Photo-Mediated Decarboxylative Ketonization of Atropic Acids with Sulfonyl Hydrazides: Direct Access to β-Ketosulfones

Article abstract of DOI:10.1002/adsc.201901525

An efficient formation of synthetically and biologically relevant β-ketosulfones via a photo-mediated decarboxylative ketonization of atropic acids was disclosed. The approach features metal-free conditions, good functional group compatibility, readily available starting materials and the use of ubiquitous dioxygen as both oxygen source and oxidant. Furthermore, mechanistic studies reveal that the decarboxylative ketonization reaction proceeds via a radical mechanism and may involve a radical chain reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.201901525

Title: Photo-Mediated Decarboxylative Ketonization of Atropic Acids
with Sulfonyl Hydrazides: Direct Access to β-Ketosulfones
Authors: Jie Chen, Zoe G. Allyson, Jing-Rui Xin, Zhi Guan, and Yan-
Hong He
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901525
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Photo-Mediated Decarboxylative Ketonization of Atropic Acids
with Sulfonyl Hydrazides: Direct Access to β-Ketosulfones
Jie Chena, Zoe G. Allysonb, Jing-Rui Xina, Zhi Guan a*, and Yan-Hong He a*
a
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering,
Southwest University, Chongqing 400715, China
[E-mails: heyh@swu.edu.cn (for Y.-H. He); guanzhi@swu.edu.cn (for Z. Guan)]
Department of Chemistry, College of Saint Benedict and Saint John’s University, Collegeville MN, 56321 USA
b
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
synthesize various ketones (I, Scheme 1).[4d] The
Abstract: An efficient formation of synthetically and
biologically relevant β-ketosulfones via a photo-mediated other is photo-mediated radical addition/decarboxy-
decarboxylative ketonization of atropic acids was
disclosed. The approach features metal-free conditions,
good functional group compatibility, readily available
starting materials and the use of ubiquitous dioxygen as
both oxygen source and oxidant. Furthermore, mechanistic
studies reveal that the decarboxylative ketonization reaction
proceeds via a radical mechanism and may involve a
radical chain reaction.
lation, i.e., the photocatalytic process first generates a
free radical, which undergoes an addition reaction
with an unsaturated acid to form a reactive
intermediate, resulting in decarboxylation. For
instance, in 2017, Noöl and co-workers reported
photo-mediated decarboxylative difluoromethylation
of cinnamic acids with ethyl bromodifluoroacetates
as CF2 sources (II, Scheme 1).[5b] Compared to
photocatalytic direct decarboxylation, the photo-
mediated radical addition/decarboxylation process is
less explored but is equally important in providing
valuable products.
Keywords: Photocatalysis; Synthetic methods; Decarboxy-
lative ketonization; β-Ketosulfones; Atropic acids
Decarboxylative reaction has received considerable
attention as a powerful synthetic technique in
contemporary chemistry, owing to its inherent
advantages, such as readily available carboxylic acid
materials, step-economy, high efficiency and
nontoxic traceless byproducts.[1] Furthermore, the
decarboxylation reaction in organic conversion also
has the advantage that the starting molecule is
synergistically activated by a carboxylic acid motif,
which can be removed from the target molecule
during a given conversion.[2] To date, in addition to
traditional metal-catalyzed decarboxylative strategies,
photo-mediated decarboxylative functionalization of
carboxylic acids has shown good prospects for
organic synthesis on account of its mildness,
cleanliness and sustainability.[3] In this field, various
types of carboxylic acids, including -keto acids,[4]
unsaturated carboxylic acids[5] and aliphatic
carboxylic acids[6] have been utilized to generate
valuable fine chemicals. These decarboxylation
processes can be divided into two types: One is
photo-mediated direct decarboxylation, i.e., the
carboxylic acid directly eliminates CO2 under
photocatalysis, thereby providing a highly reactive
radical species for further reaction. For example, in
2015, Fu and co-workers reported a photoinduced
direct decarboxylative 1,4-addition of α-keto acids to
Scheme 1. Representative examples of two types of photo-
mediated decarboxylative reactions.
