Synthesis of precursors for large-diameter hemispherical buckybowls and precursors for short carbon nanotubes

Article abstract of DOI:10.1002/ejoc.201200841

We report herein the synthesis of six precursor molecules for the rational synthesis of isomerically pure armchair, zigzag, or chiral single-walled carbon nanotubes (SWCNTs). The polycyclic aromatic hydrocarbons possess the required carbon connectivity for the generation of extended hemispherical buckybowls with predefined geometry through intramolecular cyclodehydrogenation. The precursors for the short carbon nanotubes and the large-diameter hemispherical buckybowls have potential for the rational initiation of single-chirality SWCNT growth on metal surfaces. The options for the construction of precursors for various SWCNT chiralities based on the suggested synthesis strategy are presented.

Full text of DOI:10.1002/ejoc.201200841

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FULL PAPER
DOI: 10.1002/ejoc.201200841
Synthesis of Precursors for Large-Diameter Hemispherical Buckybowls and
Precursors for Short Carbon Nanotubes
Andreas Mueller[a] and Konstantin Yu. Amsharov*[a]
Keywords: Hydrocarbons / Carbon / Nanotubes / Buckybowls / Fused-ring systems / Synthesis design / Dehydrogenation
We report herein the synthesis of six precursor molecules for
the rational synthesis of isomerically pure armchair, zigzag,
or chiral single-walled carbon nanotubes (SWCNTs). The
lecular cyclodehydrogenation. The precursors for the short
carbon nanotubes and the large-diameter hemispherical
buckybowls have potential for the rational initiation of sin-
polycyclic aromatic hydrocarbons possess the required car- gle-chirality SWCNT growth on metal surfaces. The options
bon connectivity for the generation of extended hemispheri-
cal buckybowls with predefined geometry through intramo-
for the construction of precursors for various SWCNT chirali-
ties based on the suggested synthesis strategy are presented.
chirality SWCNTs,[9–11] a complete separation of all the
formed isomers is still elusive. The post-production separa-
tion strategies are often not effective enough and are not
suitable for large-scale technological processes. Therefore
new synthetic approaches are needed for the production of
single-chirality nanotubes that are free from impurities, de-
fects, and other isomers.
Introduction
Single-walled carbon nanotubes (SWCNTs) attract con-
siderable attention due to their extraordinary electronic,
thermal, and mechanical properties, and warrant research
for their many potential technological applications.[1,2]
These materials are believed to be among the best candi-
dates for replacing the silicon-based systems used in con-
ventional microelectronic technology and for pushing it
towards the nanometer scale.[3] The virtually unlimited
number of possible isomeric nanotube structures allows for
precise tuning of the properties for specific applications and
offers vital flexibility in technological design. For instance,
the electronic structures of SWCNTs can exhibit either met-
allic or semiconducting properties depending on the orien-
tation of the hexagonal lattice relative to the tube axis and
on the diameter of the CNT, as classified by the chiral in-
dices (n, m).[4] On the other hand, applications of these ma-
terials have remained highly elusive because efficient tech-
niques for the production of SWCNTs with well-defined
structures are still lacking. Their conventional production
by graphite evaporation or by chemical vapor deposition
typically results in the formation of highly heterogeneous
materials as a result of the random nature of the formation
process.[2,5–7] Because only limited control over the structure
can be achieved,[8] the existing synthesis techniques cannot
provide access to materials in isomerically pure form, which
is crucial for many applications. Although considerable ef-
forts have been made in the past years to isolate single-
Of the several alternative approaches to controllable
SWCNT synthesis that have been suggested,[12–19] the route
based on the synthesis of hemispherical buckybowls, which
could serve as seeds for the growth of SWCNTs with prede-
fined chirality, seems to be very promising. This approach
mimics the conventional synthesis of CNTs by the root-
growth mechanism: Nanotube growth starts by the nucle-
ation of an end-cap fragment (hemispherical buckybowl) on
a catalyst nanoparticle followed by growth through the in-
corporation of carbon atoms.[20–24] The spontaneous nucle-
ation of an end-cap can be avoided by introducing a hemi-
spherical buckybowl with a predefined structure. Subse-
quent growth will finally yield isomerically pure SWCNTs.
Despite the progress made in synthesis of extended polycy-
clic aromatic hydrocarbons[25] and buckybowls,[26–29] the ef-
fective synthesis of highly strained hemispherical hydro-
carbons still remains challenging. In fact, only one example
of the synthesis of a small-diameter hemispherical bucky-
bowl, which was obtained in 2–3% yield, has been reported
so far.[30] Alternatively, buckybowl molecules can be ob-
tained very effectively from planar PAH precursors by sur-
face-catalyzed cyclodehydrogenation on Pt or Ru sur-
faces.[31,32] The cyclodehydrogenation process was found to
be very effective and highly selective,[32] which is crucial for
obtaining hemispherical buckybowls with the required con-
nectivity. Considering that the end-cap formation and
nanotube growth process can both be realized effectively on
the Pt surface,[33,34] a one-pot synthesis strategy as depicted
in Figure 1 appears to be very attractive for the preparative
[a] Max Planck Institute for Solid State Research,
Heisenbergstrasse 1, 70569 Stuttgart, Germany
Fax: +49-711-689-1502
E-mail: k.amsharov@fkf.mpg.de
Homepage: WWW: www.fkf.mpg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200841.
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synthesis of isomerically pure SWCNTs. Importantly, a only one way for intramolecular condensation, as in the
crude estimate shows that SWCNTs can be obtained on the case of precursors P1 and P6. Recently we demonstrated
kilogram scale from just 1 mg of end-cap precursor.
that intramolecular aryl–aryl coupling in related structures
can be realized very effectively on a Pt surface.[32] It was
revealed that the temperature range of buckybowl stability
on the Pt surface does not overlap with the temperature
range required for CNT growth on Pt: The decomposition
of buckybowls to graphene-like structures takes place at
720 °C whereas the lowest temperature for SWCNT growth
on Pt nanoparticles was found to be 850 °C.[33,34] The sta-
bility of the buckybowls could be enhanced by either in-
creasing the cap diameter, thus reducing the cap strain, or
by the formation of additional nanotube segments. Caps
bearing several completed CNT segments (formally short
nanotubes) are expected to have stabilities similar to nano-
tubes. The latter are known to be stable on Pt, at least up
to growth temperatures of around 850–950 °C.[33,34] The
precursor molecules presented in this work were specially
designed to allow the stability of such extended buckybowls
on metal surfaces to be investigated and to prove experi-
Figure 1. General scheme for the rational synthesis of single-walled
carbon nanotubes showing four main steps: i. precursor synthesis,
ii. deposition on the metal surface, iii. catalytic cyclodehydrogena- mentally their ability to initiate SWCNT growth.
tion to hemispherical buckybowl (end-cap), and iv. subsequent
SWCNT growth.
Up to now, only three precursor molecules have been
synthesized by such an approach.[35,36] All of them lead to
SWCNT end-caps with small diameters and without com-
pleted nanotube segments. Because both factors are crucial
for the efficiency of SWCNT growth initiation, in this work
we addressed this important issue and report herein the
synthesis of six precursor molecules to extended hemispher-
ical buckybowls, including precursors for large-diameter
end-caps and end-caps with completed nanotube segments
(short nanotube precursors). All the molecules were ob-
tained with the high purity required for the synthesis strat-
egy considered.
