Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60with α-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Article abstract of DOI:10.1021/acs.joc.0c01324

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C60 with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C60 with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C60. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of 3C60? by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C60. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C60 with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

Full text of DOI:10.1021/acs.joc.0c01324

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Article
Control of Chemoselectivity of SET-Promoted Photoaddition Reactions
60
of Fullerene C with #-Trimethylsilyl Group Containing N-alkylglycinates
Yielding Aminomethyl-1,2-Dihydrofullerenes or Fulleropyrrolidines
Suk Hyun Lim, Mina Ahn, Kyung-Ryang Wee, Jun Ho
Shim, Jungkweon Choi, Doo-Sik Ahn, and Dae Won Cho
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c01324 • Publication Date (Web): 24 Sep 2020
Downloaded from pubs.acs.org on September 24, 2020
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Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60 with
α-Trimethylsilyl Group Containing N-alkylglycinates Yielding Aminomethyl-1,2-
dihydrofullerenes or Fulleropyrrolidines
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Suk Hyun Lim, Mina Ahn, Kyung-Ryang Wee, Jun Ho Shim, Jungkweon Choi,
3
*
1*
Doo-Sik Ahn, Dae Won Cho
1Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea
(dwcho00@yu.ac.kr)
2Department of Chemistry and Institute of Basic Science, Daegu University, Gyeongsan,
Gyeongbuk 38453, Korea
3Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS),
Daejeon 305-701, Korea
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Graphical Abstract
R
N
R
R
Me Si
CO Et
2
N
3
H
Me Si
N
CO Et
2
3
CO Et
/ MB
2
hv / N2
hv / O2
(
R = alkyl)
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0
-
-
+
1
0% EtOH
toluene
10% EtOH
toluene
1
,3-dipoar
cycloaddition
via azomethine ylide
radical addition
C60
α
via -amino radical
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Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that
are utilized to produce complex organic substances. In the current study, SET-promoted photoaddition
reactions of fullerene C60 with both trimethylsilyl and various alkyl group containing glycinates, ethyl
N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl
substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction
pathways followed. The results showed that photoreactions of C60 with glycinates, performed in
deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway
involving addition of α-amino radical intermediates that are generated by sequential SET- solvent-
assisted desilylation of glycinate substrates to C60. Under oxygenated conditions, photoreactions of the
glycinate substrates, except N-benzyl substituted analogs, did not take place efficiently owing to
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*
quenching of C60 by oxygen. Interestingly, N-benzyl substituted glycinates did react under these
conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ
formed azomethine ylides to C60. The ylide intermediates were formed by regioselective H-atom
transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB) photosensitized
reactions of C60 with glycinates under oxygenated conditions took place efficiently to produce
fulleropyrrolidines independent of the nature of the N-alkyl substituents of glycinates.
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Introduction
Among the variety of organic substances that participate in photoinduced single electron transfer
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(SET) processes, aliphatic/aromatic amines have a long abundant history as readily oxidized (Eox < 1
V vs SCE) electron donors that undergo photoaddition reactions with a host of different electron
acceptors.1-5 In these photochemical reactions, amine radical cations 2 (i.e., aminium radicals)
generated by SET from amines serve as key reactive intermediates,1,3b,5 that undergo diverse reactions
6
including energy-wasting back-SET to reform ground state of donors and acceptors and base-
1b,3a,7,8
promoted deprotonation at α-carbons to form α-amino radicals 3-4 (Path A in Scheme 1).
Electron rich α-amino radicals formed in this manner participate in radical coupling or nucleophilic
4,5,9,10
addition to electron deficient olefins, cyanoarenes, and α,β-unsaturated ketones.
Another
common viable route of aminium radicals 2 is hydrogen atom transfer (HAT) from α-carbons to
produce iminium ions 5-6. (Path B in Scheme 1),5,11,12 which also arise by SET from the readily
13
oxidized α-amino radical intermediates 3-4 (Eox = ca. - 1 V) (Path C in Scheme 1). Owing to the
electrophilic nature of iminium ions, they are readily captured by a wide range of nucleophiles via
carbon-carbon or carbon-heteroatom bond forming processes.11,12,14
Scheme 1. Reaction pathways followed in SET-promoted photochemical reactions of amines
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R3
R3
R1
R2
or
R1
R2
N
N
Path A
base
hv
Acceptor
SET
3
~
H
R3
N
R3
1
1
1
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R1
R2
R1
R2
Path C
SET
N
back SET
1
~ H
2
R3
R3
Path B
R1
R2
or
R1
R2
N
N
4
Since each of pathways that are followed in SET photochemical reactions of amines have the
capability of generating interesting products, amine-based photochemical reactions have high
preparative potential, but only if approaches are available to gain control over regioselectivity and
chemoselectivity.11a,8b,15 Several strategies have been developed thus far to control these features,
especially in SET photosensitized oxidation reactions of tertiary amines where intermediate aminium
7,8
radicals generally undergo α-CH deprotonation to produce α-amino radicals or H-atom transfer
(
HAT) to external oxidant such as molecular oxygen (O
) to form iminium ions.5,11,12,15 Combined with
2
this protocol, selection of appropriate trapping agents capable of intercepting generated intermediates
is inevitable to complete reactions. Pertinent examples are found in early studies by Stephenson and
König,
which
showed
that
photosensitizer-catalyzed
photoreactions
of
N-
1
‾
=CHCOR2,
phenyltetrahydroisoquinoline 5 with nitromethane (R CH
2
NO
2
, Nu ) or enone (CH
2
E+).11a,12a,15a As depicted in Scheme 2, when the transition metal complex sensitized photoreactions of
were performed in the presence of both molecular oxygen and nucleophiles like nitromethane,
5
product formation took place through a pathway involving iminium ions 8. In contrast, the reactions
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of 7 with electrophilic enone conducted in oxygen-free condition led to final photoadduct 9 through
addition of α-amino radicals 7 to enone.
