Vinyl Sulfonium Salts as the Radical Acceptor for Metal-Free Decarboxylative Alkenylation

Article abstract of DOI:10.1021/acs.orglett.0c03074

Vinyl sulfonium salts typically act as an electrophilic Michael acceptor, thus initiating many tandem cyclization reactions. Herein, we disclosed the novel reactivity of vinyl sulfonium salts as a radical acceptor. Using redox-active ester as an alkyl rad

Full text of DOI:10.1021/acs.orglett.0c03074

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Vinyl Sulfonium Salts as the Radical Acceptor for Metal-Free
Decarboxylative Alkenylation
Yu-Lan Zhang,∥ Lei Yang,∥ Jie Wu, Chunyin Zhu,* and Peng Wang*
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ABSTRACT: Vinyl sulfonium salts typically act as an electrophilic Michael
acceptor, thus initiating many tandem cyclization reactions. Herein, we disclosed
the novel reactivity of vinyl sulfonium salts as a radical acceptor. Using redox-
active ester as an alkyl radical precursor, metal-free decarboxylative alkenylation
with vinyl sulfonium salts was realized using eosin Y as a photocatalyst. This
process features a broad substrate scope and tolerates with a broad range of
primary, secondary, tertiary carboxylic acids.
nstallation of the alkene motif in complex molecules and
synthetic mediates is fundamentally crucial not only because
position, and then undergo an ylide-initiated tandem
reaction7b,f to afford mono-cyclic or bicyclic products9
(Scheme 1A). As part of our continuing interest in using
I
of its wide presence in natural products and bioactive
molecules1 but also due to its broad synthetic applications as
a versatile functional group.2 As such, significant progress has
been made over the past decades thanks to the rapidly
developed transition-metal catalysis3 and radical chemistry4
with various alkenyl coupling partners, including vinyl metal
reagents or vinyl electrophilic partners (halogens, triflates,
etc.). In addition to traditional alkenyl electrophiles, use of
alkenyl sulfonium salts as an alkenylating reagent was largely
underexploited, despite the fact that they are readily accessible
by treating the styrene derivative with sulfoxide in the presence
of anhydride and bench stable.5 To date, only three vinyl
sulfonium salts participating in alkenylation reactions have
appeared in the literatures by Liebeskind,6a Shen and Lv,6b and
Procter,6c respectively, which underwent transition metal (Pd
or Ni) catalyzed cross coupling of electrophilic vinyl sulfonium
salts with Ar-Met (Met = Zn, B, etc.) species. Moreover, alkyl-
tethered styrene could not be obtained using the aforemen-
tioned strategy except for trans-β-methylstyrene, probably due
to the ready β-hydrogen elimination with alkyl transition-metal
intermediates. Herein, we disclosed a metal-free photo-
catalyzed protocol to access various alkyl tethered styrenes
using vinyl sulfonium salts as the alkenylating reagent for the
first time. Mechanistic studies indicate that vinyl sulfonium
salts could serve as a radical acceptor to accept the radical
addition at the α-position, which was not known until now.
Recently, sulfonium salts have attracted increasing attention
not only in light of their accessibility from simple arenes and
alkenes but also due to their versatility for many carbon−
carbon bond and carbon−heteroatom bond formation
reactions.7,8 However, the reactivities of vinyl sulfonium salts
are relatively less explored.9 In addition to their use as
electrophilic coupling partners in the transition-metal-catalyzed
coupling reaction, vinyl sulfonium salts typically act as a
Michael acceptor to react with some nucleophiles at the β-
Scheme 1. Reactivities of Vinyl Sulfonium Salts
sulfonium salts as a versatile reagent in organic synthesis,10 we
envisioned that the vinyl sulfonium salts might serve as a
radical precursor or a radical acceptor with an alkyl radical,
thus paving a new avenue to alkyl styrene derivatives, which are
prevalent in a wide variety of pharmaceuticals. Employing
redox-active N-(acyloxy)phthalimide as the alkyl radical
precursor,11 decarboxylative alkenylation12 with vinyl sulfo-
nium salts was realized in the absence of transition-metal
promoters with a broad substrate scope. Alternatively, alkyl-
tethered styrenes can be accessed via styrenyl halides,11e
Received: September 14, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03074
© XXXX American Chemical Society
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cinnamic acid derivatives,12d or alkenyl zinc reagents11f using a
decarboxylative approach with a redox-active ester.
