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DOI: 10.1039/C6RA16041G (Paper) RSC Adv., 2016, 6, 79095-79105

Facile access to 2,5-diaryl fulleropyrrolidines: magnesium perchlorate-mediated reaction of [60]fullerene with arylmethylamines and aryl aldehydes†

Hong-Yu Zhang‡ a, Hui-Juan Wang‡b, Fa-Bao Li*a, Chun-Xiang Liua, Xiao-Feng Zhanga, Li Liu*a and Chao-Yang Liu*b
aHubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, P. R. China. E-mail: lfb0615@hubu.edu.cn; liulihubei@hubu.edu.cn
bState Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. E-mail: chyliu@wipm.ac.cn

Received 21st June 2016 , Accepted 15th August 2016

First published on 15th August 2016

Abstract

The facile one-step reaction of [60]fullerene with arylmethanamines and aryl aldehydes in the presence of magnesium perchlorate under air conditions generated a series of scarce 2,5-diaryl fulleropyrrolidines in good to excellent yields. Intriguingly, all the formed 2,5-diaryl fulleropyrrolidines were confirmed only as cis isomers and thus displayed high stereoselectivity. In addition, the type of aryl aldehyde was found to have a great correlation with the formation of different 2,5-diaryl fulleropyrrolidines. Aryl aldehydes without electron-withdrawing groups always afforded exclusively the desired 2,5-diaryl fulleropyrrolidines, while aryl aldehydes connecting electron-withdrawing groups unexpectedly produced symmetrical 2,5-diaryl fulleropyrrolidines besides the anticipated fulleropyrrolidines. A plausible formation mechanism for the 2,5-diaryl fulleropyrrolidines was proposed.

Introduction

Fulleropyrrolidines, namely, pyrrolidine-containing fullerene derivatives, are a kind of important fullerene compound, which have exhibited a broad range of interesting features in materials science, biological applications, and nanotechnology, and thus have attracted extensive attention among the scientific community over the past 30 years.1–6 For example, some novel fulleropyrrolidines have been exploited recently as the acceptors of photovoltaic solar cells, and a higher power conversion efficiency (PCE) relative to that of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) under the same experimental conditions has been obtained.2 The most powerful and versatile protocol for the synthesis of fulleropyrrolidines is the well-known Prato reaction.3 Fulleropyrrolidines can also be prepared by several alternative approaches.3–5 Nevertheless, these known methods still have great difficulty in the synthesis of 2,5-diaryl fulleropyrrolidines, which may have multiple promising applications in electrochemical and photophysical fields because a wide range of diad and triad donor–acceptor systems can be readily designed and synthesized based on 2,5-diaryl fulleropyrrolidines. On the other hand, almost all of the reported 2,5-disubstituted fulleropyrrolidines are a mixture of cis and trans isomers,1 which will seriously hamper their applications in many fields such as novel organic photovoltaic materials. Therefore, the further exploration and development of new reactions to prepare 2,5-diaryl fulleropyrrolidines with high stereoselectivity is still demanding. To our delight, the synthesis of symmetrical 2,5-diaryl fulleropyrrolidines with high stereoselectivity has been recently achieved by our group via the Fe(ClO4)3-mediated reaction of [60]fullerene (C60) with arylmethanamines.6 However, the preparation of unsymmetrical 2,5-diaryl fulleropyrrolidines is still difficult by the above method.6 In addition, arylmethanamines bearing strong electron-withdrawing groups such as NO2 are very expensive and hardly available, which will lead to the extreme difficulty in the preparation of the corresponding symmetrical 2,5-diaryl fulleropyrrolidines by the Fe(ClO4)3-promoted protocol.6 Thus, it is still necessary to develop a more practical and convenient method for the preparation of 2,5-diaryl fulleropyrrolidines, especially the scarce unsymmetrical 2,5-diaryl fulleropyrrolidines together with symmetrical 2,5-diaryl fulleropyrrolidines connecting strong electron-withdrawing groups on the aryl rings.

Recently, the reactions for functionalizing fullerenes with the aid of various metal salts have received increasing attention because a large variety of novel fullerene derivatives with different structural motifs have been successfully prepared by this strategy.7 For instance, the reactions of C60 with nitriles,8a aldehydes/ketones,8b malonate esters,8c arylboronic acids,8d acid chlorides,8e β-keto esters,8f isocyanates/isothiocyanates,8g diols,8h N-sulfonyl aldimines,8i arylmethanamines,6 and amides8j in the presence of Fe(ClO4)3 afforded a series of rare fullerene derivatives including C60-fused oxazolines, C60-fused 1,3-dioxolanes, C60-fused disubstituted lactones, fullerenyl boronic esters, 1,2-fullerenols, C60-fused hemiketals, oxazolidinofullerenes/thiazolidinofullerenes, C60-fused dioxanes/dioxepanes, fulleroxazolidines, and 2,5-diaryl fulleropyrrolidines. In efforts to extend the reactions of fullerenes promoted by metal salts, we are determined to investigate the Mg(ClO4)2-mediated reactions of C60. To the best of our knowledge, the reaction of C60 under the assistance of cheap and easily available Mg(ClO4)2 has not been reported until today. In continuation of our interest in fullerene chemistry,6,8g,h,j,9 herein we describe a simple and efficient method for the stereoselective synthesis of symmetrical and unsymmetrical 2,5-diaryl fulleropyrrolidines as cis isomers by the facile one-step reaction of C60 with arylmethanamines and aryl aldehydes in the presence of Mg(ClO4)2, and also disclose the unusual behavior of electron-withdrawing aryl aldehydes resulting in the unexpected formation of symmetrical 2,5-diaryl fulleropyrrolidines.