As an important sulfur-containing motif, β-ketosul-
fone not only possesses useful biological and phar-
maceutical properties,[7] but also is widely used in
various transformations.[8] Therefore, the synthesis of
β-ketosulfones has attracted great interest from
chemists over the past few decades. The conventional
methods for the formation of β-ketosulfones mainly
focus on the direct alkylation of sodium sulfinates (I,
Scheme 2).[9] Nevertheless, these methods suffer from
1
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
limitations related to pre-functional substrates,
elevated temperatures and prolonged reaction times,
which make them unappealing, especially for large
scale preparation. In recent years, the difunctionali-
zation of alkenes or alkynes have been extensively
found to achieve β-ketosulfones (II, Scheme 2).[10]
For example, in 2014, Yadav and co-workers
demonstrated
an
AgNO3-catalyzed
synthetic
approach toward β-ketosulfones with sodium alkenes
sulfinates.[10h] In 2016, Yang and co-workers
developed the visible-light-induced oxysulfonylation
of alkenes with sulfinic acids to produce β-keto
sulfones under inert conditions using eosin Y (EY) as
a photocatalyst and tert-butyl hydroperoxide (TBHP)
as the oxidant.[10i] In 2017, Ni and co-workers
reported a visible-light-induced oxysulfonylation
reaction between alkynes and sulfonyl chlorides with
Ir(dtbbpy)(ppy)2PF6 as a photocatalyst and HCl as an
essential additive.[10j] In addition, the decarboxylative
reaction, with its huge advantages, have also made
important contributions to the synthesis of β-
ketosulfones (III, Scheme 2)[11], such as, K2O2S8-
enabled cinnamic acid’s decarboxylation, manganese-
(III)-mediated arylpropiolic acid’s decarboxylation
and CuBr2-catalyzed atropic acid’s decarboxylation.
However, from an eco-friendly and practical point of
view, there is still a need to develop more
environmentally friendly and effective methods to
avoid the drawbacks of some established approaches,
including the inconvenience of inert gas conditions,
the use of hazardous radical initiators or toxic
reactants, pre-functioned starting materials, heavy
metal residues, and so on.
Scheme 2. Synthesis of β-ketosulfones.
Furthermore, several control experiments on reaction
parameters were performed and the results were
compiled in Table 1. Replacing KI with KBr resulted
in a greatly reduced yield (entry 2, Table 2),
indicating that the role of KI is critical. In the absence
of oxygen, Fl, visible light, KI or base, the desired
reaction hardly happened, clearly confirming that all
of these factors are essential to the formation of 3aa
(entries 3-6 and 8, Table 1). Additionally, when air
was used instead of oxygen, the reaction gave a lower
yield (entry 7, Table 1).
a)
Table 1. Control experiment.
To meet this challenge, we envisioned whether β-
ketosulfones can be produced directly from readily
available atropic acids using moisture-insensitive and
green sulfonyl hydrazides[12] as a superior sulfur
source and dioxygen[13] as a natural oxygen source
under metal-free photoredox conditions (IV, Scheme
2). To the best of our knowledge, visible-light-
mediated decarboxylative functionalization of atropic
acids remains unexplored. Fortunately, we success-
fullly implemented this original design concept.
Herein, we report this novel and efficient protocol for
the formation of β-ketosulfones via photo-mediated
decarboxylative ketonization of atropic acids, using
an inexpensive and readily available organic dye as
the photocatalyst and molecular oxygen as a clean
oxygen source and oxidant.
