
triflate groups. The excess of the ditriflates 6a–d could be
Anthracene and DAA units which can be incorporated
recovered virtually quantitatively by precipitation from the into building blocks via their C-1, C-8 positions are particu-
reaction mixture with MeOH and subsequent filtration larly attractive for photochemical dimerization reactions
through a silica plug.
because shrinkage can be prevented between the com-
Compound 8 is commercially available but was neverthe- pounds that are connected to one another. This made us
less synthesized on the 10 g scale from 1,3,5-tribromoben- explore the reactivity of ditriflates 6 towards Sonogashira
zene following literature procedures (Scheme 1, b).[14] For cross-couplings. As substrate, we decided to use the not yet
the final step to the propellers 1a–d, the precursors 7a–d discussed derivative 6e which carries methyl ester groups.
were subjected to the complex cyclization required which The compound was initially synthesized in order to achieve
involved one inter- and two intramolecular consecutive re- the methyl ester monomer 1e (see Scheme S1), however syn-
action steps. For these steps copper-free Sonogashira condi- thesis of the latter failed most likely due to severe solubility
tions were applied under high-dilution (1–2 mm) with 8 as problems at the precursor stage 7e. As coupling partners for
stoichiometric coupling partner. Due to the inconvenience 6e, we chose phenylacetylene and the two silylated acetyl-
of degassing the large volumes of solvent employed for enes trimethylsilylacetylene and triisopropylsilylacetylene
these reactions, the use of copper iodide was avoided in or- (Scheme 2). In all cases high yields of the doubly substi-
der to reduce the formation of oligomers due to Glaser cou- tuted products 10, 11 and 12 were obtained under mild con-
pling. Considering the complexity of this step, yields rang- ditions. For product 12, a single crystal structure was ob-
ing from 20–34% were quite satisfactory. Compounds 1a–c
precipitated out during the reaction while the relatively
highly soluble 1d was best purified by rGPC. In the latter
case, the polarity of the intermediates is dominated by the
dodecyl chains, making difficult to separate the desired
compounds from the side products by silica gel column
chromatography. The waxy consistency of all these com-
pounds was also an undesirable characteristic in terms of
purification and for analytical purposes. Nevertheless, this
compound could be obtained in pure form. The overall
yields to the ditriflates 6a–d were between 49% and 72%,
those to the final propellers 1a–1d were between 6% and
16% over all steps.
Given the high symmetry of the compounds, analysis by
NMR spectroscopy was simple; full characterization by 1H-
NMR- and 13C-NMR spectroscopy as well as high-resolu-
tion mass spectrometry (HRMS) confirmed the proposed
structure of rotors 1c–d. For 1a–b, a 13C-NMR spectrum Scheme 2. Sonogashira cross-coupling reactions of the ditriflate 6e.
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M. Servalli, L. Gyr, J. Sakamoto, A. D. Schlüter
FULL PAPER
evaporated in vacuo and the obtained residue was washed with
warm MeOH to obtain a yellow suspension. The solid was filtered
and rinsed with additional MeOH to afford pure 6a–d as a yellow
fluffy solid.
tained (displayed in Figure S36), displaying a rare case of
DAA face-to-face stacking. The two units of 12 are engaged
in this stack not in the expected anti-parallel,[15] but rather
a parallel yet slightly set-off fashion. The crystals do not
contain solvent molecules, which proves the efficiency of
the packing. It is noted that the ditriflates 6 are clearly su-
perior to the dibromide 9 mentioned in Scheme 1 (b). While
the latter would perhaps have been the natural choice for
transformations as described in this paper, it proved to be
so poorly soluble and unreactive, that even under forcing
conditions such as microwave treatment at 150 °C, no cou-
pling products could be obtained (not further described).
General Procedure for Monomer Precursors 7a–d: An excess of tri-
flates 6a–d (6 equiv.) was suspended/dissolved in dry dioxane with
1,3,5-triethynylbenzene 8 (1 equiv.) and 2,6-lutidine (6 equiv.). The
reaction mixture was degassed and then the catalysts Pd(PPh3)4
(0.05 equiv.) and CuI (0.05 equiv.) were added. The reaction mix-
ture was then heated under nitrogen in the dark at 65 °C for 2 d.
For the isolation of 7a–b, the reaction mixture was filtered and the
obtained yellow solid was washed with dioxane and acetonitrile to
eliminate any excess of starting material and then washed with
MeOH. The obtained solid was isolated in purities high enough for
the next step. To recover the excess of ditriflate, MeOH was added
to the filtrate and the obtained precipitate was dissolved in little
DCM and passed through a short silica plug (eluent 20% ethyl
acetate in hexanes) to obtain 6a–b. For the isolation of 7c–d,
MeOH was added to the reacton mixture and the resulting yellow-
ish precipitate was collected by filtration and purified by rGPC to
afford the products as yellow solids and recover the excess starting
Conclusions
In summary, by using the very potent ditriflates 6a–d a
variety of propeller-shaped compounds were made available
in short sequences with reasonable overall yields and rea-
sonable overall effort. These propellers will be important
building blocks for laterally extended covalent structures material.
derived from photochemically induced DAA dimerization
General Procedure for Synthesis of the Monomers 1a–d: Precursors
reactions. Future reports will be devoted to this.
