Electronic and steric effects controlling efficiencies of photoaddition reactions of fullerene C60with N-α-trimethylsilyl-N-alkyl-N-benzylamines

Article abstract of DOI:10.1016/j.tetlet.2017.01.073

Single electron transfer (SET)-promoted photoaddition reactions between fullerene C₆₀and both various alkyl (Me, Et, i-Pr, t-Bu)- and para-substituted (p-Me, p-OMe, p-F, p-CF₃) arene ring containing, N-α-trimethylsilyl-N-alkyl-N-benzylamines were explored to gain information about photoproduct profiles and how the electronic and steric nature of the amine substrates influence reaction efficiencies. The results show that visible light (λ?>?540?nm) irradiation of 10% EtOH-toluene solutions containing C₆₀and N-α-trimethylsilyl-N-alkyl-N-benzylamines produce 1-aminomethyl-1,2-dihydrofullerenes as a sole photoproduct. In addition, SET-promoted photoaddition reactions of unsubstituted and para-electron donating group substituted arene ring containing N-α-trimethylsilyl-N-alkyl-N-benzylamines take place to give photoproducts more efficiently than those containing para-electron withdrawing group substituted arene rings. Moreover, although steric factors are less significant than the electronic nature of the amine substrates in governing reaction efficiencies, sterics do play a significant role in photoreactions of electron deficient amine substrates.

Full text of DOI:10.1016/j.tetlet.2017.01.073

Accepted Manuscript
Electronic and Steric Effects Controlling Efficiencies of Photoaddition Reac-
tions of Fullerene C60 with N-α-Trimethylsilyl-N-alkyl-N-benzylamines
Ho Cheol Jeong, Suk Hyun Lim, Youngku Sohn, Young-Il Kim, Hoeun Jang,
Dae Won Cho, Patrick S. Mariano
PII:
S0040-4039(17)30103-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.01.073
TETL 48576
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
14 December 2016
17 January 2017
23 January 2017
Please cite this article as: Cheol Jeong, H., Hyun Lim, S., Sohn, Y., Kim, Y-I., Jang, H., Won Cho, D., Mariano,
P.S., Electronic and Steric Effects Controlling Efficiencies of Photoaddition Reactions of Fullerene C60 with N-α-
Trimethylsilyl-N-alkyl-N-benzylamines, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.
2017.01.073
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Electronic and Steric Effects Controlling Efficiencies of Photoaddition Reactions of
Fullerene C60 with N-α-Trimethylsilyl-N-alkyl-N-benzylamines
Ho Cheol Jeong,a Suk Hyun Lim,b Youngku Sohn,b Young-Il Kim,b Hoeun Jang,b
Dae Won Cho,b* Patrick S. Marianoc*
aDepartment of Energy Convergence Engineering, Yeungnam University, Gyeongsan,
Gyeongbuk 712-749, Korea
bDepartment of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Korea
(dwcho00@yu.ac.kr)
cDepartment of Chemistry and Chemical Biology, University of New Mexico, Albuquerque,
New Mexico 87131, United State
(mariano@unm.edu)
1

Graphical Abstract
2

Abstract
Single electron transfer (SET)-promoted photoaddition reactions between fullerene C60 and both
various alkyl (Me, Et, i-Pr, t-Bu)- and para-substituted (p-Me, p-OMe, p-F, p-CF3) arene ring
containing, N-α-trimethylsilyl-N-alkyl-N-benzylamines were explored to gain information about
photoproduct profiles and how the electronic and steric nature of the amine substrates influence
reaction efficiencies. The results show that visible light (λ>540 nm) irradiation of 10% EtOH-toluene
solutions containing C60 and N-α-trimethylsilyl-N-alkyl-N-benzylamines produce 1-aminomethyl-1,2-
dihydrofullerenes as a sole photoproduct. In addition, SET-promoted photoaddition reactions of
unsubstituted and para-electron donating group substituted arene ring containing N-α-trimethylsilyl-
N-alkyl-N-benzylamines take place to give photoproducts more efficiently than those containing
para-electron withdrawing group substituted arene rings. Moreover, although steric factors are less
significant than the electronic nature of the amine substrates in governing reaction efficiencies,
sterics do play a significant role in photoreactions of electron deficient amine substrates.
3

