Chemistry on the rim of buckybowls: Derivatization of 1,2,5,6-tetrabromocorannulene

Article abstract of DOI:10.1016/S0040-4039(00)01785-8

Synthesis of a number of substituted corannulenes and dibenzo[a,g]corannulenes starting from 1,2,5,6-tetrabromocorannulene 3 is described. Since 3 is conveniently obtained by a large scale, non-pyrolytic route, it may serve as a synthon for the further elaboration of this bowl-shaped system. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01785-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9931–9934
Chemistry on the rim of buckybowls: derivatization of
,2,5,6-tetrabromocorannulene
1
Guopin Xu, Andrzej Sygula, Zbigniew Marcinow and Peter W. Rabideau*
Department of Chemistry and Ames Laboratory, Iowa State University, Ames, IA 50011, USA
Received 22 August 2000; accepted 12 October 2000
Abstract
Synthesis of a number of substituted corannulenes and dibenzo[a,g]corannulenes starting from 1,2,5,6-
tetrabromocorannulene 3 is described. Since 3 is conveniently obtained by a large scale, non-pyrolytic
route, it may serve as a synthon for the further elaboration of this bowl-shaped system. © 2000 Elsevier
Science Ltd. All rights reserved.
The synthesis of buckybowls, i.e. bowl-shaped, polycyclic aromatic hydrocarbons with carbon
1
networks represented on the surface of fullerenes, has attracted significant attention. The
smallest member of the family, corannulene (1), was first synthesized by an elaborate 16 step
2
synthesis in the ‘pre-fullerene’ era by Barth and Lawton. A major breakthrough occurred in
1
991 when the Scott group announced a much simpler route to 1 employing flash vacuum
3
pyrolysis (FVP) in the final step.
Br HC
CHBr2
CHBr2
Br
Br
Br
Br
2
Br HC
2
1
2
3
1
Since then several buckybowls have been obtained by FVP. However, despite the great success
with pyrolytic methods, the need for conventional, condensed phase protocols is critical to the
further exploration of these systems since FVP suffers from problems with scale-up, low yields,
and relatively non-volatile precursors being required for larger bowls. Only recently have such
protocols been developed, mostly utilizing transition metal catalyzed intramolecular coupling of
*
Corresponding author.
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01785-8

9932
4
bromides. However, we have just discovered that short reflux of tetrakis(dibromomethyl)-
fluoranthene 2 with NaOH in dioxane/water (3:1) produces 1,2,5,6-terabromocorannulene 3 in
5
8
3% isolated yield. This simple procedure offers an inexpensive, large scale synthetic route to
corannulene and its derivatives. In this communication we report our recent progress in
functionalization of the corannulene core utilizing 3 as a synthon. These reactions of 1,2,5,6-
6
terabromocorannulene are depicted in Scheme 1.
Debromination. Our original attempts to produce corannulene by the debromination of 3 met
with only limited success. Treatment of 3 with n-butyl- or methyllithium in THF, followed by
quenching with dilute HCl, produces mixtures of corannulene and mono- to tetraalkylated
5
corannulenes even at −78°C. Reduction with LiAlH in THF gives a mixture of 3 and di- and
4
tetrahydrocorannulenes. While this latter mixture can be converted to corannulene by DDQ, the
5
combined yield of the two steps is only ca. 30%. However, we subsequently found that refluxing
3
in ethanol for 6 h with an excess of activated zinc powder and potassium iodide produces
7
corannulene in 90% yield. This step, combined with the high yield for the formation of 3 from
2
, makes the present protocol an attractive, low-cost alternative synthesis of corannulene.
Halogen exchange and Heck reactions. The bromine atoms in 3 are replaced with iodine in
good yield by 18 h reflux with an excess of KI in DMF. The resulting tetraiodocorannulene 4
8
undergoes condensation with methyl acrylate under Heck conditions yielding the tetrakis-
methylacrylate 5; subsequent reflux with DDQ in o-xylene affords the tetrakis(methylcarboxy)
4f
derivative 6 of the known dibenzo[a,g]corannulene. Interestingly, condensation of 4 with
styrene under Heck conditions leads to the formation of tetraphenyldibenzocorannulene C H
52
30
(
7) in a single step. Notwithstanding the moderate yields of this reaction (ca. 45%, not
optimized), it nonetheless has great potential for the expansion of the corannulene system to
produce larger buckybowls.
Suzuki coupling. Coupling of 3 with phenylboronic acid was achieved under Suzuki
9
a
conditions in the solvent mixture of toluene/ethanol/water (5:5:2) which has been suggested for
9
b
sterically congested systems. Tetraphenylcorannulene 8 is formed in good yield (60–70%). The
ortho orientation of the appended phenyl groups in 8 raised an interesting possibility for further
reaction, given the well-known o-terphenyl to triphenylene cyclization, possibly affording
buckybowl 9. However, attempts at cyclization either photochemically or by oxidative coupling,
have so far been unsuccessful, presumably due to the curvature of the system. We continue to
explore other possibilities for cyclization, including pyrolytic methods.
Ph
Ph
Ph
Ph
hν
8
9
Conversion of tetraacetylene. Compound 3 couples with (TMS)acetylene to give 10 in high
5
10
yield, and further treatment with HCl in acetic acid leads to the tetrakis(1-chlorovinyl)
derivative 11. We were interested in the latter due to the possibility of generating five-membered
rings via flash vacuum pyrolysis. However, FVP of 11 at 1025°C led only to diben-
4
f
zo[a,g]corannulene 12.

