Synthesis of highly strained π-bowls from sumanene

Article abstract of DOI:10.1021/ja9031693

(Figure Presented) The short step synthesis of highly strained naphtosumanenes was successfully achieved from sumanene based on a nonpyrolytic approach.

Full text of DOI:10.1021/ja9031693

Published on Web 07/21/2009
Synthesis of Highly Strained π-Bowls from Sumanene
Toru Amaya, Takuto Nakata, and Toshikazu Hirao*
Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity,
Yamada-oka, Suita, Osaka 565-0871, Japan
Received April 21, 2009; E-mail: hirao@chem.eng.osaka-u.ac.jp
Scheme 1. Synthetic Strategy for 2 (C42H18
)
Bowl-shaped π-conjugated carbon molecules (geodesic polyare-
nes, buckybowls, or π-bowls) are considered to be another group
of key materials in addition to C60 and carbon nanotubes in the
curved π-conjugated carbon systems. Most of the investigation in
π-bowl chemistry has been performed on corannulene (C20H10,
Figure 1) and its derivatives.1 Deeper π-bowls are more interesting
because they may have properties more similar to those of fullerenes
or nanotubes. Therefore, it is important to develop their efficient
synthetic routes. Furthermore, highly strained π-bowls are still
challenging targets in synthetic organic chemistry.2 To this end,
flash vacuum pyrolysis (FVP) has proven to be an effective tool
for forming curved bonds.3 In fact, highly strained π-bowls, such
as hemifullerene (C30H12, Figure 1),4 circumtrindene (C36H12),5 and
even C60,6 were synthesized using this methodology. On the other
hand, a nonpyrolytic synthetic strategy has been strongly required
for the elaborate synthesis of this class of molecules because the
reaction conditions of FVP are quite severe and an employable
functional group is limited. Recently, Scott and co-workers reported
the synthesis of the highly strained bowl C50H20 from corannulene
using intramolecular Mizoroki-Heck reactions as a key step.2
Meanwhile, we achieved the synthesis of sumanene (1, C21H12,
Figure 1) based on the nonpyrolytic approach7 and have studied
the structure,8 dynamics,9 derivatization,10 complexation,11 and
application for organic electrical materials.12 Herein, we report the
facile synthesis of highly strained π-bowls 6, 11a, 11b, and
trinaphtosumanene 2 (C42H18) from 1.
reaction mixture was allowed to warm to room temperature,
resulting in the disappearance of the aldehyde in the 1H NMR. The
usual workup and purification through silica-gel chromatography
gave the desired 6 in 98% yield. The structure was confirmed by
1H and 13C NMR, IR, and HRMS. The structural optimization of 6
using molecular orbital calculation (B3LYP/6-31G(d,p)) showed a
deeper and more highly strained bowl structure than 1. The bowl
depth at carbon O and Haddon’s π-orbital axis vector (POAV)13
at hub carbon P are 1.33 Å and 10.36°,14 respectively (Scheme
2I). On the contrary, the corresponding bowl depth and POAV for
1 are 1.11 Å and 8.8°,15 respectively. Bowl-to-bowl inversion was
examined by the conversion to dideuterated 7a, where the inversion
is equivalent to the isomerization between the diastereomers 7a
and 7b (see ref 16). As the inversion was not observed at room
temperature, the equilibration between 7a and 7b was monitored
in mesitylene-d12 at 140 °C by 1H NMR. Growing of the peaks for
exo-protons (δ 4.42 and 4.54) was observed over a period of hours,
as shown in Figure S2a. The observed rate constant (k) and inversion
barrier (∆G‡) of 7 are 8.20 × 10-5 s-1 and 32.2 kcal/mol (for details,
see Figure S2),16 which is almost 10 kcal/mol higher than that of
1. The inversion barrier was approximately reproduced by calcula-
tions (31.4 kcal/mol from B3LYP/6-311+G(2d,p)//B3LYP/6-
31G(d,p); see Figure S3).
Figure 1. Corannulene, sumanene (1), and hemifullerene.
Our synthetic strategy for 2 possessing the hemifullerene skeleton
is outlined retrosynthetically in Scheme 1. We envisaged that 2 is
constructed via intramolecular dehydrative benzannulation of the
trialdehyde via 1,2-addition of the benzylic anions. Such a trial-
dehyde would arise from the tribromide 3a through the Suzuki-
Miyaura coupling reaction with 2-formylphenylboronic acid. Tri-
bromide 3a would be prepared from 1 under aromatic bromination
conditions.
To investigate the key benzannulation reaction, mononaphto-
sumanene 6 was chosen as the first target molecule (Scheme 2I).
Monoaldehyde 5 was prepared from monobromide 412 in 93% yield
via the Pd-catalyzed cross-coupling reaction with t-Bu3P as a ligand.
The benzannulation reaction was monitored by 1H NMR. After the
