Anna Wrona-Piotrowicz1, Anna Makal2, Janusz Zakrzewski1. 1. Department of Organic Chemistry, Faculty of Chemistry, University of Lodz, Tamka 12, 91-403 Łodź, Poland. 2. Biological and Chemical Research Center, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02-089 Warszawa, Poland.
Abstract
Triflic acid-promoted 1-adamantylation and tert-butylation of pyrene at positions 2 and 2,7 along with the synthesis of compounds having one-, two-, and three-pyrenyl groups attached to the adamantane scaffold are disclosed. Fluorescent properties of these compounds and channeled crystal structure of the 1,3,5-tris(pyren-2-yl)adamantane containing chloroform as a guest are also presented.
Triflic acid-promoted 1-adamantylation and tert-butylation of pyrene at positions 2 and 2,7 along with the synthesis of compounds having one-, two-, and three-pyrenyl groups attached to the adamantane scaffold are disclosed. Fluorescent properties of these compounds and channeled crystal structure of the 1,3,5-tris(pyren-2-yl)adamantane containing chloroform as a guest are also presented.
Pyrene 1 (Figure ) is a fascinating
fluorophore[1] and one of the most promising
starting materials for preparing a
plethora of organic optoelectronic materials.[2] Two alkyl-substituted pyrene derivatives, 2-tert-butylpyrene 2 and 2,7-di-tert-butylpyrene 3, have been widely used as useful starting materials in numerous
syntheses of dyes that feature special molecular shapes and suppressed
aggregation properties.[3] Moreover, the
latter compound proved to be an efficient acceptor for triplet–triplet
annihilation[4] and a π–donor
for semiconductive charge-transfer cocrystals.[5] To date, both compounds could be obtained via Friedel–Crafts
alkylation of pyrene with tert-butyl chloride in
the presence of a Lewis acid (aluminum chloride or bromide). This
reaction is unusual because electrophiles usually attack pyrene at
positions 1, 3, 6, or 8.[2a] Moreover, there
have been no reports on the extension of its scope to other alkyl
halides.
Figure 1
Formulae of compounds 1–3.
Formulae of compounds 1–3.
Results
and Discussion
In continuation of our earlier work on triflic
acid (TfOH)-promoted
Friedel–Crafts pyrene chemistry,[6] we became interested in elaborating a synthetic route to pyrenes
bearing other bulky tertiary alkyl groups at C(2) and C(7) using this
superacid as a promoter. The adamantyl group seemed especially interesting
because physicochemical properties of adamantyl-substituted compounds
may be different from those of their tert-butyl-substituted
counterparts.[7] Moreover, in contrast to
the tert-butyl group, the rigid adamantane scaffold
offers a possibility of attachment, via the Friedel–Crafts
chemistry, of the two, three, or four aryl group to the tertiary carbons.
It has been demonstrated that di-, tri-, and tetraaryl adamantanes
have a great tendency to form various types of supramolecular systems
in the solid state (e.g., encapsulating crystals[8] and were used in syntheses of porous aromatic frameworks).[9] The adamantane skeleton was also used as a rigid
linker between chromophoric groups.[10] Therefore,
molecules bearing several pyrenyl groups in a well-defined spatial
arrangement might be of interest for developing solid-state emitters,
sensors, or self-assembled nanomaterials.In this study, we
report the results of this research along with
the basic fluorescent properties of the synthesized pyrene derivatives 4–7 (Table ) and the crystal structure of tripyrenyladamantane 7 encapsulating molecule of the solvent (chloroform).
Table 1
Optimized Conditions for the Adamantylation
of Pyrene
entry
AdaX
pyrene/AdaX/TfOH (mmol)
reaction time
product (isolated yield, %)a
1
1-AdaBr
1:1:4
5 min
4 (93)
2
1-AdaOH
1:1:4
5 min
4 (84)
3
1-AdaBr
1:2.2:4
30 min
5 (91)
4
1-AdaCl
1:2.2:4
30 min
5 (87)
5
1-AdaOH
1:2.2:4
1 h
5 (95)
6
2-AdaBr
1:2.2:4
30 min
5 (83)
7
2-AdaOH
1:2.2:4
1 h
5 (87)
8
1,3-AdaBr2
3:1:12
4 h
6 (62)
9
1,3-Ada(OH)2
3:1:12
4 h
6 (49)
10
1,3,5-Ada(OH)3
4.5:1:18
6 h
7 (63)
Reactions were carried out in dichloromethane
(10 mL) at r.t.; Ada = adamantyl.
