Single-atom-nickel photocatalytic site-selective sulfonation of enamides to access amidosulfones

Article abstract of DOI:10.1039/d1gc00379h

Single-atom photocatalysis as an important organic transformation strategy has received increasing attention, with the performances largely depending on the design of catalysts. This protocol involves initially the fabrication of a single-atom photocataly

Full text of DOI:10.1039/d1gc00379h

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Single-atom-nickel photocatalytic site-selective
sulfonation of enamides to access amidosulfones†
Cite this: Green Chem., 2021, 23,
2756
Jianjing Yang,‡a Zongzhao Sun,‡b Kelu Yan, a Haozhe Dong,a Hongyan Dong,a
a
Jiakai Cui,a Xutao Gong,a Shilin Han,a Limin Huang *b and Jiangwei Wen
*
Single-atom photocatalysis as an important organic transformation strategy has received increasing atten-
tion, with the performances largely depending on the design of catalysts. This protocol involves initially
the fabrication of a single-atom photocatalyst Ni/TiO2 for the visible-light-induced site-selective sulfona-
tion of enamides to give amidosulfones with 36 examples up to 99% yield. The experimental results show
that this single-atom photocatalyst Ni/TiO2 can achieve site-selective sulfonation of enamide to construct
α-amidosulfones and β-propionamidosulfones under visible light. Importantly, such a single-atom photo-
catalysis-based synthesis system exhibits favorable recyclability, high turnover number (up to 18 963),
excellent tolerance of functional groups, and can be easily scaled up with good efficiency.
Received 1st February 2021,
Accepted 12th March 2021
DOI: 10.1039/d1gc00379h
rsc.li/greenchem
Recently, Manolikakes and co-workers10 successively
realized the oxysulfonylation and sulfonylation of enamides
Introduction
Sulfones, as one of the versatile building blocks, have been mediated by stoichiometric Mn(OAc)3 and Fe(NO3)3·9H2O
namely “chemical chameleons” and “pluripotent” due to their (Scheme 1c).
ability to participate in a variety of chemical processes.1
Visible-light photoredox catalysis has proven to be a power-
Among the different types of sulfones,2 the amidosulfone ful and environmentally friendly strategy for the sulfonation of
motif represents an important framework and is present in olefins. The groups of Yu11 and Zhang12 have realized the sul-
various pesticides, functional materials, and pharmaceutically fonation of secondary and tertiary enamides via visible-light
active molecules.3 Selected examples of active pharmaceutical photoredox catalysis to access β-amidovinyl sulfones. Although
ingredients containing the amidosulfone scaffold include sul- a variety of homogeneous catalytic methods have been dis-
bactam4 or tazobactam5 (as a beta-lactamase inhibitor in acute closed based on the sulfonation of enamides to access
sinusitis), Syn-1390 6 (human leukocyte elastase inhibitors), β-amidovinyl sulfones in recent years, the traditional homo-
bicalutamide7 (anticancer agent), and letaxaban8 as the anti- geneous catalysis strategies can only achieve the β-sulfonation
coagulant (Fig. 1). Consequently, substantial efforts have been of enamine to access alkenyl sulfone and β-alkyl sulfone, and
devoted to discovering efficient synthesis and functionali- these methods can encounter some intrinsic challenges such
zation strategies for amidosulfones.
as high cost, non-recyclable catalyst, poor selectivity, and
During the last decades, the construction of biologically harsh reaction conditions. To overcome these shortcomings,
active amidosulfone frameworks by powerful transition metal- the development of visible-light-induced heterogeneous cata-
mediated homogeneous strategies has attracted the interest of
many synthetic organic chemists. For example, the group of
Loh9 has reported Pd- and Cu-catalyzed direct C–H functionali-
zation of enamides with sulfonyl chlorides and with
sulfur dioxide as the sulfone source (Scheme 1a and b).
aInstitute of Medicine and Materials Applied Technologies, College of Chemistry and
Chemical Engineering, Qufu Normal University, Qufu, Shandong 273165,
P. R. China. E-mail: wenjy@qfnu.edu.cn
bDepartment of Chemistry, Southern University of Science and Technology, Shenzhen,
Guangdong, 518055, China. E-mail: huanglm@sustech.edu.cn
†Electronic supplementary information (ESI) available: Experimental procedure,
characterization data, and copies of NMR spectra. See DOI: 10.1039/d1gc00379h
‡These authors contributed equally to this work.
