Buckyball fullerenes, which are the most representative and abundant in terms of natural occurrence, possess unique physical and chemical properties that may be exploited for a wide variety of applications ranging from chemical sensors to superconductivity.[1 ] Functionalized fullerene derivatives have potential applications in the field of medicinal chemistry and have boosted the interest of the scientific community in these new molecular carbon allotropes.[2 ]
[3 ] The broad range of physical and chemical properties of functionalized fullerenes makes them interesting building blocks for superconductors, photoconductors, and semiconductors, ferromagnetic, electronic and optical devices.[4–10 ] Thus, dumbbell-like amine-functionalized bis-C60 fullerene has attracted much attention in recent years because of its unique applications in molecular electronics and in the construction of photovoltaic devices.[11 ] The functionalized bis-C60 fullerenes not only increase the solubility in common organic solvent but also tune the electronic properties. The great solubility in common organic solvent enables desirable morphological properties of the functionalized bis-C60 fullerenes to be developed.
Synthetic methods for the formation of dumbbell-like fullerene compounds have been reported by reaction of fullerene, carbonyl compounds, and diamines.[12 ]
[13 ]
[14 ]
[15 ]
[16 ] The most common approach for the synthesis of amine functionalized C60 fullerene derivatives through azomethine ylides was first reported by using the Prato reaction.[17 ] Accordingly, A. S. Konev et al. developed the reaction of bisaziridines with C60 fullerene to give dumbbell-like bis-C60 fullerene triads.[13 ] However, the reported methods have limited scope, harsh reaction conditions, require the use of transition-metal catalysts and additives, and involve tedious work-up processes and long reaction times, which strongly affect the economics as well as the environment-friendly nature of the reaction. The most common approaches derived for the preparation of dumbbell-like amine-functionalized bis-C60 fullerenes involve thermal reaction. In contrast, photoinduced single-electron transfer (SET) reactions without using photocatalyst have become attractive for the preparation of substituted fullerenes because they can be carried out under environmentally benign conditions using visible light and because they generate unique products.[18 ]
[19 ] The SET-promoted reaction involves reductive cleavage of a functional group without using any catalyst, and it is often utilized in modern organic reaction. The formation of a C–C bond through photoinduced SET reaction requires a radical ion pair in which the electron donor unit, such as tertiary amines, and electron acceptor, such as fullerene, result in the formation of complex organic molecules.
In the framework of our research concerning the development of new C–C and C–N bond-forming reactions for the synthesis of novel types of organic molecules,[20 ]
[21 ]
[22 ] compounds with two bis-α-silyl tertiary benzyl amines connected by an aliphatic linker have been developed, which allow the construction of amine-functionalized bis-C60 fullerene. We present here the synthesis and characterization of bis-α-silyl tertiary benzyl amines, and the reaction with C60 fullerene for the preparation and characterization of novel dumbbell-like amine-functionalized bis-C60 fullerene derivatives by using photoinduced SET reactions (Scheme [1 ]). The reaction is easy to perform and allows various amine-functionalized bis-C60 fullerenes to be synthesized. More importantly, the reaction was carried out in the absence of photocatalyst, which makes them green protocols. These reaction procedures are simple and can be used to isolate products with high purity and satisfactory yields. To our knowledge, this is the first report of a SET-mediated photochemical approach for the synthesis of dumbbell-like amine-functionalized bis-C60 fullerene through C–C bond formation.
