K2S2O8-catalyzed highly regioselective amidoalkylation of diverse N-heteroaromatics in water under visible light irradiation

Article abstract of DOI:10.1039/d1gc02107a

A K2S2O8-catalyzed versatile C(sp2)-C(sp3) bond formation with N-heteroaromatics and γ-lactams/amides was developed. Quinoxalin-2(1H)-one, quinoline, isoquinoline, phthalazine, and benzothiazole reacted with γ-lactams/amides to give the corresponding C(sp2)-H amidoalkylation products in moderate to good yields with high regioselectivity. This visible-light-induced photocatalyst-free reaction was conducted in H2O at ambient temperature, which comply with the principles of "green chemistry". The new K2S2O8 catalytic mechanism was investigated with control experiments.

Full text of DOI:10.1039/d1gc02107a

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K2S2O8-catalyzed highly regioselective amidoalky-
lation of diverse N-heteroaromatics in water under
visible light irradiation†
Cite this: DOI: 10.1039/d1gc02107a
Received 14th June 2021,
Accepted 12th July 2021
Jiadi Zhou,a,b Quanlei Ren,c Ning Xu,c Chaodong Wang,c Shengjie Song,c Zhi Chenc
a,c,b
DOI: 10.1039/d1gc02107a
and Jianjun Li
*
rsc.li/greenchem
A K2S2O8-catalyzed versatile C(sp2)–C(sp3) bond formation with has been reported. For example, the Ni-catalyzed oxidative
N-heteroaromatics and γ-lactams/amides was developed. coupling of the C(sp2)–H bond of benzamides with the C(sp3)–
Quinoxalin-2(1H)-one, quinoline, isoquinoline, phthalazine, and H bonds of the amide was reported by the Zhang group
benzothiazole reacted with γ-lactams/amides to give the corres- (Scheme 1a).4 The persulfate induced amidoalkylation of
ponding C(sp2)–H amidoalkylation products in moderate to good N-heteroaromatics was achieved by the Zhu5 and Cristian6
yields with high regioselectivity. This visible-light-induced photo- groups, respectively (Scheme 1b). Diverse and efficient light-
catalyst-free reaction was conducted in H2O at ambient tempera- induced organic transformations have been widely used to
ture, which comply with the principles of “green chemistry”. The achieve amidoalkylated N-heteroaromatics (Scheme 1c).7
new K2S2O8 catalytic mechanism was investigated with control However, transition metals (Fe,8 Ni,4,9 Ru,7d,10 Ag,11 etc.) and/
experiments.
or stoichiometric oxidants (TBHP,4,8,12 Na2S2O8,6 K2S2O8,5,13
DCP,14 BPO,15 etc.) and photosensitizers (benzaldehyde,7a
eosin Y,7b carbon nitride,7c etc.) need to be used to realize this
transformation. To the best of our knowledge, the studies on
persulfate catalyzed amidoalkylation of N-heteroaromatics
were rarely reported.
In recent years, visible-light-initiated photocatalysis organic
reaction has become an increasingly important subject in syn-
thetic chemistry.16 However, in most cases, photocatalysts
(transition metals or organic dyes) are usually necessary for
the process of photocatalysis. Recently, visible-light-induced
organic reactions using photoactive substrates in the absence
Introduction
Lactams and amides,1 especially γ-lactams,2 widely exist in
drugs and biologically active compounds. Direct α C–H
functionalization of lactams/amides is of great interest in
organic synthesis for the generation of potential drug candi-
dates via C–C and C–heteroatom bond formations. On the
other hand, N-heteroaromatics, such as quinoxalinones, qui-
nolines, isoquinolines, benzothiazoles, etc., play an important
role in the field of natural products, biologically active struc-
tures, and medicinally relevant compounds3 (Fig. 1).
The development of novel and efficient methods to con-
struct amidoalkylated (hetero)aromatics continues to be a very
important research area in organic chemistry. In the past few
decades, activation of α-N–C(sp3)–H bond of amides mediated
by metal catalysis, additional oxidants, and photochemistry
aCollaborative Innovation Center of Yangtze River Delta Region Green
Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P. R. of
China
bNational Engineering Research Center for Process Development of Active
Pharmaceutical Ingredients, Zhejiang University of Technology, Hangzhou 310014,
P. R. of China
cCollege of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou
310014, P. R. of China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc02107a
Fig. 1 Representative
N-heteroaromatics.
biologically
active
amidoalkylated
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Table 1 Optimization of the reaction conditionsa
Entry
Oxidant
Solvent
Yield [%]
1a
2
3
4
5
K2S2O8 (1 equiv.)