Introduction
Owing to the broad application of fullerene derivatives in the material1-5 and biological6-
9sciences, a number of studies have been performed to develop novel types of organofullerenes that
have unique photophysical properties. Specific examples are [6,6]-phenyl C61-butyric acid methyl
ester (PCBM), indene-C60 bisadduct (ICBA) and fulleropyrrolidines, all of which have been utilized
as electron acceptors in photovoltaic cells10-12 and as singlet oxygen generators for photodynamic
therapy (PDT).13
The most common approaches disclosed to date for the preparation of organofullerenes include
the Prato reaction utilizing 1,3-dipolar azomethine ylide cycloadditions,14-15 addition of
organometallic reagents,16-19 transition metal mediated radical reactions20-21 and single electron
transfer (SET) promoted photoaddition reactions.22-25 Among these synthetic protocols, SET
photochemical methods are highly promising because they utilize relatively mild reaction conditions
(e.g., usage of a potentially renewable energy source) and take place in relatively high chemical
yields. In particular, because C60 absorbs light over a wide range of the UV-visible region and it has a
high reduction potential (Ered (C60) = -0.42 V vs SCE in PhCN),26 C60 participates in SET-promoted
photochemical reactions with a variety of electron donating substrates.
SET-promoted photoaddition reactions between C60 and aliphatic/aromatic amines have been
explored intensively.27-28 The results of these studies showed that irradiation of solutions containing
C60 and tertiary amines 1 led to formation of 1-aminomethyl-1,2-dihydrofullerenes 4 (i.e., 1,2-
4

adducts) by a pathway involving thermodynamically favorable SET from the amine donors to the
* 26-28
-
ꢀ
triplet excited state of C60 (3C60 )
(Scheme 1). This process formed the C60 anion radical (C60 )
and amine cation radicals (i.e., aminium radicals) 2, the latter of which underwent subsequent α-CH
-
deprotonation (Lv = H) to form α-amino radicals 3 that couple with either C60 ꢀ or C60 to form anion
(5) or radical (6) precursors of 1-aminomethyl-1,2-dihydrofullerenes. As an alternative mechanistic
pathway, proton transfer from aminium radicals 2 (Lv = H) to fullerene anion radicals also could
ꢀ
occur to generate α-amino radicals and hydrofullerene radical (H-C60 ) which then couple to form 1-
aminomethyl-1,2-dihydrofullerenes directly.24,27b
Scheme 1. Mechanistic pathway for formation of 1-aminomethyl-1,2-dihydrofullerenes
In recent studies, we explored SET-promoted photoaddition reactions of 10% EtOH-toluene
(vol %) solutions containing C60 and tertiary aliphatic amines that possess α-trimethylsilyl
substituents 1 (Lv = SiMe3).25a The results of this effort designed to evaluate the scope and
efficiencies of these processes showed that photoreactions of C60 with α-trimethylsilyl substituted
amines 1 (Lv = SiMe3) produced 1-aminomethyl-1,2-dihydrofullerenes 4 with higher chemical
5

efficiencies than photoreaction that utilize non-trimethylsilyl containing amine analogs 1 (Lv = H).
(Scheme 1) The elevated efficiencies of these processes was a consequence of the fact that α-
trimethylsilyl substituted aminium radicals 2 (Lv = SiMe3) generated by SET undergo silophilic
solvent-assisted desilylation much more rapidly as compared to α-CH deprotonation of their non-
trimethylsilyl analogs 2 (Lv = H) to produce aminomethyl radicals. As we have discussed earlier,
aminium radical stabilization by overlap of the filled σ orbital and half-filled nitrogen p-orbital
C-Si
causes weakening of the C-Si bond and increasing electron deficiency at silicon, which facilitate
rapid transfer of the silyl group to a silophile.29-31 (Scheme 2)
Scheme 2. Efficient sequential SET-desilylation of α-trimethylsilyl substituted amines
In more recent investigations, we have explored photoreactions of C60 with arene ring
containing, N-α-trimethylsilyl-N,N-dibenzylamines in order to determine if and how the electronic
nature of arene ring substituents govern the nature and efficiencies of the processes.25b The results
showed that while α-trimethylsilyl-amines possessing para-electron donating group (e.g., p-Me, p-
OMe) substituted benzyl group(s) underwent photoaddition reactions with C60 that efficiently and
exclusively yield aminomethyl substituted 1,2-adducts, photoaddition reactions between related
para-electron withdrawing group (e.g., p-F, p-CF3) substituted benzyl amines and C60 occurred
inefficiently and generated fulleropyrollidines along with aminomethyl substituted 1,2-adducts.
6