Results and Discussion
The structures of all six nanotube end-cap precursors
synthesized in this work are summarized in Figure 2. The
precursor structures were designed in such a way that after
deposition on a metal surface the precursors should display
a conformation providing the highest interaction with the
metal surface. Intramolecular cyclodehydrogenation would
then lead to the formation of buckybowls with the desired
connectivity, as shown in Figure 2 (the corresponding hemi-
spherical buckybowls are highlighted in orange). Import-
Figure 2. Structures of synthesized SWCNT precursor molecules
antly, the possible partial condensation of precursors from
an alternative conformation (“wrong” orientation) cannot
lead to alternative hemispherical buckybowls and the re-
sulting PAHs are expected to be inactive towards the initia-
tion of nanotube growth. Thus, the selectivity of nanotube
formation should not be affected by the presence of
“wrongly” condensed precursor molecules. Although such
and the corresponding single-walled carbon nanotubes. The end-
cap structures formed by precursor condensation are highlighted
in orange. Insert: The corresponding SWCNTs are marked in the
chirality map in orange. SWCNT chiralities for which end-cap pre-
cursors can be derived on the basis of P2 and P6, as described in
the main text, are highlighted in blue.
P1 represents the precursor for a (9, 0) zigzag nanotube
a scenario seems to be possible in the case of P2 and P3, with a small diameter of about 0.73 nm. Intramolecular
in general, precursor structures can be designed to allow cyclodehydrogenation of P1 would give a CNT end-cap
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Synthesis of Precursors for Buckybowls and Carbon Nanotubes
bearing two completed nanotube segments (Figure 2). For- tained by the reaction of 2 with the truxene trianion, which
mally, further elongation of poly(p-phenylene) chains in the was obtained in situ by treating truxene with 3 equiv. of
P1 structure would give precursors for nanotubes with ad- nBuLi. In the final step, intramolecular palladium-catalyzed
ditional segments. Intramolecular cyclodehydrogenation of arylation of 5 resulted in P1. After sublimation under re-
P2 on the metal surface would give a hemispherical buck- duced pressure, the precursor P1 was obtained in pure form
ybowl that represents the end-cap of the (12, 0) zigzag nano- as evidenced by LDI-MS analysis (see the Supporting In-
tube with a diameter of 0.94 nm. After such a condensa- formation).
tion, the rim of the resulting end-cap is not seamlessly
closed, as can be seen from Figure 2. Although further con-
densation in the cove region, which would lead to a penta-
gon–pentagon junction, is highly unlikely, it cannot be ex-
cluded. To avoid this, the structurally related precursor P3
was synthesized. In P3, an additional six methyl groups are
placed in appropriate positions for the formation of a single
completed nanotube segment during condensation. P4 rep-
resents the precursor for the end-cap of the same nanotube
type but bears two additional completed ring-segments. The
third ring-segment, which is not seamlessly closed, can be
completed by the introduction of methyl groups, in analogy
to the precursor P3, to avoid possible cyclization in the cove
regions. The precursors P5 and P6 represent the first exam-
ples of precursors for large-diameter hemispherical buck-
ybowls (1.22 and 1.21 nm, respectively), which could have
the required stability without completed CNT segments. Al-
though the presented hydrocarbons include precursors for
zigzag, armchair, and chiral SWCNTs, all the resulting
nanotubes would be metallic. This is a direct consequence
of the C3 (or C6) point-group symmetry of these molecules
because all nanotubes having at least C3 symmetry satisfy
the requirement for metallic SWCNTs: (n–m)/3 = integer (n
and m are chiral indices).[4] Nevertheless, there is no limita-
tion for the construction of precursor structures with all
possible chiralities. For instance, the consecutive replace-
ment of naphthalene moieties in P2 (12, 0) by phenanthrene
will give precursors for (12, 1), (12, 2), (12, 3), (12, 4), and
tions: a) [Pd(PPh3)4], Cs2CO3, toluene/MeOH, reflux, 70%;
Scheme 1. Synthetic route to precursor P1. Reagents and condi-
(12, 5) end-caps and, in P5, a (12, 6) end-cap. Interestingly,
the relative positions of the naphthalene and phenanthrene
fragments in the precursor do not influence the chirality of
the corresponding nanotube, although different cap struc-
tures result. By analogy, precursors for the nine chiralities
(10, 6), (10, 7), (11, 3), (11, 4), (11, 5), (12, 0), (12, 1), (12,
2), and (12, 3) can be derived from P6 by replacing the
phenylnaphthalene fragments by naphthalene or phenan-
threne.
b) NBS, DBPO, CCl4, reflux, 86%; c) AcOH, H2SO4, 80 °C, 80%;
d) nBuLi/THF, –78 °C; e) 2, 91%; f) Pd(OAc)2, DMA, Cs2CO3,
Me3BzNBr, 150 °C, 37%.
Precursors for the End-Cap of a (12, 0) Zigzag SWCNT
Three precursors for the end-cap of a (12, 0) zigzag
SWCNT were synthesized based on cobalt-catalyzed tri-
merization of symmetrical diarylethynes.[41,42] The synthetic
routes to the precursors P2–P4 are summarized in
Scheme 2. In contrast to peviously reported syntheses, the
suggested strategy provides a facile route to the precursors
Precursor for the End-Cap of a (9, 0) Zigzag SWCNT (P1)
The synthetic route to the precursor P1 for a (9, 0) zigzag of hemispherical buckybowls. Thus, starting from commer-
CNT end-cap is presented in Scheme 1. It is based on the cially available 2-bromonaphthalene, the precursor P2 for
three-fold alkylation of truxene and subsequent palladium- the end-cap of a (12, 0) nanotube can be obtained in just
catalyzed intramolecular arylation.[37–39] 4-Bromo-3- two steps. 2-Bromonaphthalene was converted into the di-
methyl-p-terphenyl (1) was obtained by Suzuki coupling of arylethyne 5 by a one-pot palladium-catalyzed coupling re-
4-biphenylboronic acid and 2-methyl-4-iodophenyl bromide action with in situ generated acetylene.[43] Although the pre-
and brominated with N-bromosuccinimide (NBS) to give 4- viously described protocol gave rather low conversion to the
bromo-3-bromomethyl-p-terphenyl (2). Truxene (3) was target ethyne, it was found that the yield can be significantly
synthesized by trimerization of indanone according to a improved by exchanging the K2CO3 base with Cs2CO3.
known procedure.[40] Subsequently, compound 4 was ob- Thus, by using these modified conditions, the yield of the
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desired 1,2-di(naphthalene-2-yl)ethyne (5) was increased thesis of diarylethyne 10 from 8 by palladium-catalyzed
from 25 to 82% (see the Supporting Information). In the coupling with acetylene failed because the resulting 10
final step, ethyne 5 was cyclotrimerized by treatment with turned out to be completely insoluble and separation from
octacarbonyldicobalt to yield hexanaphthylbenzene P2 in other insoluble byproducts was not possible. Compound 10
60% yield. Importantly, the resulting precursor P2 was was obtained from 8 by two consecutive Sonogashira reac-
found to be sufficiently soluble in common organic solvents, tions. Despite the very low solubility of 10, the reaction
which remarkably simplified its purification and characteri- with octacarbonyldicobalt in dioxane/toluene gave the de-
zation. Furthermore, based on this strategy, two extended sired cyclotrimerization product P4 as a mixture with unre-
precursors for (12, 0) nanotube caps with completed nano- acted 10. Fortunately, the insoluble P4 can be obtained in
tube segments were synthesized (precursors P3 and P4). high purity by gradient sublimation, as evidenced by MS
The precursor P3 was synthesized following the same route analysis (see the Supporting Information).
from 2-bromo-7-methylnaphthalene, which was obtained by
a two-step synthesis according to a previously reported pro-
cedure.[44] A precursor for an end-cap with two additional
ring-segments was synthesized starting from 2-bromochrys-
Precursor for the End-Cap of a (12, 6) Chiral SWCNT
The strategy based on the formal [2+2+2] cyclotrimeri-
ene (8). Compound 8 was obtained by standard Wittig ole-
zation of diarylethynes can be easily extended to the synthe-
fination of 1-acetonaphthone with 3-bromobenzyl bromide
sis of precursors of various SWCNT types. Thus, trimeri-
and subsequent photocyclization. The direct one-pot syn-
zation of 1,2-di(phenanthren-3-yl)ethyne (12) led to the pre-
cursor for the end-cap of a (12, 6) chiral SWCNT (P5).