Fig. 1 Amidosulfone-based bioactive molecules.
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Morphological and structural characterization
The morphological and structural characterization of the
single-atom-nickel-containing photoredox catalyst Ni/TiO2 was
performed separately by FT-IR and UV spectroscopy, induc-
tively coupled plasma mass spectrometry (ICP-MS), scanning
electron microscopy (SEM), X-ray photoelectron spectroscopy
(XPS), and high-angle annular dark-field scanning electron
microscopy (HAADF-STEM) (Fig. S1† and Fig. 2). Firstly, the
FT-IR spectra of the dried Ni/TiO2 samples were recorded to
investigate the appropriate functional groups during the for-
mation stages of the photocatalysts. As can be seen from
Fig. 2f, two new peaks of Ni/TiO2 were observed at 1395 cm−1
and 1639 cm−1, which can belong to the C–N fingerprint
absorption peaks, indicating that Ni may be anchored on the
surface of TiO2 in the form of the Ni–N bond. To further
understand the chemical properties and loading mechanism,
Ni/TiO2 was investigated by X-ray photoelectron spectroscopy
(XPS). As shown in Fig. S1a,† the valence state of nickel in Ni/
TiO2 is NiII. Two different forms of pyrrolic-N and pyridinic-N
can be observed in the N 1s orbital spectrum, which may be
derived from formamide (Fig. S1b†). Besides, there is no
change except that the C 1s orbital drops by 0.6 eV at 288.6 eV
(belonging to C–N bonds) by comparing the binding energies
of Ti 2p, O 1s, and C 1s orbitals (Fig. S1c and d†). The
decreased binding energy of the C–N bond indicates that it is
more conducive to the flow of photogenerated electrons to
nickel. Moreover, the UV spectra indicate that Ni/TiO2 exhibi-
ted an excellent absorption range for visible light (Fig. 2g).
Subsequently, the ICP-MS measurement was performed to
determine the content of nickel in Ni/TiO2, and the result
showed that the content of Ni was 1.1 wt% (Table S1†).
Importantly, the morphology of the Ni/TiO2 photocatalyst was
characterized by HAADF-STEM imaging, showing uniformly
sized nanoparticles (Fig. 2b and e). Afterward, the energy dis-
persive X-ray spectroscopy (EDS) mapping was performed for
Ni/TiO2, and the results confirmed that Ni is uniformly distrib-
uted on the surface of TiO2 (Fig. 2c and d). More importantly,
in the aberration-corrected image with atomic resolution
(Fig. 2a), the bright spots with sizes of ∼0.10 nm can corres-
pond to the atomic dispersion of nickel on Ni/TiO2.
Accordingly, based on the above experiments, it can be con-
firmed that the photocatalyst Ni/TiO2 with an excellent visible
light absorption range has been successfully prepared, in
which nickel is uniformly distributed in a single atomic form.
Scheme 1 Representative example sulfonation of enamide.
lysts has received much attention from chemists. Therefore,
various kinds of heterogeneous photoredox catalysts have been
successfully fabricated for a variety of organic transformations in
recent years.13 However, a visible light-mediated single-atom
catalyst for organic transformations has rarely been reported.14
Therefore, it is highly desirable to develop efficient, recyclable,
low consumption, and environmentally benign heterogeneous
catalysts for photocatalysis reactions to access biologically active
compounds. In the continuation of our efforts in developing a
new type of visible-light catalyst15 and constructing sulfone-con-
taining compounds,16 herein, we wish to report an efficient and
readily available visible-light-driven single-atom photocatalyst Ni/
TiO2 for the selective sulfonation of enamides to access
α-amidosulfones and β-propionamidosulfones (Scheme 1f).
