Molecular recognition by a silica-bound fullerene derivative

Article abstract of DOI:10.1021/ja970502l

Thermal ring opening of N-[3-(triethoxysilyl)propyl]-2-carbomethoxyaziridine in the presence of C₆₀ produces a fulleropyrrolidine derivative which is then attached covalently to HPLC silica gel. The new chromatographic material is used to investigate binding affinities of potential hosts for the immobilized C₆₀ Exceptionally high size selectivities have been obtained for cyclic oligomeric compounds like calixarenes and cyclodextrins in organic and water-rich media, respectively. A number of rationally designed, helical-shaped peptides bind selectively to the grafted fullerene. The most tightly bound peptide carries two ferrocene moieties at the periphery of a hydrophobic binding cavity complementary in size to C₆₀.

Full text of DOI:10.1021/ja970502l

7550
J. Am. Chem. Soc. 1997, 119, 7550-7554
Molecular Recognition by a Silica-Bound Fullerene Derivative
Alberto Bianco,† Francesco Gasparrini,*,‡ Michele Maggini,*,† Domenico Misiti,‡
Alessandra Polese,† Maurizio Prato,§ Gianfranco Scorrano,† Claudio Toniolo,† and
Claudio Villani‡
Contribution from the Centro Meccanismi Reazioni Organiche and Centro di Studio sui
Biopolimeri del CNR, Dipartimento di Chimica Organica, UniVersita` di PadoVa,
Via Marzolo 1, 35131 PadoVa, Italy, Dipartimento di Studi di Chimica e Tecnologia delle
Sostanze Biologicamente AttiVe, UniVersita` “La Sapienza”, Piazzale A. Moro 5,
00185 Roma, Italy, and Dipartimento di Scienze Farmaceutiche, UniVersita` di Trieste,
Piazzale Europa 1, 34127 Trieste, Italy
ReceiVed February 17, 1997X
Abstract: Thermal ring opening of N-[3-(triethoxysilyl)propyl]-2-carbomethoxyaziridine in the presence of C60
produces a fulleropyrrolidine derivative which is then attached covalently to HPLC silica gel. The new
chromatographic material is used to investigate binding affinities of potential hosts for the immobilized C60.
Exceptionally high size selectivities have been obtained for cyclic oligomeric compounds like calixarenes and
cyclodextrins in organic and water-rich media, respectively. A number of rationally designed, helical-shaped peptides
bind selectively to the grafted fullerene. The most tightly bound peptide carries two ferrocene moieties at the periphery
of a hydrophobic binding cavity complementary in size to C60.
Introduction
of silica for the preparation of an HPLC microcolumn, which
was shown to separate a mixture of C60 and C70 and also a
mixture of calixarenes.2a,b (3-Aminopropyl)silyl-bonded silica
was employed to prepare a C60-based stationary phase, which
was utilized in the HPLC separation of haloaromatics.2c
In addition, recent reports on C60 derivatives as potential or
effective antiviral agents and enzyme inhibitors suggest that C60-
containing chromatographic phases, tailored for biopolymers,
may be useful as biological probes as well as in developing
therapeutical agents.3
The chromatographic separation of complex fullerene mix-
tures, obtained by resistive heating of graphite, has required an
increasing use of appropriate stationary phases. Size exclusion
chromatography, or suitably modified silica columns, have been
successfully employed. Accordingly, silica has been modified
by introducing substituted arenes, phthalocyanins, or tetraphe-
nylporphyrins.1 The basic idea is that the aromatic portion of
the stationary phase should efficiently interact with the fullerene
cage, giving rise to supramolecular complexes whose relative
stability, which determines the extent of the separations, is
highly dependent on the electronic properties of the aromatic
rings and on their spatial arrangement.
The opposite approach, namely, the use of fullerenes in the
modification of HPLC chromatographic supports, has not
received comparable attention. As a matter of fact, grafting a
fullerene, or any suitable derivative, to a solid matrix not only
would greatly facilitate the evaluation of relative affinities of
potential hosts for the immobilized fullerene from the analysis
of retention data but would also enable the use of solvents, in
which fullerenes are not soluble, in the study of host-guest
chemistry of fullerenes.
