Synthesis and structural analysis of a highly curved buckybowl containing corannulene and sumanene fragments

Article abstract of DOI:10.1021/ja2067725

Buckybowls 7, 9, and 10 were prepared from benzo[k]fluoranthene 6 and fluoranthene 8 using straightforward procedures involving key palladium-catalyzed cyclization reactions. The structures of bowl-shaped molecules 7 and 10 were determined by using X-ray

Full text of DOI:10.1021/ja2067725

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Synthesis and Structural Analysis of a Highly Curved Buckybowl
Containing Corannulene and Sumanene Fragments
Tsun-Cheng Wu,† Hsin-Ju Hsin,† Ming-Yu Kuo,‡ Ching-Hsiu Li,‡ and Yao-Ting Wu*,†
†Department of Chemistry, National Cheng Kung University, No. 1 Ta-Hsueh Road, 70101 Tainan, Taiwan
‡Department of Applied Chemistry, National Chi Nan University, No. 1 University Road, 54561 Puli, Nantou, Taiwan
S Supporting Information
b
Scheme 1. Preparation of Buckybowls 7 and 9a
ABSTRACT: Buckybowls 7, 9, and 10 were prepared from
benzo[k]fluoranthene 6 and fluoranthene 8 using straight-
forward procedures involving key palladium-catalyzed cycli-
zation reactions. The structures of bowl-shaped molecules 7
and 10 were determined by using X-ray crystallographic
methods. The observed p-orbital axis vector (POAV) angle
of 7 was found to be 12.8°.
ntroducing five-membered rings into sp2 hexagonal networks
leads to the formation of aromatic bowls. Representative
I
examples of substances in this family include corannulene (1)1
and sumanene (2),2 both of which are elementary subunits of
buckminsterfullerene. However, the structural, chemical, and
physical properties of these substances differ markedly. The so-
called mixed buckybowls, molecules that contain both corannu-
lene and sumanene fragments, have relatively high degrees of
curvature and deep bowl depths. Indeed, mixed buckybowls,
such as circumtrindene (C36H12)3 and hemifullerene (C30H12),4
exhibit these expected structural characteristics.5 In this regard,
compound 3 (C40H10) is also a mixed buckybowl that is
predicted to be the simplest capped nanotube.6
a Reagents and conditions: (a) C6H5I (1.5 equiv), Pd(OAc)2 (5 mol %),
AgOAc (1 equiv), p-xylene, 110 °C, 36 h; (b) Pd(PCy3)2Cl2 (40 mol %),
DBU, DMF, 160 °C, 36 h; (c) 2-butyne (3 equiv), Rh(PPh3)3Cl (2.5
mol %), p-xylene, 110 °C, 60 h; (d) Pd(PCy3)2Cl2 (40 mol %), DBU,
NMP, 160 °C, 36 h.
synthesis of pentaindenocorannulene 49a,c and buckybowl
C42H189b from corannulene and sumanene, respectively. Bucky-
bowl 4 is considered to be a possible precursor to an “arm-chair”
[5,5]-nanotube. Unlike the two synthetic strategies presented
above, the one employed in our recent studies involves direct
generation of highly curved buckybowls from easily accessible
planar precursors. Below, we describe the results of this effort
that focused on the preparation of the bowl-shaped substances 7
and 9 selected as targets because of their rarity10 and potential
applications to the synthesis of 3.
Chart 1. Representative Examples of Buckybowls
The first step of the route devised for the synthesis of 7
employed a recently developed procedure11 involving a Pd-
catalyzed [(2 + 2) + 2] cycloaddition reaction of 1,8-bis(aryl-
ethynyl)naphthalene 5 with iodobenzene to produce benzo-
[k]fluoranthene 6 (Scheme 1). Cyclization of 6 in the presence
of DBU and Pd(PCy3)2Cl2 then generated the desired easily
purified product 7 in 31% yield. This synthetic approach is both
simpler and more efficient than those that rely on the use of
conventional methods devised to date.
The currently available mixed buckybowls are prepared ex-
clusively by using high-temperature flash vacuum pyrolysis
(FVP) methods.3ꢀ5 The conditions required for FVP have
limited functional group tolerance, and they can potentially
cause thermal rearrangements of molecular frameworks.7 Owing
to the fact that metal-catalyzed carbonꢀcarbon bond formation
occurs under mild conditions, synthetic protocols employing
solution phase processes do not suffer from these drawbacks.7,8
Several less strained bowls have been produced in this manner.
However, for the construction of highly strained π-bowls, a
special strategy is needed that relies on extension of the backbone
of a smaller bowl.9 Examples of this approach are found in the
The results of our recent studies12 indicated that crowded
environments and the short distances between the haloaryl
and the methyl groups enhance the efficiencies of palladium
Received: July 20, 2011
Published: September 20, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja2067725 J. Am. Chem. Soc. 2011, 133, 16319–16321
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Table 1. Bowl-to-Bowl Inversion Barriera
Exptl
B3LYP/cc-pVDZ
M06-2X/cc-pVDZ
9.9
1
ca. 11
ca. 20
>40
9.1
18.2
56.2
24.1
2
9
59.1
26.9
10
>23.5
a kcal molꢀ1
.
promoted benzylic CꢀH bond activation reactions. In accord
