Regio- and diastereo-controlled synthesis of bis(formylmethano)[60]fullerenes and their application to the formation of [60]fullerene pearl-necklace polyimines

Article abstract of DOI:10.1016/j.tetlet.2006.02.148

The tether-directed method was firstly applied to the biscyclopropanation of [60]fullerene via the addition-elimination reaction of bis(sulfonium ylide)s to give bis(formylmethano)[60]fullerenes with satisfactory regio- and stereoselectivity. The equatori

Full text of DOI:10.1016/j.tetlet.2006.02.148

Tetrahedron Letters 47 (2006) 3095–3098
Regio- and diastereo-controlled synthesis
of bis(formylmethano)[60]fullerenes and their application
to the formation of [60]fullerene pearl-necklace polyimines
Hiroshi Ito, Yasuhiro Ishida and Kazuhiko Saigo*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Received 4 February 2006; revised 22 February 2006; accepted 27 February 2006
Available online 20 March 2006
Abstract—The tether-directed method was firstly applied to the biscyclopropanation of [60]fullerene via the addition–elimination
reaction of bis(sulfonium ylide)s to give bis(formylmethano)[60]fullerenes with satisfactory regio- and stereoselectivity. The equato-
rialbisadduct thus obtained was used for the polycondensation with an aromatic diamine to afford the corresponding pearl-necklace
polyimine with satisfactorily high degree of polymerization.
Ó 2006 Elsevier Ltd. All rights reserved.
The attractive structure and properties of fullerenes have
driven chemists to apply them and/or their derivatives in
various fields.1 From the viewpoint of materials science
and technology, the synthesis of fullerene-containing
polymers is a key issue to improve the processability
of fullerenes without harming their unique properties.
Fullerene-containing polymers can be roughly classified
into two categories, main-chain type and side-chain
type.2–4 Among them, polymers containing fullerene
cores in their main chain, so-called pearl-necklace poly-
mers, are of great interest owing to their unique struc-
necklace polyamides by the condensation of a regio-
and diastereo-regulated bis(carboxymethano)C60 with
aromatic diamines.4 The resultant polyamides were of
particular interest, owing to the special properties of
the bridge-head carbons of the cyclopropane rings; the
C60 cores and the carbonyl functions, embedded in the
main chain, might conjugate through the cyclopropane
carbons having an sp2-character to some extent, which
arises from the highly distorted structure of the cyclo-
propane rings. On the other hand, in order to develop
a novel building block for C60-containing functional
materials other than carboxymethano C60 derivatives,
we have investigated the synthesis and reactions of a
C60 derivative with a formylmethano group;5 the formyl-
methano C60, prepared by the nucleophilic addition
and elimination of a sulfonium ylide, could efficiently
condense with aromatic amines to afford imine bond-
connected C60-aromatic ring arrays, for which photo-
induced electron transfer between the C60 core and the
aromatic group was observed. These results indicate
that C60 derivatives with two formylmethano groups
would be attractive monomers, which should easily give
polyimines by the condensation with aromatic diamines.
Compared with the carboxyl groups in bis(carboxy-
methano)C60s, formylmethano groups might be intro-
duced to a C60 core without any protecting group, and
their sufficient reactivity would allow us to use the for-
myl groups directly for a polycondensation. Further-
more, the resultant polyimines were expected to be
ture; chromophores are arrayed in
a sequence-
regulated manner along with the direction of the main
chain. For the creation of such pearl-necklace polymers,
the preparation of bifunctionalized fullerenes is indis-
pensable. Even at present, however, pearl-necklace poly-
mers have been rarely reported, mainly because of
difficulty in the preparation of structurally ordered,
bifunctionalized fullerenes.3
As a part of our ongoing program to develop fullerene-
containing functional polymers, we have reported the
synthesis and properties of [60]fullerene (C60) pearl-
Keywords: Bifunctionalization; Formyl group; Fullerene; Polyimine;
Template synthesis.
*
Corresponding author. Tel.: +81 3 5841 7267; fax: +81 3 5802
3348; e-mail: saigo@chiral.t.u-tokyo.ac.jp
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.148

