Induced Correspondence of a Local π-Aromatic Sextet in Heteroannulenes: Synthesis and Characterization

Article abstract of DOI:10.1002/chem.201600380

Acid-catalyzed [3+3] condensation reactions of two hitherto unknown tripyrrane moieties with pentafluorobenzaldehyde has led to the formation of new generation heteroannulene (4.1.4.1) and mutant heteroannulene (1.1.1.1.1.1). Inclusion of local π-aromatic sextets, namely the N-methyl pyrrole rings through β,β-linkages and α,β-linkages, has led to the isolation of first ever heteroannulenes cross-conjugated at four points and two points respectively within the macrocycles.

Full text of DOI:10.1002/chem.201600380

DOI: 10.1002/chem.201600380
Communication
&
Heteroannulenes
Induced Correspondence of a Local p-Aromatic Sextet in
Heteroannulenes: Synthesis and Characterization
[a]
[b]
[b]
[c]
[c]
Abhijit Mallick, Juwon Oh, Dongho Kim,* Masatoshi Ishida, Hiroyuki Furuta, and
[a]
Harapriya Rath*
Dedicated to Professor Tavarekere K Chandrashekar on the occasion of his 60th birthday
[
6,7]
Mçbius aromaticity. However, there are only a handful of re-
[
8]
ports on cross-conjugated expanded porphyrins. Prompted
Abstract: Acid-catalyzed [3+3] condensation reactions of
two hitherto unknown tripyrrane moieties with penta-
fluorobenzaldehyde has led to the formation of new gen-
eration heteroannulene (4.1.4.1) and mutant heteroannu-
lene (1.1.1.1.1.1). Inclusion of local p-aromatic sextets,
namely the N-methyl pyrrole rings through b,b-linkages
and a,b-linkages, has led to the isolation of first ever het-
eroannulenes cross-conjugated at four points and two
points respectively within the macrocycles.
[
9]
by our recent findings where inclusion of b,b-linkages of N-
methyl pyrrole ring into the macrocyclic core led to the isola-
tion of hitherto unknown smallest ever metal-free Mçbius aro-
matic [20] heterocyclic macrocycles, we were curious in explor-
ing more about the connection between structure and elec-
tron delocalization on the one hand and the ability of macro-
cyclic system to sustain an induced diamagnetic or paramag-
netic ring current on the other. Thus, we report herein hitherto
unknown heteroannulene (4.1.4.1) through the connectivity of
b,b-carbons of N-methyl pyrrole into the core of macrocycles
and mutant heteroannulene (1.1.1.1.1.1) through the connec-
tivity of a,b-carbons of N-methyl pyrrole into the core of mac-
rocycles. We have examined the consequences on aromaticity
both in the free base and upon protonation, as it is a well-
known fact that electron delocalization is significantly en-
hanced in excited or charged states of cross-conjugated sys-
tems, which may disclose new applications for this neglected
type of conjugation.
p-Conjugated macrocycles have attracted considerable atten-
tion owing to their unusual optical and magnetic properties
[1]
based on their effective cyclic conjugation. Whereas through
conjugated systems have been extensively explored, cross-con-
[
2]
jugated compounds have received only relatively little atten-
tion owing to much less ability to promote electron delocaliza-
tion along the conjugation path, thus offering much less op-
portunity to tune properties by means of functionalization. Re-
cently, however, the synthesis of extended cross-conjugated
systems has seen substantial progress, allowing the develop-
To grasp the target macrocycles, a synthetic design has
been logically approached in arriving at the tripyrrane moieties
6 and 10 (Supporting Information, Scheme S1). Rational syn-
thesis of the macrocycles 11 and 12 entails a [3+3] acid cata-
lyzed condensation of N-confused N-methyl tripyrrane 6 and
10, respectively, with pentafluorobenzaldehyde followed by ox-
[3]
ment of nanometer-size cross-conjugated entities. The pecu-
liarity of cross-conjugation is the presence of competing conju-
gation paths that may be exploited for the design of functional
materials, and thus materials based on these compounds are
now being explored more rigorously as they may present new
[
10]
idation with chloranil (Scheme 1).
The best yield was ob-
tained with 0.1 equiv of p-toluene sulfonic acid as catalyst.
Column chromatographic separation over basic alumina fol-
lowed by repeated silica gel (200—400 mesh) chromatographic
separation yielded the respective macrocycle 11 in 10% yield
and macrocycle 12 in 8% yield as green solids.
