1,1,8,8-tetramethyl[8](2,11)teropyrenophane: Half of an aromatic belt and a segment of an (8,8) single-walled carbon nanotube

Full text of DOI:10.1002/anie.200806363

Angewandte
Chemie
DOI: 10.1002/anie.200806363
Cyclophanes
1,1,8,8-Tetramethyl[8](2,11)teropyrenophane: Half of an Aromatic
Belt and a Segment of an (8,8) Single-Walled Carbon Nanotube**
Bradley L. Merner, Louise N. Dawe, and Graham J. Bodwell*
Dedicated to Professor Reginald Mitchell on the occasion of his 65th birthday
Cyclacenes, for example, [12]cyclacene (1; Figure 1), and
cyclophenacenes, for example, cyclo[12]phenacene (2), are
aromatic belts that map onto the surfaces of various fullerenes
A recurring theme in the numerous unsuccessful attempts
to synthesize aromatic belts is the inability to aromatize
partially saturated belt precursors. The interplay of two major
energetic factors, strain and aromaticity, is at the heart of the
problem.[9] The generation of a nonplanar aromatic system
from a non- or partially aromatic precursor is inherently
disfavored by the build-up of strain, but favored by the
aromatic stabilization energy (ASE) of the aromatic system
being formed. Increasing the distortion from planarity causes
the strain energy to become larger and the ASE to become
smaller. As a result, strain ultimately becomes dominant and
aromatization, by standard methods such as the elimination of
water, can become unfavorable. For example, the addition of
water (2 equiv) to Schlꢂterꢁs “double-stranded cycle” has
been calculated to be exothermic by 42.2 kcalmolÀ1.[7b]
Therefore, if any approach to aromatic belts is ultimately
going to be successful it will not only have to target a
relatively stable structural motif, but also rely upon method-
ology that is capable of generating highly nonplanar aromatic
systems under relatively mild conditions.
Figure 1. [12]Cyclacene (1), cyclo[12]phenacene (2), and Vꢀgtle belts
3a–c.
and single-walled carbon nanotubes (SWCNTs).[1] Indeed,
they can be viewed as the shortest possible open-ended
(uncapped) zig-zag and armchair single-walled carbon nano-
tubes, respectively.[1e] Although interest in the synthesis of
aromatic belts, especially cyclacenes, predates the discovery
of the fullerenes[2] and SWCNTs[3] by decades,[4] the discovery
of SWCNTs and their homology to aromatic belts has
engendered more intense synthetic and theoretical interest,
not only in cyclacenes[5] and cyclophenacenes,[6] but also in
related aromatic/conjugated belts.[7] Of particular interest to
us are the Vꢀgtle belts,[8] for example, 3, which are armchair
SWCNT segments having a pyrenoid or rylenoid motif,
depending upon oneꢁs perspective.
The valence isomerization/dehydrogenation (VID) reac-
tion of [2.2]metacyclophane-1,9-diene (4) to give pyrene 6 is
such a method (Scheme 1a),[10] and it has been used for the
synthesis of [n](2,7)pyrenophanes containing nonplanar
[*] B. L. Merner, Prof. Dr. G. J. Bodwell
Department of Chemistry, Memorial University
St. John’s, NL, A1B 3X7 (Canada)
Fax: (+1)709-737-3702
E-mail: gbodwell@mun.ca
Dr. L. N. Dawe
CREAIT Network, Room IIC 1001, Memorial University
St. John’s, NL, A1B 5S7 (Canada)
[**] The Natural Sciences and Engineering Research Council (NSERC) of
Canada is gratefully acknowledged for financial support of this work.
Prof. Dr. Yuming Zhao, Memorial University, is thanked for helpful
and interesting discussions.
Scheme 1. The parent VID reaction of [2.2]metacyclophane-1,9-diene
(4) to give pyrene (6), its use in the synthesis of [n](2,7)pyrenophanes
(10) and a strategy for the synthesis of large [n]cyclophanes (12).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806363.
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5487

Communications
pyrene systems having deviations from planarity ranging from
slight to severe.[11] The large ASE of pyrene (74 Æ 1 kcal
molÀ1)[12] and the retention of the majority of this ASE,[9,11b]
even in the most highly distorted geometries, are keys to the
success of these reactions.
