Aromatic π-systems more curved than C60. The complete family of all indenocorannulenes synthesized by iterative microwave-assisted intramolecular arylations

Article abstract of DOI:10.1021/ja9031852

Syntheses and X-ray crystal structures are reported for all seven members of the indenocorannulene family, comprising indenocorannulene, both isomers of diindenocorannulene, both isomers of triindenocorannulene, tetraindenocorannulene, and pentaindenocora

Full text of DOI:10.1021/ja9031852

Published on Web 07/02/2009
Aromatic π-Systems More Curved Than C60. The Complete
Family of All Indenocorannulenes Synthesized by Iterative
Microwave-Assisted Intramolecular Arylations
Brian D. Steinberg,† Edward A. Jackson,† Alexander S. Filatov,‡
Atsushi Wakamiya,§ Marina A. Petrukhina,‡ and Lawrence T. Scott*,†
Merkert Chemistry Center, Department of Chemistry, Boston College, Chestnut Hill,
Massachusetts 02467-3860, Department of Chemistry, UniVersity at Albany, State UniVersity of
New York, Albany, New York 12222-0100, and Department of Chemistry, Graduate School of
Science, Nagoya UniVersity, Furo, Chikusa, Nagoya 464-8602, Japan
Received April 21, 2009; E-mail: lawrence.scott@bc.edu
Abstract: Syntheses and X-ray crystal structures are reported for all seven members of the indenocoran-
nulene family, comprising indenocorannulene, both isomers of diindenocorannulene, both isomers of
triindenocorannulene, tetraindenocorannulene, and pentaindenocorannulene. With each additional inde-
noannulation, the pyramidalization of the trigonal carbon atoms at the hub of the corannulene increases.
Five of the seven indenocorannulenes contain carbon atoms at the hub that are actually more pyramidalized
than the carbon atoms of C60. This work demonstrates, for the first time, that extended π-systems with
curvatures exceeding those of the most curved stable fullerenes and carbon nanotubes can be prepared
by ordinary laboratory methods in solution, without recourse to high-temperature gas phase methods, such
as flash vacuum pyrolysis. This proof of principle presages the days when scientists will be able to synthesize
isomerically pure fullerenes and single-chirality nanotubes by well-understood, controlled chemical methods.
Pentaindenocorannulene, itself, represents an attractive precursor to the C5v end-cap of an all-carbon,
single-walled, “armchair” [5,5]nanotube.
Scheme 1. Indenoannulation: Bridging peri-Positions with an
ortho-Phenylene Unit
Introduction
Any chemical synthesis of an isolable fullerene will require
the embedding of 12 fully unsaturated five-membered rings into
a lattice of condensed six-membered rings (Euler’s theorem1
and the isolated pentagon rule, IPR2). It is the five-membered
rings encircled by fused benzene rings that give IPR-fullerene
surfaces their curvature.3 Most of the strain energy in fullerenes
(all of it in C60) is also associated with the carbon atoms of the
five-membered rings, in which the internal angles (108° for a
regular pentagon) deviate significantly from the 120° natural
angle of unstrained sp2-hybridized carbon atoms. Unfortunately,
synthetic methods remain scarce even for the simple annulation
of fully unsaturated five-membered rings onto the perimeters
of appropriately constituted polycyclic aromatic hydrocarbons
(PAHs) (Scheme 1).4-6
Faced with this shortcoming in the toolbox of synthetic
organic chemistry, our laboratory has undertaken a program to
develop indenoannulation methods that are not only versatile
but also amenable to multiple indenoannulations within the same
molecule in a single laboratory operation. As an initial step in
that direction, the tandem Suzuki-Heck-type coupling cascade
† Boston College.
‡ University at Albany, State University of New York.
§ Nagoya University.
(1) Fowler, P. W.; Manolopoulos, D. E. Atlas of Fullerenes; Oxford
University Press: Oxford, U.K., 1995. A discussion of Euler’s theorem
can be found in Chapter 2.
(2) Hirsch, A.; Brettreich, M. Fullerenes; Wiley-VCH: Weinheim, 2005.
A discussion of the isolated pentagon rule, IPR, can be found in section
1.5.1.
(3) Corannulene (5), the C20H10 hydrocarbon with a five-membered ring
completely encircled by fused benzene rings, represents the smallest
subunit of a fullerene that retains a curved π-system:(a) Barth, W. E.;
Lawton, R. G. J. Am. Chem. Soc. 1966, 88, 380–1. (b) Lawton, R. G.;
Barth, W. E. J. Am. Chem. Soc. 1971, 93, 1730–45. (c) Hanson, J. C.;
Nordman, C. E. Acta Crystallogr., Sect. B 1976, 32, 1147–53.
(4) (a) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH:
New York, 1997. (b) Harvey, R. G. Curr. Org. Chem. 2004, 8, 303–
323.
(5) (a) Preda, D. V.; Scott, L. T. Polycyclic Aromat. Compd. 2000, 19,
119–131. (b) Wu, Y.-T.; Hayama, T.; Baldridge, K. K.; Linden, A.;
Siegel, J. S. J. Am. Chem. Soc. 2006, 128, 6870–6884.
(6) (a) Wegner, H. A.; Scott, L. T.; de Meijere, A. J. Org. Chem. 2003,
68, 883–887, and references therein. (b) Quimby, J. M.; Scott, L. T.
AdV. Synth. Catal. 2009, 351, 1009–1013.
(7) (a) Wegner, H. A.; Reisch, H.; Rauch, K.; Demeter, A.; Zachariasse,
K. A.; de Meijere, A.; Scott, L. T. J. Org. Chem. 2006, 71, 9080–
9087. (b) Aprahamian, I.; Wegner, H. A.; Sternfeld, T.; Rauch, K.;
de Meijere, A.; Sheradsky, T.; Scott, L. T.; Rabinovitz, M. Chem.-
Asian J. 2006, 1, 678–685. (c) de Meijere, A.; Stulgies, B.; Albrecht,
K.; Rauch, K.; Wegner, H. A.; Hopf, H.; Scott, L. T.; Eshdat, L.;
Aprahamian, I.; Rabinovitz, M. Pure Appl. Chem. 2006, 78, 813–
830.
