Stabilization of fulleropyrrolidine N-oxides through intrarotaxane hydrogen bonding

Article abstract of DOI:10.1039/b617835a

The chemical stabilization of labile fulleropyrrolidine N-oxides is achieved by encapsulation through intrarotaxane hydrogen bonding. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617835a

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Stabilization of fulleropyrrolidine N-oxides through intrarotaxane
hydrogen bonding{
Aurelio Mateo-Alonso,* Peter Brough and Maurizio Prato*
Received (in Cambridge, UK) 6th December 2006, Accepted 8th January 2007
First published as an Advance Article on the web 29th January 2007
DOI: 10.1039/b617835a
The chemical stabilization of labile fulleropyrrolidine N-oxides
is achieved by encapsulation through intrarotaxane hydrogen
bonding.
by the macrocycle increases its chemical stability and inhibits its
deoxygenation.
21
We began our research by studying thread 1, which consists of
a fullerene stopper linked to a succinamide template that is
connected to a diphenyl stopper through a long alkyl chain
(Scheme 1). The known epoxidation reactions of fullerene itself are
Since the commercialization of fullerenes in 1990, several protocols
1
,2
for their functionalization have been developed. One of the most
2
3
versatile and popular procedures is the 1,3-dipolar cycloaddition of
of some concern, and, to minimize this undesired side reaction,
we employed high dilution conditions. A dilute solution of
mCPBA was added to a dilute solution of thread 1 over one
hour. The reaction was monitored using thin layer chromato-
graphy and was never driven to completion in order to avoid the
formation of mixed N-oxide–epoxide bisadducts. Purification by
flash chromatography and reprecipitation yielded the desired
thread N-oxide 2. The solubility of thread N-oxide 2 in organic
solvents is different to its unoxidized counterpart; for
example, thread 1 dissolves well in chloroform, whereas thread
N-oxide 2 is only sparingly soluble. This is a common theme
spanning all of the molecules described in this work. However, the
N-oxide molecules are more soluble in solvent systems of greater
polarity.
3,4
azomethine ylides, which yields the highly stable fulleropyrro-
lidines. Recently, we have reported their oxidation to full-
eropyrrolidine N-oxides as
a novel family of fullerene
5
6
7
derivatives. Both fullerenes and N-oxides are well known for
their ability to stabilize negatively charged species. In principle, the
combination of both would potentially lead to groups with
7
improved ability for stabilizing radicals. This is particularly
important in molecular photovoltaics, where the lifetime of a
photogenerated radical-pair has a direct influence on solar
8
,9
photoconversion efficiency. One of the disadvantages of full-
eropyrrolidine N-oxides is their thermal instability, which compli-
cates their study and characterization. In fact, the derivatives are
only stable for days, even if stored at 220 uC. Their instability is
patent from the facile reversibility of the oxidation reaction.
Fulleropyrrolidine N-oxides can be cleanly deoxygenated to their
parent fulleropyrrolidines by heating in the presence of protic
The structure and behaviour of thread N-oxide 2 were
1
confirmed from H NMR and COSY experiments in different
solvents. The NMR spectra show the typical signals of full-
5
eropyrrolidine N-oxides. From the unoxidized thread 1 to the
1
oxidized thread 2 there is a distinct change in the H NMR
5
solvents.
A practical approach for the stabilization of labile species is
10
based on their encapsulation by other molecules. In this context,
rotaxanes have shown to be very useful since they are comprised of
a molecular axle encapsulated by a macrocycle. Encapsulation in
rotaxanes has been applied not only in the enhancement of
chemical shift of the protons D (the assignments correspond to the
lettering shown in Scheme 1) belonging to the pyrrolidine group
(Fig. 1a and 1c). For thread 1, they show as a singlet at 4.5 ppm,
owing to rapid pyramidal inversion, whereas in the case of thread
N-oxide 2, they split as an AB quartet at 5.1 ppm. The signals of
protons J and G are shifted downfield due to the inductive effect of
the partial positive charge on the nitrogen of the fulleropyrrolidine.
Remarkably, the amidic proton A of thread N-oxide 2 is shifted
downfield from 6.8 to 8.2 ppm, indicating that is hydrogen
bonded. At a closer look, proton A and the fulleropyrrolidine
N-oxide can establish an intramolecular hydrogen bond through a
highly favoured chair-like complex. N-Oxides can form stronger
hydrogen bonds than amides since they are more polarized and
11
11–14
chemical and photochemical