The general strategy for the synthesis of [n]-
(2,7)pyrenophanes (Scheme 1b) involves the tethering of
two appropriately substituted benzene rings (7) to afford the
tetrafunctionalized system 8, which is then converted into the
tethered syn-[2.2]metacyclophane-1,9-diene 9. The VID reac-
tion then affords the [n](2,7)pyrenophane 10. Similarly, the
use of an appropriately substituted aromatic “board” (11)
would be expected to afford large [n]cyclophanes (12)
containing elongated aromatic systems, which structurally
resemble large segments of the Vꢀgtle-type aromatic belts 3
and armchair SWCNTs (Scheme 1c).
Pyrene (6) was identified as a viable substrate for the
synthesis of an appropriately substituted aromatic board
because of its predictable substitution chemistry. Specifically,
electrophilic aromatic substitution occurs with very high
selectivity at the 1-, 3-, 6-, and 8-positions in the vast majority
of cases.[13] The one clear exception is Friedel–Crafts tert-
butylation, which occurs exclusively at the 2- and 7-posi-
tions.[14] Both of these characteristics were exploited in the
synthesis of the title compound.
The first synthetic task was to tether two pyrene units at
their 2-positions, which required a dichloroalkane wherein
both halides were tertiary. Dimethyl suberate (13) was
reacted with MeMgBr to afford 2,9-dimethyldecane-2,9-diol
(14; 86% yield),[15] which was then treated with a concen-
trated aqueous HCl solution to give 2,9-dichloro-2,9-dime-
thyldecane (15; 91% yield; Scheme 2).[16] Friedel–Crafts
alkylation of pyrene (6) with dichloride 15 in the presence
of AlCl3 then furnished 2,9-dimethyl-2,9-bis(2-pyrenyl)de-
cane (16; 62% yield).
The next objective was to functionalize the pyrene units
and then use the functionality to form a cyclophanediene.
Although the original plan to achieve this goal involved two-
fold functionalization of both pyrene units, it was ultimately
discovered that a stepwise functionalization/bridge-formation
approach was more effective. Furthermore, the discovery that
McMurry reactions could be used to directly install the
Z-configured alkene bridges obviated the need to proceed
through one or more thiacyclophane intermediates, as is the
case for the [n](2,7)pyrenophanes.[17] Rieche formylation of
16 afforded dialdehyde 17 (88% yield), which was subjected
to an intramolecular McMurry reaction to afford [8.2]-
(7,1)pyrenophane 18 as an inseparable mixture of E and Z
isomers.[18] This structure is the first example of a pyreno-
phane having a (1,7)-bridging motif.[19] Rieche formylation of
this mixture delivered a chromatographically separable
mixture of enedialdehydes 19 [(Z)-19 (57% yield), (E)-19
(11% yield) over 2 steps]. Subjection of (Z)-19 to another
intramolecular McMurry reaction then led to the formation of
the desired cyclophanediene 20 (41% yield). Finally, treat-
ment of 20 with DDQ in m-xylene at 1458C brought about an
efficient VID reaction, from which 1,1,8,8-tetramethyl[8]-
(2,11)teropyrenophane (21) was isolated in 95% yield as a
stable and hexanes-soluble orange solid. At just eight steps
Scheme 2. Synthesis of 1,1,8,8-tetramethyl[8](2,11)teropyrenophane
(21). Reagents and conditions: a) MeMgBr, THF, 08C to reflux, 10 h,
86%; b) 12m HCl (aq), RT, 2 h, 91%; c) pyrene (6), AlCl3, CH2Cl2, 08C
to RT, 4 h, 62%; d) Cl2CHOCH3, TiCl4, CH2Cl2, 08C to RT, 2 h, 88%;
e) TiCl4, Zn, pyridine, THF, 08C to reflux, 5 h; f) Cl2CHOCH3, TiCl4,
CH2Cl2, 08C to RT, 2 h, 11% (E)-19, 57% (Z)-19; g) TiCl4, Zn, pyridine,
THF, 08C to reflux, 4 h, 41%; h) DDQ, m-xylene, 1458C, 48 h, 95%.
DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
from dimethyl suberate (13) and 10% overall yield, the
synthesis of 21 is considerably shorter than all reported
syntheses of [n](2,7)pyrenophanes (ꢀ 12 steps) and compara-
ble in efficiency.
In addition to the remarkable structural characteristics
discussed below, teropyrenophane 21 is noteworthy because it
is just the second teropyrene system to have been synthe-
sized,[20] and also because teropyrene is now the largest
aromatic system to have been incorporated into an [n]cyclo-
phane (i.e. one aromatic system and one bridge).[21] The
teropyrene system (36 carbon atoms) in 21 contains more
than half of the carbon atoms in the D6h-symmetric Vꢀgtle
belts 3a (60 carbon atoms) and 3b (70 carbon atoms).
However, as outlined below, its structure (including the
benzylic carbon atoms) more closely resembles a substructure
of the D8h-symmetric Vꢀgtle belt 3c (80 carbon atoms), which
is, in turn, a segment of an (8,8) SWCNT.
A single-crystal X-ray structure determination of 21
revealed two independent molecules in the asymmetric unit,
one of which is shown in Figure 2 (see the Supporting
Information for other views).[26] In the packing diagram (see
the Supporting Information) molecule A sits in pairs with a
face-to-face orientation of the aliphatic chains, whereas
molecule B does not. As dictated by the eight-atom bridge,
each independent molecule has a highly nonplanar teropyr-
ene unit. In the [n](2,7)pyrenophanes, the nonplanarity of the
pyrene system is most commonly characterized by the angles
formed by the adjacent planes of atoms and, more generally,
by the angle (q)[11d] formed between the two terminal planes
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Table 1: Structural data for 21 (see Figure 3).
Metric
Molecule A
Molecule B
b1 [8]
q1 [8]
5.2
68.0
6.1
67.5
q2 [8]
95.9
92.8
q3 [8]
69.8
70.4
b2 [8]
4.7
6.5
qtot [8]
overall bend [8]
d1 [ꢁ]
d2 [ꢁ]
d3 [ꢁ]
167.6
178.5
9.033(4)
9.101(5)
9.067(6)
6.19(2)
166.4
179.0
9.084(4)
9.082(4)
9.082(5)
6.12(2)
d4 [ꢁ]
The two independent molecules of 21 have some small
local differences in their structures, but the general features
are essentially the same. The teropyrene system is not far
from being bent through 1808 (qtot = 166.4–167.68), which
invites comparison to a Vꢀgtle belt or an armchair SWCNT. It
is useful to include the benzylic carbon atoms in the analysis.
In the D8h-symmetric Vꢀgtle belt 3c (or an (8,8) SWCNT), the
Figure 2. POV-Ray ball-and-stick representation of molecule B of
1,1,8,8-tetramethyl[8](2,11)teropyrenophane (21) in the crystal.
of atoms (C(a)-C(b)-C(c) and C(x)-C(y)-C(z) in 10).[22] An
analogous analysis can be applied to 21, in which three pyrene
substructures can be identified. Thus three angles (q1 and q3
for the terminal pyrene units and q2 for the central pyrene
unit) and a total bend angle (qtot = the angle formed by the
two terminal planes of atoms in the teropyrene system, that is,
(C9-C10-C26 and C17-C18-C19 in 21; Figure 2) can be
measured (Figure 3, Table 1). The angles (b)[23] formed by
À
bonds corresponding to the two Cbridgehead Cbenzylic bonds in 21
are parallel. In 21, the observed overall bend (qtot + y1
+
y2 = 178.58 and 179.08 for molecules A and B, respectively) is
indeed very close to 1808. This near-parallel orientation is also
reflected in the values of d1, d2, and d3 (range = 9.03–9.08 ꢃ),
which should be identical in a parallel orientation.