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A R T I C L E S
Steinberg et al.
that we introduced in 20036a was successfully applied to the
first synthesis of a tetraindenopyrene (1) from pyrene in 2006.7
More recently, we succeeded in synthesizing tetraindenocoran-
nulene8 (tetra-IC) and pentaindenocorannulene9 (penta-IC) by
a powerful stepwise indenoannulation method.10 [Descriptive
nicknames that are easier to recognize than bare numbers will
be used when referring to the indenocorannulenes in this paper].
This new method has now been extended to provide all seven
members of the family of indenocorannulenes, including both
isomers of triindenocorannulene (1,2,3-tri-IC11 and 1,2,4-tri-
IC12), both isomers of diindenocorannulene (ortho-di-IC13 and
para-di-IC14), and the parent indenocorannulene (mono-IC15).
Figure 1 shows three-dimensional structural representations of
all seven indenocorannulenes, generated from the X-ray crystal
structures of the individual compounds (discussed further
below).
This sizable family of aromatic hydrocarbons provides an
exquisite opportunity for testing the reliability of modern
computational methods as applied to geodesic polyarenes, by
allowing comparisons to be made between theoretically calcu-
lated geometric parameters, NMR spectra, etc., and those
obtained experimentally.16,17 The bowl-shaped PAHs in this
family range in size from C26H12 to C50H20, and they all map
onto the geodesic framework of C60.
pyramidalization of trigonal carbon atoms at the centers of all
but two of the indenocorannulenes actually exceeds that
observed for the carbon atoms of C60 (see X-ray section). Prior
to this work, such acutely curved aromatic π-surfaces had been
accessible only by high-temperature gas phase methods such
as flash vacuum pyrolysis.18 Our findings make the impediments
to synthesizing fullerenes and carbon nanotubes entirely by
solutionphasechemicalmethodsnolongerappearinsurmountable.
Syntheses of the Indenocorannulenes
General Strategy. The indenoannulations performed in this
work all begin with the preparation of suitably halogenated
corannulenes. Suzuki-Miyaura couplings of these haloarenes
with ortho-chlorophenyl boronic acid then introduce all of the
carbon atoms required for the final target molecules, and
palladium-catalyzed intramolecular arylations close all of the
five-membered rings (Scheme 2). The details of how the
reactions were conducted to accomplish these transformations
are described in the sections below.
Halogenated Corannulenes: Functionalized Scaffolds on
Which to Build. The most reliable method we have found for
preparing bromocorannulene (2) in high yield, uncontaminated
by dibromocorannulene (3) and tribromocorannulene (4), in-
volves the direct bromination of corannulene (5) with IBr in
dichloromethane at room temperature (99% yield19). The
deactivating effect of the first bromine substituent prevents
overbromination until the ratio of product to starting material
climbs to a high level. Interrupting the reaction before all of
the starting material is consumed yields a product containing
minor amounts of corannulene as the only significant contami-
nant. Chromatographic separation can be achieved at this stage
but has proven unnecessary. Corannulene is inert during the
next reaction and can easily be recovered and recycled virtually
quantitatively after the bromocorannulene has been carried
through the Suzuki-Miyaura coupling.
Most importantly, this work demonstrates for the first time
that solution phase chemical methods can be used to synthesize
fused networks of five- and six-membered rings with curvatures
equaling, and eVen surpassing, that of the most curved, stable
fullerene, C60. As revealed by X-ray crystallography, the
(8) Complete name: tetraindeno[1,2,3-bc; 1′,2′,3′-ef; 1′′,2′′,3′′-hi; 1′′′,2′′′,3′′′-
kl]corannulene.
(9) Complete name: pentaindeno[1,2,3-bc; 1′,2′,3′-ef; 1′′,2′′,3′′-hi; 1′′′,2′′′,3′′′-
kl; 1′′′′,2′′′′,3′′′′-no]corannulene.
(10) Jackson, E. A.; Steinberg, B. D.; Bancu, M.; Wakamiya, A.; Scott,
L. T. J. Am. Chem. Soc. 2007, 129, 484–485.
(11) Complete name: triindeno[1,2,3-bc; 1′,2′,3′-ef; 1′′,2′′,3′′-hi]coran-
nulene.
Pushing the bromination of corannulene with more IBr at
higher concentrations attaches a second bromine atom onto the
rim,20 and the reaction conveniently stops there, even when a
large excess of IBr is used at room temperature (87% yield).
The complex mixture of dibromocorannulenes obtained from
this reaction may contain as many as seven possible positional
(12) Complete name: triindeno[1,2,3-bc; 1′,2′,3′-ef; 1′′,2′′,3′′-kl]coran-
nulene.
(13) Complete name: diindeno[1,2,3-bc; 1′,2′,3′-ef]corannulene.
(14) Complete name: diindeno[1,2,3-bc; 1′,2′,3′-hi]corannulene.
(15) Complete name: indeno[1,2,3-bc]corannulene. For prior syntheses of
variously substituted indenocorannulenes, see ref 5b.
(16) We have previously demonstrated that X-ray quality geometries of
relatively rigid geodesic polyarenes can be obtained from theoretical
calculations at the B3LYP/6-31G* level of theory: Petrukhina, M. A.;
Andreini, K. W.; Mack, J.; Scott, L. T. J. Org. Chem. 2005, 70, 5713–
5716.
(18) (a) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996,
118, 8743–8744. (b) Forkey, D. M.; Attar, S.; Noll, B. C.; Koerner,
R.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 1997, 119, 5766–
5767. (c) Ansems, R. B. M.; Scott, L. T. J. Am. Chem. Soc. 2000,
122, 2719–2724.
(17) Higher levels of theory with more extensive basis sets than B3LYP/
6-31G* have been used to gain more reliable insight into the properties
and dynamics of geodesic polyarenes:(a) Baldridge, K. K.; Siegel, J. S.
Theor. Chem. Acc. 1997, 97, 67–71. (b) Seiders, T. J.; Grube, G.;
Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2001, 123, 517–25.
