Synthesis of a C56H40 hydrocarbon bearing a 54-carbon framework of C60

Article abstract of DOI:10.1021/ol900749a

A C₅₆H₄₀ hydrocarbon possessing a 54-carbon framework represented on the surface of C₆₀ was synthesized by solution-phase chemistry. The structure of this hydrocarbon has a 30-carbon core, which can be regarded as a partia

Full text of DOI:10.1021/ol900749a

ORGANIC
LETTERS
2009
Vol. 11, No. 12
2527-2530
Synthesis of a C56H40 Hydrocarbon
Bearing a 54-Carbon Framework of C60
†
Kung K. Wang,* Yu-Hsuan Wang, Hua Yang, Novruz G. Akhmedov,
and Jeffrey L. Petersen
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity, Morgantown,
West Virginia 26506-6045
kung.wang@mail.wVu.edu
Received April 7, 2009
ABSTRACT
A C56H40 hydrocarbon possessing a 54-carbon framework represented on the surface of C60 was synthesized by solution-phase chemistry. The
structure of this hydrocarbon has a 30-carbon core, which can be regarded as a partially hydrogenated [5,5]circulene, a C30H12
semibuckminsterfullerene, with only one of the carbon-carbon bonds remaining unconnected. The central 30-carbon core is fused with two
indeno groups and bears two phenyl and two methyl substituents.
Construction of bowl-shaped polycyclic aromatic hydro-
carbons (buckybowls) having carbon frameworks repre-
sented on the surface of C60 by solution-phase chemistry
has enjoyed substantial progress in recent years.1 The
synthetic strategy of constructing a carbon framework
containing multiple tetrahedral sp3 carbons as a precursor
prior to the final transformation to an aromatized adduct
was employed by Barth and Lawton in the first synthesis
of corannulene, a C20H10 buckybowl possessing significant
curvature.2 More recently, this approach was adopted in
the synthesis of sumanene, a symmetric C21H12 subunit
of C60.3 We recently reported the use of the cascade
cyclization reactions of benzannulated enyne-allenes as
key steps in the construction of three buckybowls4 and a
C44H26 fullerene fragment.5 We have further extended this
strategy to the synthesis of a C56H40 hydrocarbon pos-
sessing a 54-carbon fragment of C60. There are 10
tetrahedral sp3 carbons in this 54-carbon framework,
causing the molecule to adopt an unsymmetrical, skewed
structure with several parts of the molecule in close
proximities to one another.
The synthetic sequence outlined in Scheme 1 produced
the aromatic diketone 10 as a key intermediate leading to
the C56H40 hydrocarbon 16. Cyclopentadienone 1 was
prepared in 70% yield from condensation between di-
methyl acetonedicarboxylate and benzil followed by
dehydration as reported previously.6 Cycloaddition bet-
ween 1 and 2,5-dihydrofuran (2) then gave the Diels-Alder
adduct 3 as a mixture of the endo/exo (14:1) isomers.
Treatment of 3 with water to induce decarbonylation then
† Dedicated to Professor William R. Moore on the occasion of his 80th
birthday.
(1) (a) Wu, Y.-T.; Siegel, J. S. Chem. ReV. 2006, 106, 4843–4867. (b)
Tsefrikas, V. M.; Scott, L. T. Chem. ReV. 2006, 106, 4868–4884. (c) Mehta,
G.; Rao, H. S. P. Tetrahedron 1998, 54, 13325–13370. (d) Rabideau, P. W.;
Sygula, A. Acc. Chem. Res. 1996, 29, 235–242.
(2) (a) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88, 380–
381. (b) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1971, 93, 1730–
1745.
(4) Kim, D.; Petersen, J. L.; Wang, K. K. Org. Lett. 2006, 8, 2313–
2316.
(5) Zhang, H.-R.; Wang, K. K. J. Org. Chem. 1999, 64, 7996–7999.
(6) White, D. M. J. Org. Chem. 1974, 39, 1951–1952.
(3) Sakurai, H.; Daiko, T.; Hirao, T. Science 2003, 301, 1878–1882.
10.1021/ol900749a CCC: $40.75
Published on Web 05/22/2009
 2009 American Chemical Society

