Synthesis, characterization, and theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon nanohoop structures

Full text of DOI:10.1021/ja807126u

Published on Web 12/04/2008
Synthesis, Characterization, and Theory of [9]-, [12]-, and
[
18]Cycloparaphenylene: Carbon Nanohoop Structures
†,‡
‡
‡
,†,‡,§
Ramesh Jasti, Joydeep Bhattacharjee, Jeffrey B. Neaton, and Carolyn R. Bertozzi*
Departments of Chemistry and Molecular and Cell Biology, Howard Hughes Medical Institute, UniVersity of
California, and The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received September 8, 2008; E-mail: crb@berkeley.edu
Due to their unique architectural features, the cycloparaphe-
nylenes (1, Figure 1) have attracted attention from many vantage
1
points. Their strained and distorted aromatic systems and radially
oriented p orbitals (2) have intrigued synthetic chemists, theoreti-
cians, supramolecular chemists, and materials scientists alike.
Figure 1. Cycloparaphenylenes (1) and a schematic of their radially
oriented p orbitals (2).
Despite this widespread interest, the cycloparaphenylenes remain
2
-4
an unsolved synthetic challenge.
Here we describe the first
a
synthesis of [9]-, [12]-, and [18]cycloparaphenylene. We also utilize
computational methods to understand the structural and optical
properties of these new macrocycles. As these structures correspond
Scheme 1. Preparation of Macrocyclic Precursors
5
to the shortest-possible segment of an armchair carbon nanotube,
we refer to them as “carbon nanohoops.”
The heart of the synthetic challenge for the cycloparaphenylenes
6
lies in the strain energy of the bent aromatic system. Our strategy
was to build up strain sequentially during the synthesis, using a
3
,6-syn-dimethoxy-cyclohexa-1,4-diene moiety as a masked aro-
matic ring in a macrocyclic intermediate (see structure 4, Scheme
). We reasoned that the cyclohexadiene unit would provide the
1
curvature and rigidity necessary for macrocyclization to afford a
marginally strained intermediate. Subsequent aromatization would
then provide the driving force necessary to achieve the strain
energies of target compounds, calculated to be 47, 28, and 5 kcal/
mol for [9]-, [12]-, and [18]cycloparaphenylenes, respectively (Vide
infra).
a
Reagents and conditions: (a) (i) nBuLi, THF, -78 °C, (ii) benzo-
quinone; (b) (i) NaH, THF, 0 °C, (ii) MeI, 0 °C to rt; (c) (i) nBuLi, THF,
-
3 4 2 3
78 °C, (ii) isopropyl pinacol borate (Bpin), -78 °C; (d) Pd(PPh ) , Cs CO ,
toluene/methanol (10:1), 80 °C.
The synthesis of the macrocyclic precursors to cycloparaphenylenes
is depicted in Scheme 1. Monolithiation of diiodobenzene and
7
subsequent addition of benzoquinone generated a syn diol, which was
then alkylated with methyl iodide to produce diiodide 4 in an overall
yield of 34%. In preparation for a tandem Suzuki coupling/macrocy-
clization sequence, portions of diiodide 4 were converted to diboronate
5
in a straightforward manner. Advantageously, both coupling partners
could easily be prepared on a gram scale without the need for column
chromatography. Standard Suzuki reaction conditions afforded mac-
rocycles 6, 7, and 8 in 22% combined yield (Scheme 1). Formation
of macrocycle 6 was unexpected and likely involves homocoupling
8
of diboronate 5 as the intermediate step. Although the yield of
macrocyclization products was modest, the synthetic route provided
rapid access to three different macrocycles, which were easily separated
by column chromatography. It is notable that the construction of
macrocycle 8 involved formation of 18 carbon-carbon bonds in only
Figure 2. Aromatization reaction and proposed mechanism.
10
tant alkyl shifts generating 2,4-dialkylanisole derivatives. We at-
tempted to circumvent such rearrangements with the use of strong
11
4
steps.
nucleophilic hydride sources but without success. Similarly, reducing
12
13
With the macrocyclic precursors in hand, we turned our attention
conditions employing Stephen’s reagent or low-valent titanium led
to complex mixtures due to the Lewis acidic nature of the reagents.
Lithium napthalenide, a reagent known to reduce benzylic ethers
to the aromatization reaction. To date, no efficient reactions to convert
the cyclohexadiene moiety to a para-substituted aryl ring had been
9
14
reported in the literature. Cyclohexadiene compounds similar to ours
at low temperature, provided the eventual solution (Figure 2).
have been aromatized under acidic conditions, but only with concomi-
Treatment of each macrocycle (6, 7, and 8) with lithium napthalenide
at -78 °C afforded the corresponding cycloparaphenylenes (9, 10, and
11) in moderate yields. Mechanistically, the reaction is envisioned to
proceed via an initial one-electron transfer which generates stabilized
radical intermediate 12 and an equivalent of lithium methoxide. A
†
Department of Chemistry, University of California.
‡
The Molecular Foundry, Lawrence Berkeley National Laboratory.
§
Department of Molecular and Cell Biology, University of California, and
Howard Hughes Medical Institute.
1
7646 9 J. AM. CHEM. SOC. 2008, 130, 17646–17647
10.1021/ja807126u CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
data were also rationalized with DFT methods. The large Stokes shift
observed in these spectra is governed by the degree of structural
relaxation in the optically excited state. In the smaller nanohoops,
3
enhanced curvature leads to greater sp hybridization and increasingly
asymmetric p orbitals, with their smaller lobes oriented inside the
nanohoops. This factor reduces steric interactions and facilitates a larger
decrease in dihedral angle in the excited state for the smaller nanohoop.