In this paper we report the preparation, characterization, and
chromatographic applications of a new HPLC stationary phase,
characterized by the presence of a fullerene derivative covalently
linked to silica microparticles.
Results and Discussion
The potential utility of surface-linked fullerene materials4 has
recently resulted in the development of synthetic methodologies
to covalently incorporate C60 into inorganic or organic matri-
ces.2,5 This has been achieved either by chemical modification
of a matrix with a reagent able to add to an appropriate fullerene
in solution or by preliminary introduction of a reactive function
onto the C60 spheroid, followed by a bond-forming step between
the modified fullerene and the solid support. We have adopted
the second strategy employing a preformed, well-characterized
fullerene-containing trialkoxysilane which smoothly reacts with
silica microparticles, leading to a chemically homogeneous
Since chromatography promptly responds to even subtle
variations in the structure, shape, size, and dynamics of the
analytes, C60-based HPLC materials have practical implications
in both separation science and supramolecular chemistry.
Preliminary results in this direction have been recently reported.2
Chlorosilane-functionalized C60 was used in the modification
(2) (a) Nagashima, H.; Terasaki, H; Saito, K.; Jinno, K.; Itoh, K. J. Org.
Chem. 1995, 60, 4966-4967. (b) Saito, Y.; Ohta, H.; Terasaki, H.; Katoh,
Y.; Nagashima, H.; Jinno, K.; Itoh, K.; Trengove, R. D.; Harrowfield, J.;
Li, S. F. J. J. High Resolut. Chromatogr. 1996, 19, 475-477. (c) Chang,
C.-S.; Wen, C.-H.; Den, T.-G. Huaxue 1996, 54, 11-21; Chem. Abstr. 1997,
126, 139272d.
† Universita` di Padova.
‡ Universita` “La Sapienza”.
§ Universita` di Trieste.
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) (a) Gu¨gel, A.; Beker, M.; Hammel, D.; Mindach, L.; Rader, J.; Simon,
T.; Wagner, M.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1992, 31, 644-
646. (b) Welch, C. J.; Pirkle, W. H. J. Chromatogr. 1992, 609, 89-101.
(c) Jinno, K.; Ohta, H.; Saito, Y.; Uemura, T. J. Chromatogr. 1993, 648,
71-77. (d) Kibbey, C. E.; Savina, M. R.; Parseghian, B. K.; Francis, A. H.
Anal. Chem. 1993, 65, 3717-3719. (e) Kimata, K.; Hosoya, K.; Araki, T.;
Tanaka, N. J. Org. Chem. 1993, 58, 282-283. (f) Herren, D.; Thilgen, C.;
Calzaferri, G.; Diederich, F. J. Chromatogr. 1993, 644, 188-192. (g) Saito,
Y.; Ohta, H.; Nagashima, H.; Itoh, K. J. Liq. Chromatogr. 1994, 17, 2359-
2372. (h) Xiao, J.; Meyerhoff, M. E. J. Chromatogr. A 1995, 715, 19-29.
(3) (a) Sijbesma, R.; Srdanov, G.; Wudl, F.; Castoro, J. A.; Wilkins, C.;
Friedman, S. H.; DeCamp, D. L.; Kenyon, G. L. J. Am. Chem. Soc. 1993,
115, 6510-6512. (b) Toniolo, C.; Bianco, A.; Maggini, M.; Scorrano, G.;
Prato, M.; Marastoni, M.; Tomatis, R.; Spisani, S.; Palu`, G.; Blair, E. D. J.
Med. Chem. 1994, 37, 4558-4562. (c) Jensen, A. W.; Wilson, S. R.;
Schuster, D. I. Bioorg. Med. Chem. 1996, 4, 767-779. (d) Da Ros, T.;
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9070-9072.
(4) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113-5130.