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H. Ito et al. / Tetrahedron Letters 47 (2006) 3095–3098
more fascinating materials for the development of novel
electrochemical and optical devices, considering the fully
conjugated structure of the azomethine units and aro-
matic rings.6 Here we report the regio- and diastereo-
selective synthesis of bis(formylmethano)C60s and their
successful condensation with an aromatic diamine to
generate novel C60 pearl-necklace polyimines.
to give the corresponding bis(a-formylsulfonium ylide)s
1a–c (64–99%, two steps).9
The bis(a-formylsulfonium ylide) 1a derived from
hydroquinone was used for the biscyclopropanation at
rt. Unfortunately, however, C60 was not consumed at
all (Table 1, entry 1). Even when the reaction was con-
ducted at 80 °C, only a product with high polarity was
generated. The spectroscopic analyses of the unidenti-
In order to regio- and diastereoselectively introduce two
formylmethano groups on a C60 core, we attempted to
apply the tether-directed bifunctionalization strategy,
which is known to be applicable to various kinds of
C60 functionalization reactions, such as the Bingel reac-
tion, [4+2] cycloaddition, and [3+2] cycloaddition.7,8
Up to present, however, tether-directed method has not
been applied to the biscyclopropanation with bis(sulfo-
nium ylide)s, even though some functional groups
including a formyl group can be introduced only by this
method. Therefore, we designed three bis(a-formylsulfo-
nium ylide)s 1a–c in order to achieve regio- and diaste-
reo-controlled synthesis of bis(formylmethano)C60s. As
the core part of 1a–c, a 1,4-phenylene moiety was used,
because bismalonates with a similar tethered structure
have been known to realize the highly regio- and diaste-
reo-controlled double Bingel reaction.7 The bis(a-formyl-
sulfonium ylide)s 1a–c were successfully synthesized as
follows (Scheme 1): The diols 2a–c were converted to the
dimesylates 3a–c (93–95%), which were then trans-
formed to the diiodides 4a–c (87–96%). The diiodides
4a–c were treated with a lithium enolate, generated from
(methylthio)acetaldehyde N,N-dimethylhydrazone, to
afford the bis(a-sulfenylated hydrazone)s 5a–c (64–
81%). The bis(a-sulfenylated hydrazone)s 5a–c thus ob-
tained were hydrolyzed with an aqueous hydrochloric
acid solution to give the corresponding a,a0-disulfenyl-
ated dialdehydes 6a–c (69–85%). Finally, the a,a0-disulf-
enylated dialdehydes 6a–c were bismethylated with
Me3OBF4, and the resultant bis(sulfonium tetrafluoro-
borate)s were treated with an aqueous alkaline solution
1
fied product with MALDI-TOF-MS and H NMR sug-
gested that only one of the two sulfonium ylide moieties
in 1a participated in the cyclopropanation and the other
decomposed (entry 2). The oxygens adjacent to the phen-
ylene core are likely to decrease the flexibility of the
tether part, probably due to the partial conjugation with
the aromatic ring. As a result, the monocyclopropanated
C60 generated by the first addition might be not able to
take a suitable conformation for the second intramole-
cular cyclopropanation. With the result in mind, we next
attempted the cyclopropanation with bis(a-formylsulfo-
nium ylide) 1b possessing ethylene spacers between the
phenylene core and the sulfonium ylide units. When
the reaction was conducted at rt, unreacted C60 was
recovered quantitatively once again (entry 3). To our
delight, the reaction proceeded with moderate efficiency
and excellent selectivity at
a higher temperature
(80 °C); the cis-3 bisadduct (cis-3-7b) was exclusively
afforded (17% yield), accompanied with the formation
of a trace amount of cis-2-7b (entry 4).10,11 Quite inter-
estingly, the predominant formation of the cis-3 isomer
thus observed was completely opposite to our expecta-
tion; in the tandem biscyclopropanation of C60 with a
monodentate a-folmylsulfonium ylide, the cis-1 and
cis-3 bisadducts were not generated at all among the
possible regio-isomers.5b Therefore, the notable regio-
selectivity was undoubtedly attributable to the constraint
of the tether.
In order to improve the yield of bis(formylme-
thano)C60s, the bis(a-formylsulfonium ylide) 1c with rel-
atively long spacers was then employed. As we expected,
Table 1. Biscyclopropanation of C60 with 1a–c
Entry
Ylide
Temp.
Isolated yielda
cis-2
cis-3
equatorial
trans-4
1
1a
1a
1b
1b
1c
1c
rt
80 °C
rt
80 °C
rt
80 °C
(no reaction)
2b
3
—
—
—
—
(no reaction)
4
5
6
trace
—
—
17
23
9
—
12
30
—
—
trace
Scheme 1. Synthesis of bis(a-formylsufonium ylide)s 1a–c. Reagents
and conditions: (i) MsCl, pyridine, CH2Cl2, 0 °C; (ii) NaI, acetone,
reflux; (iii) (Methylthio)acetaldehyde N,N-dimethylhydrazone, LDA,
THF, ꢀ78 °C; (iv) 6 M HCl aq., C6H6; (v) Me3OBF4, CH2Cl2; (vi)
12.5 M NaOH and satd K2CO3 aq., CHCl3.
a — Not detected.
b The monocyclopropanated C60 was generated in <5% yield.