As part of a continuing research program aimed at probing the factors that influence the nature
and efficiencies of SET-promoted photoaddition reaction between amines and fullerene, in the
current effort, we designed and prepared novel types of N-α-trimethylsilyl-N-alkyl-N-benzylamines
that possess various alkyl substituents (i.e., Me, Et, i-Pr, t-Bu) on nitrogen and various para-
substituents (i.e., H, Me, OMe, F) on the arene ring and performed photoaddition reactions to see
how steric bulkiness of alkyl group as well as electronic factor influence reaction efficiencies.
Importantly, while the studies exploring electronic factors governing reaction efficiencies were
reported in some previous studies,25,32 the study exploring the effect of sterics of amine substrates are
very rare.
Studies of photochemical reactions of C60 with these benzylamines showed that SET-promoted
photoaddition reactions of unsubstituted and electron donating group substituted N-α-trimethylsilyl-
N-alkyl-N-benzylamines took place to give photoproducts more efficiently than those containing
electron withdrawing group substituted arene rings. Importantly, although steric factors arisen by N-
alkyl substituents were less significant than the electronic nature of the amine substrates in governing
reaction efficiencies, sterics did play a significant role in photoreactions of electron deficient amine
substrates. (see below)
Results and Discussion
7

Preparation of N-α-trimethylsilyl-N-alkyl-N-benzylamines. The α-trimethylsilyl substituted
benzylamines required for this investigation were prepared by using a modification of the well-
known procedure.25,32 Specifically, commercially available p-substituted benzylamines 7-9 (p-X-
PhCH2NH2) were reacted with iodomethyltrimethylsilane (ICH2SiMe3) in the presence of K2CO3 to
yield the corresponding secondary N-α-trimethylsilyl-N-benzylamines 12-16, which were then
subjected to base-catalyzed substitution reactions with alkyl iodides to produce N-α-trimethylsilyl-N-
alkyl-N-benzylamines 17-26. (Scheme 3) The route employed for preparation of N-t-butyl substituted
amines 28-30 utilized reactions of secondary N-α-trimethylsilyl-N-t-butylamine (27) with para-
substituted benzyl iodides.
Scheme 3. Synthetic routes for the preparation of N-α-trimethylsilyl-N-alkyl-N-benzylamines
Photoreactions of C60 with N-α-trimethylsilyl-N-alkyl-N-benzylamines. To perform
photochemical reactions, N2-purged 10% EtOH-toluene (vol %) solutions containing C60 (0.28
8

mmol) and amine substrates (0.56 mmol) were irradiated by using visible light (450 W Hanovia
medium pressure mercury lamp equipped a colored flint glass filter (λ> 540 nm)) for the time periods
shown in Table 1 that bring about high (70-100%) conversions of C60. Structural assignment of
1
13
photoproducts were made by using H and C NMR and UV-vis spectroscopy along with HRMS
and by comparison spectroscopic data of closely related 1-aminomethyl-1,2-
dihydrofullerenes.25,27,28,32,33 Importantly, all of spectroscopic data of photoproducts correspond to 1-
aminomethyl-1,2-dihydrofullerenes, that result from addition across the [6,6]-ring juncture rather
than [5,6]-ring juncture.25
As can be seen by viewing the results given in Scheme 4 and Table 1, 15 min visible light
irradiation of a N2-purged 10% EtOH-toluene solution of C60 and benzylamine 17 gave rise to
formation of the 1-aminomethyl-1,2-dihydrofullerene 31 as a sole product in high yield (74%). It
should be noted that no polysubstituted photoproducts were generated in this reaction even when
high concentrations of benzylamine 17 (entry 2) or longer irradiation times (30 min) were employed
(entry 3). Moreover, no photoreaction took place when an O2 purged 10% EtOH-toluene or a N2
purged pure toluene solution of 17 and C60 were irradiated (entries 4-5).25a
Scheme 4. Photoreactions of C60 with N-α-trimethylsilyl-N-methyl-N-benzylamines 17-20
9