S0002-7863(97)00502-7 CCC: $14.00 © 1997 American Chemical Society

Molecular Recognition by a Silica-Bound Fullerene DeriVatiVe
J. Am. Chem. Soc., Vol. 119, No. 32, 1997 7551
Scheme 1
sp2 and 2 sp3) out of the possible 60 signals were detected due
to some overlapping in the overcrowded aromatic region.
Reaction of derivative 2 with HPLC spherical silica particles
in refluxing toluene afforded fulleropyrrolidine-functionalized
silica 3 (Scheme 1). Elemental analysis of the vacuum-dried,
dark brown silica 3 indicated a loading of 0.07 mmol (72 mg)
of fulleropyrrolidine per gram of silica (based on 5.86% C)
corresponding to a surface coverage of 0.4 µmol m-2; FT-IR
analysis of 3 showed distinctive bands at 2990-2940 cm-1 (C-
H), 1738 cm-1 (CO2Me), 1428 and 526 cm-1 (C60), matching
the corresponding absorptions of fulleropyrrolidine 2. Thermal
gravimetric analysis of 3 showed two initial weight losses (ca.
1.5% and 2.2%) in the 50-200 and 200-450 °C temperature
ranges, the first due to removal of physically sorbed water and
the second related to dehydroxylation of residual silanols and
removal of unreacted Si-ethoxy groups; further heating to 950
°C resulted in a ca. 9.5% loss corresponding to degradation of
the fulleropyrrolidine moiety, in fair agreement with elemental
analysis.
Modified silica 3 was packed into a 250 × 1.8 mm stainless-
steel column using standard procedures (see the Experimental
Section). Preliminary chromatographic runs revealed some
unique properties of 3 in that it allows efficient separations (N/m
in the range (4-5) × 104) of simple aromatic solutes in both
organic and water-rich media. With aqueous eluents, retention
increases with the water content and solute hydrophobicity; i.e.,
the fulleropyrrolidine phase has a retention mode resembling
that of typical reversed phase packings. However, different
selectivities are expected toward more complex solutes capable
of establishing multipoint contacts with the curved fulleroid
surface of 3.
It is known that calix[8]arenes form insoluble complexes with
C60 from toluene, benzene or CS2 solutions.7 This prompted
us to investigate the interactions between tert-butylcalix[n]arenes
(hereafter calix[n]arenes) and 3 in organic solvents. When a
mixture of calix[n]arenes (n ) 4, 6, 8) was chromatographed
on 3 using 100% toluene as eluent the six- and eight-membered
calixarenes were almost coeluted (retention times 7.3 and 7.9
min, respectively) and well separated from calix[4]arene (reten-
tion time 2.0 min). By contrast, with a CH2Cl2/i-PrOH (99.5/
0.5) mixture as eluent, 3 shows extraordinary size-selectivity
in the form of a marked preference for calix[8]arene over the
four and six-membered oligomers (Figure 1). It is interesting
to note that, under these conditions, the underlying silica does
not contribute to the observed selectivity, as all the solutes are
front-eluted when chromatographed on a bare silica column.8
In a study of the influence of eluent composition on retention,
we noted that incremental additions of alcoholic modifiers (2-
propanol or methanol) to the eluent cause the retention of calix-
material with unequivocal structure (Vide infra) that retains the
inherent high chromatographic efficiency of the starting silica
and presents only minimal modifications of the fullerene
structure.
A versatile synthesis of fullerene derivatives relies on the
1,3-dipolar cycloaddition of azomethine ylides to C60, a reaction
that, under controlled conditions, affords fulleropyrrolidines as
single and stable products of monoaddition across a 6,6 junction
of C60.6 One way of generating azomethine ylides is the thermal
ring opening of aziridines. This approach was chosen in the
present work due to the easy scaling up of the reaction.
Aziridine 1, prepared from (3-aminopropyl)triethoxysilane and
methyl 2,3-dibromopropionate (from methyl acrylate), was
heated in chlorobenzene in the presence of C60. The reaction
gave rise to fulleropyrrolidine 2 in 36% isolated yield (71%
based on C60 conversion), Scheme 1.
