Regioselective triphenylamine-tether-directed synthesis of [60]fullerene bis-adducts

Article abstract of DOI:10.1016/j.tetlet.2008.11.022

The tether-directed regioselective synthesis of equatorial bis-adducts of [60]fullerene via the 1,3-dipolar cycloaddition reaction of azomethine ylides is reported. A mono-[60]fulleropyrrolidine adduct, derived via 1,3-dipolar cycloaddition of azomethine

Full text of DOI:10.1016/j.tetlet.2008.11.022

Tetrahedron Letters 50 (2009) 398–401
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Regioselective triphenylamine-tether-directed
synthesis of [60]fullerene bis-adducts
*
Georgios Rotas, Nikos Tagmatarchis
Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
The tether-directed regioselective synthesis of equatorial bis-adducts of [60]fullerene via the 1,3-dipolar
cycloaddition reaction of azomethine ylides is reported. A mono-[60]fulleropyrrolidine adduct, derived
via 1,3-dipolar cycloaddition of azomethine ylides generated in situ upon thermal condensation of tri-
Received 29 August 2008
Revised 27 October 2008
Accepted 7 November 2008
Available online 12 November 2008
phenylamine bis-aldehyde and an
a-amino acid, was isolated and further reacted to yield, exclusively
and selectively, the equatorial bis-adduct, which is structurally characterized by appropriate spectro-
scopic means.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
serves as an efficient hole-transporter and electroluminescent
material.6–8
Tether-directed functionalization of fullerenes is a method of
choice for the regioselective formation of multi-adducts.1 The nov-
elty of the method is that it offers the possibility for the synthesis
of specific fullerene bis-adducts (or multi-adducts), in moderate to
high yields, that are otherwise difficult and/or improbable to
obtain under standard, thermodynamically and/or kinetically
controlled reaction conditions.
The 1,3-dipolar cycloaddition of azomethine ylides to fullerenes
is a powerful methodology for the preparation of nanosized hybrid
materials suitable for diverse applications.2 Therefore, it is not sur-
prising that stepwise bis-cycloaddition of symmetric azomethine
ylides onto the skeleton of [60]fullerene has led to a mixture of
all possible eight bis-adducts. HPLC separation of the bis-adduct
mixture into single isomers has also been achieved.3
2. Results and discussion
Our approach for the tether-directed regioselective formation of
[60]fullerene bis-adducts is based on a two-step mono-addition
reaction sequence of azomethine ylides onto [60]fullerene, in
which TPA itself is utilized as a rigid tether. In the first step, 1,3-
dipolar cycloaddition of azomethine ylides, generated in situ upon
thermal condensation of TPA bis-aldehyde and sarcosine onto
[60]fullerene, resulted in the mono-fulleropyrrolidine adduct hav-
ing a free aldehyde unit. In the following step, further reaction with
sarcosine resulted in the formation of the second fused pyrrolidine
ring onto the fullerene sphere, with complete regioselectivity, at
the equatorial position (Scheme 1).
The use of a tether in the 1,3-dipolar cycloaddition of azome-
thine ylides to [60]fullerene has already been employed. However,
the synthesis of mixtures containing bis-adduct isomers that have
not been separated is reported,4 while separation of such a mixture
and characterization of isolated [60]fulleropyrrolidine tethered
bis-adducts have only recently been achieved.5
Herein, we report on the regioselective synthesis of an equato-
rial [60]fullerene bis-adduct via cycloaddition of in situ generated
azomethine ylides upon thermal condensation of triphenylamine
In a typical experiment, when a toluene solution of [60]fuller-
ene was treated at reflux with 1 equiv of 4,40-diformyltriphenyl-
amine9 and 10 equiv of sarcosine for 2.5 h, the mono-adduct 1a,
having a free aldehyde, was obtained in 31% yield (76% based on
recovered [60]fullerene) after column chromatography. At this
point, it should be emphasized that performing the reaction with
either an excess of TPA bis-aldehyde (2.5 equiv) and/or higher
(TPA) bis-aldehyde and an
a-amino acid. The aim of the current
work is two-fold, namely, (i) to incorporate the rigid TPA moiety
as a tether to direct, with full regioselectivity, the introduction of
the second fused pyrrolidine unit at the fullerene sphere and, (ii)
to prepare a new push-pull hybrid material consisting of the good
electron acceptor [60]fullerene and electron donor TPA, which also
* Corresponding author. Tel.: +30 210 7273835; fax: +30 210 7273794.