also as a way to modulate the physical properties
encapsulated parts. We have described the synthesis and properties
stability of other molecules but
15–18
of the
19–22
of several fulleropyrrolidine stoppered rotaxanes
using the
benzylic amide protocol of Leigh et al. In such rotaxanes, the
macrocycle is clipped through hydrogen bond recognition between
the precursors of the macrocycle and a template comprised of
amides as binding sites. In this article, we report how the
macrocycle can be translocated from a succinamide template to
bind preferentially and encapsulate a fulleropyrrolidine N-oxide by
hydrogen bonding. Remarkably, the encapsulation of the N-oxide
24
possess a higher hydrogen bond basicity. Since no other
interactions can be identified and the NMR experiments were
2
3
carried out in very dilute solutions (%4 6 10 M) due to the low
solubility of thread N-oxide 2, we must assume the intramolecular
Italian Interuniversity Consortium on Materials Science and Technology
(INSTM), Unit of Trieste, Dipartimento di Scienze Farmaceutiche,
1
hydrogen bonded structure shown in Scheme 1. The NMR ( H
Universit a` degli Studi di Trieste, Piazzale Europa 1, 34127 Trieste, Italy.
E-mail: amateo@units.it; prato@units.it; Fax: +39 040 52572;
Tel: +39 040 5587883
{ Electronic supplementary information (ESI) available: Full experimental
procedures. See DOI: 10.1039/b617835a
and COSY) experiments carried out in DMSO-d confirm the
6
structure of thread N-oxide 2. In this case, the chemical shifts of
the amidic protons (A, B and C) correspond with those of thread 1
in the same solvent, showing that there are no hydrogen bonds. In
1
412 | Chem. Commun., 2007, 1412–1414
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Scheme 1 Synthesis and behaviour of fulleropyrrolidine N-oxides.
3 3 3
Fig. 1 400 MHz NMR spectra of (a) thread 1; (b) rotaxane 3; (c) thread N-oxide 2 in CDCl ; (d) rotaxane 4 in CDCl –CD OD 99 : 1. Peaks highlighted
with stars correspond to residual solvent signals.
fact, DMSO possesses similar hydrogen bond basicity to that of
Rotaxane 3 was assembled by simultaneous addition of
isophthaloyl chloride and p-xylylenediamine to a solution of
24
the N-oxide, breaking the intramolecular hydrogen bonds by
preferentially solvating the hydrogen bonding sites.
21
thread 1 in the presence of triethylamine. Purification by flash
chromatography to separate unreacted thread 1, followed by
reprecipitation–centrifugation, yielded rotaxane 3. NMR spectro-
scopy is the best tool to locate precisely the location of the
macrocycle along the thread. The encapsulated thread protons are
shielded by the anisotropy effect of the aromatic rings of the
Thread N-oxide 2 presents enhanced stability when compared
with other fulleropyrrolidine N-oxides and is stable for months at
220 uC. This might be due to the stabilization of the N-oxide by
intramolecular hydrogen bonding. Nevertheless, thread N-oxide 2
behaves as the previous fulleropyrrolidine N-oxides and can be
converted to the initial thread 1 by thermal deoxygenation in a
solution of toluene–MeOH (4 : 1).
3
macrocycle, revealing its position. In CDCl , protons K were
shielded by nearly 1.4 ppm relative to the thread, showing that the
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Chem. Commun., 2007, 1412–1414 | 1413

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macrocycle is located over the succinamide template (Scheme 1,
Fig. 1a and 1b).
We thank Prof. David A. Leigh for helpful discussions and for
introducing us to this field. This work was carried out with partial
support from the University of Trieste, INSTM, MIUR (PRIN
2006, prot. 2006034372 and Firb, prot. RBNE033KMA), and the
EU (RTN networks ‘‘WONDERFULL’’ and ‘‘EMMMA’’).
Rotaxane 3 was allowed to react with mCPBA using the high
dilution conditions for the preparation of thread N-oxide 2.
Rotaxane 4 was purified from unreacted rotaxane 3 by flash
chromatography, followed by reprecipitation–centrifugation. As
expected, rotaxane 4 presents the lowest solubility of the whole set
Notes and references
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3
1
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3
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2
3
4
(Fig. 1c) and are deshielded by 1.1 ppm in comparison with
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1
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months at 220 uC. Additionally, rotaxane 4 did not undergo
deoxygenation under the typical thermal activation conditions, 12 h
reflux in toluene–methanol (4 : 1). These results indicate that the
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414 | Chem. Commun., 2007, 1412–1414
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