Results and discussion

Benzylamine (1a) and benzaldehyde (2a) were first chosen as the model substrates to react with C60. At the onset, we studied the reaction of C60 with benzylamine (1a) and benzaldehyde (2a) without the addition of any additives, and found that the desired product cis-3a could be obtained in 16% isolated yield when the reaction was conducted under air conditions in a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 in chlorobenzene at 120 °C for 24 h (entry 1, Table 1). To improve the product yield of cis-3a, various reaction conditions have been examined. Gratifyingly, the isolated yield of cis-3a could be significantly improved from 16 to 60% when the reaction of C60 with benzylamine (1a) and benzaldehyde (2a) was carried out in the presence of Mg(ClO4)2 (entry 2 vs. entry 1, Table 1). Raising the reaction temperature to 130 °C did not improve the product yield of cis-3a (entry 3, Table 1), while decreasing the reaction temperature to 100 °C drastically reduced the isolated yield of cis-3a (entry 4 vs. entry 2, Table 1). Varying the amount of both 1a and 2a (from 2 to 10 equiv.) had also no benefit to the product yield of cis-3a (entries 5–8, Table 1). When the reaction was performed under the protection of nitrogen, the reduction in the isolated yield of cis-3a was observed (entry 9 vs. entry 2, Table 1) although a reasonable explanation for this phenomenon could not be provided now. Accordingly, the reagent molar ratio of C60, Mg(ClO4)2, 1a, and 2a as 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5, the reaction temperature as 120 °C together with the air conditions were chosen as the optimized reaction conditions (entry 2, Table 1). It is worth mentioning that other metal salts such as Fe(ClO4)3·xH2O, Mn(OAc)3·2H2O, FeCl3, FeCl3·6H2O, Cu(OAc)2·H2O, Pb(OAc)4, (NH4)2Ce(NO3)6, AlCl3, CuCl2·2H2O, and CuCl2 are also used to replace Mg(ClO4)2 (entries 10–19, Table 1). Unfortunately, only a little amount or no yield of product cis-3a was obtained under the optimized conditions. In addition, the addition of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and CH3COOH in place of Mg(ClO4)2 are also detrimental to get a higher product yield (entries 20–21, Table 1). For example, the addition of DBU to this reaction for 0.5 h only generated some unknown products with very large polarity and did not give any desired cis-3a although C60 has almost been totally consumed.
Table 1 Optimization of reaction conditions for the Mg(ClO4)2-mediated reaction of C60 with benzylamine 1a and benzaldehyde 2aa

image file: c6ra16041g-u1.tif
Entry Additive Molar ratiob Temp. (°C) Time (h) Yield (%) of cis-3ac
a Unless otherwise indicated, all reactions were performed under air conditions.b Molar ratio refers to C60/additive/1a/2a.c Isolated yield; those in parentheses were based on consumed C60.d The reaction was conducted under nitrogen conditions.
1 None 1[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 16 (84)
2 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 23 60 (95)
3 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 130 21 60 (90)
4 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 100 24 17 (89)
5 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]2 120 24 22 (96)
6 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10 120 8 43 (96)
7 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10 120 10 59 (82)
8 Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10 120 23 60 (62)
9d Mg(ClO4)2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 43 (93)
10 Fe(ClO4)3·xH2O 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 <5%
11 Mn(OAc)3·2H2O 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
12 FeCl3 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
13 FeCl3·6H2O 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
14 Cu(OAc)2·H2O 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
15 Pb(OAc)4 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
16 (NH4)2Ce(NO3)6 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
17 AlCl3 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 Trace
18 CuCl2·2H2O 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 None
19 CuCl2 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 None
20 DBU 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 0.5 None
21 CH3COOH 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 120 24 17 (63)

With the optimized reaction conditions in hand, we started to explore the substrate scope of the reaction. Typical arylmethanamines such as benzylamine (1a), 4-methoxybenzylamine (1b), 3,5-bis(trifluoromethyl)benzylamine (1c), 1-naphthalenemethylamine (1d), and 2-thiophenemethylamine (1e) were first selected to react with representative aryl aldehydes such as benzaldehyde (2a), 3,4-dimethoxybenzaldehyde (2b), 4-methoxybenzaldehyde (2c), 4-methylbenzaldehyde (2d), 1-naphthaldehyde (2e), and 2-thiophenaldehyde (2f), and were found to afford the desired 2,5-diaryl fulleropyrrolidines 3a–h as cis isomers. The reaction conditions and yields for the Mg(ClO4)2-mediated reaction of C60 with arylmethanamines (1a–e) and aryl aldehydes (2a–f) are summarized in Table 2.

Table 2 Reaction conditions and yields for the reaction of C60 with arylmethylamines 1 and aryl aldehydes 2 without electron-withdrawing groups in the presence of Mg(ClO4)2a

image file: c6ra16041g-u2.tif
Amine 1 Aldehyde 2 Time (h) Product cis-3 Yield of cis-3b (%)
a All reactions were performed in chlorobenzene (10 mL) under air conditions at 120 °C unless otherwise indicated, molar ratio refers to C60/Mg(ClO4)2/1/2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5.b Isolated yield, those in parentheses were based on consumed C60.
image file: c6ra16041g-u3.tif image file: c6ra16041g-u4.tif 23 3a 60 (95)
image file: c6ra16041g-u5.tif image file: c6ra16041g-u6.tif 48 3b 21 (88)
image file: c6ra16041g-u7.tif image file: c6ra16041g-u8.tif 24 3c 24 (92)
image file: c6ra16041g-u9.tif image file: c6ra16041g-u10.tif 24 3d 51 (96)
image file: c6ra16041g-u11.tif image file: c6ra16041g-u12.tif 48 3e 26 (87)
image file: c6ra16041g-u13.tif image file: c6ra16041g-u14.tif 48 3f 29 (48)
image file: c6ra16041g-u15.tif image file: c6ra16041g-u16.tif 24 3g 61 (98)
image file: c6ra16041g-u17.tif image file: c6ra16041g-u18.tif 20 3h 47 (71)

As can be seen from Table 2, all of the examined arylmethanamines (1a–e) and aryl aldehydes (2a–f) without electron-withdrawing groups could exclusively generate the expected 2,5-diaryl fulleropyrrolidines cis-3a–h in 21–61% isolated yields (48–98% yields based on consumed C60), superior to the previously reported data for most monoadducts. Intriguingly, 3,4-dimethoxybenzaldehyde (2b) and 4-methoxybenzaldehyde (2c) showed lower reaction activity than other aryl aldehydes (2a, 2d–f) probably attributed to the existence of strong electron-donating methoxyl group. In addition, the isolated yields of 2,5-diaryl fulleropyrrolidines cis-3g,h in our current work (61% and 47%) are obviously higher than those from our previous study (18% and 22%),6 indicating that Mg(ClO4)2 has played a very crucial role in the efficient synthesis of fulleropyrrolidines.

