Isolation and characterization of [5,6]-pyrrolidino-Sc3N@Ih-C80 diastereomers

Article abstract of DOI:10.1039/c4cc04946b

Reactions of Sc₃N@I_h-C₈₀ with aziridine derivatives were conducted to afford the corresponding mono-adducts. A pair of diastereomers of the mono-adduct [5,6]-pyrrolidino-Sc₃N@I_h-C₈₀ was isolated and characterized by means of mass spectrometry, vis-NIR absorption spectroscopy, and electrochemical measurements. Structural analysis of the mono-adducts was conducted by NMR and single-crystal X-ray structure determinations. This journal is

Full text of DOI:10.1039/c4cc04946b

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Isolation and characterization of [5,6]-pyrrolidino-
Sc3N@Ih-C80 diastereomers†
Cite this: Chem. Commun., 2014,
50, 12552
Yutaka Maeda,*a Masato Kimura,b Chihiro Ueda,a Michio Yamada,a Toru Kikuchi,a
Mitsuaki Suzuki,a Wei-Wei Wang,c Naomi Mizorogi,b Nikolaos Karousis,d
Nikos Tagmatarchis,d Tadashi Hasegawa,a Marilyn M. Olmstead,e Alan L. Balch,e
Shigeru Nagasec and Takeshi Akasaka*abfg
Received 28th June 2014,
Accepted 5th August 2014
DOI: 10.1039/c4cc04946b
www.rsc.org/chemcomm
Reactions of Sc3N@Ih-C80 with aziridine derivatives were conducted successful reactions in fullerene chemistry. Azomethine ylides are
to afford the corresponding mono-adducts. A pair of diastereomers organic 1,3-dipoles possessing a carbanion next to an immonium
of the mono-adduct [5,6]-pyrrolidino-Sc3N@Ih-C80 was isolated and ion and can be readily produced upon decarboxylation of the
characterized by means of mass spectrometry, vis-NIR absorption immonium salts derived from the condensation of a-amino acids
spectroscopy, and electrochemical measurements. Structural analysis with aldehydes or ketones.10 Notably, functionalized aldehydes lead
of the mono-adducts was conducted by NMR and single-crystal X-ray to the formation of 2-substituted fulleropyrrolidines, whereas the
structure determinations.
reaction with N-substituted glycines leads to N-substituted full-
eropyrrolidines. Recently, the chiral control of fulleropyrrolidine of
Recent developments in the chemistry of endohedral metallo- C60 was achieved.11 In this context, the advantages, such as good
fullerenes (EMFs) have led to increasing efforts to elucidate yields, the high regioselectivity, and availability or easy preparation
how the chemical reactivity and selectivity of empty fullerenes of the starting materials in reactions of Sc3N@Ih-C80, encourage the
are changed by endohedral metal doping and how the electronic synthesis of functionalized materials to open the materials science
properties of metallofullerenes affect chemical functionalization.1 of metallofullerenes.12
The preparation and isolation of trimetallic nitride template
Alternatively, fulleropyrrolidines can be obtained via the thermal
endohedral metallofullerenes (M3N@Cn) in macroscopic quantities ring opening of aziridines.13 Moreover, the addition of aziridines has
have facilitated the study of their structures, properties, and been extended to carbon nanotubes, which upon thermal reaction
chemical reactivities.2 To date, the functionalization of with 2-alkoxycarbonylaziridine, afforded the corresponding
Sc3N@Ih-C80 has been widely performed using the Diels–Alder pyrrolidino-derivatives.14 Since custom-synthesized aziridines, carry-
reaction,3 [2+2] cycloaddition,4 carbene addition,5 hydroxylation,6 ing groups with advanced functionality, can be prepared from
disilirane addition,7 radical addition,8 azide addition,9 and so on. commercially available reagents in high yields through only a few
1,3-Dipolar cycloaddition of azomethine ylides is one of the most steps, the applicability of this kind of functionalization deserves
more attention. Herein, we report the synthesis of [5,6]-pyrrolidino-
Sc3N@Ih-C80 from Sc3N@Ih-C80 and aziridine derivatives as well as
the formation and isolation of a pair of diastereomers. In addition,
the structural characterization of pyrrolidino-Sc3N@Ih-C80 was
obtained from electronic absorption, NMR spectroscopy and
X-ray crystallography, while the redox properties of pyrrolidino-
Sc3N@Ih-C80 were evaluated by electrochemical means.