Table 1. Products and yields of photoreactions of C60 and N-α-trimethylsilyl-N-methyl-N-
benzylamines 17-20 in N2-purged 10% EtOH-toluenea
irradiation time
(min)
Conversion
(%)b
entry
amine
product (%)c
1
2d
3
17
17
17
17
17
18
19
20
15
15
93
93
99
-
31 (74)
31 (76)
31 (71)
no rxn
no rxn
32 (69)
33 (69)
34 (53)
30
4e
5f
6
300
300
15
-
94
94
84
7
15
8
60
aC60:Amine is 0.28:0.56 mmol in 220 mL of 10% EtOH-toluene (vol %). bDetermined
based on recovered C60. cIsolated yields. d5.6 mmol (20 equivalent) of 17 in 10% EtOH-
toluene (220 mL) was used. ePhotoreaction of O2-purged 10% EtOH-toluene solution.
fPhotoreaction in pure toluene
Likewise, 15 min irradiation of C60 with p-Me and p-MeO substituted arene ring containing
benzylamines 18-19 in N2-purged 10% EtOH-toluene solutions resulted in high yielding production
of the respective 1-aminomethyl-1,2-dihydrofullerenes 32-33 as a single product. (entries 6-7).
However, photoreaction of electron withdrawing p-F substituted benzylamine 20 with C60 under the
same conditions required a much longer irradiation time (1 h) to bring about significant conversion
of C60 (> 80%). Moreover, the 1,2-adduct 34 was formed in a relatively lower yield (53%, entry 8).
The observed dependence of irradiation time (quantum efficiency) and photoproduct yields on the
electronic nature of arene ring substituents matched those made in earlier studies.25b,32,34
10

Photoreactions of C60 with more bulky alkyl (i.e., Et, i-Pr, t-Bu) substituted N-α-trimethylsilyl-
N-alkyl-N-benzylamines 21-26 and 28-30 were also explored to determine if steric factors in the
amine substrates influence the process. Irradiation times, percent conversions, photoproducts and
yields of photoreactions of 21-26 and 28-30 with C60, carried out under identical conditions are
depicted in Scheme 5 and Table 2. The results showed that visible light irradiation of 10% EtOH-
toluene solutions containing C60 and these benzylamines gave rise to formation of 1-aminomethyl-
1,2-dihydrofullerenes 35-43 solely (Scheme 5, Table 2) and that much longer irradiation times (60-
120 min) were required to bring about high conversions of the p-F substituted benzylamines 23, 26
and 30 (entries 3, 6 and 9 in Table 2). These findings demonstrate that, in contrast to effects of para-
arene ring substituents, the presence of sterically bulky N-substituents on the amines does not play a
demonstrable role in governing the efficiencies of these photoreactions.
Scheme 5. Photoreactions of C60 with N-α-trimethylsilyl-N-alkyl-N-benzylamines 21-26 and 28-30
Table 2. Products and yields of photoreactions of C60 and N-α-trimethylsilyl-N-alkyl-N-benzylamines
21-26 and 28-30 in N2-purged 10% EtOH-toluenea
11

irradiation time
(min)
Conversion
(%)b
entry
amine
product (%)c
1
2
3
4
5
6
7
8
9
21
22
23
24
25
26
28
29
30
30
30
98
100
79
94
95
78
89
91
77
35 (73)
36 (72)
37 (57)
38 (58)
39 (61)
40 (50)
41 (61)
42 (66)
43 (52)
60
30
30
120
30
30
120
aC60:Amine is 0.28:0.56 mmol in 220 mL of 10% EtOH-toluene (vol %).
bDetermined based on recovered C60. cIsolated yields.
In order to gain detailed information about how the SET-promoted photoreaction efficiencies
are influenced by the electronic and steric nature of the N-α-trimethylsilyl-N-alkyl-N-benzylamines,
relative quantum efficiencies (Φ ) were determined by measuring the amounts of photoproducts
rel
formed in photoreactions conducted under identical conditions. For this purpose, N2-purged 10%
EtOH-toluene solutions (10 mL) containing C60 (0.17 mM) and the amines (0.35 mM) were
simultaneously irradiated (λ> 540 nm) for equivalent time periods that bring about low conversions
(< ca. 10 %) of the starting materials. The amounts of photoproducts generated were then determined
by using HPLC analysis of the crude photolysates. The relative quantum efficiencies included in
12

