Buckybowls: A simple, conceptually new synthesis of C2v-semibuckminsterfullerene (C30H12, [5,5]-fulvalene circulene)

Article abstract of DOI:10.1039/a706336i

An extremely simple synthesis of decacyclic, C₃₀H₁₂ 'bucky-bowl' 2 from m-xylene, employing a strategy of wider applicability for the synthesis of diverse curved aromatic surfaces, is described.

Full text of DOI:10.1039/a706336i

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Buckybowls: a simple, conceptually new synthesis of
C2v-semibuckminsterfullerene (C30H12, [5,5]-fulvalene circulene)
Goverdhan Mehta* and Gautam Panda
Molecular design and synthesis laboratory of JNCASR, School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
An extremely simple synthesis of decacyclic, C30H12 ‘bucky-
bowl’ 2 from m-xylene, employing a strategy of wider
applicability for the synthesis of diverse curved aromatic
surfaces, is described.
recently, in 1997, the groups of Scott and Zimmermann
accomplished2b a synthesis of 2 via FVP of 4, a compound
obtained serendipitiously from cyclopenta[def]phenanthrene.
We now report a new synthesis of 2 which is conceptually
different from the earlier efforts and notable for its simplicity
and brevity.4
The onset of the fullerene era, following the discovery of
buckminsterfullerene 1, has generated a great deal of interest in
the synthesis and chemistry of ‘bowl-shaped’ polycyclic
aromatic hydrocarbons (PAHs), particularly those representing
a dominant motif on the fullerene surface.1 Such PAHs, with
curved surfaces, can be expected to exhibit novel physico-
chemical characteristics and mimic some of the properties of
fullerenes. Since buckybowls have never been encountered
during the graphite vapourization processes, the only way to
access them is through logically orchestrated synthetic method-
ologies. Efforts in this direction have been mounted in several
laboratories during the past few years1 and among the notable
successes is the synthesis of C2v-semibuckminsterfullerene 2,2
representing half the carbon content of 1 and endowed with
substantial curvature (calculated bowl depth 2.7 Å).2a
Readily available 2,7-dimethylphenanthrene 55 was trans-
formed to the bis-ylide precursor 7 via the bis(2,7-bromo-
methyl)phenanthrene 6 (Scheme 2). Two-fold Wittig coupling
between the ylide derived from 7 and p-bromobenzaldehyde 8
furnished 9 as a mixture of E- and Z-isomers. On irradiation, 9
underwent facile two-fold oxidative photocyclization with high
regioselectivity to furnish the desired 10 as the only character-
izable product. The symmetry of 10 and its structure followed
from its characteristic 1H NMR spectrum [bay-region protons:
d 9.41 (s, Ha), 9.02 (s, Hb), 8.93 (d, J 9 Hz, Hc)] and 15 line 13
C
NMR spectrum [d 132.0, 131.4, 131.3, 130.6, 130.0(2C), 129.3,
128.1, 127.2(2C), 127.0(2C), 125.7, 122.7, 120.6]. We had
planned to effect in tandem, three-fold cyclization in 10 (see
dotted lines) as the pivotal step and for that purpose strategically
positioned the two bromine atoms as promoters of such a
process.2b To our great delight, we found that 10 on FVP
furnished 2, which was isolated from the pyrolysate through
+
–
Br
PPh3Br
Me
1
i
ii
+
Me
–
Br
PPh3Br
6
5
7
CHO
MNDO minimized
structure
iii
2
Br
Rabideau et al.2a were the first to report the synthesis of 2 via
flash vacuum pyrolysis (FVP) of 3, following a variant of the
Scott’s seminal synthesis of corannulene (Scheme 1).3 Very
8
Br
Br
Br
Br
Cl
Cl
Ha
Hb
Hc
v
iv
FVP
5%
FVP
2
2.1%
2
10
9
Scheme 2 Reagents and conditions: i, NBS, CCl4, heat, 4 h, 80%; ii, PPh3,
benzene, room temp., 2–3 days, 79%; iii, K2CO3, THF, 18-crown-6, 10 h,
40%; iv, hv (450 W), Pyrex, benzene, 2 h, 60%; v, FVP, 1150 °C, 0.5 mm,
N2, 2–3%
Cl
Cl
3
4
Scheme 1
Chem. Commun., 1997
2081

