π-Conjugated macrocycles from thiophenes and benzenes

Article abstract of DOI:10.1039/b715553k

π-Conjugated macrocycles consisting of thiophenes and benzenes exhibit benzenoid features for 4nπ macrocycles, whereas (4n + 2)π macrocycles are annulenoid due to rapid interconversion between quinoid and Kekule canonical forms in the benzene units. The R

Full text of DOI:10.1039/b715553k

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p-Conjugated macrocycles from thiophenes and benzenesw
J. Sreedhar Reddya and Venkataramanarao G. Anand*b
Received (in Cambridge, UK) 9th October 2007, Accepted 11th December 2007
First published as an Advance Article on the web 11th January 2008
DOI: 10.1039/b715553k
p-Conjugated macrocycles consisting of thiophenes and benzenes
exhibit benzenoid features for 4np macrocycles, whereas
(4n + 2)p macrocycles are annulenoid due to rapid interconver-
sion between quinoid and Kekule canonical forms in the benzene
units.
Large p-conjugated macrocycles are attractive synthetic
targets due to their structural features, optical properties and
as a synthetic challenge. The electronic properties of such
macrocycles are dependent on the delocalization of the p
electrons. Annulenoid macrocycles exhibit diatropic or para-
tropic ring current effects depending on the number of p
electrons in the system. Cyclophanes1 and porphyrins2 are
Chart 1
examples of such annulene type macrocycles. A few conju-
gated macrocycles do not display significant ring current
effects and are considered benzenoid in nature. For example,
1(b)) by the acid catalyzed condensation of 1,4-bis[(penta-
fluorophenyl)(thiophen-2-yl)methyl]benzene, 6, with aryl alde-
hydes, 7a–7e, such as pentafluorobenzaldehyde (7a) and 2,6-
difluorobenzaldehyde (7b). We were also successful by using
other electron-withdrawing aldehydes such as p-nitrobenzal-
dehyde (7c), 2,6-dichlorobenzaldehyde (7d) and 2-chloro-6-
fluorobenzaldehyde (7e) (ESIw).
The composition of all the macrocycles were confirmed
from mass spectrometric analysis (ESIw). The electronic
absorption spectrum (Fig. 1) of 1 shows a strong band at
398 nm (e = 4.4 ꢁ 104) followed by two weaker absorptions at
543 nm (0.6 ꢁ 104) and 569 nm (0.6 ꢁ 104). 2a shows a split
absorption at 520 nm (30 ꢁ 104) and 540 nm (31 ꢁ 104)
followed by low energy absorption at 613 nm (4.9 ꢁ 104), 664
nm (6.9 ꢁ 104) and 721 nm (13 ꢁ 104). Replacing the
meso-pentafluorophenyl substituent by 2,6-difluorophenyl
did not change the absorption pattern, as depicted in Fig. 1.
1 and 2 have formal 20p and 30p electron counts, respectively,
along their conjugated pathway. The red shift in absorptions
for 2 is attributed to the extended conjugation and the higher
intensity is due to the aromatic nature of the macrocycle as
compared to 1.
a-conjugated
cyclo[n]thiophenes
exhibit
benzenoid3
characteristics. In general the macrocyclic property can be
tuned by the nature of a p sub-unit involved in conjugation. A
benzene in an annulenoid can adopt a Kekule or a quinoid
cannonical form, while contributing to the overall delocaliza-
tion of p electrons. Even though such a possibility has been
discussed,4 a quinoid-type benzene in air-stable aromatic
macrocycles are typically unknown in literature. Herein, we
report a straightforward synthesis of air-stable non-pyrrolic
porphyrin type macrocycles, 1 and 2. Both 1 and 2 have a
blend of thiophene and benzene p sub-units, wherein one of
the benzene rings can be forced to adopt a quinoid form in the
30p aromatic framework of 2. Such a quinoid form is not
essential to maintain conjugation with the 20p network in 1
(Chart 1).