Importantly, the as-prepared photocatalyst containing single-
atom nickel can achieve site-selective sulfonation of enamides by
controlling the position of N in the substrate. In terms of sub-
strate scope, various enamides and sulfinic acids can be compa-
tible with the present protocol, generating the desired products
with yields up to 99%. It is worth noting that the visible light-
promoted single-atom catalytic sulfonation of enamides can
easily achieve the amplification reaction with a high turnover
number (TON: up to 18 963) and excellent recyclability.
Investigation of photocatalytic performances
To explore the performances of the developed single-atom
photocatalyst Ni/TiO2, the visible light-promoted sulfonation
of enamides with sulfinic acid was carried out in air at room
Results and discussion
Synthesis of Ni/TiO2
Typically, NiCl2·6H2O (35.7 mg) and TiO2 (P25: 500 mg) were temperature, aiming to obtain a highly bioactive drug, block
added into formamide (15.0 mL) under sonication for 10 min. α-amidosulfones. Initially, N-vinyl-2-pyrrolidinone (1a) and
Then the mixture was transferred into a 20 mL Teflon-lined p-methylbenzenesulfinic acid (2b) were chosen as the model
autoclave and heated at 180 °C for 12 h. The resulting black substrates to optimize the key reaction conditions, with the
product was washed with deionized water and ethanol 3 times results summarized in Table 1. By optimizing the key reaction
and dried at 60 °C overnight.
parameters, the best results can be obtained by irradiating the
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Fig. 2 Characterization of Ni/TiO2. (a) Aberration-corrected high resolution electron microscopy images; (b) HAADF-STEM images with EDS
mapping of (c) Ni, and (d) Ti elements distributed in the same region; (e) SEM images; (f) FT-IR spectra; (g) UV spectra; (h) XRD patterns.
single-atom catalyst Ni/TiO2 at room temperature in the
solvent of CH3CN under visible-light for 3.5 h, and the desired
product 3ab can be obtained in 97% yield (Table 1, entry 1).
Table 1 Optimization of the reaction conditionsa
Accordingly, the control experiments confirm that both the
catalyst and light are essential conditions for this reaction
(Table 1, entries 2 and 3). Moreover, the presence of molecular
sieves can help increase the reaction yield due to the ability to
further disperse the catalyst and remove water from the reac-
tion system (Table 1, entries 4 and 5). Further increasing or
Entry
Deviation from standard conditions
Yieldb (%)
decreasing the concentration of the photocatalyst has no sig-
nificant effect on the yield of the reaction (Table 1, entries 6
and 7, Ni: 1.1 wt%). In particular, the desired product has not
been observed when simply mixing NiCl2·6H2O and TiO2 as
the catalyst instead of Ni/TiO2, which indicates that there is no
electron transfer between NiCl2 × 6H2O and TiO2 (Table 1,
entry 8). Nevertheless, either Ni/CN (Ni loaded on CN) or CNH
(coupling carbon nitride with hemin) as a photocatalyst has
not obtained the desired products (Table 1, entries 9 and 10).
These results suggest the superiority and necessity of introdu-
cing single-atom nickel on TiO2 to convert light energy into
electrons for subsequent reactions. Subsequently, the investi-
gation results of the reaction solvent confirmed that aceto-
nitrile was the best choice (Table S2,† entries 1–9). Shortening
1
2
3
4
5
6
7
8
None
Without Ni/TiO2
Without light
Without 4 ÅMS
H2O (4.0 equiv.), without 4 ÅMS
0.625 mg mL−1 instead of 1.25 mg mL−1
2.5 mg mL−1 instead of 1.25 mg mL−1
NiCl2·6H2O (5 mmol%) + TiO2
Ni/CN instead of Ni/TiO2
CNH instead of Ni/TiO2
N2 instead of air
98 (97)c
n.d.