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Scheme 2. Photoreactions of N-phenyltetrahydroisoquinoline with nitromethane/ enone
N
Ph
5
hv
Ru(II) or Ir(III)
N
Ph
6
path A
~ H
path B
with O
without O2
2
~ H
N
N
Ph
Ph
8
7
O
R
1CH
NO2
2
=
H, alkyl)
2
1
R
(R
=
alkyl, aryl)
2
(R
N
N
Ph
Ph
R1 NO2
O
R2
9
10
One another excellent approach to govern both regioselectivity and chemoselectivity of tertiary
amines was developed in earlier independent investigation by Mariano, and Yoon,3,4a,7a,4c,16 which
showed that α-trimethylsilyl substituted tertiary amines serve as useful precursors of α-amino radicals
owing to the fact that α-trimethylsilyl group greatly lowers oxidation potentials of amines and the
7a,17
resulting aminium radicals undergo rapid and selective desilylation to form α-amino radicals 13.
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(
Scheme 3) Based on this strategy, several photoaddition and photocyclization reactions has been
5
6
7
employed to prepare amine-functionalized photoproducts.4a,4c
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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3
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Scheme 3. Selective SET- desilyation process of α-trimethylsilyl substituted tertiary amines
R1 N CH2 SiMe3
SiMe3
R2
~ SiMe3+
SET
R1 N CH2
R2
R1 N CH2 SiMe3
R1
N
C
H
H
R2
R2
R1 N CH2 SiMe3
R2
1
1
13
=
2
alkyl, aryl)
1
(
R ,R
12
1
,3-dipolar cycloaddition reactions are a highly useful and reliable synthetic method that lead to
18
construction of five-membered heterocycles. In particular, the reaction using azomethine ylides with
dipolarophiles is a powerful method for the synthesis of diversely substituted pyrrolidines. Classical
azomethine ylide forming reactions involve deprotonation (or decarboxylation) of intermediates in situ
generated by the reaction of aldehydes with a secondary amines bearing an electron-withdrawing group,
ring opening reactions of aziridines, and metal-catalyzed α-iminoesters. Besides these thermal
synthesis, recent development of photoredox catalysis reactions also was able to provide an efficient
photochemical method for the generation of 1,3-dipolar azomethine ylide intermediates through
deprotonation of photochemically generated iminium ions.11
In our studies aimed at exploring application of amine SET photochemistry to the synthesis of
functionalized fullerenes,19,20 we observed that photoreaction of C60 with α-trimethylsilyl substituted
amines in deoxygenated condition promotes formation of aminomethyl-1,2-dihydrofullerenes via
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exclusive generation and addition of α-amino radicals produced by sequential SET-desilylation
processes from α-trimethylsilyl amines. More recently, we found that both α-trimethylsilyl and α-
alkoxycarbonyl groups containing amines 14 (i.e., ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates)
serve as precursors of not only α-amino radicals 15 but also azomethine ylides 17. (Scheme 4) What
made these reactions especially interesting was that photochemical reaction pathways open to
glycinates 14 can be effectively governed by reaction conditions employed and, in addition, resulting
intermediates can be captured by common fullerene C60 through either radical addition of α-amino
radicals (leading to aminomethyl-1,2-dihydrofullerenes 15) or 1,3-dipolar cycloaddition of ylides
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(leading to fulleropyrrolidines 18). Thus, in the current study described below, we thoroughly explored
how N-alkyl substituents of glycinate substrates (i.e., N-aliphatic alkyl, N-phenethyl and N-benzyl
moieties) and reaction conditions (i.e., N
2
vs O -purged condition, presence/absence of a
2
photosensitizer) govern the chemoselectivity of SET-promoted photoaddition reactions of fullerene
60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates.