NHPI (1 equiv.)
TBHP (1 equiv.)
DTBP (1 equiv.)
H2O2 (1 equiv.)
H2O
H2O
H2O
H2O
83
11
Trace
Trace
14
H2O
6
7
8
9
10
11
12
13
14b
15c
16d
17e
18f
K2S2O8 (2 equiv.)
K2S2O8 (0.5 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (1 mol%)
—
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
K2S2O8 (0.1 equiv.)
H2O
H2O
H2O
H2O
85
83
81
Trace
H2O
0
CH3CN
C2H5OH
EtOAc
H2O
H2O
H2O
Trace
Trace
Trace
Trace
69
0
17
H2O
H2O
74
a Reaction conditions: 1a (0.2 mmol), 2a (1 mmol), solvent (1.5 mL),
oxidant, 3 W blue LEDs, 24 h, under an air atmosphere. b 3 W green
LEDs. c 3 W white LEDs. d In the dark. e Under a N2 atmosphere.
f Under an O2 atmosphere.
Scheme 1 Amidoalkylation of the heterocyclic skeleton.
of any external photosensitizer has received extensive atten-
tion.17 This strategy is believed to be a promising research
direction. For a long period of time, we have been focusing on
the C(sp2)–H functionalization of N-heteroarenes and aro-
matics.18 Herein, we describe a K2S2O8-catalyzed highly regio-
selective amidoalkylation of N-heteroaromatics in water under
visible light irradiation.
generated without K2S2O8 (Table 1, entry 10). The effect of
various solvents was also tested, such as CH3CN, C2H5OH and
EtOAc (Table 1, entries 11–13), and H2O was found to be the
best choice to promote the reaction. The Stern–Volmer experi-
ments revealed that the excited species 1a* might be generated
much easily in H2O than CH3CN (see the ESI, Fig. S5†). In
addition, when the reaction was conducted under irradiation
with 3 W green or white LED lamps, the target product 3a was
obtained in trace and 69% yield, respectively (Table 1, entries
14 and 15). In the absence of visible light, no product was
Results and discussion
Initially, the reaction of 1-methylquinoxalin-2(1H)-one (1a) and detected (Table 1, entry 16). Only a small amount of product
1-methylpyrrolidin-2-one (2a, NMP) was performed to optimize was formed under a nitrogen atmosphere (Table 1, entry 17),
the reaction conditions. Then the template reaction was con- and the result indicated that the oxygen in air may participate
ducted in H2O by employing potassium persulfate as the in the reaction. However, under an oxygen atmosphere, a
oxidant under visible light at room temperature for 24 h higher amount of the oxidation by-product (1-methyl-1,4-dihy-
(Table 1, entry 1). The products 3a and 3a′ were isolated in droquinoxaline-2,3-dione 4a) was detected (Table 1, entry 18,
83% yields with 100 : 3 N-methylene/N-methyl-substituted see the ESI† for the mechanistic studies). Finally, the optimal
regioselectivity, which was detected by 1H NMR spectroscopy reaction conditions for the direct amidoalkylation of quinoxa-
of the crude product (see the ESI, Fig. S1†). Encouraged by the lin-2(1H)-one was identified as follows: 1a (0.2 mmol), 2a
excellent selectivity,
a
number of oxidants such as (1 mmol) and K2S2O8 (0.02 mmol) in H2O at ambient tempera-
N-hydroxyphthalimide (NHPI), tert-butyl peroxide (TBHP), di- ture for 24 hours.
tert-butyl peroxide (DTBP), and H2O2 were examined; K2S2O8
With the optimized reaction conditions in hand, we then
was still the best oxidant (Table 1, entries 2–5). Subsequently, evaluated the scope of this reaction using various quinoxalin-2
the stoichiometries of the oxidant were screened, and the (1H)-ones (1) with NMP (2a) and the results are summarized in
results showed that increasing the amount of K2S2O8 to 2 Table 2. Initially, quinoxalin-2(1H)-ones bearing various
equiv. did not obviously enhance the reaction efficiency N-protecting groups such as N-methyl, N-ethyl, N-esteryl,
(Table 1, entry 6). To our delight, a slightly lower yield of 3a N-benzyl and N-allyl groups were well tolerant, providing the
(81%) was obtained when the amount of K2S2O8 was reduced corresponding products 3a–3f in 56–85% yields. To our
to 10 mol% (Table 1, entry 8). No corresponding product was delight, N-phenylquinoxalin-2(1H)-one also could undergo the
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Table 2 Substrate scope of quinoxalin-2(1H)-onesa
Table 3 Substrate scope of lactams or amidesa
a Reaction conditions: 1 (0.2 mmol), 2a (1 mmol), K2S2O8 (0.02 mmol),
water (1.5 mL), air, rt, 24 h, 3 W blue LEDs.