[4,5]
opportunities to nanomaterial science.
Porphyrins and ex-
panded porphyrins serve as benchmarks for extreme manifes-
tation of aromaticity, antiaromaticity, pseudo aromaticity, and
The structures of these new macrocycles have been estab-
lished by a thorough characterization by spectroscopic and
single-crystal X-ray diffraction analysis. Macrocycle 11 shows
parent ion peak at 1130.436 and macrocycle 12 shows parent
ion peak at 1130.279 under positive ionization conditions in
MALDI-TOF mass spectrometry (Supporting Information, Fig-
ure S7,S8), thus confirming the composition. Substitution of N-
methyl pyrrole in the core of porphyrins alters the electronic
structure of the ring and this is reflected in the altered spectro-
scopic and structural properties. The free base form of both
the macrocycles exhibit ill-defined absorption spectra in the
Soret band region without having any Q-like bands accounted
for by nonaromatic nature of these macrocycles. Figure 1
[
a] A. Mallick, Prof. Dr. H. Rath
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2
A/2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032 (India)
Fax: (+91)33-24732805
E-mail: ichr@iacs.res.in
[
b] J. Oh, Prof. Dr. D. Kim
Department of Chemistry
Yonsei University, Seodaemun-gu, Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
[
c] M. Ishida, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry
Kyushu University Fukuoka, 819-0395 (Japan)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600380.
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Communication
Scheme 1. Synthesis of macrocycles 11 and 12.
1
Figure 2. H NMR spectra of free base (top) and protonated (bottom) 11 at
98 K (? indicates residual solvent peak or solvent impurity).
2
other pyrrole ring. The NH signal appeared as a broad signal at
3.62 ppm confirmed by D O exchange experiment (Fig-
1
2
ure S38). There are two magnetically distinct phenyl rings.
Based on the scalar coupling between the peaks as well as the
dipolar coupling with heterocyclic b-CHs, the peaks in the
range 6.93–7.15 ppm correspond to one phenyl ring whereas
the peaks in the range 7.29–7.45 ppm correspond to another
phenyl CHs. The fact that all the b-CHs and a-CHs of heterocy-
cles in the macrocycle resonate in the same region as that of
an isolated pyrrole/heterocycle, the free base 11 can be
gauged as nonaromatic. Upon lowering the temperature nei-
ther any marked difference in spectral pattern nor any confor-
mational change was observed. Only rotation of one of the
phenyl ring is locked, thereby the signals of phenyl CHs are
separated without any shift (Figure S27). Next we attempted to
find the impact of protonation on the conformational change
of the macrocycle, however not much change in the spectral
pattern was observed in the completely protonated state at
298 K. To differentiate the heterocyclic b-CHs from meso-aryl
CHs, 2D ROESY (Figure S34) spectra were recorded in addition
to 2D COSY (Figure S33) spectra at ambient temperature, as
lowering the temperature led to merging of signals and no
Figure 1. UV/Vis absorption spectra of 11 and 12.
shows the UV/Vis absorption spectra of free base and proton-
ated forms of 11 and 12. As can be seen, protonation of both
the macrocycles lead to significant change in the absorption
spectra; the generation of an intense band at 641 nm with ill-
defined band within 700–950 nm for 11 and a sharp band at
6
60 nm with broad and ill-defined band within 850–1200 nm
for 12 typifying porphyrinic nature of these macrocycles with
[
11]
lack of aromaticity.
The twisted structure for the macrocycles in free base is pre-
1
served in solution, as determined by H NMR (Figure 2). For
macrocycle 11, well-resolved peaks were observed at ambient
temperature in free base form itself, despite the unusual way
of linking N-methyl pyrrole into the conjugation pathway.