Compound 2 was fully characterized by IR, UV-vis,
1
MALDI-MS, and H and 13C NMR techniques and its purity
checked by HPLC and elemental analysis. In the proton
spectrum, the pyrrolidine ring is readily recognized by the
characteristic AB quartet of the methylene protons (4.1 and 5.0
ppm, 9 Hz) and by the singlet at 5.2 ppm for the methine proton.
Due to the C1 symmetry of 2, all the carbon atoms of the
fullerene spheroid are different. In the carbon spectrum, 39 (37
(7) (a) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994,
368, 229-231. (b) Suzuki, T.; Nakashima, K.; Shinkai, S. Chem. Lett. 1994,
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M.; Fukazawa, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 259-260.
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I.; Siedschlag, C.; Torres-Garc´ıa, G.; Mattay, J. In Fullerenes and Fullerene
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M., Roth, S., Eds.; World Scientific: Singapore, 1995; pp 137-141.
(8) Attemped resolutions of the three calixarenes on unmodified Hypersil
silica were unsuccessful. However, we found that on LiChrosorb SiO2 Si
100 (hexane/CH2Cl2 (50/50) as eluent) the capacity factors for the calix-
[8]-, -[6]-, and -[4]arenes are 0.07, 0.27, and 0.47, respectively; i.e., the
elution order is inverted compared to 3 for calix[4]- and calix[8]arenes.
(5) (a) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley,
J. P.; Smith, A. B.; Blasie, J. K.; Peticolas, L. J.; Bean, J. C. J. Am. Chem.
Soc. 1993, 115, 4383-4384. (b) Chen, K.; Caldwell, W. B.; Mirkin, C. A.
J. Am. Chem. Soc. 1993, 115, 1193-1194. (c) Geckeler, K. E.; Hirsch, A.
J. Am. Chem. Soc. 1993, 115, 3850-3851. (d) Bergbreiter, D. E.; Gray, H.
N. J. Chem. Soc., Chem. Commun. 1993, 645-646. (e) Liu, B.; Bunker, C.
E.; Sun, Y. J. Chem. Soc., Chem. Commun. 1996, 1241-1242. (f) Planeix,
J.; Coq, B.; de Menorva, L.; Medina, P. J. Chem. Soc., Chem. Commun.
1996, 2087-2088. (g) Stalling, D. L; Guo, C.; Kuo, K. C.; Saim. S. J.
Microcolumn Sep. 1996, 5, 223-235. (h) Stalling, D. L; Guo, C.; Saim. S.
J. Chromatogr. Sci. 1993, 31, 265-278.
(6) (a) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993,
115, 9798-9799. (b) Bianco, A.; Maggini, M.; Scorrano, G.; Toniolo, C.;
Marconi, G.; Villani, C.; Prato, M. J. Am. Chem. Soc. 1996, 118, 4072-
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G.; Farnia, G. Tetrahedron 1996, 52, 5221-5234.

7552 J. Am. Chem. Soc., Vol. 119, No. 32, 1997
Bianco et al.
Figure 1. HPLC trace for the separation of tert-butylcalix[n]arenes
on 3. Eluent CH2Cl2/i-PrOH (99.5/0.5), flow rate 0.3 mL/min; T ) 25
°C; UV detection at 280 nm. Numbers on top of the peaks denote the
solute’s ring size.
[8]arene to steadily increase, while retention of the other
calixarenes was almost unaffected by the amount of alcohol in
the 0-10% range in toluene or CH2Cl2 (Figure 2). These results
can be best interpreted considering the structures of calix[8]-
arene and of its complex with C60: in the solid state, and
presumably in solution as well, calix[8]arene adopts a nearly
planar, “pleated-loop” conformation with the phenolic OH
engaged in a circular array of H-bonds.9 In the solid state
complex between C60 and calix[8]arene,7 the latter adopts an
alternate-cone conformation with the original H-bond network
partly disrupted. Assuming similar conformations in the solid
state and in solution, the chromatographic behavior of calix[8]-
arene on 3, at variable alcohol concentrations, may be due to a
favorable effect of the polar modifier on the conformational
changes10 and reorganization of the intramolecular H-bond
system required for complexation with the grafted C60.