Compound 12 was synthesized in 67% yield in analogy to
the synthesis of 5 and 6 by using 3-bromophenanthrene (11;
Scheme 3). 3-Bromophenanthrene was obtained by stan-
dard Wittig olefination of benzaldehyde with 4-bromo-
benzyl bromide and subsequent photocyclization. In con-
trast to 10, compound 12 was found to be soluble enough
to allow chromatographic purification. After final cobalt-
catalyzed trimerization of 12, the precursor P5 was ob-
tained in 47% yield.
Scheme 3. Synthetic route to the precursor P5. Reagents and condi-
tions: a) PPh3, toluene, 95%; b) C6H5CHO, KOtBu, EtOH, 79%;
c) I2, hv, propylene oxide, cyclohexane, 31%; d) CaC2, Cs2CO3,
Pd(OAc)2, iPr2NPPh2, THF, 67%; e) [Co2(CO)8], dioxane, 47%.
Scheme 2. Synthetic route to precursors P2–P4. Reagents and con-
ditions: a) CaC2, Cs2CO3, Pd(OAc)2, iPr2NPPh2, THF, 5 (82%), 6
(26%); b) [Co2(CO)8], dioxane, P2 (69%), P3 (57%); c) PPh3, tolu-
Precursor for the End-Cap of a (9, 9) Armchair SWCNT
ene, reflux, 95%; d) naphthalene-1-carbaldehyde, KOtBu, EtOH,
reflux, 86%; e) I2, hv, propylene oxide, cyclohexane, 22%;
An alternative approach to the synthesis of buckybowl
f) [PdCl2(PPh3)2], CuI, PPh3, toluene, iPr2NH, TMS–CϵCH,
precursors is based on the aldol trimerization of aryl methyl
ketones. The applicability of this strategy has been demon-
80 °C;
g) KOH,
MeOH/CH2Cl2,
room
temp.,
(97%);
h) [PdCl2(PPh3)2], CuI, PPh3, toluene, iPr2NH, 8, 80 °C, 61%.
4
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Synthesis of Precursors for Buckybowls and Carbon Nanotubes
strated by the synthesis of precursor P6 (Scheme 4). The proceeded much slower in the latter case. Importantly, if
first synthetic route to P6 was based on the Lewis acid cata- debromination was suppressed, the mono- and bis-func-
lyzed cyclotrimerization of the ketone 18. 1,4-Dibromo- tionalized 19 (bearing one and two bromine atoms, respec-
naphthalene (free of 1,5-dibromonaphthalene) was synthe- tively) could be easily separated and obtained in pure form.
sized from 1-bromonaphthalene by bromination in acetoni- These molecules have potential for the synthesis of end-cap
trile. The following Suzuki coupling with phenylboronic precursors for other SWCNT types. Thus, for example, they
acid resulted in 1-phenyl-4-bromonaphthalene (14), which can be converted into four different precursors for (10, 6),
was subsequently converted into the corresponding pinacol (10, 7), (11, 3), and (11, 5) SWCNTs by a single Suzuki
ester 15 by using a known literature procedure.[45] 1-Bromo- coupling step with naphthyl- or phenanthrenylboronic acid.
2-ethylnaphthalene (16) was synthesized by bromination of In general, different combinations of naphthalene, phenyl-
2-ethylnaphthalene with N-bromosuccinimide in acetoni- naphthalene, and phenanthrene groups attached to 19 result
trile. NBS acts under these conditions as a mild brominat- in ten different end-cap precursors (Figure 3). Five of these
ing agent resulting in selective electrophilic substitution at molecules lead to the semiconducting (10, 6), (11, 3), (11,
the naphthalene core. 1-(1-Bromonaphthalen-2-yl)ethanone 4), (12, 1), and (12, 2) SWCNTs. In principle, this combina-
(17) was obtained in one step by oxidation using KMnO4 torial strategy can be extended to other substituted benz-
in acetone/H2O. Although the isolated yield was only 14%, enes allowing the derivation of end-cap precursor molecules
approximately 60% of the starting material could be recy- for a variety of different CNT chiralities (for details, see the
cled and used for repetitive oxidation. Alternatively, com- Supporting Information).
pound 17 was obtained by a four-step synthesis from 1-
bromo-2-methylnaphthalene in a combined yield of 60%.
After double radical bromination of the methyl group, the
resulting 1-bromo-2-(bromomethyl)naphthalene was con-
verted into 1-bromo-naphthalene-2-carbaldehyde with sil-
ver nitrate.[46] Reaction with methylmagnesium bromide
gave 1-(1-bromonaphthalen-2-yl)ethanol, which was con-
verted effectively into the corresponding ketone 17 by oxi-
dation with Jones’ reagent or KMnO4. Suzuki coupling of
15 and 17 resulted in the aryl methyl ketone 18, which was
used for the following Lewis acid catalyzed cyclotrimeri-
zation. In all cases, trimerization of 18 to P6 under various
conditions (TiCl4/o-dichlorobenzene, SiCl4/HCl/EtOH, p-
toluenesulfonic acid/o-dichlorobenzene) and at different
temperatures resulted in a complex mixture and all attempts
to separate P6 in pure form failed. As an alternative route,
1,3,5-tris(1-bromonaphthalen-2-yl)benzene (19) was synthe-
sized by aldol trimerization of 17 and subsequent conver-
sion into the desired precursor by three-fold Suzuki cou-
pling with compound 15. Unexpectedly, the trimerization
of 17 using SiCl4 or Si(OEt)4/HCl in ethanol[47] failed and
no product formation was detected. Condensation in o-
dichlorobenzene with TiCl4 in a closed ampoule at
160 °C[48,49] resulted in a complex mixture, which was not
further analyzed. By heating 1-(1-bromonaphthalen-2-yl)-
ethanone and TiCl4 at reflux in toluene[50] a mixture con-
taining the desired trimer and an unknown byproduct was
obtained. Unfortunately, the desired product could only be
isolated by HPLC in this case. The best results for the tri-
merization of 17 were obtained by using TiCl4 in o-DCB at
110 °C for 12 h. In this case, the formation of the hard-
to-separate byproduct was avoided and the desired 19 was
obtained in 18% yield. Subsequent three-fold Suzuki cou-
pling of 1,3,5-tris(1-bromonaphthalen-2-yl)benzene (19)
Scheme 4. Synthetic route to the precursor P6. Reagents and
conditions: a) Br2, acetonitrile, 52%; b) PhB(OH)2, [Pd(PPh3)4],
with 15 resulted in P6 in a moderate yield of 34%. Al-
though significant debromination of 1,3,5-tris(1-bromo-
naphthalen-2-yl)benzene and its single and two-fold Suzuki
coupling products was observed when using toluene/meth-
anol for the reaction, the formation of these side-products
K2CO3,
toluene/MeOH,
72%;
c) bis(pinacolato)diboron,
[PdCl2(dppf)],·KOAc, dioxane, 75%; d) NBS, DBPO, CCl4, 87%;
e) AgNO3, H2O/acetone, 80%; f) MeMgBr, THF; g) KMnO4, H2O/
acetone, 84%; h) NBS, CH3CN, 79%; i) KMnO4, H2O/acetone,
14%; j) TiCl4, o-DCB, 110 °C, 18%; k) [Pd(PPh3)4], Cs2CO3, tolu-
ene/MeOH, reflux, 53%; l) 15, [Pd(PPh3)4], Cs2CO3, toluene/
was reduced by using toluene/H2O, although the reaction MeOH, reflux, 34%.
