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Enrique E. Maroto, a Marta Izquierdo,a Michihisa Murata,b Salvatore Filippone,a Koichi Komatsu,b Yasujiro Murata*b and Nazario Martín*a,c 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A complete stereocontrol of 1,3-dipolar cycloaddition of Nmetalated azomethine ylides onto endohedral fullerene H2@C60 is reported for the first time. The stereodivergent synthesis of either the cis or the trans endohedral cycloadduct is achieved with excellent diastereo- and enantioselectivities.
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Since “molecular surgery” approach1 was made available at macroscopic scale, fullerenes derivatives enclosing small species in their cavity have attracted a wide interest for the scientific community. Endohedral fullerenes encapsulating different molecules such as hydrogen2 and water3, metal atoms4 or clusters5, are promising molecules due to their unique chemical and physical properties which dramatically differ from that exhibited by empty fullerenes. In fact, they have found applications in areas such as biomedicine6,7 or materials science.8,9 As a remarkable example, Sc3N@C80 has been used to prepare organic photovoltaic devices because of its capability to increase the open circuit voltage and, therefore, the energy conversion efficiency. 10,11 On the other hand, whereas empty fullerenes chemistry has been widely studied, the chemical reactivity of endohedral fullerenes remains much less investigated.12 Derivatization of these species is, however, decisive in order to increase their potential applications. In this regard, a fundamental aspect of endohedral fullerenes that still remains nearly unexplored is the chemical control of stereoselectivity. To the best of our knowledge, the only example of enantioselective synthesis of endofullerenes was recently carried out in our group.13 Thus, the asymmetric 1,3-cycloaddition reaction of an N-metalated azomethine ylide onto the functionalized endohedral derivative La@C72-(C6H3Cl2) afforded eight different compounds unambiguously characterized. In that non-symmetric system, the reactivity was governed by the inner atom, in this case the lanthanum. Therefore, the development of a methodology to apply onto symmetric and not
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Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Ciudad Universitaria s/n 28040 Madrid, Spain. b Institute for Chemical Research,Kyoto University, Uji, Kyoto 611-0011, Japan. c IMDEA- Nanociencia, Campus de la Universidad Autónoma de Madrid, 28049 Madrid, Spain. † Electronic Supplementary Information (ESI) available: [catalytic procedures and spectroscopic data]. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
90
previously functionalized endohedral fullerenes to synthesize enantiopure derivatives is still remaining. Thus, we decided to carry out the study on a simple and symmetric pristine endofullerene, namely H2@C60. It has recently been proved that the spin equilibrium between the ortho and para hydrogen molecules confined into the [60]fullerene is controlled by the catalytic oxygen outside the sphere.14 Therefore, it becomes even more important to determine if the H2 molecule inside the sphere is also able to rule the reactivity of the endofullerene. Herein, we report the first example of a methodology to achieve a complete stereocontrol onto the pristine and symmetric H2@C60 by catalytic enantioselective 1,3-dipolar cycloaddition of Nmetalated azomethine ylides. Recently, a succesful procedure for chiral derivatization of [60] and [70]fullerene into pyrrolidinofullerenes using different copper and silver salts was developed by Martin and coworkers.15 Extending the scope of this methodology to endohedral fullerenes, namely hydrogen endofullerene H2@C60, paves the way to the stereodivergent obtention of new pyrrolidinoendofullerenes, thus broadening the potential applications of these derivatives. The suitable combination of chiral ligands and metal salts make the 1,3-dipolar cycloaddition of azomethine ylides stereoselective16 affording either the cis or the trans adduct. Thus, the catalytic complex Cu(II)/Fesulphos 3 at -15ºC directs the 1,3dipolar cycloaddition of iminoester 1a onto H2@C60 toward the synthesis of cis-(2S,5S) 5-(p-methoxyphenyl)-2-methoxycarbonyl pyrrolidino[3,4:1,2]H2@C60 2a with an excellent asymmetric induction and yield (scheme 1a). Moreover, the enantioselectivity can be switched to the opposite enantiomer (2R,5R) of the cis adduct 2a with a similar level of enantioselectivity by changing the catalytic system to Ag(I)/BPE 4 at the same temperature (scheme 1b). In both catalysts, the counterion of the metal salt is acetate because it acts as a base and, probably, occupies the vacancy in the metal coordination sphere, thus enabling a better stereodifferentiation.14 Furthermore, in order to get a stereodivergent methodology, the efficient formation of both enantiomers of the trans adduct is required. In this regard, the complex Cu(II)/(S)-DTBM-Segphos 5 in presence of triethylamine at room temperature as a base afforded the (2S,5R)trans 2a with high enantioselectivity and good yield (scheme 1c). As expected, the use of the opposite chiral ligand (R)-5 afforded the inverted enantiomer, obtaining again the same value of enantiomeric excess in the formed (2R,5S) adduct (scheme 1d). [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
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Catalytic Stereodivergent Functionalization of H2@C60
ChemComm
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OMe
MeO H N
H N
MeO2C
CO2Me a) Cu(OAc)2/3 -15º C
b) Ag(OAc)/4 -15º C H H
H H
50 cis enantiomers
Published on 12 November 2013. Downloaded by University of Missouri at Columbia on 26/11/2013 04:29:50.
