Methanofullerene-functionalized dendritic branches

Article abstract of DOI:10.1016/S0040-4039(99)01996-6

Fullerene-functionalized dendrons containing a C₆₀ sphere at each branching unit have been prepared by a convergent approach using successive DCC-mediated esterifications followed by cleavage of a t-butyl ester moiety under acidic conditions.

Full text of DOI:10.1016/S0040-4039(99)01996-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 41–44
Methanofullerene-functionalized dendritic branches
Jean-François Nierengarten,∗ Delphine Felder and Jean-François Nicoud
Groupe des Matériaux Organiques, Institut de Physique et Chimie des Matériaux de Strasbourg,
Université Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg, France
Received 26 September 1999; accepted 21 October 1999
Abstract
Fullerene-functionalized dendrons containing a C60 sphere at each branching unit have been prepared by a
convergent approach using successive DCC-mediated esterifications followed by cleavage of a t-butyl ester moiety
under acidic conditions. © 1999 Elsevier Science Ltd. All rights reserved.
Dendrimers have attracted increasing attention in the past decade because of their unique structures
and properties.1,2 Of the various cores utilized for the synthesis of dendrimers, C60 appears to be a
versatile tecton for dendrimer chemistry.3 Effectively, the almost spherical shape of the fullerene leads to
globular systems even with low-generation dendrons. Furthermore, variable degrees of addition within
the fullerene core, especially from mono- up to hexaadducts, are possible.3 It has also been shown that
the dendritic part of amphiphilic dendrimers with a C60 core is able to prevent the aggregation of the
fullerene moieties during the preparation of Langmuir films.4 We have recently reported the efficient
preparation of dendritic branches with peripheral fullerene units5 and their attachment to a functional
core yielded monodisperse fullerene-rich macromolecules.6 We have also shown that the fullerene-rich
microenvironment is able to modulate the physical properties of the central core.7 As part of this research,
we now report the synthesis of the first examples of fullerene-functionalized dendrons containing a C60
sphere at each branching unit.
The preparation of 1 and 2, the two key building blocks for the preparation of the fullerene-
functionalized dendritic branches, is depicted in Scheme 1. Compound 1 can be considered as the first
generation dendron and diol 2 is the branching unit. N,N0-Dicyclohexylcarbodiimide (DCC)-mediated
esterification of 35 with t-butyl 2-hydroxyacetate8 in CH2Cl2 yielded malonate 4 in 93% yield. The
subsequent functionalization of C60 with 4 and iodine in the presence of diazabicyclo[5.4.0]undec-7-
ene (DBU) in toluene at room temperature is based on the Bingel reaction9 and gave methanofullerene
5 in 44% yield. Selective cleavage of the t-butyl ester moiety with CF3CO2H in CH2Cl2 then gave 1
in 97% yield. Treatment of 1,6-hexanediol with Ag2O and p-methoxybenzyl chloride (PMBCl) under
the conditions developed by A. Bouzide and G. Sauvé10 afforded the mono-protected derivative 6 in
∗
Corresponding author. Fax: 33 388 10 72 46; e-mail: niereng@michelangelo.u-strasbg.fr
0040-4039/00/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)01996-6

42
58% yield. Treatment with CBr4/PPh3 in THF at room temperature followed by the reaction of the
resulting bromide with 3,5-dihydroxybenzyl alcohol in DMF at 70°C in the presence of K2CO3 yielded
7. Subsequent reaction with 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum’s acid)5 at 120°C gave the
mono-ester 8 of malonic acid in a quantitative yield. DCC-mediated esterification of 8 with t-butyl 2-
hydroxyacetate in CH2Cl2 then yielded malonate 9, which after treatment with C60, I2 and DBU in
toluene at room temperature gave 10 in 30% yield.
Scheme 1. Synthesis of 1 and 2. Reagents and conditions: (a) t-butyl 2-hydroxyacetate, DCC, DMAP, CH2Cl2, 0°C to rt, 24 h
(93%); (b) C60, I2, DBU, toluene, rt, 6 h (44%); (c) CF3CO2H, CH2Cl2, rt, 1 h (97%); (d) PMBCl, Ag2O, CH2Cl2, rt, 4 h (58%);
(e) CBr4, PPh3, THF, rt, 30 min (79%); (f) 3,5-dihydroxybenzyl alcohol, DMF, K2CO3, 70°C, 20 h (52%); (g) Meldrum’s acid,
120°C, 3 h (99%); (h) t-butyl 2-hydroxyacetate, DCC, DMAP, CH2Cl2, 0°C to rt, 48 h (91%); (i) C60, I2, DBU, toluene, rt, 6 h
(30%); (j) DDQ, CH2Cl2, H2O, rt, 1 h (84%)
The choice of the appropriate protecting group for the two alcohol groups in 10 was the key to this
synthesis. Effectively, the deprotection conditions must not be acidic to preserve the t-butyl ester moiety
and may not be basic to preserve the other ester functions. Furthermore, the decomposition of fullerene
derivatives under reaction conditions using fluoride11 prevents the use of silyl protecting groups. The
PMB protecting groups in 10 could be removed with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) in
CH2Cl2 containing a small amount of water at room temperature.12 Under these neutral conditions, all
the ester functions remained unchanged and compound 2 was thus obtained in a good yield (84%).
Reaction of acid 1 with diol 2 under esterification conditions using DCC, 1-hydroxybenzotriazole
(BtOH) and 4-dimethylaminopyridine (DMAP) led to the t-butyl-protected second generation dendron
11 in 70% yield (Scheme 2). Subsequent hydrolysis of the t-butyl ester group under acidic conditions
afforded acid 12 in high yield (98%).