1 and 2 were synthesized from thiophene and benzene
derivatives (Scheme 1(a)). 1,4-bis[(pentafluorophenyl)(hydroxy)-
methyl]benzene, 5, was condensed with 2,5-bis[(pentafluoro-
phenyl)(thiophen-2-yl)methyl]thiophene, 3, in equimolar con-
centrations, with BF3ꢀOEt2 as the catalyst. Further, oxidation
by FeCl3 and column chromatographic purification yielded 1
in 11% yield. In an identical procedure 5-pentafluorophenyl-
dithienylmethane, 4,5 was condensed with 5 to obtain 2a
in 32% yield. 2a–2e can be synthesized in better yields (Scheme
a Chemical Sciences & Technology Division, National Institute for
Interdisciplinary Science & Technology (Formerly RRL –
Trivandrum), CSIR, Trivandrum, 695 019, India
b Department of Chemistry, Indian Institute of Science Education &
Research, 900, NCL Innovation Park, Dr Homi Bhabha Road, Pune,
411 008, India. E-mail: vg.anand@iiserpune.ac.in; Fax: +91-20-
25898022; Tel: +91-20-25898021
w Electronic supplementary information (ESI) available: Details of
synthesis, UV-Vis, FAB mass, VT NMR for 1 and 2a, and Fꢀ ꢀ ꢀF non-
bonded interactions in 2b. See DOI: 10.1039/b715553k
Scheme 1 Synthesis of 1 and 2.
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tionally, the effective p-electron delocalization hastens the
exchange between the canonical forms (Kekule 3 quinoid)
in both the phenylene rings. These two process are too fast
compared to the NMR time scale and hence the phenylene
protons appear as a singlet even at low temperatures.
Further confirmation of the proposed structure comes from
single-crystal X-ray diffraction analysis.z 2b shows a bowl type
conformation in the solid state (Fig. 2). The deviation from
planarity is attributed to the tilting of the four thiophene rings
and the two phenylene rings from the mean plane defined by
the six meso carbons. The two phenylene rings (A and B)
deviate by 27.24 and 17.191, while the four thiophene rings
(containing S1, S2, S3 and S4) deviate by 25.76, 14.99, 22.63
and 27.511, respectively from the mean macrocyclic plane. The
bond lengths between the adjacent carbon atoms along the
conjugated pathway vary from 1.34 to 1.45 A, indicative of
aromatic delocalization throughout the macrocycle. The dis-
tance between the meso carbon and paraphenylene carbons
(C17–C14; C10–C11) for ring A was equal and found to be
1.418 A, while it is 1.421 A (C28–C27) and 1.448 A (C34–C31)
for ring B. This shows the bond distances between meso
carbons and ring B are more towards single bond nature than
those of ring A.7 This crystallographic observation shows
preferential benzenoid and quinoid sites in the structure. The
above argument do not purport to say that one side of the
molecule in the crystal structure is always having benzenoid
form, and quinoid form on the other side, but it shows that
there is more probability for the benzenoid form in ring B and
quinoid form in ring A. Due to full resonance within rings A
and B, they are indistinguishable from each other. This is
consistent with the proton NMR results. Further analysis of
the crystal packing revealed intermolecular interactions
through C–Hꢀ ꢀ ꢀF hydrogen bonds.8 Two C–Hꢀ ꢀ ꢀF interac-
tions (C39–H39ꢀ ꢀ ꢀF15 (symm: x, 2.5 ꢃ y, 1/2 + z) 2.55 A,
1541 and C25–H25ꢀ ꢀ ꢀF5 (symm: x, 1 + y, z) 2.58 A, 1341) are
strong enough for the formation of a two-dimensional supra-
molecular architecture (Fig. 3). Apart from hydrogen bonds
three different Fꢀ ꢀ ꢀF non-bonded interactions9 (F8ꢀ ꢀ ꢀF5,
F12ꢀ ꢀ ꢀF19, F17ꢀ ꢀ ꢀF21) were also observed in the crystal lattice
(ESIw). The distance between the corresponding fluorine
Fig. 1 Electronic absorption spectrum of 1 (10ꢃ5 M), 2a (10ꢃ6 M)
and 2b (10ꢃ6 M) in CH2Cl2.
The effect of p-electron delocalization in 1 and 2 could be
well understood from their H NMR spectra. The phenylene
1
protons of 1 appear as a singlet at d 7.53 ppm. Due to C2
symmetry, the thiophene protons resonate as a singlet at
6.47 ppm and as two doublets at 6.69 and 6.59 ppm. The d
values for 1 signify neither diatropic nor paratropic ring
current effects and hence appear to be benzenoid in nature.