(c) Hayama, T.; Wu, Y.-T.; Linden, A.; Baldridge, K. K.; Siegel, J. S.
J. Am. Chem. Soc. 2008, 130, 1583. (d) Yao-Ting Wu, Y.-T.; Bandera,
D.; Maag, R.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem.
Soc. 2008, 130, 10729–39.
(19) The 99% yield is based on corannulene consumed (details can be found
in the Supporting Information). For alternate syntheses of bromoc-
orannulene, see: (a) Seiders, T. J.; Baldridge, 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. (c) Mack, J.; Vogel, P.; Jones, D.;
Kaval, N.; Sutton, A. Org. Biomol. Chem. 2007, 5, 2448–2452.
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Aromatic π-Systems More Curved Than C60
A R T I C L E S
Figure 1. The complete family of all seven indenocorannulenes.
Scheme 2
mentioned above, excess IBr and long reaction times fail to
attach a third bromine atom onto the rim of dibromocorannulene
at room temperature; however, gentle heating of the reaction
mixture (38 °C for 24 h) gives tribromocorannulene as a mixture
of positional isomers in 92% yield. Higher levels of bromination
by IBr pose no problem under these conditions.23 Scheme 3
summarizes these bromination reactions.
As a starting point for the synthesis of tetraindenocoran-
nulene (tetra-IC), we chose 1,2,5,6-tetrabromocorannulene
(6), a useful corannulene derivative that is readily available
by the method of Sygula et al. (Scheme 4, top).24 For the
synthesis of pentaindenocorannulene (penta-IC), 1,3,5,7,9-
pentachlorocorannulene (7) proved to be the most convenient
precursor. The direct 5-fold chlorination of corannulene with
ICl to give this symmetrical derivative (Scheme 4, bottom) was
discovered in our laboratory many years ago,25 and the
compound has found considerable use since then.19a,b,26 No
conditions for the direct bromination or iodination of corannu-
isomers;21 however, we made no attempt to separate the
individual components. Application of the indenoannulation
sequence outlined in Scheme 2 was expected to convert all seven
dibromocorannulenes into diindenocorannulenes, and the latter
has only two isomers (ortho-di-IC and para-di-IC). As
discussed below, this isomer proliferation-reconvergence plan
works just fine, and the two diindenocorannulenes can be
separated, isolated, and individually characterized at the end.
Triindenocorannulene likewise has only two isomers (1,2,3-
tri-IC and 1,2,4-tri-IC), so every positional isomer of tribro-
mocorannulene subjected to the indenoannulation sequence in
Scheme 2 would be expected to give either 1,2,3-tri-IC or 1,2,4-
tri-IC.22 Consequently, we decided to push the bromination
further and to continue ignoring the lack of site selectivity. As
(23) Under sufficiently forcing conditions with elemental bromine, even
decabromocorannulene can be prepared: (a) Prinzbach, H.; Weiler,
A.; Landenberger, P.; Wahl, F.; Wo¨rth, J.; Scott, L. T.; Gelmont, M.;
Olevano, D.; v. Issendorff, B. Nature 2000, 407, 60–63. (b) Prinzbach,
H.; Wahl, F.; Weiler, A.; Landenberger, P.; Wo¨rth, J.; Scott, L. T.;
Gelmont, M.; Olevano, D.; Sommer, F.; Issendorff, B. Chem.-Eur.
J. 2006, 12, 6268–6280.
(20) The five interior carbon atoms of corannulene are often referred to as
the “hub” carbon atoms, and the 15 outer carbon atoms comprise the
“rim”; see, for example: (a) Borchardt, A.; Fuchicello, A.; Kilway,
K. V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114,
1921. (b) Samdal, S.; Hedberg, L.; Hedberg, K.; Richardson, A. D.;
Bancu, M.; Scott, L. T. J. Phys. Chem. A 2003, 107, 411–417. (c)
Petrukhina, M. A.; Sevryugina, Y.; Rogachev, A. Y.; Jackson, E. A.;
Scott, L. T. Angew. Chem., Int. Ed. 2006, 45, 7208–7210.
(24) (a) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 2000, 122, 6323–
6324. (b) Sygula, A.; Xu, G.; Marcinow, Z.; Rabideau, P. W.
Tetrahedron 2001, 57, 3637–3644.
(25) Cheng, P.-C. Ph.D. Dissertation, Boston College, Chestnut Hill, MA,
1996.
(21) In principle, bromocorannulene could acquire a second bromine atom
at any of the nine remaining CH positions in the molecule. Because
the two substituents are the same, however, the 1,3- and the 1,9-isomers
are identical, and the 1,5- and the 1,7-isomers are also identical.
(22) The one exception to this assertion would be 1,2,3-tribromocorannu-
lene, but its formation is considered highly unlikely on both steric
and electronic grounds.
(26) (a) Mizyed, S.; Georghiou, P. E.; Bancu, M.; Cuadra, B.; Rai, A. K.;
Cheng, P.-C.; Scott, L. T. J. Am. Chem. Soc. 2001, 123, 12770–12774.
(b) Grube, G. H.; Elliott, E. L.; Steffens, R. J.; Jones, C. S.; Baldridge,
K. K.; Siegel, J. S. Org. Lett. 2003, 5, 713–716. (c) Bancu, M.; Rai,
A. K.; Cheng, P.-C.; Gilardi, R. D.; Scott, L. T. Synlett 2004, 173–
176. (d) Mori, T.; Grimme, S.; Inoue, Y. J. Org. Chem. 2007, 72,
6998–7010.
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Steinberg et al.
Scheme 3. Bromination of Corannulene Using IBra
Scheme 5. Suzuki-Miyaura Couplings with Brominated
Corannulenesa
a (a) IBr (1.9 molar equiv, 0.1 M in CH2Cl2), rt, 24 h, 99% yield; (b)
IBr (9.0 molar equiv, 0.25 M in CH2Cl2), rt, 5 h, 87% yield; (c) IBr (9.0
molar equiv, 0.25 M in CH2Cl2), 38 °C, 24 h, 92% yield.
a (a) 2-Chlorophenylboronic acid, Pd(0), K2CO3, heat, 24 h.