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SWCNT precursors with a large variety of nanotube chiral-
ities by simple exchange of aryl fragments.
In general, the structures of the precursors are suitable
for the synthesis of hemispherical buckybowls and short
nanotubes by using alternative condensation procedures.
Thus, for instance, the recently discovered Al2O3-mediated
aryl–aryl coupling reaction[51,52] seems to be an appropriate
method for the synthesis of these unique molecules in bulk.
Experimental Section
General: All reagents and solvents were used as received unless
stated otherwise. 1H and 13C NMR spectra were recorded with a
Bruker AVANCE DPX-300 SB spectrometer at room temperature
unless stated otherwise. Tetramethylsilane was used as internal
standard to determine the chemical shifts. MS analysis was per-
formed with a Shimadzu/Kratos Axima Resonance Spectrometer
in LDI (laser desorption ionization) mode. Rf values were deter-
mined on TLC/PET sheets coated with silica gel with fluorescent
indicator (λ = 254 nm, layer thickness 0.25 mm, medium pore dia-
meter 60 Å, Fluka). Flash chromatography was either carried out
by using Kieselgel 60 (0.06–0.2 mm, Roth) or with the automated
flash chromatography system PuriFlash 430 evo (Interchim).
HPLC measurements were carried out with a Shimadzu CBM-20A
system equipped with a Shimadzu SPD-M20A UV/Vis DA detec-
tor. Cosmosil Buckyprep (4.6ϫ250 mm) and 5PYE (4.6ϫ250 mm;
Nacalai Inc.,) columns were used. Single-crystal X-ray analysis was
performed with a Bruker Smart APEX II diffractometer equipped
with a Siemens X-ray source (Mo-Kα radiation, λ = 0.71073 Å)
at 100 K. The Bruker Suite software package was used for data
integration and reduction.[53] Structure determination was per-
formed by using direct methods and refinement by the least-squares
method by using the SHELXTL software.[54]
Figure 3. End-cap precursor structures based on compound 19 that
can be obtained by a combinatorial strategy using three different
aryl groups (phenylnaphthalene, naphthalene, and phenanthrene).
Conclusions
The synthesis of six precursor molecules for large-dia-
meter hemispherical buckybowls and short carbon nano-
tubes has been presented. The corresponding hemispherical
buckybowls represent end-caps of SWCNTs and are ex-
pected to exhibit enhanced stability and therefore are very
attractive for the controlled synthesis of single-chirality
CNTs. Further investigation of the large-diameter end-cap
precursors and the precursors for end-caps with additional
completed nanotube segments will reveal the main factors
responsible for cap stability on a metal surface and the abil-
ity of CNT growth initiation in a fully controllable manner
can be examined experimentally. These investigations are in
progress and the results will be reported elsewhere.
4-Bromo-3-methyl-p-terphenyl (1): 2-Bromo-4-iodotoluene (7.6 g,
20 mmol), 4-biphenylboronic acid (4.0 g, 20 mmol), and
[Pd(PPh3)4] (0.3 g, 0.26 mmol) were dissolved in toluene (200 mL)
and K2CO3 (5.6 g, 40 mmol) dissolved in water (40 mL) was added.
After degassing under dynamic vacuum the system was filled with
argon and the mixture was heated at reflux for 10 h. The mixture
was extracted with toluene, washed with H2O, dried with Na2SO4,
and concentrated. After precipitation with petroleum ether and
recrystallization from petroleum ether, 4-bromo-3-methyl-p-ter-
phenyl (1) was obtained as a white crystalline powder (4.5 g,
13.9 mmol, 70%). Rf (hexane/CCl4, 2:1)
=
0.60. 1H NMR
(300 MHz, CD2Cl2): δ = 7.68–7.51 (m, 7 H), 7.47–7.34 (m, 3 H),
7.34–7.21 (m, 2 H), 2.41 (s, 3 H) ppm. 13C NMR (75 MHz,
CD2Cl2): δ = 141.41, 141.05, 140.81, 140.35, 137.96, 133.14, 129.74,
129.54, 128.96, 127.72, 127.70, 127.41, 127.04, 118.59, 40.07 ppm.
MS (LDI-TOF): calcd. for C19H15Br 322.04 [M]+; found 321.95.
This synthetic strategy provides a facile route to C6 sym-
metrical nanotube precursors by the formal [2+2+2] cyclo-
addition of diarylacetylenes, as demonstrated by the synthe-
sis of P2–P5. Truxene and compound 19 were used for the 4-Bromo-3-bromomethyl-p-terphenyl (2): 4-Bromo-3-methyl-p-ter-
phenyl (1; 2.9 g, 9 mmol) was dissolved in CCl4 (50 mL) and NBS
(1.8 g, 10 mmol) and dibenzoyl peroxide (DBPO; 50 mg) were
added. The solution was then heated at reflux for 6 h (TLC moni-
toring). After cooling, the mixture was concentrated, filtered
through silica gel, and the solvents evaporated. After addition of
EtOH and filtration, a white powder was obtained. After purifica-
tion by flash chromatography (silica gel, hexane/CCl4, 1:4), 3.1 g
(7.7 mmol, 86%) of the pure product were obtained as a white pow-
synthesis of C3 symmetrical precursors P1 and P6. It was
shown that all insoluble molecules can be obtained in high
purity by gradient sublimation. Importantly, 19 is a pro-
spective building block and can be used for the construction
of precursor molecules for SWCNTs with different chirali-
ties. Thus, eight additional end-cap precursors can be easily
obtained based on 19 by consecutive Suzuki couplings with
naphthalene, phenylnaphthalene, and phenanthrene frag-
ments in different combinations. In addition, such a combi-
1
der. Rf (hexane/CCl4, 2:1) = 0.44. H NMR (300 MHz, CD2Cl2): δ
= 7.67 (d, J = 2.3 Hz, 1 H), 7.65–7.54 (m, 7 H), 7.43–7.34 (m, 3
natorial strategy can be extended to the synthesis of H), 7.33–7.25 (m, 1 H), 4.61 (s, 2 H) ppm. 13C NMR (75 MHz,
6
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Synthesis of Precursors for Buckybowls and Carbon Nanotubes
CD2Cl2): δ = 140.80, 140.76, 140.39, 138.19, 137.68, 133.82, 129.78,
(36 mg, 0.16 mmol), diisopropylaminodiphenylphosphane (91 mg,
128.94 (2 C), 128.65, 127.65 (2 C), 127.64, 127.32 (2 C), 127.01 (2 0.32 mmol), benzene (5 mL), and THF (5 mL) at 65 °C for 14 h.
C), 123.40, 33.60 ppm. MS (LDI-TOF): calcd. for C19H14Br2 After flash chromatographic separation (silica gel, petroleum ether/
399.95 [M]+; found 399.82.
CCl4, 1:1), 51 mg of 6 (0.17 mmol, 26%) were obtained. Rf (CCl4)
= 0.60. H NMR (300 MHz, CD2Cl2): δ = 8.02 (s, 2 H), 7.82–7.72
1
C84H57Br3 (4): Anhydrous THF, which had been freshly distilled
from sodium, was used and all steps in the synthesis were carried
out under argon by using the Schlenk technique. The THF was
first degassed and subsequently distilled under reduced pressure.
Truxene (63 mg, 0.18 mmol) was dispersed in THF (15 mL). A 1.6
m nBuLi (0.38 mL) solution in hexane (0.61 mmol) was then slowly
added to the mixture at –78 °C resulting in an orange dispersion.