To expand the scope of the reaction, aryl aldehydes bearing electron-withdrawing groups were further selected as the reaction substrates. To our surprise, the reaction of benzylamine (1a), 4-methoxybenzylamine (1b), 2-chlorobenzylamine (1f), and 4-chlorobenzylamine (1g) with 2-chlorobenzaldehyde (2g), 4-chlorobenzaldehyde (2h), 3-nitrobenzaldehyde (2i), and 4-nitrobenzaldehyde (2j) in the presence of C60 and Mg(ClO4)2 unexpectedly generated the symmetrical 2,5-diaryl fulleropyrrolidines besides the anticipated fulleropyrrolidines. The reaction conditions and yields for the Mg(ClO4)2-mediated reaction of C60 with arylmethanamines (1a,b, 1f,g) and aryl aldehydes (2g–j) are summarized in Table 3.

Table 3 Reaction conditions and yields for the reaction of C60 with arylmethylamines 1 and aryl aldehydes 2 bearing electron-withdrawing groups under the assistance of Mg(ClO4)2a

image file: c6ra16041g-u19.tif
Amine 1 Aldehyde 2 Time (h) Product cis-3 Yield of cis-3b (%) Product cis-4 Yield of cis-4b (%)
a All reactions were performed in chlorobenzene (10 mL) under air conditions at 120 °C unless otherwise indicated, molar ratio refers to C60/Mg(ClO4)2/1/2 = 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5.b Isolated yield, those in parentheses were based on consumed C60.c The reaction was conducted in a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]5.d The reaction was conducted at 160 °C in o-dichlorobenzene (6 mL).
image file: c6ra16041g-u20.tif image file: c6ra16041g-u21.tif 22 3i 29 (52) 4a 25 (45)
24c 3i 38 (93) 4a Trace
image file: c6ra16041g-u22.tif image file: c6ra16041g-u23.tif 48 3j 25 (71) 4b 7 (20)
image file: c6ra16041g-u24.tif image file: c6ra16041g-u25.tif 4d 3k 38 (57) 4c 26 (39)
image file: c6ra16041g-u26.tif image file: c6ra16041g-u27.tif 48 3l 18 (30) 4d 40 (66)
image file: c6ra16041g-u28.tif image file: c6ra16041g-u29.tif 48 3m 19 (66) 4d 8 (28)
image file: c6ra16041g-u30.tif image file: c6ra16041g-u31.tif 18 — — 4a 64 (83)
image file: c6ra16041g-u32.tif image file: c6ra16041g-u33.tif 48 — — 4b 29 (88)

It can be seen from Table 3 that arylmethanamines (1a,b, 1f,g) could readily react with aryl aldehydes (2g–j) bearing electron-withdrawing groups to produce the desired unsymmetrical 2,5-diaryl fulleropyrrolidines cis-3i–m as well as the unexpected symmetrical 2,5-diaryl fulleropyrrolidines cis-4a–d in moderate to good yields. For the synthesis of cis-3i and cis-4a, increasing the amount of benzylamine (1a) (from 5 to 10 equiv.) could selectively generate cis-3i, while raising the amount of 2-chlorobenzaldehyde (2g) (from 5 to 10 equiv.) had no benefit to the product selectivity of this reaction (see ESI†). As for the preparation of cis-3k and cis-4c, acceptable product yields could be easily obtained by increasing the reaction temperature to 160 °C although the product selectivity was not high. To improve the product selectivity of this reaction, changing the reaction temperature (from 160 to 120 °C) together with varying the amount of benzylamine (1a) and 3-nitrobenzaldehyde (2i) (from 5 to 10 equiv.) was attempted, and was found to be useless in improving the product selectivity. In addition, improving the amount of benzylamine (1a) (from 5 to 10 equiv.) usually led to the additional formation of cis-3a, which would further reduce the selectivity of the reaction of C60 with benzylamine (1a) and 3-nitrobenzaldehyde (2i) in the presence of Mg(ClO4)2 (see ESI†). It should be noted that functional groups such as Cl and NO2 in cis-3i–m and cis-4a–d could be tolerated under the current reaction conditions and could be further transformed to other moieties. In addition, the NH group of cis-3a–m and cis-4a–d could also be employed for further transformation to prepare a series of scarce N-substituted 2,5-diaryl fulleropyrrolidines as cis isomers, which would be very difficult to synthesize by traditional methods.6

The identities of known compounds cis-3a,6,10 cis-3e,6 cis-3g,h,6 cis-3l,5d and cis-4a,b6 were well established by comparing their spectral data with those reported in the previous literature. The structures of new compounds cis-3b–d, cis-3f, cis-3i–k, cis-3m, and cis-4c,d were unambiguously characterized by their HRMS, 1H NMR, 13C NMR, FT-IR, and UV-vis spectra. All new products gave the correct [M + H]+ peaks in their mass spectra. Their UV-vis spectra showed a characteristic peak at 430–432 nm for 1,2-adducts of C60. Their IR spectra displayed the absorptions at 3304–3318 cm−1 attributed to the diagnostic stretching vibrations of the NH group. In their 1H NMR spectra, all protons exhibited the expected chemical shifts as well as the splitting patterns. In their 13C NMR spectra, products cis-3b–d, cis-3f, cis-3i–k, and cis-3m displayed similar spectral patterns, and there were two peaks in the range of 75.00–75.97 ppm for the two sp3-carbons of the fullerene skeleton along with at least 34 peaks including some overlapped ones in the range of 134.73–153.54 ppm for the 58 sp2-carbons of the fullerene cage, agreeing well with the C1 symmetry of their molecular structures. However, products cis-4c,d exhibited different spectral patterns with the aforementioned cis-3b–d, cis-3f, cis-3i–k, and cis-3m. The observation of no more than 31 signals for the sp2-carbons of the fullerene moiety at 135.25–151.78 ppm together with 1 signal for the two sp3-carbons of the C60 skeleton at 75.20–75.31 ppm was consistent with their Cs molecular symmetry. It should be noted that the stereochemistry of symmetrical fulleropyrrolidines cis-4c,d could be unequivocally assigned based on their 13C NMR spectra. For symmetrical fulleropyrrolidines, the cis isomers with Cs symmetry should theoretically display 32 peaks including 4 half-intensity ones (corresponding to 1C) for the carbons of fullerene skeleton, while the trans isomers with C2 symmetry should give 30 peaks with equal intensity. In fact, half-intensity peaks were found in all of the 13C NMR spectra of cis-4c,d, and thus fulleropyrrolidines cis-4c,d were unambiguously assigned as cis isomers. As for unsymmetrical fulleropyrrolidines cis-3b–d, cis-3f, cis-3i–k, and cis-3m, the stereochemistry could not be deduced from the 13C NMR spectra because of their C1 symmetry, but could be determined by the nuclear Overhauser enhancement spectroscopy (NOESY). The NOESY spectra of cis isomers should show the cross peak between the two methine protons on the pyrrolidine ring, while that of trans isomers would lack the cross peak between them. Experimentally, the correlation between the two methine protons was clearly showed on the NOESY spectra of representative cis-3i (Fig. 1) and cis-3m (see ESI†). Hence, the assignments of cis-3b–d, cis-3f, cis-3i–k, and cis-3m were unequivocally confirmed based on their NOESY spectra.


image file: c6ra16041g-f1.tif
Fig. 1 NOESY spectrum of cis-3i, and the nuclear Overhauser effect between the two methine protons is indicated by the curved arrow.