C
Scheme 4. Photoaddition reaction of C60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates
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R
N
H
CO Et
2
R
R
N
N
CO Et
2
Me Si
CO Et
3
2
(
R = alkyl)
hv
hv
14
15
+
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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2
2
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0
1
6
SiMe3
R
N
Me Si
CO Et
2
3
N
R
C60
17
CO Et
2
1
8
Results and Discussion
Photoreactions of C60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates in
deoxygenated condition. A variety of alkyl- and trimethylsilyl group substituted glycinates 19a-19t
used to explore photoreactions with C60 were prepared by using the general synthetic procedures
shown in Scheme 5, involving base-catalyzed alkylation of respective secondary trimethylsilyl group
containing N-alkylamines19a,21a with ethyl bromoacetate (BrCH
CO
Et).21d
2
2
Scheme 5. Synthesis of ethyl N-alkyl-N-((trimethylsilyl)methyl))glycinates 19a-19t
X
Br
CO Et
2
R
R
N
Me Si
NH
Me Si
CO
Et
2
3
3
Me Si
N
CO Et
3
2
K CO /CH CN
2
3
3
(
X = H, 77%)
1
9a
19b
9c
9d
9e
CH , 78%)
CH CH , 84%)
19l
(
(
(
R = CH
R = CH
R = CH
2
(CH
2
)
4
3
19m
19n
19o
19p
19q
19r
19s
19t
2
2
3
(X = o-Me, 63%)
(X = m-Me, 74%)
(X = p-Me, 68%)
1
1
1
CH OCH , 69%)
2 2
3
(
(
R = allyl, 83%)
R = cyclohexyl, 73%)
R = CH
(
(
(
X = p-OMe, 70%)
X = o-F, 80%)
19f
CH CO Et, 77%)
(
2
2
2
X = m-F, 76%)
(X = p-F, 88%)
(X = p-CF
1
1
1
1
9g
9h
CO Et, 52%)
(R = CH
R = CH2
2
(
CH C H , 85%)
2
6
2 5
9i
9j
CH C H Me-p, 79%)
3, 61%)
(
(
R = CH
R = CH2
6
2 4
CH C H OMe-p, 81%)
R = CH2
2
6
4
(
19k
CH C H F-p, 79%)
2
2 4
6
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5
5
5
6
Photoaddition reactions of 10% EtOH-toluene (v/v) solutions containing C60 (0.28 mmol, 1 equiv)
and 19a-19t (0.56 mmol, 2 equiv) were performed by irradiation (450 W Hanovia medium pressure
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
mercury lamp equipped with flint glass filter (> 310 nm)) under deoxygenated (N -purged) conditions.
2
The photolysates were then purified by column chromatography to determine conversion of C60 and
photoproducts yields.
Scheme 6. Photoreactions of C60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates 19a-19k
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8
9
R
N
H
CO Et
2
R
N
hv (8 min) / N -condition
2
+
Me Si
CO Et
2
3
10% EtOH-toluene
(
1 equiv)
-
(2 equiv)
-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C60
19a 19k
20a 20k
O
H
N
H
N
H
N
H
N
CO Et
CO Et
CO Et
CO Et
2
2
2
2
20a
82
20b
83
20c
20d
94
aconversion yield (%):
85
b
product yield (%):
43 (53)
46 (55)
54 (63)
58 (62)
CO Et
2
CO Et
2
H
N
H
N
H
N
H
N
CO Et
CO Et
CO Et
CO Et
2
2
2
2
20e
90
20f
46
20g
4
20h
aconversion yield (%):
76
b
product yield (%):
52 (58)
23 (51)
trace
39 (51)
Me
OMe
F
H
N
H
N
H
N
CO Et
CO Et
CO Et
2
2
2
2
0i
71
38 (53)
20j
20k
aconversion yield (%):
72
36
b
product yield (%):
43 (60)
21 (57)
aConversion yields were determined by recovered C60. bProduct yields were determined after isolation. Those in
. Reaction condition: N -purged, 10% EtOH-toluene (v/v) solutions
parentheses were determined based on consumed C60
2
(
0.278 mol, 1.26 mM) and glycinate (0.556 mmol, 2.53 mM) were irradiated for 8 min.
(220 mL) containing C60
First, photoreactions of C60 with alkyl group containing glycinates 19a-19k in the N -purged
2
environment were conducted. As can be seen by viewing the results in Scheme 6, 8 min irradiation of
solutions containing C60 and 19a-19e brought about high conversion of C60 (ca. 82-94%) and efficient
formation of aminomethyl-1,2-dihydrofullerenes 20a-20e as the sole adduct. Similarly, photoreactions
of C60 with N-phenethyl substituted glycinates 19h-19j promoted by 8 min irradiation produced the
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7
8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
same types of photoproducts 20h-20j, albeit relatively lower conversion and product yields than those
of 19a-19e. It is noteworthy to mention that no photoproducts of multi-addition of the amines to C60
were formed even under conditions where an excess of glycinates (2.8 mmol, 10 equiv) was employed
and, in addition, photoreactions did not take place even by much longer irradiation time in the absence
of EtOH from the reaction medium.19a These observations show that polar protic EtOH enhances the
efficiencies of these photoaddition reactions, which proceed through a mechanistic route involving
sequential SET-desilylation of α-silylamines as well as protonation of photoadduct precursors,
aminomethylated fullerene anions, in the final reaction process (see below mechanistic reaction
pathways). In contrast to those of 19a-19e and 19h-19j, photoreactions using 19f, 19g and 19k took
place much less efficiently, mainly due to the electronic nature of electron withdrawing group near the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
nitrogen atom. Especially, when electron withdrawing ethoxycarbonyl group (-CO Et) as an alkyl
2
substituent was closer to nitrogen atom (19g), both conversion and product yields were much lowered
than that of further to nitrogen atom (19f).