Figure 2. Retention as a function of the amount of 2-propanol in the
eluent for calix[n]arenes on 3. K′ (capacity factor) defined as (t - t0)/
t0 where t and t0 are the retention time of the analyte and the void
time, respectively. 4, calix[8]arene; O, calix[6]arene; 0, calix[4]arene.
The fulleropyrrolidine-bonded phase displays large size
selectivities also in aqueous media as demonstrated by the
separation of cyclodextrins shown in Figure 3. Several groups11
have proved that γ-cyclodextrin is able to form a water soluble
complex with C60, while the smaller R- and â-cyclodextrins are
not. Spectroscopic data are in favor of a 2:1 complex in which
C60 is sandwiched between two γ-cyclodextrin molecules. The
steric bulk of the substituted pyrrolidine ring in 3 and low
concentration of cyclodextrin in the mobile phase make this
kind of complex unlikely to form in our chromatographic
system, where presumably complexation takes place with a 1:1
stoichiometry; despite this, 3 maintains a strong preference for
the γ-cyclodextrin over the R- and â-cyclodextrins (Figure 3).
(9) Gutsche, C. D., Gutsche, E.; Karaulov, I. J. Inclusion Phenom. 1985,
3, 447-451.
Figure 3. HPLC trace for the separation of R-, â-, and γ-cyclodextrins
on 3. Eluent 3 min isocratic H2O/MeOH/THF (80/10/10) and then linear
gradient to 40/30/30 in 25 min, flow rate 0.25 mL/min; T ) 25 °C;
evaporative light scattering detector, P(air) ) 2 atm, T ) 60 °C.
(10) Polar, H-bond-forming solvents like acetone and pyridine are known
to alter the conformational behavior of calixarenes: Gutsche, C. D.; Bauer,
L. J. J. Am. Chem. Soc. 1985, 107, 6052-6059.
(11) (a) Andersson, T.; Nilsson, K.; Sundahl, M.; Westman, G.; Wen-
nerstro¨m, O. J. Chem. Soc., Chem. Commun. 1992, 604-606. (b) Yoshida,
Z.; Takekuma, H.; Takekuma, S.; Matsubara, Y. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1597-1599. (c) Andersson, T.; Westman, G.; Wennerstro¨m,
O. Sundahl, M. J. Chem. Soc., Perkin Trans. 2 1994, 1097-1101. (d)
Priyadarsini, K. I.; Mohan, H.; Tyagi, A. K.; Mittal, J. P. J. Phys. Chem.
1994, 98, 4756-4759.
As expected, none of the three cyclodextrins were retained on
the octadecyl-bonded phase (ODS Hypersil) under the same
conditions.
Since the fulleropyrrolidine-modified stationary phase size
selectively recognizes tight-fitting partners in both organic and

Molecular Recognition by a Silica-Bound Fullerene DeriVatiVe
J. Am. Chem. Soc., Vol. 119, No. 32, 1997 7553
Figure 4. Schematic representation of the P2-C60 complex (see the
text) obtained with the help of the Insight II program running on a
Silicon Graphics workstation. The helical stripe is an exact geometrical
reproduction of the 310-helical peptide backbone.
aqueous eluents, we were ready to explore the recognition ability
of 3 toward small peptides containing hydrophobic cavities.
These compounds, in fact, could be considered suitable models
for the interactions between 3 and more complex biological
systems possessing hydrophobic clefts.3a The peptides inves-
tigated were the side-chain-modified tyrosine nonapeptides P1-
P4:
P1: Bz-Aib-L-Tyr(Me)-Aib2-Gly-Aib2-L-Tyr(Me)-
Aib-OMe
P2: pBrBz-Aib-L-Tyr(Bzl)-Aib2-Gly-Aib2-L-Tyr(Bzl)-
Figure 5. HPLC traces for the separation of peptides P1-P4 on
Hypersil ODS (a) and 3 (b). Eluent linear gradient from 50/25/25 to
10/45/45 H2O/CH3CN/THF in 45 min, flow rate 0.25 mL/min; T ) 25
°C; evaporative light scattering detector, P(air) ) 2 atm, T ) 60 °C.