The possible reaction pathways leading to the formation of fulleropyrrolidines via thermal tautomerization of imines have been suggested by a few research groups.5,6 On the basis of the above literature, a plausible formation mechanism for 2,5-diaryl fulleropyrrolidines 3/4 through the Mg(ClO4)2-mediated reaction of C60 with arylmethanamines and aryl aldehydes is proposed in Scheme 1. Arylmethanamine 1 first reacts with aryl aldehyde 2 under the assistance of Mg(ClO4)2 to generate Schiff-base imine intermediate I. The synthesis of imines by the reaction of carbonyl compounds with amines in the presence of Mg(ClO4)2 has been well documented in previous literature.11 Imine intermediate I can undergo either tautomerization to produce 1,3-dipole II (path a) or rearrangement to form another imine intermediate III (path b, R2 = EWGs). It should be noted that the rearrangement of imine intermediates has been previously confirmed by Troshin and co-workers.5d In the case of 1,3-dipole II, a subsequent cycloaddition to C60 results in the generation of the desired unsymmetrical 2,5-diaryl fulleropyrrolidine 3. As for imine intermediate III, hydrolysis leads to the formation of the corresponding arylmethanamine IV accompanied by the elimination of R1CHO. The subsequent condensation of arylmethanamine IV with aryl aldehyde 2 forms new imine intermediate V, followed by tautomerization to produce 1,3-dipole VI, which can undergo a concerted 1,3-dipolar cycloaddition to C60 to give the unexpected symmetrical 2,5-diaryl fulleropyrrolidine 4.


image file: c6ra16041g-s1.tif
Scheme 1 Proposed formation mechanism for 2,5-diaryl fulleropyrrolidines 3/4.

It should be noted that the formed R1CHO in path b could also react with R1CH2NH2 (1) to afford the symmetrical 2,5-diaryl fulleropyrrolidine bearing the R1 group (Scheme 1). For instance, the reaction of C60 with benzylamine (1a) and aryl aldehydes (2g–j) in the presence of Mg(ClO4)2 should form cis-3a besides the anticipated cis-3i–l and cis-4a–d (Table 3). Furthermore, the isolated yield of cis-3a should be theoretically equal to those of cis-4a–d. However, the above reaction by adopting the experimental conditions listed in Table 3 only gave a trace amount of cis-3a. We thus conjectured that R1CH2NH2 (1)/R2CHO (2), R1CH2NH2 (1)/R1CHO, R2CH2NH2 (IV)/R2CHO (2), and R2CH2NH2 (IV)/R1CHO might display different reaction activities (Scheme 1). Controlled experiments were then conducted by studying the Mg(ClO4)2-mediated reaction of C60 with benzylamine (1a), 2-chlorobenzaldehyde (2g), 2-chlorobenzylamine (1f), and benzaldehyde (2a) in a molar ratio of C60, Mg(ClO4)2, 1a, 2g, 1f, and 2a as 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 (Scheme 2). Experimental results indicated that the reaction under air conditions at 120 °C for 7 h afforded the expected cis-3i, cis-4a, and cis-3a although the isolated yield of cis-3a (5%) was much lower than cis-3i (28%) and cis-4a (22%). Decreasing the amount of both 1f and 2a from 5 to 3 equiv. could further reduce the isolated yield of cis-3a, and only 2% yield of cis-3a together with 21% yield of cis-3i and 18% yield of cis-4a were obtained when the reaction was conducted for 10 h (Scheme 2). Based on these experimental data from controlled experiments, we can come to the conclusion that the reaction activity of 1a/2a is obviously lower than 1a/2g, 1f/2a, and 1f/2g. Therefore, the symmetrical 2,5-diaryl fulleropyrrolidines bearing the R1 group are generally difficult to form in our current reaction conditions (Table 3).


image file: c6ra16041g-s2.tif
Scheme 2 Mg(ClO4)2-mediated reaction of C60 with benzylamine (1a), 2-chlorobenzaldehyde (2g), 2-chlorobenzylamine (1f), and benzaldehyde (2a).

It is noteworthy that all the obtained 2,5-diaryl fulleropyrrolidines cis-3a–m and cis-4a–d have been well confirmed only as cis isomers based on their 13C NMR and NOESY spectra and have thus displayed high stereoselectivity. We considered that the stability of azomethine ylides should play a very important role in the highly stereoselective synthesis of cis-3a–m and cis-4a–d. Imine A from the direct condensation of benzylamine and benzaldehyde was selected as a representative to elucidate the high stereoselectivity of cis-3a–m and cis-4a–d. As shown in Scheme 3, imine A can undergo a thermal tautomeric equilibration between A and syn-A, or between A and anti-A. Nevertheless, azomethine ylide syn-A is more stable than anti-A because the coplanarity of phenyl ring with nitrogen atom in anti-A is hindered by the interactions between the bulky phenyl group and the α-hydrogen atom. In addition, a B3LYP/6-31G(d) energy calculation for syn-A and anti-A also disclosed that syn-A was more stable than anti-A.6 The stable azomethine ylide syn-A can react with C60 to afford the expected 2,5-diaryl fulleropyrrolidine cis-3a, and therefore the cis isomer is expected to be the predominant product, which is in line with the experimental data.


image file: c6ra16041g-s3.tif
Scheme 3 Thermal tautomerization of imine A.

Conclusion

In summary, we have successfully achieved the highly stereoselective synthesis of scarce 2,5-diaryl fulleropyrrolidines with symmetrical and unsymmetrical structures through the Mg(ClO4)2-mediated one-step reaction of C60 with cheap and easily accessible arylmethanamines and aryl aldehydes under mild conditions. In the current method, the unusual formation of symmetrical 2,5-diaryl fulleropyrrolidines from the aryl aldehydes bearing electron-withdrawing groups on the aryl rings would provide a simple and effective route to symmetrical 2,5-diaryl fulleropyrrolidines connecting electron-withdrawing groups, especially those with strong electron-withdrawing groups such as NO2, which would be extremely difficult to prepare by known methods. A possible reaction pathway along with a plausible explanation are also provided to elucidate the stereoselective formation of 2,5-diaryl fulleropyrrolidines.

