Double head-to-tail direct arylation as a viable strategy towards the synthesis of the aza-analog of dihydrocyclopenta[hi]aceanthrylene-an intriguing antiaromatic heterocycle

Article abstract of DOI:10.1039/c5cc08716c

The first case of double head-to-tail direct arylation of aromatic compounds and the unusual photophysical properties of the resulting 2,2a^1,5b^1,7-tetraazacyclopenta[hi]aceanthrylene are reported. This molecule, comprising of two imidazo[1,2-a]pyridine units, is antiaromatic due to the changes in the efficiency of π-electron ring current and it belongs to a class of seldom encountered compounds with a dark lowest electronically excited singlet state.

Full text of DOI:10.1039/c5cc08716c

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Double head-to-tail direct arylation as a viable strategy towards
aza-analog of dihydrocyclopenta[hi]aceanthrylene – intriguing
antiaromatic heterocycle
Cite this: DOI: 10.1039/x0xx00000x
a
b
c
b
Received 00th January 2012,
Accepted 00th January 2012
Dikhi Firmansyah, Irena Deperasińska, Olena Vakuliuk, Marzena Banasiewicz,
c
b
d
Mariusz Tasior, Artur Makarewicz, Michał K. Cyrański, * Bolesław
Kozankiewicz, * and Daniel T. Gryko *
b
a,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
the direct C3-arylation,10 could be fused via dehydrogenative
coupling reaction. The required substrates 1 and 2 were obtained
in the condensation of 2-chloroacetaldehyde with 2-aminopyridine
and with 2-amino-6-bromopyridine, respectively.11 The compound
The first case of double head-to-tail direct arylation of
aromatic compounds and the unusual photophysical
2,2a1,5b1,7
properties
of
the
resulting
-
tetraazacyclopenta[hi]aceanthrylene are reported. This
molecule, comprising of two imidazo[1,2- ]pyridine units, is
3
was synthesized via direct arylation of imidazo[1,2-a]pyridine
a
(
1) with substrate 2, following the general procedure reported by
antiaromatic due to the changes in the efficiency of π-
electron ring current and it belongs to a class of seldom
encountered compounds with dark lowest electronically
excited singlet state.
Doucet et al.10e (Scheme 1).
Polycyclic aromatic hydrocarbons (PAHs)1 and their
2
heterocyclic analogues possessing -expanded structures are
of great significance in the field of organic electronics. The
potential applications of materials possessing the heteroacene
core in electronic devices such as organic light emitting
diodes (OLEDs),3a organic field effect transistors (OFETs)3b
as well as dye-sensitized solar cells4 have been widely
studied in recent years. Most of the work in this area has
focused on linear heteroacenes5 and ladder-type
heterocycles.6 Although perylene is unique in the family of
PAHs and has been extensively studied, the incorporation of
heteroatoms into its skeleton is a difficult task. Only a few
7
such perylene analogues are known because of the lack of
general synthetic methods. Imidazo[1,2-a]pyridine is
a
prototypical heterocyclic system that is known to possess
useful optical properties,8 and over the last five years,
numerous papers have described advances in its synthesis.9
The development of suitable substrates to obtain heterocycles
comprised of two imidazo[1,2-a]pyridine moieties that are
fused at the 3,5 positions by using classical methods remains
a difficult undertaking. Therefore, we aimed to develop an
efficient synthesis of this compound through a different
Scheme 1. Direct arylation of imidazo[1,2-a]pyridine and the synthesis
of compound 4.
Subsequently, singly-linked precursor 3 was subjected to the
recently optimized anion-radical coupling conditions (K, toluene,
95 °C, air).12 However, the expected bis-imidazo[1,2-a]pyridine 4
was not formed under these conditions. Moreover, neither Scholl
strategy.
reaction (AlCl
BF ∙Et
3
, NaCl, 150 °C),13 nor oxidative coupling (PIFA,
Our initial strategy towards bis-imidazo[1,2-a]pyridine 4 was