Based on the 2D COSY (Supporting Information, Figure S28)
and ROESY (Figure S29) spectra, the peak at 3.37 ppm has
been assigned to methyl peak of N-methyl pyrrole ring due to
space correlation with signals at 6.32 ppm and 7.6 ppm (Fig-
ure S29), confirming the later signals as a-CHs of N-methyl pyr-
role ring since both these signals exhibit correlation in 2D
COSY spectra (Figure S28). The broad signal at 6.02 ppm exhib-
iting bond correlation with the doublet at 6.06 ppm (Fig-
ure S28) in 2D COSY spectra confirms that these signals are b-
CHs of same pyrrole ring. The observation of correlation be-
tween doublet at 6.15 ppm with the doublet at 6.59 ppm in
1
conclusive results were obtained (Figure S32). The H NMR
spectrum of protonated 11 at 298 K showed two distinctly dif-
ferent signals at 10.90 and at 12.00 ppm accounted for -NH
groups of the pyrrolic and protonated pyrrolenic rings, as con-
firmed by a D O exchange experiment (Figure S39). Further-
2
more, both these NH signals exhibit exchange correlation in
2D EXSY spectra (Figure S34a). The b-CHs of both the pyrrolic
pyrrole rings and protonated pyrrolenic rings exbihit a typical
COSY pattern (Figure S33a) and they have one set of protons
centered at 5.88, 6.29 ppm and another set of protons cen-
tered at 6.33 and 6.79 ppm and hence the observation of two
sets of correlations in 2D COSY spectra (Figure S33a). These as-
2
D COSY spectra indicates that these signals are b-CHs of an-
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signments were further confirmed by scalar coupling that each
of the NH signals exhibits to these well-resolved doublets (Fig-
ure S33b). The spatial proximity between the signal at
3
.73 ppm and the singlets at 6.67 ppm and 7.23 ppm enabled
us to identify two sets of correlations in 2D ROESY spectra (Fig-
ure S34b) and thus these signals correspond to the a-CHs of
N-methyl pyrrole rings. Here it is worth mentioning that there
are two different magnetically distinct phenyl groups with the
observation of multiplets in the range 7.13–7.19 ppm for one
phenyl ring and a set of signals in the range 7.32–7.52 ppm for
the other phenyl group, as observed in the free base form as
well. The CH groups of meso-phenyl substituents have been
assigned based on 2D COSY spectra (Figure S33c), ROESY spec-
tra (Figure S34) and EXSY spectra (Figure S35). Further confir-
mation of the individual signals came from HSQC spectra (Fig-
ure S36). The observations of heterocyclic a-CHs and b-CHs
resonating in the region as those of isolated pyrrole/heterocy-
cle clearly indicate absence of any sizable macrocyclic ring cur-
rent, and hence the protonated 11 can be gauged as nonaro-
matic.
Figure 3. X-ray crystal structure of free base 11 (a,b) and protonated 12
(a’,b’). Top view (a,a’) and side view (b,b’).
[15]
tures of isolated N-methyl pyrrole and bond lengths are practi-
cally unperturbed; that is, CbÀCb> CaÀCb (1.438 Š vs 1.369 Š)
for 11. For macrocycle 12, failure to obtain a good single crys-
tal suitable for X-ray diffraction in free base form forced us to
resort to geometry optimization based on DFT calculations.
In our next attempt, we tried to investigate the impact of
a,b linkages of N-methyl pyrrole rings into the macrocyclic
core upon the aromaticity in macrocycle 12. Even though mac-
rocycle 12 exhibits well-resolved spectra at RT in free base
form itself, but only upon lowering the temperature to 243 K
[
12]
With the B3LYP/6-31G* basis set, a minimum energy struc-
ture turned out to be a figure-eight structure (Supporting In-
formation, Figure S51). On the other hand, single crystals suita-
ble for X-ray diffraction were obtained by slow diffusion of cy-
clohexane into TFA/dichloromethane solution of 12. Macrocy-
cle 12 upon protonation turned out to have a flat geometry
(
Supporting Information, Figure S41), the NH signal was gener-
ated along with b-CH of N-methyl pyrrole. The spectral obser-
vation (Figure S43) indicates absence of ring current and thus
non-aromaticity in the free base form of the macrocycle. The
diprotonated 12 however shows drastic change in NMR spec-
tral pattern. The diprotonated 12 was obtained by adding
+
(Figure 3a’). The interesting features were observed in 12·2H ,
7
0 mL of 10% CF COOH/CDCl owing to the fact that stepwise
where four TFA molecules were involved in the flat geometry
3
3
+
addition of 10% CF COOH/CDCl led to precipitation and,
of 12·2H . Each half of the macrocycle contains one neutral
3
3
hence an excess amount was added in one step to get a clear
pyrrole NH and one protonated pyrrole NH. The TFA molecule
which is protonating the pyrrole, the carbonyl oxygen of this
TFA is involved in strong H-bonding with carboxyl group of
other TFA. The carbonyl oxygen of this second TFA molecule is
in turn involved in H-bonding with the free pyrrole NH (Sup-
porting Information, Figure S48). The short distance for O···O
was estimated to be 2.47 Š. These features suggest that while
the highly distorted structures of 11 and 12 lead to nonaroma-
solution in the NMR tube. Upon lowering the temperature to
+
2
43 K, well-resolved spectra were obtained for 12·2H (Fig-
ure S44). The inner NH proton resonated as a singlet at
1
1
5.45 ppm, whereas the two doublets at 9.7 ppm and
0.7 ppm have been assigned to the inner b-CH protons of in-
verted pyrrole ring. Similarly the singlet at 10.3 ppm has been
assigned to the inner b-CH protons of N-confused N-methyl
pyrrole ring. The outer NH proton of inverted pyrrole ring res-
onated at 4.3 ppm, the outer a-CH proton of N-confused N-
methyl pyrrole ring resonated at 6.4 ppm and the outer b-CH
protons of non-inverted pyrrole ring resonated at 5.9 ppm and
+
ticity, planar 12·2H induced by protonation allows its antiaro-
matic character.