Experimental section

General methods

All reagents and solvents were directly used as obtained commercially without further purification. Reaction was monitored by thin layer chromatography (TLC) using carbon disulfide/toluene as developing solvent. All fullerene products were purified by flash chromatography over silica gel. Chemical shifts in 1H NMR spectra were referenced to tetramethylsilane (TMS) at 0.00 ppm, while chemical shifts in 13C NMR spectra were referenced to residual DMSO at 39.52 ppm. High-resolution mass spectrometry (HRMS) was performed by MALDI-TOF in positive-ion mode with 4-hydroxy-α-cyanocinnamic acid as the matrix.

General procedure for the reaction of C60 with arylmethylamines and aryl aldehydes in the presence of Mg(ClO4)2

A mixture of C60 (36.0 mg, 0.05 mmol), Mg(ClO4)2 (22.3 mg, 0.10 mmol), arylmethylamine 1 (0.25 mmol), and aryl aldehyde 2 (0.25 mmol) was added to a 50 mL round-bottom flask. After the added compounds were completely dissolved in 10 mL of chlorobenzene (6 mL of o-dichlorobenzene in the case of 1a and 2i) by sonication, the resulting solution was heated with stirring in an oil bath preset at 120 °C (160 °C in the case of 1a and 2i) under air conditions. The reaction was carefully monitored by thin-layer chromatography (TLC) and stopped at the designated time. The reaction mixture was filtered through a silica gel plug in order to remove any insoluble material. After the solvent was evaporated in vacuo, the residue was separated on a silica gel column with carbon disulfide/toluene as the eluent to afford first unreacted C60 and then fulleropyrrolidines cis-3/4.

Fulleropyrrolidine cis-3a

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2a (26 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 23 h afforded first unreacted C60 (13.1 mg, 37%) and then cis-3a6,10 (27.4 mg, 60%) as an amorphous brown solid.

Fulleropyrrolidine cis-3b

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2b (41.5 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (27.2 mg, 76%) and then cis-3b (10.0 mg, 21%) as an amorphous brown solid: mp > 300 °C; 1H NMR (600 MHz, CS2/DMSO-d6) δ 7.92 (d, J = 7.6 Hz, 2H), 7.47 (s, 1H), 7.38–7.34 (m, 3H), 7.27 (t, J = 7.0 Hz, 1H), 6.81 (d, J = 8.3 Hz, 1H), 5.87 (s, 1H), 5.82 (s, 1H), 4.16 (s, 1H), 3.78 (s, 3H), 3.74 (s, 3H); 13C NMR (175 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 153.39, 152.96, 152.83, 152.64, 148.36 (aryl C), 148.35 (aryl C), 146.04 (2C), 145.99, 145.89, 145.14, 145.11, 145.10 (2C), 145.08, 145.06, 144.93 (2C), 144.69 (2C), 144.61, 144.42, 144.36 (2C), 144.09 (2C), 144.07, 144.05, 144.00, 143.98, 143.57, 143.51, 143.24, 143.23, 142.02, 141.85, 141.50 (2C), 141.45, 141.44, 141.22, 141.21, 141.07, 141.06, 140.95, 140.92, 140.89 (2C), 140.85, 140.81, 140.40, 140.37, 138.82, 138.76, 138.37, 138.18, 137.41 (aryl C), 135.92, 135.57, 134.95, 134.86, 129.63 (aryl C), 127.69 (2C, aryl C), 127.64 (2C, aryl C), 127.37 (aryl C), 120.13 (aryl C), 111.60 (aryl C), 110.78 (aryl C), 75.71 (sp3-C of C60), 75.43 (sp3-C of C60), 74.01, 73.84, 54.73, 54.55; FT-IR ν/cm−1 (KBr) 3312, 2921, 2849, 1602, 1591, 1513, 1460, 1427, 1383, 1265, 1235, 1182, 1157, 1136, 1114, 1028, 855, 764, 699, 526; UV-vis (CHCl3) λmax/nm 257, 310, 431; MALDI-TOF MS m/z calcd for C76H18NO2 [M + H]+ 976.1338, found 976.1318.

Fulleropyrrolidine cis-3c

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2c (30 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 24 h afforded first unreacted C60 (26.7 mg, 74%) and then cis-3c (11.1 mg, 24%) as an amorphous brown solid: mp > 300 °C; 1H NMR (600 MHz, CS2/DMSO-d6) δ 7.92 (d, J = 7.3 Hz, 2H), 7.82 (d, J = 8.5 Hz, 2H), 7.35 (t, J = 7.6 Hz, 2H), 7.26 (t, J = 7.4 Hz, 1H), 6.86 (d, J = 8.5 Hz, 2H), 5.87 (s, 1H), 5.84 (s, 1H), 4.14 (s, 1H), 3.74 (s, 3H); 13C NMR (100 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 158.59 (2C, aryl C), 153.50, 153.35, 153.12, 152.92, 146.19 (2C), 146.10, 146.09, 145.45, 145.29, 145.25 (2C), 145.22 (2C), 145.08 (2C), 144.83 (2C), 144.80, 144.60, 144.51, 144.49, 144.25 (2C), 144.18 (2C), 144.13 (2C), 143.70, 143.68, 143.40 (2C), 142.14, 142.00, 141.64 (2C), 141.57 (2C), 141.41 (2C), 141.23 (2C), 141.09 (4C), 140.98, 140.96, 140.53 (2C), 139.01, 138.95, 138.45, 138.32, 137.73 (aryl C), 136.10, 136.02, 135.14, 135.05, 129.43 (aryl C), 128.93 (aryl C), 128.86 (aryl C), 127.92 (aryl C), 127.77 (3C, aryl C), 127.45 (aryl C), 113.19 (aryl C), 75.97 (sp3-C of C60), 75.69 (sp3-C of C60), 74.19, 73.82, 54.17; FT-IR ν/cm−1 (KBr) 3315, 2921, 2851, 1611, 1510, 1460, 1429, 1375, 1249, 1170, 1037, 829, 699, 527; UV-vis (CHCl3) λmax/nm 257, 308, 432; MALDI-TOF MS m/z calcd for C75H16NO [M + H]+ 946.1232, found 946.1210.