Scheme 4a
Scheme 6. Suzuki-Miyaura Couplings with
1,3,5,7,9-Pentachlorocorannulenea
a (a) Pd2(dba)3, 1,3-bis(2,6-di-isopropylphenyl)imidazolium, Cs2CO3, 1,4-
dioxane, 80 °C, 2 days.
a (a) NaOH, dioxane/water (3:1), 100 °C, 15 min, 83% yield; (b) ICl
(12.5 molar equiv, 1.0 M in CH2Cl2), rt, 2 days, 37% yield.
(9 and 10) both consist of collections of many isomers, for the
reasons discussed in the previous section, but we encountered
no difficulty in carrying these isomer mixtures through the final
reaction without separation.
To cross-couple ortho-chlorophenylboronic acid with the less
reactive 1,3,5,7,9-pentachlorocorannulene (7), we resorted to
the conditions introduced by Nolan et al. (catalytic Pd2(dba)3,
Cs2CO3, and 1,3-bis(2,6-di-isopropylphenyl)imidazolium chlo-
ride).29 In this manner we were able to obtain 1,3,5,7,9-
pentakis(ortho-chlorophenyl)corannulene (12)30 in 48% isolated
yield (86% average per C-C coupling) (Scheme 6).
The fact that ortho-chlorophenylboronic acid does not simply
polymerize or form cyclic oligomers, such as triphenylene, by
self-coupling under these reaction conditions deserves a com-
ment. We believe that the cross-coupling is favored because
oxidative addition to the Pd(0) occurs faster at the rim of
corannulene than oxidative addition at the Ar-Cl bond of ortho-
chlorophenylboronic acid. This difference in reactivity holds
lene have been found that exhibit the same fortuitous site
selectivity as this chlorination.17c,d,27
Adding the New Six-Membered Rings by Suzuki-Miyaura
Coupling. We have previously shown that bromocorannulene
(2) can be converted to indenocorannulene (mono-IC) in a
single step by our Suzuki-Heck-type cascade method;6a
however, the yield by that procedure was just 40%, which we
did not consider adequate for multiple indenoannulations.
Fortunately, the stepwise alternative proved highly effective.
Standard Suzuki-Miyaura conditions (catalytic Pd(PPh3)4 and
K2CO3) work well to cross-couple ortho-chlorophenylboronic
acid with the variously brominated corannulenes. Thus, ortho-
chlorophenylcorannulene (8), bis(ortho-chlorophenyl)corannu-
lene (9), tris(ortho-chlorophenyl)corannulene (10), and 1,2,5,6-
tetrakis(ortho-chlorophenyl)corannulene (11)28 were prepared
in 90%, 98%, 67%, and 91% isolated yields, respectively
(Scheme 5). The bis- and tris(ortho-chlorophenyl)corannulenes
(27) (a) Corpuz, E. M.S. Thesis, Boston College, Chestnut Hill, MA, 1997.
(b) Bancu, M. Ph.D. dissertation, Boston College, Chestnut Hill, MA,
2004.
(29) Grasa, G. A.; Viciu, M. S.; Huang, J.; Zhang, C.; Trudell, M. L.; Nolan,
S. P. Organometallics 2002, 21, 2866–2873, and references therein.
(30) Other 1,3,5,7,9-pentaarylcorannulenes have been prepared from 7 by
Siegel et al.17d,26b
(28) Sygula et al. have previously prepared 1,2,5,6-tetraphenylcorannulene
also from 6.24b
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Aromatic π-Systems More Curved Than C60
A R T I C L E S
Scheme 7. Intramolecular arylationsa
even for the chlorinated corannulene 7, because the boronic acid
group is both bulky and anionic under the basic conditions of
the reaction, and that deactivates the Ar-Cl bond of ortho-
chlorophenylboronic acid toward oxidative addition both steri-
cally and electronically. After the cross-coupling has occurred,
the Ar-Cl bond on the phenyl group loses its electronic
protection, but it gains new steric protection as an ortho-
chlorobiaryl. The differential reactivity is not perfect, but it is
enough to give us the products we needed. With all the necessary
arylated corannulenes in hand we could now explore the final
cyclization step to form the new polyarenes.
Closing the New Five-Membered Rings by Microwave-
Assisted Intramolecular Arylations. Some years ago, we found
that palladium-catalyzed intramolecular arylation reactions could
be used to impose curvature on planar PAH π-systems.31,32 By
applying this method to the ortho-chlorophenyl-substituted
derivatives of corannulene, we learned that curved PAHs can
also be made even more curved through this chemistry. Thus,
8 was cyclized to indenocorannulene (mono-IC) in 95% yield
by heating it to 160 °C for 1 h in dimethylacetamide (DMAc)
with a catalytic amount of Pd(PCy3)2Cl2 and several equivalents
of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (Scheme 7).27b
These same conditions were used to convert the mixture of
bis(ortho-chlorophenyl)corannulene isomers (9) into the iso-
meric diindenocorannulenes (ortho-di-IC:para-di-IC ) 45:55),
also in 95% yield (Scheme 7), and the two new C32H14 poly-
arenes could be separated by chromatography.27b
Unfortunately, these reactions were discovered to be highly
capricious,33 and the need for a more dependable method
became critical. Extensive efforts to stabilize and optimize these
intramolecular arylations eventually led to testing microwave-
assisted variants, and those results proved most satisfying. In
just 30 min at 170 °C in a microwave oven with excess DBU
and a catalytic amount of Pd(PCy3)2Cl2 in DMAc, the mono-
substituted corannulene 8 cyclizes to indenocorannulene (mono-
IC) in 99% yield (Scheme 7). Likewise, 9 cyclizes to the two
diindenocorannulenes (ortho-di-IC:para-di-IC ) 30:70) in 89%
yield (Scheme 7). These reactions are totally reliable and work
every time.
Molecular geometry calculations predict, and X-ray struc-
tural analyses confirm (Vide infra), that indenoannulations
of corannulene force the interior carbon atoms of the ring
system to become more pyramidalized. Consequently, we
were eager to test the limits of this chemistry and to find out
if it could be used to provide access to even larger, more
strained polyarenes. Gratifyingly, microwave heating with
the same catalytic system converts 10 into the two new
triindenocorannulene in 48% yield (1,2,3-tri-IC:1,2,4-tri-
IC ) 25:75, Scheme 7), and the two isomers could be
separated by chromatography and individually characterized.