5
(2S,5S)cis-H2@-2a e.e. = 94% 78% yield
(2R,5R)cis-H2@2a e.e. = 90% 62% yield
H2@C60 +
MeO
N
CO2Me OMe
MeO
1a H N
CO2Me
trans enantiomers
d) Cu(OTf)2/(R)-5 Et3N
c) Cu(OTf)2/(S)-5 Et3N
H H
10
S-tBu
P
55
H H
(2R,5S)trans-H2@2a e.e. = 93% 63% yield
(2S,5R)tr ans-H2@-2a e.e. = 94% 62% yield
Fe
H N
MeO2C
O
R1
O
P
R1
P
R1
60
P
PPh2
O
R1
O
65
15
3
4
Scheme 1. Stereodivergent synthesis
5
of the
R1 = 2,5-tBu,3-OMe-Ph
It is worth mentioning that both empty and endofullerenes have the same spectroscopic and analytical features such as the retention time in HPLC either for cis (14 and 16 min, Regis Whelk 02, hexane/methanol 97:3, 2mL/min) and trans enantiomers (19 and 20 min, Regis Whelk 02, hexane/methanol 97:3, 1mL/min) and the chemical shifts of the different signals in NMR (for example the pyrrolidine ring protons at 5.8 and 5.7 ppm for cis adduct and 6.4 and 5.8 ppm for the trans). Circular dichroism (CD) spectra verified the optical activity of the synthesized compounds and revealed the capability of the two new created chiral centres adjacent to the fullerene sphere to perturb the inherent symmetry of the π-system in endofullerene H2@C60. Every derivative exhibits a Cotton effect at around 430 nm as expected for a [6,6] fullerene monoadduct but with opposite signs for every pair of enantiomers (see Fig. 1). Finally, the specific optical rotation measurements for the different enantiomers were determined. Thus, 2a displayed [α]D20= +145º (2R,5R), -145º (2S,5S), +231º (2S,5R) and -231º (2R,5S) (see S.I.). Furthermore, these values resulted to be very similar to those obtained for the related empty pyrrolidinofullerenes.15
four stereoisomers of the
pyrrolidinoendofullerene 2a. The starting fullerene was a mixture of H2@C60 and
Conclusions
C60 (80:20).
20
25
Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) confirmed the formation of the target compounds, revealing a peak at 930 m/z due to the protonated [M+H]+ adduct. Moreover, NMR spectra allowed to assign unambiguously the structure of cis and trans-2a derivatives. Particularly remarkable is the signal at around -5 ppm in the 1H NMR spectra that corresponds to the inner hydrogen molecule (see S.I.).
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We have described the first stereoselective synthesis carried out on a pristine endohedral fullerene, namely H2@C60. Interestingly, the endofullerene enriched sample reveals the same reactivity and stereoselectivity than that observed for the hollow fullerene C60. These experimental findings are in line with those predicted and calculated for the related noble gas endohedral fullerenes12 showing the inert character of the H2 molecule incarcerated in the C60 cage. Work is currently in progress on other related endofullerene endowed with a polar molecule inside the sphere.
