Single electron transfer promoted photoaddition reactions of α-trimethylsilyl substituted secondary N-alkylamines with fullerene C60

Article abstract of DOI:10.1039/c6ob02069k

Single electron transfer (SET) promoted photoaddition reactions of secondary N-α-trimethylsilyl-N-alkylamines to C₆₀ were explored to gain a deeper understanding of the mechanistic pathways followed and to expand the library of novel types of organofullerenes that can be generated using this approach. The results show that photoreactions of 10% EtOH-toluene solutions containing C₆₀ and N-α-trimethylsilyl-N-alkylamines produce either aminomethyl-1,2-dihydrofullerenes or symmetric fulleropyrrolidines as major products depending on the nature of alkyl substituents. In contrast, photoreactions of 10% EtOH-ODCB solutions of these amines with C₆₀ mainly lead to the formation of symmetric fulleropyrrolidines. Based on the analysis of product distributions and the results of earlier studies, two feasible mechanistic pathways are proposed for these processes. One route is initiated by SET from the amine substrates to the triplet-excited state of C₆₀ to form the corresponding aminium radicals and C₆₀ anion radicals. EtOH-promoted desilylation of the aminium radicals then takes place to produce aminomethyl radicals which can either add to C₆₀ or couple with the C₆₀ radical anions to form respective radicals or anion precursors of aminomethyl-1,2-dihydrofullerene products. The competing pathway leading to the generation of symmetric fulleropyrrolidines also involves the formation of aminomethyl radicals by using the sequential SET-desilylation process. In this route, the aminomethyl radicals are oxidized by SET to C₆₀ to form iminium ions, which are then transformed to azomethine ylides by a pathway involving a second molecule of the secondary amine. Dipolar cycloaddition of the azomethine ylides to C₆₀ forms the symmetric fulleropyrrolidine cycloadducts. Importantly, the observation that symmetric fulleropyrrolidines are the sole products formed in photoreactions between N-α-trimethylsilyl-N-alkylamines and C₆₀ in 10% EtOH-ODCB has synthetic significance.

Full text of DOI:10.1039/c6ob02069k

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Single Electron Transfer Promoted Photoaddition Reactions of -Trimethylsilyl Substituted
Secondary N-Alkylamines with Fullerene C60
Ho Cheol Jeong,1 Suk Hyun Lim,2 Dae Won Cho,2* Sung Hong Kim,3 Patrick S. Mariano4*
1Department of Energy Convergence Engineering, Yeungnam University, Gyeongsan, Gyeongbuk
712-749, Korea
(dwcho00@yu.ac.kr)
2Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Korea
3Analysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 702-701, Korea
4Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New
Mexico 87131, United States
(mariano@unm.edu)
1

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Graphical Abstract
2

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Abstract
Single electron transfer (SET) promoted photoaddition reactions of secondary N--trimethylsilyl-
N-alkylamines to C60 were explored to gain a deeper understanding of mechanistic pathways followed
and to expand the library of novel types of organofullerenes that can be generated using this approach.
The results show that photoreactions of 10% EtOH-toluene solutions containing C60 and N--
trimethylsilyl-N-alkylamines produce either aminomethyl-1,2-dihydrofullerenes or symmetric
fulleropyrrolidines as major products depending on the nature of the alkyl substituents. In contrast,
photoreactions of 10% EtOH-ODCB solutions of these amines with C60 mainly lead to the formation
of symmetric fulleropyrrolidines. Based on the analysis of product distributions and the results of
earlier studies, two feasible mechanistic pathways are proposed for these processes. One route is
initiated by SET from the amine substrates to the triplet excited state of C60 to form the corresponding
aminium radicals and C60 anion radicals. EtOH-promoted desilylation of the aminium radicals then
takes place to produce aminomethyl radicals, which can either add to C60 or couple with the C60 radical
anions to form respective radicals or anion precursors of the aminomethyl-1,2-dihydrofullerene
products. The competing pathway leading to generation of symmetric fulleropyrrolidines also involves
formation of aminomethyl radicals by way the sequential SET-desilylation process. In this route, the
aminomethyl radicals are oxidized by SET to C60 to form iminium ions, which are then transformed to
azomethine ylides by a pathway involving a second molecule of the secondary amine. Dipolar
cycloaddition of the azomethine ylides to C60 forms the symmetric fulleropyrrolidine cycloadducts.