After stirring the mixture for 10 min it was slowly warmed to 0 °C
resulting in a deep-red solution. After 1 h a solution of 4-bromo-
3-bromomethyl-p-terphenyl (2; 267 mg, 0.66 mmol) in THF
(10 mL) was added dropwise to the solution of the truxene tri-
anion. The resulting mixture was stirred for another 2 h, diluted
with EtOAc, washed with a saturated aqueous NaCl solution, dried
with Na2SO4, and concentrated. The product was purified by flash
chromatography (silica gel, CCl4). The anti isomer (218 mg,
0.167 mmol, 91%) was obtained as a pale-yellow powder. Rf (CCl4)
(m, 4 H), 7.61 (s, 2 H), 7.56 (dd, J = 8.4, 1.6 Hz, 2 H), 7.35 (dd, J
= 8.3, 1.6 Hz, 2 H), 2.54 (s, 6 H) ppm. 13C NMR (75 MHz,
CD2Cl2): δ = 136.24, 132.84, 130.69, 130.59, 128.85, 127.53, 127.40,
127.35, 126.43, 120.22, 90.01, 21.51 ppm. HRMS: calcd. for C24H18
306.1409; found 306.1410. MS (LDI-TOF): m/z = 306.13 [M]+.
Hexa(naphthalen-2-yl)benzene (P2): Dinaphthylacetylene
5
(175 mg, 0.63 mmol) and octacarbonyldicobalt (20 mg,
0.058 mmol) were dissolved in anhydrous dioxane (10 mL). The
solution was heated at reflux under argon for 19 h after which the
solvent was evaporated under reduced pressure. The solid was re-
dissolved in dichloromethane and filtered through silica gel. The
product was recrystallized from dichloromethane/ethanol and ob-
tained as a beige powder (120 mg, 69%). Rf (hexane/CH2Cl2, 2:1)
= 0.44. 1H NMR (300 MHz, C2D2Cl4, 80 °C): δ = 7.40 (s, 6 H),
7.33–7.21 (m, 12 H), 7.13–6.99 (m, 24 H) ppm. The product was
too insoluble for 13C NMR analysis. MS (LDI-TOF): calcd. for
C66H42 834.33 [M]+; found 834.31; isotopic distribution calculated:
m/z (%) = 834.33 (100), 835.33 (71.4), 836.34 (25.1), 837.34 (5.8);
isotopic distribution found: m/z (%) = 834.31 (100), 835.32 (69.62),
836.33 (23.30), 837.33 (4.85).
1
= 0.22. The ratio of aliphatic to aromatic protons was 9:48 by H
NMR. MS (LDI-TOF): calcd. for C84H57Br3 1302.20 [M]+; found
1302.75.
C84H48 (P1): Compound 4 (218 mg, 0.17 mmol), Pd(OAc)2 (76 mg,
0.33 mmol),
trimethylbenzylammonium
bromide
(154 mg,
0.67 mmol), and Cs2CO3 (274 mg, 0.84 mmol) were dissolved in Hexakis(7-methylnaphthalen-2-yl)benzene (P3): Hexakis(7-methyl-
N,N-dimethylacetamide (15 mL). After degassing, the mixture was
stirred under argon at 150 °C for 5 d. The mixture was then allowed
to cool to room temperature and the solid precipitate was filtered
off and washed with CH2Cl2, acetone, and water. The resulting
solid was subsequently dispersed in aqueous NaCN, stirred for 3 h,
filtered, and washed sequentially with water, acetone, and CH2Cl2.
The product was obtained as a brown powder. After sublimation
under reduced pressure P1 (65 mg) was obtained as an orange pow-
naphthalen-2-yl)benzene (P3) was synthesized from 1,2-bis(7-meth-
ylnaphthalen-2-yl)ethyne (46 mg, 0.15 mmol) according to the pro-
cedure used for the synthesis of P2: Octacarbonyldicobalt (10 mg,
0.029 mmol) and dioxane (10 mL) at reflux for 19 h. P3 was ob-
tained as a beige powder (26.4 mg, 2.9 mmol, 57%). Rf (hexane/
1
CH2Cl2, 2:1) = 0.56. H NMR (300 MHz, CD2Cl2): δ = 8.12–6.54
(m, 36 H), 2.51–1.99 (m, 18 H) ppm. The product was too insoluble
for 13C NMR analysis. MS (LDI-TOF): calcd. for C72H54 918.4226
der (0.062 mmol, 37%). MS (LDI-TOF): calcd. for C84H48 1056.38 [M]+; found 918.43; isotopic distribution found: m/z (%) = 918.43
[M]+; found 1056.35; isotopic distribution calculated: m/z (%) =
1056.38 (100), 1057.38 (90.9), 1058.38 (40.8), 1059.39 (12.1),
1060.39 (2.6); isotopic distribution found: m/z (%) = 1056.35 (100),
1057.35 (90.9), 1058.36 (40.5), 1059.37 (11.1), 1060.38 (1.0). The
product was too insoluble for NMR analysis.
(100), 919.44 (77.7), 920.45 (32.1), 921.47 (7.8), 922.49 (0.7); iso-
topic distribution calculated: m/z (%) = 918.42 (100), 919.43 (77.9),
920.43 (29.9), 921.43 (7.5), 922.44 (1.4).
(E/Z)-1-[2-(3-Bromophenyl)ethenyl]naphthalene (7): 3-Bromobenzyl
bromide (10 g, 40 mmol) was dissolved in toluene (150 mL) and
triphenylphosphane (11.8 g, 45 mmol) was added. The solution was
heated at reflux for 12 h under continuous stirring. A white precipi-
1,2-Di(naphthalen-2-yl)ethyne (5): A round-bottomed flask was
charged under argon with 2-bromonaphthalene (280 mg,
1.35 mmol), calcium carbide (319 mg, 5.4 mmol), Cs2CO3 (880 mg, tate formed during the synthesis. The mixture was then allowed to
2.7 mmol; dissolved in 2–3 drops of H2O), Pd(OAc)2 (30 mg, cool to room temperature, filtered, and washed with toluene and
0.135 mmol),
diisopropylaminodiphenylphosphane
(96 mg,
hexane to yield 20.1 g (39.2 mmol, 98%) of (3-bromobenzyl)tri-
phenylphosphonium bromide. The compound was used without
any further purification in the next synthetic step. (3-Bromo-
benzyl)triphenylphosphonium bromide (10 g, 20 mmol), naphth
alene-1-carbaldehyde (3.1 g, 2.716 mL, 20 mmol), and KOtBu
(2.24 g, 20 mmol) were dissolved in EtOH (250 mL). The solution
was subsequently heated at reflux for 20 h. After cooling to room
0.34 mmol), benzene (5 mL), and anhydrous THF (5 mL). The
mixture turned black after several minutes. It was then heated at
65 °C for 12 h. After cooling, the solvent was removed and the
crude product was dissolved in CCl4 and filtered through silica gel.