The reaction of Sc3N@Ih-C80 with 1a was conducted at 180 1C
in o-dichlorobenzene (ODCB) for 1 h (Scheme 1). Subsequent
preparative HPLC separation afforded products 2a and 3a in
26% and 18% yields, respectively (Fig. S1 and S2, ESI†). The
conversion yields of 2a and 3a were calculated from the HPLC
peak area assuming that Sc3N@Ih-C80 and the mono-adducts
have the same absorption coefficients. Matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass spectra
of 2a and 3a clearly displayed the molecular ion peaks at m/z
1267, as expected for the 1 : 1 adducts of Sc3N@Ih-C80 and 1a
a Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501,
Japan. E-mail: ymaeda@u-gakugei.ac.jp
b Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba,
Tsukuba, Ibaraki 305-8577, Japan. E-mail: akasaka@tara.tsukuba.ac.jp
c Fukui Institute for Fundamental Chemistry, Kyoto University,
Kyoto, 606-8103, Japan
d Theoretical and Physical Chemistry Institute, National Hellenic Research
Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece
e Department of Chemistry, University of California at Davis, One Shields Ave,
Davis, CA 95616, USA
f College of Materials Science and Engineering, Huazhong University of Science and
Technology, 430074 Wuhan, China
g Foundation for Advancement of International Science, Tsukuba, Ibaraki 305-0821,
Japan
† Electronic supplementary information (ESI) available: Experimental details,
HPLC, CV, mass spectra, NMR spectra, Vis-NIR spectra, and optimized structures.
CCDC 1010741. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc04946b
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Scheme 1 Reaction of Sc3N@Ih-C80 with 1.
(Fig. S3, ESI†). The fragment peaks were observed at m/z 1109, which
corresponded to the mass of the fragment ion [Sc3N@Ih-C80]ꢀ.
Similarly, the reaction of 1b and Sc3N@Ih-C80 gave two products
2b and 3b in 19% and 24% yields, respectively (Fig. S10 and
S11, ESI†).
Fig. 2 Front and side views of the crystal structure of 2a with thermal
ellipsoids shown at the 30% probability level.
There are two possible position isomers (A–D) for 2 and 3 as
shown in Fig. 1 because the Ih-C80 cage has two kinds of non-
equivalent C–C bonds. Mono-addition occurs at a C–C bond located
between a five- and six-membered ring (so-called [5,6]-addition) and
the other addition takes place at a C–C bond located between two six-
membered rings (so-called [6,6]-addition). When glycine derivatives
and aldehydes were used as the starting materials, [5,6]-pyrrolidino-
Sc3N@Ih-C80 was obtained. On the other hand, when N-triphenyl-
methyl-5-oxazolidinone was used as the starting material, [6,6]-
pyrrolidino-Sc3N@Ih-C80 was obtained as the kinetically controlled
product. It was also reported that [6,6]-pyrrolidino-Sc3N@Ih-C80
is converted thermally to the more thermodynamically stable
[5,6]-adduct. As previous studies showed that the UV-vis-NIR
absorption spectra of C60 adducts reflected sensitively the addition
pattern rather than the nature of the addends,15 the UV-vis-NIR
spectra can be regarded as powerful tools to determine the addition
pattern in fullerene adducts. [6,6]-N-Tritylpyrrolidino-Sc3N@Ih-C80,
[6,6]-Sc3N@Ih-C80-(CH2)2NTrt, and the benzyne [6,6]-adduct of
Sc3N@Ih-C80, [6,6]-Sc3N@Ih-C80-(C6H4) exhibited the characteristic
absorption peak at ca. 800 nm.4a,b,12a,e In contrast, [5,6]-Sc3N@
Ih-C80-(C6H4) showed weak absorption peaks centered ca. 950 nm.
The absorption spectra of 2 and 3 show the distinctive absorption
maxima at around 960 nm. (2a: 958 nm, 2b: 959 nm, 3a: 959 nm,
3b: 959 nm). These characteristic absorption spectra suggest that 2
and 3 are [5,6]-addition products (Fig. S4, ESI†).
3.56 ppm, respectively (Fig. S5, ESI†). The diastereotopic geminal
protons in the pyrrolidine rings appeared at 4.19 and 4.02 ppm
( J = 8.8 Hz) in 2a and 4.40 and 3.56 ppm ( J = 9.5 Hz) in 3a
and are correlated with the carbon atom at 59.8 ppm in 2a and
62.6 ppm in 3a in the HMQC spectra (Fig. S8, ESI†). The 13C NMR
spectra of 2a and 3a showed a total of 78 lines assigned to
sp2-hybridized cage carbons of Sc3N@Ih-C80 (Fig. S6, ESI†). In
addition, the sp3-hybridized cage carbon atoms appeared at
62.2 and 57.6 ppm in 2a and 61.8 and 56.4 ppm in 3a, which
disappeared in DEPT-135 NMR spectra (Fig. S7, ESI†). Echegoyen
et al. suggested that the large Dd values for the geminal protons of
Sc3N@Ih-C80-(CH2)2NR are mainly due to surface ring currents on
the C80 cage.12b,d The chemical shift of the methine proton on the
pyrrolidine ring can be used for the simple and concise structural
identification of 2-substituted-[5,6]-pyrrolidino-Sc3N@Ih-C80. It is
noteworthy that the 1H NMR signals of methine protons of 2a and
2b in the pyrrolidine ring appeared downfield compared to those of
1
3a and 3b, respectively. For the assignment of the H NMR, the
1H NMR chemical shifts of 2b and 3b were calculated by the M06-2X-
GIAO method.16 The calculated chemical shift of the methine proton
of 2b (4.64 ppm) was lower than that of 3b (3.49 ppm), as shown in
Table 1, which strongly supports the structures of 2b and 3b.