7a–d (1 equiv.) were suspended in dry toluene (concentrations rang-
ing from 1 mm to 2 mm) with 8 (1 equiv.) and 2,6-lutidine
(100 equiv.). The suspension was degassed, then Pd(PPh3)4
(0.6 equiv.) was added. The reaction mixture was heated in the dark
at 80 °C under argon for 7 d. To isolate 1a–b, the reaction mixture
was filtered and the obtained brown solid was washed with toluene
and MeOH. It was then suspended in chloroform and passed
through a pad of celite to obtain yellow solution. The solution was
concentrated to saturation and put in the fridge to let the product
precipitate out as a fluffy yellow solid. Filtration afforded pure
monomers 1a–b. To isolate 1c–d, the reaction mixture was concen-
trated to about ¼ of its volume, MeOH was added and the ob-
tained yellow solid was collected by filtration. Purification by rGPC
afforded the desired compounds.
Experimental Section
General: Details about the experimental procedures, including
characterization, can be found in the Supporting Information.
General Procedure for Esterification to Compounds 3a–d: Acetylene-
dicarboxylic acid 2 (1 equiv.) was refluxed with the corresponding
alcohol (2.2 equiv.) and p-toluenesulfonic acid (0.1 equiv.) in benz-
ene using a Dean–Stark trap until the theoretical amount of water
was distilled off. The reaction mixture was concentrated and the
products were isolated by vacuum distillation in the case of 3a–b,
while silica gel column chromatography was employed for the
higher congeners 3c–d.
General Procedure for Sonogashira Cross-coupling Reactions to
Compounds 10, 11 and 12: Ditriflate 6e (1 equiv.) was suspended
in dry dioxane with the corresponding acetylenic coupling partner
(2.5 equiv.) and 2,6-lutidine (3 equiv.). The suspension was de-
gassed, then Pd(PPh3)4 (0.03 equiv.) and CuI (0.03 equiv.) were
added and the reaction mixture was heated to 65 °C under nitrogen
for 16 to 24 h, depending on the coupling partner. For the isolation
of 10, the reaction mixture was passed through a celite pad and
concentrated; the residue was dissolved in little DCM, then MeOH
was added and the formed yellow precipitate was collected and
recrystallized from acetonitrile to afford pure 10 as yellow needles.
To isolate 11, the reaction mixture was passed through a celite pad
and concentrated; the residue was subjected to silica gel chromatog-
raphy (10% EtOAc in hexane) to afford the pure product. Com-
pound 12 precipitated out during the reaction as a yellow solid;
filtration and subsequent recrystallization from boiling o-dichloro-
benzene, afforded the target compound as yellow needles, suitable
for SC-XRD analysis.
General Procedure for Aza-Michael Addition to Compounds 4a–d:
A solution of 2,6-diaminotoluene (1 equiv.) in MeOH was added
dropwise to a solution of diesters 3a–d (2 equiv.) in MeOH/CHCl3
at 0 °C. The reaction mixture was stirred at room temperature for
additional 16 h. The products 4a–b were isolated in pure form by
filtration and subsequent rinsing with MeOH, while for 4c–d, the
reaction mixture was concentrated and the oily residue was purified
by silica gel column chromatography.
General Procedure for Double Intramolecular Cyclisation to Com-
pounds 5a–d: Compounds 4a–d were refluxed for 1.5 h in degassed
diphenyl ether under high diluted conditions (ranging from 43 to
118 mm). In the case of 5a–c, the product precipitated out of the
solution upon cooling to room temperature; hexane was added to
complete precipitation and the obtained solid was then filtered and
thoroughly washed with additional hexane in order to eliminate all
traces of diphenyl ether. For the isolation of 5d, the solvent was
removed by vacuum distillation and the residue purified by silica
gel column chromatography.
General Procedure for Triflation to Compounds 6a–d: Compounds
5a–d (1 equiv.) were suspended/dissolved in dry DCM along with
DMAP (0.4 equiv.) and 2,6-lutidine (3 equiv.). The reaction mix-
ture was cooled to 0 °C and triflic anhydride (3 equiv.) was added
dropwise over 10 min. After addition, the mixture was warmed to
room temperature and stirred for additional 6 h. The solvent was
Acknowledgments
This work was supported by the ETH Zürich, Switzerland (grant
number ETH-26 10-2). The authors thank Chiara Gstrein for her
competent help with UV/Vis measurements, Dr. Bernd Schweizer
(D-CHAB, ETHZ) for solving the X-ray crystal structure and Rolf
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Building Blocks for Photoinduced Growth Reactions
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Received: April 20, 2015
Published Online: June 9, 2015
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