CCl4 was subsequently evaporated and the resulting solid was
washed with EtOH and petroleum ether. 1,2-Di(naphthalen-2-yl)-
ethyne was obtained as a yellow powder (154 mg, 82%). Rf (CCl4) temperature, the mixture was concentrated, diluted with H2O, ex-
1
= 0.58. H NMR (300 MHz, CDCl3): δ = 7.41–7.47 (m, 4 H), 7.56 tracted with CH2Cl2, dried with Na2SO4, and purified by flash
(d, J = 10.0 Hz, 2 H), 7.75–7.78 (m, 6 H), 8.03 (s, 2 H) ppm. 13C chromatography (silica gel, petroleum ether/CH2Cl2, 1:1). Com-
NMR (75 MHz, CDCl3): δ = 90.19, 120.64, 126.57, 126.69, 127.79,
127.81, 128.04, 128.45, 131.52, 132.87, 133.09 ppm. HRMS: calcd.
for C22H14 278.1096; found 278.1098. MS (LDI-TOF): m/z =
278.14 [M]+.
pound 7 was obtained as a pale-yellow viscous oil (5.3 g,
17.1 mmol, 86%). The spectroscopic data were found to be in
agreement with those previously reported.[55]
2-Bromochrysene (8): (E/Z)-1-[2-(3-Bromophenyl)ethenyl]naphth-
alene (7; 5 g, 16.2 mmol) was dissolved in cyclohexane (400 mL)
and placed in a photochemical reactor. After the addition of iodine
(4.45 g, 17.5 mmol), argon was bubbled through the solution for
about 15 min. Then propylene oxide (9.3 g, 160 mmol, 12 mL) was
1,2-Bis(7-methylnaphthalen-2-yl)ethyne (6): Compound 6 was syn-
thesized from 2-bromo-7-methylnaphthalene (280 mg, 1.26 mmol)
according to the procedure used for the synthesis of compound
5. CaC2 (295 mg, 5 mmol), Cs2CO3 (814 mg, 2.5 mmol), Pd(OAc)2
Eur. J. Org. Chem. 0000, 0–0
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A. Mueller, K. Yu. Amsharov
FULL PAPER
added and the solution was irradiated with UV light. After 20 h
irradiation, the crystalline solid formed was removed from the reac-
tor walls and recrystallized from CH2Cl2 and washed with EtOH.
2-Bromochrysene was obtained as a white powder (1.1 g, 3.6 mmol,
diphenylphosphane (114 mg, 0.4 mmol), benzene (5 mL), and THF
(5 mL). The product 12 was obtained as a yellow crystalline powder
1
(222 mg, 66.8%). Rf (CCl4) = 0.41. H NMR (300 MHz, CDCl3):
δ = 7.51–7.75 (m, 10 H), 7.83–7.89 (m, 4 H), 8.69 (d, J = 8.4 Hz,
2 H), 8.91 (s, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 90.97,
121.56, 123.03, 126.72, 126.77, 127.15, 127.22, 128.11, 128.90,
128.91, 129.55, 130.10, 130.46, 132.00, 132.55 ppm. HRMS: calcd.
1
22%). Rf (hexane/CCl4, 1:1) = 0.72. H NMR (300 MHz, CDCl3):
δ = 8.65 (t, J = 8.7 Hz, 2 H), 8.53 (dd, J = 9.0, 3.2 Hz, 2 H), 8.04
(d, J = 2.0 Hz, 1 H), 7.97–7.85 (m, 2 H), 7.80 (d, J = 9.1 Hz, 1 H),
7.73–7.51 (m, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 120.42, for C30H18 378.1409; found 378.1413. MS (LDI-TOF): m/z =
120.82, 122.47, 123.10, 125.00, 126.15, 126.62, 126.91, 127.84,
128.01, 128.26, 128.61, 129.16, 129.77, 130.44, 130.57, 132.22,
133.49 ppm. MS (LDI-TOF): calcd. for C18H11Br 306.00 [M]+;
found 306.05. UV/Vis: λmax (toluene) = 297, 308, 322, 346, 354 (s),
364 nm.
378.25 [M]+.
Crystal Data for 12: Monoclinic, space group P121/c1, a =
9.3136(11), b = 7.8691(9), c = 13.1155(15) Å, β = 100.9280(10)°, V
= 943.80(19) Å3, Z = 2; 2θmax = 54.32, –11Ͻ hϽ 11, 0Ͻ kϽ 10,
0Ͻ lϽ 16, λ = 0.71073 Å, T = 100(2) K, final R value = 0.0411
(Rw = 0.1158).
2-Ethynylchrysene (9): 2-Bromochrysene (8; 100 mg, 0.33 mmol),
[Pd(PPh3)2Cl2] (10 mg, 0.014 mmol), CuI (2.7 mg, 0.014 mmol),
and PPh3 (7.3 mg, 0.028 mmol) were dissolved in toluene (5 mL)
and diisopropylamine (0.5 mL) was added. The mixture was de-
gassed under dynamic vacuum and the system was filled with ar-
gon. The mixture was stirred for 10 min (25 °C) and then (trimeth-
ylsilyl)acetylene (50 mg, 0.5 mmol) was added. The mixture was
stirred at 80 °C for 3 h, then a saturated NH4Cl solution was
added, and the mixture was extracted with toluene, dried with
Na2SO4, and the solvents evaporated. The resulting yellow powder
was dissolved in dichloromethane (4 mL) and KOH (36 mg) dis-
solved in MeOH (1.5 mL) was added. The mixture was stirred at
80 °C for 1.5 h under argon and then water (50 mL) was added,
and the mixture extracted twice with toluene (50 mL). The organic
phases were combined, dried with Na2SO4, and the solvents evapo-
rated to give chrysene 9 as a yellowish powder (82 mg, 0.32 mmol,
97%). Rf (hexane/CCl4, 1:1) = 0.50. 1H NMR (300 MHz, CD2Cl2):
δ = 8.77–8.56 (m, 4 H), 8.09 (d, J = 1.7 Hz, 1 H), 8.00–7.87 (m, 3
H), 7.73–7.53 (m, 3 H), 3.20 (s, 1 H) ppm. MS (LDI-TOF): calcd.
for C20H12 252.09 [M]+; found 252.10.
CCDC-887712 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.
Hexa(phenanthren-3-yl)benzene (P5): Hexa(phenanthren-3-yl)benz-
ene (P5) was synthesized by trimerization of 1,2-di(phenanthren-3-
yl)ethyne (12; 100 mg, 0.26 mmol) according to the procedure used
for the synthesis of P2 by using octacarbonyldicobalt (20 mg,
0.058 mmol) and dioxane (40 mL). Hexa(phenanthren-3-yl)benz-
ene was then precipitated by the addition of EtOH, filtered, and
washed. The product was obtained as a beige powder (47 mg,
47%). MS (LDI-TOF): calcd. for C90H54 1134.42 [M]+; found
1134.51; isotopic distribution calculated: m/z (%) = 1134.42 (100),
1135.43 (97.3), 1136.43 (46.9), 1137.43 (14.9), 1138.44 (3.5); iso-
topic distribution found: m/z (%) = 1134.51 (94.73), 1135.51 (100),
1136.51 (51.78), 1137.54 (24.76), 1138.51 (7.27). Compound P5 was
too insoluble for NMR characterization.
1,4-Dibromonaphthalene (13): 1-Bromonaphthalene (50 g, 0.24 mol)
was dissolved in acetonitrile (150 mL). The solution was then co-
oled to 0 °C and Br2 (38.59 g, 0.24 mol, 12.4 mL) was slowly added
under continuous stirring. The resulting solution was further
stirred overnight at 25 °C and the white precipitate filtered off and
washed carefully with acetonitrile. 1,4-Dibromonaphthalene was
recrystallized several times from acetonitrile until no traces of 1,5-
dibromonaphthalene were detected by 1H NMR spectroscopy. The
product 13 was obtained as colorless crystals (35.5 g, 0.12 mol,
52%). The spectroscopic data are identical with those of an authen-
tic sample.
