Fullerene C60promoted photochemical hydroamination reactions of an electron deficient alkyne with trimethylsilyl group containing tertiaryN-alkylbenzylamines

Article abstract of DOI:10.1039/d1ra00166c

C₆₀-promoted photoaddition reactions of both trimethylsilyl- and a variety of alkyl group containing tertiary benzylamines (i.e.,N-α-trimethylsilyl-N-alkylbenzylamines) with dimethyl acetylenedicarboxylate (DMAD) were carried out to explore the synthetic utility of trimethylsilyl group containing tertiary amines as a substrate in the photochemical hydroamination reactions with dimethyl acetylenedicarboxylate (DMAD). The results showed that photoreactions of all the trimethylsilyl containingN-alkylbenzylamines with DMAD, under an O₂-purged environment, produced non-silyl containing enamines efficiently through a pathway involving addition of secondary amines to DMAD, the former of which are produced by hydrolytic cleavage ofin situformed iminium ions. Exceptionally, five-membered N-heterocyclic rings, pyrroles, could be produced competitively in photoreaction of bulky alkyl (i.e.,tert-butyl) group substituted benzylamines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to DMAD. Furthermore, C₆₀-sensitized photochemical reactions of non-silyl containing benzylamines with DMAD under oxygenated conditions took place in a less efficient and non-regioselective manner to produce enamine photoadducts. The observations made in this study show that regioselectivity of C₆₀-promoted photochemical reactions ofN-α-trimethylsilyl-N-alkylbenzylamines, leading to formation of secondary amines, can be controlled by the presence of the trimethylsilyl group, and that these trimethylsilyl containing tertiary amines can serve as a precursor of secondary amines for hydroamination reactions with a variety of electron deficient acetylenes.

Full text of DOI:10.1039/d1ra00166c

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PAPER
Fullerene C60 promoted photochemical
hydroamination reactions of an electron deficient
alkyne with trimethylsilyl group containing tertiary
N-alkylbenzylamines†
Cite this: RSC Adv., 2021, 11, 5914
*
Suk Hyun Lim, Hannara Jang and Dae Won Cho
C60-promoted photoaddition reactions of both trimethylsilyl- and a variety of alkyl group containing tertiary
benzylamines (i.e., N-a-trimethylsilyl-N-alkylbenzylamines) with dimethyl acetylenedicarboxylate (DMAD)
were carried out to explore the synthetic utility of trimethylsilyl group containing tertiary amines as
substrate in the photochemical hydroamination reactions with dimethyl acetylenedicarboxylate
(DMAD). The results showed that photoreactions of all the trimethylsilyl containing N-alkylbenzylamines
with DMAD, under an O2-purged environment, produced non-silyl containing enamines efficiently
through a pathway involving addition of secondary amines to DMAD, the former of which are produced
by hydrolytic cleavage of in situ formed iminium ions. Exceptionally, five-membered N-heterocyclic
rings, pyrroles, could be produced competitively in photoreaction of bulky alkyl (i.e., tert-butyl) group
substituted benzylamines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to
DMAD. Furthermore, C60-sensitized photochemical reactions of non-silyl containing benzylamines with
DMAD under oxygenated conditions took place in a less efficient and non-regioselective manner to
a
produce enamine photoadducts. The observations made in this study show that regioselectivity of C60
-
Received 8th January 2021
Accepted 26th January 2021
promoted photochemical reactions of N-a-trimethylsilyl-N-alkylbenzylamines, leading to formation of
secondary amines, can be controlled by the presence of the trimethylsilyl group, and that these
trimethylsilyl containing tertiary amines can serve as a precursor of secondary amines for hydroamination
reactions with a variety of electron deficient acetylenes.
DOI: 10.1039/d1ra00166c
rsc.li/rsc-advances
amines to electron sufficient C–C multiple bonds are not
readily available due to the repulsion between non-bonding
Introduction
electron on a nitrogen atom and p-electron on alkyne. To
overcome these thermodynamic and kinetic constraint,
The development of synthetic protocols for enamines pos-
sessing biologically interesting properties and their chemical
manipulation have been a promising research topic in
pharmaceutical and organic chemistry elds.1–3 In particular,
because enamine moieties can be readily converted to valu-
able chiral amine moieties in asymmetric catalytic hydroge-
nation reactions of unsaturated compounds, enamines have
served as useful intermediates in natural product synthesis.4
(Scheme 1).