3

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Importantly, the observation that symmetric fulleropyrrolidines are the sole products formed in
photoreactions between N--trimethylsilyl-N-alkylamines and C60 in more 10% EtOH-ODCB has
synthetic significance.
4

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Introduction
Chemical manipulations of fullerene C60 that modify its photochemical and/or photophysical
properties have attracted the wide interest of chemists carrying out studies in the material1-6 and
biological sciences.7-12 Efforts are being conducted to develop efficient methods for chemical
modification that are ready applicable to the preparation of novel fullerene derivatives. Although most
common synthetic approaches to prepare fullerene derivatives involve organometal and transition
metal catalyzed free radical reactions,13-17 single electron transfer (SET) promoted photochemical
reactions between fullerene C60 and various kinds of substrates containing electron donating groups
have been recognized as being useful for the synthesis of novel organofullerenes owing to the mild
and environmental benign conditions, and their potential use of sun light.18-21
In a variety of SET-promoted photochemical reactions studied thus far, aromatic and aliphatic
tertiary amines have been employed as substrates22-28 because of their moderately low oxidation
potentials (Eox < 1 V vs SCE)22 and the high excited state reduction potentials of the fullerenes (e.g.,
3Ered(C60) = ca. 1 V vs SCE)30,31. These redox properties lead to large rates of SET (kSET = ca. 1x1010
M-1s-1) from amines to the triplet states of the fullerenes, generated by light absorption followed by
intersystem crossing (ISC, ISC = ca. 1 and kISC = ca. 2 x 109 s-1).32 This process generates amine
radical cations 1 (aminium radicals) and fullerene radical anions 2 (e.g., C60-) (Scheme 1). Aminium
radicals formed in this way typically undergo loss of electrofugal groups (e.g., deprotonation,
decarboxylation, and desilylation) to form carbon centered aminomethyl radicals 3, which then react
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with fullerene radical anions 2 or the fullerene itself through C-C bond formation.28c,33 Especially
interesting in this regard are aminium radicals generated by SET from tertiary amines possessing -
trialkylsilyl groups, which undergo silophile promoted desilylation.28c Significantly, aminomethyl
radical formation by desilylation of -trialkylsilylaminium radicals takes place more rapidly than by
deprotonation of non-trialkylsilyl containing aminium radicals.33-35
Scheme 1.
The results of several studies have shown that primary and secondary amines also can serve as
electron donors in SET-promoted photochemical reactions with electron acceptors.36-38 In contrast to
those arising from tertiary amines, primary and secondary aminium radicals generated typically
undergo preferential N-H rather than -CH deprotonation to form nitrogen-centered aminyl radicals.
Pertinent examples of reactions in which this pathway leads to production of photoadducts are found
in recent studies by Nakamura,39 Gan40 and others.41 Specifically, these efforts demonstrate that
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photoreactions of secondary amines with C60, proceeding through sequential SET- N-DHOId: 1e0p.1r0o39t/oCn6OatBi0o2n069K
routes, yield aminyl radicals that undergo N-C bond formation to generate mono- and multi-aminated
fullerene derivatives.