Scheme 1 Synthesis dumbbell-like amine-functionalized bis-C60 fullerene
Scheme 2 Synthesis of bis-α-silyl tertiary benzyl amines 13 –16
The study was initiated by the synthesis of bis-α-silyl tertiary benzyl amines. The routes employed to prepare the bis-α-trimethylsilyl-substituted tertiary benzyl amines is outlined in Scheme [2 ]. The α-silyl benzyl amines 6 –9 were synthesized by employing potassium carbonate promoted substitution reactions of commercially available benzyl amines 1 –4 with iodomethyltrimethylsilane 5 (Scheme [2 ]).[23 ]
The known compound 2,2′-oxybis(ethane-2,1-diyl)bis(2-bromoacetate) (12 ) was easily prepared by adopting the literature precedent.[24 ] The diethylene glycol 11 was reacted with bromoacetic acid 10 under solvent-free conditions to yield the corresponding 2,2′-oxybis(ethane-2,1-diyl)bis(2-bromoacetate). The compounds with two bis-α-silyl tertiary benzyl amine units connected by an aliphatic linker have been developed by reaction of known α-trimethylsilyl-substituted secondary benzyl amines 6 –9 with 12 in the presence of K2 CO3 to yield the corresponding bis-α-silyl tertiary benzyl amines 13 –16 in high yields (80–60%).[25 ]
With bis-α-silyl tertiary benzyl amines 13 –16 in hand, we next explored the photochemical reaction with C60 fullerene for the formation of dumbbell-like amine-functionalized bis-C60 fullerene derivatives. The yields and product distributions of photoreactions of C60 with the bis-α-trimethylsilyl-substituted tertiary benzyl amines 13 –16 , which have various substituents on the para -position of the phenyl ring, were determined (Scheme [3 ]). All photochemical reactions were carried out by irradiation with a Hanovia medium-pressure mercury lamp (450 W) through a flint glass filter (λ >300 nm) by using 10% EtOH–toluene solutions containing C60 (0.28 mmol) and bis-α-trimethylsilyl-substituted tertiary benzyl amines (0.56 mmol).[26 ]
Scheme 3 Synthesis of dumbbell-like amine-functionalized bis-C60 -fullerene
The photoproducts were triturated with CHCl3 to recover C60 and the triturates were concentrated in vacuo to generate residues, which were subjected to silica gel column chromatography to generate pure photoproducts. Structural assignments to the photoproducts were made based on analysis of 1 H and 13 CNMR and on HRMS analysis.
Table 1 Products and Yields of Photoaddition Reactions of N2 -Purged Bis-α-silyl Tertiary Amines with C60 in 10% EtOH–toluenea
Entry
Amine
Irradiation time (min)
Conv. (%)b
Product (%)c
1
13
90
89
17 (49), 21 (4)
2
14
80
85
18 (51), 22 (3)
3
15
120
70
19 (34), 23 (5)
4
16
90
84
20 (53), 24 (2)
a Bis-α-silyl tertiary benzyl amine/C60 ratio 0.56:0.28 mmol in 220 mL of 10% EtOH–toluene.
b Conversion was determined based on recovered C60 .
c Isolated yield based on consumed C60 .
Table 2 Products and Yields of Photoaddition Reactions of O2 -Purged Bis-α-silyl Tertiary Amines with C60 in 10% EtOH–toluenea
Entry
Amine
Irradiation time (min)
Conversion (%)b
Product (%)c
1
13
90
90
17 (2), 21 (51)
2
14
80
90
18 (4), 22 (50)
3
15
120
72
19 (trace), 23 (42)
4
16
90
88
20 (3), 24 (47)
a Bis-α-silyl tertiary benzyl amine/C60 ratio 0.56:0.28 mmol in 220 mL of 10% EtOH–toluene.
b Conversion was determined based on recovered C60 .
c Isolated yield based on consumed C60 .