Among the well-known enamine synthetic methods, inter- or
intramolecular hydroamination reactions of alkynes with
primary- or secondary amine substrates seem to be atom and
step-economic transformations because of direct addition of
the N–H moiety of amines to an alkyne substrate.5 Typically,
hydroamination reactions through nucleophilic addition of
a
number of catalytic hydroamination reactions using
a variety of metal catalysts5–8 and alkali bases9 have been
employed, in which the use of metals facilitate amines to be
readily added to electron-rich alkyne via metal–amine
complex insertion to alkyne, while the alkali bases deprive
proton to endow more nucleophilic ability. Although these
metals and alkali base exhibit efficient catalytic activities for
hydroamination reactions of alkynes, a cost-effective and
Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea.
E-mail: dwcho00@yu.ac.kr
Scheme 1 Chiral amine synthesis via asymmetric catalytic hydroge-
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00166c
nation of enamines.
5914 | RSC Adv., 2021, 11, 5914–5922
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toxic waste-free strategy amenable for the large scale appli- those of non-silyl containing tertiary amines, aminium radi-
cations are still remained. cals generated by SET processes undergo H-atom abstraction
In the studies exploring application of amine SET photo- regioselectively at a-carbons next to trimethylsilyl substituent
chemistry for the synthesis of nitrogen-containing compounds, to produce iminium ions, which are hydrolyzed to secondary
interesting observation was made in our recent studies amines. Then, generated secondary amines are added to
aiming at exploring photochemical behaviors of both a-tri- alkyne to produce enamines. Especially, when the bulky alkyl
methylsilyl and a-alkoxycarbonyl group containing N-ben- group like tert-butyl is present on nitrogen atom, not only
zylamines (i.e., N-a-trimethylsilyl-N-benzylglycinates).10 The secondary amine but azomethine ylide intermediates could be
results showed that photoirradiation of N-a-trimethylsilyl-N- formed.
benzylglycinates in the presence of molecular oxygen (O2)
and photosensitizer (PS) could convert starting tertiary
amines to non-silyl containing secondary amines via hydro-
Results and discussion
lytic cleavage of in situ formed iminium ions. This investi-
gation suggested that a-trimethylsilyl group substituted
tertiary amines can participate in photochemical hydro-
amination reactions with alkynes to produce enamine
derivatives predominantly. To the best of knowledge,
hydroamination reactions of alkynes with tertiary amines as
a reaction substrates are not common.11
To explore substrate scope and synthetic utility of a-tri-
methylsilyl group containing tertiary aliphatic amines in the
photochemical hydroamination reactions of C–C multiple
bonds, in current study, we have prepared (trimethylsilyl)
methyl and a variety of alkyl group substituted tertiary N-
benzylamines (i.e., N-a-trimethylsilyl-N-alkylbenzylamines)
and then, the photochemical reactions of them with dimethyl
acetylenedicarboxylate (DMAD) in the presence of fullerence
C60 were carried out. Since fullerene C60 is excellent electron
C60-promoted photoaddition reactions of N-a-trimethylsilyl-
N-alkylbenzylamines and DMAD
A variety of alkyl (i.e., methyl, ethyl, isopropyl, and tert-butyl)
and (trimethylsilyl)methyl substituted N-benzylamines, where
phenyl group within benzyl moiety possesses a substituent like
H, Me and F at para-position, were readily prepared by using
earlier reported synthetic procedure.14,15 With these tertiary
benzylamines in hand, photoaddition reactions of toluene
solutions (220 mL) containing benzylamines (0.7 mmol, 1
equiv.), dimethyl acetylenedicarboxylate (DMAD) (1.4 mmol, 2
equiv.) and fullerene (C60) photosensitizer (0.035 mmol, 0.05
equiv.) were performed by irradiation (450 W Hanovia medium
pressure mercury lamp equipped with int glass lter (>310
nm)) under oxygenated (O2-purged) conditions. The photo-
lysates were then puried by column chromatography to
determine photoproducts and their yields.
First, C60-promoted photoaddition reactions of N-a-trime-
thylsilyl-N-methylbenzylamines 1a–1c with DMAD in the
oxygenated toluene were conducted. As shown in Table 1,
10 min photoirradiation of toluene solutions containing 1a–1b
and DMAD gave rise to formation of high yielding of enamines
2a–2b as the sole adduct. In the photoreaction of para-uoro
acceptor owing to its high electron affinity and relatively high
3
reduction potentials (for C60
,
3Ered ¼ 1.14 V vs. SCE),12,13
fullerene C60 as a photosensitizer can participate in rapid and
efficient SET process with a variety of amine electron donors
possess oxidation potentials lower than ca. 1.1 eV. The results
of this study, presented below, showed that in contrast to
Table 1 Products and their yields in the C60-promoted photoaddition reactions of benzylamines 1a–1c with DMAD under the oxygenated
condition
Entry
Amine
Irradiation time (min)
Conversiona (%)
Productb (%)
1
2
1a
1b
1c
1a
1a
10
10
10
30
30
100
100
93
2a (78)
2b (73)
3
2c (60), 3 (19)
4c
5e
—
—
d
d
d
d
—
—
Conversion was determined based on recovered amine. b Isolation yields. c Photoreaction in deoxygenated (using freeze–pump–thaw degassing)
a
condition. d No reaction. e Photoreaction in the absence of C60
.
