Lowering inversion barriers of buckybowls by benzannelation of the rim: Synthesis and crystal and molecular structure of 1,2-dihydrocyclopenta[b,c]dibenzo[g,m]corannulene

Article abstract of DOI:10.1021/ol016607h

(metrix presented) The title compound (3) has been synthesized by a non-pyrolytic route providing a 36% isolated yield in the final step. X-ray crystal structure determination shows that 3 crystallizes with one solvating molecule of toluene and exhibits s

Full text of DOI:10.1021/ol016607h

ORGANIC
LETTERS
2001
Vol. 3, No. 22
3527-3529
Lowering Inversion Barriers of
Buckybowls by Benzannelation of the
Rim: Synthesis and Crystal and
Molecular Structure of
1,2-Dihydrocyclopenta[b,c]dibenzo[g,m]corannulene
Zbigniew Marcinow, Andrzej Sygula,* Arkady Ellern, and Peter W. Rabideau*
Department of Chemistry and Ames Laboratory, Iowa State UniVersity,
Ames, Iowa 50011
rabideau@iastate.edu
Received August 17, 2001
ABSTRACT
The title compound (3) has been synthesized by a non-pyrolytic route providing a 36% isolated yield in the final step. X-ray crystal structure
determination shows that 3 crystallizes with one solvating molecule of toluene and exhibits slightly lower curvature than the parent
cyclopentacorannulene. DFT calculations predict a substantially lower bowl-to-bowl inversion barrier for 3 than for dihydro-cyclopentacorannulene,
and this is confirmed by dynamic 1H NMR experiments.
“Buckybowls”, or fullerene fragments, represent a novel class
of curved-surface polynuclear aromatic hydrocarbons with
carbon networks represented on the surfaces of fullerenes.1
The synthesis of these novel aromatics presents a serious
challenge, however, as a result of the considerable strain
involved. Since the early 1990s, flash vacuum pyrolysis,
employed originally by the Scott group, has been the major
methodology and has led to the successful synthesis of
several buckybowls, albeit in moderate to low yields.1 Only
recently have more practical, condensed phase protocols been
developed.2 Although “wet” syntheses of corannulene (1, the
smallest member of the family) and its derivatives have been
reported with yields in excess of 80%, similar protocols
applied to the synthesis of more strained buckybowls
exhibited a dramatic drop in yields.2 For example, solution
phase syntheses of cyclopentacorannulene (2), with only one
* Additional corresponding author e-mail: asygula@iastate.edu.
(1) For recent reviews, see: (a) Rabideau, P. W.; Sygula, A. Acc. Chem.
Res. 1996, 29, 235-242. (b) Scott, L. T. Pure Appl. Chem. 1996, 68, 291-
300. (c) Mehta, G.; Rao, H. S. P. Tetrahedron 1998, 54, 13325-13370.
(d) Scott, L. T.; Bronstein, H. E.; Preda, D. V.; Anselms, R. B. M.; Bratcher,
M. S.; Hagen, S. Pure Appl. Chem. 1999, 71, 209-219.
(2) (a) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc.
1996, 118, 2754-2755. (b) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc.
1998, 120, 12666-12667. (c) Seiders, T. J.; Baldrige, K. K.; Elliott, E. L.;
Grube, G. H.; Siegel, J. S. J. Am. Chem. Soc. 1999, 121, 7439-7440. (d)
Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 1999, 121, 7800-7803. (e)
Seiders, T. J.; Elliott, E. L.; Grube, G. H.; Siegel, J. S. J. Am. Chem. Soc.
1999, 121, 7804-7813. (f) Reish, H. A.; Bratcher, M. S.; Scott, L. T. Org.
Lett. 2000, 2, 1427-1430. (g) Sygula, A.; Rabideau, P. W. J. Am. Chem.
Soc. 2000, 122, 6323-6324. (h) Sygula, A.; Marcinow, Z.; Fronczek, F.
R.; Guzei, I.; Rabideau, P. W. Chem. Commun. 2000, 2439-2440. (i)
Seiders, T. J.; Baldridge, K. K.; Grube, G. H.; Siegel, J. S. J. Am. Chem.
Soc. 2001, 123, 517-525.
10.1021/ol016607h CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/04/2001