Aib-OMe
P3: pBrBz-Aib-L-Tyr(Me)-Aib2-Gly-Aib2-L-Tyr(Bzl)-
Aib-OMe
have prepared the [Tyr(Bzl)2,8]- and the [Tyr(ferrocenoyl)2,8]-
nonapeptides P2 and P4, respectively (see the Supporting
Information). For comparison, the related P1 and P3 nonapep-
tides were also synthesized.
P4: Bz-Aib-L-Tyr(ferrocenoyl)-Aib2-Gly-Aib2-
L-Tyr(ferrocenoyl)-Aib-OMe
Binding affinities of 3 were screened against nonapeptides
P1-P4 in aqueous media using gradient elution. As shown in
Figure 5b, the fulleropyrrolidine phase easily recognizes solute
hydrophobicities and shows a marked preference for P2 and
P4, the peptides characterized by a hydrophobic cavity (see
Figure 4). Of the latter two peptides, the ferrocene-containing
P4 is more retained, as expected on the basis of its stronger
donor ability with respect to the benzyl group. Under identical
conditions, an ODS phase revealed lower affinities for all the
peptides and a preference for P2 over P4 (Figure 5a).
The above results indicate that the C60 spheroid of 3 interacts
with (and discriminates between) the model peptide substrates
more effectively than the ODS alkyl chains do. This effect can
be expected considering the differences in the total area of
hydrocarbons engaged in the binding processes. Moreover, the
two phases show different selectivities for P2 and P4 which
are distinguished, on the fulleropyrrolidine phase, by the
π-bonding attitudes of the tyrosyl side-chain protecting groups.
The novel shape and functional group selectivities of 3
underscored by these experiments originate from the ability of
the curved surface of C60 to simultaneously establish π-π and
hydrophobic interactions.
where Bz is benzoyl, pBrBz p-bromobenzoyl, Me methyl, Bzl
benzyl, OMe methoxy, and Aib R-aminoisobutyric acid.
These terminally-blocked peptides are all characterized by a
high content (66%) of the CR-tetrasubstituted Aib residue which
is known to significantly stabilize 310- and R-helical structures.12
The 310-helix is preferred over the R-helix when the Aib content
exceeds 50%.12a This expectation was fully confirmed by the
X-ray diffraction analysis of P1.13 The two Tyr residues,
separated by five intervening amino acids, are located on the
same side of the ternary, right-handed helix after two complete
turns. This scaffold generates a cavity characterized by a
distance between the two Tyr Câ atoms of 11.5 Å, where a C60
molecule may be accommodated (Figure 4). To avoid unfavor-
able steric interactions, we have incorporated a Gly residue,
the least sterically demanding R-amino acid, on the same face
of the helix between the two Tyr residues. Using this bistyrosyl,
310-helical peptide template, for the construction of C60 receptors,
we also decided to exploit the known affinity of C60 for the
(benzyloxy)benzyl14 and ferrocenyl15 moieties. Therefore, we
(12) (a) Karle, I. L.; Balaram, P. Biochemistry 1990, 29, 6747-6756.
(b) Toniolo, C.; Benedetti, E. Macromolecules 1991, 24, 4004-4009. (c)
Toniolo, C.; Benedetti, E. Trends Biochem. Sci. 1991, 16, 350-353.
(13) Bianco, A.; Crisma, M.; Valle, G; Formaggio, F. Toniolo, C.
Unpublished results.
(14) Balch, A. L.; Catalano, V. J.; Lee, J. W.; Olmestead, M. M. J. Am.
Chem. Soc. 1992, 114, 5455-5457.