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Table 2 Products and their yields in the C60-promoted photoaddition reactions of benzylamines 4a–4c and 5a–5c with DMAD under the
oxygenated condition
Irradiation time
(min)
Entry
Amine
Conversiona (%)
Productb (%)
1
2
3
4
5
6
7
8
9
4a
4b
4c
5a
5a
5b
5b
5c
5c
10
10
10
10
30
10
30
10
50
100
100
90
34
96
39
100
11
6a (65)
6b (76)
6c (60)
7a (18)
7a (51)
7b (21)
7b (53)
7c (6)
100
7c (53)
a
Conversion was determined based on recovered amine. b Isolation yields.
phenyl containing amine 1c, non-silyl enamine 2c was either deoxygenated (freeze–pump–thaw degassing) condition
produced as a major product, along with silyl group containing or absence of C60 photosensitizer.
enamine 3 as a minor adduct. Importantly, control experiments
Similarly, photoreactions of N-a-trimethylsilyl-N-ethyl-
(entries 4 and 5 in Table 1) revealed that photoreaction did not benzylamines 4a–4c with DMAD in the presence of C60
take place in cases that the reactions were conducted under the produced the same types of enamines 6a–6c as a single product
even in the reaction of para-uoro substituted amine 4c. (Table
Table 3 Products and their yields in the C60-promoted photoaddition reactions of benzylamines 8a–8c with DMAD under the oxygenated
condition
Entry
Amine
Irradiation time (min)
Conversiona (%)
Productb (%)
1
2
3
4
5
6
8a
8a
8b
8b
8c
8c
10
30
10
30
10
90
19
60
15
63
6
9a (10)
9a (19), 10a (19)
9b (8)
9b (25), 10b (8)
9c (3)
9c (36), 10c (9)
73
a
Conversion was determined based on recovered amine. b Isolation yields.
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Table 4 Products and their yields in the C60-promoted photoaddition reactions of benzylamines 11–13 with DMAD under the oxygenated
condition
Amine
Irradiation time (min)
Conversiona (%)
Productb (%)
11
11
12
12
13
13
10
20
10
20
10
20
60
100
35
79
58
2a (15), 14 (22)
2a (28), 14 (42)
2c (14), 14 (9)
2c (35), 14 (21)
6a (16), 15 (15)
6a (35), 15 (30)
100
a
Conversion was determined based on recovered amine. b Isolation yields.
2) When N-alkyl substituent of amine substrates was changed analogs (see Tables 1 and 2), photoreactions of non-silyl group
from ethyl (N-ethyl, 4a–4c) to isopropyl (N-isopropyl, 5a–5c) tethered amines 11–13 with acetylene (DMAD) led to competi-
group, enamines 7a–7c were also produced. However, conver- tive formation of two types of enamine mixtures, 2a, 2c, 6a and
sion yields of amine substrates 5a–5c and their photoproduct 14–15 inefficiently. These observation reveal that silyl group
yields were much lower than those from 4a–4c. Especially, in resided on nitrogen atom is likely to be responsible for regio-
case of photoreactions of para-uoro phenyl tethered amine 5c, selective secondary amine formation. Especially, silyl group
much longer irradiation time was required to bring about high tethered aminium radicals undergo H-atom abstraction at a-
conversion of starting amine 5c and high yielding of photo- carbons adjacent to silyl moiety regioselectively to form
product 7c. (entries 8 and 9 in Table 2).
precursors of secondary amines (see reaction mechanism). In
The results of photoreactions of tert-butyl group containing addition, considering irradiation time vs. conversion of amine
benzylamines 8a–8c with DMAD in the presence of C60 were substrates, silyl group on amines seem to be able to enhance the
quite different from those of less alkyl group containing ben- reaction efficiencies.