Trace
78
40
85
98
Trace
n. d.
n. d.
Trace
55
9
10
11
12
Red LED: 630 nm
a Standard conditions: 1a (0.5 mmol), 2b (1.0 mmol), Ni/TiO2 (2.5 mg),
4 ÅMS (40.0 mg), anhydrous CH3CN (2.0 mL), air, blue LED (460 nm),
r.t., 3.5 h, “n.d.” = not detected. b GC yield, 2-phenylphenol as the
internal standard. c Isolated yield.
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Table 2 Synthesis of α-amidosulfonesa,b
the reaction time is detrimental to the yield of the desired
product (Table S2,† entries 10 and 11). The desired product
3ab was able to obtain a yield of 82% when the equivalent stoi-
chiometric amounts of 1a and 2b were executed in the reaction
system (Table S2,† entry 12). Besides, after replacing the
ambient conditions with a nitrogen atmosphere, only trace
amounts of the products could be obtained, indicating air can
also play a crucial role in the Ni/TiO2-photocatalytic reactions
(Table 1, entry 11). To our delight, the desired product 3ab can
also be obtained in a yield of 55% when the LED (460 nm)
lamp was replaced by red light (630 nm) under the standard
conditions (Table 1, entry 12). Notably, it is otherwise imposs-
ible to achieve these products by the conventional photo-
catalytic reactions of organic synthesis.
To test the reusability of this photocatalyst of single-atom
Ni-containing Ni/TiO2, repeated experiments were carried out
under the standard reaction conditions. As shown in Fig. 3, a
slight decrease of catalysis activity to 84% was observed after
five cycles. Meanwhile, the content of Ni in the reaction solu-
tion was measured by ICP-MS, and the results showed that
only 0.1–0.5 ppm leaching of Ni was detected in the reaction
solution. These results indicate that the decrease of the yield
of the product might be mainly attributed to the loss of the
catalyst during centrifugation (Table S1†). However, the overall
activity of the photocatalyst remains high. These results con-
vincingly confirm that the present protocol can serve as an
efficient, site-selective, and recyclable heterogeneous photoca-
talysis strategy to synthesize α-amidosulfones with desirably
high catalysis efficiencies.
a Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), 4 ÅMS (40.0 mg),
anhydrous CH3CN (2.0 mL), air, blue LED (460 nm), r.t., and 3.5 h.
b Isolated yields. c 12 h. d Sodium sulfinate, HCl (3.0 equiv.).
Substrate scope
With the optimized conditions in the hand, the scope and
limitations of the Ni/TiO2-photocatalytic reactions of various
sulfinic acids (2) were explored with N-vinyl-2-pyrrolidinone
(1a) to yield α-amidosulfones (Table 2). To our delight, the
investigation of different arylsulfinic acids suggests that all of
the substrates bearing electron-rich and electron-deficient
groups can be compatible in this photocatalytic protocol to
give the corresponding products in excellent yields (3aa–3aj).
In particular, various functional groups such as Cl, Br, CF3,
CCl3, and acetamido are more compatible with this protocol to
access the corresponding products (3af–3aj), which could be
further functionalized to synthesize other important complex
compounds.
Moreover, the yields were not negatively affected when aryl-
sulfinic acid was substituted by Cl, Br, or Me at the o- or
m-position of the benzene ring (3hb–3ib). To our delight,
naphthalene-2-sulfinic acid and thiophene sulfinic acid were
used under standard conditions, and the corresponding pro-
ducts 3ao and 3ap can be obtained in a yield of 88% and 60%,
respectively. Unfortunately, even in the presence of a stoichio-
metric amount of hydrochloric acid, sodium methylsulfinate
cannot result in the desired product. Subsequently, several
N-ethylene substituted olefins were also investigated with
p-methylbenzenesulfinic acid (2b) to obtain α-amidosulfones.