Tetraindenocorannulene (tetra-IC) is likewise obtained from
11 under the microwave conditions, but the yield drops to
a (a) Pd(PCy3)2Cl2, DBU, DMAc, 170 °C (microwave), 30 min, 99%
yield; (b) Pd(PCy3)2Cl2, DBU, DMAc, 170 °C (microwave), 30 min, 89%
yield (30:70); (c) Pd(PCy3)2Cl2, DBU, DMAc, 180 °C (microwave), 40 min,
48% yield (25:75); (d) Pd(PCy3)2Cl2, DBU, DMAc, 170 °C (microwave),
40 min, 13% yield; (e) Pd(PCy3)2Cl2, DBU, DMAc, 180 °C (microwave),
45 min, 35% yield.
only 13%. The lower yield in this case can be attributed, in
part, to the intervention of a competing side reaction, whereby
some of the proximal phenyl groups couple with each other
instead of with the peri-positions on the corannulene rim.
On the other hand, we were delighted to find that the
microwave-assisted intramolecular arylation of 12 produces
the highly strained pentaindenocorannulene (penta-IC) in
35% yield (86% average yield per C-C coupling, Scheme
7). We never encountered problems of irreproducibility with
the microwave-assisted cyclizations.34
Closing the New Five-Membered Rings by Flash Vacuum
Pyrolysis. Flash vacuum pyrolysis (FVP) has been used since
the early 1990s for the preparation of more than two dozen
geodesic polyarenes, ranging in size from corannulene to
(31) Reisch, H. A.; Bratcher, M. S.; Scott, L. T. Org. Lett. 2000, 2, 1427–
1430.
(32) For excellent reviews and leading references to other intramolecular
arylation reactions, see: (a) Echavarren, A. M.; Gomez-Lor, B.;
Gonzalez, J. J.; de Frutos, O. Synlett 2003, 585–597. (b) Pascual, S.;
de Mendoza, P.; Echavarren, A. M. Org. Biomol. Chem. 2007, 5, 2727–
2734.
(33) Cyclized materials were always formed cleanly as the only products
detectable by 1H NMR analysis; however, the reactions run with
conventional heating would often come to a halt, for reasons that we
were never able to discern, or overcome, leaving varying amounts of
unchanged starting material. The microwave-assisted cyclizations
proved to be much more reproducible.
(34) Under bone dry conditions in the winter, these intramolecular arylations
sometimes falter, but traces of water added to the reaction mixture
restore the normal reliability.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10541

A R T I C L E S
Steinberg et al.
fullerene-C60.35-38 In the present case, however, it has proven
to be inferior to the solution phase methods. FVP of the
tetrakis(ortho-chlorophenyl)corannulene 11 at 1100 °C, for
example, gives tetraindenocorannulene (tetra-IC) in only 2%
isolated yield (isolated by HPLC), along with a host of other
products. Both of the triindenocorannulenes (1,2,3-tri-IC and
1,2,4-tri-IC), both of the diindenocorannulenes (ortho-di-IC
and para-di-IC), and even the simplest indenocorannulene
(mono-IC) were also isolated from the same pyrolysate by
tedious HPLC, all in low yield. The presence of these lower
molecular weight products clearly points to aryl group loss, by
rupture of corannulene-aryl bonds, as a significant side reaction
in these FVP experiments. Such “aryl group loss” products are
not uncommon when the FVP starting material has parts of the
molecule linked by only a single bond.39
Analysis of the Stepwise Introduction of Strain Energy. In
our synthesis of pentaindenocorannulene (penta-IC), the most
strained member of this family, the severe deformation of the
trigonal carbon atoms away from their natural geometries is
not achieved all at once. Rather, it is built in gradually, in a
stepwise manner. Each new five-membered ring closure adds a
little more strain and causes a little more pyramidalization of
the interior carbon atoms (see X-ray section below). This
strategy of introducing a large amount of strain little by little
over many steps has been used numerous times before, Eaton’s
synthesis of cubane being a prime example.40 In our case, the
price for the increase in strain is paid for by the energy released
in the intrinsically exothermic reductive elimination of Pd(0)
in the final step of each intramolecular arylation. Multiplied 5
times over, that intrinsic exothermicity pays for a considerable
increase in strain.
To get a sense of how much strain is associated with each
ring closure, we have calculated minimum energy geometries
at the B3LYP/6-31G* level of theory16,17,41 for 1,3,5,7,9-
pentakis(ortho-chlorophenyl)corannulene (12) and for all of the
partially closed intermediates along the various pathways leading
from there to penta-IC. Figure 2 summarizes the results. The
strain energy increases shown over each arrow correspond to
the extra energy costs for the cyclizations indicated, compared
Figure 2. Values over the arrows indicate the extra energy cost (kcal/
mol) calculated for each cyclization at the B3LYP/6-31G* level of theory,
compared to the strain-free reference system of benzene coupling with
chlorobenzene to make biphenyl.41,42
(35) (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.; Cheng, P.-C.;
Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am.
Chem. Soc. 1997, 119, 10963–10968.
to the “strain-free” reference system of benzene coupling with
chlorobenzene to make biphenyl.42 Chlorine atoms have been
arbitrarily located exo to the bowl in this analysis to circumvent
the obfuscation of the bewildering conglomeration of nonin-
terconverting rotamers. At this level of theory, the rotamer of
ortho-chlorophenylcorannulene (8) with the chlorine atom exo
is more stable than the endo rotamer by 0.36 kcal/mol.
(36) (a) Scott, L. T. Pure Appl. Chem. 1996, 68, 291–300. (b) Scott, L. T.;
Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.;
Hagen, S. Pure Appl. Chem. 1999, 71, 209–219. (c) Tsefrikas, V. M.;
Scott, L. T. Chem. ReV. 2006, 106, 4868–4884.