43
Scheme 2. Synthesis of dendron 12. Reagents and conditions: (a) 2, DCC, DMAP, BtOH, CH2Cl2, 0°C to rt, 24 h (70%); (b)
CF3CO2H, CH2Cl2, rt, 1 h (98%)
Esterification of acid 12 with diol 2 (DCC/DMAP/BtOH) afforded 13 in 37% yield and subsequent
hydrolysis of the t-butyl ester group with CF3CO2H gave the third generation carboxylic acid 14
(Scheme 3). All of the spectroscopic studies were consistent with the proposed molecular structures
assigned to the dendrons of each generation.13
Scheme 3. Synthesis of dendron 14. Reagents and conditions: (a) 2, DCC, DMAP, BtOH, CH2Cl2, 0°C to rt, 24 h (37%); (b)
CF3CO2H, CH2Cl2, rt, 3 h (80%)

44
Whereas the FAB-MS spectra of methanofullerenes 1, 2 and 5 showed the expected molecular ion
peaks, no characteristic peaks could be observed for the dendrons of highest generation. Actually,
due to aggregation resulting probably from fullerene–fullerene interactions, high energy is required
for dissociation during FAB-MS analysis and therefore fragmentation occurs, especially on the fragile
benzylic ester functions. Furthermore, the molecular masses are quite high and no easily protonable sites
1
are present in these dendrons. Nevertheless, the H and 13C NMR, IR, UV–vis and elemental analysis
data obtained for the dendrons provide good evidence for the proposed structures.
Following the preparation of dendrimers with a fullerene core or with peripheral C60 subunits, we have
now succeeded in the preparation of fullerene-functionalized dendrons containing a C60 sphere at each
branching unit. Those new dendritic branches appear to be versatile building blocks for the preparation
of fullerene-rich macromolecules and their attachment on a functional core is now under investigation in
our laboratory.
Acknowledgements
We thank A. Van Dorsselaer, H. Nierengarten and R. Hueber for recording the mass spectra, J.-D.
Sauer for NMR measurements and L. Oswald for technical help.
References
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8. t-Butyl 2-hydroxyacetate was prepared in two steps from t-butyl 2-bromoacetate as described in: Jurayj, J.; Cushman, M.
Tetrahedron 1992, 48, 8601–8614.
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10. Bouzide, A.; Sauvé, G. Tetrahedron Lett. 1997, 38, 5945–5948.
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1
13. Selected spectroscopic data for 11: H NMR (CDCl3, 200 MHz): 0.88 (t, J=6, 12H), 1.20–1.43 (m, 52H), 1.53 (s, 9H),
1.60–1.80 (m, 12H), 3.86 (t, J=6, 4H), 3.90 (t, J=6, 8H), 4.22 (t, J=6, 4H), 4.84 (s, 2H), 4.94 (s, 4H), 5.46 (s, 6H), 6.33
(t, J=2, 1H), 6.40 (t, J=2, 2H), 6.57 (d, J=2, 2H), 6.60 (d, J=2, 4H). Anal. calcd for C264H122O24: C 86.22%, H 3.34%.
Found: C 86.07%, H 3.36%. For 13: 1H NMR (CDCl3, 400 MHz): 0.88 (t, J=6, 24H), 1.20–1.46 (m, 116H), 1.54 (s, 9H),
1.56–1.79 (m, 28H), 3.85 (t, J=6, 4H), 3.86 (t, J=6, 8H), 3.91 (t, J=6, 16H), 4.23 (t, J=6, 4H), 4.24 (t, J=6, 8H), 4.85 (s,
2H), 4.94 (s, 12H), 5.45 (s, 2H), 5.47 (s, 12H), 6.33 (t, J=2, 1H), 6.35 (t, J=2, 2H), 6.42 (t, J=2, 4H), 6.56 (d, J=2, 4H), 6.58
(d, J=2, 2H), 6.61 (d, J=2, 8H). Anal. calcd for C608H266O56·3CHCl3: C 83.16%. H 3.07%. Found: C 83.09%, H 3.18%.