H. Ito et al. / Tetrahedron Letters 47 (2006) 3095–3098
3097
the reaction proceeded smoothly at 80 °C to give mainly
a mixture of bisadducts. From the reaction mixture, the
cis-3 and equatorial bisadducts (cis-3-7c and equatorial-
7c) were easily isolated in 9% and 30% yields, respec-
tively, by using preparative TLC (entry 6).10,11 The dif-
ference in regioselectivity between the reaction with 1b
and that with 1c is in good agreement with the length
and flexibility of their spacers. Worth to note is the fact
that 1c could react with C60 even at room temperature,
which was in sharp contrast to the cases of 1a and 1b;
the cis-3 and equatorial bisadducts were again predomi-
nantly generated in 23% and 12% yields, respectively
(entry 5). Considering the fact that the reactions with
1a and 1b did not afford monocyclopropanated C60s at
a detectable level, these observations might be eluci-
dated by the difference in the inherent reactivity of the
sulfonium ylides rather than the mobility of the linkers;
the slight structural modification of a sulfonium ylide
results in the significant change in the reactivity, which is
in good agreement with our previous results.5a Further-
more, in the reactions of 1c, the regioselectivity could be
switched by changing the reaction temperature (entries 5
and 6). The observed change in regioselectivity indicates
that the cis-3 bisadduct was the kinetic-controlled prod-
uct whereas the equatorial bisadduct was the thermo-
dynamic-controlled one.12
Scheme 2. Polycondensation of bis(formylmethano)C60
s
7c with
diamine 8. Reagents and conditions: (i) TiCl4 (10 equiv), DABCO
(40 equiv), reflux in C6H6, 5 h; (ii) Filtration; (iii) Successive extraction
with CHCl3, CS2, and C6H5Cl; (iv) Reprecipitation with MeOH from a
CS2 solution.
bility of the C60 monomers significantly affected the effi-
ciency of the polycondensation; for example, the
polycondensation of cis-3-7c, of which the solubility
was lower than that of equatorial-7c, with 8 under the
same conditions caused the precipitation of oligomeric
cis-3-9c at a quite early stage of the polycondensation
to prevent further chain elongation. As a result, only a
mixture of oligomers with insufficient degree of poly-
merization was recovered in poor yield.
Thus, three kinds of bis(formylmethano)C60s with a
well-defined structure, cis-3-7b, cis-3-7c, and equato-
rial-7c, could be prepared. Considering the potential
transformation of formyl groups into other functional
groups, such as acetals, imines, alkenes, alcohols, etc.,
the bis(formylmethano)C60s thus obtained are expected
to be applied as fundamental building blocks for C60-
containing functional materials.
1
All of the bis(formylmethano)C60s thus obtained were
potential monomers for the polycondensation with aro-
matic diamines to form C60 pearl-necklace polyimines.
As the initial study on the synthesis of the polyimines,
we selected equatorial-7c as the monomer, because of
its high yield in preparation and because of its high solu-
bility compared with the other bisadducts. On the
other hand, the aromatic diamine should be carefully
selected, because the resultant polyimine was worried to
be a material with poor solubility due to the rigid struc-
tures of the C60 core, diamine moiety, and imine bond.
Therefore, we used an aromatic diamine with two long
alkyl chains, 4,400-diamino-20,50-bis(dodecyloxy)-p-ter-
phenyl (8).
The FT-IR and H NMR spectra of polymeric equato-
rial-9c, obtained by polycondensation in benzene,
clearly showed that the formyl groups were transformed
to the corresponding imino groups in high conversion.9
1
In the H NMR spectrum, every signal was observed as
a broad peak but completely assigned to the protons in
the repeating unit, as observed for polymers with a suf-
ficient degree of polymerization. In addition, no detect-
able signal was observed around 6.5 ppm, strongly
suggesting that the undesired nucleophilic attack of the
diamine to the C60 core hardly occurred.15 Worth to
note is the excellent processability of the pearl-necklace
polyimine, equatorial-9c. In general, carbon clusters are
prone to form self-aggregated domains, which is a seri-
ous obstacle in the full reflection of their characteristic
properties in carbon cluster-containing devices. How-
ever, an isotropic thin film could be easily obtained from
the pearl-necklace equatorial-9c. For example, the cast
of a solution of the polyimine equatorial-9c in CS2 on
a glass plate, followed by the slow evaporation of the
solvent, gave a film, which could be detached from the
glass plate. The homogeneity of the resultant film
allowed its characterization by UV–vis absorption
spectroscopy;9 the absorption spectrum of the polymer
thin film was quite similar to that of the polymer in a
dilute solution, but slightly red-shifted most likely owing
to the stacking of the chromophores.
For the polycondensation of equatorial-7c with 8, sev-
eral methods were attempted, and finally we found that
the combination of TiCl4 and 1,4-diazabicyclo-
[2.2.2]octane (DABCO) was quite efficient for the
polyimine formation (Scheme 2).13 By using monofunc-
tionalized analogues, we confirmed that the condensa-
tion did not cause any detectable side reactions. When
the polycondensation of equatorial-7c with 8 was carried
out in benzene, the polymeric equatorial-9c was obtained
quantitatively.9 A GPC analysis revealed that the mean
molecular weight was satisfactorily high (Mw = 24,000,
Mw/Mn = 96).14 As we had worried, however, the solu-