Table 3 were calculated by setting the quantum yield of formation of photoadduct 43 in
photoreaction of amine 30 as 1.0.
Table 3. Relative quantum efficiencies (Φ ) of SET-photochemical reactions of 17-26 and 28-30 with
rel
C60
N-alkyl group
alkyl
p-X-Ph
amine
Φ
rel
X
17
21
24
28
18
19
22
25
29
20
23
26
30
Me
Et
H
H
4.1
4.0
3.6
3.1
4.2
4.2
4.2
3.7
3.1
2.8
2.5
1.2
1.0
i-Pr
t-Bu
Me
Me
Et
H
H
Me
OMe
Me
Me
Me
F
i-Pr
t-Bu
Me
Et
F
i-Pr
t-Bu
F
F
Analysis of the relative quantum efficiency data given in Table 3 showed that SET-promoted
photochemical reactions of electron donating group arene ring substituted (p-Me and p-OMe) N-α-
trimethylsilyl-N-alkyl-N-benzylamines took place more efficiently than those of the electron
withdrawing group substituted (p-F) analogs.25b,32,34a The differences in relative quantum efficiency
13

brought by changing the para-arene ring substituents were enhanced in the N-iso-propyl and N-tert-
butyl substituted amines. Specifically, p-H and p-Me-arene ring containing N-iso-propyl- and N-tert-
butyl-amines 24-25 and 28-29 produced the corresponding 1,2-adducts with ca. 3 times higher
quantum efficiencies than did the respective p-F-arene ring containing amine analogs 26 and 30.
Based on the reasonable assumption that the oxidation potentials of all of the amines used in the
phtoreactions are nearly equal,25,32 it is reasonable to conclude that the rates of SET from the amines
to triplet excited state of C60 (3C60 ) should be near equal and, as a result, that the differences in the
*
quantum efficiencies of these reactions are a consequence of unequal rates of solvent promoted
desilylation of aminium radical intermediates, caused by competitive reactions such as deprotonation
from the benzylic position. If deprotonation process takes place more efficiently, benzyl carbon
-
ꢀ
radicals (PhĊHNRCH2SiMe3, R = alkyl) would be formed and subsequently couple with either C60
or C60 to form anion or radical precursors of novel types of 1-aminomethyl-1,2-dihydrofullerenes.
(see Scheme 1)
Furthermore, a comparison of the relative quantum efficiencies of photoreactions of the N-α-
trimethylsilyl-N-alkyl-N-benzylamines with C60 also provided interesting information about the
effects of steric bulk of the N-alkyl substituent. As the data in Table 3 show, the efficiencies of
reactions of the p-H and p-Me substituted arene containing N-alkylamines decreased when the alkyl
substituent was changed from methyl, to increasingly more bulky ethyl (Et), iso-propyl (i-Pr) and
tert-butyl (t-Bu). Moreover, increases in the steric size of the N-alkyl substituent caused a dramatic
14

decrease in the efficiencies of photoreactions of the p-F-arene ring containing N-alkylamines. For
example, the less sterically hindered alkyl (N-Me) substituted amine 20 generated the corresponding
1,2-adduct with ca. 2.8 times higher quantum efficiency than did the more hindered alkyl (N-t-Bu)
analog 30. These results show that although steric hindrance of the amine substrate has less of an
effect on quantum efficiencies than does the electronic nature of the para-arene ring substituent,
steric factors play a more significant role in govern reaction efficiencies when the electron
withdrawing p-F group is present on the arene ring. To explore this effect further, various alkyl (Me,
Et, i-Pr, and t-Bu) substituted N-α-trimethylsilyl-N-benzylamines 44-47, which possess the electron
withdrawing p-CF3 arene ring substituent, were prepared and their photoreactions with C60 in 10%
EtOH-toluene solution were probed using conditions that are identical to those employed in
photoreactions of related amines described above. (Scheme 6 and Table 4) The results showed that
while less hindered alkyl (i.e., N-Me and N-Et) substituted N-α-trimethylsilyl-N-benzylamines 44-45
underwent efficient SET-promoted photoreactions to produce 1-aminomethyl-1,2-dihydrofullerenes
48-49, no major photoproducts were formed in photoreactions of the more hindered alkyl (i.e., N-i-Pr
and N-t-Bu) substituted amines 46-47.
Scheme 6. Photoreactions of C60 with N-α-trimethylsilyl-N-alkyl-N-benzylamines 44-47a
15