The results of photoreactions of C60 with N-benzyl substituted glycinates 19l-19t, in which benzyl
groups contain Me, OMe, F and CF
substituted phenyl rings,21a are shown in Scheme 7. Again, 8 min
3
irradiation of 10% EtOH-toluene solutions containing C60 and 19l-19p, where phenyl rings possess
non (H)- or electron donating group (Me, OMe), led to formation of aminomethylated fullerenes 20l-
2
0p as single photoadducts. Likewise, presence of EtOH in the reaction medium was required to
produce photoproducts. However, conversion of C60 and product yields were much lower than those
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from 19a-19e and 19h-19j. Especially, in case of photoreactions using F and CF substituted phenyl
3
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
containing glycinates 19q-19t, no adducts were generated by using 8 min irradiation time and still low
conversions and product (20q-20t) yields occurred even after a 60 min irradiation time.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 7. Photoreactions of C60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates 19l-19t
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
X
X
H
N
CO Et
2
hv (8 min) / N -condition
2
+
Me Si
N
CO Et
10% EtOH-toluene
3
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
1 equiv)
-
(2 equiv)
C60
19l 19t
-
2
0l 20t
Me
Me
N
H
N
H
H
N
CO Et
CO Et
CO Et
2
2
2
20l
20m
20n
irradiation time :
conversion yield (%):
8 min
34
8 min
35
8 min
35
a
b
product yield (%):
18 (53)
18 (51)
17 (49)
F
Me
OMe
H
N
H
N
H
N
CO Et
CO Et
CO Et
2
2
2
2
0o
20p
20q
irradiation time :
conversion yield (%):
8 min
30
8 min
32
60 min
18
a
b
product yield (%):
16 (53)
18 (56)
7 (39)
F
F
CF3
H
N
H
N
H
N
CO Et
CO Et
CO Et
2
2
2
2
0r
20s
20t
irradiation time :
conversion yield (%):
60 min
20
60 min
22
60 min
19
a
b
product yield (%):
8 (40)
9 (39)
7 (38)
a
Conversion yields were determined by recovered C . bProduct yields were determined after isolation. Those in
60
parentheses were determined based on consumed C
.
Reaction condition: N -purged 10% EtOH-toluene (v/v)
2
60
solutions (220 mL) containing C (0.278 mol, 1.26 mM) and glycinate (0.556 mmol, 2.53 mM) were irradiated for
60
-
-
8 min (for 19l 19p) or 60 min (for 19q 19t).
The results of photoreactions described above clearly show that the reaction efficiencies are
largely influenced by the nature of N-alkyl substituents on glycinate substrates. To obtain semi-
quantitative information about the substituent effects on reaction efficiencies, relative reaction
quantum yields (Φrel) for photoreactions of C60 with selected glycinates were determined. For this
1
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purpose, N
2
-purged 10% EtOH-toluene solutions (10 mL) containing C60 (1.7 mmol) and each
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
glycinates (3.5 mmol) were simultaneously irradiated for a fixed time period that promotes an average
substrates conversion below ca. 10%. Photoproduct yields were then determined by using HPLC
analysis of crude photolysates and transformed into relative reaction quantum yields (Φrel) by setting
the Φrel for reaction of 19l to be unity (Φrel = 1). As the results summarized in Table 1 show, the reaction
efficiencies for photoreactions of electronically neutral and electron donating group substituted N-alkyl
glycinates like N-propyl 19b, N-phenethyl 19h, N-benzyl 19l, 19o and 19p analogs were much larger
than those containing electron withdrawing groups like 19g, 19s and 19t. Interestingly, in spite of the
presence of an electron withdrawing group in glycinate 19f, the reaction efficiency of 19f was nearly
equal to that of 19l. Especially, the least reaction efficiency was made in the photoreaction of C60 with
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
glycinate 19g, possessing electron withdrawing ethoxycarbonyl (CO Et) group at much close to
2
nitrogen atom.
Table 1. Relative reaction quantum yields (Φrel) of photoreactions of C60 with ethyl N-alkyl-N-
((trimethylsilyl)methyl)glycinates in deoxygenated 10% EtOH-toluene solutions.
R
Φ
rel
Me Si
N
CO Et
2
3
19b (R = CH
2
CH
CH
CO
CH
2
CH
CO
Et)
3
)
2.4
1.1
0.05
1.5
1.0
1.2
1
1
1
1
1
9f (R = CH
2
2
2
Et)
9g (R = CH
2
2
9h (R = CH
2
2
C
6
H )
5
9l (R = CH
2
C
6
H )
5
9o (R = CH
2
C
6
H
4
Me-p)
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
1
1
9p (R = CH
9s (R = CH
9t (R = CH
2
C
6
H
4
OMe-p)
F-p)
CF -p)
1.2
0.2
0.1
2
C
6
H
4
2
C
6
H
4
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The mechanism for formation of the aminomethyl-1,2-dihydrofullerenes seem to follow the
1
9
pathway suggested earlier by our studies (Scheme 8). It is assumed that because the singlet excited
1
*
state of fullerene ( C60 ) undergoes rapid and efficient intersystem crossing (ΦISC = 1) to form the
3
* 22,23
triplet excited state of C60 ( C60
)
and, moreover, the concentrations of the glycinates are in the 2.5
mM range and their oxidation potentials (Eox < 1 V vs SCE) are lower than the reduction potential of
3C60 ( Ered = 1.14 V vs SCE), (see Table S1 in Supporting Information) it is likely bimolecular SET
* 3
22
3
*
from glycinates to C60 take place favorably to form aminium radicals 21. Then, EtOH-assisted
desilylation from 21, occurring in competition with back SET, forms α-amino radicals 22 that can
•
-
couple with the radical anions of C60 (C60 ), followed by protonation of the formed anions 23 to
generate the aminomethyl-1,2-dihydrofullerenes 25. Evidence for this proposal comes from the
observation that photoreactions of C60 with N,N-dibenzyl-1-(trimethylsilyl)methanamine
(
(C
6
H
5
CH
2
)
2
NCH
2
SiMe ) performed in the deuterated 10% EtOD-toluene solution give rise to
3
formation of deuterium-incorporated fullerene products exclusively.19a Alternatively, α-amino radical
2
2 can be added to C60 to form fullerene radicals 24 which are then transformed to 25 by SET from
•
-
C
60 , followed by protonation.