based on the assumption that singly-linked dimer 3, obtained via
O)13 afforded the desired product 4 (Scheme 1). In light of
3
2
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these results we decided to investigate an alternative strategy,
namely double head-to-tail direct arylation. Thus, 5-
bromoimidazo[1,2-a]pyridine (2) was subjected to optimized
conditions reported recently by Doucet and co-workers, (based on
Another case of antiaromaticity has been recently revealed by
Tobe and co-workers.19 This unusual decline of aromaticity was
also well reflected in the dramatic decrease of magnetic
susceptibility exaltation of compound 4 as compared with the
parent imidazo[1,2-a]pyridine system.
3 5
PdCl(C H
)(dppb) as catalyst).14 To our delight, double direct
arylation occurred, leading to compound 4 with 20% yield. This
conversion constitutes the first case of double head-to-tail direct
arylation ever reported. Careful optimization studies (ESI) allowed
us to improve the yield of the formed bis-imidazo[1,2-a]pyridine 4
to 57% using Pd(OH)
2
/C, KOAc in DMA.15 Starting from 2-
bromo-4-methyl-6-aminopyridine we subsequently prepared
analogous bis-imidazo[1,2-a]pyridine 5 (Fig. 1, details in ESI). It
is important to add that substituted 2-bromo-6-aminopyridines are
neither commercially available nor easy to synthesize, which
somewhat limits the scope of this methodology. All attempts to
obtain X-ray quality crystals of compounds 4 or 5 failed.
Fig. 2. The NICS values for
1 and 4. Values of NICS1 are given in
parentheses.
Application of a simple homodesmotic approach20 (bis-
imidazo[1,2-a]pyridine + 4 x ethene => 2 x imidazo[1,2-
a]pyridine + 2 x butadiene), using the magnetic susceptibilities of
the systems (calculated at CSGT/B3LYP/6-311+G** level of
1
8
theory ) led to a reduction of 42 cgs ppm. This decrease is
noteworthy, taking into account that the value of magnetic
susceptibility for 4 is –92.6 cgs ppm, whereas a similar
homodesmotic approach for perylene (perylene + 4 x ethene => 2
x naphthalene + 2 x butadiene) is only 16.5 cgs ppm. Importantly,
this large change in magnetic structure was not accompanied by
changes in either stability or geometry. Application of the
homodesmotic approach (as above) using the total energies of the
systems (fully optimized at B3LYP/6-311+G**, corrected for zero
point energy correction; all species corresponded to minima on the
1
13
Fig. 1. H NMR and C NMR assignment for compound
4 (a); and
1
13
structure of compound
Both confirmation of the structure and unambiguous signals
assignment was performed based on additional 2D NMR-
experiments, such as COSY, C H-HSQC and C H-HMBC
Fig. 1, details in ESI).
5 (b). H and C shifts given in ppm.
18
potential energy surface with no imaginary frequencies ) of the
compounds gave only 2 kcal/mol reduced stability of compound 4
as compared with imidazo[1,2-a]pyridine. Moreover, the
1
3
1
13
1
(
21
geometry-based index of aromaticity HOMA gave almost the
same values for the imidazo[1,2-a]pyridine moiety in 4 and in the
parent heterocycle 1 (0.85 and 0.83, respectively). This result
implies that the changes of the extent of -electron delocalization
due to coupling of two imidazo[1,2-a]pyridines are almost
exclusively manifested in the magnetic properties. They are
dominated by dramatic changes in the efficiency of the -electron
ring current.
1
The analysis of the H NMR spectrum of bis-imidazo[1,2-
a]pyridine (2,2a1,5b1,7-tetraazacyclopenta[hi]aceanthrylene, 4)
revealed one striking observation. The protons of the six-
membered rings, which in the spectrum of imidazo[1,2-
a]pyridines are usually located around 7.12-8.13 ppm,16 were up-
field shifted to 5.14-6.39 ppm. There is also the 0.35 ppm up-field
shift of the Me groups in dye 5 compared to compound S2. This
observations led us to analyze the changes of the extent of cyclic
2
5000