with the earlier findings, the bis-methylene bridged buckybowl 9,
which has a backbone that is similar to that of 7, can be obtained
by treatment of the tetrachlorofluoranthene derivative 8 with
Pd(PCy3)2Cl2. The starting fluoranthene 8 was prepared by
employing a simple Rh-catalyzed cocyclotrimerization reaction
of diyne 5 with 2-butyne.9d It should be noted that the Pd-
catalyzed cyclization reaction of 8 produced a mixture of 9 and 10
in a 7:3 ratio (28% yield). Although simple chromatographic
purification did not lead to complete separation of these sub-
stances, both 9 and 10 were obtained in their pure forms. It should
be emphasized that the synthetic approach we have developed for
the preparation of methylene-bridged buckybowls differs from the
one developed earlier by Sakurai et al.2a In the 1H NMR spectra of
9 and 10, each pair of methylene protons appears with respective
coupling constants of 20.0 and 19.8Hz, which are typical values for
AB systems and also indicate that they are bowls.
Figure 1. Crystallographic structures and the POAV pyramidalization
angles of buckybowls 7 (top) and 10 (bottom). The POAV angles for 10
are average values of those in two nonequivalent molecules.
With buckybowls 7, 9, and 10 in hand, the focus was turned
toward determining accurate values for the bowl-to-bowl inver-
sion barriers of the latter two substances using variable-tempera-
ture NMR. Based on the results of earlier structure/energy
correlation studies by Siegel and co-workers,13 the order of the
bowl-to-bowl inversion barriers should be 7 > 9 > 10. We
Density functional theory (DFT) calculations were performed
to examine bowl-to-bowl inversions of buckybowls 9 and 10
(Table 1). Optimized geometries and frequencies of these
substances were calculated at the B3LYP/cc-pVDZ and M06-
2X/cc-pVDZ levels using the Gaussian 09 program. The com-
puted bowl-to-bowl inversion barriers were observed to be close
to those found experimentally.
1
observed that the resonances for protons in the 500 MHz H
NMR spectra of 9 and 10 in o-dichlorobenzene-d4 at both room
temperature and 443 K were essentially the same (see Support-
ing Information). Moreover, the inversion dynamics of 9 were
investigated by carrying out the 2D EXSY NMR experiments
with its deuterium substituted derivative exo-11, which was
prepared by using a previously described procedure14 involving
treatment of a solution of 9 in THF-d8 with t-BuLi and
subsequent quenching with methanol-d4. The 2D EXSY spec-
trum of a solution of exo-11 in o-dichlorobenzene-d4 at 443 K
(see Supporting Information) did not reveal that exchange of the
methylene protons takes place.
X-ray quality crystals of 7 and 10 were obtained by slow
evaporation of CH2Cl2/MeOH solutions at ambient tempera-
ture.16 As expected, the X-ray crystallographic structure of 7
shows that it has a deeper bowl depth compared to 10. The bowl
depths of the corannulene and sumanene cores of 7 were
determined to be 1.24 and 1.48 Å, respectively. These values
are significantly larger than those of the parent corannulene (0.87
Å)17 and sumanene (1.11 Å).18 Finally, the bowl depth for 10 was
observed to be 1.04 Å, which also exceeds that of corannulene.
The POAV pyramidalization angle is useful for quantifying the
curvature that exists in buckybowls. For example, the values of
planar benzene and C60 are observed to be 0° and 11.6°,
respectively.19 As shown in Figure 1, the maximum POAV angles
of 7 (12.75°) and 10 (11.8°) exceed that of C60. It is interesting
to note that bowl-shaped molecules rarely have such high POAV
angles.20 The maximum curvatures in 7 and 4 (12.36°ꢀ12.99°,
avg. 12.69°) are comparable. To the best of our knowledge, 4 and
7 are the most curved π-bowls observed to date. The methylene
bridge in 10 actually enhances the curvature as shown by
comparisons of the POAV angles of the central Hub ring.
The results of the studies described above demonstrate that a
simple solution phase synthetic approach exists for preparing
buckybowls 7, 9, and 10 and that these substances possess in-
teresting structural properties. Continuing investigations of the
physical properties of 7, 9, and 10 and potential applications of
The experimental results demonstrate that buckybowl 9 has a
much higher inversion barrier (>40 kcal molꢀ1) in comparison to
those of both corannulene (ca. 11 kcal mol )
ꢀ1 13,15 and sumanene
(ca. 20 kcal molꢀ1).2a,14 Due to its availability in only limited
amounts, buckybowl 10 could not be converted into an analog
that is suitable for EXSY studies. However, it is expected that its
inversion barrier is slightly higher than that of 12 (ca. 23.5 kcal
molꢀ1) because it has a higher coalescence temperature, bowl
depth, and maximum p-orbital axis vector (POAV) angle (see
below).8b,13
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dx.doi.org/10.1021/ja2067725 |J. Am. Chem. Soc. 2011, 133, 16319–16321