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3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Scheme 8. Mechanistic pathways leading to formation of aminomethyl-1,2-dihydrofullerenes from
photoreactions of C60 and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates
1. hv
. ISC
R
2
SET
*
3C60
+
C60
Me Si
N
CO Et
2
C60
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
R
N
21
Me Si
CO Et
3
2
EtOH
20
R
N
SiMe
Et O H
3
H
CO Et
2
C60
R
N
+
R
H
C60
CO Et
2
N
CO Et
2
C-C bond
forming
22
23
25
R
N
1
. SET from C60
C60
C60
CO Et
2
+
2. H
24
Based on consideration of this scenario, it is possible to evaluate possible origins of the observed
N-substituent effects on reaction efficiencies. First, it is unlikely that the effects are a consequence of
different rates of EtOH-promoted desilylation of intermediate aminium radicals 21. Specifically, in
earlier laser flash photolysis (LFP) studies with anilinium radicals originating from N-α-trimethylsilyl-
N-alkylanilines (C
6
H
5
NRCH
2
SiMe
3
), it was found that N-electron withdrawing groups (R = CO Et
2
and COMe) containing anilinium radicals cause a significant enhancement of the rates of desilylation,
compared to N-electron donating group (R = Me) tethered analog, owing to the fact that N-electron
withdrawing group make the oxidation potential of anilinium radicals increased to accelerate the
24
destabilization of anilinium radicals, resulting in a rapid desilylation process. However, this trend is
not in accord with those observed in photoreactions of C60 with ethyl N-alkyl-N-
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
(trimethylsilyl)methyl)glycinates. As an alternative, we consider N-substituent can control of the rates
of SET from glycinates to C60*. To address this possibility, ionization potential (IP) of each ethyl N-
alkyl-N-((trimethylsilyl)methyl)glycinates in ethanol and toluene were calculated by using density
functional theory (DFT) with B3LYP hybrid exchange correlation function and the 6-311++G(d,p)
basis set. As can be seen from Figure 1, ethoxycarbonyl containing glycinate 19g, showing the least
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Φ
rel (Φrel = 0.05), has the highest IPs (5.31 eV in ethanol and 5.91 eV in toluene) whereas the propyl
containing glycinate 19b, showing the highest value (Φrel = 2.4), has the lowest IPs (5.04 eV in ethanol
and 5.68 eV in toluene). These DFT calculation results demonstrate that the reaction efficiency trends
of glycinates (Φrel) parallel with IPs in both solvents, where reaction efficiencies increase as the IPs of
the glycinate substrates decrease.
Figure 1. The correlation plot of DFT-calculated ionization energies (IP) of ethyl N-alkyl-N-
((trimethylsilyl)methyl)glycinates vs their Φrel. The ionization potentials (IP) were calculated in both
ethanol (black dot) and toluene (red dot).
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Photoreactions of C60 with ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates in oxygenated
condition. To assess the effects of oxygen, photoreactions of C60 with glycinates 19a-19t were
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
conducted in the O -purged solutions under otherwise identical conditions. At the outset, we expected
2
that the efficiencies of the processes would be decreased by the presence of molecular oxygen (O )
2
3
* 22a,25
owing to rapid quenching of C60
.
Indeed, irradiation of O -purged solutions of C60 and N-alkyl
2
or N-phenethyl substituted glycinates 19a-19k did not promote formation of any products even when
a much longer irradiation time (8 h) was used. In contrast, the N-benzyl substituted glycinates 19l-19t
did react under these conditions, but only inefficiently (i.e., relatively long irradiation times
required),21a and interestingly, the reactions of 19l-19t with C60 produced pyrrolidine ring fused
fullerenes (fulleropyrrolidines 26l-26t) rather than the corresponding aminomethyl-1,2-
dihydrofullerenes 20l-20t (Scheme 9).