zylamines 1a–1c (N-methyl), 4a–4c (N-ethyl), 5a–5c (N-isopropyl)
in terms of both photoreaction efficiency and photoproduct
distribution pattern. As shown in Table 3, the photoaddition
Table 5 Products and their yields in the visible light irradiated, C60
-
reactions of 8a–8c with DMAD took place much less efficiently
and, moreover, the least reaction efficiency was made in the
photoreaction using both tert-butyl and para-uoro phenyl
groups containing benzylamine 8c (see Tables 1 and 2). More
interesting observation made in these reactions was that
when much longer irradiation time was employed for the
reactions, not only enamines 9a–9c but also the highly
substituted pyrrole derivatives 10a–10c were produced, albeit
relatively low yield. Considering our previous studies,10 it is
likely that 1,3-dipolar cycloaddition reactions of in situ
formed azomethine ylides from 8a–8c to DMAD are respon-
sible for the formation of ve membered N-heterocycles. In
here, it is noteworthy to mention that the generation of non-
stabilized azomethine ylides stimulated by sterically
hindered N-substituent is quite unusual.
promoted photoaddition reactions of benzylamines 1a–1c with DMAD
under the oxygenated condition
Irradiation time
(h)
Amine
Conversiona (%)
Productb (%)
1a
1a
1b
1b
1c
1c
1c
20
40
20
40
20
40
60
47
100
51
100
20
48
2a (32)
2a (70)
2b (36)
2b (73)
2c (10)
2c (28)
2c (45)
In order to evaluate how the trimethylsilyl group on the
amine inuence the reaction efficiencies and photoproduct
distributions, we probed C60-promoted photoaddition reactions
of non-silyl containing benzylamines 11–13. As described in
Table 4, in contrast those of silyl group containing amine
80
a
b
Conversion was determined based on recovered amine. Isolation
yields.
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Scheme 2 Plausible mechanistic pathways for the enamine and pyrrole photoadducts.
fullerene C60 to singlet state of C60, followed rapid and efficient
intersystem crossing (FISC ¼ 1) processes, the SET process from
Visible light irradiated, C60-promoted photoaddition
reactions of N-a-trimethylsilyl-N-alkylbenzylamines 1a–1c
with DMAD
3
benzylamines 16 to triplet state of fullerene C60 ð C*60Þ takes
place initially to generate aminium radicals 17 and radical ions
of C60 (C60cꢀ), latter of which are oxidized by molecular oxygen
(O2) to ground state of C60 and radical anions of molecular
oxygen (O2cꢀ). Among the several pathways open to aminium
radicals 17, O2cꢀ-promoted regioselective H-atom abstraction of
the aminium radicals seems to be the most feasible pathway.16
Then, generated iminium ions 18 undergo hydrolytic cleavage
by hydrogen peroxide anions (HOOꢀ) to produce non-silyl
containing secondary benzylamines 19, which add to DMAD
to form enamine adducts 20.5 When the sterically bulky N-
substituent is present on the starting amine substrates, another
photochemical process takes place competitively. A mechanistic
pathway for this process, which is compatible with the results of
photoreactions, is 1,3-dipolar cycloaddition reactions of in situ
formed azomethine ylides to DMAD to produce N-heterocyclic
pyrrole photoadducts. As shown in Scheme 2, the key step in
this route is that aminium radicals 17 undergo desilylation to
form a-amino radicals 21, which are consecutively converted to
iminium ions 22 through a pathway involving second SET
owing to the much lower oxidation potential of a-amino radi-
cals in the range of ꢀ1 V (vs. SCE). Non-silyl containing iminium
ions 22 formed in this manner then lose proton from benzyl
position to produce 1,3-dipolar azomethine ylides 23 which
serve as a precursor of pyrrole photoadducts 24.
Visible-light promoted photochemical reactions have attracted
great attention because of their wide application in organic
synthesis and their use of sustainable and green energy sources.
Especially, utilization of visible light is highly advantageous as
most organic compounds do not absorb in this region and
product decomposition and unwanted side products are readily
avoided. Because fullerene C60 can absorbs light in the visible
region as well as serve as an efficient electron acceptor, as
a preliminary study, we explored viability of photoreactions of
N-a-trimethylsilyl-N-alkylbenzylamines 1a–1c with DMAD using
visible light. For this purpose, C60-promoted photoaddition
reactions of benzylamines 1a–1c with DMAD under the O2-
purged condition were performed by irradiation using
commercially available 20 W compact uorescent lamp (CFL)
for xed time periods (information about emitted wavelengths
and their relative intensity from 20 W uorescent lamp is
available in ESI†). The results, given in Table 5, show that 20 h
visible light irradiation of toluene solutions containing 1a–1c
and DMAD led to production of the respective enamines 2a–2c,
although reactions of para-uorophenyl containing amine 1c
gave rise to lower conversion and product yield. Denitely,
employing much longer visible light irradiation time could
enhance the conversion and photoproduct yields in the photo-
reactions of 1a–1c with DMAD.
