Synthesis of imino[60]fullerenes using nitriles and trimethylsilylmethyl triflate

Article abstract of DOI:10.1021/ol901851g

The synthesis of a new class of fullerene derivatives, 1-imino-4- silylmethyl[60]fullerene derivatives, Is described. The anion (C ₆₀R^1- ) of an alkyl- or aryl-adduct of [60]fullerene, C₆₀R¹H (R¹ = CH

Full text of DOI:10.1021/ol901851g

ORGANIC
LETTERS
2009
Vol. 11, No. 18
4192-4194
Synthesis of Imino[60]fullerenes Using
Nitriles and Trimethylsilylmethyl Triflate
Keiko Matsuo,† Yutaka Matsuo,*,†,‡ Akihiko Iwashita,† and Eiichi Nakamura*,†,‡
Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-0033, Japan, and Nakamura Functional Carbon Cluster Project, ERATO, Japan
Science and Technology Agency, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
matsuo@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Received August 10, 2009
ABSTRACT
The synthesis of a new class of fullerene derivatives, 1-imino-4-silylmethyl[60]fullerene derivatives, is described. The anion (C60R1-) of an
alkyl- or aryl-adduct of [60]fullerene, C60R1H (R1 ) CH2SiMe3, CH2SiMe2Ph, C6H4-OMe-4, C6H4-NMe2-4, C6H4-CF3-4 and C6H4-OMe-2), was
allowed to react with a nitrilium salt [R2CNCH2SiMe3][OTf] (Tf ) SO2CF3) that was generated in situ by the reaction of Me3SiCH2OTf and a
nitrile solvent R1CN (R2 ) Ph and Me). The desired imino[60]fullerene derivative C60(R1) [C()NCH2SiMe3)R2] was produced in a yield up to 80%.
The structure of the product with R1 ) C6H4-OMe-4 and R2 ) Ph was determined by single-crystal X-ray analysis.
An imine group is a useful functionality that may serve as a
surrogate of the corresponding carbonyl compound,1 as a
precursor of nitrogen compounds, and as a ligand for metal-
assisted directed functionalization of nearby carbon atoms.2
Despite such utilities, imines are rarely found in the repertoire
of functionalized fullerene3 probably because of the hydro-
lytic instability of the imine group and the paucity of
synthetic approaches.4 We report here the synthesis of new
imino-fullerenes 3 by the reaction of a fullerene anion and
an in situ generated nitrilium ion. The new reaction was
discovered during our effort to find approaches to organof-
ullerene bearing two addends in a 1,4-relative position such
as a bis(trimethylsilylmethyl)[60]fullerene 2 (Scheme 1).5
While the synthesis of 2 can be achieved in 93% yield, as
previously reported, by the reaction of an anion derived from
the monosilylmethyl compound 1 with Me3SiCH2I in ben-
zonitrile, we obtained the imine 3a when we used
Me3SiCH2OTf in place of the iodide.
A typical procedure is given: we first synthesized the
monoadduct 1 in 93% yield by addition of Me3SiCH2MgCl
(3 equiv) in THF to a solution of [60]fullerene in a mixture
of o-dichlorobenzene and DMF (30 equiv).5 The monoadduct
1 was deprotonated with KOtBu (1.2 equiv) in PhCN at 25
°C and then treated with Me3SiCH2OTf (2.0 equiv) at the
same temperature for 1 h to obtain the imino[60]fullerene
3a in 63% yield. The bistrimethylsilyl compound 2 (9%)
and the starting material 1 account largely for the remainder.
The imine product 3a was hydrolytically rather unstable and
was quickly purified on triethylamine-impregnated silica gel
† The University of Tokyo.
‡ Japan Science and Technology Agency.
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10.1021/ol901851g CCC: $40.75
Published on Web 08/26/2009
 2009 American Chemical Society