In a recent study, we showed that secondary N--trimethylsilyl-N-benzylamines 5 participate in
SET promoted photoreactions with C60 to form aminomethyl substituted, 1,2-dihydrofullerenes 7 as
major photoproducts (Scheme 2).42 In addition, we observed that aminium radicals generated from
these types of secondary amines undergo selective desilylation rather than N-H deprotonation to form
aminomethyl radicals 4. The results show that in contrast to those of non-trimethylsilyl containing
secondary amines, aminium radicals generated from -trimethylsilyl substituted secondary amines
undergo silophilic solvent-assisted desilylation more rapidly than -CH deprotonation and N-H
deprotonation. Moreover, we observed that both symmetric (8) and unsymmetric (9)
fulleropyrrolidines are formed in solvent dependent yields through routes involving azomethine ylide
intermediates 6 (Scheme 2). These findings show that the chemoselectivities of SET photochemical
reactions of secondary amines can be dramatically altered by introduction of -trialkylsilyl group into
these substrates.
Scheme 2
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Results and Discussion
In one phase of our continuing efforts in this area, we carried out an investigation to explore the
scope of SET-photochemical reactions of -trimethylsilyl substituted secondary amines with C60. For
this purpose, a variety of secondary -trimethylsilyl substituted N-alkylamines were prepared by using
well-established N-alkylation reactions of the corresponding primary amines with
iodomethyltrimethylsilane (TMSCH2I) in the presence of K2CO3. In each case, the reaction generated
the target amine (10-25, Figure 1) in modest to high yields (48-94%).
Photochemical reactions of these substrates were carried out on nitrogen-purged 10% EtOH-
toluene or 10% EtOH-o-dichlorobenzene (ODCB) solutions (10 mL) containing C60 (0.17 mM) and
the individual amines 10-25 (0.35 mM) under simultaneous irradiation conditions using a 450W
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Hanovia medium pressure mercury lamp equipped with flint glass filter (>300 nm). The photolysates
were analyzed by using HPLC and subjected to chromatographic separation to give pure products.
Figure 1. Synthesized -trimethylsilyl substituted secondary N-alkylamines 10-25
Structural assignments of aminomethyl-1,2-dihydrofullerenes and fulleropyrrolidines formed in
these reactions were made by using NMR and UV-visible spectroscopic methods, HRMS analysis as
well as by comparison of the data to those of previously characterized analogs.28c,34,35,40,42 In the NMR
1
spectra of the aminomethyl-1,2-dihydrofullerenes, H signals for protons directly bonded to the
13
fullerene core (6.7-7.0 ppm) and C signals for sp3 carbons of the fullerene core (60-68 ppm) are
diagnostic. Moreover, the symmetric fulleropyrrolidines have simple 1H patterns in the upfield region
(4.2-4.5 ppm) comprised resonances for the methylene protons (see Supplementary information).43 In
addition, UV-visible spectra of all photoproducts contain wavelength maxima in the 433-436 nm
region that are characteristic of 1,2- rather than 1,4-dihydrofullerene photoadducts.31,34,43
The results showed that the secondary phenethylamines 10 and 11 (0.35 mM) and C60 (0.17 mM)
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underwent photoreactions to produce the respective aminomethyl substituted 1,2-dihydrofullerene 26
and 27 nearly exclusively (Scheme 3, Table 1, entries 1 and 3). However, when excess of amine 10 (3,
5, 7 and 10 equivalent) over C60 was used, the photoreaction generated not only 1,2-addcut 26 (major)
but also the symmetric fulleropyrrolidine 33 (minor) (entry 2 and Table S1 in Supplementary
Information). Furthermore, photoreaction of C60 with the p-F substituted phenethylamine 12 (0.35 mM)
required much longer irradiation times and led to high yielding formation of the aminomethyl adduct
31 and a trace amount (>7 h) of the fulleropyrrolidine 37 (entries 4-6).
To compare the above observations with those made earlier,42 photoreactions of the benzylamines
13-16 with C60 were carried out under the same conditions. The results showed that relatively short
time irradiations of solutions containing 10-13 with C60 led to formation of aminomethyl-1,2-
dihydrofullerenes 29-32, along with minor amounts of the symmetric (only from 14) and unsymmetric
(only from 16) fulleropyrrolidines 37 and 40. (entries 7-11) In addition, no change in the product
distribution occurred when an excess of benzylamine 13 was utilized (entry 8). Importantly, the product
distributions generated in photoreactions of 13-16 matched those observed in our pervious effort.42
Scheme 3.