Table 4. Products and yields of photoreactions of C60 and N-α-trimethylsilyl-N-alkyl-N-benzylamines
44-47 in N2-purged 10% EtOH-toluenea
amine
Conversion (%)b
product (%)c
44
71
48 (44)
45
46
47
68
34
11
49 (43)
d
-
d
-
aVisible light irradiation of 10% EtOH-toluene (vol %) solutions for 60 min.
bDetermined based on recovered C60. cIsolated yields. dNo major products formed.
The observations made in studies probing the steric effects of N-alkyl substituents on
photoreaction efficiencies should be contrasted to those made by Chatterjee35, De Silva36and others.37
These workers observed that increasing steric hindrance in donor-acceptor radical ion pairs (contact
radical ion pair, CRIP) generated by SET led to an increase in the efficiencies for formation of
solvent-separated radical ion pairs (SSRIP). (Scheme 7) This effect resulted in a decrease in the rate
of back-SET (kBSET) owing to increased distance between the ion radicals in the SSIRP. As a result,
one would predict that photochemical reactions of sterically hindered donors taking place by SET-
promoted pathways would be more quantum efficient than those involving less hindered donors.
Photoreactions of N-α-trimethylsilyl-N-alkyl-N-benzylamines described above do not fit this
predicted pattern. One possible explanation of this difference focuses desilylation of aminium
radicals by the silophilic solvent EtOH that produce α-amino radicals which add to C60 to generate
adducts. The rates of desilylation of aminium radicals also impact quantum efficiencies. Thus, it is
16

possible that bulky i-Pr and/or t-Bu moieties on nitrogen of aminium radicals would block access of
EtOH to the silicon center and thus slow desilylation. To gain more information about this possible
origin of the steric effects on reaction efficiencies, measurement of the rates of desilylation of
aminium radicals are currently underway.
Scheme 7. Ion radicals generated by SET processes
Conclusion
In this study, SET-promoted photoaddition reactions between fullerene C60 and various alkyl
(Me, Et, i-Pr, t-Bu)- and para-substituted (p-Me, p-OMe, p-F, p-CF3) arene ring containing N-α-
trimethylsilyl-N-alkyl-N-benzylamines were explored to gain informations about photoproducts
profiles and how electronic and steric factors in the amine substrates influence reaction efficiencies.
The results showed that visible light (> 540 nm) irradiation of 10% EtOH-toluene solutions
containing C60 and N-α-trimethylsilyl-N-alkyl-N-benzylamines produced 1-aminomethyl-1,2-
dihydrofullerenes as a sole photoproduct. In addition, SET-promoted photoaddition reactions of non-
and electron donating group-substituted N-α-trimethylsilyl-N-alkyl-N-benzylamines took place to
give photoproducts more efficiently than those of the electron withdrawing group substituted
17

benzylamine analogs. Moreover, although steric hindrance of amine substrates was less significant
than electronic nature of amine substrates in governing reaction efficiencies, it did play a significant
role in photoreaction of electron deficient (e.g., electron withdrawing group substituted) amine
substrates.
Acknowledgments
This investigation was financially supported by 2015 Yeungnam University Research Grant
(215A380034).
A. Supplementary data
Supplementary data associated with this article can be found at http://xxxxxx.
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Graphical Abstract
25

Highlights
► SET-promoted photoaddition reaction between C60 and benzylamine derivatives were explored.
► Electronic and steric effects of benzylamines influence efficiencies of photoaddition reaction.
► Reaction efficiencies are largely governed by electronic effects of benzylamines.
► Steric effects play a significant role in electron deficient benzylamine derivatives.
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