Fulleropyrrolidine cis-3d

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2d (30 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 24 h afforded first unreacted C60 (16.8 mg, 47%) and then cis-3d (23.7 mg, 51%) as an amorphous brown solid: mp > 300 °C; 1H NMR (600 MHz, CS2/DMSO-d6) δ 7.93 (d, J = 7.5 Hz, 2H), 7.80 (t, J = 6.6 Hz, 2H), 7.37 (t, J = 7.8 Hz, 2H), 7.28 (t, J = 7.3 Hz, 1H), 7.17 (d, J = 5.9 Hz, 2H), 5.90 (d, J = 8.8 Hz, 1H), 5.87 (d, J = 8.8 Hz, 1H), 3.95 (t, J = 6.5 Hz, 1H), 2.35 (s, 3H); 13C NMR (175 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 153.02 (d, J = 8.9 Hz), 152.87 (d, J = 8.9 Hz), 152.69, 152.57, 146.02, 146.01, 145.87 (d, J = 3.3 Hz), 145.84 (d, J = 3.3 Hz), 145.17, 145.08 (2C), 145.04 (3C), 144.91 (2C), 144.66 (2C), 144.59, 144.40, 144.35, 144.33, 144.07 (2C), 144.01 (2C), 143.96, 143.95, 143.50, 143.49, 143.21 (2C), 141.96, 141.83, 141.47 (2C), 141.40 (2C), 141.19, 141.18, 141.03 (2C), 140.89 (3C), 140.88, 140.78 (2C), 140.35, 140.34, 138.81, 138.80, 138.23 (d, J = 4.2 Hz), 138.20 (d, J = 4.2 Hz), 137.38 (aryl C), 136.64 (d, J = 3.3 Hz), 135.90 (d, J = 2.1 Hz), 135.85 (d, J = 2.1 Hz), 134.91 (d, J = 1.6 Hz), 134.85 (aryl C), 134.33 (aryl C), 128.38 (2C, aryl C), 127.65 (4C, aryl C), 127.60 (2C, aryl C), 127.37 (d, J = 2.6 Hz, aryl C), 75.49 (sp3-C of C60), 75.39 (sp3-C of C60), 74.02 (d, J = 1.9 Hz), 73.88, 20.67; FT-IR ν/cm−1 (KBr) 3315, 3023, 2918, 2849, 1614, 1511, 1492, 1451, 1427, 1376, 1356, 1300, 1277, 1215, 1178, 1114, 1097, 1021, 1003, 868, 818, 766, 699, 686, 611, 526; UV-vis (CHCl3) λmax/nm 257, 309, 431; MALDI-TOF MS m/z calcd for C75H16N [M + H]+ 930.1283, found 930.1260.

Fulleropyrrolidine cis-3e

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1b (33 μL, 0.25 mmol) and 2c (30 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (25.1 mg, 70%) and then cis-3e6 (12.9 mg, 26%) as an amorphous brown solid.

Fulleropyrrolidine cis-3f

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1c (60.9 mg, 0.25 mmol) and 2c (30 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (14.3 mg, 40%) and then cis-3f (15.6 mg, 29%) as an amorphous brown solid: mp > 300 °C; 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.49 (s, 2H), 7.84 (d, J = 8.6 Hz, 2H), 7.81 (s, 1H), 6.89 (d, J = 8.6 Hz, 2H), 6.04 (s, 1H), 5.87 (s, 1H), 4.72 (s, 1H), 3.76 (s, 3H); 13C NMR (125 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 158.70 (aryl C), 153.02, 152.72, 152.00, 151.79, 146.28, 146.24, 145.90, 145.34 (2C), 145.29 (2C), 145.27 (2C), 145.16 (2C), 144.91 (2C), 144.71 (2C), 144.55, 144.52, 144.39, 144.33, 144.31, 144.29, 144.25 (2C), 144.14, 143.76, 143.60, 143.49, 143.39, 142.21, 142.09, 141.75, 141.73, 141.66, 141.59, 141.47 (aryl C), 141.42, 141.35, 141.29, 141.24, 141.13 (2C), 141.06, 141.03, 140.98, 140.91, 140.63 (2C), 139.14, 139.12, 138.65, 138.46, 136.46, 135.81, 135.56, 135.01, 130.31 (q, JC–F = 33 Hz, 2C, aryl C), 129.02 (aryl C), 128.93 (2C, aryl C), 128.22 (2C, aryl C), 122.44 (q, JC–F = 272 Hz, 2C), 120.86 (aryl C), 113.22 (2C, aryl C), 75.95 (sp3-C of C60), 75.22 (sp3-C of C60), 73.84, 72.80, 54.10; FT-IR ν/cm−1 (KBr) 3305, 2922, 2849, 1610, 1512, 1462, 1428, 1365, 1276, 1250, 1175, 1135, 1034, 1007, 898, 830, 784, 706, 681, 527; UV-vis (CHCl3) λmax/nm 257, 312, 431; MALDI-TOF MS m/z calcd for C77H14F6NO [M + H]+ 1082.0980, found 1082.0953.

Fulleropyrrolidine cis-3g

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1d (37 μL, 0.25 mmol) and 2e (34 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 24 h afforded first unreacted C60 (13.6 mg, 38%) and then cis-3g6 (31.0 mg, 61%) as an amorphous brown solid.

Fulleropyrrolidine cis-3h

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1e (26 μL, 0.25 mmol) and 2f (23 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 20 h afforded first unreacted C60 (12.2 mg, 34%) and then cis-3h6 (21.8 mg, 47%) as an amorphous brown solid.

Fulleropyrrolidines cis-4a and cis-3i

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2g (28 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 22 h afforded first unreacted C60 (15.8 mg, 44%), then cis-4a6 (12.3 mg, 25%) and cis-3i (13.6 mg, 29%) as amorphous brown solid.