a Reaction conditions: 1a (0.2 mmol),
2
(0.6 mmol), K2S2O8
(0.02 mmol), water (1.5 mL), air, rt, 24 h, 3 W blue LEDs. b (NH4)2S2O8
(0.02 mmol). c (NH4)2S2O8 (0.02 mmol), H2O2 (0.4 mmol), under a N2
atmosphere.
reaction smoothly to give the corresponding product 3g in
83% yield. It should be noted that N-free protected quinoxalin-
2(1H)-one derivatives were also suitable substrates to afford the
desired products 3h and 3i in 72% and 65% yields, respect-
ively. In addition, quinoxalin-2(1H)-ones substituted with in 63% yield through further oxidation. We also investigated
methyl, fluoro, chloro, and bromo groups on the aryl ring the cyclic amide and the chain amide, 1-acetylpyrrolidine,
reacted well with NMP to provide the corresponding products which gave the corresponding product 3x in 51% yield. N,N-
3j–3q in moderate to good yields, offering a chance for further Diethylformamide, N,N-diethylacetamide, DMF, and DMAc
structural modification. A mixture of 3r and 3r′ was obtained could provide the products 3y, 3z, 3aa, and 3ab in moderate
in
a ratio of 4.5 : 5.5 with an overall yield of 79%. yields. N,N-Dimethylbenzamide could also give the corres-
Unfortunately, quinoxalin-2(1H)-one bearing a strong electron- ponding product 3ac in 54% yield. 1,1,3,3-Tetramethylurea was
withdrawing group (–NO2) on the aryl ring could not undergo also compatible with the reaction, affording the corresponding
this reaction to yield the desired product under the present coupled product 3ad in 48% yield. To our surprise, 1-vinylpyr-
conditions.
rolidin-2-one could also react with 1-methylquinoxalin-2(1H)-
After that, the scope and limitation of this reaction was one, giving the corresponding product (E)-1-methyl-3-(2-(2-oxo-
further examined using various lactams/amides (Table 3). pyrrolidin-1-yl)vinyl)quinoxalin-2(1H)-one (3ae). When hydan-
Firstly, we explored the regioselectivity by using 1-ethyl-2-pyrro- toin was used, the reaction selectively occurred on the nitrogen
lidone (NEP) and the regioisomers 3s and 3s′, which was atom, which provided the product 3af in 68% yield.
carried out in a ratio of 6.3 : 1 with an overall yield of 67%. Pyrrolidone and piperidin-2-one provided the products 3ag
1-Benzylpyrrolidone alone gave the corresponding product 3t and 3ah in 59% and 46% yields, respectively. To further show
in 65% yield. 1,3-Dimethylimidazolidin-2-one was found to be the application value for the synthesis of potentially bioactive
viable for the reaction, providing the desired product 3u in compounds, clinical drugs (Piracetam) were explored, provid-
78% yield. The reaction could also tolerate lactam with an ing the corresponding amidoalkylation product 3ai in 62%
oxygen atom, giving the corresponding product 3v in 61% yield.
yield. Interestingly, the reaction of N-methylcaprolactam regio-
To further show the applicability of this reaction protocol,
selectively occurred at the N-methyl, obtaining the product 3w we used other N-heteroaromatics to investigate the applica-
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bility of the reaction. Further screening of the reaction con-
ditions was performed to explore the optimal reaction con-
ditions (see the ESI, Table S3†). At first, diverse electron-
deficient N-heteroaromatics were tested under the standard
conditions. Unsubstituted quinolone could react with
γ-lactam, giving a mixture of ortho and para substitution pro-
ducts (3aj/3aj′, o/p = 3 : 10). 2-Methylquinoline gave the desired
product 3ak in 69% yield. Quinolines with para-substituents
such as an electron-donating group (Me) or electron-withdraw-
ing group (Br) gave the desired products 3al and 3am in 66%
and 63% yields, respectively. 3-Bromoquinoline, which has two
possible sites for amidoalkylation, afforded the desired
product 3an in 59% yield with high regioselectivity.