In our initial investigation, the reaction of atropic
acid (1a) and benzene sulfonyl hydrazide (2a) was
chosen as a model reaction. After systematical scre-
ening and optimizing various parameters that affect
the reaction, the optimal standard conditions for the
reaction were determined as follows: 3 mol%
fluorescein (Fl) as a photocatalyst, 1.0 eq. of
NaHCO3 as a base, 1.0 eq. of KI as an additive, molar
ratio of substrates 1a : 2a = 1: 1.5, and MeCN/H2O
(5.5 : 1, v/v) as a mixed solvent under O2 atmosphere
with the irradiation of a 23 W compact fluorescent
lamp (CFL) at room temperature (rt) [for details, see
the Supporting Information (SI), Tables S1-S9].
Entry Variation from standard conditions
Yield b)
1
2
3
4
5
6
7
8
None
1 eq. KBr instead of 1eq. KI
Without KI
82%
13%
Trace
Trace
Trace
Trace
35%
Without NaHCO3
Without Fl
Without light
In the air atmosphere
In the argon atmosphere
Trace
a) Unless otherwise specified, all reactions were carried out
under standard conditions: 1a (0.2 mmol, 1 eq.), 2a (1.5
eq.), Fl (3 mol%), NaHCO3 (1eq.) and KI (1 eq.) in
MeCN/H2O (5.5:1, 1.56 mL) under 23 W CFL irradiation
and O2 atmosphere (O2 balloon) at rt. b) Isolated yield.
With the optimal conditions in hand, substrate scope
of the photo-mediated decarboxylative ketonization
reaction was then explored. As can be seen in
Scheme 3, various sulfonyl hydrazides are suitable
for the reaction to provide the corresponding products
smoothly. In general, electron-rich arylsulfonyl hydr-
azides worked well, while electron-deficient ones
underwent this transformation less efficiently (3ab-
3ae). Then, the steric effects were evaluated. It was
observed that the para- or meta- substituent did not
2
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
a)
exhibit obvious steric hindrance (3af and 3ag).
However, the ortho-substituted benzene sulfonyl
hydrazide only gave the product in a moderate yield
(3ah), and more sterically hindered mesitylene
sulfonyl hydrazide resulted in a poor yield even after
a prolonged reaction time (3ai). Notably, under
standard reaction conditions, halogen substituents
such as F and Cl groups exhibited good tolerance,
which is difficult to achieve in many transition-metal-
participated reactions (3aj and 3ak). In addition,
alkyl sulfonyl hydrazide, polyarylate sulfonyl
hydrazide and heteroaryl sulfonyl hydrazide were
also amenable to this protocol, providing correspond-
ing products in moderate to good yields (3al-3an).
Encouraged by these promising results, the effect of
various substituents of atropic acids on the reaction
was tested using benzene sulfonyl hydrazide as the
counterpart. Gratifyingly, different atropic acid
derivatives with an electron-donating or electron-
withdrawing group on the aromatic ring were feasible
in this transformation, affording the corresponding
products in good to excellent yields (3ba-3fa).
Among them, 4-Cl-phenylsulfonyl hydrazide showed
the best reactivity, giving 3ba in 91% yield within 10
h. The disubstituted atropic acid also performed
pretty well and gave the corresponding 3ga in 77%
yield. However, the developed reaction is not
applicable to -alkyl substituted acrylic acids (3ha
and 3ia). Finally, it is worth mentioning that due to
the appealing applications of fluorine-containing
compounds in pharmaceutical and agrochemical
fields,[14] this protocol is of great significance in
providing good yields of β-ketosulfone products with
fluorine-containing groups in the benzene rings (3ae,
3ak, 3ca and 3fa).
Unless otherwise specified, reaction conditions: 1 (0.2
mmol), 2 (0.3 mmol), Fl (3 mol%), NaHCO3 (1 eq.) and KI
(1 eq.) in MeCN/H2O (5.5:1, 1.56 mL) under 23 W CFL
and O2 atmosphere (O2 balloon) at rt. Isolated yield. 2k
(3 eq.) was employed.
b)
The feasibility of obtaining β-ketosulfones from
sodium sulfinates through a similar photo-mediated
decarboxylative ketonization process was also
examined (Scheme 4). Under the above-optimized
conditions, sodium sulfinate (4) was found to be less
effective than sulfonyl hydrazide (2a), affording the
product 3aa in 60% yield.