(15) (a) Crane, J. D.; Hitchcock, P. B.; Kroto, H. W.; Taylor, R.; Walton,
D. R. M. J. Chem. Soc., Chem. Commun. 1992, 1764-1765. (b) Birkett,
P. R.; Crane, J. D.; Hitchcock, P. B.; Kroto, H. W.; Meidine, M. F.; Taylor,
R.; Walton, R. M. J. Mol. Struct. 1993, 292, 1.
Conclusions. We have described a new HPLC material
incorporating a fullerene derivative that combines the unique
size, shape, and function selectivity of C60 with the high
efficiency and versatility of modern chromatographic media.
On the new phase, solute dimensions dictate the retention
behavior of macrocyclic compounds like calixarenes and

7554 J. Am. Chem. Soc., Vol. 119, No. 32, 1997
Bianco et al.
mmol) of N-[3-(triethoxysilyl)propyl]-2-carbomethoxyaziridine (1) in
100 mL of chlorobenzene was stirred at reflux temperature for 30 h,
and then the solvent was removed in vacuo. The residue was purified
by flash chromatography (eluant toluene, then toluene/ethyl acetate,
(9/1), affording 51.5 mg (36%) of 2 along with 49.3 mg (49%) of
unreacted C60. FT-IR (KBr): 2972, 2924, 2886, 1755, 1736, 1462,
cyclodextrins, while shape and functionality modulate the
relative retentions of a series of protected peptides. Given the
discrimination capabilities shown by 3, we anticipate it will find
additional applications in molecular recognition studies of C60
as well as in analytical issues requiring nonconventional
selectivities.
As a final remark, the easy synthesis of optically pure
fulleropyrrolidines16 should allow the preparation of chiral
fullerene-modified stationary phases potentially useful in enan-
tioselective separations.
1431, 1387, 1167, 1103, 1087, 957, 575, 527 cm-1 1H NMR (200
.
MHz, C6D6): δ 1.04-1.17 (m, 2H), 1.29 (t, 9H, J ) 6.9 Hz), 2.20-
2.27 (m, 2H), 2.91-2.95 (m, 1H), 3.33-3.40 (m, 1H), 3.47 (s, 3H),
3.95 (q, 6H, J ) 6.9 Hz), 4.06, (d, 1H, J ) 9.1 Hz), 5.01 (d, 1H, J )
9.1 Hz), 5.16 (s, 1H). 13C NMR (62.5 MHz, C6D6): δ 8.65, 18.68,
22.61, 51.50, 55.04, 58.66, 65.0, 70.0, 73.20, 77.69, 126.97, 135.79,
136.50, 137.0, 138.23, 140.0, 140.19, 140.49, 140.56, 142.08, 142.11,
142.25, 142.36, 142.42, 142.59, 142.65, 142.93, 142.99, 143.39, 144.73,
144.78, 144.89, 144.94, 145.32, 145.58, 145.68, 145.75, 145.94, 146.16,
146.31, 146.39, 146.57, 147.55, 147.64, 151.86, 154.48, 155.17, 155.48,
170.34. UV-vis (THF): λ (ꢀ) 254 (105 200), 308 (33 900), 429 (3300),
629 (230). MALDI MS (C73H27NO5Si (MW ) 1025)) m/z 1025 [M+].
Anal. Calcd for C73H27NO5Si: C, 85.45; H, 2.65; N, 1.36. Found:
C, 85.57; H, 2.51; N, 1.40.
Fulleropyrrolidine-Based Silica 3. A suspension of Hypersil, 5
µm (900 mg), in 20 mL of toluene was heated with stirring under a
continuous stream of argon until the volume was reduced to ca. 15
mL. To this azeotropically dried mixture solid N-[3-(triethoxysilyl)-
propyl]-2-carbomethoxy-3,4-fulleropyrrolidine (2) (185 mg, 0.18 mmol)
was added, and the mixture was refluxed for 6 h. After being cooled
to room temperature, the modified silica was collected by filtration
and sequentially washed with 200 mL portions of toluene, chloroform,
methanol, chloroform, and n-hexane and dried at 60 °C under reduced
pressure. FT-IR (KBr): 2941, 2859, 1748, 1469, 527 cm-1. Anal.
