Asymmetric synthesis of a chiral buckybowl, trimethylsumanene

Article abstract of DOI:10.1021/ja802822k

The first asymmetric synthesis of a chiral buckybowl, a C₃ symmetric (C)-8,13,18-trimethylsumanene (1), was achieved by employing a synthetic strategy that translates chirality at sp³ centers into bowl chirality. The synthesis features a syn selective cyclotrimerization of an enantiopure halonorbornene derivative, tandem ring-opening/closing olefin metathesis reactions, and DDQ oxidation at low temperature. The bowl-to-bowl inversion energy of 1 was determined as 21.6 kcal/mol by circular dichroism spectra measurement. Copyright

Full text of DOI:10.1021/ja802822k

Published on Web 06/14/2008
Asymmetric Synthesis of a Chiral Buckybowl, Trimethylsumanene
†
,†,‡
Shuhei Higashibayashi and Hidehiro Sakurai*
Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787,
Japan, and PRESTO, JST, Tokyo 102-0075, Japan
Received April 17, 2008; E-mail: hsakurai@ims.ac.jp
Buckybowls, bowl-shaped π-conjugated aromatic compounds
corresponding to partial structures of fullerenes or cap structures
of carbon nanotubes, are of importance not only as model
compounds for fullerenes but also for their own chemical and
1
physical properties. The science of buckybowls has recently grown
2
,3
drastically since practical synthetic routes have been developed.
However, there are still numerous potential buckybowl targets
needed to clarify their physical properties dependent on their unique
architectures. Among such structural features, many buckybowls
Figure 1. Racemization of 1 through a bowl-to-bowl inversion.
Scheme 1. Strategy for the Asymmetric Synthesis of (C)-(M)-1
4
possess “bowl chirality” derived from their three-dimensional
5
geometry, similar to chiral fullerenes and carbon nanotubes.
Methods to control the bowl chirality can potentially be applied to
the related chiral fullerenes and carbon nanotubes as well. In
addition, chiral aromatic compounds are expected to contribute to
a variety of applications such as asymmetric molecular recognition,
homochiral crystal organic materials, and chiral ligands for orga-
nometallic catalysis. Although racemic syntheses of chiral bucky-
6
bowls have been reported, no asymmetric synthesis has appeared
since the synthetic strategies have not yet been established. In this
3
paper, we report the first asymmetric synthesis of a C symmetric
chiral buckybowl, (C)-(M)-8,13,18-trimethylsumanene (1), by a
rational synthetic approach.
bornene derivative 3 could be obtained by cyclotrimerization of a
chiral halonorbornene derivative (1S,4S)-4. By adopting this ap-
proach, the bowl chirality of 1 is controlled by the sp chirality of
4
3
Most of the approaches toward the construction of bowl-shaped
aromatic structures involve carbon-carbon bond formations in
which planar π-conjugated precursors are converted to the bowl
the starting norbornene derivative.
Selective formation of a syn-benzocyclotrimer through a regi-
oselective cyclotrimerization of a halonorbornene derivative under
palladium nanocluster conditions has been recently developed by
2
d–g,3,6
shape.
Such reactions usually require harsh conditions,
therefore achieving asymmetrization of the bowl chirality during
8
our group. The conditions were applied to the cyclotrimerization
6
the bowl formation would be quite difficult. In contrast, the
9
of (1S,4S)-4 to afford the desired syn-benzocyclotrimer 5 without
synthesis of sumanene features a stepwise conversion strategy
contamination of the diastereomer anti-5 (Scheme 2). The carbonyl
groups of 5 were converted to methyl-substituted olefins through
alkenyl phosphates by cross-coupling reactions with MeMgI,
3
through a bowl-shaped intermediate with sp carbons under milder
2
a
reaction conditions. This strategy should be more amenable to
the introduction of an asymmetric element by addition of functional
groups or substituents at the desired positions of sumanene. Based
on the synthesis of sumanene, our strategy for chiral buckybowl 1
10
giving the desired benzonorbornene derivative 3.
Treatment of 4 with Grubbs first generation catalyst resulted in
a mixture of only ring-opening reaction products. Grubbs second
generation catalyst was effective for the ring-closing reaction. Thus,
the tandem ring-opening/closing olefin metathesis reactions were
successfully achieved with successive treatment by these two
catalysts. The final aromatization step was carried out with 2,3-
dichloro-5,6-dicyano-p-benzoquinone at 0 °C in a short time (1
min). The resulting reaction mixture was quickly purified by silica
gel chromatography at low temperature (<-20 °C) to afford the
desired chiral buckybowl (C)-1.
3
relies on conversion from an sp chirality to the bowl chirality.
The key reaction steps for the asymmetric synthesis of (C)-1 are
summarized in Scheme 1. The aromatization step was designed
3
for the final stage of synthesis, where the sp chirality of 2 could
be converted to the bowl chirality of (C)-1.
