Selective synthesis of strained [7]cycloparaphenylene: An orange-emitting fluorophore

Article abstract of DOI:10.1021/ja205606p

[n]Cycloparaphenylenes, which are short fragments of carbon nanotubes, have unique size-dependent optical properties. In this communication, we describe the first synthesis of [7]cycloparaphenylene ([7]CPP), the smallest cycloparaphenylene prepared to date. In order to access this structure, we have developed a synthetic route that capitalizes on successive orthogonal Suzuki-Miyaura coupling reactions. [7]CPP has 83 kcal/mol of strain energy and an orange emission at 592 nm.

Full text of DOI:10.1021/ja205606p

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pubs.acs.org/JACS
Selective Synthesis of Strained [7]Cycloparaphenylene: An
Orange-Emitting Fluorophore
†
†
†
,†,‡
Thomas J. Sisto, Matthew R. Golder, Elizabeth S. Hirst, and Ramesh Jasti*
†
‡
Department of Chemistry and Division of Materials Science and Engineering and Center for Nanoscience and Nanobiotechnology,
Boston University, Boston, Massachusetts 02215, United States
S Supporting Information
b
ABSTRACT: [n]Cycloparaphenylenes, which are short
fragments of carbon nanotubes, have unique size-dependent
optical properties. In this communication, we describe the
first synthesis of [7]cycloparaphenylene ([7]CPP), the
smallest cycloparaphenylene prepared to date. In order to
access this structure, we have developed a synthetic route
that capitalizes on successive orthogonal SuzukiÀMiyaura
coupling reactions. [7]CPP has 83 kcal/mol of strain energy
and an orange emission at 592 nm.
Figure 1. Size-dependent fluorescence of cycloparaphenylenes, quan-
tum dots, and linear p-phenylenes.
he cycloparaphenylene (CPP) class of molecules represents
T
the smallest possible slice of an armchair carbon nanotube
1
À3
2
(
CNT). The syntheses of these strained sp -hybridized mole-
cules have received significant attention recently because of their
potential application as templates for the preparation of single-
4À10
chirality CNTs.
In addition to their potential utility in CNT
synthesis, CPPs exhibit size-dependent optical properties (Figure 1).
All synthesized CPPs possess a common absorption maximum,
but their fluorescence is conditional on size. For example, [12]-
CPP has an absorption maximum at 340 nm with a fluorescence
4
maximum at 450 nm. Interestingly, [8]CPP also absorbs light at
3
40 nm, but its emission is red-shifted dramatically, with a maxi-
6
,10
mum at 540 nm. This trend of red-shifted fluorescence corres-
ponding to smaller size is unusual in that it is the opposite of
the typical “particle in a box” phenomenon seen in semiconduc-
tor quantum dots and linear p-phenylenes. As in the case of
quantum dots, this ability to absorb at similar wavelengths yet
size-selectively emit at various wavelengths is a property that
could be exploited for multiple applications, such as biological
1
1
12
Figure 2. Previous synthetic routes are not amenable to smaller CPPs.
1
3,14
imaging.
In order to probe the size-dependent optical pro-
energy of a macrocycle with only two cyclohexadiene moieties.
Since this pioneering report, Itami has synthesized several cyclo-
paraphenylene structures utilizing a cyclohexane unit (6) as the
perties of CPPs further, synthetic access to smaller versions is
necessary. Herein we report the first synthesis and characteriza-
tion of highly strained [7]cycloparaphenylene, the smallest CPP
that has been prepared to date.
5,7À9
masked aromatic ring.
through this method is [9]CPP. [8]CPP, the smallest CPP previ-
Again, the smallest structure prepared
9
6,10
The first synthesis of a cycloparaphenylene was reported by
ously synthesized, was reported by Yamago. With 73 kcal/mol
of strain energy, this structure was prepared through square-
planar platinum complex 7, which underwent reductive elimina-
tion to deliver [8]CPP. A synthetic strategy to access even smaller
cycloparaphenylenes would be a significant addition to this emerg-
ing field.
4
Jasti and Bertozzi in 2009. The synthetic approach relied on
cyclohexadiene structures as masked benzene rings (1 and 2 in
1
5
Figure 2). In a “shotgun”-type approach, aryl halide 1 and boro-
nate 2 were subjected to SuzukiÀMiyaura coupling conditions,
leading to macrocycles 3, 4, and 5. These three macrocycles were
subsequently converted to [9]-, [12]-, and [18]CPP, respectively.
Notably, the more strained precursor to [6]CPP was not formed
in the macrocyclization step, likely because of the increased strain
Received: June 29, 2011
Published: September 13, 2011
r 2011 American Chemical Society
15800
dx.doi.org/10.1021/ja205606p
|J. Am. Chem. Soc. 2011, 133, 15800–15802

