An unexpected Diels-Alder reaction on the fullerene core rather than an expected 1,3-dipolar cycloaddition

Article abstract of DOI:10.1039/b201122k

Fullerene derivatives resulting from an unexpected Diels-Alder cycloaddition have been obtained by reaction of trans-2-stilbenecarboxaldehyde derivatives with N-methylglycine and C₆₀.

Full text of DOI:10.1039/b201122k

An unexpected Diels–Alder reaction on the fullerene core rather than an
expected 1,3-dipolar cycloaddition
Jean-François Eckert, Cyril Bourgogne and Jean-François Nierengarten*
Institut de Physique et Chimie des Matériaux de Strasbourg, Groupe des Matériaux Organiques, Université
Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg Cedex, France.
E-mail: niereng@ipcms.u-strasbg.fr
Received (in Cambridge, UK) 31st January 2002, Accepted 22nd February 2002
First published as an Advance Article on the web 5th March 2002
Fullerene derivatives resulting from an unexpected Diels–
Alder cycloaddition have been obtained by reaction of trans-
2-stilbenecarboxaldehyde derivatives with N-methylglycine
and C60.
The structure of 5a–c were deduced from their 1H-NMR
spectra with the help of 2D-COSY and 2D-NOESY experi-
ments. The typical pattern seen in the 1H-NMR spectra for the
bicyclic structure is shown for compound 5a in Fig. 2. It must be
noted that the experimental vicinal coupling constants 3J
observed in the 1H-NMR spectrum of 5a are in perfect
agreement with the theoretical ones given by the Karplus–
Conroy curve4 based on the dihedral angles deduced from the
calculated structure of 5a.† In addition, the 2D-NOESY
spectrum of 5a provided further support for the stereochemistry
of the C atom bearing the tert-butyloxyphenyl group. Effec-
tively, NOE cross-peaks have been observed for the following
pairs of protons: Ho–HA, Ho–HC and HB–HD. The 13C-NMR
spectra of 5a–c are also in full agreement with the proposed
structures. For example, in addition to the resonances corre-
sponding to the sp2 carbons, the 13C-NMR spectrum recorded
for 5a in CD2Cl2 is characterized by nine signals: d = 29.10 and
78.62 for the t-Bu group, d = 49.27 for the N-CH3, d = 70.69
and 72.32 for the fullerene sp3 carbons, d = 47.96 (CH), 55.71
(CH2), 60.26 (CH) and 78.03 (CH) for the sp3 C atoms of the
cycle. Finally, it can also be pointed out that the UV-Vis spectra
of 5a–c show all the characteristic features described in the
literature for C60 Diels–Alder adducts.5
The 1,3-dipolar cycloaddition of azomethine ylides generated in
situ from aldehydes and N-methylglycine has proven to be a
very powerful procedure for the functionalization of C60 due to
its versatility and the ready availability of the starting
materials.1 The reaction is fast, clean and fulleropyrrolidines are
obtained in fair to good yields. As part of our research program
on the synthesis and the study of fullerene-donor systems,2 we
became interested in the preparation of new fulleropyrrolidines
by reaction of trans-2-stilbenecarboxaldehyde derivatives with
N-methylglycine and C60. Surprisingly, the expected 1,3-dipo-
lar cycloaddition on the fullerene core does not take place and
fullerene derivatives resulting from an unexpected Diels–Alder
reaction have been obtained instead. This new reaction and the
characterization of the resulting fullerene adducts are now
reported.
The synthetic approach to prepare the trans-2-stilbene-
carboxaldehyde derivatives 3a–c used in this study relies upon
the Heck cross-coupling reaction3 of 2-bromobenzaldehyde (2)
with the styrene derivatives 1a–c (Scheme 1).
Treatment of 2 and 1a–c with a catalytic amount of
Pd(OAc)2 in the presence of tri-o-tolylphosphine (POT) af-
forded the corresponding trans-stilbene derivatives 3a–c in 70
to 80% yield. All of the spectroscopic studies were consistent
with the proposed molecular structures. In particular, the E
configuration of the double bond in 3a–c was confirmed by a
coupling constant of ca. 17 Hz for the AB system corresponding
to the vinylic protons in their 1H-NMR spectra. The reaction of
aldehydes 3a–c with C60 was performed under the conditions
used for the preparation of fulleropyrrolidines.1 However,
rather than the expected products 4a–c, the fullerene derivatives
5a–c were obtained in 40 to 42% yield. In a typical procedure,
a solution of 3a (194 mg, 0.69 mmol), C60 (1 equiv.) and N-
methylglycine (10 equiv.) in toluene (500 ml) was refluxed for
36 h under argon. The reaction mixture was then filtered over
celite and the solvent evaporated. Two successive chromato-
graphic separations (SiO2, toluene–hexane 6+4 then Al2O3,
CH2Cl2–hexane 3+7) yielded pure 5a (284 mg, 40%).
As shown in Fig. 1, this compound must be the product of the
Diels–Alder reaction of C60 with diene ( )-7 which itself results
from a cyclisation of the intermediate azomethine ylide 6
initially formed by reaction of 3 with N-methylglycine.
In principle, the reaction of C60 with diene ( )-7 should lead
to two diastereoisomeric pairs of enantiomers [( )-5 and ( )-8]
depending on which face of ( )-7 the Diels–Alder [4 +
2]-cycloaddition takes place. Surprisingly, this reaction is
highly diastereoselective and only compound ( )-5 was ob-
tained. Molecular modeling studies have not revealed any
particular steric hindrance to explain the preferential approach
of the C60 on one face of ( )-7. Actually, it might be possible
that p–p interactions of the fullerene sphere with the phenyl
group in ( )-7 are able to assist the approach of the C60, thus
allowing a regioselective attack and the formation of ( )-5
only.
Scheme 1 Reagents and conditions: i, Pd(OAc)2, POT, Et3N, toluene, D (70
to 80%); ii, C60, N-methylglycine, toluene, D (40 to 42%).
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This journal is © The Royal Society of Chemistry 2002