(37) (a) Boorum, M. M.; Vasil’ev, Y. V.; Drewello, T.; Scott, L. T. Science
2001, 294, 828–831. (b) Scott, L. T.; Boorum, M. M.; McMahon,
B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A.
Science 2002, 295, 1500–1503. (c) Scott, L. T. Angew. Chem., Int.
Ed. 2004, 43, 4995–5007.
According to this analysis, five indenoannulations on coran-
nulene add another 51.8 kcal/mol of strain energy to the already
strained ring system. The strain is introduced gradually, and
the first ring closure actually appears to be the most difficult
(13.1 kcal/mol of strain added). Closing the fifth ring to make
penta-IC introduces only 7.9 kcal/mol of additional strain. The
intrinsic exothermicity associated with reductive elimination of
Pd(0) from a diaryl palladium has never been determined
(38) (a) Aprahamian, I.; Hoffman, R. E.; Sheradsky, T.; Preda, D. V.;
Bancu, M.; Scott, L. T.; Rabinovitz, M. Angew. Chem., Int. Ed. 2002,
41, 1712–1715. (b) Aprahamian, I.; Preda, D. V.; Bancu, M.; Belanger,
A. P.; Sheradsky, T.; Scott, L. T.; Rabinovitz, M. J. Org. Chem. 2006,
71, 290–298.
(39) (a) Bratcher, M. S. Ph.D. Dissertation, Boston College, Chestnut Hill,
MA, 1996. (b) Peng, L.; Scott, L. T. J. Am. Chem. Soc. 2005, 127,
16518–16521. (c) Xue, X. Ph.D. Dissertation, Boston College,
Chestnut Hill, MA, 2008. (d) Amsharov, K. Y.; Jansen, M. J. Org.
Chem. 2008, 73, 2931–2934. (e) FVP of the pentakis(ortho-chlo-
rophenyl)corannulene 12 at 1100 °C produces only ca. 1% yield of
pentaindenocorannulene (penta-IC), which was positively identified
by HPLC with a UV-diode array detector but was not isolated.
(40) (a) Eaton, P. E.; Cole, T. W., Jr. J. Am. Chem. Soc. 1964, 86, 3157–
3158. (b) Eaton, P. E. Angew. Chem., Int. Ed. Engl. 1992, 31, 1421–
1436.
(42) Homodesmic reaction used for the first cyclization: 12 + Ph-Ph f
product + PhH + Ph-Cl.
(43) High-level NMR calculations on geodesic polyarenes include: (a)
Baldridge, K. K.; Siegel, J. S. J. Phys. Chem. A 1999, 103, 4038–
4042. (b) Baldridge, K. K.; Siegel, J. S. Theor. Chem. Acc. 2008, 120,
95–106.
(41) Spartan. software; Wavefunction, Inc.: Irvine, CA92612.
9
10542 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Aromatic π-Systems More Curved Than C60
A R T I C L E S
Table 1. Experimental H NMR Chemical Shifts of the
Indenocorannulenes and Theoretical Values Calculated by the
GIAO Method Using Geometries Optimized by Density Functional
Theory (B3LYP/6-31G*)
1
experimentally, to the best of our knowledge, but it must be
worth at least 13 kcal/mol (lower limit), according to our
calculations.
experimental
1H NMR (ppm)
calculated
1H NMR (ppm)
deviationa
(ppm)
Properties of Indenocorannulenes
compound
1
NMR Spectra of the Indenocorannulenes. In the H NMR
indenocorannulene
7.63
7.62
7.62
7.56
7.52
7.19
7.98
7.66
7.56
7.46
7.38
7.29
7.29
7.59
7.59
7.55
7.47
7.46
7.18
7.18
8.05
8.00
7.61
7.38
7.37
7.31
7.28
7.28
7.96
7.66
7.63
7.54
7.48
7.32
7.32
7.25
7.98
7.89
7.86
7.52
7.27
7.25
7.23
7.19
7.19
8.10
7.37
7.59
7.55
7.53
7.52
7.44
7.12
7.85
7.55
7.40
7.36
7.27
7.17
7.17
7.53
7.50
7.43
7.41
7.30
7.14
7.13
7.94
7.91
7.53
7.27
7.25
7.24
7.20
7.19
7.81
7.53
7.52
7.36
7.30
7.17
7.17
7.15
8.02
7.93
7.90
7.56
7.30
7.29
7.27
7.23
7.23
8.04
7.33
average |dev|
-0.04
-0.07
-0.09
-0.04
-0.08
-0.07
-0.13
-0.11
-0.16
-0.10
-0.11
-0.12
-0.12
-0.07
-0.09
-0.12
-0.06
-0.16
-0.04
-0.05
-0.11
-0.09
-0.08
-0.11
-0.12
-0.07
-0.08
-0.09
-0.15
-0.13
-0.11
-0.08
-0.18
-0.15
-0.15
-0.10
+0.04
+0.04
+0.04
+0.04
+0.03
+0.04
+0.04
+0.04
+0.04
-0.06
-0.04
0.09
spectrum of corannulene, the 10 equivalent protons give rise to
a single peak at δ 7.82 (CD2Cl2). One indenoannulation causes
the signals for the corannulene rim hydrogens all to shift upfield
by 0.2 ppm or more, and an even larger shielding is seen for
the corannulene rim hydrogens of the higher indenocorannu-
lenes. Whenever two indeno moieties are introduced “ortho”
to one another (i.e., attached to the same benzene ring of
corannulene), a fjord region results, and the sterically congested
hydrogens located therein become strongly deshielded. The
effect is greatest in penta-IC (δ 8.10).
ortho-diindenocorannulene
para-diindenocorannulene
1,2,3-triindenocorannulene
The rigidity of the indenocorannulenes fixes the locations of
the hydrogens and makes these molecules excellent candidates
for NMR chemical shift calculations. We have chosen to test
the B3LYP/6-31G* level of theory against our new data set,
primarily because this computational method has gained con-
siderable popularity among practicing chemists, and calibration
points such as these are valuable, despite the fact that higher
levels of theory can give more reliable results.17,43 Using the
computationally optimized geometries, we find that gauge-
invariant atomic orbital calculations (GIAO)44 reproduce the
experimental NMR spectra with remarkable accuracy ((0.09
ppm average deviation between experiment and theory, Table
1). The calculations slightly overestimate the shielding of the
hydrogens in all cases except for tetra-IC, for which it
underestimate the shielding. We have no explanation at the
moment for the nonconformity of tetra-IC. This is the first such
assessment of NMR chemical shift calculations on geodesic
polyarenes that draws from such a large experimental data set.