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (55 μL, 0.50 mmol) and 2g (28 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 24 h afforded first unreacted C60 (21.1 mg, 59%) and then cis-3i (17.8 mg, 38%) as an amorphous brown solid; mp > 300 °C; cis-3i: 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.46 (d, J = 8.0 Hz, 1H), 7.92 (d, J = 7.4 Hz, 2H), 7.40–7.34 (m, 4H), 7.28–7.23 (m, 2H), 6.45 (d, J = 3.2 Hz, 1H), 5.92 (d, J = 3.0 Hz, 1H), 4.28 (s, 1H); 13C NMR (100 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 153.54, 153.42, 153.18, 152.54, 146.21 (4C), 146.04, 145.26 (3C), 145.22 (2C), 145.06 (2C), 144.91 (2C), 144.87, 144.69, 144.66, 144.53, 144.38, 144.31, 144.25, 144.16 (3C), 143.64, 143.56, 143.47, 143.36, 142.09, 142.00, 141.72, 141.62 (3C), 141.50, 141.46, 141.33, 141.18, 141.16, 141.05, 141.02, 140.99, 140.88 (2C), 140.78, 140.57, 139.03, 138.71, 138.53, 138.42, 137.61 (aryl C), 135.92 (3C), 135.41, 134.34 (aryl C), 133.59 (aryl C), 130.44 (aryl C), 129.01 (aryl C), 128.43 (aryl C), 127.94 (aryl C), 127.86 (aryl C), 127.79 (2C, aryl C), 127.53 (aryl C), 126.44 (aryl C), 75.84 (sp3-C of C60), 75.05 (sp3-C of C60), 73.79, 69.30; FT-IR ν/cm−1 (KBr) 3306, 2917, 2845, 1455, 1427, 1371, 1266, 1182, 1033, 998, 778, 760, 698, 544, 526; UV-vis (CHCl3) λmax/nm 257, 310, 431; MALDI-TOF MS m/z calcd for C74H13ClN [M + H]+ 950.0737, found 950.0712.

Fulleropyrrolidines cis-4b and cis-3j

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2h (35.3 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (23.5 mg, 65%), then cis-4b6 (3.4 mg, 7%) and cis-3j (11.9 mg, 25%) as amorphous brown solid: mp > 300 °C; cis-3j: 1H NMR (600 MHz, CS2/DMSO-d6) δ 7.92 (d, J = 8.3 Hz, 4H), 7.37–7.33 (m, 4H), 7.26 (t, J = 7.6 Hz, 1H), 5.88 (s, 2H), 4.38 (s, 1H); 13C NMR (100 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 153.05, 152.73 (2C), 152.59, 146.17 (2C), 145.96, 145.81, 145.20 (5C), 145.06 (3C), 144.81 (2C), 144.70, 144.51 (3C), 144.22 (3C), 144.12 (3C), 143.63 (2C), 143.36, 143.35, 142.11, 141.98, 141.63 (2C), 141.55 (2C), 141.32 (2C), 141.18 (2C), 141.03 (3C), 140.98 (2C), 140.91, 140.52 (2C), 139.00, 138.95, 138.44, 138.37, 137.55 (aryl C), 136.48 (aryl C), 136.14, 135.95, 135.17, 135.04, 133.20 (aryl C), 129.26 (aryl C), 129.19 (aryl C), 127.83 (4C, aryl C), 127.77 (2C, aryl C), 127.49 (aryl C), 75.59 (sp3-C of C60), 75.50 (sp3-C of C60), 74.13, 73.33; FT-IR ν/cm−1 (KBr) 3318, 3021, 2920, 2849, 1489, 1454, 1428, 1376, 1277, 1172, 1088, 1015, 828, 780, 699, 527; UV-vis (CHCl3) λmax/nm 257, 307, 430; MALDI-TOF MS m/z calcd for C74H13ClN [M + H]+ 950.0737, found 950.0712.

Fulleropyrrolidines cis-3k and cis-4c

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2i (37.8 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) at 160 °C in o-dichlorobenzene (6 mL) for 4 h afforded first unreacted C60 (11.9 mg, 33%), then cis-3k (18.3 mg, 38%) and cis-4c (13.2 mg, 26%) as amorphous brown solid: mp > 300 °C; cis-3k: 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.84 (s, 1H), 8.33 (d, J = 7.6 Hz, 1H), 8.15 (d, J = 8.3 Hz, 1H), 7.97 (d, J = 7.4 Hz, 2H), 7.62 (t, J = 8.0 Hz, 1H), 7.39 (t, J = 7.7 Hz, 2H), 7.30 (t, J = 7.1 Hz, 1H), 6.04 (d, J = 2.8 Hz, 1H), 5.93 (d, J = 2.8 Hz, 1H), 4.76 (s, 1H); 13C NMR (175 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 152.66, 152.42, 152.01, 151.76, 147.35 (aryl C), 146.15, 146.11, 145.78, 145.38, 145.21, 145.17 (2C), 145.15, 145.04 (2C), 145.02, 144.78 (2C), 144.61, 144.53 (2C), 144.44 (2C), 144.24, 144.19 (2C), 144.10 (2C), 144.03, 143.61, 143.46, 143.34, 143.25, 142.06, 141.96, 141.60 (2C), 141.53, 141.48, 141.27, 141.23, 141.15, 141.09, 141.01, 140.98, 140.92, 140.88 (3C), 140.49, 140.48, 140.28 (aryl C), 139.06, 138.94, 138.43, 138.37, 137.24 (aryl C), 136.43, 135.82, 135.29, 134.93, 133.58 (aryl C), 128.47 (aryl C), 127.80 (2C, aryl C), 127.74 (2C, aryl C), 127.50 (aryl C), 122.44 (aryl C), 122.19 (aryl C), 75.53 (sp3-C of C60), 75.15 (sp3-C of C60), 74.10, 72.98; FT-IR ν/cm−1 (KBr) 3310, 2920, 2848, 1528, 1454, 1426, 1382, 1346, 1266, 1217, 1183, 1094, 869, 732, 700, 527; UV-vis (CHCl3) λmax/nm 258, 312, 430; MALDI-TOF MS m/z calcd for C74H13N2O2 [M + H]+ 961.0977, found 961.0952. cis-4c: 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.83 (s, 2H), 8.36 (d, J = 7.5 Hz, 2H), 8.15 (d, J = 9.2 Hz, 2H), 7.64 (t, J = 8.1 Hz, 2H), 6.07 (d, J = 3.4 Hz, 2H), 5.14 (s, 1H); 13C NMR (125 MHz, CS2/DMSO-d6) (all 2C unless indicated) δ 151.74, 151.44, 147.39 (aryl C), 146.20, 145.26 (4C), 145.24, 145.11, 144.86, 144.59 (3C), 144.48, 144.43 (1C), 144.32, 144.26, 144.14, 143.54, 143.34, 142.13 (1C), 142.06 (1C), 141.71, 141.58, 141.27, 141.17, 140.99, 140.95, 140.91, 140.59, 139.94 (aryl C), 139.17, 138.56, 136.35, 135.28, 133.60 (aryl C), 128.57 (aryl C), 122.49 (aryl C), 122.29 (aryl C), 75.31 (sp3-C of C60), 73.07; FT-IR ν/cm−1 (KBr) 3309, 2918, 2849, 1526, 1477, 1462, 1428, 1382, 1345, 1267, 1215, 1184, 1164, 1095, 897, 839, 806, 730, 698, 526; UV-vis (CHCl3) λmax/nm 257, 314, 430; MALDI-TOF MS m/z calcd for C74H12N3O4 [M + H]+ 1006.0828, found 1006.0801.