+
In contrast to 12·2H , the energy-minimized structure for
+
11·2H obtained was a substantially bent shape (Supporting
6
.4 ppm (Figure S46). All of these assignments were confirmed
Information, Figure S50). The b–b linkages of N-methyl pyrrole
1
1
by using a H– H correlation spectroscopy (COSY) experiment
Figure S45). These observations clearly indicate the presence
would provide the steric restriction on a planar structure,
+
(
which results in the bent shape of 11·2H and accounts for its
of moderate paratropic ring current and hence antiaromaticity
nonaromaticity.
+
in 12.2H . The observed spectral patterns for free base and
protonated forms of macrocycles 12 are well-supported by cal-
culated NMR spectra (Figures S59,S60).
Such nonaromaticity and weak antiaromaticity of free-base
and protonated 11 and 12 were well-described by nucleus-in-
dependent chemical shifts (NICS) and anisotropy of induced
current density (AICD) calculations in their optimized struc-
The solid-state X-ray crystallographic analysis and structure
optimization calculation revealed the involved structural
changes of macrocycles 11 and 12 by protonation. For macro-
cycle 11, its crystal structure was strongly puckered (Figure 3a).
Here, the protruding N-methyl pyrrole rings preserve all fea-
[
13]
tures. In both 11 and 12, the NICS values at the center of
the macrocycle were calculated to be d=0.46 and À2.87 ppm,
respectively (Supporting Information, Figure S53, S55), and the
AICD plot (Figure S57a, S58a) showed the localized ring current
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Communication
on the pyrrolic subunits, representing their nonaromatic char-
tively rather nonaromatic macrocycles. Research into this
promising field is currently under progress in the author’s labo-
ratory.
+
acters. Compound 11·2H having a substantially bent shape
displayed similar results to 11 and 12, where the NICS value at
the center of the macrocycle was d=À1.45 ppm (Figure S54)
and its ring current was not fully delocalized in the AICD plot
+
Acknowledgement
(
Figure S57b). In the case of 12·2H , the NICS values of d=
3
.15–4.51 ppm (Figure S56) were estimated inside the macrocy-
The work at IACS, Kolkata was supported by DST-SERB (SR/S1/
IC-37/2012), CSIR (02(0120)/13/EMR-II), New Delhi, India and
DST-SERB Ramanujan Fellowship (SR/S2/RJN-93/2011), India.
A.M. thanks CSIR for senior research fellowship. The work at
Yonsei University was supported by the Global Research Labo-
ratory Program (2013K1A1A2A02050183) funded by the Minis-
try of Science, ICT & Future, Korea. The quantum calculations
were performed using the supercomputing resources of the
Korea Institute of Science and Technology Information (KISTI).
The work at Kyushu University, Japan was supported by JSPS
Grant-in-Aid for Scientific Research (25248039, 26810024). The
DBT-funded single-crystal X-ray diffraction facility in the De-
partment of Organic Chemistry, IACS and the single-crystal X-
ray diffraction facility in IISER, Pune are acknowledged.
cle, and the AICD plot (Figure S58b) showed the counterclock-
wise ring current over the whole macrocycle. These results are
1
+
in accordance with the H NMR result of 12·2H , indicating its
weak antiaromatic character.
The excited state dynamics also reflect the nonaromatic and
weak antiaromatic nature of free-base and protonated 11 and
1
2 (Figure 4; Supporting Information, Figure S62). The femto-
Keywords: aromaticity · cross-conjugation · heteroannulenes ·
unconventional pyrrole connectivity · unorthodox tripyrranes
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[
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[
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Received: January 27, 2016
Published online on March 8, 2016
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