M?bius Aromatic Core-Modified Heterocyclic [20] Macrocycles (4.1.1) with a Protruding N-Methyl Pyrrole Ring

Article abstract of DOI:10.1002/asia.201600025

Herein, we report the first synthesis of an unorthodox tripyrrane moiety from the regioselective β-benzoylation of pyrrole and the acid-catalyzed condensation of the desired precursors. A [3+1] Mac Donald type condensation strategy for this tripyrrane has led to the exclusive isolation of two hitherto-unknown aromatic [20] heterocyclic macrocycles (4.1.1).

Full text of DOI:10.1002/asia.201600025

DOI: 10.1002/asia.201600025
Communication
Macrocycles
Mçbius Aromatic Core-Modified Heterocyclic [20] Macrocycles
(4.1.1) with a Protruding N-Methyl Pyrrole Ring
Abhijit Mallick and Harapriya Rath*[a]
Dedicated to Professor Tavarekere K. Chandrashekar on the occasion of his 60th birthday
thetic point of view, but also from an understanding of the
Abstract: Herein, we report the first synthesis of an unor-
thodox tripyrrane moiety from the regioselective b-ben-
zoylation of pyrrole and the acid-catalyzed condensation
of the desired precursors. A [3+1] Mac Donald type con-
densation strategy for this tripyrrane has led to the exclu-
sive isolation of two hitherto-unknown aromatic [20] het-
erocyclic macrocycles (4.1.1).
molecular and electronic structures of these macrocycles. In-
cluded in the continuing efforts to synthesize more and more
such fascinating macrocycles has been the exploration of im-
proved and new precursors. Herein, we report our first efforts
towards the rational synthetic design and synthesis of an un-
precedented tripyrrane moiety (5), which has led to the isola-
tion of two hitherto-unknown [20] aromatic macrocycles 8 and
9. These macrocycles have been synthesized by strategically
bringing b,b-carbon atoms of N-methyl pyrrole ring into the
core of the macrocycles.[6]
Aromaticity continues to be the topic of numerous studies
that are not only associated with its relevance in chemistry,
biology, and technology, but also with the very concept itself.[1]
Even though chemists are accustomed to thinking of nearly
planar [4n+2] Hückel topology, recently, there has been an up-
surge in interest in the exploration of twisted Mçbius [4n] an-
nulenes. According to Hückel Molecular Orbital (HMO) theory,
in a fully delocalized system with 4np electrons, Hückel topolo-
gy predicts an open-shell electron configuration, whereas
Mçbius topology predicts a closed-shell structure.[2] Porphyrins
and expanded porphyrins have been widely studied to probe
subtle variations in aromaticity, antiaromaticity, and Mçbius ar-
omaticity.[3] Ever since the successful observation of Mçbius
topology in A,D-di-p-benziporphyrin,[4a] many new promising
classes of expanded porphyrins have been rigorously investi-
gated by inducing structural variations through metal coordi-
nation, temperature changes, solvent effects, protonation with
an appropriate acid, and functionalization with meso or b sub-
stituents.[4] Notably, Rh(I) [24] N-fused pentaphyrin and Pd(II)
[44] decaphyrin[5] are the smallest and largest Mçbius aromatic
macrocycles reported to date, respectively. The recent synthe-
ses of many new and novel Mçbius aromatic macrocycles have
challenged the synthetic and theoretical chemists to quantify
the smallest-possible porphyrin macrocycle with less than
[24]p electrons in the conjugation pathway that could exhibit
distinct metal-free Mçbius aromaticity,[5c] not only from a syn-
To synthesize the target macrocycles, we designed the
three-step synthesis of (1-methyl-1H-pyrrole-3,4-diyl)bis(phenyl-
methanone) (3; Scheme 1) from pyrrole, in which bulky sub-
Scheme 1. Regioselective b-benzoylation of the pyrrole ring.
stituents on the nitrogen atom of the pyrrole moiety were em-
ployed to obstruct electrophilic attack at the a positions
through significant steric and electronic effects. In this context,
the report by Corey et al.[7a] was highly impactful, sparking the
idea that triisopropylsilyl (TIPS)-pyrrole could act as a progeni-
tor of b-substituted pyrrole 3 through kinetic electrophilic sub-
stitution of 1-(triisopropylsilyl) pyrrole. Thus, TIPS-pyrrole 1 was
readily prepared in high yield from the sodium salt of pyrrole
and triisopropylsilyl chloride.[7b] Following the literature proce-
dure for the benzoylation of pyrrole,[7c] an excess use of AlCl3
and benzoyl chloride upon refluxing for 72 hours, has led to
the isolation of 3,4-dibenzoyl pyrrole 2 due to hydrogen chlo-
ride induced desilylation in 30% yield after flash column chro-
matography on silica gel. We attributed the high regioselectivi-
ty of this reaction to the fact that the regioselective step oc-
curred before the desilylation of TIPS-pyrrole, which would
have otherwise led to the formation of 2,4-dibenzoyl pyrrole.