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extensive chromatography (alumina) and characterized through
its mass spectrum [m/z 372 (M+)] and diagnostic three line 1H
NMR spectrum [d 7.92 (d, J 9 Hz, 4 H), 7.55 (d, J 9 Hz, 4 H),
7.42 (s, 4 H)] which was found to be identical with the reported
values.2 Admittedly, the yield (2–3%) of 2 in the FVP step is
low and as yet unoptimised, but it compares well with the earlier
reports (2.1 and 5% in FVP).2
Footnote and References
* E-mail: gmchem@uohyd.ernet.in
1 R. Faust, Angew. Chem., Int. Ed. Engl., 1995, 34, 1429; P. W. Rabideau
and A. Sygula, Acc. Chem. Res., 1996, 29, 235; L. T. Scott, Pure Appl.
Chem., 1996, 68, 291; G. Mehta and H. S. P. Rao, in Advances in Strain
in Organic Chemistry, ed. B. Halton, JAI Press, London, 1997, vol. 6,
pp. 139–187.
Our approach to 2 from dimethylphenanthrene 5 involves the
following sequence: NBS bromination–Wittig olefination–
photocyclization and pyrolysis. Interestingly, the phenanthrene
precursor 5 itself is assembled from m-xylene through the same
sequence involving NBS bromination–Wittig olefination–
photocyclization.5 Thus, in an overall sense, through an
iterative three step sequence and a final pyrolysis step, m-xylene
is elaborated into the decacyclic, C30H12, aromatic ‘bowl’ 2. We
believe that the simple strategy outlined here can be adapted
towards the synthesis of several bowl-like non-planar PAHs.
Efforts along these lines are currently underway.
We thank CSIR for the research support and a fellowship to
G. P. We also appreciate the help of Mr Anirban Banerjee in
performing some important preliminary experiments. We also
thank Professor L. T. Scott and Dr S. Hagen, Boston College,
USA, for sending the 1H NMR spectrum of [5,5]-circulene for
comparison.
2 (a) P. W. Rabideau, A. H. Abdourazak, H. E. Folsom, Z. Marcinow,
A. Sygula and R. Sygula, J. Am. Chem. Soc., 1994, 116, 7891; (b)
S. Hagen, M. S. Bratcher, M. S. Erickson, G. Zimmermann and
L. T. Scott, Angew. Chem., Int. Ed. Engl., 1997, 36, 406.
3 L. T. Scott, M. M. Hashemi, D. T. Meyer and H. B. Warren, J. Am. Chem.
Soc., 1991, 113, 7082.
4 The synthetic strategy followed presently is generally along the lines
pursued previously by us for the synthesis of other fullerene fragments,
see G. Mehta, G. V. R. Sarma, M. A. Krishna Kumar, T. V. Vedavyasa
and E. D. Jemmis, J. Chem. Soc., Perkin Trans. 1, 1995, 2529; G. Mehta,
K. Venkateswara Rao and K. Ravikumar, J. Chem. Soc., Perkin Trans. 1,
1995, 1787; G. Mehta and K. Venkateswara Rao, Synlett, 1995, 317;
G. Mehta and G. Panda, Tetrahedron Lett., 1997, 12, 2145; G. Mehta,
G. Panda, S. R. Shah and A. C. Kunwar, J. Chem. Soc., Perkin Trans. 1,
1997, 2269.
5 F. B. Mallory and C. W. Mallory, J. Am. Chem. Soc., 1972, 94, 6041.
Received in Cambridge, UK, 1st September 1997; 7/06336I
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Chem. Commun., 1997