A practical, large scale synthesis of the corannulene system [19]

Full text of DOI:10.1021/ja0011461

J. Am. Chem. Soc. 2000, 122, 6323-6324
6323
A Practical, Large Scale Synthesis of the
Corannulene System
Andrzej Sygula* and Peter W. Rabideau*
Department of Chemistry and Ames Laboratory
Iowa State UniVersity, Ames, Iowa 50011
ReceiVed March 31, 2000
Corannulene (1), whose C20 carbon network may be considered
to represent the polar cap of buckminsterfullerene C60, was first
synthesized by Barth and Lawton in 1966 by a 16-step process.1
so far the largest buckybowl, a C36H12 from the Scott group.7
Most recently we have been working on the development of
solution-phase syntheses for these systems since, if they could
be prepared in substantial amounts, they could serve as synthetic
intermediates for the synthesis of even larger buckybowls and
perhaps provide an organic synthesis of buckminsterfullerene
itself.8
Serendipity reared its head as we attempted the hydrolysis of
2 in acetone/water mixtures containing either sodium carbonate
or sodium hydroxide to generate the tetraaldehyde as a precursor
to cabonyl coupling. To our surprise no aldehyde was formed
under these conditions, but rather a mixture of dibromo- and
tribromocorannulenes as demonstrated by GC/MS. Separation of
the products was not attempted, but when the mixture was treated
with n-butyllithium in THF at -78 °C, and then quenched
carefully at that temperature, corannulene was formed in yields
of 50 to 55% overall for the two steps combined! Optimization
of the reaction conditions ultimately allowed us to cleanly produce
tetrabromocorannulene (3) in 83% isolated yield (Scheme 1).9
For example, refluxing 6.5 g of 2 in 250 mL of dioxane and 100
mL of water containing 3 g of NaOH afforded 3.4 g of 3.
This simple procedure, which can provide very large amounts
of a well-defined and functionalized corannulene, has, in our
opinion, extraordinary potential. For example, production of
corannulene itself on a very large scale is already possible by
this method (Scheme 1), and we hope for considerable improve-
ment as we begin to explore methods for the inexpensive, high-
yield debromination of 3.10 Of course the presence of function-
alizable bromine in 3 represents a major feature of this synthesis.
For example, the bromine atoms can easily be replaced by methyl
groups (Scheme 2, path a)11 or TMS-acetylene (Scheme 2, path
c),12 and this will allow for further elaboration of this novel
structure.
Several attempts to improve the synthesis failed until Scott and
co-workers applied flash vacuum pyrolysis methodology (FVP)
to the ring-forming step.2 This high-temperature (ca. 1000 °C)
gas-phase process promotes intramolecular ring closures that can
lead to the formation of strained systems such as 1. In 1996 Siegel
and co-workers reported a modest yield synthesis of dimethyl-
corannulene via high-dilution, low-valent titanium coupling on
both sides of 1,6-bis(bromomethyl)-7,10-bis(1-bromoethyl)fluo-
ranthene, followed by DDQ dehydrogenation.3 Recently, we4 and
Siegel’s group5 improved the synthesis by the employment of
dibromomethyl derivatives; the low-valent titanium or vanadium
treatment of 1,6,7,10-tetrakis(dibromomethyl)fluoranthene (2)
produces corannulene in 70-80% yield. However, these reactions
require high dilution techniques with reaction times of 2-4 days,
as well as strict oxygen- and moisture-free conditions. Herein
we report an inexpensive, convenient synthesis of 1,2,5,6-
tetrabromocorannulene from 2 in over 80% yield that only requires
refluxing for 15 min in aqueous dioxane containing a small
amount of NaOH.
Curved surface polynuclear aromatics with carbon frameworks
identifiable on the buckminsterfullerene surface, sometimes called
“buckybowls”, are novel structures of considerable interest for
their chemical and physical properties. They have both concave
and convex surfaces, and aromatics in this series may possess
fullerene-like properties with increasing numbers of carbon atoms.
To date, larger buckybowls have generally been prepared by
flash vacuum pyrolysis in low yield. This includes two C30H12
semibuckminsterfullerenes, first synthesized by our group,6 and
The coupling of benzyl halides, and related compounds, is well-
known in the literature,13 and recently this protocol has been used
(7) (a) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996,
118, 8743-8744. (b) Ansems, R. B. M.; Scott, L. T. J. Am. Chem. Soc. 2000,
122, 2719-2724.
(8) For a conceptually different approach to the synthesis of fullerenes,
see: Buntz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28, 107-
119 and references therein.
(1) (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.
(2) (a) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am.
Chem. Soc. 1991, 113, 7082-7084. (b) Scott, L. T.; Hashemi, M. M.; Bratcher,
M. S. J. Am. Chem. Soc. 1992, 114, 1920-1921. (c) Scott, L. T.; Cheng,
P.-C.; Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am.