E-mail address: tagmatar@eie.gr (N. Tagmatarchis).
Scheme 1. Synthetic route of TPA-bridged [60]fullerene bis-adducts.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.022

G. Rotas, N. Tagmatarchis / Tetrahedron Letters 50 (2009) 398–401
399
temperature/shorter reaction time conditions (i.e., refluxing
chlorobenzene for 15 min) did not have a positive effect on the
yield of 1a.
chemical shift. This is in sharp contrast to the AA0XX0 patterns of
the mono-adducts as shown in Figure 1. The two pyrrolidine rings
showing characteristic patterns are discernible, for example, in the
spectrum of 2a, a pair of doublets at 4.14 ppm and 4.86 ppm
(|2J| = 9.3 Hz) and a singlet at 4.60 ppm resembling the pyrrolidine
signals of the mono-adduct 1a (as doublets at 4.30 ppm and
4.99 ppm with |2J| = 9.3 Hz and a singlet at 4.96 ppm) were evident,
while the second pyrrolidine appears as a pair of doublets at
4.82 ppm and 5.05 ppm (|2J| = 12.3 Hz) and a much less shielded
singlet at 6.17 ppm. The corresponding 1H NMR spectra of mono-
adduct 1b and bis-adduct 2b are presented in the Supplementary
data (Figs. S1 and S2). The limited solubility of 2a hindered our at-
tempts to record a suitable 13C NMR spectrum; however, we were
able to record the 13C NMR spectrum of mono-adduct 1a, as well as
that of 1b (Supplementary data, Figs. S3 and S4). However, as pre-
viously mentioned, the enhanced solubility obtained for bis-adduct
2b allowed us to record the 13C NMR spectrum (Fig. 1). A total of 71
signals in the sp2 region between 160 and 114 ppm were found, as
well as 23 signals in the sp3 region, between 81 and 28 ppm. Com-
bination of the 13C and 1H NMR measurements indicates that
bis-adduct 2b possesses C1 molecular symmetry. Apparently, the
bis-adduct is nonsymmetric, having two chiral centres in the
pyrrolidine rings, with the structure shown in Scheme 1 being a
racemate.5
Upon thermal treatment of mono-adduct 1a with sarcosine
(5 equiv) in o-dichlorobenzene (DCB) at 133 °C, bis-adduct 2a
was formed in 30% yield (41% yield based on recovered 1a). Similar
results were obtained in refluxing chlorobenzene. In general, tem-
peratures greater than 120 °C are ideal for the formation of the bis-
adduct (i.e., very poor results were obtained in toluene); however,
special caution is needed as extensive degradation occurs at tem-
peratures above 140 °C.
As 2a was insoluble in polar solvents and showed limited solu-
bility even in carbon disulfide, a more polar a-amino acid was em-
ployed in the azomethine ylide reaction with [60]fullerene. Thus,
we were able to perform a complete spectroscopic characterization
and study of the so-formed bis-adduct. Reaction of [60]fullerene
with TPA bis-aldehyde (2 equiv) and tert-butoxy carbonyl
(Boc)-((aminoethoxy)ethoxy)ethyl (AEE) N-substituted glycine10
(1 equiv), in refluxing toluene for 15 min, afforded the mono-ad-
duct 1b in 33% yield (62% based on recovered [60]fullerene). In
the following step, 1b was treated with Boc–AEE–glycine (4 equiv)
in refluxing toluene for 30 min to provide the bis-adduct 2b in 35%
yield (53% based on recovered 1b). At this stage, the following
points should be highlighted: (i) 2b is very soluble in chloroform
and DMF, in contrast to 2a which shows limited solubility, (ii) reac-
tions leading to 2b were faster and took place at lower tempera-
tures compared to those applied for the synthesis of 2a, which is
In the attenuated-total-reflectance infra-red (ATR-IR) spectra of
2b, the characteristic peaks for the protecting Boc carbonyl
(1708 cmꢁ1) and NH (3438 cmꢁ1 and 3342 cmꢁ1) groups were evi-
dent (Supplementary data, Fig. S5). Moreover, in both the mono-
most likely rationalized to the better solubility of the a-amino acid
utilized and, (iii) HPLC (Japan Analytical Industry Co Ltd, utilizing a
Buckyprep 20 ꢀ 250 column with toluene as eluent at 10 mL/min
flow rate) for the purification of 1b and 2b is preferable.