Conclusion
Reaction mechanism
On the basis of observation made in earlier studies,10,16,17 In current study, C60-promoted photoaddition reactions of both
plausible mechanistic pathways can be suggested. (Scheme 2) trimethylsilyl- and a variety of alkyl group containing tertiary
Enamine formation in these reactions seems to occur via benzylamines (i.e., N-a-trimethylsilyl-N-alkylbenzylamines) with
Michael addition of formed secondary amines 19 to DMAD. dimethyl acetylenedicarboxylate (DMAD) were carried out to
Specically, right aer the excitation of ground state of explore synthetic utility of trimethylsilyl group containing
5918 | RSC Adv., 2021, 11, 5914–5922
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tertiary amines as substrates in the photochemical hydro-
amination reactions with C–C multiple bonds. The results
showed that photoreactions of all of trimethylsilyl containing N-
alkylbenzylamines with DMAD under O2-purged environment,
produced non-silyl containing enamines efficiently through
a pathway involving addition of secondary amines to DMAD,
former of which are produced by hydrolytic cleavage of in situ
formed iminium ions. Exceptionally, ve-membered N-hetero-
cyclic ring, pyrroles, were also produced competitively in
photoreaction of those of less bulky alkyl group (i.e., tert-butyl)
substituted benzylamines through a pathway involving 1,3-
dipolar cycloaddition of azomethine ylides to DMAD. Further-
more, photoreactions of non-silyl containing benzylamines with
DMAD under oxygenated conditions took place less efficient
and non-regioselective manner to produce enamine
photoadducts.
Photoreaction of 1a
10 min irradiation (100% conversion), column chromatography
1
(EtOAc : hexane ¼ 1 : 5) to yield 2a10 (144 mg, 78%). H-NMR
d 2.72 (s, 3H), 3.61 (s, 3H), 3.90 (s, 3H), 4.27 (s, 2H), 4.65 (s,
1H), 7.19–7.34 (m, 5H); 13C-NMR d 36.8, 50.7, 52.9, 56.3, 84.6,
127.3, 127.8, 128.7, 135.5, 154.9, 166.0, 168.0; HRMS (EI) m/z
263.1154 (M+, C14H17NO4 requires 263.1158).
Photoreaction of 1b
10 min irradiation (100% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 2b (142 mg, 73%). 1H NMR
d 2.30 (s, 3H), 2.70 (s, 3H), 3.61 (s, 3H), 3.90 (s, 3H), 4.22 (s, 2H),
4.64 (s, 1H), 7.07–7.14 (m, 4H); 13C NMR d 21.0, 36.7, 50.7, 52.9,
56.2, 84.6, 127.4, 129.4, 132.5, 137.6, 155.0, 166.1, 168.1; HRMS
(EI) m/z 277.1313 (M+, C15H19NO4 requires 277.1314).
The observation made in this study show that regioselectivity
of C60-promoted photochemical reactions of N-a-trimetylsilyl-N-
alkylbenzylamines, leading to formation of secondary amines,
can be controlled by the presence of trimethylsilyl group, and
that this silyl containing tertiary amines can serve a precursor of
secondary amines for the hydroamination reactions with elec-
tron decient acetylenes.
Photoreaction of 1c
10 min irradiation (93% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 2c (119 mg, 78%) and 3 (34 mg,
19%).
1
2c: H NMR d 2.68 (s, 3H), 3.58 (s, 3H), 3.88 (s, 3H), 4.2 (s,
2H), 4.62 (s, 1H), 6.94–7.0 (m, 2H), 7.15–7.19 (m, 2H); 13C NMR
d 36.5, 50.6, 52.8, 55.5, 84.7, 115.5 (d, JC–F ¼ 21.5 Hz), 128.9 (d,
JC–F ¼ 8.1 Hz), 131.1 (d, JC–F ¼ 3.1 Hz), 154.6, 160.5, 162.1 (d, JC–F
¼ 245 Hz), 165.9, 167.8; HRMS (EI) m/z 28101066 (M+,
Experimental
General
C
14H16FNO4 requires 281.1063).
3: H NMR d 0.08 (s, 9H), 2.68 (s, 2H), 2.84 (s, 3H), 3.57 (s,
1
The 1H (300 MHz) and 13C NMR (75 MHz) spectra (obtained by
using Bruker DPX300) were recorded on CDCl3, and chemical
shis were reported in parts per million (d, ppm) relative to
3H), 3.88 (s, 3H), 4.44 (s, 1H); 13C NMR d ꢀ1.6, 50.5, 52.7, 82.5,
154.7, 166.1, 168.2; HRMS (EI) m/z 259.1242 (M+, C11H21NO4Si
requires 259.1240).