Acknowledgments
30 80
Financial support by the Ministerio de Ciencia e Innovación (MINECO) of Spain (CTQ2011-24652, PIB2010JP-00196, and CSD2007-00010 projects), the European Research Council ERC2012-ADG_20120216 (Chirallcarbon), and CM (Madrisolar-2); S.F. acknowledges MINECO and ESF for R&C grant. M.I. and E.E.M. are thankful for a research grant.
Notes and references 1 35 2 3 4
5
40 6
45
Figure 1. CD Spectra of cis-2a and trans-2a (concentration 2x10-6 M in toluene). The four stereoisomers can be seen: enantiomers formed by the Cu/3 (orange line) and Cu/(R)-5 catalyst (blue line) present the opposite Cotton effect at 430 nm than the ones obtained from the Ag/4 (green line) and Cu/(S)-5 catalyst (red line).
2 | Journal Name, [year], [vol], 00–00
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Y. Rubin, Top. Curr. Chem., 1999, 199, 67. K. Komatsu, M. Murata and Y. Murata, Science, 2005, 307, 5707. K. Kurotobi and Y. Murata, Science, 2011, 333, 6042. R. Beyers, C. –H. Kiang, R. D. Johnson, J. R. Salem, M. S. De Vries, C. S. Yannoni, D. S. Bethune, H. C. Dorn, P. Burbank, K. Harichi and S. Stevenson, Nature, 1994, 370, 196; M. N. Chaur, F. Melin, A. L. Ortíz and L. Echegoyen, Angew. Chem. Int. Ed,, 2009, 48, 7514. S. Stevenson, G. Rice, T. Glass, K. Harich, F. Cromer, M. R. Jordan, J. Craft, E. Hadjun, R. Bible, M. M. Olmstead, K. Maitra, A. J. Fischer, A. L. Balch and H. C. Dorn, Nature, 1999, 401, 55. D. W. Cagle, S. J. Kennel, S. Mizadeth, J. M. Alford and L. J.Wilson, Proc. Natl. Acad. Sci. U.S.A., 1999, 96, 5182. P. P. Fatouros, F. D. Corwin, Z. –J. Chen, W. C. Broaddus, J. L. Tatum, B. Kettenman, Z. Ge, H. W. Gibson, J. L. Russ, A. –P. Leonard, J. C. Duchamp and H. C. Dorn, Radiology, 2006, 240, 756; J. Zhang, P. P. Fatouros, C. Shu, J. Reid, L. S. Owens, T. Cai, H. W. Gibson, G. L. Long, F. D. Corwin, Z. –J. Chen and H. C. Dorn, Bioconjugate Chem., 2010, 21, 610.