Product 3ba can be obtained in an equivalent yield when
N-vinylcaprolactam instead of N-vinyl-2-pyrrolidinone was used
as a substrate. It is noteworthy that the desired products, 3ca,
can be obtained in a moderate yield when 9-vinylcarbazole was
used under standard conditions. Unexpectedly, N-ethenyl-N-
methylacetamide and N-vinylacetamide also have high selecti-
vity and products 3da and 3ea can be obtained in yields of
Fig. 3 Recycle experiments.
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95% and 92%, respectively. For these surprising results, we
speculate that the reason may be due to the lone pair electrons
of N changing the electron cloud density of the α-position of
alkene, which is more favorable for the attack of sulfone
radicals.
Subsequently, to verify this hypothesis, acylamide was used
as the substrate to react with p-methylbenzenesulfinic acid
(2b) under standard conditions (Table 3). To our delight, the
β-sulfonated product of acylamide (3fa) can be obtained with
high selectivity, which proved our conjecture. Under estab-
lished conditions, both electron-donating and electron-with-
drawing aryl sulfonic acid can react with N,N-dimethyl-
acrylamide to obtain the corresponding β-sulfonated products
in excellent yield (3fb–3fi). Moreover, 3,5-dichlorobenzenesulfi-
nic acid and naphthalene-2-sulfinic acid could be tolerated
under the model reaction conditions to obtain the desired pro-
ducts 3fj and 3fn in a yield of 68% and 66%, respectively.
Other types of enamides, such as N,N-diethylprop-2-enamide,
N-phenylprop-2-enamide, and acrylamide, can also be adapted
to the present protocol with excellent yields (3ga–3ha).
Moreover, unsaturated internal olefins can also obtain the
desired products with high selectivity and excellent yields (3ia–
3ka). Interestingly, the β-sulfonated product of diphenyl(vinyl)
phosphane can also be obtained in a moderate yield (3la).
Unfortunately, the desired product 3ma was not observed
when ethenylsulfanylbenzene was implemented under the
present protocol. In particular, we unexpectedly found that
this kind of self-colorless compound can quickly display a
variety of different colors under short-time ultraviolet
irradiation (Fig. 4b). The UV spectroscopy results also con-
firmed that obvious absorption peaks can be observed in the
Fig. 4 (a) UV results at 254 nm irradiation for 2 min in CH3CN. (b) TLC
results (irradiation at 254 nm for 2 min).
visible light absorption region after these compounds were
excited by UV light (Fig. 4a). To explore the luminescence
mechanism, the 1H NMR spectra of 3fa before and after illumi-
nation were firstly compared and analyzed, and it was found
1
that the H NMR results did not change (Fig. S2†). Based on
the preliminary experimental results and the literature,17 we
speculate that the luminescence mechanism may be caused by
intramolecular electron transfer. These interesting results indi-
cate that these compounds may have potential applications in
organic solar cells to enhance light absorption and electron
transfer.
1a and 1f were selected as substrates for gram-scale experi-
ments to investigate the superiority of the visible-light-driven
single-atom photocatalyst Ni/TiO2 for the site-selective sulfona-
tion of enamides. As shown in Scheme 2, the desired products
3ab and 3fa can be obtained in excellent yield and high TON
when the reaction was performed using a small amount of the
catalyst system. These results indicate that this protocol is a
practical and effective strategy.
Table 3 Synthesis of β-propionamidosulfonesa,b
Mechanistic studies
To gain further insights into the single-atom catalyzed oxi-
dative coupling mechanism of enamide with sulfinic acids to
form amidosulfones, a series of control experiments were
carried out using the photocatalyst of Ni/TiO2 (Scheme 3).