The low solubility of some of these indenocorannulenes, the
high density of 13C NMR signals in the aromatic region, and
the long relaxation times associated with interior carbon atoms
made it difficult to obtain a complete set of experimental 13C
NMR spectral data. A listing of all the calculated 13C NMR
chemical shifts for the seven indenocorannulenes can be found
in the Supporting Information. The hub20 carbon signals are all
predicted to fall in the narrow range 139.5-143.4 ppm (cf. 143.2
ppm for fullerene-C60). The lowest field chemical shift in each
spectrum invariably comes from one of the carbon atoms
attached to the hub, usually at the site of an indenoannulation
(147.1-157.8 ppm), whereas the highest field chemical shifts
are always associated with R-carbon atoms on the indeno groups
(122.5-127.3 ppm).
1,2,4-triindenocorannulene
tetraindenocorannulene
pentaindenocorannulene
a Deviation ) calculated - experimental.
corannulenes appear yellow in color; the triindenocorannulenes
begin to take on an orange tinge, tetra-IC is distinctly orange,
and penta-IC is dark orange. We have not examined the
emission spectra of these new chromophores.
UV-Vis Spectra of the Indenocorannulenes. As expected for
such highly conjugated systems, all of the indenocorannulenes
exhibit rich UV-vis spectra. None have absorption maxima in
the visible region of the spectrum, but all of the long-wavelength
absorptions tail well beyond 400 nm, and some extend beyond
500 nm. Figure 3 shows the UV-vis spectra of all the
indenocorannulenes superimposed for comparison; larger indi-
vidual spectra of each family member can be found in the
Supporting Information. Successive indenoannulations impart
progressive red shifts in the spectra. The mono- and diindeno-
X-ray Crystal Structures of the Indenocorannulenes. After
a considerable investment of time and energy, we were
ultimately rewarded with X-ray quality crystals of all seven
indenocorannulenes (Figure 1). Tables of experimental bond
lengths, bond angles, etc., can be found in the Supporting
Information; however, two aspects of the observed molecular
structures deserve special attention, namely, (1) the degree of
pyramidalization of the interior trigonal carbon atoms in each
molecule and (2) the depth of the corannulene bowl in each.
(44) NMR chemical shifts were calculated by the GIAO method incorpo-
rated into the Gaussian 03 suite of programs, using the chemical shift
of tetramethylsilane calculated at the same level of theory as the
reference point: Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.:
Wallingford, CT, 2004.
The pyramidalization of trigonal atoms is most conveniently
quantified using Haddon’s widely adopted p-orbital axis vector
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10543

A R T I C L E S
Steinberg et al.
Figure 3. UV-vis spectra of the indenocorannulenes in CH2Cl2.
Table 2. POAV Angles at the Interior Five-Membered Ring Carbon
Atoms of the Indenocorannulenes As Determined Experimentally
by X-ray Crystallography and Theoretically by Geometry
(POAV) analysis.45 Planar trigonal carbon atoms, such as those
in benzene, have POAV angles of 0.0°, whereas the hub20
carbon atoms of corannulene have POAV angles of 8.2°, and
the more highly pyramidalized atoms of C60 (all identical) have
POAV angles of 11.6°. The largest POAV angles in each of
the indenocorannulenes were found to range from 11.2° in
monoindenocorannulene up to 12.7° in pentaindenocorannulene,
as determined from the crystal structures. Bridging a pair of
peri-positions on the rim of corannulene or any of its derivatives
with an ortho-phenylene unit (i.e., indenoannulation) invariably
squeezes the peri-positions closer together and imposes greater
pyramidalization on the nearest hub carbon atom.17a The
B3LYP/6-31G* geometry optimizations described above do a
good job of predicting the observed POAV angles at the central
five-membered ring carbon atoms of the indenocorannulenes
((0.2° average deviation from experiment, Table 2). It is
important to note that five of the seven indenocorannulenes
contain carbon atoms that are more seVerely pyramidalized than
the carbon atoms of C60, which is the most curved of the stable
fullerenes. From high-level theoretical calculations, Baldridge
and Siegel predicted curvatures greater than that in C60 for
several other geodesic polyarenes that have a corannulene
base.17a
Corannulene is characterized by a “bowl depth” of 0.870
Å.3c,16,46 This metric corresponds to the distance between the
plane containing the five carbon atoms at the hub of corannulene
and the plane containing the 10 methine carbon atoms on the
rim. For the indenocorannulenes of lower symmetry, the mean
planes defined by these two sets of carbon atoms are used to
determine the bowl depths. Table 3 lists the experimental
bowl depths of all seven indenocorannulenes, as determined
from their X-ray crystal structures, along with the values
calculated from the B3LYP/6-31G*-optimized geometries.
It is gratifying to see that density functional theory does a
good job also of predicting these bowl depths ((0.015 Å
Optimizations Using Density Functional Theory (B3LYP/6-31G*)
compound
X-ray POAVa (deg) DFT POAV (deg) deviationb (deg)
indenocorannulene
8.91
9.83
11.15
8.87
8.96
9.85
11.00
9.44
+0.05
+0.02
-0.15
+0.57
+0.34
+0.05
-0.11
+0.16
+0.20
+0.13
+0.04
-0.04
+0.11
+0.60
+0.62
-0.26
-0.05
-0.06
-0.27
0.20
ortho-diindenocorannulene
para-diindenocorannulene
1,2,3-triindenocorannulene
1,2,4-triindenocorannulene
tetraindenocorannulene
9.79
10.13
11.80
10.30
11.09
11.40
10.41
11.85
12.37
11.20
12.12
12.63
11.17
12.02
12.39
12.42
average |dev|
11.64
11.75
10.41
10.93
11.20
10.28
11.81
12.41
11.09
11.52
12.01
11.43
12.07
12.45
12.69
pentaindenocorannulene
C60 (for comparison)
11.64
a The X-ray POAV angles reported here are the values obtained by
averaging the POAV angles of symmetry-related carbon atoms,
assuming Cs symmetry for all compounds except for pentain-
denocorannulene, which was treated as C5V symmetrical. b Deviation )
calculated - experimental.
average deviation between experiment and theory). For
comparison purposes, the bowl depth of the parent coran-
nulene molecule is included in Table 3.