Scheme 3a
a (a) 1,3-Bis(2-bromophenyl)-2-propanone, methanol, NaOH; rt,
12 h. (b) Norbornadiene, Ac2O; 160 °C, 24 h. (c) Pd(PCy3)2Cl2,
DBU, N,N-dimethylacetamide; 145 °C, 2 days.
more five-membered ring at the rim of 1, have been much
less successful with reported yields of ca. 20%.2d,e,i In the
present communication we report a non-pyrolytic synthesis
of 1,2-dihydrocyclopenta[b,c]dibenzo[g,m]corannulene (3)
with a modest 36% isolated yield, as well as the results of
crystal and molecular structure studies showing that the
annulation of two benzene rings on the rim of the parent
cyclopentacorannulene causes a reduction in curvature and,
consequently, a lowering of the inversion barrier.
Figure 1. ORTEP view of 3 (top) and 3 with the solvating
molecule of toluene at 50% probability level (bottom).
As outlined in Scheme 1, this route follows the recently
published protocol of Reisch, Bratcher, and Scott for
dibenzo[a,g]corannulene.2f It starts with known diketopyra-
cylene 4,3 which is condensed under Knoevenagel conditions
with 1,3-bis(2-bromophenyl)-2-propanone to give the cyclo-
pentadienone 5 in nearly quantitative yield. Diels-Alder
reaction of 5 with norbornadiene produces fluoranthene 6
in 88% isolated yield. Cyclodehydrohalogenation of 6 with
Pd(PCy3)2Cl2 as a catalyst in the presence of DBU produces
a mixture of 3 along with debrominated, single ring closure
byproduct 7 and diphenyl benzopyracylene 8 (59:38:3 by
1H NMR). The crude material affords 36% of 3 after flash
column chromatography (silica gel, cyclohexane/DCM,
10:1).4
The presence of the solvent molecule prevents concave-
convex stacking of 3 in the crystal. This latter interesting
crystal packing arrangement of buckybowls was observed
in 26 as well as in the largest buckybowl to date, C36H10.7
However, it is absent in the crystals of both 18 and
tetramethylsemibuckminsterfullerene C34H20.2h
As expected, 3 exhibits significant curvature. The π-orbital
axis vector (POAV)9 pyramidalization angles calculated for
the carbon atoms in the central five-membered ring are
100.7°, 100.0°, 99.6°, 98.7°, and 98.4°. These numbers are
very close to those found earlier for the crystal of cyclo-
pentacorannulene (2).6 Also, the bowl depth of 3, defined
as the average distance of the rim carbon atoms of the
Several solvent systems were used for growing a crystal
of 3 suitable for X-ray diffraction; all but one produced very
tiny needles, too small for study. However, high quality
crystals could be grown from toluene, and crystal structure
determination5 shows that 3 crystallizes with one molecule
of solvent (Figure 1).
(5) Crystal data: C37H24, FW ) 468.56; orthorhombic, P2(1)2(1)2(1);
a ) 9.0353(17), b ) 9.8916(18), c ) 26.543(5) Å; V ) 2372.2(8) Å3; Z )
4; Dcalc ) 1.312 g cm-3; F(000) ) 984; T ) 173 K; R ) 0.0385, Rw )
0.0868 for 5538 observed data. Intensity data were collected on a Bruker
CCD-1000 diffractometer with Mo-KR radiation (λ ) 0.71073 Å).
Hydrogen atoms were refined in isotropic approximation. See Supporting
Information for crystallographic data in CIF format.
(6) Sygula, A.; Folsom, H. E.; Sygula, R.; Abdourazak, A. H.; Marcinow,
Z.; Fronczek, F. R.; Rabideau, P. W. Chem. Commun. 1994, 2571-2572.
(7) Forkey, D. M.; Attar, S.; Noll, B. C.; Koerner, R.; Olmstead, M. M.;
Balch, A. L. J. Am. Chem. Soc. 1997, 119, 5766-5767.
(8) Hanson, J. C.; Nordman, C. E. Acta Crystallogr. 1976, B32, 1147-
1153.
(3) Trost, B. M. J. Am. Chem. Soc. 1969, 91, 918-923.
(4) Pale yellow solid, mp 256-258 °C. 1H NMR (300 MHz, CDCl3):
8.67-8.58 (m, 4H), 8.23 (s, 2H), 7.76 (s, 2H), 7.74-7.70 (m, 4H), 3.84,
3.09 (m, 4H). 13C NMR (75.4 MHz, CDCl3): 149.61, 144.15, 138.96,
135.92, 135.26, 134.33, 133.73, 128.18, 127.56, 127.51, 125.17, 125.15,
124.63, 120.35, 32.42. MS (EI, 70 eV) m/z (rel intensity) 376 (100). HRMS
(EI, 70 eV): calcd for C30H16 376.1252, found 376.1258.
(9) Haddon, R. C.; Scott, L. T. Pure Appl. Chem. 1986, 58, 137. Haddon,
R. C. Acc. Chem. Res. 1988, 21, 243. Haddon, R. C. J. Am. Chem. Soc.
1990, 112, 3385. Haddon, R. C. Science 1993, 261, 1543.
3528
Org. Lett., Vol. 3, No. 22, 2001