9933
MeO C
CO Me
2
2
iv
MeO C
CO Me
2
2
1
5
MeO C
CO Me
2
2
i
iii
MeO C
2
CO Me
2
Br
Br
Br
Br
I
I
ii
6
I
I
v
3
Ph
Ph
Ph
4
vi
Ph
Ref. 5
Ph
Ph
Ph
Me
Me
Me
Me
Ph
7
Ref. 5
8
13
Cl
Cl
TMSC C
TMSC C
C CTMS
C CTMS
ix
BrH C
CH Br
2
Cl
Cl
2
vii
BrH C
CH Br
10
11
2
2
viii
x
14
O
O
O
O
O
O
15
12
Scheme 1. Reagents and conditions: (i) Zn/KI/EtOH, reflux 6 h, 90%. (ii) CuI/KI/DMF, reflux 18 h, 70%. (iii)
CH CHCOOMe/P(o-tollyl) /Pd(OAc) /Bu NBr/K CO /DMF, 100°C, 48 h, 43%. (iv) DDQ/o-xylene, reflux 4 h, 55%.
2
3
2
4
2
3
(
v) as (iii), styrene instead of CH CHCOOMe, 43%. (vi) PhB(OH) /Pd(PPh ) /PhMe/EtOH/water/Na CO , reflux 24
2 2 3 4 2 3
h, 66%. (vii) AcOH/HCl, 80°C, 3 h, 30%. (viii) FVP, N (1 torr), 1025°C, ca. 5%. (ix) NBS, dibenzoyl peroxide/CCl ,
2
4
hg, reflux 4 h, 70%. (x) Maleic anhydride/KI/18-crown-6/toluene, reflux 24 h, 23%
Conversion of tetramethylcorannulene. Compound 13 is converted to 14 by bromination with
NBS. Subsequent treatment of 14 with KI in the presence of excess maleic anhydride leads to
the formation of the bis-anhydride 15, presumably through the intermediacy of a reactive

9934
quinoid species. While proton and carbon NMR suggest 15 to be a single isomer, its
stereochemistry could not be determined from these spectra.
In summary, the range and scope of corannulene chemistry has now been extended so that
this novel aromatic hydrocarbon can be employed as the starting point for new and interesting
bowl-shaped systems.
Acknowledgements
This work was supported by the Ames Laboratory, which is operated for the US Department
of Energy by Iowa State University under Contract No. W-7405-Eng-82.
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1
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