3098
H. Ito et al. / Tetrahedron Letters 47 (2006) 3095–3098
M.; Kinbara, K.; Saigo, K. Chem. Lett. 2002, 728, and
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Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature
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In conclusion, we succeeded in the regio- and diastereo-
selective synthesis of bis(formylmethano)C60, upon
firstly applying the tether-directed bifunctionalization
method for the biscyclopropanation of C60 with bis(sul-
fonium ylide)s. The addition pattern could be controlled
by the structure of the biscyclopropanating reagents and
the reaction temperature. Among the bis(formyl-
methano)C60s thus obtained, equatorial-7c was found
to be a fascinating monomer for the formation of regio-
and diastereo-regulated pearl-necklace polymers by the
polycondensation with aromatic diamines such as 8.
The corresponding high polymer could be obtained by
using a TiCl4/DABCO system. The resultant pearl-neck-
lace polyimine, equatorial-9c, showed excellent process-
ability, which would enable the creation of novel
functional devises possessing polymeric C60 arrays.
Acknowledgements
9. See Supplementary data.
A part of this work was financially supported by
Mizuho Foundation for the Promotion of Sciences.
10. The structural assignment of the C60 bisadducts 7b and 7c
was conducted based on the 1H and 13C NMR spectra,
UV–vis spectra, and the consideration of the distance
between the two cyclopropane rings of each bisadduct
with respect to the length of the tether moiety. See
Supplementary data.
Supplementary data
11. The tether-directed biscyclopropanation can afford several
diastereoisomers with respect to the relative orientation of
the formyl groups at the bridge-head carbons (in–in, out–
out, in–out). Considering the distance between the two
cyclopropane rings of 7b and 7c with respect to the length
of the tether moieties, together with the symmetry deduced
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.02.148.
1
from H and 13C NMR spectroscopy, the out–out confi-
References and notes
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alternative method to determine the Mn, end-group assay
by a 1H NMR measurement was conducted. Upon
comparing the resonances attributed to the imine bond
(ACH@NA: 8.7–8.9 ppm) and those of the unreacted end
groups (ACH@O: 10.3–10.5 ppm and ACH@CANH2:
6.6–6.7 ppm), the Mn was calculated to be 8000 g molꢀ1
(see Supplementary data). For examples of the charac-
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scattering, see: Refs. 2b and 2d.
3. For the pearl-necklace polymers using C60 monomers as a
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Chem. Commun. 2000, 1373.
4. For the pearl-necklace polymers with a regio- and
diastereo-regulated structure, see: Hino, T.; Hamada,
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K. T. Fullerene Sci. Technol. 1996, 3, 407.