Found: C, 5.86; H, 0.51; N, 0.10. Fulleropyrrolidine-based silica was
packed in a 250 × 1.8 mm i.d. glass-lined, stainless steel column by
the slurry procedure (CHCl3, 600 atm); efficiency tests gave N/m >
47 000 for 1,3-dinitrobenzene eluting with hexane/CHCl3 (90/10) and
N/m > 40 000 for anthracene eluting with H3O/CH3CN (50/50) at a
flow rate of 0.2 mL/min at 25 °C.
Experimental Section
Materials and Methods. Details regarding instrumentation used
in this paper have been described elsewhere.6 C60 was purchased from
Bucky USA (99.5%). HPLC grade spherical silica (Hypersil, 5 µm
particle diameter, 100 Å pore diameter, 170 m2 g-1) was purchased
from Shandon, U.K. All other reagents were used as purchased from
Fluka and Aldrich. All solvents were distilled prior to use. Methyl
2,3-dibromopropionate was prepared by allowing methyl acrylate to
react with bromine, according to published methodologies.17 Tetrahy-
drofuran (THF), employed for UV-vis measurements, was distilled
from LiAlH4 prior to use.
N-[3-(Triethoxysilyl)propyl]-2-carbomethoxy aziridine (1).
A
solution of 1.05 g (4.27 mmol) of methyl 2,3-dibromopropionate in
3.5 mL of dry benzene was cooled in an ice bath under nitrogen
atmosphere. A solution of 1.0 mL (4.26 mmol) of (3-aminopropyl)-
triethoxysilane and 1.2 mL (8.6 mmol) of triethylamine in 5 mL of
dry benzene was added dropwise over a period of 20 min. The ice
bath was removed and the mixture refluxed for 2 h, then filtered with
suction over a pad of Celite, and washed with toluene. The filtrate
was washed with water, and the organic layer, dried over Na2SO4, was
concentrated under reduced pressure, affording 1.0 g (77%, purity
>95% by GC-MS) of 1 as a clear oil that was used for the next reaction
without further purification. 1H NMR (200 MHz, C6D6): δ 0.76-
0.82 (m, 2H), 1.06 (dd, 1H, J ) 1.5 Hz, J ) 6.2 Hz), 1.19 (t, 9H, J )
6.9 Hz), 1.78-1.88 (m, 2H), 1.94-2.16 (m, 1H), 2.08-2.10 (m, 2H),
3.35 (s, 3H), 3.81 (q, 6H, J ) 6.9). 13C NMR (62.5 MHz, C6D6): δ
8.48, 18.53, 23.48, 34.04, 37.37, 51.32, 58.44, 63.69, 170.96. GC-
MS (C13H27NO5Si (MW ) 305)): m/z 79 (76), 119 (64), 163 (86),
200 (46), 218 (100), 259 (26), 274 (8).
Acknowledgment. We thank Dr. Roberta Seraglia for
MALDI-MS data and Mr. Stefano Zanellato for his help in
working out detailed synthetic procedures. This work was in
part developed within the Progetto Strategico Materiali Inno-
VatiVi of the CNR.
N-[3-(Triethoxysilyl)propyl]-2-carbomethoxy-3,4-fulleropyrroli-
dine (2). A solution of 100 mg (0.14 mmol) of C60, 167.6 mg (0.55
Supporting Information Available: Characterization of
compounds P1-P4 (2 pages). See any current masthead page
for ordering and Internet access instructions.
(16) Novello, F.; Prato, M.; Da Ros, T.; De Amici, M.; Bianco, A.;
Toniolo, C.; Maggini, M. Chem. Commun. 1996, 903-904.
(17) Baldwin, J. E.; Spivey, A. C.; Schofield, C. J.; Sweeney, J. B.;
Tetrahedron 1993, 49, 6309-6330.
JA970502L