The bowl-to-bowl inversion energy barrier must be taken into
account at this step, because this inversion corresponds to a
racemization process for a chiral buckybowl (Figure 1). The bowl-
to-bowl inversion energy barrier of 1 is estimated to be ca. 21 kcal/
7
The chirality of thus-obtained (C)-1 was confirmed by measure-
ment of the circular dichroism (CD) spectra. The intensity of the
CD spectra at -40 °C scarcely changed but gradually decreased at
10 °C (Figure 2). The bowl-to-bowl inversion energy barrier of 1
mol. Since the energy barrier corresponds to ca. 2 h half-life time
of racemization at 0 °C, the aromatization step must be carried out
at a low temperature. The bowl-shaped structure of 2 could be
constructed from a benzonorbornene derivative 3 utilizing a tandem
ring-opening/closing olefin metathesis approach. The benzonor-
3
in CH CN was determined to be 21.6 kcal/mol by tracing the decay
of the CD spectra at 247 and 282 nm at 10 °C (Supporting
Information). This value is in good accordance with the estimated
one.
†
Institute for Molecular Science.
PRESTO.
‡
8
592 9 J. AM. CHEM. SOC. 2008, 130, 8592–8593
10.1021/ja802822k CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
a
Scheme 2. Synthesis of (C)-8,13,18-Trimethylsumanene (1)
As described above, we have succeeded in the novel asymmetric
3
synthesis and isolation of a C symmetric chiral buckybowl 1 by
3
employing a synthetic strategy with the conversion from sp
chirality to bowl chirality. The present strategy will also provide a
3
versatile path for derivatization of C symmetric chiral buckybowls
by introduction of a variety of substituents at desired positions.
Acknowledgment. Support for this work was generously
provided by MEXT, JST, the Tokuyama Foundation, the Ishikawa
Carbon Foundation, the Asahi Glass Foundation, and the Sumitomo
Foundation. We thank Ms. Sachiko Nakano for the preparation of
5
.
Supporting Information Available: Experimental procedures and
spectral data for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
a
Reagents and conditions: (a) Pd(OAc)2 5 mol %, PPh3 10 mol %,
Bu4NOAc 1000 mol %, Na2CO3, MS 4 Å, 1,4-dioxane, 100 °C, 2 h, 55%;
References
(
b) NaN(SiMe3)2 350 mol %, (2,6-diMePhO)2P(O)Cl 350 mol %,
(
1) For recent reviews,(a) Wu, Y.-T.; Siegel, J. S. Chem. ReV. 2006, 106, 4843–
P(O)(NMe2)3 350 mol %, THF, -80 °C, 75%; (c) Pd(OAc)2 5 mol %,
PCy3 ·HBF4 10 mol %, MeMgI 400 mol %, THF, 40 °C, 2 h, 72%; (d)
Grubbs 1st generation catalyst 50 mol %, CH2Cl2, under ethylene, 40 °C,
4
867. (b) Tsefrikas, V. M.; Scott, L. T. Chem. ReV. 2006, 106, 4868–
4884. (c) Sygula, A.; Rabideau, P. W. In Carbon-Rich Compounds; Haley,
M. M., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2006;
pp 529-565.
6
2
h, then Grubbs 2nd generation catalyst 50 mol %, CH2Cl2, 40 °C, 12 h,
4%; (e) 2,3-dichloro-5,6-dicyano-p-benzoquinone 600 mol %, CH2Cl2, 0
(
2) For synthesis of sumanene,(a) Sakurai, H.; Daiko, T.; Hirao, T. Science
2
003, 301, 1878. (b) Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.; Hirao,
°
C, 1 min, 68%.
T. J. Am. Chem. Soc. 2005, 127, 11580–11581. (c) Amaya, T.; Mori, K.;
Wu, H.-L.; Ishida, S.; Nakamura, J.; Murata, K.; Hirao, T. Chem. Commun.
2
3
007, 1902–1904. For other C or C3V symmetric buckybowls, see: (d)
Abdourazak, A. H.; Marcinow, Z.; Sygula, A.; Sygula, R.; Rabideau, P. W.
J. Am. Chem. Soc. 1995, 117, 6410–6411. (e) Bratcher, H. S.; Erickson,
M. S.; Zimmerman, M. S.; Scott, L. T. Angew. Chem., Int. Ed. 1997, 36,
4
06–408. (f) Imamura, K.; Takimiya, K.; Aso, Y.; Otsubo, T. Chem.
Commun. 1999, 1859–1860. (g) Ansems, R. B. M.; Scott, L. T. J. Am.
Chem. Soc. 2000, 122, 2719–2724.
(
3) For synthesis of corannulene, see:(a) Barth, W. E.; Lawton, R. G. J. Am.
Chem. Soc. 1966, 88, 380–381. (b) Scott, L. T.; Hashemi, M. M.; Bratcher,
M. S. J. Am. Chem. Soc. 1992, 114, 1920–1921. (c) Borchardt, A.;
Fuchicello, A.; Kilway, K. V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem.