Fig. 1 Proposed mechanism for the formation of the diene intermediate and
representation of the two possible diastereoisomers resulting from its
reaction with C60
.
Scheme 2 Reagents and conditions: i, CuCN, NaI, DMF, D (89%); ii, NBS,
AIBN, CCl4, D (40%); iii, P(OEt)3, D (80%); iv, 3,4,5-tridodecyl-
oxybenzaldehyde, t-BuOK, THF, 0 to 25 °C (74%), v, DIBAL-H, CH2Cl2,
D (54%); vi, C60, N-methylglycine, toluene, D (34%).
In conclusion, a new reaction for the functionalization of C60
has been discovered and new covalent fullerene derivatives for
materials science or biological applications should become
available by this facile new methodology.
Notes and references
†
Selected analytical data. For 5a: 1H-NMR (200 MHz, CD2Cl2): 7.66
(m, 3 H), 7.56 (m, 1 H), 7.23 (d, J = 8.5 Hz, 2 H), 6.95 (d, J = 8.5 Hz, 2H),
5.37 (s, 1 H), 4.82 (dd, JBC = 11.5 Hz, JBD = 4.5 Hz, 1 H), 4.64 (s, 1 H,
it can be noted that JAB = 0), 3.04 (dd, JCD = 11.5 Hz , JBD = 4.5 Hz, 1
H), 2.87 (s, 3 H), 2.40 (t, JCD = JBC = 11.5 Hz, 1 H), 1.34 (s, 9 H). 13C-
NMR (100 MHz, CD2Cl2): 158.74, 157.96, 157.95, 156.04, 154.81, 148.03,
146.82, 146.57, 146.53, 146.51, 145.95, 145.93, 145.91, 145.80, 145.75,
145.68, 145.20, 142.99, 142.98, 142.84, 142.67, 142.62, 142.36, 142.34,
142.32, 141.95, 141.88, 140.29, 139.37, 138.20, 137.95, 136.15, 135.76,
134.54, 129.83, 129.77, 129.47, 128.67, 128.35, 124.64, 78.62, 78.03,
72.32, 70.69, 60.26, 55.71, 49.27, 47.96, 29.10. FAB-MS: m/z (%): 1028.1
(55, MH+), 719.9 (100, C60+). UV-Vis (CH2Cl2): 256 (96000), 308 (32100),
433 (2700), 706 (310).
For 12: 1H-NMR (400 MHz, CD2Cl4): 7.83 (s, 1 H), 7.78 (d, J = 1.5 Hz,
1 H), 7.60 (d, J = 1.5 Hz, 1 H), 7.24 (AB, J = 16 Hz, 2 H), 6.89 (s, 2 H),
6.61 (s, 2 H), 5.42 (s, 1 H), 4.84 (dd, J = 11.5 Hz , J = 4.5 Hz, 1 H), 4.70
(s, 1 H), 4.06 (m, 12 H), 3.20 (dd, J = 11.5 Hz , J = 4.5 Hz, 1 H), 3.01 (s,
3 H), 2.59 (t, 2J = 3J = 11.5 Hz, 1 H), 1.90 (m, 12 H), 1.82 (m, 108 H), 0.96
(m, 18 H). ES-MS: 2164.8 (MH+). UV-Vis (CH2Cl2): 254 (110000), 326
(63200), 431 (3400), 706 (300).
Fig. 2 1H-NMR spectrum of 5a (CD2Cl2, 200 MHz).
The reaction conditions developed for the preparation of 5a–c
were also used for the synthesis of 12 (Scheme 2). The aldehyde
precursor 11 was obtained in five steps from 2-bromo-p-xylene
(9). Treatment with CuCN under Rosenmund–von Braun
conditions led to the corresponding benzonitrile derivative.
Benzylic halogenation with NBS followed by treatment with
triethylphosphite then gave bisphosphonate 10. Reaction of 10
with 3,4,5-tridodecyloxybenzaldehyde under Wadsworth–Em-
mons conditions and subsequent reduction of the resulting
nitrile with DIBAL-H afforded 11. The E configuration of both
double bonds in 11 was confirmed by coupling constants of ca.
16.5 Hz for the two AB systems corresponding to the two sets
of vinylic protons in its 1H-NMR spectrum. As observed for 3a–
c, treatment of 11 with an excess of N-methylglycine in the
presence of C60 gave the fullerene derivative 12 resulting from
a Diels–Alder cycloaddition and the expected fulleropyrrolidine
derivative could not be detected. All of the spectroscopic studies
were consistent with the proposed molecular structure.† In
particular, the 1H-NMR spectrum of 12 shows the characteristic
features of the bicyclic structure discussed for 5a–c. It is also
interesting to note that the diagnostic absorption band of the
extended conjugated p-system observed at 375 nm in the UV-
Vis spectrum of 11 is shifted to 326 nm in 12. The latter
observation is in perfect agreement with the reduced conjuga-
tion length resulting from the loss of a double bond during the
transformation of 11 into 12.
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