Chem. Soc. 1997, 119, 10963-10968.
(3) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1996,
118, 2754-2755.
(9) Colorless solid (from xylenes); mp 338-340 °C dec. 1H NMR (400
MHz, CDCl3): δ 7.85 (d, J ) 8.9 Hz, 2H), 7.95 (d, J ) 8.9 Hz, 2H), 7.98 (s,
2H). 13C NMR was not obtained due to the very poor solubility of 3 in common
deuterated solvents. MS (EI, 70 eV) m/z, (rel intensity >10%) 569 (13), 568
(33), 567 (36), 566 (55), 565 (47), 564 (42), 563 (40), 562 (15), 560 (19),
407 (11), 406 (14), 405 (23), 403 (14), 283 (16), 281 (17), 250(23), 246 (100),
244 (13), 203 (15), 202 (10), 162 (19), 123 (72). HRMS (EI, 70 eV) calcd for
C20H6Br4 (M+) 565.7165, found 565.7162.
(4) (a) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 1998, 120, 12666-
12667. (b) Sygula, A.; Rabideau, P. W. Abstracts; National Meeting of the
American Chemical Society, New Orleans, Louisiana, August 1999, ORGN-
065. (c) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 1999, 121, 7800-
7803.
(5) (a) Seiders, T. J.; Baldrige, K. K.; Elliott, E. L.; Grube, G. H.; Siegel,
J. S. J. Am. Chem. Soc. 1999, 121, 7439-7440. (b) Seiders, T. J.; Elliott, E.
L.; Grube, G. H.; Siegel, J. S. J. Am. Chem. Soc. 1999, 121, 7804-7813.
(6) (a) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow,
Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891-7892. (b)
Abdourazak, A. H.; Marcinow, Z.; Sygula, A.; Sygula, R.; Rabideau, P. W.
J. Am. Chem. Soc. 1995, 117, 6410-6411.
(10) Although the mixture of di- and tribromocorannulenes is smoothly
converted to corannulene by metalation with n-butyllithium followed by
quenching with dilute HCl (Scheme 1, path b), a similar method applied to 3
produces a mixture of corannulene and mono- and dibutylcornannulenes. Under
similar conditions, methyllithium produces a mixture of di-, tri-, and
tetramethylcorannulenes. Refluxing 3 with LiAlH4 in THF gives corannulene
and some dihydro- and tetrahydrocorannulenes. This mixture can be converted
to pure corannulene with DDQ, but the yield of the two steps is only ca.
30%.
(11) Tetramethylcorannulene 4 was previously synthesized by low-valent
titanium coupling of 1,6,7,10-tetrakis(1-bromoethyl)fluoranthene, followed by
DDQ dehydrogenation to afford a 6% yield for the two steps combined.5b
10.1021/ja0011461 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/17/2000

6324 J. Am. Chem. Soc., Vol. 122, No. 26, 2000
Communications to the Editor
Scheme 1a
transformation of 2 to the corannulene core is open to speculation.
Deprotonation of dibromomethyl is certainly the first step since
refluxing 2 in acetone/water mixtures without base (or with dilute
hydrochloric acid) affords unchanged starting material. Bromocar-
bene formation from the dibromocarbanion may be a viable option
for the next step since nucleophilic attack of the latter on the
neighboring dibromomethyl group may be unlikely due to steric
congestion. The resulting bromocarbene could insert into either
the C-H or more likely the C-Br bond of the adjacent
dibromomethyl group, followed by HBr elimination. This, of
course, would lead to tetrabromocorannulene, the observed
product. However, other reaction pathways are possible and likely
responsible for the mixture of products, including di- and
tribrominated corannulenes, formed in acetone/water (Scheme 1,
path a).16 Further investigations of the reaction pathway are
underway.
In conclusion, we regard this new synthetic pathway as a major
step toward the practical, large-scale syntheses of corannulene
and related bowl-shaped aromatics. The overall process begins
with easily available 2,7-dimethylnaphthalene,17 and the sequence
of chloromethylation, cyanation, hydrolysis, ring closure, and
oxidation to produce 2,7-dimethylacenaphthenone is very straight-
forward and affords good yields.18 The conversion of this quinone
to 1,6,7,10-tetramethylfluoranthene is achieved either by the
original two-step process18b or by a one-pot reaction using the
method developed by the Scott group.2a Bromination of the
hydrocarbon gives 2 in yields above 80%,4c,5b so this key precursor
can be easily prepared, and on a very large scale. Thus we expect
tetrabromocorannulene to become a key intermediate for the
synthesis and study of larger bowl systems, and perhaps for the
synthesis of buckminsterfullerene itself. The latter process would
be important since it may provide a method of trapping atoms,
molecules, or ions within the fullerene cavity during the final
closure steps.19
a Reagents and yields: (a) acetone/water (3:1), NaOH, reflux, 1 h;
(b) n-BuLi, THF, -78 °C, 30 min, then quenched with dilute HCl,
50-55% yield for the two steps combined; (c) dioxane/water (2.5:1),
NaOH, refluxed 15 min, 83% yield after crystallization (xylenes).