1
CHCl3 (7.24 ppm for H and 77.0 ppm for 13C) as an internal
standard. High resolution (HRMS) mass spectra were obtained
by using an EI or ESI. All starting materials used in the synthetic
routes came from commercial sources. Photochemical reac-
tions were conducted by using an immersion-well photochem-
ical apparatus, consisting of a 450 W Hanovia medium pressure
mercury vapor UV lamp (aceglass cat. # 7825-34), a power supply
(aceglass cat. # 7830-61), a water-cooled quartz immersion well
(aceglass cat # 7874-27), a borosilicate reaction vessel (agceglass
cat. # 7841-03) and a int glass lter (>310 nm). Detail infor-
mation about UV lamps used in this study are provided on ESI.†
Photoreaction of 4a
10 min irradiation (100% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 6a (127 mg, 78%). 1H NMR d 1.05
(t, 3H, J ¼ 7.2 Hz), 3.12 (q, 2H, J ¼ 7.2 Hz), 3.55 (s, 3H), 3.87 (s, 3H),
4.26 (s, 2H), 4.62 (s, 1H), 7.18–7.28 (m, 5H); 13C NMR d 50.4, 52.7,
77.2, 83.9, 126.9, 127.5, 128.5, 135.4, 154.0, 165.9, 168.0; HRMS (EI)
m/z 277.1312 (M+, C15H19NO4 requires 277.1314).
Photoreaction of 4b
10 min irradiation (100% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 6b (156 mg, 76%). 1H NMR
d 1.06 (t, 3H, J ¼ 7.2 Hz), 2.28 (s, 3H), 3.12 (q, 2H, J ¼ 7.2 Hz),
3.57 (s, 3H), 3.88 (s, 3H), 4.23 (s, 2H), 4.63 (s, 1H), 7.09 (s, 4H);
13C NMR d 20.9, 50.5, 52.7, 77.2, 83.8, 127.0, 129.2, 132.4, 137.2,
154.1, 166.0, 168.1; HRMS (EI) m/z 291.1474 (M+, C16H21NO4
requires 291.1471).
General procedure of C60-promoted photoreactions of N-a-
trimethylsilyl-N-alkylbenzylamines with DMAD.
The toluene solutions (220 mL) containing benzylamines
(0.7 mmol,
1
equiv.), dimethyl acetylenedicarboxylate
(1.4 mmol, 2 equiv.), and C60 (0.035 mmol, 0.05 equiv.) that were
purged with O2 before and during irradiation were irradiated
with a 450 W Hanovia medium pressure Hg lamp surrounded
Photoreaction of 4c
by a int glass lter (>310 nm) in a water-cooled quartz 10 min irradiation (90% conversion), column chromatography
immersion well for time periods given below. Then, photo- (EtOAc : hexane ¼ 1 : 5) to yield 6c (125 mg, 60%). 1H NMR
lysates were concentrated in vacuo and then the resulting d 1.03 (t, 3H, J ¼ 6.9 Hz), 3.09 (q, 2H, J ¼ 6.9 Hz), 3.53 (s, 3H),
mixture were separated by using silica gel column chromato- 3.85 (s, 3H), 4.2 (s, 2H), 4.58 (s, 1H), 6.91–6.97 (m, 2H), 7.13–7.17
graphic isolation to give photoadducts.
(m, 2H); 13C NMR d 50.6, 52.8, 77.2, 84.2, 115.5 (d, JC–F ¼ 21.4
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Hz), 128.7 (d, JC–F ¼ 8 Hz), 131.2, 153.9, 160.4, 162.1 (d, JC–F
¼
Photoreaction of 8b
244.5 Hz), 165.9, 168.0; HRMS (EI) m/z 295.1221 (M+,
10 min irradiation (15% conversion), column chromatography
C
15H18FNO4 requires 295.1220).
(EtOAc : hexane ¼ 1 : 5) to yield 9b (18 mg, 8%); 30 min irra-
diation
(60%
conversion),
column
chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 9b (55 mg, 25%) and 10b
Photoreaction of 5a
(18 mg, 8%).
9b: 1H NMR d 1.36 (s, 9H), 2.30 (s, 3H), 3.59 (s, 3H), 3.80 (s,
3H), 4.38 (s, 2H), 4.95 (s, 1H), 7.08–7.15 (m, 4H); 13C NMR d 28.8,
50.8, 52.8, 90.8, 126.4, 129.1, 135.4, 136.6, 154.1, 167.2, 167.9;
HRMS (EI) m/z 319.1786 (M+, C18H25NO4 requires 319.1784).