Attempts to synthesize bis-adducts 2a and 2b in one-step, that
is, directly from [60]fullerene without isolating the corresponding
mono-adduct, were not successful. Competitive formation of a di-
mer, as well as degradation, polymerization and/or multiaddition
reactions occurred, resulting in the formation of inseparable and
complex mixtures in very low yields.
and bis-adducts, the C–H stretching vibrations (3030–2800 cmꢁ1
)
as well as the characteristic absorptions for fullerenes were also
present.
Matrix-assisted-laser-desorption-ionization time-of-flight mass-
spectrometry (MALDI-TOF-MS), in the negative ionization mode,
showed molecular ion peaks for both the mono- and bis-adducts
(Supplementary data, Figs. S6 and S7). As commonly observed in
fullerene derivatives, the molecular ion peak was accompanied
by an intense peak at m/z = 720, corresponding to the C60 fragment.
In the electronic absorption (UV–vis) spectra of monoadducts
1a and 1b obtained in toluene, the characteristic absorptions of a
In the 1H NMR spectra of the bis-adducts, each proton of the
TPA phenyl rings adjacent to the fullerene core showed a unique
Figure 1. 1H NMR (300 MHz) spectra of mono-adduct 1a (top, 50% CS2/CDCl3) and bis-adduct 2a (middle, CS2) and the 13C NMR (125 MHz, CDCl3) spectrum of bis-adduct 2b
(bottom).

400
G. Rotas, N. Tagmatarchis / Tetrahedron Letters 50 (2009) 398–401
position. Interestingly, the performance of the second cycloaddi-
tion reaction onto the fullerene core with a different -amino acid
a
than that utilized in the first step allows the preparation of novel
hybrid bis-fullerene adducts. Experiments along these lines are
currently underway in our laboratories and the results will be
reported in due course.
3. Experimental
3.1. Mono-adduct 1a
A mixture of C60 (100 mg, 0.139 mmol), sarcosine (124 mg,
1.39 mmol) and 4,40-diformyltriphenylamine (42 mg, 0.139 mmol)
in toluene (100 mL) was stirred at reflux for 2.5 h. After cooling,
petroleum ether (65 mL) was added, and the mixture was sub-
jected to column chromatography (silica gel) using 40% petroleum
ether/toluene to collect unreacted C60 (40 mg, 67%), toluene (traces
of bis-adduct and dumbbell dimer) and finally 2% ethyl acetate/tol-
uene to yield 1a as a dark red solid (45 mg, 31%). ATR-IR (neat):
v(cmꢁ1) 3032, 2948, 2919, 2781, 1689, 1586, 1505, 1488, 1463,
1427, 1317, 1283, 1268, 1216, 1159, 1124, 1105, 903, 821, 725,
692, 552; 1H NMR (300 MHz, 50% CS2/CDCl3): d (ppm) 2.89 (s,
3H), 4.30 (d, J = 9.3 Hz, 1H), 4.96 (s, 1H), 4.99 (d, J = 9.3 Hz, 1H),
6.92–6.97 (AA0XX0, 2H), 7.04–7.08 (AA0XX0, 2H), 7.12 (t, J = 7.2 Hz,
1H), 7.17–7.21 (AA0XX0, 2H), 7.25–7.31 (m, 2H), 7.59–7.63 (AA0XX0,
2H), 7.77 (br s, 2H), 9.77 (s, 1H); 13C NMR (75 MHz, 50% CS2/CDCl3):
d (ppm) 189.28, 156.12, 153.79, 153.20, 153.09, 152.71, 147.35,
147.32, 146.66, 146.43, 146.43, 146.35, 146.34, 146.33, 146.26,
146.23, 146.15, 146.00, 145.96, 145.75, 145.65, 145.55, 145.51,
145.43, 145.40, 145.35, 145.33, 145.29, 145.19, 145.18, 144.76,
144.67, 144.44, 144.37, 143.25, 143.09, 142.76, 142.70, 142.66,
142.66, 142.29, 142.27, 142.21, 142.14, 142.11, 142.09, 142.02,
142.02, 141.79, 141.73, 141.68, 140.29, 140.25, 140.01, 139.17,
136.84, 136.53, 136.02, 135.77, 133.69, 131.23, 130.49, 129.85,
129.68, 126.12, 126.04, 125.18, 119.96, 82.90, 77.32, 70.06, 68.89,
40.11; MALDI-TOF MS calcd for C82H20N2O: 1048, found: m/z
1049 [M+H]+.