Isoquinoline gave an inseparable mixture of regioisomers 3ao
and 3ao′ in a ratio of 12.5 : 1 with an overall yield of 57%.
Moreover, other N-heteroaromatics such as phthalazine, qui-
noxaline and benzothiazole could also give the corresponding
products 3ap, 3ap′, 3aq and 3ar in moderate yields. 1,3,5-
Trimethoxybenzene, an electron-rich arene, could also provide
the corresponding product 3as in 69% yield (Table 4).
Scheme 2 Large-scale synthesis of 3a.
proceeded through
a free-radical pathway (Scheme 3a).
Subsequently, the reaction was completely inhibited by the
addition of a singlet oxygen quencher,20 1,4-diazabicyclo[2.2.2]
octane (DABCO), indicating that singlet oxygen may be
involved in the reaction (Scheme 3b). Furthermore, hydrogen
peroxide could promote the reaction under a nitrogen atmo-
sphere, indicating that hydrogen peroxide might be produced
during the reaction (Scheme 3c). Only a small amount of the
target product was obtained under the heating conditions in
darkness, which showed that triplet oxygen (3O2) in air could
not directly participate in the reaction (Scheme 3d). Moreover,
the results of the on/off light-illumination experiment showed
that continuous illumination of light was essential for the
present reaction (see the ESI, Fig. S2†). The results of fluo-
rescence quenching (Stern–Volmer) experiments proved that
no energy transfer process between the excited species 1a* and
NMP or K2S2O8 took place in this transformation under
visible-light irradiation (see the ESI, Fig. S3 and 4†).
Based on the above experimental results and literature
reports,17,21 a possible mechanism is proposed, as shown in
Scheme 4. Initially, K2S2O8 may be decomposed to generate a
sulfate radical anion under visible-light irradiation.22
Meanwhile, 1a is irradiated by visible light to produce the
excited species 1a*,18a which acts as a photocatalyst and under-
goes an energy transfer process with triplet oxygen to give
singlet oxygen,17c,23 along with the regeneration of ground-
state 1a. Then, the sulfate radical anion and 1-methyl-2-pyrroli-
Finally, a gram-scale reaction was carried out between 1a
(6.25 mmol) and NMP, and the desired product 3a was finally
isolated in 76% yield (Scheme 2).
Several control reactions were carried out to elucidate the
possible reaction pathway. Firstly, the radical scavenger
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added to the
reaction system under the standard conditions, and the
present transformation was completely inhibited. Phenol,
which could effectively quench radical activity,19 was added to
the reaction system to quench the sulfate radical anion and/or
the hydroxyl radicals, and only a small amount of the product
was obtained. These results suggested that the reaction likely
Table 4 Scope of N-heteroarenesa
a Reaction conditions: 1 (0.4 mmol), 2a (1.5 mL), K2S2O8 (0.2 mmol),
air, rt, 24 h, 3 W blue LEDs. b K2S2O8 (0.2 mmol), 2a (2 mmol), H2O
(1.5 mL).
Scheme 3 Control experiments.
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Notes and references
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Scheme 4 Plausible mechanism.
dinone undergo an intermolecular hydrogen atom transfer to
obtain the radical 2a′ and hydrogen sulfate. The carbon radical
intermediate 2a′ selectively attacks the C3-position of 1a to
produce intermediate A, which may undergo a 1,2-hydrogen
shift to give a carbon radical B. Intermediate A or B is further
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single electron transfer. Finally, the desired product 3a is
obtained after dehydrogenation. Hydrogen peroxide produced
during the reaction may undergo homolytic cleavage24 under
visible-light irradiation to generate hydroxyl radicals (E° = 2.7
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V).25 In addition, intermediate HOO• may extract hydrogen
from 2a to produce H2O2.
Conclusions
In summary, a mild method for the direct C(sp3)–H/C(sp2)–H
coupling between γ-lactams/amides and N-heteroaromatics
through K2S2O8 catalysis has been developed. A diverse range
of N-heteroaromatics and γ-lactams/amides could be employed
at ambient temperature by using eco-friendly H2O as the reac-
tion medium. The new K2S2O8 catalytic mechanism was inves-
tigated with control experiments. Furthermore, it may be
worthwhile to provide a reasonable method for medicinal syn-
thesis in the near future.
Conflicts of interest
There are no conflicts to declare.
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We are grateful to the Natural Science Foundation of China
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