However, treatment of 13 with diiododisilane in the
presence of hydrogen iodide10 to reduce the keto groups
was successful with concomitant reduction of the central
carbon-carbon double bond to give 15. The newly formed
C-H bonds on the central cyclohexyl ring in 15 are cis
to the methyl groups as indicated by the NOE measure-
ments. The methylene hydrogens on the five-membered
rings of the benzofluorenyl substructures are relatively
acidic, making the corresponding carbanions readily
accessible. Treatment of 15 with sodium t-butoxide,
prepared from sodium hydride in mineral oil and t-butyl
alcohol, was successful in promoting the intramolecular
alkylation reactions, giving rise to the C56H40 hydrocarbon
16.
Scheme 1
The final alkylation steps appeared to be sensitive to
the reaction conditions. Removing the mineral oil from
NaH by washing with hexanes prior to using it to prepare
sodium t-butoxide for the intramolecular alkylation reac-
tions produced a more complex mixture with several
unidentified products. Attempts to remove mineral oil from
16 by silica gel column chromatography also resulted in
the formation of a more complex mixture perhaps due in
part to epimerization of the C1/C6 stereogenic centers.
However, a rapid filtration through a silica gel column to
give 16 along with the mineral oil appeared to produce
the cleanest product. Because of the presence of mineral
oil, the yield of 16 was determined by adding a known
amount of methylene chloride as an internal standard to
16 and the proton NMR signal of methylene chloride at
δ 5.30 and the signal of 16 at δ 6.34 were integrated.
led to diester 4. The structures of 3 and 4 were established
by X-ray structure analyses. Saponification of 4 to produce
the corresponding diacid followed by transformation to
the diacid chloride with thionyl chloride for the AlCl3-
catalyzed intramolecular Friedel-Crafts acylation reac-
tions then furnished diketone 5. Dimethylation from the
less hindered side opposite to tetrahydrofuran ring then
furnished diketone 6. Condensation of 6 with one equiv
of lithium acetylide 7 produced propargylic alcohol 8.
Treatment of 8 with thionyl chloride promoted a cascade
sequence of reactions leading to 9 as reported previously.5
Air oxidation of 9 then furnished diketone 10 in 87%
overall yield from 8. The structure of 10 was established
by X-ray structure analysis.
The cleavage of the tetrahydrofuran ring in 10 with
Me3SiI7 produced diiodide 11 (Scheme 2). The structure
of 11 was established by X-ray structure analysis.
Condensation of 11 with one equiv of 7 was selective in
producing propargylic alcohol 12, which on exposure to
SOCl2 followed by oxidation with MnO2 then furnished
diketone 13. In addition, chloride 14, presumably derived
from a [2 + 2] cycloaddition reaction of the in situ
generated benzannulated enyne-allene,8 was also isolated.
The structures of 13 and 14 were established by X-ray
structure analyses. Attempts to promote the intramolecular
Barbier reactions of 13 with SmI2 were unsuccessful.9
1
The structure of 16 was elucidated by H and 13C NMR
spectroscopy and high-resolution MS. A complete assignment
of the 1H NMR and 13C signals of 16 was made on the basis
of the applications of several two-dimensional experiments.
The hydrogen connectivities were established from the
COSY and 1D/2D TOCSY spectra. The cis relationship
between the hydrogens on the five-membered rings and the
hydrogens and the methyl groups on the central cyclohexyl
ring was determined by NOE experiments. These NMR data
are included in the Supporting Information.
The 1H NMR spectrum of 16 indicates that it is
unsymmetrical and adopts a skewed conformation. In
addition to two distinct methyl signals, ten different
aliphatic hydrogen signals could be discerned, indicating
a slow rate of racemization on the NMR time scale.
Furthermore, the signal of the aromatic hydrogen on C23
is shifted upfield to δ 3.98, and the endo and exo
hydrogens on C5 are shifted upfield to δ -3.32 and 0.29,
respectively. These observations could be attributed to the
adoption of a twist-boat conformation by the central
cyclohexyl ring with C4 and C21 holding the flagpole
C-C bonds as shown in the structure optimized by
MM2 calculations (Figure 1). The twist motion involves
moving C38 and C39 up and C3 and C20 down, causing
the two benzofluorenyl substructures to approach each
other and providing significant magnetic shieldings for
(7) Jung, M. E.; Lyster, M. A. J. Org. Chem. 1977, 42, 3761–3764.
(8) Li, H.; Zhang, H.-R.; Petersen, J. L.; Wang, K. K. J. Org. Chem.
2001, 66, 6662–6668.
(9) Krief, A.; Laval, A.-M. Chem. ReV. 1999, 99, 745–777.
(10) Keinan, E.; Perez, D. J. Org. Chem. 1987, 52, 4846–4851.
2528
Org. Lett., Vol. 11, No. 12, 2009

