Visible-Light-Enabled Decarboxylative Sulfonylation of Cinnamic Acids with Sulfonylhydrazides under Transition-Metal-Free Conditions

Article abstract of DOI:10.1021/acs.orglett.6b01353

Decarboxylative cross-coupling reactions of cinnamic acids with sulfonylhydrazides were explored using oxygen as the sole terminal oxidant, realizing a conceptually novel technology for vinyl sulfone synthesis under the synergistic interactions of visible light irradiation, organic dye-type photocatalyst eosin Y, KI, and Cs₂CO₃ at room temperature.

Full text of DOI:10.1021/acs.orglett.6b01353

Letter
pubs.acs.org/OrgLett
Visible-Light-Enabled Decarboxylative Sulfonylation of Cinnamic
Acids with Sulfonylhydrazides under Transition-Metal-Free
Conditions
,†,‡
†
†
†
†
†
Shunyou Cai,* Yaohui Xu, Danling Chen, Lihuang Li, Qifa Chen, Mingqiang Huang,
and Wen Weng†
†Key Laboratory of Modern Analytical Science and Separation Technology of Fujian Province, School of Chemistry and
Environment, Minnan Normal University, Zhangzhou 363000, China
‡
Key Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking
University, Shenzhen 518055, China
*
S Supporting Information
ABSTRACT: Decarboxylative cross-coupling reactions of cinnamic acids with sulfonylhydrazides were explored using oxygen as
the sole terminal oxidant, realizing a conceptually novel technology for vinyl sulfone synthesis under the synergistic interactions
of visible light irradiation, organic dye-type photocatalyst eosin Y, KI, and Cs CO at room temperature.
2
3
7
isible-light-promoted decarboxylative cross-coupling reac-
tions have emerged as powerful synthetic technologies in
oxidants or transition metal catalysts. Some examples are
depicted in Scheme 1; Guo et al. disclosed vinyl sulfones
1
V
organic synthesis because of their potential advantages, e.g.,
readily available starting materials, simple operation, and
Scheme 1. Various Synthetic Routes to Vinyl Sulfones
Formation
nontoxic and easily removed byproduct (CO as the only
2
byproduct), bringing about a range of organic transformations
involving various C−N, C−F, and C−C bond formations. A
pioneering work by Lei et al. reported an inspirational amide
synthesis via visible-light-enabled decarboxylative amidation of α-
2
keto acids and amines. Synergistic combinations of photoredox
catalysis with transition metal catalysis were also shown to be
effective to afford a new alternative cross-coupling scenario, in
which simple, readily available carboxylic acid derivatives could
be systematically employed as coupling partners under mild3
reaction conditions; such activities have been regularly reported.
However, the formation of C−S via visible-light-promoted
decarboxylation coupling strategy remains an underdeveloped
process and continues to capture considerable attention from the
synthetic community.
synthesis via copper-catalyzed decarboxylative sulfonylation of
7a
cinnamic acids with sodium sulfinates. Other transition metal
Vinyl sulfones are known to be highly versatile encountered
7
b
7c
4
complexes and strong oxidants, e.g., Pd catalyst, I , and
motifs, which have found widespread applications in many
2
7d
PhI(OAc)2, were shown effective in similar scenarios (Scheme
a). Recently, I /TBHP-catalyzed decarboxylative sulfonylation
biological researches as activity-based protein profiling (ABPP)
5
1
probes and covalent protease inhibitors. Their important
2
of cinnamic acids and sulfonyl hydrazides was reported by Singh
utilities have encouraged considerable synthetic efforts to
develop direct, mild, and efficient protocols enabling their
preparations. Synthetically, previously described methodologies
for accessing vinyl sulfones are to take advantage of the oxidation
7e
et al. for synthesis of vinyl sulfones (Scheme 1b). However,
these established methodologies suffer from one or more
drawbacks, e.g., the use of unstable and expensive reagents,
harsh reaction conditions, and employment of transition metal
catalysts or strong oxidants. It remains highly desirable to
of the vinyl sulfides, β-elimination of seleno- or halo-sulfones, o6r
condensation of sulfonylacetic acids with aromatic aldehydes.