On the other hand, 2a and 2b show diatropic ring current
effects as illustrated by the upfield shift of phenylene protons
and downfield shift of thiophene protons. The phenylene
protons of 2a resonate at 4.74 ppm, an upfield shift of more
than 3 ppm compared to an unsubstituted benzene.6 The
thiophene protons appear as two doublets at 8.82 and 8.76
ppm, revealing a downfield shift of nearly 2 ppm with respect
to an unsubstituted thiophene.6 2b also exhibits a similar
NMR pattern with a singlet for phenylene protons at
4.53 ppm and two doublets for the thiophenes at 9.0 and
8.9 ppm. In both, 1 and 2, the rapid flipping of the phenylene
ring cannot differentiate shielding and deshielding effects on its
protons at room temperature. Even upon lowering the tem-
perature to 178 K for 2a (223 K for 1) the 1H NMR spectrum
does not show any significant difference (ESIw). In 2, addi-
Fig. 2 X-Ray crystal structure of 2b. Thermal ellipsoids are drawn to 50% probability. Solvent molecules are omitted for clarity. Top view (left)
and side view (right) (meso-aryl rings are omitted in the latter view for clarity).
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Fig. 3 Supramolecular architecture of 2b due to C–Hꢀ ꢀ ꢀF intermolecular H-bonds as specified by dotted lines. Meso-aryl rings which are not
involved in hydrogen bonding are omitted for clarity.
CCDC 663304. For crystallographic data in CIF or other electronic
atoms are in the range of 2.69 to 2.82 A, which positions them
format see DOI: 10.1039/b715553k
within the range of a non-bonded interaction (van der Waals
radius for fluorine is 1.47 A).
1 (a) T. Olsson, D. Tanner, B. Thulin and O. Wenneerstrom,
In conclusion, we have devised a simple and ring-size
selective one-pot synthesis of benzene incorporated macro-
cycles 1 and 2. The extended delocalization of the macrocycle,
2, has a profound effect on the benzene aromaticity, thereby
complying the 6p arene ring current to be a part of the 30p
macrocyclic diatropic ring current. Therefore, one of the two
phenylene rings in 2 should exist in quinoid form to sustain the
30p (4n + 2 rule) aromaticity. To the best of our knowledge, 2
represents the first such example of having a quinoid form for
benzene in a p-conjugated macrocycle.
The authors thank Dr Raja Roy, SAIF–Lucknow, India for
NMR data and Dr Babu Varghese, SAIF–IIT–Madras, India
for crystallographic analysis. Funding from DST, New Delhi,
India and SRF for J. S. R. from CSIR, New Delhi, India, is
acknowledged.
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Huber, O. Wennerstrom, O. Norinder, D. Tanner and B. Thulin, J.
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derlin, T. Debaerdemaeker, E. Mena-Osteritz and P. Bauerle,
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ch. 4, pp. 209–210.
7 The torsion angle C1–C34–C31–C32 is ꢃ16.61, while the corre-
sponding value of the other side C9–C10–C11–C12 is 6.61 which
shows more non-planarity of the moiety about the C31–C34 bond
than that of the C10–C11 bond. This also indicates a more single
bond nature of the C31–C34 bond.
8 V. R. Thalladi, H. C. Weiss, D. Blaser, R. Boese, A. Nangia and G.
R. Desiraju, J. Am. Chem. Soc., 1998, 120, 8702.
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Notes and references
z Crystals were grown by slow evaporation of n-hexane into a chloro-
form solution of 2b. Crystallographic data of 2b: C70H22F24S4, Mr =
1447.15, monoclinic, space group P21/c, a = 19.7374(4), b =
19.0689(4), c = 22.0885(5) A, b = 114.818(10)1, V = 7544.6(3) A3,
Z = 4, Dc = 1.589 g cmꢃ3, T = 173 K, R1 = 0.0720, wR2 = 0.1932.
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This journal is The Royal Society of Chemistry 2008
1328 | Chem. Commun., 2008, 1326–1328