-electron delocalization in this molecule. Initially, we used
0
.6
S
0
->S14
n-hexane; RT
1
7
Nucleus Independent Chemical Shift, which we calculated at the
ring centers (NICS) and 1Å above (NICS1) using
20000
0 4
S ->S
1
8
GIAO/B3LYP/6-311+G** levels of theory. The values for 4 and
are given in Fig. 2. For the pyridine-type outer ring, NICS
S
0
->S
7
0.4
0.2
0.0
15000
1
showed a very marked drop from –6.6 ppm (in the parent
imidazo[1,2-a]pyridine) to +2.0 ppm in compound 4! The
respective NICS(1) value changed from –7.9 to –0.6 ppm. In the
five membered heterocyclic ring a similar, albeit smaller effect
was observed: the NICS decreased here by 4 ppm. Topologically,
this system resembles perylene, which has an outer naphthalene
moiety that is less aromatic than in the parent molecule, whereas
the central ring is strongly antiaromatic. Notably, here the NICS
for the central ring was even more positive (NICS = +14.1,
NICS(1) = +9.6) than in perylene (NICS = +8, NICS(1) = +3).
S0->S2
10000
0 3
S ->S
*60
5
000
0 1
S ->S
0
2
00
250
300
350
400
450
500
550
600
wavelength [nm]
2
| J. Name., 2012, 00, 1-3
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Fig. 3. Absorption spectrum of
of the electronic transitions S
4
in n-hexane and calculated energies
excitation energy relaxation.22 An example of such compounds
are diphenylpolyenes, molecules with the C2h symmetry (like
0
→S (black triangles, details in ESI).
i
Heights of the vertical lines indicate calculated oscillator strengths of
4
), where the symmetry of the lowest excited singlet state is
the allowed transitions.
dependent on the chain
length.23-25
Having the desired molecule in hand, we conducted
Tetraazacyclopenta[hi]aceanthrylene bears some structural
resemblance to s-indacenes, which are also non-fluorescent.26
Conical intersection may contribute to fast depopulation of the
photophysical studies. The absorption spectrum of compound 4 in
n-hexane (Fig. 3) revealed that the maximum of lowest energy
absorption line was located at 420.9 nm. The spectrum is simple
within the spectral range between 325 and 440 nm, and seems to
be more complicated at higher energies (at wavelengths shorter
than 325 nm). The above spectrum can be rationalized with the aid
2
S state of compound 4, and may explain the lack of radiative
emission from this state. The S
forbidden.
1 0
→S transition is symmetry
of quantum chemistry calculations. Results of calculations of S
0
→
S
i
transition energies and corresponding oscillator strengths
S
0
->S
2
cal
-1
-1