a,b
Scheme 3. Decarboxylative Alkenylation
Choosing redox-active N-(acyloxy)phthalimide 1q as the
radical precursor, we were pleased to find that 44% yield of
alkenylated product was obtained using Na2−eosin Y as the
photoredox catalyst and N,N-diisopropylethylamine (DIPEA)
as the base in dioxane under the photoirradiation of 24 w blue
LED (see the Supporting Information). After systematically
optimizing the reaction conditions, the yield was further
improved to 81% in the presence of eosin Y (4.3 mol %),
DIPEA (10 equiv), and (n-Bu)4NPF6 (2.2 equiv) in a mixed
solvent of Et2O and methyl tert-butyl ether. Control experi-
ments demonstrated that all reaction parameters are crucial for
this transformation (Scheme 2). The reaction did not proceed
Scheme 2. Evaluation of Reaction Parameters for
a,b
Decarboxylative Alkenylation.
a
Reaction conditions: 1q (0.3 mmol), vinyl sulfonium salt 2 (0.6
mmol), photocatalyst (4.3 mol %), DIPEA (10 equiv), Et2O/TBME
b
(1.4 mL/0.3 mL), (n-Bu)4NPF6 (2.2 equiv), blue LED, N2, 9 h. The
yield was determined by crude 1H NMR using CH2Br2 as the internal
a
Reaction conditions: 1 (0.3 mmol), vinyl sulfonium salt 2a (0.6
mmol), eosin Y (4.3 mol %), DIPEA (10 equiv), Et2O/TBME (1.4
mL/0.3 mL), (n-Bu)4NPF6 (2.2 equiv), blue LED, rt, N2, 9 h.
c
standard. Isolated yield.
b
c
Isolated yield. The reaction was conducted on 2.0 mmol.
in the absence of blue light, a photocatalyst of eosin Y, or
DIPEA (entries 2−4). Although a similar yield was obtained
without the addition of (n-Bu)4NPF6, this additive helped to
improve the reproducibility of this reaction, probably due to its
capability of enhancing the solubility of the sulfonium salts in
ether (entry 5). The use of ether or TBME alone resulted in
inferior results (entries 6 and 7). To our surprise, this radical
alkenylation could partially tolerate the moisture and O2,
although a lower yield was obtained under air atmosphere
(entry 8). Other organic and metal photocatalysts, solvents,
and bases all led to insufficient reaction outcomes (entries 9−
12, 15−20). Notably, the high loading of DIPEA are also
crucial for the high yield of this reaction. We have also
investigated the efficiency of sulfonium salt variants (2a−d), in
which tetrahydrothiophene-derived sulfonium salt 2a gave the
best outcome.
synthetic useful yields. The tolerance of alkyl chloride (3h),
alkyl bromide (3i), and unmasked alkyne (3j) was noteworthy
because those functionalities were normally sensitive under
photoredox conditions. In addition, this procedure is also
compatible with complex natural products and pharmaceuticals
(3x−ac), such as myristic acid, oleic acid, fenbufen,
dehydrocholic acid, isoxepac, and indomethacin, which further
demonstrated it is highly valuable for the late-stage
functionalization of bioactive molecules. The scalability of
this process was demonstrated using 1q as model substrate,
giving the desired alkenylated product 3q in 60% yield.
Next, the scope of the vinyl sulfonium salts was examined
using N-(acyloxy)phthalimide 1a as the alkyl radical source
(Scheme 4). Substituents at the ortho-, meta-, and para-
positions of the arene are all tolerant (5a−c and 5d−f), and
meta-substituted styrene-derived sulfonium salts normally
provided desired 1,2-trans-alkene in higher yields. Fluorinated
vinyl sulfonium salts are also suitable alkenyl sources for this
reaction (5g−h), while tert-butylated vinyl sulfonium salt gave
With the optimal conditions in hand, the generality of the
decarboxylative alkenylation reaction was evaluated. As
summarized in Scheme 3, primary (3a−j), secondary (3k−
t), and tertiary (3u−w) carboxylic acids are all compatible with
this procedure to afford the desired alkenylation products in
B
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a,b
Scheme 4. Scope of Vinyl Sulfonium Salts
Scheme 5. Mechanistic Study
to afford the targeted alkyl styrene 3q along with the
generation of thioether.
In conclusion, a novel reaction pathway of vinyl sulfonium
salts as a radical acceptor was disclosed for the first time.