In the final step, N-methylation of compound 2 was performed
by using methyl iodide to afford compound 3. Both com-
pounds 2 and 3 were thoroughly characterized by spectrosco-
py (see the Supporting Information, Figures S2, S3, and S11–
S14) and by single-crystal X-ray structural analysis (Figure 1).[8]
[a] A. Mallick, Prof. Dr. H. Rath
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A/2B Raja SC Mullick Road
Jadavpur
Kolkata 700 032 (India)
Fax: (+91)33-24732805
E-mail: ichr@iacs.res.in
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201600025.
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absorption spectra in the Soret band region (456.41 nm for 8;
451.28 nm for 9), with well-defined Q-like bands (624.35 and
675.45 nm for 8; 632.41 and 683.88 nm for 9), which was at-
tributed to the aromatic nature of these macrocycles. The het-
eroatom effect was clearly observed, by red-shift in the bands
with a bigger heteroatom (Se vs S). As shown in Figure 2, pro-
tonation of both the macrocycles led to the generation of
Figure 1. X-ray crystal structures of compounds 2 (left) and 3 (right).
Following reduction with lithium aluminum hydride, the corre-
sponding dicarbinol (4) was obtained in quantitative yield,
which was subsequently reacted with 80 equivalents of freshly
distilled pyrrole to yield the desired precursor (5) in 70% yield
(see the Supporting Information, Scheme S1). Notably, this is
the first report for synthesis of tripyrrane 5. Rational synthesis
of macrocycles 8 and 9 entailed a [3+1] acid-catalyzed con-
densation of tripyrrane 5 with 2,5-bis(tolylhydroxymethyl)sele-
nophene (6) or 2,5-bis(tolylhydroxymethyl) thiophene (7),[9] fol-
lowed by oxidation with chloranil (Scheme 2).[10] The best yield
was obtained with 0.1 equivalents of p-toluenesulfonic acid as
a catalyst. Purification by column chromatography on basic
alumina, followed by repeated column chromatography on
silica gel (200–400 mesh) yielded macrocycles 8 and 9 as green
solids in 10% and 8% yield respectively.
Figure 2. UV/Vis spectra of macrocycles 8 (left) and 9 (right).
sharp Soret bands with a red-shift of 30–35 nm (492.67 nm for
8; 481.86 nm for 9) and also red-shift in the Q-band region
(750.73 and 883.88 nm for 8; 719.97 and 855.67 nm for 9),
which typified the porphyrinic nature of these macrocycles
with aromaticity.[3b] Titration of these macrocycles against tri-
fluoroacetic acid was performed to isolate partially protonated
and diprotonated states (see the Supporting Information, Fig-
ures S21 and S24).
The structures of these new macrocycles were established
by thorough spectroscopic analysis. Macrocycles 8 and 9
showed peaks for their parent ions at m/z 722.404 and
674.305, respectively, under positive-ionization conditions in
MALDI-TOF mass spectrometer, thus confirming their composi-
tions. The electronic absorption spectra of both the macrocy-
cles in their free-base and protonated forms were porphyrin-
like. The free-base forms of both macrocycles exhibited sharp
The nonplanar structure of the macrocycles in their free-
base form was preserved in the solution state, as confirmed by
1H NMR spectra. For both the macrocycles, the unusual way of
linking the N-methyl pyrrole moiety into the macrocyclic con-
jugation pathway accelerated a dynamic flipping of the hetero-
cyclic rings, and so only broad peaks were observed at ambi-
ent temperature and no significant change in the spectroscop-
ic pattern was observed upon lowering the temperature (see
the Supporting Information, Figures S27 and S37). At 223 K,
the free-base form of macrocycle 9 exhibited three correlations
in the aromatic region (see the Supporting Information, Fig-
ure S38) between peaks at d=7.45 and 7.79 ppm, d=7.52 and
7.89 ppm, and d=7.65 and 7.95 ppm. The correlation between
the signals at d=2.56 and 7.45 ppm clearly indicated that the
signals at d=7.45 and 7.79 ppm corresponded to the m-CH
and o-CH protons of the meso-tolyl substituents, respectively.