Soc. 1992, 114, 1921–1923. (d) Scott, L. T.; Cheng, P.-C.; Hashemi, M. M.;
Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1997,
1
19, 10963–10968. (e) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc.
1
999, 121, 7800–7803. (f) Seiders, T. J.; Elliott, E. L.; Grube, G. H.; Siegel,
J. S. J. Am. Chem. Soc. 1999, 121, 7804–7813. (g) Sygula, A.; Rabideau,
P. W. J. Am. Chem. Soc. 2000, 122, 6323–6324.
(
4) Expressions for the chirality of buckybowls and the stereodescriptor system
for the configurations have not been officially established yet. We refer to
the chirality as “bowl chirality” for convenience. The absolute configuration
of bowl chirality of 1, 6, and 7 is shown by following two independent
stereodescriptors. (a) (C) or (A) based on fullerene nomenclature; see ref
Figure 2. (A) CD spectra of (C)-1 in CH3CN at -40 °C (red line) and
UV spectra of (()-1 in CH3CN at room temp (blue line); (B) decay of CD
spectra of (C)-1 in CH3CN at 10 °C for 3 h.
5
a. (b) (P) or (M) proposed in the following reference: Petrukhina, M.;
Andreini, K. W.; Peng, L.; Scott, L. T. Angew. Chem., Int. Ed. 2004, 43,
477–5481. (see the Supporting Information in detail)
5
a
Scheme 3. Determination of the Enantiomeric Excess of 1
(
(
5) (a) Thilgen, C.; Diederich, F. Chem. ReV. 2006, 106, 5049–5135. (b) Pen,
X.; Komatsu, N.; Bhattacharya, S.; Shimawaki, T.; Aonuma, S.; Kimura,
T.; Osuka, A. Nat. Nanotechnol. 2007, 2, 361–365, and references cited
therein.
3
6) (a) For racemic synthesis of C symmetric chiral buckybowls, see ref 2d
and e. For racemic synthesis of corannulene derivatives, see refs 1, 3b,
and following references Tsefrikas, V. M.; Arns, S.; Merner, P. M.; Warford,
C. C.; Merner, B. L.; Scott, L. T.; Bodwell, G. J. Org. Lett. 2006, 8, 5195–
5
198. (b) Wu, Y. T.; Hayama, T.; Baldridge, K. K.; Linden, A.; Siegel,
J. S. J. Am. Chem. Soc. 2006, 128, 6870–6884.
a
Reagents and conditions: (a) (()-1, LDA 1000 mol %, THF, 0 °C then
(
7) DFT calculation (B3LYP/6-311+G**) predicts inversion energy barriers
of 19.3 and 18.3 kcal/mol for 1 and sumanene, respectively. Since the
observed value for sumanene was reported to be 19.7∼20.4 kcal/mol, the
real value for 1 was estimated to be ca. 21 kcal/mol. For the observed
value of 1, see the following reference: Amaya, T.; Sakane, H.; Muneishi,
T.; Hirao, T. Chem. Commun. 2008, 76, 5–767.
Me3SiCl 1000 mol %, (()-6 45%; (b) (C)-1, LDA 600 mol %, THF, -40
C, then (R)-Ph(CF3)(MeO)CCOCl d(R)-MTPACl 1000 mol % 17%.
°
To determine the enantiomeric excess of synthetic 1, we needed
to derivatize 1 for avoiding racemization through a bowl-to-bowl
inversion. Selective introduction of trimethylsilyl groups at the
exo positions of the dibenzylic positions induces new chirality at
the sp carbons which prevents the enantiomers from racemizing
Scheme 3). However, we were unable to separate the enantiomers
of 6 using chiral HPLC. Instead, (S)-Ph(CF )(OMe)CCO groups
were introduced to enable the diastereomeric analysis. Finally the
enantiomeric excess of 1 was determined as 90% ee on the basis
(8) Higashibayashi, S.; Sakurai, H. Chem. Lett. 2007, 36, 18–19.
2
b
(9) For preparation of (1S,4S)-4 (> 99% ee), see the supporting information
and following references:(a) Hayashi, T.; Uozumi, Y. Pure Appl. Chem.
1992, 64, 1911–1916. (b) Berkessel, A.; Schroder, M.; Sklorz, C. A.;
Tabanella, S.; Vogl, N.; Lex, J.; Neudorfl, J. J. Org. Chem. 2004, 69, 3050–
3
3
056.
(10) Hayashi, T.; Fujiwa, T.; Okamoto, Y.; Katsuro, Y.; Kumada, M. Synthesis
981, 1001–1003.
11) Even the bowl-to-bowl inversion of (C)-(8R,13R,18R)-6 gives the diaste-
reomer (A)-(8R,13R,18R)-6. Trimethylsilyl groups preferring the exo
position freeze the bowl-to-bowl inversion as well (ref 2b).
11
(
1
3
(
1
of the H NMR analysis of the diastereomeric ratio of 7.
JA802822K
J. AM. CHEM. SOC. 9 VOL. 130, NO. 27, 2008 8593