The two terminal pyrene units of the teropyrene system
are less severely bent (q1 and q3 = 67.5–70.48) than the central
pyrene unit (q2 = 92.8–95.98), which means that the cross
section of the teropyrene system more closely resembles a
portion of an ellipse rather than that of a circle. In fact, the
hypothetical union of two half-belt systems (teropyrene +
À
2Cbenzylic) by fusion of the Cbridgehead Cbenzylic bonds (Figure 3)
affords an ellipsoidal segment of 3c or an (8,8) SWCNT. The
short axis of the ellipse measures 9.08 ꢃ (the average value of
d3) and the long axis measures 12.3 ꢃ (double the average
value of d4). The average of these distances is 10.7 ꢃ, which is
very close to the calculated value for an (8,8) SWCNT
(10.86 ꢃ).[24] Thus the cross section of the half-belt system
corresponds to the more curved half of an ellipse. This is also
reflected in the angles between planes of atoms in 21 that
correspond to angles between planes related by the eight-fold
symmetry in 3c or an (8,8) SWCNT. In 21, these angles range
from 34.78 to 65.48 and average 52.18 and 52.08 for mole-
cules A and B, respectively (see the Supporting Information),
which exceed the 458 angle dictated by the eight-fold
symmetry.
Figure 3. Parameters used to describe the structure of 21 and the
hypothetical joining of two half belts to afford an ellipsoidal segment
of an (8,8) armchair SWCNT.
À
the Cbridgehead Cbenzylic bonds and the terminal planes of the
1
teropyrene system have also been included in the analysis for
the calculation of the overall bend in the system (see the
Supporting Information for a full list of angles between
planes). Key distances (Figure 3, Table 1) are d1 (distance
between the bridgehead carbon atoms), d2 (distance between
the benzylic carbon atoms), d3 (distance between the cent-
The H NMR spectrum of 21 contains a set of low-field
signals at d = 8.62 (H13, H14, H22, H23), 8.39 (H12, H15,
H21, H24), 7.71 (H11, H16, H20, H25) and 7.42 ppm (H10,
H17, H19, H26) for the aromatic protons and a set of high-
field signals at d = 1.32 (CH3), 0.72 (H2, H7), À0.26 (H4, H5),
and À0.67 ppm (H3, H6) for the aliphatic protons. As with the
[n](2,7pyrenophanes,[11] the anisotropic effect of the aromatic
p system in 21 causes the aliphatic protons to resonate at
unusually high field.
À
roids of the Cbridgehead Cbenzylic bonds), and d4 (distance
between the centroid of d3 and the centroid of the central
bond of the central pyrene unit of the teropyrene system).
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The absorption spectrum of 21 in acetonitrile (Figure 4)
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489 nm. By comparison, the longest wavelength absorption
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Figure 4. Normalized absorption and emission spectra of 1,1,8,8-
tetramethyl[8](2,11)teropyrenophane (21) (9.8ꢂ10À6 m in CH3CN).
maximum of teropyrene (in 1,2,4-trichlorobenzene) is
reported to be at 537 nm,[20] which may suggest that, in
contrast to what is observed in the [n]paracyclophanes[25] and
[n](2,7)pyrenophanes,[11f] bending the teropyrene system
causes a significant blue shift. The fluorescence spectrum
(Figure 4, excitation at 370 nm) shows what appears to be two
overlapping bands with lmax = 509 and 530 nm. The fluores-
cence quantum yield (fem) is 0.11. More substantial com-
mentary on the effect of bending the teropyrene system out of
planarity on its spectroscopic properties will have to await the
synthesis of other homologs of 21 or a more suitable model
compound, for example, 2,11-di-tert-butylteropyrene, neither
of which has yet been achievable in our hands using the
approach described herein.
In summary, a teropyrene system that structurally resem-
bles a segment of an (8,8) armchair SWCNT has been
generated by linking the 2- and 11-positions with an eight-
atom aliphatic bridge. Noteworthy features of the synthesis
are 1) its brevity and efficiency (eight steps from dimethyl
suberate, 10% overall yield), 2) the first example of the use of
a McMurry reaction to form a syn-[2.2]metacyclophane-1,9-
diene system, 3) only the second example of a teropyrene
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to deliver such a large nonplanar polycyclic aromatic system.
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optimism that this methodology will ultimately be capable of
not only generating smaller homologs of 21, but also complete
aromatic belts that map onto the surfaces of (n,n) armchair
SWCNTs. Work aimed at achieving these goals is being
pursued actively in our laboratory.
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Received: December 29, 2008
Published online: March 17, 2009
Keywords: aromatic compounds · cyclophanes ·
.
strained molecules · nanotubes · valence isomerization
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[26] CCDC 718615 (21) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Angew. Chem. Int. Ed. 2009, 48, 5487 –5491
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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