In the ROESY spectra of the free-base form of macrocycle 8 at
253 K (see the Supporting Information, Figure S29), the resolu-
tion of the spectra was too low to allow for conclusive assign-
ment of all the peaks. However, the observation of methyl
signal for the N-methyl pyrrole moiety at d=3.25 ppm in the
spectra of both the macrocycles in the free-base form, strongly
downfield of the signal that regular N-substituted porphyrins
usually exhibit (e.g., d=À4.322 ppm for NCH3ÀTPPH),[11] and
the observation of two closely spaced doublets at d=
8.56 ppm, which could be assigned to the thiophene/seleno-
phene b-CH protons, provided confirmation of the structures
as shown in Scheme 3. The calculated Dd value of 6.0 ppm
suggested that the aromaticity in the free-base form was lower
Scheme 2. Synthesis of macrocycles 8 and 9.
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formation, Figure S32) at ambient temperature, because lower-
ing the temperature led to coalescence of the signals and no
conclusive results were obtained (see the Supporting Informa-
tion, Figure S31). There were two 3H singlets at d=2.70 and
2.71 ppm, which corresponded to the methyl peaks of the
meso-tolyl rings. The tolyl m-CH protons were unequivocally
assigned to the multiplet at d=7.61–7.63 ppm, based on the
correlation with the signals at d=2.70–2.71 ppm in the
2D COSY spectra (see the Supporting Information, Figure S32).
In turn, this multiplet showed a strong correlation with the
broad signal at d=8.11 ppm, thus allowing the assignment of
the later signal as the o-CH protons of the tolyl substituents
(see the Supporting Information, Figure S32). The peak at d=
À1.03 ppm was unambiguously assigned as the NH proton of
the pyrrole rings upon protonation, as confirmed by D2O-ex-
change experiments (see the Supporting Information, Fig-
ure S46). Two overlapping doublets at d=8.94 and 8.92 ppm,
which exhibited dipolar coupling with the broad signal at d=
8.11 ppm in the 2D ROESY spectra (see the Supporting Infor-
mation, Figure S33) without exhibiting any scalar coupling
with any signal(s) in the shielded/deshielded regions, were un-
ambiguously assigned to the b-CH protons of the seleno-
phene. There were four well-resolved doublets at d=8.20,
8.22, 8.38, and 8.55 ppm, which exhibited a typical COSY pat-
tern for pyrrole b-CH protons (see the Supporting Information,
Figure S32). The two sets of well-resolved doublets at d=8.22
and 8.55 ppm were unequivocally assigned to the b-CH pro-
tons on the same pyrrole ring, owing to a correlation between
them in the 2D COSY spectra (see the Supporting Information,
Figure S32) and dipolar coupling between the signal at d=
8.55 ppm with the o-CH proton of the meso-tolyl substituent
at d=8.11 ppm in 2D ROESY spectra (see the Supporting Infor-
mation, Figure S33). The doublet at d=8.38 ppm exhibited di-
polar coupling with the broad signal at d=8.21 ppm in the
2D ROESY spectra (see the Supporting Information, Figure S33),
whilst also exhibiting scalar coupling with the signal at d=
8.20 ppm in the 2D COSY spectra; thus, the doublets at d=
8.38 and 8.20 ppm corresponded to the b-CH atoms of the
other pyrrole ring, thereby confirming the assignment of the
later signal as the o-CH protons of the phenyl rings. The spatial
proximity of the signal at d=3.56 ppm and the signal at d=
7.38 ppm enabled us to identify a correlation in the 2D ROESY
spectra (see the Supporting Information, Figure S33); thus, this
later signal corresponded to both the a-CH protons of the N-
methyl pyrrole ring. These observations were consistent with
the prevalence of no pyrrole/thiophene ring-inversion, except
the protruding N-methyl pyrrole ring. Our assignment of all of
these peaks was further supported by 2D HSQC experiments
(see the Supporting Information, Figure S35). To obtain a satis-
factory interpretation of these surprising chemical shifts, we in-
voked a ring-current effect, because the manifestation of a por-
phyrin-like aromatic ring current is usually accounted for by
the downfield shifts of the peripheral substituents and up field
shifts of the inner NH/CH protons. The Dd value—that is, the
difference between the chemical shifts of the inner (NH/CH)
Scheme 3. Charge-separated resonance structures (bold lines denote the de-
localization pathways).
than that of an [18p] porphyrin system. Macrocycles 8 and 9
happened to be cross-conjugated systems, for which dipolar
resonance contributors could be written (8’ and 9’, respective-
ly; Scheme 3); that imparted a degree of porphyrinoid aroma-
ticity onto the macrocycles, thereby leading to the observation
of a weak diatropic ring current in their 1H NMR spectra. In
other word, the charge separation in the dipolar resonance
structure, as shown in Scheme 3, contributed to the reduced
aromaticity.