column chromatography or by precipitation by addition of
methanol. Some attempts to remove the imine group under
acidic conditions resulted in partial scission of the C-acyl
bond6 and did not afford the expected ketone.7
Table 1. Scope and Limitationa
We noted that the imine formation takes place under much
milder conditions (25 °C) than the formation of bis(silylm-
ethyl) adduct 2 (80-110 °C). The reaction with MeOTf8 in
place of Me3SiCH2OTf did not produce the iminofullerene
at all but gave a methylated fullerene C60(CH2SiMe3)Me
(32% HPLC area ratio as a mixture of presumably 1,2- and
1,4-positional isomers; 22% recovery of 1). The reaction with
phenyl triflate (PhOTf) resulted in 100% recovery of 1 after
protonation workup.9 Trimethylsilyl triflate and halides (e.g.,
Me3SiOTf, Me3SiCl) gave back the starting monoalkyl
fullerene. The use of a stoichiometric amount of PhCN (and
other nitriles in toluene or o-dichlorobenzene, instead of the
use as a solvent) did not afford the imine product.
Scheme 1
.
Synthesis of the 1-Imino-4-silylmethyl[60]fullerene
and 1,4-Bis(silylmethyl)[60]fullerene
a The reaction conditions are described in Scheme 1 (bottom scheme).
b Isolated yield. c 60% recovery of starting material 1.
The 1,4-addition pattern was unambiguously determined by
the X-ray crystallographic analysis of 3c (Figure 1). Single
crystals of imino[60]fullerene 3c were successfully obtained
by slow diffusion of methanol into a o-dichlorobenzene
solution of 3c. The orientation of the imine group relative
to the fullerene π-surface exposes the nitrogen lone pair in
such a direction that a suitable metal atom may be able to
coordinate both to the lone pair and to the fullerene π-surface.
In Table 1, we illustrate the scope of the reaction for some
mono(organo)[60]fullerenes and nitriles. C60(CH2SiMe2Ph)H
showed a reactivity comparable to 1 and gave the corresponding
imino[60]fullerene 3b in 80% yield (entry 2). C60(C6H4-OMe-
4)H, C60(C6H4-NMe2-4)H, and C60(C6H4-CF3-4)H differ in
the electronic properties of the R groups but showed essentially
the same reactivity to give the desired products in similar yields
(75-78%). The reaction of C60(C6H4-OMe-2)H that bears an
o-methoxy group reacted as smoothly as the less hindered
compound C60(C6H4-OMe-4)H (entry 6). We could also
synthesize a methylimino[60]fullerene 3g in 32% yield with
60% recovery of 1 (entry 7).
All products (3a-3g) were characterized by H and 13C
NMR, IR, and MS. All imino[60]fullerenes are C1 symmetric
compounds and consist of a racemic mixture of enantiomers.
1
Figure 1. Molecular structure of 3c. The ORTEP drawing with
30% level of probability level ellipsoids. The oxygen, nitrogen,
and silicon atoms are marked in red, blue, and sky blue, respectively.
The solvent molecule in the unit cell (3c:o-dichlorobenzene ) 1:1)
is omitted for clarity.
(6) Tada, T.; Ishida, Y.; Saigo, K. Org. Lett. 2007, 9, 2083–2086.
(7) Tzirakis, M. D.; Orfanopoulos, M. J. Am. Chem. Soc. 2009, 131,
4063–4069.
(8) (a) MeOTf is a more efficient alkylating reagent than iododmethane
by the factor of ∼104; see: Boche, G. Encyclopedia of Reagents for Organic
Synthesis; Paquette, L. A., Ed.; Wiley: New York, 1995; Vol. 5, pp
3617-3622. (b) Stang, P. J; Hanack, M.; Subramanian, L. R. Synthesis
1982, 8, 5–126.
The cyclic voltammogram of 3c exhibited two reversible
red
one-electron reduction processes at E1/2 ) -1.02 and
-1.61 V vs Fc/Fc+ (Figure S1, Supporting Information). The
reduction potential values of 3c shifted toward the positive
(9) The color of the monoanion (dark green) persisted until the end of
the reaction .
Org. Lett., Vol. 11, No. 18, 2009
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side of the corresponding 1,4-di(organo)[60]fullerene,
C60(CH2SiMe2Ph)2 (E1/2red ) -1.06 and -1.63 V vs Fc/Fc+),5
as one may expect in light of the electron-withdrawing
property of the imino moiety.10
Scheme 2. Proposed Reaction Mechanism
It has been reported that Me3SiCH2OTf reacts slowly with
a nitrile (in several days at 25 °C),11 and the resulting
nitrilium salt reacts readily with nucleophiles.11,12 Thus, we
assume (Scheme 2) that Me3SiCH2OTf slowly reacted with
the nitrile solvent to form in situ the corresponding nitrilium
salt, which reacted with the mono(organo)[60]fullerene anion
to produce the imino[60]fullerene 3. We can ascribe the
difference between Me3SiCH2OTf, Me3SiCH2I, and MeOTf
to the hardness/softness of the electrophiles (OTf vs I) and
to the steric and electron-withdrawing effects of the silyl
group (Me3SiCH2OTf vs MeOTf).
In summary, we have found an efficient method to
introduce an imine group to a monoadduct of [60]fullerenes
to produce a 1-imino-4-organo[60]fullerene. The simple and
mild reaction conditions (25 °C), the exclusive 1,4-regiose-
lectivity, and the synthetically useful yield are attractive
attributes of the reaction. The present synthesis is a useful
addition to the repertoire of the synthesis of 1,4-dialkyl
[60]fullerene derivatives that is less widely known than 1,2-
relative compounds that are available through a variety of
cycloaddition reactions including Prato and Bingel-Hirsch
reactions.13,14 We expected that the presence of the imine
group will give us ample opportunity to explore the
coordination chemistry of this class of compounds as well
as the applications to materials research.15
Acknowledgment. This work was partially supported by
KAKENHI (#18105004) and the Chemistry Innovation
GCOE program of the MEXT, Japan. K.M. thanks the Japan
Society for the Promotion of Science (JSPS) for a Research
Fellowship for Young Scientists.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data for all new compounds, and CV
data and CIF file for 3c. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL901851G
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