10b: 1H NMR d 1.39 (s, 9H), 2.37 (s, 3H), 3.55 (s, 3H), 3.78 (s,
3H), 7.14 (d, 2H, J ¼ 7.8 Hz), 7.22 (d, 2H, J ¼ 7.8 Hz), 7.45 (s, 1H);
13C NMR d 21.4, 31.5, 51.3, 51.6, 59.0, 112.4, 118.2, 124.4, 128.3,
129.9, 131.9, 136.5, 138.6, 164.4, 165.9; HRMS (EI) m/z 329.1625
(M+, C19H23NO4 requires 329.1627).
10 min irradiation (34% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7a (36 mg, 18%); 30 min irra-
diation
(96%
conversion),
column
chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7a (105 mg, 51%). 1H NMR
d 1.17 (d, 6H, J ¼ 6.3 Hz), 3.53 (s, 3H), 3.79 (septet, 1H, J ¼ 6.3
Hz), 3.92 (s, 3H), 4.31 (s, 2H), 4.47 (s, 1H), 7.18–7.24 (m, 3H),
7.27–7.32 (m, 2H); 13C NMR d 20.9, 47.1, 50.7, 52.8, 52.9, 85.7,
126.0, 127.1, 128.6, 136.1, 154.5, 166.3, 168.2; HRMS (EI) m/z
291.1472 (M+, C16H21NO4 requires 291.1471).
Photoreaction of 8c
Photoreaction of 5b
10 min irradiation (6% conversion), column chromatography
10 min irradiation (39% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7b (45 mg, 21%); 30 min irra-
diation (100% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7b (114 mg, 53%). 1H NMR
d 1.15 (d, 6H, J ¼ 6.6 Hz), 2.28 (s, 3H), 3.52 (s, 3H), 3.75 (septet,
1H, J ¼ 6.6 Hz), 3.91 (s, 3H), 4.26 (s, 2H), 4.46 (s, 1H), 7.04–7.11
(m, 4H); 13C NMR d 20.8, 21.0, 46.9, 50.6, 52.7, 52.9, 85.4, 125.9,
129.2, 133.0, 136.6, 154.5, 166.3, 168.2; HRMS (EI) m/z 305.1624
(M+, C17H23NO4 requires 305.1627).
(EtOAc : hexane ¼ 1 : 5) to yield 9c (7 mg, 3%); 90 min irradia-
tion
(73%
conversion),
column
chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 9c (82 mg, 36%) and 10c (21 mg,
9%).
1
9c: H NMR d 1.35 (s, 9H), 3.58 (s, 3H), 3.78 (s, 3H), 4.37 (s,
2H), 4.94 (s, 1H), 6.94–7.0 (m, 2H), 7.19–7.22 (m, 2H); 13C NMR
d 28.8, 50.8, 52.1, 52.8, 59.4, 91.9, 115.4 (d, JC–F ¼ 21 Hz), 128.1
(d, JC–F ¼ 8.3 Hz), 134.2 (d, JC–F ¼ 3 Hz), 153.7, 161.8 (d, JC–F
¼
243.8 Hz), 167.1, 167.7; HRMS (EI) m/z 323.1530 (M+,
C
17H22FNO4 requires 323.1533).
10c: H NMR d 1.39 (s, 9H), 3.55 (s, 3H), 3.79 (s, 3H), 7.01–
1
Photoreaction of 5c
7.07 (m, 2H), 7.3–7.34 (m, 2H), 7.46 (s, 1H); 13C NMR d 31.5,
51.4, 51.6, 112.6, 114.7 (d, JC–F ¼ 21.8 Hz), 118.6, 124.7, 129.0 (d,
10 min irradiation (11% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7c (12 mg, 6%); 50 min irradi-
JC–F ¼ 3.8 Hz), 133.9 (d, JC–F ¼ 7.5 Hz), 135.1, 162.9 (d, JC–F
¼
247.5 Hz), 164.3, 165.6; HRMS (EI) m/z 333.1378 (M+,
18H20FNO4 requires 333.1376).
ation
(100%
conversion),
column
chromatography
(EtOAc : hexane ¼ 1 : 6) to yield 7c (116 mg, 53%). 1H NMR
d 1.14 (d, 6H, J ¼ 6.6 Hz), 3.51 (s, 3H), 3.74 (septet, 1H, J ¼ 6.6
Hz), 3.89 (s, 3H), 4.24 (s, 2H), 4.43 (s, 1H), 6.93–6.98 (m, 2H),
7.12–7.17 (m, 2H); 13C NMR d 20.7, 46.4, 50.6, 52.7, 52.8, 85.9,
115.5 (d, JC–F ¼ 21 Hz), 127.6 (d, JC–F ¼ 8.3 Hz), 131.7 (d, JC–F ¼ 3
Hz), 154.3, 160.1, 161.7 (d, JC–F ¼ 244.5 Hz), 166.1, 167.9; HRMS
(EI) m/z 309.1378 (M+, C16H20FNO4 requires 309.1376).