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Table 1. Products and yields of photoaddition reactions of C60 with secondary amines 10-16 in N2-
purged 10% EtOH-toluene.a
Irradiation
entry
amine
Conversion (%)b
Product (%)c
Time (h)
1
2d
3
10
10
11
12
12
12
13
13
14
15
16
2
2
2
2
5
7
1
1
1
1
2
100
100
100
18
26 (49), 33 (trace)
26 (47), 33 (11)
27 (53), 34 (7)
28 (15)
4
5
59
28 (44)
6
94
28 (58), 35 (2)
29 (62),
7
98
8d
9
100
100
100
100
29 (60)
30 (61), 37 (4)
31 (72)
10
11
32 (23), 40 (31)
aAmine:C60 is 0.35:0.17 mM in 10 mL of 10% EtOH-toluene. bConversion is based on
recovered C60. cIsolated yields. dExcess amine (1.7 mM, 10 equiv.) is used.
Photoreactions of -methyl (17, R2 = Me) and -phenyl (18, R2 = Ph) benzylamines (0.17 mM)
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with C60 (0.35 mM) gave rise to nearly exclusive formation of the corresponding aminomethyl adducts
41-42. (Scheme 4). The unsymmetric fulleropyrrolidine 43 was formed in a trace quantity from 18.
Here again, reactions using 10 equivalent excesses of 17-18 gave the same product distributions
Scheme 4.
The results of photoaddition reactions of C60 with -trimethylsilyl substituted, secondary
alkylamines were interesting. While photoreactions of N2-purged 10% EtOH-toluene solution
containing allyl (19), iso-propyl (20) and tert-butyl (21) substituted amines (0.17 mM) with C60 (0.35
mM) gave rise to high yielding formation of the corresponding aminomethyl substituted 1,2-
dihydrofullerenes 44-46 as sole products (Scheme 5 and Table 2, entries 1-3), reactions of n-hexyl (22),
n-propyl (23), cyclohexyl (24) and ethyleneoxymethyl (25) substituted amines under identical
conditions occurred to generate only the respective symmetric fulleropyrrolidines 49-52 (entries 4-7).
Scheme 5.
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H
hv / N2
H
N
R3
C60
C60
+
19-25
C60
+
N
R3
10% EtOH-
toluene
or
10% EtOH-
ODCB
44 (R3 = allyl)
45 (R3 = iPr)
46 (R3 = t-Bu)
47 (R3 = allyl)
48 (R3 = iPr)
49 (R3 = Hexyl)
50 (R3 = Pr)
51 (R3 = cyclohexyl)
52 (R3 = MeOCH2CH2)
Table 2. Products and their yields of photoaddition reactions of C60 with secondary amines 19-25 in
N2-purged 10% EtOH-toluene.a
Irradiation
entry
amine
Conversion (%)b
Product (%)c
Time (h)
1
2
3
4
5
6
7
19
20
21
22
23
24
25
2
2
2
2
2
2
2
100
100
100
100
100
100
98
44 (59)
45 (57)
46 (68)
49 (60)
50 (57)
51 (51)
52 (66)
aAmine:C60 is 0.35:0.17 mM in 10 mL of 10% EtOH-toluene. bConversion isbased on
recovered C60. cIsolated yields.