Journal of the American Chemical Society
COMMUNICATION
the synthetic method to the construction of the capped tubular
(10) Buckybowl 7 was first prepared in a 0.14% yield from 7,12-
bis(2-bromophenyl)benzo[k]fluoranthene by using FVP at 1100 °C,
but its structure has not been analyzed by X-ray crystallography; see:
Clayton, M. D.; Rabideau, P. W. Tetrahedron Lett. 1997, 38, 741.
(11) Kung, Y.-H.; Cheng, Y.-S.; Tai, C.-C.; Liu, W.-S.; Shin, C.-C.;
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J. 2010, 16, 5909.
(12) Hsiao, C.-C.; Lin, Y.-K.; Liu, C.-J.; Wu, T.-C.; Wu, Y.-T. Adv.
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(13) Seiders, T. J.; Baldridge, K. K.; Grube, G. H.; Siegel, J. S. J. Am.
Chem. Soc. 2001, 123, 517.
molecule 3 are currently underway.21
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterizations, NMR spectra, crystal data, and computational
results. This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Amaya, T.; Sakane, H.; Muneishi, T.; Hirao, T. Chem. Commun.
2008, 765.
(15) (a) Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem.
Soc. 1992, 114, 1920. (b) Sygula, A.; Abdourazak, A. H.; Rabideau, P. W.
J. Am. Chem. Soc. 1996, 118, 339.
’ AUTHOR INFORMATION
Corresponding Author
ytwuchem@mail.ncku.edu.tw
(16) X-ray crystallographic data for compounds 7 (CCDC-835497)
and 10 (CCDC-835496) can be obtained from the Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/const/retrieving.html.
(17) Petrukhina, M. A.; Andreini, K. W.; Mack, J.; Scott, L. T. J. Org.
Chem. 2005, 70, 5713.
’ ACKNOWLEDGMENT
This work was supported by the National Science Council of
Taiwan (NSC 98-2113-M-006-002-MY3 and NSC 99-2113-M-
260-007-MY3). We also thank Prof. S.-L. Wang and Ms. P.-L.
Chen (National Tsing Hua University, Taiwan) for the X-ray
structure analyses and the National Center for High-perfor-
mance Computing for providing computational resources. Mr.
Yu-Sung Cheng is gratefully acknowledged for his early studies in
synthesizing 7.
(18) Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.; Hirao, T. J. Am.
Chem. Soc. 2005, 127, 11580.
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(20) For other buckybowls with maximum POAV angles larger than
12°, see: circumtrindene (ref 3c) and indenocorannulene family (ref 9a).
The central pentagon of 3 is calculated to have a POAV angle of ca. 12.2°
(ref 6).
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