Scheme 9. Photoreactions of C60 with ethyl N-benzyl-N-((trimethylsilyl)methyl)glycinates 19l-19t
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
X
Me Si
N
CO Et
3
2
X
hv / O -condition
2
+
Me Si
N
CO Et
2
3
10% EtOH-toluene
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(1 equiv)
-
(2 equiv)
C60
19l 19t
-
2
6l 26t
Me
Me
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
3
2
3
2
3
2
2
6l
26m
26n
irradiation time :
120 min
90
120 min
86
120 min
87
a
conversion yield (%):
b
product yield (%):
56 (62)
48 (56)
49 (57)
F
Me
OMe
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
3
2
3
2
3
2
2
6o
26p
26q
irradiation time :
120 min
91
120 min
89
240 min
74
a
conversion yield (%):
b
product yield (%):
56 (61)
54 (61)
44 (59)
F
F
CF3
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
2
3
2
3
2
3
2
6r
26s
26t
irradiation time :
240 min
76
240 min
82
240 min
80
a
conversion yield (%):
b
product yield (%):
43 (57)
51 (62)
50 (62)
a
Conversion yields were determined by recovered C60. bProduct yields were determined after isolation. Those in parentheses
were determined based on consumed C
. Reaction condition: O -purged 10% EtOH-toluene (v/v) solutions (220 mL) containing
60
2
C
(0.278 mol, 1.26 mM) and glycinate (0.556 mmol, 2.53 mM) were irradiated for 120 min (for
-
-
19l 19p
19q 19t
).
6
0
) or 240 min (for
26
On the basis of observation made in earlier studies by Prato as well as by Foote22c,27 and
Baciocchi,28 plausible mechanistic pathways responsible can be suggested. Fulleropyrrolidine
formation in these reactions seems to occur via 1,3-dipolar cycloaddition of azomethine ylide
2
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1
intermediates, generated by singlet oxygen ( O
2
) promoted reaction of N-α-trimethylsilyl substituted
3
*
glycinates. (Scheme 10) In this route, photochemically generated C60 participates in energy transfer
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
9
-1 -1 22a,23a
1
with O
2
(kET (O
2
) = 2 x 10 M s )
to generate singlet oxygen ( O
2
), which bind to glycinates to
form charge-transfer complexes (CT-complex) 27.29,30 Regioselective H-atom transfer (HAT) in the
•
28
complexes occur to form the α-amino radicals 28 and hydroperoxy radicals (HOO ). Oxidation of 28
produces the iminium ions 29 which undergo deprotonation to form the key ylides 30, serving as the
precursor of fulleropyrrolidine 26l-26t.
Scheme 10. Mechanistic route involving formation of fulleropyrroldines 26l-26t in oxygenated
condition
Ar
Ar
Ar
HAT
Me Si
N
CO Et
Me Si
3
N
CO Et
2
3
2
Me Si
N
-
CO Et
2
3
1O2
1O2
28
1
9l 19t
CT complex
HOO
H O
27
energy
transfer
3O
2
2
2
3C60*
C60
1. hv
Ar
2. ISC
Me Si
N
CO Et
3
2
Ar
29
~
H+
Me Si
N
CO Et
2
3
Ar
C60
Me Si
N
CO Et
2
3
1,3-Dipolar
30
Cycloaddition
26l-26t
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6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
This mechanistic route proposed for fulleropyrrolidine formation gains support from experiments
carried out using the known single oxygen quencher 1,4-diazabicyclo[2.2.2]octane (DABCO). 28,29 As
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
depicted in Figure 2, when the oxygenated 10% EtOH-toluene solutions of C60 and 19l were irradiated
for 2 h in the presence of DABCO, the presence of increasing concentrations (0-63 mM) of DABCO
caused a continuous decrease in both conversion of C60 and yields of 26l. These features represents
1
that O
2
serve as a major participant in photoreactions taking place in the oxygenated condition.
Figure 2. Plots of the both conversion- (black square) and product (red circle) yields as function of
concentration of DABCO in the photoreactions of C60 with 19l.
Reaction condition: O -purged 10% EtOH-toluene (v/v) solutions (220 mL) containing C60 (1.26 mM,
2
1 equiv) and glycinate 19l (2.52 mM, 2 equiv) were irradiated for 2 h.
1
The result of O
2
quenching experiment using DABCO provided us useful information to guess
the reason for why, in contrast to those of glycinates 19l-19t, photoreactions of C60 with N-alkyl or N-
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phenethyl substituted glycinates 19a-19k did not produce any photoproducts under oxygenated
condition. To figure out the reason, photoreactions of oxygenated 10% EtOH-toluene solutions
containing C60 and glycinate 19l (2.5 mM) were performed in the presence of N-propyl (19b) or N-
phenethyl (19h) glycinate (0, 6.3 and 12.6 mM respectively). As can be seen in Table 2, in the presence
of 6.3 mM of 19b or 19h, both conversion of C60 and yields of 26l arisen from reactions of C60 and 19l
were largely decreased. (entries 3 and 5 in Table 2) Moreover, much significant decreases in both
conversion and product yields were observed when much higher concentration of 19b or 19h (12.6
mM) were present in the solution. (entries 4 and 6 in Table 2) Clearly, no photoproducts derived from
the reactions of C60 with 19b or 19h were observed. These experimental results show that N-alkyl
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(19a-19g) or N-phenethyl (19h-19k) substituted glycinates can serve as a singlet oxygen quencher in
1
the same manner as does DABCO and possibly that they form complexes with O
2
that do not produce
the key ylide intermediates.29
Table 2. Product yields changes in the photoreactions of C60 with glycinate 19l under the oxygenated
condition.a
R
N
hv (120 min) / O2
0% EtOH-toluene
+
+
26l
C60
19l
Me Si
CO Et
2
3
1
(
(
R = CH
R = CH
1
.26 mM
19b
2
2
CH CH
2
3
)
5
19h
CH C H
(1 equiv)
2
6
)
conversion yield of
Entry reaction condition
(
%)b
26l (%)c
56 (62)
56 (62)
25 (49)
1
2
3
19l (2.5 mM, 2 equiv)
19l (6.3 mM, 5 equiv)
90
90
19l (2.5 mM, 2 equiv), 19b (6.3 mM, 5 equiv)
51
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
4
5
6
19l (2.5 mM, 2 equiv), 19b (12.6 mM, 10 equiv)
19l (2.5 mM, 2 equiv), 19h (6.3 mM, 5 equiv)
19l (2.5 mM, 2 equiv), 19h (12.6 mM, 10 equiv)
13
46
8
5 (41)
23 (50)
3 (38)
a220 ml of oxygenated 10% EtOH-toluene solutions containing C60 (1.26 mM, 1eq.) and varying
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
concentration of glycinate 19l (2.5-25 mM) and additive (19b or19h) Conversion was determined
by recovered C60. cIsolation yields. Those in parentheses were based on consumed C60
.