Scheme 4. Photocatalytic decarboxylative oxysulfonyla-
tion of atropic acid with sodium sulfinate.
In order to elucidate the reaction mechanism, a series
of experiments were carried out. It was found that the
commonly used radical scavenger 2,2,6,6-tetra-
methylpiperidine N-oxide (TEMPO) completely
inhibited the model reaction under standard
conditions (I-a, Scheme 5), indicating that the
reaction may involve a radical pathway. When the
radical acceptor 1,1-diphenylethylene was added to
the model reaction under standard conditions, its
capture of aryl sulfonyl radical (compound 5) was
detected by high resolution mass spectrometry
(HRMS) (I-b, Scheme 5), suggesting that the sulfonyl
radical may be an intermediate of the reaction. Next,
to understand the role of dioxygen in the reaction,
additional experiments were performed. Firstly, the
involvement of singlet oxygen (1O2) was excluded as
photocatalyst Fl does not produce O2.[15] And in the
presence of O2 quencher bicyclo(2,2,2)-1,4-diaza-
1
1
octane (DABCO), the desired reaction still proceeded
well with almost undiminished yield of product 3aa,
further supporting that the reaction does not involve
1O2 (I-c, Scheme 5). Then, in the presence of the
radical scavenger 2,6-di-tert-butyl-4-methylphenol
(BHT), the desired reaction was nearly shut down and
the capture of the superoxide radical anion (O2˙-) was
observed by HRMS (BHT-OOH, 6), so it can be
inferred that O2˙- is involved in the reaction (I-d,
Scheme 5). Moreover, an isotope labeling experiment
was conducted under standard conditions using
MeCN/H2O18 as the mixed solvent, and only O-3aa
16
rather than 18O-3aa was detected by HRMS,
suggesting that the carbonyl oxygen atom of β-keto-
sulfone comes from the dioxygen instead of water (II,
Scheme 5). To sum up, dioxygen probably serves as
an important oxidant and the source of oxygen in the
reaction. To confirm whether the reaction involves a
chain process, the light on/off experiment was
performed. As can be seen from Figure 1, the product
Scheme 3. Substrate scope. a)
3
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
was generated throughout the light on/off process,
indicating that a radical chain process is probably
concomitant with the photoredox process.
electron transfer (SET) to generate a highly reactive
radical cation A along with the formation of a
reduced Fl species (Fl˙-). Fl·- can be oxidized to Fl by
oxygen, while producing superoxide radical anion
(O2˙-). A deprotonates and is oxidized by dioxygen to
produce the key sulfonyl radical B with the release of
nitrogen (for the possible detailed mechanism
towards intermediate B, please see Scheme S1). The
addition of B to the double bond of the in-situ-formed
carboxylate from 1a and base furnishes the radical
intermediate C. C is captured by dioxygen to
generate the radical intermediate D. With the possible
reduction of iodine anion (Eox = 0.627 V vs. SCE,
Table S10) and subsequent protonation, intermediate
D is transformed into intermediate E, while the iodine
anion is oxidized to the iodine radical. Besides, the
intermediate E may also be obtained from the
coupling of radical C and HO2˙ which is produced by
the combination of O2˙- and the proton from radical
cation A. Finally, intermediate E is protonated to
afford intermediate F, which is converted to the final
β-ketosulfone 3aa via decarboxylative dehydration.