Figure 2. UV–vis absorption spectra of 2a (black) and 2b (grey), obtained in
toluene.
fulleropyrrolidine at 433 nm and that of the TPA unit in the UV
region were evident (Supplementary data, Fig. S8). Fullerene bis-
adducts 2a and 2b showed spectra possessing the pattern of equa-
torial bis-addition11 onto the fullerene core (Fig. 2). The latter is in
full accordance with the absence of molecular symmetry, a result
already demonstrated by the NMR measurements.
Preliminary photophysical studies of the bis-adducts were also
performed. In this context, the steady-state fluorescence emission
spectra of 2b in an apolar (toluene) and a polar (DMF) solvent were
recorded upon excitation of the fullerene moiety at 430 nm (Fig. 3).
The insolubility of 2a in DMF precluded such measurement. The
fluorescence emission of 2b observed at 778 nm with a shoulder
at 710 nm in toluene (fulleropyrrolidine bis-adducts are reported
to emit in this region12) appeared red-shifted (789 nm with a
shoulder at 720 nm), broadened and depressed in the DMF solu-
tion. Such a solvatochromic red-shift suggests stabilization of the
excited-state in polar media, whereas fluorescence emission
quenching suggests electron transfer processes towards a charge-
separated state. Further photophysical measurements, which are
currently underway, are expected to shed light on the actual mech-
anism of the charge transfer phenomena.
3.2. Bis-adduct 2a
In conclusion, the tether-directed regioselective bis-addition of
TPA bis-aldehyde onto [60]fullerene was demonstrated. It was
shown that in a two-step cycloaddition reaction sequence, the
presence of the rigid TPA moiety directs the second pyrrolidine
unit fused onto the fullerene sphere exclusively at the equatorial
A mixture of 1a (30 mg, 0.029 mmol) and sarcosine (13 mg,
0.143 mmol) in o-dichlorobenzene (30 mL) was stirred at 133 °C
for 45 min. After cooling, the mixture was subjected to column
chromatography using o-dichlorobenzene to collect 2a as a dark
red solid (9 mg, 30%), followed by 2% ethyl acetate/toluene to col-
lect unreacted 1a (8 mg, 26%). ATR-IR (neat): v(cmꢁ1) 3026, 2942,
2779, 1592, 1504, 1492, 1455, 1328, 1227, 1177, 1101, 856, 820,
739, 690, 551; 1H NMR (300 MHz, CS2, C6D6 insert as external
lock): d (ppm) 3.02 (s, 3H), 3.82 (s, 3H), 4.14 (d, J = 9.3 Hz, 1H),
4.60 (s, 1H), 4.82 (d, J = 12.3 Hz, 1H), 4.86 (d, J = 9.3 Hz, 1H), 5.05
(d, J = 12.3 Hz, 1H), 6.17 (s, 1H), 7.04 (t, J = 7.2 Hz, 1H), 7.15–7.21
(m, 3H), 7.30 (dd, J = 8.3, 1.6 Hz, 1H), 7.37–7.47 (m, 4H), 7.59 (dd,
J = 8.3, 1.9 Hz, 1H), 7.69 (dd, J = 8.4, 1.9 Hz, 1H), 7.83–7.89 (m,
2H); MALDI-TOF MS calcd for C84H25N3: 1075, found: m/z 1077
[M+2H]+.
3.3. Mono-adduct 1b
A
mixture of C60 (100 mg, 0.139 mmol), Boc–AEE–glycine
(43 mg, 0.139 mmol) and 4,40-diformyltriphenylamine (84 mg,
0.278 mmol) in toluene (100 mL) was stirred at reflux for 15 min.