Employing a redox-active ester as a radical precursor, various
alkyl tethered styrenes were obtained in synthetically useful
yields. This reaction features a broad substrate scope and
tolerates a broad range of primary, secondary, and tertiary
carboxylic acids.
a
Reaction conditions: 1a (0.3 mmol), vinyl sulfonium salt 4 (0.6
mmol), eosin Y (4.3 mol %), DIPEA (10 equiv), Et2O/TBME (1.4
mL/0.3 mL), (n-Bu)4NPF6 (2.2 equiv), blue LED, rt, N2, 9 h.
b
Isolated yield.
the desired alkenylated product 5i in 38% yield. Not
surprisingly, disubstituted arylated vinyl sulfonium salts are
compatible with the optimal conditions, providing the targeted
products (5k−n) in synthetic useful yields. Notably,
trisubstituted alkene could also be obtained using our protocol.
1,1-Diphenylethylene-derived sulfonium salt gave the trisub-
stituted alkene (5o) in 48% yield.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03074.
■
sı
Experimental procedures, complete characterization
1
data, and copies of H and 13C NMR spectra (PDF)
To explicate the mechanism of the reaction, control
experiments were conducted. No desired products were
obtained in the addition of TEMPO as a radical scavenger,
indicating a radical process involved in this decarboxylative
alkenylation reaction. Although the aryl sulfonium salts have
been proven as an efficient radical precursor under photo-
irradiation,10a,13 the tetrahydrothiophene derived salt 2a is
inert under our standard conditions, which ruled out the
formation of relatively unstable vinyl radical species. Mean-
while, treatment of vinyl sulfonium salt 2a with TEMPO under
standard conditions did not afford any alkene−TEMPO
adduct (see the Supporting Information), which further
confirmed the absence of a vinyl radical intermediate in this
transformation. Under similar conditions in dioxane, N-
(acyloxy)phthalimide 1q was converted to corresponding
protonated product via radical decarboxylation/protonation
AUTHOR INFORMATION
Corresponding Authors
■
Chunyin Zhu − School of Chemistry and Chemical Engineering,
Jiangsu University, Zhenjiang 212013, PR China;
Email: zhucycn@gmail.com
Peng Wang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis and
CAS Key Laboratory of Energy Regulation Materials, Shanghai
Institute of Organic Chemistry, Shanghai 200032, China;
orcid.org/0000-0002-6442-3008; Email: pengwang@
sioc.ac.cn
Authors
Yu-Lan Zhang − School of Chemistry and Chemical Engineering,
Jiangsu University, Zhenjiang 212013, PR China; State Key
Laboratory of Organometallic Chemistry, Center for Excellence
in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Shanghai 200032, China
Lei Yang − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute
of Organic Chemistry, Shanghai 200032, China
sequence.14 On the basis of our results, and the eosin Y
14
̈
mediated decarboxylative alkylation by Konig, a proposed
mechanism is presented in Scheme 5c. An alkyl radical Int 1
could be formed via a catalytic reductive quenching cycle eosin
Y,15 followed by radical addition to vinyl sulfonium salts at the
α-position to provide intermediate Int 2, probably due to the
formation of more stable benzylic radical species. Subse-
quently, it is assumed that the radical transfer from DIPEA
could provide the anion Int 3,16 which undergoes the β-session
Jie Wu − School of Chemistry and Chemical Engineering, Jiangsu
University, Zhenjiang 212013, PR China; State Key Laboratory
C
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of Organometallic Chemistry, Center for Excellence in Molecular
Synthesis, Shanghai Institute of Organic Chemistry, Shanghai
200032, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03074
Author Contributions
∥Y.-L.Z. and L.Y. contributed equally.
Notes
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(b) Lin, H.; Dong, X.; Li, Y.; Shen, Q.; Lv, L. Highly Selective
Activation of Vinyl C−S Bonds Over Aryl C−S Bonds in the Pd-
C a t a l y z e d C o u p l i n g o f ( E )- ( β - T r i f l u o r o m e t h y l ) -
vinyldiphenylsulfonium Salts: Preparation of Trifluoromethylated
Alkenes and Dienes. Eur. J. Org. Chem. 2012, 2012, 4675−4679.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Shanghai Institute of Organic
Chemistry, State Key Laboratory of Organometallic Chemistry,
Shanghai Rising-Star Program (20QA1411400), and National
Natural Science Foundation of China (21821002) for financial
support.
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