00 = 24145 cm
with use of TD DFT/B3LYP/6-31G(d,p)//DFT B3LYP/6-31G(d,p)
method are shown in Fig. 3 and presented with the description in
Tabs.S20 of ESI. According to calculations (see Tab. S20) bands
exp
00 =23801 cm
exp
cal
observed in the range 325-440 nm correspond to the S
transition. In the group symmetry C2h, to which molecule 4
belongs, it is the symmetry allowed A →B transition. Fig. 4
depicts both, the experimental absorption spectrum and the
simulated spectrum in the S →S region. The Franck-Condon
FC) factors for the vibronic transitions, given by the vertical
lines, were calculated for the geometry of molecule 4 optimized in
the S state. A nearly perfect fit between the calculated and
0 2
→S
g
u
3
500
3000
2500
2000
1500
1000
500
0
-500
-1
wavenumber [cm ]
0
2
(
Fig. 4. Experimental absorption spectrum (red) observed over the range
70-430 nm, which is attributed to vibronic structure of the S →S
2
2
3
0
experimental spectra was obtained when we broadened the
transition lines with the Gaussian distribution. Notably, the two
transition. Calculated FC factors for the vibronic transitions, shifted to
coincide with the experimental spectrum, are given by the vertical
lines. The simulated spectrum (black) was obtained when the lines
were broadened with the Gaussian distribution, each with fwhm = 200
-
1
main vibrations, 369 and 1608 cm , can be characterized as
stretching of molecule 4 along the C-C bonds connecting the two
subunits 1 (see vibration vectors in the inset of Fig. 4). The
relatively large values of the Franck-Condon factors for this
vibration may be understood in terms of the large changes in the
lengths of these C-C bonds when the molecule is excited from
-1
cm .
Surprisingly, cyclic voltammetry studies revealed that they are
not electrochemically active i.e. they undergo neither oxidation
nor reduction in broad range of potentials. According to the
DFT B3LYP/6-31G(d,p) calculations energies of frontier
0 2
state S to S (by 0.03A; see Fig.S24 in ESI.
molecular orbitals of compound
and E(LUMO) = −1.85 eV respectively (all energies calculated
for the geometry of optimized in neutral state).
4 are E(HOMO) = −4.98 eV
The transition S
the experimental absorption spectrum of compound
the spectral range between 430 and 600 nm we observed only a
0
(A
g
)→S
1
(A
g
) is symmetry forbidden. In
4
within
4
In conclusion, we have demonstrated that through careful
design of the substrate, it is possible to fuse two molecules of
heterocyclic bromoarene via double direct arylation. The
prepared previously unknown π-expanded system, i.e.,
-
1
weak structure (with the frequency
orders of magnitude lower intensity than that for the S
transition. We suppose that we are observing vibronically
induced transitions, like in a case of S (A1g)→S (B2u) transition
in benzene and some other molecules.22 In this mechanism the
intensity for the S (A )→S (A ) transition is borrowed from
other, symmetry allowed electronic transitions. As it was
presented in Tab. S20, in molecule there are several
symmetry allowed transitions with high oscillator strength. All
of them have symmetry B and these states can be mixed with
the S (A ) state by b promoting vibrational modes.

1450 cm ) with two
0
→S
2
0
1
1,5 1,7
2
,2a b -tetraazacyclopenta[hi]aceanthrylene, is a compound
with symmetry-forbidden transition between the ground state
and the lowest electronically excited singlet state S , which
results in no fluorescence at all. Dramatic changes in the
efficiency of the -electron ring current, as exhibited by
0
g
1
g
S
0
1
4

magnetic properties led to antiaromaticity of this compound (in
sharp contrast with aromaticity of parent imidazo[1,2-
a]pyridine). Experimental absorption spectrum and TDDFT
calculations gave compact picture of the electronic states of
u
1
g
u
Despite the use of different excitation wavelengths and a
sensitive photon detection set-up, we were not able to monitor
compound . We observed the large energy gap between the
4
any fluorescence emission of
the S state. The same optical properties have been observed
for compound (see ESI Tab. S26 and Fig. S27).
The group of molecules with a dark S state has been
1
4, neither from the S nor from
S
1
and S states, what is the characteristic feature of
2
2
antiaromatic molecules.23-25
5
This work was funded by the Foundation for Polish Science
1
(MISTRZ programme). Theoretical calculations were
extensively studied to extend our knowledge about the
mechanisms of intermolecular coupling and the paths of
performed at the Interdisciplinary Center of Mathematical and
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1
36, 9181–9189.
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| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5CC08716C
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COMMUNICATION
Table of contents entry
Straightforwardly prepared head-to-tail bis-imidazo[1,2-a]pyridine displays antiaromaticity and no
fluorescence due to the forbidden S →S transitions.
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This journal is © The Royal Society of Chemistry 2012
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