corannulene portion of the molecule from the best plane of
the central five-membered ring (1.033 Å) is similar to the
one found for 2 (1.05 Å).6 Unfortunately, the crystal structure
of the more closely related 8,9-dihydrocyclopentacoranulene
(9) (i.e., 2 with an external ethane bridge rather than ethylene)
has not yet been determined so we turned to theoretical DFT
studies to assess the effect of double benzannelation on
corannulene bowl depth and inversion barrier.10
The curvature of 3 is well described at the Becke3LYP/
3-21G level of theory. The average POAV angle calculated
for all sp2 hybridized carbon atoms is 93.4° for the DFT
optimized structure as compared to 93.5° for 3 in the crystal.
The pyramidalization angles for the carbon atoms in the
central five-membered ring (101.0°, 100.1°, 100.1°, 98.8°,
and 98.8°) are close to the ones found in the crystal (see
above). At the same level of theory, the parent 9 is predicted
to exhibit slightly but consistently higher pyramidalization
of the analogous carbon atoms (101.4°, 100.3°, 100.3°, 99.4°,
and 99.4°), indicating that the corannulene unit is flattened
as a result of dibenzannelation. The calculated bowl depth
of 9 (1.114 Å) is also slightly larger than that predicted for
3 (1.057 Å). Recently Siegel and co-workers correlated
inversion barriers with the bowl depths for several coran-
nulenes and showed that even a small change in the latter
may cause a significant change of the barrier.2i Our calcula-
tions for 3 and 9 predict inversion barriers of 19.3 and 24.1
kcal/mol, respectively, at the Becke3LYP/6-31G**//
Becke3LYP/3-21G level with zero point energy correction.
Thus, 3 is predicted to have a barrier to inversion that is
lower than 9 by almost 5 kcal/mol! Since the experimentally
determined inversion barrier for 8,9-dideuteriocyclopentac-
orannulene is 27.7 kcal/mol,11 the expected barrier for 3 is
ca. 23 kcal/mol.
Figure 2. AA′BB′ portion of the 1H NMR spectrum of 3 in
nitrobenzene-d5 at 200 MHz (top traces) with the simulated spectra
(lower traces) at (a) room temperature (kinv ) 0 s-1), at (b) 165°
(kinv ) 17 s-1), and at (c) 184° (kinv ) 48 s-1).
Although a number of inversion barriers in the range of
9-17 kcal/mol have been determined for flexible corannu-
lene derivatives,2f there is a scarcity of such data for more
strained corannulene systems. The present determination of
the barrier for 3 represents only the second example (after
9)11 of a corannulene system with ∆G* for the inversion
above 20 kcal/mol. As such it contributes to the understand-
ing of the structure/energy relationships of the fascinating
class of curved surface polycyclic aromatic hydrocarbons.
1
In contrast to 9,12 variable temperature H NMR spectra
of 3 show temperature-dependent changes in the AA′BB′
spectral system over the temperature range 160-184° (Figure
2). While the coalescence temperature could not be reached
because of spectrometer limitations, dynamic spectrum
simulation13 allowed an experimental estimation of the
inversion barrier, ∆G* ) 23.5-23.6 kcal/mol, in the
temperature range 165-184°.
These results are in very close agreement with the
theoretical prediction, and they demonstrate that the ben-
zannelation of the corannulene core indeed flattens the bowl
and consequently lowers its barrier for inversion.14
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: Experimental pro-
cedures and characterization for compounds 3 and 6; CIF
file for 3; Gaussian94 archive files for 3 and 9 (minimum
energy bowls and planar transition states for inversion). This
material is available free of charge via the Internet at
http://pubs.acs.org.
(10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision E.2; Gaussian,
Inc.: Pittsburgh, PA, 1995.
OL016607H
(11) Sygula, A.; Abdourazak, A. H.; Rabideau, P. W. J. Am. Chem. Soc.
1996, 118, 339-343.
(14) According to a personal communication from L. T. Scott, the bowl-
to-bowl inversion barrier for a dimethylcarbinol derivative of benzocoran-
nulene is ca. 9.0 kcal/mol at -90 °C (McComas, C. C., B.S. Thesis, Boston
College, 1996), and the barrier for a dimethylcarbinol derivative of dibenzo-
[a,g]corannulene is <7.5 kcal/mol at -114 °C (Bratcher, M. S., Ph.D.
Dissertation, Boston College, 1996). Both numbers are lower than the
inversion barrier of dimethylcarbinol derivative of corannulene (10.2 kcal/
mol at -64 °C. Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem.
Soc. 1992, 114, 1920-1921).
(12) Abdourazak, A. H.; Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc.
1993, 115, 3010-3011.
(13) Dynamic NMR simulations were done with the MEXICO method
(Bain, A. D.; Duns, G. J. Can. J. Chem. 1996, 74, 819-824.) implemented
into SpinWorks program package (SpinWorks, v.1.2; Kirk Marat and the
University of Manitoba: Winnipeg, MB, Canada, copyright 2001; available
from pauli.chem.umanitoba.ca.)
Org. Lett., Vol. 3, No. 22, 2001
3529