Furthermore, whereas geminal protons of 3a and 3b in their
1H NMR spectra showed a large Dd value (Dd: difference of the
chemical shifts of the geminal protons in the pyrrolidine ring, 3a:
Dd = 0.84; 3b: Dd = 1.49), 2a and 2b showed a small Dd value (2a: Dd =
0.17; 2b: Dd = 0.15). This was also found in the theoretical calcula-
tion, as shown in Table 1 (calculated value of the Dd: 2b: Dd = 0.70;
3b: Dd = 1.68). Thus, the structure of 2b (addition pattern A, the
The molecular structure of 2a was determined using X-ray
crystallographic analysis. The structural drawing shown in
Fig. 2 confirms that the addition took place at the [5,6]-bond.
The pyrrolidine ring is in an envelope conformation in which
the nitrogen atom is above the pentagon ring. The alkoxycarbonyl
group is located on the hexagon side on the C80 cage (addition
pattern: A). Thus, 3a can be assigned to the stereoisomer of 2a,
which has the alkoxycarbonyl group located at the pentagon side
on the C80 cage (addition pattern: B).
Table 1 Selected 1H NMR chemical shifts of 2-substituted pyrrolidino-Ih-
C80 adducts
The 1H NMR spectra of 2a and 3a showed protons in the
pyrrolidine rings at 4.81, 4.19, and 4.02 ppm and 4.40, 4.24, and
R1
R2
Hmethine Hgeminal Hgeminal Ddc
0
a
b
b
Fullerene
Sc3N@Ih-C80 2a
Sc3N@Ih-C80 3a
Sc3N@Ih-C80 2b
Calculated value 2b
Sc3N@Ih-C80 3b
Calculated value 3b
tBu
tBu
CO2Me 4.81
CO2Me 4.24
4.19
4.40
4.05
4.93
4.39
4.41
4.02
3.56
3.90
4.23
2.90
2.73
0.17
0.84
0.15
0.70
1.49
1.68
nOctyl CO2Et 4.58
4.64
nOctyl CO2Et 3.67
3.49
a
Methine proton in the pyrrolidine ring. b Geminal proton in the pyrrolidine
ring. c Difference of the geminal protons in the pyrrolidine ring.
Fig. 1 Possible partial structures of 2-substituted [5,6]- and [6,6]-
pyrrolidino-Sc3N@Ih-C80
.
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Table 2 Redox potentials (V) and HOMO–LUMO levels (eV) of Sc3N@Ih-
C80 and its derivatives
The Strategic Japanese–Spanish Cooperative Program funded
by JST and MICINN and the US NSF (CHE-0527015) is gratefully
acknowledged.
Compound
Eo2x
Eo1x
Er1ed
Er2ed
Er3ed
HOMO LUMO
c
Sc3N@Ih-C80 1.09 0.59 ꢀ1.26 ꢀ1.62 ꢀ2.37 ꢀ6.47 ꢀ2.58
0.64b 0.23b ꢀ1.15a ꢀ1.57a ꢀ2.33a ꢀ6.13 ꢀ2.79
0.63b 0.31b ꢀ1.11a ꢀ1.51a ꢀ2.28a ꢀ6.11 ꢀ2.80
0.60b 0.30b ꢀ1.11a ꢀ1.50a ꢀ2.23a ꢀ6.14 ꢀ2.79
0.59b 0.30b ꢀ1.10a ꢀ1.49a ꢀ2.22a ꢀ6.10 ꢀ2.80
Notes and references
2a
2b
3a
3b
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group in 2 and 3 does not affect the electronic properties of
pyrrolidino-Sc3N@Ih-C80.
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with aziridine, which affords the corresponding [5,6]-pyrrolidino-
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information for the assignment of a pair of [5,6]-pyrrolidino-
Sc3N@Ih-C80 diastereomers.
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