Journal of the American Chemical Society
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Scheme 1. Preparation of Monomers 11 and 12
1
Figure 3. (a) H NMR spectrum, (b) DFT-optimized geometry, and
2
1
(c) crystal structure of macrocycle 15.
protons (HA and H ) are much further upfield (∼6.4 and
B
∼
6.6 ppm) than is observed in the larger macrocycles 3, 4, and
Scheme 2. Orthogonal SuzukiÀMiyaura Coupling Reactions
5 (7.2 ppm). As can be seen in both the X-ray structure and the
density functional theory (DFT)-optimized geometry of 15, the
protons on the biphenyl moiety are oriented into the ring current
shielding cones of the aromatic rings across the macrocycle. The
To Prepare Strained Macrocycle 15
four protons ortho to the cyclohexadiene units (H ) are ca. 2.7 Å
A
from the face of the adjacent aryl ring (Ar ), leading us to identify
1
these as the most upfield protons. The four protons meta to the
cyclohexadiene units (H ) are ca. 4.0 and 4.8 Å from aryl rings
B
Ar and Ar respectively, placing them farther from an aromatic
1
2
21
shielding cone and thus more downfield than H . The number
A
of signals and splitting patterns in the NMR spectrum indicate
that the aryl rings in the biphenyl moiety are freely rotating. This
edge-to-face orientation places the electrostatically negative
edges of the biphenyl aryl rings into the electrostatically positive
faces of the opposing aromatic units. This interaction is known to
lead to ca. 2 kcal/mol of stabilization energy and possibly explains
Our synthetic strategy to [7]CPP relies on orthogonal SuzukiÀ
Miyaura coupling reactions (see Scheme 2). The synthesis of key
fragments 11 and 12 is shown in Scheme 1. Silyl-protected bro-
mophenol 8 underwent oxidative dearomatization in the pre-
16
sence of H O to generate ketone 9 in 68% yield. Deprotonation
2
22
of alcohol 9 with sodium hydride followed by addition of aryl-
lithium 10 and subsequent methylation yielded dimethyl ether
why the smaller macrocycle orients in this fashion.
We probed the relative strain energy of macrocycle 15 using
23
24
1
1 in a diastereomeric ratio of 19:1 favoring the syn product.
Interestingly, when alcohol 9 was alternatively treated with
equiv of aryllithium 10 (presumably proceeding through the lith-
DFT calculations and isodesmic reactions. Macrocycle 4 re-
quires a buildup of only 3.2 kcal/mol of strain energy in the cycli-
zation step, whereas macrocycle 15 requires 16 kcal/mol of strain
2
25
ium alkoxide), the dr was lower (10:1). When alcohol 9 was
methylated before the addition of the aryl nucleophile, the dia-
stereoselectivity of the reaction dropped precipitously to 3:1.
This sense of stereoselectivity is consistent with an electrostatic
model in which the anionic nucleophile approaches from the
energy. These results are consistent with the more challenging
cyclization to generate 15.
With macrocycle 15 in hand, we investigated our reductive
26
aromatization reaction. Subjecting 15 to sodium naphthalenide
at À78 °C led to [7]CPP (16) in 52% yield (Scheme 3). As the
reaction proceeded, we observed a change to a deep-purple
solution and attribute this to the radical anion of 16. We have
observed this with other CPPs as well and therefore quench these
reactions with a solution of iodine in order to avoid additional
17
opposite faceof the existing negative charge(Scheme 1). Simply
put, the more ionic the bond (NaO > LiO > CH O), the greater
3
the diastereoselectivity of the reaction. To prepare for a SuzukiÀ
Miyaura coupling, a portion of 11 was converted to boronate 12
through lithiumÀhalogen exchange followed by subsequent
trapping of the reactive species with (isopropoxy)pinacolborane.
With fragments 11 and 12 in hand, macrocycle 15 was as-
sembled utilizing orthogonal SuzukiÀMiyaura coupling reac-
tions (Scheme 2). Selective coupling of 11 and 12 using tetrakis-
27
side reactions. Impressively, this single reaction builds ca.
67 kcal/mol of additional strain energy into the [7]CPP structure
at À78 °C. The proton NMR spectrum of 16 shows a singlet at
7.48 ppm, similar to what is observed for other CPPs and indi-
4
cating free rotation of the aryl rings on the NMR time scale. The
(
1
triphenylphosphine)palladium as the catalyst yielded dichloride
3 in 84% yield. As expected, the aryl chloride functionality
DFT-optimized geometry shows dihedral angles ranging from
12.9 to 30.7°, consistent with the studies by Wong. The calcu-
lated bond lengths are similar to the experimental values for
1
8
28
remained intact under these conditions, preventing the forma-
tion of larger oligomeric structures. When a more reactive ligand
system (Buchwald’s S-Phos ) was used, dichloride 13 under-
went coupling with 1,4-benzenediboronic acid bis(pinacol) ester
larger CPPs: 1.39 Å for the CorthoÀCortho/CipsoÀC
bonds
ortho
19
and 1.49 Å for the CipsoÀCipso bonds, suggesting benzenoid
8
character.
1
4 followed by subsequent macrocyclization in one pot to deliver
The optical absorption and emission spectra of [7]CPP are
illustrated in Scheme 3. [7]CPP has a dominant absorption
2
0
29
1
5 in 8% yield.
The proton NMR spectrum of macrocycle 15 revealed some
intriguing conformational features (Figure 3). Eight aromatic
maximum at 339 nm, similar to those in all other cyclopara-
4,10
phenylenes. The insensitivity of this strong absorption to the
1
5801
dx.doi.org/10.1021/ja205606p |J. Am. Chem. Soc. 2011, 133, 15800–15802