Fulleropyrrolidines cis-3l and cis-4d

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol) and 2j (37.8 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (14.1 mg, 39%), then cis-3l5d (8.8 mg, 30%) and cis-4d (20.1 mg, 40%) as amorphous brown solid: mp > 300 °C; cis-4d: 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.25–8.21 (m, 8H), 6.05 (s, 2H), 5.03 (s, 1H); 13C NMR (125 MHz, CS2/DMSO-d6) (all 2C unless indicated) δ 151.78, 151.57, 146.78 (aryl C), 146.20, 145.36, 145.26, 145.23, 145.11, 144.99, 144.86, 144.57, 144.54 (1C), 144.49, 144.43 (1C), 144.28, 144.27, 144.13, 143.55, 143.33, 142.14 (1C), 142.05 (1C), 141.70, 141.58, 141.21, 141.15, 140.99, 140.93, 140.86, 140.56, 139.08, 138.54, 136.11, 135.25, 128.65 (4C, aryl C), 122.71 (4C, aryl C), 75.20 (sp3-C of C60), 73.08; FT-IR ν/cm−1 (KBr) 3304, 2920, 2847, 1599, 1519, 1462, 1426, 1382, 1342, 1315, 1276, 1216, 1183, 1105, 1014, 852, 839, 701, 526; UV-vis (CHCl3) λmax/nm 258, 314, 430; MALDI-TOF MS m/z calcd for C74H12N3O4 [M + H]+ 1006.0828, found 1006.0801.

Fulleropyrrolidines cis-3m and cis-4d

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1b (33 μL, 0.25 mmol) and 2j (37.8 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (25.4 mg, 71%), then cis-3m (9.6 mg, 19%) and cis-4d (4.1 mg, 8%) as amorphous brown solid: mp > 300 °C; cis-3m: 1H NMR (600 MHz, CS2/DMSO-d6) δ 8.23 (d, J = 9.0 Hz, 2H), 8.20 (d, J = 9.0 Hz, 2H), 7.84 (d, J = 8.6 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 6.01 (d, J = 3.4 Hz, 1H), 5.87 (d, J = 3.4 Hz, 1H), 4.61 (s, 1H), 3.76 (s, 3H); 13C NMR (125 MHz, CS2/DMSO-d6) (all 1C unless indicated) δ 158.51 (aryl C), 152.76, 152.54, 151.84 (2C), 146.61 (aryl C), 146.08, 146.03, 145.67, 145.42, 145.13, 145.10 (4C), 145.05, 144.95 (2C), 144.70 (2C), 144.50, 144.45 (2C), 144.34, 144.33, 144.14, 144.13, 144.09, 144.05, 144.03, 143.94, 143.54, 143.40, 143.26, 143.16, 142.00, 141.88, 141.53 (2C), 141.45, 141.41, 141.19, 141.12, 141.06, 141.01, 140.93, 140.87, 140.85, 140.82 (2C), 140.78, 140.39 (2C), 138.92, 138.89, 138.43, 138.27, 136.20, 135.63, 135.22, 134.73, 128.90 (aryl C), 128.69 (2C, aryl C), 128.48 (aryl C), 128.46 (aryl C), 122.55 (2C, aryl C), 113.04 (2C, aryl C), 75.65 (sp3-C of C60), 75.00 (sp3-C of C60), 73.68, 73.00, 53.97; FT-IR ν/cm−1 (KBr) 3309, 2918, 2849, 1617, 1526, 1477, 1462, 1428, 1382, 1345, 1267, 1215, 1184, 1164, 1095, 897, 806, 730, 698, 526; UV-vis (CHCl3) λmax/nm 258, 312, 431; MALDI-TOF MS m/z calcd for C75H15N2O3 [M + H]+ 991.1083, found 991.1060.

Fulleropyrrolidine cis-4a

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1f (31 μL, 0.25 mmol) and 2g (28 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 18 h afforded first unreacted C60 (8.3 mg, 23%) and then cis-4a6 (31.4 mg, 64%) as an amorphous brown solid.

Fulleropyrrolidine cis-4b

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1g (31 μL, 0.25 mmol) and 2h (35.3 mg, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 48 h afforded first unreacted C60 (24.2 mg, 67%) and then cis-4b6 (14.4 mg, 29%) as an amorphous brown solid.

Reaction of C60 with benzylamine (1a), 2-chlorobenzaldehyde (2g), 2-chlorobenzylamine (1f), and benzaldehyde (2a) in the presence of Mg(ClO4)2

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol), 2g (28 μL, 0.25 mmol), 1f (31 μL, 0.25 mmol), and 2a (26 μL, 0.25 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 7 h afforded first unreacted C60 (15.0 mg, 42%), then cis-4a6 (11.0 mg, 22%), cis-3i (13.4 mg, 28%) and cis-3a6,10 (2.3 mg, 5%) as amorphous brown solid.

According to the general procedure, the reaction of C60 (36.0 mg, 0.05 mmol) with 1a (27 μL, 0.25 mmol), 2g (28 μL, 0.25 mmol), 1f (18 μL, 0.15 mmol), and 2a (15 μL, 0.15 mmol) in the presence of Mg(ClO4)2 (22.3 mg, 0.10 mmol) for 10 h afforded first unreacted C60 (20.0 mg, 56%), then cis-4a6 (8.8 mg, 18%), cis-3i (9.9 mg, 21%) and cis-3a6,10 (1.0 mg, 2%) as amorphous brown solid.

Acknowledgements

The authors are grateful for the financial support from National Natural Science Foundation of China (No. 21102041), Scientific Research Foundation of Education Commission of Hubei Province (No. Q20120113), Natural Science Fund for Creative Research Groups of Hubei Province of China (No. 2014CFA015), and Innovation and Entrepreneurship Training Program for Undergraduates of Hubei Province (No. 201610512054).

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Footnotes

† Electronic supplementary information (ESI) available: Optimization results, NMR spectra of products cis-3a–m and cis-4a–d as well as HRMS of cis-3j and cis-4d. See DOI: 10.1039/c6ra16041g
‡ Authors contributed equally.
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