Alternatively, new, more efficient cross-coupling reactions of
sulfinate salts and vinyl halides, alkenyl boronic acids, alkenes, or
cinnamic acids were also achieved in the presence of strong
Received: May 9, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01353
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Identification of the Optimal Reaction Conditions
a
c
entry
2a (equiv)
PC (x mol %)
base (equiv)
solvent (v/v)
DMF
additive (equiv)
yield (%)
1
2
3
4
5
6
7
8
9
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.2
3.0
2.2
2.2
2.2
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (2)
eosin Y (1)
eosin Y (1)
eosin Y (1)
eosin Y (1)
eosin Y (1)
[Ru] (1)
Na CO (2.2)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
Kl (1.0)
trace
31
2
3
Cs CO (2.2)
DMF
2
3
Cs CO (2.2)
DMF/H O (8:1)
35
2
3
2
Cs CO (2.2)
DMF/H O (3:1)
24
2
3
2
Cs CO (2.2)
DMF/H O (15:1)
42
2
3
2
CsOAc (2.2)
Na PO (2.2)
DMF/H O (15:1)
30
2
DMF/H O (15:1)
trace
NR
decomp
36
3
4
2
K CO (2.2)
DMF/H O (15:1)
2
3
2
NaOH (2.2)
DBU (2.2)
DMF/H O (15:1)
2
10
11
12
13
14
15
16
DMF/H O (15:1)
2
CS CO (3.0)
DMF/H O (15:1)
73
2
3
2
Cs CO (3.5)
DMF/H O (15:1)
70
2
3
2
Cs CO (3.0)
ACN/H O (15:1)
56
2
3
2
Cs CO (3.0)
DMA/H O (15:1)
58
2
3
2
Cs CO (3.0)
EtOH/H O (15:1)
trace
71
2
3
2
b
2.2
Cs CO (3.0)
DMF/H O (15:1)
2
3
2
a
b
c
Eosin Y was used. [Ru] = Ru(bpy) CI ·6H O. Yield of isolated product
3
2
2
discover new and straightforward approaches for the facile
preparation of vinyl sulfones, with use of less catalysts or oxidants,
simpler substrate structures, or milder reaction conditions.
Leveraging recent remarkable achievements in uncovering
robust synthetic reactions that relied on visible-light-promoted
(see the Supporting Information). Other attempts employing
acetonitrile (ACN), dimethylacetamide (DMA), or EtOH in
place of DMF in the mixed solvent were again confirmed to
negatively influence the reaction efficiency (entries 13−15).
Finally, a transition metal photosensitizer Ru(bpy) Cl was
3
2
8
−10
photocatalysis,
it is particularly appealing to explore its
investigated under otherwise identical reaction conditions;
desired product 3 was produced in a slightly lower yield (71%,
entry 16). However, in terms of the cost and environmental
friendliness, organic dye type eosin Y seems to exhibit a superior
alternative to its transition-metal-derived counterpart Ru-
(bpy) Cl . Together these screenings built up an efficient visible
possible applications in designing and identifying useful
paradigms for alternative vinyl sulfones synthesis. As shown in
Scheme 1c, we anticipated that an oxidative denitrogenation
event might be triggered on sulfonylhydrazide under visible-light-
promoted photoredox catalysis, thereby converting it into
sulfonyl radical species, which would subsequently undergo a
sequence of radical addition, oxidation, and decarboxylation to
afford vinyl sulfones. Gratifyingly, an unusually simple and
versatile decarboxylative sulfonylation of sulfonyl hydrazides with
cinnamic acids was eventually accomplished after careful
optimization of reaction conditions.
3
2
light photoredox decarboxylative sulfonylation of cinnamic acids
with sulfonyl hydrazides relying on the synergy of 1 mol % eosin Y
photocatalyst, 3.00 equiv of Cs CO base, 1.00 equiv of KI
2
3
additive, and DMF/H O (15:1) as the solvent.
2
With the conditions in hand, a series of cinnamic acids bearing
a variety of functional groups were next attempted to react with
2a; the results were summarized in Scheme 2. Generally,
corresponding vinyl sulfone product 3 was smoothly produced in
moderate-to-excellent isolated yields (50−92%). Cinnamic acids
with a para- or ortho-methyl substituent on the aromatic ring
could be unambiguously converted into the products 3b and 3c in
54 and 76% yield, respectively. Some of the frequently
encountered functional groups, e.g., bromo (3d), chloro (3e−
3i), fluoro (3j−3l), trifluoromethyl (3m and 3n), nitro (3o), and
trifluoromethoxy (3p), were demonstrated to be well-tolerated in
With cinnamic acid 1a and benzenesulfonohydrazide 2a as the
model substrates, we started the optimization with a catalytic
11
amount of organic dye type eosin Y (2 mol %) as photocatalyst
and a 45 W household fluorescent compact bulb as the visible
light source. However, only trace amount of desired product was
detectable when the reaction was conducted in dimethylforma-
mide (DMF) in the presence of a substoichiometric amount of
Na CO (2.2 equiv) and KI (1.0 equiv) under a balloon-oxygen
2
3
atmosphere at room temperature (entry 1, Table 1). Fortunately,
12
a switch of the base Na CO to Cs CO showed dramatic
these transformations in terms of isolated yield (50−92%). The
2
3
2
3
improvement of the reactivity (31% isolated yield, entry 2). Next,
electronic characteristics of the cinnamic acid aromatic ring
a different volume ratio of DMF/H O was explored (entries 3−
substituents (R ) were found to have a crucial effect on the
reactivity, where complicated mixtures were observed with the
2
1
5
), from which a 15:1 mixed solvent of DMF and H O explicitly
2
turned out to be optimal (42% yield). Then, a series of bases
CsOAc, Na PO , K CO , NaOH, and 1,8-diazabicyclo[5.4.0]-
substrates carrying R , which is an electron-donating group (i.e.,
R1 = OMe or thienyl).