Thus smaller hoops have the potential for larger structural relaxation
and Stokes shifts. Indeed constrained DFT calculations confirmed the
observed trend that the smallest carbon nanohoop has the largest
Figure 3. (a) Absorption (solid line) and fluorescence (dashed line) spectra
of carbon nanohoops 9, 10, and 11. (b) Energy-minimized geometry of carbon
nanohoop 9 by DFT calulations.
15
relaxation in its excited state.
In conclusion, we have demonstrated the first synthesis and
characterization of [9]-, [12]-, and [18]cycloparaphenylene, the fun-
damental unit of an armchair carbon nanotube. It may be possible to
prepare extended carbon nanotubes using similar solution phase
chemistry without the need for high temperatures. Preparation of carbon
second electron transfer then produces the alkyl lithium 13 which can
aromatize via loss of a second equivalent of lithium methoxide.
Especially noteworthy is the formation of cycloparaphenylene 9, the
most strained carbon nanohoop of the series, using these low-
temperature conditions.
5
nanotubes of a specific chirality utilizing these carbon nanohoop
structures as templating agents is an intriguing prospect.
With cycloparaphenylenes in hand for the first time, we analyzed
Acknowledgment. This work was performed at the Molecular
Foundry. Experimental work was supported by the Office of
Science, Office of Basic Energy Sciences, of the U.S. Department
of Energy under Contract No. DE-AC02-05CH11231. Theoretical
work was supported by the National Science Foundation through
the Network for Computational Nanotechnology, Grant EEC-
15
their properties using a variety of methods. Interestingly, their UV/
visible absorption spectra showed symmetric peaks with a maximum
at ∼340 nm regardless of carbon nanohoop size (Figure 3a). Indeed,
the smallest carbon nanohoop (9), which has the poorest geometry
for orbital overlap and fewest number of aryl rings, has a slightly
smaller optical absorption gap than that of the larger carbon nanohoops.
This result stands in sharp contrast to the acyclic paraphenylenes in
which a narrowing of the optical absorption gap occurs with extended
0
634750.
Supporting Information Available: Procedures for the synthesis
16
conjugation. The fluorescence spectra also provided some unexpected
results. As carbon nanohoop size decreased, the Stokes shift increased.
Interestingly, the smallest carbon nanohoop 9 shows an especially broad
spectrum with a large Stokes shift of approximately 160 nm.
We further investigated the structural and optical properties of the
carbon nanohoop molecules with computational methods based on
and characterization of all new compounds and details of the compu-
tational analyses. This material is available free of charge via the
Internet at http://pubs.acs.org.
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15
density functional theory (DFT). Our calculations indicate the favored
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between two adjacent phenyl rings alternates between 33° and 34°,
respectively. This arrangement minimizes steric repulsion between
neighboring aryl rings. For [9]cycloparaphenylene, the situation is
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4
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1
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15
nanohoop (Figure 3b). We found that chiral Mobius-strip like
arrangements, where the dihedral angle is successively increased by
(
6) Jagadeesh, M. N; Makur, A.; Chandrasekhar, J. J. Mol. Model. 2000, 6, 226.
∼
35° around the ring, were higher in energy by at least 2 kcal/mol
per phenyl ring. The calculated strain energies of carbon nanohoops
, 10, and 11 were 47, 28, and 5 kcal/mol with diameters of 1.2, 1.7,
and 2.4 nm, respectively.
We also performed DFT calculations to understand the counterin-
tuitive trends in the observed optical data. We estimated the average
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9
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(
9) The one relevant example in the literature produces a mixture of para-
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(
10) Alonso, F.; Yus, M. Tetrahedron 1992, 48, 2709.
(
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in similar systems: (a) Alonso, F.; Yus, M. Tetrahedron 1991, 47, 313. (b)
Alonso, F.; Yus, M. Tetrahedron 1991, 47, 9119.
optical absorption energy gap, E
formula: E ) ionization potential (IP) - electron affinity (EA) -
e-h, where Ee-h is the electron-hole interaction energy between the
highest-occupied and lowest-unoccupied DFT one-electron wave
g
, with the following approximate
g
(
12) Newman, M. S.; Kanakarajan, K. J. Org. Chem. 1980, 45, 2301.
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E
(
(14) Liu, H.-J.; Yip, J.; Shia, K.-S. Tetrahedron Lett. 1997, 38, 2253.
15
(15) For full details, see Supporting Information.
functions. As expected, we observed that IP - EA decreases as the
number of phenyl rings increase, in both the cyclic and acyclic cases.
Remarkably, however, we found that the magnitude of Ee-h grows
more dramatically with the decreasing number of phenyl rings for
(
(
16) Nijegorodov, N. I.; Downey, W. S.; Danailov, M. B. Spectrochim. Acta,
Part A 2000, 56, 783.
17) This tendency can be directly related to the difference between linear finite
(acyclic) and closed curved (cyclic) geometries. In the cyclic system, the
electron and hole states are delocalized over the entire circumference of
the molecule. In the acyclic system, electron and hole states are localized
away from the edges, toward the middle of the molecule. The spatial
distribution of these optically active electronic states results in different
electron-hole interaction energetics.
17
carbon nanohoops than for their acyclic counterparts. This computed
trend in Ee-h more than compensates for the increase in IP - EA and
g
results in an overall decrease in E with nanohoop diameter, in
agreement with the experimentally observed trends. The fluorescence
JA807126U
J. AM. CHEM. SOC. 9 VOL. 130, NO. 52, 2008 17647