treatment of 5 with excess KN(SiMe3)2 in THF-d8 at <-80 °C, the
With the success of the monoannulation reaction, we next aimed
toward the synthesis of di- and trinaphtosumanenes. Treatment of
1 with 3.3 equiv of Br2 gave a mixture of dibromides 8 and
tribromides 3 (Scheme 2II), the separation of which being difficult
at this stage. The mixture underwent the next Suzuki-Miyaura
coupling reaction to give dialdehydes 9a,b and trialdehydes 10a,b
in two steps in 45 and 41% yields, respectively. The formation of
dialdehydes 9c,d was not observed, which might be attributable to
the bromination reaction rather than the cross-coupling reaction.
The ratio of 9b to 9a was 1.2.17 In the case of trialdehyde, the
desired 10a was obtained in a lower ratio than that estimated from
the statistics (10a/10b ) 1/5).17 These ratios of 9a/9b and 10a/
10b reflect the regioselectivity in the bromination reaction and could
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10810 J. AM. CHEM. SOC. 2009, 131, 10810–10811
10.1021/ja9031693 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2a
such as CHCl3, CH2Cl2, and toluene as well as polar solvents, such
as THF, acetone, etc. 1H NMR spectrum consists of only six peaks
(δ 8.60, 8.44, 8.04, 7.97, 7.68, 7.56 in CD2Cl2/CS2 ) 1/1). The
structure of 2 was also supported by 2D NMR (COSY, NOESY,
HMQC, and HMBC), IR, and HRMS. The structural optimization
of 11a,b and 2 using molecular orbital calculation (B3LYP/6-
31G(d,p)) showed the deeper bowl depth and POAV. The estimated
bowl depth and POAV for 2 reach 1.37 Å at carbon S and 10.75°
at hub carbon T (Scheme 1),14 which are comparable to the
corresponding values in hemifullerene.18 Accordingly, bowl inver-
sion is unlikely to occur with 2.19
In conclusion, the short step synthesis (only three steps from 1)
of highly strained naphtosumanenes was successfully achieved
through a nonpyrolytic approach. It should be noted that the
annulation reaction proceeds in quite high yield. This strategy will
allow the facile synthesis of various highly strained π-bowls.
Acknowledgment. The authors deeply thank Ms. Toshiko
Muneishi for the measurement of NMR spectra. This work was
financially supported in part by a Grant-in-Aid for Young Scientists
(B) from Japan Society for the Promotion of Science.
Supporting Information Available: Experimental details, spectral
data for 2, 3, and 5-11. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
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a Reagents and conditions: (I) (a) 2-formylphenylboronic acid, Pd2(dba)3,
t-Bu3P, Cs2CO3, dioxane, 80 °C, 24 h; (b) KN(SiMe3)2, THF-d8, <-80 °C
to rt, 1 h; (c) t-BuLi, THF-d8, <-80 °C to rt, 1 h, then excess CH3OD,
<-80 to -40 °C. (II) (d) Br2, CHCl3/CH3CN ) 4/1, 60 °C, 4.5 h; (e)
2-formylphenylboronic acid, Pd2(dba)3, t-Bu3P, Cs2CO3, dioxane, 80 °C,
24 h, 9a/9b ) 1/1.2, 10a/10b ) 1/5. (f) KN(SiMe3)2, THF-d8, <-80 °C to
rt, 1 h, 11a/11b ) 1/1.2. (g) KN(SiMe3)2, THF-d8, <-80 °C to rt, 1 h.
(12) Amaya, T.; Seki, S.; Moriuchi, T.; Nakamoto, K.; Nakata, T.; Sakane, H.;
Saeki, A.; Tagawa, S.; Hirao, T. J. Am. Chem. Soc. 2009, 131, 408.
(13) The POAV angle is widely used to gauge the curvature of a given center
in bowls: Haddon, R. C. J. Am. Chem. Soc. 1987, 109, 1676.
(14) The details for the bowl depth and POAV are shown in Figure S1c.
(15) Calculated from the crystal structure of 1.
(16) Bowl-to-bowl inversion between 7a and 7b.
be rationalized from the HOMO density of monobromide 4 and
dibromides 8, as shown in Figure S4a and S4b, respectively.
Annulation reaction of 9a,b was carried out using KN(SiMe3)2
to give dinaphtosumanene 11a,b (C35H16) almost quantitatively
(11a/11b ) 1/1.2) (Scheme 2II). For triannulation, the reaction of
10a proceeded well to afford the desired 2 (C42H18) in almost
quantitative yield based on C3 symmetric tribromide 3a, where the
corresponding dibenzannulated monoaldehyde from 10b was also
observed from the EI-MS analysis. Compensating for the strain
energy by the aromatization is likely to make the annulation reaction
proceed efficiently. The solubility of 2 is low in various solvents,
(17) Determined by 1H NMR.
(18) Bowl depth and POAV are 1.39 Å and 10.83°, which are determined from
the reported crystal structure. Petrukhina, M. A.; Andreini, K. W.; Peng,
L.; Scott, L. T. Angew. Chem., Int. Ed. 2004, 43, 5477.
(19) Estimated to be 63.8 kcal/mol by the DFT calculation (B3LYP/6-
311+G(2d,p)//B3LYP/6-31G(d,p); see Figure S3).
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