Journal of the American Chemical Society
COMMUNICATION
Scheme 3. Aromatization and Optical Characterization of
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(
25
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In conclusion, we have developed the first synthesis of [7]CPP
utilizing an orthogonal SuzukiÀMiyaura coupling strategy. This
cycloparaphenylene has a strain energy of 83 kcal/mol and an
orange fluorescence at 592 nm. The size-dependent optical be-
havior of the [n]CPPs positions them as interesting candidates
for a new class of “carbon quantum dots”. Efforts are underway in
our laboratory to prepare the more strained [6]CPP and exploit
the CPP scaffolds as interesting optoelectronic materials for a
wide range of applications.
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(
’
ASSOCIATED CONTENT
(
S
Supporting Information. Syntheses and characterization
b
of all new compounds, geometries and strain energies deter-
mined by DFT calculations, complete ref 23, and crystallographic
data for 15 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(
(
(
25) For full details, see the Supporting Information.
26) For further details, see ref 4.
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Chem. 2010, 75, 7358.
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(
’
AUTHOR INFORMATION
(
Corresponding Author
jasti@bu.edu
’
ACKNOWLEDGMENT
This work was supported by generous startup funds provided
by Boston University. The authors gratefully acknowledge
Dr. Jeffrey Bacon for crystal structure determination, Dr. Norman
Lee for instrumentation assistance, and Prof. Feng Wang for
helpful advice in regard to computational analyses.
1
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dx.doi.org/10.1021/ja205606p |J. Am. Chem. Soc. 2011, 133, 15800–15802