Scheme 2
the upfield-shifted hydrogens. The directions of the twist
hydrogen of the phenyl substituent on C27 pointing inward
toward the endohedral side of the molecule. A significant
NOE was observed between this ortho hydrogen and the
aromatic hydrogen on C8, indicating that they are in close
proximity to each other, although they are located on
opposite sides of the structure. This observation further
supports the twist-boat conformation of the central cy-
clohexyl ring, which could bring these two aromatic
hydrogens in close proximity to each other.
motion could also cause the C-H bond on C3 to become
essentially orthogonal to the C-H bond of the endo
hydrogen on C2, consistent with the observation that the
vicinal coupling between them is negligible. Heating the
sample in CDCl3 at 50 °C did not appear to cause line
broadening or coalescence of the signals, indicating the
conformation is relatively stable on the NMR time scale.
Because of nonbonded steric interactions, the two
phenyl substituents are orientated roughly perpendicular
to the benzofluorenyl substructures, and the rate of rotation
of the phenyl substituent on C27 is relatively slow on the
NMR time scale as observed previously in a related
structure.11 The signal at δ 5.45 is attributable to the ortho
The structure of 16 has a 54-carbon framework repre-
sented on the surface of C60, missing only three two-carbon
fragments. Diketone 5 provides 22 carbons of the 54-
carbon framework, while the remaining 32 carbons come
from two molecules of acetylene 7. The presence of
multiple sp3 carbons in 15 makes it possible to move the
iodo-bearing C2 and C5 close to C1 and C6 on the five-
membered rings, respectively. As a result, the intramo-
lecular alkylation reactions to connect C1 with C2 and
C6 with C5 occurred readily, giving rise to 16 in good
yield. The structure of 16 has a 30-carbon core, which
can be regarded as a partially hydrogenated [5,5]circulene,
a C30H12 semibuckminsterfullerene,12 with only the
carbon-carbon bond between C18 and C23 remaining
unconnected. The presence of 10 sp3 carbons in the 30-
carbon core appears to relieve substantial strain of the
fully aromatized [5,5]circulene.
The convergent nature of the synthetic sequence could
allow placement of bromo substituents at various positions
of the aromatic ring system of 16 by using brominated
precursors. The Pd-catalyzed intramolecular arylation
reaction for carbon-carbon bond formation could allow
further connections of the carbon atoms in 16.4,13 The
methyl groups on carbons 38 and 39 could also be replaced
(11) Dai, W.; Petersen, J. L.; Wang, K. K. Org. Lett. 2004, 6, 4355–
4357.
(12) (a) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 1999, 121, 7800–
7803. (b) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow,
Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891–7892.
(13) (a) Reisch, H. A.; Bratcher, M. S.; Scott, L. T. Org. Lett. 2000, 2,
1427–1430. (b) Wang, L.; Shevlin, P. B. Org. Lett. 2000, 2, 3703–3705.
Figure 1. MM2-optimized structure of 16 viewing from two
different perspectives.
Org. Lett., Vol. 11, No. 12, 2009
2529

by other substituents to facilitate the final aromatization
process.
EPSCoR (1002165R) for the purchase of a 600 MHz NMR
spectrometer is also gratefully acknowledged.
Supporting Information Available: All experimental
procedures, NMR spectra, MS, and X-ray crystallographic
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. K.K.W. thanks the National Science
Foundation (CHE-0414063) for financial support. J.L.P.
acknowledges the support (CHE-9120098) provided by the
National Science Foundation for the acquisition of a Siemens
P4 X-ray diffractometer. The financial support of the NSF-
OL900749A
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Org. Lett., Vol. 11, No. 12, 2009