Solvent dependent photoproduct distributions. C60 has a limited range of solvents in which it
is soluble and, more importantly, SET-promoted photochemical pathways involving formation of
radical ion pairs or ylide intermediates are highly dependent on the nature of solvents. Because of these
factors, only a limited range of solvents can be employed to probe solvent effects of the photoaddition
reactions taking place between C60 and the secondary N--trimethylsilyl-N-alkylamines. In spite of
this, our studies showed that a unique change occurred in the nature of the major reaction pathway
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followed when the solvent was changed from 10% EtOH-toluene to 10% EtOH-ODCB. It should be
noted that no photoproducts were generated when the polar protic solvent EtOH was not a component
of the solvent mixtures used for the photoreactions.28c Accordingly, the photolysates generated by
irradiation of toluene or ODCB solutions contain fully recovered amine and C60. Inspection of the data
in Table 3 showed that irradiation of solutions of secondary benzyl- and phenethyl amines 10-16 with
C60 in 10% EtOH-ODCB gave rise to generation of the respective symmetric fulleropyrrolidines 33-
39 as major photoproducts. As observed earlier (see above), photoreactions of the p-F-substituted arene
ring containing benzyl and phenethyl amines 12 and 16 required much longer irradiation times (over
10 h) to bring about high conversions. Moreover, in contrast to those arising from analogous
unsubstituted benzylamine 13, product distributions of photoreactions of solutions containing C60 and
-substituted benzylamines 17-18 did not change when the solvent was changed from 10% EtOH-
toluene to 10% EtOH-ODCB. In a similar manner, photoreactions of mixtures of C60 with secondary
N-alkylamines 19, 20 and 22-25 in 10% EtOH-ODCB also generated the corresponding symmetric
fulleropyrrolidines 47-52 solely or exclusively (Table 4). Interestingly, the t-butyl substituted amine
21 did not undergo photoreaction with C60 in the ODCB containing solvent system. We believe that
this is a consequence of the steric crowding in 21 that hinders formation of ylide intermediates (see
below fulleropyrrolidine formation mechanism).
Table 3. Products and yields of photoaddition reactions of C60 with secondary amines 10-18 in N2-
purged 10% EtOH-ODCB.a
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Product (%)c
Irradiation
entry
Amine
Conversion (%)b
Time (h)
1
2d
3
10
10
11
12
12
13
13
14
15
16
16
17
18
3
3
100
96
26 (11), 33 (53)
26 (5), 33 (55)
27 (18), 34 (44)
35 (37)
3
100
47
4
5
5
10
1
89
28 (trace), 35 (60)
29 (12), 36 (38)
29 (8), 36 (45)
30 (7), 37 (57)
31 (7), 38 (53)
32 (3), 39 (11), 40 (4)
32 (5), 39 (35), 40 (9)
41 (56)
6
98
7d
8
1
100
100
100
52
1
9
1
10
11
12
13
5
10
3
95
100
91
3
42 (54), 43 (trace)
aAmine:C60 is 0.35:0.17 mM in 10 mL of 10% EtOH-ODCB. bConversion is based on
recovered C60. cIsolated yields. dExcess of amine (1.7 mM, 10 equiv.) is used.
Table 4. Products and yields of photoaddition reactions of C60 with secondary amines 19-25 in N2-
purged 10% EtOH-ODCB.a
Irradiation
Entry
amine
Conversion (%)b
Product (%)c
Time (h)
1
2
3
4
5
19
20
21
22
23
3
3
5
3
3
100
100
47 (54)
45 (3), 48 (61)
d
d
-
-
100
100
49 (51)
50 (59)
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6
7
24
25
3
3
100
100
51 (58)
52 (49)
aAmine:C60 is 0.35:0.17 mM in 10 mL of 10% EtOH-ODCB. bConversion is based on
recovered C60. cIsolated yields. dNo reaction
Mechanistic pathways. Aminomethyl-1,2-dihydrofullerene formation. Based on analysis of
photoproduct distributions and the results of previous studies, it is possible to suggest the plausible
mechanistic pathways, outlined in Scheme 7 for the formation of aminomethyl-1,2-dihydrofullerene
55 in photoreactions of -trimethylsilyl susbstituted secondary amines with C60. As stated in the
Introduction, these reactions are initiated by photoinduced SET that produces aminium radicals 53 and
the fullerene radical anions (C60-). Then, EtOH-promoted desilylation of 53 forms neutral
aminomethyl radicals 54, which are added to C60-, followed by protonation of resulting anion 56. An
alternative sequence for product formation involves addition of 54 to another C60 to form aminomethyl
radicals 57, which are converted to adducts 55 via sequential SET from C60- and protonation.