Scheme 2b
Acknowledgment. This work was supported by the Ames Laboratory
which is operated for the U.S. Department of Energy by Iowa State
University under Contract No. W-7405-Eng-82.
Supporting Information Available: Details of the experimental
procedures for 1, 3, 4, and 5 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0011461
a Reagents and yields: (a) AlMe3, NiCl2(dppp), DME, reflux 12 h,
76% isolated yield; (b) (TMS) acetylene, Pd(PPh3)2Cl2, CuI, N(Et)3,
reflux 3 h, 86% isolated yield; (c) LiAlH4, DDQ, 30%.
(13) See for example: (a) Hauser, C. R.; Brasen, W. R.; Skell, P. S.; Kantor,
S. W.; Brodhag, A. E. J. Am. Chem. Soc. 1956, 78, 1653-1658. (b) Ledwith,
A.; Shih-Lin, Y. Chem. Ind. (London) 1964, 1867-1868. (c) Hoeg, D. F.;
Lusk, D. I. J. Organomet. Chem. 1966, 5, 1-12. (d) Hanna, S. B.; Wideman,
L. G. Chem. Ind. (London) 1968, 486. (e) Vernigor, E. M.; Shalaev, V. K.;
Luk’yanets, E. A. J. Org. Chem. USSR 1981, 17, 317-321.
(14) (a) Huber, R. S.; Jones, G. B. Tetrahedron Lett. 1994, 35, 2655-
2658. (b) Dubois, F.; Gingras, M. Tetrahedron Lett. 1998, 39, 5039-5040.
(15) Kharash, M. S.; Nudenberg, W.; Fields, E. K. J. Am. Chem. Soc. 1944,
66, 1276-1279.
for making enediynes14a and [5]-helicene.14b Generally, the
couplings are performed in anhydrous media and the mechanism
is often assumed to be carbenoid. However, the exact pathway
of these transformations is a matter of some controversy. After
the initial deprotonation step, several mechanistic routes for the
carbon-carbon bond formation have been proposed including
nucleophilic attack by the carbanion, the intermediacy of carbenes
(R-elimination), or formation of radicals by electron transfer from
the carbanionic center.13-15 Similarly, the mechanism for the
(16) Overnight reflux of tetrabromocorannulene 3 in acetone/water/NaOH
affords unchanged starting material indicating that 3 is not an intermediate in
the coupling of 2 in this solvent system.
(17) Wolinska-Mocydlarz, J.; Cannone, P.; Leitch, L. C. Synthesis 1974,
566-568.
(18) (a) Buu-Hoi; Cagniant, P. ReV. Sci. 1942, 80, 130-133. (b) Borchart,
A.; Hardcastle, K.; Gantzel, P.; Siegel, J. S. Tetrahedron Lett. 1993, 34, 273-
276.
(19) A non-pyrolytic synthesis of dibenzo[a,g]corannulene by palladium-
catalyzed intramolecular aryl-aryl coupling of 7,10-di(2-bromophenyl)-
fluoranthene has just been reported: (a) Scott, L. T.; Reisch, H. Abstracts;
National Meeting of the American Chemical Society, San Francisco, CA,
March 2000, ORGN-515. (b) Reisch, H. A.; Bratcher, M. S.; Scott, L. T.
Org. Lett. 2000, 2, 1427-1430. We thank Professor Larry Scott for sharing
the manuscript with us.
(12) Compound 5 was isolated as a yellow solid (from ethanol/benzene):
changes color above 240 °C, mp 277-280 °C dec. 1H NMR (400 MHz,
CDCl3) δ (strongly dependent on the concentration of the sample) 0.35 (s,
18H), 0.355 (s, 18H), 7.77 (d, J ) 8.8 Hz, 2H), 7.92 (d, J ) 8.8 Hz, 2H),
7.96 (s, 2H). 13C NMR (100 MHz, CDCl3) δ 0.45, 101.71, 101.84, 104.64,
104.79, 125.08, 125.36, 126.52, 126.91, 127.94, 130.57, 131.15, 131.57,
133.83, 134.57, 134.88. MS (EI, 70 eV) m/z (rel intensity) 636 (18), 635 (40),
634 (100), 633 (71), 630 (10), 618 (22), 531 (21). HRMS (EI, 70 eV) calcd
for C40H42Si4 (M+) 634.2364, found 634.2375.