1,2-Di(chrysen-2-yl)ethyne (10): 2-Bromochrysene (9; 85 mg,
0.28 mmol), [Pd(PPh3)2Cl2] (10 mg, 0.014 mmol), CuI (2.7 mg,
0.014 mmol), and PPh3 (7.3 mg, 0.028 mmol) were dissolved in tol-
uene (5 mL) and diisopropylamine (0.5 mL) was added. The mix-
ture was degassed under dynamic vacuum and stirred for 10 min
(25 °C) under argon. 2-Ethynylchrysene (9; 70 mg, 0.28 mmol) was
added and the mixture was stirred at 80 °C for 16 h. Then MeOH
(50 mL) was added and the precipitate was filtered off and washed
with MeOH, CH2Cl2, and hexane to yield ethyne 10 as a beige
powder (82 mg, 0.17 mmol, 61%). HRMS: calcd. for C38H22
478.1722; found 478.1724. MS (LDI-TOF): m/z = 478.29 [M]+. The
product was too insoluble for NMR analysis.
4-Bromo-1-phenylnaphthalene (14): Phenylboronic acid (7.5 g,
61.5 mmol), 1,4-dibromonaphthalene (27.2 g, 95 mmol), [Pd-
(PPh3)4] (1.45 g, 1.26 mmol), and K2CO3 (16.6 g, 120 mmol) were
added to a mixture of toluene (500 mL) and MeOH (250 mL). Af-
ter degassing, the mixture was heated at reflux under argon for 5 h.
The resulting mixture was extracted with toluene, washed with
H2O, filtered, dried with Na2SO4, and concentrated. Purification
was performed by column chromatography on silica gel (CH2Cl2/
petroleum ether, 1:5) to yield a white powder (11.4 g, 40.3 mmol,
Hexa(chrysen-2-yl)benzene (P4): 1,2-Di(chrysen-2-yl)ethyne (10;
30 mg) were dispersed in toluene (30 mL) and dioxane (20 mL) was
added. A catalytic amount of [Co2(CO)8] was added and the mix-
ture was heated at reflux for 48 h. After cooling, the dispersed solid
was filtered off and carefully washed with toluene, CH2Cl2, and
MeOH. A grey solid (26 mg) was obtained. Pure P4 was obtained
by gradient sublimation. MS (LDI-TOF): calcd. for C114H66
1434.52 [M]+; found 1434.63; isotopic distribution found: m/z (%)
= 1434.63 (85.6), 1435.62 (100), 1436.64 (42.7), 1437.58 (10.8),
1438.35 (3.8); isotopic distribution calculated: m/z (%) = 1434.52
(81.1), 1435.52 (100), 1436.52 (61.1), 1437.53 (24.7), 1438.53 (7.4).
The product was too insoluble for NMR analysis.
1
72%). Rf (hexane) = 0.34. H NMR (300 MHz, CD2Cl2): δ = 8.21
(ddd, J = 8.5, 1.2, 0.6 Hz, 1 H), 7.77 (ddd, J = 8.5, 1.2, 0.6 Hz, 1
H), 7.73 (d, J = 7.6 Hz, 1 H), 7.54–7.46 (m, 1 H), 7.43–7.28 (m, 7
H), 7.16 (d, J = 7.7 Hz, 1 H) ppm. 13C NMR (75 MHz, CD2Cl2):
δ = 140.74, 140.15, 133.16, 132.30, 130.21, 129.75, 128.59, 127.80,
127.54, 127.48, 127.05, 126.89, 122.32 ppm. MS (LDI-TOF): calcd.
for C16H11Br 282.00 [M]+; found 282.18.
1,2-Di(phenanthren-3-yl)ethyne (12): 1,2-Di(phenanthren-3-yl)eth-
yne (12) was synthesized from 3-bromophenanthrene (450 mg,
1.76 mmol) in analogy to the procedure used for the synthesis of
compound 5 by using CaC2 (415 mg, 7.03 mmol), Cs2CO3 (1.15 g,
3.51 mmol), Pd(OAc)2 (47.1 mg, 0.21 mmol), (diisopropylamino)-
(4-Phenylnaphthyl)boronic Acid Pinacol Ester (15): 4-Bromo-1-
phenylnaphthalene (4.39 g, 15.5 mmol), bis(pinacolato)diboron
(5 g, 17 mmol), KOAc (4.56 g, 46.5 mmol), and [PdCl2(dppf)]
8
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Synthesis of Precursors for Buckybowls and Carbon Nanotubes
(567 mg, 0.775 mmol) were dissolved in anhydrous dioxane
(60 mL). The solution was degassed and heated at 80 °C for 20 h
resulting in a black mixture. After extraction with CH2Cl2, the mix-
ture was washed three times with H2O and the organic phase was
dried with Na2SO4, filtered, and concentrated. The black oil was
first filtered through a plug of silica gel and a yellow oil was ob-
tained. Purification was performed by flash chromatography (pe-
(17; 1.69 g, 6.78 mmol), [Pd(PPh3)4] (392 mg, 0.34 mmol), and
Cs2CO3 (4.43 g, 13.6 mmol) were dissolved in toluene (30 mL) and
MeOH (15 mL). After degassing, the mixture was heated at reflux
under argon for 14 h. The resulting black mixture was extracted
with toluene, washed with H2O, filtered, dried with Na2SO4, and
concentrated. Ethanone 18 was obtained after flash chromatog-
raphy (petroleum ether/CH2Cl2, 1:2) as a yellowish powder (1.35 g,
3.62 mmol, 53%). Rf (hexane/CH2Cl2, 1:2) = 0.6. 1H NMR
(300 MHz, CD2Cl2): δ = 7.97–7.90 (m, 2 H), 7.87 (d, J = 8.2 Hz,
1 H), 7.73 (d, J = 8.6 Hz, 1 H), 7.55–7.49 (m, 2 H), 7.49–7.30 (m,
troleum ether/CCl4, 1:3) to yield
a yellow powder (3.72 g,
11.6 mmol, 75%). Rf (hexane/CH2Cl2, 5:1) = 0.32. 1H NMR
(300 MHz, CD2Cl2): δ = 8.77–8.70 (m, 1 H), 7.99 (d, J = 7.1 Hz,
1 H), 7.82–7.75 (m, 1 H), 7.47–7.26 (m, 1 H), 1.32 (s, 1 H) ppm. 7 H), 7.29–7.16 (m, 4 H), 1.83 (s, 3 H) ppm. 13C NMR (75 MHz,
13C NMR (75 MHz, CD2Cl2): δ = 143.65, 140.96, 137.47, 135.20, CD2Cl2): δ = 202.93, 141.02, 140.63, 138.59, 137.08, 135.86, 134.59,
131.49, 130.05, 128.85, 128.33, 127.48, 126.32, 126.21, 126.14,
125.69, 83.92, 24.87 ppm. MS (LDI-TOF): calcd. for C22H23BO2
329.18 [M]+; found 329.16.
133.44, 133.02, 131.73, 130.28, 128.44, 128.39, 128.16, 128.08,
127.82, 127.54, 127.46, 126.85, 126.62, 126.60, 126.56, 126.37,
126.20, 124.70, 30.20 ppm. MS (LDI-TOF): calcd. for C28H20O
372.15 [M]+; found 372.22.
1-Bromo-2-ethylnaphthalene (16): 2-Ethylnaphthalene (25 g,
160.26 mmol) was dissolved in acetonitrile (300 mL). N-Bromosuc-
cinimide (31.95 g, 179.49 mmol) was added and the mixture was
stirred at room temperature for 25 h. After the addition of an aque-
ous NaOH solution, the product was extracted with CH2Cl2 and
dried with Na2SO4. After filtration and concentration, 1-bromo-2-
ethylnaphthalene (16; 29.7 g, 127 mmol, 79%) was obtained as a
pale-yellow oil. 1H NMR (300 MHz, CD2Cl2): δ = 8.18 (d, J =
8.5 Hz, 1 H), 7.69–7.56 (m, 2 H), 7.48–7.39 (m, 1 H), 7.37–7.29 (m,
1 H), 7.22 (d, J = 8.4 Hz, 1 H), 2.86 (q, J = 7.6 Hz, 2 H), 1.16 (t,
J = 7.6 Hz, 3 H) ppm. 13C NMR (75 MHz, CD2Cl2): δ = 141.90,
133.46, 132.85, 128.26, 127.96, 127.80, 127.52, 127.29, 125.99,
123.32, 30.91, 14.53 ppm.