This journal is © The Royal Society of Chemistry [year]
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S. Kobayashi, S. Mori, S. Iida, H. Ando, T. Takenobu, Y. Taguchi, A. Fujiwara, A. Taninaka, H. Shinohara and Y. Iwasa, J. Am. Chem. Soc., 2003, 125, 8116; Y. Yasutake, Z. Shi, T. Okazaki, H. Shinohara and Y. Majima, Nano Lett., 2005, 5, 1057. T. Tsuchiya, R. Kumashiro, K. Tanigaki, Y. Matsunaga, M. O. Ishitsuka, T. Wakahara, Y. Maeda, Y. Takano, M. Aoyagi, T. Akasaka, M. T. H. Liu, T. Kato, K. Suenaga, J. S. Jeong, S. Ijima, F. Kimura, T. Kimura and S. Nagase, J. Am. Chem. Soc., 2008, 130, 450. J. R. Pinzón, M. E. Plonska-Brzezinska, C. M. Cardona, A. J. Athans, S. S. Gayathri, D. M. Guldi, M. A. Herranz, N. Martín, T. Torres and L. Echegoyen, Angew. Chem. Int. Ed., 2008, 47, 4173. See also: L. Feng, Z. Slanina, S. Sato, K. Yoza, T. Tsuchiya, N. Mizorogi, T. Akasaka, S. Nagase, N. Martín and D. M. Guldi, Angew. Chem. Int. Ed., 2011, 50, 5909; Y. Takano, M. A. Herranz, N. Martín, S. G. Radhakrishnan, D. M. Guldi, T. Tsuchiya, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2010, 132, 8048. R. B. Ross, C. M. Cardona, D. M. Guldi, S. G. Sankaranarayanan, M. O. Resse, N. Kopidakis, J. Peet, B. Walker, G. C. Bazán, E. Van Keuren, B. C. Holloway and M. Drees, Nat. Mater., 2009, 8, 208. S. Osuna, M. Swart and M. Solà, Chem. Eur. J., 2009, 15, 13111; B. A. Dicamillo, R. L. Hettich, G. Guiochon, R. N. Crompton, M. Saunders, H. A. Jinénez-Vázquez, A. Khong and J. Cross, J. Phys. Chem., 1996, 100, 9197. K. Sawai, Y. Takano, M. Izquierdo, S. Filippone, N. Martín, Z. Slanina, N. Mizorogi, M. Waelchli, T. Tsuchiya, T. Akasaka and S. Nagase, J. Am. Chem. Soc., 2011, 133, 17746. N. J. Turro, J. Y.-C. Chen, E. Sartori, M. Ruzzi, A. Marti, R. Lawler, S. Jockusch, J. López-Gejo, K. Komatsu and Y. Murata, Acc. Chem. Res., 2010, 43, 335. S. Filippone, E. E. Maroto, A. Martín-Domenech, M. Suárez and N. Martín, Nat. Chem., 2009, 1, 578; E. E. Maroto, A. de Cózar, S. Filippone, A. Martín-Domenech, M. Suárez, F. P. Cossío and N. Martín, Angew. Chem. Int. Ed., 2011, 50, 6060; E. E. Maroto, S. Filippone, A. Martín-Domenech, M. Suárez and N. Martín, J. Am. Chem. Soc., 2012, 134, 12936. For a recent review of catalytic asymmetric cycloaddition of azomethine ylides, please see: J. Adrio and J. C. Carretero, Chem. Commun., 2011, 47, 6784. See also: S. Cabrera, R. Gómez-Arrayás and J. C. Carretero, J. Am. Chem. Soc., 2005, 127, 16394; C. Nájera and J. M. Sansano, Angew. Chem., Int. Ed., 2005, 44, 6272.
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: E. E. Maroto, M. Izquierdo, M. Murata, S. Filippone, K. Komatsu, Y. Murata and N. Martin, Chem. Commun., 2013, DOI: 10.1039/C3CC46999A.
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication. Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
ChemComm
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Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication.
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COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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DOI: 10.1039/C3CC46999A
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ARTICLE TYPE
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Enrique E. Maroto, a Marta Izquierdo,a Michihisa Murata,b Salvatore Filippone,a Koichi Komatsu,b Yasujiro Murata*b and Nazario Martín*a,c 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A complete stereocontrol of 1,3-dipolar cycloaddition of Nmetalated azomethine ylides onto endohedral fullerene H2@C60 is reported for the first time. The stereodivergent synthesis of either the cis or the trans endohedral cycloadduct is achieved with excellent diastereo- and enantioselectivities.