Scheme 4 C60 furnished single-bonded bis-aminomethyl-1,2-dihyrofullerenes
The results show that photoreactions of C60 with bis-α-trimethylsilyl-substituted tertiary amines 13 –16 , carried out in N2 -purged solution, generate two types of photoproducts, bis-aminomethyl-1,2-dihyrofullerenes 17 –20 , and bis-fulleropyrrolidines 21 –24 (Scheme [3 ]). Clearly, 90 min irradiation of solutions of C60 and bis-α-trimethylsilyl-substituted tertiary benzyl amines 14 and 16 (0.56 mmol) possessing electron-donating groups (CH3 and OCH3 ) on the phenyl rings as well as the non-substituted analogue 13 , brings about high conversion of C60 and satisfactory yields of formation of the corresponding bis-aminomethyl-1,2-dihyrofullerenes 17 , 18 , 20 predominantly, along with low yields of fulleropyrrolindines 21 , 22 , and 24 (Table [1 ], entries 1, 2, and 4). In contrast, photoreactions of C60 with electron-withdrawing group (F) substituted bis-α-trimethylsilyl-substituted tertiary amines 15 (entry 3) require longer irradiation times to bring about high C60 conversions. Significantly, these reactions occur in lower yield as compared with electron-donating group (CH3 and OCH3 ).
The effect of molecular oxygen on the photochemical processes was explored. The results show that irradiation of oxygenated 10% EtOH–toluene solutions containing C60 and bis-α-trimethylsilyl-substituted tertiary benzyl amines 13 –16 under the same conditions as described above, leads to the exclusive formation of the corresponding bis-fulleropyrrolidnes 21 –24 (Table [2 ], entries 1–4) in modest yields. Longer irradiation times were also required for these photoreactions.
When photochemical addition of bis-α-trimethylsilyl-substituted tertiary benzyl amines to C60 was carried out in deoxygenated 10% EtOH–toluene, the major product obtained was the corresponding bis-adducts R-C60 H (Scheme [4 ]). In a similar manner to that of primary and secondary amines, the mechanism proposed for the photoreactions between C60 and tertiary amines starts with a SET from the bis-α-trimethylsilyl-substituted tertiary amines donor to triplet excited state of fullerene C60 to give a radical ion pair.[17 ]
[18 ] This is followed by silophile (EtOH)-induced desilylation and formation of key α-amino radical intermediates. Finally, radical ion pair fullerene radical anion (C60•− ) and α-amino radical combine to afford the final bis-adducts (Scheme [4 ]). The photoreactions of C60 with electron-withdrawing group (F) substituted bis-α-trimethylsilyl-substituted tertiary amines 15 gave a lower yield, perhaps because the fluorine atom destabilizes the cationic nitrogen atom formed during single-electron transfer (Scheme [4 ]).
Scheme 5 C60 furnished double-bonded bis-fulleropyrrolidines
Additionally, in the presence of molecular oxygen, a pathway is followed that involves singlet molecular oxygen (1 O2 ) mediated generation of azomethine ylides, which are reactive intermediates that undergo 1,3-dipolar cycloadditions to fullerene to generate the respective bis-fulleropyrrolidines (Scheme [5 ]). Thus, in the presence of molecular oxygen, the triplet state of C60 undergoes energy transfer and the formation of 1 O2 . The singlet oxygen reacts with bis-α-trimethylsilyl-substituted tertiary benzyl amines by abstraction of the α-hydrogen atom to form α-carbon radicals (Scheme [5 ]) along with hydroperoxy radical. Further hydrogen atom transfer generates the azomethine ylides, which undergo 1,3-dipolar cycloadditions to fullerene to generate the respective bis-fulleropyrrolidines. Similarly, the photoreactions of C60 with fluorine-substituted bis-α-trimethylsilyl-substituted tertiary amines 15 gave lower yield of the product, perhaps because the fluorine atom destabilizes the cationic nitrogen atom formed during single-electron transfer.
In summary, an efficient, expeditious, operationally simple, economical, and environmentally friendly method has been developed for the synthesis of dumbbell-like amine-functionalized bis-C60 fullerene. In contrast to thermal and photochemical strategies developed previously, the new photochemical approach using bis-α-trimethylsilyl-substituted tertiary benzyl amines is both mild and efficient, and, as a result, should be useful in broadening the library of substituted dumbbell-like amine-functionalized bis-C60 fullerene derivatives of type C60 -linker-C60 .