Firstly, the active species trapping experiments were conducted
to clarify the contribution of different active species in the
reactions. It was found that the yields of the desired products
were drastically decreased after separately adding ammonium
oxalate (AO: scavengers for the hole), benzoquinone (BQ: for
O2•−), and NaN3 (for 1O2) under the standard conditions
a Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), 4 ÅMS (40.0 mg),
anhydrous CH3CN (2.0 mL), air, blue LED (460 nm), r. t., and 3.5 h.
b Isolated yields. c 12 h.
Scheme 2 Gram-scale experiments.
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of the carbonyl group may cause sulfone radical B to be more
inclined to attack the β-position of 1f to generate carbanion
intermediate E. Then, finally, intermediate E obtains one H+
from the system to produce the desired product 3fb (path B).
Conclusions
In conclusion, the as-prepared single-atom photocatalyst Ni/
TiO2 was discovered to exhibit superior heterogeneous activity
and high environmental stability for the site-selective sulfona-
tion of enamide to access α-amidosulfones and
β-propionamidosulfones at atmospheric pressure and room
temperature under visible light irradiation. Under the estab-
lished reaction conditions, various enamides and sulfinic
acids can be compatible with the present strategy, and the
desired products can be generated with high selectivity and in
excellent yields. Particularly, this single-atom nickel photo-
catalytic aerobic oxidation strategy can be easily implemented
as a gram-scale reaction with a high turnover number and
excellent recyclability. Further mechanism investigation and
applications of this single-atom nickel photocatalysis-based
synthesis methodology are underway in our laboratory.
Scheme 3 Mechanism studies. (a) Active species trapping experiments.
(b) and (c) Radical inhibition experiments.
(Scheme 1a). These results indicate that the singlet oxygen and
superoxide radicals were the major reactive species for the
selective sulfonation of enamides. Moreover, it was found that
the sulfonation of enamide was almost inhibited when 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) or 2,6-di-tertbutyl-4-
hydroxytoluene (BHT) was added into the present reaction
system, suggesting the reaction presumably adopted a radical
pathway (Scheme 3b–c).
Based on the above experiment results and previous
reports,11,12,18 a possible reaction mechanism (1a, 1f, and 2b as
the models) is proposed and outlined in Scheme 4. Initially,
under visible light irradiation, the photocatalyst Ni/TiO2 has a
stronger ability to separate photo-generated electron pairs and
holes. The one part of photo-generated electrons would reduce
Conflicts of interest
There are no conflicts to declare.
•−
molecular oxygen to produce O2 and singlet oxygen 1O2,
Acknowledgements
which can oxidize p-methylbenzenesulfinic acid 2b to form
sulfone radical B. The other part will be transferred to NiII to
obtain NiI, which can be oxidized by enamide 1a to generate
NiII and radical anion intermediate A. Then, intermediate A will
be quickly attacked by sulfone radical B to obtain carbanion
intermediate C. Finally, carbanion intermediate C can be proto-
nated to obtain the desired product α-amidosulfones 3ab (path
A). Also, acylamide 1f will be reduced by NiI to give radical
anion intermediate D. However, the electron-withdrawing effect
This work was supported by the National Natural Science
Foundation of China (No. 21902083), Natural Science
Foundation of Shandong Province (No. ZR2020QB130)
Shandong, P. R. China. College Students Innovation and
Entrepreneurship Training Project of Shandong Province (No.
S202010446066). Southern University of Science and
Technology (SUSTech) start fund through the Shenzhen
Peacock Talent program, Guangdong Innovative and
Entrepreneurial Research Team Program (No. 2016ZT06N532),
and Guangdong Provincial Key Laboratory of Materials for
Electric Power (2018B030322001). This work is also supported
by the Talent Program Foundation of Qufu Normal University
(No. 6132 and 6125). The authors thank Dr W. Wang and
Prof. J. He of SUSTech for their discussions on HAADF-STEM.
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