Even though it was not possible to conclusively assign all of
the peaks for the free-base form of both macrocycles, owing
to dynamic conformational fluxionality, a profound change in
spectroscopic pattern was observed in the fully protonated
state by adding 10% trifluoroacetic acid in CDCl3 (v/v) at
298 K. This change was owing to the fact that protonation con-
strained the conformational dynamics of the macrocycles, even
at ambient temperature, thereby affording sharp signals. A rep-
1
resentative H NMR pattern of macrocycle 8 in its protonated
form is shown in Figure 3. To differentiate between the hetero-
cyclic b-CH protons and the meso-aryl CH protons, 2D NOESY
spectra (see the Supporting Information, Figure S33) were re-
corded in addition to 2D COSY spectra (see the Supporting In-
1
Figure 3. 1H NMR spectra of macrocycle 8 in 10% CF3COOH/CDCl3 (v/v) at
and outer ring protons in the H NMR spectrum—of 9.97 ppm
298 K.
implied a diatropic ring current for macrocycle 8 upon com-
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plete protonation.[12] The increased diatropic character com-
pared to the free-base form was attributed to the favorable
canonical form 8H22+, which no longer required charge separa-
tion, as in structure 8’, but instead mediated charge delocaliza-
tion. In other word, the macrocyclic aromaticity was retained
through a 20p electron-delocalization motif. The magnetic in-
equivalency of the selenophene protons and the pyrrole ring
protons unambiguously supported a lower symmetry of the
macrocyclic ring in the diprotonated form.
shift in the electronic absorption spectrum of macrocycle 8.
Notably, [20] aromatic macrocycles 8 and 9 showed significant-
ly narrower HOMO–LUMO gaps than the [18] tetraphenyl por-
phyrin (2.24 V) and the monothiatetraphenyl porphyrin
(2.09 V),[14] thus suggesting a higher degree of delocalization of
the p electrons in macrocycles 8 and 9. This result was also re-
flected in the red-shift of the Soret and Q-bands in the UV/Vis
spectra of the macrocycles 8 and 9.
In conclusion, we have reported the first synthesis of an un-
conventional tripyrrane moiety from the regioselective b-ben-
zoylated pyrrole. The unconventional 3,4-linkages of N-methyl
pyrrole unit admirably served as incentive to two different
types of highly stable [20] aromatic heteroannulenes. Most im-
portantly, the presence of the “local” aromatic unit (consider-
ing 3,4-linkages of N-methyl pyrrole) in the macrocyclic core
didn’t quench the diatropic ring-current effect of the macrocy-
The observation of four sets of signals for the b-CH protons
of the two pyrrole rings but only one NH signal clearly indicat-
ed a much-faster NH tautomerization within the NMR time-
scale, which persisted even after lowering the temperature
(see the Supporting Information, Figure S31). Another interest-
ing observation was the isolation of a single signal for the a-
CH protons of the N-methyl pyrrole ring. This result could be
owing to the fact that the protruding N-methyl pyrrole ring de-
viated from the mean macrocyclic plane (defined by the six
meso-carbon atoms) to the same extent on both sides and,
hence, both the a-CH protons experienced the macrocyclic di-
amagnetic ring current to an equal extent. A similar spectro-
scopic observation was found for the macrocycle 9 upon com-
plete protonation (see the Supporting Information, Figur-
es S37–S45).
1
cle, as confirmed by H NMR spectra. To the best of our knowl-
edge, these observations are unprecedented. The synthesis of
more such structural variants is currently underway in our lab-
oratory.