C
General procedure of C60-promoted photoreactions of non-
silyl containing N-alkylbenzylamines with DMAD
Under the same reaction conditions employed above, the
photoreactions of non-silyl containing each benzylamine 11–13
with DMAD were carried out. Then, photolysates were concen-
trated in vacuo and then the resulting mixture were separated by
using silica gel column chromatographic isolation to give
photoadducts.
Photoreaction of 8a
10 min irradiation (19% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 9a (21 mg, 10%); 30 min irra-
Photoreaction of 11 with DMAD
diation
(60%
conversion),
column
chromatography
10 min irradiation (60% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 2a (28 mg, 15%) and 14 (ref. 10)
(29 mg, 22%); 20 min irradiation (100% conversion), column
chromatography (EtOAc : hexane ¼ 1 : 5) to yield 2a (52 mg,
28%) and 14 (55 mg, 42%).
(EtOAc : hexane ¼ 1 : 5) to yield 9a (41 mg, 73%) and 10a
(41 mg, 19%).
1
9a: H NMR d 1.37 (s, 9H), 3.59 (s, 3H), 3.80 (s, 3H), 4.43 (s,
2H), 4.96 (s, 1H), 7.18–7.33 (m, 5H); 13C NMR d 28.8, 50.8, 52.8,
53.0, 59.5, 91.1, 126.5, 127.0, 128.5, 138.5, 154.1, 167.2, 167.8;
HRMS (EI) m/z 305.1629 (M+, C17H23NO4 requires 305.1627).
1
Photoreaction of 12 with DMAD
10a: H NMR d 1.38 (s, 9H), 3.52 (s, 3H), 3.78 (s, 3H), 7.32–
7.38 (m, 5H), 7.46 (s, 1H); 13C NMR d 31.3, 51.3, 51.5, 59.0, 112.3, . 10 min irradiation (35% conversion), column chromatography
118.1, 124.4, 127.4, 128.6, 131.9, 132.9, 136.2, 164.2, 165.6; (EtOAc : hexane ¼ 1 : 5) to yield 2c (28 mg, 14%) and 14 (12 mg,
HRMS (EI) m/z 315.1471 (M+, C18H21NO4 requires 315.1471).
5920 | RSC Adv., 2021, 11, 5914–5922
9%); 20 min irradiation (79% conversion), column
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chromatography (EtOAc : hexane ¼ 1 : 5) to yield 2c (69 mg,
35%) and 14 (28 mg, 21%).
Acknowledgements
14: 1H NMR d 2.79 (s, 6H), 3.54 (s, 3H), 3.84 (s, 3H), 4.50 (s,
1H); 13C NMR d 39.6, 50.5, 52.7, 84.2, 155.1, 165.9, 168.0; HRMS
(EI) m/z 187.0845 (M+, C8H13NO4 requires 187.0845).
This research was nancially supported by Yeungnam Univer-
sity Research Grant (218A380049). The authors thanks the Core
Research Center for Natural Products and Medical Materials
(CRCNM) for technical support regarding NMR spectrometer.
Photoreaction of 13 with DMAD
Notes and references
10 min irradiation (60% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 6a (31 mg, 16%) and 15 (21 mg,
15%); 20 min irradiation (100% conversion), column chroma-
tography (EtOAc : hexane ¼ 1 : 5) to yield 6a (68 mg, 35%) and
15 (43 mg, 30%).
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Photoreaction of 1a
20 h irradiation (47% conversion), column chromatography
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hexane ¼ 1 : 5) to yield 2a (32 mg, 70%).
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Photoreaction of 1c
20 h irradiation (20% conversion), column chromatography
(EtOAc : hexane ¼ 1 : 5) to yield 2b (5 mg, 10%); 40 h irradiation
(48% conversion), column chromatography (EtOAc : hexane ¼
1 : 5) to yield 2c (14 mg, 28%); 60 h irradiation (80% conver-
sion), column chromatography (EtOAc : hexane ¼ 1 : 5) to yield
2c (23 mg, 45%).
Conflicts of interest
There are no conicts of interest to declare.
© 2021 The Author(s). Published by the Royal Society of Chemistry
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