After cooling, the mixture was subjected to column chromatogra-
phy using toluene to collect unreacted C60 (46 mg, 46%), 5–10%
ethyl acetate/toluene (unreacted triphenylamine) and 20% ethyl
acetate/toluene to collect the impure product which was further
Figure 3. Steady-state fluorescence spectra of bis-adduct 2b, obtained in toluene
(black) and DMF (grey); kex = 430 nm.

G. Rotas, N. Tagmatarchis / Tetrahedron Letters 50 (2009) 398–401
401
purified by preparative HPLC (Rt = 14.37 min), resulting in 1b as a
dark red solid (58 mg, 33%). ATR-IR (neat): v(cmꢁ1) 3444, 3344,
3029, 2973, 2924, 2801, 1706, 1690, 1585, 1503, 1489, 1459,
1427, 1318, 1270, 1159, 1107, 902, 821, 725, 695, 552; 1H NMR
(300 MHz, CDCl3): d (ppm) 1.44 (s, 9H), 2.95 (dt, J = 11.8, 5.5 Hz,
1H), 3.31–3.36 (m, 2H), 3.54–3.62 (m, 3H), 3.72–3.75 (m, 2H),
3.80–3.86 (m, 2H), 4.00–4.12 (m, 2H), 4.32 (d, J = 9.6 Hz, 1H),
4.98 (br s, 1H), 5.18 (s, 1H), 5.21 (d, J = 9.6 Hz, 1H), 6.92–6.96
(AA0XX0, 2H), 7.06–7.09 (AA0XX0, 2H), 7.13 (t, J = 7.2 Hz, 1H), 7.17–
7.21 (AA0XX0, 2H), 7.26–7.32 (m, 2H), 7.61–7.65 (AA0XX0, 2H),
7.77 (br s, 2H), 9.78 (s, 1H); 13C NMR (125 MHz, CDCl3): 190.30,
156.37, 155.85, 153.99, 153.36, 153.09, 152.97, 147.30, 147.27,
146.67, 146.38, 146.26, 146.25, 146.17, 146.14, 146.08, 146.07,
145.93, 145.92, 145.67, 145.52, 145.47, 145.43, 145.41, 145.31,
145.28, 145.26, 145.20, 145.11, 144.70, 144.61, 144.38, 144.31,
143.17, 142.98, 142.67, 142.60, 142.55, 142.23, 142.20, 142.13,
142.12, 142.06, 142.03, 141.98, 141.96, 141.92, 141.67, 141.63,
141.59, 140.18, 140.10, 139.84, 139.00, 136.68, 136.42, 135.94,
135.64, 133.99, 131.23, 130.54, 129.66, 129.32, 126.17, 125.99,
125.02, 119.67, 81.72, 79.20, 76.45, 70.59, 70.45, 70.37, 70.21,
69.05, 67.63, 52.17, 40.35, 28.43; MALDI-TOF MS calcd for
C92H39N3O5: 1266, found: m/z 1267 [M+H]+.
133.98, 131.96, 130.63, 130.26, 130.03, 129.11, 128.97, 128.76,
128.54, 126.36, 118.24, 114.86, 81.25, 80.57, 79.93, 79.23, 77.21,
76.46, 75.82, 73.43, 72.19, 70.72, 70.52, 70.38, 70.37, 70.36,
70.33, 69.11, 68.04, 66.66, 65.16, 55.74, 51.74, 40.40, 28.45; MAL-
DI-TOF MS calcd for C104H63N5O8: 1510, found: m/z 1510 [M]+.
Acknowledgements
Partial financial support from the EU FP7, Capacities Program,
NANOHOST project (GA 201729) and the EU FP6, Transfer of
Knowledge (ToK) Program, FDPA project (MTKD-CT-2006-
042488) is acknowledged. We are indebted to Dr. K. Porfyrakis at
Oxford University for the MALDI-TOF-MS and to Dr. A. Panagioto-
poulou at NCSR ‘Democritos’ for some initial NMR spectra.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.11.022.
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A mixture of 1b (30 mg, 0.024 mmol) and Boc–AEE–glycine
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