Some of the indenocorannulenes arrange themselves like
stacks of bowls in the solid state, while others prefer less regular
orientations. All are characterized by significant intermolecular
π,π-contacts. A thorough discussion of these crystal-packing
patterns and the possible influences they may have on various
solid-state properties exhibited by these novel carbon-rich
(45) (a) Haddon, R. C.; Scott, L. T. Pure Appl. Chem. 1986, 58, 137. (b)
Haddon, R. C. J. Am. Chem. Soc. 1990, 112, 3385–3389. [erratum p
8217].
9
10544 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Aromatic π-Systems More Curved Than C60
A R T I C L E S
Table 3. Bowl Depths from the Five-Membered Ring Hub to the
10 Carbon Atoms of the Corannulene Rim in the
Indenocorannulenes As Determined Experimentally by X-ray
Crystallography and Theoretically by Geometry Optimizations
Using Density Functional Theory (B3LYP/6-31G*)
ambient conditions are less severely curved than C60, this work
provides assurance, for the first time, that they can all be
considered realistic targets for chemical synthesis by ordinary
laboratory methods.47
deviationa
(Å)
Finally, we note that stitching the five arms of penta-IC
together would yield a C50H10 hydrocarbon hemisphere that
represents the C5V end-cap of a [5,5]nanotube with an armchair
rim.17a Growing a long [5,5]nanotube from this end-cap has
considerable appeal as a method for obtaining single-chirality
carbon nanotubes of uniform diameter, and we are pursuing this
objective.48
bowl depth
X-ray (Å)
bowl depth
DFT (Å)
compound
indenocorannulene
1.065
1.157
1.209
1.272
1.298
1.378
1.433
1.060
1.190
1.224
1.283
1.309
1.366
1.418
average |dev|
0.858
-0.005
+0.033
+0.015
+0.011
+0.011
-0.012
-0.015
0.015
ortho-diindenocorannulene
para-diindenocorannulene
1,2,3-triindenocorannulene
1,2,4-triindenocorannulene
tetraindenocorannulene
pentaindenocorannulene
Acknowledgment. This paper is dedicated to Amir H. Hoveyda
on the occasion of his 50th birthday. We thank Dr. Bo Li for solving
the X-ray crystal structure of 1,2,4-tri-IC, Stephanie Ng for solving
the X-ray crystal structures of ortho-di-IC and 1,2,3-tri-IC, and
Dr. Mihail Bancu for early studies on conventional intramolecular
arylations. We are grateful to the National Science Foundation for
financial support of this work (grants no. CHE-0414066 and CHE-
0809494) and for funds to purchase mass spectrometers (grant no.
DBI-0619576). Generous support of our X-ray Crystallography
Facility by Schering-Plough is gratefully acknowledged. We thank
AstraZeneca for a graduate fellowship awarded to B.D.S.
corannulene (for comparison)
0.870
-0.012
a Deviation ) calculated - experimental.
materials will comprise the primary focus of our forthcoming
paper on the crystallography of these compounds.
Conclusions
The family of indenocorannulenes is now complete and offers
a wealth of new experimental data on fullerene fragments that
all map onto the geodesic framework of C60 and range in size
from C26H12 to C50H20. The rigidity of these fully unsaturated
hydrocarbons makes them ideal candidates for testing the
reliability of geometry calculations and NMR chemical shift
predictions as applied to large, conformationally locked, strongly
curved π-systems. The widely used B3LYP/6-31G* density
functional method was found to successfully predict the degree
of pyramidalization at the innermost carbon atoms of all seven
indenocorannulenes (POAV angles, average deviation ( 0.2°),
Supporting Information Available: Experimental details,
spectroscopic characterization data for new compounds, crystal-
lographic information files (CIF), XYZ coordinates for all
calculated structures, calculated 13C NMR chemical shifts, and
complete ref 44. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
JA9031852
bowl depths (average deviation ( 0.015 Å), and H NMR
chemical shifts (GIAO method, average deviation ( 0.09 ppm).
The UV-vis absorption spectra of these extended π-systems
are rich in detail but have not been analyzed.
(46) The bowl depth of corannulene has actually been determined
independently four times by X-ray crystallography, and the results
vary slightly as a function of the temperature at which the data were
collected. The reported values (Å) are as follows (temp K): 0.877
(90),16 0.876 (173),16 0.872 (203),3c 0.862 (293-303).3c The average
bowl depth from all these different temperature experiments is
calculated to be 0.870(8), accounting for the temperature effect in the
estimated standard deviation (temperature error). The crystal structures
of the seven indenocorannulenes were all determined at temperatures
between 90 and 193 K.
(47) Scott, L. T. Angew. Chem., Int. Ed. 2009, 48, 436–437.
(48) For related work, see: (a) Hill, T. J.; Hughes, R. K.; Scott, L. T.
Tetrahedron 2008, 64, 11360–11369. (b) Steinberg, B. D.; Scott, L. T.
Angew. Chem., Int. Ed. Published Online: May 28, 2009 (DOI:
10.1002/anie.200901025).
The most important conclusion to be drawn from this work
is the proof of principle that highly distorted π-systems, with
curvatures surpassing that of C60, can be synthesized by well-
understood chemical reactions in solution, without any recourse
to flash vacuum pyrolysis or other high-energy gas phase
methods. Our syntheses of the indenocorannulenes constitute
the first demonstration that the stepwise introduction of curva-
ture can be pushed all the way to produce a level of curvature
equal to that of C60 and even beyond. Since all carbon nanotubes
and fullerenes that are expected to exist as stable entities under
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10545