50
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Since “molecular surgery” approach1 was made available at macroscopic scale, fullerenes derivatives enclosing small species in their cavity have attracted a wide interest for the scientific community. Endohedral fullerenes encapsulating different molecules such as hydrogen2 and water3, metal atoms4 or clusters5, are promising molecules due to their unique chemical and physical properties which dramatically differ from that exhibited by empty fullerenes. In fact, they have found applications in areas such as biomedicine6,7 or materials science.8,9 As a remarkable example, Sc3N@C80 has been used to prepare organic photovoltaic devices because of its capability to increase the open circuit voltage and, therefore, the energy conversion efficiency. 10,11 On the other hand, whereas empty fullerenes chemistry has been widely studied, the chemical reactivity of endohedral fullerenes remains much less investigated.12 Derivatization of these species is, however, decisive in order to increase their potential applications. In this regard, a fundamental aspect of endohedral fullerenes that still remains nearly unexplored is the chemical control of stereoselectivity. To the best of our knowledge, the only example of enantioselective synthesis of endofullerenes was recently carried out in our group.13 Thus, the asymmetric 1,3-cycloaddition reaction of an N-metalated azomethine ylide onto the functionalized endohedral derivative La@C72-(C6H3Cl2) afforded eight different compounds unambiguously characterized. In that non-symmetric system, the reactivity was governed by the inner atom, in this case the lanthanum. Therefore, the development of a methodology to apply onto symmetric and not
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Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Ciudad Universitaria s/n 28040 Madrid, Spain. b Institute for Chemical Research,Kyoto University, Uji, Kyoto 611-0011, Japan. c IMDEA- Nanociencia, Campus de la Universidad Autónoma de Madrid, 28049 Madrid, Spain. † Electronic Supplementary Information (ESI) available: [catalytic procedures and spectroscopic data]. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
90
previously functionalized endohedral fullerenes to synthesize enantiopure derivatives is still remaining. Thus, we decided to carry out the study on a simple and symmetric pristine endofullerene, namely H2@C60. It has recently been proved that the spin equilibrium between the ortho and para hydrogen molecules confined into the [60]fullerene is controlled by the catalytic oxygen outside the sphere.14 Therefore, it becomes even more important to determine if the H2 molecule inside the sphere is also able to rule the reactivity of the endofullerene. Herein, we report the first example of a methodology to achieve a complete stereocontrol onto the pristine and symmetric H2@C60 by catalytic enantioselective 1,3-dipolar cycloaddition of Nmetalated azomethine ylides. Recently, a succesful procedure for chiral derivatization of [60] and [70]fullerene into pyrrolidinofullerenes using different copper and silver salts was developed by Martin and coworkers.15 Extending the scope of this methodology to endohedral fullerenes, namely hydrogen endofullerene H2@C60, paves the way to the stereodivergent obtention of new pyrrolidinoendofullerenes, thus broadening the potential applications of these derivatives. The suitable combination of chiral ligands and metal salts make the 1,3-dipolar cycloaddition of azomethine ylides stereoselective16 affording either the cis or the trans adduct. Thus, the catalytic complex Cu(II)/Fesulphos 3 at -15ºC directs the 1,3dipolar cycloaddition of iminoester 1a onto H2@C60 toward the synthesis of cis-(2S,5S) 5-(p-methoxyphenyl)-2-methoxycarbonyl pyrrolidino[3,4:1,2]H2@C60 2a with an excellent asymmetric induction and yield (scheme 1a). Moreover, the enantioselectivity can be switched to the opposite enantiomer (2R,5R) of the cis adduct 2a with a similar level of enantioselectivity by changing the catalytic system to Ag(I)/BPE 4 at the same temperature (scheme 1b). In both catalysts, the counterion of the metal salt is acetate because it acts as a base and, probably, occupies the vacancy in the metal coordination sphere, thus enabling a better stereodifferentiation.14 Furthermore, in order to get a stereodivergent methodology, the efficient formation of both enantiomers of the trans adduct is required. In this regard, the complex Cu(II)/(S)-DTBM-Segphos 5 in presence of triethylamine at room temperature as a base afforded the (2S,5R)trans 2a with high enantioselectivity and good yield (scheme 1c). As expected, the use of the opposite chiral ligand (R)-5 afforded the inverted enantiomer, obtaining again the same value of enantiomeric excess in the formed (2R,5S) adduct (scheme 1d). [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
Published on 12 November 2013. Downloaded by University of Missouri at Columbia on 26/11/2013 04:29:50.
Catalytic Stereodivergent Functionalization of H2@C60
ChemComm
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OMe
MeO H N
H N
MeO2C
CO2Me a) Cu(OAc)2/3 -15º C
b) Ag(OAc)/4 -15º C H H
H H
50 cis enantiomers
Published on 12 November 2013. Downloaded by University of Missouri at Columbia on 26/11/2013 04:29:50.