Methylene blue (MB) sensitized photoreactions of C60 with ethyl N-alkyl-N-
(trimethylsilyl)methyl)glycinates. Based on the proposal that aminomethyl-1,2-dihydrofullerenes
(
and fulleropyrrolidines forming reactions take place via pathways involving either SET from
3
*
glycinates to C60 or HAT by singlet oxygen, we explored whether these processes could be promoted
using typical triplet photosensitizers. For this purpose, we utilized an organic photosensitizer,
1
*
31
methylene blue (MB). As a well-known triplet photosensitizer (ΦISC ( MB ) = 0.52, τtriplet = 32 µs),
MB has been widely used to generate singlet oxygen and has a high excited state reduction potential
(3
E
red = 1.6 V vs SCE), which enable SET from amines to MB to take place efficiently. Indeed,
32
3
*
33
Stern-Volmer plots derived from MB luminescence quenching experiments show that triplet excited
3
*
state of MB ( MB ) is efficiently quenched by glycinates (Eox < 1 V vs SCE) even in oxygenated
solution. (see Supporting Information)
Prior to performing the reactions under the oxygenated conditions, MB sensitized photoreactions
of C60 with representative glycinates 19a-19c, 19h, 19l and 19s were carried out in the deoxygenated
conditions. As the results depicted in Table 3 show, very little difference existed in photoreactions of
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the glycinates 19a-19c, 19h, 19l, and 19s, carried out in the absence and presence of MB. These results
can be understood that because relative concentration of C60 (1.26 mM) is much higher than that of
MB (0.063 mM) and most of wavelengths emitted from medium pressure Hg lamp (i.e., 313, 365, 404,
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
*
4
35 and 546 nm) are absorbed by C60, SET taking place from glycinates and C60 seems to overwhelm
3
*
MB involved excited state deactivation processes.
Table 3. Products and yields in MB-sensitized photoreactions of C60 with glycinate 19a-19c, 19h, 19l,
and 19s under the deoxygenated condition.a
R
N
H
CO Et
2
20a
2
)
4CH
3
2
1
1
19c
19h
9a
9b
(R = CH
(R = CH
2
(CH
CH CH
)
3
20b
2
hv (8 min) / MB / N2
0% EtOH-toluene
20c
CH OCH
+
(R = CH
2
2
3
(R = CH
2
2
2
0h
0l
0s
CH C H
2
2 5
6
)
1
C H
C H F-p)
6
4
19l
( (RR == CC HH
2
6
5
)
19s
2
conversion yields (%)
entry Substrate and MB
%)b
of 20c
(
1
2d
3
19a with MB (0.063 mM)
83
82
83
83
85
85
75
76
34
34
22
44 (53)
43 (53)
45 (54)
46 (55)
55 (65)
54 (63)
38 (50)
39 (51)
17 (49)
18 (53)
10 (43)
19a without MB
19b with MB (0.063 mM)
19b without MB
4d
5
19c with MB (0.063 mM)
19c without MB
6d
7
19h with MB (0.063 mM)
19h without MB
8d
9
19l with MB (0.063 mM)
1
0e 19l without MB
19s with MB (0.063 mM)
1
1
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
2e 19s without MB
22
9 (39)
a220 ml of 10% EtOH-toluene solutions containing C60 (1.26 mM, 1eq.), glycinate (2.53 mM, 2 eq.)
and MB (5 mol %) was irradiated for 8 min. Conversion was determined by recovered C60. cIsolation
b
d
e
yields. Those in parentheses were based on consumed C60. Data from Scheme 6. Data from Scheme 7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
MB sensitized photoreactions of C60 with glycinates under oxygenated condition showed highly
interesting results. As shown in Table 4, in contrast to photoreactions of C60 with 19a in the absence
of MB, which did not produce any photoadduct (entry 7 in Table 4), the MB sensitized reaction of 19a
in O -purged 10% EtOH-toluene solution did occur to produce fulleropyrrolidine 26a in a highly
2
efficient manner. (entry 1 in Table 4) Notably, both the conversion of C60 and yield of 26a were not
affected by using high concentration of MB (from 0.063 to 0.25 mM) or 19a (from 2.53 to 6.3 mM)
(entries 2-4 in Table 4). Moreover, other sensitizers like rose Bengal (RB) (entry 5) and eosin Y (entry
6) also promoted photoinduced conversion of 19a to 26a.