On the other hand, the iodine radical may directly
abstract an electron from 2a, contributing to the
radical chain process.[10f]
In addition, the UV-vis experiments showed that
photocatalyst Fl has a maximum absorption (λmax) at
493 nm, while substrates 1a and 2a have no
absorption in the visible light range (Figures S1 and
S2). Next, the fluorescence quenching experiments of
Fl with KI and benzene sulfonyl hydrazide 2a were
performed (Figures S10-12). It was found that the
addition of KI did not quench the fluorescence of Fl,
but enhanced the fluorescence slightly. This may be
due to the slight fluorescence that KI can emit (Figure
S13). Instead, 2a can significantly quench the
fluorescence of Fl (quenching constant: 475.81 M-1,
Figure S12), indicating that light-induced electron
transfer may occur between 2a and the excited Fl.
Scheme 5. Mechanistic exploration experiments.
Scheme 6. Plausible mechanism.
In summary, motivated by the original design concept
of exploring a new protocol for β-ketosulfone synthe-
sis by means of photo-mediated decarboxylative
process, we have successfully developed this novel
and efficient method. By using this method, a range
of β-ketosulfones can be obtained directly from
commercially available atropic acids and sulfonyl
hydrazides using dioxygen as a convenient and clean
oxygen source and an oxidant under metal-free
conditions. It is worth noting that this is the first
example of applying atropic acids to photo-mediated
decarboxylative reaction with an aim of generating
valuable chemicals. Because of the broadly
appreciated importance of β-ketosulfones in the
biological and pharmaceutical fields, this discovery is
expected to find a wide range of applications in the
Figure 1. Light on/off experiments.
On the basis of the above experiments, a plausible
mechanism for the reaction was proposed in Scheme
6. Initially, under visible light irradiation, the
photocatalyst Fl is excited to Fl* (Eox = +0.87 V vs.
SCE).[16] Then, Fl* oxidizes benzene sulfonyl hydra-
zide 2a (Eox = -0.328 V vs. SCE, Table S10) by single
4
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
future and to contribute to further research of β-
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Experimental Section
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General experimental procedure for the synthsis of β-
ketosulfones 3. A round bottom flask (10 mL) equipped
with a magnetic stir bar was charged with an atropic acid 1
(0.2 mmol, 1.0 eq.), a sulfonyl hydrazide 2 (0.3 mmol, 1.5
eq.), Fl (0.006 mmol, 3 mol%), NaHCO3 (0.2 mmol, 1.0
eq.), KI (0.2 mmol, 1.0 eq.) and MeCN/H2O (v/v = 5.5:1,
1.56 mL). The mixture was irradiated by a 23 W CFL
(Philips, 220 V, 50 Hz, 400-780 nm, no any filters, placed
approximately 2.5 cm from the flask) under a balloon of O2
at rt. After completion of the reaction (monitored by TLC),
the reaction mixture was concentrated under reduced
pressure, and the resulting residue was treated with water
and extracted with EtOAc. The combined organic layers
were washed with brine, dried over anhydrous Na2SO4,
and concentrated under reduced pressure. The crude
product was purified by silica gel column chromatography
using petroleum ether/ethyl acetate as eluent to give the
desired product β-ketosulfones 3 (in 15-91% yields).
The same procedure was applied for the synthesis of
1-phenyl-2-(phenylsulfonyl)ethan-1-one (3aa) from 2-
phenylacrylic acid (1a) and sodium benzene sulfinate (4)
except using 4 instead of sulfonyl hydrazide (2), and 3aa
was obtained in 60% yield.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21672174 and 21977084), and
the Natural Science Foundation of Chongqing (No. cstc2019jcyj-
msxmX0297)
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10.1002/adsc.201901525
Advanced Synthesis & Catalysis
UPDATE
Photo-Mediated Decarboxylative Ketonization of
Atropic Acids with Sulfonyl Hydrazides: Direct
Access to β-Ketosulfones
Adv. Synth. Catal. 2019, Volume, Page – Page
Jie Chen, Zoe G. Allyson, Jing-Rui Xin, Zhi
Guan*, and Yan-Hong He*
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