Experimental Section
Synthesis of Macrocycle 8
Given a low symmetry conformation with aromaticity in
a [20] p-electron conjugation pathway, the macrocycles could
not be planar and the preferred conformation could include
a Mçbius twist. Although this structure has not been con-
firmed by single-crystal X-ray diffraction analysis, DFT level op-
timized geometries revealed the presence of a characteristic
Mçbius twist in the N-methyl pyrrole linked region of the mac-
rocycles in the free base as well as in the protonated state (see
the Supporting Information, Figures S47 and S48). The nucleus-
independent chemical shift, NICS(0), values[13] at the center of
the macrocycles were d=À4.7 and À2.9 ppm (see the Sup-
porting Information, Figures S49 and S50) for the free-base 8
and 9, respectively, which accounted for the presence of
a weak diatropic ring current, owing to dipolar resonance con-
tribution (Scheme 3). Notably, the more aromatic character
that was associated with the cross-conjugated arene unit (the
NICS(0) value at the center of the N-methyl pyrrole ring was
>À11 ppm; see the Supporting Information, Figures S49 and
S50), the lower the aromaticity that would be present in the
macrocycle.
Tripyrrane 5 (2.29 g, 5.85 mmol) and 2,5-bis(tolylhydroxymethyl)se-
lenophene (6; 2.17 g, 5.85 mmol) were added in CH2Cl2 (600 mL)
and the mixture was stirred for 15 min under a nitrogen atmos-
phere to obtain a clear solution. Next, para-toluenesulfonic acid
(0.1 g, 0.58 mmol) was added and the solution was stirred for 1 hr
in the dark. Then, p-chloranil (4.32 g, 17.56 mmol) was added and
the resulting mixture was heated at reflux for 1 hr in air and then
stirred at RT for 10 h. After removal of the solvent from the crude
mixture by rotary evaporation, the product was purified by column
chromatography on basic alumina using 20% CH2Cl2/hexane fol-
lowed by repeated column chromatography on silica gel using
1
40% CH2Cl2/hexane. Yield 43 mg (ca. 10%); H NMR (500 MHz, 10%
CF3COOH/CDCl3, 300 K): d=À1.03 (s, 2H; NH), 2.70 (s, 3H), 2.71 (s,
3H), 3.56 (s, 3H), 7.38 (brs, 2H), 7.61–7.67 (brs, 4H), 8.02–8.07 (brs,
6H), 8.11 (brs, 4H), 8.20 (d, J=4.5 Hz, 1H), 8.21 (brs, 2H), 8.22 (d,
J=4.5 Hz, 1H), 8.31 (brs, 2H), 8.38 (d, J=4.5 Hz, 1H), 8.55 (d, J=
4.5 Hz, 1H), 8.92–8.94 ppm (m, 2H); UV/Vis (CH2Cl2, 298 K): lmax
(e)=456.41 (74450), 584 (shoulder), 624.35 (9720), 675.45 nm
(11000 molÀ1 dm3 cmÀ1); UV/Vis (CH2Cl2, 1% TFA/CH2Cl2, 298 K):
lmax
(e)=492.67
(82900),
750.73
(15540),
883.88 nm
(12900 molÀ1 dm3 cmÀ1); MS (MALDI): m/z calcd for C47H35N3Se:
721.2; found:722.4.
Both macrocycles 8 and 9 were found to be robust towards
electrochemical oxidation and reduction. Electrochemical stud-
ies were performed by using cyclic voltammetry and differen-
tial pulse voltammetry with 0.1m tetrabutylammonium hexa-
fluorophosphate as supporting electrolyte. Macrocycle 8 exhib-
ited two irreversible oxidations at 0.776 and 1.104 V, followed
by a quasireversible oxidation at 1.311 V, with an estimated
HOMO–LUMO gap of 1.714 V. Macrocycle 9 exhibited two irre-
versible oxidation peaks at 0.714 and 1.089 V, followed by a re-
versible oxidation peak at 1.342 V, with an estimated HOMO–
LUMO gap of 1.778 V (see the Supporting Information, Fig-
ure S51). The slightly narrower HOMO–LUMO gap for macrocy-
cle 8 compared to macrocycle 9 also accounted for the red-
Acknowledgements
This work at IACS was supported by the DST-SERB, New Delhi,
India (SR/S1/IC-37/2012), the CSIR, New Delhi, India (02/(0120)/
13/EMR-II), and DST-SERB Ramanujan Fellowship (SR/S2/RJN-
93/2011). AM thanks CSIR for a senior research fellowship. Our
sincere thanks to Dr. Marcin Stepien, University of Wroclaw,
Poland, for helpful discussions on the NMR analysis. We are in-
debted to Prof. C. H. Suresh, NIIST, India, for performing the
theoretical calculations.
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Manuscript received: January 8, 2016
Accepted Article published: January 26, 2016
Final Article published: February 25, 2016
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