Table 4. Product and yield in MB sensitized photoreactions of C60 with 19a under oxygenated
conditiona
Me Si
N
CO Et
2
3
hv (60 min) / O2
+
Me Si
N
CO Et
2
3
Sensitizer
10% EtOH-toluene
C60
19a
26a
conversion
yields (%)
entry
amine and sensitizer
%)b
of 26ac
(
1
2
3
19a (2.53 mM), MB (0.063 mM)
19a (6.3 mM), MB (0.063 mM)
19a (2.53 mM), MB (0.126 mM)
82
81
80
48 (59)
50 (62)
43 (53)
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19a (2.53 mM), MB (0.25 mM)
81
80
43 (53)
42 (53)
40 (55)
n.rd
19a (2.53 mM), RB (0.063 mM)
19a (2.53 mM), EY (0.063 mM)
19a (2.53 mM), no MB
76
n.rd
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a220 ml of 10% EtOH-toluene solutions containing C60 (1.26 mM), glycinate (2.53-6.3 mM)
and photosensitizer like MB, RB and EY (0.063-0.25 mM) was irradiated for 60 min.
bConversion was determined by recovered C60. cIsolation yields. Those in parentheses were
based on consumed C60. dNo reaction
With the results of MB sensitized reaction of 19a in hand, we next examined the scope and
generality of these reaction protocols. As described in Scheme 11, 60 min irradiation of oxygenated
1
0% EtOH-toluene solutions containing C60 and glycinates 19b-19k in the presence of MB led to
formation of fulleropyrrolidines 26b-26k as sole photoadducts.
Scheme 11. MB-sensitized photoreactions of
(trimethylsilyl)methyl)glycinates 19b-19k
C
60
with
ethyl
N-alkyl-N-
(
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
R
N
Me Si
CO Et
2
3
hv (60 min) / MB / O2
R
+
Me Si
N
CO Et
3
2
10% EtOH-toluene
-
(2 equiv)
1
9b 19k
(
1 equiv)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C60
-
26b 26k
O
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
2
3
2
3
2
3
2
6b
26c
81
26d
87
aconversion yield (%):
83
b
product yield (%):
49 (59)
45 (55)
53 (61)
CO Et
2
CO Et
2
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
2
3
2
3
2
3
2
3
2
6e
75
42 (46)
26f
26g
26h
aconversion yield (%):
66
51
52
b
product yield (%):
33 (50)
27 (53)
31 (59)
Me
OMe
F
Me Si
N
CO Et
Me Si
N
CO Et
Me Si
N
CO Et
3
2
3
2
3
2
2
6i
26j
26k
aconversion yield (%):
58
52
56
b
product yield (%):
33 (56)
31 (61)
29 (52)
a
Conversion yields were determined by recovered C60. bProduct yields were determined after isolation. Those in parentheses
were determined based on consumed C
.
Reaction condition: N -purged 10% EtOH-toluene (v/v) solutions (220 mL) containing
2
60
(
0.278 mol, 1.26mM), glycinate (0.556 mmol, 2.53 mM) and methylene blue (MB, 0.063 mM) were irradiated for 60 min.
C60
Likewise, MB-catalyzed photoreactions of C60 with 19l-19t in oxygenated condition took place
with efficiencies that far exceed those of 19a-19k. (Table 5) Specifically, 15 min irradiation of
oxygenated solutions of C60 and non- (19l) or electron donating (ortho-Me (19m), meta-Me (19n),
para-Me (19o) and para-OMe (19p)) group containing glycinates in the presence of MB gave rise to
high yielding production of fulleropyrrolidines 26l-26p. (entries 1-5 in Table 5) Although MB
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sensitized reactions with electron withdrawing (ortho-F (19q), meta-F (19r), para-F (19s) and para-
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CF (19t)) group containing glycinates took place less efficiently than those of 19l-19p (entries 6-7 in
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 5), just 30 min irradiation of solutions containing C60 and 19q-19t brought about high
conversion of starting C60 as well as high yielding of photoproducts 26q-26t. (entries 7-13 in Table 5)
Table 5. Product and yield in MB sensitized photoreactions of C60 with glycinates 19l-19t in
oxygenated condition.a
hv / MB / O2
-
C60
+
19l 19t
26l - 26t
10% EtOH-toluene
irradiation conversion yields (%)
entry amine
time (min)
15
(%)b
of 26c
1
2
3
4
5
6
7
9
19l
19m
19n
19o
19p
19q
19q
19r
19s
100
65
15
15
15
15
15
30
30
30
30
91
94
47 (51)
48 (51)
58
100
100
32
60
16 (51)
42 (58)
43 (58)
49 (62)
42 (60)
73
75
11
79
1
3
19t
73
a220 ml of 10% EtOH-toluene solutions containing C60 (1.26 mM, 1eq.), glycinate (2.53 mM, 2 eq.)
and MB (0.063 mM). Conversion was determined by recovered C60. cIsolation yields. Those in
b
parentheses were based on consumed C60
.
On the basis of observation made in amine SET photochemistry using photosensitizer31,32a and
photoproduct distribution profiles obtained from current reactions, feasible mechanistic pathways for
29
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