1,3,5-Tris(1-bromonaphthalen-2-yl)benzene
(19):
1-(1-Bromo-
naphthalene-2-yl)ethanone (17; 4 g, 16.1 mmol) was dissolved in o-
dichlorobenzene (80 mL). TiCl4 (7.2 mL, 12.4 g, 65 mmol) in o-
dichlorobenzene (13 mL) was then added dropwise. After stirring
for 12 h at 110 °C (oil bath temperature), the reaction was stopped
by the addition of MeOH. The mixture was then concentrated,
diluted with CH2Cl2, washed with H2O, dried with Na2SO4, fil-
tered, and concentrated. Flash chromatography (petroleum ether/
CCl4, 1:2) gave 680 mg (0.98 mmol, 18%) of the desired product.
Rf (hexane/CCl4, 1:2) = 0.55. 1H NMR (300 MHz, CD2Cl2): δ =
8.36 (d, J = 8.1 Hz, 3 H), 7.86–7.79 (m, 6 H), 7.63–7.45 (m, 12
H) ppm. 13C NMR (75 MHz, CD2Cl2): δ = 141.79, 140.25, 133.88,
132.63, 130.39, 128.71, 128.24, 127.91, 127.90, 127.84, 126.80,
122.62 ppm. HRMS: calcd. for C36H21Br3 689.9193; found
689.9204. MS (LDI-TOF): m/z = 690.20 [M]+; isotopic distribution
calculated: m/z (%) = 689.92 (34.3), 690.92 (13.3), 691.92 (100),
692.92 (38.9), 693.92 (97.3), 694.92 (37.9), 695.91 (31.5), 696.92
(12.3), 697.92 (2.3); isotopic distribution found: m/z (%) = 690.2
(31.1), 691.2 (19.0), 692.2 (100), 693.2 (33.6), 694.2 (98.2), 695.2
(32.4), 396.2 (34.1), 397.2 (9.5), 398.2 (3.8).
1-(1-Bromonaphthalen-2-yl)ethanone (17)
Method A: 1-Bromo-2-ethylnaphthalene (16; 10 g, 43 mmol) was
dissolved in acetone (400 mL) and KMnO4 (42.9 g, 210 mmol) dis-
solved in H2O (50 mL) was then added. The mixture was stirred at
room temperature for 4 d, filtered through a plug of sand/silica gel/
Celite, and washed with CH2Cl2. The organic phase was washed
with H2O, dried with Na2SO4, and the solvents evaporated. A yel-
low oil (7.59 g) containing 17 and the starting 1-bromo-2-ethyl-
naphthalene (16) were obtained. Pure 17 (1.5 g, 6.1 mmol, 14%)
was obtained by flash chromatography (silica gel, petroleum ether/
CH2Cl2, 1:1) as a yellow oil.
C84H54 (P6): Compound 19 (50 mg, 0.077 mmol), 15 (254 mg,
0.77 mmol), [Pd(PPh3)4] (20 mg), and Cs2CO3 (140 mg) were dis-
solved in toluene (6 mL) and MeOH (3 mL) before the mixture was
degassed. The reaction mixture was then stirred for 16 h at 80 °C
under argon. Subsequently, the mixture was extracted with toluene
and the organic phase was washed with H2O (3ϫ20 mL), dried
with Na2SO4, filtered, and the solvent removed in vacuo. Pure P6
was obtained after flash chromatography (silica gel, petroleum
ether/CH2Cl2 3:1) as a white powder (28 mg, 0.026 mmol, 34%). Rf
(hexane/CH2Cl2, 3:1) = 0.38. 1H NMR (300 MHz, CD2Cl2): δ =
8.05–6.15 (m, 54 H) ppm. MS (LDI-TOF): calcd. for C84H54
1062.42 [M]+; found 1062.43; isotopic distribution calculated: m/z
(%) = 1062.42 (100), 1063.43 (90.9), 1064.43 (40.8), 1065.43 (12.1),
1066.44 (2.6); isotopic distribution found: m/z (%) = 1062.43 (100),
1063.44 (90.5), 1064.45 (39.1), 1065.43 (10.0), 1066.46 (1.0).
Method B: 1-Bromonaphthalene-2-carbaldehyde (4.7 g, 20 mmol)
was dissolved in anhydrous THF (50 mL). Then a 3 m solution of
methylmagnesium bromide (21 mmol, 7.0 mL) was added dropwise
and the mixture was stirred for 1 h at room temperature under
argon. Then a saturated NH4Cl solution was added and the mix-
ture was extracted diethyl ether (3ϫ 50 mL). The organic phases
were combined, dried with Na2SO4, and the solvents evaporated.
The oil obtained was dissolved in acetone (250 mL) and KMnO4
(7.0 g) in water (30 mL) was added. The resulting mixture was
stirred for 2 d at room temperature, filtered through silica gel, and
the solvents evaporated. After flash chromatography on silica gel
(petroleum ether/CH2Cl2, 1:1), 4.2 g of 17 (16.9 mmol, 84%) were
obtained as a yellow oil.
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra of key intermediates and final prod-
ucts, mass spectra of compounds P1–P6, crystallographic data for
compound 12, structures of SWCNT end-cap precursors of dif-
ferent size and chirality derived by combinatorial design.
1
Rf (hexane/CH2Cl2, 1:1) = 0.43. H NMR (300 MHz, CDCl3): δ =
8.23 (d, J = 8.4 Hz, 1 H), 7.69 (d, J = 7.8 Hz, 2 H), 7.47 (dddd, J
= 16.3, 8.0, 6.8, 1.3 Hz, 2 H), 7.25 (d, J = 8.4 Hz, 1 H), 2.58 (s, 3
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 203.21, 140.62, 134.96,
132.17, 128.57, 128.52, 128.49, 128.06, 127.99, 124.22, 119.25,
31.19 ppm. MS (LDI-TOF): calcd. for C12H9BrO 247.98 [M]+;
found 248.24.
Acknowledgments
The authors are grateful to Mr. Patrick Merz for assistance with
the syntheses and Dr. Jürgen Nuss for performing the X-ray mea-
surements.
1-[1-(4-Phenylnaphthalen-1-yl)naphthalen-2-yl]ethanone (18): Pin-
acol 15 (2.23 g, 6.78 mmol), 1-(1-bromonaphthalen-2-yl)ethanone
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Job/Unit: O20841
/KAP1
Date: 12-09-12 17:47:06
Pages: 11
A. Mueller, K. Yu. Amsharov
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Published Online: ᭿
10
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O20841
/KAP1
Date: 12-09-12 17:47:06
Pages: 11
Synthesis of Precursors for Buckybowls and Carbon Nanotubes
Hemispherical Buckybowls
The facile synthesis of six precursor mo-
lecules for large-diameter hemispherical
buckybowls and short carbon nanotubes is
presented. The corresponding hemispheri-
cal buckybowls represent end-caps of sin-
gle-walled carbon nanotubes. The mo-
lecules are attractive as seeds for the fully
controlled synthesis of single-chirality
CNTs by the root-growth mechanism.
A. Mueller, K. Yu. Amsharov* ........ 1–11
Synthesis of Precursors for Large-Diameter
Hemispherical Buckybowls and Precursors
for Short Carbon Nanotubes
Keywords: Hydrocarbons / Carbon / Nano-
tubes / Buckybowls / Fused-ring systems /
Synthesis design / Dehydrogenation
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
11