Scheme 7.
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Symmetric fulleropyrrolidine formation. The generation of symmetric fullerDoOpI:y1r0r.1o0l3i9d/Cin6OeBs02in069K
these photoreactions is unusual. In earlier studies,42 we explored mechanism of this process by
determining the origin of the extra methylene group in these cycloadducts by using isotopical amine
substrates. The results, summarized in Scheme 8, showed that the methylene group originated from
the trimethylsilyl-linked methylene group (Me3SiCH2) in the amine substrates.
Scheme 8.
This observation enabled us to propose that the key distinguishing event in the mechanistic route
for symmetric fulleropyrrolidines formation is SET oxidation of aminomethyl radical intermediates
54, generated by sequential SET-desilylation processes (Scheme 9). This process, promoted by SET
to C60 forms iminium ion intermediates 58, which react with another molecule of the secondary amine
to form germinal-diamines 60, which eliminate a primary amine to produce -trimethylsilyl-
substituted iminium ions 59. Solvent promoted desilylation of 59 then generates azomethine ylides
45
61,44 which undergo 1,3-dipolar cycloaddition to C60 to afford the corresponding symmetric
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fulleropyrrolidines 62.
Scheme 9.
In here, it should be noticeable to mention about the solvent effects. As can be seen from above,
while reactions of 10% EtOH-toluene solutions containing secondary amines and C60 led to the
formation of aminomethyl-1,2-dihydrofullerenes preferentially rather than symmetric
fulleropyrrolidines, those of 10% EtOH-ODCB solution gave rise to the exclusive/predominant
production of symmetric fulleropyrroldines. Considering SET-promoted mechanistic pathways,
preferences of photoproducts depending on solvent would be largely associated with the solvent
polarities. Specifically, owing to the fact that ODCB is more polar solvent than toluene is (dielectric
constants of them are 9.93 and 2.38 D respectively) and, as a result, formation of symmetric
fulleropyrrolidines via ionic intermediates (e.g., iminium and azomethine ylides) are much facilitated
in the polar ODCB solvent.
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In photoreactions of C60 with 17, 18 and 21 (see above), no symmetric fulleropyrrolidnes were
formed even when the processes were carried out using 10% EtOH-ODCB solvent. This finding
appears to indicate that steric hindrance offered by a bulky amine with N-alkyl substituent blocks
addition of the amines to the iminium ions 58 that are required for formation of the key geminal-
diamine intermediates 60. To gain support for this proposal, well-known thermal Prato reactions45 of
C60 with secondary amines 10, 13, 17, 18 and 21 were carried out using formaldehyde (Scheme 10).
In Prato reactions, formaldehyde reacts with secondary amines to produce azomethine ylide
intermediates, which add to C60 to form fulleropyrrolidines. The results showed that while reactions
of the -trimethylsilyl substituted N-phenethyamine 10 and N-benzyl 13 in 10% EtOH-toluene
solutions containing C60 and formaldehyde led to high yielding production of the respective symmetric
fulleropyrrolidines 33 and 36, reaction of the N--phenylbenzyl analog 18 with C60 and formaldehyde
gave rise to fulleropyrrolidine 63 in a significantly lower yield.46 Moreover, no symmetric
fulleropyrrolidines were generated in reactions of N--methylbenzyl amine 17 and N-t-butylamine 21
with C60 and formaldehyde (Scheme 10).
Scheme 10.
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As described above (Table 1, entries 2-3), when an excess of amine 10 (10 vs. 2 equiv.) was used
in the photoreaction with C60, a relatively greater amount of the symmetric fulleropyrrolidine 33 was
formed. (Table S1 in Supplementary information) This change in the product distribution suggests that
another mechanistic route might be available for formation of fulleropyrrolidines, which involves
secondary reaction of the initially formed aminomethyl-1,2-dihydrofullerenes with 10. Thus,
additional studies were performed to gain information about this mechanistic possibility. Specifically,
photoreactions of deoxygenated 10% EtOH-toluene and 10% EtOH-ODCB solutions of respective
aminomethyl-1,2-dihydrofullerenes 26, 41-42, and 45 were carried out in the presence of the
corresponding secondary amines 10, 17-18, and 20. (see Figure S1-S4 in Supplementary information).