Reactions were carried out in dichloromethane
(10 mL) at r.t.; Ada = adamantyl.We found that pyrene 1 smoothly reacts
with 1-adamantanol
or 1-bromoadamantane in the presence of excess of TfOH in dichloromethane
at room temperature to afford, depending on the reaction conditions,
compound 4 or 5. To obtain pure 4, not contaminated with 5, pyrene and adamantyl compound
were used at a molar ratio of 1:1 and the reaction was stopped after
5 min (entries 1 and 2). On the other hand, at the pyrene to adamantyl
compound molar ratio of 1:2.2 and prolonged reaction times (30 min
to 1 h), diadamantylpyrene 5 was formed in high yields
(entries 3–7). It is also worthy to note that compound 4 can also be obtained in practically the same yield using
2-bromoadamantane or 2-adamantanol as alkylating agents (Table , entries 6 and 7).
This suggests the intermediacy of the 2-adamantyl cation, which is
known to rapidly isomerize to its 1-isomer.[11,12]Having in hand an efficient protocol for the synthesis of 4 and 5, we became interested in its application
for the synthesis of compounds having two and three pyrenyl units
linked to the adamantane skeleton. To our pleasure, we found that
the reaction of 3 equiv of pyrene with 1,3-dibromoadamantane or adamantane-1,3-diol
in the presence of TfOH yielded, after 4 h, 1,3-bis(pyren-2-yl)adamantane 6 in 62 and 49% yield, respectively (entries 8 and 9). Furthermore,
the reaction of 4.5 equiv of pyrene with adamantane-1,3,5-triol gave,
after 6 h, 1,3,5-tris(pyren-2-yl)adamantane (7) in 63%
yield (Table , entry
10).We also found out that our protocol can be used for alternative
synthesis of tert-butylpyrenes 2 and 3 (see the Supporting Information for details). Both tert-butyl halides and tert-butanol afford
the expected products in high yields (86–96%). In our opinion,
this procedure is more convenient than the procedures described in
literature, which use aluminum halides as promoter.Formation
of the 2- and 2,7-substituted products in the reaction
of pyrene with tert-butyl halides is usually explained
to be due to bulkiness of the tert-butyl group (these
positions are relatively less sterically hindered).[2a] In our opinion, the same explanation may be valid for reactions
with adamantyl compounds.We studied electronic absorption and
emission spectra of compounds 2–7. The spectra
obtained for diluted solutions in
dichloromethane are shown in Figure (the table contains the photophysical data in the Supporting Information).
Figure 2
Electronic absorption
(a) and emission (b) spectra of compounds 2–7 in
argon-saturated dichloromethane solutions. Absorption
spectra were run at c = 5 × 10–6 M. Emission spectra were obtained with an excitation at 340 nm for
a solution having the same absorbance (0.10) at this wavelength.
Electronic absorption
(a) and emission (b) spectra of compounds 2–7 in
argon-saturated dichloromethane solutions. Absorption
spectra were run at c = 5 × 10–6 M. Emission spectra were obtained with an excitation at 340 nm for
a solution having the same absorbance (0.10) at this wavelength.All investigated compounds showed absorption and
emission bands
assignable to the isolated, monomeric pyrene fluorophore. There was
no evidence for the formation of intramolecular excimers by 6 and 7 (negligible emission at ∼450 nm).
This means that the rigid adamantane skeleton efficiently keeps pyrenyl
moieties electronically isolated from each other. However, there was
a significant difference in the solid-state emission of compounds 3 and 5 (normalized emission spectra in the solid
state in the Supporting Information). While
the former compound showed the structured emission characteristic
of isolated pyrene fluorophore, the latter displayed excimer emission
(λmax ∼ 450 nm). Therefore, the 1-adamantyl
group, unlike the tert-butyl group, does not protect
2,7-disubstituted pyrene against the aggregation.Unexpectedly,
we found that solution of 7 in CDCl3 in an
NMR tube slowly deposited crystals in the form of large,
yellowish elongated blocs. They tended to lose transparency and transform
into a powder within seconds after extracting from the mother solution.
Nevertheless, we succeeded in X-ray diffraction structure determination
of a single crystal suspended in a drop of the solvent. The molecular
structure of 7 is shown in Figure . The three pyrene units are differently
inclined with respect to the idealized plane containing the main adamantane
axis and C10 and H10 atoms. The inclination angles are 67.4(5)°,
15.6(6)°, and 21.9(5)° for pyrene moieties I, II, and III,
respectively. They also show some deviation from planarity and are
to some extent bent roughly along the short pyrene axis. The angles
between the average planes of the terminal rings within the pyrene
fragments are 4.2(3)°, 2.9(3)°, and 6.9(3)° for pyrenes
I, II, and III, respectively.