5
(2S,5S)cis-H2@-2a e.e. = 94% 78% yield
(2R,5R)cis-H2@2a e.e. = 90% 62% yield
H2@C60 +
MeO
N
CO2Me OMe
MeO
1a H N
CO2Me
trans enantiomers
d) Cu(OTf)2/(R)-5 Et3N
c) Cu(OTf)2/(S)-5 Et3N
H H
10
S-tBu
P
55
H H
(2R,5S)trans-H2@2a e.e. = 93% 63% yield
(2S,5R)tr ans-H2@-2a e.e. = 94% 62% yield
Fe
H N
MeO2C
O
R1
O
P
R1
P
R1
60
P
PPh2
O
R1
O
65
15
3
4
Scheme 1. Stereodivergent synthesis
5
of the
R1 = 2,5-tBu,3-OMe-Ph
It is worth mentioning that both empty and endofullerenes have the same spectroscopic and analytical features such as the retention time in HPLC either for cis (14 and 16 min, Regis Whelk 02, hexane/methanol 97:3, 2mL/min) and trans enantiomers (19 and 20 min, Regis Whelk 02, hexane/methanol 97:3, 1mL/min) and the chemical shifts of the different signals in NMR (for example the pyrrolidine ring protons at 5.8 and 5.7 ppm for cis adduct and 6.4 and 5.8 ppm for the trans). Circular dichroism (CD) spectra verified the optical activity of the synthesized compounds and revealed the capability of the two new created chiral centres adjacent to the fullerene sphere to perturb the inherent symmetry of the π-system in endofullerene H2@C60. Every derivative exhibits a Cotton effect at around 430 nm as expected for a [6,6] fullerene monoadduct but with opposite signs for every pair of enantiomers (see Fig. 1). Finally, the specific optical rotation measurements for the different enantiomers were determined. Thus, 2a displayed [α]D20= +145º (2R,5R), -145º (2S,5S), +231º (2S,5R) and -231º (2R,5S) (see S.I.). Furthermore, these values resulted to be very similar to those obtained for the related empty pyrrolidinofullerenes.15
four stereoisomers of the
pyrrolidinoendofullerene 2a. The starting fullerene was a mixture of H2@C60 and
Conclusions
C60 (80:20).
20
25
Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) confirmed the formation of the target compounds, revealing a peak at 930 m/z due to the protonated [M+H]+ adduct. Moreover, NMR spectra allowed to assign unambiguously the structure of cis and trans-2a derivatives. Particularly remarkable is the signal at around -5 ppm in the 1H NMR spectra that corresponds to the inner hydrogen molecule (see S.I.).
70
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We have described the first stereoselective synthesis carried out on a pristine endohedral fullerene, namely H2@C60. Interestingly, the endofullerene enriched sample reveals the same reactivity and stereoselectivity than that observed for the hollow fullerene C60. These experimental findings are in line with those predicted and calculated for the related noble gas endohedral fullerenes12 showing the inert character of the H2 molecule incarcerated in the C60 cage. Work is currently in progress on other related endofullerene endowed with a polar molecule inside the sphere.
Acknowledgments
30 80
Financial support by the Ministerio de Ciencia e Innovación (MINECO) of Spain (CTQ2011-24652, PIB2010JP-00196, and CSD2007-00010 projects), the European Research Council ERC2012-ADG_20120216 (Chirallcarbon), and CM (Madrisolar-2); S.F. acknowledges MINECO and ESF for R&C grant. M.I. and E.E.M. are thankful for a research grant.
Notes and references 1 35 2 3 4
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Figure 1. CD Spectra of cis-2a and trans-2a (concentration 2x10-6 M in toluene). The four stereoisomers can be seen: enantiomers formed by the Cu/3 (orange line) and Cu/(R)-5 catalyst (blue line) present the opposite Cotton effect at 430 nm than the ones obtained from the Ag/4 (green line) and Cu/(S)-5 catalyst (red line).
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