1
(
3
4
2
3
undec-7-ene (DBU)) were screened (entries 6−10), but all
resulted in either a lower product yield or complete inhibition in
reactivities. We note that a respectable 73% yield of desired
product was recorded by increasing the amount of 2a (2.2 equiv)
and Cs CO (3.0 equiv) (entry 11). Further increased loading of
To further investigate the substrate scope of this protocol, a
range of sulfonylhydrazides 2 were next exposed to the optimal
reaction conditions to react with the 1a. As shown in Scheme 3,
sulfonylhydrazides with both the electron-donating (4a−4g) and
electron-withdrawing (4h−4m) nature of the substituents on the
aromatic ring could smoothly undergo this transformation to
provide the corresponding products in various isolated yields
2
3
2
a and Cs CO was found to be disadvantageous for the product
2 3
yield (entry 12). The effect of KI was also proven fairly critical
B
DOI: 10.1021/acs.orglett.6b01353
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Organic Letters
Letter
Scheme 2. Reactivity Screenings on Cinnamic Acid 1
Scheme 5. Experimental Probes on Reaction Parameters
Scheme 6. Mechanistic Proposal
Scheme 3. Reactivity Screenings on Sulfonylhydrazides 2
Several control experiments on reaction parameters were next
carried out to shed light on the potential reaction pathways; the
results were compiled in Scheme 5. Reactions conducted without
the oxygen, visible light irradiation, eosinY, or Cs CO only led to
2
3
complete inhibition of the reactivity, thereby confirming
explicitly that the reproducible formation of 3a requires all of
these. When 2.2 equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy
(
TEMPO) was purposefully introduced into the reaction system
as a radical scavenger, the reaction was found to be totally
inhibited, suggesting a radical pathway is involved in this reaction.
Styrene 7 or ethyl cinnamate 8 was subsequently subjected to the
standard reaction conditions to couple with 2a; no desired vinly
sulfone product 3a was detected at all. Finally, a switch of 2a to
protected sulfonohydrazide 9 also resulted in failure.
To determine whether the excited photocatalyst was quenched
by iodine anion or 2a in this transformation, a number of
fluorescence quenching (Stern−Volmer) experiments on eosin Y
were therefore conducted (see the Supporting Information). A
notable decrease of fluorescence intensity ofeosin Y was recorded
with the increase in the concentration of 2a, which strongly
indicated that 2a should participate in single-electron transfer
8b
with the photocatalyst under the standard reaction conditions.
To account for the above investigations and the observed
reactivities, a plausible mechanistic network was proposed in
Scheme 6. Upon exposure to visible light stimulation,
sulfonylhydrazide 2a would be intercepted by the excited-state
species eosin Y to produce a highly active radical A, which should
undergo subsequent electron transfer with oxygen, followed by a
sequential N−H abstractions, thereby giving rise to the key
(
34−87%). Substrates possessing a naphthalenyl (4n) or a
thienyl (4o) group were also accommodated in the trans-
formations.
Finally, it is interesting that when 3-phenylpropiolic acid 5 was
employed as the substrate, only β-keto sulfone 6 was obtained in
3
8% isolated yield by using 1,2-dichloroethane (DCE) as the
solvent under otherwise identical reaction conditions (Scheme
14
1
3
sulfonyl radical D with the release of nitrogen. Then, a sequence
4
).
of radical addition, oxidation, and iodine anion capture would
15
take place to afford intermediate F. Finally, intermediate F
would undergo the elimination of carbon dioxide and iodine
anion to yield the final (E)-α,β-unsaturated phenyl sulfone
product, 3a.
Scheme 4. Decarboxylative Oxysulfonylation of 3-Phenyl-
Propiolic Acid with Benzenesulfonohydrazide
Stimulated by the original design concept of exploring a novel
technology for vinyl sulfone formation by means of visible-light
photoredox catalysis, we have disclosed herein that a diverse
C
DOI: 10.1021/acs.orglett.6b01353
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Organic Letters
Letter
series of vinyl sulfones having numerous useful functionalities
could be smoothly produced in good-to-excellent yields with
oxygen as the sole terminal oxidant, and only nitrogen and carbon
dioxide as the byproducts under considerably mild reaction
conditions. We note that the established methodology, by
contrast with the previous approaches, refrains from using
transition metal catalysts and strong oxidants and operates
efficiently at room temperature. Because of the broadly
appreciated importance of vinyl sulfone substances in biological
and pharmaceutical contexts, it is envisioned that this discovery
would find wider applications and motivate further studies in due
course.
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ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01353.
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AUTHOR INFORMATION
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Corresponding Author
caishy05@mnnu.edu.cn
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
2
̈
We thank the National Natural Science Foundation of China
21502086 and 41575118), Natural Science Foundation of
Fujian Province (2015J05028), Outstanding Youth Science
Foundation of Fujian Province (2015J06009), and Program for
Excellent Talents of Minnan Normal University (MJ14005) for
financial support. We also thank Prof. David Zhigang Wang for
helpful discussions.
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