Figure 3
Molecular structure of 7. Atomic
displacement parameters
represented at the 50% probability level. C atoms numbered 11–26
constitute pyrene I, C atoms 31–46 constitute pyrene II, and
C atoms 51–66 constitute pyrene III moieties.
Molecular structure of 7. Atomic
displacement parameters
represented at the 50% probability level. C atoms numbered 11–26
constitute pyrene I, C atoms 31–46 constitute pyrene II, and
C atoms 51–66 constitute pyrene III moieties.The crystal contains infinite channels (∼7.5% of the
unit
cell volume) spreading in the [100] direction filled with highly disordered
chloroform molecules (Figure ).
Figure 4
Crystal packing of 7 viewed in the [100] direction.
Pyrenes I, II, and III are highlighted in red, blue, and green. The
disordered molecules of chloroform occupy channels highlighted in
yellow.
Crystal packing of 7 viewed in the [100] direction.
Pyrenes I, II, and III are highlighted in red, blue, and green. The
disordered molecules of chloroform occupy channels highlighted in
yellow.Analysis of intermolecular interactions
in the crystal of 7 shows that pyrene II and III moieties
are involved in a
partial π–π stacking with their inversion-related
counterparts (interring distances are equal to 3.402(4) Å for
pyrene II and 3.489(5) Å for pyrene III). On the other hand,
pyrene I is involved in a C10–H10···π
stacking interaction with the adamantane core of the inversion-related
molecule. The distance between the planes of the symmetry-related
pyrenes in that case is as long as 5.318(4) Å (Figure ).
Figure 5
Most important intermolecular
interactions formed by 7 in the crystal structure viewed
along [100]. Interatomic distances
shorter than the sum of the van der Waals radii are presented as red
dashed lines.
Most important intermolecular
interactions formed by 7 in the crystal structure viewed
along [100]. Interatomic distances
shorter than the sum of the van der Waals radii are presented as red
dashed lines.The transformation of the crystals
of 7 into a powder
is caused by the loss of the chloroform guest, which apparently stabilizes
the crystal structure. Therefore, this compound is able to bind chloroform
in its crystal structure in chloroform solution and release it when
chloroform is absent in the surrounding medium. Such a behavior promises
possible applications of 7 and its derivatives as crystal
containers for various substances. In addition, it is worthy to note
that all of the synthesized pyrenyladamantanes are expected to be
easily functionalized by electrophilic substitution, making possible
a tuning of their photophysical and supramolecular properties.In conclusion, we have elaborated a simple and efficient synthetic
route for introduction of tert-butyl and 1-adamantyl
groups into positions 2- and 2,7- of pyrene. It allowed synthesis
of compounds having two- and three-pyrenyl groups attached to the
adamantane scaffold. They show a solvent-dependent pyrene-like fluorescence,
which may be of interest for various applications. Furthermore, a
synthesized tripyrenyladamantane is able to reversibly bind and release
chloroform in the solid state. Future work will consist of derivatization
of the synthesized pyrenyladamantanes and testing their ability to
bind various guests in the solid state.
Experimental
Section
General Remarks
Solvents were purified prior to use
by reported methods. All reagents were purchased from Sigma-Aldrich
and used without further purification. The reaction progress was monitored
by means of thin-layer chromatography, which was performed on aluminum
foil plates covered with silica gel 60 F254. Column chromatography
was performed on silica gel 60 (0.040–0.063 mm, 230–400
mesh, Fluka). 1H and 13C NMR spectra were recorded
in CDCl3 on a Bruker ARX 600 MHz (600 MHz for 1H and 151 MHz for 13C). Chemical shifts were referenced
relative to solvent signals: δ = 7.26 ppm for 1H
and δ = 77.00 ppm for 13C. Spectra were recorded
at room temperature (291 K), chemical shifts are presented in parts
per million, and coupling constants are presented in hertz. Electronic
absorption spectra were run on a PerkinElmer Lambda 45 UV/vis spectrometer.
Corrected emission spectra were obtained on a PerkinElmer LS55 fluorescence
spectrometer. The emission quantum yields were determined for argon-purged
solutions using quinine sulfate in 0.5 M sulfuric acid (ΦF = 0.546) as a reference.
General Procedure for the
Synthesis of 2–7
Alkylating agent RX
(for amounts of reactants, see Tables S1 or S2) and TfOH were added to a solution
of pyrene in 10 mL of CH2Cl2 at room temperature.
After stirring for 5 min to 6 h (Tables S1, S2), the reaction mixture was poured into water (100 mL) and extracted
several times with dichloromethane. The combined extracts were dried
over anhydrous Na2SO4 and evaporated to dryness.
The crude products were purified by column chromatography on silica
gel (hexane or petroleum ether/ethyl acetate as eluent) to give the
desired products.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5