Photoproduct production in each process was monitored by using HPLC. The results showed that as
the irradiation time of solutions containing 26 and 10 in both 10% EtOH-toluene and 10% EtOH-
ODCB were increased, the substrate gradually disappeared along with simultaneous generation of both
fulleropyrrolidine 33 and C60 in almost equal quantities. In contrast, when solutions of aminomethyl-
1,2-didhydrofullerenes 41-42 and 45 containing the respective amines were irradiated, reactions
occurred to form C60 exclusively. Similar reaction pathways were followed in dark (thermal) reactions
of the aminomethyl-1,2-dihydrofullerenes and their respective amines. For example, when 10% EtOH-
toluene and 10% EtOH-ODCB solutions containing 26 and corresponding amine 10 were stirred for
o
20 h at 110 C, both fulleropyrrolidine 33 and C60 were generated. (Figure S5 in Supplementary
20

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information) Under the same reaction conditions, however, dark reactions of 41-42 witDhOcI:o10r.r1e03s9p/Co6nOdBi0n2g069K
amines 17-18, took place to give only C60. The results of these experiments showed that, depending
on structural nature of N-substituent, initially produced aminomethyl subsutituted fullerenes could be
converted to symmetric fulleropyrrolidines. (see Figure S1-S5)
Based on the above results, another mechanistic pathway leading to N-alkyl substituted symmetric
fulleropyrrolidines is possible. (Scheme 11). Specifically, secondary photochemical reaction with of
an initially formed aminomethyl-1,2-dihydrofullerene adducts 55 with the iminium ions 58, produced
by the SET process shown in Scheme 9 could take place to generate germinate-diamines 64. Diamines
64 are then transformed to the new iminium ions 66, which cyclize to generate the symmetric
fulleropyrrolidine products 62. The cyclization reaction would be initiated by removal of the modestly
acidic proton (pKa ca. 4)47 in aminomethyl-1,2-dihydrofullerene core, followed by intramolecular
addition of the resulting anions to the iminium ion moiety.
Scheme 11.
Conclusion
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In the current study, single electron transfer (SET) promoted photoaddition reacDtiOoI:n10s.1o03f9C/C660OBa0n2d069K
secondary N--trimethylsilyl-N-alkylamines were explored in order to gain a more thorough
understanding of the reaction profiles of aminium radicals derived from -trimethylsilyl substituted
secondary amines and to broaden methods that can be employed to generate members of the
organofullerene family. The results show that photoreactions of solutions containing C60 and N--
trimethylsilyl-N-alkylamines produce either aminomethyl-1,2-dihydrofullerenes or symmetric
fulleropyrrolidines as major products depending on the nature of alkyl substituent on the amines and
solvent. Based on analysis of product distributions and the results of earlier studies, two mechanistic
pathways involving formation of the photoproducts were proposed. The major route for these
processes is initiated by SET from the N--trimethylsilyl-N-alkylamines to C60, followed by
desilylation of the formed aminium radicals to produce the key aminomethyl radical intermediates.
Coupling of these radicals with the C60 or its radical anions then leads to formation of radical or anion
precursors of the aminomethyl-1,2-dihydrofullerene products. Alternatively, the aminomethyl radicals
can be oxidized by SET to C60 to form an iminium ions which, following reaction with the starting
amine and loss of a primary amine, generate an azomethine ylides. The ylides then cycloadd to C60 to
yield the symmetric fulleropyrrolidines.
Acknowledgements
This investigation was financially supported by a grant from the Basic Science Research Program
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through the National Research Foundation of Korea (NRF) funded by the MinistrDyOIo: 1f0.1E03d9u/Cc6aOtBio02n0,69K
Science, and Technology (2015R1D1A3A01020189).
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